The conclusion of the TAP project (converting industrial waste heat into electricity) carried out last year is that the thermoacoustic process, the conversion of waste heat into acoustic power, can be scaled up in power unlimited. This however does not hold for the conversion of this acoustic power into electricity by linear alternators. In linear alternators the stroke of the moving mass is limited (by the springs). Consequently, they need to be operated at high frequencies (50-100 Hz) to get sufficient output power per unit mass. Linear alternators works fine at small systems (≈ 1 kW) but fails to perform at industrial power levels (→ 1 MW).
Size of the acoustic circuit (feedback, housing) goes up proportionally with power, not only because of the increasing system cross-sectional area but also because of longer bends. The later is a consequence from the acoustic and practical condition that the feedback tube diameter (D) << λ and bend radius (r) > 3D. Summarizing, the higher the power level of the thermoacoustic system, the lower the oscillation frequency.
For linear alternators a low oscillating frequency is disastrous because maintaining the electromagnetic induction (dΦ / dt) a larger stroke and/or a stronger magnetic field is required. This dramatically increase complexity and cost. Both because of the impossibility to maintain the seal gap requirements over large strokes and because of the increasingly cost of stronger magnets and mechanical issues associated with the high moving mass .
An alternative is to convert the acoustic power into rotation first subsequently driving an off the shelf generator. Previous work on the small bi-directional impulse turbines for converting acoustic power into electricity was very promising. Therefore, as a next step in this development, a full scale bi-directional turbine is build, and tested in the existing 100 kW TAP. The turbine is based on a classic design for an OWC plant and scaled down (69%) to fit into the open space in front of the cold hex of one of the engine stages of the TAP. The turbine rotor and guide vanes are manufactured by 3D rapid prototyping and build together with an off the shelf car generator (28V, 80A). The assembled unit is depicted below.
The generator is provided with a rotational speed sensor and loaded with a fixed dc-load of 0.5Ω. The rotor of the ac generator is disconnected from the rectifier bridge and connected to adjustable power supply to set the field current and with that the shaft torque (T). Together with the rotational speed (ω) the mechanical power (=ω.T) delivered to the shaft (by the turbine rotor) can be measured.
For the test we have only one turbine available and this one is mounted in the second engine stage (#2) of the TAP in front of the cold hex as is depicted below.
The test is performed with compressed air at 0.8 MPa as working medium. Actually the TAP is designed to operate with helium and therefore the maximum drive ratio during the test was limited to about 1%. Nevertheless the acoustic power is sufficient to test the turbine under realistic condition like an oscillating flow at elevated mean pressure (higher gas density). The measured data as a function of the field current (shaft torque) is shown below.
The highest measured mechanical output power (Pm) is 478W at a rotational speed of 377 rpm. At this point the actual drive ratio (pa/P0) was 1.14%. The acoustic power dissipated in the turbine is 629W yielding a conversion efficiency of 76%.
This result is in line with the data that in general turbine efficiency goes up with size and gas or fluid density. To illustrate this, the small bi-directional turbines (80 mmØ) tested previously at atmospheric pressure reach 40%, large (OWC) turbines (> 800 mmØ) operating at atmospheric pressure could reach 60-70% while water turbines could reach >90% due to the high fluid density. The present turbine is tested at elevated mean pressure (0.8 MPa) so its conversion efficiency should be somewhere in between which indeed is the case.
The conversion efficiency of the current bi-directional impulse turbine is somewhat less than the (theoretical) efficiency of linear alternators. Some more experiments and tests are needed to optimize the performance of this basic turbine under acoustic single frequency conditions. The experiment however proves that bi-directional impulse turbines has the potential for successfully converting acoustic power into rotation and from there into electricity at unlimited power scale.
An other, even more important, aspect is the economic efficiency. The turbine operates at ambient temperature, runs at relatively low speed and has no critical tolerances. Therefore the rotor and guide vanes can be made from plastic (e.g. injection molding). The generator is an off the shelf type. As compared to the initial cost of the linear alternators in the TAP project this concept reduce the cost per kW electric output by one order of magnitude down to less than 300€/kW. This all brings the TAP back on stage for waste heat recovery on industrial scale.