There is no doubt that the efforts that our country has put into disaster mitigation far surpass those of other countries. We are shocked, however, to find that most of these efforts were in vain during the last major disaster. Reflecting on the Great Hanshin-Awaji Earthquake and the previous huge earthquake that struck Japan, it is as if we were hit by a backlash for having underestimated the power of nature. It is obvious now that success or failure of disaster mitigation measures for our country hinges on how conservatively we predict the magnitudes of earthquakes and tsunamis rather than preparing detailed procedures for evacuation etc. With regard to the nuclear disaster, in particular, we realized that the fail-safe design concept, that should have been a fundamental basis for ultra-important nuclear facilities, had been overlooked with respect to tsunami. After this disaster, however, nuclear power plants will still be needed for some time to maintain the current high energy consuming society in Japan. Needless to say, reconfirming and strengthening the fail-safe functions in nuclear facilities is the premise, never to repeat the same scenario even when design conditions are exceeded again.
The nuclear disaster will inevitably cast a dark shadow over our country's energy policy, which has been to fulfill the international commitment of CO2 reduction by increasing the nuclear power. The policy may be forced, in the wake of the disaster, to greater use of natural energy, while its availability is generally considered insufficient to meet the primary energy demand in Japan, because of her natural environment, even if solar and wind energies are to be developed in a greater extent. Contrary to such a general view, the writer would like to indicate a possibility for a massive use of natural energy in the following.
Facing our land, the vast open sea extends as far as the Southern Hemisphere. We have to recognize a potential for exploiting solar energy in a gigantic scale by introducing a fleet towing huge solar cell rafts to generate solar energy in the Pacific Ocean. Although consensus has to be formed among international communities as well as with fishing and shipping industries, it may be said that the navigation in open seas and hence the power generation during navigation is justified according to the international law.
A huge solar cell raft of 25 km2 (5 km×5 km) in dimension, assuming 8 kW-hour/m2/day (8 hour/day of power generation) of solar energy attained per day and 12% in electrical conversion efficiency (a modest value at present) can generate 8 kW-hour/m2/day× 0.12×25,000,000m2/24h= 1,000,000 kW of energy capacity. This is comparable to a 1000 MW nuclear power plant that is in 24-hour continuous operation. If several dozen of such huge rafts were to be put into operation, our country's energy independence could be realized. The Pacific low-latitude band, east of 160 degrees east longitude, is a spacious and quiet region that is unaffected by typhoons and full of 5 to 6kW-h/day of solar energy on average (the same as the Sahara Desert) throughout the year. Energy efficiency of 8kW-h/day or more is targeted as the solar cell rafts navigate in the clear regions of the North and South Pacific using weather satellites. Traveling in search of maximum efficiency, that can only be attained on the ocean, is an advantage over a land-fixed power generation and the effects on marine life can be extremely low because of the continual moving. How to transport the generated electricity will largely depend on future technological developments, but the method under consideration is the direct transportation of electricity by a battery tanker loaded with a tremendous number of high-efficiency batteries or the transportation of hydrogen, produced by electrolysis of seawater, or its chemical compounds.
The fleet may be composed of a number of solar cell raft units, mother ships, work-ships, and tankers. If the size of the unit is chosen as 100 m × 100 m for instance, 2,500 units are hence needed. It may not be feasible to construct economically such a number of rigid floats with conventional materials such as steel or reinforced concrete. It is therefore essential to create the concept of an innovative floating unit, that includes a seamless sail clothes completely covered with flexible solar cells and a underlying float made of lightweight materials and can be folded easily. The raft units are inter-connected by wires, pressure tubes, and electrical cables and the sails should be designed in a way such that their angles can be controlled for low-speed, energy-conserving wind sail as well as solar tracking for efficient power generation.
The key to realize this system naturally includes the increased performance of solar cells (the current efficiency of compound semiconductor flexible cells with 1 square-meter modules is about 14%), reductions in the cell production costs, and further developments in electrical energy storage and transportation technology. At the same time, basic structural designs of raft units making them lightweight, flexible and highly durable, as well as minimizing the construction costs by selecting appropriate materials are extremely important. No doubt that pioneering ideas will be needed to develop feasible raft design, considering tough marine and weather conditions though the fleet is to navigate in relatively calm sea.
The solar energy reaching the earth is so great that one hour shine actually amounts to the annual energy use of human-beings. The technical and economic hurdles will inevitably be high for efficiently gathering the low-density natural energy in the vast unused open sea to use as sustainable primary energy source, whereas the significance of realizing it is immeasurable. Japan, a maritime nation with limited natural resources, should make this nuclear disaster a starting point to take the initiative in trying to reach this goal within twenty to thirty years.