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Off-grid charging with solar panels?

Discussion in 'Model S: Battery & Charging' started by commasign, Feb 21, 2015.

  1. commasign

    commasign Active Member

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    Has anyone figured out a way to charge off-grid with solar panels? I'm thinking of natural disaster situations where there might be extended regional power outage. I have a grid-connected solar system (14kW Sunpower).
     
  2. andydoty

    andydoty Member

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    You would need an off-grid or grid-interactive system such as an Outback Radian or Flex (in my case). This also requires to have batteries as a storage medium.

    I don't think you would want to have a system that didn't have a buffer of some sort. The Tesla gets pretty angry when it doesn't have clean power. Most standard solar inverters (that aren't grid-interactive) require commercial power for a buffer (the grid). Pumping DC may be an option however the voltages need to be VERY high and controlled (possibly with a high voltage charge controller).
     
  3. commasign

    commasign Active Member

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    Are there other ways to charge the car when the power is out? I guess ask a bunch of people to push it and let the regen charge it up? ;)
     
  4. rlang59

    rlang59 Member

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    Delorean_(10).jpg
     
  5. commasign

    commasign Active Member

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  6. jgs

    jgs Member

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    #6 jgs, Feb 21, 2015
    Last edited: Feb 21, 2015
    There's a market opportunity here for a way to hook one of the wheels to a stairmaster or other exercise machine. You might have to gang a few machines, mind you. For some value of "a few".
     
  7. commasign

    commasign Active Member

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    At the minimum, Tesla should build in a hand crank or several.
     
  8. andydoty

    andydoty Member

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    Not without a grid-interactive system. You need a storage medium (the grid in the case of grid-tied systems). A grid-tied system works by pushing more power back then the house requires to operate. Think of it this way, the power coming from the outlet oscillates 60 times per second. In simple terms, when the left prong is positive the right prong is negative and they switch back and forth. When you're making power and the left prong is positive (power flowing from the outlet), the solar inverter pushes MORE power on the negative pin. This causes your meter to spin in reverse. Without the power flowing INTO the inverter there's no way for the inverter to push power back. In the case of a grid-interactive or off-grid system, power can be stored in the batteries as a buffer. Also, off-grid or grid-tied inverters will produce only the power necessary to run the household critical load and/or keep the batteries charged.

    When the sun ducks behind a cloud and you're off-grid, the output from the panels plummets and the batteries kick in to subsidize the power that's not being produced by the panels to power the critical loads. DC charging for us is probably not going to happen due to the cost of the equipment necessary to produce the output. There's all sorts of battery charging stages and math that will make your head spin. I just refer to it as magic and let the system do what it does best (although I do tweak it on occasion). Most off/grid-tied inverters have a 12 to 48 volt back plane with 48 volts being the most deployed in homes. 48 volts is definitely way too low to charge the Tesla as it requires around 400 volts on average and that needs to be strictly controlled for the various stages of battery charging (bulk, absorb, float, etc.).

    That's not saying that the charge controllers in the Tesla couldn't do it. The hard part is taking 48 volts DC and converting it to 400. Again, in simple terms, when charge controllers convert power, if you increase the voltage out, it requires a great deal of amperage. Case in point, when my panels are producing 100 volts of DC @ 50 amps, the reduction down to 53.6 volts produces 75 to 80 amps at that voltage (not exact, just for informational purposes). This works in reverse as well.

    It's a tough thing to engineer. Life would be great if we could keep the DC coming out of the panels DC throughout the process as there is minimal loss. Most off/tied systems have around a 78% efficiency once the process is completed. It kind of doesn't make sense, we go from DC to AC to AC to DC...
     
  9. Evbwcaer

    Evbwcaer Member

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    You could use a generator. People seem to really like Honda's products.
     
  10. tga

    tga Active Member

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    #10 tga, Feb 21, 2015
    Last edited by a moderator: Mar 13, 2016
    Tow charging a Leaf:

     
  11. wk057

    wk057 Senior Tinkerer

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  12. Coiled

    Coiled Member

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    #12 Coiled, Mar 13, 2015
    Last edited: Mar 13, 2015
    Should be possible with a big battery bank and a 240V pure sine wave inverter like this: http://www.amazon.com/Power-PICOGLF60W24V240VS-6000W-Inverter-Charger/dp/B00HHNCQ00/ref=sr_1_5?ie=UTF8&qid=1426230877&sr=8-5&keywords=240v+inverter

    It's hard to believe this hasn't been tried before.

    Edit: I'm not too sure about 240V inverters but, worst case scenario, you could do it with a good-size 120V inverter. I'm guessing you could get by with a 5-10 kWh battery bank as a buffer. It would be a very slow and inefficient charge, but it beats trying to push the car for regen. :)
     
  13. sefs

    sefs 2012 Ford Focus Electric

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    If you're really concerned about charging when the grid is down, natural gas generators are the way to go. You can't beat the power they yield at that price. I have a Generac 22kW air cooled for this exact reason. For you guys with 85 kWh batteries, a couple hours without power isn't an issue; but for me and my 23 kWh battery, I count on charging multiple times a day. A power outage at a time where I was planning on charging for an hour can be a disaster.
     
  14. commasign

    commasign Active Member

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    Interesting idea. We have natural gas for our water heater and house heater. Is natural gas something that keeps flowing even in extended power outages? (i.e. the utilities don't shut it off for safety reasons)?
     
  15. sefs

    sefs 2012 Ford Focus Electric

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    During typical power outages, the natural gas will continue to flow. The pumping stations are backed up by, you guessed it, natural gas generators. However, since you are in California, I will state that during a severe earthquake, natural gas distribution may be ruptured and shut off. Depending on how close you are to the shutoff, though, there will still be natural gas in your local distribution lines, sometimes enough to run for a day.

    Some gas outages last year: PGE Restoring Services Impacted by Major Earthquake in South Napa Region of California
     

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