Fellows Research Group, Inc.

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

History of Development

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 heated in microseconds to create a high velocity acoustic traveling wave. In the case of the pulse jet, it is an open-cycle, reaction-mass engine. 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 acoustic wave is created by heat passing through a heat exchanger-waveguide. After the wave has performed work on a generator armature and created electricity, it is essentially "dissolved" in a cold heat exchanger that extracts the remaining energy. Another wave is generated in the hot waveguide and the cycle repeats for as long as heat energy is supplied.

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 prototype will begin trials in 2009. It has one moving part, the armature of a linear induction alternator, and will have a removable gas burner and combustor shroud around the waveguide (acoustic transformer) 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 (1 psi) is amplified by a factor of five (500%), to 5 psi, for a useful pressure excursion of 0.28 kg/cm2. At 1170 Hz, and 1 mm armature travel, the 7 cm2 armature is putting out 2.29 kg/m/sec, or 22 Watts. 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 5 kW TAR will be roughly the same size as the demonstrator, but operate at higher temperatures and pressures.

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

The 6 cm 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.

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/Lab capabilities. A production cost quote from a major manufacturing plant came in at US $7.16 for a twenty Watt device ($0.36/Watt) in quanties 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 test 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 kW research engine shown here. This engine lacks only the remaining generator optimization work to be ready for production.

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. Production costs are estimated at less than US $100 per kilowatt of generating capacity, and an amortized cost for solar power is less than US $.01 per kilowatt-hour. 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 building brick / roofing tile / paving tile with high thermal mass (42 W-hr/lb), just for this purpose. Sidewalks, roofs and parking lots can become giant solar collectors and thermal storage mass that buffers the effects of intermittent clouds, etc. Architects should note the 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 stores 97 Watt-hours per pound and never wears out. That is twice the energy density of the best lead-acid battery. The Thermal Storage Cell stores 1000 kWh per cubic meter, and costs about $35/kWh of storage capacity, not including installation. The medium alone costs approx. $8.35/kWh.

A simple solar panel may be our first market entry, in order to build the necessary funds to commercialize the TAC and MEMS-TAR. This panel should be available in 2009.

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.

We need only $5 million to tap it. Five year ROI is estimated at 10,000:1

Fellows Research Group, Inc.

THERMOACOUSTIC RESONATOR (TAR)

PATENT

Los Alamos Thermoacoustic Research

Los Alamos Thermoacoustic Engine

NASA-Ames Thermoacoustic Research

U.S. Naval Research Laboratory Post Graduate School

Penn State Thermoacoustic Research

Sandia National Laboratory Stirling Engine Research

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