Closed-Cycle OTEC Systems

The Rankine closed cycle is a process in which beat is used to evaporate a fluid at constant pressure in a “boiler” or evaporator, from which the vapor enters a piston engine or turbine and expands doing work. The vapor exhaust then enters a vessel where heat is transferred from the vapor to a cooling fluid, causing the vapor to condense to a liquid, which is pumped back to the evaporator to complete the cycle. A layout of the plantship shown in Fig. 1-2. The basic cycle comprises four steps, as shown in the pressure-volume (p—V) diagram of Fig. 4-1. 1. Starting at point a, heat is added to the working fluid in the boiler until the temperature reaches the boiling point at the design pressure, represented by point b. 2. With further heat addition, the liquid vaporizes at constant temperature and pressure, increasing in volume to point c. 3. The high-pressure vapor enters the piston or turbine and expands adiabatically to point d. 4. The low-pressure vapor enters the condenser and, with heat removal at constant pressure, is cooled and liquefied, returning to its original volume at point a. The work done by the cycle is the area enclosed by the points a,b,c,d,a. This is equal to Hc–Hd, where H is the enthalpy of the fluid at the indicated point. The heat transferred in the process is Hc–Ha Thus the efficiency, defined as the ratio of work to heat used, is: . . . efficiency(η)=Hc–Hd/Hc–Ha (4.1.1) . . . Carnot showed that if the heat-engine cycle was conducted so that equilibrium conditions were maintained in the process, that the efficiency was determined solely by the ratio of the temperatures of the working fluid in the evaporator and the condenser. . . . η=TE–Tc/TE (4.1.2) . . . The maximum Carnot efficiency can be attained only for a cycle in which thermal equilibrium exists in each phase of the process; however, for power to be generated a temperature difference must exist between the working fluid in the evaporator and the warm-water heat source, and between the working fluid in the condenser and the cold-water heat sink.

Economic, Environmental, and Social Aspects of OTEC Implementation

The financial analyses presented in Chapters 7 and 8 indicate that commercial development of OTEC will have a significant impact on the economics of U.S. energy production and use. Two scenarios for commercial development are examined in this section: 1. Development of OTEC methanol capacity sufficient to replace all U.S. gasoline produced from imported oil. 2. Development of OTEC ammonia capacity sufficient to replace all gasoline used in U.S. transportation. Commercialization of this option implies a project goal to produce methanol plantships with enough total methanol capacity to replace the gasoline used in the United States that is now produced from imported petroleum, 47 billion gallons of gasoline in 1990 (DOE/EIA, 1990). This would require a total of 427 200-MWe plantships, each producing 199 million gallons of methanol per year (1.8 gallons of methanol give the same automobile mileage as 1 gallon of gasoline. We assume financing based on an initial nominal plant investment of $960M (1990$) and an eighth plant investment of $664M. With repeated manufacture, the cost will be reduced to $438M for the 427th plantship, assuming that an experience exponent of 0.93 applies for all production of identical plantships after the first three. The average plant investment for the total production is then $507M. If financial support is maintained to complete the program, the year 2020 is a reasonable target date for achieving the full fuel production capacity. This implies construction of OTEC plantships at an average rate of 17 per year after commercial production is established. This rate could be accommodated in U.S. shipyards with feasible modifications to satisfy specific OTEC requirements. The U.S. shipbuilding facilities are discussed in Section 4.1. In addition to the investments required for OTEC, methanol automobiles must be in production, and distribution systems for methanol must be installed. The associated costs must be included in the financial analysis. Offsetting these costs are the savings resulting from: 1. Large improvements in the U.S. balance of trade through elimination of oil imports. 2. Tax receipts accruing from reinvigorated U.S. shipbuilding and associated manufacturing industries. 3. Economic benefits of stabilized world fuel prices.

OTEC Closed-Cycle Systems Cost Evaluation

Innovative technologies such as OTEC achieve commercial development when potential investors decide that the return on the investment will repay the estimated development costs plus a profit, with an acceptably low risk of cost overruns. Industrial experience shows that the estimated cost to complete development of a new technology generally increases as development proceeds from the conceptual design through pilot development, demonstration, field testing, and final commercial manufacture (Merrow et al., 1981). The ratio between final cost and initial design estimate is strongly dependent on the extent to which the manufacturing process employs already developed equipment, procedures, and facilities. New projects that require “high technology” for their success, such as jet engines or nuclear power plants, have been characterized by large underestimates of the final costs, whereas the costs of projects that are firmly based on existing technology, such as the development of “supertankers,” have been accomplished well within the usual industrial uncertainty margin of ± 15 to 20%. The accuracy of the estimate is also strongly dependent on the thoroughness of the systems engineering evaluation that is done before development proceeds. Commercial applications of OTEC have been proposed in three principal categories. The first includes OTEC power plants mounted on floating platforms that would generate 50- to 400 MWe (net) of onboard electric power. The need to minimize plant size makes it mandatory to use closed-cycle OTEC for these applications. The second category includes land-based or shelf-mounted plants designed to supply power in the 50- to 400-MWe range to municipal utilities. Either open- or closed-cycle systems could be suitable. The third category comprises small (5- to 20-MWe) land-based or shelf-mounted OTEC plants designed for island applications where electric power generation, mariculture, fresh-water production, supply of cold water for air-conditioning systems, and fuel production could be combined to offer an economically attractive OTEC system despite the relatively high cost of power for small OTEC installations. Open-cycle OTEC plants may be the preferred choice for the third category. The estimated investment costs of installed complete OTEC systems, measured in dollars per kilowatt of net OTEC electric power generated, differ significantly among the three categories.

OTEC Closed-Cycle Engineering Status

Engineering analyses and component design studies during the period 1974–977 indicated the feasibility of constructing and operating floating OTEC plants and plantships in a variety of configurations ranging in power from 40 to 500 MWe. In August 1979, an at-sea test of a complete OTEC power system (Mini-OTEC) demonstrated performance in good accord with engineering predictions and established a firm basis for scale-up to larger sizes (Owens and Trimble, 1980). Heat exchanger operation at a level equivalent to 1-MWe power generation was demonstrated 1 year later in the OTEC-1 program. In 1981, a complete land-based OTEC power plant was constructed and operated under Japanese direction at the island of Nauru on the equator in the mid-Pacific ocean. During the period 1977-1980, a U.S. plan was developed, supported by public laws PL 96-310 and PL 96-320, to demonstrate OTEC feasibility at a 100-MWe level by 1985 and 500 MWe by 1990. Testing was to start with a pilot demonstration at 40 MWe (net). Preliminary design of baseline demonstration plants at this power level for moored operation off Punta Tuna, Puerto Rico, and for grazing operation west of equatorial Brazil with on-board ammonia production was completed in 1980 (George and Richards, 1980). Conceptual designs of larger plants and power systems for demonstration at the baseline level were also completed. In accord with the requirements of the Congressional actions, a Program Opportunity Notice (PON) was issued in September 1980 by DOE that offered cost-sharing support for innovative OTEC systems designs that contractors believed would be commercially viable if government cost sharing were made available during development of demonstration vessels. The PON asked for proposals for a development program to design, construct, and test a 40-MWe (net) closed-cycle OTEC system, which would be conducted in six phases beginning with conceptual design and continuing to preliminary design, engineering design, construction, deployment and operation, and, finally, transfer of ownership and contractor operation. The schedule was set to be consistent with the goal established by PL- 96-310 of demonstration of 100-MW OTEC operation by 1985. DOE stated its intent to fund five to eight awards for the first phase, with DOE providing $900,000 as its share of each contract awarded (Dugger et al., 1983).

Open-Cycle OTEC

The historical development leading to the proposal by Claude to generate power by producing steam in flash evaporation of warm seawater has been discussed in Chapter 2. In this chapter, the thermodynamic fundamentals of the open-cycle concepts are discussed, leading to a detailed review of state of the art and commercial prospects of the process. There are several variations on the standard OTEC open-cycle (OC) system. The three major variations are “hybrid cycle” (Bartone, 1978), “mist lift cycle” (Ridgway, 1977), and “foam lift cycle” (Beck, 1975; Zener et al., 1975). These are advanced concepts that offer certain attractive features and are being investigated. The three cycles will be discussed in Sections 5.3, 5.4, and 5.5, respectively. The standard OTEC open cycle is discussed in the following. The modest but nearly steady temperature difference that exists between the warm surface water and the much colder water at great depth in some tropical regions of the world has attracted the attention of many thermodynamicists from the time that these temperature differences were first observed. From the thermodynamicist’s view, any significant temperature difference can be used to produce power. The open or Claude cycle is the forerunner of various OTEC cycles. The open cycle refers to the use of seawater as the working fluid. A schematic diagram of the system, which comprises a flash evaporator, vapor expansion turbine and generator, steam condenser, noncondensables-removing equipment, and deaerator, is shown in Fig. 5-1 (Chen, 1979). The cycle is a basic Rankine cycle for converting thermal energy of the warm surface water into electrical energy. In the cycle, the warm seawater is deaerated and then passed into a flash evaporation chamber, where a fraction of the seawater is converted into low-pressure steam. The steam is passed through a turbine, which extracts energy from it, and then exits into a condenser. This cycle derives the name “open” from the fact that the condensate is not returned to the evaporator as in the “closed” cycle. Instead, the condensate can be used as desalinated water if a surface condenser is used, or the condensate is mixed with the cooling water and the mixture is discharged back into the ocean.

OTEC System Concepts

Systems engineering is a top-down approach to program management and systems procurement. It optimizes the development process by ensuring that the operational, technical, and cost goals (and limitations) of a total proposed system are understood before development begins. The requirements for the “forest” are determined before the features of the “trees” are specified. It makes a basic assumption that a team endeavor under single-system management will be established with authority to define development goals and assign subsystem programs and funding. It recognizes that each system requires a unique management structure that is based on the qualifications of the people and organizations available for the total endeavor. Systems engineering begins with an authoritative request or requirement for a system that would provide new capabilities or would reduce existing problems in a significant technical activity. After personnel and level of effort for a preliminary assessment of the need are identified, the initial effort then involves these steps: 1. A precise definition is prepared of the specific operational need for which the proposed system must provide a solution. For example, this book addresses the present national need for a new energy system that can provide a practical, timely, cost-effective, and nonpolluting alternative to petroleum-based fuels for transportation. The need arises from three factors: a. The perception that an alternative to dependence on petroleum fuels for transportation must be developed to avoid severe disruption of world economies in the early years of the twenty-first century; b. Evidence that combustion of fossil fuels is causing a significant increase in the carbon dioxide content of the atmosphere (if not reduced, this could eventually produce a “greenhouse effect,” leading to large-scale changes in climate and an increase in sea level, with severe economic consequences); and c. The belief that solar energy can be used via OTEC to supply nonpolluting fuel in sufficient quantity, at low enough cost, and in time to become a practical alternative to dwindling or unavailable petroleum supplies. Failure to define the system need with sufficient clarity is a root cause of most system development difficulties.

Introduction and Overview

The sunlight that falls on the oceans is so strongly absorbed by the water that effectively all of its energy is captured within a shallow “mixed layer” at the surface, 35 to 100 m (100 to 300 ft) thick, where wind and wave actions cause the temperature and salinity to be nearly uniform. In the regions of the tropical oceans between approximately 15° north and 15° south latitude, the heat absorbed from the sun warms the water in the mixed layer to a value near 28°C (82°F) that is nearly constant day and night and from month to month. The annual average temperature of the mixed layer throughout the region varies from about 27°C to about 29°C (80 to 85°F). Beneath the mixed layer, the water becomes colder as depth increases until at 800 to 1000 m (2500 to 3300 ft), a temperature of 4.4°C (40°F) is reached. Below this depth, the temperature drops only a few degrees further to the ocean bottom at an average depth of 3650 m (12,000 ft). Thus, a huge reservoir of cold water exists below a depth of 3000 ft. This cold water is the accumulation of ice-cold water that has melted from the polar regions. Because of its higher density and minimal mixing with the warmer water above, the cold water flows along the ocean bottom from the poles toward the equator, displacing the lower-density water above. The result of the two physical processes is to create an oceanic structure with a large reservoir of warm water at the surface and a large reservoir of cold water at the bottom, with a temperature difference between them of 22 to 25 degrees Celsius (40 to 45 degrees Fahrenheit); this structure is found throughout the entire area of the tropical oceans where the depth exceeds 1000 m (3300 ft). The temperature difference is maintained throughout the year, with variations of a few degrees Fahrenheit due to the seasonal effects and weather, and day-to-night changes on the order of one degree. The ocean thermal energy conversion (OTEC) process uses this temperature difference to operate a heat engine, which produces electric power.

OTEC Economics

The economic contribution that OTEC will make to the solution of the nation’s energy problems depends on its perceived merits relative to existing and alternative sources of energy and fuels. The previous chapters have shown that OTEC technology is ready for large-scale demonstrations that will provide a firm basis for commercial development. OTEC can have a large impact on U.S. energy needs by supplying liquid fuels for direct use in transportation, or for electric power production via fuel cells. Its commercial development will depend finally on political and other factors that cannot be assessed quantitatively. The national security and environmental impacts of continuing dependence on oil should receive major emphasis in decisions to implement new processes for fuel production. In this chapter we review the estimated sales prices of fuels and electric power from existing and proposed sources and compare them with OTEC prices. Actual manufacturing costs are generally unavailable and are highly dependent on the financing methods and resources of the individual producer. However, an objective comparison of the sales prices of fuels produced by proposed processes can be made by using the Mossman financial analysis method to estimate the sales price of fuel or electric power that must be charged for profitable operation. This requires only information on the plant investment, input fuel costs, and operation and maintenance costs. The sensitivity of the product costs to changes in the estimates in plant investment can then be displayed in a suitable graph. With this procedure the alternatives can be equitably compared from a uniform point of view. This includes costs of related facilities as well as the plant investment. See the discussion in Section 7.3. These costs vary with time in an unpredictable manner. Past forecasts have been in error by large factors. These costs are a small part of the total, typically a few percent of plant investment for capital-intensive projects. Environmental impact costs have usually been ignored. These items are a few percent of plant investment. State and local property taxes will be zero for sea-based OTEC systems.

OTEC Historical Background

As in other branches of technology, the understanding of the physical and chemical principles underlying the operation of heat engines followed long after such systems were in commercial use. Apparently both the ancient Egyptians and Chinese were able to use steam or combustion gases to do work in special applications; however, the first practical use of a heat engine was the steam-driven piston engine for pumping water from mines, invented in 1698 by the Englishman Thomas Savery. This was followed by a better device invented in 1712 by Newcomen and further developed by Smeaton, which was widely adopted for mining operations in the tin mines of Cornwall and the British coal mines. In 1763, James Watt invented his greatly improved steam engine, which laid the foundation for the industrial revolution based on steam power. Interesting accounts of these developments are presented in Fenn (1982) and Callendar and Andrews (1958). By 1800, there were nearly 500 engines of Watt’s design emplaced throughout England for pumping water, working metal, or other uses. Steam use in ships was successfully demonstrated by Fulton on the Hudson River in New York in 1807. Railroad transportation based on steam-driven locomotives was introduced by Stephenson in 1812 following small beginnings in 1801 by Trevithick. As the steam engines of Newcomen were manufactured and installed, their performance was measured by the amount of water that could be pumped to a given height per bushel of coal burned. The heating value of the coal being used was approximately 1 million Btu per bushel. The data of Table 2–1 show how the thermal efficiency of steam engines improved with time. It is interesting to note that the industrial revolution began with engines of less than 1% efficiency and blossomed with the development of Watt’s engine of 2.7% efficiency. Watt and his predecessors related the performance of their engines in pumping water to what could be accomplished by horses engaged in the same task. An average value of the power capability of a horse was estimated by Watt, who established the unit of one horsepower as the power needed to raise 33,000 pounds 1 foot in 1 minute.