Energy Storage Heat Exchanger Research Articles

Tracking of the melting front in tube-in-tube latent thermal energy storage heat exchangers

In this study, the shrinking solid regime is investigated in a high aspect ratio tube-in-tube latent thermal energy storage (LTES) heat exchanger by tracking the phase change front evolution. In the setup, water flows through the inner tube as heat transfer fluid (HTF) and the shell contains a paraffin RT35HC as phase change material (PCM). A series of melting experiments have been performed in which the HTF mass flow rate, HTF inlet temperature and initial PCM temperature were varied. During each experiment, the movement of the phase change front was tracked using a camera placed next to the PCM tube. The front position as a function of time follows an S-shape curve. The effect of the operating conditions on the evolution of the phase change front is analysed. It is found that the temperature difference between the HTF and PCM exerts a more pronounced influence on the front position compared to the HTF Reynolds number. Finally, a correlation was developed to determine the temporal evolution of the front position. This research contributes to a deeper understanding of phase change phenomena in LTES systems and the found correlation can be a valuable tool for predicting and optimizing the performance of tube-in-tube LTES heat exchangers.

Experimental validation of a design model for latent thermal energy storage extended with sensible heat

Latent thermal energy storage can be a key technology for a green energy transition by matching fluctuating heat demand and supply. In order to implement a storage system, it needs to be designed which requires estimating the outlet temperature of a system for a given geometry and time history of the heat transfer fluid’s mass flow rate and inlet temperature. Currently, design methods are either overly simplistic, focusing solely on e.g. the phase change time or requiring the solution of partial differential equations which can be computationally expensive. The present paper proposes a novel approach where a latent thermal energy storage system is decomposed into a heat transfer fluid vessel, a sensible storage system and a storage system with only latent heat. Computationally inexpensive models are available for all three of these sub heat exchangers. A heat exchanger model is obtained by connecting the sub heat exchangers in parallel. This novel approach is used to model an industrial scale shell and tube latent thermal energy storage heat exchanger. The predicted outlet temperature is compared to the measured outlet temperature and the design model obtains good agreement.

Thermal Energy Storage Heat Exchanger Design: Overcoming Low Thermal Conductivity Limitations of Phase-Change Materials

Abstract Recently, there has been a renewed interest in solid-to-liquid phase-change materials (PCMs) for thermal energy storage (TES) solutions in response to ambitious decarbonization goals. While PCMs have very high thermal storage capacities, their typically low thermal conductivities impose limitations on energy charging and discharging rates. Extensive research efforts have focused on improving PCM thermal conductivity through the incorporation of additives. However, this approach presents challenges such as achieving uniform mixtures, maintaining high latent heat, and cost. Alternatively, it has been demonstrated that, in this study, reducing the length scale of the PCM-encasement thickness can eliminate the low thermal conductivity effect of PCMs. To illustrate this concept, a one-dimensional PCM slab was numerically simulated. The thickness of the slab was varied to represent dimensions found in flow passages of compact heat exchangers, and the heat transfer coefficient of the heating fluid was varied to represent lower and upper bounds while also including nominal values encountered in air-to-air heat exchangers. The thermal conductivity was parametrically varied from the natural value of the PCM to simulated enhanced values (potentially achieved through additives) of up to 400 times larger. Results show that reducing the PCM-encasement thickness yields substantially better performance than by improving the thermal conductivity, thereby demonstrating the potential for compact heat exchanger design to overcome the PCM thermal conductivity limitations.

Numerical simulation and optimization of compact latent heat exchanger with micro-channel plate in shape-stabilized composite phase change material

To enable the rapid provision of domestic hot water, this study presents the independent design of a plate heat exchanger featuring a large heat exchange area and a compact structure. Utilizing a sandwich structure, the thermal storage unit integrates two shaped composite phase change materials (CPCM), namely sodium acetate trihydrate (SAT)/expanded graphite (EG) blocks of identical thickness — in conjunction with a micro-channel heat exchange plate. Multiple thermal storage units are stacked to create the heat exchanger. Experimental validation of the heat transfer performance of this thermal energy storage heat exchanger is conducted, accompanied by the establishment of a heat transfer model elucidating the interaction between the phase change material (PCM) and the fluid. The investigation explores the impact of varying thicknesses of shape-stabilized CPCM, thermo-physical parameters of CPCM, and flow rates on enhancing the thermal efficiency of the thermal energy storage heat exchanger. Results indicate a competitive relationship between the phase change enthalpy and thermal conductivity of CPCM under constant density. Increasing the mass fraction of SAT effectively boosts its heat storage density. However, this improvement is counterbalanced by a reduction in CPCM's thermal conductivity, resulting in diminished heat transfer rates within the material and reduced efficiency in transferring heat between the material and the fluid. Optimal performance is achieved when the mass fraction of sodium acetate trihydrate is 80 wt% and the CPCM thickness is 20 mm. At these parameters, the thermal energy storage heat exchanger exhibits the highest heat transfer efficiency. Furthermore, the heat exchanger is capable of producing 197.86 kg of hot water in 1365 s at an inlet flow rate of 300 L/h, achieving an impressive discharging efficiency of 82.73 %. These findings underscore the feasibility of employing the mini-channel plate thermal energy storage heat exchanger for the rapid preparation of domestic hot water.

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.

A novel numerical study on the melting process of phase change materials in a heat exchanger for energy storage

In this article, a new model with a different geometry is presented for application in systems of energy storage that contain phase change materials (PCMs). The main innovation of this research is designing two tubes instead of one tube inside the heat exchanger and investigation of its impact on the melting process of the PCM. Also, the influence of extended surfaces is studied to evaluate the reduction of melting time, and the changes of related geometrical parameters, including the number and length of fins, are examined in detail. The finite volume commercial code has been employed for numerical simulation and the obtained outcomes have very good agreement with a published experimental study. Results indicate that the utilizing of two tubes and reduce the distance between these tubes has a considerable influence on energy storage and reduction of melting time. Further, it can be concluded that for a certain number of fins, the melting time of PCM reduces by enhancing the fin length.

Characterization of a latent thermal energy storage heat exchanger using a charging time energy fraction method with a heat loss model

Correlations predicting the transient behavior of latent heat thermal energy storage systems without too many computational efforts are valuable for engineering practices. This characterization of latent heat thermal energy storage systems can be done with the recently developed charging time energy fraction method. This method allows fitting a predictive model for the outlet heat transfer fluid temperature of a latent thermal storage unit as a function of the input condition. The previous application of the method neglected heat transfer to the ambient. The present paper improves the charging time energy fraction method by proposing a heat loss model to the charging time energy fraction model. A finite volume model of a high-temperature thermal battery is used to validate the proposed heat loss model. The improved charging time energy fraction method is then used to characterize a high-temperature thermal battery by calibrating a model on 36 numerical charging experiments. The charging time prediction between energy fractions of 0 and 0.96 deviates maximally 2 % from the measured charging time over all 36 calibration experiments. The deviation increases near the end of the charging process, especially for slower charging experiments. Across the 36 calibration experiments, the worst prediction has an average absolute temperature difference of 0.50 °C with a maximum deviation of 2.58 °C at the very beginning of the charging where fast transients are occurring. The calibrated model is also compared to four numerical validation experiments and four real experiments. Overall it is shown that this low computational cost model with average calculation times of 2–3 ms can accurately predict the heat transfer fluid outlet temperature of latent thermal energy storage heat exchangers.

An approximate analytical solution for the movement of the phase change front in latent thermal energy storage heat exchangers

Latent thermal energy storage (LTES) heat exchangers are being applied in a wide range of energy systems. However, there is no analytical method to determine the outlet temperature of LTES heat exchangers from its operational conditions. This hinders experimental data reduction and sizing of these components for specific energy systems. The present paper develops an analytical model for the overall phase fraction and outlet heat transfer fluid temperature for planar LTES heat exchangers. The method is developed with solutions of the Stefan problem. The analytical results are validated based on a numerical model using the same assumptions as the analytical model. The analytical solution can be reduced to a limited set of parameters which are a dimensionless time and heat exchanger surface area. This limited set of parameters can form the basis for experimental data reduction of LTES heat exchangers.

Simulation study on charging performance of the latent energy storage heat exchanger with a novel conical inner tube

The application of a latent heat thermal energy storage (LHTES) system can effectively solve the problem of the mismatch between the energy supply and demand. However, most studies focus on the traditional cylindrical configuration with a low heat storage rate, which limits the wide application of LHTES systems. The configuration optimization of a vertical shell-and-tube LHTES unit is carried out in this paper to enhance heat storage rate, and the effects of cone angle values (0–10°) in the conical shell and conical tube configurations on the heat storage performance are explored by numerical simulations. The results show that these two kinds of conical configurations both significantly improve the melting rate of the phase change material (PCM) compared with the cylindrical unit. However, when the cone angle is the same, the heat transfer enhancement effect of the latter is found to be superior to that of the former. Moreover, the complete melting time is shortened by 46 % as the cone angle of the inner tube increases from 0° to 10°. More importantly, a novel conical tube configuration design is proposed to eliminate the hard-to-melt zone at the bottom of the LHTES unit with the conical tube, then the effects of different tilting heights (60–170 mm) and bottom annulus distances (0–10 mm) are investigated. The results demonstrate that the melting rate increases with the increase in the tilting height, but the increased amplitude of that gradually reduces. Besides, the average heat storage rate shows a trend of first increasing and then decreasing with the increase in the distance of the bottom annulus. When the bottom annulus distance is 2 mm, the average heat storage rate reaches the maximum value, which is 94.84 W. The research results can provide a reference for the engineering design and configuration optimization of the vertical shell-and-tube phase change heat exchanger.

Optimizing phase change composite thermal energy storage using the thermal Ragone framework

Optimal design of thermal energy storage devices can allow systems to approach specific design objectives while maintaining desired levels of performance. The thermal Ragone framework has been applied to the design of thermal energy storage heat exchangers, specifically identifying relationships between their power and energy capabilities. In this paper, a finite-difference model is used to optimize thermal storage heat exchanger designs for three objectives given a discharge power constraint. The three objectives are maximizing energy density, minimizing energy-specific capital costs, and minimizing the levelized cost of storage. This study focuses on the design of planar thermal energy storage heat exchangers with phase change materials and thermal conductivity additives. Key design parameters identified included the conductivity additive volume fraction, spacing between heat transfer fluid tubes, and the phase transition temperature. The optimal design is found to depend strongly on the required thermal power, with higher powers requiring transition temperatures further from the use temperature, more conductivity additives, and closer tube spacing. There is also a trade-off between these design parameters, where changing one (e.g., closer tube spacing) can allow another parameter to be relaxed (e.g., less conductivity additive). To aid in future device design, models of reduced complexity were developed and evaluated for their ability to predict optimal designs. These simplified models can predict the performance of thermal energy storage heat exchangers up to 5000 times faster than the finite-difference model. For the design objectives of maximizing energy density, minimizing costs, and minimizing levelized cost of storage, the simplified models predicted optimal designs with average absolute deviations below 2 % in relation to the optimal designs chosen by the finite difference model optimization. These models can serve as tools to further study thermal storage heat exchangers incorporating the realistic trade-off between the device's power and energy.

Effect of inner-tube spacing on charging and discharging performance of latent energy storage heat exchangers

The inner-tube arrangement has an important impact on the performance of latent thermal energy storage heat exchangers, but the effect of inner-tube spacing on the performance has not been widely concerned. This study simulated numerically the charging and discharging processes of the horizontal dual-inner-tube heat exchangers and researched the effects of inner-tube spacing on the performance parameters of the heat exchangers. The studied performance parameters included the phase change time of phase change material, the charging rate, and the discharging rate. The results show, as the inner-tube spacing increases, the phase change time decreases first and then increases, and both the charging rate and discharging rate increase first and then decrease. Moreover, the inner-tube spacing has a more pronounced effect on the discharging process than the charging process. Among all cases, the performance parameters are optimal when inner-tube spacing is 20 mm. Compared with the case whose inner-tube spacing is 5 mm, the melting time mostly shortens by 5.78%, and the charging rate increases by 5.48% for the heating temperature of 80℃. Additionally, the solidification time mostly shortens by 38.68% and the discharging rate mostly increases by 60.49% when the cooling temperature is 20℃. Furthermore, the heating temperature doesn’t affect the effect of inner-tube spacing on the charging performance, and the effect of inner-tube spacing on the discharging performance also can’t be influenced by the cooling temperature. The research results have a certain reference value for the optimal design of the heat exchangers and its engineering application.

Reduced-order modeling method for phase-change thermal energy storage heat exchangers

Thermal energy storage can facilitate the effective utilization of renewable energy. To speed up the design process of thermal energy storage devices, it is critical to develop fast and accurate modeling methods for phase change material embedded heat exchangers (PCM HXs). This study developed and compared two approximation-assisted reduced-order PCM HX models for the simulation of thermal storage components and systems, which were verified against a validated finite-volume model. They are a pure black-box model, and a grey-box model based on the Number of Transfer Units (or effectiveness-NTU) approach. We used the reduced-order models to predict the performance of standalone PCM HX for 1000 cases with different fluid inlet conditions and HX designs. We also integrated the models with a vapor compression system to predict the compressor energy consumption and total charging time under various conditions. The results show that overall, the black-box model gave more accurate results than the grey-box model. On average, the mean absolute deviation in the PCM HX fluid outlet temperature was 0.05 K and 0.1 K for the black-box model and grey-box model, respectively. The grey-box model gave larger temperature deviation toward the end of the phase change process, due to the simplified two-node PCM representation. On the system level, the mean absolute deviation in compressor energy consumption was 0.2% and 0.3% for the black-box model and grey-box model, respectively. Their mean absolute deviation in total charging time was 1.1% and 2.6%, respectively. In terms of computation efficiency, the system simulation speedup ratio gained by using the reduced-order models was 11 to 57. System simulation time was decreased from an average of 1465 s to an average of 59 s. This shows that the proposed reduced-order modeling methods can be used to predict system performance of PCM HXs with less than 3% accuracy penalty, and 25 times less computational time than finite-volume models, enabling faster design and evaluation of PCM thermal storage devices.

Simulation study on the effect of fins on the heat transfer performance of horizontal dual-inner-tube latent thermal energy storage heat exchangers

Compared with the traditional horizontal double-tube heat exchangers, the horizontal dual-inner-tube latent thermal energy storage (LTES) heat exchangers can effectively shorten the melting time of the phase change materials (PCMs). This paper analyzed the liquid phase distribution, temperature field, melting rate and heat storage about finless horizontal dual-inner-tube latent thermal energy storage heat exchangers during the melting, and researched the effects of the fin position and number on melting progress and heat storage performance. The results showed that compared with the variation of fin number, the reasonable fin arrangement is better to improve heat transfer performance, and the heat transfer performance reaches to the optimal value when the single fin is placed at the bottom of the lower tube, which can shorten the melting time by 23.91% compared with the finless condition, but the increase of fin number on the basis of the optimal single fin arrangement only shortens the melting time by 1,2%. Additionally, it also obtained that the time of the high melting rate reduces by 9.14% when the fin is installed in the middle of the exchangers body, which weakens the effect of strengthening heat transfer. These results indicate that the less fins, if the arrangement is designed properly, can be as efficient as more fins in horizontal dual-inner-tube LTES heat exchangers.

Estimating the state of charge in a latent thermal energy storage heat exchanger based on inlet/outlet and surface measurements

• Estimators for the total and partial stored energy are developed and validated. • The estimator for the heat transfer fluid energy is generally applicable. • A finite volume latent thermal energy storage model is developed and fitted. The performance of latent thermal energy storage (LTES) heat exchangers is related to the stored energy (i.e. state of charge) during the (dis)charging of the energy storage system. Therefore, measuring the stored energy is crucial to understand the behavior of LTES systems. However, technical considerations often oppose the measurability of the stored energy. The present article presents a case where only measurements at the in- and outlet of the heat transfer fluid and on the outer surface of the heat exchanger were possible. Based on this sparse set of measurements, estimators for the stored energy are developed. To test these estimators, a finite volume model of the LTES heat exchanger is made. The finite volume model is fitted by comparing the predicted heat transfer fluid outlet temperature for 36 experiments. The energy estimators are compared to simulation results. The estimators for the stored energy in the heat transfer fluid, metal and phase change material obtain an average deviation of respectively 0.5 %, 3.9 % and 0.6 % with the stored energy predicted by the finite volume model. The estimator for the stored energy in the heat transfer fluid should be applicable to all LTES heat exchangers while the applicability of the estimators for stored energy in the metal and phase change material depend on small temperature differences between surface and interior of the metal and negligible heat losses.

Unsteady analysis of the cold energy storage heat exchanger in a liquid air energy storage system

Liquid air energy storage (LAES) is promising in the large scale energy storage field. The heat exchanger (Hex) in a LAES system using liquid phase working mediums for cold energy storage (CES) works discontinuously for the intermittent characteristic of the LAES. Variable temperature distribution exists in the Hex for CES (Hex-CES) in the intermittent process, and degrades the performance of the CES unit. In this paper, a computational model is constructed for the unsteady process in the Hex-CES of a 100 kW LAES system using liquid phase working mediums for CES to study the unsteady effect. The established steady temperature gradient is decreased gradually in the static process during which the LAES stops working and basically turns to zero gradient 12 h later. In the next operation cycle, it takes 34.8 min to reestablish the steady temperature gradient in the Hex-CES. Two methods to reduce the unsteady effect are proposed, the influence of key parameters on weakening the unsteady effect is investigated, and the optimal operation parameters in the static process are obtained. The results obtained in this paper are expected to provide the basis for the application of CES using liquid phase mediums and enhance the performance of LAES.

Applicability of MCDM Algorithms for the Selection of Phase Change Materials for Thermal Energy Storage Heat Exchangers

Latent heat thermal energy storage heat exchangers store heat energy by virtue of the phase transition that occurs in the thermal storage media. Since phase change materials (PCMs) are utilized as the media, there is a critical necessity for the appropriate selection of the PCM utilized. Since multiple thermo-physical properties and multiple PCMs are required to be evaluated for the selection, there arises a need for multiple criteria decision making (MCDM) algorithms to be adopted for the selection. But owing to the different weight estimation techniques employed and the voluminous quantity of selection algorithms available, there arises a need for a comparative methodology to be adopted. This study was intended to select an optimal PCM for a sustainable steam cooking application coupled with a thermal energy storage system. In this research study, six PCMs were chosen as the alternatives and five thermo-physical properties were chosen as the criteria for the evaluation. 11 different algorithms were augmented with 3 different weight estimation techniques and therefore a total of 33 algorithms were employed in this study. All of the algorithms have chosen Erythritol as the optimal PCM for the application. The outcomes of the MCDM algorithms have been validated through an intricate Pearson’s correlation coefficient study.

Melting effect in triplex-tube thermal energy storage system using multiple PCMs-porous metal foam combination

The study was performed on the melting performance of phase change materials (PCMs) inside a triplex heat exchanger with double-sided heat transfer. The purpose of numerical simulation in this paper was to investigate the effects of using three-layer PCM with different melting temperatures instead of single-layer PCM and also the effect of their combination with Al6061 metal foam to compensate for the major problem of low thermal conductivity of PCMs. According to research, despite the reduction of melting rate in pure three-layer PCM compared to pure single-layer PCM, the addition of Al-6061 metal foam to them caused the melting rate of three-layer PCM to be superior. By identifying porous three-layer PCM as a better case, the effect of changing the porosity arrangement of layers with constant average porosity of 0.95 was investigated. In the next step, this study was performed on different porosity numbers. As expected, with decreasing the porosity, the melting rate increased due to increased heat transfer. To achieve higher efficiency, the effect of heat transfer fluid (HTF) temperature was also investigated and plotted. The relevant results showed the significant effect of temperature change at the melting time of the system. Therefore, every case or combination of any of the above studies, which increased the melting efficiency, can be of great help in optimizing energy storage heat exchangers based on latent heat.

A fast reduced model for a shell-and-tube based latent heat thermal energy storage heat exchanger and its application for cost optimal design by nonlinear programming

Numerical simulation of latent heat thermal energy storage (LHTES) systems plays a fundamental role in studying the physical process and guiding the engineering design. Discretization of the PDEs describing the nonlinear solidification/melting process of phase change materials (PCMs) leads to a large-scale complex dynamics system, where the system behavior depends on a set of parameters. In a design setting, repeated model evaluations are required over the set of parameters results in significant computational burden. In this paper, an explicit analytic solution was built for the propagation of the solidification front in a cylindrical coordinate. The analytic solution approach is further employed to develop a low computational reduced model (RM) as a module for a shell-and-tube based LHTES heat exchanger. The levelized Cost of Energy (LCOE) is used as a design metric and the RM model is used to apply system-level constraints in the nonlinear programming formulation that facilitates efficient global optimal design of the PCM properties, flow conditions and tube geometries. The use of LCOE as the design metric prevents over design of the heat transfer rate and also establishes a fair ground for evaluation of different thermal storage technologies and their integrated applications with other systems. Optimal results showed that a higher effectiveness results in a higher LCOE; the velocity of the HTF and the length of the channel are highly correlated with each other; both larger PCM latent energy and conductivity result in lower LCOE.

A charging time energy fraction method for evaluating the performance of a latent thermal energy storage heat exchanger

• The charging time energy fraction method is developed. • The method allows to predict the outlet as a function of the input conditions. • The method is used to develop a model for a specific heat exchanger. • The predictions are mostly within the measurement uncertainty. Latent thermal energy storage heat exchangers have a large potential in different processes. Implementing such a heat exchanger requires evaluating the performance of the heat exchanger in the conditions of each specific process. Therefore a method to characterize the performance of a latent thermal energy storage design would be a useful tool for designing processes. Characterization methods for heat exchangers such as the effectiveness number of transfer units method are based on the steady-state operation of the heat exchanger. However, latent thermal energy storage heat exchangers do not operate in steady state and therefore the standard methods are not applicable. The present paper develops a novel method to determine the heat transfer fluid outlet state as a function of the initial and input conditions: the charging time energy fraction method. This method is used to characterize and evaluate a latent thermal energy storage channel heat exchanger. For 26 out of 30 calibration and validation experiments, the model predicts the outlet heat transfer fluid temperature within measurement uncertainty (±0.15 °C) during all but the initial 100 s of the charging process. The charging time energy fraction method is according to the authors the first approach to derive a predictive, low computational cost model of the outlet temperature of a latent thermal energy storage heat exchanger from a set of experiments.

Review on solar collector systems integrated with phase‐change material thermal storage technology and their residential applications

This article reviews the design of solar phase-change energy storage systems and their applications in residential buildings. The solar thermal collection system has high heat collection efficiency, no pollution, and it is also widely used in the field of building heating. In order to improve the efficiency of the solar collector system, the researchers found that integrating phase-change materials (PCMs) into the solar thermal collector system can achieve this goal, but there will be a problem of PCM leakage. Some researchers have solved this problem by microencapsulating PCMs, the thermal conductivity and energy storage efficiency of the PCMs after microencapsulation will be greatly affected. Adjusting the solar collector to an appropriate inclination angle can achieve the highest heat collection efficiency. Choosing the PCM in the appropriate temperature zone can improve the system efficiency. For the separated solar collector system, the parts that need to be considered include the type of heat exchanger, the structure of the energy storage heat exchanger, the shape of the heat exchanger fins, the type of solar collector, and so on. All in all, coupling PCM into the solar collector system can improve energy efficiency reduce carbon emissions and have good economics, the maximum internal rate of return of solar collectors integrated with PCMs in residential and industrial applications is 22% and 17.2%.