Impact of Structured Glass on Light Transmission, Temperature and Power of PV Modules

PV modules were fabricated using structured glass and investigated for the effect on light transmission and module temperature. Four different types of commercially available structured glass were investigated: grooves, pyramids, inverted pyramids and a very light structured type with only 5% increased surface area, along with flat glass modules for experimental control purposes. Measurements of light transmission were collected as a function of angle of incidence using an AM 1.5 G pulsed solar simulator. Results show an increase in Isc of up to 3.2% for pyramid structures with normally incident light, and the gain increases at higher angles of incidence. Wind tunnel testing was used to evaluate the effect of structured glass on module temperature. It was found that the surface structure has a significant cooling effect, increasingly pronounced at higher wind speed. In an outdoor environment, there were observed to be competing influences on module temperature: at low wind speed and high irradiance the increased light transmission associated with structured glass serves to increase module temperature, whereas at higher wind speeds, the increased convective module cooling serves to decrease the module temperature. Both the decreased temperature and increased light transmission will serve to increase the power output of the textured glass modules. Introduction and Theoretical Background Glass with a flat outer surface is most commonly used in PV modules. Structured glass has been developed as an alternative that can potentially improve module performance in two ways. First, light management in the module can be improved, leading to an increase in Isc. Second, the structured glass can enhance cooling to maintain a lower module operating temperature and a higher Voc. Approximately 4% of normally incident sunlight is reflected off the surface of a flat glass module and lost for power conversion due to Fresnel reflection. The fraction of sunlight that is reflected by flat glass increases as the angle of incidence increases, as shown in Figure 1. The angle of incidence is an important parameter for fixed solar installations, where the sunlight incidence angle changes throughout the day as the sun moves across the sky. Structured glass can trap more light by redirecting the reflected rays at the front surface and giving them a second chance to enter the glass and reach the cell (Figure 2). Additionally, the structures can recapture more light that would otherwise escape due to reflections within the module. Figure 1: Effect of angle of incidence on spectral reflection. Figure 2: Interaction of light at the air-glass interface for structured glass. Module power output decreases as the module’s temperature increases. In the case of many crystalline silicon modules, the power loss is equivalent to 0.5% / K. The thermal resistance of the air-glass interface at the front surface of the cell is known to limit module cooling. Structured glass is of potential interest as a technology that may enable a decrease in module temperature, and hence an increase in module power output, via increased surface area at the air-glass interface. In an outdoor operating environment, module performance is significantly impacted by varying environmental factors such as ambient temperature, irradiance, and wind speed, in conjunction with the design of the module itself. Light transmission and convective cooling properties of the modules are expected to be most important at high irradiance and high wind speed, respectively. We investigated the light trapping and temperature reduction effects of structured glass modules through solar simulator, wind tunnel and outdoor exposure experiments. 1 Light transmission Prior to lamination into mini-modules, the power output of several crystalline silicon solar cells was measured using an AM1.5G cell solar simulator. Commercially available glass with four different structures (Figure 3) was used to build mini modules (Figure 4). Control modules with a flat glass surface were also fabricated. The I-V curves of the mini modules were measured using an AM1.5G pulsed solar simulator (Figure 5). The electrical characteristics were measured as a function of angle of incidence between 0o (normally incident) and 80o (highly oblique incidence). Figure 3: Textured glass surfaces used in study Figure 4: Pyramid glass one cell module Figure 5: Modules on in solar simulator dark tunnel Light Transmission Results Comparison of Isc from the modules clearly demonstrates that the structured glass surfaces transmit more light to the cells than glass with a flat surface. With normally incident light (Figure 6), the greatest improvement was seen for the pyramid structures, which showed an Isc increase of 3.2% relative to the flat glass control. As the angle of incidence is increased further, the percent improvement of Isc for the structured surfaces becomes more pronounced with the greatest percent increase in Isc observed for the grooved surface structure at highly oblique angles (Figure 7). More modest increases in Isc are observed for the lightly textured glass. Figure 6: Percentage increase in Isc for normally incident light. Figure 7: Effect of textured glass on Isc as a function of angle of incidence