Electrohydrodynamic Thermal Oscillators for Waste Heat Harvesting Applications

The properties of pyroelectric materials have made them prime candidates for efficient waste heat harvesting. When placed adjacent to a thermal oscillator, structural changes occur within the pyroelectric crystal, generating a voltage and producing power through the Olsen cycle. Previously, thermal oscillators including electrowetting on dielectric (EWOD) devices and microelectronic mechanical systems (MEMS) have been used in waste heat harvesting applications. However, these devices have been hard to apply efficiently and inexpensively on a large scale. Therefore, there is a great need for a high efficiency, scalable thermal oscillator. This work attempts to design and test the efficiency of a novel liquid-based thermal oscillator which utilizes electrohydrodynamic (EHD) capillary bridging and debridging to periodically heat and cool a pyroelectric layer. The device itself consists of a parallel-plate capacitor setup with a heated top plate and a room temperature bottom plate. A droplet was placed on the bottom plate where it acted as a thermal oscillator and transferred heat from the hot plate to the cool plate by forming and breaking capillary bridges between the plates when exposed to an electric field. Preliminary testing of liquids was conducted in order to observe various modes of liquid behavior when under an electric field. It was determined that there were five distinct fluid behaviors, and these were dependent on liquid properties such as surface tension, electric conductivity, and dielectric constant, as well as the applied voltage and gap width of the device. Additionally, by testing various liquids, we were able to determine which fluids were able to successfully form and break capillary bridges when voltages were turned on and off respectively. In most cases, high molecular weight fatty acids and triglycerides, usually unsaturated and with polar groups, were most effective at forming and breaking these capillary bridges. Next, profiles of the droplets were extracted from videos of the thermal oscillator and used to calculate electrical, surface, and gravitational energy using finite element method simulations in COMSOL. It was observed that each liquid behavior exhibited unique energy configurations, indicating that differences in free energy change are the driving force in producing the various liquid behaviors. Finally, preliminary thermal tests were conducted with the pyroelectric layer and liquid droplet EHD switches, resulting in thermal cycling of 0.35-0.5°C and a power density of 12.15 W/m 2 produced through the Olsen cycle. With continued testing, this technology can overcome current roadblocks in applying pyroelectric materials for waste heat harvesting by providing efficient and scalable thermal cycling.