Heat Transfer Fluid Flow Research Articles

Parametric analysis of a solar parabolic trough collector integrated with hybrid-nano PCM storage tank

ApplicationNewly developed fluid called phase change materials (PCM) used to accumulate solar thermal energy. This paraffin-based fluid is mixed with hybrid nanoparticles composed of titanium dioxide “TiO2″ and copper oxide “CuO”. Using this fluid, we hope to improve the efficiency of industrial solar thermal installations. Purpose and MethodologyA computational fluid dynamics (CFD) model was developed using Ansys Fluent 19, coupled with a home code implemented in the C programming language as a “user-defined function”. Initially, the model was validated against experimental data, then a parametric study was conducted to investigate the impact of pure paraffin PCM and hybrid nanoparticles-paraffin PCM, as well as the effect of varying the mass fraction of nanoparticles and the heat transfer fluid flow rate. geometric parameters such as the number of fins installed in the storage tank were also considered during the analysis. Major findingsOn the basis of the obtained results, it was found that the incorporation of paraffin phase change materials (PCMs) within a storage tank has the potential to significantly prolong the operational duration of solar systems. Additionally, hybrid nanoparticles at 1% concentration increased the average storage tank temperature by 8% and decreased the melting time by 13.29% compared to pure PCM.

Thermal performance research on a novel coupled heating system combined solar air heater with ventilation PCM wall

Solar heating technology is increasingly recognized as a prominent option for clean building heating. However, challenges such as intermittent thermal energy supply and supply–demand mismatch hinder its widespread adoption. To address this issue, this paper introduces a novel coupled heating system that combines a solar air heater with a ventilation Phase Change Material (PCM) wall. A rural residential house in Tianjin was chosen as an experimental platform and comparative experiments was conducted to compare the advantages of the ventilation PCM wall with the conventional wall. Additionally, numerical simulations were utilized to address the non-uniform heat transfer in the direction of the Heat Transfer Fluid (HTF) flow by employing cascaded PCM. The experimental results demonstrated that the coupled heating system exhibited high thermal storage and release efficiency, significantly improving indoor thermal comfort. Under optimal operating conditions (air supply temperature: 35-45℃, air supply speed: 3–4 m/s), the system achieved rapid thermal storage within 4 h, reaching a maximum thermal storage efficiency of up to 87.6 %. The test room maintained a nighttime temperature of 15.7 °C, exhibiting temperatures 6.3–10 °C higher than the control room. Numerical results revealed that the cascaded PCM enhanced the thermal storage rate, particularly at the first 2.8 h of the thermal storage period, contributing to the overall efficiency and flexibility of the system. This research provides theoretical guidance for the optimal design and practical application of the ventilation PCM wall.

MHD natural convection flow in a porous medium-filled corrugated enclosure: Effect of heat sources with different heights

A two-dimensional natural convection flow problem in a porous medium-filled enclosure with electrically conducting fluids and solid heat sources of different heights is numerically investigated in this study. The top boundary is cold, and the vertical side boundaries are insulated, while rectangular-shaped corrugations at the lower boundary are generated by placing heat sources. Three distinctive cases are considered concerning the height of the solid rectangular-shaped heat sources. The nonlinear coupled transport equations are discretized by using a higher-order compact (HOC) finite difference scheme. First, existing numerical and experimental data are used to validate the developed code. Then, the effect of key parameter values, like Hartmann number (25–150), Rayleigh number (103–106), Darcy number (10−5–10−1), and Prandtl number (0.015–10), on the convective flow in the corrugated enclosure is analysed. The influence of the permeability of the porous medium is abundantly seen for a range of significant parameter values with a fixed high Rayleigh-Darcy number. The formation of primary, secondary, tertiary, and quaternary vortices affects the thickness of the thermal boundary layer on both hot and cold surfaces in both symmetric and asymmetric patterns. In addition, possible multiple steady states are observed in the Case 2 at Pr=10.0. The local and average Nusselt numbers remains constant on the bottom cold surfaces of the corrugated enclosure, while significant variations are observed on the upper surfaces of the heat sources. These findings are analysed by conducting stream-function (ψ), isotherms (T), local (Nuh,Nuv), and average (Nū) Nusselt number plots. The intensities of heat transfer rate are also examined by analysing Nusselt numbers as a function of Prandtl, Darcy, Rayleigh and Hartmann numbers. Analysing natural convective heat transfer fluid flow in an enclosure with heat sources is prominent for ample engineering applications such as solar collectors, designing nuclear reactors, chemical energy production systems, building constructions and microelectronic equipment, to name a few.

Thermal energy storage based on cold phase change materials: Discharge phase assessment

In an energy scenario characterized by strong requirements in terms of flexibility and readiness, the integration of thermal energy storage in energy systems could play an important role in demand-supply management and allows novel operational schemes. Thermal Energy Storages based on latent heat are characterized by their compactness and small temperature swing. However, there is still a lack of performance analysis on large-scale setups. This work aims to fill this gap. In this paper, a shell & tube latent heat-based cold thermal energy storage was characterized in the discharge configuration, considering different temperatures and mass flow of the heat transfer fluid, representing an opportunity to understand the behavior of a full-scale system in different operative conditions. Sensitivity analyses on heat transfer fluid flow rate, flow direction, and inlet temperature were performed. The results show that peak power increases by approximately 25 % with doubled mass flow rate, and it doubles with increased inlet temperature by 6 °C. Since the impact of the buoyancy effect occurs when the liquid phase is predominant over the solid one, there is no strong impact on the direction of the Heat Transfer Fluid, from top to bottom or viceversa. Despite no metastability phase being detected, many discontinuities in the thermal power and temperature profiles were identified and analyzed, providing new insights into full scale latent heat storage. Finally, in this work, the thermal round-trip efficiency was estimated to reach above 90 %.

Natural Convection in the Melting of Phase Change Materials in a Cylindrical Thermal Energy Storage System: Effects of Flow Arrangements of Heat Transfer Fluid and Associated Thermal Boundary Conditions

Abstract Latent thermal energy storage systems (LTESS) have received widespread attention due to their high energy density to store a significant amount of thermal energy in the form of latent heat into phase change materials (PCM) at a nearly constant melting temperature. The thermal efficiency of LTESS is usually limited by poor heat conduction in PCM but enhanced by the natural convection of molten PCM. The natural convection increases the uniformity of temperature by mixing in a PCM enclosure and therefore increases the heat transfer rates and accelerates the melting. While there is negligible natural convection, periodic reciprocation of heat transfer fluid (HTF) through the PCM enclosure has been demonstrated to increase the heat transfer rates to PCM by increasing the melt interface area and reducing temperature gradients across PCM compared to fixed-directional flow arrangements. The current study examines the effects of HTF flow direction on the strength and duration of natural convection in PCM in a vertical cylindrical shell-and-tube container. Gallium is used as the PCM because of its low melting temperature and high thermal conductivity, and water is used as the HTF. A 2-D axisymmetric numerical model developed in ANSYS Fluent was used for the study of the cylindrical LTESS. The aspect ratio of the cylindrical container is nearly 1, which allows the generation of natural convection currents of strong magnitude in molten PCM in the vertical orientation. The irregular melting front in the PCM is caused by both natural convection in molten PCM and thermal boundary conditions for different HTF flow arrangements. The temperature and melting front profiles of PCM with the reciprocating flow arrangement are compared to unidirectional flows in upward and downward directions. The influence of HTF operating parameters such as temperature, velocity, and reciprocation period on PCM melting is studied. Scale analysis is also applied to characterize the different melting regimes of PCM under different flow arrangements.

Thermodynamic performance of cascaded latent heat storage systems for building heating

Decarbonization of building space heating is essential for China to meet its carbon neutrality goal by 2060. Cascaded latent heat storage (CLHS) coupled with electric heating is a promising technology to promote renewable energy consumption, reduce carbon emissions, and save on heating bills. However, few studies have focused on the thorough investigation of the superiority of the CLHS system over a non-cascaded system. This study investigated the effects of heat transfer fluid (HTF) flow rates, HTF inlet temperatures, and number of stages on the thermodynamic performance of non-cascaded and CLHS systems based on energy and entransy analysis. Results showed that, compared with the non-cascaded system, the charged thermal energy and discharged thermal energy of the two-stage CLHS system increased by 26.5%–44.6% and 19.8%–74.5%, respectively, and the entransy increase of cold HTF increased by 20.0%–75.7%; while, in heating systems, it is not guaranteed that thermodynamic performance improves by using more stages in the CLHS system.

Predicting thermal conductivity and dynamic viscosity of nanofluid by employment of Support Vector Machines: A review

Nanofluids are employed in variety of thermal energy systems owing to their modified properties and ability of enhancement in heat transfer. Among the properties of nanofluids, thermal conductivity and dynamic viscosity have significant role in the heat transfer characteristics and fluid flow specifications. Among the intelligent techniques, Support Vector Machines (SVMs) are attractive choices regarding their significant exactness in prediction and modeling. In this article, research works on the forecasting and modeling of the mentioned properties of nanofluids with SVMs are reviewed and their outcomes are presented. In accordance with the results of the reviewed works it can be stated that the developed approaches based on the SVM technique have shown significant performance in accurate modeling of both thermal conductivity and dynamic viscosity and can have superior performance in comparison with other intelligent methods in some cases. Furthermore, it can be pointed out that the performance of these systems is dependent on some elements like the employed optimization algorithms. Some recommendations are represented for future studies to reach models with more favorable performance.

Heat transfer enhancement and performance study on latent heat thermal energy storage system using different configurations of spherical PCM balls

Thermal energy storage (TES) systems utilizing latent heat storage substances have gained significant attention recently due to their large heat storage capacity and isothermal behavior during charging and discharging processes. However, many phase change materials (PCM) used for storage exhibit relatively low thermal conductivity. Prolonged surfaces like fins are incorporated in PCM balls to improve heat transfer performance. These fins significantly improve the overall performance of the TES system. This study conducted experiments to evaluate the performance of a latent heat thermal energy storage (LHTES) system using spherical balls filled with PCM, both with and without solid internal fins. Water was used as the heat transfer fluid (HTF) at a constant temperature of 70 °C, while paraffin served as the PCM with a melting temperature of 61 °C. Time-based temperature variation of HTF and PCM has been investigated for various HTF flow rates. Moreover, the effect of non-dimensional numbers, such as Reynolds, Rayleigh, Nusselt, and Stanton numbers, has also been investigated. Performance parameters such as thermodynamic efficiencies and entropy generation are analyzed. The heat storage process aims to get a highly efficient configuration for accumulating the maximum energy possible and higher exergy efficiencies during the charging process. The charging duration is reduced by 47.37 % when comparing the solid internal fin configuration at the higher flow rate (6 L/min) with the no fin configuration at the lower flow rate (2 L/min). At 100 min, the cumulative heat stored is higher in the solid internal fin configurations than in the configurations without fins. Furthermore, the internal fin configurations have less entropy generation than without fin configurations, justifying that providing solid internal fins in PCM balls enhanced the LHTES system performance.

NTPSS-vPower: A model for system simulation of a nuclear thermal propulsion system

Nuclear thermal propulsion (NTP) with a large specific impulse and a high thrust-to-weight ratio is one of the preferred propulsion technologies for realizing mankind’s vision of interstellar exploration. The system analysis of NTP, which is a crucial part of the system design, is used to evaluate the system response during operation. This work developed the NTPSS-vPower model on the vPower simulation platform for system simulation of a typical NTP system. Specifically, a properties model considering the thermal dissociation of high-temperature hydrogen, a point neutron kinetics model, a hybrid model of two-dimensional heat transfer and one-dimensional fluid flow of the core, and simplified models of the nozzle, turbine, and pump are developed. The differences of crucial parameters between steady-state simulation results and published design data are within 5%. Furthermore, NTPSS-vPower is used to simulate four typical events to predict the transient behavior of the system, which provides a reference for subsequent system analyses and experimental studies.

Clustering-based model predictive control of solar parabolic trough plants

This paper presents a clustering-based model predictive controller for optimizing the heat transfer fluid (HTF) flow rates circulating through every loop in solar parabolic trough plants. In particular, we present a hierarchical approach consisting of two layers: a bottom layer, composed of a set of model predictive control (MPC) agents; and a top layer, which dynamically partitions the set of loops into clusters. Likewise, the top layer allocates a certain share of the total available HTF to each cluster, which is then distributed among the loops by the bottom layer in response to the varying conditions of the solar field, e.g., to deal with passing clouds. The dynamic clustering of the system reduces the number of variables to be coordinated in comparison with centralized MPC, thereby speeding up the computations. Moreover, the loops efficiencies and the heat losses coefficients, which influence the loops control model, are also estimated at the bottom layer. Numerical results on a 10-loop and an 80-loop plant are provided.

A single long NiTi tube compressive elastocaloric regenerator: experimental results

Elastocaloric cooling is an environmentally friendly alternative to the current vapor-compression refrigeration technology, and the development of an efficient operation strategy is significant for its commercialization. In this article, the cooling performance including the temperature span, specific cooling power, and coefficient of performance for a novel single long NiTi tube compressive elastocaloric regenerator (tube outer diameter 5 mm, wall thickness 1 mm, and initial length 305 mm) was comprehensively characterized under different operation parameters (operation cycle time, loading/unloading time, heat transfer fluid timing, flow time, and utilization). The single long NiTi tube compressive elastocaloric regenerator achieved maximum temperature span, specific cooling power, and coefficient of performance of 5.7 K, 135 W·kg−1, and 4.7, respectively under an applied strain of 2.5%. It was found that the most important factors for obtaining a good cooling performance of the single long NiTi tube compressive elastocaloric regenerator were a short operation cycle time, a proper heat transfer fluid timing, and a proper heat transfer fluid utilization. The dependences of the temperature span on the cycle time and heat transfer fluid utilization factor were in agreement with the existing experimental data for a parallel plate tensile elastocaloric regenerator. A thinner tube wall thickness and advanced cross-section geometry for the regenerator may further improve the cooling performance of the compressive elastocaloric regenerator.

EXPERIMENTAL AND NUMERICAL ANALYSIS OF NEW CURVED CONFIGURATION OF HEAT SINKS

In this study, the effect of changing geometry of the heat sinks on forced convection heat transfer and laminar fluid flow features in a rectangular channel has been studied experimentally and numerically of the circular fins and circular-cut fins heat sinks with straight continuous fins as reference comparison. For the experimental conditions, the Reynolds number value varied as 2418.56- 806.19 according to the variation of flow rate values. The wall heat flux was varied as 11.11, 31.11, 58.05, 83.05, 112.5, 155 kW/m2. COMSOL Multiphysics was used to solve the governing equations under laminar flow conditions with air as the working fluid. To verify the accuracy of the analytical model, the simulation's optimal design was constructed and experimentally validated. The results indicated that circular-cut fins enhanced heat dissipation. Nusselt numbers for CCFs with Reynolds numbers of 797.7 and 2393.1 are roughly 83.6 and 132.4 at a heat flux of 11.1, kW/m2.
 
 

Thermo-fluid performance enhancement in a long double-tube latent heat thermal energy storage system

Thermodynamic analysis and flow rate optimization for the long double-tube latent heat thermal energy storage systems (LHTESS) are performed. Computer modeling is carried out using created software and is based on a developed 3D non-steady non-linear coupled thermo-fluid mathematical model that combines the apparent heat capacity method and finite volume method for the sodium nitrate (NaNO3) phase change materials (PCM) and for the Syltherm800 heat transfer fluid (HTF). The mathematical model and numerical algorithm are verified through comparison with experimental data. It is found that the laminar flow of HTF ensures the uniform temperature in PCM and in HTF and this temperature is equal to the PCM melting temperature. It is proved that the PCM and HTF average temperatures along the LHTESS can be equalized using spatial and temporal velocity variations of the HTF. Mutual impact of the heat transfer and fluid mechanics parameters in the long double-tube LHTESS was studied by correlation between Nusselt numbers and Reynolds numbers. The optimal HTF flow rate parameters were analyzed based on the non-equilibrium thermodynamics method. It is discovered that the laminar regime 0 < Re < 2000 and the turbulent regime 12000<Re<15000 are the most optimal for long double-tube LHTESS of the described geometry.

Heat and mass transfer effects on fluid past an exponentially accelerated vertical plate due to time‐fractional MHD‐free convection

AbstractThis article focuses on the study of heat and mass transfer (HMT) fluid flow over an exponentially accelerated vertical plate, which is subjected to an applied magnetic field and viscous dissipation. The research has applications in various manufacturing processes such as wire/fiber drawing, hot rolling, continuous casting, and hot extrusion, where heat transfer to the ambient medium and the hot moving material are of utmost importance. The findings could also be relevant to aerospace engineering applications. The study investigates the time‐fractional natural convection phenomenon and utilizes conservation laws to derive the flow guiding equations, which are then made nondimensional. Finite difference discretization is utilized to solve the dimensionless equations implicitly. Then the flow simulation results such as concentration, temperature, and velocity profiles are discussed based on the variation in parameters such as Prandtl number (), thermal/mass Grashof number (), Eckert number (), magnetic parameter (), time‐fractional order (), and Schmidt number . Also, the HMT rate is depicted using the Nusselt number and skin friction plots. It is noted that HMT increases when increases and decreases. The change in time‐fractional order affects the velocity profiles adjacent to the wall and is more significant in the case of lower values of the Prandtl number.

Design and thermodynamic analysis of an innovative parabolic trough photovoltaic/thermal system with film-based beam splitter

This study aims to find a feasible photovoltaic/thermal (PV/T) utilization method based on film-based beam splitting technology. In contrast with previous studies, the main novelty of this paper is proposing the design of an innovative PV/T system using both parabolic trough concentrator (PTC) and film-based beam splitter (FBS). The design approach of the PV/T system is presented. A new FBS used for this PV/T system is designed by using the Needle method, and its average reflectivity and transmissivity are 27.2% and 72.8%. By using Ray Tracing and Computational Fluid Dynamics (CFD) simulation methods, the performance evaluation of the PV/T system is conducted. The evaluation results reveal that when the East-West sun tracking error is 0.2°, the optical efficiencies of the photovoltaic module and the solar thermal collector tube (TCT) can be kept at 95% and 93.9%. Moreover, when the heat transfer fluid (HTF) in the TCT is selected to be the ethylene glycol, the operation analysis results demonstrate that the photoelectric efficiency of the photovoltaic module is 25.4% and the thermal efficiency of the PV/T system is 17.24%. The thermal performance of the system can be improved when the HTF flow velocity is increased properly or when the inlet HTF temperature is decreased. The technical feasibility of the proposed PV/T system can be revealed from the analysis results of this study.

Numerical Study of Solar Chimney with Absorber at Different Locations

Heat transfer process and fluid flow in a solar chimney used for natural ventilation are investigated numerically in the present work. Solar chimney was tested by selecting different positions of absorber namely: at the back side, front side, and at the middle of the air gap. CFD analysis based on finite volume method is used to predict the thermal performance, and air flow in two dimensional solar chimney under unsteady state condition, to identify the effect of different parameters such as solar radiation. Results show that a solar chimney with absorber at the middle of the air gap gives better ventilation performance. A comparison between the numerical and previous experimental results shows fair agreement.

Thermal performance augmentation of parabolic trough solar collector using nanomaterials, fins and thermal storage material

Investigating and evaluating parabolic trough collector's (PTC) thermal performance have been shed out numerically by many researchers. Only a few researchers have focused their attention on using experiments to study the PTC's thermal performance. Consequently, the objective of this paper is to utilize an experimental method to put more shed on the PTC performance using active and passive methods. This aim is achieved by developing an experimental setup with modifications in reference parabolic trough collector (RPTC) and employing different heat transfer fluids. RPTC modifications include adding fins to the outer tube surface (FPTC), coating the absorber with Copper Oxide nanoparticles (CuO) combined with black paint (NCPTC), and filling the annulus with graphite (GPTC). Heat Transfer fluid (HTF) is changed by inserting CuO nanoparticles into water (NFPTC). All experiments are performed during August 2021 in Kafrelsheikh University. Data such as: inlet and exit temperatures of HTF, air temperature, air speed, solar irradiation, and rate of mass flow of HTF are recorded hourly for RPTC, NFPTC, NCPTC, NF-NCPTC, FPTC, and GPTC. Based on these data, thermal efficiency is calculated. FHT mass flow rate is controlled and typical values of 8, 10, 14, 16, 20, and 26 l/h are employed with RPTC in purpose to explore the optimum value based on practical considerations, HTF temperature difference, and higher efficiency. This value is found to be 20 l/h and is used in all modified collectors. All modified collectors' thermal performance is studied and compared with the traditional collector. The main finding in present study indicates that there is an augmentation in the thermal efficiency by 19.18 %, 26.23 %, 37.76 %, 44.43 %, and 54.82 % for NFPTC, NCPTC, NF-NCPTC, FPTC, and GPTC respectively when compared with RPTC.

A review of the recent advances in the heat transfer physics in latent heat storage systems

Overwhelming research interest in developing green and renewable energy systems has fostered the innovation of novel latent heat storage systems. Transport phenomena during phase transitions significantly influence the thermal energy transfer rates and the efficiency of charging and discharging such systems. Hence, a comprehensive elucidation of heat transfer physics is essential for researchers to develop the insight, model, experiment, and analyze a thermal system. This paper reviews the heat transfer mechanisms involved in phase-changing processes in various latent heat storage system configurations. At the outset, though the phase-changing process in latent heat storage systems appears simple, a critical observation unveils the complex evolution of various coupled fluid-thermal transfer processes. An astute examination of such phenomena is essential to effectively figure out the role of geometry, orientation, process variables and their dimensionless groups, boundary conditions, material property, etc., in developing accurate models and correlations. The presence of vortical flow structures and gradients of velocity and temperature lead to local variations in heat transfer coefficients in boundaries separating the heat transfer fluid (HTF) passages and phase-changing material (PCM). Gravity influences the Rayleigh–Bénard convection phase-changing process, which offers a higher energy transfer rate than heat diffusion-dominated melting/solidification. A wide variety of test cases are surveyed and conclusively figured out the role of geometry and orientation-dependent convection in the phase-changing process. The present overview of heat transfer physics shall provide a guideline for the system designers to fine-tune HTF flow path geometrical aspects and PCM confinement for improved heat transfer response and system performance.

Experimental studies on novel multi tubular reactor with shell having integrated buffer storage

This study concentrates on experimental analysis of a large scale metal hydride based hydrogen storage reactor having 50 kg LaNi5. In order to cater the heat transfer limitations of hydrogen storage in large scale systems, the present design have been developed as multi tubular reactor with shell having integrated buffer storage configuration with 7 tubes supported by means of 4 baffles having total 50 kg LaNi5 distributed equally among them and water as heat transfer fluid flows across the shell for heat transfer. The experimental study was performed for studying the absorption and desorption performance of the present hydrogen reactor wherein in case of absorption the parameters varied were HTF flow rate at 30 °C and supply pressure of hydrogen and in case of desorption which was carried out at atmospheric pressure, HTF flow rate and HTF temperature were varied. The metal hydride reactor reversibly stores 680 g of hydrogen amounting to 1.34 wt% of gravimetric capacity of metal hydride and equivalent energy storage of 10.4 MJ. In case of absorption, when the flow rate selected was 20 LPM the absorption time for 90 % reaction completion was observed to be 1286 s (21.4 min) at 30 bar H2 supply pressure. In case of desorption studies, it was observed that the varying flow rate from 15 to 25 LPM has negligible effect on hydrogen desorption hence 15 LPM was selected as flow rate for further desorption experiments. Further increasing HTF temperature from 60 °C to 80 °C improves the performance significantly.

Pore-scale numerical investigation on the thermal storage properties of packed-bed systems with phase change material microcapsules

Phase change material (PCM) microcapsules are gaining increasing attention because it can overcome the shortage of PCM macrocapsules and increase the heat transfer area, while the pore-scale thermal performance of the PCM microcapsule packed-bed systems, as well as the influence of packing methods and microcapsule sizes on thermal storage, is still poorly understood. This paper introduces various spherical PCM microcapsule pack models, including single-layered, diameter-changed two-layered, and three-layered packing models, to investigate the charging process at pore scale. The indices of thermal storage properties such as charging time, temperature distribution, phase change fraction, pressure drop and thermocline degradation are studied with the finite element method. In the single-layered thermal storage system, the results show that the charging process can be improved by reducing the capsule size and using cross-packing methods. When the capsule size is reduced from 2 to 0.5 mm with cross packing, the charging time can be shortened by 72.97%, but remarkable pressure drop is created. The results indicate that the overall thermal storage performance of multi-layered packed-bed systems is better than that of single-layered packed-bed systems. The charging time of the diameter-changed two-layered and three-layered thermal storage systems can be reduced up to 16.21% and 24.32%, respectively, compared to the single-layered in-line pack with a d = 2 mm. The filling sequence plays a key role in multi-layered thermal systems. A packed-bed system with a diameter decreasing along the heat transfer fluid (HTF) flow direction has a better capacity to reduce thermocline degradation than other packed-bed systems.