Selecting the right solar PV module: The end consumer's dilemma! - Part 2

14 May 2019 at Waaree

The first part of the article "Selecting the right solar PV module: The end consumer's dilemma!" detailed the importance of solar photovoltaic worldwide. The different available technology on cell level and at module level (bifacial type module) was also detailed. This part shall further focus on the dilemma in selecting amongst different module technologies while also acting as a guide on the parameters to be considered for selecting the right module design.

Module design

The life expectancy of the solar module has been a minimum of 25 years or above. However many degradation mechanisms were discovered (such as PID, Degradation by UV rays, Oxidation, Corrosion, etc.) which in some cases were accelerated by using either single raw material (RM) or their specific combination. Thus the RM in the solar module has been constantly evolving ensuring that the module performs as expected when in operation. However one thing that had remained more or less the same is the design (or construction) of the module i.e. they utilized 60/72 full cell. A recently introduced novel module design utilizes almost the same RM as used in traditional solar module. The primary difference is that the module used half cut cells and split type junction box. While our previous article titled "Doublet module - The magic of two!" informed in depth about the technology used and its advantages compared to traditional modules, it is important to mention briefly about the module in this article. This module can be understood as two module of 72 half cut cell connected in parallel with each other (see Figure 1 below). Such construction has the following advantages namely

  1. reduction in resistive losses (I2R) by one fourth
  2. Reduction in the nominal module operating temperature
  3. Enhanced power output due to increase in reflection of light from white spaces
  4. Enhanced performance in shadow i.e. power output only reduced by 50% when (the bottom/top half of module is) in shadow compared to 100% reduction in power output in traditional module

While such modules have enormous advantages, they do have a flip side too. Firstly, cutting the cells in half is a time consuming task. Further laser cutting or any other type of solar cell cutting may also result in junction puncture of solar cell (junction puncture means that the p-n junction of solar cell is short circuited and the cell no longer produces the desired power output) if not tested properly post cutting. Further manufacturing of such module requires a customized line given the fact that assembly of solar cells and bussing (interconnection within strings) is completely different when compared to traditional modules. Overall the entire process may add in manufacturing time right from 10% to as high as 40% (considering reworks and rejections) which may add to its overall cost. As an end customer, he/she may not have full knowledge about the manufacturing capabilities of the manufacturer which may also act as a significant hindrance while evaluating such modules.

Solar Panels
Figure 1: Traditional full cell module (on left) and half cut cell based Doublet module (on right) by Waaree Energies

Junction box

Traditionally available junction boxes (JB) performed only few specific functions i.e. to collect the generated power efficiently and to protect the module from hotspot in case of shadow. However with the market demanding increased functions from the same solar module, a lot of modifications have been made in JBs. In all the previously mentioned modules, the junction box (JB) technology more or less remained the same. However recently newer technologies such as micro inverter, optimizers and smart JB have either replaced or are being utilized in conjunction with the traditional JB (Figure 2). A micro inverter as the name suggests is a miniature inverter which converts the input DC power into grid compatible AC power. It is installed generally on or very near to the solar module which almost completely eliminated the need of DC side wiring, DC safety devices and DC combiner boxes. Unlike central and string inverter based power plant which require adequate voltages to initiate operation, micro inverters have relatively lower initial voltage requirement (when compared on basis of per module). Such lower voltage requirement can ensure that a micro inverter based plant operate for longer hours (i.e. it starts in early in the morning and is operational till late in the evening). Further with each module individually generating AC power, the end customer need not worry much about the mismatch losses. An optimizer on the other hand is a device which converts the DC power from the generating source to the desirable DC power by altering its voltage (both buck and boost is possible). The optimizer in case of solar module could either be embedded inside the module or could replace the diode inside the JB thus acting like a cell string optimizer in both the cases. Thus when the module is under shadow/ uneven soiling/ uneven irradiance distribution conditions, it extracts the maximum power from each string thus reducing the mismatch losses. Further the optimizers could also be placed after single module or in between two modules. The optimizer could be coupled with micro inverter to utilize the advantages of both the technologies. Both the above technology could enable the module to perform almost independently and the (under) performance of one module now had negligible effect on other modules in the power plant. Further both micro inverter and optimizer have an option of module level monitoring and remote DC disconnection ensuring reliability and safety of the plant. Smart JBs have been gaining popularity in recent years. They usually replace the traditional junction box (and its diodes) with the complex electronic circuits which are capable of performing function(s) ranging from data monitoring, providing safety against fire by remote DC disconnect, power optimization, enhancing length of string (in few cases) to name a few.

All the three technologies of JB mentioned above have individual advantages with no clear winner. The micro inverter produces AC output directly from the solar module thus eliminating the DC losses while the optimizer (and to some extend the smart JB) ensures that the module level mismatch losses are minimized. Further while the optimizer replace the diode in JB or are placed in conjunction with the JB, both micro inverter and smart JB are placed near the solar module. With the complex electronic housing for almost each technology, adequate care needs to be taken to ensure proper heat dissipation. While the combination of optimizer coupled with micro inverter seems to have an upper hand, multiple claims from various manufacturers regarding the efficiencies of their technology still seem to be a point of confusion. For an end customer, it means that selecting the right technology of JB or its combination based on the expected energy output may still be a challenge.

Solar modules
Figure 2: Micro inverter based AC module (on left) and smart JB based modules (on right)

All the previously mentioned technologies are the ones which are currently prevalent in the market. Starting right away from technology and structure of cell to the design of the module and further the choice of externally installed components (summed up in Figure 3) makes an end consumer sceptical about the type of module he actually wants. While there are almost endless considerations to selecting a module and there seems to be a no perfect guide for the same, we evaluate two factors which while being common are crucial & almost applicable in almost any kind of situation.

Solar Panels india
Figure 3: Technological hierarchy of available options in PV module

Climatic condition has direct impact on the performance of the module and thus needs amongst the first consideration in selecting an appropriate module technology. Considering temperature as a parameter, the half cut cells are known to have a lower operational temperature (compared to traditional modules) due to its construction. Such module would make a better match at a place with higher ambient temperatures. A bifacial module performs as expected only in the right climatic conditions. It needs what is known as high albedo (a ratio of reflected to incident light of the surface) for optimal performance. The highest known albedo is for snow (in the range of 0.4 to 0.9) and then followed by white sand (0.55 to 0.65). The countries with maximum solar installations lie mostly in tropical and sub-tropical region where the most common surfaces are sand, soil, water and grass whose albedos vary in between 0.1 to 0.4. Such surfaces while could reflect light but cannot fully reap the benefits of the bifacial module. Thus installing bifacial module in such region may need (continuous) modification of the surface over which the module is installed.

The solar power plant could be categorized into two types namely rooftop and utility scale power plant. While enough thought process and design considerations go into while erecting a rooftop plant, reconstruction/expansion of nearby buildings/ civil structures, erection of new electrical/communication lines, etc. could always cast shadow on the module which could reduce the power output of the string/ power plant. With its given advantages of better performance in shadow, half cut cell based doublet module could be the ideal choice of rooftop plant. Further utilizing optimizer in conjunction with the traditional module could also effectively enhance the lost power output by around 5~25%. The utility scale power plant is designed more aptly by considering various factors affecting the output of power plant. In such plants, utilizing the module with maximum power output makes more sense. The currently available solar module with higher power output are half cut cell from PERC based module and enhanced PERC cell size (158.75 mm) based module. In case of conditions suitable for bifacial module, the currently available highest power output module is the n-type half cut PERC cell based bifacial module which has a bifaciality of >90%.

We at Waaree Energies have understood the shifting market dynamics and have the entire product in our offering. Further, we also provide a proper technical guidance to our end consumers ensuring that he opts for the right technology understanding his exact requirements. Waaree Energies recently inaugurated its fully automated 1 GW manufacturing facility at Vapi, Gujarat which is capable of manufacturing all the kinds of products along with the traditional modules.

Let us all pledge to make solar energy the primary source of energy in the near future.