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Author Topic: Ground Based Solar PV advice  (Read 2058 times)
eabadger
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« Reply #45 on: November 28, 2018, 02:58:34 PM »

we get our data back by Ethernet over power units about 120m, the house end connects wirelessly to router to remove risk of static damage due to storms.

steve

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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
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nowty
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« Reply #46 on: November 28, 2018, 05:01:25 PM »


But I think it now seems the most sensible idea is locate the inverter near the panels. Them as 2 strings of 9 rather than one of 18 to lower input voltage and probably include a data cable alongside the a.c. cable for monitoring the inverter should I wish too.


We have had this DC verses AC over long cable runs debate before. I believe the consensuses was to run the DC over the long runs.

I think the main reasons were,

1) You can run DC at a much higher voltage than your mains AC voltage so you can achieve higher efficiency and / or use smaller cables. You could get away with 4mm or 6mm two core cable.

2) You can live with high DC voltage drops as the inverter has a large operating input voltage range and will just adjust its MPPT and still give out a stable 230v mains output. But if you run long AC cables you either need enormous cables to keep the voltage drop down or you risk the inverter tripping out by its AC output going over voltage whenever you get high solar outputs. Your going to need at least 10mm and need a 3 core cable.


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10kW of PV installed and 45+ MWh generated.
Usable battery storage of 50+ kWh.
Heat storage of 25+ kWh.
6kW Ground Source Heatpump.
200,000+ litres of water harvested from underground river.
A few cold frames and raised beds.
charlesd
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« Reply #47 on: November 28, 2018, 05:11:16 PM »


But I think it now seems the most sensible idea is locate the inverter near the panels. Them as 2 strings of 9 rather than one of 18 to lower input voltage and probably include a data cable alongside the a.c. cable for monitoring the inverter should I wish too.


We have had this DC verses AC over long cable runs debate before. I believe the consensuses was to run the DC over the long runs.

I think the main reasons were,

1) You can run DC at a much higher voltage than your mains AC voltage so you can achieve higher efficiency and / or use smaller cables. You could get away with 4mm or 6mm two core cable.

2) You can live with high DC voltage drops as the inverter has a large operating input voltage range and will just adjust its MPPT and still give out a stable 230v mains output. But if you run long AC cables you either need enormous cables to keep the voltage drop down or you risk the inverter tripping out by its AC output going over voltage whenever you get high solar outputs. Your going to need at least 10mm and need a 3 core cable.



I have to confess now to being a bit confused.
I understand that a high dc voltage will mean smaller cables and less loss but it was suggested that a typical inverter would not work with those high voltages.
I guess in simple terms can I string 18 average panels together and link those to an average with about 100m of cable or am I looking at something more complicated?
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nowty
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« Reply #48 on: November 28, 2018, 05:33:19 PM »


I have to confess now to being a bit confused.
I understand that a high dc voltage will mean smaller cables and less loss but it was suggested that a typical inverter would not work with those high voltages.
I guess in simple terms can I string 18 average panels together and link those to an average with about 100m of cable or am I looking at something more complicated?



Most inverters will only go up to around 600v although an SMA 4000TL (version 21) will go up to 750v so you could just about get away with a single string.

You can still have two strings of 9 @ 360v and connect them together in parallel so you only need a single two core cable back to the house with the inverter. You can get a pair of (MC4 parallel branch connectors) to do this.

The main problem with a long AC cable run is not inefficiency, its the AC voltage drop where the inverter will have to increase its voltage output to compensate for the long cable run and risk going over voltage.

Example, my house has an incoming mains voltage of typically 250v. For every 1kW I export, that voltage as measured at the meter tails increases by circa 1V, so at max 3.6kW export the voltage is typically 254v. But if I also had 100m of 6mm cable between the meter tails and the inverter, I would have another 8v at the inverter output so that would be 262v which would make it trip out.


« Last Edit: November 28, 2018, 05:35:34 PM by nowty » Logged

10kW of PV installed and 45+ MWh generated.
Usable battery storage of 50+ kWh.
Heat storage of 25+ kWh.
6kW Ground Source Heatpump.
200,000+ litres of water harvested from underground river.
A few cold frames and raised beds.
charlesd
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« Reply #49 on: November 28, 2018, 08:15:03 PM »

Thanks nowty
I think I'm slowly getting there. It does look like I would need a bigger cable for my 14amp 360v set up or accept a slightly higher loss a 16mm2 cable for a 1% loss or  6mm cable for a 3% loss.
The bit I'm not sure about is what is an acceptable percentage loss because 100m of 6mm swa is a lot cheaper than the 16mm2.
But I would feel much better with the inverter in the garage than in the field. In my experience most things break when it's cold dark and raining  Smiley
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marshman
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« Reply #50 on: November 28, 2018, 08:28:45 PM »

In my experience most things break when it's cold dark and raining  Smiley

Ah, but that is the great thing about PV, it doesn't work when it is dark, so if it is dark you won't know if it is broken or not  Grin

Nowty does have a very good point about the inverter tripping out because the voltage gets too high. Something I had forgotten about, but it is a problem I was plagued with for several years until UK Power Networks eventually changed the voltage tap on the local transformer. But our grid supply never fell below 240V when fully loaded with no PV feeding. On a sunny day with no load, and anything more than about 2.4kW pf PV, it would  regularly exceed 260V in the house and higher at the inverter 25m away - result inverter disconnects! So maybe D.C. to the house would be a better option.

Roger
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3.15kWpk (15xSharp ND210)/SB3000. & 3.675kWpk (15 x Suntech 245WD)/SB4000TL, 10kW GSHP driving Wirsbo underfloor heating from 1200m ground loops. 10' x 7' solar wall (experimental). Clearview 650 Wood Burning Stove. MHRV - diy retrofit. Triple glazing.
biff
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« Reply #51 on: November 28, 2018, 09:59:23 PM »

 Keen observation Nowty, fingers crossed!
                                   Biff
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An unpaid Navitron volunteer,who has been living off-grid,powered by wind and solar,each year better than the last one.
snyggapa
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« Reply #52 on: November 29, 2018, 07:29:37 AM »

If I were installing in a field and assuming no obstructions to cause shade, I would be tempted to go either 2 or 3 strings, and not orient them all the same direction - so have some on one string facing WSW, some facing ESE and some due south - in order to maximise the duration during the day of usable generation rather than the total amount. Or a SW/SE split on 2 strings

Once battery storage comes in, you may favour want max generation if you can store the excess, but a big south facing setup will generate a lot during the middle of the day which you may not be able to use - so splitting to orient some towards the morning sun and some towards the evening sun may increase the amount of usable (to you) power generated whilst both will still provide a good solid rump during the day.
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charlesd
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« Reply #53 on: November 29, 2018, 08:54:57 AM »

If I were installing in a field and assuming no obstructions to cause shade, I would be tempted to go either 2 or 3 strings, and not orient them all the same direction - so have some on one string facing WSW, some facing ESE and some due south - in order to maximise the duration during the day of usable generation rather than the total amount. Or a SW/SE split on 2 strings

Once battery storage comes in, you may favour want max generation if you can store the excess, but a big south facing setup will generate a lot during the middle of the day which you may not be able to use - so splitting to orient some towards the morning sun and some towards the evening sun may increase the amount of usable (to you) power generated whilst both will still provide a good solid rump during the day.
Planning application is in now so I don't think I'll be changing it.  Smiley
Both my wife and I work at home so I think we'll make more use of daytime generation than the average person. Also during the summer we use oil to heat the hot water so some sort if solar immersion controller (really like the idea of these) should allow us to cut our oil bill as well so it all.  I would like to think about batteries later too but I think they still have a good way to drop before I'll be buying them.
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kristen
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« Reply #54 on: November 30, 2018, 10:52:29 AM »

Without batteries you will typically use half and export half, so expect a third off your leccy bill. With batteries over half off your bill is possible, but the economics of doing so are questionable.

Both my wife and I work at home so I think we'll make more use of daytime generation than the average person.

Yes, that was my thought too (only just read the whole thread). Working from home, and assuming not an excessive-PV-capacity installation, I think likely that exports might be trivial ... but if the export is on an assumed 50% basis, rather than actually metered, that might help the Man Maths!

Maybe read your meter during the day - several times a day? - for a few days to forma  view of the run-rate when you are both working from home?
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