WO2015067848A1

WO2015067848A1 - An energy converter and a method for operating it

Info

Publication number: WO2015067848A1
Application number: PCT/FI2014/050819
Authority: WO
Inventors: Jussi HEINIMÖ; Juha Jaakko PYRHÖNEN; Pekka RÖYTTÄ; Hannu Helmer TERVONEN; Teemu TURUNEN-SAARESTI
Original assignee: Visorc Oy
Priority date: 2013-11-07
Filing date: 2014-10-31
Publication date: 2015-05-14
Also published as: FI20130325L

Abstract

An energy converter based on a thermodynamic energy conversion process comprises an electrical turbo-machine (101) for converting energy contained by vaporized working fluid into electrical energy, a condenser (104) for condensing the vaporized working fluid outputted by the electrical turbo-machine, a vaporizer (105) for receiving a main heat flow from an external heat source and for vaporizing the condensed working fluid, and a feed pump (106) for pumping the condensed working fluid to the vaporizer. The energy converter can be based on for example the Organic Rankine Cycle. The energy converter further comprises a controllable heating device (107) for generating a controllable auxiliary heat flow to the vaporizer so as to at least partly compensate for temporal variations in the main heat flow. Therefore, the energy converter is suitable for being used also with a varying main heat flow which is sometimes unable to maintain the thermodynamic energy conversion process.

Description

An energy converter and a method for operating it

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" process. Furthermore, the invention relates to a method for operating an energy converter based on a thermodynamic energy conversion process that can be, for example but not necessarily, the ORC process. 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 tech- nique 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 tur- bine 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-feeding 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. Energy converters of the kind described above are, however, not free from challenges. One of the challenges is related to the fact that the power of the heat flow received by the vaporizer may vary significantly over time. For example, the power of waste heat produced by a combustion engine of a vehicle or a working machine may be sporadically so low that the thermodynamic energy conversion process, e.g. the ORC process, of an energy converter of the kind described above cannot be maintained. Publication EP2415976 describes a system comprising a heat storage element for smoothing variations of the power of the heat flow received by the vaporizer. The required heat storage capacity depends on the magnitude of power fluctuations and especially on lengths of time periods during which the power of the heat flow is incapable of maintaining the thermodynamic energy conversion process. In some cases, it may be however challenging to provide so much heat storage capacity that is needed for ensuring reliable maintenance of the thermodynamic energy conversion process in all practical situations.

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:

- a vaporizer for receiving a main heat flow from an external heat source and for vaporizing working fluid,

- an electrical turbo-machine for converting energy contained by the vaporized working fluid into electrical energy, the electrical turbo-machine comprising a turbine section and a generator section, and

- a controllable heating device for generating a controllable auxiliary heat flow to the vaporizer so as to at least partly compensate for temporal variations of the power of the main heat flow.

The above-described energy converter is suitable for being used also with such external heat sources, e.g. combustion engines of vehicles and working machines, whose output heat flow is so fluctuating that the heat flow may sometimes be una- ble to maintain the thermodynamic energy conversion process, e.g. the ORC process, of the energy converter. The controllable heating device can be for example a burner or an electrical resistor arranged to heat the vaporizer when the power of the heat flow received from the external heat source is too low for maintaining the thermodynamic energy conversion process of the energy converter. With the aid of the controllable heating device, repetitive shutdowns and start-ups of the energy converter can be avoided.

In accordance with the invention, there is provided also a new method for operating an energy converter based on a thermodynamic energy conversion process that can be, for example but not necessarily, the ORC process. A method accord- ing to the invention comprises:

- generating, with a controllable heating device, a controllable auxiliary heat flow to a vaporizer of the energy converter so as to at least partly compensate for temporal variations of the power of a main heat flow received by the vaporizer from an external heat source, and - heating the vaporizer with the controllable heating device in response to a situation in which the power of the main heat flow received from the external heat source is too low for maintaining the thermodynamic energy conversion process of the energy converter. 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 embod- iments 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: figure 1 shows a schematic block diagram of an energy converter according to an exemplifying embodiment of the invention, and figure 2 illustrates a method according to an exemplifying embodiment of the invention for operating an energy converter based on a thermodynamic energy con- version process.

Description of exemplifying embodiments

Figure 1 shows a schematic block diagram of an energy converter according to an exemplifying embodiment of the invention. The energy converter is advantageous- ly 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 a vaporizer 105 for receiving a main heat flow 1 16 from an external heat source and for vaporizing the working fluid. The external heat source can be for example a heat- producing process or a heat-producing machine, e.g. a combustion engine. The external heat source is not shown in figure 1 . The energy converter comprises an electrical turbo-machine 101 for converting energy contained by the vaporized working fluid into electrical energy. In the exemplifying case illustrated in figure 1 , the electrical energy outputted by the electrical turbo-machine 101 is supplied to a power grid 150 with the aid of a frequency converter 1 15. The exemplifying energy converter illustrated in figure 1 comprises a condenser 104 for condensing the vaporized working fluid outputted by the electrical turbo-machine 101 and a conden- ser tank 1 17 for storing the condensed working fluid. The energy converter comprises a feed pump system for pumping the condensed working fluid from the condenser tank 1 17 to the vaporizer 105. In the exemplifying energy converter illustrated in figure 1 , the feed pump system comprises a feed pump 106, a pre-feed pump 1 18, and an ejector-pump 1 19 for supplying the pre-feed pump and operat- ed e.g. by the output flow of the feed pump 106. An energy converter according to another exemplifying embodiment of the invention comprises connection interfaces for connecting to an external feed pump system, and/or to an external condenser element, and/or to an external condenser tank.

The electrical turbo-machine 101 is advantageously a high-speed machine whose rotational speed can be as high as e.g. 10000...60000 rpm. The electrical turbo- machine 101 comprises a generator section 103 and a turbine section102. The feed pump 106 is integrated with the electrical turbo-machine 101 . The generator section 103 comprises a stator and a rotor for magnetically interacting with the sta- tor. The rotor of the generator section may comprise permanent magnets for pro- ducing 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 comprises electri- cally conductive structures so that the generator section is capable of operating as an asynchronous generator. The stator of the generator section 103 comprises a stator core structure comprising a plurality of stator teeth and stator slots. The stator core structure 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 of the generator section 103. The stator further comprises a stator winding that comprises a plurality of stator coils. The turbine section 102 comprises a diffuser, a stator nozzle ring, and an impeller suitable for operating as a turbine for rotating the rotor of the generator section. The stator nozzle ring, the im- peller, and the diffuser of the turbine section 102 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 enthalpy drop in the impeller to the static enthalpy drop in the whole turbine stage. The feed pump 106 comprises an impeller for pumping the working fluid. In the exemplifying energy converter illustrated in figure 1 , both the impeller of the turbine section 102 and the impeller of the feed pump 106 are directly coupled to the rotor of the generator section 103. The impeller of the feed pump 106 can be a straight vane radial impeller of a "Barske"-type partial emission pump. The impeller of the feed pump 106 can be provided with a screw-type inducer for reducing the risk of cavitation on the vanes of the impeller of the feed pump 106, and thereby to reduce the required pre- supply pressure. The energy converter comprises a controllable heating device 107 for generating a controllable auxiliary heat flow 108 to the vaporizer 105 so as to at least partly compensate for temporal variations of the power of the main heat flow 1 16. The controllable heating device 107 can be for example a burner adapted to use gaseous or liquid form fuel. For another example, the controllable heating device 107 can be an electrical resistor. The controllable heating device 107 can be arranged to heat the vaporizer 105 when the power of the main heat flow 1 16 received from the external heat source is too low for maintaining the thermodynamic energy con- version process of the energy converter. A situation where the main heat flow is incapable of maintaining the thermodynamic energy conversion process can be detected at least partly on the basis of one or more temperatures measured from one or more measurement points in or in the vicinity of the vaporizer 105. With the aid of the controllable heating device 107, repetitive shutdowns and start-ups of the energy converter can be avoided. The energy converter may further comprise a control system 109 configured to control the controllable heating device 107 at least partly on the basis of the one or more temperatures measured from the one or more measurement points in or in the vicinity of the vaporizer 105. The control can be for example a two-point control where the controllable heating device 107 is activated when a measured temperature goes below a first predetermined limit and deactivated when the measured temperature exceeds a second predetermined limit that is higher than the first predetermined limit. It is also possible that the control system 109 is configured to control the controllable heating device 107 at least partly on the basis of a control signal 1 10 received from the external heat source producing the main heat flow 1 16. It is also possible that the control system 109 is configured to control the controllable heating device 107 at least partly on the basis of both the control signal 1 10 and the one or more measured temperatures. The control signal 1 10 may indicate for example the loading state of the ex- ternal heat source, e.g. a position of a throttle valve of a combustion engine. In this case, the controllable heating device 107 can be controlled more proactively than in cases where the control is based merely on the one or more measured temperatures which may react with delays to the changes of the power of the main heat flow 1 16. In the exemplifying case illustrated in figure 1 , the energy converter comprises ducts 1 13 for conducting the working fluid to the bearings of the electrical turbo- machine 101 so as to lubricate the bearings with the working fluid. The electrical turbo-machine 101 and the feed-pump 106 are advantageously encapsulated with a hermetic casing for preventing the working fluid from leaking to ambient air and for preventing the ambient air from leaking to the energy conversion process run by the energy converter. The exemplifying energy converter illustrated in figure 1 comprises a recuperator 1 14 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 work- ing fluid outputted by the feed pump 106 and being supplied to the vaporizer 105.

The exemplifying energy converter illustrated in figure 1 further comprises first cooling ducts 1 1 1 for conducting cooling fluid, e.g. water, to and from the electrical turbo-machine 101 and second cooling ducts 1 12 for conducting cooling fluid to and from the condenser 104. As illustrated in figure 1 , 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 120a, 120b. However, this is only an example of cooling arrangement of the electrical turbo-machine 101 . It may be cooled also otherwise. Furthermore, the exemplifying energy converter illustrated in figure 1 comprises a turbine valve 121 and possibly other control and/or safety instrumentation.

Figure 2 illustrates a method according to an exemplifying embodiment of the invention for operating an energy converter based on a thermodynamic energy conversion process, e.g. the ORC process. The method comprises generating 201 , with a controllable heating device, a controllable auxiliary heat flow to a vaporizer of the energy converter so as to at least partly compensate for temporal variations of the power of a main heat flow received by the vaporizer from an external heat source.

A method according to an exemplifying embodiment of the invention comprises generating the controllable auxiliary heat flow in response to a situation in which the power of the main heat flow received from the external heat source is too low for maintaining the thermodynamic energy conversion process of the energy converter.

A method according to an exemplifying embodiment of the invention comprises controlling the controllable heating device at least partly on the basis of one or more temperatures measured from one or more measurement points in or in the vicinity of the vaporizer.

A method according to an exemplifying embodiment of the invention comprises controlling the controllable heating device at least partly on the basis of a control signal received from the external heat source.

A method according to an exemplifying embodiment of the invention comprises controlling the controllable heating device at least partly on the basis of both a control signal received from the external heat source and one or more temperatures measured from one or more measurement points in or in the vicinity of the vaporizer.

In a method according to an exemplifying embodiment of the invention, the controllable heating device comprises a burner for heating the vaporizer.

In a method according to an exemplifying embodiment of the invention, the controllable heating device comprises an electrical resistor for heating the vaporizer. 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.

Claims

What is claimed is:

1 . An energy converter comprising:

- a vaporizer (105) for receiving a main heat flow from an external heat source and for vaporizing working fluid, - an electrical turbo-machine (101 ) for converting energy contained by the vaporized working fluid into electrical energy, the electrical turbo-machine comprising a turbine section (102) and a generator section (103), and

- a controllable heating device (107) for generating a controllable auxiliary heat flow to the vaporizer so as to at least partly compensate for temporal variations of power of the main heat flow, characterized in that the controllable heating device is arranged to heat the vaporizer in response to a situation in which the power of the main heat flow received from the external heat source is too low for maintaining a thermodynamic energy conversion process of the energy converter.

2. An energy converter according to claim 1 , wherein the energy converter comprises a control system (109) configured to control the controllable heating device at least partly on the basis of one or more temperatures measured from one or more measurement points in or in the vicinity of the vaporizer.

3. An energy converter according to claim 1 , wherein the energy converter comprises a control system (109) configured to control the controllable heating device at least partly on the basis of a control signal (1 10) received from the external heat source.

4. An energy converter according to any of claims 1 -3, wherein the controllable heating device (107) comprises a burner for heating the vaporizer.

5. An energy converter according to any of claims 1 -4, wherein the controllable heating device (107) comprises an electrical resistor for heating the vaporizer.

6. An energy converter according to any of claims 1 -5, wherein the energy converter comprises:

- first cooling ducts (1 1 1 ) for conducting a first flow of cooling fluid to and from the generator section so as to cool at least a stator of the generator section, and

- second cooling ducts (1 12) for conducting a second flow of the cooling fluid to and from the condenser, the first and second cooling ducts constituting mutually parallel flowing paths for the cooling fluid so that both the first and second cooling ducts start at a first point where a total flow of the cooling fluid is divided into the first and second flows and end at a second point where the first and second flows join together.

7. An energy converter according to any of claims 1 -6, wherein the energy converter comprises ducts (1 13) 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.

8. An energy converter according to any of claims 1 -7, wherein the electrical turbo-machine comprises a hermetic casing for preventing the working fluid from leaking to ambient air and for preventing the ambient air from leaking to an energy conversion process run by the energy converter.

9. An energy converter according to any of claims 1 -8, wherein the energy converter further comprises a recuperator (1 14) for transferring heat energy from the vaporized working fluid outputted by the electrical turbo-machine to the condensed working fluid outputted by the feed pump.

10. An energy converter according to any of claims 1 -9, wherein the energy con- verter further comprises a frequency converter (1 15) for supplying the electrical energy outputted by the electrical turbo-machine to an external electrical system.

1 1 . An energy converter according to any of claims 1 -10, wherein the working fluid is organic working fluid.

12. A method for operating an energy converter based on a thermodynamic energy conversion process, the method comprising:

- generating (201 ), with a controllable heating device, a controllable auxiliary heat flow to a vaporizer of the energy converter so as to at least partly compensate for temporal variations of power of a main heat flow received by the vaporizer from an external heat source, characterized in that the method comprises heating the vaporizer with the controllable heating device in response to a situation in which the power of the main heat flow received from the external heat source is too low for maintaining the thermo- dynamic energy conversion process of the energy converter.