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Design and Construction of Controlled Back Reflectors for Bifacial Photovoltaic Modules

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Bifacial photovoltaic modules are a relatively new development in the photovoltaic industry which allows for the collection and conversion of light on both sides of photovoltaic modules to usable electricity.

Bifacial photovoltaic modules are a relatively new development in the photovoltaic industry which allows for the collection and conversion of light on both sides of photovoltaic modules to usable electricity. Additional energy yield from bifacial photovoltaic modules, despite a slight increase in cost due to manufacturing processes of the bifacial cells, has the potential to significantly decrease the LCOE of photovoltaic installation. The performance of bifacial modules is dependent on three major factors: incident irradiation on the front side of the module, reflected irradiation on the back side of the module, and the module's bifaciality. Bifaciality is an inherent property of the photovoltaic cells and is determined by the performance of the front and rear side of the module when tested at STC. The reflected light on the back side of the module, however, is determined by several different factors including the incident ground irradiance, shading from the modules and racking system, height of the module installation, and ground albedo. Typical ground surfaces have a low albedo, which means that the magnitude of reflected light is a low percentage of the incident irradiance. Non-uniformity of back-side irradiance can also reduce the power generation due to cell-to-cell mismatch losses. This study investigates the use of controlled back-side reflectors to improve the irradiance on the back side of loosely packed 48-cell bifacial modules and compares this performance to the performance of 48 and 60-cell bifacial modules which rely on the uncontrolled reflection off nearby ground surfaces. Different construction geometries and reflective coating materials were tested to determine optimal construction to improve the reflectivity and uniformity of reflection. Results of this study show a significant improvement of 10-14% total energy production from modules with reflectors when compared to the 48-cell module with an uncontrolled ground reflection.

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Date Created
  • 2018-05

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Potential induced degradation (PID) of pre-stressed photovoltaic modules: effect of glass surface conductivity disruption

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Potential induced degradation (PID) due to high system voltages is one of the major degradation mechanisms in photovoltaic (PV) modules, adversely affecting their performance due to the combined effects of

Potential induced degradation (PID) due to high system voltages is one of the major degradation mechanisms in photovoltaic (PV) modules, adversely affecting their performance due to the combined effects of the following factors: system voltage, superstrate/glass surface conductivity, encapsulant conductivity, silicon nitride anti-reflection coating property and interface property (glass/encapsulant; encapsulant/cell; encapsulant/backsheet). Previous studies carried out at ASU's Photovoltaic Reliability Laboratory (ASU-PRL) showed that only negative voltage bias (positive grounded systems) adversely affects the performance of commonly available crystalline silicon modules. In previous studies, the surface conductivity of the glass surface was obtained using either conductive carbon layer extending from the glass surface to the frame or humidity inside an environmental chamber. This thesis investigates the influence of glass surface conductivity disruption on PV modules. In this study, conductive carbon was applied only on the module's glass surface without extending to the frame and the surface conductivity was disrupted (no carbon layer) at 2cm distance from the periphery of frame inner edges. This study was carried out under dry heat at two different temperatures (60 °C and 85 °C) and three different negative bias voltages (-300V, -400V, and -600V). To replicate closeness to the field conditions, half of the selected modules were pre-stressed under damp heat for 1000 hours (DH 1000) and the remaining half under 200 hours of thermal cycling (TC 200). When the surface continuity was disrupted by maintaining a 2 cm gap from the frame to the edge of the conductive layer, as demonstrated in this study, the degradation was found to be absent or negligibly small even after 35 hours of negative bias at elevated temperatures. This preliminary study appears to indicate that the modules could become immune to PID losses if the continuity of the glass surface conductivity is disrupted at the inside boundary of the frame. The surface conductivity of the glass, due to water layer formation in a humid condition, close to the frame could be disrupted just by applying a water repelling (hydrophobic) but high transmittance surface coating (such as Teflon) or modifying the frame/glass edges with water repellent properties.

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  • 2012