Refrigeration has transformed the world, allowing perishable items – medicines included – to be transported across the world and inspiring migrations to hotter climates. Yet, unlike so many other technologies, the method we use to produce cold air hasn’t proven suitable for miniaturization. That could be about to change, however, with the announcement of efficient solid-state cooling, which offers the potential to one day pack a tiny fridge in your backpack on a long hike. You might even put one in your pocket if you wish.
Animals have mastered evaporative cooling for millions of years, using the fact that when water vaporizes, it removes a lot of heat. However, even when airflow over water is maximized, the potential of this is limited. Advanced cooling systems, whether for air conditioning or refrigeration, use vapor-compression, relying on the relationship between temperature and pressure in gasses. Just as compressing air, for example in a bike pump, raises temperature, allowing it to expand cools things down, and can be used to remove heat from the local environment. However, such devices are inevitably bulky.
Professor Qibing Pei of the University of California, Los Angeles, has applied a completely different approach, known as the electrocaloric effect. Applying an electric field to a dielectric material can modulate its entropy, warming or cooling the material depending on the field applied. This effect has been known for some time, but regarded as more of a curiosity than a practical cooling mechanism, since the amount of cooling that has been generated has been extremely small.
“The highest reported specific cooling power of 0.018 Watts/gram is orders of magnitude smaller than the theoretically calculated values based on intrinsic material properties,” Pei and his co-authors note in Science. This reflects the complexity of design of past electrocaloric devices, which pumped fluids past the dielectric or used motors whose heat offset much of the cooling.
Pei and co-authors showed that these features were unnecessary by using electrostatic force to move the electrocaloric material, connecting and breaking the link between a heat source and heat sink. Out of that tongue-twister, they created a device with a cooling power of 2.8 W/g – more than 100 times greater than the previous best.
Moreover, their device is six times more energy efficient than traditional refrigeration systems, dangling the potential of enormous electricity savings if it can be scaled up.
At 7-by-3-by-0.6 centimeters (2.7-by-1.2-by-0.2 inches), the test device is certainly not something that could cool your house on a blistering day. On the other hand, it is approaching the size that could be used to keep vulnerable medicines cool in transit, or provide personal body-cooling clothes. On an even smaller scale, it could be used to prevent phone batteries from overheating.
