This fall, a team from Michigan State reviewed recent developments in the important up and coming transparent photovoltaics (TPV) technology. This cool solar tech is similar to conventional PV materials, except clear as glass. If this can be made to work well, then every window and glass surface could be harvesting solar power, yet function just as well as non-generating glass.
Traverse and colleagues review the current technology in considerable detail, recounting the work of a number of labs and companies .
A key piece of the puzzle is to let visible light pass through, while harvesting wavelengths outside human vision. Conventional PV cells harvest visible and invisible light, so this restriction limits the comparative theoretical efficiency, though there is still plenty of potential.
There are several strategies that are being investigated.
- Non-wavelength-selective, spatially segmented PV. I.e., patches of opaque PV with clear spaces in between
- Non-wavelength-selective thin-film PV <<example form blog?>> semi-transparent film
- Wavelength-selective thin-film PV e.g., absorbs in near infrared, but transmits visible spectrum
Another strategy is to concentrate the light by capturing incident light and redirect it to conventional PV at the edge of the window. This might be done through dyes that capture photons and fluoresce toward the edge. Similarly, material may scatter incident light toward the edges (making a milky or foggy glass).
The paper sketches key technical problems to get useful PV out of polymers, including moving the electrons out of the film, and issues such as angle and ruggedness.
The authors are rather firm on the point that the transmission should “be reported as the integration of the trans- mission spectrum weighted against the photopic response of the human eye as accepted by the window industry”. (I gather there is considerable variability in what is reported in the literature.)
They advocate a new metric which they call “light utilization efficiency (LUE).” This metric characterizes the overall performance, including both PV and light transmission.
These technologies are promising for several applications. Obviously, building windows might contribute to the over power available for consumption. (Theoretically, this might amount to 100GW across the whole US—a significant fraction of current production.) TPV might also self-power “smart windows” that contribute to conservation and comfort.
Another application is for electronic devices, where a TPV display screen contributes to powering the device, allowing charging while in use. This is also promising for wearable electronics. I would add small robots and IoT sensor nets.
Another important possibility would be TPV windows that extend the range of electric vehicles.
These applications have varying demands, but TPV technology is already in range of some, providing that it can be made durable and mass produced.
- Christopher J. Traverse, Richa Pandey, Miles C. Barr, and Richard R. Lunt, Emergence of highly transparent photovoltaics for distributed applications. Nature Energy, 2 (11):849-860, 2017/11/01 2017. https://doi.org/10.1038/s41560-017-0016-9