In the Atlantic Ocean, warm water from beyond Cape Agulhas joins the Gulf Stream. Heated by the tropical sun it is pushed towards Northern Europe and finds it's way into the Arctic where it is cooled and sinks to the ocean floor. Flowing down past the Americas and into the Antarctic it is pulled into the Indian and Pacific oceans.
Diluted by rains and warmed by equatorial sunshine it once again rises and flows back into the Atlantic for the process to begin once again. This natural conveyor of water is known as thermohaline circulation and it is vital to the stability of climate on our planet. It has been theorized for some time that Global Warming could lead to the shutdown of thermohaline circulation, but could this miraculous natural phenomenon help turn the tide on Climate Change instead?
OTEC (Ocean Thermal Energy Conversion), as it's name suggests, uses the thermal gradient between the Ocean's warm surface water and it's cold deep water, maintained by thermohaline circulation, to generate electrical energy. OTEC systems can be categorized into two basic types: closed-cycle and open-cycle.
Closed-cycle OTEC systems utilize a secondary fluid (working fluid) with a lower boiling point than water, such as propane or ammonia. The warm surface water is pumped into a heat exchanger (evaporator) where it is used to boil the secondary liquid into a vapor.
As the vapor expands it is channeled through a turbine to generate electricity before being passed into a second heat exchanger (condenser) where cold seawater, pumped from deep in the Ocean, condenses it back into liquid form ready to be used again.
Open-cycle OTEC systems utilize the principle that a liquid in a low pressure environment has a lower boiling point. The warm ocean surface water is transferred into a low-pressure chamber (evaporator) where it boils into water vapor leaving salt and other impurities behind.
This vapor is then passed through a low-pressure turbine to generate electricity before being transferred into a heat exchanger (condenser) where it is cooled by deep ocean water and condenses into liquid once more. The water produced is almost as pure in quality as fresh water.
Hybridized versions of the technology also exist which utilize features of both closed-cycle and open-cycle OTEC systems. In a hybrid system the warm surface water is pumped into a vacuum chamber where it vaporises, as in an open-cycle system. This water vapor is then used to boil a secondary fluid with a lower boiling point, as with closed-cycle systems. The vapor from the secondary fluid then drives a turbine to generate electricity before being recycled.
OTEC is not a new concept by any means. It was first proposed by the French physicist Jacques-Arsène d'Arsonval in 1881 and the first OTEC plant was built by Georges Claude, the inventor of the neon light and a student of d'Arsonval, off Cuba in 1930. This first attempt proved that the concept worked. The plant produced 22 kilowatts of electricity, but was destroyed by weather before it could reach it's full potential. The destruction of a similar plant off Brazil resulted in plans to develop the technology further being shelved at that time. In 1956 French engineers and scientists began construction of a 3MW OTEC plant off the coast of West Africa. Construction costs proved too expensive though, and the plant was never finished.
Despite the obstacles which beset it's early development, interest in the technology remained steady and in 1974 the Natural Energy Laboratory of Hawai'i Authority (NELHA) began OTEC research. The first closed-cycle OTEC plant to generate net power (the amount of power generated after subtracting power put into the plant,) was deployed in the waters near NELHA in 1979. In 1984 the U.S. National Renewable Energy Laboratory (then called the Solar Energy Research Institute) developed an evaporator for an open-cycle OTEC plant with a staggering 97% energy conversion efficiency. In 2006 the world-record for solar to electrical conversion was only 40%.
In 1999 a 250kW closed-cycle OTEC plant was deployed by NELHA, which remains one of the leading research centers for the technology to this day. It came as no surprise then that in November 2008 Hawaiian Governor Linda Lingle announced a joint venture between the Taiwan Industrial Technology Research Institute (ITRI) and the Lockheed Martin Corporation to construct a 10MW OTEC plant off Hawai'i.
"As island economies in the Pacific, Taiwan and the State of Hawai‘i share very similar challenges of over dependence on imported petroleum for their energy needs," Governor Lingle said.
Indeed Hawai'i imports 94% of it's energy in the form of fossil fuels. Taiwan fairs even worse with over 99% of it's energy requirements being derived from fossil fuels.
The irony is that these island economies lie in the midst of one of the largest stores of renewable energy in existence, the ocean. According to the U.S. National Renewable Energy Authority: "On an average day, 60 million square kilometers (23 million square miles) of tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil." They go on to make the point that if "less than one-tenth of one percent" of this energy was to be converted to electricity it could provide more than 20 times the electricity requirements of the United states alone on a daily basis.
Electrical energy generation, despite being the obvious application of OTEC technology, is not it's only benefit. The cold deep ocean water can be used as a coolant in air conditioning systems instead of simply pouring it back into the ocean after use in an OTEC system. In fact such a system has been in operation in NELHA for some time. Deep ocean water is rich in nutrients and could be used to cultivate marine organisms in Aquaculture farms.
Chilled-soil agriculture can also benefit from OTEC. Spent deep ocean water from the OTEC system flows through pipes embedded in the ground. This cools the surrounding soil and allows plants that would normally wither in subtropical climates to thrive. NELHA point out that they maintain a chilled-soil demonstration garden taking spent deep ocean water from their OTEC facility to cultivate more than 100 different fruits and vegetables. In addition, open or hybrid-cycle OTEC plants could potentially produce vast amounts of desalinated water from sea water. The U.S. Department of Energy estimate that "an OTEC plant that generates 2-MW of net electricity could produce about 4,300 cubic meters (14,118.3 cubic feet) of desalinated water each day."
With so many additional benefits from OTEC technology what now stands in the way of large scale implementation. Well, as with almost everything, location is very important, both to maximize energy output and minimize any environmental impact. The U.S. National Renewable Energy Laboratory say that offshore locations where the temperature of surface water differs from that of deep water by just 20°C could produce significant amounts of energy. While U.S. Department of Energy maintain that appropriate spacing of OTEC plants in the tropical oceans can nearly eliminate any negative impact on marine life.
The Department of Energy points out that OTEC plants are expensive and indeed this is true, but are they really any more expensive than oil rigs or nuclear power plants. The reality is that fossil fuels are running out and nuclear power is not a sustainable alternative. As oil becomes an even more scarce commodity energy producers are starting to realize that renewable energy from systems such as OTEC is the only real alternative. Apart from the 10MW plant planned for Hawai'i an OTEC demonstrator is planned for Réunion Island and even the U.S. Navy is planning to use OTEC at facilities at Diego Garcia, Hawaii and Guam.
Other related features on Celsias:
Turning the Tide: New Energy Technology
Hydropower Roundtable: New Growth for a Mature Industry
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