Solar power satellite

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A solar power satellite, or SPS, is a proposed 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 generates no waste. 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 and they can be built in orbit from off-Earth materials
• conventional energy costs increase
• a determination is made that the disadvantages of fossil fuel use are so large they must be substantially replaced.

Contents 1 History 2 Description 2.1 Spacecraft design 2.1.1 Solar energy conversion 2.1.1.1 Comparison of PV versus SD 2.1.2 Spacecraft sizing 2.2 Earth based infrastructure 3 Advantages of SPS 4 Problems 4.1 Launch costs 4.1.1 Extraterrestrial Materials 4.1.2 Space Elevator 4.2 Safety 5 SPS's economic feasibility 5.1 Current energy price landscape 5.2 Comparison with fossil fuels 5.3 Comparison with nuclear power (fission) 5.4 Comparison with nuclear fusion 5.5 Comparison with terrestrial solar power 5.5.1 Solar Panel Mass Production 5.6 Comparison with Other Renewables (wind, tidal, hydro, geothermal) 6 Other Applications of SPS 7 Current work 8 See also 9 References 10 External links

[edit] History

The SPS concept was first described in November 1968 ^[2]. At first it was considered impractical due to the lack of an efficient method of sending the power down to the Earth for use. Things changed in 1973 when Peter Glaser was granted U.S. patent number 3,781,647 ^[3] for his method of transmitting the power to Earth using microwaves from a small antenna on the satellite to a much larger one on the ground, known as a rectenna.[1]

Glaser's work took place at Arthur D. Little, Inc., who employed Glaser as a vice-president. NASA then became interested and granted them a contract 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 congress authorized DOE and NASA to jointly perform the Satellite Power System Concept Development and Evaluation Program ^[4][5]. That study remains the most extensive performed to date. Several reports were published addressing various issues, including:

• 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 completion of these studies there was no follow up and work on the concept dwindled. Some feel that the DOE study conclusions were overly critical and the press misinterpreted this and widely reported that the concept was not feasible^[24].

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

At some price point the high construction costs of the SPS become favourable due to their low-cost delivery of power, and the varying costs of electricity 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 the SPS.^[26]

[edit] Description

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. an antenna occupying a large area on Earth to collect the power

[edit] Spacecraft design

In many ways, the SPS as a concept is simpler than most power generation systems here on Earth. This includes the structure needed to hold it together and align it orthogonal to the Sun, which in orbit can be considerably lighter due to the lack of weight (zero-g), and lack of wind or weather.

The photons from the Sun are converted into electricity aboard the spacecraft, that electricity is then fed to an array of Klystron tubes which generate the microwave beam.

[edit] Solar energy conversion

Two 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 popular heat cycles for Solar Dynamic are Brayton Cycle or Stirling Cycle. Solar Dynamic systems employ a large reflector to focus sunlight to a high concentration to achieve a high temperature for the heat cycle to operate at highest possible efficiency. ^[27]

PV uses semiconductors (e.g. Silicon or Gallium Arsenide) to directly convert sunlight photons into electric potential. Commonly known as “Solar cells”

[edit] Comparison of PV versus SD

The main problem with PV is that PV cells are reasonably high price.

SD has a much more severe pointing requirement than PV because it needs to maintain an accurate optical focus. If a PV array drifts off a few degrees, the power level drops a few percent. If a SD array drifts off a few degrees, the power level drops off to zero.

PV cells weigh between 0.5kg/kW^[28] and 10kg/kW depending on design. Designs vary also of SD but this technology seems to be heavier per kW than PV cells and thus this pushes up launch costs.

PV lifetime is limited mainly by the ionizing radiation environment which causes the cells to continuously degrade in the neighborhood of a percent or two per year, and more rapidly when exposed to particle radiation from solar flares^[29].

SD lifetimes are limited by structural and mechanical considerations, such as micrometeorite impact, metal fatigue of gas turbine blades, wear of sliding surfaces (although this might be avoided by hydrostatic bearings or magnetic bearings), and degradation or loss off lubricants and working fluids in the space vacuum and temperature extremes.

In either case, another advantage of the design is that waste heat is re-radiated back into space, instead of warming the adjacent local biosphere as with conventional sources; thus thermal efficiency is not in itself an important parameter except insofar as it affects the power/weight ratio and hence pushes up launch costs. (For example SD may require larger radiators if a lower efficiency is obtained).

[edit] Spacecraft sizing

The sizing is dictated by distance from Earth to geosynchronous orbit (22,300 miles, 35,700 km), the 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 must be circular between 1 and 1.5 kilometers in diameter and the elliptical ground rectenna around 14 kilometers by 10 kilometers. Smaller antennas would result in excessive losses due to sidelobes. For the desired (23mW/cm²) microwave intensity ^[30] this allows transfer of between 5 and 10 gigawatts of power. To be cost effective it needs to operate at maximum capacity. To collect and convert that much power, the satellite needs between 50 and 100 square kilometers of collector area using standard ~14% efficient monocrystalline silicon solar cells. State of the art and expensive triple junction gallium arsenide solar cells with a maximum efficiency of 28.3% ^[31]could reduce the collector area by half. In both cases the solar station's structure would be several kilometers wide, making it much larger than most man-made structures here on Earth. While certainly not beyond current engineering capabilities, building structures of this size in orbit has never been attempted before.

[edit] Earth based infrastructure

The Earth-based receiver antenna (or rectenna) is also key to the SPS concept. It consists of a series of short dipole antennas, connected with a diode. Microwaves broadcast from the SPS are received in the dipoles with about 85% efficiency^[32]. With a conventional microwave antenna the reception is even better, but the cost and complexity is considerably greater. Rectennas would be multiple kilometers across. Crops and farm animals may be raised underneath the rectenna, as the thin wires used only slightly reduce sunlight, so the rectennas are not as expensive in terms of land as might be supposed.

[edit] Advantages of SPS

The SPS concept arose because space has several major advantages over earth for the collection of solar power. There is no air in space, so the satellites would receive somewhat more intense sunlight, unaffected by weather. In a geosynchronous 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 an hour and a half late at night when power demands are at their lowest. This allows systems to avoid the expensive storage facilities (eg, lakes behind dams) necessary in many earth-based renewable or low impact power generation systems.

[edit] Problems

[edit] Launch costs

Without a 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), depending on whose numbers are used. Calculations show that launch costs of less than about $400-500/kg to LEO seem to be necessary.

However, economies of scale on 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 into the range where this system could be conceivably attempted.^[33] Reusable vehicles could quite conceivably attack the launch problem as well; but are not a well developed technology.

To give an idea of the scale of the problem, assuming a typical solar panel mass of 20 kg per kilowatt, and without considering the mass of the support structure, antenna or significant mass reduction of focusing mirrors, a 4 GW power station would weigh about 80,000 metric tons. This is excessive though, as a space solar-panel would not need to support its own weight, and would not be subject to earth's corrosive atmosphere. Very lightweight designs could achieve 1 kg/kW,^[34] or 4000 metric tons for a 4 GW station. This would be the equivalent of between 40 and 800 HLLV launches to send the material to low earth orbit, where it would be turned into subassembly solar arrays, which then use ion-engine style rockets to move to GEO 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 panel) and $320 billion ('expensive' HLLV, unnecessarily heavy panel). On top of this, the cost of a large assembly area in LEO and GEO (which would be spread over several power satellites) and the costs of the materials and manufacture are added.

So how much money could a SSPS be expected to make? For every one gigawatt rating, a SSPS system 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, Jan06) and an SSPS's ability to send its energy to places of greatest demand, this would equate to $1.93 billion per year or $38.6 billion over its lifetime. The example 4 GW 'economy' SSPS above could therefore generate in excess of $154 billion over its lifetime. Assuming that facilities are available, it may turn out to be substantially cheaper to recast on-site steel in GEO, than launch it from Earth. If true then the initial launch cost could be spread over multiple lifespans.

[edit] Extraterrestrial Materials

Gerard O'Neill, noting the problem of high launch costs in the early 1970s, came up with the idea of building the SPS's in orbit with materials from the Moon.^[35] Launch costs from the Moon are about 100 times lower than from Earth, due to the lower gravity. This 1970s proposal was predicated on the then advertised future launch costs of NASA's space shuttle.

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 comprising as few as thirty Solar Power Satellites of 10GW capacity each.^[36]

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 ^[37] 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.

Asteroid mining has also been seriously considered. A NASA design study^[38]produced 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 constitute traditional aerospace-grade payload. The rest would be reaction mass for the mass-driver engine; which could easily consist of the spent rocket stages used to launch the payload. Assuming that 100% of the returned asteroid was useful, and that the asteroid miner couldn't be reused, that represents nearly a 95% reduction in launch costs. The true merits of such a method would depend on a thorough mineral survey of the candidate asteroids. Once built, NASA's CEV should be capable of beginning such a survey, Congressional money and imagination permitting.

[edit] Space Elevator

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 price of construction of the elevator enough in the short term.

[edit] Safety

The use of microwave transmission of power has been the most controversial item concerning SPS development, but the incineration of anything which strays into the beam's path is an extreme misconception.

At the earth's surface, the microwave beam has a maximum intensity in the center of 23 mW/cm2 (less than l/4 the solar constant) and an intensity of less than 1 mW/cm2 outside of the rectenna fenceline^[30] (10 mW/cm2 is the current United States microwave exposure standard). According to US Federal OSHA, ^[39] the workplace exposure limit (10 mW/sq. cm.) is expressed in voluntary language and has been ruled unenforceable for Federal OSHA enforcement.

The beam's most intense section (the center) is far below the danger levels of concentration even for an exposure which has been prolonged indefinitely. ^[40] Furthermore, the possibility of exposure to the intense center of the beam can easily be controlled on the ground and an airplane flying through the beam surrounds its passengers with a protective layer of metal or Faraday Cage, which will intercept the microwaves. Over 95% of the beam will fall on the rectenna. The remaining microwaves will be dispersed to low concentrations well within standards currently imposed upon microwave emissions around the world.^[41]

The intensity of microwaves at ground level that would be used in the center of the beam can be designed into the system, but is likely to be comparable to that used by mobile phones. The microwaves must not be too intense in order to avoid injury to wildlife, particularly birds. Experiments with deliberate irradiation with microwaves at reasonable levels have failed to show any negative effects even over multiple generations.

Some have suggested locating rectennas offshore ^[42][43], but this presents problems of its own.

A commonly proposed approach to ensuring fail-safe beam targeting is to use a retrodirective phased array antenna/rectenna. A "pilot" microwave beam is emitted from the center of the rectenna on the ground to establish a phase front at the transmitting antenna, where circuits in each of the antenna's subarrays compare the pilot beam's phase front with an internal clock phase to use as a reference to control the phase of the outgoing signal. This allows the transmitted beam to be centered precisely on the rectenna and to have a high degree of phase uniformity, but if the pilot beam is lost for any reason (if the transmitting antenna is turned away from the rectenna, for example) the phase control system fails and the microwave power beam is automatically defocused.^[44] 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 the system that as much of the microwave radiation as possible is focused on the rectenna as that increases the transmission efficiency. Outside of the rectenna the microwave levels rapidly decrease, nearby towns or cities should be completely unaffected.^[44]

The long-term effects of beaming power through the ionosphere in the form of microwaves has yet to be studied.

[edit] SPS's economic feasibility

[edit] Current energy price landscape

In order to be competitive, an SPS must cost no more than existing suppliers; this may be difficult, especially if it is deployed to North America. Either it must cost less to deploy, or it must operate for a very long period of time. Many proponents have suggested that the lifetime is effectively infinite, but normal maintenance and replacement of less durable components makes this unlikely. Satellites do not, in our now-extensive experience, last forever.

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

[edit] Comparison with fossil fuels

The low price of energy today is dominated by the cheap access to carbon based fossil fuels, i.e. petroleum (crude oil), Coal and Natural Gas.

Combustion of fossil fuels emits enormous quantities of Carbon Dioxide (CO₂), which is a greenhouse gas. A large body of opinion claims that this greenhouse gas emission is causing global warming and climate change^[45]. Following the Kyoto Treaty, 141 countries introduced the first system of mandatory control via carbon credits. The ultimate direction of such policies suggest that fossil fuels might eventually be completely banned in some countries or even globally. At the same time, energy demands of third world or developing countries (e.g. China and India) are increasing steadily. As a result, there is little doubt that energy prices will continue to increase, it is only a question of how quickly.

[edit] Comparison with nuclear power (fission)

Detailed analyses of the problems with nuclear power specifically (nuclear fission) are published elsewhere^[46], they are summarized below:

• nuclear proliferation not a problem with SPS
• disposal and storage of radioactive waste, not a problem with SPS
• preventing fissile material from being obtain by terrorists or their sponsors, not a problem with SPS
• public perception of danger, problem with both
• consequences of major accident, e.g., Chernobyl or Three Mile Island, nil with SPS, save on launch during construction or maintenance
• military and police cost of protecting the public and loss of democratic freedoms, control of SPS would be a power/influence center, perhaps sufficient to translate into political power. However, this has not yet happened in the developed world with nuclear power.

On balance, SPS avoids most problems of current nuclear power, and does not have larger problems in any respect, although public perception of microwave dangers could become inflamed.

[edit] Comparison with nuclear fusion

Nuclear fusion is a process which is used in thermonuclear bombs (e.g. the H-bomb). Projected nuclear fusion power plants would not be explosive, and will likely be inherently failsafe. However, sustained nuclear fusion generators have not yet been demonstrated, despite well funded research over a period of several decades (since approx 1952^[47]). There is still no credible estimate of how long it will be before a nuclear fusion reactor could become commercially viable, yet fusion research continues to receive substantial funding by many nations. For example, the ITER facility currently under construction will cost €10 billion^[48]. There has been much criticism of the value of continued funding of fusion research^[49]. Proponents have successfully argued in favor of ITER funding^[50].

By contrast, SPS does not require any fundamental engineering breakthroughs, has already been extensively reviewed from an engineering feasibility perspective, and needs only incremental improvements of existing technology^[1]. Despite these advantages, SPS has received minimal research funding to date.

[edit] Comparison with terrestrial solar power

In the case of United Kingdom, the country is further north than even most inhabited parts of Canada, and hence receives little insolation over much of the year, so conventional solar power is not terribly competitive at 2006 per-kilowatt-hour delivered costs. (However, per-kilowatt-hour photovoltaic costs have been in exponential decline^[51] for decades, with a 20-fold decrease from 1975 to 2001.)

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

• Such a system would require a large solar array built in a well-sunlit area, the Sahara Desert for instance. An SPS requires much less ground area per kilowatt (approx 1/5th)
• The rectenna on the ground is much larger than the area of the solar panels in space.The ground-only solar array would have the advantages compared to a GEO solar array of costing considerably less to construct and requiring no significant technological advances.
• The receiving rectenna for an SPS will be quite transparent and simple and cheap, with fewer land use issues than conventional terrestrial solar. Crops could be grown beneath the rectenna, so the land would be dual-use. By comparison, solar panels would completely block sunlight thus destroying the ability of the underlying ground to support natural vegetation or crops, which in turn would result in increased soil erosion, drainage and runoff problems (increased flood risk) and loss of habitats.
• A terrestrial solar station intercepts an absolute maximum of only one third of the solar energy that an array of equal size could intercept in space, since no power is generated at night and less light strikes the panels when the Sun is low in the sky or weather interferes. A solar panel in the contiguous United States on average delivers 19 to 56 W/m² ^[52]. By comparison an SPS rectenna would deliver continuously about 23mW/cm² (230 W/m²)^[30], hence the size of rectenna required per watt would be about 8.2% to 24% that of a terrestrial solar panel array with equivalent power output.
• Further, if it is assumed that the array must supply baseload power (not true for every projected configuration), some form of energy storage would be required to provide power at night, such as hydrogen generation, compressed air, or pumped storage hydroelectricity. With present technology, energy storage on this scale is prohibitively expensive, and lossy as well.
• Weather conditions would also interfere with power collection, and can cause greater wear and tear on solar collectors than does the environment of Earth orbit; for instance, sandstorms cause devastating damage to human structures via, for example, abrasion of surfaces as well as mechanically large wind forces causing direct physical damage. Terrestrial systems are also more vulnerable to terrorism than an SPS's rectenna since they are more expensive, complex, intolerant of partial damage, and harder to repair/replace.
• Terrestrial solar panel locations are fixed, but beamed microwave power allows one to adaptively re-route the power near to where it is needed (within some limits, rectennas near the SPS's horizon (e.g., at high latitudes) will not be as efficient), while a solar generating station in the Sahara would provide power most economically only to the surrounding area, where current demand is relatively low. At least until long distance superconducting distribution becomes possible.
• A remote tropical location of a vast, centralized photovoltaic generator is a somewhat artificial scenario, and makes less sense every year as photovoltaic costs decline. The notion that ground-based photovoltaics are sensibly deployed in large, centralized arrays rather than distributed to end-use points (e.g., rooftops) should be questioned, yet it is frequently posed.

Both SPS and ground-based power could be used on-site to produce chemical fuels for transportation and storage, as in the proposed hydrogen economy.

Many advances in construction techniques that make the SPS concept more economical might make a ground-based system more economical as well. Increases in photovoltaic efficiency are an example. As well, many SPS plans are based on building the framework with automated machinery supplied with raw materials, typically aluminium. Such a system could just as easily be used on Earth, no shipping required. However, Earth-based construction already has access to extremely cheap human labor that would not be available in space, so such construction techniques would have to be extremely competitive.

[edit] Solar Panel Mass Production

Currently the costs of solar panels are too high to use them to produce bulk domestic electricity. However, the mass production of solar panels necessary to build a SPS system would be likely to reduce the costs sufficiently. As well, any panel design suited to SPS use is likely to be quite different than earth suitable panels. This may benefit as costs may be lower (see cost analysis above), but will not be able to take advantage of maximum economies of scale, and so piggyback on production of Earth based panels.

It should be noted, however, that there are also certain developments in the production of solar panels. The production of thin film solar panels (so-called "nanosolar") could reduce production costs as well as weight and therefore reduce the total cost of the project. In addition, private space corporations could gain interest in transporting goods (such as satellites, supplies and parts of commercial space hotels) to LEO, since they already are developing spacecraft to transport space tourists^[53][54].

[edit] Comparison with Other Renewables (wind, tidal, hydro, geothermal)

Other Renewables (e.g. wind energy, tidal energy, hydro-electric, geothermal) only have the capacity to supply a tiny fraction of the global demand for energy. The limitation is geography, there simply are very few sites in the world where generating systems of these types can be built. For 2005 in the USA hydro-electric power accounted for 6.5% of electricity generation, and other renewables 2.3%^[55]. The U.S. Govt. Energy Information Administration projects that in 2030 hydro-power will decline to 3.4% and other renewables will increase to 2.9%^[56].

Ocean based windpower is one possibility, but that is dominated by the high cost of long distance power transmission, in which case SPS would be highly competitive.

[edit] Other Applications of SPS

The use of microwave beams to heat the oceans has been studied. Some research has speculated that microwave beams appropriately applied would be capable of deflecting the course of hurricanes.

[edit] Current work

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

NASA's "Fresh Look" study in 2000^[57]

NASDA (Japan's national space agency) has been researching in this area steadily for the last few years. In 2001 plans were announced to perform additional research and prototyping by launching an experimental satellite of capacity between 10 kilowatts and 1 megawatt of power.^[58][59]

The National Space Society (a non-profit NGO) maintains a web page where the latest SPS related references are posted and kept current ^[60].

[edit] See also

• Energy economics
• Energy storage
• Exergy
• Energy quality
• Future energy development
• Microwave power transmission
• Renewable energy
• Wireless energy transfer

[edit] References

[edit] External links

• 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.
• 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.
• Space-Based Solar Power Efforts A synopsis of proposals and feasibility of lunar and high-orbital solar power stations, including assessments of cost.
• Japan's plans for a Solar Power Station in Space - the Japanese government hopes to assemble a space-based solar array by 2040.
• Power From Space Blog about using solar power satellites to reduce reliance on burning hydrocarbons.
• Whatever happened to solar power satellites? An article that covers the hurdles in the way of deploying a solar powered satellite.

Energy Conversion   Edit
Solar power:	Active solar | Barra system | Central solar heating plant | Energy tower | Photovoltaics | Solar cell | Solar combisystem | Solar panel | Solar pond | Solar power satellite | Solar power tower | Solar thermal energy | Solar tracker | Solar updraft tower | Passive solar | Trombe wall | Ocean thermal energy conversion
Wind power:	Wind farm | Wind turbine
Hydroelectricity: Tidal power | Water turbine | Wave power | Run-of-the-river hydroelectricity
Biological:	Mechanical biological treatment | Anaerobic digestion | Biomass
Chemical:	Blue energy | Fuel cell | Hydrogen production
Geothermal power:	Earth cooling tubes | Deep lake water cooling
Electricity generation:	Distributed generation | Microgeneration | Sustainable community energy system
Storage:	Thermal energy storage | Seasonal thermal store

Sustainability and Development of Energy   Edit Conversion | Development and Use | Sustainable Energy | Conservation | Transportation