Cost analysis of glass types

[longwolf]

I plan to go and order glass on Monday.
I haven't found any info on just how much light gets through regular verses the low iron glass.
The man I talked to said he has samples of both that I can look at.
So I plan to take my handy dandy light meter with me.
I'll post the results once I have them.

In the mean time I thought I'd do some math to see how much difference it would take to make the extra expense worth it.
If built with regular tempered, I'm looking at about $248 per panel, for 144 watt panels.
That comes out to about $1.72 per watt. The low iron would cost another $40. $40 / 1.72 would be about 23 Watts worth. That's about 16 percent of a panels worth of power.

So if there's a 16% diff between the two types of glass, it's a slam dunk. I'll go for the low iron.

Just did some more math. That 16% would apply to anyone that has plenty of space for their panels.
But in my case, there's only the space on the roof of the van. That makes the space up there worth about $25 per sq/ft.
16% of one of my panels will be about 1.7 sq/ft, or $42.
Ok, looks like I'm going LI unless there's VERY little diff in the glass types.

This was kinda thinking out loud, but it may help someone else who's trying to make up there mind.

[Mike90250]

You only need 1% better on the glass, and after 16 days, the cumulative increase is worth it. And be sure to use large enough tab wire, and interconnect wires, don't want to waste all the power in heating wires up.

[longwolf]

I went the the glass company on Monday.
As promised, he had sample of the regular glass and one of StarFire, their low iron glass.

They where both 3/8 of an inch thick.
The ambient light was 815 lux, the regular glass was 700 and (drum roll please) the starfire was 740.

Not 16%, but close enough.
On the down side, I learned that the place that quoted me $175 is the manufacturer and they only sell to retailers.
This placed wanted $230, but I got him down to $208.
Still a big ouch.

[longwolf]

Well, I wish I had done the math the other morning when I compared the samples and ordered the glass.
But I was in a hurry to get to work.

When you do the math, the regular glass is letting about 85.89% of the light through.
While the Starphire lets 90.80% through. That's a little less than a 5% diff.

5% of a 144 watt panel is about 7.2 watts. So on three panels I'd lose 21.6 watts but save almost enough to make another 144 - 7.2 watt panel.

As far as I'm concerned, all the low iron glass is good for is bragging rights.
I won't waste the money again.

[solarnoobie]

Have you calculated the number of days it will take to make up for the cost of the low-iron glass?

Maybe it will be worth it in the long run?

[longwolf]

Have you calculated the number of days it will take to make up for the cost of the low-iron glass?

Maybe it will be worth it in the long run?

Not really, if I where to take the money saved on three panels and made a forth, I'd be getting 115 watts more for about the same price as 3 panels made with the Starphire.

Another way to look at it, I payed between $10 to $13 per watt for those last 7 watts.

I'm stuck, I have to use a special glass size. You might could get a better deal by using one of the standard sizes.
You'd have to run your own numbers.

[wjgrisham]

front panel/glass

Thank you for your input on the glass. Like I said this is all new to me but slowly but sure learning about solar energy is my goal. I see a lot places online for help but a lot of them want money and at this time that is one thing we are short on.My wife was laid-off after 15 years on the same jod and yes I know right now we are not the only ones in that boat right now.
I was watching the world new last night and they said 1 out 7 people live at the poverty right now.We need some changes in this country fast.

Thank you, Bill

[wjgrisham]

sounds good

I plan to go and order glass on Monday.
I haven't found any info on just how much light gets through regular verses the low iron glass.
The man I talked to said he has samples of both that I can look at.
So I plan to take my handy dandy light meter with me.
I'll post the results once I have them.

In the mean time I thought I'd do some math to see how much difference it would take to make the extra expense worth it.
If built with regular tempered, I'm looking at about $248 per panel, for 144 watt panels.
That comes out to about $1.72 per watt. The low iron would cost another $40. $40 / 1.72 would be about 23 Watts worth. That's about 16 percent of a panels worth of power.

So if there's a 16% diff between the two types of glass, it's a slam dunk. I'll go for the low iron.

Just did some more math. That 16% would apply to anyone that has plenty of space for their panels.
But in my case, there's only the space on the roof of the van. That makes the space up there worth about $25 per sq/ft.
16% of one of my panels will be about 1.7 sq/ft, or $42.
Ok, looks like I'm going LI unless there's VERY little diff in the glass types.

This was kinda thinking out loud, but it may help someone else who's trying to make up there mind.

Looking forward to finding out!

[longwolf]

Looking forward to finding out!

I pretty well summed up my thoughts in post #6.
If you're out to make a decent, panel as inexpensively as possible, I'd recommend normal tempered glass.
If you're going to use 3x6 inch cells you can often find inexpensive glass, cut to size, on ebay.

But it does sound like you need to learn more.
If you're wanting panels to power your house, you'll want to use manufactured panels that are UL listed. Otherwise you could have have problems with insurance and the city.
The good news is, you can buy them almost as cheaply as you can make them.
You'll also need to learn about batteries, charge controllers and power inverters.

There's a lot of info on this site, and people here that can point you to other sites that can help you.
Good luck!

[longwolf]

...My wife was laid-off after 15 years on the same jod and yes I know right now we are not the only ones in that boat right now.
I was watching the world new last night and they said 1 out 7 people live at the poverty right now.We need some changes in this country fast.

Thank you, Bill

I can relate, I'm living in my van. That's why I got into this.

[DR Bruce]

Glasses aint Glasses

The glass used in standard crystalline silicon modules is a special chemically annealed low iron glass with a transmissivity of around 95%. Standard Window glass has a transmissivity of around 70% and is no annealed which means it can shatter easily. You can tell the difference between low iron glass and window glass by looking at the colour of light passing edge on through the entire length of the panel. Window glass look green i.e various important light frequencies have been absorbed by the glass and low iron glass looks white meaning most of the important light frequencies have been transmitted.

[longwolf]

I would not recommend using regular window glass.
I'm saying to use regular tempered glass.
For the size panels I'm making, it saves about $100 per panel!

And I disagree with your transmissivity figures. According to sources i've seen on the web, and my own tests with a light meter, the diff is only about 5%

As far as the color goes you may want to check one of my other threads.
Was I given the right glass?
And especially this link in that thread: Starphire (Low Iron)

The glass used in standard crystalline silicon modules is a special chemically annealed low iron glass with a transmissivity of around 95%. Standard Window glass has a transmissivity of around 70% and is no annealed which means it can shatter easily. You can tell the difference between low iron glass and window glass by looking at the colour of light passing edge on through the entire length of the panel. Window glass look green i.e various important light frequencies have been absorbed by the glass and low iron glass looks white meaning most of the important light frequencies have been transmitted.

[DR Bruce]

PV Module Glass

It would be no use to use an ordinary light meter as it will not be correlated to the light spectrum that crystalline Silicon responds to. That is why we get the large difference of transmissivity.

[longwolf]

It would be no use to use an ordinary light meter as it will not be correlated to the light spectrum that crystalline Silicon responds to. That is why we get the large difference of transmissivity.

There's a member here who's made panels with both types of tempered.
I've asked him to post reading from the two panels, but he's never responded.

If/when I get to make my next panel, I plan to use regular tempered. If/when I do, I'll post the results.

[cheapo]

my 2 bits

My first visit and first post here

Would like to thank all contributors in this thread.

I should note that I have little practical experience other than pre-made solar battery chargers.

There are 2 separate reasons glass prevents sunlight from hitting the cells.

The first is surface reflecting, and both air to glass surfaces reflect about 4%, or 8% total. Such is true regardless of the type of glass, unless it has been coated as in a camera lens. Such coatings are usually expensive and easily washed off. Low e glass (which prevents transmission) is coated on the inside surface of double pane windows where it is sealed and never washed. At least one seller claims solar glass that is coated to reduce reflection. They also claim low iron and greater transmission. Transmission would have to also be a function of thickness.

Uncoated surface reflection is largely dependent on the difference in the index of refraction (light bending) between the two materials. Air is close to 1, (vacuum is exactly1), and glass (virtually all types) and plastic have indexes of refraction about 1.5.

My point is that if one bonds the cells to the glass with plastic resin (Sylgard 184 or whatever), you eliminate 2 glass/air surfaces, and should pick up 8%, except where you have bubbles, but such should be minor.

I would not think that the transmission loss within a 1/4th inch thick piece of high iron (green from edge) glass would be that significant.

My big inspiration was to acquire used tempered glass from discarded storm doors or whatever, and build my panels to fit the glass. My difficulty is a cheap resin with good transparency, but such belongs on another thread.

Also, laminated glass may be cut and is on my office door and has survived human clumsiness for 40 years. Has anyone tried it?

Although I agree with and respect the conclusions of longwolf, I would suggest taking both volt and amp readings off a single cell behind various glasses and bonded and unbonded with resin. Not to mention encased in EVA and the other options.

Readers may well consider such to be my job, and if I ever "get it together" (currently in process of much home remodeling) i hope to do my part.