On this page you may find an explanation of thermoacoustic energy conversion. Because of the lack of moving parts thermoacoustic systems has a large freedom of implementation ranging from efficient pressurized high power systems using helium as working gas, down to low cost systems build from simple materials and using (compressed) air as medium.
Before going into the theory first a short movie of an atmospheric pressure thermoacoustic power generator build from pvc tubing and using a loudspeaker for the conversion of acoustic power into electricity.
A simple atmospheric pressure thermoacoustic power generator
Explanation, efficiency and environmental aspects
The Thermoacoustic (TA) heat pump and the Thermoacoustic (TA) engine; together they form the TA system with several unique performances. The principle can be explained in a few clear steps. For the advanced readers additional information is added below the – Advanced reading – markers in italic typeface. As we hope the explanation suits a large audience. In case there would be any questions or remarks left, please do not hesitate to contact us by email (Contact).
The TA heat pump
Comparable to a fluid pump that transfers fluid from a low to a high level, a TA heat pump can be used to transfer heat from a low to a high temperature. This way heat can be removed from a fluid circuit having a temperature of e.g. 15°C to another circuit at 40°C, and raise its temperature to 60°C.
A Thermoacoustic heat pump can also be applied for cooling purposes, for example cooling a fluid from 20°C to -20°C. In a conventional pump fluid is transferred by means of mechanical moving parts. The need for such parts, however, is eliminated in a TA heat pump. Here heat transfer takes place by means of four invisible elements: pressure, sound, temperature and a gaseous medium such as air, argon or helium. When these elements interact in a specific way, in an optimised enclosure, a thermodynamic cycle occurs.
Pressure and temperature
The image to the right shows what happens to a single parcel during a thermodynamic cycle. When stimulated by an impulse or vibration the gas will be compressed. As a result the temperature of the parcel will increase. During the next phase, when the gas expands the temperature will decrease immediately. A compressed gas parcel tends to release its heat to the environment, while an expanding one will extract heat from its surroundings. These characteristics are essential to TA heat pumps because in this way heat can be withdrawn from one location in the system and deposited at another location.
– Advanced reading – Starting at the moment of minimum pressure (t=0) the gas will be compressed by the acoustic wave. In a travelling wave the displament of the gas is one quarter of a period behind the pressure amplitude. From this the gas at the start of the compression (t=0) is in the equilibrium (middle) position (Uo). During compression the gas moves to the left (-U). Because there is a maximal heat transfer (isothermal propagation) heat (Q1) is released to the regenerator left of the equilibrium position. In the second half of the cycle the opposite occurs. During expansion the gas moves to the right (+dU) of the equilibrium position locally extracting heat (Q2) from the regenerator. A complete cycle controlled by a travelling wave therefore includes compression and heat sink (Q1) at the left side (-dU) at a high temperature (T1) followed by expansion and heat extraction (Q2) at the right (+dU) at low temperature (T2).
Imagine a column of gas as a sequence of gas parcels. When at the left side of the column a single impulse or vibration is initiated the following occurs:
- The parcel moves from the left to the right and back. The speed of this movement is called the gas velocity. During this movement the gas will be compressed and expand once. The magnitude of this variation in pressure is the pressure amplitude;
- The temperature of the gas parcel will go up and down;
- The parcel transfers the impulse to the next parcel. The speed at which the impulse is transferred is called the speed of sound or propagation speed.
The last property causes a travelling (longitudinal) wave in the direction of propagation. The number of impulses per second is the operating frequencyand is expressed in Hertz (Hz).
The thermodynamic process is in fact controlled by the travelling wave. In order to minimise losses and create as powerful a wave as possible, an acoustic resonance circuit is required. This resonator can be compared with an organ pipe.
Frequency depends on its length, the longer the pipe the lower the frequency. Power depends on the cross sectional area. In addition, the resonator functions as the enclosure for the driver part, the TA engine (at the left) and the actual heat pump (at the right).
– Advanced reading – A column of gas moves forth and back in the narrow part of the device shown. As a result, the pressure at both ends varies in an opposite way, creating periodic compression and expansion of the gas. Owing to the particular shape of the enclosure and the two bypass tubes, the relation between pressure amplitude and gas velocity is set to such a value as to optimise the thermodynamic cycle in both the regenerator and heat exchangers. The function of the resonator can be compared with the flywheel in conventional engines. Acoustic power in the system is proportional to the mean pressureand the pressure amplitude squared. Nominal mean pressure ranges from 5 to 40 bars.
Both the driver part (the engine) and theactual heat pump are each equipped witht wo heat exchangers. These heat exchangers are manufactured from a thermally efficient conducting material (e.g. copper foam or fins) embedded with copper tubes. From outside the system a fluid is pumped through these tubes; this causes heat to be extracted from the fluid in one heat exchanger (cold side) while releasing its heat at the other (hot side).
The regenerator is clamped between the two heat exchangers. It is made from a porous material in which heat can be stored for a while, for example steel wool, metal gauze or metal foam. The pore diameter of this material (the regenerator) is about 0.1 mm. The regenerator acts as a thermal buffer between the hot and cold side. Without this buffer useful temperature differences could not exist.
– Advanced reading – During a part of the thermodynamic cycle the regenerator absorbs heat, and will release this heat during another part of the cycle. At an operating frequency of 50 Hz the available time for a complete cycle is only 0.02 seconds. This temporary storage allows a large difference in temperature between both heat exchangers. A low thermal conduction through the regenerator is prerequisite. Furthermore, flow resistance should be minimised to avoid excessive wave attenuation. A characteristic parameter of the regenerator is the thermal time constant. This is defined as the time needed by the gas to assume the ambient tempera- ture of the regenerator. If this time is much less than the time for a full cycle, the thermodynamic process becomes nearly reversible, which is a prerequisite for high efficiency.
The TA heat engine (driver)
In the thermoacoustic system the TA heat engine (driver) is located at the left, theactual heat pump at the right. To power the TA heat engine energy has to be supplied to the system. Heat between 120°C and 900°C is used depending on the application. This will create a large temperature difference between the heat exchangers of the engine and thereby induce a powerful (thermo) acoustic wave. This wave is then used to power the heat pump (at the right).
– Advanced reading – The temperature difference combined with the thermodynamic process in both the regenerator and heat exchangers causes amplification of the acoustic wave. This amplification is proportional with the temperature difference. At a certain temperature difference the acoustic amplification exceeds the inherent system losses, which in principle allows oscillation. The system is self-starting because of the ever-present (minimal) vibrations in the gas. If more heat is supplied than needed to maintain oscillation, the energy surplus can be extracted from the resonator as useful output power. This output power actually drives the heat pump.
Simple construction combined with an extended range in operating temperatures permits the Thermoacoustic (TA) heat pump to cost-effectively use energy that otherwise would be lost, or for example by the associated discharge of cooling-water would harm the environment. To day, efficiency of thermo acoustic energy conversion is demonstrated to be close to 50% of the Carnot factor. The TA heat engine or heat pump can at power levels ranging from a few hundred watts up to one megawatt. These characteristics enable applications not feasiblewith conventional systems for technicalor economic reasons.
Use of the TA heat pump has several important environmental benefits. These benefits result inpart from the construction and operation of theTA heat pump, and also from the possibility this system affords to make use of heat that would otherwise be wasted. This reduces the need forfossil fuels. Heat can also be reused from, for example, environmentally harmful cooling-water discharges. Heat from solar boilers and geothermal energy(heat storage in the ground) can also be usedmore efficiently. The TA heat pump is made from durable materials and, because of the absence of moving parts, has a particularly long life span. Lubrication is unnecessary, so there are no potentially polluting oils or grease.
– Advanced reading – In addition to a lowering of the (primary) use of energy, the use of the TA heat engine reduces emissions of NOx and CO2, and limits the discharges of cooling-water. Low value waste heat (80 – 200 °C) in industry is available in large quantities (ca. 300PJ/year with a concentration near 100 °C). The TA heat pump does not use any ozone-depleting substances (CFCs and HCFCs) or greenhouse gases (HFCs). A number of existing substances for cooling (CFCs) are in fact no longer allowed. This will alsobe the case for the use of HCFCs in about 10 years.