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Assessment of retrofitting smart glass on office building Façade to enhance thermal comfort and energy consumption in hot arid climates (Case study in Egypt)

Author links open overlay panelMennatAllah Hassan, Samaa Alaa, Farah Sherief

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Abstract

Thermal comfort and energy consumption are critical factors to consider while designing spaces to improve a building’s performance. Due to economic development, an increase in the number of commercial buildings has evolved. There are various elements in office buildings’ design that directly influence thermal comfort and electrical usage, such as the building’s envelope. The Sustainable Development Goal (SDG) 7 of ensuring access to inexpensive and reliable energy is crucial when choosing building materials, particularly particle board, as its application has a significant impact on energy consumption. According to the current expansion in the number of office buildings having transparent building facades, a negative impact on indoor thermal comfort and energy consumption has been elevated. Therefore, innovative glazing technology such as smart glass is a suitable alternative to enhance the facade thermal performance. Thus, the aim of this research is to test both, the traditional and smart glass types on building’s façade through a calibrated model in a hot arid environment and optimize different smart glass alternatives to improve the indoor thermal comfort and reduce energy consumption. This is achieved through an analytical approach is conducted using quantitative simulation data using case study (The Arab Academy for Science, Technology, and Maritime Transport AASTMT) to test the impact of smart glass on the energy consumption without compromising the thermal comfort using Design Builder. The obtained results demonstrate that integrating any type of smart glass into office building transparent façade significantly contributes to the achievement of better interior thermal comfort as well as reduction in building’s electrical consumption. For instance, the best results were obtained when the Suspended Particle Devices SPD smart glass was tested, which reduced the total site energy and unmet hours by 46 % and 92 % respectively.

Keywords
Thermal Comfort
Energy Consumption
Smart Glass
Office Buildings
Design Builder Energy Simulation
Optimization

Nomenclature

SPD

ASHRAE

WWR

PDLC

Photochromic

Thermochromic

Electrochromic device

AASTMT

SDG

1. Introduction
Buildings are significant contributors to global energy consumption, with a significant amount of energy consumption from heating, cooling, and lighting systems [1]. Office buildings with large glass facades can attract more solar heat gain, leading to increased energy demand and thermal discomfort (Ahmad & Reffat, 2018). This issue increase in hot arid climates like Egypt, where high temperatures and intense solar radiation play a significant challenge to building energy performance. Specifically, the electrical consumption rate in Egypt has increased dramatically in recent decades until it surpassed 165 % as mentioned by the Ministry of electricity and renewable energy (2018). To address these challenges, innovative building technologies, such as smart glass, have emerged as promising solutions as an attempt to create smart cities. Smart cities, driven by technology, are increasingly focused on sustainability and resilience in the face of climate change. One innovative solution lies in smart building glazing. By incorporating advanced technologies into window systems, buildings can adapt to changing weather conditions, optimizing energy efficiency and occupant comfort. Smart glass offers dynamic control over solar heat gain and visible light transmittance, enabling adaptive building envelopes that can respond to changing environmental conditions. By integrating smart glass into building facades, it is possible to optimize energy consumption, improve indoor thermal comfort, and reduce reliance on traditional HVAC systems. However, the optimal selection and application of smart glass in hot arid climates remain subject to ongoing research. While previous studies have explored the potential benefits of smart glass in various climatic conditions, a comprehensive comparative analysis of different smart glass technologies in hot arid climates is still needed [2], [3], [4]. This research aims to fill this gap by evaluating the impact of various smart glass types on the energy performance and thermal comfort of office buildings in such climates. By utilizing advanced building simulation using Design Builder as a tool, this study will provide valuable insights for building designers and policymakers in promoting sustainable and energy-efficient building practices using transparent building façade.
2. Literature review

To improve thermal comfort and lower energy use in hot, dry areas is the incorporation of smart glass into building envelopes. The potential of smart glass to control solar heat gain and enhance indoor temperature control has been investigated in earlier research [5], [6].Hatem A. [7] for example, investigated how electrochromic smart glasses affected energy usage in Egyptian buildings and found that they significantly decreased cooling loads during the hottest summer months. Furthermore, Ismail Budaiwi [8] demonstrated how smart glasses could enhance occupant comfort by lowering glare and creating a comfortable indoor environment. According to these results, retrofitting buildings with smart glass technology may be a practical way of dealing with issues related to energy efficiency and thermal comfort in areas that experience intense heat. An evaluation of smart glass systems that help create a sustainable smart building envelope that addresses climate change and improves occupant thermal comfort is presented in another study using design builder simulation in hot climate [9]. Another paper assesses the most effective daylight and thermal performance of various smart glazing types and their impact on energy consumption in the climatic conditions of one of the office buildings (Diwan governorate) in Sohag governorate, one of Upper Egypt governorates [10]. While the study employs Design Builder to simulate a proposed office building, the individual changes indicated numbers lack clear analysis in relation to thermal comfort and psychometric charts. A more rigorous approach would involve a full-year simulation of a real building, incorporating detailed thermal comfort analysis and incorporating real-world energy consumption data. Existing research often relies on simplified models and limited simulation periods, neglecting the significance of year-round performance. Moreover, the integration of psychometric charts for a comprehensive understanding of indoor environmental conditions remains underutilized. Thermal comfort is a crucial aspect of architectural design, directly influencing occupant satisfaction, productivity, and overall well-being. Optimizing building designs to ensure year-round thermal comfort becomes more crucial as energy consumption. By addressing thermal comfort through innovative building materials, architects can create healthier, more sustainable, and energy-efficient buildings.ere 1 and Fig. 2 show the recent statistics for research trends on Scopus for the last 8 years. Research on smart glass and energy consumption has been broader in articles than smart glass and thermal comfort. It is widely researched in China and research is still limited in Egypt. (See Fig. 1).

Fig. 1. Scopus result for title and keywords search by country for keywords: Smart glass, smart windows, energy consumption, thermal comfort.

Fig. 2. Scopus result for title and keywords search over last 15 years.

2.1. Thermal comfort
One of the most significant conditions that users worry about in buildings is having a suitable indoor air temperature. The American Society of Heating, Refighting and Air-Conditioning Engineers (ASHRAE) has declared that the frequently used definition is stated as “the condition of mind in which satisfaction is expressed with thermal environment” (ASHRAE standard 55) as it was observed that a person may have a different level of thermal comfort than another who is present in the exact conditions of space and temperature. There are no precise limitations to thermal comfort measurement. since it depends on a variety of factors, including weight, age, gender, diet, body structure, and other individual variances [11]. However, in general, the dominant parameters that affect thermal comfort are classified as both personal including clothing and metabolic activities, while the other is environmental including air temperature, radiant temperature, humidity, and air velocity [12].
2.1.1. Importance of thermal comfort on users productivity in office building
In office buildings, interior environmental quality significantly influences occupants’ satisfaction and productivity [6]. Office building occupancy productivity significantly affects the financial performance and overall expansion of an organization [7]. Companies in industrialized countries have indicated that the cost of paying employees’ salaries is typically higher than the building’s operating expenses [6]. The productivity of occupants and the profitability of an organization might both significantly increase with an improvement in the indoor thermal comfort [13].
2.1.2. Impact of building envelope on thermal comfort and energy consumption
The structure envelope serves as a barrier between the inside of the structure and the outside environment [14]. The envelope regulates internal temperature and helps to reduce the amount of energy needed to maintain the desired thermal comfort [8]. Limiting the amount of heat gained using the building envelope is crucial to lower the amount of cooling energy needed in the hot humid climate [15]. The opaque and the transparent building envelope systems are the two most common types [9]. While clear skin systems comprise windows, skylights, and glass doors, opaque systems consist of walls, roofs, floors, and insulation [14]Since transparent façade allows more heat penetration through the building envelope, it highly contributes to increasing the indoor temperature [16]. For instance, heat gain through glazing windows can reach up to 85 % and increase in discomfort hours [17].
2.2. External building envelope
In addition to the building’s façade, there are other structural elements that can impact the internal thermal comfort [18]. Therefore, to minimize the fluctuation of thermal comfort measures of interior spaces, these elements should be standardized according to the Egyptian Code since this study is performed in Egypt. For instance, walls, floors, and ceiling all can influence a change in thermal comfort values [19]. The performance of such elements is determined according to its R-Value which is a measure of insulation and heat flow resistance of a certain material [18]. The higher the R-value levels, the higher heat transfer resistance and insulating efficiency, and thus, lowering energy consumption of the building [20]. Glazed units are a crucial component of building envelopes because they provide natural light, views, and solar heat into the interior of a structure [11]. The thermal performance of glazed units is significantly worse than that of other building components, and approximately 50 % of all energy consumed within buildings is lost or gained through windows [21]. This gain or loss has grown over the previous few decades because of a growing preference for fully glazed structures [22]. The window-to-wall ratio (WWR) is determined according to building codes and standards of each country [19]. For instance, the WWR is influenced by several factors, such as climatic zone, building location, and building’s energy performance that needs to be achieved (Y. Luo et al., 2019). WWR directly influences the quantity of solar heat gained entering the structure, which has a significant impact on the overall building's energy usage [23].
2.3. Smart glass
Smart glass is a high-tech material that, when used in a structure, it responds logically to climate variations over various seasons [22] by changing its level of transparency either manually or automatically when it is exposed to external change in environmental conditions [24]. The improvement of advanced high performing active glazing systems, intended to reduce heat loss, control incoming sun rays, increases solar gain in winter, decreasing solar heat in summer, as well as ensure the best natural lighting conditions with no glare [21].

One of the chief benefits of smart glass is that it has a much lower carbon footprint than the materials required to make and maintain the traditional blinds, making it a better environmentally friendly choice (Rezaei et al., 2019). On one hand, the initial installation cost of the smart glass might be slightly greater than that of other traditional types [23]. On the other hand, installing smart glass is a wise cost choice when weighed against the long-term advantages of this type of glass because the maintenance expenses are far lower than those associated with either blinds or conventional glass examples as the smart glass does not need to be replaced or repaired on a regular basis [23]. Durability of smart glazing can vary depending on the specific technology and manufacturer. High-quality products with proper installation and regular maintenance can last for several years. However, factors like frequent switching between states and exposure to harsh weather conditions can impact the lifespan. In addition, it can increase the building's energy efficiency, resulting in cost reduction. Finally, smart glass focuses on adding both aesthetics and functionality values to its building [25]. Intelligent glass can be classified into two types: passive glass and active glass, where each is further broken down into specific types [26]. For instance, the passive smart glass includes thermochromics and photochromic, while the active type comprises of electrochromic, Suspended particles devices (SPD), and Polymer Dispersed Liquid Crystal Devices (PDLC) [11]. Moreover, passive smart glass does not require any electrical stimulation and is controlled by external factors. On the other hand, the active glazing requires an electric current to work and is activated and controlled by the user [22]. The below Fig. 3 demonstrates the smart glass types in a diagram.

Fig. 3. Types of smart glass (Developed by Author).

2.3.1. Passive smart glass
Passive smart glazing can be categorized in two major types [11]. First is the photochromic Glass (PC) which adjusts its transparency when subjected to light, while the second is the thermochromic glass (TC) that changes its transparency because of temperature variation [18].

A.
Photochromic Glass

Photochromic glass may change its transparency level on its own depending on the amount of direct light it is subjected to [25]. This capability results from the inclusion of certain substances known as “optical sensitizers” in the glass particles, such as metal halides that react to ultraviolet light or polymers that absorb solar energy based on output color range change [21]. Once photochromic glazing is directly exposed to sunlight, a reversible process of vivid coloring is created because of the variation in spectrum absorption among the energy coatings of the glass panel and other materials resulting in changing of color from light to dark shade as shown in Fig. 4 [26]. The time it takes to switch from the colored state to the clear condition is often twice as long as the rate of response to environmental changes which takes few minutes [22]. When external brightness varies unexpectedly or when cast shadows on the structure result in uneven patches of light and shadow, these discrepancies in response to time might cause some issues [11].

B.
Thermochromic Glass

Fig. 4. Photochromic glass window [27].

Thermochromic coating could independently adjust its visual qualities responding to changes in chemical reaction between two distinct conditions caused by changes in the temperature of the external surface [21]. As a result, the material is transparent at temperatures below the transition point and turns opaque at higher temperatures as demonstrated in Fig. 5 [25].

C.
Mechanochromic / Piezochromic Glass

Fig. 5. Thermochromic glass window [27].

Mechanoresponsive smart windows operate on the rather simple principle that mechanical strain can alter the internal structures or surface morphologies of mechanoresponsive optical materials. This alteration alters the optical transmittance through visible light diffraction or scattering [28] as shown in Fig. 6.

Fig. 6. Mechanochromic glass window [27].

Photochromic, thermochromic and mechanochromic smart glasses have various features that are collected and represented in the following Table 1.
Table 1. Comparison between Passive Smart Glass types [29].

Passive smart glassPropertiesPhotochromicThermochromicMechanochromic

Response	Respond to light	Respond to temperature	Responds to external stress
Translucent State	When exposed to UV rays	above transitional temperature	When pressure is applied
Memory	Takes 30 s to darken and from 2 to 5 min to become clear	Changing of state takes hours depending on temperature transition and affected area	Varies
Changing Speed	Few minutes	Depends on affected area & transition temperature	Depends on applied pressure and mechanochromic layer thickness
Complexity	Low Simple coating or encapsulation	Low Simple coating or encapsulation	Medium The mechanical control system is required
Glare	Cannot control glare as it has same visible transmission for both cases	Cannot control glare as it has same visible transmission for both cases	Controls glare when in opaque state
Application	Applied on existing glass	Applied on existing glass	Cannot be used as a retrofitting solution
Privacy	Low privacy- can be seen through	Low privacy- can be seen through	High privacy when transitioned

2.3.2. Active smart glass
When voltage is added to the active smart glass, the characteristics of light transmission change [21]. Active intelligent glass, with the push of a button, transforms from transparent to opaque, allowing users to adjust the amount of light and heat going through while retaining a clear view of what is behind the glass [11]. Active smart glass technologies may be divided into three main categories, each of which has its own distinct features. The types are electrochromic device (EC), suspended particle device (SPD), and Polymer dispersed liquid crystal devices (PDLC) [25].

A.
Electrochromic Devices (EC)

The term “electrochromic” refers to substances that, when stimulated by an electrical current, may alter their color [25]. Electrochromic windows change from translucent to dark when voltage is applied and vice versa, while still maintaining its visibility state [11]. The user has complete control over the electrochromic glass, and it has a wide range of uses [30]. The EC glass, which can even block 99 % of sunlight, is regarded as one of the most energy-efficient glass types and greatly raises a complex's LEED rating [19]. Since energy is only needed to change the transparency level from one degree of darkness to another and not consumed to keep the acquired glass color in state, electrochromic technology is “green” technology. Also, by pressing a button, the room may be made darker in just a few minutes while still allowing light to illuminate the interior [17]. Fig. 7 illustrates the electrochromic layers and the process when it is switched on or off.

B.
Susupended Particles devices (SPD)

Fig. 7. Polymer Dispersed Liquid Crystal Devices PDLC [29].

This type is composed of 2 thin glass layers each coupled with a single energy conductor of clear thin plastic sheet where they sandwich the suspended particles in the middle as shown in Fig. 8. When the current circuit is connected, the particles of the floating rod align, light enters, and the SPD intelligent glass display becomes transparent [22]. The suspended rod particles block the light when the circuit breaks, making the glass seem dark in color. The quantity of light and heat going through may be instantly controlled because of SPD glass's ability to brighten or darken in this manner [31]. When it's dark, SPD dynamic glazing can prevent up to 99.4 % of visible radiation. Finally, SPD glass offers UV protection whether it is turned on or off [21].

C.
Gasochromic Glass

Fig. 8. Suspended Particle Devices SPD [31].

The way gasochromic windows work depends on the gas that is added to the system. A gasochromic substance, usually tungsten oxide (WO3), with a thickness of less than 1000 nm, and a thin coating of platinum, less than 15 nm, which serves as a catalyst, are applied to the inside surface of the exterior pane, which is often double glazed as shown in Fig. 9. Hydrogen and oxygen are the most used gases in gas chromic windows because of their relative ease of extraction (hydrogen via electrolysis) or abundance in the atmosphere (oxygen). The introduction of hydrogen tints the window, while the introduction of oxygen makes it more transparent [28].

D.
Polymer Dispersed Liquid Crystal Devices (PDLC)

Fig. 9. Gasochromic Glazing [32].

PDLC has 5 layers like that of the SPD [22]. However, instead of the suspended particles, this type imposes scattered small, liquefied crystal ball like spheres with a diameter as the wavelength of detectable radiation as presented in Fig. 8 [11]. When an electrical current is activated, the fluid crystals are ordered in constant order, guaranteeing the clarity of the glass sheets [21]. In the blockage of an electrical impulse, the molten crystals have a scattered layout, and the light beams go through random deflections, making glazed components appear brighter and translucent [25] as shown in glass creoss section Fig. 10. The voltage used can be utilized to regulate the transparency level. The light transmission of liquid crystal glazing in the active state frequently does not exceed 70 %, but in the off state it is only around 50 %, even though proper dyes may be applied to dim the device in the inactive phase [21]. In hot, arid regions, the PDLC system is used on residential buildings. A system was created utilizing a test cell that was put up in a Najran University room. The suggested treatment of PDLC smart system exceeds clear glass by 21 % in the south direction and 25.5 % in the west orientation, showing remarkable improvements in interior thermal performance.

Fig. 10. Polymer Dispersed Crystal Devices (PDLC) [33].

The different types of active smart glass contain various properties which need to be considered to wisely choose the most suitable type according to the design. Below are the most common properties presented in Table 2.
Table 2. Comparison between Passive Smart Glass types.

Active smart glassPropertiesElectrochromicSPDPDCLGasochromic

Power Used	Uses DC electric current.	Uses AC electric current.	Uses AC electric current.	Introduction of gas
Power Used	Voltage ranges 0–10 V	Voltage ranges 10–110 V	Voltage ranges 10–110 V	Typically, Hydrogen gas is used
Powered State	Opaque or tinted	Transparent	Transparent	Darkens to blue shades
Unpowered State	Transparent	Opaque or tinted	Opaque / translucent	Transparent
Memory	Glass stays opaque for some hours after the electric current is turned off.	Has no Memory, turns opaque when electric current is switched off	Has no Memory, turns opaque when electric current is switched off	Glass stays tinted until the hydrogen gas is dispersed
Changing Speed	Several minutes	Several seconds	Takes milliseconds	Several minutes
Glare	Can control glare	Can control glare	Can control glare	Can control glare
Intermediate State	Controllable	Controllable	Controllable	Limited
Application	Requires new glass	Requires new glass	Can be applied to existing glass or bought as new	Requires new glass
Privacy	Low privacy- can be seen through	Medium to high level of privacy	high level of privacy	Medium to high level of privacy

2.4. Active and passive smart glass durability and maintenance

Smart glass, when utilized as a building facade, offers significant advantages in energy efficiency and occupant comfort [34]. However, long-term durability and maintenance considerations are crucial. Factors such as material degradation, potential for damage, and the need for specialized cleaning and maintenance must be carefully assessed. Table 3 show the difference in the durability of the types of smart glass, the factors influencing the durability and the maintenance.
Table 3. durability and maintenance of passive and active glass [35].

Passive GlassActive glassDurabilityFactors influencing durabilityMaintenance

Empty Cell
Generally high: Passive smart glass technologies glasses typically exhibit good long-term durability.	Variable: Durability can vary significantly depending on the specific technology of glass.
• Electrode degradation: Over time, the electrodes within glass can degrade, potentially affecting switching speed and coloration (Granqvist, 2000). • Electrolyte leakage: While rare, electrolyte leakage can occur and damage the glass. • Color shift: Some glasses may experience slight color shifts over time due to exposure to UV radiation. • Performance degradation: The switching speed and temperature sensitivity of TC glass may gradually decline over its lifespan.	• Polymer degradation: The polymer matrix within films can degrade over time, affecting clarity and switching speed (Gooding et al., 2005). • Electrode wear: Electrodes can wear out, leading to reduced switching performance. • Particle settling: Suspended particles may settle over time, affecting the uniformity of the film's appearance. • Damage to the film: The film can be susceptible to damage from impact or abrasion.
• Minimal: Generally, require minimal maintenance. • Cleaning: Regular cleaning with mild soap and water is recommended. • Inspection: Periodic visual inspection is recommended to check for any signs of degradation or damage.	• Moderate: May require more frequent maintenance than passive smart glass. • Cleaning: Regular cleaning is necessary, but abrasive cleaning methods should be avoided to prevent damage to the film • Electrical components: Regular inspection and maintenance of the electrical components (e.g., control units, wiring) may be required. • Potential for repair: Some active smart glass technologies may require specialized repair or replacement of components.

Ideally, a smart window should maintain its functionality without degradation for at least 20–25 years. While commercially available smart glass solutions exist, their long-term durability, particularly under prolonged outdoor exposure, remains an area requiring further investigation. Comprehensive data on the long-term stability of these materials in real-world conditions is crucial for assessing their true potential and ensuring their viability for widespread adoption [35].
3. Methodology
The main aim of the study is to investigate the most effect smart glass to be applied on building envelope to enhance thermal comfort and energy consumption in hot climates. This study is focusing on Cairo, Egypt Climatic data as a hot arid climate. Taking into consideration the literature study, an experimental study will be carried for an existing building in Cairo for simulation modelling to measure the qualitative and analysis concerning Thermal Comfort and energy efficiency as environmental impacts affected. This experimental study will be held using design-builder as an industry-standard Building Energy Simulation tool, as the most advanced energy simulation tool.

Fig. 11 shows the research methodology map, as smart Glass is studied through the literature, thermal comfort and building envelope. The applied study carried an existing office building in Cairo modelled using Design Builder V7, used as a case study to investigate building performance. First the building will be modelled to match the existing one and compare energy consumption. After the model is validated, different iterations will be performed changing the glass façade with the new smart glass façade materials to test their performance. An optimization will be held using Design builder to evaluate optimum solution of smart glass on building envelope, minimizing energy consumption, and elevating thermal comfort.

Fig. 11. Research methodology map (Developed by Author).

3.1. The case study of Cairo, Egypt
The research case study is an administration building representing an office building managed by the Arab Academy for Science, Technology and Maritime transport (AASTMT) University. This office building is chosen based on its relevance to the research topic and represents various office buildings here in Egypt. Moreover, its façade is covered with a large area of curtain wall system which is essential to be considered as this research paper focuses on office buildings with transparent facades. In addition, the chosen case study suits the environmental conditions mentioned in this research paper. Finally, data availability plays a significant role in selecting the case study because specific information and numerical figures are required to use it as input data in the simulation. The building data and construction specifications needed were gathered through previous papers that conducted different tests on the same administration building [36].
3.2. Model Setting and input data
The empirical method of this paper follows a quantitative approach using simulation software Design Builder software. The software enables its user to assess different aspects of building performance by producing different reports for comparison which in turn could help optimize building design using Energy Plus.

The workflow for the processing simulation input data generating output results in shown in a flowchart in Fig. 12.

Fig. 12. Flowchart of simulation input and output (Developed by Author).

The case study building (AASTMT) has a two attached rectangle layout as shown in Fig. 13 and modern elevation design covered in blue transparent openings, as shown in Fig. 14. It is in Smart Village campus, Giza governorate, Egypt. Its location represents a hot arid zone where the solar radiation varies between 5.4 and 7.1 Kwh/m2 from north to south facades [36].The building is made of reinforced concrete (Skelton system), and the bottom floor is shaded from all sides. The elevations are mostly covered with tinted double glass. The roof of the structure is flat and highly insulated. The structure is made up of two basements with a combined area of 8000 m2, six typical levels with an average area of 2000 m2, and a total height of 27 m. To regulate the indoor air quality, the building features central heating, cooling, and ventilation systems. An air-cooled liquid chiller (package direct expansion unit) with electrically powered heat recovery systems is utilized as the cooling system's primary tool in the summer. Mechanical ventilation is used on the two basement floors. Moreover, 1800 people reside in the facility, which typically operates for 12 h every day from 6 am to 6 pm. The power is only shut off at the weekends, except for control rooms [36], [37].

Fig. 13. Arab Academy for science, Technology and Maritime AASMT.

Fig. 14. AASTMT Layout.

The input data are divided into two parts; the first part contains the constants which will only be inserted once into the simulation to set up the building model, which is represented in Table 3.

As for the second part, it consists of the variables of the five different types of smart glass which will be tested, analyzed, and compared to the base case, which is mentioned in Table 4 and Table 5. Table 5 show the different material specifications of the base case glass and smart glass in both conditions the original state, which is transparent and the secondary state which is dimmed. This table demonstrates specifications and sections of the glass types, including both the base case and the smart glasses, that will have changeable values when simulating the digital model.
Table 4. Building model specifications (Developed by Author).

Building DataTypeDescription

Building Type	Medium Size Office
Location	Giza, Egypt
Fuel Type	Electricity
Form
Area/floor	2000 m2
Building shape (L shape)
Number of floors	6 Floors
Floor height	4 m
Occupant Density
Number	10 m2/person
Total Number	1800 person
Lighting
Installed Lighting load	9 W/m2
Electrical System	500 Lux
Window
WWR	80 %
Window Location	Distributed equally on all facades
Sill height	0.5 m
Window Glass
External Glazing U value	0.26
Glass Solar Transmittance	0.76
Glass visible Transmittance	0.82
Exterior Wall
0.20 m Brick, 0.02 m Plaster (from 2 sides)	R. value = 0.49
Interior Partition
0.12 m Brick, 0.02 m Plaster (from 2 sides)	R. value = 1.75
Roof
0.02 m Tiles, 0.01 Cement, 0.2 m Concrete slab, 0.015 m Plaster	R. value = 0.26
HVAC System
Thermostat Setpoint	22 °C Cooling – 22 °C Heating
Thermostat Set back	28 °C Cooling – 12 °C Heating
Supply air temperature	Max. 28 °C / Min. 22 °C
Operating Hours
Weekdays	06:00 am – 06:00 pm
Weekends	Off

Table 5. Different types of traditional glass versus smart Glass specifications (Developed by author).

Base CaseModified CaseItemDescription

Glass Type
Tinted Double Glass 6 mm/13 mm air	• Thermochromic • Electrochromic • Photochromic • Suspended particles • PDLC
External Glazing U value
0.26	state	1	2	3	4	5
	Original state	2.9	1.59	1.03	1.9	2.79
	Secondary state	2.9	1.59	1.03	1.9	2.79
Glass solar Transmittance
0.76	state	1	2	3	4	5
	Original state	0.47	0.41	0.35	0.35	0.53
	Secondary state	0.2	0.09	0.26	0.05	0.39
Glass visible Transmittance
	state	1	2	3	4	5
	Original state	0.7	0.6	0.75	0.55	0.71
	Secondary state	0.25	0.01	0.2	0.05	0.27

The crucial stage in assessing energy usage in the building simulation process is to construct a suitable base case that serves as a model for the building's status. Each floor is represented by a single block, which is further divided into eight zones to reflect the four altitudes The performances of the actual monthly and yearly energy usage are utilized for validation, and the energy consumption is computed using the energy modelling software Energy Plus via the Design Builder interface. When actual energy consumption is compared to base case monthly and yearly consumption statistics, the difference is 1.4 % higher than actual consumption, which is a suitable range in simulation. Fig. 15 shows the modelled building on Design Builder and Fig. 16 shows a comparison between base case that is modeled on Design Builder software and the actual electricity consumption bills. After the model is built with current state inputs and run to predict the output of energy consumption, which is as actual building, the model is validated, and external transparent facades will be replaced to start simulation and evaluation.

Fig. 15. AAST Model on Design Builder (Developed by Author).

Fig. 16. Annual electrical consumption of AAST simulated base case model on design builder and actual energy performance from bills (Developed by Author).

4. Model results
The aim of this simulation is to test and recommend the best smart glass performance rate by comparing the base case, thermochromic glass, electrochromic glass, PCDL, and SPD to one another. For each simulated case, the end results are accompanied by in-depth analysis and visual illustration of building’s annual energy performance, heating loads, cooling loads, and unmet hours.

The simulation results are divided into four cases, each representing one of the tested parameters that was previously mentioned. Fig. 17 demonstrates the total site energy consumption of the building where each glass type was simulated once, and data was recorded. The graph presents that when the thermochromic, electrochromic, PDCL, SPD, and photochromic glass were integrated with the building façade, they reduced the total site energy by 11 %, 13.8 %, 13.9 %, 14.8 and 8 % respectively resulting in the SPD with the most total energy consumption difference compared with the base case.

Fig. 17. Total site energy for base case and 4 types of smart glass (Developed by Author).

Moreover, Fig. 18 shows the percentage decrease of the heating loads. The highest difference was achieved by the thermochromic glass with 16.5 %, while the electrochromic, photochromic, PDCL and SPD glass achieved 3.4 %, 2.9 %, 7.4 %,

Fig. 18. Heating loads for base case sand 4 types of smart glass (Developed by Author).

As for the cooling loads presented in Fig. 19, thermochromic glass had the least percentage change showing a decrease of 13.8 % while for the SPD the cooling loads decrease by 19.7 % showing the highest change. As for electrochromic glass, PDCL, and photochromic glass they were reduced by 18.2 %, 18.7 % and 9.3 % respectively.

Fig. 19. Cooling loads for base case and 4 types of smart glass (Developed by Author).

Finally, the last Fig. 20 illustrates the change in the unmet hours of the office building. The SPD has the greatest change of 45.4 %, followed by both electrochromic and PDCL with 41.6 % percentage decrease. Thermochromic glass improved the thermal comfort by only 36 %. Finally, having the least percentage change of 34 % was achieved by photochromic glass.

Fig. 20. Unmet for base case and 4 types of smart glass (Developed by Author).

The site's environment, however, has a significant impact on energy efficiency. As a result, prior to creating a structure that is climate-responsive, it is crucial to comprehend the local environment. The Psychrometric Chart is one tool that aids in this comprehension. It assists in describing the climate data and the conditions for human thermal comfort by presenting the link between air temperature and humidity in graphical form. Below are two psychrometric charts Fig. 21 demonstrating summer and winter months for the base case and the 5 smart glass types that were simulated. The blue shade in the chart represents thermal comfort; points lying within this blue range are considered acceptable as they achieved thermal comfort. On the other hand, points located outside the blue range means that the inside environment of the building has undesirable conditions and did not achieve thermal comfort. In both charts of summer and winter months, the electrochromic, PDCL, SPD, thermochromic, and photochromic glasses, when installed to the building façade, improved the indoor atmosphere by managing the humidity and air temperature levels resulting in enhancing thermal comfort. However, the base case which used double glazed conventional glass type resulted in poor outcomes and could not reach the required indoor thermal comfort range.

Fig. 21. Two psychometric chart the left present the summer season and the right present the winter season. Both charts show the base case and 4 smart glass material Thermal comfort (Developed by Author.

4.1. Thermal comfort optimization

After a comprehensive comparison was performed to conclude the best type of glass having the highest performance when placed on building’s facade, a simulation optimization between the smart glass types and WWR was conducted to know which combination will achieve the desirable results of better thermal performing office building. When the model optimization was performed, a result of 10 optimum solutions of two times PDCL and eight times SPD glass types were chosen as recorded in Table 6. Also, the selected glass type, which is SPD, from the suggested results is highlighted to perform further analysis using it.
Table 6. Cross section of Glass Layers (Developed by author).

Tinted Double Glass 6 mm/13 mm airThermochromicElectrochromicPhotochromicSuspended particlesPDLC

3mm Tinted Glass
13 mm Air Gas
3mm Tinted Glass

2mm Tinted Glass
1.2mm Thermochromic layer
2mm Tinted Glass
Argon Gas Gap
Low E- Coating
2mm Tinted Glass

3mm Tinted Glass
3 mm Transparent Conductor
2 mm Electrochromic
5mm Electrochromic Layer
3mm Transparent Conductor
3mm Tinted Glass

1mm Photochromic film
3 mm Tinted Glass
13 mm Air Gas
3 mm Tinted Glass

3mm Tinted Glass
1 mm Conductive Layer
Liquid Crystal Layer
1 mm Conductive Layer
3mm Tinted Glass

Moreover, below is a Parallel Coordinate Plot in resenting the different variables used to compare total site energy, discomfort levels, cooling loads, heating loads, window to wall ratio, and the glazing type to one another. The chart shows which combination of the window to wall ratio and smart glazing type mentioned above makes an efficient transparent facade design.

The below bar graph in Fig. 22 shows one of the optimum solutions which was selected based on the lowest WWR and compared to the base case to visualize the improvement in the achieved design.

Fig. 22. Parallel Coordinate Plot presenting the optimization objectives vertically (Total site energy, unmet hours, cooling, Heating, WWR) (Developed by Autor).

After the implementation of the SPD smart glass into the façade design of the AASTMT office building located in smart village, a decrease in the total site energy was achieved by 46 %, a reduction of the heating and cooling loads was reached by 79 % and 59 % respectively, and an improvement in the thermal comfort levels was accomplished by a 92 %, and all these were achieved through decreasing the WWR to 60 % instead of 80 % as presented in Fig. 23 and Table 7. (See Table 8.).

Fig. 23. Base case vs SPD smart glass solution (Developed by Autor).

Table 7. Optimization output of all types of Smart Glass (Developed by Autor).

Total Site EnergyUnmet hrsCooling loads (kWh)Heating loads (kWh)WWR%Glazing type

1,105,066	50.49739	627572.8	12588.64	78	SPD
1,106,224	51.39228	628454.8	12864.87	60	SPD
The upper highlighted row presents the optimum solution
1,108,724	53.78437	629373.2	14446.47	42	PDLC
1,105,473	50.71328	627893.7	12675.05	76	SPD
1,105,847	51.1895	628173.5	12769.35	74	PDLC
1,105,847	51.1895	628173.5	12769.35	74	SPD
1,106,590	51.46093	628726.1	12959.7	70	SPD
1,107,266	51.6423	629211.7	13149.87	66	SPD
1,106,939	51.61265	628981.4	13053.34	68	SPD
1,107,517	52.00627	629250.4	13362.26	62	SPD

Table 8. Base Case Vs Optimum Solution (Developed by Autor).

ParametersBase CaseSPD% Decrease

Total Site Energy kwh	2,062,378	1,106,224	46 %
Heating kwh	61,657.7	12,864.87	79 %
Cooling kwh	1,535,854	628,454.8	59 %
Unmet Hours	732	51.39228	92 %
WWR %	80 %	60 %	20 %

4.2. Findings of comparative analysis
As Egypt is a developing country, the number of office buildings is increasing. And since most users, now a days, prefer to be surrounded with a wide-open view and natural illumination of the interior space, this is reflected on the façade design by integrating the curtain wall system into the building envelope. While curtain walls represent a potential for most people, it has various drawbacks on building energy performance, indoor thermal comfort, and users’ productivity as was explained and discussed in the literature review. This leads us to the main evolving problem which is: due to the ineffective façade design and high window to wall ratio in office buildings, heat gained from the building skin negatively affects the indoor thermal comfort and energy efficiency. Therefore, a detailed data collection, sorting and analysis was performed through the literature review chapter to reach the proposed solution which is to integrate the different smart glass types into the façade design to achieve better thermal comfort and energy performance results.
Consequently, the aim of this research is to test the different types of smart glass including electrochromic, thermochromics, photochromic, SPD, and PDCL on a selected case study of AASTMT admin building façade and recommend the glass type that achieved desirable results of energy reduction and thermal comfort improvement.
After comprehensive studying of the smart glass different types and gathering the needed specifications and numerical data of each, a simulation study was done to prove the hypothesis. AASTMT office building case study, which is in smart village, Egypt, was selected based on its location, façade type, available data, WWR, and project scale. Each type of smart glass and the base case was simulated once, and results were recorded. The following results demonstrate the highest obtained percentage decrease along with its glass type that achieved it regarding each of the total site energy, heating and cooling loads, and Unmet hours. For instance, the total site energy, cooling loads and unmet hours decreased by 14.8 %, 19.7 %, and 45.5 % respectively when placing the SPD on the facade. Moreover, the heating loads were reduced by 16 % once the thermochromic glass was used.
Later, a simulation optimization was conducted between window to wall ratio and the five smart glass types to conclude which combination will result in the best indoor thermal comfort and energy performing office building. The optimum solutions that were found were 10 suggestions. 8 of them proposed SPD and the other 2 were PDCL smart glass. One of the SPD optimum solutions was selected based on the lowest WWR and compared with the base case values in order to visualize the difference. The compared results showed that the total site energy, heating loads, cooling loads, unmet hours, and WWR were decreased by 46 %, 79 %, 59 %, 92 %, and 20 % respectively. Finally, the research's aim was successfully achieved and resulted in designing an energy efficient building with better thermal comfort and lower energy usage.
5. Limitation
This study focused on active and passive smart windows, with simulations conducted in Design Builder using available material properties. Emerging technologies, such as thermotropic, Phsae chane material, gasochromic were not included due to limitations in the simulation platform. Further research is needed to explore the potential of these newer technologies“.
6. Conclusion
This research investigated the impact of smart glass integration on thermal comfort and energy consumption in office buildings, particularly in hot arid climates. The study focused on an existing office building in Cairo, Egypt, using Design Builder software to simulate various smart glass types and analyze their performance. All tested smart glass types (thermochromic, electrochromic, PDLC, SPD, and photochromic) significantly improved thermal comfort and reduced energy consumption compared to traditional glazing. SPD Glass has significant performance among the tested types, SPD glass demonstrated the most substantial improvements, reducing total site energy by 46 % and unmet hours by 92 %. Combining SPD glass with a lower Window-to-Wall Ratio (WWR) of 60 % provided optimal results, further enhancing energy efficiency and thermal comfort.
To create sustainable and energy-efficient buildings, especially in hot climates, prioritizing the integration of smart glass is essential. Among various smart glass types, SPD glass stands out as a particularly promising solution due to its superior performance in reducing energy consumption and enhancing thermal comfort. To maximize the benefits of smart glass, careful consideration of factors such as Window-to-Wall Ratio (WWR) and building orientation is crucial. Future research should delve deeper into the long-term performance and cost-effectiveness of smart glass technologies in diverse climatic conditions.
By adopting smart glass solutions and optimizing building design, architects and engineers can create more sustainable and comfortable indoor environments, contributing to a greener future.

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