Forums

*DC* from PV panels

Started by Don Y November 22, 2021
Most panels seem to want to be strung in series to develop a
higher (potential) output voltage before being fed to an
inverter -- for AC out.

What are my options if I'm looking for a *DC* output?
Ideally, I'm looking for ~40+A @ ~50VDC.  Using PV panels
in the traditional way to generate AC and then converting
that back to DC seems like its going to bear more of
an (in)efficiency cost than just trying to use the nominal
48VDC from the panel, directly.

[I have some leeway with that... maybe 10% on the voltage
with an inversely proportional change in the amperage]

OTOH, the output from the panel will vary with cloud cover,
partial shading, etc.

So, are the losses trying to redevelop ~48V from each panel
(at differing current levels based on incident sunlight)
likely to be more or less than the more traditional approach?

I should see how folks living "off grid" use DC in their
applications...

[I'm wicked busy, lately -- end of year is always rough, this year
even moreso -- so apologies if I don't reply promptly]
On Monday, November 22, 2021 at 11:47:23 PM UTC-4, Don Y wrote:
> Most panels seem to want to be strung in series to develop a > higher (potential) output voltage before being fed to an > inverter -- for AC out. > > What are my options if I'm looking for a *DC* output? > Ideally, I'm looking for ~40+A @ ~50VDC. Using PV panels > in the traditional way to generate AC and then converting > that back to DC seems like its going to bear more of > an (in)efficiency cost than just trying to use the nominal > 48VDC from the panel, directly. > > [I have some leeway with that... maybe 10% on the voltage > with an inversely proportional change in the amperage] > > OTOH, the output from the panel will vary with cloud cover, > partial shading, etc. > > So, are the losses trying to redevelop ~48V from each panel > (at differing current levels based on incident sunlight) > likely to be more or less than the more traditional approach?
Not just different current, but different voltages. Both vary with the level of light if you are shooting for maximum power. The voltage also varies with temperature significantly. Converting back and forth between DC and AC is not terribly inefficient. EVs do it all the time. I think you would have a hard time trying to match a variable source like solar panels to a DC load. The load wants to set the current. What do you plan to do with the excess? Are you going to design your own controllers? What is your thinking here? -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209
Don Y wrote:
> Most panels seem to want to be strung in series to develop a > higher (potential) output voltage before being fed to an > inverter -- for AC out.
* Not a matter of "want"; IR drop in the panel is the concern. PV panels develop DC, not your implied AC/??
> > What are my options if I'm looking for a *DC* output? > Ideally, I'm looking for ~40+A @ ~50VDC.  Using PV panels > in the traditional way to generate AC and then converting > that back to DC seems like its going to bear more of > an (in)efficiency cost than just trying to use the nominal > 48VDC from the panel, directly.
* PV panels cannot generate AC! AFAIK one PV cell produces about 1.2V, output of a "panel" can be whatever you need; connect enough in series for 200VDC.
> > [I have some leeway with that... maybe 10% on the voltage > with an inversely proportional change in the amperage] > > OTOH, the output from the panel will vary with cloud cover, > partial shading, etc.
* Yup!
> > So, are the losses trying to redevelop ~48V from each panel > (at differing current levels based on incident sunlight) > likely to be more or less than the more traditional approach? > > I should see how folks living "off grid" use DC in their > applications... > > [I'm wicked busy, lately -- end of year is always rough, this year > even moreso -- so apologies if I don't reply promptly]
On 23.11.21 5:13, Rick C wrote:
> On Monday, November 22, 2021 at 11:47:23 PM UTC-4, Don Y wrote: >> Most panels seem to want to be strung in series to develop a >> higher (potential) output voltage before being fed to an >> inverter -- for AC out. >> >> What are my options if I'm looking for a *DC* output? >> Ideally, I'm looking for ~40+A @ ~50VDC. Using PV panels >> in the traditional way to generate AC and then converting >> that back to DC seems like its going to bear more of >> an (in)efficiency cost than just trying to use the nominal >> 48VDC from the panel, directly. >> >> [I have some leeway with that... maybe 10% on the voltage >> with an inversely proportional change in the amperage] >> >> OTOH, the output from the panel will vary with cloud cover, >> partial shading, etc. >> >> So, are the losses trying to redevelop ~48V from each panel >> (at differing current levels based on incident sunlight) >> likely to be more or less than the more traditional approach? > > Not just different current, but different voltages. Both vary with the level of light if you are shooting for maximum power. The voltage also varies with temperature significantly. > > Converting back and forth between DC and AC is not terribly inefficient. EVs do it all the time. I think you would have a hard time trying to match a variable source like solar panels to a DC load. The load wants to set the current. What do you plan to do with the excess? Are you going to design your own controllers? What is your thinking here? >
A shunt regulator? With a big heatsink for killing unwanted power?
On 23/11/2021 03:47, Don Y wrote:
> Most panels seem to want to be strung in series to develop a > higher (potential) output voltage before being fed to an > inverter -- for AC out. > > What are my options if I'm looking for a *DC* output? > Ideally, I'm looking for ~40+A @ ~50VDC.  Using PV panels > in the traditional way to generate AC and then converting > that back to DC seems like its going to bear more of > an (in)efficiency cost than just trying to use the nominal > 48VDC from the panel, directly.
I suspect that your losses (~2%) with the fattest Schottky diode you can find to wire or them together will be somewhat lower than the losses in a typical active DC to DC converter. This is especially true when compared to panels in series with for example one partially shaded. Wired OR you get the current from all the ones in full light but in series your maximum current is limited by the weakest link in the chain. Also when not in direct sunlight the diodes drop will be less. I guess it depends a lot on whether your load can tolerate the variation in supply voltage that will come from the solar panels directly.
> [I have some leeway with that... maybe 10% on the voltage > with an inversely proportional change in the amperage] > > OTOH, the output from the panel will vary with cloud cover, > partial shading, etc. > > So, are the losses trying to redevelop ~48V from each panel > (at differing current levels based on incident sunlight) > likely to be more or less than the more traditional approach? > > I should see how folks living "off grid" use DC in their > applications... > > [I'm wicked busy, lately -- end of year is always rough, this year > even moreso -- so apologies if I don't reply promptly]
-- Regards, Martin Brown
On Tuesday, November 23, 2021 at 11:12:23 AM UTC+2, Martin Brown wrote:
> On 23/11/2021 03:47, Don Y wrote: > > Most panels seem to want to be strung in series to develop a > > higher (potential) output voltage before being fed to an > > inverter -- for AC out. > > > > What are my options if I'm looking for a *DC* output? > > Ideally, I'm looking for ~40+A @ ~50VDC. Using PV panels
Wouldn't an MPPT do the job? Typically they can handle up to 150-volt input a give a regulated 48V nominal output (settable up to 60-volt) Example: https://www.amazon.com/Victron-SmartSolar-Charge-Controller-150V/dp/B07B4K62LN/?th=1
On 23/11/2021 03:47, Don Y wrote:
> Most panels seem to want to be strung in series to develop a > higher (potential) output voltage before being fed to an > inverter -- for AC out. > > What are my options if I'm looking for a *DC* output? > Ideally, I'm looking for ~40+A @ ~50VDC.  Using PV panels > in the traditional way to generate AC and then converting > that back to DC seems like its going to bear more of > an (in)efficiency cost than just trying to use the nominal > 48VDC from the panel, directly. > > [I have some leeway with that... maybe 10% on the voltage > with an inversely proportional change in the amperage] > > OTOH, the output from the panel will vary with cloud cover, > partial shading, etc. > > So, are the losses trying to redevelop ~48V from each panel > (at differing current levels based on incident sunlight) > likely to be more or less than the more traditional approach? > > I should see how folks living "off grid" use DC in their > applications... > > [I'm wicked busy, lately -- end of year is always rough, this year > even moreso -- so apologies if I don't reply promptly]
Domestic PV rooftop panel assemblies intended for grid tie inverter usage have open circuit voltage typically 60-80V with strong negative temp coefficient. They are approximately current sources, if ultimate efficiency isn't needed just parallel them and load them down enough to get the target 48V. piglet
Rocky <robertgush@gmail.com> wrote:
> On Tuesday, November 23, 2021 at 11:12:23 AM UTC+2, Martin Brown wrote: >> On 23/11/2021 03:47, Don Y wrote: >> > Most panels seem to want to be strung in series to develop a >> > higher (potential) output voltage before being fed to an >> > inverter -- for AC out. >> > >> > What are my options if I'm looking for a *DC* output? >> > Ideally, I'm looking for ~40+A @ ~50VDC. Using PV panels > > Wouldn't an MPPT do the job? Typically they can handle up to 150-volt input a give a > regulated 48V nominal output (settable up to 60-volt) > Example: https://www.amazon.com/Victron-SmartSolar-Charge-Controller-150V/dp/B07B4K62LN/?th=1
Yes, that is what you need. And probably with a battery as well to cover the drops (and maybe the night). These are effectively DC-DC converters but they have an input characteritic that tries to obtain maximum power from the solar array by choosing the optimal voltage/current combination. When you don't care about that (or have no battery where the temporary surplus will be stored) a plain DC-DC converter can be used as well. Or a 115VAC to 48V DC SMPS with the AC rectifier removed.
On Tuesday, November 23, 2021 at 4:21:12 AM UTC-4, Sjouke Burry wrote:
> On 23.11.21 5:13, Rick C wrote: > > On Monday, November 22, 2021 at 11:47:23 PM UTC-4, Don Y wrote: > >> Most panels seem to want to be strung in series to develop a > >> higher (potential) output voltage before being fed to an > >> inverter -- for AC out. > >> > >> What are my options if I'm looking for a *DC* output? > >> Ideally, I'm looking for ~40+A @ ~50VDC. Using PV panels > >> in the traditional way to generate AC and then converting > >> that back to DC seems like its going to bear more of > >> an (in)efficiency cost than just trying to use the nominal > >> 48VDC from the panel, directly. > >> > >> [I have some leeway with that... maybe 10% on the voltage > >> with an inversely proportional change in the amperage] > >> > >> OTOH, the output from the panel will vary with cloud cover, > >> partial shading, etc. > >> > >> So, are the losses trying to redevelop ~48V from each panel > >> (at differing current levels based on incident sunlight) > >> likely to be more or less than the more traditional approach? > > > > Not just different current, but different voltages. Both vary with the level of light if you are shooting for maximum power. The voltage also varies with temperature significantly. > > > > Converting back and forth between DC and AC is not terribly inefficient. EVs do it all the time. I think you would have a hard time trying to match a variable source like solar panels to a DC load. The load wants to set the current. What do you plan to do with the excess? Are you going to design your own controllers? What is your thinking here? > > > A shunt regulator? With a big heatsink for killing unwanted power?
So just waste it as heat? I think the OP said something about not wanting to convert DC > AC > DC because of the efficiency issues. Why would he want to design in such losses? I think the reality is he has not given this much thought or at least not described his thinking here very well. The first point to clarify is whether there will be a battery in the circuit to provide power when the sun is clouded. But maybe he's running a water pump and doesn't care if it drops out for a bit. I don't know. -- Rick C. + Get 1,000 miles of free Supercharging + Tesla referral code - https://ts.la/richard11209
Hi Don, 
so what exactly do you need the 900W of electricity for?

If I suppose the goal is to store every bit of ENERGY collected per day, 
then we need a lossless storage medium, 
and a medium whose energy that can be efficiently drawn from at an 
arbitrary rate.

So Rick's idea of heating water is not so crazy, if the energy capacity
is high enough. Might be a cheaper start-up & more reliable than a large
bank of storage batteries. And safer than splitting water into H2 and O2.

Or, of course, some hybrid approach, using both thermal & chemical...
If you're really ambitious :-)

But simpler is probably selling excess energy to your E company and
get some $.
 
regards, RS