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Relationship between air source heat pump performance and climate
**Abstract:** The relationship between air source heat pump performance and climate is analyzed through examples.
**Keywords:** air source heat pump, climate
Under rated conditions, the COP of an air-source heat pump is approximately 3.0 in both summer and winter (air temperature 7°C and 45°C if the water temperature is 7°C). Air-source heat pumps have cooling and heating performance that is directly influenced by outdoor climate conditions. Figures 1 to 4 illustrate how the cooling and heating capacity of air-source heat pump chiller units change with variations in outdoor environment. As shown in the figures, the cooling capacity of air-source heat pump chiller units decreases as outdoor temperature increases, while power consumption increases. When the outdoor air temperature rises to 40°C, the cooling capacity typically drops by about 5-7%. The maximum allowable outdoor temperature for normal cooling operation of air-source chillers is generally 40-45°C, though some brands with condenser fan speed control systems can operate up to around 50°C. It should be noted that unlike cooling towers, relative humidity in cooling conditions does not negatively affect air-source heat pumps; in fact, higher humidity can be beneficial for cooling. In Nanjing, where summer humidity is relatively high, the difference in cooling effect between air-cooled and water-cooled systems is smaller than commonly believed.
Air-source heat pump water heating units exhibit more complex heating characteristics. When the coil surface temperature is below the dew point temperature of the air, condensation occurs on the coil surface, which enhances heat transfer and improves the heating capacity of the heat pump unit. However, when the coil surface temperature falls below the freezing point of air (below 0°C) and the relative humidity is sufficiently high, frost forms on the coil. If not defrosted promptly, frost will accumulate, obstructing airflow and hindering heat exchange on the coil. This can lead to severe frosting, causing the compressor to trip due to low voltage. Figure 5 shows three states of wet air on the coil of an air-source heat pump unit at different frontal wind speeds: ABC represents the frosting zone, ABD the condensation zone, and CBD the dry cold zone. Cream does not gel. The AB line is the frost transition curve, which closely aligns with the wet-bulb temperature line on the psychrometric chart. As shown in the figure, when the frontal wind speed is 2.5 m/s, the ambient temperature is 0°C, and the relative humidity is 73%, frosting begins on the coil. If the wind speed is increased to 4 m/s under the same ambient temperature of 0°C, and the relative humidity reaches 82%, the coil begins to frost. The frost transition line shifts left, and increasing wind speed reduces frost accumulation. Figure 6 illustrates the frost rate line at a frontal wind speed of 2 m/s. As seen in the figure, when the outdoor dry bulb temperature is between 0-5°C and the relative humidity exceeds 85%, frosting is most severe. When the temperature is below -5°C, the frosting rate slows down, as the moisture content in the air significantly decreases.
Frosting on the coil of a heat pump unit affects its normal and effective heating, necessitating defrosting. Most units currently use reverse circulation for defrosting. During this process, not only does the compressor stop heating, but it also performs cooling, thus affecting the system's heat supply. Severe frosting may require an average of 30 minutes for defrosting, with each defrost cycle lasting about 5 minutes, leading to a reduction in heat output by approximately 17%. Additionally, as the outdoor temperature decreases, the output of the heat pump significantly diminishes. At 0°C, the actual efficiency of heat pump units under rated conditions is about 70%. At -6°C, the output is only about 62% of the rated value, and at -10°C, it is roughly 55%. Cold weather, along with rain, has a significant impact on heat pump output, potentially disrupting normal operation. Some users extend defrost time or pour warm water to remove frost. When the ambient temperature is below -10°C to -15°C, air-source heat pump units generally cannot operate normally.
Nanjing is located at 118°48'E, 32°00'N, at an elevation of 8.9 meters. The annual average temperature is 15.3°C, with winter outdoor air conditioning temperatures reaching as low as -6°C, with a minimum daily average temperature of -9°C and an extreme minimum temperature of -14°C. The extreme maximum temperature is 37.4°C, and the extreme maximum humidity is 40.7°C. Table 1 and Table 2 present the BIN parameters for different periods from 1992 to 1994 in Nanjing (hours of different temperature and humidity). Table 3 shows the hours spent in the dry-cold zone, condensation zone, and frosting zone of heat pump units in Nanjing at different frontal wind speeds. According to the table, the annual frost-making time for air-source heat pumps in Nanjing is approximately 1500 hours. If operated only during the day, the total frost-accumulating time is about 680 hours. Less frost time means higher heat pump efficiency, more reliable heating, and greater energy savings.
Fig.1 Relationship between power consumption of cooling unit and inlet air temperature and water supply temperature
Fig.2 Relationship between unit heating capacity and inlet air temperature and water supply temperature
Figure 3 screw compressor heat pump refrigeration performance curve
Figure 4 screw compressor heat pump heating performance curve
Figure 5 air coil surface wet air state
Figure 6 headwind speed of 2m/s coil surface area of the cream rate
**Table 1** 1992-1994 BIN parameters (hours: h; average wet bulb temperature (ts): ℃, BIN temperature: ℃; moisture content: g / 1kg dry air)
| BIN | -6 | -4 | -2 | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 |
|-----|----|----|----|---|---|---|---|---|----|----|----|
| Hours | 1 | 7 | 31 | 66 | 164 | 209 | 267 | 230 | 211 | 194 | 218 |
| Average ts | -6.5 | -4.26 | -2.12 | 0.67 | 0.78 | 2.24 | 3.80 | 5.21 | 7.06 | 9.12 | 10.57 |
| Moisture content d | 1.97 | 2.53 | 3.11 | 3.29 | 3.53 | 3.75 | 4.09 | 4.37 | 5.07 | 6.05 | 6.57 |
**Table 2** 1992-1994 full-time BIN parameters
| BIN | -6 | -4 | -2 | 0 | 2 | 4 | 6 | 8 | 10 | 12 | 14 |
|-----|----|----|----|---|---|---|---|---|----|----|----|
| Hours | 8 | 58 | 207 | 308 | 522 | 522 | 571 | 488 | 499 | 492 | 489 |
| Average ts | -5.60 | -4.07 | -2.17 | -0.60 | 0.95 | 2.65 | 4.23 | 6.17 | 8.22 | 9.97 | 11.34 |
| Moisture content d | 2.52 | 2.65 | 3.08 | 3.34 | 3.65 | 4.05 | 4.43 | 5.15 | 6.08 | 6.84 | 7.32 |
**Table 3** Frozen season heat pump coil air surface cumulative strength hours
| The face of the wind | Frosting zone (h) | Dry cold zone (h) | Condensation area (h) | Frosting zone (h) | Dry cold zone (h) | Condensation zone (h) |
|----------------------|------------------|------------------|------------------------|------------------|------------------|------------------------|
| 4.5 m/s | 544 | 315 | 661 | 995 | 1090 | 1563 |
| 4.0 m/s | 624 | 252 | 644 | 1221 | 916 | 1511 |
| 3.5 m/s | 652 | 227 | 641 | 1293 | 825 | 1530 |
| 3.0 m/s | 728 | 156 | 636 | 1582 | 579 | 1487 |
| 2.5 m/s | 773 | 110 | 637 | 1705 | 454 | 1489 |
| 2.0 m/s | 814 | 70 | 636 | 1858 | 315 | 1475 |
**Note:** 1) The results are calculated based on charts obtained from the annual climatic operation simulation software of the heat pump developed by the Institute of Cryogenic Engineering, Zhejiang University, using meteorological data from Nanjing from 1992 to 1994.
2) The calculation assumes the heating period of the heat pump unit is from November 1 of the current year to March 1 of the next year.