Blocking diodes.

[kibi]

The site for my panels has shading issues which can't be resolved.
One way around this would be to wire the four panels up in parallel. This of course presents a problem where current will back feed into the shaded panel. Although the panels have three bypass diodes built in, I am not so sure that they have been designed to bypass the current that three other panels at full power can deliver (24 Amps).
Do you think blocking diodes in the combiner would be a good idea, or am I just being paranoid?
I am aware of the voltage drop across the diodes, but at full output power, the diodes I have in mind will dissipate about 3W for each panel, not really the end of the world for me.

[boB]

I wouldn't touch it.

The non-shaded panels won't back-feed the shaded panels unless you have a shorted panel in one of the strings.

Then, if you have too many strings in parallel with that string with the shorted panel, the series fuse or breaker should trip.

But shading isn't the same as shorted, luckily.

boB

[kibi]

Thanks for the advice Bob. 
I guess it will be dead easy to keep an eye on the current in the shaded panel with my DC clamp meter. Maybe I could show off with some hall effect sensors on each string inlet and an Arduino wired up to the internet - na, too excessive.
My combiner will have a 15A 1KV fuse per string in it.

[RossW]

Maybe I could show off with some hall effect sensors on each string inlet and an Arduino wired up to the internet - na, too excessive.

I have six arrays, each of 6, 100W series-connected panels.
Each array has a DO-220 style schottky diode to prevent backfeed, a double-pole DC-rated breaker, and a 0-10A moving coil panel meter so I can see at a glance what's happening.

I put the meters in the negative line... because it makes monitoring the current easier, using the meter coils themselves as shunts.

An inexpensive PLC board (under $50) gives me a few A/D channels and ethernet connectivity (it also happens to be the same controller for my trackers), and gives me exactly the output you speak of:



(And before you ask, yes, I also monitor via another channel, the bus volts, to work out and display actual power)

[dgd]

Rossw,

Just wondering if your graphs show that tracking panels is worth the effort/cost?
I was thinking about tracking a 1200watt array but the boxed tracking electronics at $280 and the actuators/framing/sensors/cabling etc., at close to $500 is near the price of another 600W of panels. 
So a couple more static panels would probably exceed any power gains I could expect from tracking the existing array? 

dgd

[Westbranch]

It is a near wash for trackers with PV prices so low now, a lot easier to do PV than have something you will need to do service on yearly unless you have a particular need for long hours of input to finish a full charge.
cheers.

[dgd]

yes, just what I was thinking.. the low cost of PVs must now be a factor in deciding to include wind, hydro etc., in an RE system.
I sometimes think that the $ spent on turbine/Clipper may have been better invested in a larger battery bank and a couple more KW
of PVs. But a blurred turbine in a good wind is hard to beat.

dgd

[Westbranch]

At least wind will be there when the sun is down... sometimes

[RossW]

It is a near wash for trackers with PV prices so low now, a lot easier to do PV than have something you will need to do service on yearly unless you have a particular need for long hours of input to finish a full charge.

I'd actually disagree with that!

With tracking, I get "almost" full output very shortly after sunrise, and can hold it right through to sunset.
It would be easy to say "spend the extra money on more panels" - but that doesn't help my batteries first thing in the morning - and I'd need to run the genset.

It also means that in the middle of the day, I may have far more power "available" than my charge controller can handle, or I need to buy additional charge controllers. I may then run foul of the maximum rate I can charge my batteries at.

And then at the end of the day... power tapers off quickly, my batteries start discharging earlier and I'm even further down in the morning (when I now really need the extra power to start charging them again!)

With my trackers, I "extend" the hours of power, without trying to handle it all in a peak around the middle of the day.

Some might suggest buying more panels and sticking some facing east to get the morning sun, and some to the west to get the afternoon sun... but buying panels just to use them for less than half the day makes no sense to me! Just more panels up in the weather to be potentially damaged by storms.

With the trackers, my weatherstation detects imminent (or actual) bad weather - strong wind, hail etc) and parks the arrays. It would be possible to park them in a position to minimise hail damage. I think it's also possible that the trackers, with a small amount of "give" in them may help actually save the panels some damage by absorbing some of the energy - although that's perhaps a bit of a stretch

I think I already posted this elsewhere:


The red line here is power from tracked arrays, the yellow is non-tracked....

[dgd]

I can understand what you are saying, however the graph you posted is interesting.
The red (tracked) line is consistently 150W+ above the yellow (untracked) line.  I would have expected for an hour or two that the two be almost equal as the tracking approached and then moved away from the position of the fixed PVs - 12 to 2pm.
Perhaps you have more PVs tracked compared to untracked hence the skewed graph. The bumps in output on the red line for early morning and evening show the benefits of tracking. Have  you calculated the actual daily Kw/hours?

dgd

[RossW]

I can understand what you are saying, however the graph you posted is interesting.
The red (tracked) line is consistently 150W+ above the yellow (untracked) line.  I would have expected for an hour or two that the two be almost equal as the tracking approached and then moved away from the position of the fixed PVs - 12 to 2pm.

The untracked would have "caught up" to the same output around 13:20 local time (daylight savings, plus we're about 20 minutes west of the timezone edge).... but on the day in question, my batteries hit float around 11am, which explains the sudden (and sustained) drop in power - I simply had nothing to use it!

Perhaps you have more PVs tracked compared to untracked hence the skewed graph. The bumps in output on the red line for early morning and evening show the benefits of tracking. Have  you calculated the actual daily Kw/hours?

I have 6 arrays, 4 tracking, 2 fixed. The "power" graph is just two identical arrays - one tracking, one not.
I have calculated the "potential" extra power, which I rarely actually see (because I don't have something to actually use it all during summer) - and it also varies with the season. In winter, the sun comes up and goes down further north, so the benefits of tracking are somewhat less than in winter. I'll try to find my old measurements and calculations.

[ChrisOlson]

Some might suggest buying more panels and sticking some facing east to get the morning sun, and some to the west to get the afternoon sun... but buying panels just to use them for less than half the day makes no sense to me! Just more panels up in the weather to be potentially damaged by storms.

Actually, ross, it doesn't work that way.  We recently put in panels facing east and west, all on one Classic 150 (at present).  We have 1.5 kW facing east at 22.5° tilt on the roof slope and .75 kW facing west on the other 22.5° roof slope.  3.75 kW facing due south at 30° tilt.

The array reaches the full capacity of the controller at about 9:30 AM on a good day and will hold it at full controller capacity up until about 3:30 PM.  But it never does that.  The bank is done absorbing by slightly after solar noon so the power drops way off during float stage anyway.  That's why we put in more east facing panels and less west.

But the thing is the west facing panels start getting sun by about 9:00 AM, and they are contributing some power before that just due to "sky shine".  So they're not "wasted" even in the morning.

On a cloudy day they all work the same just from "cloud shine" and it don't make any difference what direction they are pointing.

It has been working fine for us, with no moving parts, and only one controller.  I initially had some problems with shading on the south facing array from a wind turbine on a 90 foot tower this time of year.  I switched from Solar Mode to Legacy P&O Mode and that seemed to "fix" the shading problem (shadow flicker on the panels from turbine blades).

This is the graph of the amp output of the arrays when I still had .75 kW facing both east and west, and was still deciding which slope to install the final .75 kW of the east/west array on.



You can see the effect of the shading on the south facing array for about two hours starting at about 10:30 AM as the shadow flicker from the turbine blades knocked out three-panel strings one at a time.  The Legacy P&O has drastically reduced that effect.  The full capacity of the controller is about 80 amps but it will do more than that for a short while (until the FET's get hot and it de-rates from temp).
--
Chris