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.

Low temperature heat driven refrigerator

Within the framework of the THATEA project (European joint project FP7-FET) the multi-stage travelling wave engine designed and build by Aster is successfully integrated with the thermoacoustic part of the refrigerator designed and build by the French project partners Hekyom and CNRS.

The integrated system is similar to the low temperature 4-stage engine reported in an earlier post (07-11-2010) in which one of the engine stages is replaced now by the refrigerator cell. The result is a 3-stage thermoacoustic engine driving a single stage thermoacoustic refrigerator. Mutual distance between all stages equals ¼ λ yielding inherent acoustic matching.  When the 3-stage engine is powered by thermal oil at an input temperature of 211°C the cold hex temperature of the refrigerator reach -40.5°C. Cold hex cooling power is 95W.  At this temperature ice is formed rapidly on the non-isolated parts.

ice on cooler(2)

Efficiency of the thermoacoustic engine and cooler, relative to the Carnotfactors is respectively 34% and 29%. These values are measured using helium at a mean pressure of 2.7 MPa and at a drive ratio of 1.53%. In the current set-up the drive ratio or pressure amplitude is currently limited by the maximum temperature of the heat source and by the more than 40°C temperature drop across the low cost heat exchangers used in the engine stages. Reducing temperature drop is a key issue in low temperature driven thermoacoustic systems. New heat exchangers with a more close fin spacing will halve the temperature drop and improve the efficiency up to 40%. Improvement of the current refrigerator stage is expected from adapting the regenerator material.

Solar powered cooler

Aster has the intention, and has already made a start to further develop this configuration towards a solar powered cooler as add-on for vacuum tube based  solar heating systems. The output temperature of this collector type is up to 160 °C which is sufficient for powering a multi-stage thermoacoustic engine. Since the first experiments in 2004 current prices of vacuum tube collectors are reduced now by nearly one order of magnitude. Based on this developments and recent improvements in thermoacoustic the estimated return of investment now will be into the range of 5 to 8 years.  For this project Aster is working together with a Polish investment company and a manufacturer of vacuum tube collectors. First prototype and demonstration is planned for summer 2012.

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.

Hex_SKSB

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.

TAP_work1

 

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.

Arrival-TAP-300x225

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.

Components-TAP-300x225

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!

First wood fired thermoacoustic generator

On November 23th 2010, the Score team and Aster successfully tested a wood fired thermoacoustic engine with integral alternator at City University London.

Aster in collaboration with Nottingham University developed and built a planar traveling wave 2-stage thermoacoustic engine. City University London designed and built the prototype wood stove, the two were successfully integrated and power was generated on the first run. Score also includes the University of Manchester, Queen Mary University of London and the charity Practical Action.

Score 21 nov prototype A131-300x223

Score (wood fire) A26-300x225

The aim of this test was to bring all together and to demonstrate;

  • The simple construction based on the idea of a finned or corrugated back plate
  • Ease of integrating the planar design with a stove
  • Heat transfer by convection rather than by radiation between stove and regenerator
  • The rapid increase of performance at increasing mean pressure
  • Feedback by near traveling waves in order to end up with the smallest possible feedback loop.

When pressurized to 0.5 barg (150 kPa) the highest electric output measured at 350 °C regenerator temperature difference was 23 W using a standard loudspeaker as linear alternator. The measured acoustic to electric conversion efficiency of this speaker however is only 35%. A better alternator of 60% efficiency is under construction by the Nottingham University. When available, this alternator in the same configuration raise electric output to nearly 40 W. Steady state running achieved 7.5 Watts to charge a 12v Battery with atmospheric pressure acoustics.

For more information about Score see: www.score.uk.com

The team

TeamHeelRD

Record onset temperature

Record onset temperature difference of 31 K

Within the framework of the THATEA project (European joint project FP7-FET) Aster has designed, build and test a multistage thermoacoustic engine for low input temperature operation.

Heat is supplied and removed by the internal gas-fluid heat exchangers connected to a high and low temperature water circuit. Oscillation start when the high temperature water circuit reach 51°C and the low temperature water circuit is at 20 °C.  The setup for testing this four stage traveling wave engine is shown below.THATEA 4-steg (a8)

Brief explanation

At declining temperatures thermoacoustic engines become increasingly more sensitive to imperfections such as heat exchanger temperature drop, acoustic impedance matching and dissipation. Thermoacoustic power gain is proportional with the operating temperature consequently leading to less gain at abating temperatures. Because the thermoacoustic engine is a power amplifier loop power has to be increased at lower gain to maintain the same net output power. Higher loop power however will result in higher acoustic loss. Summarizing, these two effects will increase acoustic loop power (higher loss) and reduced gain and reinforce each other in a negative sense seriously degrading overall system performance at declining operating temperatures.

One way to overcome this problem, and to allow for efficient operation at declining temperatures (< 200°C), is to increase the (thermo)acoustic power gain by using multiple regenerator units (regenerator clamped between two heat exchangers). This is not as straight forward as it seems because of the thermal and acoustic interaction between the regenerator units. In the common torus geometry, as for example used in the high temperature engine, a high and near real acoustic impedance, required to drive the thermodynamic cycle properly, can not be maintained in more than two regenerator units at the same time. In addition the standing wave resonator which is part of this configuration shows relatively high acoustic losses which becomes increasingly harmful at declining operating temperatures.

For low temperature operation therefore, a novel acoustic resonance and feedback circuit is implemented which allows for inserting an arbitrary number of regenerator units while maintaining the optimal high and real impedance in each stage

The low temperature engine is build up from four identical regenerator units which are connected acoustically in series by near traveling wave loop sections and connected thermally in parallel.

Low cost aluminum brazed louvered fin heat exchangers are used to supply and remove heat from the thermoacoustic process in the regenerator. The low temperature heat exchangers are connected to a car radiator plus fan. The high temperature heat exchangers are connected to a dedicated gas fired water heater to simulate a waste or solar heat source.  Helium at 3.1 MPa bar is used as working fluid.

2010 ASME, Montreal, Canada

Novel 4-stage Traveling Wave Thermoacoustic Power Generators

Abstract

Utilizing low temperature differences from solar vacuum tube collectors or waste heat in the range 70-200 °C seems to be the most promising and commercial interesting field of applications for thermoacoustic systems. Recently a novel 4-stage “self matching” traveling wave engine is developed and tested. Beside the low acoustic loss and compactness, due to traveling wave feedback, all components per stage are identical which is beneficial from (mass) production point of view. Based on this concept a 100 kWT thermoacoustic power (TAP) generator is under construction. This project is carried out in the framework of phase two of the Dutch SBIR program. The 100 kWT TAP will be installed at a paper manufacturing plant in the Netherlands for converting part of the flue gas at 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 100kW to 1 MW thermoacoustic power generators for industrial waste heat recovery and as add-on for CHP systems is planned to begin in 2012. The same concept of the 4-stage traveling wave engine is also implemented in an atmospheric pressure operated thermoacoustic cooking device for developing countries which generate beside hot water up to 50 W electricity. Details, ongoing work and experimental results of these projects will be presented.

Read the full article:

Novel 4-stage thermoacoustic power generators

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. (www.huisman-elektro.nl) and Innoforte (www.innoforte.nl). 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.