A simple lesson in bioenergy arithmetic

In an article published in 2005 in Renewable and Sustainable Energy Reviews, Sanderine Nonhebel of the University of Groningen in the Netherlands estimated that if all of the world’s energy requirements were to be met with bioenergy under conservative agricultural yield estimates, nearly two hectares of land would be required for every person on the planet. Such a scenario is clearly untenable: given that the world population recently exceeded seven billion, and given the size of the planet (about 12,700 km across, and with three-fourth of the surface being submerged under water), with a little bit of geometry one can easily work out that two hectares per person translates to just about all the land that is actually available on Earth! Incidentally, it is also possible to work out that, with a population in excess of 90 million people occupying some 30 million hectares of land, the population density of the Philippines is more than six times that of the world average; but that’s a topic we will leave for a future installment of Star Science. In this article, let us try to revisit such limitations to global bioenergy supply with our own calculations. The figures used here are rounded off to make the computations easier to follow; nevertheless, the results are accurate enough to give us a proper sense of the magnitudes involved. If we assume that bioenergy is to be produced on a global scale through the cultivation of energy crops, the ultimate question is, how much energy can actually be supplied, and how does the figure compare with the energy demand?

First, let us look at the “demand” side of the global energy ledger. Several years ago, the global primary energy demand was about 500 exajoules (that’s 500 followed by 18 zeros; by comparison, a liter of diesel contains about 40 million joules of energy). This figure accounts for all energy sources in “raw” form, including fossil fuels (oil, coal and natural gas), nuclear energy, renewables, etc., and is of course increasing rapidly from year to year. The actual figure today is thus definitely larger. What about the “supply” side of our energy ledger? The total land area on the planet is roughly 150 million square kilometers, which translates to 15 billion hectares. Of course, not all of this land can be used to grow energy crops. In fact, it is essential that a large portion of global land area be preserved in the form of pristine ecosystems to ensure sustainability. In their celebrated 2009 Nature article entitled “A safe operating space for humanity,” Johan Rockstrom and his colleagues estimated that the sustainable limit for agriculture was the use of 15 percent of the Earth’s total landmass for cropland; they also calculated that the actual current utilization was 11.7 percent, which leaves the planet at 3.3 percent (i.e., 15 percent - 11.7 percent) below the safe limit. Let us then assume that this margin can be used entirely for cultivation of an energy crop (which is, admittedly, an overly optimistic assumption); we thus have 500 million hectares (i.e., 3.3 percent of 15 billion) of energy plantations distributed all over the world. Then, let us assume we have a crop that yields an average of 10 tons of wood per hectare per year, and that, once dried, each ton of wood gives 20 billion joules of energy. The total fuel wood production, expressed in primary energy terms, thus turns out to be 100 exajoules per year (i.e., 500 million hectares x 10 tons per hectare per year x 20 billion joules per ton). This output clearly falls short of the global primary energy demand of 500 exajoules per year.

Such “back of the envelope” calculations that we have just done here give us some sense of the resource limitations to the large-scale global production of bioenergy. The exact figures will obviously change depending on the assumptions one uses, so I encourage any interested reader to repeat these calculations using modified assumptions (for example, by assuming faster or slower growth rates for wood crops, or by assuming alternative energy crops such as sugarcane or microalgae). As a final note, even though these numbers (as well as Nonhebel’s 2005 article) show that it is highly unlikely that we can produce enough bioenergy to supply all of the world’s energy requirements, there is certainly still enough room for bioenergy to be a significant component of humanity’s future sustainable energy portfolio.

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Dr. Raymond R. Tan is a university fellow and full professor of chemical engineering at De La Salle University. He is also the current director of that institution’s Center for Engineering and Sustainable Development Research (CESDR). His main field of research is process systems engineering (PSE), which focuses on developing computational techniques to improve the efficiency and sustainability of industrial processes. He is the author of more than 70 published and forthcoming articles in ISI-indexed journals in the fields of chemical, environmental and energy engineering. Scopus lists him as having more than 80 publications to date, with an h-index of 18. He is member of the editorial boards of the journals Clean Technologies and Environmental Policy, Philippine Science Letters and Sustainable Technologies, Systems & Policies, and is co-editor of the forthcoming book Recent Advances in Sustainable Process Design and Optimization. He is also the recipient of multiple awards from the National Academy of Science and Technology (NAST) and the National Research Council of the Philippines (NRCP). He may be contacted via e-mail at [email protected].