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Capturing Ocean Heat

Ocean thermal energy conversion offers a timely renewable alternative.

Fortnightly Magazine - October 2009

later, another French team attempted to build a 3-MW open-cycle (Claude-cycle) plant for Abidjan, then the capital of Côte d’Ivoire; however, the plant never was completed because it couldn’t compete economically with local fossil-fueled power plants.

Mini-OTEC, the world’s first net-power-producing floating OTEC plant, and part of the first foray by the United States into this technology, was deployed in 1979 on a barge at Keahole Point on the Kona coast of the island of Hawaii. This proof-of-concept demonstration facility was developed by the Natural Energy Laboratory of Hawaii (NELHA) and several private firms, including Lockheed Ocean Systems. It operated for three months, generating approximately 50 kW of gross power with net power ranging from 10 to 17 kW. 5 Based on the results of Mini-OTEC, it was estimated that a 10-MW OTEC facility could achieve net-to-gross power production efficiency upwards of 75 percent, which would make it more commercially viable than the Mini-OTEC unit. Moreover, OTEC appears to be directly scalable with the applicable economies-of-scale that it implies; that is, the larger the OTEC plant, the more energy can be harvested, and the more cost-effective it is.

The results were so favorable that, in 1980, the U.S. Department of Energy (DOE) built OTEC-1, a non-power test-bed, on a converted U.S. Navy tanker to identify methods for designing commercial-scale heat exchangers, and demonstrated that OTEC systems can operate from slowly moving ships with marginal impact on the marine environment. This facility wasn’t designed to produce electricity, but rather to certify necessary technologies. DOE spent approximately $260 million on OTEC research and development between 1975 and 1982. 6

The U.S. Congress, in order to support and promote the commercial development and deployment of the nascent OTEC industry, passed Public Law (PL) 96-320, the Ocean Thermal Energy Conversion Act of 1980 , as amended by PL 98-623, National Fishing Enhancement Act of 1984 , to, among other actions, “...establish a legal regime which will permit and encourage the development of ocean thermal energy conversion as a commercial energy technology” [42 USC 9101(a)(4)]. The U.S. Congress also enacted the Ocean Thermal Energy Conversion Research, Development, and Demonstration Act , PL 96-310, which stated that “it is in the national interest to accelerate efforts to commercialize ocean thermal energy conversion by building pilot and demonstration facilities and to begin planning for the commercial demonstration of ocean thermal energy conversion technology” [42 USC 9100(a)(5)]. PL 96-310 established “as a national goal ten thousand megawatts [10,000 MWe] of electrical capacity or energy product equivalent from ocean thermal energy conversion systems by the year 1999” [42 USC 9100(b)(4)].

However, following the Mini-OTEC testing, next-generation energy costs world-wide plummeted, and continued OTEC research no longer was economically justifiable. In the 1990s, while some domestic companies and NELHA performed testing on complimentary technologies, most of the research and development work was supported by such countries as Japan and India. For in-stance, in 1981, Japan demonstrated a land-based, 100-kW closed-cycle power plant on the island Nation of Nauru, which exceeded engineering expectations by producing 31.5 kW of net electric power during continuous