Talk:Solar power satellite

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[edit] misc early comments

Any reason this article isn't at solar power satellite, which to me seems like the appropriate name according to the naming conventions? --Robert Merkel

Newcomer ignorance. Mine. Rectified a bit since. user:mirwin

By the way, the article is overly optimistic about SPS's, and needs a little neutral point of view put into it by listing some of the critical views on the topic. --Robert Merkel

Perhaps. Some of what you seek is either at the bottom or solicited there. I put a reference to it at the top to encourage interested adherents of more pessimistic or alternate worldviews to seek them their.

Another Wikipedian started to wikify it but apparently got tired of it quickly. user:mirwin

Why does the cost of building a space elevator matter for the launch cost of solar satellites? Such an elevator would not be used only to launch this particular type of satellite, so the construction costs would be spread out over far more than this. Shouldn't only the estimated launch costs of $1-10/kg matter here?

Wouldn't a satellite which directly focusses sunlight onto a solar thermal plant on earth be more efficient? The beam would be narrower than the microwave beam and the loss in thermal->electric conversion is less than that of solar cells. Clouds between the satellite and the ground station would quickly evaporate due to the intensive illumination. 193.171.121.30 02:12, 6 May 2005 (UTC)

It's not possible to use mirrors or lenses to focus the light from the sun more narrowly than the light from the sun already is. If you've ever fiddled around with a magnifying glass you should kinda know how this goes. You can make the image brighter though by collecting more light with a bigger lense. The microwave transmission technique actually gives a narrower beam, inspite of the wavelength being considerably longer. Sunlight-pumped lasers could also be used in principle; but in practice it's a weapon.WolfKeeper 02:19, 2005 May 6 (UTC)
Sorry, I don't quite understand you - you can focus sunlight from a larger mirror area onto a small spot on earth's surface where it can be used e.g. to evaporate and heat steam for a turbine which drives a generator to produce electric energy. A narrow beam is not necessary but a small spot at the earth's surface - and this spot can be made smaller at lower wavelengths with the same arperture size. I actually didn't mean narrow beam but small spot at the ground station. 193.171.121.30 12:45, 6 May 2005 (UTC)
Basically, what I'm saying is that it doesn't work. There are optical restrictions inherent in mirrors and/or lenses that prevent this from working. The angle that the light hitting the ground makes is the same as the angle that the sun makes at the satellite. This restricts how small the image it projects onto the ground can be. By making the mirror larger, you can make the image brighter, but not smaller. Because the satellite is so far away from the earth, the focused image is going to be very large. Something like that anyway... That's why the SPS was invented to circumvent this restriction. WolfKeeper
Thank you for the explanation - I should have given it a second thought before. 193.171.121.30 18:22, 6 May 2005 (UTC)

Economic feasibility - don't we need to use the cost of power at the power stations, rather than the cost to consumers? (since the SPS power is still in one place and needs to be distributed) Ojw 12:54, 22 Jun 2005 (UTC)

Please explain the distinction that you're trying to make. Afterall, ignoring sunk costs, the SPS makes power for free. The real costs are in building it, managing it, and delivering the power to the grid. --Flatline 16:54, 2005 Jun 22 (UTC)
Distinction is: we are assuming that the SPS power station can sell electricity for the same cost that you pay at home. The calculation uses the $/kWh cost taken from the back of somebody's electricity bill, and not for the price that the power stations themselves are selling at.
If the two prices were equal, then there wouldn't be any money left over to buy power lines, transformers, fat-cat salaies, electricity meter-readers and everything else that the distribution company needs to get the electricity from an SPS antenna in the middle of a desert, to the consumers scattered around the nearest city.
This is different to (for example) solar-power roof tiles, where the power is generated at the consumer's home, so you can use the retail price of electricity to calculate how much it's saving you.
Ojw 18:28, 22 Jun 2005 (UTC)

[edit] Where did these values come from!

This sentance is likely erronous: To collect and convert that much power the satellite needs between 50 and 150 square kilometers of collector area thus leading to huge satellites.

Assuming a triple junction solar cell using mirror focusing a average efficiency of 25% is reasonable and within technological limits, with a solar constant of 1370 W/m2 the solar cells would produce 342.5W/m2, assuming a antenna and power conversions efficiency of 80% and a ground rectenna efficiency of 85% that would mean a total power production of 232.9W/m2, lets add in some more inefficiency leeway and round that off to 200W/m2. For a 4GW power station it would need 20,000,000m2 of solar panels or 20km2 or a square area of ~4.5km by ~4.5 km. Using cheaper single junction silicon polymer solar cell and focusing mirrors a max efficiency of 14% and total power production of 100W/m2 we would need 40km2 or 6.3km by 6.3km. --BerserkerBen 16:59, 24 July 2005 (UTC)

I haven't done the maths or anything like that, but isn't 40% or so the new 'state of the art' efficiency record? (http://www.energy.gov/news/4503.htm) The article should be updated if so. 203.166.229.35 12:40, 6 December 2006 (UTC)

[edit] Augment photovoltaics with mirrors?

There was recently some buzz about an idea called the Sunflower. This exploits the fact that mirrors and gears are cheaper than photovoltaics. A motor drives a 5x5 array of mirrors to track the sun, so the solar panel gets hit with 25 times as much sunlight as it would normally get. Presumably the same principle could apply to a solar power satellite. Use a big parabolic mirror made from metallized mylar, or whatever is the most practical and cost-effective material for space, and put the solar panel at the focus.

Perhaps solar wind could be used to keep it lined up with the sun, like the tail fins on a windmill. But then the original SPS idea must have involved some kind of active steering to keep the beam on the rectenna. Presumably the same mechanism could be used for aligning the mirror. WillWare 03:33, 27 August 2005 (UTC)

Solar wind is much weaker than photonic pressure; by a factor of 10 or more IRC. Still, you're right, it's easy to use photonic pressure for this. If the panels are laid out in a v-shape, the panel should self orient to point towards the Sun. There's no damping in space, so you'd also need some momentum wheels or something to damp out vibrations. The other trick is that the shape needs to be such that tidal forces don't make it tend to line up with the Earth, moon or Sun. I think if the shape is carefully chosen, it should be very stable. WolfKeeper

[edit] Other benefits of SPS

"Some research claims..." - Either give a reference or take the statement out. JonathanHart 11:57, 10 July 2006 (UTC)

Does anyone know how long solar cells can last in space? They have to endure hard radiation, particles from the sun and micrometeorites. By now there should be much practical experience from existing satellites. 84.160.212.221 19:31, 13 September 2005 (UTC)

[edit] safety

Someone writes that the beam is relatively safe because if an aircraft were to fly through the beam, the passengers would be shielded by the effective Faraday cage. while that's true, I doubt the plane's electronics would be safe, as I'm pretty sure microwave beams were one method attempted with the Star Wars project. Isn't it enough that the FAA could regulate no-fly zones, as they do now? -jdw (user:71.196.216.187 on 1 Oct 2006)

If I recall correctly this question was part of the engineering evaluation done (mostly by NASA) in the decades after SPS was first proposed. The strength of the microwave broadcast carrying power to the Earth's surface was determined to be insufficient to affect birds (who do not have a metal Faraday cage surrounding them as do airline passengers). A sufficiently low power density of that beam implies a relatively large collecting antenna, and a workable design (cost effectiveness is a prime criterion in these designs) was found. So problems caused byt he beam are likely to be minimal unless the electronics involved are tuned to amplify the broadast signal.
As for the question about routing aircraft away from the beams is not necessary. The FAA currently does must this for flights over or near sensitive sites, such as military bases or high value terrorist targets such as public buildings, but each such arrangement further complicates airmans' references. They are already more complex than is really justfied. In short, it could be done but is not likdly to be a satisfacry approach. ww 06:58, 6 October 2006 (UTC)
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