Low-Grade Heat From Renewable Sources Desalinates Water

A new study conducted by McGill University researchers has highlighted a sustainable way to turn seawater into drinkable water using low-grade heat.

They studied the piston-based desalination method called thermally driven reverse osmosis (TDRO). Unlike conventional reverse osmosis, this method uses a renewable heat source, such as solar thermal energy and geothermal heat.

Jonathan Maisonneuve, associate professor of bioresource engineering and the co-author of the study, said that using heat instead of electricity comes with difficulties:

“Most desalination is done by reverse osmosis, which uses electricity to drive water through a membrane. The challenge with using heat is that it takes a lot of it to do what you could with a little bit of electricity. So, if we can find a way to harness existing heat from renewable sources, that could be very advantageous, because it’s so abundant.”

Conventional desalination typically relies on electricity and requires one to four kilowatt hours (kWh) to produce a cubic metre of fresh water. The McGill-led analysis, which refined aspects of a design originally proposed by MIT researcher Peter Godart, found that TDRO would need around 20 kWh of heat per cubic metre.

While this difference is significant, Maisonneuve noted:

“There's still a big difference when you compare it to one to four kWh, but because heat is cheaper than electricity, we don't have to totally close that gap.”

TDRO operates by cycling a working fluid through heating and cooling inside a sealed chamber. As the fluid expands and contracts, it drives a piston that pushes seawater through a reverse osmosis membrane.

Further modeling and evaluation will determine how the system functions under real-world conditions. Maisonneuve said:

“Next, we need to model it in detail, see how quickly the system can operate and introduce a number of non-ideal effects, such as heat loss through the environment.”

Read the full article here to learn more about TDRO.

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