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.
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.
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.
The project is not meant to make it entirely solar powered but extend range as a hybrid and look to extend range by 5000 km/year. Initial findings has looked promising and it’s taking to public roads now. This article quoted as source has a bit more technical details, batteries etc: https://cleantechnica.com/2023/08/31/scania-tests-its-first-...
The article states that "the system’s maximum efficiency of 13.2 kWp (kilowatt peak) and ability to deliver 8,000 kilowatt-hours (kWh) annually when operated in Sweden."
Sweden's wholesale electricity prices is ~€32/MWh in July 2023, so this is equivalent to a savings of €256/truck/year. My initial impression is that the juice is not worth the squeeze, but wondering if someone with more context would provide arguments for solar roofs on vehicles.
I know Hyundai (Ioniq 5) and Toyota (Prius) are also offering solar roof options on their cars, so there must be a good reason. The Prius, under ideal sunny conditions, can reasonably get to 1.1KWh/day (or 3.2mi of range), which is nothing to sneeze at, but still relatively small in the large scheme of things.
My personal opinion is that solar panels on vehicles don't make any sense, and probably won't _ever_ make sense.
There are two concept cars I know of that can actually get enough power from the solar panels on them to be useful. One of them is the Aptera, which is a no-compromise low energy vehicle. It's tear drop shaped and can only hold two people, so it's going to be a bit niche.
The second is the Lightyear solar electric vehicle, which is shaped a bit more like a standard car but is going to cost luxury car money for an ultra lightweight (read: no soundproofing) car.
And to what advantage? Charge per year doesn't matter, since you're recharging multiple times per year anyway there's no difference between the panels being on the car vs the ground, other than panels on the ground can be directed more optimally and don't add to the weight of the car.
Charge per day matters, and taking a 500 mile trip up to maybe 600 miles (on a perfect sunny day) isn't that big an improvement in practical terms.
Meanwhile, for us mortals: There's something to be said for using solar panels to power aircon/heaters/utilities/etc while the car is stationary outdoors.
A quick google suggests the average truck driver drives 160-180km a year. This means that this provides an extra ~3% fuel efficiency for a lot of overhead and complexity. Trailer skirts save an estimated 4-8%.
I’m not saying it’s an either/or because you can have both, but it does seem like this is potentially overly complex for minimal gains.
Wouldn’t this be the same for regenerative brakes? The amount you get back is negligible in the grand scheme but they put them on every EV/Hybrid anyway.
The individual may be minimal but the overall cumulative benefit may be worth it.
Why put a "fairly involved" solar system on top of a truck when you could just put it on the ground in a sunny spot? Maybe solar farms are too boring and effective to excite investors...
The smart investors will invest in solar farms. The greedy investors will invest in newer less effective tech. On the whole, I think it’s okay that not every investor picks one way to harness solar power.
b) put solar panels in a fixed spot, where it's easier to use big panels, maintain them, avoid nasty vibrations, and transport the energy to a charger.
In b) you get more energy, even after transferring it to the truck. So you can use the extra energy to power other truck or a home or something, and then avoid burning fossil fuel and then saving the panel.
A ~50ft trailer is pretty common, and it should be about 8ft wide and 8ft tall. So 24ft * 50ft is more than 1000sqft and it's approx 10sqft to the sqm.
Of course your best case scenario is only about 400sqft illuminated. Insolation tops out at 100w/sqft and the max efficiency of commercial panels is maybe 22-24%. Doing the math there you get something like 7-10kw which is nowhere near enough to push one of these down the highway.
A normal semi gets maybe 6-8mpg on diesel which means burning 8gal to go 60mi in 1hr. You get about 10hp/gal/hr out of a diesel so that means it takes something like 80hp or 60kw to drive a truck down the road.
It's more then enough to run the climate control, lights, and other cab loads and slow battery drain just a bit. But you'd have to leave the truck to charge for a week in order to drive for a day.
You don't really want to power the entire vehicle. The question is more like, which is the most efficient wheel, which is the best aerodynamic shape and which is the most efficient roof.
If you can have components with negative load it's all good. Say that 7-10kw / 60kw is 12% to 17% savings. 6-8mpg becomes 6.72-9.36 mpg
Or for laughs one could say every 10th car requires no fuel.
No one mentioned rolling resistance when increasing weight- i can’t say offhand where that loss becomes significant or not but i believe it’s worth consideration.
also, the energy doesn’t need to supplant the diesel engine but it could assist it.
the real math is “can it carry its self, and more” and if so, “can it’s assistance save the fleet money over the life of its service”. i have my doubts but it doesn’t sound like anyone here or there wants to show any math…
Because they will point straight up, which is only really ideal near the equator. Ideally you want to point your panels towards the equator, at about the same angle as the midday winter sun.
- 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.