OCR Text |
Show Emitter Cooling fan PV cells The emitter temperature must be high enough to generate a significant fraction of radiation above the bandgap energy of the photovoltaic cells. This system used GaSb cells manufactured at JXC. GaSb cells require an emitter temperature to be at least 2400°F to produce an acceptable power density. The P V cells must also be exposed to a uniform emitter heat flux or temperature to operate efficiently. The P V circuits have individual cells linked in series, and the cell adjacent to the lowest temperature region of the emitter limits the current output of the circuit. Furthermore, the burner must be fabricated from materials that will withstand the high operating temperatures. Finally, the burner must be the right size and shape to deliver the diesel fuel flow and be integrated into the current design concept. A commercially available burner that satisfied all of these design criteria was not found. Therefore, M T I developed a custom burner for this purpose. Meeting burner design goals depends on how well other T P V components meet their design goals. For instance, a burner that achieves high B E R efficiency is conducive to producing high emitter temperatures. However, the largest influence on the emitter temperature is P C A efficiency. Numerical simulations with a proprietary M T I combustion model ( C O M O ) and a P C A cavity model predicted that the P C A must recycle a large portion of the out-of-band energy back to the emitter. Otherwise, emitter temperatures above 2400 F will not be attained. Radiator Burner Recuperator Combustion air Figure 2: Portable TPV generator design concept The burner design is very influential in achieving a uniform emitter temperature. The emitter is heated by convection from the flue gas and radiation from the radiator. The convection component is greatest at the top, where the combustion effluent is at the highest temperature. The radiation is greatest near the burner, where the radiator is heated most intensely. The C O M O model predictions show that the system design can be tailored so that the two heat transfer mechanisms offset to yield a relatively uniform emitter temperature. Figure 3 shows computer simulations for one particular system geometry that generated a reasonably flat emitter temperature profile. The models indicated that a short flame was conducive to a relatively uniform emitter profile by preferentially heating the lower radiator. T o achieve a short flame, a burner was designed with a thermally vaporized fuel source and intense fuel / air mixing. Partial rapid premix of the combustion air with the vaporized fuel reduced the ignition delay and burnout time. High swirl velocity induced intense mixing by flow recirculation. High swirl velocity also propelled the flame nearly directly at the lower radiator wall, inducing flame impingement and a high rate of convection heat transfer. |