Under the sea: ‘Energy Bags’ add to electricity storage options

Canadian firms Thin Red Line Aerospace and Hydrostor are set to trial underwater compressed air energy storage (UW‑CAES) systems off the coast of Toronto.

As part of the process, electricity is used to pump air into giant heavy duty ‘Energy Bags’ anchored below sea‑level, causing the diaphragm expanding against the force of the surrounding water. This potential energy can later be extracted on demand and converted into electricity by driving a generator with the compressed air.

Working prototypes displace up to 40 tonnes of seawater, with the amount of energy storage dependent on the depth at which an Energy Bag is submerged. Ideally, Energy Bags would lie in the vicinity of 400-700 metres below sea level. The UW‑CAES systems are modular and, as such, can be scaled up depending on the size of the load.

Compressed air storage systems use reasonably mature technologies and there are known system inefficiencies relating to energy loss through heat. But despite the energy losses, Hydrostor founder Cameron Lewis estimates the overall storage cost for UW‑CAES to be half that of batteries. The completion of pilot projects will likely allow for more thorough project costing in future.

CAES technologies have been deployed previously for land‑based applications, using either depleted underground mining reservoirs or super‑strong metal gas containers to store the compressed air. In comparison, UW‑CAES systems have the advantage of providing constant pressure available regardless of the fill volume and considerably lower cost of storage than heavy‑duty metal containment vessels.

Like most large scale grid energy storage options, CAES systems can allow for improved operating efficiency, better utilisation of existing network generators and the provision of quick‑start electricity services. Initial drivers for the Toronto project centred on it being a method for price arbitrage and peak shifting. However, if successful, UW‑CAES will also be a strong candidate for the storage of electricity produced by renewables such as solar and wind.

As an example, one possible application floated for the technology is for the storage of off‑shore wind turbine electricity – the excess electricity produced by the wind turbines can be stored in close proximity to the floating generators. Australia’s major population centres also lie within kilometres of our coastline, providing yet another natural resource we may utilise for our energy use. If successful, offshore UW‑CAES systems may be a suitable technology for large scale storage within our own national grids.

Much like solar and wind, the idea of energy storage is gaining momentum in Australia, as evidenced by the recent formation of the Australian Energy Storage Council. In parallel with in‑house and electric vehicle battery storage, UW-CAES technology adds to the growing list of energy storage options which renewables—as intermittent electricity sources—may be able to utilise for round‑the‑clock access to clean energy. The number of energy storage innovations is growing and we are only seeing the tip of the iceberg.

Top Image Credit: Thin Red Line Aerospace

© 2014 Solar Choice Pty Ltd

John Rodriguez

John regularly contributes original technology articles to Solar Choice News. He is a PhD candidate in solar PV engineering at the University of New South Wales (UNSW), having graduated with First Class Honours in a Bachelor of Engineering (UNSW, specialising in PV). His knowledge of and passion for renewables technology led to him receiving the federally-funded Australian Postgraduate Award and Engineering Research Award for research excellence, in addition to being a Co-operative Program scholar during his undergraduate studies. John also works as an energy efficiency and process engineer and analyst.
John Rodriguez