Reliability of PMMA for CPV Lens Applications
Abstract
Polymethyl methacrylate (PMMA, PLEXIGLAS®1) has been used successfully in manifold outdoor applications like automotive lighting, signage & outdoor lighting, architectural glazing, aircraft glazing and transparent noise barriers over decades since its invention more than 75 years ago in Darmstadt, Germany. Its outstanding optical properties and unsurpassed weathering resistance in combination with robust handling make PMMA an attractive raw material for various solar energy applications and especially for optical systems in concentrating photovoltaic (CPV). With the overall photovoltaic (PV) market growing in the gigawatt range up to now the PV market dominates the building-integrated and rooftop applications, whereas solar farms are using CPV products (Kurz 2008). CPVs have gained interest since harvesting technique of direct vs. global radiation by tracking the sun has lowered the cost for each produced kWh (Miller et al. 2009). In this specific application the lens material is exposed around-the-year to extreme outdoor weather conditions of the Sunbelt region, which requires an excellent heat (temperatures of up to 85 °C) and UV resistance. CPVs in general use mirrors or lenses to focus UV-Vis and IR light onto a small area as little as 1 cm2 of active semiconductor PV material based on a combination of group III and V periodic table elements (Fraas, Knechtli 1978). These so called multi-junction cells show very high cell conversion efficiencies of over 40.7% (King et al. 2007).