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  1. 1. OCEAN THERMAL ENERGY CONVERSIONABSTRACT Ocean Thermal Energy Conversion (OTEC), to me, is the developing worldsanswer to OPEC (Organization of Petroleum Exporting Countries). We will nolonger have to depend on oil supplies from other countries. OTEC is an efficient,clean process, which utilizes the difference in temperatures between the oceanssurface and its depths to produce energy. The ocean thermal energy conversionprocess works something like a refrigerator in reverse. The warm surface watersvaporize a refrigerant and the vapour is then used to drive a turbine. Therefrigerant vapour is then condensed back into a liquid after being cooled by coldwater brought up from the ocean depths. This technology is best applied in thetropics like India, because the sun heats the ocean surface to comparably hightemperatures, creating the highest temperature differentials between surface anddepths, which translates into the largest potentials of energy production. Thispaper describes the status of the various ocean thermal energy technologies, withemphasis placed on those with a near-term potential and applicability in largenumbers.1. INTRODUCTION Ocean Thermal Energy Conversion (OTEC) technology is based on theprinciple that energy can be extracted from two reservoirs at differenttemperatures. A temperature difference as low as 20°C can be exploited effectivelyto produce usable energy. Temperature differences of this magnitude prevailbetween ocean waters at the surface and at depths up to 1000 meters in many areasof the world, particularly in tropical and subtropical latitudes between 24 degreesnorth and south of equator. This thermal gradient—the fact that the oceans layersof water have different temperatures—is effectively used to drive a power-producing cycle. The oceans are thus a vast renewable resource, with the potentialto help us produce billions of watts of electric power. This potential is estimated tobe about 1013 Watt of baseload power generation. The cold, deep seawater used inthe OTEC process is also rich in nutrients, and can be used to culture both flora &fauna near the shore or on land. 1
  2. 2. 2.HISTORY “Those hot ocean waters have a more useful purpose than just generatinghurricanes. A reverse refrigeration process generates electricity from, thedifference in temperature between surface and deep water.” -By Sterling D. Allan OTEC technology is not new. In 1881, Jacques Arsene dArsonval, a Frenchphysicist, proposed tapping the thermal energy of the ocean. But it wasdArsonvals student, Georges Claude, who in 1930 actually built the first OTECplant in Cuba. The system produced 22 kW of electricity with a low-pressureturbine. In 1935, Claude constructed another plant aboard a 10,000-ton cargovessel moored off the coast of Brazil. Weather and waves destroyed both plantsbefore they became net power generators. The United States involved in OTECresearch in 1974 with the establishment of the Natural Energy Laboratory ofHawaii Authority. The Laboratory has become one of the worlds leading testfacilities for OTEC technology.Today, with a Power Purchasing Agreement between the government of TamilNadu, India, and Sea Solar Power SSP), Anderson (President, SSP) is preparing toconstruct a test facility near Tamil Nadu coast, for the power cycle.Types of OTEC Plants: 1. Land or near the shore. 2. Platforms attached to the shelf. 3. Moorings or free-floating facilities in deep ocean water2. WORKING OF OTEC Figure 1: Schematic representation of OTEC 2
  3. 3. A Refrigerator in reverse: OTEC generates electricity by using the temperaturedifference of 20°C (36°F) or more that exists between warm tropical waters at thesun-warmed surface, and colder waters drawn from depths of about 1000 m. Toconvert this thermal gradient into electrical energy, the warm water can be used toheat and vaporize a liquid (known as a working fluid). The working fluid developspressure as it is caused to evaporate. This expanding vapour runs through a turbinegenerator and is then condensed back into a liquid by cold water brought up fromthe depth, and the cycle is repeated. Since the temperature difference between thehot and cold streams is low, the efficiency of the Rankine Cycle used for OTECsystem will be low. For very small temperature drops of around 4° to 5°C acrossthe boiler and condenser, the Rankine Cycle efficiency for most of the workingfluids will range between 2 and 3 percent. The absorption of solar energy by thewater follows Lamberts law of absorption. It states that “each layer of equalthickness absorbs the same fraction of light that passes through it.”Thus, -dl/dx = kI or I = I0 e (-kx)Where I is the intensity of radiation at a distance x below the surface and I0 is theintensity of radiation at the surface, i.e. at x = 0, k is the absorption coefficientwhich is having unit of L (-1).The values of k depend on nature of the water. The Ocean Thermal Energy is harnessed by three methods:One is Open Cycle system, known as the Claude cycle, the other is Closed Cyclesystem, also known as Anderson cycle, and the third one comprises of the variousblendings of the two. All three types can be built on land, on offshore platformsfixed to the seafloor, on floating platforms anchored to the seafloor, or on shipsthat move from place to place. 3
  4. 4. 3. CLOSED-CYCLE OTEC SYSTEM Figure 2: Closed-Cycle OTEC System In the closed-cycle system, warm seawater vaporizes a working fluid, such asammonia, flowing through a heat exchanger (evaporator). Fluids having lowboiling point temperatures like ammonia, propane; R22, etc are used as workingfluids. Warm surface seawater is passed through an evaporator with the help of apump. In the evaporator, the working fluid in the form of a high pressure liquidabsorbs heat and gets converted into Vapour. This vapour expands at moderate pressures and runs a turbine coupled to agenerator that produces electricity. The vapour is then condensed in another heatexchanger (condenser) using Cold seawater pumped from the ocean’s depththrough a cold- water pipe. The condensed working fluid is pumped back to theevaporator to repeat the cycle. The working fluid remains in a closed system andcirculates continuously. Anderson (1969) postulated this cycle design; hence thiscycle is also referred as Anderson Cycle. Since ammonia vaporizes and condensesnear atmospheric pressure at the available seawater temperatures, it provides asufficient pressure drop across the turbine so that it can achieve relatively highefficiency at modest size compared to the open-cycle system. Since this technologyis essentially similar to standard refrigeration systems, there is sufficientexperience with the components to allow straightforward scale-up to commercialsizes. 4
  5. 5. 4. OPEN-CYCLE OTEC SYSTEM Figure (3) In an Open-cycle OTEC system, warm seawater is the working fluid. The warmseawater is "flash"-evaporated in a vacuum chamber to produce steam at anabsolute pressure of about 2.4 kPa The steam expands through a low-pressureturbine that is coupled to a generator to produce electricity. The steam exiting theturbine is condensed by cold seawater pumped from the oceans depth through acold-water pipe. If a surface condenser is used in the system, the condensed steamremains separated from the cold sea- water Open-Cycle OTEC System and providesa supply of desalinated water. 5
  6. 6. 5. HYBRID OTEC SYSTEM Figure 4: Hybrid OTEC SystemA hybrid cycle combines the features of both the closed-cycle and open-cyclesystems. In a hybrid OTEC system, warm seawater enters a vacuum chamberwhere it is flash-evaporated into steam, which is similar to the open-cycleevaporation process. The steam vaporizes the working fluid of a closed-cycle loopon the other side of an ammonia vaporizer. The vaporized fluid then drives aturbine that produces electricity. The steam condenses within the heat exchangerand provides desalinated water. The electricity produced by the system can be delivered to a utility grid orused to manufacture methanol, hydrogen, refined metals, ammonia, and similarproducts. 6
  7. 7. 6. SCOPE FOR OTEC Figure 5: Global Overview for Scope of OTEC An economic analysis indicates that, over the next 5 to 10 years, ocean thermalenergy conversion (OTEC) plants may be competitive in four markets. The firstmarket is the small island nations in the South Pacific and the island of Molokai inHawaii. In these islands, the relatively high cost of diesel-generated electricity anddesalinated water make a small [1megawat (electric) (MWe)], land-based, open-cycle OTEC plant coupled with a second-stage desalinated water productionsystem. The second market can be found in American territories such as Guam andAmerican Samoa, where land-based, open-cycle OTEC plants rated at 10 MWewith a second-stage water production system. The third market is Hawaii, where alarger, land-based, closed-cycle OTEC plant could produce electricity with asecond-stage desalinated water production system. OTEC should quickly becomecost effective in this market, when the cost of diesel fuel doubles, for plants ratedat 50 MWe or larger. The fourth market is for floating, closed-cycle plants rated at40 MWe or larger that house a factory or transmit electricity to shore via asubmarine power cable. These plants could be built in Puerto Rico, the Gulf ofMexico, and the Pacific, Atlantic, and Indian Oceans. Military and security uses oflarge floating plant ships with major life-support systems (power, desalinatedwater, cooling, and aquatic food) should be included in this category. Thesepredictions are based on the cost of oil-fired power, the demand for desalinatedwater, and the social benefits of this clean energy technology. 7
  8. 8. 7. OTEC IN INDIA Conceptual studies on OTEC plants for Kavaratti (Lakshadweep islands),in the Andaman-Nicobar Islands and off the Tamil Nadu coast atKulasekharapatnam were initiated in 1980. In 1984 a preliminary design for a 1MW (gross) closed Ranking Cycle floating plant was prepared by the IndianInstitute of Technology in Madras at the request of the Ministry of Non-Conventional Energy Resources. The National Institute of Ocean Technology (NIOT) was formed by thegovernmental Department of Ocean Development in 1993 and in 1997 theGovernment proposed the establishment of the 1 MW plant of earlier studies. . Theobjective is to demonstrate the OTEC plant for one year, after which it could bemoved to the Andaman & Nicobar Islands for power generation. NIOT’s plan is tobuild 10-25 MW shore-mounted power plants in due course by scaling-up the 1MW test plant, and possibly a 100 MW range of commercial plants thereafter.NIOT has predicted, as depicted in the drawing, some 1.5 x 106km2 of theircoastlines should be technically good for total generation of 180,000 MW. NIOTplans to, construct some 1,000 OTEC power plants throughout Indian coastlines,according to their grand design. The experimental OTEC plant installed on-board aship would be towed away to the testing site some 35km off Tiruchendur coast insoutheast India.The hydrographic survey of the above mentioned sites were conducted by NationalHydrographic Office, Dehradun. Preliminary oceanographic studies on eastern siteof Lakshadweep Island offered the positive observations for the establishment ofshore based OTEC plant at the Island. For the proposed OTEC plant the waterwould be brought up from 1000m depths. The Island has large lagoons on thewestern side, which are very shallow with hardly any nutrients in the seawater. Inthe plant, a part of the cooling water, after coming out from the condenser, isproposed to be diverted to lagoons for the purpose of aquaculture.8. BENEFITS OTEC has remarkably little adverse environmental impact, especiallycompared with other energy sources of comparable size. OTEC isinherently not exothermic, so it does not adversely contribute directly to globalwarming. Unlike most other sources of renewable energy which vary with weatherand time of day, OTEC power plants can produce electricity 24 x 365 days peryear. Since the ocean doesnt change temperature at night, the solar energy stored 8
  9. 9. In the sea is readily available we can measure the value of an oceanThermal Energy Conversion (OTEC) plant and continued OTEC development byboth its economic and non-economic benefits. OTECs economic benefits include:• Helps produce fuels such as hydrogen, ammonia, and methanol (as by- products).• Produces base load electrical energy.• Produces desalinated water for industrial, agricultural, and residential uses.• Is a resource for on-shore and near-shore marineculture operations.• Provides air-conditioning for buildings.• Provides moderate-temperature refrigeration.• Has significant potential to provide clean, cost-effective electricity for the future.OTECs non-economic benefits which help us achieve global environmental goals,include:• Promotes competitiveness and international trade.• Enhances energy independence and energy security.• Promotes international sociopolitical stability.• Has potential to mitigate greenhouse gas emissions resulting from burning fossil fuels. In small island nations, the benefits of OTEC include self-sufficiency, minimalenvironmental impacts, and improved sanitation and nutrition, which result fromthe greater availability of desalinated water and mariculture products.10. OTHER APPLICATIONSFigure 6:OtherApplications ofOTECOne of thegreatest values 9
  10. 10. of OTEC plants to the developing world is precisely the by-product of energyproduction:10.1 Fresh Water The first by-product is fresh water. A small 1 MW OTEC is capable ofproducing some 4,500 cubic meters of fresh water per day, enough to supply apopulation of 20,000 with fresh water.10.2 Food A further by-product is nutrient rich cold water from the deep ocean. Thecold "waste" water from the OTEC is utilized in two ways. Primarily the coldwater is discharged into large contained ponds, near shore or on land, where thewater can be used for multi-species mariculture producing harvest yields in largevolumes.10.3 Cooling The cold water is also available as chilled water for cooling greenhouses,such as or for cold bed agriculture. The cold water can also be used for airconditioning systems or more importantly for refrigeration systems, most likelylinked with creating cold storage facilities for preserving food.11. LIMITATIONS• An OTEC facility requires a substantial initial capital outlay (in the range of $50 to $100 millions for a “small” ten-megawatt plant).• OTEC has not been demonstrated at full scale over a prolonged period with integrated power, mariculture, fresh-water, and chill-water production.• OTEC is only feasible at relatively isolated sites (deep tropical oceans); from such sites, the power and marine products must be transported to market. (In general, the fresh water--and certainly the chill-water--cannot be transported more than a few miles economically).• OTEC is ecologically controversial--at least untested--in large scale and over a long period.• The technology for OTEC, once tested and proven, will be applicable across wide geographic (and politically varied) areas; therefore, future profits will not securely inure to the adventurous capitalists who develop OTEC. 10
  11. 11. 12. NEEDED RESEARCHTo accelerate the development of OTEC systems, researchers need to: Obtain data on OTEC plant operation with appropriately sized demonstration plants. Develop and characterize cold-water pipe technology and create a database of information on materials, design, deployment, and installation. Conduct further research on the heat exchanger systems to improve heat transfer performance and decrease costs. Conduct research in the areas of innovative turbine concepts for the large machines required for open-cycle systems. Identify and evaluate advanced concepts for ocean thermal energy extraction.13. CONCLUSION Among many ocean energy prospects, OTEC offers the most near termpotential and possesses applicability for a large variety of sites. OTEC hastremendous potential to supply the world’s energy. This potential is estimated tobe about 1013 Watts of base load power generation. However, OTEC systems mustovercome the significant hurdle of high initial capital costs for construction and theperception of significant risk compared to conventional fossil fuel plants. Oceanthermal energy in multiple thousands of MW is a very promising source and needsto be exploited. Available indigenous technologies may be upgraded with detailedengineering studies on various components of OTEC plant system like cold-waterpipes, heat exchangers and innovative turbine concepts. Under such circumstances,OTEC should become the preferred renewable energy option for all the marketswhere OTEC is feasible. For developing tropical countries where OTEC isfeasible, the social benefits from OTEC might far outweigh economic factors.Some of these benefits include energy self-sufficiency, minimal environmentalimpact, and improved nutrition for inhabitants from desalinated water andmariculture products. It appears that OTEC technology might become more financiallycompetitive if it could capitalize on the many value-added byproducts that can beproduced from the deep seawater. Renewable energy technologies like OTEC arevital to the nation to meet the demands of the next century and millennia.14. REFERENCES 11
  12. 12. 1. Website under title heading “OTEC” [Brief Overview]2. Website under search keyword “OTEC + cycle + papers” [Original Technical Papers of above mentioned scientists] 12