Thesis: OTTIS Aerogel for High-Efficiency Window Applications
by Elise Strobach
Building heating, ventilation and air conditioning (HVAC) accounts for about 13.6 quadrillion BTU (“quads”) per year or 14% of the total energy consumption in the United States. Accounting for 39% of annual US carbon dioxide emissions, this consumption is directly related to the energy efficiency of the building envelopes. Windows form an essential but lossy part of building envelopes, particularly during cold weather. Thermal losses in the U.S. from controlled indoor environments to outdoors climates account for $20 billion dollars in energy each year, signifying a need for more energy efficient windows. However, insulating windows represent a thermal challenge due to the needs for optical clarity and thermal performance. Successful application of window design requires an in-depth understanding of both fundamental heat transfer and the occupant needs of our buildings.
One promising solution to these energy losses is the use of silica aerogel, a porous material with super-insulating properties. Previous studies have explored the use of aerogels for energy efficient window glazing due to its low thermal conductivity and promise of transparency. However, its adoption in the general window market has been limited by its low optical clarity characterized by a blue haze. In this work, we present the development of a high- clarity silica aerogel that is able to achieve visible transmittance > 98 % and thermal conductivity < 13 mW/mK that has been optimized for use in building windows. This performance was achieved by careful tailoring of the interconnected particle network driven by optical modeling to reduce effective scattering size within the material below 10 nm diameter. Next, clarity and thermal conductivity of the material was improved by optimization of the solution-gelation synthesis across over 300 unique samples and 80 recipes. This provided a framework for achieving a variety of low-haze aerogels with varied thermal, sound-proofing, and mechanical properties.
After achieving high-clarity through optimization of the solution-gelation chemical recipe, several 5 inch diameter double-pane prototypes were to measure the optical, thermal, and acoustic performance. Results indicate that sealing high-clarity aerogel into the gaps of existing double-pane window designs, we can achieve a center-of-glazing U-factor of 0.20 BTU/h/ft2/F, which is 35-50% more insulating than current building codes across North America. These early thermal results and a production-scale techno-economic analysis indicate the aerogel has the ability to achieve cost-effective thermal performance that is competitive with traditional double- and triple-pane windows. Additionally, the aerogel is able to withstand exposure to extreme conditions, such as temperature > 200 °C, relative humidity > 60%, and ultraviolet exposure for more than 6 months without degradation of the nanostructure or optical quality. These results show a promising proof-of-concept design for an aerogel double-pane window that is capable of state-of-the-art performance without a prohibitive cost to consumers. Successful development and commercialization of this high-clarity aerogel has the potential to save billions of dollars in annual building energy losses while satisfying the diverse and complex needs of our buildings.