|Model biogas plant in Ghana, West Africa. Burning wood is still the traditional energy source for heating and cooking purposes 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 point that could not have been foreseen even 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. Harking back a decade, 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 up to this point, 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 2011-2013. On the other 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 spawned a bioenergy blitz lining up investors with cash to build biofuel plants, mostly designed for grain feedstock (primarily corn). In North Carolina there were several such plants either planned or under construction. We even jumped into the scrum with biochar, a non-cornstarch bioenergy by-product I wrote about 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 cash-flowing 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 as having been idled in March 2011 until possibly 2016 and lists no current production there. 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, the purchase of idled former Vireol Bioenergy LLC ethanol plant by Green Plains Inc. an agro-products processor headquartered in Omaha Nebraska. Despite rising federal renewable fuel standards, the herald of bioenergy bringing prosperity to rural communities seems to have 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, although biofuels turned out not to be complicit in the 2007-2008 crisis. In fact, the biofuels link was more conjecture than real: shortages turned up for rice, which is not used in biofuel production. In the 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 did, however, rivet public debate over the production of bioenergy from cultivated agricultural crops, “food vs. fuel”.
The primary feedstock for biofuel production is corn in the USA, sugar cane in Brazil, and cassava in Southeast Asia. These commodities share in common that they can be eaten by humans as well. The link between energy and real food prices is inseparable; the question is: will the growth in energy demand divert production away from food, leading to a reduction in supply? The supply curve for agricultural commodities is an increasing function but only up to a point, mainly because arable land is finite (input costs are also a factor). Once the limit is reached, so the theory goes, substitutions will be found to satisfy demand. Thus if the demand for sugar cane ethanol exceeds the supply of the crop, other crops (perhaps corn, cassava, or cellulosic biomass) may be substituted, leading to a reduction in food supply. Or, in interesting times like these, cost-competitive oil and natural gas 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 the cost of energy. Inescapably, we must increase the supply of both food and energy for civilization to go on. 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 estimate how much energy this would create as a share of global demand. Before delving in 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 for energy production. Biomass is any organic, non-fossil material derived from living, or recently living organisms. Bioenergy is considered a renewable energy source in that it cannot be depleted within the foreseeable future. That is different from saying it is inexhaustible, but the idea is that renewable energy can be replaced ad infinitum as far ahead in time as can be foreseen. Providing that all of the plants used for bioenergy can be re-grown and there is no reduction in the 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, fuel wood, 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 collected in vast quantities to generate biogas, a type of bioenergy that can heat up your house, bring forth light, and cook your meals. The treated residuals, called biosolids, can be safely and economically used as fertilizer to grow crops.
The act of transforming the energy in wood for use in heating and cooking is perhaps one of the oldest and most important skills separating humans from animals. Every day, biomass is transformed into heat, motor fuel, and electricity through multiple processes, all of which originate from starting materials called feedstock. Figure 1 points up that bioenergy is not a single process but rather a collection of processes operating under different engineering controls. Here, we do not delve into any of these processes, but rather focus on the potential reservoir of biomass feedstock 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 whereby radiant energy is transformed into chemical energy. But where does the radiant energy come from?
From its luminous birth, all of the energy ever received by planet Earth has originated in the form of electromagnetic radiation from 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 to produce wave-like motion as they accelerate through space. The waves have two important 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).
Electromagnetic radiation is constantly bombarding the Earth. The full spectrum is divided in bands, or regions sharing similar electromagnetic properties determining how they interact with matter (Figure 3). Some of these bands are beyond the realm of human experience, whereas others are visible subjectively as colors like red, green, blue.
Plants absorb light energy in the visible wavelengths, a region that extends 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, they reflect 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 small fraction of the total energy influx represented by the 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. Through this process, energy is captured by the plant together with the release of oxygen (O2) back to 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 scratch out a rough estimate of the two quantities, but their precise values are intractable and in any case would not be predictive of anything generally. For scratching out, I borrow biophysical data from Nobel (2004) and Zhu et al. (2010), following the generalized approach given 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-latitude to estimate total bioenergy potential after Narbel et al. (2014). Note that “watt” is a unit of power, not energy3. We can use watts to represent 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) may be considered the power density per unit area.
About 30 per cent of the solar spectrum is available to drive photosynthesis (the other 70 per cent is outside of the photosynthetic spectrum 400-700 nm, reflected, transmitted, or lost via 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 was in the region that could be absorbed by plants; in reality only about 45% of radiant photons are in the photosythetically 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 needed to drive photosynthesis and downstream metabolic processes, and nutrients are needed 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 translate to a reduction in energy efficiency by 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 order of 30-40%, the most realistic estimate for net power yield averaging over all production zones is about 0.6 – 0.8 W/m2. This is in the range of 0.2-1.0 W/m2 density estimated for switchgrass, poplar, 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 taking place all over Earth’s surface, the power production is estimated:
0.6 W/m2 × 4π2Earth ~ 300 Terawatts (symbol: TW)
Oceans occupy about 70% of the planet. Ignoring oceanic capacity leaves 30% for bioenergy production. The fraction of agricultural and forest covered land is about 60%. The area that could be exploited for bioenergy production is probably no more than 10%, maximum. 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 TW. Studies by Haberl et al. (2011) concluded ~5-8 TW was realistic for 2050 bioenergy potential in considering current technology, food demand, and environmental targets (“technical potential”). Other scenarios developed by Fischer and Schrattenholzer (2001) estimate ~11 to 15 TW, pointing up that it depends on your assumptions in doing the above calculations.
What, then, does 6 TW represent? In 2013 total primary energy production by the world was ~18 TW, which is the most recent data available (IEA 2015). Bioenergy’s share is about 10% or 1.8 TW. So it appears the plausible limit of bioenergy is about 30% of world energy production, or 20% growth over 2013 levels.
Wither, or whither, bioenergy? No fate is clear.
The good news: If you’re in the bioenergy business there’s still room to grow. The bad news: bioenergy growth will flatten out in the future. This is because population growth and higher standards of living inevitably create demand for more energy. If energy consumption doubles by 2050 (a not unlikely scenario), bioenergy’s share will max out 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. Typical sunlight-to-biomass efficiency for many cultivated agricultural crops is 0.5-2%. 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 input. Sadly, there is no free lunch.
As noted earlier, human demand for energy and food is inseparable. With a global human population lurching toward 10 billion by 2050, supplying enough energy to ensure that there is sufficient food for everyone is one of humanity’s greatest challenges. At this stage it is impossible to foresee how the food vs. energy tug-of-war will play out.
I do not foresee bioenergy withering away. Bioenergy is the primordial source of human sustenance to which there is still deep attachment in many cultures. Saying that, it is, unfailingly, the ever-conjured precise knowledge of the future that is always just beyond human reach.
Cold fusion anyone?
Endnotes (For the insanely curious only!)
1Hertz (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).
2Visible 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.
3Energy and power are different, but related concepts. It is important to understand the differences to avoid ambiguity or getting tripped up. Even your faithful blogger and Acme Scientific Research Collaborative® CEO gets flummoxed now and then by all of the jargon. Following is a refresher from school fyi:
Energy is the measure of the capability 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 of force to move it a distance of 1 meter, the amount of 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 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, that is: 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 inter-convertible.
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, 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 wattsecond. 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 most 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:
•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:
Summing up: Energy is a measure of the capability to do work over a 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 it can change 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 how fast they convert energy per unit of time. Power can’t be stored and it doesn’t change form.
4The electron volt (or electronvolt) is a physics unit representing the energy release or uptake in atoms and molecules equal to the acceleration 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 centered at 680 nm and frequency 4.41 x 1014 Hz.
5When speaking of “global potential bioenergy production” the gross primary production is meant. Cropping, harvest, and transportation energy overhead, and conversion efficiency (ethanol, methane, biodiesel etc.) are not factored in.
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