2.1 Introduction

The adjective ‘sustainable’ is frequently used regarding biofuels (e. g. Abrahamson et al. 1998; Krotscheck et al. 2000; Buckley and Schwarz 2003; Bhattacharya et al. 2003; Goldemberg and Teixeira Coelho 2004; Butterworth 2006; Demirba§ 2007; Robert et al. 2007; Goldemberg et al. 2008; Karp and Shield 2008; Royal Society 2008). Also, biofuels are a regular subject in scientific journals dealing with re­newable or sustainable energy. The apparent rationale of using ‘sustainable’ and ‘renewable’ in the context of biofuels is the following: biomass may be argued to temporarily store solar energy, based on photosynthesis (see Chap. 1). In doing so, carbon is sequestered, and on burning transport biofuel, it is de-sequestered. In the meantime, photosynthesis proceeds, generating new feedstocks for biofuels. As solar irradiation and photosynthesis are expected to last for many millions of year, doing so would seem sustainable and transport biofuels renewable. However, this is not the ‘whole story’. Energy inputs in the world economy are currently, as pointed out in Chap. 1, overwhelmingly fossil fuels, and the use of fossil fuels extends to the production and distribution of transport biofuels. This is at variance with renew — ability and sustainability, as fossil fuels are non-renewables, and their use cannot be sustained indefinitely at the present level. The cumulative life cycle fossil fuel demand of biofuels will be discussed in Sect. 2.3.

For converting solar irradiation into transport kilometres, there are a variety of technologies available with widely varying efficiencies. Such efficiencies matter: they are major determinants of spatial requirements of energy supply. These spatial requirements, in turn, are important determinants of competition of energy supply with food production and habitats for living nature. Because this competition is an important matter in the current transport biofuel debate and will return later in this book, this chapter will deal with the solar conversion efficiency of transport biofuels (Sects. 2.4 and 2.5). Other methods for solar energy conversion do not involve organisms but rely on physical conversion technologies. Photovoltaic cells generating electricity are examples thereof, for which the solar conversion efficiency will be discussed in Sect. 2.6.

L. Reijnders, M. A.J. Huibregts, Biofuls for Road Transport © Springer 2009

Biofuels and the output from photovoltaic cells can be used to perform work or to deliver energy services. Work (a thermodynamic concept) or energy services (an economic concept) include, for instance, car kilometres. The performance of work often includes the use of intermediaries (e. g. power plants, batteries or motors). The energy efficiencies of such intermediaries will be discussed in Sect. 2.6. In Sect. 2.6, we will also consider the overall efficiency of a variety of methods to convert solar energy to car kilometres, giving a ‘seed-to-wheel’ perspective.

As pointed out in Chap. 1, the production of biofuels is often accompanied by by-products or co-products. For instance, in making biodiesel from rapeseed, both glycerol and an ingredient of animal feed (rapeseed cake) are produced. Before we go into the calculations of cumulative (fossil) energy demand, it should be decided how much of that demand is allotted to biodiesel and how much to glycerol and rapeseed cake. This is called ‘allocation’ and will be discussed in Sect. 2.2.