One of the staple materials of architecture is glass, but it can constitute a weak point in the thermal performance of buildings if left untreated. The optical and thermal properties of architectural glass can be improved using chemical vapor deposition or physical vapor deposition techniques.
This can lead to drastically reduced energy demand for cooling or heating systems in buildings. The optical and thermal properties of architectural glass can be finely tuned using the glass coating analysis systems from tec5USA that enable extremely accurate inline measurement of thin film thickness.
Architectural Glass’ Thermal Characteristics
Architectural glass has occupied a unique niche in buildings for thousands of years. The use of glass provides fairly robust protection against the outside environment while still allowing light into indoor spaces. New buildings commonly feature large, emphasized, dramatic glass elements, making the use of glass somewhat synonymous with modern architecture.1
Glass gets away with a number of other shortcomings because the value of its transparency is so great. Sheet glass is expensive, heavy and delicate when compared to other common architectural materials like concrete, wood and steel.
In addition, generally, the weakest points in a building’s thermal envelope are the untreated sheet glass windows; thermal envelopes are the collective structures that separate the cooled or heated interior of a building from the external environment.2
A huge amount of energy is consumed worldwide for the purpose of cooling and heating indoor spaces. The International Energy Agency produced a report in 2019 that reported that heating water and indoor spaces accounted for approximately 23% of worldwide energy consumption in 2018.3
Commercial and residential space cooling accounts for 10% of total electricity consumption in the United States.4 There is, therefore, a major opportunity to drastically reduce energy consumption and greenhouse gas emissions and improve the energy performance of buildings by improving the thermal characteristics of glass.
Enhancing Architectural Glass Performance with Coatings
The thermal and optical properties of architectural glass can be tuned by coating it with thin layers of metals or metal oxides. When considering this issue, it is important to note that there is no single correct approach to optimizing the thermal properties of architectural glass.
For example, designers in cold climates should select glass with the highest possible solar energy transmittance (characterized by a “g value”) to ensure the maximization of incident energy flux from the outside.
Simultaneously, there should be as low levels of thermal transmittance (described by various “U factors” and measured in W/m2K0) as possible to minimize the amount of heat that escapes from the building. However, in hot climates, the opposite is true, and it is important for windows to have a low g value.5
Architectural glass coatings allow building designers to use simplified methods and design guidelines to select the most appropriate types of glass according to building use-case, occupancy and climate zone, typically by providing glass panes with static values of U and g.
A wide range of materials are used to produce functional coatings for architectural glass.6
Solar transmittance can be reduced in a number of ways in hot climates. Absorption can be increased by a layer of highly absorbing coatings such as oxynitrides of titanium, while high-index dielectric materials such as titanium oxide (TiO2) are typically used to increase reflectance and decrease solar transmittance.
A more selective increase in infrared reflectance can be provided by multilayer metal-dielectric coatings.
Cold-climate coatings use multilayer silver-dielectric systems or include extrinsic semiconductors like indium tin oxide (ITO) to provide maximum transmittance over the solar spectral range and high reflectance for room temperature IR radiation.
Increasingly, there is also a demand for systems to respond to temperature (thermochromicity) by providing variable transparency.
Vanadium dioxide (VO2) has been identified as a promising candidate by research into the development of “smart coatings” for long-life thermochromic window coatings capable of passively regulating solar heat gain in response to seasonal conditions.7,8
Increasing Accuracy in Thin Film Deposition
Typically, chemical vapor deposition (CVD) or physical vapor deposition (PVD) is used to deposit coatings onto architectural glass. In fact, architectural glass was a key driver in the development of these technologies and is one of the predominant applications of large-area vacuum coating systems.9
Critical to the success of vapor deposition processes is inline analysis. Measurement techniques are used throughout the deposition process to ensure that the correct thin film layer thickness is deposited, depending on the complexity of the stack of deposited materials.
Measurement of film thickness with extremely high levels of accuracy using white-light interferometry is enabled by the tec5USA online UV-Vis spectrometer. The spectrometer system has detector options, including NMOS, CMOS and CCD, and is capable of analysis throughout the ultraviolet and visible regions of the spectrum (190 – 1100 nm).
Having low measurement acquisition times is vital because of the rapid reactions that take place during glass coating vapor deposition processes. The tec5USA UV-Vis spectrometer enables real-time monitoring of these coating processes by providing extremely high-speed acquisition (in the region of a few milliseconds).
The machine has multiplexing capabilities and can assure process uniformity; a single device can monitor up to 32 measurement locations.
The spectrometer can be easily interfaced with distributed control systems for online process control and automation through compatibility with connection protocols, including OPC, 4.20 mA and Modbus.
Users can obtain simulated spectra and material constants as well as film thickness by taking advantage of integrated control software for thin-film processes to achieve optimized coating parameters for their deposition processes.
Spectrometer systems from tec5USA have high wavelength accuracy and device resolution ranging from <2 – 10 nm ∆λFWHM, ensuring that coatings are deposited at the perfect thickness to produce the desired performance parameters.
- Amstock, J. S. Handbook of Glass in Construction. (McGraw-Hill, 1997).
- Alibaba, H. Determination of Optimum Window to External Wall Ratio for Offices in a Hot and Humid Climate. Sustainability 8, 187 (2016).
- Heat – Renewables 2019 – Analysis - IEA. https://www.iea.org/reports/renewables-2019/heat.
- Frequently Asked Questions (FAQs) - U.S. Energy Information Administration (EIA). https://www.eia.gov/tools/faqs/faq.php.
- Del Ama Gonzalo, F. Dynamic Solar Energy Transmittance for Water Flow Glazing in Residential Buildings. International Journal of Applied Engineering Research 13, (2018).
- Buffat, B. Thin films and architectural glass. Journal de Physique IV Proceedings 02, C2-11-C2-19 (1992).
- Anderson, A.-L., Chen, S., Romero, L., Top, I. & Binions, R. Thin Films for Advanced Glazing Applications. Buildings 6, 37 (2016).
- Sol, C. et al. High-Performance Planar Thin Film Thermochromic Window via Dynamic Optical Impedance Matching. ACS Appl. Mater. Interfaces 12, 8140–8145 (2020).
- Schaefer, C., Bräuer, G. & Szczyrbowski, J. Low emissivity coatings on architectural glass. Surface and Coatings Technology 93, 37–45 (1997).
This information has been sourced, reviewed and adapted from materials provided by tec5USA Inc.
For more information on this source, please visit tec5USA Inc.