## 1 Introduction
### Exergy and Buildings  
To achieve the energy-saving goal, it is very helpful to include exergy methods in the analysis of buildings, both for the building envelope and for the facilities; as done in reference [4] where the methods and the metrics of the exergy analysis in buildings are described. Above all, the recent published book of reference [5] can be considered one of the most complete works connected with exergy analysis and thermoeconomics of buildings in which all the issues related to design and exergy application in buildings is deeply explained. In order to introduce the exergy concept in buildings it is necessary to review the way in which energy is transformed from one form to another. According to the first law of thermodynamics, energy is always conserved in all processes i.e., heat can be obtained from combustion of a fuel or from an electrical source, etc. and always the quantity of energy contained in the fuel or the electrical energy is the same as the heat flux obtained [6]. However, all the energy conversions take place in a definite direction i.e., in the transformation of heat into work a part of the heat flux has to be transferred to a cold source, as a heat loss, and cannot be recovered. Moreover, if a system undergoes a reversible process, both the system and its surroundings can always be restored to their original state. It is obvious that no process is reversible and because of those irreversibilities the energy quality degrades. Indeed, this is described by the second law of thermodynamics. The quality of the energy, which is precisely what exergy reflects, is its capacity to be transformed into useful work. Depending on its nature, some energy forms can be entirely converted into useful work, such us electricity or any form of mechanical energy, while other energy forms, as heat or thermal energy, are only partially able to be converted into work [7]. Hence, electricity is 100% exergy and only a small part of a heat flux is exergy. Consequently, when the capacity of producing useful work is reduced, exergy is being destroyed. This is related to imperfections in the processes, which are called irreversibilities. Therefore, exergy allows analysis of processes and detection of real imperfections. Then, the exergy method can be implemented for design and optimization purposes, particularly in buildings. As an example, in reference [8] an exergetic methodology to promote improvements in buildings' thermal systems is explained. Accordingly, exergy application is especially interesting in buildings because different quality levels of energy [9] are mixed together to cover the energy demands. For example, on the one hand, electricity is consumed for lighting and electrical appliances. On the other hand, other forms of high-quality energy, such as natural gas, are usually consumed for heating or cooling purposes. It is worth noting that thermal comfort is acquired by tempering the indoor air a few degrees above or below the ambient temperature, so low-quality energy would be enough to cover such demands. Therefore, if a high-quality resource is used for indoor conditioning, there is a mismatch in energy qualities. As a result, significant exergy destruction occurs which is much greater than heat losses accounted for through a first law balance. Therefore, exergy methods represent more faithfully the energy conversion processes than common energy methods, which can erroneously quantify the real losses of a system. Besides, as justified in reference [10], comparison among renewable and non-renewable energy building systems can be more fairly done under this perspective. Therefore, referring to the building sector, a great potential still exists to enhance the matching between the quality of energy sources and their final destination, in order to avoid irreversibilities and achieve as a result the decrease of costs.