Thermal Characteristics of a Vernacular Building Envelope

Transcription

1 Thermal Characteristics of a Vernacular Building Envelope Priyanka Dhar, M.Tech. Department of Energy, Tezpur University, Tezpur, , Assam, India Pallavi Borah, M.Tech. Department of Energy, Tezpur University, Tezpur, , Assam, India ; Department of Electrical Engineering, Bineswar Brahma Engineering College, Kokrajhar , Assam, India Manoj Kumar Singh, PhD Local Environment Management and Analysis, Université de Liège, Chemin des Chevreuils, Liège, Belgium; Integrated Research and Action for Development (IRADe), C-80, Shivalik, Malviya nagar, New Delhi , India Sadhan Mahapatra, M.Tech. Department of Energy, Tezpur University, Tezpur, , Assam, India ABSTRACT Climate responsive building design is determined by the local micro-climate and the ability of the building envelope to regulate the indoor thermal environment. The building envelope characteristics are based on the available/accessible building materials and technology. Studies on vernacular architecture across the world showed that wall configurations and the thermo - physical properties of the building materials are used intelligently to maintain comfortable indoor comfort across the seasonal weather variations. Vernacular buildings of North-East India are naturally ventilated. Hence, it is important to find out an optimum wall configuration which will provide enough time lag, reduced discomfort hours as well as optimum thermal performance of these buildings. Various studies on building simulation/modelling show that building dynamic simulation tool can be effectively used to study the building envelope characteristics. In this study, a typical vernacular building in warm and humid climatic zone of North-East India has been considered to study the effect of thermo - physical properties of wall, thickness and material assembly on the indoor environment. Solar energy modular simulation tool TRNSYS 16 is used to carry out the simulations of this building with an objective to improve the indoor thermal environment. Building model is generated in TRNSYS and parametric simulations for different wall characteristics by varying thermo - physical properties and thickness of wall, and insulation thickness on external wall are carried out. Simulation results are obtained in terms of temperature profiles inside the different zones of the buildings. Indoor temperature profile of the building with best suggested wall configuration shows reduction in indoor temperature compared to the base case. In this study, it is also found that other climate oriented features such as shading mechanism like over hangs (very common feature in vernacular buildings) significantly influence the thermal performance of walls. INTRODUCTION Thermal performance study is one of the critical aspects of vernacular houses. These structures are evolved through generations, addressing the micro climate variations and also satisfy the needs of habitats (Singh et al., 2009a). Thermal performance of a building refers primarily to how well a building is insulated from the external weather conditions in order to achieve a comfortable indoor temperature. This can be achieved by keeping the internal temperature higher or lower than the external temperature 1

2 as per the requirement of comfort temperature. Building design is greatly influenced by the severity and climatic variations leading to the need for integrating the building thermal design with the overall design process, helping the designer to decide at the beginning of the design process to bring the built space into comfort conditions (Al-Homound, 1997; Liu et al., 2010). The important parameters required for the design of energy efficient buildings are walls, roof, placement and size of openings, and shading devices (Charde and Gupta, 2013). Building envelope like walls and roofs have important role to play in the heat transfer process between the indoor and outdoor environment of the building. Due to the quest for achieving better thermal comfort standard inside the building leads to higher heating and cooling energy requirements. Borah et al. reported that energy consumption for heating will be higher at higher base temperature and the energy consumption due to cooling of the buildings will be higher at lower base temperatures in the buildings of North-East India (Borah et al., 2015).The components of the building envelope can be used as the most effective way of controlling the indoor temperature of the building (Dutta, 2001). Simulation tools such as TRNSYS can be used effectively to study the effect of building envelope characteristics on indoor thermal environments. Singh et al. have done thermal performance simulation of three vernacular buildings at different bioclimatic zones of the north-eastern region by using TRNSYS simulation tool (Singh et al., 2009b). Simulation results concluded that the houses are fairly comfortable in pre-winter and pre-summer months compared to winter and summer months and successfully compared with the experimental results. Kalogirou et al. investigated the effects of the application of building thermal mass in Cyprus by modeling and simulating a typical house with the TRNSYS simulation program (Kalogirou et al., 2002). Jindal et al. analyzed the thermal performance of non-conditioned building of cold regions by using various insulation thicknesses at different positions of walls and roof. It is found that the thermal comfort for the three cold stations i.e. Srinagar, Shimla and Shillong cannot be ensured in the month of January if the building is not insulated (Jindal et al., 2013). The effect of insulation thickness on external walls in different seasons of the year in different climatic zones suggests that optimization of insulation thickness on external walls with respect to cooling loads is found to be more appropriate for energy savings compared to the heating loads (Bolatturk, 2008; Ozel, 2011; Yu et al., 2009). Al-Sanea et al. investigated the effects of location of thermal mass in insulated building walls on total and peak transmission loads, time lag, decrement factor, and dynamic resistance under the steady periodic conditions using climatic data of Riyadh. It is found that for a given thermal mass, a wall with outside insulation provides better overall performance than a wall with inside insulation (Al-Sanea et al., 2013). Asan investigated the effects of wall s insulation thickness and position on time lag and decrement factor. It is found that insulation thickness and position have intense effect on time lag and decrement factor (Asan, 1998). Axaopoulos et al. analyzed the thermal behavior of external walls using TRNSYS and determined the optimum insulation thickness for the external walls of a residential building in Athens, Greece, considering wall construction, orientation, wind direction and the position of insulation (Axaopoulos, 2014). Huang et al. found that the variation in the window to wall ratio for different orientation results into different economical thermal insulation thicknesses of building envelope (Huang et al., 2014). It is also found that the optimum performance can be achieved by considering the windows-to-wall ratio, house orientation, types of insulating materials and windows in addition to the impact of the windows and walls. Building energy simulation tools are widely used for thermal performance study of building. In this study, a building model generated in TRNSYS 16 is used to analyze the indoor thermal environment of a typical vernacular building at Tezpur, in warm and humid climatic zone of North-East India. Most of the houses of the region are constructed in direct response to the climatic constraints and are naturally ventilated (Singh et al., 2010). A building model is prepared based on the inputs like building construction details, thermo-physical properties of building materials, wall thickness and insulation thickness on external wall using TRNSYS simulation tool. The layout of the building considered for the base case is shown in Figure 1. Windows on the house are distributed on all the facades. It is a single zone house with flat roof and single glazed windows. Since the vernacular house is naturally ventilated, so auxiliary heating, cooling and mechanical ventilation are kept off for all the simulations and the zone air temperature is considered as the primary output parameter.the simulation provides the results in terms of hourly temperature profile inside the zone of the building. Indoor temperature variation for all the simulations is compared with the base case and the results are analyzed to obtain the optimum design parameters. An optimum thermal performance of the building has been achieved by integrating the building optimum design parameters. 2

3 METHODOLOGY Vernacular buildings are constructed across the world based on the local climatic condition. Vernacular buildings are the structures built by local people using locally available material and affordable technology to deal with the local and day-to-day needs (Singh et al., 2009a; Singh et al., 2010). Studies on vernacular architecture reveal that wall configurations and the thermo-physical properties of building materials have a great influence on indoor temperature. In recent times due to quest for achieving better thermal comfort; energy consumption is increasing in buildings. So, it is important to study thermal performance of vernacular buildings with varying building elements which will provide maximum comfortable hours inside the building. In this study, a typical vernacular building of warm and humid climatic zone of North East India is modeled in TRNSYS. Figure 2 represents the methodology followed to carry out this study. Table 1 represents the thermo-physical properties of the materials used for the simulation and Table 2 represents the wall configurations considered for the simulations. First a base case model of the vernacular house is selected to carry out the simulation. Figure 1 2D layout of the house with single zone Figure 2 Methodology of the study Table 1 Thermo physical properties of the materials used for simulation Materials Conductivity (W/m-K) Thermal capacity (kj/kg-k) Density (kg/m 3 ) Brick Aerated concrete (floor 0.127m thick) Light concrete (roof 0.127m thick) Plaster Extruded Polystyrene, Insulation Cell Glass (High density), Insulation Polyurethane, Insulation Parametric simulation studies are carried out by using TRNSYS 16 simulation tool. The standard building subroutine TYPE 56 has been used for the simulation (Singh et al., 2009b). The material properties listed in Table 1 and 2 are used as input parameters to generate the building model. For carrying out further simulations, base case model is used with various design modifications. Figure 3 shows the different scenarios for which the base case model is carried out. The different scenarios considered for the simulation are (i) increasing the wall thickness of the base case (ii) adding three different insulations on the wall of the base case (iii) placing the insulation at three different positions of the wall of the base case (iv) replacing the single glazed windows of the base case with double glazing and providing overhang on the windows (v) increasing the window to wall ratio (vi) providing insulation to the roof of the base case and (vii) considering four different orientations of the house. The infiltration 3

4 for this house is kept at 3 ACH (air changes per hour) for all the simulations.since the house considered for the simulation is naturally ventilated so the zone temperature is considered as the main output parameter. The surface temperatures of the wall are also obtained as output parameter. Table 2 Wall configuration considered for the simulation Case External wall configuration (from inside to outside) Base case Plaster (0.01m) + brick (0.127m) + plaster (0.01m) Base case + A1 Plaster (0.01m) + brick (0.254m) + (increased thickness) plaster (0.01m Base case + A2 Plaster (0.01m) + brick (0.381m) + (increased thickness) plaster (0.01m) Base case + A3 Plaster (0.01m) + brick (0.508m)+ (increased thickness) plaster (0.01m) Base case + B1 (wall Plaster (0.01m) + brick (0.127m) + with insulation 1) insulation 1 (0.05m) + plaster (0.01m) Base case + B2 (wall Plaster (0.01m)+brick (0.127m)+ with insulation 2) insulation 2 (0.05m) + plaster (0.01m) Base case + B2 (wall Plaster (0.01m) + brick (0.127m) + with insulation 3) insulation 3 (0.05m) + plaster (0.01m) Base case+ increased Plaster (0.01m) + brick (0.127m) + window area plaster (0.01m) Base case + double Plaster (0.01m) + brick (0.127m) + glazed windows plaster (0.01m) Overall heat transfer coefficient (U) (W/m 2 K) RESULTS AND DISCUSSION The vernacular house in warm and humid climatic zone is constructed with different wall configurations and various parametric evaluations have been done to analyze the effect of different wall configurations on the zone temperature. Analysis on the effect of window glazing, window to wall area ratio, providing roof with insulation on the zone temperature is carried out. Simulations are also carried out for different orientations of the building. The two very important characteristics, i.e., time lag and decrement factor, which determine the heat storage capabilities of any material, are also calculated. Simulation data for entire 8760 hours (i.e., one year) is obtained and the results in terms of temperature profiles are analyzed for January and July as the representative month for winter and summer seasons respectively. In this study, zone temperature is the main parameter around which analysis has been done. Figure 3(a) represents the variation of daily minimum zone temperature for all the days in the month of January. It is observed from the profile that variation in overall heat transfer coefficient (U value) depends on thickness and also the zone air temperature is higher in case of the wall with lower U-value, i.e., with decreasing U value, the daily minimum temperature of the zone increases which is desirable in winter. As we know that lower U value provides maximum resistance to the heat flow, hence, reducing the heat loss from inside the room to the outside air. So, it can be concluded from the Figure 3(a) that with the increase in thickness of wall, thermal inertia comes into play and consequently increases the time over which heat gain and loss takes place. This has also effect on the comfort conditions as occupants feel minimum thermal shock with noticeably varying outdoor thermal environment. So, for naturally ventilated buildings due consideration must be given to thermal inertia. Figure 3(b) represents the variation of daily maximum zone temperature for all the days in the month of July. It is observed from the profile that the temperature is lower in case of the wall with lower U-value (maximum thickness), i.e., with decreasing U value, the daily maximum temperature of the zone decreases which is reverse in the case of winter. With lower U value, heat gain from the outside air is reduced and thus the maximum temperature inside the room is low. Thus, from both the figures, it can be concluded that, the wall needs to be selected with U value as low as possible (with optimum thickness to maximize the effect of thermal inertia) to reduce the heat loss from inside the room in winter and heat gain from outside to the inside of the room in summer. Figure 4(a) and 4(b) represent the daily minimum and daily maximum zone temperatures for each day of the month of January and July respectively. It is observed from the figures that 0º and 45º and 90º and 135º orientation shows similar temperature profile. For orientation 0 and 45º, minimum and 4

5 maximum indoor temperature decreases leading to decreased indoor temperature swing. This happens because with change in orientations of the building the surface area of the external wall exposed to sun varies and so the heat gain. Since, the selected building is rectangular in shape and of single zone the effect observed here is low. It is found from literature that in the case of multi-zone building with different shapes (L, U, C and H etc.) the variation in indoor temperature is significant. Hence, it is suggested to decide the building orientation wisely before construction to maximize the use of solar passive techniques in the building thus improving energy efficiency and thermal comfort. Figure 5(a) and 5(b) represent the minimum and maximum inner surface temperatures for each day of January and July month respectively. Here, insulation is added to the base case and placed in three different positions (i.e., inside, middle and outside) of the wall. Figure 5(a) shows that in January (i.e., in winter) when insulation is placed at the outside surface of the wall, the minimum surface temperature of the wall has increased compared to the wall when insulation is placed at the middle and inside position because of thermal inertia plays its role. It is observed from figure 5(b) that in July (i.e., in summer) also when insulation is placed at the outside of the wall, the maximum temperature is lower compared to the insulation at middle and inside wall surface. Thus, it is observed that both in summer and winter months, placing insulation at the outside of the wall, shows better thermal performance, out of all the three cases (combined effect of thermal inertia and insulation). Furthermore, it is also observed that there is not much difference when insulation is placed at the middle position of the wall. Figure 3(a) Daily minimum zone temperatures for different wall thicknesses Figure 3(b) Daily maximum zone temperatures for different wall thicknesses Figure 4(a) Daily minimum zone temperatures in January for the four orientations Figure 4(b) Daily maximum zone temperatures in July for the four orientations Figure 6(a) and 6(b) represent the daily minimum and maximum zone temperatures in January and July respectively for different window to wall ratios. The windows are single glazed and window to wall ratio increases from 20 to 80%. It is observed from figure 6(a) that in January with the increasing window to wall ratio, the zone temperature is decreasing. This happens because with the increase in glazing area heat loss increase from inside to the outside environment. It is observed from figure 6(b), that the zone temperature increases with increasing window to wall ratio because heat gain is more from the outside environment. These two observations clearly indicate that both in summer and winter higher 5

6 window to wall ratio leads to uncomfortable conditions inside the room. Hence, optimum window to wall ratio needs to be chosen so that in summer there is minimum heat gain and minimum heat loss in winter. Figure 7(a) and 7(b) represent the daily minimum and daily maximum zone temperature for January and July months respectively, when single glazing on windows have changed to double glazing. It is observed from both the figures that the increase in temperature is more prominent in winter because low altitude of the sun allows direct sunlight to enter into the room and increases the thermal gain thus temperature of the indoor environment. Moreover, both the overall heat transfer coefficient and solar heat gain coefficient comes into consideration for glazing. Though, for double glazing, the SHGC (solar heat gain coefficient) is lowered by only 10 % in comparison to single glazing but the effect due to low SHGC is compensated by low U value (U value of single glazing is 4.8 and for double glazing is 2.7). So, in winter the double glazed windows perform better by trapping the absorbed heat inside the room. However, in summer, the solar heat gain coefficient allows the direct sunlight to enter the room through the window and make the room hotter. Hence, in summer it is advised to have appropriate overhang on the window to block the direct sunlight to enter inside the room. Figure 5(a) Inner surface temperatures for January for three different positions of insulation Figure 5(b) Inner surface temperatures for July for three different positions of insulation Figure 6(a) Daily minimum zone temperatures in January for different window to wall ratios Figure 6(b) Daily maximum zone temperatures in July for different window to wall ratios Figure 7(a) Daily minimum zone temperatures in January for double glazed windows Figure 7(b) Daily maximum zone temperatures in July for double glazed windows 6

7 Figure 8 represents the effect on the zone temperature when the roof is provided with insulation. It is observed from the figure that the effect of insulation on the roof is negligible on the indoor temperature profile of the zone. This may be due to high infiltration in naturally ventilated buildings leading to negligible effect on the indoor temperature swing. Figure 9 represents the variation of time lag and decrement factor for different thicknesses of the wall. The variation of time lag and decrement factor is shown for two different materials. It is observed from figure 9 that the time lag increases with the thickness of the wall, whereas decrement factor decreases with the wall thickness. Moreover, both the materials are exhibiting different time lag and decrement factor. This indicates that time lag and decrement factor not only vary with the thickness of the wall but also with the material properties. For better thermal performance, time lag should be high and decrement factor should be minimum. Figure 8 Daily minimum zone temperatures for roof (January) with insulation and without insulation Figure 9 Time lag and decrement factor for different wall thicknesses The effect of different insulation materials on inner wall surface temperature for a period of 24 hours (i.e., 1 day) in the month of January and July respectively has also been carried out. Three different insulation materials - extruded polystyrene, cell glass of high density and polyurethane are added with the base case and the inner surface temperatures of the wall are obtained. It is observed that in January (i.e., winter) month, the peak of minimum temperature is less in case of polyurethane as the insulation material, in comparison to cell glass and extruded polystyrene. It is also observed that in July (i.e., summer) month, the peak of maximum temperature for the wall with polyurethane as insulation material is lower than the wall with other two insulation materials. Moreover, the 24 hour swing in temperature throughout the day is less in case of polyurethane as insulation material. Hence, it can be concluded that with polyurethane as the wall insulation material shows better thermal performance both in summer and winter months. It is also observed that there is an increase in the temperature for the month of January (winter) when the windows are provided with overhangs. It is also observed that the maximum temperature in the month of July (summer) month is decreasing, clearly indicates that windows with overhang have a significant role to play by blocking direct sun light to enter the room in summer leading to decrease in indoor air temperature but in winter it allows sunlight to enter room thus increases the indoor air temperature. CONCLUSIONS Thermal performance analysis of building envelope is very important to evaluate indoor thermal environment of naturally ventilated buildings. The building envelope constitutes the major portion of the building through which maximum heat gain and loss occurs in both the extreme seasons respectively. In this study, a typical vernacular building of warm and humid climatic zone of North East India is considered. This study made an attempt to carry out the thermal performance analysis by varying building material parameters and their effect on the indoor environment. It can be concluded from the study that, with the increase in the wall thickness, the overall heat transfer coefficient reduces and also the overall heat transfer coefficient changes, when the thermo-physical properties of the material changes. It has been found that orientation has an important effect on the indoor temperature; because of different orientations, different area of external area is exposed to direct solar radiation leading to heat gain. It is also found that location of insulation on the wall have significant effect on the thermal 7

8 performance, showing that insulation applied on the outside surface have less temperature swing. The effect of different insulation material on the wall surface temperature has also been studied and polyurethane has been found showing the best performance. This study also reveals that windows with double glazing have maximum effect in winter when the sun s altitude is less. Hence, if the window is replaced by proper shading mechanism then overall performance can be improved. It is found that increase in the window to wall ratio has significant effect on the indoor temperature swing. Increase in the window area leads to maximum hours of discomfort inside the building in both January and July months. It can also be concluded that providing insulation to the roof has negligible effect on the indoor temperature profile. Time lag and decrement factor are also calculated for different wall thicknesses. It is found that thermal mass of the wall and thermo-physical properties of the wall have a profound impact on time lag and decrement factor. The findings of this study also relate to the policy intervention and best practice needs to be followed in these types of building design. North-Eastern region is in development process and the construction sector is rising in unprecedented way. These types of buildings are well suited to the climatic factors as well as social requirements of the people of the region. Hence, the findings of this study could be useful to the policy makers, architects, and local people for designing better thermally comfortable buildings. REFERENCES Al-Homound, M Optimum thermal design of air-conditioned residential buildings. Building and Environment, 32(3): Al-Sanea, S. A., Zedan, M. F., & Al-Hussain, S. N Effect of thermal mass on performance of insulated building walls and the concept of energy savings potential. Applied Energy, 89(1): Asan, H Effects of wall s insulation thickness and position on time lag and decrement factor. Energy and buildings, 28(3): Axaopoulos, I., Axaopoulos, P., & Gelegenis, J Optimum insulation thickness for external walls on different orientations considering the speed and direction of the wind. Applied Energy, 117(3): Bolatturk, A Optimum insulation thicknesses for building walls with respect to cooling and heating degree-hours in the warmest zone of Turkey. Building and Environment, 43(6): Borah, P., Singh, M. K., & Mahapatra. S Estimation of degree-days for different climatic zones of North-east India, Sustainable Cities and Society, 14: Charde, M., & Gupta, R Design development and thermal performance evaluation of static sunshade and brick cavity wall: An experimental study. Energy and Buildings, 60(5): Datta, G Effect of fixed horizontal louver shading devices on thermal performance of building by TRNSYS simulation. Renewable Energy, 23(3): Huang, J., Lv, H., Gao, T., Feng, W., Chen, Y., & Zhou, T Thermal properties optimization of envelope in energy-saving renovation of existing public buildings. Energy and Buildings, 75(6): Jindal, N., Ranjana, J., & Sarita, B Thermal response of a non air conditioned building by using insulation of various thicknesses at rhe different positions of the walls and roof at cold stations of India. Journal of Environmental Research and Development, 8(1): Kalogirou, S. A., Florides, G. & Tassou, S Energy analysis of buildings employing thermal mass in Cyprus. Renewable Energy, 27(3): Liu, J., Zhang, T., & Zhai, Z Considering building energy from environment perspective. Energy and Building, 42(1):1. Ozel, M Thermal performance and optimum insulation thickness of building walls with different structure materials. Applied Thermal Engineering, 31(17-18): Singh, M. K., Mahapatra, S., & Atreya, S. K. 2009a. Bioclimatism and vernacular architecture of north-east India. Building and Environment, 44 (5): Singh, M. K., Mahapatra, S., & Atreya, S. K. 2009b. Study to enhance comfort status in naturally ventilated vernacular buildings of northeast India. 29 th ISES Solar world Congress, Johannesburg October, South Africa, Vol 2, Page number Singh, M. K., Mahapatra, S., & Atreya, S. K Thermal performance study and evaluation of comfort temperatures in vernacular buildings of North-East India, Building and Environment, 45 (2): Yu, J., Yang, C., Tian, L., & Liao, D A study on optimum insulation thicknesses of external walls in hot summer and cold winter zone of China. Applied Energy, 86 (11):