About Exergy, Fruits and Coffee

As a person grown up in a little village of Spain, I used to go to the orchard with my family to harvest our fresh fruit and vegetables. Home-grown tomatoes, apples, watermelons, potatoes… I think that summer is my favourite season just because of that! However, our neighbour used to get more cherries and peaches than us, and it left a bad taste in my mouth, even though all together we had harvested much more. It was because I love cherries and peaches, so, in my opinion, they were more valuable than other fruits. So comparing both crops by their amount was not fair enough to me. Maybe my teacher at school was right when she said that we could not add up apples and oranges.

A similar problem appears when you are assessing industrial processes. You can find different types of flows like raw materials, heat, electricity, water, etc. They are measured with different units and they cannot be compared only by their quantity. For instance, 1 MW of heat and of electricity has the same amount of energy, however, everybody prefers electricity. Why? Because it is not a matter of the amount of mass or energy, but of the quality of that mass or energy, in other terms, their exergy content.

Exergy is a thermodynamic property that measures the maximum useful work that can be obtained from a system when it comes into equilibrium with its environment. Furthermore, exergy indicates the minimum work needed to “build” the system from the environment. As exergy includes the second law of thermodynamics into the analysis, the quality of the different flows of a system can be assessed.

Sometimes, the second law can be so depressing, because it states that energy can only be transformed by the consumption of quality, in other words, every process you do, will be closer to the state of maximum degradation, the death state or Thanatia [1]. Terrifying, right?

An easy example of this quality destruction is in our breakfast, as M. Reuter proposed [2]. A simple act like making a cup of coffee can be the most exergy-destroying thing that we can do day in day out. It is so easy to take coffee, milk, sugar and mix all together. However, what happens if we want to reverse the process? Could I get the coffee, milk and sugar as easily as mixing them? The answer is no. This mixing process has generated a big amount of entropy that makes the process irreversible and maybe the majority of us would choose to pour the mixture down the sink and start again. Now imagine that we have dozens of metals mixed together, the exergy destruction could be even bigger. But, should we dump them? NO! That is why we are here, aren’t we?

We can use this entropy generation or irreversibility to calculate the consumption and destruction of resources of each process and its production cost, as well as the resource efficiency of the whole system. And the best thing is that it is possible to do it through using the same measurement property to evaluate all the flows of the system, exergy. It calculates the quality of our “fruits” and now we can “compare apples with oranges”, because we are comparing the ‘quality’ of these apples and oranges, not their mere ‘quantity’.

Therefore, as one of the main targets of SOCRATES is to minimize the waste generation in industrial processes, the cost of the waste management can be evaluated. It gives us an overview of how many resources are needed to carry out the metal extraction, recovery and valorisation processes developed in our project, showing their strengths, weaknesses and where the limit of these processes is.

I hope that you are now more familiar with exergy and their strengths to assess different kind of industrial processes, specially the metallurgical ones, as well as you have a general overview about my contribution to this project. And please note that the fruits have been an example to understand in an easier way the exergy concept, so, do not go to the grocery store and ask for 10 kW of oranges!

References

About The Author

I am Alejandro Abadías (ESR 15) was born in Sariñena, a small village near to Huesca, in the north of Spain. He studied mechanical engineering at the University of Zaragoza and pursued a Master’s degree in Renewable Energies and Energy Efficiency. In beginning of 2017, he traveled to Freiberg to join the SOCRATES project. He is now a Phd student at the University of Freiberg and Helmholtz Institute Freiberg, assessing the sustainability of the metal-production value chain and the processes developed by my colleagues.