Design of Phase Change Material Wall Based on the Heat Transfer Characteristics in Summer Procedia Engineering 121 ( 2015 ) 2201 – 2208 Available online at www sciencedirect com 1877 7058 © 2015 The A[.]
Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 121 (2015) 2201 – 2208 9th International Symposium on Heating, Ventilation and Air Conditioning (ISHVAC) and the 3rd International Conference on Building Energy and Environment (COBEE) Design of Phase Change Material Wall Based on the Heat Transfer Characteristics in Summer Erlin Menga,*, Hang Yub, Chenggang Liua, Zhigao Suna, Bo Zhoua a School of Environmental Science and Engineering, Suzhou University of Science and Technology, Suzhou, China b HVAC&GAS Institute, School of Mechanical Engineering, Tongji University, Shanghai, China Abstract The thermophysical properties of the wall can greatly affect the thermal performance of the buildings When design the thermophysical properties, the choice of object function is important In this paper, two new objective functions (heat insulation factor η1 and heat absorbing factor η2) were proposed to evaluate the thermal performance of the phase change material (PCM) wall The two objective functions can describe the heat transfer of both the external and inner surface of the PCM wall The heat transfer of the PCM wall was solved by the thermal-resistance method The effect of different factors to the objective functions was analyzed Such as melting temperature, latent heat and the position of the PCM layer in the wall The model and objective functions are expected to be helpful for the design of PCM wall in active and passive room © 2015 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license © 2015 The Authors Published by Elsevier Ltd (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility the organizing committee of ISHVACCOBEE 2015 under responsibility of theof organizing committee of ISHVAC-COBEE 2015 Peer-review Keywords: PCM wall; thermal-resistance method; heat insulation factor; heat absorbing factor Introduction The utilization of phase change material(PCM) in building enclosures is an effective way to shift peak-hour cooling load of air conditioning system, reducing the system’s operating cost due to the price difference between peak and off-peak electric power; and can contribute to decreasing indoor air temperature fluctuations while improving occupants’ thermal comfort[1,2,3] Zhang YP [4] pointed out that efforts were still needed to determine suitable materials having suitable thermophysical characteristics that fit the load requirements for different forms of buildings in different climate areas and during different seasons The adequate melting temperature, latent heat and heat conduction of the PCM *correspongding author Tel: +86- 182 -511 -60960; fax: +86-0512-6824-7000 E-mail adress: m20_njnu@126.com 1877-7058 © 2015 The Authors 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 organizing committee of ISHVAC-COBEE 2015 doi:10.1016/j.proeng.2015.09.093 2202 Erlin Meng et al / Procedia Engineering 121 (2015) 2201 – 2208 should be the first concern Arnault A [5] showed that the choice of objective functions is important, and can influence to some extent the “best” design that is achieved Izquierdo BMA [6] conclude that there was not a clear optimum temperature that minimized thermal load through the PCM wall, the objective functions they used was the heat flux of the wall internal surface Halford CK [7] pointed out that as the ambient temperature increases, the optimal position for the PCM layer moved closer to the inside wall Koo J [8] found that the average phase change temperature should be close to the average room temperature to maximize the thermal heat storage in the wallboards The phase change temperature should be narrow to maximize the thermal heat storage in the PCM wallboards Jin X [9] put forward a new double layer PCM floor and concluded that the optimal melting temperatures will change with the change of the locations of the two PCM layers The objective functions they used were indoor air temperature and heat flux of the floor surface Jiang F [10] found that the optimal phase change temperature depended on the lower limit of the indoor thermal comfort, the optimal phase change temperatures were close to each other which were 1.1-3.3 ć higher than the lower limit of thermal comfort range for the passive solar rooms in different building climatic regions in China In order to determine the parameters of the PCM suitable for summer, two new indices are proposed based on the heat transfer characteristics of the PCM wall in this paper The indices are then theoretically solved by thermalresistance method This study is expected to be helpful for the design of PCM wall Objective functions of the heat transfer characteristics of the PCM wall In most cases, the PCM wall is composed of several layers In order to investigate the effect of the position of the PCM layer to the thermal performance of the wall, one three-layers wall is introduced in which one PCM layer is placed between two ordinary material layers as shown in Fig.1 Because the heat capacity of ordinary materials are quite small as compared to phase change materials, the heat capacities of the two ordinary material layers are neglected and the two layers are assumed as two thermal resistances during the heat transfer analysis Fig sketch of the heat transfer of the two cases In summer, the outdoor air temperature is assumed to be higher than indoor air temperature When the melting temperature of the PCM is between the indoor and outdoor air temperature, the heat transfer is defined as case (a) shown in Fig.1 When the melting temperature is lower than both the indoor and outdoor air temperature, the heat transfer is defined as case (b) For case(a), the PCM layer absorbs heat from outdoor which is then divided into three parts, one part is used for sensible heat storage of the PCM layer, another part is used for the latent heat storage (phase change) of the PCM layer, the other part transfers to the room In this case, the effect of the PCM wall is heat insulation The heat insulation factor η1 is defined as: K1 qin qout (1) 2203 Erlin Meng et al / Procedia Engineering 121 (2015) 2201 – 2208 Where qout is the heat flux of the external surface of the PCM layer and q in is the heat flux of the inner surface of the PCM layer In summer, in order to enhance the heat insulation performance of the PCM wall, the objective is to minimize η1 For case (b), The PCM layer absorbs heat from both indoor and outdoor The heat is then divided into two parts, one part is used for sensible heat storage of the PCM layer, the other part is used for the latent heat storage (phase change) of the PCM layer In this case, the PCM layer acts like a heat sink The heat absorbing factor η is defined as: K2 qin (2) qin qout In summer, the objective is to maximize η2 That is to increase the percentage of the heat from indoor air in the overall heat the PCM wall absorbed Mathematical model 3.1 Model of thermal-resistance method Thermal-resistance method whose accuracy is one-order can be used to solve the phase change problem when the latent heat and sensible heat of the PCM are assumed to concentrate on the phase change interface for the heat release(Chen ZS.1991) which causes the temperature distribution in the PCM layer is linear except for the interface of solid and liquid as shown in Fig.1 The temperature of the interface of solid and liquid is assumed as the melting temperature tm The heat released on the phase change interface of solid and liquid is: Q Qlate Qliq Qsol (3) Where Q is the overall heat released Qlate is the latent heat released Qliq is the sensible heat the liquid PCM released Qsol is the sensible heat the solid PCM released The sensible heat of the solid PCM can be ignored for the melting process of the PCM layer [11] Then, Q Qlate Qliq (4) Only the melting process is analyzed this paper The PCM layer is assumed to be solid initially and the temperature is tm So it can be got that: Qliq Ul cl[1T (W ) (5) Where ρl and cl are the density and specific heat of the liquid PCM.ξ1is the thickness of the outside liquid PCM is the mean excess temperature T (W ) [1 [1 ³0 T (W )dx [1 [1 ³0 (t tm )dx (6) Where is the excess temperature t is the temperature of the PCM at the x position tm is the melting temperature of the PCM Because the temperature distribution in the wall is linear, then: 2204 Erlin Meng et al / Procedia Engineering 121 (2015) 2201 – 2208 tout tm [1 R1 hout O t tm [1 x (7) O Where tout is the outdoor air temperature hout is the coefficient of convictive heat transfer for the external surface λ is the thermal conductivity of the PCM R1 is the thermal resistance of the outside ordinary material layer According to equations (5)-(7): Qliq [12 hout (tout tm ) Ul cl 2(O [1hout O hout R1 ) (8) The latent heat of the PCM layer is: Qlate Us H [1 (9) Where ρs is the density of the solid PCM H is the latent heat of the PCM The heat balance equation for case (a) is: ³ W tout tm [1 R1 hout O dW U s H [1 Ul cl W [12 hout (tout tm ) tm tin ³ dW 2(O [1hout O hout R1 ) L [1 R hin O (10) From equation (10),we can get the thickness of the outside liquid PCM (ξ1)with time(τ): The heat balance equations for case (b) are: ³ W ³ W tout tm [1 R1 hout O tin tm [2 R2 hin O dW U s H [1 Ul cl [12 hout (tout tm ) 2(O [1hout O hout R1 ) (11) dW U s H [ Ul cl [2 hin (tin tm ) 2(O [ hin O hin R2 ) (12) It can be got from (11) and (12): [ Ul cl hout (tout tm ) 2Us Hhout ][12 4Us H (O O hout R1 )[1 14.4hout O (tout tm )W [ Ul cl hin (tin tm ) 2Us Hhin ][22 4Us H (O O hin R2 )[2 14.4hinO (tin tm )W From equation(13) and (14), we can get the variation of ξ1 and ξ2 with time(τ) The heat insulation factor η1is: 0 (13) (14) Erlin Meng et al / Procedia Engineering 121 (2015) 2201 – 2208 K1 qin qout [1 R1 tm tin hout O tout t m L [1 R O hin 2205 (15) For case (b): K2 qin qin qout tin tm [2 R2 hin O tin tm tout tm [2 [1 R2 R1 hin O hout O (16) external surface heat flux ˄W/m2˅ 3.2 Validation of the model 180 thermal-resistance method results 160 140 120 100 80 60 40 10 Time15 (hour) 20 25 30 Fig validation of the model results Dates from reference [11] which were got by quasi-steady state method are used to validate the model Comparison of the two results is shown in Fig It can be seen that the two results have a good agreement with a maximum difference of 4.9% during the phase change period Results There are many factors than can affect the heat transfer characteristics of the PCM wall In order to analysis the influence of them, the following calculation parameters are assumed (Table 1) Table Parameters for the calculation parameter Value parameter Value tm H 28 ć 120 kJ kg-1 tin hin 25 ć(case(a)) 30 ć(case(b)) 8.7 W m-2 K-1 λ L ρ R1 0.5 W m-1 K-1 0.05 m 800 kg m-3 0.5 m2 K W-1 hout tout c R2 18.6 W m-2 K-1 35 ć kJ kg-1 K-1 0.5 m2 K W-1 2206 Erlin Meng et al / Procedia Engineering 121 (2015) 2201 – 2208 4.1 Influence of R1 For case (a), it can be seen from Fig that, with the increasing of R1, the exterior surface heat flux qout decreases faster than inner surface heat flux qin does which means that increasing the thermal resistance outside the PCM layer can reduce the heat flux transferring to the PCM layer, but cannot reduce the heat flux from the PCM layer to the room significantly However, when R1 increases from 0.2 m K W-1 to 0.5 m2 K W-1, the heat insulation factor η1 increases from about 0.2 to about 0.4 For case (b), the exterior surface heat flux qout decreases when R1 increases, but the absolute value of the inner surface heat flux |qin| keeps consistent When R1 increase from 0.2 m2 K W-1 to 0.5 m2 K W-1, the heat absorbing factor η2 increases from about 0.08 to about 0.15, even though the most part of the heat that the PCM needs to melt is still from the outdoor case (a) 20 qout-0.2 0.6 qout-0.3 12 η1-0.5 0.4 η1-0.4 qout-0.4 η1-0.3 qout-0.5 η1-0.2 qin-0.2 qin-0.3 qin-0.4 15 qout-0.3 10 qin-0.5 30 qout-0.5 η2-0.2 qin-0.2 η2-0.3 60 90 phase change time (hour) 0.1 0 20 120 0.2 qin-0.3 qin-0.4 qin-0.5 -5 qout-0.4 η2-0.4 η2-0.5 0.2 0.3 qout-0.2 η1 16 0.4 case (b) 20 0.8 heat flux (W/m2) heat flux (W/m2) 24 25 η2 28 40 60 80 100 120 140 phase change time (hour) -1 Notation: qout-0.2 is the value of qout when R1 is 0.2 m K W , the same is for qin-0.2 ,η1-0.2 andη2-0.2 Fig influence of R1 on η1 and η2 of PCM wall for case (a) and case (b) However, the larger the R1 is, the longer of the phase change time for both case (a) and case (b) Seen from equations (13) and (14), for case (a), the thickness of the liquid PCM ξ1 becomes larger and larger during the phase change process, so qout decreases and qin increases which lead to the increasing of η1 All of these can be reflected in Fig However, for case (b), qout and |qin| all decrease which causes it difficult to conclude the variation of η2 during the phase change process 4.2 Influence of R2 case (a) 12 2.5 qinqin-0.4 0.5 η1-0.5 η1-0.4 qout-0.3 qout-0.2 qin0.3 1.5 η1 0.2 η1-0.3 0.5 0 50 100 150 phase change time (hour) 200 250 η2-0.5 qout-0.5 qout-0.4 qout-0.3 qout-0.2 η2-0.4 qin0.2 0.25 case (b) 10 heat flux (W/m2) qout-0.5 qout-0.4 10 η1 heat flux (W/m2) 12 η2-0.3 η2-0.2 0.2 0.15 η2 14 0.1 -2 -4 50 qin-0.2 0.05 qin-0.3 qin-0.4 qin-0.5 100 150 phase change time (hour) Notation: qout-0.2 is the value of qout when R2 is 0.2 m2 K W-1, the same is for qin-0.2 ,η1-0.2 andη2-0.2 Fig influence of R2 on η1and η2 of PCM wall for case (a) and case (b) 2207 Erlin Meng et al / Procedia Engineering 121 (2015) 2201 – 2208 It can be observed from Fig 4, for case (a), the external surface heat flux q out and inner surface heat flux qin all drop when R2 rises, the variation of qin is more significant However, the heat insulation factor η1 decreases from about 0.8 to about 0.4 when R2 increases from 0.2 m2 K W-1 to 0.5 m2 K W-1 For case (b), the variation of R2 has no influence on qout, but when R2 rises, |qin| drops and the heat absorbing factor η2 rises Seen from the influence of R1 and R2, the PCM layer should be placed close to the outside of the wall for case (a) and close to the inside of the wall for case (b) 4.3 Influence of melting temperature tm qout-26 qout-27 qout-28 12 10 0.4 qin-29 0 50 0.2 150 qout-26 qout-27 qout-28 qout-29 η2-27 η2-26 0.27 0.18 qin-28 qin-27 qin-26 η2-29 qin-29 -10 200 0.36 η2-28 -5 100 0.45 case (b) 10 0.6 η1-27qin-28 qin-27 η -26 qin1 -26 15 qout-29 0.8 η1-29 η1-28 20 η1 heat flux (W/m2) 14 η2 case (a) 16 heat flux (W/m2) 18 0.09 0 30 60 phase change time (hour) 90 120 150 phase change time (hour) Notation: qout-26 is the value of qout when tm is 26 , the same is for qin-26,ă1-26 andă2-26 Fig influence of tm on and of PCM wall for case (a) and case (b) Fig shows that, for case (a), the melting temperature tm has significant effect on both q out and qin When tm is close to the indoor air temperature, the external surface heat flux q out increases and qin decreases, so the heat insulation factor η1 decreases When tm is 29ć, η1 is about 0.6 to 0.7 which means that most part of the heat entering the PCM layer from outdoor is transferring to the room, but when tm is 26ć, η1 is only about 0.1 which means that only about 10% of the heat entering the PCM layer from outdoor is transferring to the room The heat insulation performance of the PCM layer becomes better For case (b), when the melting temperature tm decreases, q out and |qin| all increase and the heat absorbing factor η2 also increases which is good for the heat absorbing performance of the PCM wall In short, a melting temperature which is close to the indoor air temperature is suggested for case (a) For case (b), a tm lower than the indoor air temperature and the outdoor air temperature is preferred 4.4 Influence of the latent heat H 13 qout-80 η1-100 η1-120 η1-80 η1 qout-80 qout-100 qout-120 qout-140 0.6 10 η1-140 0.4 0.2 qin-80 qin-100 qin-120 qin-140 40 60 80 100 phase change time (hour) 120 140 heat flux (W/m2) heat flux (W/m2) 11 20 0.2 10 0.8 12 case (b) 12 case (a) qout-100 qout-140 qout-120 0.18 0.16 η2-80 η2-100 η2-120 η2-140 η2 14 0.14 0.12 qin-80 qin-100 qin-120 qin-140 -2 50 100 phase change time (hour) Notation: qout-80 is the value of qout when H is 80 kJ kg-1, the same is for qin-80, η1-80 andη2-80 0.1 150 2208 Erlin Meng et al / Procedia Engineering 121 (2015) 2201 – 2208 Fig influence of H onη1andη2of PCM wall for case (a) and case (b) Fig illustrates that with the increasing of the latent heat H of the PCM wall, the exterior surface heat flux q out increases and inner surface heat flux qin decreases However, the heat insulation factor η1 decreases For case (b), the latent heat H seems to have no significant effect on |q in|, but qout increases when H increases which causes η2 to decrease In conclusion, a large H is suggested for case (a) and a small H is suggested for case (b) However, the latent heat H also brings notable influences to the phase change time A larger H means a longer phase change time Conclusions In this paper, two new indices were proposed to evaluate the heat transfer characteristics of the PCM wall Models were built for two heat transfer cases Heat-resistance method was used to solve the model This study is expected to be helpful for the design of PCM wall in active and passive room It is suggested that, for a active room where the indoor air temperature is well controlled by the air-condition system, the usage of the PCM wall is to minimize the heat transferring to the room In this case, it will be better to design the PCM wall according to heat transfer case(a) However, for a passive room where the indoor air temperature is far away from thermal comfort standard, the usage of PCM wall is to absorb heat from the room and decrease the indoor air temperature In this case, it will be better to design the PCM wall according to heat 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wall is composed of several layers In order to investigate the effect of the position of the PCM layer to the thermal... storage (phase change) of the PCM layer, the other part transfers to the room In this case, the effect of the PCM wall is heat insulation The heat insulation factor η1 is defined as: K1 qin qout... like a heat sink The heat absorbing factor η is defined as: K2 qin (2) qin qout In summer, the objective is to maximize η2 That is to increase the percentage of the heat from indoor air in the