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Study on Performance of Storage Tanks in Solar Water Heater System in Charge and Discharge Progress
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 48 (2014) 384 – 393
SHC 2013, International Conference on Solar Heating and Cooling for Buildings and Industry September 23-25, 2013, Freiburg, Germany
Study on performance of storage tanks in solar water heater system in charge and discharge progress Shuhong Lia,*, Yongxin Zhanga, Kai Zhanga, Xianliang Lia, Yang Lib, Xiaosong Zhanga b
a School of Energy and Environment,Southeast University,Nanjing 210096,China Shanghai Zhongjian Architectural Design Institute Company Limited, Shanghai 200233, China
Abstract Experiment was carried out to investigate the influence of position of immersed coil heat exchanger inside a storage tank on the charging and discharging performance of hot water tank. Discharging efficiency and charging efficiency were introduced to evaluate the performance of hot water tank. The experiment results showed that the discharging efficiency increased with the rising of the coil position. When the flow rate was 5L/min, the discharging efficiency of the tanks with top-coil was higher 5% and 13.1% than that of the middle-coil tank and bottom-coil tank respectively. The gap of the discharging efficiency of tank with different position HX (heat exchanger) reduced with the increasing of the flow rate. However, the charging efficiency reduced with the rising of the HX position. Numerical simulation was carried out to analyze the annual performance of SWHS (solar water heating system) with different position HX by TRNSYS. The simulation results indicated that the SWHS had the lowest annual solar fraction and annual collector efficiency when the HX was on the top level of tank. The annual performance of SWHS was greatly affected by charging performance. So the solar fraction had the same trend with charging efficiency. © 2014 2014The TheAuthors. Authors. Published by Elsevier © Published by Elsevier Ltd. Ltd. Selectionand andpeer peer review scientific conference committee SHC 2013responsibility under responsibility Selection review by by the the scientific conference committee of SHCof2013 under of PSE AGof PSE AG. Keywords: Storage tank; HX position; Annual solar fraction ; Collector efficiency
1. Introduction Solar water heating system (SWHS) has been widely used around the world. There is still something to do to
* Corresponding author. Tel.: +8613705168965; fax: +8602583792722. E-mail address: [email protected]
1876-6102 © 2014 The Authors. Published by Elsevier Ltd.
Selection and peer review by the scientific conference committee of SHC 2013 under responsibility of PSE AG doi:10.1016/j.egypro.2014.02.045
Shuhong Li et al. / Energy Procedia 48 (2014) 384 – 393
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improve the performance of SWHS yet. There are many ways, such as improving the thermal stratification of storage tank, enhancing the performance of collector, optimizing the controller and so on, to improve performance of the SWHS. Optimizing the hot water tank structure and enhancing the immersed heat exchange performance is one of the best ways to improve performance of SWHS. Currently, a lot of theoretical and experimental research about the performance of the coil heat exchanger built in the hot-water tank has been carried out by many researchers. Yan Su [1, 2] et al developed a computational fluid dynamic model for a thermal storage tank with an immersed coiled tube heat exchanger and analysed the transient heat transfer characteristics of coiled tube heat exchanger with different configuration, the mixed convection Nusselt number with the simple baffle is on average 8% greater than that without a baffle for Ra D,0=1.8h1010. The complex baffle lowers the NuM about 50% compared to the simple baffle. A TRNSYS simulation model was developed by Roman Spur to study the behaviour of a novel stratified-store and validated against measured data [3]. Results showed that the inner configurations of the tank and the immersed heat-exchangers significantly affected the performance of heat storage process. Jayanta Deb Mondol experimentally investigated the performance of a novel heat exchanger unit (‘Solasyphon’) developed for a solar hot water system. The results showed that the ‘Solasyphon’ system is more effective compared with a traditional twin-coil system for a domestic application where intermittent hot water demand was predominant under a transient solar input particularly on intermediate or poor solar days [4, 5]. András Zachár numerically investigated temperature stratification of hot water tank with a new tube-in-tube helical flow distributor [6]. The results showed that velocity field induced by buoyancy inside the helical flow distributor has a significantly secondary flow field. Vishard Ragoonanan[7] divided an indirect thermal storage vessel into two storage compartments with the heat exchanger placed in series within the compartments . The divided storage delivered a maximum of 11% more energy than the undivided storage. An experimental and numerical investigation of the thermal behaviour of mantle tanks with different positions of the mantle inlet was performed by S Knudsen and S Furbo[8]. The experiment result showed that the SDHW system with the lower mantle inlet had a little higher thermal performance in the test period. Long-term performance of SWHS has been studied by many researchers. A model for forced indirect cycle solar hot water system applied in hot summer and warm winter area was established based on simulation software TRNSYS by Zhiyong Zhou[9] to analyse the impact of heat exchanger coil area on the average efficiency of SHWS. The result showed that the coil area had greater impact on solar hot water system in winter than that in summer. Y. C. Soo Too[10] studied the performance of narrow-gap vertical mantle heat exchangers with a two-pass arrangement for use in pumped-circulation solar water heaters by TRNSYS. The annual solar contribution of a system with a mantle heat exchanger was lower than tank of a direct-coupled system. A. Carrillo Andres and J. M. Cejudo Lopez [11] developed a new TRNSYS model for solar domestic water heaters from Type 45 and 38. Results of new model are in accordance with experimental data. Han Yanmin and Wang Ruzhu et al [12] investigated the performance by TRNSYS. Different types of solar collectors, collector areas, volumes of storage tank and flow rates are compared in the TRNSYS model and to optimize the SWHS. In the aspect of optimizing performance hot water tank, the existing literatures focus on the coil structure, inlet and outlet flow rater, etc. and there is few study on the influence of the position of HX on the thermal performance of the SWHS. The present work aims to investigate the effect of immersed HX locations on the thermal performance of water tank and also the annual performance of the SWHS. 2. Experimental setup 2.1. Test facility A test facility was established to compare the performance of SHWS with different coil position. The experimental set up is shown in Fig.1. The hot water tank is a cylinder tank with a volume of 125L. The thickness of insulation is 0.05m. The inlet and outlet are in the center of bottom and top of the tank. The parameters of water tank and coil HX are list in the Table 1.There are ten temperature measure points in the tank to measure the temperature of different layers. The distance between two adjacent points is 0.1m as shown in the Fig.1. The temperatures of inlet /outlet of coil HX and water tank were also measured.
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Shuhong Li et al. / Energy Procedia 48 (2014) 384 – 393 F6
Outlet
F7
F5 EO
EI
TO O
1 2 F4
3
Circulate Pump
4 5
Electrical Heater L1
Data Logger
6
Waterr tank
7 8 Coil HX 9 10 F3
TI F2
L2
Thermocoup u le
PC Inlet
F1
Valve
Flowmeter
Fig. 1. Experimental setup of energy storage water tank. Table 1. The parameters of coil HX and water tank.
Tank
Coil HX
Diameter˄m˅ Height˄m˅ Inlet Diameter˄m˅ Outlet Diameter˄m˅ Height˄m˅ Internal/External Diameter˄m˅ Circle Diameter˄m˅ Length˄m˅ HX position(Inlet/Outlet) (m)
0.4 1 0.0192˄DN20˅ 0.0192˄DN20˅ 0.3 0.01/0.012 0.3 15 Top 0.95/0.65 Middle 0.65/0.35 Bottom 0.35/0.05
2.2. Charging process The initial temperature of every layer in water tank was kept at 14f0.5ć. Close the valve F2 and start coil heating cycle, the temperature was set to 60f0.5ć, the flow rate was 2.67 L/min, the water inside the tank was heated by the coil HX. Currently, in SWHS, a solar controller starts the pump when the difference between the temperature of water at the bottom of the tank and the heat transfer fluid at the outlet from the collector exceeds 5ć. And the pump will be shut down when the temperature difference less than 5ć. In the present work, during the charging process, when the charging time is 7200s, the temperature difference between the coil inlet and outlet is less than 3ć and the drive force for heat exchange between coil HX and water tank is quite small. So the charging time is set to 7200s. Changing the coil position, and repeat the experiment for charging. 2.3. Construction of references At the beginning of discharging process, the tank water was heated to 60f0.5ć. Close the valve F3 and F6, open other valves and turn on the electric heater and circulating pump. The hot water flow through, the coil HX and flowmeter L1, then back to the electric heater, At the same time, the cold water flow into the tank, and the hot water was released from the outlet. Changing the coil position, and repeat the experiment for charging. Thermocouples used in the experiment were T-type manufactured by Omega and was calibrated before the experiment. The accuracy is f0.1 ć. The water flowmeters were glass rotameter. The measuring range flowmeter f L1 is 0.16m3 / h, 1.0m3 / h for L2, both flowmeter’s accuracy class are 1.5. The temperature was collected by data logger Agilent 34970A; temperatures were measured every 4 seconds.
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3. Energy performance analysis 3.1. Charging efficiency At the initial time, the tank is filled with low-temperature water and the temperature of every layer was unanimous. The high temperature water flows through the coil HX to heat the cold tank water during the charging process. Keeping the coil inlet temperature constant. The average temperature of tank water at τs is N
t ev (W s )
¦t
j
(W s )
(1)
j 1
N
Where tj(τs) is the temperature of layer j at τs; N is the number of layers in vertical direction. Charging efficiency was introduced to analyze the charging performance of water tank. The charging efficiency is defined as the ratio of the actual average temperature rise to the maximum temperature difference and expressed as
H
t ev (W s ) t ev ,0
(2)
t e,i (W s ) t ev ,0
Where te,i(τs) is the coil inlet temperature at τs , ć; tev,0 is the initial average temperature of tank water, ć. 3.2. Discharging efficiency Discharging efficiency was introduced to analyze the performance of discharging at different flow rate. The discharging efficiency was defined as the ratio of energy released to the initial energy stored. Dimensionless discharging time (θd) is defined as
W use Vst / vuse
(3)
Td
(4)
W d / W use
Where Vst is the water tank volume, m3; vuse is the flow rate during discharging, m3/s; τuse is the whole scale of discharging time, s; τd is the discharging time. The initial energy stored inside the water tank is
Qd ,0 (W d
N
0) UV j c p ¦ (t j (W d
0) t d ,i )
(5)
j 1
Where ρ is the water density, kg/m3; cp is the specific heat of water, 4.18 kJ/(kg· ć); tj(τd=0) is the water temperature of layer j at τd=0, ć; td,i is the inlet temperature of water tank, ć; Vj is the volume of layer j, m3. During the discharging process, the energy released within τd is Wd
Qd (W d ) vuse U c p ³ (td ,o (W d ) td ,i )dW 0
(6)
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Where td,o(τd) is the tank outlet temperature at τd, ć; The outlet temperature td,o(τd) is measured by thermal couples and the data is discrete. The data measure interval is Δτ. During the period of τd , the number of measurement is k = τd /Δτ, the energy released may be expressed as
Qd (W d )
k
vuseUc p 'W ¦ (t d ,o (W d ) t d ,i )
(7)
n 1
The heat exchange between coil HX and tank water is
Qe,d (W d )
k
ve Uc p 'W ¦ (te,i (W d ) te,o (W d ))
(8)
n 1
Where te,i(τd) is the inlet temperature at τd, te,o(τd) is the inlet temperature at τd, ć. The discharging efficiency of the discharging process is
Kd
Qd (W d ) Qe,d (W d ) Qd ,0 (W d
(9)
0)
4. Experiment result and discussion 4.1. Analysis of charging performance The temperature of layer 1, layer 5 and layer 10 were taken to analyze the performance of charging. Fig.2 shows the bottom-coil tank has a smaller thermocline. The temperature distance between layer1 and layer10 is relatively smaller. However, for the middle-coil tank and top-coil tank, the temperature differences between layer 1 and layer 10 are bigger than that of bottom-coil tank. At the end of charging process, the temperature of layer 1 of bottom-coil tank, middle-coil tank and top-coil tank is 55.2ć, 58.1ć and 58.5ć respectively. Fig.3 shows the average temperature of bottom-coil tank is much higher than that of middle-coil tank and top-coil tank. At the end of charging process, the average temperature of bottom-coil tank, middle-coil tank and top-coil tank is 55.2ć, 46.6ć and 36.4ć. The distance of average temperature between bottom-coil tank and top-coil tank is 18.8ć.
Temperature (ć)
60 55
Layer 1 of tank A
50
Layer 5 of tank A
45
Layer 10 of tank A
40
Layer 1 of tank B
35
Layer 5 of tank B
30
Layer 10 of tank B
tank A— Bottom-coil tank tank B— Middle-coil tank tank C— Top-coil tank
25 20
Layer 1 of tank C Layer 5 of tank C
15
Layer 10 of tank C
10 0
1200
2400
3600
4800
6000
7200
Time (s) Fig. 2. Water temperatures at different layers during the charging.
The charging efficiency of water tank with different position coil HX is shown in the Fig.4. The charging performance of bottom-coil water is better than middle-coil tank and top-coil tank. When the coil HX is at the lower
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part of water tank, the average temperature of water tank is much higher. The effects of natural convection is strong, then enhanced the heat exchange between coil HX and tank water. The natural convection heat transfer effect reduced when the coil HX at the upper part of water tank. And led to a lower temperature at bottom of tank, the energy stored is less than bottom-coil tank. The charging efficiency of bottom-coil tank, middle-coil tank and topcoil tank is 89.5%, 70.8% and 48.6%. 100%
50
Charging efficiency
Average water temperature (ć)
60
40 30
20
Bottom-coil Middle-coil
10
80%
60%
40%
Top-coil
20%
0 0
1000
2000
3000
4000
5000
6000
Bottom-coil
7000
Middle-coil
Top-coil
Tank type
Time (s) Fig. 3. Average temperature of tank during the charging.
Fig. 4. The charging efficiency of water tank.
4.2. Analysis of discharging performance Fig.5 shows the outlet temperature of middle-coil and top-coil tank is much higher than bottom-coil tank. When flow rate is 5L/min, at the end of discharging process, the outlet temperature of bottom-coil tank, middle-coil tank and top-coil tank is 37.5ć, 41.5ć and 42.7ć, respectively. The outlet temperature decreased with the increasing of flow rate. When the flow rate is at 15L/min, the outlet temperature of bottom-coil tank, middle-coil tank and top-coil tank is 27.4ć, 28.3ć and 28.7ć. 65
100%
tank C,5L/min
50
tank A,10L/min
45
tank B,10L/min
40
30
tank C,10L/min tank A,15L/min
tank A— Bottom-coil tank tank B— Middle-coil tank tank C— Top-coil tank
tank B,15L/min tank C,15L/min
25 20 0.00
90%
Charging efficiency
Temperature (ć)
tank B,5L/min
55
35
Bottom-coil
tank A,5L/min
60
Middle-coil Top-coil
80% 70% 60% 50% 40%
0.16
0.32
0.47
0.63
0.79
0.95
Dimensionless time Fig. 5. The outlet temperature during discharging at different flow rate.
5
10
15
Flowrate (L/min) Fig. 6. The discharging efficiency of water tank with different position coil at different flow rate.
Fig.6 shows the discharging efficiency of top-coil tank is higher than middle-coil tank and bottom-coil tank when the flow rate is identical. When the flow rate is 5L/min, the discharging efficiency of bottom-coil tank, middle-coil tank and top-coil tank is 73.8%, 81.9% and 86.9%, respectively. The discharging efficiency of top-coil tank is 13.1% and 5% higher than that of bottom-coil tank and middle-coil tank. As the flow rate increases, the distance among different position coil tank gradually reduced. When the flow rate is 15L/min, the discharging efficiency of bottomcoil tank, middle-coil tank and top-coil tank is 71.6%, 73.1% and 76.8%, respectively.
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The charging performance decreased with the rising of coil HX position while the discharging performance increased with the rising of coil HX position. Thus, the heat exchanger should be located to take advantage of the greatest difference between the temperature of the storage fluid and the temperature of the fluid flowing through the heat exchanger [13]. In order to analyze the long-term performance of SWHS, Annual simulations of the solar water heater system with different coil HX position were carried out to analyze the effect of coil HX position on the performance of SWHS. 5. Annual simulation of SWHS 5.1. TRNSYS models A model was developed to analyze the effect of coil HX position on the performance of SWHS in TRNSYS. Shanghai meteorological data was adopted. The TRNSYS model consists of storage tank, solar collector, pump and other auxiliary control modules. The TRNSYS model is shown in the Fig.7 Type 14h Control signal for immersion heater
Type109 Weather
Type1b Collector
Type14b Tee Piece
To User
Type2b Controller Type3b Pump
Type60d Thermal Storage
Type14b Load Profile
Type 11b Diverter
Equation
Type 24 Simulation Integration
Type25c Printer
Fig. 7. TRNSYS information flow diagram for the forced circulation solar water heating systems.
A storage tank module (Type 60) was set to meet the need for different coil HX position [14]. The flat-plate collector was selected (Type 1). The hot water demand profile employed was the EU reference(EU M324EN) tapping cycle number 3, featuring 24 draw-offs with the energy output of 11.7 kWh equivalent to a total volume of 200L at 60ćdaily [15]. Fig.8 shows the volume of hot water draw-off at different times of the day.
Volume(L)
21:30
21:00
07:00
20:30
19:00
18:30
18:15
18:00
16:30
15:30
14:30
12:45
11:45
10:30
09:30
09:00
08:45
08:30
08:05
07:45
07:30
07:05
07:00
Time(h)
Fig. 8. Volume of hot water draw-off at different times of the day.
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5.2. Definition of collector efficiency, solar fraction The solar fraction is the ratio of solar heat yield to the total energy requirement for water heating, and was calculated as [15]
SF
Qsys Qsys Qaux
(10)
Where, Qsys is solar heat yield to the total energy requirement, kJ. The Qsys is expressed as
Qsys
W
qmc p ³ (te,i (W ) te,o (W ))dW 0
(11)
Where, qm is the mass flow rate of coil HX, kg/s; τ is the time of heat exchange, s; te,i(τ), te,o(τ) are the coil HX inlet and outlet temperature at τ, ć. Qaux is the heat provided by auxiliary electric heater, kJ. Collector efficiency is defined as the ratio of the heat collected by solar collector to the total solar irradiation. The equation is expressed as
Kc
W
Qc
Ac ³ G (W )dW
(12)
0
Where, Qc is the heat collected by collector, kJ; and Qc is calculated by Equation (13)
Qc
W
qmc p ³ (tc ,o (W ) tc ,i (W ))dW 0
(13)
Where, qm is the mass flow rate of collector, kg/s; tc,i(τ) , te,o(τ) are the collector inlet and outlet temperature at τ, ć; Ac is collector area, m2; G(τ) is the solar irradiation at τ, W/m2. Heat released by the water tank is calculated as
Qre
W
qmc p ³ (ttank ,o (W ) ttank ,i (W ))dW 0
(14)
Where, qm(τ) is the mass flow rate of inlet of water tank, kg/s; ttank,i(τ) , ttank,o(τ) are the water tank inlet and outlet temperature at τ, ć. 6. Simulation result and discussion 6.1. Energy analysis Annual simulation was carried out to analyze the performance of SWHS with different position coil by TRNSYS. The time step is 0.01h. The data collected once every 0.5h. The accumulated simulation time is 8760h (1 year). The annual heat variations of SWHS are shown in the Table 2.
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Shuhong Li et al. / Energy Procedia 48 (2014) 384 – 393 Table 2. Annul heat variation of SWHS. Unit: MJ Bottom-coil SWHS
Middle-coil SWHS
Top-coil SWHS
Annual heat released
10499
10458
10451
Annual coil heat exchanged
4591
3019
1646
Annual auxiliary heat
6301
7786
9130
Heat
The annual heat released almost kept the same with the rising of HX position. The annual coil heat exchanged decreased with the rising of HX position while the annual auxiliary heat had the opposite trend. There was the maximum annual HX heat exchanged when the coli HX at the bottom of hot water tank and is 2945MJ more than that of top-coil SWHS. And the auxiliary heat was 2829MJ less than that of the tank with HX at the top. The energy saved by the bottom-coil SWHS is 31% less than top-coil SWHS. 6.2. Efficiency analysis Fig.9 shows the variation of annul solar fraction and collector efficiency. Annual solar fraction and collector efficiency decreased with the rising of HX position. Therefore, the solar water heater system had a worse performance when the coil HX at upper part of hot water tank. The maximum annual solar fraction and maximum collector efficiency are 42.2% and 31.4%, and they were occurred when the coil HX at the bottom of tank. 60 Solar Fraction
Collector Efficiency
Efficiency %
50 40 30 20 10 0 Bottom
Middle
Top
Coil HX Position in Tank Fig. 9. The solar fraction and collector efficiency of SWHS.
The annual performance of solar water heater system largely relied on the charging performance of hot water tank and load to user. The HX position greatly affected the natural convective heat transfer. It’s better to have the HX at the bottom of water tank during the charging. The charging performance decreased with the rising of HX position in the tank, then led to lower annual performanceülower solar fraction and lower collector efficiency. 7. Conclusions Experiments were carried out to study the charging and discharging performance of storage tank with different position coil HX at different flow rate. The discharging efficiency and charging efficiency were introduced to evaluate the performance of discharging process and charging process. Annual simulation was also carried out to analyze the long-term performance of SWHS with different position coil HX. The conclusions are as follows. (1) The charging performance decreased with the rising of coil HX position. At the given condition, the charging efficiency of bottom-coil tank, middle-coil tank and top-coil tank is 89.5%, 70.8% and 48.6%. (2) The discharging performance increased with the rising of coil HX position. When the flow rate is 5L/min, the discharging efficiency of bottom-coil tank, middle-coil tank and top-coil tank is 73.8%, 81.9% and 86.9%, respectively.
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(3) The annual solar fraction and annual system efficiency of SWHS decreased with the rising of coil position. The SWHS had the best performance when the coil HX at the top of water tank. The annual auxiliary heat of bottom-coil SWHS was 2829MJ less than that of the tank with HX at the top. The benefit provided by SWHS with bottom-coil was much larger.
Acknowledgements This study was jointly funded by the 12th Five Year National Science and Technology Support Key Project of China (NO.2011BAJ03B05) and the Jiangsu Natural Science Foundation of China (Project NO. BK2010199). References [1] Yan Su, Jane H Davidson. Discharge of thermal storage tanks via immersed baffled heat exchangers: numerical model of flow and temperature fields [J]. Journal of Solar Energy Engineering, 2008, 130:021016-1-021016-7. [2] Yan Su, Jane H Davidson. Transient natural convection heat transfer correlations for tube bundles immersed in a thermal storage [J]. Journal of Solar Energy Engineering, 2009, 129:210-214. [3] Roman Spur, Dusan Fiala, Dusan Nevrala, et al. Performances of modern domestic hot-water stores [J]. Applied Energy, 2006,83:893-910 [4] Jayanta Deb Mondol, Mervyn Smyth, Aggelos Zacharopoulos et al. Experimental performance evaluation of a novel heat exchanger for a solar hot water storage system [J]. Applied Energy, 2009, 86:1492-1505. [5] Jayanta Deb Mondol, Mervyn Smyth, Aggelos Zacharopoulos. Experimental characterisation of a novel heat exchanger for a solar hot water application under indoorand outdoor conditions [J] Renewable Energy, 2011, 36:1766-1779. [6] András Zachár. Investigation of a new tube-in-tube helical flow distributor design to improve temperature stratification inside hot water storage tanks operated with coiled-tube heat exchangers [J]. International Journal of Heat and Mass Transfer, 2013, 63:150-161 [7] Vishard Ragoonanan, Jane H. Davidson, Kelly O. Homan et al. The benefit of dividing an indirect thermal storage into two compartments: Discharge experiments [J]. Solar Energy, 2006, 80:18-31. [8] S Knudsen, S Furbo. Thermal stratification in vertical mantle heat-exchangers with application to solar domestic hot-water systems [J]. Applied Energy, 2004, 78:257-272 [9] Zhou Zhiyong, Peng Sanbing, Fu Xiangzhao et al. Forced indirect cycle solar hot water system in hot summer and warm winter area [J]. GAS & HEAT, 2009,8(29):33-39. [10] Y C Soo Too, G L Morrison, M Behnia. Performance of solar water heaters with narrow mantle heat exchangers [J]. Solar Energy, 2009, 83:350-362. [11] A Carrillo Andres, J M Cejudo Lopez. TRNSYS model of a thermosiphon solar domestic water heater with a horizontal store and mantle heat exchanger [J]. Solar Energy, 2002, 2(72):89-98. [12] Han Yanmin, Dai Yanjun, Wang Ruzhu. Optimum analysis energy conversion for solar collector system [J]. Journal of engineering thermophysics, 2006, 27:57-60. [13] Julia F Haltiwanger, Jane H Davidson. Discharge of a thermal storage tank using an immersed heat exchanger with an annular baffle [J]. Solar Energy, 2009, 83:193-201. [14] TRNSYS 16 Documentation. 2006. [15] L M Ayompe, A Duffy. Thermal performance analysis of a solar water heating system with heat pipe evacuated tube collector using data from a field trial[J].Solar Energy,2013,90: 17-28.
ScienceDirect Energy Procedia 48 (2014) 384 – 393
SHC 2013, International Conference on Solar Heating and Cooling for Buildings and Industry September 23-25, 2013, Freiburg, Germany
Study on performance of storage tanks in solar water heater system in charge and discharge progress Shuhong Lia,*, Yongxin Zhanga, Kai Zhanga, Xianliang Lia, Yang Lib, Xiaosong Zhanga b
a School of Energy and Environment,Southeast University,Nanjing 210096,China Shanghai Zhongjian Architectural Design Institute Company Limited, Shanghai 200233, China
Abstract Experiment was carried out to investigate the influence of position of immersed coil heat exchanger inside a storage tank on the charging and discharging performance of hot water tank. Discharging efficiency and charging efficiency were introduced to evaluate the performance of hot water tank. The experiment results showed that the discharging efficiency increased with the rising of the coil position. When the flow rate was 5L/min, the discharging efficiency of the tanks with top-coil was higher 5% and 13.1% than that of the middle-coil tank and bottom-coil tank respectively. The gap of the discharging efficiency of tank with different position HX (heat exchanger) reduced with the increasing of the flow rate. However, the charging efficiency reduced with the rising of the HX position. Numerical simulation was carried out to analyze the annual performance of SWHS (solar water heating system) with different position HX by TRNSYS. The simulation results indicated that the SWHS had the lowest annual solar fraction and annual collector efficiency when the HX was on the top level of tank. The annual performance of SWHS was greatly affected by charging performance. So the solar fraction had the same trend with charging efficiency. © 2014 2014The TheAuthors. Authors. Published by Elsevier © Published by Elsevier Ltd. Ltd. Selectionand andpeer peer review scientific conference committee SHC 2013responsibility under responsibility Selection review by by the the scientific conference committee of SHCof2013 under of PSE AGof PSE AG. Keywords: Storage tank; HX position; Annual solar fraction ; Collector efficiency
1. Introduction Solar water heating system (SWHS) has been widely used around the world. There is still something to do to
* Corresponding author. Tel.: +8613705168965; fax: +8602583792722. E-mail address: [email protected]
1876-6102 © 2014 The Authors. Published by Elsevier Ltd.
Selection and peer review by the scientific conference committee of SHC 2013 under responsibility of PSE AG doi:10.1016/j.egypro.2014.02.045
Shuhong Li et al. / Energy Procedia 48 (2014) 384 – 393
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improve the performance of SWHS yet. There are many ways, such as improving the thermal stratification of storage tank, enhancing the performance of collector, optimizing the controller and so on, to improve performance of the SWHS. Optimizing the hot water tank structure and enhancing the immersed heat exchange performance is one of the best ways to improve performance of SWHS. Currently, a lot of theoretical and experimental research about the performance of the coil heat exchanger built in the hot-water tank has been carried out by many researchers. Yan Su [1, 2] et al developed a computational fluid dynamic model for a thermal storage tank with an immersed coiled tube heat exchanger and analysed the transient heat transfer characteristics of coiled tube heat exchanger with different configuration, the mixed convection Nusselt number with the simple baffle is on average 8% greater than that without a baffle for Ra D,0=1.8h1010. The complex baffle lowers the NuM about 50% compared to the simple baffle. A TRNSYS simulation model was developed by Roman Spur to study the behaviour of a novel stratified-store and validated against measured data [3]. Results showed that the inner configurations of the tank and the immersed heat-exchangers significantly affected the performance of heat storage process. Jayanta Deb Mondol experimentally investigated the performance of a novel heat exchanger unit (‘Solasyphon’) developed for a solar hot water system. The results showed that the ‘Solasyphon’ system is more effective compared with a traditional twin-coil system for a domestic application where intermittent hot water demand was predominant under a transient solar input particularly on intermediate or poor solar days [4, 5]. András Zachár numerically investigated temperature stratification of hot water tank with a new tube-in-tube helical flow distributor [6]. The results showed that velocity field induced by buoyancy inside the helical flow distributor has a significantly secondary flow field. Vishard Ragoonanan[7] divided an indirect thermal storage vessel into two storage compartments with the heat exchanger placed in series within the compartments . The divided storage delivered a maximum of 11% more energy than the undivided storage. An experimental and numerical investigation of the thermal behaviour of mantle tanks with different positions of the mantle inlet was performed by S Knudsen and S Furbo[8]. The experiment result showed that the SDHW system with the lower mantle inlet had a little higher thermal performance in the test period. Long-term performance of SWHS has been studied by many researchers. A model for forced indirect cycle solar hot water system applied in hot summer and warm winter area was established based on simulation software TRNSYS by Zhiyong Zhou[9] to analyse the impact of heat exchanger coil area on the average efficiency of SHWS. The result showed that the coil area had greater impact on solar hot water system in winter than that in summer. Y. C. Soo Too[10] studied the performance of narrow-gap vertical mantle heat exchangers with a two-pass arrangement for use in pumped-circulation solar water heaters by TRNSYS. The annual solar contribution of a system with a mantle heat exchanger was lower than tank of a direct-coupled system. A. Carrillo Andres and J. M. Cejudo Lopez [11] developed a new TRNSYS model for solar domestic water heaters from Type 45 and 38. Results of new model are in accordance with experimental data. Han Yanmin and Wang Ruzhu et al [12] investigated the performance by TRNSYS. Different types of solar collectors, collector areas, volumes of storage tank and flow rates are compared in the TRNSYS model and to optimize the SWHS. In the aspect of optimizing performance hot water tank, the existing literatures focus on the coil structure, inlet and outlet flow rater, etc. and there is few study on the influence of the position of HX on the thermal performance of the SWHS. The present work aims to investigate the effect of immersed HX locations on the thermal performance of water tank and also the annual performance of the SWHS. 2. Experimental setup 2.1. Test facility A test facility was established to compare the performance of SHWS with different coil position. The experimental set up is shown in Fig.1. The hot water tank is a cylinder tank with a volume of 125L. The thickness of insulation is 0.05m. The inlet and outlet are in the center of bottom and top of the tank. The parameters of water tank and coil HX are list in the Table 1.There are ten temperature measure points in the tank to measure the temperature of different layers. The distance between two adjacent points is 0.1m as shown in the Fig.1. The temperatures of inlet /outlet of coil HX and water tank were also measured.
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Shuhong Li et al. / Energy Procedia 48 (2014) 384 – 393 F6
Outlet
F7
F5 EO
EI
TO O
1 2 F4
3
Circulate Pump
4 5
Electrical Heater L1
Data Logger
6
Waterr tank
7 8 Coil HX 9 10 F3
TI F2
L2
Thermocoup u le
PC Inlet
F1
Valve
Flowmeter
Fig. 1. Experimental setup of energy storage water tank. Table 1. The parameters of coil HX and water tank.
Tank
Coil HX
Diameter˄m˅ Height˄m˅ Inlet Diameter˄m˅ Outlet Diameter˄m˅ Height˄m˅ Internal/External Diameter˄m˅ Circle Diameter˄m˅ Length˄m˅ HX position(Inlet/Outlet) (m)
0.4 1 0.0192˄DN20˅ 0.0192˄DN20˅ 0.3 0.01/0.012 0.3 15 Top 0.95/0.65 Middle 0.65/0.35 Bottom 0.35/0.05
2.2. Charging process The initial temperature of every layer in water tank was kept at 14f0.5ć. Close the valve F2 and start coil heating cycle, the temperature was set to 60f0.5ć, the flow rate was 2.67 L/min, the water inside the tank was heated by the coil HX. Currently, in SWHS, a solar controller starts the pump when the difference between the temperature of water at the bottom of the tank and the heat transfer fluid at the outlet from the collector exceeds 5ć. And the pump will be shut down when the temperature difference less than 5ć. In the present work, during the charging process, when the charging time is 7200s, the temperature difference between the coil inlet and outlet is less than 3ć and the drive force for heat exchange between coil HX and water tank is quite small. So the charging time is set to 7200s. Changing the coil position, and repeat the experiment for charging. 2.3. Construction of references At the beginning of discharging process, the tank water was heated to 60f0.5ć. Close the valve F3 and F6, open other valves and turn on the electric heater and circulating pump. The hot water flow through, the coil HX and flowmeter L1, then back to the electric heater, At the same time, the cold water flow into the tank, and the hot water was released from the outlet. Changing the coil position, and repeat the experiment for charging. Thermocouples used in the experiment were T-type manufactured by Omega and was calibrated before the experiment. The accuracy is f0.1 ć. The water flowmeters were glass rotameter. The measuring range flowmeter f L1 is 0.16m3 / h, 1.0m3 / h for L2, both flowmeter’s accuracy class are 1.5. The temperature was collected by data logger Agilent 34970A; temperatures were measured every 4 seconds.
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3. Energy performance analysis 3.1. Charging efficiency At the initial time, the tank is filled with low-temperature water and the temperature of every layer was unanimous. The high temperature water flows through the coil HX to heat the cold tank water during the charging process. Keeping the coil inlet temperature constant. The average temperature of tank water at τs is N
t ev (W s )
¦t
j
(W s )
(1)
j 1
N
Where tj(τs) is the temperature of layer j at τs; N is the number of layers in vertical direction. Charging efficiency was introduced to analyze the charging performance of water tank. The charging efficiency is defined as the ratio of the actual average temperature rise to the maximum temperature difference and expressed as
H
t ev (W s ) t ev ,0
(2)
t e,i (W s ) t ev ,0
Where te,i(τs) is the coil inlet temperature at τs , ć; tev,0 is the initial average temperature of tank water, ć. 3.2. Discharging efficiency Discharging efficiency was introduced to analyze the performance of discharging at different flow rate. The discharging efficiency was defined as the ratio of energy released to the initial energy stored. Dimensionless discharging time (θd) is defined as
W use Vst / vuse
(3)
Td
(4)
W d / W use
Where Vst is the water tank volume, m3; vuse is the flow rate during discharging, m3/s; τuse is the whole scale of discharging time, s; τd is the discharging time. The initial energy stored inside the water tank is
Qd ,0 (W d
N
0) UV j c p ¦ (t j (W d
0) t d ,i )
(5)
j 1
Where ρ is the water density, kg/m3; cp is the specific heat of water, 4.18 kJ/(kg· ć); tj(τd=0) is the water temperature of layer j at τd=0, ć; td,i is the inlet temperature of water tank, ć; Vj is the volume of layer j, m3. During the discharging process, the energy released within τd is Wd
Qd (W d ) vuse U c p ³ (td ,o (W d ) td ,i )dW 0
(6)
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Where td,o(τd) is the tank outlet temperature at τd, ć; The outlet temperature td,o(τd) is measured by thermal couples and the data is discrete. The data measure interval is Δτ. During the period of τd , the number of measurement is k = τd /Δτ, the energy released may be expressed as
Qd (W d )
k
vuseUc p 'W ¦ (t d ,o (W d ) t d ,i )
(7)
n 1
The heat exchange between coil HX and tank water is
Qe,d (W d )
k
ve Uc p 'W ¦ (te,i (W d ) te,o (W d ))
(8)
n 1
Where te,i(τd) is the inlet temperature at τd, te,o(τd) is the inlet temperature at τd, ć. The discharging efficiency of the discharging process is
Kd
Qd (W d ) Qe,d (W d ) Qd ,0 (W d
(9)
0)
4. Experiment result and discussion 4.1. Analysis of charging performance The temperature of layer 1, layer 5 and layer 10 were taken to analyze the performance of charging. Fig.2 shows the bottom-coil tank has a smaller thermocline. The temperature distance between layer1 and layer10 is relatively smaller. However, for the middle-coil tank and top-coil tank, the temperature differences between layer 1 and layer 10 are bigger than that of bottom-coil tank. At the end of charging process, the temperature of layer 1 of bottom-coil tank, middle-coil tank and top-coil tank is 55.2ć, 58.1ć and 58.5ć respectively. Fig.3 shows the average temperature of bottom-coil tank is much higher than that of middle-coil tank and top-coil tank. At the end of charging process, the average temperature of bottom-coil tank, middle-coil tank and top-coil tank is 55.2ć, 46.6ć and 36.4ć. The distance of average temperature between bottom-coil tank and top-coil tank is 18.8ć.
Temperature (ć)
60 55
Layer 1 of tank A
50
Layer 5 of tank A
45
Layer 10 of tank A
40
Layer 1 of tank B
35
Layer 5 of tank B
30
Layer 10 of tank B
tank A— Bottom-coil tank tank B— Middle-coil tank tank C— Top-coil tank
25 20
Layer 1 of tank C Layer 5 of tank C
15
Layer 10 of tank C
10 0
1200
2400
3600
4800
6000
7200
Time (s) Fig. 2. Water temperatures at different layers during the charging.
The charging efficiency of water tank with different position coil HX is shown in the Fig.4. The charging performance of bottom-coil water is better than middle-coil tank and top-coil tank. When the coil HX is at the lower
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part of water tank, the average temperature of water tank is much higher. The effects of natural convection is strong, then enhanced the heat exchange between coil HX and tank water. The natural convection heat transfer effect reduced when the coil HX at the upper part of water tank. And led to a lower temperature at bottom of tank, the energy stored is less than bottom-coil tank. The charging efficiency of bottom-coil tank, middle-coil tank and topcoil tank is 89.5%, 70.8% and 48.6%. 100%
50
Charging efficiency
Average water temperature (ć)
60
40 30
20
Bottom-coil Middle-coil
10
80%
60%
40%
Top-coil
20%
0 0
1000
2000
3000
4000
5000
6000
Bottom-coil
7000
Middle-coil
Top-coil
Tank type
Time (s) Fig. 3. Average temperature of tank during the charging.
Fig. 4. The charging efficiency of water tank.
4.2. Analysis of discharging performance Fig.5 shows the outlet temperature of middle-coil and top-coil tank is much higher than bottom-coil tank. When flow rate is 5L/min, at the end of discharging process, the outlet temperature of bottom-coil tank, middle-coil tank and top-coil tank is 37.5ć, 41.5ć and 42.7ć, respectively. The outlet temperature decreased with the increasing of flow rate. When the flow rate is at 15L/min, the outlet temperature of bottom-coil tank, middle-coil tank and top-coil tank is 27.4ć, 28.3ć and 28.7ć. 65
100%
tank C,5L/min
50
tank A,10L/min
45
tank B,10L/min
40
30
tank C,10L/min tank A,15L/min
tank A— Bottom-coil tank tank B— Middle-coil tank tank C— Top-coil tank
tank B,15L/min tank C,15L/min
25 20 0.00
90%
Charging efficiency
Temperature (ć)
tank B,5L/min
55
35
Bottom-coil
tank A,5L/min
60
Middle-coil Top-coil
80% 70% 60% 50% 40%
0.16
0.32
0.47
0.63
0.79
0.95
Dimensionless time Fig. 5. The outlet temperature during discharging at different flow rate.
5
10
15
Flowrate (L/min) Fig. 6. The discharging efficiency of water tank with different position coil at different flow rate.
Fig.6 shows the discharging efficiency of top-coil tank is higher than middle-coil tank and bottom-coil tank when the flow rate is identical. When the flow rate is 5L/min, the discharging efficiency of bottom-coil tank, middle-coil tank and top-coil tank is 73.8%, 81.9% and 86.9%, respectively. The discharging efficiency of top-coil tank is 13.1% and 5% higher than that of bottom-coil tank and middle-coil tank. As the flow rate increases, the distance among different position coil tank gradually reduced. When the flow rate is 15L/min, the discharging efficiency of bottomcoil tank, middle-coil tank and top-coil tank is 71.6%, 73.1% and 76.8%, respectively.
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The charging performance decreased with the rising of coil HX position while the discharging performance increased with the rising of coil HX position. Thus, the heat exchanger should be located to take advantage of the greatest difference between the temperature of the storage fluid and the temperature of the fluid flowing through the heat exchanger [13]. In order to analyze the long-term performance of SWHS, Annual simulations of the solar water heater system with different coil HX position were carried out to analyze the effect of coil HX position on the performance of SWHS. 5. Annual simulation of SWHS 5.1. TRNSYS models A model was developed to analyze the effect of coil HX position on the performance of SWHS in TRNSYS. Shanghai meteorological data was adopted. The TRNSYS model consists of storage tank, solar collector, pump and other auxiliary control modules. The TRNSYS model is shown in the Fig.7 Type 14h Control signal for immersion heater
Type109 Weather
Type1b Collector
Type14b Tee Piece
To User
Type2b Controller Type3b Pump
Type60d Thermal Storage
Type14b Load Profile
Type 11b Diverter
Equation
Type 24 Simulation Integration
Type25c Printer
Fig. 7. TRNSYS information flow diagram for the forced circulation solar water heating systems.
A storage tank module (Type 60) was set to meet the need for different coil HX position [14]. The flat-plate collector was selected (Type 1). The hot water demand profile employed was the EU reference(EU M324EN) tapping cycle number 3, featuring 24 draw-offs with the energy output of 11.7 kWh equivalent to a total volume of 200L at 60ćdaily [15]. Fig.8 shows the volume of hot water draw-off at different times of the day.
Volume(L)
21:30
21:00
07:00
20:30
19:00
18:30
18:15
18:00
16:30
15:30
14:30
12:45
11:45
10:30
09:30
09:00
08:45
08:30
08:05
07:45
07:30
07:05
07:00
Time(h)
Fig. 8. Volume of hot water draw-off at different times of the day.
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5.2. Definition of collector efficiency, solar fraction The solar fraction is the ratio of solar heat yield to the total energy requirement for water heating, and was calculated as [15]
SF
Qsys Qsys Qaux
(10)
Where, Qsys is solar heat yield to the total energy requirement, kJ. The Qsys is expressed as
Qsys
W
qmc p ³ (te,i (W ) te,o (W ))dW 0
(11)
Where, qm is the mass flow rate of coil HX, kg/s; τ is the time of heat exchange, s; te,i(τ), te,o(τ) are the coil HX inlet and outlet temperature at τ, ć. Qaux is the heat provided by auxiliary electric heater, kJ. Collector efficiency is defined as the ratio of the heat collected by solar collector to the total solar irradiation. The equation is expressed as
Kc
W
Qc
Ac ³ G (W )dW
(12)
0
Where, Qc is the heat collected by collector, kJ; and Qc is calculated by Equation (13)
Qc
W
qmc p ³ (tc ,o (W ) tc ,i (W ))dW 0
(13)
Where, qm is the mass flow rate of collector, kg/s; tc,i(τ) , te,o(τ) are the collector inlet and outlet temperature at τ, ć; Ac is collector area, m2; G(τ) is the solar irradiation at τ, W/m2. Heat released by the water tank is calculated as
Qre
W
qmc p ³ (ttank ,o (W ) ttank ,i (W ))dW 0
(14)
Where, qm(τ) is the mass flow rate of inlet of water tank, kg/s; ttank,i(τ) , ttank,o(τ) are the water tank inlet and outlet temperature at τ, ć. 6. Simulation result and discussion 6.1. Energy analysis Annual simulation was carried out to analyze the performance of SWHS with different position coil by TRNSYS. The time step is 0.01h. The data collected once every 0.5h. The accumulated simulation time is 8760h (1 year). The annual heat variations of SWHS are shown in the Table 2.
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Shuhong Li et al. / Energy Procedia 48 (2014) 384 – 393 Table 2. Annul heat variation of SWHS. Unit: MJ Bottom-coil SWHS
Middle-coil SWHS
Top-coil SWHS
Annual heat released
10499
10458
10451
Annual coil heat exchanged
4591
3019
1646
Annual auxiliary heat
6301
7786
9130
Heat
The annual heat released almost kept the same with the rising of HX position. The annual coil heat exchanged decreased with the rising of HX position while the annual auxiliary heat had the opposite trend. There was the maximum annual HX heat exchanged when the coli HX at the bottom of hot water tank and is 2945MJ more than that of top-coil SWHS. And the auxiliary heat was 2829MJ less than that of the tank with HX at the top. The energy saved by the bottom-coil SWHS is 31% less than top-coil SWHS. 6.2. Efficiency analysis Fig.9 shows the variation of annul solar fraction and collector efficiency. Annual solar fraction and collector efficiency decreased with the rising of HX position. Therefore, the solar water heater system had a worse performance when the coil HX at upper part of hot water tank. The maximum annual solar fraction and maximum collector efficiency are 42.2% and 31.4%, and they were occurred when the coil HX at the bottom of tank. 60 Solar Fraction
Collector Efficiency
Efficiency %
50 40 30 20 10 0 Bottom
Middle
Top
Coil HX Position in Tank Fig. 9. The solar fraction and collector efficiency of SWHS.
The annual performance of solar water heater system largely relied on the charging performance of hot water tank and load to user. The HX position greatly affected the natural convective heat transfer. It’s better to have the HX at the bottom of water tank during the charging. The charging performance decreased with the rising of HX position in the tank, then led to lower annual performanceülower solar fraction and lower collector efficiency. 7. Conclusions Experiments were carried out to study the charging and discharging performance of storage tank with different position coil HX at different flow rate. The discharging efficiency and charging efficiency were introduced to evaluate the performance of discharging process and charging process. Annual simulation was also carried out to analyze the long-term performance of SWHS with different position coil HX. The conclusions are as follows. (1) The charging performance decreased with the rising of coil HX position. At the given condition, the charging efficiency of bottom-coil tank, middle-coil tank and top-coil tank is 89.5%, 70.8% and 48.6%. (2) The discharging performance increased with the rising of coil HX position. When the flow rate is 5L/min, the discharging efficiency of bottom-coil tank, middle-coil tank and top-coil tank is 73.8%, 81.9% and 86.9%, respectively.
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(3) The annual solar fraction and annual system efficiency of SWHS decreased with the rising of coil position. The SWHS had the best performance when the coil HX at the top of water tank. The annual auxiliary heat of bottom-coil SWHS was 2829MJ less than that of the tank with HX at the top. The benefit provided by SWHS with bottom-coil was much larger.
Acknowledgements This study was jointly funded by the 12th Five Year National Science and Technology Support Key Project of China (NO.2011BAJ03B05) and the Jiangsu Natural Science Foundation of China (Project NO. BK2010199). References [1] Yan Su, Jane H Davidson. Discharge of thermal storage tanks via immersed baffled heat exchangers: numerical model of flow and temperature fields [J]. Journal of Solar Energy Engineering, 2008, 130:021016-1-021016-7. [2] Yan Su, Jane H Davidson. Transient natural convection heat transfer correlations for tube bundles immersed in a thermal storage [J]. Journal of Solar Energy Engineering, 2009, 129:210-214. [3] Roman Spur, Dusan Fiala, Dusan Nevrala, et al. Performances of modern domestic hot-water stores [J]. Applied Energy, 2006,83:893-910 [4] Jayanta Deb Mondol, Mervyn Smyth, Aggelos Zacharopoulos et al. Experimental performance evaluation of a novel heat exchanger for a solar hot water storage system [J]. Applied Energy, 2009, 86:1492-1505. [5] Jayanta Deb Mondol, Mervyn Smyth, Aggelos Zacharopoulos. Experimental characterisation of a novel heat exchanger for a solar hot water application under indoorand outdoor conditions [J] Renewable Energy, 2011, 36:1766-1779. [6] András Zachár. Investigation of a new tube-in-tube helical flow distributor design to improve temperature stratification inside hot water storage tanks operated with coiled-tube heat exchangers [J]. International Journal of Heat and Mass Transfer, 2013, 63:150-161 [7] Vishard Ragoonanan, Jane H. Davidson, Kelly O. Homan et al. The benefit of dividing an indirect thermal storage into two compartments: Discharge experiments [J]. Solar Energy, 2006, 80:18-31. [8] S Knudsen, S Furbo. Thermal stratification in vertical mantle heat-exchangers with application to solar domestic hot-water systems [J]. Applied Energy, 2004, 78:257-272 [9] Zhou Zhiyong, Peng Sanbing, Fu Xiangzhao et al. Forced indirect cycle solar hot water system in hot summer and warm winter area [J]. GAS & HEAT, 2009,8(29):33-39. [10] Y C Soo Too, G L Morrison, M Behnia. Performance of solar water heaters with narrow mantle heat exchangers [J]. Solar Energy, 2009, 83:350-362. [11] A Carrillo Andres, J M Cejudo Lopez. TRNSYS model of a thermosiphon solar domestic water heater with a horizontal store and mantle heat exchanger [J]. Solar Energy, 2002, 2(72):89-98. [12] Han Yanmin, Dai Yanjun, Wang Ruzhu. Optimum analysis energy conversion for solar collector system [J]. Journal of engineering thermophysics, 2006, 27:57-60. [13] Julia F Haltiwanger, Jane H Davidson. Discharge of a thermal storage tank using an immersed heat exchanger with an annular baffle [J]. Solar Energy, 2009, 83:193-201. [14] TRNSYS 16 Documentation. 2006. [15] L M Ayompe, A Duffy. Thermal performance analysis of a solar water heating system with heat pipe evacuated tube collector using data from a field trial[J].Solar Energy,2013,90: 17-28.