A comparative study of frequency-shifting strategies for uniform and energy-efficient microwave heating in solid-state microwave systems

Inhomogeneity of microwave heating is an inherent drawback for gentle and uniform microwave processing. Innovative solid-state microwave generators have the potential to address this limitation by different multi-frequency shifting strategies. Thus, we investigated different optimisation approaches derived from literature regarding uniformity and energy efficiency. We compared them to a newly-developed experimentally-derived optimisation strategy using the newly found relationship that at resonant frequencies a change of the heating pattern in the sample occurs. This was exploited by selectively exciting at neighbouring frequencies to the resonant frequencies (± 2 MHz) to result in maximum uniformity achievable by frequency variation and, at the same time, high energy efficiency (i.e. 95.0%). High uniformity and high energy efficiency are, thus, not a trade-off. This study proposes a promising optimisation strategy that relies on the individual sample feedback for frequency selection, thus it could have the potential to account for differing process and product conditions in future studies.