Harvesting energy from the ocean

Ocean thermal energy conversion, OTEC, is the process of using the heat differential between tropical surface waters of the ocean and the cool waters found at depth.

Ocean thermal energy conversion, OTEC, is the process of using the heat differential between tropical surface waters of the ocean and the cool waters found at depth.

The use of seawater’s temperature difference was first proposed by Jacques Arsene d’Arsonval in 1881. It was not until 1930, however, when one of his students, Georges Claude actually managed to get a plant operational at Matanzas, Cuba.

The process works by flash generating steam to drive a low pressure turbine and generate electricity. There are three basic types of systems for accomplishing this feat. The first is by converting the warm salt water into low pressure steam in a vacuum chamber, a process referred to as an open system. A closed loop system uses a heat exchanger to transfer water temperature to a working fluid such as ammonia, which then flashes into vapour used to drive the turbine. The third hybrid method flashes the sea water into steam and then the steam is used to flash the ammonia into vapour.

Producing electricity is not the only creation of an ocean thermal energy conversion system. As sea water is flashed into steam in the evaporator tank, all the salt and minerals it contains are left behind. Once the steam has been used to power the turbine, it is then condensed back to effectively desalinated fresh water.

Cooling is also an advantage of an OTEC plant, both for the air conditioning of buildings and the cooling of agricultural land, which allows for sub-tropical crops to be grown in tropical latitudes. The Natural Energy Laboratory of Hawaii Authority (NELHA) has researched the process and found 100 crops that do very well when grown with soil cooling technology.

Aquaculture also can benefit from ocean thermal energy conversion. Salmon, abalone clams, oysters, steelhead, lobster — all can be efficaciously grown, in pools supplied with the nutrient rich cold water pumped from a kilometre down.

Hydrogen production is also viable using OTEC technologies. Using electrolysis to convert the fresh water into hydrogen can provide hydrocarbon-poor countries with fuel for running industry and transportation.

Mineral extraction from sea water has been investigated by the Japanese. There are about 57 trace minerals in sea water and the viability of mining them is increasing with research. OTEC can provide a continuous source of recovered minerals as it operates.

Countries like Japan, India, the U.S. and China are all working on OTEC projects. Tokyo Electric Power Company’s first OTEC plant became operational in 1981. India has been working on developing and commissioning a 1MW closed loop plant since 1984. Hawaii has been working on developing various prototype ocean thermal energy conversion plants since 1974. In April of this year, China, through a company based in Hong Kong, the Reignwood Group, in co-operation with Lockheed Martin, announced the development of the world’s largest OTEC pilot power plant to date at 10 megawatts.

Ocean thermal energy conversion is a long explored method of harvesting the sun’s energy that bodes well for the future.

Lorne Oja is an energy consultant, power engineer and a partner in a company that installs solar panels, wind turbines and energy control products in Central Alberta. He built his first off-grid home in 2003. His column appears every second Friday in the Advocate. Contact him at: lorne@solartechnical.ca.

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