Thermoacoustic engines - Thermopneumatic engines - What are they?
Thermoacoustics is the term for soundwaves generated by heat.
Thermopneumatics is control of thermally-generated pressure in gases. The latter term is typically used in describing MEMS devices. FRG has developed a hybrid engine, using both, that develops high energy conversion efficiency.
One might accurately
say that all heat engines, like those in
automobiles and jet planes, generate sound and pressure from heat.
Internal
combustion (IC) engines burn fuel and generate combustion noises that
rumble or roar, depending on the type of engine. Noise and
pressure are
by-products of the engines' energy conversion process. The
pressure component usually performs the work in the system, while the
acoustic signature is an indicator of system resonance, which is
related to efficiency.
Typical internal-combustion (IC) engines burn fuel mixed
with air to produce hot, expanding gases that drive pistons
and turbines directly. A Thermoacoustic Cycle (TAC) engine, however, is a different
animal altogether. The usual arrangement consists of a closed, resonant
chamber containing two or more heat exchangers and a working medium--generally
a dry gas under pressure. Heat from outside the
resonator is injected into the internal working fluid via the hot-side heat
exchanger (HXh), causing localized pockets of increased pressure. At the other end the
resonator, a coolant circulating through the cold-side heat exchanger
(HXc) removes the heat energy from the working fluid, causing a
reduction in pressure.
Thermopneumatic Cycle
(TPC) engines use an external heat source to expand an internal dry gas
(working fluid) to drive a piston or diaphragm. Stirling
Cycle engines use a similar process, but in the Stirling Cycle engine
the working fluid is scavenged by mechanical pistons and physically
moved between heat exchangers in order to effect thermal expansion and
contraction. Our thermopneumatic engine uses traveling pressure
waves, moving through a static gas, to transfer energy from
point-to-point. The result is an engine capable of three orders
of magnitude greater power density than a typical thermoacoustic
engine, and almost double that of a Stirling engine, without the
mechanical complexity. The FRG engine is lightweight, powerful,
economical to make.
In accordance with Boyle's and Charles' Laws, a confined gas will increase in pressure by 1/273rd for each degree of increase in Celsius temperature. If the resonator cavity is pressurized to twenty atmospheres static pressure to begin with (Ps), and the temperature of the gas is increased by 600 Co, the peak dynamic pressure (Pd) will be:
Neglecting gas losses and thermal reactance in the materials of the resonator, the dynamic (fluctuating) pressure will be:
The hot
and cold heat exchangers respectively are HXh
and HXc. HXh is swept periodically by a traveling wave that
brings a cooler parcel of gas into contact with the hot internal
surface of the heat exchanger (HXh) and expands it explosively to
produce an expanding pressure wave. This periodic "cooling" of
the HXh surface
causes HXh to act as a "thermal capacitor" (Ct). HXh is Ct. The properties of Ct is
the heart of the FRG engine.
Ct acts something like a transistor oscillator, in that it teeters back and forth thermally, taking up energy from the external heat source, then giving it up to the internal working fluid. The effect is that of an acoust pump that periodically pushes in a cool parcel of gas, expands it explosively, and exhausts it into the cold-side heat exchanger (HXc). The timing of this periodic "push" is accomplished by tailoring the thermal mass and conductivity of Ct to conduct thermal energy into the working fluid at a specific rate. This permits resonator design variations not restricted by geometry. Energy acquisition and decay in Ct is illustrated below:
The result is a resonator, or engine, with a rapidly fluctuating pressure gradient and no moving parts. This oscillating pressure gradient can tapped off to drive a piston, and a linear alternator that produces electricity.
For example, a pressure swing of 44 atm (45 kg/cm2) is a significant force, and when it occurs five hundred times per second, the potential power output is quite high. For example, at this dynamic pressure, a diaphragm 10 centimeters in diameter has a surface area of 78 sq.cm. This tympanic diaphragm, with a travel of only one millimeter (0.001 meter), operating at a frequency of five hundred Hz, will produce:
Extraordinary performance, from an engine the size of a hockey puck.
The FRG engine is attractive for several reasons. It has only one moving part, and the overall part count is low. That means low manufacturing costs. The engine can be adapted to operate over a wide temperature range, so energy recover from waste heat in industrial processes and cogeneration power plants, and even power generation from resources such as solar energy and moderate thermoclines in lakes and brine ponds, becomes economically viable.
The FRG engine is a Carnot engine, in that overall efficiency is governed primarily by the temperature delta across its isothermal heat exchangers, HXh and HXc, i.e.:
Low production costs mean that lower operating efficiencies can be tolerated, and the installed cost per Watt of generating capacity remains low. Because efficiency is not necessarily a cost barrier, FRG engines may permit economical harvesting of low temperature heat sources that are not economically viable using other technologies. For instance, we intend to develop low temperature biomedical applications in which miniaturized engines will generate power for medical prostheses from body heat, at single-digit efficiencies. These microminiature (MEMS) devices are called MEMS-TARs (Micro-miniature Electro-mechanical Systems-Thermoacoustic Resonator). Hearing aids, pacemakers, baby crib monitors, personal security alarms for women and children, physiological monitors for policemen, firemen, soldiers and hospital outpatients, in the form of a ring or wristwatch, will all operate indefinitely, and without batteries, acquiring energy through contact with the body. Since they will never require maintenance, they may even be implanted permanently inside the body. One design conception illustrating the nominal size of these microchip scale devices is illustrated below:
Applications are almost limitless. The heat generated by the CPU in a laptop computer can be used by the MEMS-TAR to recharge the battery, extending operating time between charges. Automobile exhaust heat can be used to charge the vehicle battery, doing away with the alternator and reducing fuel consumption and air pollution. Passenger planes can generate power for aircraft systems from waste engine heat, saving fuel. Pets and wildlife can be monitored via implanted MEMS-TAR powered locator beacons.
More information on TARs and TAC engines can be found on FRG's website at:
http://www.io.com/~frg