Latent Thermal Energy Storage Research Articles

Numerical study of ice freezing process on fin aided thermal energy storage system

The present numerical study is based on the solidification process of water when used as a phase change material (PCM) in a latent thermal energy storage system (LHTESS). LHTESS can balance varying energy demand and supply by storing surplus energy during off-peak hours. The present study investigated a tube-in-tank storage system where the refrigerant flows inside the tube and PCM is outside the tube. Enhancement techniques like fins on the HTF tube's surface enhance the effective heat transfer rate on the PCM side. A two-dimensional transient numerical study has been carried out to optimize the number of longitudinal fins for a possible maximum decrease in solidification time, after which increasing the number of fins does not alter the rate of solidification. Moreover, a parametric study has been done to see the effect of varying temperatures of the refrigerant, tube diameter, and orientation of the HTF tubes on the phase change process. A comparison for solidifying stored PCM by applying different parametric conditions has been carried out in terms of heat transfer rate and heat transfer coefficient. Numerical results obtained indicate that 15 fins are the optimum case for achieving the balance between the solidification rate and the stored PCM mass. A maximum overall increment in the heat transfer rate is achieved around 20% more with the optimum number of fins emphasizing its vital significance over no fin case with 67.38% and 30.25% more production of ice at 500 s and 1500s, respectively. However, a least enhancement of nearly 6% is seen in the rate on changing the tube orientation from inline to staggered for given number of the tubes. In addition, a lower tube wall temperature of 257 K is found to be 89% faster than the higher temperature of 265 K in shortening the required solidification time, therefore quicker freezing of the domain is obtained.

Numerical and Experimental Investigation of Wire Cloth Heat Exchanger for Latent Heat Storages

Latent thermal energy storages (LTES) offer a high storage density within a narrow temperature range. Due to the typically low thermal conductivity of the applied phase change materials (PCM), the power of the storages is limited. To increase the power, an efficient heat exchanger with a large heat transfer surface and a higher thermal conductivity is needed. In this article, planar wire cloth heat exchangers are investigated to obtain these properties. They investigated the first time for LTES. Therefore, we developed a finite element method (FEM) model of the heat exchanger and validated it against the experimental characterization of a prototype LTES. As PCM, the commercially available paraffin RT35HC is used. The performance of the wire cloth is compared to tube bundle heat exchanger by a parametric study. The tube diameter, tube distance, wire diameter and heat exchanger distance were varied. In addition, aluminum and stainless steel were investigated as materials for the heat exchanger. In total, 654 variants were simulated. Compared to tube bundle heat exchanger with equal tube arrangement, the wire cloth can increase the mean thermal power by a factor of 4.20 but can also reduce the storage capacity by a minimum factor of 0.85. A Pareto frontier analysis shows that for a free arrangement of parallel tubes, the tube bundle and wire cloth heat exchanger reach similar performance and storage capacities.

A New Methodological Approach for the Evaluation of Scaling Up a Latent Storage Module for Integration in Heat Pumps

A clear gap was identified in the literature regarding the in-depth evaluation of scaling up thermal energy storage components. To cover such a gap, a new methodological approach was developed and applied to a novel latent thermal energy storage module. The purpose of this paper is to identify some key aspects to be considered when scaling up the module from lab-scale to full-scale using different performance indicators calculated in both charge and discharge. Different normalization methods were applied to allow an appropriate comparison of the results at both scales. As a result of the scaling up, the theoretical energy storage capacity increases by 52% and 145%, the average charging power increases by 21% and 94%, while the average discharging power decreases by 16% but increases by 36% when mass and volume normalization methods are used, respectively. When normalization by the surface area of heat transfer is used, all of the above performance indicators decrease, especially the average discharging power, which decreases by 49%. Moreover, energy performance in charge and discharge decreases by 17% and 15%, respectively. However, efficiencies related to charging, discharging, and round-trip processes are practically not affected by the scaling up.

A strategy for enhancing heat transfer in phase change material-based latent thermal energy storage unit via nano-oxides addition: A study applied to a shell-and-tube heat exchanger

Due to the scarcity of data on the industrial use of energy storage technology based on material phase change (PCM), a complete computational assessment is done in this work, where a nano-PCM technique is used to enhance the thermal energy storage in a big-scale shell-and-tube heat exchanger. Four high thermal conductive nano-oxides (i.e., Al2O3, MgO, SiO2, and SnO2) are added at various concentrations (1–5% v/v) into the PCM (i.e., paraffin RT82). A two-dimensional mathematical model is used to study the produced nano-PCM systems' heat transfer and melting rate. The model accurately predicted the PCM-melting data's observed behavior, showing that it is adequate for simulating PCM problems. The addition of nano-oxides, for up to 5%, in the PCM solid matrix dramatically enhances heat transmission and melting rate during the first melting stage. However, after a long melting period, the melting performance of the nano-PCMs decreased and became equivalent to that of the pure PCM.Furthermore, increasing the concentration of nanoparticles from 1 to 5% did not significantly improve the temperature and liquid fraction evolutions for the four studied nano-PCMs. Therefore, the use of nano-PCMs in the adopted LTES unit can effectively improve heat recuperation and melting rate, but only for a short charging time during which a part (up to 60%) of the PCM's latent heat is recovered. Consequently, the present paper's data is considered a guideline for increasing the efficacy of thermal energy storage in big-scale shell-and-tube heat exchangers using nano-PCM technology.

In situ latent thermal energy storage in underfloor heating system of building connected to the parabolic trough solar collector-an experimental study

High energy demand, and excessive consumption of natural resources, have become one of the most critical global challenges. The building sector is one of the most energy consumers, and it almost consumes one-third of final energy worldwide. Hence, it is one of the most significant CO2 emission sectors. Utilization the renewable energy in building such as solar energy connected with latent thermal energy storage (LTES) using phase change materials (PCM) is a promising solution to reduce conventional energy consumption and diminish greenhouse gas emissions. Separately, a proper heating system selection preparing a comfortable heating condition is a priority. Underfloor heating systems (UFHSs) can maintain indoor air temperature more comfortably than other heating systems. This study used parabolic trough solar collector (PTSC) as a heating source for two test cubicles equipped with underfloor heating systems. In situ latent energy storage using hydrate salt with 32 °C melting temperature is used in one test cubicle and it compared with the typical underfloor heating system embedding pipes under sand in the other same cubicle. Results revealed that the latent thermal energy stored in PCM can maintain the indoor air temperature of the cubicle in comfortable condition for more than 4 h after sunset. Furthermore, PTSC provides more than 80% of cubicles heating loads during the coldest time in Tehran, Iran.

Preliminary thermal characterisation of an active latent thermal energy storage system using PCM

Preliminary results of energy charging/discharging processes in a latent thermal energy storage system are reported. A novel design of a rotative scrapper heat exchanger has been studied. Paraffin RT44HC is employed as a phase change material. A Coriolis flowmeter is employed for measuring the mass flow through the prototype, and PT100 temperature sensors are used for measuring the inlet and exit temperature of the heat transfer fluid.

Effects evaluation of fin layouts on charging performance of shell-and-tube LTES under fluctuating heat sources

The previous studies focus on the heat storage process of shell-and-tube latent thermal energy storage (LTES) under steady heat sources, without considering the fluctuating and intermittent characteristics of the most available heat sources in real applications. The effect of different fin sizes and locations on the charging performance of shell-and-tube LTES still keeps unclear under fluctuating heat source, and it is critical to improve the heat storage rate and density due to the low thermal conductivity of phase change materials (PCM). Consequently, this study aims to investigate the charging performance of a shell-and-tube LTES considering different fin layouts. Firstly, the effects of different fin number and configurations on heat storage process are performed under fluctuating heat sources. The case with comprehensive performance of high melting rate and large heat storage capacity is obtained. Then the effects of critical parameters on the charging process, including the fin angle, fin thickness, fin height and deviation distance of tube under fluctuating heat sources are further analyzed based on this case, and the results are compared with that of steady heat source. Results indicate that the fin height is the dominant factor influencing the heat storage rate and density under both steady and fluctuating heat source conditions, while the total melting time is shorter and the heat storage capacity is smaller, and the heat storage capacity is less obvious with the variation of fin parameters under the fluctuating heat source. In addition, the effect of downward deviation distance of tube on heat storage capacity is much obvious under fluctuating heat source than that of steady heat source.

Exploration on two-stage latent thermal energy storage for heat recovery in cryogenic air separation purification system

Massive heat in the exhaust air of cryogenic air separation purification system is wasted in conventional systems. Its intermittent and dynamic characteristics are major constraints to achieve heat recovery. Cascaded latent thermal energy storage (LTES) is considered as a promising solution, but actual application is rarely reported. This paper initially investigates a two-stage LTES tank which is incorporated with a pilot plant of cryogenic air separation purification system, and a test under a real operation condition is performed. The tank contains five heat storage units in high-temperature stage and five units in low-temperature stage, which are loaded with commercial paraffin of melting temperatures 70 °C and 48 °C, respectively. Moreover, heat pipes combined with vertical fins are assembled to increase the heat transfer rate. Results show that 51369.5 kJ heat is saved in a charging-discharging cycle by introducing LTES tank to air separation system. 64.7% of waste heat in the exhaust air could be recovered based on waste heat criterion of 40 °C. Also worth noting that payback period of LTES tank is 2.3 years, and 25784.3 kg CO2 emission is reduced per year. The LTES demonstrates good performance when compared with other reported LTES for industrial application.

A Fast-Reduced Model for an Innovative Latent Thermal Energy Storage for Direct Integration in Heat Pumps

In the present paper, the numerical modeling of an innovative latent thermal energy storage unit, suitable for direct integration into the condenser or evaporator of a heat pump is presented. The Modelica language, in the Dymola environment, and TIL libraries were used for the development of a modular model, which is easily re-usable and adaptable to different configurations. Validation of the model was carried out using experimental data under different operating modes and it was subsequently used for the optimization of a design for charging and discharge. In particular, since the storage unit is made up of parallel channels for the heat transfer fluid, refrigerant, and phase change material, their number and distribution were changed to evaluate the effect on heat transfer performance.

Dominant roles of eccentricity, fin design, and nanoparticles in performance enhancement of latent thermal energy storage unit

In this paper, numerical and experimental investigations of thermal performance enhancement of horizontal longitudinal shell and tube latent heat thermal energy storage unit (LHTESU) via eccentricity (e), fin design optimization, and nanoparticles in the phase change material (PCM) of stearic acid are presented. An enthalpy-porosity-based two-dimensional, transient numerical methodology is used after validating against the experimental results and the literature. Five different eccentric positions, e = 0.14, 0.28, 0.42, 0.56, and 0.63 are investigated by modifying the design of Y-finned tube. The eccentricity and fin design modifications improved natural convection effects as compared to concentric Y-finned tube. Thermal performances of LHTESU are analyzed on the basis of melting time, heat storage capacity, heat storage rate, and performance enhancement ratio. Based on the performance analysis, an optimum eccentric configuration of LHTESU is proposed. For optimized eccentric unit (e=0.42), a reduction of 34.14% in melting time along with 30.7% improvement in thermal energy storage rate is achieved as compared to concentric (e=0) LHTESU. The effect of temperature of the tube on PCM's melting time and heat transfer is also analyzed and two important correlations are proposed. The thermal performance of the optimized eccentric unit is further enhanced by adding nanoparticles of Al2O3 and CuO ranging from 0.5% to 10% by volume in the pure PCM. The nano-enhanced PCM with 1% of Al2O3 further improves melting and energy storage rates of optimum LHTESU by 10%.

Effect of orientation of elliptic tube on the total melting time of latent thermal energy storage systems

Encapsulation of Phase Change Materials (PCM) for energy storage, thermal comfort and many other energy applications is receiving much attention due to the fact that material, physical characteristics and geometry of the container can affect drastically the thermal performance of the PCM. Phase change materials have usually low thermal conductivity which impairs their thermal charging and discharging characteristics. Different geometries were investigated including rectangular, cylindrical and spherical with and without extended surfaces to investigate the heat charge processes. Cylindrical geometries of circular sections were intensively investigated while cylinders and tubes with elliptic and elongated cross section received less attention, although they may have better thermal performance for thermal storage. The present numerical investigation is aimed at contributing to better understand the effects of the elliptic geometry and how the different geometrical and operational parameters can affect the thermal performance of the enclosed PCM. The present investigation reports the results of a numerical study on elliptic cylinders containing PCM under melting conditions. The 2D inward melting problem is modeled by using a CFD code. The numerical model is based upon the enthalpy-porosity method along with the finite control volume techniques. The numerical predictions are validated against available experimental results. The inward melting process is analyzed for two orientations of the elliptic enclosures. Due to the flow field effect namely the Rayleigh-Bénard convection, the numerical results showed that the horizontal elliptic enclosure have higher melting rate and hence lower total melting time compared to those of the vertical elliptic enclosure.

Construction strategies and thermal energy storage applications of shape‐stabilized phase change materials

AbstractPhase change materials (PCMs) are usually used in latent thermal energy storage systems to address the mismatch between heat energy supply and demand, due to their high energy storage density, high‐latent heat, and almost constant temperature during phase change process. However, the low‐thermal conductivity of PCMs and severe leakage during phase change limit their practical applications. Thus, extensive efforts have been devoted to constructing shape‐stabilized PCMs (SSPCMs). In this review, recent advances of the construction strategies and thermal energy storage applications of SSPCMs are summarized. Especially, the microencapsulated PCMs, SSPCMs based on porous scaffolds, as well as their potential applications are highlighted. Finally, the future perspectives and remaining challenges in this research field are prospected.

Computational analysis of the melting process of Phase change material-metal foam-based latent thermal energy storage unit: The heat exchanger configuration

In the current study, the melting process of phase change material (PCM) embedded with nanoparticles and metal foams (MF) in a large scale shell-and-tube based latent thermal energy storage unit (LTES) has been numerically studied. Initially, the developed model was validated with experimental data. Then, the effect of Al2O3 (5%) nanoparticles addition and several MFs, i.e. Aluminum (Al), copper (Cu), Nickel (Ni) and Titanium (Ti), of different porosity (96–99%) on the performance of the melting process has been compared based on temperature and liquid fraction evaluations during the melting operation. The MF-PCM systems showed the best performance over the nano- and pure- PCM, with a huge reduction in the melting time. The beneficial effect of MFs for the melting process followed the order Cu > Al > Ni > Ti, which is the same order of the thermal conductivity. Increasing the metal foam porosity results in shorter melting time; however, since the surface temperature of the porous-PCM unit is almost constant for different metal foam porosities, a system with higher porosity (99%) is desirable. This study optimizes the design to enhance practical application performance and to reduce waste energy.

Li2CO3 as Protection for a High-Temperature Thermoelectric Generator: Thermal Stability and Corrosion Analysis

In most steelmaking processes, huge amounts of waste heat at high temperature (700–800 °C) are thrown into the environment without any use. An alternative use for this waste heat is electricity generation through thermoelectric generators. However, these high temperatures, as well as their fluctuations over time, affect not only the conversion rate of the thermoelectric generator but also its useful lifetime. The incorporation of a latent thermal energy storage (TES) system could be a solution; nevertheless, the thermal stability and corrosive effect of the (PCM) phase change material are key aspects for the thermal storage system definition, in terms of durability. In this work, developed in the framework of the European project “PowGETEG” (RFSR-CT-2015-00028, funded by the Research Fund for Coal and Steel), a high-temperature analysis (700–800 °C) of the Li2CO3 thermal properties, thermal stability and corrosive effect on the AISI 304 and AISI 310 stainless steels is carried out. The results show that the eutectic salt Li2CO3 exhibits high thermal stability with neither change in its thermal properties nor material degradation. This work shows that lithium carbonate Li2CO3 and AISI 310 make a very good combination for the definition of a thermal storage system able to protect a high-temperature thermoelectric converter from temperature variations, making it more reliable.

Performance enhancement of double-wall-heated rectangular latent thermal energy storage unit through effective design of fins

This study aims to numerically investigate the influence of different designs of fins on the performance enhancement of a double-wall-heated rectangular thermal energy storage unit (TESU). Stearic acid is used as the phase change material (PCM) which is enclosed in a rectangular enclosure with two vertical heated isothermal walls and two adiabatic horizontal walls. The rectangular horizontal fins assist the heated walls during the melting process of the PCM. The design parameter variations of fins include angles (A), shapes (S), lengths (L) while maintaining constant PCM volume. The base case is modeled by embedding heated six horizontal fins, three on each of the heated walls. configuration-A, which incorporates three variations of fin angles, significantly increases the melting time of PCM by 16% and reduces the averaged energy storage rate by 14.2%. Whereas, the combination of straight and angled type hybrid fins of configuration-S show appreciable improvement in thermal performance of TESU by enhancing the melting rate by 18% and average energy storage rate by 19.8%. The configuration-L, encompassing combinations of straight fins with different lengths and thicknesses is found to be most effective of all. The optimum case of configuration-L augments the melting performance of the TESU by 39.5% and the average energy storage rate by 65.07%. Moreover, for optimum configuration, two new correlations of melting Fourier number and average Nusselt number as a function of Stefan number and Rayleigh number, respectively, are proposed for a wide range of wall temperatures.

Effect of Nanomaterial Inclusion in Phase Change Materials for Improving the Thermal Performance of Heat Storage: A Review

Dispersion of nanoparticles is one of the potential solutions to improve the thermophysical properties of phase change (or transition) materials (PCMs) and enhance the performance of latent thermal energy storage (LTES) systems. The PCM ought to have a high latent heat of fusion, and zero or negligible coefficient of thermal expansion. A good PCM should have melting and solidification compatibility with negligible or zero subcooling, and it should not react with the common chemical reagents. The present known PCMs possess low thermal conductivity that results into a longer solidification and melting time of PCMs. In the past two decades, researchers have reported improved thermal conductivity and heat-storing capacity of PCMs employing graphite nanoparticles/fibers, carbon nanotubes/fibers, metal, and metal oxide nanoparticles. This work reviews the reported experimental and numerical studies describing the consequences of nanoparticle inclusions of various shapes and sizes on the thermal properties of the PCMs. This review attempts to make a consolidated database of the studies related to nanoadditive inclusion into PCMs for various applications. Graphene dispersed into PCM has resulted into 14 times thermal conductivity enhancement. As far as metal oxide nanoparticles are concerned, TiO2 and Al2O3 nanoparticles outperformed others. The compatibility between the nanoadditive and PCM is necessary to tailor favorable thermal properties. This work reviews numerous studies of different nanoparticle–PCM duos.

Numerical study and multilayer perceptron-based prediction of melting process in the latent heat thermal energy storage system with a finned elliptical inner cylinder

This study numerically investigated the melting phenomena of a phase change material (PCM) in a latent heat thermal storage system with a finned elliptical inner cylinder. The enthalpy–porosity model was used to simulate the phase change process of the PCM. A parametric study was performed by varying the length of the fin in the range of 5, 6.5, and 8 mm and at temperatures of 56.15, 69.16, and 92.22°C, considering three different geometric types. The distributions of the liquid fraction, temperature, and velocity vector were numerically investigated to analyze the effects of these variables on the melting performance of the latent thermal energy storage system. The results indicate that the heat and mass transfer characteristics can be significantly affected by changes in the geometric types, length of the fin, and temperature of the inner cylinder. Based on the results from the numerical simulations, the multilayer perceptron neural network model was trained to estimate the melting time of the PCM with respect to the changes in the main variables considered. It was found that a multi-layer perceptron neural network model can estimate the melting time more accurately than the least square regression method.

Micro Encapsulated Phase Change Material for the Application in Thermal Energy Storage

Abstract The pursuit of CO2 reduction targets has increased the need of storage capacities for renewable energy or thermal energy to enhance the efficiency of industrial processes. To combine the benefits of latent and sensible thermal energy storage systems, the concept of micro encapsulated phase change material is presented. The microparticles are designed to work in a high-temperature thermal energy storage system considering economic and technical points of view. Therefore, particles of sodium nitrate are physio-mechanical coated with polyimide by using spray-drying in a fluidized bed reactor. To evaluate the influence of the process conditions, several coating experiments with different process settings are performed. Afterward, the samples are analyzed to determine their properties. Besides a microscopic and a sieving analysis, they are tested in the laboratory to define their mechanical and thermal limits. Finally, a rough layout of a thermal energy storage system using the produced particles is presented and compared to a common sensible thermal energy storage.

Influence of natural convection during melting and solidification of paraffin in a longitudinally finned shell-and-tube latent thermal energy storage on the applicability of developed numerical models

In this paper, melting and solidification processes in the shell-and-tube type latent thermal energy storage (LTES) with longitudinal fins have been numerically studied in order to determine an accurate and time-effective model for describing charging and discharging processes. Two mathematical models, one including natural convection in the liquid phase of the phase change material (PCM) and one neglecting it, have been developed. They both describe the transient three-dimensional problem of fluid flow and phase change heat transfer in the LTES. The computations, carried out using ANSYS Fluent 18.2 software, are based on an iterative, finite volume numerical procedure that incorporates enthalpy formulation for simulation of the phase change phenomenon. Results from both numerical models have been compared with data obtained by experimental investigations during melting and solidification, performed on the test LTES tank with paraffin RT 25 as the PCM and water as the heat transfer fluid (HTF). Numerical results of the model with natural convection have shown good agreement with experimental measurements, which indicates that the models could be used to accurately simulate phase change problems, while results obtained by model without natural convection have shown good agreement for solidification processes. Since numerical results obtained by both models are similar for solidification and significantly different for melting, it can be concluded that the model neglecting natural convection, with notably shorter calculation time, could be used for solidification modeling, while the model including natural convection in the liquid phase of the PCM should be used for melting modeling.

Numerical Analysis of Melting Process in a Rectangular Enclosure with Different Fin Locations

Latent thermal energy storage is regarded as an effective strategy to utilize solar energy and recover automotive waste heat. Based upon an enthalpy-porosity method, the influence characteristics and mechanism of fin location on phase change material melting behavior in vertical rectangular enclosures were explored numerically. The results show that as fin location increases, the melting time decreases before attaining the minimum at the fin location of 0.20 after which it increases and finally surpasses the no fin case, because (1) the influence range of fins for conduction is limited by the bottom surface when putting fins next to this surface, (2) the liquid flow resistance increases with moving fins up, and (3) mounting fins near the top surface accelerates melting at the upper part, facilitating thermal stratification formation and weakening natural convection. Nu is higher than the no fin case, i.e., Nu enhancement factor is a positive value, in the melting process for a lower fin location, while for other fin locations, a transition to a negative value takes place. The higher the fin location is, the earlier the transition that arises. Finally, a strategy of increasing the maximum liquid flow velocity is proposed to reinforce melting for cases with considerable natural convection.