SE541729C2

SE541729C2 - A device and a method for volume compensation and pressure control of a working medium in a WHR-system

Info

Publication number: SE541729C2
Application number: SE1751525A
Authority: SE
Inventors: Thomas Sederholm; Zoltan Kardos
Original assignee: Scania Cv Ab
Priority date: 2017-12-11
Filing date: 2017-12-11
Publication date: 2019-12-03
Also published as: WO2019117788A1; DE112018005818T5; SE1751525A1

Abstract

The present invention relates to a device and a method for volume compensation and pressure control of a working medium in a WHR-system. The device comprises a first volume compensation unit (12) comprising a first space (16) communicable with the WHR circuit (4), a valve arrangement (19a-c) able to connect and disconnect the first space (16) of the first volume compensation unit (12) to and from the WHR circuit and a second volume compensation unit (13) comprising a second space (24) communicable with the WHR circuit (4). The first volume compensation unit (12) is configured to receive working medium from the WHR circuit (4) during a start-up phase of the WHR system. The valve device (19a-c) is configured to disconnect the first volume compensation unit (12) and the received volume of working medium from the WHR circuit (4) when the start-up phase is completed. The second volume compensation unit (13) is, during a following regular operating phase of the WHR system, configured to adjust the volume of the second space (24) in order to provide a desired condensation pressure in the WHR circuit (4).

Description

A device and a method for volume compensation and pressure control of a working medium in a WHR-system BACKGROUND OF THE INVENTION AND PRIOR ART The present invention relates to a device and a method for volume compensation and pressure control of a working medium in a WHR-system according to the preamble of claims 1 and 15.

A WHR system (Waste Heat Recovery System) can be used in vehicles for recovering waste thermal energy and convert it to mechanical energy or electric energy. A WHR system includes a pump which pressurizes and circulates a working medium in a closed circuit. The circuit comprises one or several evaporators where the working medium is heated and evaporated by one or several heat sources such as, for example, the exhaust gases from a combustion engine. The pressurized and heated gaseous working medium is directed to an expander where it expands. The expander generates mechanical energy, which can be used to operate the vehicle or apparatuses on the vehicle. Alternatively, the expander is connected to a generator generating electric energy. The working medium leaving the expander is received in a condenser. The working medium is cooled down in the condenser to a temperature at which it condenses.

Conventional WHR systems comprise a compensation tank for volume compensation and pressure control of the working medium. There is a very large volume expansion of the working medium during a start-up phase of the WHR system. During a following regular operation phase of the WHR system, the volume of the working medium will fluctuate in the compensation tank but within a significantly more restricted range. The compensation tank may be designed with an outer rigid tank and an inner rubber bladder receiving liquid working medium from the WHR circuit of the WHR system. The volume of the rubber bladder and thus the volume of the working medium in the WHR circuit is changed by applying a varied compressed air pressure on the outside of the rubber bladder in the rigid tank. It is very difficult to obtain a fast response and an accurate control of the pressure in a WHR system due to the flexible properties of the rubber bladder. Thus, it is difficult to provide an accurate control of the condensation pressure in the WHR system, which is a requirement for an efficient operation of the WHR system. Furthermore, the frequent volume fluctuations of the working medium may damage the rubber bladder and reduce the operation time of the compensation tank.

US 2008/0141673 shows a system for power generation in Rankine cycle. The system comprises two tanks filled with two working fluids having different phase changing temperatures. Each tank comprises a plunger able to pull the respective fluid medium into and out of the cycle. An external source heats the mixture of the two fluid mediums in an evaporator. The concentration of the respective fluid mediums is varied in view of the temperature of the external source in order maintain a high efficiency of the Rankine cycle during different operation conditions.

SUMMARY OF THE INVENTION The object of the present invention is to provide a device for volume compensation and pressure control of a working medium in a WHR-system by which device it is possible to provide an accurate control of the condensation pressure and thus a continuously high efficiency of the WHR system.

The above-mentioned object is achieved by the device according to claim 1. The volume expansion of the working medium is very large during a start-up phase of the WHR system. The device comprises a first volume compensation unit configured to receive working medium from the WHR circuit during the start-up phase of the WHR system. The device comprises a valve arrangement configured to disconnect the first volume compensation unit and the received volume of working medium from the WHR circuit when the start-up phases is completed. During a following regular operating phase of the WHR system, a reduced amount of working medium is circulated in the WHR circuit. The device comprises a second volume compensation unit, which is configured to adjust the volume in the WHR circuit during the following regular operating phase of the WHR system. Since the second volume compensation unit handle a reduced amount of working medium, it is possible to achieve a faster adjustment of the volume of the working medium in the WHR circuit and maintain a desired condensation pressure in the condenser with a high accuracy during the whole regular operating phase of the WHR system.

According to an embodiment of the invention, the first space of the first volume compensation unit is able to receive a larger amount of the working medium than the second space of the second volume compensation unit. Since the volume expansion of the working medium is much larger during a start-up phase of the WHR system than the volume fluctuations of the working medium during the regular operating phase, it is possible to use a second volume compensation unit, which is significantly smaller than the first volume compensation unit.

According to an embodiment of the invention, the first space of the first volume compensation unit and the second volume compensation unit are connectable to positions in the WHR circuit where the working medium is in liquid phase. Thus, the volume compensation units receives and accumulates liquid working medium. Such positioning of the volume compensation units facilitates the control of the condensation pressure in the WHR circuit.

According to an embodiment of the invention, the second space of the second volume compensation unit is defined by rigid walls. In order to provide a control of the condensation pressure with a high accuracy, it is necessary that the second space does not include any flexible parts. The second volume compensation unit may comprise a second space defined by a cylinder and a piston, which is movably arranged to different positions into the cylinder by means of an actuator. In this case, the cylinder provides rigid static walls and the piston a movable rigid wall defining the second space of the second volume compensation space. By means of the piston and the actuator it is possible to change the volume of the second chamber in a quick and simple manner in order to maintain a desired condensation pressure during different operating conditions during the regular operating phase.

According to an embodiment of the invention, the valve arrangement comprises at least one valve member arranged in a connection line between the WHR circuit and the first space. Such an arrangement makes it easy to connect the first space to the WHR circuit during the start-up phase and disconnect the first space from the WHR circuit when the start-up phase is completed. Alternatively, the valve arrangement comprises a first valve member arranged in a first connection line between the WHR circuit and the first space, a second valve member arranged in a second connection line between the WHR circuit and the first space, and a third valve member arranged in the WHR circuit in a position between the connection point of the connection lines to the WHR circuit. During a start-up phase, the first valve member and the second valve member are moved to an open position and the third valve member to a closed position. During the start-up phase, the entire working medium flow is directed from the WHR circuit to the first space via the first connection line and from the first space to the WHR circuit via the second connection line. When the start-up phase has been completed, the first valve member and the second valve member are moved to a closed position and the third valve member to an open position. In this case, the accumulated amount of the working medium in the first space first volume will be isolated from the working medium flow in the WHR circuit.

According to an embodiment of the invention, the first volume compensation unit comprises a tank and the first space is defined by the interior of the tank. The tank may be a simple container receiving the expanding volume of the working medium during the start-up phase of the WHR circuit. In this case, it is not possible to provide an active control of the condensation pressure. Since the start-up phase is short in relation to the regular operating phase, it is not always necessary to provide an active control of the condensation pressure during the start-up phase of the WHR circuit.

According to an embodiment, the first volume compensation unit comprises a first space with an adjustable volume. The first volume compensation unit may comprise a tank and a flexible body arranged inside said tank and that the first space is defined by at least a wall portion of the flexible body. The flexible body may be a gas filled balloon which will become compressed when the amount of the working medium increases during the start-up phase. Alternatively, the volume of the first space is adjustable by supplying a gaseous fluid such as air to a space arranged on the opposite side of said wall portion of the flexible body. In this case, it is possible to provide an active control of the condensation pressure during the start-up phase of WHR circuit by a regulated supply and discharged of the air in said space.

According to a further embodiment, the first volume compensation unit comprises a first space defined by at least one cylinder and one piston, which is movably arranged to different positions in relation to the cylinder by means of an actuator. Since the first space has to have capacity to receive a relatively large amount of the working medium, it is favorable to use a plurality of cylinder/piston pairs, which together defines the first space. In this case, a common actuator may be used for providing movements of all pistons. Alternatively, one actuator may be used for each piston.

The above-mentioned object is also achieved by the method according to claim 15.

BRIEF DESCRIPTION OF THE DRAWINGS In the following preferred embodiments of the invention are described, as examples, and with reference to the attached drawings, in which: Fig. 1 shows a WHR system comprising a device according to the present invention, Fig. 2 shows a first embodiment of the device, Fig. 3 shows a second embodiment of the device, Fig. 4 shows a third embodiment of the device, Fig. 5 shows a fourth embodiment of the device, Fig. 6 shows a fifth embodiment of the device, Fig. 7 shows a sixth embodiment of the device and Fig. 8 shows a flow chart according to a method of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 shows a schematically disclosed vehicle 1 powered by a combustion engine 2. The vehicle 1 may be a heavy vehicle and the combustion engine 2 may be a diesel engine. The vehicle is provided with a WHR-system (Waste Heat Recovery system). The WHR- system comprises a pump 3, which pressurizes and circulates a working medium in a WHR circuit 4. In this case, the working medium is ethanol. However, it is possible to use other kinds of working mediums such as for example R245fa. The pump 3 directs the working medium to an evaporator 5. The working medium is heated in the evaporator 5 by exhaust gases in an exhaust line 6 from the combustion engine 2. The exhaust line 6 comprises a bypass line 6a and a valve 6b by which it is possible to direct a variable part of the exhaust gases past the evaporator 5. The exhaust line 6 may comprise further components, which are not indicated in Fig. 1 such as a turbine of a turbo charger and exhaust gas treatment components. The working medium is evaporated in the evaporator 5. The gaseous working medium is directed from the evaporator 5 to an expander 7.

The pressurized and heated working medium is expanded in the expander 7. The expander 7 may be a turbine or a piston. The expander 7 generates a rotary motion, which is transmitted, via a suitable mechanical transmission 8, to a shaft 9 of the power train of the vehicle 1. Alternatively, the expander 7 may be connected to a generator transforming mechanical energy into electrical energy. The electrical energy may be stored in a battery. After the working medium has passed through the expander 7, it is directed to a condenser 10. The gaseous working medium is cooled in the condenser 10 by coolant circulating in a cooling WHR circuit 11 to a temperature at which it condenses. The liquid working medium is sucked from the condenser 10 to the pump 3. The WHR system comprises a device comprising a first volume compensation unit 12 and a second volume compensation unit 13 for volume compensation and pressure control of a working medium in the WHR circuit 4. The second volume compensation unit 13 is considerably smaller than the first volume compensation unit 12.

Fig. 2 shows a first embodiment of the device. In this case, the device comprises a first volume compensation unit 12 in the form of a rigid tank 14 enclosing a flexible body 15, which may be a rubber bladder. The flexible body 15 defines a first space 16 of an adjustable volume for accommodation of working medium. A schematically indicated air regulating device 17a is used to regulate the air pressure in an air space 17 located on the outside of the flexible body 15 in the rigid tank 14. The volume of the first space 16 depends on the air pressure in the air space 17. The first space 16 is connected to WHR circuit 4 via a first connection line 18a and a second connection line 18b arranged in a position downstream of the first connection line 18a. The first connection line 18a comprises a first valve member 19a and the second connection line 18b comprises a second valve member 19b. A third valve member 19c is arranged in the WHR circuit 4 in a position between the connection points of the first connection line 18a and the second connection line 18b. The valve members 18a-c are comprised in a valve arrangement controlling the working medium flow between the WHR circuit 4 and the first space 16.

The second volume compensation unit 13 comprises a cylinder 22 and a piston 23, which is movably arranged in relation to the cylinder 22. The cylinder 22 and the movable piston 23 defines a second space 24 of an adjustable volume. An actuator 25 is used to provide movements of the piston 23. The actuator 25 may be a hydraulic or pneumatic activated power member or an electric motor. A control unit 26 control the movements of the actuator 25. The control unit 26 receives information from a first sensor 27a sensing the condensation pressure of the working medium. The first sensor 27a may be arranged in a position downstream of the condenser 10 and upstream of the pump 3. The control unit 26 also receives information from a second sensor 27b sensing the evaporation pressure or the evaporation temperature of the working medium in the WHR system. The second sensor 27b may be arranged in a position downstream of the evaporator 5 and upstream of the expander 7.

In order to maintain a high thermal efficiency of the WHR system, it is desired that the working medium is condensed at a pressure as low as possible in the condenser 10. In certain cases, it is suitable to avoid negative pressure in the WHR-system. In the latter case, it is desired to provide a condensation pressure just above 1 bar. Ethanol has a condensation temperature of 78°C at the condensation pressure 1 bar. In case the working medium is ethanol and negative pressures are avoided, it is suitable to accomplish a condensation temperature of just above 78°C in the condenser 10.

Before a cold start of the WHR system, the working medium has a temperature corresponding to the temperature of the environment and the entire amount of the working medium is in liquid phase. The control unit 26 initiates movements of the first valve member 19a and the second valve member 19b to an open position and a movement of the third valve member 19c to a closed position before start of the WHR system. When the pump 3 starts to circulate working medium through the WHR circuit 4, it defines the beginning of a start-up phase of the WHR system. When the valve members 19a-c are in the above mentioned positions, the working medium circulates from the WHR circuit, via the first connection line 19a to the first space and back to the WHR circuit 4 via the second connection line 19b. The working medium leaving the pump is heated in the evaporator 5 by exhaust gases from the combustion engine 2. During the start-up phase, the volume of the working medium increases due to the fact that the temperature of the working medium increases and especially due to the fact that the working medium evaporates in the evaporator 5. The control unit 26 determines a desired condensation temperature at which the WHR system has a high efficiency. The control unit 26 controls the air-regulating device 17a such that an air pressure corresponding to the desired condensation pressure is created in the air space 17 and in the first space 16 in the rigid tank 15. During the start-up phase, a relatively large volume of the expanding working medium is accumulated in the first space 16. During the start-up phase, the second volume compensation unit 13 may be in an inactivated state. The piston 23 may be in a predetermined fixed position in the cylinder 22 such the volume of the second space 24 is and zero or another constant volume during the start-up phase of the WHR circuit 4. However, it is not excluded to use the second volume compensation unit 13 for volume compensation and pressure control during the start-up phase of the WHR circuit 4.

The start-up phase of the WHR system may be defined as completed when the working medium has been heated to a temperature, which is defined as a predetermined regular operating temperature TA. The control unit 26 may, for example, receive information from the second sensor 27b about the evaporation temperature of the working medium. The end of the start-up phase may be achieved when the evaporation temperature has reached a predetermined evaporation temperature. However, it is possible to define the end of the start-up phase on other operating parameters. At the end of the start-up phase, the control unit 26 initiates movements of the first valve member 19a and the second valve member 19b to a closed position and a movement of the third valve member 19c to an open position. In these positions of the valve members 19a-c, the accumulated working medium in the first space 16 is disconnected from the WHR circuit 4 and the working medium flow in the WHR circuit 4 which flows past the first space 16.

During a following regular operating phase of the WHR system, it is possible to maintain a desired condensation temperature in the WHR system with relatively small volume fluctuations of the working medium. The control unit 26 determines a desired condensation pressure at which the WHR system has an optimal efficiency. The control unit 26 receives information from the first sensor 27a about the actual condensation pressure. In case there is a difference between the desired condensation pressure and the actual condensation pressure, the control unit 26 activates the actuator 25 such that it adjusts the volume of the second space 24 in order to eliminate this pressure difference.

At rapid heating changes of the working medium in the evaporator 5, it is necessary to have a fast response of the compensation space in the second space 24 in order to maintain a desired condensation pressure in the WHR circuit. During the regular operating phase of the WHR system, the amount of working medium circulating in the WHR circuit 4 is relatively small since a large part of the working medium is isolated in the first space 16. The response time for changing the volume in the second space 24 is, for example, related to the amount of the working medium in the WHR circuit 4. The step to isolate a relatively large part of the working medium in the first space 16 during the regular operating phase, makes it possible to change the compensation space with a faster response time and with a higher accuracy. The flexible properties of a conventional compensation tank deteriorates the ability to maintain a desired condensation pressure with a high accuracy. In view of that fact, the step to isolate the flexible first space 16 from the WHR circuit 4 increases further the ability to maintain a desired condensation pressure with a high accuracy.

Fig. 3 shows an alternative design of the first volume compensation unit 12. In this case, the first volume compensation unit 12 comprises a rigid tank 14 enclosing a flexible body 15, which may be a rubber bladder. The space between the flexible body 15 and an inner surface of the rigid tank 14 defines a first space 16 of an adjustable volume for accommodation of the working medium. The volume of the first space 16 depends on the air pressure in an air space 17, which is defined by the flexible body 15. A schematically indicated air-regulating device 17a is used to regulate the air pressure in the air space 17. Otherwise, the embodiment shown in Fig. 3 includes the same components as the embodiment in Fig. 2.

Fig. 4 shows a further alternative design of the first volume compensation unit 12. In this case, the first volume compensation unit 12 comprises a rigid tank 14 enclosing a flexible body 15 in the form of a balloon defining a space 17 filled with a gaseous medium such as air. The space 16 between the flexible body 15 and an inner surface of the rigid tank 14 has an adjustable volume for accommodation of the working medium. The volume of the first space 16 depends on the air pressure in the air space 17 of the flexible body 15. In this case, it is not possible to actively control the pressure in the first space 16 during the start-up phase of the WHR system. However, the pressure in the first chamber 16 and the condensation pressure will increase with the accommodated amount of working medium in the first chamber 16. In this case, only one connection line 18a is arranged between the WHR circuit 4 and the first space 16. A valve member 19a is arranged in the connection line 18a by which it is possible to connect or disconnect the first space 16 from the WHR circuit 4. Otherwise, the embodiment shown in Fig. 4 comprises the same components as shown in Fig. 2.

Fig. 5 shows a further alternative design of the first volume compensation unit 12. The first volume compensation unit 12 comprises a rigid tank 14 enclosing a flexible body 15, which may be a rubber bladder. The flexible body 15 defines a first space 16 of an adjustable volume for accommodation of working medium. The volume of the first space 16 depends on the air pressure in a surrounding air space 17 in the rigid tank 14. A schematically indicated air-regulating device 17a is used to regulate the air pressure in the air space 17. The first space 16 is connected to WHR circuit 4 via a connection line 18a. A valve member 19a controls the flow between the connection line 4 and the first space 16. A deaeration line 29 is arranged between a part of the connection line 18a connected to the first space 16 and a filling vessel 30. A deaeration valve 31 is arranged in the deaeration line 29. Otherwise, the embodiment shown in Fig. 5 comprises the same components as the embodiment in Fig. 2.

Fig. 6 shows a further alternative design of the first volume compensation unit 12. The first volume compensation unit 12 comprises a rigid tank 14 forming a first space 16 for accumulation of working medium. The rigid tank 14 is connected to the liquid side of the WHR circuit via a liquid connection line 18a and to the gas side of the WHR circuit 4 via a gas connection line 18d. The liquid connection line 18a is connected to the WHR circuit in a position downstream of the pump 3 and upstream of the evaporator 5. The gas connection line 18d is connected to the WHR circuit 4 in a position downstream of the evaporator 5 and upstream of the expander 7.

Alternatively, the gas connection line 18d is connected to the WHR circuit 4 in a position downstream of the expander 7 and upstream of the condenser 10. The liquid connection line 18a comprises a first liquid member 19a and the gas connection line 18d comprises a gas valve member 19d.

During a start-up phase of the WHR system a higher pressure is built up in the WHR circuit 4 than in the first space 16. When a desired pressure difference is reached, the liquid valve member 19a is moved to an open position and liquid working medium flows into the first space 16. The liquid valve member 19a is moved to the closed position when the start-up phase of the WHR system is completed. During the following regular operating phase, the working medium in the first space 16 is isolated from the working medium flow in the WHR circuit 4. During a shutdown phase of the WHR system, the gas valve member 19d is open and the liquid working medium in the first space will be replaced by vaporized working medium. As the working medium in the first space 16 cools down, its pressure will decrease. The second adjusting device 13 has the same design as the second adjusting devices in the embodiments above.

Fig. 7 shows a further alternative design of the first volume compensation unit 12. In this case, the first volume compensation unit 12 comprises a plurality of cylinders 321-nwhich are movably arranged in a respective cylinder 331-n. Each cylinder 321-nand piston 331-npair defines a compensation space 341-nof an adjustable volume, which is connected to a first connection line 18a via a closed circuit 18ai. A first valve member 19a is arranged in the first connection line 18a by which it is possible to connect or disconnect the compensation spaces to the WHR circuit 4. The compensation spaces 341-nform together the first space 16 for the first volume compensation unit 12. An actuator 351-nis used to provide movements of the respective piston 331-nin relation the cylinders 321-n. The actuators 351-nmay be hydraulic or pneumatic activated power members or electric motors. The control unit 26 control the actuators 351-nindividually or as a group. It is possible to use one actuator for all pistons 351-n. In the latter case, all pistons 351-nmay provide equal movements.

Fig. 8 shows a flow chart defining a method for volume compensation and pressure control of a working medium in a WHR-system. At step 40, the method starts. At step 41, the pump 3 starts the circulation of the working medium through the WHR circuit 4. The circulating working medium is heated in the evaporator 5 by exhaust gases from the combustion engine 2. At step 42, the first space 16 is connected to the WHR circuit 4. At least the valve members 19a, which are arranged in the connection lines 18a between the first space 16 of the first volume compensation unit 12 and WHR circuit 4, are moved to an open position. The working medium flow is directed through or in communication with the first space 16. At step 43, working medium is accumulated in the first space 16 of the first volume compensation unit 12. The volume of the working medium is expanded in the WHR circuit 4 due to the heating and evaporation of the working medium in the evaporator 5. The volume of the working medium may increase significantly during a start-up phase of a WHR system.

At step 44, it is evaluated if the start-up phase of the WHR system has been completed. The end of a start-up phase can be defined in different ways. It may, for example, be defined when the working medium has been heated to a regular operating temperature in the WHR circuit. If the start-up phase is not completed, the method returns to step 43. If the start-up phase is completed, the method continues at step 45. At step 45, the accumulated working medium in the first space 16 is disconnected from the WHR circuit 4. The valve members 19a, b which are arranged in the connection lines 18a, b between the first space 16 and WHR circuit 4, are moved to a closed position such that the flow communication between the first space 16 and the WHR circuit 4 in interrupted. At step 46, the volume of the second space 24 is varied in order to maintain a desired condensation pressure during a following regular operating phase of the WHR system.

The invention is not restricted to the described embodiment but may be varied freely within the scope of the claims.

Claims

1. A device for volume compensation and pressure control of a working medium in a WHR-system, wherein the WHR system comprises a WHR circuit (4) including an evaporator (5), an expander (7), a condenser (10) and a pump (3), and wherein the device comprises a first volume compensation unit (12) comprising a first space (16) communicable with the WHR circuit (4), characterized in that the device comprises a valve arrangement (19a-c) able to connect and disconnect the first space (16) of the first volume compensation unit (12) to and from the WHR circuit (4) and a second volume compensation unit (13) comprising a second space (24) communicable with the WHR circuit (4), that the first volume compensation unit (12) is configured to receive working medium from the WHR circuit (4) during a start-up phase of the WHR system, that the valve device (19a-c) is configured to disconnect the first volume compensation unit (12) and the received volume of working medium from the WHR circuit (4) when the start-up phase is completed, and that the second volume compensation unit (13) is, during a following regular operating phase of the WHR system, configured to adjust the volume of the second space (24) in order to provide a desired condensation pressure in the WHR circuit (4).

2. A device according to claim 1, characterized in that the first space (16) of the first volume compensation unit (12) is able to receive a larger amount of the working medium than the second space (24) of the second volume compensation unit (13).

3. A device according to claim 1 or 2, characterized in that the first space (16) of the first volume compensation unit (12) and the second volume compensation unit (13) are connectable to positions in the WHR circuit (4) where the working medium is in a liquid state.

4. A device according to any one of the preceding claims, characterized in that the second space (24) of the second volume compensation unit (13) is defined by rigid walls.

5. A device according to claim 4, characterized in that the second volume compensation unit (13) comprises a second space (24) defined by a cylinder (22) and a piston (23), which is movably arranged to different positions into the cylinder (22) by means of an actuator (25).

6. A device according to any one of the preceding claims, characterized in that the valve arrangement comprises at least one valve member (19a, b) arranged in a connection line (18a, b) between the WHR circuit (4) and the first space (16).

7. A device according claim 6, characterized in that the valve arrangement comprises a first valve member (19a) arranged in a first connection line (18a) between the WHR circuit (4) and the first space (16), and a second valve member (19b) arranged in a second connection line (18b) between the WHR circuit (4) and the first space (16) and a third valve member (19c) arranged in the WHR circuit (4) in a position between the connection point of the connection lines (18a, 18b) to the WHR circuit (4).

8. A device according to any one of the preceding claims, characterized in that the first volume compensation unit (12) comprises a tank (14) and that the first space (16) is defined by the interior of the tank (14).

9. A device according to any one of the preceding claims 1-7, characterized in that the first volume compensation unit (12) comprises a first space (16) with an adjustable volume.

10. A device according to claim 9, characterized in that the first volume compensation unit (12) comprises a tank (14) and a flexible body (15) arranged inside said tank (15) and that the first space (16) is defined by at least a wall portion of the flexible body.

11. 1 1. A device according to claim 9, characterized in that the volume of the first space (16) is adjustable by supplying a fluid to a fluid space (17a) arranged on the opposite side of said wall portion of the flexible body (16) which defines the inner space (16).

12. A device according to claim 9, characterized in that the first volume compensation unit (12) comprises a first space (24) at least partly defined in at least one cylinder (32) by means of a piston (33), which is movably arranged to different positions in the cylinder (32) by means of an actuator (35).

13. A WHR system, characterized in that it comprises a device according to any one of the preceding claims 1-12.

14. A vehicle, characterized in that it comprises a WHR system according to claim 13.

15. A method for controlling pressure and volume of a working medium in a WHR-system, wherein the WHR system comprises a WHR circuit (4) including a pump (3), an evaporator (5), an expander (7) and a condenser (10), a first volume compensation unit (12) comprising a first space (16) communicable with the WHR circuit (4) and a second volume compensation unit (13) comprising a second space (24) which is communicable with the WHR circuit (4), characterized by the steps of connecting the first space (16) of the first volume compensation unit (12) to the WHR circuit during a start-up phase of the WHR system such that an expanding volume of the working medium in the WHR circuit (4) is received into the first space (16), disconnecting the first volume compensation unit (12) with the received working medium from the WHR circuit (4) when the start-up phase of the WHR system is completed, and adjusting the volume of the second space (24) of the second the volume compensation unit (13), during a following regular operating phase of the WHR system, in order to provide a desired condensation pressure in the WHR circuit (4).