Zero-Vacuum-Gap Thermophotovoltaics: A Scalable and High-Power Alternative to Near-Field TPV

Thermophotovoltaic (TPV) devices are solid-state energy conversion systems that generate electricity from infrared thermal radiation emitted by a high-temperature source. These systems have significant potential for energy storage, industrial waste heat recovery, and renewable energy applications, particularly in concentrated solar power and nuclear reactors. Despite advances in TPV efficiency, the challenge of increasing power density has remained largely unaddressed, restricting the practical implementation of TPVs in moderate-temperature applications (700–1100°C). Current TPV technologies primarily rely on far-field configurations, where a macroscopic vacuum or gas-filled gap separates the thermal emitter and photovoltaic (PV) cell. However, this setup fundamentally limits power density due to Planck’s blackbody radiation law, which governs thermal emission. Near-field TPV has been proposed as a solution, leveraging evanescent wave coupling across nanometric vacuum gaps to enhance radiative heat transfer. However, near-field TPV suffers from practical challenges, including fabrication complexity, stringent surface smoothness requirements, and scalability limitations.  Another critical limitation in current TPV designs is the reliance on ultra-high temperatures to boost power output. Most TPV devices with high power density operate at emitter temperatures exceeding 1500°C, requiring materials with extreme thermal stability. These high temperatures contribute to rapid material degradation, reducing system longevity and making large-scale implementation challenging. Additionally, efficient TPV energy harvesting at lower temperatures remains an unsolved problem, preventing TPVs from being effectively integrated into many industrial processes where moderate temperatures dominate.

To this account, new research paper published in Energy & Environmental Science and conducted by Dr. Mohammad Habibi, Sai Yelishala, Yunxuan Zhu, and Professor Longji Cui from the University of Colorado Boulder together with Dr. Eric Tervo  and Dr. Myles Steiner from the National Renewable Energy Laboratory developed a novel approach: the zero-vacuum-gap TPV (z-TPV) concept. This architecture eliminates the vacuum gap and replaces it with a high-index, thermally insulating dielectric spacer, such as fused quartz. By allowing the transmission of high-wavevector modes previously inaccessible in conventional far-field devices, this method enables significant power density enhancements without requiring extreme emitter temperatures or complex nanofabrication techniques. Unlike near-field TPV, which relies on vacuum-separated nanogaps, z-TPV provides a practical solution for large-area, manufacturable devices while still harnessing high power output. To experimentally validate the zero-vacuum-gap TPV concept, the researchers fabricated and tested TPV devices using fused quartz as the spacer, tungsten and graphite as the emitters, and InGaAs as the PV cell. The experimental setup involved heating the emitter using an electrical heater while measuring the power generation and heat flux at different emitter temperatures (700–1100°C). Power output was compared between conventional far-field TPV devices and the newly designed z-TPV system. Device fabrication followed a multi-step process to ensure accurate structural integrity. Thick emitter films of tungsten and graphite were sputtered onto end-polished fused quartz rods with a roughness of approximately 1–5 nm. These rods were then bonded to the PV cells using an optically transparent epoxy layer (10 µm thick), ensuring an effective gapless interface between the dielectric spacer and the PV cell. A ceramic glue was applied to secure thermal contact between the heater and the emitter, while thermocouples were positioned to monitor temperature stability. A microchannel cold plate heat exchanger was used to maintain PV cell temperature, reducing heat accumulation that could otherwise degrade performance.

The authors demonstrated a two-fold increase in power density in z-TPV compared to the traditional far-field configuration. Remarkably, the graphite emitter surpassed the blackbody limit for conventional gap-integrated far-field TPVs, achieving power densities comparable to near-field TPV devices with a 200-nm vacuum gap. The enhancement was attributed to the ability of the dielectric spacer to convert evanescent waves into propagating waves, thereby increasing radiative energy transfer.  To ensure robustness, the researchers measured current-voltage (J–V) characteristics using a four-wire method. The maximum power output was calculated using P_max = I_sc * V_oc * FF, where I_sc represents the short-circuit current, V_oc the open-circuit voltage, and FF the fill factor. The results showed that even at lower emitter temperatures, z-TPV achieved significantly higher current densities than conventional far-field TPV, demonstrating its suitability for moderate-temperature applications. Further analysis revealed that the z-TPV concept offers a scalable and cost-effective solution for increasing TPV power density. By optimizing the spacer material, such as using amorphous silicon (a-Si) instead of fused quartz, the researchers predicted an order-of-magnitude improvement in power output. Simulations indicated that z-TPV could achieve power densities equivalent to near-field TPV with a 25-nm vacuum gap, a scale that is virtually unattainable with existing microfabrication techniques.

A key limitation identified in z-TPV was heat conduction through the dielectric spacer, which contributed to energy loss and potential PV cell heating. However, extending the spacer length from 2 cm to 10 cm significantly mitigated conductive losses, improving the overall energy conversion efficiency to nearly 19%. Additional thermal insulation layers were incorporated to minimize unwanted conduction, and alternative spacer materials with lower thermal conductivity were explored through modeling. Future optimizations include refining the thickness and optical properties of the spacer to balance power density and efficiency further.

 The findings of this study represent a fundamental shift in TPV technology, demonstrating that high power densities can be achieved without requiring ultra-high emitter temperatures or complex near-field architectures. By introducing the zero-vacuum-gap TPV concept, this work provides a scalable and manufacturable alternative to existing TPV technologies, offering a practical route for waste heat recovery and renewable energy applications.  In real-world applications, z-TPV has the potential to unlock previously inaccessible heat sources, such as moderate-temperature industrial processes (chemical, steel, and cement industries), residential heat cogeneration, and space-constrained energy storage systems. The ability to generate high power density at lower emitter temperatures can also extend TPV viability to lightweight and portable power generation systems. Given its compatibility with large-area manufacturing, z-TPV could be integrated into waste heat harvesting in industrial plants, significantly improving energy efficiency and reducing carbon emissions. Future research should focus on optimizing spacer materials to further enhance power output and efficiency. Amorphous silicon, for example, could enable a 17-fold power density enhancement, rivaling the best near-field TPV devices. Additionally, integrating high-efficiency PV cells with minimized optical and thermal losses could push z-TPV efficiency beyond 30%, making it a viable competitor to traditional energy conversion methods. Another crucial direction is improving spectral control to reduce sub-bandgap photon losses in the dielectric spacer. Selective emitters, structured metasurfaces, and embedded optical filters could help refine the emission spectrum, ensuring better spectral matching with the PV bandgap. Combined with new PV materials featuring ultra-low bandgap energy, z-TPV could be adapted for lower-temperature energy harvesting, broadening its application scope. Ultimately, the introduction of zero-vacuum-gap TPV presents a transformative opportunity for energy conversion technologies. By balancing power density, efficiency, and manufacturability, this approach provides a compelling alternative to existing TPV architectures. As further advancements in materials and device integration emerge, z-TPV could play a critical role in the next generation of sustainable energy solutions.