Solar Power World

The bulk of all solar panels made today are based on silicon wafers. In many manufacturing facilities, these wafers are produced using conventional methods such as energy-intensive casting and cutting of large silicon blocks with diamond saws; not what you would expect from a hi-tech clean energy product such as solar panels. Thus, Evergreen Solar (Marlboro, Mass.) chose a different method for manufacturing. In contrast to traditional production, Evergreen Solar designed unique equipment to produce photovoltaic cells that uses cutting-edge wafer technology, which is not only more efficient, but cleaner.

The company’s String Ribbon™ wafers originate from the natural science concept of surface tension. Simply put, the making of a String Ribbon wafer is similar to making a soap bubble — the surface tension between the soapy bubble solution and the wand is what creates the bubble. Using this principle, Evergreen replaced the “ring” inside which the bubble forms with two parallel high-temperature filaments, between which a thin film of silicon is formed. The filaments are pulled vertically through a silicon melt using a motion control system driving three types of motors. The molten silicon spans and solidifies between the filaments to form the bubble.

NoticeTo produce string ribbon silicon wafers, long hightemperature filaments are unwound from spools and run through molten silicon, where a long strip of silicon is pulled out of the melt. The wafer strip is then moved into a laser cutting box and cut into individual wafers.

The process is continuous, silent, and clean – performed inside a clean room. Long high-temperature filaments unwind from spools, run through the molten silicon, and pull a long strip of silicon out of the melt. This continuous strip of wafer material moves into a laser cutting box that cuts the strip into wafers “on the fly,” and into solar cell lengths. The process is performed inside the company’s state-of-the-art Quad furnaces. Current furnaces grow four string ribbons – 3.2-in. (80 mm) wide strips of wafer material – at a time.

Design adjustments
About 400 furnaces produce the solar cells. Each one uses a feeder drive and cam followers to move the drive shafts horizontally in and out of the furnace in a reciprocating operation where the silicon is applied. Guided by the cam followers, the drive shaft is controlled through signals coming from a motion control system. The drive shaft slides in and out of the sealed furnace area, while a layer of silicon is vibration fed down through a nozzle and onto the crucible that holds the melt. The slightest movement of air inside the furnace chamber can cause a disturbance large enough to negatively affect the silicon melt and string ribbon. Since air movement can be by pressurizing the furnace, (either positive or negative), Evergreen engineers created a way for the drive shafts to oscillate (one goes in and one out) to maintain the same internal furnace volume so that no air is stirring.

The String Ribbon Wafer furnaces operate inside a clean room so that no contamination can interrupt the proper evolution of the crystal growth process.

The furnaces operate inside a temperature and humidity controlled clean room, yet debris from the silicon as it is deposited and cut easily gets inside some of the mechanisms, particularly the cam followers used to guide the filament through the cycle. In addition, the chamber environment is highly corrosive, cutting short the wear life of the original cam followers.

These cam followers were made of two pieces, the stainless steel cam follower itself and a plastic ‘tire’ that was assembled on it to avoid the need for lubrication between the stainless steel guiding rod and the cam follower, as well as to reduce noise. Doug Miller, mechanical engineer, said, “All the standard cam followers we bought were failing. This increased down time and maintenance costs, which increased the cost of making the product. I had to find a solution.” The company first tried other plastic materials for the tire, but most of them showed significant wear in the harsh environment in less than a week.

Miller had already tried several manufacturers’ components, but continued to research his options and eventually contacted Intech (Closter, N.J.). He explained the problems he was having with the furnace breakdowns, as well as the need to maintain a contamination-free production environment.

The feeder assembly unit performs critical operations in the company’s manufacturing process. The feeder drive assembly illustrates how the filament material slides in and out of the high-temperature sealed furnace chamber simultaneously to eliminate noise.

The designed solution was for a semicustom cam follower. Intech engineers modified their standard iCamFollower® devices in several ways. First, they sealed the stainless steel ball bearings from the harsh, abrasive silicon dust. Second, the company integrated an Intech PowerCore™ tire onto the cam follower so that secondary assembly was not necessary and to eliminate another possible maintenance problem. “The new component was not only superior to what we were using, it helped to cut costs to our production,” Miller said.

Fourteen of the iCamFollowers are used in each feeder system of each Quad Furnace system. Through the slight modifications, the devices were able to be direct replacements for the cam followers already in operation in the other 400 furnaces.

iCamFollowers are lubrication free, eliminate rail wear, are resistant to flat development, greatly reduce noise, offer lower rolling resistance, and are resistant to harsh chemicals. Also, by incorporating stainless steel bearings, they resist the harsh environments in solar wafer or semiconductor foundries. The iCamFollowers can also carry axial loads, lower vibration, and can operate at extremely high and subzero temperatures.

The furnaces are run inside a clean room so that no contamination can interrupt the crystal growth process. Such contamination would destroy the efficiency of the solar cell in the long run. Besides eliminating the wear on the tires, which provided longer life to the components, the cam followers delivered smoother operation and better reliability through lower inertia and rolling resistance. This reliability was proven through the motion system, which used to have to self-correct for missed steps during the process. With the cam followers in place, missed steps happened much less often. Fewer roller failures have also cut down on maintenance considerably.

Overall, what appears to be a simple component designed inside a large machine can have an important effect on the overall operation of the machine itself as well as provide a serious savings in maintenance and repair.

Intech Corp.
intechpower.com