Design

Space-based solar power essentially consists of three parts:

1. a means of collecting solar power in space, for example via solar cells or a heat engine
2. a means of transmitting power to earth, for example via microwave or laser
3. a means of receiving power on earth, for example via a microwave antennas (rectenna)

The space-based portion will be in a freefall, vacuum environment and will not need to support itself against gravity other than relatively weak tidal stresses. It needs no protection from terrestrial wind or weather, but will have to cope with space-based hazards such as micrometeors and solar storms.

[edit] Solar energy conversion (solar photons to DC current)

Two basic methods of converting sunlight to electricity have been studied: photovoltaic (PV) conversion, and solar dynamic (SD) conversion.

Most analyses of solar power satellites have focused on photovoltaic conversion (commonly known as “solar cells”). Photovoltaic conversion uses semiconductor cells (e.g., silicon or gallium arsenide) to directly convert photons into electrical power via a quantum mechanical mechanism. Photovoltaic cells are not perfect in practice, as material purity and processing issues during production affect performance; each has been progressively improved for some decades. Some new, thin-film approaches are less efficient (about 20% vs 35% for best in class in each case), but are much less expensive and generally lighter. In an SPS implementation, photovoltaic cells will likely be rather different from the glass-pane protected solar cell panels familiar to many from current terrestrial use, since they will be optimized for weight, and will be designed to be tolerant to the space radiation environment, but will not need to be encapsulated against corrosion by the elements. They may not require the structural support required for terrestrial use, where the considerable gravity loading imposes structural requirements on terrestrial implementations.

[edit] Wireless power transmission to the Earth

Wireless power transmission was early proposed to transfer energy from collection to the Earth's surface. The power could be transmitted as either microwave or laser radiation at a variety of frequencies depending on system design. Whatever choice is made, the transmitting radiation would have to be non-ionizing to avoid potential disturbances either ecologically or biologically if it is to reach the Earth's surface. This established an upper bound for the frequency used, as energy per photon, and so the ability to cause ionization, increases with frequency. Ionization of biological materials doesn't begin until ultraviolet or higher frequencies so most radio frequencies will be acceptable for this.

William C. Brown demonstrated in 1964 (on air -- Walter Cronkite's CBS News program), a microwave-powered model helicopter that received all the power it needed for flight from a microwave beam. Between 1969 and 1975, Bill Brown was technical director of a JPL Raytheon program that beamed 30 kW of power over a distance of 1 mile at 84% efficiency.^{[citation needed]}

To minimize the sizes of the antennas used, the wavelength should be small (and frequency correspondingly high) since antenna efficiency increases as antenna size increases relative to the wavelength used. More precisely, both for the transmitting and receiving antennas, the angular beam width is inversely proportional to the aperture of the antenna, measured in units of the transmission wavelength. The highest frequencies that can be used are limited by atmospheric absorption (chiefly water vapor and CO2) at higher microwave frequencies.

For these reasons, 2.45 GHz has been proposed as being a reasonable compromise. However, that frequency results in large antenna sizes at the geostationary (GEO) distance. A loitering stratospheric airship has been proposed to receive higher frequencies (or even laser beams), converting them to something like 2.45 GHz for retransmission to the ground. This proposal has not been as carefully evaluated for engineering plausibility as have other aspects of SPS design; it will likely present problems for continuous coverage.

[edit] Laser power beaming experiments

A large-scale demonstration of power beaming is a necessary step to the development of solar power satellites. Laser power beaming was envisioned by some at NASA as a stepping-stone to further industrialization of space.

In the 1980s researchers at NASA worked on the potential use of lasers for space-to-space power beaming, focussing primarily on the development of a solar-powered laser. In 1989 it was suggested that power could also be usefully beamed by laser from Earth to space. In 1991 the SELENE project (SpacE Laser ENErgy) was begun, which included the study of laser power beaming for supplying power to a lunar base.

In 1988 the use of an Earth-based laser to power an electric thruster for space propulsion was proposed by Grant Logan, with technical details worked out in 1989. His proposal was a bit optimistic about technology (he proposed using diamond solar cells operating at six-hundred degrees to convert ultraviolet laser light, a technology that has yet to be demonstrated even in the laboratory, at a wavelength that will not easily transmit through the Earth's atmosphere). His ideas, with the technology scaled down to be possible with more practical, nearer-term technology, were adapted.

The SELENE program was a serious research effort for about two years, but the cost of taking the concept to operational status was quite high and the official project was ended in 1993, before reaching the goal of demonstrating the technology in space. However, some research is still continuing. There was some hope that an array for a laser-powered aircraft demonstration might be developed.^[40]

[edit] Spacecraft sizing

The size of an SPS will be dominated by two factors. The size of the collecting apparatus (eg, panels, mirrors, etc) and the size of the transmitting antenna which in part depends on the distance to the receiving antenna. The distance from Earth to geostationary orbit (22,300 miles, 35,700 km), the chosen wavelength of the microwaves, and the laws of physics, specifically the Rayleigh Criterion or Diffraction limit, used in standard RF (Radio Frequency) antenna design will all be factors.

It has been suggested that, for best efficiency, the satellite antenna should be circular and about 1 kilometer in diameter or larger; the ground antenna (rectenna) should be elliptical, 10 km wide, and a length that makes the rectenna appear circular from GEO (Geostationary Orbit). (Typically, 14 km at some North American latitudes.) Smaller antennas would result in increased losses to diffraction/sidelobes. For the desired (23mW/cm²) microwave intensity ^[41] these antennas could transfer between 5 and 10 gigawatts of power.

To be most cost effective, the system should operate at maximum capacity. And, to collect and convert that much power, the satellite would require between 50 and 100 square kilometers of collector area (if readily available ~14% efficient monocrystalline silicon solar cells were deployed). State of the art (currently, quite expensive, triple junction gallium arsenide) solar cells with a maximum efficiency of 40.7% ^[42] could reduce the necessary collector area by two thirds, but would not necessarily give overall lower costs for various reasons. For instance, these very recently demonstrated variants may prove to have unexpectedly short lifetimes. In either case, the SPS's structure would be essentially kilometers across, making it larger than most man-made structures here on Earth. While almost certainly not beyond current engineering capabilities, building structures of this size in orbit has not yet been attempted.

[edit] LEO/MEO instead of GEO

A collection of LEO (Low Earth Orbit) space power stations has been proposed as a precursor to GEO (Geostationary Orbit) space power beaming system(s)^[43]. There would be both advantages (much shorter energy transmission path lengths allowing smaller antenna sizes, lower cost to orbit, energy delivery to much of the Earth's surface, assuming appropriate antennas are available, etc.) and disadvantages (constantly changing antenna geometries, increased debris collision difficulties, requirement of many more power stations to provide continuous power delivery at any particular point on the Earth's surface, etc.). It might be possible to deploy LEO systems sooner than GEO because the antenna development would take less time, but it would certainly take longer to prepare and launch the number of required satellites. Ultimately, because full engineering feasibility studies have not been conducted, it is not known whether this approach would be an improvement over a GEO installation.

[edit] Earth-based infrastructure

The Earth-based receiver antenna (or rectenna) is a critical part of the original SPS concept. It would probably consist of many short dipole antennas, connected via diodes. Microwaves broadcast from the SPS will be received in the dipoles with about 85% efficiency^[44]. With a conventional microwave antenna, the reception efficiency is still better, but the cost and complexity is also considerably greater, almost certainly prohibitively so. Rectennas would be multiple kilometers across. Crops and farm animals may be raised underneath a rectenna, as the thin wires used for support and for the dipoles will only slightly reduce sunlight, or non arable land could be used, so such a rectenna would not be as expensive in terms of land use as might be supposed.