Ocean Thermal Energy Conversion (OTEC)

The greatest source of terrestrial energy is the sun and its greatest terrestrial storehouse is the ocean. The downside of this thermal buildup is thermal expansion of the oceans and the melting of polar icecaps.

Robert Stewart, of Texas A&M University points out: “Since the mid 1950s eighteen times more heat has been stored in the ocean due to global warming than has been stored in the atmosphere.”

Ocean Thermal Energy Conversion (OTEC) is a method for generating electricity, which uses the temperature difference that exists between deep ocean water 70, typically at 5oC and shallow ocean waters 71, typically about 15oC, but as high as 24oC in equatorial regions, where the largest deserts are found, to run a heat engine 72. The working fluid of the system is a low-boiling-point fluid such as ammonia 73 or or 1,1,1,2-Tetrafluoroethane, which is vaporized by the warm water 71, with the vapour driving the heat engine 72, which in turn drives a dynamo to produce electrical energy and the cold shallow water 71 then condenses the exhausted low-boiling-point fluid 73 in a condenser 74.

One aspect of the current invention would generate power using the principle of OTEC as currently practiced by the Natural Energy Laboratory of Hawaii Authority. The current invention uses the OTEC process to extract a portion of heat from the ocean that would otherwise induce thermal expansion of the ocean leading to sea level rise.

The idea for OTEC dates back to 1881 when the French Engineer, Jacques D'Arsonval first conceived of generating power utilizing the temperature differential between warm surface water and colder waters from the deep.

As with any heat engine, the greatest efficiency and power is produced with the largest temperature difference. OTEC works best when the temperature difference between the warmer, top layer of the ocean and the colder, deep ocean water 70 is about 20°C. These conditions exist in tropical coastal areas, roughly between the Tropic of Capricorn and the Tropic of Cancer where the hot deserts of the world are located.

The Earth is hit with 165,000 terawatts (TW) of solar power every moment of every day. The ocean absorbs part of this energy causing thermal expansion and sea level rise. Effectively the world’s oceans are acting like thermal batteries that are overcharging storing a potential to seriously harm low lying coastal regions and their inhabitants.

To give 10 billion people, as is the projected population by the year 2050, the level of energy prosperity the developed world is used to, a couple of kilowatt-hours per person, an additional 60 TW of power needs to be generated around the planet. The overcharging oceans are an available source of this energy shortfall.

An interesting parallel has been drawn by R Cohen from the US Dept of Energy :- " a simple comparison may lend some insight into the energy supply available from OTEC. A temperature difference is analogous to a hydraulic head; each degree centigrade corresponds to 427 m of head. Thus, a temperature difference of 21°C would correspond to having the water at a height of 8967 m if 100% conversion efficiency were attainable... Thus, a net 2.5% conversion efficiency would enable a height of 224 m to be attained... It is as if much of the world's ocean water were captured behind invisible "thermal dams" of significant heights."

The technology for producing energy by the process of OTEC is well known in the industry and does not form a part of this inventive concept. It is an objective of the current invention however to use OTEC to extract a portion of heat from the oceans that would otherwise induce thermal expansion and sea level rise.

The Energy Island Group is a partnership of experts in marine architecture and engineering, infrastructure, project design and management, applied to all forms of energies available at sea: wind, wave and solar, with a particular interest in Ocean Thermal Energy Conversion. Energy Island envisions precisely the infrastructure required to implement GWMM. Schematics of the concept and a representation of the Energy Island follow.