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Molten salt is a widely used material for high-temperature latent heat thermal energy storage, which requires a thorough understanding of heat transfer, fluid dynamics, solid-liquid interface migration and phase change during heat storage. In this study, a visual experimental platform was developed to investigate the melting process of solar salt under varying aspect ratios and wall temperatures. Key factors such as liquid fraction, temperature distribution, heat transfer dimensionless parameters and stored energy were analyzed. The results revealed that natural convection led to nonlinear migration of the phase interface and stratification in the liquid-phase temperature. Higher heating temperatures and larger aspect ratios enhanced the melting process, increasing both the energy stored and the instantaneous heat storage power. Additionally, cavities formed in the solid salt due to shrinkage during solidification, leading to irregular temperature and liquid level fluctuations during melting. Finally, a dimensionless formulation was proposed to predict changes in the liquid fraction throughout the melting process. • Visualized experiments for high-temperature molten salt were carried out. • The abnormal effect of cavities on temperature and liquid level was discussed. • A dimensionless formulation was proposed to predict the liquid fraction changes. [ABSTRACT FROM AUTHOR]

Reducing reliance on energy-inefficient mechanical cooling systems by improving building energy efficiency is critical for transforming the global energy landscape and achieving sustainability. This study introduces a novel phase change material–impregnated recycled concrete aggregate (RCA–PCM) composite concrete designed to deliver year-round energy savings and enhanced thermal regulation. The RCA–PCM concrete achieves a compressive strength of 48.7 MPa–47.1 % higher than standard RCA concrete, while providing effective latent heat storage of ∼9.2 J/g, enabling superior thermal and energy performance. Prototype testing demonstrates that this innovative material can lower peak indoor temperatures by up to 7.2 °C compared to RCA concrete and 5 °C compared to conventional concrete during summer conditions. EnergyPlus simulations indicate that incorporating RCA–PCM concrete into building walls and roofs can reduce energy consumption by 37.1 %, corresponding to an annual CO₂ emission reduction of 2.12 billion metric tons, or 5.7 % of global emissions. Manufactured using abundant recycled concrete waste and simple technologies, this sustainable material offers a scalable and practical solution for minimizing the construction industry's carbon footprint while advancing global carbon neutrality goals. [Display omitted] • Developed novel energy-efficient concrete with PCM and RCA integration. • Enhanced thermal energy storage performance of RCA-PCM composite aggregate. • Evaluated RCA-PCM concrete's energy performance using EnergyPlus simulation. • Demonstrated potential for a 5.7 % reduction in global CO 2 emissions. [ABSTRACT FROM AUTHOR]

Free cooling and waste heat recovery are vital and promising energy-saving technologies for the cooling system of data centers and the heating system of nearby buildings. The multi-heat pump system with different heat capacities is one of the most effective ways to extract heat from data centers, which need to match the cooling and heating demand in varied times and spaces. It will improve the performance of the integrated system, however, it raises their complexity and decreases the reliability. This study developed a comprehensive model from the computer room to heat consumers, mainly including information technology (IT) equipment heat dissipation, computer room air handler (CRAH), heat exchanger, water pump, heat pump, and thermal energy storage tank (TES). Based on mixed integer linear programming (MILP), an optimization control strategy was used to save energy without sacrificing the data center's reliability by regulating key operating parameters, such as the partial load ratio (PLR) of the heat pump, the ON-OFF mode, water pump's flow rate, and the heat storage and release mode of TES. Compared to the rule-based strategy, the MILP-based strategy improved the heat pump's daily average coefficient of performance (COP) and the heating system's energysaving rate by 1.19 and 15.11 % in winter, 1.21 and 14.68 % in the transition season, 1.13 and 12.91 % in summer, respectively. The related energy consumption of the cooling system in the above three seasons decreased by 3.78 %, 3.50 %, and 3.38 %, and the annual power usage effectiveness (PUE) reduced by 0.01. The annual energy-saving rate of the whole system was 8.12 %, and its global coefficient of performance (GCOP) increased from 9.96 to 10.85. • A comprehensive model for a data center integrated system from the computer room to heat consumers is established. • An optimization control strategy is developed for the integrated system without sacrificing the data center's reliability. • The whole system's annual energy consumption decreases by 8.12 % and the data center's PUE reduces from 1.16 to 1.15. [ABSTRACT FROM AUTHOR]

The concept of energy communities is drawing particular attention, given their potential to bridge energy gaps by fostering energy transition through community-led initiatives. Although the literature offers well-established business models that ensure long-term sustainability, several improvement opportunities exist to manage demand-side risks that could jeopardize the program's effectiveness. Accordingly, this paper proposes to evaluate potential customer hazards given end-users' concerns about thermal discomfort given uncertainty sources in weather forecasting and subsequent renewable production. The strategy employs a distributed energy coordination framework for residential prosumers within a hierarchical control structure governed by a benevolent coordinator. Prosumers maximize their welfare using a stochastic chance-constrained programming approach, while the coordinator minimizes planned grid costs through a cost-sharing approach. Each customer is integrated with electric thermal storage and renewable power. Extensive simulations were performed with accurate energy consumption and weather forecast data in the province of Quebec. Results demonstrate that demand-side risk-aversion preferences can significantly impact the expected aggregated demand profile, creating power congestion situations. The price of robustness as a consequence of anticipating forecasting errors ranged from 9.06 to 15.15% compared to the optimal solution with perfect information. Eventually, the proposal was validated through a cyber–physical infrastructure meeting scalability requirements. • An energy community is managed using a distributed chance-constrained approach. • Prosumers have electric baseboard heaters, thermal storage, and renewable assets. • The effects of risk preferences and cumulative forecasting errors are investigated. • The price of robustness is quantified. • The strategy was validated through an open cyber–physical infrastructure. [ABSTRACT FROM AUTHOR]

To investigate the criteria for selecting working fluids in biomass power plants coupled with pump thermal energy storage (PTES) system, two system models, HPO (heat pump only) and CP (complete PTES), were developed. Sensitivity analyses were conducted, and the performance of five commonly used working fluids was compared. The results show that the key factor for HPO is the coefficient of performance (COP) of the heat pump cycle, while for CP, the critical factor is the net power output of the thermal cycle. Roundtrip efficiency can be improved by reducing the compressor inlet temperature, minimizing the heat exchanger terminal temperature difference, and enhancing isentropic efficiency. HPO roundtrip efficiency increases with rising ambient temperatures, whereas CP roundtrip efficiency decreases. High isentropic indices and high, stable specific heat capacities are crucial criteria for selecting working fluids. Exergy analysis reveals that exergy losses primarily occur in turbomachinery for all working fluids. After parameter optimization, the highest roundtrip efficiencies for HPO and CP are achieved with carbon dioxide (54.35 %) and argon (61.01 %), respectively. Helium provides the lowest compressor and expander investment costs, at $50.86/kW for HPO and $142.23/kW for CP. • A novel energy storage solution is proposed by coupling pump thermal energy storage (PTES) with biomass power plants. • Thermodynamic, sensitivity and exergy analysis of two system configurations, namely HPO and CP, are analyzed. • The performance impacts of five commonly used working fluids in PTES systems are systematically investigated. • Practical references are provided for the selection of working fluids in heat pump energy storage systems. • The highest roundtrip efficiencies for HPO and CP are achieved with carbon dioxide (54.35 %) and argon (61.01 %), respectively. [ABSTRACT FROM AUTHOR]

Poverty in terms of conventional energy but abundant renewable energy resources coincide in solar-rich areas, so the on-site supply of solar energy is essential for alleviating energy poverty and decarbonizing buildings. We propose a novel energy structure based on a solar-driven energy system combining battery and thermal storage under the power-to-heat concept. Dual-objective optimization models were developed for PV–PT hybrid system and PV–dominant system to identify the optimal technological portfolio and design parameters with the aim of minimizing both the annual total cost (ATC) and carbon emission intensity (CEI), and the models were solved using the biologically–inspired algorithms. Considering a rural household in Lhasa as an example, the optimum design consists of 1.7 kW of photovoltaic (PV), 28.2 m2 of photothermal (PT), 4.8 kWh of battery, 1.7 m3 of thermal storage, and 3.2 kW of auxiliary heat source. The ATC is 44.5 % lower than that using conventional energy and CEI can be reduced by 94.8 %. Furthermore, the proposed system could achieve a CO 2 reduction cost of 0.3 CNY·kg−1, compared with the two PV–dominant systems, i.e., 2.8 and 0.5 CNY·kg−1 CO 2 , respectively. The proposed methodology can identify the optimal technology portfolio and balance environmental with economic. • Numerical simulation proposed for solar energy system under the power-to-heat concept. • Optimization model developed for solar energy system to identify the technological portfolio. • Optimal configuration evaluations and economic–technical and energy utilization analysis. [ABSTRACT FROM AUTHOR]

As a passive heat transfer device, thermosyphon is a promising technology for energy saving and adaptive cooling. Thermosyphon integrated latent thermal energy storage condenser (TLTESC) is intended to be applied in data center for emergency cooling and energy saving. To investigate the thermal performance of TLTESC, a quasi-steady numerical model is established. The layered thermal resistance model is used for the LTES condenser. The start-up factor is proposed to amend the start-up stage of the TLTESC. Several two-phase evaporative heat transfer formulas are compared with the experimental data; the Shah formula that obtains the least error of 6.8 % is selected. The refrigerant mass distribution is analyzed, and considering the refrigerant state in different parts, a new parameter for representing the refrigerant filling mass in thermosyphon is recommended. To compensate for parameters that cannot be measured during the experiment, the evolution of the refrigerant qualities at typical locations are clearly given and analyzed. Finally, the distribution of pressure loss in different parts of thermosyphon are investigated. Frictional pressure losses in the evaporator and condenser occupy the largest proportion. Gravity pressure loss in the evaporator is third biggest, and small vertical length of evaporator is suggested. • A quasi-steady heat transfer model is established for the thermosyphon integrated latent thermal energy storage. • Shah formula obtains the least error of 6.8 % among the two-phase evaporative heat transfer correlations. • The refrigerant in liquid pipe, heat exchangers and vapor pipe account for 60 %, 16%–19 % and 2 %, respectively. • Friction in the evaporator and condenser contribute to the main pressure loss. [ABSTRACT FROM AUTHOR]

This study explores the potential of integrating a compressed air energy storage (CAES) system, a heliostat solar field, and a biomass-driven gas turbine cycle to create an innovative cogeneration facility. The proposed system aims to deliver a dependable and eco-friendly energy solution by integrating the high energy density of CAES with the sustainable characteristics of solar and biomass resources. This study initially examines the influence of five design variables on key output parameters. Upon concluding this analysis, a bi-objective optimization is executed, concentrating on exergy round-trip efficiency and the levelized cost of the product as the primary objectives for optimization. As a realistic case study in this research, Tabriz city in Iran is selected and its solar and climatic conditions are applied in the analysis. The financial analysis reveals that the optimal scenario shortens the payback period from 6.479 years to 5.019 years and boosts net profit by 47.31 %. The optimal values of the objectives are a levelized cost of product of 0.07547 $/kWh and an exergy round-trip efficiency of 32.48 %. Therefore, the initial modeling produces 0.2376 tons of CO₂ for each MWh generated, whereas the emission for the optimized point is approximately 0.1696 ton/MWh. • Proposal of a novel hybrid plant including CAES system, heliostat solar field, and biomass-fired gas turbine cycle. • Using thermal energy storage to store hot air and use it to enter the gasifier during discharge time. • Bi-objective optimization for maximizing exergy round-trip efficiency and minimizing the levelized cost of the product. • Optimal scenario shortens the payback period from 6.479 to 5.019 years and boosts net profit by 47.31 %. • The optimal values of the levelized cost of product and exergy round-trip efficiency are 0.07547 $/kWh and 32.48 %. [ABSTRACT FROM AUTHOR]

A single-tank molten salt thermal energy storage (TES) with a supercritical CO₂ (S-CO₂) cycle is a key technology for concentrated solar power (CSP) plants to achieve efficient and cost-effective peak-shaving operation. However, the S-CO₂ cycle has to cope with complex operating conditions. This study analyzes the coupled impact mechanisms of heat source temperature and load variations on cycle performance, and optimizes parameter regulation method under off-design conditions. The benefits of the proposed cycle operation mode in enhancing TES energy utilization and improving CSP plant performance are indicated. Results show that allowing turbine inlet temperature below the design value extends operational duration by 140.74 % and increases power generation by 194.65 %, thus elevating the 24-h load fulfillment rate of a hybrid PV-wind-CSP system from 79.61 % to 99.30 %. Additionally, as turbine inlet temperature and load decrease, cycle efficiency generally experiences a downward trend, but the efficiency does not strictly decrease monotonically with the load. Under near-full load conditions, as turbine inlet temperature decreases, the minimum cycle pressure increase sharply so that CO₂ at turbine outlet remains in supercritical state, causing compressor power consumption to decrease significantly to meet high-load demands. However, these conditions raise the likelihood of compressor surge and potential safety risks. • Off-design performance of S-CO₂ cycle coupled with single-tank TES is optimized. • Impact mechanisms of heat source temperature and load variation on cycle are studied. • Proposed cycle operation mode enhances TES utilization and peak-shaving reliability. • Proposed mode increases power generation by 194.65 % by increasing operation time. [ABSTRACT FROM AUTHOR]

One of the most important preconditions for the construction and operation of lunar base is the sufficient energy supply. In this paper, a novel solar-powered closed-Brayton-cycle and thermoelectric generator (CBC-TEG) integrated energy system coupling with in-situ thermal storage is proposed for the lunar base, and the according performance assessment model including a Daytime mode and two Nighttime modes, are established. Results indicate that the optimal compressor pressure ratio (π C) for total power generation efficiency decreases with the inlet temperature of compressor (T 1), and a reasonable T 1 is necessary to balance the power, efficiency, optimal π C and the required radiator area. For the operation strategy of constant thermal storage unit (TSU) temperature drop at moon nighttime, the total power generation efficiency of Daytime mode is as high as 35.83 %, but the change of efficiency and radiator area is too dramatic. When the energy system operates at a constant radiator area, it should store more heat into TSU, rather than convert heat into electricity during the lunar daytime, to maintain a continuous running of CBC-TEG at the nighttime. This research provides an innovative solution for the all-day electricity supply of lunar base. • A solar-powered CBC-TEG integrated energy system coupling with TSU is proposed. • Optimal π C for total power generation efficiency decreases with T 1. • The total power generation efficiency of Daytime mode is as high as 35.83 %. • The radiator area change is too dramatic for constant TSU temperature drop. • It should store more heat into TSU for continuous running with constant radiator area. [ABSTRACT FROM AUTHOR]

Utilisation of combined solar energy and heat pump systems for heating has the potential to result in a notable reduction of carbon emissions. Nevertheless, the intermittent nature of solar energy and the low-temperature limitations of heat pumps still pose challenges. In order to address this issue, this paper combined the two aforementioned technologies and integrated direct phase-change thermal storage. A solar composite heat pump system with an evaporative thermal accumulator is put forth, with the objective of enhancing heating efficiency. An experimental setup was employed to examine the dynamic performance of the evaporative thermal accumulator under diverse solar thermal-electric conditions and evaluate the influence of two heat transfer media. The experimental results demonstrated that utilisation of phase change slurry as the heat transfer medium led to an enhancement in the power generation efficiency of the PV/T system, with an increase of 2.71 % compared to water-based systems. and also proved to be effective in enhancing the stability of the PV/T system. Additionally, the heat pump and composite system achieved average COPs of 4.74 and 8.23, respectively, representing a substantial 57.36 % increase compared to water-based systems. These findings demonstrate the feasibility and promising potential of using phase change slurry in solar composite heat pump systems. • A novel solar composite heat pump with evaporative thermal accumulator is proposed. • Phase change slurry increases PV/T system efficiency by 2.71 % over water-based systems. • Heat pump and composite system achieve COPs of 4.74 and 8.23, a 57.36 % improvement. • Demonstrate the feasibility and promising potential of using phase change slurry in this system. [ABSTRACT FROM AUTHOR]

Electrifying residential heat supply with rooftop photovoltaics (PV) and air-source heat pumps is essential for decarbonizing the German building stock. The potential of these technologies depends on factors such as building properties, rooftop area, weather, and resident behavior. Using a building database and statistical data, we analyzed how much of the residential heat demand in Germany could be independently met by homeowners using PV, heat pumps, batteries, and thermal energy storage. Our energy system model, based on detailed bottom-up building simulations, indicates that 35.2 % of the average annual heat demand can be covered by on-site supply. Grid savings are most significant in well-refurbished single-family and terraced houses, reaching 35 % annually. During cold periods, these houses achieve grid savings of 52%–70%, while in warm periods, they feed 155%–317% surplus electricity back into the grid. Understanding the electricity consumption and grid contributions of solar-assisted heat pumps is critical for accurate grid load forecasting and planning local grid expansions. This knowledge also supports homeowners and policymakers in evaluating the advantages and limitations of these systems, informing subsidy programs and regional energy strategies. [Display omitted] • Residential energy load profiles are generated using a building database and energy simulations. • Using PV potential, heat pumps, and storage cuts residential heating electricity demand by 35.2%. • Single-family and terraced houses can reach up to 90% self-sufficiency in energy use. • Winter grid demand is almost unchanged, but excess electricity in summer calls for grid expansion. [ABSTRACT FROM AUTHOR]

This paper studies portfolios of electricity- and hydrogen-driven heat pumps, electricity- and hydrogen-driven boilers and thermal energy storage technologies from an energy system perspective. Thermodynamic and component-costing models of heating and cooling technologies are integrated into a whole-energy system cost optimisation model to determine configurations of heating and cooling systems that minimise the overall system cost. Case studies focus on two archetypal systems (North and South) that differ in terms of heating and cooling demand and availability profiles of solar and wind generation. Modelling results suggest that optimal capacities for heating and cooling technologies vary significantly depending on system properties. Between 83 % and 100 % of low-carbon heat is supplied by electric heat pump technologies, with the rest contributed by electric or hydrogen boilers, supplemented by heat storage. Air-to-air electric heat pumps emerge as a significant contributor to both heating and cooling, although their contribution may be constrained by the compatibility with existing heating systems and the inability to provide hot water. Nevertheless, they are found to be a useful supplementary source of space heating that can displace between 20 and 33 GW th of other heating technologies compared to the case where they do not contribute to space heating. • Analysis of portfolios of end-use technologies for zero-carbon heating and cooling. • Models of end-use technologies are built into a bespoke whole-energy system model. • Some technologies can provide multiple outputs (e.g., space heating and cooling). • Model finds system-driven optimal mix of heating and cooling options in North and South. • Most heat provided by electric heat pumps (83–100 %), the rest by boilers and storage. [ABSTRACT FROM AUTHOR]

As Chile solidifies its position as a global leader in renewable energy, the intermittent and variable nature of solar and wind increasingly challenges grid stability. Concentrated Solar Power (CSP) with Thermal Energy Storage (TES) provides dispatchable, high-value electricity that can mitigate curtailment and enhance grid reliability. This study uses a Monte Carlo framework to evaluate 1,000,000 potential dispatch strategies for a 100 MW CSP facility in the Atacama, leveraging historical marginal cost data from the Cardones line (May 2023–April 2024). The best dispatch strategy achieves $60,103,253 in annual revenue, surpassing most scenarios. Specifically, the capacity factor rises 94.11% above average, while revenue increases 106.8%. Stable output under varying solar conditions underscores CSP's ability to sustain steady generation, support higher renewable penetration, and strengthen the energy supply. Granting CSP plants greater autonomy in dispatch decisions can enhance profitability and grid resilience. By leveraging data-driven optimization, CSP technology could play a pivotal role in meeting Chile's ambitious renewable goals, reducing curtailment, and bolstering overall system efficiency. • Optimized CSP dispatch using Monte Carlo simulations. • Increased annual revenue for CSP plants in Chile. • Quantified revenue impact from solar and market uncertainties. • Enhanced decision-making in CSP operations under variable conditions. [ABSTRACT FROM AUTHOR]

This study provides a comprehensive analysis encompassing experimental characterization, durability assessment and numerical modeling to evaluate the suitability of two natural magmatic rock groups —including felsic (granite and granodiorite) and mafic (gabbro) lithological rocks— for use in rock-bed thermal energy storage (TES) systems at medium (120–400 °C) and high (300–600 °C) temperatures. The main experimental findings reveal the suitability of the studied rocks as storage materials, as they exhibit greater thermal capacity (3–3.8 MJ/m3.K at 500 °C). This property is reduced after thermal cycling tests, but the impact on energy storage density remains minimal, with variations not exceeding 10 %. Both felsic and mafic rocks are appropriate for storage applications up to 400 °C, remaining stable after more than 400 cycles. However, for high-temperatures, felsic rocks are limited to 500 °C to avoid the formation of microcracks due to the α-β quartz transition. Furthermore, a numerical model was developed and validated using four experimental data sets with different heat transfer fluids. The numerical results showed high accuracy, with a mean error below 4 % compared with experimental data. Based on numerical thermal performance prediction (E st = 2306 kWh and η ovell ≈ 94 %), mafic rock was selected as the most promising storage material for the target rock-bed TES system. • Preliminary experimental characterizations of mafic and felsic rocks was conducted. • Isothermal and two thermal cycling tests at different temperatures were performed. • Cycling effects on rock's characteristics and properties were analyzed. • A validated numerical model is used to study the behavior of a rock-bed system. • Thermal behavior and performances of innovative storage system were presented. [ABSTRACT FROM AUTHOR]

In the context of the prominent energy problem, it is crucial to reduce energy consumption and improve heat utilization efficiency. Aquifer Thermal Energy Storage (ATES) is one of the promising solutions to balance the uneven distribution of seasonal energy. To improve the ATES efficiency and reduce the heat loss during the heat storage process, the novel method realized by fracturing is proposed. Through reconstruction of the upper and bottom layer around the aimed aquifer through hydraulic fracturing, then the fracturing fluid containing heat insulation material will be injected into the fracture to create an artificial insulation zone. Based on the thermal-hydraulic-mechanical coupling model, the numerical study of heat storage process and heat production process in ATES with artificial insulation zone is conducted. The results indicate that the artificial fracture insulation zone has the heat preservation effect on the aquifer thermal energy storage, which lead to a reduction in the heat loss rate during the heat storage process. When the thermal conductivity of the fractures is 0.02 W/(m·K), and the fracture density is 10 pieces/100 m2, it exhibits the most effective heat insulation ability, resulting in an 8.38 % reduction in the heat loss rate. [Display omitted] • A method of reconstructing the ATES system is proposed to reduce heat loss rate. • The heat storage effect of conventional ATES and reconstructed ATES is compared. • The heat loss rate of the ATES can be reduced by 8.38 % after reconstruction. • Effect of injection rate and injection temperature on reconstructed ATES is studied. • The continuous fractured zone is conducive to reduce heat consumption rate. [ABSTRACT FROM AUTHOR]

Ground source heat pump (GSHP) technology as an efficient and environmentally friendly solution for heating and cooling systems has gained widespread attention. However, issues such as soil thermal imbalance and high investment costs have limited its large-scale application. Hybrid ground source heat pump (HGSHP) systems integrate renewable energy sources such as solar power, alleviating environmental concerns associated with single GSHP operation and further enhancing the system performance. This review provides a comprehensive overview from the perspectives of design, configuration, and intelligent control optimization on how to enhance the coupling effects between GSHP systems and auxiliary systems including renewable energy systems and control systems to guarantee the flexibility, efficiency, and sustainability of hybrid systems. In terms of design optimization, the selection of key parameters such as heat exchanger layout and fluid circulation paths have been described in detail to ensure that the size of the HGSHP system can effectively meet the load demands. Regarding the configuration optimization, the appropriate configurations forms of ground source and auxiliary energy are selected based on the different climatic conditions and energy supply situations to maximize the system's energy utilization efficiency. For the intelligent control optimization, the advanced control algorithms and intelligent control strategies available to predict, monitor, and adjust the system's operational status have been elaborated for intelligent management and optimization of energy utilization. Through the comprehensive optimization strategies, the HGSHP system can offers the robust support for low-carbon sustainable building energy utilization. It is expected that this timely review and summaries of research progress in this field can provide some rewarding insights for future investigations of HGSHP and promote the wider application of HGSHP systems. • The key factors in the optimization of HGSHP design have been summarized. • The integration of auxiliary technologies such as energy storage systems and HGSHP is reviewed. • The series and parallel operation configurations of HGSHP are analyzed and compared. • The application of intelligent algorithms and control strategy optimization in HGSHP is discussed. [ABSTRACT FROM AUTHOR]

Thermal energy storage technology can provide better mitigation of thermal and mass fluctuations in energy systems, ensuring that an integrated system can work under the designed operating conditions. In this work, the radial temperature uniformity of steady and Gaussian-like unsteady sources is first investigated. Various thermodynamic evaluation indexes are then used to study the effects of two key unsteady conditions, i.e., temperature and inlet flow, on the thermal transport performance of the phase change packed bed. As last, the amplitude and proportion of the key factors are further analyzed by using Taguchi's method. The results show that the radial temperature inhomogeneity index (RTII) relatively increases to maximum by 140.06 % with the peak value and proportion of the unsteady mass source. As the peak value and proportion of unsteady heat sources increase from their minimum values to maximum ones, the total thermal energy storage and latent heat proportion rise by 20.04 % and 40.43 %, respectively. Similarly, for the unsteady mass sources, the total thermal energy storage and latent heat proportion increase by 46.22 % and 73.33 %, respectively. The key factors function as a whole and reinforce each other. This work provides guidelines for designing the controlled operation of thermal energy storage under unsteady conditions. [Display omitted] • Unstable factors affecting thermal storage performance in packed bed are analyzed. • The stored total heat energy is increased by 20.04 % for unsteady heat source. • The latent heat proportion is increased by 73.33 % for unsteady mass source. • Radial uniformity and thermal storage performance are analyzed using Taguchi method. • The peak and proportion of the unsteady source are mutually enhanced. [ABSTRACT FROM AUTHOR]

Heat pump drying systems are widely used in agriculture due to their high energy efficiency and cost-saving potential. This study develops a novel heat pump drying system integrated with a rock thermal energy storage bed to reduce energy consumption and enhance waste heat utilization. The system utilizes cost-effective and widely available rock materials (specific heat capacity of 2025 kJ/m³·K) that exhibit high thermal capacity, excellent durability, and low thermal expansion. These properties contribute to long-term stability and lower maintenance costs. The system was evaluated under three drying modes: intermittent, delayed-intermittent, and continuous. The experimental data were processed using thermodynamic analysis methods, and the energy performance of each mode was evaluated through regression fitting models. Compared to continuous mode, SEC decreased by 11.98 % and 25.71 % in the intermittent and delayed intermittent modes. Correspondingly, total electricity consumption was reduced by 12.37 % and 26.51 %, and COP increased by 13.71 % and 11.60 %. Additionally, annual CO 2 emissions were reduced by 11.17 % and 24.67 %. In delayed intermittent mode, extending the charging period improved thermal efficiency by 24.6 % and exergy efficiency by 9.97 %. These results demonstrate the benefits of integrating intermittent drying with rock thermal storage in improving energy efficiency and reducing environmental impact. • Explores integrating rock thermal storage with heat pump drying systems. • Evaluates rock thermal storage's impact on drying time and energy consumption. • Analyzed system performance, including energy, exergy, environmental, and economic. • Energy consumption reduced by 26.51 % while system COP increased by 12.65 %. [ABSTRACT FROM AUTHOR]

The geothermal parameters (GTPs, C and λ) are fundamental for characterizing heat storage and conduction in the soil. The measurement for these parameters requires complex mathematical models and stringent conditions. This leads to complicated measuring equipment and high time costs. To reduce the reliance on sophisticated measurement instruments and improve measurement efficiency, a multivariate prediction model of geothermal parameters based on machine learning is proposed. This method enables the simultaneous prediction of C and λ within 50 s. The approach is as follows: 1) A numerical simulation model for geotechnical materials is established under pulse heat source conditions. By setting a set of GTPs, the temperature response of geotechnical materials can be simulated. 2) More than 7000 Monte Carlo samplings of the GTPs are performed, with each sampling followed by a simulation. This results in over 7000 temperature responses of the geotechnical materials, forming a comprehensive database. 3). Machine learning algorithms are then used to train the dataset, with temperature responses as inputs and GTPs as outputs. 4). In a laboratory setting identical to the conditions simulated by the numerical model, temperature response within 50 s of the soil sample is measured. The temperature response is then input into machine learning model trained using the numerical model, allowing for real-time acquisition of the sample's C and λ. The prediction performance of the model indicates: 1) By using the first 50 s of the temperature response as input, the BP model is trained, achieving prediction accuracies with a MAPE of 0.87 % for λ and 0.3 % for C. 2) Compared to the theoretical methods, the performance of the machine learning model is far superior. Based on the multivariate prediction model presented in this study, both C and λ can be simultaneously measured within 50 s. This method can replace theoretical prediction method, eliminating errors and assumptions inherent in theoretical approaches. • Geothermal parameters are accurately interpreted from the temperature response curve. • BP algorithms trained with the first 50 s of the curve achieve 0.3 % MAPE for C. • A dataset of 7000+ temperature response curves is established and analyzed with 24 models. • The numerical simulation model's reliability is validated through experimental testing. [ABSTRACT FROM AUTHOR]

Additive-containing aqueous solutions can serve as phase change materials (PCMs) in energy storage technology. Molecular dynamics simulation used to study PCMs, is rarely applied in the cold storage field below 0 °C to explain the properties of cold storage materials. In this work, experiments and molecular dynamics simulations were used to study the effect of additive concentration and type on the cold storage performance of organic/inorganic PCMs. Results show that additives reduce the proportion of hydrogen bonds between water molecules, requiring a lower temperature for PCMs to form regular ice crystal structures, thus lowering the phase change temperature, despite the buffering effect of van der Waals forces. Additionally, additives increase intermolecular interactions, weakening the diffusion of water molecules and reducing surface tension, which decreases heat and mass transfer capabilities and contact angle, thereby extending cold storage time and reducing supercooling. Moreover, the proportion of hydrogen bonds and self-diffusion coefficients decrease with the increase in the number of hydroxyl and chlorine atoms, leading to ethylene glycol/CaCl 2 having the lowest phase change temperature, the longest cold storage time, and the lowest supercooling. • Water based ternary cold storage materials including alcohol and salt were prepared. • The research methods include experiment and molecular dynamics simulation. • Hydrogen bonding and self-diffusion coefficients link to macroscopic properties. • Additives effectively regulate the phase change temperature from 254.04 K to 268.47 K. [ABSTRACT FROM AUTHOR]

Efficient thermal energy storage systems in solar collectors require enhanced heat transfer mechanisms. This study examines convective heat transfer in a partially porous evacuated tube solar collector manifold using phase change materials, magnetic fields, and porous media. The investigation focuses on heat transfer, mass transfer, and system irreversibilities. Results show that convective intensity dominates system performance, with a threefold increase in dimensionless convective flow strength enhancing heat transfer by 138 % and mass transfer by 304 %. Phase change material concentration shows opposing effects: a 13.7 % improvement in thermal transport but an 8.3 % reduction in mass transfer at higher flow intensities. Porous media characteristics significantly affect transport processes when permeability increases. Species diffusion and buoyancy forces demonstrate complex interactions affecting system behavior. Magnetic field application enables precise performance control. These findings provide design guidelines for optimizing solar collector efficiency through balanced parameter selection. [Display omitted] • Combined NEPCM and magnetic fields enhance heat transfer by 138 % in solar collectors. • Rayleigh number dominates system performance with 304 % increase in mass transfer. • NEPCM concentration shows dual effect on heat/mass transfer and entropy generation. • Porous media and exothermic reactions offer precise control of transport processes. • Magnetic field orientation enables fine-tuning of system performance. [ABSTRACT FROM AUTHOR]

Efficient utilization of the renewable energy to meet the demands for cooling, heating, and power is an effective pathway for achieving carbon neutrality. In this paper, a novel combined cooling, heating, and power solar thermal energy storage system is proposed, consisting of a supercritical CO 2 cycle coupled with a Rankine-lithium bromide absorption cycle. System performance is evaluated from the perspectives of energy, exergy, exergoeconomic, and exergoenvironmental (4E) analysis. A multi-objective optimization method based on the multidimensional scaling dimensionality reduction algorithm for 4E analysis is introduced. The 4E analysis indicate the pressure ratio (PR) has the most significant impacts on system performance, with exergy efficiency reaching 55.30 % and thermal efficiency attaining 25.65 % as PR increases. Enhancing performance of the condenser and the evaporator is the optimal method for further improving system exergy efficiency. The solar power tower and heliostat field constitute the largest component cost portion, comprising 78.45 %. Meanwhile, the miniaturization and lightweight design of components are primary strategies for optimizing system environmental performance. Multi-objective optimization results show that, compared with single-objective optimized operating conditions, sacrificing a small portion of thermodynamic and exergy performance can reduce the unit cost of system product by 4.753 % and the unit environmental impact by 3.342 %. • A novel combined cooling, heating and power system is established. • Enhanced 4E analysis is conducted on the overall system and component performance. • A high-dimensional multi-objective optimization method is introduced. • Slightly reducing exergy performance lowers product cost and environmental impact. • Lightweight component design is the method for optimizing environmental performance. [ABSTRACT FROM AUTHOR]

The Tibet Plateau is one of the regions with the most abundant solar energy in the world. Effectively harnessing solar energy is of significant importance for energy conservation and emission reduction. However, solar energy has a significant mismatch with energy demand, for which a pit for seasonal thermal storage is utilized by the large solar district heating system proposed in this paper to solve the time-discrepancy problem of solar energy utilization. Based on the GenOpt-TRNSYS model, a hybrid optimization algorithm is used to optimize the levelized cost of heat (L C o H)under different solar fractions (SF). The optimal configuration of the system under Tibetan Plateau climate conditions is obtained, with a solar collector field area of 13502 m2 and a pit volume of 119350 m3. The levelized cost of the heat substituted by the solar part ( L C o H s o l a r ) and levelized cost of the heat generated by the boilers ( L C o H b o i l e r ) are compared from two aspects of changing natural gas price and initial investment cost, so as to evaluate the economy of the system comprehensively. • The economics of solar district heating systems is studied. • The configuration optimization is carried out for the Tibetan Plateau climate. • The economic advantages of natural gas heating and solar heating are compared. • The impact of collector field and pit costs on system economics is analyzed. [ABSTRACT FROM AUTHOR]

Harnessing thermal energy from ambient air to maintain a balanced heat extraction and release from the soil throughout the year can potentially improve the performance of ground source heat pump systems. This paper proposes and analyzes a specific configuration a coupled air-ground source heat pump system. The focus of this study is to investigate the impact of climatic variations on the feasibility of the system, as well as to analyze the economic and environmental benefits associated with the inclusion or exclusion of a thermal energy storage device. The results show that in cold regions, the ground serves as the primary heat source for heat pumps, whereas during transitional seasons, air energy is mainly utilized to transfer heat directly to the soil. As the climate warms, the fraction of heat supplied by air gradually increases, reducing electrical consumption by up to 17 % compared to the conventional air source heat pumps. Despite the negative impact of thermal energy storage on the coefficient of performance, it is still advantageous as it decreases the electricity consumption during peak price periods, leading to at least a 20 % reduction in life cycle costs in most regions. Additionally, the coupled air-ground source heat pump system is more competitive than air source heat pump systems in scenarios with longer design life, colder climates, and lower borehole heat exchanger costs. • Feasibility of CAGHP system in different regional climates and economies. • TES benefits economy but diminishes energy efficiency and CO 2 reductions. • Warmer regions can increase the proportion of air source utilization. • Economics of CAGHP systems in developed regions limited due to high overall costs. [ABSTRACT FROM AUTHOR]

To exploit the full potential of borehole thermal energy storage (BTES) systems, real-time predictive system performance modeling is required to enable heat extraction to match real-time heating demand. This study presents a data-driven modeling approach utilizing a physics-informed neural network (PINN), which can combine both the explanatory power of physical models and the expressive power of neural networks and compares it with a conventional neural network (NN). We utilized a dataset from a real-world BTES system in Chifeng, China, which included 11,947 h of continuous monitoring of fluid temperature, flow rate, and multiple soil temperature measurement points. After training, the PINN and NN achieved mean absolute errors in outlet temperature predictions of 0.3 °C and 0.6 °C, with R2 values of 0.996 and 0.984, respectively. The PINN demonstrated superior predictive accuracy compared with the conventional NN, and further experiments confirmed that the PINN exhibited robust training performance with less training data. We also assessed the impact of varying flow rates of the BTES heat transfer fluid on heat extraction, and the results highlighted the BTES system's ability to adapt to real-time changes in heating demand. • A physics-informed neural network model is presented for borehole energy storage. • It is compared with a conventional neural network. • Case studies are conducted on real-world data. • The proposed model is superior in prediction accuracy and training data efficiency. • Results of this study will help enable the real-time prediction of system performance. [ABSTRACT FROM AUTHOR]

The transient solid-liquid front movement complicates the natural convection in latent heat storage (LHS) and the heat transfer mechanism remains challenging. To address this issue, this study proposes an optimized field synergy strategy using a front-tracking algorithm, which analyzes the heat transfer in segmented regions rather than the entire domain. Four LHS units with varying shell shapes are designed and experimentally validated. The melting performance shows that the upward case improves the melting performance and heat transfer intensity by 17.44 % and 21.56 % compared to the benchmark case. Further analysis using the field synergy principle (FSP) explores the synergy and matching degree. The results indicate that heat transfer is driven by the combined effects of different regions, with heat transfer near the tube wall and the solid-liquid front dominating. The FSP analysis indicates that the heat transfer mechanism is affected by the comprehensive effect of the synergy and matching degree. Specifically, the matching degree is more crucial when the shell shape changes. The upward case improves the matching degree by 29.95 % and 33.82 % near the tube wall and solid-liquid front, respectively. This study provides a novel analysis and optimization strategy for LHS systems with natural convection. • An optimized field synergy analysis strategy is proposed. • The heat transfer is driven by the combined synergic effects of different regions. • Different units are conducted to optimize the synergic effect under same condition. • The melting time of the upward case reduces by 17.44 % with the same volume. • The heat transfer intensity of the upward rises by 21.56 % with the same volume. [ABSTRACT FROM AUTHOR]

This article presents a robust unified planning framework for a integrated energy system of electric heating gas (EHG-IES), which includes a hydrogen energy multi-scale regulation system (HEMRS). In addition to a multi-scale control system for hydrogen energy, the integrated energy system includes a power generation system comprising wind, solar, and micro gas turbines, as well as a thermal-electric storage system consisting of cogeneration units, batteries, and thermal storage tanks. To ensure the safe and stable operation of various equipment for production and daily life within the park, we propose a novel flexible and adjustable robust optimization (FARO) method to manage uncertainties arising from renewable energy fluctuations and electrical load demands. By linearizing complex nonlinear constraints, we formulated a FARO bi-level robust planning model characterized by mixed-integer linear features. This model was solved using a combined approach of stochastic bisection and the GUROBI commercial solver. Simulation results demonstrate the unique advantages of the EHG-IES with multi-scale hydrogen energy control in terms of economic costs and carbon reduction. The findings also validate the effectiveness of the FARO approach in addressing uncertainties from both supply and demand sides. • A Robust Planning Framework for Integrated Energy Systems with Bidirectional Supply-Demand Uncertainty. • The FARO Method Assists Decision-Makers in Balancing Risk and Conservatism Based on Predefined Objectives. • Proposed and developed a integrated energy system robust model incorporating multi-scale hydrogen energy management. • The FARO Method Ensures 100% Reliable Operation of HEMSR-EHG-IES under Varying Levels of Uncertainty. [ABSTRACT FROM AUTHOR]

Heat pumps, as highly efficient water heating devices, represent one of the most promising technologies for replacing coal-fired boilers and conventional electric heating systems. The global implementation of environmental protection regulations has accelerated the adoption of low global warming potential (GWP) refrigerants applications. Therefore, the use of refrigerants in heat pump water heating (HPWH) systems is initially summarized, covering conventional refrigerants, propane (R290), and R290 refrigerant blends. R290 has been demonstrated to have excellent thermodynamic properties, making it a potential low-GWP alternative refrigerant for HPWH systems. Furthermore, the use of R290 refrigerant blends with low temperature slip and non-flammable refrigerants not only enhances the system's energy efficiency but also reduces the flammability of R290, which will be the focus of future research. The performance of the current R290 HPWH systems has been unable to meet the rapid development of heat pump market demand. Therefore, the development of high-performance, safe, and reliable R290 HPWH systems offers a viable solution. In addition, a comprehensive review of potential performance enhancement methods for R290 HPWH systems are also discussed, including sub-cooler enhancements, solar-assisted enhancements, thermal energy storage enhancements, ejector enhancements, and compressor technologies. Vapor injection and two-stage compression technologies have proven to be the most effective solutions for low-temperature heating. However, hydrocarbon type compressors remain under development. Finally, given the highly flammable nature of R290, safety application methods for R290 HPWH systems are also explored. Existing research indicates that reducing the charge of flammable refrigerants is the most direct approach to improving the safety application of the system. Consequently, the development of efficient and compact heat exchangers has become critical for the future advancement of R290 HPWH systems. This state-of-the-art review is expected to provide valuable insights for the advancement of R290 HPWH technology and draw greater attention to R290 refrigerant. [Display omitted] • Reviewed the use of refrigerants in heat pump water heating (HPWH) systems. • Propane (R290) has proven to be the most promising refrigerant in HPWH systems. • Five ways to improve the performance of R290 HPWH systems are summarized. • Three methods of safety application for R290HPWH systems are discussed. [ABSTRACT FROM AUTHOR]

Thermal energy storage (TES) with phase change materials (PCM) is an interesting technology to be used to improve the energy efficiency of industrial processes, contributing to their decarbonization and the integration of renewable energy sources. Even though literature lists many materials that can be used as PCM in the temperature range between 270 °C and 400 °C, most of them lack a full characterisation, jeopardising their potential implementation by practitioners and scientists. Therefore, this paper presents a complete experimental characterisation of 24 PCMs, with melting temperature, melting enthalpy, degradation temperature, and thermal conductivity in the solid state (at room temperature) determination. Moreover, corrosion tests with two different stainless-steel fibres and with Alloy 20 fibres is presented. The findings obtained in the characterization highlight the necessity of these analyses, as notable differences were observed compared to the available data, particularly in thermal stability and thermal conductivity. Moreover, the findings obtained in the compatibility test reveals that out of the 24 selected PCMs, 11 are potentially compatible with Alloy 20, and 8 with both stainless-steel fibres under environmental conditions (air atmosphere). Finally, the results presented will allow researchers and practitioners to have very detailed data on the characterisation of those PCMs. • This paper presents a complete experimental characterisation of 24 PCMs • Corrosion tests are also included. • Recommendations on which materials are adequate as PCM are given. [ABSTRACT FROM AUTHOR]

In response to the global climate crisis, South Korea has committed to achieving net-zero emissions by 2050, requiring a transformation of its energy system. This study explores the potential sector coupling between the power and heating sectors, referred to as power-to-heat (P2H) in South Korea's 2050 net-zero energy system. Using the open-source EnergyPLAN model, we simulated the future energy scenario with government-projected data and assumptions about energy infrastructure. EnergyPLAN effectively models interactions within systems where distribution is critical, such as electricity, heat, and gas. South Korea's net-zero scenario served as the baseline input, allowing us to assess feasibility and quantify P2H's role in supporting net-zero goals. Our findings suggest that by 2050, South Korea's projected infrastructure could lead to an overbuilt system, with electricity and heating capacities exceeding demand. Variable renewable energy (VRE) capacity is expected to surpass hourly needs, even with storage and sector coupling. Annually, electricity supply may exceed demand by about 89 TWh, with a target demand of 1257 TWh. In district heating, approximately 4.7 TWh of surplus VRE could be used by P2H, meeting only 14.5 % of heating demand, indicating limited absorption of the surplus. Sensitivity analyses on flexible resources, such as electricity and thermal storage, showed limited cost-effectiveness. Increasing wind power's share rather than solar PV is recommended to enhance net-zero feasibility, given South Korea's capacity factors. • The net-zero energy system in 2050 of South Korea was evaluated. • The variable renewable energy (VRE) can be managed by sector coupling. • An increase in the share of district heating increase Power-to-Heat (P2H) potential. • The net-zero scenario of Korea has excessive solar PV capacities. • It is required to adjust the electricity mix to reinforce the future energy system. [ABSTRACT FROM AUTHOR]

As the integration and complexity of integrated energy systems (IES) continue to increase, the synergistic optimization of operation strategies and configuration schemes is encountering formidable challenges. This study presents a novel IES planning model that enables hierarchical optimization of operation strategies and configuration schemes, considering hybrid electric and thermal energy storage. In the initial phase, a population intelligence approach is employed to optimize the configuration scheme of the system. Subsequently, in the second stage, mathematical planning methods are utilized to optimize the operation strategy of the system, thereby further enhancing its performance. The case study reveals that the proposed hierarchical configuration and operation optimization model significantly improves the system's independence, energy, environmental, and economic performance compared to multiple operational strategies such as following electric load (FEL), following thermal load (FTL), and following hybrid load (FHL). Specifically, in comparison to rule-based operation strategies, the proposed model achieves a maximum annual cost-saving rate (ACSR), primary energy saving rate (PESR), pollution emission reduction rate (PERR), and performance comprehensive index (PCI) of 22.45 %, 35.73 %, 35.77 %, and 30.07 %, respectively. Additionally, through an in-depth comparative analysis of four scenarios, namely hybrid electro-thermal energy storage, electrical energy storage, thermal energy storage, and no energy storage, it is found that hybrid electro-thermal energy storage technology exhibits significant advantages in terms of enhancing energy efficiency, environmental friendliness, and operational independence of the IES. • An IES hierarchical allocation and optimization model considering hybrid energy storage is proposed. • A novel multi-objective optimization algorithm is proposed for solving the proposed model. • The proposed model is compared with a variety of rule-based operational strategies. • The performance of the proposed model in different scenarios is compared and analyzed. [ABSTRACT FROM AUTHOR]

This paper investigates the cross-scale thermal performance of biomimetic skin-flesh composite phase change material (CPCM) from material to system. The internal transient heat transfer process from the CPCM to the pack-bed latent thermal energy storage (PLTES) system is described through detailed experiments, and the effects of material characteristics, heat flow rate and unsteady boundary are evaluated. The CPCM unit size is also optimized based on a three-dimensional numerical model of transient heat transfer in PLTES. Firstly, the CPCM has excellent thermal conductivity (1.25–1.57 W m−1 K−1) and long-term stability. The experimental results show that the system with skin-flesh CPCMs has a short charging/discharging time. Higher fluid flow rates increase the exergy efficiency of the system from 0.6 to 0.62. Unstable conditions reduce the overall efficiency, but the use of CPCM can mitigate the interference caused by temperature fluctuations. The skin-flesh CPCM has a higher thermal price-performance ratio (382¥/MJ) compared with traditional CPCM. Numerical studies show that increasing unit thickness can decrease the total effective heat storage capacity and efficiency of the system, but the difference is not significant. Considering the thermal efficiency and economy of system, 25 mm thickness is the best choice with a charging efficiency of 0.87. • The thermal properties of CPCM from material to system level were studied. • The cross-scale problem of PLTES in waste heat recovery is analyzed experimentally. • The bionic skin-flesh CPCM has a higher thermal price-performance ratio. • The numerical results show that 25mmm is the optimum unit thickness. [ABSTRACT FROM AUTHOR]

This study is dedicated to improving the efficiency of the integrated system of Air Separation Unit (ASU) and Liquid Air Energy Storage (LAES) by introducing two-temperature level Cold Thermal Energy Storage (CTES). In the energy storage stage, the cold thermal energy is released from the CTES, while the ASU load increases, which increases the rate of air liquefaction and realizes the storage of liquid air. In the energy release, the ASU load is reduced, and the stored liquid air is pressurized directly into the column and used for distillation. The enriched cold thermal energy of the product gas is recovered and then stored in the CTES. The CTES of two temperature levels can make the flows in the main exchanger of ASU match well and improve the performance of the system. The presented system shows excellent efficiency, of which the exergy efficiency achieves 79.36 % and the overall efficiency can reach up to 90.66 % in our given conditions. • An ASU-LAES system with cold thermal energy storage (CTES) was proposed. • Two-temperature level thermal energy storage was introduced into the system. • CTES brings in an efficiency increase by matching the flows of the main exchanger. • Isentropic efficiency of compressors/expanders slightly affects the overall efficiency. [ABSTRACT FROM AUTHOR]

Technologies providing flexibility options to power systems, such as Power-To-X (PtX) technologies, have become more important with the increasing deployment of distributed energy resources, particularly microgrids. However, uncertainty in renewable resources creates ambiguity regarding the necessary PtX capacity to install. This paper proposes a two-stage stochastic Stackelberg approach for multi-energy microgrids, focusing on long-term investment decisions in PtX technologies and hourly operational strategies. The Stackelberg problem considers microgrids as leaders (upper level) and the independent system operator as a follower (lower level). In the first stage, investment levels for various PtX technologies are determined as one-time decisions. The second stage focuses on hourly operational decisions, including the integration of microgrids with the independent system operator with marginal endogenous prices. The results provide insights into how uncertainty in renewable generation and electric battery levels affect investment levels. Larger hydrogen and thermal storage volumes lead to more flexible and self-sufficient microgrid systems. In scenarios with higher flexibility, microgrids can: (1) satisfy up to 10% of the independent system operator demand using renewable electricity and (2) regulate supply variability by storing excess generation during peak periods and releasing it during low generation periods. • Long-term investment decisions for microgrids in connected mode. • Renewable generation uncertainty results in higher storage investment capacity. • Power-to-X technologies are a viable pathway for self-sufficient microgrid systems. • Special ordered set type 1 approach for complementarity constraints linearization. • Strong duality theory for non-convex stochastic MPEC problems. [ABSTRACT FROM AUTHOR]

Packed-bed system offers a promising solution to the uneven spatiotemporal variability of solar energy. However, challenges persist in its development, including excessive disparity in localized temperature, sluggish heat storage rate, and suboptimal efficiency. In this context, the drop structure phase change material capsule concept was proposed to enhance thermal energy storage performance, drawing inspiration from the streamlined structure such as tuna or submarine. This research delves into the comprehensive performance of drop, sphere, and shuttle structure capsules, encompassing experimental investigation and simulation analysis of the external convection, internal melting, and charging processes. Additionally, an experimental visualization setup was devised to monitor the melting process. The results indicated that replacing conventional sphere structure capsule with the drop structure capsule reduces the drag of external flow by 10.12 %. The complete melting time of the individual capsule and the packed-bed system was shortened by 5.53 % and 10.94 %, respectively. Furthermore, the standard deviations of the longitudinal and radial average temperatures decreased by 24.12 % and 29.97 %, respectively. Fig. Schematic diagram of the drop structure capsule and packed-bed system. Drop structure PCM capsule (a), source of inspiration (b, c), principle of drag reduction (d), and packed-bed (e). [Display omitted] • The idea of a drop structure phase change material capsule is proposed. • The performance of different capsules and packed-beds is compared through a combination of simulation and experiment. • Compared to sphere structure, the drop structure capsule shows a 10.12 % lower C d and a 5.53 % shorter melting time. • The novel packed-bed system constructed reduces the thermocline thickness and charging time by 10.14 % and 10.94 %, respectively. [ABSTRACT FROM AUTHOR]

The complexity and nonlinearity of components in large-scale thermal facilities have resulted in a lack of recognized energy management models, and simple rule-based energy management strategies are still the main approach, which reduces their operating efficiency. In this study, a dynamic programming (DP) method for globally optimal power distribution and operation mode decision for two-by-one combined cycle gas turbine is proposed. First, the energy management model of system is established. Then the drum pressure of the heat recovery steam generator, which indicates the thermal energy storage of the system, is chosen as the state variable, while the control variables are the gas turbine power, turbine power and operation mode. In addition, the system response time is considered to re-evaluated the mode switch command. The simulation results show that the DP optimizes the thermal storage management, which allows the gas turbine to run in the high-efficiency operating range for a longer time. The DP-based strategy saves 6.25 %, 5.89 %, and 4.92 % of fuel at initial drum pressure 8 MPa, 9 MPa, and 10 MPa, respectively, compared to the rule-based strategy. The results of this study can be used as a benchmark to evaluate online energy management strategies in future work. • An energy management model of 2 × 1 CCGT unit is established. • The state transfer equations are different for different unit's operation modes. • The response time of the thermal facility is considered in the EMS. • DP is used to obtain the optimal mode switching point and the power distribution. • The optimal value of EMS can be well tracked by the control system. [ABSTRACT FROM AUTHOR]

A coupled thermochemical energy storage (TCES) system consisting of calcium oxide (for high temperature storage) and potassium carbonate salt hydrate (for medium temperature storage) is analyzed for long-term high temperature heat storage. The coupled TCES system is expected to have smaller volume requirement as evaporator and condenser of the conventional TCES system are replaced by a second reactor. The rate expressions for hydration-dehydration reactions of both storage materials are modified to calculate the reaction completion times, which provide the rate limiting process. A thermodynamic analysis is carried out for the operating cycle of the coupled TCES system to study the impact of operating parameters on the system performance. It is observed that the charging temperature of the individual storage material has significant impact on the performance of the coupled TCES system. The analysis reveals that the coupled TCES system with optimized parameters operating in a cold ambient at 10 °C, achieves a cycle efficiency of 58.6 %, exergy efficiency of 49.1 %, specific heating power of 127.8 W kg−1 and volumetric energy storage density of 269.0 kWh m−3. • Coupled thermochemical energy storage (TCES) system using calcium oxide and potassium carbonate salt hydrate. • Condenser of conventional TCES replaced by lower temperature salt hydrate. • Hydration and dehydration reaction of calcium oxide and potassium carbonate beds. • Thermodynamic analysis of a coupled TCES system operating cycle to study the impact of various operating parameters. • Coupled TCES system designed for space heating in cold climates. [ABSTRACT FROM AUTHOR]

This paper presents the efficient process of thermal energy storage (TES) operation for heat load balancing in the domestic hot water (DHW) systems of district heating by the field experiments and prediction model. First, the TES system was developed with on-site customization for the apartment complex located in Suwon City, South Korea, and heat charge and discharge experiments were conducted hourly to reduce peak loads. The peak load balancing effect was effectively verified. Alongside the TES experiments, the heat amounts and patterns of DHW consumption of residents were monitored over a year. Then, using this monitored DHW heat consumption data, the prediction model of the deep neural network was developed and analyzed. In the prediction model, the monitored DHW consumption data served as the dependent variable, and various weather data and social factors were incorporated as independent variables. The dataset was segmented on an hourly basis, and time variables were categorized to predict DHW consumption values for each specific time. The TES system played a role in reducing peak load by approximately 25–40 % on an hourly basis during heat discharge operations, and the results of heat charge and discharge experiments of the TES system demonstrated a reduction in the dispersion of hourly DHW consumption by approximately 41.7 %, indicating the load balancing effect due to TES operation. Furthermore, the dispersion reduction effect applied with the predictive results showed a 6.8 % decrease compared to the condition with TES operations, and a 45.6 % decrease compared to the condition without TES operation. The prediction model effectively distinguished between heat charge and discharge times, enabling the effective application of the prediction model to the hourly charge and discharge process of TES operation over a day. • Thermal Energy Storage (TES) system for heat load balancing in domestic hot water (DHW) systems was developed and experimented with in an apartment complex in Suwon City, South Korea. • The peak load was reduced by 25–40 % hourly during heat discharge operations in 126 homes. • TES system decreased the deviation of hourly DHW consumption by 41.6 %. • A deep neural network (DNN) model was developed using monitored DHW consumption data, incorporating weather and social factors to predict DHW consumption patterns. • DNN model showed a high performance in predicting daily DHW consumption (R2 = 0.9681). • The prediction model contributed to a further reduction in the deviation of DHW consumption by 6.8 % compared to TES operation, and by 45.6 % as the condition without TES operation. [ABSTRACT FROM AUTHOR]

Absorption thermal energy storage (TES) is a promising technology in low-grade waste heat recovery and storage, as well as for domestic heating and space cooling. It is characterized by a high energy storage density (ESD), negligible heat loss, and high flexibility. The energy storage density is related to the concentration glide of the working fluids. To fully exploit the energy storage density potential of absorption TES, this study establishes a closed three-phase absorption TES system. The three-phase absorption involves crystallization during the charging and storage processes, and dissolution during the discharging process. Charging and discharging experiments under several typical working conditions were conducted to reveal the dynamic characteristics of three-phase absorption TES. In the charging process, the concentration glides are 38.0%–50.8 % and 38.0%–54.3 % under charging temperature of 75 °C and 85 °C , corresponding to charging heats of 902.6 kJ/kg of solution and 993.1 kJ/kg of solution. In the discharging process, the three-phase absorption TES produces heating effects, combined heating and cooling effects, and cooling effects at the evaporation temperature of 30 °C , 20 °C, and 10 °C , with respective concentration glides of 52.6%–41.0 %, 52.8%–40.2 %, and 52.2%–40.8 %. The three-phase processes enhance the energy storage densities by 19.3%–80.3 %. A dynamic absorption TES model is also established and validated using the experimental data. The energy performance of the three-phase absorption TES under a full range of working conditions is studied. The three-phase absorption can double the energy storage density, which further validates the energy storage density enhancement potential of three-phase absorption TES. Graph Abstract. [Display omitted] • Three-phase absorption thermal energy storage cycle is realized experimentally. • The energy storage density is enhanced by 19.3%–80.3 %. • Dynamic characteristics of three-phase absorption thermal energy storage are experimentally investigated. • A comprehensive performance evaluation is presented across the full range of working conditions. [ABSTRACT FROM AUTHOR]

In heating dominated regions, the flexibility obtained through coupling heating and power sectors is particularly beneficial for the integration of high shares of variable renewable energy sources. This study concerns the design of an energy system for a new neighborhood in Norway, including a seasonal thermal energy storage storing excess heat from waste incineration, a seawater heat pump, and local solar power generation. Two supply temperature scenarios are considered for the local heating network: medium-temperature (70 °C), where all heating demands are covered through the network; and low-temperature (45 °C), where booster heat pumps are applied for hot water production. Both scenarios are more cost-effective than if heat demands were to be met through import from the district heating network, however, the difference between the two scenarios is small. The low-temperature scenario has the highest degree of self-sufficiency, and the advantage of additional flexibility gained through the local heat pumps with hot water storage. Cost-optimal charging strategy for the seasonal storage was highly dependent on the pricing of excess heat with respect to the electricity prices. Unlimited sharing of electricity among all users in the neighborhood should be promoted to gain full benefits of local flexibility. [ABSTRACT FROM AUTHOR]

Although Borehole Thermal Energy Storage (BTES) technology has achieved significant progress in feasibility and sustainable energy integration, high heat loss and long preheating periods still strongly restrict its charging and storage performance. This investigation designed and constructed a laboratory-scale BTES sandbox based on similarity and symmetry principles. Detailed monitoring of sand temperature data and boundary conditions validated the effectiveness of the symmetrically simplified similarity experiment. The data comprehensively reflected the change of underground temperature fields during the charging and seasonal storage phases. The reasons for the performance limitations of BTES are thereby analyzed. During the charging phase, the prototype BTES achieved its highest average charging rate of 220W within the first 21 days, and the temperature field stabilized after 192 days. In the storage phase, the exergy destruction losses of BTES primarily occurred at the junction between the core and peripheral zones, accounting for 56 % of the total losses. An effective method to enhance BTES performance is to reconfigure the borehole layout and operation, which improves the uniformity and continuity of the underground temperature field. The experimental measurement provides fundamental data for the integrated seasonal utilization of heating and cooling, which can be essential for innovative building energy systems. • Optimized BTES sandbox experiment using symmetry principles. • Established symmetry-based invariance for the dimensionless temperature field. • Identified performance limitations during BTES charging and storage phases. • Found exergy destruction mainly at the core-periphery boundary. [ABSTRACT FROM AUTHOR]

Steel production is a highly energy-intensive industry, responsible for significant greenhouse gas emissions. Electrification of this sector is challenging, making green hydrogen technology a promising alternative. This research performs a thermodynamic analysis of green hydrogen production for steel manufacturing using the direct reduction method. Four solid oxide electrolyzer (SOE) modules replace the traditional reformer to produce 2.88 kg/s of hydrogen gas, serving as a reducing agent for iron pellets to yield 30 kg/s of molten steel. These modules are powered by 37,801 photovoltaic units. Additionally, a thermal storage system utilizing 1342 tons of steel slag stores waste heat from Electric Arc Furnace (EAF) exhaust gases. This stored energy preheats iron scraps charged into the EAF, reducing energy consumption by 5 %. A life cycle assessment, conducted using open LCA software, reveals that the global warming potential (GWP) for the entire process, with a capacity of 30 kg/s, equates to 93 kg of CO 2. The study also assesses other environmental impacts such as acidification potential, ozone formation, fine particle formation, and human toxicity. Results indicate that the EAF significantly contributes to global warming and fine particle formation, while the direct reduction process notably impacts ozone formation and acidification potential. • Pure H2 reduces pellet reduction time by 50 % in direct reduction process. • EAF exhaust preheats scrap, improving energy efficiency by 5.13 %. • EAF accounts for 35 % of GWP and drives environmental impact. • 1342 tons of slag used for scrap preheating, lowering EAF energy demand. • SOE-PV system produces 2.88 kg/s of H2, fully powered by 37,801 PV units. [ABSTRACT FROM AUTHOR]

This paper conducts an in-depth energy systems analysis on the role of thermal energy storages in Denmark's transition to a fully decarbonized Smart Energy System. Using the EnergyPLAN software and national-scale energy system scenarios, the research examines how the use and impact of thermal energy storages evolves during this transition. Findings indicate that thermal energy storages play an important role in minimizing fuel consumption, curtailing losses, and in improving the overall energy-efficiency and balance of supply and demand. Initially, it primarily lowers fossil fuel use, potentially by 3 TWh per year. As renewable energy increases in the system, its main focus shifts towards reducing excess electricity via power-to-heat and conserving biomass, cutting up to 1 TWh of excess electricity annually through added flexibility. Variable system costs potentially decrease by 17–67 million EUR yearly, though economic feasibility depends on the phase of the transition when investment costs are included. In a future smart- and fully decarbonized system, the economic feasibility is heavily affected by energy prices along with other heat- and storage alternatives and flexible consumption. This leads to the novel understanding that the role of thermal energy storage changes along with the transition of the energy system. • Smart Energy Systems: Comprehensive cross sectoral energy system analysis. • Energy Storage: Thermal Energy Storages - a pivotal component in the energy system. • Energy Transition: National energy system scenarios aligned with national targets. • Efficiency Improvement: Reduced fuel consumption, emissions and curtailment. [ABSTRACT FROM AUTHOR]

Developing clean and sustainable energy technologies has become crucial in light of the growing concern worldwide regarding energy security and climate change. Thermal energy storage technology has emerged as one of the key technologies for achieving sustainable energy development. Research on developing and improving mesoporous composite phase change materials (CPCM) with mixed nitrates as the phase change core material has gained immense attention. The objective is to thoroughly understand the transformation rules governing the phase change characteristics of mesoporous complex nitrate. This study integrates molecular dynamics simulation with an experimental approach to investigate the impact of scale, interface, and the mixing ratio of salts on the phase change properties of CPCM, exploring the competitive relationships among these factors. The results indicated that the phase change temperatures of the mixed nitrate follow the order T (9:1) > T (4:6) > T (6:4) > T (5:5) depending on the ratio of NaNO 3 and KNO 3. The latent heat associated with the phase change of the CPCM increases with the content of NaNO 3. When comparing different matrix materials with the same ratio of mixed salts, the CPCM with Al 2 O 3 as the matrix exhibits the highest phase change temperature and latent heat. For the same ratio of mixed salts, the phase change temperature initially increases and then decreases with the increase of mesopores scale, while the latent heat increases with the enlargement of scale. Thus, the interfacial effect significantly impacts the phase change temperature. When considering the impact of interfacial effects and scale effects on phase change temperature, it was evident that the interfacial effect was dominant in the scale ranges of 3–5 nm and 7.5–10 nm, whereas the scale effect is dominant in the 5–7.5 nm. We found that the ratio of salts played a dominant role in the latent heat for CPCM compared to scale and interfacial effects. • The skeleton interface properties influence the binding strength, thereby impacting phase transition characteristic. • The micro-nano scale constrains the motion of small molecules and thus influences the phase transition characteristic. • The proportion of salt changes its eutectic state and thus affects the phase transition characteristic. • Interface, scale and salt proportion have competitive effects on phase transition characteristic. [ABSTRACT FROM AUTHOR]

Traditional designs of solar greenhouse heat storage and release structures are difficult to maintain a stable thermal environment in cold desert regions. To maximize the utilization of solar energy, thereby increasing the adaptability in harsh environments such as deserts, a heating strategy that integrates surplus air heat (SAH) energy with shallow geothermal energy (SGE) within greenhouses was proposed. An efficient active heating system of air-source heat pump combined with buried water pipe (HP-BWP) for greenhouse heating was designed. Based on the energy balance method, the mathematical model that coupled greenhouse heat load with optimal heat transfer of water pipe was constructed to design the size parameters of water pipe, so as to maximize heat transfer efficiency while minimizing costs. The system was tested on the spot to verify the design theory and evaluate the heating performance of the system. After heating with HP-BWP system, the minimum temperature of indoor air and soil at night increased by 6.3 °C and 2.9 °C, respectively. The heating power at night could reach 37 kW on average. The system coefficient of performance (COP) reached an average of 4.92 and a maximum of 7.86. The energy saving rate of the system reached 76.0 %. The investment payback period of 1.7 years proved the satisfactory promotion and application value. The life cycle analysis of the heating system's carbon footprint demonstrated strong potential for sustainable development and significant environmental contributions. • A heating strategy using surplus air heat energy and geothermal energy. • An active heating system unites air-source heat pump with buried water pipes. • A mathematical model coupling heat load with optimal heat transfer of water pipe. • Minimum greenhouse air temperature rose by 6.3 °C in cold-desert regions. • Field tests confirmed excellent energy-saving performance, which can be repaid in 1.7 years. [ABSTRACT FROM AUTHOR]

The calcium hydroxide/calcium oxide (Ca(OH) 2 /CaO) system, with low cost, high energy density, and a reaction temperature range of 300–600 °C, makes it an ideal candidate for thermochemical heat storage. However, the low thermal conductivity of the material leads to poor heat transfer and low reaction efficiency. In this work, a novel reactor of horizontal biaxial stirred reactor (HBSR) was developed, the multi-physics field coupling mechanism in the reactor was revealed, and its feasibility in dehydration process enhancement was demonstrated. The results showed that the HBSR enhanced the solid fluidity and had good dehydration performance. Numerical simulations explored the effect of different reaction conditions, and it was found that the wall temperature was the key parameter affecting the dehydration process. Experimental tests further verified the excellent dehydration performance of the HBSR, with an increase of more than 2.65 times in the reaction rate and a 93.5 % increase in the bed temperature uniformity compared with the fixed bed reactor (FBR). Additionally, the powder behaviors of agglomeration and elutriation were identified, emphasizing the need for optimization of the reactor structure. Overall, this work provides a fascinating option for the design and performance improvement of future heat storage reactors. • A novel reactor of HBSR was developed for thermochemical heat storage. • Dehydration process enhancement of Ca(OH) 2 /CaO system was demonstrated. • Wall temperature was the key parameter affecting the dehydration process. • An increase of more than 2.65 times in the reaction rate compared with the FBR. • Powder behaviors of agglomeration and elutriation were identified in the HBSR. [ABSTRACT FROM AUTHOR]