Table 2.4, finally, shows overall estimated conversion efficiencies for solar irradia­tion to car kilometres, corrected for the input of fossil fuels, which are calculated by

TSCEXi = SCEx ■ СЕх,,

where TSCEx, i is the transport solar energy conversion efficiency (%) and CEx, the efficiency drive train of transport option i derived from biofuel type x (%).

According to the estimates in Table 2.4 regarding seed-to-wheel solar energy conversion efficiency, ethanol from sugar cane outperforms ethanol from European wheat by about a factor of five to ten, and biodiesel from European rapeseed by about a factor of two to three. Electrical traction from lignocellulosic biomass, how-

ever, in turn outperforms ethanol from sugar cane by roughly a factor of two to four. The relatively high efficiency of using biomass for electricity production has also been noted by other authors, such as Zhang et al. (2007). All biomass-based automotive power is, however, far less efficient than electricity from solar cells that is stored for use in electrical traction. This way of powering motor cars is roughly at least two orders of magnitude better than ethanol from Brazilian sugar cane and three orders of magnitude better than ethanol from European wheat. In calculating the values for Table 2.4, it has been assumed that solar cells and the plug-in facil­ity for cars are in the same region. When distances are large or conversion to H2 is necessary for long distance transport, the efficiency will be lower than indicated in Table 2.4 because of transport-linked losses. For instance, an estimate has been made regarding the life cycle emission of greenhouse gases linked to electrolysis powered by concentrated solar power (CSP) in the Sahara, liquefaction of H2 and transport to, and distribution of, hydrogen in Western Europe. In such a case, a re­duction of the life cycle efficiency by somewhat less than 10% has been found (Ros et al. 2009). Such a reduction applied to electricity from solar cells (last row of Table 2.4) would reduce the overall efficiency in the last column to approximately 3.5-9.4%.

A lesson from this chapter is that conversions lead to substantial reductions in solar conversion efficiency. In Chap. 1, quite a number of proposals have been sum­marized that rely on such conversions. Examples are: the conversion of methane (from the anaerobic conversion of biomass) to methanol, the conversion of lipids and ethanol to hydrocarbons or H2 and the conversion of methanol to hydrocarbons. As the starting products may in principle be used directly as transport biofuels, there is good reason to be sceptical about such sequential conversions in view of the neg­ative impact that they have on the overall solar energy conversion efficiency.

The data presented in this chapter allow for estimates of the ability of biofuels to energetically displace fossil fuels. It appears that in this respect, palm oil and ethanol from sugar cane do much better, especially when processing is powered by harvest residues, than rapeseed oil or ethanol from corn or wheat, as produced in industrial­ized countries. It should be noted, though, that the ability to energetically displace fossil fuels may be at variance with their ability to do so in the economy. The lat­ter is strongly impacted by prices and government policy. An interesting illustration thereof concerns the use of corn-derived ethanol in US gasoline, which has mainly been by E10 fuels, containing 10% ethanol and 90% conventional gasoline. The use of E10 fuels has been stimulated by a federal excise tax which in recent years led to E10 gasoline being cheaper than conventional gasoline (Tyner 2008; Vedenov and Wetzstein 2008), which in turn had an upward effect on the overall consump­tion of gasoline, thereby partly negating the downward effect of ethanol use on the consumption of conventional gasoline (Vedenov and Wetzstein 2008).

The data in this chapter also allow for estimates of land requirements linked to a large-scale displacement of fossil transport fuels by biofuels. This may be illus­trated by the following back-of-the-envelope calculation. As explained in Chap. 1, mineral oil is the dominating fossil fuel for powering transport, and about 60% of all crude oil is used for this transport. Let us suppose that all mineral oil that is currently used as an input in worldwide transport were to be replaced by vegetable oil. Corrected for the difference in lower heating value between crude oil and veg­etable oil (see Table 1.2) and the cumulative fossil fuel input into vegetable oil (estimated here at 40% of the energetic value of vegetable oil), this would require an increase of vegetable oil production by about a factor of 37.5. Part of this in­crease may be met with the increase of yields per hectare. Estimates made for the 23 most important food crops suggest that such an increase may range from 0.63- 1.76%year-1 for developing countries and from 0.59-0.79%year-1 for developed countries up to 2050 (Balmford et al. 2005), to a large extent by intensification of cropping (Tilman et al. 2001). Using intermediate values, this would allow for an increase in yield by a factor of approximately 1.75 for developing countries and by a factor of approximately 1.42 for developed countries between 2000 and 2050 (Balmford et al. 2005), far below the factor of 37.5 needed to displace all mineral oil by vegetable oil. Moreover, it may well be that the average productivity of addi­tional land is lower than that of land currently in use. Thus, even if yield increases in the future would be much larger than currently estimated, there would seem no way around large additional land requirements linked to large-scale displacement of fossil fuels by biofuels. Current policy targets are estimated to require between 55 and 166 million ha (Mha) (Renewable Fuels Agency 2008).

Moreover, expanding transport may well lead to even larger land claims in the future. Gurgel et al. (2007) studied an expansion of the production of cellulosic biofuel to supply up to 368 EJ in 2100. This, according to their scenario, would require about 2.5 x 103 Mha, an amount greater than any other land cover category. For comparison: worldwide, current cropland is about 1.6 x 103 Mha, and the land area that is currently considered fit for additional cropland is estimated at between 400 and approximately 1.2 x 103 Mha (Renewable Fuels Agency 2008).

Non-energy Natural Resource Demand

3.1 Introduction

Several stocks of natural resources are highly important to biomass-for-energy, in­cluding transport biofuel, production. These are: soil and soil organic matter, nutri­ents and water. In this chapter, these will be discussed in turn as to their current sta­tus. We will also discuss the sustainable use of such resources. Sustainable is a term that has by now many meanings. Here, the term will be used in its original meaning in the modern environmental debate, linked to a steady state economy (Daly 1973; Gliessman 1989; Hueting and Reijnders 1998; Reijnders 2006). Thus, sustainable use of biomass is defined in this chapter as a type of use that can be continued indefinitely while maintaining the supply or availability of natural resources. Sus­tainability leads to limitations regarding the use of renewable resources which are characterized by large additions to the stock of these resources and concerning the use of resources that are geochemically scarce and formed in slow geological pro­cesses (‘virtually non-renewable resources’). To allow for indefinite use, the usage of renewables should not exceed addition to stock, and resource quality should be maintained (Reijnders 2000). As to geochemically scarce virtually non-renewable resources, such as phosphate ore for which no substitution seems possible, the way to define sustainability is that wastes irretrievably lost should not substantially ex­ceed the small addition to the stock by geological processes (Goodland and Daly 1996; Reijnders 2006). This requirement corresponds with a large reduction in cur­rent wastage.