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Record Efficiency in Conversion of Heat to Sound

Researchers from the Energy research Centre of the Netherlands (ECN) have achieved a record efficiency in the conversion of heat to sound. In a so-called thermo-acoustic engine they have improved on the existing record of 41% to reach 48% of the maximum possible efficiency.

In a thermo-acoustic engine, as in every other engine, heat is converted into mechanical power. In this case, the mechanical power consists of an extremely powerful sound wave. A process is used that is similar in many ways to a Stirling cycle. Other than in a Stirling engine, though, no moving parts are used. In a thermo-acoustic engine, the sound wave controls the compression, displacement and expansion of the working medium helium. The working medium thereby undergoes a cycle that amplifies the sound wave.

The thermo-acoustic engine is part of a complete system that includes a thermo-acoustic heat pump as well as an engine. This heat pump uses the sound produced by the engine, via a reverse process, to pump heat from a low to a higher temperature level.

Substantial saving in industrial energy consumption

ECN is working with two Dutch equipment manufacturers, Bronswerk and Dahlman, on the development of a thermo-acoustic system in which industrial waste heat is used to power an engine. The sound that is produced is then used to upgrade industrial waste heat to usable heat in a heat pump. In this way, part of the useless waste heat can be usefully recycled in the industrial process. This can lead to substantial savings in industrial energy consumption.

The record that has been achieved is an important milestone on the way to the conversion yields necessary for a cost-effective system. It was well above the target yield of 30% that had been set for the engine. The present engine is driven by heat at high temperatures (500-600°C). The challenge now is to use the knowledge acquired to extrapolate this performance to engines that operate at much lower temperatures (100-150°C).

The current developments are aimed at testing an integral system in 2010 that delivers the required performance under relevant industrial conditions, although in a laboratory environment. Once this step has been completed successfully, the way will be open for further upscaling and carrying out field tests in the industrial situation.

The research described above was carried out with the help of financing from the SenterNovem programme for long-term energy research subsidies.

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