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USA Solar Journey

Discussion in 'Roadster' started by Rob360, Jan 4, 2011.

  1. Rob360

    Rob360 Member

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    Dear Tesla Enthusiasts,

    A fellow Phd student and I are setting up a project to travel across the United States by using only solar energy (www.solarjourneyusa.com). We will be using flexible solar panels that can fit in a trailer. The idea is to make daily stops every 200 miles at schools, universities and other high-profile destinations to educate the public about EV's and flexible solar panels. While we educate the people during the day, we will be charging our batteries so that we can drive off in the evening without having used any fossil fuels for charging.

    Currently, we're looking for the appropriate (commercially available) vehicle to use for this trip and we came across the Tesla Roadster. There's a couple of important questions I need to answer before moving forward, and I thought you guys could help me out with some advice.

    CHARGING
    I know a fair share about charging methods for EV's, but I was wondering:
    - Is it possible to charge the Roadster with a high-voltage DC source (without making any alterations to the car)? That is, is one of the three plugs that you can charge it with for DC? I'm asking this because we would prefer to not bring an additional inverter in the first step to go from DC (solar) to AC (input to car) back to DC (battery) to AC (drivetrain)!!!

    CONNECTING A TRAILER
    I'm not sure if this is discussed in this forum, because the Roadster is not the first car you would want to connect a trailer to:
    - Is it possible to connect a hitch to the Roadster? Have you heard of anyone who has towed anything? I know from Lotus Elise forums that people have attached a hitch to the frame under the car before: http://www.lotustalk.com/forums/931132-post106.html
    So since the Roadster has the same frame, do you think this hitch component can be attached?

    Thanks so much for your input.

    I hope to own one myself one day.

    Rob van Haaren
     
  2. doug

    doug Administrator / Head Moderator

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    #2 doug, Jan 4, 2011
    Last edited: Jan 4, 2011
    Not without changing something on the car, no.

    It has been done.
    tesla_car-pic_Sun-Frost.jpg

    See also here.
    [​IMG]
    [​IMG]

    Good luck!!
     
  3. TEG

    TEG TMC Moderator

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    JB is known for experimenting to see if something works.
    I wouldn't take that one photo of a Roadster with a trailer as any indication that it is supported, or even a good idea.
    Who knows what other changes they made to that Roadster?
     
  4. doug

    doug Administrator / Head Moderator

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    I didn't say it was recommended.
     
  5. TEG

    TEG TMC Moderator

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  6. TEG

    TEG TMC Moderator

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    Owners manual says no...
    attachment.php?attachmentid=1245&d=1294190305.jpg
     

    Attached Files:

  7. vfx

    vfx Well-Known Member

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    #7 vfx, Jan 5, 2011
    Last edited: Jan 5, 2011
    The Solar Taxi worldwide trip did exactly what you are saying. He would let anyone (including me) drive the car and his solar panel trailer was hooked to the car. http://www.solartaxi.com/ It did not make enough energy to power his very lightweight vehicle but he had a roof full of solar panels back home covering the fungible balance. (Louis is now planning a world wide EV "race". Now that sounds like fun!)

    There have been many long distance Tesla trip including the NY to CA trip (forgot the name) and Tesla's own drive of a Roadster from LA to the Detroit Auto show last year. And to me the most impressive rip was the TAG Huer cross Europe road trip with a million different plugs to contend with.

    edit to add
    The NY/CA trip was the Renew America Roadtrip http://www.teslamotorsclub.com/showthread.php/2781-RenewAmericaRoadtrip(tm)
     
  8. dpeilow

    dpeilow Moderator

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    That was worldwide - so even more plugs.
     
  9. bolosky

    bolosky Member

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    I did some back of the envelope math on this, and you're going to need some enormous solar panels to have any hope of making it work.

    Driven gently and not towing a trailer, a Roadster will use ~250 Wh/mile from the battery. It will clearly be more driving at highway speeds and (especially) towing. So, 200 miles requires > 50kWh. (Note also that this is close to the battery capacity, so you'll be on the edge with regard to range if you really plan to charge once and travel 200 miles.)

    Charging efficiency is ~85%, so you'll need to supply more than 60 kWh to the car to get that > 50kWh from the battery.

    Solar flux at ground level in cloudless daytime in mid-latitudes is about 400 W/m^2 (I got this from doubling the average flux, assuming it's averaged over day and night). It'll of course vary based on time-of-day and season and you might get some extra flux by tilting the solar cells to be more perpendicular to the sun, so I'll assume they just cancel each other out. My gut says that adding significant tilt to the array will be tough, because it'll be so big. Maybe you'll always put it on a hill facing the right way (this is the trick that the Mars Rovers used).

    If your solar cells are 25% efficient (which is good, though not the best ever demonstrated in a lab), that leaves you with 100 W/m^2. Dividing by 60 kWh needed output means that you'll need 600 m^2-h of solar cells. If you spend 6 hours/day charging, then you'll need at least a 100 m^2 array (plus the infrastructure to erect it), which is pretty darned big to haul around behind a Roadster.

    You'll also have other practical problems that will have the effect of increasing that size even further. The Roadster expects constant current AC, which is decidedly not what comes out of a solar array. So, you'll need an inverter (which will reduce efficiency and add mass and so increase needed size) and more importantly you'll need some kind of buffer (i.e. a small battery or large ultra cap) that will allow you to keep supplying probably 240V 32A AC to the Roadster as the output of the cells vary (because of clouds, etc.) That will add further mass and reduce efficiency, both of which will require larger cells.

    And that doesn't even consider clouds.

    Maybe I messed up somewhere, it wouldn't be the first time. If so, I'm genuinely curious as to where.
     
  10. TEG

    TEG TMC Moderator

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  11. Rob360

    Rob360 Member

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    I posted the same message on the teslamotors forum (thought I'd try both to reach as many as possible!) and someone else came up with that critique.

    You're right that we won't make 200 miles with a Tesla. The initial idea of the Solar Journey USA was to drive with electric motorcycles and for those it would be possible to get that range (they get ~10-15 miles/kWh).

    So lets do the math for the Tesla:
    With an average horizontal irradiation of 6.8 kWh/m2/day (from PVWatts) along the route (in summer) and 11% efficient panels, we get ~0.75kWh/m2/day. Take off 10% losses from charging and discharging the battery and you have 0.67kWh/m2/day sent to the motor. We can carry a total of 20 * 3m2 panels (each is actually 320Wp), so for the whole array we generate 0.67*60= 40 kWh/day.

    So with 250 Wh/mile we would only get 160 miles. But the reason I started thinking about a Tesla for our trip was this: http://www.teslamotors.com/blog/roadster-efficiency-and-range
    and it appears the EPA [Wh/mile] battery-to-wheel number is 133 Wh/mile (see wiki on tesla roadster)?
    So what are the true numbers that people are getting? And we'll be trying to keep a constant (rather low) speed to maximize range. Bolosky, is 250Wh/mile what you get from your Roadster?

    By the way, those new CIGS panels of Ascent are very interesting. They're as thick as a creditcard and they only weigh 6kg for 320 Watts! You can check the website for more info.

    Good point about the input electricity needed. We're also starting to look at the LEAF because that allows a 600V DC charging option. I guess significant changes would need to be made to the tesla in order to charge it with a solar array without inverter.
    Thanks for your time though and I'm looking forward to your reply.
    Rob
     
  12. Rob360

    Rob360 Member

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  13. Rob360

    Rob360 Member

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    I read about them too and that was great. When I looked into it further, I noticed that they did not generate all the electricity from the panels they carried along. Some of it was offset by an array in Switzerland or something. Didn't they plug it into the grid along the way? That sounds great but it's not quite self-sufficient, in my opinion.
    I guess it's good that more people have done it before in other variations, but we're trying it with a focus on trying to educate the people about the beauty of EV's and the possibilities of flexible PV, plus being completely self-providing our electricity.
     
  14. bolosky

    bolosky Member

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    The graph there does indeed show something 133 Wh/mile, but it's at 17 mph. Presumably if you drove that speed on the level not towing a trailer and not using HVAC, you'd get roughly that number. However, there's pretty much no place you can drive cross country at that speed.

    The energy usage that I get varies tremendously depending on what I'm doing. When I'm just driving back and forth to work running the heat in the winter and not trying to save battery (including a stretch of freeway where I drive around 80), it's well over 300 Wh/mile. This past fall I drove to Vancouver BC and was trying to optimize for efficiency, and I got 244 Wh/mile for the whole trip driving at ~60 mph most of the way. However, to get this number I had to draft trucks when I could, which isn't really a great idea. Driving on level surface streets at 35 mph seems to give numbers in the low 200s.
     
  15. Rob360

    Rob360 Member

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    Thanks for the info.. Ow and I just realized that the 133Wh/mile should be 133Wh/km! So I'm sorry for the confusion.

    Rob
     
  16. ChargeIt!

    ChargeIt! Member

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    No ... at 17mph that graph does indeed show approx 133Wh/mile not per km. And that low number makes sense given the distance that can be achieved. There are some EU Roadsters that have blogged about a 133Wh/km number ... but I don't think you were referring to those.
     
  17. Rob360

    Rob360 Member

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    No I meant the 133Wh/km on the EPA drive cycle. As on wikipedia: http://en.wikipedia.org/wiki/Tesla_Roadster. But we'll be going quicker than 17mph... I'm thinking of staying off the highway when possible and driving ~40mph.
     
  18. vfx

    vfx Well-Known Member

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    #18 vfx, Jan 6, 2011
    Last edited: Jan 6, 2011
    Yes by "back home" i meant Switzerland (or something) If you fully read up on their sojourn you will see their car was especially made for the trip. I swear i could pick it up it was so light. They must have the math somewhere. If not, I found everyone involved with that triip very generous and freely shares info. They would be a great source for car, power, breakdowns, charging and logistics info. I think they hit all the marks you are trying to achieve (except "flexible") as they stopped at schools, had heads of state drive their car (even, I believe, a president) and if you can be totally self sustaining with your trailer more power to you (pun) but I don't believe it matters if you are carrying it with you. Most people think putting a solar hardtop on a Roadster would make it self generating so you are only playing to the geek crowd who would understand that solar electricity put in the grid in one place can be drawn from another as needed.

    Also are you figuring in the weight and drag from the trailer?

    This thread has a "fun" cross country solar car video: http://www.teslamotorsclub.com/showthread.php/1980-Solar-quot-UFO-quot-Car-(Xof1)?highlight=solar+taxi
     
  19. bolosky

    bolosky Member

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    17 mph was to get 133 Wh/mile. You can probably go 40 and get 133 Wh/km (= 222 Wh/mile) if you're careful and are willing to ignore the effect of the trailer. If that's true, then you can go back to my original back-of-the-envelope SWAG on how big you'll need the solar cells to be and reduce the result by ~10%, which would still mean you'd need 90 m^2 of solar cells. Of course, my anaylsis has way, way more than 10% error bars so this difference is meaningless; all you can say is that you'd still need a whole lot of solar cells.

    Your numbers seem to indicate that you'd be using substantially less efficient cells than I assumed, but that I messed up on the available solar flux by a large factor (I got my numbers from poking around wikipedia and doubling the average flux, assuming that you wouldn't be trying to charge at night; I probably should have given some credit for being mid-day in summer).

    Still, this seems like it would be really tricky to pull off, and really impressive if you did!
     

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