More about pumped-hydro and CAES systems

Pumped-hydro storage and compressed air energy storage (CAES) systems are traditional large-scale energy-storage methods, as noted in the main article, “Integrated Energy-Storage Systems” (CE Nov. 2012). Storage plants built around these technologies did not originally have renewable energy in mind but served regional power needs. Renewable energy integration will likely be a part of these plants in the future.

By Frank J. Bartos, PE October 27, 2012

Pumped-hydro storage (PHS) requires two reservoirs at different elevations. That translates to limited natural terrain possibilities and why few new plants are being built. Still, a significant number of these plants operate worldwide. (Online Ref. 2 to CE Nov. 2012, “Integrated energy storage systems.”)

Ludington pumped-storage plant (LPSP) is one of the largest PHS facilities in the U.S. and the world (see more coverage in the main article, “Integrated Energy-Storage Systems,” CE Nov. 2012). A six-year upgrade project starting in 2013 will extend the life of the plant and increase peak generating capacity from 1,872 MW to 2,172 MW. Six reversible turbines will be replaced, one per year, with new higher power machines. LPSP has been in operation since 1973. 

Potential future plant sites are also being explored worldwide around hilly areas and coastlines with bluffs. Innovation may also play a role. One example is based on creating “reservoirs” in a large body of water (sea or a lake) using caisson construction. Water would be pumped from the reservoirs to the sea (or lake) during off-peak electricity demand by reversible turbine generators driven by wind power. During high power demand, water would be allowed to flow back to the reservoir through the turbine generators to produce electricity and refill the reservoir for the next cycle.

Other “novel” ideas include piston-and-cylinder and gravity flow designs that use renewable energy to create potential energy, some of which would be recovered as power output. These nonconventional storage schemes face an uphill path to success from the viewpoint of efficiency, cost, environmental issues, and capacity. Physical size requirements relative to what’s needed for large-scale energy storage is perhaps the biggest barrier for these approaches. 

Size does matter with CAES

Physical size requirements affect conventional CAES technology as well. While up to 80% of the U.S. has geology suitable for CAES, according to studies by the Electric Power Research Institute (EPRI), one 300-MW plant needs 22 million ft3 (623,000 m3) storage space. That volume represents about eight hours of electricity generation, EPRI noted. Other experts conclude many CAES plants of that size could be accommodated in the U.S. However, cost and environmental constraints would also apply here. 

Meanwhile R&D continues to improve CAES efficiency and management of heat associated with the air compression and expansion processes (see main article). Adiabatic as well as isothermal processes are being investigated. In the former process, which stores heat of compression for later reuse in power generation, designs with molten salt or ceramic materials are under investigation. For example, RWE Power of Germany and General Electric (among other companies) are working on a project named Adele-Stassfurt to build an adiabatic CAES demonstration plant with 360 MWh capacity and 90 MW output. 

Isothermal CAES technology (or ICAES) is based on keeping stored air temperatures close to the ambient during compression and expansion processes, which minimizes energy losses. ICAES would be deployed above-ground in systems of storage tanks, heat exchangers, conventional mechanical equipment, and process controls. Modular ICAES systems could be located near wind farms to temporarily store excess wind energy. In a more futuristic setting, an ICAES plant located near a “pneumatic manufacturing center” might directly power air motors, cylinders, and other pneumatic actuators regulated by appropriate controls. 

Present storage capacity of ICAES systems is relatively small. SustainX Inc. is working on a demonstration plant reportedly in the 1.5-2 MW output range to validate the technology; start-up is scheduled for mid-2013. A smaller previous demo plant provided valuable design and operating input. Depending on results from its in-house demonstration plant, SustainX sees field trials to follow with larger demonstration projects. In a longer time frame, commercial ICAES plants could be scaled up to 100 MW output. 

Another company, General Compression, is developing a somewhat different CAES system that is said to be highly efficient. It’s a “near-isothermal” technology but uses an underground reservoir for compressed air storage similar to conventional CAES. The company has partnered with ConocoPhillips to build a 2 MW demonstration plant in western Texas that will integrate wind energy storage. This development represents the first CAES project permitted in Texas—as well as the first repurposing of a hydrocarbon cavern to store compressed air, according to ConocoPhillips.  

CAES setback: Unsuitable site

It’s not all positive for CAES developments. An ambitious plan to build a 270 MW compressed-air energy storage facility near Des Moines, Iowa, was cancelled in July 2011, after permeability tests found the reservoir located some 3,000 ft underground unsuitable “for the scale of project envisioned.” Permeability is a measure of passage of air through pores in rock formations. Earlier evaluation of other aquifers in the region did not indicate similar conditions.

Iowa Stored Energy Park (ISEP) was a forward-looking initiative started by a group of Iowa local municipal utilities in 2003. ISEP’s objective was to “charge” the reservoir with compressed air using wind energy (or other source available on the grid) during non-peak demand periods. Then, during peak demand times, the high pressure air was to be mixed with natural gas and burned in combustion turbines to generate electricity. ISEP was slated to open in 2015.

The energy park location was ideal in view of various wind farms in the region with strong grid connectivity. Termination of ISEP is particularly ironic, given that Iowa has the second largest installed wind power capacity in the U.S., after Texas.

Sandia National Laboratories and the Iowa project team issued a “lessons learned” report on ISEP in January 2012, which included positive aspects of the initiative and prospects for a future project in another location. One of those lessons was that a bulk energy storage project of about 300 MW can be cost-effective in the Midwestern regional grid, and can be cost-competitive with natural gas-fired generating plants of the same size. 

– Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at


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