Heat Transfer Fluid Velocity Research Articles

Kinetic investigation and numerical simulation of reactor with prepared diatomite/CaCO3 for high-temperature thermal energy storage application

Calcium looping (CaL) thermochemical energy storage (TCES) gradually appears in high temperature concentrating solar power (CSP) plants. However, the CSP-Cal integrated system has encountered challenges related to the high temperature stability of calcium-based materials and reactor design. Firstly, the wet ball milling-impregnation method was used to experimentally prepare the diatomite calcium-based thermal storage materials, and a natural diatomite skeleton matrix was obtained. A modified diatomite/CaCO3 composite thermochemical energy storage material was synthesized. The material can keep maintaining an effective conversion rate of up to 85 % by repeating charging and discharging heat storage processes for 20 times. At the same time, the 3-dimensional diffusion model was applied to investigate the reaction kinetics during the heat storage process. Secondly, numerical studies were carried out to simulate the decarbonization process of the packed bed reactor used in CSP, illustrating the complex coupling mechanism of the multiple physical processes. The results reveal that increasing the temperature and velocity of heat transfer fluid (HTF) can accelerate the heat storage process and changing porosity can modify the reaction pattern. Finally, in order to enhance the heat transfer and flow rate of HTF inside the reactor, spiral metal porous channels of large permeability with high thermal conductivity are embedded in the reactor, which promotes CO2 emission and reduces the completion time during a decarbonization reaction. This study not only proposes a novel approach to prepare the calcium-based heat storage materials but optimizes the energy storage performance of packed bed reactors.

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.

Multi-objective prediction and optimization of performance of three-layer latent heat storage unit based on intermittent charging and discharging strategies

An intermittent heat charging and discharging strategy is proposed for on-demand thermal utilization in a three-layer latent heat storage unit filled with nanoparticle-enhanced phase change materials. To optimize the utilization ratio of phase change materials, and the stored and released thermal exergy amounts, a multi-objective prediction and optimization methodology combining orthogonal experimental design, range and variance analyses, multi-nonlinear regression models, and non-dominated sorting genetic algorithm-II is introduced while considering the variables of nanoparticle concentration, heat transfer fluid velocity, and intermittent time interval. Results show that the time interval presents the most significant influence. Multi-nonlinear regression models for the above three variables are established with determination factors of 0.9871, 0.9625, and 0.9253, respectively. The ultimate optimal results are 0.8, 57094.03 J, and 43066.73 J, achieved at the three variables of 44.37 min, 0.38 m s−1 and 8.99%, respectively. The maximum verification error of 5.11% indicates the reliability of this methodology. The methodology aims to enhance the overall performance of the three-layer latent heat storage system by mitigating the constraints associated with single-performance optimization.

Numerical investigation of the heat-release performance of a metal hydride reactor-integrated phase change material

Metal hydride (MH) heat storage has a relatively high energy density and can be used in concentrated solar power (CSP) plants. However, in this system, the outlet temperature of the heat transfer fluid (HTF) has large fluctuations while discharging heat, which cannot satisfy the requirement of CSP plants. Therefore, this study proposes an MH-based heat storage system integrated with phase change materials (PCMs). A mathematical model was established to investigate the effects of the thickness of PCMs, HTF velocity, and the initial state of PCMs on heat-release performance. The results showed that the fluctuations can be effectively attenuated, and the fluid outlet temperature can be kept stable for a certain period by integrating PCMs. Plateaus in the curves of the MH bed and HTF outlet temperatures were observed, and the platform temperature and time are defined as the average temperature of two inflection points of the curve and their corresponding time difference, respectively. Increasing the thicknesses of PCMs can extend the platform time. Furthermore, the liquid phase of PCMs is conducive to improving the heat-release performance of the MH reactor with PCM.

The impact of deposits in cylindrical pipes on hydrodynamics and heat transfer

The paper deals with the problems of shell-and-tube heat exchangers operation taking into account the influence of scale and corrosion deposits on heat transfer efficiency. On the basis of numerical modeling performed in the ANSYS CFX program complex, research of various geometrical characteristics of heat exchangers, as well as deposit thicknesses on the heat transfer efficiency was carried out. The simulation results showed that the increase in fouling thickness worsens heat transfer in heat exchangers with large tube diameters, while in heat exchangers with small diameters the heat transfer fluid velocity increases, compensating for the additional thermal resistance. The influence of various parameters on the operation of heat exchanger equipment is evaluated and recommendations on optimization of its operation and frequency of cleaning are offered.

Thermal performance enhancement in the receiver part of solar parabolic trough collectors

In this study, CFD analysis for thermal performance of absorber tube with inner helical axial fins which is used in solar PTC collector is considered and compared. Three-dimensional numerical model simulations were performed using flow and heat transfer interfaces in COMSOL Multiphysics software. Water is used as heat transfer fluid and copper was chosen as receiver tube material. Sun's radiation is considered as constant at around (Io=1000 W/m2) and concentration ratio is C=60. Inlet velocity of heat transfer fluid is changed from 0.2ms to 0.2 4 ms in order to define its magnitude with the length of absorber tube. Electrical analog method is utilized for mathematical modeling to evaluate heat losses through the absorber tube. Variations of surface and fluid temperature are determined to investigate the receiver which shows better performance in the given boundary conditions. In the discussion section of the article, temperature variation of the receiver tube and HTF during this process is presented.

Thermal investigation and parametric analysis of cascaded latent heat storage system enhanced by porous media

Cascaded latent heat storage (CLHS) has become a promising solution to the recovery and utilization of thermal energy. Many studies optimized the inlet temperature and velocity of heat transfer fluid to improve the heat transfer rate for the charging process of CLHS systems with pure phase change materials (PCMs), but the effect is not significant, mainly because the thermal resistance of the system comes from the PCMs side due to the low thermal conductivity of pure PCMs. Porous media composite phase change materials (CPCMs) have been shown to have promising prospects in a single PCM system, but no research precedent has been found for the application of porous media CPCM in the CLHS system. Besides, when CPCMs are encapsulated in the cavities of the CLHS system instead of pure PCMs, the difference in the degree of impact between the porosity of a porous media and boundary conditions on the thermal performance of a CLHS system is still unclear. The purpose of this article is to significantly improve the heat transfer rate by applying the CPCM to the CLHS system and to comprehensively compare the impact of porosity and HTF inlet boundary conditions on the charging/discharging time and energy/exergy charged/discharged rates of a CLHS system with CPCMs through two sensitivity analysis methods. When the porosity reduces from 0.97 to 0.92, the charging time and the discharging time decrease by 74.50 % and 73.39 % respectively; the average energy charge rate and average exergy charge rate increase by about 4.11 times; the average energy and exergy discharge rates increase by 5.76 times and 5.8 times respectively. Sensitivity analysis shows that the porosity of a porous media is the most important factor affecting the charging and discharging rates of a CLHS system, but its impact on energy and exergy charge/discharge rates is not monotonic. The effect of the inlet temperature and velocity of the HTF on a CLHS system is complex, and simply changing a single variable may not be effective in optimizing thermal performance parameters. This study provides a comprehensive performance enhancement strategy for the thermal energy storage and utilization processes of CLHS systems, which have great potential in fields such as solar energy collection, waste heat recovery, and building heating.

Design, development and performance investigations of a latent heat storage with PCM encapsulation

Encapsulation of phase change material (PCM) in cylindrical capsules has been proven to be one of the promising techniques for improving the heat transfer characteristics in latent heat storage (LHS) systems. The distribution of the capsules has a great impact on the thermal performance of these LHS systems. Therefore, in the present study, an LHS unit with cylindrical PCM encapsulation is developed numerically using COMSOL Multiphysics software. Various models (Model A, Model B, Model C, and Model D) with varying spacing between the capsules are modeled to optimize the capsule distribution inside a cylindrical shell. For this high-temperature investigation, Sodium nitrate with a phase transition temperature of 305 °C is selected as PCM, and ambient air is used as the heat transfer fluid (HTF). The performance of these models is assessed by comparing temporal variation of temperature, phase fraction in the PCM and HTF velocity in the outer shell. It is observed that the distribution of capsules greatly influences the flow dynamics of HTF which crucially altered the thermal performance of the LHS models. Model C is found to be an optimum configuration with a minimum charging/discharging time of 364/337 min. LHS module with an optimum distribution of capsules was fabricated and its charging and discharging performances were investigated in the temperature range of 270–330 °C. The heat transfer behaviour in the PCM is studied by plotting the temperature evolution at different locations in the capsules. It was observed that the LHS module with approximately 14 kg of PCM mass, stored/discharged a total heat of 3.97/3.94 MJ in the given temperature range.

Influence of heat transfer fluid on thermal performance improvement of latent heat storage unit with helical fins

The helical fins can improve the hampering effect of classical fins on the convection flows of latent heat storage units (LHSU) as an innovative design, but their thermal performance improvement was largely affected by the operational conditions of heat transfer fluid (HTF), especially for fluid temperature and velocity. Based on this, an experimental system was established to measure thermal performance of latent heat storage units with perforated and solid helical fins (PHF-LHSU and SHF-LHSU). Two inlet temperatures were considered as 65 °C and 75 °C in the charge process and 20 °C and 30 °C in the discharge process, while three velocities were employed as 0.5 L/min, 1.0 L/min and 1.5 L/min. Experimental results showed PHF-LHSU exhibited the higher thermal performance than SHF-LHSU, especially in temperature response and average Nusselt number. Increasing HTF temperature from 65 °C to 75 °C increased melting rate, the average heat flux, Nusselt number and thermal efficiency by 25.4%–31.6%, 65.9%–70.6%, 15.9–25.9% and 9.7%–11.6% in the charge process, while reducing HTF temperature from 30 °C to 20 °C lowered the solidifying rate, the average heat flux and thermal efficiency by 39.3%–47.4%, 52.5%–56.6%, and 30.7%–35.3% in the discharge process. The HTF velocity had the certain effects on thermal properties and with the HTF velocity increasing from 0.5 L/min to 1.5 L/min, the thermal efficiency was increased by 3.9%–4.7%.

Thermal performance of a plate-type latent heat thermal energy storage heat exchanger - An experimental investigation and simulation study

Heat storage is key to reducing the mismatch between energy supply and demand. The performance of thermal energy storage heat exchangers is determined by the exchanger structure and the heat transfer fluid (HTF) parameters. In this paper, the heat exchanger structure and HTF parameters of a plate-type latent heat thermal energy storage (LHTES) heat exchanger were investigated through experiments and simulations. From the experimental tests, it was observed that thermocouples accelerated the melting process of paraffin by 6 % on average for a single LHTES plate. The LHTES plate with aspect ratio of 3:1 was the optimal structure with the shortest melting time. To figure out the optimal thermal performance of the LHTES heat exchanger, a mathematical model was developed by FLUENT and verified against the experimental data. The influence of LHTES plate thickness as well as the HTF temperature and velocity was numerically studied. A performance factor was proposed to evaluate the thermal performance for the melting process of plate-type LHTES heat exchangers. Simulation analysis shows that the highest performance factor is realized when the LHTES plate thickness is 15 mm, independent to the HTF temperature and velocity. When the HTF velocity is small, it is critical to focus on the design of plate thickness of the heat exchangers rather than increasing the HTF temperature.

An absorber of parabolic trough collector for hydrogen production in a solid oxide fuel cell

Sustainable energy development is a globally attractive and indispensable research area. An effective design process and good sustainable sources are crucial for meeting the objective. This investigation utilizes solar energy by utilizing an effectively designed the integration of two-parabolic trough collector (PTC) with an area of 2.8 m2 to operate Rankine cycle-based fuel cell for hydrogen production. The heat transfer rate is optimized for maximum net energy production to operate the solid oxide fuel cell (SOFC) to maximize the hydrogen production by experimenting with the solar heating system with 3 different heat transfer fluid (HTF) flow rates such as 0.12 kg/s, 0.27 kg/s, and 0.38 kg/s. The experimental outcomes at each flow rate setting were analyzed in such a way that solar radiation received, inlet and outlet temperatures of HTF with respect to clock time, Useful heat gain and Thermal efficiency of solar heating system with respect to solar radiation received and rate of Hydrogen production (kg/s) and Net power output (kW) by sustainable solar power generation arrangement were analyzed with respect to solar radiation received. The results recommended that the flow rate of 0.38 kg/s recorded the highest thermal efficiency 82%, for the heat gain of 27589 kW. Increasing the flow velocity of HTF improves both the solar collector's efficiency and the electrical energy for the fuel cell. As a result, more solar energy is falling on the absorber and enhances the rate of hydrogen production. A preliminary cost analysis was conducted for the PTC system integrated with the hydrogen production system to determine the electric cost based on the solar advisor model (SAM). A black coating material was used to absorb more solar radiation from the PTC.

The Impact of Baffle Geometry in the PCM Heat Storage Unit on the Charging Process with High and Low Water Streams

This paper presents the effect of baffle geometry on the charging process of a low-temperature heat storage unit. Four different geometry variants were considered for this purpose. Each of them was simulated and the results were compared. The following parameters were selected as comparison criteria: the charging time of the heat storage unit, the change in the liquid and solid fractions of the phase change material, and the change in its temperature over time. The analysis showed that, independent from the heat transfer fluid velocity, the use of baffles did not significantly affect the charging time. Furthermore, the application of baffles of all studied types did not bring an essential decrease in charging time. It was found that the optimal solution was to use the simplest construction. Tuning of the HTF flow by the use of baffles is applicable to shell and tube heat exchangers; however, it adds no significant effects in the case of heat storage units of the proposed design. The abovementioned effect has been explained by the heat flux analysis, which shows that the heat transfer in the PCM is radically less intense, when comparing to the working fluid. Therefore, it is expected that enhancing the heat transfer between HTF and PCM material is possible by modifying the PCM–side design.

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.

Analysis of a falling film H2O/LiBr absorber at local scale based on entropy generation

A solution of laminar H2O/LiBr falling film flowing under the effect of gravity on a vertical plate heated by a heat transfer fluid was studied to determine the local temperature, concentration and velocity fields from a numerical resolution. In order to investigate a real absorber case, the heat transfer fluid was assumed to flow counter-current to the falling film. Different heat transfer fluid flow regimes were studied and compared with the limit cases: adiabatic and isothermal walls. A local entropy generation formulation was performed to identify the different sources of irreversibility with the aim of locating and quantifying them. The results enabled analysis of the distribution of entropy generation in the entire absorber, namely, the heat transfer fluid, the wall and the falling film. The analysis shows that an increase in the driving force generated at the film interface and of the heat transfer fluid velocity leads to an increase in the absorbed mass flow rate, but this increase is associated with higher irreversibility. Thermal entropy generation dominates near the wall for both fluids while mass entropy generation dominates at the film interface. Total entropy generation per width of absorber is mainly due to heat transfer irreversibilities through the film – more than 35% – while the irreversibilities due to mass transfer represent less than 15% under the assumptions considered.

Conceptual design of an innovative gas–gas ceramic compact heat exchanger suitable for high temperature applications

AbstractThe development of an innovative and highly efficient heat exchanger (HE) solution for gas–gas heat recovery is one of the major objectives of the HYDROSOL-beyond project which aims at enhancing the process efficiency for producing hydrogen from water dissociation with concentrated sunlight. Because of the very high temperature level of the process (up to 1400 °C), an innovative ceramic HE design was proposed with an integrated lattice structure, as secondary surface, to maximize the heat transfer. To assist the design of the HE, a multiscale approach was adopted: a 1D model based on global correlations was developed and a 3D computational fluid dynamics model of the secondary surfaces were generated. The former was applied to assess the performance of the entire HE; while, the latter was exploited to study in detail the thermo-fluid dynamics behavior of a HE core element and to provide the global correlations to be integrated into the 1D model. In this work, the effect of several parameters and operating conditions, namely, number of channels, number of lattice layers located into each channel, solid material exploited for the HE structure, parting plates thickness, heat transfer fluid velocity, radiation heat transfer and solid material emissivity, on the HE effectiveness are reported and accurately described. Furthermore, based upon the results obtained, guidelines on the HE design to maximize its performance are also provided.

Experimental analysis of a fin-enhanced three-tube-shell cascaded latent heat storage system

The charging behavior of low-temperature cascaded latent heat storage (CLHS) has been a major research gap. In order to overcome the low thermal conductivity of organic phase change materials (PCMs), a fin-enhanced three-tube-shell latent heat storage unit is proposed in this paper. A full-scale experiment is carried out to study the temperature evolution and thermodynamic performance of CLHS unit of each stage and overall system. Through a sensitivity and correlation analysis, the influence of boundary conditions on the thermodynamic performance of this CLHS system is determined. The experimental results shows that the rate of temperature rise and energy charged of the first stage of the cascaded arrangement are significantly higher than those of the latter stages. It is recommended to use a PCM with higher specific heat values in the first stages, and use a PCM with a higher latent heat value in the later stages. The sensitivity analysis shows that the enhancement effect of the heat transfer fluid (HTF) inlet temperature on the energy and exergy rate is much stronger than that of its velocity. It is recommended that the HTF inlet temperature should be maximized as much as possible, and the HTF velocity should not be too high to obtain better thermal performance of the CLHS system.

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.

A 3D resolved-geometry model for unstructured and structured packed bed encapsulated phase change material system

A 3D numerical model is developed to simulate melting in packed bed encapsulated phase change material (PCM) energy storage systems. The main novelty of the model is that it resolves the arrangement of capsules inside the system for both structured and unstructured packing and thus accurately captures the fluid flow, heat transfer and phase change in the capsules. To create the unstructured arrangement of capsules in the domain a novel dropping and rolling algorithm is implemented. The flow of heat transfer fluid (HTF) outside the capsules as well as the natural convection in the PCM within the capsules is also simulated. Initially, a single capsule study is performed, which shows that both natural convection inside the liquid PCM and forced convection outside the capsule affect the melting characteristics. Subsequently, the model is extended to simulate phase change in multiple capsules in a cubical domain arranged with structured and unstructured packing. The analysis of the effect of capsule arrangement on the melting and energy storage characteristics for both structured and unstructured packing of capsules is performed. Additionally, parametric studies are performed to analyze the effects of capsule size, fluid temperature, and fluid velocity for both types of systems. Simulation results show that the total melting time for unstructured packing is reduced on an average by 23.56% for the 2.5 mm capsules and by 10.14% for the 5 mm capsules as compared to structured packing. The effect of HTF temperature change is more prominent for structured cases, with an increase of 65.8% melting time as compared to 58.4% for the unstructured cases when the HTF temperature is reduced by 20 K. In contrast, the effect of velocity is less significant for the structured cases with a reduction of HTF velocity from 0.1 cm/s to 0.05 cm/s causing an increase of 27.3% melting time for the structured cases and 42.6% for the unstructured cases. The developed model can be used to capture the effect of different arrangements of capsules in encapsulated PCM energy storage systems and thus obtain effective designs for such systems.

Numerical simulation of the calcium hydroxide/calcium oxide system dehydration reaction in a shell-tube reactor

The Ca(OH)2/CaO system is a promising candidate for thermochemical energy storage because of its high energy density, low cost and negligible heat loss. Coupling mechanisms of multiple physiochemical processes still need further investigation and the system performance should be further improved. In this work, for the first time, a three-dimensional numerical model is developed to study the thermochemical energy storage process by Ca(OH)2 dehydration reaction in a shell-tube reactor. The turbulent heat transfer fluid flow in the shell side, and the steam flow, heat transfer and dehydration reaction in the tube side are comprehensively taken into account. Effects of operation conditions and reactor geometry parameters on the dehydration process are also investigated in detail. Five indicators are proposed to comprehensively evaluate the reactor performance, including reaction time, energy storage density, heat exchange power, heat exchange efficiency and energy storage efficiency. The results reveal that higher inlet temperature and inlet velocity of heat transfer fluid can enhance the heat transfer and accelerate the reaction. Lower porosity, although increases the energy storage density, leads to higher flow resistance and impedes the reaction. With deep understanding of the physiochemical processes, 16 more tubes are properly added in the by-pass flow region, leading to shorter reaction time and higher energy storage amount. The low values of heat exchange efficiency and energy storage efficiency reveal that the input heat cannot be efficiently transferred to the tube side. To improve the energy conversion performance of reactor, additional schemes are discussed including regenerative HTF, optimization of reactor geometry, increase of reactant thermal conductivity, reducing tube-side flow resistance and extending HTF residence time. The numerical model and simulation results in the present work could be helpful for optimizing the shell-tube Ca(OH)2/CaO reactor and improving the thermochemical energy storage performance.