Study on daylighting and overall energy performance of a novel semi-transparent BIPV system integrated with vacuum glazing

Abstract

In the context of the indoor environment and building energy consumption, windows play an essential role as they admit daylight into buildings for visual comfort and are also responsible for a larger proportion of energy consumption. Therefore, the application of high-performance windows is becoming a requisite design criterion when developing energy-efficient and environmentally friendly buildings to mitigate the high energy demand and the greenhouse gas emission. The development of vacuum glazed windows in recent decades has provided a foreseeable energy-saving opportunity in the design of low-energy consumption buildings. On the other hand, building integrated photovoltaic (BIPV) has experienced rapid development for the applications in building. When semi-transparent PV glazing is applied in a building envelope, part of visible light can penetrate into the indoor environment since it has a certain degree of transparency. The vast majority of the solar irradiation will be absorbed by the solar cells, and then part of solar energy will be converted into electricity by the photovoltaic effect. However, some deficiencies restrict the applications of the semi-transparent PV glazing. The remaining solar energy will increase the temperature of the whole glazing system. In a cooling season, this part of undesirable heat could increase the cooling demand due to the poor thermal insulation performance of the back glass sheet of the conventional PV glazing. In a heating season, the conventional PV glazing would increase the heating demand due to the reduction of solar heat gain. Since the vacuum glazing is regarded as one of the best thermal insulation glazings, it can drastically reduce the heat gain and the heat loss of the window. On account of the limitation of the PV window application and the energy-saving potential of the vacuum glazing, a novel BIPV system integrated with the vacuum glazing is proposed and studied in detail in this thesis to combine the advantage of the high thermal insulation performance of vacuum glazing and the power generation capability of PV glazing. Firstly, a preliminary study on the amorphous Silicon-based vacuum PV glazing was conducted to investigate the overall energy performance experimentally and numerically. The first prototype of the proposed vacuum PV glazing was developed with a four-layer structure, which combines an amorphous Silicon-based PV glazing with a vacuum glazing. Various experiments were conducted to evaluate the thermal performance and determine the essential characteristics of the vacuum PV glazing. A comprehensive simulation study has been conducted by EnergyPlus and Berkeley Lab WINDOW under the weather conditions of Hong Kong. The laboratory tests and outdoor tests show that the vacuum PV glazing has the best thermal insulation performance compared with other conventional window systems due to its lowest overall heat transfer coefficient (U-value) and direct solar transmittance. The simulation results of the thermal performance indicate that the west-facing façade installed with the vacuum PV glazing could reduce cooling energy by at most 25.4% and 16.5% compared with the same oriented room with single-pane windows and double-pane windows. Therefore, it is better to install the vacuum PV glazing on the west, east and south façades to achieve better building energy efficiency. Secondly, the daylight behaviour of the vacuum PV glazing has been studied comprehensively. Based on the previous chapter, the concept of the vacuum PV glazing shows an appreciable energy-saving potential. A three-layer vacuum PV glazing was proposed to integrate the vacuum glazing with a cadmium telluride (CdTe) PV glazing, which is thinner and lighter than the first prototype. Besides the energy performance of windows, the daylighting performance should be considered as another critical aspect for the requirement of modern buildings. In this respect, the daylighting performance of the CdTe-based vacuum PV glazing and the amorphous silicon-based vacuum PV glazing were investigated under different climatic conditions. Various RADIANCE models for different test scenarios were built up to determine the essential optical properties of the vacuum PV glazing and simulation parameters by validation experiments. Based on the validation model, the annual daylighting simulations of the two types of vacuum PV glazing were conducted by DAYSIM, which is a dynamic daylight simulation tool using RADIANCE-based engine. To fully evaluate the climate-based daylighting performance, the annual daylighting simulations were conducted in five different climate regions in China, including the climate zone of severe cold, cold, hot summer and cold winter, hot summer and warm winter, and moderate. The results show that the a-Si-based vacuum PV glazing can provide more useful daylight illumination in the anterior region of the room, and avoid the daylight glare for the occupants of the space. The a-Si-based vacuum PV glazing can be classified as the 'Best' fenestration product in terms of daylight visual comfort in low latitude regions, such as Wuhan, Hong Kong and Kunming. The CdTe-based vacuum PV glazing can reduce the oversupply daylight occurring compared with the vacuum glazing and provide more useful daylight in the rear half of the office compared with the a-Si-based vacuum PV glazing. On account of the preference of the occupants on the luminous environment, it is suggested that the adoption of the CdTe-based vacuum PV glazing can balance daylight availability and visual comfort compared with the vacuum glazing solely. The daylight behaviour of the glazing will affect the daylighting performance as well as the lighting energy consumption. However, Most of the available whole building energy simulations adopted the daylighting calculation method based on Daylight Factor (DF) method, which is not suitable for direct sunlight calculation of complex glazing materials. Therefore, an ANN-based daylighting prediction method has been developed to simulate the daylight behaviour of the CdTe-based vacuum PV glazing. A RADIANCE model, which can fully represent the daylight behaviour of the vacuum PV glazing, was developed and validated by laboratory tests, including the direct light tests and the diffuse light tests. The key optical parameters of the vacuum PV glazing were determined accordingly. An artificial neural network (ANN) model was trained based on the weather conditions and the RADIANCE simulation results to predict the interior illuminance. Subsequently, the predicted illuminance results of a whole year by the trained ANN model were integrated into the energy simulation made by a pre-processing coupling method to determine the lighting consumption of a typical office adopted the CdTe-based vacuum PV glazing. The performance evaluation of the ANN model indicates that it can predict the illuminance level with higher accuracy than the daylighting calculation methods adopted in EnergyPlus. Therefore, the ANN-based coupling method is a more reliable method to calculate the lighting consumption than the simulation sole with EnergyPlus. Compared with the lighting consumption determined by the coupling method, the two daylighting calculation approaches in EnergyPlus, the DElight method and the split-flux method, tend to underestimate the lighting consumption by 5.3% and 9.7%, respectively. Furthermore, the computational cost can be reduced dramatically by the ANN daylighting prediction model in comparison with the RADIANCE model. Thirdly, a new one-dimensional transient heat transfer model for the CdTe-based vacuum PV glazing with a three-layer structure has been developed to fully understand the heat transfer process and thermal behaviour of the vacuum PV glazing. Due to the non-linearity of the boundary conditions and the heat transmission through the vacuum gap, finite difference method was used to solve the transient energy balance equations for multiple layers of the vacuum PV glazing based on the assumption of a one-dimensional transient state for angular incidence of the solar radiation. The exterior and interior layer exchanges heat with the adjacent environment by convection and radiation. The solar cells absorb the majority of the solar energy and convert a small proportion into power output. The specific heat transfer through the vacuum gap and the PV power efficiency are also taken into account. Based on the numerical solutions, the temperature profile of the vacuum PV glazing can be obtained dynamically. Consequently, the heat gain of the semi-transparent vacuum PV glazing can be calculated under different outdoor and indoor conditions. The developed heat transfer model was validated by experiments and applied under four different scenarios, i.e. summer daytime, summer nighttime, winter daytime, and winter nighttime, to analyse the thermal behaviour of the vacuum PV glazing in detail. The results indicate that the vacuum PV glazing can minimize the heat gain and heat loss due to the unique features of this novel BIPV application. The fluctuation of the inner surface temperature can be controlled within a limited range away from the set point of indoor room temperature. Therefore, the vacuum PV glazing contributes to stabilizing the temperature of the controlled room despite the incident solar radiation and periodical outdoor temperature. It is suggested that the vacuum PV glazing has the potential to enhance the adaptability of BIPV windows under different climate backgrounds. Lastly, a dynamic coupling method was developed to integrate the transient heat transfer model with the building energy simulation tool, EnergyPlus, to comprehensively analyse the applicability of the vacuum PV glazing under different climatic conditions. Building Controls Virtual Test Bed was used to communicate data between Matlab and EnergyPlus at each time step during the annual simulation. Therefore, the dynamic coupling method can simulate the annual energy consumption with higher reliability than EnergyPlus solely. Five Chinese cities, Harbin, Beijing, Wuhan, Hong Kong, and Kunming, were selected as the representative cites of five different climate regions of China, namely, severe cold, cold, hot summer and cold winter, hot summer and warm winter, and moderate. Double-pane windows, double PV glazing, and vacuum glazing were also investigated for comparison purpose and the energy simulation results of the double-pane window were set as the baseline. The energy simulation by the dynamic coupling method demonstrated that the vacuum PV glazing achieves the energy-saving potential up to 43.4%, 66.0%, 48.8%, and 35.0% in Harbin, Beijing, Wuhan and Hong Kong, respectively. However, the applications of the vacuum glazing lead to additional cooling consumption for the moderate climate zone, such as Kunming. The results advanced the understanding on the applicability and limitation of the vacuum PV glazing in different climate backgrounds. Furthermore, the reversed and the reversible vacuum PV glazing were proposed to enhance the adaptability. The simulation results indicate that the reversible vacuum PV glazing can act energy response in a more efficient way and fully utilize the energy-saving potential of the integration of the PV glazing and the vacuum glazing. In conclusion, this thesis has reported a comprehensive study on the daylighting, thermal and overall energy performance of the vacuum PV glazing. The vacuum PV glazing is proposed to combine the advantages of the PV glazing and the vacuum glazing so that the integration of these two advanced glazing technologies can achieve positive complementarity for energy saving and renewable energy applications in buildings. The one-dimensional transient heat transfer model can simulate the heat transfer process of the vacuum PV glazing dynamically. The ANN daylighting prediction model can represent the daylight behaviour of the vacuum PV glazing adequately. The presented dynamic coupling method improves the accuracy of the building simulation model for the vacuum PV glazing. Therefore, the applicability and limitation of the proposed vacuum PV glazing are thoroughly investigated based on the reliable results. The work from this thesis contributes to developing the optimal utilization of the BIPV window system as a renewable energy generator and an excellent fenestration product in the world.