Heating, Hot Water With Solar PV

The efficiency of the air-source heat pumps and hybrid hot water tanks also swayed the Enoses toward their solar electric solution.

“An advantage for Dave in using a PV system is that he’s using the grid as an unlimited storage space, so when he’s away the PV system is spinning the meter backward,” says Chris Oriel, manager of the solar hot water division for the Enoses’ solar installer, New England Clean Energy in Hudson, Mass.

The apartment over the garage is heated with a Fujitsu air-source heat pump (right). (image via Green Tech Advocates)

Why Not Solar Thermal for Heat?

There’s a temptation, particularly in northern climes where forced hot water or steam radiators heaters are used, to try to heat a home with a solar thermal system, which heats water or antifreeze to heat water, principally for domestic water use. This would conceivably replace gas- or oil-fired boilers.

And while much of the northeast, where the Enoses live, uses gas or oil-fired water boilers for radiator or baseboard heat, solar thermal is often not the best option for heating, says Oriel of New England Clean Energy. One exception may be for use with hydronic heat via a radiant in-floor system that requires lower water temperatures than baseboard heating.

Yet even doing that with solar thermal can be challenging. “In the northeast, our space heating load is greatest when we have the least amount of sun and when the angle of the sun is lowest,” Oriel says.

In addition, solar thermal systems that are used for heating need more collectors, so instead of two collectors as many systems have, there could be four or more. This makes the system more expensive and more complex. And if the home’s heat is forced hot water, a boiler will still need to boost water temperatures (and should serve as a backup anyway). In addition, when all the hot water in a solar thermal tank has been heated by the system, such as in summer months, excess heat must be dealt with. Oriel says his company’s systems now all use steamback protection that voids heated antifreeze to an expansion tank to prevent the system from overheating.

Oriel says that many of the homeowners with solar thermal systems by New England Clean Energy use their systems just for hot water. Yet some only run their oil- or gas-fired boilers during the winter for heat, and leave the boiler off during the other three seasons to save on fossil fuel. They can use the boiler for backup, but they’re not buying oil or gas to heat hot water during the summer, spring or fall.

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