Cloud Edge Effect- why does it happen and how big an effect?

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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Cherokee Solar » Mon Jul 16, 2012 8:37 pm

Hi Gordon,

Gordon-Loomberah wrote:hmmm, with all those panels I would expect a cloud edge effect peak to be producing over 150A into a 24V battery!


Ha! I've never seen 150A here (yet).

I forgot to mention that that early in the morning I would usually only see about 10A, so 20A was pretty amazing. The clouds were very high and very thinly distributed.

I forgot to mention that in an earlier stage of the system with only 12x180w panels at 24v, I saw an output of 90A briefly before the regulator kicked it down to 60A. With the additional regulator and panels, I regularly see this sort of output now even during winter before the battery voltage rises too quickly and the regulators throttle the current.

I'm watching out for the effect this summer. I may have to run the microwave, vacuum, hair dryer etc all at once to make it happen though! :D

Chris
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Gordon-Loomberah » Mon Jul 16, 2012 11:05 pm

Cherokee Solar wrote:Ha! I've never seen 150A here (yet).


You'd need MPPT regulators with enough capacity to output 150A, but that many panels are quite capable of it.

Trackers help too, I saw nearly 80A from the 2.19kW array before 9am yesterday through a break in the clouds. No clouds at all today, which was a nice change, 12.85kWh from a clear-all-day sky with no edge effects at all, but only 8.3 hours of sun at this time of year.
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Cherokee Solar » Thu Jul 19, 2012 7:21 pm

Hi Gordon,

Glad to read that you are getting some sun. The past week here has been particularly clear and I've recorded 3 frosts this year so far recently which is really unusual (I haven't seen a frost for a few years now) and a sure sign that El Nino is on his way this summer.

8.3 sun hours! We are lucky to get 4.5 hours between 12pm and 4:30pm at this time of year. Before 12pm it is usually just diffuse sunlight which generates between 8A to 20A depending on cloud cover.

Still the batteries are full and hit a maximum voltage of 28.8v today before winding back to ABSORB then FLOAT on 27.7v for most of the afternoon.

As an interesting side note, the SOC % meter reads 65% which is not possible for these voltages (being too low an SOC reading). I haven't looked at it in ages and I assume that it reads this because of accumulated error in the Ah IN and Ah OUT shunt readings which it uses in the formula. Apparently it will automatically reset when the regulator is only sending 25% of the available PV charge to the batteries (It is wasting about half of PV current at present - so still a few weeks to wait yet).

I still haven't worked out how to make use of this lost energy yet, but it is a catch 22 situation in that I have to cater for the worst conditions and not the best conditions if I want to reduce my generator usage.

PS: When I install the third PL40 in my system, I'm going to move 4 x 180w panels to this regulator along with the 2 x 200w wind turbines yet to be installed. My aim is to reduce the load on the PL60 regulator which at present has 12 x 180w PV (the 4 x 180w are going to come from this array). The PL60 is only mildly warm over high summer, but I'm unsure and thought this might assist with it's longevity. What do you reckon? Please don't say get rid of those regulators though :D !

Chris

Regards

Chris
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Gordon-Loomberah » Thu Jul 19, 2012 8:26 pm

Plenty of frosts here so far this year, but no big ones. Measured at standard 1.5m, it hasn't been below -2C in past few years, and about -5C for 9 or 10 years now. Big frosts used to be quite common in the 90s.

Cherokee Solar wrote:8.3 sun hours!


That followed quite a few days with 0 hours sun in the past week!

Can you manually reset that SOC reading? It seems to be so far from reality that it is of no use at all at the moment.

I still haven't worked out how to make use of this lost energy yet, but it is a catch 22 situation in that I have to cater for the worst conditions and not the best conditions if I want to reduce my generator usage.


Pump water up the hill and run it back down through a hydro generator instead of using the stinky generator ;)


Running any electronic gear at a cooler temp is generally better for longevity, but you really need to...no I wont say it :lol: ... place those 4 panels at a different orientation.

For fixed arrays, now that panels are coming down to a relatively low cost, there is a reasonable argument for what I think of as a virtual tracker ie, placing panels facing the ecliptic across the sky, so that there is near max power (within spec) available all day, rather than the usual point everything north that people tend to do. You dont need a lot, but the north pointing panels can be supplemented with a lesser number of panels pointing E and also W at say 30degrees above the horizon.
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby offgridQLD » Fri Jul 20, 2012 7:49 am

For fixed arrays, now that panels are coming down to a relatively low cost, there is a reasonable argument for what I think of as a virtual tracker ie, placing panels facing the ecliptic across the sky, so that there is near max power (within spec) available all day, rather than the usual point everything north that people tend to do. You dont need a lot, but the north pointing panels can be supplemented with a lesser number of panels pointing E and also W at say 30degrees above the horizon.


That's interesting, In my situation my fixed array face a bit north east. It works ok for charging battery's as they get good sun early in the morning. The house has another roof area over the deck that points north west and the garage will have the same.

Kurt
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Gordon-Loomberah » Fri Jul 20, 2012 9:23 am

Virtual Tracker...remember, you read it here first ;)

You dont even really need to have any panels pointed to the north, splitting between NE and NW, or E and W, can work just as well too.

For an idealised example- clear, low horizons, sun rising in the E and setting in the W, ie near the equinoxes at a low latitude, such as in Qld:
A fixed panel array of the full, or slightly over, rating of the inverter pointing E, tilted to point 30deg above the horizon means you'd have close to full power all morning, combined with another full sized array pointing W at 60deg tilt (from horizontal) means you could have near to or full power for much of the afternoon too. At midday, each array is pointing off the sun by 60 degrees (for the case where the sun is near overhead). Cosine 60deg = 0.5, so each array is receiving about 50% of the solar radiation they would if face-on to the sun, but with 2 of them, the inverter will see ~full power input.

With the sun say 60 deg above the E or W horizon, then one array will see Cos 30deg = 87%, and the other side will see only diffuse, and no direct radiation, I think that will be the daily minimum... other than just after sunrise and just before sunset due to atmospheric extinction, of course.

The overall PV output curve will be fairly flat, always between 87% and 100% of maximum- starting at 87% when the sun rises, up to 100% a couple of hours later, then down to 87% and up to 100% at midday, mirrored through the afternoon. Not quite as good as a decent tracker, but not too far off either!

Performance of such an arrangement is best all year round at lower latitudes, and drops off somewhat in summer and winter at higher latitudes.

2 arrays is the minimum, and probably optimal (I haven't worked through any other variants) for this type of virtual tracker- by using 3 slightly smaller arrays you could lift those 87% minima up a little, but you would need more panels to do it.
Just to keep this entirely on topic- with this virtual tracker arrangement, you are in the best position to take advantage of any cloud edge effect peaks, rather than just being limited to those in the middle of the day with the typical all in one plane, north pointed array.
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby offgridQLD » Fri Jul 20, 2012 10:04 am

From a cost point ,reliability, and esthetics it sounds good. I know a lot of us are DIY and could build a tracker but what is they typical cost of a large tracking unit?

Lets say one option was for some one to purchase 2000W of PV on quality tracker installed.

Compare that to the cost of purchasing 4000w of PV 2000w of PV on the NE roof and the additional $2000w on the NW roof installed.? assuming some ones house already had ideal roof layout for this.

Kurt
Last edited by offgridQLD on Fri Jul 20, 2012 10:53 am, edited 1 time in total.
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Gordon-Loomberah » Fri Jul 20, 2012 10:16 am

Trackers are expensive, even DIY, and I'm currently building another 2kW tracker, in addition to the one I built a a few years ago. I reckon for parts the new one are going to run somewhere in the range of $3000 by the time its finished, but there was near $1000 worth of machining I had to pay for in that. A decent lathe in your own workshop would help a lot...

A tracker is still going to be better in higher latitude regions, although if you can get decent quality panels for $1/Watt, you could just add more panels in various orientations to cover the larger range of sun paths across the sky, assuming you could mount them for not too much expense. It's easier for off-grid with short string lengths, but not so easy with the long strings typically required for grid-connect.
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Cherokee Solar » Sun Jul 22, 2012 10:12 pm

Hi Gordon,

Yeah. What a great idea, virtual trackers. I'd love to see some photos of your new tracker.

Panels are getting cheaper too. I'm a bit nervous though about over loading my regulators with a virtual tracker, but you are correct about the 4 panels at the front here needing to be moved. I must learn not to argue with you! hehe! :D

I seem to be past the 3 weeks either side of the winter solstice danger period of low PV now and the system seems to be ticking along nicely. Just for your interest I counted 24 days (I previously thought it was only 20 days) of generator usage this year which is heaps better than last year (about 3 hours on average). More improvements to come though, it is just that the pareto principle (80/20 rule) is starting to bite. Once the new PL40 is installed, I'll put up two wind turbines - although they have been outlawed in my council area by the state government. :shock: Seriously, someone around here has some political clout to get that one legislated, it's not me though.

Regards

Chris
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Re: Cloud Edge Effect- why does it happen and how big an effect?

Postby Cherokee Solar » Thu Aug 02, 2012 8:04 pm

Hi Gordon,

Just for your interest, the cloud edge effect effectively doubles my PV output when there is no direct sun on the PV panels. High cloud is much more effective than no cloud at all.

Regards

Chris
Off grid solar + hot water. Heavily insulated + owner built flamezone house BAL-FZ. 300 mixed fruit trees + herbs + flowers + vegetables. Bees + heritage chickens. High up in the mountains north of Melbourne. http://ferngladefarm.blogspot.com.au/
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