Ocean Thermal Energy
Ocean develops the two types of energy :
- 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 collector. The sun’s heat warms the surface water a lot more than the deep ocean water and this temperature difference produces the thermal energy. The 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.
Ocean thermal energy conversion (OTEC)
The Ocean thermal energy conversion systems can be opened 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 store.
The OTEC systems must have a temperature difference of about 25°C to operate. In the tropical region the temperature difference is small where surface waters are very warm and there is deep cold water, OTEC systems cost high and have low efficiencies.
OTEC uses the temperature difference that exists between deep and shallow waters to run a heat engine. The temperature difference usually increases with decreasing latitude i.e. near the equator in the tropics. The small temperature difference makes energy extraction comparatively difficult and expensive due to low thermal efficiency. The heat cycle used for OTEC is Rankin cycle that uses a low-pressure turbine.
Types of OTEC Power Plants
Power Plants Depending Upon Location
Land-Based power plant
Power plants are constructed near land. Power plants can be installed in sheltered areas so that they are relatively safe from storms and heavy seas. Land-based power plants can also support mariculture. Mariculture tanks or lagoons built on shore allows workers to monitor and control miniature marine environments. One disadvantages of land-based facilities arises from the turbulent wave action in the surf zone.
Shelf Based power plant
To avoid turbulent surf zone as well as to have closer access to the cold water resource, OTEC power plants can be mounted to the continental shelf at depths up to 100 m. A shelf-mounted plant could be built in a shipyard, towed to the site, and fixed to the bottom of the sea. Problems with shelf-mounted plants include the stress of open-sea conditions and more difficult product delivery makes the shelf-mounted plants less attractive.
Floating Power plants
Floating Ocean thermal energy conversion is designed to be operate off-shore. This type of power plant is more difficult to stabilize and the difficulty of mooring it in very deep water may cause problems with power delivery. Cables attached to the floating platforms are more susceptible to damage, especially during storms cables at depth exceeding 1000 m are difficult to maintain and repair.
Floating plants require a stable base for continuous OTEC operation. Major storms and heavy seas can break the vertically suspended cold-water pipe and interrupt the intake of warm water as well. To reduce this problem pipes can be made of relatively flexible polyethylene attached to the bottom of the platform and gimballed with the joints or collars. Pipes may need to be uncoupled from the plant to prevent storm damage. Surface water can be drawn directly into the platform from a warm-water pipe. However, it is necessary to prevent the intake flow from being interrupted during violent motions caused by heavy seas.
If the floating power plant is to be connected to power delivery cables, it requires the plant to remain relatively stationary.
Power Plants Depending Upon the cycle used
The cold seawater is an integral part of each of the three types of OTEC systems: closed-cycle ,open-cycle and hybrid.
Closed-cycle OTEC Power plants
In this system, fluid with a low boiling point, such as ammonia, is used to rotate a turbine coupled to an electric generator. Warm surface seawater is pumped through a heat exchange where the low-boiling fluid is vaporized. The expanding vapor rotates the turbo-generator. Then, cold, deep water pumped through a second heat exchange condenses the vapor back into a liquid, which is then recycled through the system.
For closed-cycle systems,heat exchanger of very large size have to be designed and built and the working fluid is expensive.
Open-cycle OTEC Power plant
In this system, the tropical oceans warm surface is used to produce electricity. When warm seawater is placed in a low-pressure container it boils. The expanding steam drives a low-pressure turbine coupled to an electric generator. The steam, which has left its salt and contaminants behind the low-pressure container, is pure fresh water. From deep-ocean water, it is condensed back into a liquid by exposure to cold temperatures.
This method has the advantages of producing desalinized freshwater, suitable for drinking water or irrigation. For open-cycle, the problems associated with the design of evaporator, operation and maintenance are many.
Hybrid OTEC Power plant
A hybrid cycle combines the features of both the closed-cycle and open-cycle systems. In this system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, similar to the open-cycle process. The steam vaporizes the ammonia working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine coupled to an electric generator producing electricity. The steam condenses within the heat exchanger and provides desalinated water. This method has the advantage of producing electricity that can be supplied to a utility grid or used to produce methanol, hydrogen, refined metals, ammonia, and similar other products.
Advantages of OTEC
- It is an renewable source of energy.
- It becomes more economical, since no fuel is used in this system.
Disadvantages/Limitations of OTEC power plants
- Ocean thermal energy is not relatively clean and produces more pollutants which contributes to global warming.
- Large sized turbines are required because of low pressure(steam turbines capable of generating 10 MW or more using low pressure steam have yet to be developed).
- Due to large size of components size of plant is limited.
- Large size vacuum pumps are required for maintenance of vacuum in the pumps.
- Low temperature differences results in very low plant efficiencies, very large plant size and huge capital cost.
- A 100 MW Ocean thermal energy conversion power plant may require 1 km long and 30 m diameter pipe.
Applications of OTEC Power plants
- It can be used to produced desalinated water which can be used for irrigation and human consumption.
- A closed cycle OTEC plant can also act as a chemical treatment plant.
- It 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.
- It can be used for producing hydrogen by means of electrolysis of water.