Thermal Storage Process Research Articles

A molten salt energy storage integrated with combined heat and power system: scheme design and performance analysis

To investigate the flexibility and economic characteristics of a molten salt-combined heat and power (CHP) integrated system under different heat sources, this paper proposes a design scheme for a molten salt-CHP system based on flue gas heat storage, comparing it with main steam heat storage and reheated steam heat source schemes. First, a molten salt heat release sub-loop is designed, where the steam heated by the molten salt can either compensate for heating demands or enter the low-pressure turbine for work, meeting the flexible operation requirements of the unit. Secondly, in response to fluctuations in temperature and pressure parameters of the main steam and reheated steam in the flue gas heat storage scheme, a water-spray desuperheating device is arranged. Finally, a simulation experiment based on a 350 MW CHP was conducted, and the results show that the flue gas molten salt heat storage technology significantly reduces energy loss on the boiler side. Underrated load, compared to the main steam and reheated steam heat storage schemes, the flue gas heat storage scheme has the highest boiler exergy efficiency and total energy utilization efficiency, at 55.8% and 59.1% respectively. During the thermal storage process, the coal consumption index of the flue gas heat storage scheme decreases with increasing load, while conversely, during the heat release process, it increases with the load. The peak-shaving capacity increases with the load, reaching 78.4 MWh under the 75% THA condition. In the steam extraction heating scenario, coupling with the molten salt subsystem can expand the safe heating operation range of the original unit, increasing the maximum peak shaving range by 13.9%, and increasing the maximum heating capacity by 65.4%.

Experimental Study on Phase Change Energy Storage Flooring for Low-Carbon Energy Systems in Grassland Pastoral

Phase change energy storage technology enhances the integration of renewable resources into low-carbon energy systems for grassland pastoral settlements, further addressing the balance between energy needs and environmental sustainability. This study examines a heating system using an experimental platform in an environmental chamber, where the thermal storage and release processes of phase change energy storage flooring were monitored. The results revealed that phase change energy storage flooring exhibits higher heat transfer efficiency and faster heating rates. Under 40 °C heating conditions, the heating rate of the thermal storage layer increased by 12.5% within 1 h. The flooring also demonstrated superior heat release performance, with the peak heat flux of the thermal storage layer delayed by 15 min. Higher heating temperatures shortened the heating time and extended the heat release duration of the phase change energy storage flooring. Under 45 °C heating conditions, the heat transfer efficiency of the surface temperature of the thermal storage layer increased by 38% within 1 hour and by 24.7% over 4 h. In addition, energy consumption in different tests was analyzed, and thermal conductivity was discussed according to the heat transfer model. Phase change energy storage flooring, when coupled with the abundant solar energy resources available in grassland pastoral areas, presents a viable option for the construction of low-carbon energy systems in grassland pastoral settlements.

Evolution mechanism of interfacial multi-layer intermetallic compounds and failure behavior in the Al–Au wire bonding

Evolution mechanism of interfacial multi-layer intermetallic compounds (IMCs) and their impact on failure behavior in Al–Au wire bonding after thermal storage and thermal cycle process are investigated. Microstructural analysis shows that multi-layer IMCs form between the Al wire and barrier layer. After thermal storage and thermal cycle process, the Au8Al3 layer transforms into AuAl2, and Au pad transforms to a thicker layer containing Au2Al and AuAl. Thermodynamic analysis indicates that Au8Al3, Au2Al, AuAl, and AuAl2 have increasing stability. Small-size, equiaxed AuAl2 grains form due to high stored energy, while large-size, columnar Au2Al and AuAl grains result from slower reaction rate and limited recrystallization. The stack-like formation of AuAl2 grains is attributed to the balance between driving force and interfacial energy resistance. During thermal storage, thicker IMC layers cause cracking between IMC layers and Au pad, and micro voids form near the IMC layers/barrier layer interface due to Kirkendall effect. Thermal stress results show that plastic deformation of the Al layer slightly affects the thermal stress in other layers during the thermal cycle process. The cyclic stress at the interface with initial cracks ranges from −157 MPa to 114 MPa, facilitating crack propagation. Additionally, micro void grows under cyclic thermal input, weakening interface adhesion. Therefore, bond strength decreases and frequency of bonding lift-off with cracking along the IMC layers/barrier layer interface increases after thermal storage and thermal cycle process. These results provide the guidance for regulating the interfacial IMCs and predicting the thermal stress in Al–Au wire bonding.

Solar thermal energy and CO2 storage in saline aquifers for renewable energy space heating

There are a large number of abandoned oil and gas wells and corresponding depleted oil/gas reservoirs (DOGR) throughout the world, which are recognized as one of the best geological formations of saline aquifers for thermal energy storage and CO2 sequestration and however, the lack of innovative technique limits the efficient use of underground storage space. Here a novel scheme of storing solar thermal energy and concomitantly sequestrating CO2 in DOGR for renewable energy heating is proposed, which uses CO2 as thermal energy storage working fluid. The process of thermal energy storage and release as well as CO2 sequestration in DOGR are simulated, and thermal performance and CO2 dissolution in DOGR are analyzed. The results for twenty years of operation show that the thermal recovery efficiency reaches up to 82.93 %, which is 76 % higher than the traditional high temperature aquifer thermal energy storage. Energy storage capacity per heating season, energy storage density per unit volume of underground space and energy efficiency are respectively 57,222.27 GJ, 22,943.01 kJ/m3 and 96.19 %. Furthermore, the dissolved CO2 during thermal energy storage and extraction is 54.76 % larger than that of mere CO2 sequestration due to the repeated expansion and contraction of gas and water interface caused by the periodically injection and extraction of CO2, indicating that energy storage accelerates CO2 sequestration. To sum up, the new scheme is a high value-added technical route for solar thermal energy storage and CO2 sequestration as well as renewable energy heating.

An off-grid solar district energy system with borehole thermal energy storage: Life cycle assessment in a subarctic region

Nunavik, a remote subarctic region covering the northern third of Quebec, Canada, relies heavily on diesel to meet residential heating demand. Solar district heating (SDH) with borehole thermal energy storage (BTES) has been developed there as one of the most promising solutions that can break the dependence on fossil fuels and develop renewable energy resources locally. Developing an SDH-BTES in Nunavik is not only a technical and economic consideration, but also an environmental deliberation. Therefore, in this study, a cradle-to-grave life cycle assessment (LCA) of both SDH-BTES and conventional systems are performed for this area. The SDH-BTES system is defined as a heating system for a community comprising 1500 m2 gross solar area and 150 borehole heat exchangers with a depth of 30 m which is modeled to analyze its environmental performance. The results are presented comparatively with the conventional heating system consisting of local household diesel furnaces. Fifteen various impact categories are introduced for the comparison and contribution of main components (solar subsystem, auxiliary subsystem, transportation, and disposal) are analyzed individually. The present analyses show that SDH-BTES performs better than local diesel furnaces regarding human health, climate change, and resources. However, ecosystem quality impact of SDH-BTES system is slightly higher than the conventional domestic diesel furnaces due to the large land occupation of underground thermal heat storage and drilling process. The SDH-BTES system can improve environmental impact by 21 % compared to the conventional system and reduces CO2 emission by 22.4 %.

Analysis of heat and mass transfer in a porous solar thermochemical reactor

Solar thermochemistry shows great potential as a viable solution for addressing the challenge of energy storage. The design of the reactor is still challenging the development and rapid upscaling of solar thermal conversion and storage processes. This study presents a novel reactor that capable of gas reheating and develops a comprehensive three-dimensional model that effectively couples the processes of heat transmission, gas flow and chemical reaction for the optimization of the reactor. It is demonstrated that the maximum thermal efficiency is 43.67% as the incident solar power is 3730 W, but it decreases as the temperature rises. The re-radiation loss at the reactor front face is found as a major factor affecting the reactor thermal efficiency. The result indicates that the thermal efficiency of the reactor is significantly improved as using the cutoff wavelength coating technology. As the cutoff wavelength is 1 μm, the average temperature of porous material increases from 1510.77 °C to 1634.87 °C at 60 min, and the reactor's thermal efficiency increases from 11.65% to 13.51%. This study provided comprehensive insights into heat and mass transfer in a porous solar thermochemical reactor.

Analysis of thermal storage behavior of composite phase change materials embedded with gradient-designed TPMS thermal conductivity enhancers: A numerical and experimental study

Phase change materials (PCMs) exhibit considerable potential for utilization in energy storage and temperature regulation applications, primarily attributed to their notable latent heat capacity. Nevertheless, the intrinsically limited thermal conductivity of PCMs necessitates the use of thermal conductivity enhancers (TCEs) that possess adjustable features to compensate for greater heat storage efficiency. Porous structures with flexible design freedom have garnered growing interest in contrast to traditional random foams, owing to their significantly larger surface areas and fully interconnected pore networks. This study fabricated three polar form-designed triply periodic minimal surfaces (TPMS) porous structures using selective laser melting (SLM) based on various radial density gradients, namely the uniform density, the linear gradient, and the Boltzmann gradient. The TPMS porous structures were incorporated as TCEs within a paraffin matrix to form composite TPMS-PCMs. The melting behavior of composite TPMS-PCMs during the charging process was investigated by employing both visual experiments and numerical methods. A thorough analysis was undertaken regarding the progression of solid-liquid phase interfaces, temperature distribution, convection distribution, and heat storage rate to elucidate the mechanisms that contribute to enhanced heat transfer. The findings indicate that the configuration of the density gradient has a notable impact on the melting behavior of composite TPMS-PCMs by tuning heat transfer paths. The heat storage rate of the linear gradient case is the highest among the three, reaching 12.1 W, 1.6 times that of the uniform density case, and twice that of the Boltzmann gradient case. Although the Boltzmann gradient case has the lowest heat storage rate, it demonstrates exceptional performance in terms of temperature uniformity. The average temperature gradient along the radius during melting is 348.6 °C/m, which is only 59% observed in the linear gradient case and 56% in the uniform density case. These findings substantiate the effectiveness of the radial gradient density design in the control of the thermal storage process of composite TPMS-PCMs. They serve as a reference for optimizing thermal storage and temperature control systems that rely on latent heat storage in the future.

Thermal storage performance analysis of building envelope based on big data sensor network

In order to understand the thermal storage performance analysis of building envelope, the author proposes a research on thermal storage performance analysis of building envelope based on Big data sensor network. The author first compares and analyzes the thermal storage performance parameter systems of different theoretical methods, and determines the key characterization parameters of thermal storage performance under different working conditions based on the analysis of the thermal storage process. Second, in order to compare and verify the measured and simulated indoor temperature of the building, two groups of buildings with identical exterior wall insulation performance and distinct thermal storage performance were chosen. Three distinct thermal storage levels of the building?s exterior walls were used to simulate and analyze the internal surface temperature, heat flux density, and building cooling and heating load. Summer night ventilation was used to investigate the effect of ventilation volume on building cooling loads. In the end, a provincial office building was chosen as the research subject. The structure comprises of two stories, one underground and two over the ground, with a north hub point of 180?. The investigation object is a two-story room. The trial results demonstrate that the deliberate outcomes are in great concurrence with the mimicked results. For the district of the region, under a similar protection execution of the nook structure, the inner surface temperature of the weighty construction outside wall is higher in winter, lower in summer, with a higher intensity load in winter and a lower cooling load in summer. The proper air changes each hour of building night ventilation is 16 ach, and the effect on the cooling heap of weighty structures is huge. Demonstrated the precision of the reproduction technique and model.

Effect of Temperature Gradients and DC Electric Field on Electrothermal Convection in a Rotating Layer of Walter's-B Fluid Saturated Porous Media

Thermal convection in fluid-saturated porous media has diverse applications across geothermal energy utilisation, thermal insulation engineering, grain storage and understanding geodynamic processes. The thermal convection's physics is crucial for optimising the thermofluid system, improving energy efficiency, and enhancing our understanding of natural phenomena. The study examines how the initiation of electrothermal convection in a rotating viscoelastic fluid layer (Walter's B) is influenced by basic temperature gradients and a uniform vertical DC electric field, considering three different types of velocity boundary conditions: free-free, rigid-rigid, and lower rigid and upper free. By applying an electric field to a fluid, studying electrothermal convection helps engineers design effective cooling systems for electronics. The eigenvalue problem is solved by using the Galerkin technique. The Galerkin method is a widely used numerical technique for solving eigenvalue problems. The DC electric field effect exhibits a more stabilising effect for free-free boundaries than rigid and rigid-rigid boundaries. Moreover, the linear temperature profile exhibits a more stabilising effect than the parabolic temperature profile. The control of electrothermal convection on the system with the impact of other physical parameters is also discussed.

Performance comparison and enhancement of the thermal energy storage units under two expansion methods

To improve the performance of the basic thermal energy storage unit, two expansion methods, modular combination and linear structural expansion, are proposed and compared through numerical simulations. The impacts of the two expansion methods on the performance of the storage units are compared by investigating the thermal storage and release processes. Following the numerical study, both expansion methods can increase the heat storage capacity and airflow rates compared to a basic phase change material (PCM) storage unit linearly. However, the linear structural expansion method will increase the PCM melting time from 147 min to 367 min when the heat storage capacity of the basic unit is increased two times, which is about 2.5 times longer duration than using the modular expansion method. As for the heat release process, the results indicate that thermal release performance using linear structural expansion is lower than that of the modular combination along with a decrease in heat release efficiency of about 32.53 %. In conclusion the modular combination method is proved to be more efficient compared to the linear structural expansion method for improving the performance of the PCM storage units.

Thermal storage process of phase change materials under high humidity and laminar natural convection condition: Prediction model and sensitivity analysis

The application of phase change materials (PCMs) in renewable energy storage and building temperature regulation is considered an effective method to reduce the use of fossil fuels and carbon emissions. However, issues concerning the utilization of PCMs in high-humidity environments remain to be addressed to fill the gap in real-world applications. This paper establishes a simplified novel two-zone model, comprehensively considering the natural convection on the surface of the PCP in high-humidity environments, as well as the formation, movement, and evaporation of water films on the surface. Subsequently, numerical solutions and experimental validation are conducted, and the influence of high-humidity environments on the heat and moisture transfer is elucidated. The results indicate that a high-humidity environment can significantly suppress the sensible heat transfer with a ratio range of latent heat to sensible heat typically around 1–4. Furthermore, the increase in the height of PCP leads to an increase in the proportion of sensible heat transfer. On the contrary, increasing the thermal conductivity of the PCM can amplify the latent heat transfer. The results can help further study the thermal storage and the dehumidification potential of phase change units in application on a large scale under high humidity conditions.

Preparation and characterization of lauric acid-stearic acid/fumed silica/expanded graphite thermally conductive enhanced composites

Fatty acid and fatty acid blend phase change materials (PCM) have issues with easy leakage and low thermal conductivity in thermal storage and exothermic processes. To overcome this issue, a novel low-temperature composite material, Lauric acid - stearic acid - fumed silica - expanded graphite (LA-SA-FS-EG), was fabricated by vacuum impregnation technique using lauric acid-stearic acid (LA-SA) as the phase change material (PCM), fumed silica (FS) as the support material, and expanded graphite (EG) as the thermal conductive material. Leakage tests indicated that the maximum adsorption of LA-SA by fumed silica was 75 wt%. Scanning electron microscopy (SEM) was employed to observe the microscopic morphology of the composites. The crystal structure and chemical composition of the composites were analyzed by X-ray diffraction (XRD) and Fourier infrared spectroscopy (FT-IR). The differential scanning calorimeter (DSC) data indicates that the melting temperatures of LA-SA-FS-EG 3 %, 5 % and 7 % were 33.38 °C, 33.32 °C and 33.13 °C respectively, with melting latent heats of 110.8 J/g, 114.3 J/g and 119.9 J/g, respectively. The thermal conductivity of LA-SA-FS-EG 7 % is improved by 454 % compared to the LA-SA-FS composite without the addition of EG. LA-SA-FS-EG7% composite showed excellent heat storage capacity. LA-SA-FS-EG7% has a 46.1 % increase in heat storage rate and a 59.4 % increase in thermal release rate compared to LA-SA-FS composite. Notably, the LA-SA-FS-EG7% composite retains good thermal properties even after 1000 heating and cooling cycles. Consequently, the LA-SA-FS-EG7% composites produced have potential applications in thermal energy storage and building energy efficiency.

Effect of phase change heat storage tank with gradient fin structure on solar energy storage: A numerical study

In this paper, the heat storage process of a latent heat thermal energy storage (LHTES) tank is studied numerically. A new type of gradient fin is added to the heat storage process in a latent heat storage tank to improve the heat transfer performance of the internal phase change material (PCM). The numerical model is verified by the experimental data. The influences of different structures on the melting rate, temperature distribution, velocity distribution, and maximum melting rate of PCM in LHTES tank are investigated, and the influence of natural convection on the energy storage system is further quantified. The results show that the melting time can be significantly reduced by 16.9% by using the four straight fins with gradient shape, and the melting time can be further reduced by 41.1% by reconstruction of the gradient fin position. The deformation in the opposite direction will slow down the melting speed and extend the melting time by more than 4.66 times. The convection criterion for judging the buoyancy convection of the melt in this paper shows that natural convection is dominant in both the upper and lower part of the melt. The closer the convection coefficient is to the overall heat transfer coefficient, the better the melting efficiency and energy efficiency will be.

Study of the model of the phase transition envelope taking into account the process of thermal storage under natural draft and by air injection

The study proposed mathematical modeling of the enclosing structure of the building, which includes an energy-active panel accumulating solar radiation due to the phase transition of the heat-accumulating material. The mathematical model is based on a two-dimensional non-stationary nonlinear heat conduction equation describing the process of heat transfer in the bearing layer of the enclosing structure and the energy-active panel, taking into account the process of heat accumulation under natural and forced traction by air injection. In comparison of the calculation results, it was found. that the efficiency of the use of forced convection relative to natural was 33–44% in the question of the selection of useful heat, depending on the time (second or first day), and the value of heat accumulation showed almost the same results. Taking into account the research carried out by the authors, it is shown by calculation that enclosing structures in which a heat-accumulating material with a phase transition is used to accumulate solar energy are more energy efficient than simple heat accumulation in the carrier layer. The introduction of highly conductive inclusions into the energy-active panel and the use of forced convection instead of natural traction during heat extraction also significantly increase the energy efficiency of the structure, which can subsequently be used as an option for external fencing under appropriate climatic conditions.

Flame-Retardant and Form-Stable Delignified Wood-Based Phase Change Composites with Superior Energy Storage Density and Reversible Thermochromic Properties for Visual Thermoregulation

The development of form-stable phase change materials (PCMs) with flame retardancy and the visual thermal storage process is crucial for their application in building energy conservation. Herein, an active phosphorus/ammonium-containing non-formaldehyde flame retardant (APA) was synthesized based on the natural compound phytic acid. Then, wood-based form-stable PCM composites (PTPCMs) with high energy storage density, excellent flame retardancy, and real-time and visual reversible thermochromic properties were successfully fabricated by impregnating the thermochromic compound into the APA-grafted delignified wood. The delignified wood well supports the solid–liquid PCMs and avoids their liquid leakage during phase transition due to the high surface tension and strong capillary effect. The differential scanning calorimetry (DSC) results showed that the PTPCMs possessed high thermal energy storage density (165.3–198.6 J/g) and reliable thermal stability. With the concentration of the flame-retardant APA increased, the peak heat release rate (pHRR) and total heat release (THR) of PTPCMs reduced noticeably, demonstrating the enhanced flame retardancy of PTPCMs. Moreover, PTPCM composites had good thermoregulation properties and the visualization of the phase transition process was made possible by the reversible thermochromic properties. In summary, the novel PTPCMs show tremendous application potential for efficient building energy conservation.

Phytic acid–decorated κ-carrageenan/melanin hybrid aerogels supported phase change composites with excellent photothermal conversion efficiency and flame retardancy

Organic phase change materials (PCMs) have been widely applied to thermal energy storage systems; however, poor solar–thermal conversion performance, high flammability, and liquid leakage issues considerably limit their practical application in efficient solar energy utilization. Herein, phytic acid–decorated melanin/κ-carrageenan aerogels (PMCAs) with excellent solar–thermal effect and satisfactory flame retardancy were developed using the cationic induction method. Then, form-stable PCM composites (PMPCMs) were fabricated by integrating molten n-octacosane into PMCAs through vacuum impregnation. The PMCAs effectively supported n-octacosane and avoided liquid leakage during the solar–thermal storage process owing to their strong surface tension and capillary force. The PMPCMs exhibited considerably high encapsulated efficiency (up to 97.73%), satisfactory thermal storage ability (246.7–257.7 J/g), and excellent thermal reliability. The introduction of natural melanin (natural photothermal conversion compound) and phytic acid (bio-based phosphorus-rich compound) significantly enhanced the solar–thermal conversion efficiency (from 61.7% to 89.4%) and flame retardancy of the PMPCMs. Thus, the synthesized form-stable composites possess tremendous potential for solar energy utilization.

Dynamic thermal analysis of a coupled solar water heaters-thermal storage tank for solar powered district heating networks

The integration of solar thermal and energy storage systems into district heating networks (DHNs) has been considered as one of the most advantageous solutions to reduce the use of fossil fuels and carbon emissions. In this context, this work deals with the numerical investigation of a coupled solar water heaters-thermal storage unit for urban heating networks. The studied system contains flat plate solar collectors (FPCs) for hot water production, a shell-and-tube thermal storage unit with phase change material (PCM) for latent storage or water for sensible storage, hydraulic pumps and a mass flow rate regulation system. The functioning of the coupled system is simulated using a detailed dynamic model based on transient energy and mass balances. The reliability of the developed model is verified through comparing numerical results with measurement data from the literature and a good agreement is obtained with a maximum relative error of about 4.5 %. The validated model is used to investigate the system thermal performance under the weather conditions of the city of Pau, France, and the effects of multiple parameters such as the FPC area, the DHN target thermal power, and the storage medium type (latent or sensible) during both charging and discharging processes are presented and analyzed. Results show that the use of FPCs is beneficial for DHNs during summer as they allow the production of thermal energy for both the DHN use and the storage process. It was also shown that increasing the DHN target thermal power from 30 kW to 50 kW leads to a reduction in the thermal storage process duration by up to 40 %. For both charging and discharging operations, the use of PCM (latent storage) as a storage medium is more suitable than water (sensible storage) as it leads to an increase in the storage density and extends the duration of hot water production with constant thermal power by about 65 %.

Simulated Experimental Assessment of a Laboratory-Scale Solar Convective Furnace System

Abstract Electricity and gas-based heat treatment of metal is an energy-intensive process. To mitigate the use of such high-grade energy, the concept of an open volumetric air receiver-based solar convective furnace (SCF) system is developed for the heat treatment of metal. This system includes an in situ waste heat recovery mechanism. This paper presents a Joule heating-based, controlled, experimental assessment of a laboratory-scale, retrofitted, SCF system for generating benchmark data. The reported measurements illustrate the heat transfer for (a) the charging and discharging process of thermal energy storage and (b) the two-stage heat treatment of metal with an in situ heat recovery process. The overall system efficiency, including heat recovery, heat storage, and heat transfer, is found to be 24%. Thus, the SCF system can serve as a viable alternative to an electrical energy-based heat treatment furnace.

Thermal performance improvement of a low-temperature thermal energy storage with configuration optimization

To improve the performance of solar heat storage and reduce the energy consumption of the fresh air system in areas with poor solar conditions, this study investigates the thermal storage and release performance of a low-temperature rectangular thermal energy storage unit (RTESU). A validated Computational Fluid Dynamics (CFD) model is used to address the impacts of configuration parameters on the performance of the thermal storage process in the Phase change material (PCM) module, including the number and eccentric arrangement of tubes. Subsequently, the effect of the air channel width on the air outlet temperature and the heat transfer rate is analyzed for the heat-releasing process. The numerical results show that the number and the eccentricity of the heat exchanger tube have significant impacts on the heat storage performance, and a minimum duration of 117 min can be achieved with a tube pitch of 30 mm (corresponding to 5 water tubes) and an eccentric of 5 mm, respectively. The air channel width of 20 mm is found as the optimal width due to its higher heat release efficiency (83.24 %) and lower pressure drop (4.3 Pa). The identification of key structural impacts helped to optimize the RTESU configuration, which performs at a low heat storage time of 117 min and achieves a heat release efficiency as high as 83.24 % to satisfy the need for poor solar conditions.

Contribution degree analysis of phase change capsule size in packed bed

The phase change capsule packed bed has been widely studied by many scholars because of its simple structure and high heat transfer rate. This study aims to obtain the effect of phase change capsule size on the thermal properties of the packed bed and the variation of the contribution degree in the heat storage process. In this paper, three three-dimensional models of heat storage packed beds with different phase change capsules sizes are established and the thermal storage process is numerically simulated by Fluent. The simulation results are compared with the experimental results, and the results are in good agreement. By monitoring the temperature of the heat transfer fluid (HTF), the temperature of the phase change material (PCM), and the liquid phase fraction in the packed beds with different phase change capsule sizes, the indicators for characterizing thermal characteristics of heat storage devices, such as heat storage rate, heat storage density, instantaneous energy efficiency, and exergy efficiency, can be calculated. The comparative analysis shows that the thermal properties of the packed bed improve as the capsule size decreases. Then the contribution degree of phase change capsule size to the heat storage process of the packed bed is obtained by the chain rule. Results indicated that as the phase change capsule size decreases, its contribution degree increases gradually. Still, the degree of increase decreases, and the gain from the change in capsule size decreases. The contribution degree of the phase change capsule size is also numerically less than that of the inlet mass flow and temperature. In general, with the decrease of the phase change capsule size, the thermodynamic performance of the packed bed and the contribution degree of the phase change capsule size increase significantly, but the power consumption and cost of packed beds increase.