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以前より提案中のメガソーラー筏構想のフォローアップとして、この度ソーラー筏模型を試作しました。
30mm径のポリエチレン（PE）減圧補強ホースを丸めたφ40cmの円形をサブユニットとし、取り敢えず４個組み合わせた模型としてます。
以前の論文で提案したサブユニットのサイズは25m程度なので縮尺は1/60程度となります。風力・海流による航海のための帆や舵などはそれ専用のサブユニットを100m×100mのユニットに1～2個/16個の割合で連結することと考えています。
PEは適切な接着剤がないため強力な接着テープも多用して、試行錯誤・苦労を重ね、当初予定より大変な時間（3週間）と費用（1.5万円ほど）がかかりました。上版は安上がりのダイソーで買った金網で代用し8cmの支柱（実物高さ5m程度）で支えていますが、日の丸でも貼り付けたいとは思っています。ソーラーパネルやLEDを取り付けることも考え秋葉原にも行きましたが、意外と商品は少なく、バッテリー（電圧12V、重さ1kg)も必要で浮力の点から載せることは不可能と分かりました。計算では浮くことは確認していましたが、本日、近所の児童公園プールで4個連結で浮かしてみました。子供たちが珍しがって質問攻めにあいました。結束バンドで緩く結ばれた4基が、滝が落ちる傍では波に追従して位相差を伴って滑らかに揺れるのが見られ、大変楽しめました。
これからは、大型化を図った筏に小型ソーラーパネルは勿論、加速度計や荷重計などの小型センサー類を載せて海面に浮かべ、主要なデータの測定を行いたいと考えています。
まずは近況ご報告まで。

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Professor Emeritus, Chuo University Tokyo, Japan

Mega Solar-Module Raft Project in Low-Latitude Pacific Ocean & and Its Possibility

The most plausible strategy to meet the goal the government seems to aim is offshore wind power similar to other nations in the world. Off-shore wind power has been developed in nations in northern Europe like Denmark, UK and Germany in these decades. They are all fixed-bottom types with their foundations constructed directly on sea-floor, wherein water depth should be less than 50 m at the deepest. Unfortunately, coastal seas in most part of Japan islands tend to increase water depth in short distance from the shore, limiting the potential capacity of the fixed-bottom type. 

Hence, the floating type which can be sited in seas 200 m deep or more seemingly are considered to be promising, wherein the wind turbine is fixed on the float moored and anchored to deep sea-floor. UK is proud of this offshore technology as a front-runner in R & D making the most of the experience in the North Sea oil/gas project and planning to demonstrate the technology mature in a few years through test-site operations. The Japanese government appears to look forward largely to the floating type wind power as promising renewable energy because large sea areas may be able to be available as favorable sites.

However, the practical use of the floating wind power has not yet been started even in European front-runner countries. In Asian countries like Japan where natural environments are more hostile with severe typhoon, high seismicity and tsunami, considerable technical problems will have to be solved before the power system can serve as a major player. A couple of test sites have already been chosen and experimental floating wind power projects are tested for several years in Japan, though their clear technical perspective has not yet been recognized publicly. Furthermore, fishing industries in Japan have historically had strong voices in using oceans all along the coasts, that may impose another constraint on the off-shore wind power.

Thus, it seems too optimistic to believe that we can depend exclusively on off-shore wind power to supply huge volume of renewable energy demand in Japan to realize the carbon-neutral. Instead, possibilities of other renewable energies conceivable in our own environment have to be explored by utilizing technological development in near future.

In this respect, our research group have been proposing an innovative project for more than a decade (before the Fukushima nuclear disaster in 2011) that could be another possibility of huge renewable energy.

Huge sunshine energy affluent in low-latitude Pacific Ocean may be captured with reasonable economy by mega-solar module rafts sailing slowly. It is no doubt the right of any countries authenticated by the International Maritime Law to make a sail in international open seas for commercial purposes, wherein renewable sunshine energy is exploited on purpose. Hence, it is considered sufficiently possible to share a consensus in international forums such as IMO (International Maritime Agency) how to develop such an innovative usage of open seas as gigantic solar power generation for growing sustainable world economies by paying enough attention to minimum impact on other activities there. Fortunately, the low-latitude Pacific Ocean are remote from major commercial sea traffics in the high latitudes.

If a giant mega-solar raft as 5 km square is considered for example, the electric energy generated only during daylight hours can be equivalent to 1 GW nuclear power stations of availability 100%, by assuming daily sunshine energy per area 8 kWh/m², the energy conversion rate 12 % (of silicone solar module commercially available today). In the most part of low-latitude Pacific Ocean, the annual average of daily sunshine energy exceeds 6.0 kWh/m² among that the highest can reach 6.5～7.0 kWh/m² between the equator and 15° south in a sea expanding as vast as the Australian continent. Hence, it seems possible for the movable raft to pursue optimal sunshine depending on seasons attaining 8.0 kWh/m² (more than twice the average in the Japan island) by making an energy-saving slow wind sailing.

As for the wind condition there, it is found to be very favorable in terms of annually averaged wind speed of 3～7 m/s considerably milder than in high-latitude oceans and fixed wind directions. If wind power operational even during nighttime is integrated with solar module, the system may further boost its power generation. The waves are never rough, 1~2 m high on average in low-latitude Pacific, unlike middle/high latitude in all seasons, though the solar module raft will be designed operational in much higher waves so long as the sunshine is available on the raft for power generation.

As the greatest risk to this energy system, tropical depressions or storms named Typhoon in Japan cannot be ignored. However, it should be recognized first of all that there are two wide areas in low-latitude Pacific literally free from the risk. One is overlapping with the area of the highest sunshine energy mentioned above (due to exceptionally low temperature of sea water originated from Antarctica. Another is ±5° along the equator where tropical depressions cannot be born theoretically because of the Colioris effect. In other areas, the risk tends to increase, though not so severely as in the middle-latitude, necessitating in-advance evacuations. It may well be expected that rapidly advancing meteorological knowledge/technology will enable reliable predictions of tropical depressions in a month ahead in near future. As for one more natural disaster, tsunami, the effect may not be critical to this energy system as long as it stays remotely from shallow coastal areas.

In our scenario to realize this system contributing to the carbon-neutral initiative by 2050, three major technologies have to become practically mature in 30 years as follows;

1) The huge electric energy generated by the solar module is transformed into hydrogen gas by alkaline water electrolysis and further into MCH (Methylcyclohexane) by reacting with Toluene sequentially in real time, and MCH is transported by oil tankers (VLCC) shuffled once in two weeks between the raft and Japan.

2) Solar module (CIGS-type) seems to be promising for this project, thin (2 micron) with conversion efficiency of more than 12% and should be integrated with flexible sail clothes. Energy collection system from numerous numbers of module all over the raft should be as simplified and durable as possible.

3) The giant raft consisting of 2500 units of 100 m square, each of them further comprising 16 subunits of 25 m square on which 4 solar modules are set. All of them are connected by universal joints so as to deflect freely following wave motions. The raft are designed to be able to sail basically by wind and sea-current.

Though the hurdles to realize these widely diversed technologies seem to be too high to overcome, their technnical bases are already present actually. What is needed in the next 30 years is to practicalize the individual technical elements by scaling up capacity, modifying for higher efficiency, better performance and cost-minimizing, and then integrate them all together.

The economic feasibility has roughly been estimated on the 25 km² mega-solar raft by extrapolating current state of the arts. It indicates that subsidizing price of hydrogen by 50% will make it viable. However, more cost-cutting efforts are further required to be commercially feasible for market-competitive hydrogen price. It is particularly needed for the giant raft to employ truly innovative design concepts for drastic cost reduction. Essential chemical plants for electrolysis and hydrogenation also very costly have to be drastically economized by incorporating advanced technology, scaling-up and mass production effects.

Though further steps are still needed to reach to the gigantic 1 GW system, it seems possible to realize a smaller capacity mega-solar module raft to practically operate in low-latitude Pacific by 2050 because the basic technologies are already in our hands. Also note that such an innovative green energy initiative where international multidisciplinary cooperation of science & technology is critical will surely lead to creation of next-generation disciplines in science & technology, markets in commerce and job opportunities not only in Japan but all over the world. That will make an epoch bringing all human beings on earth to a new horizon to live a truly sustainable life.

One may wonder if such a green-energy initiative in the low-latitude Pacific Ocean may coexist with the current circumstances of US-China power struggle conducted right there. Because of that, however, it is really meaningful from a quite different perspective of world peace to start this initiative in cooperation firstly with Pacific Island countries as well as with many other interested countries.

Time surely comes when developing countries will have sufficiently developed to demand as much energy as already developed countries. Then, abundant sunshine energy in low-latitude Pacific Ocean will be targeted by many countries as indispensable natural energy resource. To prepare for that time, Japan is a right country in a right position to take the first step to this endeavor in cooperation with many other countries including Pacific Island nations.

Thus, besides offshore wind power, Japan may possibly be able to have another option of huge renewable energy. Why do not we expand our sight and investigate the possibility to make use of abundant sunshine energy in the Pacific Ocean that nobody has ever tried to do.

References： Kokusho, T., Emoto E. and Kato, T. (2012): Sailing Solar-Cell Raft Project and Weather/Marine Conditions in Low-Latitude Pacific Ocean, Journal of JSES, Japan Sunshine Energy Society, 38 (1), 49-57 (in Japanese). Kokusho, T., Emoto E. and Kato, T. (2013): Sailing solar-cell raft project and weather and marine conditions in low-latitude Pacific Ocean, Journal of Energy Engineering, ASCE, 139 (1), 2-7. Kokusho, T. (2016): Feasibility of Mega Solar Raft in Low-Latitude Pacific Ocean, 42 (6), Journal of JSES, Japan Sunshine Energy Society, 42 (6), 49-57 (in Japanese).

The following is a PPT presentation material associated with a lecture delivered in Energy Committee of Japan Society for Civil Engineers in January 2020, where a more detailed information on this energy project is available as translated into English.