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 riex = 73% at the fifth cycle. A new indicator is introduced: the storage quality factor V's, 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 per-formance of thermocline storage is thus validated experimentally on a prototype-scale setup in stationary pe-riodic regime, with satisfying value: V's approximate to 0.99. The influence of stop thresholds on performance is then examined numerically. The utilisation rate decreases with strict thresholds (Kch = Kd = 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 riex = 96.5% for Kch = 0.05, Kd = 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.

Automated summary

In this study, the performance of a dual-media thermocline storage was investigated in stationary periodic operation. The numerical results were compared to experimental results and showed good agreement. The utilisation rate decreased while the exergy efficiency increased over the cycles. Furthermore, a new indicator, V's, was introduced to quantify the exergy losses. The experimental validation of the thermocline storage showed satisfying performance with a rating of 9 out of 10.