Making a case for CAES (Compressed-Air Energy Storage) — Joseph Gaines

CAES system image via Renewable Energy World / SSS Clutch.

If you’ve even halfway been paying attention to the news in recent years, you know that, for all their power (pun intended) and potential, you still need often need someplace to put excess electricity generated by wind and solar. The Lithium battery, that problematic darling of energy storage technologies, gets all the attention these days.

But sometimes the best alternatives to the potential environmental destruction wrought by mining are literally full of hot air.

Compressed-Air Energy Storage, or CAES, is a form of energy storage that has been in use in one form or another for over 150 years. Put simply, the technology compresses and stores air at high pressures for later use. During release (expansion), the air can be used in a variety of applications for powering turbines, locomotion, and more.

Some early practical examples from 1870 onward include citywide systems installed in Paris, Birmingham (U.K.), Buenos Aires, and several cities in Germany.

An 1896 system in Paris used 2.2 MW of generation spread across 50 km of pipelines to power light and heavy industry; uses included house-delivered energy and powering machinery in dentistry, textiles, printing and baking.

The first utility-scale CAES plant however was not constructed until 1978. The 290 MW plant, located in Huntorf, Germany, provided 580 MWh energy at 42% efficiency.

A number of other utility-scale facilities have been constructed (or planned) worldwide since, with highly varying degrees of success among each.

Increased attention to the ongoing global energy transition away from fossil fuels has brought interest anew to this proven albeit sometimes limited technology.

What’s great about CAES? And what’s working?

The principal benefits of CAES systems are ostensibly reduced cost, reduced waste, and a far lighter impact on the environment overall.

Just like their peer technologies of electrochemical batteries and stored hydro, CAES systems can both mitigate overall congestion in grids and provide additional energy during periods of peak demand. All such storage systems help stabilize the unpredictable generation of wind (which is most active at night) and solar (most active during the day), thereby helping the broader transition to renewables make both more practical and economic sense.

CAES also typically produces no waste other than heat (depending on which version of the technology is employed) and does not rely on extractive technologies that potentially harm both the environment and human populations.

Since the 1990s, a number of more recent, proven applications at utility scale have either been constructed or planned worldwide, including:

US Dept of Energy-funded projects from the early 2000s onward in Kern County, CA (by PG&E) and Watkins Glen, NY (by Iberdrola USA).
Privately funded projects in McIntosh, AL; Gaines County, TX; Anderson County, TX; Cheshire, UK; and Goderich, Ontario.
Other facilities in Austria, China, Italy, and more.

In the form of pneumatic motors which drive engine pistons, turn axles, or drive turbines, the technology has also long been in use for:

Trains – especially locomotives used in mining
Cars and trucks – especially of interest for use in hybrid vehicles for regenerative braking and optimizing the efficiency of the piston engine. These can also benefit from integrated air conditioning (i.e. the cooling that’s generated as a byproduct of releasing the compressed air).
Ships – compressed air can be used to turn the crankshaft prior to fuel injection, which can basically start the ship’s engines without creating an excessive spike in demand on the ship’s electrical systems each time the engine starts. These engines can then continue to run on compressed air only in the event of a shipwide electrical failure, and generators can be restarted without an additional electrical supply.

**What are the biggest challenges? What’s not working?**

Thermal Efficiency

How you employ CAES (and thereby how much efficiency is derived) depends entirely on what you want to do with the heat: when air is compressed, it heats up. When it’s released (expanded), it cools down. What you do with the heat dictates what a CAES system can do for you in turn.

There are a variety of approaches to this process challenge, including:

Adiabatic – the system is entirely insulated such that theoretically no heat is lost during compression whatsoever (i.e. 100% efficiency). Actual RT efficiency however approaches 70%. There are no utility scale plants using adiabatic storage in operation just yet.
Diabatic – the system dissipates most of the heat using coolers; this is released into the atmosphere as waste heat. Current plants in operation have fairly low levels of efficiency (McIntosh, AL at 27%, and GE’s planned combined cycle plant at 54% efficiency).
Isothermal – the system attempts to maintain constant temperature using heat exchangers. Currently, the heat exchangers one would use for this process are only practicable at low power levels. Theoretically, this can mean near 100% efficiency in storage. However, there are no utility-scale CAES plants using this technology just yet.
Near-isothermal – compression happens with the gas adjacent to a non-compressible structure which absorbs and releases heat, stabilizing the temperature. This process can typically achieve 90-95% efficiency.
Variable-pressure series of pistons – used in mining locomotives and trams, this technology draws air over heat exchangers, moving exhaust heat from one stage to the next.

Weight

In order to be effective in transportation (especially air transport), the system must be neither too big nor too heavy. However safety considerations – like fears of catastrophic decompression – dictate the amount of pressure that a given system can safely contain. This in turn impacts both the size and weight of the system as well as how a container should be expected to perform in the event of a crash.

Location

CAES plants typically utilize either caverns or (potentially) inflatable underwater storage structures built in deep lake beds or sea areas with steep coastal dropoffs. Even underground storage, which in principle should be easier to manage than finicky undersea locations, can be problematic: salt domes for example are commonly used for this kind of storage, but they are relatively uncommon formations, and don’t always lend themselves to an optimal location on the grid.

By definition, this means there are limited locations for plant construction, which means cost efficiencies in materials and construction must be obtained by other means should the location itself prove prohibitively pricey.

CAES vs Batteries

Batteries provide more stable voltage; from an engineering feasibility standpoint, CAES would struggle to beat this level of predictability with applications like air travel.

However CAES systems don’t have the same level of toxicity inherent in their components and can also contribute to a longer lifetime use of engine components, depending on the technology.

Is CAES really “green?” It depends.

To be considered a truly environmentally friendly energy storage technology, CAES cannot be entirely source agnostic. While CAES systems certainly don’t face the ethical quandaries of, say, sourcing lithium required for electric batteries, energy stored in CAES is ultimately only as “clean” as the source from which said energy comes. If a CAES facility is storing energy from, say, a coal generator, that can muddy the “green” waters considerably.

What are the opportunities?

As mentioned above, CAES’ biggest competitor technology is the electrochemical battery.

However, the increasing pressure on global supply chains to provide ever more Lithium and similarly critical materials for the energy transition means that there’s also increasing scrutiny on the net negatives of what we need to pull out of the ground in order to make our batteries.

The devastating and highly localized impacts of Lithium mining and mining waste on communities worldwide are well documented; the health impacts on miners and those working to process those mined materials are becoming ever more visible, as well. Public pressure from consumers and the general public on governments and the global automotive industry hopefully will mean continued improvements on both fronts.

By contrast, as a proven technology with its basically negligible impact on either humans or their environment, CAES will continue to provide an attractive alternative to battery storage. However this requires that efficiencies with CAES processes and engineering constraints can still be continually improved along the way.

Lithium probably won’t be losing its spot in the limelight anytime soon, but with a little more time and research, CAES could ultimately become an even more meaningful alternative.

Why rip things out of holes in the ground when you could just fill them up with hot air, instead?