Backup solar system

talob · May 2014

I just found this forum sure glad I did looks like a wealth of info here, what I'm looking to do is set up a small solar system for an extended grid down situation, I have a 5000 w generator but I consider that short term. Guess I'm kinda going about this backwards, since this is for an emergency grid down situation I'm gonna put together all the system I can afford and taylor my needs to that, location is Southern KY,system will be 24v, what I see running on the system probably will be a 12-24 v well pump, a 12v transfer pump to a solar shower, chicken coop small green house, small barn for well water, of coarse not any two at the same time, a wash maching a couple hours a week, a few led lights, a TV DVD a couple hours a night an occasional power tool and probably some odd things I cant think of at the moment. I allready have my solar panels up but nothing else yet batterys will be about twenty feet from panels in basement at 68-70 degrees in a ventilatid battery box, heres what I'm looking at setting up please tell me where I'm going wrong I'm on a steep learning curve here.
Four sharp 250w panels wired in parellel for 1000w to a Morningstar MPPT 60 charge controller, to eight 215 ah Duracell (deka) batterys wired series parallel for 430 ah to a Samlex 2000 pure sinewave I can hardwire a circuit to it for what I want to run off 110, so does this sound feasable?

Cariboocoot · May 2014

Re: Backup solar system

Welcome to the forum.

You mean you want to know what you're doing wrong other than approaching it from the opposite direction of "loads first"?

Since you already know that let's look at what you've got, whether it will work together, and how much power you can expect. You have the back-up generator so if the system does not meet expectations you won't be completely stuck. That's good.

Four Sharp 250 Watt panels wired in parallel on an MS MPPT 60. Okay all in parallel is probably not the best way to go here. A bit about array configurations: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller

Eight Duracell/Deka 215 Amp hour 6 Volt batteries wired in series/parallel for 24: 430 Amp hours @ 24 Volts. The 1000 Watt array should produce about 32 Amps peak current or 7% charge rate. Not ideal, but viable. At 25% DOD you should get a little more than 2kW hours AC from the inverter. Not bad at all, but probably will fall far short of powering all that long list of equipment you want to run. You might want to get a Kill-A-Watt meter and measure the power usage of some of that stuff under real conditions to see what you're up against.

Samlex makes two different 24V 2kW inverters. You say yours is hard-wired so that should be the SA-2000K-124 which ought to be able to start motors (something the other series is not good at).

There's no AC powered battery charger on your equipment list. You'll want one so you can bulk the batteries up from the gen if the weather isn't co-operating with sunshine.

How are we doing so far?

talob · May 2014

Re: Backup solar system

Thanks for the reply I do appreciate the help, like I said on a steep learning curve here, so what would you recommend for a panel wireing configuration and I hadent considerd an AC charger what would you recommend for that.

Cariboocoot · May 2014

Re: Backup solar system

First thing: what are the specs on those panels?
I suspect they have a Vmp around 30 which means all in parallel would not work on a 24 Volt system anyway as the Voltage would not be high enough for full charging.
So what you do is arrange them as two parallel string of two in series. This increases the array Voltage eliminating the insufficient Vmp problem, reducing the V-drop over wiring, removing the need for individual string fuses/breakers, and does not reduce controller efficiency.

There are no ideal stand-alone battery chargers for off-grid yet. But you can use an Iota with reasonable success. It won't do 'perfect' recharging but it will allow Bulk on non-sunny days to keep the batteries above 74% SOC and ward of sulphation. You should have one about this size http://www.solar-electric.com/iota-dls-27-40-24-volt-40-amp-regulated-battery-charger.html to make most economical use of the gen without exceeding the batteries' maximum current.

It's expensive ($224), but in combination with the Samlex ($803) it still comes in cheaper than buying a full-bells-and-whistles inverter-charger like an Outback ($1027 vs. about $2000).

talob · May 2014

Re: Backup solar system

Spec's on panels.
open ckt v 37.35
max pwr v 29.98
short ckt A 9.03
max pwr 8.34
If I wire them in two parallel strings of two in seiries up's voltage cuts power in half? Does that still keep it in the 7% range?

BB. · May 2014

Re: Backup solar system

talob wrote: »

Spec's on panels.
open ckt v 37.35
max pwr v 29.98
short ckt A 9.03
max pwr 8.34
If I wire them in two parallel strings of two in seiries up's voltage cuts power in half? Does that still keep it in the 7% range?

It will double the voltage and cut the current by 1/2... But power will be the same:

29.98 volts * 4*8.34 amps (parallel) = 2*29.98 volts * 2*8.34 amps (series/parallel)

It is as close to "Magic" as modern electronics can make it.

Almost all MPPT charge controllers (and all PWM controllers) need a Vmp-array minimum voltage. For a 24 volt battery bank, that is around Vmp~35-38 volts minimum (optimum).

Above that voltage, the MPPT charge controller using a "switch mode buck type" power supply section will efficiently take the high voltage/low current from the array and "down converter" to the low voltage/high current needed by the battery bank.

It is somewhat like the DC version of an AC variable transformer stepping down the voltage to the loads (i.e., 120 VAC to adjustable low(er) voltage output).

-Bill

Cariboocoot · May 2014

Re: Backup solar system

talob wrote: »

Spec's on panels.
open ckt v 37.35
max pwr v 29.98
short ckt A 9.03
max pwr 8.34
If I wire them in two parallel strings of two in seiries up's voltage cuts power in half? Does that still keep it in the 7% range?

Definitely need two in series for 24 Volt system.
Power (Watts) remains the same; only the ratio changes from 29.98 Vmp * 8.34 Imp * 4 in parallel (1000 Watts) to 59.96 Vmp * 8.34 Imp * 2 in parallel (1000 Watts). Same max current to the batteries, same charge rate.

talob · May 2014

Re: Backup solar system

OK, thats the way it'll get wired then, now about those string breaker fuses? And speaking of which what breakers fuses do I need in the whole system?

Cariboocoot · May 2014

Re: Backup solar system

talob wrote: »

OK, thats the way it'll get wired then, now about those string breaker fuses? And speaking of which what breakers fuses do I need in the whole system?

You don't need string fuses when you only have two strings. It's a safety precaution for when you have more than two: if one shorts it won't be fed the combined Isc of the remaining strings which is almost certain to exceed its current maximum.

You should have a fuse of breaker on the output of the charge controller. Make it easy on yourself and wire it with the maximum wire size and recommended fuse/breaker for its output. A 60 Amp charger will usually take 6 AWG wire and 75 Amp fuse.

Same with the inverter. The manual should state how large and long wires should be as well as the recommended size for over-current protection. As a rule a 2kW 24 Volt inverter may run as low as 21 Volts so you could draw 95-100 Amps at maximum power and minimum Voltage. In theory that means 4 AWG minimum but that would probably result in too much Voltage sag even on a short distance. Sizing up to 2 AWG or even 0 AWG would be better. Then the fuse or breaker has to suit the wiring i.e. 150 Amp rating so that it trips before the wire fries if something goes wrong.

talob · May 2014

Re: Backup solar system

Dont need a breaker between panels and controller? Sure appreaciate the advise.

Cariboocoot · May 2014

Re: Backup solar system

talob wrote: »

Dont need a breaker between panels and controller? Sure appreaciate the advise.

Not strictly necessary, no. If the wiring is sized for the Isc as it should be there is no way panels can produce more than there Isc rating - they are self-limiting on current. People will put breakers in as a convenient disconnect (PV in should be disconnected before battery out on controllers, and reverse order for connection). But you rarely have to do this so it is not strictly necessary. You can cover the panels to knock the output down to near zero then disconnect the wire if needed.

westbranch · May 2014

Re: Backup solar system

You may not have to have it but it will come in real handy when you have to power your Charge controller off and on for repairs/ rejigging your wiring, adding new parts, etc... a CB before the CC makes for a good switch... don't ask me why I know...

Cariboocoot · May 2014

Re: Backup solar system

westbranch wrote: »

You may not have to have it but it will come in real handy when you have to power your Charge controller off and on for repairs/ rejigging your wiring, adding new parts, etc... a CB before the CC makes for a good switch... don't ask me why I know...

You know I've only ever powered mine down once in six years - to reroof.

*GLOAT*

westbranch · May 2014

Re: Backup solar system

I am trying to load updated firmware and to get the Local App, a viewer, functional on 2 classics, one of which has a breaker. The instruction requires powering down and back up at a couple of points in the process.
If everything went smoothly it would only need to be done once,.... but a misbehaving W7 Netbook has lengthened the process to 3 weekends so far... Hope to finish on Victoria Day... No Internet sure is a pain when you have a bad install to the PC... and conflicting instructions...

Plowman · May 2014

Re: Backup solar system

Cariboocoot wrote: »

Not strictly necessary, no. If the wiring is sized for the Isc as it should be there is no way panels can produce more than there Isc rating - they are self-limiting on current. People will put breakers in as a convenient disconnect (PV in should be disconnected before battery out on controllers, and reverse order for connection). But you rarely have to do this so it is not strictly necessary. You can cover the panels to knock the output down to near zero then disconnect the wire if needed.

That's interesting. I'm currently expanding my little off-grid system from one to three 12V panels (Dmsolar 158W, Isc = 8.76A). I had planned on getting a Square D QO load center and putting 15-amp circuit breakers between the panels and charge controller (after my thread here).

But you're saying I can safely skip the circuit breakers and go straight to bus bars? That'd be a lot cheaper and easier. I have a Blue Sea marine switch that I can use for a disconnect.

niel · May 2014

Re: Backup solar system

plowman,
be careful here as fuses or breakers are unnecessary only if the setup does not exceed 2 pv strings paralleled or 2 pvs paralleled. 3 or more in parallel be it pvs or strings of pvs in series need breakers or fuses.

Plowman · May 2014

Re: Backup solar system

niel wrote: »

plowman,
be careful here as fuses or breakers are unnecessary only if the setup does not exceed 2 pv strings paralleled or 2 pvs paralleled. 3 or more in parallel be it pvs or strings of pvs in series need breakers or fuses.

So with my three panels in parallel I'd need circuit protection (before or after the positive bus bar?). What are the technical differences between 2 and 3 panels in parallel?

niel · May 2014

Re: Backup solar system

each string or in your case each pv in parallel will need a fuse or breaker before it is combined. it is because a pv can take the equivalent power that it can produce, but not 2x or 3x and so on and so it's a safety thing. as an example of one possibility, that was actually reported here on the forum once, if you were to install your 3 pvs without the protection and accidentally wired one backwards you would blow out that pv. now for the record if wiring 2 pvs together and going neg to pos they will not blow out.

Plowman · May 2014

Re: Backup solar system

niel wrote: »

each string or in your case each pv in parallel will need a fuse or breaker before it is combined. it is because a pv can take the equivalent power that it can produce, but not 2x or 3x and so on and so it's a safety thing. as an example of one possibility, that was actually reported here on the forum once, if you were to install your 3 pvs without the protection and accidentally wired one backwards you would blow out that pv. now for the record if wiring 2 pvs together and going neg to pos they will not blow out.

Thanks, understood. I also read Cariboocoot's thread here, which discusses this question. If I'm not misunderstanding, every panel needs to have a fuse/CB when paralleling 3 panels, just like I had planned from the beginning.

Dang it, I thought I had found a cheap way out

Backup solar system

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