Why not solar cells built into top?

[DeafSoundGuy]

So I can't help but wonder why BMW can't implement solar cells into the top of the car. Upon my first few days of owning this incredible car, I started thinking about my beloved '92 Mazda 929. AT the time it was the first real luxury car from Mazda. It was extremely well made and was stunning inside with the best (to this day) stereo I've ever heard (or measured). But one of the coolest features of that car was it had solar cells built into the sunroof glass that powered up fans that were built into the trunk. This way, whenever the car was in the sun at a stop, the solar powered fans would suck hot air out of the car and exit them through the trunk somewhere. You could come out and hear them running faintly. If they weren't powering up the fans then they would put power back into the battery. I thought that was so cool.

The i3 has a pretty big top especially for us US buyers with no sunroof. This would be another way of regenerating power and getting back your battery life obviously if you were low, the suns out and you need a better SOC.

Is this too much a pipe dream because of the sheer amperage needed for recharging, or too expensive to implement? I can't help dream about ways of embellishing an already extremely efficient cool car.

 

[ultraturtle]

So I can't help but wonder why BMW can't implement solar cells into the top of the car...

...Is this too much a pipe dream because of the sheer amperage needed for recharging, or too expensive to implement? I can't help dream about ways of embellishing an already extremely efficient cool car.

You are correct in that amperage is indeed the issue. Both Mazda and Toyota only use roof mounted solar panels to power low draw ventilation fans for this reason. Charging the traction battery takes bucket loads of power.

The lowest charging amperage I've been able to build into a 120 volt EVSE is 6 amps, and have done so specifically for the purpose you state - for photovoltaic (PV) charging.

I have a 200 watt flexible array secured to a car cover that takes up pretty much all of the front hood, windshield, and top. It puts out only 16.7 amps of 12v DC under optimal conditions, which at 85% inverter efficiency could theoretically supply only 1.4 amps at 120 Volts AC - not even a quarter of the minimum 6 amps I need to charge. For this reason, I introduced a 4.8 kWh Lithium Ion battery system (12 Volt, 400 ah) into the mix. It easily supplies a full 12 amp 120 volt charge using the included BMW Occasional Use Cable (OUC), and on a full charge can pump as much as 16 miles of range into the i3 on a long sit, while simultaneously recovering as much as 4 miles of range potential (~1.2 kWh) per day from the sun.

I just have to remember to park with the nose of the car facing south, and I can not only put 16 miles of range into the i3 battery on the first day I park it, but can often replenish that same 16 miles of range potential over the course of my typical 4 day trips.

As for cost - yeah, it's prohibitive.

 

[DeafSoundGuy]

I have a 200 watt flexible array secured to a car cover that takes up pretty much all of the front hood, windshield, and top. It puts out only 16.7 amps of 12v DC under optimal conditions, which at 85% inverter efficiency could theoretically supply only 1.4 amps at 120 Volts AC - not even a quarter of the minimum 6 amps I need to charge. For this reason, I introduced a 4.8 kWh Lithium Ion battery system (12 Volt, 400 ah) into the mix. It easily supplies a full 12 amp 120 volt charge using the included BMW Occasional Use Cable (OUC), and on a full charge can pump as much as 16 miles of range into the i3 on a long sit, while simultaneously recovering an average of about 4 miles of range potential (~1.2 kWh) per day from the sun.

Too cool. Sounds like you have way too much time on your hands . How about a used fast food wrapper to DC conversion adapter?

 

[ultraturtle]

...How about a used fast food wrapper to DC conversion adapter?

Sounds like a worthy challenge. Send me a diagram, and I'll work on it.

 

[elptex]

I'm based in the UK and only aware of costs here. A single home PV panel that is roughly the same size as the i3 roof comes to about £500. I don't see that as inherently prohibitive. With that said, what that is capable of powering determines the true value. If it could take care of environmental power needs (somewhere between just fans and full-blown heat generation) then that would make up for the 10-15% range loss. Over the long haul that seems worth it.

If it can only run a fan, however, that's saving a couple hundred watts of energy so not really benefiting battery longevity, thus making the cost pointless.

IMHO

 

[BoMW]

What is the weight of a PV panel? It seems to me that the additional weight would counteract the additional power and therefore not feasible.

 

[ted99]

I wondered about putting a PV panel on top of a parking space cover (common in TX retirement communities). It seemed to me that the optimum path would to wire the PV cells in series to provide 400v DC, thereby avoiding the inefficiencies of DC to AC, then back from AC to DC conversion. I don't know whether the i3 DC "fast" charging connectors could be used for this purpose. Is there a similar logic circuit as the AC EVSE which has to be activated before the DC fast charging circuit in the i3 will activate? Seems to me this would be like using a small PV panel to keep a normal 12v battery charged--slow, but any little bit helps.

 

[KurtEndress]

I'm based in the UK and only aware of costs here. A single home PV panel that is roughly the same size as the i3 roof comes to about £500. I don't see that as inherently prohibitive. With that said, what that is capable of powering determines the true value. If it could take care of environmental power needs (somewhere between just fans and full-blown heat generation) then that would make up for the 10-15% range loss. Over the long haul that seems worth it.

A commercial panel like that is about 2 square meters and provides 200 to 300 watts of power when getting direct sun. When you are cruising along at highway speeds you are probably using 15 kW or more of power. So that panel would provide less than 2% of your power use on the highway. Not much of a range extender as it might not even pay back the minor weight and drag penalty. The REx will provide up to 25 kW, which is the equivalent of about a hundred solar panels.

If your car was parked in the sun all day that roof might generate a single kWh or so giving you a few miles of extra range on a sunny day. Better would be a garage or carport sized fixed solar installation that could completely recharge your car during the day.

 

[jadnashuanh]

BMW showed a (prototype/mockup) carport with the roof made of solar panels with an i8 parked underneath (if I remember correctly). Other than ventilating a parked car, the area of the roof doesn't seem like it would provide enough power to justify the costs. Some of the panels are flexible, but it seems the best ones are rigid, and aerodynamics may come to play. Protecting them from hail, or cleaning them off if it was covered in snow or ice risks damage, too.

 

[nowtta60]

I think the key benefit that do have a value are:

+ stop the battery getting bricked if you pull up in an airport car park after draining the battery and go on holiday for a couple of months.
+ keeping the 12V system powered so the central locking, alarm and telematics can keep working.
+ keeping the cabin AC running before you get in the car without using the traction battery
+ emergency option for end of the world scenario. You can lay lolw, leave the car charging for a couple of weeks and then do a 50-75 mile dash to avoid zombies / alien invading force / pandemic. More if you have the REX and don't mind syphoning the abandoned cars into a gerry can ;-)

I think the most you'll get on the roof is about 500W peak. Aero won't be affected as they'll be flat into the roof structure.

I've got an X-Trail that I use infrequently. Once or twice the battery hasn't been able to crank the engine. Even that could benefit some roof mounted solar cells, so I'm sure the i3 could.

 

[elptex]

I'm based in the UK and only aware of costs here. A single home PV panel that is roughly the same size as the i3 roof comes to about £500. I don't see that as inherently prohibitive. With that said, what that is capable of powering determines the true value. If it could take care of environmental power needs (somewhere between just fans and full-blown heat generation) then that would make up for the 10-15% range loss. Over the long haul that seems worth it.

A commercial panel like that is about 2 square meters and provides 200 to 300 watts of power when getting direct sun. When you are cruising along at highway speeds you are probably using 15 kW or more of power. So that panel would provide less than 2% of your power use on the highway. Not much of a range extender as it might not even pay back the minor weight and drag penalty. The REx will provide up to 25 kW, which is the equivalent of about a hundred solar panels.

If your car was parked in the sun all day that roof might generate a single kWh or so giving you a few miles of extra range on a sunny day. Better would be a garage or carport sized fixed solar installation that could completely recharge your car during the day.

Exactly. Well said points.

 

[Maki]

Absolutely possible. Solar panels would be very desirable. BMW actually did not look beyond one mind set. There is very inexpensive flex solar panel on EU market. I am planning to put it on roof when parked so I can gain little more energy in my i3. To do that I want to see real temperature under summer conditions, because solar panels tend to worm up. I do not want to melt my carbon fiber top.

 

[jadnashuanh]

If have to ask...where are you going to connect your solar panels? The 12vdc battery is not designed to be externally charged or jump started. THe main batteries are 360vdc versions...so, since it won't accept input via the charging port unless you can make some acv or high voltage dcv (at least 110vac and closer to 400vdc for the dc input), and then, it would need control logic to make it happen and accept it...just what do you expect to accomplish? The control logic in the car that manages recharging would likely take almost all of the output of a panel you could put on the roof, and leave little to try to recharge the batteries, and, to make the signal generator and control line logic to make the car think it had an EVSE attached would take some power and money to design (and waterproof), I just don't see it happening unless the factory decided it was worth the effort.

You might be able to hack into the ventilation fans, or maybe add your own to help cool things in the summer while sitting, but other than that, I don't have much faith you'll succeed in anything other than maybe damaging your i3.

 

[I33t]

The cost of PV is low, but the benefit of a single panel on an i3 is also low.

Firstly, the sun's radiation equates to roughly 1000w per square meter and the efficiency of PV is around 20% when tilted at the correct angle to pick up the sunlight. Less if it's horizontal.

Secondly, If you park your car in full sun, the car will be hot in summer and you will probably consume more energy making the car comfortable than you collected. In winter the sun's energy is a lot less because of the angle of the sun and the weather. There is also the problem of shading from buildings and trees, you probably won't get a reasonable stretch of full sun.

Expensive way of not charging the 12v battery considering how many cars are parked in carparks.