Experimental Investigation of Direct Heated Rock Bed Thermal Energy Storage for Application in Small Scale Power Generation

Abstract

Thermal energy storage is essential for power generation using renewable energy sources like solar and wind, addressing the intermittent nature of these resources and the fluctuating power demand by users. This study experimentally investigates using a new approach of direct heated packed rock-air bed thermal energy storage for small-scale power generation. The experimental setup comprised a cylindrical steel tank (6 mm thickness, 0.45 m diameter, 1.0 m height) with a 25 mm air gap and 200 mm (Castable, Refractory and Fiber glass) insulation. Two granite rock sizes (average diameters of 1.5 cm and 3.5 cm) were tested.  An electric heat source maintained a temperature of approximately 550 °C at the bottom of the storage for charging, while thermocouples monitored temperatures at various positions. Thermal decay characteristics were studied under no-load conditions, and discharging was tested by circulating water through an embedded coiled pipe within the storage. During the first 10 h of charging, temperatures between 100 °C and 400 °C were achieved for both 1.5 cm and 3.5 cm rocks. For the 1.5 cm rocks, temperatures reached 400 °C at the bottom, 225 °C in the middle, and 115 °C at the top, while the 3.5 cm rocks reached 395 °C, 180 °C, and 93 °C, respectively. These results show that smaller rocks (1.5 cm) provided better thermal performance, reaching higher temperatures throughout the storage than larger rocks (3.5 cm). The system reached steady state in about 10 hours, after which heat transfer slowed due to the low thermal conductivity of the rocks, with conduction as the dominant mode. During discharging without load, the 3.5 cm rocks cooled to near ambient within 40 hours, while the 1.5 cm rocks maintained 75 °C over the same period. The storage fully discharged within two days, while water circulation at 25 L/h produced steam for 5 h before temperatures dropped below boiling for the smaller rock size. Significant heat losses from all surfaces highlighted the need for better insulation. Overall, the study demonstrates the potential of packed air-rock bed thermal energy storage for small-scale applications, with recommendations to apply forced convection and improve insulation to enhance efficiency.