A working plasma jet engine

The pandemic has led to a sharp reduction in global emissions and had other positive environmental effects too. If reduction in emissions becomes more permanent, that will be one good outcome.  

Due to the lockdown, travel-related emissions have dropped, due to flights, trains and public transport shut downs. Workplaces and educational institutions have launched work from home (WFH) and online learning programmes. Online conferences and webinars have become the new normal. In addition, many industries are shut, reducing power demand.

Travel, trade and industry will recover. But some changes in paradigms such as webinars and WFH may become permanent. So far as corporates are concerned, conferencing online could prove a major saving since the technology has now been proven.

Civil airlines contribute 2.5 per cent of global carbon emissions in a normal year and reductions in commercial flights could lead to a decline in emissions. Shipping incidentally adopted new clean fuel standards in January 2020, and those will also contribute to lower emissions.

Another technology may contribute in the long term to a sharp reduction of aviation-related carbon emissions. Jet engines work on kerosene (also called aviation turbine fuel). Imagine a jet engine that uses just electricity (from clean sources) and needs no fossil fuel at all.

There’s a lot of research going into the concept of the plasma jet engine as these are called, and there have been recent breakthroughs. A conventional jet engine burns fuel, generating hot (highly polluted) air. This spurts down a tube at high pressure and hits a turbine, making the fan-blades rotate to create power as the air is forced out by exhausts at the plane’s rear. The plane moves in the opposite direction due to Newton’s Third Law (For every action, there is an equal and opposite reaction).

Now imagine that, instead of burning fuel to heat air, an electric charge is directed by microwaves at the air. This creates ionisation. Gases are stripped of their electrons and turn into plasma, with a positive charge. Given an electromagnetic field, plasma flows, creating thrust.

Plasma thrusters have been used in spacecraft with the first pulsed plasma thrusters deployed as long ago as the 1960s. But these thrusters are quite weak and not capable of operating in atmosphere and they must carry their own “fuel” in the form of stored xenon gas. In 2018, a research team from MIT demonstrated a plasma-powered microlight glider, which also has relatively weak thrust but can operate in atmosphere. (https://www.nature.com/articles/s41586-018-0707-9)/Now a new paper from the Institute of Technical Sciences, Wuhan University (https://aip.scitation.org/ doi/ 10.1063/ 5.0005814) claims a team of engineers have demonstrated thrust comparable to a conventional jet engine. They used high-temperature, high-pressure plasma generated in a 2.45 GHz microwave ionisation chamber, injecting air at high-pressure. The paper says, “propulsion forces and jet pressures comparable to those of commercial airplane jet engines can be achieved”.

The experimental setup involved putting a hollow steel ball on top of a tube. High-pressure plasma was injected into the tube and set on fire and imparted thrust to the ball, which rattled around. Normal air (not plasma) was also injected into the tube to keep the plasma flow contained and stable.

Mass was added to the steel ball in controlled fashion as the electrical field was varied in strength (the ball had a hole through which small balls could be inserted to change mass). The movement of the ball was measured, and allowances made for the non-plasma air injection.

The paper estimates that using a standard tesla battery pack with 310 Kw equivalent power, it should be possible for this experimental lab apparatus to generate a force of about 8,500 newtons. (A newton is the force required to accelerate 1 kg of mass by 1 metre per second squared. Scaling up in size, it should be comparable to a conventional jet engine with say, 10x the thrust.

The paper says, “Therefore, using a high-power microwave source or an array of multiple microwave sources, with materials resistant to high temperature and pressure, it is possible to construct a high-performance microwave air plasma jet thruster in the future to avoid carbon emissions and global warming that arise due to fossil fuel combustion.”

However there will be major challenges to solve before this proof of concept can be scaled into a working jet engine. Massive heat is also generated in this experiment. Dealing with the impact of high temperature on equipment and managing the thrust would be tough. A working plasma jet engine could move the aviation industry much closer to zero emission.  

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