Modeling of an Elastocaloric Cooling System for Determining Efficiency

When it comes to covering the growing demand for cooling power worldwide, elastocalorics offer an environmentally friendly alternative to compressor-based cooling technology. The absence of harmful and flammable coolants makes elastocalorics suitable for energy applications such as battery cooling. Initial prototypes of elastocaloric systems, which transport heat by means of thermal conduction or convection, have already been developed. A particularly promising solution is the active elastocaloric heat pipe (AEH), which works with latent heat transfer by the evaporation and condensation of a fluid. This enables a fast and efficient heat transfer in a compression-based elastocaloric cooling system. In this publication, we present a simulation model of the AEH based on MATLAB-Simulink. The model showed very good agreement with the experimental data pertaining to the maximum temperature span and maximum cooling power. Hereby, non-measurable variables such as efficiency and heat fluxes in the cooling system are accessible, which allows the analysis of individual losses including the dissipation effects of the material, non-ideal isolation, losses in heat transfer from the elastocaloric material to the fluid, and other parasitic heat flux losses. In total, it can be shown that using this AEH-approach, an optimized system can achieve up to 67% of the material efficiency

Phenomenological model for first-order elastocaloric materials [Modèle phénoménologique pour les matériaux élastocaloriques de premier ordre]

Elastocaloric cooling systems may offer a potentially more efficient as well as environmentally friendly alternative to compressor-based cooling technology. These cooling systems use stress-induced phase transformation in elastocaloric materials to pump heat. Thermodynamically consistent material models can be used to design and quantify the efficiency of these cooling systems. In this paper, we present a phenomenological material model that depicts the behavior of first-order materials during stress-induced phase transformation. This model is based on a phenomenological heat capacity equation, from which the parameters adiabatic temperature change and isothermal entropy can be derived. Hysteresis of the materials, which determines it dissipative effects, is also taken into account. Based on this model, these parameters can be calculated as a function of stress and temperature. The performance coefficients derived from the model can be used to evaluate the materials efficiency. Furthermore, the data obtained using this model coincided very closely with experimental data

Customized measuring station for Peltier modules

Peltier modules control temperatures in niche products such as wine coolers and camping fridges or are used to precisely temper processes as for example the duplication of DNA sections. Accurate characterization and long term stability of the modules is crucial in order to meet the high reliability requirements especially in critical applications such as in space or medical technology markets. While most of the measurement stations are designed for thermoelectric generators, especially in regard to long term testing, the measurement of peltier modules is rarely described. In this contribution, we show a customized measuring station specifically for the combination of the characterization of peltier modules and their long term stability. With this measurement station, it is possible to determine temperature dependent properties such as the maximum current Imax, the maximum cooling power Image 1, the maximum temperature difference Image 2 and the coefficient of performance (COP) while performing cycling tests in between the characterization measurements. In this setup, both sides consist of water driven heat exchangers with a range of Image 3 to Image 4. The heat flow through the module is measured via two graphite heat flux meters.Thereby, it is possible to completely characterize a peltier module within this measuring system. An error analysis for the measured properties is given as well. In addition to the characterization of the modules, the long-term stability of the modules can be measured not only with static current and temperature but also at cyclical stimulation of temperature or current changes meaning that application-oriented long-term tests are possible

Improved Thermal Switch Based on an Adsorption Material in a Heat Pipe

For many applications, the possibility of controlling heat flow by “thermal switching” can be very beneficial. In previous work, we presented a novel approach for thermal switching using a water-loaded adsorbent as part of the evaporator of a heat pipe. The basic idea is that the adsorbent releases water upon exceeding a certain evaporator temperature and thus “activates” the heat pipe by providing the working fluid for thermal transport. In this work, we present an improved version of the heat switch. We found that an ordinary copper heat pipe (i.e., thermosyphon) with outer diameters common in heat pipe applications (10 mm) can be used as a base of the heat switch. The reversibility of the switching effect was proven. The location of heat input relative to the adsorbent position was optimized, leading to improved switching ratios of up to 36 as calculated from thermal resistance change between evaporator and condenser. Furthermore, the role of the insulation on thermal resistance and switching ratio was examined and found to be significant

Flat-Plate PHP with Gravity-Independent Performance and High Maximum Thermal Load

In many energy-related applications, components with high heat loads, such as power electronics, play an important role. Pulsating heat pipes (PHPs) are an effective solution to deal with the increasing heat load of these components. In many real-life applications, the PHP must work against gravity and still be able to operate efficiently. However, the majority of present flat-plate PHP designs do not perform well under this condition. Therefore, this paper presents a flat-plate PHP with a conventional channel design optimized for gravity-independent operation. The PHP was capable of transmitting a heat output of 754 watts in all orientations, while the testing heater in use never exceeded a temperature of 100 °C. No indications of dryout were observed, implying that the maximum thermal load the PHP can handle is even higher. Additionally, three different condenser zone sizes were tested with the PHP. Previously published results indicated that there is a specific range of suitable condenser zone sizes, and performance problems will occur if the condenser zone size falls outside of this range. The findings from this work point in the same direction

Thermal hysteresis and its impact on the efficiency of first-order caloric materials

Cooling with caloric materials could be an option to replace compressor-based cooling systems in the future. In addition to the advantage of avoiding dangerous liquid coolants, one often cites a possible higher efficiency of the calorific cooling systems compared to compressor-based systems. But is that true? The aim of this work is to assess the efficiency potential of caloric cooling systems on a very basic material level. We placed our focus on materials with a first-order phase change since they generally show a large caloric response. We derive a relation between thermal hysteresis and the dissipative losses due to hysteresis. To predict the efficiency, this relation is integrated in a Carnot-like cycle. This approach was chosen to get access to the efficiency reduction due to hysteresis without any further losses due to other nonidealities of the thermodynamic cycle. As a main finding, we present a direct relation between thermal hysteresis and the expected maximum exergy or second-law efficiency of a caloric cooling device. These results indicate that, for many caloric materials, the thermal hysteresis needs to be further reduced to be able to compete with the efficiency of compressor-based systems

Small-Sized Pulsating Heat Pipes/Oscillating Heat Pipes with Low Thermal Resistance and High Heat Transport Capability

Electronics (particularly power electronics) are the core element in many energy-related applications. Due to the increasing power density of electronic parts, the demands on thermal management solutions have risen considerably. As a novel passive and highly efficient cooling technology, pulsating heat pipes (PHPs) can transfer heat away from critical hotspots. In this work, we present two types of small and compact PHPs with footprints of 50 × 100 mm2, thicknesses of 2 and 2.5 mm and with high fluid channel density, optimized for cooling electronic parts with high power densities. The characterization of these PHPs was carried out with a strong relation to practical applications, revealing excellent thermal properties. The thermal resistance was found to be up to 90% lower than that of a comparable solid copper plate. Thus, a hot part with defined heating power would remain at a much lower temperature level and, for the same heater temperature, a much larger heating power could be applied. Moreover, the dependence of PHP operation and thermal properties on water and air cooling, condenser area size and orientation is examined. Under some test configurations, dryout conditions are observed which could be avoided by choosing an appropriate size for the fluid channels, heater and condenser

Method to characterize a thermal diode in saturated steam atmosphere

We present a novel measurement method for the characterization of thermal diodes in a saturated steam atmosphere. A measuring setup has been developed in which two pressure sensors are integrated. Using a developed analytical model, the heat flow, the volume flow, and the cracking pressure are determined from the measured absolute pressures and the pressure difference. The analytical model was verified using a flow through an orifice. We first calculated the volume flow through the orifice, with a diameter of 3 mm, using the Reader-Harris equation and then compared it to experimentally determined values. The experimentally determined values showed a discrepancy of 9%. With the measurement setup, we have characterized a check valve developed for magnetocaloric heat pumps, which has a thermally rectifying behavior. The developed check valve consists of three spring arms, which are radially attached to a valve disk. The heat flow through the check valve in the forward direction is 166 W for water, 239 W for ethanol, and 547 W for methanol at a temperature difference of 1 K. In the reverse direction, the heat flow is −0.03 W at a temperature difference of −1 K. For methanol, this corresponds to a rectification coefficient of more than 18 000