Image courtesy NASA.

Solar Power Satellites
Tech Level: 12

One of the great enduring ideas of the near-future in space is that of enormous, orbiting arrays of solar cells that collect sunlight, convert it to energy, and beam that energy to Earth for use. The idea can be traced back to Peter Glaser of the Arthur D. Little Company, who originally suggested the concept in its modern form in 1968. NASA and the US Department of Energy did an extensive conceptual study of solar powers satellites (SPS) in the 1970s, and the idea has popped up again and again in both science fiction and in space applications studies.

These satellites are usually envisioned as large planar affairs composed of many square kilometers of high-efficiency solar cells. The satellites could be placed in geosynchronous orbits that would never pass through the Earth’s shadow, ensuring a non-stop flow of energy. The sunlight on the cells is converted into electricity, which is gathered and beamed back to Earth via microwave emitters.

Because they are in geosynchronous orbits, their microwave emitters can always be trained on one specific spot on the ground. Here, arrays of receiving antennas, also called rectennas, intercept the microwaves and convert the energy into electricity usable by average consumers. This grouping of receiving antennas is sometimes called a rectenna farm.

The original NASA/DOE study called for a rectangular satellite with a collecting array that measured 10 kilometers by 14 kilometers. It would have used a transmitting antenna roughly about a kilometer across (the larger the better, to prevent beam-spreading,) which would beam the power to Earth at a frequency of 2.45 GHz, the same frequency used by microwave ovens, but also has the advantage of allowing the beam to pass unimpeded through clouds and rain. The rectenna farm would cover an oval area roughly 13 kilometers long and 10 kilometers wide.

The peak intensity of the microwave beam would be 23 milliwatts per square centimeter; the maximum allowable leakage from a consumer microwave oven is 5 milliwatts per square centimeter. While this would not be healthy in terms of long term exposure, it would certainly be possible to walk through the entire multi-kilometer width of the naked beam without experiencing any ill effects. Since the receiving area is expected to be covered over with large, raised rectennas, anyone on the ground underneath them would receive only negligible microwave exposure. Still, rectenna farms would likely be located in remote areas such as deserts in order to allay concerns from residents about possible ill effects of the microwave exposure.

At the distance of Earth’s orbit, sunlight delivers about 1400 watts worth of power per square meter. Using the types of solar cell technology available at the time of the NASA/DOE study, this would result in a net power gain on the ground of about 5 billion watts, or about ten times the output of a typical ground-based power plant.

These estimates, however, were made with the assumption of solar cell efficiency (how much of the 1400 watts per square meter of sunlight they can convert into usable energy) of around 5%, typical for 1970s technology. Today’s space-based solar cell arrays, such as those used on the International Space Station, have an energy-conversion efficiency of about 14%. The most modern systems have efficiencies ranging between 42% and 56%. The amount of power that can be delivered to the ground would be increased proportionally as well.

The mass of the SPS in the NASA/DOE study was estimated to be between 30,000 and 50,000 metric tons. With modern composite materials and far more lightweight solar cell designs, this mass could be cut to about one half to one third that. But even so, this represents a tremendous amount of material one would have to boost into space. At a current cost of at least several thousand dollars per pound to put an object in orbit, SPSs, despite their other advantages, would remain economically unfeasible in the near future.

A recent study conducted by the Space Studies Institute (SSI) showed that 98% of the material needed to construct a SPS could be mined from materials on the Moon. This would greatly reduce the cost of construction, but it would also mean that at least the seed of a lunar manufacturing infrastructure would have to exist first before the SPS scheme became feasible.

Though the microwave beam from an SPS cannot do much harm to any individual person, it is feasible the beam could be used as an environmental damage weapon, especially if the beam intensity were increased. If trained on an area for an appreciable length of time, it could use heat damage to kill cropland, forests, swamps, and perhaps even be used to oppress the residents of a large modern city that could otherwise be under siege. This effect need not always be used to detriment, however. In the novel Fallen Angels, by Larry Niven, Jerry Pournelle, and Michael Flynn, the heat from SPS transmitters was used to keep the last Canadian city, Ottawa, ice-free and livable after the rest of the country was buried under the glaciers of a new Ice Age.

At least one nation, the perpetually power-starved Japan, has committed itself to constructing a working solar power satellite by 2040. An smaller, cheaper, but less efficient alternate design by Japanese engineers suggest an SPS with the solar cells arranged in an equilateral triangle 300 meters wide and 300 meters to a side. The satellite would sweep along the equator at an altitude of 1100 kilometers and beam its power to a long array of rectenna stations below its flight path.

SPS technology also has a secondary application, that of providing beam power to launch craft and space-borne vessels, such as Myrabo’s Lightcraft and various incarnations of solar and magnetic sails. For the latter applications, however, the energy from the satellite might be converted to laser light or frequencies other than microwaves, depending on the type of spaceship used.

Further Information

Article added 2006