Post by malewithatail on Jun 1, 2022 11:52:12 GMT 10
A question I get asked quite a lot is "how long do solar panels last for ?"
Ive collected some pictures of various failure modes.
The first is from my first solar panel I bought in 1983, and installed on the power room at the other farm to charge a set of 200 amp hour, well used, 20 plus year old telephone exchange battery's arranged in 4 parallel strings of 6 cells in series, giving 12 volts at 800 amp hours. (The original battery pak was 48 volts dc). That panel delivered 60 watts at 12 or 24 volts (2 strings of 36 cells that could be wired as 12 or 24 volts). I marveled back then that it was putting 5 amps into the battery's, from the sun, with no noise or fumes or petrol being used.
After nearly 40 years exposed to the elements, it still works and puts out around 3 amps in full sun. As can be seen from the photos, there is some out-gassing that has deposited on the tempered glass front, and the backing, probably Kevlar or similar, has parted company with the glass and some cells are now just floating between it and the glass. Ultimately, the inter-cell connections will fail and the panel will go open circuit,
Another failure mode, especially with high power panels, is hot joints on the inter-cell; connections, or on the junction box. Ive noticed an increase in this type of failure since higher powered, over 150 watt, panels have been introduced. It would seem that the connections just cant take the currents involved, especially if you don't use the correct regulator, which I will discuss below or maybe another article.
This picture shows a burnt connection in the terminal box, but a burnt inter-cell connection looks the same.
Is it fixable ? Yes and no. By carefully peeling back the Kevlar backing, and re-soldering the connection using a generous amount of no corrosive electrical type flux, it can be repaired. Then a good dollop of silicon sealant over the mess to make it water proof.
If a panel has just gone open circuit, producing no output, then by connecting a meter to one output terminal and using a pin on the other meter lead, poking it through the backing, you can narrow down the break in the inter-cell connection, if its not visible through a burnt spot. The same procedure as above can repair it and get the panel working again. If you can find the break, then placing the panel in full sun and shorting the leads together, then gently tapping the front glass may reveal the break and it may well spot weld itself together. How long that lasts is variable, but some Ive repaired are still working years later.
Occasionally, a panel tests with a low voltage output, still the correct current, but low power. This can be caused by a shorted bypass diode n the terminal box. These diodes are to prevent reverse current flowing through the cells when partly shaded. (There should ideally be a diode for each cell, and in space applications, there are, but for terrestrial panels, 3 or 4 diodes across multiple cells seems to work well enough). A panel with no output could also have a burnt up series pass diode, if fitted. The purpose of this diode is to prevent reverse current flowing from the battery to the panel in the dark, but with most panels being grid feed now, the diode is usually done away with. (Lookup on the net testing diodes).
Even a panel with smashed glass can still be used. I have several 250 watt panels that are connected to various battery's on machinery. They can still output significant power in full sun and eventually will fail due to either inter-cell connection failure from corrosion, or cell damage and cracking from thermal shock. But still useful to keep a battery on the tractor or firewood saw charged, or even to run a small system to light the firewood shed. A regulator may be needed on small battery's to prevent overcharging.
Just a short note on using grid connect panels to charge battery's.
Most grid connect panels have quite a high voltage output, well over 45 volts and don't match to a 12 volt battery. If you just connect the panel directly to the battery, it will charge, but the power will be a fraction of whats possible. The 'lost' power is dissipated as heat in the panel and can burn out connections, as a little math will show.
Assume a 45 volt, (at the maximum power point), and say 5 amps, giving a panel of around 225 watts. At 12 volts, directly connected, 5 amps will flow into the battery. (Neglecting any regulator, which will be discussed below). This is a power of 5 X 12 or 60 watts. Whats happening to the other 160 odd watts ? Its dissipated as heat in the panel, and adding to the heat load in full sun, can ruin it. The how can the panel be used ?
A linear regulator wont work properly, as it directly connects the panel to the battery and disconnects it when the battery is charged. A large external resistor could be used to dissipate the heat, expensive and rather impractical. We use a maximum power point tracking regulator (MPPT). These work by taking a higher voltage and converting it to a lower voltage, effectively turning the voltage into current to charge. From the example above, a little more maths.
225 watts at 45 volts is 5 amps, but 225 watts at 12 volts is 18.75 amps, nearly 3 times more current to charge the battery. Also, a MPPT regulator can 'track' the Maximum power point as it changes due to temperature, shading, changing sun angle etc. The real time gains are not quite as much due to switching internal losses in the regulator, but still worth while.
Be aware, warning.....Ive come across some cheap Chinese regulators (sub $100 mark), that were supposed to be MPPT type, and it was splashed across the front of the regulator etc. It wasn't, and that's what caused the burnt connection on the panel above. Reputable brands, Victron especially, work well and do what the box says it supposed to do.
To prevent panel damage, you can tap the panel down to a lower voltage in the terminal box, but you are wasting your investment as you are not getting full power. Unill I get another switching Victron regulator (we have 4 or 5 Victron 150/100's here already), that's what Ive done to prevent further panel damage.
There is more to this subject, and the article just touches the surface.
PM me for any questions. When I have time, I will do a follow up article on other solar issues, battery failure modes and so on.
I used to watch TV, then I got the internet....
