Where river meets sea we have an opportunity to extract not just tidal energy, but energy derived from the difference between salt water and fresh. Maybe if energy policies were rational we wouldn’t have such opportunities, but we are where we are.
With pressure retarded osmosis, or PRO, the idea is to take advantage of the osmotic pressure created when river water and sea water are separated by a semi-permeable membrane.
Under these conditions, water molecules from the river pass through the membrane into the sea water, creating a pressure on the sea water side of the membrane which may be used to drive turbines and generate electricity.
A pilot plant in Norway which has been generating a few kilowatts since 2009. As with so many sustainable energy technologies, the problem is one of energy density. There simply isn’t much energy to be extracted from each square metre of membrane.
Huge areas of membrane, in the order of square kilometres may be required to scale up the technology, but these membranes are also prone to fouling, so the technical issues are formidable.
A less developed but simpler and perhaps more interesting
way to extract energy from seawater and river water is Reverse Electrodialysis
In the RED approach, the osmotic energy of mixing fresh and salt water is captured by directing the solution through an alternating series of positively and negatively charged exchange membranes. The resulting chemical potential difference creates a voltage over each membrane and leads to the production of direct electric energy.
As with PRO, there are many technical hurdles and much research to be done, but a Dutch company called REDstack B V began work on a pilot plant in July. Both technologies are clean, highly modular and relatively well understood.
Could the Severn estuary be used to generate both tidal power and power from a technology such as PRO or RED?
We certainly have the ingenuity - pity about the politics.