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Stirling Engine: Free-Piston configuration
1. Stirling engine foresees several configurations: alfa, beta and gamma configurations. Free Piston operates following a different logic. Can you explain the work of a Free Piston Stirling Engine?
The free-piston Stirling is really just a means to move the moving parts. The basic idea can be applied to the alpha, beta and gamma machines. The most common free-piston Stirling is the beta configuration with the displacer and piston in a concentric cylinder. However, some workers, including myself, have built and demonstrated alpha free-piston machines with four interconnected pistons. In the free-piston alpha arrangement it is possible to have an odd number of pistons, including three and five. The gamma free-piston machine is more rare but I do not see any specific reason why it could not be driven with a free-piston mechanism.
2. How does it differ from the other configurations?
As I intimated above, the free-piston configuration differs only with the different driving mechanisms. So the differentiation is compared to a kinematic (or crank, or disciplined) mechanism and not the means of arranging the thermodynamic cycle. The free-piston mechanism uses the gas pressures and the dynamic characteristics of the moving parts to obtain the correct mechanical motions. Because the side loads on the moving parts are very small in a free-piston machine, gas bearings or other non-contact bearings are universally employed. The advantages of this approach are that no oil is used and the amplitudes of the moving parts are freely adjustable giving very efficient power control. I am personally of the opinion that the difficulties of oil lubrication and the limitation on efficient power control with crank mechanisms renders these approaches as impractical. I formed this opinion many years ago when working on the automotive Stirling engine at Mechanical Technology Inc.
3. What are its performances, even compared to other configurations of Stirling engine (alpha, beta and gamma)?
There is no basic reason for the free-piston machines to have any lower performance than crank machines. Indeed, because seal friction losses in crank machines, I suspect that the free-piston machine will be able to demonstrate higher efficiencies. However, there are limitations to the free-piston Stirlings that has prevented a clear demonstration of efficiency advantage at the high power levels. This is centrally related to two things i) the poor scaling of the linear alternator to high powers and ii) the incompatibility of the magnets used in linear alternators with hydrogen. So free-piston machines have been found to be of a more practical construction at smaller sizes. I think that the current limit of a practical free-piston machine with monocoque construction (no tubes for the heat exchanges) is around 3 to 4 kW.
4. You have written numerous articles and books about Free Piston. Why are you so interested in it?
As you know, this technology was invented 190 years ago. Except for some brief moments of success, it has never really met its promise. I believe that when Professor William Beale made the invention of the free-piston machine, he showed the way to practical realization. To me that was something that I had to embrace because it seemed to be such a breakthrough. I enjoy the challenges of developing this machine to practical reality. It is not an easily understood machine and so provides continuous surprises as it jealously reveals its secrets.
5. Currently only low power Free Piston engines (1 kW) are marketed. Why?
I think that I have answered this question above. But I believe that a further reason limiting the market to low powers applies to all Stirling engines. This is the intrinsic problem of transferring the heat into and out of the engine. If one accepts that tubular heat exchangers are costly (and some don’t), then one is limited to more-or-less a simple shell (or monocoque) design. In this case the heat transfer will scale as the square of the linear dimension and the power as the cube of the linear dimension. If we then begin to increase the pressure to reduce the size of the Stirling, the heat transfer problem just gets worse. So as I said before, if a simple monocoque design is being considered, then I believe that a practical size will be no greater than a few kW. By the way, I built a 3kW free-piston air engine in the 1980s that regularly produced about 4 kW. I still believe that this machine represents an approximate limit for practical free-piston machines (without tubular or other topologically extended heat exchangers). Perhaps I miss-answered this question, I see you are asking about marketing. I think the only reason that a 1 kW unit is marketed is because only a 1 kW has been placed into production. I don’t believe that there is a technical reason (within what I have said) why a bigger machine cannot be eventually marketed as well.
6. Do you think that the marketing phase has finally started, after a long wait?
It seems that way. I wish MEC and Infinia every success with their mass sales.
7. What are your opinions about the technical and business perspectives of Stirling engine, and of the Free Piston configuration in particular?
I believe that Stirling engines, and Stirling machinery in general (cooling machines and heat pumps) will become a common product in the near future, probably within five years. I believe that eventually all successful Stirling products will eventually be of the free-piston type. If the machine can be marketed at a reasonable price, I would imagine that the business opportunities would be tremendous. After all, it has all the technical attributes necessary to compete in a world of diminishing resources.
Chief Executive Officer of Global Cooling (CEO)
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