Facts Sheet 5b. Carbon emissions: High speed rail and air

Caveat

The abstract of a paper by Mikhail V Chester and Arpad Horvath illustrates how vital it is to have dust to dust estimates of energy consumptions and emissions.  There we see the estimates of life cycle energy inputs and emissions in the USA add 63% to the tailpipe values for road vehicles, 31% for air and 155% for rail. 

Instead of that nearly all UK emission studies deal with tail pipe emissions alone. Consequently the conclusions of all those studies may turn out to be unsustainable along with policy depending upon them.

Preamble

Those promoting high-speed rail make much of the supposed carbon emission saving that may result if internal air passengers transferred to high-speed rail.  However, the proportion of the nation’s emissions that can be attributed to air or rail is vanishingly small, see below.  Hence, any transfer of passengers from one mode to the other can only have an even more vanishingly small effect.

For that reason alone the emissions argument made by those lobbying for high-speed rail should probably be ignored.

 (This figure is from the White Paper, Delivering a Sustainable Railway, July 2007)

Summary results

The carbon emissions for rail depend on whether the average emissions from the UK generating industry are used, or those relating to coal-fired generation.  The reason for the latter is that large scale electrification may extend the life of coal-fired generation.  In this note values for both assumptions are provided, coal-fired generation emitting twice as much as the industry average.

The emissions for high speed rail are based on a 40% load factor.  The tailpipe values in grams per 100 passenger-km have the range of 4150 to 8300 for a train at 200 kph.  In contrast Ryan Air may return 8475 gms per 100 passenger-km. However, if these values are inflated to provide life cycle values, as suggested in the caveat, the range for rail is 10,600 gms to 21,200 gms and the value for Ryan Air is 11,100 gms.

Against that background it seems premature to claim that high-speed rail will have substantially lower emissions than air travel when precisely the reverse may be the case.

Data sources and detailed results

Except where stated the data which follows relates to tailpipe fuel consumptions and emissions.

Appendix 1 sourced from the BERR, provides (A) circa 150 Tonnes of carbon per GWh (550 tonnes of carbon dioxide) if we assume average emissions or (B) circa 300 Tonnes per GWh (1,100 tonnes of carbon dioxide) if emissions are related to coal-fired generation. In this note we present emission on both bases. However, the coal-fired emission rates may be the more appropriate because, as pointed out by the RSSB in its report on Traction Energy Metrics, July 2007, any large scale increase in electricity supply would prolong the life of coal-fired generation.

The emissions per passenger-km depend on occupancy assumptions.  The RSSB report Traction Energy Metrics, in section 6, assigns a  40% occupancy to intercity rail.  We have used that here although it is said to lead to overcrowding on some trains.  For comparison the average for all trains is 30%. If for service level reasons that should be the target for intercity services then the emissions per passenger-km in Table 1 should be increased by one third.

The emissions per passenger-km for the aircraft assume an occupancy of 80%.  That is above the industry average but below the 84% claimed by Ryanair.  Further, emissions by air cited in table 2 are 10% above those in the source table 3 to allow for refinery energy use and the transport of the fuel to the users.

The fuel consumptions for rail in the following table are from Professor Roger Kemp.  They have been converted to carbon emissions using the emission data cited above. 

Professor Kemp also cited a diesel HST requiring 0.8 litres per 100 seat-km at 200 kph. The specific gravity of diesel is 0.84 and the CO2 per Kg is 3,150 gms. Hence the carbon dioxide per 100 seat km is 0.8 x 0.84 x 3,150 = 2,117 gms or 5,290 gms per 100 passenger-km at 40% load factor.

Data for air is summarised in table 2.  As noted above the emissions have been increased by 10% above those in the source table 3 to allow for the energy used in refineries and in transporting the fuel to the users. The data (with the exception of Ryanair) overstates the emissions for comparison with rail because of the relatively large amounts of luggage and freight carried by passenger aircraft but not (presumably) by high-speed rail.

 Comment

The data in tables 1 and 2 could be used to claim that, if it is emission (B), coal-fired generation, that is relevant and if Ryanair is taken as the model aircraft, then air produces the lower emissions. On the other hand if it is emission (A), the generating industry average, that is relevant, then high-speed rail may claim the lower emission.

Radiative forcing

The emissions from aircraft are often weighted by a factor in the range 1.5 to 3 to allow for the greater radiative forcing arising from high altitude emissions.  However, short haul aircraft spend little time at the altitude where the higher radiative forcing factors apply. Further ground level emissions are said to carry a radiative forcing factor that is usually omitted. Hence when comparing the warming effects of rail compared with air the differences in radiative forcing can probably be ignored.

Sources and notes reference aircraft

The Institute of Energy (0207 467 7100) and others provided the following for aviation fuel:

Calorific value: gross = 46.2 giga-joules per Tonne, Net 46.2 x 0.935 = 43.2

Carbon dioxide:  3.15 Kg of CO2 per Kg of ATF (Jet 1)

Specific gravity range 0.775 to 0.84 providing an average here set to 0.8.

That together with the source data in Table 3 below enables the emission data therein to be calculated. E.g. Ref. the CRJ-700 first item: CO2 per 100 seat-km = (545 x 3.785 x 0.8 x 3,150 x 100)/(334 x 1.6 x 70) = 13,896 gms.

The Aircraft Monitor data in table 3 was recommended to us by Dr Peter Morrell of the Cranfield Institute of Technology. Dr Morrell has also been kind enough to confirm that the parameters and calculations used in this table are in close agreement with his estimate for the B747-200 (the last aircraft cited in table 3).

Fuel consumptions in this table should be increased by 10% to allow for refinery energy use and fuel used in transporting the fuel to the users

Appendix 1 Emissions from the UK generating industry

Table A1 below and the associated notes provide:

1. Ratios of primary burn to end user consumption of (a) 1024.8/345.2 = 2.97 in 2005 and (b) 1022.9/344.8 = 2.96 in 2006; leading us to use 3.0 in other calculations.
2. A fair estimate of the corresponding carbon emission from industry as a whole is 150 Tonnes per GWh delivered to end users or 50 Tonnes per GWh of primary energy burn.  Those numbers correspond to 550 and 183 Tonnes of carbon dioxide.  If it is coal-fired emissions that are relevant then these emissions should be doubled.

TABLE A1 Electricity generating industry: Energy burnt and supplied and emissions

Notes:

1. These are Mega Tonnes of Carbon per Mega Tonne of Oil Equivalent as provided by Julian Prime of BERR on 5th October 2007.
2. Table 5.1 of Energy Trends March 2007.  The Mt of oe burnt is the oil equivalent burnt by the generators.  The TWh supplied is that reaching the grid.
3. Table 5.2 of Energy Trends March 2007.   The TWh consumed is the electricity reaching users after deducting transmission losses and the generating industry use.
4. The calorific value of an Oil Equivalent is 42 GJ per tonne.  Hence TWh here are the oil equivalent values multiplied by 42 and divided by 3.6.
5. Typical calculation 293 = (33.56 billion)/(128.67 million x 345.24/387.3).
6. The BERR and others’ estimates of emissions treat the generating industry itself and sometimes the grid as end users.  That leads to lower emissions per GWh delivered than we have calculated.

Comparison with Energy Trends

Energy Trends, March 2007, provides 131 Tonnes of Carbon per GWh generated i.e. before generating industry use or transmission losses.  The electricity generated was 408,500 GWh.  The end user consumption amounted to 344.85 GWh.  Hence multiplying the carbon emission per GWh generated by the ratio 408.5/344.85 provides the carbon per GWh used by end users.  That yields 155 Tonnes which is close to the 157 Tonnes in Table A1.

Transport-watch

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