Solar Stirling Developments


The Solar Powered Diaphragm Stirling Engine

Spider Unit Motion
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Our Diaphragm Stirling Engine is a further developnment of the square arrangement of four individual Stirling cycle engines, known as a ‘Siemens’ or ‘Rinia’ type engine.
Our inventive step is replacing the double acting pistons in cylinders with special diaphragms, using a central ‘Z’ crank (Wobble-Crank), upon which a unit having hollow actuating lever arms radiating out to the diaphragms: we call this a ‘Spider unit’ and is coloured yellow on the CAD images.

Therefore besides the special diaphragms, there are only two moving parts in this engine: the central Wobble Crank (‘Z’ crank) and the ‘Spider Unit’.

The hollow arms of the Spider Unit form an ‘L’ shaped airway that contains the Regenerator heat-store, and also provide the required 90 degree phase-shift between the Hot and Cold Chambers!
Spider Arm Pair, also containing the Regenerator

So what's the secret?    ...The Diaphragms!


The Wobble Crank (Z-Crank)
The Wobble Crank (or 'Z' Crank)

Individual P-V diagram for 360 degree crank rotation Cumulative Torque produced through a 360 degree crank rotation
The area of the P-V diagram loop, represents the heat energy converted into mechanical power during each revolution of the engine, power is developed even at very marginal temperature differences between heat source and heat sink, shown by a very narrow loop in the diagram. There is no sliding friction in this engine.

The torque produced by the engine is always positive through all angles of the crankshaft (wobble-crank) therefore the engine is self-starting. The engine actually produces a greater torque when stalled (stationary crank), though no 'power' is being produced when in this stalled state.   When a sufficient temperature difference between heat source and heat sink is applied, generating an engine torque greater than the load resistance, the engine will automatically commence to run. For example: in a solar application, when the morning sun’s rays start to heat-up the engine’s solar collectors, the engine will start to run automatically.


How it Works
The Stirling Cycle Engine dates back to 1823 when Dr Robert Stirling, a Scottish minister, patented his ‘Hot Air Engine’.   Numerous engines of the same thermodynamic principle have continued to be researched, developed and produced for specialist purposes, up to the present day.   The primary advantage of the Stirling Cycle Engine is that it is the only practicable engine that, neglecting friction, can work at the theoretical Carnot efficiency!   With plentiful supplies of relatively cheap combustible fuels in the past, and scant regard for CO2 emissions, this attribute has up to now been less important than the higher power-densities possible with petrol (gasoline) or Diesel engines: When making Stirling Engines very compact and running them at high speeds, they tend to loose their efficiency advantage as the heat transfers within the engine diverge from the Isothermal of the pure engine. Besides the now very important efficiency advantage, the other significant advantages of Stirling cycle engines are that they have no valves and it is a 'closed' cycle.

A Stirling Cycle Engine contains a fixed mass of air (or other gas) which cycles within the engine between a ‘Hot’ chamber and a ‘Cold’ chamber, while being compressed and allowed to expand. Usually pistons in cylinders are used to move and compress the air, but in our engine they are replaced by special diaphragms. The sequence of the Stirling Cycle is as follows:
  1. 'Isothermal' compression, heat given out to the heat-sink.
  2. Transferring the air to the Hot Chamber increasing the pressure further
  3. 'Isothermal' expansion giving out mechanical work while absorbing heat energy
  4. transferring the air back to the Cold Chamber reducuing its pressure
If the steps of the sequence are merged together in the sinusoidal manner of a crank-shaft, it does not affect the efficiency so long as the external heat transfers are Isothermal (constant temperature volume changes). The 'Carnot' efficiency, being the maximum theoretical efficiency possible, is relative to the difference between the ‘Heat-Source’ and the ‘Heat-Sink’ absolute temperatures (degrees K).

Carnot efficiency   =   1   –     Temp of heat-sink

Temp of heat-source

In Stirling Engine terminology, the internal temporary heat-store is called the ‘Regenerator’, in our engine this is contained within the Spider Arms. Its purpose is to warm up or cool down the air, as it is transfered back and forth between Hot and Cold chambers respectively.

In ‘Alfa-type’ Stirling Engines, the mechanical arrangement of the cycle is achieved by having two cylinders with pistons in them, one cylinder being ‘Hot’ the other ‘Cold’ and an open airway between the cylinders that goes via the heat-store Regenorator. The pistons need to move out of phase to each other, typically 90 degrees.



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