Re: heat pumps

bill.sloman@xxxxxxxx wrote:

On Dec 28, 6:41=A0pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Fri, 28 Dec 2007 04:54:08 -0800 (PST), Michael <mk5...@xxxxxxxxx>
wrote:

I have wandered why heat pumps are used. In Summer it is just an AC.
In Winter they pump freon (or whatever ozone friendly stuff is used)
backwards. Wouldn't using resistive heater be more logical?? The
heaters are 100% efficient, have no moving parts... What is the point
of running compressor, defrosting coils every once in a while? Am I
missing something here??

Resistive heaters are thermodynamically very inefficient. A heat pump
pushes heat from outdoor ambient temp into the load to be heated, like
a house interior space. The amount of power needed depends on the
temperature differential times the amount of heat moved. A resistive
heater acts like a heat pump working against absolute zero, a huge
differential. So a heat pump can move far more heat per watt than a
resistive heater.

The common claim that resistive heaters are "100% efficient" is
nonsense.

Not really. They do transform all the power consumed into heat.

Heat pumps are more effective, in that they can shift three or four
joules of heat for every joule they consume - more under ideal
conditions - but it's harder to characterise their efficiency.

That also depends on where you put the reference. If the heater is
powered from nuclear energy, the efficiency is quite high. If the
electricity comes from a coal powered electricity plant, then the
efficiency is rather low.

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