A breakthrough in thermophotovoltaic technology
The latest innovation in waste heat recovery comes from the United States. A team of engineers from the University of Colorado Boulder, in collaboration with the University of Wisconsin and the National Renewable Energy Laboratory (NREL), has developed a direct-contact thermophotovoltaic system that challenges fundamental laws of thermal radiation.
According to the researchers, this device has the potential to revolutionize industries, manufacturing plants, and even power stations by harnessing heat losses without the need for extreme temperatures or expensive materials.
How thermophotovoltaics work
Thermophotovoltaic (TPV) systems operate on a simple principle: they use solar cells to convert infrared radiation emitted by a hot object into electricity. Unlike conventional photovoltaics, which rely on sunlight, TPVs can utilize thermal energy from any high-temperature heat source.
The efficiency of a TPV system depends largely on its design. Traditional thermophotovoltaic devices, known as far-field TPVs, consist of a thermal emitter and a photovoltaic cell separated by a small vacuum or air-filled gap.
In TPV technology, solar units are primarily composed of narrow-bandgap semiconductors from III-V or II-VI material families, such as indium arsenide (InAs), lead selenide (PbSe), indium antimonide (InSb), germanium (Ge), and gallium antimonide (GaSb).
The limits of Planck’s law in far-field TPVs
The two key performance metrics for TPV systems are efficiency and power density (W/cm²). While efficiency improvements have been significant – ranging between 33% and 44% – power density has received less attention. Theoretically, higher operating temperatures yield greater power densities.
However, this relationship is constrained by Planck’s law of black-body radiation. “Planck’s law, one of the most fundamental principles in thermal physics, limits the amount of thermal energy that can be extracted from a high-temperature source at a given temperature,” explains Professor Longji Cui of CU Boulder. “Researchers have attempted to reach or surpass this limit using various techniques, but current methods are too complex, costly, and not scalable.”
The direct-contact TPV: zero-vacuum-gap technology
This is where Cui’s team introduces a game-changing innovation: the direct-contact thermophotovoltaic, also known as zero-vacuum-gap TPV. Compact and highly efficient, this device surpasses the power density limits imposed by Planck’s law, achieving nearly double the power density of conventional TPVs at moderate temperatures.
According to the researchers, “By incorporating a fused quartz spacer—a high-index, infrared-transparent, and thermally insulating material—we doubled the power density compared to far-field TPVs under identical conditions. Specifically, in our experiment, the direct-contact design transformed a low-power-density far-field device into one of the highest power-density TPVs ever reported at temperatures between 700°C and 1,100°C.”
Quartz acts as a conduit, allowing infrared radiation to travel through the system without energy loss. The researchers believe that alternative materials could further enhance performance.
A breakthrough for industrial applications
“In the past, increasing power density required raising the operating temperature—sometimes from 1,500°C to 2,000°C or even higher, which becomes unsustainable and dangerous for energy systems,” Professor Cui explains. “Now, we can achieve the same energy output at lower temperatures, making the system compatible with most industrial processes while maintaining high efficiency. Our device operates at 1,000°C but delivers the same power as traditional TPVs running at 1,400°C.”
The findings are detailed in a study published in Science.