[mdlug-discuss] The Hell you say?

[Drew]

     In fact, there *are*, and have long been, devices that provide more
heating than the purchased energy that they consume. They are called
heat pumps, and are really air conditioners mounted backwards. Typical
air conditioners move around 5 times as much heat as they use electricity,
and all of it within the laws of thermodynamics.

     You see, the maximum possible efficiency of a heat moving device is
the absolute-scale low temperature divided by the temperature difference,
and in most climates the difference is small compared to the temperatures
themselves. For example, if it's 32 degrees F. outside and you're heating
the house to 80 degrees F., the absolute temperatures being 273 K and
300 K, the maximum possible efficiency will be 273K/27K, or just over 10.
So, getting twice as much heating out, or even five times as much, is
actually rather dismal performance.

     The rub is that the maximum possible efficiency for a heat *engine*
is the temperature difference divided by the absolute-scale *high*
temperature. So that if you try to run a combo heat pump/engine, the
total performance coefficient will be the low divided by the high, and
since the low is always less than the high, the heat engine can never
produce enough power to keep the heat pump going.


     I gather, though, that this is not supposed to be a heat pump. If it
*is* _creating_ the heat out of nothing, then the important question
becomes, how high a temperature can be achieved at the hot end while still
putting out double its input. Generators being around 85% efficient (and
newer ones may do better for all I know), if the hot end can get to
2 * 273K / 0.85 = 642K, or almost 700 degrees F, it can reach breakeven
in conjunction with a heat engine.

     Additional note: Cold (or hot, for that matter) fusion would also be
within the laws of thermodynamics. Hydrogen (usually heavy) is turned into
helium in the process, and it would presumably be more difficult to turn
the helium back into hydrogen than what you got out of the first process.
But while it takes at least 60598 BTUs, and possibly a bit more, to pull
a pound of hydrogen out of water, and even more if you're stuck with a
machine that needs heavy isotopes of hydrogen, turning that pound of
hydrogen into helium would release around 300 *billion* BTUs. So for any
practical purpose short of interstellar travel, a fusion reactor would
be just as good as a perpetual motion machine. The only difference is
that the fusion reactor can't be accused of violating any physical laws.


----

  - Drew.


    "When I went to school, they taught me to honor the Flag, vote the
     straight party ticket, and believe in the law of conservation of
     energy."
                 - Lazarus Long.