Optimal energy harvesting from a high-speed brushless DC generator-based flywheel energy storage system

Brushless DC (BLDC) motors are highly suitable for usage in high-speed applications like flywheel energy storage. A bidirectional power converter (BDC) interfaces the DC power source to BLDC machine which is coupled to the flywheel. The BDC acts as an energy harvesting converter (EHC) (in voltage boosting mode) during extraction of stored energy from the flywheel. However, usually an optimal energy is not extracted because of limitation on the maximum gain of the EHC and equivalent source resistance (R_s) of the generator. This study presents the effects of generator parameters on EHC and relevant analysis for the extraction of optimum energy stored in the flywheel. Existing literature on a similar system does not consider the effect R_s of the generator on the BDC´s performance. If R_L is the load resistance, then as the R_s/R_L ratio increases, maximum voltage gain and operating duty cycle range decrease. R_s can be reduced by designing machine/chokes using low loss core material like nickel-iron or cobalt-iron. This study also deals with the design and implementation of the controller for EHC taking R_s of the generator into consideration. Studies on the effect of R_s on the converter harvestable energy have been conducted and the experimental results are included.