Hey SteeMates, hope you’re all doing great! I’m back with a new episode in the Energy storage system series and I have a little question for you guys:

Aren’t batteries too mainstream?

Ah mountains!

You know the place where you go to reunite with mother nature through skiing, hiking or running away from bears. What if I told you we could use them to store energy?

You don’t believe me? The secret to it is Advanced Adiabatic Compressed Air Energy Storage (AKA AA-CAES). Let me explain!

The Technology

So, how does it work?

Basically, CAES is a means to store excess energy – often in an underground vessel– by compressing ambient air. When energy is needed, you simply have to heat up the air and expand it which will activate a turbine and drive a generator to produce electricity. The sketch below might clear that out for you.

When you compress air, it creates heat but why? It has a little something to do with kinetic energy. Indeed, the kinetic Molecular Theory states that:

The average kinetic energy of a collection of gas particles is directly proportional to absolute temperature only.

In other words, the faster the molecules, the hotter the gas.

When you compress a gas, the molecules are being pushed in a container, this increases the speed of the molecules and therefore the heat of the compressed air. The pressure at which we usually store ambient air is somewhere in between 60 and 70 bar - which is no less than approx. 70 TIMES the pressure in the atmosphere - and therefore a huge amount of heat is created - around 600°C.

What happens to the heat created?

Well, you have 2 options, store it or waste it, these 2 methods are known as adiabatic and diabatic.

The diabatic method

In this method, the heat is removed using intermediate coolers and released in the atmosphere as waste. This is a big issue as an important part of the energy stored comes from the heat produced during compression. Moreover, if the temperature of the gas is too low, you need to reheat it prior extraction.

This is generally done using gas burners, which not only emits CO2 but also reduces the energy efficiency of such a storage system with a power-to-power efficiency of only 42%. Nonetheless, this is the most used system with 2 active plants, one in Huntorf - Germany - and the other one in McIntosh - Alabama.

The adiabatic method

In this case, the heat created during compression is recovered and stored in solids like concrete or stone or in fluids such as hot oil or molten salt solutions which can respectively hold 300°C and 600°C. The heat us then used when discharging the energy storage system.

AA-CAES is based on the adiabatic method which makes it a much more attractive solution both in terms of efficiency and CO2 emission.

The Business Opportunities

ALACAES case study

ALACAES is a Swiss company, which is currently developing an AA-CAES solution for large-scale electricity storage. The technology it has developed – and patented - uses caverns in mountains to store the excess energy as compressed air and has successfully built and tested the first - worldwide - pilot plant in the Swiss Alps in 2016.

So, why use mountains?

Well, you need a large container and it is not possible to make one out of steel or any other material as it would be too expensive. ALACAES has chosen to use the weight of the mountain and that of the earth to its advantage and has been successful doing so.

The plant is a 120 m long cylinder of 5 meters of diameter and a capacity of 1MWh. To “charge”, the excess energy from the grid is pumped into an underground vessel. When passing through the thermal energy storage – AKA TES - the air drops in temperature and once the air cools down enough, it is stored in the empty volume of the cavity. To “discharge”, the process is simply reversed and the air flows through the TES once again, this time to heat up the air.

Not only was the project successful but it also received support from the Swiss Federal office of Energy who financed 40% of the project.

Should we use AA-CAES?

Advantages of AA-CAES

• 4 to 6 times cheaper than Li-ion batteries in price per KWh.

• The bigger the project, the bigger the bang for the buck. According to Dr. Giw Zanganeh, – MD at ALACAES - it would cost between 100 and 200 million USD for 200to 500 MWh output.

• It is by far one of the technologies that pollutes the less. Li-ion batteries can store 10 times the energy used to make the battery vs 250 times for compressed air.

• Such a technology could be a real asset in Europe, Canada, India, China or in other regions where mountains are abundant.

• The round-trip efficiency is over 72%, although it is less than Li-ion batteries there is still room for improvement.

Disadvantages of AA-CAES

• Not transportable, as in you won’t be able to put a mountain in your car.

• There are similar alternatives such as pumped-hydro power plants.

• Prices of Li-ion batteries might continue to fall making it a less interesting investment in the long term.

• You need to have mountains in your area.

The End

So, there you go guys! That's how and why people are trying to store energy in mountains today!

I Hope you've all enjoyed this Article, please leave a comment if you feel like it and an upvote if you like what you've felt! Resteems and new followers are also greatly appreciated!

Also, I would like to know what YOU think about this energy storage system? Do you think it's worth investing in? Or do you think it doesn't stand a chance against the competition?

N°	Type	Source
1	Article	EnergyStore AA-CAES
2	Article	EnergyStorage CAES
3	Article	Wikipedia CAES
4	Report	INERIS CAES
5	lesson	Kinetic Molecular Theory
6	Company Site	ALACAES
7	Video	ColdFusion

N°	Image
1	Mountain
2	CAES
3	logo
4	Swiss Alps
5	Bell

Fun Battery Fact:

In Oxford a battery powered bell has been ringing for over 175 years. No one knows exactly what the battery is composed of and no one dares tearing it a part. Fortunately for the neighbours, the charge is so small that the noise produced is almost non-existent.

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