Thermal cycle performance of thermocline storage: numerical and experimental exergy analysis

The performance of a dual-media thermocline storage is studied in stationary periodic operation. Successive cycles are carried out until a stabilised behaviour is obtained. Numerical results are compared to experiments realised on a 107 kWh prototype combining thermal oil and alumina. Two performance indicators are studied: utilisation rate and exergy efficiency. The utilisation rate decreases as the thermocline thickens over the cycles, while the exergy efficiency increases over the cycles, mainly due to lower heat losses improving the energy efficiency. Experimental values stabilise around UR = 53% and ηex= 73% at the fifth cycle. A new indicator is introduced: the storage quality factor Ψs, which quantifies the exergy losses due to the process. Experiments and simulations show good agreement, with a slight underestimation of heat losses by the model. Exergetic performance of thermocline storage is thus validated experimentally on a prototype-scale setup in stationary periodic regime, with satisfying value: Ψs≈ 0.99. The influence of stop thresholds on performance is then examined numerically. The utilisation rate decreases with strict thresholds (κch=κd= 0.05) because it prevents the storage to be fully loaded and because the thermocline thickens over the cycles if not extracted from the tank. The exergy efficiency globally increases for hybrid threshold (strict during charge, soft during discharge), with a maximum of ηex= 96.5% for κch= 0.05, κd= 0.95. In that configuration, the time during which the storage is hot is minimised, decreasing heat losses. Besides, setting strict thresholds limits the variation of the outlet temperature and reduces exergy destruction.