WEBVTT 00:00:00.000 --> 00:00:02.420 align:middle line:84% The following content is provided under a Creative 00:00:02.420 --> 00:00:03.600 align:middle line:90% Commons license. 00:00:03.600 --> 00:00:06.050 align:middle line:84% Your support will help MIT OpenCourseWare 00:00:06.050 --> 00:00:09.650 align:middle line:84% continue to offer high-quality educational resources for free. 00:00:09.650 --> 00:00:12.620 align:middle line:84% To make a donation or to view additional materials 00:00:12.620 --> 00:00:16.520 align:middle line:84% from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:16.520 --> 00:00:17.750 align:middle line:90% at ocw.mit.edu. 00:00:17.750 --> 00:00:21.400 align:middle line:90% 00:00:21.400 --> 00:00:24.580 align:middle line:84% GANG CHEN: Today, there are three more lectures. 00:00:24.580 --> 00:00:33.010 align:middle line:84% And we get two guest lectures on one 00:00:33.010 --> 00:00:36.860 align:middle line:84% of the reliability of the materials. 00:00:36.860 --> 00:00:40.635 align:middle line:84% I prepared this original and the last lecture. 00:00:40.635 --> 00:00:45.170 align:middle line:84% I also have to juggle a lot the schedule for everybody. 00:00:45.170 --> 00:00:50.360 align:middle line:84% So instead of what I will do is next Friday, 00:00:50.360 --> 00:00:55.690 align:middle line:84% I will be talking about the reliability of the materials. 00:00:55.690 --> 00:00:59.770 align:middle line:84% This is Professor Christopher Schuh in the Materials 00:00:59.770 --> 00:01:01.720 align:middle line:90% Department with-- 00:01:01.720 --> 00:01:05.662 align:middle line:84% has been doing research in this area for long time. 00:01:05.662 --> 00:01:08.730 align:middle line:90% [INAUDIBLE] I forget it. 00:01:08.730 --> 00:01:16.890 align:middle line:84% And then the week after last week, 00:01:16.890 --> 00:01:23.145 align:middle line:84% we will have Dr. Ivan Simonovich talk about selective surface. 00:01:23.145 --> 00:01:27.000 align:middle line:84% He has been working on several photovoltaics. 00:01:27.000 --> 00:01:33.960 align:middle line:84% And so he spent a lot of work in the spectral control area. 00:01:33.960 --> 00:01:37.320 align:middle line:84% So help you, again, better talk to me. 00:01:37.320 --> 00:01:40.860 align:middle line:84% And then see the final lecture, we will be-- 00:01:40.860 --> 00:01:43.385 align:middle line:90% 00:01:43.385 --> 00:01:45.210 align:middle line:84% the content, come back a little bit 00:01:45.210 --> 00:01:49.050 align:middle line:84% to talk about how we combine selective surface 00:01:49.050 --> 00:01:55.470 align:middle line:84% with thermoelectrics and with also photovoltaics. 00:01:55.470 --> 00:01:57.310 align:middle line:84% So we have learned thermoelectrics. 00:01:57.310 --> 00:02:01.410 align:middle line:84% We have learned photovoltaics also. 00:02:01.410 --> 00:02:07.030 align:middle line:84% So that's a solid state solar thermal energy conversion. 00:02:07.030 --> 00:02:14.470 align:middle line:84% And today, what I want to cover is the mechanical energy 00:02:14.470 --> 00:02:18.970 align:middle line:84% conversion, solar to electrical, solar, thermal 00:02:18.970 --> 00:02:21.730 align:middle line:84% but there will be mechanical means. 00:02:21.730 --> 00:02:25.720 align:middle line:84% So let's recall what I was talking 00:02:25.720 --> 00:02:28.160 align:middle line:90% in the last previous lecture. 00:02:28.160 --> 00:02:30.370 align:middle line:84% We talked about blackbody radiation. 00:02:30.370 --> 00:02:35.980 align:middle line:84% We mentioned a little bit on the Earth's motion and how we 00:02:35.980 --> 00:02:44.230 align:middle line:84% usually test solar spectrum AM0 or AM1, AM1.5, et cetera. 00:02:44.230 --> 00:02:48.640 align:middle line:84% And then we define relative property, this definition, 00:02:48.640 --> 00:02:50.120 align:middle line:90% many different definitions. 00:02:50.120 --> 00:02:54.820 align:middle line:84% So as I said, I don't expect you remember all those details. 00:02:54.820 --> 00:02:58.840 align:middle line:84% The point is for spectral control, 00:02:58.840 --> 00:03:01.450 align:middle line:84% it's very important to understand 00:03:01.450 --> 00:03:05.730 align:middle line:84% the wavelength dependence of the properties. 00:03:05.730 --> 00:03:11.070 align:middle line:84% And then with that, we discuss the maximum efficiency 00:03:11.070 --> 00:03:16.140 align:middle line:84% of solar thermal engines and maximum achievable temperature. 00:03:16.140 --> 00:03:20.640 align:middle line:84% And again, see we emphasize the silicon surface. 00:03:20.640 --> 00:03:24.690 align:middle line:84% But we do know or you do not how to do it yet, 00:03:24.690 --> 00:03:27.770 align:middle line:90% but it's important. 00:03:27.770 --> 00:03:34.700 align:middle line:84% So in today's lecture, like I said, I want to cover, 00:03:34.700 --> 00:03:38.090 align:middle line:84% really, the solar thermal mechanical part 00:03:38.090 --> 00:03:42.380 align:middle line:84% and what we'll start with is the hot water system, 00:03:42.380 --> 00:03:44.390 align:middle line:90% solar hot water. 00:03:44.390 --> 00:03:47.480 align:middle line:90% And then we'll talk about-- 00:03:47.480 --> 00:03:49.790 align:middle line:84% to bring it up to a higher temperature, 00:03:49.790 --> 00:03:53.720 align:middle line:84% we'll talk about the solar concentration. 00:03:53.720 --> 00:03:56.990 align:middle line:84% And we'll talk about the maximum, 00:03:56.990 --> 00:04:01.640 align:middle line:84% how much you can concentrate the solar radiation. 00:04:01.640 --> 00:04:05.060 align:middle line:84% And then we look at how you could 00:04:05.060 --> 00:04:10.220 align:middle line:84% in practical to do different ways of concentration. 00:04:10.220 --> 00:04:13.370 align:middle line:84% And to do concentration, you need to track the Sun. 00:04:13.370 --> 00:04:16.220 align:middle line:84% That's one of the problem challenge. 00:04:16.220 --> 00:04:19.430 align:middle line:84% So I'll talk a little bit tracking and long tracking 00:04:19.430 --> 00:04:21.860 align:middle line:90% and then go to mechanical. 00:04:21.860 --> 00:04:25.580 align:middle line:84% If I have enough time, I'll start a little bit, 00:04:25.580 --> 00:04:31.740 align:middle line:84% give you a preview of what controls the surface properties, 00:04:31.740 --> 00:04:33.010 align:middle line:90% the action of microwave. 00:04:33.010 --> 00:04:36.540 align:middle line:90% 00:04:36.540 --> 00:04:39.202 align:middle line:90% So solar hot water, right? 00:04:39.202 --> 00:04:43.920 align:middle line:84% It's very mundane, but I really appreciate this. 00:04:43.920 --> 00:04:47.310 align:middle line:84% As I probably mentioned before, when I was a kid, 00:04:47.310 --> 00:04:48.760 align:middle line:90% I grew up in China. 00:04:48.760 --> 00:04:51.630 align:middle line:84% I didn't have a chance to take a hot shower. 00:04:51.630 --> 00:04:54.180 align:middle line:84% And now when I go back to my parents, 00:04:54.180 --> 00:04:59.370 align:middle line:84% they still live in China and they use solar hot water. 00:04:59.370 --> 00:05:01.860 align:middle line:84% Concept, of course, is very simple-- 00:05:01.860 --> 00:05:03.300 align:middle line:90% solar collector. 00:05:03.300 --> 00:05:08.550 align:middle line:84% They absorb the solar radiation, and you heat up the water. 00:05:08.550 --> 00:05:11.160 align:middle line:90% Very simple concept, right? 00:05:11.160 --> 00:05:16.380 align:middle line:84% And also, I want to give you a very simple backup [INAUDIBLE]. 00:05:16.380 --> 00:05:18.000 align:middle line:84% So if you're a mechanical engineer, 00:05:18.000 --> 00:05:19.510 align:middle line:90% this will be pretty easy. 00:05:19.510 --> 00:05:24.120 align:middle line:84% But I did this for the preparation of the class, 00:05:24.120 --> 00:05:26.850 align:middle line:90% also out of my own curiosity. 00:05:26.850 --> 00:05:28.355 align:middle line:84% Of course, you are from Materials, 00:05:28.355 --> 00:05:30.360 align:middle line:84% so this is probably a little bit more, 00:05:30.360 --> 00:05:32.910 align:middle line:90% say, not very familiar with you. 00:05:32.910 --> 00:05:35.490 align:middle line:84% But it's a simple energy balance. 00:05:35.490 --> 00:05:39.600 align:middle line:84% So what I want to ask is if I want 00:05:39.600 --> 00:05:45.970 align:middle line:84% to put in my own house, what's the area I live? 00:05:45.970 --> 00:05:46.470 align:middle line:90% Right? 00:05:46.470 --> 00:05:47.850 align:middle line:90% Solar hot water. 00:05:47.850 --> 00:05:53.820 align:middle line:84% And typical household water tank is about 80 gallon. 00:05:53.820 --> 00:05:55.750 align:middle line:90% It's a larger house. 00:05:55.750 --> 00:05:59.790 align:middle line:84% If you have smaller, so ranging from 40- to 100-gallon range, 00:05:59.790 --> 00:06:00.750 align:middle line:90% I guess. 00:06:00.750 --> 00:06:04.680 align:middle line:84% And if I take a start temperature of water 00:06:04.680 --> 00:06:08.880 align:middle line:84% at 15 degrees, and I want to keep 00:06:08.880 --> 00:06:12.240 align:middle line:84% the water tank at 60 degrees Celsius-- 00:06:12.240 --> 00:06:13.650 align:middle line:90% so during the day, right? 00:06:13.650 --> 00:06:17.280 align:middle line:84% Let's suppose I deplete all the hot water during the night, 00:06:17.280 --> 00:06:20.790 align:middle line:90% and I want to store 80-- 00:06:20.790 --> 00:06:24.870 align:middle line:84% during the day, I want to heat up the water from 15 degrees 00:06:24.870 --> 00:06:27.450 align:middle line:90% to 60 degrees. 00:06:27.450 --> 00:06:34.230 align:middle line:84% And so you want to see how much was this solar surface area, 00:06:34.230 --> 00:06:36.210 align:middle line:90% hot water surface area you want. 00:06:36.210 --> 00:06:37.860 align:middle line:84% And it's simple energy balance, right? 00:06:37.860 --> 00:06:40.860 align:middle line:84% During the day, how much solar radiation is it. 00:06:40.860 --> 00:06:43.050 align:middle line:84% And, of course, you consider the efficiency 00:06:43.050 --> 00:06:46.440 align:middle line:84% of the absorber and then that balance 00:06:46.440 --> 00:06:49.980 align:middle line:84% to heat up this much water, how much energy you need. 00:06:49.980 --> 00:06:55.410 align:middle line:84% So you do that energy balance, the area, what I want, 00:06:55.410 --> 00:07:02.550 align:middle line:84% the solar radiation insulation, and time of the day efficiency. 00:07:02.550 --> 00:07:08.460 align:middle line:84% And of course, the heat capacity that's stored in the hot water. 00:07:08.460 --> 00:07:10.800 align:middle line:90% So I put in those numbers. 00:07:10.800 --> 00:07:13.200 align:middle line:84% Typically, when people calculate, 00:07:13.200 --> 00:07:15.510 align:middle line:84% you can say it's a little bit optimistic. 00:07:15.510 --> 00:07:20.460 align:middle line:84% If you live in Boston, you don't get the 1,000 watts, OK? 00:07:20.460 --> 00:07:23.870 align:middle line:84% And I took a standard calculation, 00:07:23.870 --> 00:07:26.280 align:middle line:90% 1,000 watts 5.5 hours. 00:07:26.280 --> 00:07:27.610 align:middle line:90% So that's an average. 00:07:27.610 --> 00:07:30.660 align:middle line:90% That's the whole day average. 00:07:30.660 --> 00:07:32.690 align:middle line:84% Typically, that's what people take 00:07:32.690 --> 00:07:34.830 align:middle line:90% when they test the solar cell. 00:07:34.830 --> 00:07:38.780 align:middle line:84% 1,000 watts, 5.1, we calculate solar cell output. 00:07:38.780 --> 00:07:45.330 align:middle line:84% And efficiency of solar thermal hot water system ranging from-- 00:07:45.330 --> 00:07:48.950 align:middle line:84% I'll tell you how to estimate efficiency, 00:07:48.950 --> 00:07:53.420 align:middle line:84% but I take a typical number, 60%, OK? 00:07:53.420 --> 00:07:58.130 align:middle line:90% So I need a 5 meters here. 00:07:58.130 --> 00:08:01.950 align:middle line:84% And in China, I know it's very cheap. 00:08:01.950 --> 00:08:05.720 align:middle line:84% And typically, people install somewhere 1.5, 2 meters. 00:08:05.720 --> 00:08:07.850 align:middle line:90% So it's not a very big tank. 00:08:07.850 --> 00:08:10.975 align:middle line:84% Most people live in apartment, and they use less water. 00:08:10.975 --> 00:08:14.610 align:middle line:90% 00:08:14.610 --> 00:08:16.795 align:middle line:90% OK. 00:08:16.795 --> 00:08:21.600 align:middle line:84% So I have ordered a different format. 00:08:21.600 --> 00:08:26.255 align:middle line:84% One of the format is a flat panel, right? 00:08:26.255 --> 00:08:31.320 align:middle line:84% So here, what we have is the absorber. 00:08:31.320 --> 00:08:33.990 align:middle line:84% Typically, it's a frequency selective. 00:08:33.990 --> 00:08:37.440 align:middle line:84% Meaning, they absorb solar radiation but do not 00:08:37.440 --> 00:08:42.059 align:middle line:84% or you want to re-emit a very small amount of thermal emission 00:08:42.059 --> 00:08:43.289 align:middle line:90% when it heats up. 00:08:43.289 --> 00:08:47.220 align:middle line:84% And so you can see the back side is the insulation. 00:08:47.220 --> 00:08:50.430 align:middle line:84% And this is the absorber tube, could be up on top 00:08:50.430 --> 00:08:51.780 align:middle line:90% or on the bottom. 00:08:51.780 --> 00:08:54.150 align:middle line:90% And typically, they do not-- 00:08:54.150 --> 00:08:57.120 align:middle line:90% so if we change it here. 00:08:57.120 --> 00:08:58.560 align:middle line:90% OK? 00:08:58.560 --> 00:09:03.680 align:middle line:84% And the tank could be on the top of the absorber itself. 00:09:03.680 --> 00:09:09.030 align:middle line:84% So in that case, you don't put a tank in the basement. 00:09:09.030 --> 00:09:13.710 align:middle line:84% And you can use the gravity of the water 00:09:13.710 --> 00:09:17.170 align:middle line:84% to drive the water flow, to drive the hot water down. 00:09:17.170 --> 00:09:20.610 align:middle line:84% Let's say the problem is your water flow rate depends 00:09:20.610 --> 00:09:22.770 align:middle line:90% on how high you put the tank. 00:09:22.770 --> 00:09:29.880 align:middle line:84% So if you live in low buildings, your pressure is low. 00:09:29.880 --> 00:09:32.130 align:middle line:90% And this is an example. 00:09:32.130 --> 00:09:35.530 align:middle line:84% Clearly, the water tank is not on the roof. 00:09:35.530 --> 00:09:36.750 align:middle line:90% It's in the basement. 00:09:36.750 --> 00:09:40.120 align:middle line:84% And in that case, you use a pump to pump the water. 00:09:40.120 --> 00:09:41.980 align:middle line:90% So it's a pressurized system. 00:09:41.980 --> 00:09:43.170 align:middle line:90% So they are pressurized. 00:09:43.170 --> 00:09:48.315 align:middle line:84% And this one is not a fossil system, just natural. 00:09:48.315 --> 00:09:50.860 align:middle line:90% 00:09:50.860 --> 00:09:53.500 align:middle line:90% And the one that-- 00:09:53.500 --> 00:09:55.900 align:middle line:84% what I thought was really interesting 00:09:55.900 --> 00:09:59.140 align:middle line:90% is this vacuum system. 00:09:59.140 --> 00:10:04.150 align:middle line:84% And this is predominantly used in China. 00:10:04.150 --> 00:10:09.280 align:middle line:84% And it's glass-- so it's a double-layer glass. 00:10:09.280 --> 00:10:12.940 align:middle line:90% Inside is a black coat. 00:10:12.940 --> 00:10:15.370 align:middle line:84% What looks black is actually inside. 00:10:15.370 --> 00:10:18.110 align:middle line:84% Well, outside is another glass, clear glass. 00:10:18.110 --> 00:10:19.480 align:middle line:90% So it's a circular. 00:10:19.480 --> 00:10:23.470 align:middle line:84% And the inside glass is first coated with selective surface. 00:10:23.470 --> 00:10:26.410 align:middle line:90% And then you evacuate it. 00:10:26.410 --> 00:10:27.980 align:middle line:90% You see on the other side. 00:10:27.980 --> 00:10:30.520 align:middle line:90% So it's a double jacket. 00:10:30.520 --> 00:10:37.000 align:middle line:84% And on this side, because of the vacuum so it has a getter. 00:10:37.000 --> 00:10:39.430 align:middle line:84% The getter, what does getter do is, 00:10:39.430 --> 00:10:44.680 align:middle line:84% say, the evaporation is actually collected by this metal reacted 00:10:44.680 --> 00:10:47.320 align:middle line:90% with whatever evaporates. 00:10:47.320 --> 00:10:53.860 align:middle line:84% And so you imagine so putting vacuum under the Sun, 00:10:53.860 --> 00:10:55.450 align:middle line:90% how long it lasts. 00:10:55.450 --> 00:10:58.840 align:middle line:84% And I actually visited some of this as a-- 00:10:58.840 --> 00:11:01.630 align:middle line:84% they really is the father [INAUDIBLE] 00:11:01.630 --> 00:11:03.610 align:middle line:84% the father of this technology in China. 00:11:03.610 --> 00:11:07.750 align:middle line:84% And he said that he put this stuff early 00:11:07.750 --> 00:11:09.650 align:middle line:90% in the '90s in Tsinghua. 00:11:09.650 --> 00:11:13.400 align:middle line:84% He's from Tsinghua University and still working. 00:11:13.400 --> 00:11:15.220 align:middle line:90% So after 20 years. 00:11:15.220 --> 00:11:21.100 align:middle line:84% And that's conceivable because if you think about in my house, 00:11:21.100 --> 00:11:22.840 align:middle line:90% I bought a TV. 00:11:22.840 --> 00:11:25.930 align:middle line:84% I remember my first TV is still working, 00:11:25.930 --> 00:11:27.760 align:middle line:90% about more than 20 years. 00:11:27.760 --> 00:11:29.420 align:middle line:90% And that's a CRT. 00:11:29.420 --> 00:11:31.480 align:middle line:90% It's a vacuum technology, right? 00:11:31.480 --> 00:11:37.480 align:middle line:84% Say the screen of the TV, so the cathode ray tube inside 00:11:37.480 --> 00:11:40.120 align:middle line:90% is actually ultrahigh vacuum. 00:11:40.120 --> 00:11:42.440 align:middle line:90% So the vacuum is pretty high. 00:11:42.440 --> 00:11:45.600 align:middle line:84% And you can do a calculation of 5 to 1. 00:11:45.600 --> 00:11:50.450 align:middle line:84% If the vacuum is too low, the air conduction is too much heat. 00:11:50.450 --> 00:11:54.040 align:middle line:84% And so this is an evacuated tube. 00:11:54.040 --> 00:11:55.220 align:middle line:90% That's glass. 00:11:55.220 --> 00:11:57.130 align:middle line:90% This glass is really cheap. 00:11:57.130 --> 00:12:04.450 align:middle line:84% I visited the manufacturing site and the whole tube 00:12:04.450 --> 00:12:06.700 align:middle line:90% is about this high. 00:12:06.700 --> 00:12:09.220 align:middle line:90% The diameter is 50 millimeters. 00:12:09.220 --> 00:12:14.110 align:middle line:84% And the manufacture of the double glass tube is about $1.5 00:12:14.110 --> 00:12:15.340 align:middle line:90% per tube. 00:12:15.340 --> 00:12:18.130 align:middle line:90% So it's really cheap. 00:12:18.130 --> 00:12:20.980 align:middle line:84% This one is a little bit more expensive. 00:12:20.980 --> 00:12:23.140 align:middle line:90% It's not a glass glass. 00:12:23.140 --> 00:12:27.490 align:middle line:84% And what it has is a metal absorber. 00:12:27.490 --> 00:12:32.530 align:middle line:84% So it could be a copper or aluminum. 00:12:32.530 --> 00:12:34.810 align:middle line:90% Again, this is a cost, right? 00:12:34.810 --> 00:12:36.460 align:middle line:90% This is really the-- 00:12:36.460 --> 00:12:38.830 align:middle line:90% see, the cost is crucial here. 00:12:38.830 --> 00:12:43.780 align:middle line:84% And this is selective absorptive coatings again. 00:12:43.780 --> 00:12:48.220 align:middle line:84% And so what it does this is a heat pipe. 00:12:48.220 --> 00:12:52.170 align:middle line:84% So the heat absorbed conducts to the heat pipe. 00:12:52.170 --> 00:12:55.620 align:middle line:84% So the heat pipe's diameter is not this big. 00:12:55.620 --> 00:12:57.990 align:middle line:90% That's too much copper, right? 00:12:57.990 --> 00:12:59.840 align:middle line:90% It's a small diameter heat pipe. 00:12:59.840 --> 00:13:06.620 align:middle line:84% And so the heat is evaporated, the liquid inside the heat pipe. 00:13:06.620 --> 00:13:10.040 align:middle line:84% And on this side, you have forced water, heat exchanger, 00:13:10.040 --> 00:13:11.860 align:middle line:90% forced water take the heat away. 00:13:11.860 --> 00:13:15.540 align:middle line:90% 00:13:15.540 --> 00:13:21.570 align:middle line:84% So this is one of the cheap forms unpressurized. 00:13:21.570 --> 00:13:25.210 align:middle line:84% So what it has is the double glass layer. 00:13:25.210 --> 00:13:29.520 align:middle line:84% You fill the inside of the glass with water. 00:13:29.520 --> 00:13:34.070 align:middle line:84% I go back here inside, this is empty. 00:13:34.070 --> 00:13:35.610 align:middle line:90% So you fill with water. 00:13:35.610 --> 00:13:39.210 align:middle line:84% And when it heats up, the natural convection, 00:13:39.210 --> 00:13:44.070 align:middle line:84% so the hot water goes up here, and the cold water flows down. 00:13:44.070 --> 00:13:46.540 align:middle line:90% So it's a passive system-- 00:13:46.540 --> 00:13:48.450 align:middle line:90% no pump at all. 00:13:48.450 --> 00:13:50.400 align:middle line:90% And you put on the roof. 00:13:50.400 --> 00:13:53.850 align:middle line:90% And then you can take a shower. 00:13:53.850 --> 00:13:57.480 align:middle line:84% So a typical family in China, probably, 00:13:57.480 --> 00:14:00.660 align:middle line:90% is about 15 to 20 cubes. 00:14:00.660 --> 00:14:04.230 align:middle line:84% And this one is a forced pressure, right? 00:14:04.230 --> 00:14:07.620 align:middle line:84% You put a separate tank, you put the pump through 00:14:07.620 --> 00:14:12.090 align:middle line:90% and forced cooling at the end. 00:14:12.090 --> 00:14:17.680 align:middle line:84% OK, so that's what the system looks like. 00:14:17.680 --> 00:14:20.520 align:middle line:84% Again, out of my own curiosity, I 00:14:20.520 --> 00:14:22.830 align:middle line:84% say, OK, I'm going to estimate what's 00:14:22.830 --> 00:14:28.140 align:middle line:84% the efficiency of the system, flat panel and vacuum systems. 00:14:28.140 --> 00:14:33.030 align:middle line:84% So this is a model for a vacuum system, 00:14:33.030 --> 00:14:37.340 align:middle line:84% where I have a cylinder inside the cylinder, inner cylinder. 00:14:37.340 --> 00:14:40.440 align:middle line:90% So this cylinder, there is one-- 00:14:40.440 --> 00:14:43.230 align:middle line:84% so actually, this is a self-tracking, right? 00:14:43.230 --> 00:14:46.290 align:middle line:84% No matter what Sun direction is, there 00:14:46.290 --> 00:14:50.880 align:middle line:84% is always side wall is facing the perpendicular to the Sun. 00:14:50.880 --> 00:14:54.530 align:middle line:84% So does have some advantage here. 00:14:54.530 --> 00:14:59.400 align:middle line:84% In fact, there is a startup company called Solyndra. 00:14:59.400 --> 00:15:03.400 align:middle line:84% And they are putting solar cells like this. 00:15:03.400 --> 00:15:07.890 align:middle line:84% So circle along the circle of the cylinder. 00:15:07.890 --> 00:15:12.790 align:middle line:90% And it's a vacuum in between. 00:15:12.790 --> 00:15:19.590 align:middle line:84% So incoming solar radiation is just the cross section area. 00:15:19.590 --> 00:15:22.440 align:middle line:84% That's like capital-- the outer diameter. 00:15:22.440 --> 00:15:28.740 align:middle line:84% And say the insulation, solar insulation. 00:15:28.740 --> 00:15:32.550 align:middle line:84% So absorbed, how much is absorbed? 00:15:32.550 --> 00:15:34.260 align:middle line:84% In terms of absorption, the first thing 00:15:34.260 --> 00:15:37.120 align:middle line:84% you have to go through is the outer glass layer. 00:15:37.120 --> 00:15:40.290 align:middle line:84% So that's a transmissivity [INAUDIBLE]. 00:15:40.290 --> 00:15:45.750 align:middle line:84% And then inside the surface is absorbed. 00:15:45.750 --> 00:15:48.510 align:middle line:90% Alpha is the absorptivity. 00:15:48.510 --> 00:15:53.400 align:middle line:84% So that's the absorptivity to solar radiation. 00:15:53.400 --> 00:15:56.820 align:middle line:84% Now this is a disadvantage here-- 00:15:56.820 --> 00:16:00.300 align:middle line:90% the radiation loss, right? 00:16:00.300 --> 00:16:05.640 align:middle line:84% Radiation loss, because the whole inner tube is heated up, 00:16:05.640 --> 00:16:10.770 align:middle line:84% so the surface area is actually is pi times the inner diameter. 00:16:10.770 --> 00:16:13.230 align:middle line:84% It's not just the cross-section flat. 00:16:13.230 --> 00:16:17.420 align:middle line:84% Compared to flat, it's only the flat area. 00:16:17.420 --> 00:16:19.660 align:middle line:90% Now it's the whole surface area. 00:16:19.660 --> 00:16:25.990 align:middle line:84% And then that's the emissivity And I do a simple blackbody 00:16:25.990 --> 00:16:28.990 align:middle line:90% calculation of the surface. 00:16:28.990 --> 00:16:31.810 align:middle line:84% So what I did not consider is the multiple refraction, 00:16:31.810 --> 00:16:33.220 align:middle line:84% the surface emission, I just say, 00:16:33.220 --> 00:16:36.310 align:middle line:84% OK, everything goes to the environment. 00:16:36.310 --> 00:16:40.490 align:middle line:84% And in reality, the sum of those cannot go. 00:16:40.490 --> 00:16:45.100 align:middle line:84% In fact, the glass may trap some of the radiation back. 00:16:45.100 --> 00:16:49.600 align:middle line:84% The reflection and also the glass 00:16:49.600 --> 00:16:53.740 align:middle line:84% is non-transparent in certain wavelength range. 00:16:53.740 --> 00:16:58.180 align:middle line:84% So with that, I can say this is the absorbed radiation 00:16:58.180 --> 00:16:59.530 align:middle line:90% minus loss. 00:16:59.530 --> 00:17:03.730 align:middle line:84% The difference is what I actually get into the hot pipe, 00:17:03.730 --> 00:17:06.490 align:middle line:84% and then divided by what comes in. 00:17:06.490 --> 00:17:09.069 align:middle line:90% So I have my efficiency. 00:17:09.069 --> 00:17:15.250 align:middle line:84% So this is a simple, again, back of envelope for fun. 00:17:15.250 --> 00:17:15.900 align:middle line:90% Yes? 00:17:15.900 --> 00:17:17.067 align:middle line:90% AUDIENCE: I have a question. 00:17:17.067 --> 00:17:21.130 align:middle line:84% So if the tube is treated [INAUDIBLE] I think only the 1/2 00:17:21.130 --> 00:17:24.460 align:middle line:84% the total surface get the solar radiation? 00:17:24.460 --> 00:17:26.230 align:middle line:90% GANG CHEN: That's not-- 00:17:26.230 --> 00:17:28.870 align:middle line:90% AUDIENCE: [INAUDIBLE] reflector. 00:17:28.870 --> 00:17:31.090 align:middle line:84% GANG CHEN: That's the 1/2, right? 00:17:31.090 --> 00:17:32.620 align:middle line:90% It's cross-section area. 00:17:32.620 --> 00:17:38.080 align:middle line:84% But the emission is pi D. So that's a disadvantage. 00:17:38.080 --> 00:17:38.580 align:middle line:90% OK? 00:17:38.580 --> 00:17:43.530 align:middle line:90% 00:17:43.530 --> 00:17:45.710 align:middle line:90% And so that's an idea. 00:17:45.710 --> 00:17:47.250 align:middle line:84% I'll show you the curve later on. 00:17:47.250 --> 00:17:51.720 align:middle line:84% Now I'm going to look at the hot water flat panel type. 00:17:51.720 --> 00:17:57.060 align:middle line:84% So a flat panel type in between the air. 00:17:57.060 --> 00:18:01.890 align:middle line:84% I was actually curious whether people use double glazed windows 00:18:01.890 --> 00:18:03.190 align:middle line:90% here or not. 00:18:03.190 --> 00:18:05.860 align:middle line:84% And what I found is since they do not use it, 00:18:05.860 --> 00:18:09.710 align:middle line:90% so it's just one layer of glass. 00:18:09.710 --> 00:18:14.940 align:middle line:84% So this is air and selective surface. 00:18:14.940 --> 00:18:19.020 align:middle line:84% And of course, if we think about heat transfer, 00:18:19.020 --> 00:18:21.990 align:middle line:84% this absorbs solar radiation and then will 00:18:21.990 --> 00:18:25.020 align:middle line:84% conduct through the air, conduct through the glass 00:18:25.020 --> 00:18:27.780 align:middle line:84% and outside the lateral convection. 00:18:27.780 --> 00:18:32.730 align:middle line:84% So you have the resistance in series, conduction air, 00:18:32.730 --> 00:18:35.280 align:middle line:84% conduction resistance, glass resistance, 00:18:35.280 --> 00:18:36.750 align:middle line:90% convection resistance. 00:18:36.750 --> 00:18:40.650 align:middle line:84% So that's one part of the heat through conduction 00:18:40.650 --> 00:18:41.730 align:middle line:90% and convection. 00:18:41.730 --> 00:18:45.820 align:middle line:84% And backside, the thermal insulation can still leak heat. 00:18:45.820 --> 00:18:48.580 align:middle line:90% So backside, I have conduction. 00:18:48.580 --> 00:18:52.360 align:middle line:84% And then I have radiation goes through directly. 00:18:52.360 --> 00:18:55.480 align:middle line:84% So I have three parts for the heat transfer. 00:18:55.480 --> 00:18:58.930 align:middle line:90% And this is my simple circuit. 00:18:58.930 --> 00:19:01.510 align:middle line:84% The solar radiation come to the selective surface. 00:19:01.510 --> 00:19:04.510 align:middle line:84% The energy now, one part is going directly 00:19:04.510 --> 00:19:05.860 align:middle line:90% through radiation. 00:19:05.860 --> 00:19:11.590 align:middle line:84% And the other part goes through the air-glass convection 00:19:11.590 --> 00:19:15.790 align:middle line:84% outside, and then the backside, the insulation. 00:19:15.790 --> 00:19:17.542 align:middle line:90% OK? 00:19:17.542 --> 00:19:22.090 align:middle line:84% And I put the resistance because this is assumption 00:19:22.090 --> 00:19:23.785 align:middle line:84% if you're not the mechanical engineer 00:19:23.785 --> 00:19:26.240 align:middle line:84% or didn't take heat transfer before, this is probably 00:19:26.240 --> 00:19:27.100 align:middle line:90% less familiar. 00:19:27.100 --> 00:19:29.740 align:middle line:84% But this is similar like a circuit, OK? 00:19:29.740 --> 00:19:31.420 align:middle line:90% Thermal resistance. 00:19:31.420 --> 00:19:34.120 align:middle line:84% You do the resistance calculation 00:19:34.120 --> 00:19:37.820 align:middle line:84% for air, the thickness divided by thermal conductivity. 00:19:37.820 --> 00:19:41.840 align:middle line:84% So the gap thickness divided by conductivity divided by area. 00:19:41.840 --> 00:19:44.950 align:middle line:84% And they say if you do that, why don't you use thicker? 00:19:44.950 --> 00:19:48.340 align:middle line:84% And the problem is once you [INAUDIBLE] 00:19:48.340 --> 00:19:50.990 align:middle line:84% start the circulation, convection starts to happen. 00:19:50.990 --> 00:19:54.610 align:middle line:84% So you look at the double panel window in the typical window, 00:19:54.610 --> 00:19:56.620 align:middle line:90% that thickness is not-- 00:19:56.620 --> 00:19:58.270 align:middle line:84% it's a few millimeters thickness. 00:19:58.270 --> 00:20:00.400 align:middle line:90% So you don't use too thick here. 00:20:00.400 --> 00:20:03.010 align:middle line:90% So here, I took a 10-millimeter. 00:20:03.010 --> 00:20:06.820 align:middle line:84% And then, say, glass, glass is a-- 00:20:06.820 --> 00:20:10.360 align:middle line:84% few weeks ago, there was a talk, somebody 00:20:10.360 --> 00:20:14.710 align:middle line:84% is going to install a flat panel system 00:20:14.710 --> 00:20:16.400 align:middle line:90% in one of the MIT building. 00:20:16.400 --> 00:20:19.180 align:middle line:90% MIT, I don't know. 00:20:19.180 --> 00:20:19.720 align:middle line:90% Boston? 00:20:19.720 --> 00:20:20.440 align:middle line:90% OK. 00:20:20.440 --> 00:20:22.300 align:middle line:90% I was surprised. 00:20:22.300 --> 00:20:26.620 align:middle line:84% The cost per was just a surprise for me because that's like 00:20:26.620 --> 00:20:29.950 align:middle line:90% a 13-, 15-year return. 00:20:29.950 --> 00:20:32.410 align:middle line:84% I don't understand why, but let's say 00:20:32.410 --> 00:20:35.870 align:middle line:84% it looks like they use a very tough glass. 00:20:35.870 --> 00:20:37.750 align:middle line:90% This is a steel. 00:20:37.750 --> 00:20:39.510 align:middle line:90% What's the glass like? 00:20:39.510 --> 00:20:41.870 align:middle line:90% You harden the glass. 00:20:41.870 --> 00:20:44.140 align:middle line:90% You can walk on it. 00:20:44.140 --> 00:20:46.850 align:middle line:84% But anyway, but glass resistance is very small. 00:20:46.850 --> 00:20:49.750 align:middle line:84% It's really air resistance, convection resistance. 00:20:49.750 --> 00:20:52.450 align:middle line:84% Those are the resistance that cost the total. 00:20:52.450 --> 00:20:54.580 align:middle line:84% That's this side, the resistance, 00:20:54.580 --> 00:20:57.460 align:middle line:84% and then the insulation resistance on the back side. 00:20:57.460 --> 00:20:59.710 align:middle line:84% So I put all this lumber together 00:20:59.710 --> 00:21:04.460 align:middle line:84% and did the energy loss, again, considering all the release. 00:21:04.460 --> 00:21:06.550 align:middle line:90% So I have radiation loss. 00:21:06.550 --> 00:21:12.760 align:middle line:84% I have the conduction loss in conduction/convection loss. 00:21:12.760 --> 00:21:14.980 align:middle line:84% And with this loss, I can, again, 00:21:14.980 --> 00:21:18.520 align:middle line:84% go calculate how energy comes in, 00:21:18.520 --> 00:21:23.380 align:middle line:84% absorb, mass loss divided by what comes in. 00:21:23.380 --> 00:21:26.590 align:middle line:90% And that's what I have. 00:21:26.590 --> 00:21:28.390 align:middle line:90% OK. 00:21:28.390 --> 00:21:33.220 align:middle line:84% Those numbers, I assume, for example, in the next graph, 00:21:33.220 --> 00:21:37.330 align:middle line:84% I assume 95% absorption, 5% emission. 00:21:37.330 --> 00:21:39.340 align:middle line:84% This is what the industry can do. 00:21:39.340 --> 00:21:42.830 align:middle line:84% So we have a research group on selective surface 00:21:42.830 --> 00:21:45.590 align:middle line:84% so we can see how we can do better. 00:21:45.590 --> 00:21:48.550 align:middle line:84% Industry can do that at the meters 00:21:48.550 --> 00:21:52.120 align:middle line:90% per second, selective surface-- 00:21:52.120 --> 00:21:56.710 align:middle line:90% 95% absorption, 5% emission. 00:21:56.710 --> 00:21:59.170 align:middle line:84% And this is how water is, of course-- 00:21:59.170 --> 00:22:00.850 align:middle line:84% the typical operational temperature 00:22:00.850 --> 00:22:03.370 align:middle line:90% is about 70, 80 degrees. 00:22:03.370 --> 00:22:06.890 align:middle line:84% So they do not operate at very high temperatures. 00:22:06.890 --> 00:22:09.250 align:middle line:84% And then when you go to higher temperatures, 00:22:09.250 --> 00:22:12.820 align:middle line:84% if you can go 500 degrees, still keep this, 00:22:12.820 --> 00:22:14.890 align:middle line:84% you will be the best in the world. 00:22:14.890 --> 00:22:17.290 align:middle line:90% It's very hard to do that. 00:22:17.290 --> 00:22:20.510 align:middle line:90% OK that's the curve I have. 00:22:20.510 --> 00:22:23.140 align:middle line:90% That's the vacuum system. 00:22:23.140 --> 00:22:26.530 align:middle line:84% And this is the big flat panel system. 00:22:26.530 --> 00:22:29.920 align:middle line:84% And efficiency depends, of course, 00:22:29.920 --> 00:22:32.865 align:middle line:90% the temperature of the water. 00:22:32.865 --> 00:22:34.240 align:middle line:84% Essentially, it's the temperature 00:22:34.240 --> 00:22:37.180 align:middle line:90% of the selective surface there. 00:22:37.180 --> 00:22:41.320 align:middle line:84% And I went to the web, and I found this. 00:22:41.320 --> 00:22:44.020 align:middle line:84% Depends on which website they're looking at, 00:22:44.020 --> 00:22:45.630 align:middle line:90% people have different curves. 00:22:45.630 --> 00:22:48.360 align:middle line:84% But I found the one that I thought 00:22:48.360 --> 00:22:51.660 align:middle line:90% was closer to what I have. 00:22:51.660 --> 00:22:56.340 align:middle line:84% So this is one company that a vacuum system 00:22:56.340 --> 00:22:58.470 align:middle line:90% that compared to a flat panel. 00:22:58.470 --> 00:23:01.680 align:middle line:84% A flat panel goes to almost 0, I think probably, 00:23:01.680 --> 00:23:03.020 align:middle line:90% yeah, that's probably too small. 00:23:03.020 --> 00:23:07.078 align:middle line:90% 00:23:07.078 --> 00:23:13.780 align:middle line:84% But solar thermal is efficiency compared, of course, 00:23:13.780 --> 00:23:18.250 align:middle line:84% to the photovoltaic is many times better. 00:23:18.250 --> 00:23:25.160 align:middle line:84% And so in terms of if you look at what has been used, 00:23:25.160 --> 00:23:29.650 align:middle line:84% this is a chart I got from this publication. 00:23:29.650 --> 00:23:34.090 align:middle line:84% And here is this side that's solar thermal. 00:23:34.090 --> 00:23:34.960 align:middle line:90% That's here. 00:23:34.960 --> 00:23:37.300 align:middle line:90% It's compare all the renewables. 00:23:37.300 --> 00:23:40.390 align:middle line:90% Solar thermal here is one. 00:23:40.390 --> 00:23:42.760 align:middle line:84% Here is geothermal photovoltaics, 00:23:42.760 --> 00:23:47.560 align:middle line:84% solar thermal power plant, ocean tidal power. 00:23:47.560 --> 00:23:55.510 align:middle line:84% And you find out that the total capacity, and the red 00:23:55.510 --> 00:23:57.920 align:middle line:90% is actually produced energy. 00:23:57.920 --> 00:24:03.620 align:middle line:84% So you can see the solar thermal is actually 00:24:03.620 --> 00:24:08.270 align:middle line:84% a pretty large part of how people are currently 00:24:08.270 --> 00:24:11.390 align:middle line:90% using renewable energy, OK? 00:24:11.390 --> 00:24:17.510 align:middle line:84% Just heating up the hot water mainly. 00:24:17.510 --> 00:24:23.040 align:middle line:84% And this is in terms of technology. 00:24:23.040 --> 00:24:26.520 align:middle line:84% The vacuum tube is dominant because China-- 00:24:26.520 --> 00:24:28.820 align:middle line:90% and really China use it. 00:24:28.820 --> 00:24:37.850 align:middle line:84% And flat panel, 32%, unglazed [INAUDIBLE] collector and air 00:24:37.850 --> 00:24:38.670 align:middle line:90% collector. 00:24:38.670 --> 00:24:42.200 align:middle line:84% So this is where it's being used. 00:24:42.200 --> 00:24:45.830 align:middle line:84% You can see that China is off the chart here. 00:24:45.830 --> 00:24:50.000 align:middle line:84% And it has about 100 million square meters installed. 00:24:50.000 --> 00:24:53.990 align:middle line:84% I don't understand why we didn't use them here. 00:24:53.990 --> 00:24:58.460 align:middle line:84% But I think I see Hawaii now has legislation, 00:24:58.460 --> 00:25:03.170 align:middle line:84% if you have a new house built, you 00:25:03.170 --> 00:25:05.810 align:middle line:90% have to put in hot water system. 00:25:05.810 --> 00:25:07.530 align:middle line:90% Tim Young from Hawaii, right? 00:25:07.530 --> 00:25:08.840 align:middle line:90% You can go back, check this. 00:25:08.840 --> 00:25:10.217 align:middle line:90% [LAUGHTER] 00:25:10.217 --> 00:25:13.890 align:middle line:90% 00:25:13.890 --> 00:25:17.660 align:middle line:90% So I was once talking to-- 00:25:17.660 --> 00:25:20.120 align:middle line:84% every time I have a chance, I always ask the people what-- 00:25:20.120 --> 00:25:22.050 align:middle line:90% and also, price gets very high. 00:25:22.050 --> 00:25:28.100 align:middle line:84% That's probably because in China, your typical household 00:25:28.100 --> 00:25:32.526 align:middle line:84% is $300 to put a thermal system rooftop. 00:25:32.526 --> 00:25:35.830 align:middle line:90% 00:25:35.830 --> 00:25:38.282 align:middle line:90% OK, so that's a thermal. 00:25:38.282 --> 00:25:40.240 align:middle line:84% Now we want to go to higher temperature, right? 00:25:40.240 --> 00:25:43.690 align:middle line:84% Solar thermal is 80 degrees, 100 degrees. 00:25:43.690 --> 00:25:47.870 align:middle line:84% And we want now go to higher temperature. 00:25:47.870 --> 00:25:52.690 align:middle line:84% And to get a higher temperature, you need concentration. 00:25:52.690 --> 00:25:57.260 align:middle line:84% And now we need to look at the concentration. 00:25:57.260 --> 00:26:03.880 align:middle line:84% Let me remind you, I showed this before, the Earth itself, 00:26:03.880 --> 00:26:08.290 align:middle line:84% see the orbital is not just a perpendicular of this plane. 00:26:08.290 --> 00:26:13.572 align:middle line:84% There is a 23.5 tilt of the Earth, right? 00:26:13.572 --> 00:26:18.190 align:middle line:84% So on this side, this is tilt. On this side, the same tilt. 00:26:18.190 --> 00:26:20.080 align:middle line:90% That title is fixed. 00:26:20.080 --> 00:26:22.885 align:middle line:90% That's why we have summer and-- 00:26:22.885 --> 00:26:25.870 align:middle line:90% oh, the wrong direction. 00:26:25.870 --> 00:26:29.830 align:middle line:90% We have summer and winter. 00:26:29.830 --> 00:26:30.822 align:middle line:90% OK? 00:26:30.822 --> 00:26:35.050 align:middle line:84% And the degree suspended of the Sun 00:26:35.050 --> 00:26:41.530 align:middle line:84% by, say, if you view from Earth, the degree is 32 second. 00:26:41.530 --> 00:26:49.776 align:middle line:90% So that's about 0.5-- 00:26:49.776 --> 00:26:56.750 align:middle line:84% so that's about 0.4, 0.5 degrees. 00:26:56.750 --> 00:27:00.050 align:middle line:84% And now the question I want to ask first 00:27:00.050 --> 00:27:03.970 align:middle line:84% is, if you do concentration, how much you can get. 00:27:03.970 --> 00:27:08.000 align:middle line:90% 00:27:08.000 --> 00:27:11.964 align:middle line:84% What's the maximum concentration, right? 00:27:11.964 --> 00:27:17.290 align:middle line:84% So we can look at theoretically, what's the limit? 00:27:17.290 --> 00:27:22.510 align:middle line:84% The maximum theoretical limit is I cannot heat up an object 00:27:22.510 --> 00:27:24.925 align:middle line:90% hotter than the Sun. 00:27:24.925 --> 00:27:25.425 align:middle line:90% Right? 00:27:25.425 --> 00:27:28.610 align:middle line:90% AUDIENCE: [INAUDIBLE] 00:27:28.610 --> 00:27:30.570 align:middle line:84% GANG CHEN: Now, so that, you all agree. 00:27:30.570 --> 00:27:36.260 align:middle line:84% But if I ask you, can I concentrate radiation even 00:27:36.260 --> 00:27:39.200 align:middle line:84% with the intensity hotter than the Sun? 00:27:39.200 --> 00:27:41.850 align:middle line:84% And not hotter-- the intensity higher than the Sun? 00:27:41.850 --> 00:27:45.000 align:middle line:90% 00:27:45.000 --> 00:27:48.600 align:middle line:90% Is that crazy or not? 00:27:48.600 --> 00:27:50.460 align:middle line:90% Huh? 00:27:50.460 --> 00:27:53.460 align:middle line:90% OK, you'll see later on. 00:27:53.460 --> 00:27:57.580 align:middle line:84% OK, so first, we have to look at the first 00:27:57.580 --> 00:27:59.730 align:middle line:90% or just look at energy balance. 00:27:59.730 --> 00:28:04.450 align:middle line:84% If I think about the spherical radius from the Sun 00:28:04.450 --> 00:28:11.430 align:middle line:84% and the area of the sphere times the energy flux and area 00:28:11.430 --> 00:28:12.630 align:middle line:90% of the-- 00:28:12.630 --> 00:28:14.850 align:middle line:90% see, it's a constant. 00:28:14.850 --> 00:28:17.100 align:middle line:84% Once the energy leaves the surface, 00:28:17.100 --> 00:28:20.880 align:middle line:84% the larger you draw a circle, the less is your intensity. 00:28:20.880 --> 00:28:26.010 align:middle line:84% So the intensity of this Earth, you think of a circle 00:28:26.010 --> 00:28:29.460 align:middle line:84% here, that is the capital R 4 pi R squared 00:28:29.460 --> 00:28:35.480 align:middle line:84% times whatever insulation outside the Earth's atmosphere. 00:28:35.480 --> 00:28:41.830 align:middle line:84% So this is the insulation outside the Earth's atmosphere. 00:28:41.830 --> 00:28:45.000 align:middle line:84% This is the intensity on the surface 00:28:45.000 --> 00:28:48.280 align:middle line:90% of the Sun, radius of the Sun. 00:28:48.280 --> 00:28:51.855 align:middle line:84% So that's the energy balance, conserved always. 00:28:51.855 --> 00:28:53.230 align:middle line:84% Now, of course, you say there are 00:28:53.230 --> 00:28:56.290 align:middle line:84% something in between scattered a little bit like-- 00:28:56.290 --> 00:29:03.714 align:middle line:84% assuming the space is pretty much all empty, right? 00:29:03.714 --> 00:29:07.870 align:middle line:90% So now let me concentrate. 00:29:07.870 --> 00:29:10.540 align:middle line:84% If I concentrate this concentration 00:29:10.540 --> 00:29:18.010 align:middle line:84% ratio and this intensity on the Earth's insulation, 00:29:18.010 --> 00:29:23.830 align:middle line:84% and this is the energy I put in onto an object that receives 00:29:23.830 --> 00:29:25.990 align:middle line:90% this concentrated light, right? 00:29:25.990 --> 00:29:30.460 align:middle line:84% So energy balance will tell me in this case, 00:29:30.460 --> 00:29:34.630 align:middle line:84% again, the object should be blackbody, 00:29:34.630 --> 00:29:38.110 align:middle line:84% actually because you want to absorb 00:29:38.110 --> 00:29:42.070 align:middle line:90% every drop of solar radiation. 00:29:42.070 --> 00:29:46.450 align:middle line:84% And because you absorb the solar radiation, you also reradiate. 00:29:46.450 --> 00:29:48.430 align:middle line:90% So you want to absorb maximum. 00:29:48.430 --> 00:29:50.720 align:middle line:90% You also reradiate maximum. 00:29:50.720 --> 00:29:51.680 align:middle line:90% So it's a blackbody. 00:29:51.680 --> 00:29:54.760 align:middle line:84% So actually, I put the-- so the energy absorbed, 00:29:54.760 --> 00:29:57.580 align:middle line:84% the balance the energy reradiated, 00:29:57.580 --> 00:29:59.670 align:middle line:90% I do perfectly insulation. 00:29:59.670 --> 00:30:01.940 align:middle line:90% So this is per unit area based. 00:30:01.940 --> 00:30:03.780 align:middle line:90% This is per unit area based. 00:30:03.780 --> 00:30:07.510 align:middle line:84% So that's the energy balance for the object that receives 00:30:07.510 --> 00:30:10.270 align:middle line:90% the concentrated light. 00:30:10.270 --> 00:30:14.200 align:middle line:84% And of course, as I said, my condition is that this object 00:30:14.200 --> 00:30:16.900 align:middle line:90% cannot be hotter than the Sun. 00:30:16.900 --> 00:30:18.820 align:middle line:90% Right? 00:30:18.820 --> 00:30:23.000 align:middle line:90% So this is the concentration. 00:30:23.000 --> 00:30:28.570 align:middle line:84% And from here, I get my maximum concentration 00:30:28.570 --> 00:30:31.300 align:middle line:90% happens when this is equal. 00:30:31.300 --> 00:30:35.710 align:middle line:84% And that's the solar flux intensity divided by whatever 00:30:35.710 --> 00:30:36.970 align:middle line:90% [INAUDIBLE] in the Earth. 00:30:36.970 --> 00:30:40.840 align:middle line:84% So that's R over small r based on my energy balance. 00:30:40.840 --> 00:30:45.370 align:middle line:84% And if you look at R small r, and this theta here, 00:30:45.370 --> 00:30:48.760 align:middle line:84% it's half angle sustained to Earth. 00:30:48.760 --> 00:30:51.580 align:middle line:90% So that's a sine theta squared. 00:30:51.580 --> 00:30:54.900 align:middle line:84% So that's my maximum concentration. 00:30:54.900 --> 00:30:57.990 align:middle line:84% And if you plug in the number, I said 00:30:57.990 --> 00:31:00.780 align:middle line:90% the 2 theta is a 32 seconds. 00:31:00.780 --> 00:31:08.030 align:middle line:84% So you plug in a number, that's about 46,000 times. 00:31:08.030 --> 00:31:10.990 align:middle line:90% It's the maximum you can get. 00:31:10.990 --> 00:31:12.910 align:middle line:90% It's not the maximum. 00:31:12.910 --> 00:31:15.705 align:middle line:84% People actually have achieved higher than this one. 00:31:15.705 --> 00:31:18.510 align:middle line:90% 00:31:18.510 --> 00:31:20.238 align:middle line:90% OK? 00:31:20.238 --> 00:31:27.420 align:middle line:84% This is because the emission inside the object 00:31:27.420 --> 00:31:31.770 align:middle line:90% is proportional-- 00:31:31.770 --> 00:31:34.050 align:middle line:90% I have one mistake here. 00:31:34.050 --> 00:31:38.610 align:middle line:84% Should be proportional to n square, not proportional to n. 00:31:38.610 --> 00:31:39.915 align:middle line:90% OK? 00:31:39.915 --> 00:31:43.950 align:middle line:84% So the emission inside object is proportional to n squared. 00:31:43.950 --> 00:31:51.600 align:middle line:84% So if I have a medium like a water or glass, 00:31:51.600 --> 00:31:56.400 align:middle line:84% the intensity inside could be higher than the vacuum. 00:31:56.400 --> 00:32:02.980 align:middle line:84% Blackbody intensity, n squared. n in glass is 1.5. 00:32:02.980 --> 00:32:05.280 align:middle line:90% So square is 2.25. 00:32:05.280 --> 00:32:10.120 align:middle line:84% And if you use a silicon, the n is about 3.5. 00:32:10.120 --> 00:32:12.720 align:middle line:90% The square is about 10, right? 00:32:12.720 --> 00:32:19.450 align:middle line:84% So if you do that again, your limit is the temperature limit 00:32:19.450 --> 00:32:21.340 align:middle line:90% is not the intensity limit. 00:32:21.340 --> 00:32:24.730 align:middle line:84% Your temperature of the concentrated surface cannot be 00:32:24.730 --> 00:32:26.790 align:middle line:84% higher than the temperature of the Sun. 00:32:26.790 --> 00:32:30.250 align:middle line:84% You do that, you get your maximum is n 00:32:30.250 --> 00:32:33.250 align:middle line:90% squared sine theta squared. 00:32:33.250 --> 00:32:35.680 align:middle line:84% So that's higher than the previous one. 00:32:35.680 --> 00:32:39.760 align:middle line:84% And indeed, this was achieved in the lab. 00:32:39.760 --> 00:32:43.770 align:middle line:90% This is Roland Winston. 00:32:43.770 --> 00:32:46.390 align:middle line:84% I'll talk about this in more detail. 00:32:46.390 --> 00:32:49.210 align:middle line:84% This is a compound with the focus and non-focus, 00:32:49.210 --> 00:32:52.400 align:middle line:90% and they have achieved 56. 00:32:52.400 --> 00:32:56.680 align:middle line:90% So there we go. 00:32:56.680 --> 00:32:58.210 align:middle line:90% OK? 00:32:58.210 --> 00:33:03.250 align:middle line:84% So that's from the second law perspective, 00:33:03.250 --> 00:33:06.040 align:middle line:90% your absolute upper limit. 00:33:06.040 --> 00:33:09.430 align:middle line:84% And now let's see how we can get there. 00:33:09.430 --> 00:33:10.816 align:middle line:90% OK? 00:33:10.816 --> 00:33:15.730 align:middle line:84% And I think I'm going to use a lens. 00:33:15.730 --> 00:33:25.210 align:middle line:84% And this lens, if I think about a lens, so all parabolic trough, 00:33:25.210 --> 00:33:27.580 align:middle line:84% we know that you have [INAUDIBLE] comes 00:33:27.580 --> 00:33:30.310 align:middle line:90% in, you focus on one point. 00:33:30.310 --> 00:33:35.140 align:middle line:84% So the question is, What's the concentration 00:33:35.140 --> 00:33:38.030 align:middle line:84% limit if you-- those are the image formation, right? 00:33:38.030 --> 00:33:45.880 align:middle line:84% Using a lens or using a parabolic trough, 00:33:45.880 --> 00:33:47.500 align:middle line:90% you have a focal point. 00:33:47.500 --> 00:33:49.000 align:middle line:90% That's a focal point. 00:33:49.000 --> 00:33:50.270 align:middle line:90% This is the image. 00:33:50.270 --> 00:33:53.050 align:middle line:90% So the image forming optics. 00:33:53.050 --> 00:33:55.730 align:middle line:84% And remember, the Sun sustained angle. 00:33:55.730 --> 00:33:57.520 align:middle line:90% That's the k, right? 00:33:57.520 --> 00:34:01.240 align:middle line:84% Even though we say solar radiation is a part of light. 00:34:01.240 --> 00:34:04.720 align:middle line:84% But when you want to do focusing, 00:34:04.720 --> 00:34:07.960 align:middle line:84% that angle becomes important because previously, you've 00:34:07.960 --> 00:34:09.010 align:middle line:90% already seen. 00:34:09.010 --> 00:34:12.940 align:middle line:84% So what I want to think about is if I 00:34:12.940 --> 00:34:17.679 align:middle line:84% have either lens or parabolic trough, 00:34:17.679 --> 00:34:26.650 align:middle line:84% say the opening here is D, and the focus here 00:34:26.650 --> 00:34:30.219 align:middle line:84% is a small d-- why is it big D, the other small d? 00:34:30.219 --> 00:34:33.670 align:middle line:90% And I want to find the ratio. 00:34:33.670 --> 00:34:35.830 align:middle line:90% OK, let's look at it. 00:34:35.830 --> 00:34:40.870 align:middle line:84% So if I do 2D, this one is a flat panel. 00:34:40.870 --> 00:34:44.420 align:middle line:84% So if you do two, you have a different result. 00:34:44.420 --> 00:34:49.600 align:middle line:84% But here, I'm a flat panel, and this is the half of it. 00:34:49.600 --> 00:34:54.138 align:middle line:90% And now I look at-- 00:34:54.138 --> 00:34:57.460 align:middle line:90% this is a vertical-- 00:34:57.460 --> 00:34:59.280 align:middle line:90% I say perpendicular line here. 00:34:59.280 --> 00:35:02.510 align:middle line:84% So R times sine-- see that here is 2 theta. 00:35:02.510 --> 00:35:05.570 align:middle line:84% So this is really-- if you think about parallel line comes 00:35:05.570 --> 00:35:08.610 align:middle line:84% in this way, and that's my reflection here. 00:35:08.610 --> 00:35:12.870 align:middle line:84% And this one is my this line reflection. 00:35:12.870 --> 00:35:17.990 align:middle line:84% So that's the edge of the solar cone there. 00:35:17.990 --> 00:35:24.860 align:middle line:84% And so R times sine theta gives me this one, right? 00:35:24.860 --> 00:35:26.180 align:middle line:90% Perpendicular line. 00:35:26.180 --> 00:35:28.170 align:middle line:90% And D over 2-- 00:35:28.170 --> 00:35:31.970 align:middle line:84% so that's here-- times, this is phi. 00:35:31.970 --> 00:35:33.180 align:middle line:90% And here is phi. 00:35:33.180 --> 00:35:34.910 align:middle line:90% So here is also phi. 00:35:34.910 --> 00:35:39.390 align:middle line:84% So D over 2 times cosine 5 is all giving me this segment. 00:35:39.390 --> 00:35:42.590 align:middle line:84% So I have an equation geometry here. 00:35:42.590 --> 00:35:45.710 align:middle line:90% Geometrical relation. 00:35:45.710 --> 00:35:51.780 align:middle line:84% And then I have this R, and this R is about the same. 00:35:51.780 --> 00:35:59.690 align:middle line:84% So R times sine theta gives me D over 2, the opening there. 00:35:59.690 --> 00:36:03.620 align:middle line:84% And then from here, I can get my relation, 00:36:03.620 --> 00:36:09.230 align:middle line:84% how much focus I get, capital D over small d. 00:36:09.230 --> 00:36:13.670 align:middle line:84% I have sine 2 phi divided by 2 sine theta. 00:36:13.670 --> 00:36:17.790 align:middle line:84% So this is the maximum you will get. 00:36:17.790 --> 00:36:19.550 align:middle line:84% This is the concentration you get. 00:36:19.550 --> 00:36:22.700 align:middle line:84% And you will see the maximum happens is assigned to 2 phi 00:36:22.700 --> 00:36:24.780 align:middle line:90% equals to 1. 00:36:24.780 --> 00:36:25.280 align:middle line:90% Right? 00:36:25.280 --> 00:36:27.950 align:middle line:90% So 45 degrees. 00:36:27.950 --> 00:36:35.360 align:middle line:84% So when phi equals 45 degrees here, I have the maximum here. 00:36:35.360 --> 00:36:39.630 align:middle line:84% And that maximum is 2 sine theta. 00:36:39.630 --> 00:36:43.790 align:middle line:90% That's 107, my calculation. 00:36:43.790 --> 00:36:48.000 align:middle line:90% Maybe you can go back to check. 00:36:48.000 --> 00:36:55.290 align:middle line:84% And if you 3D axisymmetric, you square that. 00:36:55.290 --> 00:36:59.640 align:middle line:90% And that's 4 sine theta squared. 00:36:59.640 --> 00:37:02.640 align:middle line:84% The problem is now you can see this 00:37:02.640 --> 00:37:06.540 align:middle line:84% is a factor of 4 smaller than the limit, right? 00:37:06.540 --> 00:37:10.140 align:middle line:84% Thermodynamically, it's 1 over sine theta. 00:37:10.140 --> 00:37:16.400 align:middle line:84% Here, if you form an image, your limit is a factor of 4 smaller. 00:37:16.400 --> 00:37:19.378 align:middle line:90% 00:37:19.378 --> 00:37:21.910 align:middle line:84% And how you can reach the maximum number. 00:37:21.910 --> 00:37:24.540 align:middle line:90% 00:37:24.540 --> 00:37:25.080 align:middle line:90% OK. 00:37:25.080 --> 00:37:30.420 align:middle line:84% So imaging optics is a little bit too constrained. 00:37:30.420 --> 00:37:34.941 align:middle line:84% Ideally, you want everything goes to one point, right? 00:37:34.941 --> 00:37:37.020 align:middle line:90% And you want to form an image. 00:37:37.020 --> 00:37:38.580 align:middle line:90% So this is a-- 00:37:38.580 --> 00:37:41.100 align:middle line:84% here, there's another slide that I just 00:37:41.100 --> 00:37:42.990 align:middle line:90% copied from Winston's book. 00:37:42.990 --> 00:37:48.840 align:middle line:84% And if you want to focus on the cylinder, that's 1 over pi 00:37:48.840 --> 00:37:52.090 align:middle line:90% smaller than the previous slide. 00:37:52.090 --> 00:37:56.060 align:middle line:90% 00:37:56.060 --> 00:38:00.140 align:middle line:84% Now there's another way to do it. 00:38:00.140 --> 00:38:02.510 align:middle line:90% That's a lung image method. 00:38:02.510 --> 00:38:05.610 align:middle line:90% This is Roland Winston's baby. 00:38:05.610 --> 00:38:12.200 align:middle line:84% And if you read, his rationale is starting from this 1 over 4, 00:38:12.200 --> 00:38:16.010 align:middle line:84% how you can get to the maximum thermodynamic limit. 00:38:16.010 --> 00:38:20.450 align:middle line:84% And here is one where there is no simple mathematic in terms 00:38:20.450 --> 00:38:22.640 align:middle line:84% of the shape of the concentrator. 00:38:22.640 --> 00:38:25.640 align:middle line:84% But this is how he would explain it. 00:38:25.640 --> 00:38:28.880 align:middle line:84% So this is where the area you want to-- this 00:38:28.880 --> 00:38:30.170 align:middle line:90% is a flat plate example. 00:38:30.170 --> 00:38:36.140 align:middle line:84% You want to focus on a flat plate and B prime to D, OK? 00:38:36.140 --> 00:38:42.770 align:middle line:84% And this is the angle of incidence, right? 00:38:42.770 --> 00:38:46.490 align:middle line:90% So that's the maximum angle. 00:38:46.490 --> 00:38:49.580 align:middle line:84% Let's suppose this is the maximum angle that you 00:38:49.580 --> 00:38:53.090 align:middle line:90% accept the radiation coming in. 00:38:53.090 --> 00:38:58.730 align:middle line:84% If you have another angle higher than this, you will not get in. 00:38:58.730 --> 00:39:00.110 align:middle line:90% You will be blocked. 00:39:00.110 --> 00:39:03.210 align:middle line:90% So this is the maximum angle. 00:39:03.210 --> 00:39:06.710 align:middle line:90% So you take the string here. 00:39:06.710 --> 00:39:09.500 align:middle line:84% At this point, you take a string. 00:39:09.500 --> 00:39:14.040 align:middle line:84% This is the opening limit A. So you draw-- 00:39:14.040 --> 00:39:15.620 align:middle line:90% you keep this string fixed. 00:39:15.620 --> 00:39:19.440 align:middle line:84% Let's fix, and you draw the curve down. 00:39:19.440 --> 00:39:21.110 align:middle line:90% So this one will slide. 00:39:21.110 --> 00:39:25.820 align:middle line:84% So the angle of incidence equals reflection. 00:39:25.820 --> 00:39:27.510 align:middle line:90% That's what it's following. 00:39:27.510 --> 00:39:32.630 align:middle line:84% This surface gives a surface such that the angle of incidence 00:39:32.630 --> 00:39:35.180 align:middle line:90% equals angle of reflection. 00:39:35.180 --> 00:39:38.630 align:middle line:84% Of course, it's a specular surface that's being assumed. 00:39:38.630 --> 00:39:42.090 align:middle line:84% And at the limit, this one when it's here, 00:39:42.090 --> 00:39:47.330 align:middle line:84% the reflection angle as [INAUDIBLE] goes to D prime. 00:39:47.330 --> 00:39:51.180 align:middle line:90% OK, so that's the image surface. 00:39:51.180 --> 00:39:54.350 align:middle line:84% This is the image method he developed. 00:39:54.350 --> 00:40:00.020 align:middle line:84% And now we can check whether this one gets to maximum or not. 00:40:00.020 --> 00:40:04.010 align:middle line:84% So this is, again, simple geometry. 00:40:04.010 --> 00:40:10.490 align:middle line:84% I have AC, this section, plus A B prime-- 00:40:10.490 --> 00:40:13.670 align:middle line:84% that's the [INAUDIBLE] length, right? 00:40:13.670 --> 00:40:16.380 align:middle line:84% And the outer limit-- so this is the edge. 00:40:16.380 --> 00:40:18.800 align:middle line:84% You look at the edge, you'll need A prime 00:40:18.800 --> 00:40:23.978 align:middle line:90% to B plus B B prime, right? 00:40:23.978 --> 00:40:26.180 align:middle line:84% So this is the [INAUDIBLE] length. 00:40:26.180 --> 00:40:30.710 align:middle line:84% This is the [INAUDIBLE] length, and the same [INAUDIBLE]. 00:40:30.710 --> 00:40:32.930 align:middle line:84% So it's actually a very convenient and simple way 00:40:32.930 --> 00:40:37.850 align:middle line:84% to do the geometry, whether the surface is harder 00:40:37.850 --> 00:40:41.510 align:middle line:90% to express analytically. 00:40:41.510 --> 00:40:47.540 align:middle line:84% And you know, say, the symmetry A B prime, 00:40:47.540 --> 00:40:51.590 align:middle line:84% A prime B from symmetry, they're equal. 00:40:51.590 --> 00:40:55.880 align:middle line:84% So you get rid of two variables in the above equation, 00:40:55.880 --> 00:41:06.230 align:middle line:84% and your AC is this 1 times sine theta. 00:41:06.230 --> 00:41:12.200 align:middle line:84% So A A prime times sine theta, and your concentration then 00:41:12.200 --> 00:41:16.040 align:middle line:90% is 1 over sine theta. 00:41:16.040 --> 00:41:22.910 align:middle line:84% Remember, before is I'll say sine 2 phi over 2 sine theta. 00:41:22.910 --> 00:41:25.280 align:middle line:84% Now you got rid of that factor of 2 00:41:25.280 --> 00:41:27.680 align:middle line:90% in the two-dimensional geometry. 00:41:27.680 --> 00:41:29.870 align:middle line:90% And then you go to 3D. 00:41:29.870 --> 00:41:34.580 align:middle line:84% You actually reach 1 over sine theta square. 00:41:34.580 --> 00:41:39.590 align:middle line:84% So if you put your theta angle of acceptance, right? 00:41:39.590 --> 00:41:46.290 align:middle line:84% If you just say I only accept the solar cone, theta S, when 00:41:46.290 --> 00:41:49.830 align:middle line:84% that's your maximum, you have to accept those solar 00:41:49.830 --> 00:41:54.840 align:middle line:90% within that 32 arcsecond. 00:41:54.840 --> 00:41:56.850 align:middle line:90% So that's the maximum there. 00:41:56.850 --> 00:42:00.060 align:middle line:84% But this isn't convenient because let's say 00:42:00.060 --> 00:42:03.150 align:middle line:84% if you think about your design, you 00:42:03.150 --> 00:42:05.070 align:middle line:84% don't need that high concentration, 00:42:05.070 --> 00:42:08.610 align:middle line:84% but you want a large angle acceptance. 00:42:08.610 --> 00:42:12.000 align:middle line:84% Particularly when you have also streetlights, right? 00:42:12.000 --> 00:42:15.250 align:middle line:84% If you think about you don't always have parallel light, 00:42:15.250 --> 00:42:18.390 align:middle line:84% you also have the scattered light. 00:42:18.390 --> 00:42:21.640 align:middle line:84% And typically, people take 80/20. 00:42:21.640 --> 00:42:23.280 align:middle line:90% It depends on where you live. 00:42:23.280 --> 00:42:28.620 align:middle line:84% And so scattered light is actually approximate isotropic 00:42:28.620 --> 00:42:29.970 align:middle line:90% in all directions. 00:42:29.970 --> 00:42:35.190 align:middle line:84% So tracking, if you track only the solar part, 00:42:35.190 --> 00:42:38.940 align:middle line:84% maximum concentration, anything beyond that angle, 00:42:38.940 --> 00:42:40.890 align:middle line:90% you can't capture it. 00:42:40.890 --> 00:42:43.070 align:middle line:84% So that's an AD-- that 20% is gone. 00:42:43.070 --> 00:42:46.940 align:middle line:90% 00:42:46.940 --> 00:42:50.660 align:middle line:84% And so with this, these are the other examples. 00:42:50.660 --> 00:42:55.010 align:middle line:84% And if you read the paper, it's interesting. 00:42:55.010 --> 00:43:01.970 align:middle line:84% This is when you concentrate to cylinder using the same string 00:43:01.970 --> 00:43:02.540 align:middle line:90% rule. 00:43:02.540 --> 00:43:05.090 align:middle line:84% You can even do this, and the effect, 00:43:05.090 --> 00:43:08.540 align:middle line:84% those curves show different acceptance angle 00:43:08.540 --> 00:43:13.130 align:middle line:84% theta and the concentration ratio, 00:43:13.130 --> 00:43:16.310 align:middle line:90% and also the height of the-- 00:43:16.310 --> 00:43:18.420 align:middle line:90% height versus the aperture. 00:43:18.420 --> 00:43:24.800 align:middle line:90% So this height, aperture, right? 00:43:24.800 --> 00:43:26.990 align:middle line:84% And if you look at it, if you want 00:43:26.990 --> 00:43:31.730 align:middle line:84% 3 times concentration, that's about the 21 degrees, 00:43:31.730 --> 00:43:34.560 align:middle line:84% and the height, the aperture is about 2 to 1. 00:43:34.560 --> 00:43:37.240 align:middle line:90% 00:43:37.240 --> 00:43:42.850 align:middle line:84% And also, what's important is you can never make 00:43:42.850 --> 00:43:45.700 align:middle line:90% a perfect surface reflector. 00:43:45.700 --> 00:43:49.150 align:middle line:84% So every time a light bounce once, 00:43:49.150 --> 00:43:51.875 align:middle line:84% you lost a fraction of a light, right? 00:43:51.875 --> 00:43:55.000 align:middle line:84% So this surface is never perfectly reflecting, 00:43:55.000 --> 00:43:57.040 align:middle line:90% 100% reflecting. 00:43:57.040 --> 00:43:59.860 align:middle line:84% So you want to minimize that level of bounce 00:43:59.860 --> 00:44:04.690 align:middle line:84% because every time it bounced, it's absorbed, a fraction of it. 00:44:04.690 --> 00:44:09.130 align:middle line:84% And let's say aluminum is-- you can get about 97% reflection, 00:44:09.130 --> 00:44:10.340 align:middle line:90% but that's 3%. 00:44:10.340 --> 00:44:13.990 align:middle line:84% If you have 10 bounces, 97 10 to the power, 00:44:13.990 --> 00:44:16.330 align:middle line:84% you'll get some more numbers, right? 00:44:16.330 --> 00:44:17.610 align:middle line:90% So you want to minimize it. 00:44:17.610 --> 00:44:23.950 align:middle line:84% So this is the average number of bounces we calculated 00:44:23.950 --> 00:44:27.190 align:middle line:90% for these geometries. 00:44:27.190 --> 00:44:32.020 align:middle line:84% OK, so this is the conservation, right? 00:44:32.020 --> 00:44:34.960 align:middle line:84% Now, you concentrate it, particularly when 00:44:34.960 --> 00:44:41.200 align:middle line:84% you go to higher concentration, you have to track it 00:44:41.200 --> 00:44:44.995 align:middle line:84% because only certain angles are accepted, right? 00:44:44.995 --> 00:44:47.200 align:middle line:90% So you have to go track it. 00:44:47.200 --> 00:44:52.750 align:middle line:84% So let's think about how we would-- 00:44:52.750 --> 00:44:58.346 align:middle line:84% typically, how you should track it, right? 00:44:58.346 --> 00:45:07.280 align:middle line:84% Again, think about the Earth has this 23.5-degree tilt. 00:45:07.280 --> 00:45:11.690 align:middle line:84% And in fact, I haven't figured out all this a little bit detail 00:45:11.690 --> 00:45:13.290 align:middle line:90% in the Earth's motion yet. 00:45:13.290 --> 00:45:17.330 align:middle line:84% That tilt of 23.5 is an average number, 00:45:17.330 --> 00:45:21.320 align:middle line:84% which it turns out the Earth is actually wobble a little bit. 00:45:21.320 --> 00:45:27.740 align:middle line:84% So that wobble happens between 22 to 24.5. 00:45:27.740 --> 00:45:28.680 align:middle line:90% OK? 00:45:28.680 --> 00:45:33.860 align:middle line:84% And this will be important when you do the high concentration 00:45:33.860 --> 00:45:37.640 align:middle line:90% ratios and the acceptance angle. 00:45:37.640 --> 00:45:43.360 align:middle line:84% So if you think about the-- so that's what I listed. 00:45:43.360 --> 00:45:52.990 align:middle line:84% In the summer, Northern hemisphere, and in the winter 00:45:52.990 --> 00:45:54.100 align:middle line:90% they come from-- 00:45:54.100 --> 00:45:56.590 align:middle line:84% so in the winter, Southern hemisphere 00:45:56.590 --> 00:45:58.530 align:middle line:90% get their share of the sunlight. 00:45:58.530 --> 00:46:02.760 align:middle line:90% 00:46:02.760 --> 00:46:08.340 align:middle line:84% And now let's think about the why 00:46:08.340 --> 00:46:11.500 align:middle line:84% it's easy to think of why you need the packet. 00:46:11.500 --> 00:46:14.850 align:middle line:84% So [INAUDIBLE] let's say your radiation coming 00:46:14.850 --> 00:46:17.070 align:middle line:90% perpendicularly. 00:46:17.070 --> 00:46:21.510 align:middle line:84% And when you come at an angle, the [INAUDIBLE] energy 00:46:21.510 --> 00:46:24.360 align:middle line:90% you get is times sine theta. 00:46:24.360 --> 00:46:26.970 align:middle line:84% So you don't get all the energy you 00:46:26.970 --> 00:46:32.010 align:middle line:84% could get if you make this plane perpendicular to the ray 00:46:32.010 --> 00:46:33.332 align:middle line:90% direction, right? 00:46:33.332 --> 00:46:36.706 align:middle line:90% 00:46:36.706 --> 00:46:44.610 align:middle line:84% But see this installation not only just a sine theta change. 00:46:44.610 --> 00:46:47.190 align:middle line:84% Also, if you think about the departments 00:46:47.190 --> 00:46:52.350 align:middle line:84% through the atmosphere, different angle of incidences, 00:46:52.350 --> 00:46:56.400 align:middle line:84% the departments going through the atmosphere changes 00:46:56.400 --> 00:47:00.030 align:middle line:84% called 1 over cosine theta thickness derived cosine theta 00:47:00.030 --> 00:47:02.340 align:middle line:90% direction relation. 00:47:02.340 --> 00:47:09.510 align:middle line:90% So your insulation also changes. 00:47:09.510 --> 00:47:13.410 align:middle line:84% And if you combine all this together, 00:47:13.410 --> 00:47:18.090 align:middle line:84% those are some of the places people measure per square meter 00:47:18.090 --> 00:47:20.070 align:middle line:90% how much energy comes in. 00:47:20.070 --> 00:47:26.910 align:middle line:84% And so this is the Northern latitude, 45 degrees. 00:47:26.910 --> 00:47:30.132 align:middle line:90% Anyone remember Boston latitude? 00:47:30.132 --> 00:47:33.990 align:middle line:90% 00:47:33.990 --> 00:47:37.760 align:middle line:84% I thought I was going to check, but I forgot. 00:47:37.760 --> 00:47:38.615 align:middle line:90% OK. 00:47:38.615 --> 00:47:40.460 align:middle line:90% AUDIENCE: I think 42. 00:47:40.460 --> 00:47:43.760 align:middle line:84% GANG CHEN: So we're close around this number. 00:47:43.760 --> 00:47:48.440 align:middle line:84% You can say this, not very good solution. 00:47:48.440 --> 00:47:51.610 align:middle line:84% So my calculation was too optimistic. 00:47:51.610 --> 00:47:57.640 align:middle line:84% I need about 7 or 8 square meters for my household. 00:47:57.640 --> 00:48:01.600 align:middle line:84% OK, so which direction is your track? 00:48:01.600 --> 00:48:05.440 align:middle line:84% If you have a solar-powered or a solar panel, 00:48:05.440 --> 00:48:11.690 align:middle line:84% should you render the source loss East-West? 00:48:11.690 --> 00:48:17.110 align:middle line:90% So first, this is the East-West. 00:48:17.110 --> 00:48:19.750 align:middle line:84% This is the rotation, the angle of rotation 00:48:19.750 --> 00:48:22.240 align:middle line:90% in the East-West direction. 00:48:22.240 --> 00:48:26.470 align:middle line:84% So of course, if you do only one x tracking, 00:48:26.470 --> 00:48:28.810 align:middle line:84% you want the maximum amount of energy 00:48:28.810 --> 00:48:33.760 align:middle line:84% received be your solar cell area or your aperture. 00:48:33.760 --> 00:48:39.820 align:middle line:84% Then you want to do the axis in the south-north direction. 00:48:39.820 --> 00:48:43.570 align:middle line:90% And then your rotation is the-- 00:48:43.570 --> 00:48:47.230 align:middle line:84% so that it's the East-West direction. 00:48:47.230 --> 00:48:51.250 align:middle line:84% And then say in the other direction, 00:48:51.250 --> 00:48:57.440 align:middle line:84% you remember the Earth is the tilt during the summer 00:48:57.440 --> 00:49:00.680 align:middle line:84% and winter, that degree will go from-- 00:49:00.680 --> 00:49:05.570 align:middle line:90% go back here, right? 00:49:05.570 --> 00:49:11.120 align:middle line:84% So this angle will fix your solar cell 00:49:11.120 --> 00:49:14.630 align:middle line:90% here this on the summer. 00:49:14.630 --> 00:49:18.830 align:middle line:84% And then when you go to the winter side, 00:49:18.830 --> 00:49:22.400 align:middle line:90% this side your angle changes. 00:49:22.400 --> 00:49:28.865 align:middle line:84% So ideally, you want to track both directions, OK? 00:49:28.865 --> 00:49:36.340 align:middle line:90% 00:49:36.340 --> 00:49:43.150 align:middle line:84% There is one case people have done before that I think 00:49:43.150 --> 00:49:47.260 align:middle line:84% it's interesting where you do not need tracking, 00:49:47.260 --> 00:49:49.630 align:middle line:90% or you do seasonal adjustment. 00:49:49.630 --> 00:49:53.320 align:middle line:84% And this is when you have a low concentration 00:49:53.320 --> 00:49:56.500 align:middle line:90% and people use a weight ratio. 00:49:56.500 --> 00:49:58.990 align:middle line:90% So these are the-- 00:49:58.990 --> 00:50:01.330 align:middle line:90% you put a solar cell here. 00:50:01.330 --> 00:50:04.870 align:middle line:84% Those surfaces are reflecting surfaces. 00:50:04.870 --> 00:50:07.120 align:middle line:90% It's a wave group. 00:50:07.120 --> 00:50:15.725 align:middle line:84% And now if you read weight in the East-West direction, 00:50:15.725 --> 00:50:17.350 align:middle line:84% if you think about East-West direction, 00:50:17.350 --> 00:50:23.920 align:middle line:84% those groups in the East-West, so you will use this one 00:50:23.920 --> 00:50:26.818 align:middle line:90% means you can do light here. 00:50:26.818 --> 00:50:28.360 align:middle line:84% You don't need to put the solar cell. 00:50:28.360 --> 00:50:32.210 align:middle line:84% So you can actually increase the flux here by a factor of 2. 00:50:32.210 --> 00:50:33.100 align:middle line:90% You use less-- 00:50:33.100 --> 00:50:36.620 align:middle line:90% 2 times less of area. 00:50:36.620 --> 00:50:45.880 align:middle line:84% So typically for this geometry, this group geometry, 00:50:45.880 --> 00:50:48.820 align:middle line:84% you can get, let's say, concentration ratio 00:50:48.820 --> 00:50:51.860 align:middle line:90% is 2.5 to 3 times. 00:50:51.860 --> 00:50:56.060 align:middle line:84% So you can operate in that range, low concentration range. 00:50:56.060 --> 00:50:59.830 align:middle line:84% And if you do a little bit, what they needed here 00:50:59.830 --> 00:51:02.770 align:middle line:84% is a seasonal change because the acceptance 00:51:02.770 --> 00:51:07.650 align:middle line:84% angle in the south-north direction will change. 00:51:07.650 --> 00:51:10.345 align:middle line:90% 00:51:10.345 --> 00:51:13.540 align:middle line:84% But there are also people now developing this local. 00:51:13.540 --> 00:51:15.430 align:middle line:84% I will check in there, people actually 00:51:15.430 --> 00:51:16.970 align:middle line:84% do tracking, a little bit tracking. 00:51:16.970 --> 00:51:20.690 align:middle line:84% So there are even people doing south-north direction tracking. 00:51:20.690 --> 00:51:24.700 align:middle line:84% But in theory, so for this low concentration 00:51:24.700 --> 00:51:27.280 align:middle line:84% looks like you put it in south-west direction. 00:51:27.280 --> 00:51:30.130 align:middle line:84% You still don't capture that sine theta, 00:51:30.130 --> 00:51:32.040 align:middle line:90% the solar installation. 00:51:32.040 --> 00:51:35.830 align:middle line:84% That's still will change because your angle coming at an angle, 00:51:35.830 --> 00:51:37.340 align:middle line:90% you project area. 00:51:37.340 --> 00:51:40.550 align:middle line:84% You're not tracking perpendicular there. 00:51:40.550 --> 00:51:42.890 align:middle line:90% But you use less. 00:51:42.890 --> 00:51:46.040 align:middle line:84% In this case, if you do this concentration, 00:51:46.040 --> 00:51:49.670 align:middle line:84% you use two or three times less solar cell. 00:51:49.670 --> 00:51:50.995 align:middle line:90% So you cut the cost. 00:51:50.995 --> 00:51:55.600 align:middle line:90% 00:51:55.600 --> 00:52:04.900 align:middle line:84% OK, so this what we talked about is the concentration 00:52:04.900 --> 00:52:06.340 align:middle line:90% and tracking. 00:52:06.340 --> 00:52:15.820 align:middle line:84% And in terms of PV, People do [INAUDIBLE] concentrated PV. 00:52:15.820 --> 00:52:19.790 align:middle line:84% There's effort, there are some companies, start-up companies-- 00:52:19.790 --> 00:52:24.220 align:middle line:84% I'm not sure how successful they have been in deploying. 00:52:24.220 --> 00:52:27.490 align:middle line:84% This is particularly when you think 00:52:27.490 --> 00:52:31.630 align:middle line:84% about the high efficient solar cells, like gallium arsenide, 00:52:31.630 --> 00:52:35.410 align:middle line:84% the maximum here you can get about 40%. 00:52:35.410 --> 00:52:36.850 align:middle line:90% That's very expensive. 00:52:36.850 --> 00:52:39.520 align:middle line:84% So what you want is to use a high concentration 00:52:39.520 --> 00:52:48.670 align:middle line:84% ratio to reduce the cost per unit area, so income and cost. 00:52:48.670 --> 00:52:51.700 align:middle line:84% But see, I'm not sure how successful 00:52:51.700 --> 00:52:53.470 align:middle line:90% they are commercially. 00:52:53.470 --> 00:52:56.360 align:middle line:84% And I know one of the problems is actually 00:52:56.360 --> 00:52:58.700 align:middle line:90% the heat transfer is a problem. 00:52:58.700 --> 00:53:01.460 align:middle line:90% And the other is it's very-- 00:53:01.460 --> 00:53:05.690 align:middle line:84% I think it's very important to keep the light uniform. 00:53:05.690 --> 00:53:08.780 align:middle line:84% So if the higher the concentration 00:53:08.780 --> 00:53:12.470 align:middle line:84% you have, if you're tracking is not accurate, 00:53:12.470 --> 00:53:17.450 align:middle line:84% you light goes off your cell, then so you're screwed. 00:53:17.450 --> 00:53:22.850 align:middle line:84% So that's the tracking accuracy becomes very important. 00:53:22.850 --> 00:53:28.610 align:middle line:84% Now I'm going to shift next to the energy conversion 00:53:28.610 --> 00:53:34.460 align:middle line:84% system and the mechanical energy conversion. 00:53:34.460 --> 00:53:38.420 align:middle line:84% And the most popular in use is the trough. 00:53:38.420 --> 00:53:44.450 align:middle line:84% So the absorber is a tube, and you concentrate the light. 00:53:44.450 --> 00:53:49.730 align:middle line:84% And so this concentration could be either 00:53:49.730 --> 00:53:56.470 align:middle line:84% by parabolic mirrors or the Fresnel arrays. 00:53:56.470 --> 00:54:00.520 align:middle line:84% In this Fresnel, so you can see the mirror is individually 00:54:00.520 --> 00:54:01.370 align:middle line:90% adjusted. 00:54:01.370 --> 00:54:05.410 align:middle line:84% So this is not a one, say, parabolic, 00:54:05.410 --> 00:54:09.610 align:middle line:84% but the angle of each mirror is adjusted so that the light 00:54:09.610 --> 00:54:11.540 align:middle line:90% focuses to the pipe there. 00:54:11.540 --> 00:54:15.220 align:middle line:90% 00:54:15.220 --> 00:54:18.190 align:middle line:84% It's called the Fresnel, but I think the Fresnel-- 00:54:18.190 --> 00:54:21.520 align:middle line:84% my understanding of Fresnel [INAUDIBLE] real 00:54:21.520 --> 00:54:23.530 align:middle line:84% object that's a diffraction side. 00:54:23.530 --> 00:54:31.270 align:middle line:84% But anyway, and this is a dish where you concentrate-- 00:54:31.270 --> 00:54:33.130 align:middle line:90% you track 2D. 00:54:33.130 --> 00:54:36.970 align:middle line:84% Here, you could just do 1D daily tracking, 00:54:36.970 --> 00:54:41.770 align:middle line:84% or I'm not sure if people do a seasonal adjustment or not. 00:54:41.770 --> 00:54:48.940 align:middle line:84% And here, you need a 2D tracking is a dish structure. 00:54:48.940 --> 00:54:50.440 align:middle line:90% Or you tau. 00:54:50.440 --> 00:54:54.160 align:middle line:84% You have, say, station the mirror 00:54:54.160 --> 00:55:01.720 align:middle line:90% reflecting and to the power. 00:55:01.720 --> 00:55:05.710 align:middle line:84% And trough is the most common use 00:55:05.710 --> 00:55:09.430 align:middle line:84% to look at the more cost structure where 00:55:09.430 --> 00:55:11.440 align:middle line:90% you have a parabolic mirror. 00:55:11.440 --> 00:55:15.470 align:middle line:84% And you have the pipe inside in the focal plant. 00:55:15.470 --> 00:55:18.430 align:middle line:90% 00:55:18.430 --> 00:55:25.820 align:middle line:84% And this is a typical pipe, the solar collector pipe. 00:55:25.820 --> 00:55:27.220 align:middle line:90% Again, it's a vacuum. 00:55:27.220 --> 00:55:33.220 align:middle line:84% So the vacuum is a pretty high vacuum here, 10 to the minus 3 00:55:33.220 --> 00:55:35.634 align:middle line:90% millibar, right? 00:55:35.634 --> 00:55:40.390 align:middle line:84% And you can imagine the challenge there 00:55:40.390 --> 00:55:43.930 align:middle line:84% because the pipe itself is a steel pipe. 00:55:43.930 --> 00:55:45.190 align:middle line:90% Steel pipe. 00:55:45.190 --> 00:55:49.640 align:middle line:84% And then outside is coated with selective absorber. 00:55:49.640 --> 00:55:57.380 align:middle line:84% So typically, you can see the absorptance is larger than 95. 00:55:57.380 --> 00:56:03.140 align:middle line:84% Emittance is less than 14% at 400 degrees C. 00:56:03.140 --> 00:56:07.670 align:middle line:84% And the outer clear glass is also 00:56:07.670 --> 00:56:13.790 align:middle line:84% coated so that it's under reflection coating. 00:56:13.790 --> 00:56:18.620 align:middle line:84% So the typical glass transmission is about 93%. 00:56:18.620 --> 00:56:21.140 align:middle line:90% I'll get to 96%. 00:56:21.140 --> 00:56:28.760 align:middle line:84% And the watts-- if you look at this, what's important, 00:56:28.760 --> 00:56:31.430 align:middle line:90% how you make the seal, right? 00:56:31.430 --> 00:56:33.950 align:middle line:84% High-temperature seal is really-- 00:56:33.950 --> 00:56:37.820 align:middle line:84% so you have a tube, 100-meter long, 00:56:37.820 --> 00:56:41.425 align:middle line:90% and thermal expansion and seal. 00:56:41.425 --> 00:56:44.040 align:middle line:84% Those are the really critical technologies. 00:56:44.040 --> 00:56:47.260 align:middle line:90% 00:56:47.260 --> 00:56:51.910 align:middle line:84% So tubular trough plant, you've got collectors. 00:56:51.910 --> 00:56:59.110 align:middle line:84% You have the steam turbine, and you also have-- 00:56:59.110 --> 00:57:01.850 align:middle line:84% very often, you have to have the gas fired. 00:57:01.850 --> 00:57:03.920 align:middle line:90% So you have to do cogeneration. 00:57:03.920 --> 00:57:07.750 align:middle line:90% What do you do in the light? 00:57:07.750 --> 00:57:12.580 align:middle line:84% Or what happens if you have clouds come in? 00:57:12.580 --> 00:57:17.650 align:middle line:84% That's actually a big headache there, particularly for PV. 00:57:17.650 --> 00:57:19.450 align:middle line:84% Solar thermal actually slightly better 00:57:19.450 --> 00:57:24.060 align:middle line:84% because the thermal inertia, thermal heat capacity damping 00:57:24.060 --> 00:57:24.560 align:middle line:90% them. 00:57:24.560 --> 00:57:28.360 align:middle line:84% That's one of the advantage that solar thermal has. 00:57:28.360 --> 00:57:34.480 align:middle line:84% And you could also do some storage, 00:57:34.480 --> 00:57:38.530 align:middle line:84% I'm not sure here as storage, but you could, 00:57:38.530 --> 00:57:41.110 align:middle line:90% you could do storage. 00:57:41.110 --> 00:57:44.860 align:middle line:84% And the solar thermal plant here, the fluid 00:57:44.860 --> 00:57:48.290 align:middle line:84% itself, you can see there's a closed loop. 00:57:48.290 --> 00:57:51.350 align:middle line:84% The fluid through the pipe is closed. 00:57:51.350 --> 00:57:53.990 align:middle line:84% Or steam generation, steam turbine, 00:57:53.990 --> 00:57:56.600 align:middle line:84% similar if the water-based steam turbine. 00:57:56.600 --> 00:58:00.770 align:middle line:84% So this is the outer loop, the steam loop here. 00:58:00.770 --> 00:58:06.110 align:middle line:84% And in fact, in the pipe, they use oil. 00:58:06.110 --> 00:58:08.190 align:middle line:90% And the oil is one problem. 00:58:08.190 --> 00:58:09.750 align:middle line:84% You can't go to too high temperature. 00:58:09.750 --> 00:58:12.920 align:middle line:84% You go to too high temperature, you decompose. 00:58:12.920 --> 00:58:17.830 align:middle line:90% And so the oil has a stability. 00:58:17.830 --> 00:58:21.680 align:middle line:84% This is a steel area where people are doing research, 00:58:21.680 --> 00:58:25.010 align:middle line:84% stability problem, and how you can develop a higher temperature 00:58:25.010 --> 00:58:26.120 align:middle line:90% oils. 00:58:26.120 --> 00:58:28.430 align:middle line:84% You can get to higher temperature, 00:58:28.430 --> 00:58:30.140 align:middle line:90% but your oil will not unknown. 00:58:30.140 --> 00:58:33.150 align:middle line:90% 00:58:33.150 --> 00:58:37.850 align:middle line:84% OK, and this is another just a picture here. 00:58:37.850 --> 00:58:45.270 align:middle line:84% If you look at it, they typically installed 00:58:45.270 --> 00:58:51.510 align:middle line:84% and typical size of the plant is around 100 megawatts. 00:58:51.510 --> 00:58:56.070 align:middle line:84% 100 megawatts is small in terms of if you compare it 00:58:56.070 --> 00:58:58.110 align:middle line:90% with the coal-fired. 00:58:58.110 --> 00:59:03.497 align:middle line:84% Coal-fired plant, typically one steam turbine is one gigawatt. 00:59:03.497 --> 00:59:03.997 align:middle line:90% OK. 00:59:03.997 --> 00:59:07.990 align:middle line:90% 00:59:07.990 --> 00:59:10.480 align:middle line:90% And this is too small. 00:59:10.480 --> 00:59:13.240 align:middle line:90% You can read your handout. 00:59:13.240 --> 00:59:17.470 align:middle line:84% So those are examples, and what I want to point out here 00:59:17.470 --> 00:59:19.780 align:middle line:90% are the concentration here. 00:59:19.780 --> 00:59:22.360 align:middle line:84% It's typically, those concentration 00:59:22.360 --> 00:59:26.920 align:middle line:90% is between 60% to 80% Stirling. 00:59:26.920 --> 00:59:32.320 align:middle line:90% That's the concentration ratio. 00:59:32.320 --> 00:59:37.600 align:middle line:84% And they look at the cost of the solar thermal, that's 00:59:37.600 --> 00:59:44.170 align:middle line:84% the one case where different generations, 30 megawatts. 00:59:44.170 --> 00:59:47.980 align:middle line:84% This is a US dollar megawatt hour. 00:59:47.980 --> 00:59:51.650 align:middle line:84% So if you do kilowatt hour, you divide it by 1,000. 00:59:51.650 --> 00:59:57.730 align:middle line:84% So that's about $0.19 per kilowatt hour. 00:59:57.730 --> 01:00:01.890 align:middle line:90% Coal fired is about $0.05. 01:00:01.890 --> 01:00:04.200 align:middle line:90% power plant. 01:00:04.200 --> 01:00:08.460 align:middle line:84% So here is the $0.16, and there's some projection to scale 01:00:08.460 --> 01:00:16.380 align:middle line:90% up that will go down to $0.10. 01:00:16.380 --> 01:00:19.650 align:middle line:84% The efficiency, again, I'm not expecting to read it. 01:00:19.650 --> 01:00:25.770 align:middle line:84% Just let me just quoted the fuel efficiency numbers. 01:00:25.770 --> 01:00:34.140 align:middle line:84% The collector efficiency here is 50%, receiver efficiency, 82%, 01:00:34.140 --> 01:00:40.110 align:middle line:84% and the cycle, that's a steam turbine cycle efficiency here, 01:00:40.110 --> 01:00:42.330 align:middle line:90% 45%, right? 01:00:42.330 --> 01:00:45.690 align:middle line:90% So right now, it's a 38%. 01:00:45.690 --> 01:00:49.440 align:middle line:84% So at the end, if you combine all this efficiency here, 01:00:49.440 --> 01:00:52.865 align:middle line:90% this is about between 13 to 17%. 01:00:52.865 --> 01:00:55.390 align:middle line:90% 01:00:55.390 --> 01:00:59.770 align:middle line:84% The collection efficiency is not good. 01:00:59.770 --> 01:01:04.690 align:middle line:84% Collector efficiency, here it's only about 50%. 01:01:04.690 --> 01:01:09.190 align:middle line:84% You lost a lot of light in the collection. 01:01:09.190 --> 01:01:13.990 align:middle line:84% So there, you want to minimize the number of reflection. 01:01:13.990 --> 01:01:16.780 align:middle line:84% The other problem is your street light. 01:01:16.780 --> 01:01:18.730 align:middle line:84% If you design a high concentration, 01:01:18.730 --> 01:01:22.160 align:middle line:84% if it's not parallel, you don't fall into the focal point. 01:01:22.160 --> 01:01:27.090 align:middle line:90% 01:01:27.090 --> 01:01:29.850 align:middle line:90% Interesting to look at the cost. 01:01:29.850 --> 01:01:37.750 align:middle line:84% The cost here, if you look at it here, we have 58. 01:01:37.750 --> 01:01:40.290 align:middle line:84% That's the collection system, 58. 01:01:40.290 --> 01:01:48.990 align:middle line:84% And structure, this one does not include the engine. 01:01:48.990 --> 01:01:51.700 align:middle line:90% So you have the storage. 01:01:51.700 --> 01:01:54.570 align:middle line:90% Thermal storage is here. 01:01:54.570 --> 01:02:00.930 align:middle line:84% And we have 14% is the power block. 01:02:00.930 --> 01:02:05.100 align:middle line:84% I do not know what exactly is that. 01:02:05.100 --> 01:02:06.060 align:middle line:90% OK. 01:02:06.060 --> 01:02:10.690 align:middle line:90% And now here is the-- 01:02:10.690 --> 01:02:13.560 align:middle line:84% so this is the parabolic power plant. 01:02:13.560 --> 01:02:20.490 align:middle line:84% And again, this is a breakdown different materials, 01:02:20.490 --> 01:02:23.400 align:middle line:90% the cost of structure. 01:02:23.400 --> 01:02:28.840 align:middle line:90% And so that was kind of fun. 01:02:28.840 --> 01:02:30.350 align:middle line:90% I thought I saw it before. 01:02:30.350 --> 01:02:34.750 align:middle line:90% Where is the cycle? 01:02:34.750 --> 01:02:36.205 align:middle line:90% AUDIENCE: [INAUDIBLE] 01:02:36.205 --> 01:02:36.830 align:middle line:90% GANG CHEN: Huh? 01:02:36.830 --> 01:02:39.190 align:middle line:90% AUDIENCE: [INAUDIBLE] 01:02:39.190 --> 01:02:39.940 align:middle line:90% GANG CHEN: Yeah. 01:02:39.940 --> 01:02:40.440 align:middle line:90% OK. 01:02:40.440 --> 01:02:42.110 align:middle line:90% So that's where it is. 01:02:42.110 --> 01:02:42.820 align:middle line:90% Yeah. 01:02:42.820 --> 01:02:47.040 align:middle line:90% So the cycle itself is not the-- 01:02:47.040 --> 01:02:51.510 align:middle line:84% AUDIENCE: Bottom one is just the soil so it's just so. 01:02:51.510 --> 01:02:52.900 align:middle line:90% GANG CHEN: Yeah, this is-- 01:02:52.900 --> 01:02:57.040 align:middle line:84% yeah, this is-- so the first one is the overall. 01:02:57.040 --> 01:03:02.330 align:middle line:84% The bottom one is the field component breakdown. 01:03:02.330 --> 01:03:06.890 align:middle line:84% So the cycle itself is not a big part. 01:03:06.890 --> 01:03:10.250 align:middle line:90% The collector-- look at-- 01:03:10.250 --> 01:03:11.690 align:middle line:90% I think it was interesting. 01:03:11.690 --> 01:03:13.460 align:middle line:90% Look at this here. 01:03:13.460 --> 01:03:15.700 align:middle line:90% Metal support structure, 29%. 01:03:15.700 --> 01:03:18.670 align:middle line:90% 01:03:18.670 --> 01:03:19.590 align:middle line:90% All right. 01:03:19.590 --> 01:03:21.490 align:middle line:90% This is a big stuff here. 01:03:21.490 --> 01:03:24.870 align:middle line:90% The steel used a lot of steel. 01:03:24.870 --> 01:03:27.940 align:middle line:90% And receiver, 20%. 01:03:27.940 --> 01:03:30.330 align:middle line:90% Mineral, [INAUDIBLE] 19%. 01:03:30.330 --> 01:03:32.140 align:middle line:90% Mass support is the big one. 01:03:32.140 --> 01:03:35.690 align:middle line:90% 01:03:35.690 --> 01:03:38.190 align:middle line:84% So if you can cut that cost, that's good. 01:03:38.190 --> 01:03:40.060 align:middle line:90% AUDIENCE: Where's the engine? 01:03:40.060 --> 01:03:41.310 align:middle line:90% GANG CHEN: The engine is here. 01:03:41.310 --> 01:03:43.880 align:middle line:90% 01:03:43.880 --> 01:03:52.400 align:middle line:84% 14, that's nothing compared to the solar side. 01:03:52.400 --> 01:03:53.660 align:middle line:90% OK. 01:03:53.660 --> 01:03:59.510 align:middle line:84% So if you were in Jacob Carney's talk a few weeks ago, 01:03:59.510 --> 01:04:03.870 align:middle line:84% he gave a mature-- he does a lot of the [INAUDIBLE]. 01:04:03.870 --> 01:04:09.230 align:middle line:84% So you have mirrors tracking inside each one 01:04:09.230 --> 01:04:13.430 align:middle line:84% and reflect to the power, the center there. 01:04:13.430 --> 01:04:21.470 align:middle line:84% So it's like a big array of mirrors reflecting the light. 01:04:21.470 --> 01:04:24.320 align:middle line:84% So there you have the solar tower. 01:04:24.320 --> 01:04:28.190 align:middle line:84% And what's interesting is that this 01:04:28.190 --> 01:04:34.670 align:middle line:84% is what I learned from Jacob Carney is that the finger size-- 01:04:34.670 --> 01:04:36.830 align:middle line:84% and you put a question mark, but I 01:04:36.830 --> 01:04:40.130 align:middle line:84% think the [INAUDIBLE] size is actually smaller than 12. 01:04:40.130 --> 01:04:43.410 align:middle line:84% The tower, you don't want to build too big. 01:04:43.410 --> 01:04:45.138 align:middle line:90% OK? 01:04:45.138 --> 01:04:48.010 align:middle line:90% And a lot of-- 01:04:48.010 --> 01:04:49.890 align:middle line:84% but the temperature is much higher. 01:04:49.890 --> 01:04:54.120 align:middle line:84% So the [INAUDIBLE] side there, efficiency should be higher. 01:04:54.120 --> 01:04:58.760 align:middle line:90% And the problem is the focus. 01:04:58.760 --> 01:05:02.190 align:middle line:84% And he showed some very interesting pictures. 01:05:02.190 --> 01:05:03.930 align:middle line:84% You don't always hit your target. 01:05:03.930 --> 01:05:05.940 align:middle line:90% You have a lot of missing light. 01:05:05.940 --> 01:05:10.260 align:middle line:84% So if you look at around the focal point, 01:05:10.260 --> 01:05:14.880 align:middle line:84% you see a lot of white light that missed the scatter. 01:05:14.880 --> 01:05:21.210 align:middle line:84% And so this is the absorber, two different absorber 01:05:21.210 --> 01:05:23.820 align:middle line:90% ways to absorb the radiation. 01:05:23.820 --> 01:05:27.910 align:middle line:90% And what in this cases-- 01:05:27.910 --> 01:05:32.190 align:middle line:84% so I showed I think the last lecture, 01:05:32.190 --> 01:05:34.650 align:middle line:84% once you get a higher concentration, 01:05:34.650 --> 01:05:38.340 align:middle line:84% your absorptivity is more important. 01:05:38.340 --> 01:05:41.940 align:middle line:84% The emissivity is less important. 01:05:41.940 --> 01:05:46.230 align:middle line:90% So I think those are pretty-- 01:05:46.230 --> 01:05:49.063 align:middle line:84% it's not a really spectral control surfaces. 01:05:49.063 --> 01:05:53.970 align:middle line:90% 01:05:53.970 --> 01:05:55.080 align:middle line:90% There are really not-- 01:05:55.080 --> 01:05:58.200 align:middle line:84% I think there are only one or two [INAUDIBLE] probably has. 01:05:58.200 --> 01:06:01.350 align:middle line:84% But in the US, there are the [INAUDIBLE] junction mostly 01:06:01.350 --> 01:06:06.000 align:middle line:84% is 12, but it does have a tower system. 01:06:06.000 --> 01:06:11.250 align:middle line:84% And so here, it gives you in terms of the cost, 01:06:11.250 --> 01:06:14.880 align:middle line:90% they feel they sell 40%. 01:06:14.880 --> 01:06:23.430 align:middle line:84% Power is 14%, p2p storage, 11%, and the engine is 32. 01:06:23.430 --> 01:06:32.880 align:middle line:84% And so here is a structure, reflector. 01:06:32.880 --> 01:06:37.230 align:middle line:90% Structure, 10%, reflector, 36%. 01:06:37.230 --> 01:06:43.540 align:middle line:84% Drive for the tracking is 30%, and the foundation. 01:06:43.540 --> 01:06:45.755 align:middle line:84% So that's the tracking system itself. 01:06:45.755 --> 01:06:50.580 align:middle line:90% 01:06:50.580 --> 01:06:51.570 align:middle line:90% OK. 01:06:51.570 --> 01:06:56.850 align:middle line:84% The car efficiency-- this is from the number here-- 01:06:56.850 --> 01:06:59.493 align:middle line:90% is not much better than the-- 01:06:59.493 --> 01:07:02.010 align:middle line:90% 01:07:02.010 --> 01:07:04.290 align:middle line:90% this one is not power, I think. 01:07:04.290 --> 01:07:05.250 align:middle line:90% Sorry for that. 01:07:05.250 --> 01:07:06.580 align:middle line:90% This one is not power. 01:07:06.580 --> 01:07:07.830 align:middle line:90% This is a steel car. 01:07:07.830 --> 01:07:12.030 align:middle line:84% So that was going to delete this one. 01:07:12.030 --> 01:07:19.710 align:middle line:84% OK, the other form of the generator is a dish. 01:07:19.710 --> 01:07:22.740 align:middle line:90% So dish, you make smaller. 01:07:22.740 --> 01:07:25.830 align:middle line:90% This one is, say, tracking. 01:07:25.830 --> 01:07:30.520 align:middle line:84% And the problem is the generator here. 01:07:30.520 --> 01:07:34.620 align:middle line:84% So you have how you can make a small power generator. 01:07:34.620 --> 01:07:39.090 align:middle line:84% And here, it is a Stirling mechanical system, so 01:07:39.090 --> 01:07:40.800 align:middle line:90% Stirling engine. 01:07:40.800 --> 01:07:43.740 align:middle line:84% And if you're a mechanical engineering student here, 01:07:43.740 --> 01:07:46.410 align:middle line:84% the second year, they always do the Stirling engine. 01:07:46.410 --> 01:07:51.120 align:middle line:84% It's a burner Bunsen based Stirling engine. 01:07:51.120 --> 01:07:54.210 align:middle line:84% So the Stirling engine is very different 01:07:54.210 --> 01:07:57.600 align:middle line:84% from an internal combustion engine that's used in the car. 01:07:57.600 --> 01:08:03.720 align:middle line:84% You can heat up one side of the Stirling engine 01:08:03.720 --> 01:08:06.930 align:middle line:84% from external-- so external heating. 01:08:06.930 --> 01:08:09.720 align:middle line:84% And then you generate the mechanical motion 01:08:09.720 --> 01:08:12.120 align:middle line:90% to generate power. 01:08:12.120 --> 01:08:15.540 align:middle line:84% The cost is much higher, I learned. 01:08:15.540 --> 01:08:19.660 align:middle line:84% So I do not know anybody who is using this. 01:08:19.660 --> 01:08:22.020 align:middle line:90% It's not mature. 01:08:22.020 --> 01:08:23.539 align:middle line:90% The most mature is the 12th. 01:08:23.539 --> 01:08:29.181 align:middle line:90% 01:08:29.181 --> 01:08:36.170 align:middle line:84% And so we have an efficiency number here. 01:08:36.170 --> 01:08:42.960 align:middle line:90% 01:08:42.960 --> 01:08:48.779 align:middle line:90% Right here is the efficiency. 01:08:48.779 --> 01:08:52.979 align:middle line:90% This efficiency is the-- 01:08:52.979 --> 01:08:54.660 align:middle line:90% I thought I would try this. 01:08:54.660 --> 01:08:55.380 align:middle line:90% I don't know. 01:08:55.380 --> 01:08:57.370 align:middle line:90% It's a pretty high efficiency. 01:08:57.370 --> 01:09:00.569 align:middle line:90% 01:09:00.569 --> 01:09:01.790 align:middle line:90% I see. 01:09:01.790 --> 01:09:06.910 align:middle line:84% But I can't believe this number is 29%, 30%, 39%. 01:09:06.910 --> 01:09:12.220 align:middle line:90% 01:09:12.220 --> 01:09:15.430 align:middle line:90% And so this is the Stirling. 01:09:15.430 --> 01:09:19.930 align:middle line:84% And that's all I have in terms of the thermal mechanical 01:09:19.930 --> 01:09:22.000 align:middle line:90% systems. 01:09:22.000 --> 01:09:23.000 align:middle line:90% Any questions? 01:09:23.000 --> 01:09:27.490 align:middle line:90% 01:09:27.490 --> 01:09:28.850 align:middle line:90% Now I will go back. 01:09:28.850 --> 01:09:34.270 align:middle line:84% So this is a big stuff, and I'll go back to a simple-- 01:09:34.270 --> 01:09:35.439 align:middle line:90% question? 01:09:35.439 --> 01:09:38.090 align:middle line:84% AUDIENCE: Are these technologies too immature to talk about 01:09:38.090 --> 01:09:41.529 align:middle line:90% in terms of [INAUDIBLE] 01:09:41.529 --> 01:09:43.750 align:middle line:84% GANG CHEN: I think the last two-- 01:09:43.750 --> 01:09:45.550 align:middle line:84% well, at least the Stirling, I don't 01:09:45.550 --> 01:09:48.640 align:middle line:84% think it's mature enough to talk about dollar per watt. 01:09:48.640 --> 01:09:51.077 align:middle line:90% And I don't have exact number. 01:09:51.077 --> 01:09:52.160 align:middle line:90% I asked the people around. 01:09:52.160 --> 01:09:56.980 align:middle line:84% This is 10 times more expensive than 12. 01:09:56.980 --> 01:10:04.540 align:middle line:84% But the heliostat, I have not looked at how many deployment-- 01:10:04.540 --> 01:10:09.910 align:middle line:84% I see there was one running together with I think the Kramer 01:10:09.910 --> 01:10:11.475 align:middle line:90% junction, the 12th system. 01:10:11.475 --> 01:10:16.780 align:middle line:90% 01:10:16.780 --> 01:10:20.470 align:middle line:84% So what I want to do in the rest of the time now 01:10:20.470 --> 01:10:24.310 align:middle line:84% is coming back to-- we have too many simple [INAUDIBLE] stuff 01:10:24.310 --> 01:10:24.860 align:middle line:90% now. 01:10:24.860 --> 01:10:28.870 align:middle line:84% Now I want to come back to a little bit more math. 01:10:28.870 --> 01:10:33.160 align:middle line:84% And the reason we want to do next is, What are you going 01:10:33.160 --> 01:10:35.770 align:middle line:90% to say is a selective surface? 01:10:35.770 --> 01:10:40.780 align:middle line:84% And it's important to have the-- in the case of concentrating 01:10:40.780 --> 01:10:44.130 align:middle line:84% parabolic mirror, it's important to have less reflection. 01:10:44.130 --> 01:10:47.970 align:middle line:84% Each reflection, you lose very small amount of energy. 01:10:47.970 --> 01:10:49.750 align:middle line:90% So you want a high reflectivity. 01:10:49.750 --> 01:10:52.760 align:middle line:84% You want a high transmissivity going through glass. 01:10:52.760 --> 01:10:55.960 align:middle line:84% And once it gets onto the surface, 01:10:55.960 --> 01:10:57.770 align:middle line:90% you absorb solar radiation. 01:10:57.770 --> 01:11:00.700 align:middle line:84% You want to spectrally absorb the solar radiation, 01:11:00.700 --> 01:11:02.350 align:middle line:90% but do not reradiate. 01:11:02.350 --> 01:11:05.260 align:middle line:84% So now this is in terms of technology, 01:11:05.260 --> 01:11:07.120 align:middle line:90% some of the key technologies. 01:11:07.120 --> 01:11:09.010 align:middle line:84% And for those technology now you have 01:11:09.010 --> 01:11:14.320 align:middle line:84% to come back to the basics, how you can understand what people 01:11:14.320 --> 01:11:16.550 align:middle line:90% have and develop further. 01:11:16.550 --> 01:11:19.950 align:middle line:84% So the basics is maximally efficient. 01:11:19.950 --> 01:11:24.950 align:middle line:84% And I'm not going to spend much time on the Maxwell equation. 01:11:24.950 --> 01:11:29.750 align:middle line:84% If you want to really learn, you should take a course. 01:11:29.750 --> 01:11:32.690 align:middle line:84% But you all heard about Maxwell equations. 01:11:32.690 --> 01:11:36.200 align:middle line:84% It's a set of four equations for electric field, 01:11:36.200 --> 01:11:39.860 align:middle line:84% for magnetic field edge, and displacement 01:11:39.860 --> 01:11:42.370 align:middle line:90% and magnetic induction. 01:11:42.370 --> 01:11:43.280 align:middle line:90% Right? 01:11:43.280 --> 01:11:47.450 align:middle line:84% So I gave you the definitions of those symbols-- 01:11:47.450 --> 01:11:50.180 align:middle line:84% electric field, magnetic field, displacement. 01:11:50.180 --> 01:11:52.610 align:middle line:90% And j is the current. 01:11:52.610 --> 01:11:55.910 align:middle line:84% And they are considered relation related displacement, 01:11:55.910 --> 01:11:59.870 align:middle line:84% electric field, magnetic induction, magnetic field. 01:11:59.870 --> 01:12:04.130 align:middle line:84% And those constants, the material properties, 01:12:04.130 --> 01:12:08.090 align:middle line:84% electric permittivity, magnetic permeability. 01:12:08.090 --> 01:12:10.710 align:middle line:90% Yeah, Maxwell equations. 01:12:10.710 --> 01:12:13.720 align:middle line:90% 01:12:13.720 --> 01:12:16.930 align:middle line:90% And it's a wave equation. 01:12:16.930 --> 01:12:19.780 align:middle line:84% It's describing electromagnetic waves. 01:12:19.780 --> 01:12:22.960 align:middle line:84% So if you go to solve those waves, 01:12:22.960 --> 01:12:26.980 align:middle line:84% and because that's important, what's important-- say, 01:12:26.980 --> 01:12:30.490 align:middle line:84% when you do the particularly selective surface, 01:12:30.490 --> 01:12:32.320 align:middle line:84% very often, it turns out they are 01:12:32.320 --> 01:12:37.930 align:middle line:84% based on controlling the phase of those waves. 01:12:37.930 --> 01:12:41.920 align:middle line:84% So if you look at a simple solution, 01:12:41.920 --> 01:12:46.600 align:middle line:84% a wave has a direction of propagation with vector. 01:12:46.600 --> 01:12:49.390 align:middle line:84% And it's an electromagnetic field. 01:12:49.390 --> 01:12:51.310 align:middle line:84% So electric field, magnetic field, 01:12:51.310 --> 01:12:53.500 align:middle line:84% turns out the solution tells you they are 01:12:53.500 --> 01:12:55.450 align:middle line:90% perpendicular to each other. 01:12:55.450 --> 01:12:58.120 align:middle line:84% And in fact, they form a right-hand rule. 01:12:58.120 --> 01:13:01.900 align:middle line:84% So you go from the electric field to magnetic field 01:13:01.900 --> 01:13:06.730 align:middle line:84% to some point to the propagation direction. 01:13:06.730 --> 01:13:14.320 align:middle line:84% And to describe a wave, we have wavelengths, and here in time. 01:13:14.320 --> 01:13:16.780 align:middle line:84% Here, the inverse is the frequency. 01:13:16.780 --> 01:13:23.050 align:middle line:84% Angular frequency is 2 pi times period, say, 2 pi times 01:13:23.050 --> 01:13:25.000 align:middle line:90% frequency. 01:13:25.000 --> 01:13:29.260 align:middle line:84% Wave vector has a direction, but it also has the-- 01:13:29.260 --> 01:13:31.240 align:middle line:90% it's numerous wavelengths. 01:13:31.240 --> 01:13:34.090 align:middle line:90% So wave number is 1 over lambda. 01:13:34.090 --> 01:13:36.940 align:middle line:84% 2 pi over wave number gives you the magnitude 01:13:36.940 --> 01:13:38.560 align:middle line:90% of the wave vector. 01:13:38.560 --> 01:13:41.320 align:middle line:84% And the direction of the wave vector 01:13:41.320 --> 01:13:43.840 align:middle line:90% is the unitary vector here. 01:13:43.840 --> 01:13:49.690 align:middle line:84% And then if you solve the equation, turns out at the end 01:13:49.690 --> 01:13:56.230 align:middle line:84% that epsilon and mu combined give you the refractive index 01:13:56.230 --> 01:13:57.120 align:middle line:90% of the material. 01:13:57.120 --> 01:13:58.420 align:middle line:90% This is a property. 01:13:58.420 --> 01:14:01.420 align:middle line:84% You can look at different materials many times people have 01:14:01.420 --> 01:14:03.010 align:middle line:90% measured. 01:14:03.010 --> 01:14:07.090 align:middle line:84% Sometimes it's called the optical constant, 01:14:07.090 --> 01:14:08.740 align:middle line:90% but it's not a constant. 01:14:08.740 --> 01:14:12.250 align:middle line:84% It's a frequency-dependent quantity. 01:14:12.250 --> 01:14:14.320 align:middle line:84% So you have the real part and your measured. 01:14:14.320 --> 01:14:18.420 align:middle line:90% 01:14:18.420 --> 01:14:22.350 align:middle line:84% And since it's a wave, so the planned wave, 01:14:22.350 --> 01:14:26.070 align:middle line:84% simple solution of this wave is the electric field 01:14:26.070 --> 01:14:29.110 align:middle line:90% is a function of location time. 01:14:29.110 --> 01:14:30.790 align:middle line:90% This is the direction. 01:14:30.790 --> 01:14:35.046 align:middle line:84% It's a vector, direction of the field, right? 01:14:35.046 --> 01:14:43.320 align:middle line:84% And the wave vector frequency, complex refractive index. 01:14:43.320 --> 01:14:45.860 align:middle line:84% So that's your solution for electric field, 01:14:45.860 --> 01:14:47.760 align:middle line:90% similarly for magnetic field. 01:14:47.760 --> 01:14:53.480 align:middle line:84% And these two, e and h, perpendicular to each other. 01:14:53.480 --> 01:14:55.080 align:middle line:84% And so that's your plan with solution. 01:14:55.080 --> 01:14:57.742 align:middle line:84% This one, you can substitute back into Maxwell equation. 01:14:57.742 --> 01:14:59.450 align:middle line:84% You find this satisfies Maxwell equation. 01:14:59.450 --> 01:15:02.980 align:middle line:90% 01:15:02.980 --> 01:15:04.410 align:middle line:90% So this is the field. 01:15:04.410 --> 01:15:05.420 align:middle line:90% What's the energy? 01:15:05.420 --> 01:15:08.650 align:middle line:84% Energy is the product of e and h. 01:15:08.650 --> 01:15:11.140 align:middle line:84% And particularly, this time area average. 01:15:11.140 --> 01:15:14.050 align:middle line:84% This is time dependent, but to do time average, 01:15:14.050 --> 01:15:17.920 align:middle line:84% you can simply calculate the e cross h complex conjugate. 01:15:17.920 --> 01:15:21.680 align:middle line:84% If you do complex conjugate, you see this i omega t cancel. 01:15:21.680 --> 01:15:24.130 align:middle line:90% There's no time there. 01:15:24.130 --> 01:15:27.370 align:middle line:90% So that's your Poynting vector. 01:15:27.370 --> 01:15:32.050 align:middle line:90% And if I do that, I will get-- 01:15:32.050 --> 01:15:35.620 align:middle line:84% for propagation in a homogeneous medium, 01:15:35.620 --> 01:15:38.740 align:middle line:84% the Poynting vector is the real part 01:15:38.740 --> 01:15:42.280 align:middle line:84% of the refractive index [INAUDIBLE] 01:15:42.280 --> 01:15:44.950 align:middle line:90% exponential decay function. 01:15:44.950 --> 01:15:48.340 align:middle line:84% So this exponential decay, that's 01:15:48.340 --> 01:15:50.890 align:middle line:84% the problem silicon we said, right? 01:15:50.890 --> 01:15:54.790 align:middle line:84% Silicon photovoltaics takes the alpha is a problem. 01:15:54.790 --> 01:15:56.960 align:middle line:90% It's too small. 01:15:56.960 --> 01:16:00.160 align:middle line:84% We have to use a very thick one to absorb. 01:16:00.160 --> 01:16:05.210 align:middle line:84% And this alpha is the absorption coefficient we mentioned before, 01:16:05.210 --> 01:16:09.590 align:middle line:84% and that's related to the major part of the complex refractive 01:16:09.590 --> 01:16:11.550 align:middle line:90% index [INAUDIBLE]. 01:16:11.550 --> 01:16:14.250 align:middle line:90% 01:16:14.250 --> 01:16:15.078 align:middle line:90% OK. 01:16:15.078 --> 01:16:21.720 align:middle line:84% So this is the, see, [INAUDIBLE] inside the one medium. 01:16:21.720 --> 01:16:25.680 align:middle line:84% Now, what I can control is really 01:16:25.680 --> 01:16:27.420 align:middle line:90% with the interface to control. 01:16:27.420 --> 01:16:29.490 align:middle line:90% So you've got the interfaces. 01:16:29.490 --> 01:16:35.100 align:middle line:84% When you have light comes in, reflection, refraction, right? 01:16:35.100 --> 01:16:37.950 align:middle line:84% High school, you will learn that. 01:16:37.950 --> 01:16:44.190 align:middle line:84% And because the light can actually have two-- 01:16:44.190 --> 01:16:47.160 align:middle line:84% see, you decompose it into two directions, 01:16:47.160 --> 01:16:50.220 align:middle line:84% it's a transverse wave which become the two directions. 01:16:50.220 --> 01:16:54.780 align:middle line:84% One is your electric field is independent of incidence. 01:16:54.780 --> 01:16:59.020 align:middle line:84% So direction of coming in norm form the plane of incidence, 01:16:59.020 --> 01:16:59.700 align:middle line:90% right? 01:16:59.700 --> 01:17:04.470 align:middle line:84% So if your electric field is within the plane 01:17:04.470 --> 01:17:08.910 align:middle line:84% or your electric field is in the other direction. 01:17:08.910 --> 01:17:13.570 align:middle line:90% So we have the electric field. 01:17:13.570 --> 01:17:16.660 align:middle line:84% If it's in the plane of incidence, 01:17:16.660 --> 01:17:20.530 align:middle line:84% the magnetic field is perpendicular, right? 01:17:20.530 --> 01:17:22.520 align:middle line:84% It's always perpendicular to electric field. 01:17:22.520 --> 01:17:25.150 align:middle line:84% So we'll be in the perpendicular plane. 01:17:25.150 --> 01:17:30.100 align:middle line:84% So in this case, we have transverse magnetic wave 01:17:30.100 --> 01:17:32.290 align:middle line:90% or parallel. 01:17:32.290 --> 01:17:35.710 align:middle line:84% That's for electric field, different lengths. 01:17:35.710 --> 01:17:38.410 align:middle line:90% I can remember this. 01:17:38.410 --> 01:17:41.370 align:middle line:84% I always just say TM, then I say, ah, transverse magnetic. 01:17:41.370 --> 01:17:44.090 align:middle line:90% 01:17:44.090 --> 01:17:47.120 align:middle line:90% That's more intuitive. 01:17:47.120 --> 01:17:48.650 align:middle line:90% OK. 01:17:48.650 --> 01:17:49.790 align:middle line:90% Edgefield. 01:17:49.790 --> 01:17:54.560 align:middle line:84% OK, that's if the magnetic field is a plane of incidence, 01:17:54.560 --> 01:17:56.270 align:middle line:84% the electric field is perpendicular. 01:17:56.270 --> 01:17:57.530 align:middle line:90% That's TE. 01:17:57.530 --> 01:18:01.080 align:middle line:84% And so perpendicular and [INAUDIBLE]. 01:18:01.080 --> 01:18:04.580 align:middle line:84% So you have expressions when you look at or calculate 01:18:04.580 --> 01:18:05.210 align:middle line:90% the interface. 01:18:05.210 --> 01:18:08.900 align:middle line:84% You have to calculate for both solar radiation. 01:18:08.900 --> 01:18:11.460 align:middle line:84% It's a typical combination of 50/50. 01:18:11.460 --> 01:18:15.256 align:middle line:84% We say we calculate, and we say, oh incident is 50 TE, 50 TF. 01:18:15.256 --> 01:18:18.220 align:middle line:90% 01:18:18.220 --> 01:18:19.210 align:middle line:90% OK. 01:18:19.210 --> 01:18:20.500 align:middle line:90% Now, what happens? 01:18:20.500 --> 01:18:23.110 align:middle line:84% First thing is reflection and if you 01:18:23.110 --> 01:18:26.560 align:middle line:90% have a smooth surface, right? 01:18:26.560 --> 01:18:29.500 align:middle line:84% Angle of incidence equal angle of reflection. 01:18:29.500 --> 01:18:31.840 align:middle line:90% So that's the reflection rule. 01:18:31.840 --> 01:18:36.040 align:middle line:84% And then refraction is the angle change 01:18:36.040 --> 01:18:37.650 align:middle line:90% based on the refractive index. 01:18:37.650 --> 01:18:42.030 align:middle line:90% 01:18:42.030 --> 01:18:44.040 align:middle line:84% And then you can actually calculate 01:18:44.040 --> 01:18:49.830 align:middle line:84% the magnitude of the field reflected and refracted. 01:18:49.830 --> 01:18:53.010 align:middle line:84% And again, I'm giving you the solution. 01:18:53.010 --> 01:18:57.570 align:middle line:84% The point is, I want to tell you, those could be calculated. 01:18:57.570 --> 01:18:58.390 align:middle line:90% OK? 01:18:58.390 --> 01:19:01.960 align:middle line:84% And if you want to learn how to calculate, 01:19:01.960 --> 01:19:05.580 align:middle line:84% you need to take a course or pick up the book, read yourself. 01:19:05.580 --> 01:19:08.280 align:middle line:84% Let's say it depends on-- what it depends on 01:19:08.280 --> 01:19:12.500 align:middle line:84% is the refractive index of the material on both sides. 01:19:12.500 --> 01:19:15.220 align:middle line:90% 01:19:15.220 --> 01:19:20.170 align:middle line:84% And so you need to know the property very often, 01:19:20.170 --> 01:19:23.140 align:middle line:84% although, you think people should list it. 01:19:23.140 --> 01:19:26.380 align:middle line:84% You go-- when you need it, you don't find it. 01:19:26.380 --> 01:19:27.220 align:middle line:90% That's one. 01:19:27.220 --> 01:19:30.200 align:middle line:84% And even people list it, you do solar thermal, 01:19:30.200 --> 01:19:33.520 align:middle line:84% you want a high temperature, you don't find it. 01:19:33.520 --> 01:19:37.000 align:middle line:84% So there are a lot of things you can do. 01:19:37.000 --> 01:19:38.350 align:middle line:90% Find them. 01:19:38.350 --> 01:19:43.640 align:middle line:84% And so when we want to design, the first step, we say, 01:19:43.640 --> 01:19:45.760 align:middle line:84% oh let's design-- let's deposit a film 01:19:45.760 --> 01:19:49.240 align:middle line:90% and try to measure properties. 01:19:49.240 --> 01:19:52.030 align:middle line:84% So those are just a few of the reflection. 01:19:52.030 --> 01:19:55.540 align:middle line:84% And the energy, that's reflectivity, transmissivity. 01:19:55.540 --> 01:19:58.630 align:middle line:84% So you do reflection coefficient squared. 01:19:58.630 --> 01:20:00.220 align:middle line:90% You get reflectivity. 01:20:00.220 --> 01:20:05.620 align:middle line:84% Transmissivity, you have to plug in some factors in the front. 01:20:05.620 --> 01:20:11.350 align:middle line:84% But again, those all depends on the complex refractive index. 01:20:11.350 --> 01:20:14.870 align:middle line:84% So the final one is just one example, 01:20:14.870 --> 01:20:19.680 align:middle line:84% where it's a calculation given the refractive index is 01:20:19.680 --> 01:20:22.580 align:middle line:84% between vacuum or air, which is one. 01:20:22.580 --> 01:20:26.480 align:middle line:84% And let's say dielectric to the constant, gold 01:20:26.480 --> 01:20:27.980 align:middle line:90% also to the constant. 01:20:27.980 --> 01:20:30.930 align:middle line:84% But remember, it's not a constant. 01:20:30.930 --> 01:20:32.580 align:middle line:90% It's wavelength-dependent. 01:20:32.580 --> 01:20:34.890 align:middle line:84% So each wavelength, this can change. 01:20:34.890 --> 01:20:39.770 align:middle line:84% But here, it's calculating only the angle dependence. 01:20:39.770 --> 01:20:48.080 align:middle line:84% So if it's a dielectric, so copper is, say, 0. 01:20:48.080 --> 01:20:52.700 align:middle line:84% And here is for the TE wave, transverse electric. 01:20:52.700 --> 01:20:56.690 align:middle line:84% And here is for the TM, transverse magnetic. 01:20:56.690 --> 01:20:59.480 align:middle line:84% And then what's interesting is this crystal angle, 01:20:59.480 --> 01:21:00.810 align:middle line:90% and there are certain. 01:21:00.810 --> 01:21:07.460 align:middle line:84% So that's how people make the polarized glass coatings. 01:21:07.460 --> 01:21:12.300 align:middle line:84% And you can choose the polarization, the certain angle. 01:21:12.300 --> 01:21:17.570 align:middle line:84% You can get, let's say, no reflection at a closer angle. 01:21:17.570 --> 01:21:20.160 align:middle line:90% Gold is a very good reflector. 01:21:20.160 --> 01:21:22.040 align:middle line:90% You can look at that. 01:21:22.040 --> 01:21:26.240 align:middle line:84% Reflectivity is almost close to 1. 01:21:26.240 --> 01:21:29.180 align:middle line:84% So when people do selective surface, 01:21:29.180 --> 01:21:32.450 align:middle line:84% very often, they start with aluminum metal. 01:21:32.450 --> 01:21:35.660 align:middle line:84% It's a good reflection in the infrared. 01:21:35.660 --> 01:21:39.020 align:middle line:84% The problem is reflected up to about 0.2 micron 01:21:39.020 --> 01:21:42.230 align:middle line:90% or 0.5 micron or 0.4 micron. 01:21:42.230 --> 01:21:43.610 align:middle line:90% But you don't want that. 01:21:43.610 --> 01:21:48.120 align:middle line:84% You do not want to reflect the solar radiation. 01:21:48.120 --> 01:21:49.550 align:middle line:90% You want to absorb it. 01:21:49.550 --> 01:21:54.110 align:middle line:84% So you try to put materials on the surface 01:21:54.110 --> 01:21:59.720 align:middle line:84% to make them absorb between the solar radiation up to 2 microns. 01:21:59.720 --> 01:22:02.300 align:middle line:84% And then you still keep the old-- 01:22:02.300 --> 01:22:04.460 align:middle line:84% say, infrared is very reflecting, 01:22:04.460 --> 01:22:08.420 align:middle line:84% which is good because that means emissivity is small. 01:22:08.420 --> 01:22:10.700 align:middle line:84% So people have developed, like I said, 01:22:10.700 --> 01:22:15.160 align:middle line:84% let's say, the different coating even for the solar thermal. 01:22:15.160 --> 01:22:20.140 align:middle line:84% Solar hot water, I said they're very cheap, right? $1.5 per two. 01:22:20.140 --> 01:22:21.730 align:middle line:84% You go through the factory, you will 01:22:21.730 --> 01:22:23.690 align:middle line:90% see their vacuum deposition. 01:22:23.690 --> 01:22:29.200 align:middle line:84% I saw one vacuum deposition is about 100 meters long. 01:22:29.200 --> 01:22:33.700 align:middle line:84% Continuously deposited in different layers 01:22:33.700 --> 01:22:36.910 align:middle line:90% and do it at that low cost. 01:22:36.910 --> 01:22:39.460 align:middle line:90% It's amazing. 01:22:39.460 --> 01:22:42.070 align:middle line:90% OK. 01:22:42.070 --> 01:22:45.000 align:middle line:90% OK, so I'll stop here.