Smart Window A Window for Dynamic Control of Building Energy Performance

Abstract

Thermochromic (TC) and Thermotropic (TT) glazing theoretically has the potential to significantly reduce the energy demand in buildings, by allowing transmission of visible light for daylighting but passively controlling solar gains; heat gains are minimised during the cooling season while allowing useful solar gains in the heating season. In this study, a thermotropic layer made of hydroxypropylcellulose (HPC) and Sodium Chloride (NaCl) was synthesized and tested by the Evolution 201 UV-VIS spectrophotometer. The developed thermotropic layer has a transition temperature of 3233˚C. Below the transition temperature visible transmission ranges between 65-95% while solar transmission ranges between 40-90%. Above the transition temperature visible transmission ranges between 5-35 % while solar transmission ranges between 10-50%. In addit ion, simulations of a typical office with the developed TT and a TC installed were carried out using simulation software Energy Plus. The chosen TC window has a transition temperature of 20-21°C, constant visible transmission of 65% and solar transmission of 80% below the transit ion temperature and 15% above the transition temperature. From an annual energy prediction, exploring how the ambient environmental conditions affected the state of the TC, it was found that the incident solar radiation plays a large role in the tinting of the window, as heat gain within the film was predominately due to radiation, and was less reliant on convection. The performance of the TC was compared to a standard double glazing unit (DG); the use of the TC in comparison to the DG resulted in a 14.4 kWh/m2 –floor, 32%, reduction in HVAC energy annually but a 3.87 kWh/m2 –floor, 30% increase in lighting energy. Overall the TC saved 21% of combined energy annually; a significant reduction in energy use. The solar heat gain coefficient (SHGC) of the TC when tinted was 0.31 almost half that of the DG at 0.56; as the majority energy demand comes from cooling loads, this reduction in transmitted radiation, at peak temperatures greatly reduces this load. The results indicate that the TC technology would work best in areas with high levels of incident solar radiation and hot climates.