A comparison of heat transfer enhancement in a medium temperature thermal energy storage heat exchanger using fins

An experimental energy storage system has been designed using a horizontal concentric tube heat exchanger incorporating a medium temperature phase change material (PCM) Erythritol, with a melting point of 117.7 °C. Three experimental configurations, a control system with no heat transfer enhancement and systems augmented with circular and longitudinal fins have been studied. The results presented compare the system heat transfer characteristics using isotherm plots and temperature-time curves. The system with longitudinal fins gave the best performance with increased thermal response during charging and reduced subcooling in the melt during discharging. The experimentally measured data for the control, circular finned and longitudinal finned systems have been shown to vindicate the assumption of axissymmetry (direction parallel to the heat transfer fluid flow) using temperature gradients in the axial, radial and angular directions in the double pipe PCM system.     

Numerical investigation of heat transfer enhancement in a multitube thermal energy storage heat exchanger using fins

The application of a phase change material (PCM) as thermal energy storage observed unprecedented growth due to its large latent heat storage capacity at a constant temperature. However, the design of an energy storage heat exchanger is a challenging task because of the poor thermal conductivity of PCMs. In an effort to improve the heat exchanger design, this paper presents a numerical performance investigation of a PCM-based multitube heat exchanger incorporated with two new fin configurations. The analysis of the results shows that the placement of fins is one of the important aspects, which needs to be cogitated in the design of heat exchangers.     

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.     

Heat transfer enhancement of high temperature thermal energy storage using metal foams and expanded graphite

Latent heat storage (LHS) can theoretically provide large heat storage density and significantly reduce the storage material volume by using the material’s fusion heat, Δh_m. Phase change materials (PCMs) commonly suffer from low thermal conductivities, being around 0.4 W m⁻¹ K⁻¹ for inorganic salts, which prolong the charging and discharging period. The problem of low thermal conductivity is a major issue that needs to be addressed for high temperature thermal energy storage systems. Since porous materials have high thermal conductivities and high surface areas, they can be used to form composites with PCMs to significantly enhance heat transfer. In this paper, the feasibility of using metal foams and expanded graphite to enhance the heat transfer capability of PCMs in high temperature thermal energy storage systems is investigated. The results show that heat transfer can be significantly enhanced by both metal foams and expanded graphite, thereby reducing the charging and discharging period. Furthermore, the overall performance of metal foams is superior to that of expanded graphite.     

Performance enhancement of high temperature latent heat thermal storage systems using heat pipes with and without fins for concentrating solar thermal power plants

A key drawback of using latent heat thermal storage systems for concentrating solar thermal power plants is the low thermal conductivity of the phase change material during the melting and solidification processes. This paper investigates an approach for reducing the thermal resistance by utilising axially finned heat pipes. A numerical model simulating the phase change material melting and solidification processes has been developed. This paper also includes the models of the evaporation and condensation of the heat pipe working fluid. The results show that by adding four axial fins and including the evaporation and condensation, the overall thermal performance of the storage system is enhanced significantly compared to having bare heat pipes. After 3 h a total of 106% increase in energy storage is obtained during the charging process. The results also show that the combined effect of incorporating the evaporation/condensation process and adding the fins leads to a threefold increase in the heat storage during the first 3 h. During the discharge process, there was a 79% increase in energy discharged and also the combined effect of incorporating the evaporation/condensation as well as adding the fins results in an almost four fold increase in the heat extracted within the first 3 h. A parametric analysis has also been carried out to analyse the effect of the finned heat pipe parameters after incorporating evaporation and condensation of the heat pipe working fluid.     

Performance enhancement in latent heat thermal storage system: A review

Phase change material (PCM) based latent heat thermal storage (LHTS) systems offer a challenging option to be employed as an effective energy storage and retrieval device. The performance of LHTS systems is limited by the poor thermal conductivity of PCMs employed. Successful large-scale utilization of LHTS systems thus depends on the extent to which the performance can be improved. A great deal of work both experimental and theoretical on different performance enhancement techniques has been reported in the literature. This paper reviews the implementation of those techniques in different configurations of LHTS systems. The influence of enhancement techniques on the thermal response of the PCM in terms of phase change rate and amount of latent heat stored/retrieved has been addressed as a main aspect. Issues related to mathematical modeling of LHTS systems employing enhancement techniques are also discussed.     

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.     

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.     

Study of a metal hydride based thermal energy storage system using multi-phase heat exchange for the application of concentrated solar power system

Thermal energy storage system is of great significance for the concentrated solar power system to keep the balance between power generation and demand. Metal hydride based thermal energy storage system is regarded as a promising method due to its good reversibility, low cost, and no by-product. Multi-phase heat exchange has much higher heat transfer coefficient than single-phase fluid heat exchange, thus facilitating the steam generation. In this study, a two-dimensional model of the metal hydride reactor using multi-phase heat exchange is proposed to estimate the performance and its feasibility of application in the concentrated solar power system. The results show that the velocity of the heat transfer fluid should match well with the thermal conductivity of the metal hydride bed to maintain the heat flux at a relatively constant value. The match of thermal conductivity of 3 or 5 W/(m·K) and fluid velocity of 0.0050 m/s results in the heat flux up to about 19 kW/m², which is increased by 3 orders of magnitude than single-phase heat exchange. In the thermal energy storage system, the reheating cycle is recommended to improve the utilization of the thermal energy. The efficiency of the system could be improved from 18.4% to 30.8% using the reheating cycle. The increased efficiency is comparable to the previously reported efficiency of 39.2%. Besides, the operation strategy of raising the steam temperature by increasing the hydrogen pressure or the superheater temperature is suggested for the system to obtain higher efficiency.     

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 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%。     

Economic evaluation of latent heat thermal energy storage using embedded thermosyphons for concentrating solar power applications

An economic evaluation of a latent heat thermal energy storage (LHTES) system for large scale concentrating solar power (CSP) applications is conducted. The concept of embedding gravity-assisted wickless heat pipes (thermosyphons) within a commercial-scale LHTES system is explored through use of a thermal network model. A new design is proposed for charging and discharging a large-scale LHTES system. The size and cost of the LHTES system is estimated and compared with a two-tank sensible heat energy storage (SHTES) system. The results suggest that LHTES with embedded thermosyphons is economically competitive with current SHTES technology, with the potential to reduce capital costs by at least 15%. Further investigation of different phase change materials (PCMs), thermosyphon working fluids, and system configurations has the potential to lead to designs that can further reduce capital costs beyond those reported in this study.     

Numerical heat transfer studies of the fatty acids for different heat exchanger materials on the performance of a latent heat storage system

Theoretical investigations of fatty acids as a phase change material (PCM) for energy storage system have been conducted in this study. The selected fatty acids were capric acid, lauric acid, myristic acid, palmitic acid and stearic acid. For the two-dimensional simulation model based on the enthalpy approach, calculations have been made for the melt fraction with conduction only. Glass, stainless steel, tin, aluminium mixed, aluminium and copper were used as heat exchanger materials in the numerical calculations. Theoretical results show that capric acid was found good compatibility with latent heat storage system. The large value of thermal conductivity of heat exchanger materials did not make significant contribution on the melt fraction.     

套管式相变蓄热器强化传热研究

蓄热技术可有效解决热能供给与需求在时间和强度上不匹配的矛盾,提高能源利用率。针对蓄热器换热效率低和蓄热器内部温度分布不均匀等问题,结合套管式和多管式蓄热器的换热特性,开发出高效蓄热换热器。研究结果表明,通过添加翅片使相变材料融化过程节省时间66.67%;增加翅片长度能够改善相变材料凝固过程中相变"死区"对整体放热时间的影响,使凝固过程节省时间73%。     

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.     

Three—Dimensional Heat Transfer Analysis for A Thermal Energy Storage Canister

IlltroductionSolar dynamic power modules (SDPM) with phasechange material (PCM) is a vital solution to ensureuninterrupted power supply for low-earth orbitapplication. The advantage of SDPM is its longerlifehme and higher efficiency. Longer lifetime results insubstanhal savings in hardware replacement, launch, andon-orbit installation costs. Because of SDPM's higherefficiency, its solar collection area is only about 25percent of that for a PV system. This would allowspacecraft operatin…     

An experimental and numerical investigation of heat transfer during technical grade paraffin melting and solidification in a shell-and-tube latent thermal energy storage unit

The latent thermal energy storage system of the shell-and-tube type during charging and discharging has been analysed in this paper. An experimental and numerical investigation of transient forced convective heat transfer between the heat transfer fluid (HTF) with moderate Prandtl numbers and the tube wall, heat conduction through the wall and solid–liquid phase change of the phase change material (PCM), based on the enthalpy formulation, has been presented. A fully implicit two-dimensional control volume Fortran computer code, with algorithm for non-isothermal phase transition, has been developed for the solution of the corresponding mathematical model. The comparison between numerical predictions and experimental data shows good agreement for both paraffin non-isothermal melting and isothermal solidification. In order to provide guidelines for system performance and design optimisation, unsteady temperature distributions of the HTF, tube wall and the PCM have been obtained by a series of numerical calculations for various HTF working conditions and various geometric parameters, and the thermal behaviour of the latent thermal energy storage unit during charging and discharging has been simulated.     

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.     

Development of a ventilation system utilizing thermal energy storage for granules containing phase change material

We have developed an experimental ventilation system that features direct heat exchange between ventilation air and granules containing a phase change material (PCM). Measurement of outlet air temperature when the inlet air temperature was periodically varied to simulate changes of outdoor ambient air temperature showed that the outlet air temperature was stabilized and remained within the phase change temperature range. This effect is expected to be useful in practical ventilation systems. The potential of such systems for reducing ventilation load was examined through computer simulation for eight representative cities of Japan. This revealed how different temperature conditions would affect required heat storage capacity.     

Development of graphite foam infiltrated with MgCl2 for a latent heat based thermal energy storage (LHTES) system

Thermal energy storage (TES) systems that are compatible with high temperature power cycles for concentrating solar power (CSP) require high temperature media for transporting and storing thermal energy. To that end, TES systems have been proposed based on the latent heat of fusion of the phase change materials (PCMs). However, PCMs have relatively low thermal conductivities. In this paper, use of high-thermal-conductivity graphite foam infiltrated with a PCM (MgCl₂) has been investigated as a potential TES system. Graphite foams with two porosities were infiltrated with MgCl₂. The infiltrated composites were evaluated for density, heat of fusion, melting/freezing temperatures, and thermal diffusivities. Estimated thermal conductivities of MgCl₂/graphite foam composites were significantly higher than those of MgCl₂ alone over the measured temperature range. Furthermore, heat of fusion, melting/freezing temperatures, and densities showed comparable values to those of pure MgCl₂. Results of this study indicate that MgCl₂/graphite foam composites show promise as storage media for a latent heat thermal energy storage system for CSP applications.