Assessment of the energy performance, economics and environmental footprint of silicon heterojunction photovoltaic technology

Abstract

To make the transition towards a more sustainable energy supply, it is necessary that we drastically increase the share of renewable electricity generation. Solar photovoltaic energy is regarded as one of the prime options to reduce the greenhouse gas intensity of our electricity supply, and many different types of photovoltaics are being investigated to develop ever cheaper and more efficient photovoltaic devices. Silicon heterojunction technology combines the experience in both crystalline and thin film silicon photovoltaics, and offers highly efficient solar cells, although its market penetration is still very limited. As main patents describing this technology have expired, many research groups are presently investigating a variety of silicon heterojunction cell designs. In this thesis, a comprehensive assessment of silicon heterojunction technology is performed, taking into account the environmental footprint and economics of different silicon heterojunction solar cell designs and complete photovoltaic systems based on them, as well as an analysis of the performance of commercial solar modules, including silicon heterojunction modules, is given. Furthermore, the development of the complete photovoltaic industry is analysed to establish the net contribution of solar energy in terms of energy generation and reduction of greenhouse gas emissions. A life cycle inventory was built, that details all the material and energy flows in and out of the production chains of the silicon heterojunction designs. The results indicate that current silicon heterojunction technology has lower environmental impact compared to crystalline silicon, but similar or higher costs. Prospective silicon heterojunction designs are able to significantly decrease environmental impact and production cost, relatively more than the prospective crystalline silicon reference design that is investigated here. In order for silicon heterojunction technology to compete with alternative photovoltaic technologies, the outdoor performance should be comparable with or better than these alternatives. Under practical operating conditions, silicon heterojunction photovoltaic modules show very similar performance compared to both mono- and polycrystalline PV modules, both in detailed measurements conducted at a single North-Western European location as well as based on performance modelling for a broad geographical range. The smaller effect of temperature on the performance of silicon heterojunction can benefit this technology compared to crystalline silicon, although other parameters also significantly affect module performance. Because of the significant variation of performance of photovoltaics, the environmental impact and costs associated with photovoltaic electricity show strong variation over large geographical ranges. The continued and fast growth of the photovoltaic industry has raised concerns that it could result in the creation of an energy sink or a net greenhouse gas emission instead of a positive energy balance and net reduction of greenhouse gas emissions. There has been a strong decrease over time of the environmental impact of photovoltaics, and as a result, it is likely that a break even has already been achieved between the cumulative energy investments made and cumulative greenhouse gas emissions released to produce photovoltaics, and the cumulative energy production and greenhouse gas emission reductions.