Thermal Energy Storage Medium Research Articles

Hybridization of CSP Plants: Characterization of a Molten Salt Heater for Binary and Ternary Nitrate Salt Mixtures Fueled with Gas/Biogas Heaters

Hybridization of CSP plants with alternative energy sources (fuels) represents a means to improve flexibility of operation, power dispatchability and utilization factor of the plant. New generation CSP plants make use of molten salts as Heat Transfer Fluid (HTF) besides Thermal Energy Storage (TES) medium. Therefore, proper interfaces should be developed to effectively transfer the heat from the back-up source to the molten salt. This paper presents the results obtained in the experimental validation of an innovative gas-fueled Molten Salt Heater (MSH) prototype. The objective of this research is to validate the MSH design, where the specific properties of molten salts (compared to other HTFs, e.g., thermal oils) have to be taken into account. The developed reduced-scale MSH (90 kW thermal) consists of a heat exchanger with the molten salt flowing inside finned tubes cross-flowed with the hot flue gas generated in an upstream combustion chamber. LPG or a biogas-like mixture has been used as gas fuel. Experimental results have been obtained with two different molten salt mixtures: the “solar salt” binary mixture (NaNO3/KNO3, 60/40%w) typically used in CSP applications (up to 565 °C) and the ternary mixture known as Hitec XL® containing sodium/potassium/calcium nitrates (NaNO3/KNO3/Ca(NO3)2, 15/43/42%w) characterized by lower freezing temperatures. Experimental tests have been carried out changing some operative parameters like the flow rate of the molten salt (0.45–0.94 kg/s), the inlet temperatures of the molten salt (303–445 °C) and of the hot gas (596–632 °C). For both molten salt mixtures, it was demonstrated that heat transfer correlations based on the Dittus-Boelter equation allow to predict experimental results with <10% deviation between experimental and theoretical values of the heat transfer coefficient.

Effect of Flow Channel Cross Section Shape on the Performance of Heat Storage Structure

A numerical simulation is carried out for the thermal storage process in a heat storage device with magnesia bricks as thermal energy storage medium. Three types of cross section shapes (square, hexagon, circle) of the flow channel are considered with the inlet air temperature as high as 1023K. The perssure drop, temperature distribution and temperature uniformity of the flow channel are obtained. The results show that the pressure drop of the flow channel with circle cross section is the lowest when the Reynolds number is less than 2300 (in this case the inlet velocity is 5m/s), While when the Reynolds number is more than 2300, the flow channel with square cross section has the lowest pressure drop. The thermal storage time of the heat storage device decreases with the increase of air velocity. The temperature distribution of the flow channel indicates that the flow channel with hexagon cross section has the best comprehensive performance.

Measurement and analysis of thermal conductivity of ceramic particle beds for solar thermal energy storage

A systematic study was performed to measure the effective thermal conductivity of ceramic particle beds, a promising heat transfer and thermal energy storage medium for concentrating solar power (CSP). The thermal conductivity of the ceramic particle beds was measured using a transient hot-wire (THW) method within a temperature range of room temperature to 700 °C, the target operating temperature of the next-generation CSP systems. Two different types of ceramic particles were examined: (1) CARBOBEAD HSP 40/70 and (2) CARBOBEAD CP 40/100 (also known as ACCUCAST ID 50) with the average particle sizes of ~405 μm and ~275 μm, respectively, and thermal conductivities ranging from ~0.25 W m −1 K −1 to ~0.50 W m −1 K −1 from 20 °C to 700 °C in both air and N 2 gas. The gaseous pressure dependence of the thermal conductivity of the ceramic particle beds was also studied in the N 2 environment to differentiate the contributions from gas conduction, solid conduction, and radiation. Calculations using the Zehner, Bauer, and Schlünder (ZBS) model showed good agreements with the measurements. Based on the model, it is concluded that the effective thermal conductivity of the packed particle beds is dominated by the gas conduction while the solid conduction and radiation contributes to about 20% of the effective thermal conductivity at high temperature. • First thermal conductivity measurement of CARBOBEAD HSP 40/70 and CP 40/100 up to 700 °C. • Analysis of thermal conductivity from temperature and gaseous pressure dependence. • Quantification of relative contributions from gas, solid contacts, and radiation at various temperature. • Gaseous conduction is found to be the dominant mechanism.

A numerical study of supercritical carbon dioxide as a medium for thermal energy storage applications under natural convection

This numerical study investigates supercritical carbon dioxide (s-CO2) as a medium for thermal energy storage (TES) systems operating under natural convection regarding heat transfer coefficient and energy density. The numerical domain comprises a horizontal heated cylinder surrounded by the supercritical fluid. The modeling employs a 2-D formulation due to the axial symmetry while the constitutive equations consider that the thermophysical properties vary spatially with temperature and pressure. The results revealed thermodynamic states that maximize the heat transfer coefficient and energy density for constant-pressure operation. Besides, considering the numerous studies on correlations for the natural convection heat transfer coefficient for s-CO2, the analysis compares simulation data with correlations based on constant and integrated-averaged fluid properties. The results suggested that the constant-property formulation fails in the thermodynamic region where properties change significantly. Thus, this work proposes a criterion for deciding when to consider integrated-averaged properties regarding the behavior of the isobaric thermal expansion coefficient. Finally, the study extended the analysis to air, helium, and nitrogen while employing operational conditions suitable for power cycles using such fluids. The comparisons revealed that, among these competitors, helium presents the highest heat transfer coefficient, but s-CO2 has the highest energy density.

Operational Performance and Load Flexibility Analysis of Japanese Zero Energy House.

ZEHs (Zero Energy House) featuring energy-efficient designs and on-site renewable integration are being widely developed. This study introduced Japanese ZEHs with well-insulated thermal envelopes and investigated their detailed operational performances through on-site measurements and simulation models. Measurement data show that ZEHs effectively damped the variation of indoor air temperature compared to conventional houses, presenting great ability to retain inside heat energy, and are expected to potentially deliver energy flexibility as a virtual thermal energy storage medium. We developed a simplified thermal resistance–capacitance model for a house heating system; response behaviors were simulated under various scenarios. Results compared the variations of indoor temperature profiles and revealed the dependence of load flexibility on the building’s overall heat loss performance. We observed that overall heat loss rate played a crucial role in building heat energy storage efficiency; a well-insulated house shortened the heat-up time with less energy input, and extended the delayed period of indoor temperature under intermittent heating supply; a high set-point operative temperature and a low ambient temperature led to lower virtual thermal energy storage efficiency. The preheating strategy was simulated as an effective load-shifting approach in consuming surplus PV generation; approximately 50% of consumed PV generation could be shifted to replace grid import electricity for room heating during the occupied period.

Evaluation of the energy storage performance of PCM nano-emulsion in a small tubular heat exchanger

PCM emulsions have attracted considerable interest as the media for thermal energy storage (TES) owing to their high thermal storage capacity, desirable fluidity and thermal conductivity. However, the direct transportation of PCM emulsions in TES systems for both charging and discharging is rarely reported. In this work, a PCM-in-water nano-emulsion was prepared with n-hexadecane and suitable surfactants for cooling energy storage at a charging and discharging temperature range of 20–5 °C and 5–15 °C, respectively. It was applied to a tubular heat exchanger system to evaluate its TES performance for a cooling panel of 0.2 m2 total surface area. The thermal storage performance was notably increased with the flow rate of emulsion through the exchanger tube. The volumetric thermal storage capacity of charging was 50% higher than that of water. The cooling energy could be rapidly released in the discharging process, 79% of the stored energy at averaging 25 W during most of the discharging period. The emulsion remained stable throughout the test period. Overall, the results demonstrated that the PCM nano-emulsion has the unique characteristics of high static stability, and high energy releasing efficiency and the promising potential for air-conditioning application in buildings.

Innovative and cost-effective nanodiamond based molten salt nanocomposite as efficient heat transfer fluid and thermal energy storage media

Novel and efficient molten salt nanocomposites as heat transfer nano-fluids and thermal energy storage materials have been developed. Cost-effective detonation nanodiamonds (NDs) of an average particle size of 10 nm were uniformly dispersed in binary nitrate molten salt using the ultrasonication process to achieve well dispersion of NDs. NDs mass loadings were varied and optimized, furthermore, the optimized mass loading of NDs was dispersed using solid state method for comparison. The thermo-physical properties of the developed ND-molten salt nanocomposites were studied in terms of melting point temperature, thermal stability temperature, thermal conductivity, volume heat capacity and thermal diffusivity and were enhanced in terms of their applications as heat transfer nano-fluids and thermal energy storage media. Therefore, melting point was reduced by 16 °C and the thermal stability was improved by 35 °C displayed enhanced temperature range compared to base binary molten salt. On other hand, the thermal conductivity and volume heat capacity were significantly improved achieving enhancement by 93 and 38% respectively, compared to base binary molten salt. Furthermore, the thermal diffusivity of developed ND based molten salt was enhanced by 43% compared to blank molten salt recorded superior values among reported nano-fluids. The surface morphology and dispersion of NDs in binary nitrate molten salt was studied and visualized using SEM and TEM.

Effect of monotonic and cyclic temperature variations on the mechanical behavior of a compacted soil

Settlement and slope stability of an embankment are mechanical aspects that play a major role in structural safety. Due to the tendency towards renewable energy, the embankments can be considered as a suitable medium for thermal energy storage. Using horizontal heat exchanger tubes, the thermal energy will be stored in different compacted soil layers in an embankment. Therefore, this structure can be subjected to daily and seasonally temperature variations due to heat extraction. Cyclic temperature variation can modify the mechanical behavior of compacted soils. Thus this study aims to investigate the effect of monotonic and temperature cycles in the range of 5 to 50 oC, on consolidation parameters and shear characteristics of a compacted sandy lean clay. To achieve this, temperature-controlled oedometric and direct shear tests were performed. Results showed that the effect of heating and cooling on mechanical properties is more pronounced under vertical stresses higher than the preconsolidation pressure. By heating, the normal consolidation line shifted to the left and consequently, the apparent preconsolidation decreased. Compression and swelling indexes could be considered independent of temperature variation. Due to the temperature cycles, the volumetric response of the compacted soil in oedometric and shear tests was stress history-dependent. Results of the direct shear tests showed that by temperature variation (heating/cooling and temperature cycles) the cohesion increased and the friction angle remained unchanged.

A novel thermal storage integrated evacuated tube heat pipe solar air heater: Energy, exergy, economic and environmental impact analysis

This study aims to present a novel thermal energy storage integrated evacuated tube heat pipe solar air heater suitable for high-temperature applications. A new heat pipe arrangement was introduced in this study by attaching all the evaporator tubes of heat pipes to a common condenser section. A thermal energy storage medium was kept in a chamber within the common condenser unit of the heat pipe to obtain uniform outlet air temperature from the system when there is fluctuation in solar radiation. Therminol 55 was used as the sensible heat storage medium in the system. The Performance evaluation of the system was conducted by varying four airflow rates 0.03 kg/s, 0.1 kg/s, 0.2 kg/s and 0.3 kg/s, respectively. 0.03 kg/s resulted in a maximum outlet air temperature of 128 °C and the system delivered more than 100 °C of hot air until 10 h of operation. The maximum thermal and exergy efficiency obtained during each airflow rate was 38%, 77%, 85%, 89%, and 5.01%, 5.21%, 5.34%, and 5.76% respectively. An average of 6200 kJ energy was collected by the present system during its operation. The total money saved per month during the operation of TS-ETHPSD was 445 INR. The Payback Period for the system is 69 months. The solar air heater can be considered as an environment-friendly device as it can save 73 kg/month of Co2 from entering the atmosphere.

Effects of Particle Size and Concentration of Magnesium Oxide on the Lubricating Performance of a Chloride Molten Salt for Concentrating Solar Power

One engineering challenge for the next-generation concentrating solar power (CSP) is to identify appropriate heat transfer and thermal energy storage media. A chloride salt mixture (20 wt % NaCl + 40 wt % MgCl₂ + 40 wt % KCl) in a molten phase is proposed as a potential fluid for carrying the heat as well as lubricating the CSP pump bearings. Due to the hygroscopic nature of MgCl₂, the presence of magnesium oxide (MgO) particles is inevitable. In order to understand the impact of MgO particles, this study investigated the tribological behavior of the molten chloride salt mixture containing MgO of various particle sizes and concentrations. Tests were conducted at 750 °C in a dry Ar environment using a zirconia ball sliding against a Haynes 244 alloy disc lubricated by the molten salt. There was a clear trend of the increased friction coefficient and wear loss for a larger particle size and/or a higher concentration of MgO. Distinct wear mechanisms were identified for the ceramic ball and alloy disc. Results provide fundamental insights for this new research avenue of molten salt lubrication for CSP.

Energy and exergy analyses of drying medicinal herb in a novel forced convection solar dryer integrated with SHSM and PCM

This article examined the performance of a low cost, economically viable and a novel forced convection solar drying system. To control the thermal fluctuations during the day-night drying process, solar air collector (SAC) was equipped with sensible heat storage medium (SHSM) while, drying unit contained paraffin RT-42 as a thermal energy storage (TES) medium. The experiment was conducted considering a sample size of 9.0 kg of Valerian rhizomes. The rhizomes were dehydrated to saturation value of 9% from preliminary moisture content of 89% on wet basis and took 120 (216) hours with (without) use of TES. The performance evaluation of each element of the dehydrating system was carried out on the basis of energy and exergy analyses. The mean values of energy and exergy efficiencies of SAC without (with) TES were computed to be 9.8(26.10) % and 0.14(0.81) % respectively. The overall drying rate without (with) applying TES was 0.028(0.051) kg/hour. The exergy efficiency of the drying unit was assessed to vary from 3.7% to 75.15% with a mean value of 30.28%. The specific energy consumption (SEC) of the rhizomes and the overall efficiency of the drying system were 11.33 kWh/ kg of moisture and 10.53%, respectively.

One-step synthesis of molten salt nanofluid for thermal energy storage application – a comprehensive analysis on thermophysical property, corrosion behavior, and economic benefit

Concentrating solar power (CSP) plants have been widely commercialized for generating electricity from solar energy. Thermal energy storage (TES) systems have been deployed as a key component in CSP plants for resolving the supply-demand gap during daily operation. Nowadays, molten salts are used as both the primary heat transfer fluid (HTF) and as TES medium in many CSP plants. However, molten salts suffer from poor thermo-physical properties, e.g., specific heat capacity is typically less than 2 J/(g•K). In addition, the practical operational performance of CSP plants running on molten salt is limited by the chemical compatibility between the heat transfer fluid and the container. Many efforts have been devoted to developing molten salt material with enhanced thermal properties and reduced corrosivity. As a typical example, doping molten salts with minute quantities of nanoparticles has shown potential as a simple approach for enhancing the thermo-physical property values and mitigating corrosion due to formation of passivation on metal surface from naturally deposited nanoparticles. Nevertheless, mixing nanoparticles in solvents is not cost-effective for industrial applications and often results in agglomeration and precipitation issue. In this study, the feasibility of using molten salt nanofluid as the TES medium has been examined comprehensively based on its thermophysical property, corrosion behavior, and the economic value. Specifically, the specific heat capacity of the solar salt (NaNO3-KNO3 eutectic)-based nanofluid prepared from both two-step and one-step methods were measured at elevated temperatures (~500°C). In addition, the corrosion behavior of SS304 in molten salt nanofluids is investigated experimentally at 565 °C for 432 hours interval. Finally, the techno-economic benefit is assessed based on the energy storage capacity and material costs. The results from the study demonstrated that the one-step synthesis protocol for nanofluids involving generation of nanoparticles in-situ from cheap additives is a viable and cost-effective approach in industrial applications (e.g., CSP) for enhancing the energy storage capacity and power rating as well as for extending the life-cycle of equipment (e.g., heat exchangers).

Investigation of an integrated solar thermochemical plant for hydrogen production using high-temperature molten salt

One of the most promising methods for producing hydrogen is the use of a water splitting process utilising the copper-chlorine (Cu-Cl) thermochemical cycle. The Cu-Cl cycle uses heat and electricity to produce hydrogen and oxygen from the decomposition of water molecules. In this paper, a new solar-powered integrated system is proposed which utilises (LiNaK)2CO3 high-temperature carbonate molten salt as both a heat transfer fluid and a thermal energy storage medium to provide the required heat for the Cu-Cl cycle reactors and heat exchangers. The system is integrated with a supercritical regenerative steam Rankine cycle (SRC) which produces the required electricity for the electrolyser unit. Thermodynamic and economic analyses were conducted to evaluate the proposed system in terms of hydrogen production cost and system performance. For the base case, the integrated system was found to be capable of producing 823.71 kg/h of hydrogen. The system is optimised for two objective parameters, overall system thermal efficiency and the levelized cost of hydrogen. The results of optimisation analysis indicated that, for the optimal Pareto solution, the overall system thermal efficiency and levelized cost of hydrogen were 29.17%, and $7.58/kg of H2, respectively.

Improved solar drying performance by ultrasonic desiccant dehumidification in indirect forced convection solar drying of ginger with phase change material

One of the vital crops used in the recent times of pandemic COVID-19 for making medicines is ginger. It is essential to reduce the drying time compared to open sun drying while maintaining the quality of the dried ginger. The current work highlights about the novel arrangement in indirect forced convection solar drying of ginger by the combined application of liquid desiccant supplied as droplets using ultrasonic atomizer for dehumidification of air stream and paraffin wax as the thermal energy storage medium. The present study is a performance comparison of forced convection solar drying of ginger against open sun drying with and without dehumidification of air stream using liquid calcium chloride as desiccant at air flow rates of 0.153 kg/s and 0.077 kg/s. Using the dryer, fresh ginger having initial moisture content of greater than 80% is reduced to nearly 10% using liquid desiccant at the flow rate of 0.153 kg/s taking 13 h less compared to open sun drying while it is 9 h less without using desiccant. At a flow rate of 0.077 kg/s, moisture content is reduced to nearly 20% with saving of 11 and 6 h, respectively. Three drying models are also presented with the experimental data, amongst which the parabolic fit is found to be the best model for both with and without using desiccant.

Performance Analysis of Multi-Energy Hybrid System Based on Molten Salt Energy Storage

This paper briefly summarizes the current status of typical solar thermal power plant system, including system composition, thermal energy storage medium and performance. The thermo-physical properties of the storage medium are some of the most important factors that affect overall efficiency of the system, because some renewable energy sources such as solar and wind are unpredictable. A thermal storage system is therefore necessary to store energy for continuous usage. Based on the form of storage or the mode of system connection, heat exchangers of a thermal storage system can produce different temperature ranges of heat transfer fluid to realize energy cascade utilization. Founded upon the review, a small hybrid energy system with a molten-salt energy storage system is proposed to solve the problems of heating, cooling, and electricity consumption of a 1000 m² training hall at school. The system uses molten-salt storage tank, water tank and steam generator to change the temperature of heat transfer fluid, in order to realize thermal energy cascade utilization. Compared to the existing heating and cooling system, the proposed system needs more renewable energy and less municipal energy to achieve the same results according to simulation analysis. Furthermore, by improving the original heating and cooling system, PMV has been improved. The comprehensive efficiency of solar energy utilization has been increased to 83%.

Performance study on customized solar still by optimum fins for production of distilled water

The present research paper performance of a only one basin and one slope solar at rest with fly ash particles is improved by cylindrical fins (0.003 m diameter, 0.03 m length) which are effectively use to add to the surface area of the basin and evaporation rate of water. The fins also reduce the time required for preheating the saline water. In this work, the fins are attached with the basin to influence the productivity of the thermal energy storage medium (fly ash partiles-3 kg) solar still and other parameters like water temperature and vapour temperature etc. The experimental works are carried out in passive solar still for various quantity of water to find the optimum value. Then, the experiments have been conducted using fly ash particles with optimum quantity of water and varying the number of fins. Finally, the performance of the solar still using fly ash particles with optimum fins is compared to without fins below the similar climate condition. The customized solar still with optimum fins configuration produces 41% more distilled water.

Mathematical modelling of heat transmission in the temperature history apparatus by using inverse method to evaluate the latent heat of high temperature PCMs

High temperature phase change materials (PCMs) are being proposed as thermal energy storage media in concentrated solar power plants. The thermophysical properties of PCMs, such as the latent heat of fusion/solidification need to be defined to enable system design and evaluate system performance and cost analysis. Several methods have been used to measure the latent heat of fusion/solidification of PCMs including the T-history method. The original assumption of the T-history method is the Biot number should be less than 0.1 where the lumped capacitance method is applicable. However, when the Biot number is more than 0.1 then the original equations are not applicable to estimate the latent heat of PCM. This paper demonstrates how a one-dimensional heat transfer mathematical model can be used by applying an inverse method to overcome the limitation of the lumped capacitance method in the original T-history method to determine the latent heat of PCMs. The mathematical model demonstrates that variation of the temperature distributions inside the samples for the discharge curves. Furthermore, this model shows the errors in the latent heat values if the T-history method is used. It is evident from the discharge curves of low thermal conductivity PCMs that the original simplified equations used for interpreting the T-history test results are not applicable for calculating the latent heat at different ambient temperature.

Combined influence of fin, phase change material, wick, and external condenser on the thermal performance of a double slope solar still

This study focuses on the enhancement of potable water productivity of an active double slope modified solar still (SS) considering the combined effect of solid rectangular fin, paraffin wax as a thermal energy storage medium, black cotton cloth as wick material, and an external condenser. The performance of three double slope SSs (modified, finned, and traditional) is evaluated considering five different cases. A comparative assessment of the performances of three double slope SSs is performed and it is perceived that the supreme productivities are 3.07, 2.70, and 2.46 L/m2 for modified, finned, and traditional SSs, respectively. The maximum efficiency of the modified SS with and without external condenser are found as 39.74% and 30%, respectively. An external condenser insertion in the modified SS boosts the productivity by approximately 10%. The average daily efficiency of the modified SS is 14.23% and 22.33% higher than the finned and traditional SS, respectively. The economic analysis is also conducted, and the cost per liter (CPL) values for traditional SS, the finned one, and the modified one are found as 0.0135, 0.0133, and 0.0117 $/L/m2, respectively. The novel modification of SS could be a propitious alternative to meet up the potable water demand in remote regions with low cost.

High-energy storage graphene oxide modified phase change microcapsules from regenerated chitin Pickering Emulsion for photothermal conversion

Microencapsulated phase change materials (microPCMs) are not only important thermal energy storage medium but can also be applied in harvesting and storage of solar thermal energy by using solar absorbent materials as the shell. In this study, high energy storage polyurea (PUA) microPCMs for photothermal storage were fabricated from a Pickering emulsion consisting of bio-derived and sustainable regenerated chitin (RCh) from shrimp shells as the emulsifier. Graphene oxide (GO) was used as the photon captor and paraffin wax as the phase change material (PCM). The chemical, physical and thermal properties of the resulting microPCMs were studied. Using 0.1% w/w RCh Pickering emulsifier and 0.1% w/w GO to modify the microPCM, a 92.3% encapsulation efficiency was achieved. The microPCM at a core/shell ratio of 9:1 stored up to 234.7 J/g heat energy, was leak proof, and thermally reliable over 100 heating and cooling cycles with efficient photothermal conversion up to 76.03% conversion efficiency. Therefore, this cost-effective green process can be embraced for fabrication of high-energy storage and reliable functional microPCMs.

Preparation and study on thermophysical properties of inorganic salt hydrate as energy storage materials

In this paper, the preparation methods of new inorganic salt hydrate mixtures and their experimentally tested thermophysical properties are presented. Two inorganic salt hydrates mixed together at different weight proportions to study their thermodynamic properties for energy storage applications. Three samples having weight proportions of salt mixtures as 70:30, 80:20 and 90:10 are prepared and Differential Scanning Calorimetry (DSC) is used as a thermal analyzer to find the thermal stability of the samples. The DSC results of the salt hydrate are observed for determining the melting temperatures, latent heats and specific heats during melting and freezing process. The melting point, heat of fusion, and heat capacity of the salt hydrate samples are observed as 46.6 °C , 133.81 J/g and 8.75 J/gK for 70:30, 47.28 °C, 169.91 J/g and 15.95 J/gK for 80:20 and 47.26 °C, 173.96 J/g and 14.20 J/gK for 90:10. The time requirements for the completion of melting and freezing process of the salt mixture results are compared. Thermal and heat transfer characteristics of the newly prepared salt mixtures are experimentally investigated for the purpose of recommending it to use as thermal energy storage medium. The inexpensive inorganic salt hydrate mixtures, with greater enthalpy, higher heat storage capacity and good thermal reliability, shows great potential for heat storage applications in the low temperature solar thermal applications like water heaters, building space heating applications, domestic solar cookers and dryers.