A new study published in Renewable and Sustainable Energy Reviews on 23 August sought to determine whether Sweden’s current willow (Salix spp.) production, as a source of bioenergy, is capable of meeting the future energy demands of the country. A team of researchers from Chalmers University of Technology, Sweden, the Swedish University of Agricultural Sciences, and Henriksson Salix AB assessed the average annual net energy yield and the ratio between the gross energy yield and total primary energy input.
Sweden is hoping to become one of the first nations to abandon fossil fuels entirely. More precisely, the Swedish government has set two goals: all vehicles fossil fuel free by 2030 and no net greenhouse gas emissions by 2045. The country already uses biofuel to meet 23% of its current power demands and its main biofuel source, willow, may play a crucial role in meeting these honourable goals.
The willow is an important crop with high biomass production potential that has been grown commercially in Sweden since the 1990s. Perennial crops like willow and poplar require less energy to produce compared to annual biomass crops such as sugar beets, maize, and wheat and therefore have less impact on the environment. Moreover, willow species offer the advantages of rapid growth, limited pest problems, high genetic diversity, short breeding cycles, and high resprouting capacity.
The study is based on the production outputs from three currently operating commercial plantations in Swedish which use three different levels of nitrogen (N) fertilization: no N fertilization, a medium level of N fertilization, and a high level of N fertilization. The highest net energy yield was attained with the highest level of nitrogen but also had the lowest energy ratio, whereas plants produced with no nitrogen had the highest energy ratio but the lowest net energy yield. What this translates to is that nitrogen fertilisation can promote intensive production and higher yields, however, the energy efficiency is lower than crops produced without nitrogen fertilisation, which were the least productive but most energy efficient.
What does this mean for the future of biofuel in Sweden? The results may help guide landowners, policy-makers, and other stakeholders in developing and further refining biomass production. The findings suggest that further improvements in energy performance and fuel-use efficiency are possible. The most important factors affecting energy performance were found to be N fertilization and biomass transport. If the area of land is a limited, high-yielding nitrogen fertilisation systems may be the best option, whereas if energy is limited, low-input systems with higher energy ratios may be preferable. In addition, the effects of N fertilisation on soil carbon dioxide emissions and other impacts on soil quality should be considered. Clearly, there must a trade-off between achieving the highest net energy yield, highest energy ratio, and lowest environmental impact.
At present, willow plantations in Sweden provide a major source of solid fuel for heat and power but could also be used to produce liquid and gaseous fuels in the future. This study and previous studies on willow cultivation offer some important insights, not only for Sweden but for other nations hoping to follow the same path, on adopting more renewable and sustainable sources of energy.
(1) Nordborg, M. et al. Energy analysis of willow production for bioenergy in Sweden. Renewable and Sustainable Energy Reviews (2018). DOI: 10.1016/j.rser.2018.05.045