ANALYSIS OF A RECIPROCATE ENGINE–BASED COGENERATION PLANT WITH HIGH TEMPERATURE HEAT RECOVERY FOR INDUSTRIAL USES

In consequence of the increasing awareness on the future scarcity offossil energy sources and the global warming impact of energyconversion processes, the European Union has been planning severalactions to enhance the efficiency of energy use and reduce theenvironmental impact. The declared goals of EU actions aresynthetized in the 20-20-20 formula, consisting of an expected 20%increase of energy efficiency, a 20% contribution to the total energysupply by renewable sources and a 20% abatement of pollutantemissions. Applications of cogeneration in process industry cansignificantly contribute to achieve these targets. In this paper areciprocate engine-based Combined Heat and Power (CHP) plant ispresented, serving a pasta factory located in Sicily and installed by anEnergy Service COmpany (ESCO) within the context of a nationalimplementation scheme of Energy Saving Certificates (or “whitecertificates”). The CHP plant, with a 650 kWe capacity, currentlycovers a relevant fraction of the electric and high-temperature heatloads during peak hours, while it is switched off during off-peak hoursbecause of the much lower electricity price. Heat content of flue gasesis recovered by two cascaded gas-diathermic oil and diathermic oilwaterheat exchangers; the superheated water obtained is then suppliedto the pasta dryers. The first part of the paper provides a detailed plantdescription and an energetic analysis of historical performance datacollected along the last two years of operation. Both the criticalanalysis of the lay-out and the evaluation of energy saving indicatorsreveal the current scheme to represent a sub-optimal solution for theparticular application. In the second part of the paper a modifiedsolution is simulated, consisting of the same CHP unit equipped withadditional heat exchangers for heat recovery from the cooling waterjacket circuit. The marginal energetic and economic benefits comparedto the current plant setup are calculated; the results are presented inanalytic and graphical form, coherently with the provisions ofDirective 2004/8/EC and accounting separately for the different costand revenues (fuel for the CHP unit and the supplementary boilers,electricity purchased from or supplied to the grid, taxes, etc.). Theimproved solution, designed to increase the thermal efficiency of theCHP unit by allowing a full exploitation of heat cascades, resulted toprovide evident benefits and to make the CHP unit to comply with allthe current legislative provisions for the assessment of highly efficientCHP plants. Margins for further improvements are also brieflydiscussed.