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The search for inexhaustible energy resources to satisfy long term needs is a high priority. Solar Power Satellites answer mankindâ„¢s energy needs in the 21st century. We can,infact directly convert solar energy into electrical energy with the use of solar cells, but sunlight diffuses at night time from the earth. If the need arises for 24 hours power supply,we are helpless. The solution is wireless power transmission from space through a system consisting of SPS(Solar Power Satellite) and RECTENNA (RECtifying anTENNA) by Microwaves. The principal advantage of the space location is the independence of weather and day-night cycle and is pollution free.Modern techniques enables us to create a platform in space carrying solar batteries, generators converting electric current worked out by them into the Electromagnetic wave beam. On the earth, the other antenna (rectenna) receives the beam and it is converted into
electric current again. Space solar power stations are costly because of the great size of their radiation and receiving antennas. It is shown that a correct choice of the field distribution on the radiation antenna allows us to increase the wireless power transmission efficiency and to lessen its cost. Antenna and Rectenna sizes are chosen such that the rectenna is situated in the antennaâ„¢s Fresnelâ„¢s area(but not in the far area as in the ordinary radio communication). This paper thoroughly describes the construction of space antenna and rectenna to increase the effectiveness of WPT.
Compared to todayâ„¢s energy sources, the SPS and rectenna system is an economically competitive large scale energy source, and in fact appears to offer a much less expensive energy source once significant space-bases infrastructure is establishes. In addition, the SPS and rectenna system has strong advantages in terms of environmental issues.
The overall configuration of the spacetenna is a triangular prism with a length of 800 m and sides of 100 m as shown in figure. The main axis lies in the north-south direction, perpendicular to the direction of orbital motion. The transmitting antenna on the horizontal under-surface faces the Earth, and the other two sides of the prism carry solar
arrays. The faces of the prism are embedded with photovoltaic cells.
These Photovoltaic cells would convert sunlight into electrical current, which would, in turn, power an onboard microwave generator. The microwave beam would travel through space and the atmosphere. On the ground, an array of rectifying antennas, or rectennas, would collect these microwaves and extract electrical power, either for local use or for distribution through conventional utility grids.
The Spacetenna has a square shape whose dimension is 132 meters by 132 meters and
which is regulary filled with 1936 segments of sub array. The sub array is considered to
be a unit of phase control and also a square shape whose edges are 3 meters. It contains 1320
units of cavity-backed slot antenna element and DC-RF circuit. Therefore, there will be about 2.6 million antenna elements in the spacetenna

The spacetenna is composed of pilot signal receiving antennas followed by the detectors finding out the location of the rectenna on the earth, power transmission antenna elements and phase control systems. The left and right hand sides in the figure below correspond to parts of power transmission and direction detection, respectively. The antenna elements receiving the pilot signal have a polarization perpendicular to the antenna elements used in the power transmission so as to reduce effectively interaction between both antenna elements. Moreover, the pilot signal frequency and a frequency for the energy transmissionare dufferent from each other. Using two kinds of frequency for the power transmission and the pilot signal prevents each other from interfering and makes it possible to find out the accurate direction of a specified rectenna.
A Rectenna can be considered as a base station for a geo-stationary satellite. Microwaves of 2.45 GHz frequency are used to transmit power from the satellite to th rectenna. It consists of a mesh of an array of dipole antennas connected to diodes to convert the radio frequency energy to DC voltage, which is then converted to regular AC electricity and wired to homes, factories, etc. A simple reflector plane could be added to the mesh to improve the efficiency to 50%.

Orbit Selection
A 1100 km altitude quatorial orbit will be used. This choice minimizes the transportation cost and the distance of power transmission from space.
The system power is defined by the microwave power transmitted from the satellite, not by the power received on earth. It also has to be in low Earth orbit, in order to be low-cost . If it was in GEO ( that is 35,800 kilometers from Earth) then the transmittin antenna would bave to be 40 times larger than a LEO one “ or else the receiving antenna would have to be 40 times larger. Consequently the satellite has to orbit above the equator in order to have frequent transmission opportunities. It transmits up to 10 Mwatts of radio-frequency power. Ten satellites placed evenly arount the orbit would require only nine minutes of storage capacity to provide continuous power.

Field Distribution
The basic drawback of a WPT system is that the essential part of the radiated energy does not reach the given area of space because of wave beam diffraction expansion. For an qual phased field distribution the focused field with peak distribution falls down to edges
as shown by the function
The peak distribution of the field at v=1 is close to cos(pi*x/2a) and v=2 is [cos(pi*x/2a)]^2 which is similar to a GUASSIAN FIELD distribution. For a circular aperture, this sort of field distribution falls down to the edges which is ineffective use of the antenna size. The way out is accommodation the receiver with irregular sub-apertures, each of which gives a distribution of field uniform and equal in amplitude.
The increase of WPT effectiveness with non-equidistant antenna array by the discrete radiating antenna is of the factor
where 2b is the length of the receptor.Size of A is increased if the amplitude of the radiating field falls down to edges and the receiver with in the Fresnelâ„¢s area. However the efficiency WPT systems depends not only on the size of the A but also on active surface of the radiating antenna. The factor of surface utilization for a square aperture is
Where U(x) is the field distribution of the radiator Um(x) is the max.allowable field.
We require receiving a high (xA2) factor of energy transfer by saving good operating active source of the antenna .Hence it is necessary to have a high product,which may be termed as Ëœgeneralised criterionâ„¢ for energy transfer.
Thus, for a discrete step distribution , we need to have concentrated sub apertures in the centre and their gradual discharge on the edges.Thus all the sub apertures are similar and have a uniform distribution of the field with the equal amplitude,which may reach mawimum admissible value.The optimal distribution form may be reached for the large radiating sub aperture clots in places, which corresponds to high field intensity and relieving sub aperture density at the edges pf the antenna.This construction allows approaching to unit the value both of the coefficients A and x.
As a result of the WPT system will be essentially increased.
1. Unaffected by day-night cycle, weather or seasons. Optimised advances may enable 21 power supply per day.
2. This is eco-friendly, renewable and maintenance free energy resource unlike the conventional fuels.
3. As the equipment is positioned in space, it is occupies no land area and remains unaffected by harsh weather conditions. Rectennas can share land with farms.
4. The spacetenna could direct energy to any rectenna on earth with in the range of its steering angle, which could satisfy the energy requirements of all the equatorial countries.
5. Waste heat is re-radiated back into space , instead of warming the biosphere.
6. Will be a boon as we are running out of fossil fuels.
Synthesizing the wireless power transmission , it can be concluded that to make the SPS concept commercially viable, it becomes a priority to improve its efficiency and the cost per watt. This can be achieved by:
1. Placing the rectenna within the Fresnelâ„¢s area of the transmitter
2. Placing the transmitting antenna in the LEO orbit to reduce installation cost and distance of transmission.
3. Using a discontinuous equidistant array with quasi Gauss distribution.
4. Using a discontinuous non-equidistant array with uniform distribution
1. Proceedings of INTERNATIONAL CONFERENCE 2004 held at IISC Bangalore.
2. P.E.Glaser An overview of the solar power satellite , IEEE Transactions on Microwave Theory and Techniques vol.40.No.6,June 1992.
3. Microwave Devices and Circuits by Samuel Y Liao.
4. spacefuture.com

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18-02-2012, 10:58 AM

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