Energy quality is the contrast between different forms of energy, the different trophic levels in ecological systems and the propensity of energy to convert from one form to another. The concept refers to the empirical experience of the characteristics, or qualia, of different energy forms as they flow and transform. It appeals to our common perception of the heat
value, versatility, and environmental performance of different energy
forms and the way a small increment in energy flow can sometimes produce
a large transformation effect on both energy physical state and energy. For example the transition from a solid state to liquid
may only involve a very small addition of energy. Methods of evaluating
energy quality are sometimes concerned with developing a system of
ranking energy qualities in hierarchical order.
Introduction
Since before antiquity there has been deep philosophical, aesthetic and scientific interest in the contrast of quality with quantity. In some respects the history of modern and postmodern thought can be characterized by the phenomenological
approach to these two concepts. A central question has been whether the
many different qualitative aspects of the world can be understood in
terms of rational quantities, or whether the qualitative and
quantitative are irreconcilable: that is, there is no "rational
quality", or quale
ratio. Many scientists and analytic philosophers say they are not, and
therefore consider some qualitative phenomena like, for instance, spirituality, and astrology to be unquantifiable, unanalysable by scientific methods,
and therefore ungrounded in physical reality. The notion of energy
quality therefore has a tendency to be linked with phenomena many
scientists consider unquantifiable, or at least incommunicable, and are
consequently dismissed out of hand.
At the same time many people have also recognised qualitative
differences in the way things can be done by different entities (both
physical and biological). Humans, for example have qualitatively
different capacities than many other mammals, due, in part, to their opposable thumb.
In the attempt to formalise some of the qualitative differences,
entities were grouped according to distinguishing features or
capacities. Different schools of thought used different methods to make
distinctions. Some people chose taxonomic and genome structure, while others chose energetic
function as the basis of classifications. The former are often
associated with biology, while the latter with the trophic food chain
analysis of ecology. These can be considered attempts to formalise
quantitative, scientific studies of the qualitative differences between
entities. The efforts were not isolated to biology and ecology, since
engineers were also interested in quantifying the amount of work that
qualitatively different sources of energy could provide.
Ohta
According to Ohta (1994, pp. 90–91) the ranking and scientific analysis of energy quality was first proposed in 1851 by William Thomson
under the concept of "availability". This concept was continued in
Germany by Z. Rant, who developed it under the title, "die Exergie" (the
exergy). It was later continued and standardised in Japan. Exergy
analysis now forms a common part of many industrial and ecological
energy analyses. For example, I.Dincer and Y.A. Cengel (2001, p. 132)
state that energy forms of different qualities are now commonly dealt
with in steam power engineering
industry. Here the "quality index" is the relation of exergy to the
energy content (Ibid.). However energy engineers were aware that the
notion of heat quality involved the notion of value
– for example A. Thumann wrote, "The essential quality of heat is not
the amount but rather its 'value'" (1984, p. 113) – which brings into
play the question of teleology
and wider, or ecological-scale goal functions. In an ecological context
S.E. Jorgensen and G.Bendoricchio say that exergy is used as a goal
function in ecological models, and expresses energy "with a built-in
measure of quality like energy" (2001, p. 392).
Energy quality evaluation methods
There
appear to be two main kinds of methodology used for the calculation of
energy quality. These can be classed as either receiver or donor
methods. One of the main differences that distinguishes these classes is
the assumption of whether energy quality can be upgraded in an energy
transformation process.
Receiver methods: view energy quality as a measure and
indicator of the relative ease with which energy converts from one form
to another. That is, how much energy is received from a transformation
or transfer process. For example, A. Grubler [1] used two types of indicators of energetic quality pars pro toto: the hydrogen/carbon (H/C) ratio, and its inverse, the carbon intensity
of energy. Grubler used the latter as an indicator of relative
environmental quality. However Ohta says that in multistage industrial
conversion systems, such as a hydrogen production system using solar energy, the energy quality is not upgraded (1994, p. 125).
Donor methods: view energy quality as a measure of the
amount of energy used in an energy transformation, and that goes into
sustaining a product or service (H.T.Odum 1975, p. 3). That is how much
energy is donated to an energy transformation process. These methods are
used in ecological physical chemistry, and ecosystem evaluation. From
this view, in contrast with that outlined by Ohta, energy quality is
upgraded in the multistage trophic conversions of ecological systems.
Here, upgraded energy quality has a greater capacity to feedback and
control lower grades of energy quality. Donor methods attempt to
understand the usefulness of an energetic process by quantifying the extent to which higher quality energy controls lower quality energy.
Energy quality in physical-chemical science (direct energy transformations)
Constant energy form but variable energy flow
T.Ohta suggested that the concept of energy quality may be more intuitive if one considers examples where the form of energy
remains constant but the amount of energy flowing, or transferred is
varied. For instance if we consider only the inertial form of energy,
then the energy quality of a moving body is higher when it moves with a
greater velocity. If we consider only the heat form of energy, then a
higher temperature has higher quality. And if we consider only the light
form of energy then light with higher frequency has greater quality
(Ohta 1994, p. 90). All these differences in energy quality are
therefore easily measured with the appropriate scientific instrument.
Variable energy form, but constant energy flow
The
situation becomes more complex when the form of energy does not remain
constant. In this context Ohta formulated the question of energy quality
in terms of the conversion of energy of one form into another, that is
the transformation of energy. Here, energy quality is defined by the relative ease with which the energy transforms, from form to form.
If energy A is relatively easier to convert to energy B but energy B is relatively harder to convert to energy A, then the quality of energy A is defined as being higher than that of B. The ranking of energy quality is also defined in a similar way. (T.Ohta 1994, p. 90).
Nomenclature: Prior to Ohta's definition above, A.W.Culp
produced an energy conversion table describing the different conversions
from one energy to another. Culp's treatment made use of a subscript to
indicate which energy form is being talked about. Therefore, instead of
writing "energy A", like Ohta above, Culp referred to "Je", to specify electrical form of energy, where" J" refers to "energy", and the "e"subscript
refers to electrical form of energy. Culps notation anticipated
Scienceman's (1997) later maxim that all energy should be specified as
form energy with the appropriate subscript.
Energy quality in biophysical economics (indirect energy transformations)
The notion of energy quality was also recognised in the economic sciences. In the context of biophysical economics
energy quality was measured by the amount of economic output generated
per unit of energy input (C.J. Cleveland et al. 2000). The estimation of
energy quality in an economic context is also associated with embodied energy
methodologies. Another example of the economic relevance of the energy
quality concept is given by Brian Fleay. Fleay says that the "Energy
Profit Ratio (EPR) is one measure of energy quality and a pivotal index
for assessing the economic performance of fuels. Both the direct and
indirect energy inputs embodied in goods and services must be included
in the denominator." (2006; p. 10) Fley calculates the EPR as the energy
output/energy input.
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Ranking energy quality
Energy abundance and relative transformation ease as measure of hierarchical rank and/or hierarchical position
Ohta
sought to order energy form conversions according to their quality and
introduced a hierarchical scale for ranking energy quality based on the
relative ease of energy conversion (see table to right after Ohta,
p. 90). It is evident that Ohta did not analyse all forms of energy. For
example, water is left out of his evaluation. It is important to note
that the ranking of energy quality is not determined solely with
reference to the efficiency of the energy conversion. This is to say
that the evaluation of "relative ease" of an energy conversion is only
partly dependent on transformation efficiency. As Ohta wrote, "the
turbine generator and the electric motor have nearly the same
efficiency, therefore we cannot say which has the higher quality" (1994,
p. 90). Ohta therefore also included, 'abundance in nature' as another
criterion for the determination energy quality rank. For example, Ohta
said that, "the only electrical energy which exists in natural
circumstances is lightning, while many mechanical energies exist."
(Ibid.).
Transformity as an energy measure of hierarchical rank
Like
Ohta, H.T.Odum also sought to order energy form conversions according
to their quality, however his hierarchical scale for ranking was based
on extending ecological system food chain concepts to thermodynamics
rather than simply relative ease of transformation . For H.T.Odum energy
quality rank is based on the amount of energy of one form required to
generate a unit of another energy form. The ratio of one energy form
input to a different energy form output was what H.T.Odum and colleagues
called transformity: "the EMERGY per unit energy in units of emjoules per joule" (H.T.Odum 1988, p. 1135).
