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Aptera's First Solar Road Trip. 300 Miles, One Charge

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Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#41

Post by _ph_ »

dontlaugh wrote: post #27

Focusing on efficiency is a good idea, but why is it so big? It’s silly that it’s over 4m long.

Because it needs to be that long to have a low air resistance. There are two determining factors for air resistance. First, the cross section A, which determines the amount of air to be moved and second the drag coefficient c, which describes how well the air flows around an object. This gets lower the more "round" a shape is, but the sphere isn't actually the best one, you want the form of a falling raindrop. For that the shape needs to be quite a bit longer than it is wide. So if your car is e.g. 2m across, it needs to be something like 4m long to approach best efficiency. See also the empty "tails" attached to some racing cars for drag reduction.

Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#42

Post by dontlaugh »

_ph_ wrote: post #41
dontlaugh wrote: post #27

Focusing on efficiency is a good idea, but why is it so big? It’s silly that it’s over 4m long.

Because it needs to be that long to have a low air resistance. There are two determining factors for air resistance. First, the cross section A, which determines the amount of air to be moved and second the drag coefficient c, which describes how well the air flows around an object. This gets lower the more "round" a shape is, but the sphere isn't actually the best one, you want the form of a falling raindrop. For th…

They could either make it narrower or slightly less efficient.

That length makes it too big in many countries, especially for just two seats.

Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#43

Post by tromp »

trhway wrote: post #22
quailfarmer wrote: post #19

What's new here? College kids have been doing this since the 90s: https://en.wikipedia.org/wiki/World_Solar_Challenge

Some look close to like a real car, that one EV + solar, 25miles/KWh https://en.wikipedia.org/wiki/UNSW_Sunswift#/media/File:Suns... I'd venture a guess - Aptera went for 3 wheeler because certification as a motorcycle is much easier than as a 4-wheel car. Unfortunately 3 wheels - 3 vertical columns resisting the upcoming air - may be less aerodynamic than 4 wheels which are only 2 vertical columns resisting the upco…

Optimal aerodynamic efficiency on a 4 wheel car still requires a narrower rear, with a corresponding shorter rear axle to make the rear wheels roll in the slipstream of the front wheels. This can be seen on two of the most aerodynamic cars of all time, the GM EV1 and Mercedes EQXX.

Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#44

Post by _ph_ »

dontlaugh wrote: post #42
_ph_ wrote: post #41

Earlier quoted context omitted.

Because it needs to be that long to have a low air resistance. There are two determining factors for air resistance. First, the cross section A, which determines the amount of air to be moved and second the drag coefficient c, which describes how well the air flows around an object. This gets lower the more "round" a shape is, but the sphere isn't actually the best one, you want the form of a falling raindrop. For th…

They could either make it narrower or slightly less efficient. That length makes it too big in many countries, especially for just two seats.

I think it is already as narrow as you can put 2 seats side by side. But then indeed it becomes a tradeoff between size and efficiency. They went for maximum efficiency. And it is still 26cm shorter than a Model 3. So while not an compact car, not overly large either.

Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#45

Post by dontlaugh »

_ph_ wrote: post #44
dontlaugh wrote: post #42

Earlier quoted context omitted.

They could either make it narrower or slightly less efficient. That length makes it too big in many countries, especially for just two seats.

I think it is already as narrow as you can put 2 seats side by side. But then indeed it becomes a tradeoff between size and efficiency. They went for maximum efficiency. And it is still 26cm shorter than a Model 3. So while not an compact car, not overly large either.

You’re comparing against a car that is difficult to use on UK roads due to its length and (to a lesser extent) width. Perhaps in the US it’s not that big, but here it is.

Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#46

Post by trhway »

tromp wrote: post #43
trhway wrote: post #22

Earlier quoted context omitted.

Some look close to like a real car, that one EV + solar, 25miles/KWh https://en.wikipedia.org/wiki/UNSW_Sunswift#/media/File:Suns... I'd venture a guess - Aptera went for 3 wheeler because certification as a motorcycle is much easier than as a 4-wheel car. Unfortunately 3 wheels - 3 vertical columns resisting the upcoming air - may be less aerodynamic than 4 wheels which are only 2 vertical columns resisting the upco…

Optimal aerodynamic efficiency on a 4 wheel car still requires a narrower rear, with a corresponding shorter rear axle to make the rear wheels roll in the slipstream of the front wheels. This can be seen on two of the most aerodynamic cars of all time, the GM EV1 and Mercedes EQXX.

>make the rear wheels roll in the slipstream of the front wheels.

thanks. That is in general what i meant by saying "4 wheels which are only 2 vertical columns resisting the upcoming air", i missed the shorter axle as being the most aerodynamic, and your description is just much better and more right detail level correct.

Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#47

Post by GavCo »

pinkmuffinere wrote: post #12

> Even on an overcast day, the team saw over 545 watts of solar input Let’s (generously) assume that was the minimum they saw, and let’s (generously) say they charged for 14 hours. That’s 7.63 kWh gained over the day, in almost ideal conditions. Flagstaff’s high altitude means stronger sunlight, and they can do regenerative braking as they come down the mountain. In my Nissan leaf, 6 kWh would get me about 20 miles.…

Fully agree. The physics of solar panels on cars just doesn't work. It's bizarre that this is actively pursued by startups and concept cars from large manufacturers when it takes just quick back-of-the-napkin math to see.

A car has about 5 m^2 of flat space on the roof/hood/trunk so that's the maximum surface area that can capture solar energy at any given time.

The total energy to hit the area is 1000 w/m^2.

The panels can't rotate to track the sun so the effective area is the cosine of the angle. So you end up with about half the amount of effective sunlight hours as the actual daylight hours. So in summer you get about 6 hours of effective sunlight.

Good panels in real world conditions can give you 22% efficiency.

So in optimal conditions you get: 5 * 1000 * 6 * 0.22 = 6.6 kwh

That will reflect your best days. It can be dramatically less if it's cloudy, overcast, winter, far from the equator, car is dirty, parked in shade, etc.

6.6 kwh is about one tenth of the battery in my Hyundai Kona EV. With very conservative highway driving, 6.6 kwh can get about 40km of range and about 50km in city driving. It's what I get from plugging into my home charger for 30 min and what you get from a fast charger in about 3 minutes.

So besides some very niche uses, there's no sense in massively increasing the cost and complexity of a car by installing solar panels. Far better to put the panel on the roof of parking and just plug in for a few minutes while you park.

Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#48

Post by anentropic »

dontlaugh wrote: post #45
_ph_ wrote: post #44

Earlier quoted context omitted.

I think it is already as narrow as you can put 2 seats side by side. But then indeed it becomes a tradeoff between size and efficiency. They went for maximum efficiency. And it is still 26cm shorter than a Model 3. So while not an compact car, not overly large either.

You’re comparing against a car that is difficult to use on UK roads due to its length and (to a lesser extent) width. Perhaps in the US it’s not that big, but here it is.

You can't be serious that the length of a Model 3 makes it difficult to use on UK roads? It is a typical mid size vehicle, there are loads of vehicles of this size and larger on the roads over here.

Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#49

Post by dontlaugh »

anentropic wrote: post #48
dontlaugh wrote: post #45

Earlier quoted context omitted.

You’re comparing against a car that is difficult to use on UK roads due to its length and (to a lesser extent) width. Perhaps in the US it’s not that big, but here it is.

You can't be serious that the length of a Model 3 makes it difficult to use on UK roads? It is a typical mid size vehicle, there are loads of vehicles of this size and larger on the roads over here.

There are definitely cars that size and bigger and I frequently see their drivers struggle to park or navigate narrow roads.

I'd call something like a Golf/Focus "mid size". There's approximately two sizes below and two sizes above.

Re: Aptera's First Solar Road Trip. 300 Miles, One Charge

#50

Post by nindalf »

trhway wrote: post #33
nindalf wrote: post #30

Earlier quoted context omitted.

The price of solar panels isn’t static though. It might not have made sense in 2015 or 2020 or even 2025. But solar panels continue to get cheaper, so it might eventually get to a point where it’s a serious idea. Even if it costs a couple thousand extra, the panels will pay for themselves by giving you basically free commutes forever.

mainly it isn't the price of solar panels. How do you put it on the car's curved metal panels (or you're going to redesign the car into something like Cybertruck?) without worsening aerodynamics, without much additional holes and additional weight and in the way that will hold the panels for the next 7-10 years, rain, snow or shine while these panels wouldn't get easily damaged by carwash, etc., and you also need to…

Damn, you're absolutely right. The aerodynamics alone kills this idea.
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