Isegoria

Heat pumps — refrigerators, air conditioners, heaters — consume 30 percent of the world’s electricity, but capacitor-based heat pumps could “pump” more efficiently:

Compressing a gas will heat it up while lowering the pressure cools it down. However, various other materials undergo similar heating and cooling in response to other external influences, including physical stress, magnetic fields, or electric fields. In many cases, these materials remain solid despite experiencing significant changes in temperature, which could potentially simplify the supporting equipment needed for heating and cooling.

In the new work, done by researchers mostly based in Luxembourg, the researchers focused on materials that change temperature in response to electric fields, generically known as electrocalorics. While a variety of configurations have been tested for these materials, researchers have settled on a layered capacitor structure, with the electric field of the material changing as more charge is stored within it. As charge is stored, an electrocaloric material will heat up. When the charge is drained, they’ll draw in heat from the environment.

This has a significant advantage regarding the power needed for the device to operate since the current generated when draining the capacitor can just be used to power something. There’s a little energy lost during the round-trip in and out of storage, but that can potentially be limited to less than one percent.

The thing that uses power is the fact that the capacitors are entirely solid-state—on their own, they’ll just sit in either the source or sink environment. So, you either have to expend energy to physically move the device between the environments or transfer heat from the electrocaloric device to some other material that does the moving. In this case, the researchers simply exchanged heat with the source and sink by pumping a liquid through the electrocaloric material.

For the electrocaloric device, the researchers created a multilayer capacitor using a lead/scandium/tantalum oxide material. This was crafted into a series of parallel plates with gaps in between them, which allowed fluid to flow through the device.

The hardware worked by adding charge to the capacitor, which would heat the fluid in its immediate vicinity. That fluid would then be pumped to exchange heat with one environment, warming it up. While that was happening, the charge was drained from the device, cooling the fresh fluid that had been pumped into place. That cooled fluid was then pumped out to exchange heat with a separate environment, allowing the cycle to be repeated. Over time, this would gradually cool the first environment while heating the second.

And it worked. Heat was effectively transferred between the two environments, and measurements suggested that the device itself was capable of changing temperature by as much as 21° C. That’s a 50 percent improvement over the best electrocaloric device previously demonstrated. The cooling power is the equivalent of 5.6 watts, which works out to be about 116 W/kg of material.

It was also quite stable. The researchers built up a voltage difference of 400 V across the capacitor without any sign of breakdown, and performance remained steady across 100,000 cycles tested for this publication. Based on accelerated aging tests, the researchers estimate that one of these devices would last over 30 years in typical conditions.

The researchers also calculated its Carnot efficiency. This was higher for tests where the total temperature difference was relatively small. Assuming the power stored in the capacitor was put to use, the hardware can reach 64 percent of the maximum theoretical efficiency, which is considerably higher than any previous electrocaloric device.

Posted in Technology | 4 Comments »