When Green Energy is Blue Energy: Power From the Sea

Posted by Steven Zweig on September 2, 2009 at 10:32 am

Where you have the right geography to make barrage plants possible, you can potentially extract a lot of energy by tapping the power of 40 – 50 foot tides! For example, the United Kingdom is considering building a 10-mile long barrage across the River Severn as part of its drive to generate one-fifth of the nation’s power from alternative sources by 2020. Environmentalists worry about the possible impacts of damming such a major river, including flooding, reduction in local fish stocks, and destruction of wildlife habitats, which is why more modest schemes, including lagoons that would only capture part of the tidal flow, are being considered as well. However, whether it’s one scheme or another, it’s a virtual certainty that some form of barrage power will be built across the River Severn—there’s simply too much potential power there to ignore.

The other way to tap tidal power is simply with turbines, set on or secured to the bottom of the river or bay. These would simply be spun by the tide as it surges past. The cost and time to build is far less than for a barrage system, as is the potential ecological disruption; however, the energy captured would not match the amount captured by a barrage plant.

Illustration of how turbines could capture the power of tides. (image: ornl.gov)

Getting All Steamed Up: Using the Ocean’s Heat Energy

Ocean Thermal Energy Conversion (OTEC) uses the temperature differential between warm surface and colder subsea waters. There are several different processes, but they all involve using warmer temperatures to boil a working fluid and using the power of its expansion (since a gas occupies dozens or hundreds of times as much space as a liquid) to turn a turbine, then using colder temperatures to condense the fluid back to a liquid and start again. For example, in a(n)

• closed-cycle system, a liquid with a low boiling point (like ammonia) is boiled by heat from warm surface water. As it boils and expands, it spins a turbine. Then cold, subsea water is used to condense the liquid back again.
• open-cycle system, warm surface water is placed under low pressure; since the boiling point drops as pressure drops, this lets it boil without any additional heat. As the water boils, it expands, and the expansion drives turbines. The boiled water is then condensed back to liquid water through exposure to cold subsurface water. One handy side effect of the open-cycle system is that it produces fresh water as it operates—the surface water loses its salt as boils, and is fresh when recondensed.