Large battery array

ggunn · January 2011

I have a potential customer who wants a battery backed up PV system with the option to go off-grid for extended periods of time for a sizable greenhouse. He has submitted the load analysis I gave him, and when I run the numbers for 3 days autonomy, 50% DOD, efficiencies, temperature compensation, etc., I calculate that he will need something in excess of 12,000 Ah @ 48V. Have you got any suggestions as to where I can get something like that? That's a lot bigger than any battery banks I have ever dealt with.

BB. · January 2011

Re: Large battery array

Can he shift loads to day time/sunlight hours?

Can he run off-grid solar when the sun is out and use a genset for extended sunless conditions (may need one anyway)?

I have heard years ago, in Israel, some operations pipe the genset exhaust into the green house to increase CO2 for better growth (yea, I am sure there are a whole bunch of issues with that).

Here is some information about Sodium-Sulfur batteries:

Sodium-sulfur battery - Wikipedia

There are some other types of liquid salt batteries discussed--and some vendor links.

Perhaps from this thread:

Lithium Iron Phosphate Batteries; LFP; LiFePO4 discussions

Poster RWB seems to have experience with LFP batteries--Perhaps not that large, but you can try PM'ing him and see if he can help you (RWB has not been on the forum since July of 2010).

From my browser bookmarks, I believe the company is www.powerenz.com

They appear to only work with portable systems--but can probably get you pointed in the right direction for a large array of batteries.

In any case, this is a huge system and may need a Power Engineer (w/ PE cert.) to even tackle a system like this.

-Bill

ggunn · January 2011

Re: Large battery array

BB. wrote: »

In any case, this is a huge system and may need a Power Engineer (w/ PE cert.) to even tackle a system like this.

Would you give me a sanity check? The customer filled out a load survey, which totaled out to 438.38 Ah/day @120VAC, which is about 52.6 kWh/day. Right so far? That's about $5/day which is not out of line for (for example) a medium sized house, so it at least seems reasonable.

52,653Wh/48V = 1096.9 DC Ah/day @48VDC

X3 days of autonomy
X2 for 50% DOD
/.76 for lumped efficiencies
X1.4 for temperature considerations

= 12,124 Ah @48VDC battery needed

Does that seem right, ballpark-wise, or have I misplaced a decimal point somewhere?

BB. · January 2011

Re: Large battery array

Hmmm,

My usual battery sizing:

AH per day * 1/0.85 inverter eff. * 1-3 days of autonomy * 1/0.50 max. discharge

The x1.4 for temperature--I don't usually bother--Small batteries can be heated the first day somebody opens the cabin--and they will usually stay warm.

The 1/0.76 is probably more of reasonable number (at least more conservative).

Some sort of active thermal control/insulation to keep the batteries warm (do you have extreme cold winters)? Unless there is some serious cold soak (built on ice/permafrost--which nobody would do)--I would probably revisit the 1.4x factor.

You can also visit the 3 days of autonomy... Is there a real functional reason for this? At some point, the person is already going to need a genset/utility connection so we are just talking about fuel/make-up utility power as the cost difference anyway.

The difference between 3 day and 1 day savings in battery costs may well pay for fuel/utility power for poor weather (and battery replacement every 8-15 years)...

What would a 3,000 AH @ 48 volt battery bank look like for the customer operation (vs weather, charge/discharge rates, possible tracking PV arrays, etc.)... Look at a "small bank" with AGM (lower internal battery resistance so larger charge/discharge currents vs flooded cell--and lower self discharge).

-Bill

Cariboocoot · January 2011

Re: Large battery array

I too will gently suggest re-examining the need for 3 days worth of batteries. The extra cost of equipment is usually better spent on a decent genset, which can provide back-up power not only if the sun isn't shining but also if the system goes down for some other reason.

For the cabins around here I suggest people plan for a battery bank that will supply 1 day @ 25% DOD, knowing that there's 25% more to go if the sunshine is inadequate on day 2. Day 3, start the gen. This usually works out cheaper than masses of batteries and panels which sit idle/unused so-to-speak most of the time (over capacity).

I would not run the generator exhaust into the greenhouse though; CO is not the same as CO2 (Bill, you must have got that wrong somehow. Or else there's some pretty stupid people out there, poisoning themselves!)

Even a 4000 Amp hour bank would be an enormous undertaking! Four parallel banks of 1100 Amp hour 2V cells, perhaps?

icarus · January 2011

Re: Large battery array

I think Marc's formula is about right, design for 1 day at ~ 20-25% Dod, then you can run the second if you must, and run the genny on the third. In reality it is rare in most places where solar is viable not to get some charge on day one and day two, so in reality you can usually stretch that to 3-5 days. I tend to charge on day three if I have to just to avoid leaving batteries more than 20% discharged. An hour or two of the genny in the morning of day three, with full sun will bring every thing back to 100% by sun down.

Tony

BB. · January 2011

Re: Large battery array

If this link works to a 1972 issue of New Scientist (don't know anything about magazine or short article)...

Ran the exhaust through a scrubber--In conjunction with University of Arizona, Rockefeller Foundation, tested in Mexico and Abu Dhabi (part of a sea water desalination + farming project). Was there a venting system for when workers went into the greenhouses--I would guess yes--but I don't know. Just one of the weird factoids I remember reading as a kid.

Pretty difficult find anything now with a quick on-line search--everything come back with global warming and greenhouse gases...

As I said, I know there are lots of issues with doing something like this. Just trying to think outside the box.

-Bill

RCinFLA · January 2011

Re: Large battery array

One advantage of battery power, for at least a half day's kWh need, is the ability to get the most fuel efficiency out of a generator. Even with battery charging loss and inverter efficiency you will use about half the fuel of a continuously run generator.

A typical residential use has a majority of time spent between 300 and 1000 watts. Peak loads may hit 15-25 kW but they are very short in time. Running a gen capable of supplying peak demand while averaging less then 1 kW is very fuel inefficient.

With the battery system you can run the gen at 80-90% loading for charging, yielding good fuel efficiency.

Putting that much battery capacity in place is going to be a lot of cost for maintanance and replacement costs.

I would suppliment with a 15-25 kW diesel or propane generator. It will take about 3-4 hrs of run time per day to replenish batteries, assuming no solar contribution. That is about 5 gallons of diesel or 12 gallons (50 lbs) of propane a day, again with no solar contribution.

Much lower cost solution.

Cariboocoot · January 2011

Re: Large battery array

In that case we can avoid the scrubbers, buy a gasoline genset, and convert it to run on ethanol. By-products of combustion: CO2 and H2O, both good for plants. Plus you can grow the necessary raw material to brew the ethanol out of in the greenhouse! Hmmm. This is starting to sound like perpetual motion ...

niel · January 2011

Re: Large battery array

i'll 2nd what coot said on the batteries, but even going to a day and a half without exceeding 50% dod i think would suffice and would cost less too. after all the generator will be there to take up any slack anyway and can be made to kick on automatically. in any case this is a huge undertaking and i agree further engineering would be needed for something of this size. most would stay on the grid and just back it up with a generator for cost reasons and this can still be done. the pvs can also still be done using a straight gt setup, but not if the goal is to go off the grid.

stephendv · January 2011

Re: Large battery array

Let's start at the beginning, since it's a greenhouse does he really need 52kWh of electrical energy, or is a large portion of that heat energy?
If any of that is meant for heating, then using solar PV + batteries is not the most economical route. It would make more sense to revisit the design of the greenhouse and think about solar water heating, combined heat and power systems (e.g. genset with heat recovery) and storing the heat in the greenhouse in large water tanks or in the ground itself.

russ · January 2011

Re: Large battery array

Stephen is on the right track.

What he wants for heat should be supplied by other than PV - that should reduce the load greatly unless there are a large quantity of grow lamps.

Build in a large volume of warm water storage and use solar thermal to supply the heat. Much more efficient than solar PV anyway. Also much cheaper with less ongoing costs than PV.

Russ

Cariboocoot · January 2011

Re: Large battery array

This may sound ridiculous, but the power usage may be for lighting. In BC greenhouses run grow lights so they can produce 24/7 and over-come our occasional 6 month bouts of darkness. Usually natural gas is used for heating as it is far, far cheaper.

But the concern about using electric for heat is legitimate, if that is the case.

techntrek · January 2011

Re: Large battery array

RCinFLA wrote: »

One advantage of battery power, for at least a half day's kWh need, is the ability to get the most fuel efficiency out of a generator. Even with battery charging loss and inverter efficiency you will use about half the fuel of a continuously run generator.

A typical residential use has a majority of time spent between 300 and 1000 watts. Peak loads may hit 15-25 kW but they are very short in time. Running a gen capable of supplying peak demand while averaging less then 1 kW is very fuel inefficient.

With the battery system you can run the gen at 80-90% loading for charging, yielding good fuel efficiency...

This is what I do during outages. I only run the genset for a few hours each day to charge the batts, run the well and make some hot water (electric water heater). Its nice still having lights, clocks and the electric blanket overnight w/o wasting fuel and listening to the genset.

blackswan555 · January 2011

Re: Large battery array

If it is for lighting you may want to have a look at Power factor, If it is lighting the "standard" 600w HPS sodium as used in commercial greenhouses, It uses a little over 700w, More on start up, tailing back,(Digital are better)
Have a look at the spec on the ballast, then do the volts times amps, (VA) that is the real number.
Have a good one
Tim

For any that don`t know

Inverters work in VA not watts as sometimes presumed, power factor http://en.wikipedia.org/wiki/Power_factor applies to all inductive loads (transformer`y /motor type things) so if you have many it is wise to work out their rating in VA (volts times amps) not watts, CFL`s are particularly bad, have a look at one, A recently purchased (supermarket) 20W VxA`s @ 36 and actually uses 39 va, Ideally when designing a system VA or direct VA measurement should be used,

ggunn · January 2011

Re: Large battery array

Thanks for all the input. I realize that the design considerations need to be revised (I have known this from the outset), but that aside, were my calculations correct?

Large battery array

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