Energy from salinity gradients could provide renewable power source, study finds

According to new research from Griffith University, harnessing the energy created from salinity gradients could provide a renewable source of power able to mitigate climate change impacts, reduce reliance on fossil fuels and improve processes within the desalination industry.

Sept. 21, 2015 -- According to new research from Griffith University (Queensland, Australia), harnessing the energy created from salinity gradients -- for example, when freshwater meets the sea -- could provide a renewable source of power able to mitigate climate change impacts, reduce reliance on fossil fuels and improve processes within the desalination industry.

A paper published by Dr. Fernanda Helfer and Professor Charles Lemckert from Griffith's School of Engineering reviews investigations into the potential of salinity gradient energy, which is released when waters with different salinities mix. In particular, the paper explores the efficacy of Pressure Retarded Osmosis (PRO) as a carbon emission-free process to extract and implement this energy.

PRO technology comprises a semi-permeable membrane that separates water flows with different salt contents, creating a solution that, once depressurized via a turbine, produces electrical energy. Broad implementation of PRO has long been hampered by issues of cost and quality, but rising energy prices and growing acknowledgment of the potential impact of climate change have brought PRO and salinity gradient energy into renewed focus.

In their paper, "The Power of Salinity Gradients: An Australian Example," Helfer and Lemckert contend that Australia is particularly suited to osmotic power production. "Australia has various sources of saline solutions that could be used as draw solutions for PRO plants," said Helfer. "These include salt lakes, brine from desalination plants and saline groundwater. The largest Australian urban centers are located near the ocean and close to river mouths -- ideal conditions for the construction of osmotic power plants."

A unique aspect of the study is the suggestion of the use of brine, which is rejected during the desalination process, as a source of osmotic energy. "Even taking into account the current inefficiencies of PRO, and based on the power generated under laboratory conditions and published by other institutions, a mixture of seawater and brine could generate power in a PRO plant adjacent to a desalination plant," said Helfer. "This power would be used in the desalination process while the PRO plant, in turn, would use the reject brine as the draw solution and seawater as the feed solution.

Helfer and Lemckert agree that significant technical and economic improvements are required to ensure the commercial viability and credibility of PRO membrane technology. However, they also consider PRO-assisted desalination a promising alternative for the industry worldwide, one that provides power to the desalination process -- thus reducing the industry's reliance on fossil fuels -- and the opportunity to minimize environmental impacts caused by the discharge of concentrated brine into the sea.

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