BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention relates to an arrangement and a method for controlling a WHR system according to the preamble of claims 1 and 11.

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 vapor 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 down in the condenser to a temperature at which it condenses.

It is known to use a compensation tank with an adjustable inner volume for volume compensation and pressure control of the working medium in a WHR system. Such a compensation tank may be designed with an outer rigid tank and an inner rubber bladder receiving liquid working medium. The volume of the rubber bladder and thus the volume of the working medium in the WHR system 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 the 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 lifetime of the compensation tank.

JP2008231981 shows a waste heat recovery apparatus including a Rankine cycle circuit. The Rankine cycle circuit includes in a conventional manner a closed circuit having an evaporator, an expander, a condenser, and a pump. The Rankine cycle circuit also includes a bypass circuit comprising a tank, a recovery valve for recovering liquid working medium from a high pressure circuit portion, a supply valve for supplying liquid working medium to a low pressure circuit portion and a circulation flow volume control means for operating/closing the recovery valve and the supply valve, depending on the supercooling degree of working medium by way of the condenser, and controlling the volume of the working medium that circulates through the closed circuit.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an accurate control of the

condensation pressure and thus a high efficiency of a WHR system comprising a compensation unit having a constant inner volume for accumulation of working medium.

The above-mentioned object is achieved by the arrangement according to claim 1. There is a large volume expansion of the working medium during a start-up phase of a WHR system. In order to provide a high efficiency of the WHR system during a following regular operation phase, it is important that said volume expansion of the working medium is accumulated in the compensation tank. In order to ensure that a sufficient amount of the working medium is directed from the main circuit to the compensation tank during the start-up phase of the WHR system, it is necessary that the pressure in the compensation tank is significantly lower than the pressure in the main circuit.

In order to provide such a pressure difference, the arrangement is configured to replace existing liquid working medium in the compensation tank with vapor working medium, during a shutdown phase of the WHR system. When the compensation tank substantially only comprises vapor working medium, the compensation tank is disconnected from the main circuit before the pump is stopped. During a following inactive period of the WHR system, ambient air cools the vapor working medium in the compensation tank to a temperature at which it condenses. Since liquid requires less space than vapor, the pressure in the compensation unit drops to a level, which preferably is lower than ambient pressure. On the other hand, ambient air also cools the working medium in the main circuit during the inactive period. This cooling results in a pressure drop in the main circuit but a significantly smaller once since the main circuit only contains a relatively small portion of vapor working medium which is able to condense. Preferably, the pressure in the main circuit is above ambient pressure when all vapor working medium has condensed and the pressure in the compensation unit is below ambient pressure when all vapor working medium has condensed. Thus, there is a pressure difference between the working medium in the main circuit and the working medium in the compensation tank before a cold start of the WHR system.

This pressure difference ensures that it is possible to direct a sufficient amount of the volume expanding working medium from the WHR system to the compensation tank during the up-start phase of the WHR system. The main circuit and compensation tank are tight components such that they are able to maintain a positive pressure and a negative pressure during a very long inactive period of the WHR system.

According to an embodiment of the invention, the arrangement comprises a first line configured to direct vapor working medium from the main circuit to the compensation tank during the shut-down phase of the WHR system, a first valve configured to control the working medium flow through the first line, a second line configured to direct liquid working medium, from the compensation tank to the WHR system during the shut-down phase of the WHR system and a second valve configured to control the working medium flow through the second line. With such a design of the arrangement, it is easy to direct vapor working to the compensation tank and remove liquid working medium from the compensation tank. When the compensation tank only contains vapor working medium, the first valve and the second valve is closed such that the working medium in the compensation tank is disconnected from the working medium in the main circuit.

According to an embodiment of the invention, the first line is connected to a main circuit portion arranged between the expander and the condenser and that the second line is connected to a main circuit portion arranged between the condenser and the pump. The first line is connected to the low-pressure side of the main circuit in a position upstream of the condenser and the second line is connected to the low- pressure side of the main circuit in a position downstream of the condenser. Inevitably, the working medium receives a certain pressure drop when it flows through the condenser. In view of this fact, the first line is connected to a main circuit portion having a somewhat higher pressure than the main circuit portion connected to the second line. Thus, when the first valve and the second valve is moved to an open position, this pressure difference provides a vapor working medium flow to the compensation tank which presses out existing liquid working medium from the compensation tank.

According to an embodiment of the invention, the arrangement is configured to maintain the disconnection of the compensation tank from the main circuit during an initial part of the start-up phase of the WHR system. This measure makes it possible to increase the pressure in the main circuit when the working medium is heated and starts to vaporize in the evaporator. The arrangement may be configured to, during the start up phase of the WHR system, direct working medium from main circuit to the compensation tank when the actual pressure in the WHR system exceeds a threshold pressure. The threshold pressure may be a predetermined pressure at which a sufficient amount of the working medium is directed from the main circuit to the compensation tank when a connection is established between the main circuit and the compensation tank.

According to an embodiment of the invention, the arrangement is configured to, during the start-up phase of the WHR system, disconnect the compensation tank from the WHR system when the compensation tank and the main circuit contain working medium of an equalized pressure. When the working medium in the main circuit and the working medium in the compensation tank has the same pressure, a maximum amount of working medium has been directed from the WHR system to the

compensation tank. This measure reduces the amount of working medium circulating in the main circuit during a following regular operating phase of the WHR system. According to an embodiment of the invention, the arrangement comprises a third line configured to direct liquid working medium from the main circuit to the compensation tank during the start-up phase of the WHR system, and a third valve configured to control the working medium flow through the third line. By means of such a design, it is easy to direct liquid working medium from the main circuit to the compensation tank during the start-up phase of the WHR system.

According to an embodiment of the invention, the arrangement comprises an additional compensation unit having an adjustable inner volume configured to accommodate working medium, wherein the additional compensation unit is configured to adjust the volume and the pressure of the working medium in the WHR system during an ordinary operation phase of the WHR system in order to provide a desired condensation pressure. Since the volume fluctuations of the working medium is considerably smaller during the ordinary operation phase than during the start-up phase, the inner volume of the additional compensation unit may be significantly smaller than the inner volume of the compensation tank. The additional compensation unit may be a hydraulic cylinder.

According to an embodiment of the invention, the arrangement is configured to, when the condensation pressure is too low during a regular operation phase, direct vapor working medium from the main circuit to the compensation tank via the first line and direct liquid working medium from the compensation tank to the main circuit via the second line. In this case, vapor working medium is replaced by liquid working medium in the main circuit which increases the amount of working medium in the main circuit. The increased amount of working medium in the main circuit increases the condensation pressure.

According to an embodiment of the invention, the arrangement is configured to, when the condensation pressure is too high during a regular operation phase, direct liquid working medium from the main circuit to the compensation tank via the third line. In this case, the amount of working medium is decreased in the main circuit. The decreased amount of working medium in the main circuit lowers the condensation pressure.

The above-mentioned object is also achieved by the method according to claim 11. 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 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 flow chart according to a start-up phase of the WHR system Fig. 4 shows a flow chart according to a regular operation phase of the WHR system and

Fig. 5 shows a flow chart according to a shutdown phase of the WHR system. 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 main circuit 4 of the WHR system. 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 portion 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 vapor treatment components.

The working medium is evaporated in the evaporator 5. The vapor working medium is directed from the evaporator 5 to an expander 7. The pressurized and heated vapor 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 vapor working medium has passed through the expander 7, it is directed to a condenser 10. The vapor working medium is cooled in the condenser 10 by coolant or air to a temperature at which it condenses. The liquid working medium is sucked from the condenser 10 to the pump 3. A first main circuit portion 4a is arranged between the pump 3 and the evaporator 5. The first main circuit portion 4a is arranged on the high-pressure side of the main circuit 4 and the working medium is in liquid state. A second main circuit portion 4b is arranged between the evaporator 5 and the expander 7. The second main circuit portion 4b is arranged on the high-pressure side of the WHR system and the working medium is in vapor state. A third main circuit portion 4c is arranged between the expander 7 and the condenser 10. The third main circuit portion 4c is arranged on the low-pressure side of the WHR system and the working medium is in vapor state. A fourth main circuit portion 4d is arranged between the condenser 10 and the pump 3. The fourth main circuit portion 4d is arranged on the low-pressure side of the WHR system and the working medium is in liquid state.

An arrangement 11 is used to control working medium flow between the main circuit 4 and a volume compensating tank 12 provided with an inner space of a constant volume for accommodation of working medium. The compensation tank 12 is manufactured by rigid walls of a suitable material. The arrangement 11 comprises a first line 13 extending between an upper portion of the compensation tank 12 and the third main circuit portion 4c, and a first valve 14 controlling the working medium flow through the first line 13. The arrangement 11 comprises a second line 15 extending between a bottom portion of the compensation tank 12 and the fourth main circuit portion 4d, and a second valve 16 controlling the working medium flow through the second line 15. The arrangement 11 comprises a third line 17 extending between a bottom portion of the compensation tank 12 and he first main circuit portion 4a, and a third valve 18 controlling the working medium flow through the third line 17.

The arrangement 11 further comprises a control unit 19 controlling the valves 14, 16,

18 and thus the working medium flow between the main circuit 4 and the

compensation tank 12 via the lines 13, 15, 17. The control unit 19 controls the valves 14, 16, 18 by means of information from a pressure sensor 20a sensing the pressure of the working medium in the fourth main circuit portion 4d and a temperature sensor 20b sensing the temperature of the working medium in the fourth main circuit portion 4d. The pressure sensor 20a senses the pressure in a position immediately downstream of the condenser 10, which is related to the condensation pressure and the condensation temperature of the working medium in the condenser 10. The temperature sensor 20b senses the temperature of the liquid working medium leaving the condenser 10. The information from the sensors 20a, 20b makes it possible to calculate the subcooling of the working medium in the condenser 10.

In this case, the arrangement comprises an additional volume compensation unit 21. The additional volume compensation unit 21 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 configured to accommodate working medium. 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. The control unit 19 control the movements of the actuator 25 and thus the volume of the second space 24 by means of information from the sensors 20a, 20b.

Fig. 2 shows a somewhat simpler embodiment of the WHR system. In this case, the WHR system comprise the same components as the WHR system shown in Fig. 1 except the additional volume compensation unit 21.

Fig 3 shows a flow chart of a start-up phase of the WHR systems shown in Figs 1 and 2. When a cold start of the WHR systems is initiated, the working medium has a temperature corresponding to ambient temperature. The working medium, which in this case is ethanol, is in liquid state in the main circuit 4 and in the compensation tank 12. The start-up phase is initiated at step 31. The compensation tank 12 has been disconnected from the main circuit 4 during a shutdown period of the WHR system and it is still disconnected from the main circuit 4 during an initial part of the start-up phase which is indicated at step 32. Thus, all three valves 14, 16, 18 is in a closed position such they prevent working medium flows, via the lines 13, 15, 17, between the compensation tank 12 and the main circuit 4. At step 33, the pump is started and thus the circulation of the working medium in the main circuit 4. The circulated working medium is vaporized in the evaporator 5 by the exhaust gases. Since the compensation tank 12 is disconnected from the main circuit 4, the heating and the vaporizing of the working medium by the exhaust gases result in a successively increased pressure in the main circuit 4. At step 34, the control unit 19 receives information about the actual pressure in a suitable part of the main circuit 4. In this case, the control unit 19 receives information about the actual condensation pressure from the pressure sensor 20a. Alternatively, the control unit 19 may receive information about the actual pressure in another portion of the main circuit, which for example, may be the first main circuit portion 4a. At step 35, the control unit 19 verifies if the actual pressure has reached a predetermined threshold pressure. The predetermined threshold pressure is a pressure at which it is suitable to direct a sufficient amount of the working medium from the main circuit 4 to the compensation tank 12. As long as the actual pressure is lower than the threshold pressure, the process returns to step 34. When the actual pressure exceeds the threshold pressure, the process continues at step 36. At step 36, the third valve 18 is moved to an open position such that liquid working medium of the actual high pressure flows from the first main circuit portion 4a, via the third line 17, to the compensation tank 12.

The volume expansion of the working medium is very large during the start-up phase of the WHR system. By means of the third line 17 and the third valve 18, it is possible to direct the entire expanding volume of the working medium from the main circuit 4 to the compensation tank 12 due to the pressure difference of the working medium in the main circuit 4 and the compensation tank 12. When the pressure of the working medium in the first main circuit portion 4a and the pressure in the compensation tank 12 have been equalized, the third valve 18 is moved to the closed position such that the compensation tank 12 again is disconnected from the main circuit 4. The start-up phase of the WHR system is completed at step 39. During a following regular operating phase of the WHR system, a significantly reduced amount of the working medium is circulated in the main circuit 4 since a relatively large part of the working medium is isolated in the compensation tank 12.

Fig 4 shows a flow chart of a regular operation phase of the WHR systems shown in Figs. 1 and 2. The regular operation phase starts at 41. At step 42, the control unit 19 receives information about the actual condensation pressure from the pressure sensor 20a. At step 43, the control unit 19 estimates or calculates a desired condensation pressure at which the WHR system has a high thermal efficiency. In order to provide 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 main circuit 4. 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. At step 44, the control unit 19 verifies if the actual condensation pressure corresponds to the desired condensation pressure. If this is the case, the process continues at step 46 where it is verified if the regular operation phase of the WHR system is to continue. If this is the case, the process is moved back to step 42. If this is not the case, the regular operation phase ends at step 47.

In case there is a difference between the actual condensation pressure and the desired condensation pressure, the process continues at step 45 at which this difference is adjusted. In the WHR system shown in Fig. 1, this difference is adjusted in the following manner. The control unit 19 activates the actuator 25 such that it adjusts the volume of the second space 24. In case the actual condensation pressure is too low, the control unit 19 activates the actuator 25 such that it moves the piston 23 to a position in which the second space 24 obtains a smaller volume. This measure increases the pressure of the working medium in the main circuit 4 and thus the condensation pressure. In case the actual condensation pressure is too high, the control unit 19 activates the actuator 25 such that it moves the piston 23 to a position in which the second space 24 obtains a larger volume. This measure reduces the pressure of the working medium in the main circuit 4 and thus the condensation pressure. The response time for changing the volume and the pressure of the working medium is, for example, related to the amount of the working medium in the main circuit 4. During the regular operating phase of the WHR system, the amount of working medium circulating in the main circuit 4 is relatively small since a relatively large proportion of the working medium is isolated in the compensation tank 12. In view of this fact, it is possible to change the volume of the second space 24 and the actual condensation pressure with a fast response time and with a high accuracy.

In the embodiment shown in Fig. 2, the control unit 19 controls the valves 14, 16, 18 during the regular operation phase in order to adjust the actual condensation pressure in the WHR system. If the actual condensation pressure is lower than a desired condensation pressure, the control unit 19 opens the first valve 14 and the second valve 16. Since the vapor working medium in the third main circuit portion 4c has a higher pressure than the liquid working medium in the fourth main circuit portion 4d, vapor working medium is directed into the compensation tank 12, which lowers the liquid working medium level in the compensation tank 12. The vapor working medium presses out the existing liquid working medium from the compensation tank 12 to the fourth main circuit portion 4d. Since liquid has a higher density than vapor, this measure increases the amount of working medium in the WHR system, which results in a higher pressure in the WHR system and a higher condensation pressure.

On the other hand, if the actual condensation pressure is higher than the desired condensation pressure, the control unit 19 opens the third valve 18 such that liquid working medium of a high pressure is directed from the first main circuit portion 4a, via the third line 17, to the compensation tank 12. This measure reduces the amount of the working medium in the main circuit 4, which results in a lower pressure in the main circuit 4 and a lower condensation pressure. Alternatively, the control unit 19 also opens the first valve 14. In this case, liquid working medium of a high pressure is directed from the first main circuit portion 4a, via the third line 17, to the

compensation tank 12 at the same time as vapor working medium is directed from the compensation tank 12, via the first line 13, to the third main circuit portion 4c. It is to be noted that it is also possible to use this adjustment of the condensation pressure in the embodiment of the WHR system shown in Fig. 1. In the WHR system shown in Fig. 1, small adjustments of the condensation pressure may be performed by the cylinder 21 and large adjustments by means of the valves 14, 16, 18. When the condensation pressure has been adjusted to the desired condensation pressure, the process continues at step 46. At step 46, it is verified if the regular operation phase of the WHR system is to continue. If this is the case, the process returns to step 42. If this is not the case, the regular operation phase ends at step 47. The end of the regular operating phase is followed by a shutdown phase of the WHR system.

Fig 5 shows a flow chart of the shutdown phase of the WHR systems shown in Figs. 1 and 2. The shutdown phase of the WHR systems is initiated at step 51. At step 52, the control unit 19 opens the first valve 14 and the second valve 16. Since the vapor working medium in the third main circuit portion 4c has a somewhat higher pressure than the liquid working medium in the fourth main circuit portion 4d, vapor working medium is directed from the third main circuit portion 4c, via the first line 13, to the compensation tank 12. The supply of the vapor working medium to the compensation tank 12 results in a lowered liquid working medium level in the compensation tank 12. At step 53, the lowered liquid medium level results in a liquid working medium flow out from the compensation tank 12, via the second line 15, to the fourth main circuit portion 4d. The flow of liquid working medium from the compensation tank 12 continues until the compensation tank 12 is emptied of liquid working medium. At step 54, the control unit 19 closes the valves 14, 16, 18 such that the compensation tank 12 is disconnected from the main circuit 4. At step 55, the control unit 19 shuts down the pump 3 such that the circulation of the working medium in the main circuit 4 is stopped.

When the compensation tank 12 has been disconnected from WHR system 4, the compensation tank 12 only contains vapor working medium. The vapor working medium in the compensation tank 12 has a higher temperature than ambient air. When the WHR system has been inactivate during a certain period, the vapor working medium in the compensation tank 12 is cooled by ambient air to a temperature at which it condenses at step 56. In view of the fact that liquid occupies a smaller volume than vapor, the pressure in the compensation tank 12 will be considerably lower when the vapor working medium has been condensed into liquid. On the other hand, the main circuit 4 is filled with a relatively large amount of liquid working medium and a small amount of vapor working medium when it is disconnected from the

compensation tank 12. In view of this fact, a corresponding cooling of the working medium in the main circuit 4 results in a significantly lesser pressure drop than in the main circuit 4 than in the compensation tank 12. Thus, the pressure in the

compensation tank 12 is significantly lower than the pressure in the main circuit 4 when all vapor working medium has been cooled down and condensed. This pressure difference is necessary for providing a sufficient working medium from the main circuit 4 to the compensation tank 12 during a following start-up phase of the WHR system. The shutdown phase of the WHR system ends at step 57.

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