Solar Leaf

In nature, leaves generate energy for the tree by photosynthesizing. So presumably, the Nissan LEAF should photosynthesize too?

This was the silly comment I made to the head of the LEAF project at Nissan's European R&D team last month. It turned into quite an interesting discussion about the potential for a solar roof for the car.

As you probably already know, the Nissan LEAF has a tiny solar panel on the rear spoiler. It really is tiny - I estimate around 5-6 watts per hour on a really good day, which is pretty much useless for everything other than maybe charging up a mobile phone.

We ended up doing some quick estimates based on the power requirements of the car and the size of the roof to see what would be possible.

The figures were quite interesting. The roof of the car is large enough to fit a 300 watt panel - possibly slightly more with some fiddling around. We calculated that this could provide enough power to drive the car. Over a period of a year, we calculated that the solar panel would produce enough energy to drive between 2,800-3,000 miles each year in California or Florida. Further north the figures are still respectable: around 2,000 miles in Chicago or Seattle, for instance.

In Europe, the figures aren't quite so good. In damp and dull London, for instance, the range would be down to around 1,400-1,500 miles a year. But then again, England doesn't ever get a summer (it had one last year but I missed it - I was in the bathroom at the time). For a car that isn't designed to be a sun powered car, with seating for five people and highway performance, I think that is pretty impressive.

The downside? Of course there has to be one and in this case its cost. The additional cost per car would be in the region of $4,000 which makes it just far too expensive to be considered as a useful option. However, solar prices are continuing to drop. Within 3-5 years, I wouldn't write off the possibilities of Nissan offering a proper solar panel on the top of their LEAF electric car at a much more reasonable cost.

I think a 300 watt panel would be more like $1000 and a low power DC-to-DC converter about $50 to $100.

In bright sunshine, not at an optimal angle to the sun, getting only 200 watts might be lucky. Six hours generates (after losses) perhaps 1 kWh, giving one only about 3 or 4 miles a day, on the average, I suspect, if one does not park in the shade.

So, perhaps something more like 1000 miles a year in "moderate" conditions?

We took most of these variables into account with our approximate calculations, but you are right - there are still a lot of variables and those figures were closer to the best possible results than what you may get in a real environment.

One comment that was made is that you could end up with an entirely different type of road rage if you were waiting at the traffic lights and a high sided vehicle came and parked next to you and blocked out the sunlight! :lol:

I've been talking to two other electric car manufacturers recently about solar power on their cars. One of them reckoned they can get up to around 18 miles per day in the right conditions based on some tests they carried out last summer whilst the other one is confidently predicting 15 miles of solar power a day in California. These are much smaller and lighter cars than the LEAF, but I still think it is impressive that solar can be used to power a car at all, let alone provide several miles range a day.

Hmmm...if it takes 10hp to maintain a steady speed, that's about 7500W. Adding more weight costs more energy. Unless one is using the solar power to run the AC and take some load off the traction pack, all of the calcs I've yet seen show it's better to leave the PV panels on the garage roof.

Andy

Correct Andy,

The "rooftop" of cars is similar to a "flat" surface, so even when the car is parked in the sunshine, the angle to the sun's rays is ... usually poor, often cutting generation in half, or worse in mornings and evenings.

Placing the same panels on a south-ish facing, non-shaded, sloped roof is often a factor of around two better at generation.

So, "bank" the home-generated "green" electrons in the needy grid during the daytime, and get it back to charge your car at night.

Adding the charge directly to the car does not increase the range significantly, or reliably (except in unusual cases).

AndyH said:

Adding more weight costs more energy. Unless one is using the solar power to run the AC and take some load off the traction pack, all of the calcs I've yet seen show it's better to leave the PV panels on the garage roof.

Click to expand...

In my case - since the car is garaged at home & underground parked at office, it is probably a total waste. Apart from the fact Seattle only gets a couple of months of sunshine.

I wonder - if someone has an option to park underground or in Sun, would be it more beneficial to park in Sun but then cool the car - or just underground and keep the car cool.

Underground will keep the whole car cool, so it would usually be much better.

You are all right - it is better to have solar panels on a south facing roof than it is to have them on a car. However, don't expect a solar roof to simply be a case of taking off-the-shelf panels and fitting them to a car, it would be a complete array design optimized for the application, built to the correct voltage and taking into account the curvature of the roof. As a consequence it is possible to significantly reduce the losses and get the efficiency to higher levels than you could otherwise achieve from off-the-shelf panels. Weight isn't a problem as a properly designed solar roof would probably add between 5-10kg to the overall weight of the car.

Whilst my calculations were fairly rough, they were based on real data from Nissan on the car itself and based on flat surface solar insolation figures from NASA, adjusted to take into account the curvature of the roof.

When all is said and done, right now it is just some idle chatter between engineers. The cost makes it not worth doing at the moment - from an end-user point of view, you would be far better off installing solar panels on a south facing roof and charging your car from that.

I still like the idea of a self-charging car however. Despite working with solar panels for five years, I still get a childish delight from the idea of generating electricity from the sky as if by magic. And despite having done it for so long, and understanding all the science behind it, it does still feel like magic.

That means I still gets flights of fancy trying to do some quite improbable things with solar power from time to time, including some really silly experiments to generate power from the moon (answer: you can, but it really isn't worth the trouble. There are better things to do with a $5000 solar array than trickle charge a 1½ volt AA rechargeable battery at two o'clock in the morning) or powering a 24/7 vending machine with the solar panels facing down in permanent shade (again, it can be done, but only really to prove a point rather than as a practical application).

On the subject of solar powered production cars, there are three companies who are either going to be launching one, or who already are selling one:

A company called Sunmotor are now selling a NEV (neighborhood electric vehicle) with a whopping great big solar panel on the roof. It looks rather strange, but full marks for trying something different. You can see pictures at http://www.sunmotor.us. I spoke to their CTO just before Christmas and he struck me as someone else who had the same fascination that I have in generating electricity from the sun. He claims that the Sunmotor can drive around 15 miles a day in the summer purely on sunshine.

Meanwhile, here in Europe, a French sportscar manufacturer called Venturi are going to launch a solar powered car called the Eclectic next year. Here is the website: http://www.venturieclectic.fr/ It's not a practical car in the same way that the LEAF is, its the sort of thing you might hire when you're on holiday for a bit of fun or for using in a gated community. Venturi are saying that solar range in France is in the region of 8-10 miles a day during the summer. I'm hoping to pop down to the factory early next year when they're putting it into production and see it for myself.

The third company who will be offering a solar roof for an electric car is REVA. Last year, they built some prototype solar vehicles based on their REVA LiON model which they drove across India on tour. The car they will be offering for production is the REVA NXR with an optional solar roof. The NXR is a 'proper' car, a similar size to the FIAT 500 that you'll be seeing in the US next year. For Europeans, this is a fairly standard sized city car but I know you guys like much bigger cars.

Never mind, you could always buy two - one for each foot! :lol:

I still like my "portable carport" idea I mentioned somewhere on this site a month or so ago. Carry some aluminum tubes in that extra-deep trunk space the Leaf has, and when you get to the office parking lot in the morning, set up a 10' by 20' framework around your car, with the top tilted toward the south. Spread some of those thin film solar sheets over the top and plug it into your portable inverter.

OK, so it'll never pay for itself, but it will protect your car from the sun, keep it cool, and possibly even cut down on dings from neighboring car doors, while convincing everyone that you are the greenest. Not to mention give you maybe as much as 10kWh on a sunny June day.

evnow said:

AndyH said:

Adding more weight costs more energy. Unless one is using the solar power to run the AC and take some load off the traction pack, all of the calcs I've yet seen show it's better to leave the PV panels on the garage roof.

Click to expand...

In my case - since the car is garaged at home & underground parked at office, it is probably a total waste. Apart from the fact Seattle only gets a couple of months of sunshine.

I wonder - if someone has an option to park underground or in Sun, would be it more beneficial to park in Sun but then cool the car - or just underground and keep the car cool.

Click to expand...

I might be just the opposite. Five miles to work, all but about a mile of which is at a top speed 45mph. probably avg 35, then the car sits in the blazing sun for 9 hours, then home.