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Experimental study of a control strategy for a cascade air source heat pump water heater
Accepted Manuscript Experimental study of a control strategy for a cascade air source heat pump water heater Qu Minglu, Fan Yanan, Chen Jianbo, Li Tianrui, Li Zhao, Li He PII: DOI: Reference:
S1359-4311(16)31535-6 http://dx.doi.org/10.1016/j.applthermaleng.2016.08.176 ATE 8975
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
25 April 2016 26 August 2016 27 August 2016
Please cite this article as: Q. Minglu, F. Yanan, C. Jianbo, L. Tianrui, L. Zhao, L. He, Experimental study of a control strategy for a cascade air source heat pump water heater, Applied Thermal Engineering (2016), doi: http://dx.doi.org/ 10.1016/j.applthermaleng.2016.08.176
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental study of a control strategy for a cascade air source heat pump water heater Qu Minglu, Fan Yanan, Chen Jianbo*1, Li Tianrui, Li Zhao, Li He School of Environment & Architecture, University of Shanghai for Science & Technology No.516, Jungong Road, Shanghai, China
ABSTRACT Cascade air source heat pump water heater is suitable for provide high temperature water under a wide range of outdoor air temperature. Most previous studies focused on the operating characteristics of cascade heat pump system, performance improvement, and the selection of optimal immediate temperature. However, no reported studies on the control strategies for cascade heat pump may be identified. This paper proposed a control strategy for cascade air source heat pump water heater to adjust the load variation. The principle that two stage compression ratio is approximately same is used to determine the intermediate pressure. The intermediate pressure, degree of superheat (DS) and the evaporating temperature were regulated by the compressor, electronic expansion valves (EEV) and the evaporator fan of the low temperature (LT) cycle, respectively, through Proportion Integration Differentiation (PID) controller or proportional controller. The control strategy was experimentally tested over a wide operational range of cascade air source heat pump water heater. *
Corresponding author. Tel.: +86 13901735531; fax: +86 021 55270686. E-mail address: [email protected] 1
The results showed that the controller developed successfully helped realize the control of the intermediate pressure, the DS and the evaporating temperature in terms of control accuracy and sensitivity.
Keywords: Cascade heat pump; Control strategy; PID; Compressor; Experimental
1. Introduction Energy saving has become a significant issue to be addressed globally for achieving not only sustainable development but also the reduction of greenhouse gas emission worldwide. Being energy-saving and environment-friendly, air source heat pump water heater (ASHPWH) have been widely used. ASHPWH can provide the same hot water with two to three times higher efficiency compared to a traditional boiler using fossil fuels or an electric water heater [1,2], However, ASHPWH unit is not suitable for extremely cold winter condition, due to its deteriorations in both operating performance and energy efficiency. Several methods that improving the performance of ASHPs under low temperature have been reported over the past years, i.e., using ejector compression heat pump [3-5], solar-assisted heat pump [6,7], two-stage compression heat pump [8, 9], cascade heat pump, etc.
Cascade heat pump system uses two pairs of compressor plants, working individually with different refrigerants to obtain the improved condensing temperature and the reduced evaporating temperature. Its advantage lies in both kinds of refrigerant work 2
within their best working temperature ranges. Cascade air source heat pump water heater is suitable for provide high temperature water under a wide range of outdoor air temperature, even severely cold outdoor air conditions. A large number of experimental and theoretical investigations on cascade heat pump have been reported. Kaushik et al. [10] presented finite time thermodynamic optimization of irreversible cascaded refrigeration and heat pump cycles having source/sink thermal reservoirs of finite heat capacitance. The effects of various operating parameters on the cooling and heating performance of the cascaded refrigeration and heat pump cycles were reported. Park et al. [11] established a steady-state cascade heat pump model to investigate the transient behavior of the system. The heating capacity, the power consumption, the COP, and the temperature distribution in a storage tank were obtained by the model. Wu et al. [12] experimentally investigated a cascade air source heat pump water heater with phase change material (PCM) for thermal storage application to ensure the reliable operation under various weather conditions. Jung et al. [13] compared cascade multi-functional heat pump with a single-stage multi-functional heat pump. The experimental results suggested that the cascade multi-functional heat pump adopting the water heating unit showed more stable air and water heating operations and higher water outlet temperatures than the single-stage multi-functional heat pump. Chae et al. [14] tested a water-to-water cascade heat pump to investigate the effects of high stage refrigerant charge amount on the performance in a steady state and heating mode operation. The variations of temperature difference between the condensing temperature of the low stage cycle and the evaporating temperature of the high stage 3
cycle at cascade heat exchanger, heat transfer rate and COP with the high stage refrigerant charge amount were reported.
The high temperature (HT) cycle and low temperature (LT) cycle are running independently in cascade heat pump, and the intermediate heat exchanger is a key component connecting to the two cycles. The selection of intermediate pressure/temperature determines the compression ratio and the isentropic efficiency of the compressor in HT and LT cycle, and meanwhile, affects the system performance. Therefore, the selection of optimal intermediate temperature has drawn great research interest of cascade heat pump. Bhattacharyya et al. [15] established a thermodynamic model of CO2/C3H8 cascade refrigeration-heat pump system to explore the optimum allocation of heat exchanger inventories in cascade refrigeration cycles for the maximization of performance and minimization of system cost. Kim et al. [16] experimentally and numerically studied the optimal intermediate temperature for the air-water cascade heat pump using R134a/R410A. It was concluded that the optimal intermediate temperature was determined by the high temperature condensing temperature, low temperature evaporating temperature, temperature difference and system efficiency of HT cycle and LT cycle.
However, to date, there has been little research about what and how to control the cascade heat pump. Jung et al. [17] experimentally tested the performance of a cascade multi-heat pump to provide simultaneous air heating, air cooling, and water 4
heating. The compressor rotation speed and EEV opening were adjusted under four operation modes for better performance. Kim et al. [18] studied the effect of the water temperature lift on the performance of cascade heat pump water heater. The cascade heat pump was operated to meet the constant heating capacity and target intermediate temperature by Proportion Integration Differentiation (PID) control of HT and LT compressor speed.
Most previous studies focused on the operating characteristics of cascade heat pump system, performance improvement, and the selection of optimal immediate temperature. However, no reported studies on the control strategies for cascade heat pump may be identified. In this study, the control strategy of cascade air source heat pump water heater was firstly introduced: the compressor speed, evaporator fan speed and EEV opening of LT cycle were adjusted by PID controller or proportional controller to meet the load variations. The control strategy was experimentally tested over a wide operational range of cascade air source heat pump water heater. Then the control performances were experimentally investigated. Experimental results revealed that the controller developed successfully helped realize the control of the intermediate pressure, the DS and the evaporating temperature in terms of control accuracy and sensitivity.
2. System description The experimental cascade air source heat pump water heater unit was specifically 5
built up for carrying out the experimental work reported in this paper. The test unit was designed to operate under condensing temperature of -40~20 oC, and output hot water temperature of 80 oC, with heating capacity of 11.0 kW. The low temperature cycle (R410A) absorbs the heat from ambient air and transfers heat to high temperature cycle (R134a). A schematic diagram of a cascaded heat pump system has been illustrated in Fig. 1. In the cascade heat pump, the HT cycle consisted of a fixed speed compressor, an intermediate heat exchanger (heat exchange between R410A and R134a), a condenser (heat exchange between R134a and water) and a thermostatic expansion valve (TEV). The LT cycle consisted of a DC inverter compressor, an intermediate heat exchanger, and an electronic expansion valve (EEV), an air cooled evaporator, and a DC inverter evaporator fan. The specifications are listed in Table 1.
Fig.1 The schematic diagram of the cascade heat pump water heater system
Table 1 Specifications of the experimental cascade heat pump
The cascaded heat pump system was placed in the psychometric room which could provide a stable outdoor condition. Meanwhile, the psychometric room also provided stable cold/hot water to the condenser according to the experimental conditions.
3. Control strategy 6
The intermediate condensing pressure was determined by the principle of same compression ratio between the high and low level temperature cycle under different outdoor conditions and hot water temperature [19]. The compressor speed, the evaporator fan speed and the EEV opening were regulated through PID controller or proportional controller to maintain the intermediate pressure, evaporating temperature and the degree of superheat (DS) in LT cycle.
3.1 Determination of the intermediate pressure
Fig. 2 shows the change of pressure-enthalpy (P-h) diagram of the cascade cycle. To determine the target intermediate pressure under different outdoor conditions and heat load conditions, the followings were assumed: 1) Pressure losses of the refrigerant pass through the heat exchanger and the pipes were neglected. 2) Heat exchanges between the heat exchanger, the pipes and the ambient air were neglected. 3) The temperature difference between the evaporating temperature in the HT cycle and the condensing temperature in the LT cycle was 5oC.
Fig.2 System logp-h diagram
The calculation equations for the target intermediate pressure are shown as follows: The refrigerant flow rate in the HT/LT cycle, mrH and mrL, can be evaluated by: 7
(1)
(2)
where QcH and QeL are the heating capacity of the HT cycle, and the cooling capacity of the LT cycle, respectively; h 2, h 3 are the enthalpies of refrigerant at the discharge point of the compressor and at the outlet of the condenser of the LT cycle, respectively; h2′ , h3′ are the enthalpies of refrigerant at the discharge point of the compressor and at
the outlet of the condenser of the HT cycle, respectively.
The power input in the HT/LT cycle, PeH and PeL, can be obtained by:
(3) (4)
where h 1 and h1′ are the enthalpies of refrigerant at the suction point of the LT cycle and HT cycle, respectively.
The pressure ratio of the HT/LT cycle, ComH, ComL, can be evaluated by:
(5) 8
(6)
Where pcH , peH are the condensing pressure and evaporating pressure in the HT cycle, respectively, and pcL, peL are the condensing pressure and evaporating pressure
in the LT cycle, respectively.
According to the condensing temperature in the HT cycle and the evaporating temperature in the LT cycle, the evaporating temperature in the HT cycle and the condensing temperature in the LT cycle can be determined based on the principle of the same pressure ratio between the high and low temperature cycle. The target intermediate pressure equals the discharge pressure according to Assumption 1).
3.2 Control method The intermediate pressure is regulated by the compressor speed of the LT cycle, realized by incremental closed-loop feedback PID controller, to maintain the stable operation of cascade heat pump system. Based on the measured intermediate pressure and the target intermediate pressure obtained from Eqs.(1-6), programmable logic controller (PLC) output the voltage signal (0.7V~4.05V) to the DC inverter (110Hz～ 25Hz) to regulate the compressor speed. The specific control flow diagram is shown in Fig.3.
The EEV used in LT cycle controls the refrigerant flow through the evaporator by 9
means of monitoring both refrigerant pressure and temperature at compressor suction. The pressure is used to determine the evaporating temperature, and then difference between the suction temperature and the evaporating temperature is the operating DS. The opening of the EEV in the LT cycle is controlled by a conventional closed-loop feedback PID DS controller with a DS setting of 15 oC.
Fig.3 Flow chart of the cascade air source heat pump water heater control flow diagram
The evaporating pressure of the LT cycle is regulated by the DC inverter fan through open loop proportional control method. When the outdoor air temperature is below -20oC, the evaporator fan is operated under its maximum speed, 1000 rpm. When the outdoor air temperature is above -20oC, the relationship of the evaporating pressure and the fan speed are illustrated in Fig.4. As seen, when the outdoor temperature is decreased, the fan speed is accelerated to enhance the heat transfer between the evaporator and the ambient air. Based on the measured evaporating temperature of the LT cycle and the outdoor air temperature, the proportional controller output DV voltage of 0~5 V, corresponding to the evaporator fan speed of 0~1000rpm.
Fig.4 The relationship between the fan speed of outdoor evaporator and the evaporating temperature
The compressor of the HT cycle controls the hot water temperature by on-off running 10
according to the setting temperature.
4. Experiments
4.1 Experiment method According to the related criteria and actual application [20], the control performance was investigated under the inlet hot water temperature of 65 oC to 80 oC, and the outdoor temperature range was from -5 oC to 15 oC. Comparing to the slowly varying environmental temperature, the water inlet temperature changes rapidly. When the water inlet temperature changes, the condensing temperature and the target intermediate temperature changes accordingly, and therefore the disturbance is introduced. Therefore, controllability test was carried out under fixed outdoor air temperatures and varied water inlet temperatures. Two groups of experiment were carried out. Group A: investigate the controllability of the HT cycle when the water inlet temperature varied. Group B: investigate the controllability of the LT cycle when the water inlet temperature varied. The experimental conditions for the cascade air source heat pump are illustrated in Table 2.
4.2 Instrumentation Pre-calibrated T-type thermocouples (of ±0.3 oC accuracy) were used for measuring the refrigerant temperatures. Refrigerant pressures were measured using pressure transmitters with an accuracy of ± 0.3% of full scale reading. The temperature sensors for the hot water at the inlet/outlet were of platinum Resistance Temperature Device 11
(RTD) type (PT100, Class A) with a pre-calibrated accuracy of ± 0.1 oC. The electric current of the system were measured using a digital power meter with a reported accuracy of ± 0.1 % of reading. The outdoor air temperature was obtained by using the existing measuring instruments of the psychometric room.
All sensors and measuring devices were able to output direct current signals of 4–20 mA or 1–5 V which were transferred to a data acquisition unit for logging and recording.
5. Experimental results
5.1 Group A Figs.5-7 illustrate the hot water outlet temperature, discharge and suction temperature of HT cycle and the system running current value when the outdoor temperature maintained at 15 oC and the inlet water temperature varied from 65 oC to 75 oC.
Fig.5 The variations of the hot water temperature in Group A1
As seen in Fig.5, the system was running stably when the inlet water temperature maintained at 65
o
C, with the steady state error within ±0.5 oC. At 30 min,
the setting inlet water temperature was changed to 70 oC, which was considered as the first heat load disturbance. It can be seen that the hot water outlet temperature increased with the increase of water inlet temperature in the initial 10 min. At 40 min, the hot water outlet temperature reached its maximum, ~ 76.2 oC, and the water inlet 12
temperature also reach its maximum, ~70.6 oC. Then, the water inlet and outlet temperatures gradually tended to be stable at 75.8 oC and 70 oC, respectively, with the same settling time of 17 minutes. At 70 min, the setting water inlet temperature was changed to 75 oC, after 2 minutes, both the water inlet and outlet temperature began to rise, until the 79th minute the water inlet and outlet temperature reached the maximum, 75.5 oC and 81.3 oC, respectively. After that, water inlet and outlet temperatures were stable at 75 oC and 81 oC, respectively, with the same settling time of 17 minutes..
Fig.6 The variations of the suction and discharge temperature of HT cycle in Group A1
As can be seen from Fig.6, when the inlet water temperature varied from 65 oC to 70 o
C and 75 oC, the discharge temperature of the HT cycle varied from 81 oC to 87 oC
and 99 oC. The suction temperature of the HT cycle varied from 25 oC to 28 oC and 30 o
C. The discharge/suction temperature was within a reasonable range and the system
operation was stable and reliable.
Fig.7 The variations of the current of the system in Group A1
Running current is an important parameter for evaluating the safety operation of a heat pump water heater. Running current may fluctuate when the operating condition 13
varies, and current overload may occur, which would shorten the service life of compressor and fan motor, and cause the aging of the electric wire and control elements, even outbreak of fire. Therefore, in the research, the running current of the system was monitored and recorded in Fig. 7. As can be seen from Fig.7, when the inlet water temperature changed, the current changed accordingly, and when the outlet water temperature was stable, the current value also achieved a stable value, indicating that the operation of the cascade air source heat pump water heater was steadily when the inlet water temperature varied from 65 oC to 75 oC.
In order to verify the performance of the control strategy under a wider range, the controllability test was also carried out under the outdoor temperature of 10 oC with the initial setting inlet water temperature at 70 oC. Fig.8 and Fig.9 show the variations of the water inlet/outlet temperatures and discharge/suction pressures of the HT cycle with time.
The set point of the water inlet temperature was decreased to 65 oC at 20 min, and then increased to 75 oC at 83 min. It can be seen from Fig. 8 the cascade heat pump system responded quickly and reached to a new steady state. As the set point of the water inlet temperature was decreased to 65 oC at 20 min, the water inlet temperature dropped to the minimum, ~64.5 oC within 7 minute. Then it increased gradually and finally reached the set value at 34 minutes. The hot water outlet temperature also dropped to the minimum, ~69.8 oC within 7 minute, and then quickly reached a stable 14
state at 70.3 oC, with the settling time of about 17 minutes. At 83 min when the setting of the inlet water temperature changed to 75 oC, the system reached a new steady state after about 17 minutes.
Fig.8 The variations of the hot water temperature in Group A2
Fig. 9 presents the variations of suction and discharge pressure of HT cycle at the outdoor temperature of 10 oC. As seen, with the change of the water inlet temperature, it required ~10 minutes for the discharge/suction pressure of HT cycle to reach stable state. And the discharge/suction pressure fluctuated (steady-state error) within 10 kPa, and the maximum overshoot was also within the requirement of 2% ~ 5%, suggesting that it was appropriate to adopt PID control method for the intermediate pressure, and the selection of proportional coefficient was reasonable.
Fig.9 The variations of suction and discharge pressure of HT cycle in Group A2
Based on the experimental results of Group A, it can be seen that the HT cycle of the cascade heat pump water heater system quickly adapted to the variation of the load as the LT compressor responded immediately to the changes in set points, and the control strategy was accurate and effective to meet the requirements of control stability, with moderate fluctuations.
5.2 Group B 15
The three controlled components of the LT cycle, i.e., compressor, EEV, evaporator fan, were interacted on each other during the operation, and different controlled gains were chosen in the control strategy. The design sequence of control process was as following: compressor - evaporator fan - EEV. The respond times and the settling times of the three controlled parameters were investigated in Group B.
Fig.10 The variations of intermediate pressure, DS and evaporating temperature of LT cycle with time
Fig.10 shows the variations of the measured intermediate pressure, DS and evaporating temperature of LT cycle with time when the outdoor temperature maintained at -5 oC. At t =9 mins, the disturbance was introduced, with the water inlet temperature varied from 80 oC to 65 oC. During the first 9 minutes of the test, the intermediate pressure, DS and evaporating temperature of LT cycle stayed stable before the disturbances were introduced. It can be seen that immediately after the water inlet temperature varied, there was a significant drop in the intermediate pressure, from 19.4 bar to below 10 bar, and then reach the new stable intermediate pressure, ~16.25 bar at 25 min, with the settling time was 16 mins. The measured DS significantly increased at ~10 min, and then reached its maximum value, ~35 oC, then reduced to restore its setting at 33 min, with the settling time was 23 mins. Evaporating temperature also fluctuated at ~11 min, resulted from the unstable compressor speed controlled by the PID controller, and reach its new stable value, ~ -8 oC at ~28 min achieved by the proportional control, with the settling time was 19 16
mins. As for the respond time of these three parameters, the responses of EEV and evaporator fan followed by the compressor speed regulation. With the decrease of differential between set value and actual value, integral time increased, the actions of compressor and EEV reduced. Based on the experimental results of control stability of the compressor, EEV and evaporator fan, the controllers showed satisfactory control performance with less fluctuation, rapid amplitude descending, small steady-state errors and short settling time.
6. Conclusions
A control strategy for a cascade air source heat pump water heater has been developed to adjust the load variation and is reported in this paper. The intermediate pressure, DS and the evaporating temperature were regulated by the compressor, EEV and the evaporator fan of the LT cycle, respectively, through PID controller or proportional controller. The control strategy was experimentally tested over a wide operational range of cascade air source heat pump water heater. The following conclusions may be drawn: 1) Under stable outdoor condition, when the water inlet temperature varied, the system reached a new stability in about 17 min, revealing accurate and stable control. 2) The sequence of control process was as following: compressor - evaporator fan EEV. Experimental results the settling times of these three parameters were agreed with the designed control sequence in the control strategy. 17
3) In this paper, the principle that two stage compression ratio is approximately same is used to determine the target intermediate pressure for cascade heat pump system. The results showed that the controller developed successfully helped realize the control of the intermediate pressure, the DS and the evaporating temperature in terms of control accuracy and sensitivity.
Acknowledgments
The authors wish to acknowledge the funding supports from The National Natural Science Foundation of China (Project No.: 51406119), Shanghai Sailing Program of Shanghai Committee of and Technology, China (Project No.: 14YF1410000), the Hujiang Foundation of China (Project No.: D14003), Key Laboratory of Refrigeration and Cryogenic Technology of Zhejiang Province (Project No.: 2016001BB).
Reference:
[1] Neksa P, Rekstad H, Zakeri GR, Schiefloe PA. CO2-heat pump water heater: characteristic, system design and experimental results. Int J Refrigeration 1998; 21 (3): 172-179. [2] Ji J, Chow TT, Pei G, Dong J, He W. Domestic air-conditioner and integrated water heater for subtropical climate. Appl Therm Eng 2003;23(5):581–92. [3] Ma GY, Zhao HX. Experimental study of a heat pump system with flash-tank coupled with scroll compressor. Energ Buildings 2008; 40(5): 697-701. 18
[4] Yan G, Jia Q, Bai T. Experimental investigation on vapor injection heat pump with a newly designed twin rotary variable speed compressor for cold regions. Int J Refrigeration 2015; 62:232-241. [5] Gang Y, Tao B, Yu JL. Energy and exergy efficiency analysis of solar driven ejector–compressor heat pump cycle. Sol Energy 2016; 125: 243-255. [6] Sterling SJ, Collins MR. Feasibility analysis of an indirect heat pump assisted solar domestic hot water system. Appl Energ 2012; 93:11-17. [7] Qu ML, Chen JB, Nie LJ, Li FS, Yu Q, Wang T. Experimental study on the operating characteristics of a novel photovoltaic/thermal integrated dual-source heat pump water heating system. Applied Thermal Engineering 2016; 94: 819–826 [8] Bertsch SS, Groll EA. Two-stage air-source heat pump for residential heating and cooling applications in northern U.S. climates. Int J Refrigeration 2008; 31(7): 1282-1292. [9] Safa AA, Fung AS, Kumar R. Performance of two-stage variable capacity air source heat pump: field performance results and TRNSYS simulation. Energ Buildings 2015; 94: 80–90. [10] Kaushik SC, Kumar P, Jain S. Performance evaluation of irreversible cascaded refrigeration and heat pump cycles. Energy Convers Manage 2002; 43(17): 2405-2424. [11] Park H, Kim DH, Kim MS. Performance investigation of a cascade heat pump water heating system with a quasi-steady state analysis. Energy 2013; 63:283-294. [12] Wu JH, Yang ZG, Wu QH, Zhu Y. Transient behavior and dynamic performance 19
of cascade heat pump water heater with thermal storage system. Appl Energy 2012; 91: 187–196 [13] Jung HW, Kang H, Yoon WJ, Kim Y. Performance comparison between a single-stage and a cascade multi-functional heat pump for both air heating and hot water supply. Int J Refrigeration 2013; 36 (5):1431-1441. [14] Chae JH, Choi JM. Evaluation of the impacts of high stage refrigerant charge on cascade heat pump performance. Renew Energ 2015; 79: 66-71. [15] Kim DH, Park HS, Kim MS. Optimal temperature between high and low stage cycles for R134a/R410A cascade heat pump based water heater system. Exp Therm Fluid Sci 2013; 47: 172–179. [16]
Bhattacharyya
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refrigeration–heat pump system: heat exchanger inventory optimization and its numerical verification. Int J Refrigeration 2008; 31(7): 1207-1213. [17] Jung HW, Kang H, Chung H, Ahn JH, Kim Y. Performance optimization of a cascade multi-functional heat pump in various operation modes, Int J Refrigeration 2014; 42: 57-68. [18] Kim DH, Kim MS. The effect of water temperature lift on the performance of cascade heat pump system. Appl Therm Eng 2014; 67: 273-282. [19] Jeong S, Smith J. Optimum temperature staging of cryogenic refrigeration system. Cryogenics 1994; 34: 929–933. [20] Heat pump water heater for commercial & industrial and similar application. China National Standard GB/T21362-2008. Beijing: China Standard 20
Nomenclature
Com
The pressure ratio
h
The enthalpy of refrigerant kJ/kg
mr
The refrigerant flow rate , kg/s
p
Pressure , bar
Pe
The power input, kW
Qc
The heating capacity, kW
Qe
The cooling capacity, kW
Subscripts 1
At the suction point of the compressor
2
At the discharge point of the compressor
3
At the outlet of the condenser
4
At the inlet of the evaporator
c
Condensing
e
Evaporating
H
HT cycle
L
LT cycle
21
Figure captions
Fig.1 The schematic diagram of the cascade heat pump water heater system Fig.2 System logp-h diagram Fig.3 Flow chart of the cascade air source heat pump water heater control flow diagram Fig.4 The relationship between the fan speed of outdoor evaporator and the evaporating temperature Fig.5 The variations of the hot water temperature in Group A1 Fig.6 The variations of the suction and discharge temperature of HT cycle in Group A1 Fig.7 The variations of the current of the system in Group A1 Fig.8 The variations of the hot water temperature in Group A2 Fig.9 The variations of suction and discharge pressure of HT cycle in Group A2 Fig.10 The variations of intermediate pressure, DS and evaporating temperature of LT cycle with time
22
Table captions
Table 1 Specifications of the experimental cascade air source heat pump Table 2 Experimental conditions
23
Table 1 Specifications of the experimental cascade air source heat pump
Parameters
Values
The compressor of LT cycle
Refrigerant
R410A
The volume of cylinder (cm3/REV)
42.4
The DC voltage of frequency converter (V)
220
Turnover number (r/min)
3450
The cooling capacity (kW)
13.2
The input power (kW)
4.22
The compressor of HT cycle
Refrigerant
R134a
Cylinder volume (cm3/REV)
57.2
The DC voltage of frequency converter (V)
380
Turnover number (r/min)
2900
The cooling capacity (kW)
6.4
The input power (kW)
2.09
The evaporator From
Fin tube heat exchanger
Longitude tube pitch (mm)
18.19
Transverse tube pitch (mm)
21
Fin thickness (mm)
0.115
Fin pitch (mm)
2
Branch number
4
Number of tube rows
2
Heat transfer area (m2)
23.145
Evaporator fan Voltage(V)
220
Frequency (Hz)
50
Maximum speed (r/min)
1000
The condenser
Form
Plate-type heat exchanger
Number of plates
30
Design pressure (MPa)
4.5 2
Effective heat transfer area (m )
0.392 24
Design heating capacity (kW)
12
Plate length (mm)
325
Plate width (mm)
95
Pressure drop (kPa)
33
Intermediate heat exchanger
Form
Plate-type heat exchanger
Number of plates
70
Design pressure (MPa)
4.5
Effective heat transfer area (m2)
0.9
Design heating capacity (kW)
23
Plate length (mm)
325
Plate width (mm)
95
Pressure drop (kPa)
40
The EEV of LT level
Orifice size (mm)
2.4
The TEV of HT level
Orifice size (mm)
2.2
25
Table 2 Experimental conditions Water inlet temperature (oC) Group A1 65-70-75 Group A2 70-65-75 Group B 80-65
26
Outdoor air temperature (oC) 15 10 -5
Highlights We propose a control strategy for cascade air source heat pump water heater. The intermediate pressure, DS and evaporating temperature of LT cycle are
regulated. The controller can control the three parameters with accuracy and sensitivity.
27
S1359-4311(16)31535-6 http://dx.doi.org/10.1016/j.applthermaleng.2016.08.176 ATE 8975
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
25 April 2016 26 August 2016 27 August 2016
Please cite this article as: Q. Minglu, F. Yanan, C. Jianbo, L. Tianrui, L. Zhao, L. He, Experimental study of a control strategy for a cascade air source heat pump water heater, Applied Thermal Engineering (2016), doi: http://dx.doi.org/ 10.1016/j.applthermaleng.2016.08.176
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental study of a control strategy for a cascade air source heat pump water heater Qu Minglu, Fan Yanan, Chen Jianbo*1, Li Tianrui, Li Zhao, Li He School of Environment & Architecture, University of Shanghai for Science & Technology No.516, Jungong Road, Shanghai, China
ABSTRACT Cascade air source heat pump water heater is suitable for provide high temperature water under a wide range of outdoor air temperature. Most previous studies focused on the operating characteristics of cascade heat pump system, performance improvement, and the selection of optimal immediate temperature. However, no reported studies on the control strategies for cascade heat pump may be identified. This paper proposed a control strategy for cascade air source heat pump water heater to adjust the load variation. The principle that two stage compression ratio is approximately same is used to determine the intermediate pressure. The intermediate pressure, degree of superheat (DS) and the evaporating temperature were regulated by the compressor, electronic expansion valves (EEV) and the evaporator fan of the low temperature (LT) cycle, respectively, through Proportion Integration Differentiation (PID) controller or proportional controller. The control strategy was experimentally tested over a wide operational range of cascade air source heat pump water heater. *
Corresponding author. Tel.: +86 13901735531; fax: +86 021 55270686. E-mail address: [email protected] 1
The results showed that the controller developed successfully helped realize the control of the intermediate pressure, the DS and the evaporating temperature in terms of control accuracy and sensitivity.
Keywords: Cascade heat pump; Control strategy; PID; Compressor; Experimental
1. Introduction Energy saving has become a significant issue to be addressed globally for achieving not only sustainable development but also the reduction of greenhouse gas emission worldwide. Being energy-saving and environment-friendly, air source heat pump water heater (ASHPWH) have been widely used. ASHPWH can provide the same hot water with two to three times higher efficiency compared to a traditional boiler using fossil fuels or an electric water heater [1,2], However, ASHPWH unit is not suitable for extremely cold winter condition, due to its deteriorations in both operating performance and energy efficiency. Several methods that improving the performance of ASHPs under low temperature have been reported over the past years, i.e., using ejector compression heat pump [3-5], solar-assisted heat pump [6,7], two-stage compression heat pump [8, 9], cascade heat pump, etc.
Cascade heat pump system uses two pairs of compressor plants, working individually with different refrigerants to obtain the improved condensing temperature and the reduced evaporating temperature. Its advantage lies in both kinds of refrigerant work 2
within their best working temperature ranges. Cascade air source heat pump water heater is suitable for provide high temperature water under a wide range of outdoor air temperature, even severely cold outdoor air conditions. A large number of experimental and theoretical investigations on cascade heat pump have been reported. Kaushik et al. [10] presented finite time thermodynamic optimization of irreversible cascaded refrigeration and heat pump cycles having source/sink thermal reservoirs of finite heat capacitance. The effects of various operating parameters on the cooling and heating performance of the cascaded refrigeration and heat pump cycles were reported. Park et al. [11] established a steady-state cascade heat pump model to investigate the transient behavior of the system. The heating capacity, the power consumption, the COP, and the temperature distribution in a storage tank were obtained by the model. Wu et al. [12] experimentally investigated a cascade air source heat pump water heater with phase change material (PCM) for thermal storage application to ensure the reliable operation under various weather conditions. Jung et al. [13] compared cascade multi-functional heat pump with a single-stage multi-functional heat pump. The experimental results suggested that the cascade multi-functional heat pump adopting the water heating unit showed more stable air and water heating operations and higher water outlet temperatures than the single-stage multi-functional heat pump. Chae et al. [14] tested a water-to-water cascade heat pump to investigate the effects of high stage refrigerant charge amount on the performance in a steady state and heating mode operation. The variations of temperature difference between the condensing temperature of the low stage cycle and the evaporating temperature of the high stage 3
cycle at cascade heat exchanger, heat transfer rate and COP with the high stage refrigerant charge amount were reported.
The high temperature (HT) cycle and low temperature (LT) cycle are running independently in cascade heat pump, and the intermediate heat exchanger is a key component connecting to the two cycles. The selection of intermediate pressure/temperature determines the compression ratio and the isentropic efficiency of the compressor in HT and LT cycle, and meanwhile, affects the system performance. Therefore, the selection of optimal intermediate temperature has drawn great research interest of cascade heat pump. Bhattacharyya et al. [15] established a thermodynamic model of CO2/C3H8 cascade refrigeration-heat pump system to explore the optimum allocation of heat exchanger inventories in cascade refrigeration cycles for the maximization of performance and minimization of system cost. Kim et al. [16] experimentally and numerically studied the optimal intermediate temperature for the air-water cascade heat pump using R134a/R410A. It was concluded that the optimal intermediate temperature was determined by the high temperature condensing temperature, low temperature evaporating temperature, temperature difference and system efficiency of HT cycle and LT cycle.
However, to date, there has been little research about what and how to control the cascade heat pump. Jung et al. [17] experimentally tested the performance of a cascade multi-heat pump to provide simultaneous air heating, air cooling, and water 4
heating. The compressor rotation speed and EEV opening were adjusted under four operation modes for better performance. Kim et al. [18] studied the effect of the water temperature lift on the performance of cascade heat pump water heater. The cascade heat pump was operated to meet the constant heating capacity and target intermediate temperature by Proportion Integration Differentiation (PID) control of HT and LT compressor speed.
Most previous studies focused on the operating characteristics of cascade heat pump system, performance improvement, and the selection of optimal immediate temperature. However, no reported studies on the control strategies for cascade heat pump may be identified. In this study, the control strategy of cascade air source heat pump water heater was firstly introduced: the compressor speed, evaporator fan speed and EEV opening of LT cycle were adjusted by PID controller or proportional controller to meet the load variations. The control strategy was experimentally tested over a wide operational range of cascade air source heat pump water heater. Then the control performances were experimentally investigated. Experimental results revealed that the controller developed successfully helped realize the control of the intermediate pressure, the DS and the evaporating temperature in terms of control accuracy and sensitivity.
2. System description The experimental cascade air source heat pump water heater unit was specifically 5
built up for carrying out the experimental work reported in this paper. The test unit was designed to operate under condensing temperature of -40~20 oC, and output hot water temperature of 80 oC, with heating capacity of 11.0 kW. The low temperature cycle (R410A) absorbs the heat from ambient air and transfers heat to high temperature cycle (R134a). A schematic diagram of a cascaded heat pump system has been illustrated in Fig. 1. In the cascade heat pump, the HT cycle consisted of a fixed speed compressor, an intermediate heat exchanger (heat exchange between R410A and R134a), a condenser (heat exchange between R134a and water) and a thermostatic expansion valve (TEV). The LT cycle consisted of a DC inverter compressor, an intermediate heat exchanger, and an electronic expansion valve (EEV), an air cooled evaporator, and a DC inverter evaporator fan. The specifications are listed in Table 1.
Fig.1 The schematic diagram of the cascade heat pump water heater system
Table 1 Specifications of the experimental cascade heat pump
The cascaded heat pump system was placed in the psychometric room which could provide a stable outdoor condition. Meanwhile, the psychometric room also provided stable cold/hot water to the condenser according to the experimental conditions.
3. Control strategy 6
The intermediate condensing pressure was determined by the principle of same compression ratio between the high and low level temperature cycle under different outdoor conditions and hot water temperature [19]. The compressor speed, the evaporator fan speed and the EEV opening were regulated through PID controller or proportional controller to maintain the intermediate pressure, evaporating temperature and the degree of superheat (DS) in LT cycle.
3.1 Determination of the intermediate pressure
Fig. 2 shows the change of pressure-enthalpy (P-h) diagram of the cascade cycle. To determine the target intermediate pressure under different outdoor conditions and heat load conditions, the followings were assumed: 1) Pressure losses of the refrigerant pass through the heat exchanger and the pipes were neglected. 2) Heat exchanges between the heat exchanger, the pipes and the ambient air were neglected. 3) The temperature difference between the evaporating temperature in the HT cycle and the condensing temperature in the LT cycle was 5oC.
Fig.2 System logp-h diagram
The calculation equations for the target intermediate pressure are shown as follows: The refrigerant flow rate in the HT/LT cycle, mrH and mrL, can be evaluated by: 7
(1)
(2)
where QcH and QeL are the heating capacity of the HT cycle, and the cooling capacity of the LT cycle, respectively; h 2, h 3 are the enthalpies of refrigerant at the discharge point of the compressor and at the outlet of the condenser of the LT cycle, respectively; h2′ , h3′ are the enthalpies of refrigerant at the discharge point of the compressor and at
the outlet of the condenser of the HT cycle, respectively.
The power input in the HT/LT cycle, PeH and PeL, can be obtained by:
(3) (4)
where h 1 and h1′ are the enthalpies of refrigerant at the suction point of the LT cycle and HT cycle, respectively.
The pressure ratio of the HT/LT cycle, ComH, ComL, can be evaluated by:
(5) 8
(6)
Where pcH , peH are the condensing pressure and evaporating pressure in the HT cycle, respectively, and pcL, peL are the condensing pressure and evaporating pressure
in the LT cycle, respectively.
According to the condensing temperature in the HT cycle and the evaporating temperature in the LT cycle, the evaporating temperature in the HT cycle and the condensing temperature in the LT cycle can be determined based on the principle of the same pressure ratio between the high and low temperature cycle. The target intermediate pressure equals the discharge pressure according to Assumption 1).
3.2 Control method The intermediate pressure is regulated by the compressor speed of the LT cycle, realized by incremental closed-loop feedback PID controller, to maintain the stable operation of cascade heat pump system. Based on the measured intermediate pressure and the target intermediate pressure obtained from Eqs.(1-6), programmable logic controller (PLC) output the voltage signal (0.7V~4.05V) to the DC inverter (110Hz～ 25Hz) to regulate the compressor speed. The specific control flow diagram is shown in Fig.3.
The EEV used in LT cycle controls the refrigerant flow through the evaporator by 9
means of monitoring both refrigerant pressure and temperature at compressor suction. The pressure is used to determine the evaporating temperature, and then difference between the suction temperature and the evaporating temperature is the operating DS. The opening of the EEV in the LT cycle is controlled by a conventional closed-loop feedback PID DS controller with a DS setting of 15 oC.
Fig.3 Flow chart of the cascade air source heat pump water heater control flow diagram
The evaporating pressure of the LT cycle is regulated by the DC inverter fan through open loop proportional control method. When the outdoor air temperature is below -20oC, the evaporator fan is operated under its maximum speed, 1000 rpm. When the outdoor air temperature is above -20oC, the relationship of the evaporating pressure and the fan speed are illustrated in Fig.4. As seen, when the outdoor temperature is decreased, the fan speed is accelerated to enhance the heat transfer between the evaporator and the ambient air. Based on the measured evaporating temperature of the LT cycle and the outdoor air temperature, the proportional controller output DV voltage of 0~5 V, corresponding to the evaporator fan speed of 0~1000rpm.
Fig.4 The relationship between the fan speed of outdoor evaporator and the evaporating temperature
The compressor of the HT cycle controls the hot water temperature by on-off running 10
according to the setting temperature.
4. Experiments
4.1 Experiment method According to the related criteria and actual application [20], the control performance was investigated under the inlet hot water temperature of 65 oC to 80 oC, and the outdoor temperature range was from -5 oC to 15 oC. Comparing to the slowly varying environmental temperature, the water inlet temperature changes rapidly. When the water inlet temperature changes, the condensing temperature and the target intermediate temperature changes accordingly, and therefore the disturbance is introduced. Therefore, controllability test was carried out under fixed outdoor air temperatures and varied water inlet temperatures. Two groups of experiment were carried out. Group A: investigate the controllability of the HT cycle when the water inlet temperature varied. Group B: investigate the controllability of the LT cycle when the water inlet temperature varied. The experimental conditions for the cascade air source heat pump are illustrated in Table 2.
4.2 Instrumentation Pre-calibrated T-type thermocouples (of ±0.3 oC accuracy) were used for measuring the refrigerant temperatures. Refrigerant pressures were measured using pressure transmitters with an accuracy of ± 0.3% of full scale reading. The temperature sensors for the hot water at the inlet/outlet were of platinum Resistance Temperature Device 11
(RTD) type (PT100, Class A) with a pre-calibrated accuracy of ± 0.1 oC. The electric current of the system were measured using a digital power meter with a reported accuracy of ± 0.1 % of reading. The outdoor air temperature was obtained by using the existing measuring instruments of the psychometric room.
All sensors and measuring devices were able to output direct current signals of 4–20 mA or 1–5 V which were transferred to a data acquisition unit for logging and recording.
5. Experimental results
5.1 Group A Figs.5-7 illustrate the hot water outlet temperature, discharge and suction temperature of HT cycle and the system running current value when the outdoor temperature maintained at 15 oC and the inlet water temperature varied from 65 oC to 75 oC.
Fig.5 The variations of the hot water temperature in Group A1
As seen in Fig.5, the system was running stably when the inlet water temperature maintained at 65
o
C, with the steady state error within ±0.5 oC. At 30 min,
the setting inlet water temperature was changed to 70 oC, which was considered as the first heat load disturbance. It can be seen that the hot water outlet temperature increased with the increase of water inlet temperature in the initial 10 min. At 40 min, the hot water outlet temperature reached its maximum, ~ 76.2 oC, and the water inlet 12
temperature also reach its maximum, ~70.6 oC. Then, the water inlet and outlet temperatures gradually tended to be stable at 75.8 oC and 70 oC, respectively, with the same settling time of 17 minutes. At 70 min, the setting water inlet temperature was changed to 75 oC, after 2 minutes, both the water inlet and outlet temperature began to rise, until the 79th minute the water inlet and outlet temperature reached the maximum, 75.5 oC and 81.3 oC, respectively. After that, water inlet and outlet temperatures were stable at 75 oC and 81 oC, respectively, with the same settling time of 17 minutes..
Fig.6 The variations of the suction and discharge temperature of HT cycle in Group A1
As can be seen from Fig.6, when the inlet water temperature varied from 65 oC to 70 o
C and 75 oC, the discharge temperature of the HT cycle varied from 81 oC to 87 oC
and 99 oC. The suction temperature of the HT cycle varied from 25 oC to 28 oC and 30 o
C. The discharge/suction temperature was within a reasonable range and the system
operation was stable and reliable.
Fig.7 The variations of the current of the system in Group A1
Running current is an important parameter for evaluating the safety operation of a heat pump water heater. Running current may fluctuate when the operating condition 13
varies, and current overload may occur, which would shorten the service life of compressor and fan motor, and cause the aging of the electric wire and control elements, even outbreak of fire. Therefore, in the research, the running current of the system was monitored and recorded in Fig. 7. As can be seen from Fig.7, when the inlet water temperature changed, the current changed accordingly, and when the outlet water temperature was stable, the current value also achieved a stable value, indicating that the operation of the cascade air source heat pump water heater was steadily when the inlet water temperature varied from 65 oC to 75 oC.
In order to verify the performance of the control strategy under a wider range, the controllability test was also carried out under the outdoor temperature of 10 oC with the initial setting inlet water temperature at 70 oC. Fig.8 and Fig.9 show the variations of the water inlet/outlet temperatures and discharge/suction pressures of the HT cycle with time.
The set point of the water inlet temperature was decreased to 65 oC at 20 min, and then increased to 75 oC at 83 min. It can be seen from Fig. 8 the cascade heat pump system responded quickly and reached to a new steady state. As the set point of the water inlet temperature was decreased to 65 oC at 20 min, the water inlet temperature dropped to the minimum, ~64.5 oC within 7 minute. Then it increased gradually and finally reached the set value at 34 minutes. The hot water outlet temperature also dropped to the minimum, ~69.8 oC within 7 minute, and then quickly reached a stable 14
state at 70.3 oC, with the settling time of about 17 minutes. At 83 min when the setting of the inlet water temperature changed to 75 oC, the system reached a new steady state after about 17 minutes.
Fig.8 The variations of the hot water temperature in Group A2
Fig. 9 presents the variations of suction and discharge pressure of HT cycle at the outdoor temperature of 10 oC. As seen, with the change of the water inlet temperature, it required ~10 minutes for the discharge/suction pressure of HT cycle to reach stable state. And the discharge/suction pressure fluctuated (steady-state error) within 10 kPa, and the maximum overshoot was also within the requirement of 2% ~ 5%, suggesting that it was appropriate to adopt PID control method for the intermediate pressure, and the selection of proportional coefficient was reasonable.
Fig.9 The variations of suction and discharge pressure of HT cycle in Group A2
Based on the experimental results of Group A, it can be seen that the HT cycle of the cascade heat pump water heater system quickly adapted to the variation of the load as the LT compressor responded immediately to the changes in set points, and the control strategy was accurate and effective to meet the requirements of control stability, with moderate fluctuations.
5.2 Group B 15
The three controlled components of the LT cycle, i.e., compressor, EEV, evaporator fan, were interacted on each other during the operation, and different controlled gains were chosen in the control strategy. The design sequence of control process was as following: compressor - evaporator fan - EEV. The respond times and the settling times of the three controlled parameters were investigated in Group B.
Fig.10 The variations of intermediate pressure, DS and evaporating temperature of LT cycle with time
Fig.10 shows the variations of the measured intermediate pressure, DS and evaporating temperature of LT cycle with time when the outdoor temperature maintained at -5 oC. At t =9 mins, the disturbance was introduced, with the water inlet temperature varied from 80 oC to 65 oC. During the first 9 minutes of the test, the intermediate pressure, DS and evaporating temperature of LT cycle stayed stable before the disturbances were introduced. It can be seen that immediately after the water inlet temperature varied, there was a significant drop in the intermediate pressure, from 19.4 bar to below 10 bar, and then reach the new stable intermediate pressure, ~16.25 bar at 25 min, with the settling time was 16 mins. The measured DS significantly increased at ~10 min, and then reached its maximum value, ~35 oC, then reduced to restore its setting at 33 min, with the settling time was 23 mins. Evaporating temperature also fluctuated at ~11 min, resulted from the unstable compressor speed controlled by the PID controller, and reach its new stable value, ~ -8 oC at ~28 min achieved by the proportional control, with the settling time was 19 16
mins. As for the respond time of these three parameters, the responses of EEV and evaporator fan followed by the compressor speed regulation. With the decrease of differential between set value and actual value, integral time increased, the actions of compressor and EEV reduced. Based on the experimental results of control stability of the compressor, EEV and evaporator fan, the controllers showed satisfactory control performance with less fluctuation, rapid amplitude descending, small steady-state errors and short settling time.
6. Conclusions
A control strategy for a cascade air source heat pump water heater has been developed to adjust the load variation and is reported in this paper. The intermediate pressure, DS and the evaporating temperature were regulated by the compressor, EEV and the evaporator fan of the LT cycle, respectively, through PID controller or proportional controller. The control strategy was experimentally tested over a wide operational range of cascade air source heat pump water heater. The following conclusions may be drawn: 1) Under stable outdoor condition, when the water inlet temperature varied, the system reached a new stability in about 17 min, revealing accurate and stable control. 2) The sequence of control process was as following: compressor - evaporator fan EEV. Experimental results the settling times of these three parameters were agreed with the designed control sequence in the control strategy. 17
3) In this paper, the principle that two stage compression ratio is approximately same is used to determine the target intermediate pressure for cascade heat pump system. The results showed that the controller developed successfully helped realize the control of the intermediate pressure, the DS and the evaporating temperature in terms of control accuracy and sensitivity.
Acknowledgments
The authors wish to acknowledge the funding supports from The National Natural Science Foundation of China (Project No.: 51406119), Shanghai Sailing Program of Shanghai Committee of and Technology, China (Project No.: 14YF1410000), the Hujiang Foundation of China (Project No.: D14003), Key Laboratory of Refrigeration and Cryogenic Technology of Zhejiang Province (Project No.: 2016001BB).
Reference:
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[4] Yan G, Jia Q, Bai T. Experimental investigation on vapor injection heat pump with a newly designed twin rotary variable speed compressor for cold regions. Int J Refrigeration 2015; 62:232-241. [5] Gang Y, Tao B, Yu JL. Energy and exergy efficiency analysis of solar driven ejector–compressor heat pump cycle. Sol Energy 2016; 125: 243-255. [6] Sterling SJ, Collins MR. Feasibility analysis of an indirect heat pump assisted solar domestic hot water system. Appl Energ 2012; 93:11-17. [7] Qu ML, Chen JB, Nie LJ, Li FS, Yu Q, Wang T. Experimental study on the operating characteristics of a novel photovoltaic/thermal integrated dual-source heat pump water heating system. Applied Thermal Engineering 2016; 94: 819–826 [8] Bertsch SS, Groll EA. Two-stage air-source heat pump for residential heating and cooling applications in northern U.S. climates. Int J Refrigeration 2008; 31(7): 1282-1292. [9] Safa AA, Fung AS, Kumar R. Performance of two-stage variable capacity air source heat pump: field performance results and TRNSYS simulation. Energ Buildings 2015; 94: 80–90. [10] Kaushik SC, Kumar P, Jain S. Performance evaluation of irreversible cascaded refrigeration and heat pump cycles. Energy Convers Manage 2002; 43(17): 2405-2424. [11] Park H, Kim DH, Kim MS. Performance investigation of a cascade heat pump water heating system with a quasi-steady state analysis. Energy 2013; 63:283-294. [12] Wu JH, Yang ZG, Wu QH, Zhu Y. Transient behavior and dynamic performance 19
of cascade heat pump water heater with thermal storage system. Appl Energy 2012; 91: 187–196 [13] Jung HW, Kang H, Yoon WJ, Kim Y. Performance comparison between a single-stage and a cascade multi-functional heat pump for both air heating and hot water supply. Int J Refrigeration 2013; 36 (5):1431-1441. [14] Chae JH, Choi JM. Evaluation of the impacts of high stage refrigerant charge on cascade heat pump performance. Renew Energ 2015; 79: 66-71. [15] Kim DH, Park HS, Kim MS. Optimal temperature between high and low stage cycles for R134a/R410A cascade heat pump based water heater system. Exp Therm Fluid Sci 2013; 47: 172–179. [16]
Bhattacharyya
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refrigeration–heat pump system: heat exchanger inventory optimization and its numerical verification. Int J Refrigeration 2008; 31(7): 1207-1213. [17] Jung HW, Kang H, Chung H, Ahn JH, Kim Y. Performance optimization of a cascade multi-functional heat pump in various operation modes, Int J Refrigeration 2014; 42: 57-68. [18] Kim DH, Kim MS. The effect of water temperature lift on the performance of cascade heat pump system. Appl Therm Eng 2014; 67: 273-282. [19] Jeong S, Smith J. Optimum temperature staging of cryogenic refrigeration system. Cryogenics 1994; 34: 929–933. [20] Heat pump water heater for commercial & industrial and similar application. China National Standard GB/T21362-2008. Beijing: China Standard 20
Nomenclature
Com
The pressure ratio
h
The enthalpy of refrigerant kJ/kg
mr
The refrigerant flow rate , kg/s
p
Pressure , bar
Pe
The power input, kW
Qc
The heating capacity, kW
Qe
The cooling capacity, kW
Subscripts 1
At the suction point of the compressor
2
At the discharge point of the compressor
3
At the outlet of the condenser
4
At the inlet of the evaporator
c
Condensing
e
Evaporating
H
HT cycle
L
LT cycle
21
Figure captions
Fig.1 The schematic diagram of the cascade heat pump water heater system Fig.2 System logp-h diagram Fig.3 Flow chart of the cascade air source heat pump water heater control flow diagram Fig.4 The relationship between the fan speed of outdoor evaporator and the evaporating temperature Fig.5 The variations of the hot water temperature in Group A1 Fig.6 The variations of the suction and discharge temperature of HT cycle in Group A1 Fig.7 The variations of the current of the system in Group A1 Fig.8 The variations of the hot water temperature in Group A2 Fig.9 The variations of suction and discharge pressure of HT cycle in Group A2 Fig.10 The variations of intermediate pressure, DS and evaporating temperature of LT cycle with time
22
Table captions
Table 1 Specifications of the experimental cascade air source heat pump Table 2 Experimental conditions
23
Table 1 Specifications of the experimental cascade air source heat pump
Parameters
Values
The compressor of LT cycle
Refrigerant
R410A
The volume of cylinder (cm3/REV)
42.4
The DC voltage of frequency converter (V)
220
Turnover number (r/min)
3450
The cooling capacity (kW)
13.2
The input power (kW)
4.22
The compressor of HT cycle
Refrigerant
R134a
Cylinder volume (cm3/REV)
57.2
The DC voltage of frequency converter (V)
380
Turnover number (r/min)
2900
The cooling capacity (kW)
6.4
The input power (kW)
2.09
The evaporator From
Fin tube heat exchanger
Longitude tube pitch (mm)
18.19
Transverse tube pitch (mm)
21
Fin thickness (mm)
0.115
Fin pitch (mm)
2
Branch number
4
Number of tube rows
2
Heat transfer area (m2)
23.145
Evaporator fan Voltage(V)
220
Frequency (Hz)
50
Maximum speed (r/min)
1000
The condenser
Form
Plate-type heat exchanger
Number of plates
30
Design pressure (MPa)
4.5 2
Effective heat transfer area (m )
0.392 24
Design heating capacity (kW)
12
Plate length (mm)
325
Plate width (mm)
95
Pressure drop (kPa)
33
Intermediate heat exchanger
Form
Plate-type heat exchanger
Number of plates
70
Design pressure (MPa)
4.5
Effective heat transfer area (m2)
0.9
Design heating capacity (kW)
23
Plate length (mm)
325
Plate width (mm)
95
Pressure drop (kPa)
40
The EEV of LT level
Orifice size (mm)
2.4
The TEV of HT level
Orifice size (mm)
2.2
25
Table 2 Experimental conditions Water inlet temperature (oC) Group A1 65-70-75 Group A2 70-65-75 Group B 80-65
26
Outdoor air temperature (oC) 15 10 -5
Highlights We propose a control strategy for cascade air source heat pump water heater. The intermediate pressure, DS and evaporating temperature of LT cycle are
regulated. The controller can control the three parameters with accuracy and sensitivity.
27