Smart control of an MCHP unit combined with thermal and electrical storage systems: A case study

Mini- and Micro Combined Heat and Power Systems (MCHP) hold advantages over conventional power generation due to their efficient and de-centralised nature. However, intermittent load conditions challenge MCHP systems currently on the market. System efficiency and run times could be improved by de-coupling generation and consumption of thermal and electric energy by including not only a thermal storage unit, but also electrical storage that can act as buffer reservoir during short-term use. Using an MCHP test stand, we explored the utility of combining such thermal and electrical storage systems under realistic load conditions. Smart system control logic was implemented and the effects on MCHP run time characteristics. The cogeneration test stand at the Technology Centre Energy of the University of Applied Sciences Landshut in Germany consists of the MCHP system itself, a thermal and an electrical energy storage system. The aim of the operational control of the MCHP unit with its coupled thermal and electric energy storage systems is twofold: (1) to minimise the amount of electricity drawn from the national grid and, (2) to minimise the thermal energy supplied by the conventional peak gas boiler, which is incorporated to ensure that thermal demands can be met at all times. To understand the effect of the implemented smart control logic, five different multi-day data sets recorded over the period August - December 2014 formed the basis of the presented analysis. The number of MCHP unit starts varied widely across the data sets, as did the MCHP run times, consistent with ambient temperatures: The implemented control logic better suited the colder months of the year with longer MCHP run times, while the MCHP unit was running more frequently and for shorter duration during the warm season. This behaviour was consistent with state of charge (SOC) levels being driven by the electrical storage system (EES) during wintertime and by the thermal energy storage system (TES) during summertime. The best capacity utilisation of the energy storage systems was observed during the shoulder season when both EES and TES fully charge and discharge repeatedly. This indicates that the combination of EES and TES can best compensate the varying loads and ambient temperatures encountered during the shoulder season.