Thermal Storage System Research Articles

Exergy Analysis of a Shell and Tube Energy Storage Unit with Different Inclination Angles

To optimize the utilization of solar energy in the latent heat thermal energy storage (LHTES) system, this study conducts exergy analysis on a paraffin-solar water shell and tube unit established in the literature to evaluate the effects of different inclination angles, inlet temperatures, original temperatures, and fluid flow rates on the exergy and exergy efficiency. Firstly, the thermodynamic characteristics of the water and the natural convection effects of the paraffin change with different inclination angles. When the inclination angle of the heat storage tank is less than 30°, the maximum exergy inlet rate rises from 0 to 144.6 W in a very short time, but it decreases to 65.7 W for an inclination angle of 60°. When the inclination angle is increased from 0° to 30°, the exergy efficiency rises from 86% to 89.7%, but it decreases from 94% to 89.9% with the inclination angle from 60° to 90°. Secondly, under the condition that the inclination angle of the energy storage unit is 60°, although increasing the inlet temperature of the solar water enhances the exergy inlet and storage and reduces the charging time, it increases the heat transfer temperature difference and the irreversible loss of the system, thus reducing the exergy efficiency. As the inlet water temperature is increased from 83 to 98 °C, the exergy efficiency decreases from 94.7% to 93.6%. Moreover, increasing the original temperature of the LHTES unit not only reduces the exergy inlet and storage rates but also decreases the available work capacity and exergy efficiency. Finally, increasing the inlet water flow rate increases the exergy inlet and storage rates slightly. The exergy efficiency decreases from 95.6% to 93.3% as the unit original temperature is increased from 15 to 30 °C, and it is enhanced from 94% to 94.6% as the inlet flow rate is increased from 0.085 to 0.34 kg/s with the unit inclination angle of 60°. It is found that arranging the shell and tube unit at an inclination angle is useful for improving the LHTES system’s thermal performance, and the exergy analysis conducted aims to reduce available energy dissipation and exergy loss in the thermal storage system. This study provides instructions for solar energy utilization and energy storage.

Evaluation of cross-contamination in indirect thermal storage system in concentrated solar plants

Solar concentration technology is fully dispatchable due to the use of thermal energy storage systems, in particular, commercial parabolic trough power plants use indirect storage systems based on molten salts. This system includes a heat exchanger to transfer energy between the thermal oil from the solar field and the salts used in storage. In addition, some content of water is considered by cross-contamination in the steam generator of the power block. This study analyses the compatibility between both fluids in the event of cross-contamination in the equipment. The degree of compatibility between a triphasic system made up of a mixture of sodium nitrate and potassium nitrate salts (60:40), heat transfer fluid (DP:DPO), and water, when they were subjected to 390 °C for a period of 312 h (two weeks). The study shows how thermal oil undergoes complete thermal-oxidative degradation without high impact on the composition of the salts.

Copper–Alumina Capsules for High-Temperature Thermal Energy Storage

High corrosivity, leakage, and oxidation of metallic phase-change materials (PCMs) have limited their applications in high-temperature thermal energy storage (TES) systems, regardless of their favorable benefits for high-temperature TES applications of over 1000 °C. To overcome these major challenges, this work presents a facile paraffin sacrificial layer approach for directly encapsulating copper (Cu) sphere PCMs with the alumina (Al2O3) shell, considering a buffer inner cavity. The cavity is formed by the decomposition of the paraffin layer through a pre-sintered process. It plays a key role in accommodating the volume expansion of the Cu core, thereby preventing the breakage of the shell and the leakage of the liquid PCM. A series of macrocapsules with different sizes (9.5–21 mm) containing the Cu core, cavity, and Al2O3 shell are successfully produced using the paraffin sacrificial layer method and deploying a two-step heat treatment. The experimental results show that the Al2O3 shell possesses a good structure, which can prevent the leakage of the Cu core. The Al2O3 shell also has strong compatibility with the Cu core without any chemical reaction between two materials. In a temperature range of 1000–1100 °C, the calculated mass and volume energy storage densities of the PCM macrocapsule with 21 mm outer diameter are found to be 222 kJ/kg and 745 J/cm3, which are 1.83 and 1.76 times, respectively, higher than those for the Al2O3 ceramic. After thermal cycle tests, the encapsulated Cu PCM shows superior shape stability, chemical stability, and thermal durability, which can be applied in long-term thermal storage systems for high-temperature TES systems.

Study of the effect of phase state on the interfacial thermal conductance between PCMs and ceramic skeletons

ABSTRACT Composite phase change materials with sugar alcohol as the phase change material and highly thermally conductive ceramics as the porous skeleton are widely used in various thermal storage systems. The interfacial thermal conductance (ITC) between the phase change materials under different phases and the skeleton is an important factor affecting the rate of heat storage (release) in thermal storage systems. The ITC between ceramics(AlN, SiC) and sugar alcohols (mannitol and galactitol) in the solid and liquid states is investigated by means of both time-domain thermoreflectance and molecular dynamics simulations. The results show that the ITC between phase change materials and ceramic is better in liquid state than in solid state, and that the ITC between mannitol and ceramic is better, and that the ITC betweenAlN and sugar alcohol is better. More low-frequency phonons are involved in the thermal transport of the sugar alcohols in the liquid state, with an average overlap energy of about 9.5% higher than that of the solid state and an average phonon participation rate of about 6.8% higher. It was also found that it isthe H atom in the sugar alcohol that is linked to the C atom that governs the ITC.

Operating mode of Brayton-cycle-based pumped thermal electricity storage system: Constant compression ratio or constant rotational speed?

Pumped thermal electricity storage (PTES) is a thermomechanical energy storage technology that offers the advantages of geographical independence, high energy density, and low-cost. It has attracted considerable interest from scholars at home and abroad. At present, research on the thermo-economic performance of this technology is relatively in-depth. However, there remains a lack of research on its operational control strategy, which is crucial for technological industrialisation. This study innovatively proposes two operation modes for compressors and expanders operating at a constant rotational speed (CRS): the CRS-CMF and CRS-VMF modes. The performances under the proposed modes were comprehensively compared with that under the traditional constant compression ratio (CCR) operation mode, and optimisation was performed. The results revealed that the proposed CRS operation mode significantly improved the system storage performance compared to that under the CCR mode. Under the design conditions, the highest round-trip efficiency of 64.67% can be obtained in the CRS-VMF mode. Better output stability was obtained under the CRS-CMF mode, and the delivery power offset ratio reduced from over 50% under the CCR mode to a minimum of 19.13%. When air was used as the working medium, the loss was relatively high; however, better output stability was obtained. Sensitivity analysis showed that a larger charging/discharging duration and upper temperature could achieve better energy storage performance. This research can provide a theoretical basis for formulating appropriate system control schemes and further optimising operational control strategies.

Solidification performance improvement of phase change materials for latent heat thermal energy storage using novel branch-structured fins and nanoparticles

In the present study, we propose the combination of novel branch-structured fins and Al2O3 nanoparticles to enhance the performance of a phase change material (PCM) during the solidification process in a triple-tube heat exchanger. The inevitable drawback of PCMs is their lower heat conductivity, which can result in a long response time during the phase change process in latent heat thermal storage systems. Therefore, any serious improvement strategy needs an optimized phase change process. A mathematical model for a two-dimensional structure composed of a PCM with paraffin RT82 and Al2O3 nanoparticles that considers the thermal conduction in metal fins, Brownian motion of nanoparticles, and natural convection in a liquid phase PCM is proposed and verified based on experimental results. The impact of various volume fractions and fin layouts on the solidification process is discussed, involving the evolution and deformation of solid–liquid interfaces and distribution of isotherms and average temperature and liquid fraction curves. The results imply that the solidification behaviour can be significantly enhanced by the application of nanoparticles and metal fins. Compared with the inherent structure of the heat exchanger, the solidification time is decreased by 8.5%, 9.3%, and 10.3% for Al2O3 nanoparticles (at 2%, 5%, and 8%, respectively) only and by 83.0%, 80.7%, 80.8%, and 82.9%, respectively, for various fin layouts only. This is attributed to increased heat transfer by thermal conduction and natural convection. It can be concluded that the impact of the use of fins is preferable compared to that for nanoparticles, and the benefit of nanoparticles is limited.

Studying carbon fiber composite phase change materials: Preparation method, thermal storage analysis and application of battery thermal management

Carbon fiber composite phase change material (PCM) can serve as an excellent material for thermal storage system. This work presents a new composite PCM prepared with two raw materials of KAl(SO4)2·12H2O (X) and Na2SO4·10H2O (Y), supporting materials activated carbon fibers (ACFs), and thermal conductivity agent nano carbon powder (C). The characterization results show that adding 1 wt% C can increase the thermal conductivity about 81.0 % of mixed materials (C, X, and Y) than that of mixed X, Y; Moreover, the addition of ACFs also increases the thermal conductivity by 130 %. DSC tests reveal that phase change temperatures of XY and CXY are respectively 36.47 °C and 35.67 °C, meantime the enthalpies are 220.84 J/g and 185.24 J/g, respectively. Thus, this new material is suitable for the lithium-ion battery thermal management system (BTMS) applying. The application results show that ACFs composited CXY (ACFs/CXY) tremendously cripples the battery's temperature rise rate after the melting point in comparison with no wrapping material, and the ACFs/CXY-based BTMS can effectively control the battery temperature below 60 °C for 1903 s at the high heating power of 6 W. The ACFs composite PCM has the advantages of low cost, simple preparation, and easy acquisition, which reduces the cost of energy storage system, and has remarkable capacity of temperature controlling.

Storage periods and heating terminals types influence on the solar heating thermal storage volume in the Tibetan Plateau regions

The scale of the thermal storage system is critical to balancing system fluctuations caused by the mismatch between solar heat collection and heating load in time and space for solar heating systems. This study investigates the relationship between solar collector area and heat storage volume under the different storage periods and heating terminals types. This research simulates the heat storage volumes using a typical day, continuous overcast days, and heating period as thermal storage periods, based on solar radiation intensity, building heating load, and cloudy days in seven typical Tibetan regions. According to the results, the storage volume per unit collector area Vs/Ac increases with increasing temperature difference and storage period in the Tibetan Plateau. It is proposed that Lhoka, Chamdo, Nagqu, Lhasa, Shigatse, and Nyingchi take continuous overcast and cloudy days as thermal storage periods, and Ngari uses the heating period as thermal storage periods, corresponding to the Vs/Ac ranges of 0.27 m3/m2–0.9 m3/m2 and 1.6 m3/m2–1.8 m3/m2. Specifically, preheating the heat storage tank can improve the solar fraction fsim by 2.5 %, with the recommended preheating temperature of 50 °C. In addition, the variation equation for design solar fraction fn and Vs/Ac was obtained. These results can be utilized to develop solar heating systems on the Tibetan plateau.

Integration of phase change thermal storage system with vertical solar Chimney In Greenhouse

The performance stability of the system can be improved by incorporating a solar chimney with a phase-change material (PCM). It is recommended that instead of using the traditional multi-curved trough air collectors for solar greenhouses, a solar phase change thermal storage wall construction system with vertical air channels be employed. Solar thermal utilization of the rear wall. In order to verify feasibility of the construction system, a multi-curved trough air collector test system for solar greenhouse and a phase-change heat storage wall test system with vertical air channels were built respectively. The air velocity in the heater, the air flow parameters (air velocity, air channel spacing, air flow direction) in the sensible heat storage wall layer in the solar greenhouse, etc. The study’s findings indicate that the collector performs at its peak level of total heat absorption when its air velocity is between 1.4 and 1.8 m/s, and that heat absorption rises as solar radiation intensity rises. The suggested system design orientation offers an efficient way to raise the thermal performance of the system with the least amount of work and expense for practical applications.

Performance evaluation of a solar thermal storage system proposed for concentrated solar power plants

This study aims to assess the performance of the Hybrid Solar Thermal Storage (HSTS) system, the storage system proposed for concentrated solar power (CSP) plants. The heat storage hybridisation concept is based on coupling latent phase change material (PCM) and thermochemical storage system containing paired metal hydride (MH) beds. One base design which uses only thermochemical heat storage mode and two designs of the HSTS system with two distinct PCM heat exchanger designs (i.e., shell-and-tube and cylinder with truncated hollow cones) are proposed and numerically investigated. To predict the dynamic behavior of the three storage systems, a bidimensional mathematical model is established and a numerical code written in Fortran-90 is developed. Key performance indicators used to evaluate the performance of the three storage systems include volumetric storage capacity, specific power, state of hydrogen charge, and energy storage efficiency. Comparing the performance of the three systems reveals that the HSTS system's energy storage efficiency increased by 36% due to the integration of a reaction heat recovery internal system based on Na3Al as PCM. The use of the PCM in truncated hollow cones resulted in a 36.4% reduction of the heat charging and discharging time, an 18.5% increase storage capacity, and a 54.1% increase in specific power. In addition, this study proves that the third design can be imlpemented as a HSTS system in a solar power plant with the following performance indexes: 128 W/kg- Mg2FeH6, 160 MJ/m3 and 88% for specific power, volumetric storage capacity and energy storage efficiency, respectively.

Numerical Analysis of the Influence of Inner Tubes Arrangement on the Thermal Performance of Thermal Energy Storage Unit

The container shape and arrangement of the thermal storage systems (TES) play a vital role in enhancing thermal performance. In the current investigation, the impact of inner tube dimensions and arrangements of TES on the thermal performance of a PCM-based triplex-tube latent heat storage exchanger (TTHX) is numerically analyzed. COMSOL Multiphysics commercial software was employed to obtain the numerical solution of the governing equations. Eight different cases with the same volume of PCM and various configurations of the inner tubes were investigated. The results of the current study were presented in terms of temperature contours, liquid fraction, Bejan number, average temperature, and average Nusslet number. The shortest melting time was 48 min, which was achieved by a single inner tube configuration with a quicker melting time of >62% compared to other cases. While for multi-tubes, the shortest time was 78 min, which was achieved by the configuration of three tubes (two horizontal and the third placed at the lower section) with an enhancement of melting time reduction of >12% compared to other cases, except for a single inner tube configuration. Regarding the entropy generation, the single tube configuration achieved the lower Bejan number. Therefore, single tube configuration was found to be the best option for maximizing the thermal performance of the studied TTHX.

Heat transfer and storage characteristics of a hexagonal close structured packed-bed thermal storage system with molten salt phase change materials

The phase change material (PCM) spheres using 316SS as the ball shell and solar salt (NaNO3-KNO3, 60–40 wt%) as the PCM were arranged in an orderly pebble-bed arrangement. The charging performance, energy and exergy of PCM spheres in the packed ball thermal energy storage system (PBTES) were investigated using CFD simulation in order to solve the problem of heat storage at 300 °C–500 °C. The results show that the total melting time of PCM spheres slightly decreases from 1455 s to 1319 s with the inlet velocity increasing. The average heat storage efficiency of PCM spheres and PBTES are improved with the inlet temperature increasing. When the inlet flow rate increases from 0.01 m/s to 0.10 m/s, the energy efficiency of PBTES increases approximately 5.06 times, and the exergy efficiency increases from 17.91 % to 99.55 %. However the heat energy storage and exergy efficiency of PCM spheres are not affected by the inlet velocity changing. The energy efficiency of PBTES increases about 2.09 times, and the exergy efficiency increases from 30.29 % to 70.93 % with the inlet temperature grows from 300 °C to 500 °C. It provides an idea for the design of the medium-high temperature and high-efficiency molten salt PCM heat storage system.

Evaluation of solar thermal energy capture and storage alternatives for indirect steam generation: A case study

AbstractThis study analyzes the integration of a solar thermal plant for indirect steam generation in a typical industrial process with thermal energy requirements. Solar irradiation is computed using a validated predictive model in Santa Fe province, Argentina. Three configurations are proposed, consisting of parabolic trough collectors and thermal storage tanks. Performance indicators, such as generated energy, resulting solar fraction (the fraction of the industrial process' thermal requirements met by solar energy), and necessary solar field area, are evaluated for each configuration. A specific energy map is generated, showing the solar generation potential at each location. For required solar fractions of 20%‐25%, incorporating a thermal storage system does not provide any benefits as the solar plant does not produce enough energy to contribute to the industrial process during winter months. For required solar fractions of 30%‐35%, the resulting solar fraction can be increased to 42%‐45% with additional capture area and thermal storage capacity. For required solar fractions of 37%‐40%, the thermal storage improves the efficiency of the solar plant, resulting in a solar fraction of 50%‐60%, allowing the industrial process to receive energy for at least 6 h a day throughout the year. It is concluded that these configurations are suitable for providing a portion of the industrial process' thermal requirements.

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.

A Review Study of Numerical Simulation of Lid-Driven Cavity Flow with Nanofluids

Perhaps the most deliberated fluid problem in the field of Computational Fluid Dynamics is the lid driven cavity flow whose simple geometry is used to study the thermal behavior of many engineering applications such as cooling of electronic equipment, solar collectors, thermal storage systems, food processing, solar ponds, crystal growth, lubrication technologies and cooling of electrical and mechanical components. Researchers have been devoting much of their time in order to discover innovative methods to enhance the thermal conductivity of conventional fluids. With the development of nanotechnology, the concept of nanofluids has gained ground considerably as a new kind of heat transfer fluid. Nanofluid is a new kind of fluid with high thermal conductivity is a mixture of solid nanoparticles and a liquid. This review recapitulates the recent progress of the various numerical methods that are used in predicting the influence of several parameters such as type of nanoparticle and host liquid, particle volume concentration, particle size and shape, Brownian diffusion and thermophoresis effect on hydrodynamic and thermal characteristics of convective heat transfer using nanofluids in a lid driven cavity.

An optimization strategy for enhancing energy consumption performance in digital currency miner's building

AbstractCryptocurrencies, as a way of extraction, are categorized into two main groups of non‐mineable and mineable. Bitcoin (BTC), the most famous mineable digital currency, utilizes the Proof of Work (PoW) algorithm for maintaining network solidarity. Its mining process is done by devices known as application specific integrated circuits (ASICs), which consume electricity and turn it into heat. They shall operate within an accepted temperature range of around 25°C; otherwise, they will face an efficiency drop; consequently, all mining sites require a proper cooling system. This study aims to investigate the problem of high energy consumption for cooling in digital currency mining sites and to present a solution for that via optimizing the capacity and operation of the chiller and ice thermal storage system (ITS). The results show the utilization of the ITS system reduces the operational costs by 10%, and the ITS system provides 1320 kWh of cooling energy during peak hours. Finally, the sensitivity analysis results considering the impact of the electricity tariff on the size and operation of the system have confirmed that with an increase in peak time tariff, the role of the ITS system in cost reduction increases.

A way to utilize SOFC systems: The energy solution for next-generation smart farm

In the field of food and agriculture, fuel cell-based energy supply technology is gaining much attention for sustainable food systems with carbon neutrality by 2050. Previous studies have focused on the electricity balance using polymer electrolyte fuel cells (PEMFCs) without considering the temperature quality to maintain the environmental conditions required in smart farms. However, this study proposes a system that can provide all four energy sources (electricity, cooling, heating, and CO<sub>2</sub>) required by smart farms by using solid oxide fuel cells (SOFC), which can utilize high-quality heat. To confirm the feasibility of the proposed idea, we demonstrate the world's first 10 kW-class SOFC-based integrated system for a smart farm in Jinju, South Korea. The system's core components consist of a SOFC system, a hot thermal storage system, a cold thermal storage system, and a CO<sub>2</sub> supply system. In this study, the applicability of the proposed system is verified by the experimental results of the effective production of cold and hot heat required by smart farms. In addition, the technical problems encountered during the demonstration are presented. In doing so, we suggest the direction of more economical and sustainable SOFC technology development for smart farm applications.

Two-stage stochastic-based scheduling of multi-energy microgrids with electric and hydrogen vehicles charging stations, considering transactions through pool market and bilateral contracts

In order to mitigate greenhouse gas emissions and improve energy efficiency, sustainable energy systems such as multi-energy microgrids (MEMGs) with the high penetration of renewable energy resources (RES) and satisfying different energy needs of consumers have received significant attention in recent years. MEMGs, by relying on renewable resources and energy storage systems along with energy conversion systems, play an essential role in sustainability of energy supply. However, renewable energies are uncertain due to the intermittent nature of solar and wind energy sources. Thus, optimal operation of the MEMGs with the consideration of the uncertainties of RES is necessary to achieve sustainability. In this paper, risk constrained scheduling of a MEMG is carried out with the presence of the PV, wind, biomass, electric vehicles (EVs) and hydrogen vehicles (HVs) charging stations, combined heat and power (CHP), boiler, hydrogen electrolyzer (HE), cryptocurrency miners (CMs), electrical, thermal and hydrogen storage systems, responsive demands. From the trading and business model side, the proposed MEMG optimized operation relies on bilateral contracts between producers and consumers and pool electricity markets. A two-stage stochastic programming method is used for considering the uncertainties of electrical, thermal and hydrogen demands, EV and HV charging stations load, CM load, PV and wind power, and the price of electricity purchased from the pool market. The proposed mixed integer linear programming (MILP) model is solved using the CPLEX solver in GAMS which guarantees to achieve a globally optimal solution. The results show that due to the certain prices of bilateral contracts, the possibility of transaction by bilateral contracts decreases the risk metric CVaR by 50.42%. The simulation results demonstrate that risk of high operation costs while considering flexibility sources, such as storages and demand response (DR) programs, is decreased by 5.45% and 4.6%, respectively. As far as operation costs are concerned, results reveal that using renewable resources decreases operation costs by 34.47%. Moreover, the operation cost is reduced by 5.94% and 4.57% in the presence of storage units and DR programs, respectively. In the same way, storages and DR programs decrease cost of purchased electricity by 13.47% and 14.46%, respectively.

Performance analysis of PCM melting in a fin-assisted thermal energy storage system – A numerical study

The study attempts to present the melting phenomena in a Latent Heat Thermal Storage System (LHTSS) where ice is used as Phase Change Material (PCM) with fins incorporated to advance the thermal performance. The PCM-based storage system finds vast potential in addressing the imbalance between energy supply and demand. Computations are carried out to find the optimum fin number to be incorporated in Heat Transfer Fluid (HTF) tube for improved heat transfer and minimum melting time. The effect of incorporating optimum fin number on different influential parameters such as HTF tube arrangement, diameter, and temperature on charging performance is evaluated. The physical phenomena occurring due to increased fin number is also explained. Criteria such as liquid fraction, mean temperature, heat transfer rate, and heat transfer coefficient are used to compare the system's performance under varying parameters. As the number of fins is augmented from 4 to 15, the generated liquid PCM is found to increase by 34.69% followed by a declination rate of 9.03% on further increment in the numbers. Geometrical alteration in the HTF tube arrangement from inline to staggered results in a better melting performance. Considering the HTF tube temperature parameter, it is found that 280 K HTF tube produces 2.08 times more liquid PCM as compared to 275 K case. Increasing HTF tube diameter is found to enhance the charging process with 20 mm diameter HTF tube system generating 2.68 times more liquid PCM as compared to that of 5 mm HTF tube.

Thermal performance of a novel high-temperature sensible heat thermal storage steam generation system using solid graphite as material

In this paper, a prototype of high-temperature sensible heat thermal storage system for direct steam generation was presented. The structure of solid graphite blocks with embed tube was applied to improve the efficient density of thermal storage. And a net thermal storage capacity of 1.038 GJ was achieved for the module with a total mass of 1480 kg solid graphite under the working temperature range of 250–600 °C. Meanwhile, a novel series-parallel tube structure was applied for discharging power regulation. A two-stage working strategy was found to be efficient for confining discharging power fluctuated in a certain range instead of monotonously decreasing. Further, three mass flow rates from 106 kg/h to 175 kg/h were performed to investigate thermal performance of the thermal storage system. The results show that the effect of mass flow rate on discharging power and heat transfer coefficient seems to be tiny due to the major thermal resistance is resulted from contact thermal resistance. The extension of working tube in stage II has improve the development of two-phase flow, which finally results in a higher heat transfer coefficient. The flow distribution in parallel tube has a significant effect on the local discharging characteristics and the two-phase flow development in tube. The manifold header for stage transition has not only enhanced the heat transfer in the second half tube by mixing effect but also redistributed the flow rate in parallel tubes.