Power from the sea a step closer

Jun Sugimori / Yomiuri Shimbun Staff Writer

Haruo Uehara speaks about ocean thermal energy conversion.

Technology developed in Japan is now able to generate electricity and produce fresh water from seawater more efficiently and at a lower cost than before, edging the technology closer to practical use.

Ocean thermal energy conversion (OTEC)--although currently cost-ineffective--is expected to be not only a source of renewable energy, but a way to collect lithium from the sea.

Former Saga University President Haruo Uehara, a pioneer in the OTEC field, has created the Uehara cycle, a technological discovery that may have opened the door to practical use of OTEC.

The Uehara cycle generates energy by making use of the difference in temperature between deep and shallow seawater. Warm, surface seawater about 25 C is used to vaporize a fluid with a low boiling point, often ammonia, which turns a turbine to generate electricity. Then, cold seawater--about 5 C--from 800 meters below the surface is used to condense the vapor back into a liquid.

The system was conceived more than 130 years ago by a French scientist. Since then, a Japanese electricity company has succeeded in generating power using OTEC on a small scale, but has found it difficult to commercialize due to inefficiency.

Uehara, 70, has researched OTEC since 1973.

"There're places in the world with no water or electricity. I wanted to do something good for other people," Uehara said.

After leaving the university in 2005, Uehara founded the nonprofit Organization for Promotion of Ocean Thermal Energy Conversion to further pursue his studies. The organization has collaborated with venture firm Xenesys Inc., hoping to improve the system's efficiency.

Kiminao Satomi, president of a company that manufactures equipment and facilities for sake breweries, founded Xenesys to help make the technology practical after being moved by Uehara's passion.

In 1994, Uehara's group developed the Uehara cycle, which is 50 percent to 70 percent more efficient than previous systems. The cycle makes use of a mixture of ammonia and water to control the evaporation temperature, raising thermal efficiency higher than when pure ammonia is used. The improvement in efficiency was confirmed by a test at a 30-kilowatt OTEC research plant at Saga University.

"OTEC is a perfect technology for resource-limited island nations like Japan. The system can provide a stable supply of power, unlike solar- and wind-powered electricity that are often affected by the weather," Uehara said.

In February, the Uehara cycle was chosen for use in a 10,000-kilowatt OTEC plant to be built in the South Pacific island of Tahiti. According to an Xenesys estimate, the cost of power generation will be about 15 yen to 20 yen per kilowatt, about the same as wind-powered electricity.

Deep seawater used in OTEC can also produce desalinized water and eventually hydrogen. Desalinized water is made when surface seawater is vaporized after its boiling point is lowered by reducing atmospheric pressure. The steam is then condensed into pure water by using deep, cold seawater. Desalinized water can be made into hydrogen by electrolysis.

It might also be possible to collect lithium, a material used in batteries, while deep seawater can be used to enrich fishing grounds with nutrients.

Sometime this fiscal year, Kumejimacho in Okinawa Prefecture will conduct a feasibility study on the ways OTEC technology can be utilized.

"At first no one believed [practical OTEC] could be achieved. I can't give up on my research until OTEC plants are built in oceans around the world," Uehara said.