Thermo-Acoustic Power Conversion

[sanman]

Here are some articles discussing a recent design for a thermo-acoustic engine, which appears to be a variant on the Stirling Engine:

http://www.technologyreview.com/energy/32267/

http://theenergycollective.com/tyha...microchp-small-biogas-and-solar-thermal-power


So I'm wondering what the limitations are on this thermo-acoustic energy conversion process, and whether it could be suitable for nuclear power conversion applications.

The design seems to be more rugged and durable than traditional Stirling engines, which would be useful in a nuclear application.

But what are the limits on the volume of heat energy it can convert into electricity?
What is the figure of merit here?

I was thinking that this type of thermo-acoustic engine could be useful for nuclear-electric propulsion applications, whether on ocean-going ships or even submarines. The high-frequency acoustic waves would probably attenuate very quickly in the surrounding ocean, still leaving a submarine undetectable - especially in comparison to noisier steam-turbine operation.

But what about even for powering a VASIMR rocket? Could thermo-acoustic power conversion provide enough power output for a long enough duration on a round trip to Mars?
What would be the power-to-weight ratio for a thermo-acoustic engine?

 

[Astronuc]

So I'm wondering what the limitations are on this thermo-acoustic energy conversion process, and whether it could be suitable for nuclear power conversion applications.

The design seems to be more rugged and durable than traditional Stirling engines, which would be useful in a nuclear application.

But what are the limits on the volume of heat energy it can convert into electricity?
What is the figure of merit here?

I was thinking that this type of thermo-acoustic engine could be useful for nuclear-electric propulsion applications, whether on ocean-going ships or even submarines. The high-frequency acoustic waves would probably attenuate very quickly in the surrounding ocean, still leaving a submarine undetectable - especially in comparison to noisier steam-turbine operation.

But what about even for powering a VASIMR rocket? Could thermo-acoustic power conversion provide enough power output for a long enough duration on a round trip to Mars?
What would be the power-to-weight ratio for a thermo-acoustic engine?

All good questions. For a propulsion system, one wants as great a specific power (kW/kg) as possible, which generally means highest possible temperature and least amount of structural material - which means pushing materials to their technical limits WITHOUT failure.

Scalability, not only of the thermoacoutic engine, but the entire system, is a key issue.

A reactor core can produce 100's of MW or into the low GW range, but then that thermal energy has to be converted. The challenge is to get that thermal energy to power conversion system - and then the heat rejection system. It turns out that the radiator (heat rejector) is the biggest mass of a large nuclear electric power system. In a direct thrust, the thermal energy is simply dumped into the coolant/propellant, with some allowance for bleed off to the power system.

The scalability is not clear from the article, nor is the useful lifetime. Fatigue of the diaphragm is a critical performance issue. Fatigue resistance generally decreases with temperature and irradiation, so the thermoacoustic generators would have to be placed away from the reactor. Then if the reactor produces 100 MW, or 500 MW, or 1 GW of thermal energy, how many TA generators are necessary? 100, 1000, 10K?

In additional, vibration of the structures would be an issue in terms of noise and effects on the structure. Out is space, there is no atmosphere to disperse the noise, and there is no massive ground to support the TA generator or absorb the vibration. In space, dynamic systems have to be balanced, e.g., opposing pistons balanced so that there is not net acceleration/vibration. Even rotating systems have to be balanced, so that there is not net torque on the spacecraft .

TA generators may be practical for small scale applications, but for large scale applications to spacecraft is not clear.

 

[sanman]

Well, it's claimed that this thermo-acoustic engine is more rugged and durable than a conventional Stirling engine, with no fragile seals, joints or valves that can break down more easily.

I was thinking that its durable design might be suitable for powering a Venus rover in connection with an RTG.

But perhaps it could be suitable for ocean-going ships too, to supply electricity for electric drive motors instead of polluting conventional diesel engines. Maybe the thermo-acoustic vibrations could be tuned to warn whales away, to minimize collisions between whales and shipping.

The spherical shell seems quite thick, and could serve a dual role as a radiation/containment shield, if the core were located at the interior of it. Additionally, any abnormal temperature buildup in the core would probably affect the thermo-acoustic electrical output, thus offering an additional form of monitoring.

I assume that for a nuclear reactor, high temperature ceramics would be required for the thermo-acoustic diaphragm. Alternatively, why not even use a heavy dense metal like Thorium, which could also absorb neutrons for breeding purposes?