Small-scale renewable polygeneration system for off-grid applications: Desalination, power generation and space cooling

A stand-alone small-scale renewable heat powered polygeneration system to provide cold for space cooling, electricity, and seawater desalination was proposed and numerically modelled. The system is based on Permeate/Conductive-gap MD modules (P/CGMD) and an ammonia/water absorption power-refrigeration system driven by heat from an evacuated flat plate solar collector field with a biomass-fired backup boiler. Energy and exergy performance of the system was analysed using climate conditions of a typical location well-endowed with solar irradiation and characterized by a potential shortage of freshwater supply and high cooling demand. In the base-case, the system provided 130 kW of cooling capacity at 11 °C, 6.4 kW of net electrical power, and 41.4 m3/day of desalinated water based on annual average weather conditions of Almeria (Spain). The overall system's resource utilization efficiency and exergy efficiency were estimated at 44.2% and 6.9% respectively. The use of CGMD reduced the required specific membrane surface area by 53% compared to the PGMD. The results of this study show that a thermally coupled absorption refrigeration system and MD process can be implemented (i) to increase the efficiency of the entire thermal energy conversion process and (ii) to cost-effectively utilize the solar thermal installation, particularly in regions where desalination is a necessity.