Rezachek & Associates'
Energy & Environmental Resources

OCEAN THERMAL ENERGY CONVERSION (OTEC)

OTEC - What Is It?

The technology for generating electricity from different ocean temperatures is known as "ocean thermal energy conversion," or OTEC. OTEC makes use of the difference in temperature between the warm surface water of the ocean and the cold water in depths below 2,000 feet to generate electricity. As long as a sufficient temperature difference (about 22o Centigrade [C]) exists between the warm upper layer of water and the cold deep water, net power can be generated.

Advantages

OTEC uses clean, abundant, renewable, natural resources. Warm surface sea water and cold water from the ocean depths replace fossil fuels to produce electricity.
Suitably designed OTEC plants will produce little or no carbon dioxide or other polluting chemicals which contribute to acid rain or global warming (the "greenhouse effect"). Extensive research indicates little or no adverse environmental effects from discharging the used OTEC water back into the ocean at prescribed depths.
OTEC systems can produce fresh water as well as electricity. This is a significant advantage in island areas where fresh water is limited.
There is enough solar energy received and stored in the warm tropical ocean surface layer to provide most, if not all, of present human energy needs.
The use of OTEC as a source of electricity will help reduce the state's almost complete dependence on imported fossil fuels.
The cold sea water from the OTEC process has many additional uses, including air-conditioning buildings, assisting agriculture, and growing fish, shellfish, kelp and other sea plants which thrive in the cold, nutrient-rich, pathogen-free water.

Disadvantages

OTEC-produced electricity at present would cost more than electricity generated from fossil fuels at their current costs. The electricity cost could be reduced significantly if the plant operated without major overhaul for 30 years or more, but there are no data on possible plant life cycles.
OTEC plants must be located where a difference of about 22oC occurs year round. Ocean depths must be available fairly close to shore-based facilities for economic operation. Floating plant ships could provide more flexibility.
Although extensive and successful testing of OTEC has occurred in experiments on component parts or small scale plants, a pilot or demonstration plant of commercial size needs to be built to further document economic feasibility.
Construction of OTEC plants and laying of pipes in coastal waters may cause localized damage to reefs and near-shore marine ecosystems.
Some additional development of key components is essential to the success of future OTEC plants (e.g., less-costly large diameter, deep sea water pipelines; low-pressure turbines and condensers for open-cycle systems; etc.).

The Basic Process

There are basically three types of OTEC processes: closed-cycle, open-cycle, and hybrid-cycle. In the closed-cycle system, heat transferred from the warm surface sea water causes a working fluid (such as ammonia, which boils at a temperature of about 26oC at atmospheric pressure), to turn to vapor. The expanding vapor drives a turbine attached to a generator which produces electricity. Cold sea water passing through a condenser containing the vaporized working fluid turns the vapor back into a liquid which is then recycled through the system.

Open-cycle OTEC uses the warm surface water itself as the working fluid. The water vaporizes in a near vacuum at surface water temperatures. The expanding vapor drives a low-pressure turbine attached to a generator which produces electricity. The vapor, which has lost its salt and is almost pure fresh water, is condensed back into a liquid by exposure to cold temperatures from deep ocean water. If the condenser keeps the vapor from direct contact with sea water, the condensed water can be used for drinking water, irrigation or aquaculture. A "direct contact" condenser produces more electricity, but the vapor is mixed with cold sea water and the discharge water is salty. That mixture is returned to the ocean. The process is repeated with a continuous supply of warm surface sea water.

Hybrid systems use parts of both open- and closed-cycle systems to optimize production of electricity and fresh water.

OTEC Projects in Hawaii

Almost all of the major U.S. OTEC experiments in recent years have taken place in Hawaii. NELHA is recognized as the world's foremost laboratory and test facility for OTEC and OTEC-related research. The facility has been funded by the State of Hawaii with significant USDOE participation.

In 1979, the first successful at-sea, closed-cycle OTEC operation in the world was conducted aboard the Mini-OTEC, a converted Navy barge operating in waters off Keahole Point. This plant operated for three months, generating approximately 50 kW of gross power with net power ranging from 10-17 kW. This was a joint effort by the State of Hawaii and a private industrial partner.

In 1980, OTEC-1, a converted Navy tanker moored in waters off Kawaihae on the Kona Coast, tested heat exchangers and other components of a closed-cycle OTEC plant and investigated the environmental effects of an ocean-stationed OTEC plant. It was not designed to generate electricity. This was a USDOE-funded project.

Ocean Thermal Corporation, under a contract from the USDOE, in 1983 designed a 50-MW OTEC pilot plant to be located on an artificial island at Kahe Point off the coast of Oahu. The State of Hawaii contributed about $2 million (US) towards this project. The design plans were completed by the end of 1984, but funds for construction of the plant were not forthcoming. The relatively low cost of oil at that time made OTEC appear to be noncompetitive with fossil-fuel powered electric generating plants.

A significant breakthrough which promises major reductions in the cost of closed-cycle OTEC plants has been achieved through research on the design of evaporators and condensers. The research has been conducted at NELHA by ALCAN Aluminum of Canada and the Marconi Division of General Electric Company of Great Britain.

Research in both closed-cycle and open-cycle OTEC and OTEC-related aquaculture continues at NELHA. International exchange of technical information between researchers in both areas is on-going.

The Pacific International Center for High Technology Research (PICHTR) in Honolulu has continued work with the State of Hawaii and the USDOE on the final design, construction, and operation of a 210-kW open-cycle OTEC plant. The plant has already set the world record for OTEC power production at 255 kW (gross). Testing continues under the leadership of PICHTR.

PICHTR is managing the construction of a 50-kW closed-cycle OTEC plant at NELHA which is scheduled for operation in 1996. This plant is funded by the State of Hawaii, PICHTR, Hawaiian Electric Company, and the Center of Excellence for Research in Ocean Science. It will use the aluminum heat exchanger recently developed by ALCAN and already tested at NELHA; demonstrate the feasibility of commercial aquaculture downstream from an operating OTEC plant; and provide data for future expansion of the facility.

Meanwhile, KAD Partners of Hawaii has proposed a 1-MW closed-cycle OTEC plant at NELHA. The project will use the electricity generated by the plant to pump water for a lobster farm and an ocean science center. NELHA will sell the remaining water to other tenants.

Private aquaculture companies are continuing and expanding operations on land leased from NELHA, using cold OTEC water for a variety of products including fish, shellfish, edible seaweeds and microalgae. NELHA was created by the state Legislature in 1990 by merging the Natural Energy Laboratory of Hawaii and the Hawaii Ocean Science and Technology Park at Keahole. This merger assists the development of OTEC, OTEC-supported aquaculture, and other related technologies at the site.

Most of information contained in this summary was excerpted from a Fact Sheet prepared by the staff of the Energy Division of the State of Hawaii-Department of Business, Economic Development, and Tourism (DBEDT) and published with funding from the U.S. Department of Energy, Grant No. DE-FG49-94R900023. This information does not necessarily reflect the views of the State of Hawaii, the United States Government, or any agency thereof.