Phase Change Material Phase Change Research Articles

Design and performance assessment of a solar photovoltaic panel integrated with heat pipes and bio-based phase change material: A hybrid passive cooling strategy

This study investigates the effectiveness of an indirect passive cooling solution for photovoltaic (PV) panels using flattened heat pipes (FHPs) and phase change material (PCM). An innovative passive cooling design is proposed to cool a PV panel using multiple FHPs with a thin graphite sheet between the PV panel and the FHPs. Five experimental cases are examined under varying heat flux loads. Case O serves as the baseline with no cooling system, while Cases 1 to 4 feature different configurations of FHPs and aluminum sheets, with PCM heat sinks added in Cases 2 and 4. The investigation focuses on temperature profiles, temperature reductions, and final temperatures of the PV panel, especially at the end of a 4-hour experimental period when the PCM is fully melted. The results show significant temperature reductions in Case 1 compared to the baseline, with further improvement in Case 2 due to the PCM heat sink. Temperature reductions in Cases 2 and 4 are consistently higher than in Cases 1 and 3, demonstrating the enhanced cooling potential of PCM. Case 4 achieves the highest temperature reduction of 37.0 °C, followed by Case 2 with 34.9 °C under a radiative heat flux of 1000 W/m2. Using 4 FHPs (Cases 1 and 2) is economically justified, offering comparable cooling performance to 8 FHPs (Cases 3 and 4), thus ensuring cost-effectiveness and reducing material usage by over 50 %. A maximum enhancement in PV electrical efficiency of 17.3 % is achieved in Case 2 during PCM phase change with a radiative heat flux of 1000 W/m2.

Modeling the thermal behavior of multifunctional syntactic foams containing phase change materials for heat management applications

AbstractSyntactic foams are lightweight porous composites obtained by the addition of hollow glass microspheres (HGMs) to a polymer matrix. This work experimentally and numerically investigates the thermal behavior of epoxy‐based syntactic foams with enhanced multifunctionality produced by incorporating microencapsulated phase change materials (PCMs) as functional fillers, providing thermal energy storage and temperature stabilization due to its large latent heat of melting. Foams are fabricated using varying ratios of HGMs (0–40 vol%) and PCM (0–40 vol%) to achieve tuned densities and thermal properties with a total filler content of up to 40 vol%. A one‐dimensional numerical heat transfer model based on the enthalpy method is presented to simulate the thermal behavior of these materials. The model accurately incorporates the specific heat and thermal conductivity of the foam components, as well as the latent heat absorption during PCM melting (up to 68 J/g). The model is experimentally validated by demonstrating good agreement with the measurements of transient temperature profiles during heating tests on the foam samples. The validated tool is then leveraged to model the thermal response of the foams under a sinusoidally varying heat source, similar to the possible real applicative scenarios. These results can help optimize foam compositions balancing energy storage density, heat transfer properties, and structural support for lightweight energy storage systems. Potential applications include high‐efficiency thermal management in battery packs, electronic cooling, solar energy storage, and sustainable temperature‐controlled buildings.Highlights Foams with 0–40 vol% PCM and 0–40 vol% HGM show up to 68 J/g latent heat. 1D numerical model accurately captures thermal conductivity and PCM phase change. Validated model simulates thermal response under sinusoidal heat, optimizing foam composition. Foams act as thermal reservoirs, maintaining up to 5°C lower surface temperatures than epoxy.

Analysis of charging performance of thermal energy storage system with graded metal foam structure and active flip method

The latent heat thermal energy storage (LHTES) technology based on solid-liquid phase change material (PCM) is characterized by high energy storage density, small volume change, and constant operation temperature, which is widely employed in waste heat recovery, solar thermal utilization, and equipment thermal management. This paper introduces active flipping and graded porous to strengthen natural convection and heat conduction to alleviate the issue of low thermal conductivity and non-uniform phase change of PCM. Based on the fixed grid system, the enthalpy-porosity method combined with time-dependent gravity and non-Darcy porous effect is employed to solve the solid-liquid phase change problem under different porosity gradients, dimensionless flipping times (t*), and modified Ra (Ra*) numbers. Furthermore, the effects of the Ra* on the optimal t* and porosity gradient are also discussed. The results show that the flipping method can significantly enhance flow and heat transfer within the container, improve temperature field uniformity, and increase the proportion of latent heat storage. With the increase of porosity gradient and t*, the melting time initially decreases and then increases and the optimal thermal performance is achieved at a 6 % porosity gradient and t* = 0.4. Compared to the uniform porous structure without flipping, it can shorten melting time by 49.37 % and improve thermal energy storage rate (TESR) by 76.23 %. Additionally, the optimal porosity gradient is 6 % under different Ra*, but the optimal t* decreases with the increase of Ra*. This paper explores the charging performance of the thermal energy storage system with the graded metal foam structure and active flip method, which can contribute to the study of heat transfer enhancement in LHTES technology.

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

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

Micro-encapsulated phase change material suspensions for heat and mass transfer: A thermo-physical characterization

Despite the growing popularity of phase change materials (PCM) and suspensions of micro-encapsulated phase change materials (mPCM) in both industrial and scientific applications, their properties characterization remains partial and mainly limited to thermal properties. The characterization of such suspension is a crucial aspect for comprehending their behavior and optimizing their performance. This paper is dedicated to a wide characterization of two suspensions containing micro-encapsulated phase change material. In pursuit of a thorough understanding, we also extend our characterization efforts to the bulk PCM encapsulated within the particles.Our primary focus is on determining the materials thermal properties such as latent heat and specific heat capacity using differential scanning calorimetry. Different cooling and heating rates have been employed in our measurements. Regarding our experiments, the bulk phase change material is predominantly identified as octadecane. Results obtained from calorimetry are then compared between the bulk PCM and the mPCM suspensions. By comparing the magnitudes of latent heat obtained for suspensions with the bulk material, we can accurately determine the mass fraction of PCM within each suspension. Noteworthy, during the solidification process an additional latent heat peak is observed only for the encapsulated PCM.The density of the materials is also measured. The phase change of PCM included in the capsules can be observed in densimetry: within the studied temperature range (283.15–306.15 K), the most significant density variations occur during intervals associated with phase change transitions. In ranges where the PCM, included in the capsules, is in a single-phase state (solid/liquid), we provide linear laws that account for density variations with temperature.Finally, our characterization work focuses on the rheological properties of the materials. While the bulk PCM in liquid phase exhibits a Newtonian viscosity, mPCM suspensions present a non-Newtonian viscosity. Both shear-thinning and shear-thickening behaviors are observed depending on the suspension particle volume fraction ϕ of mPCM. This paper concludes with a full comparison of our characterization results with models and correlations proposed in the literature.

Structure and regional optimization of a phase change material Trombe wall system

Trombe walls (TW) have been an effective passive solar technology for enhancing building energy efficiency. Considering the thermal storage and regulation capabilities of phase change materials (PCM) in solar energy utilization, TW with PCM (PCMTW) were designed and optimized to further improve building energy performance. This study investigated the combined effects of PCM phase change temperature, PCM location and ventilation strategies on a single-layer PCMTW. Moreover, a novel double-layer PCMTW was proposed and evaluated across various climate regions of China in consideration of the effect of PCM phase change temperature combinations. Results show that the single-layer PCMTW with PCM22 placed next to inner surface and ventilation achieved the lowest annual energy consumption. The selection of PCM phase change temperature was crucial for various climatic conditions. Compared to the traditional building, the highest heating energy-saving was 118.16 kWh/m2 achieved in Changchun with PCM22 + 28, and the highest cooling energy-saving was 45.71 kWh/m2 achieved in Haikou with PCM25 + 28. Additionally, the double-layer PCMTW was recommended for climate regions where the daily average temperature and the daily average solar emissivity were 20–25 °C and below 4 kWh/m2 in summer, and below 10 °C and 2–6 kWh/m2 in winter.

A thermal management design using phase change material in embedded finned shells for lithium-ion batteries

Phase change material (PCM) has recently been regarded as a promising thermal management means for lithium-ion (Li-ion) batteries. However, solid-liquid phase change of many current PCMs occurs in a narrow temperature range, and the cooling performance of their liquid phase is rather poor. To tackle these issues, this article presents a battery thermal management (BTM) design which injects PCM into an aluminum shell around each cell, and machines the shell inner surfaces into a row of straight rectangular fins immersed into the PCM and its outer surfaces into a plain-fin shape exposed in the air. In the mild thermal conditions, the finned shells of two adjacent cells can be embedded and passive PCM cooling is employed. Once the PCM completely melts, the shells will move apart and an airflow will be driven through the gap between them. The BTM design can also protect Li-ion batteries in a freezing environment, by taking advantage of PCM solidification when the finned shells return to their embedded configuration. A series of experiments were conducted to examine its effectiveness on two 50Ah lithium nickel cobalt manganese oxide (NCM) prismatic batteries, which were discharged at a capacity-rate of 2C and at room temperatures of T0=25∘C, 40∘C and −10∘C. It is shown that in the battery discharge at T0=25∘C, PCM in the embedded finned shells effectively reduced the average battery-surface temperature (T¯) and the maximum temperature difference (ΔTmax) by 21% and 36.7%, respectively, compared to bare batteries. At T0=40∘C, this BTM design successfully limited the growth of T¯ to 9.0∘C and led to ΔTmax no more than 1.5∘C. As to the low-temperature scenario, it maintained the batteries at temperatures above T0 for 55.2% longer than those without any BTM protections. Its cooling performance was also compared with that of the conventional liquid-cooling scheme using a bottom serpentine-channel cooling plate. All these results clearly demonstrate that the BTM design in this article can compensate for the deficiencies of conventional PCM-based BTM schemes. Regardless of whether the PCM phase change is available, it has well coped with different thermal management demands of Li-ion batteries.

Experimental Investigation of the Viscosity and Density of Microencapsulated Phase Change Material Slurries for Enhanced Heat Capacity and Transfer

Working fluids that incorporate solid microencapsulated phase change materials (MPCMs) can benefit from properties such as density and viscosity, which are crucial for improving heat capacity and transfer. However, limited data are available on these parameters for specific slurry and mass ratios. In this study, we present a comparative analysis of the experimental results on the viscosity of three different MPCM aqueous dispersions, namely MPCM 31-S50, MPCM 25-S50, and Micronal 5428X. Varying MPCM mass ratios of distilled water were used to obtain different mass concentrations of the phase change material (PCM), and the resulting slurries were analysed at temperatures ranging from 15 to 40 °C. Our findings showed that all slurries exhibited non-Newtonian characteristics at low shear rates, with viscosity stabilising at higher shear rates, resulting in the characteristics of a Newtonian fluid. The viscosity results were highly dependent on the type of MPCM base dispersion, particularly at high mass ratios, with the slurries having viscosities higher than those of water. Furthermore, we conducted density experiments as a function of temperature, using a flow test setup and a Coriolis flowmeter (Endress+Hauser, Reinach, Switzerland) to determine the density of two MPCMs, namely MPCM 25-S50 and Micronal 5428X. The test samples were prepared at mass concentrations of 10%, 15%, and 20% of the phase change material. We found significant differences in density and viscosity for different MPCM slurries as a result of both the PCM concentration and the material studied. Our results also revealed an apparent PCM phase change process, in which the slurry density significantly decreased in the temperature range of the phase transition from solid to liquid.

Effect of steam flow parameters on the partial load behaviour of a PCM storage

To estimate the achievable thermal power in passive latent heat thermal energy storage (LH-TES) systems and the corresponding charging/discharging times for customised storage design and operation, relevant influencing parameters and their impact must be sufficiently known and understood. These parameters include not only the well-researched parameters on the phase change material (PCM) side, but also the parameters on the heat transfer fluid (HTF) side, such as the inlet temperature, the mass flow rate and the resulting heat transfer coefficient (HTC) between the HTF flow and the tube wall. For heat exchangers, these influences have been studied well, however for highly transient LH-TES studies are scarce. This article presents the influence of relevant parameters focusing on the ones related to the HTF side, using analytical and numerical parameter studies in Matlab®. The effect of different HTCs for varying flow conditions of the HTF in different zones of the tube is utilised to introduce the dominant heat transfer surface between the storage and the HTF (DHTS) as an indirect operating parameter. It corresponds to the area where the HTF phase change occurs and where most of the total energy is transferred. The numerical study indicates that the size of the dominant heat transfer area and thus the DHTS can be adapted by the HTF mass flow rate and the temperature difference between the HTF inlet temperature and the PCM phase change temperature. Based on this knowledge, new conjugate design and operating strategies for a test rig with a nominal thermal power of 1 kW using the commercial PCM PLUSICE A133 as storage material are proposed. They lead to a peak shaving behaviour of the storage with defined charging and discharging times.

Experimental and numerical study of cylindrical encapsulated phase change material in packed bed thermal energy storage with expansion and shrinkage effects

Packed bed latent heat thermal energy storage (PBTES) is a promising technology for storing thermal energy with a relatively compact size and smaller temperature variation during phase change. In this study, a lab-scale low-cost PBTES with cylindrical shape encapsulation of phase change material (PCM) is designed and fabricated for evaluating the charging-alone and discharging-alone thermal performance for medium-temperature range (200–350 °C) latent heat storage applications. Air is used as a heat transfer fluid (HTF), and solar salt is employed as a PCM. A new numerical model incorporating shrinkage and expansion of PCM during phase change is developed by considering the packed bed as the porous medium and the melt fraction-based density variation of PCM to model the transport and solid-liquid phase change phenomena in the PBTES. The numerical model is validated with the in-house experimental results. The maximum thermal charging efficiency is found to be 67.1 % for the mass flow rate of 4.3 g/s and a maximum charging inlet air temperature of 360.9 °C. The maximum discharging efficiency of 86.1 % is obtained for the mass flow rate of 4.3 g/s and discharging inlet air temperature of 35 °C. It is found that the expansion and shrinkage effects are prominent during the phase change of PCM during the charging and discharging operations of the PBTES.

Research on thermal management system of lithium-ion battery with a new type of spider web liquid cooling channel and phase change materials

Phase change materials (PCM) possess outstanding temperature control capabilities; however, it is difficult for pure PCM to satisfy the thermal management requirements under high discharge rates (4C-5C) in electric vehicles. This study designs and numerically simulates a Battery Thermal Management System (BTMS) that combines PCM with a spider web liquid cooling channel and compares it to pure PCM cooling. The results reveal that the new hybrid BTMS exhibits exceptional heat dissipation performance. Compared to pure PCM cooling, the hybrid cooling structure can maintain the Tmax of the battery module below 40 °C. Under a 5C discharge rate, the Tmax and ΔTmax of the battery module can be controlled at 39.6 °C and 4.9 °C. Based on this, the study analyzes the effects of coolant inlet Reynolds number, inlet temperature, and ambient temperature on the battery's Tmax, ΔTmax, coolant pressure drop, and PCM phase change rate under different discharge rates. This study aims to control the liquid phase rate at around 80 % and to minimize the pressure drop in the coolant as much as possible while meeting thermal management requirements. Finally, in a low temperature environment of −30 °C, with an inlet Re of 300 and an inlet temperature of 7 °C, the heating rate of the battery module with the newly designed hybrid cooling structure can reach 3.67 °C/min.

Dual-sided melting of a one-dimensional composite wall comprising two distinct phase change material (PCM) layers

Past literature on theoretical modeling of solid-liquid phase change heat transfer mostly addresses a single phase change material (PCM), whereas, a series combination of two or more PCMs is often used in practical applications. This work presents an approximate analytical model for the melting/freezing of a one-dimensional composite of two PCMs being heated/cooled from both ends. It is shown that up to three phase change fronts may simultaneously exist in this problem. In addition to direct melting/freezing from the respective walls, it is shown that each PCM may also melt/freeze due to heat transfer through the other PCM. Using a quasi-stationary assumption, temperature field during the multi-stage phase change process is approximated by solving a multilayer transient thermal conduction problem using eigenfunction expansion. Good agreement with numerical simulations, and with the Stefan solution for a special case is demonstrated. The impact of key non-dimensional parameters on performance metrics including total time for phase change is determined. A quantitative prediction of conditions leading to the thermodynamically favorable situation of simultaneous completion of phase change of two commonly used PCMs is carried out. This work improves the theoretical understanding of phase change in composite PCMs, leading to optimization of practical thermal systems.

Enhancing melting and solidification characteristics of a triple-pipe latent heat energy storage system via a wavy central wall with a sinusoidal fixed wavelength

This study aims to improve the melting and solidification characteristics of a triple-pipe heat storage unit by introducing a wavy middle plate with a sinusoidal fixed wavelength. The inner and outer tubes facilitate the flow of heat transfer fluids (HTFs), while the middle tube houses the phase change material (PCM). Various scenarios are examined to gauge the impact of the wavy central wall on the PCM's phase change performance, comparing liquid fraction and mean temperatures with cases featuring a straight middle plate. The primary goal is to pinpoint the optimal scenario with the shortest discharging/charging times and the highest rates of heat release/storage. The investigation commences by assessing the effects of middle wall thickness (2, 4, 6, and 8 mm) on the PCM's phase change characteristics, comparing liquid fraction and mean temperature with a straight middle plate. Subsequently, the influence of various sinusoidal wavelengths (250, 125, 50, and 25 mm) on both melting and solidification performances of the PCM is numerically examined to identify the optimum case. The research also delves into the effects of various sinusoidal amplitudes (2.5, 5.0, 7.5, and 10 mm) on the PCM's phase change process. In the final step, the capability of the wavy middle plate, with various phase shifts (0°, 90°, 180°, and 270°), in improving the PCM's phase change characteristics is analyzed. Numerical results highlight that, among the tested scenarios, the configuration with a 5 mm amplitude and a 25 mm wavelength (A5λ250) exhibits outstanding performance during both melting and solidification phases. A5λ250 showcases a 19.92 % reduction in melting time and a 2.44 % reduction in solidification time compared to other cases, coupled with a 22.55 % improvement in heat storage and a 3.48 % enhancement in heat release. However, introducing a phase shift in the melting process adversely affects system performance. Conversely, adjusting the phase shift of A5λ25 to 270° leads to enhancements in both solidification time and heat release rate. Specifically, solidification time improves by 0.86 %, and the rate of heat release increases by 0.66 W compared to A5λ25–0.

Examining the Combination of a Parabolic Solar Collector with Phase Change Material (PCM) in Solar Distillation

An Experimental investigation had been executed in a solar distill unified with parabolic collector using PCM. This experiment had been compared with PCM less experimental setup. PCM and Parabolic solar collector are things embedded with the base system to increase the overall outcome. Used paraffin wax as PCM (Phase Change Material) was one of the most cost-effective approaches to store heat energy. The solar ray’s incidence had been harvested by using the solar distill in which the water passed by the water tubes. This base solar still consist of double slope and the water tube filled with PCM. These experiments were done at the water depth of 15 mm. In these experiments produce the results indicated that a solar distillation system with a Parabolic Solar Collector coupled with PCM yielded higher productivity and temperature levels compared to not utilizing PCM, and these experiments demonstrated that the introduction of PCM in DSSD with Parabolic solar collector with or without it PCM resulted the improvements of the productivity of 33.25% and 57.31% respectively.

Dynamic performance optimization of two-stage thermoelectric generator: Impact of different geometric leg shapes in each stage

This study presents a single-stage and two-stage thermoelectric generator (TEG) with different geometric shapes of the legs (rectangular, trapezoidal, inverted trapezoidal, rhomboid, and hourglass shapes). The various geometric shapes of the legs are combined between the two stages using phase change material (PCM) to cool the generator. The study's objective is to determine the optimal dynamic behavior of the TEG through the geometric design of the legs. The results show the behavior of the efficiency, the generated voltage, and the time it takes for the PCM to melt. The legs of the optimal two-stage system, Rhomboid-Rhomboid (G) and Trapezoidal-Inverted Trapezoidal (B), with a relationship γT=4, reach the best efficiency of 3.88% and 3.9% for the first and second stages, respectively, applying a heat flow of Qh=0.1W. On the other hand, the single-stage rhomboid system achieves the same maximum efficiency as the two-stage rhomboid system (G system), which is 3.9%. The temperature difference of each module is almost the same as that of the single-stage system. However, the voltage generated by the two-stage TEG is higher, and the efficiency remains practically constant compared to the single-stage system, which decreases with time. The effect of PCM's phase change process on TEG performance is analyzed, considering the characteristic heat transfer of PCM.

A novel thermal management for PEM fuel cell stack combining phase change materials with liquid cooling under low temperature condition

In this study, a low-temperature thermal management system (TMS) that combines a phase-change material (PCM) with liquid cooling for the cold start of a proton exchange membrane fuel cell (PEMFC) is introduced. The proposed system effectively utilizes the waste heat generated during the fuel cell operation to heat the PCM. After shutdown, the heat released during the PCM phase change process is transferred to the stack via a coolant pump, thus maintaining the stack temperature above the minimum startup threshold. A comprehensive thermal model of the proposed system is established and validated using numerical simulations. The temperature variations within the stack were evaluated under various environmental conditions, material properties, and coolant- pump control parameters. The results demonstrate that the TMS can sustain the stack temperature above 0 °C for 63.36 h in an environment at −20 °C. Furthermore, compared with passive thermal management, the proposed system has better temperature uniformity and an 11.71 % reduction in the stack startup time at 0 °C due to the proper loop design, which prevents the PCM from absorbing heat from the stack. This study provides innovative solutions to address the challenges associated with cold starts and is a valuable reference for future TMS designs.

Influence of upstream and downstream flow on different packed bed configurations in phase change process

One approach to storing latent heat in large quantities is through PBSS (Packed bed storage system) with spheres filled with PCM (Phase Change Materials). In these storage systems, an HTF (heat transfer fluid) flows over the spheres. So, both the sphere wall temperature and the heat flux can vary depending on the geometric conditions of the bed of spheres, the PCM thermodynamic conditions, and the HTF thermal conditions. Thus, this work aims to verify the influence of the thermal conditions of the external flow on the PBSS filled with PCM in the melting process. Cases with variable heat flux as well as uniform temperature and uniform heat flux conditions were tested. Three sphere positions, 4 sphere spacing ratios (blockage ratio), and three temperature differences between HTF and PCM phase change temperature were also analyzed, for two external flow directions (upstream and downstream), totaling 74 different configurations. The study found that the uniform temperature condition resulted in a 33 % faster total melting time than the uniform heat flow condition. Additionally, the upstream flow showed a total melting time that was approximately 24 % shorter than the flows in the opposite direction. The relationship between the total melting time and the temperature differences was found to be non-linear. Moreover, the blockage ratio showed a non-linear dependence with total melting time. The configuration with a blockage ratio of 1.125 demonstrated the shortest total melting time, with values on average 22.4 % shorter than the blocking ratio of 0.550, which had the longest melting times. These results demonstrate that optimizing external flow thermal conditions is crucial for improving the performance of PBSS with PCM-filled spheres in large-scale latent heat storage systems.

Improving thermal storage of energy screw pile groups with phase change materials

To achieve affordable housing in a carbon-neutral society, new buildings require a dual-purpose approach that comprises efficient construction and a green energy supply. Energy screw piles [1, 7] meet this demand as they combine the agility of screw pile drilling with the capability of extracting clean shallow geothermal energy. Moreover, the screw piles can be filled with phase change materials (PCM) to provide latent thermal storage. Studies regarding the use of PCMs as borehole backfill in a ground source heat pump system (GSHP) conclude that PCM implementation can improve GSHP performance. However, most commercially available PCMs have low thermal conductivities (λ) which undermine heat exchange rates [5, 10]. Besides, using PCMs as a composition of a concrete energy pile can reduce the pile structural performance since PCMs usually have a low mechanical capacity [2, 8]. The authors recently introduced PCM-sand mixtures as a core in the central hollow part of an energy screw pile, which does not impact the pile structural capacity [6]. The mixtures with a higher PCM content benefit the pile heat exchange through its latent heat, but only when the PCM does not reduce the mixture’s λ. To overcome this problem, this work tests a new underground heat exchange system where instead of mixing the PCM in the energy screw pile filling material, regular screw piles (i.e., without the heat exchange tubes) are filled with pure PCM, acting as thermal storage piles. A numerical model built via COMSOL [4] is used to evaluate how this combination of screw piles performs thermally when supplying/rejecting heat for a GSHP system operating for a whole year.
 This work uses a validated numerical model [3, 9] to simulate a grid of evenly distributed screw piles, where Energy Piles (EP) and Thermal Storage Piles (TSP) are positioned interspersed, evenly spaced 0.7 m apart. Inside the EPs, an U-loop pipe is inserted in the pile steel case and the remaining is filled with grout. In contrast, the steel case of the TSPs is filled with only PCM [11]. All other material properties and the screw pile geometry are based on [1]. The thermal load is based on the design of a building located in Melbourne, Australia (Figure 1(a)). The model considers the hourly operation of a GSHP for one year, calculating the GSHP Coefficient of performance (COP) at every time step and adjusting the ground thermal load. For comparison, two simulations (one where the TSPs are filled with PCM and a reference where they are filled with only air) are undertaken. Besides the energy stored, the COPs of both cases are compared as a relative increase/decrease percentage (COPchange = (COPPCM / COPReference) – 1).
 Figure 1(b) presents the energy stored by the PCM and its corresponding percentage in a solid state over the simulation. As the EPs reject heat underground due to GSHP summer operation for cooling, the TSPs temperature rise and store sensible heat, until they reach their phase change temperature (Tpc), when the PCM melts to store significantly more heat energy in a shorter time window due to latent heat. Conversely, the PCM solidifies when EPs extract more heat than reject due to GSHP winter operation (heating) for a certain period. The PCM melts again on the following summer, restarting the whole process. The peak energy stored in the TSPs is 190.3 MJ/m3. Even though the thermal load varies hourly, the PCM phase change process happens without immediate responses to the load variation. Figure 1(c) presents the resulting change in the hourly COP considering cooling (CCOP) and heating (HCOP) from implementing PCM in the TSPs, compared to the reference scenario (empty TSPs). The extra heat stored by the PCM lowers the circulating fluid temperature while the latent heat is engaged (months 2 to 7 and 12), which results in a higher CCOP and a lower HCOP. By plotting a monthly average of the COPchange (grey markers), it is clear that the impact of the PCM on the COP is positive when cooling is dominant and slightly negative when heating becomes dominant while the latent heat is engaged. When the PCM is implemented in the EP [6, 10], the heat exchanged by the EP drops once the phase change process is over due to the low λ of the PCM, but by positioning it outside the EP the higher CCOP is sustained even after the phase change ends. However, lowering the fluid temperature increases CCOP while lowering HCOP at the same time, which can harm thermal performance if not designed properly. The results underlie the thermal storage potential available not only in screw piles, but also in any hollow pile foundation, by simply implementing PCM in the hollow case.

Comprehensive performance analysis and structural improvement of latent heat thermal energy storage (LHTES) unit using a novel parallel enthalpy-based lattice Boltzmann model

Latent heat thermal energy storage (LHTES) utilizing phase change material (PCM) is one of the critical enablers in developing sustainable and low-carbon energy systems. To fill the knowledge gap, this paper presents an enthalpy-based solid–liquid model through the lattice Boltzmann method (LBM) with a multi-relaxation-time (MRT) approach, aiming to simulate convective phase change in LHTES units with and without porous media in Cartesian or axisymmetric domains. To improve accuracy and efficiency, the model integrates a differential scanning calorimetry (DSC) correlated equation for enthalpy modeling, couples with a 1D heat-transfer-fluid (HTF) model for boundary treatment of HTF side, and employs a parallel LBM scheme for efficient parametric studies. The validation demonstrates the model’s success in predicting PCM phase change, with errors below 10%. A comprehensive numerical analysis is then conducted to quantitatively evaluate the effect of convection on PCM melting. Novel metrics, such as acceleration rates (ac) of PCM melting and threshold Rayleigh numbers (Radc) at various aspect ratios, are introduced. Furthermore, PCM melting in the porous cylindrical unit is explored. Findings reveal up to 86% acceleration in melting compared to pure PCM at 80% energy storage, and the porous media with porosity above 0.9 is recommended for thermal enhancement. Moreover, this paper analyzes the negative effect of uneven temperature distributions caused by convection on LHTES unit efficiency. A modified LHTES unit geometry is proposed to offset this negative effect, and the study demonstrates successful mitigation of uneven temperature distributions, achieving up to 57 % acceleration in PCM melting.

Topology optimization of HCM/PCM composites for thermal energy storage

In this paper, we have applied topology optimization (TO) to the latent heat based thermal energy storage (LHTES) device design. The high conductivity materials (HCM) are added to the phase change materials (PCM) to increase the overall thermal conductivity. Distribution of HCM in PCM is optimized by the TO method to maximize the charging speed of the LHTES device at the different charging periods. The time-dependent Navier–Stokes (NS) equations, involving natural convection and PCM phase change, are chosen as the governing equations of fluid motion. The energy equation including latent heat is strongly coupled with the NS equation. The adjoint method is utilized to derive and calculate the sensitivity. Several 2D numerical examples with different charging periods and different HCM volume fractions have been studied with this TO method. The charging period showed a huge influence on the optimized TO structures. The short charging period designs exhibit more conductive structures while the long charging period designs show convective structures. A pareto front of the topologically optimized designs has been constructed, which shows up to 110.7% of improvement in the charging speed over the parallel-fin design with the same root fin width.