The future development of energy crops, to the level at which they would replace residues as the major bioenergy fuel source, will be largely dependent on regional factors such as climate change and local energy requirements and emission factors, which will determine their environmental and financial viability.
Energy production per hectare is determined by crop yields. This is in turn dependent on climate, soil, and management practices in addition to the number of harvests per year, the fraction of the biomass feedstock harvested, and the usable fraction of the harvest. The energy conversion efficiency to a useable energy form is a further important consideration.
Generally perennial crops produce higher net energy yields than annual crops, due to the lower management inputs, and therefore lower production costs, both economic and ecological. Management techniques, fertilizer application, and genetic improvements may increase the productivity by a factor of 10 compared with no inputs; however, this can result in increased environmental problems. Eutrophication of water can be associated with fertilizer runoff; soil erosion with intensive cultivation, and concerns regarding consequences for biodiversity with genetically improved crops.
There is sometimes a misconception that it takes more energy to produce biomass for bioenergy than is generated from the alternative energy biomass itself. However, it has been repeatedly shown that such systems are capable of producing significant energy ratios.
For example, the energy production per hectare of woody biomass from a commercial plantation is 20 to 30 times greater than the energy input for agricultural operations such as fertilizer and harvesting. However, liquid fuel production from energy crops may experience conversion rates that are substantially less. In Europe and the United States for example, achieving a high-energy output per unit input requires careful management at the production stage. Ethanol production in the U.S. yields about 50% more energy than it takes to produce, whereas in Brazil where ethanol is produced from sugarcane, higher yields are possible largely as a result of greater rates of biomass growth and lower auxiliary energy inputs.
