Thermoacoustic Resonators (TARs)
White Paper
 
The ThermoAcoustic Resonator (TAR) is a patented, break-through technology that converts heat into electricity, simply and efficiently. The TAR generator has one moving part.  It can be produced at low cost, and is simple to operate and maintain. Simply put, the TAR is an ultra cost-efficient generator.
 
The TAR uses heat energy to amplify acoustic pressure waves that flow through a high density gaseous medium, inside a resonant waveguide.  The heat energy increases the magnitude of these acoustic waves, or pressure pulses; in pressure, temperature and velocity. The density of the working fluid determines Specific Power.  This renders Specific Power of the TAR as almost solely a materials issue.  With lightweight alloys, the TAR can achieve Specific Power densities greater than five kilowatts per kilogram (5 kW/kg).
 
What sets the Fellows Thermoacoustic engine apart from other technologies is the ability to heat and cool a traveling wave acoustic impulse in microseconds.  It is the thunderclap-like expansion of the internal working gas; repeating hundreds, and even thousands of times per second, that converts the heat energy into mechanical power.  The acoustic wave is not only amplified thermally, it is accelerated in velocity, multiplying its kinetic energy.  The resultant is a repeating train of large, dynamic pressure fluctuations.
 
The pressure fluctuations in the internal working gas drive a spring-mounted diaphragm piston, causing it to oscillate in step with the waves.  In one respect, this is the same principle as the common internal combustion engine, where the pressure of expanding gases drives a piston.  The difference is that in an internal-combustion engine, the fuel-air mixture is burned directly inside the piston cylinder to produce those expanding gases, while in an external-combustion engine, like the TAR, the heat required to expand the gas inside the cylinder is conducted into the engine through the walls of a heat exchanger that is heated by an external source.  Just as in the automobile engine, the rapidly repeating pressure impulses push on a piston and cause it to reciprocate.
 
In the case of the TAR, the piston is attached to the armature of a linear generator.  Most generators have a rotating armature that converts that rotary motion into electrical energy.  The armature shaft turns in lubricated bearings, and the armature rotates within a magnetic field.  The armature rotation is produced by connecting the generator, by means of mechanical linkages, to an internal-combustion engine.
 
In the TAR, the armature has no linkages, and it reciprocates instead of rotating. The piston-armature is freely mounted on spring diaphragms, and it vibrates back-and-forth at high frequency, in sync with the acoustic pressure waves.  Length of stroke averages about 5mm. 
 
An example of the mechanical power in the low power demonstration engine is as follows:
 
Static pressure 4 atm.
Delta-T = 120 C
Diaphragm area = 78 cm2
Acoustic wave pressure excursion = 0.5 kg/cm2
Force on the piston = 0.5 kg/cm2 * 78 cm2 = 39 kgf
Armature excursion = 5mm
Frequency = 1200 Hz
 
39 kgf * 0.005 m * 1200 Hz = 234 kg-m/sec * 9.8 = 2.3 kW
 
Engine weight = 12 kg
Specific Power = 0.19 kW/kg
 
At 100 atm. Static pressure and 600 C delta-T, the same engine (@35 kg) will deliver 245 kW (7 kW/kg).
 
There are other benefits with the TAR.  There is no lubrication, and no friction in the usual sense.  The result is high reliability, little wear, low maintenance, and long operating life.  In combustion applications, such as a power generator or engine for a hybrid vehicle, the external burner is quiet and steady; there are no popping exhaust noises as with an internal-combustion engine.  The flame burns cleanly, directly on a catalytic heat exchanger, and emissions of nitrous-oxides (NOx) and carbon-monoxide (CO) are almost not measurable.

The difference in pollutants is so marked that people can be in a closed room with an operating TAR and suffer no more ill effects than they do with a gas kitchen stove in their homes.  By comparison, people in a closed room with a running internal combustion engine would suffer carbon-monoxide poisoning in just a few minutes.  Like the kitchen stove, the main constituents of the TAR exhaust are harmless carbon-dioxide (CO2) and water vapor.  It requires no emissions controls.
 
For solar applications, such as solar-electric power generation, there are no pollutants at all.
 
Low emissions indicate high thermodynamic efficiency, and indeed, the TAR is almost twice as efficient as the internal combustion engine in converting fuel into output work.
 
Low production cost, low maintenance, quiet operation, non-polluting, fuel efficient.  These are the attributes of the TAR.
 
To start the TAR generator, the burner ignites and the waveguide heats up in a matter of seconds.  When the waveguide reaches operating temperature, an exciter circuit initiates movement in the spring-mounted armature.  This sets the first acoustic wave in motion.  After that, the internal geometry of the waveguide and the thermal properties of the working gas create a condition of resonance, and the acoustic oscillation becomes self-sustaining for as long as heat is applied. 
 
The linear generator converts the acoustic oscillation into electricity.  The high frequency alternating current produced by the TAR is rectified into direct current.  If required, it is then sent to an inverter where it is converted back into alternating current of the correct voltage and frequency needed by the end user.
 
The TAR can be air or water cooled.  For a space power system, a black body radiator would be used.  Efficiency is highest with water, and the heated cooling water can be used for domestic hot water or space heating, or for industrial process feed-water.  The design shows the mechanical and electrical simplicity of the TAR.
 
 

 
Slide 1 Prototype TAR
 
The TAR is efficient, inexpensive to manufacture, and requires no lubricants, maintenance or emissions controls.
   
The energy efficiency of the TAR is represented as follows:
Energy efficiency for combustion driven generator = Total fuel energy x (Th – Tc)/Th x 0.63 x 0.9
Energy efficiency for solar driven generator = (Th – Tc) / Th * 0.63 * 0.9
Th is the absolute temperature of the waveguide (hot-side) heat exchanger
Tc is the absolute temperature of the cold-side heat exchanger
Heat energy consumed by the TAR = Fuel energy x (Th – Tc)/Th
Heat energy of exhaust = Fuel energy x (1 – (Th –Tc)/Th))
Heat energy converted to dynamic pressure by TAR = Fuel energy x (Th – Tc)/Th x 0.63
Heat energy rejected by cooling water = Fuel energy x (Th – Tc)/Th x 0.37
Efficiency of the linear generator = 0.9
 
As can be seen, delta T = efficiency.  In normal power generation applications, the minimum economic delta-T for the TAR is around 100C.  For other heat engines, it is closer to 600C. This is because of the superior efficiency and lower cost of the TAR.
 
Efficiency is defined by Carnot, because the power derived from an expanding gas is directly dependant on the temperature gradient of the gas between expansion and contraction. In the TAR, it is the difference in temperature between the hot and cold heat exchangers.  At low temperatures, there is less relative energy in the gas, and the heat exchangers must be larger in order to maximize the rate of energy exchange.  This drives up cost.
 
The TAR can be used as a secondary generator on gas turbine or recip gen-sets to produce additional electricity from the engine exhaust. This co-generative capability opens up a huge market for the TAR in waste-heat recovery, and is applicable to all types of combustion engines and industrial operations.
 
FRG has built a demonstration TAR engine that proves the thermoacoustic principle.  The relatively low operating temperature of the engine, and the large increase in the dynamic power of the acoustic wave when heat is applied to the waveguide, are clearly demonstrated in a CD Video that is free for the asking.
 

THE ARMATURE IS THE ONLY MOVING PART IN A TAC ENGINE
 

FRG is seeking licensees to manufacture and market the TAC generator.  For further information, please contact:
 
Lee Fellows
Fellows Research Group, Inc.
Tel:  1 512-864-2097
Email:  frg@io.com
http://www.io.com/~frg