Common control strategies used to regulate the flow of liquid through flat-plate solar collectors are discussed and evaluated using a dynamic collector model. Performance of all strategies is compared using different set points, flow rates, insolation levels and patterns (clear and cloudy days), and ambient temperature conditions. The unique characteristic of the dynamic collector model is that it includes effects of collector capacitance. In general, capacitance has a minimal effect on long term collector performance; however, short term temperature response and the energy =storage capability of collector capacitance are shown to play significant roles in comparing on/off and proportional controllers. Inclusion of these effects has produced considerably more realistic simulations than any generated by steady-state models. Simulations indicate relative advantages and disadvantages of both types of controllers, conditions under which each performs better, and the importance of pump cycling and controller set points on total energy collection. Results show that the turn-on set point is not always a critical factor in energy collection since collectors store energy while they warm up and during cycling; and, that proportional flow controllers provide improved energy collection only during periods of interrupted or very low insolation when the maximum possible energy collection is rela= tively low. Although proportional controllers initiate flow ·at lower insolation levels than on/off controllers, proportional controllers produce lower flow rates and higher average collector temperatures resulting in slightly lower instantaneous collection efficiencies
