Solar - 12 or 24 volt?greenspun.com : LUSENET : TimeBomb 2000 (Y2000) Preparation Forum : One Thread
Would someone be kind enough to instruct me - What is any benefit of a 24 volt PV system vs. a 12 volt system? Thanks.
-- lee chesson (firstname.lastname@example.org), July 26, 1999
First, most 24v inverters are (slightly) more efficient than their 12v counterparts.
This isn't really a big issue. The second issue really only applies fully to larger systems because it is a function of how much power is being used. You can read the explanation below, or accept that for the most part this is not really an issue in small systems (which is why RVs use 12 volt systems).
Summary -- the more power used, the larger the wiring must be to avoid excessive losses, and the larger the wire size the more expensive the system becomes.
Power is equal to the product of volts times the current that flows [P= E x I]. So, to produce any given amount of power at 24v requires half the current needed at 12v. This is important because the wiring has a specific resistance (R) determined by its diameter. This resistance causes a loss in voltage -- called the IR loss --along the wiring, and it heats the wiring. [Another formula: E = I x R] The voltage across the output is less than the input voltage, which means output power is less than input power. Essentially, power is being converted into heat in the wires.
In smaller systems, where only relatively small amounts of power are being generated, the answer is simply to use a larger size wire in order to avoid this loss. This is more expensive: these are large wires you're dealing with. As the power output of the system increases it simply is a lot cheaper to use 24v than to increase wire size.
Some major commercial units use 48vdc or even 96 or 120 or 144 vdc for this very reason.
Now, since the batteries are 6 volt, you can get 12 vdc by stacking 2, 24 vdc by stacking 4. Does this mean you need more batteries to operate at 24 v than at 12? Nope: you're only using half the current, so each battery will last twice as long before needing recharge when used in a 24 v system. Typically, even at 24 v a stack of 4 batteries (in series) will be connected in parallel to a second stack of 4 batteries. This only becomes an issue if you want to design the system to use 6 or 10 batteries --- a system that uses 4, 8, or 12 is fine.
-- de (delewis@Xinetone.net), July 26, 1999.
Lee, use 12 volt for a backup power system. You can get a lot more standardized appliances, light bulbs, etc. from boating stores and RV stores.
Large yachts sometimes use 24 volt because the wire runs are longer and 12 volt power experiences too much line loss for long runs.
There's a guy named Roy at Fourwinds who can advise you on this. His name is well-circulated on the web, esp. the North site.
If you're inverting to 120 volt, then with 12 volt source, you need to keep the wire run from your solar array to your inverter very short to prevent line loss. 24 volt would allow you a longer run to the inverter.
I am not an expert, but this is my layman's distillation of the 12 versus 24 issue.
-- Puddintame (email@example.com), July 26, 1999.
Being an old shirt tailed Electrican...Might I point ot, that if the inverter(s) is placed in near location of the battery bank. Then use use ordinary wire from there( I would suggest #10 stranded THHN) to the plugs you wished powered. If you wish to power the house system, then you would of course place the inverter set up at the nearest point to your main house disconnect...and install the approiate swith over tie ins Just MHO.
-- Shakey (firstname.lastname@example.org), July 26, 1999.
You got lots of good answers to your questions there...I just want to add one more thing. There are larger inverters available in the 24 volt range. 3200 watts is about the largest inverter commonly available in the 12 volt range...but you can go up to 4000 watts and larger with 24 volts, 5500 watts and up with 48 volts. For larger home sized systems the 24 volt is most commonly used...for basic to medium sized, the 12 volt system is still a contender.
-- Roy Butler (Roy@Four-winds-energy.com), July 26, 1999.
I chose 12V for 2 reasons:
1) Redundancy. If you use 6V trojan L16's for you battery bank, and one cell in one battery has a catastrophic failure, you only take 2 batteries out of service. With 24V you'd have to take 4 out.
2) I use wind generators. Have plans to make additional units this summer using plans I purchased from someone in desert SW. These units are simple, made of electrical conduit, sheet metal, and a common GM alternator. (some welding too). Finding a 24V unit from a millitary vehicle is more difficult than the autoparts store down the road!
-- embedhead (email@example.com), July 26, 1999.
Gosh, I need some help on this. I have called the one so called "solar" guy in our valley and he says he only knows how to set up the system he sells.
I have 12 volt deep cycle marines. Two bank sets of 6 (amphs differ in the sets.) Please don't go telling me I need 6v, gel, trojan, golf cart, etc. - already have the 12v, bought and paid for.
(1) how do I wire them in sequence to keep at 12v and just extend endurance by combining their capacity? Do I wire red to red/black to black in parrallel through the sequence?
I have several solar panels. What kind and size of fuse do I install betweeen the panel and the battery bank? What guage wire do I need? (Is it a function of length of run from panel to bank?) Do I absolutely need a controller and, if so, where do I get one CHEAP. (I already have all the inverters I need.)
Besides topping off the battery's energy charge, what kind of maintenance should a battery get to decrease sulfation? When do you add water? Do you clean terminals regularly? What saftey precautions should be used?
Thanks for your answers. I really am clueless in this area...
Please go slow and use common English in your instructions, lol.
-- marsh (firstname.lastname@example.org), July 26, 1999.