Rack mounted solar/PV, power generation - calculations/math help

[RainorshinePNW]

Hey friends- hoping some of the more electrically minded among us can help me confirm the potential charging power of a rack mounted PV array could be. Inspiration is for trips into the desert/high altitude playa for extending camping trips with chargers on the very fringes or out and back range.

This is just theoretical and I've not taken any steps yet to actually try and assemble this, nor am I (for the moment) thinking too much about the other components for actually connecting/charging (although admittedly they'll be important to figure out if this idea ever makes it from the whiteboard to real life).

Idea is to utilize a fairly standard work truck roof track mounted to the bed, extended over the cab, and to mount (5) SunPower X21-470 PV panels flat on top of it, oriented perpendicular to the truck. Each panel is ~42" wide, for a total length of ~18 ft.

Is the math simply 470 watts per panel, x5 panels, assuming 8 hours of sunlight a day, for a total ~19 kWh per day generation potential? Thanks!

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[GarageMahal]

Is the math simply 470 watts per panel, x5 panels, assuming 8 hours of sunlight a day, for a total ~19 kWh per day generation potential? Thanks!

Yes and no... The formula is essentially correct but you will never be able to maintain maximum output for 8 hours and you won't likely ever get to max at all.

As an example, I have a 14.5kw solar system and on a clear day in July in Minnesota (10 hours of usable sun) I produce 80kwh of electricity. Many things contribute to the loss include panel angle to the sun (noting that the sun moves and my panels don't). Panels fixed horizontally as I understand you are proposing will not come close to achieving max output as they really need to be angled toward the sun.

Every time I look into this the ROI is not there but it gets closer as panels improve (I had no idea they were up to 470; mine are 330 which was leading edge at the time)

 

[RainorshinePNW]

Thanks- and yeah, read some about efficiency loss from angle, weather, etc, with a suggestion of using 80% as the usable output. Would be interesting to build in some lifting/angling capability so that I can travel/drive with them flat, then move them into a better position once parked.

Still, taking 80% and dropping actual output to 15 kWh a day isn't too bad. Being conservative at 2.0 mi/kWh, should get ~30 miles of range per day from a setup like that.

The spot we like to go is about 120 miles from the nearest L2 charger, which theoretically would be within range for an out and back, but still makes me a little nervous pushing remaining range down to ~10%.

 

[Lightning.Dav]

Every time I look into this the ROI is not there but it gets closer as panels improve (I had no idea they were up to 470; mine are 330 which was leading edge at the time)

@GarageMahal They achieve the 470 by being physically larger. They are not more efficient than yours yet.

 

[GarageMahal]

80% is a decent number for the MAX output at the peak part of the day but don't count on it all day.

Snapshot of my system performance today. Again this 14.5kw in panels.

This shows my max at about 75% (10.75 / 14.5) and my math says I average 55% over the 10 hours of sunlight (8kwh average over 14.5; 80kwh/10 hours)

 

[sotek2345]

Thanks- and yeah, read some about efficiency loss from angle, weather, etc, with a suggestion of using 80% as the usable output. Would be interesting to build in some lifting/angling capability so that I can travel/drive with them flat, then move them into a better position once parked.

Still, taking 80% and dropping actual output to 15 kWh a day isn't too bad. Being conservative at 2.0 mi/kWh, should get ~30 miles of range per day from a setup like that.

The spot we like to go is about 120 miles from the nearest L2 charger, which theoretically would be within range for an out and back, but still makes me a little nervous pushing remaining range down to ~10%.

A couple things to keep in mind.

1) You will have losses between generation and charging. You will have to convert the Panel output to AC, and then have the truck convert back to DC. I don't have hard numbers, but I would guess about a 25% loss overall - so your 15kwh/day is down to ~11 or 12.

2) You will lose range by having them mounted to the truck due to the extra aero drag. What you are discussing seems similar (though a little better) than the slide in camper that TFL used on their lightning. They saw about an 80 to 100 mile range loss on the highway vs. EPA (about 230/miles per charge at highway speeds. (Down to about 1.8mi/kwhr). It should be noted that the Lightning ER only gets about ~260-270 on the highway anyways if you keep it at 70mph, so not all of this is from the setup.

So - if you get 12kwhr/day, that is about 20 miles (+/-), but you will lose about 40 miles of range on the highway. It would take 2 days of charging without using any power in the truck just to get back to where you would be without them.

 

[GarageMahal]

@GarageMahal They achieve the 470 by being physically larger. They are not more efficient than yours yet.

Thanks for pointing that out!

 

[hturnerfamily]

you'd need not only solar, but an inverter for at least 120v output, a battery bank for putting out a constant voltage, and then hopefully the truck would also automatically start charging each time the inverter, solar, and batteries fails to supply that... it's, unfortunately, but a losing cause, not even considering the huge expense.

 

[RainorshinePNW]

1) You will have losses between generation and charging. You will have to convert the Panel output to AC, and then have the truck convert back to DC. I don't have hard numbers, but I would guess about a 25% loss overall - so your 15kwh/day is down to ~11 or 12.

Thanks - this is some of the math I knew I'd be missing, AC to DC, plus any and all other losses as power gets from the panels and into the battery eventually

 

[tankengr]

Here's a link to a solar calculator produced by the National Renewable Energy Lab. You put in your location, roof slope (in your case 0 degrees), and system parameters. It takes into account historical weather records and will tell you how much energy your system should produce.
My home system does outperform these estimates, but not by much.

https://pvwatts.nrel.gov/

 

[sotek2345]

Here's a link to a solar calculator produced by the National Renewable Energy Lab. You put in your location, roof slope (in your case 0 degrees), and system parameters. It takes into account historical weather records and will tell you how much energy your system should produce.
My home system does outperform these estimates, but not by much.

https://pvwatts.nrel.gov/

Good link. Almost perfect results for my solar system.

 

[Firestop]

you'd need not only solar, but an inverter for at least 120v output, a battery bank for putting out a constant voltage, and then hopefully the truck would also automatically start charging each time the inverter, solar, and batteries fails to supply that... it's, unfortunately, but a losing cause, not even considering the huge expense.

Yep, and speaking of non-math limiting factors, if you go for a 120V output solar conversion for the truck, the truck will restrict the charge amperage to 11A….or, roughly 1.1 KWh/hr ….. real slow……

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