Thermodynamic and Economic Analysis of a High Temperature Cascade Heat Pump System for Steam Generation

A high temperature cascade heat pump (HTCHP) system using low-grade heat from waste water to generate steam for industrial processes is studied from the thermodynamic and economic view. The effects of intermediate temperature, heat source temperature, heat sink temperature, working fluids, and the pinch point temperature difference at the evaporator, condenser, and intermediate heat exchanger on the thermodynamic performance and cost-effectiveness of the HTCHP system are investigated. The PBP varying with the gas price and electricity price is evaluated as well. The results show that optimal intermediate temperatures exist under different operational conditions, not only for the COP, but also for the PBP. A high heat source temperature and low heat sink temperature are conducive to increasing the COP and shortening the PBP. Working fluid pair R1234Ze(E)-R1233zd(Z) shows a promising application in the HTCHP system for its relatively high COP, short PBP, and low GWP. A lower pinch point temperature difference will cause a higher COP and shorter PBP, but a higher SEC. The energy price has a great effect on the economic viability of the system. When the price ratio is lower than 1.81, the PBP is lower than 4.6 years.