The pump is on when the temperature

The pump is on when the temperature of hot water leaving the
solar collector is higher than that of the storage tank and
off otherwise. In practice, especially for the temperaturestratified
storage tank, when the solar radiation level is
not high enough if the flow rate is kept constant as the
guideline value, then the temperature of water at the
collector outlet might be less than that of the water at the
top of the storage tank. Then the pump is off and the
system could not get the benefit from the sun even the
solar radiation level is higher than the critical solar
radiation,