Abstract
Converting blackbody thermal radiation to electricity via thermophotovoltaics (TPV) is inherently inefficient. Photon recycling using cold-side filters offers potentially improved performance but requires extremely close spacing between the thermal emitter and the receiver, namely a high view factor. Here, we propose an alternative approach for thermal energy conversion, the use of an integrated photonic crystal selective emitter (IPSE), which combines two-dimensional photonic crystal selective emitters and filters into a single device. Finite difference time domain and current transport simulations show that IPSEs can significantly suppress sub-bandgap photons. This increases heat-to-electricity conversion for photonic crystal based emitters from 35.2 up to 41.8% at 1573 K for a GaSb photovoltaic (PV) diode with matched bandgaps of 0.7 eV. The physical basis of this enhancement is a shift from a perturbative to a nonperturbative regime, which maximized photon recycling. Furthermore, combining IPSEs with nonconductive optical waveguides eliminates a key difficulty associated with TPV: the need for precise alignment between the hot selective emitter and cool PV diode. The physical effects of both the IPSE and waveguide can be quantified in terms of an extension of the concept of an effective view factor.
Highlights
Thermophotovoltaics (TPV) convert heat to electricity via thermal radiation
Several materials have been proposed for selective emission, including plasmonic metamaterials,[2,3,4] refractory plasmonic structures,[5] rare earth materials,[6,7,8] and photonic crystals (PhCs).[9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]
In this work, we propose an alternative approach for high-performance TPV systems capable of simultaneously addressing problems with long-lasting selectivity and alignment, known as an integrated photonic crystal selective emitter (IPSE), and develop a corresponding framework to analyze its performance
Summary
Thermophotovoltaics (TPV) convert heat to electricity via thermal radiation. Photons with energies below the bandgap of the photovoltaic (PV) diode, resulting from the broad spectrum of the Planck blackbody distribution, are generally the dominant source of loss in TPV systems. Significant improvement can be achieved by the use of cold-side PhC filters, including plasma filters, quarter-wave stacks,[25] and rugate filters.[26] These filters essentially reflect the low-energy photons back to the selective emitter, in a process known as photon recycling.[27,28,29,30,31,32] In order to achieve sufficient photon recycling, proximity between the emitter and filter is required.[33] In the typical cold-side filter configurations, where the filter is attached to the PV diode as an entire receiver, this requirement can be quantified by the view factor from the emitter to the receiver, which is the probability that emitted photons reach the receiver Certain strategies, such as micro-gap or nanoscale-gap TPV, require extremely. The IPSE is relatively amenable to fabrication in the near term, with key features already achieved in previous experimental work.[20,21,22,25,26]
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