15th International Green Energy Conference (IGEC-XV)

A personal review of >40 years research and product development

On-line presentation held on July 13th 2023, Glasgow, UK at the  15th International Green Energy Conference


First roof top installation of a thermoacoustic chiller

Last Monday, the Sound Energy team, jointly with the installer and contractor Rouweler Installatietechniek Zelhem, installed their thermoacoustic cooler (THEAC) on the roof of the new school building IKC Magenta in Delden (Netherlands).This THEAC is a heat-powered heat pump intended to provide solar-assisted cooling to the building climate system. For that purpose, the THEAC cold output is connected via a fluid circuit to the cold buffer in the technical room.

THEAC installation4


The THEAC installed on this building is a version with increased safety for use on schools and public buildings. In this special version of the THEAC, solar energy (from the PV panels) is transferred via electricity to the internal heat exchangers of the THEAC, avoiding the use of hot liquids such as thermal oil or hot water / steam.

Read more on the soundenergy website


Soundenergy on CES

The Soundenergy team presented their vision and production model THEAC-25 at the CES in Las Vegas,


And this was the title  of the article in Forbes

“This Dutch Startup Converts Heat Into Cold Via A Stirling Engine, And Could Just Save The Planet”

What to say more?

If you want to know more? please check  www.soundenergy.nl


The THEAC-25 convert solar heat or industrial waste heat directly into cold for building, utility or process cooling based on thermoacoustic energy conversion as underlying technology.

Market introduction of the THEAC-25 will be performed by Soundenergy B.V. (soundenergy.nl an award winning start-up established in 2015.  Aster is co-founder and shareholder and is taking care of the R&D activities for Soundenergy B.V.  This year focus was on the build and test of de pre-production prototype, providing up to 25kW of cooling power when solar or waste heat with a temperature the range 100-200°C is applied.

The THEAC-25 is based on Aster’s multi-stage traveling wave technology and is assembled from two thermoacoustic heat engine stages and two thermoacoustic heat pump stages sharing an common acoustic traveling wave resonance and feedback loop.

From the application point of view, the THEAC is a waste or solar heat driven heat pump, requiring three external circuits to be connected to the application. For one of the possible applications, which is solar heat powered cooling of a building in combination with vacuum tube solar collectors as heat source, this is depicted schematically below.


Connection diagram 2

The dashed box represent the hardware and functionality of the THEAC-25. The actual prototype of a heat driven 25kW cooling system currently under test is shown below.

Prototype BTC

In this test rig all pressure vessels, containing the thermoacoustic core (heat exchangers and regenerator), are equipped with flanges and vessels are positioned vertically for reason of simple access and experimenting.

In the commercial version these flanges are obsolete and removed for saving weight and cost. Furthermore all vessels will be positioned horizontally by default to reduce system height. Finally  the THEAC-25 will fit into a box of 4.5m x 4.5m x 0.6m.


This commercial version will be pre-assembled in a frame, including all internal piping and subsystems, providing the customer with the three connections for heat supply, heat sink and cold.

Production of the THEAC-25 by Soundenergy B.V. is planned to  start early 2018 for  solar heat driven cooling in and waste heat recovery applications in industry.

>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%.

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   https://gereports.ca/ghg-ecomagination-innovation-challenge-winners.

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 (www.score.uk.com) and by initiatives supported  by the  FACT Foundation in the Netherlands (www.fact-foundation.com). 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)