Thermophotovoltaic Spectral Control

Abstract

Spectral control is a key technology for thermophotovoltaic (TPV) direct energy conversion systems because only a fraction (typically less than 25%) of the incident thermal radiation has energy exceeding the diode bandgap energy, E g , and can thus be converted to electricity. The goal for TPV spectral control in most applications is twofold:1. Maximize TPV efficiency by minimizing transfer of low energy, below bandgap photons from the radiator to the TPV diode. 2. Maximize TPV surface power density by maximizing transfer of high energy, above bandgap photons from the radiator to the TPV diode.TPV spectral control options include: front surface filters (e.g. interference filters, plasma filters, interference/plasma tandem filters, and frequency selective surfaces), back surface reflectors, and wavelength selective radiators. System analysis shows that spectral performance dominates diode performance in any practical TPV system, and that low bandgap diodes enable both higher efficiency and power density when spectral control limitations are considered. Lockheed Martin has focused its efforts on front surface tandem filters which have achieved spectral efficiencies of ~83% for E g = 0.52 eV and ~76% for E g = 0.60 eV for a 950 o C radiator temperature.

Spectral Control for Thermophotovoltaic Energy Conversion

“…Front surface spectral control using tandem filters has achieved the highest spectral efficiency of any spectral control configuration to date (References (22)(23)(24)). In contrast, both the modeled and predicted performance of frequency selective surfaces as front surface, spectral control have achieved significantly lower spectral efficiencies and above band gap transmission performance than tandem filters (Reference (25)).…”

Spheroid-based optical cavities for tunable photon recycling and emitter temperature control in robust solar thermophotovoltaic systems

“…This reflective spectral control scheme is achieved by either using an optical filter between the PV cell and the emitter 18,19 or by reflecting photons transmitted through the PV cell by coating a reflector onto its rear surface. 20,21 Design strategies for decreasing directional losses and achieving a high emitter-to-PV cell view factor include using angular selective emitters, [22][23][24] and employing a cage-type configuration wherein the emitter is completely surrounded by an array of PV cells. [25][26][27] Other technologies to significantly enhance the transfer of emitted thermal energy to the PV cell include near-field (micron gap) and light-pipe TPV systems.…”

“…Another strategy is to reflect out-of-band photons such that they are returned to the emitter. This reflective spectral control scheme is achieved by either using an optical filter between the PV cell and the emitter 18 , 19 or by reflecting photons transmitted through the PV cell by coating a reflector onto its rear surface 20 , 21 …”

Thermophotovoltaic Spectral Control

“…The highest TPV spectral performance to date has been achieved using a front surface tandem filter [9,10,11]. As shown in Figure 9, the tandem filter concept is the combination of a plasma filter with an interference filter [12,13,14].…”