If a new type of heat pump lives up to its promise, the use of environmentally damaging gases in air conditioners and refrigerators could become redundant. A prototype, described in a study published last week Science1, Uses electric fields and a special ceramic instead to alternately vaporize the refrigerant fluid and condense it with a compressor to heat or cool air.
The technology combines several existing technologies and has “excellent performance,” says materials scientist Neil Mathur of the University of Cambridge, UK.
Materials scientist Emmanuel Defay and his colleagues at the Luxembourg Institute of Science and Technology in Belvaux created their experimental device from a ceramic with a strong electrocaloric effect. Materials that exhibit this effect become hot when exposed to an electric field.
In electrocaloric materials, atoms have electrical polarization – a slight imbalance in the distribution of electrons, which gives these atoms ‘plus’ and ‘minus’ poles.
When the material is left alone, the polarization of these atoms continues to rotate in random directions. But when the material is exposed to an electric field, all the electrostatic poles suddenly align, like hair combed in one direction. Defoe explains that this transition from disorder to order means that the entropy of the electrons – physicists’ way of measuring disorder – suddenly drops.
But the laws of thermodynamics say that the total entropy of a system can never decrease, so if it falls somewhere it must increase somewhere else. “The only possibility to get rid of this extra dirt is to force the material into the lattice of its crystal structure,” he says. That extra dislocation means that the atoms themselves begin to vibrate faster, resulting in an increase in temperature.
cooled by electrical pulse
The researchers then remove the heat by flowing a fluid between slabs of electrocaloric material, while keeping the electric field on. The result is that the slab returns to the original, ambient temperature, but with less polarization entropy. If the researchers turn off the electric field, it produces the opposite effect: the polarization becomes chaotic again, and entropy flows out of the ceramic’s atomic lattice, taking the heat with it. This results in the mesh becoming cooler than the ambient temperature and can cool the fluid pumped between the slabs. The cycle starts again.
In a refrigerator or air conditioner, heat from the hot fluid will dissipate into the environment, while the cold fluid will serve to keep the interior or room cool. For heating, a heat pump will cool the outside environment, extracting heat from it and pumping it into the building.
Defoe says that although the technology is not yet ready for commercialization, with further refinement, the efficiency of his team’s electrocaloric heat pump could be competitive with existing heat pumps. This is a difficult standard to meet, because heat pumps based on compressors are already very efficient: when used to heat buildings, for example, they consume the same amount of electricity as space heaters. Can produce three or more times more heat. But unlike a standard heat pump, an electrocaloric heat pump will not require refrigerants such as hydrofluorocarbons or ammonia, which are potentially harmful to the environment. And because it removes the need for a compressor, it can potentially fit in a smaller, simpler device, DeFay says.