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An obscure question highlights why the electric railway is the future
In a PS to an e-mail, a high-ranking chum asked ‘how much of the latent heat in a trainload of coal is used by the electric locomotive hauling it to the power station?'. This is, of course, the sort of displacement activity that tempts you to stop making sense of some obscure regulatory document and lose an afternoon in research.
But, with Informed Sources to write I resisted temptation, pausing only to pass the inquiry on to that maestro of basic physics in engineering, Prof Roger Kemp of Lancaster University , for a second opinion. But come Sunday evening I pulled my old college text book (Heat engines by A C Walshaw) and a set of conversion tables from the office bookshelves, sat down on the sofa with an envelope and a pen and got to work.
According to Walshaw the calorific value of coal back in 1957 was 13500BThU/lb. So it was a simple process to go via BThu/kg and kWh/kg to MWh/tonne.
And when the back of the envelope gave an answer of around ‘3' I knew we were onto a winner. A 4000hp electric locomotive has a rating of 3MW, so a tonne of coal would sustain the maximum rating for an hour.
Of course there are losses between the power station and the wheels. Around 1.5% of the electricity generated nationally is lost in the national grid. Interestingly, according to the Railway Forum, electrified railways use 1-2% of the electric power generated.
And converting the current at the pantograph to tractive effort at the wheel tread also incurs some loss. Typically an electric locomotive is 90% efficient.
So, to be precise, the answer to the question is 2.78MWh/tonne. But, for debate, 3MWh/tonne is both near enough and easily remembered.
Roger came in from the top down, starting with a briefing note by BP which said that One million tonnes of oil produces about 4500 gigawatt-hours of electricity in a modern power station. The same source also equated 1 tonne of oil to 1.5tonnes of hard coal in energy content. Combining the two, and watching all those zeros, also gives 3MWh/tonne at the power station.
Now we have to ask, what sort of electric locomotive is hauling what sort of coal train? For the loco I propose a modern version of British Rail' projected Class 88.
Given the need to maximum use of available path, and in line with Keith Heller's Big Freight Railway), I reckon we want a 6MW Co-Co – 1MW per axle being a comfortable rating. Given that two Class 66s – say 4.5MW, have been hauling coal trains formed with 42 102 tonne HTA coal wagons from Scotland to Yorkshire , the Class 88 would certainly cut transit times.
Allowing for accelerating and braking (with the prospect of serious regeneration) plus rising and falling gradients, the average requirement would be half rated power. So, with 42 HTAs on the back the Class 88 could bowl along the East Coast Main line at 60mile/h using up just 0.03% of the trailing load an hour.
Of course, this is all fantasy railway stuff. Network Rail is incentivised not to electrify; you'd need a fair bit of electrification to avoid traction changes and no doubt the ROSCOs would quibble about residual life. But with coal coming back and electrification on the agenda. I crave reader's indulgence for old railway thinking that could return.
Just for fun, I have also worked out how much coal a real coal burning locomotive would consume, using the Class 9F 2-10-0 freight locomotive as the comparator. According to test results in Locomotive Panorama Volume 2 by E S Cox, the 9F's maximum indicated horsepower was the equivalent of 1.54MW. To achieve this it burnt coal at the rate of 2.11 tonnes/hr giving an efficiency of 8.3%. And it weighed 144 tonnes.