Latent Heat Exchanger Research Articles

Enhancement of Melting Process Inside Toroidal Tube Heat Exchanger With Different Cross-Sectional Geometries

Abstract Utilizing phase change material (PCM) in concentric tube and shell-and-tube latent heat exchangers known as latent heat thermal energy storage (LHTES) have been extensively studied due to the high ability and density in storing energy during the melting (charging) process. Inadequate melting in these systems reduces the thermal performance of LHTES systems. To facilitate and accelerate the melting process, the innovative design of such systems is a key. The present study proposes novel designs of toroidal tubes embedded in the LHTES system as a latent heat exchanger. The effect of the cross-sectional geometry of the tube on the thermal performance of the system is investigated through simulation and comparison of different cross-sectional geometric shapes. A mathematical model based on the enthalpy-porosity approach is developed and numerically solved by the finite volume method to simulate the energy transport processes inside the system. Several transient heat transfer characteristics, e.g., thermal filed, melt fraction, Nusselt number, and energy storage during phase change, are determined and compared for all cases to evaluate their thermal performance and find the optimal geometry. The results indicate that downward triangular geometry for the cross-sectional shape of the tube shows the best performance as it significantly enhances the melting process, resulting in a faster energy storage rate during the charging process. Compared with the circular toroidal tube as the base geometry, the downward triangular shape design for the toroidal tube can improve the charging power of the system by 21%.

Compact latent heat exchanger for the fast supply of hot water with serpentine tubes in shape-stabilized composite phase change material and auxiliary electric heating

• A composite phase change material-based heat exchanger is designed. • The composite phase change material owns a large enthalpy of 252 J/g. • An electric heating film is introduced to improve the heating efficiency. To realise a fast supply of hot water, this study proposes a compact latent heat exchanger. This heat exchanger consists of a certain number of heat-exchange plates. In each plate, serpentine tubes are wrapped with an inorganic composite phase change material (CPCM), Ba(OH) 2 ·8H 2 O/modified expanded graphite (MEG). An auxiliary electric heating film is attached to the face of the entire heat exchanger to improve the heating efficiency. The parameters of the plate, such as thickness, tube pass number, thermo-physical properties of the CPCM, flow rate, auxiliary heating power size, and position are numerically evaluated to achieve the best performance of this heat exchanger. The reliability of the simulation method is verified by comparing the simulated results with the experimental data in previous study. The simulation results indicate that a plate with a thickness of 15 mm, plate tube pass number of 10, and CPCM with a Ba(OH) 2 ·8H 2 O mass fraction of 85.8 wt% is the optimal choice. Based on the optimal conditions, 196.48 kg of hot water over 40 ℃ can be produced in 875 s and the average power reaches 23 kW. Furthermore, with the addition of a heating film (3600 W), the mass of heated water can be increased by nearly 50 kg.

Thermal analysis and assessment of phase change material utilization for heating applications in buildings: A modelling

In this study, a boiler heating unit of a building is used as a heat source for an economizer heat exchanger which is connected to a PCM (phase change material) latent heat storage unit-based heat exchanger used for additional heating system. The system is simulated and analyzed for 24 min. It is found that during the melting process of the RT55 PCM, the heat flux increases up to the 250th second and conduction-dominant heat transfer occurs. The flow velocities around the serpentine tube heat exchangers are higher (between 0.01 and 0.03 m/s) between the 200th and the 800th second where transmission-dominant heat transfer is dominant. At the end of the process, the average temperature of the PCM and the liquid fraction reach 339 K and 86%, respectively, with the heat transferred from the flue gas to the heat carrier fluid at 802 K. The economizer heat exchanger efficiency rises to 51% due to the temperature difference between the heat transfer fluids. In addition, when the temperature is close to the PCM melting temperature, between the 450th and 610th second, the efficiency of the PCM heat exchanger rises to 13% as a maximum rate.

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.

Effects of non-uniform fin arrangement and size on the thermal response of a vertical latent heat triple-tube heat exchanger

The aim of this study is to explore the impact of incorporating annular fins with nonuniform distribution on the thermal response of phase change material in a vertical triple-tube heat exchanger involving two counter-current flow streams of the heat-transfer fluid. The phase change material is accommodated in the middle tube, with one stream of the heat transfer fluid flowing in an outer tube in the direction of gravity and another stream flowing contrary to gravity in an inner tube. The main contribution of this study is detecting the best locations and sizes of the fins to achieve the highest storage performance of the unit by simulating five annular fins each on both the inner and outer tubes. Considering the constant relative volume occupied by the fins in all cases studied, the results show significant uniformity of the melting rate and temperature field can be achieved with the optimization of fin distribution between the inner and outer tubes of the heat exchanger. The fin distribution and size are optimized based on the melting process, and the best case is then investigated during solidification compared with the no-fin case considering different Reynolds numbers and inlet temperatures of the fluid. The best usage of five fins shows ∼29% saving in the melting time and ∼37% in the charging rate compared with the uniform fins distribution during the melting process considering the identical volume of the fins. The results based on the optimized fin arrangement show that the melting and solidification times can be reduced by ∼53% and ∼9%, respectively, compared with the no-fin case.

Construction and Performance Evaluation of a Triple-Stage Latent Heat Exchanger Packed with Three Latent Heat Storage Materials for Recovering Low-Grade Thermal Energy

Construction and Performance Evaluation of a Triple-Stage Latent Heat Exchanger Packed with Three Latent Heat Storage Materials for Recovering Low-Grade Thermal Energy

Preliminary study on air-to-air latent heat exchanger fabricated using hollow fiber composite membrane for air-conditioning applications

In this study, an air-to-air type hollow fiber composite membrane latent heat exchanger was built and its latent heat exchange characteristics were experimentally investigated under various test conditions. Based on experiments conducted on four prototypes, polyethersulfone with 9 wt% of polyvinyl-pyrrolidone was selected as the membrane material suitable for the latent heat exchanger. Using the selected material, shell-and-tube structured hollow fiber membrane modules were fabricated and tested under various operating conditions. The experimental data showed that the proposed membrane modules exhibited 35.3%–82.7% latent effectiveness and moisture removal rates of 0.027–0.124 g/s under various operating conditions; furthermore, the modules could provide latent cooling with few temperature changes. In addition, the results of the parametric analysis indicated that the inlet primary air humidity ratio was a critical factor affecting the latent cooling performance of the proposed system and the optimal ratio of the secondary to primary air flow was 1.0. The number of mass transfer units (NTUm) in the membrane module design should be higher than 1.7 to attain satisfactory latent cooling performance, that is, higher than 70% latent effectiveness.

Numerical investigation on performance improvement of latent heat exchanger with sextant helical baffles

• Numerical investigation on a novel latent heat exchanger with sextant helical baffles was implemented. • Helix angle exhibits an opposite effect on the thermal performance and comprehensive performance of shell side. • The thermal conductivity and pr number of the HTF are important factors affecting the shell-side heat transfer feature. • Internal-external fin structure shows a large impact on the melting process of PCM. • Nano-additives have a remarkable influence on the thermal conductivity and latent heat of Nano-PCMs. Latent heat thermal energy storage is a promising technology, which is becoming more and more widespread in industrial fields. A novel latent heat exchanger with sextant helical baffles is thus investigated numerically in this paper. The different helix angles (10°, 20°, 30°, 40°) of sextant helical baffle are employed to explore the shell-side thermal and comprehensive performances in latent heat exchanger. The geometric parameters of internal-external fins, including the height, thickness and shape, are studied to improve the melting rate of phase change material (PCM). Water, fuel oil JP-4, ethylene glycol and hydraulic oil are chosen as heat transfer fluids (HTFs). Nano-PCMs with various volume concentrations of TiO 2 (2%, 4% and 6%) are used to study the discharging characteristic of latent heat exchanger. The results indicate that shell-side heat transfer and integrated behaviors need to be considered comprehensively for the choice of helix angle. The longitudinal-shaped fin with 7.5 mm height and 1 mm thickness is studied as the optimal scheme. Smaller Pr number and larger thermal conductivity are applied to HTF to further enhance the heat transfer between HTF and PCM. Furthermore, the selection of Nano-PCM needs to comprehensively consider the requirements of thermal conductivity and phase change latent heat in practical application.

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.

Experimental and numerical investigations of thermal performance enhancement in a latent heat storage heat exchanger using bifurcated and straight fins

In this paper, the effects of applying straight and bifurcated fin configurations on melting behavior of paraffin inside the shell-and-tube heat exchangers are both experimentally and numerically investigated. Three different fin arrangements for each of the fin configurations were considered: (a) cross arrangement, (b) diagonal cross arrangement and (c) arranged evenly at the lower half of the heat exchanger. The evolution of the melt front was photographed to obtain the instantaneous melt fractions and gain a better insight into the effect of fin configuration on the melting process of phase change material under different Rayleigh and Stefan numbers. The impacts of fin length and thickness on the thermal performance of the heat exchangers were also investigated while the mass of fins for all cases was kept constant. It was found that the bifurcated fin configuration outperforms the straight fin configuration. Also, the fin arrangement showed a significant effect on the melting process. Numerical results revealed that the optimum fin arrangement depends on the fin length which was more pronounced when the fins located at the lower half of the annulus. For charging temperature of 95 °C (Ste = 1.08, Ra = 1.73 × 106), the maximum melting time reductions of bifurcated fin heat exchangers with dimensionless fin lengths of 0.51 and 0.89 mm, compared to those of the straight fin heat exchangers, were 19.5% for the (c)-type fin arrangement and 61.6% for the (b)-type fin arrangement, respectively.

Entropy and thermal performance analysis of PCM melting and solidification mechanisms in a wavy channel triplex-tube heat exchanger

This paper aims to perform the entropy analysis and thermal performance evaluation of a wavy-channels triplex-tube latent heat storage heat exchanger (LHSHE) during melting and solidification mechanisms. The system with different wave amplitudes was examined for different temperatures and Reynolds numbers of the heat transfer fluid (HTF). Water is passed in the inner and outer tubes in opposite directions and the PCM is placed in the middle tube. The heat exchanger was analyzed based on the temperature, liquid fraction and velocity of the PCM as well as thermal (ST‴) and frictional (Sf‴) entropy generation rates. The results show that for a higher wave amplitude, shorter melting and solidification times are achieved. Both frictional and thermal entropy generation rates increase to the maximum values and then decrease during the melting and solidification. The frictional entropy generation rate reaches almost zero quickly during the solidification. For both melting and solidification, the magnitude of ST‴ is significantly higher than Sf‴ in the phase change problem. The maximum values of ST‴ are 0.05 and 0.13 W/Km3 for the melting and solidification mechanisms, respectively, for the dimensionless wave amplitude of 0.3. The results show the crucial role of entropy generation on the performance of the LHSHE.

Heat release characteristics of a latent heat storage heat exchanger by scraping the solidified phase change material layer

Latent heat storage (LHS) is one of the key methods to level renewable energy and utilize waste heat. In this method, heat is stored as latent heat of the phase change material (PCM) during melting. Releasing the stored heat in PCM consumes considerable time because the heat release rate is slow when a solidification layer of PCM is formed on the heat transfer wall during the heat release period. The authors have proposed a new mechanism for rapid heat exchange in LHS, i.e., by scraping the solidified PCM layer on the heat transfer wall. In this mechanism, the heat transfer wall is a rotating cylindrical tube to which a fixed blade is attached. During the heat release period, PCM is solidified on the tube, and the solidified layer is immediately removed using the fixed blade. In this study, a laboratory-scale setup with 1 kg of sodium acetate tri-hydrate as PCM was developed, and the effect of rotation rate on the heat release rate was experimentally examined. The results showed that the heat release rate during rotation accelerated by more than 100 times compared with the rate without rotation, and it increased with the rotation rate of the cylindrical tube.

Experimental investigation of the effect of perforated fins on thermal performance enhancement of vertical shell and tube latent heat energy storage systems

A key challenge in the deployment of practical latent heat energy storage systems employing phase change materials is the inherent low thermal conductivity of these materials. The present research is motivated by the need to intensify the buoyancy-driven convection flow in the phase change material to enhance the thermal performance of the system. In this paper, for the first time, the effect of applying perforated fins on the thermal performance enhancement of a vertical shell and tube latent heat energy storage heat exchanger is experimentally investigated and the results are compared with those of the unfinned and solid finned heat exchangers as the base cases. Lauric acid as the phase change material is placed in the shell side and the water is passed through the inner tube. The shells of the heat exchangers were made of transparent Plexiglas tubes to enable the visual comparison of the melting processes. The fins and tubes were made of copper. The melting process of phase change material is studied under different inlet water flow rates (0.5 and 1 l/min) and temperatures (55 and 65 °C). The experimental results showed that the time-averaged Nusselt number of the perforated finned heat exchanger is about 30% higher than that of the solid finned heat exchanger due to the minor hindering effect of the perforated fins on the development of the convection flows. Moreover, the total melting time of the perforated finned heat exchanger is about 7% lower than that of the solid finned heat exchanger.

Discharge of a composite metal foam/phase change material to air heat exchanger for a domestic thermal storage unit

This paper evaluates the discharging mechanism in a PCM (phase change material) to air heat exchanger for the purpose of space heating using a composite of copper foam and PCM. The composite system is modelled with both 2-D and 3-D computational fluid dynamics approach for different inlet air temperatures to consider the effect of room temperature using the thermal non-equilibrium model for the porous medium compared with the thermal equilibrium one. The results show the significant advantages of composite heat exchanger compared with a PCM only case. For the inlet air temperature of 22 °C, the composite unit is solidified in 43% shorter time with 73% higher heat retrieval rate compared with that for the PCM only. After 10 h, the temperature variation between the inlet and outlet of the air channels for latent heat storage heat exchanger system with the composite system is 41 °C and 34 °C for the inlet air temperatures of 0 °C and 22 °C, respectively, while it is 33 °C and 29 °C for the system with PCM only. This study show the possible usage of PCMs in the energy storage heaters by introducing metal foams which is not possible using PCM only alternatives.

Numerical investigation of high temperature metal hydride water pumping system

I n the present work, a high temperature Metal Hydride Water Pumping System (MHWPS) equipped with a latent heat exchanger was investigated numerically. The operating concept of the pump was presented and the mathematical model of heat and mass transfer within the pump was established. We simulated the pump under different operating conditions using the alloy Mg2Ni as a metal hydride and the KNO3 as a Phase Change Material (PCM). The obtained results have shown that i) the developed numerical model is flexible and accurate in predicting the dynamic behavior of the pump ii) the numerical model without radiative heat transfer gives results with errors that can reach 38%, particularly for pumping time iii) the integration of the PCM provides a reduction in the pumping time of about 90% and an increase of the efficiency of the pump of about 7.6 times compared to the case without PCM which represents an improvement of 86% and iv) the Mg2Ni alloy requires high temperature and a mass of PCM about 9 times larger than the case of LaNi5 alloy to pump a volume 6.5 times greater than that pumped with LaNi5.

Improved thermal performance of annular fin-shell tube storage system using magnetic fluid

The main contribution of this paper is achieving a significant thermal performance improvement of annular fins-shell-tube heat exchanger energy storage system using magnetic fluid. The fin efficiency is increased by 20% with annular fin-magnetic fluid geometry as compared to without magnetic fluid. This improvement in thermal characteristics of annular fin system can be attributed to the formation of nano-channels in the field directions and effective increase in overall heat transfer coefficient due to the replacement of solid-gas interface with the liquid-gas interface. This replacement leads to improved convective heat transfer rate and overall thermal characteristics, which is the new knowledge contribution for various energy storage systems. In the present work, a simple and novel method for improving the thermal performance of natural convection energy storage system is reported. This novel method can improve the overall performance of the systems with natural convection extended surfaces and makes it suitable for numerous heat transfer applications. A magnetic fluid thin layer association with annular fin geometry has been proposed to improve the shell-tube heat exchangers performance, and an in-house experimental setup is used to study its dimensional and non-dimensional heat transfer characteristics. Kerosene and water-based MF were prepared by chemical co-precipitation method and employed on the top surface of the fin to maximise the heat transfer rate and to increase the effectiveness (ε) of the fin system. This thin layer of MF was maintained on the top surface of the fins using the magnetic ring of small NdFeB magnet of 1 mm thickness and 2.5 mm diameter. It is observed that a significant increase in relative heat transfer rate (∼35%) taking place using a thin MF layer over the top surface of the fin which leads to enhancement in fin performance. Also, the temperature distribution along the length of the fin shows a sharp decrease in temperature near the top surface which corroborates our observations that the magnetic ring over the surface works as a chill block which provides a directional cooling through magnetic nano-channels. These new findings can be valuable in the design of an efficient annular fin-shell tube latent heat exchanger storage system.

Performance Evaluation of a Dual-Stage Latent Heat Exchanger Packed with Two Latent Heat Storage Materials for Recovering Low-Grade Thermal Energy

Performance Evaluation of a Dual-Stage Latent Heat Exchanger Packed with Two Latent Heat Storage Materials for Recovering Low-Grade Thermal Energy

Experimental and numerical investigation on the novel latent heat exchanger with paraffin/expanded graphite composite

In this paper, a novel latent heat exchanger with two flow channels was investigated experimentally and numerically. The phase change material was filled in the annular tube, while the working fluid flow in the shell and tube side runner. The design of multi flow channels allow it apply into energy storage system with different working fluid. The paraffin/expanded graphite composite with phase changing temperature around 50 °C was filled in the annular tubes. Heating and discharging tests have been performed in different flow rate. The numerical model was built and validated with experimental data. The experimental results indicates that overall energy shows decreasing tendency with higher flow rate while the average power increases with inlet velocity. The liquid fraction and temperature distribution of PCM was studied numerically by validated simulation model. In addition, it can conclude that the heat resistance of system lay in the PCM side, the improvement of system overall thermal conductivity still need further investigation.

Heat Pump for Energy-Efficient Sugarcane Juice Freeze Pre-concentration

A freeze concentration system with two-stage heat pump was investigated for pre-concentrating sugarcane juice in a jaggery making process. Two identical vented double-wall tube-and-tube heat exchangers were used as latent heat exchangers. One latent heat exchanger was operated as an evaporator, which selectively froze water from the juice. Another latent heat exchanger was operated as a condenser, which melted ice formed in a previous half cycle. Excess condenser duty, as a result of compressor work input and heat leakage into the cold sections of the system, was rejected with the help of second-stage compressor and concentrated juice and water heater. High juice velocity in the tube-and-tube heat exchanger helped to reduce inclusion in layer freezing. Raw juice was pre-cooled in a three fluid heat exchanger using concentrated juice and cold water streams from the latent heat exchangers. Pre-concentration of sugarcane juice from 20 to 40 °Brix helped to save about 63% of bagasse. Heat removed during freezing was one-seventh of that required for evaporation. R290 was found the best option amongst refrigerants R744 (CO2), R290 (C3H8) and R22 (CHClF2). Performance of system was evaluated considering the effect of thermal mass, ambient heat gain, variation in freezing point depression and ice layer thickness. Cooling COP of 14 can be achieved in a practicable system with evaporation at − 8 °C, condensation at 3 °C and the excess heat rejection at 34 °C. This shows that specific energy consumption can be as low as 8.8 kWhe/m3 of separated water.