5. Device prototypes

EC refrigerator based on SrTiO3 was initially proposed for cryogenic application, particularly in the 4–15 K temperature range [13, 63]. In a completely solid-state EC device, the electric field and magnetothermal heat switches were cycled in a proper time sequence [63]. However, no cooling power and device efficiency were reported.

For switching from a heat-conducting to a heat-insulating state near room temperature, EC elements are placed between a pair of thermoelectric elements (Peltier elements), serving as heat switch [58]. The first switch is in thermal contact with the heat sink and the second one with the load. During the adiabatic steps, both heat switches are turned off. After the EC material was adiabatically polarized (heated), the first heat switch is turned on, transferring heat from the EC element to the heat sink. The second heat switch stays turned off. After adiabatic depolarization (cooling), the second heat switch becomes active and heat is transferred from the load to the EC element. The first heat switch stays turned off. A device prototype in this configuration using Peltier elements in the passive mode was characterized in Ref. [64]. The thermal contact conductance of Peltier elements was about 1000 W/m2 K, i.e. they do not provide an advantage compared to laminar liquid flow of a heat transfer agent [57].

For EC micro-refrigerators, heat switches were fabricated by micro-electromechanic systems (MEMS) technology [43, 46, 65]. For fast heat exchange, laterally interdigitated electrodes were considered in Ref. [46]. The weak point of such a design is the comparably high thermal resistance at the interfaces to the load and the heat sink (cf. Table 3).

Liquid crystals were proposed as prospective heat switches [66, 67]. The operation of a thin film EC refrigerator comprising such liquid crystal heat switches was theoretically investigated in Ref. [48]. Although a thermal contrast of up to about 25 was reported for liquid crystals [68], no devices were realized yet.

A fluid-based approach uses electrohydrodynamic (EHD) flows in thin films of dielectric fluids [69]. In this case, the thermal contrast K = 4.7 1.1 yields a relative efficiency Φswitch = 0.18, which is still too low for practical application.

Table 5 compiles the parameters of EC refrigerators where the transport of thermal energy from the cold to the hot side of the system is carried out by means of heat switches. The tables illustrate that commercial multi-layer capacitors described above are an attractive EC component in proof-of-concept refrigerator prototypes. MLCs are extremely reliable. They combine a suitable thermal mass with an operating voltage in the order of 100 V as well as with the high dielectric strength obtained in thin layers (typically <10 µm) [70]. MLCs can be stacked in series to achieve a higher Tspan. Moreover, MLC arrays can be operated between a common heat source and sink to increase cooling power.


<sup>3</sup> Calculated using Eq. (18).

K,

Currently, the operational frequency of EC refrigerators is limited to about 10 Hz providing

EC refrigerator based on SrTiO3 was initially proposed for cryogenic application, particularly in the 4–15 K temperature range [13, 63]. In a completely solid-state EC device, the electric field and magnetothermal heat switches were cycled in a proper time sequence [63]. However, no

For switching from a heat-conducting to a heat-insulating state near room temperature, EC elements are placed between a pair of thermoelectric elements (Peltier elements), serving as heat switch [58]. The first switch is in thermal contact with the heat sink and the second one with the load. During the adiabatic steps, both heat switches are turned off. After the EC material was adiabatically polarized (heated), the first heat switch is turned on, transferring heat from the EC element to the heat sink. The second heat switch stays turned off. After adiabatic depolarization (cooling), the second heat switch becomes active and heat is transferred from the load to the EC element. The first heat switch stays turned off. A device prototype in this configuration using Peltier elements in the passive mode was characterized in Ref. [64]. The thermal contact conductance of Peltier elements was about 1000 W/m2

i.e. they do not provide an advantage compared to laminar liquid flow of a heat transfer

For EC micro-refrigerators, heat switches were fabricated by micro-electromechanic systems (MEMS) technology [43, 46, 65]. For fast heat exchange, laterally interdigitated electrodes were considered in Ref. [46]. The weak point of such a design is the comparably high thermal

Liquid crystals were proposed as prospective heat switches [66, 67]. The operation of a thin film EC refrigerator comprising such liquid crystal heat switches was theoretically investigated in Ref. [48]. Although a thermal contrast of up to about 25 was reported for liquid crystals [68],

A fluid-based approach uses electrohydrodynamic (EHD) flows in thin films of dielectric fluids [69]. In this case, the thermal contrast K = 4.7 1.1 yields a relative efficiency Φswitch =

Table 5 compiles the parameters of EC refrigerators where the transport of thermal energy from the cold to the hot side of the system is carried out by means of heat switches. The tables illustrate that commercial multi-layer capacitors described above are an attractive EC component in proof-of-concept refrigerator prototypes. MLCs are extremely reliable. They combine a suitable thermal mass with an operating voltage in the order of 100 V as well as with the high dielectric strength obtained in thin layers (typically <10 µm) [70]. MLCs can be stacked in series to achieve a higher Tspan. Moreover, MLC arrays can be operated between a common heat

resistance at the interfaces to the load and the heat sink (cf. Table 3).

.

cooling power and device efficiency were reported.

cooling powers of a few W/cm2

5. Device prototypes

34 Refrigeration

agent [57].

no devices were realized yet.

0.18, which is still too low for practical application.

source and sink to increase cooling power.

Table 5. Characteristics of EC refrigerators comprising heat switches.

A compromise between low thermal interface resistance and a quick heat transfer is a regenerator system, i.e. a heat exchanger where the heat is intermittently stored in a thermal storage medium. In the 1980s, EC cooling near 300 K was demonstrated at an operating frequency of f = 0.4 Hz using a regenerator of helium or liquid pentane that flowed back and forth between 0.3-mm-thick PbSc0.5Ta0.5O3 plates [57, 59, 71]. The plates were rendered alternately hot and cold by electrically cycling the phase transition. The cooling power was still low at about 7.7 kg/W. Prototypes with up to 750 plates were built. In order to maintain a high temperature span in a wide temperature range, a cascade concept was realized exploiting the shift of the temperature of maximum EC activity of ceramics tailored by different sintering temperatures. A 10-fold cascade provided a temperature span of 10 K. The same operational principle can be realized using micro-electromechanical systems technology [72]. Here, the heat transfer liquid (Galden HT-70) is pumped back and forth by two diaphragm actuators, which are driven electrostatically. A small-scale EC cooling device based on an active EC regenerator with silicon oil or water as heat transfer fluids is described in Ref. [42].

The heat regeneration process is commonly used to increase the temperature span in cooling devices. Experimentally obtained regeneration factors (ratio between the temperature span established across the device and ΔTEC) are ca. 2 [71] and ca. 3.7 [42], respectively. Simulations predict an improvement by a factor of 5–6 [71] and, by optimizing also the heat transfer agent, up to a factor of 10 [42]. Thus, temperature spans of up to 20 K seem to be technically possible.

Regeneration can be realized also by heat exchange directly between EC elements that are rotating in opposite directions with different applied fields. A corresponding rotary EC refrigerator is described in Ref. [47]. It consists of stacked EC rings where each EC ring is composed of Ns (for example, Ns = 16) thermally separated EC elements. The EC rings rotate coaxially with the same rotary speed, but the rotation directions are opposite between neighbouring rings. Every two neighbouring EC rings are directly contacted to facilitate the heat exchange with each other. Heat exchangers with high thermal conductivity are placed at the circumference at opposite sides of the device to absorb or reject heat. Simulation results showed a cooling power density of 37 W/cm3 for a Tspan of 20 K for a cooling device made of P(VDF-TrFE-CFE) terpolymer.

The electrocaloric oscillatory refrigeration device (ECOR) adapts a concept known from thermoacoustic cooling [28, 73]. It consists of an EC element and a solid-state regenerator. The length of the EC module is slightly shorter than that of the regenerator, so that the EC module can move back and forth on the regenerator. Thereby, a temperature gradient is established within both and heat is transported from one side to the other. The solid-state regenerator


Table 6. Characteristics of EC refrigerators using regeneration.

might possess an anisotropic thermal conductivity highly reducing heat conduction losses. A 1 cm long device can provide a cooling density of 9 W/cm3 . The weak point of this design is the friction during the relative motion between EC element and regenerator.

Table 6 illustrates the characteristics of EC refrigerators using regeneration. Although the cooling power of experimental prototypes is still very low, modelling based on experimental results predicts cooling powers of a few W/cm<sup>2</sup> .
