Light guidance film for bifacial photovoltaic modules

Highlights

• Simulations demonstrated that an additional gain of up to 11% can be achieved.

• Suitable materials for film and adhesive were evaluated.

• Using the roll produced with laser ablation, the structured film could be created.

• The produced films could be used for different applications depending on the light guidance.

Abstract

To improve the efficiency of bifacial photovoltaic modules, the idea of light deflection was addressed. A thin film with an intended structure was applied in the space between the cells. Due to the selected structural geometry, unused light is specifically redirected to the solar cell and can be absorbed by it and converted into additional current. The effect of the light-directing film was both simulated and measured. Single PV modules without and with commercial structured films or white back sheet were measured to evaluate the results The expected gain of up to 11% from the simulation could not be achieved by measurement, by reaching about 3%.

Introduction

For the manufacturers of photovoltaic (PV) modules, the continuous improvement of efficiency is important to survive in the regional and international market. Light management and guidance within the PV module is one approach for increasing the module efficiency by increasing the number of photons coming to the solar cell. Over the years different approaches have been investigated for monofacial solar cells. For example, the light guiding effect of structured front sheets made of polycarbonate (PC) or polymethyl methacrylate (PMMA), which replace the front glass, has been tested [[1], [2], [3]]. However, none of these approaches have been adopted by the PV industry due to issues in long term stability, thermo-mechanical behaviour and compatibility with the encapsulation material. Other approaches address the optical shadowing of the solar cell grid fingers and busbars, ranging from a local change of the optical properties in the bulk of the photovoltaic module encapsulation material [4,5] to structured ribbons [6,7]. Also backscattering via a diffuse reflecting white back sheet has been exploited [8].

According to the report from “International Technology Roadmap for Photovoltaics” of 2020, the sales market for bifacial modules will grow steadily over the next 30 years [9]. In comparison to conventional modules, the solar cells are embedded between two glass sheets in EVA and not between a glass sheet and a white back sheet. This design allows reflecting light from e.g., concrete, metal, or grass to be absorbed by the back of the cell, generating up to 30% more power. The surface of a standard PV module is usually covered with 85% of cells and is directly hit by the sun's rays. In the intermediate areas between the cells and in the edge areas, the light exits on the rear side and only the rays reflected by the ground can be used to generate energy. The remaining light is lost. Several research teams have already set themselves the target of making the best possible use of this light. By using a reflective layer, a research team from England has already been able to simulate the positive effect of light guidance.

The layer was inserted and simulated at three different positions (outside back glass, inside back glass and at the height of the solar cell). Depending on the position and layer width, a gain of 1.7–2.2% could be achieved [10]. In 2017, a group from Germany was able to redirect the sun's rays by using a structured back sheet and thus achieve an additional gain of 2.5% compared to a white back sheet [11]. In 2018, the research partner Johanneum Research developed a reflective layer of silver that was laminated underneath the solar cell, which redirects the light rays to the rear side or, by total reflection, to the top side of the solar cell. Using this, a gain of 2–6% could be achieved by means of laboratory tests [4,12].

Based on the basic principle of light deflection and the geometries developed in the previous project [12] a new approach for light guidance for bifacial PV modules is presented within this paper. The light guidance is achieved by using a structured film, which is produced by extrusion on a patterned extrusion chill roll. The resulting structured film is later attached to the outside of the rear glass of a bifacial PV module using a transparent adhesive. In comparison to the other approaches, this film can also be applied to already existing modules and does not lead to shading due to the intended material. The optical behavior of the adhered structured film and the interaction with the solar cells was simulated.

A comprehensive market and literature review was done to identify and select possible candidate materials for the film and glue. The extruded and embossed film was tested for optical properties, thermal expansion behavior, structural fidelity and roughness and then bonded to the back of a test module and tested under laboratory and real conditions. Tests performed under laboratory and real conditions served as comparative measurements to the simulations.