Storing electricity using compressed air
The amount of fluctuating power generated from wind energy is increasing all the time. Compressed air energy storage can be used to balance the oversupply during periods of strong wind with periods of high demand when winds are weak. A compressed air energy storage facility is a system that can store electrical energy in the short term. It uses excess electricity – on windy days, for example – to produce compressed air and then stores this in a cavern. When this energy is needed again, compressed air is fed to a turbine, which then drives a generator and generates electricity. As air becomes very hot when compressed in an air pump and cools down to the same significant extent when expanded again, researchers are trying to store and use heat as well as pressure in adiabatic compressed air energy storage systems.
- Development of heat storage systems with a storage capacity of up to 1,200 megawatt hours at temperatures of over 600 °C. Solid storage systems based on materials such as ceramics, natural rock, concrete or cast iron could be directly charged and discharged. These systems have proven themselves in industry, have simple designs, and have a large heat transfer surface. However, solid storage systems require a pressure-resistant shell.
- Development of compressors for charging the storage unit that are able to withstand temperatures of up to 600 °C and generate pressures of up to 160 bar (with high efficiency, variable throughput and rapid availability with a start-up time of a few minutes). Design studies for appropriate radial compressors are encouraging.
- Development of air turbines that achieve outputs of up to 300 MW by expanding compressed hot air to atmospheric pressure. The challenges here include: high power density, high inlet temperatures, large volumetric flows and changes in volumetric flow. At the same time, high efficiency with low specific costs should be achieved across the entire load range.
The planned demonstration plant in Staßfurt in the German state of Saxony-Anhalt is to be the first plant in the world to be operated adiabatically. In other words, the heat energy that is inevitably released during the compression of air is not lost, but is instead stored and then used again later. This is accomplished by the heat storage device, which stores this valuable energy in an array of hot blocks behind thick insulation. This heat is then used to heat up the cold compressed air again when it is being discharged, so that it can then generate electricity in an air turbine. The heat storage device represents a new development compared to the two existing compressed air energy storage systems in Huntorf in Lower Saxony and in McIntosh, Alabama, USA. In these plants, natural gas has to be used to heat up the compressed air required for power generation. This costs valuable percentage points in terms of efficiency.
It is foreseen that heat storage will allow for efficiencies of around 70 percent. In addition, the consumption of fossil fuels will be avoided. In this way, this technology makes it possible to supply CO2-neutral peak-load electricity from renewable energy sources. The EU project AA-CAES (Advanced adiabatic compressed air energy storage) has already demonstrated that this technology is feasible.
The demonstration plant will not be built on a commercial scale, but will instead be used to further develop this technology. Innovative components such as heat storage equipment and air turbines have not yet reached technical maturity and still have to be developed for these applications. The compressed air energy storage power plant in Staßfurt is to be able to store up to 360 megawatt hours of electricity and achieve a power output of 90 megawatts.
A commercial plant should store around 1,000 megawatt hours of electrical energy in everyday charging/discharging operation and should be able to feed around 300 megawatts into the grid for a number of hours. This demonstration plant is necessary as a preliminary step along the development curve and will naturally be smaller in scale. It is planned that it will go into service in 2016 at the earliest.
Compressed air energy storage is a promising alternative for storing excess energy that is in the electricity grid. This technology is already being employed today – mainly in the use of so-called pumped storage systems: In periods of excess electricity, water is pumped to reservoirs at higher altitudes and is then released again to drive turbines when demand increases. The efficiency of these plants is between 75 and 80 percent. These values are very good. However, there is little potential for expanding this technology in Germany, and the demand for energy storage will increase as renewable energies grow.
The share of renewable energy sources in Germany is to increase from the current value of 15 to 30 percent by 2020. Almost one third of all electricity will then be generated from solar power, wind power and biomass. However, the sun doesn't shine every day and the wind doesn't blow on demand either. Renewable energy sources are subject to strong fluctuations, and the electricity grid must be able to balance out these fluctuations in a flexible manner. After all, the stability of the energy supply must be guaranteed all the time, even when the sky is overcast or on days when there is little wind. Conventional power plants, on the one hand, and pumped storage and compressed air storage, on the other, can provide sufficient flexibility and act as a backup for renewable energies. In this way, current research is aimed at making an important contribution to energy supply for Germany that will be reliable and environmentally friendly in the long term, too.
6 current research projects
ADELE is the name of the current joint project by the German Federal Ministry of Economics and Technology and RWE. This abbreviation stands for "Adiabater Druckluftspeicher für die Elektrizitätsversorgung (Adiabatic compressed air storage systems for electricity supply)". RWE Power AG is working on this project alongside research institutions and manufacturers such as the German Aerospace Center, General Electric and the German construction and engineering company Ed. Züblin. Also involved are the RWE subsidiary Erdgasspeicher Kalle and the chimney and refractory specialist Ooms-Ittner-Hof from the Züblin group of companies.
Organisations carrying out research:
Ooms-Ittner-Hof GmbH - Niederlassung Rhein-Main, Darmstadt
Erdgasspeicher Kalle GmbH, Freiberg, Sachsen