Fellows Research Group, Inc.


Phone: (512) 864-2097
e-mail Address: frg@io.com

History of Development
TERMINOLOGY

Our use of the term "thermoacoustics" with regard to our inventions may have been a poor choice, because we define the term and treat the physics differently than most researchers do.  Likewise, the term "thermopneumatics" is not usually used in the same context as we use it.  Fellows engines and resonators make use of similar physical principles found in both thermoacoustics and thermopneumatics, i.e., the physical laws of gas mechanics, but the similarity ends there.  We can demonstrate power densities two orders of magnitude greater than "thermoacoustic" engines, and we can demonstrate our technical claims.

In our philosophy, a thermoacoustic engine is a legitimate thermodynamic cycle in its own right.  It is not a "Stirling Cycle" engine.  Even so-called Stirling engines are not Stirling Cycle engines when rpm exceeds a few hundred rpm.  In the Reverend Stirling's day, circa 1812, a one horsepower engine was a cast iron monster that seldom exceeded 100 rpm.  The good Reverend's engine was no exception.  Air was physically shuttled between heat exchangers where field effect expansion and contraction from applied heating and cooling produced pneumatic pressure changes that could drive a piston and flywheel. 

Gases, including air, have inertia.  That inertia makes possible the force that results in aerodynamic lift (apologies to Bernoulli), the destructive force of hurricanes, tornados, explosives, etc.  The violent expansion of heated gases is what drives the gasoline and deisel engines in automobiles.  That same physical force, made to function in a resonant system, is what powers our engines.  The way we manage the thermal energy throughput is our trade secret, and we are currently the only entity with a "thermoacoustic" engine capable of the power densities found in petrol engines.

The first Thermoacoustic Cycle engine of record was invented by the Nazis in Germany during the 1930s.  It was the pulse-jet engine.  It became infamous as the engine that propelled the "buzz bombs" that wreaked havoc on London.  Today's cruise missile is the progeny of that weapon.  

 

The basic physics of these engines is that a quantity of gas is point-heated in microseconds to create a high velocity traveling wave.  In the case of the pulse jet, it is an open-cycle, reaction-mass engine.  It uses heat to accelerate and eject a gaseous mass, in order to propel an aircraft.  FRG's TAC engine differs from the pulse jet in that it is a closed-cycle external combustion engine, rather than an internal combustion engine, and the gaseous mass is not ejected, but instead used to drive a generator.  After the pressure wave has performed work on a generator diaphragm and created electricity, it is essentially "dissolved" in a cold heat exchanger that extracts the remaining energy and rejects it as radiant heat. 

The following images are a small part of a research program that dates back to 1978. The first engine patent was filed in 1986 by our now defunct Thermomotor Corporation.


The Mechanical TAR has one moving part, the armature of a linear induction alternator, and will have a removable gas burner and combustor shroud around the waveguide for controlled testing.  Static pressure is 35 atm, temps: 500C HXh; 80C HXc (420C delta-T).  Carnot eff. is 35%, projected actual thermal eff. is 22%.  Mass density of the static working fluid (argon) is 3.5 lb/ft3 (17.1 kg/m3), mass density in the wavefront 3.89 lb/ft3 (19 kg/m3), sonic velocity of the impulse at the mouth of the horn is 1757 ft/sec (536 m/sec).  Armature excursion ~2 mm.  At 534 Hz, the acoustic power available at the 254 cm2 armature is 50 kW.  

 


The graph below shows theoretical net efficiency relative to the temperature difference between the TAC heat exchangers. For example, if the cold side is at 60C, and the hot side at 560C, the temperature delta is 500C. On the graph this corresponds to a net thermal-to-electric conversion efficiency of 35%.  

 


This TAR was demonstrated at the NREL Industry Growth Forum in Albany, NY, October 30, 2002.  At only 45 psi (3 atm, or 3 kg/cm2) static pressure, and a 250F (140C) delta-T, the differential pressure in the acoustic wave (0.2 psi) is amplified by a factor of 5 (500%), to 1 psi, for a useful pressure excursion of 0.07 kg/cm2.  At 1170 Hz, and 1 mm armature travel, the tiny 7 cm2 armature is putting out 0.57 kg/m/sec, or 5.6 Watts.  A larger diaphragm would mean more power.  The heat source used in the demonstration is a hot air gun, similar to a hair dryer.  The demonstrator is shown in operation in a video at:  http://www.io.com/~frg

The production 1 kW TAR will be only slightly larger than the demonstrator, but operate at higher temperatures and pressures.

 


Our experimental 10 cm MicroTAC is shown here in a takedown version.  It is a precursor of the chip-size MEMS-TAR.

 



The 6.4 cm planned production version is die-formed aluminum and stamped stainless steel.  It is pressurized and sealed in a die swaging operation.  The artist rendering below shows how the finished product will look.



An economical configuration for a solar-electric panel using the MicroTAC is shown below.  The MicroTAC units are sandwiched between a hot plate and a cold plate, and encapsulated in a glazed box.  The tiny microchip-size MEMS-TAR will eventually replace the MicroTAC for solar-electric power generation.  It will be printed in ganged arrays on the back of a blackened aluminum plate, and the age of low cost solar power will have arrived.  We also plan to embedd the MEMS-TAR in a ceramic roofing shingle, and integrate solar energy into the structure of buildings.



The MicroTAC was born in 1994.  Theoretical thermal-electric conversion efficiency is very good (30% - 40%). The design and fabrication are well within common machine shop and lab capabilities.  A production cost quote from a major manufacturing plant came in at US $7.60 for a twenty Watt device ($0.38/Watt) in quantities of 100,000.  That includes the cost of tooling.  Larger production runs would amortize tooling cost even more.  This early drawing gives an idea of the nomenclature.

  


Below is the MicroTAC demonstrator shown in the video.  The miniature heat exchangers were fabricated from samples of reticulated foam, aluminum foil and freeze plugs from a truck engine--odds and ends that we had on hand. The thermal capacitors were made from powdered metal filter media. There was no way to make the numbers line up for the various junk-box parts, so the output was not impressive, but amplification exceeded 400%, and given the material metrics, that was surprisingly good.
   



This is one of the FRG series of low temperature TAR resonators (Model TAR1999). It amplifies the acoustic wave by 400%, with a thermal efficiency of 23%. The thermal input is 60 Watts from an electric cartridge heat element, the acoustic input is 4 Watts and the AC electrical output is 15 Watts at 3460 Hz, using air as the working fluid, at 4 atm. pressure.


This is an internal view of this solid-state resonator.
 
 


The TAR1999 during trials.
 


This early resonator was designed in 1989.  It has a piston-armature assembly at right angles to the resonant tube. There is a phase conflict. It can be rectified by adjusting heat exchanger metrics, but it is simpler to stick with a linear design. Static pressure = 2 atm. Eff = ~16%.
 


This early folded resonator was designed with automotive air conditioning in mind. The annular piston design proved to have excessive mechanical friction. The coil was driven by a 12 volt automotive battery. Net refrigeration efficiency was low in this particular machine.
 
 




The TAC has since progressed to the 50 kWt research engine shown here.


The generator module shown here fits inside the engine shown above.

This semiconductor-sized MEMS-TAR illustrates the direction we are going. This device has applications in solar energy conversion, waste energy recovery, biomedical applications, sensors and controls, etc. Control circuitry and power conditioning is designed into the mask and everything produced on the chip in one operation. It is designed for automated manufacture and assembly in a wafer fab, just like other MEMS devices. Production costs are estimated at less than US $250 per kilowatt of generating capacity.  An estimated amortized cost for solar-electric power production is less than US $.01 per kilowatt-hour.  Economical distributed power systems for the housing market are close to reality.  

 



The MEMS-TAR is robust, and can be embedded into a paving and roofing tile. We have developed an iron-orthosylicate roofing tile with high thermal mass (42 W-hr/lb), just for this purpose. Residential and commercial roofs can become giant solar collectors with a built-in thermal storage mass that buffers the effects of intermittent clouds, etc.  Architects should note that it can also be used for siding on high-rise office buildings, apartment complexes and shopping malls, and does away with solar panels that detract from the appearance of the structure.  The unremarkable roof of the house shown below can be the power source for the home.



For stand-alone systems, energy storage is necessary to buffer the variability of direct solar energy.  We've developed a metal alloy storage medium that never wears out.  The Thermal Storage Cell stores 1000 kWh per cubic meter, and costs about $35/kWh of storage capacity. 




The Market. Global growth in market demand for electricity is forecast by the United States Department of Energy to exceed 3000 gigawatts by the year 2020. At USD$450 per kilowatt, generating plant equipment sales will exceed USD$3 Trillion dollars in new plant capacity alone over the next 15 years.  A 1% market share is USD$30 Billion.  That doesn't include the waste heat energy recovery market (~$6 billion), or the multi-market potential of the MEMS-TAR.

We need $10 million to commercialize our technology and tap into those markets.  We can have a prototype generator running trials within 6 months from start-up, and a packaged product ready for market within 12 months. The MEMS-TAR within 18 months.  This technology can compete economically with any other power generation technology available, including fossil fuels. The five year ROI is conservatively estimated at 100:1



THERMOACOUSTIC RESONATOR (TAR)

THERMOACOUSTIC CYCLE ENGINE (TAC)


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updated March 2010