Near-Infrared Selective Plasmonic Electrochromic Windows
Residential and commercial buildings represent a prime opportunity to improve energy efficiency and sustainability worldwide. Currently, lighting and thermal management within buildings account for 20% of the United State's yearly energy consumption. Several approaches, such as solid state lighting, energy efficient HVAC systems, and improved insulation, are currently being investigated to help mitigate building energy consumption. The work described in this dissertation focuses on studying the use of dynamic window coatings on commercial and residential buildings. Specifically, this work focuses on near infrared selective electrochromic window coatings that optimize the amount of solar heat that enters a building without affecting the amount of solar light.
Electrochromic window coatings are based on an electrochemical cell architecture that is composed of two electrochromic layers separated by ion conducting electrolyte. During operation, an applied bias is used to control the optical properties of both electrochromic layers by shuffling a small amount of current between the counter and working electrode. In a negative bias, the window coatings dim to a dark state. If you reverse the bias, the window coatings go back to a transparent state. Intermediate states can be achieved by controlling the value of the bias applied. Unfortunately, traditional electrochromic windows require a change in visible transmittance to gain energy savings within buildings. This change affects the amount of solar daylighting and inadvertently leads to an increase in electrical lighting during the day. This work focuses on developing a nanocrystal based plasmonic electrochromic window that only modulates the near infrared portion of light while remaining visibly transparent. Taking advantage of localized surface plasmon absorption, this work approaches dynamic window coatings in a new fashion. To date, no near infrared selective electrochromic windows exist in the literature.
To achieve near infrared selective modulation, thin film layers of tin doped indium oxide (ITO) and alumnium doped zinc oxide (AZO) nanocrystal films were investigated as a potential electrochromic layers. A colloidal synthetic technique was used to generate concentrated inks of both ITO and AZO nanocrystals. Thin films of ITO and AZO nanocrystals where fabricated via spin casting and tested in electrochemical half cells. Prior to testing, extensive post processing techniques were investigated to develop transparent conductive films. During optimization, variations in nanocrystal size, layer thickness, dopant concentration and electrolyte were studied. For an optimized ITO film, 35% solar near infrared modulation was achieved while maintaining less than 6% modulation in solar insolation visible to the human eye. Optimized AZO nanocrystal films achieved 42% solar near infrared modulation with no change in solar insolation visible to the human eye. Extensive models where built to elucidate the physical mechanism used to achieve this solar modulation.
Computer simulations were developed to quantify the energy performance of buildings with dynamic near infrared selective windows. This model only quantifies thermal savings and establishes the ground work for potential savings in solar daylighting. The model shows that optimized near infrared selective coatings achieve 15% and 10% energy savings in warm climate regions and cold climate regions respectively. Overall, the focus of this work sets the stage for advanced plasmonic electrochromic films that not only enhance the performance of dynamic windows but introduce a new technique for reducing building energy consumption worldwide.