Proposal and thermodynamic analysis of a combined open-cycle absorption heat pump and thermal desalination system driven by high-humidity exhaust gas

A combined system based on an open-cycle absorption heat pump (OAHP) and a low-temperature multi-effect evaporation (LT-MEE) water desalination process is configured, modeled, and analyzed. The system is applicable to be run by high-humidity gases, and the flue gas from the combustion of natural gas is used here. Compared with conventional absorption heat pumps, the OAHP is characterized by a gas-solution direct-contact absorber, where not only the sensible heat but also the latent heat and water vapor in flue gas are partially recovered. The OAHP-MEE system has good internal synergy, as demonstrated by a 36%-74% water production gain over the simple system when driven by flue gas with temperature of 120 °C-250 °C. With the increase of the feed-gas temperature, the combined system becomes increasingly favorable over the simple system in energy utilization and water production. For a specified feed gas condition, an optimal water recovery rate of OAHP and a maximum number of effects of MEE exist, mutually leading to a maximum water output. Using a cooler to cool the strong solution entering the absorber not only has a favorable effect on water production, but also widens the parameters' ranges, thus offering good flexibility for design and operation.