One of the simplest designs for the removal of energy from a collector is the single stream arrangement. In this arrangement part of the incident solar radiation * Q is delivered to a user as extracted energy Qu. The remaining fraction Qo represents the heat transfer to ambient at temperature To. In practice, since it is easier to measure fluid temperature than collector temperature, an average collector temperature, Tc, is defined as the average of the inlet and outlet temperatures of fluid hence, the collector can be assumed isothermal. The heat transfer rate to ambient, then becomes, Qo=AU(Tc-To )Where U is the overall heat transfer coefficient between the collector surface and ambient based on collector area A and the average temperature of the collector, Too. 

For the isothermal collector model described above, obtain an expression for average collector temperature to operate the collector with minimum irreversibility(question doesn't include data, the answer should be an expression)