How to extract electricity from the salinity difference between rivers and oceans

Energy

Technological Innovation Website Editor – 10/28/2022

How to extract electricity from difference

The osmotic generator requires a precise combination of nanopore diameter, quantity and spacing.
[Imagem: Makusu Tsutsui et al. – 10.1016/j.xcrp.2022.101065]

entropic energy

Japanese scientists believe they have found the way out to exploit one of the greatest sources of energy on Earth: the difference in salt concentration in the water of the oceans and in the water of the rivers that flow into these oceans.

Known as “entropic energy”, this potential arises when fresh water from rivers enters the sea, leading to a change in entropy due to the difference in salinity.

This change, calculated at 2.2 kJ per liter of fresh water, is a huge potential source of renewable energy. The big challenge is extracting that energy, converting it into useful forms – electricity, for example.

Through osmosis, the spontaneous movement of dissolved ions or molecules between two media with different concentration – media separated by a semipermeable membrane – can be used to generate electricity.

However, to be practical, this membrane needs to be very thin and highly selective, to allow ions to pass through, but not water molecules – and no one has been able to do this in a practical way.

A promising path to this has just been presented by Makusu Tsutsui and colleagues at Osaka University.

How to extract electricity from difference

Several membranes built by the team, which were compared with the theoretical standard of a single ideal nanopore.
[Imagem: Makusu Tsutsui et al. – 10.1016/j.xcrp.2022.101065]

osmosis electric generator

The team used conventional microelectronics manufacturing technology to precisely control the structure and arrangement of nanopores in an ultrathin silicon membrane.

Given the high precision of these chip manufacturing techniques, it was possible to control the size and location of the pores, creating a membrane that has, in practice, the same design used in computer simulations.

And because these manufacturing methods have been around for decades, the costs and complexities of the design are far less than alternatives proposed before.

Using a single 20 nanometer (nm) nanopore, the device achieved a peak energy efficiency of 400 kW/mtwo.

However, when they went to practical projects, the researchers found that adding too many nanopores to the membrane actually reduces the energy that can be generated: The optimal configuration uses 100 nm nanopores, arranged in a grid spaced one micrometer (1000 nm), producing an osmotic power density of 100 W/mtwo.

“Many other research groups are promising ‘green’ energy that is environmentally friendly, but we went one step further and proposed ‘blue’ energy, based on ocean water, that can be applied on an industrial scale,” said Professor Tomoji Kawai, coordinator of the team.

Bibliography:

Article: Sparse multi-nanopore osmotic power generators
Authors: Makusu Tsutsui, Kazumichi Yokota, Iat Wai Leong, Yuhui He, Tomoji Kawai
Magazine: Cell Reports Physical Science
DOI: 10.1016/j.xcrp.2022.101065

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