An experimental method is described for simulating the useful energy supplied by collector arrays during tests of solar water heating systems. The method uses an electric heat source to simulate the absorbed solar energy in series with nonirradiated collectors to simulate the concurrent heat losses. This configuration maintains the collector-loop flow characteristics which are important for system tests. Expressions are developed for programming the heat source for collector arrays connected in parallel and series combinations with the heat source located either upstream or downstream from the nonirradiated array. Thermal modeling of representative arrays is used to investigate the consequences of using linearized collector efficiency curves to program the heat source and of using nonirradiated collectors to simulate heat losses. The absence of absorbed solar radiation in collector covers indoors is shown to partially cancel the effects of generally higher windspeed and increased longwave radiation loss in the outdoor environment. In typical situations, the analytical model showed that the use of nonirradiated collectors in series with an electric heat source may give up to 10 percent higher useful energy output as compared to an irradiated array. The difference, however, can be reduced by closely matching indoor and outdoor environmental conditions and by locating the heat source downstream from the nonirradiated collector array. The results of experiments to verify the performance of a nonirradiated array with a downstream electric heat source are presented. Day-long tests of a domestic solar hot water system with irradiated collectors were repeated using a nonirradiated array with a downstream heat source. The measured useful energy in the two cases was consistent with the results of the analytical investigation.

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