Getting Power from the Sea

By The Ocean Channel

image by Scott Rouch

Power from the Sea: An Overview

The ocean contains two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves.

Oceans cover more than 70% of Earth's surface, making them the world's largest solar collectors. The sun warms t he surface water a lot more than the deep ocean water, and this temperature difference stores thermal energy. Thermal energy is used for many applications, including electricity generation. There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems.

Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves are driven primarily by the winds. A barrage (dam) is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator. For wave energy conversion, there are three basic systems: channel systems that funnel the waves into reservoirs, float systems that drive hydraulic pumps, and oscillating water column systems that use the waves to compress air within a container. The mechanical power created from these systems either directly activates a generator or transfers to a working fluid, water, or air, which then drives a turbine/generator.

Wave Energy

The total power of waves breaking on the world's coastlines is estimated at 2 to 3 million megawatts. In favorable locations, wave energy density can average 65 megawatts per mile of coastline. Three approaches to capturing wave energy are:

Floats or Pitching Devices These devices generate electricity from the bobbing or pitching action of a floating object. The object can be mounted to a floating raft or to a device fixed on the ocean floor.

Oscillating Water Columns (OWC) These devices generate electricity from the wave-driven rise and fall of water in a cylindrical shaft. The rising and falling water column drives air into and out of the top of the shaft, powering an air-driven turbine.

Wave Surge or Focusing Devices These shoreline devices, also called "tapered channel" or "tapchan" systems, rely on a shore-mounted structure to channel and concentrate the waves, driving them into an elevated reservoir. Water flow out of this reservoir is used to generate electricity, using standard hydropower technologies.

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Tidal Energy

Tidal energy traditionally involves erecting a dam across the opening to a tidal basin. The dam includes a sluice that is opened to allow the tide to flow into the basin; the sluice is then closed, and as the sea level drops, traditional hydropower technologies can be used to generate electricity from the elevated water in the basin.

Some researchers are also trying to extract energy directly from tidal flow streams. The energy potential of tidal basins is large — the largest facility, the La Rance station in France, generates 240 megawatts of power. Tidal energy systems can have environmental impacts on tidal basins because of reduced tidal flow and silt buildup.

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Ocean Thermal Energy Conversion Systems

A great amount of thermal energy (heat) is stored in the world's oceans. Each day, the oceans absorb enough heat from the sun to equal the thermal energy contained in 250 billion barrels of oil. OTEC systems convert this thermal energy into electricity — often while producing desalinated water.

Three types of OTEC systems can be used to generate electricity:

Closed-cycle plants circulate a working fluid in a closed system, heating it with warm seawater, flashing it to vapor, routing the vapor through a turbine, and then condensing it with cold seawater.

Open-cycle plants flash the warm seawater to steam and route the steam through a turbine.

Hybrid plants flash the warm seawater to steam and use that steam to vaporize a working fluid in a closed system.

OTEC systems are also envisioned as being either land-based (or "inshore"), near-shore (mounted on the ocean shelf), or offshore (floating).

Offshore Wind Turbine Technology

The Wind and Hydropower Technologies Program is leading research into offshore construction of wind power plants. Several offshore wind projects on the east coast have sparked interest in building wind turbines to harvest offshore winds. Until now, the program didn't focus on offshore wind power development because there is great potential on land for wind power and offshore development is more expensive. However, on the northeast coast of the United States, offshore development is an attractive alternative because electricity costs are high and transmission line construction faces many obstacles. Europeans have some experience with offshore wind energy projects, but these have been in sheltered, shallow water sites. U.S. offshore locations with significant wind resources do not match previously developed European sites well.

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Many of the U.S. sites will require the application of technologies that have yet to be explored or seriously considered in Europe, especially those that will allow development in deeper waters, which may have greater wind, wave, and ice loading. In addition, numerous environmental, political and regulatory issues exist in the United States, which must be dealt with in the near term before significant development can get underway.

The offshore installations will see very different environments and energy density. These turbines also need to be designed with confidence in the nature of the offshore winds (higher energy, lower turbulence) in combination with wave and current loadings at the base. Both these environments need to be specified sufficiently to allow designers to optimize the system for the conditions at the site. Current approaches to design specification are not capable of providing such a complete designation of site design conditions.

Research activities will include developing technology for offshore sites, mapping coastal wind resources, tracking European projects and studies, and organizing workshops.

The most recent report on US policy and ocean energy can be downloaded here: EPRI final report