Energy crops are grown specifically for use as fuel and offer high output per hectare with low inputs. Research into which strains perform best under UK conditions, and typical outputs expected for different UK sites has been performed on a number of potential energy crops.
Classes of Energy Crops
In general the principle purpose is to maximize the output of the desired harvest. This may simply be measured in terms of tonnes of biomass per hectare, or it may be vegetable oil for conversion to biodiesel or sugar or starch for fermentation to bioethanol. High levels of biomass production need to be balanced against the potentially damaging impacts of some crop management techniques.
For example, very high outputs of biomass per hectare may be achieved from wheat, which typically yields 7.5-8 tonnes of grain per ha in the UK, (the record is 14 tonnes per ha), in addition to which there is typically 3.5-5 tonnes per ha of straw. The grain could be used to produce liquid transport fuels and the straw could be burned to produce heat or electricity.
However, in order to achieve these high yields, high levels of fertiliser inputs are required.
For wheat in the UK nitrogen fertiliser levels of 100-200 kg/ha are typically applied. This can lead to emissions of ammonia or NOx into the air and nitrogen compounds into groundwater. If this nitrogen is applied in the form of inorganic fertilisers very large quantities of carbon will be released to the atmosphere. This is because during the production of these fertilisers are Methane (CH4) is converted to ammonia (NH3) by reaction with nitrogen from and water vapour, with the release of the carbon as carbon dioxide (CO2). This is both environmentally undesirable and expensive, especially with increasing natural gas prices.
This process takes place in any system reliant on the application of inorganic fertiliser - it is not restricted to wheat production nor does all wheat production result in this process occuring.
It is therefore highly preferable for energy crops to be developed that can achieve high levels of output with minimal or zero inputs.
Energy Crops and Forestry Working Together
One further consideration that may influence policy on establishing energy crops is the area of land established with conventional forestry and the potential for harvesting fuel from this resource.
In countries with large areas of existing forest and woodland there tends to be little interest in establishing dedicated energy crops. This is becuase although conventional forestry produces much lower levels of biomass output per hectare compared to many energy crops, the cost of producing each tonne of biomass in the forest are also significantly lower. Consequently there is little attraction in establishing energy crops on high quality agricultural land.
In countries like the UK, where there is relatively low level of forest cover (11.6% of land area), demand for biomass fuels could exceed the rate of production of biomass in the existing forest and residue resource. As such it is necessary to consider whether it is appropriate to use agricultural land for biomass production. If this is required, purpose grown energy crops become an attractive option as high yields of biomass can be produced in a short time.
Research into Characterics of Potential Energy Crops
In order to identify the characteristics of potential energy crops, including biomass productivity under different conditions, susceptibility to disease or climatic events, etc., organizations such as Forest Research, Rothamsted Research and IBERS in the UK have been undertaking research over a number of years.