Compressed air energy storage (CAES) is one of the few storage options that this blog has not looked into, and here I review how this technology might contribute to an all-renewables world. A brief review of land-based CAES storage indicates limited potential (only two plants with a total capacity of 400MW/4GWh – one of which is 40 years old – are presently in commercial operation, and both require “in-ground natural gas combustion” to work). A new approach that involves storing compressed air in balloons or concrete spheres on the sea bed reportedly offers superior potential, but on closer inspection we find that this technology is costly and far from simple, that the potential is probably not as high as claimed and that large-scale commercialization, if it ever happens, is still years away.

My attention was first drawn to the question of underwater energy storage by an article in which the Fraunhofer Institute made the following claim:

The Fraunhofer Institute for Wind Energy and Energy Systems Engineering envisions spheres with inner diameters of 30m, placed 700m (or about 2,300 ft) underwater. Assuming the spheres would be fitted with existing 5 MW turbines that could function at that depth, the researchers estimate that each sphere would offer 20 MWh of storage with four hours discharge time.

I had always assumed that a 30m diameter sphere, which holds 14,000 cu m of air (at standard temperature and pressure) at a depth of 700m below the sea surface, would have the same storage potential as 14,000 cu m of sea water in a reservoir 700m above sea level, i.e. about 20MWh, and calculations confirm that this is indeed the case. But air is compressible while water is not, with heat being generated during compression and lost during decompression, and how to handle these heat gains and losses complicates an underwater compressed-air storage operation. A more detailed explanation of the complexities involved is provided in this 2016 paper by Pimm and Garvey of the University of Nottingham. Additional information is also available from Wikipedia.

Two types of underwater compressed air storage have so far been been tested at the pilot scale. The first is the “balloon” concept that Toronto Hydro recently used in a demonstration facility in Lake Ontario. The balloons are free to expand and contract to equalize pressure between the air inside and the water outside and were anchored to the lake bed by concrete-filled platforms. They were connected to the shoreline installations, which included a compressor, a generator and a “proprietary thermal store” that captured and stored heat generated during compression, by an air hose that pumped air into the balloons during periods of surplus generation and extracted air from them during deficits. A schematic of the operation are shown in Figure 1:

Figure 1: Schematic of Toronto Hydro pilot underwater CAES project in Lake Ontario

Little additional information on the project is available. The balloons were located in 55m of water three km offshore. Capacity was around 700kW with 1.5MWh of storage. Cost was not specified. Hydorstor, the company that developed the technology, nevertheless anticipates that stored energy from a 10MW system of this type will cost about $250/MWh, which will be competitive with batteries for short-term storage but prohibitively expensive for large-scale seasonal storage applications.

The second type of underwater storage uses concrete cylinders instead of balloons (Figure 2):

Figure 2: Operating concept, Fraunhofer spheres

The Fraunhofer Institute recently conducted a pilot test involving a 3m diameter concrete sphere located 200m offshore and at a depth of 100m in Lake Constance in Switzerland. No further details are available, but the test is reported to have been successful, and Fraunhofer concludes from it that a 30m diameter sphere at a depth of 700m in the ocean will indeed have 20MWh of usable storage capacity. However, it also estimates that the levelized cost of storage at between €40 and €200 ($50 to $230 U.S.) per megawatt-hour for a 400MWh offshore facility containing twenty 30m spheres. This price range is also competitive with batteries but once again prohibitively expensive for large-scale seasonal storage applications. Figure 3 shows Fraunhofer’s 3m concrete sphere:

Figure 3: Fraunhofer’s 3m concrete sphere, Lake Constance in background.

Underwater CAES nevertheless has one major advantage over land-based CAES. Assuming the sea floor is reasonably level a one-size-fits-all design can be adopted regardless of location while the designs of land-based systems must be tailored to local conditions. There are, however, limitations on where underwater CAES systems can be installed. According to the Pimm & Garvey report linked to earlier the ideal depth for such systems is 500-700m, and the system should be located within 5km of the coast to minimize losses in the air pipe that connects the spheres with land-based facilities. This raises the question of resource availability, which according to Pimm & Garvey’s study is abundant around Europe and North America, and which according to Greentechmedia amounts to 817 TWh worldwide. There are, however, questions as to how globally abundant inshore CAES resources really are. The maps that Pimm & Garvey show for California and the Mediterranean, reproduced below as Figure 4 are examples:

Figure 4: Prospective underwater CAES sites within 5km of land and more than 400m of water, Central Mediterranean and Central/Southern California

Note that only some of the selected areas are within 5km of the mainland. The majority are within 5km of islands that have no grid connections with the mainland. This is sure to add cost and complexity.

Moreover, the reason Pimm & Garvey show maps of California and the Central Mediterranean may be that with the possible exceptions of Japan and “overdeepened” Norwegian fjords these are the only two places in the world where near-shore underwater CAES potential exists within range of major centers of energy consumption. Water depths around the eastern US, northwest Europe, China, India, Russia, Canada and Australia are generally too shallow to support CAES installations within 5km of the coast. In these areas near-shore installations would have to be placed in shallow water, with a consequent loss of efficiency, or in self-contained deep-water plants a long distance from shore to which Pimm & Garvey do not give serious consideration:

Locating the compression and expansion machinery on the seabed could reduce the air transmission distances and hence increase roundtrip efficiency, but it has not been seriously considered because of the scaling requirements and, in particular, the immense difficulty this would introduce to access and maintenance.

Figure 5 compares the basic design of an inshore (<5km from the coast) and offshore (>5km from the coast) underwater CAES plant:

Figure 5: Basic designs of near-shore and offshore CAES systems

It has been suggested that the best use of undersea CAES would be to act as storage for next-generation 5MW floating wind turbines in deep water environments, as discussed by Wind Power Monthly. This approach, however, involves combining two immature technologies and is not considered further at the moment.

A few additional comments before concluding:

The Engineer quotes Garvey as stating that “the UK should aim to install 200GWh of CAES, with a cost for the energy storage alone of between £0.2bn and £2bn depending on what compressed air stores are adopted. He believes a further sum of up to £10bn may be required for the associated energy conversion equipment“. If we assume that 200GWh of CAES costs £10 billion the 35 TWh of backup storage needed to support a 100% wind + solar UK estimated by Alex Terrell & Andy Dawson in this post would cost £1.75 trillion.

Compressed air has been used to generate electricity since the 19th century, when Paris installed a system of pipes for municipal distribution of compressed air to operate generators for lighting. Over a hundred years later there are still only two CAES plants of any size (Huntorf, 330MW and McIntosh, 110MW) in operation, and neither can operate without natural gas preheating.

Finally, according to Greentechmedia CAES companies have long claimed superior cost and performance — and have not lived up to their hype. Will an underwater approach bring a different outcome?

It seems not.