The steampunk device in the title image is a model 4-cylinder Stirling engine. Developed in 1816 by Robert Stirling, the Stirling engine was an early competitor to the steam engine.
The Stirling engine is almost a unicorn. It is a near perfect and reversible Carnot cycle engine. This means that, theoretically, a Stirling engine could convert 99% of the energy fed into it into useful work.
Practically, due to friction losses and other thermodynamic design and materials issues, this resolves to operational efficiencies of 40%-60%. In cylinder efficiencies have reached as high as 95% in prototypes and bench models, and when compared to the efficiency of an internal combustion engine (ICE) the superiority of the Stirling engine is overwhelming. ICE efficiencies (including diesel) generally max out around 35%.
So why didn’t Stirling engines beat steam engines in the 1800s? Why don’t we use them to power cars and lawnmowers today? What are the numerous benefits of Stirling engines?
Most importantly, how does this poorly understood and barely discussed engine from over two centuries ago hold the key to economic freedom for the planet?
A Stirling engine is a closed-cylinder external combustion engine. Unlike an internal combustion engine the Stirling engine consists of a sealed chamber containing pressurized working gas, usually air, nitrogen, or helium. Helium permits the most efficient operation while air is the cheapest and most common. It’s generally preferable that the gas be inert and non-combustible. The engine operates off of temperature difference. There is a hot end and a cool end. The expansion and contraction of the working gas drives one or more pistons which are connected to a driveshaft. The driveshaft rotates providing power for wheels, pumps, hydraulics, etc.
Here’s what makes the Stirling engine unique, and uniquely valuable, in today’s carbon conscious world: It will run off of ANY heat source.
This can be an electric heating coil, solar thermal fluid, waste heat from a wood stove, the cooling system of a diesel generating or an automobile, exhaust, hot water, an open flame, etc.
That alone makes it worth investigation, but when you add in the extremely high theoretical efficiency limit, the Stirling engine is the only engine that:
- Runs on any and all fuel sources.
- Permits complete combustion resulting in less pollution and greenhouse gas production.
- Can be run in reverse as a heat pump. The most common commercial use today is in cryocooling.
- Can be built with hand tools out of common materials like wood, aluminum, glass, and steel.
- Allows simple heat to drive a wide array of industrial processes with no additional power source.
So, if this engine is so great, why haven’t we been using them since 1816? There are two answers to this question.
Firstly, in a small way, we have been using them. Today, the most common modern use is in cryocooling, but you can also find them in solar power, and in the 1960’s Philips Corporation made several portable Stirling powered electrical power generators which still run today.
Secondly, while Stirling engines are theoretically the most efficient, actually doing the math to improve their efficiency was pretty much beyond us until the invention of the modern computer. Now, it’s comparatively easy, but in the early days efficiency improvements were largely trial and error. Fundamentally, steam boilers were much
simpler to build and improve.
But since 1986, at least, it would be fair to say that Stirling technology has been suppressed by extant market forces. Globally, all of our infrastructure and industry is built around internal combustion engines. So, when
NASA pioneered research into replacing internal combustion engines with highly efficient and super low emission Stirling engines that required substantially less maintenance, it’s fair to say that Big Auto wasn’t interested.
Stirling engines can, like many other engines, be built and serviced with hand tools. Compared to internal combustion engines and steam engines, Stirling engines are lightweight and safe and do not require a heavy cast metal engine block.
This makes them perfect for developing countries that lack adequate energy infrastructure. A Stirling engine can run off of sunlight during the day, and off of a full-combustion and low-emissions rocket stove at night. It can power a well pump for clean water, a tractor for agriculture, or even another Stirling engine, run in reverse, for air conditioning and food / medicine refrigeration.
At Free Country, our Human Rights Tool Kit initiative is working to develop open-source Stirling engine plans, prototypes, and a distribution network for related tools and technology to literally emPOWER the poorest 4 billion people on the planet. Donate or volunteer in this initiative today.
English