Design a 24v or 48v system

Ironbull

Things I have.... 
Charge controller: Outback Flexmax 80
Batteries: 8 Trojan t105re 6v 255ah
Inverter: Samlex 24v 2000w

What I want to power....
I have 2 chest freezers, one is converted to a fridge. Combined they use roughly 1.2kwh a day. Anything beyond that is just icing on the cake.

What i need....
Solar panels sized to whatever system we come up with.
Would like to also incorporate genny support, I have 2 gennys, one is 2000w inverter and the other is 3500w inverter 

Future requirements..... 
To be able to expand my system if/when needed. 

What would you do if you already had said equipment? 

706jim

Canadian solar panels from DNM solar. Get 6 of them 330 watts and you have an honest 2kw of charge power. Want something larger? More of the same with either an increased charge controller or another FM80.

BB.

The typical voltage "limiting" hardware are solar panels (Vmp-array voltage), MPPT vs PWM charge controller (you have MPPT--Good), AC inverter input bus voltage (yours is 24 volts--Need different inverter if want 48 volts), and batteries for bank (your 8x 6 volt batteries will work fine for 12/24/48 volt bus).

If you plan on going for larger single system in the future--A 48 volt inverter + battery bank will give you more "headroom" (the Outback MPPT controller will work on 12/24/48 volts--But the size a supported 48 volt battery bus is 2x the wattage of a 24 volt bus)...

• 80 amps * 29 volts charging * 1/0.77 panel+controller derating = 3,013 Watt typical cost effective array @ 24 volt bus
• 80 amps * 58 volts charging * 1/0.77 panel+controller derating = 6,026 Watt typical cost effective array @ 48 volt bus

 Also, depends on the size of your battery bank... I suggest about 800 AH is as large as you want to go with a bank... If 800 AH at 24 volts, it would be a 400 AH @ 48 volts (same amount of energy storage -- Power=Voltage*Current).

Looking at your AC inverter (features, efficiency, Tare Losses, "search mode", remote panel, etc.) tend to help make the decision of what AC inverter (and sometimes battery voltage) limit your choices. Large Wattage inverters on a higher voltage battery bank tend to "waste" power power when driving relatively small AC loads (i.e., if the inverter draws 40 Watts * 24 hours per day just being "on", that is more power than your one of your freezers).

-Bill

Ironbull

I currently have 255ah @48v or 510ah @24v. The only reason I considered changing inverter was so I could incorporate genny charging abilities. If there is a way to do this without upgrading inverter, I will keep what I have as far as inverter goes and if at some point I decided I need more I can go to 48v. 

So I guess the question is, is there some magical charger that does this?

And next question is what is minimum panel wattage to charge what I have? 

Ironbull

My solar insolation number are 3.1 for a winter low and 4.9 average if that helps

706jim

Minimum solar to charge your bank? Well, I had 12 GC's in three series parallel strings (660Ah @ 24 volts) that I charged with about 400 watts of solar. Far too low from what is usually discussed here, but the batteries lasted 19 seasons and ran all the loads I had at the time. However, consider the 2kw I mentioned earlier. That would put you in the middle of the usual recommended charging current.

BB.

Inverter-chargers can be very nice... They can be highly programmable (compared to the simple Iota type). And the inverter can include its own internal transfer switch... Run from battery, start genset, transfer from inverter/battery power to genset AC power, limit input AC power to genset rating, vary charging current based on Genset Max - AC loads = Battery current.

Generator support mode, you can run your 1,800 Watt genset and inverter-charger with generator support can add "battery power" to AC output (genset 1,800 Watts max, AC load max load = 1,800 W genset + 3,000 Watt from battery = 4,800 Watt total AC load support--Until battery goes dead, fuel runs out, or AC load drops back to less than 1,800 Watts--Then 1,800 Watt Gen - 500 W AC load = 1,300 Watt available for battery charging from genset (all numbers made up--Just example of function).

Or, you can keep the simple AC inverter, and parallel your solar charger + AC genset charger to single battery bank. Will work fine.

Argument for separate AC inverter and AC charger--If one fails, the other still works. With Inverter-Charger, if "one fails", both Invert/Charge functions are dead until repaired or replaced.

Your system is close to supporting 3,300 WH per day (my suggested aim point for a near normal electrical existence for a full off grid cabin/very energy efficient home (using propane/wood/etc. for heating/hot water/cooking/etc.). So, using your number:

• 3,300 WH per day * 1/0.52 off grid AC system eff * 1/3.1 hours winter sun = 2,047 Watt array "winter break even"

Now... You do should break your loads into base loads (need to run every day, like fridge, LED lighting, cell phone charger, etc.). And then the optional loads that you run when the sun is out (washing machine, irrigation pumping, power tools, vacuum cleaner, etc.).

Base loads should make up around 50% to 65% of your total solar power for a "reasonable" build and forget (basic maintenance) system (vs one you have to look at the hours of sun, loads, etc. every day and figure out what you are going to "run" that day). Of course, if you have a genset--This is more the issue of fuel costs, noise, etc. to run a genset vs buying more panels.

So, you can look at your base load harvest as:

• 2,047 W array * 0.52 AC system eff * 3.1 hours of (winter) sun * 0.65 base load fudge factor = 2,145 WH of base load support (winter)
• 2,047 W array * 0.52 AC system eff * 4.9 hours of sun (summer average) = 5,216 Watt*Hour per day summer available energy

Lots of ways of slicing and dicing these numbers... It really depends on what works best for you (more panels, less genset fuel. Or fewer panels, smaller battery bank, only 9 month or less "sunny season" usage--No need to over design system--Bring a small genset+fuel for a week of winter playtime).

I try to use the math to help you decide where best to spend your money and a reasonably accurate prediction (in solar if I am within 10% of predicted output--That is pretty much "dead on" accurate for solar).

Your thoughts?

-Bill

Ironbull

Wow that's a lot to unpack. I'm a pretty handy fella with most things, but this solar stuff is like Chinese arithmetic. 

There a lot to be said about keeping things simple. I like the idea of having redundancy,  meaning having separate inverter and charger in the event one quits. I dont need anything that starts my genset automatic. I will have a look at the Iota charger. I dont plan on having to use genset except for the occasional week here and there of gloomy weather.

As far as output goes, I'm just concerned with my critical loads, the 2 freezers. I have heating, lighting and cooking taken care of. Most projects will be done in spring and summer where I will need to charge cordless tool batteries and such. Anything else will be just icing on the cake. 

So if I'm understanding correctly, 2000w array should get me there. If I remember correctly, 2000w array is the max for a 24v system?

Estragon

2000w sounds like a good number in most locations to me.  In some locations (eg warm/hot marine climate with haze), you could go a bit bigger if you want.  In others (eg cold, high altitude), a bit less.

The Iota chargers are solid, simple, relatively inexpensive units.  I use a pair of 45a on my 12v bank.  One thing to be aware of though is I believe they still aren't power factor corrected.  Probably not an issue unless you need to run other large loads on the genny while charging.

BB.

Iota chargers here:

https://www.solar-electric.com/residential/batteries-battery-storage/battery-chargers.html?manufacturer=49

And on the opposite side, high end sophisticated battery chargers:

https://www.solar-electric.com/residential/batteries-battery-storage/battery-chargers.html?manufacturer=101

The basic Iota is inexpensive and truly outputs it rated current... The IQ4 option is great for Mains connected supplies... For off grid generator power (you turn off genset when battery charging is done), not really needed. Also the simple Iota chargers do not have a power factor corrected AC front end--They are less efficient on gensets (power factor correction, can do a separate post on that, if wanted).

The Xantrex is a very nice, but not cheap, PFC (power factor corrected) unit (which allows you to pull upwards of 20-40% more Wattage from your genset) and more programmable output options.

Based on your Outback FM 80, you can run upwards of a 3,013 Watt "cost effective" array, if you wish.

And there is the "maximum" you can run based on your battery bank size... More or less, 5% to 13% to 20% is a good range. 5% for a weekend/sunny season/emergency backup system. 10%+ recommended for full time off grid system/optimum battery charging. And 13% is usually a good max cost effective array. And you can go upwards of 20% -- But watch your charging current/battery bank temperature--The batteries can get hot at that temperature (can be a bit hard on batteries). Nominal AC generator charging is typically around 10-20% range.

So, for a 24 volt @ 510 AH battery bank, recommended charging points would be:

• 510 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 960 Watts array minimum
• 510 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,921 Watt array nominal
• 510 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 2,487 Watt array "cost effective" maximum
• 510 AH * 29.0 volts charging * 1/0.77 panel+controller deratings * 0.20 rate of charge = 3,842 Watt array "practicable" maximum

No exact right/wrong answers... Just rough ranges to give you some sort of anchor point in your system design/justifications.

-Bill

Ironbull

I found a source locally for these panels and would like your thoughts....

REC Peak Energy 72 series 
Pmpp 310w
Vmpp 36.7v
Imp 8.53a
Voc 45.3v
Isc 9.02a
___________________

Canadian Solar
Model CS6X-315P

Pmax 315w
Vmp 36.6v
Imp 8.61a
Voc 45.1v
Isc 9.18a

Either of these panels could save me a chunk of change vs ordering online. I'm thinking 6 of either of these, 2 strings of 3? Or would that put me too close to max voltage of 150v that my charge controller can handle? 

Estragon

You need to find the record low temperature in your location, and adjust Voc up for that temp.  

BB.

This is where we are hitting another "gotcha" with the standard 140-150 volts max MPPT controllers, and the getting to be common higher voltage solar panels (Vmp~36 volts and higher).

Guessing you are around Mckenzie TN, with a minimum winter temperature of 22F... You would use an Outback String Sizing Tool (download and run on your computer):

http://www.outbackpower.com/resources/technical-support/string-sizing-tool

But as a rough estimate, using Midnite's online tool (because I am running Chromebook here):

http://www.midnitesolar.com/sizingTool/index.php

You will be very close to 148 volts Voc-cold at 22F... The FM 80 has a Vmax input of 150 VDC, and a maximum startup voltage of 145 VDC... Downloading the Outback tool and putting in all the actual numbers for your panels (Voc, Vmp, Isc, Imp, etc.) is really needed here--And how cold it actually gets in your region.

If you pick 24 volt battery bank and only 2x panels in series... You will be fine. 3x panels in series--Close to maximum input voltage for the FM 80 in January.

If you pick a 48 volt battery bank and 2x panels in series--It will work, but not be optimum (MPPT controllers need a higher input voltage to work "optimally"). And with 3x panels in series, you are pretty close to maximum input voltage in cold weather.

There is nothing "wrong" with your panels, or even your Outback controller--It is just the nature of the beast and evolution of the solar panels over time, where they are installed, etc.

-Bill

softdown

Going to have a well pump? What depth? 

If it was me, I'd likely go 24 volt since you have a decent inverter already. Your decisions on fridge/freezer indicate energy thriftiness. Like money, it isn't what power you have so much as how you use it. 

softdown

Estragon said:

2000w sounds like a good number in most locations to me.  In some locations (eg warm/hot marine climate with haze), you could go a bit bigger if you want.  In others (eg cold, high altitude), a bit less.

The Iota chargers are solid, simple, relatively inexpensive units.  I use a pair of 45a on my 12v bank.  One thing to be aware of though is I believe they still aren't power factor corrected.  Probably not an issue unless you need to run other large loads on the genny while charging.

After waking up to panels covered by snow (sitting at 50 volt battery bank at 11:15), I wonder how snow enters that particular gem of an equation. 

Estragon

In cold dry thin air at altitude, some folks have reported higher than STC output (vs ~75% at NOCT) from arrays.  Covered in snow, not so much 

Photowhit

I  think with the FM80  these panels would   world  fine  in strings  of  2 charging  24  volt  bank, but not in strings  of 3 charging a 48 volt  bank. I would  use what you have for  now as a  1200+ watt   array  and  your battery bank and  inverter should  support  the fridge and freezer  without need  to  worry   about generator backup in  general.

softdown

Photowhit said:

I  think with the FM80  these panels would   world  fine  in strings  of  2 charging  24  volt  bank, but not in strings  of 3 charging a 48 volt  bank. I would  use what you have for  now as a  1200+ watt   array  and  your battery bank and  inverter should  support  the fridge and freezer  without need  to  worry   about generator backup in  general.

"Future requirements..... 
To be able to expand my system if/when needed. "

That array does not allow for any real expansion. 

Ironbull

Ok so I picked up 6 of the REC 310w panels I posted above. With an option to get more when he gets more in. 

At the moment I'm planning to wire 3 strings of 2 panels. Which would give me 1860w. If my math is correct, 1860x.77=1432 usable watts? 

So if I over panel with another string that would give me 2480w. 2480x.77=1909 usable watts? Which is within the 2000w limit my CC can handle on 24v system. 

Or would I simply be better off getting another FM80 with the added benefits of having a backup if one of the CC fails. I'm heavily leaning this direction with another 6 or 8 panels. 

Estragon

As well as redundancy, having the load split between two controllers rather than one running flat out may let the pair run cooler.  In theory, this may help longevity of the pair.  In practise of course, YMMV.