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 the rated conditions, the COP of the air-source heat pump is about 3.0 in the summer and about 3.0 in the winter (air 7 ℃ and 45 ℃) if the temperature is 35 ℃ and the water is 7 ℃. 3.0 or so, air-source heat pump cooling, heating performance and the outdoor climate has a direct relationship. Figure 1 - Figure 4, respectively, reflect the air source heat pump chiller units in the outdoor environment changes in cooling and heating capacity changes. It can be seen from the figure that the cooling capacity of air-source heat pump chiller units decreases with the increase of outdoor temperature, and the unit power consumption increases with the outdoor ambient temperature. When outdoor air temperature is increased to 40 ℃, the cooling capacity will generally drop about 5-7%. Air-source chiller normal cooling maximum temperature is generally 40-45 ℃, individual brands with condenser fan speed control system, the maximum allowable outdoor temperature of up to about 50 ℃. It should be pointed out that with the cooling tower is not the same, the relative humidity in cooling conditions on the air source heat pump did not adversely affect the contrary, the relative humidity, the cooling is beneficial. Nanjing summer relatively high humidity, so in fact the difference between the cooling effect of air-cooled and water-cooled, smaller than people think.

Air-source heat pump water heating unit more complex heating characteristics, when the coil surface temperature is below the air dew point temperature, the air will be dew condensation, coil surface heat transfer occurs at this time, help to improve the heat pump unit Heating capacity, but when the coil surface temperature is below the freezing temperature of air (below 0 ℃), if the relative humidity in the air at the same time reach a certain level, the coil surface will frost, if not timely defrosting, frost Will be more knot the thicker, affecting the actual flow of air, and hindered the heat exchange on the coil, the heavier will freeze, the compressor appears low voltage protection downtime. Figure 5 shows that there are three states of wet air on the air coil outside the heat pump unit at different frontal wind speeds. ABC is the frosting zone, ABD is the condensation zone, and CBD is the dry cold zone, Cream does not gel. The AB line is the frost transition curve, which is close to the wet-bulb temperature line on the psychrometric chart. As can be seen from the figure, when the frontal wind speed is 2.5M / S, the ambient temperature is 0 ° C and the relative humidity is 73%, the frosting begins on the coil, for example, the head speed is increased to 4M / S and the ambient temperature is 0 ℃, then the relative humidity of 82%, the coil began to frost, frost condensation transition line corresponding left, increase the wind speed can reduce the accumulation of frost. Figure 6 for the front of the wind speed of 2M / S frosting rate line. As can be seen in the figure, the outdoor air dry bulb temperature 0-5 ℃, relative humidity> 85% of the frost when the worst, when tw <-5 ℃, the frosting rate slowed down, this is due to air at this time Moisture has been significantly reduced.

Heat pump unit frosting appears on the coil, will affect the unit's normal and effective heating, it must be defrosted. At present, most of the units use reverse circulation to defrost. At this time, not only the compressor stops heating operation, but also the cooling operation, so the system heat supply is obviously affected. Frosting serious, an average of half an hour of cream, a defrost time of about 5 minutes, due to defrost reduced heat about 17%. In addition, the outdoor temperature decreases, the output of heat pump significantly reduced. 0 ℃ conditions, the actual rate of heat pump units under rated conditions of about 70%. -6 ℃ case, the output is only about 62% under rated conditions, -10 ℃ under the conditions of heat supply is only about 55% under rated conditions. Cold weather, rain and heat have a significant impact on the heat pump output, while affecting the normal operation of heavy, some users to extend the defrost time, pouring warm water and other methods to remove frost. Ambient temperature is below -10 ℃ - -15 ℃, the heat pump unit generally can not be normal operation.

Its geographical location is 118 ° 48'E, latitude 32 ° 00 ', 8.9m above sea level, annual average temperature is 15.3 ℃, winter outdoor air-conditioning outdoor temperature is -6 ℃, minimum daily average temperature is -9 ℃, extreme minimum temperature is -14 ℃, The extreme minimum temperature is -8.6 ℃, the daily average temperature is ≤8 ℃, the number of days is 115 days, summer outdoor air-conditioning design and calculation temperature is 35 ℃, wet bulb temperature is 28.3 ℃, extreme maximum humidity is 40.7 ℃ and extreme maximum temperature is 37.4 ℃. Table 1 - Table 2 shows the BIN parameters of different periods from 1992 to 1994 in Nanjing (hours of different temperature and humidity). Table 3 shows the hours of dry-cold zone, condensation zone and frosting zone of heat pump units in Nanjing at different frontal wind speeds. The table shows that Nanjing area air source heat pump frost-making time throughout the year in about 1500 hours, if only in the daytime operation, the year frost-accumulating time in a total of about 680 hours. Frosting time is less, the higher heat pump heat efficiency, more reliable heating, energy-saving more obvious.

Fig.1 Relationship between power consumption of cooling unit and inlet air temperature and water supply temperatureFig.2 Relationship between unit heating capacity and inlet air temperature and water supply temperature

Figure 3 screw compressor heat pump refrigeration performance curve 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

Note: a group air-conditioning working hours: 8:00 ~ 18:00

Note: 1) The result is calculated on the basis of the charts obtained from the annual climatic operation simulation software of the heat pump developed by Institute of Cryogenic Engineering, Zhejiang University, with reference to the meteorological data of Nanjing from 1992 to 1994.

2) Calculation Suppose the heating period of the heat pump unit is from November 1 of the current year to March 1 of the next year.

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