Temperature Thermal Energy Storage Systems Research Articles

A novel approach for the active‐mode indirect drying of food grains with high temperature thermal energy storage system

AbstractThis article introduces a novel approach for the active‐mode indirect drying of food grains using an independent high‐temperature thermal energy storage (TES) system. It uses commercial software to conduct numerical analysis of a solar drying unit fused with indirect TES. Employing commercial grade phase change material X‐180, solar drying unit incorporates scheffler dishes, receiver, TES, thermic fluid pump, air blower, and dryer fitted with a radiator. Heat transfer fluid Thermenol‐66 has been used to facilitate the heat transfer from receiver to TES and dryer. Two distinct TES designs, one without fins (case A) and the other with fins (case B), have been used to encapsulate phase change material and facilitate the flow of heat transfer fluid. The dynamic performance of independent solar drying unit components was examined at various mass flow rates during charging and discharging process. The results demonstrate that adding 42 fins, i.e. (case B), reduces the charging and discharging time to 22% and 24%, respectively. Due to high charging and discharging power, the rate of temperature change in case B is 19% higher compared to case A. To maintain drying quality, the dryer should have a continual flow of hot air at a constant temperature. Therefore, utilization of the TES, case A, provides a significantly higher uniformity index and a more extended drying period during the discharge process.

Adsorption and nucleation mechanism of molten carbonate on nano-SiC surface

Molten carbonate is an important thermal storage media for high temperature thermal energy storage system. However, the specific heat capacity (SHC) of carbonate is less than 2 J/g K, which limits its application prospects. Nano-SiC was used to improve the SHC of carbonate at high temperature in our previous work. The SHC of binary carbonate increased by 83 % with the addition of nano-SiC. However, the improvement mechanism is not clear. In this study, the adsorption and nucleation mechanism of molten carbonate on nano-SiC surface were clarified. The DFT calculation results show that the adsorption energy of Li+, Na+ and CO32− is −310, −259 and −654 kJ/mol, respectively, which means the addition of nano-SiC causes a non-uniform distribution of molten carbonate ions around its surface. This affects the subsequent nucleation and results in carbonate segregation. Na2CO3 is the most thermodynamically favorable to adsorb and nucleate, followed by LiNaCO3, and Li2CO3 is the most difficult. The crystalline phase changes of carbonate on nano-SiC surface from solid to liquid phase during melting process were obtained in-situ and the results are consistent with the DFT calculations. Therefore, the adsorption and nucleation mechanism of molten carbonate on nano-SiC surface is well elucidated by the calculations and experiments of this study.

Energy, exergy and economic (3E) analysis and multi-objective optimization of a combined cycle power system integrating compressed air energy storage and high-temperature thermal energy storage

Traditional adiabatic compressed air energy storage system has a low turbine efficiency and a low power output due to the low turbine inlet temperature and high turbine outlet temperature without heat recovery. To address these issues, a combined cycle power system integrating compressed air energy storage and high-temperature thermal energy storage is proposed in this paper. The thermodynamic and economic models of the proposed system are developed considering the thermodynamic laws and life cycle assessment, respectively. The energy, exergy, economic (3E) analysis of the system are performed based on the models. The results show that under the design condition, the round-trip efficiency, exergy efficiency, energy storage density, levelized cost of energy and dynamic payback period of the system can reach 59.22 %, 62.12 %, 5.77 kWh/m3, 0.1186 $/kWh and 6.51 years, respectively. The sensitivity analysis shows that the maximum air storage pressure, minimum air storage pressure and outlet temperature of high temperature thermal energy storage system are the critical parameters impacting the system performance. Finally, the multi-objective optimization of the proposed system is carried out. The aforementioned three parameters are selected as the decision variables and the round-trip efficiency and levelized cost of energy are chosen as the objective functions. Compared with the pre-optimization system, the round-trip efficiency, exergy efficiency, and energy storage density of the system increase by 0.47 %, 0.43 %, and 1.33 kWh/m3, respectively, while the levelized cost of energy and dynamic payback period decrease by 0.0051 $/kWh and 1.55 years, respectively.

Comprehensive thermal properties of ternary eutectic molten salt/nanoparticles composite phase change materials for high–temperature thermal energy storage

Chlorine and fluorine salts are very promising thermal storage media for high–temperature thermal energy storage (TES) systems. In this study, NaCl–KCl–NaF/nanoparticles composite phase change materials (CPCMs) were employed by an advanced high–temperature and high–pressure reactor. NaCl–KCl–NaF was adopted as the BS, while CuO and Al2O3 nanoparticles have been uniformly dispersed in different ratios to generate CPCMs from the base salt (BS). Microscopic characterization, thermal properties characterization, and economic analysis of CPCMs and the BS were performed. The findings revealed that the best results were obtained when the BS was added with 0.5 wt percent Al2O3 and 0.5 wt percent CuO nanoparticles, CPCMs have improved solid/liquid thermal conductivity by a factor of 3.70 and 1.84, and TES density by a factor of 1.13, while reducing the cost per unit of TES density by approximately 26.8%. In addition, the CPCMs and BS possess excellent operating temperature range and thermal stability, whereby they are applicable in high–temperature TES systems.

Thermophysical characterization of paraffin-based PCMs for low temperature thermal energy storage applications for buildings

Trustworthy information about the thermophysical properties of phase-change-materials (PCMs) is vital for designing low temperature thermal energy storage systems for buildings and for modelling their thermal behaviour. However, data provided by manufacturers are recurrently optimistic, insufficient and/or uncertain for developing reliable numerical approaches, which requires further investigation. This paper aims to assess the key thermophysical properties of ten paraffin-based PCMs: microencapsulated and bulk paraffins.The melting/solidification peak temperatures, the enthalpy of fusion/solidification and the specific heat capacity were measured by Modulated-Differential-Scanning-Calorimetry. The Transient-Plane-Source-method was used to estimate the variations of the thermal conductivity/diffusivity with temperature. The thermal stability was assessed by High-Resolution-Thermogravimetric-Analysis. The microstructures of the microencapsulated PCMs were examined via Scanning-Electron-Microscopy.It was concluded that some information provided in the datasheets is roughly corroborated by the experimental results. However, in some cases, it was too optimistic (particularly the enthalpy of fusion/solidification), incoherent (mainly the melting/solidification peak temperatures) or scarce (e.g., the values of the thermal conductivity and the specific heat of the solid/liquid phases). The major contribution of this work is a reliable dataset for upcoming numerical modelling, saving time and resources in future studies. The results can also be used as a reference for future inter-laboratorial comparison studies.

Preparation and characterisation of binary eutectic phase change material/activated porous bio char/multi walled carbon nano tubes as composite phase change material

The primary purpose of the current work is to develop, fabricate, and characterise a novel composite phase change material for the medium temperature thermal energy storage systems. The proposed composite PCM is made up of a binary eutectic PCM (LiNO3 + NaCl), a supporting material (activated bio char), and thermal conductivity-improving particles (i.e., multi walled carbon nano tubes). XRD, FTIR, and SEM-EDS analysis was used to determine the chemical stability of the composite PCM samples. Using differential scanning calorimetry (DSC), the thermophysical parameters of the composite PCM samples, such as phase transition temperature and latent heat value, are evaluated. With the addition of activated biochar (A-BC) and multiwalled carbon nano tube (MWCNT) particles, the latent heat value of PCM is reduced, and the thermal conductivity enhancement value is increased to 116.11%. The findings demonstrate that employing A-BC and MWCNT nanoparticles ensures the stability of the eutectic chloride and prevents leak. The corrosion rate of the copper, aluminium, stainless steel, and Inconel 617 were determined by immersing it in pure PCM and composite PCM for 12 weeks. The metal specimens that were inserted in the composite PCM were found to have good corrosion stability.

Effect of expanded graphite on the thermal conductivity of sodium sulfate decahydrate (Na2SO4·10H2O) phase change composites

Inorganic salt hydrate phase change materials (PCMs) are of interest for near-room temperature thermal energy storage (TES) systems, but their low thermal conductivity, ~ 0.5 W/m-K, limits their performance. In this work, we report the thermal conductivity and bulk density of composites containing sodium sulfate decahydrate (SSD) Na2SO4·10H2O with three types of graphite: expanded graphite (EG), milled EG (MG), and graphite nanoplatelets (GnP). The effect of these thermophysical properties on TES performance is presented. The composites were made using a readily scalable one-pot synthesis procedure with graphite received as-is. A 583% increase in thermal conductivity (4.1 W/m-K) was achieved with 25 wt% EG. However, as EG fraction increases, bulk density decreases and thermal conductivity plateaus. This ultimately resulted in lower thermal performance at higher EG fractions despite higher thermal conductivity. This highlights the tradeoff between PCM composite properties and performance, and why thermal conductivity is insufficient to describe PCM thermal performance. GnP is added to EG-SSD to increase bulk density and energy storage density, but these density improvements do not offset lower thermal conductivity and thus thermal performance declined. Similarly, MG-SSD composites had a higher bulk density and energy storage density, but lower thermal conductivity and thermal performance than EG-SSD composites at similar compositions. Atomistic molecular dynamics simulations were performed to understand the structure-property relationship of graphite-SSD interfaces. The simulations support the hypothesis that atomic level contact resistance between graphite and SSD increases thermal resistance at the interfaces resulting in effectively lower bulk thermal conductivity in MG-SSD compared to EG-SSD.

Thermodynamic analysis of a hybrid cogeneration energy system based on compressed air energy storage with high temperature thermal energy storage and supercritical CO 2 Brayton cycle

The large-scale penetration of renewable energy leads to some imperative issues to the power grid. Energy storage technology is regarded as an effective method to solve these problems. In this paper, a hybrid cogeneration energy system based on compressed air energy storage system with high temperature thermal energy storage and supercritical CO2 Brayton cycle is proposed. A thermodynamic model of the system is established. Energy and exergy analysis are carried out based on a case study. It was found that under the design condition, the round trip efficiency of 58.66%, the energy storage density of 5.45 kWh/m3, and the overall exergy efficiency of 62.00% can be achieved. At the same time, the hot water about 88°C can be supplied. The influences of some key parameters on the performance are analyzed. It was found that the round trip efficiency is most sensitive to the outlet temperature of high temperature thermal energy storage system, isentropic efficiency of compressors and turbines, followed by the intercoolers effectiveness.

Bimodal particle distributions with increased thermal conductivity for solid particles as heat transfer media and storage materials

Solid particles are being considered in several high temperature thermal energy storage systems and as heat transfer media in a variety of advanced power generation systems, particularly in concentrated solar power plants. The downside of such an approach is the low overall heat transfer coefficients caused by the inherently low thermal conductivity values of the low-cost solid media when coupled to heat exchanger for the power cycle working fluid. Choosing the right particle size distribution, emittance, and material of the solid media can all make a substantial difference in packed bed thermal conductivity. Current research though exclusively focuses on continuous unimodal distributions of particles. Here, we propose the use of a binary particle system with a bimodal size distribution to significantly increase packed bed thermal conductivity by reducing packed bed porosity. This is the first study related to ceramic solid particle heat transfer that has considered the thermal conductivity of non-unimodal size distributions at room and elevated temperatures. The following study found that for the binary particle system using Carbo particles CP 16/30 – CP 70/140 where the large particle volume fraction was 50% there was an 17–47% increase in packed bed thermal conductivity when compared with a nearly unimodal particle size distribution of CP 16/30 between 50 and 300 °C. Two different porosity and effective thermal conductivity models were studied, with one providing better prediction of porosity but both effective thermal conductivity models providing less predictive capacity. Importantly this approach can have a substantial impact of thermal performance, with little to no impact on the particle cost.

Performance assessment of convective heat transfer in tubes with a temperature difference in the high‐temperature solar power generation system

Heat transfer enhancement technology provides many advantages in heat exchanger applications. Thermal energy storage is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and power generation. The experimental investigation was carried out for heat convection in tubes with significant temperature differences (400 and 600°C), which were used convective heat transfer in a tube with air-water steam and heat conduction oil in the high temperature solar thermal power generation system. From the experiment, the results recorded with an average maximum temperature for air, water, and steam up to 422.4, 413.9, and 405.7℃, respectively, with the outer wall temperature of 600℃. Similarly, for an external wall temperature of 400°C, the average maximum temperature was obtained up to 351.2, 310.9, and 308.1℃, respectively. Therefore, from the experimental results, when the diameter of the flow pipe is higher than that of pipe diameter, then the effect of flow velocity on heat flow temperature was uncertain. At last, from the results, we conducted that the empirical correlation of convective heat transfer in a tube with temperature difference exceeding 100℃ obtained. The results showed that the increase of temperature thermal energy storage system could effectively enhance heat transfer and the calculation results can be used to optimize the performance of the high-temperature thermal energy storage system. Novelty impact statement Assessment of convective heat transfer in tubes with a temperature difference in the high-temperature is performed. The increase of temperature in thermal energy storage system could effectively enhance heat transfer. Carried out the study for different temperature thermocouples tube bundles with Nu, Re, and Pr. Conducted the empirical correlation of convective heat transfer in a tube with temperature difference.

Aggressive corrosion of C-276 nickel superalloy in chloride/sulphate eutectic salt

Molten salts are often used to store thermal energy in concentrating solar power plants. The need for higher temperature thermal energy storage systems has prompted investigation into salts and structural alloys for use at higher temperatures. A eutectic mixture of 68.05 wt% Na2SO4: 31.95 wt% NaCl salt is a potential phase change material for such systems, with low cost and high latent heat of fusion. This paper investigates corrosion of Inconel C-276 nickel alloy in this salt under air and argon atmospheres.It was found that this alloy was aggressively corroded in air at a rate calculated as 64 mm/year, with complex and unique corrosion structure observed. When tested in argon, this corrosion was even more severe, with the sample completely consumed in 5 days. This result was attributed to a basic salt corrosion mechanism which attacked the chromium and iron, whilst sulphur attacked the nickel. The high molybdenum and tungsten content of C-276 was not present in significant concentration in the corrosion product, possibly because these elements formed chloride-based compounds which evaporated. This reinforces that sulphur and oxygen containing salts can be highly corrosive, even to corrosion resistant nickel superalloys under inert atmospheres.

Flexible Electricity Dispatch of an Integrated Solar Combined Cycle through Thermal Energy Storage and Hydrogen Production

In this work, the flexible operation of an Integrated Solar Combined Cycle (ISCC) power plant has been optimized considering two different energy storage approaches. The objective of this proposal is to meet variable users’ grid demand for an extended period at the lowest cost of electricity. Medium temperature thermal energy storage (TES) and hydrogen generation configurations have been analyzed from a techno-economic point of view. Results found from annual solar plant performance indicate that molten salts storage solution is preferable based on the lower levelized cost of electricity (0.122 USD/kWh compared to 0.158 USD/kWh from the hydrogen generation case) due to the lower conversion efficiencies of hydrogen plant components. However, the hydrogen plant configuration exceeded, in terms of plant availability and grid demand coverage, as fewer design constraints resulted in a total demand coverage of 2155 h per year. It was also found that grid demand curves from industrial countries limit the deployment of medium-temperature TES systems coupled to ISCC power plants, since their typical demand curves are characterized by lower power demand around solar noon when solar radiation is higher. In such scenarios, the Brayton turbine design is constrained by noon grid demand, which limits the solar field and receiver thermal power design.

High temperature interaction between Si–B alloys and Si3N4

Si3N4 is a candidate crucible material for Si–B alloys in high temperature thermal energy storage system. In this regard, the phase formation in the Si–B alloys and the interaction between the alloys and Si3N4 were investigated in an induction furnace at 1750 °C with the B addition of 2, 5, 8, and 11 mass %, respectively. Moreover, the wettability property of the Si-3.25B alloy (mass %) on Si3N4 substrate was examined at temperatures up to 1400 °C. The cross-sectional images and micro-analyses showed that BN precipitates were produced in the Si–B alloys and a BN layer was formed at the interface between the Si–B alloys and Si3N4. However, the amount of BN precipitates was negligible. Next, the equilibrium contact angle of liquid Si-3.25B alloy on Si3N4 substrate was determined to be 134 ± 1°, which indicates non-wetting behavior. The above results support that Si3N4 is a desirable refractory material for the high temperature thermal energy storage system.

Decomposition kinetics of Al- and Fe-doped calcium carbonate particles with improved solar absorbance and cycle stability

Calcium-based materials are considered to be promising heat storage methods for the upcoming 3rd generation concentrated solar power systems (CSP) due to their high operation temperatures and energy storage densities. However, pure calcium carbonate (CaCO3) particles suffer from poor solar absorptance and stability. In this work, we successfully enhance solar absorptance, cycle stability, and decrease decomposition temperature, simultaneously, based on proposed doped CaCO3 particles. A fabrication method, which is cheap and suitable for large scale applications, is proposed based on doping Al and Fe elements into CaCO3 powders via sol–gel processes. The average solar absorptance is enhanced by about 560%, and the energy storage density decay rate after 50 cycles is prominently reduced to be as low as 4.5% from 35.5%. The decomposition temperature is reduced by 15 to 24 K depending on the atmospheres, and the decomposition kinetics of both doped and pure CaCO3 particles is found to follow the equation of phase boundary controlled reaction. The activation energy increases only slightly after doping, but will have a sharp increase when switching the atmosphere from N2 to pure CO2. This work paves the way to the design of high-performance calcium-based materials for next-generation high temperature thermal energy storage system.

Investigation of low grade thermal energy storage systems with phase changing materials

• Paraffin wax is a good storage medium due to fast charging and good latent heat absorption. • The two designs follow the same heating dynamics independently of the final temperatures reached. • An addition of a second inlet yields higher final temperatures and improves the HTF distribution. • This allows a better heat transfer rate, yet the temperature increase is minimal and negligible. • Higher or lower velocities at the inlet(s) can benefit the system depending on the design type. The use of phase changing materials (PCMs) for energy storage has been in the focus of scientific research for a while, primarily focusing on building cooling/heating applications due to favourable melting temperature ranges. In this paper we simulated the suitability of encapsulated Paraffin Wax on a small scale in a low temperature thermal energy storage system using COMSOL Multiphysics. Heat absorption and heating dynamics were analysed for different inlet designs and velocities, and the thermal gradient was evaluated across the tank geometry in a number of charging scenarios. Results show that paraffin wax proves to be a good storage medium based on its fast charging and good latent heat absorption. The study found that although an addition of a second inlet to the system yields higher final temperatures and improves the heat transfer rate, it does so minimally and therefore is not notably beneficial. Both designs follow the same heating dynamics independently on the final temperatures. Lastly, higher or lower velocities benefit the system based on the design.

Analysis on Peak-shaving Energy Efficiency of Thermal Power Plant with High Temperature Thermal Energy Storage

Integration of energy storage infrastructures into electrical grids represents a crucial milestone in the transition towards energy systems with high penetration of renewables. However, the high cost of the currently available technologies is a significant barrier for their implementation on the industrial scale. High temperature thermal energy storage systems, in combination with bottom steam cycles, are being investigated as potential cost effective alternatives to traditional large-scale energy storage technologies. In this study, by adding a high temperature heat storage device in the cold (hot) section of the reheating pipeline and taking the 300MW extraction steam turbine as the research object, it is concluded that the introduction of the high temperature heat storage device can improve the consumption of wind power and greatly reduce the power supply of the coal-fired unit. The coal consumption rate is analyzed. The ideas and methods for solving the energy consumption of the renewable energy and reducing the peak energy consumption are provided.

High Temperature Interaction of Si-B Alloys with Graphite Crucible in Thermal Energy Storage Systems.

Si-B alloys are proposed as a potential phase change material (PCM) in the novel high temperature thermal energy storage systems. For successfully introducing the new PCM, the selection of proper refractory material in the PCM container is vital. At present, graphite is chosen as a potential refractory material for the PCM container, due to its high temperature stability, low thermal expansion, and high thermal conductivity. The Si-B alloys and the high-temperature interaction with graphite are hence studied. The phase formation in the Si-B alloys and the interaction with graphite at B content of 2–11 mass % and temperatures of 1450–1750 °C were investigated. Carbides were observed at the interface between the solidified alloys and the graphite. A single SiC layer was produced at B content of 2 and 3.25 mass %. Otherwise, SiC and B4C layers were generated at B content higher than 5 mass %. In the Si-B-C system, the phase formation is dependent on the B content. Moreover, the equilibrium B content is calculated to be 3.66 mass % in the molten Si-B alloys at 1450 °C in equilibrium with SiC and B4C, based on the experimental results. In this regard, the eutectic alloy (3.25 mass % B) is recommended to be used as the new PCM in the graphite container, due to that it produces simple phases and also because it is expected not to deplete any B to the B4C layer.

Concrete based high temperature thermal energy storage system: Experimental and numerical studies

This paper presents the concept of developing a cost-effective Concrete based Thermal Energy Storage (CTES) system by performing extensive experimental studies and numerical simulations. A stand-alone experiment facility to study the performance of high-temperature thermal energy storage system which operates up to 500 °C using air as the heat transfer fluid has been developed. The CTES module is made of shell and tube configuration, where concrete is filled in the shell side, and 22 air passages are provided on the tube side. The inlet air temperature and velocity are the decision parameters used for analyzing the thermal behaviour of the CTES module. From the spatial variations of temperature, it is observed that the heat transfer rate is uniform and faster along all radial planes, whereas, the heat transfer rate drops gradually along the length of the CTES module due to drop in Heat Transfer Fluid (HTF) temperature. The parametric investigation conducted shows that the charging and discharging times were reduced by approximately 48% and 29%, respectively, with a change in inlet temperature of 40 °C at a fixed air velocity of 2 m/s. A 3-D model for CTES module developed using the finite element method has been validated with experimental results. The temperature contours plotted from 3-D simulation describes the spatial variation of CTES temperature at different inlet air temperatures. Further, a 1-D dynamic model has been developed, which is fast and accurate with a maximum error of ±4.9 °C with reference to real-time experiments and also provides a substantial scope of integrating the CTES with industrial applications. The outcomes of the present studies conclude that the developed CTES module is highly recommended for the high-temperature applications.

Performance comparison of medium temperature domestic packed bed latent heat storage systems

Thermal performances of three packed bed latent heat medium temperature thermal energy (TES) storage systems are evaluated during charging and discharging cycles at low (4 mL s−1), medium (6 mL s−1) and high (8 mL s−1) flow-rates. The three phase change materials (PCMs) used in the storage systems are adipic acid, erythritol and eutectic solder (Sn63Pb67). Each TES tank comprises of a packed bed of 40 similar aluminium spherical capsules with the three different PCMs inside each tank. The thermal performance of these systems is evaluated in terms of the temperature profiles, total energy, total exergy, useful energy and useful exergy, during charging and discharging. Erythritol did not undergo phase transition during discharging at all the flow-rates due to super-cooling. The erythritol TES system shows the greatest efficiencies with regards to all the performance parameters considered with an overall energy efficiency of about 39.1%. The efficiencies of all the TES systems generally decreased with an increase in the heat transfer fluid (HTF) flow-rate. Although the eutetic solder TES stored and discharged greater quantities of energy at all the flow-rates, its performance was reduced by the lower degree of thermal stratification in the tank and longer charging and discharging times.

Design and testing of a horizontal rock bed for high temperature thermal energy storage

Integration of energy storage infrastructures into electrical grids represents a crucial milestone in the transition towards energy systems with high penetration of renewables. However, the high cost of the currently available technologies is a significant barrier for their implementation on the industrial scale. High temperature thermal energy storage systems, in combination with bottom steam cycles, are being investigated as potential cost-effective alternatives to traditional large-scale energy storage technologies. In this study, the performance of a rock bed high temperature energy storage unit is experimentally investigated. The rock bed has a storage capacity of 450 kWhth, was built to store heat at 600 °C and is characterized in terms of thermal efficiencies. Charge and discharge cycles were performed for different operating conditions and the temperature distribution across the bed was analyzed. A particular focus was set on the study of the impact of buoyancy forces on the temperature gradient inside the bed and on the storage unit efficiency. Different charging powers, flow concepts and rock bed configurations were discussed to optimize storage operations and led to an improvement in efficiency of the charging phase of 17%. A thermal round trip efficiency of around 68% was estimated for the best configurations and different improvement approaches were discussed for future research.