Model biogas plant in Ghana, West Africa. Burning wood remains the traditional energy source for heating and cooking in low-income countries. Bioenergy has promised to improve rural livelihoods, but impacts in North America have been uneven.

Let it not be said that we don’t live in interesting times, a quip that could not have been more relevant a year ago: oil sinking under US$30 a barrel; dislocated energy markets; silenced fracking rigs; and a global commodities glut showing no sign of recession any time in the near future. A decade back, things were very different. In 2005, the US Congress passed the Energy Policy Act (EPA 2005), which laid the groundwork for domestic biofuels by mandating the blending of renewable fuel with gasoline, up to 7.5 billion US gallons by 2012. Two years later, renewable fuel targets were extended to 36 billion gallons by 2022 under the Energy Independence and Security Act of 2007 (EISA 2007). Ever since, foes have lobbied unsuccessfully to repeal the ethanol mandate. Meanwhile, the bioenergy sector has languished through the Great Recession despite oil peaking above $145 per barrel in 2008 and $100 multiple times from 2011 to 2013. On the flip side, corn growers are rejoicing the ethanol mandate because it locks in demand for a product they have to sell.

Nothing loves business better than a government mandate. EISA 2007 sparked a bioenergy boom, mobilizing investors with capital to build biofuel plants, primarily designed for grain feedstock (mainly corn). In North Carolina, several such plants were either planned or under construction. We even jumped into the scrum with biochar, a non-cornstarch bioenergy by-product detailed in The Biochar Mystique. Never mind that North Carolina is a net importer of corn grain from the Midwest; the capacity must have been perceived to exist here as higher corn prices gave lift to producers. Who would have attempted to cash-flow imported grain? Right!

Today, Tyton Biofuels in Raeford is the sole survivor of that onrush. Even so, Nebraska’s Ethanol Capacity by State and Plant indicates the Tyton facility was idled in March 2011 until possibly 2016 and lists no current production there (as of September 2018). Tyton’s website extols the virtue of “energy tobacco”, a high sugar, low cellulose variant of Nicotiana and corn grain substitute they hope to begin processing for ethanol in the repurposed facility. Further north, in Hopewell, Virginia, Reuters News Agency reported on October 26, 2015, that Green Plains Inc., an agro-products processor headquartered in Omaha, Nebraska, had purchased the idled former Vireol Bioenergy LLC ethanol plant. Despite rising federal renewable fuel standards, the herald of bioenergy bringing prosperity to rural communities has fallen flat, at least here along the eastern USA seaboard.

Meanwhile, another tempest faced out: sharp increases in food prices in 2007-2008, coinciding with the 2005-2008 oil price surge, induced widespread riots and civil unrest, turning public opinion against biofuels. The increasing use of biofuels in developed countries was linked to rising competition for finite food supplies in poorer countries. However, biofuels were not found to be complicit in the 2007-2008 crisis. In fact, the biofuels link was more conjecture than reality: shortages arose for rice, which is not used in biofuel production. In that event, global rice stocks were not short. Nor were stocks of other food grains, for that matter; trade, speculation, and market distortions were blamed. The food crisis, however, riveted public debate over the production of bioenergy from cultivated crops, specifically the “food vs. fuel” debate.

Corn (Zea mays) is the primary feedstock for biofuel production in the USA. Like sugar cane, corn has an efficient photosynthetic pathway for carbon metabolism. About 50% of corn biomass is grain. Corn stover (non-grain residue) may also be harvested for its energy content.

The primary feedstocks for biofuel production in the USA are corn, in Brazil, sugar cane, and in Southeast Asia, cassava. These commodities share the commonality that they can be eaten by humans. The link between energy and real food prices is inseparable; the question is whether rising energy demand will divert production from food, leading to reduced supply. The supply curve for agricultural commodities is an increasing function, but only up to a certain point, mainly because arable land is a finite resource (input costs are also a factor). Once the limit is reached, so the theory goes, substitutions will be found to satisfy demand. Thus, if demand for sugar cane ethanol exceeds the supply of the crop, other crops (such as corn, cassava, or cellulosic biomass) may be substituted, leading to a reduction in the food supply. Or, in interesting times like these, cost-competitive fossil fuels are substituting for bioenergy crops. At some point, however, the market reaches equilibrium. The message is that, everything else being equal, real food prices will rise at the same rate as energy costs. Inescapably, we must increase the supply of both food and energy to drive modern technological civilization. What is the global bioenergy potential? What is the share of global energy production that can be recovered from biomass?

Such questions are not imponderable. We can, in fact, estimate the potential biomass available for bioenergy production. We can also calculate the amount of energy this would create as a percentage of global demand. Before delving into these matters, let us first define bioenergy. Ask the average Joe on the street, “What is bioenergy?” and you are likely to get a blank stare. Ethanol might come to mind, but bioenergy is much more than the sum of two-carbon alcohol molecules.

Broadly speaking, bioenergy is the conversion of biomass into energy. Biomass is any organic, non-fossil material derived from living or recently living organisms. Bioenergy is considered a renewable energy source because it cannot be depleted in the foreseeable future. That is different from saying it is inexhaustible, but the idea is that renewable energy can be replaced ad infinitum into the future as far as can be foreseen. Providing that all plants used for bioenergy can be regrown and that there is no reduction in total plant area, the use of biomass for energy production can also be considered sustainable and carbon-neutral.

Biomass sources are typically distinguished between solid biomass (energy crops, forestry, and agricultural crop residue, fuelwood, animal residues), liquid biomass (animal slurry, sewage, vegetable oils), and gaseous biomass (decomposing household, municipal, or industrial waste). Solid waste, of whatever provenance, may be used to generate biogas, a type of bioenergy that can heat your house, bring forth light, and cook your meals. The treated residuals, known as biosolids, can be safely and economically utilized as fertilizer to grow crops.

The act of transforming the energy in wood for heating and cooking is perhaps one of the oldest and most essential skills that separates humans from animals. Every day, biomass is converted into heat, motor fuel, and electricity through various processes, all of which start with feedstock. Figure 1 shows that bioenergy is not a single process but a collection of processes operating under different engineering controls. Here, we do not delve into these processes but rather focus on the potential biomass feedstock reservoir available for bioenergy production.

Figure 1. Various processes for bioenergy production. Source: redrawn and slightly modified from Narbel et al. (2014).

The starting point for all biomass is photosynthesis, a plant-driven process in which radiant energy is converted into chemical energy. But where does the radiant energy come from?

From its primordial birth, all the energy ever received by planet Earth has originated from electromagnetic radiation emitted by a massive, hot ball of plasma at the center of our solar system, otherwise known as the Sun. Fossil fuels are buried remnants of ancient solar electromagnetic radiation that we drill, excavate, and refine as energy products. Electromagnetic radiation is the energy carried by massless, electrically neutral “particles” called photons. Photons oscillate, producing wave-like motion as they accelerate through space. The waves have two essential properties: frequency, or the number of wave crests passing a given point per second, measured in Hertz¹; and wavelength², which is the distance between two successive oscillating crests (Figure 2a and 2b).

Figure 2. (a) Two-dimensional view of an electromagnetic wave. The frequency and wavelength of EM radiation determine how EM radiation interacts with matter. (b) Three-dimensional view of an EM wave showing coupled perpendicular electric and magnetic fields inherent to all EM phenomena. Image (b): redrawn from spie.org.

Figure 3. Principal wavebands mark the electromagnetic spectrum. Photosynthesis is driven by energy in the so-called "visible" light bands that appear to us as mixtures of red, green, and blue. Light in the visible region has nanometer-scale wavelengths, which are approximately one billionth of a meter (10^-9 meters). Low-energy wave bands above ~740 nm (infrared, radar, microwave) are not used in photosynthesis but can be exploited in remote sensing applications. Note: The label "shortwave" at the ~100-meter band refers to the upper limit of the medium frequency, first used for radio communication, not to wavelength compared with that of visible light.

Plants absorb light energy in the visible wavelength range, approximately 400 to 740 nanometers (nm). Chlorophylls are the light-sensitive molecules activated by incoming photons. Chlorophyll absorbs two principal wavebands: blue light centered on 460 nm, and red light centered on 680 nm; and it reflects green light centered on 520 nm. The 520 reflectance is the reason healthy plant tissue appears green to the human eye. Note that photosynthesis captures light energy only in two bands. This represents a tiny fraction of the total energy influx represented by the entire electromagnetic spectrum.

The fundamental process of photosynthesis is the transfer of free gaseous carbon dioxide (CO₂) from the air to energy molecules containing carbon stored in the plant. During this process, the plant captures energy and releases oxygen (O₂) back into the air. In chemical shorthand, the photosynthetic reaction is written:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

To estimate the total energy production via photosynthesis, two quantities are needed: (1) the influx of light energy impinging on Earth’s surface that is available to energize photosynthesis; and (2) the efficiency of photosynthesis energy capture and conversion.

We can estimate the two quantities roughly, but precise values are impossible and, in any case, would not be predictive of anything generally. To make this point, I have borrowed biophysical data from Nobel (2004) and Zhu et al. (2010), following the generalized approach provided by Narbel et al. (2014).

The energy of incoming solar radiation impinging on Earth’s atmosphere is about 1,366 watts per square meter (W/m²), a.k.a. the “solar constant”. For reasons of geographic latitude, season, and atmospheric conditions, the average influx of light on Earth’s surface is much smaller. Here, we’ll use 240 W/m² at mid-latitudes to estimate the total bioenergy potential, following Narbel et al. (2014). Note that “watt” is a unit of power, not energy³. We can use watts to represent the rate of electrical energy transfer through the motion of light-stimulated electrons in a conducting material, e.g., the thylakoid membranes in the plant chloroplast, where photosynthesis takes place. Watts per square meter (W/m²) is considered the power density per unit area.

About 30 percent of the solar spectrum is available to drive photosynthesis (the other 70 percent lies outside the photosynthetic spectrum of 400 to 700 nm, reflected, transmitted, or lost due to photochemical inefficiency and thermodynamic limits). A minimum of 8 photons is needed to fix one molecule of CO₂ in photosynthesis. Light centered at 680 nm has energy per photon of ~1.8 electron volts⁴ (eV). A molecule of CO₂ typically stores energy of about 5 eV, so the average energy efficiency of photosynthesis is:

5 eV/(8 × 1.8 eV) × 100 = 35%

This figure represents a theoretical quantum energy efficiency if all solar radiation were in the region that plants could absorb; in reality, only about 45% of radiant photons are in the photosynthetically active spectrum. Carbohydrate synthesis depends on the diffusion of atmospheric CO₂ into the leaf. While this process occurs instantaneously (Earth’s atmosphere is currently 0.04% CO₂ and rising), water is necessary to drive photosynthesis and downstream metabolic processes, and nutrients are required for growth. Water and nutrients must be sucked up from the ground at some cost to the plant. Collectively, photonic mismatch and metabolic overhead in the plant result in a reduction in energy efficiency of at least a factor of 10. Zhu et al. (2010) estimated 0.086 for C-3 (three-carbon metabolism) plants and 0.10 for C-4 (four-carbon metabolism) plants. Being conservative, we’ll use the 0.086 factor. The final power production is of the order:

0.086 × 0.3 × .35 × 240 W/m² = 2.2 W/m²

Accounting for the aerobic combustion efficiency of biomass, which is in the range of 30% to 40%, the most realistic estimate for net power yield, averaged across overall production zones, is approximately 0.6–0.8 W/m². This is in the range of 0.2-1.0 W/m² density estimated for switchgrass, poplar, and Miscanthus using real plant data (McKendry, 2002). In favorable places, sugar cane and other fast-growing plants may do better than 2 W/m², but these can hardly be considered average.

On a global scale, assuming photosynthesis takes place all over Earth’s surface, the power production is estimated:

0.6 W/m² × 4π²_Earth ~ 300 Terawatts (symbol: TW)

The oceans occupy approximately 70% of the planet. Ignoring oceanic capacity leaves 30% for bioenergy production. The fraction of land covered by agricultural and forest areas is approximately 60%. The area that could be exploited for bioenergy production is likely no more than 10%, at most. We are then left with a global potential bioenergy production:

300 TW × 0.3 x 0.6 × 0.1 = 5.4 TW

Everything else being equal, this number is the most plausible estimate of the power available via terrestrial photosynthesis. However, energy is also extracted from waste matter, so the final power potential may be closer to 6 TW⁵. Studies by Haberl et al. (2011) concluded that a potential of ~5-8 TW was realistic for 2050 bioenergy in consideration of current technology, food demand, and environmental targets (“technical potential”). Other scenarios developed by Fischer and Schrattenholzer (2001) estimate ~11 to 15 TW, highlighting that the result depends on the assumptions used in the model calculations.

What, then, does 6 TW represent? In 2013, the world’s total primary energy production was ~18 TW, the most recent data available (IEA, 2015). Bioenergy’s share is about 10% or 1.8 TW. It appears that the plausible limit of bioenergy is approximately 30% of global energy production, or a 20% increase over 2013 levels.

Whither bioenergy?

The good news: If you’re in the bioenergy business, there’s still room to grow. The bad news: bioenergy growth is expected to level off in the future. This is because population growth and higher living standards inevitably create demand for more energy. If energy consumption doubles by 2050 (a not-unlikely scenario), bioenergy’s share will cap at ~16% if the 6 TW limit is plausible. There is always the prospect of increasing biomass yield per area by improving photosynthetic efficiency in plants. The typical sunlight-to-biomass efficiency for many crop plants ranges from 0.5% to 2%. Transgenic and related genome-editing technologies may improve photosynthetic efficiency and downstream energy products, but increasing biomass output invariably comes at the expense of greater inputs. Sadly, there is no such thing as a free lunch.

As noted earlier, human demand for energy and food is inseparable. With a global human population expected to reach 10 billion by 2050, supplying enough energy to ensure everyone has sufficient food is one of humanity’s most significant challenges. At this stage, it is impossible to predict how the tug-of-war between food and energy will unfold.

I do not foresee bioenergy withering away. Bioenergy is the primordial source of human sustenance, to which many cultures are still deeply attached. Saying that, it is, unfailingly, the ever-conjured precise knowledge of the future that is always just beyond human reach. Cold fusion, anyone?

End Notes (for the insanely curious only!)

¹ Hertz (symbol: Hz) is a unit of time that measures frequency. It is defined as 1 cycle per second or in shorthand, 1/s or reciprocal second, s^-1. Higher orders of magnitude are represented by prefixes Kilo (10³ Hz), Mega (10⁶ Hz), Giga (10⁹ Hz), Tera (10¹² Hz), Peta (10¹⁵ Hz), Exa (10¹⁸ Hz), Zetta (10²¹ Hz).

² Visible light is typically represented by wavelengths between 400 and 740 nanometers (symbol: nm) or 1 billionth of a meter (10^-9 m). Visible wavelengths may also be represented as: 0.4-0.74 micrometers (symbol: μm), 1 millionth of a meter (10^-6 m). It is a matter of convention which of the two to use.

³ Energy and power are distinct, yet related concepts. It is essential to understand the differences to avoid ambiguity or confusion. Even your faithful blogger and Agrosphere CEO gets flummoxed by all the jargon now and then. Following is a refresher from school, fyi:

Energy is the capacity to do work over a period of time. In turn, work is the action of a force through a distance without regard to time. For example, if a load requires 1 kilogram-force to move it 1 meter, the work done is 1 kilogram-force × 1 meter, or 1 kilogram-force meter (kg-force m). A 1 kilogram-force meter is equal to 9.807 m/s², which is the force of acceleration of a 1-kilogram object in Earth’s gravitational field at sea level. The newton (symbol: N) is the metric unit of force given by:

Force = mass × acceleration (Newton's second law)

1 N = 1 kg × m/s²

Seconds are squared (s²) because acceleration is calculated by dividing the change in velocity (meters per second), by time (also measured in seconds).

The international metric unit of energy is the joule (symbol: J). The joule is equal to the work done (or energy transferred) to an object when a force of one newton acts on the object through a distance of one meter, e.g. 1-newton meter or N × m:

1 joule = 1 N = kg × m/s² × m

Therefore 1 joule = kg × m²/s²

Energy exists in many forms and is often expressed in multiple units. Energy units are interconvertible.

Power is the rate of energy transfer (or work done) per unit of time. Power implies that a certain amount of work is done every second, minute, or hour.

The international unit of electrical power is the watt (symbol: W). One watt is equal to one joule (energy) per second (time), 1 W = 1 J/s. Conversely, 1 joule = 1 watt-second. Watt is the amount of energy flowing through a system per unit of time. The kilowatt (kW) is 1,000 watts. The terawatt (symbol: TW) is 1 trillion watts (10¹² watts), the preferred way of expressing power on a very large scale.

Since we live in an age powered by electricity, the watt is, at least in name, a unit familiar to almost everyone. In electromagnetism, the watt is the amount of work done by one ampere (A) of current flowing through an electrical potential difference of one volt (V) per unit of time. Similarly, a joule can also express the amount of work done by 1 coulomb (C) of charge (=6.242×10¹⁸ electrons) flowing through a constant potential of one volt per unit of time (second). A coulomb quantity of charge is equal to one ampere, which is, by definition, equal to one coulomb of charge flowing per unit of time (second).

There is a fundamental equation that connects energy and power:

Where:

• kilowatt-hours (symbol: kWh) is energy
• kilowatt (symbol: kW) is power
• hours is time

The equation can be rearranged to find power:

And rearranged to find time:

Summarizing: Energy is the capacity to do work over a specified period of time. The international unit of energy is the joule (J). Energy can be used to heat a home, dry grain, or propel a tractor over a field. Energy can be stored and change its form.

Power is the rate of energy transfer (or work done) per unit of time. The international unit of power is the watt (W). Power is an instantaneous quantity that remains constant as long as the system is energized. Power systems are rated by their ability to convert energy at a specific rate per unit of time. Power can’t be stored, and it doesn’t change form.

⁴ The electron volt (or electronvolt) is a physics unit representing the energy release or uptake in atoms and molecules equal to the acceleration of an electron through an electric potential difference of one volt. This is a semantic dodge, but the important fact is that the electron volt has a very small energy value ~1.6 x 10^-19 joules (see endnote #3 for the definition of joule). Chlorophylls absorb light energy in the blue and red wavebands, but not necessarily centered on a single wavelength. The energy per photon, 1.8 eV, used in my calculation of photosynthesis quantum efficiency is based on published values for red light at 680 nm and a frequency of 4.41 × 10^14 Hz.

⁵ When speaking of “global potential bioenergy production,” the gross primary production is meant. Cropping, harvesting, and transportation energy overhead, as well as conversion efficiency (for ethanol, methane, biodiesel, etc.), are not factored in.

Further Diggings

Fischer, Günther, und Leo Schrattenholzer. 2001. Global bioenergy potentials through 2050. Biomass and Bioenergy vol. 20, no. 3: 151–59. Available at https://www.sciencedirect.com/science/article/abs/pii/S096195340000074X (last access: 21 January 2016)

Haberl, Helmut, Karl-Heinz Erb, Fridolin Krausmann, Alberte Bondeau, Christian Lauk, Christoph Müller, Christoph Plutzar, and Julia K. Steinberger. 2011. Global bioenergy potentials from agricultural land in 2050: sensitivity to climate change, diets and yields. Land Use Impacts of Bioenergy. Selected Papers from the IEA Bioenergy Task 38 Meetings in Helsinki, 2009 and Brussels, 2010 vol. 35, no. 12: 4753–4769. Available at https://www.sciencedirect.com/science/article/abs/pii/S096195340000074X (last access: 21 January 2016)

McKendry, Peter. 2002. Energy production from biomass (part 1): Overview of biomass. Biosource Technology vol. 83, no. 1: 37–46. Available at https://www.sciencedirect.com/science/article/abs/pii/S0960852401001183 (last access: 21 January 2016)

Narbel, Patrick A., Jan Petter Hansen, and Jan R. Lien. 2014. Energy Technologies and Economics. Springer International Publishing. Available at https://link.springer.com/book/10.1007/978-3-319-08225-7 (last access: 21 January 2016) Unique, highly commendable treatment of energy physics, technology, and economics.

Nobel, Park S. 2005. Physicochemical and Environmental Plant Physiology. Amsterdam: Elsevier Academic Press.

Vandergriff, Linda J. 2008. Nature and properties of light. Fundamentals of photonics Mod. 1.1 International Society for Optics and Photonics. Available at https://www.spiedigitallibrary.org/eBooks/PM/Fundamentals-of-Photonics/1/Nature-and-Properties-of-Light/10.1117/3.784938.ch1 (last access: 21 January 2016)

Zhu, Xin-Guang, Stephen P. Long, and Donald R. Ort. 2010. Improving photosynthetic efficiency for greater yield. Annual Review of Plant Biology vol. 61:235–61. Available at https://www.annualreviews.org/content/journals/10.1146/annurev-arplant-042809-112206 (last access: 21 January 2016)

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