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Based on the local thermal non-equilibrium theory, a modified transient two-dimensional numerical model is developed to investigate the simultaneous charging and discharging performance of a molten-salt packed-bed thermocline tank employed in concentrated solar power plants. Two different types of operation including that of stable and periodic charging with steady discharging are analyzed. Three different solid fillers are utilized, including quartzite rock, slag pebbles, and alumina ceramics. The effects of charging/discharging flow rate ratio and non-charging periods on thermocline thickness and energy storage power have been investigated. The results show significant influences of the charging/discharging flow rate ratio. When charging is steady, the discharge performance of a packed-bed tank is better than that of a pure molten-salt tank at lower flow rate ratios. The thermocline thickness of the packed-bed tank is higher than that of a pure molten-salt tank for different solid materials. Compared with quartzite rock and slag pebbles, alumina ceramics achieves a higher thermal gradient. In periodic charging mode, as the cycle's number of charging periods increases, thermocline thickness increases linearly. Operating at higher charging rates and shorter non-charging periods, the discharge outlet temperature of molten-salt becomes more stable for the steam generation process and hence, the discharge performance is improved. [Display omitted] • Simultaneous operation of molten-salt energy storage in porous packed-bed tank presented. • Local non-equilibrium thermal model for simultaneous operation adopted. • Periodic and stable charging with simultaneous discharging process of molten-salt tank investigated. • Influences of flow rate mixing ratio, non-charging period and charging flow rate revealed. [ABSTRACT FROM AUTHOR]

• Steel making slag pebble as reliable storage medium in central receiver systems. • Metallurgical by-product waste revalorized as a cost-effective storage material. • Comparison between slag pebble and other thermocline filler materials. • Slag pebble used as packed-bed filler in a solar hybrid storage configuration. • The pumping tanks allow to increase the maximum thermocline tanks storage capacity. This work focuses on the reliable use of the steel making slag pebble as filler material of thermocline tanks which are combined with conventional molten salt tanks in a hybrid thermal storage configuration. The use of this hybrid configuration not only improves the efficiency and operation of the plant when there is solar radiation fluctuation but also includes pumps in separate molten salt tanks in order to increase the maximum height of the thermocline tanks and its useful storage volume, i.e. its storage capacity. A packed-bed numerical model has been used to study the potential of the slag pebbles in this hybrid storage configuration, comparing it against other well-known potential filler materials for thermocline tanks as the taconite and quartzite rock. The study carried out considers a storage capacity of 9 full load hours for a central receiver system of 110 MWe gross power. The results show that two thermocline tanks of 20 × 20 m, filled with steel making slag pebbles, reaches a capacity of 6.61 full load hours. An economic analysis is included which shows that the hybrid storage solution using slag pebbles represents a cost reduction of 13.7% against the state of the art solution. Additionally, comparing it against other filler materials, the slag hybrid storage solution has a cost reduction of 3.8% and 13% against the quartzite and taconite hybrid storage solution, respectively. [ABSTRACT FROM AUTHOR]

Currently, energy storage systems are considered a key solution when mismatch occurs between energy supply and demand, allowing a more efficient energy deployment and use. The present paper is focused on the study of a latent heat thermal energy storage (LHTES) system based on a packed bed of encapsulated phase change material (PCM) of spherical shape, conceived as an auxiliary component of a micro-grid to be built in a Research Center located in southwestern Sardinia (Italy). The main purpose of this work was to perform numerical simulations for predicting the performance of the TES system, designed to store the surplus thermal energy produced during the weekend by a heat pump fed by a photovoltaic (PV) plant. The stored energy would then be utilized during the weekdays to integrate the air-conditioning system supply. The numerical simulations were based on a one-dimensional (1-D) two-equation transient model, able to return the thermocline profile of the water and the PCM separately. The behavior of the LHTES device during charge and discharge phases was reproduced, as well as during the standby periods. Finally, two characteristic indexes of the PV system were evaluated, to investigate the effect of TES on grid interchanges, self-consumption, and self-sufficiency.

In the study, a transient, two-dimensional and axisymmetric model of the packed-bed thermocline tank is developed. Based on the model, the cyclic thermal performance of a traditional Single-Layered and of a novel Multi-Layered Packed-Bed molten salt Thermocline Tank (SLPBTT, MLPBTT) are analyzed. First, the analysis of cyclic thermal performance of SLPBTT shows the performance can be enhanced by reducing the retention thermocline thickness. Second, this is the first time for a detailed investigation of the expanding and the shortening effects on thermocline thickness at the interface between two kinds of filler. In addition, a novel MLPBTT is designed utilizing the above interface effects for improving the performance by controlling thermocline expansion. Finally, the studies on the performance of MLPBTTs adopting three fillers (quartzite rock, cast iron, and high-temperature concrete) with different heights present that the useful energy can be increased while thermal efficiency will be reduced with the increasing cast iron's height. An optimized MLPBTT shows a significant improvement in the useful energy of 10.5% and a small drop in thermal efficiency of 2.1% in discharging process compared with those of SLPBTT using the quartzite rock. The results can be beneficial for the design and optimization of PBTT. [ABSTRACT FROM AUTHOR]

The sensible heat storage in low-cost secondary fillers using a single tank thermocline system offers a cost-effective storage option for concentrating solar power (CSP) plants. A comprehensive numerical simulation of 125 MWh t thermocline tank is performed by adopting a transient, two-dimensional, two-phase model to investigate the thermal performance of packed-bed thermocline thermal energy storage (PBTC-TES) system. The effect of relevant design and operating parameters on the performance of TES system are examined by analysing the thermocline expansion and local variation of salt and filler temperature. It is observed that the operating temperature difference (ΔT) has ample role on thermal performance of TES system as the efficiency is found to reduce by 12% with increase in ΔT from 50 K to 150 K. Discharging efficiency is estimated to be high at low operating temperature ranges and tend to decrease with increase in ΔT. Also with increase in power output, discharging efficiency tends to reduce slightly since it demands higher flow rates and operating temperature range. In case of inlet salt velocity, discharging efficiency drops by 3.5% when velocity is increased thrice the initial value. Further, a comparative study is performed to figure out the most dominant efficiency determining operating parameter. It is observed that relative to inlet salt velocity, operating temperature range seems to have more influence on the thermal performance of TES system. When the operating range is increased from 50 to 150 K, efficiency drops by about 10% whereas a reduction of only 2.5% is observed when power output is increased from 15 to 35 MW t for similar ΔT values. Hence, operating temperature range is identified as the dominant efficiency determining parameter of PBTC-TES system. [ABSTRACT FROM AUTHOR]

Highlights: [•] Dynamic thermal performance of a molten-salt packed-bed TES system using PCM capsules was investigated. [•] The temperature response and phase change process within PCM capsules were revealed. [•] A quasi-isothermal region and two thermocline regions were identified for the axial molten-salt temperature distribution. [•] PCT of PCM, molten-salt inlet velocity and capsule diameter influence the effective discharging efficiency. [Copyright &y& Elsevier]

Packed-bed single-tank thermocline system with reduced cost is an alternative to the conventional two-tank system for thermal energy storage. This work systematically explores the wall impact on thermocline behavior of packed-bed tanks. For this purpose, adapted transient models were developed and fully exploited for the first time. Two tank configurations were investigated and compared: a high-temperature pilot-scale tank with a steel wall and a low-temperature lab-scale tank with a polycarbonate wall, both tanks being insulated by mineral wool. Results showed that the maximum energy stored in the wall at fully charged state can be up to 10% of the total stored energy. This part of stored energy has a negative impact on the discharging, causing up to 15% increase of the thermocline thickness. The energy stored in the insulation is very small so that this phase can be simplified as a thermal resistance in the modeling. The optimal wall parameters for packed-bed TES tanks were obtained that a thinner wall has a smaller impact on the energy and exergy efficiencies at the discharging cutoff time. The findings of study could provide useful design guideline for pack-bed thermocline TES tanks for different industrial applications. • Wall effect in packed-bed thermocline storage tank is studied by dynamic modeling. • Stored heat inside wall before discharging can be up to 10%. • Longitudinal wall heat conduction can be ignored. • In discharging, the stored heat in wall increase thermocline thickness by up to 15%. • Decrease wall thickness can achieve higher energy and exergy efficiency. [ABSTRACT FROM AUTHOR]

Abstract: A comprehensive transient, two-dimensional, two-phase model for heat transfer and fluid dynamics within a packed-bed molten salt thermocline thermal storage system has been developed in our prior paper. In the present paper, based on the developed model, the effects of various parameters, such as flow rate and temperature of inlet molten salt, porosity and height of the system, and the thermal losses on the thermal performance of the system, are investigated. The standby behavior focusing on the effects of wall structure, ambient air velocity on the thermocline expanding behavior is also studied. The results show that both the fluid inlet velocity and the inlet temperature have negligible influence on the thermocline development and hence the effective discharging efficiency, while increasing the tank height can effectively shrink the normalized thermocline region and lead to a higher efficiency. With good insulation, the heat losses from the standby system with a uniform initial temperature can be significantly lowered, and uniformly distributed molten salt temperature in the radial direction can be achieved. However, for the standby system with a thermocline region, the interior molten salt temperature can be influenced by the insulation layers and steel wall, causing temperature gradient in the radial direction. [Copyright &y& Elsevier]

Abstract: In this paper, a comprehensive transient, two-dimensional, two-phase model for heat transfer and fluid dynamics within the packed-bed molten salt thermocline thermal storage system is presented. After model validation, the developed model is used to investigate the general thermal behavior of a discharging process of the pack-bed thermocline system and evaluate the interstitial heat transfer coefficient, the effective thermal conductivity and effect of the thermal conductivity of solid fillers. The results show that the thermocline region is moving upward with slight expansion during the discharging process. With the use of two insulation layers, a uniform cross-sectional temperature distribution is well achieved. The use of different correlations for the interstitial heat transfer coefficient or the effective thermal conductivity from the literature leads to negligible difference in the predicted thermal performance. It is also found that decreasing the heat transfer rate between fluid and solid fillers, or increasing the thermal conductivity of solid fillers, results in an increase in the thermocline thickness which finally decreases the effective discharging time and the effective discharging efficiency. [Copyright &y& Elsevier]

• Multi-criteria optimisation is performed on an industrial thermocline tank. • Objective function includes exergy efficiency and a normalised environmental indicator. • Pareto set is obtained, bounded by the exergy and LCA-optimised solutions. • Exergy optimisation leads to a tapered tank shape. • Environmental optimisation leads to a square tank shape. Thermocline thermal energy storage is one of the most promising solutions for recovering waste heat in industrial plants. This paper aims to optimise the shape of a thermal energy storage to minimise its environmental impacts and maximise its exergy efficiency. The reference storage is an existing industrial high-temperature air/ceramic packed-bed heat storage called EcoStock®. The physical model used to determine the performances of the tank is a one dimensional model with two equations: one for the heat transfer fluid and one for the filler material. The environmental impacts are analysed using a life cycle assessment through four selected indicators: cumulative energy demand, global warming potential, abiotic depletion potential and particulate matter. To solve this multi-criteria problem, a particle swarm optimisation algorithm was applied with several exergy and environmental weighting factors. A Pareto set is obtained, bounded by the single exergy or environmental optimisations. Favouring exergy efficiency reduces the volume of the tank. However, environmental footprint of the tank is increased: the indicators of cumulative energy demand and abiotic depletion potential are considerably higher. The shape of the tank evolves with the exergy weight, from a square shape (environmental optimisation) to a tapered shape (exergy optimisation). [ABSTRACT FROM AUTHOR]

• Local non-equilibrium thermal model of molten-salt packed-bed energy storage tank presented. • Different thermocline characteristics mapping of various fillers and pure molten-salt revealed. • Discharge temperature stability affected by particle size and material properties of fillers. • Obvious merits presented for packed-bed system when operating at lower flow rate. Thermocline characteristics and the discharge performance of a molten-salt packed-bed energy storage system are analyzed numerically. For this purpose, a transient two-dimensional model based on local non-equilibrium thermal theory in a porous medium is developed. Three different solid fillers are utilized, including that of quartzite rock, slag pebbles, and alumina ceramics. Molten-salt serves as a heat transfer fluid (HTF). The effects of fluid inlet flow rate and the particle size diameter on the thermocline thickness through temperature profiles of packed-bed systems have been investigated. The discharging power is discussed in a detailed description of various operating conditions through the discharge outlet temperature stability period. The results obtained are compared with that of the pure molten-salt tank. It is revealed that the thermocline thickness of the molten-salt packed-bed tank is higher than that of pure molten-salt tank, while the latter is better in thermal stratification. As the inlet flow rate increases, both the discharging power and thermocline thickness increase. The results also indicated that decreasing particle size diameter, results in a decrease in thermocline thickness, and the discharging performance becomes more stable. Slag pebbles as a filler material is more effective than quartzite rock in thermal energy storage (TES). The results can be beneficial for the design and optimization of the packed-bed thermocline tank. [ABSTRACT FROM AUTHOR]

Currently, energy storage systems are considered a key solution when mismatch occurs between energy supply and demand, allowing a more efficient energy deployment and use. The present paper is focused on the study of a latent heat thermal energy storage (LHTES) system based on a packed bed of encapsulated phase change material (PCM) of spherical shape, conceived as an auxiliary component of a micro-grid to be built in a Research Center located in southwestern Sardinia (Italy). The main purpose of this work was to perform numerical simulations for predicting the performance of the TES system, designed to store the surplus thermal energy produced during the weekend by a heat pump fed by a photovoltaic (PV) plant. The stored energy would then be utilized during the weekdays to integrate the air-conditioning system supply. The numerical simulations were based on a one-dimensional (1-D) two-equation transient model, able to return the thermocline profile of the water and the PCM separately. The behavior of the LHTES device during charge and discharge phases was reproduced, as well as during the standby periods. Finally, two characteristic indexes of the PV system were evaluated, to investigate the effect of TES on grid interchanges, self-consumption, and self-sufficiency. [ABSTRACT FROM AUTHOR]