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• Pumped two-phase loop (PTL) integrated with thermal storage (LHS) is proposed. • Thermal response behaviors of PTL-LHS under step heat load are investigated. • Fast tracking and delay response is sequentially observed as heat load increases. • Thermal response behavior regime diagram is quantitatively recognized. • Variable flow rate optimization strategy is validated by comparative experiments. Pumped two-phase loop integrated with thermal storage is an emerging solution for the short-term high-heat-flux cooling. The transient microchannel boiling heat transfer of pumped two-phase loop (PTL) integrated with latent heat storage (LHS) under step heat load are experimentally investigated, with special focuses on dynamic thermal response of two-phase loop and latent heat storage unit. The transient thermal behaviors of PTL-LHS are determined based on the wide variations of heat load and flow rate. A thermal behavior regime diagram is proposed to quantitatively recognize the transient thermal response of PTL-LHS, depending on the heat load and flow rate. The results indicate that the fast tracking mode and the delay response mode are observed and sequentially experienced for PTL-LHS as the step heat load increases. With respect to delay response mode, thermal storage efficiency is high with short thermal storage duration but cooling performance is insufficient, while the transient thermal performances are contrary under the fast tracking mode. The increase of heat load accelerates the transformation of the heat transfer mechanism in LHS from sensitive-heat-dominated to latent-heat-dominated, while increasing flow rate is conducive for the enhancements of thermal capacity and thermal stability of PTL-LHS. Increasing heat load brings about a larger critical flow rate of the transient thermal behavior regime transition, and hence the range of flow rate tends to be larger for the delay response mode. Particularly, the proposed variable flow rate optimization strategy is validated by comparative experiments, showing better cooling performance (the average evaporator wall temperature decreases by 10 °C) and more remaining load of phase change material. [Display omitted] [ABSTRACT FROM AUTHOR]

Summary: The performance enhancement of latent heat storage (LHS) units is of great consequence for the development of sustainable energy. In this article, the transient models of phase‐change heat transfer processes in LHS units with consideration of natural convection are developed and numerically analyzed, in an effort to explore the role of the fractal tree‐shaped fin in the energy charging performance. The solid‐liquid interface evolution and dynamic temperature response in the tree‐shaped finned LHS unit are presented and compared with the wheel‐shaped one. Moreover, the influence of the fin number is investigated for maximizing the energy charging performance. The results indicate that the tree‐shaped fin has merits of effective layout optimization for the point‐to‐area heat transfer and the time‐coordination of energy charging and discharging process. Compared with the wheel‐shaped fin, the melting duration time is decreased only a little for the presence of tree‐shaped fin, however, the solidification process is decreased significantly. The presence of tree‐shaped fin leads to a lower energy charging rate during the early and middle stages of the charging process due to the natural convection suppression, however, the energy charging rate is faster during the later stage due to the thermal conduction dominated interior heat transfer. For maximizing the melting performance, the appropriate fin number in practical applications is 16. [ABSTRACT FROM AUTHOR]

Summary: The current latent heat storage (LHS) units are usually poor in energy charging and discharging efficiency. Given this, a two dimensional (2D) numerical model of the energy discharging process is presented and comprehensively analyzed to predict the role of metal foam in the solidification performance of LHS units. In the model, the fractal geometry reconstructed by the fractal Brownian motion is utilized for the pore characterization of the metal foam. The proposed model is validated through a melting experiment in copper foams from the reference. The temperature dynamic response and the solidification front evolution in metal foam are analyzed and compared to those in a corresponding cavity. The roles of the fractal dimension and porosity in the solidification behaviors are quantitatively analyzed. The results report that the presence of metal foam enhances the solidification performance. For the main goal of maximizing the latent storage, the appropriate porosity of an LHS unit is dependent on the duration time for the heat discharging process in the real application of thermal energy storage. Even if the porosity is the same, the fractal dimension also affects the solidification performance. A decrease in the fractal dimension (lower degree of disorder for pore distribution) provides greater access to heat flow through the phase change material‐foam composite and thus leads to improvement in the interstitial heat transfer, which in turn accelerates the rate of heat release. The fractal dimension is expected to be less than 1.5 for superior solidification performance. [ABSTRACT FROM AUTHOR]

Inspired by the snowflake structure, an innovative Koch-fractal fin is proposed to optimize the fin geometry of the latent heat thermal energy storage (LHTES) units. A model of unsteady heat transfer accompanied with phase change is developed and numerically analyzed to investigate the effect of fin structure on the discharging process of a LHTES unit. The dynamic response of heat release, the dynamic temperature response and solidification front evolution of a LHTES unit with Koch-fractal fins are discussed and compared with the corresponding radial fins. Furthermore, a comprehensive evaluation of thermal performance of LHTES units is conducted in terms of the TES capacity, TES rate and solidification time. The results indicate that the heat release rate of a LHTES unit with Koch-fractal fins is faster than that with radial fins. Moreover, because the Koch-fractal fins have advantages of higher specific surface area, faster heat flow path from point to surface and smaller thermal resistance arising from the reasonable spatial layout, the evolution of solidification front is faster and the temperature distribution is more uniform. The results of quantitative evaluation show that although the TES capacity is identical, the TES rate of a LHTES unit with Koch-fractal fins is six times that with radial fins. [ABSTRACT FROM AUTHOR]

Latent heat thermal storage (LHTS) using phase change materials (PCMs) faces a significant challenge of poor heat transport efficiency. Fortunately, nature has evolved numerous features and functions that can enhance material properties and heat transport efficiency. This paper provides a comprehensive review of bionic studies on LHTS technology, summarizes bionic strategies for improving LHTS performance, and explores in detail the research progress and challenges faced by bioinspired PCMs and LHTS configurations. The review demonstrates that natural morphology, structure, and function can improve heat charge/discharge efficiency. However, current research mainly focuses on simple configurations of morphological and structural bionics, so more attention is needed to develop high-level functional bionics to maximize LHTS performance. The paper also emphasizes the enhancement methods of heat transfer rate in bioinspired LHTS systems, such as synthesizing bionic hierarchical porous and three-dimensional networks, constructing biomimetic shells, extending the heat exchange area, enhancing the convective heat transfer, and optimizing the heat transport path in PCM regions. Importantly, future research should focus on micro/nano-scale bionics to construct bioinspired PCM and LHTS configurations with more advanced performance. Moreover, a complete enhancement mechanism of multiscale phase-change heat transport needs to be investigated for the development of multiscale biomimetic LHTS technology. • Bionic optimization is deemed to be a pathway for highly efficient heat storage. • Bionic strategies include morphology, structure, and functional bionics. • Bioinspired materials and configurations facilitate thermal storage efficiency. • Multiscale bionic on latent heat storage is the most promising development direction. [ABSTRACT FROM AUTHOR]

• LHS is proposed to enhance thermal performance of AGHP under intermittent heating. • Thermal performance of AGHPs with three LHS modes are experimentally investigated. • Introduction of LHS unit into AGHP reduces the temperature of heat source by 27.4%. • The presence of LHS unit greatly enhances the thermostat performance of an AGHP. • Temperature homogenization of AGHP & LHS units becomes worse as heat load increases. Heat pipe coupled with phase change material (PCM) based latent heat storage (LHS) is a significant thermal management system that can efficiently control the intermittent high heat loads of electronic devices. Herein, an axially grooved heat pipe-LHS (AGHP&LHS) system has been proposed. The influence of LHS units and their location on dynamic temperature response of AGHP has been investigated both experimentally and theoretically. The effect of filling rates of phase change materials (PCMs) as well as cooling temperature on thermal response of the AGHP has been compared and analyzed. Results indicate that the LHS effectively stabilizes the heat source temperature under the intermittent heating process, enhances the thermostat performance and brings the temperature uniformity in AGHP. In addition, LHS units arranged at the evaporator section are optimal for AGHP under intermittent heat load conditions, reducing the heat source temperature by 27.4% and prolonging the melting time by 81.5%. As the heat load increases, the melting time of AGHP&LHS system becomes shorter, and the overall temperature uniformity is worsened. In particular, the melting time is prolonged by 34.8% for AGHP&LHS system with PCM filling rate of 89% than that of 44% filling rate. [ABSTRACT FROM AUTHOR]

• Bionic design of latent heat storage unit is provided using fractal tree. • An energy discharging model is developed and experimentally verified. • Transient temperature distribution and solid-liquid interface evolution are investigated. • Tree fin induces stronger temperature uniformity and hence faster solidification rate. • Length ratio of 1.3 and width index of 1 are beneficial for superior performance. The fractal tree-shaped structure has been demonstrated as a promising optimization method to maximize the point-area heat flow access. Aiming to enhance the energy discharging rate of the latent heat storage (LHS) unit, an innovative fractal-tree-shaped structure is introduced to construct the metal fin of a shell-tube LHS unit. An unsteady model of solidification heat transfer in a LHS unit with tree-shaped fins is developed and numerically analyzed using commercial CFD software, in an effort to demonstrate the improvement of the energy discharging performance for a latent heat storage unit using fractal tree-shaped fins. The transient temperature distribution, solid-liquid interface evolution, and dynamic changes of the liquid fraction and the sensible and latent heat in a tree-fin LHS unit are compared with those of a radial-fin LHS unit. The effects of length ratio and width index on energy discharge performance are examined and analyzed. The results indicate that the tree-shaped fin significantly improves the energy discharge performance of a shell-tube LHS unit. The tree-fin LHS unit possesses a faster solidification rate, higher energy discharge rate, and stronger temperature uniformity. The complete solidification time of the tree-fin LHS unit is decreased by 66.2% and its complete melting time is reduced by 4.4% when compared with the radial-fin LHS unit. For superior thermal energy discharge performance, the length of tree-shaped fins in a shell-tube LHS unit should be shorter inward and longer outward; the optimum length ratio is about 1.3 and an appropriate width index is 1. [ABSTRACT FROM AUTHOR]

• Buoyancy-driven melting/solidification performance with different PCMs is studied. • Thermal expansion and contraction properties of PCMs affect the natural convection trend. • Downward and upward flow modes have opposite effects on PCM phase change performance. • Suitable non-uniform fin layout enhances melting but weakens solidification performance. The thermal expansion and contraction property of phase change materials (PCMs) is a significant factor in affecting the buoyancy-driven melting/solidification heat transfer performance in latent heat storage (LHS) units. To reveal the intrinsic mechanisms, a mathematical model of buoyancy-driven melting/solidification processes in finned LHS units is constructed. The phase-change behaviors, thermal transport properties of different PCMs, and roles of heat transfer fluid (HTF) direction and fin layout in melting/solidification performance are explored. The results indicated that when compared to conduction-dominated cases, the buoyancy-driven melting duration of gallium and n-octadecane increases by 1.8% and decreases by 32.6%, respectively, while the corresponding solidification times decrease by 8.4% and increase by 15.2%, respectively. The downward HTF enhances the melting rate of gallium with thermal-contraction properties and the solidification rate of n-octadecane with thermal-expansion properties. Conversely, the upward HTF enhances the melting rate of n-octadecane with thermal-expansion properties and the solidification rate of gallium with thermal-contraction properties. For finned LHS units with upward HTF, the ladder-type and reverse ladder-type fins facilitate the melting performance of n-octadecane and gallium, respectively. Moreover, the ladder fins with non-uniform layouts weaken the solidification performance of PCMs with different thermal characteristics. [ABSTRACT FROM AUTHOR]

• A single-phase immersion cooling system with LHTES devices is proposed. • An innovative LHTES device with palmate leaf-shaped fins is designed by bionic. • Heat charge and discharge durations reduce by 21.0% and 38.2%, respectively. • Performance of biomimetic LHTES devices is less influenced by inclination angles. • Flow rate of 20 L/min and large heat-transfer temperature difference are suggested. Improving the power usage effectiveness of data centers has become a significant challenge for carbon peaking and carbon neutrality. To address this, here we propose a single-phase immersion cooling system with latent heat thermal energy storage (LHTES) devices to recover waste heat. Furthermore, an innovative LHTES device with palmate leaf-shaped fins is designed by bionic techniques. The phase change behavior and thermal transport patterns of biomimetic and traditional LHTES devices are investigated, and the effects of the arrangement and working conditions are revealed. Additionally, the proposed solution's application prospect and economy are presented. The results indicate that the performance of palmate leaf-shaped finned LHTES devices is significantly enhanced compared to traditional ones, whose charge and discharge durations reduce by 21.0 % and 38.2 %, respectively. Moreover, the natural convection development enhances the melting performance, but its effect on solidification processes is insignificant. Considering the trade-off between power consumption and LHTES efficiency, a horizontal biomimetic LHTES device with a flow rate of 20 L/min and a large heat-transfer temperature difference is suggested in practical engineering. Notably, the maximum reduction in annual power consumption for using biomimetic LHTES devices is 2397.4 kW·h, and the minimum payback period is only 2.0 years. [ABSTRACT FROM AUTHOR]

• Gradient fins are introduced to enhance melting performance of LHS units. • Geometric structure of gradient fin are optimized by response surface method. • Coordination between natural convection and thermal conduction is achieved. • Optimal gradients of fin thickness and central angle are 1.846 mm and 14.96°. • Complete melting time of optimal latent heat storage unit is reduced by 30.5%. Latent heat storage (LHS) is critical for the large-scale utilization of sustainable energy. The present paper designs the gradient fins (i.e., fins with gradient thickness and gradient central-angle) to enhance the melting performance of a LHS unit. A two-dimensional model of the melting heat transfer process for phase change material with Paraffin wax RT58 in a finned LHS unit is developed and numerically solved to investigate the melting behaviors. The melting front evolution and the dynamic temperature distribution are analyzed, and the effects of natural convection, fin layout, and Stefan number are investigated as well. Moreover, the fin configuration is optimized by the response surface method (RSM) with the objective function of the complete melting time. The results indicate that the natural convection seriously affects the melting behaviors in LHS units, and the melting rate is improved by 180% as compared with the case without natural convection. The LHS unit with gradient fins has a faster melting rate and a more uniform temperature distribution due to the enhancement in heat transfer synergy between natural convection and thermal conduction. Moreover, the increase of Stefan number leads to a faster melting rate and a shorter complete melting time. The RSM optimization indicates that the optimal gradients of fin thickness and central angle in this paper are 2.846 mm and 14.96°, respectively, and the complete melting time is reduced by 30.5% for the optimized LHS unit as compared with the corresponding uniform fin. Therefore, the gradient fins are recommended to be configured in a LHS unit in the engineering applications, where the fins are distributed with gradient angle, and they are thinner at the upper part and thicker at the lower part. [ABSTRACT FROM AUTHOR]