The limits of STC values to assess the behavior of a solar module

As everyone knows, the rated power of a solar module is obtained by testing the solar module under Standard Test Conditions.
The conditions stipulate the following :
– 1000W/sqm irradiation
– Air Mass: 1.5G
– Temperature: 25°C

As for every test, there is a need for setting criterions in order to define a standard and be able to perform comparisons. But what is also important is to understand the potential limits of a test especially when compared to outdoor conditions.
Recent cases in the automotive industry provided a striking illustration surrounding differences between test conditions and real conditions, in this case regarding C02 emissions.

The main limit associated with STC conditions is definitely the set irradiation level. Indeed, the modules are tested under 1000W/sqm while they will operate under a wide range of irradiance levels along the year as illustrated for the city of Paris in the graphic hereunder.

Moreover, it is considered [2] that module efficiency at low irradiation intensities is the main driver of the variation of module behavior within a PV system. Reason why it is a phenomenon worth understanding.

Low light behavior in practice
The notion of low light behavior comes to fill the gap and bring more usable input regarding the behavior one can expect from a solar module at different irradiance which will ultimately influence energy yields and power plant financial profitability.
While we know that solar panel current drops approximately linearly with decreasing irradiance and that voltage follows a logarithmic decrease, still, all the panels do not behave the same.

The graphic hereunder, taken from a report commissioned by Sunpower in 2013, provides a good visual illustration of the differences one can observe. The efficiency of solar panels, so there ability to turn sun rays into electricity, varies depending on the irradiation.

The merit of this kind of study is also to visualize that the temptation to think that a solar panel equals another one, except for its price, may be convenient but for sure over-simplistic.

Where does it come from?
The low light behavior of a solar panel is mainly dependent on the shunt resistance and series resistance of the cells, the latter being caused by the current that circulated the emitter and the base of the solar cell, the resistance caused by the metallic contact and the silicon and the resistance related to the contacts at the top and the bottom of the cell.

Not one low light behavior
Low light behavior varies depending on cell type and from cell supplier to cell supplier, even on the same cell type [2]. It can lead up to 10% difference in annual energy yields.
Also, the challenge for a manufacturer is to ensure consistancy through the selection and supply of materials.

The implication for project developers – beware of the datasheets
The first implication for project developers is that two similar dataheets from two different manufacturers do not necessarily imply the same energy yields.

Which leads to the second implication that is a challenge. It is related to the simulation of electricity production. There are plenty of softwares on the market that help installers and projects developers assess the electricity production of solar projects. While there are accuracy tolerances and one should not get to the kWh unity considering the probability of variance, the low light behavior comes and add an additional difficulty. Because current softwares models have standard shunt and series resistance models, they cannot take into account the differences between manufacturers.

So it is safe to assume that a good option for those of you who size PV projects is first to try to gather information from your module supplier around weak light performance and energy yields of his products and then to build your own real world experience with field test comparisons that will help you build more knowledge and understanding.

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