Slow Heat Transfer Research Articles

An appraisal of phase change materials: characteristics, categorization, benefits, drawbacks and utilizations. A review

A new era of energy-efficient solutions has arrived thanks to the revolutionary class of substances known as phase change materials (PCMs), which have the extraordinary capacity to store and release thermal energy during phase transitions. The defining characteristics of PCMs, such as their well-defined melting and freezing temperatures, high latent heat of fusion, and variety of material types, including organic and inorganic PCMs, as well as eutectic combinations, are thoroughly examined in this thoroughly analysis. PCMs are divided into groups according to the physical shape they take and the temperature range in which they are used. These groups include low-temperature variations, high-temperature variants, encapsulated forms, and nanocomposite forms. PCMs provide a wide range of advantages, including effective thermal energy storage, cost-effective energy use, consistent temperature maintenance and small light designs. However, they do have some restrictions, such as compatibility issues and slow heat transfer rates. However, PCMs are used in a wide range of industries, from electronics and renewable energy to building and construction, revolutionizing temperature control, thermal management and energy efficiency. The future of PCM technology offers promise, with ongoing improvements aiming at improving heat transmission, integrating with renewable energy sources and enhancing encapsulation processes. This technology's vital position in a sustainable and efficient energy future is apparent, as it continues to transform our world in the hunt for responsible and green energy solutions.

The role of phase saturation in depressurization and CO2 storage in natural gas hydrate reservoirs: Insights from a pilot-scale study

The natural gas recovery and CO2 storage in natural gas hydrate (NGH) reservoirs is a promising integrated strategy for implementing clean energy production and CCUS. However, the fundamental characteristics (phase saturation) of NGH reservoirs on the feasibility and efficiency of the above-integrated processes remain unclear. In this study, we synthesized methane hydrate-bearing sediments (MHBS) with varying hydrate saturation (SH = 18–66 %) and aqueous saturation (SA = 7–92 %) at marine NGH conditions (T = 283.8 K, P = 11.0 MPa). The effect of phase saturation on depressurization-induced hydrate dissociation, and subsequent CO2 storage and hydrate restoration by CO2/N2 injection was investigated. The water-saturated MHBS with high SA can promote faster MH dissociation during the depressurization stage and prevent secondary hydrate formation, but a slower MH dissociation during the later stage due to slow heat transfer. A relatively high SH (> 63 %) results in sustained low temperatures due to insufficient heat supply slowing down MH dissociation. The CH4/CO2/N2 mixed hydrates (Mix-H) formation and CO2 storage in depleted MHBS after CO2/N2 injection initially increase and then decrease with post-mining SA (peak at SA = 54.5 %). The rate of Mix-H formation decreases as the post-mining SA increases. Higher pre-mining SH and CO2/N2 injection rate facilitates rapid gas phase mixing within the sediments, thereby improving the kinetics and homogeneity of Mix-H formation and hydrate-based CO2 storage efficiency. MHBS with relatively low SH and SA exhibit enhanced safety for the integrated process, offering reduced depressurization-induced sediment subsidence and increased hydrate restoration ratios (∼85 %) by Mix-H formation.

Transient characterization of the mode switching process in the reversible solid oxide cell stack

Reversible solid oxide cells (RSOCs) is a promising energy storage and conversion technology requiring frequent switching between fuel cell mode and electrolysis mode in practice. Unfortunately, it may cause a large current density undershoot that has irreversible damage to the RSOC. Therefore, understanding the transient behaviors of the RSOC is critical for its efficient and durable operation. A three-dimensional transient model of a 30-cell SOC stack with an external manifold is established. From SOFC to SOEC, the mass transfer lag and slow heat transfer cause the current density of the stack requiring 2500 s to reach a steady state. And the bottom inlet way and the external manifold structure cause differences in the current density, molar fraction and temperature variation with time between different cells. The upper cells show a slower mass transfer rate and undergo greater temperature changes. The inconsistent temperature change gradients and temperature change rates among different cells will inevitably produce changing thermal stresses and bring irreversible mechanical losses to the stack. Similar transient characteristics are observed when switching from SOEC to SOFC. However, it takes longer time (3200 s) to achieve the final steady state.

Organic and inorganic sublattice coupling in two-dimensional lead halide perovskites

Two-dimensional layered organic-inorganic halide perovskites have successfully spread to diverse optoelectronic applications. Nevertheless, there remain gaps in our understanding of the interactions between organic and inorganic sublattices that form the foundation of their remarkable properties. Here, we examine these interactions using pump-probe spectroscopy and ab initio molecular dynamics simulations. Unlike off-resonant pumping, resonant excitation of the organic sublattice alters both the electronic and lattice degrees of freedom within the inorganic sublattice, indicating the existence of electronic coupling. Theoretical simulations verify that the reduced bandgap is likely due to the enhanced distortion index of the inorganic octahedra. Further evidence of the mechanical coupling between these two sublattices is revealed through the slow heat transfer process, where the resultant lattice tensile strain launches coherent longitudinal acoustic phonons. Our findings explicate the intimate electronic and mechanical couplings between the organic and inorganic sublattices, crucial for tailoring the optoelectronic properties of two-dimensional halide perovskites.

Biomimetic phase change capsules with conch shell structures for improving thermal energy storage efficiency

Latent heat storage system utilizing a packed-bed setup with encapsulated phase change materials (EPCMs) can address the issues of mismatched energy supply and demand, in addition to preventing the leakage concerns associated with unencapsulated PCMs, which has a broad application prospect. Drawing on the principles of biomimicry, this work proposes bionic-conch and bionic-mitochondrial encapsulation models for PCMs and analyzes the heat transfer performance with different structures. Significantly, the thermal energy storage efficiency of the bionic-conch structure surpassed that of the bionic-mitochondrial and straight-fin structures by 43.11 % and 14.62 %, respectively. The enhanced thermal energy storage efficiency of the bionic-conch is attributed to the spiral fins within, which considerably amplify the heat exchange surface area. Furthermore, in order to obtain the bionic-conch capsule structure with optimal thermal properties, the present work investigates the influence of fins on the melting characteristics of PCMs. The results show that increasing fins empower the thermal energy storage efficiency by 402.89 % and thermal response rate by 616.93 %, respectively. However, there is an issue of slow melting process in the annular region and incorporating an array of a slightly off-center heat-conducting ring (SOHR) is considered to enhance localized heat transfer. The suggest that it can alleviate the problem of slow heat transfer rate in the local annular region and increase the average internal temperature. Furthermore, in order to compensate for the loss of thermal energy capacity of the fin structure, this work also investigated the effect of different wall thicknesses on the thermal performance of the phase change capsule. Interestingly, decreasing the wall thickness significantly increases the heat capacity. In concrete terms, the thermal energy capacity of 0.5 mm wall thickness is increased by 22.87 % compared to 2 mm. This study may provide new enlightenment for the optimization of green energy.

Magnetic MOFs with flexibility for efficient magnetic-induced swing adsorption

Magnetic-induced swing adsorption is effective because of its easy operation, fast heat generation, and short distance of heat transfer. The energy efficiency of this adsorption method depends on the performance of magnetic adsorbents in alternative magnetic fields. In this work, we fabricated flexible MOF-based magnetic adsorbents and applied them in the magnetic-induced swing adsorption. When magnetic field is turned off, the adsorbents show open pores for C3H8 uptake at room temperature. When magnetic field is turned on, C3H8 is released because magnetic-induced heating drives the adsorbents loss their activity through conformational change of pores derived from aromatic rings tilting and rotation. The flexibility of the adsorbents decreases energy consumption to achieve desorption, which promote the energy efficiency of magnetic-induced swing adsorption. The optimal working capacity can reach 16.8 cm3 g−1, which is superior to various typical adsorbents such as F30/470 (9.32 cm3 g−1), MIL-91(Al) (3.36 cm3 g−1), and SSZ-13 zeolite (4.48 cm3 g−1). This work may open up a new avenue to address the issues caused by high energy consumption and slow heat transfer during desorption.

Analysis of Fluid-Solid coupling heat transfer characteristics of hydrogen circulating pump

The hydrogen circulating pump (HCP) is in a high-temperature state during operation due to the heat generated by the pressure change of the transported gas. The current research on the flow characteristics of the HCP has not taken into account the influence of internal heat transfer factors, resulting in significant errors between the research results and actual test results. In this article, we define the contact modes of the fluid–solid interfaces based on heat transfer theory, propose a step-by-step calculation method based on different time scales, and complete the simulation of the fluid–solid coupling model. Compared with the test flow rate of 15.44 g/s, the fluid–solid coupling model considering heat transfer factors calculated a flow rate of 15.15 g/s with an error of only 1.85 %, while the single fluid model without considering heat transfer effects calculated a value of 14.67 g/s with an error of 5.0 %. Analyzing the internal mechanism of heat transfer leading to an increase in simulated flow rate, the external surface heat dissipation and internal heat conduction on one hand lead to a decrease in gap leakage flow rate, thereby reducing leakage volume, and on the other hand, increase the density of gas transported in the rotor cavity area, thereby improving the conveying capacity. The calculation method proposed in this article solves the problem of the large time difference between instantaneous fluid flow and slow heat transfer, shortens the calculation time, has higher calculation accuracy, and plays an important role in the precise design process of HCPs.

Three-dimensional model construction of a frost-free air source heat pump evaporator and its performance analysis

This study aims to design an efficient frost-free air source heat pump evaporator for use in cold regions. The challenges of low performance, slow heat transfer, and poor energy supply efficiency associated with air source heat pump evaporators are addressed. This study analyzed the benefits of combining solar energy and air source heat pumps for heat supply. A three-dimensional model of the evaporator was constructed in ANSYS software, and optimal operating parameters were determined through variable experiments. The experimental results showed that when other factors were controlled constant, the glass cover size increased from 1.7 m to 2.9 m, and the maximum value of heat absorption of the evaporator increased from 2155 W to 2418 W and then decreased to 2340 W. When the wind speed and wind pressure increased from 150Pa to 300Pa, the maximum value of heat absorption of the evaporator increased from 1620 W to 2110 W. In summary, the maximum value of heat absorption of the evaporator was increased from 1,620 W to 2,110 W when the size of the glass cover was 2.5 m*2.5 m*2.5 m. When the wind speed and pressure were 300Pa, the heat absorption efficiency of the evaporator was the highest. This finding not only provides a new idea for the performance optimization of air source heat pump evaporator, but also opens up new possibilities for energy efficiency improvement in cold regions, reflecting the innovative and application value of this study.

Electrocaloric cooling system utilizing latent heat transfer for high power density

Electrocalorics (EC) is potentially more efficient than refrigeration and heat pumps based on compressors and does not need detrimental fluids. Current EC-prototypes use solid-state contact or forced convection with liquids to transfer the heat generated from the EC-material, which inhibits high cycle frequencies and thus limits power density. Here we present a heatpipe system solution, where the heat transfer is realized through condensation and evaporation of ethanol as a heat transfer fluid. Our prototype with lead scandium tantalate (PST) EC-material working at 5 Hz shows a specific cooling power of 1.5 W g−1. This is one order of magnitude more than previously reported for ceramic EC-prototypes. Overcoming the limits of slow heat transfer is essential to reach high specific cooling powers enabling a future commercial success of the technology.

Thermionic Vacuum Discharges for Thin Film Depositions

The thermionic vacuum discharge method is very effective in that the films obtained using this technology are characterised by a very high degree of adhesion, density and purity because the deposition technique is carried out in high, very high or, if possible, in ultra-very high vacuum conditions with no gas present. When the substrate is placed in vacuum, no heat transfer particles are present, the substrate being heated only by the ion incident on the surface. This advantage recommends the TVD method for deposits on plastics or other thermally sensitive materials. Additionally, this slow heat transfer reduces energy loss, making the deposition method industrially competitive. The paper aims to present theoretical aspects of this type of discharge, compared to typical or more popular plasmas but also to present the achievements of this method and its utility in the thin films production, layers that have specific imposed properties. The practical depositions and applications presented are in the nuclear fusion-related material science and also for obtaining materials for granular structures, used as magneto-resistive coatings.

Experimental study of performance enhancement strategies for hydrogen production-purification systems in packed-bed reactors under power fluctuations

Methanol reforming for hydrogen production and purification is a key technology in current fuel cell hydrogen source systems. The packed-bed reactor has a simple structure and is widely used, yet it has disadvantages such as slow dynamic response and limited heat and mass transfer. In this paper, an experimental approach is used to simulate the reaction characteristics of the packed bed during power fluctuation and start-up phase. It is found that the combined catalysts are the best in terms of both economical and purification effects, especially the precious metal combined copper-based catalysts. Meanwhile, the methanol conversion rate can seriously affect CO-PROX during the fuel cell start-up phase, and the use of combined catalysts can mitigate the impact of power fluctuation on the system and reduce the CO concentration. The combination of catalysts in such a way that the noble metal should be ensured in the front section, the optimal reaction temperature of copper-based combination noble metal-based catalysts is explored, with reforming temperature of 200 °C and CO-PROX temperature of 180–200 °C, and optimal O2/CO ratio between 0.5 and 1.5, when methanol conversion reaches more than 70%, CO concentration as low as 0.42% and hydrogen concentration higher than 40%.

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

Digitizing protocols into single reactors for the one-pot synthesis of nanomaterials

Seasonal Environmental Controls on Soil CO2 Dynamics at a High CO2 Flux Sites (Piton de la Fournaise and Mayotte Volcanoes)

AbstractEnvironmental parameters drive seasonal soil CO2 efflux toward the atmosphere. However, their influence is not fully understood in contexts of high CO2 fluxes where CO2 accumulates in the subsurface. A prime example are volcanoes subject to continuous CO2 diffuse degassing rising from deep magmatic reservoirs, through the subsurface and up to the atmosphere. For many of these volcanoes where soil CO2 is monitored, a seasonal influence of the atmosphere and water table is observed but not characterized. Here, we compare variations of air temperature, atmospheric pressure, rainfall and water table level with near‐surface soil CO2 concentration by performing a time‐lagged detrended cross‐correlation analysis on years‐long time series from the volcanoes of Piton de la Fournaise and Mayotte. At Piton de la Fournaise, soil CO2 variations correlate best with air temperature variations (0.81) and water table variations (0.74). In Mayotte, soil CO2 variations correlate best with atmospheric pressure variations (−0.95). We propose that at Piton de la Fournaise, the thick vadose zone and high permeability favor CO2 transfer by thermal convection. Additionally, energy transfer is decoupled from mass transfer. Slow heat transfer from the atmosphere down to the accumulated CO2 layers in the subsurface results in a delayed influence of air temperature and of the water table level on the thermal gradient between the subsurface and the atmosphere, and consequently on the efficiency of the CO2 transfer. In Mayotte, we propose that the thin vadose zone and the presence of a network of large fractures favor CO2 transfer by barometric pumping.

Development and Experimental Validation of a Dispersity Model for In Silico RAFT Polymerization.

The exploitation of computational techniques to predict the outcome of chemical reactions is becoming commonplace, enabling a reduction in the number of physical experiments required to optimize a reaction. Here, we adapt and combine models for polymerization kinetics and molar mass dispersity as a function of conversion for reversible addition fragmentation chain transfer (RAFT) solution polymerization, including the introduction of a novel expression accounting for termination. A flow reactor operating under isothermal conditions was used to experimentally validate the models for the RAFT polymerization of dimethyl acrylamide with an additional term to accommodate the effect of residence time distribution. Further validation is conducted in a batch reactor, where a previously recorded in situ temperature monitoring provides the ability to model the system under more representative batch conditions, accounting for slow heat transfer and the observed exotherm. The model also shows agreement with several literature examples of the RAFT polymerization of acrylamide and acrylate monomers in batch reactors. In principle, the model not only provides a tool for polymer chemists to estimate ideal conditions for a polymerization, but it can also automatically define the initial parameter space for exploration by computationally controlled reactor platforms provided a reliable estimation of rate constants is available. The model is compiled into an easily accessible application to enable simulation of RAFT polymerization of several monomers.

Optimization of olive leaves' thin layer, intermittent near-infrared-drying

The study aimed to examine the effect of intermittent near-infrared radiation on the quality of olive leaves (OLs) -spread as a thin layer- upon drying using response surface methodology. Temperature and time were selected as independent variables and color, total phenol, flavonoid content, antioxidant activity (DPPH●, CUPRAC assays), the concentration of oleuropein, verbascoside, luteolin-7-O-glucoside and triterpenic acids (oleanolic and maslinic) were used as dependent variables. A statistical model was found for greenness (a*), total phenol content, and antioxidant activity (CUPRAC assay), which were favored upon heating for short time at high temperature. The optimal conditions (116 °C, 3 min) were compared with freeze-drying and oven-drying (120 °C, for 3 and, 12 min). Infrared-drying was faster and superior regarding OLs greenness and oleuropein level compared to oven-drying. Freeze-drying was generally inferior and of equal performance to the two thermal techniques only regarding total flavonoid and triterpenic acid levels. Industrial relevanceThe interest in the exploitation of OLs as a source of bioactive compounds for Pharmaceutical, Food, and Feed applications is continuously growing. Drying is a key step to producing material of high quality. Since conventional approaches can be long-lasting or energy-consuming due to the slow heat transfer there is a need to seek out innovative techniques that can be fast, therefore, economical, and eco-friendly, but also efficient. Thus, in the present work, the efficiency of intermittent near-infrared radiation in producing dried OLs of high quality is discussed and optimum conditions are provided including evidence for its superiority compared to freeze- and oven-drying.

Adsorbent Coatings for Adsorption Heat Transformation: From Synthesis to Application

In recent years, growing energy demands and environmental pollution caused by the extensive use of fossil fuels have inspired considerable research interest in adsorptive heat transformation (AHT). This technology offers effective utilization of low-grade solar or waste thermal energy for cooling and heating with low environmental impact. Increasing the AHT power is a keystone for further development and dissemination of this emerging technology. The AHT power is mainly determined by ad/desorption dynamics, which is significantly hindered by slow heat transfer between the adsorbent and heat exchanger. Shaping the adsorbent bed as a coating on the heat exchanger surface is considered an effective route to enhance heat transfer and increase the AHT power. In this review, the technology of adsorbent coating for AHT is comprehensively surveyed, including coating synthesis, adsorption dynamics, and use in real AHT devices. The advantages of the coated bed configuration are considered, and its challenges are outlined. Finally, recommendations for better organization of the coating’s structure for rational control of the relative contributions of heat and mass transfer are considered.

A simplified phase change model and performance optimization of plate latent heat storage unit with graded porous

In this paper, a simplified phase change model with graded porous is proposed for the problems of slow heat transfer rate and low computational efficiency in the field of latent heat storage (LHS) technology. Moreover, the proposed simplified model is further combined with the genetic algorithm to optimize the structure parameters of graded porous for the optimal strengthening effect. Considering the solidification process in the plate phase change heat storage device, the prediction accuracy and prediction efficiency of the simplified model based on the quasi-stationary approximation method are first verified. The results show that the simplified model can quickly and accurately predict the thermal performance of the LHS unit. When the effective Stefan number is less than 0.1645, the solidification time deviation is about 5% and the temperature deviation is less than 0.12%. Then, the genetic algorithm is introduced to optimize the distribution of graded porous under the different metal foam materials, metal foam consumptions, and container structure parameters. The optimization results indicate that the thermal performance of the LHS unit under different parameters gradually improves with the increase of the graded layer number. But the strengthening effect is close to the best when the graded layer number is only 4. Compared with the uniform porous structure, the solidification time can be shortened by 7.62%, while the best strengthening effect is 8.25%. Therefore, the four-graded layer structure is preferred to achieve a balance between thermal performance and structure complexity. This work can provide a simple and fast optimization design method for the structural layout of the LHS unit strengthened by graded porous.

Techno-economic analysis of Advanced Geothermal Systems (AGS)

Advanced Geothermal Systems (AGS) generate electric power through a closed-loop circuit, after a working fluid extracts thermal energy from rocks at great depths via conductive heat transfer from the geologic formation to the working fluid through an impermeable wellbore wall. The slow conductive heat transfer rate present in AGS, compared to heat advection, makes AGS uneconomical to this date. To investigate what would be required to render AGS economical, we numerically model an example AGS using the genGEO simulator to obtain its electric power generation and its specific capital cost. Our numerical results show that using CO2 as the working fluid benefits AGS performance. Additionally, we find that there exists a working fluid mass flowrate, a lateral well length, and a wellbore diameter which minimize AGS costs. However, our results also show that AGS remain uneconomical with current, standard drilling technologies. Therefore, significant advancements in drilling technologies, that have the potential to reduce drilling costs by over 50%, are required to enable cost-competitive AGS implementations. Despite these challenges, the economic viability and societal acceptance potential of AGS are significantly raised when considering that negative externalities and their costs, so common for most other power plants, are practically non-existent with AGS.

Techno-Economic Performance of Closed-Loop Geothermal Systems for Heat Production and Electricity Generation

• Closed-loop geothermal systems evaluated for heat and electricity production • Slow heat transfer in rock and limited heat transfer area pose challenges • Targeting high temperatures, existing wells, and multi-laterals beneficial • Relatively high levelized costs for greenfields, unless drilling at low cost Closed-loop geothermal systems, recently referred to as advanced geothermal systems (AGS), have received renewed interest for geothermal heat and power production. These systems consist of a co-axial, U-loop, or other configuration in which the heat transfer or working fluid does not permeate the reservoir but remains within a closed-loop subsurface heat exchanger. Advocates indicate its potential for developing geothermal energy anywhere, independent of site-specific geologic uncertainties, and with limited risk of induced seismicity. Here, we present a technical and economic analysis of closed-loop geothermal systems using a Slender-Body Theory (SBT) model, COMSOL Multiphysics simulator, and the GEOPHIRES analysis tool. We consider a number of different scenarios and evaluate the influence of variations in reservoir temperature (100 to 500℃), well termination depth (2 to 4 km), mass flow rate (10 to 40 kg/s), injection temperature (10 to 40℃), fluid type (liquid water vs. supercritical carbon dioxide), design configuration (co-axial vs. U-loop), and degree of reservoir convection (natural, forced or conduction-only). The resulting average heat production rates range from about 2 to 15 GWh per year for cases considering a co-axial design and from 9 to 67 GWh per year for cases with a U-loop design. Assuming generous economic and operating conditions, estimates of levelized cost of heat range from ∼$20 – $110 per MWh (∼$6 – 32/MMBtu) and ∼$10 – $70 per MWh (∼$3 – $20/MMBtu) for greenfield co-axial and U-loop cases, respectively. In the scenarios in which electricity generation is considered, annual electricity production ranged between 0.12 and 7.5 GWh per year at a levelized cost of electricity from roughly $83 to $2,200 per MWh. In all scenarios, the results exhibit a large rapid drop in production temperature after initiation of operations that levels off to a steady value significantly below the initial reservoir temperature. Operating at lower flow rate increases the production temperature but also lowers the total heat production. The consistently low production temperatures hinder efficient electricity generation in most cases considered. Natural or forced convection can increase thermal output but requires sufficiently high reservoir permeability or formation fluid flow. As expected, overall system costs are heavily dependent on drilling costs; hence, repurposing existing wells could significantly lower capital and levelized costs. In comparison with other types of geothermal systems, our results for closed-loop geothermal systems predict long-term production temperatures considerably below the initial reservoir temperature, and relatively high levelized costs for greenfield closed-loop geothermal systems, particularly for electricity production, unless significant reductions in drilling costs are obtained.

An active solar water wall for passive solar greenhouse heating

A high thermal capacity north wall is essential for nighttime heating of Chinese solar greenhouses. However, thermal storage ability of the north wall using regular building materials is limited because of their slow heat conduction restricting heat transfer from wall surface to the depth. This study added an active solar water wall constructed of hollow polycarbonate sheets to the north wall, supplemented by an underground water storage tank, to increase nighttime temperature. To evaluate water wall's thermal effect, a field test was conducted, as well as a theoretical verification. Its performance was evaluated in terms of solar heat collection and release capability, nighttime temperature increase and thermal performance in adverse weather. During the 22 coldest days of winter in Beijing, daily averages of collected heat and heat release rate on the basis of floor area were 0.83 MJ m−2 and 22.6 W m−2, respectively. Average heat-collecting efficiency reached 85.8%, and 80.4% of the collected heat was released for greenhouse heating. Compared to the scenario without the water wall, minimum nighttime air temperature increased by 3.3 °C on average. In particular, nighttime temperature maintained above 6.9 °C that was 4.1 °C higher than that in the control greenhouse during three consecutive overcast days. Retrofitting the water wall into Chinese solar greenhouses can make warm-season crop production feasible throughout winter by eliminating supplemental heating. Besides, the water wall-based system can be a self-contained solar heating system, also suitable for traditional greenhouses. Thermal analysis and economic evaluation show considerable practical application prospects.