Sustainability, overall and process efficiency of energy crops

Summary in the original language of the document

A method to calculate efficiency of energy crop production including sun energy,
direct and indirect energy for cultivation, processing, and conversion into fuel is
demonstrated using rape and derived fuels as an example. Every production and
conversion step is a process and calculated separately. The overall efficiency
includes energy input and output of all processes. The process efficiency of rape
cultivation reaches in Finland up to 1100 %. However, the overall energy efficiency of
rape methyl ester (RME) is 1 to 2 ‰ only. The production of biogas from manure of
dairy fed by rape meal results in a process energy efficiency of 33 to 41 %, but the
overall energy efficiency of RME and biogas together is only 1.2 to 2.5 ‰. In
contrast, thermal or photovoltaic solar collectors improve overall efficiency 1 to 3
orders of magnitude compared to fuel production from rape. Competition for
cultivation area and the low photosynthetic efficiency limit the feasibility of fuel
production from energy crops. As a measure for sustainability of renewable fuel
production, the energy surplus of energy conversion from insolation to fuel per
resident and square meter is proposed.