Time-dependent climate impact of short rotation coppice willow–based systems for electricity and heat production

Sammanfattning: Fossil fuel use and man-made land use change has increased carbon dioxide (CO₂) levels in the atmosphere, contributing to climate impacts such as global warming. Perennial crops such as short rotation coppice (SRC) willow have received attention because of their potential to sequester carbon (C) from the atmosphere and build up soil organic carbon stocks while producing biomass which can be used to generate energy services. The aim of this thesis was to assess the climate impact of bioenergy systems and develop the methodology used to evaluate these systems. The biomass from a SRC willow plantation can be used in a number of different ways to produce energy services. Specific objectives of this thesis were to investigate the energy efficiency and time-dependent climate impact of SRC willow–based bioenergy systems using different ways of converting the biomass into electricity and heat. Life cycle assessment (LCA) methodology was used to enable the assessment of time-dependent climate impacts using a time-distributed inventory and a time-dependent indicator, i.e. the global mean surface temperature change (∆Ts). Several different ways of generating electricity and/or heat from the biomass produced at a SRC willow plantation were compared, taking biogenic C stock changes into account. The main conclusions were that SRC willow–based bioenergy systems can be truly C negative and help contribute to counteract the current trend in global warming while delivering renewable energy at the same time. The choice of energy conversion technology affects both the energy efficiency and the potential climate impact mitigation potential of the system. Biogenic C pools can have a very large influence on the climate impact in bioenergy systems. It is therefore important to take these pools into account whenever land use or management changes take place, in order to counteract global warming more effectively.