Heat Pump Efficiency
I have seen reverse-cycle air conditioners, sometimes
called heat pumps which are rated at more than 100% effifiency. Do we
have creation of energy here? It seems unfair to rate something at more
than 100%. What's going on here?
It's all a matter of accounting. Heat pumps, refrigerators, and air
conditioners all work in basically the same way - they take heat from a
cool area and transfer it to a warmer area. This is against the natural
direction of the flow of heat, of course; the only way to perform such a
magical act is to put more heat into the system (in thermodynamic terms,
we're lowering the entropy of the cooler system, so we need to increase the
entropy of the warmer system by a greater amount). This is why leaving the
refrigerator door open on a hot day will make your house hotter, not cooler.
Electrical resistance heaters are a very expensive way to heat a room, even
though just about all the electrical energy is converted into heat in the
room. Thus, even though they operate at near 100% efficiency, electricity
is such an expensive form of energy that it's not a very economical way to
heat. A heat pump, however, even though it is powered by elecgtricity, can
put more energy into the room, in terms of heat, than the electricity it
consumes. This is because it pulls heat in from outside. The ratio of
heat brought into the room to electricity consumed depends on the operating
conditions (most importantly, it decreases as the temperature difference
between the room and the outdoors increases), but unless the heat pump is
completely broken, the ratio is always greater than 100%. Does this amount
to greater than100% efficiency? By one way of looking at it, yes.
Richard Barrans Jr., Ph.D.
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Update: June 2012