Your Questions About Solar Energy Generators

admin answers:

New Horizons is powered by a Radioisotope Thermoelectric Generator. Most spacecraft are indeed powered by solar panels linked to a bank of batteries, but that would not be feasible for New Horizons. The sunlight in the outer Solar System is too feeble for solar cells to be effective. Although they are powered by the decay of Plutonium-238, they are not nuclear reactors. They are pellets of plutonium dioxide encapsulated in a heavily armored container to prevent the plutonium from escaping in the event of a launch vehicle failure that results in a crash or explosion. Plutonium 238 has a half-life of 87 years and because it emits mostly alpha particles, it it not very dangerous unless it was inhaled or ingested from a radiological point of view. Indeed, only lightweight radiation shielding is required for an RTG. Plutonium is however also a very poisonous heavy metal, which is why NASA builds as strong a container for the RTG’s as possible to reduce the risk of Plutonium escaping. That is one reason it is put in RTG’s in the form of an oxide and that is vitrified to further contain the Plutonium. Inside them are devices called thermocouples. As the Plutonium-238 decays alpha particles and heat are released. The heat is converted by the thermocouples to generate electrical power, which operates all of New Horizon’s systems. The advantage of this system over a nuclear reactor is there’s no way for it to melt down or go “runaway.” They cannot explode, nor can they cause a massive radiation release because the amount of fuel they carry is very small. Plutonium-238 cannot be used to build nuclear weapons either. They are compact, radiation proof and will generate power for decades no matter how the spacecraft is oriented with respect to the Sun. As they Voyagers have shown, they will operate even in interstellar space. They gradually produce less and less electricity over time, and eventually New Horizons will not have enough power to run it’s instruments and it’s other systems. At that point it’s mission will end and New Horizons will become mankind’s fifth piece of interstellar space junk as well as time capsule.

admin answers:

Its complex, and expensive, but a great idea. I’ve thought it before.

The only way to do it is to use a thermal cycle with a refrigerant. You need to embed a heat exchanger (tubes) in the road. They need to be able to withstand temperatures, thermal flexing, salt, mechanical loading from cars. There are some issues to overcome.

If you use the right medium it undergoes a phase-change at the operating temperature of a road, that sucks up a TON of energy. You pump in a liquid, pump out a gas. With hot gas you can either run it through some sort of generator, or use it to heat something else up then run the something else through a generator.

So you might think of putting fins on the coils and making them more like long lines. Here in Phoenix AZ the hot summer sun only radiates 1kw/sq-meter. You can compute from the radiation coefficient and the temperature of the pavement what part of that is re-radiated away. Its likely significant. Bottom line: to get meaningful power you must have large area of pavement soaking up light.

There is a good book in thermal system design by Boehme from the 1980’s. You could find it, check it out using inter-library loan, and then put the idea as something in a science fair. If someone likes it, they could support your work, if not you can still earn glory and scholarships. A good heat transfer text would also do you pretty well.