Boosting the performance of semi-transparent PV windows

High electrical efficiency and low cost are keys for the success and wide deployment of building integrated photovoltaic (BIPV) windows. Without sufficient electricity production, PV windows can hardly compete with alternative types of smart and advanced window technologies (which aim at reducing the building’s energy consumption in alternative ways, for instance with low-e coatings or vacuum double glazing) [1]. Therefore, efforts should be directed towards enhancing the electricity output of such BIPV products. On top of that, it is essential not to overlook the additional criteria that a PV window must fulfill, including maintaining a satisfactory level of transparency and, ideally, being color-neutral or aesthetically pleasing, in such a way that visual and thermal comfort are assured. In this context, the research institute TNO has teamed up with the glass company Pilkington and other partners to develop an innovative and multifunctional PV window concept, which aims to boost the energy generation of semitransparent windows while, at the same time, providing over-heating protection and visual comfort [2]. The concept that we propose is based on the combination of bifacial striped c-Si solar cells with venetian blinds [3]. The use of the bifacial technology allows to boost the energy yield thanks to the embedded blinds (also enclosed within the window glass panes) which will act as reflectors, increasing the amount of irradiance reaching the rear side of the cells. To the best of our knowledge, no bifacial PV windows exist yet. After conceptualization, design and manufacturing efforts, we installed 12 PV window demonstrators in the façade of a dummy building in the SolarBEAT outdoor facility (the Netherlands) [4]. We are investigating the effect of different designs and technological choices, namely 2x different types of PV cells, 2x different coverage ratios (CR = ratio PV active/non-active area) and 3x different types of venetian blinds, for a total of 12 PV windows variations. The goal of this work is to analyze the outdoor performance of these PV windows and show how the combination of bifacial cells and venetian blinds can boost the energy yield of this semi-transparent BIPV product. We are conducting several measurement campaigns to research on the influence of specific parameters, such as the blinds positioning and their optimal daily control strategy. It should be noticed that, thanks to the multifunctional nature of the product, optimization can be done prioritizing energy yield, or other aspects like thermal or visual indoor comfort. In this contribution, the focus is on the electrical output maximization. The monitoring period started in September 2023, therefore, by the time of the conference, 1 full year of data will be available. The proposed PV window has a power density of around 100 Wp/m2 (front-side Pmpp) when CR=60% (i.e. transparency of 40%). This is already higher than the average rated power of most commercial semi-transparent PV products, and comparable to colored opaque BIPV façade elements. Additionally, we can identify two boosting effects: the first simply given by the bifaciality of the cells, and the second given by the reflection provided by the blinds. The results gathered in the first months of measurements showed us that even without the use of the blinds, the PR was already higher than 100%, and when the blinds were deployed a boosting effect up to +25% was recorded. The exact boosting value depends on the type of blinds used and on their tilt angle (or, more precisely, the relative angle between the sun and the blinds).