Water and Power

More than two thirds of the world's cities are in coastal zones, yet few of them utilize the ocean as a power source. Nonetheless, as the environmental cost of traditional power plants grows increasingly apparent, the oceans warrant a closer look as generators. Several emerging technologies are changing the way that coastal cities can produce power, and though they hardly look like traditional dynamos, the five following systems may be the coastal energy infrastructure of the future.

1. Offshore Wind Farms

Although the concept of offshore wind farms isn't new, large scale built examples are relatively rare. The waters of Denmark are one exception. The Danish coastline is dotted with enormous turbines, and government has aggressive plans for more. In fact, the Danish Wind Industry Association estimates that by 2030, 40% of all Danish electricity will be generated by offshore wind farms.

Taking advantage of winds undisturbed by land, current offshore wind farm methods are still only cost effective in shallow waters: costs rise with deeper water and larger waves. In response, some companies are experimenting with floating turbines or turbines strung on cables between sea anchors and blimps, but there are others that are finding ways to use the waves themselves.

2. Wave Harvesting

In Agucadoura, Portugal, the Pelamis Wave Power Company has installed a series of snake-like semi-submerged tubes that can harvest energy from waves. The insides of the tubes feature hydraulic rams that extend and compress as waves move past. The rams in turn force fluid through turbines that generate electricity. Although novel, the technology can produce significant power in rough seas.

3. Tidal Turbines

Energy can also be generated by harnessing the power of the tides. In the 1800s, tides were sometimes captured by building dams that would open at low tide and close at high water to create mill ponds. (Boston's Back Bay was used for this purpose before it was completely filled in 1882.) Today, tidal power projects, like the SeaGen turbine in Northern Ireland, work not by holding back the tide, but by letting it flow past.

Unfortunately, there are also great challenges to creating successful tidal turbines. Besides the difficulty of creating machines that don't break down in such tremendous forces, there is also the issue of preexisting coastal activities. The best locations for tidal turbines are usually the narrow inlets of large bays, but these very same places are also important for shipping. Thus some locations that may seem perfect for tidal turbines, like the Verrazano Narrows in New York Harbor or the Golden Gate of San Francisco Bay, are not available for generating electricity.

4. Ocean Thermal Energy Conversion

Fortunately, there is another technology that uses a part of the ocean with little human use: the deep. Engineers in Hawaii are currently working on an Ocean Thermal Energy Conversion (OTEC) system that uses cold seawater from the depths of the ocean to create a temperature differential for a heat engine. The ocean water serves as a heat sink for a working fluid like ammonia that vaporizes at surface temperatures. As the working fluid vaporizes and expands, it is used to generate power. Cold seawater from the deep, pumped into thermal exchangers at the surface, is used to condense the working fluid and the process starts over.

The method, which is similar to a giant refrigerator run backwards, has not yet been advanced to a scale that makes economic sense. Indeed, many obstacles remain in creating a large-scale OTEC system, and the impact on the environment of such a system is unknown. Nonetheless, given the size of the oceans and the chill of their depths, OTEC systems offer great potential for the future.

5. Halophyte Ethanol

One final way to produce energy using the ocean is simple: grow it. New efforts are being focused on the potential of saltwater tolerant crops -- halophytes -- to generate biomass for energy production. Crops like Salicornia bigelovii, which can be irrigated with seawater, are now being farmed on the western coast of Mexico to produce biodiesel. Specifically, the oil-rich seeds are converted to ethanol, turning sandy deserts into fields of energy watered by the sea.

(Danish Wind Turbine photo from Flickr user Mads Prahm. Pelamis Sea Trial video from Youtube user mindseyecreative. Tidal turbine images from Daily Mail and New Scientist. Ocean Thermal Energy Conversion diagram from New Scientist. Salicornia photo from Wired Science Blog.)

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