Time for new concepts

Photovoltaics pioneers might remember the time when we asked the industry for strong vacuum cleaners and big electric irons to laminate hand soldered strings. For sure these questions were slightly provoking, but in those days nobody imagined that worldwide demand for solar panels would one day hit 20 gigawatts.
Those days are definitely over. Module producers deal with a wide range of worldwide suppliers for lamination devices today. Besides the established and well known companies such as 3S, Meier, Spire, NPC, Bürkle, Nishimbo or Ecoprogetti, more and more new providers mainly from China and India, are supplying the market, such as Boostsolar or Hindivac. Today investors can choose from a pool of more than 100 lamination unit suppliers worldwide. The quality range is as wide as the spectrum of vacuum chamber dimensions and price levels. While single production lines increased their yearly output by the factor of 15 to 20 within the last decade, laminators usually extend their capacities by a factor of four to six.

Single layer design

The major suppliers still use a single level process system. After loading the substrates into the process chamber, the air is evacuated from the module sandwich and the batch will be heated up to about 140 to 150 degrees Celsius. A membrane slightly presses the different module layers to the hot plate to heat up the glass sheet with the encapsulant to produce a weatherproofed laminate. After a certain process time of about five to fifteen minutes, the module is “backed” and can be cooled down to ambient air temperature.
To prevent quick melting of the encapsulant materials and prevent captured air bubbles, most of the lamination suppliers use lifting pins between the heating plates to separate the modules during the evacuating phase from the hot areas.
To meet the increasing demand for higher throughput in modern module factories, many systems are designed with automated loading and unloading devices as well as cooling units with air ventilation or water cooled pressing units.

Multi-stack vs. sequenced design

With the rising demand for market and production lines larger that 100 megawatts-peak per line, new concepts such as multi-stack systems are gaining market shares. On the basis of time-tested experiences in check card or chip board production, new market players such as Bürkle re-discovered three dimensions, and Meier and Nishimbo activated their dormant R&D projects. In spite of disadvantages regarding service and redundancy, multi stack systems gained high acceptance because they have a better footprint and fit better into a slim production line design. Laminators with up to ten or more levels can be built covering a lamination area of more than 45 square meters per load.
Others, like 3S, reduce the process steps to increase the throughput on single level technology, in accordance with the original two-step lamination concept of pre-lamination and curing, which could often be seen in the early continuous furnace devices of Japan and China. Another variant is the double side heated laminator from Spaleck-Stevens that is equipped with two membranes and allows a “reverse” lamination process.

Heating devices

The systems today use different heating sources such as direct electric, oil, electric/oil hybrids or, such as Komax now offers, an inductive heater device. All of them have their specific advantages but have to achieve high temperature uniformity with +/- 1 degree Fahrenheit and high heating rates to prevent temperature drops when new module batches are loaded. Experiences with bending heating plates, destroyed heater cartouches or oil leakages force module producers to change their suppliers and try different systems. The grass is always greener on the other side of the fence. You can only know the specific problems of the system you have chosen and not the ones of the systems you didn’t buy.
So apart from some essential facts, the pros and cons of different technologies sometimes seem to be merely a question of personal preference, like the philosophies of celebrity chefs who prefer either gas, electric or conductive ovens in their kitchen.
The loading of room tempered glass sheets in batches causes short temperature drops. Ineffective heating elements can easily cause a million euros of damage, if partly not cross-linked materials cannot be detected. The heating system has to be able to create reproducible conditions and processes. The ability to detect, control and operate a lamination system under constant conditions is the key factor for good lamination results. The consistency of the heat is more important than the heat sources.

Costs

The key criteria for lamination devices are reproducible processes, high uptimes with low rejects and reduced cost of ownership. Very simple small laminators with a lamination size of about two square meters are offered at about 30,000 to 45,000 euros, but they usually can’t cover the demand of 24/7 production. High end solutions with pin lifts, cooling devices, oil free vacuum pumps and automated loading/unloading devices cost between 40,000 and 65,000 euros per square meter capex.
According to the company Robert Bürkle GmbH, the cost of laminators, including procurement, energy costs, spare parts, and operating personnel, is currently between 1.70 to 3.00 euros per module, or 0.73 to 1.36 euro cents per watt-peak. Thus the direct cost of laminators only make up about two to three percent of the overall costs of embedding PV modules.
Further trends focus on increasing the process stability, precise control devices and high uptime warranties to reduce the cost of ownership rather than reducing the direct investment. Electrically driven heating systems might get under higher cost pressure compared to gas driven heating elements in countries with higher electricity costs.
Additional potential to dramatically reduce costs within the lamination process will be driven by an optimal combination between materials and processes, optimized heat and energy management, and the development of continuous coating processes such as the R2R technology. Optimally automated material specific processes, as seen in packaging or coating industries, will be able to set up lower process costs.

Materials

The main technical solutions are designed and optimized for EVA (ethylene venyl acetate) materials that cover about 85 to 90 percent of the worldwide market, followed by autoclave driven encapsulation technologies that were developed for safety glass applications and are mainly used for thin film (glass/glass) applications using PVB (polyvinyl butyral).
The best know material with more than 20 years in the PV market is the EVA. With a melting point of 70 degrees Celsius, it attains an almost liquid condition and fills out all spaces and gaps between cells and interconnection wires. It also prevents good adhesion between glass and back sheet foils and promises long time stability under outdoor conditions.
On the other hand, a lot of curing agents, UV filters and filling materials have to be mixed to support a 25 year lifetime. Also the long curing of 30 minutes to build long polymers pushed the suppliers to develop “fast” and “super fast” material. Today a typical full process cycle takes about 12 to 17 minutes.
PVB with the autoclave process is known for more than 70 years in structural glazing or windshield production processes. In the PV area it is mainly used in thin film production, but it needs process times of up to 45 minutes within high pressure and heat process batches. Within a nip roller step, a pre- lamination process at about 65 degrees Celsius is performed. Most of the air will be rolled out in this process step. Also the single module layers will stick to each other, which allows handling and storage in the autoclave in vertical orientation. The post lamination process is typically performed at about twelve bar and 140 degrees Celsius.
Other materials like TPUs (thermoplastic elastomere), with shorter process times, or ionomers, still struggle with higher costs or technological limits such as unfavorable hygroscopic attributes, and still only cover niche applications.

New concepts

Changing markets force module manufactures to reduce costs and prices or gain extra value in quality and strong customer relationships. Both developments open the market for new materials and processes. On the materials side we will see a wide range of new EVA suppliers using new material recipes or exchanging whole material compound families. With sinking market prices for embedding materials, the increasing cost for raw materials gains more and more influence.
Silicones as embedding materials could get a new role given increasing demand for poly feedstock. Using the same raw material source, companies such as Wacker or Dow Corning could supply new materials that make new processes more effective.
In addition to material cost cutting, the extension of product lifetime and warranties will shake the markets as well. Laminator technologies have to cover material and process parameters to ensure extended lifetimes of 30 or more years under different conditions. The comeback of thin film technology or R2R production on flexible substrates will allow complete new embedding technologies.
Why not use roller coating devices similar to the equipment the print industry uses today? Why not utilize roller lamination processes used in the paper and packaging industry instead of batch-wise processing as in paper production? Why not using liquids as varnish or resin and spray or dip coat the products? Focusing mainly to the c-Si 60 cell mass markets, the industry somehow got conservative in developing alternative processes. “Never change a running system” and “keep risks low” seem to be the main credos in lamination issues.

Open questions

All lamination technologies still share the same problem. How can the process be controlled within the laminators or autoclaves? They are still black boxes that don’t allow controlling material issues like out gassing, impacts on the module geometrics, material shrinkage, glass sheet bending or a direct indicator for the interlinking level and the gel content.
Also, mechanical stress to solar cells or double glass applications cannot be directly observed and controlled within the lamination step. We still run long test series that rather follow a “try and error” course than establish error-proofed processes and parameters. Also variations in materials and conditions have a direct impact to the product lifetime.
“It looks good – it might be good” seems to be a common criteria in in-line quality control after lamination. Visible “bubbles” often seem to be indicators for the vacuum system but can also be vaporized gases within the EVA component. In addition to offline gel content measurement and peel-off tests, an in-line quality system that controls 100 percent of the processed modules should be developed to get a closer feedback loop and reduce risks.
Most quality, process or material effects can only be seen in long-term applications in the field. Delamination, “fingers” or “snail trails”, all possible effects of color changes, and power losses frequently shake up the PV community.
Lamination technology today is probably the most advanced, stable and error-proofed key process in the module manufacturing industry, with relative high yields and altogether stable, high uptimes. Critical remarks notwithstanding, it could still be a benchmark for other production steps in module manufacturing, such as stringers or framing units, which are still in need of optimization.
Nevertheless, falling market prices, increasing warranty periods and a worldwide growing market allow new concepts and production technologies to be implemented. The question today is not, will it happen, but rather, who will start it. Roller lamination technologies, coating and printing or monolithic-material concepts are just waiting to be realized.
Sometimes, even simple devices like vacuum cleaners and hot plates or blow-dryers and IR spotlights can get things going.
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