Non-imaging optics in a thermophotovoltaic generator

In order to achieve high efficiency in a TPV generator, it is important that a high fraction of emitted photons with energies below the TPV cell bandgap are reflected back to the emitter. This can be accomplished in several ways. We present the idea of an internally reflecting egg-shaped double cone with the emitter at one end, an edge filter at the wide center, and the TPV array at the other end. So far, we have studied this geometry by means of both ray tracing analysis in a computer program and by means of measurements with an emulated emitter. A sharp switchover from transmission to reflection in a multiple layer edge filter can be achieved only if the angles of incident rays are confined to a fairly narrow angular interval. The two used methods both show that the studied optics can lower the angular spread of rays incident onto the filter and that some 96% of the emitted rays (in the ideal case) reach their goal without passing the filter or being reflected by the filter more than once. A suggestion of a whole wood powder fuelled TPV system with this egg-shaped double cone and edge filter implemented is also given, as well as an animation tool for modelling the optical part of the TPV system.