Low-grade heat from renewable sources could be used for water desalination

A McGill University-led research team has demonstrated the feasibility of a sustainable and cost-effective method of desalinating seawater. This method, thermally driven reverse osmosis (TDRO), uses low-grade heat from solar, geothermal, or other renewable energy sources to produce fresh water using a piston-based system.

Although previous studies have shown promise, this study is the first to analyze the thermodynamic limits of TDRO. As a result, researchers are closer to realizing technologies that improve access to water and make infrastructure more sustainable.

“Most desalination is done by reverse osmosis, which uses electricity to force water through a membrane,” said study co-author Jonathan Maisonneuve, associate professor of bioresource engineering.

“The challenge with heat utilization is that you need a lot of heat to do something that can be done with a small amount of electricity. So if we can find a way to harness existing heat from renewable sources, it could be very advantageous because heat is so abundant,” Maisonneuve said.

“Thermal-driven reverse osmosis: Thermodynamics of a new process that uses heat for desalination and water purification” by Saber Khanmohammadi, Sanjana Yagnambhatt, Dan DelVescovo, and Jonathan Maisonneuve was published in Desalination on October 15, 2025.

Addressing the water and energy crisis

Electricity-based desalination is often inaccessible in remote areas and requires approximately 1 to 4 kilowatt hours (kWh) to produce 1 cubic meter of fresh water.

A TDRO would require 20 kWh per cubic meter, according to an analysis by researchers who optimized several elements of the design proposed by MIT researcher Peter Goddert.

“There’s still a big difference compared to 1 to 4 kWh, but heat is cheaper than electricity, so we don’t need to close that difference completely,” Maisonneuve says.

TDRO works by heating and cooling a small amount of fluid in a closed chamber, known as the working fluid. This temperature fluctuation causes the working fluid to expand, driving a piston that forces seawater through the reverse osmosis membrane, effectively combining a thermodynamic cycle with water purification.

By studying and optimizing the ratio of working fluid to seawater fluid and the size of the piston, researchers demonstrated that TDRO has the potential for better performance than previously thought. They say the method compares favorably with existing thermal desalination techniques, but further research is needed.

“Then we need to model it in detail and see how quickly the system works and introduces many non-ideal effects such as heat loss through the environment,” Maisonneuve said.