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Experimental study on multi-split air conditioner with digital scroll compressor
Applied Thermal Engineering 31 (2011) 2449e2457
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on multi-split air conditioner with digital scroll compressor Qiu Tu a, b, *, Kaijun Dong a, b, Deqiu Zou c, Yongman Lin a, b a
Guangzhou Institute of Energy Conversion, Chinese Academy of Science, Guangzhou 510640, China Key Laboratory of Renewable Energy and Gas Hydrate, Chinese Academy of Science, Guangzhou 510640, China c Faculty of Maritime, Ningbo University, Ningbo 315211, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2011 Accepted 7 April 2011 Available online 16 April 2011
A set of 10 horse powers (HP) multi-split air conditioner with a digital scroll compressor and a standard compressor was designed. The control strategy of compressor switching and the control model of compressor output capacity were presented. The output ratio of the digital compressor was determined in the control strategy. Furthermore, in the control model of compressor output capacity, considering the pressure disturbance, a certain suction pressure range was used as the pressure control target to adjust the output capacity. Experiments show that the system ran unstably, the standard compressor frequently appeared oneoff phenomenon, and some operation parameters such as the output ratio of the digital compressor, the opening of electric expansive valve (EEV), the cooling capacity and the air outlet temperature fluctuated. A new control model of the output capacity was proposed, in which the average suction pressure in an adjustment period was taken as the current pressure to regulate the output capacity. Experimental results demonstrate that the control model can be used to solve the frequent oneoff phenomenon of the standard compressor and oscillation of the operation parameters, realize control accuracy of less than 0.2 C deviation of the air outlet temperature. 2011 Elsevier Ltd. All rights reserved.
Keywords: Multi-split air conditioner Variable refrigerant flow (VRF) Digital scroll compressor Output ratio
1. Background The multi-split air conditioner, i.e., variable refrigerant flow air conditioning system (VRF AC) is a highly efficient direct evaporative air conditioning system, which is controlled by adjusting compressor output capacity and refrigerant flow to timely meet the demand for indoor thermal loads. Currently, it has been favored by more and more users because of its high energy efficiency, convenient installation, easy maintenance, centralized management, diverse indoor types and flexible layout [1e3]. Now in the markets, in accordance with the compressor driven style, there exist two kinds of systems, VRF AC with the variable speed compressor and the digital scroll compressor. Due to relatively low cost and simple electronic control, some air conditioning manufacturers have developed VRF AC with the digital scroll compressor since it was first launched by Copeland Compressor Company in 2000. The characteristics of the compressor can be described as below [4]. The digital scroll
* Corresponding author. Guangzhou Institute of Energy Conversion, Chinese Academy of Science, Guangzhou 510640, China. Tel.: þ86 20 87211743; fax: þ86 20 87057795. E-mail address: [email protected] (Q. Tu). 1359-4311/$ e see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.04.010
compressor operates in two stages- the “loaded state” when the pulse width modulation (PWM) valve in the compressor is normally closed and “unloaded state” when it is open. During the loaded state, the compressor operates like a standard scroll, and delivers full capacity and refrigerant flow. However, during the unloaded state, there is no capacity and refrigerant flow discharged through the compressor. The capacity is a time averaged summation of the loaded state and unloaded state. By varying the loaded state time and unloaded state time, any capacity (10%e100%) can be delivered by the compressor. Relative to the variable speed compressor, theoretically, it is easier to realize high reliability and stability because VRF AC with the digital scroll compressor does not need complex inverterdriven technology. However, in fact, it is not the case due to the characteristics of the compressor. So it is necessary to study the performance of VRF AC with the digital scroll compressor, resolve some technical problems, improve performance, and realize stable and reliable operation. From the surveyed literatures, the performance data and control measure are very limited. H. Huang et al. [5] collected the variable input power data of the scroll compressor under 80% and 60% output ratio, and regarded the ratio of the total capacity to the total input power during a certain sampling period as a way of evaluating standards on the efficiency of the digital scroll compressor. In fact, this dealing style is normally used in the enthalpy difference
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Nomenclature COP EER EEV Pave(i,j) Pd Pd_t Ps Ps_t Ptar Px,y PWM
coefficient of performance energy efficiency ratio electric expansive valve average suction pressure in the j-th second during the i-th adjustment period (bar) discharge pressure (bar) saturation temperature corresponding to Pd, i.e., condensation temperature ( C) suction pressure (bar) saturation temperature corresponding to Ps, i.e., evaporation temperature ( C) control pressure target (bar) current suction pressure in the y-th second during the x-th period (bar), x ¼ i, i1, ., iX; y ¼ m, n,., p pulse width modulation
method. S.C.Hu et al. [6] developed a set of VRF AC with 5 indoor units. They built the regression functions of the relationship between the opening of EEVs and the compressor output ratio to form the basic control parameters of the system. And the experimental results show that the developed system has a broader range of capacity output (from 17%w100%) than that of a variable speed air conditioning system (from 48%w104%). H. Huang et al. [7] experimentally studied the effects of air volume on the heating capacity, input power, coefficient of performance (COP), air outlet temperature, discharge pressure, discharge temperature and suction pressure of the air conditioner with the digital scroll compressor. Experimental results show that the air conditioner with the digital scroll compressor is well adapted to the variable air volume running and good for the economical and reliable operation of the system. And they thought that the outlet temperature of air conditioner was stable, and the suction pressure and discharge pressure were normal. In fact these presented performance parameters are the average values during a certain running period, and these experimental results do not really reflect the characteristics of the digital scroll compressor. Similarly, D.L. Zhang et al. [8] experimentally analyzed the overall effects of outdoor air temperature and on-unit ratio on the total power consumption of VRF AC with the digital scroll compressor in 1 h, hourly COP, and obtained the variations of hourly COP and total power consumption in 1 h with part load value under different outdoor air temperatures. The results indicate that the system can keep economical and reliable operation under part load conditions. The primary objective of this study is to investigate the performance of the VRF AC with the digital scroll compressor, analyze the affected factors, and propose the control measure to ensure reliable operation. It is hoped that this study will stimulate further research on this topic.
2. Experimental phenomenon and analysis 2.1. Experimental setup In order to test the performance of VRF AC with the digital scroll compressor, a set of 10 HP VRF AC experimental setup was designed to make cooling experiment. Two compressors, one digital scroll compressor and one standard compressor, were installed in the system. The type of the outdoor unit is GMV-R300W2/B-N2, whose rated cooling capacity is 30 KW, 10 HP. The system was charged
P0 Rmin Rmax Tai Tao Tc1 Tc2 Tdi Ti Ti,1 Ti,2
DP
basic control pressure target (bar) minimal output ratio, 20% maximal output ratio, 100% ambient temperature in the indoor unit chamber ( C) ambient temperature in the outdoor unit chamber ( C) temperature in the gas pipe of the indoor unit ( C) throttled temperature in the liquid pipe of the indoor unit ( C) discharge temperature of the digital scroll compressor ( C) adjustment period (s) loaded state time of PWM valve during an adjustment period (s) unloaded state time of PWM valve during an adjustment period (s) pressure disturbance value (bar)
with 16 kg refrigerant, R22. Five indoor units, two 3 HP, two 2 HP and one 1 HP low static pressure duct indoor units, were connected in the VRF AC. The experiment was made in the enthalpy difference laboratory of Gree Electric Appliance Co. LTD. The monitoring software in the test platform was used to record the air flow, cooling capacity, power consumption, air outlet temperature, and dry bulb (DB) and wet bulb (WB) ambient temperatures of the outdoor unit chamber and indoor unit chamber of the laboratory. And the self-made monitoring software was adopted to collect the operation parameters of the outdoor unit, such as ambient temperature Tao, discharge temperature Tdi, discharge pressure Pd as well as corresponding saturation temperatures (Pd_t, i.e., condensation temperature), suction pressure Ps as well as corresponding evaporation temperature Ps_t, compressor output ratio, oneoff status of PWM valve, and some operation parameters of indoor units, such as opening of EEV, temperature Tc1 and Tc2 respectively in the gas pipe and the liquid pipe of the indoor unit. The system principle diagram, installation pattern and principle description are similar to literature [9]. 2.2. Control measure For the digital scroll compressor, the output ratio is used to evaluate the compressor output capacity, which is define as the time ratio of loaded state time divided by the period time including loaded state time and unloaded state time. It can be formulated as Ti,1/Ti in Fig. 1. The common control method used in the inverter-driven VRF AC was adopted to control compressor cooling output capacity. Namely, in cooling mode, a certain pressure value was taken as the basic parameter target to regulate the compressor output capacity to make the suction pressure of the system reach the pressure target. Considering the suction pressure disturbance, the control pressure target is revised as a certain pressure region, which is formulated as Eq. (1). The significance of this control strategy has been verified by the experiment [9]. Ti
Ti+1
Ti
1
Current Ps
ON OFF
Ti,1
Ti,2
PWM valve ON/OFF
Fig. 1. Concept diagram of period time and output ratio of digital scroll compressor.
P0
P
P0
P
Rm in
Output capacity of compressors
Q. Tu et al. / Applied Thermal Engineering 31 (2011) 2449e2457
Rm ax
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Digital and standard compressors
Digital compressor
Fig. 2. Control model diagram of VRF with digital scroll compressor.
90% 100%
20% 30% 40%
Output ratio
Ptar ˛½P0 DP ; P0 þ DP
(1)
Here Ptar, P0 and DP are respectively the control pressure target, basic pressure target and pressure disturbance. The control model diagram is shown in Fig. 2, where Rmin and Rmax are respectively the minimal and the maximal output ratio, i.e., 20% and 100%. For the system charged with refrigerant R22, the basic pressure target P0 is designed to be 4.3 bar, whose corresponding saturation temperature is 2 C. The value of the pressure disturbance cannot be larger than that of the suction pressure fluctuation amplitude. Otherwise the output capacity of the compressors cannot be adjusted. And when the lower limit value of suction pressure (P0DP) in Eq. (1) is lower than 3.96 bar, whose saturation temperature is 0 C, the gas pipe will be covered with frost. Thus the value of the pressure disturbance is determined to be DP ¼ 0.34 bar. The opening of EEV in the indoor unit was adjusted according to the superheated degree target, 1 C, where the superheated degree is defined as the temperature difference between Tc1 and Tc2. The adjustment periods under different compressor output ratios are presented by Copeland Compressor Company, shown in Table 1. Furthermore, in order to improve energy efficiency ratio (EER), in the compressor switching model, the digital scroll compressor is set as the high priority compressor. When the output ratio of the digital compressor is adjusted to be 100%, the standard compressor starts to be thermo-on. On the contrary, when the output ratio decreases to be 20%, the standard compressor stops. The switching model diagram between the digital compressor and the standard compressor is shown in Fig. 3.
2.3. Experimental phenomenon and analysis The ambient dry bulb temperature and wet bulb temperature of outdoor unit chamber and indoor unit chamber of the laboratory were adjusted to be 35 C DB/24 C WB and 27 C DB/19 C WB, respectively. The two 3 HP indoor units and only one 3 HP indoor
Table 1 Adjustment periods under different compressor output ratios. Output ratio(R) 10% 15% 25% 35% 45%
R<15% R<25% R<35% R<45% R<55%
Adjustment period(s)
Output ratio(R)
20 20 18 16 14
55% 65% 75% 85%
R<65% R<75% R<85% R<95%
Adjustment period(s) 12 10 15 25
Fig. 3. Switching model diagram between digital compressor and standard compressor.
unit were respectively turned on to make nominal cooling experiments. The experimental results are shown in Fig. 4 and Fig. 5, in which Fig. 4(a) and Fig. 5(a) are experimental data collected by the monitoring software in the test platform, and Fig. 4(b) and (c) and Fig. 5(b) and (c) are experimental curves collected by the self-made monitoring software. Considering the similar experimental results of the two 3 HP low static pressure duct indoor units, the monitored running curves of only one 3 HP indoor unit are shown in Fig. 4(c). In Fig. 4 (a), Pressure 1 and Pressure 2 are respectively collected pressure values at the liquid pipe stop valve and gas pipe stop valve of the outdoor unit. It can be seen from Fig. 4 (a) and (b) that the system run unstably, and the standard compressor periodically appeared oneoff phenomenon. When the output ratio reached 100%, namely, the PWM valve was closed, the system ran stably, and some operation parameters such as power, cooling capacity, discharge pressure and suction pressure were basically kept constant. But the pressure target could not be reached, so the standard compressor started to run and the output ratio of the digital compressor decreased. And then these operation parameters fluctuated obviously, in which the power oscillated from 5700 Ww9800 W, and the fluctuation amplitude reached 41.8%. On the other hand, when the two compressors ran, the suction pressure was lower than the pressure target. The output capacity decreased in order to reach the control target. And when the output ratio of the digital compressor dropped to be 20%, the standard compressor was thermo-off. So the standard compressor ran in frequent oneoff status. Furthermore, Fig. 4(c) shows that the temperature Tc1 and Tc2 constantly fluctuated as a result of unsteady output capacity of compressor; subsequently, the opening of the indoor unit was frequently adjusted to satisfy the superheated degree target, 1 C. Ultimately, the air outlet temperatures of the indoor units were not stable and the fluctuation amplitude was about 0.2w0.5 C. Similarly, the same unsteady running phenomenon occurred when the only one 3 HP indoor unit was thermo-on, shown in Fig. 5. In detailed, when the PWM valve was open, the power decreased to be 1500 W. But when the PMW valve was closed, the power reached 5400 W. The fluctuation amplitude attained up to 72%. And the cooling capacity vibrated slightly due to small fluctuation of the air outlet temperature. The above fluctuation phenomenon can be explained below.
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Fig. 4. (a) Experimental results when the two 3 HP indoor units were thermo-on. (b) Experimental curves of outdoor unit operation parameters when the two 3 HP indoor units were thermo-on. (c) Experimental curves of indoor unit operation parameters when the two 3 HP indoor units were thermo-on.
Q. Tu et al. / Applied Thermal Engineering 31 (2011) 2449e2457
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Fig. 5. (a) Experimental results when the only one 3 HP indoor units was thermo-on. (b) Experimental curves of outdoor unit operation parameters when the only one 3 HP indoor units was thermo-on. (c) Experimental curves of indoor unit operation parameters when the only one 3 HP indoor unit was thermo-on.
When the output ratio of the digital compressor is less than 100%, namely, the PMV valve is in frequent oneoff status, the discharge pressure and the suction pressure fluctuate constantly. Once the PWM valve is opened, the output capacity decreases, and the discharge pressure is unloaded into the suction chamber of the compressor. Subsequently, the discharge pressure decreases and the suction pressure increases. When the PWM valve is closed, the
situation is reversed. The fluctuation of discharge pressure, suction pressure and power is determined by the characteristics of the digital compressor. On the other hand, the output ratio is controlled by PID method to attain the pressure target according to the current suction pressure. The suction pressure vibrates due to the frequent oneoff action of PWM valve. So the output ratio is adjusted constantly to
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Fig. 6. Experimental curves of outdoor unit operation parameters under different output ratios.
make the variable suction pressure reach the pressure target. And the refrigerant flow oscillates as a result of the variable output capacity of the compressor. Subsequently, the opening of EEV is adjusted accordingly, then Tc1 and Tc2 vibrate in order to reach superheated degree target, 1 C. Thus it is necessary to improve the control method to solve the unstable running of the system. In order to further test the impact of the oneoff action of the PWM valve on the system operation, the output ratios from 95% to 55%, 5% interval step, and then directly to 25%, were respectively regulated from the monitoring software to make the system run for 5e6 min every 5% interval step. The experimental results are shown in Fig. 6. Thirty maximal values and minimal values of the suction pressure were drawn from the experimental curves at every output ratio. The average values of the maximal and minimal suction pressure, and the pressure difference between them are depicted in Fig. 7. Fig. 7 shows that the suction pressure difference is about 0.9 bar, larger than the double values of the pressure disturbance defined in Eq. (1). So the control method is not conducive to the stable and reliable operation of the system.
fluctuate due to the variable suction pressure, which is adopted to adjust the output capacity of the compressors. In order to solve the problem, the average value of suction pressure in a current regulation period can be used to regulate the output capacity to attain the pressure target. The improved control model of output ratio of the digital compressor is shown in Fig. 8. Assuming that the average suction pressure in the j-th second during the i-th adjustment period is Pave(i,j), the adjustment period Ti is determined by Copeland Compressor Company, shown in Table 1, the average pressure Pave(i,j) equals the ratio of the total suction pressures during the period span to the adjustment period Ti. Numerically, Ti can be regarded as the sampling number because the suction pressure is sampled once every second in the control program. Here, i, j can be respectively understood as the period sequence and the sampling sequence during the current period. Considering that the periods are different under different output ratios, the foregoing one or some suction pressures during the preceding period(s) may be adopted to calculate the average pressure. The expression of Pave(i,j) can be written as: m¼1 X
3. Improved control measure From the above analysis, the output ratio of the digital compressor, opening of EEV and air outlet temperature frequently
Suction pressure and pressure difference (bar)
Pði1;nÞ þ/ þ
p¼r X
PðiX;pÞ
p¼TiX
n¼Ti1
Ti
(2)
And
(3) And i, j, k, m, n, p, r and X are integers. Here Px,y (x ¼ i,i1,.,iX; y ¼ m,n,.,p) is the current suction pressure in the y-th second during the x-th period. Two timers are defined in the control program. The first one is used to record the period sequence and the second one is adopted to count the sampling series. When the current adjustment period finishes and the next period comes, the second timer is cleared and recounts the sampling series.
5 4
2
n¼k X
j1þðTi1 kþ1Þþ/þðTiX rþ1Þ ¼ Ti andkTi1 ; r Tik
6
3
Paveði;jÞ ¼
m¼j1
Pði;mÞ þ
Ps_max Ps_min Ps_diff
1
Ti
1
Ti
Ti
1
Current Pave(i,j)
0 25%
35%
45%
55% 65% Output ratio
75%
85%
95% Current suction pressure sampled every second
Fig. 7. The maximal, minimal suction pressure and pressure difference under different output ratios.
Fig. 8. Improved control model of output ratio.
Q. Tu et al. / Applied Thermal Engineering 31 (2011) 2449e2457
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Fig. 9. (a) Experimental results by improved control measure when the only one 3 HP indoor unit was thermo-on. (b) Experimental curves of outdoor unit operation parameters by improved control measure when the only one 3 HP indoor unit was thermo-on. (c) Experimental curves of indoor unit operation parametres by improved control measure when the only one 3 HP indoor unit was thermo-on.
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Fig. 10. (a) Experimental curves of outdoor unit operation parameters by improved control measure when the two 3 HP indoor unit were thermo-on. (b) Experimental curves of indoor unit operation parameters by improved control measure when the two 3 HP indoor units were thermo-on.
Although the approach cannot make the current average suction pressure equivalent at every moment, it can make the suction pressure fluctuation amplitude oscillate within the pressure disturbance value defined in Eq. (1). Thus the output ratio does not fluctuate frequently because the current average suction pressure is almost constant. 4. Experimental results and discussion The 3 HP indoor unit was thermo-on to make cooling experiment in terms of the improved control measure under the same ambient temperatures. The collected experimental results are shown in Fig. 9. It can be seen from Fig. 9 that although the suction pressure and the corresponding saturation temperature oscillated with oneoff action of the PWM valve, the average values of the suction pressure and those of the saturation temperature were almost stable, thus the output ratio remained at 77.4%. At the start phase, the opening of EEV, Tc1, and Tc2 were regularly adjusted. They were kept stable after the system started to run for 3 min. And the control accuracy of less than 0.2 C deviation of the air outlet temperature could be reached.
Similarly, the two 3 HP indoor units were thermo-on to make nominal cooling experiment. The experimental results are shown in Fig. 10. It can be seen from Fig. 10 that the standard compressor kept running, the output ratio of the digital compressor almost remained at 33.8%, and the other operation parameters such as Tc1, Tc2 and the opening of EEV did not fluctuate. Furthermore, other combinations of indoor units were turned on to make cooling experiments by improved control measure under different ambient temperatures of outdoor unit chamber and indoor unit chamber. A large number of experimental results demonstrate the feasibility of the control strategy. 5. Conclusions From this study, the following conclusion can be drawn. The control strategy is a key technology for VRF AC with the digital scroll compressor to ensure stable and reliable running. The common control measure of VRF AC cannot be directly applied in the system due to the characteristics of the digital compressor. The experiment tested the performance of VRF AC with the digital compressor. When the PWM valve was closed, the system
Q. Tu et al. / Applied Thermal Engineering 31 (2011) 2449e2457
ran stably, and some operation parameters such as power, cooling capacity, discharge pressure and suction pressure were basically kept constant. However, when the PWM valve was open, the system ran in unstable status, in detail, the standard compressor periodically appeared oneoff phenomenon and the operation parameters fluctuated obviously. According to the characteristics of the digital compressor, the effective control measure, including the output ratio control model and compressor switching method, was proposed. And the experiment results demonstrate that the control measure can realize the reliable running of the system and obtain the accuracy of less than 0.2 C deviation of the air outlet temperature. Acknowledgement The authors would like to thank the financial support from “863” Project (2007AA05Z224) of the Ministry of Science and Technology of the People Republic of China, and experimental support from Gree Electronic Appliance Co. LTD, Zhuhai City, China.
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References [1] J.M. Choi, Y.C. Kim, Capacity modulation of an inverter-driven multi-air conditioner using electronic expansion valves, Energy 28 (2003) 141e155. [2] Y.P. Zhou, J.Y. Wu, R.Z. Wang, S. Shiochi, Energy simulation in the variable refrigerant flow air-conditioning system under cooling conditions, Energy and Buildings 39 (2007) 212e220. [3] T.N. AYNUR, Variable refrigerant flow systems: a review, Energy and Buildings 42 (2010) 1106e1112. [4]. [5] H. Huang, Q. H.Li, Z.B. Zhang, Analysis on the performance evaluation of multiconnected air condition unit with digital scroll compressor, Journal of Nan Jing Normal University, China 8 (2008) 23e27 (engineering and technology edition). [6] S.C. Hu, R.H. Yang, Development and testing of a multi-type air conditioner without using AC inverters, Energy Conversion and Management 46 (2005) 373e383. [7] H. Huang, Qihe Li, D.X. Yuan, Z.C. Qin, Z.B. Zhang, An experimental study on variable air volume operation of ducted air-conditioning with digital scroll compressor and conventional scroll compressor, Applied Thermal Engineering 28 (2008) 761e766. [8] D.L. Zhang, X. Zhang, J. Liu, Experimental study of performance of digital variable multiple air conditioning system under part load conditions, Energy and Buildings 43 (2011) 1175e1178. [9] Q. Tu, Z.P. Feng, K.J. Dong, Heating control strategy for variable refrigerant flow air conditioning system with multi-module outdoor units, Energy and Buildings 42 (2010) 2021e2027.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on multi-split air conditioner with digital scroll compressor Qiu Tu a, b, *, Kaijun Dong a, b, Deqiu Zou c, Yongman Lin a, b a
Guangzhou Institute of Energy Conversion, Chinese Academy of Science, Guangzhou 510640, China Key Laboratory of Renewable Energy and Gas Hydrate, Chinese Academy of Science, Guangzhou 510640, China c Faculty of Maritime, Ningbo University, Ningbo 315211, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2011 Accepted 7 April 2011 Available online 16 April 2011
A set of 10 horse powers (HP) multi-split air conditioner with a digital scroll compressor and a standard compressor was designed. The control strategy of compressor switching and the control model of compressor output capacity were presented. The output ratio of the digital compressor was determined in the control strategy. Furthermore, in the control model of compressor output capacity, considering the pressure disturbance, a certain suction pressure range was used as the pressure control target to adjust the output capacity. Experiments show that the system ran unstably, the standard compressor frequently appeared oneoff phenomenon, and some operation parameters such as the output ratio of the digital compressor, the opening of electric expansive valve (EEV), the cooling capacity and the air outlet temperature fluctuated. A new control model of the output capacity was proposed, in which the average suction pressure in an adjustment period was taken as the current pressure to regulate the output capacity. Experimental results demonstrate that the control model can be used to solve the frequent oneoff phenomenon of the standard compressor and oscillation of the operation parameters, realize control accuracy of less than 0.2 C deviation of the air outlet temperature. 2011 Elsevier Ltd. All rights reserved.
Keywords: Multi-split air conditioner Variable refrigerant flow (VRF) Digital scroll compressor Output ratio
1. Background The multi-split air conditioner, i.e., variable refrigerant flow air conditioning system (VRF AC) is a highly efficient direct evaporative air conditioning system, which is controlled by adjusting compressor output capacity and refrigerant flow to timely meet the demand for indoor thermal loads. Currently, it has been favored by more and more users because of its high energy efficiency, convenient installation, easy maintenance, centralized management, diverse indoor types and flexible layout [1e3]. Now in the markets, in accordance with the compressor driven style, there exist two kinds of systems, VRF AC with the variable speed compressor and the digital scroll compressor. Due to relatively low cost and simple electronic control, some air conditioning manufacturers have developed VRF AC with the digital scroll compressor since it was first launched by Copeland Compressor Company in 2000. The characteristics of the compressor can be described as below [4]. The digital scroll
* Corresponding author. Guangzhou Institute of Energy Conversion, Chinese Academy of Science, Guangzhou 510640, China. Tel.: þ86 20 87211743; fax: þ86 20 87057795. E-mail address: [email protected] (Q. Tu). 1359-4311/$ e see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.04.010
compressor operates in two stages- the “loaded state” when the pulse width modulation (PWM) valve in the compressor is normally closed and “unloaded state” when it is open. During the loaded state, the compressor operates like a standard scroll, and delivers full capacity and refrigerant flow. However, during the unloaded state, there is no capacity and refrigerant flow discharged through the compressor. The capacity is a time averaged summation of the loaded state and unloaded state. By varying the loaded state time and unloaded state time, any capacity (10%e100%) can be delivered by the compressor. Relative to the variable speed compressor, theoretically, it is easier to realize high reliability and stability because VRF AC with the digital scroll compressor does not need complex inverterdriven technology. However, in fact, it is not the case due to the characteristics of the compressor. So it is necessary to study the performance of VRF AC with the digital scroll compressor, resolve some technical problems, improve performance, and realize stable and reliable operation. From the surveyed literatures, the performance data and control measure are very limited. H. Huang et al. [5] collected the variable input power data of the scroll compressor under 80% and 60% output ratio, and regarded the ratio of the total capacity to the total input power during a certain sampling period as a way of evaluating standards on the efficiency of the digital scroll compressor. In fact, this dealing style is normally used in the enthalpy difference
2450
Q. Tu et al. / Applied Thermal Engineering 31 (2011) 2449e2457
Nomenclature COP EER EEV Pave(i,j) Pd Pd_t Ps Ps_t Ptar Px,y PWM
coefficient of performance energy efficiency ratio electric expansive valve average suction pressure in the j-th second during the i-th adjustment period (bar) discharge pressure (bar) saturation temperature corresponding to Pd, i.e., condensation temperature ( C) suction pressure (bar) saturation temperature corresponding to Ps, i.e., evaporation temperature ( C) control pressure target (bar) current suction pressure in the y-th second during the x-th period (bar), x ¼ i, i1, ., iX; y ¼ m, n,., p pulse width modulation
method. S.C.Hu et al. [6] developed a set of VRF AC with 5 indoor units. They built the regression functions of the relationship between the opening of EEVs and the compressor output ratio to form the basic control parameters of the system. And the experimental results show that the developed system has a broader range of capacity output (from 17%w100%) than that of a variable speed air conditioning system (from 48%w104%). H. Huang et al. [7] experimentally studied the effects of air volume on the heating capacity, input power, coefficient of performance (COP), air outlet temperature, discharge pressure, discharge temperature and suction pressure of the air conditioner with the digital scroll compressor. Experimental results show that the air conditioner with the digital scroll compressor is well adapted to the variable air volume running and good for the economical and reliable operation of the system. And they thought that the outlet temperature of air conditioner was stable, and the suction pressure and discharge pressure were normal. In fact these presented performance parameters are the average values during a certain running period, and these experimental results do not really reflect the characteristics of the digital scroll compressor. Similarly, D.L. Zhang et al. [8] experimentally analyzed the overall effects of outdoor air temperature and on-unit ratio on the total power consumption of VRF AC with the digital scroll compressor in 1 h, hourly COP, and obtained the variations of hourly COP and total power consumption in 1 h with part load value under different outdoor air temperatures. The results indicate that the system can keep economical and reliable operation under part load conditions. The primary objective of this study is to investigate the performance of the VRF AC with the digital scroll compressor, analyze the affected factors, and propose the control measure to ensure reliable operation. It is hoped that this study will stimulate further research on this topic.
2. Experimental phenomenon and analysis 2.1. Experimental setup In order to test the performance of VRF AC with the digital scroll compressor, a set of 10 HP VRF AC experimental setup was designed to make cooling experiment. Two compressors, one digital scroll compressor and one standard compressor, were installed in the system. The type of the outdoor unit is GMV-R300W2/B-N2, whose rated cooling capacity is 30 KW, 10 HP. The system was charged
P0 Rmin Rmax Tai Tao Tc1 Tc2 Tdi Ti Ti,1 Ti,2
DP
basic control pressure target (bar) minimal output ratio, 20% maximal output ratio, 100% ambient temperature in the indoor unit chamber ( C) ambient temperature in the outdoor unit chamber ( C) temperature in the gas pipe of the indoor unit ( C) throttled temperature in the liquid pipe of the indoor unit ( C) discharge temperature of the digital scroll compressor ( C) adjustment period (s) loaded state time of PWM valve during an adjustment period (s) unloaded state time of PWM valve during an adjustment period (s) pressure disturbance value (bar)
with 16 kg refrigerant, R22. Five indoor units, two 3 HP, two 2 HP and one 1 HP low static pressure duct indoor units, were connected in the VRF AC. The experiment was made in the enthalpy difference laboratory of Gree Electric Appliance Co. LTD. The monitoring software in the test platform was used to record the air flow, cooling capacity, power consumption, air outlet temperature, and dry bulb (DB) and wet bulb (WB) ambient temperatures of the outdoor unit chamber and indoor unit chamber of the laboratory. And the self-made monitoring software was adopted to collect the operation parameters of the outdoor unit, such as ambient temperature Tao, discharge temperature Tdi, discharge pressure Pd as well as corresponding saturation temperatures (Pd_t, i.e., condensation temperature), suction pressure Ps as well as corresponding evaporation temperature Ps_t, compressor output ratio, oneoff status of PWM valve, and some operation parameters of indoor units, such as opening of EEV, temperature Tc1 and Tc2 respectively in the gas pipe and the liquid pipe of the indoor unit. The system principle diagram, installation pattern and principle description are similar to literature [9]. 2.2. Control measure For the digital scroll compressor, the output ratio is used to evaluate the compressor output capacity, which is define as the time ratio of loaded state time divided by the period time including loaded state time and unloaded state time. It can be formulated as Ti,1/Ti in Fig. 1. The common control method used in the inverter-driven VRF AC was adopted to control compressor cooling output capacity. Namely, in cooling mode, a certain pressure value was taken as the basic parameter target to regulate the compressor output capacity to make the suction pressure of the system reach the pressure target. Considering the suction pressure disturbance, the control pressure target is revised as a certain pressure region, which is formulated as Eq. (1). The significance of this control strategy has been verified by the experiment [9]. Ti
Ti+1
Ti
1
Current Ps
ON OFF
Ti,1
Ti,2
PWM valve ON/OFF
Fig. 1. Concept diagram of period time and output ratio of digital scroll compressor.
P0
P
P0
P
Rm in
Output capacity of compressors
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Rm ax
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Digital and standard compressors
Digital compressor
Fig. 2. Control model diagram of VRF with digital scroll compressor.
90% 100%
20% 30% 40%
Output ratio
Ptar ˛½P0 DP ; P0 þ DP
(1)
Here Ptar, P0 and DP are respectively the control pressure target, basic pressure target and pressure disturbance. The control model diagram is shown in Fig. 2, where Rmin and Rmax are respectively the minimal and the maximal output ratio, i.e., 20% and 100%. For the system charged with refrigerant R22, the basic pressure target P0 is designed to be 4.3 bar, whose corresponding saturation temperature is 2 C. The value of the pressure disturbance cannot be larger than that of the suction pressure fluctuation amplitude. Otherwise the output capacity of the compressors cannot be adjusted. And when the lower limit value of suction pressure (P0DP) in Eq. (1) is lower than 3.96 bar, whose saturation temperature is 0 C, the gas pipe will be covered with frost. Thus the value of the pressure disturbance is determined to be DP ¼ 0.34 bar. The opening of EEV in the indoor unit was adjusted according to the superheated degree target, 1 C, where the superheated degree is defined as the temperature difference between Tc1 and Tc2. The adjustment periods under different compressor output ratios are presented by Copeland Compressor Company, shown in Table 1. Furthermore, in order to improve energy efficiency ratio (EER), in the compressor switching model, the digital scroll compressor is set as the high priority compressor. When the output ratio of the digital compressor is adjusted to be 100%, the standard compressor starts to be thermo-on. On the contrary, when the output ratio decreases to be 20%, the standard compressor stops. The switching model diagram between the digital compressor and the standard compressor is shown in Fig. 3.
2.3. Experimental phenomenon and analysis The ambient dry bulb temperature and wet bulb temperature of outdoor unit chamber and indoor unit chamber of the laboratory were adjusted to be 35 C DB/24 C WB and 27 C DB/19 C WB, respectively. The two 3 HP indoor units and only one 3 HP indoor
Table 1 Adjustment periods under different compressor output ratios. Output ratio(R) 10% 15% 25% 35% 45%
R<15% R<25% R<35% R<45% R<55%
Adjustment period(s)
Output ratio(R)
20 20 18 16 14
55% 65% 75% 85%
R<65% R<75% R<85% R<95%
Adjustment period(s) 12 10 15 25
Fig. 3. Switching model diagram between digital compressor and standard compressor.
unit were respectively turned on to make nominal cooling experiments. The experimental results are shown in Fig. 4 and Fig. 5, in which Fig. 4(a) and Fig. 5(a) are experimental data collected by the monitoring software in the test platform, and Fig. 4(b) and (c) and Fig. 5(b) and (c) are experimental curves collected by the self-made monitoring software. Considering the similar experimental results of the two 3 HP low static pressure duct indoor units, the monitored running curves of only one 3 HP indoor unit are shown in Fig. 4(c). In Fig. 4 (a), Pressure 1 and Pressure 2 are respectively collected pressure values at the liquid pipe stop valve and gas pipe stop valve of the outdoor unit. It can be seen from Fig. 4 (a) and (b) that the system run unstably, and the standard compressor periodically appeared oneoff phenomenon. When the output ratio reached 100%, namely, the PWM valve was closed, the system ran stably, and some operation parameters such as power, cooling capacity, discharge pressure and suction pressure were basically kept constant. But the pressure target could not be reached, so the standard compressor started to run and the output ratio of the digital compressor decreased. And then these operation parameters fluctuated obviously, in which the power oscillated from 5700 Ww9800 W, and the fluctuation amplitude reached 41.8%. On the other hand, when the two compressors ran, the suction pressure was lower than the pressure target. The output capacity decreased in order to reach the control target. And when the output ratio of the digital compressor dropped to be 20%, the standard compressor was thermo-off. So the standard compressor ran in frequent oneoff status. Furthermore, Fig. 4(c) shows that the temperature Tc1 and Tc2 constantly fluctuated as a result of unsteady output capacity of compressor; subsequently, the opening of the indoor unit was frequently adjusted to satisfy the superheated degree target, 1 C. Ultimately, the air outlet temperatures of the indoor units were not stable and the fluctuation amplitude was about 0.2w0.5 C. Similarly, the same unsteady running phenomenon occurred when the only one 3 HP indoor unit was thermo-on, shown in Fig. 5. In detailed, when the PWM valve was open, the power decreased to be 1500 W. But when the PMW valve was closed, the power reached 5400 W. The fluctuation amplitude attained up to 72%. And the cooling capacity vibrated slightly due to small fluctuation of the air outlet temperature. The above fluctuation phenomenon can be explained below.
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Fig. 4. (a) Experimental results when the two 3 HP indoor units were thermo-on. (b) Experimental curves of outdoor unit operation parameters when the two 3 HP indoor units were thermo-on. (c) Experimental curves of indoor unit operation parameters when the two 3 HP indoor units were thermo-on.
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Fig. 5. (a) Experimental results when the only one 3 HP indoor units was thermo-on. (b) Experimental curves of outdoor unit operation parameters when the only one 3 HP indoor units was thermo-on. (c) Experimental curves of indoor unit operation parameters when the only one 3 HP indoor unit was thermo-on.
When the output ratio of the digital compressor is less than 100%, namely, the PMV valve is in frequent oneoff status, the discharge pressure and the suction pressure fluctuate constantly. Once the PWM valve is opened, the output capacity decreases, and the discharge pressure is unloaded into the suction chamber of the compressor. Subsequently, the discharge pressure decreases and the suction pressure increases. When the PWM valve is closed, the
situation is reversed. The fluctuation of discharge pressure, suction pressure and power is determined by the characteristics of the digital compressor. On the other hand, the output ratio is controlled by PID method to attain the pressure target according to the current suction pressure. The suction pressure vibrates due to the frequent oneoff action of PWM valve. So the output ratio is adjusted constantly to
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Fig. 6. Experimental curves of outdoor unit operation parameters under different output ratios.
make the variable suction pressure reach the pressure target. And the refrigerant flow oscillates as a result of the variable output capacity of the compressor. Subsequently, the opening of EEV is adjusted accordingly, then Tc1 and Tc2 vibrate in order to reach superheated degree target, 1 C. Thus it is necessary to improve the control method to solve the unstable running of the system. In order to further test the impact of the oneoff action of the PWM valve on the system operation, the output ratios from 95% to 55%, 5% interval step, and then directly to 25%, were respectively regulated from the monitoring software to make the system run for 5e6 min every 5% interval step. The experimental results are shown in Fig. 6. Thirty maximal values and minimal values of the suction pressure were drawn from the experimental curves at every output ratio. The average values of the maximal and minimal suction pressure, and the pressure difference between them are depicted in Fig. 7. Fig. 7 shows that the suction pressure difference is about 0.9 bar, larger than the double values of the pressure disturbance defined in Eq. (1). So the control method is not conducive to the stable and reliable operation of the system.
fluctuate due to the variable suction pressure, which is adopted to adjust the output capacity of the compressors. In order to solve the problem, the average value of suction pressure in a current regulation period can be used to regulate the output capacity to attain the pressure target. The improved control model of output ratio of the digital compressor is shown in Fig. 8. Assuming that the average suction pressure in the j-th second during the i-th adjustment period is Pave(i,j), the adjustment period Ti is determined by Copeland Compressor Company, shown in Table 1, the average pressure Pave(i,j) equals the ratio of the total suction pressures during the period span to the adjustment period Ti. Numerically, Ti can be regarded as the sampling number because the suction pressure is sampled once every second in the control program. Here, i, j can be respectively understood as the period sequence and the sampling sequence during the current period. Considering that the periods are different under different output ratios, the foregoing one or some suction pressures during the preceding period(s) may be adopted to calculate the average pressure. The expression of Pave(i,j) can be written as: m¼1 X
3. Improved control measure From the above analysis, the output ratio of the digital compressor, opening of EEV and air outlet temperature frequently
Suction pressure and pressure difference (bar)
Pði1;nÞ þ/ þ
p¼r X
PðiX;pÞ
p¼TiX
n¼Ti1
Ti
(2)
And
(3) And i, j, k, m, n, p, r and X are integers. Here Px,y (x ¼ i,i1,.,iX; y ¼ m,n,.,p) is the current suction pressure in the y-th second during the x-th period. Two timers are defined in the control program. The first one is used to record the period sequence and the second one is adopted to count the sampling series. When the current adjustment period finishes and the next period comes, the second timer is cleared and recounts the sampling series.
5 4
2
n¼k X
j1þðTi1 kþ1Þþ/þðTiX rþ1Þ ¼ Ti andkTi1 ; r Tik
6
3
Paveði;jÞ ¼
m¼j1
Pði;mÞ þ
Ps_max Ps_min Ps_diff
1
Ti
1
Ti
Ti
1
Current Pave(i,j)
0 25%
35%
45%
55% 65% Output ratio
75%
85%
95% Current suction pressure sampled every second
Fig. 7. The maximal, minimal suction pressure and pressure difference under different output ratios.
Fig. 8. Improved control model of output ratio.
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Fig. 9. (a) Experimental results by improved control measure when the only one 3 HP indoor unit was thermo-on. (b) Experimental curves of outdoor unit operation parameters by improved control measure when the only one 3 HP indoor unit was thermo-on. (c) Experimental curves of indoor unit operation parametres by improved control measure when the only one 3 HP indoor unit was thermo-on.
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Fig. 10. (a) Experimental curves of outdoor unit operation parameters by improved control measure when the two 3 HP indoor unit were thermo-on. (b) Experimental curves of indoor unit operation parameters by improved control measure when the two 3 HP indoor units were thermo-on.
Although the approach cannot make the current average suction pressure equivalent at every moment, it can make the suction pressure fluctuation amplitude oscillate within the pressure disturbance value defined in Eq. (1). Thus the output ratio does not fluctuate frequently because the current average suction pressure is almost constant. 4. Experimental results and discussion The 3 HP indoor unit was thermo-on to make cooling experiment in terms of the improved control measure under the same ambient temperatures. The collected experimental results are shown in Fig. 9. It can be seen from Fig. 9 that although the suction pressure and the corresponding saturation temperature oscillated with oneoff action of the PWM valve, the average values of the suction pressure and those of the saturation temperature were almost stable, thus the output ratio remained at 77.4%. At the start phase, the opening of EEV, Tc1, and Tc2 were regularly adjusted. They were kept stable after the system started to run for 3 min. And the control accuracy of less than 0.2 C deviation of the air outlet temperature could be reached.
Similarly, the two 3 HP indoor units were thermo-on to make nominal cooling experiment. The experimental results are shown in Fig. 10. It can be seen from Fig. 10 that the standard compressor kept running, the output ratio of the digital compressor almost remained at 33.8%, and the other operation parameters such as Tc1, Tc2 and the opening of EEV did not fluctuate. Furthermore, other combinations of indoor units were turned on to make cooling experiments by improved control measure under different ambient temperatures of outdoor unit chamber and indoor unit chamber. A large number of experimental results demonstrate the feasibility of the control strategy. 5. Conclusions From this study, the following conclusion can be drawn. The control strategy is a key technology for VRF AC with the digital scroll compressor to ensure stable and reliable running. The common control measure of VRF AC cannot be directly applied in the system due to the characteristics of the digital compressor. The experiment tested the performance of VRF AC with the digital compressor. When the PWM valve was closed, the system
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ran stably, and some operation parameters such as power, cooling capacity, discharge pressure and suction pressure were basically kept constant. However, when the PWM valve was open, the system ran in unstable status, in detail, the standard compressor periodically appeared oneoff phenomenon and the operation parameters fluctuated obviously. According to the characteristics of the digital compressor, the effective control measure, including the output ratio control model and compressor switching method, was proposed. And the experiment results demonstrate that the control measure can realize the reliable running of the system and obtain the accuracy of less than 0.2 C deviation of the air outlet temperature. Acknowledgement The authors would like to thank the financial support from “863” Project (2007AA05Z224) of the Ministry of Science and Technology of the People Republic of China, and experimental support from Gree Electronic Appliance Co. LTD, Zhuhai City, China.
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