Energy-efficient schemes for seawater desalination

A financing strategy for small OTEC plants

In this paper, an optimal design concept has been utilized to find the best designs for a complex and large-scale ocean thermal energy conversion (OTEC) plant. The OTEC power plant under this study is divided into three major subsystems consisting of power subsystem, seawater pipe subsystem, and containment subsystem. The design optimization model for the entire OTEC plant is integrated from these subsystems under the considerations of their own various design criteria and constraints. The mathematical formulations of this optimization model for the entire OTEC plant are described. The design variables, objective function, and constraints for a pilot plant under the constraints of the feasible technologies at this stage in Taiwan have been carefully examined and selected. The numerical optimization method called Sequential Quadratic Programming (SQP) is selected to obtain the optimum results. The main purpose of this paper is to demonstrate the design procedure with the optimization techniques for engineering and economics in the OTEC plant so that anyone else can build upon their models according to their needs.

Ocean thermal energy conversion (OTEC) plants are likely to be located some distance offshore, and several different approaches have been suggested for carrying the energy these plants produce to the energy user. Energy carriers evaluated include both chemical and electrical energy. The Institute of Gas Technology (IGT) is conducting a study to provide a technical and economic evaluation of two previously unexamined concepts for converting OTEC energy to a storable, transportable form, and shipping it to a shore-based receiving terminal. One concept deals with an onboard electrical system that is used to produce high-temperature heat and shipping this thermal energy to shore in some form of thermal storage system, such as a molten salt. The other concept is to use OTEC energy to produce carbonaceous fuels using electrolytic hydrogen produced onboard and carbon dioxide extracted from seawater or delivered from a shore-based facility. Methane, methanol, and conventional light fuels of the gasoline family will be considered for synthesis at the OTEC plant; the possibility of producing high-energy fuels, such as hydrazine, UDMH, 1-7 octadiyne, and tetrahydrodicyclopentadiene is also being analyzed. During the second quarter we concentrated our efforts on the following areas: (1) analyzing the techno-economic characteristics of producing gasoline from methanol on an OTEC platform; (2) determining carbon dioxide levels in seawater; (3) evaluating various processes for the extraction of CO sub 2 from seawater; (4) collecting information concerning OTEC energy barge and pipeline transportation systems; and (5) developing a method for analyzing thermal energy transport systems. Research progress is summarized. (ERA citation 02:051231)

Preliminary designs for first generation Ocean Thermal Energy Conversion (OTEC) plants utilizing either closed cycle (CC) or open cycle (OC) concepts are presented. These are based on existing technology and current offshore industry practices. The CC-OTEC plant utilizes pressurized anhydrous ammonia as the working fluid to drive turbine-generators to produce electricity; and, the OC-OTEC plant makes use of low pressure steam generated in flash evaporators to drive steam turbine generators to produce electricity and surface condensers for the production of desalinated water. Introduction Given that oil reserves (˜ 1400 billion barrels) can satisfy world-wide demand (> 30 billion barrels/year) for at most another 50 years, it seems sensible to envision marine renewable energy resources as additional alternatives to our oil-based economy. In theory, for example, the ocean thermal resource could be used to generate most of the energy required by humanity (Michaelis, 2002 and Nihous, 2007). We should consider the implementation of OTEC plantships providing electricity, via submarine power cables, to shore stations. This could be followed, in 20 to 30 years, with OTEC factories deployed along equatorial waters producing energy intensive products, like ammonia and hydrogen as the fuels that would support the post-petroleum era. What is pending, however, is a realistic determination of the costs and the potential global environmental impact of OTEC plants and this can only be accomplished by deploying and subsequently monitoring operations with first generation plants (Vega, 2003). In the 1990s, it was determined that to be cost competitive OTEC plants larger than about 50 MW were required in the USA market; and, that it was necessary to deploy demonstration plants as a prerequisite to commercialization (Vega, 1992). Unfortunately, development did not proceed beyond an experimental plant sized at about 0.25 MW (Vega, 1995). A number of configurations ranging from offshore to land based concepts have been proposed. Large and small waterplane platforms have been considered. In general, the former (ship shape) is ideally suited for OTEC applications. Moored offshore configurations transmit electrical power to shore via a submarine power cable. The grazing configuration operates as a self-contained factory ship on which an energy-intensive product is produced (Nihous, 1993). The grazing plantship can cruise around tropical waters essentially decoupled from land. Conceptual designs for 50 MW OTEC plants utilizing either closed cycle (CC) or open cycle (OC) technology are summarized herein. The CC-OTEC plant utilizes pressurized anhydrous ammonia as the working fluid to drive turbine-generators to produce electricity; and, the OC-OTEC plant makes use of low pressure steam generated in flash evaporators to drive steam turbine generators to produce electricity and surface condensers for the production of desalinated water. The OTEC platform must interface with both the cold water pipe (CWP) and the deep ocean mooring system. The attachment between the vessel and CWP must provide freedom in at least pitch and roll. Deep Ocean mooring systems or dynamic positioning thrusters developed by the offshore industry can be used for position keeping.

A numerical model has been developed to rigorously simulate the physical oceanographic effects of one or several 100 megawatt Ocean Thermal Energy Conversion (OTEC) plant(s). The model suggests that OTEC plants can be configured such that the plant can conduct continuous operations, with resulting temperature and nutrient perturbations that are within naturally occurring levels. This presentation will describe the development and results of the numerical model, focusing on the physical and biological effects of operating single and multiple 100 megawatt OTEC plants in Hawaiian waters, and will discuss the implications of these results towards future design and regulatory work. Detailed statistics and visualization of the thermal variation, plume dilution, and estimates of the biological growth around an OTEC plant will be presented. Studies to date suggest that by discharging the OTEC flows downwards at a depth below 70 meters, the dilution is adequate and nutrient enrichment is small enough so that 100 megawatt OTEC plants could be operated in a sustainable manner on a continuous basis.

Energy, exergy and economic analyses of a novel hybrid ocean thermal energy conversion system for clean power production

The dynamic analysis of the coldwater pipe coupled to an Ocean Thermal Energy Conversion (OTEC) Plant is one of the primary considerations that an OTEC plant designer must evaluate. By a rigorous time domain analysis the designer can understand the interaction between the OTEC hull and cold-water intake system. This understanding is required for an adequate design since, the forces and motions of both the hull and pipe are the result of this interaction. This paper presents an approach to this dynamic analysis which can be used by the designer. The method has application to .drill ship and mining vessel analyses. INTRODUCTION In the search for alternative energy sources, the oceans hold great promise. More specifically, electric power may be generated from the vast quantities of solar energy-the heat from the sun's rays stored in the ocean. This energy is in the form of a thermal gradient, a small temperature difference between the sun-heated upper ocean layers and the colder, deeper water: When the thermal gradient is sufficient-as in tropical and subtropical waters-it can be the "fuel" for an ocean thermal power-plant. This enormous "fuel" source potential has been recognized by the U.S. Government. Consequently, the Department of Energy has undertaken a program to determine the feasibility of utilizing thermal energy from offshore floating ocean-thermal power plants. The potential of this program has been described in many previous reports. An Ocean Thermal Energy Conversion (OTEC) Plant would consist of heat exchangers evaporator and condenser), turbine generators, and circulating pumps, a hull, warm-water and cold-water intakes. The cold-water intake, consisting of a large diameter pipe or group of pipes, would extend downward 1,000 to 3,000 feet below the OTEC hull. In order to test the heat exchangers in the OTEC program, the Hughes Mining Barge (HMB) was selected by ERDA (now the Department of Energy) to be used as an OTEC-I platform. A schematic HMB-CWP system is shown in Fig. 1. This paper addresses the dynamic analysis of the cold-water pipe coupled to the OTEC hull. The analysis described here approaches the problem using a time domain solution. The finite difference technique is used in the pipe analysis and the hydrodynamic coefficients used in the ship motions analysis are based upon strip theory. MATHEMATICAL MODEL An Ideal Model The OTEC platform and cold water pipe must be designed to withstand ocean environmental loads. An ideal model would be one which accounts for all the relevant physical phenomena and is sufficiently accurate within the required range of parameters. The platform is subjected to loading parameters such as surface waves, surface currents, wind, cold water discharge, warm water intake, warm water discharge, and dynamic positioning or mooring and so on. The loading parameters on the cold water pipe include surface waves, low frequency internal waves, surface current, cold water intake, internal flow and mass of the fluid in the cold water pipe, and so on.

In this paper, the results and current work efforts of the Ocean Thermal Energy Conversion (OTEC) Cold Water Pipe (CNP) program will be presented. The program, sponsored by the Department of Energy and developed and technically managed by the National Oceanic and Atmospheric Administration, is one of the largest civil ocean engineering activities underway at the present time. The uniqueness of the OTEC GNP is derived from its physical magnitude, namely, pipes that are being designed with a diameter of 9 meters" and a length of 1000 meters. Details of the three phases of the program: the analytical procedures used to predict pipe responses, the model test projects and the large scale at-sea tests, will be discussed. The procedures used in planning the program will be highlighted. Problems and their solutions will be specified. Projects currently underway or just completed will be synopsized. INTRODUCTION The cold water pipes (GNP) required in Ocean Thermal Energy Conversion (OTEC) Plants present a formidable challenge to ocean engineers and marine constructors. The GNP size alone is impressive: 1000 meters in length, 9 meters to 30 meters in diameter depending on the OTEC plant size. The magnitude of the pipe forces and the pipe structural responses, plus pipe development and deployment procedures, pose additional problems for the designer. The uniqueness of the GNP as a marine structure, however, lies not so much in its unprecedented mass as in its need to be a flexible structure which interacts dynamically with the hostile ocean environment. This factor amplifies the concern for structural fatigue to the point where only abnormally low stress levels can be tolerated and factors of safety based on stresses alone become meaningless. There is only one GNP in an OTEC plant, which must be designed to survive all environmental conditions for the duration of the plant, namely, 30 years, and not be damaged nor cause damage to the other parts of the OTEC plant. A federal program has been formulated over the past four years, directed toward the solution of the major GNP problems. The end products of the program will be designs, data and procedures that can be utilized in the final design and development of several viable alternative GNP concepts. In this paper the current status of the CWP program will be presented. The program formulation will be discussed and the major elements of the program will be explained. PROGRAM OUTLINE The original concept of an OTEC GNP was that it was a simple vertical tube appendage attached rigidly to an OTEC platform. By making the pipe large in diameter, the cold water flow could be limited to 2 meters/sec to reduce pipe head losses to a minimum. To ensure pipe survivability, pipe concepts were made rigid from such materials as concrete and steel.

Ocean thermal energy conversion (OTEC) is a form of power generation, which exploits the temperature difference between warm surface seawater and cold deep seawater. Suitable conditions for OTEC occur in deep warm seas, especially the Caribbean, the Red Sea and parts of the Indo-Pacific Ocean. The continuous power provided by this renewable power source makes a useful contribution to a renewable energy mix because of the intermittence of the other major renewable power sources, i.e. solar or wind power. Industrial-scale OTEC power plants have simply not been built. However, recent innovations and greater political awareness of power transition to renewable energy sources have strengthened the support for such power plants and, after preliminary studies in the Reunion Island (Indian Ocean), the Martinique Island (West Indies) has been selected for the development of the first full-size OTEC power plant in the world, to be a showcase for testing and demonstration. An OTEC plant, even if the energy produced is cheap, calls for high initial capital investment. However, this technology is of interest mainly in tropical areas where funding is limited. The cost of innovations to create an operational OTEC plant has to be amortized, and this technology remains expensive. This paper will discuss the heuristic, technical and socio-economic limits and consequences of deploying an OTEC plant in Martinique to highlight respectively the impact of the OTEC plant on the environment the impact of the environment on the OTEC plant. After defining OTEC, we will describe the different constraints relating to the setting up of the first operational-scale plant worldwide. This includes the investigations performed (reporting declassified data), the political context and the local acceptance of the project. We will then provide an overview of the processes involved in the OTEC plant and discuss the feasibility of future OTEC installations. We will also list the extensive marine investigations required prior to installation and the dangers of setting up OTEC plants in inappropriate locations.

Techno-economic analyses are presented of three different methods of using shaftpower derived from an ocean thermal energy conversion (OTEC) plant to obtain currently usable forms of energy. ''High-energy'' fuels were studied to determine the initial feasibility of their synthesis on-board an OTEC plant. Fuels studied were hydrazine hydrate, anhydrous hydrazine, unsymmetrical dimethylhydrazine, 1,7-octadiyne, and tetrahydrodicyclopentadiene. Carbonaceous fuels considered were methanol, gasoline, and methane. Transportation and terminal facilities for delivering and receiving the offshore-produced energy carriers were assessed. Thermal energy media were considered as both methods of storage and transportation of OTEC-derived energy. The characteristics of the storage medium, storage configuration and materials, system energy storage density, system thermal losses, heat energy efficiency, and cycle temperature are specified. The unit capital costs of storage, transport ships, and heat engines are also discussed. The methodology for the economic evaluation of the thermal energy transport system is also described. The ship transport concept description, the economic evaluation methodology, and the results for the costs for delivered thermal energy and electricity are given. Two ''electrochemical bridge'' techniques hold promise as alternate methods of OTEC electrical energy transport for long offshore distances where cables are impractical. Overall conclusions and recommendations were made based on themore » results of the entire study and are discussed.« less

A critical operating aspect of Ocean Thermal Energy Conversion (OTEC) plants is the maintenance of clean surfaces on the seawater-side of the heat exchangers. The objective of this program was to assess the state of the art of biofouling control techniques and to evaluate the potential of these existing methods for solving the biofouling problems in the OTEC system. The first task of the program involved an in-depth review and discussion of various fouling control methods including water treatment, surface conditioning, and cleaning techniques. The methods considered applicable to OTEC were identified. This volume summarizes the second task of the program. The compatibility of the various cleaning and fouling control techniques with the different proposed heat exchanger designs and materials are discussed. Also provided are conceptual illustrations for adapting and incorporating the methods into an OTEC power plant. These conceptual designs suggest means for overcoming some of the shortcomings of the techniques which are considered suitable, however, detailed designs of the modified systems are beyond the scope of this report. Chlorination, chemical cleaning, Amertap recirculating sponge rubber balls, and MAN flow-driven brushes are the methods considered applicable for tubular heat exchangers with seawater inside the tubes. Water jets are suggestedmore » for the open-cycle and the ''trombone'' (Applied Physics Laboratory) heat exchanger designs. Although none of the methods are immediately applicable to OTEC in their present configuration, in several cases only minor developmental efforts should produce designs which can satisfy the stringent OTEC cleanliness requirements. Further research and development appear warranted for a number of other methods which indicate promise for long-range applicability. Specific recommendations are included.« less

Analysis of optimization in an OTEC plant using organic Rankine cycle

Worldwide power resources that could be extracted from Ocean Thermal Energy Conversion (OTEC) plants are estimated with a simple one-dimensional time-domain model of the thermal structure of the ocean. Recently published steady-state results are extended by partitioning the potential OTEC production region in one-degree-by-one-degree “squares” and by allowing the operational adjustment of OTEC operations. This raises the estimated maximum steady-state OTEC electrical power from about 3TW(109kW) to 5TW. The time-domain code allows a more realistic assessment of scenarios that could reflect the gradual implementation of large-scale OTEC operations. Results confirm that OTEC could supply power of the order of a few terawatts. They also reveal the scale of the perturbation that could be caused by massive OTEC seawater flow rates: a small transient cooling of the tropical mixed layer would temporarily allow heat flow into the oceanic water column. This would generate a long-term steady-state warming of deep tropical waters, and the corresponding degradation of OTEC resources at deep cold seawater flow rates per unit area of the order of the average abyssal upwelling. More importantly, such profound effects point to the need for a fully three-dimensional modeling evaluation to better understand potential modifications of the oceanic thermohaline circulation.

The large volumes of water withdrawn at both the warm and cold water intakes of an OTEC plant must be screened to remove organisms and debris which could clog the heat exchangers. The recent literature on screening technology is reviewed. In addition, various screen manufacturers and coastal facilities which use large volumes of seawater were visited to determine the operating experience with present screen technology. Static screens (particularly the Johnson Division, UOP profile wire screen and the Royce Equipment Company carrousel screen) have the potential advantage for OTEC for operating in a completely submerged state and of being cheaper to operate and maintain than traveling screens. However, there is no operational history with these static screens for large intake systems. The most promising traveling screen options for OTEC are the dual flow screens. They offer more screening surface and less head loss than through flow screens of similar size. They also have been operated in seawater for large intake systems. More detailed designs of potential OTEC plants, particurlarly screen wells, conduit and surge tank construction and head losses need to be determined before the best alternative intake screen can be selected. 38 references.

The moored pipe/mobile platform (MP-Squared) approach is one in which the cold water pipe (CWP) of a floating Ocean Thermal Energy Conversion (OTEC) plant is permanently moored in the ocean at the production site. The plantship (surface platform), however, is capable of being disconnected from the moored CWP to avoid storms or for shoreside maintenance. Two conceptual versions of MP-Squared were developed through a parametric design process and both were tested in an experimental wave tank. The results indicated that MP-Squared offers a technically sound and operationally attractive option for designers of OTEC plants.