
Water has many potential uses for generating energy. The most common water based power generators are hydroelectric dams, also called “large hydro”. There are also “small” and “micro” hydro plants in use in remote locations where water is plentiful. Hydroelectric power accounts for around 3% of the world’s energy. An example is the Hoover Dam pictured above (from Wikipedia).
Hydroelectric power makes use of the potential energy stored in dammed water using it to drive a turbine and generator. The amount of energy extracted depends on the volume of water in the reservoir and the height difference between the source and the outflow (see picture on left). Hydroelectric is renewable since rainfall adds water up-river which ends up in the reservoir until it is released to generate electricity. Aside from building the dam and plant, the energy production is clean and low-cost. However, these dams can cause other environmental damage by changing the natural waterways. Some side effects include accelerated erosion, reduction of fish spawning, and water quality changes (e. g. depleted oxygen and elevated temperature). Hydroelectric dams provide fairly clean, low-cost and predictable energy, but they do have adverse effects and viable water sources are limited.
A burgeoning area of water power generation involves waves and tides. Wave power makes use of the kinetic energy in the rise and fall of waves in the ocean. A European manufacturer (as described on ZDnet), Pelamis Wave Power, makes the Wave Energy Converter (WEC). The Pelamis WEC (pictured on right) makes use of several cylindrical sections linked with hinged joints.
The wave-induced motion of the joints is resisted by hydraulic rams which pump high-pressure fluid through motors which drive electric generators. California’s PG&E is investing in the United State’s first “wave park” off the coast of Eureka, CA. This installation will make use of Finavera Renewables‘ AquaBuOY (pictured on left) to generate 2 MegaWatts of electricity. For more information on the AquaBuOY technology, check out their video or read about this deal on GreenWombat.
Tidal power makes use of the rising and falling of the water level due to tides. One way to do this is to capture water at high tide in a basin, then discharge it near low tide through a turbine. This method, also known as a barrage, has been used for a thousand years in the form of tide mills for grinding grain. Another alternative, called tidal stream power, utilizes turbines installed underwater in tidal channels. PG&E, the City of San Francisco, and Golden Gate Energy are conducting a study to assess the possibility of harnessing the tides in San Francisco Bay (from Green Car Congress) using a device like the Lunar Energy RTT Turbine (pictured right).
Ocean thermal energy conversion (OTEC) exploits the temperature difference between the warm surface and colder deep waters. The process uses something called a heat engine. A heat engine uses a device placed between a hot reservoir and a cold one. The engine extracts some of the heat in the form of work. A common example is a steam turbine where fuel is burned to create steam, which turns a turbine, and then condenses back to water to be recycles. The OTEC concept is the same, but the fuels is the sun warming the surface water, a low boiling-point fluid like ammonia is used as the steam, then deep sea-water is used to cool the ammonia back to liquid (see diagram on left). Unfortunately, the OTEC engines are not very efficient and the ocean locations with large thermal differences are limited.
The final water power technology is called osmotic (or Blue) energy. It uses the difference in salt concentration between seawater and river water to generate energy. The technology relies on osmosis through methods such as Reverse Electrodialysis (RED) and Pressure Retarded Osmosis (PRO). Osmotic energy technologies are still in early stages of development, primarily in Norway.
There are quite a few water power options. It seems like hydroelectric dams are fairly saturated and cause their own environmental problems. Wave and tidal power look pretty interesting. They should be more predictable than options like winds and solar. I wonder what sort of impact these technologies will have on marine life and shipping. Osmotic power also has potential, but it has a ways to go before it is economically viable. It certainly seems as if energy from the big blue can be green.