The ground-source heat pump is a promising solar thermal application. It will save on heating expense and reduce the cost of air-conditioning.

The ground-source heat pump is a promising solar thermal application. It will save on heating expense and reduce the cost of air-conditioning.

This system is advertised today as a geothermal heat pump, suggesting that it uses the searing heat of the earth’s mantle, which is many miles below the surface. Actually, the GSHP uses solar energy, stored in the earth’s crust, in water that was heated by the sun before leaking down into the earth. It is our critical aquifer, insulated by the earth’s crust, with a stable temperature of 50-55 degrees F, year round.

The GSHP heats and cools the building with almost the same equipment as a household heat pump, which is a central air conditioner that dumps house heat into the hot outside air in summer and, with a reversing valve, extracts heat for the house from the cold outside air in winter.

There is one important difference, for both heating and cooling. The GSHP is much more efficient. Its heat exchanger is underground piping, where its environment is cooler than the summer air and warmer than the winter air.

In an existing home the investment in new equipment, plus outside digging, is hard to justify. Builders are starting to install GSHPs in new homes, however, because potential buyers will be pleased to use renewable energy to protect the environment and to reduce the threat of rising fuel costs.

The other way to use the sun’s rays directly is, as you know, photovoltaic (PV). This technology has limitations that can add up to poor return on investment for the homeowner.

The cells are only about 15 percent efficient at converting sunlight to electricity.

Also, most household use occurs before and after the collection period, except where air conditioning is used for a large part of the time. That means the homeowner must store the PV output, or sell it to the utility.

Storage in batteries with short lives is expensive.

Selling to the utility is not a very good option, either, because the utilities will only pay about one-third of what it costs you to buy from them.

In summary, photocells on a house roof, without many months of daytime air conditioning, will rarely provide a good return on investment.

There are, however, sensible applications for PV.

Where wiring is impractical, they are great for driveway lights, garden illumination, movable highway signs and advertising displays.

Finally, there is already heavy investment in PV installations on the roofs of businesses and institutions where air-conditioning is considered essential.

There is support from local governments, also, and from the ISOs, the operators of the high-tower grids that connect the power plants to local retailers like NSTAR. They are very worried now, because summer peak loads are already outrunning their capacity, threatening blackouts.

There is true synergy here, with the PV output at peak when the building’s A/C load is at peak, and at the same time that the grid itself is in peril.

It makes me realize that I should not have been critical of Google recently for their major investment in PV at their headquarters.

In the last analysis, anything that reduces our risky dependence on fossil fuels is a gain for society.