103 Vehicles Pollution a Quantitative Analysis

Energy for electric vehicles clearly has to come from somewhere. If battery electric vehicles are widely introduced the vast majority will have to be charged from the mains grid, where at present most electricity production comes from burning fossil fuels. At present sustainable sources of energy currently provide less than 10% of the energy used in the grid, so most of the electricity used for charging electric vehicles would be obtained from burning fossil fuels, including coal, gas and oil, at the power stations.

Conversion efficiency (energy at power station outlet/calorific value of fuel) for producing electricity from fossil fuels at modern power stations is typically about 45%, much higher than motor-car engines. However, this has to be transmitted to the consumer and the average transmission efficiency, including transmission through the low voltage local networks, is around 90%. This means that the actual efficiency of converting the chemical energy of fuel at the power station to energy at consumers' electrical socket outlets is typically about 41%. This then has to be converted to energy delivered via the vehicle wheels.

For an efficient electric vehicle the efficiency of converting electrical energy supplied to the vehicle on charging to energy at the wheels will be around 50-60%. This means that the overall efficiency of converting fuel at the power station to wheel energy for electric vehicles is around 20%. The overall efficiency of internal combustion engine vehicles (energy delivered via the wheels/fuel energy) under normal driving is typically 12-18%, a very similar figure to that of electric vehicles. The result is not really surprising; basically one combustion engine, a vehicle bound diesel or petrol, with a transmission system, is being replaced with another located at the power station.

However, as discussed earlier, fuel efficiency of internal combustion engine vehicles at low speeds in heavy traffic gets considerably worse; see Figure 10.1. Under these conditions electric vehicles undoubtedly produce considerably less carbon dioxide, which in itself makes a strong environmental case for their use, particularly in towns and cities. Also, it should be borne in mind that harmful emissions from burning fuel at a power station can and should be very carefully monitored and controlled and where possible eradicated.

This study of the pollution caused by the use of a vehicle is usually called a 'well-to-wheel' analysis. In this context a coal mine, for example, is also a 'well'. There are several stages in the process, and these will depend greatly on the vehicle. In any case, every stage of the process will involve pollution and/or the consumption of energy. See Figure 10.2.

Several different fuel cycle analyses or well-to-wheel studies have been published, including ETSU (1996) and Hart and Bauen (1998). There is also a good summary of

Well-to-wheel

(a) Fuelled vehicles

Well-to-wheel

(a) Fuelled vehicles

Well-to-battery--Battery-to-wheel—

Well-to-wheel

(b) Battery powered vehicles

Figure 10.2 Energy transfers in fuelled and battery powered vehicles used in the analysis of Section 10.3

this research by Bauen and Hart in Hoogers (2003). Some of the results of their analysis are shown in Table 10.1.

For this study the emissions of the gasoline and diesel IC engine cars were taken as those for the EURO III standard. The energy consumption, and hence carbon dioxide production, figures were taken from the average consumption of the all British cars in 1998. The average engine efficiency was taken as 15%.

The car fuelled on compressed natural gas (CNG) was assumed to have the same energy use as the gasoline vehicle, but with a 10% better efficiency, i.e. 16.5% efficient.

The hydrogen fuelled IC engine vehicle was assumed to be supplied with cryogenic liquid hydrogen, as outlined in Chapter 5. This is the method most commonly used among those companies, such as BMW, that have made the most progress with this technology. It is explained in Chapter 5 that the liquefaction of hydrogen is a very energy-intensive process, and this has a significant effect on the energy use results.

For the hydrogen fuelled fuel cell electric vehicle, the hydrogen was assumed to be generated using medium-scale steam reforming plant based at refilling stations, with natural

Table 10.1 Emissions and energy use figures for different types of car, being mainly a summary of data calculated by Bauen and Hart (2003)

Vehicle type

NOx,

SOx

CO

PM

CO2

Energy

g.km-1

g.km-1

g.km-1

g.km-1

g.km-1

MJ.km-

Gasoline ICE car

0.26

0.20

2.3

0.01

209

3.16

Diesel ICE car

0.57

0.13

0.65

0.05

154

2.36

CNG ICE car

0.10

0.01

0.05

<0.0001

158

2.74

Hydrogen ICE car

0.11

0.03

0.04

0.0001

220

4.44

Gasoline fuelled hybrid

0.182

0.14

1.61

0.007

146.3

2.212

MeOH fuel cell car

0.04

0.006

0.014

0.0015

130

2.63

Hydrogen fuel cell car

0.04

0.01

0.02

<0.0001

87.6

1.77

Battery car, British

0.54

0.74

0.09

0.05

104

1.98

electricity

Battery car, CCGT

0.17

0.06

0.08

0.0001

88.1

1.71

electricity electricity gas as the feedstock. The hydrogen would then be stored at high pressure (~300bar) onboard the fuel cell vehicle, as outlined in Chapter 5. The consensus view of studies in the USA, and summarised by Ogden (2002), is that this is the most economic and least environmentally damaging method of supplying hydrogen to vehicles.

The second fuel cell vehicle uses hydrogen derived from an onboard methanol reformer, as described in Chapter 5. The performance is assumed to be broadly similar to the units described there, and realistic figures from the methanol industry were used for the feedstock and fuel stages (see Figure 10.2).

The hybrid vehicle is rather difficult to include, as there is so much variation in the degree of hybridisation, as was mentioned in Chapter 1. The emissions per kg of petrol consumed are unlikely to be very different from those of ordinary IC engine vehicles; the main benefit is that less fuel will be used, and proportionately less pollution of all types produced. So, for the hybrid vehicle the figures have been set, quite simply, as 70% of those of an ordinary gasoline powered vehicle. For the next 10 years or so, this represents a reasonable figure for the saving that might be expected from such vehicles, though of course this is highly debatable.

It is impossible to find figures for a battery electric vehicle that are not contentious. Following the very thorough study in ETSU (1998), the energy figure of the electric vehicles1 is assumed to be 0.72MJ.km-1. The simulation described in Chapter 7 can be used to show that the energy taken from the battery by the GMEV1 electric car driving the SFUDS cycle, with heater and headlights on, is about 0.4MJ.km-1. So a fair allowance is being made for a rather more harsh driving cycle, which is quite justified. Inefficiencies in the generation, distribution, and charging process mean that the actual energy use is about two and a half times this figure. In their study, Bauen and Hart (2003) proposed two different sources, and these figures have been used. One was the average figure from the current mix of UK electricity generators. This comprises a combination of generator

1 The energy figure is the energy out of the battery, so due allowance is made for losses in the motor, controller, transmission, etc.

types that is probably not very different from that of many western countries. The second vehicle was assumed to be powered by electricity from state-of-the-art combined cycle gas turbine (CCGT) generators supplied with natural gas. Such a hypothetical system makes a good comparison with the hydrogen fuelled fuel cell vehicles, which also use a somewhat idealised supply system. The efficiency of CCGT systems is about 50%, whereas that for the current British system is about 43% on average. The charging process is assumed to be about 85% efficient, which is obtainable when using NiMH batteries.

Not included in this table are three types of vehicle that are considered later in the chapter:

• battery powered vehicles, with batteries charged from renewable electricity generators;

• fuel cell vehicles, powered by hydrogen made from biomass or water electrolysed by renewable electricity;

• IC engine vehicles powered by biofuels such as ethanol.

These have zero net carbon dioxide emission, and very low production of other pollutants, though the last is the worst in this regard. However, their overall energy use and efficiency cannot sensibly be estimated at the moment, as there is not enough experience with this technology to make sensible comparisons.

The figures of Table 10.1 are shown in the graphs of Figures 10.3, 10.4 and 10.5. To a certain extent these charts speak for themselves, but several interesting points can be made:

1. Figure 10.3 shows that all the electric vehicles are noticeably better than all the IC engine vehicles.

2. The source of the electrical power for the battery powered vehicles is very important. Figure 10.5 shows that a battery powered vehicle using the current British electricity generation mix is not significantly better as regard pollutants than other vehicle types.

3. The hydrogen fuelled fuel cell vehicle comes out very well from the study.

4. The hydrogen powered IC engine appears very poor from the point of view of energy use. This is because an inefficient fuel processing system is combined with an inefficient engine. However, the non-carbon dioxide pollutants are very low.

5. The hybrid vehicle also comes out very well from the study. If hybrid technology were combined with a diesel engine, then even better performance might be expected.

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