In an energy system based on renewable and sustainable sources, different energy storage models become crucial for a stable supply to consumers. One possible solution is Isothermal Compressed Air Energy Storage (I-CAES), which could act as a buffer for fluctuating production. This is what Ilmatar’s 2023 summer intern, Linnea Karsten, has researched and summarised as follows:
During the past year, energy and electricity prices have become a matter of public interest due to the energy crisis in Europe. The situation and the sudden shortage of, for example, natural gas led to energy production being more focused on renewable energy sources, including wind power. For a country’s energy system to rely heavily on, for example, wind power makes it more vulnerable to weather changes affecting the market and electricity prices.
The system needs stabilizing
The hourly electricity prices we, as consumers, see reflect the balance between energy production and consumption during each hour. This means that the electricity prices are lower when the energy production is larger than the consumption and higher when we want to consume more than the amount of energy produced. From the perspective of wind power, stronger winds and, therefore, higher production would lead to lower prices.
Similarly, the costs would rise during little to no wind. The excess energy production would need to be moved to hours with larger consumption to stabilise the prices. One potential solution for this challenge is Isothermal Compressed Air Energy Storage, I-CAES, which could serve as a buffer for varying production.
A simple construction
A subsea I-CAES system consists of a fixed-volume container filled with air and placed on the seabed with a bidirectional pump/turbine, meaning that it works as both a pump for charging and a turbine for discharging. It is also equipped with a support frame and a gravity anchor to strengthen and stabilise the container.
This system is charged by pumping seawater from the surrounding ocean into the container with a constant amount of air. When filling the container with seawater, the existing air is forced into a smaller volume, which causes its potential energy to rise. This process converts the electric energy from the wind turbine to potential energy in the air, and the now-pressurized air stores the energy until the system is discharged.
When discharging the storage system, the potential energy is converted back into electrical energy. This is done by allowing the pressurised air to expand into its original volume in the reservoir while pushing out the seawater from the tank back into the ocean. When the water is pushed out, it again passes the pump, now working as a turbine to restore the energy as electricity.
Comparable to pumped hydro
This process can be compared to Pumped Storage Hydropower, where water is pumped from a lower reservoir to a higher one for storage when the energy prices are low, implying, as described above, that there is less consumption than production. Lifting the water higher up gives it additional potential energy equivalent to the elevation.
To recover the potential energy from the water when additional energy is needed, the water is directed through a turbine while flowing back down to the original reservoir, converting it back into electricity. In both cases of energy storage, the energy is stored as potential energy in the media, water and air. However, the difference between the two is that I-CAES is not dependent on environmental elevation differences.
The fact that the storage system is isothermal means that the temperature stays unchanged. This is possible thanks to the surrounding ocean’s ability to compensate for any changes in the heat and keep the whole system at a constant temperature. Since the sea around the system provides extra heat when the temperature wants to sink, the storage system does not need external heating by burning fuel.
Therefore, this solution can operate without continuous emissions of greenhouse gases as opposed to some other adaptations of Compressed Air Energy Storage. However, using the ocean for cooling and heating limits how fast the system can be charged and discharged. To allow the water to have time to compensate for the temperature changes, the system is best suited for balancing out differences in production between 4-12 hours rather than quicker variations within the hour.
The concept is tested
The system described above is based on the solution developed at the University of Malta and by the Dutch company FLASC. The company presented a system prototype just over five years ago to test how well the sea could compensate for temperature changes in operation. The prototype also worked as a proof-of-concept study to show that the solution could be executed.
Studies of the prototype indicated high efficiencies for both the heating and cooling and the roundtrip efficiency, showing how much energy used to charge the system can be recovered during discharging and returned to the grid for consumption.
Ilmatar’s planned offshore projects are situated all the way up the Gulf of Bothnia.
Suitable for the Gulf of Bothnia
As offshore wind power gains popularity in Nordic countries, stabilising energy storage becomes a more relevant aspect of the modern energy system. Thanks to FLASC’s pre-charging technology, this storage system does not require large sea depths and the high hydrostatic pressure that this creates to reach high energy capacities per volume unit.
Instead, the FLASC solution can be installed in shallower waters, with depths of around 40-400 m, which makes this system well-suited for the Gulf of Bothnia. In addition, the storage system introduces little to no harmful substances to the ecosystem it’s placed in since the whole system only uses normal air and local seawater to store the energy.
As part of the Finnish and Nordic power systems, an I-CAES system could contribute to stabilising the energy supply and responding to hours of high consumption, thereby minimising the risk of extreme energy prices caused by changing weather.
Linnea Karsten holds a Bachelor’s degree in Energy and Environmental Engineering from Aalto University. In autumn 2023, she will start her master’s studies in heat and power engineering for a double master’s degree at Aalto University and Chalmers University of Technology.