Thread by @Gergyl

Don't like this old calculation that I've quoted several times recently? Coal-fired power stations are thermodynamic heat engines; they do not get to break the laws of physics. The maximum achievable thermodynamic efficiency depends on the ratio of the absolute temperatures...

https://twitter.com/Gergyl/status/833505080464642048

…The ratio of the (Kelvin) temperature of the 'cold side' of the engine (~condensing steam in the condenser) to that of the 'hot side' (~the hottest steam in the boiler). Under idealised low-heat-flow conditions, the well-know Carnot formula applies:

ηₘₐₓ = 1 - Tc/Th

...But under more realistic high-flow conditions, it becomes the square root of the temperature ratio, which is what I applied:

ηₘₐₓ = 1 - √(Tc/Th)

That's an extremely simple representation, there are multiple confounding effects, but guess what the quoted numbers for the newest coal-fired power station attached to the national electricity grid give...

That's the 750 MW single-unit, supercritical-steam, dry-cooled (and highly unreliable) Kogan Creek power station near Chinchilla in SE Queensland. Quoted operating parameters are:
steam pressure 25 MPa
steam temperature 560 °C
efficiency 40%

It is direct-condensed dry cooled, so the cool-side temperature is ~that of (fan-forced) ambient air; lets say 25°C. Put those numbers into the simple thermodynamic formula and guess what (+273°C to convert to Kelvin):

ηₘₐₓ = 1 - √((25+273)/560+273)) == 40%

What a surprise. Remember this is a simplistic treatment -- there are confounding effects, but the point is the thing cannot depart far from what it says, because physics.

Now look again at what we had 5 years ago:

https://twitter.com/Gergyl/status/833505080464642048?s=20

Next time someone spruiks 'HELE' coal-fired power at you, ask them how much more efficient that will be that Kogan Creek, or Millmerran, or Callide C, or Tarong North. Answer: about 5% more (42% efficient vs 40%).

'High-efficiency low-emission' is a deliberate, semantic lie, made up of words that have been carefully chosen to obscure facts: 42% efficiency is not 'high', and a kilogram of CO₂ emitted for every kilowatt-hour of electricity is not 'low'. That's ultra-supercritical 'HELE'.

There are now (hugely expensive) designs with even higher steam temperatures and pressures -- up to as high as 700°C, for ~45% calculated efficiency. We call those 'advanced ultra-supercritical' ... just add another hyperboliser to the list.

The only qualifier there with any meaning is the 'super' in supercritical. But that doesn't mean super-good -- it means 'operating above the critical point of water' (below). The others are hype.

https://twitter.com/Gergyl/status/828017658251538432?s=20

Above the critical point, water no longer has separate liquid and gas (steam) phases -- it's all the one watery-steam fluid. Changes the boiler design, because no longer need to design to separate water and steam (in the great big steam drum of a conventional coal-fired boiler).

(Long ago, as a very junior eng., helped with a full check of the boiler house structural design for a certain major power station. Wanted to do the huge, 3m deep, steel plate girder that the steam drum hangs from. Not allowed.)