NZ248146A

NZ248146A - Rankine cycle power plant with two turbine stages; second turbine stage of higher efficiency than first

Info

Publication number: NZ248146A
Application number: NZ24814693A
Authority: NZ
Inventors: Lucien Y Bronicki; Asher Elovic
Original assignee: Ormat Ind Ltd
Priority date: 1992-07-24
Filing date: 1993-07-14
Publication date: 1995-04-27
Also published as: CN1075589C; CN1086290A; MX9304455A; IL106454A0; ITMI931662A1; ITMI931662A0; IT1265112B1

Description

Priority Date(s): . , I Complete Specification Fiiod: j Class: f7P.\Q\3k??i. j .Ik t Publication Date: ..2.7. APR J99.5 P.O. Journal, No: lv?53.l PATENTS ACT 195 3.

Number: Date: COMPLETE SPECIFICATION RANKINE CYCLE POWER PLANT UTILIZING ORGANIC WORKING FLUID I/WE ORMAT, INC., a corporation organised and existing under the laws of the state of Delaware, United States of roerica, of 980 Greg Street, Sparks, Nevada 89431-6039, 'nited States of America hereby declare the invention fcr which I/we pray that a patent may be granted to me/us and the method by which it is to be performed, to be particularly described in and by the following statement:- The following page is numbered "la" 2wA:!1394 f 1. Technical Field This invention relates to a process for converting heat to work using an expansion engine such as in a Rankine cycle 5 power plant that utilizes an organic working fluid, and more particularly a power plant to the type disclosed that operates on low grade heat sources. 2. Background US 3234734 discloses a Rankine cycle power plant that 10 operates on various organic fluids (e.g., biphenyl). The thermodynamic properties of many organic fluids are such that the saturated vapor line on the temperature-entropy (T-S) diagram for the fluids has a positive slope. As a result, working fluid that exits the vaporizer of a power 15 plant in a saturated vapor state and is applied to the inlet of a turbine, expands and exits the turbine in a superheated state. Upon its expansion to the pressure of the condenser, the working fluid will contain a substantial amount of superheat that the condenser must remove before the working 20 fluid can be condensed and returned to the vaporizer. The superheat rejected by the condenser into the ambient environment is wasted because the rejected heat, which was added to the working fluid in the vaporizer and preheater of the power plant, can not be converted to work. 2 5 The invention in the '734 patent seeks to improve the thermodynamic efficiency of a power plant of the type described by decreasing the amount of superheat in the vaporized working fluid supplied to the condenser. It achieves this end by decreasing the superheat between the stages of turbine. 30 Specifically, the patent discloses a multi-stage turbine in which vaporized working fluid exhausted from a preceding stage is withdrawn from the turbine, and mixed with liquid working fluid derived from a preheater interposed between the condenser of the power plant and the vaporizer. As a 35 result, the superheat in the working fluid applied to the succeeding stage is reduced permitting this heat to be^ : 4 8 t ' utilized for conversion to work instead of being rejected in the condenser. The result is an increase in the power produced by the power plant, and in its thermodynamic efficiency.

While the design approach in US 3234734 is one that significantly improves the performance of a Rankine cycle power plant that operates on an organic fluid using a multistage turbine configuration, it does not address the economic problem of constructing a practical system to 10 generate electricity in the range below, for example, about 10 MW with a low temperature heat source such as geothermal hot water with a temperature lower than about 400°F. Thus, the patent shows a single multi-stage turbine, or a binary cycle using two turbines, which must be connected to a 15 generator by way of a gear reducer because of the large differences in rotational speeds of the turbine and the generator. This configuration complicates construction, maintenance, and lubrication by reason of the multiple bearings and the gear reducer required. 2 0 It is therefore an object of the present invention to provide a new and improved expansion engine such as in a Rankine cycle power plant that utilizes an organic working fluid and which usually operates from a relatively low grade heat source of medium or low temperature fluid, which is 25 less complicated and more reliable.

DISCLOSURE OF INVENTION Apparatus in accordance with the present invention for producing power includes a pair of expansion engine modules, preferably in the form of multi-stage turbine modules each 30 of which has an input for receiving vaporized working fluid and an output shaft, and each of which is responsive to vaporized working fluid applied to its input for expanding the working fluid and producing work at the output shaft, and heat depleted working fluid that is superheated. An 3 5 electrical generator is located between the modules and directly coupled to their output shafts. Means are provided for supplying superheated heat depleted working fluid from one of the modules to the input of the other of the modules.

By dividing the turbine into separate modules and placing the modules on either side of a generator, the 5 physical construction of the power plant is simplified because only two bearings are necessary to support the turbines and the generator, and because no gear reduction system is required for interposition between the turbine modules and the generator.

The apparatus of the present invention further includes a vaporizer for supplying vaporized working fluid to one of the modules, a preheater, preferably separate from the vaporizer, for supplying heated working fluid to the vaporizer, and preferably, a mixer interposed between modules and through which superheated, heat depleted working fluid from the one module is transferred to the input of the other module. Preferably, means are provided for supplying liquid working fluid to the mixer for desuperheating working fluid supplied to the input of the other modules.

Preferably, the liquid working fluid supplied to the mixer is heated working fluid produced by the preheater. Alternatively, the liquid working fluid may be derived directly from the condenser output without preheating.

BRIEF DESCRIPTION OF THE DRAWINGS 2 5 Embodiments of the present invention is described by way of example with reference to the accompanying drawings wherein: Fig. 1 is a block diagram of one embodiment of a power plant according to the present invention; Fig. 2 is a temperature-entropy (T-S) diagram for a typical organic fluid; and Fig. 3 is a block diagram of a second embodiment of the present invention.

DETAILED DESCRIPTION 3 5 Referring now to the drawings, reference numeral 10 designates apparatus according to the present invention _ 4 - comprising a pair of multi-stage turbine modules 12, 14 each of which has input 16 for receiving vaporized workijig fluid, and output shaft 18. These turbine modules are responsive to vaporized working fluid supplied to input 16 for 5 expanding the working fluid and producing work at output shafts 18, and heat depleted working fluid that is superheated as described below. Apparatus 10 further includes electrical generator 20 located between modules 12 and 14 and directly coupled to their input shafts. 10 Specifically, shafts 18 are the free ends of a unitary shaft that extends from opposite ends of generator 20 and is mounted in two separate bearings 22, 24 thereby supporting the generator and the two turbine modules. Turbine module 12 includes disc 2 6A rigidly connected to output shaft 18, 15 and a plurality of turbine blades 28A on the periphery of the disc. Turbine module 14 includes disc 2 6B rigidly connected to shaft 18, and a plurality of turbine blades 28B on the periphery of the disc. Enclosing each of the modules, and preferably enclosing also generator 20 is 20 casing 30.

Depending on the capacity of the power plant, each module may include more than one stage, and preferably two stages. Inputs 16 to the turbine modules are schematically shown in the drawing but, in accordance with standard 25 practice associated with organic fluid turbines, each input is in the form of an annular nozzle ring to which vaporized working fluid is applied. Apparatus 10 further comprises means in the form of conduit 32, for supplying superheated, heat depleted working fluid from module 14 to module 12. 3 0 By reason of the serial application of working fluid to the turbine modules, the pressure ratio across each of the modules is much smaller than the pressure ratio would be were the modules a part of a single multistage unitary turbine. That is to say, if the inlet pressure of the 35 vaporized working fluid to module 14 is 200 psia, the pressure at the inlet of module 12 may be of the order of 9 / 'i /. r* ;• > , ;- 5 - ;magnitude of about 60 psia and the condenser pressure may be of the order of 20 psia. If a single, multistage turbine were used, a pressure drop of about 10:1 across the turbine would be produced. The rotational speed would be several 5 times the ideal rotational speed for a low cost generator, which is about 1800 RPM. ;According to the present invention, using two separate modules will result in a pressure drop of about 3:1 across each module. As a result, the turbine modules will operate 10 efficiently at about 1800 rpm which is a convenient, and standard rotational speed for generator 20. This eliminates the need for any gear reduction between the turbine modules and the generator. This configuration also permits the overhang shaft of generator 20 to be mounted in bearings 22, 15 24 with sufficient additional overhang to provide mounting for discs 26A and 26B of the modules. Two or more discs can be used in each module if circumstances warrant. ;While the drawing schematically illustrates a single stage in each of modules 12 and 14, the preferred 2 0 arrangement is to have two stages such that with pentane as the working fluid, generator 20 will produce about 3.5 MW under the inlet conditions specified above. ;Power plant 10 further includes vaporizer 34 for supplying vaporized working fluid to input 16 of turbine 25 module 14, preheater 36 for supplying heated working fluid to vaporizer 34, and condenser 38 for condensing heat depleted working fluid exhausted from turbine module 12 and producing condensate that is returned to preheater 3 6 using pump 40. Preferably, apparatus 10 further includes mixer 42 30 interposed between turbine modules 12 and 14 through which superheated, heat depleted working fluid from module 14 passes via conduit 32 for transfer to input 16 of turbine module 12. ;Conduit 44 constitutes means for supplying liquid 35 working fluid to mixer 42 for desuperheating the working fluid exhausted from module 14 before this working fluid is ;2 4 G I •: ;- 6 - ;applied to input 16 of module 12. Specifically, mixer 42 is a chamber into which liquid working fluid supplied by conduit 44 is sprayed into the superheated vaporized working fluid exhausted from module 14. Sufficient volume for the 5 mixer is provided so that the desuperheating of the working fluid occurs before the working fluid enters inlet 16 of module 12. The design is such that the working fluid is in a substantially saturated vapor state at the inlet to module 12. ;10 The heat required for the power plant to operate is supplied by a low temperature heat source such as geothermal hot water which is supplied to vaporizer 34. After vaporizing the working fluid in the vaporizer, the heat depleted geothermal water is then delivered to preheater 36 15 via conduit 42 where additional heat in the geothermal hot water is given up to liquid working fluid in the preheater. The further heat depleted geothermal water is then typically transported to an injection well for disposal. ;Condensate produced by condenser 38, shown as being 20 cooled by water, but which alternatively may be air cooled, is in state "a" as indicated in the T-S diagram of Fig. 2. Thus, the condensate lies substantially on the saturated liquid line of the T-S diagram, and pump 40 serves to raise the pressure of the liquid working fluid which is then moved ;2 5 to state "b" in the supercooled region of the T-S diagram. ;In preheater 36, the state of the working fluid changes from "b" to "c" as the temperature of the working fluid is raised to the temperature of the vaporizer, state "c" being on the saturated liquid line of the T-S diagram. Heat ;3 0 absorbed by the saturated liquid in vaporizer 34 causes the working fluid to boil at constant temperature and pressure as indicated in the T-S diagram of Fig. 2. At the exit of the vaporizer, the working fluid is in a substantially saturated vapor state indicated at "d", and the vaporized 3 5 working fluid is supplied to inlet 16 of turbine module 14. In this module, the working fluid expands at essentially ;- 7 - ;2 4 8 1 ;constant entropy into the superheated region ending at state "e" at a pressure intermediate the vaporizer and condenser pressure. The positive slope of the saturated vapor line in the T-S diagram for organic .fluids,, results in the 5 superheated condition of the vapor exhausted from module 14. ;Superheated, heat depleted working fluid exhausted from module 14 is applied to mixer 42 where liquid at state "c" from preheater 36 is sprayed into the vaporized working fluid. This liquid is vaporized by a heat exchange process 10 with the superheated working fluid such that the working fluid substantially is in state 11 f" at the inlet to module 12. This is to say, the vaporized working fluid moves along a constant pressure line substantially to the saturated vapor state by reason of the operation of mixer 42. Thus, 15 the mass of fluid applied to inlet 16 of module 12 is greater than the mass of fluid applied to inlet 16 of module 14 by reason of the liquid extracted from preheater 36 and applied via conduit 44 to mixer 42. ;The now substantially saturated vapor applied to module 20 12 expands in the module producing work that drives shaft 18, and producing superheated, heat depleted working fluid at state "g". This now superheated and heat depleted working fluid is applied to condenser 38 where condensation takes place returning the working fluid from state "g" to 2 5 state "a", and the cycle repeats. ;Some of the advantages derived from the apparatus shown in Fig. 1 can be appreciated from inspection of Fig. 2 where the cross-hatched area represents the amount of heat involved in desuperheating the working fluid passing from 30 module 14 to module 12. Normally, without mixer 42, this heat would have been rejected into the ambient environment in condenser 38. In the present invention, however, this heat is actually utilized within the second module for the purpose of generating power. The additional work is 35 represented by the incremental amount of liquid which is in injected into mixer 42. ;•»«n j ;- 8 - ;Further advantages of the apparatus shown in Fig. 1 lie in the increased turbine efficiency that is achieved by reason of reducing the pressure ratio across each turbine module, the resultant larger blade size, the use of improved 5 blade angles, and the increased mass flow achieved by arranging the turbine modules for serial rather than parallel flow. Combined with the slower speed of the turbine modules and the elimination of a gear reduction between the modules and the generator, as well as the use of 10 a common unitary shaft for mounting the turbine discs and the generator permitting the mechanical portion of the power plant to be supported with two bearings, a coupling is eliminated, better alignment is achieved, and less vibration stresses are present, consequently facilitating the 15 mechanical design. Furthermore, stresses in the turbine discs are also reduced because of the lower rotational speed of the turbine. Thus, a particularly efficient and simple design is achieved with a consequent reduction in maintenance. ;20 In accordance with the present invention, the temperature of the heat depleted geothermal water can be reduced to a new optimum point compared to a system where liquid working fluid is not added for desuperheating working fluid supplied to the input of the other module. This 25 permits greater amounts of heat to be input into the power plant increasing the power output at the same Carnot efficiency. ;As a result of the preferred use of an increased number of expansion stages, the Mach numbers in the turbine blading 30 used in accordance with the present invention are advantageously reduced, resulting in higher turbine efficiency. Additionally, by arranging the turbine modules in series in accordance with the present invention, the mass flow through the turbine stages is doubled in comparison to 3 5 the flow that would occur were the turbines operated in parallel, resulting in increased blade heights thereby ;achieving higher efficiency levels. Furthermore, the higher mass flow through the turbine at each stage combined with a lower pressure ratio per stage, reduces parasitic losses resulting from turbine disc seal leakage (i.e., the amount 5 of working fluid leaking from the turbine discs and flowing between the circumference of the turbine wheels and the turbine housing), as well as nozzle diaphragm losses. Also, by adding liquid working fluid to the mixer for desuperheating the working fluid supplied to the downstream 10 modules, the blade height of the turbines can be increased further in the low pressure module further increasing turbine efficiency. ;In addition, the present invention permits the use of more turbine stages than were used previously in accordance 15 with the teachings of U.S. Patent Nos. 4,578 , 953 and 4,7 00,54 3, the disclosures of which are hereby incorporated by reference. Preferably, four turbine stages (i.e., two turbine discs for each module) are utilized in the present invention whereas two modules in parallel, each using two 20 turbine stages were usually used in power plants built and operating in accordance with the teachings of the two above mentioned U.S. patents. Fig. 3 shows a schematic representation of this embodiment of the invention. By using more stages in accordance with the present invention, 25 the off-design performance of the system is greatly enhanced. ;Off design performance is mainly caused by variations in temperature of the ambient air and/or temperature of the condenser cooling medium which modify the condenser pressure 3 0 consequently changing the back pressure on the turbine. By dividing the turbine into separate modules, the Mach number in the fourth, or last turbine stage, can be made high enough (e.g., around M = 1.2) to substantially isolate the upstream discs from changes in the back pressure. 35 Consequently, substantially only the last stage is affected by changes in pressure drop across the entire turbine. For ;A ;c- ;/

Claims

1. - 13 - WHAT l/WE CLAIM IS: 1.

2. A Rankine cycle power plant for generating power from a source fluid comprising at least one system having the following components: a preheater responsive to heat in the source fluid 5 for preheating liquid working fluid; a vaporizer responsive to heat in the source fluid for vaporizing preheated liquid working fluid produced by the preheater; a first turbine module having an input for receiving vaporized working fluid produced by the vaporizer and having an output shaft, the first module being 10 responsive to vaporized working fluid from the vaporizer for expanding the working fluid to produce work at the output shaft and heat depleted working fluid that is superheated; a second turbine module having an input and having an output shaft, the second module being responsive to vaporized working fluid for 15 expanding the working fluid to produce work at the output shaft and heat depleted working fluid that is superheated; an electric generator coupled to the output shafts of the turbine modules; and a condenser for condensing heat depleted working fluid produced by the second module into condensate; and means for 20 supplying the condensate to the preheater characterized in that the generator is located between the modules, the heat depleted working fluid produced by the first module is supplied to the inlet of the second module, and the turbine efficiency of the second turbine module is higher than the turbine efficiency of 2 5 the first module. 2.

3. A Rankine cycle power plant according to claim 1 characterized in that the second turbine module comprises at least one turbine stage whose last stage is effective to substantially isolate upstream stages of said second turbine 30 module from changes in back pressure on the second module. 3.

4. A Rankine cycle power plant according to claim 2 characterized in that the Mach number of the last stage in the second turbine module is around 1.2 for substantially isolating the upstream stages of said second turbine module from changes in 35 back pressure. 4.

5. A Rankine cycle power plant according to claim 1 - 14 - £ -T u S ^ ^ characterized in that the electric generator is positioned between the modules and permits the generator to be directly driven by the output shafts of the modules at the same rotational speed as the modules at a relatively low speed. characterized in that the relatively low speed is 1800 RPM.

6. A Rankine cycle power plant according to any of the preceding claims characterized in the provision of a second system having the same components as the at least one system, and 10 means for supplying the source fluid to all of the vaporizers of the systems in series to produce heat depleted source fluid; and means for supplying the heat depleted source fluid to all of the preheaters of the systems in parallel. 15 preceding claims characterized in that the working fluid is an organic fluid. 8. A Rankine cycle power plant according to any of the preceding claims characterized in that the source fluid is a medium or low temperature fluid. 20 9. A Rankine cycle power plant according to any of the preceding claims characterized in that the source fluid is a geothermal fluid. 10. A Rankine cycle power plant according to any of the preceding claims characterized in that the source fluid is 25 geothermal hot water. 11. A Rankine cycle power plant according to any of the preceding claims characterized in the provision of a mixer interposed between the modules of each system, the superheated, heat depleted working fluid from one of the modules being 30 transferred to the input of the other of the modules through the mixer; and means supplying for liquid working fluid to the mixer for desuperheating the working fluid supplied to the input of the other of the modules. 5 5. A Rankine cycle power plant according to claim 4

7. A Rankine cycle power plant according to any of the the Applicon1's^