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THermoAcoustic Technology for Energy Applications

Final Report Summary - THATEA (THermoAcoustic Technology for Energy Applications)

Executive Summary:
The objective of the THATEA (THermoAcoustic Technology for Energy Applications) project is to advance the basic scientific and technological knowledge in the field of thermoacoustics. The project will assess the feasibility of thermoacoustic applications to achieve conversion efficiencies of:
- Heat to acoustic power of 40% of the maximum theoretical Carnot efficiency;
- Acoustic power to heating/cooling of 40% of the maximum theoretical Carnot efficiency.
In addition, integrated systems should be investigated that couple the separate components, resulting in high overall system efficiency. This project is the first thermoacoustic initiative on a European level. The partnership consists of: two SME’s (Aster, Hekyom), two research institutes (ECN, NRG) and three leading academic groups (CNRS, UNIMAN, UNIME).
A thermoacoustic system usually comprises an engine (producing acoustic power from heat), a heat pump (uses acoustic power to pump heat) and a resonator containing the engine and heat pump. The core of both engine and heat pump consist of a regenerator sandwiched between two heat exchangers. The main findings with respect to the different activities is summarized below.
Experiments were carried out to measure these heat transfer rate, pressure drop, and thermal conductivity for different regenerator materials. The conclusions that the friction factor results show good agreement with correlations from literature’s and that the heat conduction through the regenerator material is overestimated when using the commonly used value of 0.15 as the thermal conductivity degradation factor.
Two different types of thermoacoustic engines have been constructed and tested. Both engines generate acoustic power from heat. The difference between the two engines concerns the driving temperature. The high-temperature engine uses heated air up to 800°C to simulate flue gasses from a burner, while the low-temperature engine uses heat up to 200°C as heat supply. The high-temperature engine achieved an efficiency of 41.5 % of the Carnot efficiency, thereby exceeding the target performance. The low-temperature engine showed an efficiency of 33 % of the Carnot efficiency with options identified fur further improvement.
Heat pumps
Two different types of thermoacoustic heat pumping devices have been constructed and tested. These two differ with respect to the temperature level on which they operate. One device pumps heat from 10°C to 80°C while the second acts as an refrigerator and pumps heat from -40°C to ambient temperature. The first device achieved 36 % of Carnot in the component testing but 40 % of Carnot as part of the integral system. The refrigerator showed a maximum performance of 33 % of Carnot. Noteworthy is that both components showed measured efficiencies much higher than previously measured worldwide.
The scaling analysis shows that the linear approximation used for the design modelling of thermoacoustic systems functions very well. Heat losses by convection, conduction and radiation need careful attention since these are not adequately covered by the present modelling. In addition, more practical aspects with respect to scaling to larger powers were addressed. Most important issues are the construction of the heat exchangers, the manufacturability of the heat exchangers and possible more-dimensional effects.
Heat exchangers
Detailed measurements on heat exchange in oscillating flow conditions combined with numerical simulations have led to design rules (non-dimensional correlations) for thermoacoustic heat exchangers. These design rules relate to the length of the heat exchanger and the fin spacing. The practical design of heat exchangers has to be improved to arrive at the situation where the heat transfer is optimal, combined with low costs of manufacturing.
Two alternatives for the standard acoustic resonator were explored in this project. The first concept uses a traveling wave loop in combination with a multiple regenerator units. This concept runs at a low pressure amplitude (ensuring low losses) and has an inherent proper timing for each unit. The second concept replaces the acoustic resonator by a mechanical massspring system. The mechanical mass-spring could not be tested as an integral system but the component testing showed promising (low loss) results. The main conclusion is that the alignment of the cylinder in the piston is a very critical issue.
Non-linear phenomena
An experiment setup was used that is capable of measuring acoustic streaming inside the resonator. The measurement results were used to validate a Computational Fluid Dynamics (CFD) model. The qualitative agreement was reasonable but the quantitative agreement should be improved. CFD modeling proved to be a useful tool to study the time-independent phenomena that occur in oscillatory flow conditions. A thorough understanding of these phenomena is needed to identify countermeasures that stop streaming. This subject definitely requires further research.
Integral systems
Two integral systems have successfully been designed and taken into operation. Both systems show the required functionality. Both systems also show that the targeted performance is not reached. They reach about 60 % of the objective. While at the component level the efficiencies are 75 % or higher (even to 100 %) of the target, the multiplication of the efficiencies of the individual components leads to this overall result. The requirements for the optimal performance of the individual components do not always match with each other. For example, a high drive ratio leads to high useful powers compared to the heat losses and thus a high efficiency for an engine but at the same time a high drive ratio leads to high acoustic losses in the resonator. Several improvement options have been identified which should guide the way to a more efficient integral system.

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