- Email: [email protected]
Comparison of solar water tank storage modelling solutions
Solar Energy 79 (2005) 216–218 www.elsevier.com/locate/solener
Brief Note
Comparison of solar water tank storage modelling solutions K. Johannes
a,*
, G. Fraisse a, G. Achard a, G. Rusaoue¨n
b
a Laboratoire Optimisation de la Conception et Inge´nierie de l’Environnement (LOCIE), Equipe ‘‘Ge´nie Civil et Habitat’’ (GCH), Ecole Supe´rieure d’Inge´nieurs de Chambe´ry, Universite´ de Savoie Campus Savoie Technolac, 73376 Le Bourget du Lac Cedex, France b CEntre THermique de l’Institut de Lyon (CETHIL) INSA de LYON, e´quipe thermique du baˆtiment, Avenue des Arts, 69621 Villeurbanne, France
Received 7 April 2004; received in revised form 8 November 2004; accepted 24 November 2004 Available online 31 December 2004 Communicated by: Associate Editor Jean-Louis Scartezzini
Abstract In this brief note, we have carried out an analysis of the temperature field inside a solar storage tank without any specific stratification device. The purpose of this study is to verify the ability of TRNSYSÕs Types 60 and 140 to reproduce the temperature field in the storage tank, so as to use one of these type in a large solar system, and to analyze fluid motion by means of CFD simulations. This part has highlighted the mixing in the top of the tank due to the inlet configuration, but also the limits of the stratified fluid models with the multinode approach used in TRNSYS. The validation of a global model for a large solar system using Type 140 will be done later. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction As stratification is important in solar systems, we perform experiments on a traditional solar storage tank. This tank is said to be ‘‘traditional’’ because it contains no stratification device. These experiments were compared with simulations carried out with TRNSYSÕs Types 60 and 140 (Klein et al., 1996) and with the FLUENT software based on CFD (Computational Fluid Dynamics) code. These comparisons will allow us to reveal the ability of the models to reproduce the dynamics of the temperature field with some differences in the
*
Corresponding author. Tel.: +33 4 79 75 86 68; fax: +33 4 79 75 81 44. E-mail address: [email protected] (K. Johannes).
values. CFD allows to describe fluid motion inside the tank so as to find out about the stratification process.
2. The configurations studied To test the accuracy of the different tank models available in the TRNSYS library, we conducted two experiments on a traditional tank corresponding to the following two cases: supply case of the tank (Fig. 1), draw-off case of the tank (Fig. 2). The experiments are carried out on a solar 374-l tank. This natural stratification tank contains water mixed with antifreeze fluid (40%). In both cases, the tank inside is equipped, with PT 1000 temperature sensors (Figs. 1 and 2), up to the top. In the first case, 16 temperature sensors are positioned every 5 cm upwards while in the second case,
0038-092X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.11.007
K. Johannes et al. / Solar Energy 79 (2005) 216–218
217
T(°C) 60 55 50 45 40 35 30 5
0
T1_exp
Fig. 1. Supply configuration.
10
t(mn)
15
20
25
T1_Type60
T1_Type140
T6_exp
T6_Type60
T6_Type140
T13_exp
T13_Type60
T13_Type140
Fig. 3. Comparison of experimental and simulation temperatures during the supply test.
Débit (l/h)
T (°C) 70
700 T1
60
600
50
500
T4
40
400
30
300 T6
20
T16
10
100
0
Fig. 2. Draw-off configuration.
sensors are arranged from bottom to top at 10 cm intervals. Sensors are set approximately 10 cm off the tankÕs side. Each simulation is made with outside temperature of 18 °C, an overall heat coefficient of 1.44 W/K and a 34-node tank model. The first scenario consisted in injecting the fluid (700 l/h, 56 °C) in the initially stratified upper part of the tank. In the second case, 2 water tapping are made (Fig. 4) respectively 400 l/h and 620 l/h with an 18 °C inlet temperature.
200
0
500 T1_Type140 T1_exp T1_Type60 Qecs
1000
t (s)
T4_Type140 T4_exp T4_Type60
1500
2000
T6_Type140 T6_exp T6_Type60
0 2500 T16_Type140 T16_exp T16_Type60
Fig. 4. Comparison of experimental and simulation temperatures during the draw-off test.
in the TRNSYS model to reach the same temperature. In Fig. 4, we notice the store is much more stratified than at initial time but steady temperatures are not the same. Numerical sensors T4 and T6 may display a 12 °C discrepancy regarding experimental temperatures.
3. Experimental and numerical results
3.1. CFD simulations with internal heat exchanger
We obtain the following temperature distribution. Fig. 3 shows the uniform temperature of 55 °C in the upper part of the tank. The difference in T1 is probably due to the configuration of the tank. Indeed, the water supplied induces a stream, which can explain the fast temperature rise, while it takes longer for the mixing
The study is conducted using version 6.1.18 FLUENT software. The main associated choices of a numerical and physical modelling are: laminar flow and unsteady state with 20 s time step. The heat exchanger present inside the tank is introduced as a porous material with viscosity to estimate
218
K. Johannes et al. / Solar Energy 79 (2005) 216–218
its influence in the fluid dynamic behaviour. To allow sufficient precision, the tank was meshed in an unstructured way and includes 450,000 tetrahedral cells, for approximately 110,000 nodes. This code allowed us to understand fluid behaviour during simulation. For example, we could see the flow motion (Fig. 5) during the supply test. We noticed that when the warm water was injected through the inlet device, the configuration of the top of the tank made the mixing easier. So, the temperature in the upper half of the tank got homogeneous. Fig. 5 seems to validate the actual behaviour of the fluid inside the solar storage tank, however it is necessary to compare the CFD values with the experimental ones. Fig. 6 confirms the good approach of the model. The dynamic state is respected. However some differences persist in the temperature val-
ues at steady state. The difference of the time constant for sensor ‘‘T13_CFD’’ is certainly connected to the presence of the heat exchanger. Indeed, the geometry of the heat exchanger is not perfectly known and consequently, errors are made in the modelling. We can notice a difference of 4 °C between the experimental values and the simulation values at steady state probably owed to the friction coefficient taken for the porous material (modelling principle of the heat exchanger), which plays an important role in the simulation. The CFD modelling shows a bi-directional mass transfer in the layers that induce a non-uniform temperature probably due to inlet design. A zonal model would therefore be better adapted (Kenjo et al., 2002) but simulation time would increase.
4. Conclusion
Fig. 5. Temperature in the tank with heat exchanger at t = 4 mn.
We decided to analyse the capacity of TRNSYS Type 60 and 140 models to reproduce temperature field within the tank. According to the results, differences are noted in both scenarios probably due to mixing hypothesis and convective resistance which is not take into account. As Type 140 allows the integration of a stratification device, we will continue our study with this Type. However, we will have to focus our attention onto stratification which is very important to energy performances. Concerning the CFD simulations, the model allows us to appreciate the fluid motion inside the tank, and to conclude that a layer is not at uniform temperature. Consequently, it would be interesting to develop a zonal model if we wish to represent the detailed stratification over reasonable simulation time. The continuation of this study concerns the integration of Type 140 in a global model for a large traditional solar system.
Acknowledgments T ( °C)
60
This study is financed by the RHONE-ALPES regional council in the framework of the programme ÔAvenir 2002Õ.
55 50 45
T1_exp
40
T1_CFD
References
T13_exp
35
T13_CFD
t (s)
30 0
5
10
15
20
25
Fig. 6. Comparison of experimental values with CFD simulation values (with heat exchanter).
Klein, S.A. et al., 1996. TRNSYS, version 14.2, Solar Energy Laboratory, University of Wisconsin. Kenjo, L., Buscarlet, C., Inard, C., 2002. Etude du comportement thermique dÕun chauffe-eau solaire a faible de´bit. FIERÕ2002, pp. 102–107.
Brief Note
Comparison of solar water tank storage modelling solutions K. Johannes
a,*
, G. Fraisse a, G. Achard a, G. Rusaoue¨n
b
a Laboratoire Optimisation de la Conception et Inge´nierie de l’Environnement (LOCIE), Equipe ‘‘Ge´nie Civil et Habitat’’ (GCH), Ecole Supe´rieure d’Inge´nieurs de Chambe´ry, Universite´ de Savoie Campus Savoie Technolac, 73376 Le Bourget du Lac Cedex, France b CEntre THermique de l’Institut de Lyon (CETHIL) INSA de LYON, e´quipe thermique du baˆtiment, Avenue des Arts, 69621 Villeurbanne, France
Received 7 April 2004; received in revised form 8 November 2004; accepted 24 November 2004 Available online 31 December 2004 Communicated by: Associate Editor Jean-Louis Scartezzini
Abstract In this brief note, we have carried out an analysis of the temperature field inside a solar storage tank without any specific stratification device. The purpose of this study is to verify the ability of TRNSYSÕs Types 60 and 140 to reproduce the temperature field in the storage tank, so as to use one of these type in a large solar system, and to analyze fluid motion by means of CFD simulations. This part has highlighted the mixing in the top of the tank due to the inlet configuration, but also the limits of the stratified fluid models with the multinode approach used in TRNSYS. The validation of a global model for a large solar system using Type 140 will be done later. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction As stratification is important in solar systems, we perform experiments on a traditional solar storage tank. This tank is said to be ‘‘traditional’’ because it contains no stratification device. These experiments were compared with simulations carried out with TRNSYSÕs Types 60 and 140 (Klein et al., 1996) and with the FLUENT software based on CFD (Computational Fluid Dynamics) code. These comparisons will allow us to reveal the ability of the models to reproduce the dynamics of the temperature field with some differences in the
*
Corresponding author. Tel.: +33 4 79 75 86 68; fax: +33 4 79 75 81 44. E-mail address: [email protected] (K. Johannes).
values. CFD allows to describe fluid motion inside the tank so as to find out about the stratification process.
2. The configurations studied To test the accuracy of the different tank models available in the TRNSYS library, we conducted two experiments on a traditional tank corresponding to the following two cases: supply case of the tank (Fig. 1), draw-off case of the tank (Fig. 2). The experiments are carried out on a solar 374-l tank. This natural stratification tank contains water mixed with antifreeze fluid (40%). In both cases, the tank inside is equipped, with PT 1000 temperature sensors (Figs. 1 and 2), up to the top. In the first case, 16 temperature sensors are positioned every 5 cm upwards while in the second case,
0038-092X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.11.007
K. Johannes et al. / Solar Energy 79 (2005) 216–218
217
T(°C) 60 55 50 45 40 35 30 5
0
T1_exp
Fig. 1. Supply configuration.
10
t(mn)
15
20
25
T1_Type60
T1_Type140
T6_exp
T6_Type60
T6_Type140
T13_exp
T13_Type60
T13_Type140
Fig. 3. Comparison of experimental and simulation temperatures during the supply test.
Débit (l/h)
T (°C) 70
700 T1
60
600
50
500
T4
40
400
30
300 T6
20
T16
10
100
0
Fig. 2. Draw-off configuration.
sensors are arranged from bottom to top at 10 cm intervals. Sensors are set approximately 10 cm off the tankÕs side. Each simulation is made with outside temperature of 18 °C, an overall heat coefficient of 1.44 W/K and a 34-node tank model. The first scenario consisted in injecting the fluid (700 l/h, 56 °C) in the initially stratified upper part of the tank. In the second case, 2 water tapping are made (Fig. 4) respectively 400 l/h and 620 l/h with an 18 °C inlet temperature.
200
0
500 T1_Type140 T1_exp T1_Type60 Qecs
1000
t (s)
T4_Type140 T4_exp T4_Type60
1500
2000
T6_Type140 T6_exp T6_Type60
0 2500 T16_Type140 T16_exp T16_Type60
Fig. 4. Comparison of experimental and simulation temperatures during the draw-off test.
in the TRNSYS model to reach the same temperature. In Fig. 4, we notice the store is much more stratified than at initial time but steady temperatures are not the same. Numerical sensors T4 and T6 may display a 12 °C discrepancy regarding experimental temperatures.
3. Experimental and numerical results
3.1. CFD simulations with internal heat exchanger
We obtain the following temperature distribution. Fig. 3 shows the uniform temperature of 55 °C in the upper part of the tank. The difference in T1 is probably due to the configuration of the tank. Indeed, the water supplied induces a stream, which can explain the fast temperature rise, while it takes longer for the mixing
The study is conducted using version 6.1.18 FLUENT software. The main associated choices of a numerical and physical modelling are: laminar flow and unsteady state with 20 s time step. The heat exchanger present inside the tank is introduced as a porous material with viscosity to estimate
218
K. Johannes et al. / Solar Energy 79 (2005) 216–218
its influence in the fluid dynamic behaviour. To allow sufficient precision, the tank was meshed in an unstructured way and includes 450,000 tetrahedral cells, for approximately 110,000 nodes. This code allowed us to understand fluid behaviour during simulation. For example, we could see the flow motion (Fig. 5) during the supply test. We noticed that when the warm water was injected through the inlet device, the configuration of the top of the tank made the mixing easier. So, the temperature in the upper half of the tank got homogeneous. Fig. 5 seems to validate the actual behaviour of the fluid inside the solar storage tank, however it is necessary to compare the CFD values with the experimental ones. Fig. 6 confirms the good approach of the model. The dynamic state is respected. However some differences persist in the temperature val-
ues at steady state. The difference of the time constant for sensor ‘‘T13_CFD’’ is certainly connected to the presence of the heat exchanger. Indeed, the geometry of the heat exchanger is not perfectly known and consequently, errors are made in the modelling. We can notice a difference of 4 °C between the experimental values and the simulation values at steady state probably owed to the friction coefficient taken for the porous material (modelling principle of the heat exchanger), which plays an important role in the simulation. The CFD modelling shows a bi-directional mass transfer in the layers that induce a non-uniform temperature probably due to inlet design. A zonal model would therefore be better adapted (Kenjo et al., 2002) but simulation time would increase.
4. Conclusion
Fig. 5. Temperature in the tank with heat exchanger at t = 4 mn.
We decided to analyse the capacity of TRNSYS Type 60 and 140 models to reproduce temperature field within the tank. According to the results, differences are noted in both scenarios probably due to mixing hypothesis and convective resistance which is not take into account. As Type 140 allows the integration of a stratification device, we will continue our study with this Type. However, we will have to focus our attention onto stratification which is very important to energy performances. Concerning the CFD simulations, the model allows us to appreciate the fluid motion inside the tank, and to conclude that a layer is not at uniform temperature. Consequently, it would be interesting to develop a zonal model if we wish to represent the detailed stratification over reasonable simulation time. The continuation of this study concerns the integration of Type 140 in a global model for a large traditional solar system.
Acknowledgments T ( °C)
60
This study is financed by the RHONE-ALPES regional council in the framework of the programme ÔAvenir 2002Õ.
55 50 45
T1_exp
40
T1_CFD
References
T13_exp
35
T13_CFD
t (s)
30 0
5
10
15
20
25
Fig. 6. Comparison of experimental values with CFD simulation values (with heat exchanter).
Klein, S.A. et al., 1996. TRNSYS, version 14.2, Solar Energy Laboratory, University of Wisconsin. Kenjo, L., Buscarlet, C., Inard, C., 2002. Etude du comportement thermique dÕun chauffe-eau solaire a faible de´bit. FIERÕ2002, pp. 102–107.