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ABSTRACT : Wireless Power Transmission technology via microwave (Microwave Power Transmission; MPT) was advanced from 1960's and many researchers which had a dream to realize the Space Solar Power Satellite/Station. This is a concept for; a future giga watt space power system to provide electrical power by converting the sun's energy and beaming into the earth's surface. This article specifically describes about the System, advantages, design and features of SPS.
Index Terms - Microwave Power Transmission, Space Solar Power Satellite, Rectenna, GEO.
Space-based solar power (SBSP)(or historically space solar power (SSP)) is a system for the collection of solar power in space, for use on Earth. SBSP differs from the usual method of solar power collection in that the solar panels used to collect the energy would reside on a satellite in Geo stationary orbit, often referred to as a solar power satellite (SPS), rather than on Earth's surface. In space, collection of the Sun's energy is unaffected by the day/night cycle, weather, seasons, or the filtering effect of Earth's atmospheric gases. Average solar energy per unit area outside Earth's atmosphere is on the order of ten times that available on Earth's surface. The collection of solar energy in space for use on Earth introduces the new problem of transmitting energy from the collection point, in space, to the
place where the energy would be used, on Earth's surface. Since wires extending from Earth's surface to an orbiting satellite would be impractical, many SBSP designs have proposed
the use of microwave beams to transmit power wirelessly. The collecting satellite would convert solar energy into electrical energy, which would then be used to power a microwave emitter directed at a collector on the Earth's surface.
Many problems normally associated with solar power collection would be eliminated by such a design, such as the high sensitivity of conventional surface solar panels to corrosion and weather, and the resulting maintenance costs. Other problems may take their place though, such as cumulative radiation damage or micrometeoroid impacts.
Interest in space solar power has waxed and waned over the 40 years since the idea for a solar power satellite was proposed by Peter Glaser in 1968) and the 35 years since a patent was granted in 1973. In1970s, the United States Department of Energy (DOE) and the National Aeronautics and Space Administration (NASA) conducted a joint study of the feasibility of space solar power and produced a reference system design .Analysis of the joint study by the National Research Council in 1981 concluded that although technically feasible, the technology was not sufficiently mature and the initial costs were too great to warrant implementation .In the 1990s, NASA again studied space solar power this time with an added emphasis on economic viability. The most recent surge was sparked by the release of a study of space-based solar power organized by the United States Department of Defense (DOD) National Security Space Office in 2007.
In Japan, many kinds of the SPS were proposed and designed in recent ten years. The two SPS designed in Japan. One is designed by JAXA (Japan Aerospace Exploration Agency) .The other Japanese SPS is designed by METI
(Ministry of Economy, Trade and Industry) and USEF (Institute for Unmanned .Space Experiment Free Flyer). USEF-SPS is so called tethered SPS.
Fig1 Fig 2
Recent Japanese SPS (a) JAXA2004 model
(b) USEF2002 model
A proposed collector of solar energy that would be placed in geostationary orbit where sunlight striking the satellite would be converted to electricity and then to microwaves,
which would be transmitted to earth.
On the left: Part of the solar energy is lost in its way through the atmosphere by the effects of reflection and absorption.
On the right: Space-based solar power systems are an attempt to convert in space, outside the atmosphere, to avoid these losses.
The SBSP concept is attractive because space has several major advantages over the Earth's surface for the collection of solar power. There is no air in space, so the collecting surfaces would receive much more intense sunlight, unaffected by weather. In geostationary orbit, an SPS would be illuminated over 99% of the time. The SPS would be in Earth's shadow on only a few days at the spring and fall equinoxes; and even then for a maximum of 75 minutes late at night when power demands are at their lowest. This characteristic of SBSP avoids the expense of storage facilities (dams, oil storage tanks, coal dumps) necessary in many Earth-based power generation systems. Additionally, SBSP would have fewer or none of the ecological (or political) consequences of fossil fuel system
Space solar power systems appear to possess many significant environmental advantages when compared to alternative approaches.
Space-based solar power essentially consists of three parts:
1. A means of collecting solar power in space, for example via solar cells or a heat engine
2. A means of transmitting power to earth, for example via microwave or laser
3. A means of receiving power on earth, for example via a microwave antennas (rectenna)
The space-based portion will be in a freefall, vacuum environment and will not need to support itself against gravity other than relatively weak tidal stresses. It needs no protection from terrestrial wind or weather, but will have to cope with space-based hazards such as micro meteors and solar storms.
Solar energy conversion (solar photons to DC current):
Two basic methods of converting sunlight to electricity have been studied: photovoltaic (PV) conversion, and solar dynamic (SD) conversion. Most analyses of solar power satellites have focused on photovoltaic conversion (commonly known as "solar cells"). Photovoltaic conversion uses semiconductor cells (e.g., silicon or gallium arsenide) to directly convert photons into electrical power via a quantum mechanical mechanism. Some new, thin-film approaches are less efficient (about 20% vs. 35% for best in class in each case), but are much less expensive and generally lighter. In an SPS implementation, photovoltaic cells will likely be rather different
from the glass-pane protected solar cell panels familiar to many from current terrestrial use.
Wireless power transmission to the Earth:
Wireless power transmission was early proposed to transfer energy from collection to the Earth's surface. The power could be transmitted as either microwave or laser radiation at a variety of frequencies depending on system design. Whichever choice is made, the transmitting radiation would have to be non-ionizing to avoid potential disturbances either ecologically or biologically. This established an upper limit for the frequency used, as energy per photon (and consequently the ability to cause ionization) increases with frequency. Ionization of biological materials doesn't begin until ultraviolet or higher frequencies, so most radio frequencies would be feasible.
William C. Brown demonstrated in 1964, during Walter Cronkite's CBS News program, a microwave-powered model helicopter that received all the power it needed for flight from a microwave beam. Between 1969 and 1975, Bill Brown was technical director of a JPL Raytheon program that beamed 30 kW of power over a distance of 1 mile at 84% efficiency.
The size of a solar power satellite would be dominated by two factors: the size of the collecting apparatus (e.g. panels and mirrors), and the size of the transmitting antenna. The distance from Earth to geostationary orbit (22,300 miles, 35,700 km), the chosen wavelength of the microwaves, and certain laws of physics (specifically the Rayleigh Criterion or diffraction limit) will all be factors.
It has been suggested that, for best efficiency, the satellite antenna should be circular and about 1 kilometer in diameter or larger; the ground antenna (rectenna) should be elliptical, 10 km wide, and a length that makes the rectenna appear circular from GEO (Geostationary Orbit). (Typically, 14 km at some North American latitudes.) Smaller antennas would result in increased losses to diffraction/side lobes. For the desired (23mW/cmÂ²) microwave intensity  these antennas could transfer between 5 and 10 gigawatts of power.
The Earth-based receiver antenna (or rectenna) is a critical part of the original SPS concept. It would probably consist of many short dipole antennas, connected via diodes. Microwaves broadcast from the SPS will be received in the dipoles with about 85% efficiency. With a conventional microwave antenna, the reception efficiency is still better, but the cost and complexity is also considerably greater, almost certainly prohibitively so. Rectennas would be multiple kilometers across. Crops and farm animals may be raised underneath a rectenna, as the thin wires used for support and for the dipoles will only slightly reduce sunlight, or non arable land could be used, so such a rectenna would not be as expensive in terms of land use as might be supposed.
Dealing with launch costs:
One problem for the SBSP concept is the cost of space launches and the amount of material that would need to be launched.
Reusable launch systems such as the Falcon 9 are predicted to provide lower launch costs to lower Earth orbit (LEO).
Much of the material launched need not be delivered to its eventual orbit immediately, which raises the possibility, that high efficiency (but slower) engines could move SPS material from LEO to GEO at an acceptable cost. Examples include ion thrusters or nuclear propulsion.
To give an idea of the scale of the problem, assuming a solar panel mass of 20 kg per kilowatt (without considering the mass of the supporting structure, antenna, or any significant mass reduction of any focusing mirrors) a 4 GW power station would weigh about 80,000 metric tons, all of which would, in current circumstances, be launched from the Earth. Very lightweight designs could likely achieve 1 kg/kW, meaning 4,000 metric tons for the solar panels for the same 4 GW capacity station.
Increased global warming:
The entire point of a solar power satellite is to increase the amount of solar energy reaching earth. This extra energy will eventually be dissipated as heat. Depending on the scale of operations, this might or might not have a significant effect. No theories to date claim that waste heat from human power generation are a significant cause of global warming, nor would it be for the foreseeable future. The most widely promoted theory connecting human activity to global warming is that increased greenhouse gases (e.g. carbon dioxide and methane) are causing the natural heat from the Sun to be trapped so it cannot radiate to space, thus increasing the temperature of the planet. Space solar power would contribute greatly to reduction of greenhouse gases.
Rectenna power conversion efficiency would be better than 90%, so waste heat from the rectennas would be considerably less than from most other common power sources, e.g. nuclear and fossil fuels which generate much more waste heat.
Beam Collection Efficiency:
The beam collection efficiency depends on the transmitter and receiver aperture areas, the
wavelength, and the separation distance between the two antennas as shown in the section 1. For
example, it was calculated approximately 89% in the SPS reference system with the parameters as follows; the transmitter aperture is 1 kmÏ†, the rectenna aperture is 10x13 km, the wavelength is
12.24 cm (2.45GHz), and the distance between the SPS and the rectenna 36,000 km. They assume 10dB Gaussian power taper on the transmitting antenna. The beam pattern on the ground is shown in Fig
Beam Pattern on the Ground
Recent Impact by Japan:
As already said in Japan, many kinds of the SPS were proposed and Designed in recent ten years. The two SPS designed in Japan were, one designed by JAXA (Japan Aerospace Exploration Agency). The characteristics of the JAXASPS are as follows;
1] The buoyancy can be used to fly the primary
2] Formation flying mirrors to avoid the rotary
3] The whole system becomes mechanically
more stable and reliable.
4] The adoption of some wavelength selective film that could cut unusable light wavelengths is also considered.
The other Japanese SPS is designed by METI (Ministry of Economy, Trade and Industry) and USEF (Institute for Unmanned .Space Experiment Free Flyer) USEF-SPS is so called tethered SPS. The attitude stability is maintained by the gravity gradient of the field. The difference of gravity at the bus and panel make the system stabilize without active attitude stabilization system. The SPS consists of sandwich panel (solar cell - microwave transmitter - phased array antenna). The combination of solar cell and antenna makes short power distribution length inside the panel.
A conceptual design for a microwave wireless power transmission experiment from low earth orbit (the International space Station) to the ground has been developed by a team from government, industry and academia. The experiment would demonstrate retrodirective control of a phased array antenna delivering
Microwave power from space to the ground, both firsts. The project would have been funded by NASA and the Department of Defense (DOD).
The SPS with the MPT is hopeful power station in future. In Japan, many researchers continue to design new SPS and carry out experiments, especially MPT experiments. However, road to the SPS is far and steep because there is no application of the MPT on ground. The high power MPT application systems and/or weak
Power MPT application systems are hopeful milestone to clime up to the SPS.