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OCEAN THERMAL ENERGY CONVERSION
SEMINAR REPORTSubmitted in partial fulfillment of
the requirements for the award of Degree of Master of Technology in Civil Engineering
(Environmental Engineering)of the University of Kerala
Submitted by
ASHA .C. RAJUM2, Environmental Engineering
Roll No: 091202
D E PA RT M E N T O F C I V I L E N G I N E E R IN G
COLLEGE OF ENGINEERING
TRIVANDRUM .
2010
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DEPARTMENT OF CIVIL ENGINEERING
COLLEGE OF ENGINEERING
TRIVANDRUM
Certificate
This is to certify that the interim seminar report entitled OCEAN THERMAL
ENERGY CONVERSION being submitted by ASHA .C. RAJU towards the partial fulfillment of the requirements for the award of Degree of Master of
Technology in Civil Engineering (Environmental ) of the University of Kerala is a
bonafide record of the work done by her under our supervision and guidance and that
this work has not been submitted elsewhere for a degree.
Guided by P.G. Professor
Mrs. LEA MATHEW Dr. V.SYAM PRAKASHLecturer Professor Department of Civil Engineering Department of Civil EngineeringCollege of Engineering College of EngineeringTrivandrum Trivandrum
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ACKNOWLEDGEMENT
I express my deep sense of gratitude to my guide Smt. Lea
Mathew, Lecturer, Department of Civil Engineering, College of Engineering, Trivandrum for her valuable guidance, constant
encouragement and creative suggestions offered during the course of this
seminar, and also in preparing this report.
I also express my sincere thanks to Dr.S.Sreekumar, Staff
Advisor, Asst .Prof. Sheeja A. K , Smt. Sindhu. P and Smt. Lea
Mathew , Seminar Co-ordinators, Dr. V. Syam Prakash, P.G. Professor
and Dr. S. Sheela, Head of the Department, Department of Civil
Engineering, College of Engineering, Trivandrum, for their kind co-
operation during the course of this work.
I would also wish to record my gratefulness to all my friends and
classmates for the help and support in carrying out this work successfully.
Last but not the least I express my heartful gratitude to Almighty
God who made this work flawless and trouble less
Asha .C. Raju
ABSTRACT
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Almost all countries are heavily dependent on fossil fuels to meet their
increasing energy needs for economic development. The high costs associated with fossil
fuels are placing an increasing strain on the economies of the countries. Within the last two
decades warnings of global destruction and climate change have become major issues. The
growth of environmental awareness and the rising demand for energy has urged researchers
to identify other sources of renewable energy. OTEC, or Ocean Thermal Energy
Conversion , is an energy technology that converts solar radiation to electric power. OTEC
systems use the ocean's natural thermal gradientthe fact that the ocean's layers of water
have different temperaturesto drive a power-producing cycle. As long as the temperature
between the warm surface water and the cold deep water differs by about 20C (36F), an
OTEC system can produce a significant amount of power.
The present study of Ocean Thermal Energy Conversion conducted by Martin et
al., (2008) and Hoshi et al., (2009) emphasized the point that the solution lies in using ocean
thermal energy as a resource rather than be unutilized. The first case study was concerning
the OTEC facility at Keahole Point on the Kona coast of Hawaii generates up to 1.2 mega
Watt-hours of electricity and 6 gallons per minute of desalinated water. The second was that
of an OTEC plant at Kumejima Island in southern part of Japan that utilizes not only ocean
thermal energy but also solar thermal energy as a heat source which is termed as Solar
Boosted Ocean Thermal Energy Conversion. SOTEC plant can potentially enhance the
annual mean net thermal efficiency up to a value that is approximately 1.5 times higher than
that of the conventional OTEC plant. The oceans are thus a vast renewable resource, with the
potential of about 10 13 watts of baseload power generation. Thus OTEC had been
demonstrated in recent studies as one of the environmental friendly methods for renewable
energy utilisation .
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CONTENTSLIST OF TABLESLIST OF ABBREVIATIONS
1. INTRODUCTION 1
1.1 GENERAL
2. MATERIALS AND METHODS 3
3. CASE STUDIES 5
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3.1. CASE STUDY 1 5
3.1.1. OPEN CYCLE OTEC 21
3.1.2 BENEFITS OF OTEC 10
3.1.3 APPLICATIONS OF OTEC 113.2 CASE STUDY 2 13
3.2.1. CONCLUSION 21
4. CONCLUSIONS 22
5. REFERENCES 23
LIST OF TABLESPage No:
Table 1 : Simulation results of 100-kWe OTEC and SOTEC operation 19
LIST OF ABBREVIATIONS
OTEC - Ocean Thermal Energy Conversion
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SOTEC - Solar Boosted Ocean Thermal Energy Conversion
DCC - Direct Contact Condenser
1. INTRODUCTION
1.1 General
Most of the countries are heavily dependent on fossil fuels to meet their increasing
energy needs. The relatively high costs associated with fossil fuels have encouraged research
into indigenous resources as an alternative source of energy. However, the potential of
indigenous resources is yet to be fully realised and exploited. The excessive use of fossil
fuels by industrialised countries has not only increased the carbon dioxide and other ozone-
depleting gases in the atmosphere, but has also contributed to global warming, sea-level rise
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and climate change The idea of using thermal energy from the ocean is not new. The high
costs associated with fossil fuels are placing an increasing strain on the economies in the
region. The growth of environmental awareness and the rising demand for energy has urged
researchers to identify other sources of renewable energy. OTEC, or ocean thermal energy
conversion, is an energy technology that converts solar radiation to electric power. OTEC
systems use the ocean's natural thermal gradientthe fact that the ocean's layers of water
have different temperaturesto drive a power-producing cycle. As long as the temperature
between the warm surface water and the cold deep water differs by about 20C (36F), an
OTEC system can produce a significant amount of power. 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 10 13 watts of baseload power generation,
A significant factor in energy usage patterns in Pacific island countries has been
attributed to the availability of energy sources as early as the fifteenth century where
pollution control was not an issue. People have used what they have been able to get, be it
coal, oil, natural gas, wood, etc. However, within the last two decades warnings of global
destruction and climate change have become major issues. The growth of environmental
awareness and the rising demand for energy has urged researchers to identify other sources of
renewable energy. The heat stored in the ocean can be converted into electricity by means of
a process called Ocean Thermal Energy Conversion (OTEC), which uses the oceans natural
temperature gradient to drive a turbine connected to a generator which produces electricity.
Apart from electricity, there are other useful by-products from the OTEC process like fresh
water, chilled water and nutrient-rich water. To date there are basically three types of OTEC
systems developed to harness the ocean heat a closed-cycle, an open-cycle and a hybrid-
cycle. The economics of energy production have delayed the financing of permanent OTEC
plants.
There have been many periodic attempts to develop and refine OTEC technology
starting in the 1800s. In 1881, Jacques Arsene d'Arsonval , a French physicist , proposed
tapping the thermal energy of the ocean. It was d'Arsonval's student, Georges Claude who
actually built the first OTEC plant, in Cuba in 1930.The system generated 22 kW of
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electricity with a low- pressure turbine .In 1931, Nikola Tesla released "On Future Motive
Power" which covered an ocean thermal energy conversion system. Although initially
excited about the idea, Tesla ultimately came to the conclusion that the scale of engineering
required for the project made it impractical for large scale development. In 1935, Claude
constructed another plant, this time aboard a 10,000- ton cargo vessel moored off the coast of
Brazil . Weather and waves destroyed both plants before they could become net power
generators.
In 1956, French scientists designed a 3 MW plant for Abidjan , Cte d'Ivoire . The
plant was never completed, however, because large amounts of cheap oil became available in
the 1950s making oil fired power plants more economical In 1962, J. Hilbert Anderson and
James H. Anderson, Jr. started designing a cycle to accomplish what Claude had not; theyfocused on developing new, more efficient component designs. After working through some
of the problems in Claude's design they patented their new "closed cycle" design in 1967.
Although Japan has no potential OTEC sites it has been a major contributor to the
development of the technology, primarily for export to other countries. Beginning in 1970
the Tokyo Electric Power Company successfully built and deployed a 100 kW closed-cycle
OTEC plant on the island of Nauru . The plant, which became operational 1981-10-14,
produced about 120 kW of electricity; 90 kW was used to power the plant itself and the
remaining electricity was used to power a school and several other places in Nauru. This set a
world record for power output from an OTEC system where the power was sent to a real
power grid . (Hoshi et al., (2009)).
2. MATERIALS AND METHODS
The main objective of Ocean Thermal Energy Conversion (OTEC) is to turn the
solar energy trapped by the ocean into useable energy. This kind of energy is found in
tropical oceans where the water temperature differs from surface to deeper into the sea. On
the ocean surface it can be at least 20 0C hotter or cooler than the temperature at a deeper sea
level. It uses the temperature difference that exists between deep and shallow waters to run a
heat engine . As with any heat engine, the greatest efficiency and power is produced with the
largest temperature difference. This temperature difference generally increases with
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decreasing latitude, i.e. near the equator , in the tropics . Historically, the main technical
challenge of OTEC was to generate significant amounts of power, efficiently, from this very
small temperature ratio. Changes in efficiency of heat exchange in modern designs allow
performance approaching the theoretical maximum efficiency.
The Earth's oceans are continually heated by the sun and cover nearly 70% of the Earth 's
surface; this temperature difference contains a vast amount of solar energy which can
potentially be harnessed for human use. If this extraction could be made cost effective on a
large scale, it could provide a source of renewable energy needed to deal with energy
shortages , and other energy problems. The total energy available is one or two orders of
magnitude higher than other ocean energy options such as wave power , but the small
magnitude of the temperature difference makes energy extraction comparatively difficult andexpensive, due to low thermal efficiency .
A heat engine is a thermodynamic device placed between a high temperature
reservoir and a low temperature reservoir. As heat flows from one to the other, the engine
converts some of the heat energy to work energy. This principle is used in steam turbines and
internal combustion engines , while refrigerators reverse the direction of flow of both the heat
and work energy. Rather than using heat energy from the burning of fuel, OTEC power
draws on temperature differences caused by the sun's warming of the ocean surface
The only heat cycle suitable for OTEC, is the Rankine cycle , using a low-pressure
turbine. Systems may be either closed-cycle or open-cycle or hybrid cycle. Closed-cycle
engines use working fluids that are typically thought of as refrigerants such as ammonia .
Open-cycle engines use the water heat source as the working fluid . Hybrid systems use parts
of both open- and closed-cycle systems to optimize production of electricity and fresh water.
All OTEC plants require an expensive, large diameter intake pipe, which is submerged a
kilometre or more into the ocean's depths, to bring very cold water to the surface as shown in
fig: 2
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Fig :1 Pipes used for OTEC.Source : Martin et al.,(2008 )
3. CASE STUDIES
In this paper two case studies are being discussed. The first case study was
conducted by Martin et al.,(2008) concerning the OTEC facility at Keahole Point on the
Kona coast of Hawaii which generates up to 1.2 mega Watt-hours of electricity and 6
gallons per minute of desalinated water. The second conducted by Hoshi et al.,(2009)
discussed about an OTEC plant that utilizes not only ocean thermal energy but also solar
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thermal energy as a heat source which is termed as Solar Boosted Ocean Thermal Energy
Conversion. SOTEC plant can potentially enhance the annual mean net thermal efficiency up
to a value that is approximately 1.5 times higher than that of the conventional OTEC plant .
3.1 Case Study I
Fig :2 Land based OTEC facility at Keahole Point
Source : Martin et al.,(2008)
Figure 1 shows a land based OTEC plant installed at Keahole Point on
the Kona coast of Hawaii .. This plant was designed and operated for an output is 2.5 MW for
26 C warm surface water and a deep water temperature 6 C. A small fraction (10 percent)
of the steam produced was diverted to a surface condenser for the
production of desalinated water. The experimental plant is successfully
operating for six years. The highest production rates achieved were 2.35
MW (gross) with a corresponding net power of 1.2 MW and 6 gallons per minute
of desalinated water . These are world records for OTEC. Here electricity is
produced in a first-stage followed by water production in a second-stage,
to maximize the use of the thermal resource available to produce water
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and electricity. In the second-stage, the temperature difference available
in the seawater effluents from an OTEC plant (e.g., 12 C) is used to
produce desalinated water through a system consisting of a flash
evaporator and a surface condenser as shown in fig 3
Fig 3 Surface Condenser for Desalinated Water Production
Source: Martin et al.,(2008)
3.1.1 Open Cycle OTEC
The open cycle consists of the following steps: (i) flash evaporation of a fraction of
the warm seawater by reduction of pressure below the saturation value corresponding to its
temperature (ii) expansion of the vapor through a turbine to generate power; (iii) heat transfer
to the cold seawater thermal sink resulting in condensation of the working fluid; and (iv)
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compression of the non-condensable gases (air released from the seawater streams at the low
operating pressure) to pressures required to discharge them from the system. These steps are
depicted in Figure 4 and 5. In the case of a surface condenser the condensate (desalinated
water) must be compressed to pressures required to discharge it from the power generating
system. The evaporator, turbine, and condenser operate in partial vacuum ranging from 3
percent to 1 percent atmospheric pressure. This poses a number of practical concerns that
must be addressed. First, the system must be carefully sealed to prevent in-leakage of
atmospheric air that can severely degrade or shut down operation. Second, the specific
volume of the low-pressure steam is very large compared to that of the pressurized working
fluid used in closed cycle OTEC. Finally, gases such as oxygen, nitrogen and carbon dioxide
that are dissolved in seawater (essentially air) come out of solution in a vacuum. These gases
are uncondensable and must be exhausted from the system.
Fig 4. Open-Cycle OTEC Flow Diagram .
(Source: Martin et al.,(2008))
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Fig 5. Schematic diagram of Open-Cycle OTEC .
Source: Vega et al.,(2005)
In spite of the aforementioned complications, the Open cycle enjoyscertain benefits from the selection of water as the working fluid. Water, unlike ammonia, is
non-toxic and environmentally benign. Moreover, since the evaporator produces desalinated
steam, the condenser can be designed to yield fresh water. In many potential sites in the
tropics, potable water is a highly desired commodity that can be marketed to offset the price
of OTEC-generated electricity. Flash evaporation is a distinguishing feature of open cycle
OTEC. Flash evaporation involves complex heat and mass transfer processes. Here warm
seawater was pumped into a chamber through spouts designed to maximize the heat-and-
mass-transfer surface area by producing a spray of the liquid. The pressure in the chamber
(2.6 percent of atmospheric) was less than the saturation pressure of the warm seawater.
Exposed to this low-pressure environment, water in the spray began to boil. As in thermal
desalination plants, the vapor produced was relatively pure steam. As steam is generated, it
carries away with it its heat of vaporization. This energy comes from the liquid phase and
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results in a lowering of the liquid temperature and the cessation of boiling. Thus, as
mentioned above, flash evaporation may be seen as a transfer of thermal energy from the
bulk of the warm seawater to the small fraction of mass that is vaporized to become the
working fluid. Approximately 0.5 percent of the mass of warm seawater entering the
evaporator is converted into steam.
A large turbine is required to accommodate the huge volumetric flow rates of low-
pressure steam needed to generate any practical amount of electrical power. Although the last
stages of turbines used in conventional steam power plants can be adapted to Open Cycle-
OTEC operating conditions, existing technology limits the power that can be generated by a
single turbine module, comprising a pair of rotors, to about 2.5 MW. Unless significant effort
is invested to develop new, specialized turbines (which may employ fiber-reinforced plastic blades in rotors having diameters in excess of 100 m),increasing the gross power generating
capacity of a Open cycle plant above 2.5 MW. Condensation of the low-pressure working
fluid leaving the turbine occurs by heat transfer to the cold seawater. This heat transfer may
occur in a Direct Contact Condenser (DCC), in which the seawater is sprayed directly over
the vapor, or in a surface condenser that does not allow contact between the coolant and the
condensate. Direct Contact Condensers are relatively inexpensive and have good heat
transfer characteristics due to the lack of a solid thermal boundary between the warm and
cool fluids. Although surface condensers for OTEC applications are relatively expensive to
fabricate they permit the production of desalinated water. Desalinated water production with
a DCC requires the use of fresh water as the coolant. In such an arrangement, the cold
seawater sink is used to chill the fresh water coolant supply using a liquid-to-liquid heat
exchanger. Effluent from the low-pressure condenser must be returned to the environment.
Liquid can be pressurized to ambient conditions at the point of discharge by means of a pump
or, if the elevation of the condenser is suitably high, it can be compressed hydrostatically. For
a system that includes both the OC-OTEC heat engine and its environment, the cycle isclosed and parallels the Rankine cycle. Here, the condensate discharge pump and the non-
condensable gas compressor assume the role of the Rankine cycle pump.
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The analysis of the cycle yields (Figure 1):
Heat (added) absorbed from seawater (J/s) q w = m ww Cp (T wwi - T wwo)
Steam generation rate (kg/s) m s = q w/h fg
Turbine work (J/s) wT = m s (h3 - h 5) = m s T (h 3 - h 5s)
Heat (rejected) into seawater (J/s) q c = m cwCp (T cwo - T cwi)
where, m ww is the mass flow rate of warm water; Cp the specific heat; T wwi and T wwo are the
seawater temperature at the inlet and outlet of the heat exchanger; h fg the heat of evaporation;
and the enthalpies at the indicated points are given by h, with the subscript s referring to
constant entropy. The turbine isentropic efficiency is given by T. The subscript cw refers to
the cold water.
3.1.2 Benefits of OTEC
Conventional power plants pollute the environment more than an OTEC plantwould and, as long as the sun heats the oceans, the fuel for OTEC is unlimited and free
We can measure the value of an ocean thermal energy conversion (OTEC) plant and
continued OTEC development by both its economic and noneconomic benefits. OTEC's
economic benefits include these :
Helps produce fuels such as hydrogen, ammonia, and methanol
Produces baseload electrical energy
Produces desalinated water for industrial, agricultural, and residential uses
Is a resource for on-shore and near-shore mariculture operations
Provides air-conditioning for buildings
Provides moderate-temperature refrigeration
Has significant potential to provide clean, cost effective electricity for the future
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OTEC's noneconomic benefits, which help us achieve global environmental goals,
include these:
Promotes competitiveness and international trade
Enhances energy independence and energy security
Promotes international sociopolitical stability
Has potential to migrate 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.
3.1.3 Applications of OTEC
Ocean thermal energy conversion (OTEC) systems have many applications or uses.
OTEC can be used to generate electricity , desalinate water , support deep-water mariculture ,
and provide refrigeration and air-conditioning as well as aid in crop growth and mineral
extraction . These complementary products make OTEC systems attractive to industry and
island communities even if the price of oil remains low. OTEC can also be used to produce
methanol, ammonia, hydrogen, aluminum, chlorine, and other chemicals. Floating OTEC
processing plants that produce these products would not require a power cable, and station-
keeping costs would be reduced. Fig: 6 shows the overall process diagram of OTEC
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Fig 6 The overall process diagram of OTEC
Source: Martin et al.,(2008)
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3.2 CASE STUDY 2
Ocean thermal energy conversion (OTEC) is a power generation method that
utilizes small temperature difference between the warm surface water and cold deep water of
the ocean. The present case study at Kumejima Island in southern part of Japan describes the
performance simulation results of an OTEC plant that utilizes not only ocean thermal energy
but also solar thermal energy as a heat source. This power generation system was termed
SOTEC (solar-boosted ocean thermal energy conversion). In SOTEC, the temperature of
warm sea water was boosted by using a typical low-cost solar thermal collector. The results
show that the proposed SOTEC plant can potentially enhance the annual mean net thermal
efficiency up to a value that is approximately 1.5 times higher than that of the conventional
OTEC plant if a single-glazed flat-plate solar collector of 5000-m2
effective area is installedto boost the temperature of warm sea water by 20 K. The objective of the study was to
estimate the potential thermal efficiency and required effective area of a solar collector for a
100-kWe SOTEC plant, study was carried out under the ambient conditions at Kumejima
Island in southern part of Japan.
Ocean thermal energy conversion (OTEC) is a power generation method
where in the heat energy associated with the temperature difference between the warm
surface water and cold deep water of the ocean is converted into electricity .Considerable
research effort has been directed to the development of OTEC. The results of these studies
revealed that due to a small temperature difference (approximately 1525 K) between the
surface water and deep water of the ocean, the Rankine-cycle efficiency is limited to be only
35%. This results in a high cost of the electricity generated by an OTEC plant. In order to
improve the cycle efficiency, an ammoniawater mixture as the working fluid have been
developed and reported to have better thermal efficiency than the Rankine cycle at the same
temperature difference. However, it is evident that increasing the temperature difference
between the hot and cold heat sources is the most effective solution to improve the thermal
efficiency of a thermodynamic power generation cycle. In this study, an OTEC system was
described that utilizes not only ocean thermal energy but also solar-thermal energy; the latter
is used as a secondary heat source. A solar collector used in a residential application is
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simply installed to the conventional OTEC component. This power generation system is
termed as SOTEC (Solar-boosted Ocean Thermal Energy Conversion). The performance
simulation of a 100-kWe SOTEC plant with three typical low-cost solar-thermal collectors,
which increase the turbine inlet temperature of the working fluid, is carried out under the
actual weather and sea-water conditions at Kumejima Island in the southern part of Japan.
The simulation results of the SOTEC plant are discussed and compared with that of the
conventional OTEC plant.
Fig. 7. Schematics of conventional OTEC operation.
Source: Hoshi et al., (2009).
Figures. 7 and 8 show schematics of the conventional OTEC operation and the
proposed SOTEC operation, respectively; these figures show the general arrangement of the
heat exchangers, pumps, piping, turbine generator, and solar collector. In SOTEC, we present
the probable way to install solar collector into the cycle, as shown in Fig. 7. In Fig. 7, the
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warm sea water is pumped from the ocean surface and is heated by a solar collector; then, the
working fluid is indirectly heated and evaporated through the evaporator. In the simulation,
an ideal saturated Rankine cycle was assumed in order to determine the theoretical thermal
efficiency of the Rankine-cycle th.
Fig. 8. Schematics of SOTEC operation: Solar collector installed in warm-sea-water
Line
Source: Hoshi et al., (2009).
Fig. 9 shows the relationship between the theoretical thermal efficiency of the
Rankine-cycle th and the temperature difference T =T E -TC. Here, T E and T C are the
evaporation temperature and condensation temperature, The conventional OTEC has T
between 15 K and 25 K; as a result, the maximum theoretical thermal efficiency is
approximately th. = 8%. If a solar collector can additionally boost T E by 20 K, the
theoretical thermal efficiency of the SOTEC can be improved up to th. = 13%.
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Fig. 9 . Relationship between theoretical thermal efficiency of saturated Rankine cycle
th and the operating temperature range T of OTEC and SOTEC.
Source: Hoshi et al., (2009).
Fig. 10 shows a diagram of the proposed SOTEC plant. Its operation wasalternatively switched from OTEC to SOTEC by controlling valves. The heat transfer areas
of the evaporator and condenser were also ideally controlled to the optimal heat exchange
efficiency by controlling valves. Fig. 11 shows the monthly variation of the daytime net
Rankine cycle efficiency net of SOTEC operation compared with that of OTEC operation.
net of SOTEC is approximately three times higher than that of OTEC in every month.. net of
SOTEC was significantly affected by pumping powers.
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Fig :10 Diagram of the SOTEC plant .Source: Hoshi et al., (2009).
Fig. 11 . Daytime net Rankine-cycle efficiency net of OTEC and SOTEC
Source: Hoshi et al., (2009).
Figure 12 shows the monthly variations of the net power PN and the pumping
powers required for warm-sea-water PWS, cold-sea-water PCS, and the working-fluid PWF:
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Fig. 12(a) and (b) is for OTEC and SOTEC operations, respectively. These figures show a
breakdown of turbine-generated power PG = 100 kW.
Fig. 12 . Monthly variation of power of: (a) OTEC operation (b) SOTEC operation.
Source: Hoshi et al., (2009).
In SOTEC operation, the pumping power required for cold sea water was reduced to
approximately 30% of that in OTEC operation because the mass flow rate of cold sea water
was reduced in SOTEC operation due to an increase in the Rankine-cycle efficiency R by
solar boosting. Consequently, the net power of SOTEC was larger than that of OTEC. The
pumping powers required for warm seawater and the working fluid were also slightly
reduced in SOTEC. These resulted in the above-mentioned increase in the net efficiency of
SOTEC.
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Table: 1 Simulation results of 100-kWe OTEC and SOTEC operationSource: Hoshi et al., (2009).
Table 1 lists important simulated values of 100-kWe OTEC and SOTEC
operations under the annual mean daytime condition at Kumejima Island. In this table, the
result obtained for the case of 40-K solar boost is added for comparison. net of OTEC
daytime operation was 2.3%, while that of SOTEC was 6.3% and 9.5% corresponding to 20-
K and 40-K solar boost, respectively. The required effective area of the solar collector (ASC)
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of the flat-plate solar collector for the 40-K solar boost is 5333 m 2, which is 893 m 2 larger
than that of the 20-K solar boost because of the collector efficiency drop from c=63% to
48%. In contrast, ASC of the evacuated-tube collector for the 40-K solar boost is
approximately 100 m 2 less than that for the 20-K solar boost because the collector efficiency
was retained at c = 68% even for the 40-K boost.
Fig. 13 . Net Rankine-cycle efficiency net of SOTEC plant and OTEC plant
Source: Hoshi et al., (2009).
Fig. 13 shows the simulation results for the annual hourly variation of the net
Rankine-cycle efficiency net of the SOTEC plant and the conventional OTEC plant at
Kumejima Island for the 20-K solar boost. The SOTEC plant was selectively operated in the
SOTEC mode during daytime only if the thermal efficiency of SOTEC operation exceeded
that of OTEC operation. In Fig. 12, net of the SOTEC plant fluctuated up to 9% due to the
change in solar gain, while net of OTEC was within the range between approximately 1%
and 3%. Further, the net power of SOTEC fluctuated within the range between
approximately 70 kWe and 150 kWe; this is not shown in Fig. 11. The annual total SOTEC
operation time at Kumejima Island was estimated to be 2867 h (approximately 120 days), so
that the annual mean net Rankine-cycle efficiency of the SOTEC plant was finally estimated
to be approximately 3%. This value is equivalent to 145% of that of the conventional OTEC
plant. The present simulation results indicate that the SOTEC operation can potentially
increase the efficiency of OTEC by means of a combination of low-cost solar collectors. 40-
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K boost by the solar collector enhances net thermal efficiency up to several times higher than
the conventional OTEC in daytime. In terms of cost estimation for the SOTEC, the cost of a
flat-plate solar collector would be lower than that of other solar collectors to achieve a solar
boost upto 20 K and that the evacuated-tube collector would be cost-effective for the SOTEC
with 40-K solar boost. The price of evacuated-tube collectors is being dramatically be
reduced due to market expansion and the development of mass production technology. The
development of a mass-production technology for large-area solar collectors will help reduce
the cost of SOTEC.
3.2.3 Conclusion
A solar-boosted ocean thermal energy conversion (SOTEC) system was
proposed and performance simulation was carried out. The results reveal that the installation
of a solar collector enhances the thermal efficiency of an OTEC plant, particularly in daytime
operation. Net thermal efficiency of SOTEC operation with 20-K solar boost is 2.7 times
higher than that of OTEC operation under the daytime conditions at Kumejima Island. This
results in approximately 1.5-times higher annual net thermal efficiency than the conventional
OTEC plant. For future studies, the authors intend to perform more precise simulation
including other thermodynamic cycles and estimate the practical cost of SOTEC plant. Thedevelopment of an advanced OTEC system will become increasingly important and
promising with a long-term upward trend in the prices of oil and fossil fuels .
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4. CONCLUSIONS
The present study emphasises the importance of using ocean thermal energy
as a resource rather than be unutilized. Conventional power plants pollute the environment
more than an OTEC plant would and as long as the sun heats the oceans, the fuel for OTEC
is unlimited and free.
1. The main objective of Ocean Thermal Energy Conversion (OTEC) is to turn the solar energy
trapped by the ocean into useable electric energy of 10 13 watts of base load power generation
2. It also desalinate water , support deep-water mariculture, and provide refrigeration and air-
conditioning as well as aid in crop growth and mineral extraction .
3. OTEC facility at Keahole point on the Kona coast of Hawaii generates upto 1.2 mega Watt
hours of electricity and 6 gallons per minute of desalinated water.
4. SOTEC (Solar Boosted Ocean Thermal Energy Conversion) utilizes not only ocean thermal
energy but also solar thermal energy as a heat source.
5. Net thermal efficiency of SOTEC operation with 20-K solar boost is 2.7 times higher than
that of OTEC operation under the daytime conditions at Kumejima island.
6. SOTEC plant can potentially enhance the annual net thermal efficiency upto a value that is
approximately 1.5 times higher than that of the conventional OTEC plant.
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http://www.nrel.gov/otec/desalination.htmlhttp://www.nrel.gov/otec/mariculture.htmlhttp://www.nrel.gov/otec/refrigeration.htmlhttp://www.nrel.gov/otec/refrigeration.htmlhttp://www.nrel.gov/otec/mineral_extraction.htmlhttp://www.nrel.gov/otec/desalination.htmlhttp://www.nrel.gov/otec/mariculture.htmlhttp://www.nrel.gov/otec/refrigeration.htmlhttp://www.nrel.gov/otec/refrigeration.htmlhttp://www.nrel.gov/otec/mineral_extraction.html -
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5. REFERENCES
1) Hoshi, A., Yamada, N., Yasuyuki, I.,(2009) Performance simulation of solar-boosted
ocean thermal energy conversion Plant, Journal of Renewable Energy, Vol 34, pp
1752-1758
2) Martin, L, L., Moore ,F, P., (2008) A nonlinear nonconvex minimum total heat
transfer area formulation for ocean thermal energy conversion (OTEC) systems
Journal of Applied Thermal Engineering, Vol 28, pp 10151021
3) Vega, L, A., Nelson, P, M., (2005) Ocean Thermal Energy Conversion: Electricity
and Desalinated Water Production, Journal of Renewable Energy, Vol : 14, pp 785 -
794
4) Wikipedia < http://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion >
5) http://
http://en.wikipedia.org/wiki/Ocean_thermal_energy_conversionhttp://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion