Arrangement and method for starting operation of a thermodynamic system

Abstract

An arrangement for starting operation of a thermodynamic system (100) comprising an evaporator (1) in the form of a plate heat exchanger (15), a condenser (2), a turbine (3), a generator (4) and a fluid pump (5), wherein the fluid pump (5) is arranged to circulate a working fluid in the thermodynamic system (100) to alternate between a liquid phase and a gaseous phase, wherein the arrangement comprises a pipe (30) adapted to be connected in fluid communication with the fluid pump (5) and to be mounted in the upper port (21a) of the plate heat exchanger (20) to introduce liquid working fluid therein An associated method is also disclosed.

Description

DESCRIPTIONTitle of Invention:ARRANGEMENT AND METHOD FOR STARTING OPERATION OF A THERMODYNAMIC SYSTEMTechnical Field

[0001] The invention relates to an arrangement and method for starting operation of a thermodynamic system arranged to convert heat to electrical energy.Background Art

[0002] Systems for converting waste heat into electricity using a thermodynamic cycle such as the organic Rankine cycle (ORC), Kalina cycle, Carbon Carrier cycle and / or Carnot cycle are known from WO 2012 / 128715 Al and WO 2013 / 045021 A2. The thermodynamic cycle comprises a working fluid in liquid state which is heated by the waste heat until it is converted into a gas which then enters a turbine to perform work, e.g. generate electricity. The working fluid is then condensed to liquid form before being pumped back to exchange heat with the heat source. These solutions provide an efficient way of recovering waste heat from industrial processes or other sources of heat which would otherwise be dissipated to the surroundings.

[0003] When cold starting such thermodynamic systems, the piping and turbine will be cold. The turbine may also contain liquid working fluid. Therefore, the piping and turbine must be pre-heated before normal operating conditions are attained, preferably in a simple and inexpensive way. One way of pre-heating is by using electrical heaters, but this is expensive and inefficient as it is difficult to apply electrical heating in a uniform manner in the thermodynamic system. Another solution is to start the gas process by filling the evaporator with working fluid, which is heated to evaporation, and then run the hot gas through the thermodynamic system, possibly in combination with a pony motor or starter to rotate the turbine.

[0004] In the case of evaporators in the form of a plate heat exchanger, the thermodynamic system pumps liquid working fluid into the lower port, which will first be filled with working fluid. When the lower port is filled, the working fluid will float up towards the active / heated surfaces between the plates of the plate heat exchanger. Part of the liquid working fluid meets the hot surface and evaporates. This evaporation creates a smallincrease in pressure locally which creates a small pressure surge in all directions. The small pressure surge into the lower port leads to a large amount of working fluid being pushed up into the hot channels between the plates. This larger amount evaporates quickly and creates a rapid pressure surge with gas into the upper and lower ports. The gaseous working fluid will implode and create even larger surges.

[0005] In summary, when starting to pump working fluid to the evaporator, the plate heat exchanger causes large pressure surges that could destroy both pumps and the turbine. The pressure surges can also create pressure waves into the hot water system and cause other problems. As a result, it is difficult to achieve smooth initial evaporation of the working fluid which is a problem in all ORC systems and other evaporation systems.

[0006] Another important aspect during startup is to have a controlled flow of gas through the turbine, since the gas can contain droplets. Otherwise, there is a high risk of damaging the turbine.

[0007] Thus, improved solutions are needed for overcoming the disadvantages associated with the known thermodynamic systems.Summary of Invention

[0008] An object of the present invention is to provide an improved apparatus and method for overcoming all or some of the disadvantages and problems described above in connection with the state of the art.

[0009] This object is achieved by the present invention, wherein in a first aspect there is provided an arrangement for starting operation of a thermodynamic system comprising an evaporator in the form of a plate heat exchanger, a condenser, a turbine, a generator and a fluid pump, wherein the fluid pump is arranged to circulate a working fluid in the thermodynamic system to alternate between a liquid phase and a gaseous phase, wherein the plate heat exchanger comprises a lower port in fluid communication with the fluid pump via a first valve for ingress of liquid working fluid and an upper port in fluid communication with the turbine via a second valve for egress of gaseous working fluid, wherein the arrangement comprises a pipe adapted to be connected in fluid communication with the fluid pump and to be mounted in the upper port of the plate heat exchanger to introduce liquid working fluid therein, wherein a diameter of the pipe is smaller than a diameter of the upper port, and a third valve for controlling introduction of working fluid into the upper port through the pipe.

[0010] The pipe enables introduction of small amounts of liquid working fluid in the upper port which leads to an even and controlled evaporation of the liquid working fluid to prevent violent pressure surges in the evaporator as well as the piping and the turbine of the thermodynamic system. Formation of collapsing and expanding pockets of gaseous working fluid surrounded by liquid working fluid is thereby avoided. The smaller diameter of the pipe enables improved control of the amount of liquid working fluid Moreover, the pipe does not interfere with or impede the flow of evaporated working fluid through and out of the upper port as well as the connected conduit.

[0011] In one embodiment, the pipe comprises at least one nozzle adapted to spray the liquid working fluid. Spraying the liquid working fluid allows for a fine and even distribution of a layer of liquid working fluid in the upper port to further improve control of evaporation.

[0012] In one embodiment, the pipe is dimensioned to extend into the upper port. This enables introduction of liquid working fluid at a selected position in the plate heat exchanger, whereby optimal conditions for evaporation may be attained, e.g. in dependence of the temperature of the incoming hot source fluid.

[0013] In one embodiment, the pipe comprises an insulating cover. The cover prevents condensation as hot evaporated working fluid comes into contact with the outer surface of the pipe which contains relatively colder liquid working fluid. Condensation leads to undesired formation of droplets which could reach the turbine and damage the impeller.

[0014] In a second aspect of the present disclosure, there is provided a thermodynamic system comprising an evaporator in the form of a plate heat exchanger, a condenser, a turbine, a generator and a fluid pump, wherein the fluid pump is arranged to circulate a working fluid in the thermodynamic system to alternate between a liquid phase and a gaseous phase, wherein the plate heat exchanger comprises a lower port in fluid communication with the fluid pump via a first valve for ingress of liquid working fluid and an upper port in fluid communication with the turbine via a second valve for egress of gaseous working fluid; wherein the thermodynamic system further comprises an arrangement according to the first aspect.

[0015] In one embodiment, the at least one nozzle is positioned at a substantially central position between end plates of the plate heat exchanger.

[0016] In a third aspect of the present disclosure, there is provided a use of an arrangement according to the first aspect in a thermodynamic system comprising an evaporator in the form of a plate heat exchanger, a condenser, a turbine, a generator and a fluid pump, wherein the fluid pump is arranged to circulate a working fluid in the thermodynamic system to alternate between a liquid phase and a gaseous phase, wherein the plate heat exchanger comprises a lower port in fluid communication with the fluid pump via a first valve for ingress of liquid working fluid and an upper port in fluid communication with the turbine via a second valve for egress of gaseous working fluid, wherein the arrangement is used to introduce liquid working fluid in the upper port to start operation of the thermodynamic system.

[0017] In a fourth aspect of the present disclosure, there is provided a method for starting operation of a thermodynamic system, the thermodynamic system comprising an evaporator in the form of a plate heat exchanger, a condenser, a turbine, a generator and a fluid pump, wherein the fluid pump is arranged to circulate a working fluid in the thermodynamic system to alternate between a liquid phase and a gaseous phase, wherein the plate heat exchanger comprises a lower port in fluid communication with the fluid pump for ingress of liquid working fluid and an upper port in fluid communication with the turbine for egress of gaseous working fluid, wherein the method comprises: introducing liquid working fluid into the upper port of the plate heat exchanger by means of a pipe mounted in the upper port and arranged in fluid communication with the fluid pump, whilst preventing introduction of liquid working fluid into the lower port by closing a first valve in a main conduit between the fluid pump and the lower port; whereby the liquid working fluid is directed in flow channels between the plates of the plate heat exchanger to flow from the upper port toward the lower port whereby at least a portion of the working fluid evaporates; whereby the evaporated working fluid is directed through the flow channels to flow from the lower port toward the upper port; and directing the evaporated working fluid from the upper port to the turbine.

[0018] The introduction of liquid working fluid in small amounts in the upper port enables an even and controlled evaporation of the liquid working fluid to prevent violent pressure surges in the evaporator as well as the piping and the turbine of the thermodynamic system. Formation of collapsing and expanding pockets of gaseous working fluid surrounded by liquid working fluid is thereby avoided.

[0019] In one embodiment, the liquid working fluid is introduced at a predetermined position in the upper port, preferably at a substantially central position between end plates of the plate heat exchanger. By introducing the liquid working fluid at a selected position, optimal conditions for evaporation may be attained, e.g. in dependence of the temperature of the incoming hot source fluid.

[0020] In one embodiment, the liquid working fluid is sprayed into the upper port. Spraying the liquid working fluid allows for a fine and even distribution of a layer of liquid working fluid in the upper port to further improve control of evaporation.

[0021] In one embodiment, the method further comprises: closing an inlet valve of the turbine to prevent ingress of working fluid from the plate heat exchanger; synchronising the generator to an electrical grid; and opening the inlet valve of the turbine to allow ingress of evaporated working fluid from the plate heat exchanger to cause the turbine to start rotating, wherein the rotation of the turbine causes rotation of a rotor of the generator. The methods allows for build-up of pressure of the working fluid in the evaporator while the generator is synchronised to the grid in order to enable transfer of power. The synchronisation may be carried out using a pony motor or process gas to rotate the turbine.

[0022] In one embodiment, the method further comprises: measuring a pressure of the working fluid and a temperature of incoming hot source fluid in the plate heat exchanger; calculating a vapour pressure of the hot source fluid based on the measured temperature; wherein the synchronisation of the generator is initiated when the measured pressure of the working fluid exceeds a predetermined threshold relative to the calculated vapour pressure of the hot source fluid. By monitoring the pressure of the working fluid in the evaporator, synchronisation is initiated at an optimal time to ensure that conditions for power production by the turbine are attained when synchronisation is completed.

[0023] In one embodiment, the method further comprises starting introduction of liquid working fluid into the lower port and stopping introduction of liquid working fluid into the upper port. When conditions for power production are attained, the supply of working fluid is switched from the lower port to the lower port which has a higher capacity.Brief Description of Drawings

[0024] The invention is now described, by way of example, with reference to the accompanying drawings, in whichFig. 1 shows a schematic view of a thermodynamic system according to one embodiment of the present disclosure;Fig. 2 shows a perspective view of an evaporator in a thermodynamic system according to one embodiment of the present disclosure;Fig. 3 shows a cross-sectional view of a plate heat exchanger according to one embodiment of the present disclosure.Description of Embodiments

[0025] In the following, a detailed description of a method for starting operation of a thermodynamic system according to the present disclosure is presented. In the drawing figures, like reference numerals designate identical or corresponding elements throughout the several figures. It will be appreciated that these figures are for illustration only and are not in any way restricting the scope of the invention.

[0026] Referring to Fig. 1, there is illustrated an exemplary system for operating a thermodynamic cycle to convert heat to electrical energy according to the present disclosure in a schematic view. The cycle can be an organic Rankine cycle (ORC), a Kalina cycle, a Carbon Carrier cycle and / or a Carnot cycle. The thermodynamic system 100 comprises an evaporator 1, a condenser 2, a turbine 3, a generator 4 and a fluid pump 5. The fluid pump 5 is arranged downstream of the condenser 2 and configured to circulate a working fluid in the thermodynamic system 100. After exiting the condenser 2, the working fluid in liquid form is directed to the evaporator 1, via a main conduit with a first valve 721, where it undergoes a phase change to gaseous form at high pressure as a result of heat exchange with an external heat source HS, typically in the form of hot water. The gaseous working fluid is then directed to the turbine 3 to cause rotation thereof which is transferred to the generator 4 to be converted to electrical energy. After passing through the turbine 3, the working fluid is returned to the condenser 2 where it undergoes a phase change back to liquid form through cooling provided by an external cold source CS.

[0027] As shown in Fig. 1, the thermodynamic system 100 comprises an additional conduit branch from the fluid pump 5 to the evaporator 1 with a third valve 155. The main conduit is in fluid communication with a lower part of the evaporator 1, whereas theadditional conduit is in fluid communication with an upper part of the evaporator 1, as will be further elaborated below. The additional conduit may be of a smaller diameter than the main conduit, preferably approximately 20% (15-30%) of the diameter of the main conduit. Furthermore, the thermodynamic system 100 comprises a temperature sensor 6 and a pressure sensor 7 at the inlet to the fluid pump 5, a temperature sensor 8 at the inlet to the turbine 3, a pressure sensor 9 at the working fluid outlet from the evaporator 1, and a temperature sensor10 at the hot source fluid inlet into the evaporator 1. Preferably, the temperature sensor 10 is arranged upstream of the hot source fluid valve which when opened allows e.g. hot water to flow through the evaporator 1 to heat the working fluid.

[0028] Referring now to Fig. 2, there is shown a perspective view of an evaporator 1 according to one embodiment of the present disclosure. The evaporator 1 is configured as a plate heat exchanger 15 with a plurality of plates 20 stacked together and clamped in a frame11 comprising a pair of end plates 12. The end plates 12 have a thickness considerably bigger than the heat exchanger plates 20 in order to provide a sturdy frame 11. Each plate typically comprises four portholes which are aligned with corresponding portholes of other plates when the plates are stacked to form corresponding through-going ports or passages 21a, 21b; 22a, 22b. The ports thus extend in a longitudinal direction of the plate heat exchanger, perpendicular to the plates 20. The plates 20 are stacked and clamped together arranged in an alternating pattern wherein the pairs of portholes located within and outside a gasket sealed area, respectively, alternate between plates. The resulting plate heat exchanger 15 exhibits parallel flow channels 23, 24 for the working fluid and the hot source fluid, respectively, in alternating spaces between the plates 20, as shown in Fig. 3. One example of a plate heat exchanger which may be used in conjunction with the thermodynamic system of the present disclosure is described in WO 2019 / 226092 Al, which is hereby incorporated by reference.

[0029] Referring now to Fig. 3, there is shown a cross-sectional view of the plate heat exchanger 15 taken along line A-A in Fig. 2. The flow channels 23 provide fluid communication between the upper and lower ports 21a, 22a for passage of working fluid. The flow channels 24 provide fluid communication between the upper and lower ports 21b, 22b for passage of hot source fluid (e.g. hot water), whereas fluid communication between the flow channels 24 and the upper and lower ports 21a, 22a is blocked by means of gaskets arranged around the portholes of the plates 20. The lower port 22a is in fluid communication with the fluid pump 5 via the main conduit and the first valve 721 for ingress of liquid working fluid,and the upper port 21a in fluid communication with the turbine 3 via a second valve 822 for egress of gaseous working fluid.

[0030] In the upper port 21a, there is mounted a pipe 30. The pipe 30 is arranged in fluid communication with the fluid pump 5 through the additional conduit via a third valve 155 for introduction of working fluid into the plate heat exchanger 15. The pipe 30 may be dimensioned such that it extends into the upper port 21a to supply working fluid. The diameter of the pipe 30 is smaller than the diameter of the upper port 21 a as well as the process conduits which direct working fluid through the thermodynamic system 100. Consequently, the pipe 30 enables supply of liquid working fluid to the plate heat exchanger 15 in a smaller amount and in a more controlled way than what is possible through the main conduit which has a larger diameter. In one embodiment, the pipe 30 comprises one or more nozzles 35, preferably near a free end of the pipe 30, arranged to spray the liquid working fluid into the upper port 21a. In one embodiment, the pipe 30 is positioned in such a way that the liquid working fluid is introduced at a substantially central position between the end plates 12 of the plate heat exchanger 15. Other positions along the length of the upper port 21a are also possible.

[0031] In one embodiment, the position of the pipe 30 may be adjusted to arrange the nozzle(s) 35 in a desired position in the upper port 21a and thereby control the location where working fluid is introduced. The location may be selected e.g. in dependence of the temperature of the incoming hot source fluid to achieve optimal conditions for evaporation. The pipe 30 may be arranged at or near the outlet of the upper port 21a, extending through the wall of the conduit from the plate heat exchanger 15 to the turbine, as shown in Fig. 3. Alternatively, the pipe 30 may be arranged at the opposite end of the upper port 21a, extending through a blind flange. Furthermore, the pipe 30 may be provided with an insulating cover (not shown) to prevent condensation as hot evaporated working fluid comes into contact with the outer surface of the pipe 30 which contains relatively colder liquid working fluid.

[0032] The method according to the present disclosure will now be explained with reference to Fig. 3. At startup of the thermodynamic system 100, i.e. before the fluid pump 5 is operated to circulate working fluid, the plate heat exchanger 15 is substantially empty of working fluid since the hot source fluid passing through the plate heat exchanger 15 in the flow channels 24 will have evaporated all working fluid without new supply thereof. In a first step, liquid working fluid is introduced into the upper port 21a by means of the pipe 30 arranged in fluid communication with the fluid pump 5. At this time, the first valve 721 in the main conduitis closed and the third valve 155 in the additional conduit is open, so that that working fluid is only introduced in the upper port 21a of the plate heat exchanger 15.

[0033] Next, the liquid working fluid flows downward through one or more flow channels 23 from the upper port 21a toward the lower port 22a, whereby at least a portion of the working fluid evaporates upon contact with the hot surfaces of the plates 20. Since there is not a large amount of liquid working fluid in the plate heat exchanger 15 which can be pushed into the flow channels 23 from below by expanding evaporated working fluid, the risk of pressure surges is greatly reduced or even eliminated. Instead, the evaporated working fluid will be pressed down toward the lower port 22a by the liquid working fluid continuously supplied from above.

[0034] Next, the evaporated working fluid is directed from the lower port 22a toward the upper port 21a. After some time, the continued supply of liquid working fluid from the pipe 30 in the upper port 21a will block the passage of the evaporated gaseous working fluid through the channels 23. Instead, the gaseous working fluid will flow laterally in the lower port 22a, as indicated by the arrows in Fig. 3. Upon reaching empty neighbouring flow channels 23, where no liquid working fluid is present, the gaseous working fluid will flow substantially unobstructed towards the upper port 21a through these empty neighbouring flow channels 23.

[0035] Next, the evaporated gaseous working fluid is directed from the upper port 21a toward the turbine 3, via the second valve 822, thereby heating the piping and turbine 3 of the thermodynamic system 100 and allowing for a controlled flow of gaseous working fluid. By means of the method, an even evaporation of the working fluid is achieved at startup which reduces the risk of pressure surges in the evaporator 1 as well as the piping and the turbine 3 of the thermodynamic system 100.

[0036] In one embodiment, the fluid pump 5 is operated at a low fluid flow rate which is insufficient to rotate the turbine 3, i.e., the internal friction in the turbine 3 is higher than the pressure provided from the fluid pump 5. This serves to drain the thermodynamic system 100 to remove any droplets of working fluid which could damage the impeller in the turbine 3. The temperature sensor 8 at the inlet to the turbine 3 is used to monitor the temperature during the drainage phase. When a predetermined temperature at the inlet to the turbine 3 is reached, e.g. above the evaporation temperature of the working fluid, one can be certain that there are no more droplets and drainage is complete.

[0037] When drainage is complete, the second valve 822 is closed and pressure is allowed to build up in the evaporator 1 through continued supply of liquid working fluid in the upper port 21a through the pipe 30. The pressure sensor 9 and temperature sensor 10 in the evaporator 1 are used to monitor the pressure of the working fluid and temperature of the hot source fluid. Introduction of working fluid is continued until a predetermined pressure in the working fluid is reached in relation to the monitored temperature of the hot source fluid. Preferably, the pressure is built up to a level which is lower than the vapour pressure of the hot source fluid at the monitored temperature by a safety margin.

[0038] When the predetermined pressure of the working fluid in the evaporator is attained, synchronisation of the generator 4 to the electrical grid is initiated. This may be done with a pony motor which rotates the rotor of the generator 4 to match the frequency and phase and voltage to the electrical grid in order to be able to transfer power. The synchronisation may also be carried out by using process gas to rotate the turbine 3.

[0039] Once the generator is synchronised to the electrical grid, power production may be started. To this end, the second valve 822 is opened to allow flow of gaseous working fluid to the turbine 3. At this point, the rate of fluid flow from the fluid pump 5 is reduced before the first valve 721 of the main conduit is opened and the third valve 155 of the additional conduit is closed so that the working fluid now is introduced only into the lower port 22a of the plate heat exchanger 15. Thereafter, the fluid flow is increased to optimise power production.

[0040] The method according to the present disclosure may be carried out by means of a control unit being operatively connected to the valves 721, 822, 155 to control opening and closing of the valves 721, 822, 155. The control unit may further be arranged to receive measurements of temperature and pressure from the sensors 6-10 and to calculate the vapour pressure of the hot source fluid based thereon. Further, the control unit may be arranged to initiate the synchronisation of the generator. To this end, the control unit comprises a processor and a memory.

[0041] The control unit may further comprise an interface, which may be considered to comprise conventional means for communication with other units or devices. Instructions executable by the processor may be arranged as a computer program stored e.g. in the memory.

[0042] The computer program may comprise computer readable code means, which when run in the control unit causes the control unit to perform the steps de-scribed in method below.The computer program may be carried by a computer program product connectable to the processor. The computer program product may be the memory. The memory may be realized as for example a RAM (Random-access memory), ROM (Read-Only Memory) or an EEPROM (Electrical Erasable Programmable ROM). Further, the computer program may be carried by a separate computer-readable medium, such as a CD, DVD or flash memory, from which the program could be downloaded into the memory. Alternatively, the computer program may be stored on a server, or any other entity connected or connectable to the control unit via the interface. The computer program may then be downloaded from the server into the memory.

[0043] Preferred embodiments of an arrangement and method for starting operation of a thermodynamic system have been disclosed above. However, a person skilled in the art realises that this can be varied within the scope of the appended claims without departing from the inventive idea.

[0044] All the described alternative embodiments above or parts of an embodiment can be freely combined or employed separately from each other without departing from the inventive idea as long as the combination is not contradictory.

Claims

CLAIMS1. An arrangement for starting operation of a thermodynamic system (100) comprising an evaporator (1) in the form of a plate heat exchanger (15), a condenser (2), a turbine (3), a generator (4) and a fluid pump (5), wherein the fluid pump (5) is arranged to circulate a working fluid in the thermodynamic system (100) to alternate between a liquid phase and a gaseous phase, wherein the plate heat exchanger (15) comprises a lower port (22a) in fluid communication with the fluid pump (5) via a first valve (721) for ingress of liquid working fluid and an upper port (21a) in fluid communication with the turbine (3) via a second valve (822) for egress of gaseous working fluid, wherein the arrangement comprises: a pipe (30) adapted to be connected in fluid communication with the fluid pump (5) and to be mounted in the upper port (21a) of the plate heat exchanger (20) to introduce liquid working fluid therein, wherein a diameter of the pipe (30) is smaller than a diameter of the upper port (21a); and a third valve (155) for controlling introduction of working fluid into the upper port (21a) through the pipe (30).

2. The arrangement according to claim 1, wherein the pipe (30) comprises at least one nozzle (35) adapted to spray the liquid working fluid.

3. The arrangement according to claim 1 or 2, wherein the pipe (30) is dimensioned to extend into the upper port (21a).

4. The arrangement according to any one of the preceding claims, wherein the pipe (30) comprises an insulating cover.

5. A thermodynamic system (100) comprising an evaporator (1) in the form of a plate heat exchanger (15), a condenser (2), a turbine (3), a generator (4) and a fluid pump (5), wherein the fluid pump (5) is arranged to circulate a working fluid in the thermodynamic system (100) to alternate between a liquid phase and a gaseous phase, wherein the plate heat exchanger (15) comprises a lower port (22a) in fluid communication with the fluid pump (5) via a first valve (721) for ingress of liquid working fluid and an upper port (21a) in fluid communication with the turbine (3) via a second valve (822) for egress of gaseous working fluid;wherein the thermodynamic system further comprises an arrangement according to any one of the preceding claims mounted in the upper port (21a) to introduce liquid working fluid therein.

6. The thermodynamic system according to claim 5, wherein the pipe (30) comprises at least one nozzle (35) adapted to spray the liquid working fluid.

7. The thermodynamic system according to claim 6, wherein the pipe (30) is mounted in such a way that it extends into the upper port (21a).

8. The thermodynamic system according to claim 7, wherein the at least one nozzle (35) is positioned at a substantially central position between end plates (12) of the plate heat exchanger (20).

9. Use of an arrangement according to any one of the claims 1-4 in a thermodynamic system (100) comprising an evaporator (1) in the form of a plate heat exchanger (15), a condenser (2), a turbine (3), a generator (4) and a fluid pump (5), wherein the fluid pump (5) is arranged to circulate a working fluid in the thermodynamic system (100) to alternate between a liquid phase and a gaseous phase, wherein the plate heat exchanger (15) comprises a lower port (22a) in fluid communication with the fluid pump (5) via a first valve (721) for ingress of liquid working fluid and an upper port (21a) in fluid communication with the turbine (3) via a second valve (822) for egress of gaseous working fluid, wherein the arrangement is used to introduce liquid working fluid in the upper port (21a) to start operation of the thermodynamic system (100).

10. A method for starting operation of a thermodynamic system (100), the thermodynamic system comprising an evaporator (1) in the form of a plate heat exchanger (15), a condenser (2), a turbine (3), a generator (4) and a fluid pump (5), wherein the fluid pump (5) is arranged to circulate a working fluid in the thermodynamic system (100) to alternate between a liquid phase and a gaseous phase, wherein the plate heat exchanger (15) comprises a lower port (22a) in fluid communication with the fluid pump (5) for ingress of liquid working fluid and an upper port (21a) in fluid communication with the turbine (3) for egress of gaseous working fluid, wherein the method comprises: introducing liquid working fluid into the upper port (21a) of the plate heat exchanger (15) by means of a pipe (30) mounted in the upper port (21a) and arranged in fluid communicationwith the fluid pump (5), whilst preventing introduction of liquid working fluid into the lower port (22a) by closing a first valve (721) in a main conduit between the fluid pump (5) and the lower port (22a); whereby the liquid working fluid is directed in flow channels (23) between the plates (20) of the plate heat exchanger to flow from the upper port (21a) toward the lower port (22a) whereby at least a portion of the working fluid evaporates; whereby the evaporated working fluid is directed through the flow channels (23) to flow from the lower port (22a) toward the upper port (21a); and directing the evaporated working fluid from the upper port (21a) to the turbine (3).

11. The method according to claim 10, wherein the liquid working fluid is introduced at a predetermined position in the upper port (21a), preferably at a substantially central position between end plates (12) of the plate heat exchanger (15).

12. The method according to claim 10 or 11, wherein the liquid working fluid is sprayed into the upper port (21a).

13. The method according to any one of claims 10-12, further comprising: closing a second valve (822) for egress of gaseous working fluid from the plate heat exchanger (15) to prevent ingress of working fluid into the turbine (3); synchronising the generator (4) to an electrical grid; opening the second valve (822) to allow egress of evaporated working fluid from the plate heat exchanger (15) to cause the turbine (3) to start rotating, wherein the rotation of the turbine (3) causes rotation of a rotor of the generator (4).

14. The method according to claim 13, further comprising: measuring a pressure of the working fluid and a temperature of incoming hot source fluid in the plate heat exchanger (15); calculating a vapour pressure of the hot source fluid based on the measured temperature; wherein the synchronisation of the generator (4) is initiated when the measured pressure of the working fluid exceeds a predetermined threshold relative to the calculated vapour pressure of the hot source fluid.

15. The method according to claim 14, further comprising starting introduction of liquid working fluid into the lower port (22a) and stopping introduction of liquid working fluid into the upper port (21a).

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