Transmittance and Reflectance Studies of Thermotropic Material for a Novel Building Integrated Concentrating Photovoltaic (BICPV) ‘Smart Window’ System

A novel Building Integrated Concentrating Photovoltaic (BICPV) Smart Window has been designed and developed as a next generation intelligent window system. In response to climatic conditions, the smart window varies solar light transmission into the building for provision of light and heat with the reflection of light to the photovoltaic (PV) for electricity generation. This unique function is realised using an integrated thermotropic layer in conjunction with embedded PVs. As commercial PVs are readily available, the success of this novel BICPV design depends solely on the performance of the thermotropic material. This study aimed to develop a suitable reflective thermotropic layer for the proposed smart Concentrating Photovoltaic (CPV) system. A Hydroxypropyl cellulose (HPC) polymer was tested for its applicability as a potential reflective thermotropic material for this purpose. HPC concentration was systematically varied from 1 wt. % to 6 wt. % in aqueous solution so as to provide insight into the relationship between transmittance/reflectance properties, the concentration of the thermotropic material and their dependence upon the environmental temperature. The degree of hysteresis of light transmittance upon subjecting HPC to heating and cooling cycles was also investigated. Specifically, for the HPC liquid samples the measured threshold temperature/transition temperature (Ts) was observed to be approximately 40 °C for 6 wt. % HPC, increasing to approximately 44 °C for 1 wt. % HPC. No hysteresis was observed upon heating and cooling HPC samples. Reflectance below the Ts was recorded at ~10%, increasing up to ~70% above the Ts for 6 wt. % HPC. Finally, a HPC-based hydrogel membrane sample was developed and exhibited good thermotropic activity therefore demonstrating its suitability for use within the BICPV smart window. This study corroborates that HPC is a suitable thermotropic material in the application of next generation BICPV smart window systems.