Off grid with DC?

garynappigarynappi Registered Users Posts: 37 ✭✭
I'm not by any stretch thinking of going off grid, my system is purely for aesthetic / power logistics reasons, those are home exterior lighting and water fountain pumps. But, aside from those uses I provide DC to my home alarm system, and video surveillance system in lieu of using A/C for power and a UPS for backup.

Off grid users like those who spend a lot of time offshore in larger private vessels have moderate sized refrigerators, lighting, and even 12 / 24 volt DC air conditioners with PV and wind powering their system. 

I get that DC has inherent losses in power distribution but I'm thinking these would be minimal due to the short cable lengths involved.

That said, I'm wondering if folks very far from power have implemented a true DC off grid system?





Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,508 admin
    DC LED lighting--There are lamps/fixtures that will run from 9 volts to >32 VDC (internal switching power supply). Good for DC systems

    DC refrigerators--Use a "DC compressor" (Danfoss or similar)... This really have a 3 phase internal VFD (variable frequency drive). Will run from 12-24 VDC nominal just fine, and (depending on model?) will run from a solar array directly (cool during day, insulated box at night).

    There are DC only well pump motors--Some are brush motors (universal motors). Others have a VFD somewhere in the system (in the pump, well head electronics, etc.). Note that the larger pumps (like Grundfos), may need a 48 VDC input voltage, and even higher voltage to run at full power... Pumps are nice, some are poly power source capable (Battery bank, solar array, AC 120-240 VAC nominal).

    While you can find a bunch of 12 VDC powered apliances--And some 24 VDC (marine, trucks)--Please be aways that DC FLA battery banks have a very wide power acceptance range--From 10.5 volts (full load, near dead) to >16.0 volts (charging/equalizing in cold climates). Many 12 VDC devices are more designed for working off a car battery system which is something like 12.5 to 14.4 volts or so... And may not be happy with 10.5-16+ VDC power.

    And while you can use DC to DC converters to take the "wide range" VDC battery bus input and output the 13.8 volts "ideal" voltage (many HAM radios are rated at 13.8 volts minimum for full/optimum transmit power). --- You are almost just substituting a 120 VAC inverter with a 12 to 12 VDC switching power supply. Similar costs and reliability (both are switching power supplies, and there are a wide range of prices and quality)--You are still "stuck" with the "problem" of 10x the current for a 12 VDC circuit vs the same power for a 120 VDC circuit (power = Voltage * Current).

    For example, wiring up a 12 VDC large FLA battery bank--You are really looking at pulling a maximum of around 1,200 to 1,800 Watts (100 to 150 Amps) with a short/heavy copper bus. Say you design for a 0.5 volt wiring drop (a good starting point for 12 VDC power systems). Using a voltage drop calculator:
    https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.2557&voltage=12&phase=dc&noofconductor=1&distance=30&distanceunit=feet&amperes=100&x=47&y=21

    100 amps @ 30 feet for 0.47 volt drop requires 2/0 copper cabling. (1,200 Watts)

    To send 120 VAC 30 feet with 3% voltage drop (3.6 volt drop) only needs:
    https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=120&phase=ac&noofconductor=1&distance=70&distanceunit=feet&amperes=10&x=0&y=0

    14 AWG to send the power 70 Feet (again 1,200 Watts).

    Add that 12 VDC standard power is the "cigarette lighter plug" (I hate those), or you have to convert to your own. HAMs like the 
    https://powerwerx.com/anderson-power-powerpole-sb-connectors
    https://powerwerx.com/powerpole-power-distribution (power distribution systems)

    And that a 10 Amp 120 VAC plug and socket is much cheaper than a 100 Amp power and socket set... It really depends on what you need the power for and how much you need.

    In times past, 120 VAC appliances were very energy inefficient because nobody really cared. Today's AC appliances are (many times) just about as efficient as their DC counterparts, and a whole bunch cheaper too (120 VAC vs a 12/24 VDC refrigerator, and most 120 VAC will be self defrosting, vs the DC appliances).

    In the end, I always suggest getting the loads together and specified you want to run... And do paper designs with AC and DC versions of these same loads (and make sure the DC loads will operate correctly on a deep cycle battery bus with wide voltage ranges)...

    Many of these new fangled appliances (aka laptop computers, cell phones, TVs, etc.) run on DC voltage--But it may run from 5 VDC to 19 VDC (USB systems), or 12 volts +/- 10%, or other voltages (9 volt, 6 volt, 20 volt DC for battery chargers and other electronics)... You end up needing some form of adapters anyway (120/240 VAC to DC are easier to find and typical more rugged than 12 VDC or 24 VDC input adapters). And with 12 VDC--You are left with Cigarette plugs for many (or as a kid, what size coins fit in that round hole?).

    See what works out for the best.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,241 ✭✭✭✭
    WhenI transitioned from gennypower to battery power, we were deep into 12 vdc...lights, fans, water pump, fridge igniter, radio etc.  but what happens, as solar is so cheap, load always grow, and 120 vac is a good option.  A small inverter, like the Suresine 300 ,makes it a no brainer.  The simple availability of 120 vac stuff makes it a no brainer.  We still don’t use a lot of power by any normal standard, and still run 12 volt legacy stuff, like the pump and the radio on 12 vdc, but most everything else is 120 vac.  In the net it is just much simpler.  

    My only rub is I would like to have a 24 vdc battery bank, but my legacy infrastructure makes that problematic.  Additionally, as you said, 12 vdc stuff is ubiquitous, especially from the RV and marine industries.

    Tony
  • mvasmvas Registered Users Posts: 338 ✭✭✭
    garynappi said:
    I get that DC has inherent losses in power distribution but I'm thinking these would be minimal due to the short cable lengths involved.

    DC does not have inherent losses in power distribution.
    Whether AC or DC voltage, too many amps over a long enough distance will have unacceptable losses.
    In fact, DC has lower losses than AC and that is why we now have High Voltage DC transmission lines in the USA.
  • garynappigarynappi Registered Users Posts: 37 ✭✭
    Agreed on the cig lighter apliance coupling...

    I use 10 gauge SAE flange connectors on the "dashboard" I made for my tin boat, even my Minn Kota trolling motor came with a 10ga. SAE connector which is fused at 40a They can carry a lot more (double to triple or more) current than the cigarette lighter's 10 amps. The trolling motor SAE connectors go to battery switches while everything else goes to the panel.

    Here's my instrument panel, it's mostly very low current. I made it removable since I can take it to every boat I own as I trade it for another model.







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