OCEAN THERMAL ENERGY CONVERSION

ABSTRACT Ocean Thermal Energy Conversion (OTEC), to me, is the developing worlds answer to OPEC (Organization of Petroleum Exporting Countries). We will no longer have to depend on oil supplies from other countries. OTEC is an efficient, clean process, which utilizes the difference in temperatures between the oceans surface and its depths to produce energy. The ocean thermal energy conversion process works something like a refrigerator in reverse. The warm surface waters vaporize a refrigerant and the vapour is then used to drive a turbine. The refrigerant vapour is then condensed back into a liquid after being cooled by cold water brought up from the ocean depths. This technology is best applied in the tropics like I ndia , because the sun heats the ocean surface to comparably high temperatures, creating the highest temperature differentials between surface and depths, which translates into the largest potentials of energy production. This paper describes the status of the various ocean thermal energy technologies, with emphasis placed on those with a near-term potential and applicability in large numbers.

1. INTRODUCTION

Ocean Thermal Energy Conversion (OTEC) technology is based on the principle that energy can be extracted from two reservoirs at different temperatures. A temperature difference as low as 20°C can be exploited effectively to produce usable energy. Temperature differences of this magnitude prevail between ocean waters at the surface and at depths up to 1000 meters in many areas of the world, particularly in tropical and subtropical latitudes between 24 degrees north and south of equator. This thermal gradient—the fact that the ocean's layers of water have different temperatures—is effectively used to drive a power-producing cycle. The oceans are thus a vast renewable resource, with the potential to help us produce billions of watts of electric power. This potential is estimated to be about 1013 Watt of baseload power generation. The cold, deep seawater used in the OTEC process is also rich in nutrients, and can be used to culture both flora & fauna near the shore or on land.

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2.HISTORY“Those hot ocean waters have a more useful purpose than just generating

hurricanes. A reverse refrigeration process generates electricity from, the difference in temperature between surface and deep water.” -By Sterling D. Allan

OTEC technology is not new. In 1881, Jacques Arsene d'Arsonval, a French physicist, proposed tapping the thermal energy of the ocean. But it was d'Arsonval's student, Georges Claude, who in 1930 actually built the first OTEC plant in Cuba. The system produced 22 kW of electricity with a low-pressure turbine. In 1935, Claude constructed another plant aboard a 10,000-ton cargo vessel moored off the coast of Brazil. Weather and waves destroyed both plants before they became net power generators. The United States involved in OTEC research in 1974 with the establishment of the Natural Energy Laboratory of Hawaii Authority. The Laboratory has become one of the world's leading test facilities for OTEC technology.Today, with a Power Purchasing Agreement between the government of Tamil Nadu, India, and Sea Solar Power SSP), Anderson (President, SSP) is preparing to construct 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 water

2. WORKING OF OTEC

Figure 1: Schematic representation of OTEC

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A Refrigerator in reverse: OTEC generates electricity by using the temperature difference of 20°C (36°F) or more that exists between warm tropical waters at the sun-warmed surface, and colder waters drawn from depths of about 1000 m. To convert this thermal gradient into electrical energy, the warm water can be used to heat and vaporize a liquid (known as a working fluid). The working fluid develops pressure as it is caused to evaporate. This expanding vapour runs through a turbine generator and is then condensed back into a liquid by cold water brought up from the depth, and the cycle is repeated. Since the temperature difference between the hot and cold streams is low, the efficiency of the Rankine Cycle used for OTEC system will be low. For very small temperature drops of around 4° to 5°C across the boiler and condenser, the Rankine Cycle efficiency for most of the working fluids will range between 2 and 3 percent. The absorption of solar energy by the water follows Lamberts law of absorption. It states that “each layer of equal thickness 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 the intensity of radiation at the surface, i.e. at x = 0, k is the absorption coefficient which 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 Cycle system, also known as Anderson cycle, and the third one comprises of the various blendings of the two. All three types can be built on land, on offshore platforms fixed to the seafloor, on floating platforms anchored to the seafloor, or on ships that move from place to place.

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3. CLOSED-CYCLE OTEC SYSTEM

Figure 2: Closed-Cycle OTEC System In the closed-cycle system, warm seawater vaporizes a working fluid, such as ammonia, flowing through a heat exchanger (evaporator). Fluids having low boiling point temperatures like ammonia, propane; R22, etc are used as working fluids. Warm surface seawater is passed through an evaporator with the help of a pump. In the evaporator, the working fluid in the form of a high pressure liquid absorbs heat and gets converted into Vapour. This vapour expands at moderate pressures and runs a turbine coupled to a generator that produces electricity. The vapour is then condensed in another heat exchanger (condenser) using Cold seawater pumped from the ocean’s depth through a cold- water pipe. The condensed working fluid is pumped back to the evaporator to repeat the cycle. The working fluid remains in a closed system and circulates continuously. Anderson (1969) postulated this cycle design; hence this cycle is also referred as Anderson Cycle. Since ammonia vaporizes and condenses near atmospheric pressure at the available seawater temperatures, it provides a sufficient pressure drop across the turbine so that it can achieve relatively high efficiency at modest size compared to the open-cycle system. Since this technology is essentially similar to standard refrigeration systems, there is sufficient experience with the components to allow straightforward scale-up to commercial sizes.

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4. OPEN-CYCLE OTEC SYSTEM

Figure (3)

In an Open-cycle OTEC system, warm seawater is the working fluid. The warm seawater is "flash"-evaporated in a vacuum chamber to produce steam at an absolute pressure of about 2.4 kPa The steam expands through a low-pressure turbine that is coupled to a generator to produce electricity. The steam exiting the turbine is condensed by cold seawater pumped from the ocean's depth through a cold-water pipe. If a surface condenser is used in the system, the condensed steam remains separated from the cold sea- water Open-Cycle OTEC System and provides a supply of desalinated water.

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5. HYBRID OTEC SYSTEM

Figure 4: Hybrid OTEC System

A hybrid cycle combines the features of both the closed-cycle and open-cycle systems. In a hybrid OTEC system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, which is similar to the open-cycle evaporation process. The steam vaporizes the working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine that produces electricity. The steam condenses within the heat exchanger and provides desalinated water. The electricity produced by the system can be delivered to a utility grid or used to manufacture methanol, hydrogen, refined metals, ammonia, and similar products.

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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 thermal energy conversion (OTEC) plants may be competitive in four markets. The first market is the small island nations in the South Pacific and the island of Molokai in Hawaii. In these islands, the relatively high cost of diesel-generated electricity and desalinated water make a small [1megawat (electric) (MWe)], land-based, open-cycle OTEC plant coupled with a second-stage desalinated water production system. The second market can be found in American territories such as Guam and American Samoa, where land-based, open-cycle OTEC plants rated at 10 MWe with a second-stage water production system. The third market is Hawaii, where a larger, land-based, closed-cycle OTEC plant could produce electricity with a second-stage desalinated water production system. OTEC should quickly become cost effective in this market, when the cost of diesel fuel doubles, for plants rated at 50 MWe or larger. The fourth market is for floating, closed-cycle plants rated at 40 MWe or larger that house a factory or transmit electricity to shore via a submarine power cable. These plants could be built in Puerto Rico, the Gulf of Mexico, and the Pacific, Atlantic, and Indian Oceans. Military and security uses of large floating plant ships with major life-support systems (power, desalinated water, cooling, and aquatic food) should be included in this category. These predictions are based on the cost of oil-fired power, the demand for desalinated water, and the social benefits of this clean energy technology.

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7. OTEC IN INDIA

Conceptual studies on OTEC plants for Kavaratti (Lakshadweep islands), in the Andaman-Nicobar Islands and off the Tamil Nadu coast at Kulasekharapatnam were initiated in 1980. In 1984 a preliminary design for a 1 MW (gross) closed Ranking Cycle floating plant was prepared by the Indian Institute 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 the governmental Department of Ocean Development in 1993 and in 1997 the Government proposed the establishment of the 1 MW plant of earlier studies. . The objective is to demonstrate the OTEC plant for one year, after which it could be moved to the Andaman & Nicobar Islands for power generation. NIOT’s plan is to build 10-25 MW shore-mounted power plants in due course by scaling-up the 1 MW 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 their coastlines should be technically good for total generation of 180,000 MW. NIOT plans to, construct some 1,000 OTEC power plants throughout Indian coastlines, according to their grand design. The experimental OTEC plant installed on-board a ship would be towed away to the testing site some 35km off Tiruchendur coast in southeast India. The hydrographic survey of the above mentioned sites were conducted by National Hydrographic Office, Dehradun. Preliminary oceanographic studies on eastern site of Lakshadweep Island offered the positive observations for the establishment of shore based OTEC plant at the Island. For the proposed OTEC plant the water would be brought up from 1000m depths. The Island has large lagoons on the western side, which are very shallow with hardly any nutrients in the seawater. In the plant, a part of the cooling water, after coming out from the condenser, is proposed to be diverted to lagoons for the purpose of aquaculture.

8. BENEFITS OTEC has remarkably little adverse environmental impact, especially compared with other energy sources of comparable size. OTEC is inherently not exothermic, so it does not adversely contribute directly to global warming. Unlike most other sources of renewable energy which vary with weather and time of day, OTEC power plants can produce electricity 24 x 365 days per year.  Since the ocean doesn't change temperature at night, the solar energy stored

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In the sea is readily available we can measure the value of an ocean Thermal Energy Conversion (OTEC) plant and continued OTEC development by both its economic and non-economic benefits. OTEC's 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.

OTEC's 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, minimal environmental impacts, and improved sanitation and nutrition, which result from the greater availability of desalinated water and mariculture products.

10. OTHER APPLICATIONS

Figure 6: Other Applications of OTEC

One of the greatest values

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of OTEC plants to the developing world is precisely the by-product of energy production:

10.1 Fresh WaterThe first by-product is fresh water. A small 1 MW OTEC is capable of

producing some 4,500 cubic meters of fresh water per day, enough to supply a population of 20,000 with fresh water. 10.2 Food

A further by-product is nutrient rich cold water from the deep ocean. The cold "waste" water from the OTEC is utilized in two ways. Primarily the cold water is discharged into large contained ponds, near shore or on land, where the water can be used for multi-species mariculture producing harvest yields in large volumes.

10.3 CoolingThe 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 air conditioning systems or more importantly for refrigeration systems, most likely linked 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.

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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 term potential and possesses applicability for a large variety of sites. OTEC has tremendous potential to supply the world’s energy.  This potential is estimated to be about 1013 Watts of base load power generation. However, OTEC systems must overcome the significant hurdle of high initial capital costs for construction and the perception of significant risk compared to conventional fossil fuel plants.  Ocean thermal energy in multiple thousands of MW is a very promising source and needs to be exploited. Available indigenous technologies may be upgraded with detailed engineering studies on various components of OTEC plant system like cold-water pipes, heat exchangers and innovative turbine concepts. Under such circumstances, OTEC should become the preferred renewable energy option for all the markets where OTEC is feasible. For developing tropical countries where OTEC is feasible, the social benefits from OTEC might far outweigh economic factors. Some of these benefits include energy self-sufficiency, minimal environmental impact, and improved nutrition for inhabitants from desalinated water and mariculture products. It appears that OTEC technology might become more financially competitive if it could capitalize on the many value-added byproducts that can be produced from the deep seawater. Renewable energy technologies like OTEC are vital to the nation to meet the demands of the next century and millennia.

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14. REFERENCES

1. Website www.wikipedia.com under title heading “OTEC” [Brief Overview]2. Website www.google.com under search keyword “OTEC + cycle + papers” [Original Technical Papers of above mentioned scientists]

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