a load of hot air – comparing air energy storage technology

written by Craig Branch

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.

Energy storage also has an added benefit of providing a means to improve network stability. This is not new to national grid systems and the use of fast response energy systems like Dinorwig Power Station, known locally as Electric Mountain, in Snowdonia, is an example of a pumped-storage hydroelectric scheme. Pumped water storage generation offers a critical back-up facility during periods of excessive demand on the national grid.

Unfortunately, not all geographic areas come with suitable elevation change and water reserves to allow for a pumped-storage hydroelectric scheme. This is where air storage can provide a viable alternative as a high power, long duration energy storage solution.

Air energy storage solutions are based on a simple premise – when energy is cheap or available from renewable sources due to lack of demand, energy is stored in a mass of air and provides a mechanism to recover the stored energy at a time that is more convenient.

There are two types of air energy storage:

  • / CAES – Compressed Air Energy Storage
  • / LAES – Liquid Air Energy Storage

CAES stores gas at high pressure and when the time comes to get the energy back, the compressed gas flows through a series of air turbines and generates electricity via a grid connected generator. For improved energy utilisation, heat produced during compression is stored and re-used during the expansion cycle, see Figure 1. The large volume of gas is typically stored in an underground cavern and can be pressure compensated such as Hydrostor’s Advanced-CAES (A-CAES) process[1]. This provides a constant pressure cavern during the charge and discharge cycles. The cavern can be constructed in most rock types, reducing constraints on feasible locations. The CAES process is clean, robust, scalable and simple.

Figure 1 Simple A-CAES system

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.

To make liquid air, the industry standard Claude processes or one of its variants can be used. The liquefaction process is generally inefficient. The energy utilisation and liquid yield is greatly improved by integration of the liquid air regasification process within the liquefaction process. This requires the addition of cold temperature storage, see Figure 2. The LAES process is also clean, robust and scaleable but evidently more complex than the CAES process.

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.

Both processes also benefit from the availability of additional heat sources. In this case, the heat is used in the discharge phase to provide additional superheat and improved turbine power output. Due to the similarity of the processes in this section, the benefits are effectively the same for both systems.

The LAES can also benefit from additional cooling capacity. If “cold” is available, for instance from the regasification of LNG, then the inefficiencies of the air liquefaction process may be offset, and the liquid yield and round-trip efficiency improved. Claims of greater than 80% round-trip efficiency are made for this type of process integration.

So, who wins in the air energy storage market?. Both systems will have their place. Where location and construction allow CAES systems are attractive, as a reliable and well understood process. Where integration into existing “cold” process infrastructure is available, LAES can achieve impressive round-trip efficiencies. Cost and operability will be an important decider, and this could be a topic for another article.

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[1]How Hydrostor A-CAES Technology Works https://www.youtube.com/watch?v=kvyuzSto0vU

[2]Highview’s LAES Animation https://www.youtube.com/watch?v=kDvlh_aG7iA&t=134s