A facade can control the interaction between a building and its environment. Advances in control technologies and material science now offer the possibility of using smart windows in high-performance facades to improve building energy performance and user comfort, as demonstrated by the study "Performance and Users Comfort Attributes" by Zhina Rashidzadeh and Negar Heidari Matin from the Department of Interior Design, Gibbs College of Architecture, University of Oklahoma. This study provides practical recommendations for the implementation of smart windows in various climatic zones worldwide.

To pursue this objective, data published from 2013 to 2022 from architecture, engineering, and material science databases were collected. Several types of smart windows, including electrochromic, photovoltaic, gasochromic, and thermochromic windows, identified as photochromic, hydrochromic, and Low-E, were analyzed.

Moreover, the thermal properties and visual characteristics of the smart layers used in windows and their impact on energy efficiency and user comfort were recognized.

Construction, a major energy consumer

The construction industry is responsible for 40% of global annual energy consumption, leading to nearly 39% of worldwide carbon emissions. Consequently, buildings could explain global warming and other environmental disasters. Among the various building components, facades have been shown to significantly control energy consumption and carbon emissions.

Recent advances in control technologies and material science have provided opportunities for developing high-performance facades. As a type of high-performance facade, a responsive facade can change its functions, features, and behavior in response to environmental stimuli, occupant needs, and preferences, to improve thermal, visual, and acoustic performance and reduce energy consumption in buildings.

Responsive facades can be classified into different types based on the control technologies used in their structure, including mechanical, electromechanical, information-based, and material-based technologies. Considering the disadvantages of mechanical, electromechanical, and information-based types, designers and scientists have extended the application of smart materials in the design and development of responsive facades.

Smart windows and materials

Smart materials can change their inherent properties through the application of external stimuli, such as stress, temperature, humidity, pH, electric fields, and magnetic fields in a controlled manner.

Smart layers are thin films applied to the surfaces of objects, such as windows, capable of dramatically adjusting their transparency and color properties in response to various environmental stimuli.

Different types of smart coatings, such as electrochromic, gasochromic, photovoltaic, thermochromic, photochromic, hydrochromic, and Low-E, have been used to develop smart windows for responsive facade systems.

This study identified several advanced smart windows. A comparative analysis was conducted considering the properties of smart windows, including thermal transmission rate (U-value), solar heat gain coefficient (SHGC), light-to-solar gain ratio (LSG), shading coefficient (SC), visible transmission ratio (Tvis), solar transmission ratio (Tsol), visible transmittance (VT), and reflectance to explore efficient applications of smart windows in different climatic zones worldwide.

Color-coded maps associated with the Koppen climate classification were used to describe the effective types of smart windows in each climatic zone, considering the thermal properties and visual characteristics of smart coatings. A comparative analysis of existing smart windows provided a useful resource for educators, researchers, and professionals in the construction industry, benefiting their practices, educational activities, and research.

Analysis Criteria

Academic research databases, including ScienceDirect, Scopus, and Web of Science, were used to identify research articles with types of smart windows in titles, abstracts, or keywords published between 2013 and 2022.

It should be noted that the database exploration in this study was limited to a few types of smart windows, including electrochromic, photovoltaic, gasochromic, thermochromic, photochromic, hydrochromic, and Low-E. Eligibility criteria, unrelated texts, duplicates, unavailable, or abstract-only articles were manually excluded from the search results.

Of the various studies identified, research focusing on the effectiveness of smart coating technologies on energy efficiency and/or user comfort was selected for further analysis.

A meta-analysis approach was used to analyze the included articles. In the meta-analysis approach, scientists combine statistical results from previous studies focusing on a specific topic to outline patterns, relationships, or contradictions. Additionally, thermal properties, visual characteristics of smart coatings, and climatic zones were verified.

Koppen-Geiger Climate Classification System

The Koppen-Geiger system is a classification that divides all climates into five significant zones, including tropical, arid, temperate, cold/continental, and polar. The classification criteria are based on precipitation levels, temperature patterns, and vegetation.

Each climatic zone and its associated subgroups are represented by two or three letters. The first letter represents the climatic zones, including tropical (A), arid (B), temperate (C), cold/continental (D), and polar (E).

The second letter indicates the precipitation level and contains f (Af), m (Am), w (Aw), W (BW), S (BS), s (Cs, Ds), w (Cw, Dw), f (Cf, Df), T (ET), F (EF), representing Rainforest, Monsoon, Savannah, Desert, Steppe, Dry summer, Dry winter, and no dry season, Tundra, and Frost, respectively.

The third letter represents the predominant temperature as h (for B as hot), k (for B as cold), a (for C and D) as hot summer, b (for C and D) as warm summer, c (for C and D) as cold summer, and d (for D) as very cold winter. Therefore, the Koppen-Geiger system represents 31 different climatic combinations.

What are smart windows?

Smart windows refer to glass windows that implement smart coatings to react to environmental stimuli by modifying solar radiation, radiant energy, and visible light to increase energy efficiency and human comfort in buildings.

These types of windows can significantly reduce the need for cooling, heating, and electric lighting in buildings and improve the performance of windows in terms of mechanical, chemical, and physical properties.

Hence, heating, ventilation, and air conditioning (HVAC) and the building's electricity requirements decrease. Switchable smart windows respond to changes in heat, electricity, gas activation, and light through implementing various mechanisms, including electrochromic, photovoltaic, gasochromic, and thermochromic systems. Here are a few examples:

Electrochromic Windows

Electrochromic (EC) windows are classified as active smart windows. Their performance is based on the transfer of electric charge between the anode and cathode (electric conductors). The structure of an electrochromic window consists of glass (polyester), transparent electric conductors (anode and cathode), electrochromic coatings, and electrolytes.

Thus, the transfer of ions from the cathode to the anode activates the electrochromic material, changing the optical characteristics of the window using low voltage. An electron barrier and an ionic conductor layer (electrolyte) are located in the center of the structure, and the two electrochromic coatings (EC) (tungsten oxide, WO3) surround the electrolyte. One of the EC layers is connected to the anode, and the other is attached to the cathode (ion bank).

Applying a voltage between the anode and cathode activates the ion transfer, and simultaneously the electrochromic material balances this ion transfer, which is the reason for the optical changes in EC windows. Reversing the voltage and implementing a short circuit reverses the color change.

The transition time of the windows takes between 7 and 20 minutes to switch from the transparent to the colored state and vice versa, providing adequate time for the occupants' eyes to adjust to lighting conditions. Electrochromic windows are usually constructed as double or triple glazing (with two or three panels) to provide enough space for the electrolyte material.

Photovoltaic Windows

One disadvantage of EC windows is their need for an external power source to supply the activation voltage. This voltage leads to higher electricity consumption by EC windows.

Therefore, scientists have proposed photovoltaic (PVC) systems that integrate EC glazing systems with photovoltaic materials, which produce the voltage for the EC part of the PVC windows.

In the structure of PVC windows, the photovoltaic material installed on the outer surface of the glass is connected to the transparent conductors of the EC part of the window. The transfer of protons between the anode of the photovoltaic cell and the cathode occurs through the absorption of solar light energy, activating the EC materials. The windows are fully activated by the photovoltaic cells, being considered active smart windows.

Gasochromic Windows

Gasochromic (GC) windows respond to infrared (IR) rays by implementing chemichromic coatings triggered by gaseous hydrogen or oxygen. Since another system should pump the gas and activate the gasochromic material, GC windows are considered active smart windows.

Most GC windows involve porous tungsten trioxide (WO3), as well as chemichromic films that can be coated directly on the interior side of the exterior glazing. However, other gasochromic materials, including nickel oxide (NiO) and magnesium alloys, can be coated on GC windows as alternative materials.

Similar to EC windows, GC smart windows (SW) react to solar heat by absorbing sunlight rather than reflecting it. This absorption causes the overheating of the windows and prevents the overheating of interior spaces. Gasochromic windows are produced as double or triple glazing units to provide a hollow space for gas pouring.

The reaction time of gasochromic windows to changes in sunlight is 20 seconds to one minute. The structure of GC windows consists of two glass panes separated by a space filled with active gases and the gasochromic layer coated on the inner surface of the outer glass.

Hydrogen (H2) and oxygen (O2) fill the argon-filled space by sensing changes in the IR rate level. Mixing hydrogen with porous WO3 changes the color of the smart layer to dark blue. The more hydrogen the tungsten trioxide

layer absorbs, the darker the color becomes, blocking more heat and light transmission.

Thermochromic Windows

Thermochromic (TC) windows are based on thermochromic materials that change their transparency and color in response to temperature changes in a building or outdoor climate conditions.

In contrast to EC and GC windows, thermochromic windows are passive, smart windows, as the structure of thermochromic windows consists of only thermochromic layers applied to the glass surface, and no external source of power, such as electricity or gas, is required to activate the transition between the transparent and dark-colored states.

As a type of passive smart window, photochromic windows (PC) use chromic materials that react to solar light by absorbing UV rays. This process changes the transparency and color of photochromic layers without an external source of power or control unit. Similar to thermochromic smart windows, photochromic windows are considered passive smart windows.

Low-E windows

Low-E (emissivity) coatings are known as “spectrally selective” coatings, meaning that they block a particular range of light waves from entering buildings. Low-E windows block UV and IR light waves while permitting visible light to pass through. These layers are applied to window surfaces by implementing a variety of deposition methods, including chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and magnetron sputtering.

The analysis included data on the thermal and visual performance of different types of smart windows to compare their effectiveness in improving energy efficiency and user comfort in buildings.

The key parameters considered in this analysis included:

1. **Thermal Transmittance (U-value)**: The rate of heat transfer through the window. Lower U-values indicate better insulation properties.

2. **Solar Heat Gain Coefficient (SHGC)**: The fraction of solar radiation admitted through a window. Lower SHGC values indicate better solar control.

3. **Light-to-Solar Gain Ratio (LSG)**: The ratio of visible light transmittance to SHGC. Higher LSG values indicate better overall performance.

4. **Shading Coefficient (SC)**: The ratio of solar heat gain through a particular window to that through a reference window. Lower SC values indicate better shading performance.

5. **Visible Transmittance (Tvis)**: The fraction of visible light transmitted through the window. Higher Tvis values indicate better natural lighting.

6. **Solar Transmittance (Tsol)**: The fraction of total solar energy transmitted through the window.

7. **Visible Transmittance (VT)**: The amount of visible light that passes through a window.

8. **Reflectance**: The ratio of reflected light to incident light on a window's surface.

The data showed that electrochromic windows are effective in controlling solar heat gain and improving thermal comfort, while thermochromic windows are more effective in climates with significant temperature variations. Photovoltaic windows are beneficial in regions with high solar radiation, as they generate electricity while improving thermal performance. Gasochromic windows are effective in climates with variable solar radiation levels.

Low-E windows are generally effective in all climates, as they provide good thermal insulation and reduce the need for artificial lighting. Photochromic and hydrochromic windows are suitable for regions with high levels of sunlight and humidity, respectively.

The study concluded that the selection of smart windows should be based on the specific climatic conditions of the region to achieve the best energy efficiency and user comfort. Additionally, the integration of smart windows with other building systems, such as HVAC and lighting, can further enhance their performance.

Overall, the analysis provided valuable insights into the benefits and limitations of different types of smart windows, highlighting the importance of considering climatic conditions in the design and implementation of high-performance facades. The study's findings can guide architects, engineers, and building professionals in selecting the most appropriate smart windows for their projects, ultimately contributing to more sustainable and energy-efficient buildings.