Volume 1 photovoltaic solar energy 1 37 – solar power satellites

Volume 1 photovoltaic solar energy 1 37 – solar power satellites Volume 1 photovoltaic solar energy 1 37 – solar power satellites Volume 1 photovoltaic solar energy 1 37 – solar power satellites Volume 1 photovoltaic solar energy 1 37 – solar power satellites Volume 1 photovoltaic solar energy 1 37 – solar power satellites Volume 1 photovoltaic solar energy 1 37 – solar power satellites

1.37 Solar Power Satellites GA Landis, NASA Glenn Research Center, Cleveland, OH, USA Published by Elsevier Ltd 1.37.1 1.37.2 1.37.2.1 1.37.2.2 1.37.2.3 1.37.2.4 1.37.2.5 1.37.3 1.37.3.1 1.37.3.2 1.37.3.3 1.37.3.4 1.37.3.5 1.37.4 1.37.5 1.37.5.1 1.37.6 1.37.7 References Background and Historical Development Power-Beaming Fundamentals Efficiency of Microwave Transmission Spot Diameter Minimum Power Level for Microwave Beaming Can a Large Aperture Be Synthesized from Many Small Transmitters? Power Transmission by Laser Beaming Why Put Solar in Space? Continuous Sunlight Sun Pointing No Atmosphere Summation: How Much More Power Do You Get by Putting the Cells in Space? Future Evolution Is Geosynchronous the Right Orbit? The Economic Case Quick and Dirty Economics Beaming Power to Space: A First Step to SPS? Summary Points to Ponder 767 768 768 768 769 770 770 770 771 771 771 771 771 772 772 772 773 774 774 1.37.1 Background and Historical Development With the increases in energy cost and recent interest in finding ways to produce energy with reduced emission of greenhouse gasses, there has been renewed interest in the concept of producing power using solar panels in space and then beaming this power downward to provide electrical power for use on the Earth This concept, called the ‘Solar Power Satellite (SPS)’, was first proposed by Peter Glaser in 1968 [1], in a conceptual design in which the SPS was placed in geosynchronous orbit and the energy beamed to a land site using a microwave beam In revised and updated forms, the concept has been proposed many times since [2–5] as a possible solution to the energy crisis The Satellite Power System was studied in the late 1970s by NASA in collaboration with the Department of Energy, producing a conceptual ‘reference’ design [3] for a system This design was analyzed and critiqued by an Office of Technology Assessment study [4] The concept has gone by several different names and acronyms, starting as the ‘SPS’ or ‘Satellite Solar Power System’ (SSPS) and more recently studied under the name ‘Space Solar Power’ (SSP) Figure shows a summary of the 1980 ‘reference design’ for such an SPS [3] The baseline satellite concept produces about 10 GW of electrical power on the Earth, using a large (10 km by 15 km) solar array located in geosynchronous orbit The power is transmitted to the Earth by a microwave beam at 2.45 GHz, and a large (approximately 100 km2) rectifying antenna (or ‘rectenna’) array at Earth receives the beamed microwave power and converts it into DC electrical power The early study also looked at several alternate technologies for both the energy conversion and the power transmission and made attempts to predict the possible future performance improvements in order to make cost estimates of a future large-scale system In 1995, NASA headquarters initiated a ‘Fresh Look’ study of SPSs, which did not revise the original concepts, but started with a clean sheet of paper to rethink the basic concepts and come up with a new design [5] Some of the initial concepts examined included use of low Earth orbit instead of a geosynchronous orbit, gravity gradient-stabilized structures, sun-synchronous orbits, and use of large-area Fresnel lenses to focus light onto panels of concentrator cells Later evolution of the Fresh Look study (which evolved into the Space Solar Power Exploratory Research and Technology (SERT) program [6–8] in the 1999–2000 time frame) returned to the geosynchronous orbit design, but continued to look at new options for satellite design, including alternative power transmission methods such as laser beaming In addition to these NASA studies, there have been several non-NASA studies of the concept [9], as well as a number of studies that have been proposed to use power-beaming technology for other applications, including defense applications [10] and in-space applications such as satellites, electric propulsion, and lunar bases [11] It is worth noting that in 2004, the North American electrical energy generation was 4730 TWh, about 45% of which was generated by coal-fired plants At a typical production cost of ¢ (kWh)−1, this represents a revenue of $230 billion yr−1 for the potential market in North America alone Thus, although the concept requires mega-engineering on a scale that would dwarf all previous space projects, the potential revenue from it is extremely large Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00137-2 767 768 Applications km Array structure 10 km Solar cell array Transmitting antenna subarray DC−RF power amplifiers km diam Antenna waveguides High power density microwave beam Half-wave dipole antenna Open-screen ground plane Rectifying antenna 10 km × 13 km at 35 ° latitude Low power density microwave beam Figure Conceptual diagram of 1980 SPS ‘reference design’ [3] 1.37.2 Power-Beaming Fundamentals The ability to use solar power generated in space for terrestrial use requires beaming the power from the source, in space, to the user on the ground This is done by converting the electrical power to electromagnetic radiation, beam the radiation across free space, and collecting it at a receiver that converts electromagnetic radiation to electrical power In the 1980 study [3] (and most of the subsequent studies), it was proposed to this using radio frequency (RF) beams at a frequency of 2.45 GHz (i.e., microwaves in the industrial, scientific, and medical band) This was chosen because of the high demonstrated conversion efficiency of electrical power to microwave energy and because of the transparency of the atmosphere to microwave radiation 1.37.2.1 Efficiency of Microwave Transmission In the ‘ideal’ case, RF power-beaming efficiency can be quite high At these frequencies, magnetron tubes can convert DC to RF efficiency at transmitter efficiency of 90% or better, and a rectenna array (consisting of an array of GaAs Schottky diodes and quarter wave antennas) can convert RF power back to DC at receiver efficiency that has been demonstrated to be as high as 86% The product of these two efficiencies yields an overall ‘potential’ transmission efficiency, DC in to DC out, of ~77% In the real world, however, it is very easy to produce much degraded efficiency For example, Neville Marzwell estimated potential losses in a ‘real world’ estimated RF link efficiency [12], as listed in Table Multiplying each of these losses yields a much lower value of 37.5% for the overall DC-to-DC efficiency, and even this has not accounted for any losses of the transmitted beam not captured by receiver nor additional losses in power management and distribution either at the satellite or the user side of the system The reference design study assumed that the transmitter would be a phased array consisting of many millions of individual small transmitters operating in phase The phase would be controlled by a ‘beacon’ at the ground site, and hence if the beacon is not present, the beam would fail to be in phase, and hence would not be focused The requirement for a cooperative link at the ground station functions as an additional safety factor; in the event of an error, the transmitter is not capable of beaming high intensities to sites other than the ground station 1.37.2.2 Spot Diameter The minimum spot diameter of any transmitted electromagnetic beam is set by the diffraction limit The Airy diffraction disk is the diffraction pattern produced from a uniformly illuminated circular aperture [13, 14] This area contains 84% of the beam energy The ‘tails’ of the diffraction pattern outside this have little power, with a fall-off in power of approximately P ∼ exp[−r2] The diameter of the Airy diffraction pattern is Solar Power Satellites 769 Table Estimated link efficiencies for a realistic microwave transmission [12] Parameter Loss factor DC to RF efficiency EMC and diplexing Subarray aperture efficiency Subarray failures Amplitude errors and taper quantization Phase errors Electronic beam steering scan loss Clear air absorption Propagation impairments (scattering) Coupling efficiency (divergence) Polarization mismatch Rectenna aperture efficiency (reflections) EMC filtering Rectenna RF–DC conversion efficiency 0.90 0.794 0.95 0.96 0.986 0.97 0.977 0.9885 0.933 0.918 0.999 0.95 0.891 0.86 dspot ¼ 2:44 λx dtransmit ½1Š where dtransmit is the diameter of the transmitter aperture, x the source to receiver distance, and λ the wavelength This is the fundamental physics of electromagnetic power transmission – a beam can worse than this (i.e., a larger spot size) but cannot be focused into a smaller size We can use this equation to calculate the minimum spot size for a beam originating at geosynchronous orbit For this case, the distance x = 35 786 km At a frequency of 2.45 GHz, the wavelength is 122 m (12.2 cm) The diffraction-limited spot size is dspot ¼ 10:7 for d in kmị dtransmit ẵ2 Thus, for a km diameter transmitter, the minimum spot size on Earth is 10.7 km This explains why the 1980 reference design (and most successive design concepts) is so large: it is necessarily large because of the beam spot size The transmitter or receiver diameter is reduced in size directly proportional to wavelength Changing to other wavelengths has a direct effect on the size of the system Assuming that both the transmitter and receiver diameters are reduced proportionately, the spot size, and hence the receiver diameter, for a beam transmitted from geosynchronous orbit is shown in Table 1.37.2.3 Minimum Power Level for Microwave Beaming These receiver sizes, in turn, mean that there is a minimum power level at which the system needs to operate This is partly an economic issue At lower power intensity the transmitter and receiver costs will be too expensive (i.e., it would not be economically feasible to build a km diameter transmitting antenna in space to transmit a power of only kW), but it is also an effect of the physics of the receiving antenna: rectennas are inefficient at low power intensities A minimum intensity at the receiver is required in order for the diode rectifiers to convert power at high efficiency Diffraction limits thus mean that SPS is inherently big: small scales are not feasible It is clear that the minimum feasible power scales directly with the wavelength, or 1/microwave frequency Assuming minimum intensity of 100 W m−2 (100 MW km−2), this minimum power is listed in Table The data show that higher frequencies allow much smaller system sizes However, the higher frequency microwaves have a higher absorption by water vapor, that is, 10 GHz microwaves will not penetrate rain, and 100 GHz microwaves need a receiver on mountaintops Table Aperture size vs frequency for a beam transmitting from geosynchronous orbit, for case of transmit aperture diameter = 10  receiver aperture Frequency (GHz) dtransmit (km) dreceiver (km) 2.45 10 100 1.07 0.73 0.51 0.16 10.7 7.3 5.1 1.6 770 Applications Table Minimum feasible power as a function of microwave frequency, assuming power density of 100 W m−2 1.37.2.4 Frequency (GHz) Power (GW) 2.45 10 100 5.3 1.6 0.5 Can a Large Aperture Be Synthesized from Many Small Transmitters? Instead of making a single large aperture, the idea is sometimes suggested that a small spot size can be synthesized from a large array of small apertures This is effectively a suggestion that a large aperture can be ‘synthesized’ using the interferometric aperture synthesis approach, which is done, for example, for radio telescopes such as the very large array (VLA) The short answer to this question is no While high observational ‘resolution’ can be achieved by aperture synthesis, this is not effective for transmission A large transmitting array synthesized from many small arrays can beam to a smaller area, but the power density beamed into that area does not increase – in essence, the extra beam power is lost into the side lobes This effect is known as the ‘thinned array curse’ [14] 1.37.2.5 Power Transmission by Laser Beaming Power can also be beamed with a laser beam, using a photovoltaic (PV) array as a receiver [11, 15] Laser power beaming still uses electromagnetic radiation and hence is still subject to the electromagnetic limitations on beam size due to diffraction However, since laser radiation has much shorter wavelengths, smaller transmitter and receiver sizes can be used An example of μm wavelength is chosen in the near IR, close to the response peak for a high-efficiency silicon PV receiver Shorter wavelengths will reduce the size proportionately, but will require more expensive receiving arrays Assuming minimum intensity of 500 W m−2 for optical (producing an output equivalent to solar illumination, sun intensity) and assuming a transmitting lens diameter of 2.3 m, a laser beaming system operating at μm will produce a 23 m diameter minimum spot size on the ground, for an area of 0.0004 km2 This results in a minimum power level of 208 kW Laser transmission removes the problem of inherently large sizes, but lasers have their own problems First, laser efficiencies are considerably lower than microwave efficiencies, for lasers with good coherence High-power semiconductor diode laser arrays are highly efficient (50% conversion efficiency or higher), but are not mutually coherent – the net result of a high-power laser diode array is that it will have the diffraction pattern characteristic of a flashlight, not the narrow diffraction-limited spot size of a laser Existing technology lasers might have efficiency approaching ~40% (e.g., for a diode-pumped alkali laser) A second problem is that PV converter efficiencies are also relatively low The conversion efficiency is better than solar conversion efficiency, because the beam can be made monochromatic at a wavelength tuned to the optimum conversion wavelength of the cell, but is still lower than rectenna conversion efficiencies A conversion efficiency of 50% is a reasonable efficiency For laser transmission, clouds are now a problem In addition, eye safety is also a problem Overall, the use of a PV array for power receiving eliminates the signal advantage of an SSP, of putting the PV array in space, for cloud-free power Laser-transmitted space power has less power per solar array area than ground solar Despite these disadvantages, laser transmission also has advantages The laser power converters are PV arrays and so can operate both on the beamed laser power and on solar power Thus, laser power satellite could ‘fill in’ power to already existing solar arrays, either increasing the power level or allowing the array to continue to produce power when the sun is not available If a ground solar installation has already paid for the ground infrastructure, space PV power looks much more attractive! The ground infrastructure now gets double use for both solar and laser power This may provide an evolutionary route to space power from ground power However, ground solar arrays typically not cover 100% of the land area This wastes laser power (unless ground arrays are redesigned) Both clouds and eye safety are still a problem 1.37.3 Why Put Solar in Space? The critical question is, why should the solar arrays be placed in space? Why not just put the same solar cells on the ground? The proposed rationale for space solar is that, unlike the ground, space has solar power 24 h per day, 365 days per year How much more power can be achieved by putting the cells in space? Three reasons are usually given for why more power is achieved for space solar: Solar Power Satellites 771 space has continuous sunlight, 24 h per day; space solar arrays are (or can be) directly sun pointing; space has no atmosphere and no clouds 1.37.3.1 Continuous Sunlight The availability of 24 h sunlight inherently gives a factor of advantage, somewhat more in winter, less in summer 1.37.3.2 Sun Pointing Space solar arrays are (or can be) directly sun pointing The added benefit of tracking depends on pointing assumptions For solar arrays located not too far from the equator, calculating the added power using the cosine approximation gives an added factor of π/2 in the average power on a solar array This is exactly true only ignoring atmospheric effects (discussed in the next section), but is roughly true for a fixed solar array tilted south at the latitude angle It is also worthwhile to note that, in fact, ground solar arrays can be made sun pointing This is slightly less efficient in the use of land area, but allows more effective utilization of the solar cell area 1.37.3.3 No Atmosphere This comprises two advantages: the sunlight is not filtered by atmosphere, and space has no clouds The solar intensity is 1.37 W m−2 in space, compared with 1.0 kW m−2 (AM1.5 standard) The AM1.5 standard only applies for high sun angles near noon; there is more atmosphere filtering at higher sun angles, earlier and later in the day However, despite the higher intensity, solar cell efficiency in space is less than that achieved on ground by roughly a factor of 0.8 Accounting for both intensity and efficiency, solar cells produce about 10% more power at AM0 (space) than AM1.5 (ground spectrum) Clouds also are an issue for ground solar For a fair comparison of space and ground solar installations in the near term, however, we need to compare SSP with the best ground solar locations, not the average locations From NREL data on average solar power, for the best locations in the southwest United States, the annual average solar radiation is 12 kWh m−2, corresponding to kW m−2 for 12 h day−1 This is, essentially, zero cloud loss 1.37.3.4 Summation: How Much More Power Do You Get by Putting the Cells in Space? A solar array in space produces a factor of 1.1π = 3.5 times more energy per area than the solar arrays at the best ground locations, summarizing the above stated facts: Space has solar power 24 h per day: added power = factor of Directly sun pointing: added power = factor π/2 (if ground arrays are not sun pointing) Higher intensity in space: added power ~ factor 1.1 Space has no clouds: no added power (over cloud-free ground locations) The direct corollary to this is that, unless the DC-to-DC transmission efficiency of the power-beaming link is at least 29%, the net power output from the space system will be ‘less’ than the energy output from the same array placed (at the optimum location) on the ground In the most optimistic case of transmitter efficiency–conversion efficiency of 90%, Airy disk capture of 84%, and rectenna conversion efficiency of 86%, the ground DC power is 65% of the power produced in space This reduces the above numbers to an improvement of 2.25 for the space array compared with the ground array For the case that the ground array is tracking, the second factor reduces to 1, and a space array produces 2.2 times more power per unit area than a tracking array at the best site on the ground Incorporating optimistic transmission factor of 65%, this reduces to 1.4 times more power 1.37.3.5 Future Evolution The discussion so far has compared space location of a solar array with the best locations on the ground This is appropriate for the initial phases of solar power, since the first implementations of large-scale power production by PVs will, of course, be at the best locations, and not at the worst However, looking into the longer term, not all sites on the ground are ‘best’ sites Long-distance transmission lines can transmit power on the ground for some distance, but there will be large losses for transcontinental transmis­ sion – it is not feasible to produce power in the Mojave Desert and use it in New York Ground solar is worse by a factor of for areas of the United States outside of southwest and as much as a factor of 2.5 worse for New England, a significant electrical market It is worth noting, however, that the ground receivers for SSP also have significant location constraints, most notably a requirement for large areas of land in order to incorporate ‘keep-out’ zones near the beam, and thus may not be able to be located near large cities in the northeast in any case 772 Applications A caveat on this calculation is that it has implicitly been assumed that balance-of-systems cost of ground solar array (e.g., land cost) and space solar array (e.g., rectenna land area) are comparable In addition to the potentially higher total amount of power produced, SSP may have other virtues Most notable of these is that the power is continuous, 24 h power, rather than power peaking at noon and dropping to zero at sunrise and sunset At the moment, however, 24 h power is not an asset, since during the nighttime hours, the power demand is very low, and power available at night sells at very low price Ground solar produces power that is moderately well matched to the (early afternoon) peak demand Nevertheless, as solar capacity grows, this production curve will be increasingly mismatched to the demand and eventually solar will need to provide power outside peak solar hours The transition of solar power from peak to a requirement for power outside of the midday peak is typically expected to occur when ground solar reaches ~10–15% of the energy market (In the United States, this represents about $300 billion yr−1 total, although the price break occurs earlier in the areas where solar is most effectively used.) At this point, the continuous availability of power from space becomes an asset 1.37.4 Is Geosynchronous the Right Orbit? This analysis has assumed that a power satellite would be in geosynchronous Earth orbit (GEO) GEO is a location where the beaming station remains stationary (with respect to the Earth) over the equator at the longitude of the receiver This maximizes the utility of the station at a given receiver site It is interesting to look at the possibility of other orbits, in particular, lower orbits that would allow a shorter distance to beam, and hence smaller spot sizes and a smaller system size Only GEO orbit puts satellite over ground station with 100% usage fraction, and hence any lower orbit will have an immediate disadvantage that it will be out of direct beaming line of sight of the ground station for much of the time In addition to the nonstationary nature of lower orbits, another difficulty of low orbit is that these orbits will have to be nonequatorial if we want to get power to northern hemisphere users Thus, low-orbit view factors are low; for example, for an orbital altitude of 1000 km, the time in view above 10° of elevation is only 12.6 min, twice a day This results in a total use fraction of 25.2 out of 24 h, which is too low a usage fraction to be economically feasible One possible solution would be to make multiple ground stations for the power, receiving power at whatever location is in sight of the ground station, and likewise multiple power satellites, so that a satellite is available over each customer at any time However, to make this work for low orbits would require a large number of ground stations dotted uniformly around the world, including in many locations where there are few customers for the power, such as the Pacific Ocean The cost of such a system is several orders of magnitude higher than the baseline, since the number of satellites is much higher It is difficult to make this an economic case An alternative would be to reconsider the proposal to send power directly to northern hemisphere ground sites and to put the power-beaming satellites into a low equatorial orbit and beam only to sites on, or near, the equator Users at sites distant from the equator would then have to be served either by transmission lines or else by secondary beams passing through microwave relay satellites This reduces the number of power satellites and receiver stations considerably; for example, approximately 24 satellites in 1000 km orbit could provide continuous power to service sites at or near the equator Although this is a larger number of satellites than the single satellite required in geosynchronous orbit, the transmitting aperture of each satellite is 25 times smaller Another approach is to adapt the space power concept to the price structure of the electric power market, by redesigning SPS to maximize the amount of power available to service peak power markets, instead of base load If this could be done, it would roughly double the revenue in terms of dollars per kWh Analyses of this approach have been summarized in an earlier paper, ‘Reinventing the Solar Power Satellite’, NASA 2004-212743 [16, 17] 1.37.5 The Economic Case The economic return for SSP requires return on investment If an SPS is to be commercially viable, it must charge the utilities to which it is selling power (per kWh) less than the utility’s cost of generating new power Note that it is important to beat the utilities’ cost, not the customer’s electric cost This cost may include the cost for externalities (e.g., possible penalties to be imposed for generating with coal), if any As a minimum, even if the operating cost is zero (i.e., small compared with the capital) and operating lifetime is infinite, the invested money must be returned 1.37.5.1 Quick and Dirty Economics It is instructive to calculate example numbers for return on investment case The rate of return must be at least $ yr − > P  Capital cost ỵ PịT ẵ3 where P is the required rate of return on investment and T the time required for construction This must be divided by the power produced per year to get the dollars per kWh Power produced is Solar Power Satellites kWh yr − ¼ 8700 h yr −  000 000 kW Power GW ị GW 773 ẵ4 Thus $ kW − h − > P  Capital cost million $ị ỵ P ịT Power GWị 8700 ẵ5 Let us assume an investment rate of return of 8.25% (P = 0.0825) and a construction time of years (T = 5) Then the factor (1 + P) = 1.49, which raises the effective interest rate to 12.3% The equation then is T $ kWh − > 0:014  Capital cost ðbillion $Þ Power ðGWÞ This can also be expressed as a limit on the capital cost, as a function of the selling cost of electricity: � � Capital cost billion $Þ < 70:8 $ kWh ị Power GWị ẵ6 ẵ7 Assuming a power level of GW and a selling price of ¢ (kWh) , this is Capital cost < $17:5 billion This analysis has assumed an infinite lifetime of the assets, and no associated operating costs If, instead, the loan is to be paid off in a finite amount of time, a slightly higher rate of return is required For 30-year payoff of 8.25% on the loan, the payment is 1.09 times higher rate; for a 25-year payoff, the yearly payment is 1.15 times higher If operating costs are not negligible, they need to be added directly to the cost per kilowatt-hour It is useful to divide the maximum allowable capital cost ($17.5 billion for the infinite lifetime/zero operations cost case) by the power produced, GW This shows that, to sell electricity at a price of $0.05 kWh−1, the total cost of the system must not exceed $3.50 W−1 This is an extremely challenging target, since it must include not only the solar arrays but also all of the in-space structure as well as the ground assets 1.37.6 Beaming Power to Space: A First Step to SPS? The economic calculation has so far assumed that the market for beamed power is in fact the terrestrial electric power market The market on Earth, however, is currently served by relatively low-cost generating capacity It is much more likely that the first steps to demonstrating and using beamed power would be for ground-to-space or space-to-space power beaming [18] Probably the best market is to start with in-space power beaming Figure shows a schematic A ground-based power system, using a large aperture antenna, sends the energy beam (in the form of either a microwave or laser energy) to receivers in space, which could include satellites, orbital transfer vehicles using electric propulsion, or even (in the longer term) a lunar base [11] Currently, electrical power in space has an effective price tag that is 10 000 times the price of power on the ground It makes sense to beam power to the place where it is expensive from the place where it is cheap Figure Earth to space power-beaming concept, showing possible receivers in satellites, electric propulsion transfer vehicles, and lunar bases [11] Image Courtesy of NASA 774 Applications 1.37.7 Summary Points to Ponder Producing solar power in space to be sent to the surface of the Earth for use by consumers on the ground (‘solar power satellite,’ or SPS) has been proposed as a means of solving the problem of electrical supply on the surface of the Earth with a renewable, low­ carbon-emission technology, see also [19] The fundamental physics are feasible, but economical feasibility is as yet an open question Although a space location for the solar panels gets more sun than a ground location, the bottom line numbers show that it is not that much more solar energy than the best ground locations The added power mostly comes from 24 h sunlight, but much of the power may thus be produced when the need is low Electromagnetic beam diffraction limits mean that SPS is inherently big: GW minimum for GHz power beam; a higher frequency will not be able to penetrate rain Switching to laser transmission allows very small satellites, but efficiency losses are large To a rough approximation, solar arrays in space produce 3.5 times more power than nontracking arrays on the ground array or 2.2 times more than tracking arrays on the ground Accounting for transmission losses, this reduces to 1.63 times more power/solar cell area If the cost of solar arrays is less than 61% of the total SPS cost, a given solar array will produce energy at a lower cost on the ground, assuming that it can be placed in the optimum location, than if placed in space While this is only applicable if ground locations are close in location to the highest selling price electric power markets, there may also be questions as to whether receiving rectenna arrays can be located close to the highest price markets In general, SSP will be economically competitive only if ground solar is economically competitive in the best markets GEO seems to be the most reasonable choice for the orbital location Other orbits are not over the receiver continuously; storage is not (yet) competitive A final question is, can solar cells that are qualified for use in space, and are of a suitable weight and efficiency for SPS use, be produced at a cost equal to the low production cost of terrestrial solar cells? This is yet to be demonstrated Space solar cells have stringent requirements, and it is not yet clear whether the production cost can be reduced without compromising the quality References [1] Glaser PE (1968) Power from the sun: Its future Science 162: 957–961 [2] Glaser PE, Davidson FP, and Csigi KI (eds.) (1994/1998) Solar Power Satellites: The Emerging Energy Option, 2nd edn New York: Wiley [3] NASA (1980) Satellite Power System Concept Development and Evaluation Program System Definition Technical Assessment Report U.S Department of Energy, Office of Energy Research, Report DOE/ER/10035-03, December [4] U S Office of Technology Assessment (1981) Solar Power Satellites [5] Mankins J (1997) A Fresh Look At Space Solar Power: New Architecture, Concepts, and Technologies Paper IAF-97-R.2.03, 48th International Astronautical Congress, pp 347–359 Turin, Italy, 6–10 October In: Acta Astronautica, vol 41, pp 4–10 August–November [6] Howell J and Mankins JC (2000) Preliminary results from NASA’s space solar power exploratory research and technology program Proceedings of the 51st International Astronautical Congress Rio de Janeiro, Brazil [7] Dudenhoefer JE and George PJ (2000) Space solar power satellite technology: Development at the Glenn research center – An overview Paper NHTC2000–12067, 34th National Heat Transfer Conference Pittsburgh, PA, 20–22 August NASA Technical Memorandum NASA/TM—2000-210210 [8] Feingold H and Carrington C (2003) Evaluation and comparison of space solar power concepts In: Proceedings of the 53rd International Astronautical Federation Congress Acta Astronautica, vol 53, pp.4–10, 547–559 August–November doi: 10.1016/S0094–5765(03)80016–4 [9] (1997) Proceedings of the SPS ‘97 Conference, Space Power Systems: Energy and Space for Humanity Montréal, Canada, 24–28 August Organized by the Canadian Aeronautics and Space Institute and the Sociéte des Électriciens et Électroniciens France [10] Johnson WN, Akins K, Armstrong J, et al (2009) Space-based solar power: Possible defense applications and opportunities for NRL Naval Research Laboratory Report NRL/ FR/7650-09-10,179, 23 October [11] Landis GA (1991) Space power by ground-based laser illumination IEEE Aerospace and Electronics Systems6(6): 3–7 Presented at 26th Intersociety Energy Conversion Engineering Conference, vol 1, pp 1–6 Boston, MA, USA, 4–9 August [12] Marzwell N (2007) The challenge of space solar power MIT Space Solar Power Workshop 15–17 May [13] Swinburne Centre for Astrophysics and Supercomputing COSMOS: The SAO Encyclopedia of Astronomy http://cosmos.swin.edu.au/entries/airydisk/airydisk.html?e=1 [14] Forward RL (1984) Roundtrip interstellar travel using laser pushed lightsails Journal of Spacecraft and Rockets 21(2): 190 http://en.wikipedia.org/wiki/Thinned_array_curse [15] Landis GA (1991) Photovoltaic receivers for laser beamed power Proceedings of the 22nd IEEE Photovoltaic Specialists Conference, vol II, pp 1494–1502 Las Vegas, NV, USA, October NASA Report CR-189075 [16] Landis GA (2004) Reinventing the solar power satellite NASA Technical Memorandum TM-2004-212743 [17] Landis GA (2006) Re-evaluating satellite solar power systems for earth IEEE 4th World Conference on Photovoltaic Energy Conversion Waikoloa, HI, USA, 7–12 May [18] Landis GA (1990) An evolutionary path to SPS Space Power 9(4): 365–371 [19] Landis GA (2009) Solar power from space: Separating speculation from reality Presented at Space Photovoltaic Research and Technology XXI Brook Park, OH, USA, 6–8 October  ... Engineering Conference, vol 1, pp 1 6 Boston, MA, USA, 4–9 August [12 ] Marzwell N (2007) The challenge of space solar power MIT Space Solar Power Workshop 15 17 May [13 ] Swinburne Centre for Astrophysics... space solar power concepts In: Proceedings of the 53rd International Astronautical Federation Congress Acta Astronautica, vol 53, pp.4 10 , 54 7–5 59 August–November doi: 10 .10 16/S009 4–5 765(03)80 016 –4 ... Science 16 2: 95 7–9 61 [2] Glaser PE, Davidson FP, and Csigi KI (eds.) (19 94 /19 98) Solar Power Satellites: The Emerging Energy Option, 2nd edn New York: Wiley [3] NASA (19 80) Satellite Power System