The efficiency of solar collectors is dependent on several features in a solar hot water system. In addition to the amount of solar radiation absorbed by the solar collector and the temperature of the ambient air, the efficiency is also determined by the flow rates of the heat transfer fluid in the solar hot water system.
At it’s most basic level any solar collector is nothing more than an air to liquid heat exchanger. The sun provides the heating on the outside of the collector (air side) and the fluid flowing through the collector picks up the heat as it passes through. In any air to water heat exchanger the amount of heat that is transferred over time increases as the flow rate increases. Along the same lines, any heat exchanger that the solar fluid passes through (either in a tank or external) increases it’s rate of heat transfer with higher flow as well. So, higher flow rates increase both the amount of heat that is extracted from the collectors as well as the amount of heat that is passed into storage. In general, the efficiency of solar water heating systems improve as flow rates increase. The reason all systems aren’t pumped at the maximum flow rate is because as the flow rate increases the pumping power required generally increases as well. At a certain point the increased efficiency you achieve through higher flow rates is offset by the greater pumping power.
While we get the question all the time “what is the right flow rate for this collector?” The real answer is hidden in the details. We do not like to see systems that are pumped at a fluid velocity beyond what the piping can support (see previous blog). That being said adding a flow meter to a system so you can make sure it matches exactly the “recommended” flow is counter productive. Flow meters to confirm flow make sense. Flow meters to control flow don’t. Pumps come in a finite number of sizes and the best answer is to choose a pump that matches your system design. When in doubt choose a larger pump (but not beyond the flow limits of the pipe) and the little you pay extra in pump energy will more than be made up in system output.