maintenance and repairs (and the prospect of technological advances that improve performance) must also be estimated. Expectations about its resale value also play an important part.

Each potential buyer of a solar system will, for individual reasons, make certain assumptions about these cost considerations. Some may choose to compare the cost of solar heat with that of using electricity rather than natural gas, even though gas is now cheaper, because they expect gas to be unavailable or extremely expensive in 20 years. Some may assume that the price of alternatives will rise fast enough over the 20 years to make solar heating economic. Some may install solar systems for noneconomic or personal reasons, just as consumers often prefer higher-cost electricity over cheaper fuels because of its perceived advantages in convenience or cleanliness. We can state no universal conclusion as to the relative attractiveness to consumers of solar space heating and conventional sources. However, because of the expected long-term economic, social, and environmental benefits, it is appropriate for the government to grant tax advantages or other subsidies that alter market economics, thereby accelerating the introduction of this technology.

Solar Space Cooling

No technically and economically adequate solar space cooling technology is now available for small residential applications. A number of approaches that are being explored are described in the report of the Solar Resource Group.14 None of these approaches yet meets the requirements of simplicity, low cost, efficiency, and effectiveness of waste heat rejection that will likely be necessary for consumer acceptance.

A fundamental problem is that it is very difficult to operate an air conditioner from a flat-plate solar collector when the waste heat must be rejected by a dry cooling unit (the type used by most residential air conditioners). The efficiency of such a unit is very low, and at other than design conditions, the unit may fail to perform at all. To solve this problem at present, one must either (1) operate the collector at higher temperatures (which requires more sophisticated collectors and some form of energy storage other than hot water) or (2) use an evaporative, or “wet,” cooling tower (which consumes significant amounts of water and requires frequent maintenance). Neither solution is satisfactory, and a new approach to solar cooling for individual residential buildings may be required.

Solar cooling of larger buildings or clusters of residential buildings is less difficult. This is because economies of scale allow use of fairly complex units and because such facilities already have maintenance arrangements for their heating and ventilation systems. A number of systems have been demonstrated successfully, and a commercial 25-ton solar cooling unit is

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