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Thread: (Partially) Solar Powered Motorhome

  1. #21
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    Quote Originally Posted by Starjots View Post
    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.


    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.
    And just about the time you've got the concept of wetted area under your belt someone will come along an introduce super cavitation.

    Though I suspect that these low-drag designs could be improved by starting with the`wetted area' composed of the `skin' of an air hockey table then proceeding by experimenting with pores-per-area and flow rates for zones/regions of pores/vents to optimize to get dynamic drag coefficients better than comparable theoretical best-case wetted models.
    The friction of wetted areas can be reduced by de_facto lubrication via a `fluid' -- as in `fluid dynamics' -- resulting in a better emergent dynamic viscosity vis-a-vis wetted area drag calculations.
    The closer one's wetted area performance gets to the dolphin, which uses musculature to dynamically alter its `wetted area' skin profile/contour -- which is also lubricated, the closer one gets to the best case I'm aware of in nature.

  2. #22
    Senior Member Starjots's Avatar
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    Quote Originally Posted by gps View Post
    And just about the time you've got the concept of wetted area under your belt someone will come along an introduce super cavitation.
    I was interested to learn the the Russians had a torpedo that could go 200 mph underwater using this, never heard of it before. Based on your post I spent at least a couple of hours doing reading on some of the concepts to reduce skin friction. For now I don't think I can realistically count on intelligently applying any of them. 3M was supposed to have made a tape for example that mimic'd a bit the skin of sharks which reduces drag. However, I can't find it being available at this time for your typical schmoo, a racing yacht used in many years ago and won the world cup, promptly getting the stuff banned

    If there were any easy tricks they'd be worthwhile but then I'd think we'd see them in use in a lot of different applications already.

  3. #23
    just dont think about it mhc's Avatar
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    paint it red! red goes faster

    (sorry)
    Just look at the blue sky

  4. #24
    Senior Member Starjots's Avatar
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    Calculating CdA for a Specific Design 24' Long 8 Feet Wide 4.3' Tall NACA 66018 cambered

    I measured my driveway and parking area out front. I don't have much room in either place, only enough for a 24' long vehicle. Next I consulted with the mate, who re-iterated the desire for a smaller more mobile unit. We've driven the 31' motorhome 3000 miles in the last two months and I often think something six or seven feet shorter would be much easier to drive.

    I also did some calculations that showed extending the unit purely for energy capture reasons (more panels) would consume as much energy as the panels added. So no point in doing that unless something changes. I do like the idea of some panels that can be deployed when camping and will try to make that part of any detailed design.

    So the shape of the thing (not to any sort of scale) from the side would be this:



    This is a NACA 66018 airfoil (cambered) from Java Foil in case you are interested, a free program I'm dinking with to explore airfoils. The height is out of scale, it should only be 18% (4.3 feet) as tall as it is long (24 feet).

    Important point: The NACA 66 has good aerodynamic characteristics for cars and low lift. It's used as a starting point for several solar race cars and at least one Bonneville Flats 400 mph car that I'm aware of. The top down view would be a rectangle 8' wide by 24' long. The bottom is made approximately flat to minimize drag from 'ground effect.'

    Next item is to find the surface area and effect coefficient of drag (body) times the surface area (Cd*A). This will (hopefully) get me back on track for seeing if this is feasible and if I can move on to other things.

    For all the calculations I followed page 135 to 146 in The Winning Solar Car.

    1. Calculating Reynold's Number

    In fluid mechanics, the Reynolds number (Re) is a dimensionless quantity that is used to help predict similar flow patterns in different fluid flow situations.
    Re = (Velocity * Cord Length) / kinematic viscosity of the fluid (air)

    Re = 26.8 m/s * 7.3125 meters / 1.46 x 10^-5 = 13,393,000

    2. Calculate surface area

    The book has some rule of thumb sort of calculations for doing this. My answer is A = 55.84 square meters. The fat body and flat sides (slightly rounded at the edges) doesn't help. This is a lot of skin to drag through the air even if it is aerodynamically shaped.

    3. Calculate effective Coefficient of Drag for the Body

    The author states these equations were developed as rough estimates over five years by his team. There is a calculation for very smooth surfaces (usually lower results) and rough surfaces - and you pick the higher of the two.

    Now if you build the airfoil very well you get a short length of laminar flow at the front where there is very little friction. After that the air becomes turbulent and skin friction increases. You do a calculation for the top and the bottom (different lengths of laminar flow). I estimated 1 meter of laminar flow on top and 0.5 on bottom, it's a stab in the dark based on the book and should affect the results too badly. It does assume a very smooth front, no headlights sticking out, no grill, nothing like that - just smooth rounded curves. Obviously side mirrors becomes a design issue.

    Cdtop = 1.328/sqrt(Re) x sqrt(LT/L) + (LT - L)/L x [ 1.89 + 1.62 x Log10 (epsilon/L) ] ^ -2.5

    Where Re is the reynolds number, L is the length, LT is the length of laminar flow, and epsilon is the 'bump height' a measure of how smooth the surface is or the biggest bumps you might say. All distances in meters.

    I inflicted that equation on you because it turns out bump height is extraordinarily important.

    Once you calculate your Cd's for the top and bottom, you average them, multiply by your area and then by a fudge factor if you round your back end or not. I didn't and this fudge factor (called BLPL in the book with another crazy equation) for me is 1.1152.

    The first calculation I had 'bump height' of 2 millimeters. That's pretty smooth, maybe the edge of a flush mounted vent for example.

    For I got CdA = .35 square meters. That's a lot.

    Then I reduced bump height to 0.5 mm (which the book uses and I assume means very tight manufacturing and no bumps on purpose) and got

    CdA = .26 square meters. That's a HUGE difference and would equate to 25% less wind resistance and (I think) 10 to 15% more range.

    SO besides getting some figures to work with, I've learned that I have to flush mount everything if possible with no protrusions.

    Next post, a quick discussion on battery size and then maybe we can get off the aerodynamic stuff for awhile.

  5. #25
    Pull the strings! Architect's Avatar
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    May have been mentioned, but don't forget that the panels need to be perpendicular to the sun. As they are angled they lose significant wattage as they are seeing a smaller cross section. Don't underestimate this affect, I've done the calculations before and really only a +- 5-10 is as much as you want.

  6. #26
    Senior Member Starjots's Avatar
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    Calculating Battery Size/Weight/Range Tradeoffs

    Here's what's been settled so far (as far as can be settled in a conceptual design)

    Crr = .0065 with Michelin low rolling resistance tires, 16", load capacity 1600 pounds each
    Side profile based on NACA 66018 airfoil, cambered to be approximately flat on the bottom
    Dimensions 24' long x 8' wide by 4.3' tall
    Surface area approx 55.84 meters square
    CdA taking great pains to make smooth with everything flush mount .26 square; adding in 25% for exposed tires/farings 0.325
    CdA with care taken to limit bumps to 2mm .35 meters square; adding in 25% for exposed tires/farings 0.4375
    Solar panel area, approximately 15 meters square = 2700 watts rated
    Batteries - Lithium Iron Phosphate for reasons not discussed yet

    Quote Originally Posted by Architect View Post
    May have been mentioned, but don't forget that the panels need to be perpendicular to the sun. As they are angled they lose significant wattage as they are seeing a smaller cross section. Don't underestimate this affect, I've done the calculations before and really only a +- 5-10 is as much as you want.
    Yes, without testing I'm assuming coupled power is theoretical max of Peff = cos theta Pmax where theta is the angle the sun is from perpendicular. For the graphs that follow I'm assuming 80% (basically form 9 am to 3 pm) of sun or 2160 watts. This is something that should be tested before any panels are ordered in quantity. Tracking the sun while moving is a tough problem and really bad when trying to make things aerodynamic. The Cambridge team tried this but there isn't much room in there for motor home stuff - still - this is something to keep in mind in case I have to go back to drawing board.



    Last fudge factor I can think of is counting for power losses between panel/battery and motor. This depends a lot on components (motors, inverters, charge controllers etc) chosen so I put this out as a target to hit or hopefully improve on more than a real number. I believe the Tesla is 70 or 75% efficient so I'll just pick 75% efficiency for power delivered to the wheel.

    So the following two charts include a lot of calculations, all based on the formulas in the second post adding in what is known as well as the 75% fudge factor. The weight doesn't include the batteries but the batteries effect on rolling resist is in the calculations.

    One final bit, numbers assume batteries are drained no more than 90%.

    First, a best case - light weight and very smooth.


    Throwing this in for gps to show range at 55 miles per hour - substantially better.


    Second, less smooth with the 1500 kg and easier to obtain 2000 kg weights.


    Personally, I think a range of 300 miles on a sunny day is very good. Minimizing battery pack size is desirable to reduce cost and time required to fill up the batteries, whether from solar or plug in.
    Last edited by Starjots; 07-07-2014 at 10:02 PM.

  7. #27
    Senior Member Senseye's Avatar
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    Dimensions 24' long x 8' wide by 4.3' tall
    Hold it. A 4.3' high motor home? No standing up allowed and everybody sits on the floor Japanese style?

    I'm afraid the folks in product marketing aren't going to be too happy with the engineering team.

  8. #28
    Senior Member Starjots's Avatar
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    Quote Originally Posted by Senseye View Post
    Hold it. A 4.3' high motor home? No standing up allowed and everybody sits on the floor Japanese style?

    I'm afraid the folks in product marketing aren't going to be too happy with the engineering team.
    Besides the chassis, this is probably next area of investigation. One idea:


  9. #29
    Scala Mountains Resonance's Avatar
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    I realize this might be taboo but what about something hybrid (gasoline/electric)? Well-designed hybrid engines already exist, and you could probably combine that with what you're already considering in order to have another layer of reliability - falling back on the existing, extensive network of fossil fuel support for convenience or in an emergency, while still being able to go 100% solar/battery-powered when you want.

    Then again, a gasoline reservoir is going to add weight, too, I guess, but the energy density and efficiency is going to be better than solar panels.

    Also: I would worry about the panels getting damaged. If you're going to be travelling through diverse climates, maybe unpaved roads, hail storms, etc. then all it's going to take is one well-placed impact to shatter your ability to recharge...

    Anyway, those are my thoughts. Keep in mind that I live in a place where the major exports are petroleum and beef, so I may have some cultural bias impacting my risk assessment.
    Empty your mind. Be formless. Shapeless. Like water. Water can flow, or it can crash. Be water, my friend.

  10. #30
    Senior Member Starjots's Avatar
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    Quote Originally Posted by Resonance View Post
    I realize this might be taboo but what about something hybrid (gasoline/electric)? Well-designed hybrid engines already exist, and you could probably combine that with what you're already considering in order to have another layer of reliability - falling back on the existing, extensive network of fossil fuel support for convenience or in an emergency, while still being able to go 100% solar/battery-powered when you want.

    Then again, a gasoline reservoir is going to add weight, too, I guess, but the energy density and efficiency is going to be better than solar panels.
    I've looked for 'do it yourself' hybrids and have found none, whereas the EV conversion industry, while small, is real and active. This tells me hybrids are more complicated and my initial approach is to try to design something that an active hobbyist industry (kit cars, ev conversions, the whole maker culture) has most of the materials or tools available.

    Still, my wife drives a Prius and that is a car that makes a lot of sense as a primary vehicle - a feat of engineering IMO. One maybe not so crazy idea would be to take an existing hybrid vehicle and build on that assuming you could get one with enough GVWR (gross vehicle weigh rating) and wheelbase for a 24' platform and still do all the super low drag stuff.... hmmmm. Besides finding suitable target vehicle the next issue would be ability to interface with existing electronics system - but clearly there would be many advantages if this was possible. I've also though of using my Leaf as a base vehicle as I could buy it for fairly cheap once lease is up.



    We read that several manufacturers are developing hybrid platforms that the RV industry could use.*

    *Quick aside on RV industry: Many companies, large and small, use multi-purpose platforms from bus, truck and van makers to build on. None that I know of make their own chassis, drive train and all that stuff. What they can do is constrained by what is available and what is available is developed mostly for other uses I believe (such as the small delivery trucks you see everywhere).

    Also: I would worry about the panels getting damaged. If you're going to be traveling through diverse climates, maybe unpaved roads, hail storms, etc. then all it's going to take is one well-placed impact to shatter your ability to recharge...

    Anyway, those are my thoughts. Keep in mind that I live in a place where the major exports are petroleum and beef, so I may have some cultural bias impacting my risk assessment.
    Yes, this is a big concern. Solar panels on roofs rely on heavy glass to protect them, this is not an option on a vehicle because of weight and the need for a non-flat surface. Solar race cars do have some sort of protection but I doubt it is made to stand up to years of outdoor exposure.

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