Wednesday, August 14, 2013

The BMW i3 and a Solar PV Roof ?

As both a BMW customer and a SunPower customer, with a few free hours for ideation (fancy word for daydreaming,) I paired these great companies and products together and it energized my eager mind.

Is it time to start looking at a solar roof for the i3?   

The BMW i3 is lighter and more efficient than any other 4 passenger electric car in production, thus it can drive more miles per kwh (somewhere around 5 mpk.) Solar PV is ever increasing in it’s ability to crank out more electricity for less cost in less space. The SunPower X21 panel is the world’s most efficient consumer panel rated at 345 watts and is 61” by 41” in size.   

61” by 41” strikes me to be very similar  in size as the general dimensions of the roof on the BMW i3. Is it time to start looking at a solar roof for the i3 and for BMW to talk to SunPower?

SunPower X21 cells on Stanford's solar racer.
Let’s break it down and find out!

A SunPower 345 watt panel will produce 530 kwh per year at an optimum angle and orientation in California  (1kw = 1550kwh production per year.)  If you assumed the BMW i3 would have the same exposure to the sun as your house roof, you would most likely de-rate about 15% due to the flat pitch of the BMW i3 roof leaving you with approximately 450 KWh per year.   That amount of energy will power the BMW i3 for 2250 miles a year.  (450kwh times 5 miles per kwh.)   That would be around 20% of the total energy required for your typical 12,000 mile a year driver.  

A realistic scenario for a traditional 9-5 worker with a constant workplace  is that your car is driving in the morning and afternoon when the solar irradiation is very low and parked at work for most of the daylight hours.   Assuming a nice non shaded parking space, a de-rate of 33% would be about right, leaving you with 354kwh of generation annually, good for 1770 miles of driving a year.  

An even more realistic scenario that is blended, is that some days including the weekends your car is in the garage all day, some days you are in the shade but most days in the sun, and some days you’re on the road several trips a day with all sort of shading variations.  In this scenario a de-rate of 50%  or 275kwh of generation annually, good for 1375 miles of driving a year.

In my scenario,  I do park outside and there is lots of sun, I would estimate around 300kwh a year or 1500 miles a year could be powered directly by  a solar powered roof rated at 345 watts.

The SunPower panel is running about $2.00 a watt thus a retail panel cost of around $700.    I’m sure there are significant cost in affixing individual cells to the CFRP roof and mono crystalline cells are not flexible but I would imagine a custom mold where the cells could be connected and sealed and then that outer shell affixed to the roof of the car.  An Inverter of some sort would be required and some crazy ass smart engineering and software controls added. (Because I want to see how much electricity my cars roof is generating on my i-drive.)  All in all,  I see no reason why this option should cost more than $3000 for a high production vehicle.

Would I pay $3000 for this option?  Heck yes!  1000-1500 mile a year free for the 10 year life of the car.

Does it save me money over roof top solar on the house? Heck no.  That $3000 panel on the car is generating half on the electricity of the $700 panel on your roof.  

Is flexible thin film a better solution? Perhaps because cost would be greatly lowered but electricity production would be only 60% of the x21 cells.

I say bring it on as an option BMW!  Then again, it would be a shame to hide all that beautifully exposed carbon fiber on the roof of the car and not be able to have an operable sunroof.

The dawn of a true solar 
powered car is nearing.  At least in part.