Heat Engines Past and Future
Did we ever really leave the age of steam?
Perennial science fiction author Arthur C Clarke formulated what’s come to be known as Clarke’s law (sometimes Clarke’s third law):
Any sufficiently advanced technology is indistinguishable from magic
This was meant as a rule of thumb for writers. He was telling his more pedantic colleagues that even if they couldn’t figure out how some fictional technology might work, the people of the future would. Just get along with the storytelling and stop worrying about whether things are possible, he implicitly implored.
Authors of the golden age of sci-fi literature mostly got on with it. Giant starships plied the space lanes of galactic empires, buoyed by anti-gravity and protected by force shields. Future spacemen (yeah, men) communicated with interstellar radio and fought with hand-held ray guns.
This 1940’s era speculation implied harnessing a lot of energy. In some cases astronomical amounts of energy. For the purposes of fictions that’s fine — we can suspend our disbelief — but we can wonder how plausible it might be. Does Clarke’s law apply in reality?
The golden age authors imagined a future where technology accelerated the availability and use of power. I mean power in the very technical sense of energy generated or consumed over time, measured most commonly in watts.
For example, an adult person needs about 8 megajoules of energy per day (2000 kilocalories, or Calories in America). To get watts you divide the energy in joules by the time in seconds (86,400 seconds per day) and you get about 100 watts. Humans have similar power needs and heat output as a 100 watt (incandescent) light bulb. Productive human labor is a fraction of that, perhaps 30 watts.
The first technology that increased the power available for human civilization was fire. Fire allows the solar energy collected slowly by plants and stored as cellulose to be released quickly, and releasing energy fast is the definition of power. Fire can cook foods reducing the effort needed to digest it, provide heat reducing the food energy needed to keep warm, and clear bush and twigs turning impassable scrub into herbivore-rich grasslands with a fraction of the labor.
It’s hard to estimate how much fire led to more available power for human societies partly because the domestication of fire may predate our own species. The chimpanzee gut is quite different from ours, reflecting the need to digest raw plants. Our modern digestive system may be adapted to cooking.
Most societies that ever existed were powered by people: 30 watts per able-bodied adult. Tools can provide leverage which multiplies the forces involved in a task, but the available power can only be scaled by adding more people.
More energy can be applied to a task by working on it a long time. For example, the energy needed to lift all the stone blocks in the Pyramid of Khufu into place can be estimated at 2 terajoules. If that’s spread over 20 years, then it only requires 3000 continuous watts — about 100 workers. If we guess the lifting is only 1% of the work, then the project needs 10,000 workers. Add one support person for each individual doing hard labor and we get very close to the archeological estimate of 20 to 30 thousand.
The fundamental technology of civilization is cooperation and forward planning.
A few ancient technologies increased the power available to human societies. Draft animals provide an order of magnitude, running at 300 watts so man with a horse could do the work of 10 men. A large waterwheel is another order of magnitude, generating around 3000 watts. Sails can capture kilowatts of wind power, but are really only useful for boats.
The real breakthrough came only a few hundred years ago with the steam engine. Even a small steam engine produces hundreds of kilowatts, and a large steam engine generates several megawatts of power. That’s a thousand times the power of a waterwheel.
The golden age sci-fi authors extrapolated more such breakthroughs, so that future people might harness gigawatts in the palms of their hands. But that didn’t happen.
Turns out the steam engine and it’s various descendants, like the internal combustion engine and steam turbine — known collectively as heat engines — have limits. Materials science and engineering precision have improved enormously in the intervening centuries, but that only equates to a small increase in efficiency. Atomic power is no different — after all it’s still a heat engine, just with the heat coming from atomic fission instead of humanity’s oldest friend, fire.
A large 19th century locomotive could generate 5 megawatts, about half the power of a modern jet airliner. Today a bullet train, cruise liner or container ship operates at about 20 megawatts. Modern engines are somewhat more efficient, but get their higher output mostly from being larger.
The pinnacle of real space technology are heavy launch vehicles, the giant rockets that lift large satellites and other payloads into space. Like heat engines rocket motors also utilize the force of expanding gases, but instead of a piston the whole rocket moves. Rocket engines pressurize highly energetic liquid fuels and mix them with liquid oxygen, creating the fastest burning combustion possible. Only explosions are more energetic, and indeed rockets teeter on the verge of exploding.
The space shuttle produced 28 gigawatts. SpaceX’s falcon 9 and starship lie in the same range, producing 27 and 30 gigawatts respectively. That is the technological limit of generating power from fire. It can’t get hotter.
Could some other kind of future technology deliver more power than heat engines? It’s possible, but maybe not quite at the level of Clarkean magic. The ultimate limits are thermodynamic, but that’s a story for part II.
