While hot things naturally lose heat and come down to the temperature of the surroundings, extracting heat, to get them any cooler, takes energy. The demand of energy is the first impact of the cooling industry. The second is that the industry uses volatile chemicals. Those get released into the atmosphere and cause pollution. While the need for cooling will increase through the rest of the century, a report in the journal, Nature, holds out the promise of mitigating at least the second environmental cost of artificial cooling.
Bing Li, Yukinobu Kawakita, Seiko Ohira-Kawamura, Takeshi Sugahara, Hui Wang, Jingfan Wang, Yanna Chen, Saori I Kawaguchi, Shogo Kawaguchi, Koji Ohara, Kuo Li, Dehong Yu, Richard Mole, Takanori Hattori, Tatsuya Kikuchi, Shin-ichiro Yano, Zhao Zhang, Zhe Zhang, Weijun Ren, Shangchao Lin, Osami Sakata, Kenji Nakajima and Zhidong Zhang, from universities and institutes in Shenyang, Changsha, Beijing, Hefei and Shanghai in China; Tokai, Osaka and Sayo in Japan; Irvine and Tallahassee in the US, report their trials with an effective, climate-friendly and abundant alternative to the powerful greenhouse gases that conventional cooling systems use.
Cooling things below the ambient temperature essentially means inducing flow of heat, from a cool object to the surroundings, which are warmer. The outward flow of heat from a cool object, however, is the opposite of what normally happens. This has, hence, to be managed through a twostep combination of otherwise natural processes.
The first step is to increase the internal energy of a system, typically a gas, by compression, so that the additional energy shows as a rise in temperature. Once the system is at a higher temperature, natural processes reduce its total energy content by cooling to the ambient temperature. We then have a system, which has energy stored in its internal composition, which, in the case of a gas, is the pressure, but which is at the same temperature as the surroundings. The internal energy is then released. In the case of a gas, this is done by allowing expansion.
The result of expansion is that the system loses energy and there is cooling, below the initial temperature. We can see that heat had earlier been released to the surroundings, when the gas cooled down. Now, when the gas comes back to normal pressure, it amounts to giving up the same heat, resulting in cooling. The heat released at first came from the external work done, during compression. Now, it is the system that does work, in expansion, which depletes its own energy and leads to cooling.
This process, however, is not feasible in the case of a normal gas, as heat capacity of normal gases is too low. To be more effective, the substance needs more heat capacity, typically an extreme greenhouse gas. And further, a gas that would change state, turning into a liquid, releasing large energy as heat, when compressed. As we may know, when liquids vapourise, they take in a large quantity of heat, as “latent heat”. When the vapour is liquefied, by applying pressure, the energy is released as heat. When the liquid evaporates, the energy has to be recouped, resulting in cooling.
A substance that has these qualities in ample measure is the chlorofluorocarbon, sold under the brand name of Freon. Freon was the refrigerant fluid of choice for many decades, before it was realised that its release into the atmosphere caused depletion of the ozone layer. An international effort, the Montreal Protocol, resulted in CFCs being replaced by hydrofluorocarbons, or HFCs. HFCs do not affect the ozone layer but like CFCs, they are extreme greenhouse gases. When the dangers of greenhouse gases were realised, so also was the environmental downside of HFCs. And a serious downside is as a kilogram of a typical CFC has the heat storing capacity of two tones of CO2.
An alternative to using gases in refrigeration are some metals that absorb energy when they are magnetised. Just as the molecules of a gas become more confined when the gas is compressed, randomly oriented magnetic crystals, called domains, within some solids undergo a phase change to get aligned when the solid is magnetised. And there is absorption of energy and warming in the process. Now, if the solid is allowed to cool to the ambient temperature and the electric current inducing the magnetism is stopped, the solid de-magnetises.
The magnetic domains then disorient, and in the process, they absorb thermal energy. And as thermal energy is absorbed, the object cools. This method, of magnetic refrigeration, has the advantage of using no fluids of gases, which could leak to the atmosphere, nor the need for compressors, valves and sealed units. The arrangements can be compact, even miniaturised, for use in chip-based instruments or equipment, and is energy efficient and scalable.
The problem, however, is that most substances that undergo such phase changes under magnetic or electric fields do not have substantial heat capacity for practical applications. Their temperature of working is also often removed from ambient conditions. Another serious disadvantage is that the materials get denatured by repeated cycles of magnetisation. The methods are effective no doubt, in niche areas, like attaining very low temperatures, for research, but the limitations have come in the way of solid state refrigeration making a mark in the industry.
The development reported in the journal appears to be the answer sought for — a solid material, not a vapour, which undergoes internal phase transition by the application and removal of pressure, just like in the case of a gas. Some organic and some inorganic substances take a soft and easily deformable form — and are hence called, “plastic crystals” — where the molecules take fixed positions, as in crystals, but the molecules themselves are randomly oriented.
When the substance is cooled, however, the molecules orient themselves, with a large change in the total energy of the system. This transition, which is from a highly disordered to an ordered state, is hence associated with the large energy change.
But what is significant is this transition can be brought about by a small change in pressure. This is to say that like a vapour can be made to condense by increasing the pressure, the structure of a plastic crystal can be changed to the ordered state by pressure of just one to 10 atmospheres. And by applying this pressure, the energy change is of the same order as traditional refrigerant materials, like CFCs, and 30 to 40 times the change in materials that respond to magnetic or electric fields. And this effect comes about at the normal ambient temperature for usual applications.
There are, however, some negatives. One is that organic materials have low melting points, sometimes as low as 25°C, which would be a disadvantage. Another is that the materials have limited mechanical strength, to withstand continued use. The crystal structure may also undergo changes that reduce the cooling performance. These, of course, are challenges to be overcome. This instance of an effective, solid state alternative to CFCs, however, may mark the beginning of ways to prevent refrigeration and air conditioning from becoming a cause of concern because of their existence.
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