Search Results (15)

In this study, we characterize electrocaloric lead scandium tantalate (PST) samples by means of the adiabatic temperature change $\Delta T_{\mathrm{ad}}$ and the dissipative heat $q_{\mathrm{diss}}$ with a direct thermal method. The figure of merit ($FOM$), defined as the ratio between the adiabatic temperature change and the thermal hysteresis, quantifies the losses of the material. Additionally, it is also possible to draw conclusions on the efficiency of a caloric cooling system based on the regenerator or cascaded approach. The maximum adiabatic temperature change of the measured samples results in $\Delta T_{\mathrm{ad},\max }=(1.39\pm 0.02)$ K and the dissipative heat yields $q_{\mathrm{diss}}=(0.39\pm 0.05) \mathrm{J}/(\mathrm{k}\mathrm{g} \mathrm{K})$, resulting in an $FOM=(5.1\pm 0.2)$. The efficiency for an ideal cascaded system is given by ${\eta }_{\mathrm{cas}}=0.56$, and for the ideal regenerator, the efficiency is given by ${\eta }_{\mathrm{reg}}=0.84$. The results demonstrate that the PST material in this study exceeds the maximum $FOM$ in the literature by 34%. Full article

Elastocaloric cooling is recognized as a promising alternative to modern vapor-compression cooling systems, which often rely on environmentally hazardous refrigerants. Natural rubber (NR), a well-known renewable resource, stands out among elastomers exhibiting elastocaloric behavior due to a peculiar combination of nontoxicity, low cost, softness, long-life fatigue and high caloric power. Despite these properties, research on the refrigeration potential of NR is still in its early stages, and several aspects require attention. This work investigates, for the first time, the effect of crosslinking density on the elastocaloric properties of NR. Samples with three different crosslinking densities (2.9, 4.0 and 5.2 mol·10⁻⁴/cm³) were produced by internal compounding and hot pressing, and thermo-mechanically characterized. The assessment of the elastocaloric effect of the produced samples revealed that reducing the crosslinking degree significantly enhanced the elastocaloric properties. To compare the cooling capacity of the samples, a qualitative coefficient of performance (COP_mat) was evaluated as the ratio between extracted thermal energy and deformational work per unit volume. The results highlight that the least crosslinked sample achieved the higher COP_mat, equal to 2.4. These results underscore the significance of crosslinking density as one of the primary factors to be considered to enhance the refrigeration potential of NR. Full article

To accommodate the integration of renewable energy, coal-fired power plants must take on the task of peak regulation, making the low-load operation of boilers increasingly routine. Under low-load conditions, the phase transition point (PTP) of the working fluid fluctuates, leading to potential flow instability, which can compromise boiler safety. In this paper, a one-dimensional coupled dynamic model of the combustion and hydrodynamics of a supercritical boiler is developed on the Modelica/Dymola 2022 platform. The spatial distribution of key thermal parameters in the furnace and the PTP position in the water-cooled wall (WCW) are analyzed in a 660 MW supercritical boiler when parameters on the combustion side change under full-load and low-load conditions. The dynamic response characteristics of the temperature, mass flow rate, and the PTP position are investigated. The results show that the over-fire air (OFA) ratio significantly influences the flue gas temperature distribution. A lower OFA ratio increases the flue gas temperature in the burner zone but reduces it at the furnace exit. The lower OFA ratio leads to a higher fluid temperature and shortens the length of the evaporation section. The temperature difference in the WCW outlet fluid between the 20% and 60% OFA ratios is 11.7 °C under BMCR conditions and 7.4 °C under 50% THA conditions. Under the BMCR and 50% THA conditions, a 5% increase in the coal caloric value raises the flue gas outlet temperature by 32.7 °C and 35.4 °C and the fluid outlet temperature by 6.5 °C and 9.9 °C, respectively. An increase in the coal calorific value reduces the length of the evaporation section. The changes in the length of the evaporation section are −2.95 m, 2.95 m, −2.62 m, and 0.54 m when the coal feeding rate, feedwater flow rate, feedwater temperature, and air supply rate are increased by 5%, respectively. Full article

The objectives of the study are to reveal the influence of multistage fuel-oxidation chemistry, thermal radiation of soot during the combustion of a small (submillimeter size) fuel droplet, and real gas effects on the operation process of compression ignition engines. The use of the multistage oxidation chemistry of iso-octane in the zero-dimensional approximation reveals the appearance of different combinations of cool, blue, and hot flames at different compression ratios and provides a kinetic interpretation of these phenomena that affect the heat release function. Cool flames are caused by the decomposition of alkyl hydroperoxide, during which a very reactive radical, OH, is formed. Blue flames are caused by the decomposition of H₂O₂ with the formation of OH. Hot flames are caused by the chain branching reaction between atomic hydrogen and molecular oxygen with the formation of OH and O. So-called “double” cool flames correspond to the sequential appearance of a separated cool flame and a low-intensity blue flame rather than two successive cool flames. The use of a one-dimensional model of fuel droplet heating, evaporation, autoignition, and combustion at temperatures and pressures relevant to compression ignition engines shows that the thermal radiation of soot during the combustion of small (submillimeter size) droplets is insignificant and can be neglected. The use of real gas caloric and thermal equations of state of the matter in a three-dimensional simulation of the operation process in a diesel engine demonstrates the significant effect of real gas properties on the engine pressure diagram and on the NO and soot emissions: real gas effects reduce the maximum pressure and mass-averaged temperature in the combustion chamber by about 6 and 9%, respectively, increases the autoignition delay time by a 1.6 crank angle degree, increase the maximum heat release rate by 20%, and reduce the yields of NO and soot by a factor of 2 and 4, respectively. Full article

Caloric cooling systems are potentially more efficient than systems based on vapour compression. Electrocaloric cooling systems use a phase transformation from the paraelectric to the ferroelectric state by applying or removing an electric field to pump heat. Lead scandium tantalate (PST) materials show a first-order phase transition and are one of the most promising candidates for electrocaloric cooling. To model caloric cooling systems, accurate and thermodynamically consistent material models are required. In this study, we use a phenomenological model based on an analytical equation for the specific heat capacity to describe the material behaviour of bulk PST material. This model is fitted to the experimental data, showing a very good agreement. Based on this model, essential material properties such as the adiabatic temperature change and isothermal entropy change of this material can be calculated. Full article

Solid-state caloric cooling is a viable route toward a more sustainable way of refrigerating. The refrigerants are solid-state materials with a caloric effect detectable by measuring a temperature variation through an external-field intensity change. The caloric effect could be particularized depending on the properties of the material and the type of field. Magnetocaloric is the effect occurring in ferromagnetic materials through the variation of an external field. Thermodynamically, two are the possible cycles regulating the cooling process in the system: the Active Caloric Regenerative cooling cycle and the solid-to-solid heat transfer (SSHT). The former requires the involvement of an auxiliary fluid for the heat transfer processes; in the latter, the heat transfer can be regulated by thermal diodes with the capability of changing their thermal conductivity depending on the intensity of an external field. The investigation introduced is focused on an SSHT system employing magnetocaloric materials as refrigerants and thermal diodes as the vehiculation elements. The two-dimensionality of the model allows the optimization of the dimensions of both the magnetocaloric and the thermal diode elements to achieve elevated operative frequencies. A comparison between two magnetocaloric materials was performed, Gadolinium and LaFe_11.384Mn_0.356Si_1.26H_1.52. Encouraging results on the system, suitably employable in the field of electronic circuit cooling, have been found. Full article

Future small satellites will demand high-performance on-board electronics, requiring sophisticated approaches to heat rejection beyond simply increasing the radiator surface area. An interesting alternative approach is to increase the surface temperature of the radiator, using a heat pump. In this study, calculations were carried out to compute the theoretical radiator size reduction potential enacted by having a heat pump as part of a satellite’s thermal management system. The practical likelihood of a ‘typical’ vapor compression cycle (VCC) heat pump satisfying theoretical requirements was considered. In agreement with theoretical calculations, employing a ‘typical’ VCC heat pump could either increase or decrease the required radiator surface area. The choice of heat pump and its design is therefore crucial. A heat pump with a large temperature lift is essential for satellite radiator cooling applications, with the coefficient of performance (COP) being less important. Even with a low COP, such as 2.4, a ‘typical’ heat pump providing a large temperature lift, close to 60 °C, could reduce the satellite’s radiator surface area by a factor close to 1.4. This is a significant potential reduction. The decision on whether to pursue this approach compared to alternatives, such as deployable radiators, should consider the relative complexity, cost, weight, size, reliability, etc., of the two options. The focus of this study is VCC heat pumps; however, the results provide performance targets for less mature heat pump technologies, e.g., caloric devices, which could ultimately be applied in space. Full article

Although stationary models for thermal circuits have been widely used, a direct analogy of transient responses of electric circuits to thermal systems is still difficult to establish. In this work, a thermal circuit model for transient responses is developed. The model states that each thermal object is a thermal resistance and a heat capacitor in parallel. The heat capacitor is the heat capacity of the overall material plus a correction term due to the thermal contacts of all thermal objects. The transient response of three basic thermal circuits is modeled, based on the proposed method, and validated, using the heatrapy Python package: single thermal resistance, two thermal resistances in series and two thermal resistances in parallel. A more complex model of a thermal circuit involving a heat source, a heat transfer medium and convection of heat to the surroundings is also developed and validated with data from literature of a thermal switch used in caloric cooling. The proposed method tackles computational issues introduced by the majority of numerical approaches. Full article

Elastocaloric cooling is considered an environmentally friendly future alternative to vapor-compression technology. Recently, a shell-and-tube-like elastocaloric regenerator loaded in compression has demonstrated record-breaking heat-pumping performance and fatigue-resistant operation. The aim of this work is thus to present a new 1D numerical model to simulate and optimize the operation of an elastocaloric regenerator with a shell-and-tube-like design. In the first part of this work, the superelastic and elastocaloric properties of a single NiTi tube, which serve as input data for the numerical model, were determined through experimental characterization and phenomenological modeling. In the second part, the results of the numerical model were compared with the experimentally obtained results. Relatively good agreement was found regarding the temperature span, cooling and heating power, and COP values, which indicates that the developed numerical model could be used for accurate optimization of shell-and-tube-like elastocaloric regenerators. Finally, the effects of operating conditions and hysteresis losses on the performance of the shell-and-tube-like elastocaloric regenerator are modeled and discussed. This work shows that the shell-and-tube-like elastocaloric regenerator with this configuration can achieve a maximum temperature span of more than 50 K at zero-thermal-load conditions and a maximum cooling/heating power of up to 4000 W·kg⁻¹ and COP of about 4 (at zero temperature span). Full article

In this study, a homemade uniaxial strain pressure cell was designed to be directly used in the standard magnetometers whereby the magnetic properties of samples subjected to a uniaxial strain and magnetic field were characterized. Its feasibility has been demonstrated by the uniaxial strain control of the phase transition and magnetocaloric effect in Ni₄₀Co₁₀Mn₄₀Sn₁₀ (NCMS) alloys. With the assistance of a uniaxial strain of ~0.5%, the cooling temperature span of NCMS alloys is broadened by 2 K, and the refrigeration capacity under a 3 T magnetic field change increases from 246 to 277 J/kg. This research provides not only direct experimental assistance for the tuning of phase transition by the uniaxial strain but also possibilities for studying the coupled caloric effect in first-order phase transition materials under a combined uniaxial strain and magnetic field by the thermodynamic analysis. Full article

The investigation of caloric effects linked to first-order structural transitions in Heusler-type alloys has become a subject of considerable current interest due to their potential utilization as refrigerants in solid-state cooling devices. This study is mainly motivated by the possibility of developing refrigeration devices of improved energy efficiency with a reduced environmental impact. We produced partially textured and isotropic bulk samples of the Heusler-type magnetic shape memory alloy Ni₅₅Fe₁₁Mn₇Ga₂₇ by arc melting and spark plasma sintering (SPS), respectively. Their structural, microstructural, and phase transition characteristics and magnetocaloric and elastocaloric effects, associated with first-order martensitic transformation (MT), were studied. The elemental chemical compositions of both samples were close to nominal, and a martensitic-like structural transformation appeared around room temperature with similar starting and finishing structural transition temperatures. At room temperature, austenite exhibited a highly ordered L2₁-type crystal structure. The partial grain orientation and isotropic nature of the arc-melted and SPS samples, respectively, were revealed by X-ray diffraction and SEM observations of the microstructure. For the arc-melted sample, austenite grains preferentially grew in the (100) direction parallel to the thermal gradient during solidification. The favorable effect of the texture on the elastocaloric response was demonstrated. Finally, due to its partial grain orientation, the arc-melted bulk sample showed superior values of maximum magnetic entropy change (|ΔS_M|^max = 18.6 Jkg⁻¹K⁻¹ at 5 T) and elastocaloric adiabatic temperature change (|ΔT_ad^me|^max = 2.4 K at 120 MPa) to those measured for the SPS sample (|ΔS_M|^max = 8.5 Jkg⁻¹K⁻¹ and (|ΔT_ad^me|^max = 0.8 K). Full article

The multicaloric effect is defined as the adiabatic reversible temperature change in multiferroic materials induced by the application of an external electric or magnetic field, and it was first theoretically proposed in 2012. The multicaloric effects in multiferroics, as well as other similar caloric effects in single ferroics, such as magnetocaloric, elastocaloric, barocaloric, and electrocaloric, have been the focus of much research due to their potential commercialization in solid-state refrigeration. In this short communication article, we examine the thermodynamics of the multicaloric effect for solid-state heating applications. A possible thermodynamic multicaloric heating cycle is proposed and then implemented to estimate the solid-state heating effect for a known electrocaloric system. This work offers a path to implementing caloric and multicaloric effects to efficient heating systems, and we offer a theoretical estimate of the upper limit of the temperature change achievable in a multicaloric cooling or heating effect. Full article

The olive oil industry represents an important productive sector in the Mediterranean basin countries. Olive stone is an essential by-product generated in the olive oil extraction industries and it represents roughly 10% by weight of the olive fruit. The seeds of pickled olives are also a significant waste product. In the present study, we have investigated the possibility of the use of differential scanning calorimetry for the thermal characterization of seeds from green and black pickled olives from Croatia. The differential scanning calorimeter (DSC) with a normal pressure cell equipped with a cooling system was used to determine the thermal properties of seeds from olives. The following analyses were also performed: the determination of calorific values in a pressure bomb calorimeter, the determination of initial water content, the determination of changes of water content during drying at the temperatures of 30 °C, 50 °C and 80 °C, the determination of a percentage content of seeds mass to the mass of the whole olives, and the determination of ash content. Seeds from olives are characterized by very good parameters as a biomass. The analyzed olive seeds were characterized by low water content, low ash content, and a relatively high caloric value. Full article

Magnetic shape memory Heuslers have a great potential for their exploitation in next-generation cooling devices and actuating systems, due to their “giant” caloric and thermo/magnetomechanical effects arising from the combination of magnetic order and a martensitic transition. Thermal hysteresis, broad transition range, and twinning stress are among the major obstacles preventing the full exploitation of these materials in applications. Using Ni-Mn-Ga seven-modulated epitaxial thin films as a model system, we investigated the possible links between the phase transition and the details of the twin variants configuration in the martensitic phase. We explored the crystallographic relations between the martensitic variants from the atomic-scale to the micro-scale through high-resolution techniques and combined this information with the direct observation of the evolution of martensitic twin variants vs. temperature. Based on our multiscale investigation, we propose a route for the martensitic phase transition, in which the interfaces between different colonies of twins play the major role of initiators for both the forward and reverse phase transition. Linking the martensitic transition to the martensitic configuration sheds light onto the possible mechanisms influencing the transition and paves the way towards microstructure engineering for the full exploitation of shape memory Heuslers in different applications. Full article

Barocaloric cooling is classified as environmentally friendly because of the employment of solid-state materials as refrigerants. The reference and well-established processes are based on the active barocaloric regenerative refrigeration cycle, where the solid-state material acts both as refrigerant and regenerator; an auxiliary fluid (generally water of water/glycol mixtures) is used to transfer the heat fluxes with the final purpose of subtracting heat from the cold heat exchanger coupled with the cold cell. In this paper, we numerically investigate the effect on heat transfer of working with nanofluids as auxiliary fluids in an active barocaloric refrigerator operating with a vulcanizing rubber. The results reveal that, as a general trend, adding 10% of copper nanoparticles in the water/ethylene-glycol mixture carries to +30% as medium heat transfer enhancement. Full article