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Off-grid solar charging

Discussion in 'Technical' started by viccooper, Jun 7, 2013.

  1. viccooper

    viccooper New Member

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    I just reserved a Model X. Woo Hoo! Pretty excited about that. And now I have an interesting question about how to charge the car because I currently live off the grid and rely on solar panels (and a backup generator) for all my electricity. I have 4000 watts of panels and on a good day I can generate 20 Kwh per day. Obviously this will not be enough to charge the house and the 80 kWh battery of Model X in a reasonable amount of time. I see two options:

    1. Get on the grid
    2. Install more solar to meet the additional needs of the car

    To get on the grid I would need to run about 600 feet of underground service. 5 years ago I got a ballpark estimate from the utility of about $40,000. This is what led me to using solar for the house in the first place since the cost of the solar power system was less than $40K and included the added benefit of no electrical bills.
    Now, with the prospect of buying an EV on the horizon, I am faced with the same cost/benefit calculation. I like the idea of just increasing my solar capacity. PV panels were $4 per watt when I bought my system. Now they are below $1 making them even more attractive. So I am looking into what sort of setup I would need to do off-grid charging of the Model X.

    I live about 20 miles south of Monterey, CA and our typical driving pattern would be to do a round trip of about 50 – 60 miles three or four times a week. So with each trip I would typically have one to two days to add back in 60 miles of range. According to the Model S calculator that would require 17 kWh which is a reasonable amount to expect a 5000 watt PV array to produce in a day with good sun.
    In a worst case scenario if I returned home from a long trip and an empty battery I would need to generate 80 kWh which would require about 4 days of charging off a 5000 watt array. And I suppose the absolute worst case would be to return home empty on a cloudy day and then charge the battery from the generator (it is a 20 kWh propane unit) which should be able to charge enough for a round trip in about an hour.
    Another consideration for an off-grid Model X would be the charging equipment. My current off-grid system uses a single 80 amp DC charge controller to feed PV power into a bank of lead acid batteries. I then have two 4000 watt inverters which provide 240 v AC to the house. Since the Model X batteries are also DC it seems like the best way to charge them would be solar DC direct to battery DC and bypass the inverters. I believe the Model X has a direct DC input path when it is hooked up to a supercharger so I am wondering if I will be able to use the same circuitry to do direct DC charging?

    Another interesting question is if there will be a way to draw power out of the battery bank to run my house? The 80 kWh lithium battery bank would be a monster compared to my lead acid battery bank and using it to power the house would help justify the cost of the car since it would be doing double duty.

    So I am wondering if there is anyone else who is or wants to power their Telsa only from solar? And does anyone have any feedback on my calculations and assumptions about solar sizing and charging?
    Thanks!
     
  2. JRP3

    JRP3 Hyperactive Member

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    Theoretically you could use the DC connection to charge the pack directly, and get power out from the pack, but unless you know how to signal the contactors in the car to make the connection internally it won't happen.
     
  3. Raffy.Roma

    Raffy.Roma Active Member

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    @viccooper

    Are you sure that the Model X (like the Model S) has direct DC input? If it is like this would it be possible for us to do direct DC charging for Model S and Model X?
     
  4. stevezzzz

    stevezzzz R;SigS;P85D;SigX

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    This is a very interesting problem, and I hope you'll make blog entry to describe your eventual solution.

    Some initial thoughts.

    1) If you choose to stay off the grid you will probably want to charge the car at significantly less than 10kW/20kW (depending on whether you'll have one or two chargers in your X), just so you don't overtax the components of your system.

    2) Off the top of my head I don't think you can get enough new PV capacity for $40K to completely ease your worries about being able to charge the car and service the house without running the generator (and you really don't want to run the generator, if you can help it). But it's a whole lot more satisfying than spending that same chunk of change just to tie to the grid.

    3) If you can schedule some charging time when you're out and about, it could make the whole problem a lot less thorny. For $40K you can buy an awful lot of vehicle charging from third party charging points (and you may even be able to find some free charging out there on your regular rounds).

    4) If you do have to run the generator to charge the car, see if you can wire things so you don't have to run it through the whole system (batteries, inverters, etc.), but go straight to the car. This assumes (and it's a big assumption) that the power from your generator is clean enough for the Tesla's chargers to handle.

    5) I doubt Tesla will make it possible for you to send DC straight into the car from your PV system; but I would sure investigate the possibility with them. You never know what sort of mad science project might tickle Elon's entrepreneurial senses.

    6) I don't know enough about tying the Tesla to your system as a battery buffer to know if it's feasible, but again, I'd bring it up with Telsa (see item 5).

    7) Cross your fingers that the whole 'vampire load' issue is resolved before you get your X.
     
  5. Robert1969

    Robert1969 Member

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    Umm, contact Solar City? They might have a solution.
     
  6. idoco

    idoco Member

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    Why not add a second battery bank and separate DC/AC inverter just for charging the EV? Cheaper than adding additional PV capacity.

    Back of the napkin calculation for 17kWH: 10 batteries x 6v x 400AH(@20hr) = 24kWH ~$4000 + inverter.
     
  7. Bigbit

    Bigbit Member

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    #7 Bigbit, Jun 16, 2013
    Last edited: Jun 18, 2013
    If you ever find a way to vampire your X, you might consider a loading station near grid (since its only 600feet) would be nice as a backup for your generator.... Take your X battery home to power your house....
     
  8. Eberhard

    Eberhard #421 Model S #S32

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    The charger of Model S can handle DC as well (for PFC it really does not matter) but need the controller programmed to do it. But you may need more than 100V DC to do it.
    This may be a good product for SolarCity: PV-controlled charging.
     
  9. Raffy.Roma

    Raffy.Roma Active Member

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    But is SolarCity available in Europe?
     
  10. Eberhard

    Eberhard #421 Model S #S32

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    Colorado ist not located in Europe
     
  11. constraint

    constraint Member

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    Not only do you need to increase your power sources (panels, wind, generator) but you should also look at increasing the size of your house pack, your inverts, charge controllers and so on. That adds up pretty fast. I would defiantly get a quote on grid tie as 40k seems high for only a 600 foot run, however saying that i think you can make off grid work for under 40k.

    My recommendation if you can locate some charging stations close by, use those after long journeys and just use your home power for daily use energy needs. For example daily i drive about 30-40 miles per day so I would size the increase in my system(which i dont have) to maybe 10 kwh per day. If you limit your charge to 3.3 kw charging you may be able to utilize your current inverter/generator for overnight charging and just have to focus on your house batteries and power generation which would essentially need to be doubled.
     
  12. PV-EV

    PV-EV Member

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    We are off grid and charge our Roadster as much as possible with surplus solar from our 4.9KW array. The maximum rate I can reliably charge at is about 14 amps at 240. That equates to about 15KWH on a nice day. Obviously I only charge the car on sunny days and in the middle of the day. That puts some limits on it but we also use off site charging as well. I installed a 70 amp Open EVSE at my (thanks buddy) neighbors house that I use when I need to and my wife has public charging at work (thanks AISMS). Our house uses about 8KWH/day so the array is way overkill for the house 6 months of the year. Being as you are in sunny CO you should be much better off than we are here in Alaska. I've been experimenting with variable rate charging using the EVSE to track our solar output. I would like to install a larger array (another 5 KW would be super!) but I would also have to double my inverter to be able to convert it all. The battery only has to be big enough to cover my house loads when the sun isn't shining as I never charge the car from the house battery. That would be very inefficient. As far as using the DC directly from the PV array you would have to operate the array at a voltage higher than the car pack voltage which I believe is about 370 volts. I'm not sure how you would regulate that but it could probably be done. The hard part is gaining access to the car's DC circuit, not to mention pretty dangerous. High voltage DC is considerably more dangerous to work with than AC. The nice part about using an inverter (which I already have to have) and charging with AC is that you get the use of all the features of your EVSE and the charging integration that is already built into your car. Good luck with your plans,

    Jack
     
  13. constraint

    constraint Member

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    Unless you are an EE with years of experience, i wouldn't even bother with direct DC charging. Using a Pure Sine Inverter or a generator that can give a very clean Sine Wave is your only way to go charging the S or X. As for charging off that battery, i know the T-105 batteries are probably pretty inefficient to charge but I am seeing a much better use case for utilizing the Thunder Bay (whatever their name is now) LFP batteries. I know a couple places in CA that sells them for under $400/kwh with much higher efficiency rates. The reason i suggest charging at night is if i was driving an S or X, most of the daylight hours when the sun is strongest I and most people are at work. Unless you have electric heat (not sure of many off grid people that do) your inverter is not doing anything when you are asleep, but probably fairly close to maxed out when you get home. If your car charging is at night you can utilize your same inverter/generator provided you have an increase in your battery storage and generation (or run the generator for longer).
     
  14. TFMethane

    TFMethane Member

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    Why not just tap off the grid and place a charging station at the edge of your property. This avoids the cost of all the trenching, panel upgrade for you house, etc.

    - - - Updated - - -

    Why not just tap off the grid and place a charging station at the edge of your property. This avoids the cost of all the trenching, panel upgrade for you house, etc.
     
  15. widodh

    widodh Model S R231 EU

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    DC to DC would obviously be the best in terms of efficiency, but what other already mentioned, you need at least 370V DC do be able to charge the pack. I assume Tesla implemented J1772-DC as well, but we can't be sure.

    DC -> AC -> AC -> DC would be easier with Open EVSE, but it would mean you would loose some kWh's in the conversion process.

    The 600 feet you are talking about, is that from the edge of your property to the nearest grid cable or? If not, why not indeed get a grid connection at the edge of your property, build a small carpark for your X with a charging station? Charge when you need to and otherwise park at your house or even charge there very slowly? At like 110V 15A
     
  16. RFBrost

    RFBrost Member

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    Not to throw cold water on your plans, but using the car as a “stationary power source” will void your warranty. However, since Solar City is beta site testing home storage solutions (built by Tesla) you really should talk to them.
     
  17. Mark Petersen

    Mark Petersen Model S EU P71

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  18. cziegler

    cziegler Member

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    If you have sunlight, solar PV is smart.

    $40K for a 600 foot run is consistient with what I was quoted, however, in my case Edison wanted me to pay a share to upgrade transformers too so the full guesstimate was 100K. I'm going with the off-grid option on my next project (combine those facts with my suspicion that energy costs are going to climb at increasing rates in the not too distant future so solar seemed like the better risk).
    It has been my experience (2 years in Monrovia, CA - very sunny!) that charging during periods of sunshine is pracitcal for folks like us that drive onlly 40ish miles per day and since Tesla Motors vehicles charging rate is configurable, I was able to set charging rates to match my generation rate greatly reducing the requirements for inverters and batteries. As a side note, if you are halfway handy installing solar or just digging the trenches or footings can save huge amounts of money - roughly half the cost of a solar system is installation.
     
  19. bwestlak

    bwestlak Member

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    I think one of the problems you are trying to solve is a common mistake people often make. The charge you need to add to your car is only enough for your next trip so always charging to 'full' and the kw needed for that is sizing for a very unusual situation where you would have two back to back 200 mile trips with a stop overnight at home. We have a Model X on order, but we are currently charging a Focus Electric from 4 panels tied to 6 used 100 ah batteries and a 3k 120 volt inverter. The system is just fine and charges up the Focus overnight along with LED lights in my garage. I believe I will need to go to 220 volt for our X, but realizing the amount of kw needed is only increasing because the X is less efficient (heavier with more overhead energy needs) than the Focus. After all we are the same people driving the same places in an EV.
     
  20. Quantum`

    Quantum` Member

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    1. Can you dig a trench?
    2. Can you lay a conduit in a trench?
    3. Can you cover over a trench?
    4. Can you pull wires through a conduit?

    Well if so, save 80% of the cost.
     

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