SE540385C2

SE540385C2 - A WHR system for recovering heat energy from a combustion engine

Info

Publication number: SE540385C2
Application number: SE1651608A
Authority: SE
Inventors: Kardos Zoltan; Hällqvist Thomas
Original assignee: Scania Cv Ab
Priority date: 2016-12-07
Filing date: 2016-12-07
Publication date: 2018-09-04
Also published as: SE1651608A1; DE102017010816A1; DE102017010816B4

Abstract

The present invention relates to a WHR system for recovering heat energy from a combustion engine (2) in a vehicle (1). The WHR system comprises an exhaust line (3) leading exhaust gases out from the combustion engine (2), a WHR circuit (19) and a heat transfer arrangement configured to transfer heat energy from the exhaust gases in the exhaust line (3) to the working medium in the evaporator (20). The heat transfer arrangement comprises a flow channel (25) enclosing an evaporator (20) of the WHR circuit (18), flow means (26) configured to provide a flow of a heat transfer fluid through the flow channel (25) and at least one heat pipe (29, 30) configured to transfer heat energy from a first end portion (29a, 30a) in thermal contact with an exhaust line part (3a, 3c) to a second end portion (29b, 30b) which is in thermal contact with the heat transfer fluid in the flow channel (25) in a position upstream of the evaporator (20) with respect to an intended flow direction of the heat transfer fluid in the flow channel (25).

Description

A WHR system for recovering heat energy from a combustion engine BACKGROUND OF THE INVENTION AND PRIOR ART The present invention relates to a WHR system for recovering heat energy from a combustion engine according to the preamble of claim 1. The present invention also relates to a vehicle comprising such a WHR system.

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 comprises a pump which pressurizes and circulates a working medium in a closed circuit. The circuit comprises an evaporator where the working medium 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 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 directed to a condenser. The working medium is cooled in the condenser to a temperature at which it condenses. The liquefied working medium is redirected to the pump which pressurizes the working medium. Thus, the waste heat energy from, for example, the exhaust gases from a combustion engine in a vehicle can be recovered by means of a WHR-system. Consequently, a WHR-system can reduce the fuel consumption of a combustion engine.

Usually, an exhaust line of a combustion engine comprises a turbine of a turbocharger and an exhaust treatment device such as a SCR catalytic. The turbine is arranged to be powered by high pressure exhaust gases and the exhaust treatment device needs to be heated to a relatively high temperature in order to provide a desired treatment of the exhaust gases. The exhaust gases provides a relatively large pressure and temperature drop when they flow through an evaporator of a WHR system. Due to this fact, the evaporator of a conventional WHR system is arranged in a position downstream of the turbine and the heat treatment device in the exhaust line in order to not decrease the function of these components. In this position, the temperature of the exhaust gases is relatively low which decreases capacity of the WHR system. Furthermore, the temperature difference between the exhaust gases and the working medium in the evaporator is many times low which makes it fairly difficult to control the heat flow in evaporator with a desired precision.

A heat pipe is a closed evaporator-condenser system consisting of a sealed, hollow tube whose inside walls are lined with a capillary structure or wick. Thermodynamic working fluid, with substantial vapor pressure at the desired operating temperature, saturates the pores of the wick in a state of equilibrium between liquid and vapor. When heat is applied to the heat pipe, the liquid in the wick heats and evaporates. As the evaporating fluid fills the heat pipe hollow center, it diffuses throughout its length. Condensation of the vapor occurs wherever the temperature is even slightly below that of the evaporation area. As it condenses, the vapor gives up the heat it acquired during evaporation. This effective high thermal conductance helps maintain near constant temperatures along the entire length of the heat pipe.

US 8,806,858 shows an exhaust system comprising a catalytic converter. In order to protect an exhaust manifold and the catalytic converter from overheating, heat pipes are used to transfer heat from the exhaust manifold to the atmosphere.

SUMMARY OF THE INVENTION An object of the present invention is to provide a WHR system recovering heat energy from exhaust gases of a combustion engine with a low pressure drop in the exhaust line. Further objects are to provide conditions for a simple control of the heat flow in the evaporator and to protect components in contact with the exhaust gases from overheating.

The above mentioned objects are achieved by the WHR system defined in claim 1. The heat transfer arrangement comprises a flow channel enclosing an evaporator of a WHR circuit and flow means configured to provide a flow of a heat transfer fluid through the flow channel. The heat transfer arrangement also comprises heat pipes configured to transfer heat energy from the exhaust gases to the heat transfer fluid flow in the flow channel. The exhaust line is usually heated to a substantially corresponding temperature as the exhaust gases. In view of this fact, it is possible to connect an end portion of a heat pipe to a suitable surface of the exhaust line and absorb heat energy from the exhaust gases. Since the first end portion of the heat pipe does not protrude into the exhaust gas flow, it will not add any flow resistance to the exhaust gas flow in the exhaust line. Consequently, the use of heat pipes makes it possible to absorb heat energy from exhaust gases in an exhaust line with a low pressure drop in the exhaust line. A low pressure drop in the exhaust line reduces the fuel consumption of the combustion engine. The heat pipe transfers heat energy from the first end portion to a second end portion which is in thermal contact with the heat transfer fluid flow in the flow channel in a position upstream of the evaporator with respect to an intended flow direction in the flow channel. Thus, the second end portion of the heat pipe heats the heat transfer fluid flow in the flow channel before it comes in thermal contact with the working medium in the evaporator. With a suitable choice of heat pipe, it is possible to heat the heat transfer medium flow in the flow channel to a very high temperature. The area of the exhaust line thermally connected to the first end portion of the heat pipe is cooled down. Said area of the exhaust line is protected from overheating.

According to an embodiment of the invention, the exhaust line comprises a turbine of a turbocharger and that the first end portion of the heat pipe is in thermal contact with an exhaust line part located upstream of the turbine with respect to the intended flow direction of the exhaust gases in the exhaust line. In this case, the heat pipes absorbs heat energy from an exhaust line part where the exhaust gases has its highest temperature and pressure. In this case, it is possible to heat the heat transfer fluid to a significantly higher temperature than the working medium in the condenser. The large temperature different between the heat transfer fluid and the working medium in the evaporator facilitate the control of the heat transfer in the evaporator.

According to an embodiment of the invention, the first end portion of the heat pipe is in thermal contact with an exhaust manifold of the exhaust line. In certain combustion engines, the exhaust gas temperature is limited due to the material properties of the exhaust manifold. By thermally connecting of a first end portion of a number of heat pipes to suitable surfaces of the exhaust manifold it is possible to reduce the temperature of the exhaust manifold and protect it from overheating. Alternatively, it is possible to permit a higher exhaust gas temperature from the combustion engine.

According to an embodiment of the invention, the exhaust line comprises a exhaust treatment device and that the first end portion of the heat pipe is connected to an exhaust line part located upstream of the exhaust treatment device with respect to the intended flow direction of the exhaust gases in the exhaust line. An exhaust treatment device is usually arranged in the exhaust line in a position downstream of the turbocharger. The temperature and the pressure of the exhaust gases drop when they flows through the exhaust treatment device. Due to this fact, it is favorable to absorb heat energy from the exhaust line in a position upstream the exhaust treatment device.

According to an embodiment of the invention, the heat transfer fluid is air. In this case, the flow means may be a fan configured to force an air flow through the flow channel. The fan may be driven with a variable speed and force a variable air flow rate through the flow channel in order to provide a desired evaporation of the working medium in the evaporator.

According to an embodiment of the invention, the flow channel comprises an opening arranged in a front portion of the vehicle and that ram air is configured to force an air flow through the flow channel. In this case, it is possible to provide an air flow through the flow channel without a fan. The use of ram air reduces the consumption of energy for operation of the fan and the WHR system. A throttle member may be configured to control the ram air flow through the flow channel. During operating conditions with varying exhaust gas temperature, it is possible to vary the air flow rate in the flow channel by means of the throttle valve in order to optimize the evaporation process in the evaporator. The throttle valve may be a butterfly valve.

According to an embodiment of the invention, the heat transfer fluid is exhaust gases. The exhaust gases may be directed to the flow channel from an exhaust line part located downstream of the turbine and the exhaust treatment device. Almost always, the exhaust gases has a significantly higher temperature than ambient temperature in this position of the exhaust line. Due to this fact, it requires a smaller temperature rise to heat the exhaust gases to a specific temperature than a heat transfer medium at ambient temperature. In this case, less heat energy is required to heat the heat transfer medium.

According to an embodiment of the invention, the flow means is a pump and that the heat transfer fluid is a liquid. In this case, the flow channel can be made more compact. The liquid may be a suitable oil or a pressurized mixture of water and glycol.

According to an embodiment of the invention, the flow channel forms a closed circuit. During most operating conditions, the heat transfer fluid has a higher temperature than ambient temperature when it leaves the evaporator. Due to this fact, it requires a smaller temperature rise to heat a heat transfer medium in a closed circuit to a specific temperature than a heat transfer medium at ambient temperature. In this case, less heat energy is required to heat the heat transfer medium.

According to an embodiment of the invention, the arrangement comprises a control unit configured to receive information about at least one operating parameter and to control the flow of the heat transfer fluid through the flow channel by means of this information. Such parameter may be related to the temperature and/or the flow of the exhaust gases in the exhaust line. The heat energy in the exhaust gases varies during different operating conditions. In order to perform an efficient evaporation in the evaporator, it may be necessary to vary the heat transfer in the evaporator. The heat transfer in the evaporator may be varied by varying the flow of the heat transfer fluid in the flow channel.

The arrangement may comprise a temperature sensor configured to sense the temperature of the exhaust gases in at least one exhaust line part and that the control unit is configured to initiate a flow of the heat transfer fluid through the flow channel when the exhaust gases has a temperature above a specific temperature. The turbine of a turbocharger is arranged to be powered by high pressure exhaust gases and an exhaust treatment device needs to be heated to a relatively high temperature in order to provide a desired treatment of the exhaust gases. The transfer of heat energy from the exhaust gases results in a temperature and pressure drop of the exhaust gases. The heat pipe transfer heat energy from the exhaust line in a position upstream of an exhaust treatment device and/or a turbine of a turbocharger. In view of these facts, it is not appropriate to recover heat energy from the exhaust gases when they have a too low temperature. In such a case, the pressure and the temperature of the exhaust gases entering the turbine and the exhaust gas treatment device will be too low for an efficient operation of these components. In case the temperature of the exhaust gases is above such a specific temperature, the pressure and the temperature of the exhaust gases entering the turbine and the exhaust gas treatment device will be enough high for an efficient operation of these components. In this case, the WHR system recovers heat energy from exhaust gases at a high temperature which increases the efficient of the WHR system and the ability to control the WHR system. The specific temperature may be a constant temperature or a temperature which varies during different operating conditions.

BRIEF DESCRIPTION OF THE DRAWINGS In the following preferred embodiments of the invention are described, as examples, with reference to the attached drawings, in which: Fig. 1 shows a WHR system according to a first embodiment of the invention, Fig. 2 shows a WHR system according to a second embodiment of the invention, Fig. 3 shows a WHR system according to a third embodiment of the invention and Fig. 4 shows a WHR system according to a fourth embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 shows a schematically disclosed vehicle 1 powered by a superheated combustion engine 2. The combustion engine 2 may be a diesel engine. The vehicle 1 may be a heavy vehicle. The combustion engine comprises an exhaust line 3 directing out exhaust gases from the combustion engine 2. The exhaust line 3 comprises a first exhaust line part in the form of an exhaust manifold 3a receiving exhaust gases from the cylinders of the combustion engine 2. The exhaust line 3 comprises a second exhaust line part 3b delivering exhaust gases from the exhaust manifold to a turbine 4a of a turbo aggregate 4. The turbine 4a drives a compressor 4b of the turbocharger 4. The compressor 4b compresses air which is led, via a charged air line 5 to the combustion engine 2. The charged air line 5 comprises a charge air cooler 5a arranged at a front portion of the vehicle 1 in which the compressed air is cooled before it is directed to the combustion engine 2. The exhaust line 3 comprises a third exhaust line part 3c leading exhaust gases from the turbine 4a to an exhaust treatment device 6.

The exhaust treatment device 6 may be arranged in a container such as a silencer. The exhaust treatment device 6 may comprise one or several of the following components a Diesel Oxidation Catalyst DOC, a Diesel Particulate Filter PDF, an injection device for a urea solution, a Selective Catalytic Reduction SCR and an Ammonia Slip Catalyst ASC. The exhaust line 3 comprises a fourth exhaust line part receiving exhaust gases from the exhaust treatment device 6. The pressure and the temperature of the exhaust gases are successively reduced in the exhaust line 3. Thus, the exhaust gases has its highest pressure and temperature in the first exhaust line part 3a and the second exhaust line part 3b. After that the exhaust gases has expanded in the turbine 4a they have a reduced pressure and temperature in the third exhaust line part 3c. After the exhaust gases have been treated in the exhaust treatment device 6 they have in a further reduced pressure and temperature the fourth exhaust line part 3d.

The vehicle 1 comprises a cooling system comprising an engine inlet line 7 directing coolant to the combustion engine 2. The engine inlet line 7 is provided with a pump 8 circulating a coolant in the cooling system. The coolant leaving the combustion engine 2 is received in an engine outlet line 9. A first valve member 10 in the form of a three way valve 10 is arranged at an end of the engine outlet line 9. The cooling system comprises a radiator bypass line 11 and a radiator 12. The first valve member 10 is configured to distribute a part of the coolant to the radiator bypass line 11 and a remaining part of the coolant to the radiator 12. The cooling system comprises a second valve member 14 in the form of a three way valve. The second valve member 14 may receive coolant from the radiator bypass line 1 land direct it to the engine inlet line 7 or to a condenser circuit 15 in which the coolant cools a working medium in a condenser 16 of a WHR circuit. In the latter case, coolant from the radiator bypass line 11 and possible coolant from the radiator 12 are mixed and directed to the condenser circuit 15. Alternatively, the second valve memberl4 receives coolant from the radiator 12 and directs it to the engine inlet line 7. The condenser circuit 15 comprises a condenser inlet line 15a directing coolant to the condenser 16 and a condenser outlet line 15b directing coolant from the condenser 16 to the engine inlet line 7.

The vehicle 1 is provided with a WHR system for recovering heat energy from a combustion engine 2. The WHR system comprises a WHR circuit 19 (Waste Heat Recovery system). The WHR circuit comprises a pump 18 which pressurizes and circulates a working medium in the WHR circuit 19. The working medium may be ethanol, R65fa or other kind of working medium. The working medium leaving the pump 18 enters an evaporator 20. The working medium is heated by exhaust gases in the evaporator 20 such that it is evaporated and superheated before it is directed to an expander 22. The working medium expands in the expander 22. The expander 22 generates a rotary motion which may be transmitted, via a suitable mechanical transmission, to a shaft of the drive train of the vehicle 1. Alternatively, the expander 6 converts thermal energy to electrical energy which may be stored in a battery. When the working medium has passed through the expander 22, it is directed to the condenser 16. The working medium is cooled in the condenser 16 by the coolant in the condenser circuit 15 to a temperature at which it is condensed. The working medium is directed from the condenser 16, to a receiver 23. The working medium is sucked from the receiver 23 to the pump 18.

The WHR system comprises a flow channel 25. The flow channel 25 encloses the evaporator 20 of the WHR circuit 19. A fan 26 is configured to provide an air flow through the flow channel 25. The fan 26 is driven by an electric motor 27. A control unit 28 control the activation of the electric motor 27 and the fan 26. A first set of heat pipes 29 is, at a first end portion 29a, in thermal contact with the exhaust manifold 3a. A second end portion 29b of the first set of heat pipes 29 is arranged inside the flow channel 25. The second end portion 29b of the first set of heat pipes 29 is arranged in a position upstream of the evaporator 20 in view of the intended direction of the air flow through the flow channel 25. A second set of heat pipes 30 is, at a first end portion 30a, arranged in thermal contact with the exhaust line 3 in a position downstream of the turbine 4a and upstream of the exhaust treatment device 6. The second end portion 30b of the second set of heat pipes 30 is arranged inside the flow channel 25. The second end portion 30b of the second set of heat pipes 30 is arranged in the flow channel 25 in a position upstream of the evaporator 20 and the second end portion 29b of the first set of heat pipes 29. A temperature sensor 31 senses the temperature of the exhaust gases in the exhaust line 3. In this case, the temperature sensor 31 senses the temperature of the exhaust gases in the second part 3b of the exhaust line 3.

During operation of the combustion engine 2 exhaust gases flows through the exhaust line 3. The control unit 28 receives information from the temperature sensor 31 about the temperature of the exhaust gases in the second exhaust line part 3b. In case the temperature of the exhaust gases is above a certain temperature, it is possible to use heat energy from the exhaust gases to heat the working medium in the evaporator 20. In this case, the pressure of the exhaust gases entering the turbine 4a is high enough to provide a sufficient operation of the turbocharger 4. Furthermore, the temperature of the exhaust gases entering the exhaust treatment device 6 is high enough to provide an efficient treatment of the exhaust gases in the exhaust treatment device 6. Such a temperature may be about 500°C. In case the control unit 28 receives information from the temperature sensor 31 indicating that the temperature of the exhaust gases is above 500°C, the control unit 28 activates the pump 19 such that the circulation of the working medium in the WHR circuit 18 starts. The control unit 28 also initiates activation of the fan 26 such that an air flow is provided in the flow channel 25.

Since heat pipes 29, 30 have a very effective high thermal conductance, the first end portions 29a, 30a and the second end portions 29b, 30b of the respective sets of heat pipes 29, 30 have a substantially corresponding temperature. Thus, the first end portion 29a and the second end portion 29b of the first sets of heat pipes 29 have a temperature related to the temperature of the exhaust in the exhaust manifold 3a. The first end portion 30a and the second end portion 30b of the second sets of heat pipes 30 have a temperature related to the temperature of the exhaust gases in the third exhaust line part 3c. The second end portions 29b of the first set of heat pipes 29 have a higher temperature than the second end portions 30b of the second set of heat pipes 30.

Consequently, the air flow in the flow channel 25, is heated in a first step by the second end portions 30b of the second set of heat pipes 30 and in a second step by the second end portions 29b of the first set of heat pipes 29. Thus, the air entering the evaporator 20 may be heated to a relatively high temperature before it enters the evaporator 20. The hot air heats the working medium in the evaporator 20 to a temperature at which it evaporates.

Fig. 1 shows the use of two sets of heat pipes 29, 30. It is usually sufficient to use one set of heat pipes 29, 30 for heating the air flow in the flow channel 25. The use of heat pipes makes 29, 30 makes it possible to absorb heat from the exhaust gases without adding any flow resistance to the exhaust gases in the exhaust line 3. Consequently, the absorption of heat energy from the exhaust gases in the exhaust line can be performed with a relatively low pressure drop in the exhaust line 3. In this case, it is possible to provide a relatively large temperature difference between the hot air and the working medium in the evaporator 20. Such a large temperature difference facilitate the control of the heat transfer in the evaporator 20. The control unit 28 controls the heat transfer in the evaporator 20 by adjusting the speed of the fan 26 and thus the air flow rate through the flow channel 25. The use of the first set of heat pipes 29 makes it possible to cool the exhaust manifold 3a. Thus, the WHR system may protect the exhaust manifold from overheating. The use of the WHR system makes it also possible to increase the temperature of the exhaust leaving the combustion engine.

During operating conditions when the control unit 28 receives information from the temperature sensor 31 indicating that the temperature of the exhaust gases is below said specific temperature, it switches off the fan 26 and the pump 18 in the WHR circuit 18. Thus, the WHR system is shut off when the exhaust gases are below said specific temperature. This measure ensures that the turbocharger 4 and the exhaust treatment device 6 maintain a desired function at operating conditions when the exhaust gases has a low temperature.

Fig. 2 shows an alternative embodiment of the WHR system. In this case, the flow channel 25 has in opening 25a at a front portion of the vehicle 1. The flow channel 25 comprises a throttle member in the form of a butterfly valve 32 by which it is possible to regulate the air flow in the flow channel 25. In this case, the air flow through the flow channel 25 is provided by ram air. Also in this embodiment the flow channel 25 comprises a second end portion 30b of a second set of heat pipes 30 transferring heat energy from the second exhaust line portion 30c to the air flow and a second end portion 29b of a first set of heat pipes 29 transferring heat energy from the exhaust manifold 30c to the air flow. The flow channel 25 also encloses the evaporator 20 of the WHR circuit 19.

During operating conditions when the control unit 28 receives information from the temperature sensor 31 indicating that the temperature of the exhaust gases is above said specific temperature, it switches on the pump 18 in the WHR circuit 18. The ram air provides an air flow through the flow channel 25. The first set of heat pipes 29 and the second set of heat pipes 30 transfer heat energy from the exhaust line 3 to the air flow in the flow channel 25. The control unit 28 regulates the air flow in the flow channel 25 by means of the butterfly valve 32 such that the temperature and the flow rate of the air obtains values resulting in a successful heating of the working medium in the evaporator 20. During operating conditions when the control unit 28 receives information from the temperature sensor 31 indicating that the temperature of the exhaust gases is below said specific temperature, it sets the butterfly valve 32 in a closed position and it shuts off the pump 18 in the WHR circuit 18.

Fig. 3 shows a further alternative embodiment of the WHR system. In this case, the flow channel 25 has in inlet opening 25b connected to the fourth exhaust line portion 3d. The flow channel 25 comprises a throttle member in the form of a butterfly valve 32 by which it is possible to regulate the exhaust gas flow in the flow channel 25. In this case, the exhaust gas flow through the flow channel 25 is provided by the pressure of the exhaust gases in the fourth exhaust gas part 3d. Also in this embodiment the flow channel 25 comprises a second end portion 30b of a second set of heat pipes 30 transferring heat energy from the second exhaust line portion 30c to the air flow and a second end portion 29b of a first set of heat pipes 29 transferring heat energy from the exhaust manifold 30c to the air flow. The flow channel 25 also encloses the evaporator 20 of the WHR circuit 19.

During operating conditions when the control unit 28 receives information from the temperature sensor 31 indicating that the temperature of the exhaust gases is above said specific temperature, it switches on the pump 18 in the WHR circuit 18. The pressure in the fourth exhaust line part 3d provides an exhaust gas flow through the flow channel 25. The first set of heat pipes 29 and the second set of heat pipes 30 transfer heat energy from the exhaust line 3 to the air flow in the flow channel 25. The control unit 28 regulates the exhaust gas flow in the flow channel 25 by means of the butterfly valve 32 such that the temperature and the flow rate of the exhaust gases obtains values resulting in a successful heating of the working medium in the evaporator 20. During operating conditions when the control unit 28 receives information from the temperature sensor 31 indicating that the temperature of the exhaust gases is below said specific temperature, it sets the butterfly valve 32 in a closed position and it shuts off the pump 18 in the WHR circuit 18.

Fig. 4 shows a further alternative embodiment of the WHR system. In this case, the flow channel 25 is formed as a closed circuit. A pump 34 is configured to circulate a liquid in the closed circuit. Also in this embodiment the flow channel 25 comprises a second end portion 30b of a second set of heat pipes 30 transferring heat energy from the second exhaust line portion 30c to the air flow and a second end portion 29b of a first set of heat pipes 29 transferring heat energy from the exhaust manifold 30c to the air flow. The flow channel 25 also encloses the evaporator 20 of the WHR circuit 19.

During operating conditions when the control unit 28 receives information from the temperature sensor 31 indicating that the temperature of the exhaust gases is above said specific temperature, it switches on the pump 18 in the WHR circuit 18 and the pump 33 in the flow channel 25. The first set of heat pipes 29 and the second set of heat pipes 30 transfer heat energy from the exhaust line 3 to the liquid flow in the flow channel 25. The control unit 28 regulates the liquid flow in the flow channel 25 by means of the pump 33 such that the temperature and the flow rate of the liquid obtains values for a successful heating of the working medium in the evaporator 20. During operating conditions when the control unit 28 receives information from the temperature sensor 31 indicating that the temperature of the exhaust gases is below said specific temperature, it shuts off the pump 18 in the WHR circuit 18 and the pump 33 in the flow channel 25.

The invention is not restricted to the described embodiment but may be varied freely within the scope of the claims. It is, for example, also possible to provide a flow channel forming a closed circuit for a gaseous heat transfer medium such as air.

Claims

1. A WHR system for recovering heat energy from a combustion engine (2) in a vehicle (1), the WHR system comprises an exhaust line (3) leading exhaust gases out from the combustion engine (2), a WHR circuit (19) which comprises a pump (18) configured to pressurize and circulate a working medium, an evaporator (20) in which the working medium is configured to be heated to a temperature at which it evaporates, an expander (22) configured to transform thermal energy from the working medium into mechanical energy or electrical energy and a condenser (16) in which the working medium is configured to be cooled to a temperature at which it condenses, and a heat transfer arrangement configured to transfer heat energy from the exhaust gases in the exhaust line (3) to the working medium in the evaporator (20), characterized in that the heat transfer arrangement comprises a flow channel (25) enclosing the evaporator (20), flow means (26) configured to provide a flow of a heat transfer fluid through the flow channel (25) and at least one heat pipe (29, 30) configured to transfer heat energy from a first end portion (29a, 30a) in thermal contact with an exhaust line part (3 a, 3 c) to a second end portion (29b, 30b) which is in thermal contact with the heat transfer fluid in the flow channel (25) in a position upstream of the evaporator (20) with respect to an intended flow direction of the heat transfer fluid in the flow channel (25).

2. A WHR system according to claim 1, characterized in that the exhaust line (3) comprises a turbine (4a) of a turbocharger (4) and that the first end portion (29a) of the heat pipe (29) is in thermal contact with an exhaust line part (3 a, 3b) located upstream of the turbine (4a) with respect to the intended flow direction of the exhaust gases in the exhaust line (3).

3. A WHR system according to claim 2, characterized in that the first end portion (29a) of the heat pipe (29) is in thermal contact with an exhaust manifold (3 a) of the exhaust line (3).

4. A WHR system according to any one of the preceding claims, characterized in that the exhaust line comprises a exhaust treatment device (6) and that the first end portion (29a, 30a) of the heat pipe (29, 30) is connected to an exhaust line part (3a, 3b, 3c) located upstream of the exhaust treatment device (6) with respect to the intended flow direction of the exhaust gases in the exhaust line (3).

5. A WHR system according to any one of the preceding claims, characterized in that the heat transfer fluid is air.

6. A WHR system according to claim 5, characterized in that the flow means is a fan (26) configured to force an air flow through the flow channel (25).

7. A WHR system according to claim 5, characterized in that the flow channel (25) comprises an opening (25 a) in a front portion of the vehicle (1) and that ram air is configured to force an air flow through the flow channel (25).

8. A WHR system according to claim 7, characterized in that the flow channel (25) comprises a throttle member (32) configured to control the ram air flow through the flow channel (25).

9. A WHR system according to any one of the preceding claims 1 to 4, characterized in that the heat transfer fluid is exhaust gases which are delivered to the flow channel (25) from an exhaust line part (3d) located downstream of the exhaust treatment device (6).

10. A WHR system according to any one of the preceding claims 1 to 4, characterized in that the flow means is a pump (33) and that the heat transfer fluid is a liquid.

11. A WHR system according to any one of the preceding claims 1 to 6 and 10, characterized in that the flow channel (25) forms a closed circuit.

12. A WHR system according to any one of the preceding claims, characterized in that the arrangement comprises a control unit (28) configured to receive information about an operating parameter and to control the flow of the heat transfer fluid through the flow channel (25) in view of this information.

13. A WHR system according to claim 12, characterized in that the arrangement comprises a temperature sensor (31) configured to sense the temperature of the exhaust gases in at least one exhaust line part (3b) and that the control unit (28) is configured to initiate a flow of the heat transfer fluid through the flow channel (25) when the exhaust gases has a temperature above a specific temperature.

14. A vehicle comprising a WHR system according to any one of the preceding claims 1-13.