Caloric Micro-Cooling

Cooling systems based on the caloric effects of ferroic materials show high potential for various cooling and heat-pumping applications due to their potentially high efficiencies and the lack of any environmentally hazardous refrigerants. One of such applications that has recently gained the attention of the scientific community is micro-cooling, which can be applied for hot-spot cooling and thermal management in electronic components. In this study a comprehensive numerical analysis of a caloric micro-cooling system using elastocaloric and electrocaloric materials was performed in order to investigate the limits and potential of this technology. We demonstrated that a caloric micro-cooling system is able to cool down the electronic component below room temperature or at least stabilize it at lower temperatures compared to the case when only the heat sink is applied in an efficient way (with the COP values exceeding 10). The specific cooling capacity of the caloric micro-cooling device strongly depends on the heat sink accompanied with the caloric material and its heat transfer capabilities. The caloric device can cool the electronic component below room temperature at heat-flux densities of up to 0.35 W/cm2 and up to 1 W/cm2 if it is used together with air-cooled heat sinks and water-cooled heat sinks, respectively. Caloric cooling systems could therefore play an important role as an efficient micro-cooling technology for certain applications, in particular where under-cooling below room temperature (low-temperature electronics) is required

Parametric analysis of fatigue-resistant elastocaloric regenerators

Elastocaloric cooling has recently shown high potential as an environmentally friendly alternative to vapor-compression technology. Here, we have studied and analyzed the geometric characteristics of two active elastocaloric regenerators (AeCRs) that were proved to have high application potential, i.e., a shell-and-tube AeCR loaded in compression and a parallel-plate AeCR loaded in tension, with the goal of maximizing their cooling performance. For this purpose, a previously developed and experimentally verified 1D numerical model was used. We focused only on the geometries and operating conditions that allow for durable, i.e., buckling-free operation in compression and fatigue-resistant operation in tension. The results show that although the applied strain of the parallel-plate AeCR loaded in tension needs to be limited (below 2%) to ensure fatigue-resistant operation, it outperforms (in terms of cooling power and COP at 15 K of temperature span) the shell-and-tube AeCR, which due to buckling issues suffers from a poorer heat-transfer geometry, but can withstand higher strains due to compressive loading. At the maximum strain of 2%, the optimum parallel-plate AeCR can generate a maximum cooling power of 1825 W (corresponding to 7075 W kg−1 of elastocaloric material) and a COP of 9.15 at a zero-temperature span. On the other hand, due to a higher applied strain (3%) the optimum shell-and-tube AeCR can generate a higher maximum temperature span at zero cooling power (up to 50 K) but has limited cooling performance at lower temperature spans. In addition, the layering of the shell-and-tube AeCR was investigated for the first time to improve its performance. This study shows the crucial impact of the heat-transfer geometry (heat-transfer area and hydraulic diameter), which needs to be further improved in compression-loaded AeCRs to improve their efficiencies (without compromising the buckling stability). The study also shows the importance of the applied strain, which needs to be at least 2% or more to achieve a high cooling performance of the AeCR. The obtained results should serve as guidelines for designing powerful and efficient AeCRs in the future

Thermo-hydraulic evaluation of oscillating-flow shell-and-tube-like regenerators for (elasto)caloric cooling

The development of novel regenerators for caloric cooling applications requires a detailed evaluation of their thermo-hydraulic properties. Structures similar to shell-and-tube heat exchangers are one of the most promising geometries for elastocaloric technology since they exhibit high thermal performance and can be applied under compressive loading to overcome the limited fatigue life of elastocaloric materials normally experienced in tension. However, thermo-hydraulic properties of shell-and-tube-like structures at the conditions relevant for caloric cooling applications (oscillating counter-flow regime at low Reynolds numbers

A regenerative elastocaloric device

Elastocaloric cooling and heating is an alternative cooling technology that has the potential to be highly efficient and environmentally friendly. Experimental results are reported for two elastocaloric regenerators made of Ni-Ti alloys in the form of parallel plates in two plate thicknesses. For the regenerator made of 0.2 mm plates, a maximum no-load temperature span of 17.6 K was achieved for an applied strain of 4.3%. For the regenerator with 0.35 mm plates, a maximum temperature span of 19.9 K was reached for a strain of 3.5%. The 0.2 mm regenerator failed after approximately 5200 cycles and the 0.35 mm regenerator failed after approximately 5500 cycles

Latent thermal energy storage using solid-state phase transformation in caloric materials

Materials with solid-to-solid phase transformations have considerable potential for use in thermal energy storage systems. While these materials generally have lower latent heat than materials with a solid-to-liquid phase transformation, their significantly higher thermal conductivity enables rapid thermal charging/discharging. Here, we show that this propertymakes them particularly promising for thermal energy storage applications requiring highly dynamic operation. A numerical analysis (using an experimentally validated numerical model) has revealed that some materials with solid-tosolid phase transformations offer an excellent capacity-power trade-off for thermal energy storage applications compared to the corresponding conventional phase change materials. While most conventional phase change materials generally offer higher thermal capacity due to larger latent heat, some metallic materials with solid-state transformation (e.g., Ni-Ti-based alloys, Mn-Co-Ga-B alloys) exhibit up to 10 times higher thermal output powers. These results highlight a significant potential of caloric solid-state materials to outperform traditional latent thermal storage systems for certain applications

Understanding the thermodynamic properties of the elastocaloric effect through experimentation and modelling

This paper presents direct and indirect methods for studying the elastocaloric effect (eCE) in shape memory materials and its comparison. The eCE can be characterized by the adiabatic temperature change or the isothermal entropy change (both as a function of applied stress/strain). To get these quantities, the evaluation of the eCE can be done using either direct methods, where one measures (adiabatic) temperature changes or indirect methods where one can measure the stress-strain-temperature characteristics of the materials and from these deduce the adiabatic temperature and isothermal entropy changes. The former can be done using the basic thermodynamic relations, i.e. Maxwell relation and Clausius-Clapeyron equation. This paper further presents basic thermodynamic properties of shape memory materials, such as the adiabatic temperature change, isothermal entropy change and total entropy-temperature diagrams (all as a function of temperature and applied stress/strain) of two groups of materials (NiTi and CuZnAl alloys) obtained using indirect methods through phenomenological modelling and Maxwell relation. In the last part of the paper, the basic definition of the efficiency of the elastocaloric thermodynamic cycle (coefficient of performance) is defined and discussed

A shell finite element model for superelasticity of shape memory alloys

A finite element formulation for the analysis of large strains of thin-walled shape memory alloys is briefly presented. For the shell model we use a seven-kinematic-parameter model for large deformations and rotations, which takes into account the through-the-thickness stretch and can directly incorporate a fully 3D inelastic constitutive equations. As for the constitutive model, we use a large strain isotropic formulation that is based on the multiplicative decomposition of the deformation gradient into the elastic and the transformation part and uses the transformation deformation tensor as an internal variable. Numerical examples are presented to illustrate the approach

A regenerative elastocaloric heat pump

A large fraction of global energy use is for refrigeration and air-conditioning, which could be decarbonized if efficient renewable energy technologies could be found. Vapour-compression technology remains the most widely used system to move heat up the temperature scale after more than 100 yearshowever, caloric-based technologies (those using the magnetocaloric, electrocaloric, barocaloric or elastocaloric effect) have recently shown a significant potential as alternatives to replace this technology due to high efficiency and the use of green solid-state refrigerants. Here, we report a regenerative elastocaloric heat pump that exhibits a temperature span of 15.3 K on the water side with a corresponding specific heating power up to 800W1/kg and maximum COP (coefficient-of-performance) values of up to 7. The efficiency and specific heating power of this device exceeds those of other devices based on caloric effects. These results open up the possibility of using the elastocaloric effect in various cooling and heat-pumping applications