Heat Transfer Fluid Inlet Research Articles

Cascaded latent thermal energy storage using a charging control method

Several studies have used thermal energy storage systems to store renewable heat energy. Nevertheless, few studies have addressed the charging of renewable heat sources with unsteady temperatures. In this study, a cascaded latent thermal energy storage (CLTES) system utilizing a charging control method (CCM) is proposed for the efficient charging of renewable energy under unsteady temperature conditions. The CCM was used to control the inlet positions of CLTES tanks depending on the inlet temperatures. A CLTES system with a total volume of 834 L (3 × 278 L) was manufactured for the experiment using three tanks and phase change materials (PCM) with three different melting points. To verify the performance of the CCM system, three system types were considered: Single-PCM latent thermal energy storage, CLTES, and CLTES with CCM. The results show that the CCM increased the charged and discharged thermal energy by 33% and 44%, respectively, compared with the reference case under unsteady inlet heat transfer fluid temperature conditions. The results confirm that a multi-tank cascade system utilizing CCM can be used to efficiently charge unsteady renewable thermal energy sources.

Numerical investigation and experimental validation of the thermal performance enhancement of a compact finned-tube heat exchanger for efficient latent heat thermal energy storage

This paper presents a numerical study of heat transfer performance inside a compact finned-tube heat exchanger employed in latent heat storage (LHS). A three-dimensional mathematical model of the storage system was developed and validated based on experimental results of charging and discharging. The heat transfer problem during phase change was addressed by solving Navier–Stokes equations combined with enthalpy-porosity technique. Visualization of thermal process inside the storage unit, which cannot be fully realized using an experimental set-up, was carried out by exploiting simulation results. Due to the heat transfer fluid (HTF) circuit design, it was found that phase change occurs faster in the lower half of storage tank and especially in phase change material (PCM) neighboring HTF inlet. Stripes of PCM adjacent to tank walls where heat transport is slow were identified. The model was used to conduct parametric studies of the effects of the HTF inlet temperature, the flow rate, the number and the position of fins on charging and discharging performances. Concerning operation parameters, increasing HTF flow rate from 0.4 to 0.62 l/min reduces charging time by 31% and discharging time by 29%. Similarly, increasing temperature difference between HTF and PCM by 10 °C accelerates melting process by 30% and solidification process by 21%. Since charging is slower than discharging in the studied system, the influence of design parameters on charging was investigated. As a consequence, it was proven that increasing the number of fins reduces melting time and raises storage power. However, adding more than 13 complete fins to the studied device was shown to cause limited improvement. Moreover, positioning fins in the upper half of the heat exchanger has the best benefit on melting time. In addition, increasing fins thickness and changing their material from aluminum to copper was proven to reduce total melting time. Therefore, an improved design was proposed based on the previous conclusions and melting time was reduced by 9% compared to the initial configuration. Comparison of the improved compact finned-tube storage device with other LHS units found in literature showed that the non-dimensional storage time obtained with the novel configuration compares well with other well-established LHS configurations. The results of the present work provide operation and design assistance for LHS systems consisting of compact finned-tube heat exchangers intended to store heat such as solar thermal energy.

Numerical modeling and performance comparison of high-temperature metal hydride reactor equipped with bakery system for solar thermal energy storage

The current study provides an optimal design strategy for the metal hydride reactor for the thermal energy storage system for solar bakery applications. The sensitivity analysis of the annular truncated hollow conical fins type reactor was conducted by using COMSOL Multiphysics 5.3a software. The gravimetric exergy output rate and exergy output of different designs were compared. It was found that the optimal geometric parameters and operating conditions for the annular truncated hollow conical fins type reactor are fin angle (θ)=20°, fin radius (Rf) = 0.07 m, number of fins (Nf)= 32 and fin spacing (Sf) = 0.02 m, fin thickness (thf) = 0.008 m, hydrogen supply pressure (Pin, H2) = 5 MPa and inlet heat transfer fluid velocity (VHTF,in) = 20 m/s. Finally, it was concluded that the Deign IV is the most favorable design, which has a uniform reaction, faster reaction time (15,000 s), and higher values of gravimetric exergy output rate and exergy output (1.23 W/kg, 0.028 kW).

Experimental investigations on the thermal performance of an ice storage system using twin concentric helical coil

The cool thermal energy storage (CTES) offers a big advantage for peak shift of high electrical power consumed by air conditioning systems. An experimental analysis for studying the thermal performance of ice storage system with twin concentric helical coil (TCHC) is conducted in the present work. Effects of operating conditions such as the heat transfer fluid (HTF) inlet temperature and flow rate on the solidified/melted mass fraction, percentage energy stored/regained, and average charging/discharging rate during charging and discharging processes are investigated. An experimental setup composed of TCHC immersed in distilled water as a phase change material (PCM) in an insulated tank is used in the present study. Three different HTF inlet temperatures of (−8, −10, and −12 °C) and (10, 12, and 14 °C) for charging and discharging processes, respectively and HTF volume flow rates (5.0, 7.5, and 10 l/min) are investigated. The results show that, about 90% of the energy stored is achieved at 59–74% of the complete charging time according to the tested parameter range in the current study. Moreover, the HTF volume flow rates have a relatively small effect on the percentage energy stored and seems to be less effective compared to the inlet HTF temperature during charging process.

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.

Investigation of heat transfer coefficients in a liquid–liquid direct contact latent heat storage system

The study presents a latent heat storage system consisting of a phase change material (PCM) and a heat transfer fluid (HTF), placed in direct contact, for storing thermal energy derived from renewable energy sources or industrial waste heat. It is important that the two media are immiscible for the function of such a storage system. In this study the PCM was an eutectic mixture of the two salt hydrates, magnesium nitrate hexahydrate and magnesium chloride hexahydrate and the HTF was a mineral oil. The direct contact leads to intensified heat transfer compared to that in the indirect contact heat storage systems. This intensified heat transfer results in short loading and unloading periods of the storage tank with a high heat exchange performance. Furthermore, a higher storage density is observed in comparison to the sensible storage systems, particularly when combined with low temperature differences between the charged and discharged storage systems. In our study, the heat transfer between liquid PCM and liquid HTF (rises drop-shaped in the PCM) is investigated. For this purpose, a storage tank has been built with only one HTF inlet opening in the bottom. For the time and space resolved data, the axial temperature profile, and the surface temperature of the rising droplet, as well as the PCM temperature, are measured with a specially developed, fast, near infrared sensitive, and fiber-coupled technique. These measurements made it possible to determine a local heat transfer coefficient between the PCM and HTF. With the chosen parameters and materials in this study a heat transfer coefficient of 1845 W/m² K was calculated.

A numerical investigation of the melting heat transfer characteristics of phase change materials in different plate heat exchanger (latent heat thermal energy storage) systems

In this study, the melting heat transfer in plate heat exchanger LHTES(Latent Heat Thermal Energy Storage) systems was numerically investigated. In the numerical analyses, using the Finite Volume Method (FVM), the phase change process was examined for different heat exchanger geometries (Geometry a, b and c), different heat transfer fluid (HTF) inlet temperatures (52 °C, 57 °C and 62 °C), different steel plate thicknesses (0.4 mm, 0.6 mm and 0.8 mm) and different PCMs(Phase Change Materials) (RT-35 and n-octadecane). The numerical analyses were performed by simplifying the problem down to two dimensions. From the results, time-dependent average PCM temperatures, and liquid fraction contours and graphs were produced. The results showed that for the same PCM, boundary conditions and geometrical characteristics, the complete melting time of the PCM in a plate heat exchanger LHTES system with different geometries could be decreased by 75% in comparison to a cylindrical LHTES system of the same PCM volume. The maximum thermal performance achieved was recorded for the following conditions: Geometry a, a HTF inlet temperature of 62 °C, a steel plate thickness of 0.6 mm and n-octadecane as the PCM.

Experimental study on the melting behavior of a phase change material in a conical coil latent heat thermal energy storage unit

An experimental study on the melting behavior of paraffin wax used as a phase change material (PCM) in a conical coil latent heat energy storage unit (LHSU) was performed. The experiments were conducted concurrently for conical coil and normal coil LHSUs to compare their thermal performances. The effect of heat transfer fluid (HTF) inlet temperature was examined. The evolution of the temperature during charging, the spatial variation in temperatures and captured images of the melting PCM in both LHSUs were compared. The results indicated that the melting rate in the conical coil LHSU was enhanced by 20.97%, 22.13% and 28.32% in comparison with that of normal coil LHSU at HTF inlet temperature of 70 °C, 75 °C and 80 °C respectively. Moreover, the rate of energy stored in the conical coil LHSU was a notably faster than that of the normal coil LHSU. The HTF inlet temperature has a similar effect on both LHSUs by substantially decreasing their melting time.

Experimental assessment of a full scale prototype thermal energy storage tank using paraffin for space heating application

Abstract In this work, a full scale prototype latent heat thermal energy storage unit has been designed, constructed, and experimentally analysed for its performance considering heating application conditions and two energy sources, solar and geothermal energy. It comprises a rectangular tank filled with phase change material and a finned tube staggered heat exchanger. Water is used as heat transfer fluid. To study its performance, selected heat transfer fluid inlet temperatures and flow rates were applied and two commercial organic phase change materials were tested, A44 and A46. Heat transfer fluid outlet temperature, heat transfer rate and stored energy were evaluated as a function of the conditions studied. Based on the results, it is concluded that A44 is more efficient during the charging period, taking into account the two energy sources, solar and heat pump. The water outlet temperature has a more stable evolution when using A44 instead of A46 and the heat transfer rate is higher too. A44 also exhibits higher storage capacity than A46 during the discharging process and more stable heat transfer rate for both flow rates The developed methodology can be applied to study different PCMs and building

Study of the effect of tilting lateral surface angle and operating parameters on the performance of a vertical shell-and-tube latent heat energy storage system

In this paper, a comprehensive mathematical model is developed to study the thermal energy storage performance in vertical shell-and-tube latent heat thermal energy storage (LHTES) systems with different tilting lateral surface angles. This model integrates heat transfer in heat transfer fluid (HTF), HTF tube and phase change materials (PCM). It is validated using experimental data and then applied to investigate effects of tilting lateral surface angles on the enhancement of natural convection in the LHTES system. The results show that increasing the tilting lateral surface angle largely enhances heat transfer in the melting process whilst it has negative effects on the solidification process. The total melting time reduces by 45% as the tilting angle increases from 0° to 7°. An optimal tilting angle of 4° is identified based on the trade-off between melting and solidification processes. Furthermore, the effect of HTF operating parameters on the performance of the LHTES system is evaluated. It shows that the melting and solidification time of the LHTES system with a tilting angle 4° reduces by 22% and 8.5%, respectively, as the HTF flow Reynolds number increases in laminar and transition flow regions. However, increasing the HTF flow Reynolds number does not show significants effect under turbulent flow regions. The variation of the HTF outlet temperature is also investigated. The temperature difference between the HTF inlet and outlet is found small at all studied HTF flow rates, which indicates that assumption of the constant HTF surface temperature boundary in simulations in the literature is reasonable.

Performance analysis of a vertically oriented concentric-tube PCM based thermal energy storage system: Parametric study and correlation development

Latent heat based thermal energy storage systems stores energy which can be retrieved whenever required. Total melting time (or melting rate) is an important parameter which governs suitability of any system for a given application. In the present work, a numerical analysis has been performed on the vertically orientated concentric double-pipe storage system. Effect of phase change material (PCM) properties, flow parameter and geometric parameters of the storage system on total PCM melting time are studied. Enthalpy-porosity technique has been used to model the phase change process apart from governing fluid flow and energy equations. It is found that with the increase in melting temperature and latent heat, the total melting time increases whereas the total melting time decreases with increase in the heat transfer fluid inlet temperature and aspect ratio. 74.11% decrease in the total melting time has been observed when melting temperature is decreased from 343 K to 323 K with latent heat 144 kJ/kg, heat transfer fluid inlet temperature 348 K and aspect ratio 11.76. A correlation for the total melting time comprehending the effect of all studied parameters has been developed which can provide the total melting time within ±15% accuracy.

ŠILUMNEŠIO DEBITO ĮTAKOS FAZINIO VIRSMO MEDŽIAGOS VEIKIMUI TYRIMAS / INVESTIGATION OF THE INFLUENCE OF MASS FLOW RATE ON PHASE CHANGE MATERIAL BEHAVIOUR

The paper presents an experimental study of the influence of heat transfer fluid (HTF) mass flow rate on phase change materials (PCM) behaviour. The experimental study was performed on a specially designed test bench. Research object – PCM based thermal energy storage unit which consists of a stainless steel tank with dual circuit tube-fin copper heat exchanger. The tank (storage volume) was filled with phase change material RT82. The experiment was carried out using three different mass flow rates of HTF: high – 0.25 kg/s, medium – 0.125 kg/s, low – 0.05 kg/s. The analysis showed that in the case of high and medium mass flow rates the melting/solidification process highly depends on the temperature of inlet HTF. Influence of mass flow rate is higher in the case of low mass flow rate.

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.

Experimental characterisation of a novel thermal energy storage based on open-cell copper foams immersed in organic phase change material

In this paper, a novel tubes-in-tank thermal energy storage (TIT-TES) based on open-cell copper foams immersed in organic paraffin is presented and experimentally studied. The system consists of a rectangular external case which encloses 16 U-tubes and phase change material (PCM) embedded in copper foams matrix, which guarantee an average power during both heat charge and discharge phases close to 1 kW and high storing/releasing energy efficiency. With the aim of measuring internal temperature, six thermocouples have been installed at two different sections, S1 and S2, located at 1/3 and at 2/3 of the total height, respectively. In order to thermally characterize the system during the heat charge and discharge phases, a series of tests have carried out under different inlet temperatures and flow rate of the heat transfer fluid (HTF). Temperature evolutions have obtained during the tests, and directional temperature derivative profiles, time-durations, average powers and energy efficiencies have derived so to evaluate and characterize the TIT-TES performance. The results have shown how both HTF volumetric flow rate and inlet temperature play a relevant role in heat charge and discharge phases. A larger temperature difference between the HTF and the PCM and a higher HTF input flow rate improve heat transfer and consequently increase the average exchanged power and energy efficiency as well. In particular, during the thermal charge phase, when the HTF temperature increases from 80 °C to 90 °C, the energy efficiencies increase from 73.6% to 91.4% and from 78.7% to 93.8% for the minimum and maximum HTF flow conditions respectively.The analysis of directional temperature derivative profiles have allowed to define some important characteristics of the apparatus, highlighting the preferential directions of the liquefaction/solidification front paths, and also confirming the importance of the HTF inlet temperature and flow rate.

An experimental investigation of the heat transfer and energy storage characteristics of a compact latent heat thermal energy storage system for domestic hot water applications

This paper presents the experimental performance analysis of a latent heat thermal energy storage system (LHTESS) designed for domestic hot water (DHW) applications. The designed, fabricated and characterised thermal store comprised of a vertically oriented multi-pass tube heat exchanger in a rectangular cross-section container filled with phase change material (PCM) paraffin wax RT44HC. The experimental investigation evaluated the heat transfer within the system, measured the transient temperature distribution, determined the cumulative thermal energy stored, charging and discharging time and the instantaneous charging and discharging power. The experimental work was conducted under controlled experimental conditions using different heat transfer fluid (HTF) inlet temperatures and different volume flow rates for store charging and discharging. It was found that during charging natural convection in the melt played a significant role. During discharging thermal conduction dominates and natural convection has an insignificant impact on the LHTESS performance. This is due to the development of a solid layer of PCM around the heat transfer tubes which increases the thermal resistance and reduces heat transfer to the liquid PCM. Higher HTF inlet temperature during charging significantly decreased store charging time. Increasing HTF inlet temperature from 60 to 70 °C shortened the charging time by 3.5 h, a further increase to 80 °C decreased melting time by a further 2 h.This study illustrates the extent to which LHTESS, and heat exchanger designs need to be improved to meet the desired charge/discharge time requirements for most short-term storage applications and that a broad range of domestic and commercial heat demands can be fulfilled by assembling several LHTESS units to operate in parallel.

Optimisation of Parameters in Thermal Energy Storage System by Enhancing Heat Transfer in Phase Change Material

Phase change material (PCM) is used to store the heat in the form of latent heat and sensible heat. Thermal energy storage by latent heat using PCM was recognized as attractive alternative to sensible heat storage, because large quantity of thermal energy is stored in small volume. In the present experimental analysis, Beeswax is used as PCM and it is enclosed in stainless steel capsules. Beeswax exhibits slow thermal response due to low thermal conductivity value, so an effort is made to dope PCM with copper, aluminium and graphite powders separately to enhance its heat transfer. The capsules are stored in insulated tank and hot water is supplied into it. The experimental design is prepared by considering parameters as Heat transfer fluid inlet temperatures, Flow rate and Number of Capsules. Experiments are conducted based on Taguchi analysis and the responses are recorded. The effect of considered parameters on Thermal Energy Storage (TES) system is studied by analyzing experimental data to find the optimal parameters for designing the effective TES system.

Experimental investigation on the melting and solidification behavior of erythritol in a horizontal shell and multi-finned tube latent heat storage unit

Renewable energy sources are incapable of supplying continuous power due to their intermittent availability. To tackle this, energy storage systems incorporating phase change materials can be utilized. Among the several techniques, shell-and-finned-tube configurations show promising heat transfer performance and have greater engineering applicability. This study experimentally investigated the heat transfer mechanism in a horizontal shell-and-multi-finned-tube energy storage unit. Variation of average temperature and storage effectiveness as well as effects of inlet heat transfer fluid temperature and flow rate on the phase change of erythritol were experimentally investigated. It was found that the effect of inlet temperature was more significant than the flow rate (12–32% and 7–17%, respectively). Moreover, natural convection was found to significantly influence the heat transfer during melting where the melted material initially occupied the upper region of the unit and then moved from top to bottom. The outcomes of the present work help understand the effect of natural convection to be effectively utilized for the design and optimization of such storage configurations.

Second law optimization of heat exchangers in waste heat recovery

This paper numerically investigates the first and second law efficiencies of transient crossflow over tube banks. A significant number of simulations were performed using a commercial computational fluid dynamics (CFD) tool, ANSYS Fluent 18.2, with varying inlet temperature, Reynolds number, pitch ratio (PR), number of inline cylinders, and heat transfer coefficient defining a heat leakage on the system. Numerical results compare favorably with published data in terms of Nusselt number and pressure drop. Five hundred seventy-six simulations were performed, and energy efficiencies ranged from 72% to 99%, while exergy efficiencies ranged from 26% to 70%. No discernable, universal trends were observed across all data sets, but the staggering amount of results spans a wide range of operational characteristics for tube bundle waste heat recovery. To illustrate the insight such data can provide, we consider four sample cases for analysis: well-insulated low-grade heat recovery, poorly insulated low-grade heat recovery, well-insulated high-grade heat recovery, and poorly insulated high-grade heat recovery. Low- and high-grade heat recovery cases were simulated with differing inlet heat transfer fluid (HTF) values, and heat leakage was introduced as a source term in the 2D energy equation. Detailed descriptions of the efficiencies of all cases are described, with references to the immense body of simulation data included in the appendix. Overall, the highest efficiencies of the first and second laws were obtained in domains that have three cylinders and inlet temperature of 500 K and eight cylinders and an inlet temperature of 400 K, respectively. The main findings of this study could be beneficial in designing a highly efficient waste heat recovery system, especially when source temperatures vary.

Numerical analysis of phase-change material melting in triplex tube heat exchanger

Latent heat thermal energy storage (LHTES) systems have gained significant attention recently owing to their high energy-storage-density per unit volume and nearly isothermal behavior during the phase-change process. This study numerically investigates the melting performance of a phase-change material (PCM: Ba(OH)2·8H2O) in a triplex tube heat exchanger (TTHX). First, the physical and mathematical models of the TTHX are established. Then, the governing equations are discretized by the finite volume method (FVM). Finally, numerical calculations are carried out by implementing the developed numerical algorithm in FORTRAN computer code. The results reveal that the inlet temperature and flow state of the heat transfer fluid (HTF) significantly influences the PCM melting performance and exergy efficiency ratio of the investigated TTHX. A higher inlet temperature and turbulent state of the HTF can accelerate the phase-change process. However, increasing the mass flow rate of the HTF decreases the exergy efficiency ratio of the investigated TTHX. In conclusion, considering both the melting time and exergy efficiency ratio, increasing the HTF inlet temperature to a certain extent and reducing the mass flow rate of the HTF in the turbulent state can improve the thermal performance of the investigated TTHX.

Numerical investigation on the melting of nanoparticle-enhanced PCM in latent heat energy storage unit with spiral coil heat exchanger

Latent heat storage units are widely used in building heating systems due to its high energy storage density, whereas the practical performances of them are limited by the low thermal conductivities of phase change materials. In this paper, copper nanoparticles were added into paraffin to enhance the heat transfer rate of a latent heat storage unit using a coil heat exchanger. A three-dimensional numerical model was built to simulate the melting process of phase change material, and it was well validated against the experimental data. The simulation results showed that the nanoparticle-enhanced phase change material saved 19.6% of the total melting time consumed by the pure phase change material. In addition, the dispersion of nanoparticles significantly alleviated the temperature non-uniformity in the unit. Moreover, for the unit using nanoparticle-enhanced phase change material, the flow rate of heat transfer fluid was not recommended higher than 0.75 m3/h. The dispersion of nanoparticles could enlarge the optimum heat transfer fluid temperature range to 60–70 °C compared with that of pure phase change material (60–65 °C). Therefore, the application of nanoparticle-enhanced phase change material in the latent heat storage unit can significantly enhance heat transfer, and the proposed optimum inlet heat transfer fluid temperature range could contribute to higher energy efficiency.