Exergy Analysis of Energy Systems in Buildings. Methods available and lessons learned.

The energy performance of energy systems in buildings is commonly studied by applying the First Law of Thermodynamics, which allows to quantify the energy losses and to measure the energy conversion efficiency of the single components. However, different forms of energy have different potential to produce useful work, mainly depending on the temperature at which they are available. For this reason, the Second Law of Thermodynamics should also be considered, leading to the definition of exergy as the maximum amount of work that can be produced, through an ideal reversible process evolving until equilibrium with the environment, by a system or an energy flow available at a certain temperature. Exergy is thus a way to measure the 'quality' of an energy flow. This approach is particularly relevant in building applications, where thermal energy is usually exploited at low temperature. This means that the users consume 'low-quality' energy, even when this is made available by the depletion of 'high-quality' energy sources, such as fuels and electricity. The exergy analysis helps identifying such irrationalities, that cannot emerge from energy analysis. The main scope of this paper is to lay sound methodological bases for the exergy analysis of energy uses in buildings. For this purpose, a literature review has allowed to identify methods and metrics commonly used for the exergy analysis, while also highlighting discrepancies and open methodological issues. Moreover, the main lessons learned from this literature review, regarding the rational use of energy in buildings, are discussed.