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Active heat storage characteristics of active–passive triple wall with phase change material
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 110 (2014) 276–285 www.elsevier.com/locate/solener
Active heat storage characteristics of active–passive triple wall with phase change material Haoshu Ling a, Chao Chen a,⇑, Yong Guan a, Shen Wei b, Ziguang Chen a, Na Li a a
College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100124, PR China b Building Performance Analysis Group, Plymouth University, Plymouth PL4 8AA, UK Received 15 April 2014; received in revised form 12 August 2014; accepted 10 September 2014
Communicated by: Associate Editor Yanjun Dai
Abstract In passive solar greenhouses, the solar energy obtained by the wall cannot be used efficiently for heating the indoor environment, due to a thermal-stable layer inside the wall, which is caused by the limitation in heat transfer properties of building materials. In order to solve this problem, a new system combining an active–passive triple phase change material wall (APTPCMW) and solar concentrators is proposed and introduced in this paper. To investigate the active heat storage performance of APTPCMW, an experiment is designed and carried out. From the experiment, the significant contribution of the new proposed system to improving the heat storage capacity of the middle layer of APTPCMW has been confirmed. Additionally, factors, namely, the gap between air tunnels, the flow direction of heated air, the temperature and velocity of the supply air, have been identified to have influence on the active heat storage performance of APTPCMW. For real applications, optimum operational conditions of APTPCMW have also been identified from the experiment. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Active–passive triple PCM wall; Active heat storage; Heat transfer; Solar concentrator
1. Introduction Due to the high speed increase of global population in current society, promoting the productivity in food by means of modern technologies is highly required (Sethi and Dubey, 2011). In order to provide suitable micro-climate for off-season crops, solar greenhouses are popularly used by both entrepreneurs and farmers (Sethi, 2009). Generally, the micro-climate of solar greenhouses can be influenced by several factors, such as outdoor meteorological conditions, soil thermal characteristics and thermal performance of solar greenhouse envelope. In the northern hemisphere, the north wall thermal performance in the ⇑ Corresponding author. Tel./fax: +86 10 67391608 201.
E-mail address: [email protected] (C. Chen). http://dx.doi.org/10.1016/j.solener.2014.09.015 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.
solar greenhouse is particularly important (Din et al., 2003). Increasing its heat storage capacity can raise the air temperature in the solar greenhouse up to 10 °C and can cover 35–82% heating load of solar greenhouses (Sethi and Sharma, 2008). A proper solution for increasing the north wall heat storage capacity is to incorporate phase change material (PCM) into the standard wall (Berroug et al., 2011; Beyhan et al., 2013; Kumari et al., 2006; Najjar and Hasan, 2008). However, recent studies have found that the efficiency of using this method can be influenced significantly by the heat transfer properties of building materials, due to a thermal-stable layer inside the north wall, which would greatly decrease the wall heat storage capacity. Based on results from simulation, Guan et al. (2014) have suggested that for a three-layer PCM wall with a thickness
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Nomenclature Symbols q active heat storage density (W m3) T temperature (°C) V volume (m3) x coordinate or thickness (m) y coordinate or length (m) z coordinate or height (m)
ates atns ats atss hb in ou pb PCMb sa wns wss ijk
Greek letters c specific heat capacity (J kg1 °C1) e heat exchange effectiveness (%) q density (kg m3) s time (s) k thermal conductivity (W m1 °C1)
Abbreviations PCM Phase Change Material APTPCMW Active-Passive Triple PCM Wall PVC Polyvinyl Chloride
Subscripts air heated air an any level
of 900 mm, the thermal-stable layer happened at the depth of about 650 mm, as shown in Fig. 1. Zhang et al. (2012) performed a two-year study monitoring the temperature change of a 3000 mm thick cob wall in a solar greenhouse, and suggested that the wall temperature became stable after a depth of 400 mm. Chen and Liu (2006) have developed a mathematical model to predict the temperature distribution inside the north walls, based on which they suggested that the thickness of the thermal-stable layer was 300 mm for 600 mm concrete north walls, and was 200 mm for 600 mm composite heterogeneous north walls that were made of 400 mm thick concrete and 200 mm thick slag-wool. In order to improve heat storage capacity of the wall interior, Rodrigues and Aelenei (2010) have developed a naturally ventilated cavity wall, which helps to increase the wall interior temperature efficiently. Zalewski et al. (2012) have performed an exploration on the behavior of 30 Cloudy day 9:00 Sunny day 9:00 Rainy day 9:00
Temperature / °C
25 20
Cloudy day 16:00 Sunny day 16:00 Rainy day 16:00
15 10 5 Thermal-stable layer
0 -5 -10 0
100
200
east surface of air tunnel north surface of air tunnel surface of air tunnel south surface of air tunnel hollow block inlet outlet polystyrene board phase change material wallboard sand north surface of wall south surface of wall space grid node
300
400
500
600
700
800
900
Thickness / mm
Fig. 1. Temperature distributions of a three-layer PCM wall in passive solar greenhouses.
a small-scaled Trombe composite solar wall that consisted of a double glazing layer, a non-ventilated air cavity, a heat storage layer made by PCM, a ventilated air layer and an insulating panel. From a study performed in spring, 2008, in northern France, they found that the temperature of PCM inside the composite wall could reach 52 °C. Kara and Kurnuc (2012a,b) carried out a study evaluating the thermal performance of a Trombe wall using a PCM as heat storage medium. From the study, they found that the solar energy stored by the PCM wall could cover up to 70% monthly heating load of their testing room, and up to 36% daily heating load. Chen and Liu (2004) have developed a numerical model, which can be used to analyze the distribution of airflow and temperature in theirselfdeveloped composite wall. Using the model, they suggested that the temperature distribution inside the composite wall could be improved and the thermal resistance could be increased during the nighttime or when it was cloudy outdoors. Hassanain et al. (2011) have applied an 800 mm Trombe composite solar wall in a solar greenhouse located at the Suez-Canal University, Egypt. From field measurement, they found that when the average ambient temperature was 21.4 °C the maximum internal surface temperature of the solar wall could reach 50 °C, and the wall temperature at 400 mm depth could research 40 °C. Generally, the north wall of solar greenhouses can absorb solar energy using two methods: the passive method and the active method. The passive method is directly absorbing the solar energy reaching the wall surface. The active method is to use solar collectors to store solar energy in some kinds of medium, such as water and air, and then use this stored energy to increase the wall heat storage
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capacity. In this study, a new system combining an active– passive triple PCM wall (APTPCMW) and multi-surface trough solar concentrators has been developed, and it can strengthen the heat storage capacity of the wall interior using the active method and can improve the heat storage capacity of the PCM wallboards using the passive method. In our previous studies (Guan et al., 2013, 2012), the passive heat storage performance of APTPCMW has been evaluated. It was found that its passive heat storage capacity was 1.18 times as much as that of the north wall without PCM wallboards, and 77.6% of them were stored in the PCM wallboards. This paper introduced results obtained from an experiment, which evaluated the active performance of APTPCMW, influencing factors on the active performance of APTPCMW and its optimum operational conditions. 2. System design 2.1. Heat transfer of the north wall in the solar greenhouse Fig. 2 depicts the schematic diagram of the solar greenhouse. Generally, it consists of a solid north wall, a partial roof on the top of the north wall and a cover over the south part of the solar greenhouse. The cover on the south part is generally made of a thin polyvinyl chloride (PVC) film, which allows solar energy to go into the solar greenhouse during the daytime. During the nighttime, a cotton blanket will be added onto the top of the PVC film to reduce heat loss (Tong et al., 2009). The north wall heat transfer process in the solar greenhouse is complex and unstable, as shown in Fig. 2. Its south surface is affected by the indoor thermal environment of the solar greenhouse and the solar radiation, and its north surface is influenced by the outdoor meteorological conditions. During the daytime in winter, the solar radiation irradiates directly on the south surface of the north wall. Some can be absorbed, accumulated and conducted into the wall interior, and others will be reflected to the surfaces of the crop, the soil, the south roof, the north roof and other walls through radiation.
In order to improve the north wall heat storage capacity, one method is to increase the heat that can go into the north wall interior; the second method is to improve the heat absorbance ability of the south surface of the north wall so more solar energy can be absorbed and stored during the daytime; and the third method is by promoting the thermal resistance of the external surface of the north wall so less heat will be lost to the outdoor environment. 2.2. A new system combining an APTPCMW and solar concentrators In this study, a new system that combines an APTPCMW and multi-surface trough solar concentrators was proposed so as to improve the north wall heat storage capacity in solar greenhouses, as explained by Figs. 3 and 4. The APTPCMW used in this study was composed of three layers: (1) an inner (south) layer made of a specific type of PCM wallboards using in greenhouses, coded as GH-20 PCM wallboards (Chen et al., 2008), in order to directly absorb and store solar energy during the daytime and release heat at the nighttime; (2) a middle layer built with concrete hollow blocks with good heat storage and heat transfer ability; and (3) an outer (north) layer built with materials with high thermal insulation properties. Inside the APTPCMW, there were several parallel vertical air tunnels, connected with the glass pipe of solar concentrators situated on the top of APTPCMW. During the daytime, the solar energy could be absorbed by those solar collectors and then be used to heat the air in the glass pipe. The heated air was then supplied into these air tunnels to increase the interior temperature of APTPCMW, so as to increase its heat storage capacity. 3. Materials and methods 3.1. Experimental setup In this study, an experiment was designed to emulate the working of the new system introduced in Section 2.2. In this experiment, the solar energy was provided by an
Blanket
Sun
North
North Roof
PVC film (South roof)
Convection
North Wall
Convection Diffused Radiation
Soil
Crops
Surface Thermal Radiation
Fig. 2. A schematic diagram of the solar greenhouse.
Surface Thermal Radiation
H. Ling et al. / Solar Energy 110 (2014) 276–285
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Solar Collector
Inlet North
PCM Layer Block Layer (Air Tunnel) Insulation Layer Fig. 6. Structure of APTPCMW.
Outlet Fig. 3. Sectional drawing of the system.
Solar Concentrators
Return Air Duct
Supply Air Duct Air Tunnel
Fig. 4. Schematic diagram of the heated air system.
electrical heater, which was easier to control the supply air temperature than real solar systems. Additionally, in order to minimise the influence from the passive heat absorbing, the experiment was carried out in a testing room that had no solar radiation. The whole system, as shown in Fig. 5, consisted of an electrical heating air system, a fan, two anemometers, two temperature measuring devices and an APTPCMW. Fig. 6 shows the structure of APTPCMW used in this study. The length, height and thickness of APTPCMW were 1.76 m, 1.14 m and 0.28 m, respectively. Its outer layer was built with polystyrene boards with a thickness of 0.05 m, working as the insulation layer. The middle layer was built with concrete hollow blocks with a thickness of 0.19 m. In this layer, there were nine air tunnels (No. A– No. I) with a dimension of 0.13 m (length) 1.14 m (height) 0.12 m (thickness). In this study, some air tunnels were connected with the supply air ducts from the electrical air heating system, while the remaining ones were filled with sands so as to improve the heat storage and heat transfer of APTPCMW. The inner layer was built with several PCM wallboards with a thickness of 0.04 m, made by a mixture of concrete and GH-20 PCM (Chen et al., 2008).
North Fan
Anemometer
APTPCMW
Heaters Anemometer
Fig. 5. The system diagram of the experiment.
Temperature Measuring Equipments
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0
Heat flux / W·g -1
8 -0.8 6 -1.2 4 -1.6
2
DSC curve of PCM Specific heat of PCM wallboards
-2 5
10
15
20
25
Specific heat / kJ·kg -1·°C -1
10
-0.4
0 30
Temperature / °C
Fig. 7. The DSC curve of PCM and the equivalent specific heat capacity of PCM wallboards.
The GH-20 PCM is a kind of the shape-stabilized solid–liquid PCM and it is also a compound composed of a paraffin as a dispersed phase change material and a high density polyethylene as a supporting material. The differential scanning calorimetry (DSC) curve of the GH-20 PCM is shown in Fig. 7, from which it could be seen that the range of phase change temperature is between 7.1 °C and 25.9 °C, and its heat of fusion can reach 213.4 kJ kg1. To avoid fire hazard, a fireproof coating was added onto the surface of the PCM wallboards. Some important physical parameters of APTPCMW used in this study are listed in Table 1. 3.2. Data collection During the experiment, the following parameters have been monitored: the temperature of the inner and outer surfaces of APTPCMW, air temperature and velocity at the inlet of the air tunnel, air temperature and velocity at the outlet of the air tunnel, the interior temperature of APTPCMW and the ambient temperature. Two ISI IR525 infrared snapshots have been used to monitor and record the surface temperatures for the inner and outer surfaces of APTPCMW, with an accuracy of ±0.5 °C. The air velocity at the inlet and the outlet of the air tunnels were recorded by two Testo 435 hot-wired anemometers, with a measurement error lower than 0.01 m/s. Other parameters were measured and recorded by a data acquisition system, including two Agilent 34972A data loggers, 245 T-type thermocouples (accuracy of ±0.5 °C), one PC with specific software. Fig. 8 illustrates the positions of the 245 T-type thermocouples: 240 of them were placed at Table 1 Physical parameters of APTPCMW. Material
Thermal conductivity/ W (m °C)1
Specific heat/ kJ (kg °C)1
Density / kg m3
GH-20 PCM wallboard Hollow block Sand Polystyrene board
0.400 0.810 0.470 0.0420
Fig. 7 1.05 1.01 1.38
900 1800 1200 30
Fig. 8. Temperature measurement positions.
different positions either on the surface or in the interior of APTPCMW to monitor the wall surface temperature, the wall interior temperature and the air temperature inside the tunnels; one was placed at the inlet of the air tunnel to measure the supply air temperature; one was placed at the outlet of the air tunnel to measure the outlet air temperature; and the remaining three were placed in the surrounding environment to collect the ambient temperature. The measurement intervals of these parameters in the study were all one minute. 3.3. Experimental conditions The heat transfer process and the airflow conditions when the heated air flows inside the tunnels are described in Fig. 9. When the heated air flow passes through the block layer of APTPCMW, the large temperature difference between the air and the tunnel surface causes strong convective heat transfer on the tunnel surface (Chen et al., 2014). The heat is stored when the tunnel surface temperature is rising, and this heat is transferred to the interior of APTPCMW by conduction, so that the temperature of APTPCMW rises and the sensible heat storage also increases. Therefore, factors influencing the heat transfer and active heat storage of APTPCMW could be the gap
H. Ling et al. / Solar Energy 110 (2014) 276–285
Convection
Conduction Storing
Convection
Storing
z
Insulation Layer
Block Layer
Block Layer
PCM Layer
Air Tunnel
281
1.14
Storing Conduction Storing
0
between air tunnels, the flow direction of heated air, the temperature and velocity of supply air. Table 2 lists the experimental conditions of this study. When evaluating the influence of one parameter in the experiment, its value was changed gradually while keeping other factors under the same conditions (Xiao et al., 2009). Case 1 was defined for identifying the optimum value for the gap between air tunnels, and other cases were trying to figure out the optimum flow direction of heated air (Case 2), the optimum velocity of supply air (Case 3) and the optimum temperature of supply air (Case 4), respectively. The heating duration in each case was 8 h, similar to the time of effective solar radiation in winter. The electrical heating system with a fan was used to control the temperature and velocity of supply air, with accuracies of ±0.5 °C and ±0.01 m/s, respectively. Furthermore, as the average daytime indoor temperature is 22.7 °C in sunny days in winter (Guan et al., 2012), during the experiment, the temperature around the APTPCMW was kept within 23 ± 1 °C to guarantee the similar ambient conditions. 3.4. Analysis approach 3.4.1. Active heat storage density When evaluating the active heat storage of APTPCMW, a physical model of the investigated APTPCMW was used, as shown in Fig. 10.
Insulation layer
Block layer
PCM Layer
Fig. 9. Heat transfer and airflow inside air tunnels.
0.28
0.135
x
0.09
0.29
North y Fig. 10. Physical model of APTPCMW.
In the model, some assumptions had been made to treat the heat transfer of APTPCMW: (1) Since the airflow direction inside the tunnel was vertical, the heat transfer of APTPCMW in that direction could be ignored. (2) The physical parameters of APTPCMW were constant, except the equivalent specific heat of GH-20 PCM wallboards. (3) The thermal contact resistance between layers was ignorable. Based on the above assumptions, the two-dimensional heat transfer of APTPCMW at any level zan was defined by Eq. (1), k
@2T @2T @ðcT Þ þk 2 ¼q 2 @x @y @s
ð1Þ
With boundary conditions as, T jx¼0;z¼zan ¼ T wss ð0; y; zan Þ
ð2Þ
Table 2 Experimental conditions. Case
Air tunnel
Sand tunnel
1 2 3 4
E B; E; H B; E; H B; E; H
A; A; A; A;
B; C; D; C; D; F; C; D; F; C; D; F;
F; G; H; I G; I G; I G; I
Supply air temperature/°C
Supply air velocity/m s1
Flow direction
60 60 60 40; 45; 50; 55; 60
0.26 0.26 0.20; 0.26; 0.31; 0.41 0.26
Downward Downward; Upward Downward Downward
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@T j ¼0 @y 0
ð3Þ
T jx¼0:075;y60:065;z¼zan ¼ T atss ð0:075; 0; zan Þ
ð4Þ
T j0:075
ð5Þ
T jx¼0:195;y60:065;z¼zan ¼ T atns ð0:195; 0; zan Þ
ð6Þ
@T j ¼0 @y 0:195
ð7Þ
T jx¼0:28;z¼zan ¼ T wns ð0:28; y; zan Þ
ð8Þ
@T j ¼0 @y y¼0:29;z¼zan
ð9Þ
An explicit finite difference method was used to disperse aforementioned equations (Chen et al., 2012; Guan et al., 2013), and the time step and the mesh size were 60 s and 0.005 m, respectively. The active heat storage density of APTPCMW at the s time was calculated by, R T ijk ðsÞ P56 P58 P228 i¼1 j¼1 k¼1 qijk V ijk T ijk ð0Þ cijk ðT ÞdT qðsÞ ¼ ð10Þ P56 P58 P228 i¼1 j¼1 k¼1 V ijk Since the investigated APTPCMW was heterogeneous, physical parameters of materials at various positions were given as, 8 cijk > > > > > > < cijk cijk > > > > cijk > > : cijk
¼ cPCMb ; qijk ¼ qPCMb
if 0 6 x 6 0:04
¼ cpb ; qijk ¼ qpb ¼ 0; qijk ¼ 0
if 0:23 < x 6 0:28 if 0:075 < x 6 0:195 and 0 6 y 6 0:065
¼ csa ; qijk ¼ qsa ¼ chb ; qijk ¼ qhb
if 0:075 < x 6 0:195 and 0:115 6 y 6 0:245 the others
ð11Þ
3.4.2. Heat exchange effectiveness To evaluate the heat exchange efficiency between the heated air and the tunnel, the heat exchange effectiveness was adopted in this study, and it was calculated by Eq. (12), eðsÞ ¼
T air;in ðsÞ T air;ou ðsÞ 100% T air;in ðsÞ T acs ðsÞ
ð12Þ
4. Results
were filled by sands. Definitions of other parameters are listed in Table 2 for Case 1. Fig. 11 shows the average temperature in the middle section of the block layer (x = 0.135 m in Fig. 10) at different time slots during the experiment. It shows that the average temperature of those measuring positions within the block layer rises gradually at the beginning, and the increasing rate is proportional to the heating time. Furthermore, when the distance from the air tunnel increases, the average temperature rise decreases. When the distance is over 200 mm, the rise is relatively small, and when the distance reaches 490 mm, the average temperature almost remains constant. Therefore, the maximum distance from the middle section of the block layer that can be affected by the heated air can be defined as 490 mm, and the suitable distance is 200 mm. It is recommended here that the gap between air tunnels to be chosen as 400 mm. 4.1.2. Airflow direction inside the tunnel The evaluation of the influence of heated airflow direction inside the tunnel on active heat storage characteristics was carried out under the experimental conditions defined in Table 2 for Case 2, lasting for 8 h. The active heat storage density and the heat exchange effectiveness of different flow directions are presented in Fig. 12. It shows that the airflow direction has an important influence on both the active heat storage density and the heat exchange effectiveness. After eight-hour experiment, the active heat storage density of APTPCMW heated by the downward heated air was 9.4 MJ m3, 1.21 times higher than that by the upward heated air; the heat exchange effectiveness heated by the downward heated air was about 66.2%, 1.15 times higher than that by the upward heated air. The reason is that the Gr/Re2 (Gr: Grashof number; Re: Reynolds number) of heated airflow in the air tunnel is about 0.6, so the impact of natural convection on the heat transfer of APTPCMW should not be ignored (Hieber, 1982). Therefore, the heat from the heated air is transferred to the surfaces of the air tunnel by mixed convection, including both natural convection and forced convection. When the heated air enters the tunnel from the bottom, the direction of forced convection is opposite to that of natural convection. Under this condition, the
4.1. Evaluating influence of factors
4.1.1. The gap between air tunnels To evaluate the influence of the gap between air tunnels on active heat storage characteristics, the top of the air tunnel E (see Fig. 8a) was connected with the supply air duct from the electrical heating system, while the other tunnels
Temperature / ºC
50
In this study, the influence of the gap between air tunnels, the flow direction of heated air inside the tunnel, the velocity and temperature of supply air on the active heat storage performance of APTPCMW were evaluated, and the results are expressed in this paper sequentially.
45
0h
1h
2h
40
3h
4h
5h
6h
7h
8h
35 30 25 Air Tunnel
20
Block
15 0
50
100
150
200
250
300
350
400
450
500
Distance / mm
Fig. 11. Average temperature in the middle section of the block layer.
100
9
90
8
80
7
70
6
60
5
50
4
40
q-downward q-upward ε-downward ε-upward
3 2 1
30 20 10
0
0 0
1
2
3
4
5
6
7
8
Time / h
Fig. 12. Active heat storage density and heat exchange effectiveness for different airflow directions.
heat transfer performance of APTPCMW is weakened, and hence the active heat storage performance is reduced. When the heated air enters the tunnel from the top, natural convection and forced convection have consistent directions, so the active heat storage performance is promoted. Therefore, it is suggested to provide the heated air from the top of the tunnel, rather than from the bottom.
q(8h) 75
q(7h) q(6h)
70
q(5h) q(4h)
65
q(3h) q(2h)
60
q(1h) ε
55 0.2
0.25
0.3
0.35
0.4
0.45
0.5
Supply air velocity / m·s -1
Fig. 13. Active heat storage density and heat exchange effectiveness for different supply air velocities.
4.2. Performance evaluation of APTPCMW under optimum operation conditions In the following contents, the contribution of the new proposed solar system to improving the performance of APTPCMW is demonstrated and discussed, with respect to the temperature distribution and the active heat storage and release, respectively. 4.2.1. Temperature distribution Fig. 15 depicts the average temperature distribution of APTPCMW along the thickness direction, when y = 0.09 m in Fig. 10. The supply air temperature was chosen as 60 °C and other experimental conditions were based on the definitions in Table 2 for Case 4. The experimental result clearly shows that there is no thermal-stable layer inside the APTPCMW under investigated. The average temperature of all measuring positions inside the block layer has been increased greatly, with a maximum temperature rise of 14.4 °C, so the heat storage capacity of the 80
10 9
q(8h)
8
q(7h)
7
q(6h)
6
q(5h)
5
q(4h)
4
q(3h)
3
q(2h)
2
q(1h)
1
ε
75 70 65 60
Heat exchange effectiveness / %
80
11 10 9 8 7 6 5 4 3 2 1 0
Heat exchange effectiveness / %
Active heat storage density / MJ·m-3
4.1.3. Supply air velocity The influence from the supply air velocity on both the active heat storage density and the heat exchange effectiveness of APTPCMW is demonstrated by Fig. 13, given by performing the experiment under the conditions defined in Table 2 for Case 3. From Fig. 13, it can be observed that with the increase of supply air velocity, the heat exchange effectiveness decreases, due to the decrease of the temperature difference between the inlet air and the outlet air. However, when the supply air velocity increases, the volume of heated air going into the air tunnel goes up, hence the heat supplied to the APTPCMW increases. Therefore, increasing the velocity of supply air leads to a gradual rise of the active heat storage density of APTPCMW. However, when the velocity is higher than 0.26 m s1, the heat exchange effectiveness drops sharply, and the increasing rate of the active heat storage density slows down. Therefore, it is recommended to choose the supply air velocity as 0.26 m s1.
283
4.1.4. Supply air temperature Fig. 14 shows the change of both the active heat storage density and the heat exchange effectiveness with different supply air temperature, according to the experimental conditions defined in Table 2 for Case 4. It reflects that with the increase of supply air temperature, the temperature difference between the heated air and the tunnel surface increases, so the intensity of heat exchange rises. Therefore, the active heat storage density of APTPCMW can be improved significantly. However, when the supply air temperature increases, the increasing rate of the temperature difference between the inlet air and the outlet air becomes smaller than the increasing rate of the temperature difference between the inlet air and the surface of the air tunnel. Therefore, the heat exchange effectiveness goes down, but the decreasing rate slows down with the increase of supply air temperature. Compared with the heat exchange effectiveness at 55 °C, the heat exchange effectiveness at 60 °C has decreased only by 1.8%. Therefore, it is recommended that the supply air temperature to be chosen as 60 °C.
Active heat storage density / MJ·m-3
10
Heat exchange effectiveness / %
Active heat storage density / MJ·m-3
H. Ling et al. / Solar Energy 110 (2014) 276–285
55
0 40
45
50
55
60
65
70
Supply air temperature / ºC
Fig. 14. Active heat storage density and heat exchange effectiveness for different supply air temperature.
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H. Ling et al. / Solar Energy 110 (2014) 276–285 0h 5h
Temperature / ºC
40
1h 6h
2h 7h
3h 8h
is 1.6%, meaning that 98.4% heat can be released to the indoor environment.
4h Insulation Layer
35 PCM Layer 30
25 Block Layer
20 0
40
80
120
160
200
240
280
Thickness /mm
Fig. 15. Temperature (y = 0.09 m).
distribution
along
the
thickness
direction
block layer is strengthened. Additionally, the average temperature of the PCM layer has also been increased dramatically, with an average temperature rise of 5.2 °C. This demonstrates that the proposed APTPCMW-solar system can improve the storage efficiency and the heat storage capacity of the PCM layer. For the insulation layer, the maximum outer surface temperature rise is 2.3 °C, much less than that of the block layer and PCM layer. Therefore, the new proposed wall can also reduce the heat loss effectively. 4.2.2. Active heat storage and heat release The results with respect to the active heat storage and heat release of APTPCMW have been presented in Fig. 16, with supply air temperature of 60 °C and experimental definitions given in Table 2 for Case 4. Fig. 16 shows that the characteristics of the active heat storage and the heat release of APTPCMW are similar. When the heating/releasing time increases, the active heat storage and the heat release will increase as well. However, the increasing rate decreases gradually. After the active heat storage experiment was carried out for 8 h, the active heat storage density was 9.43 MJ m3, 82.3% of which was stored in the block layer, 17.3% was stored in the PCM layer and 0.4% was stored in the insulation layer. Therefore, the heat storage capacity of both the block layer and the PCM layer has been improved significantly. After the heat release experiment was carried out for 16 h, the heat release density was 9.28 MJ m3. Therefore, the discrepancy of the active heat storage and the heat release Active heat storage density / MJ·m-3
5. Conclusions Nowadays, solar greenhouses have been popularly used to help promote the food production to deal with the rapid increase of global population. For solar greenhouses, increasing the efficiency of absorbing, storing and releasing solar energy is extremely important. This paper, therefore, has introduced the theory of a new system combing an APTPCMW and multi-surface trough solar concentrators, in order to achieve the goal. To evaluate the active heat storage performance of APTPCMW, an experiment was designed and carried out, and conclusions from the experiment include: (1) Influencing factors on the active heat storage performance of APTPCMW include the gap between air tunnels, the flow direction of heated air, the temperature and velocity of supply air; (2) Optimum operation conditions of APTPCMW were 0.4 m gap between air tunnels, downward flow direction for the heated air inside the tunnel, 0.26 m s1 supply air velocity and 60 °C temperature for the supply air; and (3) Using the new proposed system, the heat storage capacity of the block layer of APTPCMW can be improved significantly. Additionally, the heat stored within the wall during the daytime can be released efficiently during the nighttime. This paper has critically demonstrated the applicability of the theory of a new system combing APTPCMWs and multi-surface trough solar concentrators. Future tests in real applications are still needed to confirm its actual efficiency of using solar energy in greenhouses. Acknowledgment This study was supported by the National Natural Science Foundation of China, Nos. 51368060 and 50978002. References
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8
6
4
2
0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Time / h
Fig. 16. Active heat storage and heat release of APTPCMW.
Berroug, F., Lakhal, E.K., El Omari, M., Faraji, M., El Qarnia, H., 2011. Thermal performance of a greenhouse with a phase change material north wall. Energy Build. 43 (11), 3027–3035. Beyhan, B., Paksoy, H., Dasgan, Y., 2013. Root zone temperature control with thermal energy storage in phase change materials for soilless greenhouse applications. Energy Convers. Manage. 74, 446–453. Chen, W., Liu, W., 2004. Numerical analysis of heat transfer in a composite wall solar-collector system with a porous absorber. Appl. Energy 78 (2), 137–149. Chen, W., Liu, W., 2006. Numerical simulation of the airflow and temperature distribution in a lean-to greenhouse. Renewable Energy 31 (4), 517–535. Chen, C., Guo, H.F., Liu, Y.N., Yue, H.L., Wang, C.D., 2008. A new kind of phase change material (PCM) for energy-storing wallboard. Energy Build. 40 (5), 882–890.
H. Ling et al. / Solar Energy 110 (2014) 276–285 Chen, C., Li, Z., Guan, Y., Han, Y., Ling, H., 2012. Effects of building methods on thermal properties of phase change heat storage composite for solar greenhouse. Trans. Chin. Soc. Agric. Eng. 28 (SUPPL. 1), 186–191. Chen, C., Liang, L., Zhang, Y., Chen, Z.G., Xie, G.Y., 2014. Heat transfer performance and structural optimization design method of vertical phase change thermal energy storage device. Energy Build. 68, 679–685. Din, M., Tiwari, G.N., Ghosal, M.K., Srivastava, N.S.L., Khan, M.I., Sodha, M.S., 2003. Effect of thermal storage on the performance of greenhouse. Int. J. Energy Res. 27 (1), 79–92. Guan, Y., Chen, C., Li, Z., Han, Y., Ling, H., 2012. Improving thermal environment in solar greenhouse with phase-change thermal storage wall. Trans. Chin. Soc. Agric. Eng. 28 (10), 194–201. Guan, Y., Chen, C., Ling, H., Han, Y., Yan, Q., 2013. Analysis of heat transfer properties of three-layer wall with phase-change heat storage in solar greenhouse. Trans. Chin. Soc. Agric. Eng. 29 (21), 166–173. Guan, Y., Chen, C., Han, Y.Q., Ling, H., Yan, Q., 2014. Experimental and modelling analysis of a three-layer wall with phase-change thermal storage in a Chinese solar greenhouse. J. Building Phys.. http:// dx.doi.org/10.1177/1744259114526350. Hassanain, A.A., Hokam, E.M., Mallick, T.K., 2011. Effect of solar storage wall on the passive solar heating constructions. Energy Build. 43 (2–3), 737–747. Hieber, C.A., 1982. Laminar mixed convection in an isothermal horizontal tube – correlation of heat-transfer data. Int. J. Heat Mass Transf. 25 (11), 1737–1746. Kara, Y.A., Kurnuc, A., 2012a. Performance of coupled novel triple glass and phase change material wall in the heating season: an experimental study. Sol. Energy 86 (9), 2432–2442.
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ScienceDirect Solar Energy 110 (2014) 276–285 www.elsevier.com/locate/solener
Active heat storage characteristics of active–passive triple wall with phase change material Haoshu Ling a, Chao Chen a,⇑, Yong Guan a, Shen Wei b, Ziguang Chen a, Na Li a a
College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100124, PR China b Building Performance Analysis Group, Plymouth University, Plymouth PL4 8AA, UK Received 15 April 2014; received in revised form 12 August 2014; accepted 10 September 2014
Communicated by: Associate Editor Yanjun Dai
Abstract In passive solar greenhouses, the solar energy obtained by the wall cannot be used efficiently for heating the indoor environment, due to a thermal-stable layer inside the wall, which is caused by the limitation in heat transfer properties of building materials. In order to solve this problem, a new system combining an active–passive triple phase change material wall (APTPCMW) and solar concentrators is proposed and introduced in this paper. To investigate the active heat storage performance of APTPCMW, an experiment is designed and carried out. From the experiment, the significant contribution of the new proposed system to improving the heat storage capacity of the middle layer of APTPCMW has been confirmed. Additionally, factors, namely, the gap between air tunnels, the flow direction of heated air, the temperature and velocity of the supply air, have been identified to have influence on the active heat storage performance of APTPCMW. For real applications, optimum operational conditions of APTPCMW have also been identified from the experiment. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Active–passive triple PCM wall; Active heat storage; Heat transfer; Solar concentrator
1. Introduction Due to the high speed increase of global population in current society, promoting the productivity in food by means of modern technologies is highly required (Sethi and Dubey, 2011). In order to provide suitable micro-climate for off-season crops, solar greenhouses are popularly used by both entrepreneurs and farmers (Sethi, 2009). Generally, the micro-climate of solar greenhouses can be influenced by several factors, such as outdoor meteorological conditions, soil thermal characteristics and thermal performance of solar greenhouse envelope. In the northern hemisphere, the north wall thermal performance in the ⇑ Corresponding author. Tel./fax: +86 10 67391608 201.
E-mail address: [email protected] (C. Chen). http://dx.doi.org/10.1016/j.solener.2014.09.015 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved.
solar greenhouse is particularly important (Din et al., 2003). Increasing its heat storage capacity can raise the air temperature in the solar greenhouse up to 10 °C and can cover 35–82% heating load of solar greenhouses (Sethi and Sharma, 2008). A proper solution for increasing the north wall heat storage capacity is to incorporate phase change material (PCM) into the standard wall (Berroug et al., 2011; Beyhan et al., 2013; Kumari et al., 2006; Najjar and Hasan, 2008). However, recent studies have found that the efficiency of using this method can be influenced significantly by the heat transfer properties of building materials, due to a thermal-stable layer inside the north wall, which would greatly decrease the wall heat storage capacity. Based on results from simulation, Guan et al. (2014) have suggested that for a three-layer PCM wall with a thickness
H. Ling et al. / Solar Energy 110 (2014) 276–285
277
Nomenclature Symbols q active heat storage density (W m3) T temperature (°C) V volume (m3) x coordinate or thickness (m) y coordinate or length (m) z coordinate or height (m)
ates atns ats atss hb in ou pb PCMb sa wns wss ijk
Greek letters c specific heat capacity (J kg1 °C1) e heat exchange effectiveness (%) q density (kg m3) s time (s) k thermal conductivity (W m1 °C1)
Abbreviations PCM Phase Change Material APTPCMW Active-Passive Triple PCM Wall PVC Polyvinyl Chloride
Subscripts air heated air an any level
of 900 mm, the thermal-stable layer happened at the depth of about 650 mm, as shown in Fig. 1. Zhang et al. (2012) performed a two-year study monitoring the temperature change of a 3000 mm thick cob wall in a solar greenhouse, and suggested that the wall temperature became stable after a depth of 400 mm. Chen and Liu (2006) have developed a mathematical model to predict the temperature distribution inside the north walls, based on which they suggested that the thickness of the thermal-stable layer was 300 mm for 600 mm concrete north walls, and was 200 mm for 600 mm composite heterogeneous north walls that were made of 400 mm thick concrete and 200 mm thick slag-wool. In order to improve heat storage capacity of the wall interior, Rodrigues and Aelenei (2010) have developed a naturally ventilated cavity wall, which helps to increase the wall interior temperature efficiently. Zalewski et al. (2012) have performed an exploration on the behavior of 30 Cloudy day 9:00 Sunny day 9:00 Rainy day 9:00
Temperature / °C
25 20
Cloudy day 16:00 Sunny day 16:00 Rainy day 16:00
15 10 5 Thermal-stable layer
0 -5 -10 0
100
200
east surface of air tunnel north surface of air tunnel surface of air tunnel south surface of air tunnel hollow block inlet outlet polystyrene board phase change material wallboard sand north surface of wall south surface of wall space grid node
300
400
500
600
700
800
900
Thickness / mm
Fig. 1. Temperature distributions of a three-layer PCM wall in passive solar greenhouses.
a small-scaled Trombe composite solar wall that consisted of a double glazing layer, a non-ventilated air cavity, a heat storage layer made by PCM, a ventilated air layer and an insulating panel. From a study performed in spring, 2008, in northern France, they found that the temperature of PCM inside the composite wall could reach 52 °C. Kara and Kurnuc (2012a,b) carried out a study evaluating the thermal performance of a Trombe wall using a PCM as heat storage medium. From the study, they found that the solar energy stored by the PCM wall could cover up to 70% monthly heating load of their testing room, and up to 36% daily heating load. Chen and Liu (2004) have developed a numerical model, which can be used to analyze the distribution of airflow and temperature in theirselfdeveloped composite wall. Using the model, they suggested that the temperature distribution inside the composite wall could be improved and the thermal resistance could be increased during the nighttime or when it was cloudy outdoors. Hassanain et al. (2011) have applied an 800 mm Trombe composite solar wall in a solar greenhouse located at the Suez-Canal University, Egypt. From field measurement, they found that when the average ambient temperature was 21.4 °C the maximum internal surface temperature of the solar wall could reach 50 °C, and the wall temperature at 400 mm depth could research 40 °C. Generally, the north wall of solar greenhouses can absorb solar energy using two methods: the passive method and the active method. The passive method is directly absorbing the solar energy reaching the wall surface. The active method is to use solar collectors to store solar energy in some kinds of medium, such as water and air, and then use this stored energy to increase the wall heat storage
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capacity. In this study, a new system combining an active– passive triple PCM wall (APTPCMW) and multi-surface trough solar concentrators has been developed, and it can strengthen the heat storage capacity of the wall interior using the active method and can improve the heat storage capacity of the PCM wallboards using the passive method. In our previous studies (Guan et al., 2013, 2012), the passive heat storage performance of APTPCMW has been evaluated. It was found that its passive heat storage capacity was 1.18 times as much as that of the north wall without PCM wallboards, and 77.6% of them were stored in the PCM wallboards. This paper introduced results obtained from an experiment, which evaluated the active performance of APTPCMW, influencing factors on the active performance of APTPCMW and its optimum operational conditions. 2. System design 2.1. Heat transfer of the north wall in the solar greenhouse Fig. 2 depicts the schematic diagram of the solar greenhouse. Generally, it consists of a solid north wall, a partial roof on the top of the north wall and a cover over the south part of the solar greenhouse. The cover on the south part is generally made of a thin polyvinyl chloride (PVC) film, which allows solar energy to go into the solar greenhouse during the daytime. During the nighttime, a cotton blanket will be added onto the top of the PVC film to reduce heat loss (Tong et al., 2009). The north wall heat transfer process in the solar greenhouse is complex and unstable, as shown in Fig. 2. Its south surface is affected by the indoor thermal environment of the solar greenhouse and the solar radiation, and its north surface is influenced by the outdoor meteorological conditions. During the daytime in winter, the solar radiation irradiates directly on the south surface of the north wall. Some can be absorbed, accumulated and conducted into the wall interior, and others will be reflected to the surfaces of the crop, the soil, the south roof, the north roof and other walls through radiation.
In order to improve the north wall heat storage capacity, one method is to increase the heat that can go into the north wall interior; the second method is to improve the heat absorbance ability of the south surface of the north wall so more solar energy can be absorbed and stored during the daytime; and the third method is by promoting the thermal resistance of the external surface of the north wall so less heat will be lost to the outdoor environment. 2.2. A new system combining an APTPCMW and solar concentrators In this study, a new system that combines an APTPCMW and multi-surface trough solar concentrators was proposed so as to improve the north wall heat storage capacity in solar greenhouses, as explained by Figs. 3 and 4. The APTPCMW used in this study was composed of three layers: (1) an inner (south) layer made of a specific type of PCM wallboards using in greenhouses, coded as GH-20 PCM wallboards (Chen et al., 2008), in order to directly absorb and store solar energy during the daytime and release heat at the nighttime; (2) a middle layer built with concrete hollow blocks with good heat storage and heat transfer ability; and (3) an outer (north) layer built with materials with high thermal insulation properties. Inside the APTPCMW, there were several parallel vertical air tunnels, connected with the glass pipe of solar concentrators situated on the top of APTPCMW. During the daytime, the solar energy could be absorbed by those solar collectors and then be used to heat the air in the glass pipe. The heated air was then supplied into these air tunnels to increase the interior temperature of APTPCMW, so as to increase its heat storage capacity. 3. Materials and methods 3.1. Experimental setup In this study, an experiment was designed to emulate the working of the new system introduced in Section 2.2. In this experiment, the solar energy was provided by an
Blanket
Sun
North
North Roof
PVC film (South roof)
Convection
North Wall
Convection Diffused Radiation
Soil
Crops
Surface Thermal Radiation
Fig. 2. A schematic diagram of the solar greenhouse.
Surface Thermal Radiation
H. Ling et al. / Solar Energy 110 (2014) 276–285
279
Solar Collector
Inlet North
PCM Layer Block Layer (Air Tunnel) Insulation Layer Fig. 6. Structure of APTPCMW.
Outlet Fig. 3. Sectional drawing of the system.
Solar Concentrators
Return Air Duct
Supply Air Duct Air Tunnel
Fig. 4. Schematic diagram of the heated air system.
electrical heater, which was easier to control the supply air temperature than real solar systems. Additionally, in order to minimise the influence from the passive heat absorbing, the experiment was carried out in a testing room that had no solar radiation. The whole system, as shown in Fig. 5, consisted of an electrical heating air system, a fan, two anemometers, two temperature measuring devices and an APTPCMW. Fig. 6 shows the structure of APTPCMW used in this study. The length, height and thickness of APTPCMW were 1.76 m, 1.14 m and 0.28 m, respectively. Its outer layer was built with polystyrene boards with a thickness of 0.05 m, working as the insulation layer. The middle layer was built with concrete hollow blocks with a thickness of 0.19 m. In this layer, there were nine air tunnels (No. A– No. I) with a dimension of 0.13 m (length) 1.14 m (height) 0.12 m (thickness). In this study, some air tunnels were connected with the supply air ducts from the electrical air heating system, while the remaining ones were filled with sands so as to improve the heat storage and heat transfer of APTPCMW. The inner layer was built with several PCM wallboards with a thickness of 0.04 m, made by a mixture of concrete and GH-20 PCM (Chen et al., 2008).
North Fan
Anemometer
APTPCMW
Heaters Anemometer
Fig. 5. The system diagram of the experiment.
Temperature Measuring Equipments
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H. Ling et al. / Solar Energy 110 (2014) 276–285 12
0
Heat flux / W·g -1
8 -0.8 6 -1.2 4 -1.6
2
DSC curve of PCM Specific heat of PCM wallboards
-2 5
10
15
20
25
Specific heat / kJ·kg -1·°C -1
10
-0.4
0 30
Temperature / °C
Fig. 7. The DSC curve of PCM and the equivalent specific heat capacity of PCM wallboards.
The GH-20 PCM is a kind of the shape-stabilized solid–liquid PCM and it is also a compound composed of a paraffin as a dispersed phase change material and a high density polyethylene as a supporting material. The differential scanning calorimetry (DSC) curve of the GH-20 PCM is shown in Fig. 7, from which it could be seen that the range of phase change temperature is between 7.1 °C and 25.9 °C, and its heat of fusion can reach 213.4 kJ kg1. To avoid fire hazard, a fireproof coating was added onto the surface of the PCM wallboards. Some important physical parameters of APTPCMW used in this study are listed in Table 1. 3.2. Data collection During the experiment, the following parameters have been monitored: the temperature of the inner and outer surfaces of APTPCMW, air temperature and velocity at the inlet of the air tunnel, air temperature and velocity at the outlet of the air tunnel, the interior temperature of APTPCMW and the ambient temperature. Two ISI IR525 infrared snapshots have been used to monitor and record the surface temperatures for the inner and outer surfaces of APTPCMW, with an accuracy of ±0.5 °C. The air velocity at the inlet and the outlet of the air tunnels were recorded by two Testo 435 hot-wired anemometers, with a measurement error lower than 0.01 m/s. Other parameters were measured and recorded by a data acquisition system, including two Agilent 34972A data loggers, 245 T-type thermocouples (accuracy of ±0.5 °C), one PC with specific software. Fig. 8 illustrates the positions of the 245 T-type thermocouples: 240 of them were placed at Table 1 Physical parameters of APTPCMW. Material
Thermal conductivity/ W (m °C)1
Specific heat/ kJ (kg °C)1
Density / kg m3
GH-20 PCM wallboard Hollow block Sand Polystyrene board
0.400 0.810 0.470 0.0420
Fig. 7 1.05 1.01 1.38
900 1800 1200 30
Fig. 8. Temperature measurement positions.
different positions either on the surface or in the interior of APTPCMW to monitor the wall surface temperature, the wall interior temperature and the air temperature inside the tunnels; one was placed at the inlet of the air tunnel to measure the supply air temperature; one was placed at the outlet of the air tunnel to measure the outlet air temperature; and the remaining three were placed in the surrounding environment to collect the ambient temperature. The measurement intervals of these parameters in the study were all one minute. 3.3. Experimental conditions The heat transfer process and the airflow conditions when the heated air flows inside the tunnels are described in Fig. 9. When the heated air flow passes through the block layer of APTPCMW, the large temperature difference between the air and the tunnel surface causes strong convective heat transfer on the tunnel surface (Chen et al., 2014). The heat is stored when the tunnel surface temperature is rising, and this heat is transferred to the interior of APTPCMW by conduction, so that the temperature of APTPCMW rises and the sensible heat storage also increases. Therefore, factors influencing the heat transfer and active heat storage of APTPCMW could be the gap
H. Ling et al. / Solar Energy 110 (2014) 276–285
Convection
Conduction Storing
Convection
Storing
z
Insulation Layer
Block Layer
Block Layer
PCM Layer
Air Tunnel
281
1.14
Storing Conduction Storing
0
between air tunnels, the flow direction of heated air, the temperature and velocity of supply air. Table 2 lists the experimental conditions of this study. When evaluating the influence of one parameter in the experiment, its value was changed gradually while keeping other factors under the same conditions (Xiao et al., 2009). Case 1 was defined for identifying the optimum value for the gap between air tunnels, and other cases were trying to figure out the optimum flow direction of heated air (Case 2), the optimum velocity of supply air (Case 3) and the optimum temperature of supply air (Case 4), respectively. The heating duration in each case was 8 h, similar to the time of effective solar radiation in winter. The electrical heating system with a fan was used to control the temperature and velocity of supply air, with accuracies of ±0.5 °C and ±0.01 m/s, respectively. Furthermore, as the average daytime indoor temperature is 22.7 °C in sunny days in winter (Guan et al., 2012), during the experiment, the temperature around the APTPCMW was kept within 23 ± 1 °C to guarantee the similar ambient conditions. 3.4. Analysis approach 3.4.1. Active heat storage density When evaluating the active heat storage of APTPCMW, a physical model of the investigated APTPCMW was used, as shown in Fig. 10.
Insulation layer
Block layer
PCM Layer
Fig. 9. Heat transfer and airflow inside air tunnels.
0.28
0.135
x
0.09
0.29
North y Fig. 10. Physical model of APTPCMW.
In the model, some assumptions had been made to treat the heat transfer of APTPCMW: (1) Since the airflow direction inside the tunnel was vertical, the heat transfer of APTPCMW in that direction could be ignored. (2) The physical parameters of APTPCMW were constant, except the equivalent specific heat of GH-20 PCM wallboards. (3) The thermal contact resistance between layers was ignorable. Based on the above assumptions, the two-dimensional heat transfer of APTPCMW at any level zan was defined by Eq. (1), k
@2T @2T @ðcT Þ þk 2 ¼q 2 @x @y @s
ð1Þ
With boundary conditions as, T jx¼0;z¼zan ¼ T wss ð0; y; zan Þ
ð2Þ
Table 2 Experimental conditions. Case
Air tunnel
Sand tunnel
1 2 3 4
E B; E; H B; E; H B; E; H
A; A; A; A;
B; C; D; C; D; F; C; D; F; C; D; F;
F; G; H; I G; I G; I G; I
Supply air temperature/°C
Supply air velocity/m s1
Flow direction
60 60 60 40; 45; 50; 55; 60
0.26 0.26 0.20; 0.26; 0.31; 0.41 0.26
Downward Downward; Upward Downward Downward
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@T j ¼0 @y 0
ð3Þ
T jx¼0:075;y60:065;z¼zan ¼ T atss ð0:075; 0; zan Þ
ð4Þ
T j0:075
ð5Þ
T jx¼0:195;y60:065;z¼zan ¼ T atns ð0:195; 0; zan Þ
ð6Þ
@T j ¼0 @y 0:195
ð7Þ
T jx¼0:28;z¼zan ¼ T wns ð0:28; y; zan Þ
ð8Þ
@T j ¼0 @y y¼0:29;z¼zan
ð9Þ
An explicit finite difference method was used to disperse aforementioned equations (Chen et al., 2012; Guan et al., 2013), and the time step and the mesh size were 60 s and 0.005 m, respectively. The active heat storage density of APTPCMW at the s time was calculated by, R T ijk ðsÞ P56 P58 P228 i¼1 j¼1 k¼1 qijk V ijk T ijk ð0Þ cijk ðT ÞdT qðsÞ ¼ ð10Þ P56 P58 P228 i¼1 j¼1 k¼1 V ijk Since the investigated APTPCMW was heterogeneous, physical parameters of materials at various positions were given as, 8 cijk > > > > > > < cijk cijk > > > > cijk > > : cijk
¼ cPCMb ; qijk ¼ qPCMb
if 0 6 x 6 0:04
¼ cpb ; qijk ¼ qpb ¼ 0; qijk ¼ 0
if 0:23 < x 6 0:28 if 0:075 < x 6 0:195 and 0 6 y 6 0:065
¼ csa ; qijk ¼ qsa ¼ chb ; qijk ¼ qhb
if 0:075 < x 6 0:195 and 0:115 6 y 6 0:245 the others
ð11Þ
3.4.2. Heat exchange effectiveness To evaluate the heat exchange efficiency between the heated air and the tunnel, the heat exchange effectiveness was adopted in this study, and it was calculated by Eq. (12), eðsÞ ¼
T air;in ðsÞ T air;ou ðsÞ 100% T air;in ðsÞ T acs ðsÞ
ð12Þ
4. Results
were filled by sands. Definitions of other parameters are listed in Table 2 for Case 1. Fig. 11 shows the average temperature in the middle section of the block layer (x = 0.135 m in Fig. 10) at different time slots during the experiment. It shows that the average temperature of those measuring positions within the block layer rises gradually at the beginning, and the increasing rate is proportional to the heating time. Furthermore, when the distance from the air tunnel increases, the average temperature rise decreases. When the distance is over 200 mm, the rise is relatively small, and when the distance reaches 490 mm, the average temperature almost remains constant. Therefore, the maximum distance from the middle section of the block layer that can be affected by the heated air can be defined as 490 mm, and the suitable distance is 200 mm. It is recommended here that the gap between air tunnels to be chosen as 400 mm. 4.1.2. Airflow direction inside the tunnel The evaluation of the influence of heated airflow direction inside the tunnel on active heat storage characteristics was carried out under the experimental conditions defined in Table 2 for Case 2, lasting for 8 h. The active heat storage density and the heat exchange effectiveness of different flow directions are presented in Fig. 12. It shows that the airflow direction has an important influence on both the active heat storage density and the heat exchange effectiveness. After eight-hour experiment, the active heat storage density of APTPCMW heated by the downward heated air was 9.4 MJ m3, 1.21 times higher than that by the upward heated air; the heat exchange effectiveness heated by the downward heated air was about 66.2%, 1.15 times higher than that by the upward heated air. The reason is that the Gr/Re2 (Gr: Grashof number; Re: Reynolds number) of heated airflow in the air tunnel is about 0.6, so the impact of natural convection on the heat transfer of APTPCMW should not be ignored (Hieber, 1982). Therefore, the heat from the heated air is transferred to the surfaces of the air tunnel by mixed convection, including both natural convection and forced convection. When the heated air enters the tunnel from the bottom, the direction of forced convection is opposite to that of natural convection. Under this condition, the
4.1. Evaluating influence of factors
4.1.1. The gap between air tunnels To evaluate the influence of the gap between air tunnels on active heat storage characteristics, the top of the air tunnel E (see Fig. 8a) was connected with the supply air duct from the electrical heating system, while the other tunnels
Temperature / ºC
50
In this study, the influence of the gap between air tunnels, the flow direction of heated air inside the tunnel, the velocity and temperature of supply air on the active heat storage performance of APTPCMW were evaluated, and the results are expressed in this paper sequentially.
45
0h
1h
2h
40
3h
4h
5h
6h
7h
8h
35 30 25 Air Tunnel
20
Block
15 0
50
100
150
200
250
300
350
400
450
500
Distance / mm
Fig. 11. Average temperature in the middle section of the block layer.
100
9
90
8
80
7
70
6
60
5
50
4
40
q-downward q-upward ε-downward ε-upward
3 2 1
30 20 10
0
0 0
1
2
3
4
5
6
7
8
Time / h
Fig. 12. Active heat storage density and heat exchange effectiveness for different airflow directions.
heat transfer performance of APTPCMW is weakened, and hence the active heat storage performance is reduced. When the heated air enters the tunnel from the top, natural convection and forced convection have consistent directions, so the active heat storage performance is promoted. Therefore, it is suggested to provide the heated air from the top of the tunnel, rather than from the bottom.
q(8h) 75
q(7h) q(6h)
70
q(5h) q(4h)
65
q(3h) q(2h)
60
q(1h) ε
55 0.2
0.25
0.3
0.35
0.4
0.45
0.5
Supply air velocity / m·s -1
Fig. 13. Active heat storage density and heat exchange effectiveness for different supply air velocities.
4.2. Performance evaluation of APTPCMW under optimum operation conditions In the following contents, the contribution of the new proposed solar system to improving the performance of APTPCMW is demonstrated and discussed, with respect to the temperature distribution and the active heat storage and release, respectively. 4.2.1. Temperature distribution Fig. 15 depicts the average temperature distribution of APTPCMW along the thickness direction, when y = 0.09 m in Fig. 10. The supply air temperature was chosen as 60 °C and other experimental conditions were based on the definitions in Table 2 for Case 4. The experimental result clearly shows that there is no thermal-stable layer inside the APTPCMW under investigated. The average temperature of all measuring positions inside the block layer has been increased greatly, with a maximum temperature rise of 14.4 °C, so the heat storage capacity of the 80
10 9
q(8h)
8
q(7h)
7
q(6h)
6
q(5h)
5
q(4h)
4
q(3h)
3
q(2h)
2
q(1h)
1
ε
75 70 65 60
Heat exchange effectiveness / %
80
11 10 9 8 7 6 5 4 3 2 1 0
Heat exchange effectiveness / %
Active heat storage density / MJ·m-3
4.1.3. Supply air velocity The influence from the supply air velocity on both the active heat storage density and the heat exchange effectiveness of APTPCMW is demonstrated by Fig. 13, given by performing the experiment under the conditions defined in Table 2 for Case 3. From Fig. 13, it can be observed that with the increase of supply air velocity, the heat exchange effectiveness decreases, due to the decrease of the temperature difference between the inlet air and the outlet air. However, when the supply air velocity increases, the volume of heated air going into the air tunnel goes up, hence the heat supplied to the APTPCMW increases. Therefore, increasing the velocity of supply air leads to a gradual rise of the active heat storage density of APTPCMW. However, when the velocity is higher than 0.26 m s1, the heat exchange effectiveness drops sharply, and the increasing rate of the active heat storage density slows down. Therefore, it is recommended to choose the supply air velocity as 0.26 m s1.
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4.1.4. Supply air temperature Fig. 14 shows the change of both the active heat storage density and the heat exchange effectiveness with different supply air temperature, according to the experimental conditions defined in Table 2 for Case 4. It reflects that with the increase of supply air temperature, the temperature difference between the heated air and the tunnel surface increases, so the intensity of heat exchange rises. Therefore, the active heat storage density of APTPCMW can be improved significantly. However, when the supply air temperature increases, the increasing rate of the temperature difference between the inlet air and the outlet air becomes smaller than the increasing rate of the temperature difference between the inlet air and the surface of the air tunnel. Therefore, the heat exchange effectiveness goes down, but the decreasing rate slows down with the increase of supply air temperature. Compared with the heat exchange effectiveness at 55 °C, the heat exchange effectiveness at 60 °C has decreased only by 1.8%. Therefore, it is recommended that the supply air temperature to be chosen as 60 °C.
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Fig. 14. Active heat storage density and heat exchange effectiveness for different supply air temperature.
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Fig. 15. Temperature (y = 0.09 m).
distribution
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block layer is strengthened. Additionally, the average temperature of the PCM layer has also been increased dramatically, with an average temperature rise of 5.2 °C. This demonstrates that the proposed APTPCMW-solar system can improve the storage efficiency and the heat storage capacity of the PCM layer. For the insulation layer, the maximum outer surface temperature rise is 2.3 °C, much less than that of the block layer and PCM layer. Therefore, the new proposed wall can also reduce the heat loss effectively. 4.2.2. Active heat storage and heat release The results with respect to the active heat storage and heat release of APTPCMW have been presented in Fig. 16, with supply air temperature of 60 °C and experimental definitions given in Table 2 for Case 4. Fig. 16 shows that the characteristics of the active heat storage and the heat release of APTPCMW are similar. When the heating/releasing time increases, the active heat storage and the heat release will increase as well. However, the increasing rate decreases gradually. After the active heat storage experiment was carried out for 8 h, the active heat storage density was 9.43 MJ m3, 82.3% of which was stored in the block layer, 17.3% was stored in the PCM layer and 0.4% was stored in the insulation layer. Therefore, the heat storage capacity of both the block layer and the PCM layer has been improved significantly. After the heat release experiment was carried out for 16 h, the heat release density was 9.28 MJ m3. Therefore, the discrepancy of the active heat storage and the heat release Active heat storage density / MJ·m-3
5. Conclusions Nowadays, solar greenhouses have been popularly used to help promote the food production to deal with the rapid increase of global population. For solar greenhouses, increasing the efficiency of absorbing, storing and releasing solar energy is extremely important. This paper, therefore, has introduced the theory of a new system combing an APTPCMW and multi-surface trough solar concentrators, in order to achieve the goal. To evaluate the active heat storage performance of APTPCMW, an experiment was designed and carried out, and conclusions from the experiment include: (1) Influencing factors on the active heat storage performance of APTPCMW include the gap between air tunnels, the flow direction of heated air, the temperature and velocity of supply air; (2) Optimum operation conditions of APTPCMW were 0.4 m gap between air tunnels, downward flow direction for the heated air inside the tunnel, 0.26 m s1 supply air velocity and 60 °C temperature for the supply air; and (3) Using the new proposed system, the heat storage capacity of the block layer of APTPCMW can be improved significantly. Additionally, the heat stored within the wall during the daytime can be released efficiently during the nighttime. This paper has critically demonstrated the applicability of the theory of a new system combing APTPCMWs and multi-surface trough solar concentrators. Future tests in real applications are still needed to confirm its actual efficiency of using solar energy in greenhouses. Acknowledgment This study was supported by the National Natural Science Foundation of China, Nos. 51368060 and 50978002. References
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Fig. 16. Active heat storage and heat release of APTPCMW.
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