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DEVELOPMENT

Computer rendering of the new heat pump design The New Mk2 Design rendered in SOLIDWORKS


But it has been a long story to get here...

Early development work with novel Stirling Engines Photo from a related earlier project


My interest in Stirling Engines began back in 1988 after returning home from working in Iran as an agricultural machinery project engineer. I wanted to explore options for a 'solar powered water pump' other than photovoltaic cells, which lead me to the Stirling Engine.   This personal project continued parallel to my employment back then, until a novel four cylinder alpha type diaphragm Stirling Engine arrangement was created with a central "Z" crank.   Two British patents (now expired) were later obtained, and in 1999 a 'Smart Award' grant was giving by the British Government's department of Trade and Industry for its continued development. However, diaphragms under pressure and elevated temperature are fraught with problems, including stiffness, and by 2002 that original project had to be adjourned.

The central "Z" crank diaphragm Stirling Engine CAD design
with black diaphragms, and yellow 'L' shaped Regenerator

Animated Gif of the original novel Stirling Engine design
Afterwards, other work and issues were pursued until April 2023, when putting together different experiences from completely different chapters of my life, the realisation came that a Wind Powered, Stirling Cycle, heat pump would be very useful!   It is the Ragnarok issue – whatever name we call it.   The current design is quite different to the previous. That was working as a motor, now the opposite way round as a Heat Pump. That previous machine was of 'alpha' type and based on diaphragms, now we have reverted to conventional pistons in a 'gamma' type arrangement.

However, a great deal of valuable Stirling Engine practical experience can be brought forward into the current project.

Development Ethos

Because the Stirling Engine is inherently more efficient than a Diesel engine, nearly all development projects in the 20th century were aimed at making the Stirling Engine more compact, so that the power density could be competitive.   Unfortunately by making the engine compact with high pressure air or gas, and running at fast revolutions brought many problems, and lost its efficiency advantage. So it remained peripheral.

The current project has a very different ethos, it is bulky and slow speed and happy with it!  So long as it is not bigger than other farm machinery, it is fine!   Also this is a green energy heat pump, not a fossil fuel engine.   Rather the aim of this design is to get affordable reliability with long life in rural applications.

The design also needs to be capable of being manufactured in small scale industry.

Development of the Present Model

It has already been found that there are special requirements to this application of the Stirling Cycle as the speed varies from zero up, so piston/gland sealing has to be very good at slow speeds, perhaps requiring the use of pneumatic seals which also need lubrication. Or lubrication itself could enhance the sealing? The use of a vertical crank may be changed through the use of a second bevel gearbox. This is ongoing development.

Storm force over-centre spring release
Bird's eye view showing the Storm force over-centre spring which protects the machine from over-speeding. When the reaction force is too much, the rudder vane will swing over against the rubber bump-stop to be parallel to the turbine blade disk, thereby forcing the turbine blades to face storm-force winds edge on and thus idle. This has been found to be very successful, though more because of turbulence than of increased torque reaction.
Storm force over-centre spring release - close up
Above shows detail 'Birds Eye View' of over-centre spring.


Modifications to Turbine Blades

Home-made press tool
Home made press tool used to curve the blade section. Notice the UNF bolt at the end, used for both location of the tool halves, and evening out the press force. All parts were oiled before the maximum force available was applied. This simple press tool was fabricated out of a piece of 4" steel pipe cut lengthways.
Curved section turbine blade end view
Now curved turbine blade viewed end on. This simple modification brings a significant improvement to turbine efficiency for minimal cost.

Modifications to the Stirling Engine

Crank Shaft Assembly
Experiments with forked connecting rods to the displacer drive, permitting longer brass glands with less overhang on the rod. Also incorporating X-ring seals (quad lobe Nitrile rings) both externally on the compression pistons, and internally within these new longer glands. The pawl radius has also been shortened. The Crankshaft is vertical on this design.
Gland, Cylinder and Flange Assembly
The longer brass glands shown soldered onto the copper flange


The Stirling Cycle machine has an unusually large range of possible variations, and all are interconnected in the performance achievable. Mathematics can determine optimum sizes; both the capacity of the whole machine and the allowable stress levels of its components. CAD such as SolidWorks is also very helpful in the design process, but there is no substitute for a physical test rig to try out options.   Doing this as a model is just a very cost effective way of getting the design viable.

modified machine in the home workshop
Modified machine now reassembled in the home workshop.   When a designer works hands-on in a workshop, design and development get blended together in a very cost effective way!   This home built model has taken the greater project forward considerably.


Mk2 DESIGN

Thermodynamic calculations, plus experience from the experimental model have shown some needed changes – Some very creative solutions are currently being worked on with the Mk2 (full size) design.   It was always intended that it would have a fully enclosed crankcase with multi-cylinders in radial formation to even out the torque requirement, but there are also some other developments: These include the compression pistons now having a longer stroke to give a higher compression ratio, while the displacer retains its large diameter with very short stroke.
The wind turbine is being increased to eight blades.
Choice of materials for the Mk2 Stirling heat pump are also very creative!

PV calculated curve
Calculated Pressure-Volume curve used in the Mk2 design

Wind power is proportional to the cube of the wind-speed, which the torque requirements of ordinary refrigerant heat pumps cannot accommodate. – The Stirling Cycle is different!   This design has variable pressurizing, such that as the wind speed increases, so does the Stirling Cycle pressure until the heat output reaches its design maximum of 4.5kW with a COP of 3.0.   While for a gentle breeze, the Stirling Cycle machine will be unpressured but still turning, giving a smaller heat output though keeping the system warm.   For stronger winds, the eight bladed turbine becomes inefficient – though this was not usable power, and it makes the turbine output match the heat pump power curve.   For storm-force winds there is the protection device shown above.

When an idea is theoretically possible such as this, development engineering is only a matter of getting it right, because it is do-able!

Line drawing of engine parts
Crankshaft and connecting rods of the new Mk2 design, which copies ideas from historic aero engines, except there is no need with Stirling Engines to have an odd number of cylinders like 4-stroke petrol radial engines.
Computer rendering of the new heat pump design
The New Mk2 Design rendered in SOLIDWORKS

Design Rationale

Stirling Engines are technically very alluring – they are the perfect engine in terms of having the theoretical maximum possible efficiency (same as Carnot Efficiency) and simplicity without valves or noise in a practical engine.   But they are limited by heat transfer – Getting enough where needed, and holding it back where unwanted.   This, plus the need to match the wind turbine output, has driven the Design.
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