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Tapping into oceanic energy

The serious physical damage caused by the Fukushima No. 1 nuclear power plant disaster, following the 3/11 earthquake and tsunami, has reminded Japan all the more of its acute shortage of natural energy sources and the need for developing alternative sources. This has led scientists and private corporations as well as the public sector to look for alternative means of generating electricity.

The key word for such efforts is “ocean.” Completely surrounded by the sea, Japan has huge potentials for utilizing oceanic power and resources. For starters, Japan is surrounded by an exclusive economic zone of about 4.47 million square km. This is the sixth-largest EEZ in the world.

The first things that come to mind when thinking of submarine energy resources are methane hydrate and natural gas. Japan is said to be blessed with methane hydrate reserves whose volume is 100 times its current annual gas consumption. But exploration technologies for this material are still in their infancy. Submarine natural gas is also abundant in the East China Sea, where Japan and China are at loggerheads.

There are a number of other potential energy sources in the oceans around Japan. One is the kinetic energy of waves washing against Japan’s 34,000 km of coastlines. A 2010 report on wave-power generation by a panel of the Tokyo Metropolitan Government estimated that at least 300 million to 400 million kilowatts of electricity could be generated by wave power. Another estimate shows that within the range of 30 km from the coastlines, at a depth of 100 meters, there are potential energy sources equivalent to more than 10 nuclear power plants.

Sea-wave power can be utilized to generate electricity in two different ways: one is to let that power run a generator directly and the other is to float a hollow box in the sea so that air movement created by the vertical motion of waves inside the box will rotate the turbine of a generator.

The Saga prefectural government, in cooperation with the institute of ocean energy of Saga University, plans to start experimentation on wave power generation this year, with a budget of ¥23 million. The institute has already accumulated some experience in both methods of wave kinetic power generation.

A group headed by Hidemi Rikuta, associate professor at the Hiroshima University Graduate School, is experimenting with special films that are laid on the sea surface and generate electricity when deformed by wave power.

Wave power has advantages over other renewable energy sources such as solar power and wind power in that waves offer much higher efficiency of power conversion per unit area and the other is that, unlike sunlight or wind, waves never stop.

Tidal power generation is another possibility of utilizing oceanic energy — rapid ocean currents that directly turn the turbine of a generator.

Ehime associate professor Takayuki Nakamura and his group are experimenting with the Imabari city government in Ehime Prefecture on this method by using the tidal current at Kurushima Straits in the Seto Inland Sea.

In addition to harnessing the kinetic energy of the ocean, Europe, North America and Japan are experimenting with temperature differences at various sea depths — the method known as “ocean thermal energy conversion.” Seawater at a depth of 1,000 meters would be kept at a low temperature.

According to one estimate, the total potential energy to be generated with this method around Japan could equal or surpass that obtained from wave power. There are two ways of implementing this ocean thermal energy conversion: “closed cycle” and “open cycle.”

In the closed cycle, ammonia is used as a medium. Since it has a boiling point of -33 Celsius, it boils when it is near the sea surface — at around 20 Celsius. Ammonia steam generated near the sea surface turns the turbines of a generator. A condenser then cools the steam, which is liquefied when cold deep seawater is supplied. This cycle is repeated to continuously rotate the turbine.

In the open cycle, warm surface seawater is led into a low-pressure container in which it boils; the resulting steam turns a generator turbine. The steam is then cooled by cold deep seawater and becomes pure fresh water. This method not only generates electricity but also desalinates seawater.

Saga University is experimenting with ocean thermal energy conversion. The Okinawa Prefectural Deep Seawater Research Center has built a facility to start testing the same method in fiscal 2013.

Another interesting attempt involves taking advantage of differences in salt concentration between fresh water and seawater. A student of a physics 101 class knows that when fresh water and seawater are separated by a osmotic membrane, fresh water flows toward the seawater through the membrane. Scientists are studying how to make use of this principle. Kyowakiden Industry Co. in Nagasaki has collaborated with Tokyo Institute of Technology, and Nagasaki University has succeeded in generating power by using treated water from a sewerage system and condensed seawater.

Another means of generating electricity known as “reverse electrodialysis” utilizes differences in salinity but does not rely on kinetic energy. In this method, freshwater and seawater are separated alternately by a series of membranes.

One type of membrane lets through positive ions and the other lets through negative ions. Electricity is generated as sodium ions and chlorine ions move in opposite directions while passing through the membranes.

Research in this field by Mitsuru Higa, professor at the Yamaguchi University Graduate School, has attracted international attention. The major bottleneck is the high cost of the special membranes.

Little attention has been paid to utilization of oceanic energy. But it is likely to receive much attention from now on as a target of future-oriented investment to generate electricity. As an island nation, Japan is in an good position to make this big dream come true, although there are many difficulties to overcome.

This is an abridged translation of an article from the March issue of Sentaku, a monthly magazine covering Japanese political, social and economic scenes.