Phase Change Thermal Storage System Research Articles

Numerical Study of an Energy Storage Container with a Flat Plate Phase Change Unit Characterized by an S-Shaped Flow Channel

China’s rapid economic development and rising energy consumption have led to significant challenges in energy supply and demand. While wind and solar energy are clean alternatives, they do not always align with the varying energy needs across different times and regions. Concurrently, China produces substantial amounts of industrial waste heat annually. Effective recycling of this waste heat could substantially mitigate energy supply and demand issues. The Mobile Thermal Energy Storage (M-TES) system is a key solution to address these challenges, as it helps manage the uneven distribution of energy over time and space. This article establishes a packaged M-TES based on a plate-type phase change unit. Based on different placement methods of the plate-type phase change unit, different inlet temperatures and phase change temperature differences, and different inlet and outlet directions, the complete charging and discharging process of the packaged phase change thermal storage system was simulated using ANSYS FLUENT 2022 R1 software. The results showed that during the heating process of the entire system, the horizontal placement of the plate-type phase change unit and the inlet and outlet methods of the heat transfer fluid (HTF) significantly improved the heating effect of the system, increasing it by 15.9%. Increasing the temperature difference between the inlet temperature of the heat transfer fluid and the melting temperature of the phase change material (PCM) from 4 K to 19 K can increase the melting rate of PCM by approximately 54.9%

Experimental and numerical analysis of cosine wave heat source on thermal storage performance of a conical spiral shell-tube energy storage system

Solar radiation is the main driving force for the energy source of solar collectors, and its intensity determines the amount of energy that solar collectors can absorb. Therefore, the periodicity of radiation intensity will lead to periodic fluctuations in the outlet water temperature of solar collectors. This study numerically investigates the effects of the heat source period, amplitude, inlet flow rate, and steady state heat source temperature on the thermal storage performance of conical spiral shell-tube energy storage systems when the inlet temperature varies periodically as a cosine function using fluent software. The results show that when heat source periods are between 5 and 300 min, compared to the constant heat source, the total energy storage capacity can be increased by a maximum of 7.32 %. The melting time can be shortened by a maximum of 16.31 %. The average energy storage rate and energy storage efficiency can be increased by a maximum of 4.08 % and 0.17 %, respectively. When amplitudes are increased from 0 to 20 K, the melting time, total energy storage capacity, and energy storage efficiency are reduced by 30.21 %, 1.19 %, and 1.64 %, respectively. The average energy storage rate increases by 41.59 %. Furthermore, compared to the constant heat source, increasing the inlet flow rate and steady state heat source temperature has more significant effects on the thermal storage performance under cosine wave heat sources. The results can provide some reference for the optimization of phase change thermal storage systems under unsteady state conditions.

Design and research of a novel type of arc fin for heat transfer enhancement in hot water oil-displacement system

Phase change thermal storage system utilizes solar energy to provide continuous heat and electricity for offshore platforms in the petroleum industry. However, the low thermal conductivity of the heat storage medium (phase change material) in the system limits the overall heat transfer efficiency. This study focuses on the design of a novel longitudinal arc fin to enhance heat transfer in the storage system. A comparison is made between the influence of the rectangular fin and the novel longitudinal arc fin structure, as well as the impact of the fin number on the heat storage performance of the heat storage unit. A numerical model is constructed and used to analyze the grid and time step in the numerical process. The resulting data is further validated through experimental comparison. The study also examines the impact of fin structure and fin number on the internal convection and temperature of this phase change heat storage unit, comparing thermal performance parameters such as heat storage time, heat storage rate, temperature response, and overall heat storage. Findings indicate that, under constant overall fin size, increasing fin number significantly enhances the internal temperature of this unit. Additionally, changing the shape of fins from rectangular to arc-shaped results in decreased heat storage performance. Specifically, when compared to the initial rectangular structure, the melting time of 8 arc fins and 10 arc fins decreased by 30.70% and 40.65%, the mean temperature response increased by 45.01% and 67.70%, and the mean rate of heat storage increased by 45.35% and 69.23%. Therefore, increasing the number of arc-shaped fins, while maintaining the same fin volume, substantially improves the heat storage properties of the unit.

Thermal performance study of a solar-coupled phase changes thermal energy storage system for ORC power generation

The current solar organic Rankine cycle power generation (ORC) system cannot run smoothly under the design conditions due to the shortcomings of solar fluctuations, and thermal energy storage (TES) can effectively buffer the fluctuations of solar energy. Cascaded heat storage (CLTES) has been shown to be more suitable for solar heat storage than single-stage heat storage (LTES). The main objective of this study is to analyze the thermal storage characteristics of thermal storage systems under real-time solar energy fluctuations, and to improve the thermal storage efficiency and total thermal storage capacity of solar phase change thermal storage systems in distributed scenarios. The current research on solar CLTES system needs to focus on the heat storage characteristics under the actual solar fluctuations to provide guidance for the design and practical operation of CLTES system. Analyzing the effect of solar fluctuations on thermal storage performance is the focus of solar thermal storage research, which can provide guidance for the design and actual operation of CLTES systems. However, the current research on solar CLTES system lacks the analysis of thermal storage characteristics under typical solar energy fluctuations. Therefore, a simplified two-dimensional enthalpy-dynamic model of heat transfer for shell-and-tube latent heat storage system is developed in this paper to simulate the heat storage characteristics of single-stage and cascade heat accumulators containing different phase change materials (PCMs) at a fixed temperature of the heat source, and the heat storage performance of the system is simulated based on the solar fluctuations of typical four seasons. Finally, based on the shortcomings of large-capacity thermal storage, a dual-tank recirculation thermal storage model is proposed and the thermal storage characteristics of the improved recirculation model are analyzed. The results show that the optimized cycle heat storage mode can bring about a minimum of 10 % and a maximum of 17.2 % thermal efficiency improvement. On a typical summer day with the most abundant solar energy resources, four times of complete phase change heat storage and one incomplete phase change heat storage were completed (melting fraction = 81.83 %), and on a typical winter day with the least solar energy resources, two times of complete phase change heat storage and one incomplete phase change heat storage were completed (melting fraction = 75.81 %). The maximum difference of heat storage between them is 45.38 %.

Experimental study on heat transfer and storage of a heating system coupled with solar flat heat pipe and phase change material unit

Using renewable energy, especially solar energy, is essential to achieve a low-carbon society. PCMs suffer from low thermal conductivity, which hinders the efficiency of phase change thermal storage systems. Heat pipes exhibit vastly superior thermal conductivity, making them a promising candidate for enhancing PCM-based systems. To solve the contradiction between building insulation and importing solar energy into buildings in winter, this study proposes a new building heating ceiling system consisting of a flat-plate heat pipe (FHP) coordinated with a phase change material (PCM), and establishes a platform for a heat stockpiling device integrating the FHP and PCM. This research investigates the impact of heating power and bending angle on the heat transfer efficiency of the heat pipe and phase change heat storage system. The experimental findings demonstrate a direct relationship between increasing heating power and the enhancement of the FHP's heat transfer efficiency. When the FHP is bent at a 5° angle and heating power is increased from 20 W to 80 W, to store heat energy into the PCM cavities, the effective thermal conductivity rises significantly, escalating from 7.26 × 104 W/(m·°C) to 15.1 × 104 W/(m·°C), marking a remarkable 108 % rise. Moreover, although increasing the bending angle leads to heightened effective thermal conductivity of the FHP under constant heat input, and the heat transfer in PCM cavities will be weakened within a bending angle of 15°, experimental results demonstrate that the melting time of PCM still decreases with the increase of bending angle; the heat transfer inertia of phase change materials is dominant.

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.

Thermal Characteristics of Metal-to-Insulator Phase Transition Materials and Its Latent Heat Utilization Technique

This article outlines the latent heat transport properties of the metal-to-insulator phase transition materials that our research group has been conducting. The metallic solid-solid phase change materials exhibit no change in shape before and after the phase change, and therefore require no container to maintain their shape when applied to heat sink devices. This can be a fundamental solution to the problems of increased thermal resistance due to the container or capsule, and leakage of the phase change material due to their breakage, which are apparent in conventional solid-liquid type phase change thermal storage systems. In this article, we introduce the thermal behavior of vanadium dioxide for a heat sink material.

Studies on performance enhancement of heat storage system with multiple phase change materials

This paper aims to improve the heat storage system's performance by using multiple phase change materials (PCMs). A heat storage device is designed with multiple PCMs firstly. The heat storage device consists of three identical shell-tube phase change thermal storage units. The numerical method is developed and verified experimentally. Two parameters, namely the latent heat utilization of PCM (ω) and the heat transfer effectiveness of heat storage device (ε), are introduced to evaluate the performance of energy storage and the heat exchange of the heat storage system with multiple PCMs. The effects of factors on ω and ε are studied, including the thermal conductivity, latent heat and melting temperature difference of PCMs, and the inlet velocity and temperature of heat transfer fluid. Results indicate that increasing the inlet velocity and temperature of HTF can greatly improve ω, but reduce ε. Increasing the thermal conductivity of PCM can obviously improve ω, but it has little effect on improving ε. Multiple PCMs with suitable melting temperature difference can improve both ω and ε. In practice, multiple PCMs can be associated with the above methods to improve the overall performance of the phase change thermal storage system. This research is used to optimize the design of phase change thermal storage device and improve its performance.

Study on the influence of tank structure and fin configuration on heat transfer performance of phase change thermal storage system

Owing to the intermittent and fluctuating nature of solar energy, thermal energy storage (TES) plays a vital role in solar energy utilization. The structural and geometric limits of the traditional heat storage systems, as well as the low thermal conductivity of phase change materials (PCMs), decrease the heat storage and exothermic efficiency of TES severely. To alleviate these shortcomings and improve the TES heat storage and release efficiency, the geometrical structure of TES and fin structure of the heat storage system need to be optimized. This study established a 2D model of the phase change TES unit with a single tank and auxiliary electric heating based on the experimental findings. The heat transfer performance of the truncated cone model with different taper configurations was also analyzed numerically. The influence of long and short fins on PCM heat transfer was analyzed under the premise of a certain fin volume. The influence of fins with different inclinations on the heat transfer process was also analyzed. The results show that the truncated cone with a taper of 0.37 performs best in the charging and discharging process. Compared with long fins, most short fins showed to have a more significant effect on accelerating the charging process. The enhancement effect of the different fin on the heat transfer process was divided into two categories, and the analysis shows that the fins with an inclination angle of −15° have the most significant enhancement effect. These findings of the study will guide the optimization design of the heat transfer structure of TES with a single tank configuration.

Optimization of phase change thermal storage units/devices with multichannel flat tubes: A theoretical study

Collector storage solar water heater systems that use phase change materials as heat storage media have attracted increasing attention because of their clean, pollution-free properties and high energy storage density. The design and application of these systems require efficient and optimized phase change thermal storage units (TSUs). This study aims to optimize a TSU by using theoretical analysis methods and to fill the research gaps attributed to the lack of theoretical analysis. The TSU in this work comprises a multichannel flat tube and rectangular fins as heat exchange elements. The performance levels of this TSU, including its effectiveness, compactness factor, gravimetric specific power, and fin efficiency, under different structures are compared. The economic viability and energy efficiency of the proposed solar collector phase change thermal storage system are preliminarily evaluated. Results indicate that the structure of the multichannel flat tube TSU is relatively reasonable when the fin aspect ratio (Hfins/x) and thickness are 8.60 and 0.001 m, respectively. The effectiveness of the optimized TSU during charge and discharge processes respectively increases by 52.0% and 16.3% relative to the original multichannel flat tube TSU. The corresponding recommendation for the use of multichannel flat tube TSUs in household baths is four groups/person.

Study on Heat Transfer Performance of Phase Change Thermal Storage Heat Pump Water Heater with Carbon Nanotubes

To explore the feasibility of applying carbon nanotubes (CNTs)/paraffin composites to heat pump water heater system, heat transfer characteristics of phase change thermal storage heat pump water heater were analyzed and discussed. It was shown that the melting and solidification rates of nanocomposite were significantly improved upon the addition of CNTs nanoparticles, and the heat pump heat water with thermal storage has good stability in heat storage and release process. There are some differences in the distribution of temperature field in phase change thermal storage system, especially in exothermic stage. Thermal “stagnation” occurs near the corner of the outlet of water medium, so it is necessary to increase the amount of materials appropriately. Compared with traditional regenerative heat pump water heater with water tank, volume of regenerator of heat pump water heater is reduced by about 1/2 with the addition of nano-composites, and the thermal efficiency can reach 86%. The running experimental results of heat pump water heater suggests the reliability of designment of the thermal energy storage and the feasibility of CNTs/paraffin using in the heat pump water heater. The temperature of CNTs-paraffin composites rises faster and the heat transfer time is shorter at higher ambient temperature. Obviously, ambient temperature has a significant impact on the property of heat pump system.

An analysis of a packed bed thermal energy storage system using sensible heat and phase change materials

Thermal energy storage is an important subsystem of a solar thermal power station. Compared with the two-tank storage system, the packed bed storage system uses a single tank to store thermal energy temporarily and release it when the energy is needed. The temperature distribution in the packed bed is generally in the form of the thermocline. This study analyzed a thermocline storage system packed with sensible heat material and phase change material. These materials were characterized separately in terms of temperature difference, outlet temperature profile, total energy storage, and charging time. Results demonstrated the presence of a maximum temperature difference in the middle of the thermocline. The maximum value of the temperature difference decreased following an increase in the charging time. The density and conductivity of the sensible material and the inlet velocity of the fluid considerably affected the temperature difference. In the phase-change thermal storage system, the temperature difference between the material and the fluid exhibited two peaks, wherein the first peak was higher than the second peak. The total storage charging time and storage energy increased following an increase in latent heat. The total storage charging time was reduced following a decrease in melting point, although the final thermal energy storage remained the same.

A methodological concept for phase change material selection based on multi-criteria decision making (MCDM): A case study

Phase change material (PCM) have been explored for different applications to increase the energy efficiency and economy. However, the proper PCM selection to meet given requirements generally requires that a compromise is struck between conflicting criteria. This paper attempts to extend multi-criteria decision making (MCDM) method to solve the PCM selection problem. Firstly, the comprehensive appraisal indexes model is established and the subjective and objective weight of the criteria are obtained based on the analytic hierarchy process (AHP) and entropy information method, respectively. Secondly, the optimized combination weight method is adopted to obtain the combination weight. Then, the pre-screened PCMs are ranked using technique for order performance by similarity to an ideal solution (TOPSIS). Finally, the information retrieval is employed to justify the rankings. To demonstrate the applicability of the proposed method, a problem to select the optimal PCM for ground source heat pump (GSHP) integrated with phase change thermal storage (PCTS) system is considered. The result showed that Ba(OH)2·8H2O is the best material for the given case and the GSHP integrated with PCTS system has better economic performance compared to the GSHP system, meanwhile, the proposed method is a simple and practical approach in solving the PCM selection problem.

Two-Phase Anaerobic Digester Combined with Solar Thermal and Phase Change Thermal Storage System in Winter

Using solar energy as the heat source for biogas improvement of the anaerobic digester is an effective method. However, intermittent solar radiation and low ambient temperature in the winter make it difficult to maintain a steady fermentation temperature. In this study, a pilot-scale two-phase anaerobic digester combined with solar thermal and phase change thermal storage (PCTS) system has been constructed. A set of comparative field studies were carried out during the winter in Maanshan City, Anhui Province, China; these periods of study included no heating mode (P1), heated with solar thermal and PCTS system (P2), and conventional solar heating mode (P3). The comparison shows that methane yields and the energy conversion rate of the substrates of P2 were 12.33 m3 and 48.6%, respectively, both of which increased by 5.65 and 1.01 times, compared with that of P1 and P3, respectively. The statistical coefficients of variation for the fermentation temperature were 2.5% in P2 and 6.6% in P3, which means that ...

Numerical analysis on performance of naphthalene phase change thermal storage system in aluminum plate-fin unit

This paper presents a numerical simulation for the prediction of time, temperature and liquid fraction during the melting and solidification process in the latent heat storage of phase changed materials (PCM). The mathematical model, regarding the solid–liquid phase change heat transfer based on the enthalpy formulation, has been proposed. Naphthalene was used as the PCM which is contained in up layer of plate-fin thermal storage. Transient simulations were performed using a commercial computational fluid dynamics package, Fluent, based on the finite volume method. This computational model endeavored to describe both the melting and solidification processes of the PCM. A series of numerical calculations have been done in order to analyses the influence of several heat transfer fluid operating conditions and several fin geometric parameters on the thermal storage unit. Numerical results, which could be used for operating conditions and geometry optimization, provide guideline for the design of the latent thermal energy storage system.

Experimental validation of a CFD and an ε-NTU model for a large tube-in-tank PCM system

An experimental validation for a computational fluid dynamics (CFD) and an effectiveness-number of transfer units (ε-NTU) model for tubes in a large phase change material (PCM) tank has been conducted. The inlet and outlet heat transfer fluid (HTF) temperatures as well as twelve temperature locations in the PCM tank were compared with the CFD results. The average effectiveness of the phase change process of each experimental point was also compared with results from the CFD as well as the ε-NTU models. From this study, it was concluded that the CFD model and the ε-NTU model developed can accurately predict the behaviour of the thermal storage system during the freezing process. There are however, discrepancies in the melting process due to the exclusion of the effect of natural convection in the models. Using the experimental results, an effective thermal conductivity has been determined to account for buoyancy for various distances of tubes. The paper gives details of the CFD model of the phase change thermal storage system, and presents results from the CFD model, experiments and ε-NTU model.

An effectiveness-NTU technique for characterising tube-in-tank phase change thermal energy storage systems

Thermal storage systems with phase change materials are predominantly designed, analysed and optimised through numerical modelling. An alternative simplified method is being proposed for the characterisation of these phase change thermal storage systems. The method is based on the effectiveness-number of transfer units ( ε-NTUs) technique. A simplified mathematical representation has been analytically developed using the ε-NTU technique for a cylindrical tank filled with phase change material (PCM), with heat transfer fluid flowing through tubes inside the tank. Experiments have been carried out on a cylindrical tank filled with PCM and with one, two and four coils of tubes to validate the technique. Experimental results for the systems with a high heat transfer area compare well with those calculated from the model. The results show that this technique can readily be used as a design tool for sizing and optimising a thermal storage unit with phase change materials. From this study, it may be concluded that the model based on the ε-NTU technique can accurately predict the average heat exchange effectiveness of the thermal storage system with a high heat transfer surface area during charging and discharging.

Performance of a thermal storage system of the vertical tube type

A transient phase change thermal storage system of the vertical tube type is studied using a fixed grid numerical model. The solution of the system consists of solving the equations of the heat transfer fluid (HTF), the pipe wall and the phase change material (PCM) as one domain. The control volume finite difference approach is used to solve the equations describing the phase change thermal storage system. The SIMPLE scheme is used to solve for the pressure and velocity fields of the HTF. The radial temperature distribution, the phase change interface position and the latent and sensible heat accumulated along the system axial length are shown for different Reynolds and Stefan numbers, phase change temperature range and time periods. Also shown in this paper is the possibility of using the numerical model to solve problems with a single temperature phase change. The results obtained are important parameters for PCM thermal system performance investigation and design.