Sustainable Development

Nano tech could get zero-carbon energy from sea water

A view of an industrial estate at Tofte near Oslo, Norway, March 2, 2008, where state power group Statkraft will build an experimental "salt power" plant in 2008. The tiny plant will generate energy by mixing salt water in the sea with fresh water from a stream, exploiting the natural energy found in osmosis -- the process used by trees to suck water from their roots to their leaves.

To date, large-scale blue energy projects such as Norway’s Statkraft power plant have been impeded by the poor efficiency of existing membrane technology. Image: REUTERS/Alister Doyle

Frederique Mezerolle
Media Relations Officer, McGill University
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  • Tip-controlled local breakdown (TCLB) is a technique to 'drill' microscopic holes into membranes to improve the effectiveness of osmotic energy.
  • Osmotic energy, also known as blue energy, takes advantage of the energy released when solutions of different salinities mix.
  • These conditions occur in locations all over the world and improving the effectiveness of the technique could lead to an abundant source of renewable energy.

A new technique could enable the production of robust, high-performance membranes to harness sea water as an abundant source of renewable energy, researchers report.

Blue energy, also known as osmotic energy, capitalizes on the energy naturally released when two solutions of different salinities mix—conditions that occur in countless locations around the world where fresh and salt water meet.

The key to capturing blue energy lies in selectively permeable membranes, which allow only one constituent of a saltwater solution to pass through—either the water molecules or the dissolved salt ions—but not the other.

To date, large-scale blue energy projects such as Norway’s Statkraft power plant have been impeded by the poor efficiency of existing membrane technology. In the laboratory, researchers have developed membranes from exotic nanomaterials that have shown great promise in terms of the amount of power they can generate relative to their size. But it remains a challenge to turn these vanishingly thin materials into components that are large enough and strong enough to meet the demands of real-world applications.

In results recently published in Nano Letters, researchers have demonstrated a technique that may open the way to overcoming this challenge.

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Osmotic energy takes advantage of the energy released when solutions of different salinities mix. Image: Nano Letters

“In our project, we aimed to remedy the inherent mechanical fragility problem while exploiting the exceptional selectivity of thin 2D nanomaterials by fabricating a hybrid membrane made of hexagonal boron nitride (hBN) monolayers supported by silicon nitride membranes,” explains lead author Khadija Yazda, a postdoctoral researcher in the physics department at McGill University.

To achieve the desired characteristic of selective permeability, Yazda and her colleagues used a technique called tip-controlled local breakdown (TCLB) to “drill” multiple microscopic holes, or nanopores, in their membrane. In an advance on previous research that focused on experimental prototypes with a single nanopore, the team was able to exploit the speed and precision of TCLB to prepare and investigate membranes with multiple nanopores in various configurations of pore size, number, and spacing.

“Our experiments on pore-pore interaction in nanopore arrays shows that the optimum membrane selectivity and overall power density is obtained with a pore spacing that balances the need for high pore density while maintaining a large extent of charged surface (≥ 500nm) surrounding each pore,” Yazda says.

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Having successfully produced an array of 20 by 20 pores on a membrane surface 40µm² in size, the researchers say the TCLB technique could be used to produce much larger arrays.

“A natural next step for this research is to try scaling up this approach not only for large-scale powerplants but also in nano- or micro-power generators,” Yazda says.

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Funding for this study came from the McGill Sustainability Systems Initiative (MSSI) Ideas Fund, the Natural Sciences and Engineering Research Council of Canada, the Discovery Grants Program, and the Fonds de recherche du Québec–Nature et technologies.

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