Summary

High-capacity hybrid thermoacoustic electric generators (HTAEGs) are ideal in different small- and micro-scale energy systems, especially in space nuclear power systems. In this work, an HTAEG with a post-positioned gas spring is proposed. To demonstrate the superiority of the gas-spring-post-positioned design on high-capacity HTAEGs, an HTAEG prototype is modeled, built, and tested from the perspective of thermoacoustics accordingly. Experimental results demonstrate an output electric power of 15.0 kW. Furthermore, it could achieve the highest efficiency of 39.2% with an output electric power of 11.1 kW. Given that a 15-kW power output is an ultra-high level on a single-piston HTAEG of this type to date, and the achieved efficiency on the prototype is also encouraging, this work marks an important milestone in the development of high-capacity HTAEGs. It also demonstrates that the gas-spring-post-positioned design has significant advantages and enormous potential for application.

Introduction

The thermoacoustic electric generator (TAEG) emerges as a promising dynamic conversion technology due to its exceptional reliability, extremely long lifespan, versatility in heat source utilization, and high intrinsic efficiency.1 These attributes render it particularly well suited for space and submarine thermoelectric applications.2,3 Within the TAEG family, a hybrid one (traditionally called the free-piston Stirling engine,4 currently recognized as a generalized thermoacoustic engine5) that incorporates a solid mass to tune the internal acoustic field transcends the conventional gas-tuning systems in the aspects of power density and thermal-to-electrical efficiency.6

Most recently, dramatic progress, e.g., 16-year non-stop operation and over 39% thermal-to-electrical efficiency,7 has stoked up the enthusiasm for the hybrid thermoacoustic electric generator (HTAEG) in space application. However, the current achievements are all confined in a rather small power level (i.e., around 100 W), not sufficient to cover various application scenarios. Scaling up to tens of kilowatt power while maintaining high reliability simultaneously is formidably challenging, and so far, there is no public literature reported on it. The main obstacle is the spring force mechanism that is imperative to tune the acoustic field. For low- and medium-power HTAEGs (ranging from tens of watts to kilowatts), planar springs are commonly employed as the spring force mechanism for their excellent compactness and radial rigidity.8 These planar springs can be positioned either at the bounce space or compression space, resulting in two distinct designs for HTAEG. Meanwhile, both designs have been extensively utilized in numerous HTAEG prototypes, with their reliability well demonstrated through ground tests.7

Results and discussion

System configuration and numerical study
Figure 1 presents the schematics of an MGS-HTAEG and a PGS-HTAEG. The PGS-HTAEG is primarily a helium-filled pressure vessel that houses a hybrid thermoacoustic engine (HTAE) and a linear alternator (LA). The HTAE comprises an ambient heat exchanger (AHX), a regenerator, a hot heat exchanger (HHX), and a displacer. Meanwhile, the LA consists of an electromagnetic conversion circuit consisting of outer and inner stators, moving magnets, and coils, along with a power piston supported by gas bearings. Within the HTAEG, a combination of the AHX, the regenerator, and the HHX constitutes its thermoacoustic core, designed to transform external thermal energy into acoustic energy. By heating the HHX while rejecting heat from the AHX, helium within the regenerator undergoes oscillatory flow due to the thermoacoustic effect. Consequently, thermal energy is converted into acoustic energy. The displacer plays a pivotal role in modulating the acoustic field and facilitating the transfer of acoustic power. When the acoustic pressure fluctuation generated by the thermoacoustic core pushes the power piston to move reciprocating in the cylinder, the moving magnets mounted on the power piston move in a stationary coil accordingly, and the acoustic power is converted to electric power through electromagnetic induction.