Sustainable freshwater production using passive membrane distillation and waste heat recovery from portable generator sets

More than two billion people live in areas affected by water stress. In some coastal regions, freshwater supply has been progressively improved by large-scale desalination systems, which are nowadays mostly driven by non-renewable energy sources. Here we discuss, and experimentally investigate, the use of small-scale desalination devices for freshwater production powered by waste heat from electric power generators. The water purification technology relies on a passive, multi-stage and thermally-driven membrane distillation device, recently proposed by some of the authors of this work. The distiller is powered by low-grade (temperature lower than 80 °C) waste heat, recovered from the coolant circuit of small diesel engines for electricity production. Field experiments show that, for the tested engine, up to 1.12 kW m-2 can be recovered in standard operating conditions, which yield a nearly 2.61 L m-2 h-1 freshwater production from seawater. A lumped parameter model, validated by experiments, shows that this productivity could be eventually enhanced by tuning the number of distillation stages. Utilization with exhaust gases, and thus higher feeding working temperatures, is also discussed. The proposed solution may provide a sustainable, simple, inexpensive and efficient means for freshwater production from recovered waste heat, which would otherwise be wasted to the ambient. Therefore it could be particularly effective, for instance, for field hospitals in remote or impoverished areas, especially in emergency situations.