• Energy Research

Experimental and numerical results from the world's first advanced adiabatic compressed air energy storage (AA-CAES) pilot-scale plant are presented. The plant was built in an unused tunnel with a diameter of 4.9 m in which two concrete plugs delimited a mostly unlined cavern of 120 m length. The sensible thermal-energy storage (TES) with a capacity of 12 MWhth was placed inside the cavern. The pilot plant was operated with charging/discharging cycles of various durations, air temperatures of up to 550 °C, and maximum cavern gauge pressures of 7 bar. Higher pressures could not be reached because of leaks that were traced mainly to the concrete plugs. Simulations using a coupled model of the TES and cavern showed good agreement with measurements. Cycle energy efficiencies of the TES were determined to lie between 76% and 90%. The estimated round-trip efficiency of the pilot plant was based on the measured TES performance and estimated performances of the other components, yielding values of 63–74%, which compares favorably with the usually quoted values of 60–75% for prospective AA-CAES plants. Journal of Energy Storage, 17 ISSN:2352-152X

The present study aims at dimensioning and modeling, by means of accurate time-dependent 3D computational fluid dynamics simulations, the behavior of a high temperature rock-bed TES system. The latter is exploited to fulfill the round-the-clock energy requirements of a reference 80 MWe industrial-scale CSP plant, based upon the Airlight Energy technology, which uses air as heat transfer fluid. The TES system behavior was analyzed through 15 consecutive charge/discharge cycles to evaluate the thickness evolution of the thermocline zone, and hence the overall thermal efficiency of the system, under cyclic conditions. The numerical model was satisfactorily validated with experimental data, gathered from a 6.5 MWhth TES system prototype, located in Biasca, designed and built by the Swiss company Airlight Energy SA. The good agreement between CFD simulations results and experimental data allowed the authors to assess the relevance of radiative heat transfer, even at relatively low temperature (300 ÷ 350 °C), on the thermodynamics behavior of the TES system. Moreover, a porosity variation, with the packed bed depth, was also observed numerically and experimentally mainly due to the own weight of the packings (25m3 of natural river pebbles with 3 cm average diameter). The CFD simulations were performed with Fluent code from ANSYS. 8th International Renewable Energy Storage Conference and Exhibition (IRES 2013) Energy Procedia, 46 ISSN:1876-6102

Combined sensible/latent heat storage allows the heat-transfer fluid outflow temperature during discharging to be stabilized. A lab-scale combined storage consisting of a packed bed of rocks and steel-encapsulated AlSi12 was investigated experimentally and numerically. Due to the small tank-to-particle diameter ratio of the lab-scale storage, void-fraction variations were not negligible, leading to channeling effects that cannot be resolved in 1D heat-transfer models. The void-fraction variations and channeling effects can be resolved in 2D models of the flow and heat transfer in the storage. The resulting so-called bypass fraction extracted from the 2D model was used in the 1D model and led to good agreement with experimental measurements.

AbstractSingle-tank thermal energy storage (TES) systems represent a valuable alternative, to the most common two-tank systems with molten slat, to effectively store thermal energy in concentrating solar power (CSP) applications.From an economic standpoint, the gap between the two TES solutions is relevant. A remarkable cost reduction can be achieved if a single-tank TES system, with a low-cost filler material, is exploited.In this kind of TES system,the buoyancydriven effects of the heat transfer fluid are exploited to establish and maintain a thermocline zone which separates the hot region on top and the cold region at the bottom of the tank.The thinner the thermocline thickness, the higher the thermodynamic quality of the stored energy.As soon as the TES is charged for the first time, i.e. startup of the system, the extent of thermal stratification may vary sharply during the first cycles before achieving a stable condition. For this reason, this study aims at evaluating, by means of accurate time-dependent 3D CFD simulations, the transient evolution of thermal stratification of a single-tank TES system exploited to fulfill the round-the-clock energy requirement of a reference 80 MWeCSP plant which uses air as heat transfer fluid.A total of 30 consecutive cycles, composed by charge/discharge phases, were simulated.Since the thermal energy stored is exploited to produce electrical energy, the performances of the TES system, operating under cyclic conditions, were qualitatively characterized by means of a stratification efficiency indexbased upon the second-law of thermodynamics.

A thermal energy storage (TES) system was designed based on a packed bed of rocks as storing material and air as heat transfer fluid. A pilot-scale 6.5 MWhth TES unit was built and tested. A dynamic numerical heat transfer and fluid flow model was developed and experimentally validated with measurements obtained from the pilot-scale TES unit. The simulation model is applied to design an industrial-scale 100 MWhth TES unit for a solar power plant currently under construction in Morocco. Proceedings of the SolarPACES 2013 International Conference Energy Procedia, 49 ISSN:1876-6102

A thermal energy storage system, consisting of a packed bed of rocks as storing material and air as high-temperature heat transfer fluid, is analyzed for concentrated solar power (CSP) applications. A 6.5 MWhth pilot-scale thermal storage unit immersed in the ground and of truncated conical shape is fabricated and experimentally demonstrated to generate thermoclines. A dynamic numerical heat transfer model is formulated for separate fluid and solid phases and variable thermo-physical properties in the range of 20–650 °C, and validated with experimental results. The validated model is further applied to design and simulate an array of two industrial-scale thermal storage units, each of 7.2 GWhth capacity, for a 26 MWel round-the-clock concentrated solar power plant during multiple 8 h-charging/16 h-discharging cycles, yielding 95% overall thermal efficiency.

AbstractIn the present work, a computational fluid dynamics (CFD) approach was followed to evaluate the extent of thermal stratification of an industrial-scale thermal energy storage (TES) system, based on a packed bed of river pebbles The TES is integrated into a reference concentrating solar power plant which uses air as heat transfer fluid. The transient evolution of thermal stratification was qualitatively evaluated according to the dimensionless MIX number based on the so-called moment of energy, or height-weighted energy, into the packed bed. The resulting stratification efficiency ranges between 0 and 1 with the theoretical threshold values given by the moment of energy of fully mixed and ideally stratified TES respectively. The 30 consecutive cycles analyzed were characterized by 12hours of charging followed by 12hours of discharging. The results obtained showed that the TES system reached a stable working condition after 20-22 cycles with an average stratification efficiency of about 0.95. The CFD simulations were performed with Fluent 14.5 code from ANSYS.