¡V An Overview
By M. Ravindran, Director, National Institute of Ocean Technonlgy, IIT Campus, Chennai, India
India is geographically well placed as far as the OTEC potential is concerned. Around 2000km of coast length along the South Indian coast, a temperature difference above 20oC through out the year is available. That is about 1.5 x 106 square kilometeres of tropical water in the Exclusive Economic Zone around India with a power density of 0.2 MW/km2. Apart from this, attractive OTEC plant locations are available around Lakshedweep, Andaman & Nicobar Islands. The total OTEC potential around India is estimated as 180,000 MW considering 40% of gross power for parasitic losses. This indicates the promise of OTEC for India and points out the urgent need to develop OTEC technology.
The Indian OTEC programme started in 1980 with the proposal of General Electrical Co. of USA to install a 20 MW plant off the Tamil Nadu coast and subsequently in 1982, an OTEC cell was formed in the Indian Institute of Technology, Madras. A preliminary design was also done in 1984 for a 1 MW closed Rankine Cycle floating plant with ammonia as working fluid. After a bathymetric survey, a land based, 1 MW capacity OTEC plant was suggested for one of the islands of Lakshadweep group and a detailed design report was also prepared.
In 1993, National Institute of Ocean Technology (NIOT) was formed by the Department of Ocean Development (DOD), Government of India to pursue the research activities on ocean energy as part of their various mission-based activities. Under this mission a major thrust was given for the technology development for OTEC. Early 1997, DOD, Government of India proposed to establish a 1 MW gross OTEC plant in India, which will be the first ever MW range plant established anywhere in the world. NIOT had been exploring the participation of national and international expertise for a joint research and development. Saga University in Japan, headed by Prof. Uehara, has been doing excellent and practically oriented R & D on OTEC for more than twenty five years and this team also showed keen interest in closely working with NIOT on OTEC technology development. Considering this, an MOU was signed in 1997 between NIOT and Saga University, Japan for a joint development of OTEC in India. NIOT conducted detailed surveys at the proposed OTEC site near Tuticorin, South India. Based on the temperature and bathymetric profiles, the optimization of the closed loop systems was done with the help of Saga University in 1998.
The bathymetry of the coast around main land of India where cold water at a depth of 1000 m is available at about 40 km from the shore necessitates the use of a floating platform to house the OTEC plant. There exist some locations where a shell mounted OTEC plant can be constructed at a depth of 200 m. However, considering the future need of large plants, it was decided to design a floating OTEC plant. NIOT aims to build a 1 MW floating OTEC plant off the coast of Tamil Nadu near Tuticorin port on the south east coast of India which is free of cyclones in the last four decades.
After arriving at the detailed specifications of power module a global tender was floated during October / November ¡¦ 98 for the various sub-systems like heat exchangers, turbine-generator, seawater pumps etc. Tenders have been analysed and orders have been placed. NIOT is currently involved in the detailed design of the power system and components on the floating barge and the assembly is expected to start on March 2000.
POWER MODULE AND SEA WATER SYSTEMS
All commercial OTEC plants are expected to be 10 - 50 MW range or larger. Therefore a 1 MW gross power output plant is selected for the present design, considering the scaling up for the future. The design of the power module is based on a closed Rankine Cycle with ammonia as the working fluid . Titanium plate heat exchangers are suitable to such an environment. A cold water pipe made of HDPE of 1m outer diameter is selected, as it is the largest diameter locally available. Axial flow turbine having a higher adiabatic efficiency is chosen for power conversion and also for easy scaling up for future. The following are the baseline design conditions and the schematic diagram is given in Figure 1.
Gross power output 1 MW
Warm water temperature 29o C
Cold water temperature 7o C
Cold water intake 1000 m
Cold water pipe (ID) 0.90 m
Figure1. The schematic diagram for Closed cycle OTEC
The evaporator and condenser consist of four modules of plate heat exchangers, which will be largest of its kind used for such an application. The ammonia side of the evaporator is coated with stainless steel powder to enhance the overall heat transfer co-efficient. The coating is expected to improve the power output by 20 ¡V 40%
Radial inflow turbines have been previously used in the US-Mini OTEC experiment. The power system flow rates and net power is very much dependent on the turbine efficiency. Parametric studies showed that a 4-stage axial turbine could improve the efficiency up to 89%. The basic aerodynamic design of the turbine is done by NREC, Massachusetts, USA. While the original intent was to operate the turbine as a direct drive at the 3000 rpm, the commercial non-availability of the generator necessitated the use of 1: 2 reduction gear box.
The seawater pumps are of vertical, mixed flow type due to the low head and high discharge combination. Two identical pumps are to be connected in parallel on both warm water and cold water side and are driven by a variable speed drive to adjust the flow rates. As the system is highly sensitive to the flow variables, a variable speed drive is an essential component in an OTEC system.
There are several configurations for cold water pipe mooring. After studying several options the concept finalized consists of a floating platform connected to a cold water pipe which itself acts as mooring for the platform as shown in the Figure 2.
Figure2. The Mooring Arrangement for 1MW OTEC Plant
Makai Ocean Engineering, Hawaii, carried out the conceptual design of the seawater systems. The platform assumes a great significance due to the fact that it houses the entire plant, accommodate the seawater pumps and the cold water pipe or mooring system. NIOT has carried out studies with the help of numerical simulation as well as model tests to arrive at the motions and stresses in the vessel and pipe. The simulation for the barge and pipe combination has been carried out using the software called Visual Orcaflex. The motion studies for the barge were carried out using software like TRIBON and WAMIT. In addition to this model studies were carried out in the model sea basin of size 30m x 30 m x 3 m at the Ocean Engineering Centre in the Indian Institute of Technology, Madras .
A brief comparison of the current design with the previous OTEC plants in the world is given in the Table 1. It could be noted that as a whole the current system is large and complicated compared to the previous demonstration plants.
Table1. Summary of OTEC Demonstration Plants in the World in Comparison with the Proposed 1MW OTEC Plant.
|S. No.||Agency||Year,Location||Power Rating(kW)||Cycle||Type of plant|
|1.||Claude (France)||1930,Cuba||22||-||Open||Shore based|
|2.||Mini OTEC (US)||1979,Hawaii||53||18||Closed (Rankine)||Floating|
|3.||OTEC-1 (US)||1980,Hawaii||1MWe||-||Closed (Rankine)||Floating|
|4.||Toshiba & TEPC (Japan)||1982,Nauru,||120||31.5||Closed (Rankine)||Shore based|
|5.||NELHA (US)||1992,Hawaii||210||100||Open||Shore based|
|6.||Saga University (Japan)||1984,Saga||75||-||Closed (Rankine)||Lab model|
|7.||Saga University (Japan)||1995,Saga||9||-||Closed (Uehara)||Lab model|
|8.||NELHA (US)||1996,Hawaii||50||-||Closed (Rankine)||Floating|
|9.||NIOT, India||2000,Tuticorin||1000||-||Closed (Rankine)||Floating|
|¡@||Turbine||Cold Water Pipe|
|Mini OTEC||Radial in-flow 28,200 rpm||HDPE||0.56||670|
|OTEC-1||No turbine||HDPE||3¡Ñ1.10 each||670|
|Nauru||Freon Axial turbine||HDPE||0.70||945|
EARLIER INTERNATIONAL OTEC PROJECTS AND LESSONS LEARNED
A complete OTEC system was constructed, depl/oyed and operated successfully at the sea off shore Hawaii in the US Mini-OTEC program during August ¡V November ¡¥ 79. In this first demonstration of OTEC power with an installed capacity of 50 kW gross, the net power was low due to fact that the turbine generator could operate with only at 53% efficiency. Also seawater and ammonia pumps were operated at roughly half the efficiency consuming 35 kW of the gross power produced.
OTEC-1 experiment, sponsored by Department of Energy, USA and conducted by Argonne National Laboratory, was to simulate OTEC heat exchanger operation. It used shell and tube titanium heat exchangers. The project gave valuable information about the operation of heat exchanger, the deployment of cold water pipe and also about the bio-fouling control. The turbo generator was replaced by a throttle value. It was operational only for four months in the year 1981 and hence seasonal variations on OTEC system could not be studied.
The Nauru plant in the Pacific region was a shore based plant with Freon-22 as the working fluid for a Rankine Closed cycle and it had optimal environment for an OTEC Plant. It was operational for nearly ten months from October 1981. The heat exchangers were of shell and tube type with titanium as material. The evaporator tubes were coated with sprayed copper particles to improve the heat transfer co-efficient. Nauru test provided accurate experimental data on the performance of the power cycle and the construction and operation of a pilot plant. It provided record for gross and net power production and the power was supplied to the grid .
In 1992 a land based open-cycle experimental plant, with funding from PICHTR, was designed, built and operated by Dr. Luis A. Vega and his team for a 210 kW gross output. This was the first open cycle plant after Claude ¡¦ s pioneering work. It could produce a record of 255 kW gross and 103 kW net. This plant provided much valuable information such as the corrosive nature of seawater, violent outgassing of cold seawater, unstable synchronous generator output due to the large inertia of the turbine etc. The equipment, which caused frequent trouble to the system, was the vacuum pump .
The OTEC power can be cost effective only if the unit cost of power produced is comparable with the fossil ¡V fuelled plants. OTEC system can also have other benefits like enhanced mari culture, desalination or even air conditioning, which might reduce the cost of electricity generated. As OTEC is capital intensive, Government agencies may provide substantial initiative in developing the technology. Dr. Luis A. Vega has done extensive work on OTEC economics for open cycle plants and closed cycle plants .
For small plants of 1 MW range the unit power generation cost is considerable compared to other conventional energy sources as shown in the Table 2. The co-production of fresh water along with power is to be considered for the estimation of unit cost for OTEC plants in islands . It is apparent from the study of Dr. Vega that OTEC is economical and production cost is comparative for higher range of plants. The unit cost of electricity is estimated for Indian conditions for a range of 1 MW to 100 MW as shown in Table 3 adopting Dr. Vega ¡¦ s calculation procedure. It could notice that OTEC plants of 100 MW range are competitive with other conventional energy sources such as coal or hydel power plants. In comparison with other renewable energy sources such as photovoltaics and windmills OTEC stands lower for unit investment cost as shown in Figure 3. There are steep cost improvements for these energy sources. The learning rate (the pattern of diminishing costs with increasing experience) is nearly 20% for photovoltaics and windmills. The same result can be expected for OTEC in future with increase in experience and development of technology.
Table2. Comparison of Unit Cost of OTEC with Conventional Energy Sources in Pacific Region (1990)
|¡@||Plant capacity (MWe)||Plant Life (Years)||Capacity factor||Annual output (GWh)||Cost of energy (US$/kWh)|
Table3. Estimation of Unit Cost of Electricity from OTEC Power in India (1999)
|Power Output Gross
|Power Output Net
|Heat Exchanger cost
|Cost of cold water pipe
|Cost of barge
|Cost of electricity
Figure3. The capital cost for 1MW OTEC plant in comparison with other renewable sources.
(Nebojsa et al., Global Energy Perspective, 1998, p,50)
CURRENT ISSUES AND FUTURE PLANS
It is postulated that most of the future commercial OTEC plants are closed-cycle, floating plants of 10-50 MW range. But plants of 200-400 MW range are also feasible and economically more attractive. The commercial plants should be proceeded by demonstration plants of smaller range for power cycle optimization and also for operational information. The design, development and operation of a power system in a hostile sea environment is a great challenge.
The capital cost of the plant is depending much on the heat exchanger cost and hence any improvement in the performance in this single component is an added advantage. Attempts are to be done to find out a proven technology for heat exchangers in seawater conditions with higher heat transfer co-efficient for considerable period of time. The design, fabrication and deployment of seawater system in the environment of the sea is a matter of considerable attention. New materials for the cold water pipe is to be developed to withstand the marine conditions and also for easy fabrication and deployment. The design of the barge also requires care so as to position the seawater pumps for the required Net Positive Suction Head (NPSH). The equipment and the piping system are to be assembled on the barge such a way that the static head and the minor losses are the least. Bio fouling on the warm water circuit and the release of the dissolved gasses in the cold water circuit is a problem to be attended for a considerably long period. As the floating plants are away from seashore under- water power transmission to the land is an area needed further study.
After the completion of testing of the 1 MW OTEC plant, NIOT plans to shift the same plant to the Andaman & Nicobar Islands for power generation. This will be a stepping stone for the proposed 10-25 MW range shore mounted power plants. The experience from 1 MW floating OTEC plant could be scaled up for the construction of 100 MW range commercial plants in the nearby future.
It has been assessed that by the year 2010 about one thousand OTEC plants will be installed of the range 1-100 MW . Now there is a considerable interest in different parts of the world though there was sluggishness in OTEC research during 1985-1995 period due to the fall in oil prices. The key problem is now no longer technological or commercial, but the establishment of reliability and confidence. There is an absolute necessity to build demonstration plants representing the nature of future commercial plants. The demonstration of 1 MW Indian OTEC program is expected to contribute in a large way to provide this confidence to the future.
|Uehara. H, and Ikegami Y, ¡§ Optimization of a closed cycle OTEC system ¡¨ , ASME Journal of Solar Energy Engineering, Vol. 112, No.4, pp. 247 ¡V 256 (1990)|
|R. Abraham et al, ¡§ Analysis of Power Cycle for 1 MW Floating OTEC plant ¡¨ , Accepted paper for IOA ¡¦ color=#000080 99 Conference IMARI (1999)|
|P. Jalihal et al, ¡§ Analysis of Integrated CWP / Barge System for 1 MW OTEC plant ¡¨ , Accepted paper for IOA ¡¦ 99 Conference, IMARI (1999)|
|Avery, W. H. and Wu, Renewable Energy from the Ocean ¡V a guide to OTEC ¡¨ , Oxford University Press, pp. 280 (1994)|
|Luis A. Vega and Donald E. Evans ¡§ Operation of a Small Open Cycle OTEC experimental facility ¡¨ , Renewable Energy Technologies, pp. 93 ¡V 117 (1992)|
|Luis A. Vega, ¡§ Economics of Ocean Thermal Energy Conversion ¡¨ , Ocean Energy Recovery, pp. 152-181 (1992)|
|George Hagerman, ¡§ Wave Power Economics ¡¨ , Ocean Energy Recovery, pp. 152-181 (1992)|
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