Hydrokinetic Technologies - OTEC

The energy from the sun heats the surface water of the ocean. In tropical regions, the surface water can be much warmer than the deep water. This difference can be used to produce electricity. Ocean Thermal Energy Conversion, or OTEC, has the potential to produce more energy than tidal, wave, and wind energy combined, but it is a technology for the future.

The warm surface water is turned into steam under pressure, or used to heat another fluid into a vapor. This steam or vapor spins a turbine to produce electricity. Pumps bring cold deep water to the surface through huge pipes. The cold water cools the steam or vapor, turning it back into liquid form, and the closed cycle begins again. In an open system design, the steam is turned into fresh water, and new surface water is added to the system.

An OTEC system is only about 3 percent efficient. Pumping the water is a giant engineering challenge. In addition, the electricity must be transported to land. OTEC systems work best with a temperature difference of at least 20°C to operate. This limits its use to tropical regions where the surface waters are very warm. Hawaii, with its tropical climate, has experimented with OTEC systems since the 1970s.

Today, there are several experimental OTEC plants, but no big operations. It will probably be 15 to 20 years before the technology is available to produce energy economically from OTEC systems. OTEC will have the potential to produce non-polluting, renewable energy.

Hydrokinetic Technologies - Wave Energy

There is also tremendous energy in waves. Waves are caused by the wind blowing over the surface of the ocean. In many areas of the world, the wind blows with enough consistency and force to provide continuous waves. The west coasts of the United States and Europe and the coasts of Japan and New Zealand are good sites for harnessing wave energy.

There are several ways to harness wave energy. The motion of the waves can be used to push and pull air through a pipe. The air spins a turbine in the pipe, producing electricity. In Norway, a demonstration tower built into a cliff produces electricity for about four cents a kWh using this method. The wail of the fast-spinning turbines, however, can be heard for miles.

Another way to produce energy is to bend or focus the waves into a narrow channel, increasing their power and size. The waves can then be channeled into a catch basin, like tidal plants, or used directly to spin turbines.

There aren’t any big commercial wave energy plants, but there are a few small ones. There are wave-energy devices that power the lights and whistles on buoys. Small, on-shore sites have the best potential for the immediate future, especially if they can also be used to protect beaches and harbors. They could produce enough energy to power local communities. Japan, which must import almost all of its fuel, has an active wave-energy program.

Hydrokinetic Technologies - Tidal Energy

The tides rise and fall in eternal cycles. The waters of the oceans are in constant motion. We can use some of the ocean’s energy, but most of it is out of reach. The problem isn’t harnessing the energy as much as transporting it. Generating electricity in the middle of the ocean just doesn’t make sense -  there’s no one there to use it. We can only use the energy near shore, where people need it.

Tidal energy is the most promising source of ocean energy for today and the near future. Tides are changes in the level of the oceans caused by the rotation of the Earth and the gravitational pull of the moon and sun. Near shore water levels can vary up to 40 feet, depending on the season and local factors. Only about 20 locations have good inlets and a large enough tidal range—about 10 feet—to produce energy economically.

Tidal energy plants capture the energy in the changing tides. A low dam, called a barrage, is built across an inlet. The barrage has one-way gates (sluices) that allow the incoming flood tide to pass into the inlet. When the tide turns, the water flows out of the inlet through huge turbines built into the barrage, producing electricity. The oldest and largest tidal plant—La Rance in France—has been successfully producing electricity since 1966.

Tidal plants have very high development costs. It is very expensive and takes a long time to build the barrages, which can be several miles long. Also, tidal plants produce electricity less than half of the time. The seasons and cycles of the moon affect the level—and the energy—of the tides. The tides are very predictable, but not controllable.

On the other hand, the fuel is free and non-polluting, and the plants have very low operating costs. The plants should run for a hundred years with regularly scheduled maintenance.

Tidal power is a renewable energy source. The plants do affect the environment, though they produce no air pollution. During construction, there are major short-term changes to the ecology of the inlet. Once the plants go into operation, there can be long-term changes to water levels and currents. The plants in operation have reported no major environmental problems.

The United States has no tidal plants and only a few sites where tidal energy could be produced economically. France, England, Canada, and Russia have much more potential. The keys are to lower construction costs, increase output, and protect the environment.

Hydropower and the Environment

Hydropower dams can cause several environmental problems, even though they burn no fuel. Damming rivers may permanently alter river systems and wildlife habitats. Fish, for one, may no longer be able to swim upstream.

Hydro plant operations may also affect water quality by churning up dissolved metals that may have been deposited by industry long ago. Hydropower operations may increase silting, change water temperatures, and lower the levels of dissolved oxygen.

Some of these problems can be managed by constructing fish ladders, dredging the silt, and carefully regulating plant operations. Hydropower has advantages, too. Hydropower’s fuel supply (flowing water) is clean and is renewed yearly by snow and rainfall. Furthermore, hydro plants do not emit pollutants into the air because they burn no fuel. With growing concern over greenhouse gas emissions and increased demand for electricity, hydropower may become more important in the future.

Hydropower facilities offer a range of additional benefits. Many dams are used to control flooding and regulate water supply, and reservoirs provide lakes for recreational purposes, such as boating and fishing.

Economics of Hydropower

Hydropower is the cheapest way to generate electricity today. No other energy source, renewable or nonrenewable, can match it. Today, it costs less than one cent per kilowatt-hour (kWh) to produce electricity at a typical hydro plant. In comparison, it costs coal plants about four cents per kWh and nuclear plants about two cents per kWh to generate electricity.

Producing electricity from hydropower is cheap because, once a dam has been built and the equipment installed, the energy source—flowing water—is free.

Hydropower plants also produce power cheaply due to their sturdy structures and simple equipment. Hydro plants are dependable and long-lived, and their maintenance costs are low compared to coal or nuclear plants.

One requirement may increase hydropower’s costs in the future. The procedure for licensing and relicensing dams has become a lengthy and expensive process. Many environmental impact studies must be undertaken and as many as 13 state and federal agencies must be consulted. It takes up to seven years to get a license to build a hydroelectric dam or a relicense to continue operations.

Hydropower for Base Load Power

Demand for electricity is not steady; it goes up and down. People use more electricity during the day when they are awake and using electrical appliances and less at night when they are asleep. People also use more electricity when the weather is very cold or very hot.

Electric utility companies have to produce electricity to meet these changing demands. Base load power is the electricity that utilities have to generate all the time. For that reason, base load power should be cheap and reliable. Hydropower meets both of these requirements.

Generating electricity with hydropower is the cheapest way to generate electricity in the U.S., and the fuel supply—flowing water—is always available.

Hydro plants are more energy efficient than most thermal power plants, too. That means they waste less energy to produce electricity. In thermal power plants, a lot of energy is lost as heat. Hydro plants are about 90 percent efficient at converting the kinetic energy of the moving water into electricity.

Hydropower Production

How much electricity do we get from hydropower today? Depending on the amount of rainfall, hydro plants produce from five to ten percent of the electricity produced in this country (the most was 10.1 percent in 1997, and the least was 5.6 percent in 2001). In Oregon, Washington, and Idaho, hydropower accounts for more than half (55 to 76 percent) of each state's electricity generation.
Today, there is about 78,000 megawatts of conventional hydro generating capacity in the United states, and about 98,000 megawatts when including pumped storage. That's equivalent to the generating capacity of 80 large nuclear power plants. The biggest hydro plant in the U.S. is located at the Grand Coulee Dam on the Columbia River in northern Washington State. The United States also gets some hydropower generated electricity from Canada.
Some New England utilities buy this imported electricity. What does the future look like for hydropower? The most economical sites for hydropower dams in the U.S. have already been developed, so the development of big hydro plants is unlikely.
Existing plants could be modernized with turbine and generator upgrades, operational improvements, and adding generating capacity. Plus, many flood control dams not equipped for electricity production could be retrofitted with generating equipment. The National Hydropower Association estimates 60,000 megawatts of additional generating capacity could be developed in the United States by 2025.