Novel thermal desalination with solar-powered driven CO2 heat pump and additional cooling benefits: theoretical and economic assessment with experimental validation

Abstract

This study proposes and analyzes the performance of an innovative thermal desalination arrangement characterized by its space-efficient, vertical configuration, akin to vertical farming. Central to this system is a CO2 heat pump, employed as an environmentally friendly technology to heat seawater. Notably, the system also explores methods to utilize the cooling effects generated by the heat pump. The energy required for the heat pump is supplied by a solar photovoltaic (PV) array, designed following agrivoltaics principles to enable dual land use for both energy production and agricultural activities. Drawing inspiration from traditional solar stills, this desalination system differentiates itself by indirectly utilizing solar energy to power the CO2 heat pump, rather than directly heating the water. Accordingly, a metal plate replaces the typical glass cover, and a water spraying or cooling mechanism is proposed to maintain the surface temperature near ambient levels, thereby enhancing the yield of desalinated water. Use of a metal plate permits stacking of such systems, further reducing land use. The water flow rate is regulated by the evaporation rate to ensure a steady state. A laboratory-scale experimental validation is provided, suggesting a substantial increment in desalinated water yield compared to conventional solar stills, producing 34.5 mL in 30 min at 60 °C water temperature versus only 8 mL in 12 h for a traditional design. The proposed system achieves a heating COP in the range 2.65–3.79, and cooling COP 1.95–3.95, for a range of water pool temperatures. A comparative economic analysis of the proposed system is also presented, indicating a payback period of 18–25 months and a reduction in CO2 emissions up to 85%, with solar PV integration. Ultimately, it addresses critical challenges of freshwater scarcity, land utilization, and cooling requirements, offering a viable solution for seawater desalination.