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Y Kuroda, M Nagatomo & P Collins, 1992, "Japanese Perspective on Power from Space for Earth", SPS Rio 92 - Space Power Systems and Environment in the 21st Century Symposium, SEE, Paris.
Also downloadable from perspective on power from space for earth.shtml

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Japanese perspective on power from space for earth
Y Kuroda *1, M Nagatomo and P. Collins *2

This paper does not represent any Japanese national organization, nor does it express any official view on this subject. Japanese activities in the field of space power systems consist of two different types: one is technology development in the context of the national space program, and the other is more basic research pursuing authorization in the near future. Power from space belongs to the second category of activities, which is conducted mainly by university researchers and voluntary participants from the private sector, including the SPS 2000 task team and supporting members.

Space is believed by some people to provide mankind with the opportunity to expand their activities into limitless growth; space industrialization has been rationalized in this way. However, few studies have examined or certified this forecast quantitatively. We present results obtained by Dr Yamagiwa showing how a World Dynamics model can be modified by introducing extraterrestrial energy and materials to human activities on earth. These figures show the future trend of key factors of human activities, such as population, carbon dioxide and others.

It should be noted that the growth rate goes down next century in the case of a closed earth system, while the trend is improved in another case in which power from space begins to be introduced at the beginning of the next century. However such improvement is only possible when it is assumed that the cost for SPS is similar to that for existing terrestrial industry, such as cars and machinery industry. The present high cost of space technology would not satisfy the required condition for power from space to benefit the earth.

The scientific principles of SPS are so well known that there is no substantial obstacle to realize power from space. The major question is how to start to build such a system. The SPS 2000 task team's conceptual study is a feasibility study on the first step of power from space to earth, following four requirements:

1, SPS 2000 is to be realized as soon as possible. there is no special reason to make it a future system, so this study assumes construction of the system should start before 2000, which means practically as soon as possible. It also means that the satellite must be in low earth orbit, and so, in order to deliver power to the receivers on earth as often as possible it must orbit the equator.

2. Even Edison's power station, the first commercial electric power station was only a few hundred horse-power in generating capacity. lt is not necessary for the first SPS to be competitive with large power stations in industrialized nations. Considering the high costs of available launch system, the first SPS should be reasonably small to be accepted by concerned organizations.

3. The cost of existing space technology is too high to be applied in the power industry. The cost of power stations must be low enough to be attractive from the standpoint of financing.

4. The first SPS should be useful not only to demonstrate technical feasibility, but also to prove to be useful to people on the earth, especially for developing nations where even small amounts of electric power are useful.

In planning to build a first power station in space, among the various design drivers of this new SysLam, we have chosen COST as the key driver. It is believed that launch cost is the most difficult target to reach. However, our study indicates that the cost of present space technolgy is the main hurdle to clear for construction of a space power station.

A rough estimation of the cost per kilowatt for construction of a 10 MW solar power satellite by the space industry is one thousand times higher than that for terrestrial power stations. You should remember the fact that the payment for a three minute telephone call via satellite is more than ten times as expensive as one kilowatt-hour of electricity. That represents the cost-ratio between the present space technology and SPS technology. The next phase is to reduce the cost for launching. We are rather optimistic on this matter since its possibilily is already under discussion.

The configuration of SPS 2000 in the initial phase of conceptual design has changed because of engineering reasons. The geometry of the SPS is a prism-shape, and it has neither active attitude controll nor sun-oriented solar panels, in order to simplify the control system. We found that a solar power station should be as simple as a terrestrial solar power station, considering the maintenance cost. The modified geometry satisfies the requirement of passive attitude stabilization. Now researchers and engineers are working to design the construction method for the system in orbit, and also to choose inexpensive technology. Antenna design, overall system dynamics and integration for launching are other important subjects. There are many other subjects concerning the satellite needing further study, but another important area of study is power reception and utilization.

The SPS 2000 microwave beam will cover an area of approximately 1500 metres in diameter at the Earth's surface, about the size of an airport. The SPS receiving antenna is known as a " rectenna", because it rectifies microwaves to DC, and the surface is a lightweight mesh transparent to sunlight, so that the land underneath can be used for other purposes.

Rectennas will be built with different sizes and power outputs. For example a circular rectenna some 500 metres in diameter will receive approximately 2 MW of power during reception periods. However, because the satellite is not stationary, the power reception lasts only a few minutes per orbit. So, by using storage a rectenna will be able to deliver a continuous output of some 50 KW. In areas where electric power is not currently available. such a level of output can have great social and economic value.

Each rectenna site will be different, both in its geography, and in the local economy. Planning the best design of rectenna, and the best means of using the power produced, will involve discussions with the local community. the local government, and the SPS 2000 project staff. Each rectenna will be a unique story. The possibility of future evolution of the SPS 2000 system to higher power levels and longer power transmission times will also require careful local planning.

Like the satellite, the rectenna must be made as cheaply as possible. However, for the SPS 2000 project. the rectenna will be less economical than for a future commercial SPS for two reasons. First, power will be received for only about 200 seconds in each orbit, or about 3% of the time. Second. the intensity of the microwave beam will be very low. Nevertheless. both the power and the information produced by SPS 2000 will be valuable.

The microwave intensity at the rectenna is very low, and will be below the strictest safety standards in the world, so the operation will be entirely safe. Using no fuel, the rectenna operation is entirely clean. Thus by designing the rectenna to use local methods of construction the environmental impact will be minimized.

At some sites it will be best to place the rectenna over water, and one attractive construction method in the warm seas near the Equator will be to use artificial coral. When a weak electric current is passed through a metal structure in the sea, it is covered in a tough layer of concrete. This process has recently been developed under UNDP funding in Colombia, and could use power from the rectenna itself to extend the area of the rectenna support structure without pollution.

For rectennas sited over land, local construction methods will be used. For example, in countries where forest has been cleared, there are plentiful supplies of durable wood, which could be used to support the rectenna surface above agricultural land, In other places bamboo may be a good construction material. In areas where the land is less valuable, a simpler design of rectenna might be used that would be laid directly on the ground, which we call the "magic carpet".

The rectennas for SPS 2000 must be sited within a few hundred kilometers of the Equator. Also, in order to receive power for as long as possible, they must be separated by some 1200 km. The range of countries which might have SPS 2000 rectennas includes countries in South America, Africa and Asia, and a number of islands. No decisions have been made yet on sites, as they will involve many delicate matters. Countries with an interest are invited to contact the SPS 2000 Working Group.

The rectenna system will be designed to require minimal maintenance. Once installed, its operation will be automatic. The most challenging part of the SPS 2000 Ground System will be the design and operation of the local electricity supply network for the maximum benefit of local communities. Some of the power produced will probably be used for communal purposes, such as for improving education and health services. Some of the power will probably be sold far domestic consumption, and some will be used commercially to aid local economic development. Representatives of the user communities will have to plan this before the system is designed.

Experience of small-scale electricity supply systems, and exchange of information between rectenna user-groups in different countries will also be useful.

The concept of collecting solar energy in space for use on Earth is more than 20 years old. The objective of the SPS 2000 Project is to make a real demonstration of the concept using available technology, and to use it to provIde useful electric power to equatorial communities. Consequently the project will require international cooperation in order to succeed. I hope that in this way we can prove that clean energy from space can be a future resource for humans on Earth.

Y Kuroda, M Nagatomo & P Collins, 1992, "Japanese Perspective on Power from Space for Earth", SPS Rio 92 - Space Power Systems and Environment in the 21st Century Symposium, SEE, Paris.
Also downloadable from perspective on power from space for earth.shtml

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