🚀 go-pugleaf

In article <1992Dec21.010417.11854@cis.ohio-state.edu> rubin@grape.cis.ohio-state.edu (Daniel Rubin) writes:
>This discussion has reminded of a solution to our potable energy problems.
>This ideal is probably as useful as when I though of my perpetual motion 
>magnet motor which could never work, but I will throw it out anyways...
>
>Solar cells work by using photons to knock out electrons in a silicon crystal
>structure correct?  Now I am wondering if something could be make to use
>the same principle of "knocking" electrons around using cosmic rays?  If you
>developed such a material you could sandwich it with a thin layer of a 
>radio active substance hence producing a battery that would literally last
>a lifetime.  If this solar cell substance worked as efficiently as current
>solar cells and could produce .5 volts and say 1 amp for a 4 inch square 
>sheet.  You could make a nice 12 volt cell that would be about 4 inches square
>and maybe 1 inch heigh that would produce power for a long long time.  
>
>Of course it would have to be enclosed in lead or something to protect the
>users from the radioactivity, but that can be done I believe...
>
>OK shoot holes thorough this idea...

Alright. There are a couple of problems. First, the mechanism of electric
current generation in a photocell requires the photon to have enough
energy to lift the electron over the band gap. Any energy contained in
the photon above that value is converted to kinetic energy in the lattice,
IE heat. If the photon has insufficient energy to bridge the band gap,
no electron is liberated. Now quantum mechanics requires that only
very specific energies can be absorbed and that fractional energies
don't add, so all the energy of the insufficient photon is also converted
to heat. Now it turns out that it's possible to make materials with a
band gap energy that falls within an order of magnitude of the range
of energies contained in normal visible light. Therefore solar cells 
with efficiencies of around 10% or slightly higher are possible.

With a radioactive material, the emission is going to be in one or
more of three basic forms. Either it's an alpha particle, a beta
particle, or a gamma ray (plus an occasional neutron that we'll
ignore for this purpose). Now most substances emit these at such
an energy, much higher than that of visible light, that most of
the energy of the emission is wasted because it's larger than the
band gap energy of the absorber material. In fact in many cases the
energy is so high that it will damage the lattice of the material
by displacing an atom. Therefore, the efficiency of the device will
be very low and it's lifetime short.

Now there's another issue. To generate an ampere of current, you
must displace a coulomb of charge. That's roughly 9E11 electrons.
Few materials are that radioactive. Plutonium for example only 
emits about 2E3 beta particles per gram per second. So to generate
an ampere of current, you need 450,000 kilograms of plutonium. Since
a critical mass of plutonium is less than 4 kilograms, you've got 
a huge atomic bomb on your hands. Of course you'd use a non-fissionable
material in your battery, but the scale still remains very large.
That's why real nuclear batteries use the heating of the lattice of
the material as the mechanism for tapping the energy. The hot lattice
can then drive a thermal engine to generate electricity. These can
convert up to 30% or so of the energy to electricity.

Gary