Thermal efficiency of FPCs can be calculated directly by the inlet and outlet temperatures of working fluid, according to Eq.14. On the other hand, thermal efficiency of CPC collectors is determined by the heat transfer process in the evaporator. The temperature relationship between working fluid and conduction oil must be established.
This section focuses on heat transfer in the evaporator, and the developed equations can easily be extended to the case of the condenser. Counter-current concentric tubes are adopted, and the parameters are listed in Table 2.
The following preconditions are assumed: 1) the influence of pressure drop on the saturated temperature arising from flow resistance in the evaporator is negligible; and 2) the two – phase flow is one-dimensional, that is, all parameters change only in the flow direction (Y).