A Multi-Physics Model of a Novel Thermal Energy Harvesting Device
Dr. Adrienne Lavine
Chair, Department of Aerospace and Mechanical Engineering
University of California, Los Angeles
Energy harvesting is critical in supporting networked systems of low-power sensors and other remote/portable devices, which are being developed for a wide variety of scientific and engineering applications. Nanostructured thermoelectric (TE) materials hold great promise in harvesting thermal energy for these devices, but the demonstrated device-level performance has been rather disappointing. This has motivated exploration of new device concepts to efficiently convert low-quality thermal energy into electrical energy.
We have developed a multi-physics model to facilitate rigorous evaluation and systematic optimization of a novel two-step thermal energy harvesting device. The device first converts thermal energy into mechanical energy by exploiting the temperature dependence of the magnetization of a soft ferromagnet. The ferromagnet is suspended via a spring-like structure between a cold surface and a permanent magnet attached to a heat source. The mechanical oscillation of the ferromagnet can then be converted into electrical energy via various possible mechanisms. A prototype device has recently been demonstrated to operate for a temperature differential as small as 10 degrees and at a power density over 10 mW/cm2.
Our multi-physics model simultaneously solves the equation of motion and the equation of conservation of thermal energy. The model incorporates the temperature and spacing dependence of the magnetic force between the soft ferromagnet and the permanent magnet. A key physical mechanism that affects the oscillation frequency and hence the power density is identified as heat conduction across solid-solid interfaces when the ferromagnet is in contact with either side.
The predicted device performance agrees reasonably with the initial experimental data. Our model has allowed a systematic parametric study to quantitatively evaluate the effects of the design parameters, including the spring constant, the geometric parameters, the contact conductance, and the properties of the ferromagnet. This in turn enables design optimization to maximize the power output and energy conversion efficiency of our thermal energy harvester.
Adrienne Lavine is currently Professor and Department Chair in the Mechanical and Aerospace Engineering Department at the University of California, Los Angeles. She began her academic career there in 1984 as an Assistant Professor after obtaining her Ph.D. in Mechanical Engineering from the University of California, Berkeley in the same year. Her research addresses various aspects of heat transfer, including thermal energy harvesting, temperature control for nanomanufacturing, thermal aspects of manufacturing processes (e.g. grinding, cutting, and plasma spray), and thermomechanical behavior of shape memory alloys. She has published over 80 papers in journals and refereed conference proceedings. Her honors and awards include the Presidential Young Investigator Award from NSF (1988), the Taylor Medal of CIRP (1990), and the Best Superconductivity Paper Award from ASME (1989). She was elected Fellow of ASME in 1999. She has served as an Associate Editor of two ASME journals: the Journal of Heat Transfer(1995-98) and the Journal of Engineering for Industry (1991-94). She is presently the Associate Director for Education and Outreach for the NSF-sponsored Center for Scalable and Integrated Nano-Manufacturing. In 2006-07 she served a term as Chair of the UCLA Academic Senate.