• Open Access

Bioenergy, sustainability, and the second law


Correspondence: Niclas Scott Bentsen, tel. + 45 3533 1714, e-mail: nb@life.ku.dk

A recent study in GCB Bioenergy (Harsono et al., 2012) analyses a number of scenarios for palm biodiesel production. The authors calculate energy and greenhouse gas balances as measures of environmental sustainability; the better balance the more sustainable. The argumentation is not unique; it has been applied in numerous studies also recently, e.g., González-García et al., 2012. We question the argumentation that the better energy balance ensures more sustainability.

Sustainable development is a common mantra, also in the debate on energy supply, and much attention is devoted to cater the development of sustainable energy systems. The Brundtland definition of sustainable development targets two generations: the current and any future. However, forecasts of future energy technologies and societal development have a long history of failure (Smil, 2005), and future generation's needs and demands for energy are known only very generally. Consequently, a sustainable approach to the development of an energy sector must target flexibility and the potential for providing a variety of options for generations to come.

A subtle concept, sustainability, and its science does not build on one particular scientific discipline. We argue that basic understanding of the laws of thermodynamics can help reducing unsustainability in our stewardship of energy resources.

The first law of thermodynamics tells that energy (E) neither can be created nor be destroyed, it can only change form. Energy is subject to the law of conservation, ΔEsystem = Ein − Eout. Take the biosphere as an example. The amount of energy received from the sun almost equals the amount radiated back to space. The biosphere is more or less in energetic equilibrium. How is life then created and maintained if it does not build on consumption of energy? The second law of thermodynamics explains this apparent paradox. The second law tells that the conversion of energy from one form to another generates entropy (S). If the process is reversible the entropy level stays constant, but in real life there are no or only very few reversible processes, ΔEsystem = ≥ 0. Entropy is a measure of the dispersedness of energy (Lambert, 2011) and may be used as an indicator of the quality of energy forms. The incoming solar radiation has lower entropy (=higher quality) than the reradiated infrared radiation, and life on earth is created and maintained not through energy consumption but through energy quality consumption.

Based on the second law a quality hierarchy of energy forms can be illustrated as a flight of stairs (Fig. 1). Energy can be moved up and down the stairs but (1) eventually it ends at the bottom step, and (2) every move comes at an ‘entropy cost’ and big steps has higher costs than smaller steps.

Figure 1.

The energy quality hierarchy indicates different energy forms' ability to be converted into other energy forms. Index 1 indicates the highest possible quality. Based on Wall, 2009; Cullen & Allwood, 2010.

The best use of biomass resources in the energy sector is intensely debated in the scientific literature; consensus does not prevail. Here, six options for using cereal straw in the energy sector are analyzed according to the first and second law. The second law analysis applies the concept of exergy (Ex). Exergy is a measure of energy's potential to be converted into mechanical energy and builds on the first and second laws of thermodynamics (Dincer & Rosen, 2009).

Material and energy balances, and energy and exergy performances of the six options are presented in Table 1. In scenarios 1, 2, and 4 the efficiency measured as energy or exergy is comparable, they rank identically. In scenarios 3, 5, and 6 the energy and exergy analysis diverge. Whereas the energy analysis finds these options very efficient, the exergy analysis finds them less efficient, most pronounced in scenario 6 with the highest energy efficiency and the lowest exergy efficiency. What characterizes these scenarios is the generation of heat, the energy form of lowest quality (cf. Fig. 1).

Table 1. Mass and energy balance of six options for using cereal straw in the energy sector. C6 = fermentation of C6-sugars, C5 = fermentation of C5-sugars, P = power production, H = production of district heating, B = biogas production on C5-sugars. Energy efficiency is calculated on the basis of material's lower heating value (LHV). Exergy efficiency builds on the exergy content of input and output of materials and energy (Shieh & Fan, 1982; Szargut, 2005; Sciubba & Wall, 2007)
Straw (10% MC)EthanolPowerMethaneHeat (85 °C)Pure CO2EnergyExergyMJ
1C6 + P1000154585  147Kravanja et al. (2012)0.400.3611 904
2C6 + C5 + P224359  214Kravanja et al. (2012)0.470.4310 625
3C6 + P + H154471 1547147Kravanja et al. (2012)0.730.4011 305
4C6 + P + B15412091 147Kravanja et al. (2012)0.590.538841
5P + H 1249 3149 Danish Energy Agency (2010)1.020.3512 063
6H   4657 Danish Energy Agency (2010)1.080.1615 574

The analysis leads to quite different conclusions, whether it is interpreted according to the first or the second law, but which makes most sense and provides most useful information? In a time where resources are considered more or less infinite or the consequences of their exploitation is not emphasized, which has been the case in the greater part of the 20th century, the first law has made sufficient sense. The last few decades, the perception of resources as finite and concern over the consequences of their exploitation have gained momentum. In that context the second law provides more useful information.

Sustainability is not a thermodynamic concept (Sciubba & Wall, 2007), and there is an inherent conflict between sustainability and the second law; sustainability indicating that something can be sustained, the second law stating that nothing can be sustained. Moreover, thermodynamics does not embrace ethical, political, or social problems. Never the less thermodynamics may be of significant help in the development of a more sustainable energy supply. A positive correlation between exergy efficiency and sustainability is proposed (Dincer & Rosen, 2005). We are not sure it holds in every case, but the opposite seems obvious; unsustainability ∝ irreversibility. Entropy generates thermodynamic irreversibility (I), inline image, and irreversibility means inflexibility. When sustainability is proportionate to flexibility, as discussed above, irreversibility generation provides a measure of unsustainability. The six scenarios generate different levels of irreversibility and thus lost flexibility (1 − ψ). Scenarios 5 and 6 represent the current, most common use of biomass for energy services in the industrialized world. Generation of heat (scenario 6) loses 84% of the inherent flexibility in the straw resource, whereas combined heat and power generation (scenario 5) loses 65% on a par with ‘conventional’ second generation ethanol and power generation, 64% (scenario 1). The most flexible combination is production of ethanol, power, and methane (scenario 4).

The best use of biomass in any given situation depends not only on conversion technologies but also on the demand for energy services, supply of biomass resources, and the competition between them. Society also demands heat in high and low temperatures and an energy system must be able to supply that as well. However, a narrow-minded quest for the better energy balance may lead the development of the energy sector onto a path of reduced flexibility. Emphasizing energy quality rather than energy quantity safeguard that small steps on the energy quality stairs are prioritized, which in turn reduces unsustainability in the stewardship of energy resources.