Life Cycle Assessment of Heterojunction Solar Cells and Modules

Abstract

Heterojunction solar cells have demonstrated high efficiencies and are also viewed as potentially highly sustainable. A life-cycle assessment (LCA) was performed to compare the sustainability of heterojunction (HJ) solar cells, as developed in the HETSI project (FP7-ENERGY-2007-1-RTD), to conventional mono-crystalline Si solar cells. This calculation resulted in a comparison of the cumulative energy demand (MJ) and the global warming potential (kgCO2 eq) for HJ cells versus the mono-crystalline cells. The energy payback times (EPBT) of modules of two different HJ cell designs was found to be 1.2 and 1.2 years, while that of a ‘standard’’ mono-crystalline Si module was 1.5 years. The carbon footprint for module fabrication for the mono-crystalline Si module, and the two modules with slightly different HJ cell designs were: 0.9, 0.8 and 0.8 kg CO2 equivalents/ Wp, respectively. Assuming a southern Europe illumination of 1700 kWh/myr, a 75% performance ratio and a 30 year lifetime and no performance degradation, these values may also be expressed as 24, 20 and 20 g-CO2 equivalent/kWh. The higher efficiency of the HJ solar cells factors into a more favourable energy payback time, and a lower carbon footprint (per Wp or per kWh). A discussion of the cumulative energy demand and environmental impact of the manufacturing processes, together with an outlook on resource depletion, points out the materials and processes which require further development for future manufacturing requirements of lower energy use and lower costs. This is the first time an LCA of c-Si/ a-Si:H HJ solar cell technology is presented.