Contemplating the solar-desalination sunrise
Desalination technologies have predominantly been powered by fossil fuels, but the recent UN Climate Change Conference (COP26) has indicated that our society needs to rapidly accelerate the phase-out of fossil fuels to keep the 1.5oC global temperature rise target within reach. Deploying solar-driven desalination technologies can help hasten—rather than hinder—the requisite emissions reductions needed to reach this target while also alleviating water scarcity in locations where fossil fuel infrastructure is inaccessible or incapacitated. The good news is that solar-driven desalination technologies are available today from the small to the large -scale.
At the large-scale end, solar photovoltaic farms can be used to provide electricity to large-scale reverse osmosis. Alternatively, the waste heat from large-scale concentrated solar power (CSP) plants can be used to drive multi-effect distillation (MED) systems. A recent study by Omar et al.  has shown that having an on-site desalination plant (with the MED configuration) can improve the CSP plant’s payback period by 5-20% (depending on the water purchase agreement price). This is due to having an additional product that could be sold to the market being produced by the free waste heat from the power block. However, a nearby feed water source (or an additional pumping cost from the coast or an in-land sea). As such, large-scale solar thermal desalination is limited to locations with high solar resources, easy access to feedwater, and markets with relatively high water and energy prices. A large-scale solar-driven reverse osmosis plant faces many of these same issues, but the water and energy plants do not need to be co-located.
At the small-scale end, a team at the University of New South Wales (UNSW) in Australia is developing portable modules that can be deployed in urban and rural areas, either permanently as off-grid solutions or temporarily for disaster relief. These technologies include photovoltaics-driven reverse osmosis (RO), solar thermal-driven membrane distillation, and solar stills. Their team’s goal is to get one or more of these technologies across the line in terms of translational research and knowledge exchange with industry partners. More information can be found on UNSW's ChallENG website.
The UNSW team believes that we need a robust pipeline of technologies at all scales to ‘solarize’ the energy inputs for desalination technologies. This will enable solar solutions to simultaneously meet the two pressing UN Sustainable Development Goals of ‘affordable and clean energy’ and ‘clean water and sanitation’ for all.
 Omar, A., Nashed, A., Li, Q., Leslie, G., Taylor, RA. “Pathways for integrated concentrated solar power-Desalination: A critical review” Renewable and Sustainable Energy Reviews, 119, 109609, 2020.
Amr Omar, Robert A Taylor and Greg Leslie