Power generation from an external or internal heat source using thermal energy conversion technologies such as solid-state thermionics and thermoelectrics or dynamic conversion with Otto, Stirling, Brayton, or Rankine technologies is fundamentally limited in maximum specific power due to either low efficiency and/or operating frequency. These solid-state technologies are low voltage and hence produce a high DC current that restricts their minimum geometry to approximately 4 A/mm2 to avoid overheating. High-power implementations of this technology class are inefficient, large, and heavy.

The DELTA Converter is comprised of multiple thermo-acoustic stages in series that form a loop or delta-shaped triangle that also contains a single two-sided piston. The piston is located at the beginning and the end of the heat exchanger stages, but since the stages form a loop, this becomes a single double-acting piston in a push/pull arrangement.

The dynamic technologies are limited to approximately 400 Hz for two different reasons. First, the oscillating piston engines such as Stirling and Otto technologies require a force on the piston that grows exponentially with frequency. That force is difficult to achieve above 400 Hz with reactive springs or rods. Second, the rotating machines such as Brayton are also limited in frequency of operation because above 24,000 RPM (400 Hz), the rotor tip speed either becomes supersonic or places too much stress on the rotor due to centrifugal forces. Hence today's space, terrestrial, and proposed aircraft power systems are unnecessarily large and heavy for the power level they provide.

A new thermo-acoustic engine technology was developed that overcomes these limitations by operating at a much higher frequency than is typically achievable. It is based on a double-acting push/pull piston engine in which an acoustic wave pushes both sides of a single piston, eliminating the need for large springs while requiring only a single piston and engine to operate. This configuration enables an order of magnitude improvement in specific power compared to conventional engines.

The Double-acting Extremely Light Thermo-Acoustic (DELTA) Converter can achieve higher than 400-Hz operation. At that frequency, the converter can produce four times more power than conventional engines operating at 100 Hz. It is comprised of multiple thermo-acoustic stages in series that form a loop or delta-shaped triangle that also contains a single two-sided piston. The piston is located at the beginning and the end of the heat exchanger stages, but since the stages form a loop, this becomes a single double-acting piston in a push/pull arrangement. The multiple stages are designed such that when the piston moves, it is simultaneously creating an acoustic wave on one side while receiving acoustic power on the opposite side. The pressure forces from the multi-staged engine push and pull on both sides of the piston, enabling much higher forces on the piston than are possible if the typically one-sided power pistons are used with only a bounce space on the opposite side.

By using the engine’s reactive forces on the single double-acting piston, eliminating the use of hot moving displacers, and using multiple stages for acoustic wave phase adjustment, the single piston can oscillate at over 400 Hz without using heavy springs. At this high frequency, the output current can be minimized and the specific power is maximized. Moreover, since the engine is essentially an empty tube filled with helium, heat exchangers, regenerators, and a single non-contacting oscillating piston, the device does not require maintenance and is expected to be extremely reliable in addition to being low-cost and lightweight.

In operation, the piston moves to the left, creating a sound wave. Then the sound wave travels through the heat exchanger and regenerator stages becoming amplified and phase adjusted. The high-power acoustic wave pushes the other side of the piston where some of the power is used to create the next acoustic wave on the other side of the piston, and the rest of the power is extracted from the moving piston via a linear alternator or other transducer. The power output of the engine is controlled by managing the piston amplitude of motion.

This work was done by Rodger Dyson of Glenn Research Center. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact http://technology.grc.nasa.gov . LEW-19387-1


NASA Tech Briefs Magazine

This article first appeared in the July, 2016 issue of NASA Tech Briefs Magazine.

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