Use of OTEC in Hawaii

Almost all of the major U.S. OTEC experiments have taken place in Hawaii. The Natural Energy Laboratory of Hawaii Authority (NELHA) has been 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 and private sector participation.

On June 2, 2006, plans for a 1-MW OTEC facility at NELHA were announced. Article: Star-Bulletin, June 3, 2006.  Updates and additional information may be available from the NELHA website and the OCEES website.

There is no OTEC facility currently producing electricity at Keahole Point. However, cold seawater is being used directly to air condition (cool) the administration and laboratory buildings. The seawater provides about 50 tons of air conditioning, offsetting the equivalent of 200 kW of peak electrical demand. Using the cold seawater for air conditioning saves NELHA nearly $4000 per month in electricity cost - and the system requires much less maintenance than traditional compressor systems.

Hawaii companies with OTEC expertise

At the link, you may access a searchable directory of ocean-related science and technology companies in Hawaii. Choose "Energy" at the Search by Capabilities input box for a listing of companies that indicate some expertise with ocean thermal technologies. http://www.hawaiioceanscience.org/

Mini-OTEC, 1979

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, from August-October 1979, and generated approximately 50 kilowatts of gross power with net power ranging from 10-17 kilowatts. Its turbine generator produced a gross output of up to 55 kW. About 40 kW were required to pump up 2,700 gallons/min of 42°F water from 2200-ft depth through a 24-in diameter polyethylene pipe and an additional 2,700 gallons/min of 79°F surface water, leaving a maximum net power output of 15 kW. This was a joint effort by the State of Hawaii and a private industrial partner.

OTEC-1, 1980

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.

Design and Research

Ocean Thermal Corporation, under a contract from the USDOE, in 1983 designed a 40-megawatt OTEC pilot plant to be located on an artificial island at Kahe Point off the coast of Oahu. 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 made OTEC noncompetitive with fossil-fuel powered electric generating plants at that time.

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.

A 210-kilowatt open-cycle OTEC Experimental Apparatus was operated for onshore at NELHA's Keahole Point facility intermittently between 1992 and 1998. providing valuable data and pointing the way for future modifications and improvements in the OC-OTEC process. The turbine-generator was designed for an output of 210 kW for 26 °C warm surface water and a deep water temperature 6 °C. It produced a maximum gross output of 250 kW during late summer when the surface water is warmest. Power requirements for pumping ashore the required 6,500 gpm of 43°F seawater through a 40-in pipe from 2700 ft depth and 9600 gpm of 76-81°F surface seawater were about 200 kW. A small fraction (10 percent) of the steam produced was diverted to a surface condenser for the production of desalinated water. The highest production rates achieved were 255 kW (gross) with a corresponding net power of 103 kW and production of approximately 6 gallons per minute of desalinated water. These are world records for OTEC. It must be noted that the net power was not optimized because pumping losses were relatively high due to the use of a seawater system that was already available. It is expected that for a commercial size plant the ratio of net to gross power will be approximately 0.7.

A preliminary design was prepared for a 1.4 MW (gross) OTEC plant to utilize the seawater available through NELHA's new 55-in diameter surface and deep pipelines, which were installed primarily to provide seawater for developing aquaculture businesses. The total pumping load for full flow (27,000 gal/min deep seawater from 3000-ft depth at 39°F, and 40,500 gal/min surface water at 76-81°F) will be about 1 MW, so there is the potential to produce up to 400 kW net output. This project has not materialized because the State (NELHA) and the OTEC proposer have been unable to find a financially viable solution to accommodate the gradual scale-up required by the seawater needs of the other users.

Research in both closed-cycle and open-cycle OTEC and OTEC-related aquaculture continues in several places, notably in Hawaii (including for application in other locations, such as Diego Garcia) and in Japan. International exchange of technical information between researchers worldwide is on-going.

Related Aquaculture Activities, Kailua-Kona, Hawaii

A new seawater system has been installed to serve the acquaculture activities of the Hawaii Ocean Sciene and Technology (HOST) Park. 55-inch diameter pipelines bring ashore 4°C deep water from 3000 ft depth and 27.5°-28.5°C surface water from 80 ft depth.

The pipelines will primarily serve aquaculture tenants in the HOST Park, but companies are discussing proposals to construct an OTEC plant that will provide electricity to power the pumps, reducing the pumping cost for all tenants.

The Basic Process

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 40 degrees Fahrenheit) exists between the warm upper layer of water and the cold deep water, net power can be generated.

Three Types

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 -28°F 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. 

Reports

Complete list of on-line publications
Current; includes reports on renewables in general, and renewable portfolio standards, which include OTEC.