Current air conditioning systems and heat pumps usually require environmentally harmful refrigerants. Researchers have now tested a possible alternative: a so-called electrocaloric heat pump, whose material changes its temperature in an electric field. With a temperature range of up to 20.9 degrees Celsius and a cooling capacity of 4.2 watts, the solid-state heat pump achieves high efficiency, as the team reports. However, further improvements are required before it becomes commercially competitive.
Refrigeration devices such as freezers, air conditioners and heat pumps consume around 20 percent of the world's electricity. Almost all previous systems are based on the so-called vapor compression process, which is relatively inefficient and requires an environmentally harmful fluorinated refrigerant. Due to their high power consumption and leaking refrigerants, cooling devices are responsible for almost ten percent of global greenhouse gas emissions.
Alternative to conventional systems?
Solid-state heat pumps, which do not require a compressor or liquid refrigerants, are considered a more environmentally friendly alternative. Instead, they are based on so-called caloric materials. These are certain metal compounds that change their temperature in a magnetic or electric field or through mechanical deformation. Although such solid-state heat pumps are physically capable of achieving significantly higher efficiencies than compression heat pumps, there is currently no commercially competitive model.
Now a team led by Junning Li from the Luxembourg Institute of Science and Technology in Belvaux has created an electrocaloric heat pump with unprecedented performance and efficiency. “Our electrocaloric cooler achieves a maximum temperature range of 20.9 Kelvin and a maximum cooling power of 4.2 watts at a moderate electric field of 10 volts per micrometer,” they report. Similar systems have previously achieved temperature differences of up to 13 Kelvin and a maximum cooling output of 0.26 watts.
Two circuits for heat and cold
The team used a porous compound made of lead, scandium and tantalum as the material. If you pass an electric current through this material, it heats up. If the current flow stops, it cools down again. In the researchers' tests, the material remained stable even after numerous runs. To take advantage of the temperature changes, Li and his team sandwiched the lead-scandium-tantalate elements between two circuits of fluid, one heating and one cooling.
Even taking into account the energy required to pump the liquid, the heat pump constructed in this way achieves an efficiency of 64 percent of the theoretically maximum possible value, as the scientists report. Systems that work with vapor compression, on the other hand, only achieve around 50 percent efficiency for technical reasons; most achieve less than 30 percent efficiency. Compared to other solid-state heat pumps that use magnetic fields or mechanical deformation, the electrocaloric system also has the advantage of being more compact because the electrical current can be passed directly through the material and no other devices that generate a magnetic field are required or exert mechanical force.
Further research required
“We believe this demonstration shows that electrocaloric refrigeration is a promising alternative to vapor compression refrigeration,” the research team writes. However, before the system is ready for the market, further improvements are required. In an accompanying commentary to the study, also published in the journal Science, Jaka Tušek from the University of Ljubljana in Slovenia points out that further research is needed to further increase the cooling performance of electrocaloric devices and to scale them up to work.
On the one hand, miniaturization is conceivable, which could make the system interesting for cooling electronic devices. “On the other hand, the prototype by Li et al. “There is significant potential for scaling to larger cooling and heating capacities,” says Tušek. “Although the performance of this system has so far fallen short of the requirements of many practical applications, which often require cooling capacities of at least several hundred watts at temperature ranges of over 20 Kelvin, the work highlights the immense future potential of electrocaloric technology.”
Source: Junning Li (Luxembourg Institute of Science and Technology, Belvaux) et al., Science, doi: 10.1126/science.adi5477