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Thread: (Partially) Solar Powered Motorhome

  1. #11
    Senior Member Starjots's Avatar
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    Quote Originally Posted by prometheus View Post
    Have you considered converting excess solar energy into hydroxy (and/or hydrogen and oxygen gas) by means of electrolysis? This would allow you to camp, drive, camp, etc....
    I assumed electrolysis was too inefficient and it takes electricity which can be directly applied to motors. However, one must keep their mind open - heating is usually done by propane (and also makes good alternate fuel source for fridge) but I wouldn't mind doing without it if possible.

  2. #12
    Senior Member Starjots's Avatar
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    Reducing Frontal Area

    Although this is supposed to be an RV I'm assuming through creative use of pop-outs and pop-ups I can have a much smaller profile while traveling. From the formula in the second post, reducing frontal area is just as important as improving the aerodynamic shape in reducing power requirements.

    Since I travel with my mate, the minimum area is limited in height by the seats and width by two abreast. The STELLA car is a good staring point.




    But even this seating looks awkward. Strangely, I think the F1 seat position could be made comfortable with some minor adjustments. It requires more length, but that isn't a problem in an RV.



    An F1 race car is 1 meter tall and since ground clearance doesn't count, I'll go with that. For width, we fit snugly in my old Scion Xb which is 5.5 feet wide. The roof solar panel assembly (only) will extend a full 8.5 feet wide, giving the whole thing a fat 'T' aspect. Total frontal area including tires, about 2 square meters. (1 x 1.68 + thing wing + a bit of exposed tires).

    So with P = 3600 watts, V = 60 mph, Crr = .0065 (tires) and A = 2 meters square the last two variables look like this:

    WEIGHT COEFFICIENT OF DRAG
    500 kilograms 0.12 <- slightly feasible, the most aerodynamic car in the world was 0.19 and a Prius is 0.26
    1000 kilograms 0.08 <- world record solar race car is 0.07
    1500 kilograms 0.04 <- impossible unless I'm an airplane...
    2000 kilograms 0.01 <- impossible

    So this looks grim. Except for something called batteries.
    Last edited by Starjots; 07-02-2014 at 11:03 PM.

  3. #13
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    Quote Originally Posted by Starjots View Post
    Reducing Frontal Area
    Please be careful with this language and the biases it injects into your cognitive framwork, Starjots.
    It's really about drag coefficient however it's influenced.
    Bucky Fuller's Dymaxian car used a tear drop shape to lower drag coefficient, but it's frontal area might be considered HIGH by those thinking in terms of `frontal area'.



    Quote Originally Posted by Starjots View Post
    1500 kilograms 0.04 <- impossible unless I'm an airplane...
    Yes, Bucky basically started with an airplane design then removed the wings and metamorphosed the landing gear.
    Last edited by gps; 07-02-2014 at 05:39 PM.

  4. #14
    Senior Member Starjots's Avatar
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    Quote Originally Posted by gps View Post
    Please be careful with this language and the biases it injects into your cognitive framwork, Starjots.
    It's really about drag coefficient however it's influenced.
    Bucky Fuller's Dymaxian car used a tear drop shape to lower drag coefficient, but it's frontal area might be considered HIGH by those thinking in terms of `frontal area'.

    You are right and I was thinking about this after I posted about 'frontal area.' Some of my conclusions are probably in error.

    I had done one read through of The Leading Edge - a book on aerodynamics of solar race cars and the author went to great pains to point out that 'frontal area' is used for conventional cars because they are bluff - not very aerodynamic.

    However, if you have a streamlined design the area of interested becomes the surface area the air flows over, not the front. So I skimmed the book again and am thinking about this some more - it may not be necessary to go to such a teeny tiny front area since I'm looking to have a large surface area for solar panels anyway.

    Thanks for catching that.

  5. #15
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    Quote Originally Posted by Starjots View Post
    For traveling a constant speed, something a motorhome does a lot of, there are two types of resistance to overcome to maintain a constant velocity.
    For now I'm ignoring power lost between the battery and the wheel and just focusing on power required at the wheel.

    The first is air resistance.



    where
    is in Watts
    is density of air, about 1.2 kg per cubic meter
    is the coefficient of drag
    A is the frontal area in square meters
    V is the velocity in meters per second


    First thing that jumps out is power increases with the cube of velocity.
    Right you are.
    Though you mentioned a constant speed/velocity below you haven't mentioned velocity as the MAJOR factor as it's cubed and other factors are lower order and thus less influential.

    Quote Originally Posted by Starjots View Post

    Second, the product of is all there is to work with in design. You give the vehicle a smaller frontal area and/or you make it more aerodynamic

    The second is rolling resistance.



    where
    is in Watts
    V is velocity in meters per second
    is the coefficient of rolling resistance which is a property of the tires used
    g is gravitational constant, 9.81 m/s^2
    m is the mass of the vehicle in kilograms

    Here there are two variables that one can tweak, the coefficient of rolling resistance of the tires and the mass of the vehicle.

    So that's the major variables in designing for low power requirement at reasonable speeds; aerodynamic shape of vehicle, frontal area of vehicle, mass of vehicle and tires used.
    Here's where you'd get better, more realistic results if you'd factor in a distinction math weenies honor: independent VS dependent variables.
    If you hold constant -- as if an independent variable -- the `reasonable speed'/velocity then you can't REASONABLY drive slower pursuant to reducing the power requirement.
    If those driving RVs were to get off the 4-lane interstates and take 2-lane `back roads' they could get better mileage by driving slower.
    A lot of energy is wasted as per both Jimmy Carter's executive mandate to limit velocity to 55 AND Sammy Hagar's mea culpa "I can't drive 55!"

    Retirees, for example, generally have more time than money, so when they're driving an RV they COULD take the slow roads to swap travel time for fuel savings.
    Last edited by gps; 07-02-2014 at 06:19 PM.

  6. #16
    know nothing pensive_pilgrim's Avatar
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    Have you thought about some kind of deployable array that you can set up to charge the batteries while stopped? I'm thinking something that folds out and can cover a larger area than the roof. With an RV where you can expect to often be stopping for periods of time in one place, you could find a good sunny spot and charge up your batteries. That still probably wouldn't cover all your needs but you could try and make it a hybrid system, just use solar as a supplement to the gas tank. I can't see how the project as a whole could have more benefit than cost financially, though, unless you're gonna be traveling a whole lot in really sunny areas - were you gonna do this for fun or to save money?

  7. #17
    Senior Member Starjots's Avatar
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    I like the idea of extra deployed panels when parked for a couple of reasons. First, morning and evening sunlight would provide little power on the flat roof array - although I've also considered making that tilt-able. Second, I'm not crazy about hard core camping, I like gadgets and lights and fans and microwaves.

    The idea is no gas tank, no hybrid. My wife likes the idea of a hybrid (she drives one) and they are economical without range worries. But they don't have a hybrid RV yet in production that I'm aware of (large hybrid vehicles don't get terribly great mileage anyway). Mostly though it's fun with a dash of environmentalism. The fun part so far is trying to do something most folks think is impossible but which I'm pretty sure is quite possible. It just won't anything like your typical Class A.

    One thing I do have on my side is time. We own our current RV for at least another year before it goes to another family member and I have several more years before I can conceivably retire which is required to make this worthwhile.

    Final random thought - I can see traveling four to six months a year. My wife and I both aren't the type to stay in one place too long and I like driving, so yes, I can see 15 to 20,000 miles a year.

  8. #18
    Senior Member Starjots's Avatar
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    Reducing Frontal Area Part II - Different Rules for Streamlined Shapes Compared to Typical Cars

    My post on frontal area was not right, not the last time that will happen. There's a very thorough treatment in this book, but I found a much easier to digest story that illustrates the point.

    Wind Tunnel Testing Stanford Solar Car Luminos

    This is the Standford Teams 2013 4th place finisher in the World Solar Challenge.



    Here's the pertinent quote from the article, I've bolded the relevant parts.

    Our best value obtained from the wind tunnel was even better than the 8.43 lbs of drag that CFD (Computational Fluid Dynamics) modeling had predicted. At our ideal average speed of 55 mph, Luminos should experience a mere 35 Newtons of drag force. For comparison, an average production vehicle experiences drag forces upwards of 230 Newtons (50 lbs). Luminos’s are roughly equivalent to that of a square foot of material held up against the wind.

    One reason for this is that Luminos was designed to minimize wetted area, or the area in contact with external air flow. Wetted area experiences skin friction—the friction between a fluid and the surface moving through it. On a streamlined vehicle, it is skin friction over the entire wetted area that contributes significantly to drag. This was something that the team learned after designing Xenith, which had minimized frontal area, rather than wetted area.
    In a nutshell, they designed their 2011 world class solar car (Xenith)based on the same assumption about frontal area and found out it was wrong.

    The article also mentions Cd*A value of about 1 ft^2 or .093 meters^2.

  9. #19
    Senior Member Senseye's Avatar
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    Quote Originally Posted by Starjots View Post
    The fun part so far is trying to do something most folks think is impossible but which I'm pretty sure is quite possible. It just won't anything like your typical Class A.
    I wouldn't gloss over this. How much not like a typical Class A? I envision something like a porta-potty on wheels.

    You might want to change your approach to considering what is the minimum amount of interior space you need for an RV you consider functional (as in: a vehicle you would be prepared to travel and live in). Once you have that, you can worry about exterior design and see if you can cook up some economically feasible way with batteries/solar to move it around.

    You seem to have discarded the no battery approach as unfeasible. Is obsessing drag really relevant once you make that leap? You are going to have so much added weight in batteries that I think a bit of extra air resistance here or there won't be an issue. It's relevant for those ultralight style vehicles, for your 31 foot behemoth (or whatever you think you can pare it down to) not so much.

  10. #20
    Senior Member Starjots's Avatar
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    Quote Originally Posted by Senseye View Post
    I wouldn't gloss over this. How much not like a typical Class A? I envision something like a porta-potty on wheels.
    Great question. So my class A (i.e., the only one I have experience with) wasn't a high end model when it was new but it does provide

    While driving - nice captain's chairs and an acre of glass to see the scenery through, good ride

    While parked - (1) a high ceiling, maybe 6' 9" or so enabling me to walk around (2) about 7 1/2 feet wide interior so people can squeeze by each other (3) a small functional bathroom (4) good storage, enough to keep resupply/laundry stops down to once every week or two (5) real furniture to get comfortable (6) a good sized fridge, sink, cooktop and microwave (7) short queen sized bed - comfortable but could be longer as my feet always hang off quite a bit (8) self contained water system. In short, it's quite liveable and creature comfortable if one has electricity.

    These are all good things I'd like to replicate or get the same experience by another means if possible. Which sounds crazy when I'm talking something 3 or 4 feet tall, but it can be up to 8 feet wide and 24 to 40 feet long.

    One thing that helps is what you have when you are driving and what your setup is when you are parked can be different.

    The Hi-Lo Trailer


    The Alto Trailer


    And of course, there is the idea of pop outs on the sides.

    You might want to change your approach to considering what is the minimum amount of interior space you need for an RV you consider functional (as in: a vehicle you would be prepared to travel and live in). Once you have that, you can worry about exterior design and see if you can cook up some economically feasible way with batteries/solar to move it around.
    We've done some window shopping of new motor homes and the answer is at least a B+, the unexpanded van conversions seem too cramped for months on the road even if they are very nice and cost 20x what I paid for my used unit. Space is a big deal, I have a spouse to convince and she's adventurous but whatever I cook up has to be compared to a newer used modern B+ or small A.

    You seem to have discarded the no battery approach as unfeasible. Is obsessing drag really relevant once you make that leap? You are going to have so much added weight in batteries that I think a bit of extra air resistance here or there won't be an issue. It's relevant for those ultralight style vehicles, for your 31 foot behemoth (or whatever you think you can pare it down to) not so much.
    Absolutely, batteries are a must and quite a bit of them. The downsides to batteries are weight, say 30 pounds per kilowatt hour of capacity and cost, say $300 to $400 per kilowatt hour. And if you go batteries only and most of the driving is on a highway you'll have to focus very hard on aerodynamics anyway to get any range.

    There's a company making a city bus of normal size and shape with a 155 mile range (didn't say what speed this was at) and they gave enough information to deduce they were packing 300 KWH's of batteries (almost 4 Teslas worth) - that's 10,000 pounds and $100,000+ of batteries! And 155 miles of range is frankly too low IMO for an RV.

    I don't want to jump to the end result (because I really don't have one yet), but a 50/50 mix of energy supplied by batteries and solar panels between 9 am and 3 pm traveling at a safe two lane highway speed would be a terrific outcome. It would be like carrying 2x the batteries.

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