Fast ramping-up system for power generation, and method

Abstract

Disclosed herein is a power generation system to supply an electric load. The system includes a first power generation unit and a second power generation unit. Both power generation units include a mechanical power generating machine and an electric generator, drivingly coupled to the mechanical power generating machine to convert mechanical power into electric power. The system further includes an energy storage arrangement adapted to store energy in form of a pressurized, liquefied or solidified fluid. An expander unit of the system includes an expander and an electric generator, which is drivingly coupled to the expander to convert mechanical power generated by the expander into electric power. The expander is adapted to receive pressurized fluid from the energy storage arrangement and generate mechanical power by expansion thereof during a transient phase, in case of sudden increase of the power demand from the electric load.

Description

FAST RAMPING-UP SYSTEM FOR POWER GENERATION, AND METHODDESCRIPTIONTECHNICAL FIELD

[0001] Embodiments disclosed herein relate to uninterruptible power supply systems and methods, e.g., for critical power applications, such as data centers or other facilities requiring uninterrupted electric power supply.BACKGROUND ART

[0002] Facilities that operate power critical applications, such as data centers, require a constant or nearly constant electric power supply, to ensure continuous operation. These facilities typically use an electric power distribution grid (referred herein shortly also as “utility grid”) as a primary source of electric power and are provided with an emergency electric power supply arrangement, adapted to provide electric power in case of failure or loss of power from the electric power distribution grid, or in case of sudden increase of the electric power required by the facility. Uninterruptible power supplies typically use diesel motors, which drive electric generators, to generate electric power in case of loss of power from the utility grid, or battery storage arrangements.

[0003] Diesel motors drivingly coupled to electric generators require time to be started, and reach an electric-energy delivering condition after quite some time. Battery storages have a very short intervention time, but are unsuitable to power facilities requiring up to 50Mw electric power, or more, since reaching these levels of power output requires extremely expensive and cumbersome storage battery arrangements.

[0004] A new power generation system adapted to provide uninterruptible power supply to critical applications with a high power demand would be welcomed in the art.SUMMARY

[0005] According to an aspect, disclosed herein is a power generation system to supply an electric load requiring continuous power, for instance a data center. The powergeneration system comprises a first power generation unit, which in turn comprises at least one first mechanical power generating machine adapted to convert thermal energy into mechanical energy, and at least one first electric generator drivingly coupled to the at least one first mechanical power generating machine to convert mechanical power into electric power. The power generation system further comprises a second power generation unit, which in turn comprises at least one second mechanical power generating machine adapted to convert thermal energy into mechanical energy, and at least one second electric generator, drivingly coupled to the at least one second mechanical power generating machine, to convert mechanical power into electric power.

[0006] Furthermore, the power generation system comprises an energy storage arrangement adapted to store energy in form of a pressurized fluid, or a liquefied fluid, or a solidified fluid, or a combination thereof. A solidified fluid as understood herein is a substance stored in a solid phase, but which at ambient temperature and pressure is in the gaseous phase. Similarly, a liquefied fluid as understood herein is usually a fluid which is stored under temperature and pressure conditions at which the fluid is in the liquid phase, but in ambient temperature and pressure conditions is in a gaseous phase.

[0007] The power generation system further includes an expander unit, which in turns comprises at least one expander and one electric generator. The expander and the electric generator are drivingly coupled to one another, to convert mechanical power generated by the expander into electric power. The at least one expander is adapted to receive pressurized fluid from the energy storage arrangement and generate mechanical power by expansion thereof. In general, if the fluid is stored in the liquid phase or in the solid phase, the physical phase is converted into a gaseous phase prior to expanding the fluid in the expander.

[0008] As will be described in more detail with reference to some embodiments, the power generation system defined above is adapted to supply additional electric power following a sudden request for an increased power supply to the load. In some embodiments, the additional power request may originate from failure of the utility grid whereto the electric load is electrically connected. In such case, the power generation system shall supply the electric power which is suddenly lost due to utility grid failure. In some cases, increased power demand is due to a fluctuation of the power requiredby the electric load. The power generation system is adapted to provide uninterrupted power to the electric load also in case of grid failure or sudden increase of electric power demand from the electric load, with a fast ramp-up procedure.

[0009] In some embodiments, one, some or all the mechanical power generating machines of the first and / or second power generation units comprise internal combustion engines. Internal combustion engines can include diesel engines, for instance. For higher power rates, a gas turbine engine can be used.

[0010] In some embodiments, the energy storage arrangement can be based on CAES (Compressed-Air Energy Storage) technology, or on LAES (Liquid-Air Energy Storage) technology.

[0011] In some embodiments, the power generation system is adapted to operate the first power generation unit and supply electric power generated therewith to the electric load, while the second power generation unit is inoperative, such that the electric load is powered in combination by a utility grid and the first power generation unit in combination. The power generation system is further adapted to respond to a ramp-up signal when additional electric power is needed, for instance in case of grid failure or sudden additional power request from the load, which cannot be supplied by the utility grid.

[0012] The power generation system is configured to respond to the ramp-up signal as follows. The first power generation unit will continue to supply power to the electric load, while pressurized fluid is delivered from the energy storage arrangement to the expander unit. The pressurized fluid, in gaseous phase, expands through the expander and generates mechanical power, which is converted into electric power by the electric generator.

[0013] Electric power generated by the expander unit in this phase is supplied to the electric load to balance, i.e., compensate for, the loss of power from the utility grid, or to meet the increased power required by the electric load.

[0014] The power generation system can further react to the ramp-up signal by increasing the electric power generated by the first power generation unit.

[0015] Additionally, in this transient phase, the second power generation unit is started. Power generation through the expander lasts the time needed until the second power generation unit reaches an operative condition and can supply electric power to the load. This phase can last a few tens of minutes, during which power is supplied by the expander. Once the second power generation unit reaches a power-generation condition, i.e. can supply electric power to the electric load, the pressurized fluid flow from the energy storage arrangement to the expander unit can be stopped.

[0016] The energy storage arrangement using pressurized, or liquefied, or solidified fluid to store energy provides sufficient energy to cover the transition phase until the first power generation unit and the second power generation units in combination can provide all the power required by the electrical load, avoiding the use of expensive storage batteries. The fluid-based energy storage arrangement is less expensive and less cumbersome than a battery storage system having the same storage capacity.

[0017] To make the power generation system more reactive to a ramp-up signal, the expander unit can be maintained in continuous rotation also when no power from the expander is needed. In this way, the expander unit can start generating power in less time following a ramp-up signal.

[0018] For instance, the expander unit can be adapted to operate selectively in a first, idle mode, in which the at least one expander is maintained in rotation by an idling arrangement while producing no power or a small amount of power; and in a second, power-generation mode, in which the at least one expander converts energy from a flow of pressurized fluid supplied by the energy storage arrangement into mechanical power.

[0019] In some embodiments, the idling arrangement comprises a fluid coupling between a discharge side of the at least one first mechanical power generating machine of the first power generation unit and an inlet of the at least one expander, such that in the idle mode flue gas from the at least one first mechanical power generating machine of the first power generation unit through the at least one expander maintains the at least one expander in idle rotation.

[0020] In some embodiments, the electric generator of the expander unit is adapted to operate in a motor mode, when the expander unit is in the idle mode, and to maintainthe expander in idle rotation.

[0021] In some embodiments, expander unit is adapted to operate selectively: in a first, low-power generation mode, in which the at least one expander unit generates a first electric power rate; and in a second, high-power generation mode, in which the at least one expander unit generates a second electric power rate by expansion of a flow of pressurized fluid supplied by the energy storage arrangement, the second electric power rate being higher than the first electric power rate.

[0022] According to a further aspect, disclosed herein is a system comprising: an electric load, electrically coupled to an electric power distribution grid; a power generation system as outlined above, wherein the power generation system is electrically coupled to the electric load to supply electric power thereto; and a control unit, adapted to generate a ramp-up signal in response to a request for additional power to be delivered by the power generation system to the electric load. The ramp-up signal is applied to the power generation system to start a ramp-up routine.

[0023] According to a further aspect, disclosed herein is a method of supplying electric power to an electric load electrically connected to an electric power distribution grid. The method comprises the following steps: supplying first electric power from the electric power distribution grid to the electric load; supplying second electric power from a first power generation unit to the electric load; wherein the first power generation unit comprises at least one first mechanical power generating machine adapted to convert thermal energy into mechanical energy, said first mechanical power generating machine being drivingly coupled to a first electric generator. The method is further configures such that, responsive to a ramp-up signal requesting additional power to be supplied to the electric load, the following steps are preformed: supplying a flow of pressurized fluid from an energy storage arrangement to an expander drivingly coupled to an electric generator and generate supplemental electric power therewith; supplying said supplemental electric power to the electric load; while supplying supplemental electric power to the electric load, starting-up a second power generation unit; wherein the second power generation unit comprises at least one second mechanical power generating machine adapted to convert thermal energy into mechanical energy, said second mechanical power generating machine being drivingly coupled to a second electric generator. Moreover, the method provides that, when the second powergeneration unit has reached a power supplying condition, supply of electric power from the second power generation unit to the electric load starts, and the expander is stopped.

[0024] Further features and embodiments of the systems and method according to the present disclosure are set forth in the dependent claims and described in connection with the attached drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Reference is now made briefly to the accompanying drawings, in which:Fig. l is a schematic of a system including a utility grid, a power supply system and a facility comprising a critical power application;Fig.2 illustrates a further embodiment of a power generation system according to the present disclosure;Fig.3 illustrates a further embodiment of a power generation system according to the present disclosure;Fig.4 illustrates a further embodiment of a power generation system according to the present disclosure;Fig.5 illustrates a further embodiment of a power generation system according to the present disclosure; andFig.6 illustrates a power-vs-time diagram illustrating the mode of operation of the system in case of grid loss.DETAILED DESCRIPTION

[0026] Fig. l shows a facility 1, which may comprise a data center 3, or another power-critical electric load, which requires uninterrupted power supply. As understood herein, an uninterrupted power supply can be a supply characterized by an availability of at least 99 % or higher, for instance of 99.9% or higher, up to 99.999%.

[0027] The facility 1 is electrically coupled to an electric power distribution grid 5, referred to herein shortly also as “utility grid”. The electric power distribution grid can be electrically connected to the facility 1 through a connection line 5.1 comprising a breaker 5.2, adapted to isolate the facility 1 from the electric power distribution grid 5 in case of loss of power from the utility grid 5, i.e. when no power or insufficient powerfrom the utility grid 5 is available, for instance. Opening of the breaker 5.2 places the facility 1 in an islanding condition, as will be explained in more detail below.

[0028] A control unit 7 is adapted to detect the availability of electric power from the electric power distribution grid 5. A sensor 9 is functionally coupled to the control unit 7 to provide information on availability of electricity on the electric power distribution grid 5. The control unit 7 is adapted to generate a ramp-up signal, responsive to a loss- of-grid signal from sensor 9. The ramp-up signal will initiate a ramp-up routine of a power generation system 11, adapted to supply power to the facility 1.

[0029] In some embodiments, the control unit 7 can be adapted to detect an increase in the electric power demand from the electric load 3 and generate a ramp-up signal if the utility grid is uncapable of covering the increased power demand, or if increasing the power absorbed from the utility grid is disadvantageous, for instance from a financial point of view, or not allowed by contractual limitations imposed by the provider to the user.

[0030] In general terms, the control unit 7 is adapted to generate a ramp-up signal in response to an increased power demand from the power generation system, regardless of the cause of the increased power demand to the power generation system.

[0031] As will become apparent from the detailed description below, in embodiments disclosed herein the power generation system 11 is adapted to generate and supply power to the facility 1 in a continuous manner, i.e., also when the utility grid 5 is available, such that the facility 1 is normally powered by a first fraction of the total power demand from the utility grid 5, and by a second fraction of the total power demand from the power generation system 11. When the facility 1 is in island condition following failure or loss of the utility grid 5, the facility 1 will be powered by the power generation system 11 only.

[0032] The power generation system 11 is connected to the facility 1 through an electric power supply line 13.

[0033] With continuing reference to Fig.1, a first embodiment of the power generation system 11 is shown in Fig.2.

[0034] The power generation system 11 comprises a first power generation unit 15 and a second power generation unit 17.

[0035] Each power generation unit 15, 17 can comprise one or more mechanical power generating machines. Each mechanical power generating machine is adapted to convert thermal energy into mechanical energy.

[0036] The mechanical power generating machines can include steam or vapor turbines, for instance. In some embodiments, the mechanical power generating machines can include internal combustion engines. As understood herein, an internal combustion engine is a machine wherein heat is generated inside the machine by converting chemical energy of a fuel into thermal energy by combustion inside the machine.

[0037] As such, an internal combustion engine can include a reciprocating internal combustion engine, such as a diesel engine, or a rotary machine, such as a gas turbine engine.

[0038] In some embodiments, the first and second power generation units 15, 17 can comprise the same kind of mechanical power generating machines, but this is not strictly mandatory. For instance, the first power generation unit can include mechanical power generating machines of a first kind, such as steam turbines, and the second power generation unit can include a second kind of mechanical power generating machines, for instance internal combustion engines, such as diesel engines or gas turbine engines. Additionally, each power generation unit can include more than one mechanical power generating machine, of different kinds. For instance, one or both said first and second power generation units can include a combination of different thermal machines, such as steam or vapor turbines and internal combustion engines.

[0039] In general, as will become apparent from the following description, it can be beneficial to have faster ramping-up mechanical power generating machines in the second power generation unit, such as diesel engines or advantageously gas turbine engines.

[0040] Gas turbine engines are presently preferred for high power rates, in the range of 50-100 Mw, but the use of reciprocating engines, such as diesel engines is not ruled out.

[0041] The mechanical power generating machines are drivingly coupled to respective electric generators, as will be described in detail in the following description of exemplary embodiments.

[0042] In the following detailed description of exemplary embodiments, both the first power generation unit 15 and the second power generation unit 17 include gas turbine engines as mechanical power generating machines. Therefore, the first power generation unit 15 and the second power generation unit 17 will be referred to herein also as “first gas turbine generation unit” and “second gas turbine generation unit”, respectively.

[0043] By way of non-limiting example, the first gas turbine generation unit 15 comprises a first gas turbine engine 15A and a second gas turbine engine 15B. In other embodiments, the first gas turbine generation unit 15 can comprise a different number of gas turbine engines, for instance a single gas turbine engine or more than two, such as three or four gas turbine engines, for instance.

[0044] Each gas turbine engine 15A, 15B comprises a compressor section 15.1, a combustor section 15.2, a turbine section 15.3, and an output shaft 15.4. A respective first electric generator 16A is drivingly coupled to the first gas turbine engine 15A through the output shaft 15.4 and a respective second electric generator 16B is drivingly coupled to the second gas turbine engine 15B through the output shaft 15.4 thereof. Each electric generator 16A, 16B is electrically coupled at 19A, 19B to the electric power supply line 13.

[0045] By way of non-limiting example, the second gas turbine generation unit 17 comprises a first gas turbine engine 17A and a second gas turbine engine 17B. In other embodiments, the second gas turbine generation unit 17 can comprise a different number of gas turbine engines, for instance a single gas turbine engine, or more than two, such as three or four gas turbine engines.

[0046] Each gas turbine engine 17A, 17B comprises a compressor section 17.1, a combustor section 17.2, a turbine section 17.3, and an output shaft 17.4. A respective first electric generator 18A is drivingly coupled to the first gas turbine engine 17A through the output shaft 17.4 and a respective second electric generator 18B is drivingly coupled to the second gas turbine engine 17B through the output shaft 17.4thereof. Each electric generator 18A, 18B is electrically coupled at 21 A and 21B to the electric power supply line 13.

[0047] In other embodiments, not shown, one or more gas turbine engines of either one, the other or both gas turbine generation units 15, 17 can be drivingly coupled to more than just one electric generator.

[0048] The electric generators 16 A, 16B, 18 A, 18B can each be a synchronous electric generator. In other embodiments, one, some or all said electric generators can be combined with a respective variable frequency drive (VFD), not shown, such that they can rotate at a speed different from the frequency of the utility grid. This is true also for other electric machines, which will be described below and form part of the power generation system 11. Generally speaking, electric generators connected to the electric power supply line 13 via a VFD can operate at a rotary speed different from the grid frequency, i.e. can be operated for instance at a lower speed and do not require a synchronization step, when connected to the electric power supply line 13. This may render the ramp-up process, which will be described in more detail below, faster.

[0049] Exhaust flue gas from the gas turbine engines 15 A, 15B of the first gas turbine generation unit 15 is collected in a flue gas discharge line 23, through which the flue gas can be delivered to a stack 25, or wherefrom the flue gas can be delivered to other machinery of the power generation system 11, as described below.

[0050] In the embodiment of Fig.2, the flue gas discharge line 23 extends through a hot side 27.1 of a waste-heat recovery heat exchanger 27 and through a waste-heat absorption chiller 29.

[0051] In some embodiments, the waste-heat recovery heat exchanger 27 delivers heat from the flue gas to a bottoming cycle 31. The bottoming cycle 31 includes a fluid circuit extending through the cold side 27.2 of the waste-heat recovery heat exchanger 27, such that waste heat contained in the exhaust flue gas from the first gas turbine generation unit 15 is at least partly transferred to a working fluid circulating in the bottoming cycle 31.

[0052] The bottoming cycle 31 can include an organic Rankine cycle (ORC), or a steam Rankine cycle, or any other suitable, low-temperature thermodynamic cycle,adapted to convert waste heat from the exhaust flue gas into useful mechanical power. As understood herein, a “low-temperature” thermodynamic cycle is a cycle operating at temperatures which are lower than the temperatures of the upper cycle, i.e. the cycle of the gas turbine engines, or other internal combustion engines, such as diesel engines, of the first power generation unit 15.

[0053] By way of example, the bottoming cycle 31 of Fig.2 includes a Rankine cycle represented in a somewhat simplified manner as comprising a turbine 31.1, a condenser 31.2, a condensate collecting tank 31.3, and a pressurizing pump 31.4. The turbine 31.1 is drivingly coupled through an output shaft 33 to an electric generator 35. The electric generator 35 can be electrically coupled to a load, not shown, and / or to the electric power supply line 13, for instance.

[0054] The waste-heat absorption chiller 29 is adapted to exploit thermal energy from the exhaust flue gas to chill a cooling fluid. In the embodiment of Fig.2, the waste-heat absorption chiller 29 is positioned downstream of the waste-heat recovery heat exchanger 27, since a higher temperature in the bottoming cycle can be beneficial. However, in other embodiments (not shown) the arrangement can be reversed, with the waste-heat absorption chiller 29 arranged upstream of the waste-heat recovery heat exchanger 27, with respect to the direction of flow of the exhaust flue gas.

[0055] In the embodiment of Figs 1 and 2, the waste-heat absorption chiller 29 is fluidly coupled through a cooling circuit 37 to the facility 1, to provide chilling capacity to the facility 1, for instance for cooling servers of the data center 3.

[0056] In the embodiment of Fig.2, the power generation system 11 further comprises an expander unit 41, which can include one or more expanders and relevant electric generators drivingly coupled to the expanders. In the embodiment of Fig.2, by way of non-limiting example, the expander unit 41 includes two expanders, namely a first expander 41A drivingly coupled to a first electric generator 43A, and a second expander 4 IB drivingly coupled to a second electric generator 43B. The first electric generator 43A and the second electric generator 43B can be electrically coupled (at 44 A and 44B) with the electric power supply line 13. A VFD can be placed between the electric generators 43 A, 43B and the electric power supply lines 13, such that the electric generators 43 A, 43B can operate asynchronously.

[0057] The power generation system 11 further includes an energy storage arrangement 51, adapted to store energy in the form of pressurized and / or liquefied fluid. In some embodiments, the fluid can include nitrogen, for instance, which can be available from another process.

[0058] In other embodiments, the fluid can be, or can include air.

[0059] In the following detailed description of exemplary embodiments, reference will be made specifically to air as the storing medium. Therefore, the energy storage arrangement 51 will be referred to herein also as an “air storage arrangement” 51.

[0060] In the exemplary embodiment of Fig.2, the air storage arrangement 51 is pictorially represented as a plurality of storage tanks 53, some of which are adapted to be fluidly coupled to the first expander 41 A, and others are adapted to be fluidly coupled to the second expander 4 IB. In other embodiments, a single storage reservoir or tank can be used, or several storage tanks, each of which is fluidly coupled to one, some or all the expanders of the expander unit 41.

[0061] Control valves 55 A, 55B can be arranged to selectively open and close a fluid connection between the air storage arrangement 51 and the respective first expander 41A and second expander 41B. In other embodiments, a single control valve can be provided to control the supply of pressurized air to the two (or more) expanders 41 A, 4 IB of the expander unit 41.

[0062] In the embodiment of Fig.2, the air storage arrangement 51 contains compressed air, i.e. pressurized air and therefore configures a compressed-air energy storage system (CAES system). Compressed air, i.e., pressurized air is supplied to the air storage arrangement 51 by a compressor unit 61. In the exemplary embodiment of Fig.2, the compressor unit 61 comprises an intercooled multi-stage compressor, including compressor stages 61.1, 61.2, an intercooler 61.3, and may further comprise an aftercooler 61.4. In other embodiments, a different compressor unit can be provided, for instance with a larger or smaller number of stages and coolers, and with no intercooling and / or no aftercooling.

[0063] In some embodiments, one or more of the coolers (the intercooler 61.3 and the aftercooler 61.4 in the exemplary embodiment of Fig.2) can be air-cooled, water-cooled, or cooled with cooling fluid from the waste-heat absorption chiller 29, or a combination thereof. In the schematic of Fig.2 the aftercooler 61.4 is cooled by cooling fluid from the waste-heat absorption chiller 29, by way of example. Reference number 61.5 indicates a motor which drives the compressor unit 61 and can be electrically coupled to the electric power supply line 13. In some embodiments, a VFD (not shown) can be interposed between the motor 61.5 and the electric power supply line 13, to operate the motor 61.5 at variable speed.

[0064] As will be explained in more detail below, in normal operating conditions, when electricity is available on the utility grid 5, the first gas turbine generation unit 15 is operating and co-generates part of the electric power required by the facility 1, the remaining power being supplied by the electric power distribution grid 5. In some operating conditions, if this is economically convenient, the electric power from the gas turbine generation unit 15 can be exported to the utility grid 5, while the entire power demand from facility 1 can be covered by electricity from the utility grid 5.

[0065] The second gas turbine generation unit 17 is inoperative. The expander unit 41 is also inoperative or in an idle condition, the valves 55A, 55B are closed and no air flows from the air storage arrangement 51 to the expanders 41 A, 41B.

[0066] In such normal operating conditions, the expander unit 41 can be inoperative, with the expanders 41A and 41B and the respective electric generators 43A, 43B at stand still. In preferred embodiments, however, the expander unit 41 comprises an idling arrangement adapted to keep the expanders 41 A, 41B and relevant electric generators 43A, 43B rotating at zero load, i.e., with the electric generators 43A, 43B at zero excitation current and isolated from the electric power supply line 13. Keeping the expanders 41A, 41B and relevant electric generators 43A, 43B in an idle rotating condition is beneficial in terms of fast ramping-up of the power generation system 11, if more power from the power generation system 11 is required following failure of the electric power distribution grid 5, or increased demand from the facility 1.

[0067] An “idling arrangement” as understood herein is any arrangement adapted to keep the expanders 41 A, 41B of the expander unit 41 in rotary motion, either at zero load or under partial, i.e. reduced load. In the embodiment of Fig.2, the idling arrangement comprises ducts to supply exhaust flue gas from the flue gas discharge line 23 tothe expanders 41A, 41B, whereby the expanders are maintained at a required rotary speed at zero load or reduced load, such that the pressure drop across the expanders 41 A, 4 IB and the counter-pressure at the discharge side of the gas turbine engines 15 A, 15B are minimized. If the electric generators 43 A, 43B are active at low excitation current, each unit formed by the expander 41 A, 41B and relevant electric generator 43 A, 43B can be maintained at a rotary speed corresponding to the grid frequency and the electric generator can be in phase with the electric power supply line 13, such that when the expander is required to ramp-up and generate more power, ramping-up can be performed with no delay.

[0068] If the electric generators 43 A, 43B are electrically disconnected from the electric power supply line 13 and not excited, when a ramp-up procedure is requested, the electric generators must be synchronized with the electric power supply line 13, which requires some additional time.

[0069] In some embodiments, flue gas ducts 23 A, 23B fluidly connect the inlets of expanders 41 A, 4 IB to the flue gas discharge line 23 through valves 24 A, 24B.

[0070] In some embodiments, a control valve 23 C can be arranged between the inlet ends of the flue gas ducts 23 A, 23B and the stack 25, to fully or partly discharge exhaust flue gas thorough the stack 25 rather than through the expanders 41 A, 4 IB, if needed. In other embodiments, the valve 23 C and the stack 25 can be omitted.

[0071] In another embodiment, the idling arrangement can comprise the respective electric generator 43 A, 43B of the expanders 41 A, 41B. When the expanders 41 A, 41B are not required to generate power, the respective electric generators 43A, 43B can operate in motor mode and maintain each expander-generator unit in a rotating motion, using a small amount of power from the electric power distribution grid 5, or from the gas turbine generation unit 15, for instance.

[0072] The flue gas ducts 23 A, 23B are further adapted to be connected to heat exchangers 63 A and 63B, which are aimed at heating pressurized air from the air storage arrangement 51, upstream of the expanders 41 A, 41B. As will be explained in more detail below, when the expander unit 41 is required to be ramped-up from an idling mode to a power-generation mode responsive to a ramp-up signal, exhaust flue gas from the first gas turbine generation unit 15 is diverted to the heat exchangers 63 A,63B to heat pressurized air being supplied by the air storage arrangement 51 to the expanders 41 A, 4 IB. Pressurized air is thus heated and the increased enthalpy obtained by exploiting waste heat from the exhaust flue gas increases the amount of mechanical power generated by the expanders 41 A, 41B. Additionally, heating the pressurized air prior to expansion prevents the temperature inside the expanders 41 A, 41B from dropping below a freezing temperature.

[0073] If needed, in order to provide more waste heat to the pressurized air delivered by the air storage arrangement 51 prior to expansion in the expander section 41, the waste-heat recovery heat exchanger 27 and / or the waste-heat absorption chiller 29 can be temporarily by-passed by the exhaust flue gas (see by-pass valves 27X and 29X, Fig.2), such that more waste heat is available for heating the pressurized air delivered to the expanders.

[0074] The system described so far operates as follows. Under normal operating conditions, when the electric power distribution grid 5 is available and a substantially constant power is absorbed by the facility 1, the latter, and specifically the electric load comprising the data center 3, is powered with electric power W1 from the electric power distribution grid 5 and with power W2 from the first gas turbine generation unit 15. By way of example, the power W2 co-produced by the first gas turbine generation unit 15 can be approximately 1 / 3 of the total power W=W1+W2. The second gas turbine generation unit 17 is inoperative and the expander unit 41 is maintained in idling mode, with the electric generators 43 A, 43B rotating under no-load condition or limited load conditions. Idling rotation is obtained by delivering exhaust flue gas from the flue gas discharge line 23 to the inlet of the expanders 41A, 41B through valves 24A, 24B.

[0075] The power generated by the first gas turbine generation unit 15 can be less than the rated power, i.e. the maximum power which the first gas turbine generation unit 15 is able to generate. For instance, the first gas turbine generation unit 15 can generate 75% of the maximum (rated) power.

[0076] Additional electric power is generated by the bottoming cycle 31, if present, and chilling of the facility 1 is at least in part achieved through circulation of the cooling fluid from the waste-heat absorption chiller 29.

[0077] The air storage arrangement 51 is filled with pressurized air, which has been compressed by compressor unit 61. The compressor unit 61 can be operated intermittently to compensate for air storage arrangement pressure losses, if needed.

[0078] Availability of electricity from the electric power distribution grid 5 is continuously monitored by the control unit 7. In case of grid failure, the control unit 7 is adapted to detect a grid failure signal generated by sensor 9. Responsive to a grid failure, the control unit 7 is adapted to generates a ramp-up signal applied to the power generation system 11. The ramp-up signal initiates a ramp-up routine, which includes the following procedures within the power generation system 11. Additionally, the control unit 7 can also be adapted to detect an increased electric power demand from facility 1 and to determine whether such additional demand can be covered by power from the utility grid 5, or whether it would be more appropriate, or indeed necessary, to generate the additional power locally through the power generation system 11. This may become mandatory, for instance, if the need for additional power would exceed a maximum power threshold set by contract with the grid operator. It may also be advisable not to draw additional electric power from the utility grid 5, for instance for mere financial reasons, for instance because the additional power may be more expensive than power generated locally by the power generation system 11. Thus, a ramp-up signal can be generated also in response to an increase in power demand from the facility 1, rather than in response to a grid failure.

[0079] Responsive to the ramp-up signal, valves 24A, 24B are closed, and valves 26A, 26B, positioned between the flue gas ducts 23 A, 23B and the heat exchangers 63 A, 63B, are opened. Exhaust flue gas starts flowing through the hot side of the heat exchangers 63 A, 63B. Pressurized air from the air storage arrangement 51 is let in the expanders 41A, 41B by opening valves 55A, 55B. Air flows through the cold side of each heat exchanger 63 A, 63B, such that inflowing air is heated by residual waste-heat contained in the exhaust flue gas. The pressurized and heated air expands in the expanders 41 A, 41B generating a torque. At the same time, excitation current of the electric generators 43 A, 43B increases such that the increasing torque generated by the expanders 41 A, 41B is balanced by the increasing braking torque applied by the electric generators 43 A, 43B.

[0080] To increase the power generated by the expanders 41A, 41B, in a modifiedembodiment of the power generation system 11 of Fig.2, a combustor can be foreseen, which can generate heat that is used to further heat the pressurized air from the air storage arrangement 51 prior to expansion thereof in the expanders 41 A, 41B. Fig.2 shows in dashed lines optional combustors 64A, 64B between each heat exchanger 63A, 63B and the inlet of the respective expander 41A, 41B. The combustors 64A, 64B can receive pressurized air and mix the air flow with a fuel, generating a flow of combustion gas which is expanded in the respective expander 41 A, 41B.

[0081] The expander unit 41 starts supplying electric power to the facility 1 and specifically to the data center 3, or another electric load thereof. Ramping-up of the expanders 41 A, 41B and electric generators 43 A, 43B is very fast, since the rotary equipment of the expander unit 41 starts from an idly rotating condition and does not require to accelerate angularly. It may be foreseen that during the idling mode the expanders 41A, 41B and the relevant electric generators 43A, 43B rotate at a rotary speed lower than the speed required in the power-production mode. In such case, the electric generators 43 A, 43B shall be electrically isolated from the electric power supply line 13. This reduces power losses due to ventilation in the idling mode. If the idling rotary speed is lower than the rotary speed of the expanders needed to synchronize with the electric power supply line 13, ramp-up of the expanders is nevertheless facilitated and made faster with respect to a condition where the expanders 41A, 41B and relevant electric generators 43 A, 43B must start from a stand still condition.

[0082] In some embodiments, as noted above, the electric generators 43 A, 43B can be inactive during idling. In such case, the ramp-up signal will also trigger a synchronization phase, to synchronize the electric generators 43 A, 43B with the electric power supply line 13. If during the idling phase the electric generators 43 A, 43B are electrically coupled to the electric power supply line 13, and are generating power, they are already synchronized, i.e. in phase, with the electric power supply line 13.

[0083] Full operability of the expander unit 41 can be achieved for instance in a 30 seconds time span starting from the ramp-up signal.

[0084] In some embodiments, the power generation system 11 can include a battery storage, shown schematically at 66 in Fig.1. The battery storage 66 can be activated by the ramp-up signal and start supplying electric power to the electric power supplyline 13 as a first step of the ramp-up routine, which covers the increased power demand for the first time span of some seconds, for instance 20-40 seconds, which are required to start-up the expander-generator units 41 A, 43A and 41B, 43B. Since the time interval during which electric energy must be supplied by the battery storage 66 is limited, a small battery storage 66 is sufficient.

[0085] In parallel with the activation of the expander unit 41, the gas turbine engines 15 A, 15B can ramp-up to full power. The time required by the first gas turbine generation unit 15 to reach maximum power output can be around 10 seconds.

[0086] After 30 seconds from detection of the utility grid failure, the power generation system 11 can thus cover the full power demand (W=W1+W2) from the facility 1. If a battery storage 66 is foreseen, the entire power demand can be covered practically instantaneously.

[0087] In addition to ramping-up the first gas turbine generation unit 15 to full power and bringing the expander unit 41 in a full power-generation mode, the ramp-up signal from the control unit 7 also initiates the start-up of the second gas turbine generation unit 17. The time needed to achieve full operative conditions of the second gas turbine generation unit 17 can be around 9-10 minutes.

[0088] This time interval is equal to or less than the autonomy of the CAES system comprised of the air storage arrangement 51, such that when the expanders 41 A, 41B are no longer able to generate electrical power due to exhaustion of the pressurized air stored in the air storage arrangement 51, the second gas turbine generation unit 17 is able to supply the facility 1 with the power previously generated and supplied by the expander unit 41.

[0089] The process described above is summarized in the diagram of Fig.6, which shows the power supplied by the power generation system 11 vs. time. In the diagram of Fig.6, tO is the instant in which the loss of electricity from the utility grid 5, or the increased power demand is detected. At instant tO, ramp-up of the first gas turbine generation unit 15 and switching of the expander unit 41 from the idling mode to the power-generation mode are started. At instant tO+lOs, the first gas turbine generation unit 15 has reached maximum power (W3), while the power generated by the expander unit 41 gradually increases. At instant t0+30s, the expander unit 41 has reachedmaximum power output. If the ramp-up signal is triggered by a grid failure, the facility 1 is now operating in island mode and is entirely powered by power generated locally by the power generation system 11. During the time interval from tO to t0+600s, the second gas turbine generation unit 17 is brought into operative conditions and at instant t0+600s will fully replace the expander unit 41.

[0090] The control unit 7, or an additional controller, not shown, can be used to for load management purposes. Load management involves detachment of low-priority electric loads, to provide more electric power to high-priority electric loads during transients involving ramp-up of the power generation system 11. Load management may improve the operation of the power generation system 11 and of the facility 1, providing additional time to achieve the required level of local power generation in case of grid failure, or increased power demand from facility 1.

[0091] With continuing reference to Figs. 1 and 2, a further embodiment of a power generation system 11 according to the present disclosure is shown in Fig.3. The same reference numbers designate the same components or elements already shown in Fig.2 and described above, and which will not be described again. The embodiment of Fig.3 differs from the embodiment of Fig. 2 mainly in that a Liquid Air Energy Storage system (LAES) is used, instead of a CAES system as shown in Fig.2.

[0092] In Fig. 3 the air storage arrangement 51 is adapted to store liquefied air, which is produced by an air compression and liquefaction unit 71, which replaces the compressor unit 61. The air compression and liquefaction unit 71 can be configured in any known manner. In the schematic of Fig.3, the air compression and liquefaction unit 71 comprises a multistage intercooled compressor, in turn comprising a plurality of stages 71.1, 71.2 (only two shown in Fig.3) with respective intercooler(s) 71.3. The delivery side of the most downstream compressor or compressor stage 71.2 is fluidly coupled with a hot side of an aftercooler 71.4, which cools the compressed air to a temperature such that subsequent expansion in an expander or in an throttling or laminating valve 71.5 causes at least partial liquefaction of the air. The resulting two-phase flow is collected in a gas / liquid separator 71.6. Liquid air is pumped therefrom in the air storage arrangement 51. Air in the gaseous phase is returned to the suction side of the most upstream compressor or compressor stage 71.1 through the cold side of the aftercooler 7.4. The multistage intercooled compressor of the air compression and liquefactionunit 71 can be driven by a driver 71.6, for instance an electric motor.

[0093] Alternative and more efficient air compression and liquefaction systems can be used instead of the simplified system shown by way of example only in Fig.3.

[0094] Operation of the power generation system 11 of Fig.3 is the same as described in connection with Fig.2. Liquefied air from the air storage arrangement 51 can be heated and brought back in the gaseous phase prior to expansion in the expanders 41 A, 4 IB using waste heat from the exhaust flue gas as described above, through heat exchangers 63 A, 63B. In some embodiments, liquid air is pressurized before being brought back in the gaseous phase and expanded in the expanders 41, 4 IB.

[0095] To supply a higher amount of thermal power to vaporize the liquefied air, the waste-heat recovery heat exchanger 27 and / or the waste-heat absorption chiller 29 can be temporarily by-passed (see 27X, 29X), if needed. In other embodiments, not shown, a combustor can be combined with the air storage arrangement 51, to supply thermal power to the air flow from the air storage arrangement 51 and facilitate conversion of liquefied air in the gaseous air prior to expansion in the expanders 41 A, 4 IB.

[0096] With continuing reference to Figs. 1, 2 and 3, a further embodiment of a power generation system 11 is shown in Fig.4. The same reference numbers are used to label the same elements and components shown in Fig.2 and described above, or components and elements equivalent thereto. These components will be described shortly again in the context of Fig.4.

[0097] The power generation system 11 of Fig.4 comprises a first gas turbine generation unit 15 and a second gas turbine generation unit 17. Each gas turbine generation unit can include one or more gas turbine engines and each gas turbine engine can be drivingly connected to a respective electric generator.

[0098] By way of non-limiting example, the first gas turbine generation unit 15 comprises a first gas turbine engine 15A and a second gas turbine engine 15B. Each gas turbine engine 15 A, 15B comprises a compressor section 15.1, a combustor section 15.2, a turbine section 15.3, and an output shaft 15.4. A respective first electric generator 16A is drivingly coupled to the first gas turbine engine 15A through the output shaft 15.4 and a respective second electric generator 16B is drivingly coupled to thesecond gas turbine engine 15B through the output shaft 15.4 thereof. Each electric generator 16A, 16B is electrically coupled at 19A, 19B to the electric power supply line 13.

[0099] By way of non-limiting example, the second gas turbine generation unit 17 comprises a first gas turbine engine 17A and a second gas turbine engine 17B. Each gas turbine engine 17A, 17B comprises a compressor section 17.1, a combustor section 17.2, a turbine section 17.3, and an output shaft 17.4. A respective first electric generator 18A is drivingly coupled to the first gas turbine engine 17A through the output shaft 17.4, and a respective second electric generator 18B is drivingly coupled to the second gas turbine engine 17B through the output shaft 17.4 thereof. Each electric generator 18A, 18B is electrically coupled at 21A, 21B to the electric power supply line 13.

[0100] Exhaust flue gas from the gas turbine engines 15 A, 15B is collected in flue gas discharge line 23.

[0101] In the embodiment of Fig.4, the flue gas discharge line 23 extends through a hot side 27.1 of a waste-heat recovery heat exchanger 27 and through a waste-heat absorption chiller 29.

[0102] In some embodiments, the waste-heat recovery heat exchanger 27 delivers heat from the flue gas to a bottoming cycle 81, to be described in more detail below.

[0103] As in Figs.1 and 2, the waste-heat absorption chiller 29 of Fig.4 can be fluidly coupled through a cooling circuit 37 to the facility 1, to provide chilling capacity to the facility 1, for instance for cooling servers of the data center 3.

[0104] In the embodiment of Fig.4, the power generation system 11 further comprises an expander unit 41, which can include one or more expanders and relevant electric generators drivingly coupled to the expanders. For the sake of simplifying the drawing, in Fig.4 the expander unit 41 is shown as including a single expander 41A, but it shall be understood that a larger number of expanders can be used, as schematically shown in the embodiments of Figs 2 and 3.

[0105] The expander 41 A is drivingly coupled to an electric generator 43 A, which iselectrically coupled at 44 A with the electric power supply line 13.

[0106] Turning now to the bottoming cycle 81, the latter includes a fluid path extending through the cold side 27.2 of the waste-heat recovery heat exchanger 27, such that waste heat contained in the exhaust flue gas from the first gas turbine generation unit 15 is at least partly transferred to a working fluid circulating in the bottoming cycle 81 and converted into mechanical power.

[0107] Differently form the bottoming cycle 31 of Fig.2, in the embodiment of Fig.4 the bottoming cycle 81 is a closed Brayton cycle, including a compressor 83, driven by a driver 84, for instance an electric motor, and a cooler 85. The turbomachine of the bottoming Brayton cycle 81 is represented by the expander 41A of the expander unit 41. If the expander unit 41 comprises more than just one expander, each expander can be included in a respective bottoming Brayton cycle, or several expanders can be arranged in the same bottoming Brayton cycle, for instance in parallel to one another.

[0108] In this embodiment, the bottoming Brayton cycle 81 operates as an idling arrangement adapted to maintain the expander 41 A or each expander of the expander unit 41 in rotation when the electric power distribution grid 5 is supplying electricity to the facility 1. The expander 41 A and the generator 43 A are maintained in rotation exploiting enthalpy transferred from the exhaust flue gas to the working fluid circulating in the bottoming cycle 81.

[0109] As such, the idling function, which in Fig.2 is performed by the exhaust flue gas flowing through the expanders 41A, 41B at no load or reduced load, is performed in the embodiment of Fig.4 by the bottoming cycle 81, such that the idling expander 41A and relevant electric generator 43A are generating some electric power, in addition to the electric power generated by the first gas turbine generation unit 15.

[0110] As noted, above, during idling the expander(s) 41 A and the relevant electric generator(s) 43 A can rotate at a speed lower than the rated speed. In such case, the electric generator(s) 43A can be connected to the electric power distribution grid and / or to the electric power distribution line 13 through a variable frequency drive (VFD).[OHl] As described with reference to Fig.2, the power generation system 11 of Fig.4further includes an air storage arrangement 51, pictorially represented as a plurality of storage tanks 53, adapted to be fluidly coupled to the expander(s) 41 A of the expander section 41. In some embodiments, connection between the air storage arrangement 51 and the expander section 41 is via the bottoming cycle 81. In the embodiment of Fig.4, an air delivery duct 91 fluidly connects the air storage arrangement 51 to the bottoming cycle 81, between a delivery side of compressor 83 and the waste-heat recovery heat exchanger 27. A valve 91 can selectively isolate the air storage arrangement 51 from the bottoming cycle 81, or connect the air storage arrangement 51 to the bottoming cycle 81 and therefore to the expander section 41, namely to the expander 41 A or each expander of the expander section 41, if more than one such expanders are provided.

[0112] The bottoming cycle 81 is configured such that, when pressurized air from the air storage arrangement 51 is delivered to the bottoming cycle 81, the latter is converted from a closed cycle to an open cycle. This is achieved by arranging valves 95, 97 downstream of the expander 41 A, or each expander, of the expander unit 41. When the bottoming cycle 81 is operating as a closed cycle, valves 93 and 97 are closed, and the working fluid in the bottoming cycle 81 is air circulating in a closed loop. When the bottoming cycle 81 is operating as an open cycle, which happens for instance in case of grid failure or increased electric power demand from facility 1, as disclosed in more detail below, the valves 93 and 97 open, and the valve 95 closes.

[0113] In the open-cycle operating condition of the bottoming cycle 81, air from the air storage arrangement 51 flows through the cold side 27.2 of the waste-heat recovery heat exchanger 27 and is heated prior to expanding in the expander unit 41.

[0114] In some embodiments, between the waste-heat recovery heat exchanger 27 and the inlet of the (or each) expander 41 A, a combustor 99 can be arranged. The combustor 99 is adapted to bum fuel and heat therewith the pressurized air prior to expansion in the expander unit 41. The enthalpy of the pressurized air is thus increased and more power can be converted in electric power through the expander(s) 41 A and electric generator(s) 43 A.

[0115] In the embodiment of Fig.4, the air storage arrangement 51 contains pressurized air and therefore configures a compressed-air energy storage system (CAES system) as in the embodiment of Fig.2. Pressurized air is supplied to the air storagearrangement 51 by a compressor unit 61, which can be configured as in Fig.2 and which is not described again. The same reference numbers used in Fig.4 and in Fig.2 designate the same components of the compressor unit 61 in the two embodiments.

[0116] As described in connection with Fig.2, in normal operating conditions, when electricity is available from the utility grid 5, the first gas turbine generation unit 15 is operating and co-generates part of the electric power required by the facility 1, the remaining power being supplied by the electric power distribution grid 5. The second gas turbine generation unit 17 is inoperative.

[0117] The expander unit 41 is maintained in rotating condition under partial load, such that the (or each) electric generator 43A driven by a respective expander 41A generates electric power exploiting waste heat from the exhaust flue gas through the closed bottoming cycle 81.

[0118] The system of Fig. 4 operates quite in the same way as disclosed with respect to Figs. 1 and 2. Under normal operating conditions, when the electric power distribution grid 5 is available, the facility 1, and specifically the electric load comprising the data center 3, is powered with electric power W1 from the electric power distribution grid 5 and with power W2 from the first gas turbine generation unit 15. By way of example, the power W2 co-produced by the first gas turbine generation unit 15 can be approximately 1 / 3 of the total power W=W1+W2. The second gas turbine generation unit 17 is inoperative and the expander unit 41 is maintained in idling mode, with the electric generator(s) 43A rotating under limited load, such that waste heat recovered through the waste-heat recovery heat exchanger 27 via the closed bottoming cycle 81 is converted into useful electric power delivered to the electric power supply line 13.

[0119] As described in connection with Fig.2, the power generated by the first gas turbine generation unit 15 can be less than the rated power. For instance, the first gas turbine generation unit 15 generates 75% of the maximum (rated) power.

[0120] The air storage arrangement 51 is filled with pressurized air, which has been compressed by compressor unit 61. The compressor unit 61 can be operated intermittently to compensate for air storage arrangement pressure losses, if needed.

[0121] When a grid failure signal is detected by the control unit 7, or an increasedpower demand from facility 1 is detected, a ramp-up routine of the power generation system 11 is initiated by a ramp-up signal from the control unit 7 applied to the power generation system 11. The ramp-up routine is substantially the same as described in conjunction with Fig.2, except for the operation of the bottoming cycle 81.

[0122] The valves 93 and 97 are opened and valve 95 is closed, such that the bottoming cycle 81 converts from a closed cycle to an open cycle. Pressurized air from the air storage arrangement 51 enters the waste-heat recovery heat exchanger 27 and receives heat from the exhaust flue gas. Additional thermal power can be transferred to the flow of pressurized air through combustor 99. Pressurized and heated air expands in the expander(s) 41 A of the expander unit 41 and is released in the environment.

[0123] Thererefore, the expander unit 41 increases the electric power generated and supplied to the facility 1 and specifically to the data center 3, or another electric load thereof. Ramping-up of the expander(s) 41 A and electric generator(s) 43A is very fast, since the rotary equipment of the expander unit 41 is already rotating and operating under partial load, i.e. the electric generator(s) 43A are already synchronized with the electric power supply line 13.

[0124] In parallel with ramping-up of the expander unit 41, the gas turbine engines 15 A, 15B can ramp-up to full power. In addition to ramping-up the first gas turbine generation unit 15 and the expander unit 41, the ramp-up signal from the control unit 7 also initiates the start-up of the second gas turbine generation unit 17 as described in detail above in connection with Fig.2.

[0125] With continuing reference to Figs. 1 to 4, a further embodiment of a power generation system 11 is shown in Fig.5. The same reference numbers are used in Figs. 4 and 5 to designate the same elements or components, which are not described again. The embodiment of Fig.5 differs from the embodiment of Fig.4 mainly in that the air storage arrangement includes a LAES system, i.e., stores liquefied air. The power generation system 11 of Fig.5 includes therefore an air compression and liquefaction unit 71 as shown in Fig.3 and described above.

[0126] In the embodiments described above an energy storage arrangement is adapted to store energy in the form of pressurized or liquefied fluid. In other embodiments, energy can be stored in the form of solidified fluid. As understood herein, asolidified fluid is a substance which is in a gaseous phase at environmental conditions, i.e. at a pressure of 1 BarA and temperature around 20°C, for instance. Solidified fluid can be for instance solidified carbon dioxide. Energy can be stored in the form of solidified fluid by pressurizing and chilling a gas until the latter reaches a solidified phase. Energy can be recovered and used in the system described herein by converting the solidified fluid into a pressurized, gaseous fluid adapted to be expanded in the expanders).

[0127] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A power generation system to supply an electric load, the system comprising: a first power generation unit comprising at least one first mechanical power generating machine adapted to convert thermal energy into mechanical energy and at least one first electric generator, drivingly coupled to the at least one first mechanical power generating machine, to convert mechanical power into electric power; a second power generation unit comprising at least one second mechanical power generating machine adapted to convert thermal energy into mechanical energy and at least one second electric generator, drivingly coupled to the at least one second mechanical power generating machine, to convert mechanical power into electric power; an energy storage arrangement adapted to store energy in form of a pressurized fluid, or a liquefied fluid, or a solidified fluid, or a combination thereof; and an expander unit, comprising at least one expander and one electric generator, which is drivingly coupled to the at least one expander to convert mechanical power generated by the expander into electric power; wherein the at least one expander is adapted to receive pressurized fluid from the energy storage arrangement and generate mechanical power by expansion thereof; wherein the power generation system is adapted to: operate the first power generation unit and supply electric power generated therewith to the electric load, while the second power generation unit is inoperative; responsive to a ramp-up signal, continuing to operate the first power generation unit, and: feed pressurized fluid from the energy storage arrangement to the expander unit, expand the pressurized fluid in the expander of the expander unit and generate electric power therewith, and supply electric power generated by the expander unit to the electric load;while supplying electric power generated by the expander unit to the electric load, start operating the second power generation unit; once the second power generation unit reaches a power-generation condition, start supplying electric power from the second power generation unit to the electric load; and subsequently, stop feeding pressurized fluid from the energy storage arrangement to the expander unit.

2. The power generation system of claim 1, wherein the at least one first mechanical power generating machine comprises an internal combustion engine, in particular a gas turbine engine.

3. The power generation system of claim 1 or 2, wherein the at least one second mechanical power generating machine comprises an internal combustion engine, in particular a gas turbine engine.

4. The power generation system of any one of the preceding claims, wherein said energy storage arrangement is adapted to store compressed or liquefied air.

5. The power generation system of any one of the preceding claims, further adapted to increase power generated by the first power generation unit in response to the ramp-up signal.

6. The power generation system of any one of the preceding claims, wherein the expander unit is adapted to operate selectively: in a first, idle mode, in which the at least one expander is maintained in rotation by an idling arrangement; and in a second, power-generation mode, in which the at least one expander converts energy from a flow of pressurized fluid supplied by the energy storage arrangement into mechanical power.

7. The power generation system of claim 6, wherein the idlingarrangement comprises a fluid coupling between a discharge side of the at least one first power generating machine of the first power generation unit and an inlet of the at least one expander, such that in the idle mode flue gas from the at least one first power generating machine of the first power generation unit flows through the at least one expander and maintains the at least one expander in idle rotation.

8. The power generation system of claim 6, wherein each electric generator of the expander unit is adapted to operate in a motor mode when the expander unit is in the idle mode, and maintain the expander in idle rotation.

9. The power generation system of any one of claims 1 to 6, wherein the expander unit is adapted to operate selectively: in a first, low-power generation mode, in which the expander unit generates a first electric power rate; and in a second, high-power generation mode, in which the expander unit generates a second electric power rate by expansion of a flow of pressurized fluid supplied by the energy storage arrangement; wherein in the second electric power rate is higher than the first electric power rate.

10. The power generation system of anyone of claims 6 to 9, wherein the expander unit is controlled to switch from the first operating mode to the second operating mode responsive to a ramp-up signal.

11. The power generation system of any one of the preceding claims, comprising a fluid pre-heating arrangement adapted to pre-heat fluid from the energy storage arrangement upstream of the at least one expander.

12. The power generation system of claim 11, wherein the pre-heating arrangement comprises a heat exchanger; and wherein the heat exchanger comprises: a hot side fluidly coupled with a discharge side of the at least one mechanical power generating machine of the first power generation unit and adapted to flow flue gas from the first power generation unit therethrough; and a cold side fluidly coupled with the energy storage arrangement and with an inletof the at least one expander.

13. The power generation system of any one of the preceding claims, further comprising a bottoming cycle; wherein the bottoming cycle is adapted to receive waste heat from the first power generation unit and convert waste heat into mechanical power.

14. The power generation system of claim 13, further comprising a waste-heat recovery heat exchanger adapted to transfer waste heat from flue gas of the first power generation unit to a working fluid in the bottoming cycle; and wherein the bottoming cycle comprises a power generating turbomachine, in which compressed working fluid of the bottoming cycle expands to generate mechanical power.

15. The power generation system of claim 14, wherein the bottoming cycle further comprises an electric generator drivingly coupled to the power generating turbomachine.

16. The power generation system of claim 13, when depending on at least claim 9, comprising a waste-heat recovery heat exchanger adapted to transfer waste heat from flue gas of the first power generation unit to a working fluid in the bottoming cycle; wherein the bottoming cycle comprises the at least one expander, and a compressor; and wherein the bottoming cycle is adapted to take a closed configuration and an open configuration.

17. The power generation system of any one of the preceding claims, further comprising at least: an air compressor unit adapted to compress air and supply pressurized air to the energy storage arrangement; or an air compression and liquefaction unit, adapted to compress and liquefy air and to supply liquefied air to the energy storage arrangement; or both an air compressor unit adapted to compress air and supply pressurized air to the energy storage arrangement, and an air compression and liquefaction unit adapted to compress and liquefy air and to supply liquefied air to the energy storagearrangement.

18. The power generation system of claim 17, further comprising a waste-heat absorption chiller, adapted to absorb waste heat from flue gas generated by the first power generation unit and to chill a flow of cooling fluid therewith; wherein the air compression unit and / or the air compression and liquefaction unit is / are fluidly coupled to the waste-heat absorption chiller, which provides chilling capacity to the air compression unit and / or to the air compression and liquefaction unit.

19. The power generation system of any one of the preceding claims, further comprising a combustor arranged between the energy storage arrangement and the at least one expander, and adapted to heat fluid from the energy storage arrangement prior to expansion in the at least one expander.

20. A system comprising: an electric load, electrically coupled to an electric power distribution grid; a power generation system according to any one of the preceding claims; wherein the power generation system is electrically coupled to the electric load to supply electric power thereto; and a control unit, adapted to generate a ramp-up signal in response to a request for additional power to be delivered by the power generation system to the electric load; wherein the ramp-up signal is applied to the power generation system to start a ramp- up routine.

21. A method of supplying electric power to an electric load electrically connected to an electric power distribution grid; wherein the method comprises the following steps: supplying first electric power from the electric power distribution grid to the electric load; supplying second electric power from a first power generation unit to the electric load; wherein the first power generation unit comprises at least one first mechanical power generating machine adapted to convert thermal energy into mechanicalenergy, said first mechanical power generating machine being drivingly coupled to a first electric generator; responsive to a ramp-up signal requesting additional power to be supplied to the electric load: supplying a flow of pressurized fluid from an energy storage arrangement to an expander drivingly coupled to an electric generator and generate supplemental electric power therewith; supplying said supplemental electric power to the electric load; while supplying supplemental electric power to the electric load, starting-up a second power generation unit; wherein the second power generation unit comprises at least one second mechanical power generating machine adapted to convert thermal energy into mechanical power, said second mechanical power generating machine being drivingly coupled to a second electric generator; and when the second power generation unit has reached a power generation condition, starting supplying electric power from the second power generation unit to the electric load and stopping the expander.

22. The method of claim 21, wherein the first mechanical power generating machine comprises an internal combustion engine, in particular a gas turbine engine.

23. The method of claim 21 or 22, wherein the second mechanical power generating machine comprises an internal combustion engine, in particular a gas turbine engine.

24. The method of any one of claims 21 to 23, wherein said flow of pressurized fluid comprises a flow of pressurized air.

25. The method of any one of claims 21 to 24, further comprising the step of maintaining the expander in rotation prior to receiving the ramp-up signal.

26. The method of claim 25, wherein the step of maintaining theexpander in rotation prior to receiving a ramp-up signal comprises one of the following steps: rotating the expander through a flow of flue gas from the first power generation unit; and idly rotating the expander through the electric generator drivingly coupled to the expander, the electric generator being operated in a motor mode.

27. The method of any one of claims 21 to 26, further comprising the step of recovering waste heat from the first power generation unit and converting waste heat into mechanical power in a bottoming cycle.

28. The method of claim 27, when depending on claim 25, wherein the step of maintaining the expander in rotation includes the steps of: transferring waste heat from flue gas of the first power generation unit to a bottoming cycle; wherein the bottoming cycle comprises the expander; and circulating a working fluid in the bottoming cycle, and expanding compressed working fluid of the bottoming cycle in the expander.

29. The method of any one of claims 21 to 28, further comprising the following step: responsive to the ramp-up signal, increasing the electric power generated by the first power generation unit and supplied to the electric load.

30. The method of any one of claims 21 to 29, wherein the step of supplying pressurized fluid to the expander comprises the step of heating said pressurized fluid upstream of the expander.

31. The method of claim 30, wherein the step of heating the pressurized fluid comprises the step of heating said pressurized fluid with waste heat from the first power generation unit.

32. The method of claim 30 or 31, wherein the step of heating the pressurized fluid comprises the step of burning fuel in a combustor and transferring heatgenerated in the combustor to the pressurized fluid.

33. The method of any one of claims 21 to 31, further comprising the following steps: recovering waste heat from flue gas of the first power generation unit and operat- ing a waste-heat absorption chiller therewith; and chilling the fluid delivered to the energy storage arrangement by a cooling fluid from the waste-heat absorption chiller.

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