Air could be the world’s next battery
Wind and sun, two unpredictable resources, are becoming ever more important as sources of energy in Europe. This means that we face a growing need for energy storage facilities, because if energy cannot be used immediately when it is generated, it needs to be stored until it is needed.
The least expensive method is to use hydropower reservoirs as ‘batteries’: i.e. generate electricity using the stored water, when power is in short supply, and subsequently pump the water back uphill when surplus renewable energy is available. However, this is a practical solution only in mountainous regions, such as we have in Norway and a few other countries.
What if less fortunate countries and regions could use air instead of water as a way of storing energy? Under the auspices of the European Union, scientists from all over Europe are attempting to turn this concept into a viable prospect, via a research project (RICAS 2020) of which SINTEF is a member. The project participants have in mind all parts of the world where sealed disused caverns could be used as storage sites.
Like a hot bicycle-tyre pump
The general principle, which has already been adopted at a few sites around the world, is essentially a matter of using surplus electric power to compress air, which is then stored in an underground cavern. When power needs to be made available, the air is released through a gas turbine that generates electricity. Existing plants of this type are often used to meet peak demand as a supplement to classical power plants, providing the right amount of electricity needed at different times during the day.
The physics governing storing energy in the form of compressed air is a result of a law of nature familiar to every user of a bicycle pump: the process of compressing air heats it up. Bicycle pumps compress air in order to increase the pressure of the tyres, and in doing so, makes the pump hot.
“The more of the heat of compression that the air has retained when it is released from the store, the more work it can perform as it passes through the gas turbine. And we think that we will be able to conserve more of that heat than current storage technology can, thus increasing the net efficiency of the storage facilities,” says Giovanni Perillo, project manager for SINTEF’s contribution to RICAS 2020.
Underground caverns as heat storage
The two largest compressed air stores in the world are in Germany and the USA. They are underground chambers created in salt formations. But these plants lose a large proportion of the potential energy of the compressed air, because they do not incorporate a system to store the heat produced during the air compression stage.
The participants in RICAS 2020 have a recipe for reducing these losses in future underground storage caverns. At the core of the recipe is an extra station that they have incorporated in their solution.
On its way down to the underground cavern, the hot compressed air passes through a separate cavern filled with crushed rock.
The hot air heats up the rock, which retains a large proportion of the heat.
The cold air is stored in the main cavern
When the air subsequently returns through the crushed rock on its way to be used to generate electricity, the flow of air is reheated by the stones.
Hot air is then expanded through the turbine generating electricity
Cheaper than batteries
SINTEF’s project manager explains that it is estimated that this technology could raise the efficiency of the system to as much as 70-80%. The corresponding figures for most of the existing storage sites are no better than 45 to 55 per cent, which means that the produced energy is only half of what was initially used to compress the air into the cavern.