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Comment by svara | Hacker News Reader

Electric aviation always strikes me as an odd idea, given the low power densities that batteries achieve.

By contrast, biofuels are available today and can be made from agricultural waste products (2nd gen), in bioreactors using salt water algae (3rd gen) or electrochemically (4th gen).

They're potentially a drop in replacement for regular fuel - for planes, but also for cars.

Due to the lower energy requirements for building ICE cars versus batteries, a biofuel powered car will be much better for CO2 emissions than an electric one, which you need to drive for many years before it beats a similar sized gasoline fueled car on sum CO2 emissions.

So why are we not heavily investing in biofuel infrastructure? Why are electric motors and batteries hyped up as being the tech of the future? That's an honest question, since I've yet to hear a well-founded physical or engineering argument against it.

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For general aviation (small prop planes), it's because so much of the cost of running one comes from the engine. The most common piston engines are supposed to make it 1500-2000 hours between overhauls, and an overhaul runs around $16K, so you have to save $10-15 per flight hour in engine reserve to pay for the overhaul when it comes due and to cover any unexpected maintenance between overhauls. Plus, there are consumables that aren't cheap -- oil changes, spark plugs, and so forth. A brushless electric motor has a lot fewer moving parts, and a lot less vibration, so overhauls could be cheaper and with a larger interval between them, and regular maintenance would be cheaper as well. Plus, no leaded avgas to mess with, no worries about reduced power at higher altitudes where the air is thinner, no worries about carburetor icing or adjusting the air/fuel mixture. Sounds like a win to me.

Unfortunately the reduced power is still a problem at higher altitudes but for different reasons. Instead of oxygen starvation, the props will have less to push against.

This is still less of an issue as the electric motor can spin faster due to the reduced resistance. Higher speeds will create higher resistances from within the motor itself (which will in turn be increase the heat generated in the motor).

That heat will also be harder to dissipate in lower pressures, but I suppose alternative cooling could be added as well.

I’m not entirely sure how you’ll cool the engines at high altitudes, combustion engines can use their exhaust to cool the engine.

Using large radiators on an electric engine kinda defeats the purpose especially when they can’t be effectively be built into the body of the engine.

I'm not sure why largeish passive radiators are a problem. If you're worried about the aesthetics you can put them inside of ducts and pass the air over them. This doesn't seem like it should be that big of an issue.

Plus you'll probably want to recover some of that waste heat to heat the cabin.

How exactly does one use exhaust to cool the engine? This is something I've never heard of in the context of cars of motorcycles, where exhaust is generally removed as soon as possible, and if it's used for anything, it's for running turbochargers that make the engine even hotter.

Not sure what GP was referring to, but they might have had in mind the practice to run engines a bit too rich, so that fuel runs through the system with insufficient oxygen to burn, ie as a liquid, which cools the engine.

There are two strategies for keeping cylinders cool in cruise: push the red mixture knob forward for a rich fuel/air mixture setting that uses unburned fuel to lower cylinder head temperatures, or pull the mixture knob out to such a lean setting that an abundance of air and reduction in engine power and heat cools the cylinders. Running aircraft engines “lean of peak” (LOP) typically reduces airspeed about 5 percent in cruise while lowering fuel consumption about 20 percent—but not all engines are capable of LOP operations, and doing it wrong can cause permanent and costly engine damage.

source: https://www.aopa.org/news-and-media/all-news/2019/may/flight...

Nope, combustion engines generate MUCH more heat than electric engines. Also they don't use exhaust to cool the engine, with all due respect, your argument is entirely backwards. It will be easier to cool electric engines and they will need smaller radiators (if any, have you seen radiators on engines in drones?). More probable is that you will need to heat batteries so that they don't freeze at higher altitudes.

But no worse of a problem than it currently is for fossil fuel engines, right?

I don't math good, so I'll just ask :

Does the RPM of the electric engine matter if the RPM of the propeller can't exceed a certain rate (because the tips of the prop will break the speed of sound, which you generally don't want for comfort reasons)? I mean this in terms of power generation and pushing air.

Gas-powered aircraft don't have transmissions, and gas engines tend to only be efficient in a narrow band, so I'd guess that the RPM limitations of an electric motor (which can generally tolerate a wider range) wouldn't be an issue as long as it's a reasonably-engineered motor.

Beyond a certain RPM range electric motors lose torque. Eventually you'd reach the limits of the bearings or the maximum centrifugal force that the rotor can withstand, but that's usually at a higher RPM than gas engines are made to survive. I would expect an electric motor designed for an aircraft would be optimized for maximum power/efficiency at whatever RPM band is optimal for the propeller, and the battery would be optimized for whatever voltage is optimal for the motor at that RPM range.

> Unfortunately the reduced power is still a problem at higher altitudes but for different reasons. Instead of oxygen starvation, the props will have less to push against.

Yes, but air-density-altitute problems are identical no matter what is spinning the prop, righ?

> This is still less of an issue as the electric motor can spin faster due to the reduced resistance. Higher speeds will create higher resistances from within the motor itself (which will in turn be increase the heat generated in the motor).

OK, so do the math for me here, I am a bit confused. I am not a motor whiz, but torque at the output converted to watts is going to be power at the input less i^2*r losses. The back-EMF will go up as the motor speeds up, so it may be at a somewhat less efficient point on the RPM-versus-torque curve, but the resistance of the actual wires doesn't change.

> That heat will also be harder to dissipate in lower pressures, but I suppose alternative cooling could be added as well.

I think that is all true. Seems to me you design cooling around the cooling capacity of the air flow at max operating altitude. Is that really a huge design constraint?

As another commenter noted, propeller speeds are limited by multiple factors, and generally complex aircraft use a constant-speed propeller that changes pitch. Because of this the increased electric speed likely isn't a factor.

Cooling the motors isn't a big problem. Their heat output is around 10% of the shaft power, compared to combustion engines that put out 200% of shaft power as heat. While piston engines need a lot of cooling air flow and often have cowls that are mostly open at the front, an electric propellor drive can have a small cooling vent.

The batteries may produce as much heat as the motor, and they have a more limited temperature range. Their service life decreases when run above 40 C. Tesla batteries are liquid-cooled.

Could low temperatures especially at higher altitudes be a problem? Aircraft probably get exposed to higher temperature gradients and in much shorter periods of time than a typical Tesla would encounter. Not sure about Lithium Sulfur batteries but reduced capacity at lower temperatures is common for many battery types.

It's certainly not a problem if the batteries are in the climate-controlled fuselage. It might be tricky to keep them insulated inside the wings (where fuel typically is.)

And the motor _should_ be a lot more reliable.

But discouragingly, just last August, in Norway, a Pipistrel Alpha Electro lost power shortly after takeoff and crashed. https://aviation-safety.net/wikibase/228207

Hopefully, these folks in Fresno, CA will get their electric aircraft flight school up and running sometime soon. But summertime in California's central valley is challenging: https://sustainableaviationproject.com/2018/10/28/we-hit-the...

"A brushless electric motor has a lot fewer moving parts"

Not to mention it makes a lot less weight to lift; the difference is huge.

But the battery would be much heavier than a fuel tank. You can't have an electric engine without a battery, so...

Well, there are fuel cells (that turn fuel into electricity).

Don't biofuels take up way too much land to be economical? A large portion of US crop area is for corn and soybeans for ethanol and biodiesel. I can see the reasoning behind biofuels for aircraft, but I don't think biofuels are sustainable if people continue driving as much as they do now.

> Don't biofuels take up way too much land to be economical?

Can we turn solar energy into hydrocarbons? (Water and CO2 are total feedstocks.)

Yes.

Hydrogenate CO2 to methanol with electrolytic hydrogen produced from clean electricity [1]. Reform methanol to hydrocarbons [2] that can burn in existing engines. (Or burn methanol itself in slightly modified engines. This offers somewhat lower energy density per tankful of fuel, higher total efficiency from electricity-to-motion.)

Synthetic fuel makes sense for fueling aircraft, rockets, long distance shipping, collectible historic cars, and other niches. It doesn't make sense for fueling everyday ground transportation. Too much of the original energy is lost in the chain of chemical transformations and the chemical processing requires facilities on the scale of conventional oil refineries. The total cost is lower to electrify ground transportation and supply it with clean electricity than to transform clean electricity into liquid fuels for equivalent legacy vehicles.

[1] https://www.frontiersin.org/articles/10.3389/fchem.2018.0044...

[2] https://en.wikipedia.org/wiki/Gas_to_liquids

> Synthetic fuel makes sense for fueling aircraft, rockets, long distance shipping, collectible historic cars, and other niches

What's driving the emphasis on lightweight batteries for aviation over better synthesisation?

For UAVs, battery/electric systems don't just reduce fueling costs. They deliver features unobtainable from hydrocarbon engines:

- Extremely long flight endurance without refueling (in large solar/battery UAVs)

- Low noise

- Odorless

- Simpler and cheaper manufacturing

The advantages are much less obvious to me for aircraft that transport passengers or significant cargo loads. I suppose that the companies developing them are hoping to make the lifetime total cost of operation cheaper than existing aircraft serving the same niche. That seems much tougher to achieve.

EDIT: Bye Aerospace, the company described in the IEEE article, says they are targeting a particular niche:

https://byeaerospace.com/eflyer/

They intend to replace trainer aircraft that burn leaded aviation gasoline. This polluting niche fuel costs significantly more than kerosene. Maybe the economics can work out for this case.

Thank you. Incredibly clarifying.

There's no emphasis on lightweight batteries for passanger aircrafts, both batteries and biofuels are pursued and both are marginal compared to traditional fossil fuels.

Combustion engines are quite inefficient and so are fuel synthesis processes. batteries have a high energy round-trip efficiency. The energy density does not matter as much in cars, which after mobile compute have been the primary drivers for technology. The density may make more sense for planes.

Additionally 1st gen biofuels have poisoned the well to some extent due to competing with food crops.

For me, the experience of operating an electric vehicle is so much better than biofuels: refueling at home, simplicity of the drivetrain, and quietness/smoothness all are huge improvements over ICE vehicles.

For your comparison between ICE and electric vehicles, are you counting the fuel processing and delivery infrastructure? Electric delivery infrastructure has very low emissions once it's in place, and production continues to get better (especially with distributed models like rooftop solar). Also, do you have a citation for "...lower energy requirements for building ICE cars versus batteries"? Yes, battery production is fairly intensive (economically and resource use), but (like I said above) the drivetrain is vastly simpler.

I can't see how electric general aviation will become widespread without a monumental breakthrough. But it's hard to beat electric for ground-based transportation, IMHO.

This article addresses your questions:

https://www.carbonbrief.org/factcheck-how-electric-vehicles-...

Check out the "lifecycle greenhouse gas emissions" figures. It's really interesting in how it shows how different the total emissions are depending on which country you charge your electric car in. Also, depending on which electric car model you have, the lifecycle emission improvement over a conventional one might be relatively slim, depending on where and how much you drive it.

By and large, electrics are at a substantial disadvantage due to both battery production and fuel cycle emissions, which then gets compensated over time as you drive the car.

Edit: There's also this super interesting article on Wikipedia: https://en.wikipedia.org/wiki/Environmental_aspects_of_the_e... which is largely consistent with the Carbonbrief article.

I've seen the graph there and I find one thing troubling:

Lifecycle greenhouse gas emissions for conventional and electric vehicles (by country) in grammes CO2-equivalent per kilometre, assuming 150,000 kilometres driven over the vehicle lifetime.

That's less than 100,000 miles - rarely do cars travel less than that during their lifetime.

Specifically in my country that would mean using the car for ~10 years, meanwhile the average passenger vehicle age is 14 years!

As for biofuels: land and water usage are major hindrances. For example you need ~1800 liters of water to grow 1kg of soybean. Sure, you can use waste products for fuels, but they need to be waste from something.

My gut feeling is electric cars may very well last significantly longer than internal combustion ones. Assuming a 100,000 mile lifetime range smells to me like someone putting their thumb on the scale.

For very short distance flights, electric aircraft are already viable. For example, Harbour Air Seaplanes operates a 20 minutes flight between Vancouver, BC and Victoria, BC. They use float planes because both cities are on the coast. Their first electric aircraft should take off in a couple months.

One nice thing about electricity is that it's much cheaper than biofuels. This is because motors are much more efficient than engines, and because large generating stations are more efficient than small aircraft engines. BC is 95% hydroelectric which makes electricity even cheaper.

>So why are we not heavily investing in biofuel infrastructure?

I think the problems are more economics than physics. There was a huge investment in algal biofuels but it never proved economic. (eg https://www.greentechmedia.com/articles/read/lessons-from-th...)

I was a fan of Algenol for a while who were supposed to have algae that gave off ethanol cheaper than normal fossil fuel but it never worked out. Maybe something like that in the future will work.

Electric motors and batteries are very efficient, and therefore extremely cheap to operate.

Biofuels make sense I think for aviation because you need that energy density to get a usable range, but the fuel is likely to be expensive for a long time. If you can figure out how to make liquid fuel for about fifty cents a gallon or so, it might be competitive with electricity for car use.

I don't know what the energy requirements are for modern batteries, but I expect it varies a lot depending on battery chemistry. Battery sizes vary a lot, too.

> Due to the lower energy requirements for building ICE cars versus batteries, a biofuel powered car will be much better for CO2 emissions than an electric one

What's the GHG emissions for the different types of biofuels ? What's the land use needed ? How abundant ? Etc.

Alternatively, we can use electricity and heat from renewable power sources to synthesize kerosene from CO2 and water.

The only reason we have electric cars being pushed as much as they are is because of Tesla, specifically Elon Musk. It is not some government program. If it weren't for them we would have the Nissan Leaf and that is about it. And it wouldn't have half the performance that even it has today.

Short answer: Monied interests don't really care, or don't think it is possible or feasible.

Biofuels always sounded wasteful to me. But this post makes a lot of good points on its merits that I hadn’t considered.

How efficient is producing biofuels using renewable energy as a way of storing power long term?

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