Thermodynamic solar panels: What you need to know

Typically, when you think about solar panels, you picture solar photovoltaics (PV): panels that are installed atop your roof or in an open space and convert sunlight into electricity. However, solar panels can also be thermal, meaning that they convert sunlight into heat as opposed to electricity. Thermodynamic solar panels are one type of thermal solar panel–also called a collector–that differ dramatically from traditional thermal panels; instead of requiring direct sunlight, thermodynamic solar panels can also generate power from heat in the air.

Thermodynamic solar panels are components of some direct-expansion solar-assisted heat pumps (SAHPs), where they serve as the collector, heating the cold refrigerant. In direct expansion SAHPs, they also serve as the evaporator: as refrigerant circulates directly through a thermodynamic solar panel and absorbs heat, it vaporizes, turning from a liquid into a gas. The gas then travels through a compressor, where it's pressurized, and finally to a storage heat exchanging tank, where it heats your water, as explained in our article about SAHPs.

Unlike photovoltaics or traditional thermal solar panels, thermodynamic solar panels don't need to be placed in full sunlight. They absorb heat from direct sunlight but can also pull heat from ambient air. Thus, while thermodynamic solar panels are technically considered solar panels, they are, in some ways, more similar to air-source heat pumps. Thermodynamic solar panels can be mounted to roofs or walls, in full sun or in complete shade–the caveat here is that if you live in a cold climate, they will probably operate most efficiently in full sunlight because the ambient air temperature may not be warm enough to meet your heating needs.

Unlike solar hot water systems, thermodynamic solar panels are still a developing technology and are not as well-tested. In 2014, one independent laboratory, Narec Distributed Energy, conducted tests in Blyth, United Kingdom, to determine the efficiency of thermodynamic solar panels. Blyth has a fairly temperate climate with heavy rainfall, and the tests were run from January to July.

The results showed that the coefficient of performance, or COP, of the thermodynamic SAHP system, was 2.2 (when you account for the heat lost from the heat exchanging tank). Heat pumps are typically considered highly efficient when they achieve COPs above 3.0. However, while this study demonstrated that, in 2014, thermodynamic solar panels weren't highly efficient in a temperate climate, they can operate more efficiently in warmer climates. Additionally, as the technology has continued to advance, thermodynamic solar panels are probably in need of a new independent testing study.

Thermodynamic solar panels are more widely available in European countries, such as the United Kingdom and Ireland, as opposed to the United States. However, some are now starting to enter the US market. One manufacturer in the United Kingdom, SAHP Ltd., has distributors in New England, Florida, and California. Its thermodynamic solar panels can be purchased with its SAHP systems, which come with either a heat exchanging tank or can be retro-fitted to your existing tank. These systems qualify for the investment tax credit (ITC), which is currently set at 26 percent, and come with a 25-year warranty.

The system by SAHP Ltd. that comes with the heat exchanging tank can also be integrated with solar PV. If you have PV panels, your SAHP installer can connect your PV inverters to your SAHP. During periods of the day when your PV panels generate more electricity than you need, your SAHP can use some of the electricity to increase the temperature of the water within the heat exchanging tank.

Looking to install PV panels to pair with your thermodynamic SAHP system? Visit the EnergySage Marketplace to receive free, custom quotes from local installers. If you're interested in thermodynamic panels because you don't get enough sunlight, you can join our community solar marketplace to still receive the benefits of solar without installing a PV system on your property.