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Thread: (Partially) Solar Powered Motorhome

  1. #1
    Senior Member Starjots's Avatar
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    (Partially) Solar Powered Motorhome

    At this point this is a theoretical sort of project, and I post it here because it is multi-disciplinary, conventionally deemed impossible and could benefit from random opinions or experience of others.

    The vision:
    Traveling hither and yon like a sailing ship of old powered by the sun instead of the wind in a fairly comfortable home on wheels for a good part of the year.

    Inspiration the First:

    I've driven a Nissan Leaf all electric car for the last four months. It is a sweet car to drive.

    Inspiration the Second:

    I've owned a twenty year old 31' Fleetwood Flair RV for the better part of the year with a few short trips and one long trip in the can. Great way to see the sights, very inexpensive to buy, fairly expensive to maintain and fuel.

    Inspiration the Third:

    Nuna 3 Solar Race Car, more generically the cars that compete in the World Solar Challenge in Australia. Probably of more interest is Stella:



    Hopefully that gives you an idea what I'm thinking about.

    Now this sounds pie in the sky, but fortunately this idea can be analyzed up front with the help of a few simple equations which, combined with practical considerations such as cost, ease of implementation, safety and being street legal.

    Next up - the equations.

  2. #2
    Senior Member Starjots's Avatar
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    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. Jimmy Carter was right, but that's another thread.
    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 to 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.

    I did a lot of calculating before I ran across this online calculator which makes it easy to plug in different numbers to see the results.

    Aerodynamic and Rolling Resistance Calculator, Power and MPG

    In the next post I'll consider converting my current motorhome to a solar powered dream machine.

  3. #3
    Senior Member Starjots's Avatar
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    So I already own this:


    Sure, it's a twenty years old but it cost less than your typical used car and all the hard work is done.

    I found that 60 mph works just fine 99% of the time on two lane highways. I also know I got a hair over 9 mpg on flat.

    Working backwards, I come up with mass of 12,000 pounds or 5454 kg, frontal area of about 7 square meters, of about 0.35 and of about .01.

    Plugging these into the calculator, at 60 mph all I need is... 43 kilowatts of power - or about 58 horsepower to maintain speed.

    The roof is 31 feet by 7.5 feet or about 20 square meters (optimistically given the vents and stuff) available for solar.

    Which brings me to solar panels. It turns out many of those cars racing in the World Solar Challenge use super high efficiency solar cells designed for use in space. The reason is it costs ten thousand dollars a pound or something like that to put an object in orbit so it pays to use really good solar panels on things like the International Space Station. But these cells would cost tens or hundreds of thousands of dollars - that is one reason solar race car teams have sponsors.

    For down to Earth, the best I could find in mass produced solar panels was 20-22% efficiency.

    This panel is one square meter, rated at 180 watts and mounted on a flexible backing. Weighing only 6.6 pounds and costs $350 each.

    Ignoring losses (which will be 15 to 30%), that's 3.6 kilowatts in direct sun (which is never since I don't live on the tropics).

    3.6 kilowatts out of 43 kilowatts... not going to cut it. It is enough to sustain 13 miles per hour though, so if I vacation exclusively in school zones I'm set.

  4. #4
    Senior Member Starjots's Avatar
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    In the last post I showed that strapping a lot of panels on the old motor home wasn't going to do much good.

    A different vehicle is required, but what would it look like and is it feasible?

    To explore this, I can combine the two equations already mentioned into:



    Setting velocity and power constant, I can graph the solution space in terms of the composite variables Cr*M (rolling resistance) and Cd*A (wind resistance).



    To put this in context, if I plotted the existing old and heavy motorhome on this chart it's Cd*A would be 2.5 and its Cr*M would be 54.

    Generating charts like this should make it easier to evaluate different design tradeoffs.

  5. #5
    Senior Member Starjots's Avatar
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    Fixing one variable, Coefficient of Rolling Resistance

    Of all the variables mentioned so far; power, mass, frontal area, coefficient of drag and coefficient of rolling resistance, the easiest to pin down is coefficient of rolling resistance.

    This is determined by the tires used.

    The solar car racers use tires with rolling resistance as low as .0025. Problem is, they are designed to support very little weight, wear out quickly and are very expensive. However, several tire companies which provide tires for solar racing have used what they learn to come out with low rolling resistance tires. Car manufacturers, responding to higher demand for better mileage as well as stricter requirements for fleet fuel economy, are putting low rolling resistance tires on their eco models such as the Prius and Honda Insight.

    So there are good solutions already available backed by megabucks worth of research, engineering and design. However, Crr isn't something that is typically quoted as a spec. Wikipedia however, comes through.

    Low Rolling Resistance Tires

    The second lowest Crr, the Michelin Symmetry P225/60R16 has a load rating of 1600 pounds, sufficient for this project and a measured Crr of .0065. The tires are well reviewed, all season and put an upper limit of 5600 pounds on the vehicle. Low mass is a goal. The Ultravan, a 1960s motor home with aluminum monocoque construction, only weighed 3000 pounds so this is in the realm of reasonable.

    Substituting in the value for Crr of .0065, the solution space graph now looks like this:


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    I've been leaning toward this solar-powered perambulation through the troposphere for years.
    I purchased an electric-assist bike last summer and I have imagined it serving as the tow vehicle for a minimalist shelter not qualifying as a POSH home.

    I'm thinking that some aftermarket modifications to the ELF might be worth some thought and possible monetary investment.

    Though I'd like my solar-powered home to float and be wind powered when on water as well.

    A great subject, thanks for starting the thread.

  7. #7
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    Quote Originally Posted by Starjots View Post

    Fixing one variable, Coefficient of Rolling Resistance


    Of all the variables mentioned so far; power, mass, frontal area, coefficient of drag and coefficient of rolling resistance, the easiest to pin down is coefficient of rolling resistance.
    Which is why steel-on-steel as found in rail/trolly systems is so damned attractive.
    I'd like to see something equivalent to the I-x5 system parallel each of those north-south interstates done with steel-on-steel rails ... no I don't need the rail made of Rearden metal.
    I'm imagining seasonal snow bird migrations of solar-powered rail-following vehicles that might take a month or so for either leg to get from the snow belt to the sun belt and back.
    I'd wager that old geezers would rather have rails guide their `RV' than to have to steer it on I-95, I-85, I-75, etc.
    Though the serial nature of rails should entail that pull-offs be built into every line to all faster vehicles to push the slow pokes off onto a siding

    If you can't float something on liquid helium, steel-on-steel is a pretty low-roll-resistance way to go.

  8. #8
    Senior Member Starjots's Avatar
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    Quote Originally Posted by gps View Post
    I've been leaning toward this solar-powered perambulation through the troposphere for years.
    I purchased an electric-assist bike last summer and I have imagined it serving as the tow vehicle for a minimalist shelter not qualifying as a POSH home.
    Very cool shelter! As you can probably tell, reducing frontal must be a major goal for the solar motor home which implies some form of popping out, popping back and popping up.

    I've also toyed with, and not totally given up on the idea of lots of bike tires and bike electric motors, sort of like a cycle centipede. The advantage being these items have a wide market already and if one fails it doesn't break the bank. Actually making something like this work I haven't attempted to work out, so for now I'm going down the more conventional design road.

    I'm thinking that some aftermarket modifications to the ELF might be worth some thought and possible monetary investment.
    Nice. I've watched a lot of videos of recumbent bicyclists who have an aerodynamic shell and the speeds those guys are getting are really high. Makes bike lanes make even more sense for urban planning.

    Though I'd like my solar-powered home to float and be wind powered when on water as well.

    A great subject, thanks for starting the thread.
    My sister and brother in law are shoe-stringing a sail boat retirement right now on a home built 44 footer. They said the number one thing that limited how far they went and where they went was the cost of petrol. I thought you just sailed around, you know? Apparently not, with giant freighters and storms and what not I guess you go from point A to B as quickly as possible.

  9. #9
    Senior Member Starjots's Avatar
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    Quote Originally Posted by gps View Post
    Which is why steel-on-steel as found in rail/trolly systems is so damned attractive.
    I'd like to see something equivalent to the I-x5 system parallel each of those north-south interstates done with steel-on-steel rails ... no I don't need the rail made of Rearden metal.
    I'm imagining seasonal snow bird migrations of solar-powered rail-following vehicles that might take a month or so for either leg to get from the snow belt to the sun belt and back.
    I'd wager that old geezers would rather have rails guide their `RV' than to have to steer it on I-95, I-85, I-75, etc.
    Though the serial nature of rails should entail that pull-offs be built into every line to all faster vehicles to push the slow pokes off onto a siding

    If you can't float something on liquid helium, steel-on-steel is a pretty low-roll-resistance way to go.
    Just for goofs I recalculated putting the big 31' RV on steel rails using Cr of .002 (from wikipedia, passenger car on steel rail) and got a power requirement of 32 Kilowatts to maintain 60 mph. That's a 25% reduction in energy from the 43 KW for it's regular tires. Crazy.

  10. #10
    a cantori Perdix's Avatar
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    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....

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