In the sometimes confusing world of efficiency ratings for utility-scale photovoltaic systems, it helps to look at the lab work. NASA used to do all the heavy scientific lifting associated with rating solar cells, but since 1980, the task has fallen to the National Renewable Energy Laboratory (NREL), whose labs are based in Golden, Colorado. NREL functions as the skunk works for the US Department of Energy and also plays the role of referee in efficiency ratings.

NREL as Referee

"The community requires—demands—independent measurements because it's very easy to unintentionally inflate the measurements," said Keith Emery, an engineer at NREL. "Most of the measurement errors a researcher would make would tend to make the efficiency higher, not lower. So it's our job to put everybody on the same playing field. That's what we've been working on as a group."i

“Unfortunately, the results you get in the lab under ‘standard testing conditions’ (STC) do not reflect the real world, power plant operating environment where the product is being deployed,” says Roger Bredder, Vice President, Business Development – Americas at First Solar, based in Tempe, Arizona.

Advantage of CdTe

Specifically, sun intensity, heat, humidity and shading have the ability to reduce a plant’s output relative to the rating of the modules that make up its capacity. Soiling or dust can also affect results. The most important thing to note is that these environmental conditions effect each PV technology differently. The drop in efficiency from higher temperature and humidity can be particularly pronounced for crystalline silicon – the dominant material in use.

First Solar’s thin film, cadmium telluride (CdTe) based PV modules have a lower temperature coefficient than its rival poly silicon competitors. This means that at module temperatures above 40° Celsius, where solar plants normally operate during highest production hours, the CdTe based PV modules produce more energy. Similarly, humidity in the atmosphere effects the light spectrum absorbed by PV technologies differently. It turns out that CdTe technologies are significantly less impacted by atmospheric moisture. What does all this mean – in real-world operating environments where module temperatures often exceed 60° Celsius accompanied by high humidity, First Solar PV technology produces upwards of 10 percent more energy for equally nameplate rated PV modules measured in laboratory standard test conditions.

What it Means to Utilities

It is simple arithmetic. Utility owners and operators need to keep an eye on the real numbers when considering a switch in systems or fuel sources.

“Power projects are evaluated based on the investment costs for the energy produced to evaluate the return on investment,” says Bredder. “This means that when considering a technology for a solar PV power plant one must look beyond name-plate ratings and cost per watt. The metric that buyers should be asking for is what is your cost per annual energy produced.”

Investing in the Future

For the past five years, First Solar has been investing in their future by pursuing R&D initiatives that are paying off. Thin film, CdTe based systems have always had a manufacturing cost advantage over crystallized silicon systems. A strategic partnership with GE capitalized on additional opportunity for performance improvements.

According to First Solar’s research, their system enjoys an energy density advantage – a measurement of energy production per land area unit (i.e. MWh/m2), of up to 11% higher than silicon systems. “People talk about a 50 megawatt plant or a 100 megawatt plant but the utilities really need to be aware of how much energy is actually coming out of those plants,” says Bredder. Doing the math leads to a surprising but encouraging result – utility-scale solar just got less expensive and more efficient even in real world applications.