The term biofuels refers to a wide range of products derived from organic material, but has generally come to mean liquid fuels intended for transport use. Bio-ethanol is the main replacement for petrol, whilst a variety of animal fats and vegetable oils can be processed into diesel replacement. Bio-ethanol can be produced by fermenting sugar or starch based material such as wheat and sugar beet. Latterly enzymes and other chemical processes have been used to break down cellulose found in plant material to produce alcohol, this opens up the possibility that a wide range of waste stalks and stems, and farming and forestry residues could become the feedstock for petrol replacements.

Bio-diesel can be produced by the trans-esterification of fatty acids from a wide range of plant oils and animal fats. This process is straightforward and the equipment can be purchased to allow small-scale local production. Some diesel engines can burn plant oil direct

Oil Seed Rape

Oil seed rape (OSR), which produces a distinctive bright yellow flower in spring, is usually grown for cooking oil. After the oil has been used for some time (e.g. in deep fat fryer), it degrades and is replaced. The old oil is collected and transported for re-processing into bio-diesel. There is a move to include 5% of bio-diesel in all mineral diesel sold at petrol stations. All diesel engines can accommodate higher levels of biodiesel, some can run on 100% recovered vegetable oil. (N.b. take professional advice before switching fuels in your vehicle).

OSR is already grown in T&D as a commercial crop for use as cooking oil. It needs a number of inputs in terms of tractor operations to prepare the land and fertilizer. The balance between energy output and input is about 2:1. Although this is a positive energy balance, the question needs to be whether such a marginal benefit is the best use of land. It therefore seems likely that OSR will continue to be grown in T&D primarily as a food crop, not as an energy crop. After use as cooking oil, the waste vegetable oil (WVO) will be available for trans-esterification into biodiesel.

Bio-ethanol

Bio-ethanol is derived from fermenting either grain (wheat) or sugar beet into alcohol. This is a tried and tested technology in other parts of the world, particularly Brazil and USA. While there was a good amount of interest in bio-ethanol for vehicle fuel at the time of the DARE report (2006), this has largely been discounted due to the need for all available arable land and food crops to be used for food rather than fuel as the latter is of lower priority can be sourced through other processes. However for the benefit of providing a comprehensive summary of all possible fuel options and to enable considered comparison, the calculations for T&D are included here.

It appears that Sugar beet will produce more than twice the energy yield of wheat per hectare.

Energy from algae

Experimentation is currently underway at Plymouth University. Information is included here for reference only as no data is currently available on the potential energy yield for their work. Algae to diesel energy has a number of steps including providing a CO₂ rich atmosphere and ponds for the algae to grow. Algae to hydrogen has a higher energy yield potential as it is a more direct process, however it requires high electricity inputs and a reactor for processing¹.

Assuming some low-level development of this technology will take place in T&D during the next 21 years. Say on 5ha of flooded land no longer viable for food growing (e.g. at Dartington and ATMOS):

N.b. This technology may become important in carbon sequestration / removing carbon from the atmosphere.

Summarising Energy Crops

A straightforward comparison of the four main annual crops is difficult, whilst energy balance data is available for some crops, it is not for all. It seems reasonable to assume that all annual energy crops will need multiple tractor operations to plough, cultivate, sow, fertilise and harvest the crop so the energy balance for annual crops with a low yield may be marginal at best or negative. Further assessment should be undertaken to gain a more detailed breakdown of the energy inputs and outputs for bio-ethanol produced from annual energy crops².

Purely from an energy balance perspective using the set aside land, Miscanthus appears to be the crop of choice (and is therefore the data included in the energy totals in Table 8) as the other energy crops would be in competition for the same land. There will be multiple tractor operations in the first year to cultivate the ground, plant the crop and spraying to prevent weeds until the crop is established. But being a perennial crop, in subsequent years, the only input will be the single tractor operation to harvest the crop. Anecdotal evidence suggests Miscanthus can survive in the same plot of ground for 20+ years.

Algae to bio-diesel and algae to hydrogen are currently experimental but do offer productive use of flooded land, which is no longer viable for buildings or food growing or building.

Energy crops are generally regarded as ‘carbon neutral’, i.e. they only release the carbon absorbed when growing and therefore do not contribute new carbon to the atmosphere, but the energy capture per hectare is low. Energy from algae however utilises an enriched carbon atmosphere pumped into the pond and could be beneficial to carbon capture processes.

Comparing the energy capture from energy crops with other renewable technologies;

• One hectare of Miscanthus could produce 62,000 kWh/y
• One hectare of PV could produce 1,250,000 kWh/y (creating a significant ‘heat island’)
• One single 1.3 MW wind turbine could produce 2,867,400 kW/y (& most of the land remain available for planting)

Leave a comment

If you wish to comment on a particular paragraph

and quote the relevant number in your comment.