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Barrowman 10-28-20 02:48 AM

Using my panels without the grid
 
Hi everyone just found this site and hope I can get something sorted out.
I have 16 panels which are wired as 2 sets of 8. I assume their output is at 192 volts.
The system is a grid tied one here in the U.K.
What I am wondering is if I can use the panels unmodified on their own off grid.
It's a 4Kw system.
If I fed one set to a PC power supply and the other to a separate PC power supply I am thinking I could wire one 12 volt output from one with a 5 volt output from the other so I could charge some batteries,
I know it would only give around 10 amps at 17 volts but that would be better than nothing and I think it might be possible to develop a more powerful system based on this.
Do you see any problems with trying this?
I am assuming that the output voltage from the panels would not rise much so as to cause any problems for the PSU input.
Forget this I didn't realise this had a transformer input and wasn't what I thought.

Barrowman 10-29-20 02:45 AM

Although the unit I originally had was not what I thought I have been looking at how the PC power supply works. I have opened up a fairly recent one and think now it would work. I guess I didn't understand fully how they do work. Also I am wondering if I could run several of the outputs in parallel with some way of load balancing.
Alongside this I have a few old laptop chargers as well as some wall warts and maybe they could be used to provide lower power chargers.

NiHaoMike 10-29-20 08:34 PM

Might want to take a look at server power supplies, commonly available into the low kW range. Modifying one to have MPPT is going to be tricky even if you're advanced with electronics.

Barrowman 10-30-20 02:49 AM

Quote:

Originally Posted by NiHaoMike (Post 63216)
Might want to take a look at server power supplies, commonly available into the low kW range. Modifying one to have MPPT is going to be tricky even if you're advanced with electronics.

I like the idea of using server power supplies.
I was thinking that the 17 volts would feed a charge controller so no need to mess with them. Can't it work like that?
We tend not to think we might be without power for long here in Britain but we lived for 11 years on a Scottish island fed by a single undersea cable from the mainland. Something went wrong one winter and the whole island was without power for 3 days.
I have been trying to find out what the lowest input voltage needs to be for a charge controller as I will have to make sure I wire up the power supplies to at least reach this level but up to now all I can find is the maximum voltage and power. Does anyone know any values please?

Robl 10-30-20 06:19 PM

I can see a few issues:
If you get a feed in tariff (as it's the UK), they won't like you messing with the DC from panel to inverter. You could inject DC here, and "make" money!
Normally a 2kW string of 8 panels would be at a higher voltage - I'd expect more like 70V per panel, so around 500V.
4kW at 12V is a lot of current - normally people would try and avoid such heavy currents, due to the losses in the copper wiring.

A much simpler option, avoiding the possible legal issues & dangerous side with the high voltage DC, would be to charge up batteries from AC, and then use them for certain loads. You could set a timer to charge the batts at mid-day, so they always get charged, and usually it's free. 12V lighting is a very suitable load, maybe power for a laptop. It won't charge up from PV when there's a power cut, but it's a lot simpler.

Barrowman 10-31-20 06:22 AM

Quote:

Originally Posted by Robl (Post 63224)
I can see a few issues:
If you get a feed in tariff (as it's the UK), they won't like you messing with the DC from panel to inverter. You could inject DC here, and "make" money!
Normally a 2kW string of 8 panels would be at a higher voltage - I'd expect more like 70V per panel, so around 500V.
4kW at 12V is a lot of current - normally people would try and avoid such heavy currents, due to the losses in the copper wiring.

A much simpler option, avoiding the possible legal issues & dangerous side with the high voltage DC, would be to charge up batteries from AC, and then use them for certain loads. You could set a timer to charge the batts at mid-day, so they always get charged, and usually it's free. 12V lighting is a very suitable load, maybe power for a laptop. It won't charge up from PV when there's a power cut, but it's a lot simpler.

I can see that I could inject DC but it would surely cost me more to generate it than I would get paid wouldn't it?
Or am I missing something?

I am surprised that you think the voltage will be so high.
I just checked an ebay listing which shows the following
Quote:

Solar panels
250 watts

Dimensions (H x W x D) in mm: 1485mm x 992mm x 40mm

Weight in kg: 15.8

Cell type: multicrystalline

Power max .: 250 watts

Open circuit voltage max .: 28.16 V

Max voltage: 33.80 volts

Max current: 8.88 A.

Short-circuit current max .: 9.16 A

Module efficiency: 16.97%
Which is in line with what I expected.

It's interesting that you suggested a timed battery charging regime. I had considered doing that and am in any case going to implement a small system to keep my spare car battery and my wife's mobility scooter battery in reasonable condition seeing as my wife and I are old and in poor health so I am rarely using my car.
Still that's no good for such a time when the grid is off for any length of time as the inverter will not run.

Robl 10-31-20 10:48 AM

Hi Barrowman - we're in the UK too, got our 4kWpk solar 6 years ago. I still have the paperwork - we have 2 strings of 10 "Kinve" panels, each panel max power is at 36V*5.5A, and 45V peak, so each string could have 450V dc from it. The "Samil" solar inverter we have can cope with up to 550V.
I think that in the 2kW-10kW range high voltages around this level are favoured, as it keeps the DC cable current lower, so cheaper, but isn't so high voltage to need specialised components. I think that ebay one that you found is intended to be connected via a PWM controller to a 24V battery, and that grid tied ones we have are usually higher voltage. You could google the particular inverter model that you have, and it will say the max voltage.

We get a "Feed In Tariff" of 17p, and 2p per kWh, so 19p/kWh overall. That is slightly more than the cost of electricity. It is odd, I agree. I would be more comfortable with just using the grid as a battery than the FIT (which is pretty much gone now in the UK).

When did you get solar - do you get the FIT or not ?

Barrowman 10-31-20 12:17 PM

We moved back to England about 5 1/2 years ago. The system had already been in place since early 2013 so we got a straight transfer from the last owners.
The cables from the panels look as if they could easily handle 10 amps. I've forgotten what the inverter is and not feeling up to climbing up into my loft right now.
In my younger years I did a lot of electronics stuff including repairing the old colour CRT TV's ( 25 KV DC on the anode ) for family, friends and workmates so well aware of the need for caution.:)

Well if the output of each string is around 500 volts then I could wire the inputs of 3 psu's in series and then I would have a wider range of combination of outputs to wire in series to give suitable voltages to feed the charge controllers.

Robl 10-31-20 06:03 PM

Wiring dc smps power supply inputs in series is unlikely to work. Maybe if they were identical and designed for it, but its a recipe for a bang I fear. Any mismatch, and one of them would get all the volts, and will instantly fail, then the next too. Those psu have turn on delays, overcurrent trips - again, mismatch, bang.
In 2013 the fit was alive and well - perhaps its a ‘rent a roof’ scheme - the panels are owned by a company, you get free electricity from them, they get the feed in fariff? If the fit is involved, you mustn't connect to the dc cables. Aside from the danger too.
There are solar controllers designed for off grid I think. I think the newer Tesla Powerwall can ‘Island’, among others.
How often do you get powercuts? We have one every few years, 5mins maybe. Fridges, freezers, laptops just ride through - maybe a few lights would be good to keep on, but that’s it. Unless you had a serious ‘islanding’ system, it won’t cope with normal house loads.
A timed battery charger is so much simpler and cheaper to the above though.

Barrowman 11-01-20 03:52 AM

Okay thanks for the information Robi. I certainly do get FIT. I get a cheque every 3 months and average about £60 per month.
I guess I will go for the timed battery charging, As you say it's very simple.
We have only 1 power cut which lasted several hours, but quite a few lasting from a few seconds up to about 5 minutes. My main reason for what I had intended was that although our home is on fairly high ground the immediate area is a flood plain.
Now with so many changes in the weather we are getting many more days with heavy rain and we have seen what happens in areas with flooding and total lost of power for long periods.

Barrowman 12-18-20 04:02 PM

More about my panels
 
I have been thinking about this some more and have more ideas as to getting this to safely work.
Assuming the approx. 500 volt no load output of the panel strings is correct several things come to mind which I believe could make this work safely.
I would plan for there to be 3 PC supplies with their primaries wired in series so able to handle 1020 volts input ( peak values of 240 v AC ) which should be safely above what would be present.

First is that if they were originally fitted at night then surely the output of the panels would rise slowly so quite possibly it would give time for the power supply units to start up without damage.

Second I could wire a 250 volt AC varistor and 220 microfarad electrolytic capacitor in parallel across the input of each unit to prevent it being damaged by over voltage. The varistor I looked at reacts in <50 ns and can handle 100 amps.

I could set up each psu to have a default minimum draw from one of it's outputs so it always placed a load on the system if it was running.

Of course I have not been able to find out what happens if the power demand is less than the panel string can deliver. As it is I believe these have an open circuit voltage specification which, as far as I can see isn't much greater than the on load voltage and so should not be a problem
Does this all sound reasonable or not?

MN Renovator 12-20-20 03:59 AM

I'm not sure wiring up 3 computer power supplies in series will work as well as you expect because you can't be completely sure all 3 supplies will carry the load evenly. That would rely on them a turning on at the same time, not faulting when the voltage goes out of range, their voltage regulation handling the varying voltages identically. Maybe it's worth the science experiment, but I don't think this will work. It all depends on the loads needed, but I'd rather try to find a charger that could handle the input voltage and output to a battery bank sized well enough to buffer the source and load variations. The only things I really care about in a power outage are to run the refrigerator, the furnace, and charge some electronics like USB power banks, phone, maybe a laptop, and AA/AAA batteries for flashlights. For me the cost to buy the equipment for the loads I mentioned above costs way more than a generator would, so I have a generator and keep enough gas to keep the furnace and refrigerator running intermittently as needed. The reality is I'll wear a coat and run long furnace cycles to make sure the pipes won't freeze, theoretically a relatively larger than standard uninterruptable power supply wired to my furnace which only uses 300 watts when the blower is running could keep things more consistent, especially at night when I don't want to run a generator outdoors.

There are periods in the winter where a solar system stands zero chance of powering critical loads. I've had weeks with zero power output due to snow covered panels and even with them clear, thick winter clouds might not even be enough to run a refrigerator for a day even if the energy produced was being stored in a battery as the day went on.

jeff5may 12-23-20 06:29 PM

Ok, I just read through the thread and I thought I would give you my 2 cents worth...

You're getting a feed in check for power generation. Why would you rob yourself?

You have a power inverter that's doing the risky power conversation for you already. Just use the usual wall outlet devices for the science experiment stuff. Safer, tested for reliability, etc.

If you feel like you just must play with the hvdc, you absolutely must make a custom device that will run reliably at 10 percent or so higher DC voltage than you'll ever be able to generate. Stepping down from 500 to 12 or 17 vdc brings some sort of buck converter to mind.

Barrowman 12-24-20 12:34 AM

Quote:

Originally Posted by jeff5may (Post 63345)

You're getting a feed in check for power generation. Why would you rob yourself?

I don't intend to rob myself. As I said in an earlier post my home is on higher ground but around it the whole area is a flood plain. In a flood the whole area including my home could be without power for a long time. That is when I would want to use the panels because that is when the inverter would not run
Quote:

You have a power inverter that's doing the risky power conversation for you already. Just use the usual wall outlet devices for the science experiment stuff. Safer, tested for reliability, etc.
The testing would only be supplied with 230 v AC but I suppose I could rectify it and end up with about 325v unregulated DC and wire 2 psu in series as they will run when only receiving about 160v feed.
Quote:

If you feel like you just must play with the hvdc, you absolutely must make a custom device that will run reliably at 10 percent or so higher DC voltage than you'll ever be able to generate. Stepping down from 500 to 12 or 17 vdc brings some sort of buck converter to mind.
I guess that would be an alternative, mosfets can now handle a high voltage and control huge amounts of power now.

jeff5may 12-24-20 01:36 AM

The power supply in series idea will eventually end up in smoke. As with the old series connected vacuum tube heaters, it will work for a little bit. Once your luck runs out, something hogs a little more juice and poof, it's all over.

Barrowman 12-24-20 02:03 AM

Quote:

Originally Posted by jeff5may (Post 63348)
The power supply in series idea will eventually end up in smoke. As with the old series connected vacuum tube heaters, it will work for a little bit. Once your luck runs out, something hogs a little more juice and poof, it's all over.

Do you think that would be true even though there would be varistors across each input?

Barrowman 01-04-23 03:35 PM

I know this is an old thread but now I am thinking I could use an IGBT controlled by a pwm signal generated by an arduino to control the rms voltage to a PC psu. my system uses a solis inverter and it's peak voltage spec is 600 volts and its rated voltage is 330 volts. I would set the pwm signal to 60% high and 40% low so it would ensure at even if 600 v is being generated the psu would see 135v rms.


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