Re: [Phys-L] Earth as a "heat engine"

[John Denker <jsd@av8n.com>]

On 10/8/19 2:34 PM, Folkerts, Timothy J wrote:
> I occasionally hear earth's atmosphere and/or oceans described as a
> "heat engine" pumping air and water and energy around.
>
> I think this is highly questionable nomenclature.

We can agree that it is open to interpretation, including a
a great many unhelpful interpretations ... but it is not
entirely crazy.

By way of analogy:  Suppose we have a practical real-world
engine hooked up to a prony brake, so as to measure its
power output.  There are a couple of ways to think about
this, depending on where you draw the boundary of "the"
system.

 a) If you draw a boundary around everything mentioned
  above, including the prony brake, then "the" system
  has zero thermodynamic efficiency.  Fuel goes in
  and nothing but heat comes out.

 b) However, it would be more conventional and more
  useful to draw a boundary that includes the engine
  but not the prony brake.  The drive shaft crosses
  the system boundary.  Now "the" system has nonzero
  thermodynamic efficiency.

So it is with the earth's oceans and atmosphere:

 a) If you look at things on a large enough scale,
  then "the" system has zero thermodynamic efficiency.

 b) However, in a very practical sense, you can stick
  a windmill into the atmosphere, in such a way that
  the shaft crosses the system boundary.  Now "the"
  system has nonzero thermodynamic efficiency. Heat
  goes in and useful power is harvested.

  There is energy in the atmosphere that is /not/ just
  heat -- that is, relatively low-entropy well-organized
  energy -- even if you don't harvest it.  And if you
  do harvest it, natural engine-like processes will
  create more.

 b') Ditto for wave-energy harvesting systems.