Abstract
In areas with substantial heating requirements, electrically heated windows present an innovative solution for alleviating the discomfort caused by cold window surfaces. While initially thought to be inefficient, a validated model comparing traditional and heated windows has shown otherwise. Heated windows have the potential to reduce overall space heating and cooling demands by enhancing thermal comfort through the prevention of cold indoor window surfaces. Despite a slight increase in heating demand compared to energy-efficient double-pane windows, there is a decrease in cooling requirements. In colder climates, it is advisable to install heated windows on the north and east (or west) facades due to their lower solar heat gain coefficient (SHGC) in comparison to standard windows. A study conducted on an early iteration of IQ Radiant Heating Glass has demonstrated a 35% enhancement in performance due to technological advancements.
Introduction
Managing thermal comfort in buildings with significant heating needs, particularly near windows, can be challenging. Cold window surfaces during winter create discomfort through cold drafts, radiation chills, and condensation or frost. High-performance windows can improve comfort, but in extremely cold climates, even these may fall short. For example, at -18°C outside, a standard double-pane window has an indoor surface temperature around 6°C, while an energy-efficient window’s surface temperature is around 11°C. This often leads to increasing the thermostat setting, raising overall energy consumption. Traditional perimeter heating systems, like baseboards, create a “thermal curtain” but can cause discomfort and inefficiency.
A more effective solution targets the problem at the source: electrically heated windows. These windows, used in industries to prevent frost and condensation, can also enhance comfort in buildings. Hydro-Québec’s Research Institute developed a model to predict the thermal performance of both conventional and heated windows, assessing their impact on energy needs.
Indoor environments have become humans’ dominant habitat as we now spend more than 90% of our time indoors (Klepeis et al., 2001). The health, well-being, and productivity of building occupants depend on four aspects of indoor environmental quality (IEQ): (1) thermal comfort, (2) visual comfort, (3) acoustics, and (4) indoor air quality (Frontczak and Wargocki, 2011). Of these, thermal comfort has been shown to be a dominant factor. Studies indicate that thermal comfort is closely related to occupant productivity (Wyon et al., 1979; Lan et al., 2010; Arif et al., 2016). For example, cognitive performance decreases due to underarousal caused by increases in indoor temperature (Provins, 1966). Monitoring occupant thermal comfort can lead to more efficient ways to regulate indoor conditions, potentially improving cognition and productivity. Energy savings from comfort-driven HVAC operations range from 4–32%, depending on building characteristics and climate (Masoso and Grobler, 2010; Vakiloroaya et al., 2014; Ghahramani et al., 2015a, 2016b; Ghahramani and Dutta, 2018). HVAC systems account for up to 10–20% of total energy consumption in developed countries (Pérez-Lombard et al., 2008), making thermal comfort-driven HVAC operations an opportunity for energy savings and productivity improvements. Interestingly, more than 20% of occupants experience thermal discomfort in buildings (Mishra and Ramgopal, 2013).
Electrically Heated Window Technology
Electrically heated windows are similar to conventional double-glazed windows but include an additional low-emissivity (low-E) film on surface #3, which heats surface #4 when powered. This setup reduces thermal losses, enhanced by filling the space between panes with argon or krypton gas. A temperature sensor on surface #4, controlled by a thermostat, maintains the surface temperature around 20-22°C, ensuring comfort even at external temperatures as low as -25°C.
Simulation Model
The simulation model, based on ISO Standard 15099, calculates the thermal performance at the center of the glass, avoiding edge effects. It uses data from tests conducted at Hydro-Québec’s Research Institute, where the model’s accuracy was validated against experimental results. The model simulates the energy balance on each glass surface, factoring in the electricity used by the heated window and the resulting temperature changes.
Impacts on the Energy Needs of Buildings
Simulation Parameters
Simulations were performed using annual weather data for Montreal, Canada, considering heating from October to May and cooling from May to October. Three window types were evaluated, maintaining the heated window surface temperature at 20°C during the heating season, and setting room temperatures at 20°C for rooms with heated windows and 21°C for those without.
Natural Ventilation and Heated Window Glass for Energy Conservation
Integrating natural ventilation with electrically heated window glass can significantly enhance energy conservation in buildings. Natural ventilation reduces the reliance on HVAC systems by facilitating passive cooling and fresh air circulation, lowering overall energy consumption. Simultaneously, electrically heated glass maintains optimal indoor temperatures by preventing cold drafts and condensation on window surfaces. This combination ensures that while natural ventilation provides cooling and air quality improvements, the heated glass minimizes heat loss and maintains thermal comfort, resulting in a balanced and energy-efficient indoor environment.
Results
During the heating season, heated windows significantly reduce thermal losses compared to conventional windows, potentially eliminating the need for perimeter heating systems. Despite the electricity consumed, heated windows save more energy than they use, particularly when compared to standard double-pane windows. They do, however, show a slight increase in energy consumption compared to energy-efficient double-pane windows.
During the cooling season, the lower SHGC of heated windows reduces the cooling load. Overall, heated windows save heating and cooling energy compared to standard double-pane windows, though they slightly increase heating load compared to energy-efficient double-pane windows while reducing cooling load. Installing heated windows on north and east (or west) walls is more advantageous than on south-facing walls due to reduced solar gains.
Electrically Heated Glass prioritizes the well-being of human occupants and surfaces, enabling a reduction in ambient room temperatures while still ensuring optimal thermal comfort. This innovative technology not only leads to substantial energy savings but also enhances the overall efficiency of the space.
Enhanced Performance with AI Energy Management Systems
The integration of electrically heated windows with an AI-driven Building Management System (BMS) can further enhance energy savings and performance. AI systems can dynamically adjust the window heating based on real-time data, occupancy, weather conditions, and energy usage patterns. By optimizing the operation of heated windows, AI systems can:
• Enhance Overall Building Performance: Integrate with other building systems (e.g., HVAC, lighting) to create a cohesive, energy-efficient environment.
• Improve Comfort and Efficiency: Maintain optimal thermal conditions and reduce energy consumption through intelligent, adaptive control.
Conclusion
Electrically heated windows provide numerous benefits, including enhanced comfort, condensation prevention, noiseless heating, and no maintenance requirements. Despite misconceptions, these windows do not necessarily increase energy consumption. In fact, they reduce energy needs compared to standard double-pane windows and only slightly increase energy use compared to energy-efficient windows. For optimal energy efficiency, heated windows should be installed on north and east (or west) walls. Integrating these windows with AI-driven Building Management Systems can further enhance energy savings and performance, making them a smart choice for modern, energy-efficient buildings. Future research should incorporate these windows into comprehensive building models to fully evaluate their impact on energy needs and overall building performance.
This study was conducted on an early version of IQ Radiant Heating Glass. Advancements in the technology have improved overall performance by 35%. This and other studies demonstrate the expected performance improvements in the top 20 U.S. states with similar winters, including Alaska, Colorado, Idaho, Illinois, Indiana, Iowa, Maine, Massachusetts, Michigan, Minnesota, Montana, New Hampshire, New York, North Dakota, Ohio, Pennsylvania, South Dakota, Vermont, Wisconsin, and Wyoming.
Using IQ Radiant Glass for Condensation Control
IQ Radiant Glass, with its electrically heated glass technology, offers a cutting-edge solution for managing condensation in buildings. By maintaining optimal surface temperatures through integrated heating, IQ Radiant Glass effectively prevents the formation of condensation, ensuring clear visibility and enhancing the aesthetic appeal of glazed surfaces. This technology is particularly beneficial in high-humidity areas and on large glass facades, where condensation can be a significant issue. Additionally, it promotes a healthier indoor environment by reducing the risk of mold and mildew, all while providing superior thermal comfort and energy efficiency.