OCEAN THERMAL ENERGY CONVERSION
A seminars report
A.Swathi P.Srivalli Sneha
PRASAD.V. POTLURI SIDDHARTHA INSTITUTE OF TECHNOLOGY
Affiliated to JNTU, Hyderabad
Ocean Thermal Energy Conversion (OTEC) is an energy technology that converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradientâ€the fact that the ocean's layers of water have different temperatures to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20Ã‚Â°C (36Ã‚Â°F), an OTEC system can produce a significant amount of power, with little impact on the surrounding environment.
The distinctive feature of OTEC energy systems is that the end products include not only energy in the form of electricity, but several other synergistic products. The principle design objective was to minimize plan cost by minimizing plant mass, and taking maximum advantage of minimal warm and cold water flows. Power is converted to high voltage DC, and is cabled to shore for conversion to AC and integration into the local power distribution network.
The oceans are thus a vast renewable energy resource, with the potential to help us produce billions of watts of electric power.
OCEAN THERMAL ENERGY CONVERSION
Oceans cover more than 70% of Earth's surface, making them the world's largest solar collectors. The sun's heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy. Just a small portion of the heat trapped in the ocean could power the world.
I. INTRODUCTION TO OCEAN ENERGY:
Most people have been witness to the awesome power of the world's oceans. For least a thousand years, scientists and inventors have watched ocean waves explode against coastal shores, felt the pull of ocean tides, and dreamed of harnessing these forces. But it's only been in the last century that scientists and engineers have begun to look at capturing ocean energy to make electricity.
The ocean can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves. Ocean thermal energy is used for many applications, including electricity generation. Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves are driven primarily by the winds. As a result, tides and waves are sporadic sources of energy, while ocean thermal energy is fairly constant. Also, unlike thermal energy, the electricity conversion of both tidal and wave energy usually involves mechanical devices.
II. OCEAN THERMAL ENERGY CONVERSION:
OTEC is a process which utilizes the heat energy stored in the tropical ocean. The world's oceans serve as a huge collector of heat energy. OTEC plants utilize the difference in temperature between warm surface sea water and cold deep sea water to produce electricity.
The energy associated with OTEC derives from the difference in temperature between two thermal reservoirs. The top layer of the ocean is warmed by the sun to temperatures up to 20 K greater than the seawater near the bottom of the ocean. OTEC energy is different from geothermal energy in that one cannot assume the cold reservoir is infinite. The physical energy of two large reservoirs of fluid at different temperatures is
in J/kg where r is the mass of warm water divided by the mass of cold water entering the plant(1). For optimal performance, r is approximately 0.5. It is assumed in this analysis that the specific heat of the two fluid reservoirs is an average value over the often small temperature difference, but varying with salinity in the case of seawater.
Thermal energy conversion is an energy technology that converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradientâ€the fact that the ocean's layers of water have different temperaturesâ€to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20Ã‚Â°C, 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 1013 watts of base load power generation, according to some experts. The cold, deep seawater used in the OTEC process is also rich in nutrients, and it can be used to culture both marine organisms and plant life near the shore or on land. OTEC produce steady, base-load electricity, fresh water, and air-conditioning options.
OTEC requires a temperature difference of about 36 deg F (20 deg C). This temperature difference exists between the surface and deep seawater year round throughout the tropical regions of the world. To produce electricity, we either use a working fluid with a low boiling point (e.g. ammonia) or warm surface sea water, or turn it to vapor by heating it up with warm sea water (ammonia) or de-pressurizing warm seawater. The pressure of the expanding vapor turns a turbine and produces electricity.
Plant Design and Location
Commercial OTEC facilities can be built on
Â¢ Land or near the shore
Â¢ Platforms attached to the shelf
Â¢ Moorings or free-floating facilities in deep ocean water
Land-based and near-shore are more advantageous than the other two. OTEC plants can be mounted to the continental shelf at depths up to 100 meters, however may make shelf-mounted facilities less desirable and more expensive than their land-based counterparts. Floating OTEC facilities with a large power capacity, but has the difficulty of stabilizing and of mooring it in very deep water may create problems with power delivery.
Commercial ocean thermal energy conversion (OTEC) plants must be located in an environment that is stable enough for efficient system operation. The temperature of the warm surface seawater must differ about 20Ã‚Â°C (36Ã‚Â°F) from that of the cold deep water that is no more than about 1000 meters (3280 feet) below the surface. The natural ocean thermal gradient necessary for OTEC operation is generally found between latitudes 20 deg N and 20 deg S.
III. TYPES OF ELECTRICITY CONVERSION SYSTEMS
There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems.
In the closed-cycle OTEC system, warm sea water vaporizes a working fluid, such as ammonia, flowing through a heat exchanger (evaporator). The vapor expands at moderate pressures and turns a turbine coupled to a generator that produces electricity. The vapor is then condensed in heat exchanger (condenser) using cold seawater pumped from the ocean's depths 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.
The heat exchangers (evaporator and condenser) are a large and crucial component of the closed-cycle power plant, both in terms of actual size and capital cost. Much of the work has been performed on alternative materials for OTEC heat exchangers, leading to the recent conclusion that inexpensive aluminum alloys may work as well as much more expensive titanium for this purpose.
Required condensate pump work, wC. The major additional parasitic energy requirements in the OTEC plant are the cold water pump work, wCT, and the warm water pump work, wHT. Denoting all other parasitic energy requirements by wA, the net work from the OTEC plant, wNP is
wNP = wT + wC + wCT + wHT + wA
The thermodynamic cycle undergone by the working fluid can be analyzed without detailed consideration of the parasitic energy requirements. From the first law of thermodynamics, the energy balance for the working fluid as the system is
wN = QH + QC
Where wN = wT + wC is the net work for the thermodynamic cycle. For the special idealized case in which there is no working fluid pressure drop in the heat exchangers,
QH = THds
QC = TCds
so that the net thermodynamic cycle work becomes
wN = THds + TCds
Subcooled liquid enters the evaporator. Due to the heat exchange with warm sea water, evaporation takes place and usually superheated vapor leaves the evaporator. This vapor drives the turbine and 2-phase mixture enters the condenser. Usually, the subcooled liquid leaves the condenser and finally, this liquid is pumped to the evaporator completing a cycle.
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) 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.
This process being iso-enthalpic,
h2 = h1 = hf + x2hfg
Here, x2 is the fraction of water by mass that has vaporized. The warm water mass flow rate per unit turbine mass flow rate is 1/x2.
The low pressure in the evaporator is maintained by a vacuum pump that also removes the dissolved non condensable gases from the evaporator. The evaporator now contains a mixture of water and steam of very low quality. The steam is separated from the water as saturated vapour. The remaining water is saturated and is discharged back to the ocean in the open cycle. The steam we have extracted in the process is a very low pressure, very high specific volume working fluid. It expands in a special low pressure turbine.
h3 = hg
Here, hg corresponds to T2. For an ideal adiabatic reversible turbine,
s5,s = s3 = sf + x5,ssfg
The above equation corresponds to the temperature at the exhaust of the turbine, T5. x5,s is the mass fraction of vapour at point 5.
The enthalpy at T5 is,
h5,s = hf + x5,shfg
This enthalpy is lower. The adiabatic reversible turbine work = h3-h5,s.
Actual turbine work wT = (h3-h5,s) Ãƒâ€” polytropic efficiency
The condenser temperature and pressure are lower. Since the turbine exhaust will be discharged back into the ocean anyway, a direct contact condenser is used. Thus the exhaust is mixed with cold water from the deep cold water pipe which results in a near saturated water.That water is now discharged back to the ocean.
h6=hf, at T5. T7 is the temperature of the exhaust mixed with cold sea water, as the vapour content now is negligible,
There are the temperature differences between stages. One between warm surface water and working steam, one between exhaust steam and cooling water and one between cooling water reaching the condenser and deep water. These represent external irreversibilities that reduce the overall temperature difference.
The cold water flow rate per unit turbine mass flow rate,
Turbine mass flow rate,
Warm water mass flow rate,
Cold water mass flow rate
Hybrid OTEC System
Another option is to combine the two processes together into an open-cycle/closed-cycle hybrid, which might produce both electricity and desalinated water more efficiently. In a hybrid OTEC system, warm seawater might enter a vacuum where it would be flash-evaporated into steam, in a similar fashion to the open-cycle evaporation process.
The steam or the warm water might then pass through an evaporator to vaporize the working fluid of a closed-cycle loop. The vaporized fluid would then drive a turbine to produce electricity, while the steam would be condensed within the condenser to produced desalinated water
IV. OTHER TECHNOLOGIES
OTEC offers one of the most benign power production technologies, since the handling of hazardous substances is limited to the working fluid (e.g., ammonia), and no noxious by-products are generated. OTEC requires drawing sea water from the mixed layer and the deep ocean and returning it to the mixed layer, close to the thermo cline, which could be accomplished with minimal environmental impact. Aquaculture is perhaps the most well-known byproduct of OTEC. Cold-water delicacies, such as salmon and lobster, thrive in the nutrient-rich, deep, seawater from the OTEC process. Micro algae such as Spirulina, a health food supplement, also can be cultivated in the deep-ocean water.
Wave energy systems also cannot compete economically with traditional power sources. However, the costs to produce wave energy are coming down, Once built, however, wave energy systems (and other ocean energy plants) should have low operation and maintenance costs because the fuel they use sea water is free. Like tidal power plants, OTEC power plants require substantial capital investment upfront. Another factor hindering the commercialization of OTEC is that there are only a few hundred land-based sites in the tropics where deep-ocean water is close enough to shore to make OTEC plants feasible.
V. BENEFITS OF OTEC
We can measure the value of an ocean thermal energy conversion (OTEC) plant and continued OTEC development by both its economic and no economic benefits. OTECâ„¢s economic benefits include the:
Â¢ Helps produce fuels such as hydrogen, ammonia, and methanol
Â¢ Produces base load electrical energy
Â¢ Produces desalinated water for industrial, agricultural, and residential uses
Â¢ Is a resource for on-shore and near-shore Mari culture operations
Â¢ Provides air-conditioning for buildings
Â¢ Provides moderate-temperature refrigeration
Â¢ Has significant potential to provide clean, cost-effective electricity for the future.
Â¢ Fresh Water-- up to 5 liters for every 1000 liters of cold seawater.
Â¢ Food--Aquaculture products can be cultivated in discharge water.
OTECâ„¢s no economic benefits, which help us achieve global environmental goals, include these:
Â¢ Promotes competitiveness and international trade
Â¢ Enhances energy independence and energy security
Â¢ Promotes international sociopolitical stability
Â¢ 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 Mari culture products
Â¢ OTEC-produced electricity at present would cost more than electricity generated from fossil fuels at their current costs. The electricity cost could be reduced significantly if the plant operated without major overhaul for 30 years or more, but there are no data on possible plant life cycles.
Â¢ OTEC plants must be located where a difference of about 40Ã‚Â° Fahrenheit (F) occurs year round. Ocean depths must be available fairly close to shore-based facilities for economic operation. Floating plant ships could provide more flexibility.
Ocean thermal energy conversion (OTEC) systems have many applications or uses. OTEC can be used to generate electricity, desalinate water, support deep-water Mari culture, 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.
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. The cold [5Ã‚Â°C (41Ã‚ÂºF)] seawater made available by an OTEC system creates an opportunity to provide large amounts of cooling to operations that are related to or close to the plant. Likewise, the low-cost refrigeration provided by the cold seawater can be used to upgrade or maintain the quality of indigenous fish, which tend to deteriorate quickly in warm tropical regions. The developments in other technologies (especially materials sciences) were improving the viability of mineral extraction processes that employ ocean energy.
VIII. CASE STUDY: (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 Rankine 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. NIOT signed a memorandum of understanding with Saga University in Japan for the joint development of the plant near the port of Tuticorin (Tamil Nadu).
It has been reported that following detailed specifications, global tenders were placed at end-1998 for the design, manufacture, supply and commissioning of various sub-systems. 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. NIOTâ„¢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.
OTEC has tremendous potential to supply the worldâ„¢s energy. It is estimated that, in an annual basis, the amount solar energy absorbed by the oceans is equivalent to atleast 4000 times the amount presently consumed by humans. For an OTEC efficiency of 3 percent, in converting ocean thermal energy to electricity, we would need less than 1 percent of this renewable energy to satisfy all of our desires for energy.
OTEC offers one of the most compassionate power production technologies, since the handling of hazardous substances is limited to the working fluid (e.g., ammonia), and no noxious by-products are generated. Through adequate planning and coordination with the local community, recreational assets near an OTEC site may be enhanced. OTEC is capital-intensive, and the very first plants will most probably be small requiring a substantial capital investment. Given the relatively low cost of crude oil and of fossil fuels in general, the development of OTEC technologies is likely to be promoted by government agencies. 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.
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2. Claude G. (1930), "Power from the Tropical Seas" in Mechanical Engineering, Vol. 52, No.12, 19, pp. 1039-1044.
3. Nihous G.C. and. Vega L.A (1991), "A Review of Some Semi-empirical OTEC Effluent Discharge Models", in Oceans Ëœ91, Honolulu, Hawaii. [The OTEC effluent models are summarized]
4. Ocean Thermal Corporation. (1984a). Ocean Thermal Energy Conversion (OTEC) Preliminary Design Engineering Report. Prepared for U.S. Department of Energy, Washington, D.C.
5. Ocean Data Systems Inc. (1977). OTEC Thermal Resource Report for Hawaii Monterey, CA: Ocean Data Systems, Inc.