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Stirling Heat Engines
NOTE: January, 2010. This page now has most of my Stirling
engine designs.
I am currently building one liquid piston Stirling engine and one metal
piston Stirling engine (see below). I hope to encourage others to build
and experiment too!
Any thoughts appreciated.
Geoff Haselhurst
PS - For the next few months I am busy trying to finish writing up the physics and philosophy on this website - so these Stirling engines will just have to wait for their completion! They are a fun hobby for me - I do think solar panels are actually the best solution for extracting energy from the sun.
Introduction
I have been fascinated by Stirling Engines for several years now and have built one prototype that did not work. But we learn a lot by doing things so the time was not wasted. (Since then I have built hundreds of designs in my head!)
The design principles are: Simplicity - Use of existing technology - Cheap and easy to build - Potentially high power output (5 kilowatts) - Suitable for third world applications. And I do wish to emphasise simplicity - I hope to build the world's simplest heat engine (a noble challenge!).
Below are two basic designs - one based on liquid pistons, the other using metal pistons (converting two 6 cylinder petrol motors to 6 connected Stirling engines).
1. Liquid Piston Heat Injection Stirling Engine
I am currently building this - I hope to finish it this weekend (it is very simple). This Stirling heat engine can be used as a water pump, or it can be connected to a metal piston - flywheel - alternator and be used as a generator. (I may be able to use a linear alternator and avoid the piston / flywheel - I need to do more research if / when machine works!)
NOTE: If the liquid piston Stirling engines are used as direct water pumps then they operate at atmospheric pressure. However, it will be possible to connect them to a metal piston or by connecting two machines together such that they can be pressurized (to say 50 Atm) and then used to drive a generator at substantial power (from 1 to 5 Kw).
How this Oil Vapor 'Explosion' Stirling Engine Works:
The two columns of water / oil act as liquid pistons.
In the diagram, the cold side is higher thus the liquid will level itself causing the hot side to rise, transferring the hot working gas through the regenerator (stores heat energy) to the cold side where it contracts and water is sucked in from inlet.
When the hot oil reaches the top of the cylinder some (~ 20 mls) of oil flows into the Heat Injector. The vaporized oil expands pushing the hot side down, the cold side up.
This transfers the cold gas back through the regenerator (heats gas) and into the heat injector where it mixes with the high temperature oil vapor and is further heated / expanded.
During this expansion water is pumped out.
Once the pressure equalizes in both cylinders then the liquid pistons will again level themselves, causing the cycle to repeat.
Some further thoughts on this Stirling Engine Design
Stirling engines do not use change of phase which is inefficient because
energy is wasted in breaking molecular bonds (latent heat). However, in
this Stirling engine we are using the change of phase (oil falling on 800
C copper and lead) to create a very hot oil vapor that then transfers its
energy to the working gas and the regenerator, thus the energy is not wasted
(some will be lost).
This has three advantages;
i) We can use copper and molten lead in the heating pot and thus heat it to around 800 C.
ii) This allows a rapid heat transfer to the working gas as it mixes with the very hot oil vapor (very large surface area due to the mixing of the hot and cold gasses).
iii) We get very rapid high pressures - more like a combustion engine.
Further design considerations;
i) It may need float valves at the outlets of each cylinder to the regenerator to stop liquid moving around the top of the Stirling engine.
ii) In terms of cooling the gas I am going to experiment with 2 methods.
a) Add copper wire inside top of cold cylinder and have intake water that flows in around the outside of the cold cylinder thus cooling it (so add another larger pipe around it with water in space).
b) In the cooling phase bubble the hot gas through cold water in cold cylinder, and then in the expansion phase this water would be pumped out (if the Stirling engine is used as a water pump).
iii) To begin I will simply use a fire to heat the hot side (wood has about
4kw/hrs of energy per kg, and a fire easily reaches temperatures of 600
degrees celsius).
The cold side will be cooled to around 70 degrees celsius by running water
(I have a lot of ponds and creeks on my property!). If they work then in
the long term I would use solar heat energy.
2. Liquid Piston Stirling Heat Engine
This is a similar design - just with the heat injection / change of phase explosion removed so it is a pure Stirling engine (it just heats and cools gas to push and pull on the liquid pistons).
How this Liquid Piston Stirling Engine Works:
The two columns of water / oil act as liquid pistons moving the working gas from hot to cold.
I have two options for heating and cooling.
i) Add cooper wire to the inside top of each cylinder such that the liquid heats / cools the copper that then heats / cools the gas (Nitrogen or Helium). Thus the heat is applied directly to the hot cylinder so this is limited to around 400 C (the boiling point of oil).
ii) Bubble the gas through molten lead on the hot side (add a heater pot between hot cylinder and re-generator half full of molten lead and copper). Likewise bubble the gas through cold water on the cold side.
I am also going to use a 40 KG lead weight hanging on springs inside the cold cylinder that acts as a heavy piston that helps the liquid to rock (stores more of the expansion energy by lifting the lead weight when the hot side expands, the weight then wants to fall which transfers gas back to cold side).
3. Six Cylinder Petrol / Diesel Motor
Converted into Six Stirling Engines
I have put this Stirling engine on its own page. It is a high pressure
(100 Atmospheres) mid temperature (350 C) combined metal piston and liquid
oil piston Stirling engine. Basically just connecting two 6 cylinder petrol
motors together 90 degrees out of phase and replacing the heads with heating
and cooling cylinders half full of oil and copper wire - connected via a
regenerator.
It is nearly finished (I worked on it about 4 months ago, but I have been
away ... ). I hope to finish it in the next few weeks.
https://www.spaceandmotion.com/inventions/convert-petrol-motor-stirling-engine.htm
And I also have an upside down version of this that uses molten lead so that it can work as a high pressure high temperature Stirling engine.
https://www.spaceandmotion.com/inventions/convert-petrol-motor-stirling-engine2.htm
(This page also has a lot of the maths physics for heat transfer which
is important to understand if you wish to build these machines).
Cheers,
Geoff Haselhurst
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