Re: [Vo]: Proof of capturing ambient temperature energy

[Jed Rothwell]

Michel Jullian wrote:

The highly OU commercial ambient air energy pumping devices I was 
talking about are those residential heating heat pumps we have been 
discussing in the 'loop closed' thread, they commonly reach COPs of 
4 (1000W out for 250W in, the excess coming from ambient air).

I thought this thread was a joke. A heat pump is NOT over unity. 
Using old-fashioned terminology, a heat pump moves heat from a cold 
"reservoir" to a hot one. When you drive a heat engine with the heat 
from the hot reservoir, the heat engine produces less energy overall 
than the heat pump consumed.

The "COP" of 4 refers to the amount of heat energy moved versus the 
amount expended by the machine, but you cannot move that same heat 
back the other direction with a heat engine and get back 4 times 
input, because the hot reservoir is only a little warmer than the 
cold one, and Carnot efficiency is low.

Let me give an imaginary example with round numbers. Suppose you live 
in Florida where you need to heat a house only a little more than the 
surroundings. You run a gasoline powered heat pump -- a very 
efficient heat pump with a COP of 6. It converts 20% of the starting 
energy into air conditioner power, which then moves 5 * 20% into the 
house = 120% of the original fuel.

You can burn 1000 joules worth of fuel in the house, or use the same 
amount of fuel in the heat pump to move 1200 joules of heat from the 
surroundings into the house. But, the temperature difference between 
the house and the surrounding is 5 deg C (6 deg C with the heat 
pump), and when you attach a heat pump to the house and let the heat 
flow through it, it is only 1% efficient, so the 1200 joules converts 
to 12 joules of useful work. It is physically impossible for any heat 
engine to convert the heat back into 1000 joules of work, or more.

If you only want to make the hot reservoir a little warmer than the 
surroundings, a heat pump can achieve very high C.O.P.s of 10 or 
more, but in no case can you reverse the process with a heat engine 
and get more out than you put in.

- Jed