Radiation-driven thermoacoustic energy conversion in absorbing media

Thermoacoustic engines rely on an instability that can convert heat into an intense acoustic wave - oscillating pressure and velocity. Being heat-driven, thermoacoustic engines are a promising technology for solar energy conversion, potentially cheap, reliable, and robust, with no moving parts and no exotic materials. Herein, we present an as-yet overlooked mode of triggering a thermoacoustic instability in such engines, driven by radiation. This mode of operation can potentially eliminate much of the viscous losses associated with high-amplitude thermoacoustic energy conversion by eliminating the need for a solid, porous medium with which to mediate heat transfer. A theoretical framework is developed for modeling such devices, with a simplified analysis illustrating that any non-uniform, temperature-dependent heat source has the potential to trigger the thermoacoustic instability. It is then further demonstrated that radiation can indeed drive such instability, with numerical results showing that energy conversion is more substantial when the oscillation period is matched with the characteristic radiative heating time of the gas. A limit-cycle analysis indicates that, for a traveling wave acoustic field, potential efficiencies of up to 90% of Carnot’s efficiency may be achieved, with power densities of up to 5 MW/m3. While several of the assumptions made in the model likely result in lower performance for a real system, the analysis demonstrates the great potential offered by this approach, for future development.