Energy storage is seen by some as the Holy Grail of the energy markets, providing a low-carbon alternative to conventional generation for balancing systems with increasing levels of intermittent renewable generation. Over a series of posts, I will explore some of the technologies under development and assess the relative strengths and weaknesses of the various projects.
Electricity cannot be stored in a meaningful way (excluding capacitors which are inefficient and very short in duration), so energy is normally stored in a potential form, as chemical potential energy in batteries and stockpiles of coal and stored gas, and gravitational potential energy in pumped hydro.
There are essentially two breeds of energy storage under development: grid storage solutions, and behind-the-meter systems. In this first post I’d like to start big, with the technology which claims to have the potential for large-scale grid level applications: compressed air energy storage (“CAES”).
Compressed air energy systems
The first air compressors were bellows used for metallurgy as far back as 2000 B.C. In the late 19th century some cities such as Paris, Birmingham and Buenos Aires had compressed air grid systems in place, initially to power clocks by providing a pulse of air every minute, but later to deliver power to homes and industry. By 1896 Paris had a 30-mile wide 80 psi compressed air system powered by over 3000hp of compressed air equipment, and it was expected by some that compressed air would take over from electricity as a cleaner energy source.
The first CAES system was constructed at Huntorf in Germany, with a capacity of 290 MW. The plant runs in a daily cycle with 8 hours of compressed air charging and 2 hours of expansion. It has achieved high levels of performance with 90% availability and a round-trip efficiency of 42%.1, 2
A 110 MW plant with a capacity of 26 hours was built in McIntosh, Alabama in 1991, at a cost of USD 65 million, which equates to USD 590 / kW of generation capacity and USD 23 / kW-hr of storage capacity. The plant is capable of delivering its full power output for up to 26 hours. The major improvement relative to the Huntorf facility is the installation of a heat recuperator that re-uses part of the waste heat, reducing fuel consumption by 22-25% and increasing efficiency to about 54%. The plant has also demonstrated high levels of availability, over 90%.1, 2, 3
Currently a number of other projects are under consideration, including a 1,200 MW CAES project at Intermountain in Utah, with a 48-hour duration.
CAES facilities consist of compressed air which is stored in large underground cavities, which is released when needed and used to turn turbines that generate electricity. They utilise an open-cycle natural gas-fired combustion turbine, without compressor blades, which are not required because the air has already been compression on injection into the cavity.
The compression of air generates heat, much of which is dissipated as waste in the current CAES installations. When the stored air is later expanded, its temperatures drops significantly, so secondary heat must be applied, typically using natural gas to warm the air. This heat loss impacts the cost-effectiveness of CAES technology, and generates emissions. For example, the Alabama plant has an efficiency in the compression phase of 82%, whereas the expansion phase requires combustion of natural gas at one third the rate of a gas turbine producing the same amount of electricity.
1. Succar S, Williams RH. Compressed air energy storage: theory, resources, and applications for wind power. Technical report, Energy Systems Analysis Group, Princeton Environmental Institute, Princeton University; 2008.
2. Chen H, Cong TN, Yang W, Tan C, Li Y and Ding Y. Progress in electrical energy storage system: A critical review. Progress in Natural Science, 2009; 19:291-312
3. Samir S. Large energy storage systems handbook. Levine G editor. CRC Press; 2011, p 112-152