System for producing compressed gas using the brayton cycle for waste heat recovery and method of operating such system

Abstract

A system (10) for producing compressed gas using the Brayton cycle for waste heat recovery is described. The system (10) includes a compressor (12) and a turbine (13) and at least one electric machine (14) coupled to at least one of the compressor (12) and the turbine (13). The at least one electric machine is operable as motor and generator. Additionally, the system (10) includes a first gas line (151) connecting the compressor (12) and the turbine (13). The first gas line (151) includes a first heat exchanger (16A) and a first gas discharging / charging device (17). The first gas discharging / charging device (17) 10 is configured to discharge / charge gas from / into the system (10). The first gas discharging / charging device (17) is arranged upstream from the first heat exchanger (16A). Further, the system (10) includes a control unit (18) connected with the at least one electric machine (14) to control the at least one electric machine (14).

Description

SYSTEM FOR PRODUCING COMPRESSED GAS USING THEBRAYTON CYCLE FOR WASTE HEAT RECOVERY AND METHOD OF OPERATING SUCH SYSTEMTECHNICAL FIELD

[0001] Embodiments of the present disclosure relate to systems for producing compressed gas using the Brayton cycle for waste heat recovery. Further, embodiments of the present disclosure relate to methods of operating such systems.BACKGROUND

[0002] The predominant method for harnessing waste heat is through power generation employing either the Clausius-Rankine Cycle (CRC) or Organic-Rankine Cycle (ORC). The CRC demonstrates optimal efficiency when utilizing heat sources exceeding 300 to 350°C, employing water or steam as the working fluid. However, efficiency diminishes below these thresholds due to the necessity for larger and more costly plant equipment, stemming from the lower pressure levels in the high-pressure segment of the cycle. Transitioning to the ORC and employing organic working fluids with lower boiling points, such as silicone oil or propane, allows for an expanded operational range encompassing lower temperature heat sources while maintaining relatively efficient processes, facilitated by the higher molecular mass of organic working fluids. Nonetheless, the efficiency of these processes is significantly contingent on the temperature levels of the heat input and discharge. Consequently, CRC steam power plants typically exhibit superior efficiencies compared to ORC power plants.

[0003] Recently, there has been a growing demand for specialized solutions involving the utilization of conventional turbochargers for waste heatrecovery in the production of compressed air and for CO2capture and storage purposes.

[0004] Gas compression, particularly the compression of air for industrial purposes, is typically accomplished by electrically powered compression plants, with the choice of compressor largely dictated by the specific requirements of the intended application. The primary differentiation lies in the underlying working principle, as illustrated in Figure 2. Dynamic compressors, commonly employed for high-volume flow rates, primarily consist of turbo compressors and can be further categorized based on the direction of flow through the compressor, including radial, axial, and mixed- flow compressors.

[0005] Numerous industrial sectors require compressed air while simultaneously generating significant waste heat expelled into the atmosphere through chimneys. Examples include the production of iron and steel, as well as the realms of non-ferrous metals and non-metallic minerals, such as the cement, concrete, or glass industry. In these sectors, compressed air serves primarily as combustion air for processes such as smelting, roasting, conversion, recycling, or purification.

[0006] However, conventional systems for producing compressed air using turbochargers have some disadvantages. For example, one drawback is the absence of self-starting capability. In other words, conventional compressed air production plans have to be initiated, for example pneumatically using an external air pressure source. Additionally, given the required amount of compressed air and pressure level, a minimum temperature at the inlet of the expansion unit (e.g., a turbine) is necessary to drive the process.

[0007] With respect to the state of the art, reference is made to document US 2023 / 265790 Al pertaining to methods and systems provided for a Brayton cycle system. In one example, a system for an air Brayton cycle includes a chamber that can receive a first energy source and a second energysource, a turbocharger, and a motor / generator coupled to a shaft of the turbocharger between a compressor and a turbine.

[0008] Accordingly, in view of the above, there is a demand for improved systems and methods for producing compressed gas, which at least partially overcome some of the problems of the state of the art.SUMMARY

[0009] In light of the above, a system for producing compressed gas using the Brayton cycle for waste heat recovery and a method of operating a system for producing compressed gas using the Brayton cycle for waste heat recovery according to the independent claims are provided. Further aspects, advantages, and features are apparent from the dependent claims, the description, and the accompanying drawings.

[0010] According to an aspect of the present disclosure, a system for producing compressed gas using the Brayton cycle for waste heat recovery is provided. The system includes a compressor, a turbine, and at least one electric machine coupled to at least one of the compressor and the turbine. The at least one electric machine is operable as motor and generator. Additionally, the system includes a first gas line connecting the compressor and the turbine. The first gas line includes a first heat exchanger and a first gas discharging / charging device. The first gas discharging / charging device is configured to discharge / charge gas from / into the system. Typically, the first gas discharging / charging device is arranged upstream from the first heat exchanger. Further, the system includes a control unit connected with the at least one electric machine to control the at least one electric machine.

[0011] Accordingly, compared to the state of the art, an improved system for producing compressed gas is provided, particularly with respect to controllability, flexibility, simplified startup, and efficiency. In particular, by providing the system with an electrified turbomachine higher levels ofefficiency and performance can be achieved compared to traditional mechanical systems. Electric motors and generators offer precise control over speed, torque, and power output, allowing for optimization of operation under varying conditions. Further, using electrified turbomachinery such as electrically-assisted turbochargers or compressors beneficially provide for a self-starting capability. Further, it is to be noted that the electrified system employing the Brayton or Joule cycle without combustion, provides for the possibility use the system for CO2capture and CO2storage processes.

[0012] Hence, the system as described herein beneficially contributes to decarbonization, reduction of CO2, and meeting sustainability targets. The approach, according to embodiments described herein, can, for example, be advantageous where power generation is not the main focus, particularly because using an electrified system for power generation from waste heat may lack practicality for various reasons. The system as described herein is particularly useful in scenarios with relatively modest heat amounts available (in the order of 500 kW or more) and where simple and robust plant architectures are needed. This stands in contrast to conventional closed-loop systems like CRC or ORC, which necessitate more complex plant setups.

[0013] Due to the electrification by employing an e-machine as described herein, process controllability is improved, enabling the adjustment of pressure level and mass flow. More significantly, a self-starting capability is provided by utilizing the e-machine motorically to initiate the process (PTE power take-in).

[0014] Further, the system according to embodiments described herein facilitates the production of energy, particularly electricity, through the e- machine, for instance during times of excess supply (PTO: power take out).

[0015] The system according to embodiments described herein offers flexibility by the possible variation in the utilization of the e-machine (PTI / PTO) and the quantity of gas flow out of the system and the quantity ofgas flow into the system. For instance, the system enables the production of compressed air and electricity generation, with the e-machine operating as a generator. Additionally, the system provides for the possibility to produce compressed air while consuming electricity, with the e-machine operated as motor. Further, the system allows for the absorption of gas, along with compression and / or electricity generation, utilizing the e-machine as a generator.

[0016] According to a further aspect of the present disclosure, a method of operating a system for producing compressed gas using the Brayton cycle for waste heat recovery is provided. The method includes compressing gas in a turbo compressor to obtain a compressed gas. Additionally, the method includes providing heat to the compressed gas via a first heat exchanger provided in a first gas line connecting the compressor and a turbine. Typically, a first gas discharging / charging device is arranged upstream from the first heat exchanger. Further, the method includes, depending on a measured turbine inlet temperature, conducting at least one of the following: (a) controlling at least one electric machine coupled to at least one of the compressor and the turbine, and (b) discharging pressurized gas via a first gas discharging / charging device provided in the first gas line.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments. The accompanying drawings relate to embodiments of the disclosure and are described in the following:Fig. 1 shows a schematic view of a system for producing compressed gas using the Brayton cycle for waste heat recovery according to an embodiment of the present disclosure;Figs. 2 to 6 show schematic views of further embodiments of the system for producing compressed gas using the Brayton cycle for waste heat recovery according to the present disclosure; andFigs. 7 and 8 show block diagrams for illustrating embodiments of a method of operating a system for producing compressed gas using the Brayton cycle for waste heat recovery according to the present disclosure.DETAILED DESCRIPTION OF EMBODIMENTS

[0018] Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in each figure. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with any other embodiment to yield yet a further embodiment. It is intended that the present disclosure includes such modifications and variations.

[0019] Within the following description of the drawings, the same reference numbers refer to the same or to similar components. Generally, only the differences with respect to the individual embodiments are described. Unless specified otherwise, the description of a part or aspect in one embodiment can apply to a corresponding part or aspect in another embodiment as well.

[0020] With exemplary reference to Figs. 1 to 6, a system 10 for producing compressed gas using the Brayton cycle for waste heat recovery according to various embodiments of the present disclosure are described.

[0021] According to embodiments, which can be combined with other embodiments described herein, the system 10 includes a compressor 12, a turbine 13, and at least one electric machine 14 coupled to at least one of the compressor 12 and the turbine 13. The at least one electric machine 14 isoperable as motor and generator. Further, it is to be noted that the at least one electric machine 14 can be operated in idling mode. Additionally, the system 10 includes a first gas line 151 connecting the compressor 12 and the turbine 13. The first gas line 151 comprises a first heat exchanger 16A and a first gas discharging / charging device 17. The first gas discharging / charging device 17 is configured to discharge / charge gas from / into the system 10. A gas discharge from the system 10 via the first discharging / charging device 17 is schematically indicated by arrow 171 and a gas charge into the system 10 via the first discharging / charging device 17 is schematically indicated by arrow 172 in the figures. Further, the system 10 includes a control unit 18 connected with the at least one electric machine 14 to control the at least one electric machine 14. In particular, the control unit 18 can be configured to control a speed and / or a torque of the electric machine.

[0022] Accordingly, an improved system for producing compressed gas is provided, particularly with respect to controllability, flexibility, simplified startup, and efficiency. In particular, embodiments of the system as described herein provides for the capability to compensate fluctuations in the turbine inlet temperature, while keeping the pressurized air generation at the desired value. Further, with the system according to embodiments of the present disclosure it is possible to switch between pressurized air generation and electricity production. This can be done by reducing the mass flow of the pressurized air and by increasing the electrical power output of the e-machine. An additional benefit arises in the starting up procedure. Since with a conventional turbocharger an additional blower is required. With an e- machine as employed in embodiments described herein, it is possible to start the whole process by itself (PTI), e.g. by driving the compressor or turbocharger with the e-machine and then adding thermal energy in the heat exchanger. Moreover, having the possibility to use heat and / or electricity gives more flexibility to design the system to its requirements and optimize efficiency. For instance, if there is already excess heat on site it is possible to use this heat in the process.

[0023] Before various further embodiments of the present disclosure are described in more detail, some aspects with respect to some terms used herein are explained.

[0024] The Brayton cycle is a thermodynamic cycle and includes four main processes, namely compression, heat addition, expansion, and heat rejection. Typically, the Brayton cycle begins with the compression, e.g. of ambient air or other gases. The compression of the gas is typically achieved using a compressor, which increases the pressure and temperature of the gas. The compressed gas is then heated, e.g. by providing the compressed gas into a combustion chamber, where it is mixed with fuel and ignited. The resulting high-temperature, high-pressure gas expands rapidly, releasing heat energy. The hot gas expands and can be provided through a turbine, causing the turbine to rotate. The rotation of the turbine can be used to drive a shaft connected to a compressor or any other external load, such as an electrical generator in a power plant. After passing through the turbine, the gas is typically expelled into the atmosphere (in case of combustion engines such as gas turbines) or cooled down in a heat exchanger (in case of closed cycles without combustion). This cooling process prepares the gas for the next cycle by reducing its temperature. The Brayton cycle operates on the principle of converting thermal energy into mechanical work. Further, the Brayton cycle is characterized by its continuous flow of working fluid (usually air), making it well-suited for continuous power generation applications. Additionally, the Brayton cycle offers high efficiency, reliability, and scalability, which is beneficial for the system according to embodiments as described herein.

[0025] The term “waste heat recovery” refers to a process of capturing and utilizing heat that is produced as a byproduct of industrial processes, mechanical work, or other operations, rather than allowing it to dissipate unused into the environment. This heat is often generated during processes like combustion, chemical reactions, or electrical power generation. The goal of waste heat recovery is to extract the thermal energy and convert it intouseful forms such as electricity, heating, or cooling, thereby improving overall energy efficiency and reducing environmental impact. By implementing waste heat recovery systems as described herein, industries can reduce their reliance on primary energy sources, lower operating costs, and decrease greenhouse gas emissions. Waste heat recovery plays a crucial role in promoting sustainable practices and enhancing energy efficiency across different sectors, ranging from manufacturing and power generation to transportation and residential buildings.

[0026] In the present disclosure, an “electric machine being operable as motor and generator” may also be referred to as “motor-generator” or “electromechanical transducer”. In particular, the electric machine as described herein has the capability to convert electrical energy into mechanical energy (as a motor) and vice versa, converting mechanical energy into electrical energy (as a generator). More specifically, the motor mode, the electric machine converts electrical energy into mechanical energy. When electric power is supplied to the motor windings, a rotating magnetic field is generated. The magnetic field may interact with permanent magnets or other magnetic components in the machine, causing the rotor (the rotating part of the motor) to turn, thereby producing mechanical motion. In generator mode, the electric machine converts mechanical energy into electrical energy. When the rotor is turned mechanically (either by an external force or by some other means), it induces an electromotive force (EMF) across the windings of the stator (the stationary part of the generator). This EMF creates an electric current in the windings, generating electrical power.

[0027] In the present disclosure, a “gas line” can be understood as a pipeline or conduit used to transport gas from one location to another. The physical structure of the gas line typically consists of pipes, for instance made from materials like steel, plastic, or composite materials.

[0028] In the present disclosure, a “heat exchanger” can be understood as a device configured to transfer heat from one fluid to another fluid without thetwo fluids coming into direct contact. It facilitates the exchange of thermal energy between fluids at different temperatures, allowing heat to be transferred from a hot fluid to a cooler fluid.

[0029] In the present disclosure, a “gas discharging / charging device” can be understood as device is capable of either release gas (discharge) from the system or introduce gas (charge) into the system as described herein. For example, the gas discharging / charging device described herein can be a multiple way valve (e.g. a three way valve). Preferably, the multiple way valve is combined with a further device enhancing separation or mixing (e.g. a swirler). Further, the gas discharging / charging device may be configured for providing one or more separation mechanisms, such as filtration, adsorption, absorption (e.g. a scrubber), electrostatic or mechanical separation.

[0030] In the present disclosure, a “control unit” can be understood as an electronic system configured for controlling the operation of one or more elements of the system described herein. Further, the control unit may be configured for monitoring parameters, such as temperatures and pressures, at various location of the system as describe herein. Typically, the control unit is capable of interpreting input data or signals, decision making based on a predefined logic or algorithms, generating output commands or signals to achieve desired actions or outcomes, particularly of the elements of the system according to embodiments described herein.

[0031] According to embodiments, which can be combined with other embodiments described herein, the control unit 18 is configured to switch the at least one electric machine 14 between a motor mode and a generator mode. Typically, the control unit 18 is configured to control at least one of a motor speed and a motor torque in the motor mode. It is to be understood, that by controlling the motor speed and / or the motor torque, the pressure of the power charged / discharged gas can be adjusted.

[0032] According to embodiments, which can be combined with other embodiments described herein, the compressor 12 and the turbine 13 are connected via a shaft 111, as exemplarily shown in Fig. 1. Typically, the at least one electric machine 14 is indirectly coupled to the shaft 111 via electromagnetic interaction. For instance, the at least one electric machine 14 can be arranged between the compressor 12 and the turbine 13. Accordingly, an electrified turbocharger can be provided. In motor mode, electrical power is provided to the at least one electric machine 14, indicated by arrow 141 symbolizing power take-in. In generator mode, electrical power is provided form the at least one electric machine 14, indicated by arrow 142 symbolizing power take-out.

[0033] With exemplary reference to Fig.2, according to embodiments, which can be combined with other embodiments described herein, the at least one electric machine 14 may include a first electric machine 143 being connected to the compressor 12, e.g. via a shaft. In particular, the first electric machine 143 can be a motor. Accordingly, an e-compressor may be provided. Additionally or alternatively, the at least one electric machine may include a second electric machine 144 being connected to the turbine 13, e.g. via a shaft. In particular, the second electric machine 144 can be a motor or a generator. Accordingly, an e-turbine may be provided.

[0034] According to embodiments, which can be combined with other embodiments described herein, the at least one electric machine 14 can be coupled to at least one of the compressor 12 and the turbine 13 via a transmission gearbox, particularly with one or more couplings.

[0035] According to embodiments, which can be combined with other embodiments described herein, control unit 18 is connected with the first electric machine 143 and / or the second electric machine 144 in order to control the first electric machine 143 and / or the second electric machine 144. It is to be understood that features described herein with respect to the at leastone electric machine 14 may also apply to the with the first electric machine 143 and / or the second electric machine 144.

[0036] According to embodiments, which can be combined with other embodiments described herein, the first gas discharging / charging device 17 is arranged upstream from the first heat exchanger 16A. "Upstream" refers to the position of the first gas discharging / charging device 17 in relation to the first heat exchanger 16A within the system with respect to the gas flow direction as indicated by the arrows in the first gas line 151. In particular, "upstream" indicates that the first gas discharging / charging device 17 is located before or earlier in the flow path compared to the first heat exchanger 16 A.

[0037] With exemplary reference to Fig. 3, according to embodiments, which can be combined with other embodiments described herein, the system 10 further includes a second gas line 152 connecting the turbine 13 and the compressor 12. Typically, the second gas line 152 includes a second gas discharging / charging device 19. The second gas discharging / charging device 19 is configured to discharge / charge gas from / into the system. A gas discharge from the system 10 via the second discharging / charging device 19 is schematically indicated by arrow 191 and a gas charge, particularly a CO2charge, into the system via the second discharging / charging device 19 is schematically indicated by arrow 192. In particular, the second gas discharging / charging device 19 is arranged downstream from the turbine 13. Here, "downstream" refers to the position of the second gas discharging / charging device 19 in relation to the turbine 13 within the system with respect to the gas flow direction as indicated by the arrows in the second gas line 152. In particular, "downstream" indicates that the second gas discharging / charging device 19 is located after or later in the flow path compared to the turbine 13.

[0038] According to embodiments, which can be combined with other embodiments described herein, the second gas line 152 further includes asecond heat exchanger 16B, as exemplarily shown in Fig. 3. Typically, the second heat exchanger 16B is arranged downstream from the second gas discharging / charging device 19. In other words, the second heat exchanger 16B is located after or later in the flow path compared to the second gas discharging / charging device 19.

[0039] The system 10 as shown in Fig. 3 can beneficially be used for CO2capturing. For instance, the CO2to be captured can temporarily be stored in a solid. To release the CO2from the solid heat is needed supplied by the working gas. In the exemplary system 10 shown in Fig. 3, the heat is supplied by the turbine exhaust. To keep the charged CO2pure the working gas is preferably CO2. After the CO2is charged into the working gas of the system 10 (see arrow 192 in Fig. 3), it is cooled down and compressed by the compressor. After the compression, a part of the CO2can be discharged (see arrow 171 in Fig. 3), e.g. via the first gas discharging / charging device 17 and can be used for downstream processes such as direct use of CO2or storage. To keep the process going heat might be added in front of the turbine. If there is excess or too little heat added in front of the turbine the e-machine can be used to compensate with electricity, by either PTO or PTI.

[0040] In comparison to compressed air production it is beneficial from an efficiency point of view to use the system as described herein compared to an e-compressor, if the turbine inlet temperature is lower than the temperature TTCneeded to drive the turbocharger when there is zero pressurized air produced and neither PTI nor PTO on the e-machine. The difference to the topology producing compressed or pressurized air is the reference system comprising an e-compressor only. In the pressurized gas generation system (i.e. when the system as described herein is used for producing compressed gas, the main purpose is gas compression. E.g. if 10% of the compressed gas (mass flow share) is discharged, a fair comparison on economic scale would be an electrically driven compressor purely delivering this amount, i.e. 10%, of pressurized air. Therefore, if only heat is used to drive the plant there is abenefit from an efficiency point of view. The remaining 90% of the flow are needed to keep the process running. Hence, if the turbine inlet temperature is too low additional power is required to even keep the 90% of flow running. On the contrary, in the case of the CO2discharging 100% of the flow is needed, since there is the need to supply heat and a carrier gas which is preferably the same as the captured gas. Therefore, the purely electric counterpart would be an e-Compressor compressing 100% of the flow. In this case it is always beneficial from a thermodynamic point of view to use a turbine as long as the turbine generates mechanical power. Additionally, in case excess heat is available, the process can furthermore deliver electricity.

[0041] With exemplary reference to Fig. 4, according to embodiments, which can be combined with other embodiments described herein, the system 10 further includes a first bypass 123 bypassing the compressor 12. For instance, the first bypass 123 may be connected to the compressor inflow 121 and the compressor outflow 122. Further, the first bypass 123 may include a first valve 124 for controlling the flow through the first bypass 123.

[0042] With exemplary reference to Fig. 5, according to embodiments, which can be combined with other embodiments described herein, the system 10 further includes a second bypass 163 bypassing the first heat exchanger 16A. For instance, the second bypass 163 can be connected to an inflow of the first heat exchanger 16A and an outflow of the first heat exchanger 16 A. Further, the second bypass 163 may include a second valve 165 for controlling the flow through the second bypass 163.

[0043] With exemplary reference to Fig. 3, according to embodiments, which can be combined with other embodiments described herein, the system 10 further includes a third bypass 164 bypassing the second heat exchanger 16B.

[0044] For instance, the third bypass 164 can be connected to an inflow of the second heat exchanger 16B and an outflow of the second heat exchanger16B. Further, the third bypass 164 may include a third valve 166 for controlling the flow through the third bypass 164.

[0045] With exemplary reference to Fig. 6, according to embodiments, which can be combined with other embodiments described herein, the system 10 further includes a turbine waste gate 133. Typically, the waste gate 133 is connected to the first gas line 155 between the first heat exchanger 16A and the turbine 13. Further, the waste gate 133 can be connected to the turbine outflow 132. Typically, the waste gate 133 includes a fourth valve 134 for controlling the flow through the wastegate 133.

[0046] With exemplary reference to Fig. 3, according to embodiments, which can be combined with other embodiments described herein, the system 10 further includes a temperature sensor 181 for measuring a turbine inlet temperature. Typically, the temperature sensor 181 is connected with the control unit 18.

[0047] With exemplary reference to the block diagrams of Figs. 7 and 8, embodiments of a method 20 of operating a system for producing compressed gas using the Brayton cycle for waste heat recovery according to the present disclosure are described.

[0048] According to embodiments, which can be combined with other embodiments described herein, the method 20 includes compressing (represented by block 21 in Figs. 7 and 8) gas in a compressor 12 to obtain a compressed gas. Additionally, the method 20 includes providing heat (represented by block 22 in Figs. 7 and 8) to the compressed gas via a first heat exchanger 16 A provided in a first gas line 151 connecting the compressor 12 and a turbine 13. Further, the method includes, depending on a measured turbine inlet temperature TTin, conducting at least one of the following: (a) controlling (represented by block 23 in Figs. 7 and 8) at least one electric machine 14 coupled, particularly indirectly via electromagnetic interaction, to at least one of the compressor 12 and the turbine 13, and (b) discharging(represented by block 24 in Figs. 7 and 8) pressurized gas via a first gas discharging / charging device 17 provided in the first gas line 151.

[0049] According to embodiments, which can be combined with other embodiments described herein, controlling (represented by block 23 in Figs. 7 and 8) the at least one electric machine 14 includes switching the at least one electric machine 14 into a generator mode (represented by block 231 in Fig. 8) when the turbine inlet temperature TTinis above a predetermined turbine inlet temperature TTinin an idle state. Typically, the predetermined turbine inlet temperature TTinin the idle state is 200°C < TTin< 600°C, particularly 400°C < TTin< 600°C.

[0050] According to embodiments, which can be combined with other embodiments described herein, controlling (represented by block 23 in Figs. 7 and 8) the at least one electric machine 14 includes switching the at least one electric machine 14 into a motor mode (represented by block 232 in Figs. 7 and 8) when the turbine inlet temperature is below the predetermined turbine inlet temperature TTinin an idle state.

[0051] For better understanding, an example is described in the following. In steady-state operation there are three different operation modes possible which all depend on the turbine inlet temperature and thus heat transferred to the working fluid, e.g. gas. TTCis the temperature needed to drive the turbocharger when there is zero pressurized air produced and no PTI nor PTO on the e-machine.

[0052] First operation mode: Turbocharger operation (turbine inlet temperature = TTC): In this situation there is exactly enough energy available on the turbine side to just drive the compressor without producing any bleed off pressurized air nor any power on the e-machine (neither PTI nor PTO).

[0053] Second operation mode: High energy operation (turbine inlet temperature > TTC): In this situation there is excess energy available on the turbine side. This energy can either be used to bleed off pressurized air or energy can be recovered with the e-machine generating electric power (PTO).

[0054] Third operation mode: Low energy operation (turbine inlet temperature < TTC): In this situation there is not enough energy on the turbine to drive the compressor. In general, this situation should be avoided since electrical energy is required to even drive the compressor in the desired operation point.

[0055] According to embodiments, which can be combined with other embodiments described herein, the method 20 further includes releasing gas (represented by block 25 in Fig. 8) by discharging gas via a second gas discharging / charging device 19 provided downstream from the turbine 13. Alternatively, the method 20 may include charging gas (represented by block 27 in Fig. 8), particularly CO2, into the system via the second discharging / charging device 19 for CO2capturing.

[0056] According to embodiments, which can be combined with other embodiments described herein, the method 20 further includes cooling the gas (represented by block 26 in Fig. 8) by a second heat exchanger 16B provided in a second gas line 152 connecting the turbine 13 and the compressor 12. In particular, the second heat exchanger 16B is arranged downstream from the second gas discharging / charging device 19.

[0057] It is to be understood, that typically the method 20 is conducted by using a system 10 according to embodiments described herein.

[0058] In view of the above, it is to be understood that embodiments of the present disclosure are improved compared to the state of the art. In particular, embodiments described herein provide the capability to compensate for fluctuations in turbine inlet temperature while maintaining pressurized airgeneration at a desired value. Furthermore, with embodiments of the present disclosure, it is possible to switch between pressurized air generation and electricity production. This can be achieved by reducing the mass flow of pressurized air and increasing the electrical power output of the e-machine. An additional benefit arises in the starting-up procedure. Since an additional blower is required with a conventional turbocharger, with an e-machine as employed in embodiments described herein, it is possible to start the whole process by itself (PTI), for example by driving the turbocharger with the e- machine and then slowly adding thermal energy in the heat exchanger. Moreover, having the possibility to use heat and / or electricity provides more flexibility to design the system according to its requirements and optimize efficiency.

[0059] While the foregoing is directed to embodiments, other and further embodiments may be devised without departing from the basic scope, and the scope is determined by the claims that follow.REFERENCE NUMBERS10 system111 shaft12 compressor121 compressor inflow122 compressor outflow123 first bypass bypassing the compressor124 first valve13 turbine131 turbine inflow132 turbine outflow133 turbine waste gate134 fourth valve14 electric machine141 power take in142 power take out143 first electric machine144 second electric machine151 first gas line16A first heat exchanger16B second heat exchanger161 heat intake162 heat output163 second bypass bypassing the first heat exchanger164 third bypass bypassing the second heat exchanger165 second valve166 third valve17 first gas discharging / charging device171 gas discharge from the system172 gas charge into the system18 control unit181 temperature sensor19 second gas discharging / charging device191 gas discharge from the system192 gas charge into the system20 method21, 22, 23, 24, 25, 26, 231, 232 blocks of block diagram for illustrating the method

Claims

CLAIMS1. A system (10) for producing compressed gas using the Brayton cycle for waste heat recovery, comprising:- a compressor (12) and a turbine (13);- at least one electric machine (14) coupled to at least one of the compressor (12) and the turbine (13), the at least one electric machine being operable as motor and generator;- a first gas line (151) connecting the compressor (12) and the turbine (13), wherein the first gas line (151) comprises a first heat exchanger (16A) and a first gas discharging / charging device (17), the first gas discharging / charging device (17) being configured to discharge / charge gas from / into the system (10), and the first gas discharging / charging device (17) being arranged upstream from the first heat exchanger (16A); and- a control unit (18) connected with the at least one electric machine (14) to control the at least one electric machine (14).

2. The system (10) of claim 1, wherein the control unit (18) is configured to switch the at least one electric machine (14) between a motor mode and a generator mode.

3. The system (10) of claim 2, wherein the control unit (18) is configured to control at least one of a motor speed and a motor torque in the motor mode.

4. The system (10) of any of claims 1 to 3, wherein the compressor (12) and the turbine (13) are connected via a shaft (111), and wherein the at least one electric machine (14) is coupled to the shaft (111).

5. The system (10) of any of claims 1 to 4, further comprising a second gas line (152) connecting the turbine (13) and the compressor (12), the second gas line(152) comprising a second gas discharging / charging device (19) being configured to discharge / charge gas from / into the system, the second gas discharging / charging device (19) being arranged downstream from the turbine (13).

6. The system (10) of claim 5, wherein the second gas line (152) further comprises a second heat exchanger (16B).

7. The system (10) of claim 6, wherein the second heat exchanger (16B) is arranged downstream from the second gas discharging / charging device (19).

8. The system (10) of any of claims 1 to 7, further comprising one or more of a first bypass (123) bypassing the compressor (12); a turbine waste gate (133), a second bypass (163) bypassing the first heat exchanger (16A), and a third bypass (164) bypassing the second heat exchanger (16B) according to claim 7.

9. The system (10) of any of claims 1 to 8, further comprising a temperature sensor (181) for measuring a turbine inlet temperature, the temperature sensor (181) being connected with the control unit (18).

10. A method (20) of operating a system for producing compressed gas using the Brayton cycle for waste heat recovery, comprising- compressing (21) gas in a compressor (12) to obtain a compressed gas;- providing (22) heat to the compressed gas via a first heat exchanger (16A) provided in a first gas line (151) connecting the compressor (12) and a turbine (13), wherein a first gas discharging / charging device (17) is arranged upstream from the first heat exchanger (16A); and depending on a measured turbine inlet temperature conducting at least one of(a) controlling (23) at least one electric machine (14) coupled to at least one of the compressor (12) and the turbine (13), and(b) discharging (24) pressurized gas via the first gas discharging / charging device (17) provided in the first gas line (151).

11. The method (20) of claim 10, wherein controlling (23) the at least one electric machine (14) comprises switching (231) the at least one electric machine (14) into a generator mode when the turbine inlet temperature is above a predetermined turbine inlet temperature TTinin an idle state, particularly the predetermined turbine inlet temperature TTinin the idle state being 200°C < TTin< 600°C, particularly 400°C < TTin< 600°C.

12. The method (20) of claim 10 or 11, wherein controlling (23) the at least one electric machine (14) comprises switching (232) the at least one electric machine (14) into a motor mode when the turbine inlet temperature is below a predetermined turbine inlet temperature TTinin an idle state, particularly the predetermined turbine inlet temperature TTinin the idle state being 200°C < TTin< 600oc, particularly 400°C < TTin< 600°C.

13. The method (20) of any of claims 10 to 12, further comprising releasing (25) gas by discharging gas via a second gas discharging / charging device (19) provided downstream from the turbine (13), or charging (27) gas, particularly CO2, into the system via the second discharging / charging device (19).

14. The method (20) of claim 13, further comprising cooling (26) the gas by a second heat exchanger (16B) provided in a second gas line (152) connecting the turbine (13) and the compressor (12), particularly the second heat exchanger (16B) being arranged downstream from the second gas discharging / charging device (19).

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