want to design a 3.5 kw off grid system with cheap panels

Txrancher · April 2013

I currently have a leased 6.44kw system grid tie. I am wanting to build a new off grid system myself as the price of some panels has fallen to 50 cents/watt on craigslist.
There are 2 brands of panels avbl at this price with different specs, this is where I could use some advise.

my solar system makes almost 1000kwh per month but my last bill was still for 2500 kwh. I will use much more in the summer.

I am considering buying a pallet of 20 panels, they are either 175w or 187w.

Schott SAPC-175 w VOC 44.4 or Sharp ND187U1 w/ VOC 32.7

again, these panels are avbl at 50 cents/watt

what would be the best design and components to use close to 20 of either panel ?

doesnt seem like prices could get much lower ?

thanks

westbranch · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

what kind of warranty do they have?

icarus · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

You do realize that off grid comes at twice the price per net installed watt, with about 1/2 the efficincy don't you?

Why in the in the world, with a 2500 kwh/month bill why wouldn't you just install more grid tie?

Tony

Cariboocoot · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Okay, the numbers you want to use for evaluating panels are Vmp and Imp.
The Schott panels sound like "true 24 Volt" with a Vmp around 35-36 and an Imp of about 5.
The Sharp panels sound like "GT panels" with a Vmp around 26 and an Imp of about 7.

The Schott panels could be used on a 24 Volt system with a PWM type controller or an MPPT controller (required for 48 Volt). The Sharp panels would need an MPPT controller to maximize power no matter what system Voltage is used.

20 * 175 Watts = 3500 Watt array, 20 * 187 = 3740 Watt array (6.8% larger).
Either would overload a controller on 24 Volts.
Put either through MPPT on a 48 Volt you get 56 Amps and 60 Amps respectively, enough for a very big 48 Volt battery bank (usually 10X the current rating, plus or minus a bit).
Output using the Icarus formula: 7.2 kW hours and 7.7 kW hours respectively (with 4 hours sun).

All things considered, I'd look at what sort of batteries you can get a good deal on for a bank around 600 Amp hours +/- @ 48 Volts and then pick the panels based on which are best suited to charging that bank.

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

icarus wrote: »

You do realize that off grid comes at twice the price per net installed watt, with about 1/2 the efficincy don't you?

Why in the in the world, with a 2500 kwh/month bill why wouldn't you just install more grid tie?

Tony

because when the grid is down I have no solar avbl
because the grid tie is leased I can not increase the size
because I dont want 2 seperate grid tie systems, 1 which is "homemade"

thanks

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Okay, the numbers you want to use for evaluating panels are Vmp and Imp.
The Schott panels sound like "true 24 Volt" with a Vmp around 35-36 and an Imp of about 5.
The Sharp panels sound like "GT panels" with a Vmp around 26 and an Imp of about 7.

The Schott panels could be used on a 24 Volt system with a PWM type controller or an MPPT controller (required for 48 Volt). The Sharp panels would need an MPPT controller to maximize power no matter what system Voltage is used.

20 * 175 Watts = 3500 Watt array, 20 * 187 = 3740 Watt array (6.8% larger).
Either would overload a controller on 24 Volts.
Put either through MPPT on a 48 Volt you get 56 Amps and 60 Amps respectively, enough for a very big 48 Volt battery bank (usually 10X the current rating, plus or minus a bit).
Output using the Icarus formula: 7.2 kW hours and 7.7 kW hours respectively (with 4 hours sun).

All things considered, I'd look at what sort of batteries you can get a good deal on for a bank around 600 Amp hours +/- @ 48 Volts and then pick the panels based on which are best suited to charging that bank.

So really no way around an mppt charger with either type correct?
are those mppt types like twice as much as a regular? what would you expect a new one that size to cost ?

also you lost me on the 7.2 kwh output, I would expect a 3.5 kw system to give me about 20 kwh per day as my current 6.44kw system makes as much as 40kwh/day

thanks for the help

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

westbranch wrote: »

what kind of warranty do they have?

my guess would be none. they are new but off craigslist

westbranch · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

What loads do you want to power? Once the batteries are fully charged they will not accept any more charge other than a small (tiny) float charge.

You have to use it or lose it...

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

westbranch wrote: »

What loads do you want to power? Once the batteries are fully charged they will not accept any more charge other than a small (tiny) float charge.

You have to use it or lose it...

good point
what I am thinking, is that here in Texas, about 8 months/year during daylight, I run lots of A/C so I will always use at least what the new 3.5kw system will produce.
I built the house and also wired it myself so I made an extra subpanel with some of the loads in that panel. still not exactly sure how I would set the new system up as far as using the power. I would probably like the ncapability to also charge the batteries via the grid so they never discharge much, except in emergency

sounds like Im rambling, sorry......

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

icarus wrote: »

You do realize that off grid comes at twice the price per net installed watt, with about 1/2 the efficincy don't you?

Why in the in the world, with a 2500 kwh/month bill why wouldn't you just install more grid tie?

Tony

Tony, thanks,
is the reason that its 1/2 as efficient due to the batteries ? compared to a gridtie system a battery system would only produce 1/2 the kwh ?
what if anything could be done to get better results ?

thanks

Cariboocoot · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

So really no way around an mppt charger with either type correct?
are those mppt types like twice as much as a regular? what would you expect a new one that size to cost ?

The only way around it is with a 24 Volt system and the array divided up into two through PWM controllers. Probably not worth it. Otherwise a 60 Amp MPPT controller is roughly $500-$600 for a good one. As an example, the PWM version of the 60 Amp Tristar is roughly $200 and the MPPT version is roughly $525. A top-quality MidNite Classic MPPT is $610, but they have a "no display" version for about $100 less.

also you lost me on the 7.2 kwh output, I would expect a 3.5 kw system to give me about 20 kwh per day as my current 6.44kw system makes as much as 40kwh/day

thanks for the help

I am using the minimal 4 hours equivalent good sun and 52% over-all efficiency. So 3500 Watts * 4 hours * 0.52 = 7280 Watt hours. This could be higher with more sun hour and/or better system efficiency. Looks like you have about 6 hours of sun, so possibly 10.9 kW hours. Keep in mind a battery-based system will not be as efficient as a straight grid-tie.

westbranch · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

from what you say it might work for you to have another panel with just the A/C wired to and us the PV to keep it powered during the day. You need the load info for the A/C..... the PV would be used as an opportunity load so to speak as you would want to recharge the battts first/

yup DC in AC out is about 52% efficient so best to use an opportunity load...

icarus · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Here is e short answer to why battery based systems are less efficient.. (yes, they are more expensive because of batteries and Charge controllers, and inverters and the batteries have a comparitivley short life)

First, batteries take ~ 20% more energy to charge them than they give, due to basic battery chemistry. Next, you can almost never use 100% of the pow avaialbe. As the batteries come near full, the CC will dial down current such that tend up "leaving power on the table" . Even if ycan load shift yooads to these times, it is never as efficinent as grid tie. Not to mention, as you begin to draw load shifted loads to whethe batteries are nearly full, you can bring to confuse the controller by dropping the apparent voltage of h battery bank.

So e general rule of thumb is that off grid PV costs about twice as much per kw, at about half the harvest in the net, leading to a kwh price that is as much (or more) than 4 times as expensive per kwh. With grid tie nearly 100 harvest, as all the power goes to the grid, to off set a load somewhere.

Hope this helps,

Tony

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

All 3 of you have given me great info, I probably should have put this thred in the beginers are, as I certainly am one.

I had no idea the losses were quite THAT high just in charging the batteries.
I was hoping that a 3.5kw system on batts would give me about 20kwh/day like a grid tie would....

I guess I am looking to use it more as though it was a grid tie and have not that many batteries and mainly use the power as it is produced rather than store it.

If I had a very small battery bank for the number of panels, the excess power would be lost correct ?
you can not hook panels up to an inverter or another "box" directly and just use the power as it is produced once the batt bank is fully charged, right ?

no way to get 120v AC from the DC panels without going through the batts first then to the inverter ?

So I guess that just means I need 2 pallets of panels....ha ha holy smokes

Photowhit · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

The only way around it is with a 24 Volt system and the array divided up into two through PWM controllers. Probably not worth it. Otherwise a 60 Amp MPPT controller is roughly $500-$600 for a good one. As an example, the PWM version of the 60 Amp Tristar is roughly $200 and the MPPT version is roughly $525. A top-quality MidNite Classic MPPT is $610, but they have a "no display" version for about $100 less.

If you wanted to use a basic charge controller, I'd actually recommend the 35VMP panel and a 48 volt system, You could use the panels in pairs and likely get all 20 on a single charge controller, with out losing much other than cold days, if then...

Lots of things to consider with an off grid system. Do you understand that the balance of the off grid system will likely cost much more than the panels?

There is rack and mounting, there is a combiner box, Charge controller, Likely an E-panel or at least a DC disconnect box, an inverter, and a substantial battery bank. The $1800 spent on array, 1600-2500(or more) on a battery bank, $2000 on an inverter, throw in $1000 for fusing and wiring. $500 up for mounting...

There are other reasons to go with a MPPT Charge controller like the Midnite Classic(and since there are no others like the Classic, I mean the Midnite classic) You can have the charge controller turn on things like your air conditioners when the batteries reach a specific point in the charge cycle.

icarus · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Asside from the access to power when the grid is down, at see zero advantqge of using a hybrid off grid systems. If you wish to hqve emergency power en the grid is don simply buy a cheap genny. Unless your grid is VERY unstable, a 3kw genny is WAAAAY cheaper than a 3 kw battery based PV system.

As I Sid before, if it were me, i would simple add a second grid tie array, pocket the tax credit and the net metering and b ble to use nearly 100% of the power.

If you really wish to build a hybrid system ( and there are those out thee that hve, and do run them) then you could use a somewhat smaller battery bank, and consume as much of the load in house as possible, and use the battery bank to even out the system, but such a syst is for from perfect. With too small a battery bank, you risk dumping too much current into it if you don't have the loads figured properly. Also, managing those loads with cloud events, (not just cloudy days, but partially cloudy days) too small a battery and you can't effectively power those loads, so you will spend times switching loads from one circuit to another.

In short, it sounds like way more headache (and expense) than it is worth. 3.5 kw of grid tie, might cost $2-5/watt all in. The same battery bsed system might cost $5-8. You can buy a lot of PV and or a lot of genny and fuel for the price difference.

Tony

PS. A pretty good rule of thumb for off grid PV is this. Take the name plate rating of the PV, divide by two to account for all cumulative system loses (as discussed before) then multiply that by 4 to account for the averqge hours of good sun you can reasonably expect, over the course of the year, pr day on averqge, figuring in weather, and season changes. (look at, and do a PV watts calc and use a derate of ~50% to get your real number)

So, 3500/2=1750*4=7000 WH/day. A far cry from your hope of 20kwh/day.

A 3.5 kw grid tie system, according to PV watts in San Angelo TX will deliver ~13-14kwh/day on average,

http://rredc.nrel.gov/solar/calculators/pvwatts/version1/US/code/pvwattsv1.cgi
The Same 3.5 kw system, according to PV watts will averqge ~8 kwh/day. (assuming you can manage and use all the power you'd produce, hence my estimate (that has been born out in 15 yeqr of off grid living) of ~7 kwh/day.

http://rredc.nrel.gov/solar/calculators/pvwatts/version1/US/code/pvwattsv1.cgi

vtmaps · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

I had no idea the losses were quite THAT high just in charging the batteries.

Those losses are nothing! Wait until you discover the losses in buying batteries. And remember, you don't get to buy them just once. In your hot climate batteries (and electronics) will not last as long as up here in VT.

--vtMaps

Cariboocoot · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Txrancher wrote: »

I guess I am looking to use it more as though it was a grid tie and have not that many batteries and mainly use the power as it is produced rather than store it.

If I had a very small battery bank for the number of panels, the excess power would be lost correct ?
you can not hook panels up to an inverter or another "box" directly and just use the power as it is produced once the batt bank is fully charged, right ?

Here you're looking at what off-gridders call opportunity loads or load-shifting; making use of the otherwise unharvested solar power by having things run when the batteries are full. It isn't easy. And there are minimum battery sizes for any given inverter as well; if the battery is too small you can have other problems.

By turning things on when the batteries are full (or nearly full) more power will be pulled from the panels and the system's over-all efficiency goes up. It's just difficult to manage as there are limits to how you can control this. MidNite Classic controllers have a "waste not" function which aids in managing 'extra' available power.

no way to get 120v AC from the DC panels without going through the batts first then to the inverter ?

That is what a standard grid tie inverter does: converts any available panel power to AC current and feeds to the household wiring or grid. No grid = no power.

solar_dave · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Here is my reasoning for Grid tie Vs Off Grid.
http://forum.solar-electric.com/showthread.php?19383-Newbie-with-a-question-about-personal-Wind-Turbines-for-the-home!-First-post!&p=152018#post152018

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

OK, now you all have me rethinking the idea of having 2 seperate grid tie systems.
the first is the leased 6.44kw system which can not be modified.

maybe I can add another grid tie system DIY without involving the utility or anyone else.
I live in an area with zero permits or inspections required for anything,

let me explain my electric setup and see if it might be simple to do this.

My grid power comes in underground to a meter with double lugs located on an outbuilding.
from this meter, I have power going out to 2 panels,
the 1st lug goes to a 125amp panel for the outbuilding
the 2nd lug sends power thru a transfer sw for my diesel gen, and then underground to the main house 200 amp panel, where the grid tie system is also attached.

what Im wondering, is if I could add a second grid tie system to the 125amp panel on the outbuilding, with the only connection being from the new inverter into the 125 amp panel using a cb not having the utility turn off the power into the main lugs of the 125 amp panel

on right is 125amp panel for the outbuilding
on left of meter is transfer sw to main house 200amp panel

Attachment not found.

main house setup with gridtie, you cant see it well, but there is a 125 amp sub panel to the right of the main 200 amp panel

Attachment not found.

what would be easiest way to install a 2nd grid tie system ?

THANKS ALL

Cariboocoot · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

maybe I can add another grid tie system DIY without involving the utility or anyone else.
I live in an area with zero permits or inspections required for anything

Don't.
Sometimes there's darn good reasons for all that legal mumbo-jumbo.
Add a non-permitted system and if anything goes wrong you are on the hook for all liability whether or not the system is to blame.

NEC rules about back-feeding limits (120% of bus rating) are there to prevent overloading the circuitry. A 125 Amp panel could only be fed about 30 Amps @ 240 Volts (7kW). The 200 Amp panel already has 6kW on it but could have 9.6kW max. Depending on the wiring some of the current rating may be allocated for the smaller load panel (seems like an odd way to wire from your description - it's the equivalent of a 325 Amp service attached to the meter?) Possibly you could add another 3kW here.

I find it hard to believe that NE Texas exempts itself from NEC. Practically unheard of in this age when every little government office is out to grab a buck wherever they can. :roll:

Please understand that we have now entered the dicey guessing game of advising on system configuration and addition. Without precise knowledge of what's there already we can only generalize. I know some of the forum members are in Texas and they may have more insight for you on this.

mikeo · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

North Texas may be like North Arkansas in that most county's have no code enforcement or permit requirements. That still doesn't mean the power company will not have certain standards to be met. Here we have Entergy and for grid tie you have to present them a system plan, can't be larger the 10KW and sign a agreement with them for net metering. They will do a final inspection before allowing the system to be connected to their grid. They don't care who installs it, only that their requirement for labeling, disconnects, grounding and that the the grid tie inverter is on their list of approved equipment. He may already be limited by his current leased system to not exceed their 10KW limit if they have one.

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Don't.
Sometimes there's darn good reasons for all that legal mumbo-jumbo.
Add a non-permitted system and if anything goes wrong you are on the hook for all liability whether or not the system is to blame.

NEC rules about back-feeding limits (120% of bus rating) are there to prevent overloading the circuitry. A 125 Amp panel could only be fed about 30 Amps @ 240 Volts (7kW). The 200 Amp panel already has 6kW on it but could have 9.6kW max. Depending on the wiring some of the current rating may be allocated for the smaller load panel (seems like an odd way to wire from your description - it's the equivalent of a 325 Amp service attached to the meter?) Possibly you could add another 3kW here.

I find it hard to believe that NE Texas exempts itself from NEC. Practically unheard of in this age when every little government office is out to grab a buck wherever they can. :roll:
Please understand that we have now entered the dicey guessing game of advising on system configuration and addition. Without precise knowledge of what's there already we can only generalize. I know some of the forum members are in Texas and they may have more insight for you on this.

well you got the gvnmt figured out alright.
I live on 23 acres in my county there are no permits or inspections required. Exception is a new law about 3 years ago that you need 3 inspections for a new house, foundation, electric, plumbing.
before that time, none required.

There is still an exemption if you built the house yourself, actually built it, not just acting gen contractor.
thats my situation, My wife and I built 2 houses on the property by ourselves. I did all the elctrical also, went over code, not even one 15 amp circuit or 14 ga wire, its all at least 12ga with 20amp outlets. so yes it took a lot of research to do it right,

Yes the meter is 325 amps with double lugs, I will look at the NEC backfeeding, but you think I could add 3.5kw to the 125 amp panel ?

it may not look like a "subdivision" house but having NO mortgage = priceless right?

we are about as "free" in my county as possible these days

main house

Attachment not found.

Attachment not found.

outbuilding where grid comes in at back

Attachment not found.

.

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

mikeo wrote: »

North Texas may be like North Arkansas in that most county's have no code enforcement or permit requirements. That still doesn't mean the power company will not have certain standards to be met. Here we have Entergy and for grid tie you have to present them a system plan, can't be larger the 10KW and sign a agreement with them for net metering. They will do a final inspection before allowing the system to be connected to their grid. They don't care who installs it, only that their requirement for labeling, disconnects, grounding and that the the grid tie inverter is on their list of approved equipment. He may already be limited by his current leased system to not exceed their 10KW limit if they have one.

correct, I already have the agreement with the utility for the 6.44kw leased system. I will have to look at the agreement and see what it says about over 10kw total, although adding 3.5kw more would be just under 10kw

Cariboocoot · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Crikey! Here I'm having a major war with local dimwits over a bloody storage shed! You guys get to do as you please? I am living in the wrong place.:grr

Vic · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Coot Marc touched on this earlier;

The 52% of STC rating for overall off-grid efficency is total round-trip when recharging batteries after a certain amount of power has been removed from the batts.

For daytime opportunity loads, the efficency is on the order of 77-ish percent. In Texas with warm/hot weather this efficency value will be lower. But for the off grid 3.15 KW STC system here can get 20 to 21 Kwh from this array on a 45-ish F day with a bit of wind. This is not scientific data, but shows that on a good sunny day one can count on more than four hours if there are no clouds, and very good exposure to the sun.

Managing opportunity loads can require some time every day, but are well worth the effort.
Nice looking homestead there txrancher. Opinions, Vic

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Cariboocoot wrote: »

Crikey! Here I'm having a major war with local dimwits over a bloody storage shed! You guys get to do as you please? I am living in the wrong place.:grr

Too funny,
if it makes you feel better, I did have to have an inspection for the septic system, that was it.
many places here in the states are just as you are dealing with. I researched the rules on permits before buying my 23 acres.

and the elec company did come out both to install the electric and then to change the meter to a run backwards type, but they did not "inspect" it

Txrancher · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Vic wrote: »

Coot Marc touched on this earlier;

The 52% of STC rating for overall off-grid efficency is total round-trip when recharging batteries after a certain amount of power has been removed from the batts.

For daytime opportunity loads, the efficency is on the order of 77-ish percent. In Texas with warm/hot weather this efficency value will be lower. But for the off grid 3.15 KW STC system here can get 20 to 21 Kwh from this array on a 45-ish F day with a bit of wind. This is not scientific data, but shows that on a good sunny day one can count on more than four hours if there are no clouds, and very good exposure to the sun.

Managing opportunity loads can require some time every day, but are well worth the effort.
Nice looking homestead there txrancher. Opinions, Vic

thanks,its no mansion, but the wife and I do take a certain pride it having built it ourselves with no mortgage, something maybe our great grandparents would have done, but not much lately

OK, plz try to educate me more on this "opportunity load" situation.

is this something like hooking panels up to a inverter or charger and whatever power is avbl after charging the batt bank would somehow be avbl for use at 120v AC ?

so during daylight I could charge up a somewhat small batt bank and then use the remainder at that time to power whatever I wanted with 120v AC ??

that might work, I am always home and could switch circuits or whatever to take advantage.

just dont want to deal with elec company, thats why I moved out here in the country...ok ok Im anti-social.....

so I either need to hook up at 2nd grid tie, if that can be done DIY or maybe the opportunity load way

icarus · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Opportunity loads (or load shifting ) is simply a matter of powering loads when the batteries re full or near full. For example, when my batteries are near full, I will pump extra water, top up the hot tub, charge portable tool batteries, lap tops etc. The problem with most load shifting is that many or most of these loads may b time sensitive. For example you might be able to crank you freezer extra cold for a few extra hours in the afternoon, and then turn it of or down for some number of hours, but you couldn't do that with a frige. You could run power tools, but if you need the shop tools before the batteries are charged, it doesn't help much.

Tony

Eric L · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

OK, plz try to educate me more on this "opportunity load" situation.

As someone mentioned above, some of the recent charge controllers like the Midnite Classic have a pretty sophisticated opportunity load function that allows you to scale some loads to match the available surplus power from the panels (up to the limit of the inverter/load), by sending a pulse width modulated signal to a relay that powers the load. However, this function only works with resistive loads that can take a wide range of input voltages (which in practice means electric heating elements, like hot water heaters; it will not work with things like air conditioners). This effectively makes the hot water tank an extension of your battery bank. Some of us have been able to combine this with more traditional opportunity loads (that turn on/off with changes in battery bank voltage) to get close to full capacity utilization from our battery-based pv solar system.

However, most of the above remarks still apply: you'll always still have the efficiency losses of the batteries, plus the cost of the batteries themselves. So while the battery based systems can, with enough tweaking, get a lot better than in the past, they will never match grid-tie efficiency.

northerner · April 2013

Re: want to design a 3.5 kw off grid system with cheap panels

Eric L wrote: »

However, most of the above remarks still apply: you'll always still have the efficiency losses of the batteries, plus the cost of the batteries themselves. So while the battery based systems can, with enough tweaking, get a lot better than in the past, they will never match grid-tie efficiency.

Not to mention that even with a grid tie system there are losses as well. Does anyone know what those losses are (ie through the inverters, distribution lines, transformers, etc...)??? I know that a grid tie system is more efficient, but you can't assume it's 100 % either.

want to design a 3.5 kw off grid system with cheap panels

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