Energy Balance and Energy Use Efficiency of Annual Bioenergy Crops in Field Experiments in Southern Germany

Abstract

The main objective of the cultivation of energy crops is the production of renewable energy, the substitution of fossil energy resources, and a substantial contribution to energy supply. Thus, energy yield and energy efficiency are the most important criteria for the assessment of energy crops and biomass-based renewable energy chains. Maize is the energy crop with the highest cultivation acreage in Germany because of its high energy yields, but is the subject of controversial debate because of possible detrimental effects on agro-ecosystems. This raises the question as to which energy crops and production systems could be used instead of maize, in order to increase crop diversity and lower environmental impacts. We examined yields, energy inputs, energy outputs, and energy efficiency of alternative energy crops (combinations of catch crops and main crops) compared to maize in four-year field experiments at three southern German sites by means of process analyses. Maize showed moderate energy inputs (11.3–13.2 GJ ha−1), with catch crops ranging from 6.2 to 10.7 GJ ha−1 and main crops ranging from 7.6 to 24.8 GJ ha−1. At all three sites, maize had the highest net energy output compared to the other crops (x¯ = 354–493 GJ ha−1), but was surpassed by combinations of catch and main crops at some sites (winter rye/maize: x¯ = 389–538 GJ ha−1). Although some combinations yielded higher net energy outputs than maize, no other crop or combination of crops outperformed maize regarding energy use efficiency (energy output/energy input: x¯ = 32–45).