Inlet Velocity Of Heat Transfer Fluid Research Articles

Experimental study on the thermal response of a metal foam dual phase change unit thermal storage tank

Adding fins or metal foam to phase change units can effectively enhance the heat storage efficiency of thermal energy storage (TES) tanks, thereby improving the reliability of TES tanks in coordinating renewable energy supply and demand. In this study, an experimental TES tank platform was designed and constructed to investigate how the addition of straight fins, metal foam, and straight fin/metal foam composite materials to paraffin affects the temperature response during the heat storage process. High-temperature water was selected as the heat transfer fluid (HTF) and injected from the top into the TES tank at the same paraffin fill level, and T-type thermocouples were arranged along the central axis of the phase change unit. TES tanks filled with either pure paraffin or composite phase change materials were tested by varying the HTF inlet temperature and velocity, and temperature response data were captured and recorded using a KEYSIGHT DAQ970A data logger. The results indicated that increasing the HTF inlet temperature significantly enhanced the heat storage rate while increasing the HTF inlet velocity had varying effects on different phase change units. A comprehensive analysis suggested that an inlet velocity of 1.3 m3/h (from a range of 0.8–1.8 m3/h) was reasonable. The addition of enhancement measures improved the temperature drift phenomena and effectively increased the temperature response rates and distribution uniformity. Moreover, the straight fin/metal foam composite material exhibited the best performance.

Charging performance of structured packed-bed latent thermal energy storage unit with phase change material capsules

A mathematical model of the charging process for a structured packed-bed latent thermal energy storage unit with phase change material capsules is established. The thermal-hydrodynamic characteristics of the unit are investigated. The impacts of the heat transfer fluid inlet velocity, heat transfer fluid inlet temperature, initial temperature of the latent thermal energy storage unit, and diameters of the phase change material capsules are analyzed. The charging performance of a scaled-up latent thermal energy storage unit is studied. The numerical results indicate that the structured packing configuration of the structured packed-bed latent thermal energy storage unit influences the periodic flow characteristics of the latent thermal energy storage unit. The melting behaviors in each phase change material capsule are synchronous due to the excellent heat transfer capacity of the latent thermal energy storage unit. With the increasing initial temperature of the latent thermal energy storage unit, the duration of the charging process varies little, but the total heat storage capacity of the unit decreases. The decreasing diameter of the phase change material capsule significantly shortens the charging process of the latent thermal energy storage unit. The total heat storage capacities of the latent thermal energy storage unit with different phase change material capsule diameters are nearly the same. The heat storage capacity of the phase change material unit can be easily scaled up by adding more phase change material capsules and extending the phase change material capsule zone. The scale-up of the structured packed-bed latent thermal energy storage unit does not affect the charging time of the latent thermal energy storage unit.

LSTM-BP neural network analysis on solid-liquid phase change in a multi-channel thermal storage tank

Latent heat thermal storage (LHTS) system is a crucial technology for achieving carbon neutrality and alleviating energy stress. Although metal foam can ameliorate the thermal storage rate of the LHTS system, the impact of the inlet velocity and temperature of heat transfer fluid (HTF) on the phase transition of phase change material (PCM) needs to be properly designed to achieve optimal performance. A novel multi-channel LHTS tank with metal foam is designed. A three-dimensional numerical model is established to describe the transient melting process in the LHTS tank. Besides, a new LSTM-BP neural network is developed, in which the HTF inlet velocity, temperature and time are employed as input data. Simulated results are consistent with the previous measurement data, verifying the correctness of the numerical methods. Results suggest that the whole melting time of the PCM is diminishing with increasing HTF velocity (or temperature). The machine learning prediction results show minor differences with the simulation results. The developed machine learning model provides new adaptive approaches for thermal storage design and operation.

Energy storage and exergy efficiency analysis of a shell and tube latent thermal energy storage unit with non-uniform length and distributed fins

This work proposes a novel type of shell and tube latent thermal energy storage unit (LTESU). Effects of the thermal conductivity of PCM, the inlet temperature of heat transfer fluid (HTF), the inlet velocity of HTF and fin layout (fin length and distribution) on the thermal performance and exergy efficiency of the LTESU are numerically investigated. The transient numerical computation of the PCM phase transition process incorporates iterative by the finite volume method based on enthalpy-porosity technique. The results showed that the role of fin layout for enhancing the energy storage performance of LTESU is more predominant than that by the PCM thermal conductivity. The LTESU with fins has better heat storage intensity compared with the case of no-fins, where the heat storage intensity increased by 104.33% with non-uniform fin length and distribution and 61.06% with uniform fin length and distribution. In addition, the exergy efficiency of LTESU with reduced fin length and uniform distribution along the HTF flow direction (Case 1) has the greatest improvement, which is increased by 8.53% for Case 1 relative to the no-fins case. The findings reveal that adopting non-uniform length and distributed fins is an effective solution to attain higher thermal performance in LTESU.

Performance improvement of metal hydride hydrogen storage tanks by using phase change materials

In metal hydride based hydrogen storage tanks, heat transfer fluid (HTF) has been extensively used to continuously transfer the reaction heat for promoting the reaction via heat exchangers. In this study, the phase change material (PCM) is integrated with the tank to enhance heat transfer and recycle the reaction heat. A novel storage tank with a simple concentric straight-tube heat exchanger surrounded by PCM is put forward to improve the hydrogen storage performance. A numerical model is built to track the transfer and reaction process. By comparison, the new tank shows better heat transfer and storage performance, and the hydrogen absorption time is shortened by 60.2% than that of the tank without PCM. For the new tank, the optimal amount of PCM is obtained, based on which the increased absorption pressure could effectively accelerate the heat discharge and reaction rate during the absorption process. However, the increased inlet velocity of HTF has a limited improvement effect on heat transfer and reaction performance. Furthermore, on the PCM side of the tank, the addition of fins and increasing the thermal conductivity of PCM had little effect on the performance of the tank.

Design and investigation of single tank phase change thermal storage domestic hot water system

Thermal energy storage (TES) is extensively applied in production and daily life. As a basic work, we designed a single tank phase change TES domestic hot water system using night valley power. Based on the single tank TES experimental platform, a 2D numerical model of the system was established. To optimize the performance of a single tank TES system, the effect of heat transfer fluid (HTF) inlet velocity and temperature on the heat release of the storage tank was analyzed based on simulation. The enhanced heat transfer effect of the radial cascade system and axial cascade system was also compared. The results showed that 0.8 m/s is the optimal HTF inlet velocity in the calculation range. The rule of the heat release about the HTF inlet temperature just meets the different hot water requirements in winter and summer. The heat release capacity of the radial cascade system and axial cascade system is higher than that of the conventional single-stage system, and the proportion of latent heat release in radial cascade is larger. This study will provide theoretical guidance for the optimal design of the single tank TES domestic hot water system in the future.

Performance analysis of an air rock thermocline TES tank for concentrated solar power plants using the coupled DEM–CFD approach

One of the most common solutions currently available to meet future energy needs in the world is concentrated solar power (CSP) plants combined with thermocline thermal energy storage (TES) tank subsystems. The air rock thermocline TES tank asserts to be an effective and inexpensive TES subsystem for CSP plants. In this study, a discrete element method (DEM) combined with a numerical model of computational fluid dynamics (CFD) was adopted to investigate the thermal performance of a scale model of an air rock thermocline TES tank. The study is carried out to optimize TES thermocline tank's thermal behavior by changing the heat transfer fluid (HTF) inlet velocity. The DEM–CFD coupled model is validated against analytical and experimental data. The results show that the use of the distributor reduced the influence of the wall effect by distributing the HTF in a regular way as the fluid passes through it, especially at high inlet velocity. Moreover, the results show that each TES tank subsystem has a critical velocity at which the tank operates most efficiently. Furthermore, the results demonstrated that the maximum value of the overall cycle efficiency was equal to 70.15% at 10 m/s, while the minimum value was equal to 42.5% at 2 m/s. The highest capacity and utilization ratio are 81.31 and 72.81% at 16 m/s, while the lowest capacity and utilization ratio are 52.01 and 44.75% at 2 m/s. The pumping energy needed to overwhelm the pressure drop rises when the HTF inlet velocity increases. This study can provide an effective reference for the efficient operation regulation and optimal design of the thermocline tank.

Effect of Inlet and Geometrical Parameters on the Melting of PCM Capsules of Elliptical Cross Section.

This paper focuses on the melting of phase change material capsulated in the elliptical cross-section horizontal cylinder under convective boundary conditions. Different parameters are discussed, namely, the HTF inlet temperature and velocity and the axes ratio of the capsule cross-section. The effect of HTF inlet temperature, inlet velocity, and the capsule axes ratio was studied using the CFD software FLUENT 6.3.26. A comparison between the numerical and experimental results was made for the system. The experimental results matched well with the heat transfer model. It is shown that the inlet temperature of the heat transfer fluid (HTF) has a great effect on the process of paraffin wax melting. The increase of HTF inlet temperature increases the Phase change material( PCM) liquid fraction at constant charging time and decreases at the same time the total time of the capsulated paraffin wax melting. The geometry of the capsule cross-section represented by its axes ratio has a sensible effect on the process of paraffin wax melting. Increasing the axes ratio of the capsule, i.e. elongated the capsule in the perpendicular direction of flow increasing the PCM liquid fraction at constant charging time and decreases the total time of the capsulated paraffin wax melting. The increase of HTF inlet velocity has a weak effect on the process of paraffin wax melting in the inlet velocity range 0.003-0.012 m/s.

Experimental and Computational Study of Melting of Paraffin Wax Inside a Cylindrical Capsules of Elliptical Cross Section

This paper focuses on the melting of phase change material capsulated in the elliptical cross-section horizontal cylinder under convective boundary conditions. Different parameters are discussed namely, the HTF inlet temperature and velocity and the aspect ratio of the capsule cross-section. The effect of HTF inlet temperature, inlet velocity, and encapsulant aspect ratio was studied using the CFD software FLUENT6.3.26. A comparison between the numerical and experimental results was made for the system. The experimental results matched well with the heat transfer model. It is shown that the inlet temperature of the heat transfer fluid (HTF) has a great effect on the process of paraffin wax melting. The increase of HTF inlet temperatures increases the PCM liquid fraction and decreases at the same time the total time of melting the capsulated paraffin wax. The geometry of the capsule cross-section represented by its aspect ratio has a sensible effect on the process of paraffin wax melting. Decreasing the aspect ratio of the capsule, i.e elongated the capsule in the direction of flow increasing the PCM liquid fraction and decreases the total time of melting the capsulated paraffin wax. The increase of HTF inlet velocity has a very weak effect on the process of paraffin wax melting.

Analysis of a latent heat thermal energy storage unit with metal foam insert in both the HTF and PCM sides

Latent heat thermal energy storage (LHTES) systems using phase change material (PCM) have received significant research attention in numerous engineering fields. The transient heat transfer phenomenon inside a vertical LHTES unit is numerically investigated, with the paraffin as the PCM and water as the heat transfer fluid (HTF). As a performance enhancement technique, the metal foam insert is applied in both the HTF and PCM sides. The conjugate thermal model for the HTF/foam-wall-PCM/foam system is built, considering the non-equilibrium effect between the solid and fluid phases with two-temperature energy equation and the natural convection inside the PCM with Boussinesq approximation. The enthalpy-based method is employed to account for the solid-liquid phase change problem. The overall performance is compared with other three cases: no foam insert, foam insert in HTF side and foam insert in PCM side. Besides, parametric study is also conducted on the melting features, including the foam structural parameters and inlet conditions of HTF. The results show that foam insert in both sides accelerates the PCM melting effectively. The foam porosity and HTF inlet temperature play important roles in the overall heat transfer, whereas the pore density and HTF inlet velocity have limited effects on the melting rate. The findings can provide referential information for the design of a LHTES system.

Carbonate salt based composite phase change materials for medium and high temperature thermal energy storage: From component to device level performance through modelling

This paper concerns the thermal performance of composite phase change materials (CPCMs) based thermal energy storage (TES) from component to device levels. The CPCMs consist of a eutectic salt of NaLiCO3 as the phase change material (PCM), an MgO as the ceramic skeleton material (CSM) and graphite flakes as the thermal conductivity enhancement material (TCEM). A computational model describing the transient heat transfer within the CPCMs module and component is developed and validated by experiment. Extensive modelling is then performed to investigate the performance of TES from component to device levels. Firstly, the influences of CPCMs properties, module size and surface roughness on component performance are evaluated. The results show that a higher mass loading of TCEM gives a higher thermal conductivity at CPCMs module level and hence a shorter charging/discharging process at component level. A larger CPCMs module and a higher heat transfer fluid inlet velocity achieve a remarkable improvement on component performance. The CPCMs based device is then investigated with a focus on the effect of the component arrangements. The results indicate that the trapezoidal configuration offers the best performance with the total charging process being shortened by 55.6% and 34.8% in comparison with the parallel and interlaced configurations.

The Characteristics of Heat Transfer in Plate Phase Change Energy Storage Unit

Abstract In order to study the heat transfer characteristics of the plate-type phase change energy storage unit, the Fluent Enthalpy method was used to simulate the heat storage and release process of the plate phase change materials (PCM) energy storage unit. The simulation results show that the main heat transfer zone will advance along the direction of the flat plate (air flow direction), and the speed of the main heat transfer zone is proportional to the air velocity with the continuous heat transfer process. In heat storage, the speed of the main heat transfer zone will accelerate with the increase of air velocities; while in the heat release process, the main heat transfer zone will accelerate with the increase of air velocities. Meanwhile, the liquid phase ratio of PCM material is also changing continuously which increases with time and is affected by the liquefaction or solidification speed. The faster the liquefaction and solidification speed are, the faster it will change. As the fluid inlet velocity increases, the heat storage exothermic efficiency also increases as the heat transfer fluid inlet velocity increases. Overall, the total heat transfer rate is gradually reduced compared with the initial heat transfer, and then tends to be consistent with the temperature of the heat transfer fluid.

Computational Investigation of Thermal Interaction Phenomena Between Two Adjacent Spheres Filled with Different Phase Change Materials (PCMs)

One of the discussed techniques in improving the performance of latent thermal energy storage applications is to use different phase change materials (PCMs) encapsulated in spherical nodules. However, the large number of spheres used in the storage applications pushes the researchers to neglecting the natural convection effect inside spheres. Consequently, they cannot observe the real thermal interaction effect between spheres. The objective of this work is to carry out a computational study on the thermal behavior of two adjacent different PCMs inside aluminum spherical capsules and taking into account the natural convection effect inside the spheres. Water is used as heat transfer fluid (HTF) at constant inlet temperature. The study shows that for the HTF inlet velocity lower than 10[Formula: see text][Formula: see text]m/s, the thermal interaction between two adjacent different PCMs cannot be neglected. However, the thermal interaction appears more clearly in the discharging mode. The thermal interaction can accelerate the discharging of one of the PCMs by reducing the discharging of the other one.

Effects of parameters on performance of high temperature molten salt latent heat storage unit

In order to investigate the performance of high temperature molten salt latent heat storage (LHS) unit under variable conditions, the effects of heat transfer fluid (HTF) inlet temperature, velocity and tube geometric parameters on melting time, melting fraction, heat storage rate and solid–liquid interface were numerically investigated. The results show that within the studied parameters, the HTF inlet temperature has the largest effect on LHS rate. With the HTF inlet temperature increasing from 1070 K to 1110 K, the melting time reduces 53.0%. However, the larger inlet temperature will result in more non-uniform melting rate and solid–liquid interface distribution. The second important influential factor is the HTF inlet velocity. When inlet velocity increases from 10 m/s to 20 m/s, the melting time reduces 45.4%. And the effect of velocity on the solid–liquid interface distribution is uniform. LHS tube diameter has the lowest effect on performance. With outer tube radius increasing from 24.0 mm to 28.0 mm, the melting time only augments 16.3% and the solid–liquid interface distribution becomes more uniform. In a general conclusion, when the heat load of the heat source is larger, a larger HTF mass flow rate is suitable to maintain a moderate HTF temperature. And then for the LHS unit, a larger tube diameter is recommendable.

Simulation studies on heat and mass transfer in high-temperature magnesium hydride reactors

Magnesium hydride is one of the most promising materials for hydrogen storage and high-temperature heat storage. The heat and mass transfer processes occurring in magnesium hydride reactors are very complicated. In the present study, a mathematical model for a magnesium hydride reactor is developed to investigate the mass and heat transfer characteristics under various operating conditions. The distributions of gas velocity, gas pressure, temperature and reacted fraction are obtained by solving the rigorous model. Two versions of the model, which are respectively applied on two different computational domains, are investigated. The simplified version of the model is proved capable of predicting the performance of the magnesium hydride reactor, with high computation efficiency and acceptable accuracy. The simulation results indicate that an increase in hydrogen supply pressure (pin) accelerates the absorption process due to enhanced absorption reaction kinetics. The hydriding time decreases when heat transfer fluid (HTF) velocity (uf) goes up, because the convection heat transfer coefficient between HTF and the magnesium hydride rises with increasing HTF inlet velocity. Moreover, an increase in HTF inlet temperature (Tin) leads to an increase in the equilibrium pressure of the magnesium hydride which decelerates the hydrogen absorption reaction. The optimal setting of the three operating parameters for the magnesium hydride reactor is determined based on the mathematical model, and the corresponding values of the parameters are as follows: pin=2.6MPa, Tin=473K and uf=5ms−1.

Numerical study on performance of molten salt phase change thermal energy storage system with enhanced tubes

Based on enthalpy method, numerical studies were performed for high temperature molten salt phase change thermal energy storage (PCTES) unit used in a dish solar thermal power generation system. Firstly, the effects of the heat transfer fluid (HTF) inlet temperature and velocity on the PCTES performance were examined. The results show that although increasing the HTF inlet velocity or temperature can enhance the melting rate of the phase change material (PCM) and improve the performance of the PCTES unit, the two parameters will restrict each other for the fixed solar collector heat output. Then three enhanced tubes were adopted to improve the PCTES performance, which are dimpled tube, cone-finned tube and helically-finned tube respectively. The effects of the enhanced tubes on the PCM melting rate, solid–liquid interface, TES capacity, TES efficiency and HTF outlet temperature were discussed. The results show that compared with the smooth tube, all of the three enhanced tubes could improve the PCM melting rate. At the same working conditions, the melting time is 437.92min for the smooth tube, 350.75min for dimpled tube which is reduced about 19.9% and 320.25min for cone-finned tube which is reduced about 26.9% and 302.75min for helically-finned tube reduced about 30.7%. As a conclusion, the thermal performance of PCTES unit can be effectively enhanced by using enhanced tube instead of smooth tube. Although, the HTF pressure drops for the enhanced tubes are also larger than that of the smooth tube, the largest pressure drop (1476.2Pa) is still very lower compared with the working pressure (MPa magnitude) of the dish solar generation system. So, the pressure drops caused by the enhanced tubes could almost be neglected.