Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 103 (2016) 22 – 27
Applied Energy Symposium and Forum, REM2016: Renewable Energy Integration with Mini/Microgrid, 19-21 April 2016, Maldives
Numerical modelling of thermal performance of activepassive ventilation wall with phase change material Chao Chen*, Haoshu Ling, Nan Yu, Na Li, Mingxing Zhang, Yin Li College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100124, P R China
Abstract In order to improve the utilization of solar energy in buildings, a new system combining an active-passive ventilation wall with phase change material (PCM) and solar concentrators was proposed, and a two-dimensional unsteady numerical heat transfer model of the proposed wall was established and validated using data collected from field measurement. Using this model, influencing factors, namely, velocity and inlet temperature of air, thickness and length of air tunnel, have been identified to have influence on its thermal performance. Then optimum operational conditions have also been identified to guide the optimal design of the proposed wall and the application of solar energy in buildings. © by Elsevier Ltd. This an open access ©2016 2016Published The Authors. Published byisElsevier Ltd. article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of REM2016 Peer-review under responsibility of the scientific committee of the Applied Energy Symposium and Forum, REM2016: Renewable Energy Integration with Mini/Microgrid. Keywords: Ventilation wall with solar energy; Phase change material; Thermal performance; Numerical model; Optimal design
1. Introduction Phase change material (PCM) has been widely integrated in the building envelopes to reduce the mismatch between energy supply and demand, thanks to its high energy-storage density within a relatively narrow temperature range [1, 2]. Compared with the conventional passive envelopes, the active ventilation envelopes can receive more heat, due to the flowing air. Therefore, incorporating PCM in ventilation envelops has exerted a tremendous fascination, and it has been considered as a more attractive technology to improve thermal comfort and reduce the energy consumption of heating and air conditioning systems. Diarce et al. [3] have developed a ventilated active facades with PCM, which was installed on the outer surface of envelops. Their experimental results showed that use of the ventilated facade in the wall would render an average indoor temperature 1.1 °C higher. Kara and Kurnuc [4] carried out a study evaluating
* Corresponding author. Tel.: +86-010-67391608-201; fax: +86-010-67391608-201. E-mail address:
[email protected] (C. Chen).
1876-6102 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the Applied Energy Symposium and Forum, REM2016: Renewable Energy Integration with Mini/Microgrid. doi:10.1016/j.egypro.2016.11.243
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Chao Chen et al. / Energy Procedia 103 (2016) 22 – 27
Nomenclature length of air tunnel, m ȡ density, kg/ m3 specific heat capacity, J/(kg°C) IJ time, s equivalent diameter, m ¨t air temperature difference between inlet friction coefficient of flow in pipe and outlet, °C forced convection heat transfer coefficient, Subscripts W/(m2°C) Air air k thermal conductivity, W/(m°C) bBr hollow block behind the air tunnel ks absolute roughness, mm fBr hollow block front the air tunnel Q cumulative heat exchange capacity, MJ In polystyrene board t temperature, °C id indoor v velocity, m/s il intlet x horizontal coordinate od outdoor y vertical coordinate ol outlet Į natural convection heat transfer coefficient, PCM PCM wallboard W/(m2°C) w wall į thickness, m ws surface of air tunnel the thermal performance of a Trombe wall with PCM. From the study, they found that the solar energy stored by the wall could cover up to 70% monthly heating load, and up to 36% daily heating load. In order to improve the utilization of solar energy in buildings, a new system that combined an activepassive ventilation wall with PCM and multi-surface trough solar concentrators was proposed [2], which is shown in Fig.1. In this study, a two-dimensional unsteady numerical heat transfer model of the proposed wall was established and validated using data collected from field measurement. Using this model, influencing factors, namely, velocity and inlet temperature of air, thickness and length of air tunnel, have been identified to have influence on the thermal performance of the proposed wall. Then optimum operational conditions have also been identified to guide the optimal design and operation. a c d f h
2. Numerical modeling The heat transfer process of the proposed wall is described in Fig.2. When air flows inside the air tunnel, the large temperature difference between air and the surface of air tunnel causes a strong forced convective heat transfer. The heat is stored with rise of the air tunnel surface temperature, and this heat is transferred to the interior of the wall by conduction, so that the wall temperature rises. Owing to the temperature difference between the wall surface and environment, there is a natural convection taking place on the wall surface. Therefore, the two-dimensional unsteady numerical heat transfer model of the Solar collector
y
GPCM
GfBr
GAir
Convection
Storing
Storing
Insulation layer
Fig. 1. Schematic diagram of the new system
Conduction
Air channel Convection
Outlet
Convection
PCM layer Block layer (Air tunnel) Insulation layer
Storing
Convection
North
Block layer
Block layer
PCM layer
Inlet
Conduction Storing
GbBr
GIn
x
Fig.2. Heat transfer process of the proposed wall
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Chao Chen et al. / Energy Procedia 103 (2016) 22 – 27
proposed wall is defined by Eqs.(1)-(11). Wall part: Uw
w ( cw t w ) wW
kw (
w 2tw w 2 tw ) wx 2 wy 2
(1)
Air part: U Air c Air (
wt Air wt v Air ) wW wy
h
2( a G Air ) (tws t Air ) aG Air
(2)
With boundary conditions as, x =0,
wtw Did (tw tid ) wx wt k w w h(t Air tw ) wx wtw kw h (tw t Air ) wx wt k w w Dod (tod tw ) wx kw
x =G PCM G fBr , x =G PCM G fBr G Air , x =G PCM G fBr G Air G bBr G In ,
(3) (4) (5) (6)
Where h is the forced convective heat transfer coefficient. Since the air tunnel is made of concrete and its surface is rough, the forced convective heat transfer coefficient can be calculated by Eqs.(7)-(10). 2aG Air d (7) a G Air f St
h
[2 lg(
d ) 1.74]2 2ks
(8)
f 2/3 Pr 8
(9)
St U Air cAir v
(10) Since the proposed wall is heterogeneous, the physical parameters of materials at different positions are given as, cw ° °cw ° ®c f °c ° w °cw ¯
cPCM , U w cBr , U w
U PCM , kw kPCM U Br , kw kBr
if G PCM x d G PCM G fBr
cAir , U f
U Air , k f
if G PCM G fBr x d G PCM G fBr G Air
cBr , U w
U Br , kw kBr
if G PCM G fBr G Air x d G PCM G fBr G Air G bBr
cIn , Uw
U In , kw kIn
if G PCM G fBr G Air G bBr x d G PCM G fBr G Air GbBr G In
k Air
if 0 d x d G PCM
(11)
An alternating direction implicit method is used to disperse aforementioned equations, which can be calculated by Matlab program. 3. Model validation In order to verify the established model, an experimental wall was built, and experimental conditions were measured and embedded in the numerical model in Section 2. Then the measured and calculated outlet air temperature were compared. The experimental setup consisted of an electrical heating air system, a fan, two anemometers, one temperature measuring device and a proposed wall, which is shown in Fig.3. Dimensions of the proposed wall were 1.76m (length)×1.14m (height)×0.28m(thickness). Its outer layer was built with 0.05m-thick polystyrene boards. Its middle layer was built with 0.19m-thick concrete hollow blocks. In the middle of this layer, there was an air tunnel with a dimension of 0.13m (length) ×1.14 m (height)×0.12m (thickness). Its inner layer was built with 0.04m-thick PCM wallboards, whose physical parameters are given in Fig.4
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Chao Chen et al. / Energy Procedia 103 (2016) 22 – 27
Wall Anemometer
Fan
0.950
Heaters 0.190
Temperature measuring equipments
Anemometer
Fig. 3. Experimental setup: (a) Experimental system; (b) The proposed wall and Table 1. Other important physical parameters of materials are also listed in Table 1.During the experiment, two Testo 435 hot-wired anemometers (accuracy of ±0.01m/s) were installed at the inlet and the outlet of air tunnel to accurately measure the air velocity. Four T-type thermocouples (accuracy of ±0.5°C) were used to monitor the inlet, outlet and environment air temperature. To verify the model, an experiment was carried out. During the experiment, the air velocity was collected and it was 0.41m/s. The environment temperature, inlet and outlet air temperature were recorded, and they are shown in Fig.5. Then the air velocity, environment temperature, inlet air temperature were embedded in the numerical model in Section 2, and the outlet air temperature was calculated, whose result is shown in Fig.5. From Fig.5, a good agreement between the measured and calculated temperature of outlet air has been observed, with an average temperature difference of 0.12°C. 4. Influence of various factors According to the heat transfer analysis of the proposed wall in Section 2, factors influencing the thermal performance of the proposed wall could be temperature and velocity of air, thickness and height of air tunnel. In the section, the effect of these factors on the thermal performance of the proposed wall was evaluated, according to indicators, namely, air temperature difference between inlet and outlet and cumulative heat exchange, defined as Eqs.(12)-(13). Table 1. Physical parameters of wall materials Thermal conductivity (W/(m·°C)) 0.40 0.81 0.042 0.0276
Material PCM wallboard Hollow block Polystyrene board Air
Specific heat ( kJ/(kg·°C)) Fig.4 1.05 1.38 1.005
Density (kg/m3) 900 1800 30 1.128 70
10 Temperature (ć)
Specific heat (kJ/kgć)
12
8 6 4 2 5
10
15 20 25 30 Temperature (°C)
35
40
Fig. 4. Equivalent specific heat capacity of PCM wallboard
't (W ) til (W ) tol (W )
Q(W 0 )
³
0
Outlet_simulation Inlet
Pr 0.699
Outlet_experiment Environment
50 40 30 20
0
W0
60
Absolute roughness (mm) 0.04 -
cAir U Airabv(til (W ) tol (W ))dW
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 Time (h)
Fig.5. Measured and calculated temperature of outlet air
(12) (13)
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Chao Chen et al. / Energy Procedia 103 (2016) 22 – 27
4.1. Air velocity To evaluate the influence of the air velocity on the thermal performance of the proposed wall, the outlet air temperature was simulated, then the air temperature difference between inlet and outlet and the cumulative heat exchange capacity were calculated. As shown in Fig.6, the air velocity had an important influence on the thermal performance of the proposed wall. With the increase of air velocity, the air temperature difference between inlet and outlet decreased, but the cumulative heat exchange capacity rose. Remarkably, its growth rate decreased. When the air velocity was 0.6m/s, the growth rate after six-hour simulation was 11.59%, more than 10%, and it was less than 10%, when the air velocity was more than 0.7m/s. Therefore, it is recommended to choose the air velocity as 0.6m/s. 4.2. Inlet air temperature The air temperature difference between inlet and outlet and the cumulative heat exchange capacity of different inlet air temperature are presented in Fig. 7. From the figure, it could be observed that with the increase of inlet air temperature, both air temperature difference between inlet and outlet and cumulative heat exchange capacity obtained a significant increase. This was principally because that with the increase of inlet air temperature, the temperature different between air and wall went up. Therefore, for the design and operation of the proposed wall, it should be as much as possible to improve the air inlet temperature. 4.3. Thickness of air tunnel The effect from thickness of air tunnel on the air temperature difference between inlet and outlet and the cumulative heat exchange capacity is demonstrated by Fig.8. With the increase of thickness, the amount of air increased, and the contact surface between air and the proposed wall also rose. Therefore, the cumulative heat exchange capacity went up, but the air temperature difference between inlet and outlet was decreased. When the thickness was less than 0.1m, the amount of air flowing through the air tunnel was less than that required by wall. So the air temperature difference between inlet and outlet changed fiercely. When the thickness was more than 0.15m, the proposed wall received enough amount of air, so the air temperature difference changed smaller. Therefore, it is recommended to choose the thickness as 0.1m-0.15m. 25
6 20
3 2
15
1 10
0 0.1
0.2
0.3
0.4
0.5
0.6
Air velocity(m/s)
0.7
Fig.6. Influence from air velocity
0.8
Q(1h) Q(4h) ¨t(1h) ¨t(4h)
7
5
Q(2h) Q(5h) ¨t(2h) ¨t(5h)
40
Q(3h) Q(6h) ¨t(3h) ¨t(6h)
35 30 25
4
20
3
15
2
10
1
5 0
0 40
50
60
70
80
Inlet temperature of air(ć)
Fig7. Influence from inlet air temperature
90
¨t (ć)
Q(3h) Q(6h) ¨t(3h) ¨t(6h)
¨t (ć)
Q (MJ)
4
Q(2h) Q(5h) ¨t(2h) ¨t(5h)
Q (MJ)
Q(1h) Q(4h) ¨t(1h) ¨t(4h)
5
27
Chao Chen et al. / Energy Procedia 103 (2016) 22 – 27 Q(3h) Q(6h) ¨t(3h) ¨t(6h)
12 35
25
4
Q(1h) Q(4h) tol(1h) tol(4h)
10
Q(3h) 80 Q(6h) tol(3h) tol(6h) 60
Q(2h) Q(5h) tol(2h) tol(5h)
8
¨t (ć)
Q (MJ)
6
Q(2h) Q(5h) ¨t(2h) ¨t(5h)
2
15
0 0.05
5
tol(ć)
Q(1h) Q(4h) ¨t(1h) ¨t(4h)
Q (MJ)
8
6 4
40
2 0.1
0.15
0.2
0.25
0.3
Thickness of air tunnel(m)
Fig.8. Influence from thickness of air tunnel
0
20 0
1
2
3
4
Height of air tunnel(m)
5
6
Fig. 9. Influence from height of air tunnel
4.4. Height of air tunnel Fig.9 shows the change of both outlet air temperature and cumulative heat exchange capacity with different height of air tunnel. It reflected that the height of air tunnel had a significant influence on the thermal performance of the proposed wall, when the height was less than 4m. With the increase of height of air tunnel, the outlet air temperature dropped dramatically, and the air temperature difference between inlet and outlet rose sharply, so the cumulative heat exchange capacity also surged. However, when the height was more than 4m, the outlet air temperature varied slowly and was similar with the initial temperature of wall, and the cumulative heat exchange capacity also increased slowly. Therefore, it is recommended that the height of air tunnel to be chosen as 4m. 5. Conclusions In this study, a new system combining an active-passive ventilation wall with PCM and solar concentrators was proposed, and a two-dimensional unsteady numerical heat transfer model of the proposed wall was established. Then the proposed model was validated using data collected from field measurement. According to the simulation results, optimum operation conditions of the proposed wall were 0.6m/s air velocity, 0.1m-0.15m thickness of air tunnel, and 4m height of air tunnel. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 51368060 and 51578012) and Doctoral Innovation Foundation of Beijing University of Technology. References [1] Ling H, Chen C, Wei S, Guan Y, Ma C, Xie G, et al. Effect of phase change materials on indoor thermal environment under different weather conditions and over a long time. Appl Energ. 2015;140:329-37. [2] Ling H, Chen C, Guan Y, Wei S, Chen Z, Li N. Active heat storage characteristics of active–passive triple wall with phase change material. Sol Energy. 2014;110:276-85. [3] Diarce G, Campos-Celador Á, Martin K, Urresti A, García-Romero A, Sala JM. A comparative study of the CFD modeling of a ventilated active façade including phase change materials. Appl Energ. 2014;126:307-17. [4] Kara YA, Kurnuc A. Performance of coupled novel triple glass and phase change material wall in the heating season: An experimental study. Sol Energy. 2012;86:2432-42.
