It is relatively easy to find scientific, rigorous, and mathematical definitions of entropy and exergy. However, it is not so common to find colloquial explanations of these terms, which are possibly unknown to most people for that very reason. In this article, we will try to explain why these concepts are important and what influence they have in engineering and everyday life.
First, we define entropy as a thermodynamic magnitude that determines which part of a system’s energy cannot be used to perform work. On the other hand, exergy is a complementary term that indicates the part that can be used to perform work. These two concepts express a reality: given an energy source, not all the available energy can be converted into work.
This introduces what could be called the “quality” of energy. Given two fluids, the one with the higher temperature has higher exergy and, therefore, is capable of performing more work. Imagine a house by a lake, with an ambient temperature of 15 ºC and the lake temperature at 16 ºC. Although, as a whole, the lake has an enormous amount of energy, due to the small temperature difference, it has little exergy. Therefore, if I want to warm myself, I will use a bag of water at 60 ºC, even though the bag actually contains a smaller amount of energy than the lake.
Another example is electrical energy, which is almost entirely exergy. This is the reason why electrical resistance heaters should never be used to heat a fluid or as a heating system. The correct way is to use this high-exergy electrical energy to perform work (move a motor, a compressor…) and use the cooling water to obtain heat. This is one of the principles of cogeneration.
Finally, let’s give an example in solar energy. In a solar thermal installation, a solar collector can generate about 700 W/m² of fluid at 100-110 ºC. However, a photovoltaic panel of the same surface area would deliver approximately 150 W/m². Apparently, it has a much worse performance. The difference is that the latter are electrical Watts, i.e., of very high exergy. If we now take this electrical energy and use it to run a heat pump (which typically have an EER between 4 - 4.5), we would have similar performance to that of a solar collector.
