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A practical solar car for $15k

Solar transportation for self-sufficiency.

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For several months, I've been modeling in 3d a solar car that could be built as a kit for around $15,000 USD. It would also cost next to nothing to run. In the back would be a work area with outlets, drawers, and a fold-out bench top. This car could really help self-employed people live their dreams, not to mention helping to save the environment. The car is a five-seater with twin hub motors powered by lithium batteries charged with solar panels and an on-board AC unit.

The frame, suspension, and body will be made of wood, mostly douglas fir. Its superior stiffness-to-weight ratio allows much lighter construction compared to steel or aluminum. While the metal suspension components, hub motors, and other electricals are from aftermarket sources, most of the car could be built at home relatively inexpensively. The solar car will have 5 mph expanded polypropylene bumpers and fenders to minimize damage in accidents. The wood parts will be sealed against water, fire, and insects.

What can you do in a car beside going places? Not much. Modern automobiles enable us to travel quickly in style but are expensive to maintain and degrade the environment. They are not designed for self-employed people, nor do they encourage sociability in neighborhoods.

My proposed solar car would be an economical solution to these issues and would establish a benchmark of environmentally-friendly urban transportation. It would charge up from a robust photovoltaic array, and for increased mileage and range, the superior stiffness-to-weight of wood keeps weight to a minimum. Wood is already manufactured by nature, unlike the metal that comprises most cars, and the tooling and assembly of the vehicle would require far less energy. Wood is also 100% recyclable.

This is a car for the self-employed: rental property owner, taco stand operator, artist, craftsman, plumber, or even juggler of hats and bowling pins. Pack the tools of your trade in four locking drawers, fold down a large work surface when you arrive, plug in your tools chargers and USB devices, and then roll up your sleeves. After work, as a food and beverage support vehicle for social gatherings, it would excel wherever parked. This is a car design that would enable people professionally, save them money, and bring them together.

I've given a lot of thought to the use of wood as an engineering material for cars. It's not a perfect material but neither is metal. Here's my comparison:

Stiffness is critical for car frames and bodies, to minimize their twisting when navigating bumps and corners. Metal is stiffer than wood, but only if you compare volume of the material. A metal nail is much stiffer than a wood nail of the same dimensions. However, if you compare by weight, wood is much stiffer. For example, a steel bar 1/4" thick, 1/2" wide, and 24" long can be bent over your knee, albeit with some effort. Douglas fir, which is commonly used for house framing and boat stringers, is 1/15th the weight of steel. Imagine bending a chunk that's roughly 1" thick, 2" wide, and 24" long. Even with the grain flat against your knee, it would be extremely difficult for all but dedicated bodybuilders.

Regarding environmental damage of cars left out in weather, steel and wood are prone to rust and rot, respectively. However, by coating wood with waterborne polyurethane on all its exposed surfaces, water and insect damage is for all practical purposes eliminated. Water content also fluctuates much less, with the result that dimensional stability is greatly improved.

On the subject of body panel or frame damage in a collision, the repair expense is significant for cars constructed of either wood or metal. To minimize damage, the solar car of wood will be protected at exposed points with expanded polypropylene, a recyclable material which has the excellent property of shape memory.

In the case of damage inside the car from friction of moving parts, metal wins the day. For this reason, a wooden car's suspension and drive components are best made of metal or composites.

Another concern with wood construction is fire damage, though it's actually difficult to start a wood fire in a car, even by holding a lighter against a wood panel. Would you feel safer in an accident in a fossil-fuel car, sitting a few feet away from several gallons of gasoline in the tank? Some might counter that lithium batteries in electric vehicles are a fire hazard, but the use of lithium iron phosphate chemistry in batteries has minimized that risk. There also exists the danger of arcing and starting a fire at the high voltages involved in any electric car, but for that reason, multiple circuit breakers are routinely employed by electric car builders, and my project will be no exception.

If wood is so great as an engineering material, then why is it no longer used in mass produced vehicles? The reason is that metal parts can be stamped or extruded quickly in an automated factory, but not wood parts, which have to be...

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  • 1 × Frame, suspension, steering, brakes The frame would be a box beam of Douglas fir sides and plywood top and bottom, a very strong structure. The double wishbone suspension would be a bolt-in modules, front and back, consisting of a box-beam cross member and aftermarket A arms and coilovers. An aftermarket manual rack and pinion steering system will be installed. Disc brakes at all four corners, probably motorcycle aftermarket units considering the light weight of the vehicle.
  • 1 × Body and doors Body panels constructed of plywood. two doors instead of four, for body stiffness, lower weight and cost. The doors would have three 1/4" ply on the outside, to provide a window track, beams inside the door for crash protection, and ply or paneling on the inside. Door hinges are metal piano hinges. Window and lock mechanisms are mechanical and use white oak.
  • 1 × Windows Windshield would be an aftermarket laminated glass safety windshield. Side windows would be of polycarbonate.
  • 1 × Solar panels and charge controller Five 240 watt banels in series would recharge the battery bank rather than drive the car directly. A special charge controller will be used that boosts or bucks voltage, depending on solar conditions.
  • 1 × Lithium battery bank and AC charger A bank of twelve 40-amp lithium iron phophate cells and an onboard 12 amp AC charger. These cells have a great track record in EVs..

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  • Blondes have more fun.

    Joel Fairstein06/10/2016 at 08:09 0 comments

    I've decided the wood finish should be more of the natural blonde or strawberry blonde color of douglas fir rather than a stained cherry finish. This will save some time and money and also keep the car cooler inside.


    I've renamed the project A practical solar car for $15k to emphasize its low cost rather than its wood construction.

  • Image tweaks and build plans

    Joel Fairstein05/23/2016 at 10:14 0 comments

    I changed the wood finish to a dark cherry and made some better scene renders, which I hope are a little more pleasing to the eye than my earlier Sketchup exports. As far as building this thing, first I need to move my family back to the US from China this Fall and find a house with a decent garage or workshop area. If I can secure some modest funding, I'll start the build, which I figure will take about eight months. After that, another four months of testing and corrections, then I can release the final plans free and clear to the public.


    I'm anticipating some areas of difficulty:

    1) Door and work area clearances. Temperature doesn't affect wood dimensionally as much as does humidity, so I'll measure clearances between the door and body on different days in order to track and predict changes. If I use marine-grade plywood, this won't be much of an issue.

    2) Integration or solar charging and AC charging. Not sure if there will be a switch between the two or if they be rigged to can charge concurrently. Also, the MPPT (maximum power point tracker) has to be set up just right for the solar charging.

    3) Suspension tuning. I hope the suspension design parameters are close enough that I can take the car to a good alignment shop and have them do their thing. I'm using adjustable coilover shocks with the lightest springs available, but the ride may still be stiff since the design is so lightweight.

    4) Hub motor setup and performance. The car may be under-powered with only two 3000w motors, but there's not much range of product out there that's affordable. I'd rather have a lower top speed in exchange for good starting torque, if that solves the issue.

  • Floated the front bumper

    Joel Fairstein05/21/2016 at 07:29 0 comments

    A real issue with any car is the likelihood extensive damage even after low-speed collisions. I totaled a Ford Taurus once in a parking lot doing only 5 mph. The bumper and front molding cracked, various metal parts behind that were bent, and the radiator burst. I couldn't believe how bad the damage was. So, with the solar car of wood I've designed a bumper that floats on four inches of expanded polypropylene foam, which has shape memory. That's in addition to the existing 2" foam block in front of the bumper. There's also another 2" hump in front of that as well as 1" foam above and below the lights. The fender corners are also floated on four inches of foam and the fenders themselves are polypropylene over wood. The front bumper will be held by heavy-duty spring fasteners, since polypropylene adheres poorly to other materials. My hope is this car can withstand a 5 mph collision with no damage and possibly a 10 mph collision with minimal damage. This, I believe, is really important in a city car!

  • Cargo area improvements

    Joel Fairstein05/12/2016 at 03:55 0 comments

    The front cargo area (light colored wood) now has a diagonal board for better fore-aft bracing. In back, a 48"-wide drawer for long items or artwork replaces three desk drawers . When pulled out, it can also help support the fold-down work surface. The lower drawers on either side are now standard file-size for carrying documents--handy for doing business or real-estate. The larger drawer in the middle can be insulated for food and beverage. All drawers are equipped with high-quality slides, and the lower drawers have locking latches.


  • Batteries, safety features, and other improvements

    Joel Fairstein05/08/2016 at 05:54 0 comments

    I made several design changes for safety and practicality:

    • Batteries moved lower and forward.
    • Reinforced electrical area under the hood and opened up air channels.
    • Reshaped fenders for wheel clearance.
    • Enlarged front bumper and added vent slots.
      Added rear strut rods and emergency brakes.


    With these changes, I hope I've addressed most of the practical and safety concerns of making a solar car out of wood. My thought process now will be about the wood joinery and how to reduce weight as much as possible without sacrificing structural integrity.

  • Redesigned rolling chassis

    Joel Fairstein05/05/2016 at 01:15 0 comments

    The rolling chassis has been re-designed to integrate the new suspension geometry. An under-carriage trapezoidal box frame connects the front and rear suspension modules, which are themselves boxed into the chassis. Longitudinal torsional stiffness, my main concern before, should be more than adequate. The idea from here on out is keeping vehicle weight down, currently around 1300 pounds, as any weight savings translates into increase battery range.

    Several other changes were made since the last update:

    Integrated hub motors into rear suspension with custom metal brackets.
    Wheel diameter increased 2" to accommodate modern disc brakes.

    Increased track width by 5".
    Raised body 5".

  • Suspension designed

    Joel Fairstein04/22/2016 at 02:43 0 comments

    The suspension design has so far been the most challenging task. I've designed a front double-wishbone suspension with a triple-laminated cross member. Rear suspension will be narrowed to accommodate the width of the hub motors. The shock tower boxes will be fabricated from 3/16" steel and attached to each side of the crossmember. The brakes are Wilwood drag-racing brakes to save weight. The QA1 coilover shocks have ride height and rebound damping adjustments. This baby should really handle! The next challenge will be to rigidly integrate the suspension to the rest of the frame.


  • Waiting for suspension measurements

    Joel Fairstein03/17/2016 at 02:46 0 comments

    At this point, I've designed the entire car in 3d but with only an approximated suspension. Suspensions are notoriously difficult to design, so I'm waiting on a set of plans based around the Mustang II double wishbone suspension, which is the go-to suspension for street rods. I've even seen it used in VW type 1 and type 2 bolt-on kits. It's a proven design and aftermarket parts are readily available.

    I'll build the cross members to spec using box beams and then attach all the metal parts in the correct locations. This way, front and rear suspensions are modules that can be attached to the box beam frame member. That said, I will probably extend the suspension side walls longitudinally to form a longer bond surface to the frame.

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Joel Fairstein wrote 06/10/2016 at 14:03 point

Thanks Mike!

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Mike Maluk wrote 06/10/2016 at 08:07 point

Looking forward to the build!

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Paulo Andrade wrote 03/18/2016 at 10:56 point

«For all practical purposes, it happens when you let off the throttle.», Basically it just reverses the polarity and charges the batteries, It doesn't  brake that much but allows you to recover up to 20% of the energy. (I think I reed it somewhere). You need a H-bridge that is capable of prevent reflow or u will fry your car electronics. I had a similar project but I've been giving up on projects like that to improve my grades. The best University in Portugal is only 20 km (12 miles) from my grandparents house so I have to get at least an 16/20 to one engineering course and I have 14/20.

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Joel Fairstein wrote 03/17/2016 at 01:15 point

The controller and motor in the solar car of wood are regen capable. But after doing more research, I see just how muddled regen is. For all practical purposes, it happens when you let off the throttle. I'm not sure I understand why you would need a brake switch to kick it in, unless you want to reverse polarity and apply voltage to increase stopping power. But hey, then it's no longer regen!

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Joel Fairstein wrote 03/17/2016 at 01:06 point

Paulo, I've been following the OSV EV platform for a few months and like their business model. That said, their platform use McPherson strut suspension, which raises the shock towers thereby requiring lots of vertical frame elements in the absence of a body. That  sort of contradicts the platform concept. My design uses a double wishbone suspension, which can be fastened to the lower frame rails as a module, has a much lower profile, and automatically adjusts for camber when cornering. Also, a metal frame is going to be heavier.

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Paulo Andrade wrote 03/16/2016 at 15:53 point

K I get it, In Europe things work differently, What about regenerative braking? Check out https://www.osvehicle.com/ (get here some ideas for the car itself) and (something from self parking to self-driving with a few cams) http://opendavinci.cse.chalmers.se/www/

And if u want a personal assistant: http://sirius.clarity-lab.org/

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Joel Fairstein wrote 03/16/2016 at 01:06 point

Pauolo, thanks for commenting. Ugliness, like beauty is in the mind of the beholder. Most modern cars look like tennis shoes to me. But aside from that, regarding acrylic for windows, it is not only dangerous (acrylic shatters, especially when cold) but is prohibited in the NHTSA standards. I will use laminated safety glass for the windshield and polycarbonate for the side windows, to save weight. Regarding oak as a frame material, Douglas fir is much stiffer by weight, but I am considering using white oak where the suspension joins the frame, for it's toughness (fasteners won't hog out). I've sourced solar panels from China, and they are flexible but not that pliable. I'm not interested in laminating or steam bending curves for the body, except for the roof front, it's too labor intensive--I know this from my boat building experience. I am using EPP foam for some of the details, you can see this in the pics. Thank you for the suggestion to use Blender for rendering. 

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Paulo Andrade wrote 03/15/2016 at 15:47 point

Hi! It's an awesome Idea but the car is ugly as hell, I recommend that y use acrylic for the windows, oak for the frame and 4-8mm plywood to the bodywork, Wood can easly make better designs than metal, and you can always use fiberglass and foam to some details (mirror, etc).

Most cheap Chinese solar panels are flexible btw. 

ohh and use blender to render (Internal is fast and easier to beginners), sketchup-looking designs aren't that easy on the eyes.

Good luck :)

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