INFRARED (IR) THERMOGRAPHY MEASUREMENT OF U-VALUE Siliang Lu

Transcription

1 INFRARED (IR) THERMOGRAPHY MEASUREMENT OF U-VALUE Siliang Lu Supervisor: Itai Danielski Sumission date: June, 6 th, 2015 The Department of Ecotechnology and Sustainable Building Engineering Mid Sweden University

2 Abstract U-value (thermal transmittance) is an important parameter for predesigning the heating and cooling systems of a building. U-values of many materials have been measured in the laboratory in the past. However, these values are hard to be applied in practice since the experimental conditions are hard to be realized on-site. Therefore, many in-situ measurement methods of U-values have been developed and ISO provides a clear guideline to measure U-values under non-steady state. Since Infrared (IR) thermography has become a reliable technique to analyze building status qualitatively, many researchers have recently begun to test the applicability of IR thermography to measure U-values quantitatively. This report measured U-values of three massive wooden walls and a ceiling of a small wooden house in Mid Sweden University. The results illustrate that IR thermography is applicable to measure U- values. However, further research has to be done to improve the measurement method with IR thermography and increase its accuracy. Key words: U-value, IR thermography, ISO

3 Table of Contents 1 Introduction Background: Review of IR thermography: Goals Related Theories Heat transfer: Definition of thermal resistance R and thermal transmittance U: Comparisons of measurement methods of U-value Imposing steady-state by use of guarded hot box In-situ measurement from ISO Apparatus Installation IR thermography Methodology The case study In-situ set-ups: Apparatus Internal set-up: Measurement of emissivity Measurement of the reflected temperature: Calculation method with IR thermography Sensitivity method Results The northern exterior wall Convective heat transfer coefficient Conductivity U-value The eastern exterior wall Convective heat transfer coefficient Conductivity U-value The western exterior wall Convective heat transfer coefficient U-value The ceiling Convective heat transfer coefficient U-value Sensitivity analysis Discussion Conclusion... 31

4 9 Further research References Acknowledgement... 35

5 1 Introduction 1.1 Background: Large amount of energy consumption due to heating and cooling in buildings has recently drawn much attention around the world. In a US typical home, heating and cooling accounts for more than half of the energy use, which makes it the largest energy expense for most homes. (U.S. Department of Energy). In China, the energy consumption in the building sector has accounted for 30% of the total energy consumption. Meanwhile, heating and cooling system has contributed half of total energy consumption to the building sector (China Center and Energy and Development, 2013). Moreover, among European countries, electricity, transport as well as heating and cooling are three main sectors where effective policies aims to promote renewable sources (Renewable Energy Policy in the EU, 2012). The 2012 Energy Efficiency Directive (EED) establishes a set of binding measures to help the EU reach its 20% energy efficiency target by Particularly, the EED sets energy saving requirements on EU s buildings and requires each country to establish their own national plans (Energy Efficiency Directive, 2015). Therefore, it is necessary to investigate the energy performances of buildings. Since the knowledge of U-value, also known as thermal transmittance, is a precondition for the classification of the energy performances of existing buildings, this information is of great importance (Fokaides PA. and Kalogirou SA., 2011). Standard ISO describes an approach to measuring U-value with Heat Flow Meter (HFM) and temperature sensors. Even if it has been a standard method, improvements are still needed. Hence, the currently commercialized Infrared (IR) thermography has become a new research area which provides a new way to measure U-value. 1.2 Review of IR thermography: IR thermography utilizes the emitted infrared radiation from an object. Based on the fact that radiation is a function of the temperature, IR camera is able to get the temperature of an object by capturing its emitted infrared radiation. IR thermography has increasingly been widespread to study surface temperature over the past few years. This method is especially important when considering its applications in giving spatially resolved surface temperature distributions (Fokaides PA. and Kalogirou SA., 2011). In addition, it will be more convenient to study certain parts of buildings, e.g. ceiling, which is hard to be accessible for equipment like temperature sensors or HFMs. IR thermography can also be used in building envelopes to diagnose the building weaknesses, e.g. air leakage, missing insulations. The discussion on applications and development of IR thermography for building diagnostics has been held since this technique was commercialized in the early 90s (Fokaides PA. and Kalogirou SA., 2011). Matthew Fox et al. extended studies on thermography from single image 1

6 to a series of images over a period, which is called time-lapse thermography. Besides qualitative analysis of buildings using IR thermography, researchers have been involved in its applications into quantitative analysis, especially the determination of U-values. Fokaides and Kalogirou used IR thermography for the determination of U-values of the wall, roof and glazing. The percentage absolute deviation has a range of 10-20%. Since measurements of U-value with IR thermography is significantly different from that with HFM and temperature sensors and is still a developing technique, researchers have not reached a consensus on many aspects regarding IR thermography measurements. 2

7 2 Goals This report has two goals. The first goal is to compare different approaches to measuring U-value based on previous studies. The second goal is to test the applicability of IR thermography in quantitative analysis by both using IR thermography and in-situ measurements with HFMs and temperature sensors to measure the overall U-values of four exterior massive wooden walls and the ceiling as well as conductivities of massive wooden walls. 3

8 3 Related Theories 3.1 Heat transfer: Heat transfer is energy transferred because of a temperature difference (ASHRAE Fundamental Handbook, 2013). In temperate climate regions like in Sweden, residential houses are equipped with radiators. Indoor temperature is usually higher than outdoor temperature. However, by virtue of temperature differences, energy loss happens by the following three modes in buildings: conduction, convection and radiation. Conduction Conduction is the mode when heat transfer occurs at rate Φ through the solid like walls from warmer side (Ts1) to the cooler (Ts2). The conduction rate Φ can be calculated according to Eq. 3-1 obtained from ASHRAE (ASHRAE Fundamental Handbook 4, 2013). Λ thermal conductivity L distance between the two surfaces A surface area Convection Φ = λ (T s1 T s2 ) A (3-1) L Convection occurs when heat transfers from the fluid like indoor moving air at temperature of T to the surface like walls at temperature of T s. The convective heat flow rate Φ can be written as: Φ = h c (T s T )A (3-2) h c convective heat transfer coefficient A surface area Radiation Radiation is the mode that any matter emits at its surface when its temperature is above absolute zero. A surface at temperature of T s that absorbs all radiation incident upon it is called a black surface. The heat emission from a black surface is given by the Stefan-Boltzmann law: 4 Φ black surface = AσT s (3-3) σ = , Stefan Boltzmann constant A surface area However, most of surfaces in reality are grey surface, which means they do not absorb all incident radiation. For nonblack surface, the absorbed radiation is 4 Φ gray surface = εaσt s (3-4) ε emissivity, [0,1] A surface area Two surfaces at different temperatures can exchange energy through radiation. The net radiation exchange from a gray surface with surface area A at temperature T s with the surroundings at temperature T surr can be written as: Φ net = Φ emitted Φ absorbed = εaσt s 4 ασt surr 4 = εaσ(t s 4 T surr 4 ) (3-5) α absorptivity, [0,1] 4

9 Where α is ε for the gray surface. Therefore, net radiation between different building elements can be calculated with the equation Definition of thermal resistance R and thermal transmittance U: ISO states that the thermal properties of the building elements have the following definitions: R is the thermal resistance of an element, surface to surface and is given by: R = T i T e q (3 6) q heat flux(or density of heat flow rate) = Φ/A T i interior surface temperature T e exterior surface temperature In addition, the thermal transmittance of U is defined in ISO-7345 as the Heat flow rate in the steady state divided by area and by the temperature difference between the surroundings on each side of a system and is given by: q U = (T i T e ) = 1 (3 7) R T T i internal ambient temperature T e external ambient temperature Where R T is the total thermal resistance which is given by: R T = R si + R + R se (3 8) R si internal surface thermal resistance R se external surface thermal resistance 5

10 4 Comparisons of measurement methods of U-value 4.1 Imposing steady-state by use of guarded hot box The guarded box is used to impose steady state conditions for the measurement of U-value. In the guarded hot box (see Fig.1), the metering box is surrounded by a guarded box. Ideally, the total heat flow ϕ 1 through the specimen, for instance, a piece of wooden wall, will be equal to the heat input ϕ p if lateral heat flow in the specimen ϕ 2 and heat flow through the metering box wall ϕ 3 are both equal to 0. In that way, thermal transmittance U can be calculated as heat flux through the specimen divided by temperature differences between the environmental temperature T n1 (hot side) and the environmental temperature T n2 (cold side). Fig.1 The guarded hot box This method is commonly used in the laboratory (ISO-8990) and can obtain accurate steadystate transmission properties. However, it is cumbersome to be applied into measuring U-value under non-steady state. 4.2 In-situ measurement from ISO In principle, U-value can be obtained under steady state condition. However, steady state are unlikely to be encountered on a site in practice. Since it is appreciated to have on-site data and to place a certain level of agreement between the empirical values and measurement in the context of the overall energy performance of a building element, several approaches are presented in ISO to overcome this problem: a) Assuming the mean values of the heat flow rate and temperatures are over a sufficiently long period of time give a good estimate of the steady state, which is called the average 6

11 method. The average method is a simple method that assumes U-value can be obtained by dividing mean density of heat flow rate by mean temperature difference which has been taken over a long enough period of time. This method is conditionally acceptable that the thermal properties of the materials and heat transfer coefficients must be constant over the range of temperature fluctuations occurring during the test and the change of amount of heat stored in the element is neglected. b) Using a dynamic theory to take into account the fluctuations of the heat flow rate and temperatures in the analysis of the recorded data. These two measurements are both require data recorded from on-site experiments with apparatus and installation methods described below. More details are in ISO Apparatus ISO has put forward a measurement of thermal resistance (R) and thermal transmittance (U) by using apparatus of heat flowmeter (HFM) and temperature sensors. Most HFMs or heat flux sensors are thin, thermally resistive plates with temperature sensors arranged in such a way that the electrical signal given by the sensors is directly related to the heat flow through the plate. Temperature sensors are transducers giving an electrical signal which is a monotonic function of its temperature. Two main types of temperature sensors are used in different measurements. The one is the ambient temperature sensors (for solely U-value measurements). The other is surface temperature sensor (for R-,Λ or U-value) From ISO , the data from HFMs and temperature sensors shall be recorded continuously or at fixed intervals over a periods of complete days. The minimum test duration is 72h if the temperature is stable around the HFMs. Otherwise, this duration may be more than 7 days. Besides, it is recommended that recordings are made at fixed time intervals which are the average of several measurements sampled at shorter intervals Installation ISO states that HFMs shall not be installed in the vicinity of thermal bridges, cracks or similar sources of error. In addition, the internal surface temperature sensor shall be mounted on the internal surface either under or in the vicinity of the HFMs. The external surface temperature sensors shall be mounted on the external surface opposite the HFMs. For the purpose of measuring U-values properly, the exterior surface of the element should avoid rain, snow and direct solar radiation. Due to strict location of the measured area, U-value measured with HFMs and temperature sensors are solely U-value of the intact and small part of the whole building element. In other words, no crack or damage on the element is allowed during the measurements. 7

12 4.3 IR thermography Unlike standard measurements from ISO-9869, the surface temperature from IR thermography is no longer focused on such small points. Instead, it is average temperature of the surface within the camera. Hence, U-value with this measurement can be seen as the overall thermal transmittance of the whole element (Fokaides PA. and Kalogirou SA.., 2011). Recently, IR thermography has been applied to obtain two-dimensional quantitative imaging of U-value. During measurements, the total heat flux q has to be obtained. From Ohlsson and Olofsson, here are two different approaches to get q: The first approach is given by Vavilov et al. The heat flux is firstly measured at a reference point on the surface using HFMs. Then the heat flux in other locations is calculated based on the surface temperature from thermography measurements. Meanwhile, they assumed that the thermal resistance was constant but unknown. The second approach is to obtain the heat flux completely based on thermography. However, the convective heat transfer coefficient is a known value. Tanner et al. suggested a standardization of the thermography method for measuring U-value, where they adopted a constant value of the h c =8.7 W/ (m 2 *K). Ohlsson and Olofsson also took the example of the often used model of Jurges to point out that it is necessary to select an appropriate model for h c for each particular case. During the measurement, the radiation measured by the camera does not only depend on the temperature of the object but is also a function of the emissivity. (Flir tool ix series, User s manual). Therefore, it is of significance to set the correct emissivity and reflected temperature before measuring U-value. 8

13 5 Methodology 5.1 The case study The full scale experiment was carried out from mid-january until the end of March, 2015 in Mid Sweden University, Ostersund, during which Sweden transited from winter to early spring. The indoor temperature was set to be 24 by a thermostat. However, the outdoor temperature was varying a lot during the day and the night, thus the measurements being under non-steady state. The wooden house (Fig.2) was built adjacent to the building for the Department of Ecotechnology and Sustainable Building Engineering. The layout is shown in Fig.3. Fig.2 The wooden house Fig.3 the house layout The thickness of the northern, eastern and western wall is 140mm, 165mm and 190mm, respectively, as illustrated in Fig.3. As shown in Fig.4, the house was constructed by sliding pieces of massive wood one by one. The typical logs are also shown in Fig.5, which illustrates 9

14 that the log contains random marks on the surface. The exterior wall is not a completely flat surface. Instead, it is a rough surface and has random weaknesses. Fig.4 Construction with logs (Loglock) Fig.5 Glulam (Loglock) 5.2 In-situ set-ups: Apparatus Heat Flow Meter: The heat flux was measured by three HFP01 heat flux plates (Fig.6). The sensitivity of the sensor is 61.96μV/(Wm 2 ),61.66μV/(Wm 2 ),62.06μV/(Wm 2 ), respectively. In addition, LI 19 loggers were programmed to record the data every 15mins. Fig.6 Heat flux sensor Surface temperature sensor: The interior and exterior surface temperatures were measured by surface temperature sensors (Fig.7) from Elcoplast Oy. Three or two sensors were installed to measure the internal surface of the wall. Meanwhile, two temperature sensors were installed to measure the temperature of the external surface of the wall, as shown in Fig.8. All surface temperature sensors recorded temperature every 15mins, which corresponded with the 10

15 HFM record interval. Fig.7 Surface temperature sensor Fig.8 the exterior surface temperature sensors Ambient temperature sensor: The indoor ambient temperature was measured by five ambient temperature sensors of RHTemp 1000 (Fig.9). The outdoor ambient temperature was measured by one ambient temperature sensor, as shown in Fig.10. The ambient temperature between the roof and the ceiling was measured by one ambient temperature sensor. All the ambient temperature sensors recorded average temperature for every 15mins, which corresponded with the HFM record interval as well. 11

16 Fig.9 Ambient temperature sensor Fig.10 the outdoor ambient temperature sensor Thermal imaging camera of Flir tool IR thermography was conducted with the thermal camera of Flir ix series. The photos were taken manually for three times every day on average Internal set-up: Fig.11a, 11b, 11c and 11d show the internal set-up of the northern, eastern and western wall as well as the ceiling, respectively. A corrugated aluminum foil was used to measure the reflected temperature. In addition, the measured area is in the middle part of the exterior wall to get rid of the effects from the ground and corners, which may result in appreciable temperature 12

17 stratifications. Therefore, the measured part has relatively uniform temperature distribution. Fig.11a the northern wall setting Fig. 11b the eastern wall setting Fig. 11c the western wall setting Fig.11d the ceiling setting 5.3 Measurement of emissivity Before measuring U-value with IR thermography, it is necessary to measure the emissivity (ε) of the exterior wall. Here are procedures which follow the user s manual (Flir ix series, User s manual) to measure it: 1 Measure the outdoor reflected temperature (described in next section in detail) by using a crumpled aluminum foil put outside where the temperature is much lower than the indoor temperature. 2 Put a piece of black tape with known emissivity (ε = 0.95) on the sample of the wall 3 After the tape has the same temperature as the sample, put the sample outside. 4 Focus and auto-adjust the camera, and take the image quickly to ensure the sample still has much higher temperature (usually 20K) than the outdoor temperature 5 Set emissivity to that of the tape 13

18 6 Write down the exact temperature of the tape 7 Create a box on the portion of the wall in the picture to measure its temperature, as shown in Fig.12 8 Change the emissivity setting until the temperature of the sample is the same as the previously written one. 9 The final emissivity is the emissivity of the wall Fig.12 the emissivity measurement 5.4 Measurement of the reflected temperature: Reflected temperature can be seen as surrounding temperature in Eq Reflected temperature is used to compensate for the radiation reflected by the object (Flir ix series, User s manual). From ASTM E , two methods can be used to measure the reflected temperature. The one is the direct method, the other is the indirect method. This report used the latter one to measure the emissivity following the user s manual (Flir ix series, User s manual): 1 Crumble up a large piece of aluminum foil 2 Unfold the foil and attach it to a piece of cardboard 3 Put the cardboard in front of the wall and the side with the foil points to the camera, shown as Fig. 11a~11d. 4 Set the emissivity to Measure the temperature of the aluminum foil, which is the reflected temperature 14

19 5.5 Calculation method with IR thermography The calculation of U-value with IR thermography is based on the assumption that the conduction from the interior surface to the exterior surface is equal to the sum of radiation absorbed by the target element and the convective heat flux between the target and the indoor ambient air: q = q cond = q conv + q rad (6 1) q conv = h conv (T in T s,in ) (6 2) q rad = εσ((t ref + 273) 4 (T s,in + 273) 4 ) (6 3) q total heat flux, W/m 2 q cond conductive heat flux, W/m 2 q rad radiative heat flux, W/m 2 q conv convective heat flux, W/m 2 T s,in interior surface temperature from thermal camera or temperature sensors, T in indoor ambient temperature, T ref reflected temperature, When calculating convective heat transfer coefficient, conductive heat flux can be known from HFMs. Thus, convective heat transfer coefficient can be calculated by using Eq. 6-1 and Eq. 6-2: h c = q cond q rad T in T s,in (6 4) Then, the convective heat transfer coefficient is used to calculate the conductivity of the exterior wall based on Eq For conductivity from thermography: Λ = εσ((t ref + 273) 4 (T s,in + 273) 4 ) + h c (T s,in T in ) T s,in T s,out thickness (6 5) T s,out exterior surface temperature from temperature sensors, For conductivity measured with HFMs: q cond Λ = thickness (6 6) T s,in T s,out In order to validate the conductivity with method from ISO-9869, only a small part of the wall within the thermal camera which has similar conditions (i.e. size, uniform temperature distribution) to that measured with HFMs and surface temperature sensors are used to be compared, as shown in Fig.134a. In addition to conductivity, U-value of the small part of the wall is also used to be compared with that measured with HFMs. Fig.13b shows the setting of measuring large part of the wall. Based on Eq. (3-7), the overall U-value of the small or large part of the wall can be calculated by: U = εσ ((T ref + 273) 4 (T s,in + 273) 4 ) + h c (T in T s,in ) T in T out (6 7) 15

20 Fig.13a measurement of small part Fig.13b measurement of big part In this report, convective heat transfer coefficient, conductivity and U-value are all calculated as slopes of zero-crossing linear fitting lines where numerous heat flux values are over the corresponding temperature differences according Eq. (6-4), (6-5), (6-7), respectively. For each regression line, coefficient of determination (R 2 ) is calculated to indicate how well data fits the model. 5.6 Sensitivity method The partial derivatives f x i are often called sensitivity coefficients, describing how the output estimate y varies with changes in the values of the input estimates x 1, x 2,, x N. The corresponding variation u(y i ) of the standard uncertainty of the estimate x i is f x i u(x i ). In addition, the combined variance u c 2 (y) can be viewed as a sum of terms, each of which represents the estimated variance associated with the output estimate y. This can be written as: (JCGM 100: 2008) u 2 c (y) = 4 i=1 )u(x i ) 2 ) (6-8) 16 ( y 2 x i In this project, the output of calculated conductive heat flux q has five independent inputs, which are the surface emissivity, the convective heat transfer coefficient, the interior surface temperature the reflected temperature and the indoor ambient temperature. The sensitivity coefficient of each input are expressed as: q q q = σ(t 4 ε ref T 4 s,in ) (6-9) h c = T s,in T in (6-10) T s,in = 4σε(T s,in + 273) 3 h c (6-11)

21 q 3 = 4σεT T ref (6-12) ref q T in = h c (6-13) 17

22 convective heat flux/(w*m 2 ) 6 Results The northern, eastern and western exterior walls as well as the ceiling are analyzed. The values in the figures were all measured without strong solar radiation. In this report, the northern and eastern exterior walls were firstly tested in order to be verified with results using HFMs and temperature sensors. For these two walls, convective heat transfer coefficient, conductivity, U- value were all measured. Then, the IR thermography were further applied in the measurement of U-values of the western exterior wall and the ceiling. Therefore, for the western exterior wall and the ceiling, convective heat transfer coefficient and U-value were both measured 6.1 The northern exterior wall Convective heat transfer coefficient Fig.14 shows the convective heat transfer over the temperature difference between the indoor ambient temperature and the interior surface temperature y = x R²= temperature difference/ Fig.14 convective heat flux vs temperature difference From the figure, convective heat transfer coefficient is around 2.6 W/(m 2 *K) and the R 2 is

23 conductive heat flux/(w*m 2 ) calculated convective heat flux/(w*m 2 ) Conductivity Fig.15a shows the calculated conductive heat transfer with IR thermography over the temperature difference between the interior surface and the exterior surface y = x R²= temperature difference/ Fig.15a calculated conduction vs temperature difference From the figure, it can be seen that the slope is 0.72 W/(m 2 *K) and the R 2 is After multiplying with the thickness, the conductivity can be calculated, which is 0.1 W/(m*K). Fig.15b shows the conductive heat transfer from HFMs over the temperature difference between the interior surface and the exterior surface y = x R²= temperature difference/ Fig.15b conductive heat flux with HFMs vs temperature difference As seen in the figure, the slope is 0.71W/(m 2 *K) and the R 2 is After multiplying with the thickness, the conductivity can be calculated, which is 0.1 W/(m*K). 19

24 calculated conductive heat flux/(w*m 2 ) calculated conductive heat flux/(w*m 2 ) U-value Fig.16a and Fig.16b show the calculated conductive heat transfer from the big or small part of the wall over the temperature difference between the indoor ambient temperature and the outdoor ambient temperature, respectively y = x R²= temperature difference/ Fig.16a calculated conductive heat flux of big part vs temperature difference y = 0.614x R²= temperature difference/ Fig.16b calculated conductive heat flux of big part vs temperature difference From the figures, it can be seen that U-value of the big part of the wall is around 0.66 W/(m 2 *K) while that of the small part of the wall is around 0.61W/(m 2 *K). The R 2 of both figures are around 0.5. Fig.16c shows the conductive heat transfer using HFMs over the temperature difference between the indoor ambient temperature and the outdoor ambient temperature 20

25 convective heat flux/(w*m 2 ) conductive heat flux/(w*m 2 ) y = x R²= temperature difference/ Fig.16c conductive heat flux with HFM vs temperature difference As seen in the figure, the U-value using HFMs is around 0.6W/(m 2 *K) and the R 2 is The eastern exterior wall Convective heat transfer coefficient Fig.17 shows the convective heat transfer over the temperature difference between the indoor ambient temperature and the interior surface temperature y = x R²= temperature difference/ Fig.17 convective heat flux vs temperature difference From the figure, convective heat transfer coefficient is around 2.5 W/(m 2 *K) and the R 2 is

26 conductive heat flux/ (W*m 2 ) calculated conductive heat flux/(w*m 2 ) Conductivity Fig.18a shows the calculated conductive heat transfer with IR thermography over the temperature difference between the interior surface and the exterior surface y = x R²= temperature difference/ Fig.18a calculated conduction vs temperature difference From the figure, it can be seen that the slope is W/(m 2 *K) and the R 2 is only After multiplying with the thickness, the conductivity can be calculated, which is 0.11 W/(m*K). Fig.18b shows the conductive heat transfer using HFMs over the temperature difference between the interior surface and the exterior surface y = x R²= temperature difference/ Fig.18b conductive heat flux with HFMs vs temperature difference As seen in the figure, the slope is 0.6 W/(m 2 *K) and the R 2 is only After multiplying with the thickness, the conductivity can be calculated, which is 0.1 W/(m*K). 22

27 calculated conductive heat flux/(w*m 2 ) calcualted conductive heat flux/(w*m 2 ) U-value Fig.19a and Fig.19b show the calculated conductive heat transfer from the big or small part of the wall over the temperature difference between the indoor ambient temperature and the outdoor ambient temperature, respectively y = x R²= temperature difference/ Fig.19a calculated conductive heat flux of big part vs temperature difference y = 0.581x R²= temperature difference/ Fig.19b calculated conductive heat flux of small part vs temperature difference From the figures, it can be seen that the U-value of the big part of the wall is around 0.68W/(m 2 *K) while that of the small part of the wall is around 0.58W/(m 2 *K). R 2 of both figures are around 0.5. Fig.19c shows the conductive heat transfer using HFMs over the temperature difference between the indoor ambient temperature and the outdoor ambient temperature 23

28 convective heat flux/(w*m 2 ) conductive heat flux/(w*m 2 ) y = x R²= temperature difference/ Fig.19c conductive heat flux with HFMs vs temperature difference As seen in the figure, the U-value using HFMs is around 0.55W/(m 2 *K) and the R 2 is The western exterior wall Convective heat transfer coefficient Fig.20 shows the convective heat transfer over the temperature difference between the indoor ambient temperature and the interior surface temperature y = x R²= temperature difference/ Fig.20 convective heat flux vs temperature difference From the figure, convective heat transfer coefficient is around 2.82 W/(m 2 *K) and the R 2 is 24

29 calculated conductive heat flux/(w*m 2 ) calculated conductive heat vlux/(w*m 2 ) U-value Fig.21a and Fig.21b show the calculated conductive heat transfer from the big or small part of the wall over the temperature difference between the indoor ambient temperature and the outdoor ambient temperature, respectively y = x R²= temperature difference/ Fig.21a calculated conductive heat flux of big part vs temperature difference y = x R²= temperature difference/ Fig.21b calculated conductive heat flux of small part vs temperature difference From the figures, it can be seen that the U-value of the big part of the wall is around 0.54W/(m 2 *K) while that of the small part of the wall is around 0.50W/(m 2 *K). The R 2 of the Fig.21a is 0.19 and that of the Fig.21b is

30 convective hat flux/(w*m 2 ) 6.4 The ceiling Convective heat transfer coefficient Fig.22 shows the convective heat transfer over the temperature difference between the indoor ambient temperature and the interior surface temperature y = x R²= temperature difference/ Fig.22 convective heat flux vs temperature difference From the figure, convective heat transfer coefficient is around 2.76 W/(m 2 *K) while the R 2 is U-value Fig.23a and Fig.23b show the calculated conductive heat transfer from the big or small part of the wall over the temperature difference between the indoor ambient temperature and the outdoor ambient temperature, respectively. 26

31 calculated conductive heat flux/(w*m 2 ) calculated conductive heat flux/(w*m 2 ) y = x R²= temperature difference/ Fig.23a calculated conductive heat flux of big part vs temperature difference y = x R²= temperature difference/ Fig.23b calculated conductive heat flux of small part vs temperature difference From the figures, it can be seen that the U-value of the big part of the wall is around 0.084W/(m 2 *K) while that of the small part of the wall is around 0.066W/(m 2 *K). The R 2 of the Fig.23a is and that of the Fig.23b is The on-site data and the reference values (Hüttemann, 2007) are given in Table 1. 27

32 Building envelope Table 1 Conductivity and U-values with IR thermography and HFM during winter and spring convective heat transfer coefficient/ (W/m 2 *K) Conducti vity Referenc e/(w/m* K) Conductivity/(W/m*K) U-value(W/m 2 *K) IR thermo graphy HFM Diffe rence IR thermography Big Small HFM differe nce northern wall % eastern wall % % western wall ceiling * Source: huettemann, Sensitivity analysis Here is the sensitivity analysis of the wooden walls for the heat flux q within 15±0.5 W/m 2, the sensitivity coefficient was 4.6, 4.1, -7.72, 5.13 and 2.63, respectively. In addition, for combined variance analysis, the output of y is the conductive heat flux q while inputs x represents the surface emissivity, the convective heat transfer coefficient, the reflected temperature, the surface temperature and the indoor ambient temperature. Since the surface emissivity were set constant and the variance of the CHTC was merely 0.01, for q within 15±0.5 W/m 2, u c 2 (q)was only correlated with the reflected temperature and the interior surface temperature. The relative contribution of the interior surface temperature and the reflected temperature tou c 2 (q), expressed as( q x i u(x i ) u c (q) )2, was 66% and 34%, respectively. 28

33 7 Discussion One of the biggest advantages of measuring U-value with IR thermography is that it is the overall U-value of a certain element since the average temperature of the area captured by the camera is applied into U-value calculation. Unlike HFMs, it can be applied to rough surfaces as well. This report features on-site measurements. Due to outdoor conditions in Ostersund, the unsteady-state conditions are inevitable. Besides, it is assumed that the thermal properties of the wooden walls and the ceiling are constant. Under such assumption, the heat balance is only related with radiation, heat convection between the target element and conduction between the interior and the exterior surface. By comparing the conductivity of the northern wall and the eastern wall with the HFMs and the thermal camera, respectively, it is found that the both results around 0.1W/(m*K) and the reference value is 0.13W/(m*K). In addition, the relative difference of results of U-value of the northern wall and the eastern wall measured with the HFMs and the thermal camera is 1.67% and 5.45%, respectively. These results illustrate that the IR thermography is applicable to some extent. However, some factors could still affect the accuracy of results. For instance, the accuracy of the apparatus (HFM, temperature sensor and IR camera). What is more, the heat lag of the material, heat convective heat transfer coefficient and calculation of radiation might also contribute to some unacceptable values. Because of heat lag of the material, the heat fluxes measured from indoor or outdoor might be different from each other. Apart from the heat lag, the convective heat transfer coefficient and the calculated radiation will be discussed in detail. Convective heat transfer Coefficient (CHTC) Convective heat transfer has impacts on performance of building components and affects heating and cooling load. However, in the overview of earlier studies on convective heat transfer coefficient (CHTC), significant differences between the existing CHTC correlations have been found (Thijs Defraeye et al., 2010). Therefore, these correlations are only applicable under specific conditions. It is suggested that the CHTC is measured in-situ and simultaneously with the measurement of the other input quantities (Ohlsson and Olofsson, 2014). The fundamental non-dimensional quantities describing forced convection are Nusselt Number (Nu), Prandtl number (Pr) and Reynolds number (Re). These three dimensionless groups can be related together with the following equation (de Dear Richard J et al., 1997): Nu = KPr a Re b (8-1) Therefore, the CHTC correlations have been determined in the past using wind tunnel experiments with the above equation, full-scale experiments on buildings and numerical simulations. In this project, the average CHTC used in calculating convection between the wooden wall and the indoor ambient air is 2.63W/(m 2 *K) and that used between the ceiling and the indoor ambient air is 2.76W/(m 2 *K) despite with negative R 2.Since this project was conducted under a small, single-story and rectangular building, previous research results under similar conditions are expected to be compared with. Yazdanian and Klems got a 29

34 correlations of CHTC: CHTC = (0.84 T 1 2 3) + (2.38U ) 2 (8-2) where T is the temperature difference between the environment and the exterior surface, U10 is the wind speed at a height of 10m above the ground. Zhang et al. also studied the CHTC of a small building (3x3x3m). They got the correlation of CHTC as a function of Us, which is the wind speed near the building surface: CHTC = U s U s (8-3) Given the CHTC is 2.63W/(m 2 *K), the Us is calculated as 5.96 m/s. That value is higher than the expected wind speed near the interior wall surface in this project. However, compared the CHTC used in Ohlsson and Olofsson s paper (2014), the CHTC of this project is relatively much too low. Calculated radiation In this report, the radiation is calculated with the reflected temperature and the surface temperature. However, it has not been fully explained why the U-value of the ceiling is so low. When analyzing the U-value of the ceiling, it was found that the heat flux transferred from the exterior surface into the interior surface. However, the indoor temperature was much higher than the outdoor temperature, which means that the convective heat flux transferred from the interior surface to the exterior surface. Therefore, the net radiation was toward inside and ought to be higher than the convection and sometimes larger than the convective heat flux. That might be caused by the low temperature of the ground. Since the ground is opposite to the ceiling and with lower temperature, the net radiation is from the ceiling towards the ground. However, more investigations will be required to explain such low U-value of the ceiling. 30

35 8 Conclusion In this project, the applicability of IR thermography in measuring the U-value of the building elements is verified by the in-situ measurement according to ISO From the results, the U- value of the northern, eastern and western wooden wall as well as the ceiling with IR thermography was 0.66W/(m 2 *K), 0.68W/(m 2 *K), 0.54W/(m 2 *K) and 0.084W/(m 2 *K), respectively. The conductivity of the wooden walls with IR thermography was 0.1W/(m*K). The calculation method of the radiative heat flux and the convective heat flux under non-steady state needs further research to guarantee the accuracy when measuring U-value. Besides, the convenience of measuring the U-value with IR thermography also needs further investigations. 31

36 9 Further research During the full-scale measurement with IR thermography, the surface emissivity and the reflected temperature are both of great importance. However, since the reflected temperature is changing, the analysis of the U-value needs to manually change the reflected temperature every time the conditions are changing. Therefore, further research is expected to improve the IR thermography by realizing the auto-generation of reflected temperature. Last but not least, the project is under non-steady state. Though it has been conducted over a long period, it is still recommended to use the dynamic algorithm to verify the results. 32

37 References Chapter 4: Heat Transfer. In: ASHRAE Fundamental Handbook de Dear, Richard J., et al. "Convective and radiative heat transfer coefficients for individual human body segments." International Journal of Biometeorology40.3 (1997): Defraeye, Thijs, Bert Blocken, and Jan Carmeliet. "Convective heat transfer coefficients for exterior building surfaces: Existing correlations and CFD modelling." Energy Conversion and Management 52.1 (2011): Energy Efficiency Directive, 2015, Energy, European Commission. [online] Available at: [Accessed 27 April, 2015] Evaluation of measurement data-guide to the expression of uncertainty in measurement. [2008]JCGM 100 Fokaides PA., Kalogirou SA., Application of infrared thermography for the determination of the overall heat transfer coefficient (U-Value) in building envelope. Applied Energy 88(2014): Fox Matthew, et al Time-lapse thermography for building defect detection. Energy and Buildings 92 (2015): GLULAM TIMBER ELEMENTS, [pdf] Huttemann. Available at: pdf [Accessed 30 March 2015] Heating and cooling, Department of Energy,.[online]Available at: [Accessed 27 April 2015] Loglock R. [pdf] GLULAM. Available at: [Accessed 30 March 2015] Ohlsson K.E.A., Olofsson T Quantitative infrared thermography imaging of the density of heat flow rate through a building element surface. Applied Energy: 134(2014): User s Manual, Flir ix series, FLIR R Renewable Energy Policy in the EU, 2012, European Renewable Energy Council.[online] Available at: [Accessed 27 April 2015] 33

38 Thermal Insulation-Building elements-in-situ measurement of thermal resistance and thermal transmittance. [2014] ISO Thermal Insulation-Physical quantities and definitions. [1987] ISO-7345 Thermal Insulation-Determination of steady-state thermal transmission properties-calibrated and guarded hot box. [1994] ISO-8990 Yiting Dong, 2013, Investigations on China s building energy consumption.[pdf] China Center for Energy and Development, Peking University. Available at: [Accessed 27 April 2015] 34

39 Acknowledgement This project was carried out under PhD Itai Danielski s instructions. Firstly, great thanks for his help and support throughout the whole project. Besides, thanks to all my friends who helped me a lot from mid-january until the beginning of April. Last but not least, thanks to Linda, the coordinator of this project course. It was her that helped me improve my final presentation a lot. Without all of your help, it was impossible for me to finish this project. 35