Solar power satellite

[neil]

A possibly big customer for space elevators is SPS = solar power satellite. These can beam power to earth cities from space. Japan is planning a demonstration project. USA did a $25 million dollar study about 1985. In theory, infrared lasers can beam the energy to earth, but present lasers are low efficiency. The sunlight could be sent down as a beam of sunlight, but apparently this is also low efficiency unless done very large scale such as a 100 kilometer mirror, and a solar energy receiving array perhaps 20 kilometers in diameter to receive many gigawatts of electricity. Most locations can not use more than a few gigawatts, and high voltage power lines lose more than half the energy when they are 400 kilometers long. Room temperature super conductors would allow electricity to be sent with low loss for 1000 kilometers or more, which might make the beam of sunlight the best option.
Since the superconductor power lines may not happen, most people think magnetrons, which make microwaves at about 75% efficiency. I like the klysitron, about 50% effeciency as the beam can carry broadband data which can be received with a small antenna over about 1/2 of planet earth and about 1/2 of the solar system. This communications could prove more profitable than the electricity we beam down. Usually we think GEO staionary orbit = 36,000 kilometers altitude, but solar synchronous semipolar orbit, would serve perhaps a dozen rectenas around the world during their peak demand period when the wholesale price of electricity doubles. My guess is 15,000 kilometers altitude for sun sychronous semi polar orbit. This is a small disadvantage for the space elevator, but the minimum dimentions of the transmitting antenna (or sun beam mirror)  and the rectenna = are reduced by about 1/2. Please add details, suggest alternatives and make corrections.   Neil

[windemut]

The sunlight could be sent down as a beam of sunlight, but apparently this is also low efficiency unless done very large scale such as a 100 kilometer mirror, and a solar energy receiving array perhaps 20 kilometers in diameter to receive many gigawatts of electricity.

You can not beam sunlight, because of its bad collimation. Such a beam alway diverges at least 1%, meaning with a flat mirror of 360 km you'd get a peak of one extra sun intensity, with a spot size of 720 km. The one sun would be in the center of the spot, it would drop off towards the edge from there. With a perfectly parabolic 360 km mirror of 36,000 km focal length, you would get a circular spot 360 km wide, with one sun intensity throughout. This would be an image of the sun, and you'd be able to make out sunspots and coronary mass ejections, most likely. People living in the area would probably not appreciate the extra sun, unless you focussed it on Northern Canada, Greenland, or Great Britain.

Andreas

[A_M_Swallow]

My guess is 15,000 kilometers altitude for sun sychronous semi polar orbit.

To go into orbit at 15,000 km with a velocity of 4.32 km/s the satellite needs dropping at 31,450.513 km and a circularizing Delta_V of -0.563 km/s applied.

[neil]

Hi Andreas: Even if 90% of the energy misses the 20 kilometer solar receiving site, the semi-parabolic mirror may be less costly than an equivelent SPS of other types. If we assume the 100 kilometer mirror produces a 10 trillion 10E13 watt beam = 10,000 gigawatts on the average; what percentage is aimed at the 20 kilometer solar receiving site? Assume 18,000 kilometers distance from mirror to solar receiving site. There wll ibe additional atmospheric scattering. Montana, USA on a steep south facing site might be a good location for a solar receiving site. The scattered sunlight might be appreciated at higher elevations, especially in winter.  Neil

[neil]

Hi AM SWallow: 0.563 kilometers per second is not a large penelty to circularize at 15,000 kilometers altitude, but the multiple rectennas will be very costly for receiving energy perhaps 10% of a typical day, unless the public can be convinced that 10 kilowatts per square meter, wont fry their hide. The international maximum for microwave oven leakage is 1/10 watt per square centimeter = one kilowatt per square meter = one gigawatt per square kilometer, and that assumes there are no hot spots in the illuminated area. We can turnoff the beam less than one second after it pans off the rectenna, but that will not satisfy some people, since the turnoff freature can be disabled or malfunction. Eventually a beam will accidentally illuminate a city, unless it is turned off at first signs of trouble. My guess is serious injuries will rare at 10kw per square meter, unless the hot spot happens to track the person and the person is unable to find shelter and/or the person was close to heat exhaustion just before the hot spot got them. Neil

[A_M_Swallow]

In the case of a microwave system there are multiple ways of turning it off.  If you make them independent you can produce a highly reliable system.  Require all to report active before the beam starts.

Possible controls.

• Gyroscopic sensor system to ensure that the beam is pointing in the correct direction.
• Radio activation message and deactivation message.
• Continuous signal (radio or laser) from Earth receiver to transmitter.
• On board physical ON/Off switch.

If this was an Earth based system I would also suggest a physical safety key similar to an ignition key but since this is in space and there is no one to turn it such a switch is a waste of time.

[neil]

Hi Andreas: Most light sources have poor collimation, so we use collimators for overhead projectors and other image projectors. Perhaps CNT will make a 10 kilometer collimator practical.
Almost anything seems better than a million times a million = 10E12 photovoltaic cells, in a series-parallel array, powering 100,000 magnetrons in series parallel. At 1/10th watt per photovoltaic cell, that's 25 gigawatts at 30% efficiency and the overall efficiency may be less than 10% delivered to customers of the power grid of Earth. A single very fast hydrogen nucleus can short or open a photocell. Both shorts and opens cause disproportionate power loss in a series parallel array.  Neil

[neil]

Hi AM: I agree multiple turn off increases certainty and all those you suggested are workable. We can also turn the the solar array and or collimator, until there is no light on the photovoltaic cells (or the light beam misses Earth).
As a last resort, explosives on the power line can be detonated to open the circuit. That may be the only fast reliable option for 10,000 amps at ten million volts, dc = 100 gigawatts which should make a spectacular arc even in the vacuum of space.   Neil

[publiusr]

Gene Meyers at http://www.spaceislandgroup.com is talking to some politicos and they seem receptive

[neil]

Gene Myers of space Island group is regarded as over optimistic by most experts, but that has been the expert opinion of all who have done great things in the past. Gene Myers thinks 10 cents per kilowatt hour is possible without any major advances of 2006 technology. That might be competitive, if very reliable.  Neil

[publiusr]

He is making in-roads and contacts