Abstract:
The application discloses an organic Rankine Cycle system with a generating unit, a condenser for condensing an organic work fluid, a feeder pump for circulating the organic work fluid and an evaporator ( 14 ) for evaporating the organic work fluid. The generating unit comprises a high-pressure screw expander and a low-pressure screw expander, which are connected in series, wherein the high-pressure screw expander and the low-pressure screw expander are mechanically connectable to a generator, which is provided between the high-pressure screw expander and the low-pressure screw expander. The ORC system comprises a by-pass line for bypassing the high-pressure screw expander. The bypass line comprises a control valve for opening and closing the by-pass line.

Description:
TECHNICAL FIELD 
     The present disclosure relates to thermal systems for converting thermal energy, specifically systems utilizing organic Rankine cycles. 
     BACKGROUND 
     An Organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. The fluid allows Rankine cycle heat recovery from lower temperature sources such as biomass combustion, industrial waste heat, and waste heat from various small-scale heat engines, fuel cells or electric devices, geothermal heat, solar ponds, energy from combustion or decomposition of biodegradable materials. The low-temperature heat is converted into useful work that can itself be converted into electricity. 
     An idealized Clausius-Rankine cycle, also known as Rankine cycle, is characterized by an isentropic expansion, a heat dissipation at constant temperature, isentropic compression and an isobaric heating, which may be followed by a superheating. In a real Rankine cycle, the steps of expansion and compression are not exactly isentropic but the entropy increases slightly. 
     In the case of a “dry fluid”, the Rankine cycle can be improved by the use of a regenerator. In a temperature—entropy diagram, a dry-fluid is characterized by an overhanging mixed phase to vapour phase boundary. The dry fluid, which has not reached the two-phase state at the end of the expansion, has a temperature that is higher than the condensing temperature. The higher temperature fluid can be used to preheat the work fluid before it enters the evaporator. 
     SUMMARY 
     It is an object of the application to provide an improved ORC cycle system for operation under high and low temperature conditions. 
     The application discloses an organic Rankine Cycle system with a generating unit, a condenser for condensing an organic work fluid, a feeder pump for circulating the organic work fluid, and an evaporator for evaporating the organic work fluid. It is advantageous to use organic work fluids for low temperature applications. By choosing suitable organic compounds, they can be tailored to the application with respect to the boiling point and other thermal properties. Organic liquids are also usually less corrosive than water. 
     According to the application, the generating unit comprises a high-pressure screw expander and a low-pressure screw expander which are connected in series. Thereby, the heat can be used effectively without the need to make the expanders very big. Screw expanders are often better than turbines for use in compact machines rather as compared to large scale steam engines for atomic power plants, for example. The high-pressure screw expander and the low-pressure screw expander are mechanically connectable to a generator which is provided between the high-pressure screw expander and the low-pressure screw expander. The arrangement of the generator between the expanders according to the application yields a compact and robust design. 
     The ORC system further comprises a by-pass line for bypassing the high-pressure screw expander, wherein the bypass line comprises a control valve for opening and closing the by-pass line. Under conditions when the temperature of a heat source to which the evaporator is connected is low it can be advantageous to disconnect the high-pressure turbine and to use just the low-pressure turbine. 
     In a waste recovery system for an engine such a condition can occur, for example, when the engine has just started and does not provide sufficient exhaust heat to drive both of the expanders. On the other hand, it can also be advantageous to disconnect a previously closed bypass and use both expanders in situations when a temperature of a heat source is higher than it was before. This could occur in a geothermal power station, when a bore hole is drilled to a greater depth with a higher temperature or when the ORC machine is moved or connected to a different geothermal heat source. If heat from volcanic heat sources or from geysers is harnessed, the temperature may also change over time. These changes may be periodic changes, especially for geysers, or also long-term changes. 
     According to the one embodiment, the high-pressure expander is mechanically connectable to the generator via a freewheeling device. This provides a simple way of disconnecting from the high-pressure expander from the low-pressure expander. 
     According to the application, the bypass can be activated and deactivated by providing an input control valve and an output control valve. The high-pressure expander is arranged in the flow path of the work fluid between the input control valve and the output control valve. The high-pressure expander can be shut off from the work flow current from the input and the output side by closing the input control valve and the output control valve. 
     Furthermore, the Organic Rankin Cycle system may comprise one or more gear sets in order to adapt the generator speed to the output speed of the expanders for operating the generator close to a desired working point. 
     According to one embodiment, the generating unit comprises a first spur gear that is connected to the high-pressure expander and a second spur gear ( 64 ) that is connected to the low-pressure expander ( 24 ). 
     According to another embodiment the ORC system comprises a first planetary gear set that is connected to the high-pressure expander a second planetary gear set that is connected to the low-pressure expander. While a spur gear is easy to realize, a planetary gear can provide a higher reduction ratio for a given dimension. 
     In yet another embodiment of the ORC system, the generating unit comprises a planetary gear set, wherein a sun gear of the planetary gear set is connected to the high-pressure expander and a planetary carrier of the planetary gear set is connected to the low-pressure expander. With a planetary gear set which is connected in this way, the output of the two expanders can be used simultaneously. 
     According to one embodiment, the ORC system comprises a work fluid which is an azeotropic mixture, the azeotropic mixture comprising a first organic fluid with a normal boiling point in a temperature above 35° C. and a second organic fluid which is of low flammability. The boiling point of above 35° C. is advantageous when the temperature of the cooling fluid is as warm as 30° or even slightly warmer. This situation occurs for a ship in tropical latitudes. The low flammability is important for security reasons to prevent a fire on board a ship in which the ORC system is installed. This also applies to other environments, in which flammable substances such as oil are close to the ORC system. 
     In order to achieve these desired properties, the first organic fluid of the work fluid may comprise a pentafluorobutane and the second organic fluid may comprise a perfluoropolyether. 
     In particular, the condenser and/or the evaporator may comprise a plate heat exchanger. This type of heat exchanger is advantageous for situations in which the ORC engine is moving around. Movements of the fluid within the heat exchanger are constrained by the geometry of the plate heat exchanger. 
     In particular, the expanders may be realized as oil-free expanders. Thereby, it is not necessarily to mix oil into the working fluid which could deteriorate the properties of the working fluid. 
     Furthermore, the application discloses a ship engine with an aforementioned ORC system wherein the evaporator of the ORC system is connected to an exhaust of the ship engine via a pipe. The pipe may be filled with a circulating thermal oil that acts as a heat transporter. 
     In another embodiment, the application discloses a geothermal power station with the aforementioned ORC system wherein the evaporator of the ORC system is connected to a pipe for a brine of the geothermal power station, the pipe being connected to the geothermal heat source via a borehole. The brine is injected into the heat source or, in the case of a geyser, it may also be provided by the geothermal heat source itself. 
     Moreover, the application discloses a method for operating an ORC system with a high-pressure expander and a low-pressure expander. Therein, the ORC system comprises a bypass line that extends, in a flow direction of the working fluid, from a branching point before the high-pressure expander to the low-pressure expander. The ORC system furthermore comprises an input control valve before the high-pressure expander. 
     In a high temperature operating mode the bypass line is closed and the input control valve is opened. In a low temperature operating mode the bypass line is opened and the input control valve is closed. Thereby, the ORC system can be adapted to different temperature conditions without the need to provide to different ORC systems. 
     Furthermore, the method may comprise steps of measuring a temperature of a heat source and automatically selecting one of the high-pressure operation mode and the low-pressure operation based on the temperature of the heat source. This embodiment is advantageous, when a temperature of the heat source can change more rapidly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter of the application will now be explained in further detail with reference to the following Figures in which 
         FIG. 1  shows an ORC system according to the application, 
         FIG. 2  shows the ORC system of  FIG. 1  in a high-pressure operation mode, 
         FIG. 3  shows the ORC system of  FIG. 2  in a low-pressure operation mode, 
         FIG. 4  shows a generation unit with a clutch, 
         FIG. 5  shows a generation unit with a clutch with an alternative orientation of the low-pressure expander, 
         FIG. 6  shows a generation unit with two freewheeling devices, 
         FIG. 7  shows a generation unit with two freewheeling devices with an alternative orientation of the low-pressure expander, 
         FIG. 8  shows a first view of an evaporator, 
         FIG. 9  shows a second view of an evaporator, 
         FIG. 10  shows a first view of a condenser, 
         FIG. 11  shows a second view of a condenser, 
         FIG. 12  a schematic process diagram of the ORC system of  FIG. 1 , 
         FIG. 13  shows a further embodiment of an ORC system having a separator chamber between first and second expander stages, 
         FIG. 14  shows a generation unit with a spur gear, 
         FIG. 15  shows a generation unit with two planetary gearsets, and 
         FIG. 16  shows a generation unit with a planetary type overriding drive. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, details are provided to describe the embodiments of the application. It shall be apparent to one skilled in the art, however, that the embodiments may be practised without such details. In the following description, the same reference numbers refer to the same or similar parts and primed reference numbers refer to similar parts. The expression “generating unit ( 11 ,  111 )” refers to all embodiments of the generating unit. 
     Similar parts have the same reference numbers. Different embodiments of similar parts are marked with primes. 
       FIG. 1  shows an Organic Rankine Cycle (ORC) system  10 . The ORC system  10  comprises, in the sense of a work fluid flow, a generating unit  11 , a condenser  12 , a feed pump  13 , and an evaporator  14 . Respective work fluid pipes connect a work fluid outlet  15  of the generating unit  11  with a work fluid inlet  16  of the condenser  12 , a work fluid outlet  17  of the condenser  12  with a work fluid inlet  18  of the feeder pump  13 , a work fluid outlet  19  of the feeder pump  13  with a work fluid inlet  20  of the evaporator  14  and a work fluid outlet  21  of the evaporator  14  with a work fluid inlet  23  of the generating unit  11  such that a closed work flow loop is formed. 
     The expander screws are oriented such that the high-pressure expander  23  and the low-pressure expander  24  turn in the same direction when they are pressurized via their respective work fluid inputs. The freewheel clutch  27  is connected such that the freewheel clutch is disengaged when the low-pressure expander  24  turns faster than the high-pressure expander  23  and is engaged when the high-pressure expander  23  turns faster than the low-pressure expander  24 . 
     The generating unit  11  comprises a high-pressure expander  23  and a low-pressure expander  24 . The high-pressure expander  23  and the low-pressure expander  24  are connected to a generator  25  via a shaft  26 . According to the application, the generator  25  is provided by an alternating current generator  25 , for example by a three phase generator such as a cylindrical rotor generator, a salient pole generator, or a claw pole generator. 
     The generator  25  is arranged between the high-pressure expander  23  and the low-pressure expander  24 . A freewheeling device  27  is provided between the generator  25  and the high-pressure expander  23 . The shaft  25  comprises at least a first section, which connects the high-pressure expander  23  and a first input of the freewheeling device  27 , and a second section, which connects a second input of the freewheeling device  27  and the generator  25 . 
     At a first branching point  28  behind the work fluid input  22  of the generating unit  11 , the fluid pipe branches off into a high-pressure supply line  29  that is connected to a work fluid input of the high-pressure expander  23  and into a low-pressure supply line  30  that is connected to a work fluid input of the low-pressure expander  24 . A high-pressure exhaust line  31  that is connected to a work fluid output of the high-pressure expander  23  leads into the low-pressure supply line at a second branching point  32 . A bypass control valve  33  is provided between the first branching point  28  and the second branching point  32  at the low-pressure supply line  30 . An input control valve  34  is provided between the first branching point  28  and the work fluid input of the high-pressure expander  23 . An output control valve  35  is provided between the work fluid output of the high-pressure expander  23  and the second branching point  32 . A flow direction of the work fluid is indicated by arrows. 
     The condenser  12  is realized as a plate heat exchanger that comprises one or more channels  39  for cooling water and one or more channels  40  for the work fluid. The channel for cooling water  39  is connected to a cooling water source  36  at one end and to a cooling water sink  37  at the other end. For a ship, the cooling water source and sink can be realized by input and output ports which are connected to sea water. For a geothermal energy plant, the cooling water can be provided by a freshwater source or by circulating cooling water, which is recycled after it has cooled down. The cooling process of the cooling fluid may be accelerated by using a cooling tower or other heat exchangers. 
     Similar to the condenser  12 , the evaporator  14  is realized as a plate heat exchanger that comprises a heating fluid inlet  43 , one or more channels  41  for a heating fluid, a heating fluid outlet  44  and one or more channels  42  for the work fluid. For waste energy recuperation, the heating fluid can be provided by a thermal oil which takes up heat from a heat source  38  and which is circulated in a closed loop. In the case of a geothermal heat source, the heating fluid, also known as “injection brine”, is provided by a heated water or steam which is pumped out from the geothermal heat source and injected back into it. 
     According to the application, the expanders  23 ,  24  are preferentially realized as essentially oil-free expanders. In this context “oil-free” expanders refers to expanders in which the screw surfaces are lubricated through the work fluid. In one embodiment, the freewheeling device  27  is realized as a sprag type freewheel which has low friction. In addition or alternatively, a clutch may be provided, for example an electromagnetic clutch to decouple the motion of the low-pressure expander  24  from the motion of the high-pressure expander  23 . 
     By using a two-stage expander, the expanders can be made smaller, such that they fit into the limited space of a ship&#39;s engine room. When a work fluid according to the application is used, the dimensions of the expanders can be made such that they each provide an expansion ratio of about 5. 
     In  FIG. 1 , measuring locations for thermodynamic state quantities of the work flow are marked by squared numbers “1” to “4”. The thermodynamic state quantities comprise directly measurable state quantities, such as pressure and temperature, as well as derived state quantities, such as specific enthalpy and entropy. A “1” denotes a measuring location between the generating unit  11  and the condenser  12 , a “2” denotes a measuring location between the condenser  12  and the feeder pump  13 , a “3” denotes a measuring location between the feeder pump  13  and the evaporator  14  and a “4” denotes a measuring location between the evaporator  14  and the generating unit  11 . The measuring locations correspond to start and end points of process sections of a Clausius-Rankine cycle. 
       FIG. 2  shows the ORC system  10  of  FIG. 1  in a high-pressure mode, also referred to as high temperature mode. The high-pressure mode is especially advantageous for waste heat recovery from heat sources which have substantially higher temperatures than 100° C. These conditions apply to combustion motors but also to some geothermal heat sources, for example. 
     In the high-pressure mode, the bypass valve is closed and the input valve and the output valve of the high-pressure expander  23  are open. During operation, work fluid in the gaseous phase is supplied to the high-pressure expander  23 , the work fluid, which is still in the gaseous phase, is expanded in the high-pressure expander  23  and discharged through the output valve of the high-pressure expander. Then, the work fluid flows to the low-pressure expander  24  and is expanded in the low-pressure expander. The work fluid, which is still in the gaseous state, is then discharged from the generating unit  11 . The faster revolving one of the high-pressure expander  23  and the low-pressure expander  24  drives the shaft  26  and there by the rotor of the electricity generator  25 . 
     If the high-pressure expander  23  turns faster than the low-pressure expander  24 , the freewheel  27  engages and the high-pressure expander  23  turns the rotor of the generator and the low-pressure expander  24 . If, on the other hand, the low-pressure expander  24  turns faster than the high-pressure expander, the freewheel  27  disengages and the low-pressure expander  24  turns the rotor of the generator. Thereby, the low-pressure expander  24 , which is now under load, will slow down again. 
     The remaining cycle of the work fluid is similar to a standard ORC cycle and is omitted here for brevity. 
       FIG. 3  shows the ORC system  10  of  FIG. 1  in a low-pressure mode, also referred to as low temperature mode. The low-pressure mode is especially advantageous for waste heat recovery from heat sources which have temperatures of only about 100° C. or lower. These conditions apply, for example, to low temperature geothermal sources or to the decomposition of biodegradable substances. 
     In the low-pressure mode, the bypass valve is opened whereas the input valve and the output valve of the high-pressure expander is closed. 
     During operation, work fluid in the gaseous phase is supplied to the low-pressure expander  24 . The work fluid, which is still in the gaseous phase, is expanded in the low-pressure expander  24  and discharged through the output valve of the low-pressure expander. The remaining cycle of the work fluid is similar to a standard ORC cycle and is omitted here for brevity. 
     The work fluid of the Rankine cycle system according to the application is an organic fluid in the form of an azeotropic mixture. Preferentially, the working fluid fulfils the following criteria.
     1. non-toxic   2. non-flammable   3. non-corrosive and fouling resistant   4. material compatibility and suitable fluid stability limits   5. high latent heat and high density   6. low environmental impact   7. acceptable pressure range for screw expanders   8. safety   

     In particular, SES36 is a suitable work fluid according to the application. SES36 is an azeotropic mixture of 365 mfc (1,1,1,3,3 pentafluorobutane) and PFPE (perfluoropolyether). While 365 mfc on its own already provides a good efficiency as an ORC work fluid, the addition of PFPE to 365 mfc has the benefit of reducing the reactiveness significantly. 
     The following table lists thermodynamic properties of SES36: 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Liq. 
                   
               
               
                 Tem- 
                   
                 Liq. 
                 Vap. 
                 Liq. 
                 Vap. 
                 Entropy 
                 Vap. 
               
               
                 perature 
                 Pressure 
                 Density 
                 Density 
                 Enthalpy 
                 Enthalpy 
                 [kJ/(k 
                 Entropy 
               
               
                 [° C.] 
                 [bar] 
                 [kg/m3] 
                 [kg/m3] 
                 [kJ/kg] 
                 [kJ/kg] 
                 gK)] 
                 [kJ/(kgK)] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0.263 
                 1422.04 
                 2.00 
                 200.00 
                 349.88 
                 1.000 
                 1.549 
               
               
                 10 
                 0.395 
                 1400.14 
                 2.89 
                 207.26 
                 359.64 
                 1.026 
                 1.564 
               
               
                 20 
                 0.579 
                 1377.21 
                 4.12 
                 215.92 
                 369.63 
                 1.056 
                 1.580 
               
               
                 30 
                 0.833 
                 1353.24 
                 5.77 
                 226.05 
                 379.70 
                 1.090 
                 1.597 
               
               
                 40 
                 1.174 
                 1328.22 
                 8.00 
                 237.62 
                 389.66 
                 1.127 
                 1.613 
               
               
                 50 
                 1.622 
                 1302.16 
                 10.97 
                 250.50 
                 399.25 
                 1.168 
                 1.628 
               
               
                 60 
                 2.200 
                 1275.04 
                 14.90 
                 264.36 
                 408.23 
                 1.210 
                 1.642 
               
               
                 70 
                 2.932 
                 1246.85 
                 20.08 
                 278.74 
                 416.31 
                 1.252 
                 1.653 
               
               
                 80 
                 3.845 
                 1217.59 
                 26.82 
                 293.04 
                 423.29 
                 1.293 
                 1.662 
               
               
                 90 
                 4.964 
                 1187.26 
                 35.45 
                 306.69 
                 429.05 
                 1.331 
                 1.668 
               
               
                 100 
                 6.316 
                 1155.82 
                 46.31 
                 319.35 
                 433.67 
                 1.365 
                 1.671 
               
               
                 110 
                 7.929 
                 1123.24 
                 59.79 
                 331.04 
                 437.38 
                 1.396 
                 1.673 
               
               
                 120 
                 9.831 
                 1089.48 
                 76.45 
                 342.12 
                 440.51 
                 1.424 
                 1.674 
               
               
                 130 
                 12.055 
                 1054.38 
                 97.20 
                 353.16 
                 443.37 
                 1.451 
                 1.675 
               
               
                 140 
                 14.636 
                 1017.66 
                 123.68 
                 364.78 
                 446.20 
                 1.479 
                 1.676 
               
               
                 150 
                 17.622 
                 978.55 
                 158.94 
                 377.61 
                 449.10 
                 1.509 
                 1.678 
               
               
                 160 
                 21.072 
                 934.72 
                 209.17 
                 392.33 
                 451.93 
                 1.542 
                 1.680 
               
               
                 170 
                 25.067 
                 875.19 
                 289.01 
                 410.03 
                 454.18 
                 1.582 
                 1.681 
               
               
                   
               
             
          
         
       
     
     Further characteristics of the work fluid SES36 are summarized in the following table, in which “NBP” denotes the normal boiling point, cp′ the liquid heat capacity for constant pressue, cp″ the vapour heat capacity for constant pressure and cv″ the vapour heat capacity for constant volume. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                   
                 Physical Property 
                 Unit 
                 Value 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Molecular mass 
                 g/mol 
                 184.53 
               
               
                   
                 NBP 
                 ° C. 
                 35.64 
               
               
                   
                 Tcrit. 
                 ° C. 
                 177.55 ± 0.5 
               
               
                   
                 pcrit. 
                 Bar 
                  28.49 ± 0.24 
               
               
                   
                 crit. Density 
                 kg/m_3 
                 538 
               
               
                   
                 liq. density @ NBP 
                 kg/m_3 
                 1339.25 
               
               
                   
                 vap. density @ NBP 
                 kg/m_3 
                 6.95 
               
               
                   
                 cp′ @ NBP 
                 J/(kg K) 
                 1167.2 
               
               
                   
                 cp″ @ NBP 
                 J/(kg K) 
                 641.9 
               
               
                   
                 cp″/cv″@ NBP 
                 — 
                 1.01 
               
               
                   
                 heat of vaporisation 
                 kJ/kg 
                 152.94 
               
               
                   
               
             
          
         
       
     
     By using SES36, which has a high boiling point of over 35° C., it is possible to use sea water as cooling fluid in a condenser, even under tropical conditions where the sea water may have temperatures as high as 30° C. Moreover, SES36 has a high vapour density. Thereby the expanders can be made more compact and with smaller expansion ratios. 
     In a first example, the ORC system in the high-pressure configuration of  FIG. 2  is used in a ship to produce electric current from waste heat of a ship engine using SES36 as work fluid. In this example, the work fluid has a temperature T — 1 of 40.15° C., a pressure p — 1 of 1.26 bar and a specific enthalpy h — 1 of 389.66 kJ/kg between the low-pressure expander  24  and the condenser. 
     Assuming typical water conditions in the tropics, the condenser takes in seawater at a temperature of 30° C. and ejects heated sea water at a temperature of about 35° C. Between the condenser and the feeder pump, the work fluid has a temperature T — 2 of about 35.64, a pressure p — 2 of 1 bar and a specific enthalpy h — 2 of 236.72. These conditions correspond to the normal boiling point (NBP) of the work fluid SES36. Here, the specific enthalpy h — 2 is the liquid enthalpy under the assumption that the condenser liquefies all of the work fluid. 
     The feeder pump circulates work fluid at about 0.345 liter/sec. Between the feeder pump and the evaporator, the work fluid has a temperature of 35.64° C., a pressure of 25 bar and a specific enthalpy h — 3 of 239.62. The evaporator is fed by a thermal heat transfer oil which is heated up by the ship engine to about 230° C. When the thermal oil is ejected again from the evaporator it has an outlet temperature of about 80° C. The high-pressure expander  23  and the low-pressure expander  24  drive the generator such that an output power P_gen of about 20 kW=20 kJ/sec is produced. 
     A first estimate of the thermal efficiency η_th under the conditions of the first example is given by the quotient 
     
       
         
           
             
               η 
               th 
             
             ≈ 
             
               
                 P 
                 gen 
               
               
                 
                   ( 
                   
                     
                       h 
                       4 
                     
                     - 
                     
                       h 
                       2 
                     
                   
                   ) 
                 
                 * 
                 
                   
                     m 
                     . 
                   
                   pump 
                 
               
             
           
         
       
     
     Assuming a work fluid temperature T — 4 of 170° C., a pressure p — 4 of 25.06 bar and a corresponding specific vapour enthalpy of 454.18 kJ/kJ at the inlet of the high-pressure expander  23 , and with a liquid density of 1339.25 kg/m 3  at NBP, the quotient yields an approximate thermal efficiency of: 
     
       
         
           
             
               η 
               th 
             
             ≈ 
             
               
                 20 
                 ⁢ 
                 
                   kJ 
                   sec 
                 
               
               
                 
                   ( 
                   
                     454.18 
                     - 
                     236.72 
                   
                   ) 
                 
                 ⁢ 
                 
                   kJ 
                   kg 
                 
                 * 
                 0.345 
                 ⁢ 
                 
                   l 
                   sec 
                 
                 * 
                 1.33925 
                 ⁢ 
                 
                   kg 
                   l 
                 
               
             
             ≈ 
             
               20 
               ⁢ 
               % 
             
           
         
       
     
     In a second example, the ORC system in the low-pressure configuration of  FIG. 3  is used in a geothermal energy plant to produce electric current geothermal heat using SES36 as work fluid. It is assumed here that the geothermal heat is sufficient to heat water to the boiling point. Higher or lower temperatures may be achieved as well, depending on the nature of the geothermal source and the water injection process. 
     In this example, the work fluid has a temperature T — 1 of 40.15° C., a pressure p — 1 of 1.26 bar and a specific enthalpy h — 1 of 389.66 kJ/kg between the low-pressure expander  24  and the condenser. Between the condenser and the feeder pump, the work fluid has a temperature T — 2 of about 35.64° C., a pressure p — 2 of 1 bar and a specific enthalpy h — 2 of 236.72. These conditions correspond to the normal boiling point (NBP) of the work fluid SES36. Here, the specific enthalpy h — 2 is the liquid enthalpy under the assumption that the condenser liquefies all of the work fluid. Between the feeder pump and the evaporator, the work fluid has a temperature of T — 3 of 35.64° C., a pressure p — 3 of 6.136 bar and a corresponding specific enthalpy h — 3 of 239.62 kJ/kg. Between the evaporator and the inlet of the low-pressure expander  24 , the work fluid has a temperature T — 4 of 100° C., a pressure of 6.136 bar and a corresponding specific enthalpy of 433.67 kJ/kg. A work fluid mass flow through the low-pressure expander is 0.4544 kg/sec. 
     These enthalpy values yield a theoretical thermal efficiency of 
     
       
         
           
             
               η 
               th 
             
             = 
             
               
                 
                   
                     ( 
                     
                       
                         h 
                         4 
                       
                       - 
                       
                         h 
                         1 
                       
                     
                     ) 
                   
                   - 
                   
                     ( 
                     
                       
                         h 
                         3 
                       
                       - 
                       
                         h 
                         2 
                       
                     
                     ) 
                   
                 
                 
                   ( 
                   
                     
                       h 
                       4 
                     
                     - 
                     
                       h 
                       3 
                     
                   
                   ) 
                 
               
               = 
               
                 
                   
                     
                       ( 
                       
                         433.67 
                         - 
                         389.66 
                       
                       ) 
                     
                     - 
                     
                       ( 
                       
                         239.62 
                         - 
                         236.72 
                       
                       ) 
                     
                   
                   
                     ( 
                     
                       433.67 
                       - 
                       239.62 
                     
                     ) 
                   
                 
                 ≈ 
                 
                   21 
                   ⁢ 
                   % 
                 
               
             
           
         
       
     
     For the real process one needs to consider that the feeder pump and the expanders have efficiencies below 1, for example the pump may have an efficiency of only 0.8 and the expander an efficiency of only 0.75. This yields 
     
       
         
           
             
               η 
               th 
             
             = 
             
               
                 
                   
                     
                       ( 
                       
                         433.67 
                         - 
                         389.66 
                       
                       ) 
                     
                     * 
                     0.75 
                   
                   - 
                   
                     
                       ( 
                       
                         239.62 
                         - 
                         236.72 
                       
                       ) 
                     
                     * 
                     
                       10 
                       / 
                       8 
                     
                   
                 
                 
                   ( 
                   
                     433.67 
                     - 
                     239.62 
                   
                   ) 
                 
               
               ≈ 
               
                 15 
                 ⁢ 
                 % 
               
             
           
         
       
     
     In summary, the thermal efficiency of the ORC system using SES36 fluid according to the application is as high as 20% for the high temperature example and still at about 15% for the low temperature example. 
       FIG. 4  shows another embodiment of a generating unit  11 ′ in which a clutch  50  is provided between the generator  25  and the high-pressure expander  23  and a freewheel device is provided between the generator  25  and the low-pressure expander. 
       FIG. 5  shows an embodiment of a generating unit  11 ″ which is similar to the embodiment of  FIG. 4  but in which the low-pressure side of the low-pressure expander  24 ′ faces towards the generator. 
       FIG. 6  shows an embodiment of a generating unit  11 ′″ in which a first freewheel device  27  is provided between the generator  25  and the high-pressure expander  23  and a second freewheel device  27 ′ is provided between the generator  25  and the low-pressure expander  24 . 
       FIG. 7  shows an embodiment of a generating unit  11 ″″ which is similar to the embodiment of  FIG. 4  but in which the low-pressure side of the low-pressure expander  24 ′ faces towards the generator. 
       FIG. 8  shows an evaporator  14  for use in the embodiments of the application. The evaporator  14  comprises a plate heat exchanger  51  with a preheater portion  52  or first heat exchanger  52  and an evaporator portion  53  or second heat exchanger  53  as well as a separator chamber  54 , also referred to as liquid receiver tank  54 . The separator chamber  54  comprises a liquid outlet  55  at the bottom, a liquid inlet  56  and a vapour inlet  57  at the top. According to the embodiment of  FIG. 8 , the height of the separator chamber  54  is higher than the height of evaporator  53 . Thereby, the vapour region remains separate from the liquid region under movements of a ship. 
     The liquid outlet  55  is connected to a liquid inlet  58  at the bottom of the evaporator section  53 . The liquid inlet  56  is connected to a liquid outlet  59  at the bottom of the evaporator portion  53 . The vapour inlet  57  is connected to a vapour outlet  60  at the top of the heating portion  53 . Furthermore, the evaporator  14  comprises an inlet  43  and an outlet  44  for a heating fluid such as thermo oil, water steam or hot water. 
       FIG. 9  shows a side view of the evaporator  14  of  FIG. 8 . A work fluid in a liquid state is referred to in  FIGS. 8 and 9  as “SES 36 Liquid” and a work fluid in a coexistence region of liquid and vapour is referred to as “SES 36 vapour+ liquid”. 
     During operation of the evaporator  14 , a feeding liquid, also known as work fluid, is pre-heated by the first heat exchanger  52  up to the boiling temperature and is fed to the liquid receiver tank  54 . The work fluid is then fed to the second heat exchanger  53  and converted into vapour. The vapour is fed back to the top of the receiver tank  54  and the vapour from the top of the receiver tank  54  is supplied to the expander  23  or  24  via the outlet  21 . 
     A flow of the work fluid through the evaporator is as follows: From the first heat exchanger inlet  20  to the outlet  59  to the tank inlet  56  to the tank outlet  55  via the second heat exchanger inlet  58  and the outlet  60  to the tank inlet  57  to the tank vapour outlet  21  to an inlet of a turbine or expander. A flow of a heating oil, which is referred to as “thermo  32  oil” in  FIGS. 8 and 9 , extends from the inlet  43  via pipes of the evaporator to the outlet  44 . 
       FIG. 10  shows a condenser  12  for use in the embodiments of the application. The condenser  12  comprises a cooling fluid inlet at a lower left portion, a cooling fluid outlet at an upper right portion, a work fluid inlet  16  at a top portion and a work fluid outlet  17  at a bottom portion. 
     For use in a ship, it is advantageous if the condenser and evaporator comprise plate heat exchangers due to conditions in the ship. However, for a geothermal or other ground based power station or waste recovery system, the ORC system is not moving and other types of heat exchangers may be used as well. 
       FIG. 11  shows a side view of the condenser  12  in which the cooling fluid outlet is shown in a frontal view. In the interior of the heat exchanger, the fluid channels of the work fluid and/or of the cooling fluid may branch off into several channels which are in thermal contact with each other. Another embodiment comprises undulating channels. While many undulations and/or fluid channels improve the heat exchange, fewer undulations and/or fluid channels have a smaller flow resistance. 
       FIG. 12  shows a schematic view of an idealized Clausius-Rankine cycle of the ORC system of  FIG. 1 . In  FIG. 12 , the boundary of the vapour liquid coexistence region is indicated by a line  49 . In an isentropic expansion step  4 - 1 , the work fluid is expanded to a lower density while mechanical work is generated. In a cooling step  1 - 2  the work fluid is cooled down to the liquid phase. In the coexistence region, the heat loss is provided by removing the condensation heat such that this portion of the cooling step is isothermal. In an isentropic pumping step  2 - 3 , the temperature of the work fluid is increased isentropically. In a heating step  3 - 4  the work fluid is heated until it reaches the vapour phase. In the coexistence region, the heat increase is absorbed by the vaporization heat such that this portion of the heating step is isothermal. 
       FIG. 13  shows a second embodiment of an ORC engine, in which the outlet of the high-pressure expander  23  is connected to an inlet of a second separator chamber  54 ′ in a vapour region of the separator chamber  54 ′, and a vapour outlet of the separator chamber  54 ′ in the vapour region is connected to an inlet of the low-pressure expander  24 . A liquid outlet of the separator chamber  54 ′ in the liquid region is connected to a supply line for the feeder pump  13  at a branching point. A control valve  62  is provided between the liquid outlet of the separator chamber  54 ′ and the branching point  61 . Furthermore, a second feeder pump  13 ′ is provided between the branching point  61  and the outlet  17  of the condenser  12 . 
       FIG. 14  shows a further embodiment of a generating unit  111  in which a first spur gear  63  is provided between an output shaft of the high-pressure expander  23  and a generator shaft, an a second spur gear  64  is provided between an output shaft of the high-pressure expander  24  and the generator shaft. 
       FIG. 15  shows a further embodiment of a generating unit  111 ′ in which a first planetary gear set  65  is provided between an output shaft of the high-pressure expander  23  and a generator shaft, and a second planetary gear set  66  is provided between an output shaft of the low-pressure expander  24  and the generator shaft. 
     In the first planetary gear set  65 , a ring gear is connected to a casing of the generating unit  111 ′, a planetary carrier is connected to the output shaft of the high-pressure expander  23 , and a sun gear is connected to the generator shaft. Likewise, in the second planetary gear set  66 , a ring gear is connected to a casing of the generating unit  111 ′, a planetary carrier is connected to the output shaft of the low-pressure expander  24 , and a sun gear is connected to the generator shaft. 
       FIG. 15  shows a further embodiment of a generating unit  111 ″ in which a rotor of the generator is connected to a hollow shaft. The hollow shaft is connected to a ring gear of a planetary gear set  67 . A planetary carrier of the planetary gear set  67  is connected to an output shaft of the low-pressure expander  24  and a sun gear of the planetary gear set  67  is connected to an output shaft of the high-pressure expander  23  which passes though the hollow shaft. A brake clutch  68  is provided to fix the sun gear of the planetary gear set  67  when the high-pressure expander  23  is not in operation. 
     During operation in a high temperature configuration in which valves  34  and  35  are open and the bypass valve  33  is closed, the planetary gear is driven as an overriding gear by both the sun gear and the planetary carrier. The direction of rotation of the expanders is designed such that the rotation speed of the hollow shaft is increased. 
     During operation in a low temperature configuration in which valves  34  and  35  are closed and the bypass valve  33  is closed, the planetary gear is driven by the planetary carrier and the sun gear is fixed by the brake clutch  68 . 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Reference numbers 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 10 
                 ORC system 
               
               
                   
                 11-11″″, 
                 Generating unit 
               
               
                   
                 111-111′″ 
                   
               
               
                   
                 12 
                 condenser 
               
               
                   
                 13 
                 feeder pump 
               
               
                   
                 14 
                 evaporator 
               
               
                   
                 15 
                 work fluid outlet 
               
               
                   
                 16 
                 condenser inlet 
               
               
                   
                 17 
                 condenser outlet 
               
               
                   
                 18 
                 feeder pump inlet 
               
               
                   
                 19 
                 feeder pump outlet 
               
               
                   
                 20 
                 evaporator inlet 
               
               
                   
                 21 
                 evaporator outlet 
               
               
                   
                 22 
                 generating unit inlet 
               
               
                   
                 23 
                 high-pressure expander 
               
               
                   
                 24, 24′ 
                 low-pressure expander 
               
               
                   
                 25 
                 generator 
               
               
                   
                 27, 27′ 
                 freewheel device 
               
               
                   
                 29 
                 high-pressure supply line 
               
               
                   
                 30 
                 low-pressure supply line 
               
               
                   
                 32 
                 branching point 
               
               
                   
                 34 
                 input control valve 
               
               
                   
                 35 
                 output control valve 
               
               
                   
                 36 
                 cooling fluid source 
               
               
                   
                 37 
                 cooling fluid sink 
               
               
                   
                 38 
                 heat source 
               
               
                   
                 39 
                 cooling water channel 
               
               
                   
                 40 
                 work fluid channel 
               
               
                   
                 41 
                 heating fluid channel 
               
               
                   
                 42 
                 work fluid channel 
               
               
                   
                 43 
                 heating fluid inlet 
               
               
                   
                 44 
                 heating fluid outlet 
               
               
                   
                 49 
                 coexistence region boundary 
               
               
                   
                 50 
                 coupling/clutch 
               
               
                   
                 51 
                 plate heat exchanger 
               
               
                   
                 52 
                 preheater portion 
               
               
                   
                 53 
                 evaporator portion 
               
               
                   
                 54, 54′ 
                 separator chamber 
               
               
                   
                 55 
                 liquid outlet 
               
               
                   
                 56 
                 liquid inlet 
               
               
                   
                 57 
                 vapour outlet 
               
               
                   
                 58 
                 liquid inlet 
               
               
                   
                 59 
                 liquid outlet 
               
               
                   
                 60 
                 vapour outlet 
               
               
                   
                 61 
                 branching point 
               
               
                   
                 62 
                 control valve 
               
               
                   
                 63 
                 first spur gear 
               
               
                   
                 64 
                 second spur gear 
               
               
                   
                 65 
                 first planetary gear 
               
               
                   
                 66 
                 second planetary gear 
               
               
                   
                 67 
                 planetary gear 
               
               
                   
                 68 
                 brake clutch