Double-Acting Extremely Light Thermo-Acoustic (DELTA) Convertor

Glenn's thermoacoustic power converter reshapes the conventional Stirling engine from a toroidal shape into a straight colinear arrangement. Instead of relying on failure-prone mechanical inertance and compliance tubes, this design achieves acoustical resonance by using electronic components. In a typical Stirling engine, the acoustical wave travels around a toroid and reflects back, forming a standing wave. In Glenn's device, by contrast, the wave instead travels in a straight plane where a transducer receives the acoustical wave and electrical components modulate the signal. A second transducer on the diametrically opposed side reintroduces the acoustic wave with the correct phasing to achieve amplification and resonance. Glenn's design allows the transducers to operate at high frequency while presenting a mass rather than stiffness impedance. Glenn's magnetostrictive alternator uses stacked magnetostrictive materials under a biased magnetic and stress-induced compression. The acoustic energy from the engine travels through an impedance-matching layer (which can be formed from aerogel materials) that is physically connected to the magnetostrictive mass. Compression bolts keep the structure under compressive strain, allowing for the micron-scale compression of the magnetostrictive material and eliminating the need for bearings. The alternating compression and expansion of the magnetostrictive material creates an alternating magnetic field that then induces an electric current in a coil wound around the stack. This alternator produces electrical power from the acoustic pressure wave and, when the resonant frequency is tuned to match the engine, can replace the linear alternator to great effect.

Aircraft thermal management systems typically comprise over half the mass associated with full electric power propulsion systems, with significant negative impact on fuel efficiency. In addition, the traditional method of using jet fuel to cool aircraft generators does not provide enough cooling for use in flight-weight cryogenic systems. Lastly, the much higher bus voltages required for flight-weight systems (4.5 kV vs. 270 V) introduce additional spark-ignition hazards associated with alternative cryogenic cooling fuels, including liquid methane or liquid hydrogen. The Glenn flight-weight thermal management system addresses all of these problems by using the considerable waste heat energy from turbogenerators to create a pressure wave thermoacoustically. This wave can then be delivered quietly and efficiently via routed ductwork to hollow pulse-tube coolers located near any component in the aircraft that requires cooling. The tubes can be fabricated in any length and can be curved to fit any space. This technology also allows waste heat energy to be used in at least four ways: 1) the waste heat energy can drive a thermoacoustics-based ambient or cryogenic heat pump; 2) it can be channeled directly into a thermoacoustic engine that generates power; 3) it can convectively preheat the fuel/ or air supplied to the aircraft engine; 4) it can drive a pulse-tube generator providing power. The delivered thermoacoustic power can provide cabin cooling as well as ambient/cryogenic cooling of converter, cables, and motors. In addition, this power can be converted to local electric power through the use of a transducer (such as a linear alternator) or piezoelectrics. Further, the efficient thermal management system enables the size, mass, and resultant cost of the radiating fins to be reduced. Glenn's system offers an efficient method of cooling next-generation flight-weight electric aircraft with significant benefits for fuel efficiency and safety.

Solid state power conversion devices, such as thermoelectrics, depend upon temperature gradients for their operation. For example, aeronautic gas turbine engines maintain the necessary temperature gradients throughout their systems due to the enthalpic processes of combustion, which offers the possibility of generating electrical power for use in primary and secondary electrical systems in the aircraft. However, until now thermoelectric materials have not been able to withstand the combination of high temperatures and oxidative environments present in gas turbine engines. Glenn's innovation overcomes these limitations by using a doped oxide pyrochlore (crystal compound) semiconductor as the thermoelectric material. The material has a low thermal conductivity, which allows it to maintain a thermal gradient and sufficient electrical conductivity to produce an electromotive force. The pyrochlore allows the thermoelectric material to be present within a gas turbine engine, converting heat directly into electricity and functioning at high temperatures without oxidizing in air. Glenn's innovative thermoelectric material permits the benefits of solid-state power conversion devices to improve fuel efficiencies for a broader range of applications than has ever been possible. This innovation is in the early stages of development, and Glenn welcomes opportunities for co-development.

Researchers at JPL have developed, built, and tested an innovative heat exchanger that offers reduced thermal expansion, increased structural strength, low pressure drop, and improved thermal performance while lowering the weight associated with typical heat exchangers. This innovation would benefit the commercial thermoelectric generator, aircraft, and industrial processing (i.e., glass, steel, petrochemical, cement, aluminum) industries by improving energy management/efficiency, reducing carbon dioxide emissions, and increasing system durability due to the reduced stress from thermal expansion. The Problem Thermoelectric generator systems require high-performance hot-side and cold-side heat exchangers to provide the temperature differential needed to transfer thermal energy while withstanding temperatures up to 650 °C. Because the hot-side heat exchangers must have a high heat flux, they are often made of metals such as stainless steel or Inconel® alloys. Although these materials can operate at high temperatures, resist corrosion, and are chemically stable, they also have several drawbacks: (1) Their lower thermal conductivity negatively affects their thermal performance. (2) Their higher thermal expansion leads to stresses that compromise system structural integrity. (3) Their high mass/volume reduces the power density of generator systems into which they are integrated. As a result, they are difficult to integrate into viable energy recovery systems. They also make the systems unreliable, non-durable, and susceptible to failures caused by thermal-structural expansion. The Solution JPL researchers chose to replace the metal in traditional heat exchangers with graphite, which offers an improved conductivity-to-density ratio in thermal applications as well as a low coefficient of thermal expansion. In addition, they used a mini-channel design to further increase thermal performance. Combining more advanced materials with the innovative thermal design has yielded significant improvements in performance. For example, a 200-cm3, 128-g version of JPL's exchanger successfully transported 1,100 W from exhaust at nearly 550 °C with approximately 20 W/cm2 thermal flux and a pressure drop of only 0.066 psi. JPL's technology combines lightweight, high-strength graphite material with a mini-channel design that offers high thermal performance. Further development and testing are underway. Inconel is a registered trademark of Special Materials Corporation.

Energy management is one of the most challenging issues in the world today. Accordingly, various energy harvesting technologies have gained attention, including harvesting energy from ambient vibration sources using piezoelectric materials. However, conventional piezoelectric energy harvesting transducer (PEHT) structures have effective piezoelectric constants that are lower than about 10^4 pC/N, (resonant mode). These low piezoelectric constants lead to conventional PEHTs not being able to harvest electric power effectively. Further, for a specific vibration/motion source, it would be advantageous to maximize the mechanical energy captured from the vibration structure into the piezoelectric device and to convert a greater fraction of that mechanical energy into electrical energy more efficiently. This invention is a system and method using multistage force amplification of piezoelectric energy harvesting transducers (MFAPEHTs) to increase the effective piezoelectric constant to >10^6 pC/N and to increase the mechanical energy input to the device. The invention utilizes 33 mode PZT to permit maximum coupling between the input mechanical energy with the piezoelectric material, and multilayer construction of single crystal PMN-PT material to significantly amplify the voltage/charge generation and storage from the applied mechanical force.