Solar Power Satellite

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Wireless Power Transmission for Solar Power Satellite (SPS) (Second Draft by N. Shinohara) 1. Theoretical Background It is known that electromagnetic energy also associated with the propagation of the electromagnetic waves. We can use theoretically all electromagnetic waves for a wireless power transmission (WPT). The difference between the WPT and communication systems is only efficiency. The Maxwell’s Equations indicate that the electromagnetic field and its power diffuse to all directions. Although we transmit the energy in the communication system, the transmitted energy is diffused to all directions. Although the received power is enough for a transmission of information, the efficiency from the transmitter to receiver is quiet low. Therefore, we do not call it the WPT system. Typical WPT is a point-to-point power transmission. For the WPT, we had better concentrate power to receiver. It was proved that the power transmission efficiency can approach close to 100%. We can more concentrate the transmitted microwave power to the receiver aperture areas with taper method of the transmitting antenna power distribution. Famous power tapers of the transmitting antenna are Gaussian taper, Taylor distribution, and Chebychev distribution. These taper of the transmitting antenna is commonly used for suppression of sidelobes. It corresponds to increase the power transmission efficiency. Concerning the power transmission efficiency of the WPT, there are some good optical approaches in Russia[5][6]. Future suitable and largest application of the WPT via microwave is a Space Solar Power Satellite (SPS). The SPS is a gigantic satellite designed as an electric power plant orbiting in the Geostationary Earth Orbit (GEO). It consists of mainly three segments; solar energy collector to convert the solar energy into DC (direct current) electricity, DC-to-microwave converter, and large antenna array to beam down the microwave power to the ground. The first solar collector can be either photovoltaic cells or solar thermal turbine. The second DC-to-microwave converter of the SPS can be either microwave tube system and/or semiconductor system. It may be their combination. The third segment is a gigantic antenna array. Table 1.1 shows some typical parameters of the transmitting antenna of the SPS. An amplitude taper on the transmitting antenna is adopted in order to increase the beam collection efficiency and to decrease sidelobe level in almost all SPS design. A typical amplitude taper is called 10 dB Gaussian in which the power density in the center of the transmitting antenna is ten times larger than that on the edge of the transmitting antenna. The SPS is expected to realize around 2030. Before the realization of the SPS, we can consider the other application of the WPT. In resent years, mobile devices advance quickly and require decreasing power consumption. It means that we can use the diffused weak microwave power as a power source of the mobile devices with low power consumption such as RF-ID. The RF-ID is a


radio IC-tug with wireless power transmission and wireless information. This is a new WPT application like broadcasting. Table 1.1 Typical parameters of the transmitting antenna of the SPS [7] Model Frequency Diameter of transmitting antenna Amplitude taper Output power (beamed to earth) Maximum power density at center Minimum power density at edge Antenna spacing Power per one antenna (Number of elements) Rectenna Diameter Maximum Power Density Collection Efficiency Old JAXA model 5.8 GHz 2.6 kmφ 10 dB Gaussian 1.3 GW 63 mW/ cm 2

JAXA1 model 5.8 GHz 1 kmφ 10 dB Gaussian 1.3 GW 420 mW/cm 42 mW/ cm 2

JAXA2 Model 5.8 GHz 1.93 kmφ 10 dB Gaussian 1.3 GW 114 mW/cm2 11.4 mW/cm2 0.75 λ Max. 1.7 W (1,950 million) 2.45 kmφ 100 mW/cm2 87 %

NASA/DOE model 2.45 GHz 1 kmφ 10 dB Gaussian 6.72 GW 2.2 W/ cm 2

6.3 mW/ cm



0.22 W/ cm


0.75 λ Max. 0.95 W (3.54 billion) 2.0 kmφ 180 mW/cm2 96.5 %

0.75 λ Max....
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