Comprehensive analysis of a novel power and cooling cogeneration system based on organic Rankine cycle and ejector refrigeration cycle

Abstract

A novel combined power and cooling system based on the organic Rankine cycle and the ejector refrigeration cycle for highly efficient utilization of low-grade heat is presented, in which the endothermic process adopts a dual-pressure evaporation approach and the two vapor generators are connected in series. A mathematical model is developed to evaluate the system thermodynamic and exergoeconomic characteristics. The effects of key parameters on system performance are evaluated. Results show that a higher low-pressure evaporation temperature and a higher vapor fraction at the low-pressure vapor generator are conducive to increasing the system cooling output. An optimal high-pressure evaporation temperature exists that gives the maximum exergy efficiency and the minimum sum unit cost of product. Compared with the net power output of the system, the cooling output is more sensitive to the variation of condensation temperature. Among the system components, the ejector has the highest exergy destruction rate and the lowest exergy efficiency. Furthermore, optimization of the system’s performance and working fluid selection for fixed cooling outputs was conducted. The results show that reducing the exergy destruction in the endothermic process is the key to improving system performance, while perfluoropropane was found to be the most suitable working fluid for the proposed system. In the cooling output range of 300–700 kW, a minimum sum unit cost of product of 45.79–58.87 $/MWh can be achieved, and corresponding ranges of net power output, energy efficiency and exergy efficiency are 614.93–430.58 kW, 14.32–19.25%, and 32.3–22.62%, respectively. Finally, the performance of the proposed system is compared with two typical systems for a cooling output range of 300–700 kW. The results show that the sum unit cost of product is reduced by 7.9–11.1%, and the net power output increased by 23.6–40.6% compared with the system with parallel vapor generators. Compared to the system with the ejector installed after the turbine, the sum unit cost of product is increased by 9.16–13.28%, and the net power output is increased by 129.73–118.38%.