Compressed air: the battery of the future?

The last thing you would want on a hot, sunny day in the not too distant future is for your solar-powered air conditioning to switch off every time a cloud gets in the way of the sun.

This mundane problem points at a much larger challenge we face as we shift towards renewable energy sources – reliable and efficient storage of power.

If left unsolved, it could threaten not only your personal comfort, but also the stability of the power grid as a whole.

At Enairys, an EPFL start-up, we are preparing for our first outdoor pilot test of what we believe to be a truly sustainable battery – one that is based on compressed air.

Compressed air is full of energy. Just watch it drive a jackhammer through layers of asphalt and cement.

Motors running on compressed air were first used to propel vehicles in the 19th century, before falling out of fashion due to the rise of the combustion engine. More recently, interest in compressed air has been rekindled, but this time as a sustainable way to store power.

But there is one physical phenomenon that has consistently thwarted efforts to use compressed air for energy storage as a battery.

When air is compressed, its temperature increases dramatically. For example, increasing the pressure of ambient air quickly from 1 bar (atmospheric pressure) to 2.5 bar (2.5 times atmospheric pressure) will increase its temperature from 20C to more than 115C.

Why is this a problem? Imagine compressing air into an underground cave. When air is pumped in, the pressure and the temperature inside the cave rise in parallel.

Then, while the air is stored, its heat is lost to the cave walls and the opposite happens: the drop in temperature leads to a drop in pressure. At the end of the day, only a small fraction of the energy that was used to compress the air can be harvested for use.

Engineers have been working on ways around this problem for decades. One way around it involves heating up the compressed air again before using it to produce power. But because this requires fuel, it comes at a cost to the overall efficiency of the storage device.

At Enairys, we are pursuing the commercialisation of an alternative approach based on a solution that all but eliminates temperature fluctuations in compressed air and the energy loss that follows them.

The technology behind our approach was developed as part of two doctorate projects at EPFL in Switzerland.

The key to our solution is a liquid piston. Rather than compressing the air with an ordinary solid piston, we use water to compress and cool it simultaneously in a dedicated chamber with an integrated heat exchanger.

Because water can absorb a lot of heat without significant changes in temperature, it can prevent change in the temperature, and therefore pressure, of the compressed air.

Once stored, the air has to be expanded again to generate electricity. To maintain a constant temperature, we again pass the compressed air through the same chambers, this time letting it take up heat from the water. This heat translates to an increase in the pressure that drives the electricity-generating turbine. At the end of the day, for every 10 units of electricity used to compress the air, seven units of electricity can be harvested.

Our current set-up is tailored to the needs of individual households and is designed to maximise storage efficiency, simplicity of use and longevity.

This system uses ordinary bundles of gas cylinders to store the compressed air. One such bundle can store up to 25 kWh of energy – the average needs of a family in Abu Dhabi for 2 days. An integrated control system automatically monitors power supply and demand, quickly switching between energy storage and energy generation.

Because every piece of our system can be easily replaced if needed, its 20-plus-year lifespan exceeds that of conventional batteries by more than a decade. In addition, its modular design means that energy storage capacity can be increased by simply adding more gas cylinders.

Sylvain Lemofouet is the CEO of Enairys, an EPFL start-up based in Lausanne, Switzerland