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From: KevinJ93 <kevin_es@whitedigs.com>
Newsgroups: sci.electronics.design
Subject: Re: Solar panels
Date: Fri, 31 May 2024 18:18:51 -0700
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On 5/31/24 1:43 PM, Don Y wrote:
> On 5/31/2024 12:56 PM, KevinJ93 wrote:
>> On 5/30/24 10:00 PM, Don Y wrote:
>>> But, the individual wafers (on a panel) are probably wired in a
>>> series-parallel configuration with a nominal 48VDC output.
>>
>> Not usually true - I don't know of any panels where there is a 
>> series-parallel configuration. As wafer sizes increase the panel 
>> current increases.
> 
> I count ~72 wafers on a panel I have here.  How does *it* get to a 48V
> nominal output?

Are you sure that isn't the open-circuit voltage(Voc)?

For the couple of 72 cell panels I looked Voc is 49.0V +/-

The peak power voltage is about 41.0V.

Each cell has a peak voltage of ~0.7V and a peak power voltage of ~0.56V.

That voltage is not enough to charge a nominal 48V battery which 
probably needs up to ~54V.

Interestingly I did find out that the 400W Q-Cell panels are in fact 144 
cell. They look they are configured as two 72 cell strings in parallel. 
There isn't any provision for reconfiguring as far as I could see.

>> It is common for the panel to be divided electrically into three 
>> series sections with a reverse diode across each so that if one 
>> section is shaded or damaged the panel will still give output at 
>> reduced voltage and the MPPT controller will adapt.
> 
> Yes.
> 
>>> To increase the ampacity from an array of such panels, I assume
>>> simply wiring in parallel would not be as effective as installing
>>> an MPPT controller on each and then combining to a 48VDC output?
>>
>> Wiring panels in parallel would require heavier gauge wiring - it is 
>> usually more cost effective to go with a higher voltage.
> 
> Yes.  But, if your goal is 48VDC, you then need to regulate that ~500VDC
> back down to 48VDC.
> 
> It then boils down to where the various components are located (long,
> high amperage runs suffering higher IR drops; conversion losses for
> long high VOLTAGE runs)
> 
>>> I.e., absorbing the cost of the conversion inefficiency in return
>>> for being able to eek a bit of extra power out of an underperforming
>>> panel?
>>
>> Residential installations commonly use micro-inverters with one per 
>> panel. This minimizes issues with individual panels being shaded or 
>> being placed on different facets of a roof.
> 
> Yes.  Or "power optimizers" in lieu of inverters.

Some inverter vendors (in particular SolarEdge) use optimizers with each 
panel - to perform part of the MPPT function. As far as I know they only 
work with the same vendor's inverter.
> 
>> Having each panel dealt with separately also avoids a problem with 
>> having high voltages on the roof where it could endanger emergency 
>> personnel in the case of fire.
> 
> Thus arguing against a high string voltage.
> 
>> Electrical code in the US requires that where panels are placed on a 
>> residence that there be no more than 80V DC present when disabled.
> 
> Local electronics at the panel ensure that -- whether microinverter or 
> power optimizer.

Or a Mid-Circuit Interrupter (MCI) as in the case of systems such as Tesla.
> 
>> Micro-inverters usually have a anti-islanding protection so that when 
>> the grid is not-present they stop producing leaving the roof safe.
> 
> I'm not looking for grid connection.  So, the grid is never present.
> 
>> In the case of DC systems this may require rapid-shutdown mid-circuit 
>> interrupters to meet these requirement.
> 
> Yes.
> 
>> Commercial solar farms don't have to meet these rules so they can go 
>> to higher voltages and avoid the expense of additional interrupters.
>>>
>>> And, that this would be preferable to stacking them and then
>>> down-regulating to 48VDC?
>>
>> Why the conversion to 48V? Residential applications usually convert to 
>> direct to 240V AC.
> 
> The straight forward approach is to AC then BACK to a lower voltage DC
> that can then be used, as is, and directly backed up with a low voltage
> (48) battery pack to carry over through periods of cloud cove > When "dark", I expect nothing from the array, having *consumed* all
> available power during the illuminated period.
> 
> I'm not trying to power AC loads.  So, going to AC means an inverter
> followed by a BIG "power supply".
> 
>> Even batteries for residential are commonly AC-in/AC-out with their 
>> own bidirectional inverters. (eg Tesla Powerwall and Enphase)
> 
> But, those try to back up the entire array's capability.  Give me 3-4KW
> during daylight and I can get by on < 100W for the rest of the day (night).
> So, a modest battery can carry the dark load and act to bridge small
> variations in output during full illumination.  Adjusting the load
> accomplishes the rest.

There are many "Solar Charge Controllers" on Amazon that could do the 
voltage conversion and MPPT to charge a 48V battery at up to 60A (~3kW).

They seem to want a string voltage of up to 100V or 150V which would 
equate to 2-3 panels in series. Provided all panels have the same aspect 
and sun exposure you could put strings in parallel to get the power 
level you need.

Not sure how to avoid excessive charge currents into the battery if it 
can't take the full solar output.

https://www.amazon.com/dp/B0D2KRL34V/ref=sspa_dk_detail_0?pd_rd_i=B0D2KRL34V&pd_rd_w=3xPB3&content-id=amzn1.sym.386c274b-4bfe-4421-9052-a1a56db557ab&pf_rd_p=386c274b-4bfe-4421-9052-a1a56db557ab&pf_rd_r=TKMP47AV732XMSDXGQRM&pd_rd_wg=bRMca&pd_rd_r=4b973d61-537b-40d4-8e1c-560122e38f7d&s=sporting-goods&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWxfdGhlbWF0aWM&th=1

kw