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Comment by irq | Hacker News Reader

there's just no way this works out in terms of physics:

- the weight a semi-truck is expected to carry vs the surface area available to absorb photons vs motor efficiency. the energy obtainable via the sun exposed surface area available is multiple orders of magnitude lower than the amount of energy required by motors of current or near future efficiency to propel such a vehicle, with typical weight, to the even 1/2 the speed of normal traffic.

- the only way this works is if for some reason the truck is unmoving for long periods of time and has a massive battery system installed that can be charged up while the vehicle is immobile, which drives the cost up a lot, because any truck that's not moving is a capital asset that's losing money.

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When calculating energy requirements on a flat plane of travel, the weight of the load is only relevant* when accelerating. On a flat plane, once the load is up to speed, it doesn’t take any significant increase of energy to maintain speed, regardless of load weight. The main reason fuel is required to maintain speed on a flat surface is overcoming air resistance. At the speeds that road traffic travels, only the front and back shapes of the vehicle contributes substantially to air resistance, not “air friction” from an elongated middle body. This means that on a flat plane like a highway, doubling the length of a vehicle to accommodate double the load, does not require anywhere near double the energy to maintain speed. It does however provide much more space to capture solar energy.

By the way, you can take advantage of this yourself on long highways by driving close (but safe) to large trucks, driving in their slip stream. It cuts fuel requirements substantially because the truck is doing some of the work of moving the air for you.

One puzzling thing is why trucks aren’t designed to be more aerodynamic, instead of a giant box shape. Anyone got any thoughts on this?

*aside from small things like added friction on bearings, changed tire geometry etc.

>aside from small things like added friction on bearings, changed tire geometry etc

This is rolling resistance, it's generally proportional to weight, and it's usually not negligible for heavy vehicles: it can easily make a large fraction of the energy usage of a truck, though at highway speeds it's rare for it to be the majority of it.

Afaik, it has partially to do with regulations. In Europe the max length of a truck includes the cab, thus it's as short and upright as possible to leave max room for cargo (cab over engine), whereas US trucks have the long "hood". Old trucks were quite angular but the new ones are a lot more aerodynamic.

Second, those "tapered" add-ons in the back can help a lot to approximate a teardrop shape better given that there are a lot more constraints in general in the front.

> The main reason fuel is required to maintain speed on a flat surface is overcoming air resistance.

Yeah, 200 horsepower's worth.

Wind resistance at semi-truck highway speeds is very significant. Multiply the numbers here[0] by 9.5x to convert from BMW to Semi-Truck (accounts for increase in Cd and frontal area). So semi trucks might take on the order of 180hp at the wheel just to maintain speed. With drivetrain losses you'd need to be generating 200hp.

0: https://www.e31.net/resistance.html

Your "physics" based comment seems to hinge on the totally invented axiom that the solar needs to provide all the power, rather than just contribute some in a way that is cost effective and/or more convenient.

Imagine wearing a Garden Cress hat 24/7. It grows fast and new batch is ready to be eaten every 2-4 days. Thats ~10 calories for free per day, FREE sun energy! Hat weights a pound and you cant take it off, even at night.

We seem to be moving further away from "physics" based reasoning with this analogy. Do trucks not like wearing hats? Do they get crushed against their pillows when they sleep?

All this time and I never realised Pixar's Cars has been spreading misleading physics.

Also extra energy lost by accelerating/decelerating the extra mass. Even with regenerative braking this surely adds up to a lot of wasted energy.

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