The different salinity levels between salt water and fresh water can be used to create a potential difference, with the aid of a membrane between the fresh water and the salt water. Salt water contains positively charged Na+ atoms and negatively charged Cl- atoms. Placing a membrane between the salt water and the fresh water that only allows one type of atom to pass causes a potential difference between the two types of water. This technique can be used to produce electricity directly, without a turbine.

In theory, this method can be used wherever fresh water flows or is pumped into salt water. However, it requires a clear demarcation between the salt water and the fresh water: it can be used at pumps and locks, but not at open river mouths. This technology, which is still in an experimental stage, appears to offer great potential for use in deltas, which are the most urbanised areas worldwide.

A pilot project has been launched on the Enclosure Dam (Afsluitdijk) in the Netherlands. The power station which will be constructed at some point in the future is expected to be able to generate 200MW, enough to supply the provinces of Groningen, Friesland and Drenthe with electricity. www.agentschapnl.nl

Literature

• Agentschap.nl; Portfolio energiezuinige stedenbouw; Utrecht, 2011