Energy efficiency of different modes of transport

The following table (from [1]) illustrates the wide variation in energy efficiency of different modes of transport.

Mode                 Fuel use (L/100km)   #commuters   Energy use

                     or electricity use               (MJ/person-km)

Automobile                   15                1           4.74

                                               6           0.79

                             10                1           3.16

                                               4           0.79

                              7                1           2.21

                                               4           0.55

Van                          20               15           0.42

                             10                7           0.45

Diesel bus                   56               40           0.52

Subway                        2.61 kWh/km     75(per car)  0.13

GO Rail                     761              810           0.35

Note that the fullest, most fuel efficient car is still less efficient than a diesel bus without standees. Also note that a subway car without standees is rated as being 4 times more efficient than a diesel bus without standees. Typical loads on the Toronto subway during rush hour are 200 per car, making the energy use 0.048 MJ/person-km - less than one tenth that of the most efficient car.

For a sample of actual BC Transit figures for the Vancouver region, see the article on operating statistics.

Importance of energy efficiency

The best way to decrease the environmental impact of transportation is to decrease the overall energy use. Given the same sources of energy, using less energy means producing less pollution and depleting fewer resources. (The issue of the pollution generated by different sources is covered in a separate article.)

What's the quickest, easiest way to improve overall energy efficiency?

The current system of transportation in British Columbia, which involves high numbers of cars carrying only a driver (commonly referred to as single occupant vehicles, or SOVs) results in a very high energy usage. The GVRD has estimated that the average occupancy of all cars during the peak periods is 1.28 people per car; overall it is 1.43. Clearly, increasing the average occupancy would have a dramatic impact on overall energy use. One cannot assume a direct correlation, however, since travel distances often increase as the number of occupants of a vehicle increase (it may be necessary to go out of one's way to pick up extra passengers).

The ultimate in high occupancy vehicles (HOV), of course, are mass transit vehicles such as ferries, buses and trains. The biggest gains in energy efficiency (and hence reductions in pollution) can be obtained by switching as many trips as possible from cars to public transport, or even better, to walking or cycling trips. Improvements in the efficiency of cars are often balanced by a continuing requirement to travel longer distances and/or longer times due to sprawl development.

The issues of land use, transportation planning, energy efficiency and pollution are all intertwined. It would seem that governments have yet to learn these lessons, since they still allow developments which practically force the use of cars, then focus their efforts on reducing emissions from cars, rather than attempting to switch some car use to more environmentally friendly modes.

A B.C. Transit study done in 1990 ([2]) showed that cars made up 98.4% of downtown Vancouver rush hour traffic but carried only 62.6% of the commuters. It would seem there is a lot more room for improvement here than there is in legislating the production of slightly more efficient cars.

The three components of energy efficiency

When analyzing the energy efficiency of a particular vehicle, it is convenient to consider three issues separately:

• the efficiency of the vehicle in translating the energy delivered to it into useful work
• the efficiency of energy delivery to the vehicle
• the energy cost of the vehicle and associated systems

The table at the start of this article refers to the first issue only. The other two issues are more difficult to analyse. We will consider the three issues in order.

Efficiency once energy source is at the vehicle

There are two common methods of propelling a vehicle: electric motors, and internal combustion engines plus gearing.

Electric motors may be fed energy from an overhead wire or third rail (for vehicles powered directly by electricity such as trolley buses or electric trains), from an array of batteries (in the case of electric cars), or from a "prime mover" which generates electricity (in the case of diesel-electric locomotives, and "hybrid" road vehicles such as the Ballard fuel cell bus).

The most common type of "direct drive" system is the internal combustion engine plus manual or automatic transmission. Internal combustion engines may be designed to use many different fuels, including gasoline, diesel fuel, propane, and natural gas.

These different propulsion modes very roughly compare as follows:

(Sources: [3],[4])

The ranges above are intended to include most vehicles of that type; there will of course be differences due to different weights, performances, etc. (Note in particular that electric cars are likely significantly less efficient than vehicles supplied directly by electricity due to the very large mass of batteries required). The figure for fuel cell efficiency refers to the current performance of the Ballard fuel cell bus.

The energy "lost" by each propulsion mode is generally heat which is radiated away from the vehicle. For electric motors, energy is lost as heat in the power conversion electronics and the electric motors. Internal combustion engines generate plenty of heat which must be dissipated in order to prevent the engine from getting too hot. Heat is also generated in the transmission. Vehicles that use rubber tires generate heat due to higher friction (25 to 100 times higher than the friction between steel rolling on steel [5]) with the road and within the tire itself as it deforms.

All vehicles also radiate heat from braking. A substantial amount of energy is lost every time a vehicle comes to a stop. Note, however, that electric motors can be used as generators to convert the energy of motion back into electricity which may be

• dissipated as heat through resistors (termed "dynamic braking")
• fed back into the distribution system to be used by other vehicles or used to charge batteries (termed "regenerative braking")

Dynamic braking does not increase efficiency but does double the life of brake pads. Regenerative braking, however, increases energy efficiency noticeably, since approximately a third of the energy normally lost to braking can be recovered.

Clearly, then, electric motors are the hands down winners in terms of energy efficiency once the energy source is at the vehicle.

Efficiency of energy delivery to the vehicle

An analysis of the energy consumed in delivering an energy source to a vehicle is quite complicated and depends on local circumstances. Here again, the issue may be broken down into two components: the energy required to construct the infrastructure, and the energy required to operate it.

Electricity generation and distribution

In the case of British Columbia, the energy required to operate the electricity delivery system is quite low. Most power in the province is generated by hydroelectric stations ([6]) - the natural gas fired Burrard Thermal station supplied 7.5% of BC Hydro production in 1994/95. (Burrard Thermal was intended as a backup station only. The construction of a fifth generating unit at the Revelstoke dam will increase BC Hydro's generating capacity by 500 MW, which is more than half the current output of Burrard Thermal).

The standard estimate of energy lost in transmission from generating stations to users is approximately 10%. Hence the overall electricity distribution system is quite efficient (i.e. 90% efficient).

In locations which rely much more heavily on non-renewable energy sources for energy production (e.g. coal or natural gas fired generating stations), the picture is much less rosy. I do not have figures on the efficiency of such stations, however. Note that the efficiency of the station must also include the cost of obtaining and shipping the non-renewable resource (e.g. coal mining and coal trains).

The energy required to construct the infrastructure includes the energy used in constructing the dams, the long distance high voltage transmission facilities, and local substations. Note that most of this infrastructure is required anyway for the electricity used by homes and businesses which are the reason for urban transportation demands in the first place.

Non-renewable resource extraction and distribution

The energy expended to discover non-renewable resources, extract them, process them, and distribute them, is undeniably large. In the case of locations which import the vast majority of their liquid fuels, such as British Columbia, it is especially easy to see that there are large costs involved, since oil must often be shipped from half way across the world. Large amounts of energy are expended daily searching for oil, operating oil wells and oil rigs, operating refineries, and shipping products hither and yon.

The energy expended to construct the infrastructure includes the energy used in constructing a fleet of vehicles for oil exploration, the oil wells and oil rigs themselves, the vehicles (e.g. helicopters) that serve them, the ships and trucks and trains which transport oil, the refineries which process them, the trucks which distribute finished product from the refineries, and the refuelling stations themselves.

Note that I have merely enumerated the energy costs and have not attempted to assign any numbers, since doing so is probably sufficient work to justify a graduate thesis!

Generation of hydrogen

There is no naturally occuring source of free hydrogen, and the production of hydrogen from natural gas or water (by electrolysis) is certainly not 100% efficient. In fact, electrolysis is approximately 60% efficient.

Energy cost of the vehicle and associated systems

It has been estimated that as much as 20% of all the energy consumed throughout the life of a car is that consumed during its manufacture ([1]). Thus, the energy cost of the vehicle itself (on the order of 100 GJ, or the energy equivalent of approximately 3000L of gasoline) is an important factor.

Here again, mass transit vehicles win hands down, simply due to the fact that many fewer of them are needed to carry the same number of people, and they are used much more intensively (i.e. they are not driven from A to B, parked for 8 hours, then driven from B to A). Certainly there is some inefficiency in the operation of mass transit with fixed routes, but this pales in comparison with the order of magnitude decrease in number of vehicles required due to their more intensive use. For example, there are fewer than 1200 vehicles (bus, SkyTrain, SeaBus) in the Vancouver Regional Transit System, which carries 105 million people per year - that's an average of about 240 people per day per vehicle.

Turning to transit vehicles themselves now, there is a difference in the energy required to operate vehicles with internal combustion engines versus the energy cost of operating vehicles with electric motors. The former require rebuilding approximately every 5 years, whereas the latter, especially in the case of the now-popular AC motor, essentially do not wear out.

There is also a difference in the energy cost of road vehicles versus rail. It seems likely that there is a larger energy investment in constructing a rail line than a road, however this is likely more than offset by the increased life of rail vehicles as compared to road vehicles. This increased lifespan is mostly due to the relative lack of vibration aboard rail vehicles as compared to road, especially when comparing road vehicles with internal combustion engines versus rail vehicles with electric motors.

Comparing transit lines with electric infrastructure to those without, it is clear that there is a significant energy cost in the construction of electrical substations, poles and overhead wire or third rail. This initial investment is mitigated by increased operating efficiencies.

Conclusions

In terms of energy use in operation, mass transit vehicles supplied directly with electricity are the clear winners. This is especially true in British Columbia, since very little energy is expended in operating the electricity generation system (just that required to maintain the stations and transmission lines), and typically only 10% of energy is lost in transmission.

The heralded Ballard bus, which cynicism suggests BC Transit would like to use to replace the trolley network, is on the order of 30% efficient (46% times 60%) as compared to more than 70% for trolley buses (90% times 80%).

Regardless of mode, however, it is clear that mass transit is a big winner in terms of improving energy efficiency. It is the author's opinion that efforts should be directed at ending automobile subsidies, constructing more pedestrian and cyclist friendly urban environments, and improving mass transit service as the most important steps towards decreasing energy use.