Exergy Analysis of an ORC Fed by a Thermal Storage with Variable Ambient Conditions

In a recent paper the derivation of flow and non-flow exergy if ambient reference temperature (T°) and pressure (P°) vary in time has been revised, highlighting that, in the expression of the exergy balance of a generic control volume, besides the well-known terms, two additional terms appear, that take into account the available work (exergy) destruction related to the variation of ambient condition during the considered time interval. In the present paper, that result is applied to a system made up by a solar collector with a thermal storage and a cogeneration unit, based on an Organic Rankine Cycle (ORC). The storage is regarded to be charged during day time, then the stored heat is used to feed the ORC during the night time, in order of producing electric energy during night also. The different evaluations of exergy destruction, with and without taking into account the actual ambient reference temperature variations during the whole process, are highlighted in the analysis. The results show that neglecting ambient reference temperature variations implies an error in the evaluation of exergy destruction and efficiency up to 10% for the system considered.