Supercritical carbon dioxide turbine

Abstract

A high density fluid turbine 12 is described. The high density fluid turbine includes an impeller 16 connected to a rotor shaft 18. A working fluid flows through an inlet volute 20 within a volute casing 22. The inlet volute 20 is spiral shaped with a reducing cross-sectional area. The inlet volute 20 and a nozzle 24 directs the working fluid towards the impeller 16 and rotates the impeller 16 which in turn rotates the rotor shaft 18 to generate power. A method of operating the high density fluid turbine 12 is also provided.

Description

Supercritical Carbon Dioxide Turbine

[0001] Supercritical Carbon Dioxide Turbine

[0002] The invention relates generally to the field of turbo-machinery and, more specifically, to turbines employing high density fluids.

[0003] Currently available standard turbines usually employ steam as the working fluid for power generation. In certain industrial applications, air has also been employed for power generation. The efficiency levels in generating power using steam and air have however reached peak levels. Increasing the efficiency levels of these turbines from the current levels is very difficult. In small scale power generation applications, such as waste heat recovery, concentrated solar power generation and energy storage, etc. the conventional fluids are unsuitable due to their low efficiencies.

[0004] Another drawback in using the conventional fluids in turbines is that such turbines cannot be used for energy storage applications. In applications up to 10MW power generation, employing high density fluids other than steam or air can improve the power generation efficiency. However, when high density fluids are used, designing the turbomachinery becomes highly challenging. For example, as the density of the working fluid is very high, smaller blades have to be used. This may result in high fluid path losses and leakages of the working fluid. Additionally, as the turbomachinery size shrinks, the rotor rpm increases significantly.

[0005] Furthermore, due to the high density of the working fluid, there is a possibility of increased rotor-dynamic excitations. High rotor-dynamic excitation can in turn affect the rotor stability and increase vibrations.

[0006] Therefore, there is a need for a novel design of turbines that can utilize high density fluids, while also maintaining stability in operation.

[0007] A high density fluid turbine 12 is described. The high density fluid turbine includes an impeller 16 connected to a rotor shaft 18. A working fluid flows through an inlet volute 20 within a volute casing 22. The inlet volute 20 is spiral shaped with a reducing cross-sectional area. The inlet volute 20 and a nozzle 24 directs the working fluid towards the impeller 16 and rotates the impeller 16 which in turn rotates the rotor shaft 18 to generate power. A method of operating the high density fluid turbine 12 is also provided.

[0008] These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings.

[0009] is a diagrammatic representation of a power generation unit in accordance with an embodiment of the invention.

[0010] is a schematic representation of a high density fluid turbine in accordance with an embodiment of the invention.

[0011] is a schematic representation of a cooling system and a leakage extraction system employed in the high density fluid turbine in accordance with an embodiment of the invention.

[0012] Currently available turbines employ steam or air as working fluids for power generation. These turbines are reaching peak levels of efficiency in generating power. Using a high density fluid such as supercritical carbon dioxide (sCO2) produces superior efficiency levels compared to conventional fluids such as steam or air. For example, employing high density fluids can significantly improve the power generation efficiency and also aid in energy storage applications. Moreover, due to the high density of the fluid used, the turbines can be manufactured with a compact size and footprint. However, there are several challenges that have discouraged the use of high density fluids. For example, when high density fluids are used in place of conventional fluids such as steam or air, the stability of the turbine rotor is compromised due to the high levels of excitations produced by the fluid. Furthermore, another challenge while using sCO2as the working fluid is that, CO2at the operational temperature and pressure tends to have very high density with very low viscosity. This makes it difficult to control leakages between clearances. Therefore, the turbine has to be optimally designed considering the above-mentioned challenges. These and various other aspects of a high density fluid turbine will now be described in greater detail through the following embodiments.

[0013] is a diagrammatic representation of a power generation unit 10 in accordance with one embodiment of the invention. The power generation unit 10 includes a high density fluid turbine 12 and a gearbox assembly 14. The high density fluid turbine 12 uses a working fluid with high density, such as but not limited to, carbon dioxide in supercritical phase, or supercritical carbon dioxide, or supercritical CO2, or simply sCO2. Other suitable high density fluids known in the art, such as but not limited to hydrocarbons such as Propane, Ethane, Isobutane, Ammonia, R-134a, etc. may also be used in various embodiments and the teachings below would be equally applicable with those fluids as well. The high density fluid turbine 12 may use a working fluid with pressure of around 210 bara and temperature of around 550℃. The high density fluid turbine 12 includes an impeller or a rotor that rotates due to the flow of the working fluid. The rotational energy of the rotor drives the gearbox assembly 14 to generate power. Similarly, the rotor may drive an alternator, a compressor, a generator, and the like in various applications.

[0014] is a schematic representation of a high density fluid turbine 12 used in the power generation unit 10 of. The high density fluid turbine 12 includes an impeller 16 coupled to a rotor shaft 18. The impeller 16 of the high density fluid turbine 12 is the rotary part which rotates about an axis. In one embodiment, the impeller 16 is a centrifugal shrouded impeller. The impeller 16 may be constructed using any material that is known in the art to have high strength and high corrosion resistance.

[0015] The high density fluid turbine 12 also includes an inlet volute 20 for delivering the working fluid or the process gas to the impeller 16. The inlet volute 20 is constructed within a volute casing 22, which also includes a nozzle 24. The inlet volute 20 has a spiral shape with progressively reducing cross-sectional area. This design ensures that the exit velocity of the working fluid remains uniform throughout with minimum swirl velocity. The spiral shape of the inlet volute 20 accelerates the working fluid, and distributes the fluid flow around the outlet periphery. The working fluid enters the turbine through the inlet volute 20 and the volute directs the fluid flow through the nozzle 24 on the impeller 16. In one embodiment, the nozzle 24 is a vaned nozzle that further increases its kinetic energy while directing the fluid flow towards the impeller 16. This transfers the energy of the working fluid to the impeller 16 thereby rotating the impeller. Since the impeller 16 is mounted rigidly on the rotor shaft 18, the rotating action of the impeller causes the rotor shaft to rotate at the same rpm as the impeller. The rotor shaft 18 is mounted on bearings and is connected to the gearbox 14. The gearbox 14 will be connected to an alternator or a generator for generation of power. The gearbox 14 ensures that the very high rotational speed of the rotor shaft 18, close to 40000 rpm, is stepped down to a suitable speed for power generation. Thus, the mechanical work obtained by the rotating impeller 16 is converted into electrical energy via an alternator or a generator. The working fluid exits from an exhaust diffuser 26 after expansion by the impeller 16. The exhaust diffuser 26 is aerodynamically designed for pressure recovery with minimum losses.

[0016] As noted earlier, sCO2as a working fluid has very high density but with low kinematic viscosity. Due to the very low viscosity, there is a high chance of leakage of the working fluid. Any leakage of working fluid would not only reduce the fluid pressure that is required to rotate the impeller 16 but also increases the risk of damaging the stationary components. Therefore, the impeller 16 includes seals to minimize leakage of working fluid. For example, a combination of labyrinth seals 28 and honeycomb seals are utilized at a front side 30 and a rear side 32. The labyrinth seals 28 are designed to reduce any leakage of working fluid, and are located on the stationary components to ensure safety. Similarly, the very high density of the working fluid causes high amounts of rotor dynamic excitations or vibrations in the rotor shaft 18. By utilizing honeycomb seals, the excitations caused in the rotor shaft 18 is reduced. Any working fluid leaked from the rear side 32 passes through a series of labyrinth seals 28 on the volute casing 22.

[0017] In a turbine, overhang is the length of the rotor shaft 18 that extends from the rotor side bearing closer to the impeller 16 to the impeller exit end. In integrally geared turbines, the overhang of the rotor shaft 18 is highly critical. This is because a large overhang can cause high deflections and vibrations, making it unsuitable for use in turbomachinery. In the current design, the ratio of overhang span to bearing span is less than 1 so as to minimize deflections and vibrations. In other words, the length of the rotor overhang span is less than the bearing span length. This is achieved by optimizing the span of the rear casing 50 so that it may not exceed around 0.55 to 0.6 times the rotor overhang span. In addition, the volute casing 22 span from butting face with rear casing 50 till impeller 16 nose is designed to not exceed around 0.4 to 0.45 times the rotor overhang span.

[0018] Due to the operating conditions of the turbine, the temperatures in the turbine are very high. The thermal expansion of the volute casing 22 and other turbomachinery parts becomes crucial due to the very high temperatures involved. The thermal expansion in the volute casing 22 is controlled by providing a guide key 34 to the volute casing 22. This prevents misalignment of the volute casing 22 with rotor shaft 18 in the transverse direction. The guide key 34 it restricts the movement to only 2 directions – axial and perpendicular. Thus, the thermal expansion is controlled to the axial direction and perpendicular to the axis and base frame.

[0019] is a schematic representation of a cooling system and a leakage extraction system employed in the high density fluid turbine 12 in accordance with another embodiment of the present technique. In the high density fluid turbine 12, there are two leakage paths – one in the front side 30 and another in the rear side 32. The leakages from both the front side 30 and the rear side 32 consists of high temperature working fluid, such as for example CO2, leaking from the gap between the rotating impeller 16 and the stationary nozzle 24. The working fluid leaking from the front side 30 mixes into the main flow of the turbine exit, and therefore this leaked fluid is not required to be separately extracted. However the working fluid leaking from the rear side 32, shown by arrow 36, is directed into a mixing region 38 between the front and rear casing. A cold fluid 40, which may be the same as the working fluid such as CO2, is directed into the mixing region 38 where it mixes with the leaked working fluid 36 that exits from the impeller 16. This reduces the temperature of the leaked working fluid 36. This leakage mixture 42 is directed outward through a labyrinth path 44, which consists of labyrinth teeth 46 to control pressure drop and mass flow. The labyrinth path 44 also prevents conduction of the heat from the volute casing 22. This leakage mixture 42 is directed outward via a leakage outlet 48 where it mixes with the main flow downstream of the diffuser 26. This placement prevents the leaked working fluid from being lost while also cooling the volute casing 22.

[0020] The above described embodiments provide a high efficiency fluid flow path for the very high density fluids such as sCO2. Moreover, the embodiments exhibit high rotor stability even when the high density working fluid has the potential to create excitations in the leakage path. Additionally, an integrally geared system is used to have a compact shape and overall footprint. Furthermore, the axial thrust that may be exhibited is balanced by the efficient design of the impeller 16 and placement of the combination of labyrinth seals 28 and honeycomb seal.

[0021] While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

A high density fluid turbine 12, comprising:an impeller 16 coupled to a rotor shaft 18 and configured to rotate the rotor shaft 18 to generate power via a working fluid;an inlet volute 20 within a volute casing 22, wherein the inlet volute 20 is spiral shaped with reducing cross-sectional area; anda nozzle 24 within the volute casing 22 and configured to direct the working fluid to the impeller 16.The high density fluid turbine of claim 1, wherein the impeller 16 comprises a centrifugal shrouded impeller.The high density fluid turbine of claim 1, wherein the impeller 16 comprises a plurality of labyrinth seals 28.The high density fluid turbine of claim 1, wherein the impeller 16 comprises a plurality of honeycomb seals.The high density fluid turbine of claim 1, wherein the inlet volute 20 is configured to generate a uniform exit velocity in the working fluid.The high density fluid turbine of claim 1, wherein the nozzle 24 comprises a vaned nozzle configured to increase the kinetic energy of the working fluid.The high density fluid turbine of claim 1, wherein the volute casing 22 comprises a guide key 34 configured to restrict movements due to thermal expansion.The high density fluid turbine of claim 1, wherein the ratio of an overhang span to a bearing span is less than 1.The high density fluid turbine of claim 1, further comprises a leakage extraction system.A method of operating a high density fluid turbine 12, comprising:directing the flow of a working fluid to an impeller 16 via a nozzle 24;rotating the impeller 16 via the working fluid;rotating a rotor shaft 18 coupled to the impeller 16 for generating power; andgenerating a uniform exit velocity in the working fluid via an inlet volute 20.The method of claim 10, further comprises sealing the impeller 16 via a plurality of labyrinth seals 28.The method of claim 10, further comprises sealing the impeller 16 via a plurality of honeycomb seals.The method of claim 10, further comprises restricting movements due to thermal expansion via a guide key 34.The method of claim 10, further comprises maintaining the ratio of an overhang span to a bearing span in the rotor shaft 18 less than 1.The method of claim 10, further comprises minimizing leakage of the working fluid.The method of claim 15, wherein minimizing leakage of the working fluid comprises mixing a leaked working fluid 36 with a cold fluid 40 in a mixing region 38 for generating a leakage mixture 42.The method of claim 16, wherein mixing the leaked working fluid 36 comprises cooling the leaked working fluid 36.The method of claim 15, comprises directing the leakage mixture 42 via a leakage outlet 48 for mixing the leakage mixture 42 with a main working fluid.

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