WEBVTT 00:00:03.600 --> 00:00:08.709 If you put water in the freezer, you'll end up with ice. If you leave ice on your countertop, 00:00:08.709 --> 00:00:13.119 you'll end up with liquid water. You've almost certainly seen these phase changes in your 00:00:13.119 --> 00:00:18.230 everyday experience. But there's more to freezing and melting than meets the eye, and we can 00:00:18.230 --> 00:00:22.410 use these seemingly simple phenomena to make buildings significantly more energy efficient. 00:00:22.410 --> 00:00:27.480 In this video, we'll explain the concept of "latent heat" and see how it can dramatically 00:00:27.480 --> 00:00:31.980 reduce heating and cooling costs in homes and skyscrapers alike. 00:00:31.980 --> 00:00:34.980 This video is part of the Conservation video series. 00:00:34.980 --> 00:00:39.329 In order to analyze or modify a system, it is important to understand how the laws of 00:00:39.329 --> 00:00:42.420 conservation place constraints on that system. 00:00:42.420 --> 00:00:47.440 Hi. My name is Stephen Ray and I am a graduate student in the Department of Mechanical Engineering 00:00:47.440 --> 00:00:53.969 at MIT. My research in the Building Technology Lab under the guidance of Professor Leon Glicksman 00:00:53.969 --> 00:00:57.079 focuses on energy efficient buildings. 00:00:57.079 --> 00:01:01.789 In order to understand the topic of this video, you should be familiar with the Law of Conservation 00:01:01.789 --> 00:01:07.719 of Energy. You should also be familiar with the effects of intermolecular forces on phase 00:01:07.719 --> 00:01:08.710 transitions. 00:01:08.710 --> 00:01:13.750 After watching this video, you should be able to describe the energy transformations that 00:01:13.750 --> 00:01:20.750 occur during a phase change and apply the law of conservation of energy to phase changes. 00:01:20.899 --> 00:01:23.549 Let's start with a demonstration. 00:01:23.549 --> 00:01:28.158 Here we have a container of water to which we are adding table salt. 00:01:28.158 --> 00:01:31.439 Quite a bit of table salt, actually. 00:01:31.439 --> 00:01:33.090 Let's stir that up. 00:01:33.090 --> 00:01:38.000 Next, we are going to crush up some ice to add to the salt water. 00:01:38.000 --> 00:01:42.219 Our goal is to lower the temperature of the ice bath. 00:01:42.219 --> 00:01:44.210 Let's add some more salt. 00:01:44.210 --> 00:01:46.579 Freezing point depression, yeah! 00:01:46.579 --> 00:01:51.649 Alright, minus 8 degrees Celcius. That's pretty good. 00:01:51.649 --> 00:01:56.950 Let's place a smaller container of liquid water into this ice bath. We've added green 00:01:56.950 --> 00:02:01.340 food coloring to the water so that it is easier for you to see. 00:02:01.340 --> 00:02:05.909 After a few minutes, we'll insert a thermometer and watch the temperature drop as the water 00:02:05.909 --> 00:02:08.410 is cooled by the ice bath. 00:02:08.410 --> 00:02:13.910 The temperature reads about -5 degrees Celsius. The green water is still a liquid even though 00:02:13.910 --> 00:02:19.910 the temperature is below the normal freezing point of water. This is called supercooling. 00:02:19.910 --> 00:02:24.050 In the next step of this demo, we are going to add a couple of small pieces of ice to 00:02:24.050 --> 00:02:28.579 the green water. The supercooled water will crystallize rapidly with the addition of the 00:02:28.579 --> 00:02:30.240 ice. 00:02:30.240 --> 00:02:34.200 When this happens, what do you predict will happen to the temperature on the thermometer? 00:02:34.200 --> 00:02:38.990 Will it increase, decrease, or stay the same? 00:02:38.990 --> 00:02:43.920 What reasoning supports your prediction? Pause the video here to discuss your ideas with 00:02:43.920 --> 00:02:50.920 the person beside you, then continue playing the video to see what happens. 00:02:53.100 --> 00:02:59.160 Okay, ready to see what happens? Be sure to keep your eye on the digital display. 00:02:59.170 --> 00:03:04.840 Now we'll drop a couple of small pieces of ice into the container of green water. 00:03:04.840 --> 00:03:07.390 Watch what's happening. 00:03:07.390 --> 00:03:12.700 The green water froze. There is still a little bit of liquid, but we can turn the container 00:03:12.700 --> 00:03:16.900 upside down and you can see that the ice remains inside. 00:03:16.900 --> 00:03:22.260 So, when the liquid froze, what happened to the temperature? 00:03:22.260 --> 00:03:28.920 The temperature went up! Is this what you predicted? How can we explain what happened? 00:03:28.920 --> 00:03:34.010 Well, let's think about what happens at a molecular level when water changes phase from 00:03:34.010 --> 00:03:41.010 a liquid to a solid. In the liquid state, water molecules are moving around a lot. As 00:03:41.040 --> 00:03:45.210 it gets colder, the water molecules slow down. 00:03:45.210 --> 00:03:50.090 Generally speaking, as the water cools and solidifies, there is an increase in hydrogen 00:03:50.090 --> 00:03:55.290 bonding, the dominant intermolecular force amongst the water molecules. 00:03:55.290 --> 00:03:59.680 With this hint, can you now explain why we observed a temperature increase when the water 00:03:59.680 --> 00:04:05.730 froze? Pause the video, take a moment to think about it on your own and then discuss your 00:04:05.730 --> 00:04:12.730 idea with a classmate. Then, continue playing the video for an explanation. 00:04:16.320 --> 00:04:21.040 As we said before, when water transitions from liquid to solid, there is an increase 00:04:21.040 --> 00:04:26.500 in the number of hydrogen bonds that are formed between water molecules. Does bond formation 00:04:26.500 --> 00:04:30.740 release energy or require energy? 00:04:30.740 --> 00:04:33.880 Bond formation releases energy. 00:04:33.880 --> 00:04:39.820 So then, is the process of ice forming exothermic or endothermic? 00:04:39.820 --> 00:04:42.790 Ice formation is exothermic. 00:04:42.790 --> 00:04:46.810 If we were to do an energy balance on the system and the surroundings, we would see 00:04:46.810 --> 00:04:51.820 that the energy that was released by the water freezing is equivalent to the thermal energy 00:04:51.820 --> 00:04:57.930 that caused the temperature to increase. This is the Law of Conservation of Energy. 00:04:57.930 --> 00:05:03.540 Generally speaking, when a substance transitions between phases, intermolecular forces between 00:05:03.540 --> 00:05:08.530 neighboring molecules are either formed or broken. When a substance transitions from 00:05:08.530 --> 00:05:14.560 a liquid to a solid, intermolecular forces, or bonds, are formed and energy is released 00:05:14.560 --> 00:05:20.060 to the surroundings. This is called the latent heat of fusion. 00:05:20.060 --> 00:05:25.260 Going in the other direction, when a substance transitions from a solid to a liquid, energy 00:05:25.260 --> 00:05:31.340 is required to overcome intermolecular forces, so this is an endothermic process. Energy 00:05:31.340 --> 00:05:37.710 is absorbed from the surroundings. This is called the latent heat of melting. 00:05:37.710 --> 00:05:42.270 The latent heat of melting is equivalent in magnitude to the latent heat of fusion, but 00:05:42.270 --> 00:05:44.659 opposite in sign. 00:05:44.659 --> 00:05:51.659 This phenomenon of latent heat is used in a variety of ways to heat and cool buildings. 00:05:51.680 --> 00:05:56.940 Using melting ice to absorb thermal energy from the surroundings saves some buildings 00:05:56.940 --> 00:06:00.710 thousands of dollars a year in cooling costs. 00:06:00.710 --> 00:06:05.389 Consider a large office building during the middle of a hot summer day. In order to keep 00:06:05.389 --> 00:06:10.169 the building comfortable, the air conditioning is running at full power, requiring a lot 00:06:10.169 --> 00:06:15.340 of electricity from the utility company. The utility companies can't easily shut down their 00:06:15.340 --> 00:06:20.510 power plants, so nearly the same amount of electricity is produced during the night as 00:06:20.510 --> 00:06:27.510 is during the day. However, nighttime demand for electricity is very low, so utility companies 00:06:27.639 --> 00:06:33.620 sell this electricity at a lower price. This is common practice by utility companies across 00:06:33.620 --> 00:06:36.370 the U.S. 00:06:36.370 --> 00:06:40.010 Engineers have thought of a way to buy the electricity during the night when it is very 00:06:40.010 --> 00:06:46.000 cheap, but use it during the day when they need to run the air conditioning. This strategy 00:06:46.000 --> 00:06:51.580 is called peak load shifting. Think about what you have learned so far. There are a 00:06:51.580 --> 00:06:58.580 variety of ways this can be done. How do you think they do this? Pause the video here, 00:06:58.840 --> 00:07:04.120 take a moment to think about it, and discuss your idea with the person next to you. Continue 00:07:04.120 --> 00:07:11.120 playing the video to hear about one way they do this. 00:07:13.389 --> 00:07:17.990 One way we can decrease the amount of electricity needed in the daytime to cool a building is 00:07:17.990 --> 00:07:23.650 to use the energy storage capacity of ice, or its latent heat of melting. 00:07:23.650 --> 00:07:28.310 Large tanks, such as these in the basement of the Bank of America Tower in New York, 00:07:28.310 --> 00:07:33.630 store water that is frozen over night using cheap electricity. During the day, the ice 00:07:33.630 --> 00:07:38.370 melts and absorbs energy from the cooling fluid running through the building's air conditioning 00:07:38.370 --> 00:07:44.740 system. Each of the tanks in the Bank of America building holds approximately 1600 gallons 00:07:44.740 --> 00:07:51.720 of water which translates to roughly 570 kilowatt hours of cooling capacity. Bank of America 00:07:51.720 --> 00:07:57.919 reports that these ice tanks supply 25% of their cooling energy annually. 00:07:57.919 --> 00:08:02.169 The desire to harness latent heat has led to the development of a class of materials 00:08:02.169 --> 00:08:07.740 called phase change materials. These materials have been specifically designed to change 00:08:07.740 --> 00:08:12.699 phase at desirable temperatures so that they can store and release energy in a way that 00:08:12.699 --> 00:08:19.580 is useful to consumers. This slide shows some examples of these materials. The materials 00:08:19.580 --> 00:08:26.580 on this slide fall into three classes of phase change materials: inorganic salt hydrates, 00:08:26.590 --> 00:08:31.560 paraffinic hydrocarbons, and organic fatty acids. 00:08:31.560 --> 00:08:37.169 Some ordinary building materials such as concrete, dry wall, or insulation have been specially 00:08:37.169 --> 00:08:44.149 engineered over the past 50 years to contain microscopic pellets of phase change materials. 00:08:44.149 --> 00:08:50.600 Cellulose insulation, shown here, is commonly used to insulate attics and walls. Researchers 00:08:50.600 --> 00:08:55.410 at the Oak Ridge National Laboratories in the United States have impregnated small paraffin 00:08:55.410 --> 00:09:00.879 pellets in this common type of insulation to increase its performance. 00:09:00.879 --> 00:09:05.730 Because these pellets are microscopic, the phase change insulation looks exactly like 00:09:05.730 --> 00:09:12.339 the ordinary insulation to our naked eye. However, under a Scanning Electron Microscopic, 00:09:12.339 --> 00:09:16.720 the clusters of paraffinic pellets are easy to spot. 00:09:16.720 --> 00:09:20.899 Now lets look at the measured performance of a similar type of phase change insulation 00:09:20.899 --> 00:09:26.220 that is installed in a typical residential roof. The chart here shows the heat transfer 00:09:26.220 --> 00:09:32.959 into a house on the vertical axis and time on the horizontal axis. The two large black 00:09:32.959 --> 00:09:38.170 peaks, which occur during two consecutive summer days, correspond to the large amount 00:09:38.170 --> 00:09:44.029 of heat transfer into a home that a conventional roof allows. The smaller green peaks show 00:09:44.029 --> 00:09:48.819 how changing the roof surface material and venting the attic significantly help lower 00:09:48.819 --> 00:09:55.329 the heat transfer into a home by approximately 70%. This reduction in heat transfer leads 00:09:55.329 --> 00:10:00.999 to less energy required for cooling the home. The purple curve shows that if phase change 00:10:00.999 --> 00:10:06.459 insulation is used in addition to these roof modifications, we can reduce the required 00:10:06.459 --> 00:10:10.559 cooling energy by 90%! 00:10:10.559 --> 00:10:16.509 How does phase change insulation help save energy? Pause the video, take a moment to 00:10:16.509 --> 00:10:23.509 think about it, and share your thoughts with a classmate. 00:10:26.079 --> 00:10:32.709 Consider two homes with and without PCM roof insulation. Both homes are exposed to the 00:10:32.709 --> 00:10:37.790 same amount of solar energy from the sun. Although not all of that energy enters the 00:10:37.790 --> 00:10:43.230 home without PCM insulation, a significant portion is transferred through the roof into 00:10:43.230 --> 00:10:49.529 the house. The house with PCM insulation reduces this amount because some of the energy that 00:10:49.529 --> 00:10:55.360 would otherwise enter the house is used to melt the phase change material in the roof. 00:10:55.360 --> 00:11:00.720 In order for this effect to occur each day, the phase change material must solidify every 00:11:00.720 --> 00:11:04.649 night before it can melt again the next day. 00:11:04.649 --> 00:11:10.439 Let's take a look at this chart of example phase change materials again. What phase change 00:11:10.439 --> 00:11:16.459 material would you use in a home or building in Singapore? Why? What additional information 00:11:16.459 --> 00:11:22.149 might you need to know to help make your decision? Pause the video here and discuss your choice 00:11:22.149 --> 00:11:29.149 with the person beside you. 00:11:31.790 --> 00:11:38.160 On average, year-round temperatures in Singapore hover between 23 and 32 degrees Celcius. This 00:11:38.160 --> 00:11:43.490 is important when making our decision. Some of you may have selected the calcium chloride 00:11:43.490 --> 00:11:48.179 hydrate as a candidate phase change material because its melting temperature falls within 00:11:48.179 --> 00:11:53.920 the range of daytime high temperatures in Singapore. You may have selected octadecane 00:11:53.920 --> 00:12:00.869 for a similar reason. If you eliminated paraffin wax and palmitic acid, it was probably because 00:12:00.869 --> 00:12:05.189 their melting points are so much higher than the highest outdoor temperatures reached in 00:12:05.189 --> 00:12:11.209 the day. However, these phase change materials could be used on a conventional dark-colored 00:12:11.209 --> 00:12:18.209 roof where temperatures can exceed 70 C because of all the energy absorbed from the sun. Caprylic 00:12:18.369 --> 00:12:24.019 acid, on the other hand, has such a low melting temperature, that few, if any, building components 00:12:24.019 --> 00:12:27.019 would ever drop below this temperature. 00:12:27.019 --> 00:12:33.800 So, let's go back to calcium chloride hydrate and octadecane. Both would melt and solidify 00:12:33.800 --> 00:12:38.839 within the temperature range of the environment, but octadecane has a higher heat of fusion, 00:12:38.839 --> 00:12:44.239 which leads to greater potential energy savings. While it is beyond the scope of this video, 00:12:44.239 --> 00:12:48.220 we also need to think about the heat transfer properties of the other building materials 00:12:48.220 --> 00:12:53.329 used in the roof and the cost and stability of the phase change materials. 00:12:53.329 --> 00:12:59.019 Today, you learned about the concept of latent heat and how the energy transformations that 00:12:59.019 --> 00:13:04.769 occur during a phase change are a consequence of the law of conservation of energy. You 00:13:04.769 --> 00:13:10.379 also saw how engineers have used the concept of latent heat to design phase change materials 00:13:10.379 --> 00:13:16.610 that can allow us to cool and heat homes more efficiently. The incorporation of phase change 00:13:16.610 --> 00:13:21.739 materials in building materials is still an active area of research. There are still engineering 00:13:21.739 --> 00:13:27.220 challenges to address, perhaps by you, in order to make these technologies cost-effective 00:13:27.220 --> 00:13:31.640 and more widely used.