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408 A COmpREHENSIvE GUIDE TO SOLAR ENERGy SySTEmS

Net energy alone can be misleadingly large when evaluating an abundant energy
resource of poor quality, such as ethanol from corn, or shale oil for which the energy cost of
fuel production is slightly less than the energy gained from burning it. EROI analysis when
used with NEA can help to assess the quality of fuels and this gives it the added ability of
ranking different fuels within given system boundaries over time. By evaluating the inputs
for the extraction of a resource and the value of the output in the same units, EROI can
interpret the difference between the effects of technology (which would increase EROI)
and those of depletion (which would decrease EROI) over time. As it pertains to fuels for
a society, such as solar pv electricity, EROI is the ratio of the amount of energy delivered
to society as a useful energy carrier by a chain of processes exploiting a primary energy
source, to the total energy invested in exploring, extracting, processing, and delivering that
energy [12,13]. A primary energy source is a natural form of energy (solar radiation, fos­
sil fuels, waterfalls, etc.) that can be used to create energy carriers (electricity, gasoline,
steam, etc.), which are used to fuel work in society [14]. EROI is represented in its simplest
form by the following equation:

EROI=Eoutput/Einput EROI = E output / E input (21.1)
Once the EROI reaches a ratio of unity, 1:1, or lower, in other words when the inputs of
energy are equal to or greater than the outputs of energy, it is no longer considered use­
ful to society, unless it is somehow being used to produce a higher quality fuel. Above 1:1,
however, the impact is nonlinear and as illustrated in Fig. 21.1, shifts in high EROI values
may have little impact on society, whereas those in low EROI values, especially below 5:1,
may have far greater impacts.
Historically, it seems that EROI for oil and gas have been decreasing and that for coal
has had its ups and downs. Assuming financial costs are related to energy costs, it has been
estimated that the EROI of global fossil fuels from 1800 to 1920 was 30–40:1, but increased
to 60:1 for oil and coal and to more than 100:1 for gas during the 1960s. All declined during
the 1970s and then increased again into the 1990s [15]. Oil and gas subsequently declined
irregularly to 10:1 and 20–40:1 respectively, while coal increased, also irregularly, in more
recent years [16]. Current ranges for such fuels are illustrated in Fig. 21.2 and those for
modern electric power generation, including pv, are in Fig. 21.3.
It is important to note that the values in Figs. 21.2 and 21.3 should not necessarily be
taken at face value. This is because of differences in the methods, data, and boundaries
of studies used to generate them [16]. Also, there is inherently a significant amount of
assumptions necessary to generate global values of EROI for fuels. Ideally, EROI is most
informative when given specific and well­defined geographic, temporal, and technolog­
ical boundaries. It can also be very useful to follow the trend over time. In this regard,
US oil/gas and oil in general have the only robust body of EROI information currently
available. Unfortunately, data, research, and personnel for other EROI analyses have been
scarce and the body of work has been small until 2011 [17]. Since then, however, there has
been a growing interest in EROI and more analyses are available, in part due to a growing
body of LCA publications in Europe. This is especially true for solar pv.