Stirling Engine

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, this is the first one that I can not find any problems with so I have decided to build it in reality!
The Stirling Engine should be built over the next week (by 25th March, 2009). I wonder if anyone can tell me why it should not work or how it can be improved!

The principles behind it are;

1. Simplicity.

2. Use of existing technology.

3. Cheap and easy to build.

4. High power output ~ 20kilowatts.

5. Suitable for third world applications.

The Haselhurst-Stirling engine design is a high pressure (60 atmospheres) low temperature (300 degrees celsius hot side, 100 degrees celsius cold side) design. It has a theoretical efficiency of 35% (solar cells have a maximum efficiency of about 20%).

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 100 degrees celsius by running water (I have a lot of ponds and creeks on my property!). If it works then in the long term I would use solar energy to heat it.

The two petrol motors are 6 cylinder 3.9 litre 120 Kw Ford Falcon motors from 1990 (yes, I live in Australia!). They cost me $40 dollars each from the local salvage yard. Each cylinder has a volume of 650cc so we are effectively making 6 small stirling engines connected together by the motor's crankshaft.
Most of the cost is in using high pressure rated seamless carbon steel ($2,200) See: Carbon seamless steel pressure ratings and the welding (estimated at $2,000). To give a quick comparison of cost, the WhisperGen Stirling engine costs $20,000 dollars and is 1Kw.

Theoretically this design should produce 12Kw using Nitrogen (500 rpm) and 25kw using helium (1200 rpm). See: Stirling Engine Performance Calculator

Below is the rough diagram, followed by a short explanation (from my provisional patent application).

The Haselhurst-Stirling Engine

Stirling Engines are theoretically the most efficient heat engines available, where the efficiency is 1 - Tmin/Tmax (in degrees kelvin). Thus the engines are more efficient the greater the temperature difference from the hot side to the cold side.

However, in the past it has been difficult to overcome design problems relating to the required rapid flow of heat energy in the hot and cold pistons, sealing the system at high pressure, and building the motor at a cost efficient price.

The Haselhurst-Stirling engine overcomes these problems by enclosing two petrol engines in two high pressure containers (seamless carbon steel pipe, 400mm by 20mm wall) and connecting the motors 90 degrees out of phase. One motor is on the hot side, the other motor is on the cold side. The pipes which contain the motors are filled with oil and pressured to 60 atmospheres using Nitrogen Gas. (Warning - never use pressured air with oil - it is highly explosive!)

The head and sump of both motors are removed so that we have the block, pistons and crankshaft only. The bore for each piston (90mm) is then connected to the same diameter pipe (heating / cooling pipes) that extend out of the pressurized container (see above diagrams). The heating and cooling pipes are then connected at the top by a 25mm pipe for each respective cylinder - half filled with copper pipe to act as a regenerator / heat storage.

The heating / cooling pipes are half filled with oil (to make liquid pistons) and above the oil the volume is pressurized with the working gas (nitrogen or helium). Further, the heating and cooling pipes have three times the volume of the cylinders, so they can be two thirds filled with copper plate to improve heat transfer.

When the crankshafts are rotated the oil moves up and down the heating / cooling pipes causing the working gas to be moved from the hot side to the cold side and back again. In one complete cycle the hot gas expands and does work, then it is transferred to the cold side, then contracts and does work, then is transferred back to the hot side to complete the cycle.

This is an animated diagram of the standard Alpha Stirling Engine cycle.

Advantages of the Haselhurst-Stirling Engine

Because we are using liquid pistons we can add sheets of copper and pipes which run through the heating / cooling pipes to increase heat flow.

By using existing technology of petrol engines we are able to build cheap efficient Stirling engines with high power output.

Effectively we are building 6 stirling engines, each with a volume of 650cc, connected by the motor's crankshaft, thus increasing heat transfer (rather than having one big cylinder).

By enclosing the petrol motors (their block and pistons) in a pressurized container we eliminate problems of loss of pressure and working gas.

Because this is an external combustion engine it can use any heat source, including the sun, thus producing a carbon / pollution free energy supply.

Geoff Haselhurst (March 18th, 2009)

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