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OTEC- Ocean Thermal Energy Conversion

OTEC or Ocean Thermal Energy Conversion is a method by which the thermal energy difference between the deep and shallow ocean currents can be converted into useful energy. Usually, this energy is converted into electrical energy. This is a renewable form of energy and can generate up to 88,000 WTh/yr without disturbing the ocean's thermal structure. This process has a high capacity factor. This means that the ratio of the power it generates to the maximum amount of power it is capable of generating is very high.

Currently, the most prominent pilot OTEC plant is located in Japan, situated in Okinawa. It is overseen by Saga University.

History

The first attempts at making the thermal energy of oceans useful for human use were made in the 1880s by Jacques Arsene d'Arsonval. His student, George Claude, was successful in making an OTEC plant in Matanzas in Cuba. The plant was completed in 1930. It produced 22 kW of power by using a low-pressure turbine. Storms destroyed the plant. In 1935, Claude again tried in building an OTEC plant in Brazil this time. But it was again destroyed before it could even start working. Closed cycle OTEC plants were invented around 1963 by J. Hilbert Anderson and his son. Starting in 1970, Japan began researching OTEC technologies. Tokyo Electric Power Company built the first such plant in Japan on an island called Nauru. Japan soon became a leader in technology. In 1994, Saga university built an OTEC plant in Okinawa. This used the Uehara cycle, which was more efficient than previous processes.

Efficiency

In thermodynamics, heat engines give maximum efficiency when the temperature difference is large. Therefore, for the OTEC technology to work, it must operate in a place where large temperature differences are available. The maximum temperature difference between the deep ocean and the surface ocean can be found in the tropics. Therefore, if the OTEC technology is properly deployed in tropical regions, it has the potential to outperform other ocean energy technologies by 10-100 times (like wave energy).

The biggest issue that OTEC technology faces is to achieve maximum efficiency with small temperature differences. Earlier systems had an efficiency of close to 1-3 percent. The maximum possible efficiency that could be achieved with those systems was approximately 6-7 percent. Modern OTEC systems are capable of achieving the maximum Carnot efficiency.

Power Cycle

OTEC technology uses several different power cycles to achieve energy conversion. In all of the schemes, cold seawater plays a very important role. The seawater has to be brought to the surface by some means. This can be done by processes like active pumping or desalination. Desalinating cold water near the ocean floor decreases its density and causes it to rise upwards.

Some prominent water cycles are:

  • Closed Cycle: In the closed cycle, the seawater vaporises and condenses a working fluid (usually ammonia) to work in a turbine. Ammonia has a low boiling point. The warm seawater is pumped near the ammonia chamber. This causes ammonia to vaporise through heat exchange. The vaporised ammonia is then used to power a turbine which is connected to an electric generator. Then cold seawater is pumped to condense the vaporised ammonia. This condensed ammonia is then recycled through the system to be vaporised again. And the cycle continues like this.
  • Open Cycle: Open cycle uses the warm seawater directly. The warm seawater, which is at a higher temperature, is pumped into a vacuum chamber. Due to very low pressure in the vacuum chamber, the warm seawater is vaporised. This vaporised seawater is then used to power a low-pressure turbine which is connected to an electric generator. The vaporised seawater has turned into freshwater. This freshwater is condensed by using heat exchange with the cold ocean water. This freshwater can be used for drinking, agriculture and other such activities.
  • Hybrid: The hybrid cycle combines both the closed cycle and the open cycle. In this, warm seawater is pumped into a vacuum chamber-- just like the open cycle. This vaporised seawater is then, in turn, used to vaporise a working fluid like ammonia. This working fluid is connected to a separate closed cycle. The vaporised ammonia is used to turn a turbine and generate electricity. The vaporised seawater is converted into freshwater by exposing it to cold ocean water.

The most popular choice of working fluid is ammonia. This is because ammonia is easily transportable. It is easily available. It also costs very little. There is a danger associated with ammonia. It is toxic and flammable. Other fluorinated carbons like CFCs and HCHCs are not toxic, but they are detrimental to the Earth's ozone layer.

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