Solar module manufacturers one-up each other on a regular basis in the quest for the world’s most efficient panel. The results usually come out of laboratories, where the modules undergo precise testing in a controlled environment under standard test conditions (STC).  

In the real world, however, the picture is more nuanced. Field conditions are often a far cry from artificially stable lab environments.

New results from a recent series of field tests highlight the variation that occurs when technologies are tested side-by-side in different regions, where array design and siting, temperature, humidity, and a variety of other factors influence performance.

Three module test labs, DNV GL, TÜV Rheinland Photovoltaic Testing Lab and Celestica, tracked the performance of 2-kilowatt systems that use one of three technologies: thin-film cadmium telluride (CdTe), n-type silicon bifacial, and crystalline-silicon (c-Si). Modules were selected at random by the labs.

Test sites in Davis, Calif., Tempe, Arizona and Toronto each assessed eight or nine systems with a mix of the different technologies. Data from the arrays were collected for at least a year.

Source: TUV Rheinland

The CdTe panels, supplied by First Solar, and the first-generation B245 bifacial panel (bifacial-1) produced by Prism Solar performed best in the two hotter environments. The results of the field study lend credibility to technologies that are gaining traction in the market beyond the c-Si, which makes up the majority of solar panels today.

In Tempe, more than 90 percent of the energy yield was produced at temperatures higher than 25 degrees Celsius, and the highest energy generation happened above 45 degrees Celsius.

Note: Array 1 Prism bifacial; Array 3 unnamed Bifacial; Array 2, 4, 5 c-Si; Array 6-9 CdTe

Source: TUV Rheinland

The two different bifacial panels show that performance varies by supplier and location, with Prism’s bifacial panel far outperforming the unnamed competitor in Tempe, while the other panel provided a slightly higher performance ratio in Toronto. Bifacial panels can produce power from both sides of the module, capturing direct and indirect sunlight.

There are various factors that drive bifacial performance, with albedo (the amount of solar energy reflected off of a surface) and row spacing being two important contributors. There is currently no industry consensus on rating methodology for bifacial modules, and manufacturers are using different methods.  

At SNEC 2017 in Shanghai in April, bifacial panels were seemingly everywhere, according to GTM Research solar analysts who attended. Editors at PV Tech commented that given the major players showing off their bifacial panels at SNEC, “a recent niche product could be set for the mainstream, very shortly.” 

However, the industry needs to solve several key issues that are limiting the product's acceptance prior to bifacial modules being adopted for large-scale usage.

The field study also backs up First Solar’s competitive advantage claims over traditional crystalline silicon technology. CdTe modules performed well at higher temperatures compared to C-Si modules in Tempe due to the better temperature coefficient of CdTe, the TÜV Rheinland study found. The CdTe modules had STC efficiencies close to those of c-Si, ranging from 13.2 to 14.7 percent. The variability between the First Solar array performance represents the improvement in efficiency and energy yield from Series 3 Black Plus through its Series 4-2.

Even in Davis, where temperatures are not quite as hot as in Tempe, most of the arrays spent at least half their time at temperatures of 25 degrees Celsius or above. Because of those high temperatures, the Davis CdTe arrays also had a performance ratio higher than the average of c-Si panels.

Note: Array 1 Prism bifacial; Array 2, 7-9 c-Si; Array 3-6 CdTe

Source: TÜV Rheinland

The picture was more muddled in the relatively cold environment of Toronto. The unnamed bifacial panel had the highest average annual DC efficiency of 17.17 percent among all the arrays, while CdTe-1 had the lowest efficiency of 12.82 percent, and another CdTe system had the second-highest efficiency of 14.45 percent. 

Note: Array 1 Prism bifacial, Array 2 unnamed bifacial, Array 3 c-Si, Array 4-7 CdTe

Source: TÜV Rheinland

The findings from the reports do not upend conventional wisdom, but they do highlight the regional variation for each technology in real-world conditions. The results also suggest the potential threats that traditional c-Si products could face as technologies continue to fall in price due to higher volumes, lower balance-of-system costs, and more widespread market recognition.


This article was sponsored by TÜV Rheinlanda global leader in independent inspection services. TÜV Rheinland inspects technical equipment, products and services, oversees projects, and helps to shape processes and information security for companies. Since 2006, TÜV Rheinland has been a member of the United Nations Global Compact to promote sustainability and combat corruption.