Numerical simulation of a multi-layer latent heat thermal energy storage system

A computational model for the prediction of the thermal behaviour of a compact multi-layer latent heat storage unit is presented. The model is based on the conservation equations of energy for the phase change material (PCM) and the heat transfer fluid (HTF). Electrical heat sources embedded inside the PCM are used for heat storage (melting) while the flow of an HTF is employed for heat recovery (solidification). Parametric studies are performed to assess the effect of various design parameters and operating conditions on the thermal behaviour of the unit. Results indicate that the average output heat load during the recovery period is strongly dependent on the minimum operating temperature, on the thermal diffusivity of the liquid phase, on the thickness of the PCM layer and on the HTF inlet mass flowrate and temperature. It is, on the other hand, nearly independent of the wall thermal diffusivity and thickness and of the maximum operating temperature. Correlations are proposed for the total energy stored and the output heat load as a function of the design parameters and the operating conditions. © 1998 John Wiley & Sons, Ltd.     

Numerical simulation of a shell-and-tube latent heat thermal energy storage unit

A theoretical model was developed to predict the transient behavior of a shell-and-tube storage unit with the phase change material (PCM) on the shell side and the heat transfer fluid (HTF) circulating inside the tubes. The multidimensional phase change problem is tackled with an enthalpy-based method coupled to the convective heat transfer from the HTF. The numerical predictions are validated with experimental data. A series of numerical experiments are then undertaken to assess the effects of various thermal and geometric parameters on the heat transfer process and on the behavior of the system. Results show that the shell radius, the mass flow rate, and the inlet temperature of the HTF must be chosen carefully in order to optimize the performance of the unit.     

Numerical analysis on the thermal behavior of high temperature latent heat thermal energy storage system

High temperature latent heat thermal energy storage technology is a promising option for future cost reduction in parabolic trough or tower power plant. However, low thermal conductivity of phase-change material (PCM) is the major shortage of latent heat thermal energy storage. This paper proposed a new thermal energy storage system (TESS) that metal foam and fins were used to enhance the effective conductivity of PCM. Three-dimensional physical model was established for representative element extracted from TESS. Considering the natural convection in the liquid part of PCM, volume-averaged mass and momentum equations were employed with the Brinkman–Forchheimer extension to Darcy law to simulate the porous resistance. A local thermal equilibrium model was developed to obtain temperature field. The governing equations were solved with finite-volume approach and enthalpy method was employed to account for phase change. The model was firstly validated against low temperature experiments from the literature and then used to predict the charging and discharging behavior of the present TESS. The position of solid/liquid interface was explored and the effects of design parameters, including that of metal foam pore density and porosity, configuration of fin and Rayleigh number, on melting and solidifying rate and energy stored in each time step were revealed and discussed. The results indicate that metal foam and fins can effectively improve the heat transfer performance for thermal storage system and decrease charging and discharging time.     

High temperature latent heat thermal energy storage using heat pipes

A thermal network model is developed and used to analyze heat transfer in a high temperature latent heat thermal energy storage unit for solar thermal electricity generation. Specifically, the benefits of inserting multiple heat pipes between a heat transfer fluid and a phase change material (PCM) are of interest. Two storage configurations are considered; one with PCM surrounding a tube that conveys the heat transfer fluid, and the second with the PCM contained within a tube over which the heat transfer fluid flows. Both melting and solidification are simulated. It is demonstrated that adding heat pipes enhances thermal performance, which is quantified in terms of dimensionless heat pipe effectiveness.     

用于储存太阳能的相变蓄热器蓄热性能研究

利用计算流体力学软件Fluent的凝固/熔化模型,模拟了用于储存太阳能的两种相变蓄热器的蓄热过程,得到了蓄热过程中相变区的温度云图,分析了两种蓄热器的蓄热能力和传热方式。研究结果表明,合理增加内管数量可以提高相变过程中的对流换热强度和蓄热能力。     

Numerical simulations on performance enhancement of a cross-flow latent thermal energy storage heat exchanger

基于列管式换热器具有传热面积大、结构紧凑、操作弹性大等优点,使其在相变储能领域具有广阔的应用前景。本文建立一种新型列管式相变蓄热器模型,在不考虑自然对流的情况下,利用Fluent软件对相变蓄热器进行二维储热过程的数值模拟。本文主要研究斯蒂芬数、雷诺数、列管排列方式、肋片数以及相变材料的导热系数对熔化过程的影响,并对熔化过程中固液分界面的移动规律进行了分析。模拟结果表明,内肋片强化换热效果明显,特别是对应用低导热系数相变材料[导热系数小于1 W/(m·K)]的列管式蓄热器,相对于无肋片结构,加入肋片（N_fn=2）可缩短熔化时间52.6%。     

Numerical analysis of latent heat thermal energy storage using encapsulated phase change material for solar thermal power plant

Thermal energy storage improves the load stability and efficiency of solar thermal power plants by reducing fluctuations and intermittency inherent to solar radiation. This paper presents a numerical study on the transient response of packed bed latent heat thermal energy storage system in removing fluctuations in the heat transfer fluid (HTF) temperature during the charging and discharging period. The packed bed consisting of spherical shaped encapsulated phase change materials (PCMs) is integrated in an organic Rankine cycle-based solar thermal power plant for electricity generation. A comprehensive numerical model is developed using flow equations for HTF and two-temperature non-equilibrium energy equation for heat transfer, coupled with enthalpy method to account for phase change in PCM. Systematic parametric studies are performed to understand the effect of mass flow rate, inlet charging system, storage system dimension and encapsulation of the shell diameter on the dynamic behaviour of the storage system. The overall effectiveness and transient temperature difference in HTF temperature in a cycle are computed for different geometrical and operational parameters to evaluate the system performance. It is found that the ability of the latent heat thermal energy storage system to store and release energy is significantly improved by increasing mass flow rate and inlet charging temperature. The transient variation in the HTF temperature can be effectively reduced by decreasing porosity.     

Low temperature latent heat thermal energy storage: Heat storage materials

Heat-of-fusion storage materials for low temperature latent heat storage in the temperature range 0–120°C are reviewed. Organic and inorganic heat storage materials classified as paraffins, fatty acids, inorganic salt hydrates and eutectic compounds are considered. The melting and freezing behaviour of the various substances is investigated using the techniques of Thermal Analysis and Differential Scanning Calorimetry. The importance of thermal cycling tests for establishing the long-term stability of the storage materials is discussed. Finally, some data pertaining to the corrosion compatibility of heat-of-fusion substances with conventional materials of construction is presented.     

A semi-analytical solution for time-varying latent heat thermal energy storage problems

Latent heat thermal energy storage (LHTES) problems include a lot of boundary conditions that could not be solved by exact solution, so new approaches to solving such problems could revolutionize the advanced energy storage devices. This paper focuses on reformulating the generalized differential quadrature method (GDQM) for a one-dimensional solidification/melting Stefan problem as a fundamental LHTES problem and solves some practical cases. Convergence and comparisons demonstrate that the proposed approach is sufficiently reliable. By checking the accuracy of the proposed approach for the LHTES problem (where Stefan number is below 0.2), it was demonstrated that for all Stefan numbers, the maximum error is less than 3.81% for temperatures. As the usual range of thermal energy storages, for Stefan numbers up to 0.2 the solution yields errors less than 0.2%. Then, the proposed approach is very ideal for such applications. In comparison, GDQM has a more accurate response than an integral solution for Stefan numbers less than 0.2. When this priority of GDQM comes with its low computational cost, it would undoubtedly be preferable.     

Numerical investigation on latent heat thermal energy storage in a phase change material using a heat exchanger

The use of a heat exchanger using phase change material (PCM) is an example of latent heat thermal energy storage (LHTES). In this study, the charging of PCM (RT50) is studied in a double pipe heat exchanger. The designing of the heat exchanger needs to be optimized for operating and boundary conditions to store latent heat efficiently. The size of the equipment and the amount of PCM are also important to calculate the latent heat storage capacity of the LHTES device. In this study, the amount of PCM taken is quite high to avoid sensible heat transfer and to maximize the heat content of PCM. The charging process of PCM is numerically simulated using an enthalpy-porosity model. The study includes the effect of inlet temperature and flow rate of high-temperature-fluid (HTF) and concludes that both play an important role in determining the charging time. The continuous increase in inlet temperature of HTF can decrease the charging time of PCM in the heat exchanger. However, the continuous increase in the HTF flow rate cannot show the same effect. The charging time can only be minimized with a specified flow rate regime for a specific inlet temperature of HTF. These factors consequently affect the efficiency of the heat exchanger.     

Heat transfer characteristics of the latent heat thermal energy storage capsule

The characteristic variation of the rate of heat transfer to and from a latent heat thermal energy storage capsule was investigated analytically and experimentally. Basic experiments were carried out to simulate a solar energy storage capsule, using a horizontal cylindrical capsule (300 mm length, 40 mm o.d.) filled with naphthalene as the phase change material. The variation of heat flux during the processes of heat storage and removal was measured by a heat flow meter wrapped around the capsule, as the capsule was subjected to stepwise variations of the surface temperature. Finite difference calculations based on heat conduction were also carried out to compare with the experimental results. For the heat removal process, the experimental results and the calculated heat flux agreed well with each other. They showed different characteristic trends for the heat storage process, due to the effects of natural convection.     

Numerical simulation of solar assisted ground-source heat pump heating system with latent heat energy storage in severely cold area

Solar assisted ground-source heat pump (SAGSHP) heating system with latent heat energy storage tank (LHEST) is investigated. The mathematical model of the system is developed, and the transient numerical simulation is carried out in terms of this model. The operation characteristic of the heating system is analyzed during the heating period in Harbin (N45.75°, E126.77°). From the results of the simulation, the average coefficient of performance (COP) of the heating system is 3.28 in heating period. In the initial and latter heating period, the COP of the heating system is higher, and the highest value is 5.95, because the system can be operated without heat pump. During the middle heating period the COP of the heating system and the operation stability of the system are improved due to solar energy and soil alternately or together as the heat source of heat pump. LHEST is a very important role in operation of the system. The system can be operated more flexibly, effectively, and stably by the charge and discharge heat of LHEST, and the effect becomes especially obvious in the initial and latter heating period.     

A model for latent heat energy storage systems

In this study, a theoretical approach is proposed for the prediction of time and temperature during the heat charge and discharge in the latent heat storage of phase changed materials (PCM). By the use of the average values of the mean specific heat capacities for the phase‐changed materials, analytical solutions are obtained and compared with the available experimental data in the literature. It is shown that decreasing the entry temperature of the working fluid from ?4 to ?15°C has a very dominant and strong effect on the PCM solidification time. The effect of the working fluid flow rate and the material of PCM capsules on the time for complete solidification and total charging is also investigated. The agreement between the present theoretical model results and the experimental data related to the cooling using small spheres and the heat storage using rectangle containers is very good. The largest difference between the present results and the experimental data becomes about 10% when the fluid temperature approaches the phase change temperature at high temperatures. Copyright © 2006 John Wiley & Sons, Ltd.     

Constructal enhancement of thermal transport in metal foam-PCM composite-assisted latent heat thermal energy storage system

The aim of this study is to augment thermal transport in latent heat thermal energy storage (LHTES) system by the optimum allocation of metal foam-phase change material (PCM) composite. This study emphasizes on the optimal volume and distribution of metal foam-PCM composite (MFPC) to enhance melting performance without delay in the total melting time. Therefore, a MFPC is designed according to constructal theory. The fundamental principle of the theory is to configure high thermal conductivity agents at optimal thermal energy flow path for effective heat exchange. A numerical code based on local thermal nonequilibrium approach equipped enthalpy porosity method is formulated, and evaluated. The results of the proposed configuration show that the provision of MFPC only at high local temperature gradient enhances the conductive transport with improvement in the overall thermal transport. It is derived that the elimination of metal foam volume at low temperature gradient incorporates the advantageous effect of natural convective transport, which is seen to be suppressed. Additionally, the proposed configuration may increase the volume of PCM, thus, the TES capacity. It also reduces the total weight and economy of energy storage system. The overall melting rate is improved by 11.11% in comparison with the LHTES with full volume of this high thermal conductivity agent.     

An analysis of a packed bed latent heat thermal energy storage system using PCM capsules: Numerical investigation

This paper is aimed at analyzing the behavior of a packed bed latent heat thermal energy storage system. The packed bed is composed of spherical capsules filled with paraffin wax as PCM usable with a solar water heating system. The model developed in this study uses the fundamental equations similar to those of Schumann, except that the phase change phenomena of PCM inside the capsules are analyzed by using enthalpy method. The equations are numerically solved, and the results obtained are used for the thermal performance analysis of both charging and discharging processes. The effects of the inlet heat transfer fluid temperature (Stefan number), mass flow rate and phase change temperature range on the thermal performance of the capsules of various radii have been investigated. The results indicate that for the proper modeling of performance of the system the phase change temperature range of the PCM must be accurately known, and should be taken into account.     

Analytical solution of heat transfer in a shell-and-tube latent thermal energy storage system

An analytical solution of a latent heat storage unit (LHSU) consisting of a shell-and tube was obtained by using the Exponential Integral Function and the variables separation technique. The working fluid (water) circulating by forced convection inside the inner tube charges and discharges the storage unit. The comparison between analytical predictions and experimental data shows good agreement. Extensive parametric studies were conducted in order to examine the effect of the pertinent parameters (such as natural convection, mass flow rate of HTF, outer tube radius, pipe length etc.) on the melting and solidification processes of paraffin as a PCM. In order to provide guidelines for system performance and design optimisation, unsteady temperature distributions within PCM during melting/solidification, energy stored, position of moving interface and thermal efficiency have been obtained by a series of numerical calculations and represented graphically.     

Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and for power generation. TES has recently attracted increasing interest to thermal applications such as space and water heating, waste heat utilisation, cooling, and air conditioning. Phase change materials (PCMs) used for the storage of thermal energy as latent heat are special types of advanced materials that substantially contribute to the efficient use and conservation of waste heat and solar energy. This paper provides a comprehensive review on the development of latent heat storage (LHS) systems focused on heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy, and the formulation of the phase change problem. The main categories of PCMs are classified and briefly described, and heat transfer enhancement technologies, namely dispersion of low‐density materials, use of porous materials, metal matrices and encapsulation, incorporation of extended surfaces and fins, utilisation of heat pipes, cascaded storage, and direct heat transfer techniques, are also discussed in detail. Additionally, a two‐dimensional heat transfer simulation model of an LHS system is developed using the control volume technique to solve the phase change problem. Furthermore, a three‐dimensional numerical simulation model of an LHS is built to investigate the quasi‐steady state and transient heat transfer in PCMs. Finally, several future research directions are provided.     

Numerical simulation of single spiral heat storage tank for solar thermal power plant

Thermal energy storage is a key technology for the solar thermal power plants. This paper set up a single spiral heat storage tank using concrete as heat storage material and Cu-water nano-fluid as heat transfer fluid. In this paper, temperature distribution and the effects of the parameters i.e., inlet temperature and inlet velocity, on the charge time of the storage tank have been investigated based on numerical simulation. The results show that the charge time of the storage tank is about 72,000 s before the other factors have been changed. The charge time of the storage tank increases with the decrease of the inlet temperature and inlet velocity.     

Enhancement of latent heat energy storage using embedded heat pipes

Latent heat thermal energy storage (LHTES) utilizing heat pipes or fins is investigated experimentally. Photographic observations, melting and solidification rates, and PCM energy storage quantities are reported. Heat pipe effectiveness is defined and used to quantify the relative performance of heat pipe-assisted and fin-assisted configurations to situations involving neither heat pipes nor fins. For the experimental conditions of this study, inclusion of heat pipes increases PCM melting rates by approximately 60%, while the fins are not as effective. During solidification, the heat pipe-assisted configuration transfers approximately twice the energy between a heat transfer fluid and the PCM, relative to both the fin-assisted LHTES and the non-heat pipe, non-fin configurations.     

Exergetic and performance analyses of two-layered packed bed latent heat thermal energy storage system

In concentrating solar power (CSP) plant, a novel method involving the use of thermocline can be employed to augment the capability of the thermal energy storage system (TES). The rate of thermocline degradation can be reduced by packing encapsulated phase change material (PCM) in the TES. The thermal performance of the packed bed latent heat thermal energy storage system (PBTES) can be further enhanced by employing different diameters of PCM capsules arranged in multiple layers. In this paper, the thermal and exergetic performance of single-layered and two-layered PBTES is evaluated for varying mass flow rate, PCM capsule diameter and bed height of larger PCM capsules using a dynamic model based on simplified energy balance equations for PCM and heat transfer fluid (HTF). The single-layered PBTES has a lower TES latent charging rate than the two-layered PBTES. The charging efficiency and charging time of two-layered PBTES are increased by 15.85% and 16.85%, respectively for reducing the HTF mass flow rate by 14.29%. A higher stratification number can be achieved by using a two-layered PBTES instead of a single-layered PBTES filled with the corresponding larger diameter PCM capsules. The second law efficiency of the two-layered PBTES is found to be less than that of the single-layered PBTES. A decrease in the bed height of larger PCM capsules decreases the exergetic efficiency of the two-layered PBTES by 3.27%. The findings from this study can be used in further designing and optimising the multi-layered PBTES.