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.     

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.     

Thermal performance simulations of a packed bed cool thermal energy storage system using n-tetradecane as phase change material

The objective of this paper is to study the thermal performance of latent cool thermal energy storage system using packed bed containing spherical capsules filled with phase change material during charging and discharging process. According to the energy balance of the phase change material (PCM) and heat transfer fluid (HTF), a mathematical model of packed bed is conducted. n-tetradecane is taken as PCM and aqueous ethylene glycol solution of 40% volumetric concentration is considered as HTF. The temperatures of the PCM and HTF, solid and melt fraction and cool stored and released rate with time are simulated. The effects of the inlet temperature and flow rate of HTF, porosity of packed bed and diameter of capsules on the melting time, solidification time, cool stored and released rate during charging and discharging process are also discussed.     

Developing microencapsulated 12‐hydroxystearic acid (HSA) for phase change material use

Phase change materials (PCM) have an increasingly more important role as a thermal energy storage (TES) media. However, leakage problem of PCM causes limitation during their integration in TES systems. Therefore, the encapsulation of PCMs is attracting research interest to extend usage of PCMs in real TES applications in recent years. In this study, hydroxystearic acid (HSA) was encapsulated with polymethyl methacrylate (PMMA) and different PMMA comonomer shells via emulsion polymerization method for the first time in literature. HSA with high melting temperature range (74–78°C) can widen the scope of using PCMs, and the encapsulated form can make it more versatile. The chemical structures, morphologies, and thermophysical properties of capsules were determined by FT‐IR, SEM, DSC, TGA, and thermal infrared camera. Among the produced HSA capsule candidates, PMMA‐HEMA is the most promising with latent heat of 48.5 J/g with melting range of 47 to 85°C. SEM analysis indicated that the capsules have spherical shape with compact surface at nano‐micro (100–440 nm) size range; however, some capsules exhibited agglomeration.     

Effect of internal void placement on the heat transfer performance – Encapsulated phase change material for energy storage

The effect of an internal air void on the heat transfer phenomenon within encapsulated phase change material (EPCM) is examined. Heat transfer simulations are conducted on a two dimensional cylindrical capsule using sodium nitrate as the high temperature phase change material (PCM). The effects of thermal expansion of the PCM and the buoyancy driven convection within the fluid media are considered in the present thermal analysis. The melting time of three different initial locations of an internal 20% air void within the EPCM capsule are compared. Latent heat is stored within an EPCM capsule, in addition to sensible heat storage. In general, the solid/liquid interface propagates radially inward during the melting process. The shape of the solid liquid interface as well as the rate at which it moves is affected by the location of the internal air void. The case of an initial void located at the center of the EPCM capsule has the highest heat transfer rate and thus fastest melting time. An EPCM capsule with a void located at the top has the longest melting time. Since the inclusion of a void space is necessary to accommodate the thermal expansion of a PCM upon melting, understanding its effect on the heat transfer within an EPCM capsule is necessary.     

Numerical Analysis on a Latent Thermal Energy Storage System with Phase Change Materials and Aluminum Foam

Abstract

A latent heat thermal energy storage system with phase change material (PCM) is numerically studied. To enhance the heat transfer inside the system, a highly conductive metal foam is employed with ceramic nanoparticles. The latter method of enhancement leads to a new class of material called Nano-PCM. The system under investigation is a 70-L tank filled up with pure PCM or Nano-PCM and several pipes are situated where the heat transfer fluid (HTF) flows. The pipe surfaces are assumed at constant temperature above the PCM melting temperature to simulate the heat transfer from the HTF. The enthalpy-porosity theory is applied to simulate the PCM phase change, while the porous media formulation is assumed to describe the metal foam behavior. The nano-PCM is modeled with single-phase model where the properties are the weighted-average between the fluid base and the nanoparticles. The simulations are accomplished for charging-discharging process at different porosities and nanoparticle concentration. The results are given in term of average melting fraction evolution, average temperature as function of time, average stored energy. The metal foam significantly improves the heat transfer between PCM and HTF respect to the addition of nanoparticles, reducing the charging and discharging time more than one order of magnitude.     

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.     

Experimental and numerical investigation of thermal energy storage with a finned tube

A latent heat thermal energy storage system using a phase change material (PCM) is an efficient way of storing or releasing a large amount of heat during melting or solidification. It has been determined that the shell‐and‐tube type heat exchanger is the most promising device as a latent heat system that requires high efficiency for a minimum volume. In this type of heat exchanger, the PCM fills the annular shell space around the finned tube while the heat transfer fluid flows within the tube. One of the methods used for increasing the rate of energy storage is to increase the heat transfer surface area by employing finned surfaces. In this study, energy storage by phase change around a radially finned tube is investigated numerically and experimentally. The solution of the system consists of the solving governing equations for the heat transfer fluid (HTF), pipe wall and phase change material. Numerical simulations are performed to investigate the effect of several fin parameters (fin spacing and fin diameter) and flow parameter (Re number and inlet temperature of HTF) and compare with experimental results. The effect of each variable on energy storage and amount of solidification are presented graphically. Copyright © 2005 John Wiley & Sons, Ltd.     

Geometric design of solar-aided latent heat store depending on various parameters and phase change materials

The cylindrical latent heat storage tanks considered here are part of a domestic heating system. In this study, the performances of such energy storage tanks are optimized theoretically. Two different models describing the diurnal transient behaviour of the phase change unit were used. The first is suited to tanks where the phase change material (PCM) is packed in cylinders and the heat transfer fluid (HTF) flows parallel to it (mode 1). The second is suited to tanks where pipes containing the fluid are embedded in the PCM (mode 2). The problem (treated as two-dimensional) is tackled with an enthalpy-based method coupled to the convective heat transfer from the HTF. A series of numerical tests are then undertaken to assess the effects of various PCMs, cylinder radii, pipe radii, total PCM volume in the tank, mass flow rates of fluid, and inlet temperatures of the HTF on the storing time. In addition, optimal geometric design of the store depending on these parameters and PCMs is presented.     

Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancement

The present paper describes the analysis of the melting process in a single vertical shell‐and‐tube latent heat thermal energy storage (LHTES), unit and it is directed at understanding the thermal performance of the system. The study is realized using a computational fluid‐dynamic (CFD) model that takes into account of the phase‐change phenomenon by means of the enthalpy method. Fluid flow is fully resolved in the liquid phase‐change material (PCM) in order to elucidate the role of natural convection. The unsteady evolution of the melting front and the velocity and temperature fields is detailed. Temperature profiles are analyzed and compared with experimental data available in the literature. Other relevant quantities are also monitored, including energy stored and heat flux exchanged between PCM and HTF. The results demonstrate that natural convection within PCM and inlet HTF temperature significantly affects the phase‐change process. Thermal enhancement through the dispersion of highly conductive nanoparticles in the base PCM is considered in the second part of the paper. Thermal behavior of the LHTES unit charged with nano‐enhanced PCM is numerically analyzed and compared with the original system configuration. Due to increase of thermal conductivity, augmented thermal performance is observed: melting time is reduced of 15% when nano‐enhanced PCM with particle volume fraction of 4% is adopted. Similar improvements of the heat transfer rate are also detected. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamic discharging characteristics simulation on solar heat storage system with spherical capsules using paraffin as heat storage material

The dynamic characteristics of solar heat storage system with spherical capsules packed bed during discharging process are studied. According to the energy balance of solar heat storage system, the dynamic discharging processes model of packed bed with spherical capsules is presented. Paraffin is taken as phase change material (PCM) and water is used as heat transfer fluid (HTF). The temperatures of PCM and HTF, solid fraction and heat released rate are simulated. The effects of inlet temperature of HTF, flow rate of HTF and porosity of packed bed on the time for discharging and heat released rate are also discussed. The following conclusion can be drawn: (1) the heat released rate is very high and decreases rapidly with time during the liquid cooling stage, it is stable at the solidification cooling stage, then it decreases to zero at the solid cooling stage. (2) The time for complete solidification decreases when the HTF flow rate increases, but the effect is not so obvious when the HTF flow rate is higher than 13 kg/min; (3) compared to the HTF inlet temperature and flow rate, the influence of porosity of packed bed on the time for complete solidification is not so significant.     

An Analytical Technique of Transient Phase-Change Material Melting Calculation for Cylindrical and Tubular Containers

In this study, an analytical model for a class of heat storage that utilizes latent heat of a phase-change material (PCM) is developed. Two basic shell-and-tube configurations are considered, one in which the PCM melts inside the tubes while the heat transfer fluid (HTF) flows in the shell along it, and the other in which HTF flows inside the tubes while PCM melts outside. A system of partial differential equations, which describes heat transfer and melting of the PCM and heat transfer in the HTF, is derived with some simplifying assumptions, while still capturing and preserving the essential features of the processes involved. These equations are solved analytically, yielding the overall heat exchange parameters, like instantaneous heat transfer rate, stored energy, and overall operation time of the system. The present work shows that the use of the proposed analytical technique and its modifications for the practical PCM arrangements is beneficial. Proper application of the model makes it possible to obtain the parameters of a real PCM melting process in the form of algebraic formulas, both for the transient values of variables over time, and for the overall process characteristics. A comparison with the results of numerical calculations of transient melting, made using computational fluid dynamics, confirms the validity of analytical findings and allows to assess the degree of accuracy of the results of our analytical method in various practical cases.     

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.     

A comparative study on the performances of different shell-and-tube type latent heat thermal energy storage units including the effects of natural convection

Numerical modeling was performed to simulate the melting process of a fixed volume/mass phase-change material (PCM) in different shell-and-tube type latent thermal energy storage units with identical heat transfer area. The effect of liquid PCM natural convection (NC) on the latent heat storage performance of the pipe and cylinder models was investigated using a 3D numerical model with FLUENT software. Result shows that NC can cause a non-uniform distribution of the solid–liquid interface, which accelerates PCM melting rate. The PCM melting rate and heat storage rate in the horizontal cylinder model are higher than those in the horizontal pipe model because of the combined effects of heat conduction and NC. A comparative study was conducted to determine the effects of horizontal and vertical shell-and-tube models with different heat transfer fluid (HTF) inlets including the effects of NC. The results indicate that the vertical model with an HTF inlet at the bottom exhibits the highest PCM melting rate and heat storage rate for the pipe models. For the cylinder models, the horizontal model and the vertical model with an HTF inlet at the bottom can achieve nearly the same completed melting time. In addition, NC has minimal effect on any model with an HTF inlet at the top.     

The value of thermal radiation in assessing the charge/discharge rate of high‐grade thermal energy storage using encapsulated phase change materials (PCMs)

The charge/discharge rate of a spherical phase change material (PCM) capsule was assessed in consideration of phase change phenomenon and the combined effect of thermal radiation and heat convection in the charging/discharging processes. The heat transfer model was developed based on a single PCM capsule. The equivalent heat flux was evaluated by using the thermal resistance method. In consideration of the thermal radiation, the equivalent charge/discharge rate was improved, and the temperature rising of the PCM was actually much faster in the charging/discharging processes. It was indicated that the influence of the thermal radiation became more significant for PCM capsules under a small Re number (constant air velocity) and for high‐grade thermal energy storage. The analytical results showed that the highest heat flux contributed by cold thermal radiation occupied 30% and 62% of that by heat convection for PCM capsules with radius of 10 and 40 mm, respectively. This illustrated the crucial value of thermal radiation on the charge/discharge rate of PCM capsules with a large radius. However, for smaller size PCM capsules, the equivalent heat flux was larger under the same fluid flow velocity, and it decreased more promptly with time, because the heat convection that played the dominant role in charge/discharge processes was sensitively affected by the radius of the PCM capsules. Copyright © 2016 John Wiley & Sons, Ltd.     

Analysis and optimization of a latent thermal energy storage system with embedded heat pipes

Latent thermal energy storage system (LTES) is an integral part of concentrating solar power (CSP) plants for storing sun’s energy during its intermittent diurnal availability in the form of latent heat of a phase change material (PCM). The advantages of an LTES include its isothermal operation and high energy storage density, while the low thermal conductivity of the PCM used in LTES poses a significant disadvantage due to the reduction in the rate at which the PCM can be melted (charging) or solidified (discharging). The present study considers an approach to reducing the thermal resistance of LTES through embedding heat pipes to augment the energy transfer from the heat transfer fluid (HTF) to the PCM. Using a thermal resistance network model of a shell and tube LTES with embedded heat pipes, detailed parametric studies are carried out to assess the influence of the heat pipe and the LTES geometric and operational parameters on the performance of the system during charging and discharging. The physical model is coupled with a numerical optimization method to identify the design and operating parameters of the heat pipe embedded LTES system that maximizes energy transferred, energy transfer rate and effectiveness.     

Microencapsulated dimethyl terephthalate phase change material for heat transfer fluid performance enhancement

Micro‐phase change materials (micro‐PCMs) are proposed to increase the thermal conductivity and the thermal energy storage capacity of a heat transfer fluid (HTF). In this work, we have selected dimethyl terephthalate (DMT) to be used as a PCM for performance enhancement of a synthetic oil in the temperature range of approximately 100 to 170 °C. Silicon dioxide (SiO₂) was used as the microencapsulant, because of its desirable properties as containment material, including thermal stability. The SiO₂‐coated DMT micro‐PCM was characterized to determine relevant properties and its suitability for HTF performance enhancement. The SiO₂‐coated DMT was found to completely disperse in the synthetic oil, Therminol SP, silicone oil, at and above 100 °C. FTIR, thermal diffusivity and differential scanning calorimetry measurements were carried out on the materials, and these tests demonstrated that the coated particles can be used for HTF enhancement in the temperature range of 100–170 °C and potentially higher temperatures if pressurized pipes/vessels are utilized. Using the measured thermal diffusivity and known data for density and specific heat capacity, the thermal conductivity of the micro‐PCM was calculated. Our calculations indicate that both the thermal conductivity and the thermal energy storage heat capacity of the HTF would be enhanced by the addition of this micro‐PCM. It is expected that the thermal conductivity increase will enhance the heat transfer of the fluid when in use at temperatures above and below the melting temperature of the PCM. At the melting point, the latent heat of the PCM will increase the thermal energy storage capacity of the fluid. Copyright © 2016 John Wiley & Sons, Ltd.     

Numerical heat transfer analysis of encapsulated ice thermal energy storage system with variable heat transfer coefficient in downstream

During charging and discharging processes, the heat transfer behavior of the encapsulated ice thermal energy storage (TES) system changes during downstream case and this should be taken into account since the temperature of heat transfer fluid (HTF) and especially the heat transfer coefficient varies considerably around each capsule. This requires a careful study of the problem with variable heat transfer coefficient to contribute to the state-of-the-art. This has been the primary motivation behind the present study. Here, we first develop a new heat transfer coefficient correlation by simulating a series of 120 numerical experiments for different capsule diameters, mass flow rates and temperatures of HTF and second undertake a comprehensive numerical analysis using the temperature based fixed grid solution with control volume approach for studying the heat transfer behavior of an encapsulated ice TES system. Thirdly, we validate the present numerical model and the new correlation with some experimental data obtained from the literature, and hence a good agreement is obtained between the model results and experimental data. The results indicate that the heat transfer coefficient varies greatly during downstream and highly affects the heat transfer taking place during the process. So, the solutions with constant heat transfer coefficient appear to be unreliable for analysis and system optimization. The results also show that the solidification process is chiefly governed by the magnitude of Stefan number, capsule diameter and capsule row number.     

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.