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Simulation Performance of Building Wall with Vacuum Insulation Panel
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 180 (2017) 1247 – 1255
International High- Performance Built Environment Conference – A Sustainable Built Environment Conference 2016 Series (SBE16), iHBE 2016
Simulation performance of building wall with vacuum insulation panel Jin-Hee Kima, Sang-Myung Kimb, Jun-Tae Kimc,* a Green Energy Technology Research Center, Kongju National University, Cheonan Chungam 330-117, South Korea Graduate school, Dept of Architectural Engineering, Kongju National University, Cheonan Chungam 330-117, South Korea c Department of Architectural Engineering, Kongju national University, Cheonan Chungam 330-117, South Korea
b
Abstract The energy consumption in buildings has continuously increased, and in some countries it reaches almost 40% of the total energy use. Therefore, there are various efforts to minimize energy consumption in buildings, and the regulations on building envelope insulation have been tightened up gradually. For vacuum insulation panel (VIP) in buildings, various researches have been conducted over the last several years and its application has been extended in building industry. VIP application in buildings enable thinner building envelope while maintaining the same thermal performance by a greatly lower thermal conductivity than conventional insulation materials. On the other hand, VIP applied in buildings may cause thermal bridges, which reduce the thermal performance of VIP envelope due to the very low thermal conductivity compared with other building materials. Therefore, the thermal performance of building envelope with VIP depends on how the VIPs are applied to the building envelope and that is affected according to the installation method of VIP with other building materials. This study aims to analyze the thermal performance of VIPs, and to evaluate the effective thermal transmittance (U-value) of building walls with VIPs. For this study, the VIPs-applied walls were designed to minimize thermal bridge effect and their linear thermal transmittances were evaluated by BISCO program. ©2017 2017The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This Peer-review under responsibility of the organizing committee iHBE 2016. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee iHBE 2016 Keywords: Vaccum insulation panels(VIPs); thermal bridge effect; linear thermal transmittance; effective thermal transmittance(U-value)
* Corresponding author. Tel.: +41-521-9333; fax: +41-562-0310. E-mail address: [email protected]
1877-7058 © 2017 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 iHBE 2016
doi:10.1016/j.proeng.2017.04.286
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Jin-Hee Kim et al. / Procedia Engineering 180 (2017) 1247 – 1255
1. Background As the entire world suffers from environmental problems such as climate change and global warming, international societies have made a variety of efforts to reduce greenhouse gases such as Rio Convention on Climatic Change and Kyoto Protocol. To keep abreast of such efforts, the government of Republic of Korea has set the target for zeroenergy house by 2025 by reinforcing the regulation on energy consumption in construction sector, which takes more than 25% of the total energy consumption [1]. To reduce the energy consumption in architectural sector, it is necessary to reinforce thermal insulation of building envelope and minimize energy loss from the envelope. Expanded Polystyrene Board (EPS) or glass-wool can be added to existing insulating materials for greater thickness, or high efficiency-insulating materials such as vacuum insulation panels (VIPs) and aerogel; the thermal conductivity of the later is lower than current conventional insulating materials and can be used to improve the thermal insulation of walls. However, increasing the thickness of existing insulating materials has an accompanying problem; as the thickness increases, the effective area of indoor space decreases [2]. Therefore, VIPs which enable slim and efficient construction solutions bearing from its lower thermal conductivity as compared to existing insulating materials is on high demand. As seen in Figure 1 [3], VIPs consists of a barrier envelope that encapsulates a core material (e.g. fumed silica and glass-wool), and heat seal flanges. In addition, getter and desiccant are additionally used to absorb moisture and gas permeating into barrier envelope at atmospheric pressure [2]. The barrier envelope could be one of metalized envelope or laminated aluminum (Al) foil envelope. The envelope is prone to heat loss mainly in the thermal bridge of vacuum insulation panel (VIP) themselves because it is composed of heterogeneous materials with high thermal conductivity as compared to the core material [4]. Furthermore, heat loss takes place at anchor, wood, and plastic on which VIPs are fixed to a wall. Moreover other bundling wall components have relatively higher thermal conductivity than VIPs; heat loses through installation components contribute to thermal bridge of VIPs retrofitted wall. Thermal bridge occurring to VIPs varies depending on the material of barrier envelope or the thermophysical properties of adjacent installation components, which has a great impact on the heat insulation performance of a VIPs retrofitted wall.
Fig. 1. Vacuum insulation panel (VIP) construction.
Significant contributions in literature have focused on estimating the impact of linear thermal transmittance of standalone VIP and VIP retrofits. For instance, considering an Al foil (8 μm thick) and two metalized films (300 nm, 90 nm thick) used as barrier envelope materials, the thermal conductivity at the center of the panel was 0.0039 ~0.0041 W/m K and there was no significant difference by envelope material. However, the linear thermal transmittance ranged from 0.007 W/m K to 0.052 W/m K, or more than 7 times, for envelope material configurations [5]. In [6], linear thermal transmittance at thermal bridge of VIPs was characterized according to installation method. Linear thermal transmittance was evaluated by joiner type, wooden lath type, anchor fixation type, and supporting pin type. Linear thermal transmittance by joiner type was 0.023 W/m K, which was lower than wooden lath type (0.073 W/mK), supporting pin type (0.074 W/m K), and anchor fixation type (0.266 W/m K). Another study evaluated the insulation performance of VIPs-applied as dry exterior insulation and finishing system in an effort to design passive and zero energy apartment housing. The study set the alternatives for dry-vacuum exterior insulation and finishing system, thereafter analyzed the energy performance, construction cost, and construction performance of each alternative as
Jin-Hee Kim et al. / Procedia Engineering 180 (2017) 1247 – 1255
evaluation item. Weight was given to the score of each evaluation item and assessment was carried out. The results showed that integrating VIPs with added conventional insulating material to both its front and back sides had higher insulation performance than EPS molding-based VIPs and VIPs with added conventional insulating material the back side [7]. VIPs can reduce the thickness of wall more than existing insulating material while satisfying heat insulation performance. However, thermal bridge occurs by installation method or due to barrier envelope that wraps panel, which lowers the heat insulation performance of retrofitted wall. Therefore, it is a vital challenge to reduce thermal bridge to use VIPs more effectively. In addition, when the heat insulation performance of VIPs retrofitted wall is evaluated, effective thermal transmittance (8eff) should be measured rather than unidimensional thermal transmittance. In the meantime, existing insulating materials or other materials such as rubber pad have also been applied to VIPs to prevent inner vacuum from being destroyed by external impact as well as thermal bridge of VIPs (Figure 2) [8].
Fig. 2. Sandwich vacuum insulation panels (VIPs).
In this study, encapsulated VIP was designed using existing insulating materials to prevent damage to barrier envelope and to reduce thermal bridge, and the performance was evaluated. Simulation for heat transfer was carried out to determine linear thermal transmittance and effective thermal transmittance (U-value) of VIPs-wall using BISCO two-dimensional steady state heat transfer computer program (version 10.0 W). BISCO calculates thermal quantities as defined by European standard: temperature factor, linear thermal transmittance, equivalent thermal transmittance, and thermal transmittance of a window frame [9]. 2. Characteristics of VIPs 2.1. Heat insulation performance of VIPs
Fig. 3. Comparative thickness of different insulation material to achieve the same U-value.
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The thermal conductivity at the core of freshly fabricated VIPs range from 0.003 W/m K to 0.007 W/m K, and this heat insulation performance is 3 to 9 times higher than that of existing conventional insulating materials. Therefore, thinner VIPs can be used while maintaining the same thermal transmittance as compared to conventional insulating materials (see Figure 3 [10]). Consequently, when VIPs are applied to a wall in place of conventional insulating materials, it can significantly reduce insulation wall thickness of 200 mm ̴ 300 mm required for passive house or zero energy houses [7]. 2.2. Thermal bridge Thermal bridge is an area where heat loss occurs due to the gap of thermal conductivity among the components of a structure. In a building, heat loss usually occurs toward where heat resistance is less due to the discontinuity of insulating materials. When VIP is applied to wall, thermal bridge ensues and it lowers the heat insulation performance of wall. As seen in Figure 4 [11], thermal bridge occurs at the metal envelope that wraps the core and the gaps of the panels when applied to a building. Also, thermal bridge can take place in the adjacent materials (to VIPs) such as steel anchor, PVC, and wood, or occurs by the difference of thermal conductivity among VIPs.
Fig. 4. Conceptual diagram of thermal bridge between two vacuum insulation panels (VIPs).
3. Simulation model (VIP-applied wall) 3.1. Design of encapsulated VIPs In this study, VIPs (450 mm x 450 mm x 25 mm) and Al foil envelope (8 μm thick) were modeled; Al foil is the main element to analyze for heat loss. As seen in Figure 5, EPS was used to design encapsulated VIP to reduce the heat loss of Al-foil envelope. VIPs were covered with 25mm thick EPS on front and back and 2mm thick EPS on the top, bottom and sides.
Fig. 5. Casulized vacuum insulation panel (VIP) detail.
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3.2. Simulation model (Outer wall) To compare and evaluate the performance of encapsulated VIPs, this study considered both encapsulated VIP and general VIP retrofits; models were simulated and compared to estimate the heat insulation performance of walls. To analyze the heat loss of VIPs metal envelope, the heat insulation performance of VIP wall retrofits with and without metal envelope was compared and analyzed. Considering heat loss that occurs in the gap between the panels, 1 mm air gap was modeled between the interfaces (joints) of each wall.
(a)
(b)
(c)
(d)
Fig. 6. Schematic diagrams of simulation models for VIP retrofits: (a) VIPs without envelope thermal bridge, (b) VIPs with envelope thermal bridge, (c) VIPs with backside EPS layer, and (d) VIPs encapsulated with EPS.
The modeling conditions for baseline VIP retrofitted wall without envelope are schematically shown in Fig. 6a; VIPs (length 450 mm, thickness 25 mm), concrete (thickness 200 mm), and wall length (2,420 mm of vertical direction). VIP retrofitted wall considering heat losses by metal envelope are schematically depicted in Fig. 6b. The modeling schematic of VIP retrofitted wall with expanded polystyrene are illustrated in Fig. 6c; VIPs (length 450 mm, thickness 25 mm), EPS (thickness 50 mm), concrete (thickness 200 mm), wall length (2,420 mm of vertical direction). Encapsulated VIP retrofitted wall design are represented in Fig. 6d; VIPs (length 450 mm, thickness 25 mm), EPS (25 mm of vertical direction, 2 mm of horizontal direction), concrete (thickness 200 mm), and wall length (2,420 mm of
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vertical direction). When compared with Figs 6a and 6b, wall length of Figs 6c and 6d increased by 500 mm, due to the addition of EPS. Thermal conductivity of materials by area for simulation were set, as summarized in Table 1, in accordance with BS EN ISO 6946:1997 [12] and DIN V 4108-4 [13] of German Standardization Organization (Deutsches Institut for Normung: DIN). Indoor/outdoor surface resistance of heat transfer and temperature was set, as shown in Table 2 according to Energy Conservation Standard for Building. [14]. Table 1. Material properties. Materials
Thermal conductivity(W/mŘk)
Concrete Adhesive VIPs Aluminum Expanded Polystyrene Cement mortar Plaster
2.500 0.353 0.005 230.0 0.040 1.400 0.196
Table 2. Boundary conditions. Temperature(Ȕ)
Heat transfer resistance (m2K/W)
Interior
20.0
0.11
Exterior
-11.3
0.043
4. Results According to the simulation, the linear thermal transmittance of general VIPs-applied wall was 0.035 W/m and effective thermal transmittance (U-value) is 0.216 W/m2K. Linear thermal transmittance (ψ) is the heat transfer per unit length and unit temperature that is lost through the region of the concerned linear thermal bridge at normal state. It can be mathematically expressed by [15]: <
j
L
2d
¦U j x l W / mK
(1)
1
ψ L2d Uj lj
Linear thermal transmittance(W/mŘK) Heat conduction coefficient calculated by two dimensional computation(W/mŘK) Thermal transmittance of components j(W/m2ŘK) Length to thermal bridge(m)
On the other hand, the effective thermal transmittance, U-value, (Ueff) is the heat transfer per unit area and unit temperature that is lost through the region of the concerned linear thermal bridge at normal state. Likewise, it can be represented by Equation 2, at boundary condition (j). The Ueff of a wall when linear thermal transmittance is considered has a higher value than that of thermal transmittance calculated by one-dimensional computation [11]. j
U
eff
Ueff ψ U1d lj
U
1d
¦< / l j 1
Effective thermal transmittance (U-value) (W/m2 K) Linear thermal transmittance (W/mŘK) Thermal transmittance calculated by one-dimensional computation (W/m2 K) Length to thermal bridge (m)
(2)
Jin-Hee Kim et al. / Procedia Engineering 180 (2017) 1247 – 1255
Figure 7 shows simulation results of wall models of four types. When heat loss that occurred to VIPs barrier envelope is excluded from the linear thermal transmittance of general VIPs-applied wall, linear thermal transmittance was 0.001 W/m K and Ueff was 0.188 W/m2 K. In other models the linear thermal transmittances of wall with EPS and encapsulated VIPs-applied wall were 0.027 W/m K and 0.023 W/m K, and Ueff was about 0.172 W/m2 K. Table 3 summarizes the results.
Fig. 7. The simulation results of wall models of four types. Table 3. Comparison of modeling results. Method
U1d(W/m2K)
Ψ(W/m K)
Ueff (W/m2 K)
Wall with VIPs without envelope Wall with VIPs Wall with VIPs and EPS Wall with capsulized VIPs
0.186 0.202 0.161 0.161
0.001 0.035 0.027 0.023
0.188 0.216 0.172 0.172
The linear transmittance of VIPs-applied wall without metal envelope was 0.001W/m K. When metal envelope was considered in the VIPs-applied wall the linear thermal transmittance increased by 0.035 W/m K. In addition, when comparing Ueff (0.216 W/m2 K) of VIPs-applied wall with Al foil envelope to that of retrofitted wall without Al foil envelope (0.188 W/ m2K), the former increased by 15%. Moreover, the Ueff of general VIPs-applied wall was 6.9% higher than one-dimensional thermal transmittance (0.202 W/m2 K). The Ueff of encapsulated VIPs applied wall was 0.172 W/m2 K, which was 6.8% higher than one-dimensional thermal transmittance (0.161 W/m2K). Ueff of encapsulated VIPs was lower than that of general VIPs in terms of incremental rate, and this is attributed to the reduced linear thermal transmittance that generates in a wall when encapsulated VIPs were applied. The linear thermal transmittance of encapsulated VIPs-applied wall was 0.023 W/m K, i.e. 34% lower than that of general VIPs-applied wall (0.035 W/m K). Ueff of encapsulated VIPs-applied wall was 0.172 W/m2 K, i.e. 26% lower than that of general VIPs-applied wall (0.216 W/m2 K). Figure 8 shows the temperature distribution of VIP retrofitted walls. As heat loss occurred in the metal envelope of general VIPs, heat loss also broke out in the gaps of the panel interfaces, which changed the temperature distribution of wall at a considerable rate and caused thermal bridge.
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(a) Wall with VIP without envelope (b) Wall with VIP
(c) Wall with VIP and EPS
(d) Wall with Capsulized VIP
Temperature(Ȕ)
Fig. 8. Temperature gradient of modeling walls.
5. Conclusion To cut down on energy consumed in the building sector, which is responsible for 25% of the total national energy consumption in the Republic of Korea, the government has gradually stepped up building energy regulation. Accordingly, it is considered vital to introduce VIP, which has far better heat insulation performance than conventional insulating materials, as well as save space. To do so, the challenge posed by thermal bridges is a limitation worth investigating in VIP applied to walls. In this respect, this study analyzed the heat insulation performance of VIPsapplied wall and brought forth conclusion as follows in an attempt to reduce thermal bridge: (1) The total heat lost at the end part of metal envelope of VIPs has a great impact on the heat insulation performance of VIPs-applied wall. (2) The linear thermal transmittance of encapsulated VIPs-applied wall decreased by 34% more than that of general VIPs-applied wall. This can be explained by the reduction in heat loss from barrier envelope (8 μm Al foil thick) when VIPs were encapsulated. (3) The effective thermal transmittance (Ueff) of encapsulated VIPs-applied wall decreased by 26% more than that of general VIPs-applied wall. This is because encapsulated VIPs with EPS reduced thermal bridge that occurs to VIPs metal envelope and also reduced the one-dimensional thermal transmittance of wall. (4) Ueff and one-dimensional thermal transmittance of the VIPs applied wall with EPS and encapsulated VIPs applied wall were same as 0.172 W/m2 K and 0.161 W/m2 K. However, linear thermal transmittance of encapsulated VIPs applied wall decreased by 17% than that of VIP applied wall with EPS; indicating that VIP installation method affects the heat insulation performance of the wall. Since VIPs are prone to damage from outside, they should be carefully installed. Thus, encapsulated VIP applied wall has dual advantage of protection VIPs and better thermal performance. (5) The inner vacuum of a VIP deteriorates when the barrier envelope is damaged, which degrades the heat insulation performance obviously. In addition, the heat insulation performance of wall decreases due to the gaps that are made when VIPs are applied to wall and the thermal characteristics of barrier envelope. When encapsulated VIPs are applied to wall, it is expected to minimize the damage risk to barrier envelope and thermal bridge of wall. With the findings of this study, additional research needs to follow the present study to conduct mock-up test on VIPs-applied wall and 3-dimensional thermal performance analysis.
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Acknowledgements This research was financially supported by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2016H1D5A1910875) and the Basic Science Research Program (No. 2015R1D1A01059050). References [1] Korean Ministry of Land, Infrastructure and Transport, Activation Plan of Zero Energy Building Response to Climate Change, 2014. [2] M. Alam, H. Singh, M.C. Limbachiya, Vacuum Insulation Panels (VIPs) for building construction industry - A review of the contemporary development and future direction, Applied Energy. Vol. 88 No. 11 (2011) 3592-3602. [3] S.H. Park, J.H. Lim, S.Y. Song, Performance evaluation of adhesively fixed external insulation and finish system using Vacuum Insulation Panels for apartment building, Journal of the Korea Institute of Ecological Architecture and Environment, Vol. 13 No. 6 (2013) 45-53. [4] H. Simmler, S. Brunner, Vacuum insulation panels for building application: Basic properties, aging mechanisms and service life, Energy and Building. Vol. 37 No. 11 (2005) 1122-1131. [5] K. Ghazi, R. Bundi, B. Binder, Effective thermal conductivity of vacuum insulation panels, Building Research & Information. Vol. 32 No. 4 (2004) 293-299. [6] S.H Park, J.H. Kim, J.T. Kim, Linear thermal transmittance of thermal bridge at building envelopes with VIPs, Korean Society for Emotion and Sensibility. (2012) 49-50. [7] S.Y. Song, S. Park, B.K. Koo, J.H. Lim, S.R. Ryu, Performance evaluation of EIFS Using Vacuum insulation panels for passive and zeroenergy apartment building, Architectural institute of Korea. Vol. 29 No. 9 (2013) 219-229. [8] Vacutherm, http://www.vacuum-panels.co.uk/service/vacupor-ps/, [accessed on 4th November 2016] and The Oxford Institute for Sustainable Development, http://oisd.brookes.ac.uk/ivisnet/images/VacuporR-RPsml.jpg, [accessed on 4th November 2016] [9] RIBA Product Selector, http://www.ribaproductselector.com/docs/9/26829/external/images/GA5247.jpg, [accessed on 4th November 2016] [10] H. Simmler, S. Brunner, U. Heinemann, H. Schwab, High Performance Thermal Insulation IEA/ECBCS Annex 39(Subtask A-B), ECBCS Annual report, 2005. [11] EN ISO 10211-1, Thermal bridges in building construction, Calculation of heat flows and surface temperatures, 2007. [12] PHYSIBEL, BISCO version10.0W computer program to calculate two-dimensional steady state heat transfer in free-form objects, 2012. [13] BS EN ISO 6949, Building Components and Building Elements - Thermal Resistance and Thermal Transmittance - Calculation Method, 1997. [14] DIN V 4108-4, Thermal Insulation and Energy Conservation in Buildings - Part 4: Characteristic Value relating to Thermal Insulation and Protection Against Moisture, 1997. [15] Korean Ministry of Land, Infrastructure and Transport, design standard of energy conservation in buildings, 2014.
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ScienceDirect Procedia Engineering 180 (2017) 1247 – 1255
International High- Performance Built Environment Conference – A Sustainable Built Environment Conference 2016 Series (SBE16), iHBE 2016
Simulation performance of building wall with vacuum insulation panel Jin-Hee Kima, Sang-Myung Kimb, Jun-Tae Kimc,* a Green Energy Technology Research Center, Kongju National University, Cheonan Chungam 330-117, South Korea Graduate school, Dept of Architectural Engineering, Kongju National University, Cheonan Chungam 330-117, South Korea c Department of Architectural Engineering, Kongju national University, Cheonan Chungam 330-117, South Korea
b
Abstract The energy consumption in buildings has continuously increased, and in some countries it reaches almost 40% of the total energy use. Therefore, there are various efforts to minimize energy consumption in buildings, and the regulations on building envelope insulation have been tightened up gradually. For vacuum insulation panel (VIP) in buildings, various researches have been conducted over the last several years and its application has been extended in building industry. VIP application in buildings enable thinner building envelope while maintaining the same thermal performance by a greatly lower thermal conductivity than conventional insulation materials. On the other hand, VIP applied in buildings may cause thermal bridges, which reduce the thermal performance of VIP envelope due to the very low thermal conductivity compared with other building materials. Therefore, the thermal performance of building envelope with VIP depends on how the VIPs are applied to the building envelope and that is affected according to the installation method of VIP with other building materials. This study aims to analyze the thermal performance of VIPs, and to evaluate the effective thermal transmittance (U-value) of building walls with VIPs. For this study, the VIPs-applied walls were designed to minimize thermal bridge effect and their linear thermal transmittances were evaluated by BISCO program. ©2017 2017The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This Peer-review under responsibility of the organizing committee iHBE 2016. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee iHBE 2016 Keywords: Vaccum insulation panels(VIPs); thermal bridge effect; linear thermal transmittance; effective thermal transmittance(U-value)
* Corresponding author. Tel.: +41-521-9333; fax: +41-562-0310. E-mail address: [email protected]
1877-7058 © 2017 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 iHBE 2016
doi:10.1016/j.proeng.2017.04.286
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Jin-Hee Kim et al. / Procedia Engineering 180 (2017) 1247 – 1255
1. Background As the entire world suffers from environmental problems such as climate change and global warming, international societies have made a variety of efforts to reduce greenhouse gases such as Rio Convention on Climatic Change and Kyoto Protocol. To keep abreast of such efforts, the government of Republic of Korea has set the target for zeroenergy house by 2025 by reinforcing the regulation on energy consumption in construction sector, which takes more than 25% of the total energy consumption [1]. To reduce the energy consumption in architectural sector, it is necessary to reinforce thermal insulation of building envelope and minimize energy loss from the envelope. Expanded Polystyrene Board (EPS) or glass-wool can be added to existing insulating materials for greater thickness, or high efficiency-insulating materials such as vacuum insulation panels (VIPs) and aerogel; the thermal conductivity of the later is lower than current conventional insulating materials and can be used to improve the thermal insulation of walls. However, increasing the thickness of existing insulating materials has an accompanying problem; as the thickness increases, the effective area of indoor space decreases [2]. Therefore, VIPs which enable slim and efficient construction solutions bearing from its lower thermal conductivity as compared to existing insulating materials is on high demand. As seen in Figure 1 [3], VIPs consists of a barrier envelope that encapsulates a core material (e.g. fumed silica and glass-wool), and heat seal flanges. In addition, getter and desiccant are additionally used to absorb moisture and gas permeating into barrier envelope at atmospheric pressure [2]. The barrier envelope could be one of metalized envelope or laminated aluminum (Al) foil envelope. The envelope is prone to heat loss mainly in the thermal bridge of vacuum insulation panel (VIP) themselves because it is composed of heterogeneous materials with high thermal conductivity as compared to the core material [4]. Furthermore, heat loss takes place at anchor, wood, and plastic on which VIPs are fixed to a wall. Moreover other bundling wall components have relatively higher thermal conductivity than VIPs; heat loses through installation components contribute to thermal bridge of VIPs retrofitted wall. Thermal bridge occurring to VIPs varies depending on the material of barrier envelope or the thermophysical properties of adjacent installation components, which has a great impact on the heat insulation performance of a VIPs retrofitted wall.
Fig. 1. Vacuum insulation panel (VIP) construction.
Significant contributions in literature have focused on estimating the impact of linear thermal transmittance of standalone VIP and VIP retrofits. For instance, considering an Al foil (8 μm thick) and two metalized films (300 nm, 90 nm thick) used as barrier envelope materials, the thermal conductivity at the center of the panel was 0.0039 ~0.0041 W/m K and there was no significant difference by envelope material. However, the linear thermal transmittance ranged from 0.007 W/m K to 0.052 W/m K, or more than 7 times, for envelope material configurations [5]. In [6], linear thermal transmittance at thermal bridge of VIPs was characterized according to installation method. Linear thermal transmittance was evaluated by joiner type, wooden lath type, anchor fixation type, and supporting pin type. Linear thermal transmittance by joiner type was 0.023 W/m K, which was lower than wooden lath type (0.073 W/mK), supporting pin type (0.074 W/m K), and anchor fixation type (0.266 W/m K). Another study evaluated the insulation performance of VIPs-applied as dry exterior insulation and finishing system in an effort to design passive and zero energy apartment housing. The study set the alternatives for dry-vacuum exterior insulation and finishing system, thereafter analyzed the energy performance, construction cost, and construction performance of each alternative as
Jin-Hee Kim et al. / Procedia Engineering 180 (2017) 1247 – 1255
evaluation item. Weight was given to the score of each evaluation item and assessment was carried out. The results showed that integrating VIPs with added conventional insulating material to both its front and back sides had higher insulation performance than EPS molding-based VIPs and VIPs with added conventional insulating material the back side [7]. VIPs can reduce the thickness of wall more than existing insulating material while satisfying heat insulation performance. However, thermal bridge occurs by installation method or due to barrier envelope that wraps panel, which lowers the heat insulation performance of retrofitted wall. Therefore, it is a vital challenge to reduce thermal bridge to use VIPs more effectively. In addition, when the heat insulation performance of VIPs retrofitted wall is evaluated, effective thermal transmittance (8eff) should be measured rather than unidimensional thermal transmittance. In the meantime, existing insulating materials or other materials such as rubber pad have also been applied to VIPs to prevent inner vacuum from being destroyed by external impact as well as thermal bridge of VIPs (Figure 2) [8].
Fig. 2. Sandwich vacuum insulation panels (VIPs).
In this study, encapsulated VIP was designed using existing insulating materials to prevent damage to barrier envelope and to reduce thermal bridge, and the performance was evaluated. Simulation for heat transfer was carried out to determine linear thermal transmittance and effective thermal transmittance (U-value) of VIPs-wall using BISCO two-dimensional steady state heat transfer computer program (version 10.0 W). BISCO calculates thermal quantities as defined by European standard: temperature factor, linear thermal transmittance, equivalent thermal transmittance, and thermal transmittance of a window frame [9]. 2. Characteristics of VIPs 2.1. Heat insulation performance of VIPs
Fig. 3. Comparative thickness of different insulation material to achieve the same U-value.
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The thermal conductivity at the core of freshly fabricated VIPs range from 0.003 W/m K to 0.007 W/m K, and this heat insulation performance is 3 to 9 times higher than that of existing conventional insulating materials. Therefore, thinner VIPs can be used while maintaining the same thermal transmittance as compared to conventional insulating materials (see Figure 3 [10]). Consequently, when VIPs are applied to a wall in place of conventional insulating materials, it can significantly reduce insulation wall thickness of 200 mm ̴ 300 mm required for passive house or zero energy houses [7]. 2.2. Thermal bridge Thermal bridge is an area where heat loss occurs due to the gap of thermal conductivity among the components of a structure. In a building, heat loss usually occurs toward where heat resistance is less due to the discontinuity of insulating materials. When VIP is applied to wall, thermal bridge ensues and it lowers the heat insulation performance of wall. As seen in Figure 4 [11], thermal bridge occurs at the metal envelope that wraps the core and the gaps of the panels when applied to a building. Also, thermal bridge can take place in the adjacent materials (to VIPs) such as steel anchor, PVC, and wood, or occurs by the difference of thermal conductivity among VIPs.
Fig. 4. Conceptual diagram of thermal bridge between two vacuum insulation panels (VIPs).
3. Simulation model (VIP-applied wall) 3.1. Design of encapsulated VIPs In this study, VIPs (450 mm x 450 mm x 25 mm) and Al foil envelope (8 μm thick) were modeled; Al foil is the main element to analyze for heat loss. As seen in Figure 5, EPS was used to design encapsulated VIP to reduce the heat loss of Al-foil envelope. VIPs were covered with 25mm thick EPS on front and back and 2mm thick EPS on the top, bottom and sides.
Fig. 5. Casulized vacuum insulation panel (VIP) detail.
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3.2. Simulation model (Outer wall) To compare and evaluate the performance of encapsulated VIPs, this study considered both encapsulated VIP and general VIP retrofits; models were simulated and compared to estimate the heat insulation performance of walls. To analyze the heat loss of VIPs metal envelope, the heat insulation performance of VIP wall retrofits with and without metal envelope was compared and analyzed. Considering heat loss that occurs in the gap between the panels, 1 mm air gap was modeled between the interfaces (joints) of each wall.
(a)
(b)
(c)
(d)
Fig. 6. Schematic diagrams of simulation models for VIP retrofits: (a) VIPs without envelope thermal bridge, (b) VIPs with envelope thermal bridge, (c) VIPs with backside EPS layer, and (d) VIPs encapsulated with EPS.
The modeling conditions for baseline VIP retrofitted wall without envelope are schematically shown in Fig. 6a; VIPs (length 450 mm, thickness 25 mm), concrete (thickness 200 mm), and wall length (2,420 mm of vertical direction). VIP retrofitted wall considering heat losses by metal envelope are schematically depicted in Fig. 6b. The modeling schematic of VIP retrofitted wall with expanded polystyrene are illustrated in Fig. 6c; VIPs (length 450 mm, thickness 25 mm), EPS (thickness 50 mm), concrete (thickness 200 mm), wall length (2,420 mm of vertical direction). Encapsulated VIP retrofitted wall design are represented in Fig. 6d; VIPs (length 450 mm, thickness 25 mm), EPS (25 mm of vertical direction, 2 mm of horizontal direction), concrete (thickness 200 mm), and wall length (2,420 mm of
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vertical direction). When compared with Figs 6a and 6b, wall length of Figs 6c and 6d increased by 500 mm, due to the addition of EPS. Thermal conductivity of materials by area for simulation were set, as summarized in Table 1, in accordance with BS EN ISO 6946:1997 [12] and DIN V 4108-4 [13] of German Standardization Organization (Deutsches Institut for Normung: DIN). Indoor/outdoor surface resistance of heat transfer and temperature was set, as shown in Table 2 according to Energy Conservation Standard for Building. [14]. Table 1. Material properties. Materials
Thermal conductivity(W/mŘk)
Concrete Adhesive VIPs Aluminum Expanded Polystyrene Cement mortar Plaster
2.500 0.353 0.005 230.0 0.040 1.400 0.196
Table 2. Boundary conditions. Temperature(Ȕ)
Heat transfer resistance (m2K/W)
Interior
20.0
0.11
Exterior
-11.3
0.043
4. Results According to the simulation, the linear thermal transmittance of general VIPs-applied wall was 0.035 W/m and effective thermal transmittance (U-value) is 0.216 W/m2K. Linear thermal transmittance (ψ) is the heat transfer per unit length and unit temperature that is lost through the region of the concerned linear thermal bridge at normal state. It can be mathematically expressed by [15]: <
j
L
2d
¦U j x l W / mK
(1)
1
ψ L2d Uj lj
Linear thermal transmittance(W/mŘK) Heat conduction coefficient calculated by two dimensional computation(W/mŘK) Thermal transmittance of components j(W/m2ŘK) Length to thermal bridge(m)
On the other hand, the effective thermal transmittance, U-value, (Ueff) is the heat transfer per unit area and unit temperature that is lost through the region of the concerned linear thermal bridge at normal state. Likewise, it can be represented by Equation 2, at boundary condition (j). The Ueff of a wall when linear thermal transmittance is considered has a higher value than that of thermal transmittance calculated by one-dimensional computation [11]. j
U
eff
Ueff ψ U1d lj
U
1d
¦< / l j 1
Effective thermal transmittance (U-value) (W/m2 K) Linear thermal transmittance (W/mŘK) Thermal transmittance calculated by one-dimensional computation (W/m2 K) Length to thermal bridge (m)
(2)
Jin-Hee Kim et al. / Procedia Engineering 180 (2017) 1247 – 1255
Figure 7 shows simulation results of wall models of four types. When heat loss that occurred to VIPs barrier envelope is excluded from the linear thermal transmittance of general VIPs-applied wall, linear thermal transmittance was 0.001 W/m K and Ueff was 0.188 W/m2 K. In other models the linear thermal transmittances of wall with EPS and encapsulated VIPs-applied wall were 0.027 W/m K and 0.023 W/m K, and Ueff was about 0.172 W/m2 K. Table 3 summarizes the results.
Fig. 7. The simulation results of wall models of four types. Table 3. Comparison of modeling results. Method
U1d(W/m2K)
Ψ(W/m K)
Ueff (W/m2 K)
Wall with VIPs without envelope Wall with VIPs Wall with VIPs and EPS Wall with capsulized VIPs
0.186 0.202 0.161 0.161
0.001 0.035 0.027 0.023
0.188 0.216 0.172 0.172
The linear transmittance of VIPs-applied wall without metal envelope was 0.001W/m K. When metal envelope was considered in the VIPs-applied wall the linear thermal transmittance increased by 0.035 W/m K. In addition, when comparing Ueff (0.216 W/m2 K) of VIPs-applied wall with Al foil envelope to that of retrofitted wall without Al foil envelope (0.188 W/ m2K), the former increased by 15%. Moreover, the Ueff of general VIPs-applied wall was 6.9% higher than one-dimensional thermal transmittance (0.202 W/m2 K). The Ueff of encapsulated VIPs applied wall was 0.172 W/m2 K, which was 6.8% higher than one-dimensional thermal transmittance (0.161 W/m2K). Ueff of encapsulated VIPs was lower than that of general VIPs in terms of incremental rate, and this is attributed to the reduced linear thermal transmittance that generates in a wall when encapsulated VIPs were applied. The linear thermal transmittance of encapsulated VIPs-applied wall was 0.023 W/m K, i.e. 34% lower than that of general VIPs-applied wall (0.035 W/m K). Ueff of encapsulated VIPs-applied wall was 0.172 W/m2 K, i.e. 26% lower than that of general VIPs-applied wall (0.216 W/m2 K). Figure 8 shows the temperature distribution of VIP retrofitted walls. As heat loss occurred in the metal envelope of general VIPs, heat loss also broke out in the gaps of the panel interfaces, which changed the temperature distribution of wall at a considerable rate and caused thermal bridge.
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(a) Wall with VIP without envelope (b) Wall with VIP
(c) Wall with VIP and EPS
(d) Wall with Capsulized VIP
Temperature(Ȕ)
Fig. 8. Temperature gradient of modeling walls.
5. Conclusion To cut down on energy consumed in the building sector, which is responsible for 25% of the total national energy consumption in the Republic of Korea, the government has gradually stepped up building energy regulation. Accordingly, it is considered vital to introduce VIP, which has far better heat insulation performance than conventional insulating materials, as well as save space. To do so, the challenge posed by thermal bridges is a limitation worth investigating in VIP applied to walls. In this respect, this study analyzed the heat insulation performance of VIPsapplied wall and brought forth conclusion as follows in an attempt to reduce thermal bridge: (1) The total heat lost at the end part of metal envelope of VIPs has a great impact on the heat insulation performance of VIPs-applied wall. (2) The linear thermal transmittance of encapsulated VIPs-applied wall decreased by 34% more than that of general VIPs-applied wall. This can be explained by the reduction in heat loss from barrier envelope (8 μm Al foil thick) when VIPs were encapsulated. (3) The effective thermal transmittance (Ueff) of encapsulated VIPs-applied wall decreased by 26% more than that of general VIPs-applied wall. This is because encapsulated VIPs with EPS reduced thermal bridge that occurs to VIPs metal envelope and also reduced the one-dimensional thermal transmittance of wall. (4) Ueff and one-dimensional thermal transmittance of the VIPs applied wall with EPS and encapsulated VIPs applied wall were same as 0.172 W/m2 K and 0.161 W/m2 K. However, linear thermal transmittance of encapsulated VIPs applied wall decreased by 17% than that of VIP applied wall with EPS; indicating that VIP installation method affects the heat insulation performance of the wall. Since VIPs are prone to damage from outside, they should be carefully installed. Thus, encapsulated VIP applied wall has dual advantage of protection VIPs and better thermal performance. (5) The inner vacuum of a VIP deteriorates when the barrier envelope is damaged, which degrades the heat insulation performance obviously. In addition, the heat insulation performance of wall decreases due to the gaps that are made when VIPs are applied to wall and the thermal characteristics of barrier envelope. When encapsulated VIPs are applied to wall, it is expected to minimize the damage risk to barrier envelope and thermal bridge of wall. With the findings of this study, additional research needs to follow the present study to conduct mock-up test on VIPs-applied wall and 3-dimensional thermal performance analysis.
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Acknowledgements This research was financially supported by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2016H1D5A1910875) and the Basic Science Research Program (No. 2015R1D1A01059050). References [1] Korean Ministry of Land, Infrastructure and Transport, Activation Plan of Zero Energy Building Response to Climate Change, 2014. [2] M. Alam, H. Singh, M.C. Limbachiya, Vacuum Insulation Panels (VIPs) for building construction industry - A review of the contemporary development and future direction, Applied Energy. Vol. 88 No. 11 (2011) 3592-3602. [3] S.H. Park, J.H. Lim, S.Y. Song, Performance evaluation of adhesively fixed external insulation and finish system using Vacuum Insulation Panels for apartment building, Journal of the Korea Institute of Ecological Architecture and Environment, Vol. 13 No. 6 (2013) 45-53. [4] H. Simmler, S. Brunner, Vacuum insulation panels for building application: Basic properties, aging mechanisms and service life, Energy and Building. Vol. 37 No. 11 (2005) 1122-1131. [5] K. Ghazi, R. Bundi, B. Binder, Effective thermal conductivity of vacuum insulation panels, Building Research & Information. Vol. 32 No. 4 (2004) 293-299. [6] S.H Park, J.H. Kim, J.T. Kim, Linear thermal transmittance of thermal bridge at building envelopes with VIPs, Korean Society for Emotion and Sensibility. (2012) 49-50. [7] S.Y. Song, S. Park, B.K. Koo, J.H. Lim, S.R. Ryu, Performance evaluation of EIFS Using Vacuum insulation panels for passive and zeroenergy apartment building, Architectural institute of Korea. Vol. 29 No. 9 (2013) 219-229. [8] Vacutherm, http://www.vacuum-panels.co.uk/service/vacupor-ps/, [accessed on 4th November 2016] and The Oxford Institute for Sustainable Development, http://oisd.brookes.ac.uk/ivisnet/images/VacuporR-RPsml.jpg, [accessed on 4th November 2016] [9] RIBA Product Selector, http://www.ribaproductselector.com/docs/9/26829/external/images/GA5247.jpg, [accessed on 4th November 2016] [10] H. Simmler, S. Brunner, U. Heinemann, H. Schwab, High Performance Thermal Insulation IEA/ECBCS Annex 39(Subtask A-B), ECBCS Annual report, 2005. [11] EN ISO 10211-1, Thermal bridges in building construction, Calculation of heat flows and surface temperatures, 2007. [12] PHYSIBEL, BISCO version10.0W computer program to calculate two-dimensional steady state heat transfer in free-form objects, 2012. [13] BS EN ISO 6949, Building Components and Building Elements - Thermal Resistance and Thermal Transmittance - Calculation Method, 1997. [14] DIN V 4108-4, Thermal Insulation and Energy Conservation in Buildings - Part 4: Characteristic Value relating to Thermal Insulation and Protection Against Moisture, 1997. [15] Korean Ministry of Land, Infrastructure and Transport, design standard of energy conservation in buildings, 2014.
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