A recent paper in NPJ | Nanophotonics introduced a hybrid metal-dielectric non-Hermitian emitter (NHE) developed for thermophotovoltaic (TPV) systems. This emitter is designed to improve the efficiency of thermal energy conversion to electricity, particularly from low- and mid-grade heat sources. This approach addresses key limitations of conventional TPV systems and offers potential advancements in waste heat recovery and renewable energy applications.
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Thermophotovoltaic Energy Conversion
TPV is an emerging technology that converts thermal energy into electricity by emitting thermal photons. Unlike thermoelectric systems, which rely on temperature gradients, TPV systems utilize light for energy conversion. Their efficiency depends on the ability to manage the spectral emission of thermal photons, particularly by minimizing emissions outside the energy range useful for conversion.
Selective thermal emitters are essential for optimizing TPV performance. These emitters are engineered to radiate thermal photons within an energy range matching the bandgap of the photovoltaic (PV) cells. The primary challenge is enhancing the spectral efficiency of these emitters while maintaining thermal stability at high operating temperatures.
NHE: A Novel Approach for TPV Applications
In this paper, the authors designed, fabricated, and tested the NHE to achieve high spectral efficiency for TPV applications. The structure combines metallic and dielectric materials to optimize performance. Tungsten, used as the metallic substrate, functions as both a heat exchanger and a plasmonic material, while the dielectric component improves spectral selectivity.
The design features cylindrical silicon resonators placed on a 0.5 mm thick tungsten substrate, separated by a silica spacer. This configuration leverages the non-Hermitian properties of thermal emitters to support distinct modes with varying damping rates. This enables the simultaneous achievement of high brightness and spectral selectivity, two traditionally conflicting properties.
The cylindrical resonators are arranged in a hexagonal lattice, creating a quasi-bound state-in-the-continuum (q-BIC) mode through the destructive interference of electric and magnetic dipolar modes. This allows for highly localized, low-loss thermal emission.
The NHE was fabricated using standard planar nanofabrication techniques, including plasma-enhanced chemical vapor deposition and electron-beam lithography. The thickness of the silica spacer was carefully optimized to improve coupling between resonator modes, enhancing overall performance.
The researchers measured the NHE's emissivity spectra at various temperatures, focusing particularly on 1273 K. Testing was conducted in a vacuum chamber to minimize thermal degradation and preserve the emitter's integrity.
Key Outcomes and Insights
The experimental findings demonstrated that the hybrid metal-dielectric NHE achieved a spectral efficiency exceeding 60 %, significantly surpassing the limitations of conventional selective thermal emitters. At 1273 K, the NHE maintained a spectral selectivity efficiency of 45 % with minimal degradation, highlighting its thermal stability and practical applicability. This efficiency is notably higher than traditional metal-based emitters, which typically range between 30 % and 50 %. The design effectively separates photon generation and storage, thus enhancing both brightness and spectral selectivity.
The study also showed that the NHE's spectral efficiency was significantly higher than that of traditional blackbody emitters across a wide range of bandgap energies. When the bandgap energy ranged from 0.65 to 0.75 eV, the NHE demonstrated a spectral efficiency over 300 % higher than that of a blackbody emitter. This finding is significant for optimizing TPV systems, highlighting the NHE's ability to improve the conversion of thermal energy into electricity.
The authors developed a complete TPV system by integrating the NHE with a commercially available gallium antimonide (GaSb) PV cell. This system produced a maximum power output of 10.8 mW at an emitter temperature of 1173 K, corresponding to an overall conversion efficiency of approximately 0.2 %. While this efficiency is relatively modest, it underscores the potential for improvement through further optimization of both the PV cell and the thermal emitter.
Potential Applications in Energy Conversion
This research has significant potential for energy conversion applications, particularly waste heat recovery. Efficiently converting low- and mid-grade thermal energy into electricity could support the creation of more sustainable energy systems. This is especially relevant in industrial settings where excess heat is often wasted.
The hybrid NHE design could improve solar TPV devices by concentrating sunlight to achieve high temperatures, thereby enhancing electricity generation efficiency. These findings highlight the importance of optimizing the spectral properties of thermal emitters to improve performance in energy harvesting applications and support broader adoption in sustainable technologies.
Conclusion
The NHE demonstrated notable advancements for TPV systems by achieving high spectral efficiency and maintaining thermal stability at elevated temperatures, both essential for practical applications. As the demand for efficient energy conversion technologies increases, this study provides a foundation for further innovations in TPV systems. Addressing challenges related to PV cell performance and incorporating advanced materials could further enhance system efficiency.
The successful implementation of such technologies could contribute to sustainable energy solutions, reducing reliance on fossil fuels and supporting global efforts to address energy and environmental challenges.
Journal Reference
Samuel Prasad, C., Naik, GV. (2024). Non-Hermitian selective thermal emitter for thermophotovoltaics. npj Nanophoton. DOI: 10.1038/s44310-024-00044-3, https://www.nature.com/articles/s44310-024-00044-3#ref-CR3
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