Solar PV or Solar Thermal?

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Hybrid hot water tanks like this GeoSpring model from GE take in surrounding warm air to help heat water efficiently.

For this homeowner, using a solar electric system for heat and hot water made sense.


Oct. 19, 2012 — by .(JavaScript must be enabled to view this email address)

Can you get all of a home’s electricity, heating and hot water from a solar photovoltaic (PV) electric system?

That’s what Dave and Suzy Enos of Bedford, Mass., are doing with their home and a 1,300-square-foot addition.

The panels for a 10-kilowatt Sanyo solar electric array line the roof over the Enoses’ new three-car garage, which is wired for 220-volt electric vehicle chargers. Also under the roof is a rental apartment in the making, which will be heated by a Fujitsu AOU12RLS air-source heat pump that can also cool the space. The heat pump, which resembles a mini-split air conditioning unit, is powered by the solar electric array—and this is tied to the electric grid to effectively sell power back to the utility, thereby cutting or eliminating the Enoses’ electric bills.

An 80-gallon Kenmore hybrid water heater will heat water with a built-in air-source heat pump and electricity from the solar array, Enos says. The heat pump unit uses warm air around it to help heat the water. An Energy Star-rated hybrid water heater that uses heat pumps and electricity—either from the grid or a solar array—can save you $300 a year or more in energy costs.

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Dave Enos of Bedford, Mass., in front of his addition with a 10-kw Sanyo solar photovoltaic (PV) array.

The Enoses paid $55,000 for the solar system, $51,000 or which was out of pocket. After federal, and state tax credits and other incentives, they’ll pay about $31,000 and look to pay off the system in six to eight years.

Energy savings will not just result from the solar array. The addition is super-insulated, with a thick, R60 level of insulation under the roof, R40 lining the walls, R20 in the basement and sides of garage walls and R10 under the concrete garage floor. The addition clad in an insulating HardiePlank cement fiber siding that has the weight and feel of tile.

The existing part of the house will undergo a deep energy retrofit, with additional insulation under the roof and in the exterior walls. Once the rental unit is completed, the Enoses will move into that and renovate their existing house. That will be heated and cooled by two more air-source heat pumps.

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The 1,300-square foot apartment above the garage will be heated with an electric an PV-powered air-source heat pump (right).

Using a solar electric system to provide heat and hot water for a home is somewhat unusual, but in the Enoses’ case, it made sense. Solar thermal systems, which produce domestic hot water—and in some cases home heating—by heating water or antifreeze behind glass plate collectors, often provide “better bang for the buck,” and generally cost around $10,000 to $12,000, which is far less than most PV systems. But Enos was concerned about a solar thermal system’s maintenance as a more mechanical system, especially if the couple is traveling or if renters are confronted with system issues. The Enoses have lived abroad for months at a time, though Dave says he pulled the trigger on this big energy renovation because “my wife and I are planning to stay here” and will reap the energy saving benefits from the system after it is paid off.

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.

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.

“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,” he 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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