@S1-A.ARPA,@MIT-MC.ARPA:redford%avoid.DEC@decwrl.ARPA (07/07/85)
From: redford%avoid.DEC@decwrl.ARPA (John Redford) Dietz of Rutgers suggests using the solar wind as a power source, since it could be potentially be gathered with much lighter and simpler collectors than sunlight could. Interesting idea! My Encyclopedia Brittanica says that the solar wind consists of protons and electrons travelling at 300 to 700 km/s. The protons have energies of about 1000 eV (the electrons of 10 eV), and densities of one particle per cm^3. The flux density is 10^8 to 10^9 particles per cm^2 per second. All of these numbers vary wildly with solar activity. Say there are 10^8 protons/cm^2/s with energies of x 10^3 eV/proton and there are x 1.6 x 10^-19 joules/eV and x 10^4 cm^2 / m^2 --------------- then the power flux is 0.16 mW / m^2 This is ten million times less than the power flux of sunlight (1600 W / m^2). This doesn't sound good. Our collector will have to be extraordinarily light compared to solar cells to make up for a 10^7 difference in power flux. In fact, if solar cells are 10% efficient, and our collector were 100% efficient, our collector would have to be a million times lighter per unit area in order to get the same power per unit mass of collector. However, the collecting electrodes can be just a fine mesh of wires, so it might be possible. One other way to improve the system might be to exploit the shock wave in the solar wind produced by the earth's magnetic field. The earth's field produces a teardrop shaped bubble in the solar wind, with the tail of the teardrop pointing away from the sun. As protons slam into the field, many get absorbed into the earth's Van Allen belts, but some must flow around the teardrop. The particle density at the boundary of the teardrop is probably a lot higher than normal, so that's the place to put our collector. It could be sited in a sun-facing polar orbit so that it is always moving around the circumference of the teardrop. This would be a nasty place to work because of the radiation levels. The collector could probably not be assembled by people, and even rad-hard electronics would have trouble. I don't think, by the way, that the Van Allen belts themselves could be used as a power source. The particles in them are very energetic, but are moving in all directions. Getting power from them would be like getting power from the thermal energy of the atoms in a hot gas; you violate the second law of thermodynamics. However, the particles are constrained by the earth's field, so their velocities might not be completely isotropic. Any anisotropy could theoretically be exploited to produce power. One last thought. In the early sixties, several nuclear tests were conducted in the ionosphere. They produced belts of charged particles which are still perturbing the magnetosphere twenty years later. Suppose that we put our MHD collector around such an explosion. Some fraction of the bomb's energy could be collected and beamed down to earth as microwaves. A continuous series of small explosions could keep the charge reservoir pumped up. We could use the entire earth's field as the magnetic bottle for a fusion reactor! The reactor would sit out in geosynchronous orbit, so there'd be little direct danger from radiation. It would be a small extra sun in the sky. We'd get lots of pretty aurora, too. Of course, this would violate certain arms control treaties, as well as potentially damaging Earth's first line of defense against cosmic radiation, but hey, anything for a few terajoules. John Redford DEC-Hudson