> I'd like to see the the energy density of petrol for comparison.
Petrol's higher energy density doesn't matter as much as people think.
Electric vehicles are around four times as efficient as petrol. In a petrol car, only 20% of the energy is converted to motion. In electric cars, this is around 80% (with some variation dependent on regenerative braking). I wrote about this extensively in a previous article: https://www.sustainabilitybynumbers.com/p/electrification-en...
That is not even comparing apples to oranges, that is comparing apples to steel. You are correct in that energy density does not matter very much for weight-insensitive generation such as grid-scale generation, but energy density matters for weight-constrained applications such as airplanes and rockets as the poster mentioned.
However, assuming that the renewable generation cost curve continues to improve exponentially then the most likely outcome for a carbon-neutral or carbon-negative future will be using electricity to manufacture high density combustible fuels out of atmospheric carbon, effectively using it as a high density "battery" for use cases that demand high energy density.
To the extent that your analysis is relevant to the concerns of the poster, all it means is that batterys are actually ~4x better than the raw energy density would indicate. As to the specifics, Wikipedia claims petrol is ~12,888 W*h/kg or ~24x the battery energy density in the article, so ~6x better with respect to car motion. Note that the current curve has only gone from ~100 W*h/kg to ~500 W*h/kg, so we would need to see density growth comparable to the last 30 years to happen again.
Excellent question! A sufficiently advanced battery can theoretically beat gasoline.
Any given energy storage technology can store a maximum amount of energy in a fixed volume or mass. Behold one of my favorite plots: [1]
From lowest to highest energy density:
- springs, which use mechanical elastic potential energy, are kinda horrible
- capacitors, which use electric permittivity, aren't great
- next are both batteries and combusted fuels, which both use chemical reactions.
- nuclear gets us another few orders of magnitude
- finally, antimatter (E=mc^2) is a ways beyond that
Both batteries and fuels rely on the energy difference between unreacted molecules, so their theoretical energy density is the same. Well, actually, fuels are burnt to create heat which is converted to energy, and this heat->energy conversion is fundamentally thermodynamically inefficient (only ~tens of percent), whereas batteries are the same sorts of reaction but much more controlled. A sufficiently clever battery, which moves atoms around to react in the right places at the right time, is thus more efficient and thus energy-dense than fuel. However, moving atoms around like this to make a more efficient battery is much more advanced nanotech than what we currently have. But it's theoretically possible.
This is what biology does: us humans are powered by chemical storage (sugar/fat/glucose), which is used more efficiently than current batteries but without combustion. (lithium-ion is ~0.8 MJ/kg, glucose is ~16 MJ/kg, gasoline ~46 MJ/kg)
One thing I wanted to add is that fat (lipids) are much more energy dense than glucose. ~38 Mj/kg, though I am not sure what fraction of that the body recovers. Which makes sense, you want to maintain your long-term storage in a denser format.
What makes one chemical more able to store a greater energy per unit mass than another? Wouldn't the theoretical limit be a volume of pure electrons compressed in the densest unit volume possible? Say, stored in a magnetic field?
Compressing neon gas won't do much, aside storing energy as compressed fluid.
My point was the traditional fuels (incl. the edible ones) use more material/weight than their own. So it is very likely they'll be more efficient. The batteries require a reversible action by just applying current - this is quite the climb compared to most chemical reactions.
We have not done much since the li-ion inception, using FePO4 instead of cobalt is more sensible from an economic point of view but the energy density is even lower.
>Petrol's higher energy density doesn't matter as much as people think.
When vehicles uphill, ramp, and fight with the increasing wind resistance due speed, it is needed a high torque for to motion.
The petrol's energy density is translated in high torque, that the gearbox latter transforms progressively.
In electric vehicles, generating high torque and cooling the overheated coils for to obtain such high torque drains the battery quickly, the range drops quickly.
And for to increase the range, more weight is added (more batteries), that requires higher torque for motion, that requires more energy again, and so on.
This is why the energy density it is important, in batteries are the watts hour per kilogram. As also it is important the number of cycles before such batteries start to drop energy density until to fail (to note the weight keeps being the same along all of this degradation).
With the current technology, due the magnetic fields strength generated in the coils, and the energy density of the batteries, EVs just can not compete with petrol vehicles. It is about torque, among other things.
What is needed? batteries with bigger energy density ( higher Wh/Kg with higher number of recharge cycles), and/or higher efficiency generating magnetic fields of high strength (ambient superconductivity, also stronger magnets would help some coil's topologies).
Regardless of theorical efficiency or not of the energy, in practice the ICE gets the torque needed by the vehicle for driving on all types of slopes (and speed, by transforming part of that high torque with the gearbox).
This is why electric and combustion vehicles have such different ranges at even the same weight. Nowadays.
Such high torque is needed at the same moment the vehicle doesn't circulate on a flat terrain, or when have to reach highway speeds.
For to get at least the same ranges, the electric vehicle must reduce the energy consumption for the generation of the required torque (and speed), or needs to increase the energy density of the battery in companion of increasing the recharge cycles.
will be possible to achieve this? of course.
(The losses cooling or heating the battery and avoid the self-discharge should get the same advances, as it's counted as stored energy but it's not used for motion)
By "significantly heavier" it is often a difference of a few hundred pounds on a few thousand pound vehicle. A 2L engine is about 400lbs, an automatic transmission is another 220lbs, 20 gallons of gas is 120 lbs, add another 100ish pounds for a much larger cooling system. So sure, the battery is like 1,000lbs but you traded 840 pounds for 200 pounds of EV motors (assuming two of them!) so in reality you're up like 360lbs.
Combined with regenerative braking, it doesn't make that big of difference in total energy usage. A massive chunk of the energy used in an EV is aero drag which makes little difference about weight. Weight makes a bigger impact with stop and go traffic on non-regen cars as slowing down that extra mass turns more energy into heat. An object in motion wants to stay in motion and all, once you're up to speed you're using about the same energy. This is why a lot of the EV trucks have close to the same range if the bed is full or not assuming it has the cover on the bed, but towing even a small trailer becomes a massive range hit.
I get on average 3.5mi/kWh in my EV, ~1MJ/mi. A gallon of gas is like 120 MJ, an average hybrid will get like 40mpg, so 3 MJ/mi being burned. You'd need to get like 120mpg to match my average efficiency of energy usage, and my EV isn't even that incredibly efficient of an EV.
We don't have billions of people living in the wilderness. And technology has reached a price level where off-grid solar is actually an affordable and superior alternative to propane and diesel for household use in rural Africa.
Heat pump heat, in the wilderness or otherwise, is about 4x as efficient as resistance or fire.
This is somewhat silly, since a gas-fired heat pump can be very efficient, but gas-fired heat pumps are quite rare.
(California has a pricing/policy problem here, IMO. Electricity is absurdly expensive, gas is somewhat reasonable, and the result is that electric heating is not nearly as economical as it should be.)
Gas-fired heat pumps are neither economical nor efficient for single-apartment or single-house systems. Internal combustion engines with shaft power output of 1-2 kW are inefficient, loud and maintenance intensive.
They can start to make sense from around 50 kW heating/cooling capacity and upwards, so the smallest units are suitable for 8-15 apartments depending on size.
“Heat in the wilderness” must be so small in terms of global emissions footprint to be almost irrelevant, no? The wilderness implies ultra low density sort of by definition.
Doesn't matter for WHAT? You start out talking about energy density, and then cite some numbers regarding efficiency. What does one have to do with the other? You've done nothing to support your opening claim here.
"Energy" value of gasoline and "energy" value of a battery pack are measuring two very different things even though they are both units of energy. When you burn gas in an engine, the engine has a theoretical upper limit on its efficiency which is FAR below 100%, and electric vehicle does not. So saying that gasoline has an energy content of 115,000 BTU/gal doesn't mean much since you'll be lucky to see 30% of that be turned into useful work.
Well, acording to wikipedia gasoline has an energy density of 46.4 MJ/Kg and LiPo batteries have an energy density of 0.36–0.875 MJ/Kg. So if your electric drivetrain has a 100% efficiency and your gas drivetrain has a 30% efficiency then the gasoline car would be able to do 16 times more work per unit of fuel.
A Tesla Model 3 has 1060 pounds of batteries. If you replace those with a 177 gallon gas tank (at 6 lbs/gal) you should be able to go 4,780 miles at 27 MPG. It’s just that nobody wants to give up that much volume (visualize a 6x6x5 cube of gallon milk jugs).
I’m not accounting for a gas engine having more heavy parts, that mostly matters in city driving.
Not enough for what? The energy densities of both LiPo batteries and gasoline are enough to make a passenger vehicle with an acceptable range. For gasoline it is more than enough and for LiPo it’s barely enough.
Yea it doesn't really follow. I think the idea is that the energy density being 50 times lower doesn't matter because the battery is efficient enough that you can make an EV with a 300 mile range which is good enough for most people. Of course if anyone wanted one, you could build a gas car with a 5,000 mile range if it had a gas tank as massive as an EV battery.
Petrol's higher energy density doesn't matter as much as people think.
Electric vehicles are around four times as efficient as petrol. In a petrol car, only 20% of the energy is converted to motion. In electric cars, this is around 80% (with some variation dependent on regenerative braking). I wrote about this extensively in a previous article: https://www.sustainabilitybynumbers.com/p/electrification-en...