Compressor for co2 cycle with at least two cascaded compression stages for ensuring supercritical state

Abstract

A compressor (1000) for processing a CO2 flow; a first compressor stage (200) having a first row of blades (250) with a first number of blades and a second compressor stage (300) downstream of the first compressor stage (200) having a second row of blades (350) with a second number of blades; the number of blades of the first compressor stage (200) being smaller than the number of blades of the second compressor stage (300); there being an annular gap (400) between the first row of blades (250) and the second row of blades (350); the first compression stage (200) being designed to ensure that the CO2 flow is in a supercritical state at its outlet, preferably close to the critical point of CO2, and such that the second compressor stage (200) processes CO2 in a supercritical state.

Description

TECHNICAL FIELD

[0001] The subject matter disclosed herein relates to a CO2 stream compressor, a CO2 cycle energy generation system and a method for compressing a CO2 stream. BACKGROUND

[0002] Supercritical carbon dioxide (sCO2) is a fluid state of carbon dioxide in which it is held at or above its critical temperature and critical pressure. This fluid exhibits interesting properties promising a substantial increase in the efficiency of traditional power plant systems.

[0003] The sCO2 cycle is a closed cycle in which the fluid is compressed by one or more compressors, heat is introduced into the cycle by a first heat exchanger, the fluid is expanded by one or more expanders and heat is released to the environment through a second heat exchanger. Advantageously, after expansion and before releasing heat to the environment, the fluid passes through a third heat exchanger, i.e. a recuperation heat exchanger, to increase the efficiency of the cycle. SUMMARY

[0004] Generally, the first compressor of the sCO2 cycle operates with a CO2 stream close to the critical point. The sCO2 cycle then exhibits a reduced work of the CO2 compressor, which takes advantage of the real gas behavior of the working fluid close to the critical point. This feature enhances the overall thermal efficiency increase of the sCO2 cycle. However, close to the critical point, the CO2 properties have large variations, which have technical implications for the design of turbomachinery and heat exchangers.

[0005] In particular, due to the size of the blade passages of the compressor impeller, the local acceleration upstream of the impeller leading edge and at both ends, the CO2 stream reaches the impeller of the first compressor in a multiphase state. In the multiphase region, i.e. under the saturation dome, the sound speed drops sharply, leading to the creation of a sonic region, thus limiting the operating range of the compressor.

[0006] When a fluid flowing at a given pressure and temperature passes through a contraction, the fluid velocity increases. At the same time, the Venturi effect causes the static pressure, and thus the density, to decrease at the contraction. This can lead to the creation of a sonic region, which can limit the operating range of the compressor.

[0007] This problem becomes more serious in the presence of a large number of compressor blades, i.e. in the presence of a large number of contractions at the compressor inlet due to the blade passages.

[0008] The large number of blades at the stage inlet, due to the Venturi effect, increases the local flow acceleration, which, combined with a greater deviation from the ideal gas behavior close to the critical point, can promote the phase change phenomena of CO2, thus reducing the compressor efficiency and the cycle efficiency.

[0009] According to one aspect, the subject-matter disclosed herein relates to a compressor arranged to handle a CO2 stream, the compressor comprising a first compressor stage and a second compressor stage downstream of the first compressor stage; the first compressor stage comprises a first row of rotating blades having a first number of blades and the second compressor stage comprises a second row of rotating blades having a second number of blades; the first number of blades is smaller than the second number of blades; the CO2 stream is in a supercritical state at the outlet of the first compressor stage.

[0010] In particular, the trailing edge of the blades of the first stage discharges the CO2 stream directly into the annular gap and the leading edge of the blades of the second stage receives the CO2 stream directly from the annular gap, the CO2 pressure at the trailing edge of the first compressor stage being equal to or higher than the saturation pressure plus a predetermined pressure margin related to the pressure drop within the second compressor stage.

[0011] According to another aspect, the subject-matter disclosed herein relates to an energy generation system based on a supercritical CO2 cycle and comprising a compressor having at least two cascaded compression stages for ensuring a supercritical state and an annular gap therebetween.

[0012] According to yet another aspect, the subject-matter disclosed herein relates to a method for compressing a CO2 stream; a first compression step for compressing the CO2 stream to a supercritical state by a first compressor stage to generate a supercritical CO2 stream and a second compression step for compressing the supercritical CO2 stream by a second compressor stage; the first compression step is such that at the end of the compression, the CO2 is close to the critical point; there is an isenthalpic step between the first compression step and the second compression step, which maintains both the total pressure and the static pressure substantially constant, in other words: low loss of total pressure and recovery of static pressure. BRIEF DESCRIPTION OF DRAWINGS

[0013] A more complete understanding of the disclosed embodiments and the attendant advantages will be readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

[0014] Figure 1 a schematic view of a CO2 system is shown;

[0015] Figure 2A a perspective view of the compressor of Figure 1 is shown;

[0016] Figure 2B a side view of the compressor of Figure 1 is shown;

[0017] Figure 3 an enlarged view of a portion of Figure 2A is shown;

[0018] Figure 4 a cross-sectional schematic of a compression system for a CO2 stream cycle is shown; and

[0019] Figure 5 an example of CO2 compression on a T-s diagram is shown. DETAILED DESCRIPTION

[0020] The subject matter disclosed herein relates to compressors and CO2 systems working with a CO2 stream, methods for compressing a CO2 stream, and compressor assemblies for a CO2 stream cycle.

[0021] The efficiency of a gas turbine cycle depends mainly on its pressure ratio (i.e. the ratio between the pressure of the gas stream at the inlet of the compressor and at the outlet of the compressor). The maximum pressure is limited due to costs related to the piping and measuring systems; therefore the minimum pressure of the sCO2 cycle significantly affects the cycle efficiency.

[0022] At the same time, the efficiency of the cycle is also affected by the state of the gas stream, in particular at the inlet of the compressor. In fact, due to cost reasons, the maximum cycle pressure is fixed, it is advantageous to work close to the critical point, as this allows a reduction of the compression work, with the result of increasing the cycle efficiency.

[0023] However, under CO2 conditions close to the critical point, shock waves can occur, limiting the operating region of the compressor and reducing the efficiency.

[0024] To overcome this problem, the compression system disclosed herein aims to increase the cycle efficiency by increasing the pressure of the fluid just enough to allow the compressor wheel to work away from the critical point while maintaining a high pressure ratio.

[0025] This is achieved by having a flow guide stage that compresses the fluid with a small pressure ratio and is designed with a small number of blades to limit the shock wave problem that causes a collapse of the performance. Advantageously, the flow guide stage with a small number of blades avoids the compressor inlet to become a sonic speed throat of the component.

[0026] Reference will now be made in detail to the embodiments of the present disclosure, one or more examples of which are illustrated in the drawings.

[0027] Examples are provided by way of explanation of the present disclosure but are not intended to limit the present disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made to the present disclosure without departing from the scope or spirit of the present disclosure.

[0028] According to one aspect and with reference to Figure 1The subject matter disclosed herein provides energy generation systems based on supercritical CO2 cycles, i.e. gas turbine plants working with CO2 as working fluid, mainly in supercritical state. Typically, in this type of cycle, the working fluid at minimum cycle pressure is in supercritical state, but also working fluid at minimum cycle pressure between 80-100% of the critical pressure is allowed in subcritical multiphase state.

[0029] Figure 1 The CO2 system comprises two heat exchangers 2000A, 2000B, an expander 3000 and a compressor 1000; advantageously, the compressor and the turbine are driven on the same shaft 1010. The shaft 1010 determines an axis A corresponding to the prevailing development direction of the shaft 1010. As used herein, the terms "axial" and "radial" refer to directions parallel and perpendicular to the axis A, respectively.

[0030] With reference to Figure 1 , the CO2 flow flows in a clockwise direction: compressed by the compressor 1000, heated in the first heat exchanger 2000A, expanded by the expander 3000, cooled in the second heat exchanger 2000B and finally restarts the cycle. In other words, the CO2 system is a closed cycle gas turbine.

[0031] According to a preferred embodiment, the CO2 system comprises a third heat exchanger 2000C, also called "recuperator"; the third heat exchanger 2000C is suitable to increase the thermal efficiency of the cycle, receiving the CO2 flow as cold fluid at the outlet of the compressor 1000 and receiving the CO2 flow as hot fluid at the outlet of the expander 3000. The recuperator 2000C allows to recover the waste heat from the expander exhaust CO2 flow and to use it to preheat the compressed CO2 flow from the compressor 1000 before further heating the compressed CO2 flow in the heat exchanger 2000A, thus reducing the external heat required.

[0032] In the example of Figure 1 , the expander 3000, in particular the shaft 1010 driving the expander 3000, is coupled with an electric generator 4000, in particular an alternator; alternatively, the expander 3000 can also be connected to an external load, not shown in the figure.

[0033] It should be noted that the number of machines and heat exchangers and the number of shafts driving the machines can vary depending on the design of the cycle.

[0034] According to one aspect and with reference to Figures 2A-2B and Figure 3 , the subject matter disclosed herein provides a compressor 1000, for example used in a supercritical CO2 system, for generating electric energy or for supplying an external load.

[0035] The compressor 1000 comprises a first compressor stage 200 and at least a second compressor stage 300, downstream of the first compressor stage 200. It is noted that "stage" here refers to a single row of vanes, which can be fixed or rotating. For example, if there is a first row of rotating vanes and a second row of stationary vanes, the first row of rotating vanes is the first stage and the second row of stationary vanes is the second stage.

[0036] The first compressor stage 200 comprises a first row of rotating vanes 250; the second compressor stage 300 comprises a second row of rotating vanes 350. Preferably, the first row of vanes 250 has a guide vane type vane, and the second row of vanes 350 has an outlet guide vane type vane. In Figures 2A-2B and Figure 3 In the preferred embodiment shown, the first number of vanes is less than the second number of vanes.

[0037] Preferably, the first number of vanes is about half or about one third of the second number of vanes. For example, if the second row of vanes 350 has a number of vanes equal to 18, the first row of vanes 250 can have a number of vanes equal to, for example, 11 or 10 or 9 or 8 or 7 or 6. It is noted that the ratio between the two numbers can be any number in general different from an integer; for example, it can be greater than 1 and less than 2, or greater than 2 and less than 3. Thus, the number of vanes can be freely chosen independently according to the mechanical design and performance desired for the two compression stages.

[0038] The compressor 100 generally works with a CO2 flow, and the first compressor stage 200 provides at the outlet a CO2 flow in supercritical state, wherein a "supercritical state fluid" is defined as a fluid having a pressure higher than its critical point, i.e. a pressure higher than its critical pressure.

[0039] In other words, at the outlet of the first compressor stage 200, the CO2 flow has a pressure higher than about 7.37 MPa.

[0040] In particular and with reference to Figure 3 The first compressor stage 200 is arranged to provide a pressure increase between the leading edge 210 and the trailing edge 220 of the first row of vanes 250; such pressure increase is sufficient to bring the CO2 flow at the trailing edge 220 to a supercritical state.

[0041] Preferably, the CO2 flow has a higher pressure at the trailing edge 220 with respect to the pressure at the leading edge 210. The ratio between the outlet pressure and the inlet pressure of the flow through a compressor stage is called "pressure ratio" or "compression ratio".

[0042] Preferably, the leading edge 210 of the first row of blades 250 corresponds to an inlet section of the compressor 1000, which receives the suction CO2 flow. This CO2 flow is then discharged corresponding to the trailing edge 220 of the first row of blades 250.

[0043] Preferably, the first row of blades 250 has a predominantly axial development with respect to the direction determined by the axis A. In particular, the axial development of the first row of blades 250 is such that the CO2 flow flows mainly in the axial direction.

[0044] With reference to Figures 2A-2B and Figure 3 , the compressor 1000 comprises a second compressor stage 300 downstream of the first compressor stage 200. In particular, the second compressor stage 300 is arranged to provide a pressure increase between a leading edge 310 and a trailing edge 320 of a second row of blades 350, such a pressure increase being much higher than the pressure increase provided between the leading edge 210 and the trailing edge 220 of the first compressor stage 200.

[0045] In other words, the pressure ratio of the first compressor stage 200 is much smaller than the pressure ratio of the second compressor stage 300, i.e. the second compressor stage 300 provides the main pressure ratio of the overall pressure ratio of the CO2 cycle. Preferably, the pressure ratio of the first compressor stage 200 is less than 70% of the pressure ratio of the second compressor stage 300, and possibly greater than 3% of the pressure ratio of the second compressor stage 300; for example, the first pressure ratio can be equal to about 1.1 and the second pressure ratio can be equal to about 1.7.

[0046] In a preferred embodiment and with reference to Figures 2A-2B , Figure 3 and Figure 4 , the second compressor stage 300 is a centrifugal compressor stage, having both an axial development and a radial development with respect to the direction determined by the axis A. In particular, the flow path between the leading edge 310 and the trailing edge 320 defines a substantially twisted surface with respect to the direction determined by the axis A. In particular, the leading edge 310 and the trailing edge 320 are located at different radial distances from the axis A.

[0047] The first compressor stage 200, in particular the first row of blades 250, is arranged to provide the CO2 flow directly to the second compressor stage 300, in particular the second row of blades 350, without any stationary component, in particular any stator blade, in between passing through a hollow axial annular gap. In particular, the CO2 flow flows from the first row of blades 250 to the second row of blades 350 without any (significant) pressure change (change in static and total pressure), for example due to stator blades between the trailing edge 220 and the leading edge 310. The Applicant has realized that stator blades between two consecutive rows of rotor blades, which are very common in turbomachinery, can appear beneficial; however, in the present case, in order to avoid a system throat, the "blade solidity" should be lower and the benefit on static pressure recovery would be negligible.

[0048] The second row of blades 350 is axially spaced apart from the first row of blades 250. In particular, an axial annular gap (developing around the axis A) is located between the trailing edge 220 of the first row of blades 250 and the leading edge 310 of the second row of blades 350. In this way, the wake is relaxed, avoiding strong aeromechanical interactions between the two rows.

[0049] Preferably, the length of the axial gap between the trailing edge 220 and the leading edge 310 is comprised between one and two times the height of the trailing edge 220 of the first row of blades 250.

[0050] With reference to Figures 2A-2B and Figure 3 , the trailing edge 220 of the first row of blades 250 and the leading edge 310 of the second row of blades 350 can not be aligned along the axial direction. In particular, the leading edge 310 of the second row of blades 350 can have a different circumferential position with respect to the trailing edge 220 of the first row of blades 250 (this arrangement is referred to as "timing effect").

[0051] In a preferred embodiment, the compressor 1000 comprises a rotor, the first row of blades 250 and the second row of blades 350 being part of the rotor.

[0052] With reference to Figure 4 , the rotor is preferably driven by a shaft 1010, so that the first row of blades 250 and the second row of blades 350 rotate at the same angular velocity.

[0053] In an alternative embodiment, the compressor 1000 comprises a first rotor and a second rotor, the first row of blades 250 being part of the first rotor and the second row of blades 350 being part of the second rotor.

[0054] Advantageously, the first rotor is driven by a first shaft and the second rotor is driven by a second shaft, the first shaft and the second shaft rotating at different angular velocities.

[0055] With reference to Figure 4The compressor 1000 can also comprise an inlet guide vane 100 upstream of the first row of vanes 250. Advantageously, the inlet guide vane 100 comprises a stator row of vanes; the stator row of vanes can be fixed or can vary the vanes' angle of attack, thereby regulating the CO2 flow drawn in by the compressor 1000.

[0056] According to another aspect, the subject-matter disclosed herein relates to a method for compressing a CO2 flow using a compressor, for example similar or identical to the compressor 1000 described above; such a method can be implemented in a supercritical CO2 cycle-based energy generation system, for example similar or identical to the energy generation system described above.

[0057] The method comprises an initial step of compressing the CO2 flow to a supercritical state by a first compressor stage 200 and a subsequent step of compressing the supercritical CO2 flow by at least a second compressor stage 300; between the first and second compression steps, there is a low-loss isenthalpic step (in particular within a hollow axial annular gap) which maintains both the total pressure and the static pressure substantially constant.

[0058] The initial step of compressing the CO2 flow to a supercritical state is such that, at the end of the compression, the thermodynamic state point of the CO2 lies outside the saturation dome, on a T-s diagram or equivalent, approximately close to the CO2 critical point Pc, Tc.

[0059] With reference to Figure 5 , a CO2 temperature-entropy diagram is shown, in which the CO2 critical point Pc, Tc is highlighted as a black point at the top of the saturation dome. According to the method disclosed herein, after the initial step of compressing the CO2 flow, the thermodynamic state point of the CO2 (i.e. the point representative of the thermodynamic state of the CO2 defined by at least two state variables, for example temperature and pressure) lies outside the saturation dome, in particular around the highlighted area 800 above the CO2 critical point Pc, Tc.

[0060] In a preferred embodiment, the pressure at the outlet of the first compressor stage 200 is equal to or higher than the saturation pressure plus a predetermined pressure margin related to the pressure drop within the second compressor stage 300.

[0061] It must be noted that the initial step of compressing the CO2 flow to a supercritical state can be followed by one or more of the following steps of compressing the supercritical CO2 flow; preferably, the initial step of compressing the CO2 flow has a much smaller pressure ratio than each of the following steps.

[0062] According to another aspect, the subject-matter disclosed herein relates to a compressor arranged to handle a CO2 flow, the compressor comprising:

[0063] a first rotary compressor stage comprising a first row of stator vanes extending mainly axially, having a leading edge 210 and a trailing edge 220;

[0064] a second rotary compressor stage comprising a second row of outlet stator vanes extending mainly axially or mainly radially or both, having a leading edge 310 and a trailing edge 320;

[0065] an annular gap between the first rotary compressor stage and the second rotary compressor stage.

[0066] In a preferred embodiment, the stator trailing edge 220 discharges the CO2 flow directly into the annular gap, and the outlet stator leading edge 310 receives the CO2 flow directly from the annular gap. Preferably, the CO2 flow pressure at the stator trailing edge 220 is higher than the CO2 flow pressure at the stator leading edge 210. In particular, the CO2 flow pressure at the trailing edge 220 is equal to or higher than the saturation pressure plus a predetermined pressure margin related to the pressure drop within the second rotary compressor stage.

[0067] The above-mentioned pressure margin is intended to avoid the CO2 flow within the second compressor stage to reach saturation conditions. In theory, there is no pressure drop within a compressor stage. However, in practice, there can be some pressure drop shortly after the leading edge 310 of the second row of outlet stator vanes; from this point of view, the most critical area is on the suction side of the outlet stator vanes, close to the leading edge 310.

[0068] The minimum pressure value inside the second row of outlet stator vanes strongly depends on the design choices, and is typically comprised between 90% and 50% of the total inlet pressure at the second rotary compressor stage, i.e. at the leading edge 310.

Claims

1. A compressor (1000) arranged to process a CO2 stream, said compressor comprising: A first compressor stage (200) includes a first row of rotating blades (250) having a first number of blades. A second compressor stage (300) includes a second row of rotating blades (350) having a second number of blades, and the second compressor stage (300) is fluidly connected downstream of the first compressor stage (200); Where the number of the first blades is less than the number of the second blades, and The first compressor stage (200) is arranged to provide a supercritical CO2 stream at the outlet; The first row of rotating blades (250) is arranged to directly supply CO2 flow to the second row of rotating blades (350); The second row of rotating blades (350) is axially spaced from the first row of rotating blades (250), thereby forming an annular gap (400) between the first row of rotating blades (250) and the second row of rotating blades (350). No stationary component passes through the annular gap, wherein the axial length of the annular gap is between one and two times the height of the trailing edge of the first row of rotating blades (250).

2. The compressor (1000) according to claim 1, characterized in that, The pressure ratio of the first compressor stage (200) is less than the pressure ratio of the second compressor stage (300).

3. The compressor (1000) according to claim 1, characterized in that, The pressure ratio of the first compressor stage (200) is greater than 1.0 and less than 1.

2.

4. The compressor (1000) according to claim 1, characterized in that, The ratio between the number of the second leaf and the number of the first leaf is a number greater than 1 and less than 2 or greater than 2 and less than 3.

5. The compressor (1000) according to claim 1, characterized in that, The compressor also includes an inlet guide vane (100) upstream of the first row of rotating blades (250).

6. The compressor (1000) according to claim 1, characterized in that, The first row of rotating blades (250) has a major axial development.

7. The compressor (1000) according to claim 1, characterized in that, The compressor includes a first rotor and a second rotor, wherein the first row of rotating blades (250) is part of the first rotor and the second row of rotating blades (350) is part of the second rotor.

8. The compressor (1000) according to claim 1, characterized in that, The compressor includes a rotor, wherein the first row of rotating blades (250) and the second row of rotating blades (350) are parts of the rotor.

9. An energy generation system based on a supercritical CO2 cycle, the energy generation system comprising two heat exchangers, an expander and at least one compressor, said at least one compressor being a compressor according to any one of claims 1 to 8.

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