Compressed air energy storage (CAES) offers a method of storing compressed air within a sealed enclosure. Storage in a compressed air system allows users to supplement energy usage during high-demand periods, enhances air quality, and maintains system stability.
CAES can be used for large-scale energy storage, in which the air is stored in pressurised storage tanks or underground caverns. The expanding air cools significantly and must be heated before it passes through the turbine. This can be effected using either diabatic (a process that involves the gain or loss of heat), and a change in entropy or adiabatic (occurring without loss or gain of heat) methods.
A compressed air energy storage (CAES) plant may look like an ordinary field, but underneath the field is an extraordinary process happening. Ambient air is compressed and stored in underground caverns that store energy. When that energy is needed, the pressurised air is heated and expanded in a turbine, driving a generator for power production. Tapping into the pressing need to store renewable energy from intermittent sources such as wind and solar, but on a grid-scale.
Source: Sino Voltaics
When energy is to be stored, off-peak electricity powers the motor, which drives the compressor. The compressor draws in air and compresses it, increasing the pressure of the air. The compression of air releases heat, leaving cooler, compressed air. The cold, compressed air is stored in a sealed underground chamber. Pre-existing structures such as disused salt mines can be used. When energy is to be recovered, the high-pressure air is first released through a heating system to start the expansion of the cold, compressed air. The expanding air spins the blades of expansion turbines, Some plants warm the air at this point using natural gas. The generator converts the spinning motion of the turbines into electricity.
Simply put, electricity from the public electrical grid or renewable power sources is used to power a compressor, which pumps air into a storage container. When electricity is needed, the compressed air is released from the holding tank and heated using either stored heat from the compression process or heat generated from burning fossil fuels. The pressurised and heated air expands to turn a turbine. In turn, the turbine enables a generator to produce electricity.
Although CAES is a mature, commercially available energy storage technology, there are only two CAES operated all over the world. One is in Huntorf in Germany, another is in Mclntosh, Alabama in the USA. The CAES plant in Huntorf, Germany is the oldest operating CAES system.
- Increased energy savings.
CAES is used to enhance storage for peak demand periods, reducing the load on the electricity grid. In addition, CAES stores pressurised air, mitigating the need to purchase and power a completely separate compressor.
- Enhanced air quality.
CAES produce significantly fewer CO2 emissions than other energy production methods. Using adiabatic processes, emissions can be reduced to nearly zero.
- Improved system stability.
Using CEAS as a supplemental energy source helps to enhance power grid stability during peak times or surges in electricity use.
- Reduced maintenance costs.
When used in small-scale energy production, compressed air storage reduces compressor maintenance frequency, which reduces maintenance costs.
- CAES plants are capable of a black startup.
This means that the CAES plant has a fast startup time. If a CAES plant is operated as a hot spinning reserve, it can reach the maximum capacity within a few seconds. The emergency startup times from cold conditions are about 10 to 12 minutes.
- Extended air compressor lifetime.
Since the CAES system stores compressed air, the compressor does not have to run as frequently. This reduces overall wear on the compressor and extends its service life.
- The CAES technology can be easily optimized for specific site conditions and economics.
- CAES is a proven technology and can be delivered competitively by several suppliers.
According to an EPRI(Electric Power Research Institute) study in 2002, the primary reason for the currently limited market penetration of this technology is likely to be the lack of awareness of this option by utility planners. In addition, the underground geology is likely perceived as a risk issue by utilities.
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