What on earth is exergy?

Preface. This is one of the best explanations of exergy I’ve been able to find.  This paper makes the case that exergy ought to be considered by just about every industry and government to achieve greater energy efficiency, and makes the case that in many ways exergy is a more valuable measure than energy use when combined with mineral depletion.

My favorite example was:

“The need to take the quality of energy into account can be shown with a simple everyday example.  Take an office space and a car battery. The energy contained in the movement of air molecules in a 68 degree 20 cubic meter office is more than the energy stored in three standard 12 volt car batteries. But you can only use the energy in the air to keep yourself warm, while the energy in the batteries will start your car, cook your lunch, and run your computer.  The reason is that even if their quantities are the same, the quality – or usefulness – of the energy in the air and in the battery is different. In the air, the energy is randomly distributed, not readily accessible, and not easily used for anything other than keeping you warm.  But the electric battery energy is concentrated, controllable, and available for all sorts of uses. This difference is taken into account by exergy.”

But you really ought to go to the original source: https://www.scienceeurope.org/wp-content/uploads/2016/06/SE_Exergy_Brochure.pdf since I’ve left out the explanatory charts, graphs, and about a quarter of the information, especially the pages of how exergy should be used in policy-making, which those of you who are trying to slow down or lessen the impact of the Great Simplification might find the most interesting.

Alice Friedemann   www.energyskeptic.com  author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer and “Crunch! Whole Grain Artisan Chips and Crackers”. Podcasts: Practical Prepping, KunstlerCast 253, KunstlerCast278, Peak Prosperity , XX2 report


Brockway, P., et al. 2016. In a resource-constrained world: Think Exergy, not energy. Science Europe.


It’s necessary to measure improved energy and resource efficiencies, but how? Of course, the amount of energy and raw materials that go into making something, or that go into services such as heating, communication, or transport, can be easily measured.  However, that does not consider the quality of the energy nor the rarity of the materials used. In order to account for the quality and not just the quantity of energy, as well as factoring in the raw materials used, we need to measure exergy.

Exergy can be considered to be useful energy, or the ability of energy to do work. Exergy can be measured not only for individual processes, but also for entire industries, and even for whole national economies. It provides a firm basis from which to judge the effect of policy measures taken to improve energy and resource efficiency, and to mitigate the effects of climate change.

Exergy as a Measure of Energy Quality

The need to take the quality of energy into account can be shown with a simple everyday example.  Take an office space and a car battery. The energy contained in the movement of air molecules in a 68 degree 20 cubic meter office is more than the energy stored in three standard 12 volt car batteries. But you can only use the energy in the air to keep yourself warm, while the energy in the batteries will start your car, cook your lunch, and run your computer.  The reason is that even if their quantities are the same, the quality – or usefulness – of the energy in the air and in the battery is different. In the air, the energy is randomly distributed, not readily accessible, and not easily used for anything other than keeping you warm.  But the electric battery energy is concentrated, controllable, and available for all sorts of uses. This difference is taken into account by exergy.

Thermodynamics is the Science of Energy

The concept of exergy is inextricably contained within the basic physical laws governing energy and resources, called thermodynamics.  These laws cannot be ignored: they are fundamental . Two of the basic laws in thermodynamics need to be considered:

First – Energy is conserved.

Second – heat cannot be fully converted into useful energy.  This second law concerns the concept of exergy. Every energy-conversion process destroys exergy. Take for example a conventional fossil fuel power station. Such a station transforms the chemical energy stored in coal to produce steam in a boiler, which is then converted by a turbine into mechanical energy and finally by a generator into electricity. in this process, only 30–35% of the chemical energy contained in the coal is converted into electrical energy; the remaining 65–70% is lost in the form of heat. Exergy analysis of this power generation plant identifies the boiler and turbine as the major sources of exergy loss. In order to improve the exergy efficiency, the boiler and turbine systems need to be altered through technical design and operational changes.

Exergy as a Measure of Resource Quality

Exergy can also be applied in order to take the quality of resources into account. A diluted resource is much more difficult to use than a concentrated one, as it first has to be collected or refined. The measure to take the concentration of a resource into account is its chemical potential (or chemical exergy).  The chemical potential of pure iron is much higher than the chemical potential of an iron ore diluted by other rocks.

An exergy consideration of any process takes into account the chemical potential of the resources used in the process. The problem with chemical potentials, however, is that it is only possible to measure their difference. In order to study the chemical potential of a specific resource, a reference point is needed. An interesting proposal as a reference point for natural minerals is the concept of ‘thanatia’, a hypothetical version of our planet where all mineral deposits have been exploited and their materials have been dispersed throughout the crust. Using thanatia as a model, it is possible to determine the exergy content of the Earth’s resources. By adding up all exergy expenditures, the rarity of resources and their products can be assessed.

Exergy Destruction in the Process Industry

Industry is a large user of both material and energy resources. Typically, an industrial production process needs the input of materials and of energy to transform those materials into products. Much of these inputs end up being discarded: in the case of materials as waste, and in the case of energy as heat. This is exergy destruction, since – recalling the Second Law of thermodynamics – not all inputs can be fully recovered as useful energy.

Methanol, for example, is a primary liquid petrochemical manufactured from natural gas. It is a key component of hundreds of chemicals that are integral parts of our daily lives such as plastics, synthetic fibers, adhesives, insulation, paints, pigments, and dyes. Before methanol production even begins, 10% of the natural gas is used to warm the chemical reactor. Subsequently, during production further reactor losses amount to 50%. This contributes to the exergy destruction footprint of methanol production and of all its products.

How can we Increase the Energy Efficiency of Production?

While exergy destruction for any process is never zero, it can be minimized. Every process has a characteristic exergy-destruction footprint. Knowledge of this footprint can be used to rationalize resource choices before production begins and to monitor the use of energy and resources during production. In a full life-cycle approach, it can be used to consider the total energy and resource ‘cost’ of a product: essentially its exergy-destruction footprint.

An example of a process where reducing exergy destruction can increase energy efficiency is distillation. Distillation is the most commonly applied separation technology in the world, responsible for up to 50% of both capital and operating costs in industrial processes. It is a process used to separate the different substances from a liquid mixture by selective evaporation and condensation. Commercially, distillation has many applications; in the previous example of methanol production, it is used to purify the methanol by removing reaction byproducts from it, such as water. The conventional separation of chemicals by distillation occurs in a column that is heated from below by a boiler, with the desired product (referred to as the condensate) produced from a condenser at the top. The exergy efficiency of this distillation setup is about 30%.

The obvious question is whether the same distillation results can be achieved with a higher exergy efficiency by operating the column differently. The answer to that question is yes, as there are better ways to add heat to the column than by a boiler. The boiler and condenser can be replaced by a series of heat exchangers along the column, producing a more exergy-efficient heating pattern. This arrangement minimizes the exergy destruction in the system, reducing the exergy footprint of the process. In this way, the same product can be obtained with only 60% of the original exergy loss.  This of course requires investment in replacing or retrofitting the technology, but in the long run such costs are compensated by lower operating costs.  Financial benefits aside, the potential impact of technological development driven by exergy analysis on the energy  and material efficiency of industry,  is enormous.

The Exergy Destruction Footprint – Developing More Environmental friendly Technologies

When exergy analysis is performed on a process, the exergy losses can be identified and the exergy-destruction footprint can be minimized. In the fossil fuel industry, single- and two-stage crude oil distillation are used to obtain materials from crude oil for fuels and for chemical feedstocks.

A single-stage system consists of a single heating furnace and a distillation column; a two-stage system adds another furnace (to heat the product of the first unit) and a second column.  Tests have shown that the two stage system has a much higher efficiency – 31.5% versus 14 for a single stage process.  This is because the two stage system can be better controlled than the one-stage system.  Adding more stages gives even better control.

It is important to keep in mind that there is no production without an exergy destruction footprint. 

A Large-scale Problem Needs a Common-scale Solution

In 2013, industry accounted for 25% of the EU’s total final energy consumption, making it the third-largest end-user after buildings and transport. Over 50% of industry’s total final energy consumption is attributed to just three sectors: iron and steel, chemical and pharmaceutical, and petroleum and refineries.

Between 2001 and 2011, EU industry reduced its energy intensity by 19%. However, significant efficiency potential remains. As previous examples of several industrial processes have shown, exergy analysis offers a guide to the development of more energy-efficient technologies and provides an objective basis for the comparison of sustainable alternatives. Energy analysis explains that electric and thermal energy are equivalent according to the First Law of thermodynamics, and that heating by an electric resistance heater can be 100% efficient. Exergy analysis, however, explains that heating by an electric heater wastes useful energy. When we know about this kind of waste, we can start to reduce it by minimizing exergy destruction. While the given examples have focused on industrial processes, exergy analysis can also tackle the energy and resource efficiency of larger consumers of energy, such as the buildings and transport sectors. It is important to highlight that exergy analysis can be used not only to quantify the historical resource use, efficiency and environmental performance, but also to explore future transport pathways, building structures and industrial processes.

As explained in the Opinion Paper “A common Scale for Our common Future: Exergy, a thermodynamic metric for Energy”, a major roadblock for implementing – or even finding – solutions to our societal challenges is the fact that energy and resource efficiency are commonly defined in economic, environmental, physical, and even political terms. Exergy is the resource of value, and considering it as such requires a cultural shift to the thermodynamic-metric approach of energy analysis. Exergy provides an apolitical scale to guide our judgement on the road to sustainability. Exergy is first step to a common-scale solution  to our large-scale problems.


Energy Efficiency as a Key Climate Policy: the Need to Measure Progress with Exergy

Improving the efficiency of energy use and transitioning to renewable energy are the two main climate policies aimed at meeting global carbon-reduction targets. The 2009 renewable Energy Directive mandates that 20% of energy consumed in the EU should be renewable by 2020.  At the same time, the EU’s 2012 Energy Efficiency Directive sets a 20% reduction target for energy use. Progress towards the renewable-energy target is straightforward to measure, since national energy use by renewable sources is collected and readily available. Indeed, for many citizens, the proportion of domestic electrical energy generated from renewable sources appears clearly defined on their electricity bills. In contrast, national-scale energy efficiency remains unclear and a qualitative comparison of renewable sources is lacking. A central problem is that there is no single, universal definition of national energy efficiency. In this void, a wide range of metrics is inconsistently adopted, based on economic activity, physical intensity or hybrid economic– physical indicators.

None of these methods are based on thermodynamics, however, making them inherently incapable of measuring energy efficiency in a meaningful way. As such, they are unable to contribute to evidence-based policy making or to measure progress towards energy efficiency targets. The EU is not alone, there is currently no national-scale thermodynamic based reporting of energy efficiency by any country in the world. Second-law thermodynamic efficiency – in other words, exergy efficiency – stands alone in offering a common scale for national, economy-wide energy efficiency measurement, applicable at all scales and across all sectors.


From Gaia to Thanatia: How to Assess the Loss of Natural Resources

As technology today uses an increasing number of elements from the periodic table, the demand for raw materials profoundly impacts on the mining sector. As ever lower grades of ore are being extracted from the earth, the use of energy, water and waste rock per unit of extracted material increases, resulting in greater environmental and social impact. Globally, the metal sector requires about 10% of the total primary energy consumption, mostly provided by fossil fuels. By 2050, the demand for many minerals, including gold, silver, indium, nickel, tin, copper, zinc, lead, and antimony, is predicted to be greater than their current reserves. Regrettably, many rare elements are profusely used, with limited recycling.

The loss of natural resources cannot be expressed in money, which is a volatile unit of measurement that is too far removed from the objective reality of physical loss. Neither can it be expressed in tonnage or energy alone, as these do not capture quality and value. Exergy can solve such shortcomings and be applied to resource consumption through the idea of ‘exergy cost’: the embodied exergy of any material, which takes the concentration of resources into account measured with reference to the ‘dead state’ of thanatia.

Thanatia – from the greek  personification of  Death – is a hypothetical dead state of the anthroposphere, conceiving an ultimate landfill where all mineral resources are irreversibly lost and dispersed, or in other words, at an evenly distributed crustal composition. If our society is squandering the natural resources that the Sun and geological evolution of the Earth have stored, we are converting their chemical exergy into a degraded environment that progressively becomes less able to support usual economic activities and eventually will fail to sustain life itself. The end state would be thanatia, a possible end to the ‘anthropocene’ period. It does not represent the end of life on our planet, but it does imply that mineral resources are no longer available in a concentrated form.

An Essential approach to making better use of our mineral resources: the application of mineral exergy rarity

The exergy of a mineral resource as calculated with thanatia as a reference can be measured as the minimum energy that could be used to extract that resource from bare rocks, instead of from its current mineral deposit. This is an essential approach, since the European commission’s communication ‘towards a circular Economy: a Zero Waste Programme for Europe’, states that “valuable materials are leaking from our economies” and that “pressure on resources is causing greater environmental degradation and fragility, Europe can benefit economically and environmentally from making better use of those resources.” Applied to minerals we can define a ‘mineral Exergy rarity’ (in kWh) as “the amount of exergy resources needed to obtain a mineral commodity from bare  rocks, using prevailing technologies”. Tthe ‘exergy rarity’ concept is thus able to quantify the rate of mineral capital depletion, taking a completely resource exhausted planet as a reference. This rarity assessment allows for a complete vision of mineral resources via a cradle-to-grave analysis. Exergy rarity is, in fact, a measure of the exergy-destruction footprint of a mineral, taking thanatia as a reference.

Given a certain state of technology, the exergy rarity is an identifying property of any commodity incorporating metals. Hence, exergy rarity (in kWh/kg) may be assessed for all mineral resources and artefacts thereof, from raw materials and chemical substances to electric and electronic appliances, renewable energies, and new materials. Especially those made with critical raw materials, whose recycling and recovery technologies should further enhance. Such thinking is a step towards “a better preservation of the Earth’s resources endowment and the use of the Laws of Thermodynamics for the assessment of energy and material resources as well as the planet’s dissipation of useful energy”. More than ever, the issue of dwindling resources needs an integrated global approach. Issues such as assessing exhaustion, dispersal, or scarcity are absent from economic considerations. An annual exergy content account of not only production, but of the depletion and dispersion of raw materials would enable a sound management of our material resources. Unfortunately, similar to the problem of inconsistent national energy-efficiency measurement, there is also a lack of consistency in natural-resource assessment, which is necessary for effective policy making.

It is time to charge for exergy use rather than for energy use. in the future, consumers should be informed about products and services in terms of their exergy content and destruction footprints in much the same way as they are about carbon emissions, and pay the price accordingly. that gives a scientific basis for charging for loss of valuable resources.

The energy and exergy used in production, operation and destruction must be paid back during life time in order to be sustainable. LCEA shows that solar thermal plants have much longer exergy payback time than energy payback time, 15.4 and 3.5 years respectively. Energy based analysis may lead to false assumptions in the evaluation of the sustainability of renewable energy systems.


  1. Science Europe Scientific committee for the Physical, chemical and mathematical Sciences, “a common Scale for Our common Future: Exergy, a thermodynamic metric for Energy, http://scieur.org/op-exergy
  2. a. valero capilla and a. valero Delgado, “thanatia: the Destiny of the Earth’s mineral resources, a thermodynamic cradle-to-cradle assessment”, World Scientific Publishing: Singapore, 2014.
  3. S. Kjelstrup, D. bedeaux, E. Johannessen, J. gross, “non-Equilibrium thermodynamics for Engineers”, World Scientific, 2010, see chapter 10 and references therein.
  4. h. al-muslim, i. Dincer and S.m. Zubair, “Exergy analysis of Single- and two-Stage crude Oil Distillation units”, Journal of Energy resources technology 125(3), 199–207, 2003. 5. SEt-Plan Secretariat, SEt-Plan actiOn n°6, DraFt iSSuES PaPEr, “continue efforts to make Eu industry less energy intensive and more competitive”, 25/01/2016,    https://setis.ec.europa.eu/system/files/issues_paper_action6_ee_industry.pdf
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  12. Federal nonnuclear energy research and development act of 1974,Public Law 93–577, http://legcounsel.house.gov/comps/Federal%20nonnuclear%20 Energy%20research%20and%20Development%20act%20Of%201974.pdf
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2 Responses to What on earth is exergy?

  1. Jeff W. says:

    An interesting paper. It’s curious that there is no mention of enthalpy nor entropy; I guess the authors consider those concepts too confusing, hence the effort to promote “exergy” as a measure that basically incorporates the latter so as to create an easier to grasp “apples to apples” means of comparing energy conversion methodologies.

  2. steve c says:

    While it is harder to understand and quantify, I think emergy as studied by H.T. Odum is also very useful at taking the whole impact of our economy in to account.