Space-based solar power

From Wikipedia, the free encyclopedia

Space-based solar power (SBSP) (or historically space solar power (SSP)) is a system for the collection of solar power in space, for use on Earth. SBSP differs from the usual method of solar power collection in that the solar panels used to collect the energy would reside on a satellite in orbit, often referred to as a solar power satellite (SPS), rather than on Earth's surface. In space, collection of the Sun's energy is unaffected by the day/night cycle, weather, seasons, or the filtering effect of Earth's atmospheric gases. Average solar energy per unit area outside Earth's atmosphere is on the order of ten times that available on Earth's surface^{[citation needed]}.

The collection of solar energy in space for use on Earth introduces the new problem of transmitting energy from the collection point, in space, to the place where the energy would be used, on Earth's surface. Since wires extending from Earth's surface to an orbiting satellite would be impractical, many SBSP designs have proposed the use of microwave beams to transmit power wirelessly. The collecting satellite would convert solar energy into electrical energy, which would then be used to power a microwave emitter directed at a collector on the Earth's surface. Dynamic solar thermal power systems are also being investigated.^[1][link broken]

Many problems normally associated with solar power collection would be eliminated by such a design, such as the high sensitivity of conventional surface solar panels to corrosion and weather, and the resulting maintenance costs. Other problems may take their place though, such as cumulative radiation damage or micrometeoroid impacts.

Producing electricity from sunlight in space is not a new or untried technology. Many space-faring craft, such as rovers and shuttles, are covered in solar cells, and hundreds of operating satellites use solar energy as their main source of power. What has never been tried before is transmitting that power back to Earth for our use^[2], or building any space structure even remotely as large as would be required.

Being a clean and safe energy design, space-based solar power has the potential to play a significant role in solving global energy and environmental problems if the basic issues of cost and engineering can be addressed. It utilizes space outside of Earth's ecological system, and may essentially produce no by-products.

Contents 1 Timeline 2 History 2.1 SERT 3 Design 3.1 Solar energy conversion (solar photons to DC current) 3.2 Wireless power transmission to the Earth 3.2.1 Laser power beaming experiments 3.3 Spacecraft sizing 3.4 LEO/MEO instead of GEO 3.5 Earth-based infrastructure 4 Advantages 5 Problems 5.1 Launch costs 5.1.1 Building from space 5.1.2 Non-Conventional Launch Methods 6 Myths 6.1 Safety 6.2 Atmospheric damage due to launches 6.3 Increased global warming 7 In fiction 8 See also 9 References 10 External links

[edit] Timeline

• 1968: Dr. Peter Glaser introduces the concept of a large solar power satellite system of square miles of solar collectors in high geosynchronous orbit (GEO is an orbit 36,000 km above the equator), for collection and conversion of sun's energy into an electromagnetic microwave beam to transmit usable energy to large receiving antennas (rectennas) on earth for distribution on the national electric power grid.

• 1973: Dr. 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, now known as a rectenna.^[3]

• 1970s: DOE and NASA examines the Solar Power Satellite (SPS) concept extensively

• 1994: The United States Air Force conducts the Advanced Photovoltaic Experiment using a satellite launched into low Earth orbit by a Pegasus rocket.

• 1995–1997: NASA conducts a “Fresh Look” study of space solar power (SSP) concepts and technologies.

• 1998: Space Solar Power Concept Definition Study (CDS) identifies commercially viable SSP concepts which are credible, with technical and programmatic risks identified.

• 1999: NASA's Space Solar Power Exploratory Research and Technology program (SERT see section below) program initiated.

• 2000: John Mankins of NASA testifies in the U.S. House "Large-scale SSP is a very complex integrated system of systems that requires numerous significant advances in current technology and capabilities. A technology roadmap has been developed that lays out potential paths for achieving all needed advances — albeit over several decades.^[4]

• 2001: PowerSat Corporation founded by William Maness.

• 2001: Dr. Neville Marzwell of NASA states "We now have the technology to convert the sun's energy at the rate of 42 to 56 percent... We have made tremendous progress. ...If you can concentrate the sun's rays through the use of large mirrors or lenses you get more for your money because most of the cost is in the PV arrays... There is a risk element but you can reduce it... You can put these small receivers in the desert or in the mountains away from populated areas. ...We believe that in 15 to 25 years we can lower that cost to 7 to 10 cents per kilowatt hour. ...We offer an advantage. You don't need cables, pipes, gas or copper wires. We can send it to you like a cell phone call—where you want it and when you want it, in real time."^[5]

• 2001: NASDA (Japan's national space agency) announced plans to perform additional research and prototyping by launching an experimental satellite of capacity between 10 kilowatts and 1 megawatt of power.^[6][7]

• 2007: The Pentagon's National Security Space Office (NSSO) issued a report^[8] on October 10, 2007 that states they intend to collect solar energy from space for use on Earth to help the United States' ongoing relationship with the Middle East and the battle for oil. The International Space Station is most likely to be the first test ground for this new idea, even though it is in a low-earth orbit.

• 2007: In May 2007 a workshop was held at MIT to review the current state of the market and technology.^[9]

• 2009: A new company, Space Energy, Inc., plans to provide space-based solar power commercially. They say they have developed a "rock-solid business platform" and should be able to provide space-based solar power within a decade.^[10]

• 2009: Pacific Gas and Electric (PG&E) announces it is seeking regulatory approval for an agreement with Solaren to buy 200 MW of solar power, starting in 2016. PG&E spokesman Jonathan Marshall stated that "We've been very careful not to bear risk in this."^[11][12][13]

• 2009: PowerSat Corporation files a patent concerning ganging multiple power satellites to form a single coherent microwave beam, and a mechanism to use the solar array to power ion thrusters to lift a power satellite from LEO to GEO. ^[14]

[edit] History

The SPS concept, originally known as Satellite Solar Power System ("SSPS") was first described in November 1968 ^[15]. In 1973 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, now known as a rectenna.^[16]

Glaser then worked at Arthur D. Little, Inc., as a vice-president. NASA 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 ^[17].

Between 1978 and 1981 the US Congress authorized DOE and NASA to jointly investigate. They organized the Satellite Power System Concept Development and Evaluation Program ^[18][19]. The study remains the most extensive performed to date. Several reports were published investigating possible problems with such an engineering project. They include:

• Resource Requirements (Critical Materials, Energy, and Land)^[20]
• Financial/Management Scenarios^[21][22]
• Public Acceptance^[23]
• State and Local Regulations as Applied to Satellite Power System Microwave Receiving Antenna Facilities^[24]
• Student Participation^[25]
• Potential of Laser for SPS Power Transmission^[26]
• International Agreements^[27][28]
• Centralization/Decentralization^[29]
• Mapping of Exclusion Areas For Rectenna Sites^[30]
• Economic and Demographic Issues Related to Deployment^[31]
• Some Questions and Answers^[32]
• Meteorological Effects on Laser Beam Propagation and Direct Solar Pumped Lasers^[33]
• Public Outreach Experiment^[34]
• Power Transmission and Reception Technical Summary and Assessment ^[35]
• Space Transportation^[36]

The Office of Technology Assessment^[37] concluded

Too little is currently known about the technical, economic, and environmental aspects of SPS to make a sound decision whether to proceed with its development and deployment. In addition, without further research an SPS demonstration or systems-engineering verification program would be a high-risk venture.

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"^[38]. In assessing "What has changed" since the DOE study, this study asserts that

Another important change has occurred at the US national policy level. US National Space Policy now calls for NASA to make significant investments in technology (not a particular vehicle) to drive the costs of ETO [Earth to Orbit] transportation down dramatically. This is, of course, an absolute requirement of space solar power.

[edit] SERT

In 1999 NASA's Space Solar Power Exploratory Research and Technology program (SERT) was initiated for the following purpose:

• Perform design studies of selected flight demonstration concepts;
• Evaluate studies of the general feasibility, design, and requirements.
• Create conceptual designs of subsystems that make use of advanced SSP technologies to benefit future space or terrestrial applications.
• Formulate a preliminary plan of action for the U.S. (working with international partners) to undertake an aggressive technology initiative.
• Construct technology development and demonstration roadmaps for critical Space Solar Power (SSP) elements.

It was to develop a solar power satellite (SPS) concept for a future gigawatt space power systems to provide electrical power by converting the Sun’s energy and beaming it to the Earth's surface. It was also to provide a developmental path to solutions for current space power architectures. Subject to studies it proposed an inflatable photovoltaic gossamer structure with concentrator lenses or solar dynamic engines to convert solar flux into electricity. Collection systems were assumed to be in sun-synchronous orbit.

Some of SERT's conclusions include the following:

• The increasing global energy demand is likely to continue for many decades resulting in new power plants of all sizes being built.
• The environmental impact of those plants and their impact on world energy supplies and geopolitical relationships can be problematic.
• Renewable energy is a compelling approach, both philosophically and in engineering terms.
• Many renewable energy sources are limited in their ability to affordably provide the base load power required for global industrial development and prosperity, because of inherent land and water requirements.
• Based on their Concept Definition Study, space solar power concepts may be ready to reenter the discussion.
• Solar power satellites should no longer be envisioned as requiring unimaginably large initial investments in fixed infrastructure before the emplacement of productive power plants can begin.
• Space solar power systems appear to possess many significant environmental advantages when compared to alternative approaches.
• The economic viability of space solar power systems depends on many factors and the successful development of various new technologies (not least of which is the availability of exceptionally low cost access to space) however, the same can be said of many other advanced power technologies options.
• Space solar power may well emerge as a serious candidate among the options for meeting the energy demands of the 21st century.^[39]

[edit] 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.

[edit] Advantages

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^[45] 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 are reluctant to sell to developing nations, where political pressures might lead to proliferation of nuclear technology. Whether biofuels 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.

The longer-term potential power production 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.

[edit] Problems

[edit] Launch costs

One problem for the SPS concept is the current cost of space launches. Current rates on the Space Shuttle run between $6,600/kg and $11,000/kg to low Earth orbit, depending on whose numbers are used. Alternative vehicles, such as the Falcon 9 Heavy, are predicted to launch to LEO for approximately $2,900/kg. Calculations^[which?] show that launch costs of less than about $400–500/kg to LEO (Low Earth orbit) are 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.^[46] 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,^[47], 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, and total launch costs for alternative HLLVs at $78 million, total launch costs would range between $11.3 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.

[edit] Building from space

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.^[48] 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.^[49]

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 ^[50] 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 study^[51] evaluated 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.

[edit] Non-Conventional Launch Methods

It is possible that a solar power satellite could be reduced in cost if a means of putting the materials into orbit were developed that did not rely on rockets. Some possible technologies to do this include ground launch systems such as Mass drivers or Lofstrom loops, which launch using electrical power, or the geosynchronous orbit space elevator. Such non-conventional launch techniques could make construction of an SPS considerably less expensive, possibly making them competitive with conventional sources. However, these all require some technology development, and many of these require additional advances in materials science, such as development of high-strength carbon nanotubes.

Advanced techniques for launching from the moon may reduce the cost of building a solar power satellite from lunar materials. Some proposed techniques include the lunar mass driver and the Lunar space elevator, first described by Jerome Pearson.^[52] It would require establishing silicon mining and solar cell manufacturing facilities on the Moon, as discussed above.

[edit] Myths

[edit] 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^[41] (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)^[53], 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. ^[54] 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 metal shell (ie, a Faraday Cage), which will intercept the microwaves. Other aircraft (balloons, ultralight, 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.^[55]

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. ^[56]

Some have suggested locating rectennas offshore ^[57][58], 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.^[59] 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.^[59]

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.

[edit] Atmospheric damage due to launches

When rockets launch through the atmosphere the hot rocket exhaust reacts with the atmospheric nitrogen and can form nitrogen compounds. In particular these nitrogen compounds are problematic when they form in the the stratosphere as they can damage the ozone layer. However, the environmental effect of rocket launches is negligible compared to higher volume polluters, such as airplanes and automobiles.

[edit] Increased global warming

The entire point of a solar power satellite is to increase the amount of solar energy reaching earth. This extra energy will eventually be dissipated as heat. Depending on the scale of operations, this might or might not have a significant effect. No theories to date claim that waste heat from human power generation are a significant cause of global warming, nor would it be for the foreseeable future. The most widely promoted theory connecting human activity to global warming is that increased greenhouse gases (e.g. carbon dioxide and methane) are causing the natural heat from the Sun to be trapped so it cannot radiate to space, thus increasing the temperature of the planet. Space solar power would contribute greatly to reduction of greenhouse gases.

Rectenna power conversion efficiency would be better than 90%, so waste heat from the rectennas would be considerably less than from most other common power sources, e.g. nuclear and fossil fuels which generate much more waste heat.

[edit] 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".
• In Sid Meier's Alpha Centauri, an endgame 'building' that fulfills the same function as an SPS is the 'Orbital Power Transmitter' which provides every city that you own with a unit of energy per satellite launched, providing the city has an Aerospace Command building or your faction controls the Space Elevator. Building multiple Orbital Power Transmitters provides massive bonuses to energy generation and soon pay for themselves many times over.
• Much of the conflict in the anime series Mobile Suit Gundam 00 arose from the construction of three space-based solar power systems and orbital elevators in a post-fossil fuels society.
• 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.

[edit] See also

• Future energy development
• Photovoltaics
• Project Earth
• Satellite
• Solar power
• Solar panels on spacecraft
• Orbital station-keeping in GEO
• Attitude control
• Friis transmission equation equation for beamed wireless power efficiency

[edit] References

[edit] 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.
• 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.
• PowerSat Corporation / PowerSat Limited website Commercial space based solar power companies in Europe and the United States.
• "Conceptual Study of A Solar Power Satellite, SPS 2000" Makoto Nagatomo, Susumu Sasaki and Yoshihiro Naruo
• Researchers Beam 'Space' Solar Power in Hawaii (Wired Science)
• http://www.nss.org/settlement/ssp/library/index.htm — The National Space Society's Space Solar Power Library