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.     

Study on the efficiency of effective thermal conductivities on melting characteristics of latent heat storage capsules

Improvement of the thermal conductivity of a phase change materials (PCM) is one effective technique to reduce phase change time in latent heat storage technology. Thermal conductivity is improved by saturating porous metals with phase change materials. The influence of effective thermal conductivity on melting time is studied by analyzing melting characteristics of a heat storage circular capsule in which porous metal saturated with PCM is inserted. Numerical and approximate analyses were made under conditions where there are uniform or non-uniform heat transfer coefficients around the cylindrical surface. Four PCMs (H₂O, octadecane, Li₂CO₃, NaCl) and three metals (copper, aluminum and carbon steel) were selected as specific materials. Porosities of the metals were restricted to be larger than 0.9 in order to keep high capacity of latent heat storage. Results show that considerable reduction in melting time was obtained, especially for low conductivity PCMs and for high heat transfer coefficient. Melting time obtained by approximate analysis agrees well with numerical analysis. A trial estimation of optimum porosity is made balancing the desirable conditions of high latent heat capacity and reduction of melting time. Optimum porosity decreases with increase in heat transfer coefficient.     

Experimental studies on the behaviours of hydride heat storage system

Experimental studies on the heat transfer characteristics of the heat storage vessel are conducted. Mg₂Ni is used as the heat storage medium. The structure of the heat storage vessel is a single tube in which a number of thermocouples are installed for temperature measurement. The temperature gradient along the axial direction of the tube was not found at the steady state for both heat generation and absorption period. Temperature gradient, however, exists along the radial direction. The overall heat transfer coefficient obtained is about 10 Kcal/m²h deg which implies that heat transfer through the powder bed is a controlling step. The lumped parameter model to describe the performance of the heat storage vessel was derived and it could explain the heat transfer characteristics of the vessel qualitatively.     

Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system

The phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as phase change material (PCM) during the melting and solidification processes were determined experimentally in a vertical two concentric pipes energy storage system. This study deals with three important subjects. First is determination of the eutectic composition ratio of the palmitic acid (PA) and stearic acid (SA) binary system and measurement of its thermophysical properties by differential scanning calorimetry (DSC). Second is establishment of the phase transition characteristics of the mixture, such as the total melting and solidification temperatures and times, the heat transfer modes in the melted and solidified PCM and the effect of Reynolds and Stefan numbers as initial heat transfer fluid (HTF) conditions on the phase transition behaviors. Third is calculation of the heat transfer coefficients between the outside wall of the HTF pipe and the PCM, the heat recovery rates and heat fractions during the phase change processes of the mixture and also discussion of the effect of the inlet HTF parameters on these characteristics. The DSC results showed that the PA–SA binary system in the mixture ratio of 64.2:35.8 wt% forms a eutectic, which melts at 52.3 °C and has a latent heat of 181.7 J g⁻¹, and thus, these properties make it a suitable PCM for passive solar space heating and domestic water heating applications with respect to climate conditions. The experimental results also indicated that the eutectic mixture of PA–SA encapsulated in the annulus of concentric double pipes has good phase change and heat transfer characteristics during the melting and solidification processes, and it is an attractive candidate as a potential PCM for heat storage in latent heat thermal energy storage systems.     

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.     

The use of phase-change coatings for estimating the heat transfer characteristics in a rotating disc system

Phase-change coatings have been applied to the axial-clearance rotor-stator cavity for the estimation of the transient heat transfer characteristics of the surface of the rotating disc. The tests were conducted for an air mass flow coefficient C_w = 1220, a gap ratio G = 0.1, an axial-clearance ratio G_ca = 0.05 and for rotational Reynolds numbers of Re? = 1 × 10⁵ and 2 × 10⁵. The phase-change coating used had a melting point of 38°C. From a video recording of the transient movement of the melt-line on the rotor (coated with the phase-change material) blown with heated air, it was possible to compute the heat transfer coefficients. The data reduction was made using the ‘semi-infinite slab’ approximation to the governing one-dimensional transient heat conduction equation.     

Thermal conductivity enhancement in a latent heat storage system

Latent heat storage systems especially those employing organic materials have been reported to exhibit a rather slow thermal response. This is mainly due to the relatively low thermal conductivity of organic latent heat materials. In this study, experiments were carried out to investigate a method of enhancing the thermal conductivity of paraffin wax by embedding aluminum powder in it. The size of the aluminum powder particles was 80 μm. The tested mass fractions in the PCM-aluminum composite material were 0.1, 0.3, 0.4, and 0.5 of aluminum. The used mass fraction in the experimental work was 0.5.The experiments were conducted by using a compact PCM solar collector. In this collector, the absorber-container unit performed the function of absorbing the solar energy and storing the phase change material (PCM). The solar energy was stored in the PCM and was discharged to cold water flowing in pipes located inside the PCM. Charging and discharging processes were carried out. The propagation of the melting and freezing fronts was studied during the charging and the discharging processes. The time wise temperatures of the PCM were recorded during the processes of charging and discharging. The solar intensity was recorded for the charging process. It was found that the charging time was reduced by approximately 60% by adding aluminum powder in the wax. In the discharging process, experiments were conducted for different water flow rates of 9-20.4 kg/h. It was found that the useful heat gained increased when adding aluminum powder in the wax as compared to the case of pure paraffin wax. The heat transfer characteristics were studied.     

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.     

潜热储热系统强化传热研究进展

近年来,潜热储热系统在太阳能和工业废能的利用中发挥着极其重要的作用,因此用于潜热储热的相变材料受到普遍关注.文章对国内外潜热储热系统众多强化传热技术进行了综述与讨论.     

Correlation of the volumetric heat transfer coefficient for air-rock bed thermal storage system - considering humid air

Rock bed thermal storage with humid air green house or regenerative air cooling application are becoming widely investigated nowadays. The effect of the mass transfer in the circulated air has not received much attention. In this experimental study, the effect of the mass in the operated air is studied and new correlation for calculating the coefficient of volumetric heat transfer for humid air is presented.     

CFD investigation of the flow and heat transfer characteristics in a tangential inlet cyclone

This work presents a computational fluid dynamics (CFD) calculation to investigate the flow field and the heat transfer characteristics in a tangential inlet cyclone which is mainly used for the separation of the dens phase of a two phase flow. Governing equations for the steady turbulent 3D flow were solved numerically under certain boundary conditions covering an inlet velocity range of 3 to 30 m/s. Finite volume based Fluent software was used and the RNG k −  turbulence model was adopted for the modeling highly swirling turbulent flow. Good agreement was found between computed pressure drop and experimental data available in the literature. The structure of the vortices and variation of local heat transfer were studied under the effects of inlet velocity.     

Enhancement of heat and mass transfer characteristics of metal hydride reactor for hydrogen storage using various nanofluids

The execution of metal hydride reactor (MHR) for storage of hydrogen is greatly affected by thermal effects occurred throughout the sorption of hydrogen. In this paper, based on different governing equations, a numerical model of MHR filled by MmNi_4.6Al_0.4 is formed using ANSYS Fluent for hydrogen absorption process. The validation of model is done by relating its simulation outcomes with published experimental results and found a good agreement with a deviation of less than 5%; hence present model accuracy is considered to be more than 95%. For extraction or supply of heat, water or oil is extensively used in MHR during the absorption or the desorption process so as to improve the competency of the system. Since nanofluid (mixture of base fluid and nanoparticles) has a higher heat transfer characteristics, in this paper the nanofluid is used in place of the conventional heat transfer fluid in MHR. Further the numerical model of MHR is modified with nanofluid as heat extraction fluid and results are presented. The Al₂O₃/H₂O, CuO/H₂O and MgO/H₂O nanofluids are selected and simulations are carried out. The results are obtained for different parameters like nanoparticle material, hydrogen concentration, supply pressure and cooling fluid temperature. It is seen that 5 vol% CuO/H₂O nanofluid is thermally superior to Al₂O₃/H₂O and MgO/H₂O nanofluid. The heat transfer rate improves by the increment in the supply pressure of hydrogen as well as decrement in temperature of nanofluid. The CuO/H₂O nanofluid increases the heat transfer rate of MHR up to 10% and the hydrogen absorption time is improved by 9.5%. Thus it is advantageous to use the nanofluid as a heat transfer cooling fluid for the MHR to store hydrogen.     

Performance investigation of the capric and lauric acid mixture as latent heat energy storage for a cooling system

The thermal performance of the capric acid and lauric acid mixture (C–L acid) in the respective composition of 65% and 35% by mole was investigated for its cooling capacity. Pentadecane was used for comparison. A vertical cylindrical storage capsule was employed for the study. The temperature distribution of the C–L acid during charging and discharging inside a vertical tube was experimentally determined in both radial and axial directions. A melting point of 18–19.5°C was observed. This value corroborates with the DSC-obtained values for the C–L acid. The calculated stored energy based on the radial temperature distribution during charge and discharge processes indicates that the C–L acid is a potential latent heat storage material.     

Thermodynamics and kinetics analysis of hydrogen absorption in large-scale metal hydride reactor coupled to phase change material-metal foam-based latent heat storage system

Phase change materials (PCMs) have recently been coupled with metal hydride storage tanks (MHSTs) to store adsorption heat and subsequently deliver it for hydrogen desorption through melting and solidification cycles. This method might reduce process costs by eliminating the use of HTF (i.e. heat transfer fluid). However, thermodynamics and kinetic data are scarce for large-scale MH-PCM applications, particularly when PCM is loaded in metal foams (MFs) to promote heat transfer. The current work aimed to develop a 2D model for simulating H₂-absorption in a LaNi₅-metal bed integrating a PCM-MF unit in a large-scale tube-and-shell heat exchanger. The constructed model (via Fluent 15.0 CFD-platform) was first-validated using referenced experimental data. The resulting heat transfer was analyzed for different MFs [aluminum, copper, nickel and titanium] of different porosities (0.1–1.0). Excellent outcomes were retrieved. By trapping the H₂-absorption heat, the MF-PCM unit improved the LaNi₅ hydriding. The LaNi₅ charging was achieved after ～500 s, independently of the MF type and porosity. The PCM melting rate depends on tube position, porosity and the MF type. It increased with MFs incorporation (order of enhancement: Cu > Al > Ni > Ti) and MF-porosity decrease (from ε = 100% to 10%). Besides, the PCM tube above the H₂-feeding pipe melts more quickly than the other tubes, presumably to the gravitational-force effect. Longer times (i.e. 9 000 s to >16 000 s, depending on tube position) were recorded for complete melting of the PCM when ε_MF = 100%; however, when ε_MF is less than 80%, the required time for total melting was tremendously reduced to less than 500 s. Nevertheless, the MFs porosity could not be decreased considerably to avoid a huge loss of material storage (PCM), thereby diminishing the thermal storage performance of the PCM matrix.     

Heat transfer coefficient and latent heat of martensite in a medium-carbon steel

In this paper the method used in a recently published paper to measure the latent heat of the martensite transformation during quenching of a steel is analysed. The arrangement of the experiment made it possible to obtain reasonable values for the latent heat of transformation, but cannot be expected to produce reasonable values for the heat transfer coefficient. Improved methods are discussed, including the novel probe design of the present authors.     

Hydrogen storage system based on hydride materials incorporating a helical-coil heat exchanger

Metal hydrides offer the potential to store hydrogen at modest pressures and temperatures with high volumetric efficiencies. The process of charging hydrogen into a metal powder to form the hydride is exothermic. The heat released by the reaction must be removed quickly in order to maintain a rapid charging rate. An effective method for heat removal is to embed a heat exchanger within the metal hydride bed. Here, we investigate the effectiveness of a helical coil heat exchanger tube to remove the heat generated during the absorption process. This paper presents a three-dimensional mathematical model formulated in Ansys Fluent 12.1 to evaluate the transient heat and mass transfer in a cylindrical metal hydride tank embedded with a helical-coil cooling tube. We present results from a parametric study of hydrogen storage efficiency as a function of helical coil pitch and convective heat transfer coefficient (h) within the cooling tube. We also explore the effect of adding aluminum foam to enhance the thermal conductivity of the metal hydride. The parametric study reveals that the mass of stored hydrogen is less sensitive to the coil pitch when aluminum foam is added. It is also found that the absorption rate increases with h as expected, although the rate of improvement diminishes at high values of h. Results were examined at filling times of 3 and 6 min to draw conclusions about the overall effectiveness of this hydrogen storage system. At 3 min, it is found that the addition of 5% Al foam is optimal, and h = 1000 W/m²-K is sufficient to bring the metal hydride to saturation; under these conditions a non-dimensional pitch of 0.5 maximizes the hydrogen absorption. Adding Al foam beyond 5% does not improve volumetric efficiency as the Al foam begins to displace the active hydrogen-absorbing material.     

Heat and mass transfer and fluid flow in cryo-adsorptive hydrogen storage system

Compared to room temperature adsorption, cryo-adsorptive hydrogen storage capacity has been greatly improved, and has become the central issue of the hydrogen storage research. Accurate simulation and optimization for cryo-adsorptive hydrogen storage has important guidance and application value to the experimental research, and the finite element software Comsol Multiphysics™ and system analysis software Matlab/Simulink™ can be used to simulate the cryo-adsorptive hydrogen storage. However, the computational fluid dynamics (CFD) software Fluent™ can provide more information on the heat and mass transfer and the fluid flow than above softwares. Based on the mass, momentum and energy conservation equations, this paper uses the modified Dubinin–Astakhov (D–A) adsorption isotherm model, linear driving force (LDF) model and dynamic thermal boundary condition which are implemented by means of CFD software Fluent to simulate the hydrogen adsorption processes of charging and dormancy in the case of liquid nitrogen cooling. We study the variations of temperature and pressure during the processes of charging and dormancy. The results show that the experimental data is in good agreement with the simulation results. We also analyze the effect of variable specific heat and anisotropic thermal conductivity on the heat and mass transfer and the fluid flow in cryo-adsorptive hydrogen storage system.     

Modelling heat transfer in a circulating fluidized bed

A model for the bed-to-wall heat transfer under low temperature condition in a circulating fluidized bed (CFB) was developed based upon a simplified cluster renewal concept. The age of clusters in contact with the wall at different locations along the height of the CFB was estimated as the weighted average age considering their formation and disintegration. One set of experimental data on heat transfer in a 4.5-metre high, 0.15-metre diameter CFB under low temperature condition (67–77°C) was chosen for comparison with prediction of local heat transfer coefficient. The experimental observation and prediction have shown a qualitative agreement.     

Experimental volumetric heat transfer characteristics between oil and glass pebbles in a small glass tube storage

A small glass tube containing oil and glass pebbles is constructed to evaluate rapidly the volumetric heat transfer characteristics during charging. An electrical hot-plate in thermal contact with a steel spiral coil is used to charge the oil/pebble-bed storage under different average charging flow rates. The heat transfer characteristics are presented in terms of the average temperature of the oil, the average temperature of the pebbles, the average charging power and the volumetric heat transfer coefficient. The results reveal that an increase in the average charging flow rate results in a linear increase in the volumetric heat transfer coefficient. An expression for the correlation between the superficial mass flow velocity, the average pebble diameter and the volumetric heat transfer coefficient is also formulated.