Heat Storage Medium Research Articles

A novel 3-D model of an industrial-scale tube-fin latent heat storage using salt hydrates with supercooling: A model validation

Latent heat storage using salt hydrates based on tube-fin heat exchanger is a promising heat storage approach for distributed building heating. In spite of abundant experimental investigations, there is quite a lack of numerical works focusing on the industrial-oriented design and optimization on this technology. The present work develops a reasonably accurate 3-D model for the evaluation of the thermal behavior of a pilot-scale tube-fin latent heat storage device using salt hydrates, considering the balance between computing effort and simulation accuracy. The availability and accuracy of the proposed model are validated through comparisons with experimental results in which numerical discrepancies and model applicability are comprehensively discussed. The results show that the model is able to portray a clear heat transfer process of a 100 kWh-level pilot-scale latent heat storage device during a continuous 24-h charge-discharge operation, with considering detailed thermal behaviors of PCMs including non-isothermal melting and solidification with supercooling effect. The calculation errors on the transient temperature variation of the heat transfer fluid and storage medium, as well as the overall thermal performances of the storage device are less than 7%. The CPU time consumed for the calculation is around half of the actual operation time.

Experimental investigation and performance evaluation of a novel solar dryer integrated with a combination of SHS and PCM for drying chilli in the Himalayan region

This work presents an experimental analysis of a novel indirect solar dryer for drying chillies in the Western Himalayas. Iron scrap assorted with gravels are kept on the absorber plate and engine oil transported through the copper tube, have been applied as sensible heat storage (SHS) medium in the solar air collector (SAC). Paraffin wax has been employed as a phase change material (PCM) in the drying compartment. The performance evaluation of the drying system has been carried out on the basis of physical, thermal and quality attributes. The moisture level of chillies is reduced to 4.85% from 86.50% (wet basis) in 21 h as compared to 96 h and 150 h by using solar drying without thermal storage and in the natural sun, respectively. Outcomes of this work conclude that the dehydrating time is reduced by 78.12% and 86.00% as against drying conducted without the use of SHS and PCM, and natural sun; respectively. Using this drying system, the colour, texture, pungency (sensory parameters) and rehydration capacity of the dehydrated chillies have been found better. From the experimental results vitamin C, capsaicin, carotene and thiamin of the chillies were found 49.25 mg, 0.52%, 341.5 µg and 0.912 mg which were 32.25 mg, 0.23%, 217.8 µg and 0.534 mg, in case of natural sun drying.

Thermal Performance Analysis of Solar Dryer Integrated with Heat Energy Storage System and a Low-Cost Parabolic Solar Dish Concentrator for Food Preservation

Solar energy has become a viable alternative energy because it is a clean type of energy that converts solar radiation into heat energy for various applications such as heating water, power generation, cooking, and food drying. The solar dryer, integrated with the heat energy storage system, uses nitrate salt as a heat storage medium which was designed and tested by drying 1000 grams of red pepper at 19.6 to 62.4°C. The average ambient temperature ranged from 19.3 to 37.4°C, and the maximum temperature of the heat storage media ranged from 87.8 to 125°C. The solar drying process was compared to open sun drying system loaded with 1000 grams of red pepper. The findings showed that the solar dryer maintained color and flavor and lowered the original moisture content from 86% to 10% for 24 hours compared to 36 hours of drying in open air. In this study, nitrate salt is shown to be the perfect heat storage medium for drying food products; it preserved heat for about 4 hours when there is no active sunlight.

Cyclic Mechanical Behavior of Two Sandy Soils Used as Heat Storage Media

In this research, the cyclic mechanical behavior of two heat storage sandy soils is experimentally studied using a cyclic thermo-mechanical triaxial device. The results of the tests, which were performed under controlled temperature conditions between 20 and 60 °C, show a significant dependence of the mechanical response of the sandy soils with the amplitude of the cyclic loading and medium temperature. The mechanical performance and accumulation of plastic strains of the soils with an increasing number of loading cycles are discussed in view of the intrinsic soil behavior.

Nucleation promoting effect of fat shell on phase change material particles dispersed in an emulsion for thermal energy storage medium

Abstract Phase change material emulsions (PCMEs) can be prepared in which a paraffin type phase change material (PCM) is dispersed as small particles in water by the action of a surfactant. Using the latent heat of PCM particles in the PCME, the PCME can be used as an efficient heat storage and transport medium. PCM particles in the PCME are stabilized by the action of a surfactant, but they irreversibly coalesce over time. When the PCM particles change from liquid to solid, they can also show large supercooling because of their small size. These problems must be overcome for PCMEs to be used as efficient thermal media. In this study, PCM particles are emulsified with a high-melting-point fat containing a hydrophobic group consisting of single bonds, in an attempt to overcome these problems. The high-melting-point fat exists as fat shell around PCM. The stability and supercooling characteristics of the PCM particles are investigated. The amount of fat affects the stability of the emulsion. Adding 1.0–5.0 wt% fat is the optimum amount for stabilizing the emulsion. The fat promotes the nucleation of the PCM particles, and the degree of supercooling is suppressed to 3.0 K. Furthermore, the latent heat retained by the emulsion emulsified with fat is about 10% larger than the value obtained by multiplying the latent heat of the PCM by the content of the PCM. These features indicate that PCME prepared with high-melting point fat is suitable as high performance thermal energy storage medium for thermal energy storage and transport system.

Experimental study on forced convection heat transfer of KNO3–Ca(NO3)2 + SiO2 molten salt nanofluid in circular tube

Molten salt is a major heat storage medium for medium–high temperature solar thermal power applications. The heat storage density and heat transfer efficiency of molten salt are important factors of solar thermal power efficiency. Recently, many studies have reported that molten salt nanofluid, which is formed by adding nanoparticles in molten salt, can remarkably enhance the specific heat capacity of molten salt. However, research on the forced convection heat transfer of molten salt nanofluid is inadequate. In this study, the forced convection heat transfer experiments were carried out in a circular tube of a previously reported promising molten salt nanofluid composed of KNO3–Ca(NO3)2 + 1 wt% of 20 nm SiO2 nanoparticles (K-C-Snm). Results showed that compared with its base pure molten salt, the K-C-Snm molten salt nanofluid exhibited substantially better forced convection heat transfer performance under the same working condition. The Nusselt number and the convective heat transfer coefficient of K-C-Snm molten salt nanofluid were 16.3% and 39.9% higher than its base pure molten salt, respectively. The experimental data of K-C-Snm molten salt nanofluid largely differed from the classical correlation equations. Hence, a new empirical heat transfer correlation equation was set for K-C-Snm molten salt nanofluid, and the deviation between the experiment data and new correlation was within ±7%.

Experimental and thermal performance investigations on sensible storage based solar air heater

Abstract The thermal performance of the solar collector for air heating purpose is investigated with and without packed bed through experimental and theoretical methods. A simple flat plate solar air heater was developed with 2m2 corrugated absorber plate and integrated with porous pebble bed in the airflow path to act as a sensible heat storage medium. The experimental studies were carried out for different mass flow rates of air from 0.014 to 0.087 kg/s with the increment of 0.0095 kg/s. Steady-state theoretical models have been established to predict the thermal performance of the air heaters and the results indicated that they are well enough to predict the outlet air temperature with acceptable error percentage. The collector with packed bed storage medium is more efficient than without packed bed in terms of energy efficiency as well as in reducing temperature fluctuation of air at the outlet of the collector. The energy and exergy efficiency of the collector with packed bed veried from 20.35% to 50.92% and +3.97% to −2.807% for the airflow rates 0.014 kg/s to 0.087 kg/s respectively. Solar collectors with packed bed are capable to deliver the hot air in the range of 45 to 60 °C for a longer period and which can be preferred for achieving a better quality of the dried products as well as for space heating applications.

Compatibility of Phase Change Materials and Metals: Experimental Evaluation Based on the Corrosion Rate.

The construction and maintenance of building stock is responsible for approximately 36% of all CO2 emissions in the European Union. One of the possibilities of how to achieve high energy-efficient and decarbonized building stock is the integration of renewable energy sources (RES) in building energy systems that contain efficient energy storage capacity. Phase Change Materials (PCMs) are latent heat storage media with a high potential of integration in building structures and technical systems. Their solid-liquid transition is specifically utilized for thermal energy storage in building applications. The typically quite old example is the use of ice that serves as long-term storage of cold. Large pieces of ice cut in winter were stored in heavily insulated spaces and prepared for cooling of food or beverages in summer. In the contemporary use of the principle, the PCMs for building applications and tested in this study must have a melting range close to the desired temperature in the occupied rooms. As the PCMs need to be encapsulated, several types of metal containers have been developed and tested for their thermal conductivity and resistance to mechanical damage, which enhances the performance of these so-called latent heat thermal energy storage (LHTES) systems. Long-term compatibility of metals with PCMs depends, i.e., on the elimination of an undesirable interaction between the metal and the specific PCM. Heat storage medium must be reliably sealed in a metal container, especially if the LHTES is integrated into systems where PCM leaks can negatively affect human health (e.g., domestic hot water tanks). The aim of this study is to evaluate the interactions between the selected commercially available organic (Linpar 17 and 1820) and inorganic (Rubitherm SP22 and SP25) PCMs and metals widely used for PCM encapsulation (aluminum, brass, carbon steel, and copper). The evaluation is based on the calculation of the corrosion rate (CR), and the gravimetric method is used for the determination of the weight variations of the metal samples. The results show good compatibility for all metals with organic PCMs, which is demonstrated by a mass loss as low as 2.1 mg in case of carbon steel immersed in Linpar 1820 for 12 weeks. The exposure of metals to organic PCMs also did not cause any visual changes on the surface except for darker stains, and tarnishing occurred on the copper samples. More pronounced changes were observed in metal samples immersed in inorganic PCMs. The highest CR values were calculated for carbon steel exposed to inorganic PCM Rubitherm SP25 (up to 13.897 mg·cm−2·year−1). The conclusion of the study is that aluminum is the most suitable container material for the tested PCMs as it shows the lowest mass loss and minimal visual changes on the surface after prolonged exposure to the selected PCMs.

Energy and exergy analysis of a natural convection dryer with and without sensible heat storage medium

Abstract In the present work, thermodynamic analysis of a natural convection dryer with and without sensible heat storage material (pebbles) has been studied. The purpose of this study is to enhance the performance of the dryer by using pebbles as a sensible heat storage medium. Energy and exergy analysis has been done to investigate the magnitude of losses whether the pebbles will be appropriate to use as a sensible heat storage material. The experiments were performed under different experimental conditions, without pebbles (case-I), pebbles filled up to the furnace height (case-II), pebbles filled up to the paraffin wax tray (case-III). In the case-IV, pebbles filled up to the paraffin wax tray but hot air is not allowed to flow from the rectangular chamber to the drying chamber. From the 1st three cases (I to III), it was observed that the energy losses and exergy destruction in the rectangular chamber are lower in the case-III. From the result, it can be seen that the use of sensible heat storage material and restricting the hot air in the rectangular chamber is promising in contributing to the performance of the dryer.

Convective heat transfer performance of solar salt in an inclined circular tube

Solar salt is the most common heat storage medium in solar power system, In order to understand convective heat transfer performance of solar salt, and testify the validity of the well-known classical and four literature convective heat transfer correlations, an experimental study on turbulent convective heat transfer of solar salt in an inclined tube was conducted in this paper. Various experiments were respectively carried out within the experimental data of solar salt in the range of Reynolds between 10,000 and 36,000 and Prandtl between 4.75 and 8.0. Based on the experimental data, its local heat transfer coefficient, overall heat transfer coefficient had been studied in detail. Besides, good agreement was observed between the experimental data of solar salt and part classical and literature correlations. Conversely, other correlations over predicted the heat transfer performance of solar salt, Finally, the fitted forced convective heat transfer correlation of solar salt was obtained by using multiple linear regressions within a deviation of ±10%.

Effect of porous material as heat storage medium on fuel consumption in a turbocharged gasoline engine

Exhaust gas energy recovery systems have shown great potential in improving internal combustion engine’s fuel consumption. The use of porous Si-SiC material as heat storage medium to improve the turbine performance and fuel consumption of the engine has not been investigated. In order to study the effect of porous Si-SiC material as heat storage medium on the fuel consumption for a turbocharged gasoline engine, steady and transient engine conditions were considered. A one-dimensional flow dynamic model of the porous heat storage material was established and comprehensively validated by experimental data. Then, this model was applied to predict the performance of the engine with heat storage medium under various conditions, including steady operation condition at targeted torque and transient operation conditions following the worldwide harmonized light vehicles test cycles. As a result, under steady operation conditions, using heat storage medium, a fuel consumption reduction of about 1.1% was obtained due to increased turbine efficiency and reduced pumping loss. Moreover, the simulation under worldwide harmonized light vehicles test cycle driving cycle achieved fuel consumption saving up to 6.6% at medium vehicle speed due to the reuse of recovered exhaust gas energy in the heat storage medium. Thus, to use of the porous Si-SiC material as a kind of heat storage medium can be deemed as a prominent way to improve the turbine efficiency and engine performance of turbocharged gasoline engines.

Heat and mass transportation properties of binary chloride salt as a high-temperature heat storage and transfer media

Molten binary chloride salts are considered as promising heat transfer and storage mediums in a Concentrating Solar Power (CSP) Station. The properties of them over the entire operating temperature are required to be calculated, since they are difficult to be measured under high-temperature conditions. Reliable numerical method is an alternative tool to accurately predict thermal properties of molten salts from the viewpoint of micro-structure. Based on the effective Born-Mayer-Huggins (BMH) pair potential model, the local structures and thermodynamic properties of lithium-potassium chloride were computed by using Molecular Dynamics simulations (MD) and the results are consistent with available experimental literature data. The simulation results prove that BMH force field is reasonable to be adopted to predict the properties of various molten chloride salts and their mixtures. The minimum errors of thermal properties above are all within 11.1%, which is accurate and reliable. The results show that Li+ promotes the diffusion of the system efficiently. In addition, the specific heat capacity and thermal conductivity are significantly improved with the increase of Li+. The database of thermal properties and structures of this work are significant for accurate design of material structure and regulation of thermal storage system performance.

Enhanced performance figures of solar cookers through latent heat storage and low-cost booster reflectors

Abstract Solar box cookers draw attention of many researchers across the globe as a promising renewable energy application for cooking purposes. Compared to other types available in literature, solar box cookers are more in the centre of interest owing to their simple and low-cost design, emerging thermodynamic performance figures, high durability and reliable cooking processes without any risk of burning food. On the other hand, cooking power and overall thermal performance of solar box cookers are still somewhat challenging to compete with the conventional cooking systems for the climatic conditions with low solar radiation potential like the Black Sea Region of Turkey. Within the scope of this research, a novel solar box cooker is devised, fabricated and tested to overcome the said shortcomings of traditional solar box cookers through natural and recycled materials. Double-glazed structure having 16-mm-thick air between two 6-mm-thick thermally resistive clear glasses is considered for aperture glazing with an area of 0.16 m2. The oven area has a depth of 350 mm, and it is entirely painted matte black for maximum solar absorption. The oven body made of stainless steel sheets is encountered by a latent heat storage medium filled by natural beeswax product, waste of propolis. Propolis is a resinous mixture that is used for protection of beehives, from either climatic changes or diseases. Polyphenols rich in balsamic part of the structure is extracted by alcohol than used for many apitherapic purposes. The remaining pulp or waste is not considered. In this study, it is used as green chemistry agents. It is found that water temperature in the cooking pot is kept over 40°C till very late hours as a consequence of latent heat storage. First figure of merit is determined to be 0.08, and the thermal efficiency of the cooker varies from 7.47 to 4.54%.

A trans-critical carbon dioxide energy storage system with heat pump to recover stored heat of compression

In this paper, the heat pump system is used as the thermal storage system to reheat the heat of compression of the trans-critical CO2 energy storage system based on the underground gas storage reservoir, and the thermodynamic analysis and sensitivity analysis of the main equipment of the energy storage system are carried out. Considering the limitation of the thermal properties of the heat storage medium, the influence of the energy storage system composed of different heat storage medium on the system performance is studied. Results have shown that the round-trip efficiency, energy storage efficiency and heat storage efficiency of the system are 66%, 58.41%, and 46.11%, respectively. With the increase of energy storage pressure, the round-trip efficiency and energy storage efficiency increase gradually, while the heat storage efficiency decreases first and then increases. When the energy storage pressure is 23 MPa, the system has the lowest heat storage efficiency. The round-trip efficiency, energy storage efficiency and heat storage efficiency increase with the increase of the energy recovery pressure. The study also shows that the energy storage system with water as the heat storage medium has the highest efficiency and the best performance.

Thermoeconomic analysis on a molten salt parabolic trough‐based concentrated solar organic Rankine cycle system

Solar thermal electricity generating technology is an alternative solution to energy crises and environmental problems, which has caused wide concern in recent decades. In this paper, a molten salt parabolic trough-based concentrated organic Rankine cycle system is proposed and investigated. A quadribasic nitrate salt with low melting temperature is employed as a heat transfer and storage medium. A stable heat transfer and economic model is established with Matlab. The radial and axial temperature distributions in the collector tube are obtained, and the impact of condensation and evaporation temperatures on the heat transfer area is analyzed. Results show that the temperature along the axial direction linearly increases, and the temperature at the collector tube exit decreases with the increase of molten salt mass flow rate. The maximum temperature difference along the radial direction of the collector tube happens in the annular gap. Heat transfer and thermodymanic analysis indicates that condensation temperature has a more evident effect on heat transfer area than that of evaporation temperature. An increase in condensation temperature leads to a decrease in the evaporator area, the condenser area increases, and the total area decreases. Economic analysis indicates the collector cost plays a predominant role in total capital costs, and decreasing molten salt mass flow rate can considerably reduce collector cost. Levelized energy cost sensitivity analysis indicated that operation time per year has a more evident effect than that of the four factors. Heat transfer and economic analysis on the system helps in the selection of operation parameters.

Experimental analysis and comparison of flat plate solar air heater with and without integrated sensible heat storage

In the present work, a novel flat plate solar air heater (SAH) has been designed to yield a good outlet air temperature irrespective of fluctuations in solar radiation. It was achieved with the help of a specially designed, integrated absorber plate cum storage unit, together acted as a single component. The integrated unit consists of a set of copper tubes with black painted copper foil, welded longitudinally on two main header tubes. High quality synthetic oil (Therminol-55) had been contained in those copper tubes, which acted as a sensible heat storage medium. To understand the characteristics of the novel SAH, a comparison study was made with a conventional SAH of similar dimensions experimented without any thermal storage. Both the SAHs were tested for three different mass flow rates (0.017 kg/s, 0.02 kg/s and 0.028 kg/s) at one of the hottest cities in India, Madurai. The results show that the novel SAH had operated at a maximum efficiency of 67.7% when the mass flow rate was 0.028 kg/s, before the solar radiation started to decrease around 14:00 h Indian standard time. It also concluded that sensible heat storage at the absorber plate improves the thermal output, therefore leads to a consistent performance.Thus, the SAH with integrated storage unit performed with better efficiency compared to the conventional SAH with no storage.

Form-stable phase change material based on fatty acid/wood flour composite and PVC used for thermal energy storage

This work focused on a novel form-stable phase change material (FSPCM) for building energy saving. Capric acid (CA) and palmitic acid (PA) were selected to prepare binary eutectic. The CA-PA/wood flour (WF) composites with different mass ratios were prepared by the vacuum impregnation method. After the shape-stabilized properties were investigated, the results indicated that there were the good encapsulate capacity of WF to CA-PA eutectic and the slight effect of WF on the crystallization. Phase change wood plastic composites (WPCs) using CA-PA /WF composites as the latent heat storage medium and PVC as the matrix were prepared. The relations between mass ratios of CA-PA /WF composites and mechanical properties of WPCs were discussed. The results revealed that the WPCs with appropriate mass fraction of CA-PA eutectic had desired mechanical properties for practical applications. The prepared composites were examined by the scanning electron microscope (SEM), differential scanning calorimetry (DSC), fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG) and accelerated thermal cycling test. SEM images showed that the WPC prepared by the CA-PA /WF composite with the mass ratio of 0.4:1(WPC0.4) was more compact and uniform, but as the amount of CA-PA eutectic increased, more gaps and cracks appeared. DSC results showed that the WPC0.8 melted at 22.03 °C with a latent heat of 28.16 J/g and froze at 20.06 °C with a latent heat of 29.77 J/g, which was more applicable to the thermal energy storage systems for buildings. FTIR results demonstrated that there was no chemical reaction between CA-PA eutectic and components in WPC. The prepared FSPCM exhibited excellent thermal stability for the building materials from the TG analysis. Furthermore, the results of accelerated thermal cycling test showed that the composite had a good thermal reliability after 500 thermal cycles.

Study on enhancing heat transfer performance of spiral groove tube in phase change heat storage for solar energy

Based on the energy storage problem for solar energy utilization and the advantages of spiral groove tube heat exchanger, spiral groove tubes were used in the solar energy phase change heat storage. The thermal storage process of heat reservoir was simulated numerically. Firstly, the simulation method and the reliability of the used model are verified experimentally with smooth tube. Using spiral groove tube as water flow pipe and phase change material as heat storage medium, The three-dimensional model of heat storage was built by Gambit software and the grids were divided by ICEM. The heat storage process in the spiral groove tube and smooth tube heat storage were numerically simulated and the heat transfer enhancing effect was investigated. The influence of structural parameters such as groove pitch and groove depth on the heat storage process is simulated numerically and the influence rules are analyzed. The results show that the convective heat transfer intensity and heat transfer capability are enhanced when the smooth tubes are substituted by spiral groove tubes in the phase change heat storage and the heat storage time becomes shorter. In the range of this paper, the optimal structural parameters of spiral groove tube is groove pitch p=7mm and groove depth e=0.4mm.

Evaluation of water and paraffin PCM as storage media for use in thermal energy storage applications: A numerical approach

Abstract Two different heat storage media, namely water and paraffin phase change material, are studied in order to evaluate and compare them for use in thermal energy storage systems. Using commercial computational fluid dynamics software, the charging process of the two materials is simulated and the results focus on the energy stored, the storage material temperature and heat transfer fluid outlet temperature. The geometric characteristics of the computational domain are selected in accordance with commercially used heat exchangers for thermal storage applications. Two different tube lengths and two different feed rates are used for the comparison of the charging process, while the heat transfer fluid temperature and the initial temperature of the components of the system are the same for all the simulations. Results show that for the same volume of storage mediums, the phase change material can store more energy than water, for the same temperature of the heat transfer fluid, as expected. As the feed rate is doubled, the same energy is stored in the media, but in approximately half the time. The simulation yielded an increase of the storage capacity of the examined system by approximately 4.1 times when phase change material is used instead of water and a significantly lower storage medium temperature change during the process. It is also shown that the stored energy is about 6.6 times greater for the 6.6 m long tube than the 1 m long tube and is proportional to the heat storage medium mass. However, the duration of the process is not proportional to the heat storage medium mass and when using water as heat storage medium, it almost doubles for a 6.6 times longer tube, while when using phase change material the duration increases approximately only 1.5 times.

Heat discharging and natural convection heat transfer performance of coil heat exchanger in single molten salt tank

There has been an increasing attention on single-tank thermal energy storage, attracted by the low cost, high efficiency and compactness in the operation. The most researches on single-tank thermal energy storage focused on temperature stratification to improve its heat discharging efficiency using water as heat storage media. The present research aims at investigating heat discharging performance by improving natural convection heat transfer of molten sat around coil heat exchanger in single heat storage tank. The effect of different parameters of the coil heat exchanger on the heat discharging performance, flow field and temperature distribution are analyzed and discussed in the single molten salt tank. The results show that the heat discharging performance of the coil heat exchanger can be enhanced by increasing the buoyancy-driven flow around the coil heat exchanger. The heat discharging efficiency can be improved by the optimal arrangement of coil heat exchanger for the highest velocity can be induced around it, which results in a fully mixed of molten salts and its temperature turns to uniform, instead of temperature stratification. Based on the results, a correlation among the natural convection Nu number with Ra number and other heat exchanger parameters in a single molten salt tank is obtained by numerical results. The research results provide a theoretical basis and paradigm for the design of single thermal storage tank energy storage system.