Experimental study on frost growth and dynamic performance of air source heat pump system

Transcription

1 Available online at Applied Thermal Engineering 28 (2008) Experimental study on frost growth and dynamic performance of air source heat pump system Xian-Min Guo a,b, *, Yi-Guang Chen b, Wei-Hua Wang b, Chun-Zheng Chen a a Energy and Power Engineering, Xi an Jiaotong University, Xi an , PR China b School of Mechanical Engineering, Tianjin University of Commerce, Tianjin , PR China Received 11 April 2007; accepted 6 January 2008 Available online 18 January 2008 Abstract The effect of the frost growth and frost morphology on the performance of an air source heat pump was investigated experimentally. The frost thickness, frost accumulation and the dynamic performance of the heat pump were measured. It is found that the frost growth can be divided into three stages according to the frost morphology. In the initial stage, condensed water freezes and forms a transparent thin ice layer on the fins and tubes firstly, then the granular ices appear and grow gradually on the ice layer, and the column-shaped ice crystals are formed at last. The growth rate of the frost thickness, the heating capacity and COP of the heat pump increase with the frosting time significantly until the column-shaped frost layer is formed. In the second stage, the column-shaped ice crystals grow in its radius rather than in its length, and the frost thickness growth rate decreases or remains to be constant. However, the heating capacity and COP of the heat pump are only slightly affected by frosting on the outdoor coils. In the third stage, the ice crystals mainly grow in its length, and become gradually of an acerose-shaped one, finally a fluffy frost layer is formed. The frost thickness growth rate is about 2 4 times of that in the second stage. The drops per minute in the heating capacity and COP are increased by several times of those in the second stage. In addition, it is found that the frost growth rate and the drop in the performance of the heat pump are highest when the outdoor air temperature is about 0 C with various relative humidity. The experimental results are in agreement with the corresponding simulation data except in the third frosting stage. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Air source heat pump; Frosting; Dynamic performance; Frost growth; Frost thickness; Experiment 1. Introduction Frost formation on the outdoor heat exchanger of an air source heat pump (ASHP) unit occurs when the surface temperature is both below the dew point of the moist air and the freezing point. Frost accumulation on the surface of the evaporator results in a performance degradation of the ASHP system because the frost layer leads to an increase in thermal resistance and air flow pressure drop across the heat exchanger, hence, causes a decrease in the air flow rate for a fan-driven heat exchanger. When the * Corresponding author. Address: Energy and Power Engineering, Xi an Jiaotong University, Xi an , PR China. Tel./fax: address: xmguo@tjcu.edu.cn (X.-M. Guo). evaporator heat transfer rate is reduced, the evaporation temperature, the suction pressure and the system capacity drop accordingly. The lower evaporation temperature may create mechanical problems in the system such as liquid refrigerant flood back to the compressor and periodic defrost is required. Therefore, it is important to investigate the frosting process and its effects for improving the performance and operational reliability of the ASHP system under frosting conditions. A number of theoretical and experimental studies [1 10] on the performance of heat exchangers under frosting conditions were reported, and the effects of ambient and geometry parameters on the performance of the heat exchangers were discussed comprehensively. In most of these experimental [1 4,7] and numerical [5,6] studies the air flow rate /$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi: /j.applthermaleng

2 2268 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) Nomenclature A face area, m 2 d absolute humidity, kg/kg dry air h air specific enthalpy, J/kg M fr frost accumulation mass, kg q a air volume flow rate, m 3 /s p pressure, N/m 2 P consumption power, W Q h heating capacity, W COP coefficient of performance RH relative humidity T temperature, C t time, s V velocity, m/s d fr frost thickness, m q a air density, kg/m 3 Subscripts i inlet o outlet av average a air c condenser e evaporator fr frost p pipe wall 1 6 state point number and refrigerant temperature were assumed as constant during the frosting process, the changes inside a practical evaporator, such as the variation of the evaporation temperature and refrigerant liquid dry out position, was not taken into account, and only in a few of the studies [8 10] the decrease in the air flow rate due to frost accumulation was considered. However, the constant flow rate constraint is unlikely for a fan-driven heat exchanger used in ASHP unit. Tso et al. [11,12] developed a general distributed model with two-phase flow for refrigerant coupled with a frost model for studying the dynamic behavior of an evaporator validated experimentally. In fact, frost growth will cause the changes of cycle parameters of the ASHP system, such as the evaporating and condensing temperatures, which will in turn affect frost formation. Therefore, it is necessary to investigate the performance of ASHP unit by coupling the frosting process with the operating process of ASHP system. In despite of a large amount of studies on the performance of the heat exchangers under frosting conditions, the published literatures for study on the performance of ASHP system under frosting conditions are limited. Senshu et al. [13] and Yasuda et al. [14] studied the performance of a heat pump air conditioner under JIS frosting conditions numerically and experimentally. Miller [15] tested an airto-air split-system residential heat pump under variable frosting conditions. The experimental results suggested that the performance of the heat pump was slightly affected by frosting. The present authors [16,17] developed a model, which coupled the frost growth model with the refrigerant-side parameters variation of ASHP system due to the frost formation, to predict the performance of ASHP unit under frosting conditions. The simulated results were compared with the experimental data, but in that work the frost thickness was not measured. The purpose of this paper is to observe the frost growth process and investigate its corresponding effect on the performance of ASHP system. An ASHP unit with 11.2 kw rated refrigeration capacity was tested under various frosting conditions, and the transient performance of the heat pump, frost accumulation mass, and frost thickness were measured. 2. Experimental apparatus and prototype The experiments were conducted in two psychrometric rooms, in which the air temperature and relative humidity could be controlled respectively to be the desired values. The experimental prototype was a split-type heat pump air conditioner with 11.2 kw rated refrigeration capacity. The test outdoor and indoor units were connected by a pair of 5 m long well insulated pipes. The indoor unit was located in the indoor psychrometric room and its discharge air was fed into a chamber, in which the discharge air was adequately mixed and the pressure and temperature were measured. To determine the air flow rate through the indoor heat exchanger, the discharge air was exhausted into the room through the nozzles by a variable speed fan. The outdoor unit was located in the outdoor psychrometric room. The schematic diagrams of the experimental apparatus and the test ASHP system with measuring points are shown in Figs. 1 and 2, respectively Psychrometric rooms As shown in Fig. 1, to obtain the air temperature and relative humidity in the rooms, the air dry/wet bulb temperatures in both of the psychrometric rooms were measured by using Pt 100 X RTD sensors (±0.1 C) in the sampling ducts, in which the velocity was about 5 m/s. The air temperature and relative humidity in the outdoor and indoor rooms were adjusted by two sets of air handling units (AHU). Each AHU contained a cooling coil, an electrical heater, and a steam humidifier, which were controlled by a computer program using PID control algorithm to keep the ambient air parameters at the desired values. The air flow rate through the indoor exchanger was measured by the multiple nozzles, across which the pressure drop was

3 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) Fig. 1. Schematic diagram of experimental apparatus. Fig. 2. Schematic diagram of test ASHP system and measuring points. detected in terms of a precision difference pressure transducer (±0.08%). The air flow rate was controlled to be a fixed value over all the experiments by adjusting the rotation speed of the variable speed centrifugal fan. The outlet air dry/wet bulb temperatures of the indoor heat exchanger were measured by using Pt 100 X RTD sensors (±0.1 C). The power consumption of the test heat pump was measured in terms of a precision power transducer (±0.5%). From the measured inlet and outlet parameters of the indoor heat exchanger, the heating capacity and COP of the ASHP unit was estimated from Q h ¼ q a;c q a;c ðh c;o h c;i Þ for condenser side ð1þ COP ¼ Q h =P ð2þ where the air density and the air specific enthalpy were obtained from the psychrometric chard according to the measured dry/wet bulb temperatures Parameters measurement of the outdoor heat exchanger and HP system The entering air velocity, dry-bulb temperature and relative humidity were measured with twelve velocity sensors (±0.5%) and twelve dry-bulb temperature/relative humidity multi-sensors (±0.1 C, 2%RH), which were located on the 4 by 3 sensor grid in the upstream of the outdoor heat exchanger. Four dry-bulb temperature/relative humidity multi-sensors (±0.1 C, 2%RH) were located in the downstream of the outdoor coil to measure the outlet air temperature and relative humidity. The airflow rate through the outdoor heat exchanger (evaporator), the frost accumulation mass, and the average inlet and outlet air enthalpies were determined from the average measured parameters. The frost accumulation mass in the period of Dt was calculated from

4 2270 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) DM fr ¼ q a;e A e V av Dt ðd e;i d e;o ÞðkgÞ ð1 þ d e;i Þ The heating capacity of the ASHP unit was estimated from energy balance of the ASHP system as Eq. (4) Q h ¼ q a;e A e V av ðh e;i h e;o Þ þ P for evaporator side ð4þ where the air density, the absolute humidity, and the air specific enthalpy were obtained from the psychrometric chard according to the measured dry/wet bulb temperatures. During the experiments the error between the calculated heating capacities from Eqs. (1) and (4) was maintained within 5%. The pipe wall temperatures of the outdoor heat exchanger were measured in terms of forty pre-calibrated T-type Table 1 Component parameters of test ASHP unit Parameters Values Heat exchangers Indoor Outdoor Tube length (mm) Number of columns Number of rows 4 2 Air flow rate (m 3 /h) Fin type Wavy fin Wavy fin Out tube diameter (mm) Fin thickness (mm) Fin pitch (mm) Tube spacing (mm) Scroll compressor Refrigeration capacity (kw) Input power (kw) 4.28 Capillary tube Diameter (mm) 2.96 Length (mm) 700 ð3þ copper-constantan thermocouples (±0.1 C) which were stuck on the copper elbows of the evaporator. The refrigerant-side pressures and temperatures were measured in terms of precision pressure transducers (±0.08%) and Pt 100 X RTD sensors (±0.1 C), respectively. The measuring points are shown in Fig. 2. The frost on the fins was observed and recorded by a microscope and a CCD camera every 5 min. The frost thickness was obtained from processing the images captured with the CCD camera through a microscope with a magnification of 90 times on a front view of the outdoor heat exchanger Component parameters of the prototype and experimental procedure The schematic diagram of the test ASHP system with measuring points is shown in Fig. 2. The component parameters of the test ASHP unit are shown in Table 1. There are eight and six refrigerant circuits in the outdoor and indoor heat exchanger, respectively. The tube arrangements are shown in Fig. 3. In this work the air temperature and the relative humidity in the outdoor environmental room were controlled in the ranges of 15 to 5 C and 55 90%, respectively. It was found that the frost formation did not occur or was of no significance on the cold surfaces under the conditions of 55% outdoor air relative humidity. The air temperature and the relative humidity in the indoor environmental room were controlled to be 20 C and 60%, respectively, and the air flow rate across the indoor heat exchanger was kept at 2500 m 3 /h over all the experiments. Before performing the experiments, the AHUs were operated to adjust the indoor and outdoor air parameters. When the air conditions in the rooms reached to the steady state, the frosting experiments were started by running the Fig. 3. Schematic diagram of heat exchangers of test ASHP unit.

5 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) test ASHP unit. The experimental data were recorded every 6 s by the data acquisition system and the computer. 3. Results and discussion 3.1. Frost growth pattern The measured frost thickness and frost growth rate under various outdoor air conditions are shown in Fig. 4. It is noted that the frost layer grows in a manner of three stages. In the initial stage, of which the duration is about min, and the frost thickness growth rate increases with the frosting time. In the second stage the frost thickness growth rate decreases with the frosting time when the outdoor air relative humidity is above 65%, and is nearly invariable when the outdoor air relative humidity is 65%. In the period of last min of the frost growth cycle, the frost thickness rises rapidly with the frosting time, and at the end of the third stage the frost thickness growth rate can reach to times of that in the second stage. For the conditions of a fixed outdoor air temperature, the durations of the initial and third stages are nearly invariable under the outdoor air relative humidity ranging from 65% to 85%, and the duration of the second stage decreases with the increase of relative humidity. For the conditions of a fixed outdoor air relative humidity, the duration of the second stage is minimum when the outdoor air temperature is about 0 C. As shown in Fig. 4, the trend of frost growth with frosting time presents the concave-up curves in the third stage over all test conditions. This frost growth pattern is different from the simulated or experimental concave-down curves reported by Yang [5], Yao [9], Tso [11,12], and Qu [18]. In some ways the present experimental results are similar to the frost thickness curve from images reported by Xia [10], but the frost thickness grows faster in the third stage. The growth pattern of the frost thickness can be explained by the morphological variation of frost, for an example, as shown in Fig. 5 for test condition of 0 C outdoor air temperature and 75% relative humidity. In Fig. 5 Fig. 4. Frost thickness and frost thickness growth rate with frosting time under different conditions.

6 2272 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) Fig. 5. Photographs of frost layer on the fins under condition of Ta,i = 0 C, RH = 75%. the photographs (a) (c), (d) (e) and (f) (i) are corresponded to the initial stage, the second stage, and the third stage, respectively. It can be seen that in the initial stage the condensed water freezes and a transparent thin ice layer is formed on the fins firstly (Fig. 4a), after about 6 min from the beginning of frosting the granular ices appear and grow gradually on the surface (Fig. 4b), and then the columnshaped crystals are formed (Fig. 4c). In this stage the frost thickness growth rate increase with time until the columnshaped ice is formed. In the second stage, some melted water droplets seemed to be visible on the top of the ice columns, so the column-shaped crystals grow in its radius rather than in its length (Fig. 4d f). As a result, the frost thickness rises slowly. In the last about 20 min of the frost growth cycle, the column-shaped ice crystals grow mainly in its length, and the ice crystals become gradually of an acerose-shaped one (Fig. 4g i), finally a fluffy frost layer through which air can still pass across the coil is formed, so in the period the frost thickness grows very rapidly. Therefore, the three stage patterns of frost growth may be mainly attributed to the morphological variation of the frost crystals. Qu [18] reported that the frost layer thickness increased stepwise during the frost formation process on a cold plate,

7 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) and the phenomena was attributed to the column-shaped ice crystals growing alternately in its length and radius. In this work, we found the similar stepwise growth phenomena of the frost thickness, but the alternateness of slow and rapid increasing in the frost thickness occurred only once in the second and third stages. This is different from the multi-cycle pattern reported by Qu [18]. However, it is considered that the morphological variation of frost in the third stage is caused by the rapid decrease in the wall temperature and air flow rate as shown in Figs. 6 and 7. It is seen that the wall temperature and air flow rate in the third stage is significantly reduced due to the insulating and blocking effects of the frost layer, and the drops will in turn affect the frosting process. As a result, a vicious spiral of the frost growth and the decrease in the wall temperature and the air flow rate is formed. Hence the frost thickness increased rapidly in the third stage Effects of frosting on dynamic performance of ASHP system Figs. 8 and 9 show the heating capacity and COP, and the evaporating and condensing pressures of the test ASHP system as a function of the frosting time under conditions of a fixed outdoor air temperature with various relative humidity. It can be seen from Fig. 8 that the effects of the frost formation on the performance of the ASHP unit are different Fig. 6. Dynamic wall temperature of evaporator in frost growth cycle. Fig. 7. Dynamic face velocity of evaporator in frost growth cycle.

8 2274 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) in the three stages. In the initial stage the heating capacities of the ASHP unit increase with the frosting time, and reach to the peak point by the end of the initial stage. The COP curves present a similar trend with the heating capacity. The improvement in performance of the ASHP unit may be caused by two factors. Firstly, the change of the air-side conditions of the evaporator from dry to wet improves the performance. Secondly, the initial frost deposition makes the surface roughly (Fig. 5b), but its effect on the air flow rate is of no significance (Fig. 7), so the heat transfer coefficient on air-side of the evaporator is increased. Furthermore, the thermal resistance of the frost layer can be ignored due to its very small thickness. As a result, the performance of the ASHP unit is improved in the initial stage. In the second stage, the evaporating pressure and the air flow rate are slightly reduced due to the slow increasing in frost layer (Figs. 9 and 7). Therefore, the heating capacity and COP of the ASHP unit are slightly affected by frosting in this stage. As showing in Fig. 8, in the third stage, just corresponding to the rapid increase in frost thickness, the heating capacity and COP of the ASHP unit reduce rapidly. The drops per minute of the heating capacity and COP can reach to several times of these in the second stage respectively. The fact can be explained as follows. The frost morphological variation in the third stage causes the rapid increase in the frost thickness as showing above, and results in a decrease in outdoor air flow rate and a rapid increase in thermal resistance between moist air Fig. 8. Dynamic heating capacity and COP in frost growth cycle. Fig. 9. Dynamic evaporating and condensing pressures in frost growth cycle.

9 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) and cold surface. As a result, the evaporating pressure and the wall temperature decrease rapidly. The lower wall temperature will in turn result in a faster increase in the frost thickness. In this way a vicious spiral is formed. Thus the performance of the ASHP unit degrades rapidly. Miller s experimental data [15] showed that the performance of the ASHP unit decreased only marginally despite the frost accumulation on the outdoor heat exchanger. However, in our experiments it was not found that the outdoor fans operated through the surging or stalling region as observed in experiments by Miller [15]. Therefore, it is suggested that the frost morphological variation mainly caused the rapid performance degradation in the third stage Effects of outdoor air parameters on frost growth Fig. 10 presents the frost thickness and frost accumulation mass after 35 min from the beginning of the frost-cycle under various test conditions. It is found that the frost thickness and the frost accumulation mass is maximized when the outdoor air temperature is about 0 C with a fixed relative humidity. Further more, the maximum frost thickness and the frost accumulation mass are formed at the same outdoor temperature for different outdoor air relative humidity. In other words, there is a peak frosting outdoor air temperature, under which the frost growth rate is maximum. The experimental peak frosting outdoor air temperature for the test ASHP unit is about 0 C at all outdoor air relative humidity level. The phenomena can be explained as follows. When the outdoor air temperature is increased with a fixed relative humidity, both of the air absolute humidity and the wall temperature of the evaporator will increase correspondingly. The increase in the air absolute humidity is benefit to frost formation on the cold surface, but the increase in the wall temperature makes against frost formation. Therefore, there is a most favorable condition for frost formation. Fig. 10. Comparison of frost accumulation mass and frost thickness after 35 min operating.

10 2276 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) Comparison of experimental data with simulated results The present authors developed a quasi-steady model to predict the dynamic performance of a heat pump system under frosting conditions [16,17]. The two-phase flow distributed model was used for the analysis of refrigerant flow in heat exchangers, and was coupled with a frost model for studying the dynamic behavior of the evaporator. The flow in the capillary tube was considered as homogeneous, and the delay of flash point was included quantitatively. The fitted performance curves of the compressor and the outdoor heat exchanger fans were used in the model to determine the refrigerating capacity, the power consumption, the refrigerant mass flow rate of the compressor, and the air flow rate of the fans, respectively. The model and simulation method were described in detail by Chen et al. [16] and Guo et al. [17]. In this paper the dynamic performance of the test ASHP unit under frosting conditions was predicted by using the presented model. The simulated results under the condition of 0 C outdoor air temperature and 75% relative humidity were compared with the experimental results as shown in Figs. 11 and 12. Fig. 11 presents a comparison between the experimental and simulated frost accumulation mass and frost thickness. It is noted that the simulated frost accumulation mass increases linearly with the frosting time, and is in agreement with the experimental data. The experimental frost accumulation mass increases faster within the prior Fig. 11. Comparison of experimental frost accumulation mass and frost thickness with simulated results under condition of T a,i =0 C, RH = 75%. Fig. 12. Comparison of experimental heating capacity and COP with simulated results under condition of T a,i =0 C, RH = 75%.

11 X.-M. Guo et al. / Applied Thermal Engineering 28 (2008) min and then slower than simulated data. The simulated frost thickness grows linearly with the frosting time before its rapidly increasing at about 57 min. It is found that the experimental frost growth rate in the first and third stages is greater than the simulated one, especially in the third stage, the experimental frost growth rate is about 2 3 times of the simulated one. As described above, in the third frosting stage the ice crystals on the fins and pipes changes gradually to an acerose-shaped, finally a fluffy frost layer, therefore the frost thickness grows rapidly. However, the morphological variation of the frost crystals is not taken into account in the simulation model. As a result, the maximum error between the simulated and experimental frost thickness reaches to about 25% in the third frosting stage. It is found from Fig. 12 that the trend of the simulated dynamic performance of ASHP unit is consistent with the experimental results. The error between the experimental data and simulated results is within 8%. In the initial frosting stage, the experimental heating capacity and COP increase faster than simulated one. The causes may be that in the simulation model the frost growth is assumed in a fully developed period and the frost formation starts from the steady state of the system. In the experiments frost formation occurs before its reaching to the steady state of the test HP system. In the third stage the experimental performance of the ASHP unit decline faster than simulated results, the decreasing rate of the experimental heating capacity is about two times of that of the simulated data. As mentioned above, the rapid falling in the performance of ASHP unit can be attributed to the rapid frost growth caused by the variation of frost morphology. 5. Conclusions The frost growth on the outdoor heat exchanger and the dynamic performance of an ASHP unit under frosting conditions was experimentally investigated with consideration of the fan performance curves and the variable refrigerantside parameters of the system. The effects of the outdoor air parameters on the frost growth as well as the performance of the ASHP unit are discussed. The experimental results are compared with the simulated data. (1) The frost layer on the outdoor heat exchanger grows in a manner of three stages. In the initial stage, the frost layer goes through a transparent thin ice layer and a granular ice layer, at last form a columnshaped ice crystal layer. The frost thickness growth rate increase with the frosting time in this stage. The performance of the ASHP unit is improved due to the rough surface formed by the initial frost layer. After the first stage, the column-shaped ice crystals on the frost surface grow in its radius rather than in its length, therefore, the frost thickness growth rate decreases or keeps nearly at a constant value with the frosting time. The heating capacity and COP of the test ASHP unit are only slightly affected by frosting on the outdoor coils due to a slow frost thickness growth. In the third stage the column-shaped ice crystals on the frost surface grow in its length rather than in its radius, thus the acerose-shaped ice crystals on the frost layer are formed, and the frost thickness growth rate increases rapidly to about times of that in the second stage. The performance of the ASHP unit decays rapidly, and the decreasing rates of the heating capacity and COP can reach to a level of several times of these in the second stage. The experimental results suggested that the rapid performance degradation is mainly caused by the morphological variation of the frost layer in the third stage. (2) The duration of the initial and the third stages are nearly a constant value under conditions of variable outdoor air relative humidity, but the duration of the second stage is longer with lower outdoor air relative humidity. (3) The experimental results indicate that there is a peak frosting outdoor air temperature, under which the frost growth rate is maximized under variable outdoor air relative humidity conditions. For the test ASHP unit the peak frosting outdoor air temperature is about 0 C at all outdoor air relative humidity level. (4) Experimental results are in agreement with the corresponding simulation data except in the third frosting stage. Acknowledgement The financial support of this work by Tianjin Municipal Science and Technology Commission, China, under Grant No. 06YFJMJC05500 is gratefully acknowledged. 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