Quote:
Originally Posted by ITD
.... Also heat pump output diminishes below 40 degrees, regardless of technology, there is just less heat available in the air.
|
For a heat pump in heating mode, the term to be used is not efficiency, as for a combustion device, but "Coefficient of Performance" (COP), which is the total heat delivered divided by the heat equivalent of the electric power input (3412 BTU/hr per KW). There is a theoretical limit to COP, for infinite heat exchange surface area on both heat absorption and heat release sides of the system and for an engine operating isentropically, which is T(hot)/[T(hot)-T(cold)], for T on the absolute scale (t in F +460). Thus for any heat pump the COP must drop as the temperature difference from hot to cold increases). This comes from here:
http://hyperphysics.phy-astr.gsu.edu.../heatpump.html.
It isn't as much that there is less heat in the air; one just has to move more air to grab the same amount of heat from it. It's more a matter of "how high" that heat has to be pumped, temperature-wise. It's like pumping water uphill; the higher the lift, the more power it takes to deliver a given water flow. Or, for a given pumping power (compressor input), the flow decreases as the "lift" (temperature difference) increases.
The refrigerant fluid used for absorbing heat from the outside air must evaporate at some temperature below that of the outside air, and after compression of the vapor to some high enough pressure it must condense at some temperature greater than that of the inside room air. A heat pump can be designed for almost any range of outside air temperatures by selection of the right fluid and operating pressures, and for any unit there is a lower limit below which it can't move any heat, because there is no temperature difference to drive heat into the evaporating refrigerant.