Ocean Thermal Energy Conversion

For many years, scientists have been aware of one enormous reservoir of energy on the earth's surface: the oceans. As sunlight falls on the oceans, its energy is absorbed by seawater. The oceans are in one sense, therefore, a huge "storage tank" for solar energy. The practical problem is finding a way to extract that energy and make it available for human use.

The mechanism suggested for capturing heat stored in the ocean depends on a thermal gradient always present in seawater. Upper levels of the ocean may be as much as 36°F (20°C) warmer than regions 0.6 mile (1 km) deeper. The technology of ocean thermal energy conversion (OTEC) takes advantage of this temperature gradient.

An OTEC plant would consist of a very large floating platform with pipes at least 100 feet (30 m) in diameter reaching to a depth of up to 0.6 mile (1 km). The "working fluid" in such a plant would be ammonia, propane, or some other liquid with a low boiling point.

Warm surface waters would be pumped into upper levels of the plant, causing the working fluid to evaporate. As the fluid evaporates, it will also exert increased pressure. That pressure can be used to drive a turbine that, in turn, generates electricity. The electricity could be carried to shore along large cables or used directly on the OTEC plant to desalinize water, electrolyze water, or produce other chemical changes.

The Open Cycle Ocean Thermal Electric Generator requires a water temperature differential of at least 36°F (20° C) to produce both fresh water and electricity. (McGraw-Hill Inc. Reproduced by permission.)

In the second stage of operation, cold water from deeper levels of the ocean would be brought to the surface and used to cool the working fluid. Once liquified, the working fluid would be ready for a second turn of the generating cycle.

OTEC plants are attractive alterative energy sources in regions near the equator, where surface temperatures may reach 77°F (25°C) or more. These parts of the ocean are often adjacent to less developed countries, where energy needs are growing.

Wherever they are located, OTEC plants have a number of advantages. For one thing, oceans cover nearly 70 percent of the planet's surface so that the raw material OTEC plants need—seawater—is readily available. The original energy source—sunlight—is also plentiful and free. Such plants are also environmentally attractive since they produce no pollution and cause no disruption of land resources. Planners suggest that a by-product of OTEC plants might be nutrients brought up from deeper ocean levels and used to feed "farms" of fish or shellfish.

Unfortunately, many disadvantages exist also. The most important is the enormous cost of building and maintaining the mammoth structures needed for an OTEC plant. Also, the temperature differential available under even the best of conditions means that an OTEC plant will not be more than about 3% efficient.

Currently, the disadvantages of OTEC plants are so great that none has even been built. Research continues in a number of countries, but some experts believe that the low efficiency of OTEC means that this technology will never be able to compete economically with other alternative sources of energy.