Ocean-Thermal-Energy Conversion

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 19979, "Ocean-Thermal- Energy Conversion (OTEC): Technical Viability, Cost Projections, and De velopment Strategies," by T.J. Plocek and M. Laboy, Offshore Infrastructure Associates, and J.A. Marti, Technical Consulting Group, originally prepared for the 2009 Offshore Technology Conference, Houston, 4-7 May. The paper has not been peer reviewed. Ocean-thermal-energy conversion (OTEC) is a renewable-energy source particularly suited for tropical zones. It uses the temperature difference between the warm surface ocean water and the cold deep ocean water to generate electricity and, if desired, potable water. This alternative energy source does not depend on fossil fuels, is not vulnerable to world market fluctuations and has less environmental impact than other energy sources. Introduction OTEC is a renewable-energy technology that is applicable to most parts of the world's deep oceans between 20° north and 20° south latitude, including the Caribbean Sea and the Gulf of Mexico; the Pacific, Atlantic, and Indian Oceans; and the Arabian Sea, where the temperature difference between the warm surface ocean water and the cold deep ocean water is equal to or greater than 68°F. OTEC basically recovers part of the solar energy absorbed by the ocean. Its main application is in tropical zones where deep ocean water is available at a short distance from the shore (less than 6 miles). In addition, the potential site must have a marine environment that allows the operation of a stable system. One of the greatest advantages of OTEC is that it allows the coproduction of potable water in addition to electric power. It is possible to produce as much 0.5 million gal/D for each MW of electricity generated. Because OTEC does not use fossil fuel, the produced electricity has a fixed cost; thus, it is not susceptible to the cost volatility that affects other energy sources such as petroleum, coal, and natural gas. Moreover, the environmental impact is less than that of other sources of energy because no products of combustion are generated during the power-production process. All of these aspects have caused a revival of interest in OTEC. Basic Principles An OTEC plant consists of a heat engine that converts thermal energy into mechanical work by use of the temperature gradient between a heat source and a heat sink. Although this temperature gradient is relatively small compared to a steam engine, the principle is the same. OTEC technology is divided into three categories: closed cycle, open cycle, and hybrid cycle.