What does Sustainable Energy say about bus transportation?


In Sustainable Energy, buses and electric trains are present day technologies with better energy usage than autos.

At its best, shared public transport is far more energy-efficient than individual car-driving. A diesel-powered coach, carrying 49 passengers and doing 10 miles per gallon at 65 miles per hour, uses 6 kWh per 100 p-km -13 times better than the single-person car.

In 1979, when I asked a Los Angeles public transit planner what they used as facts for energy efficiency, the planner said a typical 52 passenger bus got 5 miles per gallon of diesel fuel.

From, MacKay's appendix, a U.S. gallon of diesel fuel computes to 40.48 kWh. With a further conversion of 1.609 km/mile how does my 1979 BusBook energy calculation compare with the energy usage reported in Sustainable Energy?

Here is the calculation. The unit of transportation energy usage to be computed is kWh per 100 passenger kilometers.

40.48 kWh/ (5 miles x 1.609 km/mile x 52 passengers) = 9.68 kWh per 100 passenger km.

McKay recognizes in his transportation analysis that buses and trains do not run fully loaded.

In 2006–7, the total energy cost of all London’s underground trains, including lighting, lifts, depots, and workshops, was 15 kWh per 100 p-km – five times better than our baseline car. In 2006–7 the energy cost of all London buses was 32 kWh per 100 p-km. Energy cost is not the only thing that matters, of course. Passengers care about speed: and the underground trains delivered higher speeds (an average of 33 km/h) than buses (18 km/h). Managers care about financial costs: the staff costs, per passenger-km, of underground trains are less than those of buses.
That is a really interesting value, his real world total energy cost of 32 kWh per 100 p-km is 5.3 times as much energy as the ideal fully loaded bus with no extraneous energy charges applied.

Taking the reciprocal, the value of .19 crudely compares to a 19% "load factor". Now we are being really crude here.... but what it suggests is the London bus system probably has a working load factor of perhaps 30 or 40%.

That is a lot of empty bus available for carrying freight.



Sustainable Energy defines a consistent set of energy and CO2 emission units. Presenting the entire energy problem with a consistent set of units is a great contribution.

In 1979 I struggled with the energy units problem and I wound up settling on BTU per passenger mile. The consistent metric units used in David MacKay's analysis help greatly with seeing the various parts of the energy problem in their proper proportion.




1 comment:

David MacKay said...

One difference between the ideally-loaded "coach" mentioned above, and the real "buses" of the London transport system is that the "coach" was, I assumed, going at a steady 65 mph. A "coach" in the UK is what one would call a Greyhound in the USA. The real "buses" in London never get anywhere near 65 mph; they spend all their time starting and stopping with an average speed of 10 mph or so. If anyone can provide accurate numbers about the average performance of real-life coaches, I'd love to receive them. The book's website includes links to a wiki where such data can be placed and discussed. Thank you for your support!