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Closing the gap: Converting heat to electricity at nanometer distances


Researchers at the University of Michigan have demonstrated an exceptionally efficient system for converting waste heat to electricity. 

“Depending on the exact energy system, typically 20-50% of the energy consumed in the industrial sector is lost as waste heat to the environment, which for US manufacturing alone represents more than 3 trillion kWh or 10 quadrillion Btu each year. With the growing need for sustainable energy solutions to tackle global climate change, it is essential that we convert this otherwise wasted heat into useful electricity,” said Edgar Meyhofer, Professor of Mechanical Engineering (ME) at the University of Michigan (U-M).

Thermophotovoltaic (TPV) devices that can harvest photons emitted by a hot object and convert them to electricity using a photovoltaic cell are a promising approach to converting heat to useful electricity. Thermophotovoltaic systems are in principle similar to a roof top photovoltaic system but instead use a hot object in place of the sun to generate electricity.

All hot objects radiate light in the form of photons into their surroundings. However, a large amount of energy is trapped on their surfaces in evanescent waves that are attenuated within a short distance from the surface and is not accessible to conventional photovoltaic cells. The researchers demonstrated that by placing the hot object very near to the photovoltaic cell (below 100 nm, or about a thousand times smaller thickness than a human hair), it is possible to capture that otherwise trapped heat and convert it into electricity.

“We have demonstrated an unprecedentedly high electrical power extraction. To do so we had to overcome a number of really challenging problems by developing custom devices and methods not available anywhere else. For example, we designed an ultra-flat, silicon heater (or emitter) that can endure temperatures up to 1000 degrees Celsius. By carefully controlling the emitter’s temperature and positioning it into known, nanometer-sized distances away from a unique InGaAs-based cell, we were able to systematically measure the TPV devices heat-to-electricity conversion as a function of temperature and gap size.,” said Rohith Mittapally, a U-M ME PhD student who led the project. 

Rohith Mittapally, a PhD candidate in mechanical engineering at U-M, works on exploring thermophotovoltaic energy conversion at the nanoscale. The custom-fabricated emitter and the photovoltaic cell are aligned in a nanopositioner and the gap size is controlled with nanometric precision. The assembly is then maintained at a high vacuum and experiments are performed to measure the TPVpower output.

The efficiency of a TPV device is characterized by how much of the total energy transfer between the emitter and the PV cell is used to excite the electron-hole pairs in the depletion region of the PV cell. While increasing the temperature of the emitter increases the number of absorbed photons above the band-gap of the cell, the number of sub band-gap photons that can heat up the PV cell need to be minimized.

“This was achieved by fabricating thin-film TPV cells with ultra-flat surfaces, and with a metal back reflector. The photons above the band-gap of the cell are efficiently absorbed in the micron-thick semiconductor, while those below the band-gap are reflected back to the silicon emitter and recycled,” said Stephen Forrest, Professor of Electrical and Computer Engineering at U-M.

Another important metric of a TPV system is the power density or power per unit area. In the experiments, the power output was measured to increase by 7 times when the emitter at 800 K was brought from ten micrometers distance to hundred nanometers. Both the devices need to be devoid of any nanoscale contamination for this to be possible.

“We grew thin-film InGaAs PV cells on thick semiconductor substrates, and then peeled off the very thin semiconductor active region of the cell and transferred it to a silicon substrate. A key point was to develop methods to make the cell atomically flat to allow such close approach to the heat source,” explained Byungjun Lee, a doctoral student in U-M’s electrical and computer engineering department.

All these innovations in device design and experimental approach resulted in a near-field TPV system that could not be explored before.

“The team has achieved a record ~5 kW/m2 power output, which is an order of magnitude larger than systems previously reported in the literature,” said U-M ME Professor Pramod Reddy.

Researchers also performed state-of-the-art theoretical calculations to estimate the performance of the PV cell at each temperature and gap size. The short-circuit current and the open-circuit voltage of the PV cell follow a trend that is in good agreement with the theoretical model.

“This demonstration meets theoretical predictions of radiative heat transfer at the nanoscale, and directly shows the potential for developing near-field TPVs for efficient energy conversion,” said Dr. Pani Varanasi, Program Manager at the Army Research Office that funded this work.

The paper, “Near-field thermophotovoltaics for efficient heat to electricity conversion at high-power density,” is now available on Nature Communications’ website.

Mittapally works in the labs of U-M ME professors Pramod Reddy and Edgar Meyhofer. He collaborated closely with Lee from Professor Stephen Forrest’s lab in the U-M Department of Electrical and Computer Engineering on this project.

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