The oceans and rivers cover about 70% of the earth’s surface which receives, store and dissipates the vast amount of thermal energy. Thus they act as huge reservoirs of thermal energy. The temperature gradient across the depth of sea can be used to generate electrical power. It is known as ocean thermal energy conversion system.
Since the temperature difference is very low, the efficiency of energy conversion in such plants is very low, associated with high capital cost. These plants can operate indefinitely and their best location is in the tropical belt of 20oN and 20oS latitude.
When solar radiations fall on the sea water, the most of solar heat absorption takes place at the surface just below the water. About 95% heat energy takes place within 6 m depth.
Since warm water is lighter compared to cold water as no convection currents are set and the deep water in the ocean remains cool.
In tropical regions, this temperature gradient across the depth of sea becomes very significant. Here the temperature of seawater at its surface is on average of 25oC and at the depth of 1000 m is about 5oC. Therefore, this temperature gradient can be utilized to run a heat engine coupled to a generator to generate electrical power.
In this system, the warm water layer of sea acts as a reservoir of heat source and cool water at the depth act as a reservoir of heat sink.
There are two basic types of ocean thermal energy conversion (OTEC) systems which are as under:
Open Cycle Ocean Thermal Energy Conversion System
- In this system, the warm water from the ocean surface is fed to the deaerator. It removes the dissolved non-condensable gases from water and fed it to the evaporator.
- The flash evaporator is maintained under high vacuum. As a result, a low-pressure steam is produced due to throttling effect and the residue liquid is released back to the ocean at high depth.
- In the next step, this low-pressure steam is fed to the turbine where it expands and rotates the turbine. The mechanical power so developed is converted into electrical power by the generator.
- The exhaust steam from the turbine is released into a direct contact type heat exchanger where it mixes with the cold water drawn from the ocean depth. It condenses the exhaust steam and the mixture is discharged into the ocean.
If a surface condenser is employed instead of the direct contact type condenser, we can get the condensate as desalinated water.
Closed Cycle Ocean Thermal Energy Conversion System
In this system, working fluids having a low boiling point like ammonia, Freon-12, butane gas are used for heat engine because the working temperatures of sea water are small. The schematic diagram of a Closed OTEC power plant is shown in Figure.
- The warm water from the surface of the sea is circulated through a heat exchanger with help of a pump.
- In the heat exchanger, the heat of seawater is absorbed by Freon and Freon vapors are generated at high pressure.
- This vapor expands in the turbine to drive it and a coupled generator with it.
- The Freon vapor from the turbine condensed in a condenser with the help of cold water.
- The Freon condensate is pumped again into the heat exchanger and the complete cycle is repeated.
The overall efficiency of such a plant is very low in the range of 2 to 3% only.
Ocean Thermal Energy Conversion Advantages and Disadvantages
- It is a clean and pollution free renewable source of energy.
- Power generated by this system is continuous. Only a small variation in output takes place from season to season. In simple words we can say, the system is independent of weather.
- By making only small changes in design, conventional power plants can be used in this system.
- It can produce the desalinated water and nutrients for mariculture at the same time.
- The capital cost is very high.
- The overall efficiency of the system is very low.
- The open cycle system needs very large sized turbine due to the low pressure of steam.
- In the closed cycle system, the working fluid is very expensive.
- The cost of power generation per kWH is very high.
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