Solyndra: Its technology and why it failed
Don Scansen, IP Research Group- November 22, 2011 inShare0 Save Follow
Save to My Library
Follow Comments
Follow Author
PRINTEMAIL
Solyndra was a darling of clean tech but filed for bankruptcy. Department of Energy loan guarantees of $535M sparked an investigation. No wrongdoing has yet been uncovered despite a huge waste of government money. Lower production costs in China have been singled out as the critical factor in Solyndra's demise. Will this story become a parable for America's waning competitiveness?

With its clever 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. It is reminiscent of making oats cereal in small toroids, the shape of the letter O. We need to determine if there was more to Solyndra than just good public relations.

Figure 1: Solyndra's elegant design is evident in this 200 series panel with well-spaced 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. With no need to physically anchor the panel to the roof, Solyndra certainly delivered on the promise of simpler installation.

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? As electronics professionals, we tend to focus on this rather than the secondary aspects of marketing or mechanical design. 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.

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.

Without data to compare side-by-side, we should be slow to judge. But to a first order, it appears questionable that the cylindrical modules could actually achieve what was advertised. 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.

Solyndra's process of forming the solar cell material over 360° of the tube along with the gaps between the tubes enabled the harvesting of light passing through the panel and reflected from the surface behind. 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 to be gained 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°.

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 the crystalline panel. The net effect was an increase in total daily energy supplied by the Solyndra panel, but the improvement was only 7%.

Without an independent comparison test, we cannot be entirely certain, but the comparison of outputs even as shown in Solyndra's own marketing collateral was not compelling enough to warrant a much higher price tag for their panels. With feed-in-tariffs in effect, the goal is to sell the most power back to the utility company. That was Solyndra's prime market, but their design offered only a mild performance benefit.

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 angle was the energy collected after reflection from the surface behind the panels.

Looking to situations where there is value in allowing some light through, Solyndra attempted to market the tubular module concept to greenhouses. They took that one step further to position their product for greenhouses in climates where partial shade is advantageous. Solyndra panels would provide the perfect protection for plants growing underneath while producing electrical energy for greenhouse operations (http://www.solyndra.com/technology-products/greenhouse/).
Non-technical factors
As engineers, though, we should not let the purely technical rule out the other advantages. For the case of installations on large warehouses and other commercial rooftops, the tubes with gaps may have made sense regardless of the effect on performance. The Solyndra design allowed building owners to claim roof repairs as part of the solar panel installation for clean energy tax benefits.

The tax incentive of claiming the roof as part of the solar installation misses the broader implications of the tubular design. The commercial rooftop is a large potential market in the middle of the size range of solar power installations. But it is just one segment. Unfortunately, the Solyndra design had no way to adapt to the markets at either end of the size range.

For example, large utility-scale installations could not benefit from the idea of a reflective surface at the installation site. Solar is a commodity, and it's all about producing and selling large volumes of product. You can ignore the utility segment, but think about the number of panels used. The 290-MW Agua Caliente Solar Project will contain five million panels when it is completed in 2014. That's a lot of rooftops.

At the other end of the size range, the Solyndra panels were ill-suited to residential installations. The alleged villains in this case - SunTech Power and other Chinese manufacturers - viewed this as an important market. You need only browse their web sites to confirm their interest in residential customers.

Manufacturing complexity
Thin film photovoltaic materials are more commonly deposited onto flat sheets of glass, a fact that is regularly cited as the differentiator in Solyndra's marketing. They were different. They used tubes instead of sheets, and the added complexity was obvious. A tube is a higher cost substrate than a flat sheet. The volume of the photovoltaic deposited is not a concern, but the deposition equipment was specialized compared to the rest of the industry.

Furthermore, 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 seems an odd approach. The optical layer was then encased inside a second, outer glass tube. The bill of materials was rising at the module level before taking into account final assembly of the panel.

Assume for a moment that Solyndra's competitors were only other thin-film manufacturers. Whether other CIGS producers, cadmium-telluride, or amorphous silicon, thin films are always coated onto a flat substrate. The processes of scribing, contacting and interconnecting the individual solar cells in a module are simpler compared to the cylindrical format. To make matters worse, the final Solyndra panel required physical mounting of 40 individual cylindrical modules. Solyndra panels may have been easy to install on a rooftop, but they were not easy to get out their factory door.

The corollary of Moore's Law for solar panels is increasing Watts per dollar rather than transistors per micro-cent. To create any sort of scale comparable to integrated circuits requires efficient manufacturing methods for depositing very large areas of (preferably cheap) photovoltaic semiconductors at very high unit volumes.

Starting with a relatively inexpensive material-CIGS-and complicating the module manufacturing and panel assembly was a bad mix. It makes sense to use complicated assemblies for solar but only for concentrating photovoltaic panels that use focusing techniques to reduce the surface area coverage required for the very high efficiency cell materials such as Ge or GaAs that are often multi-junction stacks.
Production costs are key
It is not an easy task to get to the bottom of comparing production costs between manufacturers, but looking at retail prices provides some insight. Solyndra is at the top end of current prices for panels of comparable output (peak near 200 W).

Figure 4: Typical current retail prices for Solyndra and several competitor panels in dollars per watt show Solyndra at the top end of panel prices.

With the focus on China as the culprit in the downfall of another American enterprise, it may come as some surprise that another U.S. solar company appears comfortably near the bottom of the price comparison. First Solar demonstrates that American inventiveness can still compete in high-volume manufacturing for a commodity product.

Solyndra and First Solar panels are both based upon non-silicon thin-film processes. Solyndra used copper-indium-gallium-diselenide (CIGS). First Solar uses a combination of cadmium-telluride (CdTe) and cadmium-sulfide (CdS). In July, First Solar broke an efficiency record that stood for more than a decade by pushing the CdTe/CdS technology to 17.3%.

First Solar developed the underlying technology in the cell material and processes for making electrical contact. The effort appears to be paying off.

From a marketing point of view, it might be more challenging to explain these improvements since they lack the visual appeal of Solyndra. It was easy to tout the latter's unique panel design composed of multiple cylindrical modules to a non-technical audience. They seemed Apple-like in their sophisticated industrial design. Solyndra assembly and installation were elegant and slick. Perhaps these qualities played better at dinner parties than discussion of efficiency ratings, production costs or manufacturing scalability. But reaching the $1/W threshold (before tax credits) should attract at least some attention.

For manufacturing costs, Solyndra used a design that required more inputs and more steps. No Chinese manufacturers imitated the tubular module, so a direct comparison is difficult. Perhaps this point is telling in itself. There was no motivation to create a cheap Solyndra knock-off. There were a few technical benefits, but these were ultimately less important than the higher production costs they imposed.

In the end, solar is not about building a better mousetrap. It's about reducing costs and increasing scale.

About the author:
Don Scansen is a partner at IP Research Group, a technology consulting firm delivering unparalleled service to intellectual property clients. He has devoted the last 12 years to technology and intellectual property analysis including 10 years at Semiconductor Insights (now UBM TechInsights). He holds a Ph.D. in Electrical Engineering from the University of Saskatchewan and is a licensed professional engineer in the Province of Ontario. Don can be contacted at don.scansen@ipresearchgroup.com.