Through a series of single-day experiments, we have compared in detail the performance of on/off and proportional control strategies for solar hot water systems. The experiments encompass a variety of meteorological conditions, and include start-up with both warm and cool storage tanks. Insolation, tank temperatures, and pump energy use are monitored nearly continuosly throughout the day. Results show a consistent advantage in thermal energy gain for proportional control, especially under conditions of low or variable insolation, although proportional control requires considerably more parasitic pumping energy under these conditions.

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