By Wayne E. Boor
For years, solar module producers and other solar technology users have pushed glass manufacturers to develop affordable, heat-strengthened Photovoltaic (PV) glass that is thinner than 3.2 millimeters for use in their products. This is due to the many potential advantages thin glass can provide to advance solar technology, such as higher light transmittance for more efficient energy conversion, as well as incidental benefits related to lighter weight, increased flexibility and potentially lower costs.
Recent developments in furnace technology, along with newfound capabilities using traditional heat-treating lines, are now making thin, high-strength glass for various solar technologies more feasible. While that is good news, solar module manufacturers and other solar technology users must understand the trade-offs and misconceptions associated with thin glass so they can make fully informed decisions before switching from traditional, thicker glass designs.
The purpose of this article is to detail the current state of thin, strengthened PV glass technology so solar module manufacturers can understand its advantages and disadvantages, and determine the true benefits and value of converting to a thin-glass design.
Chemically Strengthened Glass
The availability of high-strength, thin glass for solar, electronic and transportation applications is not a new development. Chemically strengthened thin glass, which is six to eight times stronger than standard, annealed float glass, has been successfully used in solar modules as well as many other applications where exceptional strength and extreme flatness are required such as photocopiers, scanners, high-end display screens, and automotive and aerospace applications.
In the chemical strengthening process, glass is submerged in a bath of potassium salt at temperatures of up to 450°C. The glass is fortified when larger potassium ions from the bath replace smaller sodium ions that reside in the original surface layer, creating surface compression and core tension conditions that are typical to those of heatstrengthened glass.
In addition, because chemically strengthened glass is submerged in a bath, it does not have surface contact with rollers, as glass does during conventional heat-strengthening processes; therefore, the resulting product has very uniform stress distribution, exceptionally low levels of distortion and very good flatness characteristics.
Unfortunately, while chemical strengthening is ideal for producing thin glass that is both flat and strong, it has two major disadvantages: first, the process usually requires special glass compositions to facilitate ion exchange; and second, the glass must be submerged in the bath for a significant period of time. That means it is not well-suited to high-volume production. Both issues make chemically strengthened glass significantly more expensive to produce than other forms of thin, strengthened glass.
New Heat-Strengthening Glass Technology
Thanks to recent advances in furnace technology, most notably by the Austrian company LiSEC, the ability to heat-strengthen─and possibly fully temper─float glass in thicknesses as low as 2.0 mm is now a reality.
This development is significant because it addresses the two primary concerns associated with chemically strengthened glass. The first is affordability. Since the new furnace is designed to accommodate conventional float glass, thin glass can be heat-treated affordably and in large quantities.
The second benefit is derived specifically from the design of the furnace itself. Instead of using a roller to convey glass through the tempering oven, the new technology incorporates a roller-less hearth with a perforated design that moves it through the chamber on a bed of heated air.
Because the glass passes through the unit at a slight angle, only one edge actually makes contact with a conveyor roll; the rest avoids any type of mechanical impingement. The result is a strong, thin glass that has minimal warp, bow or distortion caused by roller contact.
Conventional Equipment to Produce Thin Tempered Glass
In addition to roller-less furnace technology, glass manufacturers are pursuing new developments in conventional roller-hearth furnaces and processing techniques that are enabling the production of fully tempered (>10,000 psi surface compression) thin glass. This new generation of equipment and techniques already has generated full-panelsized pieces of 2.5-millimeter nominal (2.26-millimeter actual) glass that meets ASTM C1048 strength requirements for fully tempered safety glass, as well as architectural standards for overall bow and warp, and roll-wave distortion.
The functionality of this new thin-glass technology is currently being evaluated by solar technology manufacturers, including a large crystalline-silicon PV producer that is measuring the strength and flatness of samples to determine their acceptability. To meet the needs of the industry, the glass must be flat enough to permit good, long-term laminating quality and adequate visual aesthetics. It also must be strong enough to allow finished modules to pass UL-testing for hail impact, static loading and other environmental durability tests.
The Benefits of Heat-Strengthened Thin Glass
As mentioned earlier, one of the benefits of using thin glass in solar applications is increased light transmittance. By allowing more light to reach the active layer of a solar module or the reflective layer of a mirror, thin glass enhances the conversion of sunlight into usable energy.
For PV technologies, this translates into more power output per module and, for solar thermal applications, it can mean more heat generation per mirror. Regardless of the technology, improved levels of light transmittance and energy conversion can lower the capital investment required to achieve a specified level of energy output for a given solar installation.
Lower Module Costs
Most c-Si modules feature a 3.2-4.0-millimeter glass cover plate with a PolyVinyl Fluoride (PVD, or other plastic) back plate that protects the silicon wafers and functions as a moisture barrier. With the availability of stronger, thinner glass, many module manufacturers can now consider the possibility of replacing the existing PVD back-barrier with a second panel of less-expensive, conventional, clear glass. The resulting glasson- glass solar module may not only be less expensive to produce than a traditional mono- glass module, but it also has the potential to be even stronger and more durable.
Some manufacturers considering glasson- glass module designs also are evaluating their compatibility with frameless mounting systems. This would not only eradicate the material cost associated with aluminum frames, but would also eliminate a manufacturing step, which also could result in significant cost savings.
Other potential advantages associated with glass-on-glass module design include lighter weight, reduced packaging, higher shipping density and easier installation. For rooftop and other mounted arrays, the lighter weight also has the benefit of reducing the load factor for each module, which can increase the surface area available for solar collection if roof-load rating is an issue.
In addition to replacing ‘hard’ costs related to aluminum framing, glass-on-glass solar modules also may enable manufacturers to cut inventory expenses (denser storage), eliminate suppliers and potentially improve cash flow. Finally, because the finished modules are lighter and easier to handle, labor costs and the potential for worker injury may be reduced.
For other solar technologies like Concentrated Solar Power (CSP) and Concentrated Photovoltaics (CPV), thinner, stronger glass adds to the benefits listed above by being more flexible than thick glass.
Some reflector designs for CSP and CPV applications feature thicker glass that is coldbent, then bonded to a back frame that already is molded to the required curvature. Thin, strong glass can be cold-bent to more severe radii without breaking, which could conceivably open the door to less-expensive alternatives to thermally bent-and-tempered, thick-glass designs. One concept already in development is to ship thin, heat-strengthened, flat mirrors to the field where they can be flexed and ‘snapped’ into a formed support frame that already has the required profile. This has the potential to significantly reduce the cost of reflector technology.
The Limitations of Heat-Strengthened Thin Glass
Despite the drawbacks associated with its higher cost, chemically strengthened glass still represents the standard for applicability in the solar glass industry. With surface compression ranges of 24,000 to 40,000 psi, chemically strengthened glass is typically four to six times stronger than even the most robust heat-treated glass, which can fracture under stresses of 7,000 to 12,000 psi, depending on the level of strengthening. Many solar module manufacturers have yet to determine if these lower strength levels are adequate for their thin-glass designs to meet the performance requirements set forth in IEC 61464.
Higher Distortion Levels
Although thin glass strengthened with new furnace technologies offers distortion quality that is acceptable for architectural applications, it still cannot achieve the levels associated with chemically strengthened glass. For that reason, heat-strengthened thin glass may not meet the established aesthetic standards of some module designs.
Individual solar module manufacturers must also evaluate the potential impact of higher distortion and diminished flatness on glass lamination during the manufacturing process, as well as their potential for delamination after long-term environmental exposure in the field.
One potential misconception about thin glass is that it is proportionally less expensive (relative to the thickness) when compared to 3.2 or 4.0 mm pricing. Unfortunately, that is not always true. For glass manufacturers, losses associated with thin glass usually are higher than with thick glass due to the increased potential for defects and breakage.
Although the number of defects per-ton may be constant whether producing thin or thick glass, more of these defects can affect the surfaces when the glass is thin. This increases rejection rates and lowers yield. There also is more loss in the transportation and fabrication of thin glass due to its fragility prior to heat-strengthening.
Like the solar companies it serves, the glass industry is in a constant state of evolution. With the potential for widespread availability of affordable tempered (or nearly tempered) thin glass on the horizon, glass manufacturers and their customers are inching ever closer to the achievement of a long-sought goal.
As this article hopefully demonstrates, a number of obstacles remain in the way of an ultimate solution, but recent developments are creating additional motivation and incentives for glass suppliers and their solar industry partners to overcome them. With open exchange and marriage between requirements and capabilities, accomplishing this goal might be just around the corner.
Wayne E. Boor is Manager of Solar Technology Transfer at PPG Industries (www.ppg.com).