Study and modeling of regenerative solid/fluid heat storage systems

Cette étude porte sur les systèmes de stockage thermique régénératif dont le principe consiste à stocker de l’énergie sous forme de chaleur sensible dans un lit fixe. Le système est chargé et déchargé à l’aide d’un fluide caloporteur circulant à travers le lit fixe. Ce type de système est prometteur pour réduire le coût des infrastructures de stockage, par exemple dans les centrales solaires thermodynamiques. Cependant, le pilotage de ces systèmes est relativement complexe car leur fonctionnement est régi par divers phénomènes et met en jeux plusieurs modes de transfert de chaleur. Leur identification a permis de développer un modèle numérique monodimensionnel constitué d’une équation de continuité et de trois équations d’énergie : une pour le fluide caloporteur, une pour le solide du lit fixe et une pour les parois du réservoir. Les études expérimentales réalisées sur trois systèmes différents (un système huile/roches+sable, un système gaz/roches et un système gaz/céramique structuré en canaux), ainsi que des résultats issus de la littérature ont permis de valider le modèle proposé dans une large gamme de configurations. Le modèle s’avère notamment capable de traiter les fluides caloporteurs liquides ou gazeux et les lits fixes structurés en canaux ou constitués de milieux granulaires à granulométrie simple ou double.This study deals with regenerative heat storage systems which aim to store sensible heat in a packed bed. The system is charged and discharged thanks to a heat transfer fluid which circulates through the packed bed. This kind of system is promising to reduce the cost of heat storage facilities, for example in concentrated solar power plants. However, the operation of these systems is relatively complex because their thermal behavior is governed by several phenomena and heat transfer modes. Thanks to the identification of these mechanisms, a one-dimensional numerical model consisting of one continuity equation and three energy conservation equations has been developed. There is one energy equation for the heat transfer fluid, one for the packed bed and one for the walls of the tank. The experimental studies carried out on three different systems (an oil/rock+sand system, a gas/rock system and a structured gas/ceramic system) and experimental results from the literature have enabled to validate the model in various configurations. The model is able to deal with liquid or gaseous heat transfer fluids and with structured packed beds with channels or granular packed beds with uniformly sized particles or particles of two different sizes

Étude et modélisation des systèmes de stockage thermique de type régénératif solide/fluide

This study deals with regenerative heat storage systems which aim to store sensible heat in a packed bed. The system is charged and discharged thanks to a heat transfer fluid which circulates through the packed bed. This kind of system is promising to reduce the cost of heat storage facilities, for example in concentrated solar power plants. However, the operation of these systems is relatively complex because their thermal behavior is governed by several phenomena and heat transfer modes. Thanks to the identification of these mechanisms, a one-dimensional numerical model consisting of one continuity equation and three energy conservation equations has been developed. There is one energy equation for the heat transfer fluid, one for the packed bed and one for the walls of the tank. The experimental studies carried out on three different systems (an oil/rock+sand system, a gas/rock system and a structured gas/ceramic system) and experimental results from the literature have enabled to validate the model in various configurations. The model is able to deal with liquid or gaseous heat transfer fluids and with structured packed beds with channels or granular packed beds with uniformly sized particles or particles of two different sizes.Cette étude porte sur les systèmes de stockage thermique régénératif dont le principe consiste à stocker de l’énergie sous forme de chaleur sensible dans un lit fixe. Le système est chargé et déchargé à l’aide d’un fluide caloporteur circulant à travers le lit fixe. Ce type de système est prometteur pour réduire le coût des infrastructures de stockage, par exemple dans les centrales solaires thermodynamiques. Cependant, le pilotage de ces systèmes est relativement complexe car leur fonctionnement est régi par divers phénomènes et met en jeux plusieurs modes de transfert de chaleur. Leur identification a permis de développer un modèle numérique monodimensionnel constitué d’une équation de continuité et de trois équations d’énergie : une pour le fluide caloporteur, une pour le solide du lit fixe et une pour les parois du réservoir. Les études expérimentales réalisées sur trois systèmes différents (un système huile/roches+sable, un système gaz/roches et un système gaz/céramique structuré en canaux), ainsi que des résultats issus de la littérature ont permis de valider le modèle proposé dans une large gamme de configurations. Le modèle s’avère notamment capable de traiter les fluides caloporteurs liquides ou gazeux et les lits fixes structurés en canaux ou constitués de milieux granulaires à granulométrie simple ou double

A versatile one-dimensional numerical model for packed-bed heat storage systems

International audienceThanks to their versatility and their relatively low cost, packed-bed sensible heat storage systems are promising for various applications like in central solar power plants, adiabatic compressed energy storage and pumped thermal energy storage. A versatile one-dimensional numerical model able to describe many packed-bed configurations is developed and presented. This model is able to treat liquid or gaseous heat transfer fluids. The packed bed can include a mixture of large and small solid particles such as rocks and sand, commonly encountered in the literature due to the advantages it procures. The model is compared and validated with specific experimental data and results from the literature covering wide ranges of configurations and operating conditions: several heat transfer fluids (molten salts, thermal oil, air), solid materials (rocks, sand, ceramics), fluid velocities, temperature levels and packed bed configurations are successfully tested. This shows the versatility of the developed model. The influence of the fluid velocity on heat losses, thermal diffusion and fluid/solid heat exchange are analysed. It enables to determine the optimal velocity which maximizes the performance of the storage system

A review on experience feedback and numerical modeling of packed-bed thermal energy storage systems

International audienceSolar 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

Experimental study and numerical modelling of high temperature

International audience12 13 Two pilot-scale regenerative heat storage systems have been tested by the French Alternative Energies and Atomic 14 Energy Commission (CEA). The first one is a 1.1-MWhth structured packed bed consisting of ceramic plates forming 15 corrugated channels. The second one is a 1.4-MWhth granular packed bed consisting of basaltic rocks enclosed by refractory 16 walls. The two regenerators were tested over a hundred of thermal cycles between 80°C and 800°C with different fluid mass 17 flows. Both systems showed their ability to store heat efficiently and to provide thermal energy at a stable temperature for the 18 most part of the discharge process. The granular packed bed exhibited large transverse thermal heterogeneities due to flow 19 channelling in the corners of the cross section. However, this phenomenon appears not to have degraded significantly the 20 thermal performances, and the average one-dimensional thermal behaviour of the system may be assessed thanks to the surface 21 weighted average of the temperature over the bed cross section. Compared to the granular packed bed, the structured bed 22 showed comparable thermal performances while inhibiting flow heterogeneities and reducing by up to 54% the average 23 pressure drop. Furthermore, at the end of the test campaign, the packed beds were observed and compared from a mechanical 24 point of view. The thermal results were successfully simulated over numerous charge/discharge cycles thanks to a one-25 dimensional numerical model. This is significant since the discrepancies between experimental and numerical results are likely 26 to accumulate from a cycle to the other. The model considers the packed beds as continuous and homogeneous porous media 27 but takes account of the conduction resistances within the solid filler and the walls. The pressure drop of the beds was 28 computed using a correlation developed thanks to a previous CFD study for the structured packed bed, and the Ergun equation 29 for the granular packed bed. Compared to experimental data, these correlations enabled to estimate the order of magnitude and 30 the evolution trend of the pressure drop with an average deviation ranging from-7.2% to +61.9%. For the granular packed bed, 31 these deviations are ascribed to the flow heterogeneities and the shape of the rocks which are not taken into account in the 32 Ergun equation. 33 34 In order to tackle fossil fuel depletion and climate change, the renewable energy share in the energy mix has to be 40 increased, especially for electricity generation (IPCC, 2014). However, some promising renewable energy sources like wind 41 and solar are intermittent. That's why energy storage is one of the technical solutions enabling the development of a 42 continuous and controllable renewable energy supply. 43 Nowadays, electricity storage is largely dominated by Pumped Hydroelectric Energy Storage since this technology is 44 mature and offers low loss storage capacities. However, it has a very low energy and power density (Sabihuddin et al., 2015), 45 and requires specific locations with high altitude difference to be implemented. That's why this technology is unlikely to 46 respond the increasing need for large scale energy storage. Several promising alternative technologies have been proposed like 47 Adiabatic Compressed Air Energy Storage (Hartmann et al., 2012; Sciacovelli et al., 2017a), Pumped Thermal Energy Storage 48 (Desrues et al., 2010; Garvey et al., 2015), Liquid Air Energy Storage (Sciacovelli et al., 2017b) and Thermal Energy Storage 49 in Concentrated Solar Power (CSP) plants (to shift the conversion of heat into electricity). In all these thermodynamic 50 processes, there is a need for large scale Thermal Energy Storage systems. It is preferable to store/recover the energy at very 51 high or very low temperature (for Pumped Thermal Energy Storage and Liquid Air Energy Storage) to ensure good 52 thermodynamic efficiency. Regenerative sensible heat storage using gas (usually air) as heat transfer fluid is a relevant and 53 mature technology which meets this requirement. 54 It consists in storing sensible heat in a solid packed bed enclosed in an insulated tank. The packed bed may be either 55 granular or structured. The thermal energy is conveyed by a gaseous heat transfer fluid in direct contact with the solid. During 56 charging process, the hot fluid is injected by the top of the tank, heats the solid and exits at cold temperature from the bottom. 57 To discharge the system, the flow is inverted: the cold fluid is injected by the bottom, is heated up by the solid and exits at hot 58 temperature from the top. Thanks to buoyancy forces, this configuration preserves thermal stratification with hot and cold 59 regions well separated by a thermal gradient as stiff as possible. 60 This technology is already used in steel and glass industries (to preheat air in blast furnaces), and in industrial air 61 purification systems. However, to use it in the above mentioned thermodynamic processes, the design and the operation 62 strategy should be adapted to each particular case to ensure optimal performances. This optimization requires numerical 63 models validated with experimental data obtained on representative pilot-scale setups. 6