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Real World Physics Problems Newsletter - Travel Efficiency, Issue #27
January 27, 2016
The Energy Efficiency Of Transportation
Transportation is a key part of our lives. A large share of the energy use of the world goes into transportation. The common ways we travel are by land, air, or water. What I want to talk about in this newsletter is the energy efficiency of these different modes of travel. It is a fact that the most energy efficient way to travel is not necessarily the fastest way to travel.
The most common way to travel is by land, using vehicles such as bikes, cars, trucks, buses, etc. A really efficient car can travel, say, 1000 km on a tank of gas, at a travel speed of 100 km/h or so. If the car can carry 5 passengers and the tank holds 50 liters of gas then this means that the energy cost is 10 liters per person per 1000 km. A bike on the other hand, would require much less energy than this. In fact, the bicycle is the most energy efficient way to travel under human power, and is much more efficient than walking or running. The average person riding a bicycle at 15 km/h would consume about 100 megajoules of energy to ride 1000 km. For the car, the energy use per person (over the 1000 km distance) is about 340 megajoules. This is the amount of energy in the fuel consumed, and it is more than three times the energy use for a bicycle, traveling the same distance. Keep in mind that these energy comparisons are for total energy consumed, and not the mechanical energy used to move the bicycle or car forward. This is a consequence of there not being a 100% conversion of chemical energy (from food or gasoline) into mechanical energy. The energy conversion efficiency of humans and cars are similar at about 20-25%.
Now let's look at planes. I'm going to look at the Airbus A380 since it is one of the most fuel efficient commercial passenger planes in the world. It has a cruising speed of about 900 km/h, and a range of 15,000 km using 320,000 liters of jet fuel. It can carry 850 people. If you work through the numbers you find that the energy use per person (over 1000 km distance) is about 830 megajoules (of fuel energy). This is between 2-3 times the energy use for the car mentioned previously, assuming both car and plane are carrying the maximum number of passengers. Flying by air is among the most energy inefficient, although fastest, way to travel. And this trade off is one we are perfectly willing to make, because even though it takes a lot more energy to use a plane, it gets us to our destination much faster than all the other viable options we currently have available. It is air resistance that affects fuel efficiency for these options. Air resistance increases as speed increases, and as speed increases more of the travel energy is used to overcome air resistance. If there was no air resistance then energy use would be much lower. But if there was no air resistance then we couldn't fly using wings as we know them, because part of air resistance (in addition to producing drag) is deliberately used to generate lift underneath specially designed wings which keeps planes in the air. Air resistance arises from the friction that exists between a fluid (such as air) and a body immersed in a fluid (such as a plane). Paradoxically, the very thing that resists the motion of an object flying through the air is also the thing that enables the object to stay airborne, through the generation of lift.
Then there are trains, such as high speed trains. These can be significantly more energy efficient than cars are, per passenger. This is due to their very streamlined shape and their steel wheels rolling on steel tracks which creates much less rolling resistance than rubber tires on the road.
Traveling by water can actually be the most energy efficient of all, as long as you're only using the force of the wind to move your vessel along, using sails, in which case the source of your energy (wind) costs nothing. But if you're using propeller powered engines to move the vessel, such as a large ship, then the energy consumption from fuel can be very high. After doing some quick calculations I found that a cruise ship has an energy efficiency per passenger that is comparable to a commercial passenger plane, such as the Airbus A380. Now this is very interesting given that a cruise ship travels much slower. That said, a propeller driven cruise ship is probably the worst way to travel in terms of energy efficiency, made worse by the relatively slow travel time compared to a plane. It is the drag resistance of water exerted on a ship's hull which makes it such a great energy consumer, for relatively low speeds. This is due to the high density of water, which is much greater than the density of air, which causes high drag resistance at low speeds.
If you look at all of this you might start to wonder what the best travel method is, considering all the possibilities that exist. Well, in essence it would have to be energy efficient and fast. To be
energy efficient you have to find a way to minimize resistance to motion, such as from air or water. The way I see it this leaves only two possible alternatives, the first of which is travel through space above the atmosphere using a space-plane, which is a large technological challenge. The second alternative is travel through a tunnel using a high speed train in which the air is (mostly) removed, which drastically cuts down on air resistance. This would also be very challenging to do. In both cases the costs are very high and certainly wouldn't be cheap to use. The train idea is probably the best one given that you can probably carry more passengers than a space-plane could and it would probably require less energy than a space-plane, given that a space-plane needs a lot of energy to overcome gravity and fly up through the air before reaching a high enough altitude where air resistance is absent. There are in fact proposals for such a train. It's called a vacuum tube train
which would use maglev (magnetic levitation) technology. The lack of air resistance in the tunnel could permit these trains to travel at very high speeds up to 6500 km/h, which is five times the speed of sound. This speed could be achieved using relatively little power, resulting in a very low energy (and very high speed) trip.
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