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EROI of Different Fuels and the Implications for Society

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All forms of economic production and exchange involve the use of energy directly and in the transformation of materials. Until recently, cheap and seemingly limitless fossil energy has allowed most of society to ignore the importance of contributions to the economic process from the biophysical world as well as the potential limits to growth. This paper centers on assessing the energy costs of modern day society and its relation to GDP. Our most important focus is the characteristics of our major energy sources including each fuel's energy return on investment (EROI). The EROI of our most important fuels is declining and most renewable and non-conventional energy alternatives have substantially lower EROI values than traditional conventional fossil fuels. At the societal level, declining EROI means that an increasing proportion of energy output and economic activity must be diverted to attaining the energy needed to run an economy, leaving less discretionary funds available for “non-essential” purchases which often drive growth. The declining EROI of traditional fossil fuel energy sources and the effect of that on the world economy are likely to result in a myriad of consequences, most of which will not be perceived as good.
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EROI of different fuels and the implications for society
Charles A.S. Hall
, Jessica G. Lambert, Stephen B. Balogh
State University of New York, College of Environmental Science and Forestry, 1 Forestry Dr., Syracuse, NY 13210, USA
For nations examined, the EROI for oil and gas has declined during recent decades.
Lower EROI for oil may be masked by natural gas extracted/used in oil production.
The EROI trend for US coal is ambiguous; the EROI for Chinese coal is declining.
Renewable energies lack desirable fossil fuel traits, including often higher EROI, but create fewer pollutants.
Declines in EROI of main fuels have a large impact on economies.
article info
Article history:
Received 7 May 2013
Accepted 16 May 2013
Available online 31 October 2013
Limits to growth
All forms of economic production and exchange involve the use of energy directly and in the transformation of
materials. Until recently, cheap and seemingly limitless fossil energy has allowed most of society to ignore the
importance of contributions to the economic process from the biophysical world as well as the potential limits
to growth. This paper centers on assessing the energy costs of modern day society and its relation to GDP. Our
most important focus is the characteristics of our major energy sources including each fuel's energy return on
investment (EROI). The EROI of our most important fuels is declining and most renewable and non-
conventional energy alternatives have substantially lower EROI values than traditional conventional fossil
fuels. At the societal level, declining EROI means that an increasing proportion of energy output and economic
activity must be diverted to attaining the energy needed to run an economy, leaving less discretionary funds
availab le for non-essentialpurchases which often drive growth. The declining EROI of traditional fossil fuel
energy sources and the effect of that on the world economy are likely to result in a myriad of consequences,
most of which will not be perceived as good.
&2013 The Authors. Published by Elsevier Ltd. All rights reserved.
1. Introduction
Energy has played a critical role throughout human society's
demographic, economic and social development. The availability
and quality of various energy and material resources to a society is
linked to the general trend of the settlement, growth, and eventual
decline experienced by each civilization (White, 1959;Tainter,
1988). A society must have an energy surplus for there to be
division of labor, creation of specialists and the growth of cities,
and substantially greater surplus for there to be wide-spread
wealth, art, culture and other social amenities. Economic uctua-
tions tend to result, directly or indirectly, from variations in a
society's access to cheap and abundant energy (Tainter, 1988;
Cleveland et al., 1984). Today, fossil fuel resources are among the
most important global commodities and are essential for the
production and distribution of the rest. Fossil fuels supply greater
than 75% of the total energy consumed by societies (EIA data for
various years as discussed in Hall et al., 2009). The prosperity and
stability of modern society is inextricably linked to the production
and consumption of energy, especially oil (Odum, 1973;Hall et al.,
1986;Hall and Klitgaard, 2012; Tverberg, 2012).
Economic production, exchange and growth requires work and
consequently a steady and consistent ow of energy to do that work.
Longer intervals of sustained economic growth in countries and the
world have been punctuated by numerous oscillations; i.e. there are
periods of economic expansion but also recession. In general, the
growth of real GDP is highly correlated with rates of oil consumption
(Murphy et al., 2011). Four out of the ve recessions experienced
since 1970 can be explained by examining oil price shocks (Hamilton,
2009;Hall and Groat, 2010;Jones et al., 2004). During periods
of recession, oil prices tend to decline, eventually encouraging
increased consumption. Alternatively, during periods of expansion,
oil prices usually increase and higher energy consumption and
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0301-4215/$ - see front matter &2013 The Authors. Published by Elsevier Ltd. All rights reserved.
This is an open-access article distributed under the terms of the Creative
Commons Attribution-NonCommercial-No Derivative Works License, which per-
mits non-commercial use, distribution, and reproduction in any medium, provided
the original author and source are credited.
Corresponding author. Tel.: +1 315 469 7271.
E-mail address: (C.A.S. Hall).
Energy Policy 64 (2014) 141152
economic expansion are eventually constrained by these higher
prices (Jones et al., 2004). Economic growth and stability is depen-
dent on not only the total quantity of energy accessible to society but
also the cost of this energy to different sectors of that society. Jones
et al.'s (2004) article, Oil Price Shocks and the Macroeconomy,
demonstrates a clear relation between oil price and GDP. The main
conclusions drawn from this and similar assessments are:
(1) Decreases in GDP during the post WWII period are chiey
attributable to oil price shocks, not government policy.
(2) Oil price shocks are the only novel or surprising price move-
ment observed in a two-year window of time prior to a
(3) Oil price shocks lead to costly reallocations of people and
industries as well as uctuations and pauses in investment.
This inuences industrial output and subsequently GDP (Jones
et al., 2004).
1.1. Economic cost of energy
The ratio of the monetary cost of energy compared to the GDP
generated for the same year gives a quantitative index of how much
money is invested in energy on average to generate a unit of wealth.
This can be calculated by dividing the money required to buy energy
by the total gross domestic product. When this ratio is low, typically
around ve percent, economies grow strongly (Hall and Klitgaard,
2012). When this ratio is high, about ten percent (and, historically, up
to fourteen percent), recessions tend to occur. A sudden climb
(followed by a subsequent decline) in the proportion of the GDP
spent for energy occurred during the two 1970s and the mid-2008
oil price shocks(Hall and Cleveland, 1981;Hamilton, 2009;Hall and
Klitgaard, 2012). Rapid increases in the economic cost of energy (e.g.
from ve to ten percent) result in the diversion of funds from what is
typically devoted to discretionary spending to energy acquisition
(Hall and Klitgaard, 2012). Consequently, large changes in energy
prices inuence economies strongly.
The energy and economic communities currently host strongly
polarized views about whether the quantity of fossil fuel resources
ultimately available to society is declining and, if so, the potential
repercussions of this for societal well-being and economic growth.
Much of the argument used by the energy community revolves
around the concepts of net energyand energy return on investment
(EROI). For example, while most energy scientists accept the econo-
mists'argument that there is a lot of oil left in the ground and that
higher prices will encourage its extraction and production, they also
point out that when more money is required more energy is required
too, and that there is a limit to how much we can pay for oil that
occurs as one approaches using a barrel of oil to extract another barrel
of oil. Such net energy analysis is sometimes called the assessment of
energy surplus, energy balance, or, as we prefer, EROI.
1.2. Types of EROI analyses and associated boundaries
Energy return on investment (EROI) is a means of measuring the
quality of various fuels by calculating the ratio between the energy
delivered by a particular fuel to society and the energy invested in the
capture and delivery of this energy. Much of the current EROI analysis
literature tends to focus on the net or surplus for a given project,
industry, nation, fuel, or resource, for example recent discussions on
the energy break evenpoint of EROI for corn based ethanol, i.e.
whether the EROI is greater than 1:1. The apparently different results
from this seemingly straightforward analysis generated some con-
troversy about the utility of EROI. But, the variation in these ndings is
mostly the result of the choice of direct and indirect costs associated
with energy production/extraction included within the EROI calcula-
tions: i.e. the boundaries of the denominator (Hall et al., 2011). The
possible boundaries of the various net energy assessments evaluated
in this study are illustrated in Fig. 1.
These and other boundary issues are addressed in Murphy
et al.'s recent paper, Order from Chaos: A Preliminary Protocol for
Determining the EROI of Fuels (Murphy et al., 2011). We clarify
further the boundaries used in the EROI calculations given here
into the following categories derived from Hall et al. (2009):
(1) Standard EROI (EROI
): A standard EROI approach divides the
energy output for a project, region or country by the sum of
the direct (i.e. on site) and indirect (i.e. offsite energy needed
to make the products used on site) energy used to generate
that output. It does not include e.g. the energy associated with
supporting labor, nancial services and the like. This EROI
calculation is applied to fuel at the point where it leaves the
extraction or production facility (well-head, mine mouth, farm
gate, etc.). This approach allows for the comparison of differ-
ent fuels even when the analysts do not agree on the rest of
the methodology that should be used (Murphy et al., 2011).
(2) Point of Use EROI (EROI
): Point of use EROI is a more
comprehensive EROI that includes additionally the costs asso-
ciated with rening and transporting the fuel. As the boundaries
of the analysis are expanded, the energy cost of getting it to that
point increases, resulting in a reduced EROI (Hall et al., 2009).
(3) Extended EROI (EROI
): This expanded analysis considers the
energy required not only to get but also to use a unit of energy.
In other words, it is the EROI of the energy at the mine mouth
required for that energy to be minimally useful to society, for
example to drive a truck (Hall et al., 2009).
(4) Societal EROI (EROI
): Societal EROI is the overall EROI that
might be derived for all of a nation's or society's fuels by
Fig. 1. Boundaries of various types of EROI analyses and energy loss associated with the processing of oil as it is transformed from oil at the well-headto consumer ready
fuels (gure from Lambert and Lambert (in preparation) based on calculations by Hall et al. (2009)).
C.A.S. Hall et al. / Energy Policy 64 (2014) 141152142
summing all gains from fuels and all costs of obtaining them.
To our knowledge this calculation has yet to be undertaken
because it is difcult, if not impossible, to include all the
variables necessary to generate an all-encompassing societal
EROI value (Hall et al., 2009). We develop a preliminary
method for deriving EROI
at the national level in another
paper in this series (Lambert et al., 2013).
We next present the historical and ongoing trends in EROI
ndings for various energy sources and discuss the potential
impact of low EROI fuels on the continuance of a high EROI society.
2. Meta-analysis of EROI values for various fuel sources
Our research and that of Dale (2010) summarizes EROI esti-
mates for the thermal energy delivered from various fossil fuels
and also the electric power generated using fossil fuel and various
other energy technologies. These initial estimates of general values
for contemporary EROI provide us with a beginning on which we
and others can build as additional and better data become
available. We have fairly good condence in the numbers repre-
sented here, in part because various studies tend to give broadly
similar results. Values from different regions and different times
for the same fuels, however, can give quite different results. Given
this, we present these values with considerable humility because
there are no government-sponsored programs or much nancial
support to derive such numbers.
EROI values for our most important fuels, liquid and gaseous
petroleum, tend to be relatively high. World oil and gas has a mean
EROI of about 20:1 (n of 36 from 4 publications) (Fig. 2)(see
Lambert et al., 2012 and Dale, 2010 for references). The EROI for the
production of oil and gas globally by publicly traded companies has
declined from 30:1 in 1995 to about 18:1 in 2006 (Gagnon et al.,
2009). The EROI for discovering oil and gas in the US has decreased
from more than 1000:1 in 1919 to 5:1 in the 2010s, and for
production from about 25:1 in the 1970s to approximately 10:1 in
2007 (Guilford et al., 2011). Alternatives to traditional fossil fuels
such as tar sands and oil shale (Lambert et al., 2012) deliver a lower
EROI, having a mean EROI of 4:1 (n of 4 from 4 publications) and 7:1
(n of 15 from 15 publication) (Fig. 2). It is difcult to establish EROI
values for natural gas alone as data on natural gas are usually
aggregated in oil and gas statistics (Gupta and Hall, 2011;Murphy
and Hall, 2010).
The other important fossil fuel, coal, has a relatively high EROI
value in some countries (U.S. and presumably Australia) and shows
no clear trend over time. Coal internationally has a mean EROI of
about 46:1 (n of 72 from 17 publications) (see Lambert et al., 2012
for references) (Fig. 2). Cleveland et al. (2000) examined the EROI
values for coal production in the United States. They found a
general decline from an approximately 80:1 EROI value during the
mid 1950s to 30:1 by the middle of the 1980s. Coal, however,
regained its former high EROI value of roughly 80:1 by 1990. This
pattern may reect an increase in less costly surface mining. The
energy content of coal has been decreasing even though the total
tonnage has continued to increase (Hall and Klitgaard, 2012). This
is true for the US where the energy content (quality) of coal has
decreased while the quantity of coal mined has continued to
increase. The maximum energy from US coal seems to have
occurred in 1998 (Hall et al., 2009;Murphy and Hall, 2010).
Meta-analysis of EROI values for nuclear energy suggests a mean
EROIofabout14:1(nof33from15publications)(seeLambert et al.,
2013 for references) (Fig. 3). Newer analyses need to be made as
these values may not adequately reect current technology or ore
grades. Whether to correct the output for its relatively high quality is
an unresolved issue and a quality correction for electricity appears to
contribute to the relatively high value given here.
Hydroelectric power generation systems have the highest
mean EROI value, 84:1 (n of 17 from 12 publications), of electric
power generation systems (see Lambert et al., 2012 for references)
(Fig. 3). The EROI of hydropower is extremely variable although
the best sites in the developed world were developed long ago
(Hall et al., 1986).
We calculate the mean EROI value for ethanol from various
biomass sources using data from 31 separate publications covering
a full range of plant-based ethanol production (e.g. EROI of 0.64:1
Pimentel and Patzek, 2005 for ethanol produced from cellulose
from wood to EROI of 48:1 for ethanol from molasses in India (Von
Blottnitz and Curran, 2007)). These values result in a mean EROI
value of roughly 5:1 with an n of 74 from 31 publications (Fig. 2). It
must be noted, however, that many of the EROI gures (33 of the
74 values) are below a 5:1 ratio (see Lambert et al., 2012 for
references) and diesel from biomass is also quite low (2:1 with an
n of 28 from 16 publications) (see Lambert et al., 2012 for
references). The average is skewed in a positive direction by a
handful of outliers (four EROI gures are above 30:1) (Von
Blottnitz and Curran, 2007;Yuan et al., 2008 in Dale, 2010).
Fig. 2. Mean EROI (and standard error bars) values for thermal fuels based on known published values. Values are derived using known modern and historical published
EROI and energy analysis assessments and values published by Dale (2010). See Lambert et al. (2012) for a detailed list of references. Note: please see text for discussion as all
these values should not necessarily be taken at face value.
Fig. 3. Mean EROI (and standard error) values for known published assessments of
power generation systems. Values derived using known modern and historical
published EROI and energy analysis assessments and values published by Dale
(2010). See Lambert et al. (2012) for detailed list of references. Note: please see text
for discussion as all these values should not necessarily be taken at face value.
C.A.S. Hall et al. / Energy Policy 64 (2014) 141152 143
We believe that outside certain conditions in the tropics most
ethanol EROI values are at or below the 3:1 minimum extended
EROI value required for a fuel to be minimally useful to society.
Wind power has a high EROI value, with the mean perhaps as
high as 18:1 (as derived in an existing meta-analysis by
Kubiszewski et al., 2010) or even 20:1 (n of 26 from 18 publica-
tions) (see Lambert et al., 2012 for references) (Fig. 3). The value in
practice may be less due to the need for backup facilities.
An examination of the EROI literature on solar photovoltaic or PV
energy generation shows differences in the assumptions and meth-
odologies employed and the EROI values calculated. The values,
assumptions, and parameters included are often ambiguous and differ
from study to study, making comparisons between PV and other
energy EROI values difcult and fraught with potential pitfalls. Never-
theless, we calculated the mean EROI value using data from 45
separate publications spanning several decades. These values resulted
in a mean EROI value of roughly 10:1 (n of 79 from 45 publications)
(see Lambert et al., 2012 for references) (Fig. 3). It should be noted that
several recent studies that have broader boundaries give EROI values
of 2 to 3:1 (Prieto and Hall, 2012;Palmer, 2013;Weissb ach et al., 2013),
although these are not weighted for the higher quality of the
electricity when compared with thermal energy input. Geothermal
electricity production has a mean EROI of approximately 9:1 (n of 30
from 11 publications) (see Lambert et al., 2012 for references) (Fig. 3).
A positive aspect of most renewable energies is that the output of
these fuels is high quality electricity. A potential draw back is that the
output is far less reliable and predictable. EROI values for PV and
other renewable alternatives are generally computed without con-
verting the electricity generated into its primary energy-equivalent
(Kubiszewski et al., 2009) but also without including any of the
considerable cost associated with the required energy back-ups or
storage. EROI calculations of renewable energy technology appear to
reect some disagreement on the role of technological improvement.
Raugei et al. (2012) attribute some low published EROI values for PVs
to the use of outdated data and direct energy output data that
represents obsolete technology that is not indicative of more recent
changes and improvements in PV technology (Raugei et al., 2012).
EROI values that do reect technological improvements are calcu-
lated by combining top-of-the-linetechnological specications from
contemporary commercially available modules with the energy out-
put values obtained from experimental eld data. Other researchers
contend that values derived using this methodology do not represent
adequately the actualenergy cost to society and the myriad energy
costs associated with this delivery process. For example Prieto and
Hall, 2012 calculated EROI values that incorporate most energy costs,
with the assumption that where ever money was spent energy too
was spent. They use data from existing installations in Spain, and
derived EROI values of roughly 2.4:1, considerably lower than many
less comprehensive estimates. Similarly low EROI values for roof top
PVs with battery back up were found by Palmer (2013),althoughit
should be noted that the outputs of both systems were higher quality
electricity. Nearly all renewable energy systems appear to have
relatively low EROI values when compared with conventional fossil
fuels. A question remains as to the degree to which total energy costs
can be reduced in the future, but as it stands most renewable
energy systems appear to be still heavily supported by fossil fuels.
Nevertheless they are considerably more efcientatturningfossil
fuels into electricity than are thermal power plants, although it takes
many years to get all the energy back.
3. Methodology
We summarize existing studies of EROI while attempting to
understand the differences among them. Specically, published
values of EROI for similar fuels sometimes are substantially different
leading to large differences within the published data for EROI
assessments. To reduce these differences Murphy et al. (2011) derived
astandard protocolfor calculating EROI. While recognizing the
uncertainties involved in, and inherent to, all EROI calculations,
Murphy et al. (2011) proposed that these differences largely can be
reduced when similar boundaries are used for the assessment. The
generation of EROI values is best developed using industry- or
government-derived data on energy outputs and energy costs in
physical units, or by using a "process energy method" based on
measured energy costs of components. But, more commonly part or
all of the data is only in nancial units. Energy cost values can be
derived from nancial costs that can be translated into energy costs
using energy intensities (i.e. energy used per monetary unit for that
type of activity). Unfortunately, most companies consider their costs
proprietary knowledge.
Different boundaries and variables differ between nations and
may result in conicting or inconsistent data (Lambert et al., 2013).
Only a few countries, including the US, Canada, the UK, Norway,
and China keep the necessary industry-specic estimates of
energy costs required to perform an EROI analysis. Fortunately,
this data, taken as a whole and within a given country, seems to be
relatively consistent with information available from various non-
governmental sources e.g. Gagnon et al. (2009), or the differences
make logical sense. A short description of our methodology for
each respective fuel follows.
3.1. Oil and gas
Oil and gas EROI values are typically aggregated together. The
reason is that since both often are extracted from the same
Fig. 4. Gagnon et al. (2009) estimated the EROI for global publicly traded oil and gas. Their analysis found that EROI had declined by nearly 50% in the last decade and a half.
New technology and production methods (deep water and horizontal drilling) are maintaining production but appear insufcient to counter the decline in EROI of
conventional oil and gas.
C.A.S. Hall et al. / Energy Policy 64 (2014) 141152144
wells, their production costs (capital and operations) are typi-
cally combined, and therefore the energy inputs for EROI
calculations are very difcult to separate. Obtaining reliable
data on global petroleum production and its associated invest-
ment costs can be very difcult since most production is from
national oil companies, whose records tend not to be public.
Gagnon et al. (2009) estimatedglobaloilandgasEROIfrom
1992 to 2006 using data from most publicly-traded oil compa-
nies summarized by John Herold Company. Cleveland et al.,
1984;Hall et al., 1986 and Guilford et al., 2011 used time series
data for oil and gas production in the US from several sources
(mostly the U.S. Energy Information Agency and Census of
Mineral Industries) going back to 1919. Relatively good time
series data is available for Norway (Grandell et al., 2011), Mexico
(Ramirez, in preparation), Canada (Freise, 2011;Poisson and
Hall, in press)andChina(Hu et al., 2011,2013).
3.2. Coal
We evaluate the EROI
for US and Chinese coal, two of the
world's largest producers. For both nations direct energy gains and
costs were derived in physical and energy units available from
government sources (Balogh et al., unpublished data;Hu et al.,
2013). In the U.S. analysis, direct energy consumption and indirect
costs were derived from physical and nancial data published in
the U.S. Census of Mineral Industries and the U.S. Economic Census
reports (various years, 1919 through 2007). Direct energy con-
sumption was converted from physical units to joules, and the
energy equivalent of the indirect monetary costs was derived
using the average energy intensity for the entire economy for a
given year multiplied by the nominal dollar cost. A sensitivity
analysis (for China) was derived using the energy intensity for all
engineeringsectors (see Hu et al., 2013 for details).
Table 1
Published EROI values for various fuel sources and regions (adapted from Murphy et al. (2011)).
C.A.S. Hall et al. / Energy Policy 64 (2014) 141152 145
4. Results
EROI for various fuels varies from 1:1 to 100:1 with, in general,
the highest values being for coal in the US and oil and gas from 1970
to 1990. There is a tendency for EROI for oil and gas production to
increase during earlier years of development and then decline over
time and with rate of exploitation. We organized existing published
and unpublished EROI values by fuel type, year and individual study.
This information, presented in Table 1, summarizes our existing
knowledge of EROIs for various energy sources by EROI value,
geographic region, and time.
4.1. Global oil and gas (petroleum)
The EROI for petroleum production appears to be declining
over time for every place we have data. Gagnon et al. (2009) were
able to generate an approximate globalEROI for private oil and
gas companies using the upstreamnancial database maintained
and provided by John H. Herold Company. These results indicate
that the EROI for publicly-traded global oil and gas was approxi-
mately 23:1 in 1992, 33:1 in 1999 and 18:1 in 2005 (Fig. 4). This
dome shapedpattern seems to occur wherever there is a long
enough data set, perhaps as a result of initial technical improve-
ments being trumped in time by depletion.
4.1.1. United States oil and gas
Three independent estimates of EROI time series for oil and gas
production for the United States are given in Fig. 5 along with
some important oil-related historical events (Cleveland et al.,
1984;Hall et al., 1986;Guilford et al., 2011).
The data show a general pattern of increase and then decline in
EROI over time except as impacted by changes in exploration
(drilling) intensity (in, for example, the late 1970s and early
1980s). During the mid 1970s1980s and late 2000s, the price of
oil increased as did exploration intensity, as measured by
increased feet drilled and energy used. EROI values tend to decline
when there is an increase in the energy required for exploration
and drilling. But, usually increased drilling was linked to little or
no additional oil discoveries; hence EROI values declined (Fig. 6).
The greatly increased amount of money being spent for oil and gas
development in the US in recent years suggests that despite the
recent increases in production the EROI may continue to decline.
4.1.2. Canadian oil and gas
Two independent EROI estimates for Canadian production of oil
and gas (blue line, from Freise, 2011)(Fig. 7) and oil, gas and tar
sands combined (red line, from Poisson, in press) demonstrate that
the EROI of conventional oil and gas in Canada has declined
considerably in recent decades. Freise (2011) estimates the EROI
of western Canadian conventional oil and gas over time from 1947
to 2010. Freise agrees with Poisson that the later values are
probably more accurate.
Poisson and Hall (in press) found that the EROI of conventional
oil and gas has decreased since the mid-1990s from roughly 20:1
to 12:1, a 40% decline. The EROI of conventional combined oilgas
tar sands has also decreased during this same period from 14:1 to
7.5:1, a decline of 46% (Fig. 8)(Poisson and Hall (in press)). Poisson
and Hall's estimated EROI values for Canadian oil and gas are about
half those calculated by Freise and their rate of decline is some-
what less rapid (Freise, 2011;Poisson and Hall, in press)(Fig. 8).
Poisson and Hall's estimate of the EROI of tar sands is relatively
low, around 4.5 (a conservative (i.e. high) estimate, using only the
front end of the life-cycle); incorporating tar sands into total oil
and gas estimates decreases the EROI of the oil and gas extraction
industry as a whole (Poisson and Hall, in press). These estimates
would be lower if more elements of the full life-cycle
(e.g. environmental impact) were included in the calculation.
4.1.3. Norwegian oil and gas
Norwegian conventional oil and gas elds are relatively new
and remain protable both nancially and with regard to energy
production. Grandell et al. (2011) estimate that the EROI of oil and
gas ranged from 44:1 (during the early 1990s) to 59:1 (1996), to
approximately 40:1 (during the latter half of the last decade)
(Fig. 9)(Grandell et al., 2011).
4.1.4. Mexican oil and gas
Ramirez's trends for the EROI of Mexican oil and gas suggest
that this country may have peaked twice in the past decade. The
EROI for conventional oil and gas production in Mexico declined
from roughly 60:1 in 2000 to 47:1 the following year, but returned
Fig. 5. Time series analyses of oil and gas production within the US including several relevant oil relatedhistorical events. Each analysis demonstrates a pattern of general
increase then decline in EROI with an additional impact of increased exploration/drilling.
Fig. 6. US oil and gas values published by Guilford et al. (2011) from 1992 to 2007.
C.A.S. Hall et al. / Energy Policy 64 (2014) 141152146
Fig. 9. Time series data on EROI for oil and gas for Norway, Mexico and the Daqing oil eld in China based on several papers published in the 2011 special issue of the journal
Sustainability and works in progress.
Fig. 10. Two published studies on the EROI of dry (not associated with oil) natural gas: Sell et al. (2011) examined tight natural gas deposits in western Pennsylvania in the
US, and Freise (2011) analyzed all convention natural gas wells in western Canada.
Fig. 7. Two independent estimates of EROI for Canadian petroleum production: oil and gas (blue line, from Freise, 2011) and oil, gas and tar sands combined (red line, from
Poisson and Hall, in press). (For interpretation of the references to color in this gure legend, the reader is referred to the web version of this article.)
Fig. 8. Canada oil and gas and oil, gas and tar sand values by Freise (2011) and Poisson and Hall (in press).
C.A.S. Hall et al. / Energy Policy 64 (2014) 141152 147
to 59:1 by about 2003 (Fig. 9)(Ramirez, in preparation). This was
followed by a steady decline over the following six years reaching
45:1 by 2009. The collapse of production from the Cantarell eld
in the Gulf of Mexico, once the world's second largest, appears
largely responsible for this decline.
4.1.5. Chinese oil and gas
The EROI for the Daqing eld, China's largest conventional oil
eld, has declined continuously from 10:1 in 2001 to 6:1 in 2009
(Fig. 9)(Hu et al., 2011).
4.2. Dry natural gas
The data represented in Fig. 10 includes analyses for a portion
of the US and for all of Canada. Since most published numbers
combine data on natural gas with that of oil, it is usually difcult
or impossible to assess the production costs of these fossil fuel
resources independently. Sell et al.'s (2011) trends for EROI of
natural gas in Pennsylvania (US) has an undulating decline; from
roughly 120:1 in 1986 to 67:1 in 2003. This value is probably a
high value as some important indirect costs were not included.
Freise (2011) estimated the EROI of western Canadian natural gas
from 1993 to 2009 and found that the EROI of natural gas has been
decreasing since 1993 through 2006, from roughly 38:1 to 14:1.
This trend shifted in 2006 as exploitation intensity declined
resulting in a steady increase and an EROI of roughly 20:1 by
2009 (Fig. 10)(Freise, 2011). Aucott and Melillo (2013) found
values similar to Sell et al. for "fracked" shale gas from Pennsylva-
nia from, presumably, "sweet spots".
4.3. Coal
The only EROI analyses for coal production are from the US and
China because information on the energy expended to extract coal
in other areas of the world appears unavailable. Time series of
EROI for coal production for the United States and China are given
in Fig. 11. A great deal of variability in EROI is evident in these
gures. This data, however, have signicant holes (e.g. no data is
reported for approximately 30 years, from the mid 1950s to the
mid 1980s). Cleveland's work provides additional information for
three noncontiguous years that is only partly consistent with
Balogh et al.'sndings. Hu et al. (2013) establishes annual data
for Chinese coal production for the years 1994 through 2009.
These show very little variation in EROI values.
5. Research challenges
There are four major challenges for calculating the EROI of
various fuels at the national, regional and global scale. First is the
lack of data on fuel used during the extraction process. Data on on-
site non-traded fuels generally is not readily available, although
ideally they are reported to government agencies undertaking
census work. Often indirect energy cost calculations must be
derived from nancial data. This requires converting currency into
an energy equivalent (e.g. mega joules used per dollar of GDP).
Methods for accomplishing this conversion usually assume that
expenditure for inputs to the energy industry are the same as for
society more generally or for an engineering component. They
may be less accurate but sensitivity analyses can be undertaken to
address uncertainties. Ideally Inputoutput analysis is undertaken
which can give much more accurate results. This analysis used to
be done by a team at the University of Illinois but these results are
seriously outdated. A more recent analysis may have been done at
Carnegie-Mellon's Green Building Program, from which useful
general values can be obtained (See Prieto and Hall, 2012,
chapter 4). Second is the issue of variation in scale. Are studies
at the regional level comparable to those at the national level and
how do these size upwhen presented next to international
studies that include a small subset of representative countries?
Various variables and boundaries often vary with the scale of
investigation making it difcult to compare data among diverse
analyses. Third, energy analysts are not in agreement on what
indirect costs should and should not be included in an EROI
assessment. When complete systems are analyzed for solar PV
installations, their nancing, their operations and maintenance
costs and their backups are included the energy costs are about
three times larger than for just the modules and inverters. One
very contentious indirect cost is the inclusion or exclusion of the
energy cost of supporting human labor (Murphy et al., 2011). This
can result in varying and potentially controversial assessments
especially when assessing fuels where small differences may
determine whether that fuel is perceived as a viable energy option
(e.g. corn-based ethanol). Fourth, is that the quality or utility of
these various fuels is represented differentially within different
data sets. Total primary energy consumption values at the global
level published by the U.S. Energy Information Agency, International
Energy Agency (IEA), and BP (Hayward, 2010); Smil, 2008,tendtobe
similar. They occasionally vary, however, in their method of addres-
sing primary energyconversion. For example, EIA data includes the
heat generated by nuclear power in its energy output assessments.
Various researchers, government agencies and industry organiza-
tions present data from a variety of sources using various assess-
ments, e.g. national (EIA), global (IEA) and industrial (BP). Laherrère
Fig. 11. EROI for US and Chinese coal production derived from Cleveland (1992),Balogh et al. (unpublished data) and Hu et al. (2013).
C.A.S. Hall et al. / Energy Policy 64 (2014) 141152148
addressed this issue at the 2011 ASPO conference (Laherrère, 2011).
He also noted that IEA data is presented as the direct electricity
generated for nuclear and hydropower, while EIA data includes waste
heat produced by nuclear ssion.
There is a broadly consistent pattern to our results, as indicated
by the similar temporal patterns of different studies, all of which
(except coal) have declined over time (and with increased effort)
and by the fact that regions developed for oil and gas for a longer
period (e.g. US, China or anywhere over time) have lower EROIs,
while newer developments (e.g. Norway) tend to have higher
values. If and as the Murphy et al. (2011) protocol is more
universally followed we expect even greater consistency in results.
6. Discussion
Our research summarizes EROI estimates for all industrial fuels,
and for the three major fossil fuels, coal, oil and natural gas over
time. These initial estimates of general trends in EROI provide us
with a beginning on which we and others can build as additional
and better data become available.
6.1. Historical perspective (19001939)
The industrial revolution was in full swing by the early 1900s
(Fig. 2). Abundant high quality coal from relatively thick seams
(with high EROI), capable of generating an enormous amount of
energy, was harnessed by humans to do all kinds of economic
work including: heating, manufacturing, the generation of elec-
tricity and transportation. Biomass energy, in the form of wood
burning for domestic use (heating and cooking), remained an
important contributor to the world's energy portfolio (Perlin,
1989). During this period the oil industry was in its infancy and
was primarily used for transportation and lighting (in the form of
kerosene in non-urban/non-industrial regions). High quality oil
remained a small contributor to the energy mix until the end of
the 1930s although it was increasing rapidly on a global scale (Hall
and Klitgaard, 2012).
6.2. Historical perspective (19401979)
The massive WWII war effort during the 1940s saw increased
use of coal and oil for the manufacture and use of war machinery.
During the post-war era, the great oil discoveries of the early
twentieth century found a use in global reconstruction and
industrialization. Throughout the 1950s and 1960s the repair of
war-torn Europe and the proliferation of western culture resulted
in massive increases in the manufacturing and transport of goods
and the oil necessary for their production and use. By the late
1960s the EROI of coal (mostly from deep mines) began to decline
while the EROI of oil remained high. The EROI for coal production
in the US declined from 80:1 in the 1950s to 30:1 in the 1970s
(Cleveland et al., 1984). During this time period, coal was mined
almost exclusively in the Appalachian mountain region areas of
the US using a combination of room and pillar mines with
conventional and continuous mining methods. The coal initially
extracted from these locations was a combination of anthracite
and high quality bituminous coal, coal with high BTUs/ton. As the
best coal was used rst, the EROI for coal decreased over time.
The quality of coal being produced was decreasing while world oil
production was increasing. The peak of US oil production in 1970 and
subsequent peak of US conventional natural gas in 1973 meant an
increased reliance on OPEC oil. Increases in oil prices reected in part
increased energy required to purchase this fuel. The price of other
economic activities increased at similar rates (Hall and Klitgaard,
2012). After the oil shocks of the 1970s, oil prices surged in the US
and around the world, stimulating both increased drilling activity and
greater interest in the exploitation of more marginal resources (those
with higher production costs) (Guilford et al., 2011). Increased drilling
activity in the U.S. did not result in increased production but caused a
sharp decline in the EROI for conventional oil and gas for the US
between the early 1970s and mid 1980s. The oil shocks of the 1970s
temporarily halted a long period of increased oil use. It also generated
a global oil market and price, destroying an advantage once held by
the US. Recovery of production did not occur in the continental US
where oil production had declined since its peak in 1970; until the
rather recent uptick in 2008 following the introduction of the new
technologies of horizontal drilling and hydraulic fracturing. New work
on the EROI for oil and gas produced by horizontal drilling and rock
fracturing indicate that the EROI can be very high, in part because it is
not necessary to pressurize the elds (e.g. Aucott and Mellilo 2013;
Moeller and Murphy personal communication; Waggoner personal
communication) but that these high values are likely to decline
substantially as production is moved off the "sweet spots".
6.3. Historical perspective (19802009)
In the 1980s post energy price shockera, oil that had been found
but not developed suddenly became worthy of developing. Many
world oil resources, incentivized by higher prices, were developed;
some were over developed. Important gains in oil and gas production
occurred in some non-OPEC countries including Norway, Mexico, and
China. Heating and transportation, historically fueled by coal, was
transformed to oil and gas. Energy from coal production shifted to, and
remained essential to, manufacturing and increasingly the production
of electricity. The EROI of US coal returned to 80:1 by about 1990. This
pattern reects a shift in the quality of coal extracted, the technology
employed in the extraction process and especially the shift from
underground to surface mining. A shift in mining location, from
Appalachia to the central and northern interior states of Montana
and Wyoming and extraction method, from underground to surface
mining (area, contour, auger, and mountain top mining techniques)
have resulted in less energy required to mine and beneciate coal. The
energy content of the coal extracted, however, has decreased. The coal
currently mined is lower-quality bituminous and sub-bituminous coal
with much lower BTUs/ton (Hall et al., 1986;Hall and Klitgaard, 2012)
The increased efciency of surface mining seems to just about
compensate for the decline in the quality of the coal mined. See
Section 7 for a consideration of environmental externalities.
Between 1985 and the early 1990s international oil and gas
prices fell, then remained stable until 2000 while drilling effort
declined until the mid 2000s. Thus the 1990s was a period of
abundant oil and plummeting oil prices bringing the real cost of oil
back to that of the early 1970s (Hall and Klitgaard, 2012). Discre-
tionary spending in the US and other western nations, often on
housing, increased. The late 1990s was a time of reduced oil
exploration efforts apparently resulting in an increase in EROI.
The mid 2000s marked an increase in global oil and gas exploration
efforts (Smil, 2008). Discretionary spending decreased with the
energy price increases from 2007 to the summer of 2008. Oil prices
hit an all time high of $147 per barrel in the summer of 2008 (Read,
2008). This extra 510% taxfrom increased energy prices was
added to the US (and other) economy as it had been in the 1970s,
and much discretionary spending disappeared (Hall et al., 2008).
Speculation in real estate (in the US) was no longer desirable or
possible as consumers tightened their belts because of higher
energy costs (Hall and Klitgaard, 2012). The stock market crashed
in September 2008 reducing market value by $1.2 trillion and
forcing the Dow to suffer its biggest single-day point loss ever
(Twin, 2008), and most Western economies have essentially
stopped growing since. In general there has been a decade-by-
decade decline in growth of the US economy since 1935, in step
C.A.S. Hall et al. / Energy Policy 64 (2014) 141152 14 9
with the decline in the annual rate of change for all oil production
liquids globally (Hall et al., 2012). Even though the global EROI for
producing oil and gas continues to be reasonably high, it is probable
that the EROI of oil and gas will continue to decline over the coming
decades (Gagnon et al., 2009). The continued pattern of declining
EROI diminishes the importance of arguments and reports that the
world has substantially more oil remaining to be explored, drilled
and pumped. HighEROI values for oil and gas production are
increasingly attributable to the inclusion of high EROI natural gas (e.
g. the EROI of Norwegian oil is about half that for oil and gas
combined) (Grandell et al., 2011). The recent declining trend is
described by Grandell et al. as probably due to aging of the elds.
It is likely that varying drilling intensity has had minimal impact on
the net energy gain of these elds. Grandell et al. (2011) expects the
EROI of Norwegian oil and gas production to deteriorate further as
the elds become older.Meanwhile,China's use of oil has
expanded enormously so that China has been importing a larger
and larger proportion of its oil from the rest of the world. Recently,
China has increased its oil exploitation efforts tremendously, both
inside and outside of China. Even so, Hu et al. (2011) suggest that
China appears to be approaching its own peak in oil production.
Since 2008 producers have shifted increasingly to non-conventional
oil and gas resources (tar sands, shale oil and gas) which have
increased production but also costs. New technologies such as
horizontal drilling and hydro-fracturing are currently keeping the
total levels of non-conventional and conventional natural gas
production in the US at rates similar to those of 1973 from
conventional natural gas alone. Given the numerous shifting
environmental variables and social issues surrounding horizontal
drilling and fracking,itisdifcult to predict the future of non-
conventional oil and gas (Hall and Klitgaard, 2012).
Already some areas of production from the Barnett and
Haynesville formations appear to have reached a production
plateau (Hughes, 2013). Recent analyses by Hughes (2013) argue
against assuming production will continue to increase. Much of
the discussion about peak coal(e.g. Patzek and Croft, 2010)
involves changing mining technology and capacity, rather than
the quantity and quality of coal that remains available for extrac-
tion. Peak coal will likely have the greatest impact on the world's
largest coal user, China. Nations with abundant untapped coal
resources (i.e. the US, Australia and Russia) are likely to be less
affected. The total recoverable coal estimated for the US alone is
approximately 500 billion tons. US coal production in 2009 was
about one billion tons. Although it is difcult to predict future
production technology, environmental issues, consumption pat-
terns and changes in EROI, it appears that coal may be abundantly
available through the next century.
6.4. Renewable energy sources
Alternative renewable energies lack many of the undesirable
characteristics of fossil fuels, including direct productions of
carbon dioxide and other pollutants, but also lack many of the
highly desirable traits of non-renewable fossil fuels. Specically,
renewable energy sources:
are not sufciently energy dense,
tend to be intermittent,
lack transportability,
most have relatively low EROI values (especially when correc-
tions are made for intermittency), and
currently, lack the infrastructure that is required to meet
current societal demands.
If we were to replace traditional nonrenewable energy with
renewables, which seems desirable to us in the long run, it would
require the use of energy-intensive technology for their construc-
tion and maintenance. Thus it would appear that a shift from non-
renewable to renewable energy sources would result in declines in
both the quantity and EROI values of the principle energies used
for economic activity.
Although wind, apparently relatively favorable from an EROI
perspective and photovoltaic (PV) energy, are currently the world's
fastest growing renewable energy sources, they continue to account
for less than one percent of the global energy portfolio (REN21, 2012).
Nevertheless there are many informal reports of PV reaching price
paritywith fossil fuels and to many the future of PV is very bright.
Proponents of EROI assessments using actual operational instal-
lations (rather than laboratory estimates) believe that, in order to
portray renewable energy technology accurately, it is necessary to
make note of the fact that these technologies are dependent upon
(i.e. constructed and maintained using and therefore subsidized by)
high EROI fossil fuels. Higher EROI values found in conceptual
studies often result from assumptions of more favorable conditions
(within simulations) than those actually experienced in real life. For
example, English wind turbines were found to operate considerably
fewer hours per month than anticipated (Jefferson, 2012).
Kubiszewski et al. (2010) infer that variations in EROI values, in
the case of reported EROI values for wind energy, (between process
and input output analyses) stems from a greater degree of sub-
jective system boundary decision-making by the process analyst,
resulting in the exclusion of certain indirect costs. Other researchers
believe that the focus of EROI assessments must be on net energy
produced from existing installations and variables associated with
wind and PV modules once they have entered the infrastructure
rather than extrapolating into the future. Examination of concrete
input and output data from operational facilities, e.g. wind turbines
(Kubiszewski et al., 2010), appears to offer the best opportunity to
calculate wind and PV EROI values accurately.
Also of concern is that wind and PV technology are not base load
technologies, meaning that future large scale deployment, beyond 20
percent of the grid capacity, will likely require the construction of
large, energy intensive storage infrastructures which, if included
within EROI assessments, would likely reduce EROI values consider-
ably. In the case of wind, the cost for inclusion within a wind EROI
analysis requires not only the initial capital costs per unit output but
also the backup systems required for the 70 or so percent of the time
when insufcient wind is blowing. Thus, the input for an EROI analysis
of wind and PV technology is by and large upfrontcapital costs. This
is in sharp contrast to the less well known returnover the lifespan of
the system. Therefore, a variable referred to as energy payback time
is often employed when calculating the EROI values of wind and other
renewable energy sources. This is the time required for the renewable
energy system to generate the same amount of energy that went into
the creation, maintenance, and disposal of the system. The boundaries
utilized to dene the energy payback time are incorporated into most
renewable EROI calculations. Other factors inuencing wind and PV
EROI values include energy storage, grid connection dynamics and
variations in construction and maintenance costs associated with the
installation location. For example, off-shore turbines, while located in
wet salty areas with more reliable energy-generating winds, require
replacement more often. Turbines located in remote mountainous
areas require long distance grid connections that result in energy loss
and reduced usable energy values (Kubiszewski et al., 2010).
7. Policy implications
In conclusion, the EROI for the world's most important fuels, oil
and gas, has declined over the past one to two decades for all
nations examined. It remains possible that the relatively high EROI
values for the natural gas extracted during, and often used for, the
C.A.S. Hall et al. / Energy Policy 64 (2014) 141152150
production of oil may mask a much steeper decline in the EROI of
oil alone. Declining EROI is probably already having a large impact
on the world economy (Murphy and Hall, 2010;Tverberg, 2012).
As oil and gas provide roughly 6065% of the world's energy, this
will likely have enormous economic consequences for many
national economies. Coal, although abundant, is very unevenly
distributed, has large environmental impacts and has an EROI that
depends greatly on the region mined. A general decline in the
energy content of US coal resource over time may be compensated
by a shift from energy-intensive underground mining of relatively
high quality (but declining) Eastern US coal resources to lower-
cost surface mining of lower energy-content Western US coal
resulting in no clear trend in EROI for coal.
The decline in EROI among major fossil fuels suggests that in
the race between technological advances and depletion, depletion
is winning. Past attempts to rectify falling oil production i.e. the
rapid increase of drilling after the 1970 peak in oil production and
subsequent oil crises in the US only exacerbated the problem by
lowering the net energy delivered from US oil production (Hall
and Cleveland, 1981). Increasing prices, thought by most econo-
mists to negate depletion through increasing incentives for
exploitation, cannot work as EROI approaches 1:1, and even now
has made oil too expensive to support the high economic growth it
once did.
It would be tempting, from a net energy perspective, to
recommend that we replace fossil fuels with renewable energy
technologies as the EROI for fossil fuel falls to a level where these
technologies become competitive. While EROI analyses generate
numerical assessments using quantitative data that include many
production factors, they do not include other important data such
as climate change, air quality, health benets, and other environ-
mental qualities that are considered externalitiesto these ana-
lyses. The energy intensive carbon capture and sequestration (CCS)
required to reduce fossil fuel emissions to levels equivalent with
that of wind or PV electricity production would reduce the nal
coal EROI value considerably ((e.g. Akai et al. 1997 in Dale, 2010
and Lund and Biswas, 2008). EROI gures do not take into account
the high life-cycle greenhouse gas emissions from thermal elec-
tricity production, and coal-red systems in particular (Raugei
et al., 2012). This could, with difculty, be worked into future,
more comprehensive EROI calculations. Most alternative renew-
able energy sources appear, at this time, to have considerably
lower EROI values than any of the non-renewable fossil fuels.
Wind and photovoltaic energy are touted as having substantial
environmental benets. These benets, however, may have
lower returns and larger initial carbon footprints than originally
suggested (e.g. the externalities associated with the mining of
neodymium and its subsequent use in wind turbine construction).
The energy costs pertaining to intermittency and factors such as
the oil, natural gas and coal employed in the creation, transport
and implementation of wind turbines and PV panels may not be
adequately represented in some cost-benet analyses. On the
positive side, the fact that wind and PV produce high quality
electricity needs to be considered as well.
Thus society seems to be caught in a dilemma unlike anything
experienced in the last few centuries. During that time most
problems (such as needs for more agricultural output, worker
pay, transport, pensions, schools and social services) were solved
by throwing more technology investments and energy at the
problem. In many senses this approach worked, for many of these
problems were resolved or at least ameliorated, although at each
step populations grew so that more potential issues had to be
served. In a general sense all of this was possible only because
there was an abundance of cheap (i.e. high EROI) high quality
energy, mostly oil, gas or electricity. We believe that the future is
likely to be very different, for while there remains considerable
energy in the ground it is unlikely to be exploitable cheaply, or
eventually at all, because of its decreasing EROI. Alternatives such
as photovoltaics and wind turbines are unlikely to be nearly as
cheap energetically or economically as past oil and gas when
backup costs are considered. In addition there are increasing costs
everywhere pertaining to potential climate changes and other
pollutants. Any transition to solar energies would require massive
investments of fossil fuels. Despite many claims to the contrary
from oil and gas advocates on the one hand and solar advocates on
the otherwe see no easy solution to these issues when EROI is
considered. If any resolution to these problems is possible it is
probable that it would have to come at least as much from an
adjustment of society's aspirations for increased material afuence
and an increase in willingness to share as from technology.
Unfortunately recent political events do not leave us with great
optimism that such changes in societal values will be forthcoming.
This research has been supported by the UK Department for
International Development (No. 59717) and the Santa Barbara
Family foundation. We are very grateful to several experts who
provided us with very helpful unpublished data on on-site fuel
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... Fossil fuel-derived electricity is significantly more lifecycle greenhouse intensive than renewable sources due to direct combustion ( [29], Figure 7.6); [86]. On the other hand, renewable and nuclear electricity possess a higher embodied material and energy content [86,87]. Wind, ocean and CSP require more steel and cement than fossil fuel plants, per unit of electricity generated ( [29], p. 549). ...
... Given that generating electricity using hydrogen turbines or fuel cells loses 40-65% of the embedded energy [88], the energy return on the energy investment of hydrogen would be even lower than the presented values. To be economically sustainable, the minimum energy return on energy investment is three [8,42]. ...
... v The extraction of algal oils is an energy-intensive process. However, the return on investment (ROI) can be improved [219,220] by the biorefinery approach [221,222] vi Approaches followed for strain selection/ engineering [223], method(s) of growing, harvesting, biomass pre-treatment, extraction of desired products, biomass hydrolysis, and fermentation of the residual mass is tricky and varies from species to species [224]. vii Depending on the feedstock availability, the price of fuel-grade bioethanol and FAME varies at different locations across the globe. ...
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... The EROI for hydropower is usually given as greater than that for all other energy sources: FF, RE, or nuclear [52,53]. Unlike other RE, future dams are assumed to have a very long operational life, which they need to recover their high input energy costs. ...
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In this research, symmetrical and inexpensive flexible electrodes were fabricated from Ethaline deep eutectic solvent (DES) on a graphite substrate. The growth potentials in DES and cycling performance of Fe- and Ni-based films in KOH were investigated. The modified electrodes had two redox peaks separated from each other due to the difference between the electroactive potential of Fe- and Ni-based reactions. Iron- and nickel-based redox reactions occur on the negative and positive sides, respectively. Furthermore, they do not affect each other when they are active in different potentials. Therefore, only one straightforward fabrication step can be applied to obtain electrodes which can be used as both positive and negative electrodes. The maximum areal capacitance of Fe–Ni alloy electrodes reached a maximum value of 79.6 mF cm−2. Charge–discharge curves and self-discharge performances of the films electrodeposited potentiostatically by the application of different deposition voltages were investigated.Graphical abstract
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