High Temperature Heat Pump Cascade Systems With Water (R718) as Refrigerant

Abstract

Abstract The heat demand of industrial processes is still mostly covered by fossil fuels, especially at temperatures above 160°C. The installation of high temperature heat pumps (HTHP) that can operate at these temperatures is an effective way to reduce industrial CO2-emissions and primary energy consumption. Water can be an attractive refrigerant due to its high critical point and environmental sustainability. However, using water as refrigerant also requires a waste heat source of high temperature, since any heat source below 100°C would result in sub-atmospheric evaporation, leading to more expensive equipment. If low temperature heat sources or even ambient heat should be utilized, the HTHP would consist of two coupled heat pumps, a cascade system. In this theoretical study a cascade high temperature heat pump process is presented and simulated operating with water and R1336mzz(Z)e. The heat pump consists of multiple compression stages with intercooling in order to increase the COP. Heat sink temperatures of up to 250°C were investigated and an optimal process design was identified for several heat sink and -source temperatures. The calculated COPs ranged from 1.5 under challenging conditions to up to 2.2 at moderate temperature lifts. The crucial evaporation pressure of the water heat pump has been close to one bar for most of the studies, so the installation of vacuum equipment could be avoided.