During the 3th International Workshop on Thermoacoustics held on October 26-27 at the Twente University in the Netherlands, Aster had presented some results of ongoing work on bi-directional turbines and the interface to thermoacoustic engines.

Since introduction on the Aster website by the end of 2012, and more recently as presented at the 2014 Pamir Conference, held in Riga,  the thermoacoustic community has recognized the concept of bi-directional turbines as an alternative for linear alternators in particular when power levels goes up. Experimental results are encouraging and had proven that

• bi-directional turbines can be scaled up in power unlimited
• efficiency raise with gas density.

In addition, emplacement of multiple stages in series is found to be a way to match the turbine circumferential speed to the available acoustic gas velocity and generator parameters.

Based on these results a design procedure is in progress allowing to characterize, scale and match bi-directional turbines to any type of thermoacoustic heat engine.

Some details may be found in the presentation

Beyond acoustic to electric conversion limits (Keynote)

Bi-directional turbines for the conversion of acoustic power into electricity as they were  introduced by Aster early 2012, has turned out be a successful approach both in scaling up in power levels and in reduction of complexity and manufacturing cost.

To demonstrate the simplicity of the concept you may have a look at the video below, which shows a small bi-directional turbine plus generator, driven by the periodic acoustic flow from a loudspeaker.

What is interesting to see, is that the electric output (light) is hardly dependent of the acoustic frequency. This is an additional benefit of the bi-directional turbine concept because of the oscillation frequency of thermoacoustic engines varies with temperature which in case of resonant linear alternators could lead to detuning and performance degradation.

Since the first experiments a lot of progress is made in the understanding, design, modelling and construction(e.g. 3D-printing) of this type of A-E conversion and at present a full scale bi-directional turbine operating in 3.6 MPa helium is under construction to be applied in a 14kWe thermoacoustic power generator.

Last year some details and intermediate results on this A-E conversion concept were presented during the 9th International Pamir conference and Summer School, both held on June 16-20,2014 in Riga, Latvia. The paper and presentation may be viewed  here.

Paper Bi-directional turbines

Presentation Acoustic to electric conversion