Energy futuresNorway looking to osmotic power generation

Back in the early days of experimental science in pre-Revolution France, a priest-turned-physicist called Jean-Antoine Nollet performed an odd, if not perverse, experiment. For reasons that must surely have baffled anyone visiting his laboratory, he filled a pig’s bladder with wine and then dunked it into a barrel of water. He was surprised, as anyone would have been, when the submerged bladder swelled up and finally burst.

Nollet would have been castigated for wasting perfectly good wine, but he had discovered the phenomenon of osmotic pressure: the tendency for the concentration of two solutions, separated by a permeable membrane, to equalize. Fresh water had diffused through the walls of the bladder to dilute the wine inside.

This phenomenon is currently being investigated as a new form of marine power. Water-based energy generation conjures up pictures of towering hydro-electric dams, submerged tidal turbines, and bobbing wave-energy converters. The energy embodied in moving water — which all of these technologies exploit — is one way to make electricity from the sea, but it is not the only one. In Scandinavia, a pilot power station is demonstrating that another of the sea’s defining characteristics, saltiness, could also be harnessed to provide electricity.

The Engineer reports that the plant, built and operated by Norwegian energy company Statkraft not far from Oslo, employs a technique known as osmotic power generation, which uses the difference in salinity between seawater and fresh water to drive an electricity-generating turbine. First discovered in the 1970s, osmotic power was considered an interesting but impractical phenomenon for commercial energy production for many years, but increasing energy prices and advances in materials are now making it a technology worth watching.

Osmotic power is a blanket term for two techniques: reverse electrodialysis (RED) and pressure-retarded osmosis (PRO). Both rely on harnessing the energy dissipated when fresh water flows into salt water. The other factor they have in common is the use of membranes, but here the techniques differ significantly.

RED

RED uses two types of membrane to create what is essentially a battery, with the differences between ion concentration between seawater and river water setting up a small electrical voltage across each membrane. Stacking the cells up with alternating cathode and anode membranes — that is, membranes that only let positively or negatively charged ions through — creates a system capable of generating reasonable voltages. A stack the size of a shipping container could generate 250kW.

Small-scale studies of RED