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Thermomagnetics

 

In renewable energy, approaches exist to harvest energy from mechanical (wind and water) and solar sources at multiple scales but a successful approach to harvest thermal energy at similar levels is unavailable. In 2017, the US generated 66.7 quadrillion BTU of waste heat where 75% was low-grade (≤230°C) [1]. Further, low-grade waste heat accounts for 45% of the work potential of total waste heat and assuming a cold side temperature of 25°C, a Carnot efficiency of ~40% [2]. Therefore, developing thermal energy harvesting that can operate at this temperature level is an important area of opportunity. Thermomagnetic (TM) materials convert heat to electricity using magnetic transduction. The “magnetic” and “non-magnetic” state of a TM material is temperature dependent and can be switched via thermal cycling.  I use this capability to investigate how material selection, device design and thermal parameters affect power generation and efficiencies. In [3], a method to determine the conversion efficiency of TM materials was introduced and various thermomagnetic materials were evaluated. This led me to introduce the concept for an active thermomagnetic device that can harvest low-grade waste heat [4,5]. My device is comprised of gadolinium (TM material) traveling between a heat source/permanent magnet and heat sink surrounded by a copper coil. Using FEA simulations, I identified optimal device configuration, operating conditions and the effects of contact conductance and applied magnetic field. Though preliminary optimal operating parameters have been assessed, further attention must be paid towards developing materials with higher conversion efficiencies, implementing varied applied fields and system engineering.

 

[1]       Lawrence Livermore National Laboratory, 2017, “Energy Flow Charts: Charting the Complex Relationships among Energy, Water, and Carbon” [Online]. Available: https://flowcharts.llnl.gov/. [Accessed: 30-Jun-2018].

[2]       ARPA-E, D. of E., 2016, “Request For Information on Lower Grade Waste Heat Recovery DE-FOA-0001607.”

[3]       Wetzlar, K. P., Keller, S. M., Phillips, M. R., and Carman, G. P., 2016, “A Unifying Metric for Comparing Thermomagnetic Transduction Utilizing Magnetic Entropy,” J. Appl. Phys., 120(24), p. 244101.

[4]       Phillips, M.R., Carman, G. P., 2017, “Investigating Thermomagnetic Materials for Energy Harvesting,” International Mechanical Enegineering Congress and Exposition, Tampa.

[5]       Phillips, M.R., Carman, G. P., 2018, “Numerical Analysis of an Active Thermomagnetic Device for Thermal Energy Harvesting,” ASME J. Energy Resour. Technol.