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Whitepaper, Solaria Shading Performance | MKT-RPT-0001 Rev 02 04-2020
© 2020 The Solaria Corporation 1700 Broadway Oakland, CA 94612 510.270.2507 www.solaria.com
Improved Shade Tolerance of
Series-Parallel Interconnected
Solaria PowerXT Panels
THE EFFECT OF SHADING ON PV
PANEL OUTPUT
The output power of a PV panel is strongly dependent
on the amount of irradiance it receives. Under normal
operating conditions, the power increases with irradiance
in a more or less linear way. However when the irradiance
isn’t uniform, as is the case when the panel is partially
shaded (see Figure 1), the response is usually no longer
linear, and the shading of small areas can lead to a large
loss in power. The way a given panel responds to shading
strongly depends on how the cells in the panel have
been connected together. In this paper we’ll look at how
the series-parallel interconnection in Solaria’s PowerXT
panels can mitigate power loss under many commonly
encountered shading conditions.
Let’s start by considering the shading response of a
single solar cell. When a single cell is shaded, its current
and voltage is reduced. Since power is the product of
current and voltage, the consequence is that the power
is also reduced. If this shaded cell is connected to other
cells in series to form a string (Figure 2), this enforces
a restriction on the string current: the current that ows
through the string must be the same at every point.
Therefore, if one cell is shaded, it will act as the current-
limiting cell, and the current of the entire string will be
limited to the current of the shaded cell. Moreover, the
optimum operating point of the string can occur at a
voltage that corresponds to forward bias for the unshaded
cells and a large reverse bias for the shaded cell. In such a
situation, the shaded cells can heat up considerably - this
is referred to as hotspot formation – and result in damage
to the PV panel.
Conventional PV panels usually contain 60 or 72 series-
connected cells. It is clearly an undesirable situation that
the shading of just one cell would limit the output of the
entire panel and put it at risk of hotspot damage.
This problem is partially resolved by the use of bypass
diodes. In conventional panels, three diodes are usually
connected as shown in Figure 3 to divide the string into
three sub-strings. Under normal conditions, the diodes
are in the ‘OFF’ state i.e. they pass negligible current and
are hence effectively not involved in the circuit. If one sub-
string is shaded to such an extent that the corresponding
diode becomes forward-biased, the diode will switch to
the ‘ON’ state. In this state, the diode has considerably
less resistance than the sub-string, and hence panel
current will tend to ow through the diode, bypassing the
shade-affected sub-string. This greatly mitigates hot-spot
risk and prevents the shaded sub-string from limiting the
current from the rest of the panel.
Figure 1: Dormer shading, photo courtesy of Mass Renewables
(top); shading from partial snow coverage (bottom).
Solaria Proprietary and Confidential Information ©2019
1
Unshaded
50% shaded
 Current reduced by 50%
Figure 2: Shading a single cell reduces the current for the entire
string.
WHITEPAPER