Latent Heat Exchanger Research Articles

Heat release efficiency Betterment inside a novel-designed latent heat exchanger featuring arc-shaped fins and a rotational mechanism via numerical model and artificial neural network

Thermal energy storage (TES) units featuring phase change materials (PCMs) have a crucial impact on efficient energy management. Since energy demands keep changing, and the world increasingly relies on sustainable energy sources, innovation is continuously required. New technologies, materials, and methods for energy storage must be developed to meet these shifting needs effectively. In this study, a triplex-tube heat exchanger (TTHE) was employed as a TES medium, utilizing PCM within the middle tube. Because of its high thermal energy storage capacity, RT50 was selected as the PCM. Four innovative arc-shaped fins were strategically integrated within the PCM space to accelerate heat release. The system was subjected to rotational speeds of 0.1, 0.3, 0.5, 1, and 1.5 rpm. The investigation was carried out in two stages: initially, the impact of varying rotational speeds on the discharging process of the PCM in a finless TTHE was explored; subsequently, the influence of the same speeds on the solidification behavior of the finned TTHE was analyzed. Natural convection and PCM’s solidification process were examined through the enthalpy-porosity method. The findings represented that in the absence of fins, the device’s rotation had a more noticeable impact on the solidification behavior; however, incorporating fins was much more influential. Up to 2850 s, all the finned systems had solidified entirely in the presence or absence of the rotational mechanism, while in the finless system with a rotational speed of 1.5, 30.52 % of the material was still unsolidified until this time. Finally, the influence of three key structural parameters of the fins: α (outer fins arc angle), β (inner fins arc angle), and S (space between fins) on the discharging time of the PCM was analyzed using an artificial neural network (ANN) model. The study offered critical insights for optimizing fin configuration by systematically varying these parameters within defined ranges. An ANN predictive model was also proposed to assist TES system manufacturers and developers in future design and optimization efforts. The results demonstrated that the optimal setting of the TTHE (with the parameters of α = 60°, β = 60°, S = 10 mm) achieved the liquid fraction of 0.1, 80.33 % faster than the finless system.

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.

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.

Performance investigation and optimization of latent heat exchanger with pin fins based on non-uniform deployment strategy

Latent heat storage technology is an essential way of efficiently utilizing industrial residual heat, which helps to the objective of carbon neutrality in all countries. In this work, latent heat exchangers with pin fins are numerically investigated based on non-uniform deployment strategy and then optimized according to response surface method. The influences of rotation angle, non-uniform fin pitch, non-uniform fin length and combination of phase change materials with different melting temperatures on thermal storage performance are studied. Results show that the increment of rotation angle facilitates the reduction of complete melting time, and the optimal angle accelerates the melting speed by 10.3% compared to the 0° angle. The arrangements of gradually decreasing fin pitch and gradually increasing fin length along flow direction of heat transfer fluid improve the heat storage performance, which can reduce the complete melting time by 7.3% and 22.2% respectively, compared to the uniform arrangement. The multiple phase change materials latent heat exchanger with high melting temperature phase change material placed in high temperature region and low melting temperature phase change material in low temperature region can make better utilization of heat from heat transfer fluid, which can improve the melting speed by 16.8% compared with the single phase change material latent heat exchanger. The optimized latent heat exchanger based on response surface method reduces the complete melting time by 23.4% compared to the original model. In conclusion, the heat storage performance of latent heat exchanger with pin fins effectively improved by adopting the non-uniform deployment strategy.

Toroidal tube latent heat exchanger: An experimental study

Concentric shell and tube latent heat exchangers integrated with phase change material (PCM) are widely used in many heat storage/transfer applications specially for building energy management due to their high energy storage density and fair simplicity in design. However, inadequate phase change in such systems reduces the thermal performance. The innovative geometric design is key in addressing this issue and accelerating the phase change process. The present work reports an experimental effort to investigate the progression of the melting process of PCM inside a novel design of toroidal tube embedded in the system as a shell and tube latent heat exchanger. The experimental setup consists of a toroidal enclosure filled with a bio-based PCM (i.e., coconut oil) and a constant temperature bath as a heat exchange environment. Digital images of melting progression and melting front at three different boundary temperatures (high, middle, and low temperature differences) are captured. Moreover, the time dependent temperature at a selected prob. point is also measured during the phase change process. As expected, decreasing temperature difference, decrease the melting rate, as the experiments show that the completed melting times are 20, 32, and 75 min for the high, middle, and low temperature differences. The melting front is sharper and more distinct to track for higher temperature differences as well. To observe the melting pattern and melting front inside the system (cross-sectional area of the tube) better, the melting progression in different selected times in a half-toroidal tube is also investigated. Furthermore, an extensive uncertainty analysis associated with measuring devices is reported.

Geometry modification of a vertical shell-and-tube latent heat thermal energy storage system using a framed structure with different undulated shapes for the phase change material container during the melting process

This study aims to modify the melting performance of a vertical latent heat double-pipe heat exchanger by geometry modification of the phase change material (PCM) container. To modify the configuration of the PCM container placed in the outer tube, a framed structure is embedded in the inner surface of the outer tube. Different framed structures i.e. smooth, arc-shaped, reverse arc-shaped, and zigzag-shaped structures are examined and compared with that of the unframed enclosure. Moreover, different numbers of pitches are also examined. The results show that the melting duration drops by almost 55 % and the thermal energy storage rate improves by 115 % using a smooth framed structure for the PCM container compared with that of using an unframed structure. The thermal energy storage rate and melting time are ∼9.5 W and 244.4 min, respectively, for the system using an unframed structure. The analysis of the pitch number for the zigzag-shaped structure reveals that increasing the number of pitches from 0 to 11 enhances the thermal energy storage rate by 36.5 %. The evaluation of different shapes of the framed structure shows that the case with reverse arc-shaped structure is the most enhanced unit which has the thermal energy storage rate of ∼34 W and melting time of 63.15 min heated by water at the Reynolds number 1000 and inlet water temperature of 50 °C. The sensitivity analysis of the HTF characteristics reveals that by increasing the Re from 1000 to 2000, the thermal energy storage rate increases by around 6 W. Furthermore, by increasing the inlet temperature of HTF from 50 °C to 55 °C, the thermal energy storage rate enhances by around 10 W. This paper shows the capability of geometry modification in enhancing the thermal energy storage rate of the systems.

Numerical study on discharging process of a latent heat triple-tube heat exchanger in the presence of a central plate using the enthalpy–porosity approach

Numerical study on discharging process of a latent heat triple-tube heat exchanger in the presence of a central plate using the enthalpy–porosity approach

Genetic algorithm optimization for double pipe heat exchanger PCM storage system during charging and discharging processes

Nowadays, world faces an energy crisis and residential buildings in developed countries account for 20–40% of the world's total energy demand. Therefore, reducing energy consumption in this sector is crucial for sustainable energy systems. Using phase change materials (PCMs) in energy systems is one of the most reasonable solutions. This study investigates PCMs in a dual-pipe latent heat exchanger system for thermal energy storage. The outer pipe of the exchanger contains the salt hydrate TH29 PCM, while the inner pipe carries water. The system is charged using hot water at 70°C and discharged with cold water at 8°C. An adiabatic time interval is introduced to optimize the charging and discharging operations during which the pump power is cut off and the water flow is stopped. The stored energy and exergy destruction during one hour of system operation is investigated by varying the adiabatic time interval. The Genetic algorithm is implemented to maximize the stored energy or minimize the destruced exsergy in the heat exchanger. The algorithm starts with 20 initial samples and 50 number of samples per iteration. The results show that the stored energy is a function of adiabatic time and maximizes to 25,312 J with an adiabatic time interval of 2150 s (out of 1 hour). Furthermore, the exergy destruction is minimized by an adiabatic time interval of 2249 s.

Energy saving potential of latent heat exchanger-integrated dual core energy recovery ventilator

High-efficiency energy recovery ventilators (ERVs) have a significant impact on the conservation of building energy. In particular, the importance of the latent cooling performance of ERV which used in hot, humid climates has been growing. To enhance the latent heat recovery performance of the existing ERVs and attain satisfactory sensible heat recovery performance as well, a dual-core energy recovery ventilation unit combining a hollow fiber membrane-based latent heat exchanger (M-LHX) and a sensible heat exchanger (SHX) was proposed in a previous work. This study evaluated the energy saving potential of the proposed ventilation unit integrated air-conditioning system via a series of building energy simulation. As the reference system, a conventional ERV with a flat-plate membrane enthalpy exchanger was chosen. The M-LHX and SHX in the proposed ventilation unit enable decoupled sensible and latent heat recovery of outdoor air. Energy simulations were conducted for a single-story office building located in South Korea, which is one of the humid regions during the summer. The simulation results showed that the proposed ventilation unit could reduce 15.9, 14.4, and 14.1% of building air-conditioning loads in summer, intermediate season, and winter, respectively, compared with the reference ERV. This is because the proposed ventilation unit provided energy benefits by recovering more latent heat from the M-LHX and comparable amount of sensible heat from the SHX than the reference system. Accordingly, the proposed system presented the annual primary energy savings of 10.6% over the reference system, although the proposed system consumed more fan energy.

Numerical feasibility study of improving the melting performance of horizontal and vertical double-pipe latent heat storage systems using ultrasonic field

The present research is devoted to the numerical study of the effect of ultrasonic waves emitted from an ultrasonic transducer on the melting performance of a double-pipe latent heat exchanger system (LHS). In the considered LHSs, hot water passes through the inner tube and the phase change material (PCM) located in the annulus melts by taking heat from the water. The performance of the LHS systems including the ultrasonic waves were compared with the performance of common LHS systems in both horizontal and vertical situations. In contrast to conventional methods for enhancing the heat transfer, integration of ultrasonic source with LHS is the novelty of current research. It is followed by evaluating the effect of transduce position and power on thermal energy absorption. The ultrasonic transducers were installed in various positions to find the most suitable location for improving the heat transfer. The melting time of PCM decreased up to 56.63 % and 74.59 % by applying the ultrasonic field for horizontal and vertical LHS, respectively. In addition, the effect of the transducer power on the performance of the best horizontal and vertical LHS cases was investigated. It was found that the increase in the transducer power from 20 W to 150 W is associated with a decrease up to 40.83 % in the charging time of PCM. The obtained improvement of heat charging of LHS can be attributed to the intensifying the convective heat transfer through ultrasonic field.

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.

Investigation on the heat discharge performance of a bioinspired latent heat exchanger with leaf-like fins

The low energy efficiency of phase-change thermal storage technologies leads to slow progress in promoting sustainable energy. To address this, here constructs a creative leaf-like fin and designs a bioinspired latent heat exchanger (LHE) using phase change materials (PCMs). A two-dimensional model of solidification processes considering natural convection is developed and solved numerically. The phase-change behaviors and dynamic thermal characteristics of different LHEs are investigated. Moreover, material selections and operating conditions of LHEs are studied, focusing on their effects on discharge performance. The results imply that the innovative leaf-like fin strengthens discharge performance. Compared to conventional LHEs, bioinspired LHEs shorten complete solidification time by 41.6% and increase average discharge power by 69.9%. Since the solidification procedure is driven by heat-conduction, the temperature stratification is only apparent in the early stage, while the solid–liquid phase interface and temperature distribution are almost axisymmetric for most solidification processes. For material selections, aluminum is recommended based on the trade-off between cost and thermal enhancement gain, and Stearic acid has apparent advantages in achieving satisfactory heat discharge performance for PCMs selected in this paper. Moreover, the lower temperature of the heat transport fluid facilitates the discharge rate enhancement for practical engineering.

Analysis of direct expansion heat recovery ventilation devices by orthogonal optimization method.

Heat recovery ventilation devices include rectangular plate cross-flow, hexagonal plate combined counter and cross-flow, rotary wheel sensible, sorption rotor hybrid sensible and latent heat exchanger. Currently, existing studies have produced no clear findings on which climatic conditions latent heat recovery would be optimal, and therefore sought to determine in which climatic conditions it would be suitable to use devices that perform latent heat recovery. This study analysed the performance of different heat recovery devices in different climatic conditions in a ventilation project of a sample hotel building. In the case study, while there is a useful heat recovery between 44.01 and 58.68kW at low outdoor temperatures in devices with only sensible heat transfer, this value increases up to 158.42kW, as the outdoor temperature rises. In the heat recovery device providing latent heat transfer, the amount of useful heat recovery varies between 51.34 and 352.16kW at low outdoor temperatures, depending on the outdoor relative humidity, while this amount increases to 411.26 from 773.25kW at high outdoor temperatures. Outdoor temperature and humidity levels required for latent heat recovery was also determined by orthogonal optimization method. By using the orthogonal optimization, the study found that under conditions of high temperature that exceeds 35°C in outdoor ambient temperature, and high humidity that exceeds 60% relative humidity, usage of latent heat recovery devices caused significant differences in total heat recovery ratio. Analysis also concludes that these devices can be used under these conditions.

Experimental analysis on energy recovery ventilator with latent heat exchanger using hollow fiber membrane

A novel energy recovery ventilation unit with a hollow fiber membrane-based latent heat exchanger is proposed and built to experimentally evaluate its operational performance and energy efficiency. The proposed system consists of a latent heat exchanger fabricated by hollow fiber membrane to accommodate the latent load of introduced outdoor air and a flat-plate sensible heat exchanger to eliminate the sensible load. Annual operating strategies are established based on outdoor air conditions, and experiments are conducted under each operation mode. Further, the thermal behaviors of air (including temperature and humidity) under each operation mode are described according to the measured data. The proposed system exhibits a sensible, latent, and enthalpy effectiveness of 53.6–80.1%, 68.1–73.1%, and 76.1–89.6%, respectively. As for the energy efficiency of the proposed ventilation unit, the coefficients of energy are 29.63 and 15.29 under the summer and winter operation modes, respectively, which meet the minimum energy performance requirements of the local test standard. Finally, the potential reduction in the ventilation load for each operation mode is estimated using the measured data. The proposed unit could reduce the ventilation load by 75.4%and 77.3% in summer and winter operation modes, respectively. These results reveal that the proposed ventilation unit demonstrates significant application potential in improving the operating and energy performance of buildings.

Investigation of Latent Heat Thermal Energy Storage Type Direct-Contact Heat Exchanger

Investigation of Latent Heat Thermal Energy Storage Type Direct-Contact Heat Exchanger

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.

Thermodynamic Performance Analysis of New Paper Machine Air Hood Heat Recovery System

In this paper, a new heat recovery system of hot air hood of paper machine is proposed, the thermodynamic model of the system is established, and the effect of different thermal parameters on the heat recovery efficiency of the system is studied. The results indicate that both the recovered energy and the proportion of exhaust air passing through the latent heat exchanger attenuate as fresh air temperature increase and fresh air moisture content decrease. The system heat recovery increasing with rising of exhaust temperature. Exhaust temperature and exhaust relative humidity have different effects on sensible and latent heat recovery. The maximum value of proportion of exhaust air passing through latent heat exchanger appears when the exhaust relative humidity is 35.8%. Overall, the fresh air temperature and the exhaust relative humidity have great influence on the heat recovery efficiency.

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.

Experimental and numerical investigation on heat transfer enhancement of vertical triplex tube heat exchanger with fractal fins for latent thermal energy storage

Latent thermal energy storage (LTES) technology applied in the recovery of industrial waste heat, solar thermal applications, and energy conversation of buildings, etc. can solve the mismatches of thermal energy in supply and demand, space and time, and thus increasing the efficiency of thermal energy utilization. However, the heat transfer process in a latent heat exchanger(LHE) is limited due to the poor thermal conductivity of phase change material (PCM). Therefore, a vertical triple tube heat exchanger with fractal fins was applied to enhance the heat transfer process of the LTES system. The performance of the LHE with fractal fins was discussed and compared to that with traditional rectangular fins. The results manifest that fractal fins can effectively improve the heat transfer rate of the LHE by providing more heat flow channels and heat transfer areas. The total melting time of PCM in the LHE with fractal fins is reduced by 27.3% compared to that with rectangular fins during the heat charging process. Furthermore, the structure of fractal fin is optimized through a heat transfer model and numerical simulation. It was apparent from the simulation results that the PCM melting time of the LHE with optimized fractal fins is reduced by 35.6% compared with that of rectangular fins. It was also proved that the dual-channel structure is used to reasonably arrange the charging and discharging processes, which can make the LTES system achieve better results.

Development of empirical models to predict latent heat exchange performance for hollow fiber membrane-based ventilation system

In this study, a hollow fiber membrane-based dual-core ventilation system is proposed to reduce space heating and cooling energy requirements. Simple prediction models were developed for the seasonal latent heat exchange performances of the proposed system. A mock-up model of the hollow fiber membrane module that serves as a latent heat exchanger in the proposed system was fabricated. A total of 32 experimental sets for the cooling and heating seasons were designed based on five operating parameters (outdoor air temperature and corresponding relative humidity, room air temperature and corresponding relative humidity, and outdoor air face velocity). The R-squared values of the derived models exceeded 96%, and the models were validated using additional measured data. Empirical correlations to predict the latent and sensible effectiveness were derived based on the response surface methods. A sensitivity analysis of each operating parameter on the latent heat exchange performance of the hollow fiber membrane module was performed. Finally, a guideline for the implementation of the developed models for energy simulation in building applications was presented. The proposed ventilation system improved the energy saving over that of the conventional energy recovery ventilator and reduced the ventilation load by 54.8% and 19.9% in summer and winter, respectively.