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From: loren@dweasel.llnl.gov (Loren Petrich)
Newsgroups: sci.energy,sci.electronics,sci.physics,sci.space
Subject: Re: solar cells
Message-ID: <88637@lll-winken.LLNL.GOV>
Date: 3 Jan 91 00:56:48 GMT
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	The issue of chemical vs. nuclear poisons was brought up yet
again. I feel that this anti-nuclear allergy that too many people have
will someday be remembered as one of the irrational phenomena of our
time. But I doubt that it is worse that the turn-of-the-century
enthusiasm for patent medicines made from radium and other radioactive
materials.

	As to chemical poisons being decomposable, that depends on
what kind of chemical poison. Heavy metals cannot be chemically
decomposed. And some chemical poisons are difficult to decompose, such
as chlorinated hydrocarbons. The persistence of certain pesticides
like DDT should be well known. True, DDT and other such
non-biodegradable substances can be burned at high temperature, but
burning at high temperature is just that.

	I remember some years back that the EPA was hoping to burn
some toxic wastes in a ship at sea, but some environmentalists didn't
like that idea very much.

	I keep on being amazed by the anti-RTG movement. They complain
that those who send up RTG's on spacecraft have not done comprehensive
studies of possible alternatives. Yet I wonder if the anti-RTG people
have done anything similar. Consider the difficulties of doing
maintenance on a spacecraft, which usually cannot be brought back to
its designers. Millions of dollars and months of work go into
designing some spacecraft, so it is important that they be likely to
keep on working. One should try to use components that need as little
maintenance as possible, and RTG's fit the bill very well. They are
continuously "on" and have no moving parts. Solar cells are one common
alternative, but they tend to degrade over time and they cannot be
used in the outer Solar System, due to the extreme dilution of
sunlight there. A focused-sunlight system would have several problems.
A mirror would have to be kept pointing at the Sun, and the generating
system has an abundance of moving parts, which are an all-too-familiar
maintenance headache. There is also the problem of replenishing leaked
working fluid. And I am not aware of any focused-sunlight system that
has ever been used in a spacecraft.

	Chemical reactions are out of the question. Buth fuel and
oxidizer would have to be taken along, which would add a serious
amount of weight for a months-long mission. The power sources usually
have an abundance of moving parts, and would have to be made redundant
for the sake of safety (if one breaks down, the others could keep on
moving). Batteries have a minimum of moving parts, but they usually
have a very low available power to mass ratio (ask any designer of a
battery-powered car). Fuel cells are relatively efficient, but even
they have moving-part problems, and they require liquid hydrogen and
oxygen, which must be kept away from heat. Systems using combustion
can use fuels and oxidizers that are liquid at room temperature, but
they also suffer from problems with moving parts -- consider typical
turbines and piston engines.

	So either solar cells or RTG's are the way to go for
spacecraft. I presume that this is the standard argument.

	In fairness to opponents of nuclear energy, I think that there
is a sociological question to be considered. Most nuclear energy has
been handled as large-scale projects. Simply consider how big a
typical nuclear reactor is and how long it takes to build one. Big
organizations have to justify their policies, and they often make
excuses for keeping on doing what they have been doing. And they
sometimes seem insensitive and arrogant. It's just what computers have
seemed like in their early years, before personal computers became
common.

	And on the issue of safety, one should ask what kinds of
critical tests are possible. It is much easier to perform really tough
tests on an RTG than on a nuclear reactor, so one may feel more
confidence in their safety.

	And another possible difficulty with solar cells -- how much
energy does it take to make them? They would not be too good if the
amount of energy needed to make them was only equal to their output
for several years of running. Has that question ever been addressed?


$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
Loren Petrich, the Master Blaster: loren@sunlight.llnl.gov

Since this nodename is not widely known, you may have to try:

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