The potential of waste heat utilization in solar multistage membrane distillation: An alternative to PV + RO systems?

Solar membrane distillation (MD) is an emerging technology for addressing water scarcity in arid regions with access to terrestrial water bodies. Recent studies highlight that vaporization enthalpy recycling enhances both distillate flux and solar-to-water efficiency. However, due to high material costs, scaling beyond 10 stages under one-sun solar illumination becomes economically unfeasible. A novel waste heat utilization concept for continuous freshwater production is introduced, supported by a generalized Python-based mathematical model and COMSOL Multiphysics simulations that consider simultaneous heat and moisture transfer. The findings highlight the potential of low-grade waste heat to enhance the efficiency of MD processes. With a solar-to-waste heat ratio of 0.4, a 9-stage MD system can match PV + RO water production per exergy use (36.2 L kWh^-1), while even a 3-stage MD system operating solely on waste heat surpasses PV + RO performance. This highlights the importance of cross-sectoral integration, where polygeneration, water, and electricity production play a key role. Techno-economic analysis indicates that such a system in Spain could achieve a 6-year payback period, reinforcing its potential as a competitive alternative to PV + RO for freshwater production. This work contributes to UN Sustainable Development Goals 6 and 7 (SDG-6, SDG-7), promoting clean water, sanitation, and sustainable energy solutions.