On 06/11/2010 15:14, Sean Breheny wrote:
> Perhaps my understanding is wrong but I usually think about this in
> the following way:
>
> To get net heat flow of Pf from source at temperature T0 to sink at
> temperature T1 requires power P1.
>
> If T0>T1, the heat flow is spontaneous, P1 is negative, and you can
> actually get some useful work out of the system by using a heat
> engine.
>
> If T0<T1, then you need to input a positive P1 power in order to
> "pump" the heat against the temperature gradient. However, the power
> P1 which you input to the system also gets converted to heat and
> contributes to Pf. So, you actually get Pf+P1 heat flow out into the
> sink at temperature T1.
>
> If you are going to heat a house using electricity, you could use a
> pure resistive heater and be 100% efficient (i.e., all of your input
> power is converted to heat). You could alternatively use a heat pump,
> in which case you input P1, which allows you to push Pf heat flow
> against the gradient from the cold outside to the warm inside, so the
> total thermal input power to the house is Pf+P1. There is still strict
> conservation of energy because Pf came from heat taken from outside,
> it didn't just appear from nowhere. The air outside got colder when Pf
> was taken from it.
>
> It also isn't a violation of thermodynamics because the heat didn't
> flow against the gradient spontaneously - power P1 had to be input to
> cause the reverse heat flow.
>
> It's the same thing as saying that an air conditioner heat sink must
> dissipate the sum of the electrical input power AND the heat flow from
> the cold side. But we have turned the air conditioner unit around so
> that we are cooling the outside and heating the inside. In both cases
> we are moving heat against the temp gradient.
>
> Sean
>

To my thinking the "efficiency" of a heat pump is how close the energy=20
input is to the theoretical minimum. A regular compressor/evaporator +=20
two radiators machine such as Fridge/Air-conditioner has a particular=20
efficiency as does a solid state heat pump. To compare you need the=20
efficiency for a given energy transferred up a T1 -> T2  If it's a=20
fridge or air-conditioner then T1 is desired lower temperature and T2 is=20
ambient. If it's a heat pump then T1 is ambient and T2 is desired higher=20
temperature.

You can't transport energy from T1 ->T2 for free. There is a theoretical=20
minimum for given (T2-T1) and transfer rate.

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.