It’s been a torrid summer in Europe with wildfires raging across the continent – and dangers extending to the air with a helicopter crash involving firefighters in Greece.
Autonomous and remote technologies are being courted with renewed vigour, in an attempt to reduce human risk and improve accessibility. And cryogenics may also provide part of the solution.
Northeastern University has developed a prototype of a firefighting robot which uses a cryogenic liquid to snuff out hotspots.
It’s been a long journey for Yiannis Levendis. About 20 years ago the professor of mechanical and industrial engineering was working on an experiment to find an alternative to coal – grinding up old car and truck tires and burning them.
When he was igniting rubber tyre bits in his lab, on impulse, he fetched a spoonful of liquid nitrogen and threw it on the flames.
“Liquid nitrogen, when thrown on fire, quickly evaporates and expands into gas almost 1,000%,” he said. “That suffocates the fire and creates a cooling effect on the fuel.”
And that’s exactly what happened. “It was almost instant extinguishment,” Levendis said.
© Matthew Modoono / Northeastern University
He remains hopeful about its potential because, unlike other retardants used to fight in wildfires, liquid nitrogen leaves no chemical residue that could damage the soil or harm the environment.
Many fire retardants used on wildfires today contain phosphorus, a chemical used in fertilisers, and according to the US Geological Survey, in heavy concentrations, it can run off into streams and ponds.
That can cause algae blooms that can kill fish and other aquatic lifeforms. Some retardants also contain sodium ferrocyanide, a chemical that can hurt the soil.
Tom Ryden, Executive Director of MassRobotics in Boston, said that Levendis’ invention just might catch the eye of firefighting agencies or an industrial partner.
“A perfect use for robots is to send them into dangerous spots,” said Ryden, whose Boston-based company serves as an incubator firm fostering more than 100 startup companies in the Boston area involving robots.
Police use robots now to retrieve and defuse bombs, and also to go in close on hostage situations to assess the danger.
“Even if we lose a robot, it’s just the cost of the machine that’s lost, not a person,” Ryden said. The prototype is being tested but Levendis said he envisions other applications.
“I believe it can be converted to vehicles as large as firetrucks,” Levendis said. “We are fairly close.”
Cryogenic challenges
While cryogenic liquid nitrogen offers an attractive, residue-free approach to smothering fires by freezing the fuel bed and displacing oxygen, scaling it to tackle vast wildfires presents massive thermodynamic, logistical, and safety challenges.
While liquid nitrogen is incredibly cold, its actual physical heat-absorption capacity is significantly weaker than water.
Pound for pound, water absorbs roughly 11 times more energy just to convert into steam than liquid nitrogen does to vaporise.
The infrastructure, production, and specialised equipment needed for liquid nitrogen make it exponentially more expensive than utilising natural water resources or standard chemical retardants.
Another major challenge is liquid nitrogen expands roughly 700 to 1,000 times its volume when converting to a gas. In a wildfire setting, a sudden, massive blanket of nitrogen gas will completely displace oxygen. This creates an immediate suffocation hazard for ground crews, nearby residents, and local wildlife.
As a result of these limitations, liquid nitrogen is therefore ineffective as a standalone agent for open-air forest fires. However, it may be successfully used in highly localised, confined fire scenarios.
As climate challenges intensify, one thing is for sure: firefighters are going to need every tool available in their armoury.
