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The energy
balance for a particular form of energy is the amount of energy that is
put in, compared to the amount that comes out. Two or three decades ago it
took more energy to produce a gallon of ethanol from corn than the amount
of energy contained in that gallon. This is what is known as a negative
energy balance, and it made the process difficult to justify. So it is
legitimate to ask what the energy balance is of different bioenergy
technologies, and also, what the energy efficiency is, relative to other
crops.
Corn
Ethanol
The energy
balance for production of ethanol from corn takes into account the energy
for producing the corn (such as tillage, weed control, fertilizer,
combining, etc.), transport of the corn to the plant, and production of
the ethanol at the plant. While this was indeed negative several decades
ago, it is no longer the case.
In 2002,
the USDA concluded that the output to input ratio was 1.35 to 1, but in
June 2004, it updated its 2002 analysis of the issue and determined that
the net energy balance of ethanol production is 1.67 to 1. (For every 100
BTUs of energy used to make ethanol, 167 BTUs of ethanol are produced.)
The
improved energy balance is due to more efficient field production methods,
such as no-till planting of corn and genetically improved corn varieties,
and improvements of efficiency at the ethanol plants. According to the
National Corn Growers Association, compared to just five years ago, today’s
ethanol plants produce 15 percent more ethanol from a bushel of corn and
use 20 percent less energy in the process.
What is
also important is that energy from ethanol is not the only result of
ethanol production. Coproducts such as distillers grains, gluten feed,
carbon dioxide, and corn sweeteners are also produced at modern ethanol
plants. This means that not all the energy used by an ethanol plant is
directed at manufacturing only ethanol, thus further improving the net
energy balance of the overall ethanol production process.
The
National Corn Growers Association also points out that ethanol opponents
(who are mostly linked to the oil industry) frequently cite a study by
Cornell University’s Dr. David Pimentel, who concluded that it takes 70
percent more energy to produce ethanol than it yields. Pimentel’s
findings have been consistently refuted by USDA and other scientists who
say his methodology uses obsolete data and is fundamentally unsound.
Energy
from Switchgrass
Technologies
are available to produce both electricity and ethanol from perennial crops
such as switchgrass. Several studies have examined the energy balance of
switchgrass production only, separately from converting the biomass
produced to energy. Typically, the result is about 14 to 1. In other
words, for every Btu of input in the form of fertilizer, harvesting,
baling, etc., 14 Btu’s are produced in the form of switchgrass.
This is
more efficient than for corn production, mainly because switchgrass is
perennial, and therefore does not have to be planted and treated for weeds
every year, and because fertilizer requirements are very low compared to
corn. The result is that conversion technologies for switchgrass can be
relatively inefficient, but will still result in a positive energy
balance. For example, co-firing switchgrass with coal is about 30%
efficient. This will mean that the energy balance will be approximately
0.3 x 14 = 4.2 to 1.
If
switchgrass is used for co-generation of both electrical power and heat,
the conversion efficiency can be 80 percent or better. This would result
in an energy balance of approximately 0.8 x 14 = 11.2 to 1, which is
impressive. Because there are no commercial technologies available for
producing ethanol from switchgrass yet, energy balance studies for this
option have apparently not been conducted. However, even if the conversion
efficiency of such a technology were very inefficient, say 20%, the energy
balance would still be positive at around 0.2 x 14 = 2.8 to 1.
Energy
Efficiency
Energy
efficiency of different crops can be expressed in several ways. For the
purpose of this article, energy efficiency is defined as the proportion of
the energy contained in the above ground portion of the crop that is
ultimately used by the consumer. But remember that crops produce a lot of
biomass in the form of roots below the ground too, sometimes as much as
they do above the ground. So if this is taken into account, energy
efficiency figures quoted here would be substantially lower.
The most
inefficient agricultural production chains are those involving animals.
For example, feeding corn to cattle involves 50% harvest efficiency of the
corn crop (the grain constitutes only 50% of the total crop) x 14%
conversion efficiency by a feedlot steer (only 14% of the corn consumed by
the steer is turned into animal tissue) x a dressing percent of 63% at the
packing house (only 63% of the total carcass is retained for sale) x 50%
consumption by the consumer (we only eat 50% of the carcass that is sold
from the packing house – the rest is bone, etc.).
This comes
to a total of only 2.21% of the energy in the corn crop that is actually
used by the consumer as meat if corn grain is fed to cattle. Corn fed to
broilers has a better efficiency (6.5%) because broilers have a much
higher feed conversion rate than cattle (52% compared to 14%)
Cotton
looks better. It has a 17% harvest efficiency for lint, and all of this
goes to the consumer. In addition, cotton seed is fed to cattle, and this
adds another 1.3%, for a total efficiency of 18.3%. Clearly, if corn grain
is consumed directly by people, it has an efficiency of 50%, because grain
forms about half of the above ground portion of the crop.
If
switchgrass is used to produce electricity by co-firing it with coal, then
total efficiency is calculated as about 80% harvest efficiency x 30%
conversion efficiency = 24% total efficiency. In other words, about 24% of
the energy contained in the switchgrass crop reaches the consumer in the
form of electricity. If switchgrass is used in the co-generation of both
heat and electricity, the efficiency is then 80% harvest efficiency x 80%
conversion efficiency = 64% total efficiency. If wood were used instead of
switchgrass, efficiencies would be about the same. In general, therefore,
energy crops stack up favorably against other cropping systems in terms of
total energy efficiency. |
