Participants: C. B. Clemons (Theo and Applied
Math), G. W. Young (Theo and
Applied Math)
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.
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