A refuelling station for compressed air cars

In order to offer an alternative to electric cars, EPFL scientists are looking closely into compressed air vehicles. A prototype of a quick-stop refuelling station was recently developed in EPFL’s Industrial Electronics Laboratory.

Another step has been made in the optimization of compressed air vehicles. Researchers in Professor Alfred Rufer’s Industrial Electronics Laboratory (LEI) at EPFL have developed a quick-stop refuelling station for cars made at the French company MDI, which are already circulating in some airports. Now it’s possible to fill up on compressed air in less than three minutes, no longer than it takes to fill up a regular gas tank, enough fuel for about 70 km. “Individual mobility is an important consideration in the energy debate,” Rufer reminds us. “We need to study all the possible options to deal with exhaust emissions and the shortage of available resources on the planet.”

Better than electric cars?
In this context, compressed air motors have some real advantages over electric cars, which have a host of technical and environmental problems: Electrochemical accumulators have a limited lifetime, world lithium resources are far from unlimited, and refuel time is enormous compared with gas-powered cars – often equivalent to driving time, and there is no technical solution to this problem (see figure). In addition, batteries for electric cars are drained even when the car is parked. In opposition, the compressed air car is not made up of elements that need to be regularly replaced. In addition, the pressure in the reservoir doesn’t diminish while the car is parked over a long period of time. And thanks to recent research done at EPFL, the refuel time has been drastically reduced, going from several hours to less than three minutes, depending on the type of compressor used.

Less heat, more autonomy
This new rapid refueling process has given EPFL researchers something to get their teeth stuck into.  It must be done in two stages (see schematic below): first the air is compressed in a high-performance compression station and transferred to an intermediate storage station. This operation, which allows for constant supply to the intermediate station, takes place slowly, in nearly isothermal conditions, meaning that the heat arising from compressing the air is continually evacuated via an integrated heat exchanger. In the second stage, the compressed air is transferred from the intermediate storage station to the car’s tank, this time very rapidly, using a process of flow rate limitation, which only emits a small amount of energy.

The only problem, which is caused by this rapid compression, is that the air heats up in the tank, reducing the density. When the vehicle leaves the station, it cools down slowly, reducing the pressure in the reservoir and, consequently, decreasing the distance it can go without needing another refuelling (its autonomy).

Detailed explanation
When you increase a gas temperature, it loses density and gains volume. The fast filling of compressed air cars poses a problem. During the refueling, the heating of the air due to compression makes it impossible to obtain the required air density in the tank. Then, when the air cools down and loses volume, we see that the tank is not 100% full.

In order to fill the tank in an optimal manner, Rufer’s team designed a system that recirculates the air after the equalization of the pressure during fill-up, which instantly cools the reservoir. The air that’s warmed up by pressurization is sent back to the intermediate station, and replaced by cooler air. The fill-up is  done in two steps, increasing the density of the air in the tank and stabilizing the temperature. “Using this method, the yield from the transfer is theoretically close to 95%,” notes Rufer.

Are we heading towards degrowth?
Because of their restricted autonomy, compressed air cars can not rival petrol-powered cars for the moment. However, they could become a serious option in the zero-emission vehicle lineup, and would be perfectly adequate for short trips in town –delivering mail, for example or in airports. Rufer is of the opinion that “people need to reduce their energy consumption and rethink the fundamentals. Some people who are accustomed to the comfort of internal combustion vehicles deny the utility of more energy-efficient but less high-performance solutions, such as compressed air cars. They will nonetheless have to reconsider when there are no longer any available resources. In this context, there’s no need to push actively for degrowth. We’re going to be subjected to it sooner or later.”


"Car of the Future", from Ideaslab, the World Economic Forum at Davos