Thermoacoustic engines (sometimes called "TA engines") are thermoacoustic devices which use high-amplitude sound waves to pump heat from one place to another (this requires work, which is provided by the loudspeaker) or use a heat difference to produce work in the form of sound waves (these waves can then be converted into electrical current the same way as a microphone does).
These devices can be designed to use either a standing wave or a travelling wave.
Compared to vapor refrigerators, thermoacoustic refrigerators have no coolant and few moving parts (only the loudspeaker), therefore require no dynamic sealing or lubrication. [1]
The ability of heat to produce sound was noted by glassblowers centuries ago. [2]
In the 1850s experiments showed that a temperature differential drove the phenomenon, and that acoustic volume and intensity vary with tube length and bulb size.
Rijke demonstrated that adding a heated wire screen a quarter of the way up the tube greatly magnified the sound, supplying energy to the air in the tube at its point of greatest pressure. Further experiments showed that cooling the air at its points of minimal pressure produced a similar amplifying effect. [2] A Rijke tube converts heat into acoustic energy, [3] using natural convection.
In about 1887, Lord Rayleigh discussed the possibility of pumping heat with sound.
In 1969, Rott reopened the topic. [4] Using the Navier-Stokes equations for fluids, he derived equations specific for thermoacoustics. [5]
Linear thermoacoustic models were developed to form a basic quantitative understanding, and numeric models for computation.
Swift continued with these equations, deriving expressions for the acoustic power in thermoacoustic devices. [6]
In 1992 a similar thermoacoustic refrigeration device was used on Space Shuttle Discovery. [2]
Orest Symko at University of Utah began a research project in 2005 called Thermal Acoustic Piezo Energy Conversion (TAPEC). [7]
Niche applications such as small to medium scale cryogenic applications. Score Ltd. was awarded £2M in March 2007 to research a cooking stove that also delivers electricity and cooling for use in developing countries. [8] [9]
A radioisotope-heated thermoacoustic system was proposed and prototyped for deep space exploration missions by Airbus. The system has slight theoretical advantages over other generator systems like existing thermocouple based systems, or a proposed Stirling engine used in ASRG prototype. [10]
SoundEnergy developed the THEAC system that turns heat, typically waste heat or solar heat into cooling with no other power source. The device uses argon gas. The device amplifies sound created by the waste heat, converts the resulting pressure back into another heat differential and uses a Stirling cycle to produce the cooling effect. [2]
A thermoacoustic device takes advantages of the fact that in a sound wave parcels of gas adiabatically alternatively compress and expand, and pressure and temperature change simultaneously; when pressure reaches a maximum or minimum, so does the temperature. It basically consists of heat exchangers, a resonator and a stack (on standing wave devices) or regenerator (on travelling wave devices). Depending on the type of engine a driver or loudspeaker might be used to generate sound waves.
In a tube closed at both ends, interference can occur between two waves traveling in opposite directions at certain frequencies. The interference causes resonance and creates a standing wave. The stack consists of small parallel channels. When the stack is placed at a certain location in the resonator having a standing wave, a temperature differential develops across the stack. By placing heat exchangers at each side of the stack, heat can be moved. The opposite is possible as well: a temperature difference across the stack produces a sound wave. The first example is a heat pump, while the second is a prime mover.
Creating or moving heat from a cold to a warm reservoir requires work. Acoustic power provides this work. The stack creates a pressure drop. Interference between the incoming and reflected acoustic waves is now imperfect. The difference in amplitude causes the standing wave to travel, giving the wave acoustic power.
Heat pumping along a stack in a standing wave device follows the Brayton cycle.
A counter-clockwise Brayton cycle for a refrigerator consists of four processes that affect a parcel of gas between two plates of a stack.
Travelling wave devices can be described using the Stirling cycle.
Engines and heat pumps both typically use stacks and heat exchangers. The boundary between a prime mover and heat pump is given by the temperature gradient operator, which is the mean temperature gradient divided by the critical temperature gradient.
The mean temperature gradient is the temperature difference across the stack divided by the length of the stack.
The critical temperature gradient is a value that depends on characteristics of the device such as frequency, cross-sectional area and gas properties.
If the temperature gradient operator exceeds one, the mean temperature gradient is larger than the critical temperature gradient and the stack operates as a prime mover. If the temperature gradient operator is less than one, the mean temperature gradient is smaller than the critical gradient and the stack operates as a heat pump.
In thermodynamics the highest achievable efficiency is the Carnot efficiency. The efficiency of thermoacoustic engines can be compared to Carnot efficiency using the temperature gradient operator.
The efficiency of a thermoacoustic engine is given by
The coefficient of performance of a thermoacoustic heat pump is given by
The most efficient thermoacoustic devices have an efficiency approaching 40% of the Carnot limit, or about 20% to 30% overall (depending on the heat engine temperatures). [11]
Higher hot-end temperatures may be possible with thermoacoustic devices because they have no moving parts, thus allowing the Carnot efficiency to be higher. This may partially offset their lower efficiency, compared to conventional heat engines, as a percentage of Carnot.
The ideal Stirling cycle, approximated by traveling wave devices, is inherently more efficient than the ideal Brayton cycle, approximated by standing wave devices. However, the narrower pores required to give good thermal contact in a travelling wave device, as compared to a standing wave stack which requires deliberately imperfect thermal contact, also gives rise to greater frictional losses, reducing practical efficiency. The toroidal geometry often used in traveling wave devices, but not required for standing wave devices, can also boost losses due to Gedeon streaming around the loop.[ further explanation needed ]
A Carnot heat engine is a theoretical heat engine that operates on the Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824. The Carnot engine model was graphically expanded by Benoît Paul Émile Clapeyron in 1834 and mathematically explored by Rudolf Clausius in 1857, work that led to the fundamental thermodynamic concept of entropy. The Carnot engine is the most efficient heat engine which is theoretically possible. The efficiency depends only upon the absolute temperatures of the hot and cold heat reservoirs between which it operates.
An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.
A heat engine is a system that converts heat to usable energy, particularly mechanical energy, which can then be used to do mechanical work. While originally conceived in the context of mechanical energy, the concept of the heat engine has been applied to various other kinds of energy, particularly electrical, since at least the late 19th century. The heat engine does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat source generates thermal energy that brings the working substance to the higher temperature state. The working substance generates work in the working body of the engine while transferring heat to the colder sink until it reaches a lower temperature state. During this process some of the thermal energy is converted into work by exploiting the properties of the working substance. The working substance can be any system with a non-zero heat capacity, but it usually is a gas or liquid. During this process, some heat is normally lost to the surroundings and is not converted to work. Also, some energy is unusable because of friction and drag.
A Stirling engine is a heat engine that is operated by the cyclic expansion and contraction of air or other gas by exposing it to different temperatures, resulting in a net conversion of heat energy to mechanical work.
The Stirling cycle is a thermodynamic cycle that describes the general class of Stirling devices. This includes the original Stirling engine that was invented, developed and patented in 1816 by Robert Stirling with help from his brother, an engineer.
Thermoacoustics is the interaction between temperature, density and pressure variations of acoustic waves. Thermoacoustic heat engines can readily be driven using solar energy or waste heat and they can be controlled using proportional control. They can use heat available at low temperatures which makes it ideal for heat recovery and low power applications. The components included in thermoacoustic engines are usually very simple compared to conventional engines. The device can easily be controlled and maintained.
The Rijke tube is a cylindrical tube with both ends open, inside of which a heat source is placed that turns heat into sound, by creating a self-amplifying standing wave, due to thermo-acoustic instability. It is an entertaining phenomenon in acoustics and is an excellent example of resonance.
Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time.
The Ericsson cycle is named after inventor John Ericsson who designed and built many unique heat engines based on various thermodynamic cycles. He is credited with inventing two unique heat engine cycles and developing practical engines based on these cycles. His first cycle is now known as the closed Brayton cycle, while his second cycle is what is now called the Ericsson cycle. Ericsson is one of the few who built open-cycle engines, but he also built closed-cycle ones.
A hot air engine is any heat engine that uses the expansion and contraction of air under the influence of a temperature change to convert thermal energy into mechanical work. These engines may be based on a number of thermodynamic cycles encompassing both open cycle devices such as those of Sir George Cayley and John Ericsson and the closed cycle engine of Robert Stirling. Hot air engines are distinct from the better known internal combustion based engine and steam engine.
A thermodynamic cycle consists of linked sequences of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and other state variables within the system, and that eventually returns the system to its initial state. In the process of passing through a cycle, the working fluid (system) may convert heat from a warm source into useful work, and dispose of the remaining heat to a cold sink, thereby acting as a heat engine. Conversely, the cycle may be reversed and use work to move heat from a cold source and transfer it to a warm sink thereby acting as a heat pump. If at every point in the cycle the system is in thermodynamic equilibrium, the cycle is reversible. Whether carried out reversible or irreversibly, the net entropy change of the system is zero, as entropy is a state function.
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc.
This timeline of heat engine technology describes how heat engines have been known since antiquity but have been made into increasingly useful devices since the 17th century as a better understanding of the processes involved was gained. A heat engine is any system that converts heat to mechanical energy, which can then be used to do mechanical work.They continue to be developed today.
Atmospheric thermodynamics is the study of heat-to-work transformations that take place in the Earth's atmosphere and manifest as weather or climate. Atmospheric thermodynamics use the laws of classical thermodynamics, to describe and explain such phenomena as the properties of moist air, the formation of clouds, atmospheric convection, boundary layer meteorology, and vertical instabilities in the atmosphere. Atmospheric thermodynamic diagrams are used as tools in the forecasting of storm development. Atmospheric thermodynamics forms a basis for cloud microphysics and convection parameterizations used in numerical weather models and is used in many climate considerations, including convective-equilibrium climate models.
A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynamic engine during the conversion of heat into work, or conversely, the efficiency of a refrigeration system in creating a temperature difference through the application of work to the system.
Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical models for heat pump, air conditioning and refrigeration systems. A heat pump is a mechanical system that transmits heat from one location at a certain temperature to another location at a higher temperature. Thus a heat pump may be thought of as a "heater" if the objective is to warm the heat sink, or a "refrigerator" or “cooler” if the objective is to cool the heat source. The operating principles in both cases are the same; energy is used to move heat from a colder place to a warmer place.
The pulse tube refrigerator (PTR) or pulse tube cryocooler is a developing technology that emerged largely in the early 1980s with a series of other innovations in the broader field of thermoacoustics. In contrast with other cryocoolers, this cryocooler can be made without moving parts in the low temperature part of the device, making the cooler suitable for a wide variety of applications.
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