U. of Minnesota researchers demonstrate new method for direct conversion of heat to electricity
28 June 2011
Researchers at the University of Minnesota have demonstrated a new method for the direct conversion of heat to electricity using a multiferroic alloy, Ni45Co5Mn40Sn10, which they had discovered earlier (Srivastava 2010). An open access paper on the work appears in the journal Advanced Energy Materials.
The researchers say the material could use waste heat from a car’s exhaust to produce electricity for charging the battery. Other possible future uses include capturing rejected heat from industrial and power plants or temperature differences in the ocean to create electricity. The research team is looking into possible commercialization of the technology.
Multiferroic materials combine unusual elastic, magnetic and electric properties. The alloy Ni45Co5Mn40Sn10 achieves multiferroism by undergoing a highly reversible phase transformation where one solid turns into another solid. During this phase transformation the alloy undergoes changes in its magnetic properties that are exploited in the energy conversion device.
Specifically, this alloy undergoes a low hysteresis, reversible martensitic phase transformation from a nonmagnetic martensite phase to a strongly ferromagnetic austenite phase upon heating. When biased by a suitably placed permanent magnet, heating through the phase transformation causes a sudden increase of the magnetic moment to a large value.
As a consequence of Faraday’s law of induction this drives a current in a surrounding circuit. Theory predicts that under optimal conditions the performance compares favorably with the best thermoelectrics. Because of the low hysteresis of the alloy, a promising area of application of this concept appears to be energy conversion at small ΔT, suggesting a possible route to the conversion of the vast amounts of energy stored on earth at small temperature difference.
—Srivastava 2011
Even an optimized version of the demonstration has low efficiency, Srivastava et al. note in their paper. However, the predicted voltage output for an optimized device is comparable to that of good thermoelectric materials and the predicted power density of an optimized device is more than an order of magnitude greater than thermoelectrics with the highest known ZT value, they said.
| In the lab, University of Minnesota researchers show how the multiferroic material begins as a non-magnetic material then suddenly becomes strongly magnetic as the piece of copper below is heated a small amount. When this happens, it jumps over to a permanent magnet. |
Funding for early research on the alloy came from a Multidisciplinary University Research Initiative (MURI) grant from the US Office of Naval Research (involving other universities including the California Institute of Technology, Rutgers University, University of Washington and University of Maryland), and research grants from the US Air Force and the National Science Foundation. The research is also tentatively funded by a small seed grant from the University of Minnesota’s Initiative for Renewable Energy and the Environment.
Resources
Srivastava, V., Song, Y., Bhatti, K. and James, R. D. (2011), The Direct Conversion of Heat to Electricity Using Multiferroic Alloys. Advanced Energy Materials, 1: 97–104. doi: 10.1002/aenm.201000048
Vijay Srivastava, Xian Chen, and Richard D. James (2011) Hysteresis and unusual magnetic properties in the singular Heusler alloy Ni45Co5Mn40Sn10, Appl. Phys. Lett. 97, 014101 doi: 10.1063/1.3456562
The energy conversion efficiency is so low (1% to 3% ?) that it could not be mentioned.
Posted by: HarveyD | 28 June 2011 at 12:46 PM
It seems the value here lies not only in the inductive current - but in the material's kinetic potential. It should be relatively easy to build a motor. In the mold of Sterling where instead of a piston a stator/rotor is alternately heated and cooled (by convection) causing the rotor to turn. If the T delta is small enough almost any source of heat would turn the motor.
We can think of many good sources of waste heat starting with Earth's magmatic core.
Posted by: Reel$$ | 28 June 2011 at 01:20 PM
I'm all for searching breakthroughs on recovery otherwise wasted energy, but I wonder what the minimum conversion efficiency has to be to justify the weight, cost, volume of these schemas? So far these seem to either be variations on a steam-turbine, or an infrared PV cell, or such, and they either require very high temperatures or return tiny 1-3% conversion efficiency.
Posted by: HealthyBreeze | 29 June 2011 at 02:27 PM