ENERGY USE EFFICIENCY IN BIOMASS PRODUCTION SYSTEMS - Economics of Sustainable Energy in Agriculture - page 77

4.1 Use of fossil energy
The energy required to grow biomass crops is higher for the high-input
systems than for the low-input systems. The relative increase of the inputs
is larger than the relative increase in yield so that the resource use efficiecy
declines (less biomass per input). This picture is in accordance with results
found in Naylor (1996), Kramer et al. (1999), Schroll (1994) and Conforti
and Giampietro (1997).
More detailed study of the inputs shows that 'fertilisation' covers a
large amount of the total fossil energy inputs of the crop. This is because of
the large energy requirements of artificial fertilisers. Crops can also obtain
nitrogen from other sources (for instance, manure or compost) and the
energy requirements of this nitrogen are much lower. Production systems
using non-artificial nitrogen sources will have a high fossil fuel use
efficiency, as is shown in this study for the low-input systems and
This implies that the value of fossil energy use efficiency for a
production system is strongly determined by the nitrogen source used in
that system. Consequently, high-input systems using manure as nitrogen
source will have a high fossil fuel use efficiency (however, since artificial
nitrogen is cheap and easy to apply, high-input systems using manure or
compost are very scarce and no data exist).
4.2 Use of solar energy
In biomass production systems, solar energy is converted into plant
material, which is used later on as an energy carrier. Therefore, the
efficiency with which solar energy is converted into plant material is of
interest. In Table 6.4, a comparison is made between energy inputs (both
fossil and solar) and the energy yield (the heating value of wood: 18
MJ/kg). The solar energy irradiated on a hectare is about 1000 times as
high as the fossil energy used for the cultivation of the crops.