How Cold Are Your Winters?
Abstract
In regions with significant heating needs, electrically heated windows offer an innovative solution to mitigate the discomfort caused by cold window surfaces. At first glance, heated windows might seem inefficient due to the perception that the energy used is lost to the outdoors. However, a validated model comparing conventional and heated windows reveals that heated windows can actually reduce overall space heating and cooling loads. This model shows that heated windows increase thermal comfort by preventing cold indoor window surfaces. While there is a slight increase in heating load compared to energy-efficient double-pane windows, the cooling load decreases. In cold climates, it is more beneficial to install heated windows on the north and east (or west) walls, rather than the south, due to the lower solar heat gain coefficient (SHGC) of heated windows compared to conventional ones. This study was conducted on an early version of IQ Radiant Heating Glass. Advancements in the technology have improved overall performance by 35%.
1. 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.
2. Description of 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.
3. 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.
4. Impacts on the Energy Needs of Buildings
4.1 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.
4.2 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.
5. 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.
6. 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.
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