Environmental Engineering Reference
In-Depth Information
When looking at transformation or conversion, energy sources are classified as primary and
secondary.
Primary sources
are those that already exist in nature, for instance crude oil, coal,
uranium, sun, wind, biomass, and tides. (Kydes and Cleveland, 2007).
Secondary sources
are
forms of energy produced from primary sources through a conversion process, for example,
electricity, refined fuels, and hydrogen.
Energy return on the investment
Energy accumulated in primary sources always needs some type of transformation, extraction,
or the development of special equipment to harvest that energy and make it available for con-
sumption. And to do that it takes energy. For instance, production of electricity by harvesting
wind power requires the construction of windmills, generators, and distribution lines. During
the construction phase, energy is spent in the manufacture, transport, and installation of the
equipment. During the operational phase, not 100 percent of the electricity produced goes to
the consumers. An important fraction of the energy is used to operate the electronics and con-
trols, offices and maintenance, and losses during distribution. Because windmills do not last
forever, after a life span of 20 to 25 years, the installation needs to be decommissioned, which
takes energy. At the end of the project life, the total net energy produced by the windmill is not
just the output delivered to customers, instead it is the energy produced during its lifetime
minus the energy spent during construction, operation, and decommissioning.
In the case of wind power, the primary source of energy does not need any transformation,
so there is no energy expenditure on this. For a coal-fired plant, however, other expenditures
need inclusion, such as the energy spent to dig the coal from the ground (e.g., shovels, jack-
hammers, and dump trucks), energy spent during preparation for burning (e.g., crushing and
sorting), transportation of the coal from the mine to the power plant, as well as the energy
spent to make all the machines involved in the processes (Homer-Dixon, 2006).
Often the energy produced by the source is contrasted with the energy consumed in the
process in a ratio that is called the energy return on investment, or EROI (Fig. 11.1). Basically,
the EROI compares the energy output of a source with the energy input.
A source of energy is sustainable only when the energy produced is more than the amount
of energy invested, which means EROIs larger than 1. An EROI of 1 represents the break-even
point, and when the EROI falls below 1 the source becomes a sink (Fig. 11.2). From the prac-
tical point of view, however, EROIs need to be more than just slightly higher than 1. Hall et al.
(2009) established in a review paper that the minimum practical EROI needed to develop a
sustainable society based on biofuels is 3:1, otherwise biofuels would still be subsidized by
fossil fuels.
It is estimated that in 1930, the EROI for production of oil and gas was at least 100:1, and
by 1970, it fell to 30:1 (Cleveland et al., 1984; Cleveland, 2005). EROIs for other nonrenew-
able sources such as nuclear and coal energy are 5:1 and 8:1, respectively, whereas for renew-
able sources including hydro, geothermal, wind, and photovoltaic (PV) solar energy are
approximately 12:1; 8:1; 18:1; and 8:1, respectively (Kubiszewski and Cleveland, 2008). See
Table 11.1 for comparison. It is important to clarify that these are averages that have signifi-
cant ranges of variation as a result of different technologies and project sizes.
In the biofuels arena, some sustain that biofuels will substitute oil in the future; however,
others are unsure. The EROI for corn-based ethanol in the United States is 1.12 to 1. Brazil's
ethanol program from sugarcane generates ethanol with an EROI of 8:1 according to a report
from the University of Campinas in Brazil (Smeets et al., 2006). However, in a recent paper,
Pimentel and Patzek (2007) estimated an EROI for Brazilian ethanol of 1.38:1. An EROI of
8:1 is close to the 10:1 for gasoline; however, 1.12: and 1.38:1 are marginal. At this level of
