As our energy sector transitions from fossil fuels to cleaner alternatives, there is a pressing need to provide storage capacity with the power networks. Energy storage provides a mechanism that mitigates the untimely generation from variable renewable sources. In essence our energy demands do not always coincide with the sun shining or wind blowing.
There are two types of air energy storage:
- CAES – Compressed Air Energy Storage
- LAES – Liquid Air Energy Storage
The LAES process [2] stores liquid air at low pressure. The advantage of this is that liquid air has a density of around 870kg/m3 and as such the storage volume is significantly reduced and above ground. The low temperature of the liquid air, -193°C, makes it easy to re-gasify for the discharge cycle. Water and carbon dioxide removal is generally required to prevent solids formation in the low temperature sections.
Figure 2 Simple LAES system
It should be clear from Figures 1 and 2 that both processes are similar in their general transfer of energy between supply and demand. They differ in their respective means of storage. Both systems have industrial scale facilities operating that demonstrate their respective designs and can scale to over 1 GWh.
Metrics to quantify the efficiency of a given process include round-trip efficiency (RTE) and charge to discharge cycle time ratio.
For the processes described above, the stand-alone round-trip efficiency and charge to discharge time ratio are as follows:
- A-CAES – 55% to 65%, 10hr/6hr
- LAES – 50% to 60%, 10hr/4hr
These values reflect realistic and consistent electrical, mechanical and thermodynamic efficiencies for the import and export motors and generators, grid connection losses, compressors and turbines. One may see claims of a round-trip efficiency of 70% or more for some highly optimised and proprietary air storage systems. Care must be taken when comparing such systems and making sure they are presented on a like for like basis.
We can see that the LAES system typically has the lower round-trip efficiency and a lower available energy storage for a like for like charge cycle duration. Alternatively, the same discharge cycle may be set, and the charge duration required is 50% longer for the LAES system. This observation simply reflects the liquid air yield for the liquefaction process.
The impact of having a longer charge cycle or a shorter discharge cycle is not necessarily a negative. It depends on the energy market in which the system is to be integrated. If the low cost or renewable energy is only available within a specific window, or the grid requires an extended energy supply for longer duration, then these considerations become important. It would be addressed on a system by system basis.
Optimisations can be undertaken on both processes that may improve these comparisons, but they serve as baseline yardstick at the time of writing.
A subtle point is the ease by which the CAES system achieves its round-trip efficiency and charge to discharge ratio. In contrast, LAES must invoke addition process complexity to bring the two systems within a comparable range. However, the complexity is made up with well understood and generally accepted off the shelf technology.
References:
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- How Hydrostor A-CAES Technology Works https://www.youtube.com/watch?v=kvyuzSto0vU
- Highview’s LAES Animation https://www.youtube.com/watch?v=kDvlh_aG7iA&t=134s