Thermodynamic, exergoeconomic and multi-objective optimization analysis of new ORC and heat pump system for waste heat recovery in waste-to-energy combined heat and power plant

Abstract

Waste-to-energy (WTE) technology is regarded as the most promising way to deal with municipal solid waste because it has the advantages of saving land area and reducing the emission of pollutants. However, the electrical efficiency of WTE combined heat and power plant is low. One factor that leads to such result is the large heat loss of the boiler exhaust gas. Besides, the amount of high-temperature steam used for power generation decreases because a part of high-temperature steam is used to provide district heating (DH), resulting in a low electricity output. In this study, a novel combined organic Rankine cycle (ORC) and heat pump cycle (ORC-HP) system is analyzed. The waste heat of the exhaust gas is recovered by the ORC and generates mechanical work to drive the HP system, which absorbs the waste heat of low-temperature and low-pressure vapor for the DH. Considering the environmental compatibility, different organic working fluids are compared to select a suitable working fluid for the combined system. Comprehensive thermodynamic and exergoeconomic analyses are performed to identify the effects of different parameters on combined system. And the economic benefits of the system are considered from the perspective of the investment payback period (PBP). Furthermore, based on sensitivity analysis, multi-objective optimization analysis was applied in the combined system to determine the optimal working conditions. The results indicate that butane and ammonia are the most suitable working fluids. Sensitivity analysis results show that for SIC, the evaporator pressure of ORC and the superheat degree of HP evaporator have a greater impact, and for ORC-HP system efficiency, the evaporator temperature of HP, the superheat of HP evaporator and ambient temperature have a greater impact. Optimization results show that, the optimal PBP and the specific investment cost (SIC) of the new combined system are 0.48 years and 325.94 $/GJ. After optimization, SIC reduces from 333.15 $/GJ to 325.94 $/GJ, with a 2.2% reduction and the product unit cost of DH reduces to 33.97 $/GJ.