Analytical Computation of Thermal Response Characteristics of Homogeneous and Composite Walls of Building and Insulating Materials Used In India

Transcription

1 Analytical Computation of Thermal Response Characteristics of Homogeneous and Composite Walls of Building and Insulating Materials Used In India ABSTRACT Saboor S, Research scholar [NITK, Surathkal, India] Ashok Babu T.P., Ph.D [NITK, Surathkal, India] Climate responsive building design involves the study of the thermal response of building and insulating materials exposed to periodic changes of environmental conditions. For calculation of such unsteady thermal characteristics, one dimensional heat flow diffusion equation under periodic boundary conditions was solved using matrix algebra and thermal characteristics like admittance, transmittance, decrement factor, time lag, surface factor and surface factor time lags were calculated for homogeneous and composite walls. In this study, ten building and ten insulating materials were studied. Optimum wall thicknesses of building and insulating materials were calculated. From the results, it was concluded that mud phuska and coconut pitch are the most recommended homogeneous building and insulation materials respectively, among studied building and insulating materials, from lower decrement factor and higher time lag point of view. It is found that the insulation materials are highly responsive to short wave radiation than that of building materials. From the study, ultimately it is concluded that Burnt brick composite walls with Coconut pitch insulation is the best composite wall among all studied walls for reduced cooling loads and the most energy efficient building construction. INTRODUCTION The building sector represents about 33% of power consumption in India, with the commercial sector and residential sector accounting for 8% and 25% respectively (ECBC, 2009). Buildings are also responsible for carbon dioxide emissions with a consequential impact on global warming. The building envelope is the physical barrier that separates the interior of the building from the outdoor environment. The purpose of the envelope of a building is to act as a passive climate modifier to help in maintaining an indoor environment more suitable for habitation than the outdoors. Previously, The EN ISO 13786:2007 procedure has been compared with fast Fourier transform analysis (Gasparellaa et al., 2011). The effects of thermo physical properties and thickness of a wall of a building on time lag and decrement factor have been investigated using crank Nicolson method by many researchers (Asan et al., 1998). Numerical computations of time lag and decrement factor for different building materials were also investigated (Asan, 2006) and also Effects of Wall s insulation thickness and position on time lag and decrement factor were studied in detail (Asan, 1998). The present study focuses on the cyclic response admittance method to calculate unsteady state thermal characteristics of the homogeneous and composite walls for more energy efficient building design. ANALYTICAL SOLUTION FOR WALL THERMAL RESPONSE CHARACTERISTICS The admittance procedure is used (CIBSE, 2006) to calculate the unsteady state parameter values which use matrices to simplify the temperature and energy cycles for a composite building fabric 1

2 element that is subjected to sinusoidal temperature variations at the sol air node. The temperature distribution in a homogeneous wall subjected to one dimensional heat flow is given by the diffusion equation, (,) = (, ) (1) Fourier equation can be written as shown in Equation 2. (,) = h(+)+h(+)(2 ) (2) Where, =! " # When the conducting medium finite thickness slab X, temperature and flows at the two surfaces are considered then the above equation can be written as, $ % h()+)) *h()+))+, '=(.$ / ' (3) & % *h()+))+, h()+)) & / Where, cyclic thickness ())= " #= 2 and Characteristic admittance of slab(,) = 2# " 3 = 2 2. Transmission matrix of single layer can be written as (Davies, 2004), + ( +4), $ ( 4+)., + ' (4) Where, constants = cosh())cos ()), = sinh())sin ()), = cosh())sin())+sinh())cos ()) 2 and 4 = cosh())sin()) sinh())cos ()) 2. Transmission matrix of surface internal (R si ) and external (R se ) film resistances can be written as, = = > AB 0 1 D,E = = AF 0 1 D (5) Transmission matrix for composite wall can be written as, $ B ' = > AB & B 0 1 D> / G D> H / H / H G H D.> AF / 0 1 D$ F ' (6) & F Where, x and y represent number of layers of the wall. Transmission matrix can be further reduced as follows, $ B '=$ / '$ F ' (7) & B G K & F From the above matrices, the following unsteady state thermal characteristics can be calculated. Thermal Transmittance (U) This is the steady state heat flow through the element per unit degree of temperature difference between the internal and external environmental temperatures per unit area. Thermal Admittance (Y) This is the amount of energy leaving the internal surface of the element into the room per unit degree of temperature swing. L =ǀN & B O P ǀ=ǀ / ǀ (8) Q RST Time lead for thermal admittance (ω) It is the time difference between the timing of the peak heat flow at the internal surface and timing of the peak internal temperature. U = 12 WX( / Y (9) / ) Decrement factor (f) It is the attenuation of sinusoidal wave as it progresses through the wall. Z = ǀ 1 ǀ (10) [ Decrement delay (ϕ) It is the time lag between the timing of the internal temperature peak and the peak heat flow out of the external surface. \ = 12 1 WX(,2,V [ 1 Y (11) ) [ 2

3 Surface factor (F) It is the ratio of the swing in heat flow from the internal surface of the element to the swing in heat flow received at the internal surface of the element. ] = ǀ / AB^ _ ǀ (12) Time lag for the surface factor (ѱ) It is the time lag between the timing of the peak heat flow entering the surface and peak heat flow leaving the surface into the room. ` = 12 ABN,2,V AB N Y (13) / P ) Optimum wall thickness (d) It is the thickness of the wall at which the wall has its maximum heat storage. E = c/U (14) UNSTEADY STATE THERMAL CHARACTERISTICS OF HOMOGENEOUS AND COMPOSITE WALLS Table 1 shows the thermo-physical properties of Building materials at 50 o C and Table 2 shows the thermo-physical properties of Insulating materials considered for the study at 50 o C (SP:41, 1987). Ten building and ten insulation materials were selected for the study from Indian standard guide for heat insulation of non industrial buildings as per IS code The methods used to measure thermal conductivities (k) of building and insulating materials in IS code are guarded hot plate method and ASTM heat flow methods (IS 3792, 1978). Thermal properties k, Cp and α represent thermal conductivity specific heat capacity and thermal diffusivity respectively. ρ is the density of material. Table 1. Thermo Physical Properties of Building materials at 50 o C (*Experimental values) k ρ Cp α x10-7 Building material Code (W/mK) (kg/m 3 ) (/kgk) (m 2 /s) Malabar Laterite Stone* BM Madras Black clay BM Indore Black clay BM Slate BM Burnt brick BM Mud brick BM Reinforced brick BM Brick tile BM Mud phuska BM Cinder concrete BM Plaster* P / Table 2. Thermo Physical Properties of Insulating materials at 50 o C k ρ Cp Insulating material Code (W/mK) (kg/m 3 ) (/kgk) α x10-7 (m 2 /s) Saw dust IM Rice husk IM Coir board IM ute felt IM ute fiber IM Coconut pitch insulation IM Straw board IM Asbestos fiber IM Wall board IM Chip board IM

4 The computer program was developed and used to calculate the unsteady state thermal characteristics of homogeneous and composite walls. Figure 1 shows the images of building and insulating materials considered for the study. The building materials are coded from BM1 to BM10 whereas the insulating materials are coded from IM1 to IM10. Plaster was represented by code P. Thermal properties of Laterite stone (BM1) and plaster (P) were measured experimentally using ISO transient plane source method at K-Analys AB, Sweden. Table 3 and Table 4 show the unsteady state thermal characteristics of building and insulating materials, respectively. The nominal thickness of the homogeneous wall was taken as 0.2m. Table 3. Unsteady state thermal characteristics of Building materials Code U (W/m 2 K) f Φ Y (W/m 2 K) BM BM BM BM BM BM BM BM BM BM Code ω Table 4. Unsteady state thermal characteristics of Insulating materials U Φ Y ω (W/m 2 f K) (W/m 2 F K) IM IM IM IM IM IM IM IM IM IM F Ѱ Ѱ Configuration C.W-1 C.W-2 C.W-3 C.W-4 C.W-5 Table 5. Configuration of Composite walls Thickness (m) P BM IM BM P P BM IM BM P P BM IM BM P P BM IM BM P P BM IM BM P One dimensional diffusion equation was solved under periodic boundary conditions using matrix algebra. The transmission matrix for a homogeneous wall was shown by Eqs.(4). The transmission matrix for internal and external surface resistances is shown by Eqs. (5). In the present study, the walls are considered as external walls therefore external and internal surface resistances selected are 0.04 m 2 K/W and 0.13 m 2 K/W respectively as per CIBSE standards. Among all the studied insulating materials, coconut pitch insulation (IM6) is found to be energy efficient from lowest decrement factor (0.22) and highest time lag (11.80h) point of view. Hence this insulation is used to frame the composite walls with the most commonly used building materials in South India. Five composite walls are coded from C.W-1 to C.W-5. Figure 2 shows the configuration of composite wall. Table 5 shows the configuration of 4

5 composite walls with thicknesses of plaster, building materials and Insulation materials. Table 6 shows the unsteady state thermal characteristics of composite walls. Images of homogeneous materials ( Images of building materials ( Images of Figure 1 insulating materials Table 6. Unsteady state thermal characteristics of Composite walls U Code C.W-1 C.W-2 C.W-3 C.W-4 C.W-5 Figure 2 Plaster) (W/m2K) f Φ Y (W/m2K) ω F Ѱ Configuration of composite wall ( BM: Building material, IM: Insulating material, P: RESULTS AND DISCUSSIONS Wall thickness greater than optimum does not give any additional energy storage benefits rather it reduces energy storage. The physical explanation given is that as heat stored in the fabric from previous days tries to escape, it meets with the current heat flow attempting to enter the fabric. Also, as d increases, after the peak Y value has been achieved, the thermal transmittance, U and volumetric heat capacity, continue to increase. Optimum wall thicknesses for building and insulating materials were calculated using Eqs. (14) (Magyari et al., 1998). Figure 3 (a) and Figure 3 (b) show the variation of admittance and transmittance of the building materials with thickness. From figures, it is observed that for thin cross section fabrics admittance is equal to the transmittance. The values of A, B, C, D, E, F, G, H, I and represent the optimum fabric thicknesses of the building materials from BM1 to BM10 respectively. Figure 4 (a) and Figure 4 (b) show the variation of admittance and transmittance of the insulating materials with thickness. The values of a, b, c, d, e, f, g, h, i and j represent the optimum fabric thickness 5

6 of the insulating materials from IM1 to IM10 respectively. The results show that among all the ten building materials studied, Mud phuska (BM9) has least optimum fabric thickness value I (0.118m) and slate (BM4) has higher optimum fabric thickness value D (0.169m). It is observed that among all the ten insulating materials studied, Coconut pitch insulation (IM6) material has least optimum fabric thickness value f (0.064m) and rice husk (IM2) has higher optimum fabric thickness value b (0.127m). At an optimum fabric thickness all the building and insulating materials have the maximum thermal heat capacity. Figure 3 (a) Optimum wall thickness of Building materials (BM1 to BM5) (b) Optimum wall thickness of Building materials (BM6 to BM10) Figure 4 (a) Optimum wall thickness of Insulation materials (IM1 to IM5) (b) Optimum wall thickness of Insulation materials (IM6 to IM10) Figure 5 (a) Decrement factor of Building materials (b) Time lag of Building materials Figure 5 and Figure 6 show the effects of wall thickness of the homogeneous building and insulating materials on the decrement factor and it s time lag respectively. The decrement factor of the building material decreases with an increase in the wall thickness. The time lag of building materials 6

7 increases with an increase in the wall thickness. The smaller decrement factors and larger time lags are the more effective for the walls at suppressing temperature swings. Figure 6 Figure 7 materials (a) Decrement factor of Insulation materials (b) Time lag of Insulation materials (a) Surface factor of Building materials (b) Surface factor Time lag of Building Figure 8 (a) Surface factor of Insulation materials (b) Surface factor Time lag of Insulation materials From Figure 5, it is apparent that mud phuska (BM9) has least decrement factor (0.5) and higher decrement time lags (6.74) whereas cinder concrete (BM10) has higher decrement factors and lower time lags among ten building materials studied. From Figure 6, it is observed that coconut pitch insulation (IM6) has least decrement factor (0.24) and higher decrement time lags (11.41) whereas rice husk (IM2) has higher decrement factors and lower time lags among ten insulating materials studied. From Figure 7, it is observed that among ten studied building materials, slate (BM4) is slow responsive to short wave radiation due to its lowest surface factor (0.33) and highest surface factor time lags (2.09h) whereas mud phuska (BM9) is fast responsive to short wave radiation due to its higher surface factor (0.55) and lower surface factor time lags (1.46). 7

8 From Figure 8, it is noticed that among ten insulating materials studied, chip board insulation (IM10) is slow responsive to short wave radiation due to its low surface factor (0.86) and high surface factor time lags (0.49h) whereas coir board (IM3) is fast responsive to short wave radiation due to its higher surface factor (0.95) and lower surface factor time lags (0.17h). From Figure 7 and Figure 8, it is seen that surface factor and it s time lag do not depend on the thickness of the wall, but they depend only on thermal conductivity of the building or insulating material and the insulating materials are fast responsive to short wave radiation than the building materials due to their higher surface factors and lower surface factor time lags. In practice, building walls are composite i.e., they are constructed with the combination of two or more homogeneous materials. Hence the best insulation material (coconut pitch (IM6)) among ten studied insulating materials was used as insulation material to frame composite walls with the most commonly used building materials (among ten building materials), Laterite stone (BM1), burnt bricks (BM5), mud bricks (BM6), reinforced brick (BM7) cinder concrete (BM10) and plaster (P). The insulation material was placed at the center of the composite wall as shown in Figure 2. From Table 6, it is observed that laterite stone composite walls (C.W-1) with Coconut pitch insulation (IM6) give lowest decrement factor values (0.32) and burnt brick composite walls (C.W-2) give highest decrement time lags (9.1h) whereas cinder concrete composite walls (C.W-5) with coconut pitch insulation give highest decrement factor (0.42) and lowest decrement time lags (8.09h). CONCLUSION Laterite stone and burnt brick composite walls with Coconut pitch insulation are the best composite walls for reduced cooling loads due to smaller decrement factors (0.30) and higher time lag values (9.1h) respectively, among studied composite walls. Mud phuska and coconut pitch have the least optimum fabric thickness (0.118m & 0.064m) among all the building and insulating materials studied respectively. Using these materials in construction, energy can be saved with smaller thicknesses of the walls. Mud phuska is the best homogeneous building material from the least decrement factor (0.5) and the highest time lag (6.74h) point of view among studied building materials and coconut pitch is the best homogeneous insulation material due to its lowest decrement factor (0.24) and highest time lag (11.41h) values among studied insulating materials. Hence, these are recommended for energy efficient building construction among studied materials. The insulating materials are fast responsive to short wave radiation than the building materials due to their higher surface factors and lower surface factor time lags. Hence insulation materials should not be exposed to direct radiation. REFERENCES Asan, H Numerical computations of time lag and decrement factors for different building materials. Building and Environment 41(5): Asan, H Effects of Wall s insulation thickness and position on time lag and decrement factor. Energy and Buildings 28(3): Asan, H., and Y.S. Sancaktar Effects of Wall s thermo physical properties on time lag and decrement factor. Energy and Buildings 28(2): CIBSE, CIBSE Environmental Design Guide-A. 7 th ed., chartered Institution of Building services engineers, London. pp Davies, M.G Building Heat Transfer, ohn-wiley & sons Ltd. pp ECBC Energy conservation building code, Bureau of energy efficiency, India, pp.1-2. Gasparellaa, A., Pernigottob, G., Baratieri, M. and P. Baggioc Thermal dynamic transfer properties of the opaque envelope: Analytical and numerical tools for the assessment of the response to summer outdoor conditions. Energy and Buildings, 43(9): IS: Indian standard guide for heat insulation of non industrial buildings, Indian standards institution, New delhi, India., pp Magyari, E., and B. Keller The storage capacity of a harmonically heated slab revisited. International journal of heat and mass transfer, 41(10): SP:41 (S&T) Handbook on functional Requirement of Buildings other than industrial buildings. Bureau of Indian Standards, India., pp