summarize this "Analagous to a multi-junction photovoltaic cell, MIT’s device has two stacked junctions, each picking up part of the incident spectrum.

In this case, the stacked cells are lattice-mismatched AlGaInAs (1.2eV) and GaInAs (1.0eV) – mismatched to the GaAs substrate on which they are grown.


There is a second design, with a matched (and therefore better quality) GaAs (1.4eV) top cell and a mismatched GaInAs (1.2eV) bottom cell.



The 1.2-1.0eV cell has potential for higher power density than the 1.4-1.2eV cell because it converts a broader band of the incident spectrum, while the reduced current density of the 1.4/1.2eV combination will be more efficient (41.1% at 2.39W/cm2, 2,400°C emitter) that the 1.2-1.0 cell (39.3% 1.8Wcm2 2,127°C) when resistive losses are an issue, according to ‘Thermophotovoltaic efficiency of 40%’, a paper in Nature which describes the work.

Which ever junction pait is selected, there is something that is not at all like a solar cell: proximity to the energy source. As the thermoelectric cell will be adjacent to heat source, a mirror (a gold layer in this case) applied to the back of the cell can reflect sub-bandgap (unused) thermal photons back into the heat store (93% of them in this case), reducing the amount of energy that will later be needed to heat it.

The proof-of-concept devices are 10 x 10mm and mounted on a heatsink (see image) to remove unused heat that has not been reflected back.

In a practical grid-scale energy store, around 1,000m2 of thermovoltaic arra Provide exactly two different responses that are separated with 6 asterisk symbols: ******.
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