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.
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.