High flow rates have been used in pumped circulation solar water heaters
to improve the heat removal factor, FR, and thus maximize the collector efficiency. If the complete system performance is considered, however, rather than
the collector as an isolated element of the system, it is found that the solar fraction can be increased if a low flow rate through the collector and a thermally
stratified tank are used. Stratification can also be promoted by the use of flow
diffusers in the tank and collector loop heat exchangers; for maximum effect,
however, it is necessary to combine these features with low flow rate.