Comparative energy and exergy performance assessments of a microcogenerator unit in different electricity mix scenarios

The Directive 2004/8/EC on the promotion of cogeneration proposes a comparative indicator based onprimary energy savings, neglecting some important thermodynamic aspects, such as exergy. This studyaims to compare and discuss the usefulness of a set of complementary indicators for performance assessmentsof cogeneration systems, concerning thermodynamic principles based on first and second law (theexergy approach). As case study, a 6 kW electric output micro-combined heat and power unit was experimentallytested and a model of the unit was developed in TRNSYS. Considering as reference a set of differentheat and electricity scenarios, including the actual electric mixes of Portugal and Italy, theindicators Primary Energy Savings (PES) and Primary and Total Irreversibilities Savings (PIS and TIS), aswell as energy and exergy renewability ratios were assessed and discussed. The results show that theuse of MCHP has higher advantages for the Italian electric grid, than an equivalent scenario consideringthe Portuguese electric network as reference. As result, for a particular scenario analysed, PES and PIShave 3% and 6% for Portugal, and 10% and 18% for Italy, respectively. Furthermore, for one particular scenarioevaluated, the indicators energetic and exergetic renewability ratios have 23% and 14%, respectivelyfor the Portuguese electric grid, and 19% and 10% for the Italian electric system.