System for storing energy by means of compressed air

1. A plant for storing energy by means of compressed air, in which:

a storage volume accommodates air at elevated store pressure p H ;

for energy storage, ambient air is compressed and introduced into the storage volume;

for removal from storage, compressed air is extracted from the storage volume and discharged into the environment, performing work;

at least one low-pressure turbomachine for alternate compression and expansion respectively compresses ambient air to a medium pressure p M and expands said ambient air from said medium pressure p M ;

at least one high-pressure machine for alternate compression and expansion respectively compresses air from the medium pressure p M to the store pressure p H and expands said air from the store pressure p H , wherein if there are a plurality of such high-pressure machines, then the plurality of such high-pressure machines are connected in parallel in terms of flow perform said task; and

said at least one low-pressure turbomachine and said at least one high-pressure machine are connected in series in terms of flow and are mechanically coupled to in each case one, or to one common, generator/motor, wherein the generator/motor is an electric machine that operates selectively as a motor or a generator,

wherein the at least one low-pressure turbomachine has a fixed rotational speed or a virtually fixed rotational speed both during expansion and during compression, said rotational speed being defined by the frequency in a country-wide electrical grid,

wherein the at least one low-pressure turbomachine provides more than half of a compression power of the at least one low-pressure turbomachine and the at least one high-pressure turbomachine combined,

wherein the store pressure p H can be changing from between a minimum store pressure p H2 and a maximum store pressure p H1 ,

wherein the medium pressure p M is kept constant irrespective of the changing store pressure p H during loading the storage volume, and also during discharging the storage volume in normal operation, except for short overload periods with increased p M , and

wherein the medium pressure p M during normal expansion operation amounts to between 1.05 times and 1.3 times the value of p M during compression operation.

2. The plant as claimed in claim 1 , wherein the at least one high-pressure machine is a positive-displacement machine.

3. The plant as claimed in claim 1 , wherein the at least one high-pressure machine is a positive-displacement machine with a store-pressure side and a medium-pressure side and is constructed such that, on the store-pressure side, during expansion operation the at least one high-pressure machine closes a connection from the storage volume at the store pressure p H to a working chamber at varying working chamber volume, and during compression operation the at least one high-pressure machine opens the connection from the working chamber to the storage volume at the store pressure p H at varying working chamber volume, and thus permits different working chamber charges during expansion operation and different discharge timing during compression operation.

4. The plant as claimed in claim 1 , wherein the minimum store pressure p H2 in normal storage operation amounts to less than one third of the maximum store pressure p H1 , and the medium pressure p M is kept equal in each case independently of the store pressure p H during expansion and compression by control of the at least one high-pressure machine.

5. The plant as claimed in claim 1 , wherein the at least one low-pressure turbomachine has a virtually fixed rotational speed having a deviation of +/−5% both during expansion and during compression.

6. The plant as claimed in claim 1 , comprising multi-stage heat storage and maximum temperatures of between 200° and 400° C.

7. The plant as claimed in claim 2 , wherein, in the positive-displacement machine, during operation, a mechanical, power-transmitting element is moved, owing to the construction of the drive and independently of the store pressure, with always approximately the same frequency and with a deviation of up to +/−5%.

8. The plant as claimed in claim 1 , comprising at least one directional coupling, between the generator/motor and the low-pressure turbomachine coupled thereto or between the generator/motor and the high-pressure machine coupled thereto.

9. The plant as claimed in claim 6 , wherein the exchange of heat for the multi-stage heat storage takes place in intermediate stages of the at least one low-pressure turbomachine.

10. The plant as claimed in claim 6 , wherein the exchange of heat for the multi-stage heat storage takes place between the at least one low-pressure turbomachine and the at least one high-pressure machine.

11. The plant as claimed in claim 6 , wherein at least one of the at least one high-pressure machine is a positive-displacement machine, and wherein the exchange of heat for the multi-stage heat storage takes place in the positive-displacement machine.

12. The plant as claimed in claim 6 , wherein the exchange of heat for the multi-stage heat storage takes place in a lubricant circuit of a positive-displacement machine.

13. The plant as claimed in claim 6 , wherein the exchange of heat for the multi-stage heat storage takes place between the at least one high-pressure machine and the storage volume.

14. The plant as claimed in claim 1 , wherein the plant operates without any combustion in normal operation.

15. The plant as claimed in claim 1 , wherein the medium pressure p M is a value of 15-30% of the maximum store pressure p H1 , such that the at least one low pressure turbomachine provides more than half of the compression power of the plant.

16. The plant as claimed in claim 1 , wherein the at least one low-pressure turbomachine has a spiral without any guide apparatus.

17. The plant as claimed in claim 2 , wherein in the positive-displacement machine, at least during expansion operation, different volumetric displacements per unit of time can be set.