Modeling and energetic-exergetic evaluation of a novel screw expander-based direct steam generation solar system

This paper proposes an innovative solar electricity generation system based on the steam Rankine cycle, which employs screw expanders and parabolic trough collectors, with water as heat transfer and working fluid. Owing to its good efficiency at low-medium heat source temperatures, the solar generation system can meet the need to design renewable energy power plant with simple technical requirements in heat storage and collection. Screw expander-based solar thermal electricity systems have further advantages: avoidance of overheating, satisfactory applicability in energy conversion with liquid-steam mixtures, low operating pressure and good applicability for distributed power generation. In this paper a thermodynamic mathematical model on the polytropic expansion process is first investigated, revealing variation of isentropic efficiency with the operating pressure ratio and built-in volume ratio of the screw expander. Basic criteria are then provided to evaluate the energetic and exergetic benefits of such a power plant, developing a macro balance to define optimum operating conditions. Water evaporation temperature and condensation pressure were thus optimized with respect to heat-to-power conversion efficiency, solar thermal power generation efficiency and exergetic efficiency. Our results indicate that the optimum evaporation temperature ranges from about 204 degrees C to 276 degrees C with increasing solar radiation, whereas the corresponding maximum solar thermal power efficiency and exergetic efficiency are around 12.6% and 17.8%, respectively.