Ocean thermal energy conversion (OTEC) is an energy technology that makes use of the ocean’s temperature difference between the warm surface water and the cold deeper water to drive a turbine connected to a generator to produce electricity.
OTEC systems are operated based on the principle that the temperature difference of the fluid can be used to generate an increase in pressure which is in turn applied for generating mechanical work.
OTEC works best when the difference in temperature between the warm top layer and cold deep layer of the sea is around 20 °C (36 °F). This condition commonly exists in tropical coastal areas.
OTEC plants require an expensive, large-diameter intake pipe submerged to more than a mile into the ocean to bring cold water to the surface. Cold water obtained as a by-product from OTEC can be used for air conditioning and refrigeration.
Some energy experts suggest that OTEC can be used to produce billions of watts of electric energy, if it becomes cost-effective when compared to other conventional power technologies.
Ocean Thermal Energy Conversion Technologies
There are basically three types of OTEC systems:
Closed-cycle systems use working fluids with low boiling point, such as ammonia for rotating a turbine to produce electricity. Warm surface seawater is pumped via a heat exchanger where vaporization of the low-boiling-point fluid takes place.
The turbo-generator is rotated upon the expansion of vapor. Cold deep seawater is, in turn, pumped via a second heat exchanger where the vapor is condensed back into a liquid recycled through the system.
The Natural Energy Laboratory along with several private-sector partners carried out the mini OTEC experiment in 1979 which successfully achieved the production of net electrical power at sea with the help of closed-cycle OTEC.
In 1999, the Natural Energy Laboratory experimented with a 250-kW pilot OTEC closed-cycle plant, which is the largest of its kind.
Open-cycle systems make use of warm surface water of the tropical ocean to generate electricity. The expanding steam formed due to the boiling of warm seawater in a low-pressure container drives a low-pressure turbine connected to an electrical generator. The steam is then condensed back into a liquid upon exposure to cold temperatures from deep-sea water.
The Solar Energy Research Institute developed a vertical-spout evaporator in 1984 for the conversion of warm seawater into low-pressure steam using open- cycle plants and achieved 97% of energy conversion efficiencies.
An open-cycle OTEC plant developed at Keahole Point, Hawaii, in 1993 generated 50,000 W of electricity during a net power-producing experiment.
Hybrid system is a combination of open- and closed-cycle systems. In this system, warm seawater is flash-evaporated into steam in a vacuum chamber like the open-cycle evaporation process. The steam is then vaporized into a low-boiling-point fluid that in turn drives a turbine for producing electricity.
Benefits of OTEC
The key benefits of OTEC include the following:
- OTEC systems produce fresh water and electricity as well, which is highly beneficial for island regions where fresh water availability is limited.
- It makes use of renewable, clean, natural resources. Fossil fuels are replaced by warm surface seawater and cold water from deep sea to generate electricity.
- OTEC plants do not pollute the environment by releasing carbon dioxide emissions or other polluting substances.
- It helps reduce the country’s dependence on imported fossil fuels.
Disadvantages of OTEC
Some of the major drawbacks of OTEC include the following:
- Construction of OTEC plants and pipes in ocean may cause damage to onshore marine ecosystems and reefs.
- As this technology has been tested only in small-scale, it is not feasible for an energy company to invest in this project.
- Electricity produced from OTEC would currently cost more than that produced from fossil fuels.
- Discharging of cold and warm sea water needs to be carried out several metres away from the shore to avoid any dwelling impact on marine ecosystems.
- Energy required to pump the sea water from depths may be huge, which otherwise need a diesel generator.
Power generated from OTEC technology will be inexpensive when compared to the unit cost of power from other plants like hydro, diesel and wave. As a result, several proposals for the development of OTEC in Pacific island countries have been submitted to the investors for consideration.
Therefore, the OTEC technology is the best solution to meet the world’s increasing energy demands thereby reducing the need for importing petroleum products.
Some of the areas related to the OTEC technology currently being considered in research include the development of new materials from cold water pipe and the improvement of the heat transfer coefficient for heat exchanges for a period of time.
However, the OTEC researchers believe that involvement of the private sector firms is very limited owing to the need of enormous initial investment. Another major factor that hinders the OTEC plant development is the limited availability of land-based sites in the tropical regions where deep-sea water is close enough to the shore to make the operation of the plant feasible.
Upon resolving these issues related to the OTEC development, the technology could easily change the face of energy consumption by causing a shift from fossil fuels.