PV modules for utility scale plant – Importance of module rating and binning!
Solar energy market in India has seen tremendous growth within a short span of 5 years. The two major modes of installation are at rooftop and ground mounted (or utility scale power plants). Out of these two, utility scale power plants has been growing significantly and a similar trend is expected to continue (as shown in Figure 1). Additionally now with both the central and state(s) government supporting the idea of solar park and ultra-mega solar power plant with its target upgraded to 40GW (from previously 20GW), such plants would grow exponentially. The solar PV module used in such utility plants are in huge quantities. Let us take an example to get a glimpse of the number; the currently announced largest solar park (power wise) in the world would be in Kutch with a capacity of 30,000 MW. Considering the currently available solar module with power output of 540 Wp, the plant would have close to 56 million PV modules in it. With such numerous module interconnected to form a string and the strings in-turn connected in parallel, it is crucial that their output characteristics match exactly. However any slightest change in manufacturing parameters could cause a notable change in electrical characteristics of solar module. Additionally the power output is poised to vary even with the best manufacturing equipment due to the sensitivity of raw materials. Modules with significant difference in power output if interconnected as we explained you in our previous blog "Mismatch in solar cell and modules” would have a devastating effect on the power plant. It is for those reason that accurate rating of the module is important. Additionally with such deeper understanding, module manufacturers nowadays also bin the modules. This blog aims to educate its readers on such process and its on-field advantages.
Figure 1: Cumulative capacity addition of utility scale PV plants (Source: Waaree research and MNRE)
As we explained above, a utility scale power plant would have immense number of modules. Thus identification and knowing the exact characteristics of module is important. This is where module rating and labelling comes into play. The module rating as the name suggests is determining the maximum power output (Pm) along with other electrical parameters namely open circuit voltage (Voc), short circuit current (Isc), of the module, maximum voltage (Vm) and maximum current (Im), module efficiency (η), fill factor, etc. It is crucial that such characteristics are determined at standard test condition (STC) i.e. at 25 °C cell temperature, irradiance of 1000 W/m2 and air mass of 1.5. This is primarily to ensure that a layman could easily gauge its performance. Additionally it also ensures that modules with similar characteristics could be grouped easily and interconnected as and when needed.
In order to ease the process of identification, a label (sample shown in Figure 2 below) is affixed at the back side of module. The label in addition to the critical information on electrical parameters also includes safety instructions and the necessary certifications of modules. The safety instruction must include the warning for electricity hazard. It may also encompass the system voltage level which the module is feasible with. These information ensures that the module received on-field are sufficiently safe and have passed adequate quality standards for usage. It also ensures that on-field team have adequate knowledge about its ideal operations.
Figure 2: A typical label used for PV module (Source: Waaree Energies)
As evident from above, we now understand the exact importance of rating and labeling in solar PV module. While this data is available along with the necessary certification, safety instruction and the contact details, for a plant with large number of modules identifying and sorting these modules accordingly onsite would be a challenge. Additionally as we mentioned above, with there would be slight variance in electrical parameters of PV module even if manufactured with best available equipment. This is where module binning comes into picture. While checking the output characteristics of the module i.e. at the time of flashing (at STC conditions), the modules are classified (binned) according to preset values. There are two ways by which this binning is done i.e. by power and current. The power binning is done to asses and confirm the grade of the modules manufactured. Grade A modules are the ones which have the required power output and have no defects in them. The rest modules are termed as NG grade modules and may be used for internal testing/ other purposes.
The current binning is done based on the actual current output from the module. The bins are preset at a particular range with interval of say 0.1A (say 9.61 to 9.70A, 9.71 to 9.80A & so on) or a fixed percentage. While this may seem simple, the range (or tolerance) are decided based on the value of mismatch losses for such range of current. Few manufacturers have also started giving closer tolerances which leads to reduced mismatch losses (as shown in Figure 3). Thus technically, this reduced mismatch would in-turn reduce the probability of hotspots. Such systems also generate increased energy compared to plants which use random and/or un-binned modules in their strings and arrays.
Figure 3: Mismatch loss vs current binning tolerance (Source: Solar Professional)
We have understood this trend of binning have been doing it for a few years now. We consider strict tolerance ranges and ensure that the customer gets the modules from the same bin (as shown in Figure 4). Further we ensure that the same modules are put in one pallet and are labelled properly so that it could be easily identified when the modules reach at the end customer’s site. Further with our best in class equipment we ensure that the output of the solar module is as anticipated. Such cutting edge technology gives Waaree an upper hand over its competitors in international markets.
Figure 4: A glimpse of current sorting and labelling on traditional solar modules