Multifunctional thermochromic smart window for energy saving application

• Main Author: Wang, Shancheng
• Other Authors: Long Yi
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/149226
• Institution: Nanyang Technological University

Building consumes more than 40% of global energy while window as the least energy efficient part in the building envelope, is responsible for 60% energy loss in building. Therefore, improving energy efficiency of window becomes a crucial task. Chromogenic technology is intensively researched technique to cut off the energy consumption. Among the different chromogenic techniques such as electrochromic and photochromic, thermochromic technique is the highly competitive technology with the advantages of passivity, rational stimulus-response and no extra energy input required. The well-researched thermochromic materials can be categorized as inorganic and organic materials. The well-known inorganic thermochromic material is vanadium oxide (VO2). At room temperature, VO2 shows high transmittance to both visible and near-infrared (NIR) light. While when temperature is higher than its transition temperature, VO2 blocks NIR and keeps its visible transparency. On the other hand, organic thermochromic material is mainly referred to the thermochromic hydrogel such as Poly(N-isopropylacrylamide) (PNIPAm) and the hydroxypropyl cellulose (HPC). Organic thermochromic hydrogel adjusts transmittance of visible light. At room temperature, the hydrogel is highly transparent; while once heated, the visible transmittance drastically decreases. Although current thermochromic smart window has shown promising light regulating performance. Energy-saving in building is a complicated issue. There are several factors such as heat radiation and thermal energy storage that have rarely been considered for windows. In this thesis, functions such as emissivity regulation, and thermal energy storage were added onto thermochromic smart window to further understand their effects on energy saving performance. In Chapter 4, the synthesis of VO2/carbon hybrid material via plasma enhanced chemical vapor deposition technique was discussed. The material shows a best reported thermal emissivity contrast (0.44) among the experimental value, a largely lowered τc (45 °C), promising near IR-regulation ability with enhanced photothermal effect in smart windows applications. The theoretical calculation results proved that the improvement of emissivity contrast is due to the hybridization of carbon. This work provides a new approach to synthesis VO2 hybrid material which could extend its application to camouflage and sensing. Followed by the VO2 based emissivity regulation material, HPC hydrogel based tuneable emissivity thermochromic window (TET smart window) was introduced in Chapter 5. The TET smart window showed large luminous transmission (Tlum, 72%), solar modulation ability (ΔTsol, 51%) and switchable front side long-wave infrared emissivity (εFront, 0.98 to 0.1). By combining the suppression of solar heating and enhancement of radiative cooling, the hydrogel-based TET smart window achieved a ~35 °C temperature reduction compared with the normal glass in summer outdoor demonstration in Singapore. While in cold environment indoor demonstration, TET smart window showed higher inner surface temperature and commercial energy-saving low-E window. The TET window opens a new venue for energy-saving buildings for dynamic climate conditions. Thermochromic window with large thermal energy storage capability was discussed in Chapter 6. Its excellent thermo-responsive optical property (90% of luminous transmittance and 68.1% solar modulation) together with outstanding specific heat capacity of PNIPAm based thermos-responsive liquid gives the smart window excellent energy conservation performance. Simulations suggested that the novel window can cut off 44.6% heating, ventilation and air-conditioning (HVAC) energy consumption compared with the normal glass in Singapore. In outdoor demonstrations, the smart window showed promising energy-saving performance in summer daytime. This thesis for the first time investigated the synergetic effects of emissivity, thermal heat capacity with light modulation and understands their effects on the energy saving performance.