The Irish target for 2020 for renewable energy supply in electricity (RES-E) is 40%, while the EU target for renewable energy supply in transport (RES-T) is 10%. In Ireland transport is expected to account for 40% of final energy demand in 2020 while electricity is expected to account for 20%. Renewable transport is far further from a green solution than renewable electricity. Peak oil is a problem for transport more so than for electricity. The search for suitable renewable transport fuels must be intensified.
Methane is suggested as a vector for transport fuel. Natural Gas Vehicles (probably better termed as methane vehicles) are a mature technology; many countries have a natural gas infrastructure. Renewable methane (biomethane) is upgraded, cleaned biogas; again a mature technology. The sources of biogas vary from country to country and range from slurries, food wastes, sludges to energy crops. Germany has embraced maize as a source of biogas. However in temperate oceanic climates (such as Ireland) maize does not have the same yield as in drier, warmer continental Europe. Grass yields tend to be higher in wet temperate oceanic Europe. An analysis suggests that 2% of pastureland in Ireland could allow compliance with the EU target of 10% RES-T by 2020 through grass biomethane. This may be compared with EV technology; 10% of the private transport fleet could satisfy only 1.6% RES-T.
Yields of transport energy per hectare are a combination of yield of crop per hectare and yields of methane per unit of crop. Technology can influence yield of methane per unit of crop (expressed as L CH4 / kg volatile solid (VS)). A single phase system was compared with a two phase digestion (hydrolysis and acidogenisis separated from methanogenisis) in digesting grass silage. The single phase system (continuously stirred tank reactor (CSTR)) effected 90% destruction of volatiles and 450 L CH4 / kg VS at a 50 day retention time. The two phase system (leach beds followed by an Upflow Anaerobic Sludge Bed (UASB) reactor) effected 341 L CH4 / kg VS at a 30 day retention time. The two phase system could be cheaper as it is smaller. It produced a biogas richer in methane than the one phase system (71% versus 52%); this would reduce upgrading costs. The two phase system did however produce less methane per unit of crop. Two phase systems are as yet not commercially mature. More research is required to optimise hydrolysis of crops such as grass which may be classified as lignocellulosic. Good research could lead to two phase digestion, smaller more efficient digester technology, cheaper biomethane and a suitable renewable transport fuel for Europe and beyond.