Thermal Energy Storage Performance Research Articles

Enhanced thermal energy storage performance of salt hydrate phase change material: Effect of cellulose nanofibril and graphene nanoplatelet

Thermal energy storage (TES) has attracted intense attention because of its positive contribution to sustainable energy utilization. To improve the TES performance of sodium acetate trihydrate (SAT), the combined use of cellulose nanofibril (CNF) and graphene nanoplatelet (GNP) was investigated to tackle the phase separation problem and to improve the thermal conductivity of SAT. Phase stability and rheology tests revealed that adding 0.8% of CNF to SAT increased viscosity, enhanced solid-like rheological behavior, and successfully eliminated phase separation. Meanwhile, the amphiphilicity of CNF facilitated the dispersion of GNP. Sodium phosphate dibasic dodecahydrate (DSP; Na2HPO4·12H2O) was selected as the nucleating agent, which reduced the supercooling degree of SAT to 2.1 °C. Phase change materials (PCMs) were prepared by simply blending GNP pre-dispersed with CNF, DSP, and SAT. Due to the excellent thermal performance of GNP and its good dispersion by CNF, the prepared PCM composites showed enhanced thermal conductivity compared with that of pure SAT. Thermal reliability testing indicated that the melting point and enthalpy of the prepared PCM composites decreased after 100 melting/freezing cycles. Overall, PCM composites with enhanced performance were fabricated based on renewable, bio-based, and biodegradable nanocellulose. These composites can be used for certain TES applications.

A novel composite for thermal energy storage from alumina hollow sphere/paraffin and alkali-activated slag

This study proposes a novel thermal energy storage composite (TESC) with an alumina ceramic-based form-stable phase change material (FSPCM) as the phase-change aggregate and alkali-activated slag as the matrix material. FSPCM was prepared with alumina hollow sphere as the supporting material and paraffin as the phase change material (PCM). A series of tests were conducted to investigate the thermo-mechanical properties of the TESC. The results show that the phase transition peak temperature of FSPCM is 23.6 °C, the latent heat value is 69.8 J/g, and the paraffin adsorption mass fraction with respect to the alumina hollow sphere can be as high as 82.33%. As the mass fraction of FSPCM increases from 0% to 80%, the latent heat value increases from 0 J/g to 19.18 J/g, while the compressive strength can still be maintained above 24.04 MPa. The effective thermal penetration depth and the thermal energy storage performance of the TESC were mathematically analysed and experimentally tested. Meanwhile, the influence of FSPCM dosage on the mechanical strength of the TESC and the microstructure characteristics of the interface transition zone between FSPCM and the matrix were investigated. A mathematical model with a maximum relative error of 12.08% was established for predicting the 28-day compressive strength of TESC.

Enhancing the melting of phase change material using a fins–nanoparticle combination in a triplex tube heat exchanger

A combination of heat transfer augmentation techniques is highly necessary to enhance the performance of Thermal Energy Storage (TES) systems employed in a wide range of applications. The major issue is that many of the Phase Change Materials (PCMs) possess low thermal conductivity (k≤0.2 W/m K), resulting in an inefficient melting process. Triplex Tube Heat Exchanger (TTHX) based TES system is both numerically and experimentally studied using Paraffin (RT82) with Alumina (Al2O3) nanoparticles that has a charging temperature in the range of 78.15–82.15 ∘C. The experimental findings indicate that the Paraffin is not completely melted within the required time of four hours for the inside heating method at 97 ∘C. The Paraffin is successfully melted for both sides heating at 90 ∘C in lesser time and average temperature than the outside heating. With different charging temperatures, the Paraffin melting was consumed a short time for the non steady state at the mass flow rate of 29.4 kg/min, compared with the 16.2 and 37.5 kg/min for inner and outer tubes. Other outcomes were that with the fins–nanoparticle combination, an improved performance for melting the Paraffin, compared with those that occurred without nanoparticle. Furthermore, in the numerical study, compared with the pure Paraffin case, the melting time was minimized for TTHX with longitudinal fins (12%) and TTHX with triangular fins (22%) for the PCM having 10% nanoparticle, respectively. Close agreement is found between the numerical and experimental findings.

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%.

Experimental study on the charging and discharging behaviour of capric–lauric acid/oleic acid mixture in a cold thermal energy storage system for cold storage applications

In this study a capric–lauric acid/oleic acid mixture was synthesized to form a low temperature PCM for cold storage applications. An experimental investigation was carried out in a double helical coil cold thermal energy storage unit (CTESU) to study the charging and discharging behaviour of capric–lauric acid/oleic acid mixture. The inlet temperature and flow rate of the heat transfer fluid (HTF) were selected as experimental parameters to analyse the cold thermal energy storage performance of PCM. The analysis includes charging and discharging behaviour, energy dissipation rate, energy storage rate, energy charging efficiency, energy discharging efficiency and influence of HTF inlet temperature and flow rate under varied experimental parameters. With the increased contact area, the developed double helical coil considerably improves the heat transfer performance of the cold thermal energy storage unit. The discharging rate was 18% faster than the charging process due to predominance of natural convection. The PCM solidification during charging process is controlled by conduction. With increasing HTF flow rate during the discharging process, the melting rate was 16% higher than lower flow rates. In contrast, as flow rates increased, the charging rate did not change significantly. The role of natural convection and stratification which leads to unnecessary mixing of PCM consequently lowering the cooling energy stored. Also results suggest that capric–lauric acid/oleic acid can be utilized as a potential PCM for low temperature storage applications.

Environmental-friendly electrospun phase change fiber with exceptional thermal energy storage performance

Electrospun phase change fiber (PCF) releases more than 85 wt% of organic solvents yet suffers from long-term thermal cycling-induced leakage. In this contribution, a novel PEG/PVA composite PCF was obtained by electrospinning their aqueous solution instead of using organic solvent in traditional method, which makes design of PCF towards green and cost-effective direction. Meanwhile, a simple surface crosslinking technology was applied to prevent leakage of PEG during the long-term service, and improve the thermal stability and tensile strength of the obtained crosslinked PCF (CPCF). The CPCF-50 shows the optimal morphological structure and exhibits the a relative high latent heat of 72.3 J/g. Owing to the confinement effect, CPCF exhibits robust thermal, chemical, and morphological stability with respect to 1000 thermal cycling. The CPCF also shows exceptional temperature regulation capability. The thermoregualting times of heating and freezing processes of CPCF-50 are 59.0% and 89.5% longer than those of the control and CPCF-0, respectively. Therefore, the eco-friendly and cost-effective prepared CPCF in this work, which exhibits relatively high latent heat and long-term reliability, paves a new way for the large-scale production of phase change fiber for thermal energy storage application.

Analysis of different operating strategies of thermal energy storage with radiant cooling system

SummaryThermal energy storage systems in building cooling applications have been explored extensively as a peak load‐shifting technology. Thermal energy storage performance has been recognized and studied from an energy cost‐savings point of view because of peak‐valley price differences, but not many studies have been conducted from an energy savings viewpoint. This study experimentally investigates the performance of the energy storage‐retrofitted to a ceiling‐type radiant cooling system. To study the performance, a water‐based storage system was designed and developed for an academic office building equipped with a radiant cooling system. The water in the storage tank was cooled to a certain storage temperature in the nighttime, and the same water was used during the daytime for meeting the cooling load. Different combinations of charging and discharging schedules were analyzed. The key objective of the study was to achieve energy savings and energy‐cost savings simultaneously. This objective was accomplished by identifying the major factors contributing to the energy consumption of the storage‐retrofitted cooling system and devising novel operating strategies, leading to an enhanced energy savings potential. Two operating strategies comprising 24 operating scenarios were compared and the storage was used to dispatch the load for 3 hours of the day as a full storage unit. Results showed that in hot and dry climate conditions, using the storage with the radiant cooling system offered energy savings of 3% to 14%. The energy‐cost analysis was also performed using a time‐of‐day electricity tariff plan. The energy‐cost savings varied from 17.5% to 22.4% for these operating scenarios.

Exploring thermodynamic potential of multiple phase change thermal energy storage for adiabatic compressed air energy storage system

Adiabatic compressed air energy storage provides a promising solution for matching intermittent sources of renewable energy with customer demand during the daily cycle. In this work, a novel thermal energy storage strategy — using multiple phase change materials — are proposed to achieve less thermal energy loss in an adiabatic compressed air energy storage system, and the thermal energy storage performance of this type of thermal accumulators is discussed in detail by constructing the off-design thermodynamic mathematic model. The obtained results indicate that, compared with single-phase change material, a smaller amount of thermal energy loss appears while using multiple phase change materials stores compressed heat. Besides, in the initial cycles, there is still considerable residual heat remaining in the proposed thermal accumulator, however this will gradually fade away after 4 cycles, resulting in the thermal energy storage efficiency increases to 91.5%. It is confirmed that the round-trip efficiency increases by 4.7% if a thermal accumulator with multiple phase change materials was used for a conventional compressed air energy storage plant — Huntorf.

Design of stearic acid/graphene oxide-attapulgite aerogel shape-stabilized phase change materials with excellent thermophysical properties

The large-scale commercial application of phase change materials (PCMs) was seriously limited by the leakage, poor heat storage capacity and slow thermal response behavior. To address these issues, stearic acid/graphene oxide-attapulgite aerogel (SA/HGA-ATP) shape-stabilized PCMs were fabricated via hydrothermal method. The morphology, structural characteristics, thermal properties were determined and the effects of ATP percentage on the thermal properties of confined SA were studied synchronously. The nanofibers of ATP intercalated among GO sheets via grafting modification and formed fiber-bridging 3D-network. Due to the enhanced loading interspaces and suppressed volumetric shrinkage of hybrid matrix, SA/HGA-ATP exhibited excellent thermal energy storage capacity (190.9 J g−1) and ultra-high SA contents (approx. 98 wt%) without leakage. The intercalated nanofibers sheltered the oxygen-containing groups of matrices, leading to the promotion of thermal energy storage performance with increase of ATP contents. Besides, the as-prepared PCMs displayed outstanding thermal response behavior because the interconnected and fiber-bridged matrices provided continuous thermal transfer pathway. Due to the protection of matrix, the composite PCMs also exhibited superior structure stability and incomparable thermal energy storage/release reliability. Considering the outstanding thermal-physical properties and low-cost of ATP, the SA/HGA-ATP has the potential to be applied in the fields of thermal energy storage, conversion and utilization.

Thermal conductivity enhancement of a sodium acetate trihydrate–potassium chloride–urea/expanded graphite composite phase–change material for latent heat thermal energy storage

To create an energy–efficient heat pump latent heat thermal energy storage (HPLHTES) system, a novel sodium acetate trihydrate (SAT)–potassium chloride (KCl)–urea/expanded graphite (EG) composite phase–change material (CPCM) was developed in this study. EG was introduced into the base composite salt to prepare the novel CPCM, SAT–KCl–urea/EG. The thermal properties, crystalline phase, and morphology of the resulting CPCM were experimentally characterized. It was found that the SAT–KCl–urea/EG CPCM comprising 9 wt% EG was optimal for affording improved thermal–energy storage performance without compromising phase–change properties. The obtained CPCM exhibits an appropriate melting point of 47.5 °C and a high phase–change enthalpy of 200.3 kJ/kg. The thermal conductivity of the SAT–KCl–urea/EG CPCM was 1.48 W/m·K, approximately 5.3 times that of the SAT–KCl–urea composite salt (0.28 W/m·K). The required phase–change duration of the SAT–KCl–urea/EG CPCM was approximately 30% less than that of the SAT–KCl–urea composite salt. After 100 thermal–cycle tests, the SAT–KCl–urea/9 wt% EG CPCM retained a desirable phase–change temperature of 45.9 °C, high latent heat of 190.8 kJ/kg, and an acceptable supercooling degree of 1.56 °C. These thermal feature results showed that the SAT–KCl–urea/9 wt% EG CPCM exhibited great potential for use in the HPLHTES systems.

Solid–solid phase‐change materials cross‐linked by carbon quantum dots: Address leakage and high super‐cooling degree simultaneously

AbstractLeakage and super‐cooling are two main disadvantages of polyethylene glycol (PEG) when used as phase‐change materials (PCMs) for thermal energy storage, which seriously restrict the practical applications of the materials. In this study, we employ carbon quantum dots (CQDs) as cross‐linkers to fabricate PEG‐based solid–solid PCMs, which not only address the leakage issue of PEG during melting process, but also have much lower super‐cooling degree compared with PEG, exhibiting extraordinary thermal energy storage performance. CQDs serve as a heterogeneous nucleating agent in the crystal domains of these PEG‐based PCMs according to the characterizations. Additionally, CQDs are also beneficial to the thermal stability of the PCMs. And these PEG‐based PCMs have high phase‐change enthalpies (94.4 J/g) and applicable phase‐change temperatures (25–37°C), showing potential for thermal energy storage.

Numerical Evaluation of the Transient Performance of Rock-Pile Seasonal Thermal Energy Storage Systems Coupled with Exhaust Heat Recovery

This study seeks to investigate the concept of using large waste rocks from mining operations as waste-heat thermal energy storage for remote arctic communities, both commercial and residential. It holds its novelty in analyzing such systems with an experimentally validated transient three-dimensional computational fluid dynamics and heat transfer model that accounts for interphase energy balance using a local thermal non-equilibrium approach. The system performance is evaluated for a wide range of distinct parameters, such as porosity between 0.2 and 0.5, fluid velocity from 0.01 to 0.07 m/s, and the aspect ratio of the bed between 1 and 1.35. It is demonstrated that the mass flow rate of the heat transfer fluid does not expressively impact the total energy storage capacity of the rock mass, but it does significantly affect the charge/discharge times. Finally, it is shown that porosity has the greatest impact on both fluid flow and heat transfer. The evaluations show that about 540 GJ can be stored on the bed with a porosity of 0.2, and about 350 GJ on the one with 0.35, while the intermediate porosity leads to a total of 450 GJ. Additionally, thermal capacity is deemed to be the most important thermophysical factor in thermal energy storage performance.

Performance of laboratory scale packed-bed thermal energy storage using new demolition waste based sensible heat materials for industrial solar applications

Thermal energy storage (TES) is essential for cost-effective use of solar energy in industries. The most energy intensive processes in industry operate below 200 °C. This study tested a new sustainable and low-cost sensible thermal energy storage material (STESM) based on demolition wastes in a lab-scale packed bed TES system, specifically built to analyze its performance in industrial solar applications below 200 °C. This system was investigated both experimentally and numerically under different operating conditions. One-dimensional continuous phase model used verified that experimental results were in good agreement with numerical ones during charging and discharging steps. Performance of demolition waste STESM was compared with Therminol 66 synthetic oil as a liquid heat storage media. The maximum system energy efficiency of the packed-bed filled with demolition waste STESM was 67% at charging temperature of 150 °C and superficial fluid velocity of 0.95 mm s−1, while it was 63% for Therminol 66. Packed bed TES system with demolition waste STESM showed good performance up to 180 °C in fully laminar flow regime (Rep < 10) and Bi number < 0.1. The material cost of demolition waste STESM is at least tenfold cheaper than the alternative natural rock based packing materials. Packed bed TES systems using demolition wastes can be recommended for low cost and sustainable solar applications in industry.

A Mini-Review on the Effect of Design Parameters on the Performance of Latent Heat Thermal Energy Storage

A Mini-Review on the Effect of Design Parameters on the Performance of Latent Heat Thermal Energy Storage

Enhanced thermal energy storage of nitrate salts by silica nanoparticles for concentrating solar power

SummaryDispersing nanomaterials can effectively improve the thermal energy storage performance of molten salts. However, research on such improvement mechanism is still immature, as the understanding of the performance improvement levels induced by different nanoparticles has not been consistent. Using silica nanoparticles as additives, this work prepares nine molten salt nanofluids by aqueous solution method with the use of potassium nitrate, sodium nitrate, and their binary mixture as base salts. The key thermal performance of these nanofluids are evaluated through different characterization approaches, such as differential scanning calorimetry, thermal gravimetric analysis, laser flash analysis, and scanning electron microscopy for microstructural characteristics. The possible performance enhancement mechanism caused by silica nanoparticle is also explored. Results show that, for these nine molten salt nanofluids containing silica nanoparticles, the change in the melting point is negligible. The potassium nitrate salt dispersed with 20‐nm silica nanoparticles achieve the highest performance improvement not only in latent heat but in specific heat and thermal conductivity, while sodium nitrate salt nanofluids reach maximum improvement at their decomposing temperature. The improvement level can be associated with the electronegativity of alkali ions. Cloud nuclei cause microstructure differences among molten salt nanofluids and may be the main reason for the thermal performance enhancement of the molten salt nanofluids.

Research on long-chain alkanol etherified melamine-formaldehyde resin MicroPCMs for energy storage

Aiming at the characteristics of melamine-formadehyde resin with poor toughness and high brittleness owing to its unique structure of trizine ring, long-chain alkanol etherified melamine-formaldehyde prepolymer was selected as shell material to synthesize MicroPCMs. The study mainly explored the effect of etherification and relevant influencing factors in the etherification process on performance of microcapsules. Through a series of characterization, the alkyl chain of dodecanol can be grafted into the methylolated melamine-formaldehyde prepolymer via etherification, contributing to enhancing the adsorption of core material and encapsulation effect of microcapsules. The optimal molar ratio of melamine, formaldehyde and n-dodecanol was 1:7:2 in etherification process. N-dodecanol-etherified melamine-formaldehyde prepolymer was synthesized via one-step method as shell material for encapsulation n-dodecanol to obtain MicroPCMs4-2. The MicroPCMs4-2 finally obtained presented complete microstructure, uniform particle size distribution, well dispersibility and satisfactory thermal energy storage performance with high enthalpy value (△Hm = 166.1 J/g, △Hc = 160.2 J/g) and effective loading (72.2%) for potential applications to thermal-regulating textiles and energy storage fibers.

Performance mapping of packed-bed thermal energy storage systems for concentrating solar-powered plants using supercritical carbon dioxide

Thermal energy storage is extremely important to power plants that rely on intermittent heat sources. Additionally, the interest in power cycles operating with supercritical carbon dioxide (s-CO2) is increasing recently. Therefore, this study investigates the performance of a packed-bed thermal energy storage (TES) system using s-CO2. The 1-D computational porous medium-based model accounts for the heat exchange between the solid matrix and heat transfer fluid through a two-temperature formulation, thus allowing the calculation of the pressure drop and the heat loss to the environment, which are derived from the fluid flow through the thermal energy storage tank. The thermal-hydraulic analysis investigates, from parametric and optimization standpoints, the effects on the performance of the TES system of design and operational parameters, such as the storage tank volume and length/diameter ratio, porous medium particle size and porosity, charging and discharging mass flow rates, charging temperature, and the charging-discharging cycling. The results clearly suggest design trends for s-CO2-based systems, as well as show that not only the thermal-hydraulic charging-discharging combined efficiency of the TES unit is highly dependent on the above-mentioned parameters, but also and more importantly, that it can be optimized with respect to dimensional and operational parameters.

Experimental and numerical investigation of sebacic acid/expanded graphite composite phase change material in a double-spiral coiled heat exchanger

• The thermal performance of sebacic acid /expanded graphite composite material in a double-spiral coiled heat exchanger was studied. • Experimental and numerical tests about charging performance were presented. • The inlet temperature, flow rate as well as thermal conductivity of phase change material were investigated. Solar energy is considered to be one of the most prospective sources of energy, but it needs suitable energy storage technology for efficient usage. Among various applications of thermal energy storage of solar heating, the medium-high temperature (over 120 °C) heat accumulation has a greater market potential since energy with higher temperature has higher quality. The medium-high temperature solar energy utilization has wide application prospect, including building heating system, solar water heaters, et al. This work studies the thermal energy storage performance and phase change behavior of sebacic acid (SA)/expanded graphite (EG) composite material in a double-spiral coiled heat exchanger, while the heat transfer oil works as thermal fluid concerning with high temperature of system. SA is a promising phase change material to store medium-high temperature thermal energy. Results show that the system with SA/EG composite has uniform temperature distribution inside the energy storage tank. The effect of the flow rate and inlet temperature of heat transfer fluid on charging efficiency is explored. The numerical method is adopted to further study the thermal conductivity of SA/EG and the thermal performance of system.

Enhanced thermal energy storage of a paraffin-based phase change material (PCM) using nano carbons

Thermal energy storage performance of a paraffin-based phase change material (PCM) enhanced by nano graphite and nano coconut shell charcoal was investigated. The nano carbon concentration was 0.02, 0.06, and 0.10 wt%, respectively. To understand the influence of nano carbons on the thermal behavior of PCMs, the melting process of nano-enhanced PCMs (NePCMs) was monitored using thermocouples and an infrared camera, in comparison with the melting process of pure paraffin. It was found that the melting process of NePCMs without dispersant was accelerated with nano carbons at concentration of 0.02 wt%, although the thermal conductivity of NePCMs with higher concentrations was higher. With increasing concentration, the melting process was decelerated because of the agglomeration of nano carbons and the suppression of natural convection. To eliminate the agglomeration, Span 80 and oleic acid were used as dispersants for the PCMs with 0.06 wt% and 0.10 wt% nano graphite. The melting process was accelerated further with the increasing concentration of the nano graphite. The shortest melting time was realized using 0.06 wt% nano graphite and 2.0 wt% oleic acid, which was 21% shorter than the pure paraffin. Moreover, it was noticed that the data from thermocouples were not able to represent the melting process accurately because of the limited measuring points, especially for the melting process with agglomeration and sedimentation.

The influence of reinjection and hydrogeological parameters on thermal energy storage in brine aquifer

When brine aquifer is used as thermal energy storage medium, pore structure and permeability of brine aquifer may change owing to the difference of salinity and temperature between the reinjected water and underground. Consequently, heat transfer characteristics and thermal energy storage performance in brine aquifer may vary correspondingly. In present work, heat transfer in brine aquifer is investigated, which considers the effect of reinjected water salinity, temperature, effective porosity and hydraulic gradient. Based on the temperature variation of pumping water, thermal energy storage efficiencies are analyzed under different conditions. The results indicate that the heat transferred from reinjection well to pumping well decreases with the reduction of reinjected water salinity, temperature difference and effective porosity of brine aquifer. The occurrence of thermal breakthrough delays when reinjected water salinity and effective porosity reduce, while thermal breakthrough is almost unaffected by reinjected water temperature and hydraulic gradient. The effective porosity of brine aquifer has the greatest influence on performance of thermal energy storage. When effective porosity of brine aquifer increases from 0.33 to 0.43, cold storage efficiency and heat storage efficiency decreases about 56.56% and 40%, respectively. Reducing reinjected water salinity and hydraulic gradient result in the increase of thermal energy storage efficiency. Reinjected water temperature has opposite effect on heat and cold energy storage efficiency. Sensitivity analysis indicates that the sensitivity of heat energy storage efficiency to considered parameters is greater than that of cold energy storage efficiency does.