Ocean Thermal Energy Conversion: Heat Exchanger Evaluation and Selection

Abstract

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