By Chengjiang (CJ) Fu and Hongbin Fang, LONGi Solar
Large-format modules are the “big” topic in the industry right now because they can produce big results. At first glance, the higher the power each module produces, the larger the cost saving for utility-scale projects with benefits ranging from reduced balance of system (BOS) costs to lower installation costs. But there’s a reason that the top three module manufacturers — LONGi Solar, JinkoSolar and JA Solar — are pushing for the “large-size,” 1.13-m-wide module, rather than the “oversized,” which are greater than 1.20 m wide. When you look beyond the module cost and power output to the lifetime systems cost, it’s clear that the 1.13-m, large-size module (using 182-mm/M10 wafers) is the most cost-effective solution to optimize lifetime returns.
Why isn’t bigger always better? BOS savings generally increase as module power increases, but only to certain levels before other factors kick in to diminish the savings or bring additional reliability risks. In reliability, transportation, handling and full system cost, large-size modules outperform oversized modules (using 210-mm/G12 wafers). Large-size modules are not just an interim solution to reach larger, more powerful module sizes, but instead are the optimum choice to strike a balance between system cost and reliability. Modules and solar systems are built to last 25 to 30 years, or even longer, and performing well against extreme weather conditions becomes a necessity. Module reliability performance throughout the lifetime of PV projects is important to ensure consistent energy yield and return on investment for all PV projects.
As we face extreme weather much more often, module reliability is becoming more critical to ensure long-term returns. Increased precipitation, storms with higher wind speeds and extreme heat all put stress on modules. Analyzing just two of these elements — wind and static load — we see advantages to large-format module when compared to the oversized modules.
In wind tunnel tests conducted by LONGi Solar and TÜV Nord, large-size modules experienced less acceleration during the change of wind speed and less vibration amplitude during consistent wind speed. These results can be attributed to the large-size module’s greater structural stiffness, meaning higher energy required for wind-induced module vibrations, lower risk of failures and ultimately, better reliability. In thresher wind tunnel tests, large-size modules passed the test when wind speed continuously increased to 134.2 mph, while oversized modules failed with deformed and broken bolted screw holes at only 100.7 mph.
Perhaps the most compelling test results come out of the static load tests conducted by LONGi Solar on both monofacial and bifacial modules. When -2,400 Pa pressure was applied to the large-size and oversized monofacial modules, cracks of the oversized module were nearly six-times that of the large-size modules, even with reinforcement ribs installed on the back of the oversized modules. In the same test with bifacial modules, the deformation of the oversized module was 65% greater than that of the large-size module.
With oversized modules, you can certainly pack more watts per shipping container — but at a risk to the modules. When taking into account the container door height and appropriate accommodation for forklift unloading, the maximum module width is 1.13 m. In fact, shipping container dimensions drove the optimal design of large-sized modules. Oversized modules simply won’t fit in a standard shipping container using the vertical landscape double stacking technique — an industry best practice.
It is possible to place bigger modules in standard shipping containers using other loading methods, such as flat landscape and vertical portrait stacking, but both methods have disadvantages. Flat landscape packaging increases cracking risk and vertical portrait packaging increases falling risk because the package is too high and may need the application of a large counterweight for anti-falling brackets.
Shipping container weight is also a consideration. The double-glass that’s part of the bifacial module design significantly increases its weight, not to mention the added surface area for potential microcracking. A 40-ft container fully packed with oversized, bifacial modules will exceed the weight limit on U.S. freeways and add constraints and extra costs to transportation.
Using the vertical landscape packing method with large-size modules minimizes module damage during shipping and ensures the container is not overweight.
There are also advantages to handling a large-size module over an oversized module. A large-side module can be carried by two people safely, but the oversized module size and weight adds risk to module handling.
When two people are lifting an item, the general rule is that the weight should not exceed two-thirds of the total sum of their individual lifting capabilities. Large-size modules weigh in at 71.2 lbs, which is less than the maximum weight-handling limit of two people (73.4 lbs) according to the general industry practice and the Health and Safety Executive (HSE). Oversized modules on the other hand weigh in at 77.8 lbs or above, which exceeds the HSE maximum limit. The added weight results in increased installer fatigue, which increases the risk of damaging the module and personal injury. Similarly, the width of oversized modules further increases fatigue.
Oversized modules do not necessarily reduce cost at the module level because of higher wafer and module bill of materials (BOM) cost. At the system level, you can achieve lower total cost with large-size modules when considering cable and tracker costs.
For cable, the total cost is lowest when the module current is between 14 and 15 A, which is the working current of the large-size bifacial module. Cable cost decreases as module maximum working current increases with a decreasing slope, but the resistive cost increases linearly as module maximum working current increases.
And with trackers, we’re bound by the tracker length. With a 1P tracker, oversized modules will reduce the number of strings from three to two, meaning the total power carried by a single tracker is lower and the cost per watt is higher. With a 2P tracker, oversized modules will reduce one string of modules. The total power carried by the tracker will be equivalent to that of a large-size module, but there will be a string of modules arranged on separate sides of the main axis, leading to power loss due to mismatch.
Larger modules create exciting opportunities for utility-scale projects to realize lower BOS cost and improved levelized cost of energy (LCOE). With limited strength using the same 2+2-mm glass or 3.2-mm glass + backsheet, module size cannot go indefinitely larger without compromising on reliability performance. Choosing large-size modules can reduce reliability risk and overall cost, while still achieving impressive power output.
So what’s next for the innovation race? We can’t keep making modules bigger without risking reliability and long-term cost-savings and benefits. Module manufacturers must refocus on module efficiency and field performance to increase power output, help customers to achieve lower BOS cost and lower LCOE, and push us closer to our climate goals.
Chengjiang (CJ) Fu is the director of technical services, and Hongbin Fang is the director of product marketing at LONGi Solar.