Experimental Investigation on Thermal Retention and Evaporative Loss Suppression in Hybrid Rock-Water Packed Beds: Aspect Ratio and Headspace Boundary Effects

Solid-medium packed beds offer a robust, chemically stable solution for thermal energy storage (TES); however, their limited volumetric heat capacity and susceptibility to internal natural convective circulation constrain their thermal retention performance. While introducing a liquid phase (such as water) markedly enhances volumetric thermal capacitance, it introduces an acute, frequently neglected vulnerability: rapid latent heat dissipation via free surface evaporation at elevated temperatures. This paper presents an extensive experimental investigation into the transient thermal discharge dynamics of packed pebble beds across dry and hybrid (rock-water) media under varying geometric aspect ratios (H/D = 0.50 to 1.25) and headspace boundary conditions (hermetically sealed versus open-to-atmosphere). Utilizing dual-probe immersion thermocouples and calibrated infrared thermography, thirteen distinct configurations were evaluated across six core experiments and two exploratory phases. The experimental findings establish that free evaporative discharge exerts a catastrophic penalty on thermal retention: an open hybrid canister lost 24.0 g of water, dissipating 54.24 kJ purely through latent heat of vaporization (h_fg = 2260 kJ/kg), which accelerated the thermal degradation rate by 198% relative to its sealed counterpart. The sealed unit lost only 2.0 g of moisture (4.52 kJ), extending the system thermal time constant to tau = 24.50 min. In dry beds, increasing the aspect ratio from 0.50 to 1.00 suppressed internal natural convective chimney circulation, establishing a core retention advantage of +19.0 °C. Crucially, a novel Thermal Crossover Phenomenon was captured, wherein a half-height (50 mm) sealed canister possessing a 55% water mass deficit completely inverted an initial -6.0 °C deficit to finish +3.0 °C hotter than an open full-height (100 mm) canister. Kline-McClintock uncertainty analysis validated experimental precision within +/- 5.65%, and twin-canister repeatability demonstrated a mean absolute deviation of <= 0.58 °C (relative error < 1%). These results provide definitive design guidelines for headspace vapor containment and aspect ratio optimization in packed-bed solar thermal collectors and industrial waste heat recovery systems. Keywords: Packed bed thermal energy storage, Hybrid sensible-latent storage, Natural convection suppression, Aspect ratio, Evaporative loss suppression, Thermal crossover phenomenon, Kline-McClintock uncertainty analysis