A Study on Effect of Tuning Capacitor in Dynamic Radioisotope Power Systems

This study presents the effect of the tuning capacitor on the stability and electrical power output of a gas spring-based free-piston Stirling convertor (FPSC) when the FPSC runs on a forced-oscillation type controller. It has been widely accepted that the tuning capacitor is an important circuit component of the FPSC to stabilize piston amplitude and maximize electrical power output by correcting the power factor. Recent modeling analysis, however, suggests that while the effect on the stability is significant, the effect on the electrical power output is almost negligible as long as the operating frequency the FPSC is fixed by a forced-oscillation controller and operating conditions, such as the hot-end/cold-end temperatures, piston amplitude, and pressure, are unchanged. To verify the modeling analysis results, Advanced Stirling Convertor (ASC) is controlled by an AC power supply and the piston amplitude, hot-end temperature, cold-end temperature, and pressure are actively controlled to 4.3mm, 760˚C, 40˚C and 485psig, respectively, while the tuning capacitor value is changed from 840μF to 1040μF and 1240μF. Furthermore, the vibration test results of the ASC with the three tuning capacitor values are also presented to show the effect of the tuning capacitor on stability. The test results suggest that when the FPSC operates at a fixed frequency, the effect of tuning capacitor on electrical power output is negligible while the effect on stability is rather significant. Therefore, the tuning capacitor value should be selected toward the highest stability.