Study of electrostatic actuation for electrocaloric cooling devices

Solid-state refrigeration offers potential advantages over traditional cooling systems, but few devices offer high specific cooling power with a high coefficient of performance (COP); they fall short of the well-established performances of gas-based refrigerant devices. Electrocaloric (EC) polymers present various specificities : they have a high adiabatic temperature change (~10K) and a low mass density (1g/cm3). Thus, using these materials could lead to refrigeration with high specific power (W/g), and high COP. Even more importantly, polymers are softer than other caloric materials; therefore they are surface-conformable (E~1GPa), which allows one to increase the effective contact surface between the polymer film and the heat reservoir. Additionally, films can be very thin and therefore show a remarkable surface/volume ratio. Because of their properties (softness and thinness), these films are well-suited to enhance the heat transfer and achieve the power density that is required in cooling devices. However, the handling of such films is not easy—precisely because they are soft. Ma et al. present an EC cooling device based on a polymer film, showing a COP up to 13, a specific cooling power of 2.8W/g, and working at a frequency of 0.8 Hz. In their device, the EC film is electrostatically moved from the hot reservoir to the cold one and vice-versa, thus allowing one to control the heat transfer. Using this method, they outperform most of the existing magnetocaloric and elastocaloric cooling devices. Using a similar setup, we [Almanza et al. (2018)] have recently shown that the heat transfer can be significantly enhanced, which enables a significant increase in the working frequency—up to 100Hz for a 20um thick film. The resulting output power, which is estimated at around 200W/g, would be of the same order of magnitude as in conventional cooling devices. The key to achieve such working frequencies is a wise balance between film displacement (mechanical switching) and heat transfer characteristic times. Here, we focus on how to reduce the switching time down to a few milliseconds. This issue has been studied in the MEMS field, particularly in RF switches, and micro-pumps. It combines several branches of physics, such as electrostatics, thin film mechanics and hydrodynamics. Here, the main challenge is to understand and harness the airflow and squeeze film damping which limits the film’s dynamic. In this work, we investigate different approaches in order to increase the frequency of our previously-presented electrostatic thermal switch, using an EC film as active material. The measurements will be discussed with regard to a simple model, taking into account the way air affects the dynamic of the moving part. Our results will provide insight on the potential of EC polymers as caloric substance for high cooling power devices