Ocean Thermal Energy Conversion (OTEC) is one of the two primary types of ocean energy, It utilizes the temperature gradient from ocean surface to deeper waters in order to convert heat energy to electricity. IEA-OES 2006 Policy Report has shown that OTEC resources have the ability to produce 10000 TWh per year, which is greater than other types of ocean energy such as tides, marine currents and salinity gradient. OTEC functions best when there is a temperature difference of 36° Fahrenheit (20°C). Most of the good resources for this technology are located around the equator. Hawaii, the Philippines and some parts of the Atlantic ocean have good resources.
OTEC uses two different technologies, which are the open loop cycle technology and the closed loop cycle technology.
Open loop cycle technology uses the warm surface ocean water placed in pressurized receptacle causing it to boil and then the steam turns a turbine that is connected to a generator, which is used to generate electricity. The used steam is almost fresh water and it is condensed with the cold ocean water. Open cycle plants must be located on land within 3 km access to cold water and these plants are small plants from 1-10 MW due to the limit of the low-pressure turbine at 2.5 MW. Natural Energy Laboratory (NEL) constructed and operated a 210 kW plant from 1992-1998.
Closed loop cycle technology uses the warm surface ocean water to heat a working fluid that has a low boilint point. Ammonia is a common working fluid for this process. NEL commissioned a successful closed system in 1979 on a ship off Hawaii but the system had problems with metals utilized in heat exchanger reacting with ammonia. Utilizing other metals increased cost of project significantly.
OTEC uses clean, abundant environmentally friendly renewable sources and OTEC plants have very low carbon emissions. It produces fresh water and it generates electricity. In island areas such as Hawaii, the Philippines, fresh water is limited and OTEC can help these areas by providing fresh water. The cold deep seawater is rich in nutrients and therefore it can be used for aquaculture.
OTEC also has some disadvantages and the major disadvantage is the cost of electricity generation. Electricity from OTEC cost more than electricity from fossil fuels. OTEC's performance can be affected by seasonal variations as well as by natural calamities. OTEC is usable only if the deep water is fairly close to shore-based OTEC plants because transmission and distribution losses make systems less efficient. Environmental impact is also concerning because the construction of plants and laying of pipes may cause damage to reefs and marine ecosystems.