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ECOS 2019 - 32nd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, 23-28 June 2019 | Wroclaw - Poland


University of Zaragoza
Research Centre for Energy Resources and Consumption (CIRCE - Institute)
Zaragoza, Spain

Alicia Valero Delgado is head of the group of industrial ecology and resource efficiency at the Research Centre for Energy Resources and Consumption (CIRCE - Institute) and is lecturer at the University of Zaragoza in the Mechanical Engineering Department. Her more than fifteen years research activity has been focused on the identification of resource efficiency measures and the application of thermodynamics in the evaluation of resource depletion, subject from which she has received four international awards. She is the author or co-author of over 50 publications in international journals and book chapters and over 60 international conference papers, all of which relate to the analysis and optimisation of energy and the use of raw materials. She acts as advisor for the European Commission in the EIP on Raw Materials and participates in international round tables for criticality assessments of raw materials.

Beyond exergy: thermodynamic rarity as a tool for resource efficiency and recyclability assessments

Exergy has been traditionally used to optimize energy systems. Thanks to exergy, we are able to assess in a fair and objective way, any system considering its physical “distance” from the reference environment, meaning that the far removed the system in terms of temperature, pressure, height, chemical composition, etc. from the reference environment, the more exergy it has. In this way, we are able to assign a physical value to resources, considering their quality. This has a very important advantage over other measuring parameters such as mass or price based approaches. Mass analyses, even if universal and objective, have the problem of “mixing apples with oranges”. In turn price, which has not that problem, is volatile and not universal. Moreover, exergy allows for a more rational cost allocation among coproducts in any system and is used as the unit of measure for thermoeconomic analyses to assess costs and identify irreversibilities in industries.

That said, exergy is a property that is still poorly understood by non thermodynamicists. Even industry often relies on mere first law assessments and exergy and thermoeconomic analyses remain on the academic level.

Yet since exergy is capable of providing rigorous and reliable answers to key issues, it is slowly finding its way in not only industrial applications, but also environmental assessments and for policy making. One example is the incorporation of exergy as a measuring unit and allocation procedure in common Life Cycle Assessment software to evaluate the environmental performance of processes.

Exergy has also much to say in raw material assessments and this lecture will deal with that. However, in order to do so, it is important to understand what makes minerals valuable. The chemical composition of minerals is one of the features that make a given raw material valuable, but it is not the only parameter. Minerals are also valuable because they are scarce in the crust. Moreover, they are valuable because they require useful energy to be extracted and refined. Accordingly, to assess the exergy value of minerals, we have defined an exergy-based indicator called thermodynamic rarity. The Thermodynamic Rarity method incorporates two aspects: embodied exergy and exergy replacement costs (ERC). The first evaluates the exergy cost required to mine and beneficiate a given commodity with prevailing technologies, assuming current average concentrations of mineral deposits. The second aspect relates to the fact that having minerals concentrated in ore bodies (and not dispersed throughout the crust) represents a “free bonus” provided by nature, which reduces the otherwise required energy costs of mining. The reduction of this bonus when mines are depleted is quantified as so-called Exergy Replacement Costs (ERC). These are defined as the cumulative exergy consumption that would be needed to re-concentrate a mineral from a completely dispersed state (denoted Thanatia) to the conditions of concentration and composition found in the original mines using prevailing technology. Hence, ERC can be seen as the ultimate future effort that society would need to put into play when all mines become depleted. In contrast to the Szargut approach, the Thermodynamic Rarity method does not include a reference state in the form of reference compounds, but rather uses the composition and the average concentration of the 294 most abundant minerals found in the earth’s crust from which the concentration exergy is calculated.

Minerals can then be ranked according to their intrinsic thermodynamic rarities and with it, raw materials and fossil fuels have now the same order of magnitude (note that otherwise, the exergy of a fossil fuel would be much higher than that of any metal if solely evaluated through chemical exergy). Thanks to this approach, we have been able to assess the thermodynamic criticality of the elements in the periodic table and so give advice to policy makers for protecting the mineral capital. We have been also able to assess the resource efficiency of renewables, the electric vehicle or other electric and electronic equipment and so identify critical parts which might be affected by material supply issues and so advise for improving eco-design. In order to facilitate the use of the thermoeconomic analysis and thermodynamic rarity indicator in metallurgy, we have developed a module in HSC Chemistry (a prominent software used in metallurgy). With this, recyclability and circular economy assessments can be performed to any process or product where not only energy flows, but also chemical substances come into play. This lecture will show the intricacies of the methodology and the whole spectrum of real applications that have been opened up and that are currently under development.