Double Glass, Glass On Glass, and Bifacial Panels — What The Hell Are They?

Glass is a vital part of almost every solar panel installed on Australian roofs.  The sheets of glass used to protect solar cells are long lasting, non-reactive, very tough, and — above all else — it’s transparent.  As a result glass functions far better than wood, cheese, iron, or mucus. 

While not as strong as diamond or transparent aluminium¹ it does have a significant cost advantage over those materials.

Because glass is such a great material for the front of solar panels, at some point a bright spark decided it would be even better to have glass on the back as well.  And you know what?  He or she was right.  Glass backing is superior to the plastic backsheet used in standard solar panels, but has the disadvantages of being heavier and more expensive.

Too Much Terminology

Another disadvantage of using two sheets of glass to make a solar panel sandwich is there is no good agreement on what to call them.  Terms used in the order of most Google love to least Google love are:

• Glass-glass
• Double glass
• Glass-on-glass, and…
• Dual glass

The term I tried to popularize, “crunchy silicon sandwich” has precisely zero hits, so clearly I am not the funky trendsetter I always thought I was.  Because I think the popularity of the term glass-glass is due to its use in Germany², I will use the more euphonious name double glass, which I think rolls off the tongue of English speakers a little better.

Bifacial Solar Panels

Another important term is “bifacial” which is used for solar panels that can use of light coming from the front as well as the rear.  All bifacial panels are double glass, but not all double glass panels are bifacial.  I think the term bifacial sounds a little strange, but if they had called them Janus panels very few people would get the joke and if they called them two-faced panels they would carry a bad connotation.  Namely Tommy Lee Jones’s acting in Batman Forever:

 Double Glass Panels Are Durable

Glass doesn’t have a great reputation for resilience.  We say delicate things are fragile as glass, we describe a boxer with an easily broken mandible as having a glass jaw, and I have a heart of glass because I am sensitive yet quick to love.  Also because I eat chocolate coated bacon three times a day.

I thought people’s estimation of the strength of glass would increase because so many of us carry around an incredibly tough sheet of it less than a millimeter thick on the front of our smart phones.  But because we all too often pay ridiculous amounts of money for our electronic squawk boxes, when it does break it makes us completely forget about all the times it didn’t.

The glass used on solar panels is strong enough to withstand 2.5 cm hail, which is as large as it normally gets.  Unfortunately, things aren’t always normal in Toowoomba and during one storm fist sized hail started smashing through our roof.  Solar panels won’t survive rare, giant, horse killing³ hail stones, but they will stop a hole being punched through your roof, so their sacrifice won’t be in vain.

Solarwatt is a German company that only makes double glass solar panels.  At just 2 mm thick they use the thinnest sheets of glass in the solar industry I know of, which allows their panels to be no heavier than standard ones.  But despite only having a total of 4 mm of glass, they can survive being jumped on or smashed with a hammer, as this short film demonstrates:

Unfortunately, this video has no sound and so I strongly suggest everyone write to Solarwatts⁴ and demand they pay Arnold Schwarzenegger to dub in the word “boing” every time someone jumps on the panel or hits it with a hammer.  The great thing about this suggestion is his ridiculous Austrian accent will also make it funny for Germans⁵.  For the slow motion shots he should say, “BOOIIIINNNNNNNGGG!”

While the thin glass panel does not break, despite all the abuse it is given, you can see being smashed with a mallet does leave some marks and they are not good for the panel.  Also, all that flexing can damage the solar cells and wiring inside.  But according to Solarwatts, the damage will be less than with a conventional panel with a plastic backsheet.

Flexing Causes Fewer Microcracks

Solarwatts says when a standard solar panel is flexed by wind, snow piling up, or people jumping on it while Arnold Schwarzenegger says “boing”, the solar cells won’t only be bent, but will also be stretched, contributing to microfractures that degrade panel performance.  But stretching apparently does not occur with double glass panels, as this Solarwatts diagram demonstrates.  (For some Germanic reason they call standard panels “glass-foil”.):

Of course, you’d expect a company that only makes double glass panels to say good things about them, but if you can’t trust Germans who can you trust⁶?

Glass Is Non-Reactive

Glass is very non-reactive and so, provided it is not smashed, under most conditions can remain almost unchanged for thousands of years⁷.  For example, this wine bottle is around 1,700 years old.  Because glass is non-reactive, chemical reactions won’t occur between it and the solar cells or the epoxy that bonds panels together.  While plastic backsheets are not very reactive, glass is in a whole different class.

Double Glass Can Reduce PID

Because glass is non-reactive and two layers does a great job of keeping water out of the panel, double glass panels can reduce, or possibly eliminate, damage caused by Potential Induced Degradation or PID.  This is caused by stray electrical currents going where they shouldn’t.  It’s  worse is humid climates with high temperatures, causes marks that look like snail trails, degrades panel performance, and can eventually cause them to fail.

Bifacial Solar Panels Like It Both Ways

Nearly all solar panels have a reflective layer on the bottom of their solar cells so light hitting the bottom of the cell will bounce back through it again to give it a second chance to be absorbed and generate electricity.  In a bifacial panel, because the bottom of the solar panel is glass, this reflective layer can be left off to allow light coming from behind the panel as well as the front generate electricity.  Even among double glass panels, bifacial ones are still a minority, but they are gaining acceptance and in the future they may be used in solar farms on a large scale.

Bifacial Panels In Racks Produce More Power

When bifacial panels are mounted in tilted racks, light can reflect off the ground and pass through the panel from behind, increasing their performance above conventional ones.  According to LG Solar, under perfect conditions their bifacial panel can generate 27% more energy.  But I don’t think perfect conditions exist in Australia, unless you happen to own a field full of mirror balls to install them over.  Also, their results are based on one clear summer day in Germany so we can only guess how well they’ll perform in reality in Australia.

This graph I stole from LG Solar shows their results when racked up over a variety of different surfaces:

The worst performance is obtained over soil or green grass, where LG’s bifacial panel only produces 5.5% more kilowatt-hours over a sunny day than an otherwise identical standard panel.  But grass is a type of living solar panel and doesn’t reflect much light except for green⁸.  Of course, in Australia green grass is an abomination and only exists in Victoria and an insignificant portion of the rest of the mainland.  Normally it’s either yellow, brown, grey, or turned to dust and blown away.  So in this country bifacial performance over grass may be closer to the figure given for dirt or sand.  Unless you put effort into preparing a bright surface and keeping it clean, it may be difficult to get performance much higher than for sand, unless perhaps it is Lucky Bay beach sand which is so white it will make your eyeballs sizzle.  In this country don’t go putting hope into “snow” boosting output.

One useful feature of bifacial solar panels is they have improved performance in cloudy conditions compared to standard ones.  Or at least they are supposed to.  I’m starting to wonder now, because you would think LG would have included cloudy days in their comparison if there was a significant improvement over conventional panels, wouldn’t you?  Assuming it is true, this is useful because reduced solar generation can make electricity more valuable when it’s cloudy, especially in winter when clouds and high electricity use for heating go together.

They Like It Both Ways — But The Front Way Is Better

Because there is normally more wiring on the back of a solar cell than the front, and also because there is a junction box and cables on the back that are aren’t transparent, bifacial panels are more efficient at turning light into electricity from the front than from the rear.  For example, Longi Solar’s bifacial panel has the best performing rear in the business and, when exposed to equal amounts of light, it will produce 82.15% as much as power as the front.  Having one side less less efficient than the other is mostly unavoidable, but isn’t a major problem problem provided you remember not to install them upside down⁹.

Don’t Install Bifacial Panels On A Normal Roof

Bifacial solar panels will only produce more electricity if light can hit the back of the panel and that is not going to happen when installed flat on a roof.  The gap between the panel and the roof is what we scientists call very small and not much light is going to get in there.  It doesn’t matter how pearly white your roof is, you will produce more electricity with panels that have a reflective layer on the bottom of the solar cells than bifacial ones.  Despite this, I have had a solar panel salesperson tell me a bifacial panel will still generate additional energy when used for a normal roof installation.  He seemed nice enough, so either he was dim or a two faced, bifacial bastard.

If you have a flat roof and are mounting your panels in tilted racks, then bifacial solar panels can produce extra energy, especially if you have a shiny white roof.  Unfortunately roofs usually don’t stay shiny white for long.  They often get grubby because rain doesn’t wash flat roofs clean as well as tilted ones and their material isn’t normally as dirt resistant as glass panels.

Bifacial Panels And Inverter Limits

An interesting feature of bifacial panels is while they can produce extra energy compared to standard ones, this does not appear to affect their capacity rating.  So bifacial panels can allow people who are limited in the amount of panel capacity they can install generate more electricity they they would using conventional panels.  If the solar panel capacity is at or close to the maximum of 133% of the inverter capacity, then a small amount of the extra energy produced by bifacial solar panels can be lost, but it’s not likely to be much.

The inverter will need to be capable of handling the extra power, but because of the usual 133% limit, it’s very unlikely to be a problem for on-grid installations in Australia.

Double Glass Panels Can Be Used As Roofing Material

Double glass panels have been used as roofing material and this can help offset their cost.  For open structures, such as verandah roofs and carports, bifacial panels can allow for extra generation.  Double glass panels can also be used for closed structures, but a lot of thought needs to be given to the design because solar panels can get very hot.  While it doesn’t happen often, on a hot sunny day panels can hit 65 degrees.  Also, sunlight can enter through the gaps between solar cells and this can turn a room into a greenhouse.

The CSIRO library in Newcastle, NSW installed double glass solar panels as a roof without a ceiling underneath and they ended up using retractable tarpaulins to try to control the light and heat.

Double Glass Warranties Are Better Than Average

Because of their slow rate of deterioration, it is normal for double glass panels, including bifacial ones, to have a 30 year performance warranty that says for up to three decades they will still produce over 80% of their original output.

Solar panels normally have product warranties that are shorter than their performance warranty.  Solarwatts offers a 30 year product warranty. That is as long as their performance warranty, but they are Germanic warranty gods and offer the best set of warranties I am aware of.

Here is a list of some double glass product warranties from various manufacturers:

• Solarwatt 30 years
• Solarworld Bisun¹⁰ 20 years
• LG Solar NeON 2 Bifacial 12 years
• Longi Solar 10 years
• Seraphim Solar 10 years
• Trina Solar Duomax 10 years

As you can see there is a wide range, with the 10 year product warranties being kind of disappointing given they are willing to boost the performance warranty to 30 years.  But slapping a long product warranty on something a manufacturer isn’t sure will go the distance isn’t a good idea either.

While I am not a lawyer, it seems clear to me that under Australian Consumer Law a manufacturer will be required to replace panels that fail, through no fault of their user, for their entire performance warranty period which will normally be 30 years for double glass panels.  But it is possible you will have to fight the manufacturer and take them to small claims court or a consumer tribunal to get a satisfactory resolution if their product warranty is over.  (At the moment we are waiting to see how they respond.)  But if you want long lasting panels, because ones with longer product warranties are more likely to be reliable, go with them.

The Future Is Glass — But One Sheet Or Two?

Solar panels are going to have glass on top of them for a long time to come.  Even if something revolutionary and new is developed it will probably still need glass on top.  Whether or not we’ll see more panels with glass on the bottom is harder to say.  While both solar panel and TV screen manufacturers have already put a lot of effort into lowering the cost of tough, high quality, sheets of glass, further reductions seem likely.  Also, as more efficiency gains are slowly wrung out of solar cells, the extra cost of double glass per watt of solar panel capacity will decrease.  So it is possible it will become common, at least among premium panels.

While bifacial panels won’t see a great deal of residential use, they could still be popular for carports and above patios.  I do think there is a good chance bifacial panels will see a lot of use in solar farms, but I’ll save my thoughts on that for another article.