Theory 097/Energy & Environment /HVAC
Heat Pumps Fail at a Lift
Performance is quoted as a coefficient. The curve may have a cliff.
Heat pump performance falls sharply above a threshold temperature lift, so the same unit can be efficient in a mild climate and marginal in a cold one, and the threshold is set by the compressor rather than by the refrigerant.
01The evidence
Measured performance degrades faster than the theoretical cycle predicts at high lift, with the shortfall attributed to compressor losses.
02The tension
Performance ratings use a single test point, which predicts efficiency from the rated condition and misses the cliff that appears in service.
03The mechanism
As lift rises, the compressor pressure ratio rises and its volumetric and isentropic efficiencies fall. Beyond a threshold the losses compound and the coefficient drops faster than the cycle model predicts.
That makes lift the determinant, and predicts a cliff that a single-point rating cannot represent.
04What this is not
Ratings extrapolate from one condition. This predicts a threshold, which changes cold-climate suitability.
05Predictions
- P1Coefficient should fall sharply above a lift threshold.
- P2Units rated alike should diverge in cold climates.
- P3Compressor design should shift the threshold.
06Refutation
Smooth performance fall-off across lifts would remove the threshold.
07What it changes
It would change cold-climate heat pump specification.