Dutch researchers’ findings could treble yield of power from salinity gradients

Physicists at Utrecht University in The Netherlands have calculated that that energy producers theoretically could achieve a much higher yield when generating electricity by a version of so-called blue energy production – which harnesses the energy generated when saline and non-saline water mix. Conversely their findings suggest a much more efficient means for desalinating brackish water.

The researchers calculated that warming the non-saline water component to 50∘C prior to mixing can double the yield. Heating it to 80∘C could treble the output according to a report by René van Roij, Mathijs Janssen and Andreas Härtel published in December 2014 in Physical Review Letters.

The gains at the lower temperature indicate promise in the use of industrial waste heat according to lead researcher Janssen: “By heating the water using waste heat, we kill two birds with one stone: blue-energy installations generate more clean, sustainable energy, and industrial waste heat is put to better use,” he said.

The Utrecht team’s findings hold implications for the reverse of blue energy – the desalination of brackish water for drinking water production. According to van Roij, the theory “predicts that the desalination of cold brackish water is considerably less expensive than that from warm brackish water.”

The researchers were investigating capacitative blue energy technology that harnesses the energy generated on charging a porous electrode alternately with saline and non-saline water. To date the technology has been largely experimental.

More established forms of blue energy include pressure retarded osmosis - so-called osmotic power - where energy is harvested from a salinity gradient across a semi-permeable membrane that separates a low-saline (feed) solution and a high-saline (draw) solution). Electricity can be generated when the osmotic pressure of the transported water is used to drive a hydro-turbine. Reverse electrodialysis is another form of blue enemy that deploys alternate cells containing feed and draw water each separated by alternating anion and cation exchange membranes.

The greatest potential for the use of blue energy is viewed as being at the points where river water runs into the sea to provide natural reserves of feed and draw water. It has been estimated that The Netherlands’ abundance of estuaries means that blue-energy was capable of providing 30% of the country’s electricity demand.

A number of laboratories, including those as Utrecht University, are understood to be testing the theoretical findings.