[R-G] [BillTottenWeblog] The Conservation Imperative

[Bill Totten]

Energy Limits to Growth and the Path to Sustainability

by Richard Heinberg

MuseLetter 201 (January 2009)

Post Carbon Institute (January 09 2009)

Dear readers: This issue of MuseLetter is Part I of a brief series of
articles about energy alternatives, net energy, and the best options for
an energy transition. Later this spring the articles will be published
as a booklet by Post Carbon Institute and the International Forum on
Globalization.

You are no doubt keenly aware that the economy is and will be the matter
of overwhelming concern for the coming year (or years), for policy
makers as well as for most nearly every individual and family. Following
this current project, my research and writing will be devoted to
identifying strategies that can help families and communities adapt to
the new economic conditions while laying the groundwork for a truly
sustainable society.

Executive summary

This report is intended as a non-technical overview of the prospects for
known energy sources to supply society's energy needs at least up to the
year 2100. It serves as a layperson's introduction to the concept of net
energy, or energy returned on energy invested (EROEI). It also discusses
energy transition scenarios, showing why many that have been published
up to this time are overly optimistic because they do not address all of
the relevant limiting factors to the expansion of alternative energy
sources. Finally, it shows why energy conservation (using less) and
humane, gradual population reduction must be key strategies to achieving
sustainability.

Overview

The world's current energy regime is unsustainable. This is the explicit
conclusion of the International Energy Agency, and it is also the
substance of a wide and growing public consensus ranging across the
political spectrum. One broad segment of this consensus is concerned
more about the climate impacts of society's reliance on fossil fuels;
the other is moved more by questions regarding the security of future
supplies of these fuels - which, as they deplete, are increasingly
concentrated in only a few countries.

To say that our current energy regime is unsustainable means that it
cannot continue and must therefore be replaced with something else.
However, replacing the energy infrastructure of modern industrial
societies is no trivial matter. Decades have been spent building the
current oil-coal-gas infrastructure, and trillions of dollars invested.
Moreover, if the transition from current energy sources to alternatives
is wrongly managed, consequences could be severe: there is an undeniable
connection between per-capita levels of energy consumption and economic
well-being (see Robert Ayres and Benjamin Warr, Two Paradigms of
Production and Growth). A failure to supply sufficient energy, or energy
of sufficient quality, could undermine our global economic future.

It is a commonly held assumption that alternative energy sources capable
of substituting for conventional fossil fuels are readily available,
whether fossil (tar sands or oil shale), nuclear, or renewable. All that
is necessary is to invest sufficiently in them, and life will go on
essentially as it is.

But is this really the case? Energy sources have varying
characteristics. And it is the characteristics of our present energy
sources (principally oil, coal, and natural gas) that have enabled the
creation of a society with high mobility, large population, and high
economic growth rates. Will alternative energy sources perpetuate this
kind of society?

While it is possible to point to innumerable successful alternative
energy production installations within modern societies (ranging from
small home-scale photovoltaic systems to large "farms" of three-megawatt
wind turbines), it is not possible to point to the example of an entire
modern industrial society obtaining the bulk of its energy from sources
other than oil, coal, and natural gas. The energy transition is still
more theory than reality.

But if current primary energy sources are unsustainable, this implies a
daunting problem. The transition to alternative sources must occur, or
the world will lack sufficient energy to maintain basic services.

Thus it is vitally important that energy alternatives be evaluated
thoroughly according to relevant criteria, and that a staged plan be
formulated and funded for a systemic societal transition away from oil,
coal, and natural gas and toward the alternative energy sources deemed
most fully capable of supplying economic benefits similar to those of
conventional fossil fuels.

Many readers will probably assume that this has already been done
adequately. After all, it is possible to assemble a bookshelf (as this
author has done) filled with reports from nonprofit environmental
organizations and books from energy analysts, dating from the early
1970s to the present, all attempting to illuminate alternative energy
pathways for the United States and the world as a whole. These plans and
proposals vary in breadth and quality, but especially in their success
at identifying limiting factors that could prevent specific alternative
energy sources from adequately replacing conventional fossil fuels.

A limiting factor that is most frequently omitted from energy transition
plans is net energy, or energy return on energy invested (EROEI). One
reason for its omission is, as we shall see in more detail below, that
it suffers from lack of standard measurement practices. Nevertheless,
for the purposes of large-scale and long-range planning, net energy may
be the single most important criterion for evaluating energy sources.

This report is not intended to serve as an authoritative analysis of
available energy options, nor as a comprehensive plan for a nation-wide
or global transition from fossil fuels to alternatives. While such
analyses and plans are needed, they will require institutional resources
and ongoing re-assessment to be of value. The goal here is simply to
identify the primary criteria that should be used in such analyses and
plans, with special emphasis on net energy, and to offer a cursory look
at some currently available data on alternative energy sources, so as to
provide a general, preliminary sense of whether such alternative sources
are up to the job of replacing fossil fuels - and if not, what should be
the fall-back plan of governments and the other responsible institutions
of modern society.

As we will see, this preliminary survey yields the disturbing conclusion
that all alternative energy sources are subject to limits of one kind or
another, and that there is no clear scenario in which the energy from
conventional fossil fuels can be replaced with energy from alternative
sources without (1) enormous investment, (2) significant time for
build-out, and (3) significant sacrifices in terms of energy quality and
reliability.

Thus there is a strong likelihood that neither conventional fossil fuels
nor alternative energy sources can reliably be counted on to provide the
amount and quality of energy that will be needed to sustain economic
growth - or even current levels of economic activity - during the
remainder of the current century.

This preliminary conclusion in turn suggests that a sensible transition
energy plan will have to emphasize energy conservation above all. It
also raises questions about the sustainability of growth per se, both in
terms of human population numbers and economic activity.

Limiting Factors: Energy Evaluation Criteria

In evaluating energy sources, it is essential first to give attention to
the criteria being used. Some criteria merely give us information about
an energy source's usefulness for specific applications: for example, an
energy source (like oil shale) that is a solid and has low energy
density per unit of weight and volume is unlikely to be a good transport
fuel unless it can first somehow profitably be turned into a liquid fuel
with higher energy density. Other criteria offer essential information
about the suitability of an energy source for powering large segments of
an entire society: micro-hydro power, for example, can be
environmentally benign, but simply cannot be scaled up to provide a
significant portion of the national energy budget of the US or other
industrial countries.

In general, society will be better off with energy sources that have
high economic utility, that are capable of being scaled up to produce
large quantities of energy, and that have minimal environmental impacts.

Economic utility and scalability are determined by, among other things,
energy density, the nature and quantity of other resources needed in
order to employ the energy source in question, and the size of the
resource base. Economist Douglas Reynolds, in a paper discussing the
energy density of energy sources (which he terms "energy grade"), writes:

Higher-grade energy resources have more potential for being productive
than lower grade energy resources. Energy is the driving force behind
industrial production and is indeed the driving force behind any
economic activity. However, if an economy's available energy resources
have low grades, that is, low potential productivity, then new
technology will not be able to stimulate economic growth as much. On the
other hand, high-grade energy resources could magnify the effect of
technology and create tremendous economic growth. High-grade resources
can act as magnifiers of technology, but low grade resources can dampen
the forcefulness of new technology. This leads to the conclusion that it
is important to emphasize the role of the inherent nature of resources
in economic growth more fully. (Should EROEI be the most important
criterion our society uses to decide how it meets its energy needs?)

But economic utility is not the only test an energy source must meet. If
there is anything to be learned from the ongoing and worsening climate
crisis, it is that the environmental impacts of energy sources must be
taken very seriously indeed. The world cannot afford to replace oil,
coal, and gas with other energy sources capable of posing a survival
challenge to future generations.

Here then, are some primary energy evaluation criteria. The first three
together define energy density.

Weight density refers to the amount of energy that can be derived from a
standard weight unit of an energy resource. For example, if we use the
British Thermal Unit (Btu) as a measure of energy and the pound as a
measure of weight, coal has about twelve thousand Btu per pound, natural
gas about ten thousand Btu per pound, and oil almost twenty thousand Btu
per pound. However, an electric battery typically is able to store and
deliver only about 100 Btu per pound, and this is why electric batteries
are problematic in transport applications: they are very heavy in
relation to their energy output. Thus electric cars tend to have limited
driving ranges and electric aircraft (which are quite rare) are able to
carry only one or two people.

Consumers and producers are willing to pay a premium for energy
resources with a higher energy density by weight; therefore it makes
economic sense in some instances to convert a lower-density fuel such as
coal into a higher-density fuel such as synthetic diesel, even though
the conversion process entails both monetary and energy costs.

Volume density refers to the amount of energy that can be derived from a
given volume unit of an energy resource (eg, Btu per cubic foot).
Obviously, gaseous fuels will tend to have lower volumetric energy
density than solid or liquid fuels. Natural gas has about one thousand
Btu per cubic foot at sea level atmospheric pressure, and 177 thousand
Btu per cubic foot at 3000 pounds per square inch. Oil, though, can
deliver about one million Btu per cubic foot.

In most instances weight density is more important than volume density;
however, for certain applications the latter can be decisive. For
example, fueling airliners with hydrogen, which is a highly diffuse gas
at common temperatures and surface atmospheric pressure, would require
very large tanks; indeed, this would be true even if the hydrogen were
super-cooled and highly pressurized.

The greater ease of transporting a fuel of higher volume density is
reflected in the fact that oil moved by tanker is traded globally in
large quantities, while the global tanker trade in natural gas is
relatively small. Consumers and producers are willing to pay a premium
for energy resources of higher volumetric density.

Area density expresses how much energy can be obtained from a given land
area (eg, an acre) when the energy resource is in its original state.
For example, the area energy density of wood as it grows in a forest is
roughly one to five billion Btu per acre. The area grade for oil is
usually tens or hundreds of billions of Btu per acre where it occurs,
though oilfields are much rarer than forests (except perhaps in Saudi
Arabia).

Area energy density matters because energy sources that are already
highly concentrated in their original form generally require less
investment and effort to be put to use. Reynolds makes the point:

If the energy content of the resource is spread out, then it costs more
to obtain the energy, because a firm has to use highly mobile extraction
capital [machinery], which must be smaller and so cannot enjoy
increasing returns to scale. If the energy is concentrated, then it
costs less to obtain because a firm can use larger-scale immobile
capital that can capture increasing returns to scale.

Thus energy producers will be willing to pay an extra premium for energy
resources that have high area density - such as oil that will be refined
into gasoline - over ones that are more widely dispersed - such as corn
that is meant to be made into ethanol.

Other resources needed: A very few energy sources come in an immediately
useable form; for example, without exerting effort or employing any
technology we can be warmed by the sunlight that falls on our shoulders
on a spring day. But most energy sources, in order to be useful, require
some method of gathering or converting the energy. That usually entails
some kind of apparatus, made of some kind of material (for example,
oil-drilling equipment is made from steel and diamonds); and sometimes
the extraction or conversion process uses some resource (for example,
the production of ethanol from corn requires land, and the production of
synthetic diesel fuel from tar sands requires water and natural gas).
The amount or scarcity of the material or resource, and the complexity
and cost of the apparatus, thus constitute limiting factors on energy
production.

The requirements for ancillary resources in order to produce a given
quantity of energy are largely reflected in the price paid for the
energy. But this is not always the case. For example, thin-film
photovoltaic panels use materials (such as gallium and indium) that are
non-renewable, rare, and depleting quickly. While the price of thin-film
PV panels reflects and includes the current market price of these exotic
materials, it does not give indication of future limits to the scaling
up of thin-film PV.

Environmental impacts: Virtually all energy sources entail environmental
impacts, but some have greater impacts than others. These may occur
during the acquisition of the resource (in mining coal, for example), or
during the release of energy from the resource (as in burning wood,
coal, oil, or natural gas), or in the conversion of energy from one form
to another (as in converting the kinetic energy of flowing water into
electricity via a dam and hydro-turbines).

Some environmental impacts are indirect, and occur in the manufacture of
the equipment used in energy harvesting or conversion. For example, the
extraction and manipulation of resources used in manufacturing wind
turbines or solar panels may entail significantly more environmental
damage than the operation of the turbines or panels themselves.

Renewability. If we wish our society to continue using energy at
industrial rates of flow not just for years or even decades into the
future, but for centuries, then we will require energy sources that can
be sustained more or less indefinitely. Energy resources like oil,
natural gas, and coal are clearly non-renewable because the time
required to form them through natural processes is measured in the tens
of millions of years, while quantities available will power society
reliably for only a few decades into the future at current rates of use.
In contrast, solar photovoltaic and solar thermal energy sources rely on
sunlight, which for practical purposes is not depleting and will
presumably be available in equal quantities a thousand years hence.

It is important to note, however, that the equipment used to capture
solar or wind energy is not itself renewable, and that both depleting
raw materials and non-trivial amounts of energy are required to
manufacture such equipment.

Some energy sources are renewable yet are still capable of being
depleted. For example, wood can be harvested from forests that
regenerate themselves; however, the rate of harvest is crucial: if
over-harvested, the trees will be unable to re-grow quickly enough and
the forest will shrink and disappear.

Even energy sources that are renewable and that do not suffer depletion
are nevertheless limited by the size of the resource base (as will be
discussed next).

Potential size or scale of contribution. Estimating the potential
contribution of an energy source is obviously essential for
macro-planning purposes, but such estimates are always subject to error
- which can sometimes be enormous. With fossil fuels, amounts that can
be reasonably expected to be extracted and used on the basis of current
extraction technologies and fuel prices are classified as reserves,
which are always a mere fraction of resources (defined as the total
amount of the substance present in the ground). For example, the US
Geological Survey's first estimate of national coal reserves, completed
in 1907, identified 5000 years' worth of supplies. In the decades since,
most of those reserves have been reclassified as resources, so that
today only 250 years' worth of US coal supplies are officially estimated
to exist - a figure that may still be much too optimistic. Reserves are
downgraded to resources when new limiting factors are taken into
account, such as (in the case of coal) seam thickness and depth,
chemical impurities, and location of the deposit.

On the other hand, reserves can sometimes grow as a result of the
development of new extraction technologies, as has occurred in recent
years with US natural gas supplies. While the production of conventional
American natural gas is declining, new underground fracturing
technologies have enabled the recovery of gas from low-porosity rock,
significantly increasing the national production rate and expanding US
gas reserves.

Reserves estimation is especially difficult when dealing with energy
resources that have little or no extraction history. This is the case,
for example, with methane hydrates, with regard to which various experts
have issued a very wide range of estimates of both total resources and
extractable future supplies; it is also true of oil shale, and to a
lesser degree tar sands, which have limited extraction histories.

Estimating potential supplies of renewable resources such as solar and
wind power is likewise problematic, as many limiting factors are often
initially overlooked. With regard to solar power, for example, a cursory
examination of the ultimate resource is highly encouraging: the total
amount of energy absorbed by Earth's atmosphere, oceans, and land masses
from sunlight annually is approximately 3,850,000 exajoules (EJ) -
whereas the world's human population uses currently only about 428 EJ of
energy per year from all sources combined, an insignificant fraction of
the previous figure. However, the factors limiting the amount of
sunlight that can potentially be put to work for humanity are numerous,
as we will see in more detail below.

Consider the case of methane harvested from municipal landfills. In this
instance, using the resource provides an environmental benefit: methane
is a more powerful greenhouse gas than carbon dioxide, so harvesting and
burning landfill gas (rather than letting it diffuse into the
atmosphere) reduces climate impacts while also providing a local source
of energy. If landfill gas could power the US electrical grid, then the
nation could cease mining and burning coal. However, the potential size
of the landfill gas resource is woefully insufficient to support this.
Currently the nation derives about 400 trillion Btu per year from
landfill gas for commercial, industrial, and electric utility uses. This
figure could probably be quadrupled if more landfills were tapped. But
US electricity consumers use over twenty-five times as much energy as
that. There is another wrinkle: if society were to become more
environmentally sensitive and energy efficient, the result would be that
the amount of trash going into landfills would decline - but this would
reduce the amount of energy that could be harvested from future landfills.

Location of the resource. The fossil fuel industry has long faced the
problem of "stranded gas" - natural gas reservoirs that exist far from
pipelines and that are too small to justify building pipelines to access
them. Renewable resources often face similar hurdles.

The location of solar and wind installations is largely dictated by the
availability of the primary energy source; often, this is in sparsely
populated areas. For example, in the US there is large potential for the
development of wind resources in Montana and North and South Dakota.
However, these are some of the least-populous states in the nation.
There are also good wind resources offshore along the Atlantic and
Pacific coasts, nearer to large urban centers, but taking advantage of
these resources will entail overcoming challenges having to do with
building and operating turbines in deep water and connecting them to the
grid onshore. Similarly, the nation's best solar resources are located
in the Southwest, far from population centers in the Northeast.

Thus taking advantage of these energy resources will require more than
merely the construction of wind turbines and solar panels: much of the
US electricity grid will need to be reconfigured, and large-capacity,
long-distance transmission lines will need to be constructed.

Reliability. Some energy sources are continuous: coal can be fed into a
boiler at any desired rate, as long as the coal is available. But some
energy sources, such as wind and solar, are subject to rapid and
unpredictable fluctuations. Wind often blows at greatest intensity at
night, when electricity demand is lowest; the sun shines for the fewest
hours per day during the winter - but consumers are unwilling to curtain
electricity usage during winter months, and power system operators are
required to assure security of supply throughout the day and year.

Intermittency of energy supply can be managed to a certain extent
through storage systems - in effect, batteries. However, this implies
extra infrastructure costs as well as energy losses. It also places
higher demands on control technology. In the worst instance, it means
building electricity generation capacity much larger than would
otherwise be needed. (See: Wind: intermittent Power: continuous)

Transportability of energy is largely determined by the weight and
volume density of the energy source, as discussed above. But it is also
affected by the state of the material - whether it is a solid, liquid,
or gas. In general, a solid fuel is less convenient to transport than a
gaseous fuel, because the latter can move by pipeline. Liquids are the
most convenient of all because they take up less space than gases.

Some energy sources cannot be classified as solid, liquid, or gas. The
energy from sunlight or wind cannot be directly transported; it must
first be converted into a form that can - such as hydrogen or electricity.

Electricity is highly transportable, as it moves through wires, enabling
it to be delivered not only to nearly every building in industrial
nations, but to many locations within each building.

Transporting energy always entails costs - whether it is the cost of
hauling coal (which may account for over seventy percent of the
delivered price of the fuel), the cost of building and maintaining
pipelines and pumping oil or gas, or the cost of building and
maintaining an electricity grid. These costs can be expressed in
monetary terms or in energy terms.

The energy costs of transporting energy affect net energy - which we
will discuss next in a separate section because it is such an important
aspect of the overall discussion, and because it will be a principal
focus of this report.

Net Energy (EROEI)

Energy must be invested in order to obtain energy, regardless of the
nature of the energy resource or the technology used to obtain it, and
society relies on the net energy gained from energy-harvesting efforts
to operate all of its manufacturing, distribution, and maintenance systems.

If the net energy produced is a large fraction of total energy produced,
this means that a relatively small portion of societal effort must be
dedicated to energy production, and most of society's efforts can be
directed toward other purposes. This is the situation we have become
accustomed to as the result of having access to cheap, abundant fossil
fuels.

If the net energy produced is a small fraction of total energy produced,
this means that a relatively large portion of societal effort must be
dedicated to energy production, and only a small portion of society's
efforts can be directed toward other goals. For example, in a society
where energy is acquired principally through agriculture - which yields
a low and variable energy profit - most of the population must be
involved in farming in order to provide enough energy to fund the
maintenance of a small hierarchy of full-time managers, merchants,
soldiers, et cetera, who make up the rest of the societal pyramid.

In the early decades of the fossil fuel era, the quantity of both total
and net energy liberated by efforts to mine and drill for these fuels
was unprecedented, and it was this abundance of cheap energy that
enabled the growth of industrialization, urbanization, and globalization
during the past two centuries. It took only a trivial amount of effort
in exploration and drilling to obtain an enormous energy return on
energy invested (EROEI). But the energy industry understandably followed
the best-first or "low-hanging fruit" policy of exploration and
extraction. Thus the coal, oil, and gas that were highest in quality and
easiest to access tended to be found and extracted early on, and so with
every passing decade the net energy (as compared to total energy)
derived from fossil fuel extraction has declined. In the early days of
the US oil industry, for example, a 100-to-one net energy profit was
common, while it is estimated that current US exploration efforts are
approaching an averaged one-to-one (break-even) energy payback. {Hall
and Gagnon}

In addition, as we will see in some detail later in this report,
alternatives to conventional fossil fuels generally have a much lower
EROEI than coal, oil, or gas did in their respective heydays. For
example, industrial ethanol production from corn is estimated to have at
best a 1.5-to-one positive net energy balance; it is therefore nearly
useless as a primary energy source.

If the net energy available to society declines, more of society's
resources will have to be devoted directly to obtaining energy, and less
will be available for all of the activities that energy makes possible.
Thus increasing constraints will be felt on economic growth, and also
upon the adaptive strategies (which require new investment - for
example: the building of more public transport infrastructure) that
society would otherwise deploy to deal with energy shortages. The
immediately noticeable symptoms will include rising costs of bare
necessities and a reduction in job opportunities in fields not
associated with basic production.

Net energy can be thought of in terms of the number of people in society
engaged in energy production. If energy returned exactly equals energy
invested (EROEI = 1), then everyone is involved in energy production and
no one is available to take care of society's other needs. If EROEI =
100, then one person is involved in energy production and 99 can do
other things - build houses, teach, take care of the sick, cook, write
advertising copy, and so on. If there are two energy workers and 98
people doing other things, then EROEI = 50; and similarly with four
people obtaining energy and 96 doing other things, EROEI = 25. With
eight getting energy and 92 doing other things (EROEI = 12.5) there may
begin to be problems finding enough workers who are trained at getting
energy while others build the tools and infrastructure (drilling rigs or
assembly lines for making solar panels) that enable these energy workers
to do their jobs. With sixteen getting energy and 84 doing other things
(EROEI = 6.25), serious problems may become apparent, since 84 people
may not be enough to provide for all of the needs of the sixteen, given
that half of the larger group may consist of children, the elderly, and
disabled persons. With sixteen energy workers and 42 others providing
everything else, an industrial mode of societal organization may not be
viable.

Archaeologist Lynn White estimated that hunter-gatherer societies
operated on a ten-to-one net energy basis (EROEI = 10). Since
hunter-gatherer societies are the simplest human groups in terms of
technology and degree of social organization, ten should probably be
regarded as the minimum sustained average societal EROEI required for
the maintenance of human existence (though groups of humans have no
doubt survived for occasional periods, up to several years in duration,
of lower EROEI). Since industrial society entails much greater levels of
complexity, its minimum EROEI must be substantially higher.

However, in this report we will not be discussing the EROEI of society
as a whole, but of individual energy sources.

Both renewable and non-renewable sources of energy are subject to the
net energy principle. Fossil fuels become useless as energy sources when
the energy required to extract them equals or exceeds the energy that
can be derived from burning them. This fact puts a physical limit to the
portion of resources of coal, oil, or gas that should be categorized as
reserves, since net energy will peak and decline to the break-even point
long before otherwise extractable fossil energy reserves are depleted.

Therefore the need for society to find replacements for fossil fuels may
be more urgent than is generally recognized. Even though large amounts
of fossil fuels remain to be extracted, the transition to alternative
energy sources must be negotiated while there is still sufficient net
energy available to continue powering society while at the same time
providing energy for the transition process itself.

Because this report is a layperson's guide, we cannot address in any
depth the technical process of calculating net energy. However, it is
important to note that the process is complex and is subject to ongoing
controversy. Most of this controversy centers on system boundaries: what
should be counted as an energy cost for a specific instance of energy
production? For example, should we count the energy expended in the
manufacturing of shoes worn by the workers on an oilrig?

The use of net energy or EROEI as a criterion for evaluating energy
sources has been criticized on several counts. As just mentioned, there
is difficulty in establishing system boundaries that are agreeable to
all interested parties, and that can be easily translated from analyzing
one energy source to another. Moreover, the EROEI of some energy sources
(such as wind, solar, and geothermal) may vary greatly according to
location. Advances in technology can also affect net energy. All of
these factors make it difficult to calculate figures that can reliably
be used in energy planning.

This difficulty only increases as the examination of energy production
processes becomes more detailed. Does the office staff of a drilling
company actually need to drive to the office to produce oil? Is the
energy spent filing tax returns actually necessary to the manufacture of
solar panels?

Yet despite challenges in precisely accounting for the energy used in
order to produce energy, net energy acts as an absolute constraint in
human society, regardless of whether we ignore it or pay close attention
to it. EROEI will determine if an energy source is able successfully to
support a society of a certain size and level of complexity. In
situations where EROEI can be determined to be low, even though there is
dispute as to the exact figure, we can conclude that the energy source
in question cannot be relied upon as a primary source.

Many criticisms of net energy analysis boil down to an insistence that
other factors that limit the efficacy of energy sources should also be
considered. EROEI does not account for limits to non-energy inputs in
energy production (inputs such as water, soil, or the minerals and
metals needed to produce equipment); it does not account for undesirable
non-energy outputs of the energy production process - most notably,
greenhouse gases; it does not account for energy quality (the fact, for
example, that electricity is an inherently more versatile and useful
energy medium than the muscle power of horses); and it does not reflect
the scalability of the energy source (recall the example of landfill gas
above).

However, just because net energy is not the only important criterion for
assessing a potential energy source, this is no reason to ignore it.
EROEI is a necessary - though not a sufficient - basis for evaluating
energy sources. It is one of five criteria that we should regard as
having make-or-break status (the others, discussed above, are
renewability, environmental impact, size of the resource, and the need
for ancillary materials). If a potential energy source cannot score well
with all of these criteria, it cannot realistically be considered as a
future primary energy source. Stated the other way around, a potential
energy source can be disqualified by doing very poorly with regard to
just one of these five criteria.

It should be noted, however, that an energy source with a low or
negative EROEI can still be useful as a medium or carrier to make other
energy sources easier to use. In an energy system with many source
inputs, common energy carriers are extremely helpful. Electricity serves
this function well in our current energy system: it would be difficult
for consumers to make practical use of coal, nuclear, and hydropower
without it. But convenient negative-EROEI energy carriers need to be
connected to high-EROEI energy sources - otherwise the system cannot
function.

In the following discussions of specific energy sources, data on EROEI
are drawn from the work of Dr Charles Hall, who, working with his
students at the State University of New York in Syracuse, has for many
years been at the forefront of developing and applying the methodology
for calculating net energy.

Following this consideration of known energy sources case-by-case, we
will explore the prospects for combining non-fossil sources into a
workable future energy system.

(c) 2004, 2005, 2006, 2007, 2008 Post Carbon Institute

Post Carbon Institute is a 501(c)3 non-profit organization incorporated
in the United States.

http://globalpublicmedia.com/museletter_201

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