PV modules are the final commercial product that customers buy from manufacturers and that require some data provided from manufacturers to allow customers to evaluate the performance of these modules in terms of electrical power rating, safety, and reliability measures. For that purpose, the module’s performance is set by test standards. These standards vary according to the location where the modules are to be used. For the purpose of our class, we will focus on the US standards -- and some of these standards apply internationally.
PV modules should be approved/listed by Underwriter Laboratories (UL), which is a nationally recognized test laboratory accepted by OSHA that assures that manufacturers comply with electrical safety standards.
What is the UL safety standard that relates to PV modules?
ANSWER: UL 1703 Safety standards for flat-plate photovoltaic modules and panels.
PV modules are expected to meet certain quality measures that are set by the international Electrotechnical Commission (IEC). These standards are specific to the technology, e.g., IEC has different standards for Thin-film and Crystalline Silicon.
What are the IEC reliability standards that relate to PV Crystalline Silicon modules?
ANSWER: IEC 61215 Safety standards for flat-plate photovoltaic modules and panels.
PV Module Datasheet Parameters
PV module manufacturers are required by NEC and IEC to provide their product with performance information such as the electrical characteristics measures that have to be labeled on nameplates. These parameters include maximum power, open-circuit voltage, short-circuit current, maximum overcurrent device rating, and maximum permissible system voltage. However, these numbers vary according to operating conditions such as temperature and irradiance. For that reason, test standards are needed to set reference operating conditions when taking the measurements at the laboratory.
PV Module and Test Standards
There are various test standards that differ mainly by the operating condition used when taking the measurement. For example, Standard Test Conditions (STC) is an international and most widely used test standard that rates PV modules at solar irradiance of 1000 W/m2, spectral conditions Am1.5, and cell temperature of 25 °C (77 °F). However, in practice, modules rarely operate in these conditions, and that's the main reason behind other test standards that try to simulate real-world operating conditions.
What are the other test standards that are used to measure PV performance and how they differ?
ANSWER: The following table shows various standards and the reference values used for each test.
|Name||Abbreviation||Solar Irradiance (W/m2)||Wind Speed (m/s)||Temperature °C (°F)|
|Standard Test Conditions||STC||1000||N/A||25°C Cell Temperature|
|Nominal Operating Conditions||NOC||800||N/A||Nominal Operating Cell Temperature (NOCT)|
|Standard Operating Conditions||SOC||1000||1||Nominal Operating Cell Temperature (NOCT)|
|PVUSA Test Conditions||PTC||1000||1||20°C (68°F) ambient Temperature|
So to put everything we have learned so far together, let’s take a look at a PV modules’ datasheet for Trina Solar Allmax 250 W. As can be seen, most module manufacturers have series of modules with different power ratings, and they all appear in the same specification sheet. We can see the main electrical parameters and power ratings at STC and NOCT, efficiency, power tolerance, temperature coefficients, warranty, mechanical dimensions and testing certifications such as UL and ISO and more.
Some considerations when selecting PV modules
Since PV systems are large in size and consist of multiple panels, we expect these systems to be visible to clients, and there will be an aesthetic factor involved in the design process. For example, a client may have a color preference for the PV panels that should be taken into account when selecting the panels, since it might be affected by the availability of that color. Another aesthetic factor is the dimension of each panel and the layout on the roof, for example. A designer should discuss with clients all possible options for best design to maximize the solar utility at that location while keeping the system aesthetically acceptable. Other selection factors are more technical, such as the number of cells in each panel, protection fuses and bypass diodes, degradation, and warranty. All these factors should be taken into account when selecting the right module.