High Temperature Cycling Research Articles

High-temperature resistant electromagnetic protection bilayer structure based on the low-reflection metasurface and wave-absorbing material.

In this paper, we propose a high-temperature resistant bilayer structure for electromagnetic protection with low reflection, consisting of a metasurface and an absorbing layer. The bottom metasurface decreases the reflected energy by using a phase cancellation mechanism to make electromagnetic wave scattering in the 8-12 GHz range. While the upper absorbing layer assimilates the incident electromagnetic energy through electrical losses and simultaneously regulates the reflection amplitude and phase of the metasurface to enhance scattering and expand its operating bandwidth. Research shows that the bilayer structure achieves a low reflection of -10 dB in the range of 6.7-11.4 GHz due to the combined effect of the above two physical mechanisms. In addition, long-term high-temperature and thermal cycling tests verified the stability of the structure in the temperature range of 25-300°C. This strategy provides the feasibility of electromagnetic protection in high-temperature conditions.

Experimental study on the nonlinear flow characteristics of fractured granite after high-temperature cycling

To understand the influence of temperature on the flow characteristics of fractured granite, high-temperature cyclic thermal treatment and flow tests on the fractured rock sample with different joint roughness coefficients and intact rock samples were conducted. The larger confining pressure and larger joint roughness coefficient will increase the resistance of fluid flow and affect the flow characteristics of the fluid. With the temperature increasing, the aperture of the fractures, the number of micro-fractures, and micropores increase which forms a large number of new connected hydraulic channels in the matrix. Forchheimer's law and Izbash equation can well describe the nonlinear flow characteristics, and the fitting coefficients are greater than 0.99. As the increasing temperature, the slope of the curve between the volumetric flow rate and pressure gradient gradually decreases, and the coefficients in Forchheimer's law and the Izbash equation decrease. The transmissivity decrease with the increasing Reynolds number and the change range of that increase with the increasing temperature. When the temperature is at the lower level (T = 200 ~ 600 °C), the contribution of split fracture to the permeability is greater than that of the matrix. When the temperature continuously increases to 800 °C, the contribution of the matrix to the permeability gradually rises and then exceeds that of split fracture. The results indicate that 400 °C is the critical temperature, after which the flow characteristics of fractured granite after high-temperature cycling change more obviously.

Investigation of shear strength of sand–bentonite mixtures with boron additives at high temperature for energy geo-structures

The soils surrounding energy geo-structures are exposed to high temperatures and temperature cycles. Changes in the engineering properties of soils should be investigated under thermal effects and soils that are highly durable against temperature changes are needed for thermo-active geo-structures. Generally, bentonite or sand–bentonite mixtures (SBMs) are preferred as natural barrier soil materials. Hence, the engineering properties of these natural soil materials against high temperatures should be improved. Boron, which has high thermal resistivity, reduces the heat expansion of materials, when added to soils may increase the durability of buffer materials at high temperatures. In the present study, the effects of tincal and ulexite additives were observed on the shear strength behavior of SBMs at 80 °C and room temperature. The general results showed that with the contribution of boron, the shear strength of the SBMs increased with increasing temperature. The effect was more pronounced for 20% SBMs at high temperature. Tincal and ulexite can be used to increase the shear strength of SBMs at high temperatures.

Analysis of the Mechanical Properties of 3D-Printed Plastic Samples Subjected to Selected Degradation Effects.

The Fused Filament Fabrication (FFF) method is an additive technology that is used for the creation of prototypes within Rapid Prototyping (RP) as well as for the creation of final components in piece or small-series production. The possibility of using FFF technology in the creation of final products requires knowledge of the properties of the material and, at the same time, how these properties change due to degradation effects. In this study, the mechanical properties of the selected materials (PLA, PETG, ABS, and ASA) were tested in their non-degenerate state and after exposure of the samples to the selected degradation factors. For the analysis, which was carried out by the tensile test and the Shore D hardness test, samples of normalized shape were prepared. The effects of UV radiation, high temperature environments, high humidity environments, temperature cycles, and exposure to weather conditions were monitored. The parameters obtained from the tests (tensile strength and Shore D hardness) were statistically evaluated, and the influence of degradation factors on the properties of individual materials was assessed. The results showed that even between individual manufacturers of the same filament there are differences, both in the mechanical properties and in the behavior of the material after exposure to degradation effects.

Comparative Study of High-Temperature Cycling and Thermal Stability of LLOs and NCMs under Medium–High Voltage

Lithium-rich layered oxides (LLOs) are considered as one of the most potential layered cathode materials due to their high discharge specific capacity, low cost, and environmental friendliness. However, the decomposition of electrolytes at high voltages (4.7 V) limits the commercial application of LLOs. Furthermore, to meet the demand for high energy density, lithium nickel cobalt manganese oxides (NCMs) need to improve their working voltage. In limited conditions, all LLOs and NCMs need to be used under the medium–high (4.4–4.6 V) cutoff voltage. Herein, we compare the performance of LLOs and NCMs in the voltage window with a medium–high cutoff voltage. Results show that after 100 cycles at high temperature (45 °C), the capacity retention rate of LLO is 98.68%, while that of NCMs is 78.15–96.93%. Differential scanning calorimetry reveals that the exothermic temperature of LLO is increased by 60–100 °C, and thermal release is reduced by 33–93%. LLOs have obvious advantages under medium–high voltages and a promising application prospect. This work systematically analyzes and compares LLOs and NCMs under medium–high voltage, providing guidance for the industrialization of LLOs.

Corrosion Behaviour of NiCr/TiO2 coated 316L stainless steel before and after thermal shock exposure

ABSTRACT Understanding the impact of high-temperature thermal cycle on coating composition and its corrosion characteristics is essential for coating development. The present study investigated NiCr/TiO2-coated 316L austenitic stainless steel (316L SS) under thermal shock exposure and their corrosion behaviour in 3.5% NaCl solution. The electrochemical parameters were evaluated using potentiodynamic polarisation and electrochemical impedance spectroscopy techniques to understand the corrosion characteristics. Various techniques, viz. field emission scanning electron microscope, energy-dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy, were employed to examine the morphology, elemental composition, crystalline and oxidation characteristics of all the samples. The NiCr/TiO2-coated 316L SS demonstrated thermal resistance up to 9-h oxidation period and five thermal cycles. Afterwards, with gradual weight loss, TiO2 spallation appeared on the coated samples. The as-deposited sample revealed an increased pitting potential of 118.76% and improved corrosion resistance over ten times in magnitude than the bare sample. This improvement in the coated sample is attributed to high charge transfer resistance and chemically stable coating layer between the metal matrix and corrosive solution. In contrast, the delaminated sample revealed over four times greater resistance than the bare sample, which is credited to the limited surface area for possible redox reactions and passive NiCr bond coat layer over the metal matrix.

Cogeneration of Fresh Water and Electricity with High-Temperature Power Cycles: Comparative Assessment of Multi-Effect Distillation and Reverse Osmosis

The pressing problems of water scarcity in many parts of the planet make water desalination one of the technological solutions for guaranteeing the fresh water supply. However, desalination processes require high energy consumption, mainly provided by fossil fuels. The integration of renewable energy sources into desalination processes is a promising option for decarbonizing the desalination sector. As most water-scarce regions with access to seawater frequently have high solar irradiation levels, it seems appropriate to exploit the sun to power the desalination process. This work presents the assessment of two integrated solar power and desalination systems regarding efficiency and water production. Two desalination processes (multi-effect distillation and reverse osmosis) are studied for potential coupling with the combined cycle of a central receiver solar plant to produce electricity and freshwater. In the case of the multi-effect distillation plant, it is integrated by replacing the Rankine cycle condenser of the combined cycle. In the case of the reverse osmosis plant, it is powered by the electricity generated from the combined cycle. For this comparison, the 21st of March has been considered as the design point and Almería (in the Southeast of Spain) as the plant location. The results show that the thermal cogeneration option renders a worse outcome (thermal efficiency of 50.2% for LT-MED case) than the decoupled generation of electricity and water (thermal efficiency of 53.3% for RO case), producing 18% less fresh water than the RO configuration (3831 m3/d vs. 4640 m3/d), due to the 6% penalty in the efficiency of the Rankine power cycle in the MED configuration as a result of increasing the condensation temperature from 42.6 °C to 70 °C.

Comparison of composite yarns fabricated with ring spinning technologies for extreme environment

In this work, polyamide 66/meta-aramid composite yarn was manufactured for applications involving extreme temperatures. Three types of spinning methods were chosen, including siro core-spun, sirofil and embedded and locatable spinning (ELS) systems. Under the discussion of the filament motion feature and the staple fiber trapping effect separately, it is concluded that the siro core-spun system has the lowest trapping ability for the staple fibers due to the loose interaction force between the filament and the rovings. By contrast, the twist effect between the filament and the rovings in sirofil method prevents the fiber migration, resulting in a more stable yarn. Finally, ELS system further enhances the wrapping effect of the fibers outside. For the specific properties, hairiness tests and SEM photos of the yarns prove that the ELS system has the best control of the surface fibers, contributing to better uniformity. Similarly, the tensile property for the ELS yarn is also better than the other two. Finally, the tensile modulus and thermal property of the the ELS composite yarn fabric almost remain the original level after the high and low temperature cycling tests, demonstrating promising application prospect in the extreme environment.

Influence of lithium plating on lithium-ion battery aging at high temperature

Lithium plating is an important issue for lithium-ion battery safety and cycle life that can be caused by cycle at low temperature. In this study, we investigated battery aging over an extended low-temperature cycle and at high temperature after the low-temperature cycle. Nondestructive and destructive analysis methods were used to study battery electrochemical dynamics and changes to the materials, and the results indicated that battery aging over the extended low-temperature cycle could be divided into two stages. The mechanism of the first stage is that of “dead lithium,” which causes significant capacity fading, whereas that of the second stage is solid electrolyte interphase (SEI) growth. It was also found that plated lithium accelerates battery aging at high temperature; part of it is intercalated into anode, another part of it reacts with the electrolyte, thus consuming the plated lithium. Thus, electrolyte decomposition and SEI growth are the primary aging mechanisms at high temperature after low-temperature cycling. The plated lithium content and charge/discharge rate affected plated lithium consumption during the high-temperature cycle. Plated lithium was not depleted for the battery with significant amounts of plated lithium or upon charging with high current and discharging with both high and low current when the state of health (SOH) decreased to 70%. The factors affecting battery aging ranked in order of importance are charge current > plated lithium content > high temperature > discharge current.

XLi2MnO3·(1-x)LiMeO2 and Li4Ti5O12 cell chemistry for Behind-the-Meter Storage applications

Li- and Mn-rich layered oxide material (xLi2MnO3·(1-x)LiMeO2, Me = Ni, Mn, and etc., LMR-NM) is paired with Li4Ti5O12 (LTO) in a full cell and evaluated for the Behind-the-Meter Storage (BTMS) applications. The LMR-NM/LTO full cell shows very high capacities and excellent long-term cycle life. It delivers 192 mAh g−1 after 500 cycles at C/2 and 45 °C with a capacity retention of 75 % and coulombic efficiency higher than 99.95 %. It also has impressive rate capabilities. A capacity of 220 mAh g−1 is achieved at 2C which is 88 % of the initial capacity at C/10. The high cycling temperature clearly enhances electrochemical kinetics and activates more Li2MnO3 component, which gives high capacities, low cell impedance, and better rate capabilities. Moreover, it helps to form a relatively thick cathode-electrolyte interphase (CEI) film to suppress transition metal dissolution from the cathode surface. The upper cut-off voltage (UCV) of 3.0 V keeps the structural integrity of the cathode during cycling. A higher UCV of 3.2 V accelerates structural instabilities of the cathode as well as growth of the solid-electrolyte interphase (SEI) via transition metal dissolution and deposition on the anode surface. It results in higher cell impedance, worse capacity retention and faster capacity fade.

High-affinity decoy protein, nFD164, with an inactive Fc region as a potential therapeutic drug targeting CD47

CD47, as an innate immune checkpoint molecule, is an important target of cancer immunotherapy. We previously reported that a high-affinity SIRPα variant FD164 fused with IgG1 subtype Fc showed a better antitumor effect than wild-type SIRPα in an immunodeficient tumor-bearing model. However, CD47 is widely expressed in blood cells, and the drugs targeting CD47 may cause potential hematological toxicity. Herein, we modified the FD164 molecule by Fc mutation (N297A) to inactivate the Fc-related effector function and named it nFD164. Moreover, we further studied the potential of nFD164 as a candidate drug targeting CD47, including the stability, in vitro activity, antitumor activity of single or combined drugs in vivo, and hematological toxicity in humanized CD47/SIRPα transgenic mouse model. The results show that nFD164 maintains strong binding activity to CD47 on tumor cells, but has weak binding activity with red blood cells or white blood cells, and nFD164 has good drug stability under accelerated conditions (high temperature, bright light and freeze-thaw cycles). More importantly, in the immunodeficient or humanized CD47/SIRPα transgenic mice bearing tumor model, the combination of nFD164 and anti-CD20 antibody or anti-mPD-1 antibody had a synergistic antitumor effect. Especially in transgenic mouse models, nFD164 combined with anti-mPD-1 significantly enhanced tumor suppressive activity compared with anti-mPD-1 (P < 0.01) or nFD164 (P < 0.01) as a single drug and had fewer hematology-related side effects than FD164 or Hu5F9-G4. When these factors are taken together, nFD164 is a promising high-affinity CD47-targeting drug candidate with better stability, potential antitumor activity, and improved safety profile.

Experimental Investigation on the Effect of TiO2 Nanoparticles Emulsion in Water on Emissions and Performance Characteristics of DI Diesel Engine

The world is presently confronted with the twin crisis of resource restriction and environmental degradation. The search for solutions that promise a harmonious correlation with sustainable development, energy conservation, efficiency, and environmental preservation has become highly important. The main purpose of innovative studies on fuel refinement and combustion engines is to improve fuel properties by adding fuel additives. In this study, the impact of Titanium dioxide, TiO2, nanoparticles solution blended with diesel fuel on the performance and emission characteristics of four-stroke combustion engine OM 364 EU III, manufactured by IDEM Co and licensed by Daimler Benz, has been investigated. The selection of TiO2 nanoparticles is based on the easy access in the market and the gap recognized; in previous literature, these nanoparticles were added to biodiesel or n-butanol blends. The proposed combined fuel in this study contains 2.5 ppm TiO2 nanoparticles dissolved in 1200 [ml] water and added to 60 [Lit] base diesel fuel. The results of the aforementioned combined fuel have been compared with the base diesel fuel. It has been observed that applying nano-additives improves the mechanical performance of the diesel engine, such as power, torque, brake-specific fuel consumption, and thermal efficiency. Moreover, soot, unburned hydrocarbons, and carbon monoxide have declined by 2.78%, 3.55%, and 3.32%, respectively, due to TiO2 nanoparticles' catalytic effect on fuel combustion. However, the amount of NOx has increased up to 3.09% because of the high cycle temperature.

Strengthening mechanism in thermomechanical fatigue properties of Ti6Al4V titanium alloy by laser shock peening

In this study, the role of laser shock peening (LSP) on the thermomechanical fatigue properties of Ti6Al4V titanium alloy was investigated. The effects of LSP with different coverage layers on the microstructural evolution, residual stresses distribution, and thermomechanical fatigue lives were systematically analyzed by scanning electron microscopy, transmission electron microscope, residual stresses tests, microhardness tests, and thermomechanical fatigue tests. Results showed that the thermomechanical fatigue lives of the LSP1 and LSP2 specimens at room temperature increased by 48.00 % and 114.95 %, respectively. While the fatigue life of the LSP2 specimen increased by 68.91 % even under high-temperature heat cycle. Meanwhile, the locations of crack initiations in the LSPed specimens were significantly shifted to the sub-surface with a smaller fatigue initiation region area compared to the unLSPed specimen. Furthermore, the fracture surface was essentially occupied by equiaxed dimples and small-spaced fatigue striations. LSP-induced considerable depth of severe plastic deformation in the near-surface of Ti6Al4V titanium alloy, generating gradient compressive residual stresses, high-density dislocation structures (dislocation lines, dislocation tangles, dislocation walls, and dislocation cells), and grain refinement (nanograins and nanotwins). These three main factors contributed to inhibiting the sprouting and propagation of cracks during the thermomechanical fatigue tests and played a primary role in improving the thermomechanical fatigue properties.

Effects of Al and Co doping on the structural stability and high temperature cycling performance of LiNi0.5Mn1.5O4 spinel cathode materials

The poor structural stability and capacity retention of the high-voltage spinel-type LiNi0.5Mn1.5O4 (LNMO) limits their further application. Herein, Al and Co were doped in LNMO materials for a more stable structure and capacity. The LNMO, LiNi0.45Al0.05Mn1.5O4 (LNAMO) and LiNi0.45Co0.05Mn1.5O4 (LNCMO) were synthesized by calcination at 900 °C for 8 h, which was called as solid-phase method and applied universally in industry. XRD, FT-IR and CV test results showed the synthesized samples have cation disordering Fd-3m space group structures. Moreover, the incorporation of Al and Co increased the cation disordering of LNMO, thereby increasing the transfer rate of Li+. The SEM results showed that the doped samples performed more regular and ortho-octahedral. The EDS elemental analysis confirmed the uniform distribution of each metal element in the samples. Moreover, the doped samples showed better electrochemical properties than undoped LNMO. The LNAMO and LNCMO samples were discharged with specific capacities of 116.3 mA·h·g−1 and 122.8 mA·h·g−1 at 1 C charge/discharge rate with good capacity retention of 95.8% and 94.8% after 200 cycles at room temperature, respectively. The capacity fading phenomenon of the doped samples at 50 °C and 1 C rate was significantly improved. Further, cations doping also enhanced the rate performance, especially for the LNCMO, the discharge specific capacity of 117.9 mA·h·g−1 can be obtained at a rate of 5 C.

The effect of multiple thermal cycles on Ti-6Al-4V deposits fabricated by wire-arc directed energy deposition: Microstructure evolution, mechanical properties, and corrosion resistance

Thermal cycles have an important effect on the microstructure and properties of the components fabricated by wire-arc directed energy deposition (wire-arc DED). In this study, a Gleeble thermal-mechanical simulator was adopted to create closer-to-reality thermal cycles with the assistance of a numerical simulation model and experimental Ti-6Al-4V deposition. Step-by-step microstructure evolution, including αm, retained β, and GB α, microhardness gradual variation, and the corrosion resistance change before and after the entire thermal cycle were investigated. Therefore, combining phase orientation and high-magnification morphology, transformed and untransformed α that occurred in low- and medium-temperature thermal cycles can be distinguished. After the entire thermal cycle, αm laths coarsened from ∼1 µm to ∼1.2 µm, and the content of retained β phase became more and more. The αm formed around grain boundaries partially disappeared and was occupied by α laths from the inner grain. GB α was more continuously distributed along prior β grain boundaries due to its lower formation temperature during the subsequent thermal cycles that were occurring incomplete α→β transformation. The severe preferential orientation of α phases formed after the deposition and high-temperature thermal cycle was also alleviated through the twice low-temperature thermal cycles. Besides, the microhardness decreased from 318.78 ± 7.5 HV to 285.17 ± 5.3 HV after the high-temperature thermal cycle but eventually increased significantly to 330.5 ± 6.4 HV after experiencing the final low-temperature thermal cycle. The corrosion resistance decreased after the entire thermal cycle, indicating a performance difference between the top and bottom regions of the Ti-6Al-4 V component fabricated by wire-arc DED.

Design and optimization of a radial-axial two-stage coaxial turbine for high-temperature supercritical organic Rankine cycle

High-temperature supercritical organic Rankin cycle (SORC) has great application potential but has not been fully studied. To achieve a large expansion ratio in the high-temperature SORC system, a radial-axial two-stage coaxial turbine with Siloxane MM as working medium is designed and optimized in this paper. This newly-structured turbine, which has simple and compact structure and relatively light weight, is more suitable for large commercial ORC system with medium–high temperature. The whole design flow of this turbine is described, including input, design, optimization, modeling and simulation processes. The turbine performances at both design and off-design conditions are discussed. The total-to-total isentropic efficiency of 86.55% is achieved at the expansion ratio of 15.37 and it can also maintain at a relatively high level within a wide range of expansion ratio and rotating speed. The results prove that the proposed methodology can be applied to designing the radial-axial turbine for SORC system. The turbine configuration and the design method proposed in this study can provide the reference for the research and development of high-power SORC turbine.

Experimental investigation on high-temperature flow boiling heat transfer characteristics of R245fa in a horizontal circular tube

Working fluid is the lifeblood of thermodynamic cycle for its heat transfer behaviour is critical to the design of heat exchangers in a thermodynamic cycle. Though being widely applied in organic Rankine cycles and high-temperature heat pump cycles, R245fa has witnessed little research on its high-temperature flow boiling heat transfer characteristics by far. In the present study, an experimental investigation on the flow boiling heat transfer characteristics of R245fa with a mass flux of 248–460 kg·m−2∙s−1, a heat flux of 5.5–17.3 kW·m−2 and an evaporating temperature of 80–115 °C in an inner diameter of 10 mm horizontal smooth circular tube is conducted. The influences of vapor quality, mass flux and evaporating temperature on the flow boiling heat transfer coefficient are also analyzed. The comparison of experimental heat transfer coefficients and the predicted heat transfer coefficients based on five heat transfer coefficient correlations is conducted and it is discovered that Gungor and Winterton's correlation with a mean deviation of 20.4% gives the best fit to the experimental data. Thus, based on Gungor and Winterton's work, an improved heat transfer coefficient correlation with a mean deviation of 12.8% is developed, in which 95% of the data points fall within the range of ±30%.

Effect of Glutamine on the Growth Performance, Oxidative Stress, and Nrf2/p38 MAPK Expression in the Livers of Heat-Stressed Broilers

The purpose of this work was to study the effects of glutamine (Gln) on the growth performance, oxidative stress, Nrf2, and p38 MAPK pathway in the livers of heat-stressed broilers. In total, 300 broilers were divided into five groups, including a normal temperature (NT, without dietary Gln) group and four cyclic high temperature groups (HT, GHT1, GHT2, and GHT3) fed with 0%, 0.5%, 1.0%, and 1.5% Gln, respectively. High temperature conditions increased (p < 0.05) liver malonaldehyde (MDA) concentration, but decreased (p < 0.05), body weight gain (BWG), feed intake (FI), liver superoxide dismutase (SOD), total antioxidant capacity (T-AOC), glutathione peroxidase (GSH-Px), glutathione S-transferase (GST), and glutathione (GSH) levels in broilers. Nrf2 and p38 MAPK protein and mRNA expression levels were lower (p < 0.05) in the NT group than that in the HT group. However, dietary 1.5% Gln decreased (p < 0.05) liver MDA concentration, but increased (p < 0.05) BWG, FI, liver SOD, T-AOC, GSH-Px, GST, and GSH levels in heat-stressed broilers. Nrf2 and p38 MAPK protein and mRNA expression levels were higher (p < 0.05) in the GHT3 group than that in the HT group. In summary, Gln improved oxidative damage through the activation of Nrf2 and p38 MAPK expression in the livers of heat-stressed broilers.

Effect of shape-stable phase change materials on heat transfer characteristics of ground source heat pump backfill materials

Ground source heat pump (GSHP) systems can utilize shallow geothermal energy to realize low-carbon buildings. The backfill material used in GSHPs directly affects the heat exchange between the ground heat exchanger and soil. Phase change materials (PCMs) can utilize the phase change process to realize both the storage and release of energy. Applying PCMs to GSHP backfill materials may be beneficial towards improving efficiency and saving energy. In this study, decanoic acid and lauric acid were used as composite PCM and graphite powder was added to enhance thermal conductivity. Expanded perlite was used as absorbent and silica sol was used to shape and encapsulate PCM. A shape-stable phase change material (SSPCM) was prepared for GSHP backfilling. Basic characteristics including thermal conductivity of SSPCMs were assessed and analyzed, and one-dimensional heat transfer experiments were carried out. The results showed that at a decanoic acid to lauric acid mass ratio of 7:3, the phase change temperature of binary organic acids is 19.9 °C, and the latent heat of phase change is 124.03 kJ/kg. At a 60% mass content percentage of PCMs, the amount of added graphite powder was 4%, the thermal conductivity of the SSPCM encapsulated by silica sol was 0.1431 W/(m·K), and the mass loss rate of SSPCM was less than 5% after 25 high and low temperature cycles. Materials with the above parameters have suitable comprehensive properties and can be used as preferred phase change backfill material for GSHP. The thermal conductivity of the backfill material is negatively correlated with the mixing ratio of SSPCM, and positively correlated with water content. At an intermittent ratio of 10:14, after 10 h of heating, the temperature range of the phase change backfill sample was 0.38–7.45 °C lower than that of the original soil backfill sample; after 14 h of cooling, the temperature of the phase change backfill sample was closer to the initial temperature. Therefore, adding SSPCM to the backfill material is beneficial to the long-term intermittent operation of GSHPs.

Enhancement of conventional concrete mix designs for sensible thermal energy storage applications

The study examines twenty-nine different concrete mix designs with varying constituents to improve thermal performance for energy storage at elevated temperatures up to 420 °C. Effects of variables including the type and volumetric percentage of coarse and fine aggregates, the type and replacement content of supplemental cementitious materials, water-to-cement ratio, the type and quantity of steel fiber reinforcement, and the inclusion of iron oxide powder are experimentally investigated by measuring the mechanical and thermal properties of each mix, with the overall goal of enhancing thermal performance and mitigating mechanical degradation for thermal energy storage (TES) applications. It is found that the use of siliceous aggregate with a high volumetric percentage produces the highest improvement (23 %–37 % at elevated temperatures) of concrete thermal conductivity, while silica fume replacement of Type I cement provides the highest improvement (4 %–8 % at elevated temperatures) of concrete specific heat. To simulate the operation of a TES system, thermal cycling was conducted to examine the change in concrete properties as a function of repetitive heating (during and after 50 total cycles to 420 °C). Based on the results of testing, concrete mix designs with high aggregate percentages (up to 72 % by volume), siliceous coarse aggregate, silica fume replacement of Type I cement (15 % by weight), and steel fiber reinforcement (up to 2 % by volume) facilitate enhanced TES performance by providing consistent thermal conductivity of ~2.2 W/m·K and specific heat of ~3.2 MJ/m3·K under cyclic high temperature exposure.