Archive for november, 2011

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.


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.