Archive for the ‘ THATHEA ’ Category

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.

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.