Compressed air, the new stationary battery



Lead acid stationary batteries have been in use since 1859 and despite several disadvantages, such as their low energy-volume-ratio their low cost still makes them a firm favourite for a number of applications for example as stationary batteries. In  the meantime, new battery technologies have come and gone and although the latest favourite, namely the Ni battery is far superior in every way, cost still limits their use as stationary batteries.

Enter compressed air storage. Compressed air storage can make a name for itself in this arena as the new stationary battery in that it eliminates all the lead acid battery disadvantages but at lower cost in terms of energy to volume ratio than Li-ion batteries. As a matter of fact, in this regard it can compete with lead acid batteries, especially in terms of lifecycle cost. The LIGE compressed air energy storage system being an example of that.

Stationary batteries

Although lead acid batteries have evolved over the years they have in essence remained the same. The disadvantages that they started out with, viz. their low energy-to-volume ratio, limited depth of discharge, and being environmentally unfriendly, remains with them to this day. Furthermore, if care is not taken, acid burns or explosions can result. In reality, they have only two advantages: their large power to weight ratio which allows large surge currents to be supplied and low cost which, in spite of short life span and sensitivity to adverse operating conditions, still makes them a firm but reluctant favourite.

A number of new battery technologies have been introduced such as the NiCad, NiMH, Alkaline and the Li-ion. However, not all have stayed and even the current new ones on the block may have a limited life in that new pretenders are being developed all the time to eliminate disadvantages. It is also important to note that not all battery technologies can readily make the transition from the original application to that of stationary batteries, mainly because of cost.

Fig. 1: Side views of the LIGE compressed air energy storage system.

To take this thought further, what is a stationary battery. It is a battery or a battery bank which is typically used as a backup energy source during power outages to ensure continuous power for data centres, communication networks or other loads of strategic importance. It is therefore important to note that their presence would vital to their application and reliability is critical. On the other hand, their duty cycle is limited and during their lifetime would see limited use other than remain on floating charge. Conditions which typical lead acid batteries do not adapt to very well. It is not uncommon for a power outage to occur to find that one or all of the batteries in the bank reached the point where they cannot function anymore as a result of sulfation.

Compressed air energy storage

Although compressed air energy storage has been around for some time and has the potential to be scaled up from micro systems to 300 MWe, it has not become commonplace yet because efficiencies have always been relatively low namely 40 to 50% as opposed to 70 to 85% for batteries. Designing a compressed air energy storage system that combines high efficiency with small storage size is not obvious but can be done. This has been achieved in the LIGE compressed air energy storage system which performs at efficiencies rivalling that of stationary batteries.

In theory a compressed air energy storage can achieve efficiencies of 100% however, that is not possible because of losses in the peripheral components of the system such as the motor/generator and the hydraulic system, should it have a hydro-pneumatic compressor as the LIGE compressor, illustrated in Fig 1. The LIGE hydro-pneumatic compressor has three cylinders namely the low-pressure on the left for speed, the high-pressure on the right, with the hydraulic cylinder in the middle as the compressor’s main driver. Most of the losses are typically incurred by compressing and expanding the air because, as air is compressed, the temperature increases in proportion to the pressure while the opposite is true when the air is expanded to do work.

The objective is therefore to compress and expand the air adiabatically where the heat energy produced, when the air is compressed, is captured by means of a regenerator and released into the air when it is expanded and in so doing recycling the heat energy which would otherwise be dissipated and lost. Figure 2 provides a clear image of  the coils on the low-pressure side and the high-pressure side which functions as heat exchangers to capture and release the heat energy, stored in the regenerator during the charge discharge cycles respectively. Once the efficiency question was resolved compressed air energy storage is a sustainable and resilient alternative to batteries, with much longer life expectancy, lower life cycle costs, technical simplicity, and low maintenance.

Fig. 2: The LIGE hydro-pneumatic compressor.

Other than this, a level of flexibility can be designed into a compressed air energy storage system which far outperforms that of battery banks and can be applied in any application where stationary batteries are used. The hydro-pneumatic compressor, for example, is able to compress air up 250 bar and above allowing operation at a range of energy densities. From an energy storage perspective energy density is very important and in a compressed air energy system the higher the pressure the higher the energy density which allows for smaller receivers to be used or alternatively more receivers to be stalled in the same area. This ability to ramp up compressed air pressures also makes that the energy density of compressed air energy storage has the ability to far exceed that of typical stationary batteries. Compressed air stored at 250 bar has potential energy density depending on ambient temperature of approximately 0,16 MJ/l whereas stationary batteries has approximately 0,006 MJ/l. Finally, micro-compressed air energy storage systems has no self-discharge, is tolerant of a wider range of environments, and promises to be cheaper than chemical batteries, especially the more exotic types such as Li-ion.

Most importantly, a distributed network of compressed air energy storage systems would be much more sustainable and environmentally friendly because they do not require rare or toxic materials, and the hardware is easily recyclable. Stationary batteries, charge discharge cycle life, ranges between 200 to 1000 depending on operating conditions, whereas compressed air energy storage systems do not have any limits. In addition, decentralised compressed air energy storage does not need high-tech production lines and can be manufactured, installed and maintained without sophisticated machines and equipment, unlike an energy storage system based on chemical batteries. In other words, the manufacturing process of compressed air energy storage system components and assembly of the system are such that production and assembly would have to be done by hand or hand operated machines, an aspect which have a significant job creating potential.

Conclusion

Compressed air energy storage systems has the potential to take over as the new stationary battery, an area where lead acid batteries are still clinging to for what ever it is worth. However, the one aspect which still allowed lead acid batteries to remain, namely its low cost is, not a factor anymore and in addition their relatively high life cycle costs counts against them. Compressed air energy storage systems have the upper hand in a number or areas where it out performs batteries by a significant margin.

Contact Andries, Afri-Engineering, Tel 082 825-0985, andries@afriengineering.africa

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