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SUMMARYBy combining heat and power generation, mini-combined and micro-combined heat and power systems (MCHP) provide an efficient, decentralised means of power generation that can complement the composition of the electricity generation mix. Dynamic tools capable of handling transient system behaviour are required to assess MCHP efficiency beyond a mere static analysis based on steady-state design parameters. Using a simulation of a cogeneration system, we combine exergetic definitions for different operational system states to quantify the overall system efficiency continuously over the whole period of operation. The concept of exergy allows direct comparison of different forms of energy. A sensitivity analysis was performed where we quantified the effect on MCHP overall performance under varying engine rotational speed, thermal energy storage size and fluid storage temperature in a range of MCHP simulations. We found that the exergetic quantity of natural gas used by the MCHP decreased slightly at higher engine speeds (À2% to À4%). While the total amount of electricity generated is almost constant across the range of different engine output, more thermal exergy (up to +21%) can be recovered when the engine is operating at elevated speeds. Furthermore, selection of specific optimal thermal storage fluid temperatures can aid in improving system efficiency.

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Micro-combined heat and power (MCHP) systems generate heat and electricity concurrently, making them an ideal addition for home and small/medium business owners to generate their own electricity and replace conventional natural gas-burning boilers. Combining MCHP units with thermal and electric storage systems can aid in decoupling supply and demand of energy. In such a combined setup, MCHP units can run for prolonged periods when they not only cover existing demand but charge storage systems for deferred consumption of energy. In the present work, we analyzed such an MCHP system, with a particular focus on integrating electrical storage systems and the resulting degree of electrical self-sufficiency achievable under realistic working conditions. We implemented a system control logic to optimize MCHP unit run time geared towards taking energy storage system charging levels into account. We demonstrate that an MCHP unit and electrical storage system can complement each other benefitting overall system performance. Separating days according to their respective degree of electrical self-sufficiency enabled us to identify supply composition characteristics that result in higher electrical load coverage by MCHP-generated electricity.