When thinking about battling climate change, did you know that a major electricity consumer in your house is your refrigerator? The International Institute of Refrigeration (yes, there are institutes for just about everything) estimates that about 20% of all electricity used is expended on vapour compression refrigeration. The refrigerator works by compressing a vapour, often some sort of freon, or in industry, with ammonia, and when compressed it gives off heat. When you ship it somewhere else and expand it, the somewhere else gets colder. This is also the principle of the heat pump and various air conditioning units. Compress gas here to add heat from a heat exchanger; ship the gas there and expand it, where it cools a heat exchanger. The compression and expansion of gas moves heat from A to B, hence the name heat pump. Also, the refrigerant gases tend to be powerful greenhouse gases. One kilogram of R410a has the same greenhouse impact as two tonne of carbon dioxide. Refrigerants leak into the atmosphere from faulty equipment or when equipment is not properly disposed of.
It is possible to heat or cool without any gas through the Peltier effect. Basically, when electric current passes between two conductors, heating or cooling effects may be generated. There are commercial solid-state such cooling systems, but they suffer from high cost and poor efficiency, in part because the effect is restricted to the specific junction.
There is an alternative. Some solid state materials can cool when they are subjected to strain, which is generated from an external field, such as the electric or magnetic fields, or simply pressure. So far most efforts have been focused on the magnetic field, and one material, Mn3SnC apparently gives significant cooling, but the magnetic field has to be greater than 2 tesla. That means expensive and bulky magnets, and additionally the refrigerator, if it used them, would have to be a “no-go” area for credit cards, and possibly people with a pacemaker. Even aside from direct messing with the pacemaker, losing all those bitcoin just because you wanted a cool beer could lead to medical problems.
However, there has been an advance. Wu et al. (Acta Materiala 237: 118154) have taken the Mn3SnC and coated it with a piezoelectric layer of lead zirconate titanate. Don’t ask me how they come up with these things; I have no idea, but this is certainly interesting. They probably do it by looking through the literature to find materials already known to have certain properties. Thus a piezoelectric effect is where you generate an electric voltage by applying pressure. Such effects are reversible so if you can generate a voltage by applying pressure to something, you can generate pressure by applying a potential difference. Recall that pressure also can generate a cooling effect. Accordingly, by applying an electric field to this coated material a cooling effect was obtained equivalent to that of a 3 tesla magnetic field. When the electric field is removed, the temperature returns to where it was. How useful will this be? Hard to tell right now. The temperature drop when applying a field of 0.8 kV/cm was slightly under 0.6 of a degree Centigrade, which is not a huge change while the voltage is tolerably high. Interestingly, if you apply a magnetic field you also get a temperature change, but in the opposite direction – instead of cooling, it heats. Why that is is unclear.
As you might guess, there is still a significant distance to go before we get to a refrigerator. First, you have to get the cooling into some other fluid that can transport it to where you need it. To do that you have to take the heat out of the cooling fluid, but that will heat up your unit, so you need another fluid to take the heat to where it can be dissipated. We have very roughly the same cycle as our present system except we are not compressing anything but we have two fluids. Except I rather think we will be, because pumping a fluid involves increasing its pressure so it flows. The alternative is to put the material across the rear wall of the refrigerator thus to cool the interior and have the heat dissipated out the back. The problem now is the change of temperature is rather small for the voltage. This is not so much a problem with fluids transporting it, but if the solids transport it, the solids are always heated by the environment so your temperature drop is from the room temperature. Half a degree is not very helpful, although you could increase the electric field. Unfortunately, to get a big enough temperature change you might be into the spark jumping region. Lightning in the kitchen! Finally, do you want the back of your refrigerator to be carrying even a kilovolt electric field? My guess is this effect may remain a curiosity for some length of time.