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.

Effect of temperature on TAP output power

Performance of the TAP or any other low temperature heat driven thermo acoustic system like the thermo acoustic solar cooler depends heavily on the available input temperature and the temperature at which heat can be rejected. This is due to the second law of thermodynamics, which state that the theoretical amount of heat that can be converted is proportional with the temperature difference supplied.

To show this dependency an simulation is made for the TAP.

ta power

With the left side slider input temperature can be set between ambient and 160°C. Above about 60°C oscillation will start and at 85°C temperature the acoustic power level is high enough to switch on the counterbalanced alternators in the middle for converting acoustic output power of each engine stage into electricity. Full electric output power is reached at 160°C heat input temperature.

Asterdemo  (seperate window).

Acoustics08, Paris, France

Low operating temperature integral thermo acoustic devices for solar cooling and waste heat recovery

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 field of applications for thermo acoustic systems.At these reduced temperatures overall system performance is increasingly affected by the ratio between amplified (useful) power and acoustic power in the resonance circuitry.Well known is that this ratio can be improved by deploying multiple regenerator units (hex-reg-hex).However, in commonly used torus or coaxial bypass configurations the correct timing (real and high acoustic impedance) is hardly to realize inside more than two regenerator units (soft spot).Acoustic losses in the standing wave resonator together with a high regenerator impedance account for another fundamental limitation because of the relatively low transferred and loop power at a given pressure amplitude.Therefore a novel acoustic geometry will be presented in which a near real impedance can be maintained in even more than two regenerator units and in which acoustic feedback is performed by a near traveling wave.This approach improves the overall performance of low operating temperature thermo acoustic systems.

Read the full article

Low operating temperature integral thermoacoustic devices for solar cooling and waste heat recovery

or view the presentation

Acoustics08_kees_de_blok

Patent list

C.M. de Blok. “Multistage traveling wave thermoacoustic engine with phase distributed power extraction”. PCT/NL2010/050057

C.M. de Blok. “Meertraps thermoakoestische warmtemotor met fase gedistribueerde vermogensuitkoppeling”, 2009, NL 1036630

C.M. de Blok. “Multi stage heat driven thermoacoustic engine” . PCT/NL2004/000405, WO2005/001269 A1

C.M. de Blok, N.A.H.J. van Rijt. Thermo acoustic system, 1997, WO 99/20957, US 6,314,740 B1.

C.M. de Blok, N.A.H.J. van Rijt. “Device for utilizing heat via via conversion into mechanical energy, in particular cooling device”. PCT/NL89/00012, WO1989NL0000012

Measuring technology

In the beginning we at Aster realized that representing and measuring acoustic components was essential to understand the physics and to be able to develop and optimize the  acoustic configuration and interaction of thermoacoustic engines and  heat pumps or other loads like linear alternators.

The first article describes the representation of acoustic components based on N-ports and scattering (S) parameters.  The second paper deals with the design and construction of direct reading 1-port acoustic network analyzer for measuring complex acoustic reflection in the range of 1-100 Hz.

Characterization of acoustic networks

Acoustic network analyzer