Solar charging DIY

Discussion in 'Prime Main Forum (2017-Current)' started by BrickedKeyboard, Dec 27, 2019.

  1. BrickedKeyboard

    BrickedKeyboard Junior Member

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    So I worked out a pretty simple paper solar charging system.

    As many high efficiency solar mats as will fit. You need the 25% efficient ones or better. An MPPT charge controller. Connect the charge controller to a fake battery made of banked capacitors, probably configured for 48 volts. (3 modules of caps in series). This will store approximately 30 watt hours. And an inverter.

    Main idea : the Prime needs 1400 watts to charge at 120 volts. That's a lot of power. So you slowly charge up with solar mats your capacitor bank. If you can find a place on the body of the car for 3 mats, that's 330 watts of input capacity. You can store just 26 watt hours, or just 66 seconds of charging time.

    So the system trickle charges up for approximately 4.5 minutes in peak sunlight, enables the inverter for about a minute, which means the car will detect 120V AC available, charge for a minute, then shut off.

    So the last piece is inside a metal box where the electronics are you solder in the usual arduino and SSR relay to glue this thing together: have the arduino monitor the ultracapacitor voltage through a divider and cycle on the SSR relay at one voltage and cycle off at another. The SSR relay would be connected to the input lead to the power inverter.

    How well this will really work depends on losses - how much energy gets used "setting up" to begin charging the battery? Also, cycling this often might be bad for the components, certain components like the relays involved might exceed their MTBF much faster than intended by Toyota.

    My other though is I wonder how you attach such a mat to the car without damaging the paint. And yes, I recognize that even in the best case, you would gain about 1.65 kwh per day. A system would take a long time to pay for itself, if ever. Without cost-optimizing, it would be $200 (inverter) + $600 (solar mats) + $420 (capacitors) + $90 (charge controller).

    Plus a metal box to contain the obvious fire hazard of bodged together electronics and some wire and about $20 of electronics.

    Questions for those knowledgeable about the Prius:

    a. What happens when the Prius Prime gets AC power. Does it actually cause any relays to switch? What do you think of the idea of charging for a minute at a time in terms of wear on the charging systems?

    b. How would you secure a solar mat without destroying the paint?
     
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  2. bisco

    bisco cookie crumbler

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    nice
     
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  3. Diemaster

    Diemaster Active Member

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    i often have thoughts of getting a flexible solar panel for an RV (or 2 depending on if it will fit on the roof,) or equivalent and then getting the solar roof components from a JDM or EDM Prime.
     
  4. fuzzy1

    fuzzy1 Senior Member

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    Only in perfect full sunshine, only when the panels are squarely pointed at the sun. Clouds, haze, and angled sun will all reduce that. As will high summer temperatures.

    Any car-mounted solar panels will suffer sub-optimum solar exposure conditions, i.e. more shade and poorer exposure angles than typical home solar systems. Car-mounted systems will also suffer additional air and weight drag.

    It generally seems that the most powerful, productive, and cost effective way to 'put solar' on a plug-in car is to put grid-tied solar modules on a home rooftop, bank the energy with utility net metering, then use the credits for ordinary plug-in charging. This way all the hardware is always working at the full production capacity available in the local climate zone, without causing extra vehicle drag. And when the system is producing energy that the car cannot use or store, all the available production capacity still gets put to use for something useful, instead of tossed overboard and wasted.
     
    #4 fuzzy1, Dec 28, 2019
    Last edited: Dec 28, 2019
  5. BrickedKeyboard

    BrickedKeyboard Junior Member

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    Oh absolutely you are correct. Note that any fixed mount solar panel will get "less than full sunshine" results, however. The quickest way to estimate this is with a "sun hours" map: these maps usually assume a panel pointed South at a fixed angle of 30 degrees. Example: Solar Insolation Map: How Many Sun Hours Do You Get?

    The location I am considering is Southern California, which you can see from the map is a good location for solar with 5 hours a day.

    This setup wouldn't be intended to a be a serious product, and it might marginally save money, if any. The reason to do it is for the fun of it and also a "talking point".

    I'm actually considering doing this on a RAV4 Prime when it comes out this summer. By my rough figuring, the RAV4 prime has enough surface area to fit roughly 1 kW of capacity on the roof and hood.

    So with 5 "sun hours" a day, that would be 5 kW on an average day in Southern California.

    There is a loss of efficiency when the panels are pointing straight up, so the real number might be 4 per day, for 4 kW gained with 1 kW of panels.

    Electricity is very expensive in San Diego and San Francisco, about 19 cents a kWh for EV chargers. So this setup would technically save $277 a year, assuming the vehicle battery doesn't fill up.

    The reason to use supercapacitors instead of batteries is cycle longevity, and also since all the equipment would be in a metal box in the back of the vehicle, supercaps are more efficient than batteries and thus don't produce as much heat per cycle.

    The actual mounting of the panels, I think I would have to go to a professional shop that does custom cars and ask. Someone will know and have the skills to do this. Stainless steel screws with sealant around them are probably what you'd have to use, with a bit of patching of the paint when they are removed.
     
  6. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    If you're using flexible mats, do it all with adhesives. If they're rigid panels, the hardware for decklid spoilers and roof racks is more appropriate.

    I think your insolation and conversion estimates are optimistic. I think I would first experiment with one such mat, duct-taped to an old car hood and balanced on sawhorses in the driveway. You could use that as a 1/3 scale model to figure out the delta between optimum collection angle and what the car contour mount would give.

    The car hood is suggested mostly for ease of holding the project material but it would also give you experience working with paint less valuable than what's on your car. Additionally it shows you the honest effect of a convex curved mounting surface for fit, finish and power production.
     
    #6 Leadfoot J. McCoalroller, Dec 28, 2019
    Last edited: Dec 28, 2019
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  7. Tha_Ape

    Tha_Ape Active Member

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    Id put paint protection film on the surface and use neodymium magnets to secure the panel.

    If you want to drive with them on, you can use a basic aerodynamic equation to estimate the suction placed on them.

    1/2 the air density * the surface area * speed^2

    (pv^2S/2)

    Make sure all the units match up. For air density use kg/m^3, speed use m/s, and area use m^2.

    That will leave you with kg*m/s^2. Force, in this case, lift.

    That's just a back of the envelope calculation, but it'll give you an idea about how much pull they're gonna get.

    Keep in mind lift goes up exponentially with speed so going from 65 to 70 is a bigger deal than going from 55 to 60.
     
  8. BrickedKeyboard

    BrickedKeyboard Junior Member

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    This is a good idea. Ironically, the cost of the film dwarfs the cost of the solar stuff. Maybe I could just apply a couple rolls of the stuff crudely myself, it won't matter if it's imperfect and peeling since the panels will go on top.
     
  9. Rob43

    Rob43 Senior Member

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  10. fuzzy1

    fuzzy1 Senior Member

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    Panels mounted on the car won't have the same 30 degree tilt to the south used for the home solar production estimates. And if the car is used the same as many other cars, the panels won't won't be in shade-free locations all day every day, further cutting the total solar harvest.

    I lean towards home roof-top systems because of the greater collection duty cycle of the PV modules, and the less investment needed in support hardware (per kWh harvested), at least for systems large enough to make a difference.

    ===============

    Separately, I should point out that the above site's advice on optimum tilt angle (for maximum annual harvest) is too simplistic:

    "Optimal Tilt Angle (No Adjustments)
    If you never want to bother with adjusting your panels, set them at a tilt angle that is equal to your latitude."


    Unfortunately, that supposed 'optimum' fails to account for seasonal differences in cloudcover and length of daylight. Online calculators at NREL will provide more detailed results. Here is a chart I made from an older NREL calculator for my location, clearly showing that a more shallow tilt produces better results here, weighting more towards long clear summer days over short cloudy winter days. And that simply putting my panels in the same plane as my roof (4:12 pitch), to minimize mounting difficulty and wind loading, causes only a few percent loss.
    [​IMG]
     
    #10 fuzzy1, Dec 28, 2019
    Last edited: Dec 28, 2019
  11. BrickedKeyboard

    BrickedKeyboard Junior Member

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    Hmm. Couple of interesting details here. My first thought was look at how with a 0 degree tilt (flat on a car roof) it is 85 percent as good as the best fixed tilt. Making this idea viable.

    But on the other hand, If I use flexible mats flexed to the vehicle roof, which isn't flat, at any given time only some of the mat will be at the correct angle. So further losses perhaps. Might be 75 percent as good.

    Another factor is charging at just 1400 watts, some significant percentage of the energy would go to the charger electronics. Also wonder what happens at the end of each cycle when the power is cut - do the charger electronics just lose power?
     
  12. fuzzy1

    fuzzy1 Senior Member

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    Two more things to consider:

    - The Prius charger can be turned down to just 8 amps, or 960 watts. If this mode is persistent (not needing to be hand selected each time), then it may ease your design a bit.

    - Does the charger start filling the battery immediately, or is there some significant delay before it starts giving some charge to the battery? I have vague memories of a problem here (or is it just old age creating false memories:cool:), but you'll need to ask real Prime owners about it. A delay could eat up a bunch of the available energy each cycle.
     
  13. m8547

    m8547 Senior Member

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    There is some delay while the current ramps up, but it's not too long, probably less than a minute. I think it wastes some power during that time, but it's maybe wasting around a hundred watts at most not the full charging current.

    I think it does switch off the battery relays while not charging.

    The current limit is set by the EVSE (but the car takes the current it needs up to the limit). It might be possible to modify one to have a lower current limit.
     
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  14. Deoc

    Deoc Member

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    Here are things to keep in mind.
    The charging system in the prius plug in/prime uses significant amount of power about 100-200 watts just to charge, plus add the inverter you need to use and any losses in the middle you need at least 250 watts just to run the charging system, you need an inverter of at least 1800 watts and a battery big enough to power the inverter, if the inverter doesnt get enough power it will simple shut down, if you really want a solar panel the best way is to connect it to the 12v battery
     
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  15. Rob43

    Rob43 Senior Member

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    I find all of this a moot point...

    If this were doable, Toyota would offer it for Extra $$ in some top level trim package. But they don't because Toyota can't offer a worthwhile* charging benefit from some sort of highly efficient rood panel system.


    Rob43

    * My definition of worthwhile is at least a 70% charge from 0% during the course of one day when parked outside in full sun.
     
  16. heiwa

    heiwa Active Member

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    Mr. Chris Jones posted something like op is proposing. The pictures of his system was #16 on the thread "Estimated Miles / Best Actual Miles on Ev Charge". He also gave us his part list for his system at #23.

    Posted via the PriusChat mobile app.
     
  17. Diemaster

    Diemaster Active Member

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    o_Ouhh...toyota DOES offer a solar charge system. Its the pussy lawmakers here in the USA that couldn't bear a "non rollover safe" solar roof and made Toyota eliminate the solar roof. why else does JDM and EDM primes have solar roofs?

    And your definition of worthwhile is physically impossible in the footprint of a car today. Solar technology in 50+ years might but by then, we'll be driving nuclear powered EV's anyway.:ROFLMAO:
     
  18. BrickedKeyboard

    BrickedKeyboard Junior Member

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    Thanks. Looking at his system, it is of course very similar to my proposal. He has 400 watts worth of panels, and does not have the hood covered where another 150 watt panel would easily fit. So 550 watts is possible.

    The 3 large lead acid batteries he used are not what I would choose: a similar setup (avoiding possible damage if ultracaps are used), but with probably 1-2 used Valence U27-12XP, which weigh 46 lbs each. And he used 3M VHB tape, which sounds like the right stuff to use. Apparently you can get it off the paint later.

    My reason for doing this is partly the satisfaction of getting something like this working, partly so I won't have to ever plug in just to have to go unplug before a deadline (I live in San Diego in an apartment, where electricity is both crazy expensive and EV chargers usually have a 2-4 hour limit where you have to go unplug your car after), and partly as a "talking point" among my embedded systems buddies.

    I plan to use the rav4 prime as the target vehicle, which by my figuring can support much more solar. Let's say it's 750 watts mounted, and I get 4 "sun hours" per day in Southern California. 90 mpge means the vehicle needs 37.4 kwh/100 miles, or 374 watt-hours per mile. So it would regain 8 miles of free range per day. Or produce 1095 kWh per year, which at 19 cents a kWh is $208 a year.

    Technically this would even pay itself off in about 8 years, assuming I could build the system for $1500, which seems roughly doable.
     
    #18 BrickedKeyboard, Dec 29, 2019
    Last edited: Dec 29, 2019
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  19. Rob43

    Rob43 Senior Member

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    ...And that's why its no good ...


    Rob43
     
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