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In order to perform the thermoelectric conversion, open and closed Rankine cycles are adopted. In the open cycle, the operating fluid is the warm sea water itself, while in the closed cycle a fluid like ammonia or propane, which can produce high vapor pressure at relatively low temperatures, is selected as working fluid. Open cycle OTEC Systems are still in the research and development phase. The principal disadvantages of the open cycle OTEC System are the low pressure difference available to operate the turbine, 2.8 kPa, compared with 270 kPa for the closed system using ammonia as working fluid, and the large specific volumes that must be used by the steam turbine. Another disadvantage of open cycle operation is the need to provide vacuum pumps to remove fixed gasses and steam that are removed from the sea water along with the steam that forms the working fluid to drive the turbine. Otherwise, the accumulated fixed gasses degrade the operation of the condenser. So, the installation of vacuum pumps significantly reduce the net output. With all these disadvantages, an open cycle system promises a competitive performance if mariculture products and fresh water are produce along with electric power. So, generally a closed-Rankine cycle is used for heat exchangers of OTEC system which is meant for generation of only electric power.

In the closed cycle, the working fluid is evaporated at 21¢J with a vapor pressure of 8.7 atmospheres, whereas, seawater, in case of open cycle, is evaporated at 26¢J. After driving the turbine, the ammonia is recondensed at 10.7¢J by a heat exchanger with 5¢J water drawn from 700 to 900 m depth. Because the overall ocean temperature difference on which this system operates is only 20¢J+4¢J( 20¢J-4¢J)depending on plant location, temperature differences in the evaporator and condenser are 5¢J each and only 10¢J is left for the turbine.

So, the theoretical Rankine cycle efficiency is only 3.3 %. When the pumping power required for the warm sea water, cold sea water, and the ammonia, plus the efficiencies of the pumps, the turbines, and generators are taken into account, this theoretical value drops to a net value near 2.5 %. The fact that the efficiency is small does not mean that it is in danger of going to zero; it simply means that the heat exchangers will be large and that large volumes of seawater will have to be pumped through them. With all these factors, it is considered that the overall ocean temperature difference is about 20¢J for the operation of OTEC system working at probable maximum efficiency.

2.0 OTEC POTENTIAL FOR INDIA

The potential of ocean thermal energy depends mainly on the temperature difference between surface and deep sea water. So, it is necessary to study the temperatures of surface and deep sea water.

2.1 Sea Surface Temperatures in India's EEZ

Satellite derived sea surface temperatures between the 600 m depth contour and the boundary of India's EEZ were obtained from National Remote Sensing Agency, Hyderabad. These temperatures are average values over 10¡Ñ10 grids in the EEZ and over a month for a typical year. There are 160 grids between the 600 m depth contour and the EEZ boundary. From these monthly averages of sea surface temperatures, yearly averages of sea surface temperatures are calculated. Statistical representation of these values is presented in Table. 1. From Table 1, it can be understood that the yearly average temperature varies from 28.0¢J to 28.8¢J over 85% (136 Grids) of the area between the 600 m depth contour and the EEZ boundary. The statistical average of yearly sea surface temperatures of this range is 28.39¢J.

2.2 Deep Seawater Temperature in India's EEZ

Deep sea water temperatures were obtained only during limited oceanographic surveys. It may be very difficult to obtain temperatures of deep seawater similar to satellite derived sea surface temperatures. Data of deep sea water temperatures available to the authors is tabulated in Table 2. Measurements of deep sea temperatures at a location in a region do not vary with time, either summer or winter. Therefore, typical average values shown in the Table 2 are used for further computations. From this table, it may be inferred that the average temperatures of deep seawater at the 800 m and 1000 m depth contours are 7.65 ¢J and 6.45¢J, respectively.

2.3 Thermal Difference For OTEC in India's EEZ

From the sea surface temperatures and deep seawater temperatures discussed above, it can be estimated that available thermal difference for OTEC is about 20.74¢J and 21.94¢J at 800 m and 1000 m depth contours. With this, it may be concluded that a minimum thermal difference of about 20¢J is available in the entire area between 800 m depth contour and boundary of India's EEZ.

2.4 OTEC Potential

Based on the minimum thermal difference discussed earlier, the area of OTEC resource within EEZ of India is identified as the area between the 800 m depth contour and boundary of India's EEZ. This potential area is shown in Figure 3. This area is estimated as 1 533 803 km², calculated by taking an average area of the 10¡Ñ10 grid as 10 302 km². Avery and Wu (1994) note that a study by Brin on the effect of continuing OTEC operation on the cold water reservoir below 1000 m in depth showed that the impact would be insignificant if OTEC plants were sited to produce no more than 0.2 MWe /km² of ocean surface area. So, it is assumed that the acceptable spacing for OTEC plants in the OTEC potential area is about 0.2 MWe /km² . With this acceptable spacing, the gross OTEC potential of India's EEZ is 306760 MWe. Allowing 40% of this power to be used for the parasitic components of OTEC system, the net OTEC potential is 184056 MWe, say 184000 MW.

3.0 CONCEPTUAL STUDIES ON OTEC

The OTEC project group at IIT Madras (IITM) was the first team to start activities in Ocean Thermal Energy in 1980. But, the Ministry of Non-conventional Energy Sources(MNES), Government of India has been responsible for initiating activities at various organisations or institutions. Conceptual studies on OTEC plants for Kavaratti, in Andaman-Nicobar Islands and off Tamilnadu coast at Kulasekarapattinam were done.

3.1 OTEC Plant for Kavaratti

M/S. Metallurgical & Engineering Consultants(India) Ltd. (MECON) prepared a detailed project report on a 1 MW (net) experimental OTEC plant for Kavaratti, part of the Lakshadweep islands, in 1985 at the request of MNES. While preparing this report, MECON considered various aspects like negative pressure, cold water pipeline design, temperature rise in the cold water intake pipe, friction losses, OTEC cycle, lagoon temperature stabilization and optimization studies. Pipeline route for this plant was suggested by the OTEC project group of IITM. MECON came out with two alternatives for this OTEC plant. The details of these alternatives are shown in Table 3.

3.2 OTEC Plant in Andaman & Nicobar Islands

The OTEC project group of IITM submitted a report on preliminary studies for installing OTEC plants in these islands in May 1984. The project group has identified four possible sites based on bathymetry, distance from shore to OTEC plant and temperature profile of seawater. To prepare this report, the data on bathymetry and temperature profile was obtained from Naval Hydrographic office, DehraDun and National Institute of Oceanography, Goa respectively. This project group has also suggested the type of OTEC plant depending on the site chosen for installation of a plant.

3.3 OTEC Plant off Tamilnadu Coast at Kulasekarapattinam

A proposal for setting up a 600 MW OTEC plant off Tamilnadu Coast at Kulasekarapattinam was submitted to Tamilnadu State Government in 1985 on a Build, Own, Operate (BOO) basis by Sea Solar Power Inc., USA (SSP). Later SSP submitted the project report for this proposal in 1994, reducing the power capacity to 1 MW. As per this report, this plant rests on a floating platform with eight 12.5 MW units. Cold water intake and warm water intake of this plant are 126 m/s and 248 m³/s respectively. The working fluid of this plant is propylene. It is reported that the sea surface temperature varies from 26 ¢J to 29 ¢J during the year and 1000 m depth is available at a distance of 46 kM from the shore at Kulasekarapattinam. It is also mentioned that this location is not prone to hurricanes. The estimated cost of this project is Rs 800 Crores as per the report submitted in 1994.

4.0 OTEC FUTURE FOR INDIA

The estimated total potential of ocean thermal energy cannot be fully extracted for various practical reasons. So the actual estimate of extractable ocean thermal energy needs further scientific and R&D input with respect to India's EEZ. However, the following OTEC programme may be proposed to begin OTEC development. This programme is proposed for the next four decades as shown in Table 4. This table envisages the extraction of ocean thermal energy to about 14% of net potential of the country over the next four decades. Available technology for OTEC needs to be scaled up for higher capacities of power plants. Presently proved technology for power capacities up to 220 kW is available. Before implementation of the above proposed programme, it is suggested that India's OTEC programme should start with a 1 MW demonstration plant and then with 5 MW pilot plant, so that these results can be extended to higher capacities proposed in the OTEC programme for the country. The proposed programme looks ambitious in comparison with the total installed power capacity of 81750 MW (Hydro Power = 22072 MW, Thermal Power = 57225 MW and Others (Nuclear, Wind) = 2453 MW; as of March 1993) achieved over many decades. However, this is being presented only to highlight the promises of OTEC and to emphasise the need for the support and funding to develop technology for OTEC in our country.

It should be noted that OTEC plants with a capacity of around 100 MW only are going to provide energy at a unit cost equal to that of conventional fossil fuels. But, technology is not yet available to build plants of such large capacities. Therefore, one has to start with demonstration plants of low capacities like 1 MW and 5 MW. But the cost of building such OTEC plants of small capacities will be at least 10 times the cost of other conventional plants. Many of the policy planners seem to forget the longterm advantages of such large renewable energy plants when large investments are required today for necessary technology development. What India needs today is a bold decision on OTEC technology development, only then can targets as listed in this paper be realised.

5.0 CONCLUSIONS AND RECOMMENDATIONS

Ocean thermal energy in multiple thousands of MW is a very promising source for India and needs to be exploited. Available indigenous technologies may be upgraded with detailed engineering studies on various components of an OTEC plant system like the cold water pipe. It is suggested for the establishment of 1 MW and 5 MW OTEC plant to get scalable results and to understand environmental impact of OTEC plants.

6.0 ACKNOWLEDGMENTS

The authors are thankful to NRSA, Hyderabad and NIO, Goa for providing suitable data to prepare this paper.