Capacitor-based heat pumps see big boost in efficiency

Various forms of heat pumps—refrigerators, air conditioners, heaters—are estimated to consume about 30 percent of the world's electricity. And that number is almost certain to rise, as heat pumps play a very large role in efforts to electrify heating to reduce the use of fossil fuels.

Most existing versions of these systems rely on the compression of a class of chemicals called hydrofluorocarbons, gasses that were chosen because they have a far smaller impact on the ozone layer than earlier refrigerants. Unfortunately, they are also extremely potent greenhouse gasses, with a short-term impact several thousand times that of carbon dioxide.

Alternate technologies have been tested, but all of them have at least one major drawback in comparison to gas compression. In a paper released in today's issue of Science, however, researchers describe progress on a form of heat pump that is built around a capacitor that changes temperature as it's charged and discharged. Because the energy spent while charging it can be used on discharge, the system has the potential to be highly efficient.

Pumping heat

Heat pumps are a great choice for heating since it's possible to move heat between a sink and a source using far less energy than it takes to simply heat the source up by, for example, burning natural gas. And, since the processes that are used for this work just as well in reverse, the same basic approach can be used for cooling.

It may sound counterintuitive that you could somehow extract heat from a room-temperature environment and export it into blazing hot desert air. But the process relies on creating a temperature difference between a working material (like those hydrofluorocarbons) and the environment. As long as the hydrofluorocarbons can be made hotter than the outside air, they can export heat to it. Similarly, they just have to get colder than whatever Arctic blast they're working against when being used for heating.

For hydrofluorocarbons, the difference in heat content can be controlled by altering the pressure. 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.

The details

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.

Next steps

While this is a major step forward for electrocalorics, it falls far short of what we'd need for practical use. One of the key limits is the 20° C swings that are possible. While there are probably some contexts where that limited range can still be sufficient, a lot of uses will need to manage much larger temperature differences—think about the contrast between a warm house and an Arctic winter, for example. The other issue is that the cooling power, in the area of about 5 watts, is much lower than we'd want for a practical system.

Part of that can be handled simply by scaling up the size of the system. The test device was only 4 centimeters long, with a width and depth of a centimeter each. Obviously, anything commercial will be much larger and can generate more power.

The other issue with the current iteration is its reliance on a heat-exchange fluid that's also an electrical insulator. That's necessary to prevent the breakdown of the current storage in the capacitor, but it means the exchange of heat is slower than it might be otherwise. A simple liquid like water would work much better if the capacitors could be made fully waterproof.

That, however, may involve a trade-off. Thinner capacitor layers would also boost the efficiency of heat transfer but may not be compatible with the addition of waterproofing. In the same way, increasing the voltage across the layers of the capacitor would boost performance, but may be difficult to do with that much water around.

It's also important to acknowledge that some of the other technologies, like magnetocalorics, have outperformed this device on a number of these measures. But it's much more difficult to recycle the energy used in altering a magnetic field, so they're likely to underperform in energy efficiency, making figuring out the practical limits of electrocaloric devices so important.

Science, 2023. DOI: 10.1126/science.adi5477  (About DOIs).