Dossier Ocan et nergie - nergie Thermique des Mers

Sommaire IOA News Letters

Assessment of OTEC Potential and Promises for India

By
Prof. M. Ravindran , Dr. B. Nagendra Kumar
National Institute of Ocean Technology
(Dept. of Ocean Development, Govt. of India)
IIT Campus, Madras

ABSTRACT

Renewable energy contributions to the available energy resources are very important to the country as it is presently facing an energy resources crunch. One of the important untapped renewable ocean energy resources in our country's Exclusive Economic Zone (EEZ) is ocean thermal energy. Since India is a tropical country, the country's EEZ has a very high potential for exploitation of our ocean thermal energy resources. This resource is due to the temperature difference between surface and deep sea water.

This paper presents the estimates of the area in our EEZ where a minimum temperature difference of 20 is available tetween the surface and deep sea water which will be necessary for establishing an Ocean Thermal Energy Conversion (OTEC) plant. The sea surface temperatures have been provided by the National Remote Sensing Agency (NRSA) Hyderabad. The temperature at water depths around 800 meters have been taken from the available measured data for the Arabian Sea and Bay of Bengal. The initial theoretical estimates of the OTEC resource in EEZ of India is around 180000 MW. But the realizable power from OTEC is currently limited due to large technology gaps and limited resources.

This paper also presents some highlights of the earlier work done relating to the establishment of an OTEC plant in Lakshadweep as well as identification of possible sites in the EEZ around the main land and Andaman and Nicobar Islands.

1.0 INTRODUCTION

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India has a land mass of about 3.28 million square kilometers and has a long coastline of around 7000 kilometers with its EEZ that extends 200 nautical miles from the coastline. India's EEZ is approximately 2.56 million square kilometers and India has exclusive right for exploiting the living and non living resources from this EEZ area which can contribute enormously to the economic growth of our country. India is also a power deficient country, importing oil at a very high cost. Renewable energy contributions will help very much to reduce the import cost of energy. Among the many forms of ocean energy resources, wave energy conversion to electricity has been demonstrated in India. But one of the important untapped renewable energy resources is ocean thermal energy stored in the ocean water mass of the EEZ.

This paper estimates the potential of ocean thermal energy. This paper also presents some highlights of the ealier work done relating to the establishment of OTEC plant in Lakshadweep as well as identification of possible sites in EEZ around the main land and Andaman and Nicobar Islands.

1.1 OCEAN THERMAL ENERGY SYSTEM

A large part of the incoming solar energy, about 1016 Watts, is stored in the form of heat in the upper part of oceans, thus creating thermal gradients between the cold deep water and the surface water. A typical temperature profile measured in the Bay of Bengal is shown in Figure 1. Such gradients exist throughout the tropical belt around the earth. These gradients vary form 15 to 25 J over the water columns extending from 600 to 1000 m in depth. Such temperature differences are enough to support a thermodynamic cycle and to run a heat engine producing mechanical energy which can finally be converted to electrical energy. Though the practical efficiency of this conversion is as low as about 2 to 3 %, it does not constitute a serious problem since the energy reservoir is of considerable volume and is permanently fed by the incoming solar radiation.

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Although some technical problems exist due to the huge volume of water which has to be handled and the dimensions of some parts of the system like heat exchangers and turbines, this concept appears to be extremely promising since its influence on the environment or relatively small and possibilities for integrating a range of industrial operations are great. The major advantages of OTEC plants are thermal mixing and continuous power generation. The slow rate of vertical mixing between surface and deep waters due to an OTEC plant enriches the surface marine ecosystem by artificial transfer of nutrients from deep to surface water. Since the ocean thermal resource always exists, OTEC plants can generate power continuously and can work as a base load power supply system. The major disadvantage of all ocean energy systems is that they suffer from very high initial cost even though the operating cost is less.

For determining reasonable sites for an OTEC plant, it is necessary to know the criteria for efficient plant operation and combine this information with ocean data like temperature difference, currents, waves, meteorological conditions including hurricanes and bottom topography.

1.2 TYPES OF OTEC PLANTS

Depending on the distance from the nearest shoreline where 650 to 700 m water depth is available, the type of OTEC plant could be Land based or Shelf mounted or Floating Plant. Schematic views of the three plants are shown in Figure 2.

A land based system is technically the simplest, since all the components like the heat exchanger, turbines, generator, and pumps will be on land. The cold water pipe will be lying on the ocean bed to reach a water depth of around 700m.

If the bed profile near the shore is flat for 3 to 5 kM before it starts into a steeper slope, then a tower could be built in a water depth around 50 to 100 m where the bed starts sloping down steeply. The power plant will be installed on this tower and the cold water pipe go down the ocean bed slope from the tower. Such a plant is known as a shelf mounted plant. The generated power will be transmitted to shore by overhead or submarine cables.

If the continental shelf is very shallow and the depth of 650 m is reached at a distance of 10 to 15 kM from the shoreline, a floating platform system is considered. From this platform, the cold water pipe will descend vertically. Keeping the position of the floating platform with the action of waves, winds and currents is difficult. The cyclones in the sea make it all the more difficult to install such plants. This platform should also have a dynamic positioning system to maintain stability.

1.3 THERMODYNAMIC CYCLE AND EFFICIENCY OF OTEC SYSTEM

>T

>26.3

>26.5

>26.7

>26.9

>27.1

>27.3

>27.5

>27.7

>27.9

>28.1

>28.3

>28.5

>28.7

>28.9

>n

>1

>0

>0

>0

>2

>2

>4

>2

>6

>24

>45

>48

>19

>7

@

@

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 21J with a vapor pressure of 8.7 atmospheres, whereas, seawater, in case of open cycle, is evaporated at 26J. After driving the turbine, the ammonia is recondensed at 10.7J by a heat exchanger with 5J water drawn from 700 to 900 m depth. Because the overall ocean temperature difference on which this system operates is only 20J+4J( 20J-4J)depending on plant location, temperature differences in the evaporator and condenser are 5J each and only 10J 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 20J 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 1010 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 28.0J to 28.8J 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 28.39J.

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 6.45J, 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 20.74J and 21.94J at 800 m and 1000 m depth contours. With this, it may be concluded that a minimum thermal difference of about 20J is available in the entire area between 800 m depth contour and boundary of India's EEZ.

2.4 OTEC Potential

@

Position

@

Deep seawater Temperature

@

Place

Latitude

Longitude

Time

At 800 m Contour

At 1000 m Contour

Source/Ref.

Bay of Bengal

120 00' N

810 00' E

7'84

7.490 J

6.480 J

NIO Goa

Arabian Sea

110 30'

720 30' E

9'95

9.050 J

7.790 J

NIO Goa

Arabian Sea

120 30' N

740 01' E

9'95

8.460 J

7.210 J

NIO Goa

Markanam Comples

100-120 N

800-900E

-

6.480 J

5.590 J

SSP Report

Kulasekara patnam

00-100 N

700-800 E

-

6.000 J

5.22 0 J

SSP Report

Kavaratti

100 50' N

0 54' E

12'83

8.400 J

6.400 J

IIT Madras

@ @

Average

7.650 J

6.450 J

@

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 1010 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

Alternative No.

Cold water intake Temp.

Depth of Cold water Intake (m)

Distance from shore to plant (m)

Project Cost (million Rs) (1985 Rates)

Unit Cost (Rs/kWh)

I

8.7

765

2200

309

3.69

II

10.0

620

1500

287

3.12

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.

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