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US scientists have developed a thermophotovoltaic cell that could be combined with cheap thermal storage to supply energy on demand. The indium gallium arsenide (InGaAs) thermovoltaic cell absorbs most of the in-band radiation to generate electricity, while acting as a near-perfect mirror.
Thermophotovoltaics (TPV) is a power generation technology that uses thermal radiation to generate electricity in photovoltaic cells. A POS system generally consists of a thermal emitter that can reach high temperatures, close to or greater than 1,000 ºC, and a diode photovoltaic cell that can absorb photons from the heat source.
The technology has been arousing the interest of scientists for decades, because it can capture sunlight across the entire solar spectrum and has the technical potential to overcome the Shockley-Queisser limit of traditional photovoltaics. However, the efficiencies recorded so far have been too low to make it commercially viable, as POS devices continue to suffer from optical and thermal losses.
In this sense, a group of researchers from the University of Michigan (United States) has developed TPV cells that, apparently, solve these problems and reach an energy conversion efficiency of 44%.
“This level of efficiency could allow thermal battery systems to reach the price needed to run most of the grid on wind and solar energy,” lead author of the research Andrej Lenert told pv magazine . “These systems have to continually extract energy from a hot storage material, such as graphite, as it cools from its maximum allowable temperature. Achieving 40% efficiency at storage temperatures as low as 1,300ºC, compared to the 2,000ºC needed previously, means that these batteries could possibly obtain twice as much energy per kg of graphite.”
According to Lenert, this result represents a significant improvement in TPVs and in solid-state heat-power generation in general. “It is the culmination of several years of intense research to understand how to minimize power losses and mechanical issues in airlift POS cells, which we originally reported on in 2020,” she added. “Those cells had an efficiency of 32% and were relatively fragile; Now we are closer to 44% and we have much more robust technology.” Although not yet at the kW or MW scale, this result demonstrates what is possible with single-junction TPV cells, fulfilling theoretical predictions made decades ago by the TPV community.”
In the study “ High-efficiency air-bridge thermophotovoltaic cells ,” recently published in Joule , Lenert and colleagues describe the cell as a bridged indium gallium arsenide (InGaAs) device. of air that can absorb most of the radiation within the band to generate electricity. It can also serve as an almost perfect mirror, with a reflectance of almost 99%.
The cell was constructed with a silicon substrate, a 570 nm thick air bridge structure, a gold (Au), titanium (Ti) back contact, an n-doped InGaAs layer, a 1 µm thick, an InGaAs absorber and a front contact of Au, Ti, platinum (Pt) and p-doped InGaAs. Three different absorbing layers were tested with energy bandgaps of 0.74 eV, 0.90 eV and 1.1 eV, respectively.
The air bridge layer is embedded between the three active layers and the Au back mirror to improve the back reflectance and out-of-band photon recovery. The membrane support layer is intended to minimize buckling of the independent semiconductor membrane and ensure a single cavity mode within the air layer.
“The combination of a nanoscale air layer and a relatively high coverage of conductive back electrodes ensures that the thermal resistance of the airbridge is small compared to that of the Si substrate,” the scientists emphasize. “In addition, the design includes a membrane support layer to minimize buckling of the independent semiconductor membrane and ensure a single cavity mode within the air layer.”
The researchers found that the cell with an absorbing bandgap of 0.90 eV had the best performance. It achieved an energy conversion efficiency of 43.8% at 1,435 ºC. “It exceeds the 37% achieved by previous designs within this temperature range,” Lenert stated. “We have not yet reached the efficiency limit of this technology. “I am confident that we will surpass 44% and reach 50% in the not too distant future,” added Stephen R. Forrest, co-author of the research.
These results, according to the research group, also promise significant improvements in the devices round-trip efficiency. “It is a form of battery, but very passive. You dont have to extract lithium like with electrochemical batteries, which means you dont have to compete with the electric vehicle market,” Forrest explained. “Unlike pumped water for hydroelectric energy storage, you can put it anywhere and you dont need a nearby water source.” |
