Due to the diurnal cycle, both the cooling load and the storage tank temperature vary through the day. Therefore, designing such a system presents appreciable difficulties. When a Rankine heat engine is coupled with a constant-speed air conditioner, the output of the engine seldom matches the input required by the air conditioner. Therefore, auxiliary energy must be supplied when the engine output is less than that required; otherwise, excess energy may be used to produce electricity for other purposes.