Techno-economic feasibility of absorption heat pumps using wastewater as the heating source for desalination

Abstract

The utilization of low-grade heat could significantly reduce energy consumption in thermal desalination plants. This paper studies the feasibility of using low-grade heat for thermal desalination via a hybrid absorption heat pump system, in which ammonia–water is the working fluid. A thermodynamic model was proposed to simulate the operation for freshwater production of 5000T/day using multiple effect distillation (MED) technology. Results from the evaluation showed that the performance and waste-water usage are affected by operating pressures and temperatures. Based on Net Present Value evaluation, the economic profitability could be achieved at recommended operating conditions of absorption pressure (<25bar) and generation temperature (>30°C) at a generation pressure 5bar. A case study is presented in which water at 90°C, discharged from process industry in the Sheffield region, is used for generating 20,000T/day of freshwater. The system proposed consisted of four-stage sequential heat pumps with different operating configurations. These served as individual MED stages. The coefficients of performance were evaluated to be 3–11. According to the specific power consumption, the proposed system is a competitive technology compared with reverse-osmosis technology. Economic profitability could be achieved in 3years.