Regenerators
What They Are and How They Work
Regenerators are part of what makes Stirling engines interesting and efficient. Unfortunately, they can also be difficult to understand.
Regenerators are an internal heat exchanger that makes Stirling engines more efficient.
What Regenerators are and How They Work
A regenerator is a component in a Stirling engine that stores heat from one cycle so it can be used in the next cycle. Regenerators are often made of sheets of foil, steel wool, or a metallic sponge.
The hot working gas flows over the regenerator (storing some of its heat there) on it’s way to the cold zone. When the cold gas returns, it flows back over the regenerator and is pre-heated before it goes to the hot zone.
The result is higher power and more efficiency from a given Stirling engine.
Click on a Link to Jump to that Section:
- Regenerators Explained With a Clear Model Engine
- Purpose of a Regenerator
- Invisible Regenerators
- Internal Aerodynamic Drag
- Effects of Different Gases
- Variable Compression Ratio Engine
- History of the Regenerator
Regenerators in a Clear Model Engine
To understand how regenerators work in all Stirling engines, let’s first learn how they work in one particular engine.
Yellow foam regenerator / displacer in a model Stirling engine cylinder.
This is a good example of a regenerator because the working cylinder is clear and the regenerator/displacer is bright yellow.
It’s Not a Piston
The yellow part inside the engine looks a lot like a piston, but in fact, it’s not a piston.
Regenerators are Porous – Pistons Aren’t
The original model MM-1 Coffee Cup Stirling Engine by American Stirling.
A piston is defined as something that a gas pushes or pulls on.
But this yellow part is porous, made of a material very similar to air conditioning filter foam for window’s air conditioners.
In this engine, the air flows through and around the displacer.
When it does either of these things, either flowing through it or around it, some of the heat from that cycle is being stored in the yellow piece of foam and saved for the next cycle and the yellow foam is working as a regenerator.
You can see where how and where the regenerator is used in the completed engine above by clicking on the page for this coffee cup engine and then clicking on the picture once more to get the largest possible image.
A Moving Yellow Regenerator
The yellow part inside the engine is both the regenerator and the displacer. Air flows through it and around it.
In most Stirling engines, the regenerator is fixed and the gas moves. But in this engine, the regenerator and the gas both move.
This Engine Uses a Regenerative Displacer
This engine has what’s called a “regenerative displacer,” so the yellow part that moves the air back and forth inside the engine also functions as a regenerator.
Purpose of a Regenerator – Economy
To operate as an engine, a Stirling engine needs to absorb heat, expand the gas, reject waste heat, and then compress or contract the gas.
A regenerator works by storing some of the heat that would otherwise have to be rejected to the environment in the regenerator until the working gas flow reverses and the heat can be used in the next cycle.
The purpose of a regenerator is to make the engine more economical. In fact, Robert Stirling originally called it the “economizer” and today they are known as regenerators.
The result of the regenerator is that the engine puts out more power with less fuel consumed.
So, What Does it Do?
The regenerator in a Stirling engine works as an internal heat exchanger, located between the hot and cold parts of the engine. The working fluid flows over it in both directions, storing heat from one cycle to be used in the next cycle.
A regenerator is meant to recycle the heat within the engine, as opposed to wasting the heat to the atmosphere. This improves overall efficiency and power output.
With a regenerator, there is less wasted heat, because some heat is stored in the regenerator for use in the next cycle.
Invisible Regenerators
Anybody who’s looked at many different designs of Stirling engines will notice that some of them don’t have anything that is an obvious regenerator.
Are these engines still, in fact, Stirling engines?
Well, yes, they are Stirling engines because regenerators don’t have to be a separate named part to work as regenerators.
Even when an engine does not have a separate regenerator, the edges of the displacer will have a fair amount of heat flow around them and they will function as a regenerator.
Anytime there’s a long, narrow passage the working fluid must flow down, that passage will function as a regenerator.
An Almost Free Lunch
On the positive side, a regenerator is a component that can improve the operation of a Stirling engine – sometimes by a lot.
On the negative side, they can also increase the internal aerodynamic drag and dead space.
In well-developed Stirling engine designs, regenerators dramatically increase the performance of the engine.
Internal Aerodynamic Drag
Designers have lots of tradeoffs to make when designing a new Stirling engine. When the internal aerodynamic drag gets too high, the engine will run badly because of too much drag.
But internal drag helps the engine transfer heat to the working gas and if the internal aerodynamic drag gets too low, the engine will not have enough heat transfer to run.
Somewhere in the middle, wonderful engine designs can be made.
Effects of Different Gases
One of the ways helium and hydrogen work to improve the power output of some Stirling engine designs is by reducing the internal aerodynamic drag and improving heat transfer.
Obviously, the internal heat transfer to the regenerator is influenced by your choice of working gas.
Regenerators Affect Everything
Designers of new Stirling engines need to remember that adding a or changing a component, like a regenerator, always changes more than just the regenerator.
For example when you add a regenerator usually the dead space and the internal drag inside the engine will increase.
Whether this is beneficial to the overall engine performance or not will depend on how accurately the compression ratio and the rest of the engine design, was optimized before this.
Experiment with Regenerators
Stirling engines are complex, inter-related systems where everything you do effects everything else in the engine
Improving one thing in the engine will not necessarily improve the performance of the overall system.
If you make an improvement on the hot side to get more heat into the working gas, you should plan on adjusting the compression ratio of your engine.
You will also need to figure out how to reject more waste heat on the cold side because everything affects everything.
Make Compression Ratios Easy to Change
Say, for example, you want to experiment with different materials for regenerators in a new Stirling engine you are building.
If you possibly can, you should design a method for changing compression ratios quickly and easily.
Capturing Your Engines Power
If your new regenerator material works much better you will be hoping that it will produce a bigger power pulse.
But if your engine design doesn’t let you easily change the compression ratios, you might not know that your new regenerator was an improvement.
If you build your new design with a variable compression ratio, then any new component you test, like a different regenerator, heater, or cooler, can be easily optimized for the proper compression and expansion ratio.
History of the Regenerator
The first patent of the regenerator, known then as an “economizer,” was issued in 1816. Two years later an engine based on this design was used to pump water in a quarry.
An illustration of Robert Stirling’s original patent from 1816.
Subsequently, Robert Stirling and his brother produced patents with enhancements to the original configuration, which included pressurization meant to increase the overall power output of the engine.
Ultimately, the materials used to build these early engines (typically cast iron) were unable to withstand the continuous high temperatures that Stirling engines require on the hot side and the factories that first used Stirling engines eventually when back to steam engines to power their machinery.
Modern stainless steels can maintain their high operating temperatures indefinitely so this is not a problem with modern Stirling engines.
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