Efficiency is a key metric for air conditioners, heat pumps and dehumidifiers. Before selecting equipment and matching a system, a thermodynamic calculation can show which parameters efficiency is most sensitive to. Taking R290 as an example, this article looks at how subcooling, superheat, evaporating temperature and condensing temperature each affect cooling COP, and then asks which parameter to move first when resources are limited.
Calculation conditions
We use a single-stage vapor-compression theoretical cycle, with properties from the open-source CoolProp library. Baseline: evaporating temperature 0°C, superheat 10°C, condensing temperature 40°C, subcooling 5°C, compressor isentropic efficiency 75%. Pipe pressure drop is ignored, as are heat-transfer constraints such as heat exchanger area and airflow. Baseline cooling COP is about 4.29. Each case below changes one parameter only; the rest stay at baseline.
1. Subcooling: a steady, linear gain
Raising subcooling from 5°C to 15°C lifts COP from 4.29 to about 4.69. Across this range, each additional 1°C of subcooling adds roughly 0.04 to COP, almost linearly.
2. Superheat: very little effect in theory
Raising superheat from 10°C to 20°C moves COP only from 4.29 to about 4.32, less than 0.01 per 1°C. In the R290 theoretical cycle, superheat is hardly an efficiency lever.
3. Evaporating temperature: the higher, the bigger the gain
Raising evaporating temperature from 0°C to 10°C lifts COP from 4.29 to about 6.18, and the gain per degree grows: about +0.15 from 0→1°C and about +0.24 from 9→10°C.
4. Condensing temperature: the lower, the bigger the gain
Lowering condensing temperature from 40°C to 30°C lifts COP from 4.29 to about 6.07. Again each degree is worth more: about +0.14 from 40→39°C and about +0.23 from 31→30°C.
In selection, which parameter do you move first?
The results above look at one parameter at a tim