Generation of compressed air by overdriven free-displacer thermocompressors – experimental investigation of a single stage

Abstract

A thermocompressor cascade of identical stages has been identified as a promising approach to utilize waste heat for the direct generation of compressed air. This contribution presents the first design, realization and experimental investigation of a single-stage prototype featuring a free displacer oscillating in overdriven mode, which has so far only been devised and analyzed theoretically and by simulations. It is self-excited by the p,V-work generated by its rod, which periodically plunges into the cold cylinder volume. In general, the experimental performance confirms the expectations, as a stable, self-controlled operation is possible within a wide range of pressure ratios and inlet pressure levels. However, the theoretical maximum pressure ratio, at which the machine would inevitably stop, is not reached experimentally. Instead, operation continues at a lower pressure ratio, where the net mass flow drops to zero due to leakage effects. This behavior could be confirmed by an enhanced numerical model. In addition, an automatic, self-excited start by decreasing the pressure ratio could be experimentally confirmed. This is a typical industrial operating scenario, when compressed air is fed to a reservoir and consumption increases. These promising findings strongly suggest the realization and testing of a multi-stage cascade as the next step.