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Japanese Archipelago has little fossil energy. However, the seas surrounding
Japan are blessed with an abundant energy resource: wave energy of
the sea. The wave energy is renewable and relatively safe to the environment.
Japan cannot afford to disregard wave energy. The world's wave energy is estimated at 1 to 10 teraWatts, large enough to match the world's total electric energy demand. Wave energy intensity is expressed in terms of energy quantity per meter of coastline or sea area. The European sea has an energy intensity as high as 40kW per meter of coastline, on average. There, wind always blows strong because of higher latitudes, and therefore waves hit the coastline hard. In Japan, the average wave energy intensity is 6 to 7kW per meter of coastline, on average. Even in the roughest sea of Japan, the wave energy intensity is 10kW, at most, per meter of coastline. Although waves to Japan do not contain great energy compared with Europe, Japan has a very long coastline, extending to a total of about 35,000 kilometers. The total wave energy to Japan amounts to 31 to 36 GW, an amount corresponding to one-third of Japan's annual power generation. Naturally, it is not possible to convert all wave energy to electric energy. The MIGHTY WHALE, a floating type wave power device, on which JAMSTEC has been conducting open sea tests, can convert wave energy to electric power at a maximum efficiency of 15 percent. The annual average wave energy intensity of the open sea tests area of the MIGHTY WHALE is 4kW per meter. The width of the MIGHTY WHALE exposed to waves is 30 meters; therefore, the MIGHTY WHALE receives 120kW of wave energy. The maximum electric power the MIGHTY WHALE can produce is calculated to be 18kW by simple arithmetic. |
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An important issue has emerged as to how to store the generated electric power. The MIGHTY WHALE stores generated electricity in batteries. But batteries are expensive and not suited to storing large amounts of electricity. Storing electricity in the form of pneumatic air or spinning fly wheels is proposed, rather than in the form of electric energy.
It is also suggested, as a future measure, that electricity from wave power generation be used to electrolyze water, to generate hydrogen, and then the hydrogen would be stored. The 21st century will be, in a way, the era of hydrogen energy. Hydrogen can generate a large amount of energy upon reacting with oxygen, producing nothing but water. The produced water may be recycled for electrolysis to generate hydrogen again. This is a clean and renewable energy cycle.
If seawater is simply electrolyzed, chlorine is generated instead of hydrogen. Some technical study will be needed to solve this problem. If the concept explained above becomes a reality, it may become possible to provide an unlimited amount of ultimate clean energy.
Cost is always a key issue in realizing new energy. Wave power generation is technically close to realization, but not so in cost.
Photovoltaic power generation and wind power generation have already been commercialized. They took a long time, in research and development, before the costs were reduced to realistic levels. The history of research and development of wave power generation is still short. The hurdle of cost will be reduced as more study is done on this subject.
Hopefully, the ocean will be an important source of energy for the clean energy society that mankind will have realized in 30 years from now.
| JAMSTEC found the fact that a large quantity of methane gas is released from the seafloor off Kuroshima Island in Okinawa Prefecture, and has since been conducting its survey. The photo shows processing of deposit of the outer layer of the seafloor off Kuroshima which was gathered by the SHINKAI 2000, a manned research submersible. First of all, the deposit taken by the pillar-shaped mud collector is divided into clods at a several-centimeter interval. The pore water which is squeezed from each clod with a hydraulic squeezer. Is geo-chemically analyzed, thus, the nature of fluid contained in a deposit can be understood. |  |
 Methane hydrates are sherbet-like substances where methane molecules are trapped in spaces surrounded by ice molecules. Methane is a clean energy that generates, upon combustion, a lesser amount of carbon dioxide, a greenhouse gas causing global warming, compared with petroleum or coal. Moreover, methane does not emit such offensive gases as sulfur oxides or nitrogen oxides. Methane hydrates exist in the permafrost and below the seafloor. The reserves worldwide are estimated at between 1,000 trillion and 50,000 trillion cubic meters. This amount is several times larger than the known reserves of petroleum. The Unites States, Canada, and Germany has been actively promoting exploration of methane hydrates, telling that they make an important resource that could replace petroleum. Japan also is keen on commercializing methane hydrates. The Nankai Trough is a sea area around Japan where methane hydrates were found. The reserves are estimated at 77 trillion cubic meters. Now, Japan depends mostly on imports for its supply of fossil fuels. Japan could be an exporter of energy someday, should it succeed in commercializing the resource. |
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There are a number of problems that must be overcome before methane hydrates can be commercialized.
Existence of methane hydrates can be detected by sound. The seafloor beneath which methane hydrates exist exhibits a distinct plane conspicuously reflecting acoustic waves, called "bottom simulating reflector" (BSR), parallel to the seafloor. The global distribution of methane hydrates has been studied by acoustic means. However, BSRs alone do not indicate their concentrations, or reserves. The present estimated reserves have been calculated based on the concentrations of specimens obtained from only several locations. Therefore, it is possible, at present, that a drilling venture could hit deposits too poor in methane concentrations to make the venture economically feasible.
There is no established method for mining methane hydrates, either. Being a liquid, petroleum can be extracted by pushing it by pressure. Methane hydrates, however, are in the form of ice, and therefore pressure does not work. The digging methods could be to inject steam or seawater into the methane hydrate layer, to melt the methane hydrates, and to extract methane, or to extract methane gas existing beneath the methane hydrate layer. There is concern about collapse of layers above a methane hydrate layer, if methane hydrates are artificially melted.
Detailed scientific studies are indispensable to the commercialization of methane hydrates. Studies on methane hydrates are an important objective of the Integrated Ocean Drilling Program (IODP). The IODP will scientifically study how methane hydrates are formed, their properties, their reserves, and their impacts on environmental changes. Achievements to be made by the IODP will greatly contribute to commercialization of methane hydrate reserves.
Methane hydrates are fossil resources that have been produced over a long period of time through biodegradation of organic substances. After thirty years from now, mankind may become able to produce methane hydrates. The method can be to inject carbon dioxide beneath the seafloor, through boreholes, and to let methanogens decompose carbon dioxide into methane. Depending on the temperature and pressure conditions, the methane thus produced may form hydrates. To facilitate mining, methane may preferably be in the gas form. With advancement of genome analysis of methanogens, it would become possible to produce methane more efficiently.
Great is the expectation placed on the study of methane hydrates. To make this expectation come true, however, will require steady basic research. Fortunately, JAMSTEC is particularly strong in such basic observations and experiments as study of the conditions of the seafloor, and those just below it, and conditions of fluids flowing through the apertures of rocks on the seafloor.
Methane hydrates have immeasurably great possibility as new energy. However, methane and carbon dioxide, both being greenhouse gases, could cause environmental disruptions if they are incorrectly handled. Again, basic scientific surveys and studies must precede any practical business venture attempt. (This article was prepared based on the interview.)