Simulation Report
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: Solid Works 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. The COP value decreases by approximately 1–3%.
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.
