Heating Things Up With Stirling Engines

A Stirling Engine

Rushing waters and strong winds turn turbines and photons from the Sun to knock electrons free from their atoms. Hydro, wind, and solar energy are among the most common forms of sustainable energy derived from our natural world. As the need for sustainability grows, so do the innovations. Newer ways to create energy without the need to burn fossil fuels, such as perovskite solar cells, are being continuously engineered and new innovations like black silicon photodetectors are just over the horizon. With all of these awesome innovations and developments making headlines in science journals, it’s easy to lose sight of some of our older technologies that are (in my opinion at least) just as cool. A prime example of this is the Stirling Engine.

No steam, no Sun, and no fancy chemistry is needed to get these things up and running. Stirling Engines basically use nothing but heat as fuel to create mechanical work. More specifically, they take advantage of great temperature differences. Whether it be from the Sun itself or from a hot cup of java, the heat is what’s key here. Before I get into the nitty-gritty of a Stirling Engine, some history.

The Story Behind the Stirling Engine

Dr. Robert Stirling

As factories erupted all over England and locomotives filled the air with thick black smoke, the Industrial Revolution really started to pick up steam in the 1800s. The main staple that came out of this epoch was none other than the steam engine. Spiffied up by James Watt in 1775, the steam engine was the driving force behind the Industrial Revolution, but it wasn’t without its caveats. High-pressure steam boiler explosions were happening all over the place since they weren’t made with the correct materials or even built right to begin with. Because of all the injuries and damages this was causing, Scottish visionary Robert Stirling decided it was due time for some inventing. He decided to takeit upon himself to invent a power cycle that didn’t need a boiler. In 1816, he succeeded in doing so with the birth of the Stirling Engine.

Ok that’s great and all, but what exactly are they and how do they work?

Well, I’m glad you asked.

A Stirling Engine is an external combustion heat engine. What this means is that the engine’s internal working fluid is heated by an external source outside the engine itself. This is different from internal heat engines that, as the name suggests, are engines that burn fuel within the engine itself. Stirling Engines work off of the heat differentials between the hot and cool chambers located within the engine. They then make use of a cycle of constant expansion and compression of a gas to power a set of pistons in a closed system. Before I get into any more of the specifics of how they work, it’s important to know what these things are made up of in the first place. The main components of a simple Stirling Engine are the:

  • hot and cool reservoirs
  • heat source
  • engine cylinder
  • working fluid
  • displacer and power pistons
  • flywheel

The three main concepts of a heat engine are:

  1. some form of heat is supplied to the engine from one of the reservoirs
  2. the heat is used to perform work
  3. the heat not used to create work is sent to the cold reservoir.

But how does this all look like in action?

A fixed volume of gas goes through this process as it is pushed back and forth throughout the engine, being heated, expanded, cooled and compressed along the way. The hot reservoir is in contact with the heat source and the cold reservoir is in contact with a cooling mechanism. This could be cooling fins and even the surrounding atmosphere. As the gas is in a closed cylinder, once the working fluid is heated, it expands. This pushes the power piston upwards and turns a crank system.

Once the heated working fluid reaches the cold reservoir at the other end of the engine, its temperature drops. The aforementioned crank system is attached to two things, the flywheel and the displacer. The flywheel is what creates mechanical work through its rotation. Without a means to create mechanical work, you wouldn’t be able to do anything with an engine in the first place. As the crank turns, it pulls the displacer piston from the hot to the cold reservoir forcing the now cool air back down. Now, the working fluid is back where it began at the hot reservoir, ready to be reheated.

In short, the Stirling Engine functions off of the temperature differential between its two reservoirs. This creates a cycle of heating, expanding, cooling and compressing a fixed volume of gas and converting it into mechanical work. In other words, the Stirling Cycle. (video explanation)

Like I had mentioned at the beginning of this article, heat engines like the Stirling only need a heat differential, giving them a wide range of potential energy sources. They are also much quieter than your average internal combustion engine.

If Stirling Engines are all that great, why haven’t I heard of them before?

Well, there are actually some pretty good reasons as to why these things aren’t commonplace in our lives. Firstly, there are problems with the size and cost of these engines. As the system needs a consistent temperature differential present at all times to work, the engine’s materials need to be able to take all that heat in the first place. Power output is also a problem. Unlike the more popular internal combustion engines, Stirling Engines are relatively quite large in relation to their power density.

You might also wonder if we’ve tried putting these engines in cars? Stirling Engines in cars have been experimented with since the ’70s. In 1979, NASA along with AM General had done just that and had gotten varying levels of success. Stirling Engines just aren’t at home when it comes to things such as cars, that have constantly changing velocities. This is because it takes them time to warm up to get the engine going in the first place, making them ideal only for constant speed engines instead. Even though there aren’t any cars whizzing about with a Stirling under the hood, Stirling Engines aren’t out of the picture quite yet.

Stirling powered car designed by NASA and AM General

Where do Stirling Engines stand now?

Despite their shortcomings, they still have some practical applications even today. For example, Stirlings are used to power submarines, as they can be built to be practically silent. Using the cold depths of the ocean for the temperature differential, they can extend the operational time of subs when used in tandem with the submarine’s batteries. Along with this, they can even be modified and turned into cooling devices such as fridges and AC units. They would still pale in comparison to machines solely dedicated to cooling in terms of performance and price.

Stirling Engine used in submarines designed by Swedish company SAAB

They can even be used in power plants where excess heat is created such as nuclear power plants. Stirling Engines would be able to take some of the heat produced by these plants and convert them into useful work so that energy isn’t needlessly wasted. They can also shine as bright as the Sun in solar farms. In conjunction with the arrays of the solar panels, heat engines can be set up alongside them, creating power with help from the heat of the Sun.

Even though Stirling Engines are a technology all the way from 1816, there are still countless potential applications for them. The general ignorance surrounding them has proven to be a problem for the future of the Stirlings. If more and more people were to learn about these, I’m sure we’d see them everywhere, especially since the need for clean energy is constantly on the rise.

Key Takeaways:

  1. The Stirling Engine was made in hopes of it becoming a safer alternative to the Steam Engine during the Industrial Revolution
  2. Although they’re more efficient than internal combustion engines, in theory, they still fall short when it comes to cost and power density
  3. Despite their limitations, Stirling Engines still have tons of potential, especially with some more innovation and awareness.

16-year-old learning about the nuanced field of haptics as well as the future of emerging technologies. @tks.world