The culture around science is obsessed with novelty. This is understandable, but in my view regrettable. And nowhere is this clearer than in that most unfashionable of scientific fields: thermodynamics. The name combines thermo – to do with temperature and heat – and dynamics – to do with forces. So very roughly, thermodynamics is the study of what causes heat to move.
Thermodynamics emerged in the early to mid-nineteenth century as scientists around Europe struggled to understand the limits of steam engines. In the latter part of the 18th Century, just getting any amount of mechanical work ‘for free’ was a benefit, but as the miracle of steam became more workaday, engineers struggled to optimise the devices and get the most work output for the least coal input. It must have frustrated engine-owners that they could see hot water and steam leaving their machines – so clearly some energy was not being utilised!
After several years a solution emerged, generally credited to the french engineer Nicolas Léonard Sadi Carnot – even though his work was essentially ignored druing his short life. His conclusion was that despite all the astonishing complexity of heat engines – the maximum possible efficiency was determined by one simple formulae that depended only on the relative temperatures of the hot and cold parts of the engine. And absolutely nothing else. No clever device, new fuel, or intricate design could overcome this limit.
I want to write lots of things for completeness, but since I have to go bed shortly, I will restrict myself to just one aspect of his description of how heat engines work: they work like a water mills.
The first result of Sadi Carnot’s analysis is that ‘waste’ heat is not ‘wasted’. In fact it is essential to the operation of the device.
- In a watermill – it is the flow of water which generates mechanical power. Reducing the amount of”waste’ water by preventing water leaving the mill causes the ‘water level’ at the output to rise and eventually no water would flow.
- In a heat engine – it is the flow of heat which generates mechanical power. Reducing the amount of”waste’ heat by preventing heat leaving the engine cause the ‘heat level’ – or temperature – at the output to rise and eventually no heat would flow
The second result of his analysis concerns the maximum possible efficiency.
- In a water mill – the maximum amount of mechanical work per litre of water flow that may be extracted is determined just by the difference in ‘water levels’ between the input and the output i.e. the height difference between the input and the output.
- In a heat engine – the maximum amount of mechanical work per joule of heat flow that may be extracted is determined just by the difference in ‘heat levels’ between the input and the output i.e. the temperature difference between the hot part and the cold part.
This is astonishing. It says that no matter how clever a device one builds: whether it is a steam engine, a gas turbine or a nano-engineered thermoelectric generator, the maximum achievable efficiency doesn’t depend on any details of the device. For sure it it is possible to have one design that is better than another, but nothing can beat Sadi Carnot’s limit
I love results like this: simple insights that transcend the particular details of their discovery. And the law that Sadi Carnot discovered? It is called the Second law of Thermodynamics.