The 2014 Joseph W. Richards Summer Research Fellowship -- Summary Report: Thermogalvanic Waste Heat Recovery in Transportation Energy Systems

Abstract

Waste heat recovery remains an inviting subject for research. Solid state thermoelectric devices have been widely investigated for this purpose, but their practical application remains challenging due to high cost and the inability to fabricate them in geometries that are easily compatible with heat sources. An alternative to solid-state thermoelectric devices are thermogalvanic cells.1-7 The temperature difference between the hot and the cold electrodes creates a difference in electrochemical potential of the redox couples at the electrodes. Once connected to a load, electrical current and power is delivered, converting thermal energy into electrical energy. The aim of this summer project is to extend ongoing research to study the feasibility of incorporating thermogalvanic systems into automobiles. A climate-controlled wind tunnel (Fig. 1) was built to provide equivalent conditions to the ambient air stream under the car. Temperature was controlled using a window air conditioner, while an air mover drove the flow up to 6 m s−1. A heat gun provided the equivalent of a low-temperature exhaust gas stream (~110 oC). The annular cell was bound by concentric copper pipes (electrodes) and CPVC bulkheads; all sealed with silicone. K-type thermocouples were attached with epoxy onto electrode surfaces to measure the electrode temperature, which were monitored and recorded by a Campbell Scientific CR23X Micrologger. The cell potential (E) was probed using a Fluke 8846A Digital Multimeter. An Elenco RS-500 resistor box (Rext) was connected in parallel to measure power output, P = E/Rext. A 0.7 M CuSO4 aqueous electrolyte was used, with 0.1 M H2SO4 as the supporting electrolyte. The Tcold was varied based on the quad-monthly average ambient air temperature (Tambient) in Phoenix, AZ of 31.6, 22.5, and 14.1 oC. Because thermal resistance from the hot air stream to the inner copper pipe is large relative to the thermal resistance of the cell, the Thot was measured to be less than the hot air stream’s temperature. Reducing this resistance would greatly improve the system performance. The experimental values of Thot and Tcold are shown next to their resultant plots in Fig. 2. These results showed that higher Tambient yielded a higher P, because of the higher average cell temperature, Tavg = (Thot+ Tcold)/2. This trend agreed with our previous study.8 Since the resistor could only be Fig 1. 3D CAD drawings of the wind tunnel and the cross-sectional diagram of the annular Cu/Cu2+ thermogalvanic cell.