Assessment of the impacts of window-to-wall ratio and overhangs on building energy performance – Kabul, Afghanistan

. Windows have the potential to save energy in a significant way. Therefore, the analysis and optimization of the Window-to-Wall Ratio (WWR) play an important role in improving energy efficiency in buildings. Windows are always beneficial to utilize daylight and reduce lighting energy demand. However, solar heat gain is advantageous during cold days and disadvantageous on hot days. As the configuration of the window areas varies according to the geographical location, the objective of this study is to evaluate the impact of WWR on the energy performance of buildings in the cold semi-arid climate of Kabul, Afghanistan. In this study, various energy simulations were conducted for three types of building shapes (Square, Rectangle, and “L”-Shape) to determine the most energy-efficient proportion of windows and overhang sizes in different cardinal directions. Based on the simulation results, windows on south façades have notable energy-saving potential in Kabul's climatic conditions. However, increasing the window size on north, east, and west façade leads to an increase in the total heating and cooling energy consumption. The optimal WWR of the south façade window for square, rectangular, and “L”-Shaped is 0.7, 0.55, and 0.55, respectively. Additionally, the findings indicate that overhangs in all orientations slightly affect the total energy demand as they only reduce cooling load. The study concludes that optimizing the windows on the south façade reduces energy demand by up to 8.13% if no windows are provided in other directions, while the total energy consumption rises by up to 36% as the WWR on the north, east, and west façades increase from 0 to 0.8.


Introduction
During the past few decades, buildings energy consumption contributes significantly to global energy demand and greenhouse gas emissions (GHG) [1]. Therefore, reducing the building's energy consumption and promoting sustainable energy resources for building industries are of the utmost importance nowadays. The buildings industry as one of the "top three" industries in terms of global energy consumption, accounts for 36% of global energy consumption and 37% energy related CO2 emissions [2]. Only residential buildings utilize 22% of this total, and the majority of that amount is mainly spent on spaces heating and cooling to maintain comfortable indoor space [2], [3]. As overall efficiency increases, the energy demand for the industrial and transportation sectors tends to decline; however, this is not the case for the construction sector. The construction industry's energy demand will continue to rise as a result of population expansion and prompt urbanization, which will have a substantial impact on the environment and offer great energy savings potential [4]- [6]. * Corresponding author: mustafa7karimi@gmail.com Due to the rapid urbanization and population growth in the last two decades, the housing demand skyrocketed in Afghanistan's major cities particularly the capital city, Kabul [7]- [9]. To accommodate this rapid growth many informal settlements popped up around the city causing serious social and environmental issues [7], [10], [11]. A study about post-2000 buildings in Kabul city rates these buildings as significantly low in terms of sustainability level [11]. In addition, energy dependence and the use of coal, wood, LPG, and other biomass as an alternative source of energy exacerbate economic and environmental issues in Kabul [12]. Although Afghanistan is one of the lowest energy consuming countries in the world and has a large energy generation capacity, it imports 78% of the total electricity consumption [13], [14].
To address these issues, it is crucial to improve energy efficiency in buildings by the fundamentals of design rather than using energy-efficient mechanical services. This method is a cost-neutral energy-saving approach that not only does not increase building initial costs but also reduces the building's life cycle costs [15]- [17]. Architectural design parameters that effect the energy performance of building such as orientation [18], [19], building form [20], [21], fenestration ratio [22], [23], shading devices [24], zoning [25], [26] and etc. have extensively been studied. Among all the aspects involved in the design process of buildings, the window-to-wall ratio (WWR) is important as it influence both the aesthetics and the energy consumption [27], [28].
According to the previous studies, the optimal WWR of a building varies according to its geographical location and the climatic condition. Muhaisen et al. [29] stated that the optimal window area for all facades of a building in the hot and humid climate of Gaza strip is 10% of the total wall area. Moreover, In Gaza south windows are considered the worst in terms of energy efficiency. A study of building located in Vancouver, Canada by Kim et al. [22] confirmed that by increasing the WWR, the annual energy consumption of the building increases regardless of window location. It was observed that the minimum energy demand of the building occurs when WWR is 0% and it reaches the maximum at WWR of 100% which is 45% more than the minimum value. Another study by Goia et al. [30] investigated the optimal WWR of office buildings in different European climates. They concluded that WWR of larger than 0.3 and smaller than 0.45 are considered as ideal WWR for all orientations except southorientation, and energy saving of 5-25% is achieved by WWR optimization. Rana et al. [31] suggested the WWR range between 30% to 40% as energy efficient WWR for the air-conditioned office buildings in Bangladesh, and added that an energy saving of up to 9.4% is achieved by incorporating the optimal WWR. Feng et al. [32] stated that in severe cold regions, windows on east and west façades have the greatest impact of on energy consumption followed by south and north façades respectively. They confirmed that the most energy efficient WWR for east and west façade is between 10-15%, for south 10-22.5%, and for north is 0%, unless required for lighting or ventilation purposes.
As fenestration ratio is influenced by geographical location and climatic conditions, it is important to study the impact of window size on energy efficiency of buildings in the semi-arid climate of Kabul. Moreover, the window area of different shapes of a same size room varies for a given WWR. Therefore, this paper aims to find the optimal WWR in different orientations that provide adequate thermal comfort and high energy efficiency in buildings, resulting in a reduction in energy consumption of three same-sized rooms with different shapes in the climatic condition of Kabul.

Methodology
Within the scope of this study, three common building shapes such as square, rectangle, and L-shaped are developed and simulated in a dynamic energy simulation software of THERB to evaluate the impact of WWR and overhangs on energy performance. To investigate the impacts of WWR in all four cardinal directions, the WWR of the models is change from 0 to 0.8 while keeping the other variables constant. To study in impact of overhangs on energy demand, the size of overhangs is changed from 0 to 160cm for windows on all four cardinal directions.

Simulation tool
Building Energy Simulation Tools (BEST) are essential for building energy efficiency studies since they are cost and time saving tool. BESTs are employed not only for evaluation of buildings energy efficiency, but also used for the optimization of parameters influencing the energy performance [33]. THERB energy simulation software is used for this study.
THERB (Simulation software of the thermal environment of residential buildings), is a dynamic simulation software that calculates indoor temperature, humidity, sensible temperature, as well as heating and cooling load for buildings with multiple zones [34]. Moreover, THERB is a Heat, Air and Moisture (HAM) simulation software developed for the purpose of estimating the hygrothermal environment within buildings. This software has complete HAM features including principles of moisture transfer within walls and is one of the official energy simulations software approved by Japanese government [35]. Generally, simulation software that predicts temperature, humidity, and heating/cooling load of an indoor environment does not consider the moisture transfer in wall assemblies. In most of the software, the calculation of humidity level is simply affected by ventilation focusing on just the building spaces. However, THERB can simulate humidity conditions in both building spaces and wall assemblies in detail [35].

Study model
To simplify the simulation process, three shapes of room such as square, rectangle, and L-shaped are considered for this study as they are the basic and common spaces for residential and hotel buildings. The floor area, ceiling height, door's width and heigh for all the models are equal. The ceiling height is kept 2.75m which is common height for residential building in Kabul and provides better thermal comfort for cold regions [36]. Moreover, widely used construction materials with high thermal properties are considered for this study. One of the most significant personal variables of thermal comfort that can be directly influenced by an individual is thermal insulation from clothing [37]. As full body covering clothing is commonly worn year-round in Afghanistan, particularly during winters. Therefore, the heating set point is set at 20 °C and the cooling set point is set at 24.5 °C ( Table  1). THERB offers the option to alter the airflow rate throughout the day. Therefore, the room ventilation is considered higher during hot season and lower in cold season. For this study, ACH (Air Change Hour) rate varies between 0.5 to 10. To provide adequate indoor air quality, a residential building should have an ACH of 0.5 or higher [38]. Fig. 2Fig. 3 show the hourly forced ventilation of the room in a day during cooling and heating periods. ACH is kept minimum during cold season as most people rarely or never open the windows and the air exchanges via infiltration or the door openings.

Climatic condition
According to the Köppen climate classification, Kabul's climate is categorized as continental, cold, semi-arid type BSk. Kabul has its greatest amount of precipitation during spring and winter (in form of snow). While summer is dry [39], [40]. Fig. 4 shows the average high and low temperature in Kabul.

Results and discussion
The results shown below presents the impacts of WWR and overhangs on energy performance of a room in climatic condition of Kabul, Afghanistan.

Window-to-Wall ratio (WWR)
This section explains the impacts of the change in WWR of all four façades of the study model on its energy demand. The graphs in the following figures (Fig. 5Fig.  6Fig. 7Fig. 8) show the annual energy demand for heating, cooling, and total of heating and cooling. As shown, heating load is dominant in climate condition of Kabul as the city experience very cold winters. According to the findings, the annual cooling energy demand rises by increasing the WWR on south façade, while the annual heating energy demand decreases after a slide increase between WWR 0 to 0.05. The rise in heating energy demand between the WWR 0 to 0.05 is because the heat loss through small windows (WWR=0.05) in winter is greater than the solar heat gain compared to rooms without any window. Since the solar heat gain correlates directly with the size of the fenestration, by increasing the size of windows, the solar heat gain rises. Thus, the cooling energy demand also goes up. On the other hand, the large windows allowing more solar heat gain will decrease the heating energy demand during cold seasons. However, all three graphs of total cooling and heating energy demand forms a Ushaped curve after a rise between WWR 0 to 0.05 with the minimum values when the WWR is 0.7, 0.55, and 0.55, respectively. This is due to the steady decrease of the heating load of WWR from 0 to 0.7 and 0.55. Increasing the WWR above those values have little effect on the heating energy demand, while the cooling load increases consistently. Therefore, the total energy demand increases if the WWR rises beyond 0.7 and 0.55. The difference between the total energy consumption of WWR 0.05 and WWR 0.7 and 0.55 for square, rectangular, and L-shaped rooms is 8.1%, 8.13%, and 4.2% respectively. As a result, the ideal WWR for windows on the south façade without shading devices is between 0.55 and 0.7. Fig. 6 shows the cooling, heating, and total energy demand of different WWR of windows on north façade of the study models. The graphs demonstrate that increasing the WWR on the north façades of the shapes result in an increase in the energy demand for heating and cooling. This is due to the fact that the solar radiation never penetrates from the north side of buildings located in the northern hemisphere. Therefore, avoiding any kind of openings in the north façade unless required for lighting or ventilation purposes would be more energy efficient. As per results, the total energy demand of square room rises by 12%, rectangular by 18.4%, and L-shaped by 13.3% as WWR of north façade windows increases from 0 to 0.4.

WWR of East and West façade
As shown in the Fig. 7 Fig. 8, larger windows on both east and west façade results in higher total energy consumption. It is mainly because of the sun's movement. In the northern hemisphere during summer, the sun rises from the northeast and sets in the northwest. On summer mornings the sun moves from the northeast to its highest altitude in the south, and on evenings it moves from the highest altitude towards the northwest at a low altitude which allows sunlight to penetrate through windows on either the east or west façade resulting in higher cooling energy demand. The difference between the total energy demand of east façade windows with WWR 0 and 0.8 is 36%, 28%, and 29.3% for square, rectangular, and L-shaped rooms respectively. Moreover, the increase of WWR from 0 to 0.8 for west façade windows increase the total energy demand by 26% for square, 19% for rectangular, and 17% for L-shape room. The cooling load for windows on both east and west façades increase by the increase in window area but the rise is more rapid in case of windows on east façade compared to west façade windows. On the other hand, increasing WWR results in reduction of heating energy demand for windows on east façade while it increases the heating load for west façade windows. As both the heating and cooling energy demand increase by increasing the size of windows on west façade, therefore windowless façade on west orientation would be more energy efficient. However, windows on east façade with effective shading device that can protect windows from solar radiation during summers can reduce energy demand.

Overhangs
Following figures illustrates the impacts of various overhang depth on energy performance. A window of 2.5m X 2m (h x l) for all four façades is considered for this study. Fig. 9 presents the annual energy demand of the square shape room with variation in its overhang depth of south façade windows. The graph shows that as the overhang depth is increased, the heating load rises while the cooling load decreases. It is because overhangs block sunlight and minimize solar heat gain, which is only beneficial during the summer. Since overhangs on south façade block the winter sun, the total energy demand increases.

Overhang of North façade
According to Fig. 10,overhangs on north façade windows have no impact on total energy consumption. Therefore, avoiding overhangs of any size in the north orientation would be advantageous.  depth increases, the shadow size also gets bigger resulting in lesser cooling demand during the summer. However, overhangs of east and west façades do not significantly affect heating loads since the solar radiation from east and west orientation is only for a short time during cold seasons.

Conclusion
According to this study, the windows area of various façades, as well as the overhang size, affect cooling, heating, and total energy demand of a building in the climatic condition of Kabul. A short summary of the findings is as follows: x Providing windows without shading device only on the south façade have the potential to save energy. The optimal WWR changes as the width-length ratio of the rooms varies. For a 1:1 ratio the WWR is 0.7 and for 1:2 it is 0.55. Moreover, an energy saving of up to 8.14% can be achieved by optimization of WWR on south orientation. x Windowless façades on north, east, and west orientations would be more energy efficient compared to façades with any size of windows. x Windows on the east façade are comparatively better than the west façade windows in terms of energy efficiency as they contribute to heating load reduction. Furthermore, windows with appropriate shading on east façade can reduce total energy consumption.
x Overhangs only on the east façade can reduce energy consumption, while they increase the energy demand if provided on the south façade. x East and west façades receive solar radiation at a very low angle during the hot season, requiring long projection of overhangs that are difficult and inefficient to construct. Therefore, vertical shading devices on both the east and west façades would be more effective.
x Windows of smaller hight on the east and west façade are more efficient as they require small overhang projections.

Heating
Cooling Total