Are you getting all you can out of your water? Statkraft in Norway would tell you the right answer is no, as they are trying to tap into a new source of energy, osmotic power. Osmotic power plants utilize concentration differences inherent between 2 fluids (say fresh and salt water) to create power. In the type of plant that Statkraft has prototyped (Pressure-Retarded-Osmosis or PRO), this works by separating the 2 fluids with a semi-permeable membrane that only allows water to pass through. Osmotic forces from the concentration differences pull fresh water through the membrane into the salt water, and the excess in pressure this causes can be used to drive a turbine and create energy. It is pretty much reverse osmosis water purification…well…reversed.
There are a lot of benefits to this form of energy generation. It is clean, no combustion, no mining for the fuel source, etc. renewable, and compared to the other source of power from water (hydroelectric) has a very low impact environmentally (no giant lake, though there are reports of fish getting caught in the pumps). Essentially, in theory, this is tapping into the power being lost as rivers dump into the oceans everyday, meaning the impact on water usage upstream is also minimal (if at all).
Nothing is perfect though. For starters, the membranes used for the power plants are very expensive. The price has come down over the years as people have researched this more, in part due to the need in reverse osmosis desalinators, but it is not at a price point that makes these plants extremely viable yet. Also, the amount of power you get per area of membrane is also quite low right now, meaning that you need to by a LOT of membranes. Maintenance is also an issue, as membrane clogging of any form reduces their efficiency further. And also, there could be issues in choosing proper locations for such a plant.
All of that aside though, in 2009 Statkraft built a prototype PRO plant in Norway that has been running successfully since. Though the plant is extremely small, it has been a very promising field test, and Statkraft has plans to build a 25MW plant by 2015. They also claim that the potential for this form of energy production is quite large as well moving into the future. Statkraft claims that 1,600-1,700TWh could be produced globally every year using this type of power plant.
To me however, I don’t imagine that potential will ever be fully realized, and goes back to that last con, location, location location. In order for this type of power generation to be particularly viable, you’d probably need to place the plant right at a river delta where you have an easy supply of fresh and salt water. And while there are a lot of river deltas in the world, I can’t see people getting extremely comfortable with a large power plant on every one of them (disturbing delta ecosystems, beaches, etc.) But the Statkraft prototype plant is certainly an exciting advance in this area, and I will be interested to see how the technology fairs as the plant size is scaled up and advances continue in the realm of membrane design and maintenance. To my mind, if locations are planned out properly, and the technology can get a little bit better, this great green (blue?) solution to help satisfy our electricity demands.