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

Abstract

Abstract: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 (T s ) 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 T s was recorded at~10%, increasing up to~70% above the T s 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.

Thermoresponsive, Freezing-Resistant Smart Windows with Adjustable Transition Temperature Made from Hydroxypropyl Cellulose and Glycerol

“…HPC fibers swelled by taking in water at low temperature, but increasing the temperature drove out water from the HPC fibers and shrank them, causing scattering. 30 In addition, increasing the glycerol concentration decreased the temperature of the cloud point (where the transmittance is 10% of the transparent state) 31 from 50 to 30 °C. These results show that glycerol promote the release of water from the HPC fibers even at low temperature because of the strong hydrogen bonding between glycerol and water (Figure 1b).…”

Optical aspects and energy performance of switchable ethylene-tetrafluoroethylene (ETFE) foil cushions

“…Ray tracing is a computational method used to calculate the trajectory of light and the refraction, reflection and scattering effects through optical systems by the representation of beams. The theoretical base for ray-tracing was initially developed by Spencer and Murty [68] and is now a widely accepted technique to aid the design of solar energy systems and solar façades [69][70][71][72]. The use of ray-tracing for the analysis of the optical behaviour of the multilayer cushion system seemed, therefore, appropriate in this study.…”

A Review of Transparent Insulation Material (TIM) for building energy saving and daylight comfort

“…In response to evolving standards of building regulation and to satisfy the growing expectations of occupant comfort, the performance of building envelopes has undergone sustained improvement over a period of several decades [12][13][14]. Increasing the thermal resistance of the building envelope is one of the most effective approaches to reduce heat loss (or gain) and in so doing reduce the required energy consumption for maintaining a desired level of indoor thermal comfort [15].…”