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Today, buzzwords in economics, science and culture include “green,” “global warming,” “recycling,” “sustainable” and “conservation.” With large bodies of water literally disappearing (the Earth Policy Institute reports that more than half of the world's 5 million lakes are endangered), glaciers receding, arctic ice melting and the global population exploding, water is a central theme in discussions and debates about the Earth’s future. Of primary concern: Will we have enough potable water to sustain our civilization?
Enter IOES, the Institute of Ocean Energy, Saga University, in the city of Imari on Kyushu, dedicated to the study of ocean energies. A key part of its activities is Ocean Thermal Energy Conversion (OTEC). “OTEC is trying to solve five major issues modern human societies are currently facing: energy, environment, water, food and population,” says Dr. Yasuyuki Ikegami, an associate professor of engineering at the Institute. “Desalination of ocean water is one area of our research.”
OTEC’s primary function is to convert ocean thermal energy into electric power, but it also works to desalinate water.
“Between northern and southern latitudes of 40 degrees, the surface of the sea is heated by sunlight up to 25ºC to 30ºC (77ºF to 86ºF),” Dr. Ikegami says. “On the other hand, the sea temperatures at 600 and 1,000 meters (2,000 and 3,300 feet) deep are around 5ºC to 10ºC (41ºF to 50ºF). These temperature differences are used to generate electricity by OTEC and the same technology can be employed for desalination.”
The OTEC desalination system was designed with the environment in mind. The OTEC’s basic design does not use fossil fuels to generate electricity, so it has no energy and pollution issues. In fact, it’s a “green” role model. It produces clean energy, creates vast amounts of potable water and does not harm the environment.
OTEC and Desalination: How It Works
In the OTEC system, liquid ammonia, which has a low boiling point of 33ºC (92ºF), circulates in pipes. The liquid ammonia changes into gas in evaporators, where it’s heated by warm, surface seawater (25ºC to 30ºC or 77ºF to 86ºF). This gaseous ammonia rotates turbines that generate electricity before it changes back to liquid, cooled by cold water (5ºC to 10ºC or 41ºF to 50ºF) pumped up from the deep sea.
In the desalination system, the warm seawater from the surface changes into steam as soon as it enters into a depressed flash chamber. The steam is sent to a condenser, where it's transformed into potable water by the cold seawater. Research and experiments are underway to make both systems workable in combination operations.
The desalination barge off Chennai produces 1,000 tons/day of drinkable water.
(Courtesy of NIOT)
Desalination is yielding substantial results for communities struggling with critical water-supply issues. One case study is in India, the world’s second most populous country. Its National Institute of Ocean Technology (NIOT) approached Saga University in 1997 to collaborate on an OTEC system. “In March 2005,” Dr. Ikegami says, “a low-temperature thermal desalination (LTTD) system was set up on Kavarotti, one of 36 islands in the Lakshadweep chain off the subcontinent’s southwestern coast; since then it has been providing 100 tons of drinking water every day.”
The success at Kavarotti inspired NIOT to set up an experimental barge off the coast of Chennai in the Bay of Bengal in April 2007. During a two-week period, the system produced 1,000 tons of potable water each day. Following this achievement, NIOT is planning a new operation to expand capacity to 10,000 tons per day, with the local government stating that it wants this to be the primary source for the public water supply. Furthermore, it is planned that NIOT will shift from using diesel fuel for power generation and instead utilize OTEC’s environmentally friendly system.
“With the success at Chennai, the desalination business has really taken off,” Dr. Ikegami notes. “As the number of facilities grows in India, people will start recognizing the benefits; the presence and importance of desalination will be widely acknowledged.”
