Optimal design of the sensible heat storage system of a central receiver solar power plant operating with gas cycles

The project intends to optimize the thermal energy storage (TES) system of Concentrated Solar Power (CSP) plants. A detailed analysis of the performance of a novel TES system design of sensible heat based on granular material will be done experimentally and numerically. Several configurations of the granular material of the TES system will be studied including the fixed bed configuration, the operation as a fluidized bed, and the performance of a novel design of confined granular material. The novel design will maximise the exergy available of the working fluid beyond its  urrent operation limits. The ultimate aim is to increase the global performance of CSP plants.

Thermo-economic optimization of a novel confined thermal energy storage system based on granular material

The effect of the thermocline thickness was combined with a thermo-economic analysis of a confined bed TES system proposed for a case of study. The new confined bed here proposed was optimized considering thermodynamics aspects, namely the fluid exergy increment in the bed, and economic factors, specifically the total investment cost of the TES system. The optimization resulted in low values of the fluid velocity, between 0.2 and 0.4 m/s, but still higher than the minimum fluidization velocity of sand particles of 750 μm, justifying the requirement of a confined bed, and low bed aspect ratios, between 0.25 and 0.9, to prevent excessively high fluid pressure drops. However, the bed aspect ratio increases significantly for higher granular material particle sizes, up to a ratio of bed height to diameter of 3 for a particle size of 10 mm and a TES demand time of 6 h.

Applied Thermal Engineering Volume 224, April 2023, 120123

Total investment cost per unit of fluid exergy increment.

Modeling sensible thermal energy storage in solid blocks for concentrating solar power

In concentrating solar power plants, the mismatch between solar energy availability and energy demand requires the development of thermal storage systems. This study analyses a solid thermal energy storage module made of alumina. The block has a honeycomb pattern where the air flows through hexagonal channels. A transient 1D model, based on the finite difference method, is implemented and verified against CFD simulations. Such a simplified model considers the temperature dependence of thermodynamic properties for air and alumina. With the same boundary conditions, a 3D RNG k-ε turbulence CFD model is built, whose mesh and time step are assessed by means of sensitivity analyses. The average temperature difference between both models is as low as 14.8 K, even though the simplified one is around 300 times faster than the detailed CFD model.

Results in Engineering Volume 18, June 2023, 101051

Contour maps of temperature along the longitudinal section of the module at four instants of time during the charging process.

Experimental analysis of a novel confined bed system for thermal energy storage

Thermal energy storage (TES) is an essential subsystem for the uniform operation of concentrated solar power (CSP) plants. A sensible heat storage system based on a novel confined bed of small particle size granular material was experimentally evaluated. The bed of regular silica sand was mechanically confined between a bottom and a top perforated plate gas distributor, to prevent the motion of particles even for gas velocities above the minimum fluidization velocity of the solids, operating the bed under a fixed or packed bed regime. The discharge process of the confined bed was experimentally analyzed, preheating the granular material at 300–320 °C and supplying various volumetric flow rates of cold air through the bottom distributor. During the discharge process, the temperature of the bed was segregated, obtaining a high temperature zone at the top region and a low temperature zone at the bottom region of the bed. These regions are separated by a thermocline that evolves in the upwards direction as the discharge process progresses. The temperature distribution in the bed and the total pressure drop of the TES system were monitored during the tests. For all cases, the temperature of the bed at different heights evolves as in a fixed bed, confirming the proper confinement of the granular material. The thermocline velocity depends on the volumetric flow rate of cold air, obtaining a discharge time for the temperature located at the exit of the system of around 50, 40, and 30 min for air volumetric flow rates of 700, 900, and 1100 Nlpm, respectively, using 55 kg of regular silica sand as granular material. The experimental results of the evolution and distribution of temperature in the bed were compared with an analytical model of the process for a fixed/packed bed regime of operation, resulting in a good agreement between the experimental measurements and the numerical predictions.

    Journal of Energy Storage Volume 69, 1 October 2023, 107972

    Experimental (Exp.) and numerical (Mod.) evolution of the distribution of temperatures along the confined bed height during the discharge process.

    Experimental study of the discharge process of a thermal energy storage system based on granular material operated as a fluidized or confined bed

    The stable generation of green electricity by Concentrated Solar Power (CSP) plants is subjected to the proper performance of Thermal Energy Storage (TES) systems, which constitute a critical subsystem of the plant. TES systems based on granular material have already been tested for industrial scale, allowing the operation at high temperature. Depending on the particle size of the granular material and the velocity of the working fluid, the bed conforming the TES system may be operated in a bubbling fluidized or a fixed bed regime. Further, beyond minimum fluidization conditions the bed can operate as fixed if the particles are mechanically confined. The performance of the discharge process of a lab-scale TES system (20.5 cm of diameter, 1 m bed height and 55 kg of silica sand as bed material) of granular material operated as a fluidized bed and as a confined bed was experimentally analyzed for various air volumetric flow rates. For the fluidized bed configuration, the bed temperature was found to be uniform along the bed height due to the high axial mixing rate caused by gas bubbles. During the discharge process of the fluidized bed TES system, the bed temperature decreased exponentially from the initial bed temperature to the temperature of the cooling fluid, with a faster temperature reduction for higher flow rates of fluid. The confined bed operation resulted in a stable temperature of the fluid at the outlet, close to the maximum temperature of the system, for around 45 min when discharging the bed with 700 Nlpm of cold air, whereas after 45 min of the discharge process, the outlet temperature of gas for the fluidized bed was roughly half the temperature obtained from the confined bed. However, the average air pressure drop during the discharge of the TES system of granular material operated as a fluidized bed is stable at around 0.16 bar, while the pressure drop of the confined bed is significantly higher, decreasing from 0.65 to 0.30 bar as the discharge process of the bed progressed.

        Journal of Energy Storage Volume 73, Part C, 15 December 2023, 109173

        Comparison of the evolution of the temperature distribution in the TES systems during the discharge process with an air volumetric flow rate of 700 Nlpm operating as a fluidized (top) and confined (bottom) bed.

        Time evolution of the temperature measured by various thermocouples during the discharge of the fluidized and confined bed TES systems.


        Fluidization XVII, May 21, 2023 to May 25, 2023, Edinburgh, Scotland, United Kingdom

        • Soria-Verdugo, J. F. Guil-Pedrosa, F. Hernández-Jiménez, L.M. García-Gutiérrez, E. Cano-Pleite , N. García-Hernando. Experimental Analysis of a Confined Bed of Granular Material As Thermal Energy Storage System

         SolarPACES Conference, October 10 – October 13, 2023, Sydney, Australia

        • Soria-Verdugo, I. Jiménez-Montero, E. Cano-Pleite, F. Hernández-Jiménez, L.M. García-Gutiérrez, A. Sánchez-González. Experimental Characterization of Sensible Heat Storage in Solids: Granular Material vs. Solid Blocks

        XII National and III International Conference on Engineering Thermodynamics. Universidad Carlos III de Madrid. 29th June – 1st July 2022. Spain.

        • Hernández-Jiménez, L.M. García-Gutiérrez, E. Cano-Pleite, A. Soria-Verdugo, Exergetic and economic optimization of a novel thermal energy storage system based on granular material.
        • Díaz-Alonso, S.; Sánchez-González, A.; Hernández-Jiménez, F.; Soria-Verdugo, A. Modeling sensible thermal energy storage in solid blocks for concentrating solar power. Oral presentation.

        WCPT9 – World Congress on Particle Technology. 18th-22nd, 2022, Madrid (Spain).

        • E. Cano-Pleite, F. Hernández-Jiménez, L.M. García-Gutiérrez, A. Soria-Verdugo, Thermo-economic optimization of a novel confined thermal energy storage system based on granular material. Oral presentation and poster.

        SolarPACES Conference September 26-30, 2022, Albuquerque, New Mexico

        • Sánchez-González, A.; Erasmus, D.J.; Georgiou, M.C. Aiming strategy for circular aperture receiver: Experimental validation at PROTEAS. Online oral presentation.