A versatile one-dimensional numerical model for packed-bed heat storage systems (original) (raw)
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Extended modeling of packed-bed sensible heat storage systems
An original modeling approach for packed-bed heat storage systems based on the combination of an intra-particle conduction model and a single-phase model is presented. The intra-particle conduction approach enables to model the solid phase whatever the dimensionless Biot number of the particles of the packed bed. The single-phase model approach is used when a part of the packed bed consists of very small particles like sand. This approach assumes thermal equilibrium between the fluid and the small particles. In addition, an energy equation dedicated to the tank wall enables to model small storage systems in which the influence of the wall is significant. The resulting one-dimensional three phase model has been validated with the liquid/solid and the gas/solid heat storage systems of the French Atomic Energy Commission (CEA). Since the model has been validated on two very different setups with various cyclic operating conditions, it is likely to enable accurate and affordable modeling of a wide range of packed-bed heat storage configurations. MODEL DESCRIPTION General Principle The one-dimensional numerical model is an original combination of an intra-particle conduction model and a single-phase model. Intra-particle conduction models were introduced by [4] and consist in modeling the packed-bed with at least two energy equations: one for the fluid and the other for the solids (e.g. rocks). With this model, the
A review on experience feedback and numerical modeling of packed-bed thermal energy storage systems
Solar Energy, 2017
Solar thermal energy is a clean, climate-friendly and inexhaustible energy resource. It is therefore promising to cope with fossil fuel depletion and climate change. Thermal storage enables to make this intermittent energy resource dispatchable, reliable on demand and more competitive. Nowadays, most of the concentrated solar power plants equipped with integrated thermal storage systems use the two-tank molten salt technology. Despite its relative simplicity and efficiency, this technology is expensive and requires huge amounts of nitrate salts. In the short to medium term, packed-bed thermal energy storage with either liquid or gaseous heat transfer fluid is a promising alternative to reduce storage costs and hence improve the development of solar energy. To design reliable, efficient and cost-effective packed-bed storage systems, this technology, which involves many physical phenomena, has to be better understood. This paper aims to sum up some key aspects about design, operation, and performances of packed-bed storage systems. In the first part, most representative setups and their experience feedback are presented. The controllability of packed-bed storage systems and the special influence of thermal stratification are pointed out. In the second part, the various numerical models used to predict packed-bed storage performances are reviewed. In the last part, some useful correlations enabling to quantify the main physical phenomena involved in packed-bed operation and modeling are presented and compared. The correlations investigated enable to calculate fluid/solid and fluid/wall heat transfer coefficients, effective thermal conductivity and pressure drop in packed beds.
A high temperature sensible heat thermal energy storage (TES) system is designed for use in a central receiver concentrating solar power plant. Air is used as the heat transfer fluid and solid bricks made out of a high storage density material are used for storage. Experiments were performed using a laboratory scale TES prototype system and the results are presented. The air inlet temperature was varied between 300 0 C to 600 0 C and the flow rate was varied from 50 CFM to 90 CFM. It was found that the charging time decreases with increase in mass flow rate. A 1D packed bed model was used to simulate the thermal performance of the system and was validated with the experimental results. Unsteady 1D energy conservation equations were formulated for combined convection and conduction heat transfer, and solved numerically for charging/discharging cycles. Appropriate heat transfer and pressure drop correlations from prior literature were identified. A parametric study was done by varying the bed dimensions, fluid flow rate, particle diameter and porosity to evaluate the charging/discharging characteristics, overall thermal efficiency and capacity ratio of the system.
Modeling, Simulation and Optimal Operation of Multi-Extraction Packed-Bed Thermal Storage Systems
2020
Solar thermal power technologies require storage systems to mitigate the natural variability of solar irradiation. Packed bed thermal storage systems (PBTES) offer a cost-effective solution using air as heat transfer fluid and rocks as a storage medium. Compared to its alternatives, however, PBTES presents a limited flexibility of operation due to the conventional unidirectional flow, which involves the progressive reduction of the outlet temperature during discharge and thus lowers the thermodynamic efficiency of the power cycle. The present study summarizes the progress on the design and optimal operation of a novel multi-extraction PBTES, a project that aims at mitigating its typically poor operational flexibility for solar power applications. To this end, a one-dimensional model with a high spatial resolution of a PBTES was developed, which includes four intermediate outlet points along the axial direction to investigate the benefits of optimal extraction operation. In order to ...
Transient response of a packed bed for thermal energy storage
International Journal of Heat and Mass Transfer, 1984
An analysis, with experimental results, for the transient response of a packed bed thermal storage unit is presented. The analysis is in two spatial dimensions and considers the influence of both axial and radial thermal dispersion for arbitrary time and radial variations in the inlet fluid temperature. Both charging and recovery modes are included. Spatial variations in void fraction are found to have significant influence on the dynamic response of both fluid and solid temperatures. An unconditionally stable numerical model is developed that predicts the two-dimensional transient response of both solid and fluid phases. The influence of radial velocity variations, wall heat capacity and wall thermal losses are considered. The analysis is valid for fluids of various Prandtl numbers. Experimental measurements of temperature distributions in a randomly packed bed ofuniform spheres, with air as the working fluid, compare favorably with the analytical results over a broad range of Reynolds numbers. Results are presented for full-size rock beds which indicate pronounced effects of void distribution in such systems.
Analytical simulation in heat storage systems
W�rme - und Stoff�bertragung, 1982
Packed bed heat exchangers for thermal energy storage systems are investigated by means of two phase heat transfer models. The paper is mainly aimed at deriving analytical solutions to the thermal balance equations relevant to different kinds of packed beds, taking into account the roles played by heat capacity and conduction effects. The results are shortly discussed and some graphs are shown for situations typical of various operational modes. Analytische Simulation yon Wiirmespeichern Zusammenfassung. Festbett-Wiirmeaustauscher ffir Systeme yon W~irmespeichern werden mit Hilfe yon zweiphasigen W~irmeiibergangsmodellen untersucht. Haupts~ichlich werden analytische L6sungen ftir die Wgrmebilanzen unterschiedlicher Festbettarten abgeleitet unter Ber~cksichtigung der Einfltisse yon W~irmekapa-zit~it und W~irmeleitung. Die Ergebnisse werden kurz diskutiert und in einigen Diagrammen typische Arbeitsweisen gezeigt.
Parametric analysis of a packed bed thermal energy storage system
2017
Even if the packed bed thermal energy storage concept has been introduced as a promising technology in the concentrated solar power field in the last years, its full deployment in commercial plants presents a clear improvement potential. In order to overcome the under-development of this storage technology, this work attempts to show the great capabilities of packed bed heat storage units after a successful design and operational parametric optimization procedure. The obtained results show that a correct design of this type of facilities together with a successful operation method, allow to increase significantly the storage capacity reaching an overall efficiency higher than 80 %. The design guideline obtained as a result of this work could open new objectives and applications for the packed bed storage technology as it represents a cost-effective and highly performing storage alternative.Even if the packed bed thermal energy storage concept has been introduced as a promising techn...
Numerical modeling of thermal energy storage system
Thermal energy storage in the form of latent heat of fusion of phase change material gained considerable attention in solar energy applications since it significantly increases the energy density and reduces the storage tank size compared to the sensible heat storage system. Several numerical and experimental studies have been conducted to enhance the performance of the system. In this study, 2-D continuous solid phase and effective packed bed models are developed to study the behavior and performance of a thermal energy storage system for high temperature applications, which is composed of spherical capsules encapsulated by phase change material (Sodium nitrate) and high temperature synthetic oil (Therminol 66) as heat transfer fluid. Temperature distribution, fluid flow, melting, solidification and thermocline behavior of the system are predicted and the influence of capsule size on the performance of the system is studied.
IJERT-Effect of Thermal Properties of Storage Material on Packed Bed Performance
International Journal of Engineering Research and Technology (IJERT), 2020
https://www.ijert.org/effect-of-thermal-properties-of-storage-material-on-packed-bed-performance https://www.ijert.org/research/effect-of-thermal-properties-of-storage-material-on-packed-bed-performance-IJERTV9IS070425.pdf For concentrated solar power plants, packed bed of rock represents a good alternative to two-tank molten-salt thermal energy storage system. In this study, a two-phase numerical model is developed and successfully validated with experimental data. A parametric study was carried out to assess the effect of thermal properties of storage material on the thermal behavior and performance of rock bed energy storage system. The results obtained show that the thermal capacity and conductivity of storage material have a great effect on thermocline zone, thermal stratification, stored energy during charging, recovered energy during discharging and on the efficiency of the storage system.
Solar Energy, 2012
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 MWh th 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 GWh th capacity, for a 26 MW el round-the-clock concentrated solar power plant during multiple 8 h-charging/16 h-discharging cycles, yielding 95% overall thermal efficiency.