Glass Manufacturing, Science & Standards
Is a decline in the compressive strength of solar module glass partly to blame for some of the unique field failure modes associated with new ultra-large-format module designs? To find out, we interviewed an expert in glass reliability science.
JAMES WEBB, CORNING
Dr. James Webb is a senior research manager for reliability sciences at Corning, a pioneering glass manufacturer founded in 1851. Today, Corning provides state-of-the-art products for a variety of applications, such as optical communications, mobile consumer electronics, displays, automotive supplies, and life sciences. Dr. Webb’s research, development, and engineering activities are foundational to the company’s product and process innovations for ceramics and specialty glass.
RETC: What are the challenges associated with using glass as a structural material?
JW: Glass is a unique material that is largely used for two main properties: chemical stability and visual transparency. Certainly, glass is not commonly used as primary beams supporting a building, but people do use it, in a sense, to manage applied loads. What is interesting about glass as a structural material is that its strength is largely an extrinsic property, meaning it is a function of something you do to the glass that changes its strength. The strength of best-in-class commercially available glass optical fiber is on the order of 10 to 14 gigapascals, which approaches theoretical limits. By comparison, the practical strength of plate glass is multiple orders of magnitude lower, on the order of 70 to 100 megapascals (MPa). What dictates glass strength is largely what manufacturers do to the surface. For example, heat tempering adds compressive strength to the surface of the glass. This compressive strength provides protection against surface flaws that would otherwise change the glass’s inherent strength. Flaws resulting from manufacturing processes, handling processes, or environmental exposure will reduce structural strength in glass.
RETC: How does heat-strengthened glass differ from tempered glass?
JW: The best way to understand these differences is to refer to ASTM C1048 Standard Specification for Heat-Strengthened and Fully Tempered Glass. This standard differentiates between fully tempered and heat strengthened based on surface stress levels and breakage patterns. For example, the standard defines tempered glass as having a minimum surface tension of 69 MPa, whereas heat-strengthened glass will have a surface compressive strength of 24 to 52 MPa. In terms of breakage patterns, the stress level in tempered glass is intended to ensure that for improved safety, the glass breaks into small cube-shaped pieces of glass rather than sharp jagged shards. While fully tempered glass meets building code requirements for safety glass, heat-strengthened glass does not. Annealed glass [ordinary float glass] has a surface compression of less than 24 MPa and may fracture as sharp shards, which can pose a risk of injury.
RETC: Is there a relationship between glass thickness and heat strengthening?
JW: When a piece of glass is tempered, its temperature is raised to between the annealing and softening temperatures, where the glass can adjust its own stress quickly. Then it is quickly cooled. During cooling, the glass surface shrinks first, while the interior of the glass still has the ability to flow. Surface compression then develops as the interior completes its process of cooling, shrinking, and solidifying. This results in tensile stress in the center of the glass and compressive stress on the exterior surface. What drives the tempering process is the manufacturer’s ability to develop a temperature differential through the thickness of the glass.
RETC: Is it possible to fully temper thinner sheets of glass?
JW: Yes, it is. However, the temperature differential in the glass decreases as the physical distance between the interior and the exterior decreases. Unless something is done to quickly extract more heat from the surface of thinner glass, the temperature differential is not going to be enough to develop tempering stress. With 2.0-mm [0.08-in.] glass, some form of accelerated cooling—such as forced air—is needed to get it to temper. However, this may require different manufacturing equipment and potentially creates challenges with respect to cost. When using existing manufacturing infrastructure to manufacture thinner glass for architectural applications, you may only be able to produce heat-strengthened glass.
RETC: Are there practical alternatives to heat strengthening for solar glass?
JW: Chemical tempering is one alternative to heat tempering. Corning uses chemical tempering to produce its Gorilla Glass product, which is used for products in the mobile consumer electronics and automotive spaces. With chemical tempering, you are submerging glass into a molten potassium salt bath and exchanging the sodium ions for potassium. Since potassium is much larger than sodium, the process basically shoves a larger ion into a small hole. Chemical tempering develops compressive stresses that can be much higher than with thermal tempering. While this is an effective process for producing glass with an impressive strength profile, a cost-benefit analysis is required to determine its suitability for solar applications. Thermal tempering provides a good equilibrium between cost and strength. Another option is for manufacturers to maximize glass strength by considering panel, process, and system design strategies that lower glass stress, improve handling, and reduce strength-limiting damage.
“Unfortunately, simply buying strong glass and putting it into a manufacturing process will not necessarily result in a strong solar module. The glass needs to be kept strong during module assembly, which is not a benign process.”
RETC: Is using thinner glass a good way for module companies to reduce costs?
JW: There is often an assumption that glass gets cheaper as it gets thinner, which is not necessarily true. Certainly weight can be reduced by manufacturing with thinner glass. Similarly, more glass can be produced pending the amount of material that goes into the tank. However, the most cost-effective glass thickness is somewhat process dependent. Manufacturing float glass is a complicated process. The process speed needs to be right. There are rollers involved to stretch the glass out. Rollers are also used to get the AR [anti-reflective] texture utilized on solar glass. As the glass gets thinner, handling requirements also change. Handling, pick and place, and rolling equipment need to be adjusted to account for the increased flexibility of thinner glass. Common adjustments include decreasing the spacing of rollers and suction cups. Thin glass edges will have smaller radii, which can have implications for tolerance to impact events during the manufacturing process. There may be short-term increases in handling issues as solar glass is getting thinner. While it is possible to manufacture thinner glass that is very strong, it is also a learning process.
RETC: How can the solar industry mitigate against glass breakage issues?
JW: All glass manufacturers have a relatively structured process for getting glass off their manufacturing lines and into a box for shipment. They will all have strategies for packing, separating, and delivering the glass to the customer. While there may be some subtle differences from one manufacturer to another, it is reasonable to assume that the glass coming out of those boxes generally meets the compressive strength specifications in ASTM C1048. Unfortunately, simply buying strong glass and putting it into a manufacturing process will not necessarily result in a strong solar module. The glass needs to be kept strong during module assembly, which is not a benign process. If module manufacturers are not paying attention to what they are doing during the module assembly process, they can introduce flaws that reduce the strength of the glass. The same thing is true during installation or operations and maintenance. It is important to pay attention to maintaining the strength of the glass.
This interview with James Webb originally appeared in the industry trends section of the 2024 PV Module Index Report.
