Study on start-up characteristics of single piston free piston linear generator for small-scale compressed air energy storage system

This study presents a new idea of applying single piston free piston linear generator (FPLG) to small-scale compressed air energy storage (CAES) system. Firstly, a single piston FPLG prototype for CAES is built. Then, the thermodynamic, motion and output characteristics of FPLG during start-up process are studied based on the experimental data. The results show that the start-up process of single piston FPLG can be divided into three stages, and the order of achieving a steady state is movement and output > in-cylinder pressure > the in-cylinder temperature. The in-cylinder temperature increases in a spiral pattern, and the temperature difference between adjacent cycles at the same state point decreases. In the 0th cycle, when the free piston assembly (FPA) moves from top dead center (TDC) and bottom dead center (BDC), the peak current and power output are greater than the other cycles. The peak current and power output when FPA moves from TDC to BDC are greater than those when FPA moves from BDC to TDC. The maximum current and power output can reach 0.8 A and 62.25 W. At the beginning of the expansion stage, as the intake duration (t(in)) increases, the in-cylinder pressure increases first and then decreases. As the intake pressure (p(in)) increases, the in-cylinder pressure continues to increase. In addition, the in-cylinder temperature of working chamber A (T-A) is lower than in-cylinder temperature of working chamber B (T-B), and the temperature difference between T-A and T-B can reach 12.4 K. At higher p(in) condition, the intake point cannot be too close to the two ends and t(in) cannot be too long. At lower p(in) condition, the intake point cannot be too close to both ends and t(in) cannot be too short to ensure the system starts. The purpose of this paper is to provide valuable insights and guidance for the practical application of small-scale CAES system.