8 Energy balance of bioenergy
The fuel energy ratio (FER) is the ratio of useful energy in the fuel compared to the total amount of fossil fuel used in producing that fuel, including that used to construct equipment. Where the ratio is less than one, more energy is used to produce the fuel than is available from that fuel.
Woody energy crops perform well, with FERs between 10:1 and 20:1 on a heat output basis, but biodiesel may achieve only 3:1. Ethanol from grain may barely break even at just over 1:1 in the worst case, though there is little consistency among estimates (Davies et al., 2009).
Table 4 The range of fuel energy ratios for selected bioenergy systems reported in the literature to 2016
|Fuel energy ratio||Lowest||Highest|
|Lignocellulosic alcohol (generalized)||0.75||1.24|
|Miscanthus (rhizome to pellet combustion)||1.77||9.23|
|Only one set of data for miscanthus systems was available.|
Energy ratios can be improved by good design of the production and processing systems to ensure that no energy is wasted. Co-products, such as straw or bagasse, associated with biofuels can also be used to replace fossil fuels in supplying heat for the processes involved.
How much does bioenergy cost? You’ll look at this next.