The intercept by the Earth of solar energy exceeds the present input of fossil and uranium fuels into the world economy by a factor of about 10,000 (Lewis and No- cera 2006). The average daily solar irradiation varies, dependent on latitude, climate and season. When on the equator, maximum irradiation is on a horizontal plane, but away from the equator, for the maximum intercept of solar radiation by a fixed plane, the plane should have an angle corresponding to latitude (e. g. Qelik 2006). Average daily solar irradiation (measured on a horizontal surface) that may support feedstock for biofuel production varies roughly between 7 and 25 MJm~2. The daily worldwide average irradiation is about 15.5MJm~2, or 180 Wm~2. Differences be­tween days can be large. For instance, in Amsterdam (52°21′ N), the average daily irradiation is approximately 3 MJm~2 in January and 17 MJ m~2 in July (Akkerman et al. 2002). The greatest annual input of solar radiation tends to occur in subtrop­ical regions at latitudes between 20 and 30° and little cloud cover. Humid tropical regions have somewhat lower irradiation (Sinclair and Muchow 1999). When go­ing poleward from a latitude of about 30°, solar irradiation tends to decrease. As for major areas for current biofuel production, in Brazil, where sugar cane ethanol is produced, daily solar irradiation is on average about 220 W m~2 (approximately 19 MJday-1 m~2 or 694 x 102 GJyear-1 ha-1). In the US, average daily irradiation varies between 12 and 22 MJm~2, whereas in the US Midwest, where there is large — scale corn ethanol production, solar irradiation is about 170 Wm~2 (approximately 14.7MJday-1m~2 or 536 x 102GJyear-1ha-1) (Kheshgi et al. 2000; Vasudevan and Briggs 2008).

In establishing the overall conversion efficiency of technologies for the conver­sion of solar energy, there should be a correction for the cumulative energy demand associated with the biofuel life cycle and the life cycle of physical conversion tech­nologies (Reijnders and Huijbregts 2007). For instance, if the lower heating value of fossil fuel inputs amounts to 20% of the lower heating value of a biofuel, the solar energy conversion efficiency will be corrected by this percentage. The result thereof is the overall energy efficiency of the biofuel. This is summarized in the following equation:

SCEX = Yx ‘Ex’FEx • 100 x F

-r^solar

where SCEX is the solar energy conversion efficiency of biomass or biofuel type x (%), Yx is the yield of biomass type x (kg/ha/year), Ex the energy content of biomass or biofuel type x (MJ/kg), FEx the correction factor for fossil fuel input in the life

cycle of biomass or biofuel type x (MJ/MJ), and Esolar is the yearly solar irradiation (MJ/ha/year). SCEx is a measure that can help in estimating the ability of biofuels to displace fossil fuels.

As pointed out in Chap. 1, conversion of solar energy into biomass occurs by photosynthesis. Harvestable biomass that can be used for energy generation (yield) depends on a number of factors. At the present atmospheric CO2 concentration for C4 terrestrial plants, the maximum conversion efficiency is estimated at 5.5-6.7% and for C3 plants at 3.3-4.6% (Hall 1982; El Bassam 1998; Heaton et al. 2008b). A 6.7% solar energy conversion efficiency would correspond with a dry biomass yield of approximately 250Mgha-1year_1 at 40° latitude (El Bassam 1998). Ac­tual yields are much lower than theoretical yields, because there are factors — such as in the case of terrestrial plants, the absence of a full canopy, shading, photosatu­ration and limited availability of nutrients and water — which in practice reduce the efficiency. Due to such factors, the theoretical differences in conversion efficiency between, for instance, C3 and C4 plants may not materialize in real life differences in conversion efficiency. For instance, sorghum is a C4 plant that tends to be roughly as efficient as the C3 cereals. And C3 plants such as sugar beet and oil palm are in practice often more efficient in converting solar radiation into biomass than the C4 plant Miscanthus.