Sizing Off-Grid Systems And Using Gen Support

I hope Mike doesn't see this as off-topic because it as the basis for how I propose an off-grid setup should size the battery bank, and how we have sized ours.

Correct me if I'm wrong, but I don't think these charts take into account charging efficiency. I developed a spreadsheet about 5 1/2 years ago to figure this out and I can't find it. I have switched computers in the mean time and it may have gotten deleted or lost along the way.

But the basis of my design is that lead-acid batteries are more efficient on charging during bulk stage than they are during absorb, i.e. if you run long deep cycles you will waste less of your RE energy you produce in heat in the battery. Versus cycling shallow and absorbing more often, you put energy into the battery that you never get back out. So the net result is that when you cycle on the 20 - 80% DoD part of the curve the battery's efficiency is higher, gaining you more kWh out for what you put in. When you cycle on the 0 - 20% DoD part of the curve the efficiency is horrible and 10-15% of your harvested energy goes up in smoke in heat loss in the battery.

The other factor on life expectancy is that batteries don't like heat. Our logging system logs battery temp once per minute, 24 hours a day. My logged data shows that overall average operating temp of our batteries is 5-7° C cooler cycling deep vs shallow cycling with frequent absorbs.

So I have become a firm believer in that what GB Industrial says is true - do not charge that battery and cycle it unless it absolutely needs it. Our logged data shows that we are cycling our battery once every 3.26 days on average. Some cycles go 7-10 days, some are cycled every day when our loading is too light and RE production is high. But it comes out to 112 cycles per year with our management regime. My belief is that the major design flaw in most off-grid systems is to cycle the batteries too shallow and cycle them every day, and the resultant efficiency is horrible.

You got it and has always been my point from the start. You can pay me now or pay me more later. In addition to fuel cost of a genny, there is maintenance and replacement cost to factor in. It is not easy to weigh in all the factors.

Sandia National Labs, John Wiles (the father of off-grid solar and who writes NEC 690), and IEEE all recommend 5 day minimum reserve capacity, and C/8 gen backup. I prefer to stick with that group and use it in my practice.

SunKing,

Thanks for the clarification. I can see the benefits of both ways (high and low DOD implementation). Like you say... Anyway you slice it there ain't no free lunch.


Chris,

Your thought pattern and setup is along the same line as what I was originally assembling for our retirement cabin (we have since gotten grid) except I was using a Magnum with a NiFe bank. The batts were really only used for light loads. When I ran the bank down or pulled amps hard (and got voltage sag) the generator would kick in.

A huge difference (make that potential improvement) in your setup is that the generator and battery bank add together. That gives you a continuous kW capability higher than either of the two systems alone. I didn't know there was an inverter capable of synching with a genny. Very slick.

Sunking - I am going to submit that Sandia and John Wiles have neither the off-grid living experience, nor the equipment that I have, to develop and prove that there are more efficient methods. For instance, there are lots of off-grid homes up here and I know of nobody that has a large generator sized at C/8 that doesn't check their pockets first to see if they got enough left to be able to afford to even stick the key in the switch. The end result is that these off-grid folks avoid running the fuel sucking thing at all costs and the battery ends up be abused instead of managed.

You might want to check into a "study" done by Sandia back when they developed what they called their ACONF generator management system for remote off-grid hybrid solar/generator installations. They used a DC genset and came up with a "revelation" on what I had been doing for years before they did their one year "pilot study" on it.

Basically Sandia's ACONF used the genset for prime power to reduce cycling on the battery and keep it in its bulk charging range, and let the solar do the actual charging. They ran two systems side by side, the reference system designed in the "conventional" terms, and the ACONF system designed identical to mine (except I use AC gensets these days because they are more efficient than DC when you have AC loads).

The results of their "pilot study" was that the managed system used the generator 17% more frequently than the reference system did (same here with prime power). However, the total generator run time was reduced by 38% vs the reference system. Fuel consumption reduced by 25% vs the reference system. Pretty much identical to what I had come up with years before, and have been doing all along. Only difference is that when the government does it they proclaim they just discovered something new!

I fined tuned it because real off-grid homes have peak loads that requires a genset of larger capacity, and prime loads that require a genset of lesser capacity, further increasing the system efficiency over using one genset. The systems they tested had a simple static DC load with a resistor bank, and is not near as complicated as a real off-grid home as far as managing demand from batteries with wildly varying loads. Like I said, they don't have the off-grid experience I got, and IIRC they were testing all this for the USCG.

Chris I am not going to debate the merrits of Sandia, IEEE, or John with you. Every pro in the biz knows who they are and stake their careers on them and their collective research. Kind of like a college student arguing with NASA on rocket engine design. Nor am I going to debate your method with you if you are satisfied with it. I agree with some of your points just not all of them. I am not saying using a generator is a bad thing OK?

My only question, and I hope I read it wrong, is are you saying the XW series inverters sync with a generator? That cannot be and I hope I misunderstood. The XW does have generator input, but does not sync with the generator to my knowledge, it uses the built in AC charger to rectify to DC power. It would be almost impossible to sync a small generator to an inverter. Generators outputs are way to unstable to do that on a small scale. Can be done with Prime Movers like dual 2 mw units, but not 2 kw units. A small change in load will change frequency and voltage by a significant magnitude the inverter could not follow. Basically works like any hybrid EV, UPS, or battery plant.

Yes, the XW syncs its sine waves with the generator on L1 and L2 and keeps synchronized with it all times during operation. If loads are lower than available genset power the genset power is passed thru to loads and the balance is used for battery charging if the charger is enabled. As loads increase the XW infinitely dials the charger down to zero when loads match gen power. When loads exceed gen power the inverter switches to invert mode and begins to assist the generator so the generator remains at maximum output (or whatever you have it set at) and the inverter supplies the difference to the loads

Trace and Xantrex inverters have done this for 20+ years.

Chris I do not think it works like you think it does. True the XW series has generator input, but that input is converted to DC to power the inverter. If the load exceeds what the genny can produce then the batteries make up the missing power up to a limit. That is the basis for all UPS and back up battery systems.

In hybid mode and commercial AC power fails, the batteries take over to run the inverter. When battery gets low, the generator runs to supply the load and recharge the batteries just like any hybrid PHEV works.

No, it is not. When a XW brings a genset online it synchronizes with it during qualification of the generator. This usually takes about 30 seconds if the genset has clean power output. It can take a minute or two on a genset with "dirty" power, a diesel that is cold with the governor surging, etc.. When it makes the switch to genset power it initially very lightly loads the generator, allowing the genset to take some load off the AC output section of the inverter. Then it switches 100% to the genset to accept full load and stops inverting. The transition from inverter to genset power is so seamless that your most sensitive things like CFL's don't even flicker.

It remains sync'd with the genset at all times during operation and can therefore bring the inverting section online at any time to assist the genset with loads. The principle it uses to sync with the generator is identical to parallel stacking inverters, and the XW can be stacked four deep to a 24 kW cluster. Here is a photo of the inside of a XW:



On the left you can see the AC board and there are five relays on it - the XW has dual AC inputs for grid and generator or dual gensets. The four relays in a square pattern are 60A and are for the AC1 L1, AC2 L1, AC1 L2 and AC2 L2 inputs respectively. The transfer relay that connects either AC1 or AC2 to the internal charger is on the left. The main processor board is on the upper right and the DC section and FET board is below it. There is a quite large toroid transformer that weighs approx 60 lbs behind the DC section.

On the upper right of the AC board you can see the transformer leads where they go the connection block. Those can be altered by paralleling the transformer and switch the unit from 120/240V split-phase output to 120V single-phase output. Three XW's can be stacked in 120V single phase configuration to supply 120/208V three-phase power @ 18 kW, and the cluster will sync with a three-phase generator set.

I have worked directly with Schneider engineering on field testing for quite some time, am bound by NDA on some things, and other things like schematics of the unit I hesitate to post lest they fall into the wrong hands. But I assure you genset power is not rectified to DC to power the inverter during load function. The inverter is bi-directional, meaning it can accept AC input and convert it to DC to charge batteries, or seamlessly switch directions and convert DC to AC to power loads, or assist either AC input. Sync'ing sine waves with gensets, that you think is so impossible, is actually a very trivial function.

Edit:
I will add one bit of trivia that few people know about the XW-series; it can be AC coupled with GT inverters or prime and peaking generator sets. This is normally used to build standalone mini-grids for community electrification projects in remote areas.

What model number?

Schneider builds six versions of the XW-series and they all function identically. Model base numbers are XW4024, XW4548 and XW6048. The three models are built in 120/240V split-phase versions for the North American market. The other three are 230V 50Hz output versions for overseas markets.

I've also used the Gen Support, and it is robust enough to sync to the power pulse of a 640RPM genset.

I do it a bit differently than Chris does, I load the snot out of the genset (95%), and what it's not feeding into loads, it's changing the battery with. When the toaster comes on, the charger throttles back, and if the big 240V pump comes, on the battery flow on the Conext diagram, switches from charge to discharge, and the batteries are aiding the genset while the peak load is on. And when the load reduces, the batteries are back to charging. Very fancy (and fast) sensors in the XW that it can track these events and not disqualify a lugged down genset.

And the L1 inputs could do the same thing too, their input voltages and freq are quite variable, but the intent is Line1 = grid, and Line2 = generator [ not the same as Phase 1 & Phase 2 of a split phase system].


My question to Chris is regarding battery sulfation.
Does the irreversible build up of hard sulfate deposit on the cells outweigh the hit in cycle life if you charge off the generator ?

I've seen some diesel sets and LPG fueled Generacs that have governor problems where the XW will struggle with them to get and maintain a "lock" on the sine wave sync. Last fall I worked on a Yanmar diesel generator that the XW would not sync with and qualify it at all. One injection pump on it had a sticky rack so it would tend to "hunt" under no load and you could hear the XW's processor varying the freq by the tone from the transformer and it couldn't quite "grab" it and qualify it. It was varying on freq from 56 to 63 Hz and meanwhile everything in the house that had motors in it was speeding up and slowing down while the XW was trying to get a lock on that renegade generator.

I fixed the bad injection pump and then she grabbed her in 30 seconds.

So there is some limits you have to stay within when you are using a genset on a sync'ing inverter. They won't qualify a really bad one. Inverters like Magnums will because the Magnum just looks at voltage and freq and if it's within allowable limits it flips its transfer switch and simply passes the genset power thru to loads. And whatever happens, happens.

On the sulfation issues, we don't have any. Rolls told me it takes 30 days before sulfate crystals harden to where they are a problem. Steve Higgins (same Steve Higgins that used to work for Outback and now works in Rolls tech support) said as long as you get a full charge once every 30 days you're fine. There's actually a new standard for off-grid battery testing called IEC 61427 that deals with PSOC (Partial State of Charge) cycling. Trojan was the first manufacturer to certify their RE batteries to it.

I guess Rolls is re-writing the the gospel on sulfation, the old version says after 48 hours you start having issues with irreversible deposits. I wonder if they have done any tweaks to the chemistry to change this from 2 days to 30 days - if this applies to all new batteries, or just Rolls and Trojan?

I don't really know, Mike. Steve had mentioned once that one of the things he was hired to do at Rolls was to re-write the battery manual for RE applications. All their charging specs are based on the use of a IUIa profile constant-current industrial charger and not voltage-based RE chargers. Steve said voltage-based chargers are cheap to build compared to constant-current chargers but you have to use higher Absorb V because they're not as efficient as constant-current chargers.

Rolls has sort of re-designed their tech support section of their website and you have to do a search to pull up the different tech bulletins and stuff that they always used to have available as PDF download. So I think it is a work-in-progress. Maybe go to their tech support page and do a search on sulfation or something might pull up some info on it? I haven't tried it, as I always just gotten on the phone and talked to a human there instead.

Well, actually they are claiming 3-4 weeks in their tech support stuff for batteries that get cycled like ours do. I just did a search on it:

Hardened sulfate also forms in a battery that is constantly being cycled in the middle of its capacity range (somewhere between 80% charged and 80% discharged), and is never recharged to 100%. Over time, a portion of the plate's active materials turns into hard sulfate. If the battery is continually cycled in this manner, it will lose more and more of its capacity until it no longer has enough capacity to perform the task for which it was intended. An equalizing charge, applied routinely every three to four weeks, should prevent the sulfate from hardening.


For us, using that 2.58VPC Absorb V totally prevents the problem with partial state-of-charge cycling. I have not run a EQ on our batteries for like 4 1/2 years. It is interested to note that Rolls tends to take a dim view of voltage-based chargers and instead of addressing the problem with them (not using high enough Absorb V), they just describe it. This is called CYA tech support.

In my opinion, Rolls does not address the problem of absorb charging good enough. If you have a 1,200ah battery and 100A charge rate their formula (t = 0.42 x (C/I)) says 5 hours in absorb @ 2.4 VPC. This might work for somebody living in Rio De Janeiro where you get those kinds of hours of sunlight. But it doesn't even come close to working here in the winter time when we're lucky to get 2.5 hours in a day. Therefore, we have to use 2.58 VPC and in the summer we can drop it to 2.50 VPC. The hydrometer tells you whether or not your settings are working with however you cycle your batteries and what you got for charging capacity. Not some chart or generic recommendation. That's why they call them "recommendations". Too many people think the recommendations are absolute law over what the hydrometer says, and a system operated at high latitudes is going to be different than a system operated at low latitudes, simply due to difference in daylight hours.