Converting acoustic power from thermoacoustic engines into electricity using resonant linear alternators is a common approach but has severe limitations in terms of cost and scalability. The increase of moving mass with increasing power finally sets a practical limit caused by the extreme periodic forces in the construction and impossibility to maintain clearance seals stable over a large stroke.

An option, in particular for low cost generators for rural areas (e.g. FACT, Score) and for high power industrial applications( e.g TAP), is to convert the acoustic wave motion into rotation first. This allows for deploying standard generators. Rotational speed could be made arbitrary high. Therefore this type of alternators require much less magnetic material which significantly reduce size, weight and cost.

Linear alternators with pistons or membranes make use of the pressure variation of the acoustic wave. There is however no physical reason why not using the periodic velocity component of the acoustic wave. A way to convert such a bi-directional flow into rotation is known from shore and off-shore electricity production plants based on an oscillating water column (OWC). In this type of power stations, waves force a water column in a chamber to go up and down. This chamber is connected to the open atmosphere and the periodic in- and outflow of air drives a bi-directional turbine of which the rotation direction is independent of the flow direction.

This class of “rectifying turbines” is explored extensively and in principle they can be deployed for conversion of periodic acoustic wave motion as well. Common to uni-directional turbines they could lift or impulse based. For this application the impulse version seems to be most appropriate because of they are self-starting and could operate over a large amplitude range.

For testing and validating this option both an radial and axial bi-directional turbine designed and build using 3-D rapid prototyping. Below the 3D design and the “3D printed” result for the radial turbine. The radial turbine could be positioned at the junction between at the end of a feedback tube and the volume in front of the cold heat exchanger (e.g. TAP).

In addition, an axial bi-directional turbine is designed  and produced because of such a type could  be placed in line with the acoustic feedback tube (FACT)

Both turbines are provided with a brush less electro motor acting as generator. For the measurements the turbines are acoustically driven by the loudspeaker of the impedance measuring set which allows for both setting the frequency and measuring the acoustic input power of the turbines. The results are encouraging. The conclusion from these preliminary experiments is that this small turbine,

• convert acoustic wave energy into rotation
• can be place in-line with the thermoacoustic section
• operate at acoustic frequencies up to > 100 Hz
• shows an acoustic to mechanical  (rotor) efficiency in the order of 40% under small signal conditions

For such small turbines (80 mmØ) a rotor efficiency of 40% is according to theory. The same rotor scaled up to 300 mmØ will have an efficiency of 70%. Because of the periodic flow is at a fixed frequency, acoustic manipulation of the flow around the in- and outer vanes could raise the efficiency over 80%. This is for further study.

In order to validate the concept, a full scale radial bi-directional impulse turbine is under construction now for testing in the TAP (see previous posts) replacing the current linear alternators.

Thermoacoustic electricity generation is still under development. The emphasis hereto is on  increasing power level and efficiency and in reducing the cost per Watt electric output power.

Early May, Aster together with the Nottingham Score team managed to get the demo2.2 Score thermoacoustic engine to produce 36 Watts electricity continuously, and 45 Watts peak. The working gas was air at an absolute pressure of nearly 200 kPa. A high power speaker was used as alternator. Its acoustic to electricity conversion efficiency is about 50% which means that the engine delivers 70-90 Watt net acoustic output power. Increasing the mean pressure up to 250 kPa and/or using a more efficient alternator will suffice to achieve the 50 Watt project target.

This result was obtained by modifying the acoustic circuitry toward a serial connection of two individual (acoustic) matched engine stages. This yield both a higher power density and a reduction of feedback tube length. The test set-up is shown below.

The current (messy) shape of the tubing is for measuring purposes and for easy access. In a final version the tubing can be folded to fit into, or be part of, the wall of the stove.  Note the relatively short feedback tube length which allows for a higher frequency and proportionally higher electric output of the alternator.