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.