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Sound that Chills! – Thermoacoustics

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Juhajit Chakraborty Assoc, AIA, LEED AP WSP Built Ecology

Energy Efficient Building HVAC systems have been gaining grounds in the United States since the last decade. One of the areas that have achieved significant improvement is the refrigerant emissions. We all know that refrigerants contribute greatly to global warming. A project cannot be LEED certified if they use CFC based refrigerants. Almost all state and local building codes have banned CFC use and mandates use of only low ozone depleting potential (ODP) and global warming potential (GWP) refrigerants. But they still contribute to global warming. Refrigerants are an integral part of the HVAC process as they transport heat thru continuous compression and expansion cycles.

Xerox-owned Palo Alto Research Center (PARC) has developed a technique to enable sound or thermoacoustic cooling technology for air conditioning applications. In a nutshell, what they have been develop-ing is an air conditioner but it works like a loudspeaker. Interesting, isn’t it?

What is Thermoacoustics?
It is the interaction between temperature, density and pressure variations of acoustic waves. A thermoacoustic HVAC system compresses and expands gases with high intensity sound waves. When compressed, heat is generated because of the pressure applied and during expansion, cooling happens. More so like the chill we get when a carbon dioxide cartridge or canister is suddenly discharged and the gas is allowed to expand. The thermoacoustic system can potentially allow the compressor to complete 10,000 compression/expansion cycles, way more than a mechanical compressor system. Thermoacoustic compressors have been traditionally used mainly in labs and cryogenic cooling to turn on atmospheric gases like nitrogen into chilly liquids and therefore they usually perform best under extreme conditions and not efficient for commercial use for maintaining ambient temperatures (70°F -75°F) – this is what PARC has developed a prototype system, for ambient temperatures. With this technology hitting the com-mercial market, not only will result in zero emissions, but also it will save tremendously on energy. Cooling applications represent 25% of all electricity use in the United States, consuming over seven quadrillion BTUs of energy and generating nearly 600 million metric tons of CO2 emissions annually. If widely used, this prototype technology can save up to 13% reduction in energy consumption in US, double the efficiency of air conditioning and reduce CO2 emissions by 311 million metric tons annually.

How does it work?
Gases are first inserted into a tube filled with mesh membranes which are called regenerators. As the sound wave passes through the regenerators, a low-pressure, low-temperature/high-pressure, high-temperature gradient begins to form. One end gets hot while the other gets cool. Heat exchangers are then used to extract and exploit the cooling or the heating as required. The prototype’s increased efficiency comes from the electromechanical couplings generated in the “transducers” (device that converts variations – in this case pressure and temperature) through the interconnected thermoacoustic chambers in series such that unused energy is efficiently passed from one chamber to another. The image below shows a basic system (left) which is used in labs and the improvised prototype developed by PARC for ambient temperatures (below).

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