WO2015028708A1

WO2015028708A1 - An electrical turbo-machine and an energy converter

Info

Publication number: WO2015028708A1
Application number: PCT/FI2014/050642
Authority: WO
Inventors: Juha Jaakko PYRHÖNEN; Jussi HEINIMÖ; Janne Nerg; Pekka RÖYTTÄ; Hannu Helmer TERVONEN; Teemu TURUNEN-SAARESTI
Original assignee: Visorc Oy
Priority date: 2013-08-26
Filing date: 2014-08-21
Publication date: 2015-03-05
Also published as: AT15743U1; FI11339U1; DE212014000175U1; FI20130244A

Abstract

A high-speed electrical turbo-machine for an Organic Rankine Cycle energy converter is presented. The electrical turbo-machine comprises a stator (101), a rotor (102) for interacting with the stator, and an impeller (103) at an end of the rotor, the impeller being suitable for operating as a turbine for rotating the rotor. The stator comprises a stator core (104) having stator teeth (111, 112) and a stator winding having stator coils (105, 108). The width of each stator coil is one stator slot pitch and each stator coil is arranged to surround only one of the stator teeth so as to minimize the length of the stator end-windings in the axial direction of the rotor. The short end-windings enable the rotor to be shorter, and thus the natural frequencies of the rotor are higher. As a corollary, the critical speeds that excite the natural frequencies of the rotor are higher too.

Description

An electrical turbo-machine and an energy converter

Field of the invention

The invention relates generally to rotating electrical machines. More particularly, the invention relates to an electrical turbo-machine suitable for operating as a turbo-generator of an energy converter.

Background

Electrical turbo-machines, wherein the turbine is directly coupled with a rotor of an electrical machine, can be used in many different applications such as high-speed turbo-compressors and energy converters. For example, small-size energy converters based on the Organic Rankine Cycle "ORC" process can be used for converting the thermal energy of waste heat into electricity which is easily used for different purposes. The waste heat can be received from various heat-producing processes or heat-producing machines, e.g. a combustion engine, where, due to the temperature of the waste heat and/or due to the circumstances of the environment, the waste heat cannot be used as such or by means of conventional heat exchangers or corresponding means.

It can be shown thermodynamically that the ORC process is an applicable technique for this kind of energy conversion. The heat of vaporization of organic work- ing fluid is low in relation to e.g. the heat of evaporation of water, and its fall of specific enthalpy in the turbine is small and the mass flow rate in relation to the output is high, wherein it is possible to reach high turbine efficiency even in a range of small capacity. The utilization of high-speed technology, wherein the turbine is directly coupled with a generator rotating at the same speed and thus pro- ducing high-frequency current, has made it possible to further simplify the process in a way that e.g. a separate reduction gear required by conventional processes is not needed.

Publication EP0090022 describes an energy converter that comprises a boiler, a radial turbine, a condenser, a feed pump, and a high-speed generator. The energy converter may further comprise a recuperator and a pre-feeding pump. The thermal energy supplied to the boiler is arranged to maintain the Organic Rankine Cycle process driving the generator and thus producing electricity. The radial turbine and the feed pump are directly connected to the rotor of the generator. The rotor is rotatably carried with gas-dynamic bearings utilizing the organic working fluid in gaseous form. The back-surface of the radial turbine is arranged to serve as one abutment surface of a gas-static thrust bearing.

The high-speed technology is, however, not free from challenges. One of the challenges is related to the fact that all rotating objects deflect during rotation. Any un- balance in the mass of a rotating part causes deflection that may create resonant vibration at certain critical speeds that excite the natural frequencies of the rotating part. In order to avoid problems with e.g. bearings, there is an endeavor to design the rotating part of a high-speed turbo-generator so that the lowest critical speed is not exceeded during the operation of the high-speed turbo-generator, i.e. the rotat- ing part should be sub-critical instead of being supercritical.

Summary

The following presents a simplified summary in order to provide a basic understanding of some embodiments of the invention. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention.

In accordance with the invention, there is provided a new electrical turbo-machine that is suitable for being used, for example but not necessarily, as a high-speed turbo-compressor or as a high-speed turbo-generator of an Organic Rankine Cycle "ORC" energy converter. An electrical turbo-machine according to the invention comprises a stator, a rotor for magnetically interacting with the stator, and an impeller directly connected to an end of the rotor, wherein the impeller is suitable for operating as a turbine for rotating the rotor and wherein the stator comprises: - a stator core structure comprising a plurality of stator teeth and stator slots, and

- a stator winding comprising a plurality of stator coils.

The width of each stator coil is one stator slot pitch and each of the stator coils is arranged to surround only one of the stator teeth so as to minimize the length of the stator end-windings in the axial direction of the rotor.

The short end-windings enable the rotor to be shorter, and thus the natural frequencies of the rotor can be higher. As a corollary, the critical speeds that excite the natural frequencies of the rotor are higher too. Therefore, the upper limit of the rotation speed can be increased, which in turn increases the maximum power of the electrical turbo-machine.

In conjunction with the present invention, it has been surprisingly noticed that the stator structure where each stator coil is wound around a single stator tooth is suitable for a high-speed electrical machine where a turbine is directly connected to the rotor of the electrical machine even though the use of the above-mentioned stator structure in a high-speed electrical machine is against the traditional design principles according to which the air-gap harmonics of the magnetic flux distribution should be better and better minimized when increasing the rotational speed.

In accordance with the invention, there is provided also a new energy converter that is advantageously, but not necessarily, an Organic Rankine Cycle "ORC" energy converter. An energy converter according to the invention comprises:

- a boiler for vaporizing working fluid,

- an electrical turbo-machine for converting energy contained by the vaporized working fluid into electrical energy, - condenser for condensing the vaporized working fluid outputted by the electrical turbo-machine, and

- a feed pump system for pumping the condensed working fluid to the boiler. The electrical turbo-machine of the energy converter comprises a stator, a rotor for magnetically interacting with the stator, and an impeller at an end of the rotor, wherein the impeller is suitable for operating as a turbine for rotating the rotor and wherein the stator comprises: - a stator core structure comprising a plurality of stator teeth and stator slots, and

- a stator winding comprising a plurality of stator coils.

The width of each stator coil is one stator slot pitch and each of the stator coils is arranged to surround only one of the stator teeth so as to minimize the length of the stator end-windings in an axial direction of the rotor.

A number of non-limiting and exemplifying embodiments of the invention are described in accompanied dependent claims.

Various exemplifying embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying drawings.

The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless other- wise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.

Brief description of the figures

The exemplifying embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figure 1 a shows a section view of an electrical turbo-machine according to an exemplifying embodiment of the invention, figure 1 b shows a perspective view of a stator of the electrical turbo-machine illustrated in figure 1 a, and figure 2 shows a schematic illustration of an energy converter according to an exemplifying embodiment of the invention. Description of exemplifying embodiments

Figure 1 a shows a section view of an electrical turbo-machine 100 according to an exemplifying embodiment of the invention. The electrical turbo-machine comprises a stator 101 and a rotor 102 for magnetically interacting with the stator. The rotor 102 may comprise permanent magnets for producing a magnetic flux penetrating the air-gap between the rotor and the stator. In this case, the electrical turbo- machine is capable of operating as a permanent magnet synchronous generator "PMSG". It is also possible that the rotor 102 comprises electrically conductive structures so that the electrical turbo-machine is capable of operating as an asynchronous electrical machine. The rotor is rotatably carried with bearings 103 and 131 . In an exemplifying case where the electrical turbo-machine 100 is used as a turbo-generator of an Organic Rankine Cycle "ORC" energy converter, the bearings 103 and 131 are advantageously lubricated with the organic working fluid of the ORC process.

Figure 1 b shows a perspective view of the stator 101 . The stator 101 comprises a stator core structure 104 comprising a plurality of stator teeth and stator slots. In figures 1 a and 1 b, some of the stator teeth are denoted with reference numbers 1 1 1 , 1 12, and 1 13. The stator core structure 104 is preferably made of steel sheets that are electrically insulated from each other and that are stacked in the direction parallel with the axial direction of the rotor 102. The axial direction is paral- lei with the z-direction of a coordinate system 199. In the exemplifying case illustrated in figure 1 b, the stator core structure 104 is composed of segments 1 14, 1 15, 1 16, 1 17, 1 18, and 1 19 each of which comprises one of the stator teeth. The stator 101 comprises a stator winding that comprises a plurality of stator coils 105, 106, 107, 108, 109, and 1 10. The width of each stator coil is one stator slot pitch and each of the stator coils is arranged to surround only one of the stator teeth. As illustrated in figure 1 b, the end-windings of the stator coils do not crisscross with each other. Therefore, the length of the end-windings in the axial direction of the rotor is minimized. The length of the end-windings in the axial direction is denoted with L in figure 1 a. In order to facilitate the assembly of the stator 101 , the stator teeth can be shaped to allow each stator coil to be installed by pushing the stator coil to surround the stator teeth under consideration. Electrically series connected turns of the stator coils are electrically insulated from each other, and the required strength of the insulation depends on the voltage level being used.

The electrical turbo-machine 100 comprises a first impeller 103 at a first end of the rotor 102. The impeller 103 is suitable for operating as a radial turbine for rotating the rotor. The electrical turbo-machine 100 further comprises a diffuser 1 15 containing the impeller 103 as illustrated in figure 1 a. The diffuser 1 15 comprises an inlet 1 16 and an outlet 1 17 for gaseous fluid that is in energy-exchange relation with the impeller 103. The impeller 103 and the diffuser 1 15 are advantageously suitable for operating as a radial turbine stage whose degree of reaction is less than 50 % e.g. 30 %. Thus, the axial height of the impeller vanes can be increased and, as a corollary, the ratio of the axial clearance to the axial height of the impeller can be made smaller, and thus the efficiency can be improved. The degree of reaction or reaction ratio is defined as the ratio of the static pressure drop in the impeller to the static pressure drop in the whole turbine stage or as the ratio of the static enthalpy drop in the impeller to the static enthalpy drop in the whole turbine stage.

In the exemplifying case illustrated in figures 1 a and 1 b, the electrical turbo- machine 100 further comprises a second impeller 1 18 at a second end of the rotor 102. The impeller 1 18 is suitable for pumping liquid and, in an exemplifying case where the electrical turbo-machine 100 is used as a turbo-generator of an Organic Rankine Cycle "ORC" energy converter, the impeller 1 18 can be used as a feed pump of the ORC energy converter. The impeller 1 18 can be a straight vane radial impeller of a "Barske"-type partial emission pump. The impeller 1 18 can be provided with a screw-type inducer 1 19 for reducing the risk of cavitation on the vanes of the impeller 1 18, and thereby to reduce the required pre-supply pressure. The electrical turbo-machine 100 may further comprise a third impeller 150 for cooling the electrically active parts of the electrical turbo-machine. Figure 2 shows a schematic illustration of an energy converter according to an exemplifying embodiment of the invention. The energy converter is advantageously an Organic Rankine Cycle "ORC" energy converter that uses suitable organic fluid as the working fluid. The organic fluid can be, for example but not necessarily, toluene. It is also possible that the energy converter uses suitable non-organic fluid as the working fluid. The energy converter comprises a boiler 220 for vaporizing the working fluid. The boiler can be operated by e.g. waste heat received from a heat-producing process or a heat-producing machine, e.g. a combustion engine. The energy converter comprises an electrical turbo-machine 200 for converting the energy contained by the vaporized working fluid into electrical energy. The electrical energy is supplied to a power grid 242 with the aid of a frequency converter 241 . The electrical turbo-machine 200 comprises a generator section 225, a turbine section 226, and a pump section 222. The generator section 225 comprises a stator and a rotor for magnetically interacting with the stator. The stator comprises a stator core structure having a plurality of stator teeth and stator slots, and a stator winding having a plurality of stator coils. The width of each stator coil is one stator slot pitch and each of the stator coils is arranged to surround only one of the stator teeth so as to minimize the length of the stator end-windings in the axial direction of the rotor. The turbine section 226 comprises a diffuser and a first impel- ler suitable for operating as a turbine for rotating the rotor. The pump section 222 comprises a second impeller for pumping liquid. The first and second impellers are directly coupled to the rotor of the generator section.

The energy converter comprises a condenser 221 for condensing the vaporized working fluid outputted by the electrical turbo-machine 200, and a feed pump sys- tern for pumping the condensed working fluid to the boiler 220. In the exemplifying case illustrated in figure 2, the feed pump system comprises the pump section 222 arranged to operate as a feed pump and a pre-feed pump 223.

In the exemplifying case illustrated in figure 2, the energy converter comprises channels 224 for conducting the working fluid to the bearings of the electrical tur- bo-machine 200 so as to lubricate the bearings with the working fluid. Advantageously, the electrical turbo-machine 200 comprises a hermetic casing for preventing the working fluid from leaking to ambient air. Furthermore, the energy convert- er may comprise a recuperator 227 for increasing the efficiency of the energy conversion and a condenser tank 228.

The specific examples provided in the description given above should not be construed as limiting. Therefore, the invention is not limited merely to the embodi- ments described above.

Claims

What is claimed is:

1 . An electrical turbo-machine (100) comprising a stator (101 ), a rotor (102) for magnetically interacting with the stator, and a first impeller (103) directly connected to a first end of the rotor, wherein the first impeller is suitable for operating as a turbine for rotating the rotor and wherein the stator comprises:

- a stator core structure (104) comprising a plurality of stator teeth (1 1 1 -1 13) and stator slots, and

- a stator winding comprising a plurality of stator coils (105-1 10), characterized in that width of each stator coil is one stator slot pitch and each of the stator coils is arranged to surround only one of the stator teeth so as to minimize a length of stator end-windings in an axial direction of the rotor.

2. An electrical turbo-machine according to claim 1 , wherein the stator core structure is composed of segments (1 14-1 19) each comprising one and only one of the stator teeth.

3. An electrical turbo-machine according to claim 1 or 2, wherein the electrical turbo-machine further comprises a diffuser (1 15) containing the first impeller and having an inlet (1 16) and an outlet (1 17) for gaseous fluid being in energy- exchange relation with the first impeller.

4. An electrical turbo-machine according to claim any of claims 1 -3, wherein the electrical turbo-machine further comprises a second impeller (1 18) at a second end of the rotor, the second impeller being suitable for pumping liquid.

5. An electrical turbo-machine according to claim 4, wherein the second impeller is a straight vane radial impeller.

6. An electrical turbo-machine according to claim 4 or 5, wherein the second impeller is provided with a screw-type inducer (1 19) for reducing a risk of cavitation on vanes of the second impeller.

7. An electrical turbo-machine according to any of claims 1 -6, wherein the rotor (102) comprises permanent magnets for producing a magnetic flux penetrating an air-gap between the rotor and the stator.

8. An electrical turbo-machine according to any of claims 3-7, wherein the first impeller and the diffuser are suitable for operating as a radial turbine stage whose degree of reaction is less than 50 %.

9. An energy converter comprising:

- a boiler (220) for vaporizing working fluid,

- an electrical turbo-machine (200) according to any of claims 1 -8 and for converting energy contained by the vaporized working fluid into electrical energy,

- condenser (221 ) for condensing the vaporized working fluid outputted by the electrical turbo-machine, and

- a feed pump system (222, 223) for pumping the condensed working fluid to the boiler.

10. An energy converter according to claim 9, wherein the energy converter comprises channels (224) for conducting the working fluid to bearings of the electrical turbo-machine so as to lubricate the bearings of the electrical turbo-machine with the working fluid.

1 1 . An energy converter according to claim 10, wherein the electrical turbo- machine comprises a hermetic casing for preventing the working fluid from leaking to ambient air.

12. An energy converter according to any of claims 9-1 1 , wherein the working fluid is organic working fluid.