Author: Robert Luo
The performance of the bifacial BIPV module is affected by installation factors such as the distance between the PV module and the inner wall and by the reflective properties of the inner wall. It is also affected by module design elements such as the solar cell formation method used and the transparent space ratio. To evaluate the effects of various elements, it is necessary to select the elements which have a relatively high impact. Identifying such elements can be accomplished through an analysis of variance (ANOVA). Therefore, the ANOVA approach was used to identify the impact of the respective design elements on the bifacial BIPV module performance capabilities. Moreover, with the key design elements derived through the ANOVA as independent variables and the performance levels of the bifacial BIPV module as the dependent variable, a multiple regression analysis was carried out to analyze the performance of the bifacial BIPV module.
1. Analysis of the Design Elements
This study identified three design elements that have an effect on the bifacial PV performance levels. These are shown in Figure 1.
Key parameters affecting the bifacial PV performance.
The first was the transparent space ratio, as determined by the space between the cells. The maximum transparent space ratio was set to 50% considering the surface area output of the PV module; hence, the transparent space was not larger than the cell surface area. Five different transparent space ratios of 10%, 20%, 30%, 40%, and 50% were selected. Additionally, two types of cell placements were selected: grid and line types.
The second design element was the reflectivity of the wall on which the bifacial PV module was installed. Usually, the inner surfaces of exterior walls on buildings are clad with waterproof sheets or reflective insulation, as shown in Figure 2.
Wall section detail.
The spandrels in curtain walls are usually installed with finishing materials such as aluminum composite panels or stone, with insulation panels inside. If BIPV modules are used as building envelopes, the inner wall surface can be covered with a variety of reflective materials, the selection of which has a significant impact on the performance capabilities of bifacial PV modules. The experimental variables of a black reflector, a white reflector, a red reflector, and an aluminum reflector were selected for this design element. The aluminum reflector was further classified into two types based on the surface properties: mirror reflection and diffuse reflection.
The reflectivity was measured using a Shimadzu UV-3600 spectrometer. Accounting for the spectral response characteristics of the bifacial cell, the average reflectivity in the wavelength range of 300 nm~1200 nm was utilized as the reflectivity measurement. As shown in Figure 3, the reflectivity levels of the black, red, white, and aluminum reflectors used were 5%, 47%, 72%, and 82% respectively.
Reflectivity by reflector type.
The third design element was the distance between the bifacial PV module and the reflective surface (inner wall). Experimental distances were selected by considering the curtain wall frame thickness and insulation panel thickness in the curtain wall systems, where BIPV modules are typically installed.