Trying to setup Solar energy at office

Re: Trying to setup Solar energy at office

I am not sure I understand what "it" is:

I am okay if the computers would restart once in 10 failures. Employees will not be happy if it will restart at every time the grid goes down ( atleast 2-3 times in a day )

We do have a separate small cheap UPS powering each computer for 10-20min backup which is helping for now to be a buffer between the grid/diesel generators .

but we plan to remove it when we go with a larger UPS /Inverter system as they would draw the battery bank unnecessarily and is less efficient/ and heats the room.

If we ditch it its going to be just sitting around taking up space and if we sell them they would be going for scrap value. now would you suggest doing something useful with them or are they better of ditching?.

I guess you are talking about that to do with the smaller UPS systems (personal computer size?).

Other than the usual sell/scrap--Two things I would suggest. Give them (or scrap them) to your employees for home use. Or, try some experimenting with them for conversion with external battery banks/solar charging/backup generator charging. May regions of the world use various battery backed systems for home use (fridge/lights/TV/etc.) for areas with unreliable power. You may find some inexpensive units are good for conversion (cheap, rugged, reliable). Many may have too small of battery charger for a large external battery bank, etc. May be worth "blowing them up" for experimentation, see how long the UPS/battery banks last in home use, etc. I assume there is a big market in your region for home UPS systems that are between the small 30 minute and big industrial units. Either for personal education or further business opportunities--Your call.

I can PM our website to you if you wish to look at but I do not want to spam here .

We do allow "personally connected commercial signatures" here for folks that actually participate in the forum. You are more than welcome to put a link and short description to your business in your Sig.

Wouldn't the uneven draw from the bank make the cells unbalance?

Is it possible to like draw a 96V and 48V from the 144V bank? without future issues?

No, you should not take 48 volts from a string of 144 volt batteries in a bank. This "unbalances" the batteries. When you recharge them, some batteries are "near full" and others are discharged. When recharging you have to "force" lots of current through the "full batteries" to recharge the discharged ones. Over charging is very hard on Lead Acid Batteries and can knock years off the battery life.

By the way--It is possible to pull (for example) 12 volts from a 24 volt battery bank. There are (electrically isolated) DC to DC converters (bi-directional DC power supplies) where you attach one between the "high 12 volt" and "low 12 volt" batteries. These "balance" the voltage between the two sets of batteries. Essentially, when the "low bank" is being discharged, the DC to DC converter takes current from the high bank and uses to supplement/recharge the low bank under load. And, conversely, when the 24 volt bank is being recharged, the DC converter again balances the voltage between the two different 12 volt halves of battery bank. If one battery is fully charged, the "higher voltage" will be drained down (converted) to provide current for the lower voltage battery.

This is done quite often in recreational (caravan/bus conversion) type installation where you have a 24 volt battery to start/run the vehicle and still have 12 volts for other loads.

There have been all sorts of designs that go down even to the cell (2 volt) to charge/control charging (central charger with basically 6/12/24/etc. separate isolated battery chargers). I think they are pretty rare--But have been done before (I fall across links to such charging systems from time to time, but can never find them to reference in a post when I want too).

There are sometimes vendors that make battery top "diversion" chargers. Wire the diversion "controller" on one battery and connect a pair of sense leads to the battery next to it. Repeat for each battery/cell in the bank. If one battery has a higher voltage vs the one next to it, then start bypassing a couple of amps around the "higher voltage battery". This allows batteries to be "balanced" without over charging other batteries in the string.

Not to suggest that you want to do any of this--But to give you ideas of what people have done in the past to address specialized needs/try to get longer life from their expensive batteries.

it takes 12 Battery in serial to get this voltage.. how many strings I could go ? , I think financially speaking I could afford only 1 more string.

I personally try to advise people to get higher AH rated cells/batteries (i.e., 400 AH cell/battery vs 4x 100 AH cells/batteries in parallel). There are lots of issues with paralleling battery banks. Batteries are very low impedance and who's voltage (charging/discharging) is dependent on chemistry/temperature/stray resistance/age/etc. It is very difficult to get batteries to "evenly share" current over multiple parallel paths. Balanced wiring (same wire lengths) and monitoring (with a DC current clamp meter under load/charge) to look for problems (poor electrical connection, cells going open/shorted, etc.) can help keep things in control--But it does require constant attention to find issues before they take out a string or a whole bank.

Plus, if you have flooded cell batteries, you have more cells to check every month for electrolyte levels, more electrical connections to maintain, and more costs (each battery string should have its own fuse/circuit breaker for fault protection).

My recommendation is 1 string as optimum. 2-3 parallel battery strings maximum unless you have no other choice (because of above issues).

So 24 batteries total.. 200AH each . would be 400AH at 144v ? , I think can give about 24-26 KwH theoretical power a day at 50% DOD?

Yes, that would do it. 12x 12 volt batteries in series, and two strings in parallel.

144 volts * 400 AH * 0.50 discharge = 28,800 Watt*Hours = 28.8 kWH of energy

I could be wrong but from my memory for just computers and lightings I may just need that per day.

Also I am going to look for any one selling their UPS installation around here.

You have several parameters to "balance here"... First is total storage. You have that nailed (remembering to add inverter/battery/charging losses).

Next is current output. Batteries are more efficient if discharged slowly (Peukert factor). Flooded cell tend to see this effect more. AGM tend to see this effect less (AGM are more efficient under heavy loads per AH rating). For typical flooded cell batteries, discharging at C/8 (8 hour till battery dead) is about the fastest you would want (batteries can over heat if discharged faster, more losses, etc.). For Off Grid use, we usually aim at C/20 discharge rates. If you have surge draw (pumps, A/C compressors, etc.)... C/2.5 is about the highest surge current you would want to plan on. I.e., a 400 AH battery bank / 2.5 rate of discharge = 160 amps maximum surge current.

And then there is recharging. Again, for flooded cell batteries, roughly C/8 is a "comfortable" maximum recharging rate. Higher recharging current should have temperature monitoring of the battery bank to avoid thermal run away issues (hot batteries have lower charging voltage, remote battery temperature sensor will tell charge controller to reduce voltage set point to prevent over charging a hot battery bank and boiling/cooking the battery bank).

More or less, this works out to around 100 AH @ 48 volts per 1 kW of constant load and even solar array for reliable/long term operation. I.e., a 48 volt @ 400 AH battery bank would be 4kW inverter and 4kW of solar power--A 400 AH @ 144 volt battery bank would be 3x as much or 12 kW inverter + 12 kW solar maximum (the limit for solar panels seems to be based on the fact that nearly fully charged battery bank with a MPPT type charge controller can "sweep" the array looking for Vmp*Imp--The full current surge appears to "over charge" a "too small" battery bank and can take a 48 volt battery bank to over 72 volts in some conditions.

AGM batteries can supply much higher surge currents--But cannot "absorb" high charging currents when full (same limitations as flooded cell).

Note--The above are rules of thumb we use around here and have worked well (reliable systems over life of battery bank--and great to use for quick back of the envelope calculations). Your needs may differ. And if you are "cost sensitive", then you may choose to push the rules based on your design needs and actual components/batteries selected.

For example, many folks look at the discharge characteristics of their system, but overlook the recharging needs. A "smaller bank" that is taken to 50% state of charge may supply the loads well and be cost effective (i.e., 50% maximum discharge "wears out" the batteries faster, but a 2x larger bank may last 2.2x longer with 75% SoC discharging--So the costs over time are similar). But, the deeper you discharge the batteries, the longer it takes to recharge them. Deep cycling a battery bank may take upwards of 16 hour charging cycle to completely recover (again for flooded cell).

Solar power is limited (daylight only, summer vs winter sunlight reduction for non-equatorial installations, etc.). And if you operate down to 50% discharge in normal operation, what happens if you have an unplanned second/longer outage (yes, this is where the backup genset fills that need).

And other battery constructions can have their own limitations. For example GEL batteries have very good surge capabilities (same as AGM), but many (most) GEL batteries are limited to C/20 (5%) rate of charge--A 50% discharge may take 10-20 hours to recharge--Not great for a solar powered installation where you get ~6-8 hours maximum of charging time per day.

Some battery mfg. recommend 5% minimum rate of charge... Others recommend 10% minimum rate of charge. And others recommend C/8 or ~13% maximum rate of charge (thermal issues). Again more rules of thumb--Check with actual specifications when looking for buying parts.

Break into two posts--Too long:

-Bill

Re: Trying to setup Solar energy at office

And, forgot to add... Look at Midnite Solar's Classic MPPT Charge Controllers. I believe they have the ability to operate a (somewhat) higher input/output voltages. Or higher output current at "nominal" battery bank voltages.

www.solar-electric.com/misoclchco.html
www.midnitesolar.com

They also have their own forum too for further technical discussions.

-Bill

Re: Trying to setup Solar energy at office

paimpozhil wrote: »

Regarding batteries I dont think I have a lot of choice, We have to work with what we have in my city and due to tons of power outages, its very high demand to even get the standard size batteries.. here 180AH/200AH are the larges for 12V and I do not see any other volts at all.. may be 6V very rarely.

I am buying all the other things from other big cities/other countries. Batteries we cannot pay the huge shipping cost so we need to find it local.

You might look for forklift and other industrial battery suppliers (if they exist in your region). Your in southern India? Perhaps more options near a port facility?

Yeah we are cost sensitive at this point of time. due to tons of fuel costs and electricity over the last 2 years , we do not have lot of money to spare.

At some point, you will have to model battery life vs depth of discharge vs "battery quality" vs costs... In the end, you probably want to come up with a $$$/kWH (or kWH/$$$) cost and life of battery (i.e., downtime+costs to replace a "cheap bank every 3 years vs a horribly expensive bank every 15-20 years (or perhaps, your system will only have a 2-4 year useful life anyway--then you either expand, contract, move, etc. and need to start over).

I will write up my load calc with morning usage (9-5PM) as well as night usage.. ( night(6-2 AM) usage is LOT less than our day time)

This is where things "get interesting". It is very easy to model "charge when the sun is up and discharge when the sun is down).

If the model is charge/use power when the sun is up--Potentially you don't need much in the way of battery capacity (just enough to account for morning/evening use + any surge capacity). You size the bank for that, then figure out your average mid-day power usage (say 60 amps @ 48 volts), then your solar array would be 60 amps load sustaining+5-13% rate of charge to supply enough energy to recharge the bank for morning/evening use). You could start small (5%) and see what happens, then add solar array+charge controllers if 5% is "not enough" and move towards 10-13% rate of charge (25% would probably be the ceiling).

Please help me size the battery where I can use the less possible number of batteries at the same time take maximum advantage of the Morning time bleeding energy after battery is charged and choose the best system voltage ( I believe this is directly related to the Inverter used ).

For you, the best voltage will probably be the highest voltage you can find equipment to support. For Solar RE, that is usually 48 volts. If you use UPS systems, you could look at 144 volts or higher. Charging becomes a bit of an issue, finding AC Battery Chargers and some sort of DC Solar Charger will be a bigger issue (you can charge 48 volt blocks of batteries with standard solar RE controllers--But you have to watch for grounding issues. Most controllers assume battery minus is earth ground--Those 48 volt blocks will have offset negative lead voltages--And connecting RS 232/422 voltage isolation, wiring up arrays with multiple +/- fuses/breakers all add to complexity and costs.

You should be able to support a 6-10 kW inverter with 48 volt--But more than that will really push up the copper content (and high current breaker/fuses/switches) for the system costs/difficulty to find devices.

At my home I am taking full advantage of this morning usage and I will simply cut off my EB supply and everything sems to run off the solar without discharging the battery until early evening when I will switch on the grid back.. I would love to do something at office too.

Since you have AC Power available during (at times) random parts of the day--Then you can always make up for the time/cost limited issues of pure Solar Panel system. I.e., the solar will keep the system reasonably charged for the better part of a week (don't take the batteries below ~50% SOC and try to get, at least, back above 75% a couple times a week (that would be my initial goals).

Then use the weekend to fully recharge the battery bank back over 90% SOC. And during times of poor weather/heavy power usage, use available utility power to help recharge the bank (say at night, or even help during the day). And use the diesel genset when both sun and utility fail to keep the bank "happy".

And regarding the type of batteries, I am inclined to use the flooded cell because its cheap and have experience handling them . Here they have tall tubular batteries, Do you think they are any better than the normal standard flooded LA batteries for solar applications.

I don't have any real "large battery" experience--Stephen and others here do--So they can probably answer the questions better for you.

Personally, I would only use more expensive batteries if they give you something you need. Better surge capacity, higher rates of charge/discharge, and/or longer life in your application.

Sealed batteries cycling--I have to wonder if they will do much better than 2-3 year life unless they have an enforceable warranty that gets you out 6+ years. One poster in another forum asked the question why AGMs (for example) seem to have shorted useful life... one suggestion was that flooded cell electrolyte kept Oxygen from the battery grids and reduced corrosion vs the ability for "battery oxygen" to directly attack grid/supports in an AGM battery (don't know if true--but and interesting idea trying to explain the apparent life differences).

Our battery manufactures offers zero support and the distributors that sell them do not know whats a rate of charge

Generally people here do not seem to really care about any tech specs.

I will try to get the spec/data sheet before deciding on which one to do.

Thanks to the Web--Most everything is available somehow/somewhere.

As usual, thanks for all the info. I am very close to commit into the system and start working towards making it real.

And please come back and document/let us know how everything works out. It is nice to get feed back on our suggestions to see if we were right or wrong.

paimpozhil wrote: »

NO I would never exceed the 25% because I wouldnt plan to have enough panels to do so at first place.

AC Backup charger will be 100% disconnected during mornings to ensure the grid is not used .

If you use solar+ac backup charger (when cloudy and/or during periods of heavy loads)--Your solar+AC Backup charging could easily exceed 13%--You should have a temperature display/alarm on the battery bank "just in case".

I think some Tristar models can sync remember reading it somewhere.

Probably--And Outback, Midnite, Xantrex/Schneider MPPT controllers do too (I believe).

Both the controllers will sense the battery voltage to decide the charge stage right? but if it by mistake senses the each other's voltages this may get in-efficient? because we are connecting all the wires together in to the battery posts anyway?

And why I always recommend "home running" each charge controller back to the battery bus. If you run a cable from battery to charger A then to Charger B (daisy chaining), the voltage drop from multiple controllers (and the "electrical noise" of the different controllers/charging modes) can confuse the chargers--Reducing their charging current.

But, if you have two solar MPPT controllers running at 50% vs one running at 80% and the other running at 20%--It really does not make a difference to the battery bank.

-Bill