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Pros and cons of 12V, 24V, 48V system. - Page 2

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Posted by Crappy on August 13, 2008, 8:44 pm
 
Ron,

I know I am being vague, but my requirements are such :)

I figure I will stick with 24V system to start as stage one, I will
only run the lights. Once this is settled, I will expand it in both
batteries, inverter and PV.
I will run 3 inverters ultimately. One for lighting, the other two for
plugs. (Possible one for plugs)

As for the cable from PV to batteries, I will run the fattest I can
buy now as I plan to expand.

Question on the charge controllers:
I suspect they are slightly more advanced than simple regulators. I
suspect they would be switch mode to help bring your 72 down to 48.
Will they act as a voltage doubler if the voltage is below the desired
or is that just lost time. Or, is there a sharp climb in PD as light
hits it. (Am I making sense?) Can a panel produce 6v, 1A in dim light
or does it rapidly climb to full voltage?

Are the cheap 30 regulators on ebay  a false economy?

Thanks for the help - I am writing my shopping list now. Might publish
it for comment :)

Cheers,
Crispin

Just thought of another question:
If I plan on using 24V, would it be better to have an array producing
36v? Or would 24v array be ok? Would it dip below "chargability" to
often. Would the 12 (36-24) be wasted or usable?

Posted by Bruce Harvey on August 14, 2008, 8:30 am
 
Ron,

I know I am being vague, but my requirements are such :)

I figure I will stick with 24V system to start as stage one, I will
only run the lights. Once this is settled, I will expand it in both
batteries, inverter and PV.
I will run 3 inverters ultimately. One for lighting, the other two for
plugs. (Possible one for plugs)

As for the cable from PV to batteries, I will run the fattest I can
buy now as I plan to expand.

Question on the charge controllers:
I suspect they are slightly more advanced than simple regulators. I
suspect they would be switch mode to help bring your 72 down to 48.
Will they act as a voltage doubler if the voltage is below the desired
or is that just lost time. Or, is there a sharp climb in PD as light
hits it. (Am I making sense?) Can a panel produce 6v, 1A in dim light
or does it rapidly climb to full voltage?

Are the cheap 30 regulators on ebay  a false economy?

Thanks for the help - I am writing my shopping list now. Might publish
it for comment :)

Cheers,
Crispin

Just thought of another question:
If I plan on using 24V, would it be better to have an array producing
36v? Or would 24v array be ok? Would it dip below "chargability" to
often. Would the 12 (36-24) be wasted or usable?


The voltage (nominal voltage of your PV array/panels needs to be the same as
the inverter input. Keep in mind that the actual output of the panel will
vary depending on sun angle/intensity etc but the charge controller will
take that and transform it into a usable voltage for your batteries/inverter
(this is complicated and needs further information). For example a 24V
(nominal) PV solar panel will output voltage from 0 to about 36V depending
on sunlight. This information is available from the solar panel
manufacturers via their web sites in the form of tables and graphs of their
output at given insolation (sun intensity) values. Go here
http://www.kyocerasolar.com/products/spec_module.html  to see a sample of the
info from Kyocera Solar.
Hope this helps a little.


Posted by Crappy on August 14, 2008, 9:57 am
 Thanks again Bruce,

As for location, I am in the UK, just north of London. (Stop Laughing.
Ambitious for solar?)
Here is me: http://maps.google.com/maps?f=q&hl=en&geocode=&q=al9+7pa&ie=UTF8&z 


As I don't have the means to tell the power company right now  to go
away, I have to build this slowly.
As such, I plan on isolating certain circuits and feeding them from
the PV / batteries / Inverter. I do not intend (now in any case) to
have this grid tied. (As is becoming common)
My steps are:
1) Lights
2) Plugs - This will be the big jump as I don't want to chance it with
modified sine wave, just go all about and get a big w inverter.

At this point, I can include a disconnect from the grid.

Thanks for the answers on the controller.
The array will be fixed and pointed optimally. (I have a clear view of
the sun from about 8am (summer) to about 8pm. Winter, not sure, not
hoping on much :) Wind will help me here.
As for the controller, it was answered thanks. So when the panels are
producing say 20w @ 18v (instead of 24+) the controller will bring
that up to 24 to continue charging the batteries, albeit at a lower
current.

So far, I have opted for this:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item20286111891&ru=http%3A%2F%2Fsearch.ebay.co.uk%3A80%2Fsearch%2Fsearch.dll%3Ffrom%3DR40%26_trksid%3Dm37.l1313%26satitle%3D320286111891%26fvi%3D1

It is cheapish, is a starting point. I will get two and series them. I
have opted for a 24v system.
As for batteries, I have opted for two 150ah floated. (They are deep
cycle, marine apparently) (I thought only sealed and Gel were deep
cycle capable)
While the PV will be very underrated for the batteries, they a good
starting point (or not?)

Thanks again, you've answered some of the questions I have swirling
around in my head. (There are many more...)
As I have got bye-in from the boss (aka: Wife) I don't particular want
to end up with a white elephant...



Why are you not on the grid? No possible or just because you liked the
idea of free power. (Free after paying a sh1t load for it....)


Cheers,
Crispin

Posted by wmbjkREMOVE on August 14, 2008, 2:42 pm
 On Thu, 14 Aug 2008 02:57:56 -0700 (PDT), Crappy


Controllers such as the MX60 only boost current, not voltage.


Negatives for that seller
http://toolhaus.org/cgi-bin/negs?User=%09koekraf&dirn=Recd&v=b1

Wayne

Posted by Bruce Harvey on August 15, 2008, 6:16 am
 
 Yes and no. The controller scans the entire pre-programmed voltage range
available from the solar panels and uses the voltage that gives the maximum
POWER (VoltsxAmps) available anywhere in that range. Obviously this is more
efficient to some extent than setting the charger at the theoretical Maximum
Power Point and expecting it to stay at the same voltage throughout the day.
Frequency of scans can be programmed from 1 min to almost any amount of
time. Mine scans every 10 minutes)When the batteries are fully charged or
close to it the controller raises the voltage to a point where the output
power is reduced to maintain the batteries at a 'float' charging level. The
other thing that the MX60 (and many other MPPT charge controllers) does is
give flexibilty to you panel array wiring. The MX60 has a maximum input of
140VDC @ 60A (This can be programmed up to 80A but is not recommended).
Therefore a number of different panels can be arranged in series to give a
usable voltage as long as the Voc total in each string of the array does not
exceed 140VDC. There are some limitations on this but you get the idea.

What  has been said in other replies to this thread is also true. The output
voltage from the solar panels must be higher than the charging voltage of
the batteries. For example a 24VDC battery array would normally need to
charge to about 29.2V (dependent on the batteries used) to reach the fully
charged level of the batteries and hold the batteries on 'float' once they
are fully charged. The output voltage open circuit (Voc) of a 24V solar
panel or 2 12V solar panels in series is around 32-36VDC depending again on
the panels used. Some solar panels have also been designed for grid
connected sytems and have a lower Voc bur depending on how you set up the
panel array and the charger these can be used in similar ways as long as the
panel voltage is higher than the battery voltage.
Hope this helps more than it adds to the confusion.



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