Going green is no simple matter when it comes to energy crops. Should we grow sugarbeet and wheat for ethanol production or rape seed for biodiesel? There are also the options of elephant grass and coppice willow for biomass in power plants.
University College Cork's Dr Jerry Murphy has studied these options intensively. Based in the Civil and Environmental Engineering Department, he believes all of these could work but we might get the best results from doing what we have always been good at - growing grass.
The impetus to develop biofuels and biogas sources comes from the EU Biofuels Directive. This has set a target of having 5.75 per cent of transport fuel as an energy equivalent coming from these biofuels by 2010, says Murphy.
He believes we could get at least three per cent of this from growing biofuel crops, but deciding which ones is a complex business. Land used now for food crops will have to be diverted to growing energy crops, and this might require importing foods formerly grown here.
"It is important to choose a biofuel that minimises land requirement and to set policy that promotes optimisation of land used," he says.
Rape seed oil has been held up as a useful biodiesel crop and is one of the cheapest fuels to produce, says Murphy. Yet it is one of the worst crops on the basis of energy return per unit of land required.
He has produced sets of calculations based on national requirements and on the requirements of a single Dublin city bus. A city bus burns about 27,000 litres of diesel fuel a year, he says, providing a target for biofuel replacement.
Replacing this with rape seed biodiesel would require 26 hectares (ha) a year for a single bus. "But rape seed is a one in five year crop, you can't grow it on the same ground every year," Murphy points out. A single bus would therefore require the availability of 130ha of arable land.
Sugarbeet and wheat are being assessed as crops for bioethanol production, but rotation cropping allows the same land to be used over and over. Bioethanol production with these crops would require about 13ha to power that city bus, Murphy calculates.
Silage is one of the best energy crops, but requires the bus to run on biogas, rather than biodiesel. Buses in many countries already run on biogas so it is not a technical challenge.
"We have fantastic returns from silage, there is no plant giving a better energy return," he says. The 65 tonnes of silage from a single hectare of land is processed into biogas in a digester, and that same citybus can be kept running for a year on the crop from just 6.5ha.
The total land farmed here in 2003 was 4.45 million hectares, and 400,000ha of this was used as arable land.
On a national basis, Murphy calculates that reaching the 5.75 per cent EU Biofuel target using rape seed as the biocrop, given rotation, would require 1.3 million ha, more than three times the current arable land used in Ireland.
If wheat and sugarbeet are rotated as a bioethanol crop then 138,000ha would be required, demanding more than a third of all arable land.
Meeting the biofuels target using silage would need 73,000ha of land or just under 20 per cent of arable land. Silage does not need rotation or replanting and is a trouble-free crop.
"Another big advantage to biogas from silage is the fact that 10 times the land area is available to grow silage, 400,000ha of arable land, four million ha of grass land," Murphy adds.
Dick Ahlstrom