Phase Change Material Capsules Research Articles

Experimental results of embedded phase change material capsules for increasing the performance of a wrapped heat pump water heater

To enhance the capacity of heat pump water heaters (HPWHs), high energy density phase change material (PCM) was used in two experimentally tested HPWH prototypes. Preliminary findings showed that in the PCM, stored thermal potential energy could be effectively converted into sensible heat in the water over a slight temperature difference, with the goal of increasing the capacity of hot water above a desired cut-off temperature. This exchange of energy results in an increased amount of hot water necessary for mixing at the location utilized by the consumer. It was also found that for a wrapped HPWH, there was an average increase of 27.7% in the capacity when 7.6 L of novel PCM filled spherical capsules were added to a 22.7 L tank downstream of the HPWH. In the second design (Beta), PCM capsules were immersed into the tank with a cylindrical geometry, and the UEF was increased by 5.5% over the baseline HPWH test result. As for the Beta design, heat loss was reduced significantly compared to a typical HPWH and the Alpha design. Food-grade PCMs were selected as appropriate. The novel encapsulation design increased the effective conductivity inside the capsule by adding metal fins.

Numerical simulation of heat transfer and melting process in a NEPCM: Using new fin shape

Nowadays, phase change materials (PCMs) have gained further importance as they are the main substance in thermal energy storage systems. The observations indicated that the rate of thermal storage is related to the rate of melting and thermal distribution within these materials, where it has been concluded in almost works that convection is an effective contributor to boosting heat transfer, mainly in the upper layers of PCM capsules unlike in the lower layers. This article aims to accelerate heat distribution and fusion rates of NEPCM inside rectangular capsules by using novel fins in the shape of dendrites planted in the lower region of the capsule. Three different cases are considered and analyzed; in the first case without fins, in the second case fin branches length is decreasing downward (type 1), and in the third case, fin branches length is increasing downward (type 2). The porosity enthalpy method is used in modeling heat transfer during phase change. The impact of nanoparticles volume fraction is investigated by three different concentrations (ϕ = 0, 3%, and 6%). The findings indicated that enriching the weight of the nano-elements and using fins yield improving the thermal conductivity characteristic of the base PCM, besides, integrating the fin in the lower region of the PCM, i.e. under the heat source extends the heat transfer area, particularly where the dominated heat transfer in the low region is conduction, thereupon, as a result, both the thermal resistance and fusion time diminish. The maximum heat transfer rate and shorter fusion time were achieved by case 3 (fin type 2) with ϕ = 6%, where, fin-type 2 increased the melting rate by 33% compared to no fin case due to the largest branch of the fin is located in the bottom area, whereas the conduction is more dominated as approaching the lower wall. Finally, it is recommended to further investigate the dimension characteristic of this new kind of fins, for instance, the number of branches, spacing between the fin branches, and the branches' angle.

Wet spinning of strong cellulosic fibres with incorporation of phase change material capsules stabilized by cellulose nanocrystals

Incorporating a phase change material (PCM) into fibres allows the fabrication of smart textiles with thermo-regulating properties. Previously, such fibres have been made from thermoplastic polymers, usually petroleum-based and non-biodegradable, or from regenerated cellulose, such as viscose. Herein, strong fibres are developed from aqueous dispersions of nano-cellulose and dispersed microspheres with phase changing characteristics using a wet spinning technique employing a pH shift approach. Good distribution of the microspheres and proper compatibility with the cellulosic matrix was demonstrated by formulating the wax as a Pickering emulsion using cellulose nanocrystals (CNC) as stabilizing particles. The wax was subsequently incorporated into a dispersion of cellulose nanofibrils, the latter being responsible for the mechanical strength of the spun fibres. It was possible to produce fibres highly loaded with the microspheres (40 wt%) with a tenacity of 13 cN tex−1 (135 MPa). The fibres possessed good thermo-regulating features by absorbing and releasing heat without undergoing structural changes, while maintaining the PCM domain sizes intact. Finally, good washing fastness and PCM leak resistance were demonstrated, making the fibres suitable for thermo-regulative applications. Continuous fabrication of bio-based fibres with entrapped PCMs may find applications as reinforcements in composites or hybrid filaments.

Optimization design and performance investigation on the cascaded packed-bed thermal energy storage system with spherical capsules

Most of the previous researches on the cascaded packed-bed latent heat storage (CPTES) system are the thermal performance optimization based on numerical calculation. Experimental studies are few and mainly focus on single design parameter at low-temperature conditions. Lack of the detailed temperature - flow field experimental data and optimization scheme for the CPTES system. In this paper, an optimized two-layered filling structure of packed-bed heat storage system (OT-PTES) was proposed, which considers melting temperature of phase change material (PCM), capsule diameter, and filling volume ratio. The heat transfer process of PCM capsule and heat transfer fluid (HTF) during charging/discharging process were studied in detail by experiments. Under consistent working conditions, the average charging/discharging rate, total heat capacity, overall efficiency, and exergy efficiency was evaluated. And the influences of HTF inlet flow and the volume filling ratio of two-layered on thermal performance were analyzed. Finally, the concentric diffusion model was used to simulate the system to further optimize its hierarchical structure. The results are concluded as follows: (1) The temperature difference between PCM capsule and HTF at the interface changes continuously, and there is also an obvious temperature gradient in the radial direction of HTF, which improves the heat transfer efficiency. (2) Through experiments, the packed-bed with single large/small size PCM capsules are compared with the OT-PTES system. The overall efficiency of the latter is 74.2%, increased by 3.38% and 8.28%, and the exergy efficiency is 62.7%, increased by 7.5% and 52%. It is proved that the OT-PTES system has better charging/discharging performance. (3) When the volume filling rate = 1/2, the system has the best thermal performance (75.04%) and exergy efficiency (67.1%). (4) The numerical simulation results show that the overall performance of the packed-bed is optimal when the capsule size composition is 20–30 mm and the temperature difference of PCMs melting point is 60 ℃. In summary, OT-PTES is an optimized system with better heat storage performance. This study can provide experimental data support for the optimal design of the PLTES system and useful guidance for industrial applications.

PCM-based ceiling panels for passive cooling in buildings: A CFD modelling

To relieve the upsurge of the energy demand associated to building indoor cooling, passive cooling employing phase change materials (PCMs) has been proposed as a promising technique to stabilise indoor air temperature fluctuation and reduce building energy demand. PCMs can exchange a large amount of latent heat during the melting and solidification processes, which can be utilised in line with the discharging and charging processes of a cooling system. The use of encapsulated PCM ceiling panels in buildings has immense application potential for passive cooling, which has attracted investigations in recent years. The geometry selection of the PCM ceiling panel is vital as it determines the exposed surface area to the surrounding air and the natural convection, and therefore the heat transfer rate to/from the PCM, which generally suffers from low thermal conductivity. This paper builds on the geometric optimisation of enclosed PCM capsules driven by natural convection, aiming to investigate the phase change behaviour and thermal performance of encapsulated PCM ceiling panels of different geometries. Here, the presented study focuses on the conjugate heat transfer phenomena through the surrounding air, panel shell, and PCM. A mixture of lauric, capric, and oleic acids was selected as PCM encapsulated in a thin-shell enclosure made of acrylonitrile butadiene styrene plastic, which forms the aesthetic panels attached to the ceiling. A computational fluid dynamics model was validated using full-scale experimental data and further employed to analyse the discharging process and design considerations of the PCM ceiling panels. The effects of various influencing factors were explored, such as panel volume, the initial temperature difference between PCMs and surrounding air, shell thickness, and geometrical features. The results indicate that the panel comprised of a pyramid array is more advantageous than the tetrahedral-based panel in terms of thermal performance, where the average melting rate is 20.8 % higher when the panel volume is 250 mL. In addition, tweaking the geometric features of the panel will largely impact its thermal performance as the effective heat transfer area and natural convection conditions are varied. Finally, it was found that hanging the panel to the ceiling will effectively enhance the heat transfer compared to directly attaching it to the ceiling. The results complement the knowledge body of macro-encapsulated PCM ceiling panels in terms of thermal performance and geometric optimisation. The effects of geometric features on the thermal performance of the PCM ceiling panels are studied in a more holistic manner.

THERMAL MANAGEMENT STRATEGY WITH PHASE-CHANGE MATERIAL CAPSULES FOR PULSED-POWER SOURCE CONTROLLER IN EXTREME OIL-WELL THERMAL ENVIRONMENT

The application of a pulsed-power source controller provides high energy and strong penetrability, which can effectively improve the shale oil exploitation efficiency. However, the harsh high-temperature oil-well environment will significantly affect the safe operation of electronic devices. This study compares the thermal protection performance of the insulation layer with microscale phase-change material (PCM) capsules and macroscale PCM capsules composed of paraffin within a nylon shell and fabricated by 3D printing technology. Results show that the microscale PCM capsules demonstrate considerable thermal protection performance when operating under a high temperature of 100°C, which can prolong the operating time of the electronic device by only 64% in comparison to the case without the thermal protection strategy. However, the amount of PCM still shows dominant influence. On the other hand, the macroscale PCM capsules contain a relatively higher capacity of PCM, which can further extend the operating time by 84%, up to 92 min, within the maximum junction temperature of 80°C. In addition, a simplified numerical model was developed and validated with the experimental data. Eventually, the thermal protection performance of macroscale PCM capsules with orderly stacked configuration as well as the effect of thermal protection thicknesses are numerically investigated.

Heat transfer analysis of hollow channel in phase change materials spherical capsule

• A PCM spherical capsule with simple and low-cost hollow channel is presented. • The conduction and natural convection in the center of capsule are enhanced. • The effect of angle and diameter are investigated experimentally and numerically. • The heat storage time reduced by 28% with a dimensionless diameter of 0.25. The packed bed thermal energy storage device (PBTESD) filled with spherical phase change capsules can overcome the mismatch between the energy supply and the heat demand, which has broad application prospects. In this paper, a spherical phase change material capsule with a hollow channel with a simple, low-cost heat transfer enhanced structure has been proposed and studied. Experiments were carried out to measure the temperature of the typical capsule and capsule with hollow channel during the melting process to study the feasibility of the structure. Moreover, a three-dimensional numerical model based on the enthalpy method is established and verified by experiments. In order to optimize the structure and study the effect of random placement on the performance of hollow channels, the effects of the angle and diameter of the hollow channel on the thermal performance are analyzed by numerical simulation. A 24% increase in melting rate is found through experimental temperature profiles preliminarily. Besides, it is found that the performance decreased by 26.7% when the angle of the hollow channel decreasedfrom 30° to 0°, and the performance is similar when the angle is between 30° and 90°. In addition, the energy storage time decreases linearly with the increase of the diameter when the dimensionless diameter d/D is bigger than 0.05. Increasing the dimensionless diameter can improve the performance of the hollow channel, but it also reduces the phase change material filling rate. The melting time is shortened by about 28% when d/D=0.25. This paper indicated the transient thermal properties of the spherical capsule with hollow channel during constraint melting, which laid a foundation for further structural optimization and practical application.

Parametric investigation of charging and discharging performances of a cascaded packed bed thermal energy storage system

The packed bed thermal energy storage (PBTES) system employing cascaded phase change material (PCM) is useful for low-grade waste heat recovery and utilization. The performance of a cascaded PBTES system is parametrically investigated in the present study. A one-dimensional concentric dispersion model is developed and validated by the experimental results. The average heat transfer rates of a cascaded PBTES system at various volumetric ratios of PCM capsules are numerically investigated, and the cascaded PBTES system of a volumetric ratio of hydroquinone: adipic acid: erythritol = 3:6:1 is found to be the best configuration in the cyclic charging and discharging processes. Furthermore, the comparison of the thermal behaviors of the cascaded and non-cascaded PBTES systems shows that the PCMs in each layer of a cascaded PBTES system complete the phase change at nearly the same time, leading to a larger heat transfer rate compared to the non-cascaded one. The average heat transfer rate of a cascaded PBTES system with the volumetric ratio of 3:6:1 is about 6.96 % higher than that of the non-cascaded one for the cyclic process. In addition, the results show that the average heat transfer rate of the cyclic process of the PBTES system is improved by employing the cascaded PCMs, while the enhancement ratio of average cyclic heat transfer rate is found to decrease from 10.57 % to −1.35 % as the mass flow rate increases from 20 kg·h−1 to 100 kg·h−1 and to decrease from 10.32 % to 0.91 % as the capsule diameter increases from 20 mm to 100 mm.

Low temperature energy storage by bio-originated calcium alginate-octyl laurate microcapsules

Octyl laurate phase change material (PCM) was microencapsulated by calcium alginate for eco-friendly low temperature energy storage. The PCM microcapsules were prepared by repeated interfacial coacervation followed by crosslinking method. In order to enhance the antibacterial properties of the as prepared capsules, the calcium alginate shell was functionalized by Ag nanoparticles. Calcium alginate-octyl laurate microcapsules possessed high latent heat of fusion values (130.8 and 128.6 J g−1 on melting and cooling, respectively) which did not significantly change when Ag nanoparticles were entrapped in the shell (127.5 and 125.2 J g−1 for melting and freezing enthalpy changes). Based on these values 71.0 and 69.0% maximal PCM content in the microcapsules were determined by the differential scanning calorimetry method. Both of the Ag-loaded and unloaded calcium alginate-octyl laurate PCM capsules maintained the high heat storing capacity after 250 warming and cooling cycles, which proved they did not suffer from leakage after the accelerated thermal test.

Effect of variable capsule size distribution for unstructured packed bed encapsulated phase change material system

Packed bed encapsulated phase change material (PCM) energy storage systems have considerable potential for energy storage applications involved in renewable energy systems. In this paper, a 3D unstructured packed bed system consisting of spherical encapsulated phase change material with varying sizes of spheres is modelled. The main novelty of the work is that the geometry of the capsules and the flow between the capsules are resolved, even with unequal capsule sizes arranged in an irregular manner. The size of the spheres is varied by changing the radius of the spheres following specified normal distribution curves. Then, by changing the mean and standard deviation values, different size distributions of spheres are generated which are arranged in unstructured packed bed form. The model incorporates the flow of heat transfer fluid (HTF) through the gaps between the capsules, the heat transfer in both the HTF and the PCM capsules, the melting of PCM in each capsule, and the natural convection in the melted PCM. Based on the simulations, important parameters such as the melting time and energy storage rates are compared for different size distributions. It is found that for a normal distribution of capsule radius, increasing the mean radius by 2 times while keeping the standard deviation constant increases the melting rate considerably with an increase in melting time by 29.4%. The effect of standard deviation of radius was found to be significantly less as compared to that of the mean radius.

In-situ copper ion reduction and micro encapsulation of wood-based composite PCM with effective anisotropic thermal conductivity and energy storage

This work develops metallic wood-based phase change materials (MWPCM) with high-performance anisotropic thermal conductivity by impregnating wood with phase change microcapsules and subsequent in situ chemical deposition of copper inside the wood cell lumen. Phase change material (PCM) is encapsulated by polymer to form phase change microcapsules, which solve the leakage problem effectively. Benefited with the well-aligned and hierarchical porous structure of wood, phase change microcapsules coated with copper layer are orderly confined inside wood vessels and fibers, developing a continuous and anisotropic heat transfer network along the highly oriented transport tissues of wood. The morphology, chemical structure and crystallization of microcapsules and MWPCM are investigated using scanning electronic microscope (SEM), flourier transformation infrared spectroscopy (FTIR) and X-ray diffractometer (XRD), and the thermal storage capacity, thermal stability and thermal conductivity of the developed PCM composites are examined using differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and laser flash diffusivity apparatus (LFA) respectively. The results show that PCM capsules were effectively accommodated in orderly porous wood as a carrier; the radial and longitudinal thermal conductivity of MWPCM reached 0.37 W/(m*K) and 0.53 W/(m*K), which increased by 362% and 211% compared to pure wood, respectively. The anisotropic thermal conductivity of MWPCM enabled an efficient heat transfer along the longitudinal direction and reduced the heat loss in the transverse direction. MWPCM exhibited good thermal energy storage capacity (92.9 J/g and 94.6 J/g) and suitable phase change temperature (11.4 °C and 29.4 °C) for indoor thermal energy management. MWPCM also displayed outstanding shape-stability, excellent thermal stability and temperature regulation, which has a great potential for building energy collecting, storage and management.

Numerical investigation of the thermal protection mechanisms of phase change capsules with orderly stacked layouts applied to pulse power sources in deep wells

<p indent="0mm">Currently, the reliance of oil and gas consumption on imported resources in China is indicative of a serious energy supply concern. The mainland of China has been found to contain abundant medium- and low-maturity shale oil resources that represent substantial exploitable reserves of energy supply. The exploitation of medium- and low-maturity shale oil resources, which are comparable to conventional oil and gas resources, can alleviate the problem of energy import dependence. However, low-maturity shale oil resources are mainly enriched in the deep crust, with temperatures reaching 150°C at a 2-km depth. This high-temperature environment might severely limit the operation of electronic equipment. This paper focuses on the thermal protection of pulsed power source equipment in the harsh thermal environment of deep wells. The performance of phase change materials (PCMs) with capsule-like structures having various orderly stacked orientations is numerically investigated. By calculating the inner wall temperature and heat flow, the influence of phase change capsule size and stacked layout on the thermal protection efficiency is analyzed, and the evaluation criteria considering the heat flux density as well as the prediction model for the inner wall temperature are proposed. The results show that the case of phase change microcapsule-glass wool composite structures exhibits improved thermal protection performance, with the inner wall temperature rising rate reduced by 13.8% and 2.35% compared to the case of the glass wool material and microcapsule, respectively. However, the small packed volume of PCMs in the microcapsules restricts further improvement of the thermal insulation. Consequently, PCM capsules of larger size are applied, increasing the PCM proportion and natural formation of closed porous material. Compared to the case of microcapsule-glass wool composite structures, for the case of larger PCM capsules, the temperature at the inner wall is further reduced by 19.3%, and the total heat flow during heating is reduced by <sc>373.7 W/m².</sc> By changing the stacked structure of the capsules, the case of single-layer orientation with large-scale PCM capsules illustrates the best thermal protection performance. The temperature rise rate during the heating period decreases by 1.25% and 5.84% compared to that of the staggered and double stacking structures, respectively, and the total inflow heat flux is also 1.09% and 4.19% lower compared to that of the staggered and double stacking structures, respectively. Based on the results, the prediction model for the inner wall temperature is proposed when applying the single-layer stacked PCM capsules. Note that the average deviation between the prediction model and numerical simulations is &lt; 7%. The maximum temperature difference can be controlled to within ±6°C during 84% of the operating time.

RETRACTED:Economic cost and numerical evaluation of cooling of a cylindrical lithium-ion battery pack using air and phase change materials

In this paper, the thermal management (THM) of a battery pack (BPK) with 9 cylindrical batteries is numerically modeled. The batteries are 18,650 lithium-ion (LIN) type, which are placed in an air duct. A capsule of Phase Change Materials (PCM) is placed around all the LIN battery cells. The PCM used was CaCl2.6H2O, which was oval in the PCM capsule. This study was performed when the entire PCM was molten, and the air stream was trying to solidify it. At all times, the LIN battery is working and generating heat. By changing the horizontal distance of the battery cells from each other, this study has been done for different times of battery operation. An economic analysis has also been performed on the cost of electricity consumed to generate airflow in the battery pack for different battery distances. For numerical modeling, multi-physics COMSOL software has been used. The enthalpy method was used to simulate the PCM freezing front. Examination of the PCM solidification front showed that the PCM solidification was formed from the air inlet side. The highest amount of solid PCM occurred at fixed times for the first column of the LIN battery, while the last column batteries had the lowest amount of frozen PCM. Increasing the distance of the LIN battery cells horizontally from each other causes the amount of PCM temperature (TPCM) and LIN battery temperature (TBat) to decrease. At 120 min and at a horizontal distance of 8 cm from each other, the amount of molten PCM is 22.98% and at a distance of 11 cm the amount of molten PCM It has reached 8.73%. Changing the distance between the batteries may thus affect the quantity of molten PCM by 14.25% at this time. It was also found that increasing the distance between the batteries reduces the cost of electricity used to solidify the PCM.

Apparent heat capacity method to describe the thermal performances of a latent thermal storage system during discharge period

In this work, a novel detailed dynamic model for latent cold thermal storage unit containing water as phase change material (PCM) is presented. The proposed dynamic model is based on transient energy balances for both PCM capsules and heat transfer fluid (HTF). The apparent heat capacity approach is used to model phase transition processes in PCM, where the PCM heat capacity is given as a temperature-dependent function. This method can be simply integrated into a variety of software to assess the energy efficiency of latent storage. The dynamic model of the PCM storage unit is validated using measured results from the literature and it is used to investigate the influence of multiple parameters on the storage thermal performance. The obtained result show that the PCM melting process varies in both axial direction and radial direction in the storage unit. It was also observed that the higher the height of the tank, the longer the discharge period. Increasing the diameter of capsules from 47 mm to 77 mm leads to increase the discharging period by about 30%. In addition, varying the storage tank initial temperature between −8 °C and −4 °C does not have a significant effect of on the discharging time. Finally, it was recommended that for improving the storage performance and minimizing water pumper energy consumption, a compromise between the HTF flow rate and the capsule diameter must be determined.

Application of Monodisperse Encapsulated Phase Change Materials in Building Thermal Energy Storage

Monodisperse encapsulated phase change materials (PCMs) are fabricated via microfluidic technology. To evaluate the thermoregulation ability of PCM capsules applied in building thermal energy storage, a gypsum model house with PCM capsules embedded is prepared. The temperature revolutions outside and inside the gypsum model house with different PCM capsules filling ratios are investigated. The effect of the filling ratio of the PCM on the thermoregulation performance is discussed. The potential application of monodisperse encapsulated PCMs in building thermal energy storage is verified. Attributing to the PCM capsules, the thermal response of the gypsum model house to the varying environmental temperature is retarded. As the filling ratios increase, the thermoregulation performance of the gypsum model house is better.

Biomimetically calabash-inspired phase change material capsule: Experimental and numerical analysis on thermal performance and flow characteristics

The propositions of bionics have led to the evolution of engineering technologies, such as solar energy utilization and thermal energy storage. Inspired by the idea of the natural calabash structure concept, the present study designed and proposed the bionic-calabash-shaped capsule to enhance the thermal performance of phase change material (PCM) capsules used in thermal energy storage (TES). The melting properties and flow characteristics of three capsules with different shapes were investigated numerically and experimentally. A visualized experimental system was established to observe the melting front of the bionic-calabash-shaped capsule during the constrained melting process. Experimental and numerical results indicated that the throat of bionic-calabash encapsulation accelerated the constrained melting process, and the constrained melting time of bionic-calabash-shaped capsules was reduced by 18.2% compared to that of the sphere capsule. The drag coefficient of the novel structure capsule was 34.2% lower than that of the spherical capsule. The bionic-calabash-shaped capsule, with geometric parameters of L/R = 1.53 and L/r = 2.53, was inferred to be the optimal structure for heat storage based on the optimization analysis of the thermal behavior and flow peculiarities.

Effect of steel-wires and paraffin composite phase change materials on the heat exchange and exergetic performance of salt gradient solar pond

The addition of Phase Change Materials (PCM) to the salt gradient solar pond can significantly increase the heat capacity of the solar pond. Paraffin wax has a suitable range of phase transitions temperature and is feasible to be applied in solar pond. However, the poor thermal conductivity of paraffin is detrimental to thermal storage and release. In this study, the heat transfer behavior for the composite PCM of paraffin and steel wires has been studied firstly, the results shows that adding steel wires in paraffin improve the heat transfer performance obviously. With 10wt% adding percent, the maximum of heat transfer coefficient is 18.17 W/m2 °C−1 with increment of 18.84 % in melting process; and the maximum is 21.59 W/m2 °C−1 with increment of 20.88% during freezing process. Secondly, experiment study has been conducted in an experimental solar pond, with 20 PCM capsule tubes added, adding RT 50 and RT 60 made the day–night temperature differences 2.87 °C and 2.53 °C smaller than that of without any PCM conditions. In this study, adding RT 50 and RT 60 in solar pond achieve 9.34% and 14.13% exergy increments respectively. The study suggested that adding PCM to solar pond improves the quality of the heat energy of solar pond, and further extends the heat application scope of solar pond.

Modeling and optimization of a thermal energy storage unit with cascaded PCM capsules in connection to a solar collector

In the current study, a 2-D concentric dispersion model with apparent heat capacity method was developed in order to investigate the thermal reactions of a solar system with thermal energy storage. The system is consisted of a phase change material (PCM) or sensible, encapsulated spherical capsules and a synthetic oil which was operated as the heat transfer fluid (HTF). In order to resolve the low thermal conductivity of the PCM for the energy storage, increasing the charging and also storage efficiencies of the system, an optimization was conducted using the Genetic Algorithm (GA) with a novel approach of combination possibility of sensible-latent materials as the optimal configuration with two different scenarios (scenario 1: outlet temperature of the unit with no temperature limitation, scenario 2: outlet temperature of the unit with threshold constraint). Results showed that by 5% increase (decrease) in the input HTF temperature, the charging time is lowered (increased) the by 25% (32%) and also the total stored energy was increased by 20% (decreased by 17%). Also, 50% increase (decrease) of the HTF flow rate results in 12% increase (25% decrease) in the charging time. 4940 and 4725 kWh of energy is stored by the optimal configuration during the charge for the 1st and 2nd optimization scenarios respectively which is at least 14% higher compared to the single PCM/sensible studied cases.

Experimental investigation on performance improvement of latent heat storage capsule by oscillating movement

Benefiting from the high availability of various phase-change materials (PCMs), latent heat storage is becoming one of the most important ways in the tune of energy production and demand. However, the charge and discharge power of latent energy stored in PCMs is impeded because of the low heat conductivity of the solid phase. Based on the principle that the relative motion of phase interface can enhance the heat transfer, this study presents a new way of enhancing heat transfer inside the PCM capsule by moving the capsule in an oscillating way. A crank and rocker reciprocating device was built to generate the vertical capsule oscillation, by which a stroke distance between 12 and 25 mm and an oscillation frequency between 1 and 3.7 Hz can be achieved. The capsule was made with hexadecane as the PCMs and brass as the shell. It is found that both the increase of the stroke distance and the increase of the frequency produce positive effects on the heat charge/discharge process. In the case of stroke distance of 25 mm and frequency of 3.7 Hz, the charge duration can be reduced from 36 mins to 26 mins and the discharge duration can be reduced from 153 mins to 127 mins. By analyzing the temperature variations inside the capsule, it is found that the melting and solidifying heat transfer coefficients could be improved maximally by 50% and 94%, respectively.

Experimental and numerical study of combining encapsulated phase change material to sensible heat storage material in one-tank pilot scale thermal energy storage

This work presents the design, experimental and numerical results related to a pilot-scale one-tank (thermocline) thermal energy storage (TES) combining latent and sensible heat storage materials with synthetic oil as heat transfer fluid (HTF). The layer of phase change material (PCM) is used to enhance the thermal performance of the TES. Alumina spheres are used as sensible heat storage materials, while the PCM is NaNO3 (sodium nitrate). The PCM is encapsulated inside 140 stainless steel tubes. The volume of the PCM represents 5.5% of the storage volume. NaNO3 was found safe to use with the synthetic oil at the designated operating conditions in case of leaks in the tank. The influence of the phase change material on HTF temperatures were observed experimentally during charge, discharge and stand-by. A numerical model that couples two one-dimensional (1D) physical methods is developed. It simulates natural convection within the PCM capsules by modifying the thermal conductivity of the material. The enthalpy porosity method is applied to simulate the phase changing behavior and a single equation estimates the liquid fraction of the PCM at each time step. The model is validated from the experimental results. This validation reveals that there is still a high fraction of unsolidified PCM in the tubes during discharge, which indicates that the performance of the combined TES solution is limited by heat transfer within the encapsulation tubes.