Help me help a friend (sanity check)

ggunn · February 2013

A friend of mine is attempting to pull together a feasibility study for an off-grid application. His client has specified a steady state 24hr/day load of 11 amps at 120VAC and three days of autonomy.

When I ran through some rough numbers with him (and I hasten to admit that this is a departure from what I normally do), we came up with a necessary battery bank size somewhere significantly north of 4000 Ah @ 48V (50% max DOD, 91% inverter efficiency).

The numbers look right to me, but can someone give me a sanity check in case I dropped a decimal place somewhere?

Cariboocoot · February 2013

Re: Help me help a friend (sanity check)

Well let's do some math (always fun!)

11 Amps @ 120 Volts is 1320 Watts, times 24 Hours is 31.68 kW hours. Call it 32.
Leaving out the inverter load for now, that's 666 Amp hours used on 48 Volts.
Now how do you want to define 3 days autonomy?
3 times 666 is 1998 Amp hours, and if you want it to be 50% SOC after that it's as close to 4,000 Amp hours as makes no never mind.

Sounds to me like you did the math right.

Other interpretations would include allowing for some of the power to come directly from the panels (at least on good days), and/or using 25% DOD per day and forklift batteries that can stand 80% total DOD (the 1998 then becomes 75% of the total battery bank, saving a whole 1336 Amp hours).

That would be one mighty massive system. I can't help but think it would be better broken up into smaller systems, but I don't know the actual AC application so it may not be possible.

BB. · February 2013

Re: Help me help a friend (sanity check)

Is this an off grid radio repeater?

After he is peeled off the floor (thanks to the rules of thumb that save a whole lot of work to get an order of magnitude system sizing)--Go back to loads and conservation.

For HAM and such, I would think that spending money/time on better antenna, adding some directionality, etc. can easily reduce the power requirements on transmit a lot. Even using existing setup and reducing power by transmit power 1/2 (-3db) is going to be un-noticable by almost all users.

Playing games with adjustable transmit power (higher power during day, low power at night, cutting power in bad weather, etc.) could help a lot too. Improved passive cooling/low power fans instead of large fans-cooling system/shading against sun/better insulation/etc. could all help.

Trying to think what-else would need 24x7 relatively constant off grid power....

-Bill

westbranch · February 2013

Re: Help me help a friend (sanity check)

Also there is a type of load shifting that could be included with extra PV to supply that daytime load, not much but it helps, and using a tracked array to extend the daylight inputs, early AM late PM. Mainly conservation though.

add Oh and Li batteries for more discharge %.

ggunn · February 2013

Re: Help me help a friend (sanity check)

Cariboocoot wrote: »

Well let's do some math (always fun!)

11 Amps @ 120 Volts is 1320 Watts, times 24 Hours is 31.68 kW hours. Call it 32.
Leaving out the inverter load for now, that's 666 Amp hours used on 48 Volts.
Now how do you want to define 3 days autonomy?
3 times 666 is 1998 Amp hours, and if you want it to be 50% SOC after that it's as close to 4,000 Amp hours as makes no never mind.

Sounds to me like you did the math right.

Other interpretations would include allowing for some of the power to come directly from the panels (at least on good days), and/or using 25% DOD per day and forklift batteries that can stand 80% total DOD (the 1998 then becomes 75% of the total battery bank, saving a whole 1336 Amp hours).

That would be one mighty massive system. I can't help but think it would be better broken up into smaller systems, but I don't know the actual AC application so it may not be possible.

Thanks for the backup. Next question: How large would a PV system have to be to add one more day to the time before batteries hit 50% DOD, assuming insolation of 5 suns/day? Our numbers came up to about 1.3kW, but intuitively that seems too small.

Cariboocoot · February 2013

Re: Help me help a friend (sanity check)

ggunn wrote: »

Thanks for the backup. Next question: How large would a PV system have to be to add one more day to the time before batteries hit 50% DOD, assuming insolation of 5 suns/day? Our numbers came up to about 1.3kW, but intuitively that seems too small.

Not sure I understand the question.
How much PV would supply the 32 kW hours in a 5 hour day? That would be about 12 kW +/-:
12,000 Watts * 5 hours good sun * 0.52 % system efficiency = 31,200 kW hours.
How much PV would you need to recharge 4000 Amp hours @ 48 Volts? Hideously large amount:
400 Amps @ 48 Volts / 0.77 = 25 kW array. And of course 5 charge controllers.

You know this project uses twice the power per day of my house?

BB. · February 2013

Re: Help me help a friend (sanity check)

Where will the system be installed? Will the power usage be the same winter/summer?

-Bill

niel · February 2013

Re: Help me help a friend (sanity check)

i'm with coot as i don't understand the question. you don't want to design a solar setup to just add another day before it hits the 50% dod point as this is designing it to be deficit charged. does he plan to drag out a generator to bulk charge the batteries every so many days?

that is a huge amount of power and you've got me curious what it is that will be that is constantly powered.

ggunn · February 2013

Re: Help me help a friend (sanity check)

niel wrote: »

i'm with coot as i don't understand the question. you don't want to design a solar setup to just add another day before it hits the 50% dod point as this is designing it to be deficit charged. does he plan to drag out a generator to bulk charge the batteries every so many days?

that is a huge amount of power and you've got me curious what it is that will be that is constantly powered.

I can't go into detail because I have signed an NDA, but yes, I know it's not a sustainable charging situation. He wants three days of autonomy stretched to four days with solar, and after that, something else happens.

Cariboocoot · February 2013

Re: Help me help a friend (sanity check)

ggunn wrote: »

I can't go into detail because I have signed an NDA, but yes, I know it's not a sustainable charging situation. He wants three days of autonomy stretched to four days with solar, and after that, something else happens.

Ah, so it's a grow-op!

BB. · February 2013

Re: Help me help a friend (sanity check)

Is the 4 days an "emergency" operation condition--Or just every winter?

With the phone company, they define emergency operation as when everything goes south--No external power, no fuel deliveries, no A/C--Operate until the equipment/battery bank is a smoldering pile (i.e., keep in operation for required time until power/thermal conditions become impossible). After the initial emergency is over (hurricane, earthquake, etc.), then replace all equipment that has failed.

-Bill

westbranch · February 2013

Re: Help me help a friend (sanity check)

well if it comes down to beating the snot out of the batteries I would go with Wet NiCd's.
At least they are proven to be recoverable from heavy discharge without issues, as opposed to Li types.
And a honkin' big Array.

What about wind to supplement, using one of Chris' 3.2 kw turbines?

solar_dave · February 2013

Re: Help me help a friend (sanity check)

ggunn wrote: »

I can't go into detail because I have signed an NDA, but yes, I know it's not a sustainable charging situation. He wants three days of autonomy stretched to four days with solar, and after that, something else happens.

The missiles launch?

BB. · February 2013

Re: Help me help a friend (sanity check)

Don't make me moderate your behinds.

He said he is under NDA.

-Bill "Moderator" B.

Help me help a friend (sanity check)

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