THEAC-25

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.

 

theac-schematics

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.

theac-testlab

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 3.5m x 3.5m x 0.6m.

theac-final-version

 

 

 

 

 

 

 

 

 

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.

The first tests showed an onset input temperature less than 50°C. Above this onset temperature, acoustic loop power and cooling power rise proportional with input temperature difference applied to the heat engine stages.

However, within the design and test temperature of 160°C applied,  a cooling power of 25kW is not reached yet due to the under performance of the heat exchangers (aluminium brazed finned tube) exchangers in combination with thermal oil, causing the TA process effectively runs at less than 120°C.

For that reason the commercial versions will make use of 2-phase heat transfer (looped heat pipe) reducing the temperature loss between heat source and thermoacoustic process drastically.

Production of the THEAC-25 by Soundenergy B.V. is planned to  start mid 2017 for  industrial waste heat recovery applications in industry, followed by solar heat driven cooling in 2018 for more southern countries.

>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

turbine-cross-section

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

turbine-open

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.

graph-impedance

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)

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.

demo_nf_web

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.

generator_front_web

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.