1) What is Space Solar Power (SSP)?
The basic idea is to put a large array of solar electric panels in space. Then there is no problem with weather and if the SSP plant is in GEO, then it receives sunlight 24 hours per day. The solar electric panels (also called photovoltaics) convert sunlight to electricity. The electricity powers some magnetrons, which convert it to radio waves (RF) at 2.45 GHz. So, essentially the SSP is an array of photovoltaics and magnetrons. The transmitter is a phased array.
The frequency is 2.45 GHz. The reasons are that a) the atmosphere is very transparent to this frequency and no, rain is not a problem. b) magnetrons that work at this frequency are a dime a dozen. Millions of them are made for microwave ovens.
The space solar power plant is probably the only way that solar energy will be able to supply baseload (and continuous) solar power. While gathering solar energy on Earth is very good for rural and residential users, the problem of storage makes it too expensive and difficult for powering large cities and factories.
2) What are the environmental and health hazards of the RF beam?
* At the receiver on Earth the power density of the beam is 20 milliwatts per square centimeter. This is so low, that you cannot feel any heating from walking through the beam.
* This beam power density is 1/4 the power density of natural sunlight.
* Radio waves at 2.45 GHz are often called microwaves, since compared to other types of RF, their wavelength is short (about 1 centimeter). Microwaves are NON-ionizing radiation. They are 1 MILLION times too weak to rip electrons from atoms. This means that the only effect of them is heating. Heating is known to be hazardous to eyeballs at power densities greater than 100 milliwatts per square centimeter. There is no known hazard below that level. This is not to say that we shouldn't still be careful though. So the IEEE has declared certain safety standards for exposure. The power density at the edge of the SSP receiver is within this standard limit.
* The small power density of the beam means that as a weapon, the SSP is less effective than a squirt gun!
* If the beam strays from the receiver, it defocuses, then turns off (with loss of pilot from the receiver).
* Interference with communications is an important concern for SSP. Work is being done on transmitters which minimize harmonics and bandwidth of the beam. RADAR, TV and Radio stations all have this same problem, and those put higher power signals into the local area than an SSP would. While air traffic may be more concerned about it, (since a plane can fly through the beam), they would simply avoid that air space.
3) Why has no one built one of these yet? Are there technical problems?
* The primary difficulty in building an SSP plant is that our national energy priorities are to a) import over 50% of all the oil we use, b) continue to use nuclear fission and c) develop nuclear fusion. While development of nuclear fusion power is a worthy goal, it has many more technical problems than SSP. The main components (photovoltaics, magnetrons and phased array transmitters) of SSP have already been demonstrated on Earth and in space. In fact, these components were invented for use in space!
* One of the problems not yet explored for SSP is how to build fairly large structures in space.
* The first SSPs will probably have to be transported from Earth to LEO or GEO. Space transportation systems are still poorly developed and expensive.
* SSP plants will have to be built in the same manner as other types of power plants, in order to keep the costs reasonable. This looks to be possible, but plenty of work remains to be done in this area.
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