A discussion of “Heat pumps as a source of heat energy for desalination of seawater”

The paper, “Heat pumps as a source of heat energy for desalination of seawater”, was published in Desalination [Vol. 139, 2001]. In the paper, heat pumps using agent R12 or water and vapor were introduced to be used as a source of heat energy for desalination. The cyclic processes of heat pump were illustrated in T-S and P-H thermodynamic diagrams under the conditions of condensation temperature at 120°C and an evaporative temperature of 20°C, but there are some questions that need to be discussed. First, thermodynamic diagrams of the transcritical cycle of heat pumps using agent R12 are given. Second, the ratio between mass flows of the water vapor and agent R12 is discussed and the heat balance equations of desalination are given. Third, calculations from a 5 t/d desalination plant are given as an example.

By analysis of heat pumps as a heat source of energy for the desalination of seawater, heat pumps using agent R12 are not feasible. When a heat pump using agent water and vapor is used as the heat source of energy for desalination, it is feasible. The heat pump cycle is under the critical temperature (374°C). We have calculated the cycle of heat pumps using agent water and vapor at the evaporation temperature of 100°C and the condensation temperature at 140°C. It was shown that power consumption is lower than electronic compression desalination, as illustrated in Table 1. The conclusion was reached that the efficiency of heat pumps as an energy source for desaliTable 1 Comparison of power consumption for different kinds of desalination Type Production of Power fresh water, consumption, t/d kW Heat pump type Electronic compression 5 vapor type Electronic heating type 5 nation is higher than the other two types, but there are still many questions such as reducing the volume, lubrication, etc., which need to be researched for a compressor using agent water and vapor.