Improved performance and efficiency in photovoltaic systems have traditionally focused on advances in battery technology or charge controllers. Recently, however, solar module makers are looking at specialty glasses for better performance.

Both crystalline silicon and thin-film module makers have long known that low-iron soda lime glass can provide higher conversion efficiency relative to standard soda lime glass. Standard soda lime glass has been used in PV panels up until now, largely due to availability.

Low-iron glass provides higher optical transmittance as compared to standard soda lime glass

Float glass manufacturers throughout the world produce a range of thicknesses, with 3.2 millimeter thick soda lime being the most common due to its use in applications such as architecture, transportation and now solar modules. Though a number of factors contribute to increased solar cell efficiency, low-iron glass provides higher optical transmittance as compared to standard soda lime glass. Corning’€™s engineered glass, for example, provides optical transmittance performance that exceeds both.

If one considers the 400 nm to 900 nm wavelength range of the solar spectrum, measurements show that standard soda lime glass transmittance decreases rapidly from just below 90% at 400 nm to less than 80% at 900 nm. Low-iron soda lime glass performs better, exceeding 90% transmittance at 400 nm, though the transmittance declines to less than 90% at 900 nm.

High optical transmittance is only one factor which contributes to higher solar cell efficiency. Iron-free, engineered glass has been proven to increase thin-film cell efficiency even further.

High conversion efficiency creates significant value.

Specialty glass further enables high efficiency through its ability to withstand high absorber layer deposition temperatures. While soda lime glass is readily available for photovoltaic applications, the ability of this glass to withstand high temperature (up to 600°C and beyond) is a limiting factor. New engineered glass from Corning presents the opportunity to raise absorber deposition temperatures, with demonstrated absolute efficiency increases of greater than +1% achieved by depositing thin film absorber layers at high temperature. The use of increased absorber deposition temperature results in a higher quality semiconductor film, and hence, higher solar cell efficiency.

Raising cell efficiency should be looked at as more than just a technical measure of solar industry progress. Increased efficiency creates higher energy output for a given system size, and can reduce overall balance of system (BOS) costs.

Consider a side-by-side comparison of a hypothetical thin-film module with an area of one square meter. It’€™s reasonable to assume that soda lime glass enables a module efficiency of 10%, whereas the use of a specialty glass could potentially increase this to 12%. A 100 W module would now produce 120 W when manufactured with engineered glass. Efficiency and power output are correspondingly increased by 20%. Reduced weight reduces costs

A secondary benefit of using thinner, specialty glass is weight reduction. Specialized glass can be produced in different thicknesses to meet customer specifications. Instead of the traditional 3.2 mm soda lime glass, module makers will find engineered glass to be significantly thinner, no greater than 2 mm.

The same one square meter module described above using one sheet of 1.5 mm specialty glass combined with 3.2 mm soda lime glass weighs 28% less than the same module using two pieces of 3.2 mm soda lime. The result is lower BOS costs by reducing transportation and installation expenses.
More efficient, lighter, thinner but is it reliable?

Corning specialty glass for thin-film photovoltaic solar panels. The majority of solar module warranties cover a period of 25 years, and depending on location, the installation may be exposed to wind, rain, hail, snow and even blowing sand. Despite being much thinner, the special nature of engineered glass makes it reliable for solar installations. Engineered glasses made by Corning meet or exceed International Electronic Commission (IEC) standards.

This includes withstanding a 25 mm ice ball impact at 23 m/s, wind load resistance of 2,400 Pa, and heavy snow load of 5,400 Pa.
Looking ahead.

As the trend indicates, the glass of choice used in solar modules is changing as new, engineered glasses are being developed and customized to achieve higher conversion efficiency. Corning is tailoring glasses for each of the leading thin-film technologies: cadmium telluride (CdTe), copper indium gallium di-selenide (CIGS), and Si-Tandem. Corning’€™s research has produced consistently high cell efficiencies for CdTe, and achieved a world record 11.9% cell efficiency for Si-Tandem.

Independent of technology, increased conversion efficiency and lower cost per watt is vital for the long-term success of the PV market.

Written by Dr. Mark Krol | 10 August 2011
www.solarnovus.com

About the Author
Dr. Mark Krol is Commercial Technology Director at Corning Photovoltaic Glass Technologies in Corning, New York.