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Ocean thermal energy conversion (OTEC) is a baseload 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 alternate energy source does not depend on fossil fuels, is not vulnerable to world market fluctuations, and has less environmental impact than other energy sources. During the 1970's and 1980's R&D projects such as Mini-OTEC and OTEC-1 in Hawaii and the Japanese 100-kW land-based pilot plant at the Republic of Nauru demonstrated the technical viability of OTEC, specifically with a closed-cycle system to generate electric power. Between 1993 and 1998, the Natural Energy Laboratory of Hawaii (NELHA) built and operated a 210-kW open-cycle pilot plant for the co-production of electricity and potable water. The facility was shutdown by the federal government. Today, this facility is used primarily for aquaculture and desalinated bottled Deep Ocean Water. Due to the recent progress in systems design, heat exchangers efficiency, the high costs of fossil fuels experienced in 2008, combined with the fluctuations in the oil world market, several companies have re-evaluated the use of OTEC. At present, a number of projects focused on the commercial implementation of OTEC at various sites are under consideration. Puerto Rico possesses specific conditions that make it an ideal site to implement OTEC. Conceptual design and capital cost estimates for the proposed 75-MWe closed-cycle plant for Puerto Rico are based on commercially available components and manufacturing practices. A modular and integrated design has been applied which is readily adaptable to other plant sizes. OTEC is a benign and environmentally compatible technology. Its potential impact to the environment can be minimized through proper design and engineering & construction best practices. The first plant would include a periodical program to study long-term environmental effects of OTEC, to optimize the design and operation of future plants. This paper summarizes efforts dedicated to commercial implementation of OTEC in Puerto Rico and other locations, concentrating on the technical and economical viability, and the associated environmental and socio-economical implications. Introduction Ocean thermal energy conversion (OTEC) is a renewable energy technology that is applicable to most parts of the world's deep oceans between 20o North and 20o South latitude including the Caribbean and 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 or greater than 20 oC. In essence, 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 short distance from the shore (less than 6 miles or 10 km). In addition, the potential site must have a marine environment that allows the operation of a stable system (Avery et al. 1994). One of OTEC's greatest advantages is that it allows the co-production of potable water, in addition to electric power through desalination. It is possible to produce up to 2 million liters per day (0.5 million gallons per day) for each megawatt of electricity generated (Cohen 1982). Since OTEC does not utilizes fuel, the produced electricity has a fixed cost, thus, it is not susceptible to the volatility of costs that affects other energy sources such as petroleum, coal and natural gas. Moreover, the environmental impact is less than other sources of energy since no products of combustion are generated during the power production process. All of these aspects have caused a revival of interest in OTEC.

This paper summarizes work conducted in design and development of commercial and demonstration-scale Ocean Thermal Energy Conversion (OTEC) powerplants, and discusses conditions required for commercial implementation, expanding on the information provided by the same authors in the 2009 OTC conference. It documents that commercial OTEC plants for power and potable water generation, developed from commercially available equipment and techniques, are technically feasible today. If favorable financial and market conditions are present, such plants are also economically viable. Key factors for economic viability are long-term financing and stable and favorable interest rates, both of which in turn require a long term commitment for use of the power generated by the plant. Introduction Ocean thermal energy conversion (OTEC) is a renewable energy technology that is applicable to most parts of the world's deep oceans in tropical and sub-tropical areas, where the temperature difference between warm surface and the cold deep water is equal or greater than 20 °C (63 °F). In essence, the technology works by recovering the solar energy absorbed by the ocean. Since the temperature differential suffers very little fluctuation, OTEC is able to generate power on a continuous (baseload) basis, as opposed to some renewable technologies, such as solar and wind. OTEC is most attractive for tropical locations where deep water is available at short distance from shore (less than 6 miles or 10 km), and the marine environment is sufficiently stable to allow operation. [1]. A feature that is particularly attractive is that, if desired, OTEC can be used to co-produce potable water through desalination, in addition to electric power. It is estimated that up to 2 million liters per day (0.5 million gallons per day) can be produced for each megawatt of electricity generated [2] OTEC requires no fuel. Thus, cost of producing electricity and water is not susceptible to the volatility that affects other energy sources such as petroleum, coal and natural gas. Energy can be generated from purely local sources at a cost that is essentially fixed and predictable. Further, since no fuels or radioactive materials are used, environmental impacts (including greenhouse gas generation) are much less than those of conventional methods of power generation. Basic Principles The basic principles of OTEC have been presented in multiple publications (such as [1], [2] and [3] among others). All OTEC plants are heat engines that convert heat into work through the energy gradient between a " source?? and a " sink.?? The basic principle is the same of a steam engine, although in the case of OTEC, the temperature gradient is much smaller. This makes OTEC plants much larger than steam plants of comparable generating capacities.

This study summarizes available data on heat exchanger requirements for closed-cycle OTEC power systems obtained during over thirty years of R&D work and technology demonstration programs, and presents how these requirements can be met using commercially-available heat exchangers used today in other applications by a variety of industries. The study focuses on the following design criteria: configuration (shell-and-tube, compact-type, and others), process performance, surface enhancement, corrosion resistance, biofouling control, manufacturability, ease of operation and maintenance, and over-all cost-effectiveness. Selection of the appropriate working fluid will also be discussed. Data evaluated include previously developed power system designs such as those completed during the I970's and 1980's by GE, JHU/APL, ANL, and engineering reports from OTEC technology demonstration programs such as the Nauru, Mini-OTEC and OTEC-1 test projects. A critical performance assessment is made between the use of stainless-steel plate heat exchangers and aluminum-brazed plate-fin heat exchangers in the context of present day technology. Alternatives to mitigate and control the adverse effects of biofouling are discussed. INTRODUCTIONOcean thermal energy conversion (OTEC) is a base-load renewable energy source that uses the temperature difference between the warm surface ocean water and the cold deep ocean water to generate electricity. OTEC is applicable to most parts of the world's deep oceans between 20° North and 20° South latitude including the Caribbean and 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 or greater than 20 °C. In essence, OTEC recovers part of the solar energy continuously absorbed by the ocean and converts it into electric power. OTEC does not utilize any fuel. The electricity generated has a fixed cost, thus, it is not susceptible to the volatility resulting from world market fluctuations that affects other energy sources such as petroleum, coal and natural gas. Moreover, environmental impacts are less than those of conventional sources of energy since no products of combustion and no solid or toxic wastes are generated during the power production process. In addition, effluents are essentially similar to receiving waters. All of these aspects have caused a revival of interest in OTEC*". An OTEC power system consists of a heat engine cycle that converts thermal energy into mechanical work through the temperature difference between a "heat source" and a "heat sink". Although this temperature difference is relatively small compared to a steam engine, the principle is the same (Rankine thermodynamic cycle). OTEC technology is divided into three major categories: closed, open and hybrid cycles. In the closed-cycle, the temperature difference is used to vaporize (and