In this article we will discuss about:- 1. Introduction to Ocean Thermal Energy Conversion (OTEC) System 2. Energy Conversion of OTEC Plants 3. Advantages and Limitations 4. Applications.
Introduction to Ocean Thermal Energy Conversion (OTEC) System:
The ocean can develop two types of energy viz., thermal energy from the sun’s heat and mechanical energy from tides and waves.
Ocean covers 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 (or temperature gradient) produces thermal energy. The ocean thermal energy is immense.
In tropical regions, the surface water can be 40 or more degree warmer than the deep water. This temperature difference can be utilized in a heat generator to generate electric power. This is called ocean thermal energy conversion (OTEC). OTEC has the potential to generate more energy than tidal, wave and wind energy combined.
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The OTEC systems can be open or closed. In a closed system, an evaporator turns warm surface water into steam under pressure. This steam rotates a turbine coupled to an electric generator. Water pumps bring cold deep water through pipes to a condenser on the surface. The cold water condenses the steam and the closed cycle begins again. In an open system, the steam is turned into fresh water and new surface water is added to the system. A transmission cable carries the electricity to the shore.
The OTEC systems must have a temperature difference of about 25°C to operate. This restricts OTEC’s application to tropical region where the surface waters are very warm and there is deep cold water.
The temperature difference is small even in tropical regions, therefore, OTEC systems have very low efficiencies and consequently have very high capital cost.
The first plant for utilizing the Ocean Thermal Energy had a capacity of 40 kW and was tested in Cuba on 1930 by Georges Claude. The first modern OTEC power plant, which was located near Hawaii (USA), had a thermal capacity of 2.8 MW and electric power output of 50 kW (gross). However, the net power output of the plant was 18 kW. Hence, the overall efficiency was only 0.643%.
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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 in 1981, produced about 120 kW; 90 kW was used to power the plant itself and the remaining was used to power a school and several 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.
India piloted a 1-MW floating OTEC plant near Tamil Nadu. Its government continues to sponsor various researches in developing floating OTEC facilities.
Today there are several experimental OTEC plants, but no large operations. There are many challenges to widespread use. The OTEC systems are not very energy efficient. Pumping of water is a giant engineering challenge. In addition, the electricity is to be transported to land. It will be 10 to 20 years before the technology is available to generate and transmit electrical power economically from OTEC systems.
Energy Conversion of OTEC Plants:
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OTEC 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 developed with the largest temperature difference. This temperature difference usually increases with decreasing latitude, i.e., near the equator, in the tropics.
Historically, the main technical challenge of OTEC was to produce 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 small temperature difference makes energy extraction comparatively difficult and expensive, due to low thermal efficiency.
Earlier OTEC systems had an overall efficiency of only 1 to 3% (the theoretical maximum efficiency lies between 6% and 7%). Current designs under review will operate closer to the theoretical maximum efficiency. The energy carrier, sea water, is free, though it has an access cost associated with the pumping materials and pump energy costs. Although an OTEC power plant operates at a low overall efficiency, it can be configured to operate continuously as a base load power generation system.
A heat engine is a thermodynamic device placed between a high temperature reservoir and a low temperature reservoir. As heat flows from one reservoir to another reservoir, the engine converts some of heat energy to work energy. This principle is employed in steam engines and steam turbines. While in refrigeration, direction of flow of both the heat and work energy is reversed. 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.
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The most commonly used heat cycle for OTEC is the Rankin cycle that uses a low-pressure turbine. OTEC systems may be either open-cycle or closed-cycle. Open-cycle engines use vapour from the sea water, itself as the working fluid while closed-cycle engines use working fluids that are typically thought of as refrigerants like ammonia or R-134a.
Advantages and Limitations of OTEC Plants:
The variety of products and services are the major advantage of OTEC plants. Ocean thermal is not relatively clean and produces more pollutants that contribute to global warming. The open cycle eliminates the need for a surface heat exchanger. The major advantage of OTEC for power generation is that the ocean energy is renewable source. Since no fuel is used, the OTEC system may become more economical in future.
Limitations of OTEC Power Plants:
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Problems facing commercial development of OTEC systems are enormous:
i. Because of low pressure, large sized turbines are required. Steam turbines capable of generating 10 MW or more using low pressure steam have yet to be developed.
ii. Size of plant is limited due to large size of components.
iii. The maintenance of vacuum in the pumps requires very large size vacuum pumps.
iv. Low temperature differences result in very low plant efficiencies, very large plant size and huge capital cost.
v. For open-cycle systems, the problems associated with the design of evaporator, operation and maintenance are many.
vi. For closed-cycle systems, heat exchangers of very large size have to be designed and built and the working fluid is expensive.
vii. Pollution can be caused by closed-cycle OTEC if intermediate chemicals leak into ocean.
viii. OTEC plant can alter and may damage the surrounding ecosystem.
ix. Pumps handling large amounts of water have to be developed.
x. A 100 MW OTEC power plant may require 1 km long and 30 m diameter pipe.
xi. The whole plant has to be stationed and moored at large depth.
xii. Construction of floating power plant is difficult.
xiii. Cost of electrical energy generated from open-cycle OTEC is very high.
xiv. The development of the electrical cables required to carry the power to shore needs special care because it is subjected to severe stresses from its own weight and ocean currents and eddies.
xv. An OTEC power plant must be capable of withstanding severe ocean storms over its lifetime, corrosion by seawater salts, erosion due to large volume flows, bio-fouling due to algae growth by various marine lives such as barnacles.
Applications of OTEC Plants:
In addition to generation of electrical energy, open-cycle plants can be used to produce desalinated water, which can be used for irrigation and human consumption. A closed-cycle OTEC plant can also act as a chemical treatment plant. An OTEC plant can also be employed for pumping deep seawater. The cold water then can be used for air-conditioning and refrigeration if it is brought back to shore. In addition, while the sea water surrounding the plant is being moderated, the enclosing area can be used for aquaculture and mariculture.
The power generated in the OTEC plants can be used for producing hydrogen by means of electrolysis of water.