# Designing of a battery as a back up storage unit

Registered Users Posts: 2
Hello,
I have to design a battery as a back up unit for my load (data center in my example) which is in connection with a photovoltaic module. As I know the load profile of my load and the generation values of my photo voltaic system, I have to design a battery for this. In this designing calculations, how do I need to consider days of autonomy? Beacuse my study is related to find the resiliency of the back up system between PV+battery and diesel generator. So at the end I am going to compare these both on how long these would work when there is a blackout in my system. In this scenario I can't assume my days of autonomy to be X days.

Can someone help me with this?
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• Solar Expert Posts: 448 ✭✭✭✭
You know what your loads are, you know what your available power production is. You want to know how to consider days of autonomy, but then you say that you cannot assume how many days of autonomy.

I am unclear what your question really is?

I always have more questions than answers. That's the nature of life.
For various reasons, we typically use 1-3 days of storage for Lead Acid battery banks, and a maximum of 50% discharge (for longer battery life). Assuming 2 days as a typical optimum for an off grid storage system and lets say you are using 3.3 kWH per day (this is ~100 kWH per month, and is enough to run a very conservation minded off grid home with near normal electric lifestyle--Refrigerator, LED lighting, clothes washer, well pump, TV, laptop computer):
• 3,300 WH per day * 1/24 volt battery bank * 1/0.85 AC inverter eff * 2 days of storage * 1/0.50 maximum discharge = 647 AH @ 24 volt battery bank
Then there is the rate of charge... For a typical lead acid battery bank, we assume 5% to 13% rate of charge. 5% is OK for seasonal/weekend use. 10%+ is good for full time off grid:
• 647 AH * 29 volt charging * 1/0.77 solar panel+charge controller derating * 0.05 rate of charge = 1,218 Watt array minimum
• 647 AH * 29 volt charging * 1/0.77 solar panel+charge controller derating * 0.10 rate of charge = 2,436 Watt array nominal
• 647 AH * 29 volt charging * 1/0.77 solar panel+charge controller derating * 0.13 rate of charge = 3,168 Watt array "cost effective" maximum
Note--If you have a day time and/or 24 hour per day load, you should up size the solar array to support both charging the battery bank (5-13% rate of charge), plus the loads. Lets say your constant daytime load is 800 Watts (i.e., well pump for irrigation):
• 800 Watts * 1/0.77 panel+controller deratings * 1/0.85 AC inverter eff = 1,222 Watt array to support daytime pumping loads (plus above array for charging)
And you have to also look at how much power you need vs the amount of sun (per season):

### MagdeburgAverage Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 38° angle from vertical:
(For best year-round performance)  Jan Feb Mar Apr May Jun 1.30 2.18 2.81 3.70 4.24 4.13 Jul Aug Sep Oct Nov Dec 4.12 4.02 3.15 2.25 1.36 1.10
Assuming that you are near Magdeburg Germany--You do not get much sun, especially during winter. You have to decide what your power needs will be (by season, if it matters)... But, assuming you will have a genset, toss out the bottom 3 months and use 2.18 hours of "average sun" for February as the "break even month":
• 3,300 WH per day * 1/0.52 typical off grid AC end to end system efficiency * 1/2.18 hours of sun = 2,911 Watt array for "break even February"
Say you decide on 10% rate of charge (recommended minimum rate of charge for typical flooded cell lead acid battery bank) and support an 800 Watt daytime load:
• 2,436 Watt array charging + 1,222 Watt daytime load = 3,658 nominal array (based on battery bank charging needs)
Which is > 2,911 February minimum array. So, assuming you use 3,658 Watt array, the typical system output for a few different months would be:
• 3,658 Watt array * 0.52 system eff * 1.10 Hours of sun (December) = 2,092 WH per day (December)
• 3,658 Watt array * 0.52 system eff * 2.18 Hours of sun (February) = 4,147 WH per day (February)
• 3,658 Watt array * 0.52 system eff * 4 Hours of sun (four months of "summer" minimum) = 7,608 WH per day ("summer")
In general, solar powered system do not make a lot of sense for backup power... The alternative is genset+battery bank, with the battery bank sized large enough to support the load until the primary (or backup) AC genset is started. Say 1 hour of storage with AGM/GEL/LiFePO4 batteries:
• 3,300 WH per day * 1/24 hours per day * 1/0.85 inverter eff * 1/24 volt battery bank = 6.7 AH @ 24 volt battery bank
Assumes your AC inverter runs for 30 minutes (enough time to get primary or backup genset running), batteries are only used for a few cycles and typically replaced (and replaced every 2 years).
• 3,300 Watt load * 1/0.67 Power Factor * 1/0.80 AC genset derating * 1/24 hours per day = 257 VA genset average load (during power failure)
If you can justify the genset (fumes, fuel storage, noise, etc.)--For outages that are random/few and last for ~1 week per year, a genset + short term UPS is difficult to beat.

If your outages last > 1 month--Possibly an off grid solar power system + generator backup makes sense (if this is a commercial installation, you will always need a backup genset or two anyway--And for a larger battery bank, you will have to up size the genset to manage the 24x7 loads and recharge the battery bank (10% rate of charge is optimum for typical lead acid battery bank).

Anyway--You did not give a lot of information--And because numbers are easier to explain and show in an example equations (rather than using lots of variables)--I chose a very efficient off grid home to show what happens and how quickly things scale up if you want to use full time off grid solar (or for multi-week backup power).

Problem is that computers (i.e., data center) really suck down the energy. Normally, for off grid solar folks, first thing we push for is conservation--It is almost always cheaper to conserve than it is to generate power.

Remember your battery bank may last 3-5 years (if "cheap" batteries) or 10-20+ years (if expensive traction type industrial batteries), your electronics (charge controllers, AC inverter, etc.) may last 10+ years, and solar array 20+ years. Plus add cost for maintenance (distilled water+labor once per month to keep battery cells filled, genset test and maintenance, etc.).

And if you have a data center, you have HVAC (heating ventilation and air Conditioning), lighting, security, networking support equipment, remote monitoring, spares, etc.... All adding to the cost.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 2
@Marc Kurth
Basically, I have to find resiliency of my backup systems. So in this case if I implement a battery, I wnat to know for how many hours it would last? If I get to know that it would supply for about X hours then I could find resiliency of this system in terms of %. But while designing a battery, we have a term called days of autonomy that means before designing itself I am making sure that my battery works for X hours. But this isn't what I want. So now how do I size my battery? My total energy per day is 2180kWh.
• Solar Expert Posts: 8,011 ✭✭✭✭
2.2Mwh per day ! That's 91Kw load !  Your battery will be sized to last 10 minutes, just enough to get the backup generator started if the grid does not return after 5 minutes.  Then things depend on the size of the fuel tank. Any larger battery and you will need the entire ground floor for it and then move all electronic gear upstairs.
You may have to split your datacenter loads to "Critical" and the other ones that can fail and go dark for the blackout.
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Sorry for the late reply, but as Mike says--That is a lot of battery bank. While anything is possible, what is the end game here?

Is it to design a solar powered Data Center, or is it purely looking at a possible backup power alternative?

There is a 380 VDC Data Center consortium that is intended to make data centers more efficient and easier to integrate solar power+battery power into data centers (skip the whole DC to AC conversion step). It turns out that for many computer type AC input power supplies that are rated for 120/240 VAC, you can feed them 380 VDC directly and all will operate fine.