High-efficiency solar installation photo, courtesy of Solaria and Solar Rising.
Solaria’s PowerXT panels are among the most technologically advanced residential solar panels. Key to its performance is its unique series-parallel cell interconnection structure, which can mitigate power loss in common shading conditions. The company went in-depth on the topic recently, publishing the results of internal shade tolerance testing it conducted last year. Here’s Solar Builder‘s exclusive synopsis.
Brief overview
When a panel is partially shaded, irradiance on that panel is no longer uniform, which leads to a loss in power. But the way a given panel responds to shading depends on how the cells themselves are connected. When PV cells are connected to one another in series, the current that flows through the string must be the same at every point.
So, when a single cell is shaded, its current and voltage (power) is reduced, which then limits the entire string to that shaded cell output. That’s obviously not great for system output, but this is also a big factor in hotspot formation that can damage PV panels. This is how just one shaded cell could wreck an entire panel.
Conventional panels overcome the hotspot issue with bypass diodes that sub-divide the cell string. Solaria explains:
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 flow 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.
So, better for sure, but room for improvement.
Series-parallel concept explained
PowerXT panels are constructed by “shingling” cells — full cells are diced into strips and reconnected in a way that achieves higher panel efficiency. The PowerXT panels are then assembled from units of series-connected PowerXT Cells. Here’s how a panel consisting of 20 PowerXT Cells arranged in four quadrants of five cells each would be constructed.
Like the sub-strings in a conventional panel, the quadrants are series-connected and fitted with bypass diodes, allowing them to be bypassed if necessary. The big impact of four panel segments over three is just simple math:
[W]hen a small object shades a conventional panel such that power from an entire sub-string is lost, the panel can be expected to drop by one third, or 33%. If the same shading causes the power from an entire quadrant of a Solaria PowerXT panel to be lost, the panel power will only drop by one quarter, or 25%.
But Solaria adds in another layer of protection against shading losses by the parallel connecting the five Cells within each quadrant. If a shading object were to reduce current flow for one of the five Cells in a quadrant, the presence of four other current paths can help to mitigate overall power loss.
Experiment 1 – Indoors
A conventional panel and a Solaria PowerXT panel are compared for some commonly encountered shading scenarios under controlled indoor laboratory conditions. Panel shading was achieved by adhering pieces of opaque material directly to the glass on the front side of the panels. The results are definitely notable:
Experiment 2 – Outdoors
For each panel type, an 8-panel, single-string system was installed on a single south-facing mock roof with a 20° tilt. Each system was connected to an identical string inverter. Testing was performed over a 70 day period in late Summer/early Fall. A dormer-like shading object was positioned over the panels such that it created approximately 12 inches of shading on the top row of panels at solar noon on the first day of the test.
By the end of the 70-day test period, the Solaria system had generated 37% more energy than Competitor 1 and 45% more energy than Competitor 2.
Big key to remember
These are impactful results, but since this shade tolerance is so unconventional, the expected performance is not factored into common PV design software tools. Solaria says they are not aware of any tools capable of capturing the shade-tolerance benefits of PowerXT panels that have laid out in this research.
— Solar Builder magazine
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