Thermal and mechanical properties of bio-based PCMs encapsulated with nanofibrous structure

An environmentally friendly phase change material (PCM) was successfully prepared by encapsulating natural soy wax into polyurethane (PU) nanofibers using coaxial electrospinning technique. The morphology and the structure of the wax/PU composites were characterized. Thermal behaviors as well as mechanical properties of the composites were also investigated. The results indicated that coaxial electrospinning produced uniform fiber morphology with a core–shell structure and a homogeneous wax distribution throughout the core of the fibers. The soy wax was successfully encapsulated into PU fibers without being miscible with PU fibers. Thermal analysis results show that the enthalpy increases as the wax content increases. The fibrous structures exhibited balanced thermal storage and released properties for thermo-regulating function. The thermal properties were unaltered after 100 heating–cooling cycles, demonstrating that the composite fibers had good thermal stability and reliability. Tensile tests also indicate that the presence of wax enhanced the modulus and lowered the tensile strain.     

A numerical investigation of heat transfer in phase change materials (PCMs) embedded in porous metals

The effects of metal foams on heat transfer enhancement in Phase Change Materials (PCMs) are investigated. The numerical investigation is based on the two-equation non-equilibrium heat transfer model, in which the coupled heat conduction and natural convection are considered at phase transition and liquid zones. The numerical results are validated by experimental data. The main findings of the investigation are that heat conduction rate is increased significantly by using metal foams, due to their high thermal conductivities, and that natural convection is suppressed owing to the large flow resistance in metal foams. In spite of this suppression caused by metal foams, the overall heat transfer performance is improved when metal foams are embedded into PCM; this implies that the enhancement of heat conduction offsets or exceeds the natural convection loss. The results indicate that for different metal foam samples, heat transfer rate can be further increased by using metal foams with smaller porosities and bigger pore densities.     

Performance enhancement of a solar dynamic LHTS module having both fins and multiple PCMs

Latent heat thermal storage units span a wide and varied range of applications in the domestic, industrial and space based activities. Numerical investigations on the performance enhancement of a solar dynamic latent heat thermal storage (LHTS) unit employing multiple phase change materials (PCM) and fins are made. The LHTS unit has been studied for the charging mode alone. Enthalpy based formulation of the energy equations governing the behaviour of the LHTS system has been made and compared with the response of a single PCM unit. The governing conjugate equations have been solved employing finite difference techniques. The results show an appreciable enhancement in the rate of melting of PCM and nearly uniform exit temperature of heat transfer fluid (HTF) in the multiple PCM LHTS unit.     

Extending thermal response test assessments with inverse numerical modeling of temperature profiles measured in ground heat exchangers

Thermal response tests conducted to assess the subsurface thermal conductivity for the design of geothermal heat pumps are most commonly limited to a single test per borefield, although the subsurface properties can spatially vary. The test radius of influence is additionally restricted to 1–2 m, even though the thermal conductivity assessment is used to design the complete borefield of a system covering at least tens of squared meters. This work objective was therefore to develop a method to extend the subsurface thermal conductivity assessment obtained from a thermal response test to another ground heat exchanger located on the same site by analyzing temperature profiles in equilibrium with the subsurface. The measured temperature profiles are reproduced with inverse numerical simulations of conductive heat transfer to assess the site basal heat flow, at the location of the thermal response test, and evaluate the subsurface thermal conductivity, beyond the thermal response test. Paleoclimatic temperature changes and topography at surface were considered in the model that was validated by comparing the thermal conductivity estimate obtained from the optimization process to that of a conventional thermal response test.     

Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications

The influence of expanded graphite (EG) and carbon fiber (CF) as heat diffusion promoters on thermal conductivity improvement of stearic acid (SA), as a phase change material (PCM), was evaluated. EG and CF in different mass fractions (2%, 4%, 7%, and 10%) were added to SA, and thermal conductivities of SA/EG and SA/CF composites were measured by using hot-wire method. An almost linear relationship between mass fractions of EG and CF additives, and thermal conductivity of SA was found. Thermal conductivity of SA (0.30 W/mK) increased by 266.6% (206.6%) by adding 10% mass fraction EG (CF). The improvement in thermal conductivity of SA was also experimentally tested by comparing melting time of the pure SA with that of SA/EG and SA/CF composites. The results indicated that the melting times of composite PCMs were reduced significantly with respect to that of pure SA. Furthermore, the latent heat capacities of the SA/EG and SA/CF (90/10 wt%) composite PCMs were determined by differential scanning calorimetry (DSC) technique and compared with that of pure SA. On the basis of all results, it was concluded that the use of EG and CF can be considered an effective method to improve thermal conductivity of SA without reducing much its latent heat storage capacity.     

Study on the efficiency of effective thermal conductivities on melting characteristics of latent heat storage capsules

Improvement of the thermal conductivity of a phase change materials (PCM) is one effective technique to reduce phase change time in latent heat storage technology. Thermal conductivity is improved by saturating porous metals with phase change materials. The influence of effective thermal conductivity on melting time is studied by analyzing melting characteristics of a heat storage circular capsule in which porous metal saturated with PCM is inserted. Numerical and approximate analyses were made under conditions where there are uniform or non-uniform heat transfer coefficients around the cylindrical surface. Four PCMs (H₂O, octadecane, Li₂CO₃, NaCl) and three metals (copper, aluminum and carbon steel) were selected as specific materials. Porosities of the metals were restricted to be larger than 0.9 in order to keep high capacity of latent heat storage. Results show that considerable reduction in melting time was obtained, especially for low conductivity PCMs and for high heat transfer coefficient. Melting time obtained by approximate analysis agrees well with numerical analysis. A trial estimation of optimum porosity is made balancing the desirable conditions of high latent heat capacity and reduction of melting time. Optimum porosity decreases with increase in heat transfer coefficient.     

Experimental and numerical analysis of dynamic metal hydride hydrogen storage systems

This paper examines the dynamics of metal hydride storage systems by experimentation and numerical modelling. A specially designed and instrumented metal hydride tank is used to gather data for a cyclic external hydrogen load. Thermocouples provide temperature measurements at various radial and axial locations in the metal hydride bed. This data is used to validate a two-dimensional mathematical model previously developed by the authors. The model is then used to perform a parametric study on some of the key variables describing metal hydride systems. These variables are the equilibrium pressure, where the tails and concentration dependence are investigated, and the effective thermal conductivity of the metal hydride bed, where the pressure and concentration dependence are analyzed. Including tails on the equilibrium pressure curves was found to be important particularly for the accuracy of the initial cycles. Introducing a concentration dependence for the plateau region of the equilibrium pressure curve was found to be important for both pressure and temperature results. Effective thermal conductivity was found to be important, and the inclusion of pressure and concentration dependence produced more precise modelling results.     

Dimensionless temperature at the wall of an infinite long cylindrical source with a constant heat flow rate

The differential diffusivity equation for an infinitely long cylindrical source with a constant heat flow rate in a homogeneous and isotropic medium has a solution in complex integral form. This integral cannot be expressed in terms of known functions. At present the temperature at the wall of the cylindrical source is determined by means of numerical integration techniques. However, in many cases the heat flow rate varies with time. In these cases the principle of superposition should be used, thus requiring an analytical solution for the constant heat flow rate case. In this paper we present a semi- analytical equation that can be used to approximate the transient dimensionless wall temperature. The accuracy of the equation proposed is also given.     

Thermal performance of a meso-scale liquid-fuel combustor

Combustion in small scale devices poses significant challenges due to the quenching of reactions from wall heat losses as well as the significantly reduced time available for mixing and combustion. In the case of liquid fuels there are additional challenges related to atomization, vaporization and mixing with the oxidant in the very short time-scale liquid-fuel combustor. The liquid fuel employed here is methanol with air as the oxidizer. The combustor was designed based on the heat recirculating concept wherein the incoming reactants are preheated by the combustion products through heat exchange occurring via combustor walls. The combustor was fabricated from Zirconium phosphate, a ceramic with very low thermal conductivity (0.8 W m⁻¹ K⁻¹). The combustor had rectangular shaped double spiral geometry with combustion chamber in the center of the spiral formed by inlet and exhaust channels. Methanol and air were introduced immediately upstream at inlet of the combustor. The preheated walls of the inlet channel also act as a pre-vaporizer for liquid fuel which vaporizes the liquid fuel and then mixes with air prior to the fuel–air mixture reaching the combustion chamber. Rapid pre-vaporization of the liquid fuel by the hot narrow channel walls eliminated the necessity for a fuel atomizer. Self-sustained combustion of methanol–air was achieved in a chamber volume as small as 32.6 mm³. The results showed stable combustion under fuel-rich conditions. High reactant preheat temperatures (675 K–825 K) were obtained; however, the product temperatures measured at the exhaust were on the lower side (475 K–615 K). The estimated combustor heat load was in the range 50 W–280 W and maximum power density of about 8.5 GW/m³. This is very high when compared to macro-scale combustors. Overall energy efficiency of the combustor was estimated to be in the range of 12–20%. This suggests further scope of improvements in fuel–air mixing and mixture preparation.     

Thermal analysis of a direct-gain room with shape-stabilized PCM plates

The thermal performance of a south-facing direct-gain room with shape-stabilized phase change material (SSPCM) plates has been analysed using an enthalpy model. Effects of the following factors on room air temperature are investigated: the thermophysical properties of the SSPCM (melting temperature, heat of fusion and thermal conductivity), inner surface convective heat transfer coefficient, location and thickness of the SSPCM plate, wall structure (external thermal insulation and wallboard material) etc. The results show that: (1) for the present conditions, the optimal melting temperature is about 20 °C and the heat of fusion should not be less than 90 kJ kg⁻¹; (2) it is the inner surface convection, rather than the internal conduction resistance of SSPCM, that limits the latent thermal storage; (3) the effect of PCM plates located at the inner surface of interior wall is superior to that of exterior wall (the south wall); (4) external thermal insulation of the exterior wall obviously influences the operating effect and period of the SSPCM plates and the indoor temperature in winter; (5) the SSPCM plates create a heavyweight response to lightweight constructions with an increase of the minimum room temperature at night by up to 3 °C for the case studied; (6) the SSPCM plates really absorb and store the solar energy during the daytime and discharge it later and improve the indoor thermal comfort degree at nighttime.     

Thermal cycle testing of calcium chloride hexahydrate as a possible PCM for latent heat storage

In order to study the changes in latent heat of fusion and melting temperature of calcium chloride hexahydrate (CaCl₂·6H₂O) inorganic salt as a latent heat storage material, a thousand accelerated thermal cycle tests have been conducted. The effect of thermal cycling and the reliability in terms of the changing of the melting temperature using a differential scanning calorimeter (DSC) is determined. It has been noticed that the CaCl₂·6H₂O melts between a stable range of temperature and has shown small variations in the latent heat of fusion during the thermal cycling process. Thus, it can be a promising phase change material (PCM) for heating and cooling applications for various building/storage systems.     

Thermal management of electronic components with thermal adaptation composite material

Thermal adaptation composite material is a kind of composite material with required thermal conductivity or coefficient of thermal expansion through the selection and design of its components. A kind of thermal adaptation composite material that has excellent thermal conductivity and heat storage capacity is prepared by absorbing paraffin into expanded graphite. An electronic cooling experimental system based on the thermal adaptation composite material is built. The temperature variations of the simulative chip are respectively measured in this system and the traditional cooling system to investigate the effect of the thermal adaptation composite material on electronic cooling. At the same time, the impacts of composite material dosage and combining active cooling manner on the performance of electronic cooling are also studied. The experimental results show that the apparent heat transfer coefficients of the electronic cooling experimental system are 1.25–1.30 times higher than those of the traditional cooling system. It also can be found that the dosage of composite material has positive impact on the performance of electronic cooling. By combining active cooling manner, it can compensate the deficiency of cooling capacity in phase change thermal control.     

Latent heat energy storage characteristics of building composites of bentonite clay and pumice sand with different organic PCMs

In the present work, six new kinds of building composite PCMs (BCPCMs), PS/octadecane, BC/octadecane, PS/CA–MA, BC/CA–MA, PS/PEG1000, and BC/PEG1000 composites, were prepared by using vacuum impregnation method. The maximum percent of PCM in the composites was assigned to be 12, 13, 18, 23, 30, and 42 wt%, respectively. The form‐stable BCPCMs were characterized using SEM, FT‐IR, DSC, and TG analysis techniques. The characterization results showed the existence of homogenous dispersion of the PCM into the PBM matrixes. The DSC measurements indicated that the melting temperatures of the form‐stable BCPCMs are in the range of 20–33°C while they have latent heats of melting in the range of about 28–55 J/g. These results make them promising BCPCMs for low temperature‐passive TES applications in buildings. Thermal cycling test indicated that the prepared BCPCMs have good thermal reliability and chemical stability. TG analysis proved that the prepared BCPCMs have good thermal durability. In addition, the thermal conductivity of BCPCMs was enhanced considerably by addition of expanded graphite (EG). The improvement in thermal conductivity of the BCPCMs caused appreciably reduction in their melting times. Copyright © 2014 John Wiley & Sons, Ltd.     

Combustion and heat transfer at meso-scale with thermal recuperation

Results are presented from successfully designed and fabricated meso-scale ceramic combustors that incorporate internal thermal energy recirculation. The combustor provided sustained operation using propane and air as the reactants. Flames could be obtained well below the normal quenching distance. The development required examination of several different combustor designs and materials. Flammability limits of these combustors have been determined experimentally. Experimental investigations have been performed on the effects of flame holder geometry, material conductivity, equivalence ratio, and inlet Reynolds number on the combustor performance. Measurement of the reactant preheating and product exhaust temperatures was performed using K-type thermocouples which were installed with minimal intrusion to the flow. The reactant preheating temperatures were observed to be in the range 700 K–1000 K. However, the combustor suffered significant overall heat loss (50–85%) which was implied by the low exhaust temperatures (500 K–750 K). For a constant fuel flow rate, the exhaust temperature increased monotonously with decrease in equivalence ratio until the blow-off condition implying that the combustor’s maximum thermal efficiency occurs at its lean blow-off limit. Thermal imaging of the combustor walls was performed using infrared camera to obtain the temperature distribution within the combustor. Numerical simulations were performed with the aid of CFD software using a heat loss coefficient chosen so as to give best correlation with experimental results. These CFD simulations helped to obtain better insight of the dependence of combustor performance on thermal conductivity of the material and heat load.     

Hydrogen storage and thermal conductivity properties of Mg-based materials with different structures

Mg-based hydrogen storage materials can be very promising candidates for stationary energy storage application due to the high energy density and low cost of Mg. Hydrogen storage kinetics and thermal conductivity are two important factors for the material development for this kind of application. Here we studied several types of Mg-based materials with different structure-micrometer scale Mg powders, Mg nanoparticles, single crystal Mg, nanocrystalline Mg₅₀Co₅₀ BCC alloy and Mg thin film samples. It seems the Mg materials with good kinetics usually are the ones with nanostructure and tend to show poor thermal conductivity due to electron/phonon scattering resulting from more interfaces and boundaries in nanomaterials. Based on this work, good crystallinity Mg phase incorporated in carbon nano framework could be one promising option for energy storage.     

Optimization design of slotted fin by numerical simulation coupled with genetic algorithm

Using a novel method that couples genetic algorithm (GA) with numerical simulation, the geometric configuration for a two-dimensional slotted fin has been optimized in this paper. The objective of optimization is to maximize the heat transfer capacity of slotted fin, and minimize the pressure drop penalty of fluid flow through the fin. The key of this method is the fitness function of GA, which were (j/j₀)/(f/f₀) and j/j₀. In this complex multiparameter problem, the numerical simulation is a crucial step to calculate the Colburn factor j and friction factor f. The results showed that for two-dimensional slotted fin considered, the j factor is increased by 229.22%, the f factor is increased by 196.30%, and the j/f ratio was increased by 11.11% at Re = 500 based on optimal integrated performance (j/j₀)/(f/f₀); the j factor is increased by 479.08% at Re = 500 based on optimal heat exchange capacity j/j₀. The feasibility of optimal designs was verified by the field synergy principle.     

Simple method for estimation of effectiveness in one tube pass and one shell pass counter-flow heat exchangers

In one tube pass and one shell pass counter-flow heat exchangers, when both streams change temperature by different amounts, the effectiveness is defined as the temperature change for the stream with lower capacity divided by the maximum possible change and the effectiveness depends on the number of transfer units and the thermal capacity ratio. In this paper, an attempt has been made to formulate a simple-to-use method which is easier than existing approaches, less complicated and with fewer computations for accurate and rapid estimation of effectiveness in one tube pass and one shell pass counter-flow heat exchangers as a function of number of transfer units and the thermal capacity ratio. The proposed method permits estimating the exit temperature for a one tube pass and one shell pass counter-flow heat exchanger without a trial-and-error calculation. The average absolute deviations between the reported data and the proposed correlations are found to be less than 2% demonstrating the excellent performance of proposed correlation. The tool developed in this study can be of immense practical value for engineers and scientists to have a quick check on the effectiveness in one tube pass and one shell pass counter-flow heat exchangers at various conditions without opting for any experimental measurements. In particular, practice engineers would find the predictive tool to be user-friendly with transparent calculations involving no complex expressions.     

Three-dimensional numerical simulation of heat and fluid flow in noncircular microchannel heat sinks

A three-dimensional model of heat transfer and fluid flow in noncircular microchannel heat sinks is developed and analyzed numerically. It is found that Nusselt number has a much higher value at the inlet region, but quickly approaches the constant fully developed value. The temperature in both solid and fluid increases along the flow direction. In addition, the comparison of thermal efficiencies is conducted among triangular, rectangular and trapezoidal microchannels. The result indicates that the triangular microchannel has the highest thermal efficiency.     

Improvement of thermal characteristics of latent heat thermal energy storage units using carbon-fiber brushes: experiments and modeling

Brushes made of carbon fibers with a high thermal conductivity are inserted on the shell side of a heat exchanger to enhance the conductive heat transfer rates in phase change materials. The experimental results show that the brushes essentially improve the heat exchange rate during the charge and discharge processes even when the volume fractions of the fibers are about one percent. A three-dimensional model describing the heat transfer in the heat exchanger is numerically solved. The model predicts well the experimental outlet fluid temperatures and the local temperatures in the composite.