SE541700C2

SE541700C2 - An arrangement and a method for controlling of a WHR system

Info

Publication number: SE541700C2
Application number: SE1850071A
Authority: SE
Inventors: Thomas Hällqvist; Zoltan Kardos
Original assignee: Scania Cv Ab
Priority date: 2018-01-24
Filing date: 2018-01-24
Publication date: 2019-11-26
Also published as: DE112019000288T5; SE1850071A1; WO2019147179A1

Abstract

The present invention relates to an arrangement and a method for controlling a WHR system arranged in a vehicle powered by a combustion engine (2), The WHR system (17) comprises an evaporator (19) in which a working fluid is heated and evaporated by exhaust gases from the combustion engine (2) and an expander (20) mechanically connected to a power train (22) of the vehicle (1). The vehicle (1) comprises a system (4, 28, 32) with a circulating medium which is heated during at least some operating conditions of the vehicle (1). The arrangement comprises a control unit (12) configured to determine when the exhaust gases are unable to evaporate the working fluid in the evaporator (19), to determine if said medium has a higher temperature than the working fluid, and if these two conditions are met to direct medium from the system to a heat exchanger (27, 30, 37) in which the medium heats the working fluid in the WHR system when the exhaust gases are unable to evaporate the working fluid in the evaporator (19).

Description

An arrangement and a method for controlling of a WHR system BACKGROUND OF THE INVENTION AND PRIOR ART The present invention relates to an arrangement and a method for controlling of 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 fluid in a closed circuit. The circuit comprises an evaporator where the working fluid is heated and evaporated by a heat source such as, for example, the exhaust gases from a combustion engine. The pressurized and heated gaseous working fluid 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. The working fluid leaving the expander is directed to a condenser. The working fluid is cooled in the condenser to a temperature at which it condenses. The liquefied working fluid leaving the condenser is returned to the pump. Since waste heat energy from, for example, the exhaust gases from a combustion engine can be recovered by the WHR system, the fuel consumption of the combustion engine can be reduced.

However, during certain operating conditions of a vehicle, the temperature of the exhaust gases from the combustion engine is too low for evaporating the working fluid in the evaporator. In such a case, the WHR system can operate in an idle mode, in which the pump circulates the working fluid in liquid phase through the WHR system without evaporation in the evaporator and without generation of mechanical energy in the expander. In the idle mode, the working fluid may flow past the expander via a bypass line. If the temperature of the exhaust gases is too low during a long period of time, the working fluid in the WHR system is cooled down to a temperature considerably lower than its regular operating temperature. This may occur when there is no or a very low load on the combustion engine during a relatively long period of time which, for example, is the case when the vehicle is running down a long hill. After such a period of time, it can take a relatively long time for the working fluid to be heated up to a temperature at which it is possible to start the generation of mechanical energy in the WHR system.

US 2009/0211253 shows an organic Rankine cycle system comprising a turbine connected to a generator for generation of electrical energy and an evaporator in which a working fluid is heated and evaporated by exhaust gases from a combustion engine. The working fluid is preheated in several heat exchangers by engine intake air, coolant, oil, and EGR before it is evaporated by the exhaust gases in the evaporator.

JP 2010 077901 shows a waste heat recovery device for a vehicle provided with a retarder. The device comprises a first cooling water heater in which cooling water of a cooling water circuit is heated by the retarder and a second cooling water heater in which the cooling water of the cooling water circuit is heated by exhaust gases from an engine. The cooling water is heated in the first cooling water heater when the retarder is in operation and in the second cooling water heater when the retarder is not in operation. The heated cooling water is used to heat and evaporate a working fluid in an evaporator before the working fluid expands in an evaporator connected to a generator for generation of electrical energy.

SUMMARY OF THE INVENTION The object of the present invention is to provide an arrangement and a method for controlling of a WHR by which it possible to increase the operating time of WHR system.

The above mentioned object is achieved by the arrangement defined in claim 1. During operating conditions when the exhaust gases are unable to evaporate the working fluid in an evaporator of a WHR system, the working fluid may be cooled down to a lower temperature than its regular operating temperature. If the vehicle, for example, runs down a long hill, the working fluid may be cooled down to a significantly lower temperature than its regular operating temperature. The arrangement comprises a control unit determining when the exhaust gases are unable to evaporate the working fluid in the evaporator. The control unit also determines if a medium in another system of the vehicle has a higher temperature than the working fluid. A vehicle powered by a combustion engine usually comprises a number of systems in which a circulating medium is heated during operation of the vehicle. If said medium has a higher temperature than the working fluid, the control unit initiates a flow of the medium to a heat exchanger where the medium heats the working fluid. Such an additional heating of the working medium results in that the working medium can maintain its operating temperature even during operating conditions when the heating from the exhaust gases is to poor for evaporation of the working medium. The additional heating of the working fluid results in that the working fluid may already have a regular operating temperature when the exhaust gases again has capacity to evaporate the working fluid. Consequently, no time needs to be used for heating the working fluid to a temperature at which it is possible to start the generation of mechanical energy. Thus, it is possible to increase the operating time of the WHR system and convert a greater amount of waste thermal energy to mechanical energy.

According to an embodiment of the invention, the control unit is configured to receive information about at least one parameter indicating when the exhaust gases are unable to evaporate the working fluid in the evaporator. The control unit may be configured to receive information from a sensor detecting the exhaust gas temperature. The temperature of the exhaust gases may be used to determine when the exhaust gases are unable to evaporate the working fluid in the evaporator. The exhaust gas flow rate may be a parameter used in combination with the exhaust gas temperature for determining if the exhaust gases are able to evaporate the working medium. Alternative parameters may be the actual load on the combustion engine or information from a GPS unit about the topography of the road ahead.

According to an embodiment of the invention, the control unit is configured to receive information from a sensor detecting the working fluid temperature and a sensor detecting the medium temperature. In view of this information, it is easy for the control unit to determine when the medium has a higher temperature than the working fluid in the WHR system.

According to an embodiment of the invention, said heat exchanger is arranged in a part of the WHR system arranged in a position downstream of a pump and upstream of the expander. Such a positioning of the heat exchanger makes it possible to evaporate the working fluid in the heat exchanger by means of the medium and expand the gaseous working fluid in the expander despite the fact that the exhaust gases are unable to evaporate the working medium in the evaporator. In case the medium has a very high temperature, it is not excluded that it is possible to evaporate the working fluid in the heat exchanger by means of the medium. In case the heat exchanger is arranged in a position immediately upstream of the evaporator, it is also possible to use the heat exchanger and pre-heat the working fluid when the exhaust gases are able to evaporate the working fluid in the evaporator. If the medium is only used to heat the working fluid when it circulates in the WHR system without evaporation, it is possible to arrange the heat exchanger in a substantially arbitrary part of the WHR system.

According to an embodiment of the invention, the arrangement comprises a heat transfer loop connected to said system, flow means by which it is possible to temporarily direct a medium from the system, via the heat transfer loop, to the heat exchanger. Such a design of the arrangement makes it easy to temporarily direct medium from an ordinary part of the system to the heat exchanger for heating of the working fluid in the WHR system.

According to an embodiment of the invention, said heat transfer loop comprises an inlet connected to a part of the system where the medium has its highest temperature. In this case, the possibility increases to maintain a regular operating temperature of the working fluid by means of the medium during a relatively long period of time when the exhaust gases are unable to evaporate the working fluid. Said flow means may comprise a valve device which in a first position directs the medium flow in the system to the heat transfer loop and in a second position directs the medium flow in the system past the heat transfer loop. Such a valve device may be a three -way valve.

Alternatively, it is possible to use a valve device comprising two two-way valves. Alternatively or in combination said flow means may comprise a pump configured to circulate the medium is the heat transfer loop. In case the medium is not continuously circulated in the system, it may be necessary to use a separate pump for circulating the medium to the heat exchanger.

According to an embodiment of the invention, said system is a hydraulic retarder system and the medium is retarder oil. The retarder oil may have a temperature within the range 120-170°C. During a retarder braking process, there is no need to supply mechanical energy for propulsion of the vehicle. During the retarder braking process, the retarder oil temperature is usually significantly higher than the temperature of the working fluid in the WHR system. Consequently, it is usually no problem to maintain a regular operating temperature of working fluid by means of the retarder oil during a retarder braking process. Furthermore, it can be possible to evaporate the working fluid in the heat exchanger by the retarder oil when it has a very high temperature. The retarder oil leaving the retarder may be cooled in a retarder cooler by coolant circulating in a cooling system cooling the combustion engine. The cooling of the retarder oil in the heat exchanger reduces the load on the ordinary retarder cooler and makes it possible to avoid over heating of the coolant during a heavy retarder braking process.

According to an embodiment of the invention, said system is an engine oil system and the medium is an engine oil. The engine oil may have a temperature within the range 90-130°C during normal operation of a combustion engine. Thus, it is usually possible to maintain a regular operating temperature of the working fluid by means of the engine oil during period of times when the exhaust gases are unable to evaporate the working fluid in the evaporator.

According to an embodiment of the invention, said system is a cooling system and the medium is coolant. The coolant may have a temperature within the range 90-115°C. Especially if the coolant is used to cool the retarder oil in a retarder cooler, the coolant may have a very high temperature during a retarder braking process. Thus, it is usually possible to maintain a regular operating temperature of the working fluid by means of the coolant during period of times when the exhaust gases are unable to evaporate the working fluid in the evaporator.

According to an embodiment of the invention, the arrangement comprises at least two systems each provided with a circulating medium which is heated during at least some operating conditions of the vehicle, and that the control unit is configured to use the medium which has the highest temperature to heat the working fluid in the WHR system. It is of cause possible to use several mediums having a higher temperature than the working fluid for simultaneously heating the working fluid in the WHR system. In this case, the working fluid may be heated in different steps by the different mediums in different heat exchanger.

According to an embodiment of the invention, the arrangement comprises a bypass line and a bypass valve by it is possible to direct the working fluid past the condenser and or a compensation tank of the WHR system during operating conditions when said medium heats the working fluid in the heat exchanger. In this case, it is possible to avoid undesired cooling of the working fluid in the condenser and in the compensation tank during operating conditions when said medium is used for maintaining the temperature of the working fluid.

The above mentioned object is also achieved by the method defined in claim 15.

BRIEF DESCRIPTION OF THE DRAWINGS In the following a preferred embodiment of the invention is described, as an example, with reference to the attached drawings, in which: Fig. 1 shows an arrangement for controlling of a WHR system according to the invention and Fig. 2 shows flow chart defining a method for controlling of the WHR system.

DETAILED DESCRIPTION OF AN EMBODIMENT 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 combustion engine 2 comprises an exhaust line 3. The exhaust line 3 may comprise not indicated components such as a turbo compressor and components for after treatment of the exhaust gases. The vehicle 1 comprises a cooling system 4 comprising an engine inlet line 5 provided with a pump 6 circulating a coolant in the cooling system. When the coolant has circulated through the combustion engine 2, it is received in an engine outlet line 7. The engine outlet line 7 comprises a retarder cooler 8. A first three-way valve 9 is arranged at an end of the engine outlet line 7. The first three-way valve 9 directs coolant to a radiator 10 or a radiator bypass line 11. The first three way valve 9 is controlled by a control unit 12. The first three way valve 9 is adjustable in a stepless manner. Thus, it is possible for the first three way valve 9 to distribute a part of coolant flow to the radiator 10 and a remaining part of the coolant flow to the radiator bypass line 11.

The cooling system comprises a second three way valve 13 which is controlled by the control unit 12. The second three way valve 13 is also adjustable in a stepless manner. The second three way valve 13 may receive coolant from the radiator bypass line 11 and direct a part of it to a condenser inlet line 14a and a remaining part of it to a second bypass line 15. Alternatively, the second three way valve 13 receive a part of the coolant flow from the radiator 10 and direct it to the condenser bypass line 15. A remaining part of the coolant flow from the radiator 10 is directed, via the condenser inlet line 14a, to a condenser 16 of a WHR system 17. A condenser outlet line 14b directs coolant from the condenser 16 to the engine inlet line 5. The engine inlet line 5 directs a mixture of coolant from the condenser bypass line 15 and the condenser outlet line 14b to the combustion engine 2.

The WHR system 17 comprises a pump 18 which pressurizes and circulates a working fluid through the WHR system 17. In this case, the working fluid is ethanol. However, it is possible to use other kinds of working fluids such as for example R145fa. The pump 18 pressurizes and circulates the working fluid in the WHR system 17. In a first part 17a of the WHR system, which is arranged between the pump 18 and an evaporator 19, the working fluid is in liquid state and it has a high pressure when the WHR system is in an active mode. The working fluid is heated in the evaporator 19 to a temperature at which it evaporates. In a second part 17b of the WHR system, which is arranged between the evaporator 19 and an expander 20, the evaporated working fluid is in gaseous phase and it has a high pressure when the WHR system is in an active mode. The expander 20 may be a turbine or a piston. The pressurised and heated working fluid expands in the expander 20. The expander 20 generates a rotary motion which may be transmitted, via a suitable mechanical transmission 22, to a shaft 23 of a power train of the vehicle 1. Thus, the expander 20 converts thermal energy to mechanical energy for propulsion of the vehicle 1. The WHR system 17 comprises a first bypass line 20a provided with a first bypass valve 20b. The first bypass valve 20b is controlled by the control unit 12. The existence of the first bypass line 20a and the first bypass valve 20b makes it possible for the working fluid to flow past the expander 20 in an idle mode of the WHR system.

In a third part 17c of the WHR system, which is arranged between the expander 20 and the condenser 16, the working fluid is in gaseous phase and it has a low pressure in the active mode of the WHR system. The working fluid is cooled in the condenser 16 by coolant from the cooling system to a temperature at which it condenses. The temperature and the flow rate of exhaust gases and thus the heating effect of the working fluid in the evaporator 19 fluctuates during different operating conditions. In order to maintain a substantially continuously high thermal efficiency in the WHR system 17, it is favourable to establish a condensation pressure as low as possible. However, it is suitable to avoid negative pressure in the WHR system by practical reasons. In view of these facts, it is suitable to provide a cooling of the working fluid in the condenser 16 to a condensation pressure just above lbar. The WHR system 17 comprises a second bypass line 16a provided with a second bypass valve 16b. The second bypass valve 16b is controlled by the control unit 12. The existence of the second bypass line 16a and the second bypass valve 16b makes it possible for the working fluid to flow past the condenser 16.

In order to maintain a high thermal efficiency, the control unit 12 controls the three way valves 9, 13 such that coolant of a variable temperature and flow rate cools the working fluid in the condenser 16 in a manner such that the condensation pressure will be just above 1 bar. The working fluid ethanol has a condensation temperature of 78°C at 1 bar. In this case, it is suitable to accomplish a condensation temperature of just above 78°C in the condenser 16. In a fourth part 17d of the WHR system, which is arranged between the condenser 16 and the pump 18, the working fluid is in liquid phase and it has a low pressure in the active mode of the WHR system. A compensation tank 24 for volume compensation and pressure control of the working fluid is arranged in the fourth part 17d of the WHR system. The WHR system 17 comprises a third bypass line 24a provided with a third bypass valve 24b. The third bypass valve 24b is controlled by the control unit 12. The existence of the third bypass line 24a and the third bypass valve 24b makes it possible for the working fluid to flow past the condenser 16 as well as the compensation tank 24.

The vehicle comprises a first heat transfer loop 25. The first heat transfer loop 25 comprises a first valve device 26 and a first heat exchanger 27. The first valve device 26 in controlled by the control unit 12. The first valve device 26 is positionable in a non-heating position in which it directs the coolant past the first heat transfer loop 25 and in a heating position in which it directs the coolant, via the first heat transfer loop 25, to the first heat exchanger 27. The first heat exchanger 27 is connected to the WHR system 17 in a position downstream of the pump 18 and upstream of the evaporator 19. The vehicle comprises an engine oil system 28. A not visible oil pump circulates the engine oil in the engine oil system 28. The engine oil system 28 comprises a second heat transfer loop 29 comprising a valve device 31 and a second heat exchanger 30. The second valve device 31 is positionable in a non-heating position in which it directs the engine oil past the second heat transfer loop 29 and a heating position in which it directs the coolant, via the second heat transfer loop 29, to the second heat exchanger 30. The second heat exchanger 30 is connected to the WHR system 17 in a position downstream of the pump 18 and upstream of the evaporator 19.

The vehicle comprises a hydraulic retarder system 32. The hydraulic retarder system 32 comprises a hydraulic retarder 33 provided with a stator and a rotor. When the control unit 12 initiates a braking process of the hydraulic retarder 34, retarder oil is circulated in the hydraulic retarder system 32 between the hydraulic retarder 33 and the retarder cooler 8. The hydraulic retarder system 32 comprises a third heat transfer loop 34. The third heat transfer loop 34 comprises a pump 35, a valve device 36 and a third heat exchanger 37. The third valve device 36 is positionable in a non-heating position in which it directs the coolant past the third heat transfer loop 29 and a heating position in which it directs the coolant, via the third heat transfer loop 36, to the third heat exchanger 37.

The pump 35 is used during operating conditions when the hydraulic retarder 33 is not activated and the third valve device is positioned in the heating position. The third heat exchanger 37 is connected to the WHR system 17 in a position downstream of the pump 18 and upstream of the evaporator 19. The control unit 12 controls the valve devices 26, 31, 36 in view of information from a first temperature sensor si sensing the exhaust gas temperature T1in the exhaust line 3, a second temperature sensor s2sensing the coolant temperature T2in the engine outlet line 7 in a position downstream of the retarder cooler 8, a third temperature sensor s3sensing the engine oil temperature T3in the engine oil system 28, a fourth temperature sensor s4sensing the retarder oil temperature T4in the hydraulic retarder system 32 and a fifth temperature sensor s5sensing the working fluid temperature T5in the WHR system 17.

Fig. 2 shows a flow chart indicating a method for controlling of the WHR system 17. The method starts at step 41. During operation of the vehicle, the control unit 12 receives information from the first temperature sensor si about the exhaust gas temperature Ti in the exhaust line 3. In view of this information, the control unit 12 determines, at step 42, if the exhaust gases are able to evaporate the working fluid in the evaporator. If this is the case, the control unit 12 switches, at step 42, the WHR system to an active mode. In the active mode, the control unit 12 positions the bypass valve device 20b in a closed position such that the evaporated working fluid from the evaporator 19 is directed to the expander 7. The control unit 12 positions the bypass valve device 16b in a closed position such that the working fluid is directed to the condenser 16 and the bypass valve device 24b in a closed position such that the working fluid flows in contact with the compensation tank 24. Furthermore, the control unit 12 controls the three way- valves 9, 13 such that the coolant cools the working fluid to a desired condensation temperature in the condenser 16. After that, the method restarts at step 41.

During operation conditions when the exhaust gas temperature Ti is not high enough to evaporate the working fluid in the evaporator 19, the method continues at step 44. At step 44, the control unit 12 switches the WHR system to an idle mode. In the idle mode, the control unit 12 opens the first bypass valve 20b such that it is possible to circulate the working fluid, via the first bypass line 20a, past the expander 20. The control unit 12 opens the second bypass valve 16b such that the working fluid flows via the second bypass line 16a past the condenser 16. This measure prevents cooling of the working fluid in the condenser 16. Alternatively, the control unit 12 opens the third bypass valve 24b such that the working fluid flows via the third bypass line 24a past the condenser 16 and the compensation tank 24. This measure prevents cooling of the working fluid in the condenser 16 as well as in the compensation tank 24. In the idle mode, the working fluid is continuously in liquid phase since the working fluid is not evaporated in the evaporator 19.

At step 45, the control unit 12 receives information from the second sensor s2about the coolant temperature T2in the cooling system 4, from the third sensor s3about the engine oil temperature T3in the engine oil system 28, from the fourth sensor S4about the retarder oil temperature T4in the hydraulic retarder system 32 and from the fifth sensor s5about the working fluid temperature T5in the WHR system 17. The control unit 12 determines if any one of the coolant, the engine oil or the retarder oil has a higher temperature than the temperature of the working fluid in the WHR system. If none of these mediums have a higher temperature than the working fluid, it is not possible to heat the working fluid in the WHR system by any of these mediums. In this case, the method restarts at step 41. However, it is very rare that it is not possible to heat the working fluid by any one of these mediums. Usually, at least one of said mediums have a higher temperature than the working fluid in the WHR system. In this case, the process continues at step 46. At step 46, said medium is used to heat the working fluid in the WHR system 17. If several mediums have a higher temperature than the working fluid, the medium having the highest temperature can be used to heat the working fluid. After that the method restarts at step 41.

The heating of the working fluid makes it possible to maintain a regular operating temperature of the working fluid when the WHR system is in the idle mode. This measure makes it possible to substantially immediately switch the WHR system from the idle mode to the active mode when the exhaust gases again are able to evaporate the working fluid in the evaporator 19. Thus, there is no need for heating the working fluid up to a regular operating temperature before it is possible to switch the WHR system to the active mode. Since such a heating period is eliminated, it is possible to increase the operating time of the WHR system and convert a greater amount of waste thermal energy to mechanical energy.

The invention is not restricted to the described embodiment but may be varied freely within the scope of the claims. The exhaust gases heating the working fluid in the evaporator may be exhaust gases directed from a combustion engine, via an exhaust line, to an environment but it may also be recirculating exhaust gases which are directed back to the combustion engine. It is not excluded to heat the working fluid in a heat exchanger by, for example, the retarder oil to a temperature at which it evaporates. In such a case, it is possible to switch the WHR system to an active mode despite the fact that the exhaust gases are unable to evaporate the working fluid in the evaporator.

Claims

1. An arrangement for controlling of a WHR system for a vehicle (1) powered by a combustion engine (2), wherein the WHR system (17) comprises an evaporator (19) in which a working fluid is adapted to be heated and evaporated by exhaust gases from the combustion engine (2) and an expander (20) mechanically connected to a power train (22) of the vehicle (1), and wherein the vehicle (1) comprises a system (4, 28, 32) with a circulating medium which is heated during at least some operating conditions of the vehicle (1), characterized in that the arrangement comprises a heat exchanger (27, 30, 37) and a control unit (12) which is configured to determine if the exhaust gases are unable to evaporate the working fluid in the evaporator (19), to determine if said medium has a higher temperature than the working fluid, and if these two conditions are met to initiate a medium flow from the system to the heat exchanger (27, 30, 37) in which the medium heats the working fluid.

2. An arrangement according to claim 1, characterized in that the control unit (12) is configured to receive information about at least one parameter indicating when the exhaust gases are unable to evaporate the working fluid in the evaporator.

3. An arrangement according to claim 2, characterized in that the control unit (12) is configured to receive information from a first sensor (si) detecting the exhaust gas temperature (T1).

4. An arrangement according to any one of the preceding claims 1 to 3, characterized in that the control unit (12) is configured to receive information from a sensor (s5) detecting the working fluid temperature (T5) and a sensor (s2-s4) detecting the medium temperature (T2,T3,T4).

5. An arrangement according to any one of the preceding claims, characterized in that said heat exchanger (27, 30, 37) is arranged in a part of the WHR system arranged in a position downstream of a pump (18) and upstream of the evaporator (19).

6. An arrangement according to any one of the preceding claims, characterized in that the arrangement comprises a heat transfer loop (25, 29, 34) connected to said system (4, 28, 32), flow means (26, 31, 35, 36) by which it is possible to temporarily direct a medium from the system (4, 28, 32), via the heat transfer loop (25, 29, 34), to the heat exchanger (27, 30, 37).

7. An arrangement according to claim 6, characterized in that said heat transfer loop (25, 29, 34) comprises an inlet connected to a part of the system (4, 28, 32) where the medium has its highest temperature.

8. An arrangement according to claim 6 or 7, characterized in that said flow means comprises a valve device (26, 31, 36) which in a first position directs the medium flow in the system to the heat transfer loop (25, 29, 34) and in a second position directs the medium flow in the system past the heat transfer loop (25, 29, 34).

9. An arrangement according to claim 6 or 7, characterized in that said flow means comprises a pump (36) configured to circulate the medium in the heat transfer loop (34).

10. An arrangement according to any one of the preceding claims 1 to 10, characterized in that said system (4, 28, 32) is a hydraulic retarder system and the medium is a retarder oil.

11. 1 1. An arrangement according to any one of the preceding claims 1 to 10, characterized in that said system (4, 28, 32) is a cooling system and the medium is a coolant.

12. An arrangement according to any one of the preceding claims 1 to 10, characterized in that said system (4, 28, 32) is an engine oil system and the medium is an engine oil.

13. An arrangement according to any one of the preceding claims, characterized in that the arrangement comprises at least two systems (4, 28, 32) each provided with a circulating medium which is heated during at least some operating conditions of the vehicle (1), and that the control unit (12) is configured to use the medium which has the highest temperature to heat the working fluid in the WHR system.

14. An arrangement according to any one of the preceding claims, characterized in that the arrangement comprises a bypass line (16a, 24a) and a bypass valve (16b, 24b) by which it is possible to direct the working fluid past a condenser and or a compensation tank (24) of the WHR system during operating conditions when said medium heats the working fluid in the heat exchanger (27, 30, 37).

15. A method for controlling of a WHR system arranged in a vehicle (1) powered by a combustion engine (2), wherein the WHR system (17) comprises an evaporator (19) in which a working fluid is adapted to be heated and evaporated by exhaust gases from the combustion engine (2) and an expander (20) mechanically connected to a power train (22) of the vehicle (1), and wherein the vehicle (1) comprises a system (4, 28, 32) with a circulating medium which is heated during at least some operating conditions of the vehicle (1), characterized by the steps of determining if the exhaust gases are unable to evaporate the working fluid in the evaporator (19), determining if said medium has a higher temperature than the working fluid, and if these two conditions are met to heat the working fluid in the WHR system by means of the medium.