Air source heat pump enclosure – flow simulation results
Flow simulations of a protective housing and defrost water guide plate for an air source heat pump outdoor unit
Date: 7 May 2022
Prepared by: Tommi Haavisto
Simulation software: SolidWorks Flow Simulation
1. Model and initial data
Ambient temperature: 0 °C
Heating capacity: P = 4 kW
Wind speed: 0 m/s and 3 m/s
Maximum airflow of the outdoor unit fan: 0.6 m³/s
Comparisons:
- Metal housing + open bottom ↔ Metal housing + defrost water guide plate (wind 0 m/s)
- OSB housing + open bottom ↔ OSB housing + defrost water guide plate (wind 0 m/s)
- OSB housing + open bottom ↔ OSB housing + defrost water guide plate (wind 3 m/s)
2. Results
The effect of the defrost water guide plate on the heat pump’s coefficient of performance (COP) is very small. In the simulated comparisons, adding the guide plate reduces COP by approximately 1–3%. These comparisons assess the guide plate’s effect relative to an enclosure with an open bottom; they do not compare an enclosure with an uncovered outdoor unit.
According to average COP tables of different units, the percentage change remains of the same magnitude even at lower temperatures than 0 °C, for example -5 °C to -25 °C.
At an average wind speed of 3 m/s, the impact on the COP value is slightly smaller.
Average COP values at different temperatures
| Heat pumps | COP +7 °C | COP 0 °C | COP -5 °C | COP -7 °C | COP -15 °C | COP -25 °C |
|---|---|---|---|---|---|---|
| Panasonic | 4.4 | 3.3 | 2.5 | 2.15 | 2.1 | 1.84 |
| Average of three other units | 4.2 | 3.2 | 2.8 | 2.6 | 2.0 | - |
| Average COP | 4.25 | 3.23 | 2.73 | 2.49 | 2.03 | - |
| Average COP change per 1 °C | 0.15 | 0.10 | 0.12 | 0.06 | - | - |
Simulation results
| Model | Wind speed | Flow rate (m³/s) | Incoming air temperature (avg) | Air temperature leaving the fan (avg) | COP (avg) | Effect on COP (%) |
|---|---|---|---|---|---|---|
| Metal housing | 0 m/s | 0.543 | -3.4 | -7.2 | 2.86 | - |
| Metal housing + defrost water guide plate | 0 m/s | 0.541 | -3.7 | -7.4 | 2.83 | -1.0 |
| OSB housing | 0 m/s | 0.531 | -4.7 | -8.5 | 2.73 | - |
| OSB housing + defrost water guide plate | 0 m/s | 0.528 | -5.4 | -9.3 | 2.66 | -2.6 |
| OSB housing | 3 m/s | 0.477 | -1.9 | -6.4 | 3.01 | - |
| OSB housing + defrost water guide plate | 3 m/s | 0.498 | -2.2 | -6.6 | 2.98 | -1.0 |
3. Analysis and conclusions
- Only very small differences can be observed in the flow distribution and in the temperature of the air entering the outdoor unit between the different simulation cases.
- The effect of the defrost water guide plate on volumetric flow and flow distribution is practically negligible.
- Although the OSB housing has a smaller open area compared to the metal housing, it does not significantly increase the back pressure on the fan.
- The average temperature of the incoming air decreases slightly, by approximately 1 °C, indicating a minor increase in air recirculation. This does not significantly reduce the COP value.
- The results indicate that side wind may have a positive effect on the operation of the outdoor unit when used with a protective housing.
- Based on the simulation results, the defrost water guide plate can be integrated into the housing without causing any significant reduction in performance.
Download the original simulation report (Finnish)
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