Direct-contact counterflow gas-particle heat exchanger for pumped thermal energy storage: Experimental performance analysis

Particle-based Pumped Thermal Energy Storage is a promising pathway for long-duration energy storage technology. However, its commercial viability requires efficiently transferring heat from hot gas into solid particles without using expensive indirect-contact heat exchangers during the charging process. This study presents the first laboratory-scale demonstration of a direct-contact, counterflow gas-particle heat exchanger. By directly exposing gravity-fed silica sand to the hot air, this first of its kind prototype design maximizes the heat transfer area and eliminates the need for costly high-temperature alloy tubes. The core challenge is preserving axial thermal stratification against the disruptive mixing effects caused by upward flowing hot air. Testing across various airflows (12.7–14.5 g/s) and particle sizes (600 μm and 825 μm) revealed a delicate operational balance. Smaller 600 μm particles were highly susceptible to over-fluidization; a 9% increase in air flow rate induced turbulent mixing that collapsed thermal stratification, dropping effectiveness from 0.79 to 0.4. Conversely, larger 825 μm particles expanded the stable operational window, enabling a “minimally fluidized” regime that successfully preserved thermal stratification. This configuration achieved a peak effectiveness of 0.90, with approach temperatures of 43.5 °C (hot end) and 11.1 °C (cold end). A 1-D transient two-phase model captured axial temperature evolution and offered design guidance, while underpredicting mixing at higher velocities. The results establish critical design and operating principles such as selecting particles near Geldart B/D boundary and maintaining minimal fluidization to balance local convective heat transfer with global thermal stratification. These principles inform the design of direct-contact heat exchangers for Particle-based Pumped Thermal Energy Storage, a step toward scalable and cost-effective long duration energy storage.