Plant Design and Location

Commercial ocean thermal energy conversion (OTEC) plants must be located in an environment that is stable enough for efficient system operation. The temperature of the warm surface seawater must differ about 20°C (36°F) from that of the cold deep water that is no more than about 1000 meters (3280 feet) below the surface. The natural ocean thermal gradient necessary for OTEC operation is generally found between latitudes 20 deg N and 20 deg S. Within this tropical zone are portions of two industrial nations—the United States and Australia—as well as 29 territories and 66 developing nations. Of all these possible sites, tropical islands with growing power requirements and a dependence on expensive imported oil are the most likely areas for OTEC development.

Commercial OTEC facilities can be built on

• Land or near the shore
• Platforms attached to the shelf
• Moorings or free-floating facilities in deep ocean water.

Land-Based and Near-Shore Facilities

Land-based and near-shore facilities offer three main advantages over those located in deep water. Plants constructed on or near land do not require sophisticated mooring, lengthy power cables, or the more extensive maintenance associated with open-ocean environments. They can be installed in sheltered areas so that they are relatively safe from storms and heavy seas. Electricity, desalinated water, and cold, nutrient-rich seawater could be transmitted from near-shore facilities via trestle bridges or causeways. In addition, land-based or near-shore sites allow OTEC plants to operate with related industries such as mariculture or those that require desalinated water.

Favored locations include those with narrow shelves (volcanic islands), steep (15-20 deg) offshore slopes, and relatively smooth sea floors. These sites minimize the length of the cold-water intake pipe. A land-based plant could be built well inland from the shore, offering more protection from storms, or on the beach, where the pipes would be shorter. In either case, easy access for construction and operation helps lower the cost of OTEC-generated electricity.

Land-based or near-shore sites can also support mariculture. Mariculture tanks or lagoons built on shore allow workers to monitor and control miniature marine environments. Mariculture products can be delivered to market with relative ease via railroads or highways.

One disadvantage of land-based facilities arises from the turbulent wave action in the surf zone. Unless the OTEC plant's water supply and discharge pipes are buried in protective trenches, they will be subject to extreme stress during storms and prolonged periods of heavy seas. Also, the mixed discharge of cold and warm seawater may need to be carried several hundred meters offshore to reach the proper depth before it is released. This arrangement requires additional expense in construction and maintenance.

OTEC systems can avoid some of the problems and expenses of operating in a surf zone if they are built just offshore in waters ranging from 10 to 30 meters deep (Ocean Thermal Corporation 1984). This type of plant would use shorter (and therefore less costly) intake and discharge pipes, which would avoid the dangers of turbulent surf. The plant itself, however, would require protection from the marine environment, such as breakwaters and erosion-resistant foundations, and the plant output would need to be transmitted to shore.

Shelf-Mounted Facilities

To avoid the turbulent surf zone as well as to have closer access to the cold-water resource, OTEC plants can be mounted to the continental shelf at depths up to 100 meters. A shelf-mounted plant could be built in a shipyard, towed to the site, and fixed to the sea bottom. This type of construction is already used for offshore oil rigs. The additional problems of operating an OTEC plant in deeper water, however, may make shelf-mounted facilities less desirable and more expensive than their land-based counterparts. Problems with shelf-mounted plants include the stress of open-ocean conditions and more difficult product delivery. Having to consider strong ocean currents and large waves necessitates additional engineering and construction expense. Platforms require extensive pilings to maintain a stable base for OTEC operation. Power delivery could also become costly because of the long underwater cables required to reach land. For these reasons, shelf-mounted plants are less attractive for near-term OTEC development.

Floating Facilities

Floating OTEC facilities could be designed to operate off-shore. Although potentially preferred for systems with a large power capacity, floating facilities present several difficulties. This type of plant is more difficult to stabilize, and the difficulty of mooring it in very deep water may create problems with power delivery. Cables attached to floating platforms are more susceptible to damage, especially during storms. Cables at depths greater than 1000 meters are difficult to maintain and repair. Riser cables, which span the distance between the sea bed and the plant, need to be constructed to resist entanglement.

As with shelf-mounted plants, floating plants need a stable base for continuous OTEC operation. Major storms and heavy seas can break the vertically suspended cold-water pipe and interrupt the intake of warm water as well. To help prevent these problems, pipes can be made of relatively flexible polyethylene attached to the bottom of the platform and gimballed with joints or collars. Pipes may need to be uncoupled from the plant to prevent damage during storms. As an alternative to having a warm-water pipe, surface water can be drawn directly into the platform; however, it is necessary to locate the intake carefully to prevent the intake flow from being interrupted during heavy seas when the platform would heave up and down violently.

If a floating plant is to be connected to power delivery cables, it needs to remain relatively stationary. Mooring is an acceptable method, but current mooring technology is limited to depths of about 2000 meters (6560 feet). Even at shallower depths, the cost of mooring may prohibit commercial OTEC ventures.

An alternative to deep-water OTEC may be drifting or self-propelled plantships. These ships use their net power on board to manufacture energy-intensive products such as hydrogen, methanol, or ammonia (Francis, Avery, and Dugger 1980).

NOTE: Many less developed countries have access to energy obtained through exploitation of the differences in water temperatures. They must be within 25 kilometers (15.5 miles) of an ocean region where there is a temperature difference of about 20°C (36°F) in the first 1000 meters (3280 feet) below the surface.

Electricity generated by plants fixed in one place can be delivered directly to a utility grid. A submersed cable would be required to transmit electricity from an anchored floating platform to land. Moving ships could manufacture transportable products such as methanol, hydrogen, or ammonia on board.

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