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In the last years, it has been experimentally proven that the precursors of synthetic fuels can be produced exploiting the thermochemical water and CO2 splitting process, driven by concentrated solar radiation. This process requires large amount of thermal energy at very high temperature levels (1300-1400°C) far from being obtainable with conventional commercial receivers. For this reason, Synhelion SA is developing an innovative receiver design capable of operating at very high temperatures (in the order of 1500°C). To assist the design phase of the receiver, a CFD model was developed and exploited to run some CFD simulations campaigns. At first, an analysis on the effect of modeling the receiver behavior with different turbulence models (realizable k-ε and k-ω SST) was conducted with the aim of determining the most suitable for the cases under investigation. The CFD model was then applied to study the receiver behavior under different operating conditions. In particular, in terms of receiver operating pressure, two different cases were considered: ambient pressure and 10 bar. For each of them, a specific receiver design was described, and a relative simulations campaign, assuming different values of heat transfer fluid (HTF) mass flow rates, was performed not only to evaluate its thermo-fluid dynamics behavior but also to determine the most promising HTF inlet configuration between the two proposed of single and double inlets. Based upon the results obtained, the absorbing gas receiver resulted to have promising performance for all the working conditions evaluated. In the case of receiver operating at ambient pressure, the best resulting thermal efficiency values were in the range of 0.78 and 0.43, with relative HTF outflow temperature range of 1200°C and 1800°C. While, in the case of pressurized receiver, the best resulting efficiency values were in the range of 0.88 and 0.54, with relative HTF outflow temperature range of 1000°C and 1670°C. [ABSTRACT FROM AUTHOR]

The concept of combined sensible/latent heat thermal energy storage (TES) has been exploited to mitigate an intrinsic thermocline TES systems drawback of heat transfer fluid outflow temperature reduction during discharging. In this study, the combined sensible/latent TES prototype under investigation is constituted by a packed bed of rocks and a small amount of encapsulated phase change material (AlSi12) as sensible heat and latent heat sections respectively. The thermo-fluid dynamics behavior of the combined TES prototype was analyzed by means of a computational fluid dynamics approach. Due to the small value of the characteristic vessel-to-particles diameter ratio, the effect of radial void-fraction variation, also known as channeling, was accounted for. Both the sensible and the latent heat sections of the storage were modeled as porous media under the assumption of local thermal non-equilibrium (LTNE). The commercial code ANSYS Fluent 15.0 was used to solve the model's constitutive conservation and transport equations obtaining a fairly good agreement with reference experimental measurements. [ABSTRACT FROM AUTHOR]

The purpose of this study is to evaluate, by means of a computational fluid dynamics approach, the effect of performing an initial charging, or pre-charging, on thermal stratification of an industrial-scale thermocline TES unit, based on a packed bed of river pebbles. The 1 GWhth TES unit under investigation is exploited to fulfill the energy requirement of a reference 80 MWe concentrating solar power plant which uses air as heat transfer fluid. Three different scenarios, characterized by 4 h, 6 h and 8 h of pre-charging, were compared with the reference case of TES system operating without pre-charging. For each of these four scenarios, a total of 30 consecutive charge/discharge cycles, of 12 h each, were simulated and the effect of TES pre-charging on thermal stratification was qualitatively evaluated, by means of a stratification efficiency, based on the second-law of thermodynamics. On the basis of the simulations results obtained, the effect of pre-charging, more pronounced during the first cycles, is not only relevant in reducing the time required by the TES to achieve a stable thermal stratification into the packed bed but also to improve the performance at startup when the system is charged for the first time. [ABSTRACT FROM AUTHOR]