FIELD OF THE INVENTION
The invention relates generally to energy converters for converting thermal energy into electricity. More particularly, the invention relates to an energy converter that can be based on, for example but not necessarily, the Organic Rankine Cycle “ORC”.
BACKGROUND
Small-size energy converters, which can be based on for example 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 or a gas turbine, 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 working 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 producing 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. Also, the high speed technology makes it possible to provide a hermetic process, which means significant savings in the operational expenses.
Publication EP0090022 describes an energy converter that comprises a vaporizer, i.e. 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-feed pump. The thermal energy supplied to the vaporizer 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 feed pump connected to the rotor of the generator requires pre-supply pressure in order to avoid cavitation on the vanes of the impeller of the feed pump. Therefore, an energy converter of the kind described above is, in many cases, provided with a pre-supply pump for generating the required pre-supply pressure for the feed pump. However, also the pre-feed pump may require pre-supply pressure in order to avoid cavitation on the vanes of its impeller. In order to produce sufficient pre-supply pressure for the pre-feed pump, the pre-feed pump can be placed at a lower level than the condenser tank so as to have a sufficient hydrostatic pressure ρ×g×h at the suction side of the pre-feed pump, where ρ is the density of the working fluid in condensed form, g is the acceleration of the gravity, and h is the height difference between the pre-feed pump and the liquid surface in the condenser tank. An inherent advantage of this arrangement where the pre-feed pump is placed at a lower level than the condenser tank is that the pre-supply pressure, i.e. the hydrostatic pressure, is generated for the pre-feed pump irrespective of the operation of the energy converter. Therefore, there is sufficient pre-supply pressure for the pre-feed pump during also a starting phase and other situations in which the turbo-generator and thereby the feed pump are not fully operating.
Energy converters of the kind described above are, however, not free from challenges. One of the challenges is related to the mechanical dimensions of the energy converters which, in order to be suitable for different operation sites, should be as small as possible. Especially the total height of an energy converter installation can be a critical dimension in conjunction with many energy converters in which the pre-feed pump is placed at a lower level than the condenser tank.
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 energy converter that can be based on, for example but not necessarily, the Organic Rankine Cycle “ORC” process. An energy converter according to the invention comprises:
- an electrical turbo-machine for converting energy contained by vaporized working fluid into electrical energy, the electrical turbo-machine comprising a turbine section and a generator section,
- a feed pump for pumping the working fluid in a condensed form to a vaporizer, the feed pump being arranged to be operated by the electrical turbo-machine,
- a pre-feed pump for pumping the working fluid in the condensed form to the suction side of the feed pump, the pre-feed pump being arranged to be operated by a motor capable of running independently of operation of the electrical turbo-machine, and
- an ejector-pump for pumping the working fluid in the condensed form to the suction side of the pre-feed pump, the ejector-pump being arranged to be operated by a part of the output flow of the pre-feed pump.
The ejector-pump facilitates generating sufficient pre-supply pressure for the pre-feed pump. Thus, the pre-feed pump can be located, with respect to the condenser tank, higher than in cases where the pre-supply pressure of the pre-feed pump is based merely on the hydrostatic pressure. Therefore, the total height of an energy converter installation can be lower than in the cases where the pre-supply pressure of the pre-feed pump is based merely on the hydrostatic pressure. As the pre-feed pump is operated by the motor capable of running independently of the operation of the electrical turbo-machine and the ejector-pump is operated by a part of the output flow of the pre-feed pump, the required pre-supply pressure can be generated for the pre-feed pump during also a starting phase and other situations in which the electrical turbo-machine and thereby the feed pump are not fully operating. Also, it should be noted that an ejector pump is very simply without any moving parts, and very inexpensive to manufacture.
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 dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does as such 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:
FIG. 1a shows a schematic block diagram of an energy converter according to an exemplifying embodiment of the invention, and
FIG. 1b shows a section view of an electrical turbo-machine of the energy converter illustrated in FIG. 1a.
DESCRIPTION OF EXEMPLIFYING EMBODIMENTS
FIG. 1a shows a schematic block diagram 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, one of the siloxanes. It is also possible that the energy converter uses suitable non-organic fluid as the working fluid.
The energy converter comprises an electrical turbo-machine 101 for converting energy contained by vaporized working fluid into electrical energy. The electrical turbo-machine 101 is advantageously a high-speed machine whose rotational speed can be as high as e.g. 10000 . . . 60000 rpm. FIG. 1b shows a section view of the electrical turbo-machine 101. The section plane is parallel with the yz-plane of a coordinate system 199 shown in FIG. 1b. The electrical energy outputted by the electrical turbo-machine 101 is supplied to a power grid 119 with the aid of a frequency converter 118. A vaporizer, i.e. a boiler, 115 which vaporizes the working fluid can be operated by e.g. waste heat 120 received from a heat-producing process or a heat-producing machine, e.g. a combustion engine. In the exemplifying case illustrated in FIGS. 1a and 1b, the energy converter comprises a piping interface 116a, 116b for connecting to the vaporizer 115 which is not an integral part of the energy converter but an external element. In an energy converter according to another exemplifying embodiment of the invention, the vaporizer is an integral part of the energy converter.
The exemplifying energy converter illustrated in FIGS. 1a and 1b comprises a condenser 104 for condensing the vaporized working fluid outputted by the electrical turbo-machine 101. An energy converter according to another exemplifying embodiment of the invention comprises a piping interface for connecting to an external condenser element that is not an integral part of the energy converter. The energy converter comprises a feed pump 105 for pumping the condensed working fluid to the vaporizer 115. The feed pump 105 is operated by the electrical turbo-machine 101. The energy converter comprises pre-feed pump 106 for pumping the condensed working fluid to the suction side of the feed pump 105. The pre-feed pump 106 is operated by a motor 107 that is capable of running independently of the operation of the electrical turbo-machine 101. The motor 107 can be for example an electrical motor. The electrical motor can be such that the air-gap between the rotor and the stator of the motor is filled with the working fluid, and also bearings are exposed to the working fluid. In this case, the motor 107 and the pre-supply pump 106 can be arranged to form a hermetic entity without a need for seals between the parts constituting the motor 107 and the parts constituting the pre-supply pump 106. The energy converter comprises an ejector-pump 108 for pumping the condensed working fluid to the suction side of the pre-feed pump 106. The ejector-pump 108 is operated by a part of the output flow of the pre-feed pump 106. The ejector-pump 108 facilitates generating the required pre-supply pressure for the pre-feed pump 106. Thus, the pre-feed pump can be located, with respect to a condenser tank 129, higher than in cases where the pre-supply pressure of the pre-feed pump is based merely on the hydrostatic pressure. Therefore, the height of the energy converter can be smaller. As the pre-feed pump 106 is operated by the motor 107 capable of running independently of the operation of the electrical turbo-machine 101 and the ejector-pump 108 is operated by a part of the output flow of the pre-feed pump 106, the required pre-supply pressure can be generated for the pre-feed pump 106 during also a starting phase and other situations in which the electrical turbo-machine 101 and thereby the feed pump 105 are not fully operating. The pre-feed pump 106 can be started by increasing gradually the rotational speed. Then the pressure produced by the ejector-pump 108 increases in same phase as the feeding pressure required by the pre-feed pump, and danger of cavitation during pre-feed pump start is avoided. The energy converter may comprise a controller 133 that is arranged to start the pre-feed pump so that the rotational speed of the pre-feed pump is increased gradually according to a pre-determined speed profile. The controller 133 can be implemented for example with the controller elements of the frequency converter 118 or the controller 133 can be a separate element. The exemplifying energy converter illustrated in FIGS. 1a and 1b comprises the condenser tank 129. An energy converter according to another exemplifying embodiment of the invention comprises a piping interface for connecting to an external condenser tank that is not an integral part of the energy converter.
The electrical turbo-machine 101 comprises a generator section 103 and a turbine section 102. The feed pump 105 for pumping the working fluid to the vaporizer 115 is integrated with the electrical turbo-machine 101. As illustrated in FIG. 1b, the generator section comprises a stator 121 and a rotor 110 for magnetically interacting with the stator. The stator comprises a stator core structure 122 having a plurality of stator teeth and stator slots, and a stator winding 123 having a plurality of stator coils. The stator core structure 122 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 110. The axial direction is parallel with the z-direction of the coordinate system 199. The rotor 110 of the generator section may comprise permanent magnets for producing a magnetic flux penetrating the air-gap between the rotor and the stator. In this case, the generator section is capable of operating as a permanent magnet synchronous generator “PMSG”. It is also possible that the rotor 110 comprises electrically conductive structures so that the generator section is capable of operating as an asynchronous generator. The rotor 110 is rotatably carried by bearings 127 and 128. In the exemplifying energy converter illustrated in FIGS. 1a and 1b, the bearings 125 and 126 are lubricated with the working fluid of the thermodynamic energy conversion process. As illustrated in FIG. 1a, the energy converter comprises ducts 114 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. The ducts 114 can be connected to the delivery side of the pre-feed pump 106 so that the bearings can be lubricated also in situations where the feed pump 105 is not operating. In some cases it is however also possible that the ducts 114 are connected to the delivery side of the feed pump 105.
As illustrated in FIG. 1b, the turbine section of the electrical turbo-machine 101 comprises a diffuser 124, a stator nozzle ring 125, and an impeller 126 suitable for operating as a turbine for rotating the rotor 110 of the generator section. The feed pump comprises an impeller 109 for pumping the working fluid. In the exemplifying energy converter illustrated in FIGS. 1a and 1b, both of the impellers 126 and 109 are directly coupled to the rotor 110 of the generator section. The stator nozzle ring 125, the impeller 126, and the diffuser 124 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 vanes of the impeller 126 can be increased and, as a corollary, the ratio of the axial clearance to the axial height of the impeller vanes 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 enthalpy drop in the impeller 126 to the static enthalpy drop in the whole turbine stage. The impeller 109 of the feed pump can be, for example, a straight vane radial impeller of a “Barske”-type partial emission pump. The impeller of the feed pump can be provided with a screw-type inducer 111 for reducing the risk of cavitation on the vanes of the impeller 109, and thereby to reduce the required pre-supply pressure of the feed pump. The electrical turbo-machine 101 may further comprise an impeller 130 for generating cooling flow inside the frame of the generator section.
The exemplifying energy converter illustrated in FIGS. 1a and 1b comprises a recuperator 117 for increasing the efficiency of the energy conversion. The recuperator is a heat exchanging element arranged to transfer heat energy from the vaporized working fluid outputted by the electrical turbo-machine 101 to the condensed working fluid outputted by the feed pump 105 and being supplied to the vaporizer 115.
The exemplifying energy converter illustrated in FIGS. 1a and 1b further comprises first cooling ducts 112 for conducting cooling fluid, e.g. water, to and from the electrical turbo-machine 101 so as to cool the generator section, and second cooling ducts 113 for conducting cooling fluid to and from the condenser 104. As illustrated in FIG. 1a, the first and second cooling ducts constitute mutually parallel flowing paths for the cooling fluid. The first and second cooling ducts can be connected to an external cooling fluid circulation system with the aid of a piping interface 131a, 131b. However, this is only an example of a cooling arrangement of the electrical turbo-machine 101. It may be cooled also otherwise.
Furthermore, the exemplifying energy converter illustrated in FIGS. 1a and 1b comprises a turbine valve 132 and possibly other control and/or safety instrumentation.
The specific examples provided in the description given above should not be construed as limiting. Therefore, the invention is not limited merely to the embodiments described above.
Patent Application
20160298499
