Offgrid solar project

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David57
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Re: Offgrid solar project

Post by David57 » Mon, 06 May 2019, 19:56

Here we can see how greeat are "The Accords de Paris"
COP are just a joke. Industrials drive the world. And financials benefits give the direction.
Wednesday, i receive a first part of 32x310Wc panels, for beginning and testing how i arrange and connect it.
I reuse 3 toroidal 230V/50V transformers with redresser and some capacitors.
If that run, i can purchase Meanwell power source.

Richo
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Re: Offgrid solar project

Post by Richo » Fri, 10 May 2019, 12:47

David57 wrote:
Fri, 03 May 2019, 19:53
Two panels in series, one voltage limiter, one DC output, for each pair, why is this not possible ?
it's the same thing than many battery in parallel, no ?
While each branch is protected to never run as receiver, all branch are added and giving the current that is ask by inverter, no more
Two panels in series is 66V-80V 620W.
You need an output of 48V to feed the inverter.
So the peak would be 66V-48V= 18V @ 9A = 162W of loss through a voltage limiter.
24 pairs of panels is total loss of 3888W [26%] (HEAT).

So a linear regulator is not practical.
It has to be a switching device (PWM).

Ideally it would be an interleaved multiphase buck converter.
But to be honest the system may still be unstable without batteries.
And it is not a device you can go to your solar shop for.

I haven't come up with a better solution, yet, than a small LTO battery pack and linkable MPPT charge controllers.
And I haven't looked for cheaper alternatives to the MPPT charge contollers.
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Richo
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Re: Offgrid solar project

Post by Richo » Fri, 10 May 2019, 13:38

ONE panel with ONE boost converter might be cheaper.
So the input would be 33Vpk 40V max and output set to 48V.
You may need an output diode, fuse and a heat sink on each.
Then ALL the panels are in parallel on the 48V line.
Its the DC equivalent of the microgrid inverter system for grid feed systems...

I'm skeptical if this would work.

https://www.ebay.co.uk/itm/600w-10A-DC- ... SwtrJbSKr6
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Rusdy
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Re: Offgrid solar project

Post by Rusdy » Fri, 10 May 2019, 22:40

I've been using 'el cheapo buck converter for mine (no MPPT) and has been working well for more than two years so far.

Since I got the PV on the cheap, not worth spending the money on the MPPT controller. Utilisation factor is very poor, but not a concern for my purpose (I just need extra Wh for winter coverage).

I did use boost converter in the past. It didn't work.

As @Richo already suggested above, the only difference is: 2 strings in series with cheap high powered buck converter. Set in CC CV mode at 48V. Extremely modular!

I've been using this supplier. No disappointment so far: https://www.prodctodc.com

Unless, you're super keen (like me :) ) building your own boost MPPT controller.

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Re: Offgrid solar project

Post by David57 » Sat, 11 May 2019, 14:52

Hi,
I try to not thinking the system as a on grid system.
Here, the max solar power is not the max usable power, but is here to cover more days of the year.
If all panel are under full sun on summer, my water is hot, I don't need warming, maybe a clim, fridge, computer, just need in continuous 1 or 2 kVA, 3 or 4 if I use oven for cooking.
So in practice, the continuous loss are nor near 4000W.
I just see for the more simple, and the more low cost system (cost, not quality)
MPPT are great for system that use all the power.
Give money in a MPPT system that's not use the gain of power is a non sense.
Ow run the MPPT with just a 0.5A output power from panel ? How run buck and boost converter in case Pout>Pin ?
Whit high cost panels, or systems that must be reduced for place problem, each watt must be save.
At 100€ for 310W, adding panel can be cheaper than electronics devices. Same for solar trackers. for a time, it was interesting, now, is just for fun.
Other thing, I can use the inverter at more than 48V, if I use it at 55V, middle of voltage input, i reduce the voltage and the power to dissipate.
Normally, for greater input voltage in inverter, he take lower current at same output power, not ?

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Re: Offgrid solar project

Post by Richo » Mon, 13 May 2019, 12:43

Rusdy wrote:
Fri, 10 May 2019, 22:40
As @Richo already suggested above, the only difference is: 2 strings in series with cheap high powered buck converter. Set in CC CV mode at 48V. Extremely modular!

I've been using this supplier. No disappointment so far: https://www.prodctodc.com
TWO 310W panels is 66V peak power and 80V open (620W peak)
Is there a buck converter that does say 96V in to 48V out with 600W+?

I had a quick look and didn't see one.
The other issue I have is a simple buck converter has a tendency, when they fail, to supply Vin to Vout.
So 80V input to a 48V inverter would be bad.
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Richo
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Re: Offgrid solar project

Post by Richo » Mon, 13 May 2019, 12:46

David57 wrote:
Sat, 11 May 2019, 14:52
just need in continuous 1 or 2 kVA, 3 or 4 if I use oven for cooking.
So in practice, the continuous loss are nor near 4000W.
So the "typical" loss in your situation is 250W-1000W.

Someone suggested that you use the loss to heat the water..
I thought it would turn to steam though :lol:
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Richo
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Re: Offgrid solar project

Post by Richo » Mon, 13 May 2019, 12:56

David57 wrote:
Sat, 11 May 2019, 14:52
I just see for the more simple, and the more low cost system (cost, not quality)
MPPT are great for system that use all the power.
Give money in a MPPT system that's not use the gain of power is a non sense.

At 100€ for 310W, adding panel can be cheaper than electronics devices.
Same for solar trackers. for a time, it was interesting, now, is just for fun.
So you still prefer the bare minimum black box to connect your panels to the 48V line.
Ideally if the old 7805 linear regulator existed as a 7848 with 10A and a fat heat sink would be the way to go.
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francisco.shi
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Re: Offgrid solar project

Post by francisco.shi » Mon, 13 May 2019, 17:49

There is a way to do that with an extra transistor and a few resistors to adjust the output voltage. I am just not sure if it can run up to 48v.

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Re: Offgrid solar project

Post by francisco.shi » Mon, 13 May 2019, 17:53

To protect the output against failure in a buck converter you can use a crowbar circuit. It is basically an SCR that puts a short across the output if the voltage goes above a certain limit. This effectively blows the fuse.

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Re: Offgrid solar project

Post by Rusdy » Mon, 13 May 2019, 19:40

Richo wrote:
Mon, 13 May 2019, 12:43

Is there a buck converter that does say 96V in to 48V out with 600W+?

I had a quick look and didn't see one.
The other issue I have is a simple buck converter has a tendency, when they fail, to supply Vin to Vout.
So 80V input to a 48V inverter would be bad.
Ah yes, you're right, the only one available is this one and not cheap :(

True that, regarding failure mode. Mine is low power system (only a single 200W panel). Definitely don't want that failure mode when you have large panels. I like the crowbar cct idea :geek:

Looks like can't have it all (cheap and safe) :twisted:

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Re: Offgrid solar project

Post by David57 » Tue, 14 May 2019, 02:28

Making a regulator with a darlington power transistor and a resistor in serial with zener is not a problem, power dissipation must be see in practice. With that, the Ampere output still the same, only voltage is reduced. If power is not enough to answer, voltage fall down to the AC/DC converter level, and give the current that he can giving.
Other possibility, reduce the voltage with PWM regulated power transistor. Average Ampere stay the same as entry, voltage is reduced, low power dissipation, need an microcontroller to control PWM output. On cloudy days, if PWM is at 100%, the voltage fall to AC/DC converter level. All current is use.
Third, buck or boost converter, low dissipation power, more power in output on good condition, but how it run on cloudy days ? If solar input is not enough to answer to the inverter asking ?
If I simplify a solar panel, for a giving sun power (W/m2), I have a maximum intensity that can give the solar panel (A). If I pull less that this intensity, i have the full voltage panel, if I pull more, the voltage panel fall rapidly, intensity stay at maximum. That's ignore the curve with mppt point. Just to simplify.
How is it with buck/boost converter when the source is not a constant voltage source and work on the vertical line of the solar panel characteristic ?

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Re: Offgrid solar project

Post by Rusdy » Tue, 14 May 2019, 10:42

David57 wrote:
Tue, 14 May 2019, 02:28
...
How is it with buck/boost converter when the source is not a constant voltage source and work on the vertical line of the solar panel characteristic ?
I had an experiment with boost converter, it didn't go well when Pin < Pout. The boost converter couldn't recover even if the Pin > Pout (so, a bird's shadow on the panel quickly throw the boost converter to chaos).

With buck converter, no issue. I've done this in the last 2 years and counting. Noting the failure mode as previous conversation above.

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Re: Offgrid solar project

Post by francisco.shi » Tue, 14 May 2019, 10:47

You need to run the boost converter in a different way so that the draw current Does not overpower the cells.
Boost converters tend to be unstable when they get too close to 100% duty cycle.

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Re: Offgrid solar project

Post by Richo » Tue, 14 May 2019, 12:59

The way I see it is a Micro with Boost circuit that runs both MPPT from the solar and PID Spread spectrum PWM on the output.
There is about AUD$15 in parts including the PCB.
I'm a bit surprised that this doesn't exist.
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David57
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Re: Offgrid solar project

Post by David57 » Tue, 14 May 2019, 15:01

But solar power and needed current are totally unpredictable, with big jump, such oven, electric plate (all in the kitchen...), washing...

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Re: Offgrid solar project

Post by Rusdy » Tue, 14 May 2019, 16:12

My favourite is still from this guy:
https://www.youtube.com/watch?v=jPtBYg6N29E

Who needs solar controller or batteries when you can connect your appliances directly to solar PV! :lol:

David57
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Re: Offgrid solar project

Post by David57 » Tue, 14 May 2019, 20:31

Resistor directly on solar panel won't run correctly.
I have in past a water heater 2000W/230VAC with 26 ohm resistor, connected on 160V/6,5A string solar panel.
At 6A, it's work fine, i receive 1kw.
Little less sun, at 5A, it's fall to 650W
At 4A, around 400W
At 3A, 230W
2A, 100W, 1A, 26W.

Richo
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Re: Offgrid solar project

Post by Richo » Thu, 16 May 2019, 12:47

David57 wrote:
Tue, 14 May 2019, 15:01
But solar power and needed current are totally unpredictable, with big jump, such oven, electric plate (all in the kitchen...), washing...
Yes which is why, at the beginning, I suggested a small battery pack.
But it needs to handle the peak output from the meanwell ie 9kW or peak output from solar ie 15kW.
So a 5kWh pack would need to handle a recharge of 3C, hence the use of LTO.
Obviously if at some point you want a big pack, say, 30kWh then the recharge is only 0.5C so LiFePO4 etc is okay.
You could then sell the 5kWh pack.

But if there are 32 panels each with its own boost I would think the response time would be in the micro seconds.
Some of that would be buffered from the 48V to 240V inverter anyway.
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T1 Terry
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Re: Offgrid solar project

Post by T1 Terry » Thu, 23 May 2019, 10:04

If you needed a bigger battery pack, why wouldn't you add cells in parallel to the existing LTO pack? Are the different to LFP/LYP cells in a way you can't mix old with new?

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Richo
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Re: Offgrid solar project

Post by Richo » Thu, 23 May 2019, 12:40

Normally I suggest all cells are parallel first then those put in series to a make a string.
So LTO/LiFePO4/Lipo all have different voltages so this is not possible.

A series of small LTO performance cells then in parallel with a large general pack of LiFePO4 may end up "balanced".
So 10A draw would be 5A from the small pack and 5A from the large pack.
The LTO pack would be depleted well before the LiFePO4 pack. 5kWh vs 30kWh.

You would then need a string top module to control the connect/disconnect of individual packs.
Not pretty.

Its easier to have them all the same - if they aren't sell the odd/unwanted pack.
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Re: Offgrid solar project

Post by Richo » Thu, 23 May 2019, 13:18

David57 wrote:
Mon, 06 May 2019, 19:56
i receive a first part of 32x310Wc panels
Do you have the actual data on the 310W panels you have as there seems to be some differences between brands.
Vp ??
Voc ??
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T1 Terry
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Re: Offgrid solar project

Post by T1 Terry » Thu, 23 May 2019, 14:40

Richo wrote:
Thu, 23 May 2019, 12:40
Normally I suggest all cells are parallel first then those put in series to a make a string.
So LTO/LiFePO4/Lipo all have different voltages so this is not possible.

A series of small LTO performance cells then in parallel with a large general pack of LiFePO4 may end up "balanced".
So 10A draw would be 5A from the small pack and 5A from the large pack.
The LTO pack would be depleted well before the LiFePO4 pack. 5kWh vs 30kWh.

You would then need a string top module to control the connect/disconnect of individual packs.
Not pretty.

Its easier to have them all the same - if they aren't sell the odd/unwanted pack.
Misunderstanding, I mean adding more LTO cells to pack of LTO cells already there, just adding new cells in parallel with the older cells and joining these packs up in series to build the same output voltage but a larger capacity pack. We do this quite often with the Winston LYP cells where an RV owner gets the taste for the power they can get out of a lithium battery pack and want to run more appliances.

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Richo
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Re: Offgrid solar project

Post by Richo » Fri, 24 May 2019, 12:19

Ah LTO's are too expensive for home storage at this stage.
The only reason I suggested them is its a small pack that has to handle high charge/discharge.
Really only there to stabilise the system when using the MPPT charge controllers.
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Richo
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Re: Offgrid solar project

Post by Richo » Fri, 24 May 2019, 12:34

IF the system has NO battery AND using diodes from the panels direct onto the DC bus.
For the 310W panels "typically" they have an output voltage range of 32-40V.
Now this doesn't suit the existing 48Vdc/240Vac inverter.

However there are some "36V" to 240Vac inverters that have an input range of 31.5-45.0V.
This would closely match the panels running through a 10A diode.
Not very common though.

BUT you have to be careful as some 300W+ panels have higher open circuit voltages.

NOW the only problem is the 3000W-48V Meanwell AD/DC Power supplies are WRONG.
You would need a 32V power supply.
That way if the voltage drops below 32V from the solar panels the mains supplies the power.
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