As useful as refrigerators and air conditioners are, the gases they use as refrigerants can seep into the atmosphere and be a major contributor to climate change. Now engineers at Harvard University have demonstrated a prototype of a new cooling device that uses a solid material as a coolant.
Appliances such as refrigerators and air conditioners gain cooling capacity through the material’s phase change cycle. In gaseous form, the refrigerant, usually hydrofluorocarbon (HFC), absorbs heat from the environment, cooling the room or the interior of the refrigerator. That gas then goes to a compressor, which, well, compresses it, turns it into a liquid, and releases the stored heat, which is thrown out. When the pressure is turned off, the liquid can turn back into a gas to start the cycle again. This is known as the air pressure effect.
It’s an efficient process that has served us well for decades, but these HFCs can leak out of equipment during use, if damaged, or when moved or disposed of. When released into the atmosphere, they become a more powerful driver of climate change than carbon dioxide, so finding ways to reduce these emissions is a key part of environmental action plans.
This can be helped by solid refrigerants, which can still withstand barometric influences. The new study identifies a promising material from a class of materials known as metal halide perovskites that are already well established as next-generation solar cell materials. In this case, the material can be used for cooling in much the same way as a material that switches between liquid and gas phases, except that it remains solid in both phases.
The secret of these barometric solids lies in their atomic structure. They are usually composed of long, flexible molecular chains that are disordered and somewhat loose, but solidify under pressure into a more ordered state, releasing heat in the process. Release the pressure and they will be able to absorb heat from the environment again. Although both states are solid, the team likens the transition to a partial melting of the wax.
The team demonstrated the idea on a prototype device. The solid refrigerant is packed in metal tubes along with an inert liquid such as water or oil. The hydraulic piston exerts pressure on the fluid, which in turn transfers it to the refrigerant. The fluid also helps transfer heat out of the system.
The system is showing promise, but the team says there are still some issues that need to be fixed. First, the hydraulic system may be pressurized, but it is too high for consumer equipment – about 2900 psi, compared to the maximum pressure of 150 psi in a conventional air conditioner. Other special materials can also conduct heat better.
Other scientists have explored the use of plastic crystals or shape memory alloys as potential solid state refrigerants. With more work, this new material could help make refrigeration equipment more environmentally friendly.
The new study was presented at the American Chemical Society’s fall meeting, and the team’s presentation can be seen in the video below.