With its combination of solar and cylinder, the “O” with rays striking it, and the tag line, “The new shape of solar,” marketing for Solyndra’s unique tubular module design was at the top of the class. Why did it fail?
Figure 1: Solyndra’s design is evident in this panel with cylindrical modules and simple mounting system.
Tubular module design
The design for Solyndra panels was based on a series of tubular modules mounted parallel to each other inside a frame. The generously spaced tube structure allowed airflow through the panel thereby reducing wind loading.
Where large-area flat panels might fly off a roof in strong winds, Solyndra claimed its panels could withstand 130-mph winds without specialized mounting.
On the other hand, the danger of strong winds pulling flat panels off a roof is more of an issue for the partially upright mounts typical of panels angled to the optimum azimuth for the particular geographic location. As topical and “green” as solar panels are, they are a commodity and cannot avoid economic realities. Any benefits must outweigh the cost, and standard flat panels can still be mounted horizontally to save installation costs.
A second indisputable aspect of the tubular design was that the modules could shed snow and debris that often obscure standard flat panels. In many locations, solar power output can be significantly lower while waiting for the wind to clear snow from the panels. But again, the loss of power on temporarily shaded cells needs to be weighed against the higher cost of the tubular construction.
Electrical performance
What about the electrical performance of the solar cells? In terms of energy harvesting performance, Solyndra promised that cylindrical modules required only their shape to track the sun rather than costly mechanical systems. They reasoned that a curved surface would collect rays at all the sun’s angles throughout the day. But, only a small portion of the cylindrical shaped cell is exposed to the sun at any time, and the angle the exposed surface sees is rarely perpendicular to the cylindrical surface. The cell’s back side will never see sunlight.
Figure 2: The module construction employed by Solyndra is revealed in this cross-section view showing the inner tube coated with the photosensitive CIGS thin film covered with a coupling element to provide a light concentrating effect.
The sun passing across a fixed flat surface definitely produces a constantly varying angle of incidence. Peak output occurs during a relatively short portion of the day as the ray angle is perpendicular to the solar cell surface. Solyndra cited this as their key differentiator.
There is no disputing the fact that the sun strikes the surface of a curved solar cell at right angles throughout much of the day. However, the surface area subjected to this optimal angle is extremely small compared to a flat panel, leaving less photovoltaic material exposed to strong sunlight for energy conversion.
Fill Factor
The amount of cell coverage compared to the surface area of the array is known as the fill factor. Although a reflective backdrop improved power output from the Solyndra panels, it is difficult to make a case for their design compared to a panel with 100% fill factor capturing only incident light. There would be little gained and much lost by leaving large gaps and collecting reflections compared to simply covering the area receiving the direct sunlight.
Figure 3: This Solyndra marketing graphic was used to highlight the ability of the tubular design to accept light through a full 360°. What surface is the reflected light coming from?
Solyndra marketing materials pointed to the improvement in solar energy harvesting early and late in the day compared to a crystalline silicon flat panel. If the solar panels were directly tied to a load, a longer window of useful energy would be a big advantage.
Solyndra’s datasheet compared output from a standard crystalline flat panel with 15° tilt to their tubular design. Although the Solyndra output was strikingly higher both early and late in the day, peak output at midday was lower than a standard flat panel.
The tubular design for Solyndra solar panels forced them to focus on some niche markets. There is no doubt that their panels allowed light through. One justification was the energy collected after reflection from the surface behind the panels. Adding a special reflective material to the back surface would be costly, and depending on just the existing roof material would generate only a tiny amount of additional power, if any at all.
Solyndra tubular modules included an extra layer of an optical coupling material. This was intended to bend light hitting the module at oblique angles to focus the beam more directly toward the photovoltaic layer underneath. Solyndra claimed this optical coupling agent increased the active solar cell surface. Mild as this effect was, it was similar in concept to a concentrating photovoltaic cell.
Adding complexity to save a solar cell material that was chosen for its relatively lower cost is nonsense. The optical layer was then encased inside a second, outer glass tube. The bill of materials was rising quickly.
Keep It Simple
My design principle of “Keep It Simple” is ringing loud and clear in the financial failure that was Solyndra.
In the final analysis, how did this complex design with poor performance make it through a critical design review? The decision to move forward was not based on basic engineering good sense. The cost of this failure to the U.S. taxpayer was entirely avoidable!
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org