The Impact of Building Orientation and Window-to-Wall Ratio on the Performance of Electrochromic Glazing in Hot Arid Climates: A Parametric Assessment

: The signiﬁcant increase in construction industry demand with its negative effects on energy consumption is particularly evident in areas with severe climatic factors. Here, the designers aim at providing comfort with the least amount of energy, and consequently have resorted to using different software tools to choose the optimal solution in the early phase of design to avoid time and cost losses. The use of smart innovative technologies such as electrochromic (EC) glazing may provide an important contribution in reducing consumptions while providing good thermal and visual comfort conditions. Nonetheless, as shown in the literature, such glazing should be used and managed carefully to avoid any adverse effects or low performance. Thus, a parametric simulation was carried out using Rhinoceros and Grasshopper to assess the advantages in terms of energy use resulting from use of EC glazing in residential buildings as a function of façade orientation and window-to-wall ratio (WWR) with reference to the city of Biskra, located in the northeastern region of Algeria. Eight main orientations and ten WWR scenarios were investigated in order to evaluate the beneﬁt of using this technology in the selected climatic region. The research results proved the beneﬁt of using EC glazing in all orientations, with energy savings ranging from 35.88% in the northern façade to 60.47% in the southwestern façade.


Introduction
Two compelling challenges of current times are the timely fight against climate changes and fossil fuels dependency.Since the construction sector is responsible for 35% of worldwide CO 2 emissions [1], and residential buildings in Algeria consume approximately 33.6% of total electricity demand [2], an adequate choice of building materials envelope can play a major role in minimizing the negative impacts of unsustainable energy resources on the environment.The largest effects in terms of reduction of energy consumption of buildings can be obtained by improving the weakest link in the building envelope: the windows.In hot climates, the window-to-wall ratio was determined to be the most important indicator of total load, particularly for cooling loads, whereas it was negligible for heating loads [3].
Among the diverse solutions that can be used to control window behavior, one of the most promising is represented by dynamic smart windows based on EC glazing (Figure 1).An EC glazing is a device resembling an electrical battery, generally including five superimposed layers, between two glass or plastic substrates, covered with transparent conductive oxides (e.g., indium tin oxide or fluorine-doped tin oxide): the middle layer is a transparent electrolyte (containing small cations, such as lithium or hydrogen ions).This layer is in direct contact with at least one EC material and an ion accumulation layer, or a further EC layer [4,5].The most widely investigated EC materials are inorganic transition metal oxides, such as tungsten trioxide (WO 3 ) or nickel oxide (NiO).The former is a cathodic EC material, because it reversibly colors upon electron insertion and ion intercalation, by applying a small bias, in a redox reaction.Anodic EC materials, such as NiO, show a "complementary" behavior, because they color upon charge extraction and ion deintercalation.Moreover, they effectively work as ion storage layers when devices are bleached.Due to their behavior, upon ion intercalation/deintercalation, these two materials have been often used jointly to enhance visual contrast of EC devices [6,7].
Different manufacturing processes have been used in the fabrication of EC devices (Table 1), where the main aspects are the EC materials and the deposition of the ion accumulation material onto a transparent electrode.These depositions can be made either in liquid, vapor, or solid phase [9].
According to Jud Einstein et al., the sol-gel process was firstly used in the fabrication of EC devices in 1988.It is a chemical solution process that is one of the simplest techniques, compatible with spraying, spin-coating, and dip-coating deposition [10].
In comparison to sol-gel, other techniques that are widely used, but have more advantageous characteristics (Table 1), are the physical and chemical vapor depositions, that take place under vacuum conditions [11].
Physical vapor deposition is a vaporization coating method in which a solid material is heated to produce vapors that cover the surface of the substrate [12].In chemical vapor deposition, the precursor heated is in gaseous form and reacts chemically [13].Several research activities state the importance of window orientation, type of glazing, and window-to-wall ratio (WWR) for the energy efficiency of buildings.A study conducted by Mehaoued and Lartigue [35] in Algeria showed that in hot regions, augmenting the WWR led to a rise in the cooling demand caused by an increase in the temperature of the air surrounding the building due to multiple reflections of the heat flux.In the same context, Leskovar et al. [36] found that the impact of increasing WWR is less critical in the north than in the south orientation.
Hassouneh et al. [37] studied the impact of eight types of window glazing on thermal behavior of an apartment located at latitude 32 • N in Amman.The results indicated that a larger window with clear glass facing east, west, and south reduces heating load.
According to Friess et al. [38], in hot climates, the building orientation is the first factor that must be taken into account in passive design.
Yang et al. [39] studied the impact of WWR on the heating and cooling energy consumption of residential buildings in the hot summer and cold winter zone in China.They reported that the most important parameters for determining the optimum WWR are air conditioning system, window orientation, and glazing types.
Qiong et al. [40] tested twenty types of glazing, in three different Chinese climates, using Design Builder software.It was found that 6 mm double-glazed units (equipped with tinted glass panes) embodying 13 mm air interspace showed advantages in a climate characterized by hot summer and cold or warm winter, compared to single glazing and double-glazed units filled with argon, which was more expensive, while the same glazing (without tinting) was more suitable in cold climate.
Sibilio et al. [41] concluded-in their review paper dealing with smart windows for residential applications-that when using EC glazing, the energy saving may range from 39% to 59% compared to conventional static glazing, depending on the building orientation, climatic conditions, and control strategy.
Piccolo et al. [42] used an experimental and theoretical model to investigate the behavior of EC glazing based on WO 3 in a cooling-dominated climate.The results showed that the performance of EC glazing in dynamic mode was comparable to that of lowemissivity double glazing (about 31% savings) in comparison with clear glass.
Another study [43], based on EnergyPlus dynamic simulation software, was conducted in an office building located in Milan, with the goal of determining the best option among three different types of glazing (EC glazing, common glass, and one equipped with an external venetian blind system), as compared to standard glazing.EC glazing decreased energy consumption by 39.5% and by 26.2% when using an external venetian blind device.
Using the same software as Ref. [43], Cannavale et al. [44] compared the efficiency of innovative solid-state EC devices with transparent glazing in commercial buildings in three different locations (Rome, London, and Aswan): the best scenario was discovered in Rome, using innovative EC devices, with energy savings ranging from 20.7% to 28.7%.
A study using the eQuest building simulation tool [45] analyzed three different United States climate zones; the model was a commercial office building with 60% WWR, according to a comparison of static and EC glazing efficiency: energy savings of up to 45% were observed in various climate zones.
Aldawoud [46] also analyzed the effect of EC glazing on building energy consumption in a hot and dry climate, using Design Builder software to compare EC glazing to various glazing types.The reduction in solar heat gains varied from 53% to 59%.
According to Piccolo et al. [47], the performance of EC glazing in residential buildings has been investigated in a few simulation studies, one of which was conducted in a cooling dominated climate, using Grasshopper software.The results show that larger EC window openness area on the exterior can be used without affecting the energy [48].
Tavares et al. [49] used the ESP-r software to estimate the energy savings for heating and cooling in the case of using a commercial EC glazing in a Mediterranean climate.The case study was an old building in Portugal, which was simulated in various window orientations with different WWR and different control strategies (outdoor and indoor air temperature, solar irradiance); the results indicated that the energy savings ranged from 20.28 to 36.94 kWh/m 2 year (per square meter of glazed area), and the best saving was obtained in the west orientation.
According to previous studies, we observed that several interactive parameters affect the energy performance of buildings, and this point confirms the importance of using a parametric approach to simulate the building behavior.On the other hand, the lack of research on the impact of using EC glazing in Northern Africa, as well as the absence of building energy regulations in Algeria, complicate the choice of designers, hence the importance of highlighting the performance of such new technology in this country.

Climatic Condition
The case study of this work was located in the northeast part of the Algerian desert, at latitude 34.80 • N, longitude 5.73 • E, and an altitude of 82 m above sea level, classified as a hot and dry climate according to Köppen-Geiger classification [50].
Climatic data were taken from the international weather for energy calculations database for the location of Biskra [51], where the climate is influenced by solar radiation, given in Figure 2b, that represents a summary of the different monthly average solar irradiation values for different exposures.

Building Description
In the present study, the building context was a typical residential project located in the city of Biskra consisting of four floors, each one hosting two apartments, composed of two bedrooms, kitchen, bathroom, hall, and a living room; based on the Algerian national office of statistics [52], it represents a typical apartment in collective housing model in Algeria.
The case study deals with a southwest-oriented living room with 8.5% Window-to-Wall Ratio (WWR) and a surface area of 19.09 m 2 , representing about a quarter of the apartment total area (80 m 2 ), located on the ground floor (Figure 3).Based on typical characteristics of similar buildings, the material specifications of the boundaries of the space to be analyzed were defined and are listed in Table 2.As said above, there is no building regulation in Algeria that gives reference values for U-values.The opaque building envelope data and characteristic have been fixed for all the case studies to appreciate the effect of glazing.With reference to transparent elements (listed in Table 3), three different options were evaluated, assuming a laminated glazing based on the characteristics of the base case, and then it was compared to EC glazing to estimate the amount of energy saved when used in the real-life building.
Due to the significant differences in the properties of both glazing, an intermediate case (double vacuum glazing) was considered, taking into account the similarities between the double vacuum and the EC glazing in terms of thermal characteristics, cost, layer number, and pane glass thickness, with one exception: the thickness and materials of the space between panes.
As the EC glazing is significantly more expensive than the standard static glazing used, the vacuum glazing was chosen based on the comparable price range of 300 to 500 €/m 2 according to the market information in 2021 [53,54].The high cost of both glazing is due to their manufacture processes, which both need high vacuum conditions.
For the EC glazing, it was assumed based on its commercial availability by the manufacturer Sage Glass [55], even though it is less efficient than other types of EC glazing.Moreover, all glazing properties were calculated using LBNL Window 7.5 [56].

Simulation Data
The main aim of the research is to assess the efficiency of EC glazing in reducing solar heat gain and providing new design alternatives without increasing energy consumption, in order to address the problem of limited building openness and orientation flexibility in hot climates.
There are two basic simulation tools for evaluating building energy performance: static, which uses a simple mathematical procedure, and dynamic, which uses circumstances that vary over time [57].Recently, software based on parametric simulation has been developed, giving designers the ability to work out possible combinations between the different parameters, resulting in more variables.Another advantage of these tools is the possibility to use many plugins adding on the same platform; for example, Grasshopper links to Rhinoceros, Honeybee, EnergyPlus, Open studio, Ladybug, and Radiance [58].
In the present work, once the building and site specifications were defined, the model was implemented in Rhinoceros; afterwards, it was exported to Grasshopper.
In order to specify the zone properties, another plugin, named Ladybug, was used to identify the envelope materials used in the existing building and listed in Table 2, as well as to create the windows with the assigned ratio variation, with glazing properties (Table 3), calculated using LBNL Window 7.5 and then exported as an input data file.
In this step, all parameters were kept as in the reference building (Table 4) except the orientation, which was changed in eight different directions, with an increment of 45 degrees, with 0 • corresponding to north, 90 • to east, etc.
Because of the specific environmental conditions of the case study and in reference to literature review [59], the boundary conditions have been set as follows: •  The occupation period of the space has been defined as 12:00 to 24:00, according to the authors' own observations and to previous investigation [60].Indeed, the living room as a main space in the apartment is used mostly in the afternoon after work or school for family gathering and for recreative and pleasure activities in the night, such as watching television, playing, reading studying, etc.
For the building infiltration, an average building was considered, with an infiltration rate of 0.0003 m 3 /(s•m 2 ), the equipment load per area was supposed to be 5 W/m 2 , and 5 W/m 2 for lighting density [61].
Ventilation was calculated according to the windows opening, assumed to be two hours a day from 9 a.m. to 11 a.m. in order to simulate a realistic window-opening behavior of occupants that are supposed to be four individuals.
Using Honeybee plugin, the city's weather data were imported as an EPW file from the international weather for energy calculations database [51].
The component "EP output" was used to control the type of calculation and results, analyzed in terms of cooling and heating, that were calculated assuming an "ideal loads air system" converted to equivalent electric energy by a COP of 2.7 [62], as well as in terms of electrical lighting and overall consumption.
Through a parametric simulation, the WWR was varied from 8.5% to 10% and then, with 10% steps, to 90%, thus including 10 alternatives, with three different types of glazing (Table 3), where the laminated glass was replaced by a double vacuum glazing and commercial EC glazing, controlled by the "interior illuminance" strategy that ensures a 300 lux target over a virtual sensor, located at the center of the zone, 2.8 m above the floor.The living room window was also rotated in eight different orientations, obtaining 240 scenarios, summarized in Figure 4.
The consumption of the base case was then compared to the other variants, which were normalized with reference to floor surface area to allow for greater generalization of results.

Results and Discussion
The electric energy consumption of the reference case with laminated glazing as a function of orientation and WWR (Figure 5) illustrates that minimizing the WWR decreases the energy consumption and the disparity between the different orientations.
Furthermore, when the WWR is less than 10%, the difference between the eight orientations is insignificant, due to the decrease in sun radiation entering the building, and the heat transfer from outside to inside, while in the case of WWR 90%, the difference between the north and southwest facades reaches 39.31 kWh/(m 2 .y).
Due to the long hot season, the overall consumption is largely influenced by the cooling needs, with the maximum values corresponding to the SW exposure and minimum to the north exposure.It is clear from the results that the energy consumptions were affected by the solar irradiation (Figure 2b).The highest heating needs were found in the case of north orientation, which receives the least amount of irradiation for the whole year.On the other hand, the southwest façade receives convergent irradiation during the different seasons (maximum in spring, 3678.33 Wh/m 2 , and autumn, 3579.66Wh/m 2 ), which increases the cooling needs periods.
In all the orientations, increasing the WWR decreases the lighting demand, while in the case of north orientation, the lighting needs are slightly higher because of the relatively minimum amount of irradiation.Moreover, all the other orientations have approximately the same lighting consumptions due to the type of building, where the majority of consumption occurs at night, therefore the natural lighting has a negligible effect compared to artificial lighting.
Figure 4 summarizes results from the investigated scenarios, showing that the highest energy consumption was found in the base case (laminated glazing), and for both alternative glazing, the results show a positive correlation between the WWR and the energy consumption.However, the increment is small compared to laminated glazing, due to a decrease in the amount of radiation entering the building.
Moreover, in the case of EC glazing, the disparity of cooling needs between different orientations is less than 3 kWh/(m 2 .y).This results from the combination of both reduced solar heat gain coefficient (SHGC) and visible light transmittance (T vis ) that characterize the EC glazing in its colored state, which greatly reduce the amount of thermal radiation entering the building.In fact, during the cooling season, increased daylight illuminance contributes to set the EC glazing in tinted status in most of the time.
On the other hand, it was found that using double vacuum glazing reduces heating needs (to nearly zero from WWR 70% up, in the south orientation).This is because the high SHGC combined with low thermal transmittance decreases heat loss.Consequently, an increase in cooling needs can be observed.
In terms of energy expenditures for artificial lighting, EC glazing increased the consumption, compared to the reference glazing.This increase was inversely related to the WWR, due to the decreasing sunlight entering the space, while the consumption decreases when using double vacuum glazing, because the T vis of laminated glass is greater than T vis of the former.This slight difference is a result of the material used in laminated glazing between the two glasses (this type of glass is widely used in residential buildings in Algeria).In addition, the use of lighting during night hours results in a slight difference of consumption between the different types of glazing.
Figures 6 and 7 report the amount of energy saved after using the EC glazing compared to the double vacuum glazing and the reference window, respectively.Either way, the improvement was higher in terms of cooling consumption (a), due to the long cooling season, which is why the EC glazing outperforms vacuum glazing in terms of overall consumption reduction (d), even though the vacuum glazing is more efficient in reducing the heating needs because of the high SHGC value.
In Figure 7b, which represents the variation in energy required for heating, a small increase was found in the south orientation, which receives the highest irradiation in December (4635 Wh/m 2 ), after using EC glazing as a result of reduced solar radiation entering the space.Conversely, in the north orientation a slight decrease was found, thanks to the improved U-value of the EC glazing compared to simple laminated glazing.
Moreover, the best saving observed for overall consumption was about 62.87% compared to laminated glazing and about 60.47% compared to double vacuum glazing, which is consistent with previous research that found improvements up to 59% [41], while the findings are higher than the energy saving reported in [42] (31%), in [43] (39.5%), and in [44] (28.7%).

Conclusions
This research activity focuses on the energy savings that can be obtained using EC windows in residential buildings located in a hot arid climate.The chosen glazing has already been commercialized by a well-known manufacturer and we are strongly confident that using EC glazing with improved characteristics may yield even better results.
As a consequence of the yearly long period of overheating, the cooling is more important, and heating shows minimum loads.
Furthermore, the results led to some design recommendations; since the space function impacts the choice of orientation, it is recommended, in the case of laminated glazing, to orient the living room toward north with WWR up to 90 as it is a space that needs a panoramic view.This orientation also prevents the interference of intense luminance with the television; however, it may be oriented toward the south to ensure a warmer ambient in winter plus more of the natural light, without exceed WWR 50%, which has a similar overall consumption with WWR 90 north-facing.
Meanwhile, it is preferred to orient the bedroom toward the east so it obtains the advantage of facing the sunrise plus keeping it cool at the end of the day.
In the kitchen, the internal heat gains are high due to cooking, so north orientation is an adequate choice; however, the zone may be dimly lit; therefore, a double orientation northeast to southeast will increase the benefit of the natural light in the morning.
As the bathroom and garage do not require an outside view, a double northwest or southwest orientation choice will be sufficient.
The best advantage resulting from the use of EC glazing was found for the orientations with high solar radiation, where it allowed obtaining significant savings compared to laminate and double vacuum glazing.
The best improvement, compared to double vacuum glazing, was obtained in the case of WWR 90% for southwest facing windows, with 60.47% savings for overall yearly energy consumption, followed by south orientation with 59.85%, southeast orientation with 59.83%, west orientation with 55.81%, east orientation with 55.71%, northwest orientation with 45.89%, northeast orientation with 45.65%, and north orientation with 35.88%.
The results show that, starting from WWR 40%, in the case of laminated or vacuum glazing, the influence of orientation is greater than the influence of WWR.While in the case of EC glazing, the influence of WWR and orientations is negligible, due to the significant reduction in solar heat gain, thus solves the problem of limited WWR and orientation choices.
However, because of the relatively high cost of EC glazing, it is recommended that it be used when savings greater than a quarter of initial overall consumption are achieved, such as in the southwest orientation from WWR 20%, from 30% in southeast and south, from 50% in the northwest or east, or from 70% in the north or northeast.
The major limitation that obstructs large-scale market introduction of the EC glazing is the high cost of the production process that can be developed in the future to be less expensive, such as the (NPDS) process mentioned in the introduction.Even with the high cost, it is considered as a useful technology to reduce energy consumptions and, therefore, the environmental consequences obtained through CO2 emission to the atmosphere.
At this stage, visual and thermal comfort parameters were not included in the analysis, although other studies show that EC may significantly contribute to improve it.However, it will be interesting for future research work on this subject to concentrate on parametric optimization, in order to find a suitable balance between daylighting, thermal comfort, and energy consumption.

Figure 2 .
Figure 2. (a) Ambient air temperature ( • C).(b) Incident global irradiation on a vertical plane.According to the ambient air temperature of the city of Biskra shown in Figure 2a, the period from June to September represents the hot season with average maximum temperature of 35.5 • C and minimum of 29.5 • C, respectively; the hottest month is August, with an average maximum temperature of 42 • C and minimum temperature of 29 • C. While the cool season extends from the second half of November to the first week of March, with an average maximum of 19.5 • C and minimum of 13 • C, the coldest month is January, with an average maximum of 18 • C and minimum of 8 • C.

Figure 4 .
Figure 4. Energy consumption as a function of WWR, glazing window, and orientation for Cooling.Heating.Lighting.Overall consumption.

Figure 6 .
Figure 6.Variation in energy demand resulting from use of EC glazing instead of double glazing window, as a function of WWR and orientations for (a) cooling.(b) Heating.(c) Lighting.(d) Overall consumption.per unit area kWh/(m 2 .y).

Figure 7 .
Figure 7. Variation in energy demand resulting from use of EC glazing instead of reference window, as a function of WWR and orientations for (a) Cooling.(b) Heating.(c) Lighting.(d) Overall energy consumption per unit area kWh/(m 2 .y).

Table 1 .
Advantages and disadvantages of fabrication processes for electrochromic devices.

Table 2 .
Thermal characteristics of the building envelope.

Table 3 .
Thermal and optical characteristics of windows.

Table 4 .
Boundary conditions for the base case.