§ 5.12.6 SPS Technical Issues
This section deals with miscellaneous technical items which I didn't want to put in the appropriate part of the chapter because it may lose some people. This section can be skipped.
The rectenna consists 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. While DC to AC conversion can occur at the rectenna, if the consumers are a long distance away, e.g., in another state, it may be more efficient to transmit by DC power lines and then to convert to AC at a local power grid.
The efficiency of the SPS is often stated in terms of "DC to DC efficiency", i.e., from the DC input at the solar cells to the DC output of the rectenna. The DC to DC efficiency is generally estimated at 63%, with losses shown in the figure below.
The satellites can have a useful lifetime of many decades. As space development takes off, they are more likely to become obsolete than to be taken out of service due to problems. Old SPSs can be either upgraded (e.g., the transmitter or the solar array) or sold off to a less developed country and moved to that country's space in geosynchronous orbit.
Micrometeors would knock out only about 1% of electricity production over 30 years, and the only remedy is replacement of the damaged cells.
Thick silica glass would protect the solar cells from most solar and cosmic radiation. If there were no silica glass or thin glass, then the solar cells would slowly degrade in performance over time due to solar wind proton and cosmic proton radiation, and we could expect up to 30% to 40% degradation over 30 years if left alone. The protons (i.e., hydrogen nuclei) fracture the solar cell crystal thereby reducing its efficiency, and become an impurity. The solution in this case would be to perform "annealing" by electron beam guns mounted on a gantry that automatically travels back and forth across the array, between glass and the cells, on occasion, e.g., after the end of solar sunspot and flare activity which happens on an 11 year cycle.
Catastrophic failure of the satellite or rectenna is almost impossible, unlike a nuclear power or coal fired power plant where a giant generator can go down due to mechanical problems. Each satellite and rectenna is modular, consisting of a large number of the same power producing part. Individual solar cell and transmitter tube components can go out and not significantly affect overall power output. Failed or failing elements can be replaced by standard robots without turning off the satellite.
The satellite will be eclipsed around March 21 and September 22, for up to 71 minutes. These will be scheduled outages, and will occur around midnite. The thermal affects on the satellite during this time would not be serious if designed properly. Perhaps flywheels at the satellite could store energy during the day and provide power to the transmitter during eclipses so that the power outage is not total and so the local grid is not dependent upon importing power from the national grid's fossil fuel power generators for these brief occasions. Flywheels and/or other energy storage mechanisms may be employed anyway to vary power output according to fluctuating demand.
Satellite array and antenna stability (vs. bending and oscillating) and control systems have been thoroughly modelled (42-45). Structural response to stationkeeping can be dampened by small thrusters, but many alternative means have been studied for large space structures. SPS designs have taken into account such phenomena as spacecraft differential charging and discharging, electrical transients, space plasma interactions, etc., though other satellites have experienced unexpected anomalies and ameliorative designs will evolve as we gain experience with large space structures.
Construction of the satellite and rectenna will entail a high degree of automation, due to the repetitiveness and simplicity of the tasks. This makes the SPS an attractive product in the fairly early years of space development. On Earth, too, the feasibility of automated rectenna construction reduces costs compared to alternative energy sources. Rectenna construction "involves the placement or assembly of a large number of identical structural elements in a very simple environment", requiring "simple, monotonous, and repetitive tasks", and "low skill level requirements". (68) Building one in a less developed country using local low-skilled labor and materials will make rectennas cheaper for developing nations.
Alternative designs for rectennas have abounded over time. Rectenna design and construction scenarios "should be subjected to careful cost effectiveness sensitivity studies ... the panel believes that the final system will probably not look much like the present reference system and urges [planners] to recognize this in all future planning. Work on novel concepts is encouraged." (74)
In addition to the rectenna, alternative designs also exist for the SPS, both for specific subsystems and for the whole satellite.
Studies have shown that there will be no problem finding rectenna sites in the USA for the current design of rectenna, assuming the low power density of the beam (mainly for birds) and hence large rectennas, and lower frequency (longer wavelength) beam.
Europe and Japan have less space, and offshore antennas look more attractive. More than 80% of the major electricity consumption centers of Europe are within 300 km (200 miles) of a coastline. Large parts of the relevant offshore regions are relatively shallow, with depths between 10 and 50 meters. European industry has extensive experience in more difficult environments for construction of offshore structures, from deep North Sea oil and natural gas exploration and recovery. There are already underwater DC power lines from mainland Europe to England and Scandinavia for import and export of electricity, so that a rectenna in one country can export power to other countries. With cross-border pollution issues as regards coal power and acid rain, rectennas could be part of a larger cross-border solution.
Daily electricity consumption rate varies by time of day, peaking in the industrial workday. Coal provides most of the constant "baseload" electricity, and hydroelectric and nuclear energy add to baseload. Peak power is usually provided by natural gas and oil fired generating stations. SPS provides baseload electricity, so there would be a need to level out the load, e.g., running some major power consumption processes at night, including synthetic fuels plants and electric vehicle battery chargers. Storage of electricity is probably easier at the SPS, e.g., spinning flywheels in the zero gravity of space, which can be tapped at peak times during the day to supplement the constant output of the solar cell array. Excess SPS power can also be beamed to electric powered interorbital vehicles (ion drive) and space industry at nite.
For brevity, I have stuck with the most conservative SPS designs based on old technologies. The scenario presented in this book is probably close to a worst case scenario, with much room for improvements.
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