The flow rate of the refrigerant in the air-refrigerant heat exchanger is controlled by a thermostatic expansion valve together with a feeler bulb. The flow stops when the degree of superheat at the exit of the heat exchanger reaches to the stationary degree of superheat, Ats.

Therefore, the heat exchanger can absorb heat from the ambient air only when the evaporation temperature is less than ta-Ats.

Using the panels and the heat exchanger as the evaporators, which are arranged in parallel, only the refrigerant flows through the panels if the evaporation temperature is higher than ta-Ats.

In the case that the evaporation temperature is less than ta-Ats, the refrigerant flows through both evaporators. Assuming that there is no pressure loss at the evaporators, the evaporation temperatures become the same in the two evaporators. Then,

AF'{S — U(te — ta )} + K(ta — te ) — Q