Untitled Document


The practical use of Ocean Thermal Energy Conversion was first presented in 1979 by the state of Hawaii and a group of American companies; the plant only produced a net power gain of 18 kW and was too small to be commercially applicable, but the viability and practicality of the OTEC system was proven with this system. The system was used in equatorial waters because of the large (roughly 20ºC difference between temperatures of 25ºC and 8ºC waters at the water surface and 1000 meters below the sea level) temperature gradients when compared to more moderate climates. Most of the plants that have been constructed have been based on the Rankine cycle, some employing an open cycle while most used a closed cycle with anhydrous ammonia used as the working fluid of the system. While both the closed and open cycles have comparable efficiencies and power outputs, we will only focus our research on the closed cycle due to the fact that this was the type of cycle specified by our industry partner – Lockheed Martin. The closed cycle uses a pump to pressurize the anhydrous ammonia at liquid state and then utilizes the warm top level sea water the boil the high pressure fluid, this high pressure and high temperature vapor powers a turbine-generator to produce useful electrical power; after the vapor passes through the turbine where it drops in pressure and temperature, it is cooled and condensed in the heat exchanger that is cooled by the cold lower level sea water. The working fluid is then re-circulated through the pump again to repeat the cycle. The maximum physical efficiency possible from the Rankine cycle that operates between hot and cold reservoirs of 26ºC and 5ºC is approximately 8%, but due to irreversible processes, frictional losses from the working fluid, and heat loss and gain of the system, most systems in actuality only have efficiencies of 3% to 4%. These numbers are low compared to other conventional modern power plants that are based on the Rankine cycle, but the energy within the ocean is constantly replenished by the radiation from the sun, therefore we are basically getting energy from an energy resource that has previously been unused. The measured efficiency of a thermodynamic cycle is simply the ratio between the power output of the system and the heat input to the system. It is therefore not logical to use the overall cycle efficiency to characterize the performance of an OTEC system because the heat input is constantly replenished from the sun’s radiation that is absorbed by ocean water. Unlike fossil fuels or coal used in most modern power plants, the source of heat input to an OTEC cycle is entirely renewable and monetarily free of charge. The characterization of the performance within an OTEC cycle would be more aptly described by measuring the back work ratio of the Rankine Cycle that it employs. This is simply the ratio between the work input from the system pump compared to the power output from the turbine. The change in work caused by a pressure change in a fluid or gas is characterized by the equation: from this is can be seen that the work is inversely proportional to the density of the fluid, therefore given a certain processes that takes a working fluid through the same pressure differential the change in work will be much greater for a gas than for a liquid. Due to this property the work input to the pump pressurizing the condensed working fluid is far less than the work output from the turbine that expands the working fluid through the same pressure differential. There is enough energy within the ocean’s waters to provide the world power for all of their power needs; this energy source is also constantly replenished and therefore provides a viable and powerful option to enhance our ability to create renewable energy that has a minimal impact on the natural environment.

Copyright © 2008 by "Lockheed Martin - Team 17 " · All Rights reserved · E-Mail: cnt03c@fsu.edu

The practical use of Ocean Thermal Energy Conversion was first presented in 1979 by the state of Hawaii and a group of American companies; the plant only produced a net power gain of 18 kW and was too small to be commercially applicable, but the viability and practicality of the OTEC system was proven with this system. The system was used in equatorial waters because of the large (roughly 20ºC difference between temperatures of 25ºC and 8ºC waters at the water surface and 1000 meters below the sea level) temperature gradients when compared to more moderate climates. Most of the plants that have been constructed have been based on the Rankine cycle, some employing an open cycle while most used a closed cycle with anhydrous ammonia used as the working fluid of the system. While both the closed and open cycles have comparable efficiencies and power outputs, we will only focus our research on the closed cycle due to the fact that this was the type of cycle specified by our industry partner – Lockheed Martin. The closed cycle uses a pump to pressurize the anhydrous ammonia at liquid state and then utilizes the warm top level sea water the boil the high pressure fluid, this high pressure and high temperature vapor powers a turbine-generator to produce useful electrical power; after the vapor passes through the turbine where it drops in pressure and temperature, it is cooled and condensed in the heat exchanger that is cooled by the cold lower level sea water. The working fluid is then re-circulated through the pump again to repeat the cycle.
            The maximum physical efficiency possible from the Rankine cycle that operates between hot and cold reservoirs of 26ºC and 5ºC is approximately 8%, but due to irreversible processes, frictional losses from the working fluid, and heat loss and gain of the system, most systems in actuality only have efficiencies of 3% to 4%. These numbers are low compared to other conventional modern power plants that are based on the Rankine cycle, but the energy within the ocean is constantly replenished by the radiation from the sun, therefore we are basically getting energy from an energy resource that has previously been unused. The measured efficiency of a thermodynamic cycle is simply the ratio between the power output of the system and the heat input to the system.

It is therefore not logical to use the overall cycle efficiency to characterize the performance of an OTEC system because the heat input is constantly replenished from the sun’s radiation that is absorbed by ocean water. Unlike fossil fuels or coal used in most modern power plants, the source of heat input to an OTEC cycle is entirely renewable and monetarily free of charge.
            The characterization of the performance within an OTEC cycle would be more aptly described by measuring the back work ratio of the Rankine Cycle that it employs.

This is simply the ratio between the work input from the system pump compared to the power output from the turbine. The change in work caused by a pressure change in a fluid or gas is characterized by the equation:

from this is can be seen that the work is inversely proportional to the density of the fluid, therefore given a certain processes that takes a working fluid through the same pressure differential the change in work will be much greater for a gas than for a liquid. Due to this property the work input to the pump pressurizing the condensed working fluid is far less than the work output from the turbine that expands the working fluid through the same pressure differential.
            There is enough energy within the ocean’s waters to provide the world power for all of their power needs; this energy source is also constantly replenished and therefore provides a viable and powerful option to enhance our ability to create renewable energy that has a minimal impact on the natural environment.