One of the perceived problems with all biofuels is that if they require too much energy for their production then this makes them uneconomic and unsustainable. The energy input is measured by the net energy value (NEV), the ratio of the energy obtained versus the energy required to generate the fuel. If this is less than one it indicates an unsustainable energy source.

Energy balances for biomass

The net energy balance or NEV for biomass combustion or use in electricity generation is shown in Fig. 8.33. Electricity generated from fossil fuel had an NEV of 0.32 and electri­city generation with all forms of biomass had much higher values. This indicates that these are more sustainable sources. Electricity generated from SRC and Miscanthus, either directly or after gasification, had NEV values of around 5. Combustion of willow and the perennial grasses had NEV values of 15 and above. The differences between combustion and electricity generation reflect the losses that occur in electricity generation.

Ethanol

Fig. 8.31. The introduction of xylose-metabolizing enzymes into Saccharomyces cerevisiae. The shaded area indicates the introduced enzymes. (From Prasad et al, 2007.)