Gasoline has great energy density. For common non-nuclear fuels, I believe only diesel beats it. Will batteries ever be able to get into the same ball park as gasoline? I understand that batteries powering an electric motor are on the order of 90% efficient whereas internal combustion engines are on the order of about 30% efficient, so I guess the density only needs to be about a third of gasoline.
Rob
No. Non-rechargable lithium batteries have approximately 1/20 the energy density of gasoline by weight and 1/8 the density by volume. Nothing comes close, and there aren’t really any promising battery technologies on the horizon, despite what tech magazines will say. The most likely battery innovations, by my guess, are lithium ion batteries that are cheaper and recharge more quickly.
Possibly, but no chemical battery comes close at this point. If someone has a battery chemistry that’s better than Li-ion they’re keeping it a secret. That’s what’s used for modern electric cars.
That’s one of the problems with trying to find replacements for gas and diesel - it is so energy-dense for the purpose nothing else comes close. To make vehicles that compete with gas cars takes huge compromises somewhere - or everywhere.
All I know is that in 150 years or so, with pretty much a maximum amount of motivation and potential rewards, battery efficiency by weight density has remained almost static. I believe that the ratio of gas to the best batteries available today is still greater than the ratio of those batteries to basic liquid lead-acid types from the 1880s.
Aluminum-air batteries that were in the news lately with Renault-Pinergy experimental vehicle claim the energy density of 8KWh/kg. Which is getting close to 12.2 KWh/kg of gasoline. Considering that the electric motors are simpler and lighter than the equivalent IC engines, it may be achieving parity right there.
Electrochemical batteries can be compared to gasoline by thr use of the gasoline gallon equivalent (GGE) metric; one GGE is about 33.4 kWh/gal (114,000 BTU/gal) or about 7600 kCal/liter. One gallon of gasoline masses about 6.3 lbm or about 2.9 kg, so the energy mass density of gasoline is about 11.8 kWh/kg. Lithium-ion (Li-ion) and nickel metal hydride (NiMH) batteries, which are the best deep cycle batteries in use today, top out at a little over 0.3 kWh/l and and 0.1 kWh/kg, so by most measures electrochemical batteries have a GGE of about 0.01; in other words, they have about 1% total energy density capacity. There are higher density batteries (e.g. so-called thermal batteries) but these produce high power output for a short period following introduction of the electrolyte and essentially consume themselves in operation, making them fundamentally unusable for reuse.
Will better battery technology equalize this metric? The best potential technologies (lithium phosphate, lithium sulfur, and prismatic/polymer matrix lithium) only best this by around 20% volumetric density and 30% mass density, which still leaves more than an order and a half of energy density difference. Even accounting for the thermodynamics of combustion-powered heat engines (more on that in a second) there is still an order of magnitude gulf in capability. And despite the pretensions of some advocates, there just aren’t any scaling or efficiencies that are going to make any known electrolytic energy storage technomagically improve to the same order of magnitude as hydrocarbon fuels such as gasoline, kerosene, propane, methane, or even alcohols and ethers. The costs of manufacturing batteries
However, there difference in application should be considered from both a cost and power throughput standpoint. The o.p. notes that internal combustion engines are “about 30% efficient”; it should be noted that this is in relation to the thermal efficiency (i.e. how much of the heat energy can be extracted as useful mechanical work between the combustion chamber temperature and the ambient environment); most car engines are better than 70% of the theoretical Carnot efficiency. Combustion efficiency under steady load is typically better than 98% efficient. There are mechanical losses in the transmission, et cetera, which a direct wheel driven electric vehicle won’t see, as well. However, an electric vehicle is penalized by carrying the large mass of batteries and the high sprung weight of electic motors on the wheel hub (which has impacts on handling and cruising efficiency). On the other hand, while all expended work with a combustion engine is lost to the environment, an electric vehicle can recover at least some modest proportion via regenration in braking and potentially from the suspension.
Cost is another consideration; anelectric vehicle has a high upfront cost in terms of the battery (the cost of a Tesla Model S is about 40% battery) and the cost of copper and rare earth-based components in the motors, which is significant. However, the cost of energy as electricity is much less than production cost of fuel (even petrochemicals extracted from ground reserves, and even more over synthesized hydrocarbon fuels) and is fungible, easily shifted from coal or natural gas to solar and wind power production. Electric vehicles also have fewer moving parts and (in normal operation) generate less heat, which leads to less maintenance. However, the need to periodically replace and recycle the battery cells can offset or even exceeds operating costs, depending on lifecycle assumptions and cost of recycling.
There are also the operating considerations, which weigh heavily toward liquid fuels in many applications; not only the speed of refueling versus the hours required to fully recharge a large electric battery pack and the difference in range for long distance travelling, but the power output of internal combustion engines in cold ambient temperatures (in which the system is essentially self-heating) versus the low power throughput of a battery in the same conditions. Using an electric car in a cold Michigan winter is just not viable without supplementary heating, which requires more energy and therefore further reduces range.
The most compelling argument for the use of batteries isn’t cost or that they save the environment (which depends on the total production-to-wheel cycle efficiency and pollution) but that they are fungible with regard to energy supply; you can pick the cheapest way to produce electricity to recharge the battery regardless of what is going on in the Middle East or whether farmers are being subsidized to grow crops for biofuels. And for moderate range commuting in temporate climes, the total operating cost and total efficiency is probably better than hydrocarbon fuel options. But for long range, heavy haul, and cold temperature use electric vehicles have some major limitations that are fundamentally restricted by basic chemical kinetics, and no, there is no magic formula waiting around the corner that will make electrochemical battery energy density or power throughput comparable to hydrocarbon-powered internal combustion engines.
Stranger
Good luck recharging them.
Unfortunately there’s a lot more to vehicle power than battery energy density - charge rate, charge-holding ability, discharge current, number of charge cycles, and some fundamental safety concerns (see: exploding laptops) all have to be considered as well.
I am/would be unsurprised if there laboratory batteries that have much higher densities. Putting them into practical vehicles is an entirely different problem.
Nearly all batteries now available or under evaluation seem to be variations of long-known techniques and materials; I don’t know when the last fundamental improvement in battery tech was made. I’d bet it’s been a while.
Ethanol has a similar (but slightly lower, 30% lower-ish if I recall) energy density to gasoline, and does compete with gasoline to some extent (E-85 anyone?). Since it’s made from plants, increased use can have the side effect of increasing food prices. Good for farmers, bad for everyone else.
Ethanol might not be a “winner”, but it’s certainly in the same general ballpark as gasoline and diesel.
There are also petroleum-based fuels other than gasoline and diesel. Kerosene (i.e. Jet Fuel) is a big one. Bunker fuel oil as used in some heavy oceangoing ships is another.
You could certainly make a kerosene-burning car or truck, but it’s probably not practical because of the lack of research and infrastructure into kerosene cars. After burning through a few million in R&D, you’d probably end up with a car that didn’t offer much in the way of fuel savings.
Also, fwiw, home heating oil is Diesel, more or less. Sometimes HHO has additives that aren’t allowed in road Diesel, and is dyed to indicate that no road fuel taxes were paid. In certain jurisdictions, if you get caught with HHO in your tank, you’re in Big Trouble ™.
We could manufacture any arbitrary organic molecule we wanted for transportation fuel. The reason ethanol, natural gas, gasoline and diesel are the current favorites is that we have cheap sources for them. If in the future we have to synthesize fuel ethanol, octane, or methane might not be the optimal choices.
We may never have a mostly electric vehicle fleet, even when it’s no longer economical to extract petroleum out of the ground we might use electricity to create fuels that are distributed and burned in conventional IC engines rather than use the electricity to charge the batteries of electric cars and trucks. We’re never going to have battery powered aircraft for one thing, so if we’re making arbitrary liquid organic molecules for air transport we can just crank up the production to handle the surface fleet.
While fuels like ethanol are techically carbon-neutral, any hydrocarbon fuel brings with it problems we really need to get rid of. So swapping out gas for ethanol is only a partial solution anyway. Non-fossil-fuel electrical production is the long-term solution for a sustainable civilization on the planet.
keep in mind that the efficiency numbers frequently bandied about for IC engines is “peak,” meaning the thermal efficiency at wide open throttle (WOT) with the engine producing peak torque.
when you’re going down the highway at 65 mph with the throttle barely cracked open, the pumping losses of the throttle plate knock that number down a good bit (the engine is spending a good bit of energy just trying to breathe.)
They are not rechargeable, but recyclable. But replacing them should not take longer than full-tank fueling up today, and the cost from what I have read is not much more either. Of course, the difficulty becomes complete standartization of the battery, otherwise replacing it as a “refuel” is not feasible.
In any case, the question was about battery energy density.
Combustion efficiency does not change much across the power band in modern engines. The whole question does illustrate how slippery “efficiency” can be.
In any case, car-sized batteries that need to be recycled for each use are never going to be in the same time zone as ‘efficiency.’
Combustion efficiency specifically refers to what fraction of the fuel’s chemical energy is released as heat. For engines built in the last couple of decades, it’s upwards of 97% when fully warmed up.
A relatively small portion of that energy makes it to the flywheel; a lot gets lost as heat to the combustion chamber walls, and a lot gets tossed out as hot exhaust gas. A portion goes to fight friction in all of those sliding surfaces, and yet another portion is dedicated to sucking air past the throttle plate. Take away some more to drive the fuel pump and coolant pump. As noted upthread, the final percentage of energy that makes it to the flywheel as you’re cruising down the interstate is probably on the order of 10-15%.
The overall efficiency of a combustion-based power plant can be much higher, more like 50%; they are large-scale, optimized to run at a very specific condition (instead of a broad range of conditions, like your engine), and run at a large fraction of their design load. Take the transmission efficiency to get electrical power to your house (75%?), charge the car battery (90%?), and use the battery to power the car’s wheels (90%?), and the overall efficiency for an electric vehicle using combustion-based power generation is more like 30%. This of course does not consider the ability to employ regenerative braking, or the ability to use renewable power generation sources such as wind/solar/hydro.
To be honest, the energy density of gasoline is a bit of a cheat; if you took an internal combustion engine to the Moon, you’d have to take a supply of oxygen with you. To get the true figure of the energy density you have to factor in the oxygen tankage as well.
I don’t care about “combustion efficiency” here. If I was dealing with emissions compliance, I would. The only efficiency I care about in this discussion is how much of the potential energy in the fuel actually goes towards pushing the car down the road.
Okay. It’s a murky topic because a vast number of both fraudulent and commercial devices have made a big deal about how inefficient IC engines are, and then waved wands and made bunnies appear to solve an essentially non-existent problem. Even a good carbureted engine has combustion efficiency over 90% and modern engines approach 100% across the power and load bands - so miracle carburetors and cow magnets could not possibly deliver more than a fractional increase in MPG.
That whole topic of internal efficiency gets mixed up with energy-to-the-road efficiency, even in the hands of writers who should know better. (I’ve seen some very respected automotive journalists start out talking about net efficiency and then detour into magic carburetor territory as if it’s the same thing.)
We have pretty much reached the limits of gasoline/diesel road efficiency; there are cars on the road whose net efficiency could not be improved by any reasonable means. (There are also many cars on the road that ignore points of efficiency for other reasons like pavement-ripping acceleration.)
We have only two routes for greater overall fuel-based car efficiency. The first is to take all the automotive engineering of the last 25 years that’s put 300-600 HP under the hood of a great many models, and apply it to much smaller powerplants - 150 HP out of 1 liter or so, something that could not be done short of a racing engine just a few years back. These engines will still lack some acceleration zip, but would be adequate for most drivers in most situations. It’s a matter of selling it as thoroughly as Dodge sold the “Hemi.”
The other approach is to build significantly lighter vehicles overall, which will mean safety compromises. We could cut the weight pf almost any vehicle on the road by 1/3 or more - but not while preserving the accepted standard of safety. The solution is to start dividing roads, partitioning off some lanes for existing vehicles (especially trucks), and an increasing number for lightweight, high-efficiency models. It doesn’t really matter what powers these vehicles - we have to get away from 3500 pound econo cars if we’re going to make any further progress in overall system efficiency, and despite wishful thinking, you can’t put a Lotus Seven full of kids on a highway.
Battery technology has probably nearly reached the limit of what’s possible, but do note that batteries aren’t the only option for fueling electric cars. You could also make an electric car that used fuel cells or capacitors.
Fuel cells share the advantage that internal combustion engines have that they can use oxygen from the air. Combine this with the increased thermodynamic efficiency you get from using an electric engine instead of combustion, and you could, in principle, far exceed current cars, possibly using the same fuels. We’re not there yet, but there’s a lot of room for development.
Capacitors also aren’t there yet, but there are no fundamental limits to the energy density you can get from capacitors (or, rather, there are, but they’re so incredibly high that you might as well ignore them entirely). And capacitors have been advancing at breakneck speed. When I was an undergraduate in the 90s, a 1 farad capacitor would have been the size of a garbage can and cost tens of thousands of dollars-- Now, they’re a couple of bucks and fit in the palm of your hand.
When either of these technologies really arrives, they’ll probably be called “batteries” by the general public. But their underlying principles are quite different, allowing them to bypass the limitations of chemical cells.