Energy Harvesting Based on Bistable Shape Memory Film Actuation

Most microsensors, microdrivers, and low-power devices for wearable devices require continuous power supply through batteries. However, with the increase in the number and density of electronic devices, the demand for the number of batteries will increase rapidly, which brings many severe problems such as battery recycling and waste battery pollution. In order to overcome these drawbacks, it is of practical significance to develop micro-energy technology to harvest energy from the surrounding environment. Among the different energy sources, thermal energy is widely present in industrial production but often neglected and not effectively used. Direct thermoelectric conversion requires large temperature gradients and it is difficult to sustain sufficient temperature gradients for practical applications. Besides, miniaturization is necessary to enable thermoelectric energy harvesting technology to be more competitive compared to batteries. There are still many difficulties in the miniaturization of current thermoelectric technology. When the volume is miniaturized to a certain extent, heat dissipation by air convection will not be able to meet the cooling demand and large heat sinks are needed, thus limiting the miniaturization. ... mehrThermoelastic materials that can produce work by heating or cooling open up new approaches to overcome the limitations of miniaturized thermal energy harvesting systems.
Shape memory alloy (SMA) microactuators enable the conversion of modest heat quantities into usable work output within a limited temperature range due to high power/weight ratio, low cost and easy machining. Additionally, SMA film becomes an attractive candidate for thermal energy harvesting, since the SMA film produces substantial stress on heating and promises efficient heat transfer property due to the high surface-to-volume ratio. At a certain critical temperature, the SMA undergoes a phase transformation dominated by shear distortions of the crystal lattice, which can be utilized to drive mechanical systems or generate electrical energy through oscillation induced by the shape memory effect.
This work presents a SMA-based bistable actuator and a miniature-scale thermal energy harvester, which can be applied in a low-grade temperature range for waste heat recovery. The starting material is an unstructured free-standing TiNiCu film of 15µm thickness, which has been prepared by magnetron sputtering followed by crystallization by rapid thermal annealing. The bistable SMA film actuator consists of two antagonistically coupled pre-deflected SMA beams, a spacer and two heat sources. The antagonistic coupling beams achieve a bi-directional snapping motion by alternately heating, which results in an oscillatory snapping motion. The stationary force-displacement characteristics of monostable actuators consisting of single buckling SMA beams and bistable actuators are characterized with respect to their geometrical parameters. The dynamic performance of bistable actuation is investigated by selectively heating the SMA beams via direct mechanical contact with a heat source below the temperature of 190°C. The bistable actuation is characterized by a large stroke up to 3.65 mm, corresponding to more than 30% of the SMA beam length. Operation frequencies are in the order of 1 Hz depending on geometrical parameters and heat source temperature.
When the bistable SMA actuator is located in the magnetic field induced by a miniature permanent magnet and the spacer is replaced with a miniature coil, the pick-up coil can convert the mechanical oscillation to electricity according to Faraday's law. After optimization of the magnet position, the heat source position, the heat source temperature, the distance between two beams, and the turns of the pick-up coil, thermal energy harvesting demonstrated a maximum output voltage of 57 mV with a thermal actuation frequency of 1.2 Hz. By reducing the number of turns of the pick-up coil to 100 turns, the oscillation frequency of the bistable energy harvester can be increased to 1.56 Hz, an improvement of 30%. After optimizing the external resistor, the maximum peak power density reaches 16.2 μW/cm2. In addition to actuators, SMA beams can also be used as electrodes for triboelectric nanogenerators (TENG). The SMA beams are able to contact and separate from a Kapton film attached to the heat source during selective heating of the bistable actuator. Relying on the principles of contact initiation and electrostatic induction, the contact-separation mode TENG generates a peak power density of 2.4 μW/cm2. Different energy harvesting units can either work independently and simultaneously or be connected in series or parallel to increase the power density, which can increase the total output of the energy harvesting device.
The results of this work provide the basis for the design of novel waste-heat thermal actuators operating at low-temperature differences. The bistable thermoelastic energy harvester presents an environmentally friendly approach for converting waste heat energy from the surrounding environment into reusable electrical energy.