Astro Fisika: Solar Power Satellite III

Wednesday 12 June 2013

Solar Power Satellite III

Satelit tenaga surya adalah satelit yang diusulkan dibuat di orbit Bumi tinggi yang menggunakan transmisi tenaga gelombang mikro untuk menyorotkan tenaga surya kepada antena sangat besar di Bumi yang dpaat digunakan untuk menggantikan sumber tenaga konvensional.

Working lifetime:

The lifetime of a PV based SPS is limited mainly by the ionizing radiation from the radiation belts and the Sun. Without a protection method, this is likely to cause the cells to degrade by about a percent or two per year. Deterioration is likely to be more rapid during periods of high exposure to energetic protons from solar particle outburst events. If some practical protection can be designed, this also might be reducible (eg, for a CPV station, radiation and particle shields for the PV cells out of the energy path from the mirrors, of course).

Lifetimes for SD based SPS designs will be similarly limited, though largely for structural or mechanical considerations, such as micrometeorite impact, metal fatigue of turbine blades, wear of sliding surfaces (although this might be avoidable by hydrostatic bearings or magnetic bearings), degradation or loss of lubricants and working fluids in vacuum, from loss of structural integrity leading to impaired optical focus amongst components, and from temperature variation extremes.

As well, most mirror surfaces will degrade from both radiation and particle impact, but such mirrors can be designed simply (and so to be light and cheap), and replacement may be practical.

In either case, another advantage of the SPS design is that waste heat developed at collection points is re-radiated back into space, instead of warming the adjacent local biosphere as with conventional sources, though some care will likely be required to provide for the radiation of this waste heat.

Thus thermal efficiency will not be in itself an important design parameter except insofar as it affects the power/weight ratio via operational efficiency and hence pushes up launch costs. (For example SD may require larger waste heat radiators when operating at a lower efficiency).

Earth based power handling systems must always be carefully designed, for both economic and purely engineering reasons, with operational thermal efficiency in mind.

One useful aspect of the SPS approach is that, at the end of life, the material does not need to be launched a second time, at least in principle. In theory, it would be possible to recycle much of the satellite 'on-site', potentially at a significantly lower cost than launching an SPS as new.

This might allow a very expensive launch cost to be paid for over multiple satellite lifetimes, but does require an in orbit re-processing facility which doesn't currently exist.

Energy Payback:

Clearly, for an SPS system (including manufacture, launch and deployment) to provide net power it must repay the energy needed to construct it.

Solar satellites can pay back the lift energy in a remarkably short time. It takes 14.75 kWh/kg for a 100% efficiency system to lift a kg from the surface of the earth to GEO; no such launch system exists and so energy costs are always higher. If the satellite generated a kW with 2kg of mass, the payback time would be 29.5 hours. Assuming a much less efficient (and more realistic) 3% efficient rockets, the energy payback time is only extended to about 6 weeks for such an SPS.

For current silicon PV panels, production energy requirements are relatively high, and typically three-four years of deployment in a terrestrial environment is needed to recover this energy.

With SPS, net energy received on the ground is higher (more or less necessarily so, if the system to be worth deploying), so this energy payback period would be reduced to about a year. Thermal systems, being made of conventional materials, are more similar to conventional power stations and are likely to be less energy intensive during manufacture. They would be expected to give quicker energy break even, depending on construction technology. The relative merits of PV vs SD is still an open question.

Clearly, for a system (including manufacture, launch and deployment) to provide net power it must repay the energy needed to construct it. For current silicon PV panels this is relatively high. With an SPS, the net energy received on the ground is higher so this energy payback period would be somewhat reduced; however an SD based SPS, being made of conventional materials, are more similar to conventional powerstations and are likely to be less energy intensive during production and would be expected to give a quicker energy break even, depending on construction technology and other variations.

Wireless power transmission to the Earth:

Wireless power transmission was early proposed to transfer energy from collection to the Earth's surface. The power could be transmitted as either microwave or laser radiation at a variety of frequencies depending on system design. Whatever choice is made, the transmitting radiation would have to be non-ionizing to avoid potential disturbances either ecologically or biologically if it is to reach the Earth's surface. This established an upper bound for the frequency used, as energy per photon, and so the ability to cause ionization, increases with frequency. Ionization of biological materials doesn't begin until ultraviolet or higher frequencies so most radio frequencies will be acceptable for this.

William C. Brown demonstrated in 1964 (on air Walter Cronkite's CBS News program), a microwave-powered model helicopter that received all the power it needed for flight from a microwave beam. Between 1969 and 1975, Bill Brown was technical director of a JPL Raytheon program that beamed 30 kW of power over a distance of 1 mile at 84% efficiency.

To minimize the sizes of the antennas used, the wavelength should be small (and frequency correspondingly high) since antenna efficiency increases as antenna size increases relative to the wavelength used. More precisely, both for the transmitting and receiving antennas, the angular beam width is inversely proportional to the aperture of the antenna, measured in units of the transmission wavelength. The highest frequencies that can be used are limited by atmospheric absorption (chiefly water vapor and CO2) at higher microwave frequencies.

For these reasons, 2.45 GHz has been proposed as being a reasonable compromise. However, that frequency results in large antenna sizes at the GEO distance. A loitering stratospheric airship has been proposed to receive higher frequencies (or even laser beams), converting them to something like 2.45 GHz for retransmission to the ground. This proposal has not been as carefully evaluated for engineering plausibility as have other aspects of SPS design; it will likely present problems for continuous coverage.

Spacecraft Sizing:

The size of an SPS will be dominated by two factors. The size of the collecting apparatus (eg, panels, mirrors, etc) and the size of the transmitting antenna which in part depends on the distance to the receiving antenna. The distance from Earth to geostationary orbit (22,300 miles, 35,700 km), the chosen wavelength of the microwaves, and the laws of physics, specifically the Rayleigh Criterion or Diffraction limit, used in standard RF (Radio Frequency) antenna design will all be factors.

For best efficiency, the satellite antenna should be circular and for the probable microwave wavelength, about 1 kilometers in diameter or larger; the ground antenna (rectenna) should be elliptical, 10km wide, and a length that makes the rectenna appear circular from GSO. (Typically, 14km at some North American latitudes.) Smaller antennas would result in increased losses to diffraction/sidelobes. For the desired (23mW/cm²) microwave intensity these antennas could transfer between 5 and 10 gigawatts of power.

To be most cost effective, the system should operate at maximum capacity. And, to collect and convert that much power, the satellite would require between 50 and 100 square kilometers of collector area (if readily available ~14% efficient monocrystalline silicon solar cells were deployed). State of the art (currently, quite expensive, triple junction gallium arsenide) solar cells with a maximum efficiency of 40.7% could reduce the necessary collector area by two thirds, but would not necessarily give overall lower costs for various reasons.

For instance, these very recently demonstrated variants may prove to have unacceptably short lifetimes. In either cases, the SPS's structure would be essentially kilometers across, making it larger than most man-made structures here on Earth. While almost certainly not beyond current engineering capabilities, building structures of this size in orbit has not yet been attempted.

LEO/MEO instead of GEO:

A collection of LEO (Low Earth Orbit) space power stations has been proposed as a precursor to GEO (Geostationary Orbit) space power beaming system(s). There would be advantages, such as much shorter energy transmission path lengths allowing smaller antenna sizes, lower cost to orbit, energy delivery to much of the Earth's surface (assuming appropriate antennas are available), etc. And disadvantages, including constantly changing antenna geometries, increased debris collision difficulties, many more power stations to provide continuous power delivery at any particular point on the Earth's surface, etc.

It might be possible to deploy LEO systems sooner than GEO because the antenna development would take less time, but it would certainly take longer to prepare and launch the number of required satellites. Ultimately, because full engineering feasibility studies have not been conducted, it is not known whether this approach would be an improvement over a GEO installation.

Earth Based Infrastructure:

The Earth-based receiver antenna (or rectenna) is a critical part of the original SPS concept. It would probably consist of many short dipole antennas, connected via diodes. Microwaves broadcast from the SPS will be received in the dipoles with about 85% efficiency. With a conventional microwave antenna, the reception efficiency is still better, but the cost and complexity is also considerably greater, almost certainly prohibitively so. Rectennas would be multiple kilometers across. Crops and farm animals may be raised underneath a rectenna, as the thin wires used for support and for the dipoles will only slightly reduce sunlight, so such a rectenna would not be as expensive in terms of land use as might be supposed.