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Assessment of thermal performance of different types of masonry bricks used in Saudi Arabia
Applied Thermal Engineering 29 (2009) 1123–1130
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Assessment of thermal performance of different types of masonry bricks used in Saudi Arabia Luai M. Al-Hadhrami *, A. Ahmad Center for Engineering Research, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e
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Article history: Received 2 October 2007 Accepted 2 June 2008 Available online 13 June 2008 Keywords: Thermal conductivity Thermal resistance Building materials Masonry bricks Red clay brick Concrete brick
a b s t r a c t This paper presents the assessment of thermal performance of nine types of clay brick and two types of concrete brick in use in Saudi Arabia. The results are based on the experimentally measured values of equivalent thermal conductivity. The analysis of the measured data showed that the addition of insulation material either within the masonry brick mix to make the brick more lightweight or through filling insulation material into the holes of masonry bricks increases the thermal resistance significantly. Red clay brick were found more thermally effective than concrete brick. Use of insulating mortar in building walls increases the thermal resistance compared to that of the walls prepared with ordinary concrete mortar. An economic analysis shows that the insulated clay brick (type 7) is the most effective among the types of brick studied as it has the lowest net present value (NPV) of 36.9 $/m2. The load due to thermal transmission through a building envelope (walls and roof) constitutes an appreciable percent a building’s total thermal load. Therefore, long-term studies of the thermal performance of building materials should be conducted to establish thermally optimal brick for usage in building construction in the Kingdom of Saudi Arabia. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Energy/electricity requirements are increasing in line with population growth and globalization effects. Electricity generation facility development is not in pace with demand. So, efforts are being made world-wide to conserve energy resources and optimize the utilization of energy/electricity. At present, the Kingdom of Saudi Arabia has adequate energy/electricity capacity. Efforts are being directed toward optimization of energy resources. However, this is recognized as a good step and should be welcomed and encouraged. One such direction is to use thermally efficient building materials in the construction industry so as to minimize the consumption of electricity. About three-quarters of the total electricity sold in Saudi Arabia is consumed in buildings [1]. Available data show that the total energy consumption reached about 145,469 GWh in 2004. The annual increase in energy consumption averaged 6.2% between 2000 and 2004. Air-conditioning requires about 73% of total electricity consumed in residential buildings according to another of our studies [2]. In Saudi Arabia the environment is hot and in some places also humid, thus, knowledge of the thermal properties of building materials is essential in building design and in selecting air-conditioning systems [3,4]. The load due to thermal transmission through a building envelope (walls and roof) * Corresponding author. Tel.: +966 3 860 2888; fax: +966 3 860 3996. E-mail address: [email protected] (L.M. Al-Hadhrami). 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.06.003
constitutes an appreciable percent of the total thermal load. Proper selection of building materials could reduce considerably this thermal transmission load. Accordingly, experimental and numerical work to study the ways of reducing heat transfer through brick/ wall assemblies is of interest [5–9]. Recently, del Coz Diaz et al. [5] carried out an experimental and numerical (finite element) study to investigate the thermal transmittance coefficient, U, of a wall made of Arliblock bricks. The transmittance ‘V’ provides information of the thermal insulation of an enclosure. Numerical results were compared with experimental ones and a very good agreement was reported. The findings show that wall insulation decreases with corresponding increase in the mortar and material conductivities. The temperature distribution depends strongly on the mortar conductivity. Changing the profiles of hollow bricks alters the rate of the heat transfer through the bricks. Al-Hazmy [6] considered heat transfer through a common hollow building brick. The insulation assessment of the building blocks was examined based upon the heat transfer rate. Three different configurations for building bricks were studied including a gas-filled and insulation-filled cavity. Results showed that the cellular air motion inside bricks cavities contributes significantly to the heat loads. Insertion of polystyrene bars reduced the heat transfer rate by a maximum of 36%. Using hollow polystyrene bars reduced the heat transfer rate by only 6% due to the air motion inside the cells. If the varying temperature is considered, then the use of polystyrene bars may reduce the heat transfer rate by 25%.
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Nomenclature A dds dts e i I kds keqv,ts keqv n N NPV
main heater surface area (m2) thickness of dummy sample (m) thickness of test sample (m) energy inflation rate (%) discount rate (%) heater current (A) thermal conductivity of dummy sample (W/m K) equivalent thermal conductivity of test sample (W/m K) equivalent thermal conductivity of the brick (W/m K) lifetime of the brick (yr) correction factor of the equipment net present value ($/m2)
Antar and Thomas [7,8] addressed the heat transfer across a hollow building brick and showed the effects of two-dimensional heat transfer estimation across the brick. The transfer of heat by conduction throughout the ceramic block by convection and radiation through the cavity were considered. The authors demonstrated a numerical finite-difference analysis for steady-state heat transfer in a composite wall with a two-dimensional rectangular gray body radiating cavity with and without natural convection circulation of air. The primary purpose of this analysis was to provide a basis for evaluating the accuracy of the first-order twodimensional method for modeling the effects of radiation in practical applications of this type. Recently, Antar [9] investigated the significance of multidimensional effects in estimating the rate of heat loss and identified cases in which simple one-dimensional convection/radiation analysis may be adequate. It was reported that dividing the single slot into two slots having the same total width as well as decreasing the wall surface emissivity result in a significant increase in the thermal resistance of the whole brick. Choi et al. [10] measured the effect of composite insulation thickness on thermal properties by using a Lambda heat flow meter and performing quantitative evaluation on the measurement precision and uncertainty of composite materials. Thermal resistance changed from 3% to 4% when the thickness increased by about four times, this was attributed to heat loss from the side. The results showed a difference of 3% between the theoretical value and the actual value for three types of composite insulation whereas a 5% difference was observed for four other types of composite insulation. Studies involving thermal properties of building materials in use in Saudi Arabia have mainly concerned measurement of thermal conductivity of locally manufactured thermal insulation [11– 13] using different measurement techniques. Testing of bricks for thermal conductivity is less commonly reported. Abdelrahman et al. [3] measured thermal conductivity of some of the commonly used bricks manufactured locally in Saudi Arabia using a guarded hot plate. The thermal conductivity measurements were conducted at 40 °C mean test temperature. The results showed that the measured values lie on the higher side of those reported in handbooks (mainly at mean temperature of 24 °C). Three basic mechanisms affect heat transfer through hollow bricks. These are solid conduction through the materials forming the brick, radiation transfer through the voids and convective transfer in and through the voids. The three interact to affect heat transfer. Because of that, in such a case, an equivalent thermal conductivity (Keqv) is determined for the brick samples. An appropriate selection of masonry products for walls and roof reduce the energy consumption of air-conditioning. To achieve this
Q Qds Qts Th,ds Tc,ds Th,ts Tc,ts V ds ts
heat flow through the main heater (W) heat flow through dummy sample (W) heat flow through dummy sample (W) hot side surface temperature of dummy sample (°C) cold side surface temperature of dummy sample (°C) hot side surface temperature of test sample (°C) cold side surface temperature of test sample (°C) heater voltage (V) Subscript for dummy sample for test sample
objective, the thermal properties of these materials have to be known. Hence, the purpose of the present study is to assess the thermal performance of locally manufactured masonry bricks for the local environmental conditions. The assessment includes studying the effects of geometry, type and method of use of thermal insulation, and mortar types on equivalent thermal conductivity (Keqv) and thermal resistance (R) of the bricks. These kinds of data are not widely available and will be useful as a basis for selection of brick types for buildings in Saudi Arabia. 2. Test method and sample preparation The equivalent thermal conductivity values presented in this paper were measured under steady-state heat flow conditions using a guarded hot plate that conforms to ASTM Standard C 177-85 [14,15]. The Guarded Hotplate thermal conductivity measuring equipment (Dynatech, model TCFG-R4-6) is suitable for testing masonry building materials – clay brick, concrete brick, solid concrete, gypsum, pozzolan, composites, etc. and insulating building materials – extruded polystyrene, rock wool, perlite, vermiculite, rubber, plastic, etc. The accuracy of the Dynatech test equipment is about ±4% of the true value of the thermal conductivity. The guarded hotplate thermal conductance measuring system requires two samples (different or identical) of maximum 15 cm thickness. Since samples had more than this thickness, a thin calibrated dummy sample was used on the upper side of the plate in order to accommodate test samples of larger thickness on the lower side of the plate. The types of wall samples tested are shown in Figs. 1-1, 1-2,1-3. The test wall samples of 61 cm 61 cm were prepared in the same way as they are assembled on walls. During the tests, the average temperature of the samples was kept at about 35.0 ± 2 °C with the cold surface maintained at about 25 °C. These are realistic values for the outside and inside weather conditions in Saudi Arabia. The surfaces of the samples must be flat and parallel to minimize contact resistance between these two surfaces and the corresponding hot and cold plate surfaces. Due to the rough surfaces of the samples it was not possible to get the flat and parallel surfaces. An uneven surface would result in a significant temperature difference between the hot plate and the corresponding sample surface. To overcome this problem, the thermocouple wires were fixed on both sides of the sample and the surface temperatures were monitored by a Campbell Scientific 21X automatic data acquisition system. The samples were covered with a blanket on both sides to have smooth contact with the plate surfaces. The sample temperatures were monitored till steady-state conditions were obtained.
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2.1. Calculation of equivalent thermal conductivity The test sample position on the guarded hot plate is shown in Fig. 2. The following equations were used for calculation of equivalent thermal conductivity of the samples. The power input at steady-state condition, Q = N V I. The heat flow, Qds, through the dummy sample was calculated by
Q ds ¼
kds AðT h;ds T c;ds Þ : dds
The heat flow, Qts, through the test sample was calculated by
Q ts ¼ Q Q ds ¼ NVI
kds AðT h;ds T c;ds Þ : dds
The equivalent thermal conductivity, keqv,ts, for the test sample was calculated by
h keqv;ts ¼
NVI
Fig. 1-1. Tested red clay brick types 1–4.
kds AðT h;ds T c;ds Þ dds
i
AðT h;ts T c;ts Þ
dts :
2.2. Uncertainty in equivalent thermal conductivity Taking into the consideration the measured values, which have significant uncertainty, the equivalent thermal conductivity is a function of
keqv;ts ¼ f ðV; I; kds ; T h;ds ; T c;ds ; T h;ts ; T c;ts ; dds ; dts Þ: The standard uncertainty in the equivalent thermal conductivity, neglecting the covariance, is calculated using the following equation [16]:
ðU c;keqv;ts Þ2 ¼
Fig. 1-2. Tested red clay brick types 5–9.
2 2 2 okeqv;ts okeqv;ts okqv;ts ukds uV þ uI þ oV oI okds 2 2 2 okeqv;ts okeqv;ts okeqv;ts þ uT h;ds þ uT c;ds þ uT h;ts oT h;ds oT c;ds oT h;ts 2 2 2 okeqv;ts okeqv;ts okeqv;ts þ uT c;ts þ udds þ udts : oT c;ts odds odts
Uncertainty propagation for the dependent variable in terms of the measured values is calculated using the engineering equation solver (EES) software [16]. The measured variables x1, x2, etc. have a random variability that is referred to as its uncertainty; this uncertainty is displayed as a ± ux. The input to EES for calculating the uncertainty of a dependent variable is the magnitude (a) of the measured variable (x) and the uncertainty (ux) in each measured variable. The sample uncertainty results obtained using EES display the partial derivative of the calculated variable with respect to each measured variable and the percentage of the total uncertainty in the calculated variable resulting from the uncertainty in each measured variable as shown in Table 1. The estimated uncertainty in equivalent thermal conductivity lies in the range from ±4.65% to 5.65%. The total uncertainty (equipment + experimental) in equivalent thermal conductivity of the tested samples lies in the range from 6.13% to 6.92%. 3. Results and discussions
Fig. 1-3. Tested concrete brick types 10 and 11.
The equivalent thermal conductivity values obtained experimentally are used to calculate the thermal resistance. The thermal performance of these bricks is discussed in terms of mortar type, brick size, method and type of insulation used on thermal conductivity. Fig. 1 shows the different types of masonry
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Fig. 2. Sample position on the guarded hot plate.
Table 1 Sample uncertainty calculations of equivalent thermal conductivity Variable ± uncertainty Equivalent thermal conductivity, dds = 0.03021 ± 0.00018 dts = 0.199 ± 0.00047 I = 0.7265 ± 0.001 kds = 0.04001 ± 0.0012 Tc,ds = 29.46 ± 0.5 Tc,ts = 24.84 ± 0.5 Th,ds = 46.78 ± 0.5 Th,ts = 43.83 ± 0.5 V = 7.864 ± 0.005
Partial derivative Keqv,ts = 0.382 ± 0.01883 okeqv,ts/odds = 7.955 okeqv,ts/odts = 1.921 okeqv,ts/oI = 0.8571 o keqv,ts/okds = 6.007 okeqv,ts/oTc,ds = 0.01388 okeqv,ts/odc,ts = 0.02014 okeqv,ts/oTh,ds = 0.01388 okeqv,ts/oTh,ts = 0.02014 okeqv,ts/oV = 0.07918
% of uncertainty 0.58 0.23 0.21 14.65 13.57 28.57 13.57 28.57 0.04
brick tested for assessment of thermal performance in the present study. Table 2 summarizes the types of masonry brick wall samples tested. As listed in Table 2, nine types of clay brick and two types of concrete brick wall samples were tested and analyzed for thermal performance. The equivalent thermal conductivity of the tested red clay brick wall samples varies between a minimum of 0.262 and to a maximum of 0.389 W/ m K, Table 2. The brick sample, with holes filled with mineral wool insulation, prepared with insulating mortar (type 7) has the lowest equivalent thermal conductivity whereas the brick samples prepared with ordinary concrete mortar (type 8) and holes filled with perlite–cement mix have the highest equivalent thermal conductivity. The equivalent thermal conductivity for
the lightweight concrete brick sample (type 11) is 0.489 W/m K whereas for the normal hollow concrete brick sample (type 10) it is 0.976 W/m K. The concrete brick samples were prepared with ordinary concrete mortar. The thermal resistance for normal, lightweight and insulated red clay bricks; and the normal and insulated concrete bricks is compared in Fig. 3. The thermal resistance of normal clay bricks is of type 1 while for lightweight and insulated clay bricks it is the average of types 2 and 3 and types 4 and 9, respectively. It can be seen from Fig. 3 that the lightweight and insulated clay bricks have about 12.7% and 6.3% higher thermal resistance as compared to the normal clay brick, respectively. However, the insulated concrete brick showed an increase of thermal resistance of about 99.5% over the uninsulated. Among the types of bricks tested, the lightweight clay brick has the highest thermal resistance. The thermal resistance of different types of clay bricks is compared in terms of percent increase relative to normal concrete brick (type 10) and the results are presented in Fig. 4. The normal, lightweight and insulated clay bricks have 155.6%, 188.0% and 171.7%, higher thermal resistance as compared to normal concrete brick, respectively. However, making the normal concrete brick lightweight resulted in an almost 100% increase in thermal resistance. The results show that the clay bricks are more effective than concrete brick. However, the thermal performance of lightweight concrete brick is comparable to that of the insulated and lightweight clay bricks.
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L.M. Al-Hadhrami, A. Ahmad / Applied Thermal Engineering 29 (2009) 1123–1130 Table 2 Equivalent thermal conductivity and thermal resistance values for 20 cm thick red clay and concrete brick samples Sample typea
Description
Equivalent thermal conductivityb (W/m K)
Thermal resistance (m2 K/W)
Type Type Type Type Type Type Type Type Type Type Type
Normal hollow red clay brick, ordinary concrete mortar Type 1 with body added polystyrene insulation Type 1 with body added perlite insulation Type 1 with center holes filled w/polystyrene insulation Normal hollow red clay brick, insulating mortarc Type 5 with center hole filled w/perlite + cement mix Type 5 with center hole filled w/mineral wool insulation Type 5 with center hole filled w/perlite + cement mix, Ordinary concrete mortar Type 5 with center hole filled w/mineral wool, ordinary concrete mortar Normal hollow concrete brick, ordinary concrete mortar Type 10 with body added perlite insulation
0.382 0.330 0.348 0.339 0.347 0.316 0.262 0.389 0.382 0.976 0.489
0.524 0.606 0.575 0.590 0.576 0.633 0.763 0.514 0.524 0.205 0.409
a b c
1 (clay) 2 (clay) 3 (clay) 4 (clay) 5 (clay) 6 (clay) 7 (clay) 8 (clay) 9 (clay) 10 (concrete) 11 (concrete)
See Fig. 1 for graphical description of different types of bricks. Uncertainty lies in the range from 6.13% to 6.92%. Samples were prepared with insulating mortar.
Fig. 3. Comparison of average thermal resistance for different types of bricks.
Fig. 5. Effect of body-addition of insulation material on thermal resistance of the tested brick samples.
Fig. 4. Percent increase in thermal resistance of bricks compared to normal concrete brick (type 10).
3.1. Effect of thermal insulation and method of use The recent trend in the brick manufacturing industry shows an increased use of thermal insulation both as mixing insulation material with the brick mix (or body-addition of insulation) and/ or filling of holes with insulation material. The purpose of mixing the insulation material with the brick mix is to make the bricks lightweight and more thermally resistive as compared to normal bricks. Some manufacturers are producing bricks with insulation inserted in the holes as well as adding the insulation material with the brick mix.
The effect of body-addition of insulation material on thermal resistance of the tested brick samples was studied and the test results are presented graphically in Fig. 5. It can be observed from the figure that the thermal resistance of the body-mixed thermal insulation clay brick samples (types 2 and 3) increased by as much as 9.7–15.6% compared to that of normal clay brick sample (type 1). In other words, the thermal resistance shows an increasing trend with the body-addition of insulating material compared to the normal bricks. The thermal resistance of body added polystyrene mixed concrete brick sample (type 11) increased by about 99.5% compared to that of the normal concrete brick sample (type 10), as shown in Fig. 5. This shows that the body-addition of insulating materials causes significant increase in thermal resistance of concrete block samples whereas the increase in thermal resistance for clay brick samples is moderate. The effect of filling of thermal insulating material into the holes of the brick is presented graphically in Fig. 6. The center holes of the brick types 6 and 7 were filled with perlite–cement mix and mineral wool insulating material, respectively. It was observed that the thermal resistance increased approximately by 10% for the brick samples with holes filled with perlite–cement mix (type 6) and about 32% for the brick samples with holes filled with mineral wool insulation inserts (type 7) as compared to that of the normal hollow brick samples (type 5). This shows that the type of insulating material filled into the holes of the brick also affects the thermal resistance of the bricks. In order of preference, bricks
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Fig. 6. Effect of filling of insulation material into the holes of the brick on thermal resistance of the tested brick samples.
filled with mineral wool insulating material are more thermally resistant than the perlite–cement mix filled bricks. The thermal resistance of brick samples prepared by mixing the insulation material with the brick mix (body-mixed insulation) was found to be lower than that of the bricks having holes filled with insulating materials (perlite–cement mix or mineral wool inserts), as observed in Figs. 5 and 6. In selecting bricks, strength characteristics as well as insulating properties should be considered. Bricks made with a body-mixed procedure have lower strength as a result of lower density, while bricks made with holes filled with insulation inserts are stronger. The other factors such as the water absorption and cost of the bricks should also be considered (see Section 3.4). High water absorption of bricks will adversely affect thermal performance. 3.2. Effect of mortar type The mortar used to join the bricks also affects the thermal performance of the wall assemblies. The mortar prepared with sand and cement is called ordinary concrete mortar and the mortar prepared with sand and cement along with insulating material is known as insulating mortar. Normally, ordinary concrete mortar is used for building walls and it constitutes approximately about 5–7% of the total surface area (as measured for the tested wall samples). The ordinary concrete mortar has high thermal conductivity compared to the masonry bricks. The mortar joints are continuous and act as thermal bridges and hence affect the thermal performance of all brick types. The results show that thermal resistance of the wall samples prepared with insulating mortar increased by about 23–46% compared to that of the samples prepared with ordinary concrete mortar, as shown in Fig. 7. The holes of brick of sample types 6 and 8 were filled with perlite–cement material while in case of types 7 and 9 they were filled with mineral wool insulation. The above variation in thermal resistance is due to the combined effect of the type of filling material into the holes of the bricks and the type of the mortar used for sample preparation. 3.3. Effect of brick size The brick size also plays a role when the thermal performance of the walls is concerned. The smaller the brick size the higher will be the ratio of mortar to wall surface area, which in turn provides more thermal bridging for heat flow. The mortar used to join bricks acts as a thermal bridge across the wall thickness, which reduces the overall thermal resistance of the wall assemblies. The ordinary
Fig. 7. Effect of type of mortar on thermal resistance of the tested brick samples.
concrete mortar has a higher thermal conductivity as compared to the concrete or clay bricks. Therefore, use of smaller sized bricks will increase the mortar area of the wall and hence will affect the thermal performance of the wall assemblies. Therefore, use of either large sized bricks or insulating mortar will result in obtaining a higher thermal resistance. 3.4. Cost analysis A cost analysis has been carried for different types of brick samples considered in the present study. In order to calculate the simple back payback period for alternative brick wall materials in comparison to concrete brick (type 10), the electrical energy cost needs to be determined for different types of brick walls. Since the thermal load on a building is cumulative, the energy used (E) by the air-conditioning equipment to pump out the wall thermal transmission load is calculated by the following equation [17]:
E¼
0:024 D kWh=m2 ; RC
where D is the degree-days of the location (D = 2185); R is the thermal resistance of the brick sample, and C is the coefficient of thermal performance of the air-conditioning system (C = 2.16). These D and C values are taken from Ref. [17]. The total discounted cost (energy and brick) or a net present value (NPV) per square meter of brick surface area is calculated using the following equation:
NPV ¼ E PWF EC þ BC ð$=m2 Þ; where E is the annual energy consumption, EC is the energy cost (4 cents/kWh), PWF is the present worth factor and BC is the brick cost. The present worth factor, PWF, is calculated using the equation [18]
PWF ¼
n 1 1þe for e–i; 1 ie 1þi
PWF ¼ n ði þ eÞ for e ¼ i; where e is the energy inflation rate (3%); i is the discount rate (6%) and n is the lifetime of the brick (30 years). The cost analysis along with NPV for different types of brick is presented in Table 3. It can be seen from the table that the annual energy cost lies in the range from 1.27 (clay brick type 7) to 4.74 $/ m2 (concrete brick type 10). The simple payback period analysis is carried out to obtain the payback period for different types of brick
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L.M. Al-Hadhrami, A. Ahmad / Applied Thermal Engineering 29 (2009) 1123–1130 Table 3 Energy and cost analysis for the tested brick samples Sample type
Description
Annual energy (kWh/m2)
Annual energy cost ($/m2)
Annual energy savinga ($/m2)
Brick cost ($/m2)
Increase in brick costa ($/m2)
Simple pay back period (yr)
NPVb ($/m2) (n = 30 yrs)
Type Type Type Type Type Type Type Type Type Type Type
Normal Lightweight Lightweight Insulated Normalc Insulatedc Insulatedc Insulated Insulated Normal Lightweight
46.33 40.06 42.22 41.15 42.15 38.35 31.82 47.23 46.33 118.43 59.36
1.85 1.60 1.69 1.65 1.69 1.53 1.27 1.89 1.85 4.74 2.37
2.88 3.13 3.05 3.09 3.05 3.20 3.46 2.85 2.88 – 2.36
8.25 9.38 10.75 11.50 9.13 12.38 12.38 11.50 11.50 6.63 8.00
1.63 2.75 4.13 4.88 2.50 5.75 5.75 4.88 4.88 – 1.38
0.56 0.88 1.35 1.58 0.82 1.80 1.66 1.71 1.69 – 0.58
43.9 40.2 43.3 43.2 41.6 41.9 36.9 47.9 47.2 97.8 53.7
1 (clay) 2 (clay) 3 (clay) 4 (clay) 5 (clay) 6 (clay) 7 (clay) 8 (clay) 9 (clay) 10 (concrete) 11 (concrete)
a Energy saving and increase in brick cost are obtained with reference to normal concrete brick (type 10) values. Energy cost is taken as 4 cents/kWh. Also, the energy and cost values are normalized to brick wall surface area. b NPV = Net present value or the total discounted cost (discount rate, i = 6%, energy inflation rate, e = 3%) over the lifetime of the brick (n = 30 years). c Samples were prepared with insulating mortar.
as compared to concrete brick (type 10). The analysis shows that the clay brick (type 1) has the lowest payback period of 0.56 years while the clay brick (type 8) has highest payback period of 1.71 years. From simple payback analysis, it is difficult to say which brick is the most effective among the brick types studied. As the simple payback analysis gives only the recovery period of an additional investment, it ignores the influence of discount rate and cost escalation. Net present value analysis is the correct approach for economic analysis as it takes into account of the life period of the brick, time value of money (discount rate) and energy escalation cost. The NPV analysis shows that the insulated clay brick (type 7) is the most effective among the types of brick studied as it has the lowest NPV of 36.9 $/m2. The second most economical brick is the lightweight clay brick (type 2) with NPV of 40.2 $/m2. The concrete bricks are not suitable from an economic consideration point of view due to higher NPV as compared to the NPV of the clay brick types. 3.5. General observations The advantage of using normal brick over the lightweight brick is that the normal bricks are denser than the lightweight brick and are less susceptible to high moisture absorption. Also, the higher the moisture absorption by a brick the lower will be its thermal insulation value. The red clay brick can be used in buildings located in regions with humid climate whereas lightweight brick can be used in dry regions. The lightweight bricks have lower strength as a result of lower density while the normal bricks and bricks made with holes filled with insulation inserts are stronger (more dense). 4. Conclusions and recommendations In the present study, the thermal performance of nine types of red clay and two types of concrete bricks were studied. It is concluded that: 1. The maximum percentage increase in thermal resistance was found for the insulated red clay brick sample, prepared with insulating mortar, compared to the normal concrete brick. 2. The addition of insulation either by body-mixing or through filling insulation materials into the holes of the bricks increases the thermal resistance significantly. 3. The use of insulating mortar for sample preparation increases the thermal resistance significantly compared to the ordinary concrete mortar.
4. The method of insulation used to make the bricks also affects thermal performance of the bricks. As observed in the present study, the bricks insulated by using a body-mix method were found to have less thermal resistance than those insulated by inserting the insulating material into the holes of the bricks. 5. The size of the bricks also plays a minor role on the thermal performance of the bricks. This is dependent on the type and the amount of mortar used to construct the wall. 6. Care should be taken in selecting the bricks because the bricks made from body-mixed procedure to make them lightweight have lower strength as a result of lower density while bricks made with holes filled with insulation inserts are stronger (more dense). 7. An economic analysis shows that the insulated clay brick (type 7) is the most effective among the types of brick studied as it has the lowest net present value (NPV) of 36.9 $/m2. It is recommended that a long-term detailed study should be conducted to establish thermally optimal brick assemblies for usage in building construction in the Kingdom of Saudi Arabia. Acknowledgement The authors acknowledge the support of the Research Institute at King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia. References [1] Ministry of Industry and Electricity, Annual report for the year 2004, Information and Statistics Department, Saudi Electricity Company, Riyadh, Kingdom of Saudi Arabia, 2005. [2] M.A. Elhadidy, M. Ul-Haq, A. Ahmad, Electric energy consumption in selected residential buildings at KFUPM, Dhahran, Saudi Arabia, Proceedings of the Mediterranean Conference for Environment and Solar, Beirut, Lebanon, November 16–18, COMPLES 2000, vol. EX493, IEEE, 2001. [3] M.A. Abdelrahman, S.A.M. Said, A. Ahmad, et al., Thermal conductivity of some major building materials in Saudi Arabia, Journal of Thermal Insulation 13 (April) (1990) 294–300. [4] M.A. Abdelrahman, S.A.M. Said, A. Ahmad, A Comparison of energy consumption and cost-effectiveness of four masonry materials in Saudi Arabia, Energy, The International Journal 18 (11) (1993) 1181–1186. [5] J.J. del Coz Diaz, P.J. Garcia Nieto, A. Martin Rodriguez, A. Lozano MartinezLuengas, C. Betegon Biempica, Non-linear thermal analysis of light concrete hollow brick walls by the finite element method and experimental validation, Applied Thermal Engineering 26 (2006) 777–786. [6] M.M. Al-Hazmy, Analysis of coupled natural convection conduction effects on the heat transport through hollow building blocks, Energy and Buildings 38 (2006) 515–521. [7] M.A. Antar, L. C Thomas, Heat transfer through a composite wall with enclosed spaces: a practical two-dimensional analysis approach, ASHRAE Transactions 106 (2001) 318–324.
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Energy Conservation and Management in Buildings, King Fahd University of Petroleum & Minerals, 2002, pp. 159–173. Slaeh A. Al-Aijlan, Measurements of thermal properties of insulation materials by using transient plane source technique, Journal of Applied Thermal Engineering 26 (2006) 2184–2191. Dynatech, Guarded Hot-Plate Thermal Conductance Measuring System, Dynatech Model TCFG-R4-6, Dynatech R/D Company. ASTM Standard C-177-85, Standard test method for steady-state heat flux measurements and thermal transmission properties by means of the guarded hot plate apparatus, Annual Book of ASTM Standards, 4.06, 1985. Engineering equation solver (EES), version 7.966, 1992–2007. M.A. Abdelrahman, A. Ahmad, Cost-effective use of thermal insulation in hot climates, Building and Environment 26 (2) (1991) 104–189. S.A. Hejazi, C. Ferguson-Perim, A simplified technique for estimating economic thickness of insulation, ASHRAE Journal (1987) 20–38.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Assessment of thermal performance of different types of masonry bricks used in Saudi Arabia Luai M. Al-Hadhrami *, A. Ahmad Center for Engineering Research, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 2 October 2007 Accepted 2 June 2008 Available online 13 June 2008 Keywords: Thermal conductivity Thermal resistance Building materials Masonry bricks Red clay brick Concrete brick
a b s t r a c t This paper presents the assessment of thermal performance of nine types of clay brick and two types of concrete brick in use in Saudi Arabia. The results are based on the experimentally measured values of equivalent thermal conductivity. The analysis of the measured data showed that the addition of insulation material either within the masonry brick mix to make the brick more lightweight or through filling insulation material into the holes of masonry bricks increases the thermal resistance significantly. Red clay brick were found more thermally effective than concrete brick. Use of insulating mortar in building walls increases the thermal resistance compared to that of the walls prepared with ordinary concrete mortar. An economic analysis shows that the insulated clay brick (type 7) is the most effective among the types of brick studied as it has the lowest net present value (NPV) of 36.9 $/m2. The load due to thermal transmission through a building envelope (walls and roof) constitutes an appreciable percent a building’s total thermal load. Therefore, long-term studies of the thermal performance of building materials should be conducted to establish thermally optimal brick for usage in building construction in the Kingdom of Saudi Arabia. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Energy/electricity requirements are increasing in line with population growth and globalization effects. Electricity generation facility development is not in pace with demand. So, efforts are being made world-wide to conserve energy resources and optimize the utilization of energy/electricity. At present, the Kingdom of Saudi Arabia has adequate energy/electricity capacity. Efforts are being directed toward optimization of energy resources. However, this is recognized as a good step and should be welcomed and encouraged. One such direction is to use thermally efficient building materials in the construction industry so as to minimize the consumption of electricity. About three-quarters of the total electricity sold in Saudi Arabia is consumed in buildings [1]. Available data show that the total energy consumption reached about 145,469 GWh in 2004. The annual increase in energy consumption averaged 6.2% between 2000 and 2004. Air-conditioning requires about 73% of total electricity consumed in residential buildings according to another of our studies [2]. In Saudi Arabia the environment is hot and in some places also humid, thus, knowledge of the thermal properties of building materials is essential in building design and in selecting air-conditioning systems [3,4]. The load due to thermal transmission through a building envelope (walls and roof) * Corresponding author. Tel.: +966 3 860 2888; fax: +966 3 860 3996. E-mail address: [email protected] (L.M. Al-Hadhrami). 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.06.003
constitutes an appreciable percent of the total thermal load. Proper selection of building materials could reduce considerably this thermal transmission load. Accordingly, experimental and numerical work to study the ways of reducing heat transfer through brick/ wall assemblies is of interest [5–9]. Recently, del Coz Diaz et al. [5] carried out an experimental and numerical (finite element) study to investigate the thermal transmittance coefficient, U, of a wall made of Arliblock bricks. The transmittance ‘V’ provides information of the thermal insulation of an enclosure. Numerical results were compared with experimental ones and a very good agreement was reported. The findings show that wall insulation decreases with corresponding increase in the mortar and material conductivities. The temperature distribution depends strongly on the mortar conductivity. Changing the profiles of hollow bricks alters the rate of the heat transfer through the bricks. Al-Hazmy [6] considered heat transfer through a common hollow building brick. The insulation assessment of the building blocks was examined based upon the heat transfer rate. Three different configurations for building bricks were studied including a gas-filled and insulation-filled cavity. Results showed that the cellular air motion inside bricks cavities contributes significantly to the heat loads. Insertion of polystyrene bars reduced the heat transfer rate by a maximum of 36%. Using hollow polystyrene bars reduced the heat transfer rate by only 6% due to the air motion inside the cells. If the varying temperature is considered, then the use of polystyrene bars may reduce the heat transfer rate by 25%.
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Nomenclature A dds dts e i I kds keqv,ts keqv n N NPV
main heater surface area (m2) thickness of dummy sample (m) thickness of test sample (m) energy inflation rate (%) discount rate (%) heater current (A) thermal conductivity of dummy sample (W/m K) equivalent thermal conductivity of test sample (W/m K) equivalent thermal conductivity of the brick (W/m K) lifetime of the brick (yr) correction factor of the equipment net present value ($/m2)
Antar and Thomas [7,8] addressed the heat transfer across a hollow building brick and showed the effects of two-dimensional heat transfer estimation across the brick. The transfer of heat by conduction throughout the ceramic block by convection and radiation through the cavity were considered. The authors demonstrated a numerical finite-difference analysis for steady-state heat transfer in a composite wall with a two-dimensional rectangular gray body radiating cavity with and without natural convection circulation of air. The primary purpose of this analysis was to provide a basis for evaluating the accuracy of the first-order twodimensional method for modeling the effects of radiation in practical applications of this type. Recently, Antar [9] investigated the significance of multidimensional effects in estimating the rate of heat loss and identified cases in which simple one-dimensional convection/radiation analysis may be adequate. It was reported that dividing the single slot into two slots having the same total width as well as decreasing the wall surface emissivity result in a significant increase in the thermal resistance of the whole brick. Choi et al. [10] measured the effect of composite insulation thickness on thermal properties by using a Lambda heat flow meter and performing quantitative evaluation on the measurement precision and uncertainty of composite materials. Thermal resistance changed from 3% to 4% when the thickness increased by about four times, this was attributed to heat loss from the side. The results showed a difference of 3% between the theoretical value and the actual value for three types of composite insulation whereas a 5% difference was observed for four other types of composite insulation. Studies involving thermal properties of building materials in use in Saudi Arabia have mainly concerned measurement of thermal conductivity of locally manufactured thermal insulation [11– 13] using different measurement techniques. Testing of bricks for thermal conductivity is less commonly reported. Abdelrahman et al. [3] measured thermal conductivity of some of the commonly used bricks manufactured locally in Saudi Arabia using a guarded hot plate. The thermal conductivity measurements were conducted at 40 °C mean test temperature. The results showed that the measured values lie on the higher side of those reported in handbooks (mainly at mean temperature of 24 °C). Three basic mechanisms affect heat transfer through hollow bricks. These are solid conduction through the materials forming the brick, radiation transfer through the voids and convective transfer in and through the voids. The three interact to affect heat transfer. Because of that, in such a case, an equivalent thermal conductivity (Keqv) is determined for the brick samples. An appropriate selection of masonry products for walls and roof reduce the energy consumption of air-conditioning. To achieve this
Q Qds Qts Th,ds Tc,ds Th,ts Tc,ts V ds ts
heat flow through the main heater (W) heat flow through dummy sample (W) heat flow through dummy sample (W) hot side surface temperature of dummy sample (°C) cold side surface temperature of dummy sample (°C) hot side surface temperature of test sample (°C) cold side surface temperature of test sample (°C) heater voltage (V) Subscript for dummy sample for test sample
objective, the thermal properties of these materials have to be known. Hence, the purpose of the present study is to assess the thermal performance of locally manufactured masonry bricks for the local environmental conditions. The assessment includes studying the effects of geometry, type and method of use of thermal insulation, and mortar types on equivalent thermal conductivity (Keqv) and thermal resistance (R) of the bricks. These kinds of data are not widely available and will be useful as a basis for selection of brick types for buildings in Saudi Arabia. 2. Test method and sample preparation The equivalent thermal conductivity values presented in this paper were measured under steady-state heat flow conditions using a guarded hot plate that conforms to ASTM Standard C 177-85 [14,15]. The Guarded Hotplate thermal conductivity measuring equipment (Dynatech, model TCFG-R4-6) is suitable for testing masonry building materials – clay brick, concrete brick, solid concrete, gypsum, pozzolan, composites, etc. and insulating building materials – extruded polystyrene, rock wool, perlite, vermiculite, rubber, plastic, etc. The accuracy of the Dynatech test equipment is about ±4% of the true value of the thermal conductivity. The guarded hotplate thermal conductance measuring system requires two samples (different or identical) of maximum 15 cm thickness. Since samples had more than this thickness, a thin calibrated dummy sample was used on the upper side of the plate in order to accommodate test samples of larger thickness on the lower side of the plate. The types of wall samples tested are shown in Figs. 1-1, 1-2,1-3. The test wall samples of 61 cm 61 cm were prepared in the same way as they are assembled on walls. During the tests, the average temperature of the samples was kept at about 35.0 ± 2 °C with the cold surface maintained at about 25 °C. These are realistic values for the outside and inside weather conditions in Saudi Arabia. The surfaces of the samples must be flat and parallel to minimize contact resistance between these two surfaces and the corresponding hot and cold plate surfaces. Due to the rough surfaces of the samples it was not possible to get the flat and parallel surfaces. An uneven surface would result in a significant temperature difference between the hot plate and the corresponding sample surface. To overcome this problem, the thermocouple wires were fixed on both sides of the sample and the surface temperatures were monitored by a Campbell Scientific 21X automatic data acquisition system. The samples were covered with a blanket on both sides to have smooth contact with the plate surfaces. The sample temperatures were monitored till steady-state conditions were obtained.
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2.1. Calculation of equivalent thermal conductivity The test sample position on the guarded hot plate is shown in Fig. 2. The following equations were used for calculation of equivalent thermal conductivity of the samples. The power input at steady-state condition, Q = N V I. The heat flow, Qds, through the dummy sample was calculated by
Q ds ¼
kds AðT h;ds T c;ds Þ : dds
The heat flow, Qts, through the test sample was calculated by
Q ts ¼ Q Q ds ¼ NVI
kds AðT h;ds T c;ds Þ : dds
The equivalent thermal conductivity, keqv,ts, for the test sample was calculated by
h keqv;ts ¼
NVI
Fig. 1-1. Tested red clay brick types 1–4.
kds AðT h;ds T c;ds Þ dds
i
AðT h;ts T c;ts Þ
dts :
2.2. Uncertainty in equivalent thermal conductivity Taking into the consideration the measured values, which have significant uncertainty, the equivalent thermal conductivity is a function of
keqv;ts ¼ f ðV; I; kds ; T h;ds ; T c;ds ; T h;ts ; T c;ts ; dds ; dts Þ: The standard uncertainty in the equivalent thermal conductivity, neglecting the covariance, is calculated using the following equation [16]:
ðU c;keqv;ts Þ2 ¼
Fig. 1-2. Tested red clay brick types 5–9.
2 2 2 okeqv;ts okeqv;ts okqv;ts ukds uV þ uI þ oV oI okds 2 2 2 okeqv;ts okeqv;ts okeqv;ts þ uT h;ds þ uT c;ds þ uT h;ts oT h;ds oT c;ds oT h;ts 2 2 2 okeqv;ts okeqv;ts okeqv;ts þ uT c;ts þ udds þ udts : oT c;ts odds odts
Uncertainty propagation for the dependent variable in terms of the measured values is calculated using the engineering equation solver (EES) software [16]. The measured variables x1, x2, etc. have a random variability that is referred to as its uncertainty; this uncertainty is displayed as a ± ux. The input to EES for calculating the uncertainty of a dependent variable is the magnitude (a) of the measured variable (x) and the uncertainty (ux) in each measured variable. The sample uncertainty results obtained using EES display the partial derivative of the calculated variable with respect to each measured variable and the percentage of the total uncertainty in the calculated variable resulting from the uncertainty in each measured variable as shown in Table 1. The estimated uncertainty in equivalent thermal conductivity lies in the range from ±4.65% to 5.65%. The total uncertainty (equipment + experimental) in equivalent thermal conductivity of the tested samples lies in the range from 6.13% to 6.92%. 3. Results and discussions
Fig. 1-3. Tested concrete brick types 10 and 11.
The equivalent thermal conductivity values obtained experimentally are used to calculate the thermal resistance. The thermal performance of these bricks is discussed in terms of mortar type, brick size, method and type of insulation used on thermal conductivity. Fig. 1 shows the different types of masonry
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Fig. 2. Sample position on the guarded hot plate.
Table 1 Sample uncertainty calculations of equivalent thermal conductivity Variable ± uncertainty Equivalent thermal conductivity, dds = 0.03021 ± 0.00018 dts = 0.199 ± 0.00047 I = 0.7265 ± 0.001 kds = 0.04001 ± 0.0012 Tc,ds = 29.46 ± 0.5 Tc,ts = 24.84 ± 0.5 Th,ds = 46.78 ± 0.5 Th,ts = 43.83 ± 0.5 V = 7.864 ± 0.005
Partial derivative Keqv,ts = 0.382 ± 0.01883 okeqv,ts/odds = 7.955 okeqv,ts/odts = 1.921 okeqv,ts/oI = 0.8571 o keqv,ts/okds = 6.007 okeqv,ts/oTc,ds = 0.01388 okeqv,ts/odc,ts = 0.02014 okeqv,ts/oTh,ds = 0.01388 okeqv,ts/oTh,ts = 0.02014 okeqv,ts/oV = 0.07918
% of uncertainty 0.58 0.23 0.21 14.65 13.57 28.57 13.57 28.57 0.04
brick tested for assessment of thermal performance in the present study. Table 2 summarizes the types of masonry brick wall samples tested. As listed in Table 2, nine types of clay brick and two types of concrete brick wall samples were tested and analyzed for thermal performance. The equivalent thermal conductivity of the tested red clay brick wall samples varies between a minimum of 0.262 and to a maximum of 0.389 W/ m K, Table 2. The brick sample, with holes filled with mineral wool insulation, prepared with insulating mortar (type 7) has the lowest equivalent thermal conductivity whereas the brick samples prepared with ordinary concrete mortar (type 8) and holes filled with perlite–cement mix have the highest equivalent thermal conductivity. The equivalent thermal conductivity for
the lightweight concrete brick sample (type 11) is 0.489 W/m K whereas for the normal hollow concrete brick sample (type 10) it is 0.976 W/m K. The concrete brick samples were prepared with ordinary concrete mortar. The thermal resistance for normal, lightweight and insulated red clay bricks; and the normal and insulated concrete bricks is compared in Fig. 3. The thermal resistance of normal clay bricks is of type 1 while for lightweight and insulated clay bricks it is the average of types 2 and 3 and types 4 and 9, respectively. It can be seen from Fig. 3 that the lightweight and insulated clay bricks have about 12.7% and 6.3% higher thermal resistance as compared to the normal clay brick, respectively. However, the insulated concrete brick showed an increase of thermal resistance of about 99.5% over the uninsulated. Among the types of bricks tested, the lightweight clay brick has the highest thermal resistance. The thermal resistance of different types of clay bricks is compared in terms of percent increase relative to normal concrete brick (type 10) and the results are presented in Fig. 4. The normal, lightweight and insulated clay bricks have 155.6%, 188.0% and 171.7%, higher thermal resistance as compared to normal concrete brick, respectively. However, making the normal concrete brick lightweight resulted in an almost 100% increase in thermal resistance. The results show that the clay bricks are more effective than concrete brick. However, the thermal performance of lightweight concrete brick is comparable to that of the insulated and lightweight clay bricks.
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L.M. Al-Hadhrami, A. Ahmad / Applied Thermal Engineering 29 (2009) 1123–1130 Table 2 Equivalent thermal conductivity and thermal resistance values for 20 cm thick red clay and concrete brick samples Sample typea
Description
Equivalent thermal conductivityb (W/m K)
Thermal resistance (m2 K/W)
Type Type Type Type Type Type Type Type Type Type Type
Normal hollow red clay brick, ordinary concrete mortar Type 1 with body added polystyrene insulation Type 1 with body added perlite insulation Type 1 with center holes filled w/polystyrene insulation Normal hollow red clay brick, insulating mortarc Type 5 with center hole filled w/perlite + cement mix Type 5 with center hole filled w/mineral wool insulation Type 5 with center hole filled w/perlite + cement mix, Ordinary concrete mortar Type 5 with center hole filled w/mineral wool, ordinary concrete mortar Normal hollow concrete brick, ordinary concrete mortar Type 10 with body added perlite insulation
0.382 0.330 0.348 0.339 0.347 0.316 0.262 0.389 0.382 0.976 0.489
0.524 0.606 0.575 0.590 0.576 0.633 0.763 0.514 0.524 0.205 0.409
a b c
1 (clay) 2 (clay) 3 (clay) 4 (clay) 5 (clay) 6 (clay) 7 (clay) 8 (clay) 9 (clay) 10 (concrete) 11 (concrete)
See Fig. 1 for graphical description of different types of bricks. Uncertainty lies in the range from 6.13% to 6.92%. Samples were prepared with insulating mortar.
Fig. 3. Comparison of average thermal resistance for different types of bricks.
Fig. 5. Effect of body-addition of insulation material on thermal resistance of the tested brick samples.
Fig. 4. Percent increase in thermal resistance of bricks compared to normal concrete brick (type 10).
3.1. Effect of thermal insulation and method of use The recent trend in the brick manufacturing industry shows an increased use of thermal insulation both as mixing insulation material with the brick mix (or body-addition of insulation) and/ or filling of holes with insulation material. The purpose of mixing the insulation material with the brick mix is to make the bricks lightweight and more thermally resistive as compared to normal bricks. Some manufacturers are producing bricks with insulation inserted in the holes as well as adding the insulation material with the brick mix.
The effect of body-addition of insulation material on thermal resistance of the tested brick samples was studied and the test results are presented graphically in Fig. 5. It can be observed from the figure that the thermal resistance of the body-mixed thermal insulation clay brick samples (types 2 and 3) increased by as much as 9.7–15.6% compared to that of normal clay brick sample (type 1). In other words, the thermal resistance shows an increasing trend with the body-addition of insulating material compared to the normal bricks. The thermal resistance of body added polystyrene mixed concrete brick sample (type 11) increased by about 99.5% compared to that of the normal concrete brick sample (type 10), as shown in Fig. 5. This shows that the body-addition of insulating materials causes significant increase in thermal resistance of concrete block samples whereas the increase in thermal resistance for clay brick samples is moderate. The effect of filling of thermal insulating material into the holes of the brick is presented graphically in Fig. 6. The center holes of the brick types 6 and 7 were filled with perlite–cement mix and mineral wool insulating material, respectively. It was observed that the thermal resistance increased approximately by 10% for the brick samples with holes filled with perlite–cement mix (type 6) and about 32% for the brick samples with holes filled with mineral wool insulation inserts (type 7) as compared to that of the normal hollow brick samples (type 5). This shows that the type of insulating material filled into the holes of the brick also affects the thermal resistance of the bricks. In order of preference, bricks
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Fig. 6. Effect of filling of insulation material into the holes of the brick on thermal resistance of the tested brick samples.
filled with mineral wool insulating material are more thermally resistant than the perlite–cement mix filled bricks. The thermal resistance of brick samples prepared by mixing the insulation material with the brick mix (body-mixed insulation) was found to be lower than that of the bricks having holes filled with insulating materials (perlite–cement mix or mineral wool inserts), as observed in Figs. 5 and 6. In selecting bricks, strength characteristics as well as insulating properties should be considered. Bricks made with a body-mixed procedure have lower strength as a result of lower density, while bricks made with holes filled with insulation inserts are stronger. The other factors such as the water absorption and cost of the bricks should also be considered (see Section 3.4). High water absorption of bricks will adversely affect thermal performance. 3.2. Effect of mortar type The mortar used to join the bricks also affects the thermal performance of the wall assemblies. The mortar prepared with sand and cement is called ordinary concrete mortar and the mortar prepared with sand and cement along with insulating material is known as insulating mortar. Normally, ordinary concrete mortar is used for building walls and it constitutes approximately about 5–7% of the total surface area (as measured for the tested wall samples). The ordinary concrete mortar has high thermal conductivity compared to the masonry bricks. The mortar joints are continuous and act as thermal bridges and hence affect the thermal performance of all brick types. The results show that thermal resistance of the wall samples prepared with insulating mortar increased by about 23–46% compared to that of the samples prepared with ordinary concrete mortar, as shown in Fig. 7. The holes of brick of sample types 6 and 8 were filled with perlite–cement material while in case of types 7 and 9 they were filled with mineral wool insulation. The above variation in thermal resistance is due to the combined effect of the type of filling material into the holes of the bricks and the type of the mortar used for sample preparation. 3.3. Effect of brick size The brick size also plays a role when the thermal performance of the walls is concerned. The smaller the brick size the higher will be the ratio of mortar to wall surface area, which in turn provides more thermal bridging for heat flow. The mortar used to join bricks acts as a thermal bridge across the wall thickness, which reduces the overall thermal resistance of the wall assemblies. The ordinary
Fig. 7. Effect of type of mortar on thermal resistance of the tested brick samples.
concrete mortar has a higher thermal conductivity as compared to the concrete or clay bricks. Therefore, use of smaller sized bricks will increase the mortar area of the wall and hence will affect the thermal performance of the wall assemblies. Therefore, use of either large sized bricks or insulating mortar will result in obtaining a higher thermal resistance. 3.4. Cost analysis A cost analysis has been carried for different types of brick samples considered in the present study. In order to calculate the simple back payback period for alternative brick wall materials in comparison to concrete brick (type 10), the electrical energy cost needs to be determined for different types of brick walls. Since the thermal load on a building is cumulative, the energy used (E) by the air-conditioning equipment to pump out the wall thermal transmission load is calculated by the following equation [17]:
E¼
0:024 D kWh=m2 ; RC
where D is the degree-days of the location (D = 2185); R is the thermal resistance of the brick sample, and C is the coefficient of thermal performance of the air-conditioning system (C = 2.16). These D and C values are taken from Ref. [17]. The total discounted cost (energy and brick) or a net present value (NPV) per square meter of brick surface area is calculated using the following equation:
NPV ¼ E PWF EC þ BC ð$=m2 Þ; where E is the annual energy consumption, EC is the energy cost (4 cents/kWh), PWF is the present worth factor and BC is the brick cost. The present worth factor, PWF, is calculated using the equation [18]
PWF ¼
n 1 1þe for e–i; 1 ie 1þi
PWF ¼ n ði þ eÞ for e ¼ i; where e is the energy inflation rate (3%); i is the discount rate (6%) and n is the lifetime of the brick (30 years). The cost analysis along with NPV for different types of brick is presented in Table 3. It can be seen from the table that the annual energy cost lies in the range from 1.27 (clay brick type 7) to 4.74 $/ m2 (concrete brick type 10). The simple payback period analysis is carried out to obtain the payback period for different types of brick
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L.M. Al-Hadhrami, A. Ahmad / Applied Thermal Engineering 29 (2009) 1123–1130 Table 3 Energy and cost analysis for the tested brick samples Sample type
Description
Annual energy (kWh/m2)
Annual energy cost ($/m2)
Annual energy savinga ($/m2)
Brick cost ($/m2)
Increase in brick costa ($/m2)
Simple pay back period (yr)
NPVb ($/m2) (n = 30 yrs)
Type Type Type Type Type Type Type Type Type Type Type
Normal Lightweight Lightweight Insulated Normalc Insulatedc Insulatedc Insulated Insulated Normal Lightweight
46.33 40.06 42.22 41.15 42.15 38.35 31.82 47.23 46.33 118.43 59.36
1.85 1.60 1.69 1.65 1.69 1.53 1.27 1.89 1.85 4.74 2.37
2.88 3.13 3.05 3.09 3.05 3.20 3.46 2.85 2.88 – 2.36
8.25 9.38 10.75 11.50 9.13 12.38 12.38 11.50 11.50 6.63 8.00
1.63 2.75 4.13 4.88 2.50 5.75 5.75 4.88 4.88 – 1.38
0.56 0.88 1.35 1.58 0.82 1.80 1.66 1.71 1.69 – 0.58
43.9 40.2 43.3 43.2 41.6 41.9 36.9 47.9 47.2 97.8 53.7
1 (clay) 2 (clay) 3 (clay) 4 (clay) 5 (clay) 6 (clay) 7 (clay) 8 (clay) 9 (clay) 10 (concrete) 11 (concrete)
a Energy saving and increase in brick cost are obtained with reference to normal concrete brick (type 10) values. Energy cost is taken as 4 cents/kWh. Also, the energy and cost values are normalized to brick wall surface area. b NPV = Net present value or the total discounted cost (discount rate, i = 6%, energy inflation rate, e = 3%) over the lifetime of the brick (n = 30 years). c Samples were prepared with insulating mortar.
as compared to concrete brick (type 10). The analysis shows that the clay brick (type 1) has the lowest payback period of 0.56 years while the clay brick (type 8) has highest payback period of 1.71 years. From simple payback analysis, it is difficult to say which brick is the most effective among the brick types studied. As the simple payback analysis gives only the recovery period of an additional investment, it ignores the influence of discount rate and cost escalation. Net present value analysis is the correct approach for economic analysis as it takes into account of the life period of the brick, time value of money (discount rate) and energy escalation cost. The NPV analysis shows that the insulated clay brick (type 7) is the most effective among the types of brick studied as it has the lowest NPV of 36.9 $/m2. The second most economical brick is the lightweight clay brick (type 2) with NPV of 40.2 $/m2. The concrete bricks are not suitable from an economic consideration point of view due to higher NPV as compared to the NPV of the clay brick types. 3.5. General observations The advantage of using normal brick over the lightweight brick is that the normal bricks are denser than the lightweight brick and are less susceptible to high moisture absorption. Also, the higher the moisture absorption by a brick the lower will be its thermal insulation value. The red clay brick can be used in buildings located in regions with humid climate whereas lightweight brick can be used in dry regions. The lightweight bricks have lower strength as a result of lower density while the normal bricks and bricks made with holes filled with insulation inserts are stronger (more dense). 4. Conclusions and recommendations In the present study, the thermal performance of nine types of red clay and two types of concrete bricks were studied. It is concluded that: 1. The maximum percentage increase in thermal resistance was found for the insulated red clay brick sample, prepared with insulating mortar, compared to the normal concrete brick. 2. The addition of insulation either by body-mixing or through filling insulation materials into the holes of the bricks increases the thermal resistance significantly. 3. The use of insulating mortar for sample preparation increases the thermal resistance significantly compared to the ordinary concrete mortar.
4. The method of insulation used to make the bricks also affects thermal performance of the bricks. As observed in the present study, the bricks insulated by using a body-mix method were found to have less thermal resistance than those insulated by inserting the insulating material into the holes of the bricks. 5. The size of the bricks also plays a minor role on the thermal performance of the bricks. This is dependent on the type and the amount of mortar used to construct the wall. 6. Care should be taken in selecting the bricks because the bricks made from body-mixed procedure to make them lightweight have lower strength as a result of lower density while bricks made with holes filled with insulation inserts are stronger (more dense). 7. An economic analysis shows that the insulated clay brick (type 7) is the most effective among the types of brick studied as it has the lowest net present value (NPV) of 36.9 $/m2. It is recommended that a long-term detailed study should be conducted to establish thermally optimal brick assemblies for usage in building construction in the Kingdom of Saudi Arabia. Acknowledgement The authors acknowledge the support of the Research Institute at King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia. References [1] Ministry of Industry and Electricity, Annual report for the year 2004, Information and Statistics Department, Saudi Electricity Company, Riyadh, Kingdom of Saudi Arabia, 2005. [2] M.A. Elhadidy, M. Ul-Haq, A. Ahmad, Electric energy consumption in selected residential buildings at KFUPM, Dhahran, Saudi Arabia, Proceedings of the Mediterranean Conference for Environment and Solar, Beirut, Lebanon, November 16–18, COMPLES 2000, vol. EX493, IEEE, 2001. [3] M.A. Abdelrahman, S.A.M. Said, A. Ahmad, et al., Thermal conductivity of some major building materials in Saudi Arabia, Journal of Thermal Insulation 13 (April) (1990) 294–300. [4] M.A. Abdelrahman, S.A.M. Said, A. Ahmad, A Comparison of energy consumption and cost-effectiveness of four masonry materials in Saudi Arabia, Energy, The International Journal 18 (11) (1993) 1181–1186. [5] J.J. del Coz Diaz, P.J. Garcia Nieto, A. Martin Rodriguez, A. Lozano MartinezLuengas, C. Betegon Biempica, Non-linear thermal analysis of light concrete hollow brick walls by the finite element method and experimental validation, Applied Thermal Engineering 26 (2006) 777–786. [6] M.M. Al-Hazmy, Analysis of coupled natural convection conduction effects on the heat transport through hollow building blocks, Energy and Buildings 38 (2006) 515–521. [7] M.A. Antar, L. C Thomas, Heat transfer through a composite wall with enclosed spaces: a practical two-dimensional analysis approach, ASHRAE Transactions 106 (2001) 318–324.
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