generally one-thirtieth the intensity of sunlight. Furthermore, the peak power is over the collecting aperture and tapers to much lower levels at the exclusion fences. Higher frequency systems that could have much more intense beams such as millimeter power beaming facilities as shown in Fig. 1 must be located in high and dry sites chat arc generally not readily assessable to the public. This is because the propagation loss is too much at sea level through the Earth's atmosphere. Particularly onerous is the fact that we cannot see RF power beams. The capability to do so would go a long way toward assuring the public that they are not being irradiated. Therefore in the design engineering of WPT systems we must have the beam position determining equipment and the power density monitoring equipment made visible to the affected populace, preferably with indicators that they can interpret and understand. Glowing lamps or other visible indicators around the periphery of power transmitters and receivers are a possibility. A small fraction of the beam power may have to be allocated for this function. “How am I doing?” spot monitors around the systems may be used to assure safety. Signs extolling "Please call toll free I -800-WPT-BEAM to report if this indicator is above the green zone” might be utilized. Restricted airspace may be obtained for system sites so that intentional trespass by aircraft and aerostats is prohibited. However, additional radar surveillance or scattered sidelobe detecting systems should be installed and be continuously operating to assure that no harmful radiation interferes with the avionics of even an unplanned emergency overflight. WPT systems would not like to be socked with a similar Food and Drug Administration letter from Lillian J. Gill. Director of Compliance, Center for Devices and Radiological Health, dated Dec. ll. 1995. which required a cessation of operations for outdoor laser displays in all of Clark County. NV within 20 miles of any airport (Bigness. 1995). If restricted airspace cannot be obtained at the desired power beam site and the beam may traverse a controlled airspace then the system design must be such as to accommodate the beaming interruptions occasioned by having to dim, divert, or douse the power beam. This obviously has ramifications for those systems designed for uninterrupted electric power delivery, meaning that energy storage must be provided for the beam outages in that case. THE SPECTRUM ISSUE The attenuation due to scattering of radio waves propagating through the atmosphere is related to the wavelength of radiation compared to the size of meteors in the atmosphere such as dust particles, raindrops, cloud droplets, ice crystals, sleet, hail stones, and snow. The larger the wavelength compared to the meteors, the lower the propagation loss. Therefore if one wants to design a WPT system to be nearly independent of the weather then low frequencies below 3 GHz (Brown. 1981) are a must. However, the long wavelengths mean that in order to focus a tight beam a very large diameter aperture is needed. The beam spot size (-3 dB width) is approximately the product of the range and wavelength divided by the projected aperture diameter. Large aperture area requirements translate into large capital costs. Tight beams can be obtained with smaller diameter apertures if short wavelengths arc employed. However, the atmosphere has severe absorption lines at the high frequencies. There are windows of reasonable propagation loss in between the two regions. A system cost-performance trade is evident between large-diameter aperture all-weather performance and acceptable propagation loss small-diameter apertures. This unencumbered trade would be readily done but for the fact that the RF spectrum is already occupied by many diverse users with many different services (Radio location. Broadcasting. Inter-satellite. Astronomy; etc.). There is no beamed power service. The closest service is the Industrial. Scientific and Medical Bands (ISM) which do not currently prohibit wireless power transmission. With regard to out-of-band interference, the reradiation of harmonics of the fundamental beamed power frequency may be inhibited by engineering good low-pass and band-stop fillers into the converter arrays. Ideally one requires harmonic energy to be generated in the conversion process and in order to yield high conversion efficiency it should be used and not lost. Also, the possibility of lower RF frequencies mixing in the nonlinear converters can be prohibited by the use of halfwave slot aperture coupling of the fundamental into and out of the convener. The waveguide beyond cutoff function is a very effective filter. Mixing of higher than the fundamental frequencies and subsequent reradiation must be stopped with narrow-band pass filters. Cost, complexity, breakdown voltage margin, insertion loss, and mass are converter system engineering trades for achieving electromagnetic compatibility. The spectrum allocation issue remains and will probably only be resolved in the future when there is a compelling economic rationale fur international cooperation. The International Telecommunications Union (ITU) is an agency of the United Nations that recommends radio frequency allocations world wide. Allocations within the United States are determined by the FCC for civilian commercial applications and the National Telecommunications and Information Administration (NTIA) for government agencies in the US. Costly spectrum auctions may be a pan of the marketplace answer to this issue. THE AFFORDABILITY ISSUE It is a hallmark of electric power that generally the more power a facility can handle, the cheaper the cost of selling a kWh of energy. Microwave systems are in the electric category. Couple this with the laws of diffraction regarding beam spot size and WPT systems tend to want to be large. Large systems cost a lot Thus, affordability is an issue. One of the outcomes of the DOE-NASA study of the Peter Glaser concept was that the entire solar power satellite (SPS) had to be constructed before significant electric power could be delivered to the ground. This meant that considerable capital and the interest on it had to be committed before any revenue arose from delivered power. Additionally, the SPS could only deliver power from space to ground. The same equipment al the same locations could not be used in the future to also deliver power from the Earth to geosynchronous orbit loads. Existing ground-based electric power transmission systems can deliver power in either direction, especially the dc-ac conveners. This fealure provides the system- level flexibility to produce revenue for power delivery in either direction as source and load requirements change over lime with industrial buildup on the ground and in space.
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