A muffler that suppresses low frequencies: indispensable for the energy transition

Twenty percent of the European population is exposed to noise that is harmful to their health for long periods of time. That problem is only threatening to get worse in the world of the new, green energy system. As it happens, low-carbon systems, such as the heat pump, produce more noise than the boilers we have in our homes at present because of the fan on the outdoor unit.

Lean combustion processes

Future low-carbon systems like gas boilers and hydrogen boilers need to be safe with less emissions, explains Mohammad Kojourimanesh, PhD candidate in mechanical engineering at the Eindhoven University of Technology (TU/e). “To minimize the release of nitrogen oxides, the flame temperature is kept as low as possible in new combustion systems. You then get lean combustion processes that way, which are accompanied by an increase in the likelihood of thermoacoustic instability. The unique combustion properties of hydrogen amplify this effect, resulting in a higher level of noise (above 120 dB) and vibrations.”

Because of the thermoacoustic instability, sound does not always have the same frequencies and these can be shifted by varying the parameters. Current mufflers can only suppress a very limited range of frequencies. Therefore, multiple mufflers are now often used in a system. Moreover, those mufflers are large in size. In short, the industry has problems designing combustion systems that can include silencers.

ABC muffler

Consequently, there is a need for compact, robust mufflers that can suppress the sound of appliances such as boilers, heat pumps and air conditioners. Kojourimanesh is about to enter the market with a silencer that does exactly that. Although the name of the Anechoic Broadband Compact (ABC) attenuator is quite complicated, when it comes to its construction, the attenuator is “simple and cheap,” the PhD student promises.

The muffler that Kojourimanesh designed

The system comprises a stack of plates separated by thin spacers. When the space between the plates is adjusted correctly, the air viscosity and thermal losses in the narrow channels are able to absorb noises. A major advantage is that the damper can be adjusted to all frequencies between 20 and 800 Hz. “The amount of acoustic wave reflection from a material is called the acoustic reflection coefficient. The reflection coefficient of the ABC attenuator is less than 0.1, which means the absorption coefficient is 99 percent. Even at low frequencies, which are the most difficult sounds to absorb.”

Low frequencies, such as a bass tone, are difficult to absorb because they have long wavelengths.

Variety of applications

The ABC silencer is designed to work in a variety of devices. In addition to the noise reduction of heat pumps, air conditioners and hydrogen boilers, the damper is also suitable for reducing noise from motor vehicles and highways. The attenuator also enables more optimal use of devices. Kojourimanesh: “Take a wind turbine, for example. The noise limit for turbines is about forty decibels, depending on the location of the wind turbine. Suppose we can reduce the noise by two decibels with the muffler, then rotation of the wind turbines can be scaled up and up to twenty percent more energy can be gained.”

In the right place at the right time

With the grant in his pocket, Kojourimanesh has all the pieces of the puzzle falling into place. He has almost completed his PhD research and wants to bring his invention to the market as soon as possible.

The EU is also cooperating. For example, the Dutch government has drawn up new rules for the noise that heat pumps and air-conditioning units may produce, in response to the EU's 'Sustainable and Smart Mobility Strategy'. From 1 April 2021, the noise level of these devices may not exceed forty decibels. Companies must therefore take action. The entrepreneur-to-be is talking to companies that manufacture boilers for consumers and is about to officially register his company as a spin-off. He is not stressed, but he is enthusiastic. "I am very optimistic. I know it works and the options are endless."