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.     

Heat transfer characteristics in low-temperature latent heat storage systems using salt-hydrates at heat recovery stage

The heat transfer characteristics of a low temperature latent heat storage system have been determined for circular finned and unfinned tubes using sodium acetate trihydrate as a phase change material (PCM). In the heat recovery stage, supercooling of PCM in the finned-tube system is larger than that in the unfinned-tube system. The heat-transfer coefficient between the PCM and the heat-transfer tube surface can be predicted from the steady-state heat conduction equation except the beginning of freezing with some degree of supercooling. The heat transfer is significantly reduced by the void cavities upon shrinkage of PCM in the finned-tube system. The enhancement of heat transfer by thin finned-tube over the unfinned-tube is found to be negligible. The heat-transfer coefficient in the thick finned-tube system is approximately two times higher than that in the unfinned-tube system. The heat transfer coefficients for the unfinned-tube and thick finned-tube systems are found to be 45 150 W/m²-K and 90 250 W/m²-K, respectively. The thermal performance for three different tube systems is found to be strongly affected by the inlet temperature but not by the flow rate of the heat transfer fluid. The amount of heat recovered has been correlated in terms of the Fourier, Stefan, and Reynolds numbers.     

A review of materials,heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS)

This paper reviews the development of latent heat thermal energy storage systems studied detailing various phase change materials (PCMs) investigated over the last three decades, the heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy and the formulation of the phase change problem. It also examines the geometry and configurations of PCM containers and a series of numerical and experimental tests undertaken to assess the effects of parameters such as the inlet temperature and the mass flow rate of the heat transfer fluid (HTF). It is concluded that most of the phase change problems have been carried out at temperature ranges between 0 °C and 60 °C suitable for domestic heating applications. In terms of problem formulation, the common approach has been the use of enthalpy formulation. Heat transfer in the phase change problem was previously formulated using pure conduction approach but the problem has moved to a different level of complexity with added convection in the melt being accounted for. There is no standard method (such as British Standards or EU standards) developed to test for PCMs, making it difficult for comparison to be made to assess the suitability of PCMs to particular applications. A unified platform such as British Standards, EU standards needs to be developed to ensure same or similar procedure and analysis (performance curves) to allow comparison and knowledge gained from one test to be applied to another.     

Developed boiling heat transferTransfert thermique pour une ebullition developpeeWärmeübertragung beim ausgebildeten blasensiedenТеплообмен при развитом кипении01

In conformity with the heat transfer model considered, the developed boiling temperature conditions of the heating surface are mainly governed by the processes which are responsible for multipe star-ups of the different mechanisms of heat release. The analysis of the model yields a universal correlation for the developed boiling heat transfer on surfaces of known microgeometry and on commercial heating surfaces for all classes of heat transfer agents (water, organic liquids, cooling and cryogenic agents and liquid metals).     

Channel formation and visualization of melting and crystallization behaviors in direct-contact latent heat storage systems

Thermal storage systems are an essential component for increasing the share of renewable energies in residential heating and for the valorization of waste heat. A key challenge for the widespread application of thermal storage systems is their limited power-to-capacity ratio. One potential solution for this challenge is represented by direct-contact latent heat storage systems, in which a phase change material (PCM) is in direct contact with an immiscible heat transfer fluid (HTF). To demonstrate the applicability of the direct-contact concept for domestic hot water production, a PCM with a phase change temperature of 59°C is chosen. To enable cost-efficient implementation of the storage system, a eutectic mixture of two salt hydrates, magnesium chloride hexahydrate and magnesium nitrate hexahydrate, is chosen as the PCM. One key aspect for the direct-contact concept is that, during discharge, the HTF channels in the PCM do not become clogged during the solidification of the PCM. In this study, the formation and topology of the channels in direct-contact systems under an optimized flow condition are investigated via visual observation and X-ray computed tomography. The elucidation of the channel structure provides information on the melting and crystallization behaviors of the PCM, which are shown schematically.     

Intrinsic scales in thermohydrogasdynamicsEchelles intrinseques en hydrothermodynamique des gazEigenmasstäbe der thermo-hydro-gasdynamicСобственные масштаы в термохидрогазодинамике

Intrinsic scales and similarity criteria are considered as characteristics of local interactions in flows of homogeneous and inhomogeneous mixtures. Thereby in the analysis of similarity an important class of values is separated whose elements have been used previously, as a rule, at an intuitive level.     

Electrochemical systems for converting solar energy

Photoelectrochemical currents and potential distribution at the semiconductor/electrolyte interface for an electrochemical semiconductor solar energy converter have been analysed. Reactions involving minority and majority charge carriers in semiconductors and intermediate stages of electrocatalytic reactions involving electronic surface states were taken into account. The efficiency of semiconductor electrodes exposed to concentrated sunlight has been studied, and ways of increasing the photoconverter efficiency by the chemical modifications of the electrode surface have been considered for gallium arsenide and A^IIB^VI-type compounds. Characteristics of the cells of solar batteries are given.     

Parametric study of cascade latent heat thermal energy storage (CLHTES) system in Concentrated Solar Power (CSP) plants

Recently, Concentrated Solar Power (CSP) is attracting numerous research attentions, and thermal energy storage (TES) system filled with energy storage media is a critical component in all CSP plants. To realize a high energy storage efficiency ($\xi$) and exergy efficiency ($\eta$), a comprehensive study to the cascade latent heat thermal energy storage (CLHTES) system is necessary from the perspective of heat transfer. In this study, a dimensionless parametric study was presented using an enthalpy-based 1D transient model for energy storage/extraction in CLHTES system. A dimensionless parameter space (${\tau }_r,H_{CR},St{f}^{\text{*}}$) was constructed by considering $\xi$ and $\eta$ as the objective functions to explore the effects from dimensionless material properties (such as latent heat, specific heat at solid and liquid phases) and dimensionless operational parameters (such as charging/discharging time period, TES tank height and diameter). It is recommended that when $H_{CR}$<0.5 and ${\Pi }_c/{\Pi }_d$<1.0, the system performance is very sensitive to $H_{CR}$ and ${\Pi }_c/{\Pi }_d$, furthermore for the same TES tank volume (H/D = D/H = 1.0), the sensitivity by varying its diameter alone is double than that from changing its height. The novelty of this study is to provide the design criteria for the CLHTES system, so that it can easily be designed and its efficiencies can be competitive to sensible heat thermal energy storage (SHTES) system. The results from this parametric study and sensitivity analysis are expected to benefit the solar thermal research and industry community to design the CLHTES system.     

Experimental investigation on a combined sensible and latent heat storage system integrated with constant/varying (solar) heat sources

The objective of the present work is to investigate experimentally the thermal behavior of a packed bed of combined sensible and latent heat thermal energy storage (TES) unit. A TES unit is designed, constructed and integrated with constant temperature bath/solar collector to study the performance of the storage unit. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the constant temperature bath/solar collector to the TES tank also acts as sensible heat storage (SHS) material. Charging experiments are carried out at constant and varying (solar energy) inlet fluid temperatures to examine the effects of inlet fluid temperature and flow rate of HTF on the performance of the storage unit. Discharging experiments are carried out by both continuous and batchwise processes to recover the stored heat. The significance of time wise variation of HTF and PCM temperatures during charging and discharging processes is discussed in detail and the performance parameters such as instantaneous heat stored and cumulative heat stored are also studied. The performance of the present system is compared with that of the conventional SHS system. It is found from the discharging experiments that the combined storage system employing batchwise discharging of hot water from the TES tank is best suited for applications where the requirement is intermittent.     

High performance storage composite for the enhancement of solar domestic hot water systems: Part 1: Storage material investigation

This work aims to evaluate the performance of a solar domestic hot water (SDHW) system including a latent storage material. The originality of our approach consists to place a composite made of compressed expanded natural graphite (CENG) and phase change material (PCM) directly inside a flat plate solar collector in order to replace the traditional copper-based solar absorber. According to this target, the study is composed of two steps: the composites preparation and characterization; and the analysis of the system to achieve optimal integration of the material in the process.The present paper is focused on the selection of the most promising composite to implement in the solar collector. In order to reach this objective, several composites based on CENG and various storage materials (paraffin, stearic acid, sodium acetate trihydrate and pentaglycerin) have been elaborated and characterized. The synthesis of all these measurements allowed us to select three composites whose characteristics match their integration into a solar thermal collector.     

Study on a heat pipeEtude sur les tubes a chaleurStudium über heizungsröhreИccлeдoBaниe тeплoпpoBoднoй тpyбы

The objective of this study was to obtain an understanding of heat pipe operating limits.Sintered powders were used as the wick, and pure water and Freon 113 as the working fluid. In this study, two types of experiments were undertaken. The first involved independent studies of wick characteristics, friction losses and capillary properties. The second involved the measurement of maximum heat transfer rates. The simplified model was developed for predicting the maximum heat transfer rates of capillary limits.The agreement between predicted and experimental maximum heat-transfer rates was excellent.     

Dispersed flow heat transferTransfert thermique des ecoulements dispersesWärmeübergang in dispersionsströmungenПeceнoc тeплa в диcпecнoм пoтoкe

An experimental and theoretical analysis of the dispersed flow heat transfer has been performed. The transient experimental technique included a long tubular preheater section for creating a dispersed flow and a short tubular transient test section for collecting the heat-transfer data (heat flux vs wall superheat data). Liquid nitrogen was used as a test fluid. The mass velocities varied from 80 to 300 kg/s m².The theoretical study included: the analysis of the structure of a dispersed flow (the analysis of a drop size and drop size distribution); the analysis of the deposition motion of liquid drops (the migration of drops toward the wall); the analysis of the possible successive states of drop-wall interaction, and heat transfer to a drop deposited on the heated wall.Based on the above analyses the expression for the heat flux from the wall to dispersed flow has been developed and it has given good agreement with the experimental data.     

Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage

Fatty acids such as stearic acid (SA), palmitic acid (PA), myristic acid (MA), and lauric acid (LA) are promising phase change materials (PCMs) for latent heat thermal energy storage (LHTES) applications, but high cost is the most drawback which limits the utility area of them in thermal energy storage. The use of fatty acids as form-stable PCM will increase their feasibilities in practical LHTES applications due to reduced cost of the energy storage system. In this regard, a series of fatty acid/poly(methyl methacrylate) (PMMA) blends, SA/PMMA, PA/PMMA, MA/PMMA, and LA/PMMA were prepared as new kinds of form-stable PCMs by encapsulation of fatty acids into PMMA which acts as supporting material. The blends were prepared at different mass fractions of fatty acids (50, 60, 70, 80, and 90% w/w) to reach maximum encapsulation ratio. All blends were subjected to leakage test by heating the blends over the melting temperature of the PCM. The blends that do not allow leakage of melted PCM were identified as form-stable PCMs. The form-stable fatty acid/PMMA (80/20 wt.%) blends were characterized using optic microscopy (OM), viscosimetry, and Fourier transform infrared (FT-IR) spectroscopy methods, and the results showed that the PMMA was compatible with the fatty acids. In addition, thermal characteristics such as melting and freezing temperatures and latent heats of the form-stable PCMs were measured by using differential scanning calorimetry (DSC) technique and indicated that they had good thermal properties. On the basis of all results, it was concluded that form-stable fatty acid/PMMA blends had important potential for some practical LHTES applications such as under floor space heating of buildings and passive solar space heating of buildings by using wallboard, plasterboard or floor impregnated with a form-stable PCM due to their satisfying thermal properties, easily preparing in desired dimensions, direct usability without needing an add encapsulation and eliminating the thermal resistance caused by shell and thus reducing cost of LHTES system.     

Thermal activated (thermal) battery technology: Part II. Molten salt electrolytes

This article gives an overview of the important properties and design characteristics of electrolyte used in thermally activated (thermal) batteries. The basic physical properties of the main compositions are reviewed. The properties of electrolytes such as melting point, ionic conductivity, surface tension, density, thermal characteristics, and moisture sensitivity were analyzed in relation with the functioning of the batteries. Solubility data of alkali metals, sulphides, and oxides were compiled and analyzed. The important parameters of separator pellets are discussed in terms of both electrical and mechanical properties as they pertain to thermal-battery design and functioning. A number of lower-melting electrolytes are presented along with key physical properties for possible use in applications requiring lower operating temperatures such as borehole power supplies.