Space solar power

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It has been suggested that this article or section be merged with Solar power satellite . (Discuss)

It has been suggested that this article or section be merged with Solar panels on spacecraft . (Discuss)

The sun as seen from Earth orbit

On the left side: Part of the solar energy is lost in its way through the atmosphere by the effects of reflection and absorption. On the right side: Space-based solar power systems are an attempt to convert in space, outside the atmosphere, to avoid these losses.

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, the 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.

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.

[edit] History

1829 and 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 complementary 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]

[edit] 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 Pentagon's 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.

[edit] 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 1980s 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]

[edit] 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.

[edit] 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.

The device necessary to convert the solar energy collected by the satellite to microwave form is called a transmitter. Good transmitters will convert energy from DC to radiofrequency (RF) form efficiently, with adequate control and minimal losses. NASA’s SERT program (discussed above) mandated that transmitters be 500 meters in diameter and emit at a frequency of 5.8 GHz. Three main types of transmitters are frequently considered: klystron, magnetron, and solid-state amplifiers. All of these options have relatively similar specific masses (mass per unit area) at a given power output and thus are approximately equally suitable for these applications. Solid-state amplifiers offer the most opportunity for improvement, as the materials used are capable of a range of different power amplified efficiencies (PAEs) when converting to different frequencies. Materials currently being explored for use in solid-state amplifying transmitters are InGaAs, GaN, and SiC ^[12]. Of these, GaN appears have the highest efficiencies. Further improvements on GaN and other materials for these devices are required to improve efficiencies. Goals include reducing contact and channel resistances in the devices as well as reducing the cost of the substrate, surface traps, and charge and interface effects. With sufficient funding and research efforts, these challenges can be overcome.

A 1996 estimate^[13] 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.

[edit] 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).^[14] 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.

[edit] 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,^[15] or other planets.^[16] 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.^[17]) 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.^[18] In addition, they would be unaffected by the Earth's day-night cycle.

[edit] Possible environmental impact

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The idea for space based solar power has been around since the late 60's, but little has been done to put this idea into use, due to massive costs, insufficient ground to space lift capability, and ease of use with Earth-based systems. The possible environmental impact for this process is high, however. 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 of Earth, some experts have said humans need a clean, non-expendable source of energy. The incoming microwaves would heat up the atmosphere slightly, but the absence of harmful emissions would greatly outweigh a small addition to the global warming effect if they could replace enough terrestrial power plants. Terrestrial solar cells require all of their materials to be mined on Earth, however the "powersats" (solar power satellites) can be built using lage amounts, or, as the industrialization of space progresses, exclusively from lunar materials.

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. It's still many years before the cost-per-kW of space based solar power cells becomes balanced, 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 necessary power for the growing population.

Although SSP and LSP are many years in the future many aspects of the technology are with us today, and many interested parties are pursuing SSP ideas.

[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.^[19]

[edit] 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 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 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.^[20]

[edit] 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.^[21]

[edit] 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).

[edit] See also

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

[edit] References

[edit] External links

• "Conceptual Study of A Solar Power Satellite, SPS 2000" Makoto Nagatomo, Susumu Sasaki and Yoshihiro Naruo