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Reliability verification of a solar–air source heat pump system with PCM energy storage in operating strategy transition
Renewable Energy xxx (2015) 1e10
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Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition Dehu Qv a, Long Ni a, *, Yang Yao a, **, Wenju Hu b a
School of Municipal and Environmental Engineering, Harbin Institute of Technology, 150090 Harbin, China Beijing Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering, Beijing University of Civil Engineering and Architecture, 100044 Beijing, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2015 Received in revised form 14 May 2015 Accepted 10 July 2015 Available online xxx
In the recent decade, with solar energy assisted heat pump systems have increasingly developed. In the previous studies, a hybrid air source heat pump (ASHP) system was proposed, which coupled with latent heat thermal energy storage (LHTES) and solar thermal collector, for operating in various types of configurations. This paper describes the approach and principle for organizing the hybrid system in detail. Thereafter, a phase change material (PCM) based solareair source heat pump (PCM-SAHP) prototype was set-up and implemented under variant testing conditions. Experimental results demonstrate that the PCM-SAHP system presented remarkable advantages on correcting the mismatch between supply and demand of thermal energy and electricity. Further, when the ambient temperature was higher than 38 C, cooling COP of the hybrid system enhanced by 17%, compared with that of ASHP system under same surroundings. During the days that outdoor air temperature was below 10 C, heating COP of the PCM-SAHP system rose by 65% comparing with that of ASHP system. In additional, switching operating strategies during system running will scarcely result in the violent or continuous fluctuations on the operating parameters. Therefore, the efficiency of the PCM-SAHP systems can be improved with capacity lapse avoiding, and exhaust controlling as well. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Hybrid heat pump system Solar thermal energy Latent heat thermal energy storage Efficiency improving Exhaust controlling
1. Introduction Recent two decades, a growing number of individuals and communities incline space heating and cooling by heat pumps (HP) rather than conventional boiler system paralleling air conditioning (AC) system on account of higher energy efficiency and milder environment impact. Advanced cycle designs, improved cycle components and expanded cycle applications are the main development channels in HP technology [1]. Recently, latent heat thermal energy storage (LHTES) within HP system for space/water heating and cooling has received increasingly interests because of the flexibility, efficiency and stability through charge and discharge of thermal energy [2]. Phase change material (PCM) within passive heat storage building may be the simplest application for LHTES. PCM thermal board (PCMTB) and PCM thermal shield (PCMTS) can balance
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Ni), [email protected] (Y. Yao).
thermal energy substantially inside building [3e6]. Building structure, orientation, situated climatic region, as well as thermal property of building affects the effect of PCMTB and PCMTS [7,8] subtly. Active and-or passive heat storage with PCM for AC do not only slash energy consumption but improve adaptability of PCM [9e11]. Yet application of PCM defrosting reflects a brilliant improvement in system overall efficiency versus lower investment [12]. On the other hand, free cooling technique shows a nice potential in energy saving, efficiency enhancement and environment friendliness [13e16]. However ice storage might be a longer historical technology, which recently focuses on improving synthetic energy efficiency for the whole system [17e20]; by contrast, PCM with higher phase transition temperature within AC/HP seems more promising to rise evaporating temperature, therefore the optimistic capacity and efficiency can be anticipated [21,22]. On the other lens, PCM within AC for condensing heat recovery seems flexible, efficient and convenient [23,24] also. Actually, applications of PCM for residential cooling and heating are much more than that. This paper presents a PCM based hybrid solareair source heat pump (PCM-SAHP) system for building cooling and heating applications. Basic design concept, structure and components, and
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Please cite this article in press as: D. Qv, et al., Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.030
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
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operating principle of the PCM-SAHP system are described prior. Sequentially, a testing prototype are organized and produced. Then details of testing methods and materials are serviced. Finally, performance of the system proposed during typical conditions are proposed and discussed; reliability and stability of that at the instant of operating strategy transition is carried out additionally. Experimental performances include compressor power consumption, cooling and heating capacity and COP, as well as temperature features of PCM in the LHTES facility. 2. Methods or materials 2.1. The prototype of the PCM-SAHP system A PCM-SAHP system consists of four elements: ASHP, solar thermal collector, the triplex tube heat exchange (TRTHE) system encompassing PCM, as well as data acquisition and control sector. Briefly, this hybrid system could charge chilled energy in the TRTHE system during cool summer night with profit of the special discount tariff in electricity, and discharge chilled energy from the TRTHE system during peak energy period with high ambient temperature. Further, during low temperature days, this coupled system can store heating energy in the TRTHE system with solar thermal energy assisted and supply hot water or air, which avoids decrease in both heating capacity and COP, and refrains from increasing in compressor pressure ratio the ASHP system suffering. Therefore, the PCM-SAHP system could charge and discharge cooling/heating energy by a same facility, TRTHE which is the key component of the PCM-SAHP system. Details about the system and TRTHE were presented in literature [25,26]. Niu et al. primarily presented the TRTHE system based PCM-SAHP system. And feasibility of the TRTHE system within compression vapor cycle has been proved. Schematic diagram of the PCM-SAHP system and structure of the TRTHE unit is shown in Fig. 1, while Specifications of the prototype are listed in Table 1. In this study, the TRTHE units and a prototype of the PCM-SAHP system are redesigned and organized. Additionally, nine operating modes and corresponding ten groups of tests for building cooling and heating are projected and implemented. The TRTHE units within PCM-SAHP systems can fulfill nine operating modes, including space cooling by ASHP mode (M1), cold storage mode (M2), space cooling by TRTHE mode (M3), space cooling by TRTHE assisting ASHP mode (M4), space heating by ASHP mode (M5), heat storage mode (M6), space heating by TRTHE mode (M7), space heating by TRTHE assisting ASHP mode (M8) and space heating by TRTHE with solar hot water assisted mode (M9). All of these modes are listed in Table 2. During the summer night the hybrid system charges chilled energy into the TRTHE system using off-peak electricity (M2). When the cooling load of chamber is lower, the TRTHE system can provide stable chilling water for space cooling without compressor operation (M3). In the highest cooling load environment, the TRTHE system assists ASHP to serve for space cooling (M4). In winter, on the days of cloud or rain/snow free, solar hot water (about 30 C) from the solar thermal collector stores heat into the TRTHE units that service as the low grade heat source for the heat pump system to avoid awkward performance of the ASHP system under low ambient temperature surroundings. Moreover, the TRTHE units within PCM-SAHP systems just perform as an evaporator in the compression vapor cycle, which eases oil return in the refrigerant circulation. The TRTHE unit is made up of three concentric copper tubes and PCM that embedded in the space where the inner tube and the medium tube encompassed. Refrigerant circulates inside the inner tube driving by compressor, and water passes through the space between the outer tube and the medium tube as heat transfer fluid
E6 S4
S3
E4
2 P
3 P
P
R1 E1
9
P
R2 S2
E5
6
5
E2
S1
E9 B A E8
4
1
E7
A B
P
R3 E3
F1 V1 7
Solenoid Valve Flowmeter
Check Valve
Thermometer
Manual Valve
Throttle
Piezometer
Distributor
(a) Schematic diagram of PCM-SAHP system
Water Outlet Refrigerant Outlet
Key Thermal Points
PCM Wat. 35mm×1.2mm PCM 22mm×1.0mm Refr. 9.52mm×0.5mm
Refrigerant Inlet Water Inlet
(b) Structure of TRTHE unit Fig. 1. Schematic diagram of PCM-SAHP system and structure of TRTHE unit.
(HTF). The material of RT5HC, as the PCM embedded in the TRTHE system was used in this equipment. Three concentric copper tubes are weld together after spinning process, and thermally insulated envelope was used to reduce heat loss. Now, thermal energy storage capacity is the overriding parameter for the TRTHE system, which depends on the operating strategy of the hybrid system and the characteristics of air conditioning load for building. In this paper, a virtual building situated in Shanghai (China) was selected as the reference of air conditioning load and operating strategy for the devisal of thermal energy storage capacity in the TRTHE units, in where cooling/ heating load is approximate balance in general buildings. According to the initial cost of equipments and the special discount tariff in Shanghai, partial thermal energy storage is appropriate, meaning that thermal energy discharge from the TRTHE system has a responsibility to eliminate part of thermal energy load during a whole day. At the beginning and end of space cooling season (less than 90 days) the percentage of cooling load to maximum value is about 30%, the TRTHE system can maintain thermal comfort level in chamber without any other chilled energy source operation from 10:00 to 18:00. On the other hand, during high ambient temperature days (about 100e120 days), the TRTHE units assisting ASHP system can discharge cooling energy from 10:00 to 18:00 to control the temperature and humidity in indoor space.
Please cite this article in press as: D. Qv, et al., Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.030
D. Qv et al. / Renewable Energy xxx (2015) 1e10
3
Table 1 Specifications of the prototype. Component
Configurations
Remarks
Compressor TRTHEs FTHE PHE Throttle Refrigerant
Rated load current of 11.4 A and voltage of 220e240 V Details in Table 4 Total length of heat transfer tube: 56 m; face velocity: 2.5 m s1 Heat transfer area: 0.045 m2 per piece; total number of heat transfer plate: 38 pieces Thermal expansion valve R22
Scroll compressor 3 groups Be outside Brazed plate heat exchanger e e
2.2. Experimental set-up
Table 2 Operation modes of the prototype. Mode
M1 M2 M3 M4 M5 M6 M7 M8 M9
Manual valve V1
Solenoid valve E1eE9
Pump 5
Pump 7
On
Off
On
Off
On
On
C C
2, 3 4, 2, 1, 6, 3, 1, 3,
1, 3e7 1, 2, 4e9 1e3, 6e9 1, 3, 6, 7 2e7 1e5, 8, 9 1, 2, 4e7 2, 4e7 1, 2, 4,5
C
C C C C C C C
8, 9 5 4, 5, 8, 9 8, 9 7 8, 9 3, 8, 9 6e9
Off C C
C C
Off C C
C C C
C C C C
C C C C
Table 3 Capacity design for three groups of TRTHE. Capacity design
The quotient about cooling capacity for TRTHE to total capacity
Mass consumption of PCM (kg) Chilled energy storage (MJ) Duration of combined space cooling (h d1) The number of combined space cooling days (d) The number of space cooling by TRTHE days (d)
24%
28%
32%
6.37 1.56 9 20 20
8.12 1.99 9 42 44
10.17 2.49 9 42 60
Table 3 shows the effect of the quotient of cooling capacity for the TRTHE system on the mass consumption of PCM, chilled energy storage capacity, duration of combined space cooling, the number of combined space cooling days, and the number of space cooling by TRTHE days. Finally, quotient 32% of total cooling capacity is adopted. Thereafter, structure design for the TRTHE units is the key in hybrid system designing, and highlight of structure design is ascertaining the thickness of PCM in the TRTHE units. In this paper, diameter of refrigerant tube is given, and diameter of the medium tube is the core. Combining heat transfer equations with thermodynamic equations can be achieved. Considering demand of cooling charge fast, and heating discharge mild and steady, thickness of PCM in the TRTHE units should be controlled in 6 mm. Geometric characteristics of the TRTHE unit are given in Table 4, and Thermodynamic characteristics of the PCM are listed in Table 5.
In this paper, experimental platform comprises a prototype of hybrid system proposed and a large enthalpy difference laboratory that was set up according to the National Standard of Unitary Air Conditioners in China (GB/T 17758-2010). The laboratory includes outdoor space, indoor space and control space. The air temperature and humidity at the outdoor/indoor space could be controlled accurately by the air conditioning system. Three groups of TRTHE unit for testing were prepared, and each group of that comprises three rotor flow meters installed at three units of water pipe side, respectively. Other measuring points including pressure points for observing refrigerant circulation, temperature points installed at refrigerant pipe side for observing and controlling compression vapor cycle, temperature points at water pipe side for measuring and manipulating operation of the system and modes, and volume flow points on the hot/cold water supply pipe, have been plotted in Fig. 1. Additionally, electric parameters and time are observed and recorded likewise. Particularly, thermistors to measure temperature of PCM in the TRTHE unit are installed along the direction of fluid flow per 0.8 m. All sensors are connected to a stand-alone data logger unit to record all measured values, and the data logger is connected to a PC, therefore, the operating data could be downloaded to the PC for storage and subsequent analysis. All test parameters except electric parameters are measured and recorded every 30 s, and electric parameters are observed and recorded every 6 s. Details about the apparatus and instruments used in the experiment are listed in Table 6. In additional, it is necessary to confessed that the solar hot water is prepared by the hot water tank to avoid negative impacts from outdoor weather-setting. To perceive performances of the PCM-SAHP system during severely high/low ambient temperature conditions, as well as stability and reliability at the instant of switching heat exchange elements, a series of tests were designed and implemented. Tests comprehend severe setting experiments as seen in Table 7 and switchover experiments. Each test lasted for 60 min after the hybrid system reached an asymptotic state. Each experimental unit repeated for 7 times, and the test unit can be terminated when the deviation of experimental results controlled availably below 5%. Being an evidence of the reliability for the prototype during operation, all coefficients of dispersion for variant coefficients of performance under different tests have been discussed in Appendix. As
Table 4 Geometry details of TRTHE. Items
Unit
Number
Diameter of the inner tube (outer diameter/wall thickness) Diameter of the medium tube (outer diameter/wall thickness) Diameter of the outer tube (outer diameter/wall thickness) Length of the tube (available heat transfer section) Total length of the tube per unit (available heat transfer section) Total number of unit Total mass of the PCM per group Amount of thermal energy storage per group
mm mm mm m m e kg kJ
9.52/0.5 22/1.0 35/1.2 0.8 64 3 3.5 857.5
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D. Qv et al. / Renewable Energy xxx (2015) 1e10 Table 5 Thermodynamic characteristics of the PCM. Item
Unit
Typical values
Melting area Congealing area Heat storage capacity Density solid Density liquid Volume expansion in phase change range Heat conductivity Kin. Viscosity at 20 C Flash point Corrosion
5e6 6e5 245 0.88 0.763 12 0.2 3.01 115 Chemical inert with respect to most materials
C C kJ kg1 kg L1 kg L1 % W m1 K1 mm2 s1 C e
Table 6 Experimental instruments. Parameters
Name
Measuring range
Measuring accuracy
Remarks
Temperature Pressure Pressure Volume flow Volume flow Electric power Time
NTC thermistor Pressure transmitter Pressure transmitter Water meter Rotameter e Stopwatch
40 to 60 C 0e1.5 MPa 0e3.0 MPa 0e106 m3 10e100 L h1 0.4e12 kW e
±0.1 C ±0.001 MPa ±0.001 MPa ±105 m3 ±2.25 L h1 ±0.4% 0.06 s
e Except discharge pressure Discharge pressure For plate heat exchanger For TRTHE e e
Table 7 Experiments during diverse operating modes. Mode Experimental conditions M1 M2 M3 M4 M5 M6 M7 M8 M9
The ambient temperature is 30, 32, 35, 38, 40 C respectively; volume flow of water for space cooling is 800 L h1; temperature of return water for space cooling is 12 ± 0.5 C. The ambient temperature is 15, 20, 25, 30, 35 C respectively; the activation condition is PCM average temperature of 15 ± 0.5 C; the termination criterion is the PCM minimum temperature of below 0 C. Temperature of return water for space cooling is 18 ± 0.5 C; volume flow of water through TRTHE is 40 L h1; the termination criterion is the supply chilling water temperature of above 17 C. The ambient temperature is 35, 38, 40, 43 C respectively; volume flow of water through the plate heat exchanger is 800 L h1; volume flow of water through TRTHE is 40 L h1; temperature of return water for space cooling is 12 ± 0.5 C; The ambient temperature is 7, 10, 12, 15, 17 C respectively; volume flow of water for space heating is 1260 L h1; temperature of return water for space heating is 31 ± 0.5 C. The temperature of solar hot water is 28 C, the volume flow of solar hot water is 300 L h1, the termination criterion is the temperature of PCM at the outlet of TRTHE above 20 C. Volume flow of water for space heating is 1260 L h1; temperature of return water for space heating is 31 ± 0.5 C. The ambient temperature is 17 C; volume flow of water for space heating is 1260 L h1; temperature of return water for space heating is 31 ± 0.5 C. The temperature of solar hot water is 28 C; volume flow of solar hot water is 300 L h1; volume flow of water for space heating is 1260 L h1; temperature of return water for space heating is 31 ± 0.5 C.
seen in Table A2, the maximum coefficient of dispersion is 0.029, which is acceptable in this study. In these tests, chilling water to TRTHE was simulated by chilling water tank, and hot water into condenser for space heating was prepared by cooling water tank. Volume flow of water through the TRTHE units, and that through the plate heat exchanger (PHE) should be controlled at the setting point prior. Next, ensure that water temperature at the inlet of the TRTHE units and that of PHE should be steady at the setting point. And then, coordinate temperature and humidity in the indoor/ outdoor space in the given conditions. To simplify the data processing and analysis, this paper takes the amount capacity for cooling charge as constant and measures the duration and energy consumption for cooling charge. The COPs for performance analysis are needed to calculate by Eqs. (1) to (5); paraphrases for each symbol are listed in Nomenclatures.
¼ Qe Wcomp
(1)
COPc$ASHP ¼ Qc Wcomp
(2)
COPe$ASHP
COPperi$TRTHE ¼ Qaccu Wcomp$accu COPperi$comb ¼ Qaccu
Wcomp$accu þ WTRTHE$accu
COPcomb ¼ Qe$ASHP þ Qe$TRTHE Wcomp
(3)
(4) (5)
Further, related error analyses of these COPs are described in Appendix. From the Table A1, the maximum relative error is 4%, which is acceptable uncertainty value in engineering applications. 3. Results and discussion 3.1. Cooling tests during diverse cooling modes Cooling charge and discharge sequentially constituting a cooling cycle for the TRTHE system, and cooling cycle COP for the TRTHE system has been defined as Eq. (3). Similarly, combined cooling cycle COP considering compressor power consumption during cooling charge period has been presented in Eq. (4). According to National Standard of Unitary Air Conditioners in
Please cite this article in press as: D. Qv, et al., Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.030
D. Qv et al. / Renewable Energy xxx (2015) 1e10
China (GB/T 17758-2010), cooling tests during different cooling modes were organized and carried out. First, compressor power consumption and cooling capacity at typical condition of ambient temperature of 35 C achieves 2.08 kW and 5.408 kW respectively, which fulfill the expectation in design approximately. On the other lens, cooling charge duration at the typical testing condition of ambient nighttime temperature of 25 C is 17 min, approaching the design target. Encouragingly, the amount of cooling charge capacity achieves 2.49 MJ in three groups of TRTHE unit that fulfills the anticipation in design. Fig. 2 shows the effect of ambient temperature on the average COP during M1, M3 and M4 respectively. It can be seen that, the coefficient of performance during space cooling application declines with rising ambient temperature no matter which cooling mode is operating. The cooling COP for the ASHP system decreases markedly from 2.9 to 2.3 with increasing outdoor temperature from 30 C to 40 C. The cooling cycle COP for the TRTHE system elapses away from 3.7 to 3.0 with ascending ambient temperature at night from 15 C to 25 C, yet, when the dry bulb temperature outside arrives at 30 C, the cooling cycle COP plunges to 1.8. Encouragingly, when the ambient temperature rises from 35 C to 43 C, the combined cooling cycle COP declines mildly from 3.0 to 2.3, which is 17% higher than the cooling COP for the ASHP system under the same ambient temperature conditions. On the other lens, the mean of cooling capacity for the TRTHE units and total cooling capacity during M4 achieves 2.264 kW and 7.242 kW respectively. The percentage of the cooling capacity for three groups of TRTHE unit to the total cooling capacity during M4 is about 31% which almost approaches design value of 32%. Generally, in the vapor compression cycle, increasing condensing temperature means compressor power consumption rising and cooling capacity lapse, which is intelligible to us. While the effect of outdoor air temperature on the cooling cycle COP for the TRTHE system can be understood though the effect of air
5
temperature outside on the performance of cooling charge for the TRTHE system. Table 8 reflects the effect of ambient temperature on the compressor power consumption, cooling charge duration, as well as temperature characteristics of PCM in the TRTHE units. It can be seen from that both of compressor power consumption and cooling duration rise obviously with increasing outdoor temperature. Consequently, the amount of compressor power consumption during cooling charge period rockets from 1.17 MJ to 3.35 MJ with ascending ambient temperature at night from 15 C to 35 C in one group of TRTHE unit. Therefore, when the cooling discharge capacity is terminate, night ambient temperature dominates the accumulating compressor power consumption, as well as the cooling cycle COP certainly. On the other lens, it can be seen from Table 8 that the average temperature of PCM in one unit of the TRTHE system increasing from 1.3 C to 2.4 C with rising night ambient temperature from 15 C to 35 C, in the meanwhile, the mean square temperature error amplifies from 3.8 C to 5.5 C which may imply that PCM in the TRTHE units have not achieved the optimal state for saturating chilled energy storage. Nevertheless, the mean value and square error of temperature for PCM seems abnormal because of the higher values in temperature. It is plausible to attribute this phenomenon to undervaluation of the condensing temperature which results in an instable hydrodynamic and heat transfer state for refrigerant. Overall higher ambient temperature (may be 30 C or more) at night might cause huge energy input for cooling charge, therefore cooling discharge application just by the TRTHE units alone at daytime should be deliberated during high outdoor air temperature nights. 3.2. Heating tests during variant heating modes In the tests of space heating for building, space heating by ASHP alone (M5), by the TRTHE system alone (M7), by the ASHP system
3.2 COPe.ASHP in M1
COP
3.0
COPperi.comb in M4
2.8 2.6 2.4 2.2 29
30
31
32
33
34 35 36 37 38 39 Dry bulb temperature outside (°C)
40
41
42
43
44
30
31
COPperi.TRTHE in M3
4.0 3.6 3.2 2.8 2.4 2.0 1.6 14
15
16
17
18
19 20 21 22 23 24 25 26 Dry bulbtemperature outside at night (°C)
27
28
29
Fig. 2. Average coefficient of performance during M1, M3 and M4 respectively.
Please cite this article in press as: D. Qv, et al., Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.030
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
Table 8 Operating parameters in M2. Dry bulb temperature outside at night ( C)
Operating parameters
Compressor power consumption (kW) Cooling charge duration (min) Amount of compressor power consumption (MJ) Average temperature of PCM ( C) Mean square temperature error of PCM ( C)
15
20
25
30
35
1.22 16 1.17 1.8 4.5
1.27 16 1.22 1.3 3.8
1.41 17 1.44 2.4 4.7
1.56 26 2.43 2.5 5.1
1.69 33 3.35 2.4 5.5
paralleling the TRTHE system (M8), as well as by the TRTHE system with solar hot water assisting (M9) was considered, organized and implemented respectively. First, M5 was operated at the typical testing condition of ambient dry bulb temperature of 7 C according to National Standard of Unitary Air Conditioners in China (GB/T 17758-2010). Sequentially, heating performances during the different heating modes were analyzed, compared and discussed here. In the typical condition testing of M5, the average compressor power consumption and heating capacity achieves 1.52 kW and 4.256 kW respectively, which fulfill intended design. However, the average compressor power consumption and heating capacity decreases steadily with the ambient temperature ebbs away, and what worst is the heating COP of the ASHP system plunges with the outdoor air temperature lapse especially when the temperature below 10 C. Table 9 reflects the effect of ambient temperature on the compressor power consumption and heating capacity. It can be seen that heating capacity elapses away from 2.980 kW to 2.325 kW with descending ambient temperature from 10 C to 17 C, in the meanwhile, power consumption of the compressor declines from 1.42 kW to 1.37 kW. Thereafter, M7, M8 and M9 were carried out at the same testing condition respectively, and the testing results shows in Table 10. It can be seen from Table 10 that space heating by the TRTHE system with solar hot water assisting (M9) shows the maximum heating capacity as well as the minimum compressor power consumption among those three different heating modes. Heating COP during M5 and M9 achieves 1.7 and 2.8 under the same ambient temperature of 17 C, which means that energy efficiency of M9 is 66% higher than that of M5. Heating discharge for the TRTHE units alone (M7) reflects higher power consumption of the compressor yet weak capacity in space heating. Although heating discharge for the TRTHE units with the ASHP system assisting (M8) demonstrates the highest compressor power consumption, the capacity in space heating for M8 seems the second lowest. Some perspective of these phenomena might be given like that the optimum in space heating capacity comes from better condition of heat transfer, which should attribute to the positive and steady heat flux from moving solar hot water via the TRTHE units (during M9). While the solar hot water omitted mode (M7) shows the second highest heating capacity because of the mild evaporation environment comparing with severely ambient temperature surroundings of 17 C. However, distribution of the mass flow rate for refrigerant between the TRTHE units and the outdoor finned tube heat exchanger (FTHE) might result in insufficient heat transfer in both of the TRTHE side
and the FTHE side, which causes the negative performance of the system in heating capacity. On the other hand, the average temperature and mean square temperature error for PCM in one unit of the TRTHE system may indicate some about heat transfer capacity or efficiency between refrigerant and PCM. In the three different heating modes, the minimum average temperature and mean square temperature error of PCM both occur in the M7, when solar hot water assisting heat energy discharge (M9), the average temperature of PCM rising certainly, yet the mean square temperature error for PCM increases also. It seems that the solar hot water brings positive and steady heat flux to PCM, while also disturbs temperature distribution simultaneously. In the M8, the mass flow rate of refrigerant via the TRTHE units is undermined by the FTHE. The evaporating area is amplified though, the available ones is diminished actually. Therefore the average temperature of PCM in the M8 is higher than that in the M7, and the mean square temperature error achieves climax in the meanwhile. Overall the space heating capacity of the ASHP system on the usual ambient temperature setting is available and optimistic. Yet when the outdoor air temperature slumps below 10 C, switching the outdoor FTHE to the TRTHE units is acceptable and advisable. And it will be perfect for the heating capacity as well as energy efficiency if the solar hot water is available. Nevertheless, space heating applications for the TRTHE system paralleling the ASHP system should be deliberated over first on the coupling approaches and strategies.
3.3. Operating strategy transition test during space heating period According to National Standard of Unitary Air Conditioners in China (GB/T 17758-2010), operating strategy transition test within running system at a typical condition was implemented. testing condition was controlled at ambient dry bulb temperature of 17 C; volume flow of water for space heating of 500 L h1; supply/return water temperature for space heating of 45.3/40.9 C. Fig. 3 reflects the effect of switching the outdoor FTHE to the TRTHE units on the compressor power consumption, suction/discharge pressure of the compressor, water temperatures for space heating and the suction/discharge temperature of the compressor within the operating system. It is needed to note that zero point of the time axis in Fig. 3 marks the instant of switchover. It can be seen that the compressor power consumption rises
Table 10 Operating parameters in comparison among diverse space heating mode. Table 9 Operating parameters in M5 during low ambient temperature condition. Operating parameters
Heating capacity (kW) Compressor power consumption (kW)
Operating parameters
Space heating mode M5
M7
M8
M9
Compressor power consumption (kW) Heating capacity (kW) Average temperature of PCM ( C) Mean square temperature error of PCM ( C)
1.37 2.325 e e
1.40 2.931 4.8 4.8
1.56 2.656 5.5 6.9
1.27 3.580 16.8 5.2
Dry bulb temperature outside ( C) 10
12
15
17
2.980 1.42
2.660 1.40
2.363 1.39
2.325 1.37
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
1.76
7
48
Suction pressure Discharge pressure
1.60
0.32
Temperature (°C)
Pressure (MPa)
47 46
Return water Supply water
45
42 41
0.16
0
60
120
1.76
180 Time (s)
240
300
40
360
0
60
120
78
180 Time (s)
240
300
360
76 74
Temperature (°C)
Power (kW)
Suction temperature Discharge temperature
72
1.72
1.68
1.64
70 68 66 64 0 -2 -4 -6
1.60
0
60
120
180 Time (s)
240
300
360
-8
0
60
120
180 Time (s)
240
300
360
Fig. 3. Impact of operating strategy transition on the suction/discharge pressure, compressor power consumption, water temperatures for space heating and the suction/discharge temperature during the building heating period.
from 1.61 kW to 1.75 kW at the instant of switchover, and this power value lasts for 6 s then reduces steadily. Finally, the compressor power consumption achieves 1.68 kW and keeps stable at 108 s after switching until the end of test. The suction pressure of the compressor increases from 0.19 MPa to 0.39 MPa during the first 60 s after switching, and then the value starts decreasing steadily. Finally, the suction pressure achieves 0.35 MPa at 240 s after switching. The discharge pressure rises from 1.66 MPa to 1.76 MPa during the first 60 s after switchover, then the value declines steadily, and the value of the discharge pressure achieves 1.74 MPa at 120 s after switching. The suction temperature of the compressor rises from 6.2 C to 0.7 C during the first 120 s after switching, and then the value starts to ascend steadily. Finally, the suction temperature achieves 1.7 C at 300 s. The discharge temperature drops from 76.9 C to 65.7 C during the first 120 s, then the value declines steadily, and finally achieves 65.2 C at 240 s. On the other hand, change of water temperature for space heating seems tardy comparing with the alteration of the operating pressures. The temperature of the return water for space heating increases from 40.9 C to 41.0 C during the second 60 s after switching, then the value keeps rising during the next 120 s, and the temperature achieves 42.1 C at 240 s after switching finally. The temperature of supply water for space heating rises from 45.3 C to 47.7 C during the second 60 s after switchover and achieves relative stability to 47.4 C at 180 s after switchover finally. Therefore, the maximum percentage change of the compressor power consumption is less than 9%, the maximum percentage change of the suction pressure and discharge pressure is 105% and 6%, respectively. And the maximum percentage change of the temperature of return/supply water is 3% and 5%, respectively. The
outdoor FTHE and the TRTHE units as the evaporator of the integrated system during space heating period, switching among heat exchange elements leads to acute change in the evaporating temperature, resulting in the most obvious alteration occurs in the suction pressure rather than the other parameters. Further, the obvious increasing of the suction pressure reflects the improvement in the heat transfer conditions by switching off the FTHE outside and switching on the TRTHE units simultaneously under low ambient temperature conditions. Besides that, the change of water temperatures for space heating lags behind the alteration of the operating pressures, which should be attributed to the greater thermal capacity of water. Moreover, the change of the temperature of return water for space heating seems tardy than that of supply water for space heating, the reason may be that the supply water recovers heat from exhaust of compressor directly, whereas the return water discharges heat into the indoor space where is an invariably temperature controlled room. 4. Conclusions A PCM based solareair source heat pump system (PCM-SAHP) has been developed. A prototype of the system was built up. Ten groups of tests were organized and implemented on the experimental platform. Operating performance of PCM-SAHP system proposed were also measured and analyzed. From the present study, the following conclusions might be drawn. (1) Novel LHTES facility e the TRTHE units, within heat pump systems can be operated more flexibly, effectively, and stably through thermal energy charge and discharge.
Please cite this article in press as: D. Qv, et al., Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.030
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
(2) Ambient temperature has a strong effect on the cooling COP. The combined cooling COP for the TRTHE system assisting the ASHP system is about 17% higher than that of the ASHP system at the same environment. During high ambient temperature days, the TRTHE units assisting the ASHP system for space cooling is an advisable choice to enhance cooling capacity and efficiency. (3) Night ambient temperature has a significant effect on the compressor power consumption and duration of cooling storage, resulting in the dominant impact on the cooling cycle COP for the TRTHE system. (4) Comparing with space heating by the ASHP system during low ambient temperature days (when the outdoor air tem-
Five Year National Science and Technology Support Program (No. 2011BAJ05B04) and Beijing Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering (No. NR2013K06). Suggestions and support on experiments by Guangdong Jirong Airconditioning Company are highly appreciated. Appendix. Error analysis The maximum error associated with pressures, temperatures, volume flows, electric power and time has been listed in Table 6. Therefore, the propagation of uncertainties associated with the calculated COPs can be evaluated as follows:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 2 ! V$DT 2 2 r$cp$w u 1 Vo w 2 $ ðdðT ÞÞ2 þ ðdðT ÞÞ2 þ $dðVo Þ þ dlim ðCOPe$ASHP Þ ¼ $tðDTw Þ2 $ $dðtÞ $ d Wcomp þ V R S t Wcomp Wcomp t2 vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 2 ! V$DT 2 2 r$cp$w u 1 Vo w 2 t 2 $ ðdðT ÞÞ2 þ ðdðT ÞÞ2 þ $dðVo Þ þ $ ðDTw Þ $ $dðtÞ $ d Wcomp þ V dlim ðCOPc$ASHP Þ ¼ R S t Wcomp Wcomp t2
(A1)
(A2)
r$cp$w dlim ðCOPcomb Þ¼ $ Wcomp 0vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 u 2 2 2 ! V u 1 V $DT 2 2 o$ASHP 2 2 ASHP w$ASHP 2 BtðDT C $dðVo$ASHP Þ þ $dðtÞ $ d Wcomp þV ASHP $ ðdðTS$ASHP ÞÞ þðdðTR$ASHP ÞÞ þ w$ASHP Þ $ B C t Wcomp t2 B C B C B C ffiC B vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! 2 2 2 B u C B u 1 V VTTHE $DTw$TTHE 2C 2 2 2 @þtðDTw$TTHE Þ2 $ A $dðVo$TTHE Þ þ o$TTHE þ $dðtÞ $ ðdðT ÞÞ þðdðT ÞÞ $ d W þV comp R$TTHE S$TTHE TTHE t Wcomp t2 (A3)
dlim COPperi$TTHE
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 DT 2 2 r$Vo $cp,w w $ ðdðTS ÞÞ2 þ ðdðTR ÞÞ2 þ $ t$d Wcomp þ Wcomp $dðtÞ ¼ Wcomp $t Wcomp $t
(A4)
dlim COPperi$comb ¼
r$Vo $cp,w $ Wcomp$I $tI þWcomp$II $tII ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 2 DTw 2 2 ðdðTS ÞÞ þðdðTR ÞÞ þ $ tI $d Wcomp$I þ Wcomp$I $dðtI Þ þ tII $d Wcomp$II þ Wcomp$I $dðtI Þ Wcomp$I $tI þWcomp$II $tII (A5)
perature slumps below 10 C), the TRTHE units applying for space heating seems viable and acceptable. When the solar hot water is available, heating discharge capacity and efficiency becomes better. (5) Parameter fluctuation owing to switching among heat exchange elements is petty and transitory that will not impact the reliability or stability of the PCM-SAHP system. Acknowledgment This work was financially supposed by the National Natural Science Foundation of China (No. 51178133, 51476049), the Twelfth
By calculating from Eqs. (A1) to (A5), the maximum error (absolute values and relative values) of the COPs can be evaluated as Table A1 shown, which suggests that measuring data and calculated data are acceptable uncertainty values in engineering applications. Additionally, in this study, each experimental unit has been repeated for 7 times, and the test unit can be terminated when the deviation of experimental results controlled availably below 5%. All coefficients of dispersion for the variant coefficients of performance during tests have been shown in Table A2, which verify the reliability of the prototype proposed.
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
9
Table A1 Error analysis for indirect observing data. Name
Limit error
Relative error (%)
Qe Qc Qaccu Wcomp·accu WTRTHE$accu Qaccu$ASHP Qaccu$TRESE COPe$ASHP COPc$ASHP COPperi$TRTHE COPperi$comb COPcomb
0.2 kW 0.2 kW 0.2 MJ 4 kJ 4 kJ 2 MJ 2 MJ 0.1 0.1 0.1 0.1 0.1
4 4 4 0.3 0.3 4 4 3 3 3 3 3
Table A2 Coefficients of dispersion for variant coefficients of performance COP
COPe$ASHP
Operating mode
Outdoor temperature ( C)
The repeated number for each testing unit 1
2
3
4
5
6
7
M1
30.0 32.0 35.0 38.0 40.0 35.0 38.0 40.0 43.0 7.0 10.0 12.0 15.0 17.0 17.0 15.0 20.0 25.0 30.0 35.0 35.0 38.0 40.0 43.0 35.0 38.0 40.0 43.0 e
2.8 2.6 2.6 2.4 2.4 2.6 2.4 2.4 2.1 2.8 2.1 1.9 1.7 1.7 1.6 3.8 3.5 3.1 1.8 1.3 3.1 2.7 2.5 2.3 3.8 3.5 3.3 3.0 2.8
2.9 2.7 2.6 2.3 2.3 2.6 2.3 2.3 2.1 2.8 2.0 1.9 1.6 1.6 1.7 3.7 3.6 3.1 1.9 1.3 3.0 2.7 2.5 2.3 3.7 3.6 3.3 3.1 2.8
2.9 2.6 2.5 2.4 2.3 2.5 2.4 2.3 2.1 2.8 2.1 1.9 1.7 1.6 1.6 3.7 3.5 3.0 1.9 1.4 3.0 2.8 2.5 2.2 3.7 3.5 3.4 3.0 2.8
2.8 2.7 2.6 2.4 2.4 2.6 2.4 2.4 2.0 2.7 2.1 2.0 1.7 1.7 1.7 3.8 3.5 3.1 1.8 1.3 3.1 2.7 2.5 2.3 3.8 3.5 3.3 3.0 2.9
2.9 2.6 2.6 2.3 2.3 2.6 2.3 2.3 2.1 2.8 2.1 1.9 1.6 1.7 1.7 3.7 3.6 3.1 1.8 1.4 3.0 2.7 2.6 2.3 3.7 3.6 3.3 3.1 2.8
2.8 2.8 2.6 2.4 2.3 2.6 2.4 2.3 2.1 2.8 2.0 1.9 1.7 1.7 1.6 3.7 3.5 3.0 1.9 1.3 3.0 2.7 2.5 2.2 3.8 3.5 3.3 3.0 2.8
2.9 2.7 2.6 2.3 2.4 2.6 2.3 2.4 2.0 2.8 2.1 1.9 1.7 1.7 1.7 3.7 3.5 3.0 1.8 1.3 3.1 2.8 2.5 2.3 3.7 3.6 3.4 3.0 2.8
M4
COPc$ASHP
M5
COPperi$TRTHE
M8 M2
COPperi$comp
M2
COPcomp
M2
COPc$TRTHE
M9
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0.019 0.016 0.015 0.023 0.023 0.015 0.023 0.023 0.024 0.014 0.023 0.020 0.029 0.029 0.032 0.013 0.014 0.017 0.029 0.037 0.018 0.018 0.015 0.021 0.014 0.015 0.015 0.016 0.013
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Nomenclatures AC: air conditioning ASHP: air source heat pump COP: coefficient of performance COPc·ASHP: heating COP of ASHP COPe·ASHP: cooling COP of ASHP COPperi·TTHE: cooling COP of TTHE during a cooling cycle COPperi·comb: combined cooling COP during a cooling cycle COPcomb: combined cooling COP FTHE: finned tube heat exchanger HP: heat pump HTF: heat transfer fluid LHTES: latent heat thermal energy storage PHE: plate heat exchanger Qaccu: amount of cooling capacity, MJ Qc: heating capacity, kW Qe: cooling capacity, kW Qe·ASHP: cooling capacity of ASHP, kW Qe·TTHE: cooling capacity of TTHE, kW TES: thermal energy storage TRTHE: triplex tube heat exchanger TR: temperature of return water for space cooling or heating, C TS: temperature of supply water for space cooling or heating, C TR·ASHP: temperature of return water for ASHP, C TS·ASHP: temperature of return water for ASHP, C TR·TRTHE: temperature of return water for TRTHE, C TS·TRTHE: temperature of supply water for TRTHE, C V: volume flow of water for space cooling or heating, m3 s1 VASHP: volume flow of water for space cooling or heating by ASHP, m3 s1 VTRTHE: volume flow of water for space cooling or heating by TRTHE, m3 s1 VO: volume of water for space cooling or heating, m3 VO·ASHP: volume of water for space cooling or heating by ASHP, m3 VO·TRTHE: volume of water for space cooling or heating by TRTHE, m3 Wcomp: compressor power consumption, kW Wcomp·accu: amount of power consumption for compressor, MJ Wcomp·I: compressor power consumption for cold storage, kW Wcomp·II: compressor power consumption for space cooling by TTHE assisting ASHP, kW WTTHE·accu: amount of compressor power consumption for cooling charge, MJ cp·w: specific heat at constant pressure, kW kg1 K1 r: density of water, kg m3 t: duration of cooling or heating, s tI: duration of cold storage, s tII: duration of space cooling by TRTHE assisting ASHP, s DTw: temperature difference between supply and return water for space cooling or heating, C DTw·ASHP: temperature difference between supply and return water for ASHP, C DTw·TRTHE: temperature difference between supply and return water for TRTHE, C
Please cite this article in press as: D. Qv, et al., Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.030
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition Dehu Qv a, Long Ni a, *, Yang Yao a, **, Wenju Hu b a
School of Municipal and Environmental Engineering, Harbin Institute of Technology, 150090 Harbin, China Beijing Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering, Beijing University of Civil Engineering and Architecture, 100044 Beijing, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2015 Received in revised form 14 May 2015 Accepted 10 July 2015 Available online xxx
In the recent decade, with solar energy assisted heat pump systems have increasingly developed. In the previous studies, a hybrid air source heat pump (ASHP) system was proposed, which coupled with latent heat thermal energy storage (LHTES) and solar thermal collector, for operating in various types of configurations. This paper describes the approach and principle for organizing the hybrid system in detail. Thereafter, a phase change material (PCM) based solareair source heat pump (PCM-SAHP) prototype was set-up and implemented under variant testing conditions. Experimental results demonstrate that the PCM-SAHP system presented remarkable advantages on correcting the mismatch between supply and demand of thermal energy and electricity. Further, when the ambient temperature was higher than 38 C, cooling COP of the hybrid system enhanced by 17%, compared with that of ASHP system under same surroundings. During the days that outdoor air temperature was below 10 C, heating COP of the PCM-SAHP system rose by 65% comparing with that of ASHP system. In additional, switching operating strategies during system running will scarcely result in the violent or continuous fluctuations on the operating parameters. Therefore, the efficiency of the PCM-SAHP systems can be improved with capacity lapse avoiding, and exhaust controlling as well. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Hybrid heat pump system Solar thermal energy Latent heat thermal energy storage Efficiency improving Exhaust controlling
1. Introduction Recent two decades, a growing number of individuals and communities incline space heating and cooling by heat pumps (HP) rather than conventional boiler system paralleling air conditioning (AC) system on account of higher energy efficiency and milder environment impact. Advanced cycle designs, improved cycle components and expanded cycle applications are the main development channels in HP technology [1]. Recently, latent heat thermal energy storage (LHTES) within HP system for space/water heating and cooling has received increasingly interests because of the flexibility, efficiency and stability through charge and discharge of thermal energy [2]. Phase change material (PCM) within passive heat storage building may be the simplest application for LHTES. PCM thermal board (PCMTB) and PCM thermal shield (PCMTS) can balance
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Ni), [email protected] (Y. Yao).
thermal energy substantially inside building [3e6]. Building structure, orientation, situated climatic region, as well as thermal property of building affects the effect of PCMTB and PCMTS [7,8] subtly. Active and-or passive heat storage with PCM for AC do not only slash energy consumption but improve adaptability of PCM [9e11]. Yet application of PCM defrosting reflects a brilliant improvement in system overall efficiency versus lower investment [12]. On the other hand, free cooling technique shows a nice potential in energy saving, efficiency enhancement and environment friendliness [13e16]. However ice storage might be a longer historical technology, which recently focuses on improving synthetic energy efficiency for the whole system [17e20]; by contrast, PCM with higher phase transition temperature within AC/HP seems more promising to rise evaporating temperature, therefore the optimistic capacity and efficiency can be anticipated [21,22]. On the other lens, PCM within AC for condensing heat recovery seems flexible, efficient and convenient [23,24] also. Actually, applications of PCM for residential cooling and heating are much more than that. This paper presents a PCM based hybrid solareair source heat pump (PCM-SAHP) system for building cooling and heating applications. Basic design concept, structure and components, and
http://dx.doi.org/10.1016/j.renene.2015.07.030 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
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2
D. Qv et al. / Renewable Energy xxx (2015) 1e10
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operating principle of the PCM-SAHP system are described prior. Sequentially, a testing prototype are organized and produced. Then details of testing methods and materials are serviced. Finally, performance of the system proposed during typical conditions are proposed and discussed; reliability and stability of that at the instant of operating strategy transition is carried out additionally. Experimental performances include compressor power consumption, cooling and heating capacity and COP, as well as temperature features of PCM in the LHTES facility. 2. Methods or materials 2.1. The prototype of the PCM-SAHP system A PCM-SAHP system consists of four elements: ASHP, solar thermal collector, the triplex tube heat exchange (TRTHE) system encompassing PCM, as well as data acquisition and control sector. Briefly, this hybrid system could charge chilled energy in the TRTHE system during cool summer night with profit of the special discount tariff in electricity, and discharge chilled energy from the TRTHE system during peak energy period with high ambient temperature. Further, during low temperature days, this coupled system can store heating energy in the TRTHE system with solar thermal energy assisted and supply hot water or air, which avoids decrease in both heating capacity and COP, and refrains from increasing in compressor pressure ratio the ASHP system suffering. Therefore, the PCM-SAHP system could charge and discharge cooling/heating energy by a same facility, TRTHE which is the key component of the PCM-SAHP system. Details about the system and TRTHE were presented in literature [25,26]. Niu et al. primarily presented the TRTHE system based PCM-SAHP system. And feasibility of the TRTHE system within compression vapor cycle has been proved. Schematic diagram of the PCM-SAHP system and structure of the TRTHE unit is shown in Fig. 1, while Specifications of the prototype are listed in Table 1. In this study, the TRTHE units and a prototype of the PCM-SAHP system are redesigned and organized. Additionally, nine operating modes and corresponding ten groups of tests for building cooling and heating are projected and implemented. The TRTHE units within PCM-SAHP systems can fulfill nine operating modes, including space cooling by ASHP mode (M1), cold storage mode (M2), space cooling by TRTHE mode (M3), space cooling by TRTHE assisting ASHP mode (M4), space heating by ASHP mode (M5), heat storage mode (M6), space heating by TRTHE mode (M7), space heating by TRTHE assisting ASHP mode (M8) and space heating by TRTHE with solar hot water assisted mode (M9). All of these modes are listed in Table 2. During the summer night the hybrid system charges chilled energy into the TRTHE system using off-peak electricity (M2). When the cooling load of chamber is lower, the TRTHE system can provide stable chilling water for space cooling without compressor operation (M3). In the highest cooling load environment, the TRTHE system assists ASHP to serve for space cooling (M4). In winter, on the days of cloud or rain/snow free, solar hot water (about 30 C) from the solar thermal collector stores heat into the TRTHE units that service as the low grade heat source for the heat pump system to avoid awkward performance of the ASHP system under low ambient temperature surroundings. Moreover, the TRTHE units within PCM-SAHP systems just perform as an evaporator in the compression vapor cycle, which eases oil return in the refrigerant circulation. The TRTHE unit is made up of three concentric copper tubes and PCM that embedded in the space where the inner tube and the medium tube encompassed. Refrigerant circulates inside the inner tube driving by compressor, and water passes through the space between the outer tube and the medium tube as heat transfer fluid
E6 S4
S3
E4
2 P
3 P
P
R1 E1
9
P
R2 S2
E5
6
5
E2
S1
E9 B A E8
4
1
E7
A B
P
R3 E3
F1 V1 7
Solenoid Valve Flowmeter
Check Valve
Thermometer
Manual Valve
Throttle
Piezometer
Distributor
(a) Schematic diagram of PCM-SAHP system
Water Outlet Refrigerant Outlet
Key Thermal Points
PCM Wat. 35mm×1.2mm PCM 22mm×1.0mm Refr. 9.52mm×0.5mm
Refrigerant Inlet Water Inlet
(b) Structure of TRTHE unit Fig. 1. Schematic diagram of PCM-SAHP system and structure of TRTHE unit.
(HTF). The material of RT5HC, as the PCM embedded in the TRTHE system was used in this equipment. Three concentric copper tubes are weld together after spinning process, and thermally insulated envelope was used to reduce heat loss. Now, thermal energy storage capacity is the overriding parameter for the TRTHE system, which depends on the operating strategy of the hybrid system and the characteristics of air conditioning load for building. In this paper, a virtual building situated in Shanghai (China) was selected as the reference of air conditioning load and operating strategy for the devisal of thermal energy storage capacity in the TRTHE units, in where cooling/ heating load is approximate balance in general buildings. According to the initial cost of equipments and the special discount tariff in Shanghai, partial thermal energy storage is appropriate, meaning that thermal energy discharge from the TRTHE system has a responsibility to eliminate part of thermal energy load during a whole day. At the beginning and end of space cooling season (less than 90 days) the percentage of cooling load to maximum value is about 30%, the TRTHE system can maintain thermal comfort level in chamber without any other chilled energy source operation from 10:00 to 18:00. On the other hand, during high ambient temperature days (about 100e120 days), the TRTHE units assisting ASHP system can discharge cooling energy from 10:00 to 18:00 to control the temperature and humidity in indoor space.
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
3
Table 1 Specifications of the prototype. Component
Configurations
Remarks
Compressor TRTHEs FTHE PHE Throttle Refrigerant
Rated load current of 11.4 A and voltage of 220e240 V Details in Table 4 Total length of heat transfer tube: 56 m; face velocity: 2.5 m s1 Heat transfer area: 0.045 m2 per piece; total number of heat transfer plate: 38 pieces Thermal expansion valve R22
Scroll compressor 3 groups Be outside Brazed plate heat exchanger e e
2.2. Experimental set-up
Table 2 Operation modes of the prototype. Mode
M1 M2 M3 M4 M5 M6 M7 M8 M9
Manual valve V1
Solenoid valve E1eE9
Pump 5
Pump 7
On
Off
On
Off
On
On
C C
2, 3 4, 2, 1, 6, 3, 1, 3,
1, 3e7 1, 2, 4e9 1e3, 6e9 1, 3, 6, 7 2e7 1e5, 8, 9 1, 2, 4e7 2, 4e7 1, 2, 4,5
C
C C C C C C C
8, 9 5 4, 5, 8, 9 8, 9 7 8, 9 3, 8, 9 6e9
Off C C
C C
Off C C
C C C
C C C C
C C C C
Table 3 Capacity design for three groups of TRTHE. Capacity design
The quotient about cooling capacity for TRTHE to total capacity
Mass consumption of PCM (kg) Chilled energy storage (MJ) Duration of combined space cooling (h d1) The number of combined space cooling days (d) The number of space cooling by TRTHE days (d)
24%
28%
32%
6.37 1.56 9 20 20
8.12 1.99 9 42 44
10.17 2.49 9 42 60
Table 3 shows the effect of the quotient of cooling capacity for the TRTHE system on the mass consumption of PCM, chilled energy storage capacity, duration of combined space cooling, the number of combined space cooling days, and the number of space cooling by TRTHE days. Finally, quotient 32% of total cooling capacity is adopted. Thereafter, structure design for the TRTHE units is the key in hybrid system designing, and highlight of structure design is ascertaining the thickness of PCM in the TRTHE units. In this paper, diameter of refrigerant tube is given, and diameter of the medium tube is the core. Combining heat transfer equations with thermodynamic equations can be achieved. Considering demand of cooling charge fast, and heating discharge mild and steady, thickness of PCM in the TRTHE units should be controlled in 6 mm. Geometric characteristics of the TRTHE unit are given in Table 4, and Thermodynamic characteristics of the PCM are listed in Table 5.
In this paper, experimental platform comprises a prototype of hybrid system proposed and a large enthalpy difference laboratory that was set up according to the National Standard of Unitary Air Conditioners in China (GB/T 17758-2010). The laboratory includes outdoor space, indoor space and control space. The air temperature and humidity at the outdoor/indoor space could be controlled accurately by the air conditioning system. Three groups of TRTHE unit for testing were prepared, and each group of that comprises three rotor flow meters installed at three units of water pipe side, respectively. Other measuring points including pressure points for observing refrigerant circulation, temperature points installed at refrigerant pipe side for observing and controlling compression vapor cycle, temperature points at water pipe side for measuring and manipulating operation of the system and modes, and volume flow points on the hot/cold water supply pipe, have been plotted in Fig. 1. Additionally, electric parameters and time are observed and recorded likewise. Particularly, thermistors to measure temperature of PCM in the TRTHE unit are installed along the direction of fluid flow per 0.8 m. All sensors are connected to a stand-alone data logger unit to record all measured values, and the data logger is connected to a PC, therefore, the operating data could be downloaded to the PC for storage and subsequent analysis. All test parameters except electric parameters are measured and recorded every 30 s, and electric parameters are observed and recorded every 6 s. Details about the apparatus and instruments used in the experiment are listed in Table 6. In additional, it is necessary to confessed that the solar hot water is prepared by the hot water tank to avoid negative impacts from outdoor weather-setting. To perceive performances of the PCM-SAHP system during severely high/low ambient temperature conditions, as well as stability and reliability at the instant of switching heat exchange elements, a series of tests were designed and implemented. Tests comprehend severe setting experiments as seen in Table 7 and switchover experiments. Each test lasted for 60 min after the hybrid system reached an asymptotic state. Each experimental unit repeated for 7 times, and the test unit can be terminated when the deviation of experimental results controlled availably below 5%. Being an evidence of the reliability for the prototype during operation, all coefficients of dispersion for variant coefficients of performance under different tests have been discussed in Appendix. As
Table 4 Geometry details of TRTHE. Items
Unit
Number
Diameter of the inner tube (outer diameter/wall thickness) Diameter of the medium tube (outer diameter/wall thickness) Diameter of the outer tube (outer diameter/wall thickness) Length of the tube (available heat transfer section) Total length of the tube per unit (available heat transfer section) Total number of unit Total mass of the PCM per group Amount of thermal energy storage per group
mm mm mm m m e kg kJ
9.52/0.5 22/1.0 35/1.2 0.8 64 3 3.5 857.5
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D. Qv et al. / Renewable Energy xxx (2015) 1e10 Table 5 Thermodynamic characteristics of the PCM. Item
Unit
Typical values
Melting area Congealing area Heat storage capacity Density solid Density liquid Volume expansion in phase change range Heat conductivity Kin. Viscosity at 20 C Flash point Corrosion
5e6 6e5 245 0.88 0.763 12 0.2 3.01 115 Chemical inert with respect to most materials
C C kJ kg1 kg L1 kg L1 % W m1 K1 mm2 s1 C e
Table 6 Experimental instruments. Parameters
Name
Measuring range
Measuring accuracy
Remarks
Temperature Pressure Pressure Volume flow Volume flow Electric power Time
NTC thermistor Pressure transmitter Pressure transmitter Water meter Rotameter e Stopwatch
40 to 60 C 0e1.5 MPa 0e3.0 MPa 0e106 m3 10e100 L h1 0.4e12 kW e
±0.1 C ±0.001 MPa ±0.001 MPa ±105 m3 ±2.25 L h1 ±0.4% 0.06 s
e Except discharge pressure Discharge pressure For plate heat exchanger For TRTHE e e
Table 7 Experiments during diverse operating modes. Mode Experimental conditions M1 M2 M3 M4 M5 M6 M7 M8 M9
The ambient temperature is 30, 32, 35, 38, 40 C respectively; volume flow of water for space cooling is 800 L h1; temperature of return water for space cooling is 12 ± 0.5 C. The ambient temperature is 15, 20, 25, 30, 35 C respectively; the activation condition is PCM average temperature of 15 ± 0.5 C; the termination criterion is the PCM minimum temperature of below 0 C. Temperature of return water for space cooling is 18 ± 0.5 C; volume flow of water through TRTHE is 40 L h1; the termination criterion is the supply chilling water temperature of above 17 C. The ambient temperature is 35, 38, 40, 43 C respectively; volume flow of water through the plate heat exchanger is 800 L h1; volume flow of water through TRTHE is 40 L h1; temperature of return water for space cooling is 12 ± 0.5 C; The ambient temperature is 7, 10, 12, 15, 17 C respectively; volume flow of water for space heating is 1260 L h1; temperature of return water for space heating is 31 ± 0.5 C. The temperature of solar hot water is 28 C, the volume flow of solar hot water is 300 L h1, the termination criterion is the temperature of PCM at the outlet of TRTHE above 20 C. Volume flow of water for space heating is 1260 L h1; temperature of return water for space heating is 31 ± 0.5 C. The ambient temperature is 17 C; volume flow of water for space heating is 1260 L h1; temperature of return water for space heating is 31 ± 0.5 C. The temperature of solar hot water is 28 C; volume flow of solar hot water is 300 L h1; volume flow of water for space heating is 1260 L h1; temperature of return water for space heating is 31 ± 0.5 C.
seen in Table A2, the maximum coefficient of dispersion is 0.029, which is acceptable in this study. In these tests, chilling water to TRTHE was simulated by chilling water tank, and hot water into condenser for space heating was prepared by cooling water tank. Volume flow of water through the TRTHE units, and that through the plate heat exchanger (PHE) should be controlled at the setting point prior. Next, ensure that water temperature at the inlet of the TRTHE units and that of PHE should be steady at the setting point. And then, coordinate temperature and humidity in the indoor/ outdoor space in the given conditions. To simplify the data processing and analysis, this paper takes the amount capacity for cooling charge as constant and measures the duration and energy consumption for cooling charge. The COPs for performance analysis are needed to calculate by Eqs. (1) to (5); paraphrases for each symbol are listed in Nomenclatures.
¼ Qe Wcomp
(1)
COPc$ASHP ¼ Qc Wcomp
(2)
COPe$ASHP
COPperi$TRTHE ¼ Qaccu Wcomp$accu COPperi$comb ¼ Qaccu
Wcomp$accu þ WTRTHE$accu
COPcomb ¼ Qe$ASHP þ Qe$TRTHE Wcomp
(3)
(4) (5)
Further, related error analyses of these COPs are described in Appendix. From the Table A1, the maximum relative error is 4%, which is acceptable uncertainty value in engineering applications. 3. Results and discussion 3.1. Cooling tests during diverse cooling modes Cooling charge and discharge sequentially constituting a cooling cycle for the TRTHE system, and cooling cycle COP for the TRTHE system has been defined as Eq. (3). Similarly, combined cooling cycle COP considering compressor power consumption during cooling charge period has been presented in Eq. (4). According to National Standard of Unitary Air Conditioners in
Please cite this article in press as: D. Qv, et al., Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.030
D. Qv et al. / Renewable Energy xxx (2015) 1e10
China (GB/T 17758-2010), cooling tests during different cooling modes were organized and carried out. First, compressor power consumption and cooling capacity at typical condition of ambient temperature of 35 C achieves 2.08 kW and 5.408 kW respectively, which fulfill the expectation in design approximately. On the other lens, cooling charge duration at the typical testing condition of ambient nighttime temperature of 25 C is 17 min, approaching the design target. Encouragingly, the amount of cooling charge capacity achieves 2.49 MJ in three groups of TRTHE unit that fulfills the anticipation in design. Fig. 2 shows the effect of ambient temperature on the average COP during M1, M3 and M4 respectively. It can be seen that, the coefficient of performance during space cooling application declines with rising ambient temperature no matter which cooling mode is operating. The cooling COP for the ASHP system decreases markedly from 2.9 to 2.3 with increasing outdoor temperature from 30 C to 40 C. The cooling cycle COP for the TRTHE system elapses away from 3.7 to 3.0 with ascending ambient temperature at night from 15 C to 25 C, yet, when the dry bulb temperature outside arrives at 30 C, the cooling cycle COP plunges to 1.8. Encouragingly, when the ambient temperature rises from 35 C to 43 C, the combined cooling cycle COP declines mildly from 3.0 to 2.3, which is 17% higher than the cooling COP for the ASHP system under the same ambient temperature conditions. On the other lens, the mean of cooling capacity for the TRTHE units and total cooling capacity during M4 achieves 2.264 kW and 7.242 kW respectively. The percentage of the cooling capacity for three groups of TRTHE unit to the total cooling capacity during M4 is about 31% which almost approaches design value of 32%. Generally, in the vapor compression cycle, increasing condensing temperature means compressor power consumption rising and cooling capacity lapse, which is intelligible to us. While the effect of outdoor air temperature on the cooling cycle COP for the TRTHE system can be understood though the effect of air
5
temperature outside on the performance of cooling charge for the TRTHE system. Table 8 reflects the effect of ambient temperature on the compressor power consumption, cooling charge duration, as well as temperature characteristics of PCM in the TRTHE units. It can be seen from that both of compressor power consumption and cooling duration rise obviously with increasing outdoor temperature. Consequently, the amount of compressor power consumption during cooling charge period rockets from 1.17 MJ to 3.35 MJ with ascending ambient temperature at night from 15 C to 35 C in one group of TRTHE unit. Therefore, when the cooling discharge capacity is terminate, night ambient temperature dominates the accumulating compressor power consumption, as well as the cooling cycle COP certainly. On the other lens, it can be seen from Table 8 that the average temperature of PCM in one unit of the TRTHE system increasing from 1.3 C to 2.4 C with rising night ambient temperature from 15 C to 35 C, in the meanwhile, the mean square temperature error amplifies from 3.8 C to 5.5 C which may imply that PCM in the TRTHE units have not achieved the optimal state for saturating chilled energy storage. Nevertheless, the mean value and square error of temperature for PCM seems abnormal because of the higher values in temperature. It is plausible to attribute this phenomenon to undervaluation of the condensing temperature which results in an instable hydrodynamic and heat transfer state for refrigerant. Overall higher ambient temperature (may be 30 C or more) at night might cause huge energy input for cooling charge, therefore cooling discharge application just by the TRTHE units alone at daytime should be deliberated during high outdoor air temperature nights. 3.2. Heating tests during variant heating modes In the tests of space heating for building, space heating by ASHP alone (M5), by the TRTHE system alone (M7), by the ASHP system
3.2 COPe.ASHP in M1
COP
3.0
COPperi.comb in M4
2.8 2.6 2.4 2.2 29
30
31
32
33
34 35 36 37 38 39 Dry bulb temperature outside (°C)
40
41
42
43
44
30
31
COPperi.TRTHE in M3
4.0 3.6 3.2 2.8 2.4 2.0 1.6 14
15
16
17
18
19 20 21 22 23 24 25 26 Dry bulbtemperature outside at night (°C)
27
28
29
Fig. 2. Average coefficient of performance during M1, M3 and M4 respectively.
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
Table 8 Operating parameters in M2. Dry bulb temperature outside at night ( C)
Operating parameters
Compressor power consumption (kW) Cooling charge duration (min) Amount of compressor power consumption (MJ) Average temperature of PCM ( C) Mean square temperature error of PCM ( C)
15
20
25
30
35
1.22 16 1.17 1.8 4.5
1.27 16 1.22 1.3 3.8
1.41 17 1.44 2.4 4.7
1.56 26 2.43 2.5 5.1
1.69 33 3.35 2.4 5.5
paralleling the TRTHE system (M8), as well as by the TRTHE system with solar hot water assisting (M9) was considered, organized and implemented respectively. First, M5 was operated at the typical testing condition of ambient dry bulb temperature of 7 C according to National Standard of Unitary Air Conditioners in China (GB/T 17758-2010). Sequentially, heating performances during the different heating modes were analyzed, compared and discussed here. In the typical condition testing of M5, the average compressor power consumption and heating capacity achieves 1.52 kW and 4.256 kW respectively, which fulfill intended design. However, the average compressor power consumption and heating capacity decreases steadily with the ambient temperature ebbs away, and what worst is the heating COP of the ASHP system plunges with the outdoor air temperature lapse especially when the temperature below 10 C. Table 9 reflects the effect of ambient temperature on the compressor power consumption and heating capacity. It can be seen that heating capacity elapses away from 2.980 kW to 2.325 kW with descending ambient temperature from 10 C to 17 C, in the meanwhile, power consumption of the compressor declines from 1.42 kW to 1.37 kW. Thereafter, M7, M8 and M9 were carried out at the same testing condition respectively, and the testing results shows in Table 10. It can be seen from Table 10 that space heating by the TRTHE system with solar hot water assisting (M9) shows the maximum heating capacity as well as the minimum compressor power consumption among those three different heating modes. Heating COP during M5 and M9 achieves 1.7 and 2.8 under the same ambient temperature of 17 C, which means that energy efficiency of M9 is 66% higher than that of M5. Heating discharge for the TRTHE units alone (M7) reflects higher power consumption of the compressor yet weak capacity in space heating. Although heating discharge for the TRTHE units with the ASHP system assisting (M8) demonstrates the highest compressor power consumption, the capacity in space heating for M8 seems the second lowest. Some perspective of these phenomena might be given like that the optimum in space heating capacity comes from better condition of heat transfer, which should attribute to the positive and steady heat flux from moving solar hot water via the TRTHE units (during M9). While the solar hot water omitted mode (M7) shows the second highest heating capacity because of the mild evaporation environment comparing with severely ambient temperature surroundings of 17 C. However, distribution of the mass flow rate for refrigerant between the TRTHE units and the outdoor finned tube heat exchanger (FTHE) might result in insufficient heat transfer in both of the TRTHE side
and the FTHE side, which causes the negative performance of the system in heating capacity. On the other hand, the average temperature and mean square temperature error for PCM in one unit of the TRTHE system may indicate some about heat transfer capacity or efficiency between refrigerant and PCM. In the three different heating modes, the minimum average temperature and mean square temperature error of PCM both occur in the M7, when solar hot water assisting heat energy discharge (M9), the average temperature of PCM rising certainly, yet the mean square temperature error for PCM increases also. It seems that the solar hot water brings positive and steady heat flux to PCM, while also disturbs temperature distribution simultaneously. In the M8, the mass flow rate of refrigerant via the TRTHE units is undermined by the FTHE. The evaporating area is amplified though, the available ones is diminished actually. Therefore the average temperature of PCM in the M8 is higher than that in the M7, and the mean square temperature error achieves climax in the meanwhile. Overall the space heating capacity of the ASHP system on the usual ambient temperature setting is available and optimistic. Yet when the outdoor air temperature slumps below 10 C, switching the outdoor FTHE to the TRTHE units is acceptable and advisable. And it will be perfect for the heating capacity as well as energy efficiency if the solar hot water is available. Nevertheless, space heating applications for the TRTHE system paralleling the ASHP system should be deliberated over first on the coupling approaches and strategies.
3.3. Operating strategy transition test during space heating period According to National Standard of Unitary Air Conditioners in China (GB/T 17758-2010), operating strategy transition test within running system at a typical condition was implemented. testing condition was controlled at ambient dry bulb temperature of 17 C; volume flow of water for space heating of 500 L h1; supply/return water temperature for space heating of 45.3/40.9 C. Fig. 3 reflects the effect of switching the outdoor FTHE to the TRTHE units on the compressor power consumption, suction/discharge pressure of the compressor, water temperatures for space heating and the suction/discharge temperature of the compressor within the operating system. It is needed to note that zero point of the time axis in Fig. 3 marks the instant of switchover. It can be seen that the compressor power consumption rises
Table 10 Operating parameters in comparison among diverse space heating mode. Table 9 Operating parameters in M5 during low ambient temperature condition. Operating parameters
Heating capacity (kW) Compressor power consumption (kW)
Operating parameters
Space heating mode M5
M7
M8
M9
Compressor power consumption (kW) Heating capacity (kW) Average temperature of PCM ( C) Mean square temperature error of PCM ( C)
1.37 2.325 e e
1.40 2.931 4.8 4.8
1.56 2.656 5.5 6.9
1.27 3.580 16.8 5.2
Dry bulb temperature outside ( C) 10
12
15
17
2.980 1.42
2.660 1.40
2.363 1.39
2.325 1.37
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
1.76
7
48
Suction pressure Discharge pressure
1.60
0.32
Temperature (°C)
Pressure (MPa)
47 46
Return water Supply water
45
42 41
0.16
0
60
120
1.76
180 Time (s)
240
300
40
360
0
60
120
78
180 Time (s)
240
300
360
76 74
Temperature (°C)
Power (kW)
Suction temperature Discharge temperature
72
1.72
1.68
1.64
70 68 66 64 0 -2 -4 -6
1.60
0
60
120
180 Time (s)
240
300
360
-8
0
60
120
180 Time (s)
240
300
360
Fig. 3. Impact of operating strategy transition on the suction/discharge pressure, compressor power consumption, water temperatures for space heating and the suction/discharge temperature during the building heating period.
from 1.61 kW to 1.75 kW at the instant of switchover, and this power value lasts for 6 s then reduces steadily. Finally, the compressor power consumption achieves 1.68 kW and keeps stable at 108 s after switching until the end of test. The suction pressure of the compressor increases from 0.19 MPa to 0.39 MPa during the first 60 s after switching, and then the value starts decreasing steadily. Finally, the suction pressure achieves 0.35 MPa at 240 s after switching. The discharge pressure rises from 1.66 MPa to 1.76 MPa during the first 60 s after switchover, then the value declines steadily, and the value of the discharge pressure achieves 1.74 MPa at 120 s after switching. The suction temperature of the compressor rises from 6.2 C to 0.7 C during the first 120 s after switching, and then the value starts to ascend steadily. Finally, the suction temperature achieves 1.7 C at 300 s. The discharge temperature drops from 76.9 C to 65.7 C during the first 120 s, then the value declines steadily, and finally achieves 65.2 C at 240 s. On the other hand, change of water temperature for space heating seems tardy comparing with the alteration of the operating pressures. The temperature of the return water for space heating increases from 40.9 C to 41.0 C during the second 60 s after switching, then the value keeps rising during the next 120 s, and the temperature achieves 42.1 C at 240 s after switching finally. The temperature of supply water for space heating rises from 45.3 C to 47.7 C during the second 60 s after switchover and achieves relative stability to 47.4 C at 180 s after switchover finally. Therefore, the maximum percentage change of the compressor power consumption is less than 9%, the maximum percentage change of the suction pressure and discharge pressure is 105% and 6%, respectively. And the maximum percentage change of the temperature of return/supply water is 3% and 5%, respectively. The
outdoor FTHE and the TRTHE units as the evaporator of the integrated system during space heating period, switching among heat exchange elements leads to acute change in the evaporating temperature, resulting in the most obvious alteration occurs in the suction pressure rather than the other parameters. Further, the obvious increasing of the suction pressure reflects the improvement in the heat transfer conditions by switching off the FTHE outside and switching on the TRTHE units simultaneously under low ambient temperature conditions. Besides that, the change of water temperatures for space heating lags behind the alteration of the operating pressures, which should be attributed to the greater thermal capacity of water. Moreover, the change of the temperature of return water for space heating seems tardy than that of supply water for space heating, the reason may be that the supply water recovers heat from exhaust of compressor directly, whereas the return water discharges heat into the indoor space where is an invariably temperature controlled room. 4. Conclusions A PCM based solareair source heat pump system (PCM-SAHP) has been developed. A prototype of the system was built up. Ten groups of tests were organized and implemented on the experimental platform. Operating performance of PCM-SAHP system proposed were also measured and analyzed. From the present study, the following conclusions might be drawn. (1) Novel LHTES facility e the TRTHE units, within heat pump systems can be operated more flexibly, effectively, and stably through thermal energy charge and discharge.
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
(2) Ambient temperature has a strong effect on the cooling COP. The combined cooling COP for the TRTHE system assisting the ASHP system is about 17% higher than that of the ASHP system at the same environment. During high ambient temperature days, the TRTHE units assisting the ASHP system for space cooling is an advisable choice to enhance cooling capacity and efficiency. (3) Night ambient temperature has a significant effect on the compressor power consumption and duration of cooling storage, resulting in the dominant impact on the cooling cycle COP for the TRTHE system. (4) Comparing with space heating by the ASHP system during low ambient temperature days (when the outdoor air tem-
Five Year National Science and Technology Support Program (No. 2011BAJ05B04) and Beijing Key Lab of Heating, Gas Supply, Ventilating and Air Conditioning Engineering (No. NR2013K06). Suggestions and support on experiments by Guangdong Jirong Airconditioning Company are highly appreciated. Appendix. Error analysis The maximum error associated with pressures, temperatures, volume flows, electric power and time has been listed in Table 6. Therefore, the propagation of uncertainties associated with the calculated COPs can be evaluated as follows:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 2 ! V$DT 2 2 r$cp$w u 1 Vo w 2 $ ðdðT ÞÞ2 þ ðdðT ÞÞ2 þ $dðVo Þ þ dlim ðCOPe$ASHP Þ ¼ $tðDTw Þ2 $ $dðtÞ $ d Wcomp þ V R S t Wcomp Wcomp t2 vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2 2 ! V$DT 2 2 r$cp$w u 1 Vo w 2 t 2 $ ðdðT ÞÞ2 þ ðdðT ÞÞ2 þ $dðVo Þ þ $ ðDTw Þ $ $dðtÞ $ d Wcomp þ V dlim ðCOPc$ASHP Þ ¼ R S t Wcomp Wcomp t2
(A1)
(A2)
r$cp$w dlim ðCOPcomb Þ¼ $ Wcomp 0vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 u 2 2 2 ! V u 1 V $DT 2 2 o$ASHP 2 2 ASHP w$ASHP 2 BtðDT C $dðVo$ASHP Þ þ $dðtÞ $ d Wcomp þV ASHP $ ðdðTS$ASHP ÞÞ þðdðTR$ASHP ÞÞ þ w$ASHP Þ $ B C t Wcomp t2 B C B C B C ffiC B vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! 2 2 2 B u C B u 1 V VTTHE $DTw$TTHE 2C 2 2 2 @þtðDTw$TTHE Þ2 $ A $dðVo$TTHE Þ þ o$TTHE þ $dðtÞ $ ðdðT ÞÞ þðdðT ÞÞ $ d W þV comp R$TTHE S$TTHE TTHE t Wcomp t2 (A3)
dlim COPperi$TTHE
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 DT 2 2 r$Vo $cp,w w $ ðdðTS ÞÞ2 þ ðdðTR ÞÞ2 þ $ t$d Wcomp þ Wcomp $dðtÞ ¼ Wcomp $t Wcomp $t
(A4)
dlim COPperi$comb ¼
r$Vo $cp,w $ Wcomp$I $tI þWcomp$II $tII ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 2 DTw 2 2 ðdðTS ÞÞ þðdðTR ÞÞ þ $ tI $d Wcomp$I þ Wcomp$I $dðtI Þ þ tII $d Wcomp$II þ Wcomp$I $dðtI Þ Wcomp$I $tI þWcomp$II $tII (A5)
perature slumps below 10 C), the TRTHE units applying for space heating seems viable and acceptable. When the solar hot water is available, heating discharge capacity and efficiency becomes better. (5) Parameter fluctuation owing to switching among heat exchange elements is petty and transitory that will not impact the reliability or stability of the PCM-SAHP system. Acknowledgment This work was financially supposed by the National Natural Science Foundation of China (No. 51178133, 51476049), the Twelfth
By calculating from Eqs. (A1) to (A5), the maximum error (absolute values and relative values) of the COPs can be evaluated as Table A1 shown, which suggests that measuring data and calculated data are acceptable uncertainty values in engineering applications. Additionally, in this study, each experimental unit has been repeated for 7 times, and the test unit can be terminated when the deviation of experimental results controlled availably below 5%. All coefficients of dispersion for the variant coefficients of performance during tests have been shown in Table A2, which verify the reliability of the prototype proposed.
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D. Qv et al. / Renewable Energy xxx (2015) 1e10
9
Table A1 Error analysis for indirect observing data. Name
Limit error
Relative error (%)
Qe Qc Qaccu Wcomp·accu WTRTHE$accu Qaccu$ASHP Qaccu$TRESE COPe$ASHP COPc$ASHP COPperi$TRTHE COPperi$comb COPcomb
0.2 kW 0.2 kW 0.2 MJ 4 kJ 4 kJ 2 MJ 2 MJ 0.1 0.1 0.1 0.1 0.1
4 4 4 0.3 0.3 4 4 3 3 3 3 3
Table A2 Coefficients of dispersion for variant coefficients of performance COP
COPe$ASHP
Operating mode
Outdoor temperature ( C)
The repeated number for each testing unit 1
2
3
4
5
6
7
M1
30.0 32.0 35.0 38.0 40.0 35.0 38.0 40.0 43.0 7.0 10.0 12.0 15.0 17.0 17.0 15.0 20.0 25.0 30.0 35.0 35.0 38.0 40.0 43.0 35.0 38.0 40.0 43.0 e
2.8 2.6 2.6 2.4 2.4 2.6 2.4 2.4 2.1 2.8 2.1 1.9 1.7 1.7 1.6 3.8 3.5 3.1 1.8 1.3 3.1 2.7 2.5 2.3 3.8 3.5 3.3 3.0 2.8
2.9 2.7 2.6 2.3 2.3 2.6 2.3 2.3 2.1 2.8 2.0 1.9 1.6 1.6 1.7 3.7 3.6 3.1 1.9 1.3 3.0 2.7 2.5 2.3 3.7 3.6 3.3 3.1 2.8
2.9 2.6 2.5 2.4 2.3 2.5 2.4 2.3 2.1 2.8 2.1 1.9 1.7 1.6 1.6 3.7 3.5 3.0 1.9 1.4 3.0 2.8 2.5 2.2 3.7 3.5 3.4 3.0 2.8
2.8 2.7 2.6 2.4 2.4 2.6 2.4 2.4 2.0 2.7 2.1 2.0 1.7 1.7 1.7 3.8 3.5 3.1 1.8 1.3 3.1 2.7 2.5 2.3 3.8 3.5 3.3 3.0 2.9
2.9 2.6 2.6 2.3 2.3 2.6 2.3 2.3 2.1 2.8 2.1 1.9 1.6 1.7 1.7 3.7 3.6 3.1 1.8 1.4 3.0 2.7 2.6 2.3 3.7 3.6 3.3 3.1 2.8
2.8 2.8 2.6 2.4 2.3 2.6 2.4 2.3 2.1 2.8 2.0 1.9 1.7 1.7 1.6 3.7 3.5 3.0 1.9 1.3 3.0 2.7 2.5 2.2 3.8 3.5 3.3 3.0 2.8
2.9 2.7 2.6 2.3 2.4 2.6 2.3 2.4 2.0 2.8 2.1 1.9 1.7 1.7 1.7 3.7 3.5 3.0 1.8 1.3 3.1 2.8 2.5 2.3 3.7 3.6 3.4 3.0 2.8
M4
COPc$ASHP
M5
COPperi$TRTHE
M8 M2
COPperi$comp
M2
COPcomp
M2
COPc$TRTHE
M9
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0.019 0.016 0.015 0.023 0.023 0.015 0.023 0.023 0.024 0.014 0.023 0.020 0.029 0.029 0.032 0.013 0.014 0.017 0.029 0.037 0.018 0.018 0.015 0.021 0.014 0.015 0.015 0.016 0.013
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Nomenclatures AC: air conditioning ASHP: air source heat pump COP: coefficient of performance COPc·ASHP: heating COP of ASHP COPe·ASHP: cooling COP of ASHP COPperi·TTHE: cooling COP of TTHE during a cooling cycle COPperi·comb: combined cooling COP during a cooling cycle COPcomb: combined cooling COP FTHE: finned tube heat exchanger HP: heat pump HTF: heat transfer fluid LHTES: latent heat thermal energy storage PHE: plate heat exchanger Qaccu: amount of cooling capacity, MJ Qc: heating capacity, kW Qe: cooling capacity, kW Qe·ASHP: cooling capacity of ASHP, kW Qe·TTHE: cooling capacity of TTHE, kW TES: thermal energy storage TRTHE: triplex tube heat exchanger TR: temperature of return water for space cooling or heating, C TS: temperature of supply water for space cooling or heating, C TR·ASHP: temperature of return water for ASHP, C TS·ASHP: temperature of return water for ASHP, C TR·TRTHE: temperature of return water for TRTHE, C TS·TRTHE: temperature of supply water for TRTHE, C V: volume flow of water for space cooling or heating, m3 s1 VASHP: volume flow of water for space cooling or heating by ASHP, m3 s1 VTRTHE: volume flow of water for space cooling or heating by TRTHE, m3 s1 VO: volume of water for space cooling or heating, m3 VO·ASHP: volume of water for space cooling or heating by ASHP, m3 VO·TRTHE: volume of water for space cooling or heating by TRTHE, m3 Wcomp: compressor power consumption, kW Wcomp·accu: amount of power consumption for compressor, MJ Wcomp·I: compressor power consumption for cold storage, kW Wcomp·II: compressor power consumption for space cooling by TTHE assisting ASHP, kW WTTHE·accu: amount of compressor power consumption for cooling charge, MJ cp·w: specific heat at constant pressure, kW kg1 K1 r: density of water, kg m3 t: duration of cooling or heating, s tI: duration of cold storage, s tII: duration of space cooling by TRTHE assisting ASHP, s DTw: temperature difference between supply and return water for space cooling or heating, C DTw·ASHP: temperature difference between supply and return water for ASHP, C DTw·TRTHE: temperature difference between supply and return water for TRTHE, C
Please cite this article in press as: D. Qv, et al., Reliability verification of a solareair source heat pump system with PCM energy storage in operating strategy transition, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.030