Published on March 11, 2014
11.EARTH BASED INFRA STRUCTURE(RECTENNA) RECTifying anTENNA rectifies received microwaves into DC current. A rectenna comprises of a mesh of dipoles and diodes for absorbing microwave energy from a transmitter and converting it into electric power. Its elements are usually arranged in a mesh pattern, giving it a distinct appearance from most antenna. A simple rectenna can be constructed from a Schottky diode placed between antenna dipoles as shown in Fig. 1. The diode rectifies the current induced in the antenna by the microwaves. Rectenna are highly efficient at converting microwave energy to electricity. In laboratory environments, efficiencies above 90% have been observed with regularity. In future rectennas will be used to generate large-scale power from microwave beams delivered from orbiting SPS satellites. 12.SOLAR POWER SATELLITE OF SPS 2000 SPS2000 is a model of solar power satellites with microwave power output of 10 MW, which was proposed by the SPS working group of the Institute of Space and Astronautically Science (ISAS). The primary objective of SPS2000 research is to show whether SPS could be realized with the present technology and to find out technical problems. The conceptual study of SPS2000 is now being carried out under the assumption that the first construction will be started before the beginning of the twenty first century. 12
SPS2000 transforms the DC power generated by huge solar arrays to microwave power at 2.45 GHz and transmits it to the rectennas on the earth while it moves from west to east in an equatorial low earth orbit (LEO) of 1100 km altitude. Transmission is possible when the rectenna can be in the field of view of the controllable microwave beam from SPS2000. Therefore, SPS2000 should always detect the location of the rectenna and direct a microwave beam toward the rectenna. In order to perform the beam scan, the spacetenna should have a function of a phased-array antenna. We discuss a configuration of spacetenna of SPS2000. On the basis of the spacetenna proposed above, we design a functional system model of SPS2000 as a demonstration model and construct microwave circuits employing silicon (Si) semiconductors since there are many advantages in Si technology compared with others in terms of cost reduction, robustness of the system and extraterrestrial resources. 13.C0NFIGURATION OF SPACETENNA Fig.13.1 Spacetenna 13
The general configuration of SPS2000 has the shape like a triangular prism as shown in Figure 2 The power transmission antenna, spacetenna, is built on the bottom surface facing to the earth, and the other two surfaces are used to deploy the solar panels. SPS2000 moves on an equatorial LEO at an altitude of 1100km. The choice of the orbit minimizes the transportation cost and the distance of power transmission from space. The spacetenna is constructed as a phased-array antenna. It directs a microwave power beam to the position where a pilot signal is transmitted from a ground-based segment of power system (RECTENNA). Therefore, the spacetenna has to be a huge phased-array antenna in size with antenna in size with a retro directive beam control capability. So, microwave circuits are connected to each antenna element and driven by DC power generated in the huge solar panels. A frequency of 2.45 GHz is assigned to transmit power to the earth. Figure 2 also shows a scheme of microwave beam control and rectenna location. SPS2000 can serve exclusively the equatorial zone, especially benefiting geographically isolated lands in developing nations. Fig.13.2. Configuration of the Spacetenna 14
Fig. 13.2 illustrates a configuration of the Spacetenna. The Spacetenna has a square shape whose dimension is 132 meters by 132 meters and which is regularly 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 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 Fig.13.2 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 interactions between both antenna elements. Moreover, the pilot signal frequency and a frequency for the energy transmission are different 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. 15
14.ADVANTAGES The SPS concept is attractive because space has several major advantages over the Earth's surface for the collection of solar power. 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 SPS based power generation systems to avoid the expensive storage facilities (eg, lakes behind dams, oil storage tanks, coal dumps, etc) necessary in many Earth- based power generation systems. Additionally, an SPS will have none of the polluting consequences of fossil fuel systems, nor the ecological problems resulting from many renewable or low impact power generation systems (eg, dam retention lakes). Economically, an SPS deployment project would create many new jobs and contract opportunities for industry. Space solar power would be the only means of acquiring direct solar energy to supplement the burning of fossil fuels or nuclear energy sources under the most extreme conditions of a global catastrophic volcanic winter (or similarly, nuclear winter 16
Table no.14.1 Comparision Table 15.FUTURE SCOPE 1. To store electricity during off peak demand hours. 2. The frequency of beamed radiation is planned to be at 2.45 GHz and this frequency is used by communication satellites also. 3. Minimizing the entire size as it will be massive. 4. To reduce high initial cost and time for construction 5. Reduce radiation hazards associated with the system. 17
16.CONCLUSION In the future Rectenna stands as a milestone among non conventional energy resources. This technology is more reliable than ground based solar power. In order for SPS to become a reality several things have to happen: – Cheaper launch prices – Involvement of the private sector The economic case for a solar power satellite is most compelling if it can generate power that sells at peak, rather than average, price. Several new designs for solar power satellites were considered, in an attempt to maximize the amount of power produced at peak rates This study has given researchers a remarkable insight into uncertain future of development of power from space. 17.REFERENCES: Glaser, Peter E. (22 November 1968). "Power from the Sun: Its Future" (PDF). Science Magazine 162 (3856): 857–861. a b Glaser, Peter E. (December 25, 1973). "Method And Apparatus For Converting Solar Radiation To Electrical Power". United States Patent 3,781,647. Glaser, P. E., Maynard, O. E., Mackovciak, J., and Ralph, E. L, Arthur D. Little, Inc., "Feasibility study of a satellite solar power station", NASA CR-2357, NTIS N74-17784, February 1974 18
Satellite Power System Concept Development and Evaluation Program July 1977 - August 1980. DOE/ET-0034, February 1978. 62 pages Satellite Power System Concept Development and Evaluation Program Reference System Report. DOE/ER-0023, October 1978. 322 a b c Statement of John C. Mankins U.S. House Subcommittee on Space and Aeronautics Committee on Science, Sep 7, 2000 Satellite Power System (SPS) Resource Requirements (Critical Materials, Energy, and Land). HCP/R-4024-02, October 1978. Satellite Power System (SPS) Financial/Management Scenarios. Prepared by J. Peter Vajk. HCP/R-4024-03, October 1978. 69 pages Satellite Power System (SPS) Financial/Management Scenarios. Prepared by Herbert E. Kierulff. HCP/R-4024-13, October 1978. 66 pages. Satellite Power System (SPS) Public Acceptance. HCP/R-4024-04, October 1978. 85 pages. Satellite Power System (SPS) State and Local Regulations as Applied to Satellite Power System Microwave Receiving Antenna Facilities. HCP/R-4024-05, October 1978. 92 pages. Satellite Power System (SPS) Student Participation. HCP/R-4024-06, October 1978. 97 pages. 19
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