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Abstract
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. However, intermittent load conditions, transient system behaviour and ensuring self-sufficiency represent challenges that MCHP systems currently on the market need to address. System efficiency and run times can be increased by combining MCHP units with thermal and electric energy storage (EES) systems, decoupling energy supply and demand. When local generation of energy is complemented with energy storage during times of high supply and with discharge of stored energy during periods of high demand, self-sufficiency can be increased. Reducing demand at peak times makes the electricity grid more resilient and potentially lowers electricity costs for consumers.
Two simulation models of MCHP systems based on thermodynamic principles were developed and combined with data from an MCHP test stand at the Technology Centre Energy (TCE), Univ. Applied Sciences Landshut, Germany. 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. A range of analysis tools were used, such as exergy, degree of self-sufficiency, peak time coverage and load duration curves.
The concept of exergy allows direct comparison of different forms of energy. Exergetic definitions for different operational system states were combined to continuously quantify overall system efficiency. A sensitivity analysis assessed the effect on MCHP performance under varying engine speed, thermal energy storage (TES) size and fluid storage temperature. Selecting specific TES fluid temperatures can improve system efficiency.
A quantitative assessment was made on how the existing TCE control logic behaved and could be improved. It had been implemented to optimise MCHP unit run time, taking energy storage charging levels into account. It was determined how the MCHP unit and EES system can complement each other to benefit the system performance. Supply composition characteristics were identified that lead to higher load coverage by MCHP-generated electricity. Incorporating an EES system has a significant impact on balancing supply and demand and also ensures longer MCHP run times. Improvements to the system control logic were proposed to increase self-sufficiency and enhance electrical demand coverage during peak time.