Battery system demo with grid backup

aglicknmex · December 2011

Thanks in advance to the solar experts here-- Ive been reading !!

Im in the early stages of planning a system here at my repair shop/home. Purpose is to provide a demo to create interest/visibility at my business location, and to produce power during the day. I need no more battery than to act as a sink for using the panels during the day. I intend to auto revert to grid after panels quit producing. My big motor loads will stay on grid. Load will be strictly domestic, no motor starting other than fridge/freezer.

Some issues have already been decided
NO Grid tie - wish I could but its just not an option here
Genset already available for long grid outages, and it happens her in mtns.
Not sizing battery for long use. I have the genset and its cheaper than a huge battery

Here are the goals:
I would be happy to produce and use 10-20kw/hr daily. I have been watching the meter and that would be a starting point.
Daylight use only

Components below are just a starting point, open to all suggestions!!

48 volt battery system open to all suggestions as to amp hr sizing
Midnite Classic 150 CHARGE CONTROLLER
Qty 18-25 units Trina Solar TSM-230PA05 230-watt panels @$300 each
open to all suggestions for other panels
Mini Magnum MS4448PAE Power Center/Inverter

I realize this whole idea is not cost effective, but I really want to do this!!! Everyone in my area is sitting on their solar butts. I already heat the house with 10 solar thermal panels and 2 panels for solar domestic water . I want to show what can be done!

Many thanks to any who would like to contribute to discussion/ideas
Dont hesitate to use math etc, I can speak the language as a former Motorola Tech, Amateur Radio operator, etc.

AL from the boonies of SW New Mexico

Cariboocoot · December 2011

Re: Battery system demo with grid backup

What you seek can be done, but there are one or two issues.

The first is with this statement: "I need no more battery than to act as a sink for using the panels during the day. " This is essentially an off-grid system, and off-grid doesn't work the same as grid-tie. You need the "full" battery pack to regulate power during use, and you don't want to leave it discharged come the night.

The second is with this statement: "I would be happy to produce and use 10-20kw/hr daily." That is huge in off-grid terms. With the over-all system losses and counting on a minimal 4 hours of equivalent good sun you're looking at a 10 kW array to get 20 kW hours of power per day.

If you really want to demonstrate what can be done, start with the #1 off-grid dictum: conservation, conservation, conservation. Reduce the load potential as much as possible. Get a Kill-A-Watt meter and make accurate measurements of what you do want to run and size the system appropriately.

Running out and buying a bunch of equipment willy-nilly will only demonstrate that it is possible to spend a lot of money of solar equipment and not have a viable system. For instance if you did buy 24 of those 300 Watt panels that would be 7500 Watts and even at 48 Volts one Midnite 150 will not handle them all. You could expect 120 Amps from all that panel, which would require two 80 Amp charge controllers and a 1200 Amp hour 48 Volt battery bank (that's up to 28 kW hours, before losses). You are looking at whole-house power with your suggested array size. Are you sure that's what you need just for demonstration purposes? It's 10 times the capacity I use to run our cabin, including refrigerator.

As is so often the case with off-grid, you need to get a better handle on the usage numbers before proceeding.

BB. · December 2011

Re: Battery system demo with grid backup

One way to start... Assume 20x 230 watt solar panels and 5% to 13% rate of charge... Take 10% as a goodly sized array:

20*230watt panels * 0.77 panel derating * 1/0.10 rate of charge * 1/59 volts charging = 600 AH battery bank @ 48 volts

If you assume 2 days of no-sun and 50% maximum battery discharge (1-3 days recommended):

600 AH * 48 volts * 1/2 days * 0.50 max discharge = 7,200 WH per day nominal load on battery bank

From PV Watts, a 4,600 watt (4.6 kW for PV Watts) solar array with 0.52 end to end efficiency (another rule of thumb, yes 50% losses from panel name plate to AC inverter output) using El Paso Tx as close to your weather:

"Station Identification"
"City:","El_Paso"
"State:","Texas"
"Lat (deg N):", 31.80
"Long (deg W):", 106.40
"Elev (m): ", 1194
"PV System Specifications"
"DC Rating:"," 4.6 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 2.4 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 31.8"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 9.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 5.39, 379, 36.76
2, 6.13, 379, 36.76
3, 6.96, 481, 46.66
4, 7.33, 469, 45.49
5, 7.20, 462, 44.81
6, 7.25, 437, 42.39
7, 6.82, 423, 41.03
8, 6.65, 417, 40.45
9, 6.82, 418, 40.55
10, 6.72, 444, 43.07
11, 5.67, 376, 36.47
12, 5.43, 387, 37.54
"Year", 6.53, 5071, 491.89

So, around 380 kWH per month minimum or:

380 kWH per month / 30 days per month = 12.7 kWH per day average minimum for 9+ months of the year

Of course, you will need a backup power source... A Genset that can supply ~10% rate of charge and 50% minimum electrical loading on genset (fuel economy and smallest practical genset):

600 AH * 59 volts * 1/0.80 typical charger eff * 0.10 rate of charge * 1/0.50 loading = 8,850 Watt genset

There are lots of options and variables (a better battery charger could allow you to use a 5kW genset pretty easily for example). This is a "rule of thumb" designed system that is fairly conservative in its ratings (you should meet if not exceed the above performance numbers in your application).

As always, numbers for solar power are to within 10-20% accuracy (weather and equipment variations). I just carried out the digits so you can reproduce the math (and catch any of my mistakes :roll:).

Now, does this meet your needs--You can "adjust" to a smaller or larger battery bank and more or less solar panels... Then there is always conservation which is usually a much better investment than a pure solar system.

-Bill

niel · December 2011

Re: Battery system demo with grid backup

let's start off with the pvs. you should design for the worst time of the year (winter) and expect roughly about 77% of the stc rating to be utilized. you will also need such things as combiner boxes with associated circuit breakers in addition to wire, but that wire size will depend on the current passed and roughly what pv voltage is utilized. now with that much power you will need 2 classics and what model may depend on the pv voltage you go with and that depends on the distances from pvs to cc and cc to batteries as you may opt for even higher voltages if those distances prove to be uneconomical to spend that much on wire.

our normal recommendations for charge rates are 5%-13% of the ah capacity. a smaller battery bank would then minimally be at the 13% rate. for example a 13a charge going to a 100ah battery from a pv capable of about 16.88a with the 77% figured into it. now you could get an even smaller battery bank capacity if you go with an agm style battery as they can take at least double the current that standard fla batteries do and concorde sunxtenders can take several times their ah capacity rating with a higher efficiency in charging, but at a higher cost. in any case you will need to determine your minimum loads to be sure your battery bank will meet its needs.

this is general speak and particulars do need to be hashed out, but much of that will need to be determined and addressed by you. the further you specify the further we can comment on it all.

aglicknmex · December 2011

Re: Battery system demo with grid backup

Thanks guys,

I need to reinforce couple things given in my post. My needs are really quite different from the average system. I know you guys are really smart and can help!!

DAYLIGHT USE ONLY - auto revert to grid when panels quit producing.

I DONT WANT 1-3 days backup for battery. Just enough to allow the system to work during the day.

I have grid power for after dark, I have genset for power outages.

Lets think of this as driving down the road in my car with lights on, wiper etc using some juice. The battery is there and needs to be, I understand this. I dont want to park the car and keep using the accessories, I just want it work good while driving and of course KEEP THE BATTERY FULL FOR THE MOST PART!!!

lets use 5.5 hrs DAILY as this has proven to very accurate in Silver City,NM close to where I live.

Hope that helps explain what Im looking at. I know its not cost effective, but bear with me I'd like to see what it takes to do this.

aqain, THANKS AL

I already have several backup generators for grid down situations. (I fix and resell them as part of my business)

solar_dave · December 2011

Re: Battery system demo with grid backup

I am confused here a bit, I thought grid tie was acceptable in NM. What prevents that?

mike95490 · December 2011

Re: Battery system demo with grid backup

If you are only setting this up for daytimes & demos, here's what I'd do, and shop for.

600W of PV
Rogue MPPT controller
12V 200Ah of battery (in sexy red cases), plexiglas battery box, vented outside
Morningstar Suresine 300W inverter (12V)
A couple of shop lights
A ceiling fan (vari-speed/reverse/lights) top quality, not junk.
Timer to switch the inverter ON/OFF circuit, to match your shop & sun hours.

Set it up, very visible, inside the shop, on display, running. Use it to run it's own set of lights and fan daytime. That will inspire clients more - seeing it, rather than a note on a switch saying SOLAR

Just an idea

aglicknmex · December 2011

Re: Battery system demo with grid backup

Thanks Ted,

No utility incentives--(could change down the road.) Still need to sign production contract.
Other reasons not suitable for discussion here 8)

Maybe we could just look at this as a discussion to sharpen our skills and not try to change the parameters too much.. Not trying to be difficult just want to look at this option without changing the basic idea. Figure out the equipment to do it, etc.

Thanks again!!!!!!!!!!

al

Cariboocoot · December 2011

Re: Battery system demo with grid backup

There's a difference between the way your car's alternator can put out power and the way PV's do it. The alternator has 'unlimited' input potential from the engine and can adjust its input (by varying the Voltage to the rotor windings) to accommodate output. PV's deliver whatever they can according to how much sun they get and how much load is applied.

There is also a difference between what the panels can deliver at any one time and what they can deliver over time. The batteries are there to even this out, as you know. But there's a big difference between the relative small loads of a 12 Volt automotive system and a 240 VAC inverter system. Lots more losses, for one thing, converting PV to AC.

So let's look at some of the potential "bottlenecks".

Say you want to have full power available on that Magnum. When? Noon? Okay: peak output at noon of 4kW less inverter efficiency, wiring losses, controller efficiency, and panel efficiency. So you end up putting a 8kW array up to supply that 4kW for a few minutes at noon. Minimal batteries means it would not be available at any other time, unless you increase the array size dramatically.

Which brings us to #2 bottleneck: the charge controller. Niel mentioned that different batteries can take different charge rates. One 80 Amp controller "maxed out" would be 13% on 615 Amp hours of battery. That's about 5kW of array @ 48 Volts, which means the 8kW would lose 3kW potential; it simply won't come through the charge controller, needed or not. With that, you're peak AC Watts drops to about 2.5k

So you double up the charge controllers and run the full 8kW array. Now you've got 128 Amps potential charge current; enough for a "minimal" battery of 984 Amp hours (or a bit less if you push some AGM's really hard).

On the other hand, with 5.5 hours of good sun available you have pretty good harvest potential for a decent size system. A 4kW array would come up with around 11 kW hours under best-case conditions. If you were to use the standard method of determining battery bank size, at 25% DOD that would be a 916 Amp hour 48 Volt bank and about 7% peak charge current. Or you could go for 50% DOD, cut the bank size in half (458 Amp hours) and have about 14% peak charge current potential.

As you can see, there are a lot of options and a lot of compromises and issues to be considered. It's one of those things where you have to pick the most important aspect of the system and shoot for that. Usually this is being able to supply he kW hours need and maximum draw. Designing based on other aspects tends to leave you switching to grid or generator when you don't want to be, or buying a lot of equipment that spends most of its time unused.

In terms of automatically switching to grid, buy the right inverter and this is easy as you can have it programmed to pick up utility power based on battery Voltage. Or you can get fancy with a battery monitor controlling a relay based on SOC.

BB. · December 2011

Re: Battery system demo with grid backup

Do you have any loads that can take variable solar power?

The gold standard is water/well pumping with pumps and electronics optimized to operate from solar only panels.

What you appear to be asking for is a grid tired inverter that does not feed back to the grid because of local utility requirements.

Certainly possible, but nobody builds such an inverter. You could do the same thing by having the utility install a one way billing meter (any extra power you generate goes free to the utility). Wonder if a utility would accept such a deal.

Otherwise, there is a ratio between battery/inverter/and solar panels for all of them to work together correctly. Some of the issues of ratios appear to be engineering tradeoffs which could be fixed, but may cost more and/or raise other issues.

-Bill

Cariboocoot · December 2011

Re: Battery system demo with grid backup

Perhaps the question we should be asking here is: "What exactly are you wanting to demonstrate?"

aglicknmex · December 2011

Re: Battery system demo with grid backup

Points well taken,

Car illustration was just to get the idea across of not needing long period of backup from a battery. I know they behave differently. Might need a power dump at times. Understand. I spent years as Motorola tech fixing all things electrical and I now do auto electric repair!!

"Power over time" good point, dont want to waste a lot of solar, so need more battery than absolute minimum.

Im not stuck on full power out of the Magnum. I was looking for expansion room here

If I went with the pallet of Qty 25-- 230 w panels (cost $7500 yesterdays quote) that would be looking at about 6k

How would you set it up based on the 25 panels.......

tnx al

[PHP]Say you want to have full power available on that Magnum. When? Noon? Okay: peak output at noon of 4kW less inverter efficiency, wiring losses, controller efficiency, and panel efficiency. So you end up putting a 8kW array up to supply that 4kW for a few minutes at noon. Minimal batteries means it would not be available at any other time, unless you increase the array size dramatically.[/PHP]

Cariboocoot · December 2011

Re: Battery system demo with grid backup

I probably wouldn't set it up based on the 25 panels because I'm an old stick-in-the-mud coot.

Panel specs on the Trina 230PA05:
Voc 37, Vmp 29.8, Imp 7.72, Isc 8.26

Right away we see these are "grid tie panels" (odd Vmp) and will need to be three in series minimum to get enough Vmp for a 48 Volt system (should be above 70 Volts). Which doesn't divide into 25 well. Likewise neither does four, and using five in series would require the Midnite 200 at least (Voc of string 185) or the 250 if cold temps are an issue.

So if you use the Classic 200 and five strings of five panels each you get:
Voc 185, Vmp 149, Imp 38.6, Isc 41.3, total Watts 5750 for the array.

What you could expect for output: @ 77% efficiency 4427 Watts;
92 Amps peak charge current on 48 Volt system (more than one controller can handle) which would handle a battery bank from about 700 Amp hours to 1840 Amp hours and about 12kW hours per day AC. (Usual disclaimer: not precise calculations or system design recommendation.)

aglicknmex · December 2011

Re: Battery system demo with grid backup

Hi Bill,

I didnt see your response right away- thanks

Nice idea of one way meter, but at this point, the less I need to deal with the utility the better. It is very small coop and its very hard to do anything out of the ordinary! I will ask them however.

My well is on a different utility meter, 1/4 mile away.. Ive some experience with the Grundfos systems. Great, but pricey. My well is too deep anyway.

Id like to just stick with the idea of putting up roughly 5-6K of panels and optimizing the inverter, charge controller and battery size, with the idea of not storing a lot of excess energy and using it mostly by day with evening use as available. Then cut to grid.

I realize I will need to tailor loads, maybe use timers for certain loads. But thats the fun of having such a system . Born tinkerer!

al

ggunn · December 2011

Re: Battery system demo with grid backup

aglicknmex wrote: »

Hi Bill,

I didnt see your response right away- thanks

Nice idea of one way meter, but at this point, the less I need to deal with the utility the better. It is very small coop and its very hard to do anything out of the ordinary! I will ask them however.

My well is on a different utility meter, 1/4 mile away.. Ive some experience with the Grundfos systems. Great, but pricey. My well is too deep anyway.

Id like to just stick with the idea of putting up roughly 5-6K of panels and optimizing the inverter, charge controller and battery size, with the idea of not storing a lot of excess energy and using it mostly by day with evening use as available. Then cut to grid.

I realize I will need to tailor loads, maybe use timers for certain loads. But thats the fun of having such a system . Born tinkerer!

al

You realize that any power that your system produces will cost you at least ten times what grid power costs, right?

mikeo · December 2011

Re: Battery system demo with grid backup

and not try to change the parameters too much.

Can the inverter be set up to switch to pass through mode when the battery SOC drops 10%? I am thinking he could just rewire some of his building circuits to power those loads that he would want to be off solar, then when the clouds go over or there are cloudy days he would recharge from the grid while passing grid AC to the loads. With a 4 kw inverter his loads are limited to he inverter rating, so only enough panels to smooth his peaks during the day. I think avg loads shouldn't exceed about 2500 watts with such an inverter. The trick is getting enough panels to meet avg demand and maybe 4kw of panels. The battery only would be used to smooth out switching between grid and solar too often.

aglicknmex · December 2011

Re: Battery system demo with grid backup

Bill,
You are right of course,

Actually pretty silly to work so hard, to save maybe $500 a year. The payback is not real fast if ever

I can only guess my rate will go up not down!! Im pretty spoiled with low rates I have now and could really pare my usage if I had the incentive...

Id really like to go grid tied since I have backup generators coming out my ears.

I havent called the electric coop in a while, maybe i will, and see if rationality has seeped in!!
al

Cariboocoot · December 2011

Re: Battery system demo with grid backup

Perhaps a Xantrex XW inverter would be a better choice here. Leave "sell to grid" off and use the load sharing to pull extra power from the utility as needed? Then if grid-tie does become viable at your location a simple programming change sends any surplus PV power to the utility.

aglicknmex · December 2011

Re: Battery system demo with grid backup

Yes Mike!!!!!

You've got the picture!!! I just gotta know the best way to do it!

BB. · December 2011

Re: Battery system demo with grid backup

Fairly easy to do... A Battery Monitor with a programmable output (say a Xantrex or Victron) will provide the State of Charge signaling. And if you want, you can use a daylight sensor and/or timer to only enable the inverter from 9am-3pm (or whatever) for optimum use.

Then choose the Inverter with remote control inhibit/fail over (if there is one) or an external relay to cut the 120/240 VAC mains and have the inverter fail over.

From a cost point of view--Even just doing this without solar, working out the cost of power from a system that just time shifts power consumption, it works out to something like $0.45 per kWH to run (without solar panels or cost of Utility power and additional losses of ~30% or so).

You could use AGM batteries which will support way more surge current (upwards of C*4 vs the C/2.5 typical used for flooded cell batteries). You will have to watch the charge controller picked for the job--It appears that at least one inverter/charge controller (Xantrex XW system) has a possible issue with a large solar array and a small battery bank (AGM as I recall).

Which comes back to the typical Rules of Thumb that seem to work well:

6kW inverter (with 12kW surge) needs a 600 AH @ 48 volt battery bank (or similar ratios for smaller inverters and systems).
6kW of solar panels appear to be happier on a 600 AH @ 48 volt battery bank (a ~200 AH battery bank appeared to, perhaps, have >72 Volt over voltage issues when MPPT Charge Controller went into MPPT scan--just guesses on my part).

If your loads do not have large surges (standard A/C, induction motors, well pumps, etc.), then, perhaps you can use a 1/2 sized battery bank. If you have A/C, the Mini-Splits from Sanyo, and others, have virtually zero surge current and can (Sanyo) be programmed to operate at low cooling and draw no more than ~300 Watts.

Still not quite sure it really meets any of your requirements.

-Bill

ggunn · December 2011

Re: Battery system demo with grid backup

aglicknmex wrote: »

Bill,
You are right of course,

Actually pretty silly to work so hard, to save maybe $500 a year. The payback is not real fast if ever

You can only look at it that way (saving $500/yr) if you don't count the expense of the system. An off grid system will never pay for itself when compared to grid power; it will never even come close.

I don't mean to discourage you; if you've got the money to spend and don't mind having an expensive hobby, then go for it. Just don't expect it to be a financially advantageous thing to do. If the grid is available, then an off grid system will never be anywhere near economically competitive with it.

BB. · December 2011

Re: Battery system demo with grid backup

If you are looking to demo a "useful" system for off grid living--Around 100 kWH per month (or ~3.3 kWH per day) is a pretty useful system maximum. Should be large enough to run a full time cabin/small home with an Energy Star rated refrigerator, lights, radio/laptop computer, and even a well pump. Possibly even a washer a few times a week too.

The system is fairly inexpensive and small/simple enough that a person with a bit of training can install and do most of the maintenance themselves.

Call it 1,200 watts of panels for your area, a 150 AH @ 48 volt or 600 AH at 12 volts battery bank and a smaller, efficient true sine wave inverter... And if a larger inverter is needed, use a big and cheap MSW inverter to run the the hand tools (just a quick estimate--should define your loads/needs and run the numbers again).

If your needs are on the small side (1,200 watt or smaller inverter, probably no refrigerator) then a 12 VDC system may be a good fit. And look at the MorningStar 300 watt TSW 12 VDC inverter.

If you go with higher power requirements, then a 24 volt system and a 1,500 watt to 2,000 watt 24 volt TSW inverter would be a good starting point.

Also, I would suggest a Battery Monitor (or Victron Battery Monitor) would be very nice to have... For you, a demonstration of how the battery bank is performing and for a "real" cabin/home--A much easier and more accurate way for a non-technical person to manage their battery bank and avoid the typical battery killing mistakes (under charging, deficit charging, something left on and killing the bank, etc.). Of course, if you have a flooded cell bank--then a Hydrometer is pretty much manditory for monitoring your bank's health (but not possible for AGM or other sealed cell type battery).

-Bill

aglicknmex · December 2011

Re: Battery system demo with grid backup

Ive studied the XW some but all of its capabilities are not real clear. I will do some more reading. I really like the idea of using grid capable inverter with charging capabilities. I was not aware you could program "sell to grid off"

I wont have ANY big surge loads.

Talking typical residential loads
NO water pumping.
Washer, fridge, 2 freezers, one mini split, standard kitchen appliances. Solar circulation pumps, lighting, computers,etc.

RandomJoe · December 2011

Re: Battery system demo with grid backup

I do exactly what you describe with my semi-off-grid system. I actually switch to battery a while before dawn to give the panels "something to do" recharging the batteries once the sun is up (my load is otherwise a low but steady 150-250W during most days), then run off-grid until the charge controller goes to sleep or the battery SOC drops to 75% (if it's a really cloudy day).

I think you'd find this sort of thing much easier to do with a properly integrated system such as the Outback (what I used) or Xantrex systems. Since everything communicates, it's easy to get it to all cooperate together. The Xantrex has the added feature, as someone else mentioned, of doing "load sharing" - if your loads are higher than solar can supply, the inverter draws partial power from the grid at the same time. With the Outback kit I have to either switch fully back to grid or draw down the battery bank.

I really liked that last feature of Xantrex, but in the end went with Outback because I'm a computer nerd who likes to hack his systems. Outback provides documentation on their serial communications protocol so I could interface the system to my home servers (you can see my system on the links in my sig). It's possible to get a functional system with fairly simple controllers, and the Outback system has some fairly complex control available, but with the computer interface I can make it do anything I want for any reason - I've even considered getting the solar irradiance sensor for my weather station and using that to tell the system when to switch to/from grid. (I have way too many projects and ideas - never seem to complete much!

)

And as has already been said, this is NOT a cost-effective or money-saving endeavor! My primary motivator was for a bit of power in case of another long outage, without having to feed a generator 24x7. It's also a fun hobby for me!

I once figured payback to be about 56 years - IF NOTHING DIED. Don't think the batteries will last quite that long! :roll: Sure has been nice on the few multi-hour outages since I put it in though - only reason I notice is the overhead lights go out, but my computers keep going! :cool:

n3qik · December 2011

Re: Battery system demo with grid backup

I am doing about the same as what the OP wants. Switch the AC loads to grid at night and only run the LED lights at night. This gives the batteries something to do.

My system is cobbled together, with an PLC as the brains.

jeffkruse · December 2011

Re: Battery system demo with grid backup

I use the XW4548 with grid sell off and grid support on. I don't sell to the grid at all and I set the battery voltage for the grid support. I could set it so I use the grid when the voltage is below say 52V. That sounds like what your asking for. Very easy.

aglicknmex · December 2011

Re: Battery system demo with grid backup

Jeff
Are you happy with documentation, performance etc of the 4548?? Its sounds pretty ideal, if I could be convinced it would last, and have support if problems occur.
Would solve a lot of problems for me.

jeffkruse · December 2011

Re: Battery system demo with grid backup

Mostly yes. I was not so happy on the way the grid support worked and the way it is documented. When using grid support the grid is always being used to some degree. I made a thread about it here. I cant remember exactly how many watts it uses but I wanted it to use zero when the battery was above the threshold I set.

But over all I am very happy with this inverter.

Battery system demo with grid backup

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