ORC BINARY CYCLE GEOTHERMAL PLANT AND PROCESS

Abstract

An ORC binary cycle geothermal plant, including at least one ORC closed-cycle system and a geothermal system. The geothermal system includes at least one intake line of a geothermal fluid connected to at least one geothermal production well, wherein the fluid includes non-condensable gases; one interface line connected to the intake line, coupled to the ORC system in an interface zone, wherein the fluid exchanges heat with the organic working fluid; one reinjection line connected to the interface line and to at least one geothermal reinjection well. Further at least one separator device configured to separate at least the gases from the fluid; one expander connected to an outlet of the gases by the separator device; and one auxiliary generator connected to the expander. The expander is for interfacing with the system to receive and expand at least the gases after they have exchanged heat with the organic working fluid.

Claims

1. An ORC binary cycle geothermal plant, comprising: at least one ORC closed-cycle system comprising at least: one vaporizer; one expansion turbine; one generator operatively connected to the expansion turbine; one condenser; one pump; ducts configured to connect the vaporizer, the expansion turbine, the condenser and the pump according to a closed cycle in which an organic working fluid (OWF) circulates; a geothermal system comprising at least: one intake line for a geothermal fluid (GF) connected to at least one geothermal production well, wherein the geothermal fluid (GF) comprises non-condensable gases (NCGs); one interface line connected to the intake line and operatively coupled to the at least one ORC closed-cycle system in an interface zone, wherein the geothermal fluid (GF) exchanges heat with the organic working fluid (OWF) of said ORC closed-cycle system; one outlet line connected to the interface line; wherein the geothermal system further comprises: at least one separator device configured to separate at least the non-condensable gases (NCGs) from the geothermal fluid (GF); an expander operatively connected to an outlet of the non-condensable gases (NCGs) by the separator device; an auxiliary generator operatively connected to the expander; wherein the expander is located downstream of the interface zone for interfacing with the ORC closed-cycle system so as to receive and expand at least the non-condensable gases (NCGs) after they have exchanged heat with the organic working fluid (OWF).

2. The plant according to claim 1, wherein the at least one separator device is also located downstream of the interface zone.

3. The plant according to claim 1, comprising a high pressure ORC closed-cycle system and a low pressure ORC closed-cycle system positioned operatively downstream of the high pressure ORC closed-cycle system.

4. The plant according to claim 3, wherein an interface zone of the low pressure ORC closed-cycle system receives the geothermal fluid (GF) after the geothermal fluid (GF) has exchanged heat in the interface zone of the high pressure ORC closed-cycle system.

5. The plant according to claim 4, wherein the expander is located downstream of the interface zone of the low pressure ORC closed-cycle system and/or of the interface zone of the high pressure ORC closed-cycle system.

6. The plant according to claim 5, wherein the at least one separator device is operatively located downstream of the interface zone of the low pressure ORC closed-cycle system and/or of the interface zone of the high pressure ORC closed-cycle system.

7. The plant according to claim 1, wherein an inlet pressure (P.sub.in) of the expander is comprised between about 2 bar and about 16 bar.

8. The plant according to claim 1, wherein a discharge pressure (P.sub.out) of the expander is comprised between about 0.8 bar and about 1.3 bar.

9. The plant according to claim 1, wherein an enthalpy change (H) through the expander is comprised between about 80 kJ/kg-K and about 200 kJ/kg-K.

10. The plant according to claim 1, wherein a percentage of water (H.sub.2O %) in the expander is comprised between about 2% and about 25% of the mass flow (MF).

11. The plant according to claim 1, wherein the expander is a multi-stage counter-rotating centrifugal radial turbine.

12. An ORC binary cycle geothermal process, comprising: circulating an organic working fluid (OWF) in an organic Rankine cycle (ORC), wherein the organic working fluid (OWF) is heated and vaporized, expanded in a turbine connected to a generator, condensed and again heated and vaporized; extracting a geothermal fluid (GF) comprising non-condensable gases (NCGs) from a geothermal production well; operatively coupling the geothermal fluid (GF) to the organic working fluid (OWF) of the organic Rankine cycle (ORC) in order to exchange heat with the organic working fluid (OWF) and heating and vaporizing the organic working fluid (OWF); discharging the geothermal fluid (GF); wherein the process further comprises: separating at least the non-condensable gases (NCGs) from the geothermal fluid (GF), and expanding the non-condensable gases (NCG) in an expander connected to an auxiliary generator; wherein the expansion of the non-condensable gases (NCGs) in the expander is carried out after the non-condensable gases (NCGs) have exchanged heat with the organic working fluid (OWF).

Description

DESCRIPTION OF THE DRAWINGS

[0078] This description will be given below with reference to the attached drawings, provided solely for illustrative and therefore non-limiting purposes, in which:

[0079] FIG. 1 illustrates a binary cycle geothermal plant in accordance with the present invention;

[0080] FIG. 2 illustrates the plant of FIG. 1, which is also representative of other plants according to the present invention, with a schematically illustrated portion thereof;

[0081] FIG. 3 illustrates a variant embodiment of the plant of FIGS. 1 and 2;

[0082] FIG. 4 illustrates a further variant embodiment of the plant of FIG. 2;

[0083] FIG. 5 illustrates a further variant embodiment of the plant of FIG. 2;

[0084] FIG. 6 illustrates a sectional view of an expander usable in the plants of the preceding figures;

[0085] FIG. 7 schematically represents an element of the expander of FIG. 6;

[0086] FIG. 8 is a schematic representation of an expander usable in the plants of the preceding figures associated with the element of FIG. 7;

[0087] FIG. 9 schematically represents a variant of the element of FIG. 7;

[0088] FIG. 10 is a schematic representation of an expander usable in the plants of the preceding figures associated with the element of FIG. 9;

[0089] FIG. 11 illustrates an enlarged detail of the expanders of FIGS. 8 and 10;

[0090] FIGS. 12 and 13 schematically represent a further variant of the element of FIG. 7 in respective operating configurations;

[0091] FIG. 14 is a schematic representation of an expander usable in the plants of the preceding figure associated with the element of FIGS. 11 and 12;

[0092] FIGS. 15 and 16 are likewise schematic representations of variants of the expander of FIG. 14.

DETAILED DESCRIPTION

[0093] With reference to the aforesaid figures, the reference number 1 denotes in its entirety an ORC binary cycle geothermal plant. With particular reference to FIG. 1, the plant 1 comprises an ORC closed-cycle system (Organic Rankine Cycle) 2 and a geothermal system 3.

[0094] The ORC closed-cycle system 2 comprises: a vaporizer 4, an expansion turbine 5, a generator 6 operatively connected to the expansion turbine 5, a condenser 7, a pump 8, and a preheater 9. Ducts 100 connect the vaporizer 4, the expansion turbine 5, the condenser 7, the pump 8 and the preheater 9 according to a closed cycle. A high molecular weight organic working fluid OWF is circulated in the closed cycle. The organic working fluid OWF is preheated, heated and vaporized in the preheater 9 and in the vaporizer 4. The organic working fluid OWF in the vapour state exiting the vaporizer 4 enters the expansion turbine 5, where it expands, causing the rotation of the rotor(s) of the expansion turbine 5 and of the generator 6, which thus generates electricity. The expanded organic working fluid OWF subsequently enters the condenser 7, where it is brought back to the liquid phase and from here pumped by the pump 8 back into the preheater 9.

[0095] The heating and vaporization of the organic working fluid OWF take place by virtue of a heat exchange with a geothermal fluid GF coming from the geothermal system 3.

[0096] The geothermal system 3 comprises an intake line 10 for the geothermal fluid GF connected to a geothermal production well 11, an interface line 12 connected to the intake line 10 and operatively coupled to the ORC closed-cycle system 2 in an interface zone 13 and an outlet line consisting of a reinjection line 14 connected to the interface line 12 and to at least one geothermal reinjection well 15. In the embodiment in FIG. 1, the interface zone 13 comprises the vaporizer 4 and the preheater 9. More in general, in the present description and in the appended claims, the term interface zone 13 means the set of devices (e.g. vaporizers, preheaters) in which the geothermal fluid GF and the organic working fluid OWF exchange heat. The ORC closed-cycle system 2 and interface zone 13 are schematically illustrated in FIG. 2.

[0097] The geothermal fluid GF comprises geothermal brine GB and a geothermal mixture GM comprising geothermal vapour GV (water steam) and non-condensable gases NCGs. Typically, the non-condensable gases NCGs are almost totally made up of carbon dioxide CO.sub.2 (e.g. 70%-98%) and hydrogen sulphide H.sub.2S (e.g. 0.6%-24%), and to a small extent of other gases (e.g. nitrogen N.sub.2, hydrogen H.sub.2, methane CH.sub.4).

[0098] Downstream of the interface zone 13, relative to the flow of the geothermal fluid GF, the geothermal system 2 represented in FIGS. 1 and 2 comprises a separator device 16 configured to separate the geothermal vapour GV and the non-condensable gases NCGs from the geothermal fluid GF. The separator device 16 is, for example, a flash separator or surface-type heat exchanger, known per se. The flash separator consists of a tank into which the liquid supply (geothermal fluid GF) is introduced through an expansion device. The tank has a first outlet 17 at the top for the geothermal mixture GM comprising the geothermal vapour GV and the non-condensable gases NCGs, which is freed of entrained liquid by means of a demister (drop separator), and a second outlet 18 at the bottom for the geothermal brine GB, collected at the bottom of the tank.

[0099] The separator device 16 is located in the reinjection line 14, which is thus made up of a first section 14a, which connects the interface zone 13 to the separator device 16, and a second section 14b which connects the second outlet 18 to the reinjection well 15 in order to reinject the geothermal brine GB into said well 15.

[0100] The geothermal plant 1 further comprises an expander 19, operatively connected to the first outlet 17 of the geothermal mixture GM (comprising the non-condensable gases NCGs and the steam GV) by the separator device 16, and an auxiliary generator 20 operatively connected to the expander 19. The expander 19 is located downstream of the interface zone 13 where it interfaces with the ORC closed-cycle system 2 so as to receive and expand the non-condensable gases NCGs and the geothermal vapour GV on exiting the separator device 16, i.e. after the geothermal mixture GM has already exchanged heat with the organic working fluid OWF of the ORC cycle.

[0101] The expander 19 is connected to the first outlet 17 of the separator device 16 through one or more inlet conduits 21.

[0102] The expander 19 is, for example, a centrifugal radial (outflow) turbine, for example, of the counter-rotating type, such as the one illustrated in FIG. 6. In unillustrated variant embodiments, the expander 19 can be another type of turbine (single-rotating centrifugal radial, centripetal radial, axial, etc.).

[0103] The counter-rotating centrifugal radial turbine 19 of FIG. 6 comprises a first supporting disk 22 having a first face bearing a plurality of first radial rotor stages 23a, 23b, each made up of a series of blades disposed in succession along a respective circular path and with a first orientation. A first rotation shaft 24 is integral with the first disk 22. A second supporting disk 25 has a second face bearing a plurality of second radial rotor stages 26a, 26b, each made up of a series of blades disposed in succession along a respective circular path and with a second orientation, opposite the first. A second rotation shaft 27 is integral with the second disk 25. The first disk 22 is facing the second disk 25 so as to delimit an expansion volume and the blades of the first disk 22 are radially alternated with the blades of the second disk 25.

[0104] The first and second rotation shafts 24, 27 are connected to a single auxiliary generator 20 or else each to a respective auxiliary generator 20.

[0105] Each of the disks 22, 25 has admission channels 28, 29 located in a radially internal position relative to the series of blades of the radial rotor stages 23a, 23b, 26a, 26b. The admission channels 28, 29 are connected to the first outlet 17 of the separator device 16 by means of the inlet conduits 21. The first and the second disk 22, 25 are free to rotate together with the respective shafts 24, 27 about a common rotation axis X-X and rotate in opposite directions under the action of the geothermal mixture GM entering through the admission channels 28, 29.

[0106] The first and second supporting disks 22, 25 are housed in a fixed casing 30. The first and second shafts 24, 27 are rotatably supported in the casing 30 by means of bearings 31.

[0107] The counter-rotating centrifugal radial turbine 19 further comprises a sealing device 32 (schematically illustrated in FIG. 6) operatively disposed about each of the rotation shafts 24, 27 at the respective supporting disk 22, 25. Every sealing device 32 is configured to prevent the leakage of the non-condensable gases NCGs or of the geothermal vapour GV with non-condensable gases NCGs towards said shaft 24, 27, i.e. in the passage delimited between the shaft 24, 27 and a sleeve 33 that accommodates it.

[0108] The structure of the sealing device 32 can be seen in FIG. 7. In this embodiment, the sealing device 32 comprises: three sealing elements 34a, 34b, 34c delimiting two annular chambers 35, 36 disposed about the rotation shaft 24, 27.

[0109] A first sealing element 34a is adjacent to the internal volume of the centrifugal radial turbine 19 occupied by the gases. A third sealing element 34c is adjacent to an environment in communication with the outside, i.e. at atmospheric pressure. A second sealing element 34b separates the two chambers 35, 36. A first annular chamber 35 is delimited by the first and second sealing elements 34a, 34b. A second annular chamber 36 is delimited by the second and third sealing elements 34b, 34c.

[0110] An ejector 37 is operatively connected to said two annular chambers 35, 36.

[0111] The ejector 37, known per se, comprises (FIG. 11) a motive fluid inlet 38, a nozzle 39 connected to the motive fluid inlet 38, a suction inlet 40, and a diffuser 41.

[0112] The first annular chamber 35 of the sealing device 32 is in fluid communication with the motive fluid inlet 38 of the ejector 37 by means of a first conduit 42. The second annular chamber 36 is in fluid communication with the suction inlet 40 of the ejector 37 by means of a second conduit 43 (FIGS. 7, 8 and 11). The diffuser 41 is in fluid communication with a discharge outlet 44 of the centrifugal radial turbine 19 by means of a third conduit 45, as schematically illustrated in FIG. 8 (which for the sake of simplicity shows a single-rotating centrifugal radial turbine).

[0113] The ejector 37 generates a pressure lower than atmospheric pressure in the second annular chamber 36, exploiting the non-condensable gases NCGs or the geothermal vapour GV with non-condensable gases NCGs present in the centrifugal radial turbine 19. The negative pressure in the second annular chamber 36 draws in air from the outside environment, preventing the leakage of the air and non-condensable gases NCGs it contains. For this purpose, the ejector 37 exploits, as a motive fluid, the non-condensable gases NCGs or the geothermal vapour GV with non-condensable gases NCGs, which pass through the first seal (and are thus present in the first annular chamber 35) so as to draw in a mixture of the gases present, together with the air that has entered from the outside environment, into the second annular chamber 36. This mixture is then introduced into the discharge outlet 44 of the centrifugal radial turbine 19.

[0114] In a variant embodiment illustrated in FIGS. 9 and 10, the sealing device 32 comprises a third annular chamber 46 axially interposed between the first annular chamber 35 and the second annular chamber 36. In this case, two second sealing elements 34b delimit said third annular chamber 46. The third annular chamber 46 is in fluid communication with the discharge outlet 44 of the centrifugal radial turbine 19 by means of a fourth conduit 47. In this manner it is possible to improve tightness, thus limiting the amount of non-condensable gases drawn by the ejector 37 into the mixture of air and non-condensable gases present in the second chamber 36.

[0115] In a further variant embodiment illustrated in FIGS. 12, 13 and 14, the sealing device 32 further comprises an auxiliary annular chamber 48 set between the second chamber 36 and the outside environment, i.e. next to the second chamber 36. Said auxiliary chamber 48 can be selectively connected, by means of a fifth conduit 49 fitted with a proportional valve 50, to a source 51 of gas under pressure (air).

[0116] The sealing device 32 of this additional variant embodiment is configured to operate under two conditions. If the motive fluid (non-condensable gases NCGs or geothermal vapour GV with non-condensable gases NCGs) of the ejector 37 is at a pressure such as to be able to create negative pressure in the second chamber 36, the auxiliary chamber 48 will be disconnected from the source of gas under pressure 51 (FIG. 13, valve 50 closed). If the motive fluid of the ejector 37 is at a pressure such as not to be able to create negative pressure in the second chamber 36, the auxiliary chamber 48 will be connected to the source of gas under pressure 51 and will accordingly be at a pressure higher than atmospheric pressure (FIG. 12).

[0117] In order to switch automatically from the first condition to the other one it is sufficient to measure the pressure differential between the auxiliary chamber 48 under pressure and the second chamber 36 by means of a pressure sensor 52 and adjust the pressure differential with the proportional valve 50 controlled by a controller 53 (PLC). In this manner, when the turbine 19 enters a phase in which the ejector 37 is able to create a sufficient vacuum, the proportional valve 51 will close so as to avoid using up air pointlessly.

[0118] FIG. 15 schematically illustrates the counter-rotating centrifugal turbine 19 of FIG. 6 with the two sealing devices 32 configured as in FIGS. 12 and 13. In the solution in FIG. 15 there are two injectors 37 and two sources of gas under pressure 51 (with a respective valve 50, pressure sensor 52 and controller 53), one for each sealing device 32. In the variant in FIG. 16, by contrast, there is only one injector 37 and only one source of gas under pressure 51 (with a respective valve 50, pressure sensor 52 and controller 53) connected to both the sealing devices 32.

[0119] The sealing device 32 with the above-described variants thereof can also be used in expanders/turbines other than the one dedicated to the expansion of non-condensable gases and thus form the subject matter of an independent invention. In use, in accordance with the process of the invention and with reference to FIGS. 1 and 2, the geothermal fluid GF extracted from the geothermal production well 11 passes, in sequence, into the evaporator 4 and into the preheater 9, where it exchanges heat with the organic working fluid OWF and brings about the preheating and evaporation thereof. Subsequently, the geothermal fluid GF, which has transferred heat to the organic Rankine cycle ORC, is introduced into the separator device 16.

[0120] The separator device 16 separates the non-condensable gases NCGs and the geothermal vapour GV from the geothermal fluid GF. The non-condensable gases NCGs and the geothermal vapour GV exit from the top, through the first outlet 17, and are introduced into the expander 19. The geothermal brine GB exits from the bottom, through the second outlet 18, and is reinjected underground through the reinjection well 15. The expander 19 receives and expands the geothermal mixture GM comprising the geothermal vapour GV and the non-condensable gases NCGs after it has transferred heat to the organic working fluid OWF of the ORC cycle. The typical inlet thermodynamic conditions of the expander 19 are shown in the following Table 1.

TABLE-US-00001 TABLE 1 Min Max Pressure [bar] 2 16 Temperature [ C.] 90 160 Mass flow rate [kg/s] 6 20 Volumetric flow rate [m.sup.3/s] 0.4 2.5 H.sub.2O [% Mass flow] 2% 25%

[0121] The typical discharge conditions of the expander 19 are shown in the following Table 2.

TABLE-US-00002 TABLE 2 Min Max Pressure [bar] 0.8 1.3 Volumetric flow rate [m.sup.3/s] 3 15 Titer [%] 85% 100%

[0122] With regard to the specific enthalpy change and power, the typical values are shown in the following Table 3.

TABLE-US-00003 TABLE 3 Min Max Enthalpy change [kJ/kg-K] 80 200 Power [kW] 500 4000

[0123] If the counter-rotating centrifugal radial turbine of the above-described type is adopted as an expander 19, supporting disks 22, 25 of the same will rotate with an angular velocity comprised between about 2000 RPM and about 4000 RPM. The shafts 24, 27 of the counter-rotating centrifugal radial turbine 19 can therefore be connected directly to the auxiliary generator(s) 20 without the interposition of any reduction gear.

[0124] The variant embodiment of the plant 1 illustrated in FIG. 3 comprises a first separator device 16 positioned upstream of the interface zone 13 and a second separator device 16 positioned downstream of the interface zone 13. An auxiliary expander 54 is moreover connected to the first separator device 16, through a first branch 10 of the intake line 10, and is mechanically connected to an additional auxiliary generator 55. The first separator device 16 separates the geothermal fluid GF coming from the intake line 10 into geothermal vapour GV with non-condensable gases NCGs and geothermal brine GB.

[0125] The geothermal vapour GV with non-condensable gases NCGs exit from the top, through a first outlet 17, and are introduced into the auxiliary expander 54. In the auxiliary expander 54, the geothermal vapour GV and the non-condensable gases NCGs expand without having first exchanged heat with the ORC cycle, i.e. in the manner according to the prior art. The geothermal brine GB exits from the bottom, through a second outlet 18, and flows into a second branch 10 of the intake line 10.

[0126] The expanded geothermal vapour GV together with the non-condensable gases NCGs exiting the auxiliary expander 54 flow into a first line 12 of the interface line 12 through the vaporizer 4 and then the preheater 9 of the ORC system 2 and are subsequently sent to the second separator device 16, through the first section of a first branch 14a of the reinjection line 14. The second separator device 16 has a first outlet 17 connected by means of the inlet conduit 21 to the expander 19. The second separator device 16 has a second outlet 18 connected by means of the second section of the first branch 14b to the reinjection well 15. The second separator device 16 separates the mixture of geothermal vapour GF and non-condensable gases NCGs coming from the interface zone 13 (i.e. after it has exchanged heat with the ORC cycle) into a liquid part (condensed geothermal vapour GV) and a gaseous part (uncondensed geothermal vapour GV and non-condensable gases NCGs). The liquid part is introduced into the reinjection well 15. The gaseous part exits through the first outlet 17 and expands in the expander 19 in the same manner as described above with reference to the expander 19 of FIGS. 1 and 2.

[0127] The geothermal brine GB coming from the second outlet 18 of the first separator device 16 flows through a second line 12 of the interface line 12 and through the preheater 9 of the ORC system 2, and then it is introduced into the reinjection well 15 through a second branch 14 of the reinjection line 14.

[0128] The further variant embodiment of the plant 1 illustrated in FIG. 4 comprises a high pressure ORC closed-cycle system 2 and a low pressure ORC closed-cycle system 2 positioned operatively downstream of the high pressure ORC closed-cycle system 2. The low pressure ORC closed-cycle system 2 receives the geothermal fluid GF after said geothermal fluid has exchanged heat in the high pressure ORC closed-cycle system 2.

[0129] The first separator device 16 is positioned upstream of the high pressure ORC closed-cycle system 2 but no auxiliary expander is present. The geothermal vapour GV with non-condensable gases NCGs which exit from the top through the first outlet 17 exchange directly heat with the high pressure ORC closed-cycle system 2 and then enters the second separator device 16 (which is a reboiler or direct contact heat exchanger) connected to the expander 19. The geothermal brine GB coming from the second outlet 18 of the first separator device 16 exchanges heat with the high pressure ORC closed-cycle system 2 and is then sent to the low pressure ORC closed-cycle system 2. The liquid part separated in the second separator device 16 flows into the second section of the first branch 14b, which joins up with the second branch 14 before entering the low pressure ORC closed-cycle system 2. On exiting the low pressure ORC closed-cycle system 2, the geothermal brine GB is in part introduced into the reinjection well 15, through the reinjection line 14, and in part recirculated, through a recirculation line 56, in the second exchanger 16 (reboiler) so as to extract heat from the mixture of geothermal vapour GV and non-condensable gases NCGs.

[0130] The further variant embodiment of the plant 1 illustrated in FIG. 5 comprises two ORC closed-cycle systems 2, 2 which operate in parallel. The first outlet 17 of the first separator device 16 is connected to a first ORC closed-cycle system 2. The geothermal vapour GV with non-condensable gases NCGs which exit from the top through the first outlet 17 exchange heat directly with the first ORC closed-cycle system and then enters the second separator device 16 (which is a surface-type heat exchanger) connected to the expander 19. The geothermal brine GB coming from the second outlet 18 of the first separator device 16 enters a third separator device 16 through the second branch 10 of the intake line 10, together with the liquid part separated in the second separator device 16 through the second section of the first branch 14b of the reinjection line 14. In the third separator device 16 a further separation takes place. The gaseous part exiting through the first outlet 17 of the third separator 16 is sent to a further auxiliary expander 57 connected to a respective generator 58. The expanded gases exiting the further auxiliary expander 57 are condensed in an auxiliary condenser 59 and introduced into the reinjection well 15. The liquid part exiting through the second outlet 18 of the third separator 16 enters the second ORC closed-cycle system 2 and exchanges heat with the respective organic working fluid OWF in order then to be introduced into the reinjection well 15 together with the condensed gases coming from the auxiliary condenser 59.