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Didcot A Power Station

Technology, Travel — July 2012

Last week we went on a tour of Didcot A. Commissioned around 1970, this is a coal-fired power station capable of generating around 2GW, with four 500MW units. The tour lasted more than two hours and we got to see at least something of most of the parts of the station. (We also got to see an introductory video which was aimed more at school students and dragged on a bit. This did mention global warming, but spent more time talking about nPower's renewable energy projects than the pressing need to stop burning coal...)


Didcot A and B from the Wittenham Clumps

The guts of the station consists of four coupled systems.

The first involves the combustion of coal, augmented with up to 5% biomass, to produce heat. The coal is shipped from Canada and Siberia to the Severn, and thence via rail. We got to see a train moving very slowly through the unloading station, where the bottom of the carriages opens and dumps 70 tons of coal in a few seconds into an underground hopper/conveyor system that takes it to the storage area, while a man walked alongside manually locking the carriage bottoms closed again once they'd emptied.

The stockpiles were not so large at the moment, but still impressive. The coal from there is first milled down to a fine power and then injected into the boiler (which is started on propane and then run on oil up to 250MW, before being switched to coal). The station can burn up to 18,000 tons of coal a day.

The second is a closed system containing highly purified water, which is heated to steam in the boiler and then used to drive the turbines, in several stages (high, medium and low pressure). We didn't really see any of this, as we couldn't look inside the boilers or the turbine housings.

photoThe third is a cooling system using river water (up to 70 cubic metres a second) which includes the most prominent feature of Didcot A, the six huge cooling towers. These are actually completely empty, and just cool hot water from around 32 degrees to below 27 (the warmest the station is allowed to return it to the river); the white clouds coming from their tops are water vapour.

There's a weird perspective illusion with the cooling towers. They're about 100 metres high and seem huge seen from a distance - as in the top photo - where they loom over the entire Thames Valley. From the car park right next to them they seemed much smaller. And then at the end of the tour, after drizzle for two hours, the sun came out and we had a grand sunset behind them, in which they again seemed impressively large. (We weren't allowed to take photos at all inside the plant, unfortunately; the photo above is from the car park outside the fence.) The chimney on the boiler is the tallest part of the station, at 200 metres, but doesn't seem as impressive.

The fourth system is the actual electrical generation, which we saw something of in the rather noisy generator room. Small exciter generators are used to drive the central electromagnets in the rotors, which are driven by the turbines in hydrogen-cooled generators. The resulting electricity is transformed up to 400kV before being shunted to the National Grid operated substation just next door. (I need to go back and reread Wildi, but my copy is unfortunately in storage in Sydney.)

We got to see the control room, which looks like something out of an sf film. There's also a set of four Royals-Royce jet engines which can be started from batteries and provide black-start capability both to Didcot A and to the power "island" centred on it. (They can also be called on for peaking power when the prices are high enough.) And we got decent views of Didcot B next door. This is a more modern 1.5GW combined-cycle gas plant, which lacks large cooling towers because they couldn't get planning permission for them and instead uses 5MW driving fans in much smaller cooling towers.

Didcot A is due to be shut down within three years due to EU pollution regulation. Here it's not the carbon dioxide that is the problem (though the possibility of a higher price for carbon permits in the future must have weighed on the economics), but the sulphur dioxide, which nPower would have to remove from the emissions using limestone slurry if they they wanted to continue operating. That would only have made economic sense if the plant could have been expanded, apparently, but the railway system isn't capable of bringing in the additional coal that would require.

With a number of other coal-fired power stations due to close for the same reason, the United Kingdom needs to build nearly 8GW of new power generation just to replace them.


  1. This is as good an introduction to coal-fired power generation as any I can get. I thought coal is simply shoved into boilers to heat water, water turns into steam and in turn drives turbines, and condenser collects the residual steam to drive other turbines, and bingo electricity. There are technical terms I don't fully understand. Jet engines for black-start? Not sure how it works.

    I have never been to a thermal fire power station. Even there was an open day, the nearest coal-fired station is simply too far away. ( I visited a crematorium on an open day.) I did visit the Snowy Mts. hydroelectric plant back in 1966 or 67 on excursion. I think there was just whirring of the turbines in the housing, nothing much exciting about that. It was so long ago.

    It was on a late night fishing trip to Budgewoi that I gained a little knowledge of discharged cooling water from a power station. The fishing spot at Budgewoi is next to Lake Munmorah Power Station. The cooling water is discharged into the estuary. On that particular night over thirty years ago, I could feel the seawater was pleasantly warm. But the fishing was good, and there were many, many anglers. The warm water didn't do any harm to fishing activity. Like Didcot, Munmorah is also slated for closure.

    Why there is a need to ship coal from Canada and Siberia? I thought there is plenty of high grade coal to be mined in Wales. What about Newcastle? A Welsh workmate told me coal is still plentiful in Wales. His grandfather, a coal miner lived to 70, a ripe old age for a coal miner. But he was bedridden, and had tubes and tubes to help him breathe. There was no compensation. I asked him about the pit ponies. This workmate told me the ponies were taken above ground only once per year. What a dreadful lot for men and beasts down in the mine shafts.

    Comment by DL — July 2012
  2. There is coal in Wales, but it's too expensive to extract. There are too many people - and national parks - for large-scale open-cut mining to be practical. (We went down the Big Pit coal mine at Blaenavon, which was quite an experience; we heard there about the horses that spent almost their entire lives underground.)

    Comment by danny — July 2012
  3. 'Black Start' is an issue for all large power stations - as the amount of power required by the power station itself is considerable, many 10s of MW. If the grid is unavailable 'ie black', many power stations can't be started.
    Presumably the 'jet' turbines (read gas turbines) can generate enough electricity that they can power all the required auxiliary systems to get one of the main steam turbines on line. And remember a 'cold' turbine may take many hours or even days to bring on-line. (They have to wind up the speed slowly to allow the sag to be 'spun' out of the turbine shafts among other things), so that 'black start' power has to be sustained for possibly several days. It's not something you could do with batteries!

    I seem to recall from a tour of the Eraring power station on Lake Macquarie, NSW that each 660Mw turbine there used 60Mw of it's own capacity just to keep itself running. Eraring can't black start - it requires power from the grid to boot-strap.

    Comment by Matthew — July 2012
  4. The pumped storage system at Dinorwig, which we visited last year, is also a black-start system - I guess hydro is easier because you don't need power to warm up a boiler, just to operate the sluice gates and maybe power stator electromagnets. The 50MW hydro station at the bottom of Warragamba Dam only runs when the dam is full, but it is capable of black-starting the NSW grid. (Vic or Hansen could probably tell us what the power from that would be used to start next.)

    Comment by danny — July 2012
  5. I am still learning, for example, I had no idea that almost 10% 0f the power generated is consumed on site. That is a big chunk. Still, it is better than the incandescent light or internal combustion which powered the family car.

    Comment by DL — July 2012
  6. Matthew, the black-start system is actually made up of jet engines, burning aviation fuel, and not gas turbines, at least according to this source.

    "To cover black start conditions the station has four 27 MW Rolls Royce Avons. These can be at full load in less than 5 min ( just over 2 minutes is possible). They can also be used for peak lopping. Fuel is kerosene."

    Comment by danny — August 2012
  7. On aviation kerosene : I spent a lot of time in Hong Kong in the 1970s. One of my favourite past time was plane-spotting at a park looking directly on to the old Kai Tak Airport runway. Depending on the wind direction, I could smell the exhaust from the jets. The smell was quite unlike to kerosene used in household stoves. Is there much difference to the two grades of kero?

    Comment by DL — August 2012
  8. In Australia, where the National Electricity Market operates (that is QLD, NSW, ACT, Vic, Tas, and SA), System Restart Ancillary Services (SRAS or black start capability) is procured by competitive tender. The costs are apportioned on a 50/50 basis between market generators and market consumers. SRAS providers must by capable of starting or operating indefinitely without a grid connection and supplying at least 100MW to the grid within 60 minutes of a request from AEMO. At 50MW, the hydro generator at Warragamba Dam does not meet the requirements for SRAS currently (the operator, Eraring Energy says it currently isn't even connected to the grid).

    BTW, I would have said jet engines are gas turbines, no matter the fuel they run on. In static power use, they are typically referred to as gas turbines and it's quite common for them to be able to accept both liquid and gas fuels, depending on what's cheaper and/or available.

    Comment by AndrewM — July 2013
  9. Aha, they must have upgraded the requirements since I was told about Warragamba - that was probably thirty years ago. So how do modern SRAS work?

    Comment by danny — July 2013
  10. The last cold start of the NSW grid occurred on the 10th of June 1964 using the now-decommissioned 330MW Wangi Wangi coal-fired power station, which was the last of the coal-fired power stations that was small enough that its auxiliary loads could be met by diesel generators. The station was kept mothballed for many years "just in case", before finally being stripped in the 1990s. After that, the Snowy Scheme was typically considered the primary system restart option. I'm not convinced the Warragamba generator could ever have been used to cold-start - it came too late, is too small, and wasn't operated by the Electricity Commission.

    SRAS works by diving the national grid (except NT and WA) up into seven islands and taking offers from the generating companies to provide a certain level of cold-startable generating capacity within those islands. Winning tenderers have to demonstrate their capability once a year, with a long list of tests performed and overseen by the AEMO.

    As for how it's done, at least in NSW: there are several pumped storage systems around, in particular: Snowy (maybe 3.5GW), and Kangaroo Valley (about 240MW) - and there have also been a lot of stand-alone gas turbine plants built in recent years, some quite large: Colongra 660MW, Tallawarra 430MW, Uranquinty 640MW. Most of the gas turbine plants run intermittently starting when the market price gets high enough and helping to meet peak demand. Being able to claim SRAS is money for nothing for those guys. Also, some of the coal-fired power stations have gas turbine units that are only used to meet their own auxiliary loads to allow them to do a cold restart of their main units.

    The SRAS price has doubled or tripled over the last decade - in 2012-13, the AEMO spent $55 million on SRAS across the market. There has only been one significant cold start within a region of the NEM in the last decade, and that was performed by importing power from neighbouring regions, rather than calling on SRAS.

    Comment by AndrewM — July 2013

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