Solar power satellite

An artist's depiction of a solar satellite, which could send energy wirelessly to a space vessel or planetary surface.

A solar power satellite, or SPS or Powersat, as originally proposed would be a satellite built in high Earth orbit that uses microwave power transmission to beam solar power to a very large antenna on Earth. Advantages of placing the solar collectors in space include the unobstructed view of the Sun, unaffected by the day/night cycle, weather, or seasons. It is a renewable energy source, zero emission after putting the solar cells in orbit, and only generates waste as a product of manufacture and maintenance. However, the costs of construction are very high, and SPS will not be able to compete with conventional sources (at current energy prices) unless at least one of the following conditions is met:

• Sufficiently low launch costs can be achieved
• A determination (by governments, industry, ...) is made that the disadvantages of fossil fuel use are so large they must be substantially replaced.
• Conventional energy costs increase sufficiently to provoke serious search for alternative energy

In common with other types of renewable energy such a system could have advantages to the world in terms of energy security via reduction in levels of conflict, military spending, loss of life, and avoiding future conflict over dwindling energy sources.

History

An artist's concept of a solar power satellite, 1976. (NASA)

The SPS concept was first described in November 1968 . At first it was regarded as impractical due to the lack of a workable method of sending power collected down to the Earth's surface. This changed in 1973 when Peter Glaser was granted U.S. patent number 3,781,647 for his method of transmitting power over long distances (eg, from an SPS to the Earth's surface) using microwaves from a very large (up to one square kilometer) antenna on the satellite to a much larger one on the ground, which came to be known as a rectenna.

Glaser then worked at Arthur D. Little, Inc., as a vice-president. NASA became interested and signed a contract with ADL to lead four other companies in a broader study in 1974. They found that, while the concept had several major problems -- chiefly the expense of putting the required materials in orbit and the lack of experience on projects of this scale in space, it showed enough promise to merit further investigation and research .

During the period from 1978 - 1981 the US Congress authorized DOE and NASA to jointly investigate. They organized the Satellite Power System Concept Development and Evaluation Program . The study remains the most extensive performed to date. Several reports were published addressing various issues, together investigating most of the possible problems with such an engineering project. They include:

• Resource Requirements (Critical Materials, Energy, and Land)
• Financial/Management Scenarios
• Public Acceptance
• State and Local Regulations as Applied to Satellite Power System Microwave Receiving Antenna Facilities
• Student Participation
• Potential of Laser for SPS Power Transmission
• International Agreements
• Centralization/Decentralization
• Mapping of Exclusion Areas For Rectenna Sites
• Economic and Demographic Issues Related to Deployment
• Some Questions and Answers
• Meteorological Effects on Laser Beam Propagation and Direct Solar Pumped Lasers
• Public Outreach Experiment
• Power Transmission and Reception Technical Summary and Assessment
• Space Transportation
• Office of Technology Assessment

After these studies were published, there was no follow up work as the political climate had shifted against such projects. The DOE study conclusions were critical of the project's possibilities. Confused press reports claimed, incorrectly, that the concept had been found infeasible .

More recently, the SPS concept has again become interesting, due to increased energy demand, increased energy costs, and emission implications, starting in 1997 with the NASA "Fresh Look" however funding is still minimal.

In 2007, the US Department of Defense expressed interest in the concept.

At some cost point, the high initial costs of an SPS project will become favourable due to the low-cost delivery of power. By some estimates, this has already happened in some locations, as a result of the widely varying costs of electricity which sometimes approach (or even exceed) this point. In addition, continued advances in material science and space transport continue to whittle away at the startup cost of an SPS.

The SPS essentially consists of three parts:

1. a solar collector, typically made up of solar cells
2. a microwave antenna on the satellite, aimed at Earth
3. one or more paired, and much larger, antennas (rectennas) on the Earth's surface

Spacecraft design

In many ways, the SPS is a simpler conceptual design than most power generation systems previously proposed. The simple aspects include the physical structure required to hold the SPS together and to align it orthogonally to the Sun. This will be considerably lighter than any similar structure on Earth since it will be in a freefall, vacuum environment, will not need to support itself against gravity other than relatively weak tidal stresses, and needs no protection from terrestrial wind or weather.

Solar photons will be converted to electricity aboard the SPS spacecraft, and that electricity will be fed to an array of Klystron tubes which will generate the microwave beam.

Solar energy conversion (solar photons to DC current)

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

SD uses a heat engine to drive a piston or a turbine which connects to a generator or dynamo. Two heat cycles for solar dynamic are thought to be reasonable for this: the Brayton cycle or the Stirling cycle. Terrestrial solar dynamic systems typically use a large reflector to focus sunlight to a high concentration to achieve a high temperature so the heat engine can operate at high thermodynamic efficiencies; an SPS implementation will be similar. A major advantage of space solar is the ease with which huge mirrors can be supported and pointed in the freefall and vacuum conditions of space. They can be constructed from very thin aluminum or other metal sheets with very light frames, or from materials available in space (eg, on the Moon's surface).

PV uses semiconductor cells (e.g., silicon or gallium arsenide) to directly convert sunlight photons into voltage via a quantum mechanical mechanism which evades the thermodynamic limitations on heat engines. Photovoltaic cells are not perfect in practice as material purity and processing issues during production affect performance; each has been progressively reduced for some decades. These are commonly known as “solar cells”, and will likely be rather different from the glass pane protected solar cell panels familiar to many which are in current terrestrial use. They will, for reasons of weight, probably be built in a membrane form not suitable to terrestrial use where the considerable gravity loading imposes structural requirements on terrestrial implementations.

It is also possible to use Concentrating Photovoltaic (CPV) systems, which like SD are a form of existing terrestrial Concentrating Solar Energy approaches which convert concentrated light into electricity by PV, again avoiding the thermodynamic constraints which apply to heat engines. On Earth, these approaches use solar tracking systems, mirrors, lenses, etc to achieve high radiation concentration ratios and are able to reach efficiencies above 40% Concentrating Photovoltaic Technology. Because their PV area is rather smaller than in conventional PV, the majority of the deployed collecting area in CPV systems is mirrors, as with most SD systems. They share the advantages of building and pointing large (simple) mirror arrays in space as opposed to more complex PV panels.

Comparison of PV, CPV, and SD

The main problems with non-concentrating PV are that PV cells continue to be more expensive relative to the other approaches, and require a relatively large area to be acceptable for a significantly sized power station. In addition, semiconductor PV panels will require a relatively large amount of energy to manufacture; amorphous-silicon designs require much less energy to produce but have been substantially less efficient. CPV designs with a small area of 40%+ efficient cells and large reflector area are expected to be less expensive to produce. As well, the materials used in some PV cells (eg, gallium and arsenic) seem to be less common in lunar materials than is silicon; this may be significant if lunar manufacturing is involved.

SD is a more mature technology, having been in widespread use on Earth in many contexts for centuries. Both CPV and SD systems have more severe pointing requirements than PV, because most proposed designs require accurate and stable optical focus. If a PV array orientation drifts a few degrees, the power being produced will drop a few percent. If an SD or CPV array orientation drifts a few degrees, the power produced will drop very quickly, perhaps to near zero. Aiming reflector arrays requires much less energy in space than on Earth, being without terrestrial wind, weather, and gravitation loads, but it has its own problems of gyroscopic action, vibration, limits on usable reaction mass (though electrically powered gyros would avoid that problem), solar wind, and meteorite strikes on control mechanisms.

Currently, PV cells weigh between 0.5kg/kW and 10kg/kW depending on design. SD designs also vary but most seem to be heavier per kW produced than PV cells and thus have higher launch costs, all other things being equal. CPV should be lighter; since it replaces the thermal power plant (except for a radiator for waste heat) with a much lighter PV array.

Working lifetime

The lifetime of a PV based SPS is limited mainly by the ionizing radiation from the radiation belts and the Sun. Without a protection method, this is likely to cause the cells to degrade by about a percent or two per year. Deterioration is likely to be more rapid during periods of high exposure to energetic protons from solar particle outburst events. If some practical protection can be designed, this also might be reducible (eg, for a CPV station, radiation and particle shields for the PV cells -- out of the energy path from the mirrors, of course). Lifetimes for SD based SPS designs will be similarly limited, though largely for structural or mechanical considerations, such as micrometeorite impact, metal fatigue of turbine blades, wear of sliding surfaces (although this might be avoidable by hydrostatic bearings or magnetic bearings), degradation or loss of lubricants and working fluids in vacuum, from loss of structural integrity leading to impaired optical focus amongst components, and from temperature variation extremes. As well, most mirror surfaces will degrade from both radiation and particle impact, but such mirrors can be designed simply (and so to be light and cheap), and replacement may be practical.

In either case, another advantage of the SPS design is that waste heat developed at collection points is re-radiated back into space, instead of warming the adjacent local biosphere as with conventional sources, though some care will likely be required to provide for the radiation of this waste heat. Thus thermal efficiency will not be in itself an important design parameter except insofar as it affects the power/weight ratio via operational efficiency and hence pushes up launch costs. (For example SD may require larger waste heat radiators when operating at a lower efficiency). Earth based power handling systems must always be carefully designed, for both economic and purely engineering reasons, with operational thermal efficiency in mind.

One useful aspect of the SPS approach is that, at the end of life, the material does not need to be launched a second time, at least in principle. In theory, it would be possible to recycle much of the satellite 'on-site', potentially at a significantly lower cost than launching an SPS as new. This might allow a very expensive launch cost to be paid for over multiple satellite lifetimes, but does require an in orbit re-processing facility which doesn't currently exist.

Energy payback

Clearly, for an SPS system (including manufacture, launch and deployment) to provide net power it must repay the energy needed to construct it.

Solar satellites can pay back the lift energy in a remarkably short time. It takes 14.75 kWh/kg for a 100% efficiency system to lift a kg from the surface of the earth to GEO; no such launch system exists and so energy costs are always higher. If the satellite generated a kW with 2kg of mass, the payback time would be 29.5 hours. Assuming a much less efficient (and more realistic) 3% efficient rockets, the energy payback time is only extended to about 6 weeks for such an SPS.

For current silicon PV panels, production energy requirements are relatively high, and typically three-four years of deployment in a terrestrial environment is needed to recover this energy.

With SPS, net energy received on the ground is higher (more or less necessarily so, if the system to be worth deploying), so this energy payback period would be reduced to about a year. Thermal systems, being made of conventional materials, are more similar to conventional power stations and are likely to be less energy intensive during manufacture. They would be expected to give quicker energy break even, depending on construction technology. The relative merits of PV vs SD is still an open question.

Clearly, for a system (including manufacture, launch and deployment) to provide net power it must repay the energy needed to construct it. For current silicon PV panels this is relatively high. With an SPS, the net energy received on the ground is higher so this energy payback period would be somewhat reduced; however an SD based SPS, being made of conventional materials, are more similar to conventional powerstations and are likely to be less energy intensive during production and would be expected to give a quicker energy break even, depending on construction technology and other variations.

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.

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 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.

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.

For best efficiency, the satellite antenna should be circular and for the probable microwave wavelength, about 1 kilometers in diameter or larger; the ground antenna (rectenna) should be elliptical, 10km wide, and a length that makes the rectenna appear circular from GSO. (Typically, 14km at some North American latitudes.) Smaller antennas would result in increased losses to diffraction/sidelobes. For the desired (23mW/cm²) microwave intensity 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% 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 unacceptably short lifetimes. In either cases, 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.

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). There would be advantages, such as 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, including constantly changing antenna geometries, increased debris collision difficulties, 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.

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. 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, so such a rectenna would not be as expensive in terms of land use as might be supposed.

Advantages of an SPS

The SPS concept is attractive because space has several major advantages over the Earth's surface for the collection of solar power. There is no air in space, so the collecting surfaces would receive much more intense sunlight, unaffected by weather. In geostationary orbit, an SPS would be illuminated over 99% of the time. The SPS would be in Earth's shadow on only a few days at the spring and fall equinoxes; and even then for a maximum of 75 minutes late at night when power demands are at their lowest. This characteristic of SPS based power generation systems to avoid the expensive storage facilities (eg, lakes behind dams, oil storage tanks, coal dumps, etc) necessary in many Earth-based power generation systems. Additionally, an SPS will have none of the polluting consequences of fossil fuel systems, nor the ecological problems resulting from many renewable or low impact power generation systems (eg, dam retention lakes).

Economically, an SPS deployment project would create many new jobs and contract opportunities for industry, which may have political implications in the country or region which undertakes the project. Certainly the energy from an SPS would reduce political tension resulting from unequal distribution of energy supplies (eg, oil, gas, etc). For nations on the equator, SPS provides an incentive to stabilise and a sustained opportunity to lease land for launch sites.

An SPS would also be applicable on a global scale. Nuclear power especially is something many governments would be reluctant to sell to developing nations in which political pressures might lead to proliferation. Whether bio-fuels can support the western world, let alone the developed world, is currently a matter of debate. SPS poses no such problems.

Developing the industrial capacity needed to construct and maintain one or more SPS systems would significantly reduce the cost of other space endeavours. For example, a manned Mars mission might only cost hundreds of millions, instead of tens of billions, if it can rely on an already existing capability.

More long-term, the potential power production possible is enormous. If power stations can be placed outside Earth orbit, the upper limit is vastly higher still. In the extreme, such arrangements are called Dyson spheres.

Problems

Launch costs

Without doubt, the most obvious problem for the SPS concept is the current cost of space launches. Current rates on the Space Shuttle run between $3,000 and $5,000 per pound ($6,600/kg and $11,000/kg) to low Earth orbit, depending on whose numbers are used. Calculations show that launch costs of less than about $180-225 per pound ($400-500/kg) to LEO (Low Earth orbit) seem to be necessary.

However, economies of scale for expendable vehicles could give rather large reductions in launch cost for this kind of launched mass. Thousands of rocket launches could very well reduce the costs by ten to twenty times, using standard costing models. This puts the economics of an SPS design into the practicable range. Reusable vehicles could quite conceivably attack the launch problem as well, but are not a well-developed technology.

Much of the material launched need not be delivered to its eventual orbit immediately, which raises the possibility that high efficiency (but slower) engines could move SPS material from LEO to GEO at acceptable cost. Examples include ion thrusters or nuclear propulsion. They might even be designed to be reusable.

Power beaming from geostationary orbit by microwaves has the difficulty that the required 'optical aperture' sizes are very large. For example, the 1978 NASA SPS study required a 1-km diameter transmitting antenna, and a 10 km diameter receiving rectenna, for a microwave beam at 2.45 GHz. These sizes can be somewhat decreased by using shorter wavelengths, although they have increased atmospheric absorption and even potential beam blockage by rain or water droplets. Because of the thinned array curse, it is not possible to make a narrower beam by combining the beams of several smaller satellites. The large size of the transmitting and receiving antennas means that the minimum practical power level for an SPS will necessarily be high; small SPS systems will be possible, but uneconomic.

To give an idea of the scale of the problem, assuming an (arbitrary, as no space-ready design has been adequately tested) solar panel mass of 20 kg per kilowatt (without considering the mass of the supporting structure, antenna, or any significant mass reduction of any focusing mirrors) a 4 GW power station would weigh about 80,000 metric tons, all of which would, in current circumstances, be launched from the Earth. Very lightweight designs could likely achieve 1 kg/kW,, meaning 4,000 metric tons for the solar panels for the same 4 GW capacity station. This would be the equivalent of between 40 and 80 heavy-lift launch vehicle (HLLV) launches to send the material to low earth orbit, where it would likely be converted into subassembly solar arrays, which then could use high-efficiency ion-engine style rockets to (slowly) reach GEO (Geostationary orbit). With an estimated serial launch cost for shuttle-based HLLVs of $500 million to $800 million, total launch costs would range between $20 billion (low cost HLLV, low weight panels) and $320 billion ('expensive' HLLV, heavier panels). Economies of scale on such a large launch program could be as high as 90% (if a learning factor of 30% could be achieved for each doubling of production) over the cost of a single launch today. In addition, there would be the cost of an assembly area in LEO (which could be spread over several power satellites), and probably one or more smaller one(s) in GEO. The costs of these supporting efforts would also contribute to total costs.

So how much money could an SPS be expected to make? For every one gigawatt rating, current SPS designs will generate 8.75 terawatt-hours of electricity per year, or 175 TW•h over a twenty-year lifetime. With current market prices of $0.22 per kW•h (UK, January 2006) and an SPS's ability to send its energy to places of greatest demand (depending on rectenna siting issues), this would equate to $1.93 billion per year or $38.6 billion over its lifetime. The example 4 GW 'economy' SPS above could therefore generate in excess of $154 billion over its lifetime. Assuming facilities are available, it may turn out to be substantially cheaper to recast on-site steel in GEO, than to launch it from Earth. If true, then the initial launch cost could be spread over multiple SPS lifespans.

Extraterrestrial materials

Gerard O'Neill, noting the problem of high launch costs in the early 1970s, proposed building the SPS's in orbit with materials from the Moon. Launch costs from the Moon are about 100 times lower than from Earth, due to the lower gravity. This 1970s proposal assumed the then-advertised future launch costing of NASA's space shuttle. This approach would require substantial up front capital investment to establish mass drivers on the Moon.

Nevertheless, on 30 April 1979, the Final Report ("Lunar Resources Utilization for Space Construction") by General Dynamics' Convair Division, under NASA contract NAS9-15560, concluded that use of lunar resources would be cheaper than terrestrial materials for a system of as few as thirty Solar Power Satellites of 10GW capacity each.

In 1980, when it became obvious NASA's launch cost estimates for the space shuttle were grossly optimistic, O'Neill et al published another route to manufacturing using lunar materials with much lower startup costs This 1980s SPS concept relied less on human presence in space and more on partially self-replicating systems on the lunar surface under telepresence control of workers stationed on Earth. Again, this proposal suffers from the current lack of such automated systems on Earth, much less on the Moon.

Asteroid mining has also been seriously considered. A NASA design studyevaluated a 10,000 ton mining vehicle (to be assembled in orbit) that would return a 500,000 ton asteroid 'fragment' to geostationary orbit. Only about 3000 tons of the mining ship would be traditional aerospace-grade payload. The rest would be reaction mass for the mass-driver engine; which could be arranged to be the spent rocket stages used to launch the payload. Assuming, likely unrealistically, that 100% of the returned asteroid was useful, and that the asteroid miner itself couldn't be reused, that represents nearly a 95% reduction in launch costs. However, the true merits of such a method would depend on a thorough mineral survey of the candidate asteroids; thus far, we have only estimates of their composition. There has been no such survey. Once built, NASA's CEV should be capable of beginning such a survey, Congressional money and imagination permitting.

Lofstrom launch loop

A Lofstrom loop could conceivably provide the launch capacity needed to make a solar power satellite practical. This is a high capacity launch system capable of reaching a geosynchronous transfer orbit at low cost (Lofstrom estimates a large system could go as low as $3/kg to LEO for example). The Lofstrom loop is expected to cost less than a conventional space elevator to develop and construct, and to provide lower launch costs. Unlike the conventional space elevator, it is believed that a launch loop could be built with today’s materials.

Space elevators

More recently the SPS concept has been suggested as a use for a space elevator. The elevator would make construction of an SPS considerably less expensive, possibly making them competitive with conventional sources. However it appears unlikely that even recent advances in materials science, namely carbon nanotubes, can make possible such an elevator, nor to reduce the short term cost of construction of the elevator enough, if an Earth-GSO space elevator is ever practical. A variant to the Earth-GSO elevator concept is the Lunar space elevator, first described by Jerome Pearson in 1979. Because of the ~20 times shallower (than Earth's) gravitational well for the lunar elevator, this concept would not rely on materials technology beyond the current state of the art, but it would require establishing silicon mining and solar cell manufacturing facilities on the Moon, similar to O'Neill's lunar material proposal, discussed above.

Safety

The use of microwave transmission of power has been the most controversial issue in considering any SPS design, but any thought that anything which strays into the beam's path will be incinerated is an extreme misconception. Consider that quite similar microwave relay beams have long been in use by telecommunications companies world wide without such problems.

At the earth's surface, a suggested microwave beam would have a maximum intensity, at its center, of 23 mW/cm² (less than 1/4 the solar irradiation constant), and an intensity of less than 1 mW/cm² outside of the rectenna fenceline (10 mW/cm² is the current United States maximum microwave exposure standard). In the United States, the workplace exposure limit (10 mW/cm²) is at present, per the Occupational Safety and Health Act (OSHA), expressed in voluntary language and has been ruled unenforceable for Federal OSHA enforcement.

The beam's most intense section (more or less, at its center) is far below dangerous levels even for an exposure which is prolonged indefinitely. Furthermore, exposure to the center of the beam can easily be controlled on the ground (eg, via fencing), and typical aircraft flying through the beam provide passengers with a protective shell metal (ie, a Faraday Cage), which will intercept the microwaves. Other aircraft (balloons, ultra-light, etc) can avoid exposure by observing airflight control spaces, as is currently done for military and other controlled airspace. Over 95% of the beam energy will fall on the rectenna. The remaining microwave energy will be absorbed and dispersed well within standards currently imposed upon microwave emissions around the world.

The microwave beam intensity at ground level in the center of the beam would be designed and physically built into the system; simply, the transmitter would be too far away and too small to be able to increase the intensity to unsafe death ray levels, even in principle.

In addition, a design constraint is that the microwave beam must not be so intense as to injure wildlife, particularly birds. Experiments with deliberate microwave irradiation at reasonable levels have failed to show negative effects even over multiple generations.

Some have suggested locating rectennas offshore , but this presents serious problems, including corrosion, mechanical stresses, and biological contamination.

A commonly proposed approach to ensuring fail-safe beam targeting is to use a retrodirective phased array antenna/rectenna. A "pilot" microwave beam emitted from the center of the rectenna on the ground establishes a phase front at the transmitting antenna. There, circuits in each of the antenna's subarrays compare the pilot beam's phase front with an internal clock phase to control the phase of the outgoing signal. This forces the transmitted beam to be centered precisely on the rectenna and to have a high degree of phase uniformity; if the pilot beam is lost for any reason (if the transmitting antenna is turned away from the rectenna, for example) the phase control value fails and the microwave power beam is automatically defocused. Such a system would be physically incapable of focusing its power beam anywhere that did not have a pilot beam transmitter.

It is important for system efficiency that as much of the microwave radiation as possible be focused on the rectenna. Outside of the rectenna, microwave intensities would rapidly decrease, so nearby towns or other human activity should be completely unaffected.

The long-term effects of beaming power through the ionosphere in the form of microwaves has yet to be studied, but nothing has been suggested which might lead to any significant effect.

Defending solar power satellites

Solar power satellites would normally be at a high orbit that is difficult to reach, and hence attack.

However, it has been suggested that a large enough quantity of granular material placed in a retrograde orbit at the geostationary altitude could theoretically completely destroy these kinds of system and render that orbit useless for generations.

Whether this is a realistic attack scenario is arguable, and in any case at the present time there is only a small list of countries with the necessary launch capability to do this, such an attack would probably be considered an act of war by every single nation (except the attacker, which would lose its satellites, too) with satellites in geostationary orbit, and an attack with more conventional anti-satellite weapons would probably be considered an act of war by the nation whose satellite was attacked. In any case, the receiving stations on the ground, and conventional power generators (which are unlikely to be completely replaced by solar power satellites), are more easily attacked.

Computer security may be a bigger issue than physical defense, since launch capabilities aren't necessary to hack a satellite for purposes of malicious orbital "corrections", extortion (by threatening to destabilize its orbit) or outright "grand theft satellite".

SPS's economic feasibility

Current energy price landscape

In order to be competitive on a purely economic level, an SPS must cost no more than existing supplies. (Such costs must include the costs of cleaning waste from construction, operation and dismantling of the generating systems--including lifestyle and health costs.. Currently(2007) most Earth-based power generation does not include these costs. The cost figures below are undated, but are obsolete as of 2007. This greatly reduces the prices paid for power currently reducing the apparent benefits of SPS'.) This may be difficult, especially if it is deployed for North America, where energy costs have been relatively low. It must cost less to deploy, or operate for a very long period of time, or offer other advantages. Many proponents have suggested that the lifetime is effectively infinite, but normal maintenance and replacement of less durable components makes this unlikely. Satellites do not, in our now-extensive experience, last forever. (But with regular maintenance there is no reason that a high orbit satellite has to 'die.' Currently (2007) the majority of such satellites--weather and communications, fail due to correctable maintenance issues which we do not correct because we have no repair people on site. Common failures are: running out of station keeping fuel or dead batteries-no longer holding a charge. Neither of these failure modes is much of a problem if service is available. With available refueling and battery replacement, the life of a satellite can be greatly increased. Structural components, which make up the largest percentage of mass, seldom fail. Nearly all of the other components can be modularized for easy replacement/upgrade.)

Current prices for electricity on the public grid fluctuate depending on time of day, but typical household delivery costs about 5 cents per kilowatt hour in North America. If the lifetime of an SPS is 20 years and it delivers 5 gigawatts to the grid, the commercial value of that power is (5,000,000,000 watts)/(1000 watts/kilowatt) = 5,000,000 kilowatts, which multiplied by $.05 per kW•h gives $250,000 revenue per hour. $250,000 × 24 hours × 365 days × 20 years = $43,800,000,000. By contrast, in the United Kingdom (October 2005) electricity can cost 9–22 cents per kilowatt hour. This would translate to a lifetime output of $77–$193 billion for power delivered to the UK.

Comparison with fossil fuels

The relatively low price of energy today is entirely dominated by the historically low cost of carbon based fossil fuels (e.g., petroleum, coal and natural gas).

There are several problems with existing energy delivery systems. They are subject to (among other problems)

• political instability for various reasons in various locations -- so that there are large hidden costs in maintaining military or other presence so as to continue supplies
• depletion (some well regarded estimates suggest that oil and gas reserves have been in net decline for some time and that price increases and supply decreases are inevitable)
  • oil prices rose from around $20/bbl in the early 2000s to over $130/bbl in early 2008, despite no major disruptions in supply, suggesting to some industry observers (e.g., Matthew Simmons) that the days of cheap oil are over
• greenhouse pollution -- fossil fuel combustion emits enormous quantities of carbon dioxide (CO₂), a greenhouse gas, contributing to global warming and climate change.

Following the Kyoto Treaty, 141 countries introduced the first system of mandatory emissions control via carbon credits. The ultimate direction of such policies is to increase efficiency of fossil fuel use, perhaps to the point of elimination in some countries or even globally. But, the energy requirements of Third World or developing countries (e.g., China and India) are increasing steadily. Because of the net increase in demand, energy prices will continue to increase, though how fast and how high are less easily predicted. hi

Comparison with nuclear power (fission)

Detailed analyses of the problems with nuclear power specifically (nuclear fission) are published elsewhere. Some are given below, with some comparative comments:

• nuclear proliferation -- not a problem with SPS
• disposal and storage of radioactive waste -- not a problem with SPS
• preventing fissile material from being obtained by terrorists or their sponsors -- not a problem with SPS
• public perception of danger -- problem with both SPS and nuclear power
• consequences of major accident, e.g., Chernobyl -- effectively zero with SPS, save on launch (during construction or for maintenance)
• military and police cost of protecting the public and loss of democratic freedoms -- control of SPS would be a power/influence center, perhaps sufficient to translate into political power. However, this has not yet happened in the developed world with nuclear power.
• installation delays. These have been notoriously long with nuclear power plants (at least in the US), and may be reduced with SPS. With sufficient commitment from SPS backers, the difference may be substantial.

On balance, SPS avoids nearly all of the problems with current nuclear power schemes, and does not have larger problems in any respect, although public perception of microwave power transfer (ie, in the beams produced by an SPS and received on Earth) dangers could become an issue.

Comparison with nuclear fusion

Nuclear fusion is a process used in thermonuclear bombs (e.g., the H-bomb). Projected nuclear fusion power plants would not be explosive, and will likely be inherently failsafe. However, sustained nuclear fusion generators have only just been demonstrated experimentally, despite well funded research over a period of several decades (since approximately 1952). There is still no credible estimate of how long it will be before a nuclear fusion reactor could become commercially possible; fusion research continues to receive substantial funding by many nations. For example, the ITER facility currently under construction will cost €10 billion. There has been much criticism of the value of continued funding of fusion research. Proponents have successfully argued in favor of ITER funding.

By contrast, SPS does not require any fundamental engineering breakthroughs, has already been extensively reviewed from an engineering feasibility perspective over some decades, and needs only incremental improvements of existing technology to be deployed. Despite these advantages, SPS has received minimal research funding to date.

Comparison with terrestrial solar power

In the case of the United Kingdom, the country as a whole is further north than even most inhabited parts of Canada, and hence receives little insolation over much of the year, so conventional solar power is not competitive at 2006 per-kilowatt-hour delivered costs. However, per-kilowatt-hour photovoltaic costs have been in exponential decline for decades, with a 20-fold decrease from 1975 to 2001, so this situation may change.

Let us consider a ground-based solar power system versus an SPS generating an equivalent amount of power.

• Such a system would require a very large solar array built in a well-sunlit area, the Sahara Desert for instance. An SPS requires much less ground area per kilowatt (approx 1/5th). There is no such area in the UK.
• The rectenna on the ground is much larger than the area of the orbiting solar panels. A ground-only solar array would have the advantage, compared to a GEO (Geosynchronous orbit) solar array, of costing considerably less to construct and requiring no significant technological advances. A small version of such a ground based array has recently been completed by General Electric in Portugal.
• The receiving SPS rectenna will be quite simple, cheap, and even transparent, with fewer land use issues than a conventional terrestrial solar array. Crops could be grown beneath the rectenna, so the land needed could be dual-use. By comparison, ground-based solar panels would completely block sunlight thus destroying vegetation and having a considerable effect on local ecology, which in turn would result in increased soil erosion, drainage and runoff problems (increased flood risk) and loss of habitats, though this would be reduced somewhat for desert installations.
• A terrestrial solar station intercepts an absolute maximum of only one third of the solar energy an array of equal size could intercept in space, since no power is generated at night and less light strikes the panels when the Sun is low in the sky or weather interferes. A solar panel in the contiguous United States on average delivers 19 to 56 W/m² . By comparison an SPS rectenna would deliver about 23mW/cm² (230 W/m²) continuously, hence the size of rectenna required per collected watt would be about 8.2% to 24% that of a terrestrial solar panel array with equivalent power output, neglecting weather and night/day cycles. Assuming, of course, current levels of solar cell efficiency.
• Further, if it is assumed that a ground-based solar array must supply baseload power (not true for every projected configuration), some form of energy storage would be required to provide power at night, such as hydrogen generation/storage, compressed air, or pumped storage hydroelectricity. With present technology, energy storage on this scale is prohibitively expensive, and will incur energy losses as well.
• Weather conditions would also interfere with power collection, and will cause wear and tear on solar collectors which will be avoided in Earth orbit; for instance, sandstorms cause devastating damage to human structures via, for example, abrasion of surfaces as well as mechanically large wind forces causing direct physical damage. Terrestrial systems are also more vulnerable to terrorism than an SPS's rectenna since they are more expensive, complex, intolerant of partial damage, and harder to repair/replace. Wear and tear on orbital installations will be of very different character, for quite different reasons, and can be reduced by care in design and fabrication. Long experience with terrestrial installations shows that there is substantial, inescapable maintenance for any economically feasible electrical installation.
• Terrestrial solar panel locations are inherently fixed, but beamed microwave power allows one to adaptively re-route delivered power near to places it is needed (within limits -- rectennas near the SPS's horizon (e.g., at high latitudes) will not be as efficient). A station in the Sahara could provide practical power only to the surrounding area; current demand is relatively low there. That is, at least until long distance superconducting distribution becomes possible, which will make remotely sited Earth surface collection systems more practical, and distribution of generated power equally so, including that from an SPS.
• Remote tropical location of an extensive photovoltaic generator is a somewhat artificial scenario, as photovoltaic costs continue to decline. Deployment of ground-based photovoltaics can be distributed (say to rooftops), but nevertheless, the required acreage (at any credible solar cell efficiency) will remain very large, and maintenance cost and effort will increase substantially compared to a large centralized design. In any case, dispersed installation is not possible for some terrestrial solar collectors.
• Energy payback time for the capital costs of terrestrial PV cells has been typically in the 5-15 year range, depending largely on existing local cost structures. Payback for an orbital installations is likely to be quicker due to the higher total insolation rate, which will, of course be essentially continuous, without interruptions during nighttimes or bad weather. While it is true some of the potential energy available would not be collected (cell inefficienies will assure this in any case), that some would be lost internally at the SPS (no equipment is loss free), and that still more would be lost in transmission back to the Earth, the engineering feasibility studies have established that none of these losses will be large enough to make an SPS project infeasible on those grounds. Losses due to conventional fossil fuel generation are of larger magnitude than in an SPS design, and are more than merely lost efficiency as such losses all contribute to pollution (eg, exhaust gases).

Both SPS and ground-based solar power could be used to produce chemical fuels for transportation and storage, as in the proposed hydrogen economy. Or they could both be used to run an energy storage scheme (such as pumping water uphill at a hydropower generation station).

Many advances in solar cell efficiency (eg, improved construction techniques) that make an SPS more economically feasible might make a ground-based system more economic as well. Also, many SPS designs assume the framework will be built with automated machinery supplied with raw materials, typically aluminium. Such a system could be (more or less easily) adapted for operation on Earth, no launching required. However, Earth-based construction already has access to inexpensive human labor that would not be available in space, so such construction techniques would have to be extremely competitive to be significant on Earth.

Solar panel mass production

Currently the costs of solar panels are too high to use them to produce bulk domestic electricity in most situations. However, mass production of the solar panels necessary to build an SPS system would be likely to reduce those costs sufficiently to change this -- perhaps substantially -- especially as fossil fuel costs have been increasing rapidly. But, any panel design suited to SPS use is likely to be quite different than earth suitable panels, so not all such improvements will have this effect. This may benefit earth based array designs as costs may be lower (see the cost analysis above), but will not be able to take advantage of maximum economies of scale, and so piggyback on production of Earth based panels.

It should be noted, however, that there are also frequent developments in the production of solar panels. Thin film solar panels and so-called "nanosolar" might increase collection efficiency, reduce production costs as well as weight, and therefore reduce the total cost of an SPS installation. In addition, private space corporations could become interested in transporting goods (such as satellites, supplies and parts of commercial space hotels) to LEO (Low Earth orbit), since they already are developing spacecraft to transport space tourists. If they can reduce costs, this will also increase the economic feasibility of an SPS.

Comparison with other renewables (wind, tidal, hydro, geothermal)

Main article: Renewable energy

Most renewable energy sources (for example, tidal energy, hydro-electric, geothermal, ethanol), have the capacity to supply only a tiny fraction of the global energy requirement, now or in the foreseeable future. For most, the limitation is geography as there simply are very few sites in the world where generating systems can be built, and for hydro-electric projects in particular, there are few sites still open. For 2005, in the US, hydro-electric power accounted for 6.5% of electricity generation, and other renewables 2.3%. The U.S. Govt. Energy Information Administration projects that in 2030 hydro-power will decline to 3.4% and other renewables will increase to 2.9%.

Comparison with biofuels

Main article: Biofuel

Ethanol power production depends on farming in the case of corn or sugar cane, currently the two leading sources of ethanol fuel. There is insufficient farming capacity for both significant energy production and food production. Corn prices have risen substantially in 2006 and 2007, partly as a result of nascent ethanol production demand. Due to the high energy cost of industrial agriculture as well as the azeotropic distillation necessary to refine ethanol, serious questions remain about the EROEI of ethanol from corn. Ethanol from cellulose (eg, agricultural waste or purpose collected non-cultivated plants, eg, switchgrass) is not practicable as of 2007, though pilot plants are in development. Processing improvements (eg, a breakthrough in enzyme processing) may change this relative disadvantage.

Comparison with wind power

Main article: Wind power

Wind power is somewhat unique among the renewables as having emerged as competitive with fossil fuels on cost (similar to hydro), but unlike hydro has significant potential for expansion. Wind power has been the fastest-growing form of renewable energy throughout the 2000s, growing at an annual rate of approximately 30%. As of 2008, wind power's share of global energy output remained small, but wind power accounted for a large share of new power generation capacity in several countries including the United States and the United Kingdom. Improvements in technology, especially the trend toward larger wind turbines mounted on taller towers, has reduced the cost of wind power to be competitive with fossil fuel. The potential for wind power appears to be very large. For example, just the four windiest states in the United States (North Dakota, Texas, Kansas, and South Dakota), have wind resources that could equal the current electricity consumption of the entire country. Offshore wind resources appear to be even larger than on-shore wind resources. One advantage of wind farms is their ability to expand incrementally; individual wind turbines can be assembled on site at a typical rate of approximately one per week, and begin generating electricity (and thus revenue) as soon as they connect to the transmission grid. This gives wind power a lower capital risk compared to large-scale power generation schemes that require heavy investment for years before they become operational (e.g., hydroelectric power, nuclear power).

Ocean-based windpower offers access to very large wind resources (there being large areas for potential installations, and winds tend to blow stronger and steadier over water than over land due to reduced surface friction), but it is strongly affected by two factors: the difficulty of long distance power transmission as many regions of high demand are not near the sea, and by the very large difficulty of coping with corrosion, contamination, and survivability problems faced by all seaborne installations.

Some potential locations for offshore wind turbines suffer less from these problems, such as the Great Lakes of the United States and Canada, which are surrounded by well-developed power grids and large populations of electricity consumers. The lakes, being fresh water, would pose fewer corrosion problems, and construction in these environments is well-understood.

Current work

For the past several years there has been no line item for SPS in either the NASA nor DOE budgets, a minimal level of research has been sustained through small NASA discretionary budget accounts.

NASA's "Fresh Look" study in 2000

NASDA (Japan's national space agency) has been researching in this area steadily for the last few years. In 2001 plans were announced to perform additional research and prototyping by launching an experimental satellite of capacity between 10 kilowatts and 1 megawatt of power.

The National Space Society (a non-profit NGO) maintains a web page where the latest SPS related references are posted and kept current .

In May 2007 a workshop was held at MIT in the U.S.A. to review the current state of the market and technology

In 2007 the U.S. Department of Defense expressed interest in studying the concept.

On 10/10/2007 The National Security Space Office of the US Department of Defense, published an assessment report . The report was released at a press conference which simultaneously announced the formation of the Space Solar Alliance for Future Energy which intends to pursue the recommendations of the NSSO-Led Study.

In fiction

Space stations transmitting solar power have appeared in science-fiction works like Isaac Asimov's Reason (1941), that centers around the troubles caused by the robots operating the station.

Solar Power Satellites have also been seen in the work of author Ben Bova's novels "Powersat" and "Colony".

The anime series Gundam 00 explores the effects and politics of space based solar power.

In both SimCity 2000 and 3000, plants that improvised solar satellite technology called microwave powerplants were available in the future. The plant was discontinued in SimCity 4 but several fan-made microwave powerplants were available on various SimCity 4 fan-sites.

Solar Sats are used in the online browser-based game ogame. They are a means to supply power to planet production.

See also

• Space solar power
• Energy economics
• Energy storage
• Exergy
• Energy development
• Energy quality
• Microwave power transmission
• Renewable energy
• Spectrolab
• Thinned array curse
• Wireless energy transfer

References

• Solar Power Satellites (Hardback) Glaser, P. E., Frank P. Davidson and Katinka Csigi, 654 pgs, 1998, John Wiley & Sons ISBN 0-471-96817-X
• Rodenbeck, Christopher T. and Chang, Kai, "A Limitation on the Small-Scale Demonstration of Retrodirective Microwave Power Transmission from the Solar Power Satellite", IEEE Antennas and Propagation Magazine, August 2005, pp. 67–72.
• The above sites Solar Power Satellites Office of Technology Assessment, US Congress, OTA-E-144, August 1981.

External links

• The World Needs Energy from Space Space-based solar technology is the key to the world's energy and environmental future, writes Peter E. Glaser, a pioneer of the technology.
• Reinventing the Solar Power Satellite", NASA 2004-212743, report by Geoffrey A. Landis of NASA Glenn Research Center
• Japan's plans for a Solar Power Station in Space - the Japanese government hopes to assemble a space-based solar array by 2040.
• Power From Space Blog about using solar power satellites to reduce reliance on burning hydrocarbons.
• Whatever happened to solar power satellites? An article that covers the hurdles in the way of deploying a solar power satellite.
• Solar Power Satellite from Lunar and Asteroidal Materials Provides an overview of the technological and political developments needed to construct and utilize a multi-gigawatt power satellite. Also provides some perspective on the cost savings achieved by using extraterrestrial materials in the construction of the satellite.
• A renaissance for space solar power? by Jeff Foust, Monday, August 13, 2007 Reports on renewed institutional interest in SSP, and a lack of such interest in past decades.