When it comes to transportation, no battery technology even comes close to the energy density of liquid hydrocarbon fuels. Whether they be derived from ancient or recent biomass, the energy density of hydrocarbon fuels is many times that of even the most advanced battery technology. And energy density is one of the most important metrics when it comes to transportation energy, because you have to carry the energy with you.
For the sake of this article, I am only focusing on mass energy density, not volumetric energy density. Volumetric energy density doesn’t usually come into energy discussions until the topic of hydrogen comes up, which has terrific mass energy density, but extremely poor volumetric energy density. But that’s a completely different topic that I won’t get into here. There are some who will point out that not all transportation modes require carrying fuel onboard, such as electric trains and subway systems that pick up their power from overhead cables or tracks. However, these forms of transportation are typically only suitable for high population density environments like large cities. And we’ve run this experiment in many cities before. When given the choice, people generally choose flexible, personal transportation over public transportation. To those who would seek to proselytize, incentivize, or cajole the general populace in demanding more and better public transportation, well, good luck with that.
Energy density is important because the weight of a vehicle largely determines its fuel economy. Aerodynamic drag plays a role too, of course, but not until you get up to highway speeds. Rolling resistance, which is proportional to vehicle weight, still has the largest influence on a vehicle’s overall energy requirements.
A typical passenger car holds approximately 15 to 20 gallons of fuel weighing between 90 to 120 lbs, making it account for less than 4% of a vehicle’s overall weight. And, unlike batteries, the fuel weight declines as the tank empties making its average weight contribution even less than 4%. In the case of electric cars, the battery weight is a substantial portion of a vehicle’s weight at all times. Even in the Tesla Roadster, which uses the most advanced battery technology available, the battery weighs over 900 lbs., making it responsible for more than a third of the 2550 lb. weight of the vehicle. It seems incredible but this 900+ lb. battery only gives the Roadster as much range as the equivalent of about 4 gallons (24 lbs.) of gasoline. You don’t have to be a math whiz to see that the energy density of even the most advanced battery technology is only about 3% that of gasoline. If you go with the most economical battery chemistry, namely lead-acid, which has been around for 150 years, the energy density is 4 times worse than lithium-ion, or less than 1% that of gasoline. To store the equivalent energy of 24 lbs. of gasoline, you’d be looking at a battery that weighed nearly 2 tons!
The reason I even bring up lead-acid batteries is because they are still the workhorses of the electric energy storage industry, despite their low energy density compared to the much more modern lithium-ion technology. A major advantage lead acid batteries have over their competition is that their materials are relatively low cost and very recyclable. In fact, about 98% of lead acid battery materials can be recycled. They actually have enough value after they’ve worn out that someone will pay you to take it off your hands, which is rare in the world of recycling where there are usually drop off fees associated with getting products recycled because it’s cheaper to bury them in a landfill than it is to recycle them. In the case of lithium-ion batteries, their materials and construction makes them difficult and costly to recycle. For the sake of worker safety, lithium battery recycling needs to be performed at cryogenic temperatures. To complicate matters, there is a finite supply of lithium in the world. In fact, if all vehicle production were to switch to lithium-ion technology, there are some who say we’d completely exhaust all known reserves of lithium in the world in less than a decade. The topic of how much lithium we have available is a subject of debate, but no one is in disagreement that about 80% of the world’s lithium reserves are in politically unstable regions in South America like Bolivia and Chile. The last thing any manufacturer wants is a material whose primary supply is located in politically unstable regions.
Lithium batteries are also quite expensive relative to other battery types. They cost about 5 times as much per unit of energy stored as lead acid batteries, which helps explain why lead acid batteries are still used even in some mobility applications today, where keeping weight to a minimum is critical. Unlike semiconductors whose cost has effectively dropped by half every 18 months, lithium-ion battery cost has remained relatively flat for the past decade. The demand for lithium-ion batteries has been substantial during that time, since they power nearly all portable electronics today. This means we are not at the beginning of some nascent learning curve that will inevitably allow us to reduce manufacturing costs as we begin to understand the technology better. If one were to extrapolate the previous 10 years of progress curve into the future, it’s likely that lithium ion battery cost will remain flat or even increase. In other words, we should not expect some manufacturing breakthrough to cause lithium batteries to go into a downward price spiral. Downward cost spirals usually occur when a technology is new and has a steep learning curve and a material cost that comprises just a small portion of its overall cost. In the case of a lithium battery, the majority of its costs can be traced back to the commodity pricing of its raw materials. After all, this isn’t an integrated circuit with just a few grams of materials. It requires hundreds of pounds of materials, some of which are exotic and expensive, to make a lithium-ion battery large enough to power a car.
Another advantage of internal combustion engines is that they have value even after they are worn out. The majority of an engine’s materials are made of metal, which is highly recyclable. In some cases, for example, the light aircraft reciprocating engine, it’s very common to just replace the wear items such as the engine’s rings and bearings, allowing an engine to last for decades since the non-wear items, which comprise the majority of the materials that make up the engine, can be used indefinitely. In fact, the wear items account for just a few percent of a typical 300 lb. aircraft engine. Although the internal combustion engine is often thought to be an environmental menace, it has many features related to recyclability that EV propulsion systems cannot easily match.
As enthused as everyone is about seeing EVs in every garage, when you combine the internal combustion engine with biofuels, you have a winning combination in nearly every transportation category, including economy, range, existing infrastructure compatibility, and sustainability.
