Exergy Control for a Flat-Plate Collector/Rankine Cycle Solar Power System

A performance study is presented of a Rankine organic cycle powered by a low temperature solar collector. In this work a two-phase collector is considered where the heat transfer fluid is vaporized and its saturated vapor expands in a turbine according to a Rankine cycle. The collector system is divided into a boiling and a nonboiling (subcooled) part: The limit between the two depends upon the value of flow rate and radiation. A modified form of the Bliss equation is used to model the thermal performance of the collector in terms of thermal efficiency versus DTI [DTI= (Absorber average temperature-Ambient temperature)/ Solar Radiation]. The system is analyzed by second-law analysis, and it includes several exergy losses of different types (heat transfer, heat loss, etc.) which determine the overall exergy balance. Different working fluids are considered, and optimization to a certain extent is demonstrated from this point of view. In order to minimize irreversibilities and guarantee the most efficient conversion processes, the most important point is the right selection of the collector operating pressure level, which depends on the instantaneous value of radiation and ambient temperature (as well as on the collector thermal performance). The choice of the optimal pressure level is done by means of second-law arguments; the flow rates across the collector, the turbine, and the condenser are consequently determined. A simulation over a typical sunny day in Florence, Italy allows the calculation of the expected daily performance.