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)

Measuring acoustic power

Common practice in the thermoacoustic community for measuring acoustic velocity and from there acoustic power is the so called 2-microphone method. This method use the complex signals of two or more pressure sensors for reconstructing  forward and backward traveling waves and from there the acoustic power. This method however has some practical drawbacks.

  • a relatively large distance ( » 0.6 m) between both sensors
  • requiring an empiric correction term to compensate for loss between both sensors.
  • tube section between both sensors need to be uniform
  • lock-in amplifiers need to be used to get sufficient accuracy (<0.01°) for the phase measurements.

A practical and more robust method for measuring acoustic power in thermoacoustic systems is proposed based on measuring the acoustic pressure gradient in the gas rather than on reconstructing back and forward traveling waves. This makes the acoustic power measurement less sensitive for phase inaccuracies. In addition, the short mutual distance between the pressure sensors allows also for acoustic power measurements in short non-uniform tube sections like the inertance tube in thermoacoustic engines.

Details and how to calculated the acoustic power from the measured signals can be found in the article

Measuring acoustic power

 

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.

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).

rad_turb_3Drad_turb_printed

 

 

 

 

 

 

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)

axial_turb_rotor

axial_turb_gen

 

 

 

 

 

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

On the way to 50W electric output

Thermoacoustic electricity generation is still under development. The emphasis hereto is on  increasing power level and efficiency and in reducing the cost per Watt electric output power.

Early May, Aster together with the Nottingham Score team managed to get the demo2.2 Score thermoacoustic engine to produce 36 Watts electricity continuously, and 45 Watts peak. The working gas was air at an absolute pressure of nearly 200 kPa. A high power speaker was used as alternator. Its acoustic to electricity conversion efficiency is about 50% which means that the engine delivers 70-90 Watt net acoustic output power. Increasing the mean pressure up to 250 kPa and/or using a more efficient alternator will suffice to achieve the 50 Watt project target.

This result was obtained by modifying the acoustic circuitry toward a serial connection of two individual (acoustic) matched engine stages. This yield both a higher power density and a reduction of feedback tube length. The test set-up is shown below.

Score rig May7

The current (messy) shape of the tubing is for measuring purposes and for easy access. In a final version the tubing can be folded to fit into, or be part of, the wall of the stove.  Note the relatively short feedback tube length which allows for a higher frequency and proportionally higher electric output of the alternator.

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.

dPac_dT

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.