Rapid progress is occurring in hot fusion; happily, rapid progress is also occurring in cold fusion. Very repeatable, irrefutable results have just been published in Nature (arguably the world's leading multidisciplinary peer-reviewed science journal). A commercially viable cold fusion heat source may be just around the corner (perhaps in 5 to 10 years).

The original cold fusion experiment was that of Stanley Pons and Martin Fleischmann in 1989, using electrolytic cells. In later experiments, most physicists used a metal, such as palladium or nickel, in either bulk, thin films, or powder; and deuterium, hydrogen, or both, in the form of water, gas or plasma. The deuterium and/or hydrogen is loaded into the metal to create a fusion reaction. Experiments are notoriously finicky; they are not reliably repeatable. Work is continuing, but a highly repeatable, commercially viable approach has not yet emerged.

In 2002 a very different approach to cold fusion was reported -- liquid cavitation. Quoting from Wikipedia (Bubble fusion - Wikipedia): "In the March 8, 2002, issue of the peer-reviewed journal Science, Rusi P. Taleyarkhan and colleagues at the Oak Ridge National Laboratory (ORNL) reported that acoustic cavitation experiments conducted with deuterated acetone (C₃ D₆ O) showed measurements of tritium and neutron output consistent with the occurrence of fusion. The neutron emission was also reported to be coincident with the sonoluminescence pulse, a key indicator that its source was fusion caused by the heat and pressure inside the collapsing bubbles.” (see Figure 1)

Sonoluminescence was first discovered in 1934 at the University of Cologne while doing work on sonar. It occurred when a sound wave of sufficient intensity induced a gaseous cavity within water which collapsed quickly, emitting a burst of light (Sonoluminescence - Wikipedia). Cavitation can be made to occur in many different ways. One of these is heating a liquid in a small volume. About 10 years ago a team of engineers under the leadership of Distinguished Professor Bin-Juine Huang, Department of Mechanical Engineering, National Taiwan University (NTU), Taipei, began designing and conducting "fusion by cavitation" experiments. (See Notes)

They first built a simple heat exchanger designed to generate large quantities of cavitation bubbles in water (completed in 2018). This first "reactor" was a triple-pipe (three concentric copper tubes) heat exchanger about 30 meters long, bent into a box shaped structure (see Figure 2). The heat exchanger used hot freon gas from a freon compressor (3 kW electrical input) as the heat source to heat pressurized water (about 21 times atmospheric pressure) flowing through a tiny channel about 2 mm wide (see Figure 3).

The freon gas entered the heat exchanger at 146C, which is 46C above the boiling temperature of water. The water entered the heat exchanger between 10C and 55C, depending on the experiment (room temperature is about 20C). In passing through the heat exchanger, the water was converted fully to steam, reaching a temperature of 150C, which was hotter than the entering freon gas, indicating excess heat was being produced! A term called the coefficient of performance (COP) is used to quantify the performance of heat exchangers (especially heat pumps and refrigeration systems).   It is simply the ratio of energy out to energy in. The COP for the triple-pipe heat exchanger often exceeded 2, and for some experiments exceeded 4.

The triple-pipe heat exchanger worked well but was constantly under repair. The pressures from the cavitation bubbles often buckled and ruptured the copper tubes. The team next designed a two-pipe (two concentric copper tubes) heat exchanger and switched the heating gas from freon gas to steam (130C). The COP was routinely about 1.5. Buckling of the interior copper tube still occurred. In addition, they noticed the buckled copper tube was covered in hard shiny carbon! They had no idea where the carbon came from. They wondered if oxygen (8 protons, 8 neutrons) had been transmuted to carbon (6 protons, 6 neutrons).

In the past two years, the team has discovered much more going on. The abstract of their latest paper (see link above to paper in Nature) says the following: “Recently, we have tested another eight reactors. Interestingly, these reactors produce non-condensable gas. ... [We detected] isotope ²²Ne and regular CO₂ ... and isotope gases … H₂O-17 (heavy-oxygen water), isotope O₂ (¹⁶O–¹⁷O), and isotope CO₂ (¹²C–¹⁶O–¹⁷O).  ...  In the excess heat producing reactors, all these gases were detected by mass spectrometry in the absence of ²⁰Ne and ²¹Ne (it is very curious that ²⁰Ne and ²¹Ne are not present, but ²²Ne is -- Kevin Z). The observed isotope gases produced from reactors having excess heat verifies that water can trigger a peculiar nuclear reaction and produce energy." NOTE: regular Neon (²⁰N) has 10 protons and 10 neutrons, and regular oxygen (¹⁶O) has 8 protons and 8 neutrons.

If Huang's team can achieve a highly reliable COP of about 10, they can power a steam turbine generator and produce electricity. The team needs to roughly double their best COP. This will require roughly doubling the number of cavitation bubbles per unit volume. The team must also find a way to prevent fouling, and the buckling and rupturing of the heat exchanger tubes.    

Given its simplicity and low-cost, the NTU "fusion by cavitation" experiment is well suited for attempted replication by college engineering programs. The experiment would be an ideal senior design project for mechanical engineering students. Given there are over 300 accredited engineering schools across the United States, over 300 experiments can be underway at the same time. A loosely organized national endeavor can avoid duplication of effort and tackle a host of design options for the NTU experiment. Some of these design options are the following:  

1. stronger and different materials for the heat exchanger, e.g., steel, aluminum, titanium, and carbon composite, 
2. if possible, transparent heat exchanger sections to determine if sonoluminescence is occurring,
3. different geometries and surfaces for inducing cavitation, 
4. different working fluids and flow conditions, 
5. adding nanoparticles and/or different molecules, e.g., deuterium and tritium, to the cavitation-hosting liquid.

With over 300 experiments underway, some of the engineering seniors are bound to make a hugely beneficial contribution to the world. Can anything be more fun?

Figure 1. Cavitation bubble producing sonoluminescence and possibly fusion (Wikipedia)

Figure 2.  Exterior view of the triple-pipe heat exchanger (BJ Huang)

Figure 3. The three concentric copper tubes of the triple-pipe heat exchanger (BJ Huang)

Notes:

Huang’s January 2024 paper in Nature: Water can trigger nuclear reaction to produce energy and isotope gases | Scientific Reports (nature.com)

Huang’s 2021 ICCF conference paper: http://ikkem.com/iccf23/orppt/ICCF23-IA-21%20Huang.pdf

Huang’s 2021 ICCF conference presentation video: http://ikkem.com/iccf23/MP4/3b-IN22.mp4