Clean Power Resources, Inc.
 
 

A New Kind of Waste Heat Generator

The piezoelectric effect, the ability of some materials to generate electricity when a mechanical stress is applied, has been known since 1880.  It has been used to detect submarines since WWI, provide the spark in cigarette and grill lighters, and generate the high voltages needed for backlit LCD screens.  So far, however, it has not been used for large scale electric power production.  Unlocking this source of power will be possible by coupling a novel flexible-diaphragm piezoelectric alternator to a thermoacoustic engine.  A large version of this thermoacoustic-piezoelectric generator will be capable of producing 1 MW of electricity with high efficiency in only a cubic meter of volume, with few if any moving parts.  The generator will be simple, reliable, low maintenance, robust, and mechanically balanced for quiet operation.  Thermal to electrical efficiency of 20% is expected when operating with a 800 ºF (427 ºC) heat source.

In a device to generate four-phase electric power, shown in Fig. 1, four thermoacoustic engines and four piezoelectric alternators are arranged in a vertical column.  The engines convert heat from nearly any thermal source—waste heat, a burner, or solar—into a high amplitude acoustic wave that propagates down the pressurized helium gas that fills the column and up a feedback tube around the column.  This high amplitude acoustic wave actuates the piezoelectric material to generate electrical power.  Each of the four co-linear engine/alternator pairs consists of an ambient heat exchanger, a regenerator, a hot heat exchanger and an alternator.  The sketch in Fig. 1 shows a device that captures waste heat from gaseous combustion products that pass horizontally through four hot heat exchangers.  Input and output manifolds are not shown.

The thermoacoustic engine is an acoustical version of the classic mechanical Stirling engine that in its idealized limit is capable of Carnot efficiency.  Being acoustical, however, the thermoacoustic engine is able to perform the energy conversion with no moving parts and at a much higher frequency and thus at a much higher power density.  As the acoustic wave propagates through the engine, the motion and pressure swings of the gas act to shuttle heat from the hot to ambient heat exchangers and amplify the amplitude of the wave.  The wave, in the presence of the temperature gradient in the regenerator, causes each gas parcel to undergo the Stirling thermodynamic cycle, converting the acoustically induced flow of heat from hot to cold into acoustical work in the form of a more powerful wave. 

The extra power of this wave is then extracted by the piezoelectric alternators.  The wave oscillates the flexible spring steel diaphragm of an alternator, causing it to flex like a drumhead.  With each flexing of the diaphragm, at the extremes of either direction, the diaphragm pulls inward and compresses a ring of piezoelectric material attached to the rim of the diaphragm.  The piezoelectric material transforms the fluctuating compressive stress into electrical power, extracting a portion of the wave energy converting it to electricity, while allowing the remainder to travel to the next engine-alternator pair.  Vibration balance of the generator as a whole is achieved by arranging the dimensions such that an integral number of 360º phase shifts occur within the straight line column.  For example, in the four-phase design of Fig. 1, each alternator generates power that is phase shifted from its neighbors by 90º. 

The engines are quite simple, relying on geometry and physics to induce the gas into the Stirling cycle rather than pistons and linkages to force the motion.  We believe that thermoacoustic-piezoelectric energy conversion is poised to be the preferred waste heat recovery technology in the 5 kW-500 kW power range.  A preliminary version of the device has been built and is undergoing testing.  We are eager to see how far this technology can be pushed.

 

 

 

Figure 1. Thermoacoustic-piezoelectric generator, consisting of four thermoacoustic-Stirling engines driving four piezoelectric alternators.  Cutaway view on right.