Progress made on bi-directional turbines

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)

Aster Thermoacoustics announced as one of the winners of the GE Ecomagination Innovation Challence

During a ceremony held on February 3th 2015 at the GE innovation Centre in Calgary,  Aster Thermoacoustics was announced as one of four winners of the first phase of General Electric’s two phase  “Ecomagination Innovation Challenge” with winning submissions addressing alternative uses for waste heat from SAGD (Steam Assisted Gravity Drainage), ultimately reducing greenhouse gas(GHG) emissions in Canadian oil sands.

in Calgary, Alberta, February 3, 2015. Photograph by Todd Korol               in Calgary, Alberta, February 3, 2015. Photograph by Todd Korol

Image credits: Todd Korol

During the ceremony  Kees de Blok received the challenge award from Elyse Allan, President and CEO of GE Canada and John Rice, vice chairman of GE.

Beside a cash prize of $25,000 Aster Thermoacoustics and the other winners get an offer for another $100,000 in co-development funding to bring their idea’s to application in the oils sands.

Details and background of the challenge and of the other winners may be found on

The  event also get large attention of the press as can be read for example in this copy from an article in the Daily Oil Bulletin and links to the COSIA newsletter and Benelux press release

Bi-directional turbines for converting acoustic wave power into electricity

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


Waste or combustion heat driven gas liquefaction

The concept of thermoacoustic heat driven gas liquefaction has been brought back on  stage by a joint experiment of Qdrive and Aster-Thermoacoustics, combining a high performance pulse tube and a multi-stage traveling wave feedback thermoacoustic engine.

At a cold head temperature of -160°C the engine input temperature was reduced from 900°C down to less than 300°C which allows for extracting more heat from the combustion, prior to exhausting,  and for the use of ordinary construction materials.

While initially the concept was intended for liquefaction of natural gas from oil wells, nowadays small and medium scale liquefaction of locally produced biogas for storage and transportation  could be the main application.

The results of the experiments were presented on the “Second Workshop on Thermoacoustics” held on May 23 -25, 2014, at the Sendai  Tohoku university in Japan.

The abstract and presentation may be found by the links below

Thermoacoustic heat driven cooling

Heat driven cooling(1)

Blue print for a simple, reliable and low cost heat driven thermoacoustic generators for rural areas producing 20-50W electric power.

Thermoacoustics is a conversion technology in which the compression, expansion and displacement of the working gas is driven by acoustic wave motion rather than by pistons, valves  and displacers. Using acoustic wave motion eliminates mechanical friction and wear and therefore drastically increase lifespan and minimize maintenance. Because of the lack of moving mechanical parts in the thermodynamic process the construction tolerances and material requirements are relaxed allowing for (potential) low production and investment costs. These properties makes thermoacoustics not only a second generation energy conversion technology but also a candidate for low cost, small scale conversion technology for rural and developing areas.

These opportunities were recognized in, for example, the Score project in England ( and by initiatives supported  by the  FACT Foundation in the Netherlands ( The aim of these projects, started a few years ago, is to develop low cost thermoacoustic devices generating electricity combined with wood stoves for cooking or heating water.  These projects also address the social and economic aspects involving charities and local communities. This type of devices could  contribute to improving local living conditions by the use of small scale air operated multi-purpose devices for preparing hot water, cooking and generating some electricity. It could also stimulate  labour related to local production installation and maintenance of these devices.

The document below describes the design approach and underlying physics of a pre-production version of a small scale near atmospheric air operated  thermoacoustic generator and provide a blueprint for the further (local) development and production of such generators in and for rural areas and developing countries.  To make this happen, background information and construction drawings are available from Aster as input for local projects, students, scientist and construction companies who will intend to do experimenting and furthering this technology.

Design and build of a 50W thermacoustic generator(2)

Solar powered thermoacoustic cooling

Solar powered thermoacoustic cooling is presumably one of  the first commercial application of thermoacoustics on short term. To day onset and operating temperatures of multi-stage traveling wave thermoacoustic engines are at such a level that heat from vacuum tube solar collectors (120°C-160 °C) can be utilized effectively to power thermoacoustic (heat driven) heat pumps for cooling in domestic and rural applications. Main assets of the concept are the lack of environmental issues, absence of mechanical moving parts and the linear relation between cooling power and solar irradiation.

Market introduction is in preparation by a joint venture together with Solar collector manufacturer Watt Sp. z o.o and Thermo Acoustic Solutions Sp. z o.o both established in Poland. One of the activities within the framework of this collaboration is the build of two prototypes. (1) a representative prototype equipped with fluid-gas heat exchanger for testing in combination with vacuum tube collectors under realistic conditions and (2) a transportable device  for demonstration purposes. The demonstration prototype is completed by last week and is depicted below.


For simplicity this demonstrator operates without any fluid circuit and is externally powered by cartridge heaters keeping the hot heat exchanger at 160ºC (simulating  input heat from the vacuum tube collectors) and is cooled by forced convection keeping the cold heat exchanger around 40ºC, which both are representatives temperatures for our applications.

Ice_webTo allow visual inspection of the construction, no isolation is applied at all. Nevertheless the temperature lift of the cooler section is  more than 40ºC and ice is formed quickly at the refrigerator cold heat exchanger as is shown at the left.

The working gas in the demonstration set-up is helium at a mean pressure of 1 MPa, the pressure amplitude is 35 kPa and the oscillation frequency 138 Hz.  Net cooling power at -5ºC is about 50W at a net heat input of 380W at 160ºC. After correction for the absence of any isolation the exegetic efficiency of both the engine section and the refrigerator section is found to be in the order of 0.35 .

Demonstration of the thermoacoustic water heater-generator

At the “Bioenergy Innovaton Program 2012 partner day”organized by the FACT foundation and held last week in the Netherlands, Aster demonstrated a functional prototype of the combined water heater and thermoacoustic generator for use in rural area’s. This thermoacoustic generator utilize the temperature difference between an arbitrary heat source (wood, gas) and the water to be heated for generating electricity. This is shown schematically below.

FACT principe

The single stage thermoacoustic engine is constructed coaxially with the high temperature heat exchanger positioned at the lower end for thermal contact with the heat source. The low temperature heat exchanger actually becomes part of the bottom of the water tank. A potential cost reduction in this concept is the use of a small bi-directional impulse turbine equipped with standard rotating generator for the conversion of acoustic power into electricity (see previous posts).

FACT demo 27 novFACT details

The left picture shows the thermoacoustic section.  This unit will be placed on the bottom of the water tank in such a way that the hot hex protrude the bottom for interacting with the heat source beneath the tank. In the final version the whole thermoacoustic device is immersed in the water (not shown on this picture) for keeping the cold hex temperature low without the need for a circulation pump. The demonstration was performed with air at atmospheric pressure which allows for visual observation of the turbine rotor. In this setting only 5 W electricity was generated for powering the led lights.

Normal operation is with compressed air at 2-3 barg at which 50W electric output is aimed. In order to make this become true we are working now on an improved bi-directional turbine which can be pressurized as well. An option for further increasing output power or reducing the dimensions of the acoustic tubing is to apply a 2-stage thermoacoustic unit.

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.

Acoustic loss, a key factor in overall performance of TA systems

Since the early days of thermoacoustics, research en development had its focus on the thermodynamic process in the regenerator and heat exchangers. Thanks to this effort, nowadays the thermoacoustic process in itself is well understood and many publications can be found reporting thermoacoustic engines and heat pumps showing exegetic efficiencies over 40%.

However, overall or integral system performance, defined as the ratio between acoustic power delivered to a useful load and engine thermal input power is still far from that. In most experiments published, at least about one third of the net engine output power is dissipated in the acoustic circuitry or resonator and therefore is not available to the load, consequently degrading the overall performance proportionally.

Read more about this  in the paper and presentation given on the ICSV19 in Lithuania

Multi-stage traveling-wave feedback thermoacoustics in practice (Kees de Blok)

Presentation_Multi-stage traveling wave feedback