The invention relates to a compressor-expander system and to a method for operating such a system.

DE 10 2019 133 904 Al discloses a compressor-expander system having a compressor that can be driven by an electric machine and having an expander coupled to the compressor, wherein via the compressor air can be sucked in from an environment in order to compress the air, and wherein via the expander compressed air can be expanded and discharged into the environment. The compressor-expander system disclosed there, further, comprises a first heat exchanger arranged, seen in the flow direction of the air to be compressed, downstream of the compressor, in order to extract heat from the compressed air and transfer thermal energy to a storage medium. Further, a second heat exchanger is provided upstream of the compressor which is equipped in order to transfer thermal energy from the compressed air conducted via the first heat exchanger to the air to be compressed in the compressor.

Although with the compressor-expander system known from DE 10 2019 133 904 Al it is already possible for example in a power plant to utilise excess electric energy for increasing the efficiency, there is a need for operating a compressor-expander system even more efficiently.

Starting out from this, the present invention is based on the object of creating a new type of compressor-expander system and a method for operating such a system which has a high efficiency.

According to a first aspect of the invention, this object is achieved through a compressor-expander system according to Claim 1. Accordingly, the second heat exchanger comprises multiple heat tubes connected in series, namely multiple series-connected heat pipes or multiple series- connected two-phase thermosyphons.

According to the first aspect of the invention, the second heat exchanger includes multiple heat tubes connected in series. The heat tubes are preferentially heat pipes. By way of this, a particularly efficient compressor-expander system can be provided. In the region of the second heat exchanger, a particularly efficient heat transfer can be provided .

This object according to a second aspect of the invention is achieved by a compressor-expander system according to Claim 3. Accordingly, a heating device is connected between the compressor and the first heat exchanger which is equipped in order to heat the air compressed in the compressor upstream of the first heat exchanger.

With the second aspect of the invention, the efficiency of the compressor-expander system can likewise be increased. If sufficient electric energy for driving the compressor is not available, downtimes can be particularly advantageously bridged by means of the heating device.

Preferentially, both aspects are employed combined with one another. In particular when both aspects are utilised combined with one another on a compressor-expander system the efficiency can be increased in a particularly advantageous manner.

Preferentially, a bypass to the compressor is provided in order to conduct uncompressed air past the compressor to a heating device designed as burner. Further, a switching valve is then preferentially provided which is equipped in order to conduct compressed air conducted via the first heat exchanger and via the second heat exchanger either via the expander or discharge the said compressed air via a chimney into the environment. Preferentially, a fan is provided upstream of the first heat exchanger.

A fired burner is typically operated without pressure. In this case, the compressor of the compressor-expander system works either as fan or supplies the burner with pressureless combustion air. Alternatively, the combustion air is supplied to the burner via the fan bypassing the compressor. In this case, there is then preferentially no flow through the expander and the air is preferentially discharged via the chimney instead. This takes place via the switching valve.

Preferentially, at least one stage of the expander is assigned a condensate separator and/or between at least two stages of the expander a further heat exchanger is connected which is equipped in order to heat the partially expanded air. It is possible via the condensate separator to extract moisture from the expanded or partially expanded air and counteract a formation of ice.

Preferentially, a cold extraction device is provided downstream of the expander which is equipped in order to extract cold from the expanded air. By way of this it is possible to extract utilisable cold from the expanded air and further increase the efficiency of the compressorexpander system.

The method according to the invention for operating the compressor-expander system according to the invention is defined in Claim 11.

Preferred further developments of the invention are obtained from the subclaims and the following description. Exemplary embodiments of the invention are explained in more detail by way of the drawing without being restricted to this. There it shows:

Fig. 1: a first compressor-expander system according to the invention,

Fig. 2: a second compressor-expander system according to the invention,

Fig. 3: a third compressor-expander system according to the invention,

Fig. 4: a fourth compressor-expander system according to the invention,

Fig. 5: a fifth compressor-expander system according to the invention,

Fig. 6: a sixth compressor-expander system according to the invention,

Fig. 7: a further compressor-expander system according to the invention,

Fig. 8: a detail of the compressor-expander system according to Fig. 1, Fig. 3, Fig. 4, Fig. 5, Fig. 6 and Fig. 7,

Fig. 9: a P-h diagram for illustrating the invention.

Fig. 1 shows a compressor-expander system 10 according to a first aspect of the invention present here. The compressorexpander system 10 comprises a compressor 12 that can be driven by an electric machine 11, which is equipped in order to suck in air LUI to be compressed from an environment .

The compressor-expander system 10 comprises downstream of the compressor 12 a first heat exchanger 13 which is equipped in order to transfer thermal energy from the air LV compressed in the compressor to a storage medium SM. The storage medium SM can be for example liquid salt or thermal oil or the like. Upstream of the compressor 12 the compressor-expander system 10 comprises a second heat exchanger 14.

On the one hand, the air LV compressed in the compressor 12 and conducted via the first heat exchanger 13 and on the other hand the air LUI to be compressed in the compressor 12 can be conducted via the heat exchanger 14. The compressor 12 can be of the single-stage or multi-stage type .

This second heat exchanger 14 is equipped in order to transfer thermal energy from the compressed air LV conducted via the first heat exchanger 13 to the air LUI to be compressed in the compressor.

The intake of the air LUI to be compressed can take place via a silencer and/or air filter. Silencer and/or air filter are arranged preferentially upstream of the compressor 12, in particular upstream of the second heat exchanger 14.

Furthermore, the compressor-expander system 10 comprises an expander 15. The expander 15 is equipped in order to expand compressed air LV conducted via the first heat exchanger 13 and the second heat exchanger 14 and discharge it as expanded air LU2 into the environment.

As already explained, the compressor 12 can be driven by the electric machine 11 in order to for example utilise excess electric energy. Compressor 12 and expander 15 are coupled via a shaft 16. Between compressor 12 and expander 15 a clutch (see Fig. 7) and/or a gear can be connected. Mechanical energy obtained during the expansion of the air LV1 in the expander 15 can thus be utilised for driving the compressor. Downstream of the expander 15, a silencer can be arranged. According to the first aspect of the invention, the second heat exchanger 14 includes multiple heat tubes connected in series, which in the shown exemplary embodiments are formed as heat pipes 17.

Alternatively, the heat tubes can also be two-phase thermosyphons .

Fig. 8 shows the second heat exchanger 14 with multiple heat pipes 17 in greater detail. The heat pipes 17 are connected in series.

The heat transfer from the compressed air LV1 into the air LUI to be compressed is accomplished by way of the multiple series-connected heat pipes 17. By way of such heat pipes 17, a highly efficient heat transfer is possible.

The heat pipes are tubes filled with a refrigerant, wherein as refrigerant water can be utilised for example. On the hot side, i.e. in the region of the compressed air LV1, the refrigerant is evaporated.

The evaporated refrigerant rises in the heat pipes 17 and in the upper region gives off heat as condensation energy to the air LUI to be compressed. In the upper region, the respective heat pipe 17 is gas-filled.

An air duct 18 conducting the compressed, hot air LV is positioned below the air duct 19 conducting the cold air LUI to be compressed.

Outsides of the heat pipes 17 are fitted with fins 20 for better heat transfer between the heat pipes 17 and the respective air LV, LUI.

Fig. 2 shows a compressor-expander system 10 according to a second aspect of the invention, wherein for avoiding unnecessary repetitions same reference numbers as in Fig. 1 are used for same assemblies. Details, by which the compressor-expander system 10 of Fig. 2 differs from the compressor-expander system 10 of Fig. 1, will be discussed in the following.

In the exemplary embodiment of Fig. 2, the second heat exchanger 17 is not formed of multiple heat tubes connected in series, in particular heat pipes 17, but by a conventional heat exchanger instead.

According to the second aspect of the invention it is provided that between the compressor 12 of the compressorexpander system 10 and the first heat exchanger 13 a heating device 21 is connected. The heating device 21 is equipped in order to heat the air LV1 compressed in the compressor 12 upstream of the first heat exchanger 13.

In the exemplary embodiment of Fig. 2, the heating device 21 is designed as burner. With such a burner 13 it is possible for example in particular when insufficient electric energy is available in order to drive the compressor 12 via the electric machine 11, to maintain the operation of the compressor-expander system 10.

In Fig. 2, the heating device 21 formed as burner is integrated in an air duct extending between the compressor 12 and the first heat exchanger 13 while the burner is then a so-called duct burner.

The burner generates a temperature rise which during normal operation develops by way of the compression in the compressor 12.

The heating device 21 designed as burner is employed without pressure. In this case, the compressor 12 operates either as fan or is stationary. Preferentially, a bypass 22 is present which is equipped in order to conduct air LUI from the environment past the compressor 12 in the direction of the heating device 21 designed as burner.

In particular when the compressor-expander system 10 comprises the heating device 21 designed as burner, the compressor-expander system 10 preferentially continues to have a switching valve 23 in an air duct extending from the first heat exchanger 13 in the direction of the expander 15.

Dependent on the switching position of the said switching valve 23, the compressed air conducted via the first heat exchanger 13 can be conducted either via the expander 15 or be discharged in a chimney 21. Accordingly, when the heating device 21 formed as burner is operated, the air LV1 is preferentially discharged into the chimney 24 via the switching valve 23 and not conducted via the expander 15.

As soon as the control valve 23 opens the connection to the chimney 24, no backpressure is present any longer in the compressor-expander system 10 and the system 10 with the heating device 21 formed as burner is operated atmospherically or approximately atmospherically.

Fig. 2 shows a further switching valve 25. The same is integrated in the air line running between the compressor 12 and the first heat exchanger 13. Dependent on the switching position of this switching valve 25, the bypass 22 is either opened or closed.

Furthermore, Fig. 2 shows a fan 26. In the event that the air, when employing the heating device 21 formed as burner, cannot be conducted via the compressor 12, the fan 26 can ensure an adequate air flow to the burner. Fig. 3 shows a compressor-expander system 10 which combines the aspects of Fig. 1 and Fig. 2, i.e. in the case of which the second heat exchanger 14 comprises multiple heat tubes connected in series, namely multiple series-connected heat pipes 17 or two-phase thermosyphons, and in the case of which between the compressor 12 and the first heat exchanger 13 a heating device 21 is connected, which is equipped in order to heat the air compressed in the compressor 12 upstream of the first heat exchanger 13.

A further compressor-expander system 10 is shown by Fig. 4. The compressor-expander system 10 of Fig. 4 is a further development of the compressor-expander system 10 of Fig. 1, which is why again same reference numbers are used for same assemblies. In the following, again such details by which the exemplary embodiment of Fig. 4 differs from the exemplary embodiment of Fig. 1 are discussed.

In the exemplary embodiment of Fig. 4, as in the exemplary embodiments of Fig. 2 and 3, a heating device 21 is connected between the compressor 12 and the first heat exchanger 13, but which is not designed as a burner, but as an electric heater or conventional heat exchanger. It is thus possible to regulate the temperature of the air upstream of the first heat exchanger 13 to a set-point value .

Following the compression in the compressor 12 heat can be again supplied to the compressed air LV. This can be necessary in order to achieve a higher temperature level upstream of the first heat exchanger 13 than directly at the outlet of the compressor 12.

A further difference of the exemplary embodiment of Fig. 4 compared with the exemplary embodiment of Fig. 1 consists in that in the exemplary embodiment of Fig. 4 the expander 15 comprises two expander stages 15a and 15b. Between the two expander stages 15a and 15b , a further heat exchanger 27 is connected . Between the two stages 15a and 15b of the expander 15 , waste heat or ambient heat can be supplied to the proces s . In the place between the two expander stage s 15a and 15b , an additional heat supply via a heat pump or a heat supply of waste air or ambient air can al so be provided in order to increase the eff iciency of the expander 15 and/or regulate the outlet temperature of the air downstream of the expander 15 . This can take place intermittently for example during a defrost mode or as a permanent temperature regulation . It is thus pos sible to regulate the temperature of the air downstream of the expander 15 to a set-point value .

Water contained in the air can result in ice forming at the outlet of the expander 15 under certain operating conditions . In Fig . 4 , a condensate separator 28 each interacts with both stages 15a and 15b of the expander 15 . By way of the respective condensate separator 28 , condensate can be discharged in order to counteract ice forming at low temperatures .

In order to counteract a formation of ice other measures are al so pos sible which can be employed in all exemplary embodiments . Accordingly, regulating the outlet temperature on the expander 15 via a guide blade adj ustment of the expander 15 is pos s ible . Alternatively or additionally, a heating of components threatened by ice formation such as for example an expander hous ing , dif fuser , s ilencer and/or air duct can take place . Alternatively or additionally, a defrosting section and/or mixing chamber can be provided after the outlet of the expander 15 in order to separate ice , again defrost ice and/or admix warm air if required .

Alternatively, ice formation can al so be permitted and a defrost mode run intermittently . In the exemplary embodiment of Fig. 4, a cold extraction device 29 is provided downstream of the expander 15. By way of the cold extraction device 29, cold can be extracted from the expanded air prior to the same being discharged into the environment. The cold extraction device 29 is a heat exchanger which is equipped in order to extract utilisable cold expanded in the expander 15.

Further, a heat exchanger 30 is provided in Fig. 4 upstream of the second heat exchanger 14 in order to supply heat to the air upstream of the second heat exchanger 14. By way of this, heat can be supplied to the air at the process entry. It is thus possible to regulate the temperature of the air at the process entry, i.e. upstream of the second heat exchanger 14, to a set-point value.

Fig. 5 shows a modification of the compressor-expander system 10 of Fig. 1 in which the first heat exchanger 13 is part of a heat storage system 31, which is integrated in the compressor-expander system 10 downstream of the compressor 12. In Fig. 5, the heat extraction takes place directly into the heat storage system 31, which includes the first heat exchanger 13 and a heat reservoir 32.

Preferably, the heat reservoir 32 is embodied as a solid heat reservoir of concrete, volcanic rock, bulk, etc. In such a sensitive solid heat reservoir, the solid storage material is penetrated by tubes. By way of the media in the tubes, the solid storage material is heated and cooled. The heat storage in and the heat dispensation from the heat reservoir 32 can, in the case of a bulk, be also effected directly by the air on the solid storage material, when the bulk is flowed through by process air.

The heating of the solid storage material for storing thermal energy takes place via the air LV compressed in the compressor 12. The cooling of the solid storage material for dispensing thermal energy takes place via a process medium NM. The process medium NM can be steam, air, thermal oil or the like. By way of the process medium NM, process heat can be extracted in a temperature range between in particular 100°C and 400°C.

In Fig. 5, two separate tube systems are shown for the heat reservoir 32, a tube system for the air LV compressed in the compressor 12 for storing thermal energy and a tube system for the process medium NM for dispensing thermal energy. A single tube system exclusively for the air LV or three tube systems, a tube system for the air LV for heat storage and two further tube systems for heat storage and heat dispensation is/are also conceivable.

The heat reservoir 32 can also be embodied as a latent heat reservoir, with a storage material such as for example a salt, metal or the like, which in a temperature range between in particular 100°C and 300°C perform a phase transition at constant temperature.

Fig. 6 shows a further development of the compressorexpander system 10 of Fig. 5, in which, as in the compressor-expander 10 of Fig. 2 and 3, between the compressor 12 and the first heat exchanger 13 or the heat storage system 31 comprising the first heat exchanger 13, the heating device 21 is connected, which is embodied as a burner. Further, the compressor-expander system 10 of Fig. 6 comprises the switching valve 23 in the air duct emanating from the first heat exchanger 13 or from the heat storage system 31 comprising the first heat exchanger 13 extending in the direction of the expander 15.

In conjunction with the heating device 21 formed as burner, a backup can be provided in order to ensure the heat supply of the heat storage system 31 in the event of power shortage. When using the burner, the process proceeds approximately atmospherically, the supply air is preferentially supplied via the compressor 12 in at idle, alternatively via a compressor 12 with bypass 22 and fan 26 similar to Fig. 2 and 3. In the burner mode, the exhaust gas is discharged before the expander 15 via the switching valve 23 or a flap to the chimney 24.

Fig. 7 shows a further development of the compressorexpander system 10 of Fig. 5 or 6. Accordingly, a clutch 33 is connected in the shaft 16 between the compressor 12 and the expander 15. The first heat exchanger 13 is designed as bulk material storage device. Further, Fig. 7 shows a further heat exchanger 34, further switching valves 35, 36 and 39, an optional heating device 37 and an optional filter 38. With the compressor-expander system 10 of Fig. 7, different operating modes can be advantageously provided .

In a first operating mode of the compressor-expander system 10 of Fig. 7, which corresponds to a first heat pump operation, heat can be advantageously stored via the first heat exchanger 13, which is designed as bulk material storage device, in the storage medium SM or bulk material of the bulk material storage device. In the process, the switching valve 35 is opened so that the outlet of the compressor 12 is connected to a hot side of the bulk material storage device, which in Fig. 7 is shown at the top. In Fig. 7, the optional heating device 37 is connected between the compressor 12 and the switching valve 35, which is designed as electric heater or conventional heat exchanger. The switching valve 36 then connects a cold side of the bulk material storage device, which is shown at the bottom in Fig. 7, to the second heat exchanger 14, which comprises the heat pipes 17. The switching valve 36 in the process closes an outlet side of the further heat exchanger 34, which is then not flowed through. Then, the switching valve 39 is closed. Then, the switching valve 23 connects the second heat exchanger 14 with the expander 15 while the connection to the chimney 24 is then closed.

In a second operating mode of the compressor-expander system 10 of Fig.7, which corresponds to a second heat pump operation, a direct heat utilisation takes place. In this case, the switching valve 35 is opened so that the outlet of the compressor 12 is connected with an inlet side of the further heat exchanger 34. Then, the switching valve 36 connects the outlet side of the further heat exchanger 34, which is in particular a steam generator, with the second heat exchanger 14, which comprises the heat pipes 17. Then, the switching valve 36 blocks the cold side of the bulk material storage device. Then, the first heat exchanger 13 formed as bulk material storage device is not flowed through while the switching valve 39 is then closed. The switching valve 23 then connects the second heat exchanger 14 with the expander 15, while the connection to the chimney 24 is then closed.

Via a regulation of the switching valve 36, the first operating mode and the second operating mode can then be also utilised simultaneously in a mixed operation.

In a third operating mode of the compressor-expander system 10 of Fig. 7, heat can be advantageously dispensed out of the bulk material via the first heat exchanger 13. In this case, the compressor 12 works as fan, while the expander 15, with opened clutch 33, is decoupled. In this case, the switching valve 35 is opened so that the outlet of the compressor 12 is connected with the cold side of the bulk material storage device. Then, the switching valve 36 connects the outlet of the further heat exchanger 34 with the inlet of the compressor 12, wherein the switching valve 39 is then opened. The second heat exchanger 14, which comprises the heat pipes 17, is bypassed and the air circulates. The switching valve 23 then connects preferentially the second heat exchanger 14 with the expander 15, while the connection to the chimney 24 is then closed, but this is not relevant to the third operating mode .

In a fourth operating mode of the compressor-expander system 10 of Fig. 7, a so-called emergency operation, the compressor 12 again works as fan, while the expander 15, with the clutch 33 opened, is decoupled. The heating device 21 embodied as burner heats the air. In this case, the switching valve 35 is opened so that the outlet of the compressor 12 is connected to the cold side of the bulk material storage device. The switching valve 36 then connects the outlet of the further heat exchanger 34 with the second heat exchanger 14, which comprises the heat pipes 17. Then, the switching valve 39 is closed. Then, the switching valve 23 connects the second heat exchanger 14 with the chimney 24 while the connection to the expander 15 is then closed. Exhaust gases, which develop during the operation of the burner, can be discharged into the chimney 24.

The first heat exchanger 34 in Fig. 7 is a bulk material storage device, in which bulk material, preferentially concrete or gravel or rock is contained.

The compressor-expander system 10 according to the invention is an open system which can be operated particularly efficiently. In particular, heat can be provided at a high temperature level in a temperature range between 200°C and 600°C. The invention is a power-to-heat application in order to convert excess electric energy into heat and store the same in a storage medium SM.

For the compressor-expander system 10 of Fig. 1, Fig. 9 shows a pressure (P) -enthalpy- (h) diagram, wherein the open cyclic process can be taken from Fig. 9. The arrows of Fig. 9 illustrate the heating of the uncompressed air LUI in the second heat exchanger 14, the compression of the same in the compressor 12, the step-by-step cooling of the same in the region of the first heat exchanger 13 and of the second heat exchanger 14 as well as the subsequent expansion of the same in the expander 15. The temperature level of the expanded air LU2 discharged into the environment is lower than the temperature level of the air LUI to be compressed taken in from the environment.

Further, the invention relates to a method for operating a compressor-expander system 10 according to the invention. The method relates in particular to the operation of the compressor-expander system 10 of Fig. 2, 3 in which the heating device 21 connected between the compressor 12 and the first heat exchanger 13 is embodied as burner. When the burner 21 is operated, air is supplied to the same preferentially via the bypass 22 and the compressed air discharged into the chimney 24 via the switching valve 23. When the burner, by contrast, is not operated, the bypass

22 is preferentially closed, the compressor 12 is operated and the compressed air is supplied by the switching valve

23 to the expander 15 downstream of the second heat exchanger 14.

List of reference numbers

10 Compres sor-expander system

11 Electric machine

12 Compres sor

13 First heat exchanger

14 Second heat exchanger

15 Expander

15a Expander stage

15b Expander stage

16 Shaft

17 Heat pipe

18 Air duct

19 Air duct

20 Fin

21 Heating device

22 Bypa s s

23 Switching valve

24 Chimney

25 Switching valve

26 Fan

27 Heat exchanger

28 Condensate separator

29 Cold extraction device

30 Heat exchanger

31 Heat storage system

32 Heat reservoir

33 Clutch

34 Heat exchanger

35 Switching valve

36 Switching valve