Abstract
Daylighting control technologies have become an essential part of sustainable building design to reduce overheating, glare and energy consumption in buildings. In this paper, a smart glazing system where a Building Integrated Photovoltaic (BIPV) glazing is coupled with an optically switchable thermotropic hydrogel layer is proposed to improve the daylighting control and electricity generation performance of traditional BIPV glazings. Thermotropic (TT) hydrogels made of various weight percentage combinations of hydroxypropyl cellulose (HPC) polymer, gellan gum and sodium chloride (NaCl) salt were synthesised and first evaluated by visible-near-infrared spectroscopy. Subsequently, small-scale prototypes of the proposed BIPV thermotropic (BIPV-TT) laminated glazing based on these TT hydrogels were fabricated and characterised experimentally under controlled laboratory conditions. The TT hydrogel, which was synthesised of 6 wt% HPC, 0.5 wt% gellan gum and 4.5 wt% NaCl, was selected for further experimental characterisations in a dynamic outdoor environment, due to its appropriate transition temperature of 30.7 °C for use in mild climates with a wide modulation range of solar transmittance from 85.8% (in the transparent state) to 9.6% (in the light-scattering state). The outdoor tests were conducted in Nottingham, the UK, on typical summer days with sunny and partial cloudy conditions. The results showed that using the prototype BIPV-TT laminated glazing can reduce up to 80% of the solar radiation transmitted into the outdoor test cell, while providing up to 12% higher electrical power outputs, compared to its counterpart system with no thermotropic hydrogel applied.
Highlights
Large glazed windows and facades are widely used in contemporary architecture to enhance aesthetics and maximise the benefits of view and daylight availability [1,2]
The results showed that using the prototype Building Integrated Photovoltaic (BIPV)-TT laminated glazing can reduce up to 80% of the solar radiation transmitted into the outdoor test cell, while providing up to 12% higher electrical power outputs, compared to its counterpart system with no thermotropic hydrogel applied
The above results demonstrate that adding NaCl salt to an HPCgellan-gum hydrogel can effectively reduce its transition temperature while providing higher optical reflectance at temperatures above the Ts and a wider modulation range of solar transmittance, which would benefit for electricity generation and passive overheat protection of the proposed BIPV thermotropic (BIPV-TT) laminated glazing
Summary
Large glazed windows and facades are widely used in contemporary architecture to enhance aesthetics and maximise the benefits of view and daylight availability [1,2]. The hydrogel film at 240 μm thickness has a transition temperature of 32 ◦C, visible light transmittance of 87.2% at 25 ◦C and solar trans mittance modulation of 81.3% As another promising candidate for application in smart windows, HPC hydrogels fulfil demands on cost, availability and environmental capability, with high transition temperatures [45,46]. Thermotropic hydrogels with varying con centrations of HPC, gellan gum and sodium chloride (NaCl) were developed and first analysed by spectroscopy (Section 3) These ther motropic hydrogels were used for fabrication of the proposed BIPV-TT laminated glazing (Section 4) and further compared in terms of their effects on the system’s optical and electrical performance by an indoor experiment (Section 5). Prototypes of smart glazing incorporated with the selected thermotropic hydrogel were fabricated and their performance was demonstrated in an outdoor environment in Nottingham (52.9◦ N, 1.2◦ W), the UK (Section 6)
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