The quest for alternative “green” power sources has taken some surprising twists, but news of the latest technology is as interesting as it is surprising. In Holland and Norway, scientists and engineers are planning to use the difference between saltwater and fresh water found at river mouths to generate electrical power.
The two technologies take slightly different approaches. In Holland a team from Wetsus–the Centre for Sustainable Water Technology–plans to create a salinity power plant at the mouth of the river Rhine. By channelling water from the North Sea and fresh water from the Rhine into a unique kind of battery, the team estimates the estuary could generate a gigawatt of electricity, or enough to power around 650,000 homes.
They’ve successfully built a lab-demonstration of the “Blue Energy” tech. It uses a sequence of membranes (similar to those in kidney dialysis machines) to separate the two types of water. While negative chlorine ions from the salt water flow through one membrane into the fresh water, positive sodium ions from the sea water flow through another membrane in the other direction–the ions’ movements constitutes an electrical current and set up a voltage across electrodes at the ends of the device, creating a chemical battery.
The Norwegian team is exploiting an idea devised by Sidney Loeb back in 1973 dubbed “pressure retarded osmosis.” This system is more mechanical than the Dutch plans, since it relies on water molecules moving through a membrane. In this instance the membrane is semi-permeable, and due to the physical process of osmosis if salt water is contained on one side, while fresh water is on the other, fresh water is osmotically drawn into the salty side. This drives up the pressure in the “salty” chamber, and the sea water can then be sent through a turbine, that it would propel around to generate power. Norwegian power firm Skatkraft will, in the next few months, turn on an experimental station using this system at the Sodra Cell paper pulp factory, and it will be the world’s first of its kind. The company calculates that up to 10% of Norway’s power needs could come from saline power.
The innovation in both of these techniques is in the materials science of the membranes: the Blue Energy system has to be impermeable to water, but let ions through, while the PRO system has to let pure water molecules through with high efficiency while resisting the increased pressure the process generates.
And the environmental benefit of both is pretty startling: fresh river water would flow into the sea in any case–both “saline power plants” simply interrupt a natural process to exploit it to generate power.