Archive for the ‘ Thermo Acoustic Power (TAP) ’ Category

>10kW bi-directional turbine

In the framework of a joint project together with Hekyom (Fr), Aster designed and build a  6-rotor, 150mm diameter bi-directional turbine aimed to generate >10kW shaft power at 3000 rpm. Converting this shaft power into electricity is performed by standard 10kW brush-less electro-motor, used  as generator having an mechanical to electric efficiency close to 90%.  The layout of the turbine plus generator placed in a pressure vessel is shown here


Shown in detail below is the first guide vane with central bearing and the subsequent rotor-guide vane sets made using 3D printing.


Unfortunately  the thermoacoustic engine to drive the turbine is not available yet. For that reason the turbine is preliminary tested and characterised at atmospheric pressure by measuring flow resistance and complex acoustic impedance at continuous and at alternating flow respectively. The measured impedance at the 130mm input tube as a function of rotational speed is shown below.


At low frequency and blocked rotor the absolute value of the impedance is complex induced flow resistance and acceleration of the gas between blades (inertance). At increasing rotational speed, the absolute values of the acoustic impedance rise and become more and more a real load to the acoustic source (output of the thermoacoustic engine)

The impedance is measured for the 6-rotor turbine but the effect of rotational speed on the impedance as described is typical for this type of bi-directional turbines.

As soon as the high power thermoacoustic source becomes available, tests with argon at elevated pressure will be performed. We expect  the heavy gas (argon) at elevated mean pressure (3.5MPa) will improve acoustic to electric conversion efficiency up to 80%.

Breaktrough in upscaling thermoacoustic power generation

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.

The acoustic to electric conversion issue

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.

Pilot Thermo Acoustic Power (TAP) completed!

Demonstration TAP pilot

On March 7 the project team presented the results and a full functional pilot of the TAP (Thermo Acoustic Power) to the public. The TAP is build and installed at the premises of Smurfit Kappa Solid Board located in the north of the Netherlands. For a selected group of about 25 representatives from industry, economic affairs (SBIR) and press, the operation of the TAP generating electricity from the waste heat in the flue gas of the STEG was successfully demonstrated.

Project results

The aim of this pilot was to prove the scalability of thermoacoustics from lab to industrial temperature and power levels, and to prove the economic feasibility of the TAP. The first goal, the scalability of the thermoacoustic process, converting heat into acoustic (= mechanic) power, up to industrial levels was proven successful, showing an exegetic efficiency of 38% (see previous post). The economic target however is still behind, consequently delaying the intended market introduction for this year. For the conversion of acoustic power (generated from the waste heat) into electricity, linear alternators were deployed. The costs of these alternators was, and is still, found too high. Current cost of the alternators only, already exceeds the target cost of the complete TAP installed (3000 € / kWe ).

One of the conclusions from the pilot is that, based on cost reduction figures for the alternators and the other components, the power level of the TAP should be scaled up to  least 50-100 kW electric output power to become economic viable. More details can be found in the final report (in Dutch)

Eindrapportage TAP (SBIR09313)

The pilot has made clear that the conversion from waste heat into acoustic power is efficient and scalable. In the pilot set-up however the limited capacity of the linear alternators prevented utilizing the full acoustic output generated by the engine. Therefore, the further development and cost reduction on the TAP, and thermoacoustic waste heat recovery in general, will be continued on two tracks;

  1. Development of a scalable and cost effective  alternative for the conversion of acoustic power into electricity for the TAP. Beside improvements on the linear alternators, a serious cost reduction could be obtained by converting the acoustic output power into mechanical rotation allowing for the use of standard generators. Recent experiments with a small scale bi-directional impulse turbine were encouraging. A full scale turbine with generator is under construction for testing in the current pilot set-up.
  2. Utilizing the concept as a heat transformer, eliminating the need for alternators or other mechanical moving parts completely.  A heat transformer is a (multi-stage) themoacoustic engine acoustically coupled with a  thermoacoustic heat pump by sharing the same traveling wave acoustic resonance or feedback circuit.

For use as a heat transformer, the waste heat from an industrial process is split in two flows. One flow is supplied to the engine stages. The other flow is supplied to the heat pump to raise its temperature above the pinch.  In other words, a part of the useless waste heat of a too low temperature is converted into heat with a higher temperature so it can be reused in the industrial process. This way, primary energy is saved and CO2 emission reduced.  Advantages of a thermoacoustic heat transformer over existing heat pumps are the lack of exotic cooling refrigerants, the flexibility and the extreme large operating temperature range. For example, the output of the same device could be cold as well by simply commute the heat pump high and low temperature heat exchangers connections.

TAP converts 20 kW waste heat into 1.64 kW acoustic output power

At an input temperature of 99°C and heat rejection at 20°C this corresponds with 38% efficiency relative to the Carnotfactor. This is an encouraging result because it proves that the thermoacoustic process can be scaled from lab size up to power levels in real applications.

For this measurement the TAP did not run at the proposed configuration of a 4-stage engine driving four loads (alternators). This is because on the moment no correct linear alternators are available. Therefore, to be able to judge the performance of the thermoacoustic part of the TAP, one of the four engine stages is used as an artificial acoustic load. This is done by disconnecting the high temperature hex of this stage from the heat source.  Net acoustic output power of the remaining three engine stages in that case is the  difference between the acoustic power at the in- and output of the disconnected stage.

The measured values for a 3-stage engine at these low power levels (relative to the design values) are found to agree well with the simulations. The 1.64 kW output power is reached with helium at a mean pressure of 750 kPa and at only 1.7% drive ratio. At this pressure amplitude the acoustic power in the feedback loop is nearly 6 kW.  Simulation for the initial 4-stage engine running at a drive ratio of 5% shows that for an input temperature of 140 °C the acoustic output power will be about 11 kW.

An important parameter to judge the performance of an thermoacoustic engine is the relation between applied temperature difference and acoustic power in the feedback loop.  The “steepness” of the curve versus the temperature difference across the regenerator is a measure for the internal viscous losses in the acoustic feedback or resonance circuit, the heat exchangers and regenerator. Plotting acoustic power versus the input temperature difference also includes the effect of temperature drop across the high and low temperature heat exchangers. For the 4-stage engine the measured data for both air and helium as working medium is plotted below.


The plot shows that the onset temperature depends on the mean pressure and that for helium at 750 kPa oscillation start at 40°C temperature difference. For air the onset temperature is even below 30°C but the less steep curve indicate higher viscous losses with respect to the acoustic loop power (which for air at the same pressure amplitude is lower than for helium due to the higher value of ρ.c). Note that these figure holds for the current implementation only and that, for example,  inserting alternators or changing heat exchangers or regenerator material or structure will modify this figure.

In the current TAP the first energy conversion step, from heat into acoustic power, is demonstrated to be feasible at relevant thermal input powers according to the project plan.  The second energy conversion step, of acoustic power into electricity, however fails to convert the acoustic output power available into the planned 10 kW level electric output power.

For the travelling wave multi-stage configuration the acoustic impedance at the pistons of the linear alternators needs to be real in order to extract maximum power at minimum pressure amplitude. A real acoustic impedance means that the mechanical resonance of the alternators should be close to the acoustic oscillation frequency set by the feedback loop length.

Initially the TAP was designed for running at 70-80 Hz.  In the end however, the high moving mass (magnet + springs) limits the mechanical resonance frequency of linear the alternators to no more than 40 Hz. This mismatch dramatically reduce the load to the engine and from that the amount of acoustic power that could be extracted. This issue and some other necessary re-engineering found from the construction of the current set-up will delay commercializing the TAP for the moment.  Next months focus will be on exploring and testing alternatives for converting acoustic power into electricity which can be  scaled up in power as well.

TAP installed

By the end of September the installation of the TAP was completed. The flue gas heat exchanger is inserted into the STEG exhaust ( cross-section 3 x 3 m) and  connected by an isolated water/steam circuit down to the intermediate heat exchanger located near the TAP.


From the intermediate heat exchanger a thermal oil circuit is used to transfer heat to the high temperature heat exchangers in the engines stages of the TAP.  The use of an intermediate hex for separation of both the thermal oil and water circuits was required here for concession reasons. In future systems this intermediate heat exchanger and pump has to be avoided because of the additional temperature drop which is harmful at low operating temperatures.

Installation SKSB(4)The low temperature heat exchangers in the TAP are connected to an existing  cold water storage tank in a stand about 80 meters away. All fluid circuits are equipped with pumps,   flow sensors and thermocouples for measuring thermal powers. Piezo resistive pressure sensors on the TAP are used for measuring mean pressure, pressure amplitude frequency, phase and acoustic power.

Testing the periphery was done by running the TAP, without alternators and with pressurized air (up to 750 kPa) to generate maximum heat flow trough all heat exchangers. At maximum heat flow the temperatures at the various sections are measured.

Conclusion from this test was that the effective temperature for the TAP to run at is limited mainly, but not only, by the heat transfer in the thermal oil circuit. Also the air sided temperature drop in the flue gas heat exchanger is higher than expected. As a result the effective input temperature of the TAP is not more than 100°C  which means that about 60 °C is lost in the periphery.

This observation stresses again the conclusion that at low and medium temperatures the design and performance of external circuits, to supply and reject heat to the TAP, are as important as the performance of the TAP itself.

TAP transported to the pilot location

On July 11, the thermoacoustic power (TAP) unit was transported from the assembly hall of Huisman in Elst to the final pilot location at Smurfit-Kappa Solid Boards in Nieuweschans. During this transport the TAP was split in two halves to be able to pass the door and to manoeuvre the whole in position.

Heat has to be supplied to the TAP. Therefore a dedicated +100 kW flue gas heat exchanger was installed at the turbine-steam boiler exhaust at the roof of the building and completed with a hot water circuit from flue gas hex to the intermediate hex near the TAP. For heat removal a cooling water circuit  was prepared and installed.

During next weeks these high and low temperature circuits will be connected to the heat exchangers of the TAP. For testing and validating the TAP is also provided with pressure and temperature sensors for measuring thermal and acoustic power levels and a real time data-acquisition and monitoring system.

TAP Installation-SKSB

This picture shows the final set-up at the pilot location. In the back the connections to the high an low temperature and the intermediate heat exchanger are visible.

The four balanced alternator sets for converting acoustic power into electricity are still under construction and will be installed inside the pressure vessels afterwards when thermoacoustic characterization is done.

Build and first test of the 100 kW Thermo Acoustic Power unit

After a lot of preparative work by the consortium, finally  the construction of the thermoacoustic  part of the TAP was completed by last week. The high- and low temperature heat exchangers and regenerators are assembled to single units, provided with thermocouples, mounted in the pressure vessel and connected to the high and low temperature fluid circuits. Also the data-acquisition system was installed for real time measuring temperatures and acoustic power.  The complete test setup of the TAP is depicted below.

test_equipment1 TAP-300x225

At the construction location no 100 kW (waste) heat is available therefore a controllable heat source (9 kW electric oil heater) is used simulating the flue gas heat exchanger. This temporarily heat source allows for measuring static heat loss, onset temperature  and the increase of acoustic loop power with   temperature difference in the low power range.



Based on well known scaling rules the power levels in the TAP will be one order of magnitude less but the system will be thermoacoustically similar to the final system (using helium at 600 kPa) when filled with air at 240 kPa. This “similitude” allows for judging the performance of the TAP and for validating the simulations.

As a first result, the onset temperature difference  of the TAP was measured to be 29 K between the high temperature (49°C) thermal oil and the low temperature (20°C) water circuit which agrees well with the simulated values.

This is an encouraging result  because a low onset temperature is a key parameter and absolute requisite for successful operation of waste heat recovery systems. So the first test is passed and more test will follow.

Build of the 100 kW TAP started

Last week,  with the delivery of the pressure vessels and feedback loops at the assembling hall, the construction of the  100 kW TAP (Thermo Acoustic power) unit  is started.


The housing is build in two sections and will be filled with helium at 600 kPa. Because of the large volume the design, construction and testing of the pressure vessels takes a lot of effort and is done according to the PED 97/23/EC category IV requirements.

During the last months most parts and components like heat exchangers, regenerator, valves, pumps etc. are prepared, manufactured or ordered and are ready for assembling now.


After the build is completed the TAP will be tested functionally at low mean pressure  and low power first before transporting the system to the paper factory plant in the north of the Netherlands. There the TAP will be coupled to the flue gas heat exchanger which will be installed during the next production stop.

According to the plan we expect to have  TAP in full operation for testing by the end of May.

To be continued!

Pilot 100 kW TAP

Converting 100 kW waste heat at 130-150°C into electricity

This project is funded by the Dutch Economic Affairs within the framework of the Small Business Research Programme (SBIR).

Based on the four stage traveling wave concept, a 100 kWT thermoacoustic power (TAP) generator is under construction now.

For the TAP project Aster collaborate with Huisman Innovations B.V. ( and Innoforte ( Recently the first phase of this project which include also identification of component supplier and launching customers in industry is finished.

This project is carried out in the framework of phase two of the Dutch SBIR program. The 100 kW TAP will be installed at a paper manufacturing plant in the Netherlands for converting part of the flue gas at 130-150°C from the paper drying process into electricity. Emphasis in this project is on production and cost aspects lowering the investment per kWe to a level competitive to ORC’s. After successful completion of this pilot, commercialization and delivery of 100kWT to 1 MWT thermoacoustic power generators for industrial waste heat recovery and as add-on for CHP systems is planned to begin in 2012.