power and voltage ??,s

shockman · January 2013

Okay! bit of a newbie, but learning fast! My ?,s are understanding voltage versus efficiency. I keep hearing about 12v systems being for toys, mostly pertaining to line loses. So my system started with 6 old Arco M55 panels that claim 15.2 volts at 47*C and 3.37A running 100ft on #10 wire to an old 30A pmw controller attempting to charge 12 225ah 6v batts. So we all know how well that worked, I had to waste the 1.5 yr old batts to figure it out!! The best I ever seen was 12A of chargeing. So I have since added a classic 150 and 2 125 watt panels, Vmp 17.39 Imp 6.93A. I rewired old panels in pairs and the 2 new ones as a pair. I have uped the supply line to #4. I have seen amps as high as 36 in winter sun and have reduced batts to 4 of the semi pooched originals. SO my question is where and why are the inefficiencys of 12V system a problem?? I don't think there is much line loss from array, so is the problem from batts to inverter (old trace 2012 1500-2000w??) and onward?? pls explaine?
I want to expand the system from here but don't know if I should take the plunge and buy a 24v inverter. should I buy 12 or 24v panels?? to increase power?? I'm running a 1000sq.ft lake cabin with not much more than lights and TV, except would like to use auto coffee maker and run a small freezer in July and August (never enough ice for those month's in the Kamloops B.C.)
There was also talk of the old panels being a problem because of the lower voltage that I don't understand?? do they drag down the new ones and why???

solarix · January 2013

Re: power and voltage ??,s

Voltage is a measure of strength. Power is a measure of how fast work can be done. Volts x Current (flow) = Power. The problem with low voltage (12v) systems is the voltage is low and the current is high tending to have high resistive losses in the wiring. Resistance x current = Voltage, therefore resistance x current x current = power loss. Notice that as current doubles the power losses quadruple. (Voltage is good / Current is bad).
Doesn't sound like to me though that your system is so large that it is worth it yet to convert to a higher voltage.

Cariboocoot · January 2013

Re: power and voltage ??,s

Welcome to the forum.

You should read this bit: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

Basically a lower Voltage system uses more current to produce the same amount of power as measured in Watts. As such it is less efficient; current produces heat which means power being lost instead of going to work.

Your old panels @ 15 Vmp probably aren't adding much to the mix. After line loss they may not be producing enough Voltage to charge at all (if wired in parallel). 47C is not the standard temp for a Vmp rating, by the way. It should be 25C. As such the Voltage may actually be higher than 15.2 for those panels.

Since you are using a Classic 150 on a 12 Volt system you could wire your panels in series: the two 55's as one string with a Vmp of about 30 and the two 125's as another string (parallel to the first) with a Vmp of about 35. It's a bit of stretch even so (Voltage difference greater than 10%).

So you have many Voltage issues here. First is lower Voltage leads to greater line loss; less power at the end of the wire due to its resistance. Second is the low Voltage panels as a result may not actually supply enough Voltage to charge the batteries (12 Volt systems actually charge around 14 to 15 Volts). Third is the power loss to heat generated by the higher current needed for the same output.

Does that make any sense?

BTW, I was in Kamloops this week. You had plenty of ice then!

waynefromnscanada · January 2013

Re: power and voltage ??,s

Well as a start re 12 volt problems question - - - Regardless of the voltage, higher amps result in higher voltage drop over the length of wire carrying those amps. The greater the amps and the smaller the wire, the greater the voltage drop. Now lets add to that. Lets say a current of 20 amps on a given length of wire results in a 2.4 volt drop. On a 12 volt system, that's a 20% drop. Major! On a 24 volt system, it would be "only" a 10% drop. But lets take it a step farther. 20 amps @ 12 volts would be 20 X 12 = 240 Watts. But on a 24 volt system, a 240 Watt load would only draw 10 amps. 10 X 24 = 240 Watts. Another step: Since now we're only dealing with 10 amps, not 20, the voltage drop on that length of wire will now be cut in half, reducing it to 1.2 volts, which on a 24 volt system, is "only" 5% drop. So you can now see that with the identical size and length of wire, a 240 watt load has dropped it's losses from the totally horrible loss of 20%, all the way down to the less horrible loss of 5% by switching from 12 volts, to a 24 volt system.
Hope this simplistic example helps with understanding at least part of your question.
PS: I'm slow composing answers, so as usual, others have likely by now provided answers. But hey, better late than never

BB. · January 2013

Re: power and voltage ??,s

You can break the system design into several parts...

First there is the battery bank to load--I tend to suggest if your peak continuous loads are 1,200 watts or less, then 12 volts is A OK--especially if you have some 12 volt centric loads (car radio, car appliances, existing inverters, etc.).

12 volts * 100 amps = 1,200 watts...

So, you can see you are already looking at some pretty large cables to carry that amount of current with around 0.5 to 1.0 volt maximum drop.

If your loads remain low, the MorningStar 300 watt (600 watt for 10 minutes) 12 volt TSW inverter with remote on/off and "search mode" is a really nice unit for a cabin or even a small home where you have some smaller loads that need clean TSW (true sine wave) power (cell phone/laptop computers/chargers, tv/dvd players, etc.).

Even a 1,200 watt AC inverter, if you wire it for its full capabilities, the copper requirements become pretty large:

1,200 watts * 1/10.5 volt cut off * 1/0.85 inverter eff * 1.25 NEC derating (wire+breaker) = 168 amp minimum branch circuit+breaker ratings

So, look at your loads and power requirements. Note that solar really makes a lot of sense if your loads are for 9+ months of the year. If you have a 2 month load requirement for a refrigerator, you are probably better off getting a used RV fridge that runs from LP gas. Otherwise, you are "wasting" power for the other 10 months of the year.

Refrigerators are one of the most power hungry loads you can put on a solar power system... And one of the reasons is because of high starting loads which require an inverter+battery system that can output ~1,200-1,500+ watts for starting. Once the fridge is running, the power needs are not as bad.

And, if you are looking at refrigeration only, a converted chest freezer can drop the daily power requirements from around ~1+ kWH per day down towards ~0.25 kWH per day.

And while you can run a microwave or coffee maker on smaller solar power systems, frequently it is just easier to buy a stove top or propane Coleman Coffee system (I don't drink coffee, but others here who do find these to work fine) for those occasional needs (or get a Honda eu2000i genset for doing meal time, backup AC battery charging).

Note, as your load requirements go up--The size of battery bank does too... Which requires more money for battery replacement every 3-7 years or so, more solar panels for adequate charging, etc...

Next, there is the charging system... And if you use an MPPT type charge controller, you can run upwards of 40-100+ VDC for the Vmp-array and use much smaller gauge wiring from the array to the charge controller/battery box, and still stick with 12/24/48 volts (your choice) on the battery bank.

With "mixed" panels (different Vmp/Imp ratings), you may end up having to purchase different solar MPPT charge controllers for each different array configuration (you can put two or more charge controllers on one battery bank--assuming proper wiring and sizing of equipment). The downside of this option is that MPPT charge controller are not cheap. Around $225/$300/$500/$600+ depending on size and optional equipment.

-Bill

shockman · January 2013

Re: power and voltage ??,s

Okay some claification, the old panels also say on the label, max voltage at 0*C & open ckt, 24 volts DC ???? whatever that tells us?? and there are 6 of them so they are 3 sets of them and one set of the new ones all paralled to controller, and I wish I could remember what I seen for volts at the classic but can't.
So I understand the math (I think) about line loss, but I beleive that with #4 wire and series wireing of pannels to up the voltage that the line loss to controller should be hopefully below acceptable limits. My confusion is, is there a price to pay from batteries to inverter or inverter itself compared to 24v ???

Cariboocoot, next time you pass Tobiano golf course heading east, look north across Kamloops Lake and wave :>)
Marc

BB. · January 2013

Re: power and voltage ??,s

The inverters are roughly the same price for 12/24/48 volts... The bigger problem tends to be that higher DC input voltage inverters are larger units (2kW-4kW minimum for many of the "nice ones") and they have more features (search mode, remote on/off, internal AC battery charger/ups function from AC main or AC generator).

Start by defining (measuring) your loads/power needs first. In some cases, you can make changes that will reduce your power needs (well pumps tend to take a lot of power for a few minutes per day, changing to lower power devices, pumps designed for solar use, installing pump+variable frequency drive, etc.).

In general, I recommend conservation because I am "cheap"... It is almost always cheaper to conserve power than to generate it.

-Bill

NorthGuy · January 2013

Re: power and voltage ??,s

With 150 MPPT controller, the voltage between the panels and the controller doesn't have to be 12V. It can be anywhere between 16V (must be above your battery) up to 120V (cannot really go to 150 because your voltages will increase in cold weather). You can use longer strings, so that the sum of the voltages of individual panels is close to 120V. Compared to pairs, which are only 30V, you will dramatically reduce power loss (16 times to be exact).

Your system voltage (12V) lives only between charge controller and batteries and between batteries an inverter. These cables are really short (at least they should be) and by making them as thick as possible you will eliminate any significant power loss even with 12V.

If you use DC wiring to your loads then cables are long and thin. In this case, going from 12 to 24V will help a lot, although if your loads are 12V you'll need to replace them with 24V loads.

Cariboocoot · January 2013

Re: power and voltage ??,s

Well the Voc rating also isn't helpful because what matters is the Voltage and current under load: Vmp & Imp. The main function of Voc is to calculate maximum Voltage for input to the controller so that its limit is not exceeded.

Trouble is the whole array Vmp is less than <20 Volts, so line losses can be substantial. An example of those six 55 Watt panels in parallel on #10 wire:

Vmp 15.2, Imp 3.6 (*6 = 21.7). In 10 feet the Voltage drop will still be over 3%.

I just drove that way on Monday, and back again Tuesday.

westbranch · January 2013

Re: power and voltage ??,s

I would say save your money by putting those old panels aside and buy 1 or 2 more new 120 Watters, put them in series (36 or 48V) and let that Classic do its thing by still using your 12 V setup, you will be happy when the , ''Oh I think I will add a few more XXXX's " and you CAN!
Solar Rule # 1 is 'Loads Grow over time"
Once you get the new panels sorted you can play with bringing the old ones back with another CC without having to rely on them, see Rule #1

hth

shockman · January 2013

Re: power and voltage ??,s

Okay so tell me what happens to volts, watts,amps. Im wondering about the math here.

Two hypothetical panels which are rated for 12v. 55watts each.

If wired in parrallel config.
Volts=?
Watts=?
Amps=?

If wired in series config.
Volts=?
Watts=?
Amps?

Cariboocoot · January 2013

Re: power and voltage ??,s

That's why it's in my signature.

Watts = Volts * Amps.

55 Watts @ 12 Volts = 4.58 Amps. So two in parallel (110 Watts) would be 12 Volts and 9.16 Amps.
Double the Voltage haves the Amperage: 110 Watt = 24 Volts @ 4.58 Amps.

shockman · January 2013

Re: power and voltage ??,s

What about part 2 in series?? what happens

shockman · January 2013

Re: power and voltage ??,s

Oops!!! I think I got it!

westbranch · January 2013

Re: power and voltage ??,s

simply put:
series = add Volts
Parallel = add amps

ADD: when using panels with the same (+- 5-10% ) Vmp and Imp

shockman · January 2013

Re: power and voltage ??,s

westbranch wrote: »

simply put:
series = add Volts
Parallel = add amps

Okay pretty easy and I assume I can add watts as a total of all panels?

BB. · January 2013

Re: power and voltage ??,s

Yes--But you have to combine the panels in an array that "makes sense"... You should add currents (Imp) that are within ~10% of each other. And Parallel panels that have Vmp within ~ 10% of each other too...

Putting a 10 amp panel in series with a 5 amp panel will not workout well (limits to around 5 amps maximum, or overheat panel if >>5 amp). Similar for parallel. A 17.5 volt panel in parallel with a 30 volt panel does not share energy production evenly either.

The devil is in the details. Systems with PWM vs MPPT controllers will behave differently, as will the choice of battery bank voltage.

-Bill

Cariboocoot · January 2013

Re: power and voltage ??,s

shockman wrote: »

Okay pretty easy and I assume I can add watts as a total of all panels?

Yes.
Provided the panels' Voltage and current specs aren't too far different.
If you parallel a panel that has a Vmp of 17.5 with one that has a Vmp of 35 the actual Voltage ends up somewhere inbetween and the current from the lower Voltage panel will be reduced because it is operating above Vmp.
Likewise if you put higher Voltage panels on a lower Voltage system without using an MPPT type controller the panel Voltage is pulled down to battery Voltage and much of the power (measured in Watts) is lost to the Voltage difference.

inetdog · January 2013

Re: power and voltage ??,s

Cariboocoot wrote: »

...
If you parallel a panel that has a Vmp of 17.5 with one that has a Vmp of 35 the actual Voltage ends up somewhere inbetween and the current from the lower Voltage panel will be reduced because it is operating above Vmp.

Actually, with a Vmp to Vmp difference that big, chances are excellent that the low voltage panel will contribute nothing, since the Vmp of the high voltage panel will be above the Voc of the lower voltage panel. Even if the MPPT controller scans below the Voc of the lower voltage panel, its current contribution will not be large enough to make up for the lowered voltage on both panels.
You can test this with any particular brand of MPPT controller to see what happens, but I think you will measure absolutely no difference when you add a 17 volt panel of roughly equal current output in parallel with a 35 volt panel. On the other hand, if you parallel a 17 volt and a 35 volt panel of equal wattage (W), then there will be two relative maxima in the power curve. One at the higher Vmp (with output at W) and the other very close to the lower Vmp (with output as 1.5 W).

If you have several high voltage panels and add only one lower voltage panel, then the higher voltage relative maximum power point will also be the absolute maximum, leaving the lower voltage panel contributing nothing.

If you put a single lower current panel (say 5 amps) in series with several higher current panels (say 10 amps), then the absolute maximum power point will be at 10 amps with the low current panel's bypass diodes carrying the full 10 amp current (and probably not surviving it.) Once the bypass diodes fail, the lower current panel is likely to be driven into reverse breakdown and damaged.
For one 5 amp panel and one 10 amp panel of equal wattage, the absolute MPP will be at the 5 amp point.

The devil is so much in the details in these cases that you really have to do the math to see what happens instead of relying on rules of thumb, and even then there can be a variation from one model CC to another. :-(

Cariboocoot · January 2013

Re: power and voltage ??,s

inetdog wrote: »

The devil is so much in the details in these cases that you really have to do the math to see what happens instead of relying on rules of thumb, and even then there can be a variation from one model CC to another. :-(

No, I don't have to do the math when merely demonstrating a principle thank you. :roll:

power and voltage ??,s

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