Reliable systems for the storage of energy are a central component of energy supply systems using a high fraction of renewable energy.

Here we propose energy storage using two reservoirs of water with different salt concentrations. Storage of excess (electrical) energy takes place by reverse osmosis increasing the salt concentration in the solution contained in one reservoir. The produced fresh water will be stored in a second reservoir (or discarded, e.g. in a river). When necessary the stored energy will be released by running an osmosis power station (OPS), where the osmotic pressure of the high concentration reservoir is exploited. The energy storage density can exceed 11 kWh/m3 and can thus be up to an order of magnitude higher than in typical pumped-storage hydroelectricity (PSH) at comparable efficiency. In fact, the osmotic pressure of some readily available salt solutions can exceed 1000 bar, which would correspond to an altitude difference of more than 10000m in a PSH.

The technology of such a system is readily available: Reverse osmosis for production of fresh water from ocean water is in widespread use and the technical components (large area membranes and pressure exchangers) are commercially available. Also, the principle of OPS has been realized in demonstration plants. Compared to PSH our new approach requires no altitude difference of reservoirs, therefore large storage capacities can be realized very economically, almost everywhere and with little intrusion in the landscape. A series of different realization schemes and sample calculations of power and energy densities are provided.

Thus we feel that energy storage in salt concentration gradients can be realized in the near future and therefore should be seriously considered for large scale energy storage.

Ulrich Platt and Florian Dinger