Lucid Motors' Peter Rawlinson Talks E-Car Efficiency
spectrum.ieee.org
spectrum.ieee.org
Protection from the elements and accessibility are two legitimate concerns that might be addressed by bike-cars like the PEBL in the future: https://www.better.bike/
Outside of urban environments, velomobiles with aerodynamic shells have some potential, and would allow travel at highway speeds while using much less energy. Low-Tech Magazine suggests "If all 300 million Americans replace their car with an electric velomobile, they need only 25 % of the electricity produced by existing American wind turbines": https://solar.lowtechmagazine.com/2012/10/electric-velomobil... - Part 2 (link in footer is broken) https://solar.lowtechmagazine.com/2012/10/electric-velomobil...
https://solar.lowtechmagazine.com/2010/09/the-velomobile-hig...
The main problem with high-speed velomobiles is they're too fast for bike trails, but are too small and low to the ground to safely share highways with cars. Most pictures/videos I see of them are on rural roads with almost no traffic. In order for them to become popular, we probably would have to completely reimagine our transportation infrastructure.
They do show that huge efficiency improvements are possible, but they are not there yet. I feel like there's probably a happy medium: a larger somewhat heavier car with safety features built around active immobilization and advanced materials. It wouldn't be quite as efficient as these, but it would be more efficient than a conventionally built car.
I also must point out that weight and aerodynamics are more significant for efficiency than size in an absolute sense. Aerodynamics matters the most at highway speeds. You can lengthen the vehicle and add a trunk for cargo without losing too much.
At any rate, people ride motorcycles with no shell despite serious safety disadvantages, and society has no problem with people making that choice. I have to believe that velomobiles with a shell could be made significantly safer than motorcycles.
Of course, the biggest threat to velomobiles, bicycles etc. is the automobile, and if collisions with cars/trucks aren't dealt with in some reasonable fashion, all other safety concerns seem rather secondary.
Must easier to just get good rain gear, a lot lighter too (and probably cheaper).
A top of the line PEBL would cost ~$13,000. Still roughly double the price, although mass production could probably reduce the price of the PEBL.
That said, I've made an alarming/annoying discovery: rain gear doesn't stay rainproof forever! Gore-tex style rainproof gear eventually must be refreshed using e.g. Nixwax at ~$20 a pop plus your washing labor, and it doesn't always work, meaning you have to junk the clothes or accept that they are no longer rainproof. It seems prudent to have multiple redundant rain shells since you can't always tell when your rain gear is going to stop being rainproof until you go out in the rain and get wet. This means that you may have to do significantly more maintenance and replacement for rain gear than you would have to do for the enclosure of the PEBL. In other words, the cost/hassle may depend on how often you have to travel in the rain.
If you try to synchronize the system (e.g. have one central master server controlling all cars on high-speed roads), then I have a feeling that a lot of weight can be shed.
More like 25-100x more energy. And that's quite generous still.
In a city, bikes are indeed multiple orders of magnitude more efficient than cars for personal transportation.
If you can charge your car from the sun, who cares?
It might be that a bike is more efficient, but you have more risk of accidental injury or death. It's harder on your human body for say a 5 mile commute. You will arrive at work more tired, or more sweaty, or rained on.
On the other hand, if you decide to use a bike for exercise, that might change the equation again.
(I'm not trying to argue with you, just thinking from a different viewpoint)
I blame mornings and lack of coffee. :)
Something with better visibility and ergonomics is going to need more power, but 2,500w is plenty to get into seriously unsafe speeds in a small lightweight vechile.
Another thing that you have to keep in mind is that electric motors on bicycles are almost always limited in their top speed by their kv rating, that is to say, by their gearing. Once you get up to high speeds, power tends to drop off quite significantly as the battery and controller can't keep up with the motor and the ever-higher voltages required. So unless your ebike is specifically geared for that task or that you use a mid-drive with a ridiculous 72 speed transmission and gigantic pedal sprocket, you're not hitting 55mph on an ebike with a 250w motor, or with a 1000w motor for that matter. If it was actually useful to go that fast we would already do so.
So in the end, the average power of someone riding an ebike around town is of 100w-250w, whereas cars use in the order of 10000w+ in town. This means bikes are on the order of 25-100 times more efficient than cars.
If you want to build any sort of enclosed structure that can pass crash safety, it will likely have 4-5 times the frontal area of an ebike, and weigh the better part or upwards of a ton. And in the end, you will have an average power consumption, not peak, in a city in the multikillowatt range. Essentially, you would make an electric Smart Car, which already exists :)
For comparison and ignoring other forms of drag, a 1,000 kW Koenigsegg Agera RS does 447.19 km/h should only need 2.42 kWh to do 60kph. But, that’s a 4 seater we are talking about something significantly smaller, lighter, and possibly more aerodynamic.
The current path for an IGBT looks largely like a diode; it has a fixed voltage drop (0.6V in silicon), so the losses in the "On" state are linear with current. A MOSFET, however looks largely like a resistor in the "On" state, so the losses are quadratic with current.
There are also switching losses, which are a bit more complicated, but largely scale quadratically with frequency (for IGBTs there usually aren't actual formulae but just a curve from the datasheet; MOSFETs are a bit more amenable to calculation). For relatively low-frequency and high-current switching like a high-power permanent magnet motor, you will tend try to keep switching losses low relative to conduction losses; in any event IGBTs are low-frequency devices so if IGBTs are in discussion typically switching losses will be low.
As anyone who has studied asymptotic performance will know, for low-frequency applications, as current goes towards infinity, IGBTs are better. At low currents, however, MOSFETs trounce IGBTs; A resistor that drops 0.6V at 1A would have 600mOhms of resistance, and a $0.10 Power MOSFET can beat that in low voltage (<120V) applications, with on resistances on the order of 1mOhm. At 100A there are similar conductive losses.
As you design power MOSFETs for higher voltage handling though, the on resistance goes up. There are ways to design power MOSFETs with lower on-resistance, but they will make them much harder to switch and increase the switching losses to the point where that bit I said earlier about conduction losses dominating becomes not true. For switching 900V you will want a MOSFET rated for at least 1200V and now you are looking at 100s of mOhm of on resistance, so even at say 2A, the IGBT is looking quite good.
Enter SiC. I know absolutely nothing about semiconductor design, but the way it was explained to me is that the design parameters of an SiC MOSFET are such that it can block a larger voltage than a silicon MOSFET, so that the envelope shifts "up" for working voltage at any given amount of on-resistance ant gate-charge (which relates to switching losses).
Lastly, if you aren't a racecar driver, most of the time the power draw will be far below the max, so even if an IGBT beats an SiC MOSFET handily under hard acceleration (it almost certainly does), for highway cruising there is a good chance the MOSFET will win.
It's the gate capacitance (and gates resistance) that leads to switching losses. Usually the biggest part of the switching losses is because the transistor is not fully "on" or "off" for the whole time, but is in between when in the process of switching. The gate capacitance is probably one of the biggest problems (more for FET, but also for bipolar), that influences the speed of the transistor (presented as frequency or as slew rate). Otherwise you could just put a bunch of MOSFET-s in parallel and get much lower resistance, thus greater efficiency (and it works, to a point).
Another thing to consider is that FET-s let the current go back and a motor is an inductive load, while bipolar transistors do not. I forgot how much that matters, but it probably does plenty.
Thankfully electronics is one of those fields of engineering where you can get all the information you need from a datasheet. Unlike from those damned headphone manufacturers (embarrassments to engineering, 99.9% of them).
PS "..you will tend try to keep switching losses low relative to conduction losses.." I think you meant the other way around.
A 6 pole motor running at 10000 RPM has a switching frequency of 1kHz. Any application where you are considering an IGBT is probably under 20kHz. Cruising on the freeway uses about 15kW. Conduction losses are probably going to dominate.
Yea, 1kHz is nothing for modern transistors. FET, IGBT or bipolar. Not that it's not worth considering them, ofc.
From a power handling perspective, power IGBTs and FETs act similar just with their own challenges.
Is that supposed to be 900 V? Considering the car can push out 750 kW, a "900-watt architecture" doesn't mean much. I would think that the IEEE would be the one place I could trust to not mess up electrical units...
This whole article is kind of terrible though... they're talking about reducing losses by using SiC MOSFETs and using higher voltage architecture... except they fail to recognize that higher voltage MOSFETs inherently have higher impedance when on, and it causes a bunch of other potential issues.
Also, they're comparing themselves against the Porsche Taycan... which is one of the worst performing (in kWh/efficiency value) cars out there. Why not compare against the Model 3? The Model 3 uses SiC MOSFETs (although, 400V rated ones) just like they're talking about here.