"bi-facial" solar cells have a transparent back that allows light striking the cell from below to give a boost
to cell yield from indirect light. Panels using bi-facial cells are typically installed some distance off the
ground, and often have the ground or surface below them colored white to reflect light up at the underside of
the panels. (sometimes they are placed over unfinished ground, and only produce a noticeable increase in power
in the winter when the ground is covered with snow) The theoretical max additional power is +50%, though most
panels seem to be around 20-25% under ideal conditions and installations.
Due to the higher mounting of the panels, it can be possible to place them above plants and crops that don't
prefer (or can't tolerate) full direct light for the entire day, increasing land utilization. Green leaves
don't produce as much of an increase in solar yield though, and the presece of the frames can hinder harvest.
They're also well-suited to areas where partial shade and protection from rain are desirable, such as canopy
shelters over open patios.
Bi-facial panels also offer additional panel mounting and orienting options, such as mounting flat, or even
vertically. This can affect the daytime energy profile, with vertically mounted panels producing more power
in the morning and afternoon and less power around noon. Vertical panels may also be used to increase land
usage by installing them in garden and crop areas where crops don't need continuous direct lighting but will
require access for harvest.
Except in very niche situations, it looks to me to be more of a gimmick than a useful feature, because in most
cases you'd get better results by installing additional regular panels with the money you could have otherwise
spend on bi-facial panels. The panels also have to be installed some distance off the ground, which increases
wind load as well as offsetting the expense of additional mounting hardware for additional panels.
