A thermally driven reverse osmosis process that eschews reliance on fossil fuels has been demonstrated as a sustainable seawater desalination route by researchers from McGill University (Canada) and Oakland University.

Low-grade heat harnessed from solar thermal, geothermal heat and other sources of renewable energy drives the piston-based system to produce fresh water. A small amount of working fluid — preferably saturated water — is heated and cooled in a sealed chamber. The working fluid expands in response to temperature fluctuations, activating it to drive a piston and push seawater through a reverse osmosis membrane.

The research published in the journal Desalination confirms that a properly designed thermally driven reverse osmosis system can achieve specific heat as low as 20 kWh/m³, first law thermodynamic efficiency of 7.9%, second law efficiency of 18.1% and a gain output ratio of 33. This performance is achieved at reasonable working pressure and temperature of less than 40 bar and 250° C.