Thermal Energy Storage Performance Research Articles

Influence of convection on the thermal storage performance of energy tunnels

The decarbonization of the built environment requires rapid growth in energy storage solutions due to the intermittent nature of most renewable energy sources. This paper focuses on the efficacy of so-called energy tunnels (i.e., tunnels equipped with pipe heat exchangers) used for underground thermal energy storage. By harnessing a 3-D thermo-hydraulic finite element model validated against full-scale experimental data, this work specifically explores seasonal, medium-temperature, thermal energy storage operations of energy tunnels. Numerical simulations are performed to unravel the influence of convection resulting from groundwater flows and airflows on the thermal energy storage performance of energy tunnels. The analyses address the impact of different groundwater flow velocities, air temperatures, and airflow velocities on the thermal losses and storage efficiency of energy tunnels used as thermal batteries. The study discourages underground thermal energy storage in the presence of convection due to significant heat losses. It shows that thermal energy storage operations via energy tunnels are feasible in site conditions characterized by no groundwater flow, limited temperature differentials between the heat carrier fluid circulating in the pipe heat exchangers and the surroundings, and thermal insulation on the tunnel intrados.

Triplet-Sensitized Switching of High-Energy-Density Norbornadienes for Molecular Solar Thermal Energy Storage with Visible Light.

Norbornadiene-based photoswitches have emerged as promising candidates for harnessing and storing solar energy, holding great promise as a viable solution to meet the growing energy demands. Despite their potential, the effectiveness of their direct photochemical conversion into the resulting quadricyclanes has room for improvement owing to (i) moderate quantum yields, (ii) poor overlap with the solar spectrum and (iii) photochemical back reactions. Herein, we present an approach to enhance the performance of such molecular solar thermal energy storage (MOST) systems through the triplet-sensitized conversion of aryl-substituted norbornadienes. Our study combines deep spectroscopic analyses, irradiation experiments, and quantum mechanical calculations to elucidate the energy transfer mechanism and inherent advantages of the resulting MOST systems. We demonstrate remarkable quantum yields using readily available sensitizers under both LED and solar light irradiation, significantly surpassing those achieved through direct excitation with photons of higher energy. In contrast to the conventional approach, light-induced back reactions of the high-energy products do not play any role, allowing quantitative switching within minutes. These results not only underscore the potential of triplet-sensitized MOST systems to leverage the high energy storage capabilities of multistate photoswitches but they might also stimulate the broader usage of sensitization strategies in photochemical energy conversion.

Triplet‐Sensitized Switching of High‐Energy‐Density Norbornadienes for Molecular Solar Thermal Energy Storage with Visible Light

Norbornadiene‐based photoswitches have emerged as promising candidates for harnessing and storing solar energy, holding great promise as a viable solution to meet the growing energy demands. Despite their potential, the effectiveness of their direct photochemical conversion into the resulting quadricyclanes has room for improvement owing to (i) moderate quantum yields, (ii) poor overlap with the solar spectrum and (iii) photochemical back reactions. Herein, we present an approach to enhance the performance of such molecular solar thermal energy storage (MOST) systems through the triplet‐sensitized conversion of aryl‐substituted norbornadienes. Our study combines deep spectroscopic analyses, irradiation experiments, and quantum mechanical calculations to elucidate the energy transfer mechanism and inherent advantages of the resulting MOST systems. We demonstrate remarkable quantum yields using readily available sensitizers under both LED and solar light irradiation, significantly surpassing those achieved through direct excitation with photons of higher energy. In contrast to the conventional approach, light‐induced back reactions of the high‐energy products do not play any role, allowing quantitative switching within minutes. These results not only underscore the potential of triplet‐sensitized MOST systems to leverage the high energy storage capabilities of multistate photoswitches but they might also stimulate the broader usage of sensitization strategies in photochemical energy conversion.

Numerical analysis of hydrophobic surface effects on cavitation inception and evolution in high-speed centrifugal pumps for thermal energy storage and transfer systems

This study explores the influence of wettability surfaces on cavitation inception and evolution in high-speed centrifugal pumps used for thermal energy storage and transfer systems through numerical simulations. The simulations were conducted using the Kunz mass transfer model implemented in Fluent, combined with the Eulerian multiphase flow approach and the shear stress transport k–ω turbulence model. The cavitation dynamics were analyzed across contact angles ranging from superhydrophilic to superhydrophobic conditions. The results demonstrate that superhydrophobic surfaces delay cavitation onset compared to hydrophilic ones, reducing the critical cavitation coefficient by at least 28%. At flow rates of 1.11 Q0 and 0.89 Q0, cavitation numbers show distinct trends, with superhydrophobic surfaces enhancing cavitation stability and reducing the frequency of cavitation shedding. The reentrant jet dynamics are also affected, with increased hydrophobicity weakening the jets and stabilizing cavitation zones. This research aims to advance the understanding of using surface wettability to manage cavitation in high-speed centrifugal pumps, thereby improving the performance and reliability of thermal energy storage and transfer systems.

Titanium dioxide/graphene oxide synergetic reinforced composite phase change materials with excellent thermal energy storage and photo-thermal performances

The development of advanced composite solid-solid phase change materials (SSPCMs) is urgent to explore for improving solar energy harvesting and storage. Herein, novel composite SSPCMs with synergetic cross-linking structure were fabricated through polymerization using GO and TiO2 constructed on the polyurethane framework skeleton. GO and TiO2 synergetic enhanced polymer framework played a role as skeletal structure to encapsulate PEG in the molecular chains, and provided as highly thermal conductive pathways for the composite SSPCMs. TiO2 nanoparticles performed as extended surface on the skeletal structure for further improvement in thermal conductivity. The composite SSPCMs exhibited a remarkably improved thermal conductivity as high as 0.7 W/(m‧K) and fast thermal response rate. The good light adsorption property of TiO2 enhanced the light absorbance efficiency of the composite SSPCMs by 94.4%. And the photo-thermal conversion efficiency of the composite SSPCMs highly reached 93.5%. Meanwhile, the composite SSPCMs exhibited excellent anti-leakage performance and shape stability under high temperature. Consequently, the as-prepared composite SSPCMs possessed a potential for applications in thermal energy storage and solar energy utilization systems.

Prediction of thermo-physical properties and evaluation of thermal energy storage performance of phase change materials based on silicone rubber/graphite (Q/G) composites

ABSTRACT Phase change composite materials (PCCMs) are a type of thermal energy storage system known for their high latent heat of fusion. In this study, PCCM samples based on peroxide-cured silicone rubber (Q) were prepared using a simple soaking procedure in molten Paraffin Wax (PW). At any fixed amount of peroxide, the rubber base samples were filled with varying amounts of graphite powder (G), 4 to 8 Wt.%, to improve the thermo-physical properties. The effect of peroxide and graphite content was then evaluated on the PW loading, PW leakage, and thermal storage performance of the resulting Q/G/PW composites. It was found that the highest ratio of the PW loading to PW leakage occurs at a peroxide content of 6 Wt.%. Moreover, the graphite particles demonstrate a significant role in PW trapping inside the rubber structure and lowering the PW leakage. Finally, the thermal studies showed that the Q/G/PW composite containing 6 wt.% of peroxide and 4 wt.% of graphite has the highest latent heat of 111.5 J/g with a PW loading of 15.2%. This sample also had the lowest thermal diffusivity and the slowest cooling and heating rate.

Multifunctional sandwich composites with optimized phase change material content for simultaneous structural and thermal performance

This work aims at developing novel multifunctional sandwich composites with optimized thermal energy storage and structural load-bearing performance by incorporating microencapsulated phase change materials (PCMs) into polyurethane (PU) foam cores. The optimized foam containing 20 wt% PCM, balancing good latent heat storage (up to 29 J/g) with low thermal conductivity (up to 0.035 W/(m·K) at 50 °C), was used to produce sandwich panels with high-performance epoxy/carbon fiber laminate skins. Microstructural characterization confirmed excellent interfacial adhesion between the core and skins. The multifunctional panels exhibited comparable flexural strengths (150 kPa) and facing stresses (25 MPa) to neat PU sandwich controls. By integrating thermal regulation from PCM phase change with structural reinforcement from the sandwich architecture in a single multifunctional material system, this work contributes to the development of lightweight, energy-efficient composite materials suitable when weight reduction and thermal management are paramount, such as in aeronautics and cold chain logistics.

Simultaneous improvements in energy density and thermal conductivity of PAN polymer nanocomposites enabled by core-shell structure BZCT@BNNS nanofibers

If the heat generated by electronic devices in high-power applications cannot be dissipated in a timely manner, it will not only affect the use of the equipment but also reduce its service life. Therefore, there is an urgent need to develop thin films of dielectric nanocomposites with high thermal conductivity, high energy storage, and excellent flexibility and electrical insulation performance. In this study, we reported the successful preparation of BZCT nanofibers (BZCT NFs) and core-shell structures BZCT@BNNS nanofibers using electrospinning and coaxial electrospinning techniques, respectively, and there were used to composite with polyacrylonitrile (PAN) polymer matrix to prepare BZCT NFs/PAN and BZCT@BNNS/PAN dielectric nanocomposites thin film. Research has found that when the shell thickness of BNNS is 15 nm and BZCT@BNNS (15 nm) volume fraction is 20 vol%, the comprehensive performance of the dielectric nanocomposites is the best, with thermal conductivity and energy storage performance of 3.36 W m−1K−1 and 12.23 J/cm3, respectively. Therefore, the nanocomposites thin film can be used as a potential high-performance dielectric material for thermal management applications in high-power density electronic devices.

Thermal energy storage performance of liquid polyethylene glycol in core–shell polycarbonate and reduced graphene oxide fibers

Thermal energy storage is a promising, sustainable solution for challenging energy management issues. We deploy the fabrication of the reduced graphene oxide (rGO)–polycarbonate (PC) as shell and polyethylene glycol (PEG) as core to obtain hydrophobic phase change electrospun core–shell fiber system for low-temperature thermal management application. The encapsulation ratio of PEG is controlled by controlling the core flow rate, and ~ 93% heat energy storage efficacy is apparent for 1.5 mlh−1 of core flow rate. Moreover, the prepared fiber possesses maximum latent melting and freezing enthalpy of 30.1 ± 3.7 and 25.6 ± 4.0 Jg−1, respectively. The transient dynamic temperature vs. time curve of the rGO-loaded phase change fiber demonstrates the delay of fiber surface temperature change compared to pristine fiber. We indeed show that the tunable heat transfer and thermal energy storage efficacy of phase change fiber is achieved via controlled liquid PEG delivery and the addition of rGO in shell architecture. Notably, the effectiveness of unique phase change material (PCM)–based core–shell fibers is concluded from advanced scanning thermal microscopy (SThM) and self-thermoregulation tests.

High temperature polyimide nanocomposites containing two-dimensional nanofillers for improved thermal stability and capacitive energy storage performance

High temperature polyimide nanocomposites containing two-dimensional nanofillers for improved thermal stability and capacitive energy storage performance

Numerical and Experimental Investigation on Performance of Thermal Energy Storage Integrated Micro-Cold Storage Unit

Numerical and Experimental Investigation on Performance of Thermal Energy Storage Integrated Micro-Cold Storage Unit

Experimental and Numerical Investigation of Macroencapsulated Phase Change Materials for Thermal Energy Storage.

Among the different types of phase change materials, paraffin is known to be the most widely used type due to its advantages. However, paraffin's low thermal conductivity, its limited operating temperature range, and leakage and stabilization problems are the main barriers to its use in applications. In this research, a thermal energy storage unit (TESU) was designed using a cylindrical macroencapsulation technique to minimize these problems. Experimental and numerical analyses of the storage unit using a tubular heat exchanger were carried out. The Ansys 18.2-Fluent software was used for the numerical analysis. Two types of paraffins with different thermophysical properties were used in the TESU, including both encapsulated and non-encapsulated forms, and their thermal energy storage performances were compared. The influence of the heat transfer fluid (HTF) inlet conditions on the charging performance (melting) was investigated. The findings demonstrated that the heat transfer rate is highly influenced by the HTF intake temperature. When the effect of paraffin encapsulation on heat transfer was examined, a significant decrease in the total melting time was observed as the heat transfer surface and thermal conductivity increased. Therefore, the energy stored simultaneously increased by 60.5% with the encapsulation of paraffin-1 (melting temperature range of 52.9-60.4 °C) and by 50.7% with the encapsulation of paraffin-2 (melting temperature range of 32.2-46.1 °C), thus increasing the charging rate.

Multifunctional phase change film with high recyclability, adjustable thermal responsiveness, and intrinsic self-healing ability for thermal energy storage

Phase change materials (PCMs) present promising potential for guaranteeing safety in thermal management systems. However, most reported PCMs have a single application in energy storage for thermal management systems, which does not meet the growing demand for multi-functional materials. In this paper, the flexible material and hydrogen-bonding function are innovatively combined to design and prepare a novel multi-functional flexible phase change film (PPL). The 0.2PPL-2 film exhibits solid-solid phase change behavior with energy storage density of 131.8 J/g at the transition temperature of 42.1 °C, thermal cycling stability (500 cycles), wide-temperature range flexibility (0–60 °C) and self-healing property. Notably, the PPL film can be recycled up to 98.5% by intrinsic remodeling. Moreover, the PPL film can be tailored to the desired colors and configurations and can be cleverly assembled on several thermal management systems at ambient temperature through its flexibility combined with shape-memory properties. More interestingly, the transmittance of PPL will be altered when the ambient temperature changes (60 °C), conveying a clear thermal signal. Finally, the thermal energy storage performance of the PPL film is successfully tested by human thermotherapy and electronic device temperature control experiments. The proposed functional integration strategy provides innovative ideas to design PCMs for multifunctionality, and makes significant contributions in green chemistry, high-efficiency thermal management, and energy sustainability.

Enhanced heat transfer and thermal storage performance of molten K2CO3 by ZnO nanoparticles: A molecular dynamics study

Molten salt serves as a popular medium for thermal storage and transfer in concentrated solar power (CSP) systems. The existing experimental studies have found the capacity of enhancing the heat storage property of the molten carbonate by doping ZnO nanoparticles. However, it has been rarely reported to conduct simulations of ZnO/molten carbonate nanofluids, and the underlying mechanism of how ZnO nanoparticles enhance the heat transfer and storage capacity has less been explored. In this study, molecular dynamics (MD) simulation has been used to investigate the heat transfer and storage properties of a molten K2CO3 nanofluid containing ZnO nanoparticles. It was found that as the nanoparticle mass ratio in molten salt based nanofluids rises, their specific heat capacity initially increases but subsequently diminishes. The specific heat capacity peaks when the ZnO mass ratio is at 1 wt%, resulting in an increase of 17.83 % compared with the base salt. The thermal conductivity in nanofluids based on molten salt increases with the increasing of ZnO mass ratio. Multi-perspective analysis of the mechanism how ZnO nanoparticles enhance the thermal energy storage performance of molten salt, including the microstructural analysis, number density, vibrational density of state, and heat flow decomposition, is conducted. In addition, results of the number density illustrates that a compressed layer of K+ is formed surrounding the ZnO nanoparticles, and the change in the potential energy leads to an increasing heat capacity. Analysis of the heat flow decomposition suggests that the enhanced nonbonded interaction is the main reason for the improvement of thermal conductive performance in the nanofluids based on molten salt. The findings in the present study provide a new perspective of how ZnO nanoparticles enhance the thermophysical properties of molten salts.

A comparative analysis on charging performance of triplex-tube heat exchanger under various configurations of composite phase change material

In present work, the melting performance of triplex-tube latent heat thermal energy storage (LHTES) unit was numerically studied using equal volumes of PCM and metal foam composite PCM (CPCM) in various arrangements. For the n-eicosane (as PCM), the study was conducted at the fixed Rayleigh number (Ra) = 4.08x107, Prandtl number (Pr) = 62.9, and Stefan number (Ste) = 0.14. The results showed that positioning the metal foam on the bottom side and distributing segmented CPCM with alternating PCM zones effectively improved the system performance. Moreover, this also prevents the overheating of thermal layers in the LHTES unit. While the model labelled M2 exhibited the highest economic efficiency among all isotropic models, its low dimensionless thermal energy storage (TES) density (i.e., q’ ∼ 0.6) led this study to focus on models falling under the category having a TES density of ∼ 0.8. Compared to a pure PCM model, the configurations under equal volume ratio category demonstrated up to ∼ four times higher TES rate (p’) and the significant reduction of ∼ 75 % in melting time. The optimized isotropic model achieved the highest TES rate per unit cost with peak value of ∼ 3 at a price ratio (N) of 1. Lastly, the testing of metal foam anisotropy on the chosen design showed a substantial increase in melting/heat storage rates. The largest drop of ∼ 33 % in the total melting time was noticed for model M2 as compared to isotropic case.

Insights into effect of lignocellulosic biomass framework types on bio-based shape stable composite phase change materials

The utilization of biomass waste as framework materials to inhibit the leakage of phase change material (PCMs) has gained widespread attention. In this work, three new kinds of shape stable composite phase change materials (sscPCMs) were prepared which combined with stearic acid (SA) and peanut shell powders (PSP), poplar wood powders (PWP), and corn straw powders (CSP), respectively. The influence of lignocellulosic biomass types on the thermal energy storage performance of sscPCMs was studied by comparing three types of sscPCMs. The research results indicated that SA/CSP exhibited superior properties in terms of load capacity and thermal energy storage performance. According to leakage experiments the load capacity of SA/PSP, SA/PWP and SA/CSP was 31.67 %, 44.69 %, and 50.09 %, respectively. The phase change latent heat of SA/PSP, SA/PWP, and SA/CSP was 55.78, 84.27 and 98.47 J/g, respectively. In addition, the relationship between biomass component content and load rates was studied. The results showed that there was a positive correlation between the loading rates of SA and the ratio of holocellulose (cellulose + hemicellulose) to lignin. The results provided a promising approach for the selection of lignocellulosic biomass framework materials.

Machine-learning-assisted long-term G functions for bidirectional aquifer thermal energy storage system operation

Optimization of aquifer thermal energy storage (ATES) performance in a building system is an important topic for maximizing the seasonal offset between energy demand and supply and minimizing the building's primary energy consumption. To evaluate ATES performance with bidirectional operation, this study develops an analytical solution-based model to simulate the spatiotemporal thermal response in an aquifer. The model consists of three temperature response functions, similar to the G functions in borehole thermal energy storage (BTES), to estimate the transient temperature profile in the aquifer during seasonally varying injection and extraction of hot/cold water. Applying machine learning (ML) based data classification and regression techniques to the results of a series of finite element (FE) benchmark simulations of typical ATES configurations, model input parameters are linked to the subsurface thermal, hydrogeological, and ATES operational properties. Compared to the benchmark simulation results, the errors of the proposed model in estimating the annual energy storage and locating the thermally affected area are about 3 % and 1 %, respectively. The model was applied to a previous short-term case study, and the error in the transient production temperature estimation is about 1 %. The long-term heat recovery ratio estimated from the model also compares well to those calculated from the previous study and the validated numerical model. Because of its fast computation, the proposed model can be coupled with the individual building system simulation and used for preliminary ATES design, and this will allow for greater exploration of ATES operational space and, therefore, better choices of ATES operating conditions. The proposed model can also be coupled with the district heating and cooling network simulation for computationally efficient city-scale long-term ATES potential assessment.

Modeling of heat and solute transport in a fracture-matrix mine thermal energy storage system and energy storage performance evaluation

Repurposing groundwater-filled mine cavities for thermal energy storage has demonstrated promising potential to buffer the imbalance of energy supply and demand. Fractured formations are widespread in old mines due to previous excavation activities, which dominates the performance and efficiency of the mine thermal energy storage (MTES) system. Therefore, understanding the mechanisms of fluid flow, heat, and solute transport in fractured reservoirs in the MTES system is essential for its application and the assessment of environmental impact. In this study, fluid flow, heat, and solute transport in a 3D fracture-matrix MTES system are modeled simultaneously based on site-specific data in Freiberg, Germany. The simulations are conducted with a coupled Hydro-Thermo-Component process newly developed in the open-source software OpenGeoSys (OGS). To stabilize the simulation in the fracture-matrix hybrid system, a flux-corrected transport scheme is specifically implemented and applied in the model. Thus, heat and mass exchange between the embedded fractures and the matrix in the formation of MTES can be accurately simulated on the basis of the conforming mesh. In a hypothetical short-term scenario of a 15-day heating and cooling cycle of the MTES operation, the solute is transported faster than the heat in the fractured formation, indicating that disturbance of mineral compositions in the original mine water can affect a larger region than heat. The embedded fracture network in the surrounding formation is also found to increase the thermal energy storage capacity by 44%, while decreasing the recovery ratio by 14% compared to those of the unfractured formation. More energy will be transported and stored in the more fractured formation from heated mine water, but less energy can be recovered. The behavior of the MTES system strongly depends on local geology and hydraulic conditions and therefore needs to be evaluated in a site- and application-specific manner. This study provides preliminary insights into the performance and efficiency, as well as the environmental impact of the MTES operation in the short term.

Experimental and simulated evaluation of inverse model for shallow underground thermal storage

This study employs an inverse grey-box (IGB) modeling approach, which combines measured data and the physics of systems to predict the performance of shallow underground thermal energy storage (UTES) with top and side insulation. A simplified IGB model of the shallow UTES was developed using thermal network analysis. Experimental studies were conducted for shallow vertical and horizontal UTES configurations. Detailed models representing the field experiments were developed using the TRNSYS simulation tool and calibrated with the measured data from the experimental setup. The 4 Resistance 2 Capacitance (4R2C) IGB model was trained and tested in MATLAB using field experimental data and the data from the calibrated TRNSYS model. In comparing experimental data with the TRNSYS model for UTES, the prediction of outlet water temperature showed good agreement, with root mean square error (RMSE) and coefficient of variation of RMSE (CVRMSE) values of 0.94 °C and 3.16 % for vertical, and 0.99 °C and 2.97 % for horizontal configurations. The IGB model also aligned well with the experimental data, showing CVRMSE of 7.91 % and 3.17 % for vertical and horizontal systems, respectively. Sensitivity analysis revealed that model performance improves with longer training durations and closer testing times to training periods. However, a convergence point of 20 weeks of training data was achieved for making long-term performance predictions.

Performance evaluation and optimization of compact tube-fin latent heat storage system in phase change nanoemulsion discharge mode

The low thermal conductivity of phase change materials (PCMs) and the reduced heat transfer temperature difference along a tube both limit the thermal performance of latent heat thermal energy storage (LHTES). Herein, a compact tube-fin LHTES system based on phase change nanoemulsions was developed and the heat transfer process of nanoemulsion and PCM was numerically investigated. The effects of the heat transfer structure of the heat exchanger and the thermophysical parameters of the heat transfer fluid (HTF) on the cold energy discharge performance of the system were quantified. The optimized model reduced the discharge time by 24.4 % in nanoemulsion mode relative to water. Heat transfer improvement on the PCM side could be achieved by decreasing the fin spacing and tube spacing and by increasing the fin thickness. Such improvement reinforced the heat transfer enhancement of the nanoemulsion. The heat transfer mechanism was explained by the temperature distributions of the HTF and PCM along the tube. The phase-change thermostatic properties of the nanoemulsion and PCM contributed to the formation of dual-phase-change coupled heat transfer (i.e., the heat transfer process when the fluid inside the tube and the PCM filled outside the tube undergo a simultaneous phase change) within the LHTES, increasing the heat transfer temperature difference and the magnitude of the driving force. Increasing the phase change temperature difference accelerated the onset of dual-phase-change heat transfer, while increasing the PCM content prolonged the process. Briefly, this study provided operational and design assistance for LHTES systems in nanoemulsion mode with the intention to accelerate heat transfer rates.