Comparing the amount of energy required to move people and stuff around by different methods. And, road trip or fly?
One of the critical pieces of our civilization is being able to move ourselves and material goods over a wide range of distances. Transport is a driver of economic development and personal freedom. But transport requires energy to move matter from one place to another, and that energy use often results in air pollution and CO2 emissions.
Considering the amount of energy required to transport people and material goods is important for understanding what transportation methods to use now and designing transportation methods for the future that are economically and environmentally sustainable.
With the wide variety of methods of transport, from airplanes to bicycles, and the difference in energy sources required to power them, it's difficult to make comparisons. What we need is a common energy unit to compare methods of transport regardless of energy source. Here, I've done extensive research, drawn from personal test data, and converted all energy forms into common units of transport.
For transport of goods, we will use Watt Hours per Kilogram of goods per Mile (Wh/kg•mi).
For transport of people, we will use Watt Hours per Passenger per Mile (Wh/pass•mi).
These units can be derived from other units easily, such as miles per gallon of gasoline.
1 mpg = 33,705 Wh/mile. 30 mpg = 1,124 Wh/mi, for example.
The reason for using these units is because watt-hours (or kilowatt-hours) are the units we use to measure the energy in electricity, and miles are a unit familiar to Americans, which is most of you reading this.
Before we get into the data, a few assumptions are made. For goods transport, we use average-size vehicles for the type, such as average ship and train sizes. We assume these transport vehicles operate at or near their maximum capacity for carrying goods. For passenger transport, we are using common ridership numbers for vehicles designed to carry many people and are not owned by an individual: trains, buses, and jets, for example. For personal vehicles, we are assuming only one passenger for comparison. So if you have two people commuting in a car, the Wh/pass•mi would be half.
Transporting Material Goods
The chart below shows the primary methods of goods transport, from large vessels for shipping huge quantities to last-mile delivery.
We can clearly see economies of scale at work. As a general trend, the more goods that can be transported at a time, the less energy is required to move them. So, let's remove drone delivery from this chart to get a better look at the more common transport methods for goods.
It's clear that container ships are the most energy-efficient way to move goods. The ship itself uses a staggering 3,847 kWh of energy per mile, but it carries a lot of stuff. This chart uses an average-sized container ship of 8,000 TEU (Twenty-foot container Equivalent Units) carrying 100,000,000 kg of goods, which is only about half of its maximum weight capacity. But it's logical to assume that each container isn't filled with the maximum weight of goods if those goods are of lower density. There is a push to make container ships larger and larger since the larger the ship, the less energy is required to move each container. The largest ships now are over 24,000 TEU.
Trains are the most efficient way to move goods per mile over land because they carry a lot of stuff, they are aerodynamic, and they run on relatively smooth, flat rails.
Long-haul trucking uses about 80% more energy per kg per mile than rail but is still almost 10 times as efficient as smaller last-mile delivery vehicles.
We also have air freight, which is a replacement for trucking when customers want goods delivered fast. Air freight uses over 18 times the energy as trucking per good. Consider that when you select "2nd Day Air" as your shipping option.
Transporting People
The chart below shows the energy required to move people by different means. The blue bars show fossil-hydrocarbon-powered vehicles, the green lines are vehicles powered by electricity, the pink shows a hydrogen fuel cell vehicle, and the orange shows human/animal power that comes from respiration (metabolism).
Personal Transportation
The charts below show passenger transport of vehicles designed for personal use.
The chart below zooms in on the most efficient personal transport modes.
Car vs SUV/Truck
We can use a Honda Civic as a proxy for an average car in the US, which gets 35 mpg, and uses 963 Wh/mile. Larger vehicles use more energy than smaller ones. Regarding personal transportation, a Suburban, F-350, or any large SUV or truck uses more energy than pretty much anything else at over 2,000 Wh/mi.
Motorcycle vs Car
While a motorcycle might seem much more efficient than a car, it depends on the type. A very efficient motorcycle, like a 250cc dual sport, uses 535 Wh/mi, but a large model Harley-Davidson uses almost as much energy as a sedan.
Electric vs Gas
Electric vehicles use much less energy than gas or diesel-powered ones. A Tesla Model 3 uses less than 1/4 the energy of a Honda Civic, and a Tesla Cybertruck uses less than 1/4 the energy of a Chevy Silverado. This is because gas-powered engines are inherently inefficient. Only about 20% of the energy they use goes towards moving the vehicle, whereas that's close to 90% for EVs.
Hydrogen Cars
Hydrogen fuel cell vehicles (FCV) use almost twice the energy of EVs and about half that of gas vehicles. However, the process of creating hydrogen for these vehicles is often highly energy-intensive. When an EV is charged by renewable electricity, over 90% of that electricity ends up in the car. When that renewable electricity is used to make hydrogen, compressed, and used to fill a hydrogen FCV, less than half of the original electrical energy makes it into the car.
Muscle Power
You'll notice I included the most primitive means of transport in these charts: using animal muscle power from humans and horses. The energy, in this case, comes from the process of cellular respiration, which is only about 40% efficient at converting energy from oxygen and food into motion. There are a few interesting points here. Walking uses about 160 Wh/mi. Kayaking uses more energy because of the energy required to push a dense fluid out of the way. However, bicycling uses less than 1/4 of the energy of walking because of the efficiency of the wheel (and smooth paved surfaces). Riding a horse uses almost 4 times the energy of walking since the horse must not only use enough energy to carry the rider but also itself, which is a lot of weight.
Micromobility
Interestingly, the most efficient way to move around is with electric micromobility solutions such as e-bikes and e-scooters. This is due to several factors. First, they're small, and small things use less energy. Second, they have wheels, and wheels are very efficient. Third, they're electric. Electric powertrains are close to 90% efficient, whereas gas is only about 20%, and animal respiration is only about 40%. That's why electric bicycles use less energy than good old-fashioned human-powered ones.
Road Trip: Drive or Fly?
Say you're going on a vacation or a long-distance trip and want to know which method of transport will use the least amount of energy. These three charts below show how much energy will be used per mile for one person, two people, and four people traveling by different means.
You'll notice that the most efficient mode of transport is dependent on how many people are traveling. A Tesla Model 3 and many other road vehicles are capable of carrying several passengers, so they don't use any more energy for four people than for one person.
The energy intensity of public transport methods such as buses, trains, and airplanes is calculated here using the total energy per mile the vehicle uses divided by the number of seats it has. That means for two travelers on a bus, train, or plane, the total amount of energy required needs to be multiplied by two. An Airbus A320 with 150 seats can carry 150 parties of 1, but only 75 parties of 2.
The more efficient the vehicle, the less energy is required for the trip. EVs are more efficient than gas vehicles. The more people traveling in a road vehicle, the more attractive that option becomes relative to air travel. For one traveler, air travel and trains use a similar amount of energy as a large EV. However, for four travelers, air travel and trains use much more energy overall, similar to taking a gas SUV or truck. In general, smaller planes that carry 50 passengers use more energy per seat than the larger ones that carry 150.
Design Principles
From this energy of transport data, several key design principles can be concluded and used to design transportation systems of the future. The following design principles are all prefaced by "All else being equal."
For mass goods transport:
Vessels, vehicles, and aircraft with larger payload capacities are more efficient.
Ground and water-based transport is more efficient than air transport. Aircraft require a lot of energy to keep aloft. In contrast, the ground or water provides a normal force or buoyant force sufficient to hold the craft in place with no additional energy requirement.
Slower is more efficient because much of the energy loss in moving something is due to drag, which is proportional to the third power of velocity.
For passenger transport:
Smaller vehicles are more efficient than larger ones.
Ground-based transport is more efficient than air since a lot of energy needs to be used to keep aircraft aloft. In contrast, the ground provides a normal force sufficient to hold the vehicle in place with no additional energy requirement. (Sorry, flying cars)
Slower is more efficient because much of the energy loss in moving something is due to drag, which is proportional to the third power of velocity.
Wheels are very efficient.
Electric powertrains are very efficient.
Putting this together, the most efficient vehicles of the future will be ground-based, small, electric, and have wheels. In cities, electric micromobility is a great option for reducing energy use and traffic congestion.
Questions for you:
What is the most surprising thing about this data?
What is one mode of transport not included here that you'd like to know about?
How will this information affect how you think about travel or shipping?
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