Volt-ampere ratings in electronically tuned linear alternators for thermoacoustic engines

Linear alternators (LAs) coupled to thermoacoustic engines (TAEs) provide a viable solution to extract energy from a heat source in a variety of applications such as waste heat, energy harvesting, solar thermal and biomass power generation. For the electrical power to be maximised, the acoustic impedances of LA and TAE have to match. This requirement cannot, in general, be met by relying only on the design of the LA, but can be achieved at the control level, by using a fraction of the LA inverter current to create `electronic stiffness' which contributes to the overall stiffness tuning the resonance frequency. The same concept can, in principle, be used to replace part of the mechanical spring stiffness in order to overcome the limitations in the mechanical design, at the expense of an increase in LA and inverter ratings. The impact of electronic stiffness on LA power capability and ratings is analysed here. Two meaningful scenarios are considered in the analysis: the LA derating for resonance frequency tuning and the oversizing when springs are partially replaced by electronic stiffness. The study is supplemented with experiments on a small-scale LA test rig.