Waste heat is generated whenever work is performed. Harvesting such waste heat can increase the efficiency of engines, be used to power numerous devices eliminating the need for auxiliary power sources, and significantly reduce power requirements. Thermoelectric materials, however, are usually most efficient at relatively high temperatures. The ability to harvest usable energy from either low quality waste heat (< 100 C) or from small vibrations such as generated by human motion is a long standing problem in need of a solution.
This approach harvests both mechanical and thermal energy by combining nanowires and phase change materials. These devices were fabricated on Kapton® polyamide films and used ZnO nanowires with the same growth direction to assure alignment of the piezoelectric potentials of all of the wires. The circuit was designed as long, parallel electrode arrays perpendicular to the nanowire axis. Good-alignment of the nanowires in this configuration should enable scale-up of the output. Ideally, the total output voltage is the sum of the voltage from individual wires in the vertical direction because they are connected in series. Mechanical harvesting from these devices was demonstrated using a periodic application of force, modeling the motion of the human body. Tapping the device from the top of the device with a wood stick, for example yielded an Open Circuit Voltage (OCV) of 0.2 - 4 V, which is in an ideal range for device applications. To demonstrate thermal harvesting from low quality heat sources, a commercially available Nitinol (Ni-Ti alloy) thin film attached to the nanowire piezoelectric device was bent at room temperature. Upon heating above 50`C, Nitinol thin film restored to its original flat shape, which yielded an output OCV of nearly 1 V. In this work we utilized the piezoelectric properties of ZnO for thermal harvesting as well by combining the ZnO piezoelectric with the shape memory alloy NiTi, sometimes known as nitinol, to create a compound thermoelectric.
For more information see Harvesting Mechanical and Thermal Energy by Combining ZnO Nanowires and NiTi Shape Memory Alloy; Advanced NanoMaterials and Technologies for Energy Sector; 2017:1(1): 13-20.
In the expanding world of small scale energy harvesting, the ability to combine thermal and mechanical harvesting is growing ever more important. In this work, the feasibility of combining a piezoelectric nanowire such as ZnO with a phase change material to harvest both mechanical and thermal energy has been demonstrated. In both cases, optimization of the nanowire device, materials selection and interface geometry bode well for significant improvement over these initial results.
Traditional thermoelectrics do not work by the same principle. These traditional thermoelectrics have a figure of merit which depends on the temperature differential. The greater the range of temperature experienced, the more effective the thermoelectric. Our thermoelectric operates over a very limited temperature across the (tailorable) phase change temperature of the one of our materials. This different operational mode is not produced by any other thermoelectric. Therefore even though many thermoelectrics exist, the ability to produce electric energy from low quality waste heat or small motions is not addressed by the thermoelectrics currently available and may be a competitive advantage in situations where the temperature gradient is low.
We visualize a market for a device that charges sensors from low quality waste heat in remote or difficult to access locations, or for charging devices from human motion.
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