Solar panel mounting (on roof)

[WotEver]

15 minutes ago, stegra said:

However, this did get me thinking about the feasibility of using mirrors to increase the available light, rather than tilting the panels.

http://thebackshed.com/forum/forum_posts.asp?TID=507

[Johny London]

On 18/07/2017 at 15:53, Chewbacka said:

At this time of year the sun at it's peak is about 60 degrees above the horizon, which is 30 degrees below vertical.  This means that the effective area of a solar panel at mid day is reduced by the ratio of cosine 30 degrees - which is a reduction down to about 85%.  Now your panel pair rated output is 300W which at 12v is a current of 25A ignoring mppt controller losses, but as they are flat the panel collecting area is effectively reduced as explained to 85%.  So 300 x 0.85 = 255W which at 12v is 21A.   So your flat panels at mid day in June will not go above 21A at 12V and that assumes a perfectly cloudless sky etc, a bit of haze and it will reduce.  Obviously earlier or later in the day the sun is lower and so the panel output reduction is greater.  Flat panels can also get a bit grubby, and that can cause a significant reduction in output as well.

So you said you briefly saw 15A, but did not give a voltage, but if it was 13.5V, that would be 202W which doesn't sound too bad to me.

 

Good science there. It would seem to confirm my thought that the panels are not outputting fully though. It was a perfect clear mid summers day the other day at 1pm ish. The controller will only give 20a, and I would have expected to see that for a time (instead of just touching 15a occasionally). Thats 25% lower than rated - not insignificant. However, the manufacturer specifies them as max current of 8.13a. So going on 16.26a a pair max they seem ok but that doesn't quite tally with 300w does it?

The controller seems to sit them at 32v (the pair in series). The manufacturer rates them as 18.45each.

Another thing is - I thought they had confused the smart gauge. Even with 10-15a coming in for a few hours, the soc never seems to rise, though the battery voltage does (though I appreciate it is being propped up by the instantaneous panel voltage). But I found myself with very flat batteries one evening (<40%) so I guess what the smart gauge had been trying to tell me was right. I think my batteries hold about 10% of their original capacity (so 40 or so amps) I would expect them to be fully charged after a good days sun but I guess it comes down to the same old thing - fritzed batteries causing screwy results. I will replace once I have a good means of charging - ie genny and maybe more panels. Don't want to knacker the next lot quite as quickly.

Right now (noon in the sun) I'm getting 15a at 13v but with 61% soc the batteries might be pulling the voltage down a bit?

[rusty69]

14 minutes ago, Johny London said:

However, the manufacturer specifies them as max current of 8.13a

At what temperature though?

[Chewbacka]

3 minutes ago, Johny London said:

Good science there. It would seem to confirm my thought that the panels are not outputting fully though. It was a perfect clear mid summers day the other day at 1pm ish. The controller will only give 20a, and I would have expected to see that for a time (instead of just touching 15a occasionally). Thats 25% lower than rated - not insignificant. However, the manufacturer specifies them as max current of 8.13a. So going on 16.26a a pair max they seem ok but that doesn't quite tally with 300w does it?

The controller seems to sit them at 32v (the pair in series). The manufacturer rates them as 18.45each.

Another thing is - I thought they had confused the smart gauge. Even with 10-15a coming in for a few hours, the soc never seems to rise, though the battery voltage does (though I appreciate it is being propped up by the instantaneous panel voltage). But I found myself with very flat batteries one evening (<40%) so I guess what the smart gauge had been trying to tell me was right. I think my batteries hold about 10% of their original capacity (so 40 or so amps) I would expect them to be fully charged after a good days sun but I guess it comes down to the same old thing - fritzed batteries causing screwy results. I will replace once I have a good means of charging - ie genny and maybe more panels. Don't want to knacker the next lot quite as quickly.

Right now (noon in the sun) I'm getting 15a at 13v but with 61% soc the batteries might be pulling the voltage down a bit?

You are mixing up various parameters here made more confusing by the MPPT controller.

So your two panels are rated at 18.45V giving 36.9V in series, the max current you quote from the spec being 8.13A.  So 36.9 x 8.13 = 299.997W 

Solar panels are strange beasts and the max power (panel volts times panel current) will vary with the sunshine, so the MPPT controller being clever will adjust the load on the panels to run the panels at the max power by finding the optimum voltage and current, which when you were looking was 32V.

As to 15A at 13V which is about 200W which if it was noon (13:00BST) with clean panels and a clear sky does seem a bit low, as I said in my previous post the best you will get is 255W, which at 13V would be about 20A.  As the batteries charge up my MPPT controller will sit at 14.4V, so with yours at 13V it is not the batteries that don't want the charge otherwise they would be at 14V or so.  So I do conclude that your panels are down a bit.  This could be because the trim of the boat means the panels are not flat and are leaning away from the sun.  Also you only need to block out a very small area on one of the panels to have a big impact on output, so make sure there are no shadows or dirt on you panels.  Other than that I don't know what else to suggest to get back your missing power..........

[Johny London]

I thought you guys might be interested to see the panels in situ - I think they look very smart, it is possible to walk past them but I will add the grippy stuff and in any case try and use the gunnels more. I think the panels look in proportion with the boat and I like their low profile with the Z brackets. The silicon seems to be holding too

[MHS]

Very smart. Looks like you've done a good job. 

[Johny London]

Thanks! I'll add another two pairs (each pair with their own controller) as and when I can afford.