Sizing Off-Grid Systems And Using Gen Support

[ChrisOlson]

For us, it depends on how deep they've been cycled. If they've been pulled below 50% DoD it can take two days to get a full recharge. On the first day it gets them up to absorb and absorbs for awhile and then the sun runs out for the day. They start the next day at a higher SOC and can usually finish charging. I'd say, sometimes, total 7-8 hours spent in absorb, but not all at the same time, to bring them from 70% DoD to 100% charged.

If they're shallow cycled, like on several nice sunny days in a row, it only takes 1-2 hours usually. The system exits the absorb as soon as the battery current gets to 2% of C/20.

Your 1.260 SG is not low. 1.255-1.275 is the normal full charge SG range for 4000-series.

[paulcheung]

Thank you, another question, how long your battery get full charge from absorb start? According to the manual my 4000 series Rolls should take 4 to 4.5 hours to absorb charge. how long yours take? I am asking because it seem mine never get fully charged even I use the grid to charge it for over 6 hours. The SG reading never past 1.260.

Thanks again.

[ChrisOlson]

paulcheung - they actually get fully charged about 10 times a month. The solar panels charge them up with no problem if we get a couple sunny days. What we do different than most folks is that when poor solar conditions set in for a long period of time we do not run our batteries down and then recharge them with a genset. We will only do that if the poor conditions last for 10 days or more and they get down to 80% DoD. When that happens the big genset automatically starts and bulk charges them, but the system stops it when they reach absorb voltage.

We manage how fast they go down during these periods by running our little generator periodically to take the load off them and make them last until the next sunny day.

As I stated originally, this is a very, very hard thing for people used to the "conventional" way of doing it to understand. But it has to do with efficiency and energy conversions. Our loads are AC. We run a little generator at peak efficiency directly powering AC loads. This is much more efficient than running a large generator supplying AC to an inverter that converts it to DC to charge batteries, then the inverter converts the DC back to AC again before it gets to the loads. When you charge batteries with a generator to power loads on a AC system, you lose in the range of 30-40% of the energy you produced with your generator in conversion losses.

The key to making all this work is an inverter with generator support. An inverter with generator support simply kicks in and limits the load on the generator to it's max rating and helps it out while the heavy load is on. So the key to getting peak efficiency from your generator is to run it at 80% rated load or better all the time it is running, and only use it to power AC loads. This requires a generator that is smaller than your base loads, and a regulating system (inverter with generator support) that can fill in the difference.

It was actually Heart Ackerman and Trace Engineering that invented this concept 20+ years ago with the Trace SW. And the Trace SW-series became legendary in off-grid power. Trace was eventually bought by Xantrex in Canada. Xantrex enhanced the SW and it became the SW Plus, which was even better than the SW Series II. Meanwhile, there was lots of contention between the guys at Xantrex and the guys that formed Trace. Those guys left and formed Outback Power. They invented the Outback FX-series, which again became legendary in off-grid power. However, they were competing against Xantrex and the Outback FX was scaled to meet a price point. Generator support made it into the menus in the Mate but it never made it into the hardware because it became a thing where whoever can sell the most inverters wins. The off-grid market was getting pretty slim and the Outback FX was a $1,800 inverter while the SW Plus was a $3,800 inverter. So the Outback is missing things like integrated generator controllers and generator support, but lots of people buy them because they're reliable and much cheaper, and most off-grid systems are quite small.

In the meantime, Xantrex designed and built a completely new split-phase hybrid inverter in 2005 - and the XW-series was born. The market had gone primarily away from off-grid by that point and more to grid-tie. The XW could do both with split-phase power that did not require using transformers or stacking inverters on split-phase grid-tie systems. The XW literally kicked Outback's butt because they didn't have anything that could compete with it. Outback went broke and was eventually bought by the Alpha Group who owns it today.

Meanwhile, Schneider Electric, one of largest manufacturers of industrial control systems and electrical equipment on earth (Schneider is the maker of SquareD equipment) decided to get into the renewable energy business in 2009. They bought Xantrex to get their technology and expertise in inverters and solar power equipment. Today the Xantrex brand is concentrated on the RV, marine and mobile markets. The XW-series is now being rebranded into the Conext brand. The Conext inverters range from the SW-series (small brother to the XW) in battery based, to the 1,250 kW Core XC used in solar power generating plants.

So that is how it has progressed over the years, and to this day there is just a handful of inverters capable of doing generator support for off-grid. You can just about count the inverters that can do it with the fingers on one hand.

[paulcheung]

Good morning Chris,

You said you don't do EQ on your batteries, can you tell me how do you keep your batteries full charge at least once a month?

[Naptown]

great now I get to relearn everything i thought I knew
Thank you guys (no sarcasm intended)

[ChrisOlson]

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.

http://support.rollsbattery.com/support/solutions/articles/5964-battery

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.

[ChrisOlson]

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.

[Mike90250]

...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. ....

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?

[ChrisOlson]

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.

[Mike90250]

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 ?

[ChrisOlson]

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.

[Sunking]

What model number?

[ChrisOlson]

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.

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.

[Sunking]

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.

[ChrisOlson]

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.

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.