6 ms·

Predictably, the math does not work. They say that at 3.7KW it gains range at 35km/h, and from solar it gains range at 12km/h. This means that they expect to g

Predictably, the math does not work. They say that at 3.7KW it gains range at 35km/h, and from solar it gains range at 12km/h. This means that they expect to get 1.23KW of power from their solar cells. Top theoretical efficiency possible for any solar cell is about 33% (Shockley-Queisser limit), most efficient you can buy today is 22%. So that means they expect 5.59KW of sun energy to fall on the car per square meter. A typical car is 4.5x2 meters. Let's assume they tile the whole damn thing with highest efficiency cells except the windshield (1x2m) They then have 7m^2, so they expect solar energy to provide 799W/m^2. And it is true, sunlight reaching earth is about 1KW/m^2 at the equator in noon, but that is in perfect conditions and just there, and when cell is perfectly aimed at the sun. The car will not tilt, and some days there are clouds or haze, and we do not all live at the equator. Also, in fact, not the entire car can be tiled with solar cells, and dirt exists, especially on the road. So the numbers they promise are pretty much at the very limit of theoretical possibility in ideal conditions, and not even remotely likely to be hit in real life.

bkanber 7y ago
There's this claim on the site: > In fact, our solar cells provide about 20% more energy than traditional ones. The numbers they provide are best-case. There's also this: > Someone driving the national average of 20,000 km/year in the cloudy Netherlands would get about 40% of their mileage from solar energy. They've clearly thought about this. I agree that solar roof on a car, as it stands today, is a gimmick at best, but the fact that your envelope calculation actually _confirms_ that solar charging is possible is enough for me to not dismiss them out of hand. This isn't utter bunk, at very least. Edit to add: we're probably doing ourselves a disservice thinking about solar charging in terms of range; instead, consider that parking the car in the sun for a day could offset the cost of running climate control for the next 3-4 days. Someone needs to try it, sometime, so I'm glad they are.
- dmitrygr 7y ago
  20% more than leading top efficiency cells would be 26% efficiency mass-produceable solar cells. IF they had that, they would not waste their time making a car, they'd be to busy diving into huge pools of money, having just made the world's most efficient mass-produceable solar cell by far, and thus become instant billionaires!
  - bkanber 7y ago
    There's no claim that the cells are operating at 26% efficiency; they just claim that, in aggregate, the system is 20% more efficient than traditional systems. There is an important distinction there.
    - dmitrygr 7y ago
      modern DC-DC is already 96+% efficient. LiPo charging is well understood too. Where is the extra efficiency coming from, other than the hot air out of the marketing department room?
  - syntaxing 7y ago
    Totally agree how the thermodynamics and the economics doesn't add up. Solar panels on cars never really made sense for a reason. The view factor and efficiency of a solar panel on a car would produce very little energy. I would be surprised if it's more than 500W without some sort of breakthrough technology. People forget that 50 kWh (the standard range of a Model 3) is a shitton of energy. A full roof can barely keep up with those number, let alone a car amount of solar panels.
nudded 7y ago
I think I miss something in your numbers. They say 83 Wh/km. which means that they expect to get 996Wh in an hour. How did you reach 5.59KW?
- dmitrygr 7y ago
  i am analyzing charging rates. they cite charge rate in km/h for an outlet providing 1.3KW and they cite a charge rate for solar, from which we can work out the power they expect to get from solar
  - nudded 7y ago
    I see, but that does not answer my question though. How is my math wrong? the expected range from solar in 1 hour equates to 996 Wh. You suggest they need 5.6kW of solar to achieve this?
    - dmitrygr 7y ago
      you are forgetting that solar cells are not 100% efficient, and charging is not either. so you MUST use charge rates cites, not consumption rates
      - nudded 7y ago
        No, you must not. The energy required for the car to drive 12kms is 996 Wh. let's round to 1kWh. Solar is DC. for instance Tesla powerwall is 97% efficiency for DC/DC conversion for charging its battery. So you need basically 1kW of solar to produce the needed energy to drive 12km (per hour). I think you are off by at least a factor of 3. edit: at 20% solar efficiency that would require 5m^2.
        dmitrygr 7y ago
        so then your claim is they lose 66% on rectification of AC when they charge from a wall? :)
gene-h 7y ago
We have solar cells which have exceeded the Shockley-Queisser limit. It's possible to exceed the Shockley-Queisser limit by having a multiple junction solar cell. In fact the world record for conversion of sunlight is 46%[0]. Of course this was with concentrated sunlight. Without concentrated sunlight multijunction solar cells have achieved 30.2% efficiency[1]. Multijunction solar cells are somewhat expensive though. Based on the current rate of advancement of perovskite solar cells it's possible we could see perovskite silicon tandem solar cells with 30% or higher efficiency in the coming months. In december, perovskite silicon tandem solar cells achieved an efficiency of 28%[2]. Still, something's fishy about this vehicle. [0]https://www.ise.fraunhofer.de/en/press-media/press-releases/2014/new-world-record-for-solar-cell-efficiency-at-46-percent.html https://www.ise.fraunhofer.de/en/press-media/press-releases/... [1]https://www.ise.fraunhofer.de/en/press-media/press-releases/2016/30-2-percent-efficiency-new-record-for-silicon-based-multi-junction-solar-cell.html https://www.ise.fraunhofer.de/en/press-media/press-releases/... [2]https://www.oxfordpv.com/news/oxford-pv-perovskite-solar-cell-achieves-28-efficiency https://www.oxfordpv.com/news/oxford-pv-perovskite-solar-cel...
- dmitrygr 7y ago
  my comment remains: if they commercialized such efficient cells, why bother making a car? Why not take the money and go buy a small-to-medium-sized country to retire in?
  - gnode 7y ago
    More efficient conversion of sunlight electricity doesn't mean the cells are cost effective for putting on a house. On a luxury car like this, area is constrained and money is no object, so it may make sense to use otherwise prohibitively expensive cells to win a few extra percent efficiency. Similarly, such multi-junction cells are used on spacecraft, where the lower mass per watt makes it worthwhile.
slg 7y ago
On the flip side it is worth pointing out that 83 w/km (133.6 w/mi) is a little over half the power that most of the best selling EVs like the Model 3 or Bolt use. That rate would probably make it the most efficient non-ultra compact EV on the market. So the panels aren't the only place they expect to make large gains on efficiency.
- AWildC182 7y ago
  That too should probably be viewed with skepticism. Unless the car has a way lower weight and frontal cross section it's unlikely to be vastly different than existing vehicles. The motor and supporting equipment isn't going to me any more efficient.
  - slg 7y ago
    Also after doing a little math the battery is larger than the entry level Model 3 and the same size as the Bolt. Plus this car is longer and wider than both the Model 3 and Bolt (although it is shorter than both so I guess it could be somewhat more aerodynamic). I just don't see where they cut any substantial amount of weight. I am rather dubious on all their claims.

bkanber 7y ago

There's this claim on the site: > In fact, our solar cells provide about 20% more energy than traditional ones. The numbers they provide are best-case. There's also this: > Someone driving the national average of 20,000 km/year in the cloudy Netherlands would get about 40% of their mileage from solar energy. They've clearly thought about this. I agree that solar roof on a car, as it stands today, is a gimmick at best, but the fact that your envelope calculation actually _confirms_ that solar charging is possible is enough for me to not dismiss them out of hand. This isn't utter bunk, at very least. Edit to add: we're probably doing ourselves a disservice thinking about solar charging in terms of range; instead, consider that parking the car in the sun for a day could offset the cost of running climate control for the next 3-4 days. Someone needs to try it, sometime, so I'm glad they are.

dmitrygr 7y ago
20% more than leading top efficiency cells would be 26% efficiency mass-produceable solar cells. IF they had that, they would not waste their time making a car, they'd be to busy diving into huge pools of money, having just made the world's most efficient mass-produceable solar cell by far, and thus become instant billionaires!
- bkanber 7y ago
  There's no claim that the cells are operating at 26% efficiency; they just claim that, in aggregate, the system is 20% more efficient than traditional systems. There is an important distinction there.
  - dmitrygr 7y ago
    modern DC-DC is already 96+% efficient. LiPo charging is well understood too. Where is the extra efficiency coming from, other than the hot air out of the marketing department room?
- syntaxing 7y ago
  Totally agree how the thermodynamics and the economics doesn't add up. Solar panels on cars never really made sense for a reason. The view factor and efficiency of a solar panel on a car would produce very little energy. I would be surprised if it's more than 500W without some sort of breakthrough technology. People forget that 50 kWh (the standard range of a Model 3) is a shitton of energy. A full roof can barely keep up with those number, let alone a car amount of solar panels.

nudded 7y ago

I think I miss something in your numbers. They say 83 Wh/km. which means that they expect to get 996Wh in an hour. How did you reach 5.59KW?

dmitrygr 7y ago
i am analyzing charging rates. they cite charge rate in km/h for an outlet providing 1.3KW and they cite a charge rate for solar, from which we can work out the power they expect to get from solar
- nudded 7y ago
  I see, but that does not answer my question though. How is my math wrong? the expected range from solar in 1 hour equates to 996 Wh. You suggest they need 5.6kW of solar to achieve this?
  - dmitrygr 7y ago
    you are forgetting that solar cells are not 100% efficient, and charging is not either. so you MUST use charge rates cites, not consumption rates
    - nudded 7y ago
      No, you must not. The energy required for the car to drive 12kms is 996 Wh. let's round to 1kWh. Solar is DC. for instance Tesla powerwall is 97% efficiency for DC/DC conversion for charging its battery. So you need basically 1kW of solar to produce the needed energy to drive 12km (per hour). I think you are off by at least a factor of 3. edit: at 20% solar efficiency that would require 5m^2.
      - dmitrygr 7y ago
        so then your claim is they lose 66% on rectification of AC when they charge from a wall? :)

gene-h 7y ago

We have solar cells which have exceeded the Shockley-Queisser limit. It's possible to exceed the Shockley-Queisser limit by having a multiple junction solar cell. In fact the world record for conversion of sunlight is 46%[0]. Of course this was with concentrated sunlight. Without concentrated sunlight multijunction solar cells have achieved 30.2% efficiency[1]. Multijunction solar cells are somewhat expensive though. Based on the current rate of advancement of perovskite solar cells it's possible we could see perovskite silicon tandem solar cells with 30% or higher efficiency in the coming months. In december, perovskite silicon tandem solar cells achieved an efficiency of 28%[2]. Still, something's fishy about this vehicle. [0]https://www.ise.fraunhofer.de/en/press-media/press-releases/2014/new-world-record-for-solar-cell-efficiency-at-46-percent.html https://www.ise.fraunhofer.de/en/press-media/press-releases/... [1]https://www.ise.fraunhofer.de/en/press-media/press-releases/2016/30-2-percent-efficiency-new-record-for-silicon-based-multi-junction-solar-cell.html https://www.ise.fraunhofer.de/en/press-media/press-releases/... [2]https://www.oxfordpv.com/news/oxford-pv-perovskite-solar-cell-achieves-28-efficiency https://www.oxfordpv.com/news/oxford-pv-perovskite-solar-cel...

dmitrygr 7y ago
my comment remains: if they commercialized such efficient cells, why bother making a car? Why not take the money and go buy a small-to-medium-sized country to retire in?
- gnode 7y ago
  More efficient conversion of sunlight electricity doesn't mean the cells are cost effective for putting on a house. On a luxury car like this, area is constrained and money is no object, so it may make sense to use otherwise prohibitively expensive cells to win a few extra percent efficiency. Similarly, such multi-junction cells are used on spacecraft, where the lower mass per watt makes it worthwhile.

slg 7y ago

On the flip side it is worth pointing out that 83 w/km (133.6 w/mi) is a little over half the power that most of the best selling EVs like the Model 3 or Bolt use. That rate would probably make it the most efficient non-ultra compact EV on the market. So the panels aren't the only place they expect to make large gains on efficiency.

AWildC182 7y ago
That too should probably be viewed with skepticism. Unless the car has a way lower weight and frontal cross section it's unlikely to be vastly different than existing vehicles. The motor and supporting equipment isn't going to me any more efficient.
- slg 7y ago
  Also after doing a little math the battery is larger than the entry level Model 3 and the same size as the Bolt. Plus this car is longer and wider than both the Model 3 and Bolt (although it is shorter than both so I guess it could be somewhat more aerodynamic). I just don't see where they cut any substantial amount of weight. I am rather dubious on all their claims.