The role of engineering in organic farming : case energy crops

Abstract

Energy self-reliance and a closed nutrient cycle are basic principles of organic farming ever since. Engineering sciences methods in energy accounting may support efforts to introduce these principles into praxis. 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 effi-ciency of rape cultivation reaches in Finland up to 1100%. However, the overall en-ergy 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, because the process efficiency of photosynthesis attains about 0.6% whereas solar collector s efficiency reaches about 90%. However, for the time being solar energy based techniques are more expensive than the use of fossil energy sources since environmental benefits in terms of GHG mitigation, reduction of nutrient run off and use of renewable energy do not create cash income in both organic and main stream production. This and the low photosynthesis efficiency in Finland encourage bio-refinery enterprises to purchase energy crop produce for fuel production from the tropics. Mineral fertilisers as well as genetic modification increase the technical efficiency of photosynthesis. Thus, envi-ronmental pollution of mainstream agriculture is exported to developing countries in the tropics.