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Pros and cons of 12V, 24V, 48V system.

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Posted by Crappy on August 13, 2008, 9:18 am
 
Hi All,

I am planning on putting up some PV panels on my house to, initially,
run lights and some of the plug circuits but ultimately, run
everything (barring the stove) (Heating is gas)

The panels will be on the first story roof, all the electronics etc,
directly below on the ground floor. Cable run would be about 10m (33
foot).

My question is, all things considered, which is best. To combat the
losses on the low voltage cable, obviously 48V would be better. Or
would 24v system still be ok?

I want to  start with 2 or 4 panels. Two would force the 24v option
but if need be, can do 4. I plan to expand it in the near future to
have more panels as well as a wind turbine.

Thinking out loud here, expanding a 12v system means the possibility
to buy one panel at a time. 24 or 48v means 2 or 4 at a time.
Likewise, the turbine will be more expensive.

Is the effort worth going higher source voltages or not really.
Inverters (UK is 230V) are cheap for 24v and have a high enough
rating. I would use two or 3 to run different circuits in the house.
12V inverters are higher cost for high powered ones.
Advantages of 24 and 48 would be less current on the DC side of things
as well. Fewer losses there.

So, after rambling and thinking out loud, 12V is not a good idea (or
is it), 24V is better but 48 would be best provided I can get the
inverters etc.

Is 48 really much better than 24? Can I stick with 24?


Thanks,
Crispin

Posted by Bruce Harvey on August 13, 2008, 2:38 pm
 

The length of run you are talking about would be ok for 24v assuming you
have sized your cables correctly for the job. Also because you are using 24V
the cables can be smaller diameter (read cheaper) but to avoid the problem
of needing to upgrade 2 panels at a time you can get 24V (nominal) panels.
So, 24V would be a good option for you to upgrade slowly over time. Would
need more info about loads to help you more but you get the idea. BTW I use
48V system in Australia but is attached to 2.8kW solar PV and MPPT charge
controller. Runs my entire house.


Posted by Crappy on August 13, 2008, 3:21 pm
 Thanks Bruce,


What kind of inverter do you have? One or a few of them.
I plan on using at least two 3kw ones.Possibly one 3kw one and a
smaller pure sine wave one for certain plugs.
I'm curious what you run a whole house off. Modified sine wave or tsw?

Regarding the MPPT controller, I had not heard of them before but a
quick dose of wikipedia, they make sense. Are they worth it? Cost a
bit more than "normal" controllers. Me being in the UK, every ounce I
can suck out will help.

I doubt I will have a 2.8KW array anytime soon. Might approach it once
I have wind up and running as well.

Regards,
Crispin


Posted by Bruce Harvey on August 14, 2008, 7:41 am
 
First let me make it clear that I am not connected to the grid. I use a
Xantrex (Formerly Trace Engineering) SW4548E Inverter charger. This is a
4.5kW 48VDC input 230VAC 50Hz Output unit. These units can be paralleled up
to get up to about 11kW continuous output and somewhere around  30kW for
very short periods if you use the largest SW5548 Inverter chargers. There
are of course smaller models available but keep in mind that you will pay a
lot more for 2 separate 2.5kW units than one 5.5kW unit for example. Anyway
to have a look at Xantrex go here
http://www.xantrex.com/web/id/47/p/1/pt/5/product.asp . To answer your other
question it is true sine wave output. The other thing that will affect your
choice of inverter/charger will be grid or no grid and battery backup or
not.
I and anyone else would need to know what you plan here so that they can
work out what you need. Size of the units will depend on the exact
loads/usage and your GPS location (to work out sun availability. etc. etc.
as you can see this can become very complicated very quickly.
As for the MPPT (Maximum Power Point Tracking) charge controller mine is an
Outback MX60. They can improve the total output by about 10-15% on fixed PV
panels but the gains will be less for the charger and more for the solar
panels if you are using a solar tracker on your panels.


Posted by Ron Rosenfeld on August 13, 2008, 5:06 pm
 On Wed, 13 Aug 2008 02:18:14 -0700 (PDT), Crappy


In order to make a recommendation, one has to know what your ultimate size
will be, and also some idea about the price of the wiring.  Copper is
pretty expensive here in the US.

These numbers are rough, only to give you an idea of some of the tradeoffs.

Assume that your ultimate goal is to have 960 watts of PV, and that you can
tolerate up to a 3% loss in the wiring.

If these were to provide that power at 48V, you would be running 20A in the
line, and could run #10 copper wire.

In your 24 V system, you would require #4 copper wire to produce similar
results.

Here, I could probably obtain the #10 copper as scrap from an electrician
for free or for a nominal price.  #4 copper I'd have to purchase.

------------------------------

In terms of setting up for what your ultimate size will be, also consider
whether or not your 24V inverter will carry the load that you think you
might want.

The optimum design is driven by energy and power specifications.  Without
those, it's difficult to analyze your particular situation.  Cost certainly
plays an important role also, and can tilt the decision to one design or
another.  But you need the power and energy specifications in order to
evaluate the costs of different approaches.

By the way, I have a 48V system, but my solar panels are nominal 24V and
the array is wired as a nominal 72V array.  (I use an Outback MX60 MPPT
controller to get the voltage correct for my system).

One of my considerations was to keep the wiring hidden.  I had a longer run
than you, and not exceeding the capability of #0 copper was important to
me.
--ron

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