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And how big a plant will be required to replace a single power station?
Where is all this fresh water to salt water conversion going to take place?
In reply to JMW, I think this news article from 2009 answers your two questions:
The article describes the world's first 'osmotic power' prototype plant, built in Norway. (I think the osmotic power idea was originated back in the 1970s, like a lot of renewable energy ideas, in response to the 1973 Oil Crisis) According to the firm called Statkraft that run the test plant, an area the size of a football field can generate 25 MW, so a conventional power station size of about 1 GW would presumably require about 40 football pitches in area. It looks like these plants would be built by the seaside, and they would require access to fresh water which might be piped in from a nearby river or lake.
Like Brad says, this type of renewable energy actually doesn't look too bad. It appears that it would not suffer from anything like the intermittency or power variation problems that wind and solar power do, and if you choose to believe the Statkraft figures given in the article, it seems to be comparatively cheap with a quoted break-even cost of 70 to 100 Euros/MWh (59 to 84 £/MWh or 9.6 to 13.8 cents/kWh).
It wouldn't surprise me if there was a reasonably viable type of renewable energy in existence that is being underplayed, if it didn't tick exactly the right boxes for the Greenies. I've always thought that a major reason the Greenies are so enthusiastic about wind and solar power is that they have a high visual impact, sending out a statement that the Green movement is a force to be reckoned with. Windfarms are almost like a modern-day equivalent of church spires, and if somebody in your street installs solar panels on their roof, that also communicates some symbolic significance. Osmotic power plants are nondescript buildings by the sea that wouldn't have the same visual impact. Another problem for osmotic power plants in the UK is that the UK government happens to have backed another horse in terms of trying to get energy out of seawater for the last 40 years, namely wave power. I can imagine the wave power lobby being resistant to any R&D spending on these osmotic power plants.
The only real problem is the fresh water. The Uk sees to suffer frequent hosepipe bans in hot weather, which the Greenies assure us will become ore so in the future.
So presumably a conflict of use will arise.
No doubt Scotland will be keen on this idea as their objective is to become the leading Green Energy nation on the planet.
I think you may be confusing fresh water with drinking quality water. Fresh water pretty much just means that it has a significantly lower concentration of dissolved salts present in it than seawater. It comes in two types - drinkable and non-drinkable, where the technical definition of drinkable can vary quite widely from one country to another, also known as potable and non-potable (which comes from the Latin word potabilis, which means drinkable). The water subject to hosepipe bans in the UK that is distributed to domestic consumers by water utilities is actually drinking quality water. I don't think I've heard of any water shortages in the UK for non-potable water.
In regard to Scotland being keen on the osmotic power version of renewable energy, I've not heard of that. A quick check of the Wikipedia article "Renewable energy in Scotland" (http://en.wikipedia.org/wiki/Renewable_energy_in_Scotland) suggests they don't currently recognise osmotic power at all, and the article talks up wave power, the UK's favoured method for getting energy out of seawater. The 'strike price' for wave power in the UK is currently £305/MWh (50 cents/kWh), about three times higher than onshore wind and about twice as high as offshore wind.
My view of the Scottish enthusiasm for renewable energy is that this could all change if Scotland votes for independence next year, which I regard as a more likely possibility than most people seem to do. It is very helpful to the Scottish Nationalist cause for renewable energy to be hyped up as much as possible, to bluff the Scottish public into thinking they have some sort of golden future as a renewable energy powerhouse country, as this might further encourage the public into voting for independence. The poor economics of renewable energy will not currently be of much concern to the Scots Nats as the bill is largely being footed by the English, so it's mainly English money that is being wasted, not Scottish money. I suspect that if Scotland became independent, they would scale back the renewable energy ambitions substantially.