Advances in Gas-Fired Thermophotovoltaic Systems

In a first and completely new approach, a vacuum plasma-spray coating technique was used to deposit selective emitting rare-earth oxide films of ytterbia $(Yb2O3)$ on porous silicon-infiltrated silicon carbide foams (Si–SiC). The plasma-spray coating technique offers a new and promising way to produce selective emitting coatings on different refractory substrates with complex geometries. The adhesion and thermal shock stability were tested until a film thickness of $130μm$ was achieved; the selective emittance of the oxide coating has been found to be dependent on the film thickness. The material combination Si–SiC and $Yb2O3$⁠, however, needs some major improvement regarding high-temperature stability and high thermal cycling loads. In a different approach, the advantage of low emitting $Al2O3$ fibers and good thermal matching was combined with $Yb2O3$ slurry coating of flexible alumina $(Al2O3)$ fiber bundles, formed into a cylindrical shape. The thin fiber structure tried to imitate the famous incandescent mantle emitters of Auer von Welsbach, but with a more rugged structure. Even though the fibers of the woven emitter were thin, the low thermal conductivity of $Al2O3$ led to a distinct reduction of the surface temperature and emittance, and a shielding effect of the radiation emanating from the hot inner walls by the cooler outer grid structure was inevitable. Optical filters consisting of a water film and of transparent conducting oxides (TCO) have been developed and tested to protect the photocells against overheating and to reflect nonconvertible off-band radiation back to the emitter. The water film led to a significant reduction of the cell temperature and increased cell performance, whereas with the TCO filters only a reduction of the cell temperature was observed.