My grid "offload" system

dr.diesel

I'm beginning to gather my thoughts on my somewhat unusual grid offload system. The primary reasons for this are simply hobby, self interest/reliance, ROI is not of primary concern. I realize most of this could easily be accomplished with a generator and good sized fuel tank, but that's too easy and no fun. I will remain grid tried to some degree because of central AC and ultra heavy shop equipment.

My current load is right at 1KW continuous, already running off a 24V battery bank and grid tied chargers/power supplies.

Thanks to Dan at Wind/Sun for quickly answering my stock/shipping questions. My current plan is for (20) ~250watt panels and (12) Surrette 2430AH batteries. The initial (20) panels is about all I'm comfortable loading my pole barn roof with. (8' center trusses with a ceiling and 2' of blown fiberglass insulation already weighing it down. < how to calculate this with no known build/manufacture info?)

Overall goal would be to add (10) panels of solar generation each year (pole mounted via ground), while adding more DC load at 24V, with a total of probably (40) panels and perhaps one additional string of lead.

Charge controllers will be MPPT. I have not yet picked a specific model, choice will be determined by which ever has the most open data protocol. (I plan to write my own data logging software, no Windows PCs/VMs found on my property!)

Winter and low sunlight days will be supplemented by grid power at first, likely followed by automatic diesel generation. I currently have a 13HP two-cylinder kubota diesel and a 10HP single cylinder lister-petter, just need to finish the build by mounting 24v alternators and control electronics.

I don't plan to include any inverter setup at this time. I guess my point of posting, first to say Hi! And second to ask the following:?

- Any method to estimate my pole barn roof capacity? Without manufacture or build info?
- Any MPPT manufactures with documented serial/IP protocols?

Thanks

Cariboocoot

Re: My grid "offload" system

Welcome to the forum.

Who sold you on 2,430 Amp hours of 24 Volt battery? Someone needs a kicking. That's 29kW hours of stored power. If you really need that kind of capacity go to 48 Volts now, not later.

Twenty 250 Watt panels is 5kW. On 24 Volts that's 160 Amps peak current, and will need two charge controllers of 80 Amps. It is also insufficient for charging 2430 Amp hours of battery (6.5% peak current).

So far the numbers are badly balanced. If you add solar capacity it will be better. If you go up to 48 Volts it will make more sense. If you define how much power you need to supply in the first place you will be starting off on the right foot.

If you don't have any inverter, how are you going to use all that power? You will be logging -zero- on your computer.

To make this set-up practical I'd suggest you look into grid-tie, even with battery back-up. That way all that money in panels will be contributing something no matter what your loads are as any 'surplus' power will get sold back to the grid.

You realize a 13 HP engine will put out about 9kW? That's 400 Amps @ 24 Volts. How are you going to produce/handle that? Much better to use conventional 120/240 VAC generator and utilize the full functionality of an inverter-charger.

No one should hazard a guess on the weight capacity of your barn roof without knowing the structural specifications. And then you need to know the wind loading requirements for your area.

Could you explain what your end goal for this system is? At this point it sounds like a very expensive experiment.

dr.diesel

Re: My grid "offload" system

I plan to use the generated solar power via DC "stuff", I already have 1KW of continuous DC draw today and plan to add more, such as DC frig/freezer and garage/house lighting etc. So right off the bat I'd be logging the generation used to power my current DC load. I don't want to go to 48v cause then I'll have to drop it down to 24v to power what I have already. No reason to invert it to 120AC just to inefficiently convert it back down to 24VDC as most stuff is switch mode buck these days.

So far I've had no advice given, so not completely sold yet. Reason for the huge battery bank is less bank discharge and simple future planning.

I suppose my end goal could be the somewhat classified as the slow offload of 120VAC stuff to 24v DC. If I bought an inverter, the sole reason would be for banking KWs during periods of excess generation, or offsetting home AC etc, Indiana doesn't currently buy back power at any price.

Here is one example of my current DC load.

- Large Storage Area Network computer system that requires 0% downtime. Currently 24VDC powered (95%+ efficiency power supply) vs 75% efficient 120VAC supply coupled with a UPS and separate battery set. (with limited backup duration)

I'd like to take the above and completely remove the grid component.

Cariboocoot

Re: My grid "offload" system

dr.diesel wrote: »

I plan to use the generated solar power via DC "stuff", I already have 1KW of continuous DC draw today and plan to add more, such as DC frig/freezer and garage/house lighting etc. So right off the bat I'd be logging the generation used to power my current DC load. I don't want to go to 48v cause then I'll have to drop it down to 24v to power what I have already. No reason to invert it to 120AC just to inefficiently convert it back down to 24VDC as most stuff is switch mode buck these days.

So far I've had no advice given, so not completely sold yet. Reason for the huge battery bank is less bank discharge and simple future planning.

I suppose my end goal could be the somewhat classified as the slow offload of 120VAC stuff to 24v DC. If I bought an inverter, the sole reason would be for banking KWs during periods of excess generation, or offsetting home AC etc, Indiana doesn't currently buy back power at any price.

Here is one example of my current DC load.

- Large Storage Area Network computer system that requires 0% downtime. Currently 24VDC powered (95%+ efficiency power supply) vs 75% efficient 120VAC supply coupled with a UPS and separate battery set. (with limited backup duration)

I'd like to take the above and completely remove the grid component.

Okay, the problem with DC-only loads is that a DC power system runs in a Voltage range which may not be acceptable to the loads. As in a "24 Volt" system will run from 24 Volts minimum up to perhaps 32 Volts if EQ charging, and about 30 Volts normal charging. Will that be a problem for your DC loads?

Another problem with DC is that it is low Voltage. As such the Voltage drop in wiring over significant distances with heavier current draws is greater. Sometimes it is greater than the loss of powering an inverter and dropping down to lower Voltage again. You may have to use much heavier (and more expensive) wire than you would with a 120 VAC system and loads. If you can keep everything close together this won't be so much of a problem.

As for the huge bank being less discharged, this is not necessarily a good thing. With deep cycle batteries they want to be discharged. An average DOD of 25% works out well for cost/lifespan effectiveness. Discharging them 10% or less is pretty much wasting money on batteries. They are going to die over time anyway, and the time is not much different between 10% DOD and 25% DOD. The time does diminish rapidly when you go over about 30% DOD. That proposed battery bank is much too large for the proposed array in any case.

BTW there's no reason you can't use both DC and AC loads at the same time. For example you can use a small inverter that doesn't draw much power to supply 120 VAC for things like lighting that may be at the end of long wire runs, thus eliminating the extra expense of large wire and special DC lighting while at the same time running DC loads directly to avoid the 'double conversion' losses.

dr.diesel

Re: My grid "offload" system

Yes Sir, all of my current DC stuff has been designed (I'm an EE, majority been designed and built by me) for a max of 34v, hard clamping starts around ~36v. My battery array will be located in the middle of the house, I got lucky here, most runs will be less than 20', the already DC computer stuff is < 10'.

As for the roof loading, I've seen many picts here with 20+ panels, I really wonder if those were properly calculated or simply done via the place and pray method! Heck my house is only 11 years old and it didn't come with any prints to speak of. I suppose I could compare trusses of the same physical build dimensions (top/bottom cord, web etc) and see how the calcs come out.

Cariboocoot

Re: My grid "offload" system

dr.diesel wrote: »

Yes Sir, all of my current DC stuff has been designed (I'm an EE, majority been designed and built by me) for a max of 34v, hard clamping starts around ~36v. My battery array will be located in the middle of the house, I got lucky here, most runs will be less than 20', the already DC computer stuff is < 10'.

Perfect!
Now here's the thing; I can see where the big battery bank comes from based on 1kW steady over 24 hours as that would be 24 kW hours and use roughly 1000 Amp hours on 24 VDC. At that rate the batteries will be discharged near 50%. Not a bad thing in and of itself, but the 5kW array will be insufficient to recharge that much battery from that DOD.

What I would suggest is more like 7500 to 8000 Watts of array. Big batteries like big current. Those Surrettes will probably also want big Voltage, like around 30 for Absorb.

Here's a possible alternative: forklift battery. You can actually take them to a deeper discharge than standard flooded cells and they tend to be slightly less expensive. The down side is that they are huge monoliths and therefor not easy to move about. Here is one example: http://www.solar-electric.com/crinba24vo13.html You might be able to find them locally for a good price, possibly even used (then tend to be pulled from service long before their lifespan is up).

One thing I'm certain of is that for a system of this size you will want MidNite Classic charge controllers. There is no better choice when you have to parallel multiple controllers (you're looking at using three here I think), as they have a "follow me" function where one acts as master setting Voltage and charge stage and the others simply add their current. I think you will find them best for open protocol and logging too. Plus the guys who build them are very good with customer service and quite willing to help with new project ideas. http://www.solar-electric.com/misocl.html

As for the roof loading, I've seen many picts here with 20+ panels, I really wonder if those were properly calculated or simply done via the place and pray method! Heck my house is only 11 years old and it didn't come with any prints to speak of. I suppose I could compare trusses of the same physical build dimensions (top/bottom cord, web etc) and see how the calcs come out.

I've seen some pretty awful roof structures in my day that 'passed' inspection. Including dumb things like 3/8 OSB for sheathing. Gee, the snow is only 24" deep what could possibly go wrong, eh?

The good news is that with a pole barn you can see what's there and beef it up as needed. You can calculated the panel loading based on the weight of the panel * the number of panels / square footage. Then remember that the weight is concentrated at the point where the mounts attach. Sometimes it's as easy as running two-by between trusses to give a solid bolt foundation and spread the weight out between the uprights (redistribution). As always, check with the local AHJ because they have the final say (rhyme unintentional).

BB.

Re: My grid "offload" system

I hope you are not "clamping" battery bank voltage (i.e., output clamp/shunt/crowbar on a charger output)... That is common over current protection for a DC power supply (to computer/electronics) to prevent too high of output voltage (controller failure or similar).

Clamping (crowbar'ing) a large lead acid battery bank--That would be a sight to behold.

-Bill

dr.diesel

Re: My grid "offload" system

Cariboocoot wrote: »

Perfect!
What I would suggest is more like 7500 to 8000 Watts of array. Big batteries like big current. Those Surrettes will probably also want big Voltage, like around 30 for Absorb.

Yes, and my apologies, in my profession proper communication is often lacking! The main reason for the (20) panel starting point is that's all I'm likely to have time for this season. In addition to that is my quite serious character flaw, I'm incapable of hiring help, I do 100% of everything myself.

I will research the batteries and the Midnight controllers, if I can talk to them then no problem!

BB. wrote: »

I hope you are not "clamping" battery bank voltage (i.e., output clamp/shunt/crowbar on a charger output)... That is common over current protection for a DC power supply (to computer/electronics) to prevent too high of output voltage (controller failure or similar).

Clamping (crowbar'ing) a large lead acid battery bank--That would be a sight to behold.

-Bill

Most of the wiring for each branch is already completed, and properly fused at the appropriate amperage.

Cariboocoot

Re: My grid "offload" system

dr.diesel wrote: »

Yes, and my apologies, in my profession proper communication is often lacking! The main reason for the (20) panel starting point is that's all I'm likely to have time for this season. In addition to that is my quite serious character flaw, I'm incapable of hiring help, I do 100% of everything myself.

The problem is if the batteries don't get enough charging from the start they end up dead much quicker.

I was thinking that the 5kW array could produce 160 Amps @ 24 Volts, enough for a large forklift battery (1500 Amp hours for example) that could be discharged up to 80% which would provide the power needed. The FL battery would be about half the cost of the Surrettes.

The major problem is that a 5kW array isn't going to 'harvest' 24kW hours in a reasonable amount of time (hours of good daylight). Most of the power will need to be stored in the batteries and released, and that means a very large array indeed. If you have viable other charge sources that will help. But from pure solar your up against a few obstacles, not the least of which is that even when it's Summer the sun doesn't always shine.

dr.diesel

Re: My grid "offload" system

Yes, much agreed. At first the bank will be tied to the current 1KW grid tied charger already in place, basically floating providing nothing when the sun's out. In the near future the battery bank would be tied to not only the grid charger but also the diesel generator. (which I need to get busy finishing)

zoneblue

Re: My grid "offload" system

dr.diesel wrote: »

My current load is right at 1KW continuous, already running off a 24V battery bank and grid tied chargers/power supplies.

Out of curiosity what's drawing 1kW 24/7 ? Around here im known as Mr DC, but nothing i have draws that sort of power. Every DC branch circuit has a 5amp CBI breaker on it. I run 4 computers, refridgeration, water pumps ,a range of DC electronics and all our lighting.

My current plan is for (20) ~250watt panels and (12) Surrette 2430AH batteries.

Have you done a load budget?

westbranch

Re: My grid "offload" system

dr.diesel wrote: »

Here is one example of my current DC load.

- Large Storage Area Network computer system that requires 0% downtime. Currently 24VDC powered (95%+ efficiency power supply) vs 75% efficient 120VAC supply coupled with a UPS and separate battery set. (with limited backup duration)

I'd like to take the above and completely remove the grid component.

don't know what an LSAN is but it sounds BIG...

BB.

Re: My grid "offload" system

Sounds like a several servers and disk arrays. Does not have to be physically large to consume that much power (if using high speed drives/fast processors/lots of caching of data accesses). One 6' x 19" rack could probably be 1 kWatt of power pretty easily.

-Bill

jcheil

Re: My grid "offload" system

BB. wrote: »

Sounds like a several servers and disk arrays. Does not have to be physically large to consume that much power (if using high speed drives/fast processors/lots of caching of data accesses). One 6' x 19" rack could probably be 1 kWatt of power pretty easily.

-Bill

The cooling requirements for that type of equipment usually consumes more power than the equipment itself.

dr.diesel

Re: My grid "offload" system

Yes, the majority of the DC stuff I want to move off the grid is computer equipment, around 100TB worth in just one box!

As for a load calc, if I did it all I'd need about 150kw/day! My plan is to slowly move equipment off as I add/build up my solar array.

These cases are custom built my me, lots of area around each disk, slow fans just to move stagnant air away.

zoneblue

Re: My grid "offload" system

dr.diesel wrote: »

Yes, the majority of the DC stuff I want to move off the grid is computer equipment, around 100TB worth in just one box!

As for a load calc, if I did it all I'd need about 150kw/day! My plan is to slowly move equipment off as I add/build up my solar array.

These cases are custom built my me, lots of area around each disk, slow fans just to move stagnant air away.

Is this for your own use or youre providing some sort of backup storage for a business? That sort of thing more often found in a datacenter where the bandwidth is to be had. Unless youre building the worlds first solar powered datacenter.

150kW/day. Hmm. Gona be a big array then! Any way you can use more power efficient gear?
If thats just for hobby, she'll be an expesnive hobby.

jcheil

Re: My grid "offload" system

If this is all just for you (the data storage) then I say go for it. However, if you are hosting anything for anyone else, I would have a concern.
Companies host off-site for security and guaranteed up-time reasons.

There is no guarantee with solar because you never know how much the sun is going to shine. So, I would hope that you plan to have a LARGE backup generator, and then a backup to THAT one when you finally do go "off-grid" otherwise you will have some very unhappy customers! If I knew my data was being hosted at a facility that DEPENDED on solar power entirely, I'll pull my data in a heartbeat.

dr.diesel

Re: My grid "offload" system

Yeah, I'll never get it all off-loaded, simply too much consumption. All of this stuff is personal, but it's still quite serious. I have an expert that is supposed to stop by soon, hopefully he can help with my roof loading calcs, I don't plan to purchase anything till I get that part figured out.

I've also been reading the Midnight forums, I think that suggestion was absolutely correct, Midnight has their act together and serial data is available!

mike95490

Re: My grid "offload" system

dr.diesel wrote: »

.... quite serious. I have an expert that is supposed to stop by soon, hopefully he can help with my roof loading calcs, ......

And don't forget the floor load (PSF) calcs for when all the drives are full, and the servers are at full speed. You don't want the floor to cave in if the bit bucket drain clogs up while you are away (never clogs when someone is home)