Compressed air storage power plant

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Compressed air storage power plants are storage power plants in which compressed air is used as energy storage . They are used to regulate the network such as the provision of control power : If more electricity is produced than is consumed, the excess energy is used to pump air under pressure into a storage tank; When there is a need for electricity, the compressed air is used to produce electricity in a gas turbine.

After the English term Compressed Air Energy Storage , these power plants are also abbreviated as CAES power plants .

So far, only a few storage power plants of this type have been operated worldwide: The Huntorf power plant in Germany and the McIntosh power plant in the USA, as well as a test facility in the Gotthard tunnel in Switzerland.

functionality

Compressed air storage power plants have so far been designed as a hybrid in combination with a gas turbine power plant to provide electricity at times of peak load . In contrast to a pumped storage power plant developed for the same purpose , in which water flows down from a high-lying reservoir via downpipes and thereby drives turbines and thus generators , a compressed air storage power plant uses the energy contained in compressed air. In off-peak times, compressed air is stored in an underground cavern using an electrically powered compressor . In times of high electricity demand at peak loads, the compressed air is fed into a gas turbine, which transfers its output to a connected generator. As heat has to be added again when the air expands in order to prevent the turbines from icing up, a combination of compressed air storage and gas turbine power plant is used.

Since peak load power plants are idle most of the time or, in the case of storage power plants, are charged, they can only be operated profitably if the electricity is drawn at high prices during peak load times. The total costs are therefore very much determined by the installation costs of the system.

An essential feature of a compressed air storage power plant - as of other peak load and storage power plants - is that they can be started very quickly. Within three minutes z. B. in Huntorf near Elsfleth 50% and after about ten minutes 100% of the power available.

In addition, the system is black-start capable , which means that the system can help rebuild network operations after a large-scale blackout .

Efficiency

It is difficult to state the efficiency of compressed air storage power plants because two different input factors (gas and electricity) are required. These cannot simply be equated. There are two extremes of interpretation:

At comparatively high gas prices, it would be possible to convert the air compressed in the cavern into electricity without the additional supply of heat from gas. The calculation of the degree of utilization of this power plant would be analogous to that of a pumped storage power plant . Due to the high losses resulting from the unused compression heat, the efficiency is well below that of a modern gas turbine .

At very low gas prices, the cavern pressure could be reduced in order to save valuable electrical energy for compression. By burning more gas, this deficit can be compensated within limits by higher turbine inlet temperatures. However, to a certain extent this contradicts the actual purpose of a storage power plant, namely to store energy. It also approaches the conventional gas turbine process.

For technical reasons, real compressed air storage power plants move between these two extremes. For example, to generate 1 kWh of electrical energy in the Huntdorf power plant , 0.8 kWh of electrical energy and 1.6 kWh of gas have to be used.

meaning

Compressed air storage power plants could become more important in the future due to the energy turnaround and the resulting expansion of wind power plants and photovoltaic systems , which is associated with increased energy storage requirements . You can as pumped storage power plants for electricity finishing be used. In order to provide a similar amount of balancing energy , pumped storage power plants either need large volumes and thus a great deal of area or large differences in height.

Compressed air storage facilities require hollowed-out, airtight salt domes and are therefore tied to geologically suitable locations just like pumped storage power plants. On the German North Sea coast there are many salt domes that could be flushed out in order to create caverns for compressed air storage systems.

In view of the constant expansion of wind energy and photovoltaics, the widespread use of energy storage systems is expected to stabilize these fluctuating (volatile) "supply-dependent forms of energy". A lack of wind or solar energy could be supplemented from the storage facilities, and the short-term but very high production peaks could be fed into the storage facilities, which increases the grid compatibility of wind and solar energy. It is beneficial that there are many geologically suitable sites in the vicinity of the current (coastal area) and future (North and Baltic Sea) production centers for wind power generation. The EU is funding a Europe-wide research project on compressed air storage.

Systematization

In the strict sense of the word, a compressed air storage power plant consists only of an electricity-driven compressor, the compressed air storage as a large-volume, airtight container, a turbine and a generator. The compressor compresses air and presses it into the container. The compressed air flowing out later drives the turbine. The generator connected to it generates electricity. This elementary design would lead to considerable problems and inefficient operation in practice. It is therefore always expanded to include additional components.

Compressed air gas combined cycle power plant

The compression of the air automatically leads to an increase in its temperature, see: Adiabatic change of state . Depending on the pressure, well over 1,000 ° C can be reached. In order to protect the system from this, its thermal energy is released into the environment via heat exchangers and is thus lost for further use. A further cooling takes place due to the inevitable temperature equalization with the wall of the compressed air storage (cavern) in the time up to the re-extraction. Both processes result in considerable efficiency losses.

Conversely, the (already cooled) air would cool down considerably during expansion in the turbine. The residual moisture in the air would freeze and the turbine would ice up. To avoid this, the compressed air is mixed with a combustible gas as it enters the turbine and the mixture is ignited. The resulting hot exhaust gases drive the turbine.

It is therefore a gas turbine power plant in which the work of the compressor belonging to the turbine is taken over by the stored compressed air. The Huntorf power plant falls into this category. Accordingly, no energy is stored in such a power plant; the special benefit of such a power plant lies in the provision of control energy. By operating the turbine without a connected compressor, a significantly higher output can be made available than can be done in a conventional gas power plant.

Adiabatic compressed air storage power plant

In the adiabatic compressed air storage power plant (Advanced Adiabatic Compressed Air Energy Storage - AA-CAES), the compressor is not cooled and the heat from the compressed air is temporarily stored in a heat store . This is designed as a solid storage tank, similar to a cowper .

When the air is relaxed again, it first passes through the heat accumulator and is heated up again. No natural gas is needed to heat the air. In this way a significantly higher degree of utilization could be achieved. The prerequisite, however, is that the time between charging and discharging the storage tank is only short. During the expansion in the turbine, the air cools down to ambient temperature.

In power plants that have been implemented up to now, the heat generated when the air is compressed is dissipated to the environment and can therefore no longer be used. Thus, an adiabatic compressed air storage power plant works closer to an adiabatic change of state , the efficiency is higher.

Development projects

For physical reasons, however, the significantly higher efficiency of pumped storage power plants of around 80% can probably never be achieved by compressed air storage power plants. Nevertheless, there are currently efforts in Germany and also in the USA to build several new compressed air storage power plants in salt dome caverns or in the cavities of closed mines.

Between 2003 and 2005, Alstom Power Switzerland worked on the further development of compressed air storage technology (discontinued) as part of a European research project . The construction of a compressed air storage power plant in Staßfurt (RWE, DLR) is being discussed .

Isobaric compressed air storage

While the compressed air is extracted from the compressed air reservoir, the pressure in the reservoir and thus the inlet pressure of the turbine are reduced. However, conventional turbines are designed for largely constant pressure. In order to keep the pressure in the compressed air reservoir constant during extraction, it can be positioned below a column of liquid. This can be achieved by connecting the (underground) cavern with an above-ground storage basin. While the air is being blown into the cavern, it displaces the liquid (water or brine) from there into the reservoir, while the compressed air is extracted, the water flows back into the cavern. The change in pressure is reduced to the fluctuation of the surface of the lake. The functional principle is already being used in practice in natural gas cavern storage facilities , for example by the natural gas company Keyera in Fort Saskatchewan (Canada).

In the case of submarine compressed air storage, the pressure changes are automatically reduced to a minimum due to the positioning on the seabed.

Projects

Executed systems

Huntorf power plant

Huntorf power plant as a model

The Huntorf power plant - the world's first CAES power plant - was built in Germany in the late 1970s, in Huntorf near Elsfleth , and put into operation in 1978. The power plant originally had an output of 290 MW; this was increased to 321 MW through a retrofit in 2006.

The power plant is a combined compressed air storage and gas turbine power plant , i.e. the gas turbine is not a pure gas expansion turbine , but an internal combustion engine in which, in addition to energy from compressed air, energy is also converted through the combustion of natural gas.

McIntosh Power Plant

Another compressed air storage power plant is located in the USA in the state of Alabama. It was put into operation in 1991. As a further development of the CAES technology, a recuperator has been integrated that uses the hot exhaust gases from the gas turbine to preheat the air, thereby reducing fuel consumption. This power plant can provide an output of 110 MW over 26 hours. The air is stored here in a single cavern with a volume of 538,000 m³. The long running time of 26 hours shows that it is not a pure peak load power plant.

In order to generate 1 kWh of electrical energy, 0.69 kWh of electricity and 1.17 kWh of gas must be invested beforehand.

In this plant, an energy volume of 2860 MWh is extracted from a storage volume 1.8 times that of Huntorf. This corresponds to a storage density that is (2860/642) / 1.8 = 2.5 times higher.

Sesta power plant

At Sesta in Italy, a test and demonstration system with an electrical output of 25 MW was in operation from 1986, which stored compressed air in porous rock. After an earthquake, the plant was shut down in the early 1990s.

Compressed air storage at the Gotthard Base Tunnel

In mid-2016, a prototype of an adiabatic compressed air storage system developed by scientists from ETH Zurich went into test operation. In the project, which was funded by Switzerland with 4 million francs, an old supply tunnel , which was used to build the Gotthard Base Tunnel , is filled with compressed air up to a pressure level of 33 bar. The resulting heat is stored in stones, which heat up to over 500 ° C during this process. During reconversion, the compressed air is passed back through the heat storage system, so that no additional heat has to be added when the air is expanded and a high level of efficiency can be achieved. The aim is an efficiency of approx. 72%.

Planned plants

Compressed air storage facility Staßfurt (ADELE)

From 2013, a pilot and test system based on the ADELE principle (adiabatic storage) was to be built in Staßfurt in the Salzland district in Saxony-Anhalt. The concept was developed by RWE , General Electric , Züblin and the DLR . After completion, the power plant should have an output of around 90 MW el and a storage capacity of around 360 MWh and be operated by RWE. In spring 2015, RWE announced that the planning for the pilot plant in Staßfurt had been discontinued due to a lack of concrete market prospects.

Norton Energy Storage

In Ohio (USA), the company hopes Norton Energy Storage build the largest ever built compressed air energy storage. It is said to store air in a ten million cubic meter limestone mine 700 meters below the surface. The first power stage is expected to have between 200 MW and 480 MW and cost between 50 and 480 million US dollars . In four further stages, the output is to be increased to around 2,500 MW.

Iowa Stored Energy Park

In Iowa (USA), a compressed air storage power plant with a capacity of around 270 MW el was to be built by 2015 . In contrast to other systems, the compressed air should not be stored in a cavern, but in an aquifer . It was hoped that the hydrostatic pressure of the groundwater would be able to use the advantages of isobaric storage described above in a weakened form. Completion was planned for 2015. The project has now been discontinued because the geological conditions at the intended location are not met.

Web links

Individual evidence

  1. ↑ Compressed air storage power plants
  2. ↑ dlr.de
  3. ↑ Adele could come to Staßfurt ( Memento from June 25, 2013 in the Internet Archive )
  4. ↑ Lasse Nielsen, Dawei Qi, Niels Brinkmeier, Reinhard Leithner : Compressed air storage power plants for the grid integration of renewable energies - ISACOAST-CC . Lecture at the conference on decentralization and network expansion , 4th Göttingen conference on current issues relating to the development of energy supply networks, 22. – 23. March 2012. Technical University of Braunschweig, Institute for Heat and Fuel Technology, Braunschweig 2012 ( full text online as PDF ).
  5. ↑ MPI: power storage
  6. ↑ Experience with CAES power plants (PDF; 264 kB)
  7. ↑ Andrei G. Ter-Gazarân: Energy Storage for Power Systems . Ed .: Institution of Electrical Engineers (=  Iee Energy Series . Volume 6 ). IET, 1994, ISBN 0-86341-264-5 ( limited preview in Google Book Search).
  8. ^ Frank S. Barnes, Jonah G. Levine (Eds.): Large Energy Storage Systems Handbook (=  CRC Press Series in Mechanical and Aerospace Engineering ). CRC Press, 2011, ISBN 978-1-4200-8601-0 ( limited preview in Google Book Search).
  9. ↑ How Swiss use compressed air in the Gotthard tunnel to store energy . In: ingenieur.de , August 3, 2016. Accessed August 4, 2016.
  10. ↑ Falk Rockmann: plans for the compressed air storage system discontinued. In: Magdeburger Volksstimme (Volksstimme.de). March 31, 2015, accessed August 10, 2015 .
  11. ^ Iowa's Innovative Energy Solution. Iowa Stored Energy Park, accessed March 8, 2011 .
  12. ^ Lessons from Iowa. Retrieved March 14, 2012 (English).