Monte-Carlo optical model coupled with Inverse Adding-Doubling for Building Integrated Photovoltaic smart window design and characterisation
Xiao Liu, Yupeng Wu
Abstract
Building Integrated Photovoltaic (BIPV) glazings are promising technologies with the benefits of electricity generation, solar shading and building energy savings. A new approach is to integrate BIPV windows with thermotropic materials such as Hydroxypropyl Cellulose (HPC) hydrogel, which offers adaptive advantages for the systems to respond to time-varying weather conditions but also enhances the PV electricity generation. To design such systems, knowledge of the temperature-dependent scattering properties of the selected thermotropic materials is of significance. In this study, a Building Integrated Photovoltaic (BIPV) smart window system consisting of a thermotropic membrane synthesised using HPC and gellan gum gelling agent for electricity generation and also solar control has been designed and investigated. An advanced optical model, which combines a Monte-Carlo ray-tracing technique with an Inverse Adding-Doubling (IAD) method, has been developed for characterising the thermotropic membrane in terms of angular scattering distribution under various membrane temperatures and HPC concentrations. Then the developed optical model has been validated by comparison with experimental measurements. Subsequently, the validated optical model has been used to design and optimise the proposed BIPV smart window. The effects of HPC concentration, geometric concentration ratio, thermotropic membrane thickness and glass refractive index on PV power outputs have been evaluated. Finally, a prototype of the BIPV smart window with a 6 wt% HPC membrane has been manufactured and tested under indoor conditions. From the experimental tests, it was found that the total transmittance of the double-pane glass sample with a 6 wt% HPC membrane layer decreases from approximately 90%to 14%, when the membrane temperature increases from 27 °C to 56 °C. The measured short-circuit current for the prototype BIPV smart window is up to 1.15 times higher than that of its counterpart system with a similar PV area but no membrane.