My 0.0002, could be wrong..
I have a 160 watt panel and when the sun is shining and the panel is aimed
right, and Im using an MPPT controller, Im getting reasonably close to 160 watts
out of it - like maybe 150..
What Ive seen are calcutions based on the rated power of the PV panel and
assuing that you get 5 hours of sunlight per day. On good sunny days, this
possibly underestimates the power. On cloudy days, you might get a lot less.
Using current to compare things is ok but it doesnt take into account voltage.
What really is important is power.
Here are some rough calcs..
At 450 watts PV power and assuming using MPPT (which works and improves
efficiency for either 12 or 24 volt systems), the output current at full power
is (power = voltage * current)
15 amps for 24 volts system (15 = 450/30 volts - assumes MPPT allows panel to
operate at 30 volts rather than battery voltage)
30 amps for 12 volts system (30 = 450/15 volts - assumes MPPT allows panel to
operate at 15 volts)
So if this current were available for an average of 5 hours per day, you would
have
12 volts system : 5hours * 30 amps = 150 amp hours
24 volt system: 5 hours * 15 amps = 75 amp hours
Note that the amp hours is different but the power is the same - which is why
you should really look at watt hours and not amp hours to see what capacity is.
The watt hours is then:
12 volt system: 5 hours * 30 amps * 12.5 volts = 1875 watt hours
24 volts system: 5 hours * 15 amps * 25 volts = 1875 watt hours
To consider what long wire runs do, a lot of PV runs use 10 guage wire which has
an impedance of "about" .00118 ohms per foot. So if you had for exampe 40 foot
of wire between the PV array and the battery, the resistance which disipates
power would be the combination of the two wires (ie, the total wire length is 80
foot) so the power disipating resistance of the wire is 2 * 40 foot * .00118
ohms/ft = .0944 ohms.
So the power lost by the long wires would be ( power = current**2 * R)
12 volt system: 30**2 * .0944 ohms = 84.96 watts
24 volt system: 15**2 * .0944 ohms = 21.24 watts
For long wires and higher current, the 24 volts system looks a fair amount
better to me. You would be loosing nearly 20% just by the wire if you used 12
volts with this system. MPPT will help and I think a good application here but I
personally dont think MPPT is about 12 or 24 volts - its about better utilizing
either voltage system.
I could be wrong or have bad calculation so dont take any of this seriously (and
Im sure someone can find something wrong with it..)
Wally Hall
Denver
Skip Gundlach wrote:
Hi, Y'all,
I'm getting a little out of my depth, my last physics classes having been
nearly 40 years ago, but...
"Ken Heaton" wrote in message
news:xh05e.4996$VF5.1702@edtnps89...
Hi Skip, I'm looking at your math and trying to work out how you came up
with 170-125AH/day generated by your solar panels.
I'm not so much doing that as passing on the conventional and experienced
(that is, by folks who have SPs installed) wisdom that one may take the
wattage of current technology solar panels and divide by 3 (typical) to 4
(cloudy, etc.) for a real-world daily AH result. Not having same installed
on our boat (where there will be minimal or no shading), I can't do more
than speculate.
Will this 500 watt panel array ever actually put out 500watts? Or, as you
said above, will it really be only a quarter to a third of this ? If so,
how many hours a day will this panel put out this kind of wattage?
Perhaps
a couple of hours on either side of noon with diminished output before and
after this time?
500 watt panel /14.5 volt supply = 35 amps of current.
Ah - but I'm going with the higher voltages, for a couple of reasons.
One is easier transmission - smaller wires - for the same voltage drop.
The other is for more flexibility when tied to an MPPT. Typical peak
voltage would be in the 30s. So, presumably, at least, more amps result.
A third of this is only about 12 amps or so. For about 4 hours is 4 hours
x
12 amps = 48 AH per day. Perhaps you will get some useful power out of
the
rest of the available daylight, perhaps another 22 AH or so, that still
only
gives you 60 AH per day or so, no? Which doesn't seem to come close to
filling your anticipated daily power budget, let alone leave anything to
replenish the batteries from the drawdown on cloudy days.
Agreed. However, that (your scenario above) would make for a very small
output. I believe the 3 or 4 divider isn't at any given minute - else, why
would they be rated at any given wattage? - but, rather, a full-day
experience, ameliorated by darkness, clouds, shadows, and other impedimentia
to a 24-hour, full output, result.
Perhaps I've missed somthing here?
I dunno - perhaps *I've* missed something in all the research I've done. I
could swear that various posters to these fora have provided real-world
experiences on which I base my assumptions, backed up by various vendor
claims (as taken with the requisite grain of salt).
That I'll be in the Caribbean has got to help, too, but it's not the
presumption with which I started.
L8R
Skip, home for a couple more days before - this time - a 4 week working
visit to the boat-on-the-hard
--
Morgan 461 #2
SV Flying Pig
http://tinyurl.com/384p2
"And then again, when you sit at the helm of your little ship on a clear
night, and gaze at the countless stars overhead, and realize that you are
quite alone on a wide, empty sea, it is apt to occur to you that in the
general scheme of things you are merely an insignificant speck on the
surface of the ocean; and are not nearly so important or as self-sufficient
as you thought you were. Which is an exceedingly wholesome thought, and one
that may effect a permanent change in your deportment that will be greatly
appreciated by your friends."- James S. Pitkin