Performance analysis and optimization of cascade waste heat recovery system based on transcritical CO2 cycle for waste heat recovery in waste-to-energy plant

Abstract

The improvement in energy efficiency of the waste-to-energy plant (WTE) is limited due to the heat loss of flue gas and bottom ash. Many scholars have conducted the optimization studies on waste heat recovery system in WTE plant from the perspective of determining optimal system working parameters. However, few researchers pay attentions to reducing the cost of the system by cutting down the number of system components while guaranteeing the system performance.In this study, a novel waste heat recovery system combining a transcritical CO2 system, an organic Rankine cycle (ORC), and a compression heat pump/refrigeration system is proposed. The compression heat pump system can be converted into a compression refrigeration system without replacing any equipment, which is beneficial to reducing the cost of the system. Comprehensive thermodynamic, economic, and environmental analyses are performed to examine the performance of the system. Multi-objective optimization (NSGA-II) is utilized to obtain the optimal working parameters in both district heating and space cooling modes. Results indicate that the maximum energy and exergy efficiencies of the WTE coupled with waste heat recovery system are 71.75% and 67.92%, respectively. The value of ecological efficiency and dynamic payback period are 94.05% and 4.33 years, respectively. The maximum pressure of transcritical CO2 has positive effects on transcritical CO2 cycle while it has negative effects on the whole waste heat recovery system. The increment in evaporator pressure of ORC leads to a decrement in energy efficiency in district heating mode, but it improves the economic performance in space cooling mode. The optimal working parameters of both district heating and space cooling modes are determined. The maximum net present value (NPV) and minimum cost of the system are 20.40 M$ and 2.44 M$, respectively. The cost of the system descends greatly while the NPV slightly decreases. The comprehensive and sufficient theoretical analysis reveals the huge potential of waste heat recovery in WTE plant and provides a strategy to reduce the energy consumption as well as improving the economic and environmental performance.