Minimum battery sizing for PV system with prime generators

[pucksr]

So, this may be a little abnormal for the typical discussions here.
I am an EE trying to design some off-grid systems(or rather augment some off-grid systems) in the 50-150kva range.

We currently operate as generator prime systems with 24/7 generator operation(or very near it). I am trying to make an argument for converting to PV prime with standby generator operation. We have had success with this type of design at smaller facilities, but not at larger facilities.
I know that IEEE 1562 typically recommends a minimum sizing of 5 days for batteries for off-grid applications. I know I can shrink that number if I have a standby EG. I also know that if I shrink that number too much, the "standby EG" will be required to operate on a daily basis.

Example: If I only have 20 hours of battery, my EG will not be a standby generator.
If I have 36 hours of battery, I can be fairly certain I will have a standby generator.

I am trying to figure out, from a design perspective, where IEEE or similar puts that number.

Any thoughts?

[SunEagle]

Something you have to also figure in the calculation for the battery size is that if you discharge more than the mfg says you should you will shorten the life of the battery system.

That 5 days is really based on not needing to discharge more than 20% a day but still allows you a little space if you have to discharge more a few times over the life of the battery.

If you want to save money then IMO build a generator system that is fueled from different sources and forget about using batteries. You will save money in the long run.

[bcroe]

One consideration is your weather. Clear skies are nice, but frequent clouds will bring
your generator back on much more often. Bruce Roe

[pucksr]

If you want to save money then IMO build a generator system that is fueled from different sources and forget about using batteries. You will save money in the long run.

Well, not really.
We are talking about islands in the middle of the pacific or mountains in Middle Earth. If I design a dual-fuel system, that just means I have to pay someone to barge two different fuels out to a remote location. It doesn't really help with anything.

The idea behind using PV-prime and generator backup is that it reduces EG operation hours, which reduces maintenance hours. Most of these sites are highly inaccessible. One big trip every 5 years to replace batteries isn't bad. One trip every 3 months to deliver diesel is a big deal.

[Sunking]

If solar was a good option, professionals would use it. There is not a single Telco, utility, or data center using solar. That would be foolish and a waste of money. Solar is not reliable and very limited supply.

But here is your tecnical problem and why IEEE recommends 5 days. I know this for fact because I sit on IEEE battery standards committee. Batteries have Internal Resistance, and thus limits how much current the batteries can supply without significant voltage and power losses. For true deep cycle FLA batteries C/6 is about the limit until you hit the magic 3 to 5% voltage and power losses.

Now there are some AGM's that can handle 1C and greater current demand. Those are made to that and called Station Batteries like you see used on UPS. Catch is they are very expensive and are not Deep Cycle. At best 50 to 300 cycles or less than one year service if pressed into cycle service. Reality is true deep cycle batteries Specific Power will be low as it is just the naturre of the design.

True deep cycle batteries have thick heavy plates, so they can last a long time. SLI, and special purpose batteries like station batteries have more and much thinner plates to increase surface area and thus lower resistance. You cannot have both.

So what I think you will run into, is if you use a true deep cycle battery will take a lot larger battery than you want to pay for. If you use a AGM made for fast discharge and use it every day, you wil replace them every year. Again no economical way to make it work.

What proos do is use a UPS with AGM Station batteries made to discharge in 30 minutes. When power goes out, load is on the batteries for just long enough for the Generators to start up and take over to run the loads and recharge. That is the most economical, effective, and reliable way to do it.

FWIW I have designed hundreds of solar systems for Cellular Tower Applications where no commercial powers is feasible. Those are mission critical systems where power outages are unacceptable. The design comes straight from IEEE, myself, and John Wiles the God Father of Solar.

1. 10-day battery reserve capacity.
2. Panel wattage = C/10 charge current. So if you have 1000 AH batteries, enough panel wattage for 100 amps.
3. Generator and Charger sized for C/6 charge current. Enough to run the equipment and charge the batteries fast as possible.

There is 4th rule that applies to any system. Use a high enough battery voltage to keep load and discharge currents under 80 to 100 amps or whatever the maximum size charge controller you can get your hands on.

[J.P.M.]

Well, not really.
We are talking about islands in the middle of the pacific or mountains in Middle Earth. If I design a dual-fuel system, that just means I have to pay someone to barge two different fuels out to a remote location. It doesn't really help with anything.

The idea behind using PV-prime and generator backup is that it reduces EG operation hours, which reduces maintenance hours. Most of these sites are highly inaccessible. One big trip every 5 years to replace batteries isn't bad. One trip every 3 months to deliver diesel is a big deal.

Would have been nice to know your situation. Such knowledge will probably change the comments you get back.

[SunEagle]

Well, not really.
We are talking about islands in the middle of the pacific or mountains in Middle Earth. If I design a dual-fuel system, that just means I have to pay someone to barge two different fuels out to a remote location. It doesn't really help with anything.

The idea behind using PV-prime and generator backup is that it reduces EG operation hours, which reduces maintenance hours. Most of these sites are highly inaccessible. One big trip every 5 years to replace batteries isn't bad. One trip every 3 months to deliver diesel is a big deal.

Fair enough. I agree that transporting fuel of any type will be expensive. Yet installing a battery system that is not sized properly could result in a short life and another big expense just to ship, transport and replace them.

Just don't undersize the battery so you think you are saving a few dollars. It the system is critical you want to make sure you have belts and suspenders to keep the pants from falling.

[pucksr]

Would have been nice to know your situation. Such knowledge will probably change the comments you get back.

Sorry, I was trying to explain, but I will try to be a bit more verbose:

We have existing off-grid systems that use generators as their only source of power(prime generators). This means that they only run on fuel which we carry to them via boat/truck/helicopter(depending on the site). This is expensive and a big problem. Generators have a lot of moving parts and they require regular maintenance. They also don't have predictable failure rates(like batteries). This means that we frequently have to either install UPS functionality into the system or operate multiple generators in parallel with redundancy.(N+1) Sometimes we do both if this system is highly critical.

The solution?
We are thinking about PV.
PV could reduce operational hours on our generators and reduce the amount of fuel we need to truck out to the site.

In fact, we could skip the PV altogether and just go to generators+batteries. We already do this at some facilities. At the very least, it gives you some warning if an generator fails before you lose the equipment. You might even have time to get someone out there to fix it! Also, it allows you to do some nifty things with putting in bigger generators and getting some efficiency gains.

[pucksr]

What proos do is use a UPS with AGM Station batteries made to discharge in 30 minutes. When power goes out, load is on the batteries for just long enough for the Generators to start up and take over to run the loads and recharge. That is the most economical, effective, and reliable way to do it.

FWIW I have designed hundreds of solar systems for Cellular Tower Applications where no commercial powers is feasible. Those are mission critical systems where power outages are unacceptable. The design comes straight from IEEE, myself, and John Wiles the God Father of Solar.

1. 10-day battery reserve capacity.
2. Panel wattage = C/10 charge current. So if you have 1000 AH batteries, enough panel wattage for 100 amps.
3. Generator and Charger sized for C/6 charge current. Enough to run the equipment and charge the batteries fast as possible.

I figured you would be the guy to talk to about this kind of thing.

So, my question is this:
Why go with a 10-day battery reserve capacity if you have a generator? Is that just to reduce heavy cycling of the battery? Once again, we are talking about prime-rated generators, not cheap air-cooled junk. I was assuming we could easily put a few hundred hours on these generators per year and perform annual generator maintenance + battery maintenance. We might have someone working at the site for a week or two each year.

I am trying to base a lot of this off of the IEEE 1562 and 1013 and your recommendations match up perfectly, so I am not arguing with you at all. Just trying to get a bit of a good justification going forward.


I am currently going into meetings where I spend half of the meeting explaining to people why we can't just use 1-hour batteries(because that is what we use for sizing a grid-connected UPS system) in an off-grid application. This is uncharted territory for a lot of my co-workers.

[Sunking]

I am currently going into meetings where I spend half of the meeting explaining to people why we can't just use 1-hour batteries(because that is what we use for sizing a grid-connected UPS system) in an off-grid application. This is uncharted territory for a lot of my co-workers.

Hey I hear you, did that for 40 years and still doing it today. Happens here a dozen times a day. Look you asked good question and as a professional courtesy I will help you out as best I can.

OK remote telemetry or remote cell sites are Mission Critical systems. In our biz that means power availability must exceed 99%. I think we both agree on that point. OK the 5-day reserve capacity we speak of here are Consumer Level or Consumer Grade and only have at best 90% availability even with a consumer grade genny. In reality a 5-Day Reserve Capacity is only 3 cloudy days before you must shut down and eith wait for 3 or 4 days for solar to recharge or use a genny for a day. You are not going to obtain +99% on a 5 day battery and a solar panel only sized to generate 1 days use. Not going to happen.

So what do you do for commercial Mission Critical System. Well two things. First I will not go into, but involvers using 2-redundant generators and each sized to handle full load plus growth. Add in a UPS with 1-hour battery and you are set..

So you throw away all the design rules used here.

Size the battery for 10-days. That gives you 5-days run time before the Genny must start. Note however the genny will not wait 5-days, after 24 hours with no charge, the genny will start and fully recharge.

Panel wattage is determined by the battery AH, rather than watt hour used in a day. With a 10 day battery, and a C/10 to C/6 charge rate, the panels are capable of generating 2 to 3 days of power in a single day with Sun. So if you have been clouded up 3 or 4 days with a genny will not start buys you time. One good sunny day buys you 3 days run time.

Electric Power and Batteries are like fast cars. How fast you go is only limited by your wallet. How fast and far can you afford to go? It is all about trade-offs. If it is a data center generating a million cash per hour or Hospital, you can afford a lot to protect it, and if you loose it there is hell to pay and/or someone dies . At a home you loose what is in your freezer and no one gives a damn.

It comes down to your cost/benefit analysis.

[Mike90250]

solar gives you 5 or 6 hours a day you can idle the fossil fuel generators. can the cost of solar pv + electronics, save $ over fuel for generators ?

next step is 50KW+ wind generators, but they gave huge maintenance options.

[pucksr]

solar gives you 5 or 6 hours a day you can idle the fossil fuel generators. can the cost of solar pv + electronics, save $ over fuel for generators ?

Well, doing the math really quickly: I will use 100kW (though that is just for convenience)
8760 hours* 100 kW = 876,000 kWh per year
A 550kW generator will burn about 44 gal/hr of fuel under full load(I just happen to have this data handy). Extrapolate and get 70,000 gallons of fuel per year.
$4/gal for diesel in the middle of nowhere and it costs about $280,000/year to run the generators.
That is about $0.30/kWh. That seems about right, considering that is what you pay on small islands for electricity.

The cost cost of electronics can be considered negligible, because I am going to have to buy inverters or buy generators. BOS equipment can be cancelled out as well.

At a cost of $0.30/kWh, PV is pretty darn attractive. That is the cost of electricity in Hawaii and HECO is pulling their hair out with all of the PV projects.

[pucksr]

Size the battery for 10-days. That gives you 5-days run time before the Genny must start. Note however the genny will not wait 5-days, after 24 hours with no charge, the genny will start and fully recharge.

...

Electric Power and Batteries are like fast cars. How fast you go is only limited by your wallet. How fast and far can you afford to go? It is all about trade-offs. If it is a data center generating a million cash per hour or Hospital, you can afford a lot to protect it, and if you loose it there is hell to pay and/or someone dies . At a home you loose what is in your freezer and no one gives a damn.

It comes down to your cost/benefit analysis.

Well, I thought about the way I was phrasing my question. I completely agree with you. I am very familiar with the cost/benefit analysis.

Using my math from earlier, the balance of the equation clearly tips against batteries at a certain point. 10-days worth of batteries for a 100kW system is a 240,000 kWh battery. Assuming $500/kWh for a battery, that is a $12 million battery. In 10 years(the max life of the battery), I am only going to blow $2.8 million in fuel.

1) This is a critical application, ergo it is going to have a battery backup system
2) We are still going to have a generator
3) We want to integrate PV to reduce fuel consumption

Option 1:
The simplest system would just shutdown the generators while PV is available.
That is 4 hours a day* 365 * 8(gal/hr)=11,680 gallons of fuel @ $4 which would save us about $50k and 1/6th of our generator's run time. overhaul every 10,000 hours
We would use a standard UPS system

Option 2:
10 day battery(per IEEE 1562)
Build our PV array to capture enough energy so that the generator never needs to run.
That is 24*365*8(gal/hr)=70,000 gallons of fuel @ $4 which would save us about $280k and our generator would be nearly pristine.

Option 3(what we have done at smaller cell towers)
12-24 hour battery
Build our PV array to be large enough. However, expect regular runs of your generator. However, because you have batteries that can run site for 12-24, your generator can be 500kW.
You get better efficiency with a larger generator running at 100% loading. Plus, you put fewer hours on your generator.
You also get to put about 50-75% of your kWh on the PV
This now means I save about 40,000 gallons of fuel= $160k
I bought a $1.2 million battery(paid for in about 5-6 years of fuel use)
I am only putting about 2-4 hours of run time on my generator per day = 1,000 hours per year.(lasts 10x as long)

Option #3 is really what I am picturing, but I can't really find any design guidance. I don't know where it shifts from #3 to #2. I don't even know what the professional terminology is for #3 vs #2.

[Mike90250]

have you calculated how much solar & real estate you will need. I'd suggest GridTie inverters and shave load off the generators. skip batts.

[Sunking]

Puck me thinks you might be dealing with a low PF load. Battery Inverters and low PF loads do not play well together. So consider that so you do not shoot yourself in the foot like a DIY would do then wonder why it does not work.