Modelling of energy and carbon budgets of wood fuel coppice systems

The development and application of a standard methodology for evaluating the energy and carbon budgets of biofuel production systems is described, with emphasis on wood fuel production from short rotation coppice. Five major tasks were involved: definition of the system boundary; estimation of energy benefits; estimation of carbon sequestration; estimation of energy costs; estimation of carbon emissions. Calculation of overall energy and carbon budgets required a set of ‘standard’ assumptions about practices and resultant energy inputs to be made. These standard assumptions accounted for all activities involved in production and delivery of biofuel within the immediate vicinity of the farm (3.2 km). The energy ratio, that is the ratio of energy produced to energy consumed by the biofuel producing system, was estimated to be much greater than 1, typically around 30. This energy ratio proved to be very sensitive to assumptions about crop management and wood processing, as well as associated energy inputs, varying from 20 to 64 when model input assumptions were varied between extremes. The carbon emissions coefficient exhibited similar sensitivity to input assumptions. The gross non-renewable energy requirement, that is the total consumption of non-renewable energy associated with the direct consumption of 1 MJ of energy in the form of wood fuel, was estimated to be 0.035 MJMJ−1. The carbon emissions coefficient, that is the carbon emitted in producing 1 MJ of energy in the form of wood from short rotation coppice, was estimated to be 0.0013 kgCMJ−1. Further research is needed to validate input assumptions and to estimate budgets for complete, practical, energy generation systems.