Thermodynamic and thermoeconomic performance analyses and optimization of a novel power and cooling cogeneration system fueled by low-grade waste heat

Abstract

The ejector refrigeration cycle (ERC) is often used as the sub-cycle of power and cooling cogeneration systems due to its advantages of simple structure design, few moving parts, less system investment, good stability and reliable operation. Considering the potential of organic Rankine cycle (ORC) in the utilization of low-grade heat, and the advantages of simple structure and many choices for working fluid, a novel power and cooling cogeneration system was designed, based on dual-pressure evaporation ORC (DORC) and ERC. In this system, a common condenser was used by DORC and ERC, and a part of working fluid separated from low-pressure steam generator outlet in DORC was used as the primary flow of ejector to drive ERC. A mathematical model for calculating the thermodynamic and thermoeconomic performances of the system was established. Sensitivity analysis was performed to determine the key parameters of the system, the results showed that the low-pressure evaporation temperature (TLPSG,out), high-pressure evaporation temperature (THP,E), vapor fraction of the low-pressure steam generator outlet (xLPSG,out) and working fluid mass flow ratio of high-pressure stage to low-pressure stage (k) were the four key parameters of the system. Parametric analysis showed that higher TLPSG,out and lower xLPSG,out were beneficial to increase the cooling output and thermal efficiency, while higher THP,E and larger k were helpful to increase the net power output and exergy efficiency. The net power output and exergy efficiency can be optimized by TLPSG,out and xLPSG,out. Using genetic algorithm (GA), multi-objective function optimization was carried out with THP,E, TLPSG,out, xLPSG,out and k as the decision variables. Moreover, the system's adaptability for working fluids R245fa, R236ea, R600, R600a, R601 and R601a was investigated. According to the optimization results, R236ea was the most suitable working fluid. When THP,E was 402.43 K, TLPSG,out was 392.42 K, xLPSG,out was 0.357 and k was 0.321, the calculated net power output, cooling output, thermal efficiency, exergy efficiency and total cost of unit exergy product (SUCP) were 273.00 kW, 121.80 kW, 14.52%, 44.03% and 42.62$/MWh, respectively. Exergy analysis was conducted using R236ea as working fluid. The results showed that the two steam generators and condenser were the main components responsible for most of the exergy destruction of the system in both basic case and optimized case. Compared to the basic case, the exergy destruction of the steam generators decreased by about 32.3%, and the total exergy destruction of the system decreased by about 5.0% in optimized case.