RIMAS’ Technology Manager Eugene Widlak says, “At the PV SEC exhibition in Hamburg Germany, a prototype of this interconnection tester was presented. Thanks to a hand scanner model people could try it first hand on a prepared demo module. The response was extremely useful. Based on the feedback, we will finalize the technology, getting from a prototype to a more customer friendly solution. Obviously one of the models will be an inline device which can run 24/7, connected to the data management system that correlates data with the efficiency data of the sun simulator.”

Enormous amounts of money are invested in stringer equipment and in the development to make them better and more reliable. An inline technology to measure the quality as a reference, seems logical both for stringer manufacturers as well as their customers, the solar module factories. But could the hand scanner become success as well? Widlak says, “Apart from the flexibility of such hand scanner in the production process, we learned that there is more potential. The management of solar energy are eager to learn about the cause for efficiency (value) degradation of their modules in the field. The panel efficiency is monitored by computers. But in case of a fallback, you want to point out what causes it.”

Most contracts between module producers, installers and end users hold an efficiency warranty clause. All parties, including the financial and insurance parties involved, look for ways to manage the risk. Users need assurance that modules are in good shape, where producers want to minimize the risk of having to replace modules in the field. Managing these quality aspects during production, in a stage that flaws can be corrected, is, therefore, extremely relevant.  

What to Do about Cracks?

Another aspect that causes efficiency degradation, is the presence of micro cracks in solar cells. Cracks can propagate in the field, among others, due to temperature variation, potentially leading to loss of efficiency and even the loss of a complete string. To protect the customer from such quality degradation, Electroluminescence (EL) is sometimes used in production to make high resolution images. Mostly this is done on a matrix level but nowadays also string testing is on the market as a useful option. Although some EL units are used after lamination, it is advisable to use it before the laminator when cells with visible cracks still can be replaced. The downside is that such repairs are expensive, it has to be done manually.

Ronald Ceulemans, RIMAS head of solar sales, says, “Obviously we are a supporter of using (EL) test equipment inside module lines. It protects the customer in the field from cracks inside modules and also it protects manufacturers─our customers─from potential quality claims. However, there is one thing that is astounding. The majority of the production companies do these EL checks during the production process, at a string or matrix level,  but neglect to test the individual incoming cells. In case of inferior cell quality this is what we call in The Netherlands--mopping with open tabs─which means it is counterproductive.” Crack-related problems refer to inline breakage and module efficiency degradation during the production process. Research at several cell and module manufacturers, has revealed that most of the crack-related problems can be solved by doing incoming inspection. Ceulemans says, “It means that the majority of these problems are already in the cells. Some process managers refer to stress as cause of breakage, for example due to handling and stringing. Yes, stress makes things worse. But a crack-free cell is quite strong and can handle quite an amount of stress. And even if stringer manufacturers succeed in making stringers that cause less stress, it doesn’t really solve the problem. On the contrary, if 1% of the cells has a hazardous crack that survives the production process, every second module in the field is in the danger zone.”     

If you look at things from this perspective, it comes as a surprise that incoming inspection is still not common in module production. It also might have something to do with the unavailability of technology. The inspection of a matrix with EL is often done by the human eye. In a production flow, the operator has 60 to 90 seconds to inspect the EL image of a matrix, which is enough. However, at a cell level it has to be done in 1 second. Especially with multicrystalline cells, it is kind of hard to automize this because of grey boundaries and other irregularities that hurt the reliability of optical imaging.

After studying the market, RIMAS decided to adopt a technology for incoming inspection of wafers and cells, that is not based on optical inspection. Scientists from Ultrasonic Technologies Inc. in Tampa Florida, developed an ultrasonic-based technology which is very reliable while not sensitive to visual irregularities. This Resonance Ultrasonic Vibrations (RUV) technology has been in use at several production sites since 2010.

According to Ceulemans, the combination of EL at a string or matrix level and RUV for incoming inspection might be perfect. Why? EL is and remains a valuable quality protection. But looking at the financial aspect; by investing in EL the repair and scrap rates go up. So producers invest in equipment that leads to an increase of cost. At that specific production stage, it is too late for quality claims, the supplier will hide behind stress infliction in the stringer. However, by establishing the crack-related quality before stringing and filtering out the unstable cells with RUV prior to putting them in production, the costs go down again. Repair and scrap rates go down drastically and EL simply produces nicer pictures that go along with modules as a quality passport. This way everybody wins.

Lean Production

These smart solutions that RIMAS is investing its energy in, serve a clear purpose; improve the yield of the line and the reliability of the module. But most importantly, they serve the specific needs for an individual customer. For example, when it comes to the level of automation, the company strives to find the most lean solution, instead of aiming for the highest degree of automation. Obviously, it depends on issues like the necessary output capacity and the hour rate of operators, leading to the optimum cost per module or watt peak. Verstraten says, “But when you focus on emerging solar regions, where a lot of our customers are, considerations change. Companies might be supported by local, national or international economic support programs in order to boost employment in certain regions. In such case, you obviously do not suggest a high number of robots that eliminate the need for manual labor. Instead we help them with smart tools to work semi-automatically or even fully manual, but as efficiently as possible. And apart from that, the cost of robots can be so high that a return on investment sometimes isn’t realistic anyway.”

Future Developments

Recently, RIMAS took over VITRODODI Solar, specialized in glass washers. This equipment, in the 3D image visible at the right bottom, is the first unit in each line. Verstraten says, “We recognize a lot of potential for smart integrated solutions at the pre-stringer stages of production. I cannot reveal our plans but you will see it when we get there. We are now moving Vitrododi from Milan to The Netherlands. So first things first.”