| Author: | Pu, Jihong |
| Title: | Spectral regulation of liquid-filled windows using novel spectrally selective materials and mechanical structures |
| Advisors: | Lu, Lin (BEEE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Subject: | Windows -- Thermal properties Buildings -- Energy conservation Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xxv, 188 pages : color illustrations |
| Language: | English |
| Abstract: | Windows are the weakest links of building energy saving. Heat loss and gain through windows account over 30% of the total HVAC energy consumption in buildings. Among the various heat transfer mechanisms, radiative exchange plays a dominate role, contributing to over 50% of the total energy exchange through windows. Passive solar radiative heat gain through windows can exceed 600 W/m². Developing novel window technologies that combine superior optical performance and solar radiation management capabilities is crucial for enhancing building energy efficiency. Solar spectrum coupled visible (VIS) and near-infrared (NIR) components, where VIS provides daylight while NIR contributes to indoor solar heat gain. To achieve on-demand regulation of VIS and NIR through windows, this study focuses on two core scientific problems: (1) the VIS/NIR decoupling of solar spectrum and (2) on-demand modulation of windows' optical properties, and series of spectrally selective window technologies was proposed and validated. First, a novel spectrally selective material was developed to achieve effective solar VIS/NIR decoupling. Specifically, a water-based fluid was prepared using biomass dye, i.e. sodium copper chlorophyllin (SCC) and CuSO₄, capable of selectively absorbing NIR while transmitting VIS. This material demonstrated a high visible transmittance over 56%, and significantly reduced solar heat gain below 25%, achieving a maximum figure of merit (FOM=Tlum/Tsol) of 3.06, superior to all reported counterparts, including cesium tungsten bronze and transparent conductive oxide nanomaterials. Remarkably, the optical properties remained fairly stable over a 9-month weathering cycle and 48 hours UV exposure (300 W), confirming excellent long-term stability and high light/heat decoupling performance. Subsequently, a triple-layered spectrally selective liquid filled window (LFW) was designed. A mathematical model was developed to analyze the window's optical and thermal properties and the model was validated through experiments. The model was then used to assess the energy-saving performance of the triple-layered spectrally selective LFW under Hong Kong's climatic conditions. With varying solar incidence angles and varying solar spectral power distribution throughout the year, the average monthly visible transmittance of the triple layered LFW exceeded 40%, while the solar heat gain coefficient (SHGC) remained below 0.26, effectively balancing daylighting and solar heat insulation. Compared to conventional triple glazing, the triple layered LFW reduced solar heat gain by approximately 65%, with an annual reduction of about 1717.8 MJ/m² in solar energy input under typical Hong Kong weather conditions, demonstrating significant energy-saving potential. Furthermore, leveraging the fluidic nature of the working medium, a passive ammonia-pressure-powered thermos-responsive smart window (APPs) was developed. This window automatically adapts its transmittance in response to temperature change, exhibiting high visible transmittance (84% in bleached state, >50% in colored state) and up to 70% solar modulation ability. The switching temperature can be flexibly tuned between 20 to 80°C, by adjusting the ammonia concentration, enabling compatibility with diverse building environments. Field tests showed that under sunny conditions, APPs windows reduced indoor air temperatures by approximately 4.3°C compared to traditional double glazing, while performing comparably under cloudy conditions. Energy simulations conducted in different cities confirmed that APPs windows offer strong energy-saving potential across different climate zones. Compared to conventional low-e double glazing, the APPs window reduced HVAC energy consumption by 15 to 40 MJ/m2, in both hot-humid and cold climates. Additionally, an actively switchable rotatochromic bi-static (RB) window was proposed and fabricated for actively regulating indoor solar heat gain. Utilizing liquid paraffin (LP) and an SCC/CuSO₄-based solution as working fluids, the window employs gravity-induced fluid flipping to reversibly switch between two optical modes. Optical testing revealed excellent performance of the RB window: solar modulation exceeding 60%, visible transmittance of up to 90% in the bleached state and 56% in the colored state, haze below 1%, and a color rendering index (CRI) above 80. Outdoor tests during summer showed that this window reduced indoor floor temperatures by up to 11°C, and alleviated indoor glare. Energy simulations in four cities across different climate zones demonstrated that the window effectively reduced heating and cooling energy consumption by 30% to 40%. Finally, to address the windows' secondary thermal radiation from spectrally selective glazing to indoors, and to enhance sky radiative cooling in summer, a Janus bi-static (JB) window was developed. In summer, the window exhibits excellent shading (Tsol < 30%) and high radiative cooling potential (IR emissivity > 0.9); in winter, it provides high solar transmittance (Tsol > 70%) and thermal insulation (IR emissivity < 0.3), achieving broadband modulation of solar and thermal radiation. Detailed optical characterization confirmed its superior solar heat gain control, IR emissivity modulation, and visible light transmission compared to state-of-the-art smart window technologies. Field tests in Hong Kong under hot-humid conditions showed a peak indoor temperature reduction of over 10°C. Energy simulations conducted across 1929 global sites demonstrated that it exhibits superior energy-saving performance compared to traditional low-e glazing in nearly all regions worldwide with latitudes below 60°. This work begins with the design of spectrally selective materials and systematically builds an integrated multi-functional smart window system capable of dynamic regulation across visible, near-infrared, and mid-infrared bands. Through modeling and field testing, the energy-saving performance of these windows was thoroughly evaluated across various climate zones. The findings may provide some technical support for advancing green and sustainable development of buildings. |
| Rights: | All rights reserved |
| Access: | open access |
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