Westinghouse OTEC power systems

OTEC R&D in the U.S. has been focused mainly on the closed cycle with ammonia as the working fluid. The open cycle offers a number of advantages, including cost competitiveness. The two important features are in turbine protection in case of load loss and in the absence of evaporator biofouling. The Westinghouse open-cycle concept departs from earlier approaches which locate deaeration ahead of the flash evaporator. Westinghouse proposes to allow all noncondensibles to flow into the condenser. This paper summarizes the main features of both the closed- and open-cycle concepts and provides systematic discussion of performance features and cost.     

Issues in OTEC commercialization

The commercialization of a new capital intensive technology requires coordination between government and industry. Government regulation of the energy industry can hinder its commercial development. On the other hand, government financial incentive is essential to induce industry to invest in a new technology such as OTEC. Incentives will be necessary in the early phases of development both for the manufacturers of OTEC systems and components and for the owner/operators of OTEC power plants. The various incentives are analyzed and their impact on manufacturers of, investors in, and users of OTEC technology is discussed. Analysis shows that OTEC is technically and economically ready to enter the electric utility market in tropical islands. For the larger U.S. mainland market, economies of scale are expected to reduce the capital cost to a low enough level where OTEC can be competitive.     

The mist-lift OTEC cycle

The thermodynamics and implementation of the mist-lift concept for the generation of power from thermal gradients in warm oceans are analyzed. The main feature of this concept is that it permits an open cycle to operate on the ambient sea-water using state-of-the-art hydraulic turbines. An experimental facility being completed at UCLA is briefly described.     

A foam OTEC system

Experiments are described which attempt to incorporate the main features of a foam OTEC System. These features include foam generation, foam rise, foam breaking, and finally separation of liquid and vapor. Our foam generator formed foam at rates as high as would be desired in a commercial plant, ~1 g/cm² sec. The rise of the foam, accompanied by a drop in temperature, was as expected by theory. The foam breaking, and the subsequent separation of liquid and vapor, presented no problem. Totally unexpected was the dominant role played by the wall drag in our 4 in diameter, 30 ft high column. Experiments were consistent, however, with a very simple expression for the variation of the drag coefficient with the foam parameters of mass flow rate and density. For the large diameter columns envisioned for commercial plants, wall drag will play only a minor role.Likewise unexpected was the large surfactant concentration (~1000 ppm) required to maintain complete foam stability to the top of the column. The periodic appearance of large bubbles at lower concentrations may be generated by our high rates of shear strain, greater than 20 sec.Another problem which must be solved is common to all open cycle systems. This is deaeration.     

OTEC research in Japan

Research on ocean thermal energy conversion (OTEC) has been going on in Japan since 1974. The R&D program is sponsored by the Ministry of International Trade and Industry in cooperation with a number of universities and industrial organizations. To date, the focus has been on the engineering of the power system and on the end use of OTEC-produced electric power. Although not all of Japan can benefit from electricity cabled directly to shore, Japan, because of its heavy dependence on foreign sources of energy, considers OTEC to offer other opportunities. In particular, OTEC-produced electricity may be used to extract uranium to fuel nuclear power plants located on land. Mariculture is another potential benefit of OTEC. In the paper, a summary of the technical, economic, environmental and resource factors of OTEC is presented.     

Performance study of an OTEC system

This paper describes a series of studies carried out to analyse the performance of an Ocean Thermal Energy Conversion (OTEC) system. The objective function, $\text{A}\text{W}{}_{\mathrm{net}}$, where A is the total heat exchanger area and W_net is the net work output of the system, was used for the parametric and optimisation studies. By using $\text{A}\text{W}{}_{\mathrm{net}}$ the heat exchangers were directly related to the remaining OTEC components. Since changes in one component of the system invariably affect the rest of the system, it was thus possible to evaluate the combined effects on the OTEC power plant.The effects of the following parameters on system performance were investigated: ocean fluid velocity through the exchanger, log-mean temperature differences of the heat exchangers, heat transfer enhancement and cold seawater pipe diameter. It was concluded that for a 1 MW_e OTEC power plant, the net output of the plant becomes zero when ΔT (the temperature difference between the hot and cold ocean streams) approaches 12·80°C. The power cycle used in this study was a simple closed Rankine cycle with ammonia as the working fluid.     

Performance characteristics of barometric-type open-cycle OTEC system

Open Cycle-Ocean Thermal Energy Conversion (OC-OTEC) system has a merit to use a heat exchanger of direct contact type without a heat transfer tube. Therefore, it is expected that the capital cost of OC-OTEC is reduced by use of this simply designed heat exchanger. However, non-condensable gas is released in the system, because in a direct contact evaporator, the steam driving a turbine is generated by surface sea water including air, and existing air causes a reduction of system performance. From the above point of view, we carried out an experimental study on the operating characteristics and the effect of structure of a heat exchanger and the existence of non-condensable gas on the performance of a direct contact heat exchanger in an experimental apparatus of barometric type OC-OTEC. As a result, the performance of the direct contact evaporator can be satisfied. However, we cannot get satisfactory results in a direct contact condenser. Therefore, we conducted further experiments through the improvement of the structure of the condenser and the control of the non-condensable gas. Finally, we concluded that the major factor affecting the system performance is the non-condensable gas, but its effect can be ignored when its concentration is below 8%. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res 25(4): 226–237, 1996     

An update of OTEC baseline design costs

This paper discusses the relative economics of several OTEC mission concepts. The present and projected costs of alternative energy sources and manufactured products are compared to those for products manufactured by potential OTEC systems. Since the efficiency and relative economic performance of an OTEC system is site specific, a generalization is made regarding site characteristics. Tropical grazing OTEC plant-ships are assumed to operate in relatively benign ocean waters with an average 43.2 °F (24 °C) temperature differential between the surface and 3281 ft (1000 m) depth. Several potential locations in U.S. waters are examined for the grid-connected OTEC concept.The primary competitors for OTEC are baseload and intermediate electric power plants, coal-produced synthetic products, and products made with oil and gas. Certain unusual circumstances lead to specific opportunities for OTEC for island complexes, which are discussed. Other markets are also discussed and the relative potential for OTEC examined.There are technical as well as economic uncertainties regarding OTEC commercialization which have direct impact. In order for OTEC to acquire a substantial share of energy markets, these uncertainties must be resolved. Examination of the potential impacts due to these uncertainties yields the conclusion that OTEC technology must be refined by extensive R&D before any commercialization can take place.     

Heat and Mass Transfer in Open-Cycle OTEC Systems

The temperature difference between surface and deep water in the oceans represents a vast resource of thermal energy. A promising method of harnessing this resource is the open-cycle ocean thermal energy conversion (OC-OTEC) system, which utilizes steam evaporated from the surface water to power the turbine. In this paper the state of the art of heat and mass transfer related to evaporation and condensation of steam at low pressures in OC-OTEC is summarized and relevant research issues are discussed.     

Synchronous power output fluctuations for an experimentalopen-cycle OTEC plant

A 210 kW experimental open-cycle ocean thermal energy conversion plant was completed in Hawaii in 1993, and equipped with a synchronous generator to test its connection to the local power grid. During shakedown tests, large power output fluctuations were observed. Linear mathematical models of the system were developed and numerical simulations reproduced observations well, for a given line frequency input, confirming in particular the resonant nature of a massive turbine rigidly connected to a small generator. The frequency-domain algorithm was extended to analyze the effect of inserting a fluid coupler between the turbine and the generator to eliminate large power output fluctuations. The actual installation of a fluid coupler in early 1994 allowed a validation of the model predictions     

Photovoltaics and electric utilities

We determine the long-term value of a grid-connected, residential photovoltaic (PV) systems. The value of the PV electricity is defined as the full avoided cost, consistent with the Public Utilities Regulatory Policies Act of 1978. A case study approach to three utility systems is used. The avoided cost is computed using a long range utility planning approach to measure revenue requirement changes in response to the time-phased introduction of PV systems into the grid. The changing value of PV electricity over a 20-year period from 1985 is presented, and the fuel and capital savings due to PV are analyzed and translated into a measure of breakeven capital investment.     

Design and performance analysis of radial-inflow turboexpander for OTEC application

Ocean thermal energy conversion (OTEC) is a potential source of renewable energy. In order to design a turbine for maximizing the output power for very low working temperature application like OTEC, careful one-dimensional design followed by detailed three dimensional simulation is required. In this work a radial-inflow turbine with R-22 as working fluid is designed for a closed-cycle ocean thermal energy conversion plant of 2 kWe capacity. Design speed of the turbine is 34000 rpm. Inlet and outlet temperatures of designed turbine are 24.5 °C and 14 °C respectively. Three-dimensional fluid flow analysis inside the turbine at design and off-design conditions were carried out. Important dimensions of the turbine are: rotor tip and shroud radii of 24 mm and 19 mm respectively; blade widths at rotor inlet and outlet of 6 mm and 11 mm respectively; axial length of 17.5 mm; diffuser of 62 mm long. Volute casing designed has a circular cross section. The importance of the number of blades, blade filleting and stagger angle from the point of view of turbine performance are reported.     

Economics of Ocean Thermal Energy Conversion (OTEC)

Ocean thermal energy conversion (OTEC) has for many years been recognised as a possible energy of the future. Recently, the emphasis has changed towards combination plants offering not only power, but also freshwater, mariculture and other deep ocean water applications (DOWA).The models used in this modified cost-benefit analysis indicate under what economic circumtances OTEC, which is presently only competitive in a few remote islands, may become more widely competitive in the near future. Among the model variables are the plants' capital cost, operation and maintenance costs, the price of oil and desalinated water as well as a hypothetical carbon tax.     

On the selection of fin profiles for OTEC plate-fin evaporators

This paper considers the plate-fin heat exchanger for an OTEC power plant on account of its compactness. Ammonia is selected as the working fluid for a plant of 1 MW gross output. Choice of seawater side velocity is made on the basis of pressure drop and biofouling. Fourteen different fin profiles are considered in order to choose a suitable configuration. The selection of the plate-fin heat exchanger depends on minimum exchanger volume, minimum pumping power requirement and low ammonia side pressure drop.     

Modelling the competitiveness of first generation commercial OTEC power plants

First generation OTEC plants are expected to be used mainly for baseload electricity generation in the United States Gulf Coast region. In this application, OTEC plants would compete directly with nuclear and coal-fired power plants. The prospective competitiveness of OTEC is evaluated by comparing the delivered cost of electricity generated by the three types of plant for a geographical scenario typical of the region. The comparison is carried out using a modified version of the cost of energy model developed by the Jet Propulsion Laboratory and current estimates of future construction, operating and maintenance costs for the three power plant types. Four main independent variables are considered in this study: OTEC plant capital costs, real fuel escalation rates, real cost of capital resources, and OTEC plant operating capacity factors. The first two factors are found to be prime determinants of OTEC competitiveness. The values commonly forecasted suggest that OTEC plants are likely to deliver electricity at roughly the same cost as nuclear and coal-fired power plants by the year 2000. By contrast, variations in the real cost of capital resources and in OTEC plant capacity factors are found to have only a minor impact on the competitiveness of OTEC with conventional modes of electricity generation.     

Thermal properties of the Florida Current as related to Ocean Thermal Energy Conversion (OTEC)

The thermal properties of the Florida Current are presented and analyzed for the available ocean thermal energy. For a cold water intake depth equal to or greater than 600 m, potential sites for Ocean Thermal Energy Conversion (OTEC) power plants appear to exist in the Straits of Florida and, to a lesser extent, off the coasts of Georgia and South Carolina. The maximum thermal differences occur on the continental shelf because of the geostrophic motion of the Gulf Stream. An estimate of the total available ocean thermal energy from the Florida Current, delivered in the form of electricity, is 3.5 × 10¹² kWh yr⁻¹. For a cold water suction depth of 600 m or more, seasonal variability in the ocean thermal resource is approx. ±35 per cent of average annual output.     

MODEST-An energy-system optimisation model applicable to local utilities and countries

MODEST, an energy-system optimisation model is described. It has been applied to a typical local Swedish electricity and district-heating utility and to the national power system. Present and potential installations and energy flows should be considered and their best combination can be obtained through optimisation. MODEST uses linear programming to minimise the capital and operation costs of energy supply and demand-side management. Seasonal, weekly, and diurnal variations of, for example, demand, costs, and capacities are considered. MODEST may be used to decide which investments to make, the dimensioning of new installations, and the operation of all system components. The municipal utility under study should now expand its heat production using woodchips. Electricity export or nuclear phase-out will probably raise the Swedish electricity prices. In this case, cost minimisation is achieved by introducing combined heat and power (CHP) production in the municipality. Fossil fuels should be used in the cogeneration plant at current taxation levels but biofuels are favourable if higher environmental fees are imposed for CO₂ emissions. Biomass capacity expansion could decrease local CO₂ emissions by 80%. Efficiency improvements for electricity use have robust profitability at high electricity prices. The Swedish electricity demand may be satisfied without nuclear power and fossil fuels through massive biomass use, wind-power supply, and energy conservation.     

On the selection of working fluids for OTEC power plants

This paper analyzes the effect of three different working fluids (ammonia, propane, and freon-114) on the size of OTEC heat exchangesrs and system performance. Seven different combinations of shell-and-tube heat exchangers are considered. For each combination, a simple computer model of the OTEC power system is used to compare the three fluids. The comparison is made on the basis of A/W_net, where A is the total heat transfer area (evaporator plus condenser) and W_net is the net power output of the plant. Overall, ammonia is shown to be the best fluid (i.e. it yields the lowest value of A/W_net), although in some cases only by a small margin. The thermophysical property that gives ammonia its general superiority is its relatively high thermal conductivity. The paper also discusses heat exchanger design problems associated with liquid entrainment and boiling liquid superheat.     

海洋温差能向心透平的气动设计及性能研究

通过对7.5 kW海洋温差能向心透平的蜗壳、喷嘴和叶轮进行气动设计,模拟研究了透平在设计工况及非设计工况下的气动性能。采用经验参数及遗传算法优化方法对透平的一维参数进行设计,得到一维设计结果,并据此对蜗壳、喷嘴和叶轮进行三维设计,得到透平的气动结构造型。利用CFD技术模拟研究了透平的三维流场及性能,得到透平在设计工况及非设计工况下的性能,模拟结果表明:在设计工况下,透平效率为86.5%;在非设计工况下,透平效率随着叶轮转速的增加而增大,但增加至设计转速后,透平效率增加幅度较小;随着进口温度的升高,透平效率逐渐增大;当进口压力为设计工况压力时,透平效率存在最大值;非设计工况下的透平功率基本与叶轮转速、进口压力和进口温度均呈正相关;设计工况下的最佳喷嘴-叶轮相对径向间隙为0.05,可变喷嘴叶片安装角为35～40°。