Latent Heat Storage System Research Articles

Energy and Exergy Analyses of Simultaneous Charging and Discharging Latent Heat Storage System at Different Inclination Angles—An Experimental Study

Abstract In this study, the effect of inclination on the thermal performance of a shell and tube latent heat storage system (LHSS) is investigated. Due to its practical applicability, a simultaneous charging and discharging (SCD) condition is considered. The SCD process for the LHSS involves the circulation of the hot fluid from one side and at the same time cold fluid from the other side. The inclination angles considered for this study are 0 deg (horizontal), 30 deg, 60 deg, and 90 deg (vertical). The effect of inclination on the temperature distribution at various locations of LHSS, melting behavior, energy stored, exergy efficiency, heat input, and recovery are presented. The experimental results indicate that the orientation of an LHSS has a great impact on the melting behavior and performance of LHSS. The maximum energy stored is obtained for the horizontal (θ = 0 deg) configuration and shows 27.42% higher value than the vertical. This study reveals that during the initial stage, higher melting rate is observed in horizontal configuration due to the efficient buoyancy-induced thermal transport. After attaining the steady-state condition, the energy storage is not significant, and the thermal transport is noted between the hot and cold heat transfer fluids.

Correlating the thermal contact resistance between metal/erythritol interfaces with surface roughness and contact pressure

The thermal contact resistance (TCR) between the interfaces of phase change materials (PCMs) and their encapsulation materials, usually thin-walled metals, have a great impact on the heat storage/retrieval rates of PCM-based latent heat storage (LHS) systems. However, the TCR data beteween metal/PCM interfaces are rarely reported in the literature. In this work, an improved self-designed steady-state apparatus was adopted to measure the TCR between the interfaces of erythritol, a cost-effective sugar alcohol with great potential for medium-temperature LHS, and multiple metals including SS304, nickel silver (a copper-nickel-zinc ternary alloy), and 1060 Al. The metal/erythritol composites were prepared by naturally solidifying molten erythritol on the surface of thin metal plates. The variation of the TCR between metal/erythritol interfaces was measured as a function of the surface roughness (of the metal plates) and the contact pressure applied (between the interfaces). With the decrease of surface roughness, the TCR also decreases due to the better contact between the interfaces as observed by a microscope. Comparing to SS304 and nickel silver, the 1060 Al/erythritol composites exhibit a significant reduction of TCR by ~40% when rising the contact pressure from 0 MPa to 7 MPa, for all surface roughness levels tested. The in situ measured sample thickness variations confirmed that upon pressurization, the softer metal, i.e., 1060 Al, experiences more micro-deformation at the interface. In addition, correlations of the TCR data between metal/erthyritol interfaces were proposed with the surface roughness and contact pressure. The results and correlations attained in this work could serve as a reference for the thermal design of LHS units using erythritol.

Silicon as high-temperature phase change medium for latent heat storage: A thermo-hydraulic study

Latent heat storage (LHS) using high-temperature phase change medium (PCM) can provide cost-competitive solutions for dispatchable solar power and accumulate surplus Photo-voltaic (PV) and wind power. Moreover, at a sufficiently high temperature, the round trip efficiency of LHS system may approach that of electrochemical storage system. A conceptual LHS system utilizing high-temperature silicon as the phase change medium (PCM) is presented in the article. Silicon is chosen due to its high melting point (1414 °C), latent heat (1.8 × 106 J/kg), and thermal conductivity (25–50 W/mK). The thermo-hydraulic behavior during melting of silicon is modeled using a combined numerical methodology in COMSOL Multiphysics. The thermal performance of proposed system is compared against high-temperature salt-based system for different aspect ratios (AR). Results indicate that silicon systems are significantly superior to high-temperature salts under the same operating conditions. The anomalous behavior of silicon melting is established by demonstrating natural convection pattern in molten silicon. A generalized correlation is developed to predict the melting fraction as a function of Rayleigh number, Stefan number, and Fourier number for various domain sizes.

Simulative Investigation of Thermal Capacity Analysis Methods for Metallic Latent Thermal Energy Storage Systems

Latent heat storage systems are a promising technology for storing and providing thermal energy with low volume, mass and cost requirements, especially when operated at high temperatures. Metallic phase change materials are particularly advantageous for high thermal input and output, which is especially important for mobile applications. When designing a storage system, it is essential to have precise knowledge about the potential storage capacity. However, the system’s storage capacity is typically calculated from material properties determined at lab scale, although systemic boundary conditions can have a considerable influence. Systemic influences can result from thermal and reactive interfaces or from the storage design. In order to consider these influences, we propose three calorimetric procedures to thermally analyse high-temperature metallic latent energy storage systems at an application scale. We examined the procedures in a transient simulation environment, monitoring the storage capacity of the system. The procedure, based on adiabatic conditions, shows the least deviation from the simulation input parameters, but is limited to the heating process of the storage. Discharging the storage can be represented by isoperibolic conditions with controlled heat exchange. The precision of the procedures depends on the evaluation routine, the calibration routine, the heat extraction rate and the thermal inertia of the test bench.

Investigation of Sr-based perovskites for redox-type thermochemical energy storage media at medium-high temperature

Thermochemical heat storage system exhibits extraordinary properties compared with sensible or latent heat storage system. Owing to the versatility of perovskite, Sr-based perovskites were investigated for redox-type thermochemical energy storage at medium-high temperature. In this study, SrCoO3-δ, SrFeO3-δ and SrMnO3-δ were prepared via a modified Pechini method. The oxidation enthalpy of each sample was measured using a combination of tubular furnace and DSC. Composition analysis, redox and cyclability tests revealed that SrFeO3-δ possesses favorable behavior validating the design principles.

Heat-Storage Performance Optimization for Packed Bed Using Cascaded PCMs Capsules

The design, in which the capsules are packed in the bed at different sections based on the Phase Change Material (PCM) melting temperature, is an effective method to improve the heat-storage performance of the latent heat energy storage system. A latent heat storage system was established in the present study in order to optimize the arrangement of cascaded PCM capsules. Four kinds of paraffin with different melting temperatures were selected as the PCM for the experiment. The effects of stage number, cascade ratio and melting temperature on the heat-storage performance of the latent heat storage system were investigated during the charging and discharging operations. The complete time of charge and discharge, heat storage-release capacity, energy efficiency, exergy efficiency and entransy dissipation (based on a new entransy theory) were computed for different arrangement cases, in order to analyze and evaluate the heat-storage performance of the system under different stage numbers and cascade ratios of PCMs. The results revealed that the arrangement of cascaded PCM capsules has an obvious effect on the heat-storage performance of the system. When more high melting PCMs are adopted, the heat storage-release capacity, energy efficiency and exergy efficiency increases, while the entransy dissipation decreases. However, the temperature increase becomes slow, and the charging or discharging process becomes long.

THE EFFECT OF EXPANDED GRAPHITE ON THE CHARGING TIME OF STEARIC ACID PHASE CHANGE MATERIAL IN A HOT WATER TANK

The present work is to investigate the thermal conductivity of the phase change material (PCM) of stearic acid (SA) by using the supporting material of expanded graphite (EG). The EG is used with the mass ratio of 10%, 15% and 20%. SA is commercial grade and the EG is formed by thermal method using natural graphite powder and crystalline zinc nitrate. The composite materials of (SA/10% EG, SA/15% EG and SA/20% EG) are prepared by direct impregnation method. Thermal conductivity of pure SA and composite PCMs are investigated by laboratory apparatus of heat transfer base unit TD1002. The thermal conductivity of the SA/10%EG composite PCM has been improved to 0.64 W/mK from that of SA (0.28 W/mK) with the percentage enhancement of 129%. To verify the morphology of the materials, scanning electron microscopy (SEM) analysis is utilized. The results show that the EG is formed in multilayers with pores in which stearic acid is well absorbed. The charging characteristics of pure SA, SA/10% EG and SA/15% EG for low temperature latent heat storage (LHS) system are studied experimentally. The system is applied for domestic water heating. The PCM is filled in a 33 mm diameter cylindrical copper tube and placed in hot water storage tank. The water is filled in tank from the top section through the distributor to uniform entry. The phase change phenomena of the PCMs are measured by inserting thermocouples at two layers of PCM tube. The result shows that the melting time decreases with the increasing mass ratio of expanded graphite in composite PCM. Although the melting time of both layers are nearly the same at the experiment of composite PCMs, the lower layer takes longer time than upper layer for pure PCM. The charging time is decreased by 67% for SA/10%EG and 79% for SA/15% EG composite PCM from that value of pure SA.

Thermal Energy Storage by the Encapsulation of Phase Change Materials in Building Elements-A Review.

The energy sector is one of the fields of interest for different nations around the world. Due to the current fossil fuel crisis, the scientific community develops new energy-saving experiences to address this concern. Buildings are one of the elements of higher energy consumption, so the generation of knowledge and technological development may offer solutions to this energy demand, which are more than welcome. Phase change materials (PCMs) included in building elements such as wall panels, blocks, panels or coatings, for heating and cooling applications have been shown, when heating, to increase the heat storage capacity by absorbing heat as latent heat. Therefore, the use of latent heat storage systems using phase change materials (PCMs) has been investigated within the last two decades. In the present review, the macro and micro encapsulation methods for construction materials are reviewed, the former being the most viable method of inclusion of PCMs in construction elements. In addition, based on the analysis of the existing papers on the encapsulation process of PCMs, the importance to pay more attention to the bio-based PCMs is shown, since more research is needed to process such PCMs. To determine its thermophysical and mechanical behavior at the micro and macro levels, in order to see the feasibility of substituting petroleum-based PCMs with a more environmentally friendly bio-based one, a section devoted to the excellent PCM with lightweight aggregate (PCM-LWA concrete) is presented due to the lack of description given in other reviews.

Effect of Twisted Fin Array in a Triple-Tube Latent Heat Storage System during the Charging Mode

This study aims to assess the effect of adding twisted fins in a triple-tube heat exchanger used for latent heat storage compared with using straight fins and no fins. In the proposed heat exchanger, phase change material (PCM) is placed between the middle annulus while hot water is passed in the inner tube and outer annulus in a counter-current direction, as a superior method to melt the PCM and store the thermal energy. The behavior of the system was assessed regarding the liquid fraction and temperature distributions as well as charging time and energy storage rate. The results indicate the advantages of adding twisted fins compared with those of using straight fins. The effect of several twisted fins was also studied to discover its effectiveness on the melting rate. The results demonstrate that deployment of four twisted fins reduced the melting time by 18% compared with using the same number of straight fins, and 25% compared with the no-fins case considering a similar PCM mass. Moreover, the melting time for the case of using four straight fins was 8.3% lower than that compared with the no-fins case. By raising the fins’ number from two to four and six, the heat storage rate rose 14.2% and 25.4%, respectively. This study presents the effects of novel configurations of fins in PCM-based thermal energy storage to deliver innovative products toward commercialization, which can be manufactured with additive manufacturing.

Characterization of size effect of natural convection in melting process of phase change material in square cavity* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51908197 and 12072107), the Tackle Key Problems in Science and Technology Project of Henan Province, China (Grant No. 202102310262), the Program for Innovative Research Team of Science & Technology of Henan

The accelerating effect of natural convection on the melting of phase change material (PCM) has been extensively demonstrated. However, such an influence is directly dependent on the size and shape of domain in which phase change happens, and how to quantitatively describe such an influence is still challenging. On the other hand, the simulation of natural convection process is considerably difficult, involving complex fluid flow in a region changing with time, and is typically not operable in practice. To overcome these obstacles, the present study aims to quantitatively investigate the size effect of natural convection in the melting process of PCM paraffin filled in a square latent heat storage system through experiment and simulation, and ultimately a correlation equation to represent its contribution is proposed. Firstly, the paraffin melting experiment is conducted to validate the two-dimensional finite element model based on the enthalpy method. Subsequently, a comprehensive investigation is performed numerically for various domain sizes. The results show that the melting behavior of paraffin is dominated by the thermal convection. When the melting time exceeds 50 s, a whirlpoor flow caused by natural convection appears in the upper liquid phase region close to the heating wall, and then its influencing range gradually increases to accelerate the melting of paraffin. However, its intensity gradually decreases as the distance between the melting front and the heating wall increases. Besides, it is found that the correlation between the total melting time and the domain size approximately exhibits a power law. When the domain size is less than 2 mm, the accelerating effect of natural convection becomes very weak and can be ignored in practice. Moreover, in order to simplify the complex calculation of natural convection, the equivalent thermal conductivity concept is proposed to include the contribution of natural convection to the total melting time, and an empirical correlation is given for engineering applications.

Solar Cell Cooling with Phase Change Material (PCM) for Enhanced Efficiency: A Review

This literature aimed to explain recent studies related to the passive cooling of solar cells using Phase Change Material (PCM). Cooling is done to reduce operating temperature and to prevent a decrease in efficiency in an unfavorable environment because the efficiency of the solar cell system decreases when the operating temperature rises and can damage the PV module. The successful use of phase change materials (PCM) from latent heat storage systems is highly dependent on the thermal reliability and stability of the phase change materials used. In conclusion, the overall energy utilization ratio of cooled PV using a PCM system can be increased through a thermal regulation strategy, further research of this system is still needed to achieve maximum results in PV efficiency.

Latent heat energy storage system using phase change materials and techniques for their performance improvement: A Review

Entire world is struggling to explore newer methods to overcome the energy crisis. With limited and exhausting energy resources researchers around the world are finding efficient ways to store thermal energy. One of the efficient ways is to store thermal energy in the form of latent heat energy using phase change materials (PCMs). Latent heat storage (LHS) units have been widely adopted owing to superior energy storage density and constant operating temperature. But the thermal performance of these systems is limited due to low thermal conductivity of the PCMs. Researchers across the globe working on approaches to improve the thermal performance of these systems. The present review paper is concerned with a review on PCMs, methods to improve performance of LHS system using PCMs with special emphasis on use of various additives such as nano particles and porous materials to increase the thermal performance of PCMs and types of PCMs for specific applications. The factual data presented in this paper would be helpful in selection of appropriate PCM for specific application, and to adopt suitable performance enhancement technique to obtain most optimal LHS system

ANALYSIS OF A HYBRID CASCADED LATENT/SENSIBLE STORAGE SYSTEM FOR PARABOLIC-TROUGH SOLAR THERMAL PLANTS

The molten-salt two-tank system is the state-of-the-art thermal storage technology employed in the more mature parabolic-trough solar thermal power generation using synthetic oil as the heat-transfer fluid (HTF). This storage technology requires high storage-material inventory, making it very expensive. The use of latent-heat storage (LHS) system offers smaller storage size and material inventory. However, such a storage system faces two challenges: there are limited number of commercially-available phase-change materials (PCMs) are suitable in the operating temperature range; and these materials have very low thermal conductivities. The use of finned tubes, nevertheless, can overcome the later shortcoming. In this study, the analysis of a hybrid storage system, consisting of a three cascaded finned-tube LHS stages and a sensible concrete tube register stage, was carried out through modelling and simulation. A procedure for the design of the finned-tube cascaded LHS stages was developed. For a typical 50 MW parabolic-trough solar thermal power plant, the dimensions of a storage system with 6 hours of operation at full load were obtained. The three-stage cascaded LHS sub-system provides 45.5% of the total storage capacity of the entire system and has a volumetric specific capacity of 54% greater than that of the two-tank system. The volumetric specific capacity of the entire storage system is 9.3% greater than that of the two-tank system.

The effect of the inlet temperature of the heat transfer fluid in a rectangular latent heat energy storage

Solar energy is available during the day only. The excess heat energy collected during sunshine hours can be stored in the thermal energy storage and it can be reused during off sunshine hours. Latent heat energy storage systems by using phase change materials (PCMs) can be used effectively for storing thermal energy. It has high-energy storage density and the constant temperature storage process. In this paper, the performance of the latent heat storage system and phase change characteristics of PCM inside a rectangular latent heat energy storage unit will be presented. In this study, purified water is used heat transfer fluid (HTF) which is passed through the copper tubes inside the test unit and commercial-grade paraffin wax is used as PCM and indirect contact heat transfer method is used to heat exchange from the HTF to PCM. It was found that the PCM at the upper layer of the test section first melted during the melting process. Moreover, the effects of inlet temperature of HTF on the charging process was investigated. It can be concluded that increasing the inlet temperature of HTF effectively decreases the total charging time of the melting process. The amounts of heat storage for the inlet temperature of HTF 70 °C and 80 °C are 7400 kJ and 6555 kJ respectively during the melting process.

High Anisotropic Thermal Conductivity, Long Durability Form-Stable Phase Change Composite Enhanced by a Carbon Fiber Network Structure

To address the drawback of low thermal conductivity of conventional organic phase change materials (PCMs), a paraffin-wax-based phase change composite (PCC) was assembled via a vacuum impregnation method, using a new type of carbon fiber network material as the supporting matrix. The carbon fiber sheet (CFS) material exhibited a network structure comprising high-thermal-conductivity carbon fibers, beneficial for enhancing the heat transfer properties of the PCC. The sheet-shaped carbon fiber material was stacked and compressed, and then impregnated with the liquid paraffin wax PCM to form the composite. The thermal conductivity, durability, shape stability, chemical stability, and heat storage characteristics of the PCC specimen were carefully analyzed. The maximum thermal conductivity of the PCC was 11.68 W·m−1·K−1 (4670% compared to that of pure paraffin) in the radial direction, and 0.93 W·m−1·K−1 in the axial direction of the sample, with 17.44 vol % of added CFS. The thermal conductivity retention rate after 200 thermal cycles was 78.6%. The PCC also displayed good stability in terms of chemical structure, shape, and heat storage ability. This study offers insights and a possible strategy for the development of anisotropic high-thermal-conductivity PCCs for potential applications in latent heat storage systems.

Compound charging and discharging enhancement in multi-PCM system using non-uniform fin distribution

In the present study, a numerical model of a vertical shell and tube latent heat storage system validated with the experimental data is presented. The developed model comprises of three blocks of phase change materials (PCMs) having melting point temperatures (Tm) 360 °C, 335.8 °C and 305.4 °C, respectively. A non-uniform distribution of fins in three PCM blocks is initially employed to study the performance of the single PCM system (Tm=335.8 °C). The effect of inlet heat transfer fluid temperature on the charging and discharging performances of the single PCM and multiple PCM (m-PCM) systems is analysed by varying a Stefan number (Steref) parameter, calculated based on the single PCM system. The charging and discharging times for the m-PCM system are either similar or lesser than the single PCM system for Steref≥1, however, there is an improvement of 21–25% in the specific power charged and discharged by the m-PCM system for all the Sterefvalues (0.5, 1, 1.5 and 2) considered. By employing a compound enhancement technique which is a combination of non-uniform fin-distribution and PCM blocks length ratio optimization for the m-PCM system, 30% and 9% reduction in the charging and discharging time, respectively, over the single PCM system is achieved.

Strategies for the emergence of a dominant design for heat storage systems

ABSTRACT An important component of sustainable home energy systems is the self-sufficient generation and usage of energy. Although sustainable solutions to both generation and usage of energy in homes have been extensively studied in the past, the storage of energy has only scarcely been studied. This paper focuses specifically on thermal energy storage. Three competing designs are currently available: sensible, latent and thermochemical heat storage systems. The question is which will become the dominant design. Relevant antecedents for design dominance are explored and applied to this case in order to determine their weights. Furthermore, it is assessed which of these three alternatives will have the highest chance of achieving market dominance. Technological characteristics are most important and latent heat storage technology has the highest likelihood of achieving design dominance. The paper contributes to ongoing research that attempts to assign weights to factors for technology dominance in different arenas.

Exergy analysis and optimization of charging–discharging processes for cascaded latent heat storage system

The use of exergy analysis provides theoretical guidance for the cascaded latent heat storage system (CLHSS). However, the exergy analysis of the CLHSS charging–discharging processes is imperfect with two problems to be solved. One is the lack of exergy flow analysis, the other is the inaccurate expressions of the overall charging–discharging processes exergy efficiency. This paper aims to solve the above two problems. First, the exergy flow of the CLHSS charging–discharging processes was revealed, composed of one or more of the three exergy flow paths. Second, the overall exergy efficiency was derived by determining exergy produced and consumed. Only by satisfying the constraint that the exergy change of each PCM is equal to zero, the product of exergy efficiencies of charging and discharging processes can represent the overall exergy efficiency. On this basis, previous models that used the product of exergy efficiencies were modified by adding constraints. Compared to models without constraints, the model with constraints has more accurate and complete optimization results, which is conducive to the CLHSS stable operation. Finally, the model with constraints was adopted to guide the application of the CLHSS in the solar power tower.

Design and characterisation of a bi-modal solar thermal propulsion and power system for small satellites

Small satellites with increased capabilities in terms of power and propulsion are being demanded for future missions. This paper addresses an alternative bi-modal solution which consists of a solar thermal propulsion system coupled with a micro-Organic Rankine Cycle system, to co-generate thrust and electrical power. Current literature on bi-modal systems is limited to static power conversion systems such as thermionic conversion processes. Therefore, this paper expands the research of bi-modal systems to dynamic power conversion systems and latent heat storage systems. The paper documents the design process, key design parameters, and feasibility of this system for a Geostationary Transfer Orbit to Lunar Orbit insertion mission. The results of a single-objective optimisation show the system is most suitable on-board small satellites with a gross mass above 300 kg. The propellant accounts for 50% of the total system mass. The final design uses Silicon as the latent heat energy storage system due to its high specific energy of more than 250 Wh/kg. Additionally, the enthalpy method is used to describe the dynamic behaviour of the phase change material and results show the insulation thermal conductivity has the largest effect, up to 17%, on the receiver’s maximum achievable steady-state temperature.

Numerical study of an Evacuated Tube Solar Collector incorporating a Nano-PCM as a latent heat storage system

A new Evacuated Tube Solar Collector (ETSC) incorporating a Nano-PCM with fins is presented and studied. The numerical mathematical 2D model of phase change heat transfer is highlighted. The effect of adding nanoparticles of copper (Cu) to paraffin wax on system performance was investigated. The heat transfer during the energy storage process is simulated using the Ansys-Fluent. Moreover, the solid-liquid phase change characteristics of the paraffin within the system are analyzed. Additionally, an optimization analysis of fin parameters (i.e. fin thickness and fin spacing) is performed. Finally, a series of comparative simulations are carried out to investigate the performance of the ETSC system with and without the Solar Parabolic Trough Reflector (SPTR). The results showed that adding fins has a great effect on the phase change heat transfer of the paraffin in the ETSC. It was noted that the PCM melts faster as the thickness of the fins gets thinner. Also, the addition of 1% of Cu to the PCM was found to be the optimum mass concentration at which the HTF outlet temperature increased by 2 K. Moreover, it was found that the ultimate flow rate that caused the entire mass of the PCM to melt is 0.003 kg/s.