In the past, solar energy has been prohibitively expensive, with payback over 20 years for most applications. Photovoltaics are still quite expensive, while parabolic collectors and other concentrating means are not much better. The machinery and controls, in either case, are still too complicated for all but the largest installations or installations that are remote from conventional power sources.
Solar hot water panels, on the other hand, are nearly up to the threshold of affordability. More effective glass glazing to trap solar energy has boosted performance, while less expensive materials, higher-quality artistry, and mass production have combined to reduce the cost per unit. Finally, increasing energy costs and government incentives for alternative energy technologies and energy-conserving products have given solar applications a respectable payback in many common circumstances.
The ideal situation uses a bank of solar panels to augment an existing system, i.e., to temper cold water rather than to heat it to the temperature of use. The integral, small system circulating pump and thermostat controls are dependable and efficient, even for residential applications. The existing water-heater tank serves as the storage vessel and as the auxiliary heat source on cloudy days or at times of extreme demand.
The objective of such an arrangement is to use the panel at its most efficient state, i.e., to warm the coldest water by 10 to 15°. As the water temperature increases the heat-transfer rate drops off, and the system becomes less effective. An ideal application of this process is warming swimming-pool water, such as for a therapeutic hospital pool or a health club pool. The water is always cool, so it absorbs the maximum possible solar energy for the pump energy expended. The result is a copious quantity of warm water, using solar panels in a project that can have a payback of 10 years or less. An additional advantage of such a system, in very warm latitudes, is that in the summer the pool water can continue to be circulated through the panels at night, to radiate heat and to cool unseasonably warm water to more comfortable levels.
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Excerpted from Going Green, available as an eBook at this list of eBook retailers online.