JX Crystals

Mirrors in Space for Low-Cost Terrestrial Solar Electric Power at Night

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ABSTRACT — A constellation of 18 mirror satellites is proposed in a polar sun synchronous dawn to dusk orbit at an altitude of approximately 1000 km above the earth. Each mirror satellite contains a multitude of 2 axis tracking mirror segments that collectively direct a sun beam down at a target solar electric field site delivering a solar intensity to that terrestrial site equivalent to the normal daylight sun intensity extending the sunlight hours at that site at dawn and at dusk each day. Each mirror satellite in the constellation consists of a linear string of mirror elements and each terrestrial solar electric field site has a 10 km diameter and can produce approximately 5 GW per terrestrial site. Assuming that in 10 years, there will be approximately 40 terrestrial solar electric field sites evenly distributed in sunny locations near cities around the world, this system can produce more affordable solar electric power during the day and further into the morning and evening hours. The typical operating hours or power plant capacity factor for a terrestrial solar electric power site can thus be extended by about
30%. Assuming a launch cost of $400/kg as was assumed in a recent NASA Space Power Satellite study for future launch costs, the mirror constellation pay back time will be less than 1 year. A logical continuation of this space mirror satellite concept can potentially lead to solar electric power at a cost under 6 ¢ per

Index Terms — photovoltaics, satellite, solar power system, space power.

The idea of using minors in space to beam sunlight clown to earth for terrestrial solar electric power generation is not new. Dr. Krafft Ehricke first proposed this idea in 1978 [1. 2] as shown in figure 1 under the title Power Soletta. Because of the simplicity of minors compared to the complexity of the Space Power Satellite concept, his idea was brilliant particularly for the time in which it was first proposed.

Specifically. Ehricke proposed a constellation of satellites in an orbit 4200 km in altitude beaming power down to a 1200 sq km site in Western Europe. Deflecting sunlight down to earth where it is then convened to electricity is conceptually much simpler than converting it to electricity in space and then microwave beaming it down to earth and then converting it to electricity as per the Solar Power Satellite concept.

The key physical limitation for this concept relates to the size of the sun's disc as viewed from earth. The sun's disc subtends an angle. 9, of 10 mrads. This means that the minimum size of a sun spot produced on the earth's surface from a minor in space at an altitude. A, is:

Applying this formula for a minor in orbit at an altitude of 4200 km gives a sun spot diameter on earth of 42 km with a corresponding area of 1385 sq km. This explains the 1200 sq km solar field size for the Power Soletta concept. This also means That in order to produce an intensity of sunlight on earth equivalent to the normal daylight sun intensity, the area of the 3 mirrors shown beaming power down in figure 1 would have to be 1385 sq km and the area of the 10 mirror satellites in the constellation in figure 1 would have to be 4617 sq km. Unfortunately, the enormous task of placing this minor area in orbit was somewhat discouraging in 1978.

In addition, there are two other problems with this concept as Ehricke proposed it. One problem is that this orbit falls in the Van Allen radiation belt. A second problem will reside with the size of the earth solar electric power field and the resulting problem of then distributing the power produced throughout Europe. Ehricke assumed that the 1200 sq km solar field would produce electricity at 15% efficiency implying a 180 GW central power station which then implies enormous distribution problems.

While This Power Soletta concept was intriguing, given the problems just described. NASA has focused much more attention over the subsequent years on the Space Power Satellite (SPS) concept [3. 4]. A recent version (2003) of this SPS concept is shown in figure 2. This Integrated Symmetrical Concentrator (ISC SPS) concept is of interest here because it also utilizes mirrors [3. 4], As shown in figure 2, in this concept, two sets of 36 mirrors with each mirror approximately 0.5 km in diameter are used to beam sunlight to a central PV convener platform that then generates electricity and beams microwave energy to an earth generating station. This satellite is assumed to be located in Geosynchronous Orbit at an altitude of approximately 36.000 km. The special 8 km diameter earth receiver / generator station is assumed to generate 1.2 GW of electricity.

There are also problems with this ISC SPS concept. One problem is its complexity. More than just mirrors are now required and it now no longer uses a potentially existing terrestrial solar electric power station.

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