Solar Glass Types

Solar flat plate collector glass is normally made up with a low-iron content glass, this makes the glass far more transparent to Infra-red light as well as the visible radiation, this can increase the amount of energy transmitted through the glass up to about 86%. The energy passing through the collector glazing as a percentage of the overall energy striking the collector is known as the zero-loss coefficient or the optical efficiency.

Vacuum tube collectors with higher operating temperatures need to use glass with thermal shock resistant characteristics. Generally borosilicate and soda-lime Pyrex type glasses are deemed to be the best performing materials in this regard.

Low-Iron Solar Glass

Low iron glasses have similar chemical composition and properties as soda-lime float glass except the iron oxide content is significantly reduced, providing a less "greenish" tint. This allows a higher percentage of the solar spectrum to penetrate giving a higher optical efficiency.

For safety and strength reasons, the glass cover is made with tempered glass. Toughened or tempered glass is glass that has been processed by controlled thermal or chemical treatments to increase its strength by about a factor of 6 compared with normal glass. It is made by processes which create balanced internal stresses which give the glass strength. It will usually shatter into small fragments instead of sharp shards when broken, making it less likely to cause severe injury and deep lacerations. All collectors using a glass cover (sometimes plastic is used) will have tempered glass.

Soda-Lime Glass

Soda-Lime type float glass is the standard glass utilized in windows, doors, mirrors and most architectural applications. Available in standard thicknesses, it is readily available and relatively inexpensive. It offers advantages over plastics such as acrylics and polycarbonates in clarity, scratch-resistance, flatness and rigidity. The glass is produced by "floating" the glass on a bath of molten tin and mechanically manipulated to produce the right thickness.

In chemistry and physics, Dalton's law (also called Dalton's law of partial pressures) states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture. For example oxygen makes up 21% of the earth's atmosphere. The pressure that Oxygen exerts is ONLY 21% of atmospheric pressure. There is some debate in the solar industry about very small helium molecules being able to penetrate a vacuum tube made from borosilicate glass. (soda lime is better). However Helium makes up 5.24 ppmv (0.000524%) of the atmosphere, so the maximum vacuum degradation due to helium absorption is 0.000524% of 1 bar.

Helium absorption by borosilicate glass

Tests carried out by G.L. Harding of the University of Sydney looked at the time-constants for the helium to the maximum level of 0.000524% within the borosilicate tube. However depending on the temperature of the tube when pressurisation is equalised, this figure changes. If pressure equalises at 200 °C then at this temperature the helium pressure is 0.000524% of 1 bar. However during normal operation when the tube is cooler, this pressure drops. See table below for exact figures.

Thus a helium pressure which may accumulate after a long period of collector stagnation will result in heat losses of only ~10 Watts/Meter² for subsequent operation of the collector near 100°C. The maximum output of a collector in normal operation at these temperatures is about 400 Watts/Meter². Hence the maximum degradation is of the order of about 2.5%. Generally collectors run much cooler for water heating applications, so in reality the thermal losses can really be considered next to negligible.

Further information can be found in

Scanned Document - G.L. Harding - Helium Permeation in Solar Tubes

"HELIUM PERMEATION IN ALL-GLASS TUBULAR EVACUATED SOLAR ENERGY COLLECTORS" by G. L. HARDING
School of Physics, University of Sydney, NS W2006, Australia