Engineering analysis of uncertainties in the performance evaluation of CHP systems

From January 1, 2011, in all UE countries the combined production of electric (or mechanical) and thermal energy (also called Combined Heat and Power, CHP, or cogeneration) is recognized as a high efficiency technology only when it is able to ensure a minimum value of energy saving with respect to the separate production of the same energy flows. The Directive 8/2004/EC, and a few successive Decisions of the European Commission, introduced a methodology to establish whether any cogeneration plant, existing or new, can be acknowledged as a high-efficiency CHP plant, and can therefore be supported from the UE member states. In the paper, such methodology, based on the evaluation of a standard Primary Energy Saving (PES) index, is briefly described, and then a metrological analysis is presented, in order to evaluate the uncertainties affecting the field evaluation of such index. Three numerical examples are also presented and discussed, referred to natural gas plants, showing that the evaluation of the PES index can be quite critical, especially for values close to the minimum limit fixed by the Directive, and in particular for small and medium scale CHP units, mainly due to the low accuracy that usually affect, in such cases, the measures of the fuel input.

► Recognition of high-efficiency cogeneration (CHP) can lead to incentives.
► CHP is considered efficient when it is able to ensure a minimum energy saving.
► The accuracy of energy flow measurements becomes crucial for CHP evaluation.
► For small scale CHP, great attention must be paid to the evaluation of the fuel LHV.
► For large plants, the measurement accuracy of thermal energy becomes more important.