IMPACT ANALYSIS OF WINDOWS IN BUILDING THERMAL PERFORMANCE Jorge Filipe da Cruz Sirgado Lisbon, October 2010

Transcription

1 IMPACT ANALYSIS OF WINDOWS IN BUILDING THERMAL PERFORMANCE Jorge Filipe da Cruz Sirgado Lisbon, October 2010 EXTENDED ABSTRACT 1. INTRODUCTION In Portugal, the consumption of energy in the sector of the buildings is estimated in about 30% of the total consumption of energy, being the housing buildings responsible for 17%. In the future this percentage tends to grow [1]. To oppose this trend it has appeared in Portugal some regulations, namely the RCCTE [2], the RSECE [3] and the SCE [4]. It is in this context that appears this study, which focuses in the impact analysis of windows in thermal performance of buildings. Windows represent a large area in the facades of buildings and are one of the most important elements responsible by the occurrence of heat exchanges. Thus, heat losses and heat gains that occur through this element may represent a significant slice in the energy consumed by the buildings for air conditioning. In such way, the objectives of this study pass by the definition of a decision-making process for window design for housing buildings and by the analysis of the criteria that constitute this process. For such, it will be used two computer programs with special interest in this knowledge area EnergyPlus [5] and WINDOW5 [6]. EnergyPlus is dynamic simulation software of thermal performance that allows the user to analyze buildings energy use for air conditioning. WINDOW5 is a program to calculate some optical and thermal properties of many different solutions of windows, such as U-value, solar heat gain coefficient (SHGC) and visual transmittance (τ v ). Finally, it is important to note that this study contributes to a more energy-efficient design of the windows of residential buildings. 2. HEAT TRANSFER ASSOCIATED TO WINDOWS The heat flow that occurs through a window unit happens in three distinct ways: conduction, convection and radiation. Then, it will be explained how the heat transfer occurs when a window unit is subjected to temperature difference and solar radiation. 1

2 2.1. HEAT TRANSFER IN A WINDOW UNIT CAUSED BY TEMPERATURE DIFFERENCE When a window unit is subjected to a temperature difference between the exterior and the interior space, there is heat transfer for the local that has lower temperature by three distinct mechanisms: conduction through glass and frame, convection through air spaces and longwave radiation between the interior surfaces of glass in double or triple glazing systems (Figure 2.1). There is a parameter designated U-factor or U-value that quantifies the heat flow through a window unit caused by the temperature difference between exterior and interior. The U-factor definition can be understood as follows: it represents the heat flow per hour (in watt) per square meter of window for a temperature difference of 1 o C between inside and outside. A lower U- factor will cause a better window insulating. Figure 2.1 Components related to the U-factor that constitute the heat transfer through a window unit [7]. The U-factor for all window depends on glazing system and frame material, and respective areas, and it can be determinate with the following expression [8]: = (W/m 2o C) (2.1) where U f and U g are the U-values of frame and glazing system in W/m 2o C, respectively, A f and A g are the visible areas of frame and glazing in m 2, respectively, L g is the perimeter of visible glazing in m, and ψ is the linear U-value in W/m o C SOLAR GAINS THROUGH A WINDOW UNIT The radiation that achieves a glazing element is decomposed in three parts: one part that is instantly transmitted to the interior, other part that is immediately reflected to the exterior and a third part that is absorbed by the glazing (Figure 2.2). Of this third part, which is absorbed and represents energy stored in glazing, there is still a portion that is later sent to the interior and other portion that follows to the exterior, by convection and radiation mechanisms. The ratios between each of these parts and total incident radiation represent the optical solar 2

3 properties of glazing and are designated respectively by transmittance (τ s ), reflectance (ρ s ) and absorptance (α s ). Figure 2.2 Decomposition of solar radiation after to achieve a glazing element [9]. There is a parameter that represents the percentage of total incident radiation that crosses a window unit to the interior of buildings. This parameter is designated by solar heat gain coefficient (SHGC). The SHGC counts the energy incident part instantly transmitted through the glazing to the interior plus the energy incident part absorbed by the glazing that passes later to inside, and also takes into account the energy part that is retained by the frames. 3. WINDOW MATERIALS Windows are composed by two different elements: glazing and frame. Often windows still present attached shading devices to reduce the harmful effects of solar radiation in building thermal performance. In addition to represent a large area in a window unit with special importance in heat loss caused by the temperature difference between inside and outside, and in heat gain caused by the solar radiation, the glazing system provides visible light and outside views for the occupants of buildings. Initially, the glazing system consisted only in one pane of glass. However, nowadays it is common practice to use glazing with two or three panes separated by spaces that can be filled with air, or with less viscous gases, such as krypton and argon. Thus, it is possible to reduce the U-factor of window units. 3

4 There are glazing systems, which because of their properties, have a special behavior. It is the case of the glazing with solar control and the glazing with low emissivity coating. The solar control glazing is made with at least one pane in tinted glass that reduces its SHGC. Thus, the windows with this type of glazing reduce the amount of solar gain, improving the thermal performance of housing during the summer. The glazing with low emissivity coating allows the passage of short-wave radiation from the sun, but prevents the passage of long-wave radiation emitted by objects inside the homes. Thus, heat is stored inside buildings leading to an excellent thermal performance of this glazing system along the colder seasons. The window frames perform many important functions. They have as primary objective to ensure the tightness and the operability of windows and they are responsible by glazing system support. There are several frame materials. Today, aluminum with thermal break, PVC and wood are the frame materials more used in housing buildings. During the summer it is essential to control solar gains to improve the thermal performance of buildings. To this end, there are several ways. The use of shading devices, which can be applied inside or outside the building, is the most common way. Blinds, screens and shutters are examples of such devices. Currently, it is also common to shade windows using horizontal overhangs and vertical fins that can be hided in the building geometry. 4. DECISION-MAKING PROCESS FOR WINDOW DESIGN The decision-making process for window design and selection includes several criteria. This process should be initiated by the criteria that have a greater impact in building thermal performance. If a condition is defined, the factor immediately below should be the next. Figure 4.1 represents a schematic illustration of this process. 5. WORK METHODOLOGY To evaluate the impact of criteria presented in Section 4 for the thermal performance of buildings, it was necessary to develop a simple work methodology. Briefly, this methodology will be presented in this section. The work methodology used in this study passes by obtaining the needs of useful energy for heating and the needs of useful energy for cooling using simulations with software EnergyPlus [5]. For such, it was necessary to create a simulation model with simple base that seeks to reproduce a common dwelling. The base of simulation model goes suffering changes, becoming increasingly complex. With this changes, it is possible evaluate the different criteria climate zone, orientation, window area, shading condition, glazing system and frame material. During heating and cooling seasons, the lower values for needs of useful energy represent better solutions. Finally, needs of useful energy for heating and cooling are converted in needs of primary energy for air conditioning. For such, it was adopted the methodology presented at RCCTE [2], using the reference equipments for heating and cooling stipulated by this regulation. 4

5 Thus, it will be possible to make an annual analysis of different solutions, as presented in the following section. Figure 4.1 A decision-making process for window design and selection. Based in process shown at [7]. 6. ANALYSIS OF THE CRITERIA FOR WINDOW DESIGN The objective of the analysis undertaken in this section consists in to assess the application of criteria, which integrate the decision-making process for window design, on energy use of housing buildings. This analysis was performed independently for heating and 5

6 cooling seasons. However, in this section, it will be presented only the results for the annual review CLIMATE ZONE For the annual analysis of climate zone influence, the simulation model was placed in three regions of Portugal with different climates Lisbon, Porto and Faro. It was verified that the climate zone which presents a better thermal performance depends substantially of the window analyzed. Table 6.1 shows the annual thermal performance by climate zone. This table allows concluding that Lisbon region leads to lower values of primary energy use for air conditioning for four of the five windows analyzed. The Faro region presents worst annual thermal performance for three of the five windows used in simulations. Window A B C D E Better Lisbon Lisbon Lisbon Faro Lisbon Medium Faro Porto Porto Lisbon Porto Worse Porto Faro Faro Porto Faro Table 6.1 Comparison of the annual thermal performance by climate zone. The values presented in this table for primary energy use spent on air conditioning are in kgep/m 2.year. The characteristics of windows analyzed can be found in Figure 6.1. Simulations were realized with 30% of window area orientated to south. In neither case was used any shading device ORIENTATION Figure 6.1 shows the primary energy use for air conditioning of the simulation model by orientation of glazing system north, south, east and west. Independently of window used, it is possible to verify that better orientation for windows is the south orientation, while the orientation that leads to worst thermal performance is the north orientation WINDOW AREA With the glazing system orientated to south, it is possible to obtain values quite acceptable for primary energy use spent on air conditioning even for a high window area. For this orientation, the primary energy use for air conditioning by window area shows the presence of a minimum in the function. Figure 6.2 shows that the area of window which leads to a lower primary energy use is 30% in case of windows A, B, C and E, and 45% in case of window D (tinted glazing) SHADING CONDITION In summer, it is essential control the solar gains inside the dwellings. As described in Section 3, there are several ways to shade the windows. In Figure 6.3, it is shown a comparison between two shading forms exterior shading device and horizontal overhangs with the nonexistence of any shading type. It is possible verify that these two type of shading reduce 6

7 significantly the primary energy use for air conditioning of the simulation model. However, the exterior shading device presents the better thermal performance. It is important to note that the exterior shading device is activated to 100% when exterior temperature exceeds 25 o C during the cooling season GLAZING SYSTEM The Figure 6.4 presents a comparison in consumption of primary energy spent in heating and cooling between several glazing systems. As can be observed in this figure, the glazing systems that present the better thermal performance are: triple and double clear glazing and double clear glazing with low-e coating. For the other side, the glazing systems that lead to the worst thermal performance of the simulation model are the tinted glazing. It were also analyzed other parameters related with the glazing system, such as the thickness and separation of glass panes and the fill gas of space between glass panes. However, these parameters presented minimal differences in the thermal performance of simulation model. Figure 6.1 Comparison of primary energy use for air conditioning by orientation. All cases have 30% of window area and were simulated for Lisbon climate. In neither case was used any shading device. 7

8 Figure 6.2 Primary energy use comparison by window area. The characteristics of windows analyzed can be found in Figure 6.1. All cases were simulated for Lisbon climate. In neither case was used any shading device. The N t value represents the limit of primary energy use for air conditioning imposed by Portuguese regulation. Figure 6.3 Primary energy use comparison by shading condition. The characteristics of windows analyzed can be found in Figure 6.1. All cases have 30% of window area orientated to south and were simulated for Lisbon climate. 8

9 Figure 6.4 Primary energy use comparison by glazing system. All cases have 30% of window area orientated to south and were simulated for Lisbon climate FRAME MATERIAL Frame material of windows is the parameter that has less influence in thermal performance of buildings. The annual analysis undertaken in this review shows minimal differences between the materials used in window frames aluminum, PVC and wood. However, it was observed that PVC and wood lead to a better thermal performance of the simulation model. 7. CONCLUSIONS The main conclusion that can be made from this study is presented in Figure 7.1. This figure shows two distinct solutions for windows definition a good solution and a bad solution. As can be seen by the values shown in Figure 7.1, a careful design of windows can greatly reduce the primary energy use for heating and cooling of the housing buildings. In case of the simulation model used in this study and for the two solutions presented in Figure 7.1, this reduction is about 0.67 kgep/m 2.year. 9

10 Figure 7.1 Comparison of the primary energy use for air conditioning between two solutions: a good solution and a bad solution. At green, it is shown the result for the favorable solution and at red, the result for the solution notrecommended. Results were simulated for the Lisbon climate. Looking for the large scale of housing buildings construction that occurs in these days, the correct definition of windows can reduce significantly the energy use of buildings and, with this, some problems, such as the scarcity of fossil fuels and the high emissions of CO 2 to the atmosphere. REFERENCES [1] Perdigoto, J., Portugal Eficiência 2015 Plano de Eficiência Energética nos Edifícios em Portugal, Seminary CYTED, INETI, Lisbon, October [2] Decreto-Lei nº80/2006 de 4 de Abril Aprova o Regulamento das Características de Comportamento Térmico dos Edifícios (RCCTE). [3] Decreto-Lei nº79/2006 de 4 de Abril Aprova o Regulamento dos Sistemas Energéticos de Climatização em Edifícios (RSECE). [4] Decreto-Lei nº78/2006 de 4 de Abril Aprova o Sistema Nacional de Certificação Energética e da Qualidade do Ar Interior nos Edifícios (SCE). [5] Department of Energy; Energy Efficiency and Renewable Energy, EnergyPlus Manual, US Department of Energy, USA, April

11 [6] Windows & Daylighting Group; Building Technologies Program; Environmental Energy Technologies Department; Lawrence Berkeley National Laboratory, Window 5.0 User Manual For Analyzing Window Thermal Performance, LBNL, USA, November [7] Carmody, J.; Selkowitz, S.; Lee, E. S.; Arasteh, D.; Willmert, T., Window System for High- Performance Buildings, W. W. Norton & Company, New York, [8] Lopes, N. V., Reabilitação de Caixilharias de Madeira em Edifícios do Século XIX e Início do Século XX Do Restauro à Selecção Exigencial de uma Nova Caixilharia: o Estudo do Caso da Habitação Corrente Portuense, FEUP, Porto, December [9] Moret Rodrigues, A.; Canha da Piedade, A.; Braga, A. M., Térmica de Edifícios, Edições Orion, Lisbon, March