Sun over Earth

Space-based solar power (SSP) is the conversion of solar energy into power, usable either in space or on earth, from a location in space such as geosynchronous orbit (GSO). Photovoltaics (PV) would generally be utilized for energy conversion and microwave technology could be applied for wireless energy transmission through space. Dynamic solar thermal power systems are also being investigated.^[1] In space, sun shines constantly and has greater intensity than on earth. Many problems associated with weight and atmospheric corrosion are eliminated. On earth, diurnal rotation and the associated change from day to night necessitates collection only during daylight hours. Outside of earth's atmosphere, average solar energy per unit area is on the order of ten times that available on earth and increases as the sun is approached, although there are increased maintenance problems beyond acceptable solar radiation limits.

Spacecraft operating in the inner solar system usually rely on the use of photovoltaic solar panels to derive electricity from sunlight. In the outer solar system, where the sunlight is too weak to produce sufficient power, radioisotope thermal generators (RTGs) are used as a power source^[1].

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^[1]

Producing electricity from sunlight in space is not a new or untried technology. It has been utilized by hundreds of operating satellites. The major difference would be that SSP would capture much more energy and beam it to earth for our use.^[2]

Future space solar power has the potential to solve global socioeconomic and environmental problems associated with reliance on finite fossil fuels and nuclear energy. It promises to use space outside of the earth's ecology system and has essentially no by-product waste, once established.

History

1829 & 1830: Francesco Zantedeschi publishes papers on the production of electric currents in closed circuits by the approach and withdrawal of a magnet, thereby anticipating Michael Faraday's classical experiments of 1831.

1831: Michael Faraday began experiments leading to his discovery of electromagnetic induction, though the discovery may have been anticipated by the work of Francesco Zantedeschi. His breakthrough came when he wrapped two insulated coils of wire around a massive iron ring, bolted to a chair, and found that upon passing a current through one coil, a momentary electric current was induced in the other coil. He then found that if he moved a magnet through a loop of wire, or vice versa, an electric current also flowed in the wire. He then used this principle to construct the electric dynamo, the first electric power generator. He proposed that electromagnetic forces extended into the empty space around the conductor, but did not complete that work. Faraday's concept of lines of flux emanating from charged bodies and magnets provided a way to visualize electric and magnetic fields. That mental model was crucial to the successful development of electromechanical devices which were to dominate the 19th century. His demonstrations that a changing magnetic field produces an electric field, mathematically modelled by Faraday's law, would subsequently become one of Maxwell's four equations. These consequently evolved into the generalization of field theory.

1865: James Clerk Maxwell publishes his landmark paper A Dynamical Theory of the Electromagnetic Field, in which Maxwell's equations demonstrated that electric and magnetic forces are two complementary aspects of electromagnetism. He shows the associated complimentary electric and magnetic fields of electromagnetism travel through space, in the form of waves, at a constant velocity of 3.0 × 10⁸ m/s. He also proposes that light was a form of electromagnetic radiation and that waves of oscillating electric and magnetic fields travel through empty space at a speed that could be predicted from simple electrical experiments. Using available data, he obtains a velocity of 310,740,000 m/s and states "This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws." Also see his "A Treatise on Electricity and Magnetism, 1873."

1881: Nikola Tesla conceives of wireless power transmission and stated "Throughout space there is energy. If kinetic, it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature."

1888: Heinrich Hertz demonstrates the existence of electromagnetic waves by building an apparatus that produced and detected UHF radio waves (or microwaves in the UHF region). He also found that radio waves could be transmitted through different types of materials and were reflected by others, the key to radar. His experiments explain reflection, refraction, polarization, interference, and velocity of electromagnetic waves.

1903: Konstantin Tsiolkovsky publishes The Exploration of Cosmic Space by Means of Reaction Devices, arguably the first academic treatise on rocketry. He calculates the escape velocity from Earth into orbit at 8 km/second and that a multi-stage rocket fueled by liquid oxygen and liquid hydrogen would be required. He is considered the father of human space flight and the first man to conceive the space elevator. During his lifetime he published over 500 works on space travel and related subjects, including science fiction novels. Among his works are designs for rockets with steering thrusters, multi-stage boosters, space stations, airlocks for exiting a spaceship into the vacuum of space, and closed cycle biological systems to provide food and oxygen for space colonies. He also delved into theories of heavier-than-air flying machines, independently working through many of the same calculations that the Wright brothers were performing at about the same time.

1928: Herman Potočnik publishes his sole book, The Problem of Space Travel - The Rocket Motor, a plan for a breakthrough into space and a permanent human presence there. He conceived of a space station in detail and calculated its geostationary orbit. He described the use of orbiting spacecraft for detailed peaceful and military observation of the ground and described how the special conditions of space could be useful for scientific experiments. The book described geostationary satellites (first put forward by Tsiolkovsky) and discussed communication between them and the ground using radio, but fell short of the idea of using satellites for mass broadcasting and as telecommunications relays.

1945: Arthur C. Clarke publishes the article Wireless World which conceives of the possibility for mass artificial communication satellites. He examines the logistics of satellite launch, possible orbits and other aspects of the creation of a network of world-circling satellites, pointing to the benefits of high-speed global communications. He also suggested that 3 geostationary satellites would provide coverage over the entire planet.

1957: Sputnik 1, the first artificial satellite is launched by Soviet Union.

1968: Dr. Peter Glaser introduces the concept of a large solar power satellite system of square miles of solar collectors in high geosynchronous orbit (GSO 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.

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

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... Ongoing and recent technology advances have narrowed many of the technology gaps, but major technical, regulatory and conceptual hurdles continue to exist... This NASA-funded SSP activity has made significant contributions to narrowing the technology gap (e.g. a three-fold reduction in mass at the solar array level over current state-of-the-art)... An incremental and evolutionary approach to developing needed technologies and systems has been defined, with significant and broadly applicable advances with each increment... The technologies and systems needed for SPS have highly leveraged applicability to needs in space science, robotic and human exploration, and the development of space... The decades-long time frame for SPS technology development is consistent with the time frame during which new space transportation systems, commercial space markets, etc. could advance... Power relay concepts appear technically viable using space solar power technologies, but may depend upon higher frequency power beaming... The question of ultimate large-scale solar power satellite economic viability remains open." ^[3]

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." ^[4]

Future

From 1995 through 1997, NASA undertook a study effort utilizing teams from government, academia and industry to develop space transportation vehicle concepts intended to drive recurring operations costs $200 or less per pound of payload delivered to low earth orbit(LEO).^[5] The cost of transporting materials for the construction of future space solar power systems may be significantly reduced^[6] by implementing reusable launch systems^[7] such as the proposed Maglifter or Star Tram. These launch systems would utilize superconducting magnetic propulsion and levitation to propel reusable launch vehicles RVL's into orbit. Maglifter could reduce launch cost per pound of payload to one tenth of that of the Space Shuttle's $10,000 per pound. Star Tram might reduce the cost to as low as $100 per pound. The Mag Lifter would eliminate the weight of the first stage of the reusable flight vehicle, using permanent ground based magnetic propulsion to reach a horizontal speed of 550 mi/hr, then use onboard propulsion systems for the remaining acceleration to reach orbit. The Star Tram system would accelerate (using gigawatts of electrical power in superconducting magnetic energy storage) a vehicle to 8 km/second (17,800 mi/hr) in 5.3 minutes through a 1,000 mile long evacuated tube that lies on the ground for the first 800 miles, with the remaining tethered (for stability) portion magnetically levitated above the ground tangentially to the earth, rising to its 72,000 ft. end where the space vehicle exits the tube. ^[8]

Supplying materials from the Moon is more than ten times easier than lifting materials out of the gravity well of the Earth, and the lunar base could easily become the supplier of power sources for commercial applications in geosynchronous or even low Earth orbit. Later, the moonbase could provide solar arrays for solar power systems for satellites, for missions to Mars, for prospecting missions to the near-Earth asteroids, and beyond.^[9]

The Pentagons’ National Security Space Office (NSSO) issued a report^[10] on October 10th, 2007 that states they intend to collect solar energy from space to help the United States' ongoing relationship with the Middle East and the battle for oil. Solar power is a clean source of energy that has no effect on the environment. 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.

Military personnel could particularly gain from this technology as they are as of 2007 paying around $1/kWh. In principle by beaming power on demand to a location it could eliminate the need to deliver fuel for the field, and hence significantly reduce supply line issues. The American government is eager that private companies will get involved with the launching of the new scheme. Companies that get involved will receive tax and other political benefits that have not been disclosed.

Space solar cells

Although costs are generally reduced for obtaining off the shelf products, solar cells utilized in space may have some different or additional requirements than typical terrestrial application solar cells. Due to the high cost of payload delivery into space, an important measure of power system performance is the specific power (power output per unit mass). This is typically specified for Earth orbit conditions. It is possible to measure specific power at the cell level, blanket level, array level or power system level. Specific power at the cell level does not include array structure and is many times higher than array level specific power. At the blanket level, specific power includes the coverglass, interconnections, and the backing material, but not the array structure.

Total power generation system mass (excluding storage) in currently designed space power systems may be described as follows. Photovoltaic blanket weight is only about a quarter of the total. The array structure and the power management and distribution (PMAD) system account for three-quarters of the power system mass. If the system were to include conversion and transmission of microwave power to earth or other receivers, both total weight and proportions would differ due to additional PMAD hardware and larger arrays required.

Thin solar cells have greater flexibility and so are better suited to construction of a flexible or semi-flexible array capable of being unrolled and/or inflated, to reduce both transportation packing space and weight. In the 1980's considerable research was devoted to development and commercialization of thin-film photovoltaics for terrestrial power generation. Here, thin films of photovoltaic material are deposited on a supporting substrate. This approach has lower conversion efficiencies, but due to the low amount of the active material used, has the potential for high specific power.

In addition to low mass, thin-film photovoltaics are also projected to have considerably lower costs. Materials cost is reduced due to the small amount of materials required; the cost of labor and assembly is reduced by the fact that large-area, integrated assemblies are produced directly on the substrate sheet. One option is to use a thin layer of photovoltaic material deposited onto a flexible substrate.

Efficiencies over 10% have been achieved with three thin-film materials: amorphous silicon (a-Si), copper indium diselenide (CuInSe2), and cadmium telluride (CdTe). However, very little current research is aimed at depositing thin-film cells on lightweight substrates, since most of the applications being considered are terrestrial, where weight is not as critical. To enable their use in space, technology for deposition on extremely lightweight substrates will need to be developed. Thin film solar cells have not yet been demonstrated in space.^[11]

Concentrator systems

Another alternative is to use a concentrator system to focus light onto small, extremely high efficiency solar cells. This approach has been tested in space only on small-scale experiments. Conversion efficiencies of over 30% have been demonstrated using such concentrator systems. Concentrator systems will not be practical on planets such as Mars, where under worst-case conditions most of the incident sunlight is diffuse, and concentrator systems can focus only the direct component of the solar radiation.

Solar power satellites

Solar power satellites would be vast assemblies of large solar modules for producing large scale space solar power. Energy generated from sunlight could be converted into microwaves and beamed to a rectifier-antenna, called a rectenna on earth, where the microwave power is rectified and converted to electric power.

A 1996 estimate^[12] for the production of 5 billion watts (equivalent to five large nuclear power plants) would require several square km of solar collectors (weighing approximately 5 million kg) and an earth-based antenna 5 miles in diameter.

Terrestrial solar energy

An average of approximately 0.1 and 0.2 kW/m² of solar energy can be received from the Sun on the Earth's surface. Solar energy (total global insolation) striking the earth's surface consists of 2 components, direct and diffuse (diffuse light may be further subdivided into several other categories).^[13] Due to influences of the atmosphere (reflection, absorption and scattering), including man made gases and particulates only 10% to 13% of the total incident energy approaching the earth's cross sectional area from the sun is available on earth.

Extraterrestrial solar energy

Extraterrestrial solar power is that collected outside of the earth's atmosphere. Besides man-made satellites in GSO, locations for this conversion may be sun-synchronous (near-polar, always facing the Sun) orbit, space probes, the moon,^[14] or other planets.^[15] There is little loss of microwave energy passing through the Earth’s atmosphere and there is no contribution to the global warming problem by the addition of CO₂ during the production stage. In addition, the orbit of rotation can be selected such that sunlight is received by the satellite ~96% of the time. In near Earth space the average ~1 to 2 kW/m² of energy that can be collected is approximately ten times as much the solar energy available on earth. (Earth's orbit causes varying extraterrestrial S flux between approximately 1329 and 1421 W/m². 1370 W/m² is the solar constant, i.e., mean flux perpendicular with the solar beam in outer space, at the mean distance from the Earth to the Sun.^[16]) Unaffected by atmospheric gases, particulate matter and cloud cover, photovoltaic arrays in a geostationary Earth orbit (at an altitude of 22,300 miles) would receive, on average, eight times as much sunlight as they would on Earth's surface.^[17] In addition, they would be unaffected by the Earth's day-night cycle.

Possible environmental impact

The idea for space based solar power has been around since the late 60's, but little has been done to put this groundbreaking idea into use. The possible environmental impact for this process is astronomical. What sets this apart from terrestrial solar cells is the utilization of the earth's orbit to obtain eight times more energy. There is a lot of criticism surrounding SSP (space based solar power), mainly involving the initial cost and general complexity.

To keep up with the growing population we need a clean non-expendable source of energy. The incoming microwaves would heat up the atmosphere slightly, but the absence of harmful emissions greatly outweighs a small addition to the global warming effect. Terrestrial solar cells require all of their materials to be mined on Earth, however the "powersats" (solar power satellites) can be built exclusively from lunar materials. Only the "rectennas" (rectifying antennas) materials would need to be mined on Earth.

Space based solar power would eliminate the need for complex transcontinental grids. This would reduce the number of blackouts because there is less chance for a laser or microwave beam to be interrupted. Another advantage is that the power source is 22,300 mile away. This makes it nearly impossible for terrorist attacks. This system also provides a relatively easy way to replace one beam with another.

Recent developments have made solar power satellites a bit more feasible, such as advances in microwave signals, and cheaper production costs for more efficient solar collection panels. We are still many years away from balancing the cost and energy output of space based solar power, but the National Security Space Office (NSSO) has announced that they hope to start testing on SSP in the next 20 years.

Another possible plan for SSP is to place power generating stations on the moon. Lunar Solar Power (LSP) bases could supply Earth with all the necessary power for the growing population. By creating bases on opposite sides of the moon, Lunar Solar Power would create a constant stream of electrical energy to any number of rectennas on Earth.

Although SSP and LSP are many years in the future all aspects of the technology are with us today. The positive environmental impact surely outweighs the enormous initial costs. As our current sources of energy are further depleted, it is obvious that SSP is a viable alternative that must be pursued.

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.^[18]

Laser power beaming

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 1980's 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 a 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 was still continuing. There was some hope that an array for a laser-powered aircraft demonstration might be developed.^[19]

Energy in global winters

Space solar power would be the only means of acquiring direct solar energy to supplement the burning of fossil fuels or nuclear energy sources under the most extreme conditions of a global catastrophic volcanic winter (or similarly, nuclear winter). This could include the massive energy increases necessary to grow food crops and for increased heating requirements under ice age conditions. Such could be the case after a rhyolitic supervolcano at one the earth's few dozen hotspots. One at Lake Toba, Indonesia 75,000 years ago caused the Millennial Ice Age lasting 1000 years, wiping out 60% of the global population. Ejecta on this scale could occur at the Yellowstone Caldera which 640,000 years ago (one also occurred 2.2 million years ago), released 800 times more (but only one third of that released at Lake Toba and one fifth of that released at the world's largest known at La Garita Caldera in the San Juan Mountains of Colorado 27.8 million years ago) ejecta than Mount St. Helens did in 1980.^[20]

Extraterrestrial intelligence

The Sun is the Earth's ideal nuclear energy (fusion) generator. It utilizes a type of stellar nucleosynthesis particular to its spectral type (type G stars have the sun's characteristic yellow color and include stars such as Capella and Alpha Centauri A). Space solar power production can potentially utilize many diverse methods of harnessing energy from the light of the Sun, some of which are not yet feasible. Nearly 100% of the Sun's energy is radiated into space in directions other than that of earth's cross sectional area. In the distant future there may be a way to tap into a portion of this vast amount of "lost" energy that is directed away from our planet earth and into the dark universe beyond.

In the search for extraterrestrial intelligence, some speculations claim that the harnessing of a significant portion of this type of this "lost" energy from a star might be a detectable indicator of the quantum leap in the energy available to a stable, high energy consuming, advanced civilization. It is very difficult to identify planets outside of the solar system which are capable of sustaining intelligent life, but identifying a star with light modified by a civilization's large scale application of space solar power might someday provide a clue in the search for the existence of extraterrestrial life (see Dyson sphere).

See also

• Future energy development
• Photovoltaics
• Satellite
• Solar power
• Solar power satellite
• Solar panels on spacecraft

References

Links

• "Conceptual Study of A Solar Power Satellite, SPS 2000" Makoto Nagatomo, Susumu Sasaki and Yoshihiro Naruo from SpaceFuture.com
• A Fresh Look at Space Solar Power: New Architectures, Concepts and Technologies John C Mankins
• NASA Looks For New Ways to Harness Sun's Energy for Earth and Space June 10, 1999 Tyson, Tim
• Space-Based Solar Power Efforts (php) from solarpanelinfo.com
• Space Solar Power - An Earth to Orbit Challenge
• "The World Needs Energy from Space" Peter E. Glaser, Feb 2000 @Space.com
• NSSO Backs Space Solar Power

Links (NASA Articles)

• Space Solar Power articles from NASA @Google Search Engine
• KSC Next Gen Site
• Reinventing the Solar Power Satellite from NASA
• Space Solar Power - An Earth to Orbit Challenge

Spacecraft operating in the inner solar system usually rely on the use of photovoltaic solar panels to derive electricity from sunlight. In the outer solar system, where the sunlight is too weak to produce sufficient power, radioisotope thermal generators (RTGs) are used as a power source^[1].

History

The first spacecraft to use solar panels was the Vanguard 1 satellite, launched by the US in 1958.

Implementation

Solar panels need to have a lot of surface area that can be pointed towards the Sun as the spacecraft moves. More exposed surface area means more electricity can be converted from light energy from the Sun. Since spacecraft have to be small, this limits the amount of power that can be produced ^[1].

Spacecraft are built so that the solar panels can be pivoted as the spacecraft moves. Thus, they can always stay in the direct path of the light rays no matter how the spacecraft is pointed. Spacecraft are usually designed with solar panels that can always be pointed at the Sun, even as the rest of the body of the spacecraft moves around, much as a tank turret can be aimed independently of where the tank is going. A tracking mechanism is often incorporated into the solar arrays to keep the array pointed towards the sun^[1].

Sometimes, satellite operators purposefully orient the solar panels to "off point," or out of direct alignment from the Sun. This happens if the batteries are completely charged and the amount of electricity needed is lower than the amount of electricity made; off-pointing is also sometimes used on the International Space Station for orbital drag reduction.

Types of solar cells typically used

Gallium arsenide-based solar cells are typically favored over silicon in industry, due to the fact that they have a higher efficiency. The most efficient solar cells currently in production are multi-junction cells. These use a combination of several layers of both gallium arsenide and silicon to capture the largest spectrum of light possible. Leading edge multi-junction cells are capable of nearly 29% efficiency under ideal conditions. ^[2]

Spacecraft that have used solar power

To date, solar power, other than for propulsion, has been practical for spacecraft operating no farther from the sun than the orbit of Mars. For example, Magellan, Mars Global Surveyor, and Mars Observer used solar power as does the Earth-orbiting, Hubble Space Telescope. The Rosetta space probe, launched March 2, 2004, will use solar panels as far as the orbit of Jupiter (5.25 AU); previously the furthest use was the Stardust spacecraft at 2 AU. Solar power for propulsion was also used on the European lunar mission SMART-1 with a Hall effect thruster.

The upcoming Juno mission will be the first mission to Jupiter to use solar panels instead of the traditional RTGs that are used by previous outer solar system missions^[3].

Power Available

In 2005 Rigid-Panel Stretched Lens Arrays were producing 7 kW per wing. Solar arrays producing 300 W/kg and 300 W/m² from the sun's 1366 W/m² energy near the Earth are available. Entech Inc. hopes to develop 100 kW panels by 2010 and 1 MW panels by 2015. ^[4]

Future Uses

For future missions, it is desirable to reduce solar array mass, and to increase the power generated per unit area. This will reduce overall spacecraft mass, and may make the operation of solar-powered spacecraft feasible at larger distances from the sun. Solar array mass could be reduced with thin-film photovoltaic cells, flexible blanket substrates, and composite support structures. Solar array efficiency could be improved by using new photovoltaic cell materials and solar concentrators that intensify the incident sunlight. Photovoltaic concentrator solar arrays for primary spacecraft power are devices which intensify the sunlight on the photovoltaics. This design uses a flat lens, called a Fresnel lens, which takes a large area of sunlight and concentrates it onto a smaller spot. The same principle is used to start fires with a magnifying glass on a sunny day.

Solar concentrators put one of these lenses over every solar cell. This focuses light from the large concentrator area down to the smaller cell area. This allows the quantity of expensive solar cells to be reduced by the amount of concentration. Concentrators work best when there is a single source of light and the concentrator can be pointed right at it. This is ideal in space, where the Sun is a single light source. Solar cells are the most expensive part of solar arrays, and arrays are often a very expensive part of the spacecraft. This technology may allow costs to be cut significantly due to the utilization of less material.

It has been proposed that it may be possible to develop space-based solar plants — solar power satellites with large arrays of photovoltaic cells-- that would beam the energy they produce to Earth using microwaves or lasers. This could, in principle, be a significant source of electrical power generated using non-fossil fuel sources. Japanese and European space agencies, among others, are analyzing the possibility of developing such power plants in the 21st century.

See also

• Main article on space solar power
• Main article on solar cells
• Main article on Photovoltaic arrays
• For solar arrays on the International Space Station, see ISS_Solar_Arrays#Solar_arrays or Electrical system of the International Space Station

References

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^[1]. It is a renewable energy source, zero emission, 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:

• Low launch costs can be achieved
• A space-based manufacturing industry develops that is capable of building solar power satellites in orbit, using off-Earth materials

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 ^[2]. 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 ^[3] for his method of transmitting power over long distances (eg, from an SPS to the Earth's surface) using microwaves from a, perhaps square kilometer, antenna on the satellite to a much larger one on the ground, which came to be known as a rectenna.[1]

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 ^[1].

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 ^[4][5]. 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)^[6]
• Financial/Management Scenarios^[7][8]
• Public Acceptance^[9]
• State and Local Regulations as Applied to Satellite Power System Microwave Receiving Antenna Facilities^[10]
• Student Participation^[11]
• Potential of Laser for SPS Power Transmission^[12]
• International Agreements^[13][14]
• Centralization/Decentralization^[15]
• Mapping of Exclusion Areas For Rectenna Sites^[16]
• Economic and Demographic Issues Related to Deployment^[17]
• Some Questions and Answers^[18]
• Meteorological Effects on Laser Beam Propagation and Direct Solar Pumped Lasers^[19]
• Public Outreach Experiment^[20]
• Power Transmission and Reception Technical Summary and Assessment ^[21]
• Space Transportation^[22]
• Office of Technology Assessment^[23]

After these studies were published, there was no follow up work and the concept dwindled. The DOE study conclusions were critical of the project's possibilities. Confusing press reports widely reported that the concept had been demonstrated to be infeasible ^[24].

More recently, the SPS concept has again become interesting, due to increased energy demand, increased costs, and emission implications, and starting in 1997 with the NASA "Fresh Look"^[25] however funding is still minimal.

In 2007, the US Department of Defense expressed interest in the concept^[26].

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.^[27]

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 zero-g, vacuum environment and will not need to support itself against a gravity field 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 is expected to be similar. ^[28] A major advantage of space solar is the efficiency with which huge mirrors can be supported and pointed in zero gravity and vacuum conditions of space. They can be constructed with very thin aluminum or other metal sheets and very light frames, easily constructed from materials available in space.

PV uses semiconductor cells (e.g., silicon or gallium arsenide) to directly convert sunlight photons into voltage via a quantum mechanical mechanism. These are commonly known as “solar cells”, and will likely be rather different from the glass panel protected solar cell panels familiar to many and in current terrestrial use. They will, for reasons of weight, probably be built in membrane form, not suitable to terrestrial use which is subject to considerable gravitational loading.

It is also possible to use Concentrating Photovoltaic (CPV) systems, which like SD are a form of Concentrating Solar Energy that convert concentrated light into electricity by PV rather than heat engines. They also use tracking systems, mirrors, and lenses to achieve high concentration ratios and are able to reach efficiencies above 40% Concentrating Photovoltaic Technology. Because their PV area is much smaller, the majority of the area of CPV systems is mirrors, as with SD systems; so they share the advantages of building and pointing large mirror arrays in space versus the Earth's surface.

Comparison of PV, CPV, and SD

The main problems with non-concentrating PV are that PV cells continue to be relatively expensive, and require a relatively large area to be acceptable. In addition, semiconductor PV panels will require a relatively large amount of energy to produce; amorphous-silicon designs require much less energy to produce but are less efficient. CPV designs with a small area of 40%+ efficient cells and large reflector area can be built with relatively small amounts of energy. Unfortunately, the materials for high-efficiency photovoltaics, like gallium and arsenic, are much less common than silicon in lunar materials.

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

Currently, PV cells weigh between 0.5kg/kW^[29] and 10kg/kW depending on design. SD designs also vary but most seem to be heavier per kW produced than PV cells and thus this pushes up launch costs. CPV should be lighter; since it replaces the thermal power plant (except for the radiator for waste heat) with a much lighter PV array.

Lifetime

The lifetime of a PV based SPS is limited mainly by the ionizing radiation from the radiation belts and the Sun. Without some method of protection, this is likely to cause the cells to continuously 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 events^[30]. If some practical protection can be designed, this also might be reducible.

Lifetimes for SD based SPS designs will be limited by structural and 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 extreme effects. As well, most mirror surfaces will degrade from both radiation and particle impact, but such mirrors can be designed simply (and so light and cheap), so 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; 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 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.

Energy payback

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 the energy needs are relatively high, and typically several years of deployment in a terrestrial environment is needed to recover this energy.^{[31][32][31][33]} With SPS net energy received on the ground is higher (more or less necessarily so, for the system to be worth deploying), so this energy payback period would be somewhat reduced; however SD, being made of conventional materials, are more similar to conventional powerstations and are likely to be less energy intensive and would be expected to give quicker energy break even, depending on construction technology.

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 CBS news with Walter Cronkite, a microwave-powered model helicopter that received all the power 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 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. 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 water vapor and CO2 absorption of air 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. The proposal has not been as carefully evaluated for engineering plausibility as other aspects of SPS design.

Spacecraft sizing

The sizing will be dominated by 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.

For best efficiency, the satellite antenna should be circular and about 1 kilometers in diameter or larger; the ground antenna (rectenna) should be elliptical and around 14 kilometers by 10 kilometers. Smaller antennas would result in increased losses to diffraction/sidelobes. For the desired (23mW/cm²) microwave intensity ^[34] these antennas could transfer between 5 and 10 gigawatts of power. To be most cost effective, the system needs to operate at maximum capacity. And, to collect and convert that much power, the satellite would need 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% ^[35] could reduce the necessary collector area by two thirds, but would not necessarily give overall lower costs. In either cases, the SPS's structure would be kilometers wide, 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 LEO system of space power stations has been proposed as a precursor to GEO space power beaming system(s)^[36]. There would be advantages, (much shorter path length allowing smaller antenna sizes, lower cost to orbit) and disadvantages (constantly changing antenna geometries, increased debris collision difficulties, etc). It might be possible to deploy LEO systems sooner than GEO because the antenna development would take less time. Ultimately, because full engineering feasibility studies have not been conducted, it is not known whether this 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^[37]. 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^[38] when power demands are at their lowest. This allows the power generation system to avoid the expensive storage facilities (eg, lakes behind dams, oil storage tanks, etc) necessary in many Earth-based power generation systems. Additionally, an SPS will avoid entirely the polluting consequences of fossil fuel systems, the ecological problems resulting from many renewable or low impact power generation systems (eg, dams).

More long-term, the potential amount of 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.

Problems

Launch costs

Without doubt, the most obvious problem for the SPS concept is the currently immense 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.^[39] 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 SSPS 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,^[40], 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 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. On 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.^[41] 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.^[42]

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 ^[43] 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^[44]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.

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 reduce the cost of construction of the elevator enough in the short term, 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^[45] in 1979. Because of the ~20 times shallower (than Earth's) effective potential well for the lunar elevator, this concept would not rely on materials technology beyond the current state of the art, but it would require the establishment of Si mining and solar cell manufacturing facilities on the Moon, similar to the lunar material proposal of G.K. O'Neill, as described in the preceding section.

Lofstrom launch loop

An alternative to the carbon nanotube based Space Elevator could be a Lofstrom loop. 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.)^[46] Unlike the conventional space elevator it is believed that a launch loop could be built with today's materials.

Safety

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