Chapter 19 - Investigating Sonoluminescence as a Means of Energy Harvesting
Chapter 19 -Investigating Sonoluminescence as a Means of Energy Harvesting
Notes by David A Roffman on Chapter 19 of
FRONTIERS IN PROPULSION SCIENCE
Chapter by John D. Wrbanek, Gustave C. Fralick, Susan Y. Wrbanek, and Nancy R. Hill
NASA Glenn Research Center, Cleveland, Ohio
Sonoluminescence uses sound to create light in cavitations fluids. This process also generates bubbles that can produce temperatures higher than thousands of degrees (Celsius). A spherical collapse of a bubble could have a peak release temperature of 3 X 108 K. The bubbling heat/light flashes were discovered in the 1930s during sonar research. A possible model for sonoluminescence is a two stage plasma bubble in which there is a low-density halo, and a high-density core.
It may be that fusion can be achieved through sonoluminescence. The heat associated with the process may be enough, although fusion has yet to be observed. However, it has been noted that different liquids produce varying temperature bubbles. Some have projected that temperatures of one million Kelvin can occur in acetone, whereas only half a million Kelvin is projected for water. Deuterium in water (heavy water) typically results in better bangs from sonoluminescence.
Taleyarkhan’s group has recorded neutron and gamma ray flux. These results have been confirmed by others, although there is not enough evidence to say that nuclear reactions are taking place. To be effective for power generation, devices need to be compactable. Calculations in the book show that the minimal cell size is 4.6mm in diameter.
NASA has launched broad ranging experiments to study sonoluminescence. One of their tests confirmed that brightness increases by 20% in 0 g. Standard equipment is: flask containing liquid, ultrasonic transducers, piezoceramic amplifier, and a generator. If the gas is not saturated in the liquid, then a single bubble is produced. However, if the gas is saturated, then multiple bubbles are produced.
For experiments with varying flask sizes, the smaller sized containers worked best. This may be because there was less opportunity for dissipation. As for the experiments with water versus heavy water, heavy water was needed to observe what is described in the next paragraph.
Plates that combined different materials were used in the experiment. Through zooming in, it became apparent that some metals fused over the course of the experiment. Temperatures required for this welding were around several thousand Kelvin. This cannot be acquiesced with certainty (that temperature was the cause).
In the case of fusion, it is necessary to find its products. Scintillation detectors may be used to find the products of a fusion reaction. To make use of energy produced, it is a good idea to transform thermal energy into electrical energy. A heat gradient can produce electricity via the Seebeck coefficient. Thermoelectric power has been used in space missions. The hot side uses radioisotope generators, while the cold side is space.
NASA is currently researching film ceramic thermocouples for hot environments. There needs to be a way to reduce resistance in order for a device to work, as each individual unit (as shown in the book) loses a lot of power to it. Despite successes with some applications of sonoluminescence, there is still much to be learned. NASA and (more likely) other organizations will continue to study this curious concept.