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Ocean Thermal Energy Conversion (OTEC)
Covering over 70% of the planet’s area, the Earth’s oceans could potentially be utilized as a source of virtually inexhaustible renewable energy. Ocean Thermal Energy Conversion (OTEC) is a method that employs naturally occurring temperature differences between warm surface water and colder deep seawater. To be effective a minimum temperature difference between the ocean surface layers is 20°C . These temperature gradients exist primarily in specific tropical regions near the equator originally proposed by French Engineer Jacques Arsene d’Arsonval in 1881, OTEC is not a new technology. Since then many advancements have been made in the development of this technology. The three most common OTEC systems are: open-cycle, closed-cycle and hybrid cycle, all requiring a working fluid, condenser and evaporator within the system. These three systems all employ the thermodynamics of a working heat exchanger and use the temperature differences naturally occurring in the ocean as the driving force. Concerns with efficiency losses due to befouling, system power requirements and heat exchanging systems have lead to exploration through case studies and analysis. While OTEC systems have been studied since 1881 there have been few full-scale implementations. There are still, however, a number of studies being conducted, especially in Japan, regarding the implementation of this renewable large scale technology
History
The first known Ocean Thermal Energy Conversion (OTEC) system was proposed by a French Engineer Jacques Arsened’Arsonval, in 1881 (Takahashi and Trenka, 1996). Recognizing the tropical oceans as a potential source of energy, through the natural temperature differences between the ocean’s surface water and deep water, D’Arsonval built a closed-cycle OTEC system, with ammonia as the working fluid that powered an engine (Takahashi and Trenka, 1996). Ammonia was chosen as the best fluid available to accommodate the pressure differences between the two temperatures of water assuming that the temperature of the boiler was 30°C and the condenser was 15°C (Avery and Wu, 1994). The pressure differences in the OTEC system design was one of the challenges D’Arsonval had to overcome. Ammonia was selected because it had such a low boiling point allowing it to become vaporized by the small temperature gradients when by the pumps in the system. In similar cycles where the Rankine cycle is followed there is usually a higher pressure gradient in which to generate energy i.e. combustion driven engines. In the case of OTEC the temperature gradients are maximum 22°C therefore a working fluid that was able to change phases with such as small gradient was chosen. This proposed technology was never tested by d’Arsonval himself.A student of d’Arsonval named George Claude soon took on the challenge of properly designing and building a working OTEC system. Claude, however, took a different approach to the design. He stated that corrosion and bio fouling of the heat exchanger in an OTEC system would be a problem in the closed-cycle design. Claude suggested using the warm seawater itself as the working fluid in an open-cycle, now better known as the Claude cycle (Aver Ougree-Marhaye in Belgium by creating an engine fueled by water temperature differences.
Using the 30°C cooling water from a steel plant as the source for warm water for the boiler (evaporator) and 10°C water from the Meuse River as the condensing fluid, Claude successfully demonstrated the feasibility of the open-cycle concept (Avery and WU, 1994). This water from the steel plant was the cooling water sprayed on the steel during fabrication in order to prevent flaws in the steel when still malleable. In 1930 George Claude designed and built a fully operational closed loop system OTEC power station in Matanzas Bay in Northern Cuba (Takahashi and Trenka, 1996). This power station generated 22 kilowatts (kW), but had a negative energy balance, consuming more power then it produced.