AC/DC micro grid design questions

BB.
BB. Super Moderators, Administrators Posts: 33,431 admin
This discussion was created from comments split from: 120VDC battery bank ?.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset

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  • TexasGEEK
    TexasGEEK Registered Users Posts: 6 ✭✭
    edited July 2016 #2
    I am in the process of setting my sights on a AC/DC microgrid for homestead and shop in a rural setting. I read some of the Arcing comments and about 120Vdc battery banks. My perplexing issue is that I have a surplus Eaton/Best 10Kva online ups 220v/240v 1ph which happens to run off a 120Vdc battery bank. I had orginally thought to use that NOT off my AC mains, but to run the unit in DC ups mode as my primary supply. It's built to run 24/7 not like standby UPS units that would overheat in full time use. What I was hoping is to extend the battery bank for longer run times, use wind and solar to charge.

    However 120Vdc chargers (3-5 stage are non existant) plus many common solar and wind systems only cross at the 24V-48Vdc range. Using 12Vdc or 24Vdc is just too much heavy wiring for the DC grid. But at 48Vdc there is a big savings.

    The DC microgrid was primarily to power lighting, ventilation and small DC electronics with DC-DC converters instead of heat generating inefficiencies of standard 3 prong AC/DC supplies. It may also run a DC air conditioner or freezer/fridge. No double conversion losses either this way, boosted overall efficiency, lower power consumption.

    Now what to do about Codes, NEC and Arc issues, specing custom DC-DC wall warts or replacing power supplies and planning for safety and sanity in a Hybrid world? Insights? Cautions?

    What happens to normally 3 prong grounded AC/DC chassis items in Direct DC supplied world?

    I do Arc weld so yes I have seen what my welder @17Vdc at @80-120A can do with Stick, Mig and Tig.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    TexasGeek,

    I have moved your questions to your own thread... Should make the Q&A less confusing as we focus just on your questions.

    If you are working with "modern" (Energy Star Rated) appliances--The AC versions can be almost as efficient as the DC versions--Plus you can get 120 VAC (or 240 VAC, if needed) standard devices/appliances and wire them up just like normal behind a standard off grid AC inverter...

    In theory, the inverter will be about 85% efficient--And DC to DC converters are about 85-95% efficient themselves--So you are not really saving much.

    Using a UPS type inverter with 120+ VDC battery bank--Can be done, but look into the details of the efficiency of the UPS. Frequently they are not that efficient and many are designed for running for a 1/2 hour until the genset kicks in (heat, efficiency, ability to charge deep cycle batteries, etc. are all questions to be addressed).

    You can get nice off grid AC inverter (and those that have integrated backup genset controllers, and some very nice other features)--But they are not cheap.

    If you are already running an AC inverter--Not sure why one would go to the trouble of DC to DC converters/DC devices unless DC appliance(s) was all you could purchase for your needs.

    My first response anyway.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TexasGEEK
    TexasGEEK Registered Users Posts: 6 ✭✭
    I was just thinking the DC microgrid would save 50-60% efficiency if I ran some electronics off straight DC with minimal changes to supply 5V-9V-12V-24V-48V loads. Most AC use in the home/workplace today is used for tech and other electronics; Computers, TVs, Radios, Alarms, Phone, Internet, Wifi, Appliances. It used to be refrigeration and cooling/heating.

    Otherwise it's DC-AC, then AC-DC (20-40% loss) at each device conversion vs. just a voltage regulator at the load side. How is that not more efficient?

    I already have the surplus online UPS, but yes efficiency might be poor vs. newer inverter units. The 10Kw unit has ~800w overhead sitting idle. At output likely closer to 8Kva. Still that's a very expensive newer inverter to buy or many smaller bridged units to replace it. Most don't really put out a full real load that is Sine wave clean at their ratings or they OVERHEAT and fail. Many cheap AC inverters are in this boat. The online UPS is designed to run 24/7 weighing in at over 298Kg (600lbs+).
  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    I can see running a few things off of a DC-DC converter instead of DC->120VAC->DC and saving maybe 15%.   Laptop, wifi router, etc.  LEDs are most efficiently run directly from battery voltage to a current limited DC-DC controller (> 100 lumens/watt is doable).

    I am available for custom hardware/firmware development

  • TexasGEEK
    TexasGEEK Registered Users Posts: 6 ✭✭
    edited July 2016 #6
    Only 15% savings... I didn't think typical wall warts and electronics with space heater AC supply modules were near that efficient and if these could be bypassed I thought the savings would be much higher.

    Add to that new construction and solar water heating and solar assised A/C cooling, then the total loads could be cut way down.

    The only issue on the DC grid side is distribution and at what voltage 12V, 24V or 48V. What NEC socket type to use at the wall socket. Some DC ground fault engineering with a parallel tied DC breaker to trip on a low current the much larger main load line at the DC main breaker panel.

    After some reading, the 12V cig plugs are crap, I'm talking about a higher DC voltage, some leeway is given for using 220/240 euro plugs for various DC voltage levels if no AC is supplied at that voltage on site (not a local standard), or is it by location (a bit fuzzy as worded)?

    Of course lighting and some other loads could be direct wired with conduits (no need for receptacles). But I want most the small AC devices to convert over to DC direct bypassing the AC cords and efficiency losses an drawing a lot less of a load.

    I will need to run some tests to compare AC plugged power supplies and DC voltage regulator alternates for total device draw and consumption. My idea is both using less power and generating less ambient heat is a big win.
  • TexasGEEK
    TexasGEEK Registered Users Posts: 6 ✭✭
    Here is a good video that illustrates Arc over with DC at the higher voltages AND currents.

    https://youtu.be/Zez2r1RPpWY

    At what voltage should we concern ourselves in a DC system; 370V, 220V, 110V, 48V, 24V...or even 12V?

    And aside from just voltage should we adopt a different type of circuit loading mentality for max amps in a current loop. Its not just about the voltage.

    My arc welder is 17Vdc @ upto 140A (Mig, Tig, Stick) just for reference. But below a certain amp level its darn near impossible to maintain an arc, or strike one for that matter.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    About 12-15 VDC is the minimum arcing voltage (exact number depends on specific metal). Current wise--You can probably see that with your welder pretty effectively.

    Remember that DC circuits do not have a current limiting device, unlike AC (final distribution transformers are would to limit to 10,000 Amps maximum for home/small business). DC will sustain to the maximum of the battery bank's capabilities (lead acid--100's to 1,000's of amps) and the resistance of the cabling.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TexasGEEK
    TexasGEEK Registered Users Posts: 6 ✭✭
    edited July 2016 #9
    I guess maybe its time to take a step back and re-look at a stradegy. Asking for suggestions. The big issue is a mismatch between 48Vdc and 120Vdc in terms of hardware platforms.

    I try to aquire surplus when available in hopes of meeting the offgrid (& maybe ongrid of selling back) objectives of energy independence of living comfortably within my self sufficient means.

    1. 24Vdc, 48Vdc or 120Vdc microgrid?

    Why these choices?

    A.

    I have an Eaton/Best 10kW/8kVa online UPS 220/240 1ph uses 120Vdc battery bank. With big enough battery storage extension it can run continuously when other inverters would drop or overheat. 2'x4'x3' @ 185kg (400lbs+) it is made to be run continously under load.

    Hoping to use this as my AC microgrid inverter. Easily a one stop inverter for all my loads. With external battery cabinet (sans batteries currently) and AC transfer switch. Plays nice with all load types and inductive motors. Pure computer grade Sine wave power. But with ~800W overhead. Otherwise I would have to replace it with one or multiple bridged inverters to bank enough watts.

    B.

    Alcatel/Eltek M4NA 48Vdc 8kVa DC power distribution and battery unit. Normally used with DC rectifiers for the telecomm industry but could be reconfigured for Solar/Wind power and DC distribution. 48Vdc is good value for both solar and wind generation. 3'x4'x5' @ 285Kg(600lbs+)

    Hoping to use this as my starter DC microgrid basis and build it out. As configured its all breakered out with a DIN rail, has a sealed neg airflow IP67 enclosure with ventilation and heater, battery shelf and usage monitoring CAN bus controller and temp sensors but sans batteries. Accepts 220/240Vac 1ph grid power like the UPS.

    C.

    I'm still looking for panels, wind turbine, charge conroller, batteries.

    I am considering GC2 208ah 6v as my first battery SLA as these are more forgiving and I'm likely to make mistakes tweeking and configuring as I go. Ruining a set of more expensive SGMA batteries first time out is hard to swallow repeatedly while the lead acid ones can be recovered to use another day.

    This is a video of another Eaton/Best UPS (same unit) in the Virgin Islands that uses a ton of 2Vdc cells to get really long run times and is charged by solar. It was the inspiration to repurpose an older UPS for full time use for my own system.

    https://youtu.be/x7cSiiu8fI4

    I plan to build my AC/DC grid system into a 10' or 20' cargo container and place out back of the house/shop. That way if I ever relocate I can pickup my power system and go.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    You really have not told us about your power/energy needs in detail...

    Just to give you an idea, around 40 Amp*hours @ 120 VDC battery bank per 1kWatt of inverter... For your 8 kW inverter, you would need about 320 AH @ 120 VDC for a flooded cell lead acid deep cycle battery bank to support 8,000 Watt load for a few hours (4-5 hours would take that bank dead--Something you really do not want to do with lead acid batteries--Taking a bank dead, or less than 20% state of charge, will usually ruin the batteries).

    A 400 AH @ 120 VDC battery bank would take 20 golf cart batteries in series (6 volt @ 200 AH), and two parallel strings--Or 40 G.C. Batteries. Not a small battery bank (close to $4,000 in "cheap batteries" to run your inverter at full capacity).

    Once you have a system large enough to justify running any AC inverter 24x7--Generally, you are better off finding/buying Energy Star rated devices to run at 120/240 VAC.

    The problem with running 120 VDC directly from a battery bank, is that the bank is probably capable of nearly 1,000 amps into a dead short. And you need fuses/breakers rated for >1,000 Amps AIC (amp interupt current) at >120 VDC--Certainly these devices are available, but not something you get from the local hardware store.

    Also, with DC systems, you lose the very nice GFI (Ground Fault Interruption) breakers that are so cheap. They limit the 120 VAC leakage current to ~0.005 amps maximum--Very nice for use around water/sinks/outside.

    There are, sort of, DC versions of this equipment, but their limit is something like 2 amps of leakage current:

    http://www.morningstarcorp.com/products/ground-fault-protection-device/

    There is no easy way to limit current and provide ground fault safety with high voltage DC circuits. Not sure there are very many DC Arc Fault breakers out there either (Midnite may have, someday, True arc fault detection/faulting. It presently is built into some of their solar charge controllers).

    In the end, unless you have very specific 120 VDC needs (or even 12/24/48 VDC)--I would not design the battery bank base voltage to support those needs. You are, sort of, reinventing the DC Power distribution system again. Can be done, but usually is not really required these days. Many of the DC items (fridge, electronics, etc.) that used to run more efficiently on DC power are now available in AC versions that run, very closely, as efficiently on 120 VAC--Without the nightmares of large DC power distribution.

    https://en.wikipedia.org/wiki/War_of_Currents

    There are, even today, some very neat uses of DC for power distribution (even over 100's of miles or more)--But for the "average user"--Not really needed these days. If you are concerned about the 85% efficient AC inverter--Just add 1/0.85=1.25x more solar panels+battery capacity.

    For small to medium systems, the tare losses for an AC inverter should run around 6-20-40 watts. And can run 90-95% efficient under "optimum" conditions.

    Any boost type switching DC to DC converter is only going to be slightly more efficient than the "optimum" AC inverter equivalent. And beware, a 120 VDC battery bank can easily run from 105 VDC to 150 VDC or a bit more--That is a very wide range to run a "native" 120 VDC nominal device with.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • TexasGEEK
    TexasGEEK Registered Users Posts: 6 ✭✭
    edited July 2016 #11
    OK...here goes...

    I currently have a 2000 sqft home in town on the grid no renewables. My overall loads are (from bill actual smart metering):

    High Jul/Aug @ 1200/1280 kwh mo
    40/43 kwh day

    Low Apr/May @ 600/700 kwh mo
    20/23 kwh day

    Moving to a rural setting and either staying at 2000 sqft or down sizing to 1500 sqft. Heavy use of LED lighting, solar water heater, solar assisted A/C, passive air flows, using concrete or earth derm construction, with wine/root/tornado lower cellar, gas cooking, so basically higher efficiency construction improvements overall. Also a solar well pump, other small solar standalone item for gate/landscape and path/road lighting not included in figures below.  

    Now this is all hypothetical, but a best guess for planning at this point.

    A/C 4 ton*      est. 4.7kwh
    Refrigeration. est. 1.2kwh
    TV/Audio.       est. 0.6kwh
    Computer(s)   est. 1.2kwh
    Lighting          est.  0.5kwh
    Security          est.  0.2kwh
    Misc               est.  0.2kwh

    Total Use        est. 8.6kwh daily

    *may change to inverter A/C or split system use. Also replacement of 40w-65w fans with DC versions. Replacing destop pc's with laptops going DC for additional savings. Still have ocassional laser printer(s) but intermittent. I could have easily forgot to count a category or under estimate some. But based on improvements I'm hoping that I am being realistic.

    Considering dropping about 6 standby UPS units in use now if generating my own power These now present a constant charging load to maintain for only short term transit protection.

    Goals:

    Self Sufficiency (or shtf independence)
    Better stability than rural AC
    Not suspect to others sags and spikes
    Improved appliance lifespan
    Known power cost vs. variable PUC
    No transport costs, production fees, etc.

    Optional:

    Ability to sell back to grid
    Assist in rural stability of grid

    I expect buying in now, in the long term I will be better off that continued rate increases. Higher urban population growth and energy demands in Texas in next 2-5-10 years along with increased likelyhood of rolling (planned or demand) outages in peak excess times like in other parts of the country (ERCOT grid is old) I expect my costs to exceed $10K-15K but not a turnkey of $40K from Solar City or someone else. I plan to pay up front and take a tax incentive while still available.