Solar Power Satellites

The concept of Solar Power Satellites (SPS) was invented in 1968 by Dr.Peter Glaser of Arthur D. Little Corporation.

They would be solar arrays in Geosynchronous orbit around Earth, beaming power to the ground via microwaves. The late Dr.Gerard K. O'Neill determined that these could most cheaply be built from lunar materials. Some have proposed beaming down power via lasers instead of microwaves.

The receiving antenna (rectenna) is quite large, several square miles. The conversion efficiency of a rectenna is about 95%, compared to 20% or less for photovotaic cells. So SPS rectennas would require a lot less land area than conventional solar cells.

Radio Frequency Issues

The inverse quare law does not apply to a focussed beam.

The equation of beam spreading is a function of the transmitting antenna aperture versus the frequency.

The bigger the aperture, the tighter the beam.

Beam spreading does not cause significant power loss. The size of the antenna is set to precisely match the dimensions of the beam as it intersects the Earth's surface.

A little bit of energy is lost due to sidelobes which are caused by diffraction. It is typically not worth it to make the receiving antenna large enough to catch all the sidelobes.

As for problems of radio interference: that has little to do with the size of the rectenna, but is certainly an issue which needs to be addressed in the system design. The problem of radio interference is solvable.

SPSes normally would be about 2.4 GHz. This would not affect Ku band at all. There would have to be some review of the effect of harmonics. But the SPS sends a narrow beam, and the Ku band downlink receiving station would have to be within a few kilometres of the rectenna to even notice the harmonics within the sidelobes.

Interference between comm-sats of the same frequencies is a much bigger problem than interference between them and the SPS.

The biggest problem is that the best frequencies for SPS have now been allocated to cell phone services.

Maintenance

As for maintenance: very little maintenance is required for a rectenna system, it is essentially passive with no moving parts.

Economics

Deceptively cheap (subsidized) energy can continue to be readily available using nuclear power and fossil fuels for a couple of centuries at least. Solar power Satellites (SPS) will not compete head to head on price alone in the foreseeable future.

On the other hand....

If we assume (however hypothetically) that the world decides that "Fossil Fuels Are Bad", and mandates Zero emission of greenhouse gases...then what forms of power will be used ? Does this mean a widespread increase in the use of nuclear power ? Is this a good thing or a bad thing ?

Is SPS better than nuclear power ?

The collateral damage caused by fossil fuels and nuclear fission far outweighs their deceptively low price. The real price for these fuels is very high when you consider these factors:

Next steps

Lunar L-1 is the best place to put an initial solar power satellite demonstrator. We can place the rectenna on the Moon and the solar PV arrays at L-1. The distance from L-1 to the Moon (50,000 km) is the similar to the distance from GEO to Earth (40,000 km), so it will validate the engineering design perfectly, and prove that useful power can be beamed over that distance. This is also the cheapest way to deliver large scale power to the lunar surface, as rectennas are light weight and PV cells area heavy. Soft landing hardware on the Moon from Earth is very expensive.

Alternatives to Solar power Satellites

Fossil fuel (coal, oil, gas):

- military cost of securing sources of supply and supply channels, with associated geopolitical problems and trouble with the local insurgents 
- huge balance of trade deficits from importing them 
- cost of the war on terror (unfriendly regimes and terrorists funded by oil revenues) 
- global warming exacerbated by greenhouse gases 
- air pollution - reduced life expectancy / healthcare costs 
- water pollution (e.g. Mercury from coal) - reduced life expectancy / healthcare costs 

SPS will reduce heat pollution, not increase it.

Assuming the world is supplied by 200 SPS at 5 GW each. Each SPS loses 1 % into the atmosphere, a total of 10 GW of atmospheric heating caused by all the world's SPSes.

10 / 1.2 x 10E14 = 8 x 10e8 GW

So the entire losses of all the world's SPSes would be 8 parts in a hundred million.

The present power stations of the world are injecting about thirty times as much into the atmosphere right now even as we type. A total of 2000 GW.

And even that is a drop in the bucket compared to global warming. According to the NASA GSFC website (in 2002), the imbalance due to greenhouses gases is 2.45 W/m2, which the Earth is absorbing and not radiating to space. Of this, 1.56 W/m2 is due to CO2, 0.47 to methane and 0.14 to N2O.

This equates to an energy absorption rate of 12 million GW.

SPS will reduce the problem of global warming, because it will replace the 12 million GW due to greenhouse gases, and the 2000 GW due to nuclear and fossil fuels, and replace it with a more tolerable 10 GW (worst case) of direct atmospheric absorption and 100 GW of waste heat at ground level.

Nuclear fission

- military cost of securing supply chains against theft 
- military cost of securing waste sites against theft 
- cleanup cost of decommissioning power stations 
- cost of meltdown - reduced life expectancy / healthcare costs 
- cost of waste leakage - reduced life expectancy / healthcare costs 
- social cost of draconian global security regimes (big brother)

Terrestrial Solar power

Lack of 24 hour coverage (ignoring weather) means that terrestrial solar power systems need some means of supplying consumers during the night time. Night time load is usually less than daytime load, but it is not zero, far from it. In winter time especially, the working day extends substantially into dark time.

Many industries and transportation systems need to operate on a 24 by 7 basis. (If you do not know what that means then your have not worked in private industry recently).

This means that one of two systems are needed, either

a) a global power grid to pass power from the daylit side to the night side, or

b) power storage systems.

Both solutions exceed the cost of the solar cells themselves and are conveniently ignored by most proponents of terrestrial solar power.

Solution a) also suffers from political problems, similar to the international wrangling going on about the Tengiz oil field pipeline.

Pipelines and power lines are political hot potatoes, nobody wants a hostile neighbor to have the ability to cut off their power or their oil.

Another issue: Ironically, terrestrial solar power has a more severe impact on terrestrial ecosystems and land usage than SPS rectennas. Permanent shadowing of the soil from a solar panel kills the local flora and results in a dustbowl. But agriculture can continue unabated beneath an SPS rectenna and soil erosion is thus mitigated. SPS rectennas can be sited on prime agricultural land, terrestrial solar panels cannot.

Other Renewables (wind, tidal, hydro)

Other Renewables (e.g wind, tidal, hydro) only have the capacity to supply a tiny fraction of the global demand for energy. The limitation is geography, there simply are very few sites in the world where generating systems of these types can be built.

Ocean based windpower is one possibility, but that is dominated by the high cost of long distance power transmission, in which case SPS would be highly competititve.

Nuclear Fusion

For the past few decades humanity has been "ten years " away from achieving nuclear fusion breakthrough. At this time there is no credible timeline for when nuclear fusion power plants will come on line. So until then, nuclear fusion is not a credible competitor to solar power satellites.

Conclusion

If society ever reaches the conclusion that fossil fuels and nuclear fuels are undesirable for the above reasons, then there is no remaining alternative to SPS for clean inexhaustible power on a global scale.

But as long as society is willing to continue subsidizing fossil fuels and nuclear systems, then SPS is not an option.

See Also

Solar Power

NASA TM-2004-212743 - "Reinventing the Solar Power Satellite" and "Peak Power Markets for Satellite Solar Power" from the Houston IAF Congress (International Astronautical Federation). Author: Dr. Geoffrey A. Landis