modelling of conduction transfer functions for typical ... · pdf filemodelling of conduction...

MODELLING OF CONDUCTION TRANSFER FUNCTIONS FOR TYPICAL THERMAL BRIDGES IDENTIFIED IN BIM DATA

Piotr Narowski1, Jacek Stasierski1, and Piotr Wereszczyński2

1Warsaw University of Technology, Faculty of Environmental Engineering, Warsaw, Poland [email protected], [email protected]

2Sankom Ltd, Warsaw, Poland [email protected]

ABSTRACT

The Building Information Modelling (BIM) was introduced nearly ten years ago to distinguish the information rich architectural 3D modelling from the traditional 2D drawing. The BIM makes a reliable digital representation of the building available for design decision making, high-quality construction document production, construction planning, and performance predictions like building energy performance calculated by energy simulations programs. BIM involves representing a design as 3D objects, that carry their geometry, relations and properties and attributes. Because building models 3D objects are machine readable, it becomes practical to use data they bring in many ways. On the other hand whole building simulation programs still prefer to use the simplified, one-dimensional description of heat transfer through partitions of building envelope. This paper describes simple method that allows to model the conduction transfer functions for typical thermal bridges that can be found in every building. This development goal is to improve building energy calculation results obtained from dynamic simulations by incorporating thermal bridges CTFs correction factors into building simulation application that can automatically recognize thermal bridges in BIM data.

INTRODUCTION Building envelopes commonly include walls, roofs and slabs on ground and their connections with intermediate floors and internal wall which create typical thermal bridges. In case of steady-state analysis, methods for determining the heat transmission coefficients for thermal bridges are available. For dynamic analyses such as heat and cool load calculations or energy consumption analysis, the effects of such elements on building envelope heat transfer have been the subjects of a number of research projects. However, dynamic analysis procedures for multi-dimensional building components still have not found their way into commonly used simulation application or energy analysis procedures. This is the reason why practitioners have to make some kind of simplifying approximation, and somehow describe the building components joints as a set of one-dimensional layers

in order to use a one-dimensional analysis which is implemented in majority of simulation applications.

In past several works (Strachan et al., 1995; Kośny and Christian, 1995; Carpenter et al., 2003; Kośny et al., 1997; Brown et al., 1998; Thomas and Antar, 1998; Kośny and Kossecka, 2002) have pointed out that 1-D models cannot precisely determine the heat transmission through building envelopes with thermally massive elements and large diversity in thermal conductivity of the wall or thermal bridge elements. A study made by Kośny and Christian (1995) shows the thermal bridge effect of metal stud walls can reduce the thermal resistance of the centre of cavity values by nearly 50%. A similar study made by Kośny and Kossecka (2002) shows that 1-D parallel path approximations of the overall resistances for concrete and steel frame walls with 20% frame factor and 80% insulation were about 27% and 44%, respectively. These values were higher than those obtained from detailed analysis using the finite difference method. Numerical and experimental investigations by Davies, et al. (1995) show that ignoring the edge effects of the linear thermal bridges can lead to under estimating the overall heat loss by nearly 10%. Two attempts were made to provide more fundamental treatment of thermal bridges in simulation applications. A study made by Strachan et al. (1995) on development and implementation of 3-D multigridding routines implemented in dynamic thermal simulation program ESPr. Another study provided by Renon (2002) was an attempt to integrate thermal bridge models into EnergyPlus building energy simulation program with focus on how the existing code can be modified to incorporate thermal bridges models.

In this paper, two parts can be found. The first is a description of current state of the art concerning thermal bridges in the European standards related to Energy Performance Building regulations and these standards influence on the software used to calculate energy performance certificates. The second part is devoted to description of the methods that allows gathering correction factors or correction functions coefficients for wall conduction transfer functions to model typical linear thermal bridges heat transfer in simulation applications that use 1-D time domain response functions.

Proceedings of Building Simulation 2011: 12th Conference of International Building Performance Simulation Association, Sydney, 14-16 November.

- 1320 -

THERMAL BRIDGES STANDARDS, SOFTWARE AND CATALOGUES As the thermal bridges must be taken into account in the Energy Performance Building Directive (EPBD) regulations of most European Member States the detailed information of the linear thermal transmittance Ψ and point thermal transmittance χ values should be available for practitioners who assess energy consumption of buildings. There are two approaches to deliver the proper information on thermal bridges: the first is numerical calculation software and the second one are thermal bridge atlases. One can find variety of computer tools for evaluating thermal bridges available on the market. Numerical calculation of thermal bridges should be performed using validated software which is consistent to regulations given by European / International standard EN ISO 10211. This International Standard sets out the specifications for a 3D and a 2D geometrical model of a thermal bridge for the numerical calculation of steady state heat flows to assess the overall heat loss from a building or its part and minimum surface temperatures in order to assess the risk of surface condensation. In this standard one can find set of important rules relating to the dimensioning of the thermal bridges domain. EN ISO 10211 defines minimum distances between the cut-off planes and the investigated thermal bridge. These distances depend on the particular building element investigated. In most cases a distance of 1 meter from the thermal bridge is required, but when a symmetry plane exists at a closer distance then this plane is used as the cut-off. Another set of rules present in the standard apply to the boundary conditions of the numerical solution and the material parameters. The EN ISO 10211 can be used to validate the software prepared to calculate the linear and point thermal transmittance of thermal bridges. Annex A of this standard defines four test cases for software validation purposes. Two of them are two-dimensional models and other two are three-dimensional. In the framework of EPBD-regulations, it is marked that all European Member States should require that software used for thermal bridge calculations used to issue Energy Performance Certificates fulfils at least standard test cases. During last 20 years specialised software for numerical calculation of thermal bridges has been developed in many countries. These software tools are in most cases available on the market with commercial licensing but there are few of them with free of charge. The capabilities of the software is different and varies from simple 2D steady state heat transfer only applications to transient 3D multi-physics application packages used to model wide range of phenomena.

Although the determination of linear and point heat transmittance values can be done with the specialised computer software as described above, a number of

printed catalogues or atlases with thermal bridges properties are available in European countries. These publications are twofold. One type points to the problems caused by thermal bridges and give general guidance and methods to avoid thermal and condensation problems with different type of building construction details. Another type if focused on delivering detailed information of varied thermal bridges steady state parameters. The key advantage of using thermal bridges catalogues is that there are no calculations need, so the values can be obtained fast and without expert knowledge in numerical modelling. It is much easier to get the linear or point thermal transmission values for typical construction details from printed atlas than from specialised FEM application even equipped with user friendly interface. The main disadvantage of printed catalogues is their inflexibility, which means that the number of buildings construction details necessary to cover real buildings situations is huge because of the combination of dimensions and materials used in designed or erected buildings. The flexibility of catalogues is additionally reduced by the difficulties one can meet to obtain the quantitative data of thermal bridges such as surface distribution temperature or heat flow rate which may be useful for some calculation for example mould growth risk. In addition, when using atlas values, one must make sure that conventions used for their obtaining were in accordance with the conventions set by the national EPB rules.

The European / International EN ISO 14683 standard is the example of thermal bridges atlas. This standard deals with simplified methods for determining heat flows through linear thermal bridges which typically occurs at connections of building components. This standard also specifies requirements relating to thermal bridge databases and manual calculation methods of steady state heat flows. The number of construction details or building components junctions in this standard is relatively small and the cases are simplified and on the safe side as the of thermal transmission values can be used in many simplified design calculations.

One special case of the thermal bridges catalogue is available on European market – it is the “Eurokobra” database with its management application “Kobra. This electronic atlas was fully described by Strachan et al.( 1995). It is flexible electronic catalogue which allows to modify the dimensions, thermal properties of materials and boundary conditions for predefined thermal bridges classes and instances and then recalculate their linear thermal transmittance values. This software catalogue is a combination of an atlas and numerical modelling program which does not require numerical modelling knowledge from user to obtain proper values.


- 1321 -

INFLUENCE OF THERMAL BRIDGES STANDARDS ON SOFTWARE DEVELOPMENT It is important to emphasize, that both methods, described above, used to bring valid linear and point thermal transmission values of thermal bridges, are focused on steady state heat transfer parameters, although transient 3D heat transfer solvers are often used to obtain them. It is also important to mention that thermal bridges parameters printed in catalogues are very frequently adapted by computer applications which main goal is to calculate the heat and cool loads for designed or modernised building and calculate the consumption of energy use, delivered and final energy during year according to EPD regulations. At the moment there are many free of charge and commercial licensed programs used to issue EPCs in accordance to local regulation on Member States free markets. The graphical user interface of such computer application is presented on figure 1. In few cases Member States decided to deliver to their authorised energy advisors and assessors one officially issued and validated computer programs utilised to calculate and register EPCs.

Figure 1 EPC application – Audytor OZC 3D

All these programs utilise calculated energy consumptions to assess and classify building in compliance with the EPC regulations of Member States. These local regulations are very often based on CEN and ISO standards with EN ISO 13790 standard in the centre. This standard offers three methods that may be used to calculate building energy needs for heating and cooling – pseudo-dynamic monthly method , simply hourly coupled resistance and capacitance method that take into consideration heat dynamics and full building dynamic simulation. As the monthly or simple hourly methods use to calculate energy use in one time step using overall transmission heat transfer coefficient which one component among others is heat transfer due to thermal bridges it is now obvious that EPC application must utilize linear and point thermal transmission values. It is very rare or even unprecedented that EPC software has got implemented simple FEM or FDM solvers to

calculate steady state values for thermal bridges. In most cases these programs use the catalogue values for typical construction details and allows users to enter or change default values. The user interfaces of such programs are improved continuously, especially for commercial licensing applications, and it is natural that the programmers try to use the BIM standards date to make input data process more and more efficient. Geometric information input required for the building energy modelling sometimes is so big that energy advisors resign to use such tools because of the tedious work. This is the reason why the BIM data is so attractive to use in modern energy simulation application – the building model was prepared by architects and engineers as design stage . The example of BIM data processing from Autodesk “Architecture Revit” and rendering analyzed objects by one of EPC program is shown on figure 2

Figure 2 BIM data imported with automatic thermal

bridges recognition - Audytor OZC 3D app.

This application has implemented interesting functionality – automatic recognition of typical thermal bridges in building construction. Within this application sophisticated computational geometry analysis of BIM data is performed to find the junctions of construction. This solution allows to assign the default values for recognized thermal bridges and enable to change this values by user.

MODELLING OF THERMAL BRIDGES CONDUCTION TRANSFER FUNCTION As the user interfaces become more and more sophisticated and allows to operate on BIM data it is cleared that the heat losses through the 2D or 3D thermal bridges should be integrated into full dynamic simulation applications. An attempt has been made to improve the calculation of typical linear thermal bridges. The proposed method consists in determining thermal bridges correction factors for use with flat wall conduction transfer functions. The correction factors can be calculated from thermal bridges surface response functions induced by temperature unit impulses on both internal and external sides of thermal bridge. The correction factors presented in this paper were calculated for four types of typical thermal bridges that can be found in every building construction:

corner of two external walls,


- 1322 -

intermediate floor connection with external wall,

internal wall joint with external wall,

window and external wall connection.

The external wall that consist of insulated masonry with plaster on both sides was the common component of the example thermal bridges calculations. The wall materials parameters used to solve transient heat conduction in thermal bridges is displayed in table 1. In the case of intermediate floor connection with the external wall the parameters of structural floor with concrete ring in the wall were used. Connection of the internal wall with the external wall forming common thermal bridge in building constructions is typical as masonry joint. The window openings thermal bridge calculated was typical with window frame levelled in the connection plane of masonry and partition insulation with additional insulation of external reveal. The window in the modelled thermal bridge was double glazed with standard glass panels and air between them. Table 2 shows all additional materials parameters of modelled thermal bridges..

Table 1 Material parameters of wall constituting calculated

thermal bridges

NAME LAYER CONDU-CTIVITY

DENSI-TY

SPECIFIC HEAT

- m W/mK kg/m3 J/kgK Plaster 0.01 0.820 1850 840 Insulation – EPS

0.10 0.045 25 1460

Masonry 0.32 0.460 1200 840 Plaster 0.01 0.820 1850 840

The total thickness of external wall is equal d=0.44 m. This is important for configuration factors described below as the 1-D models commonly use the dimensions of partitions in the symmetry axes of building envelope and internal partitions.

Table 2 Material parameters specific to calculated thermal

bridges

NAME LAYER CONDU-CTIVITY

DENSI-TY

SPECIFIC HEAT

- m W/mK kg/m3 J/kgK Int. floor 0.32 1.060 1200 840 Int. floor - ring

0.32 x 0.30

1.700 2500 840

Int. wall 0.32 0.210 600 840 Window frame

0.09 x 0.08

0.160 550 2510

Glass 0.006 0.800 2500 840 Air 0.02 8.000 1.300 1005

The most basic time depended solution of the 1-D heat conduction through multilayer partition is the response functions equation consisting of two convolutions which relates the flux at one surface of

an element to a temperature history function at both sides as shown by equations (1) and (2):

0

2

0

0

W/m

i ii i ii i

ie e

q h T g T d

g T d

(1)

0

2

0

0

W/m

e ee e ei i

ee e

q h T g T d

g T d

(2)

where q is heat flux, T is temperature, i signifies the internal of the building element, e signifies the external side of the building element, τ represents time, h are unit step response functions and g are unit impulse response functions. In most cases the values in the pulse response functions decay fairly rapidly, the infinite number of terms needed for an exact response factor solution makes it less than desirable. The similarity of higher order terms can be used to replace them with flux history terms. The new solution contains elements that are called conduction transfer functions (CTFs). In the case of discrete functions of heat flux, temperature and partition CTF functions with declared time step Δτ, the heat fluxes on internal and external sides of partition may be calculated with equations (3) and (4):

0 0

( ) ( ) ( ) ( ) ( )n n

i ii i ie ej j

q g j T j g j T j

(3)

0 0

( ) ( ) ( ) ( ) ( )n n

e ei i ee ej j

q g j T j g j T j

(4)

These convolution equations state that the heat flux at either face of the surface of any generic building element is linearly related to the current and some of the previous temperatures at both the interior and exterior surface as well as some of the previous flux values at the interior surface. The CTF solution of 1-D transient heat conduction is elegant and powerful. Two linear, relatively simple, equations with constant coefficients, the conduction heat transfer through an element can be calculated. The coefficients (CTFs) in the equation are constants that only need to be determined once for each construction type. The only storage of data required are the CTFs themselves and a limited number of temperature and flux terms. The formulation is valid for any surface type and does not require the calculation or storage of element interior temperatures.

One of the methods that allows to obtain CTFs coefficients is solving the ordinary differential heat conduction equation for multilayered building element. This can be done with finite differential method which leads to linear equation system with tridiagonal coefficients matrix and the temperature depended column vector. This linear equation system can be solved with fast Thomas’ method. This system


- 1323 -

of equations may be used to find the transient heat conduction for unit step excitation on one side of the partition and then on the another. The next step is differentiation of calculated series of response heat flux on both sides of the partition which leads to CTF. This is very elegant solution and is used in many simulation programs. The main disadvantage of this solution is difficult to easily implement it for 2-D or 3-D thermal bridges.

It is possible to take into consideration the transient heat flow through the thermal bridges by modification of 1-D modelling using CTFs for flat partition. This method depends on modification of known CTF coefficients of 1-D model with correction functions which are acquired from comparing and fitting 1-D CTFs of flat partition and CTFs of thermal bridges.

The most difficult part of this solution is calculating the CTFs coefficients for linear or point thermal bridges. In the case of linear thermal bridges as connection of walls or walls and floors transient 2-D heat conduction problem have to be solved. With 3-D flanking elements to find the CTFs coefficients one have to solve 3D transient heat flow problem.

Figure 3 Example of 2D meshing with quadrangle elements used in FEM analysis -window opening

This paper presents solutions for typical linear thermal bridges. The CTFs coefficients were obtained from solving 2-D transient heat conduction problem with unit step temperature function on internal side and then on the external side of the thermal bridge. The time depended field temperature in the cross section of considered thermal bridges were calculated with the Finite Element Analysis Program - Personal Version (FEAPpv) developed by Taylor, (2005). The quadrangle elements with 4 nodes were used to model domains of the considered thermal bridges. The transient boundary condition was 1 and 0 on both sides of thermal bridges.

The FEAPpv allows to save the solution function values in selected nodes at every time step. These time dependent temperatures for rows of elements adjacent to the internal or external surface of thermal bridges were used to calculate surfaces unit step response heat fluxes for each boundary element. As this fluxes are dependent on the spatial independent variables it is necessary to determine unit step response functions as heat flow rate for the internal and external surfaces according to equation:

1

1 mn

i i iiA

Q qdA q L

(5)

were q is local heat flux, A – thermal bridge surface area, L – length of element from FEM analysis, (1 m) – unit height in 2-D FEM analysis.

Integrated time dependent unit step response functions were used to calculate the CTFs coefficients for internal and external surfaces. These surface unit pulse response functions can be convoluted with independent surface temperatures to calculate the heat fluxes on both sides of linear thermal bridges. The example of calculated conduction transfer functions for flat multilayered wall and corner are shown on figures 4 and 5.

Figure 4 FEM calculated CTFs for flat wall and

corner at internal surface induced by internal impulse temperature

Figure 5 FEM calculated CTFs for flat wall and corner at external surface induced by internal

impulse temperature

The CTFs calculated for wall and for thermal bridges have the same nature. This property of thermal bridges CTFs allows to calculate combined correction factors or combined correcting functions for flat wall CTFs and use them with equations 7,8 to calculate heat fluxes for thermal bridges in simulation programs. The combined correction factors or functions should take into account the geometry configuration factors and fitting factors or functions. Configuration factors used for thermal bridges analysed in this paper are presented in

12

3

4

5

8

9

1011

1213

14

15

16

19

20

2122

2324

25

26

27

30

31

3233

3435

36

37

38

41

42

4344

4546

47

48

49

52

53

5455

5657

58

59

60

63

64

6566

6768

69

70

71

74

75

7677

7879

80

81

82

85

86

8788

8990

91

92

93

96

97

9899

100101

102

103

104

107

108

109110

111112

113

114

115

118

119

120121

123

124

125

126

129

130

131132

134

135

136

137

140

141

142143

145

146

147

148

151

152

153154

156

166

167

168

171

172

173

174

1

2

3

4

6

8

9

10

11

12

13

14

16

18

19

20

21

22

23

24

26

28

29

30

31

32

33

34

36

38

39

40

41

42

43

44

46

48

49

50

51

52

53

54

56

58

59

60

61

62

63

64

66

68

69

70

71

72

73

74

76

78

79

80

81

82

83

84

86

88

89

90

91

92

93

94

96

98

99

100

101

102

103

104

106

108

109

110

112

113

114

116

118

119

120

122

123

124

126

128

129

130

132

133

134

136

138

139

140

5 15 25 35 45 55 65 75 85 95 105 115 125 135

7 17 27 37 47 57 67 77 87 97 107 117 127 137

111 121 131

141

142

143

145

147

148

149

144

146

151

153

154

155

150

152

157

159

160

156

158

162

161

163

165

164

166

168167

169

171170

172174173

175177176

178180179

181183182

184186185

187189188

190192191

193195194

196198197

199

6

7

17

18

28

29

39

40

50

51

61

62

72

73

83

84

94

95

105

106

116

117

127

128

138

139

149

150

169

170

122 133 144 155

157

158

159

162

163

164165

160

161

175

178

179

180

176

177

184

182

183

185

188

186

187

189

192

190

191

193196

194

195

197206198

205

207

210

208

209

211

214

212

213

215

218

216

217

219

222

220

221

223

226

224

225

227

230

228

229

231

234

232

233

235

238

236

237

181

199

202

200

201

203

204

‐200,0000

‐150,0000

‐100,0000

‐50,0000

0,0000

50,0000

100,0000

150,0000

200,0000

1 3 5 7 9

11

13

15

17

19

21

23

25

27

29

31

33

35

37

39

41

43

45

47

49

51

53

55

57

59

61

63

65

67

69

71

73

75

77

79

81

83

85

87

89

91

93

95

97

99

101

g_ii [W

/K]

Time [h]

wall g_ii

corner g_ii

‐0,0020

0,0000

0,0020

0,0040

0,0060

0,0080

0,0100

0,0120

1 3 5 7 9

11

13

15

17

19

21

23

25

27

29

31

33

35

37

39

41

43

45

47

49

51

53

55

57

59

61

63

65

67

69

71

73

75

77

79

81

83

85

87

89

91

93

95

97

99

101

g_ie[W

/K]

Time [h]

wall g_ie

corner g_ie


- 1324 -

table 3. The fitting function can as simple as constant or it may take any form – e.g. polynomial of n-th order (6).

2 12 1 0( ) n

nc a a a a (6)

Coefficients an to a0 of the fitting function for flat wall CTF can be determined using data modelling techniques e.g. last square approximation for difference series between flat wall CTFW and thermal bridge CTFTB. In case of the simplest correction function as constant its value can be calculated using linear optimization to minimize sum of differences absolute values between thermal bridge CTFTB and scaled flat wall CTFW. It means that one ought to minimise error function with the fitting factor a according to the equation (7).

1

minn

TB Wi ii

Err CTF a CTF

(7)

Finally correction factors for thermal bridges were calculated for analysed building construction details and presented in table 4. In this table one can find the sum of differences for correction factors that minimise the error function.

Table 3 Configuration factors for analysed thermal bridges

Corner external wall

1.0-d/2 = = 0.78

Intermediate floor or internal wall

1.0

Window opening

1.0

The correction factors may be used to calculate heat flux on internal and external surfaces of thermal bridges that are cut off 1 m from the edge in case of corners and window openings and 1 m from the axis of the intermediate floor and internal wall that joints the external wall of building. The heat flux value on the internal (qTB)i or external (qTB)e surface of thermal can be calculated with equations (7) and (8):

(7)

0 0

( ) ( ) ( ) ( ) ( )n n

TB W ii i W ie eij j

q a g j T j b g j T j

(8)

0 0

( ) ( ) ( ) ( ) ( )n n

TB W ei i W ee eej j

q c g j T j d g j T j

where a, b, c, d are correction factors for corresponding flat wall conduction transfer functions presented in table 4.

The calculation of correction factors pointed out that it is possible to approximate the thermal bridges CTFs with CTFs calculated for flat wall that creates construction detail using one correction factor. Although for most CTF functions the approximation total error is relatively small in some cases this error has quite large value. These misfits apply mostly to response functions on internal surfaces induced by unit temperature impulse at internal surface. The misfit can be find in the first two coefficients of the thermal bridge and flat wall CTFs. This problem may be solved by using correction factors for first few coefficients against to whole CTF correction.

Figure 6 Sample of correction function as polynomial

external wall corner cross CTF

Table 4 Correction factors for wall CTFs and overall errors

calculated for analysed thermal bridges

THERMAL BRIDGE

CORRECTION FACTOR

OVERALL ERROR

Corner

a 0,71795 0,46 b 0,94935 0,05 c 0,99716 0,05 d 1,28205 0,09

Intermediate floor

a 0,66500 12,11 b 0,96722 0,01 c 0,89764 0,01 d 0,47499 0,28

Internal wall

a 0,66500 2,35 b 0,91905 0,02 c 0,82278 0,02 d 0,47500 0,29

Window opening

a 1,10900 79,79 b 3,02318 1,70 c 2,11279 0,25 d 1,08999 0,67

Presented correction factors can be calculated for any type of thermal bridges and may be presented in thermal bridges catalogues. Additionally such atlases can be complemented with coefficients of correction functions that may be implemented in simulation

1000

1000

c(t) = 9E‐14t6 ‐ 4E‐11t5 + 9E‐09t4 ‐ 8E‐07t3 + 4E‐05t2 ‐ 0,0007t + 0,0009

‐0,0050

‐0,0040

‐0,0030

‐0,0020

‐0,0010

0,0000

0,0010

0,0020

1 3 5 7 9

11

13

15

17

19

21

23

25

27

29

31

33

35

37

39

41

43

45

47

49

51

53

55

57

59

61

63

65

67

69

71

73

75

77

79

81

83

85

87

89

91

93

95

97

99

Time [h]

CTF difference

Correction function


- 1325 -

applications which uses convolutions of response functions and internal and external surface temperatures to determine heat fluxes conducted near these surfaces. It is also possible to implement simple FEM or lumped parameters methods for transient problems into pre-processors of energy simulation programs. As simulation programs will browse and analyse BIM data and geometrical information saved in the objects in future, it will be possible to automatically identify the thermal bridges together with the materials parameters and then obtain correction factors or correction functions form internal solvers.

CONCLUSIONS During last two decades whole building simulation programs has been intensively developed. Most of them is used to calculate the energy required for heating and cooling a building using variety of systems and energy sources using integrated solution engines. Many user interfaces were created to help engineers, energy auditors and assessors to effectively work with such complicated systems. The geometrical data input of the simulated building is probably the most arduous work. The redundant building data can be significantly reduced by using BIM. The objects saved in the BIM databases carry enough information to prepare proper input for energy simulation systems. Although the energy simulation software has been developed during last years it is still focused on solving 1-D transient heat conduction in the building components with help of convolution CTFs with temperature functions or solving the lumped parameters circuits in time variant domain.

The development of CTFs correction factors and correction functions has been described. When detailed whole building energy simulation is required it is necessary to take 2D or 3D thermal bridges effects into account. The methodology presented in this paper shows how to calculate correction factors and correction functions for linear thermal bridges with external or internal transient 2D heat conduction solver and use them to modify CTFs in building energy simulations. This attempt of improving the treatment of typical linear thermal bridges recognized in BIM data has been made for energy simulation engines that utilise time-domain response functions such as Blast, DOE-2 or EnergyPlus. This methodology needs more analyses to find the best form of correction functions for wide range of thermal bridges that occur in buildings. Results from this exercise can be successfully applied in simulation programs but further improvement is necessary.

REFERENCES Carpenter S. C., Kośny J., Kossecka E., 2003,

Modelling transient performance of two–dimensional and three-dimensional building

assemblies, ASHRAE Trans., 109 (1), p.: 566-571.

Carpenter S. C., Schumacher C., 2003, Characterization of Framing Factors for Wood-framed Low-rise Residential Buildings, ASHRAE Transactions, 109(1), p.: 101-108.

Clarke J. A., 2001, Energy Simulation in Building Design (2nd Edn), Butterworth-Heinemann.

Davies M, Tindale A., Little J., 1995, Importance of Multi-dimensional Conductive Heat Flows in and Around Buildings, Buildings Services Engineering Research and Technology, 16(2), p.:83-90.

Kośny J., Christian J. E., 1995, Thermal Evaluation of Several Configurations of Insulation and Structural Materials for Some Metal Studs, Energy and Buildings, 22(2), p.: 157-163.

Kośny J., Petrie T. W., Christian J.E., 1997b, Thermal Bridges in Roofs Made of Wood and Light Gauge Steel Profiles, ASHRAE Transactions, 103(1), p: 537-549.

Kośny, J., Kossecka E., 2002, Multi-dimensional Heat Transfer Through Complex Building Envelope Assemblies in Hourly Energy Simulation Programs, Energy and Buildings, 34(5), p: 445-454.

Renon O., 2002, Thermal Bridge Modeling In EnergyPlus, Building Energy Simulation User News, Vol. 23, No. 4.

Strachan P. A., Nakhi A., Sanders C., 1995, Thermal Bridge Assessment, Proceedings of Building Simulation '95, p.: 563-570.

Taylor R. L., 2005, FEAPpv - A Finite Element Analysis Program - Personal Version 2.0 User Manual

EN ISO 10211: Thermal bridges in building construction – Heat flows and surface temperatures – detailed calculations (ISO 10211:2007), CEN, 2007.

EN ISO 13790: Energy performance of buildings — Calculation of energy use for space heating and cooling (ISO 13790:2008), CEN, 2008.

EN ISO 14683: Thermal bridges in building construction – Linear thermal transmittance – Simplified methods and default values (ISO 14683:2007), CEN, 2007.

Physibel n.v., KOBRA v3.0w, 2008. www.cstc.be/go/kobra (French) or www.wtcb.be/go/kobra (Dutch)

FEAPpv A Finite Element Analysis Program:, www.ce.berkeley.edu/projects/feap/feappv/

Audytor OZC – Energy Performance Certificate and Building Design Heat and Cool Load Application:, http://www.sankom.eu/


- 1326 -

Table 5 Transient temperature fields calculated with FEAPpv for analysed thermal bridges Beginning of the process – at time = 2,5 h End of the process – steady state – at time = 147,5 h

Cor

ner

– in

tern

al

unit

step

C

orne

r –

exte

rnal

un

it st

ep

Inte

r. fl

oor

– in

tern

al

unit

step

In

ter.

floo

r –

exte

rnal

un

it st

ep

Win

dow

– in

tern

al

unit

step

W

indo

w -

exte

rnal

uni

t st

ep


- 1327 -

modelling of conduction transfer functions for typical ... · pdf filemodelling of conduction...

Documents