Thermophotovoltaics (TPV) is a promising energy conversion technology for producing electricity from sources of thermal energy. The efficient recovery of waste heat is a key goal of TPV. The TPV process comprises three conceptual stages, in which an emitter first converts a source of thermal energy into infrared light, which is propagated to a collector stage and is subsequently converted into electrical energy. Effective TPV emitters must produce light that couples efficiently to the PV cell. Since PV cells only respond well to light in a limited frequency range near their bandgap, emitters can minimize losses by radiating light only in that frequency range. The resulting losses are much lower than those from conventional blackbody emitters, which radiate much of their light in the wrong spectral regions for efficient energy conversion by the collectors.
New approaches to TPV emitters use electrospun titania nanofibers modified with rare-earth oxides to produce emitters with spectral outputs tuned to emit near the bandgap of GaSb and other commercially available photovoltaic cells. Nanofiber emitters provide the additional benefit of having a geometry that maximizes the ratio between surface area (providing net emission) and volume (causing re-absorption). Furthermore, nanofiber emitters are essentially isothermal, preventing cooler regions from re-absorbing the light radiated by warmer regions.
However, despite the advances in material development, little is known about the individual and interrelating effects of the device properties and overall effectiveness. Pertinent questions are: What influence do geometry and temperature have on the net emission? Is there an optimal concentration and distribution of the rare-earth material for maximum emission?
Our approach to developing a global model of the TPV system is to develop simple models for each of the three system components: the emitter, which converts thermal energy into infrared light; the radiation transport, which determines the proportion of radiated light arriving at the collector; and the collector, itself, which converts this light into electricity. We will then piece the three submodels together, forming a comprehensive model that will predict system efficiency and power density.
Our initial efforts address some aspects of these issues through a series of parametric studies, using a one-dimensional model of a device proposed for automotive applications. A nanofiber-based TPV system could be easily incorporated into a filter structure for the generation of energy from vehicle exhaust streams. The emitter material is wrapped around the outside of the exhaust pipe, which provides the thermal energy source, and emits light radially from the emitter surfaces. We examine the total net irradiance (power density) produced by this device under a number of conditions. An optimal distribution and concentration of rare-earth material is found to exist, and geometric and temperature effects are also observed. The maximum possible power generated from the device under investigation is estimated to be less than 50 Watts.