It may seem a novel concept to heat your domestic water supply with a refrigerant, but it's not so odd when you really think about it. After all, the area behind the refrigerator in the kitchen is pretty warm, so surely there's a way to turn this to your advantage?

Refrigerant-based heaters work on the principles of physics as applied to liquids. When a liquid changes into a gas (vaporizes), it absorbs a load of energy or heat to power the change; when it converts back into a liquid (condenses), it releases that stored heat. If an appropriate liquid can be found – one which vaporizes at low temperatures and stores a ton of energy in doing so – there's the potential to use it to heat a domestic water supply.

The ideal liquid for this setup is Freon, the trade name of one of DuPont's halocarbon products. It is nontoxic, nonflammable and boils at around -22°F (-30°C) under normal pressure. In a refrigerant-based solar thermal system, the Freon travels in a closed loop to the collectors and is heated, vaporizes and returns to the water tank where it condenses and transfers all its stored energy to the domestic supply as heat.

Because Freon works so well at very low temperatures, water heaters of this type can work with extremely low energy inputs and at low temperatures, making them attractive in areas where the sun's radiation rarely achieves full strength. Some manufacturers claim that refrigerant-based systems work so well, they don't even need sunlight because they can absorb heat from the surrounding air!

Refrigerant-based heat pumps are often touted by electrical utilities as an energy-efficient alternative to natural gas or standard electrical heaters because they have a high "coefficient of performance" (COP). The COP measures how many Btus of energy a heat pump delivers for every Btu of electricity it consumes, and many refrigerant-based models have impressive ratios reaching 2.5 or 3 Btus returned for every Btu consumed.

However, there's more to this story. A heat pump uses about 1kW/h of electricity to generate 10,000 – 13,000 Btus of heat. 1kW/h is 3,400 Btu, so that looks impressive until you figure in the generation and transmission losses inherent in grid-tied power plants, which run at more than 66%. With that utility-side loss included in the calculation, the ratio comes out at 1:1 instead.

There's also the cost element to consider. Refrigerant-based heat pumps cost a lot to operate and, compared to the low cost of running any other kind of solar thermal installation, that makes them a far less attractive option. Add the facts that refrigerant-based pumps lose efficiency as the temperature drops and that the equipment can be damaged by operating in very low temperatures and they're even less attractive. Plus, Freon is a CFC that can affect the ozone layer, depending on which type is used.

In the end, a refrigerant-based heat pump system isn't a viable option at this time. Perhaps, in the future, advancements will be made that will make them a better return on investment.