01-03-2013, 02:20 PM
WIRELESS POWER TRANSMISSION AND RECEPTION USING SPS & RECTENNA
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ABSTRACT
The search for “inexhaustible” energy resources to satisfy long term needs is a high priority. Solar Power Satellites answer mankind’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 its 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 field energy of ten centimeter range (microwaves) and antenna forming 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 radiating 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 antenna’s Fresnel’s area (but not in the far area as in the ordinary radio communication). This paper thoroughly describes the construction of spacetenna and rectenna to increase the effectiveness of WPT.
INTRODUCTION
Compared to today'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-based infrastructure is established. In addition, the SPS and rectenna system has strong advantages in terms of environmental issues.
OVERALL VIEW
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.
Orbit Selection
A 1100km altitude equatorial 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 transmitting antenna would have 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 10Mwatts of radio-frequency power. Ten satellites placed evenly around 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 equal 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 at 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 accommodating the receiver with irregular sub-apertures, each of which gives a distribution of field uniform and equal in amplitude.
Conclusion
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 Fresnel area of the transmitter.
2. Placing the transmitting antenna in the LEO orbit to reduce installation cost and . the distance of transmission.
3. Using a discontinuous equidistant array with quasi Gauss distribution.
4. Using a discontinuous non-equidistant array with uniform distribution.