Operating characteristics of a standing-wave, liquid-piston thermoacoustic engine with phase-change

Low-grade heat from industrial waste streams, solar-thermal and geothermal, represents a significant energy source currently under-utilized. Phase-Change Thermoacoustic Engines, also known as Wet Thermoacoustic Engines (WTE) consist of a stack (porous medium) sandwiched between two heat exchangers, and an acoustic resonator. In this configuration, the stack walls are wet - soaked with a liquid, whose periodic evaporation and condensation drive acoustic oscillations, augmented by the latent heat transfer. However, implementation at standard acoustic frequencies involves the use of small stack pores ( < 1mm) that are not easy to wet consistently and without clogging. A possible solution is enlarging the stack pore size, which then mandates a reduction of the operating frequency. In an attempt to achieve this, the present work examines the operating characteristics of a water-based standing-wave WTE, coupled with a liquid column - in order to reduce its operating frequency. The results demonstrate a drive ratio of 5% in the wet mode, with an onset temperature difference as low as 20 ∘C, compared to 200 ∘C in the dry mode. This onset temperature difference is lower than any previously reported in similar systems, wet or dry. Operating a thermoacoustic engine under these conditions makes it suitable for various low-temperature heat sources, and the significantly larger pores (1 cm) help prevent issues related to clogging or inconsistent wetting. Experimental results are complemented by a theoretical model, obtaining fair agreement with the results and predicting high efficiency (>20% of the Carnot efficiency) and a power density >10 kW/m3 when operating at higher pressures.