Abstract
Thermal and stratification number profiles of three sensible heat TES systems are presented during charging cycles with three different flow-rates. Charging cycles are performed with flow-rates of 4 ml/s, 8 ml/s and 12 ml/s. The three systems evaluated are an oil only TES system, a packed-bed system of big pebbles (average diameter: 31.9 mm, void fraction 0.43) and a packed-bed system of small pebbles (average diameter: 10.5 mm, void fraction 0.31). The oil storage system charges up faster than the other storage systems due to its lower thermal mass. The small pebbles TES has a lower void fraction as compared to the larger pebbles TES making its rate of temperature rise faster. The flow-rate affects the rate of temperature rise. Higher flow-rates result in faster rates of temperature rise in the TES systems. Higher TES temperatures are achieved in shorter intervals with higher flow-rates, but the overall charging period is longer due to more heat losses. Stratification number profiles are seen to rise and drop during charging cycles. The higher the flow-rate, the faster is the rate of rise and drop. The highest flow-rate (12 ml/s) shows stratification number profiles dropping in a non-smooth manner possibly due to turbulence in the TES tank introduced at this particular flow-rate. The oil TES system shows the fastest rise and drop of the stratification number due its lower thermal mass, followed by the smaller pebbles and lastly by the larger pebbles. Generally, the slowest rate of drop in the stratification number profiles is seen with the large pebbles TES system for all flow-rates suggesting more quality useful energy is stored in the system with the large pebbles regardless of the fact that it charges up with the slowest rate
