BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention relates to a WHR system for a vehicle according to the preamble of claim 1. A WHR system (Waste Heat Recovery System) can be used in vehicles for recovering waste thermal energy and convert it to mechanical energy or electric energy. A WHR system includes a pump which pressurizes and circulates a working medium in a closed circuit. The circuit comprises one or several evaporators where the working medium is heated and evaporated by one or several heat sources such as, for example, the exhaust gases from a combustion engine. The pressurized and heated gaseous working medium is directed to an expander where it expands. The expander generates mechanical energy which can be used to operate the vehicle or apparatuses on the vehicle. Alternatively, the expander is connected to a generator generating electric energy. The working medium leaving the expander is directed to a condenser. The working medium is cooled down in the condenser to a temperature at which it condenses. The liquefied working medium is redirected to the pump which pressurizes the 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, a condenser in a WHR system is made by stainless steel in order to be resistant against high pressures, high temperatures and a corrosive working medium such as ethanol. Stainless steel is a relatively expensive material and it heat transfer properties are not exceptional. Consequently, a condenser of stainless steel is heavy and expensive.

US 2013/0186087 shows a waste heat recovery system for use with an internal combustion engine. The system comprises a recuperator operatively connected to a working fluid circuit downstream of an expander and upstream of a condenser to recover heat from the working fluid before the working fluid flows through the condenser. The recuperator delivers heat energy from the expanded working fluid to the working fluid downstream of the condenser. SUMMARY OF THE INVENTION

The object of the present invention is to provide a WHR system including an inexpensive condenser which has a low weight.

The above mentioned object is achieved by the WHR system according to claim 1. Aluminum is a relatively inexpensive material and it has exceptional heat transfer properties. In view of these facts, aluminum is a very suitable material to use in a condenser. However, it is not suitable to use an aluminum condenser at too high temperatures and pressures. Furthermore, aluminum may corrode in contact with certain working mediums. The corrosion resistance of aluminum is strongly related to the temperature of the working medium. It has been found that the corrosive resistance is acceptable when the temperature of the working medium entering the condenser is not too high. The WHR system comprises a cooling arrangement configured to cool the working medium such that the temperature of the working medium is prevented from rising to a level where it can be harmful to the aluminum condenser. With a suitable restriction of the temperature of the working medium directed to the condenser, the WHR system can be provided with a condenser of aluminum. In view of the initially mentioned properties of aluminum, such a condenser can have a low weight and be inexpensive.

According to an embodiment of the invention, the cooling arrangement is configured to prevent that the working medium entering the condenser at a temperature above a maximum allowable temperature. It is possible to determine a maximum temperature where the working medium is harmless to the aluminum condenser. In this case, the cooling arrangement can be activated during operating conditions when the temperature of the working medium approaches the maximum allowable temperature. During remaining operating conditions when the temperature of the working medium is well below the maximum allowable temperature, the cooling arrangement is not activated.

According to an embodiment of the invention, the cooling arrangement comprises a cooling component configured to cool the working medium in a position downstream of the expander and upstream of the condenser. In order to prevent that the working medium entering the condenser at a too high temperature, it is suitable to cool the working medium in said position.

According to an embodiment of the invention, the cooling arrangement is able to cool the working medium with a variable cooling capacity. In this case, it is possible to control the temperature of the working medium entering the condenser with a high precision. The cooling arrangement may comprise a temperature sensor sensing the temperature of the working medium, a control unit configured to receive information from the temperature sensor about the temperature of the working medium and a to control the supply of a cooling fluid to the cooling component in order to restrict the temperature of the working medium entering the condenser. By means of such a cooling arrangement it is relatively easy to control the temperature of the working medium entering the condenser. According to an embodiment of the invention, the cooling component may be a heat exchanger in which the working medium is cooled by a coolant flow. In this case, it is possible to use coolant circulating in an existent cooling system cooling a combustion engine in a vehicle. Alternatively, the cooling component is a cooling coil in which the working medium is cooled by a cooling air flow. In this case, the cooling arrangement has a simple and reliable design. Preferably, such a heat exchanger and cooling coil are manufactured by stainless steel. The heat exchanger and the cooling coil may be relatively small in relation to the condenser since they only have the task to reduce the overheating of the gaseous working medium before it enters the condenser. The condensation process of the working medium has to be performed in the condenser. The control unit may be configured to control the speed of a fan forcing an air flow towards the cooling coil. In this case, it is possible to vary the cooling capacity of the working medium by varying the cooling air flow rate towards the cooling coil.

According to an embodiment of the invention, the working medium in the WHR- system is ethanol. Ethanol has an evaporation temperature of about 78°C at 1 bar. It is relatively easy to accomplish a coolant temperature at a suitable level below the evaporation temperature of ethanol and cool the ethanol in a condenser to a condensation temperature just above 78°C. However, ethanol is very corrosive to aluminum at high temperature. In this case, it is very important to restrict the temperature of the working medium to a suitable maximum temperature where the ethanol is harmless to the aluminum condenser. A maximum acceptable temperature at about 150°C is suitable in this case. During the most operating conditions, the temperature of the working medium leaving the expander is lower than 150°C. Higher temperature of the working medium may, for example, occur when the expander is pre-heated for a start up to avoid condensation on a cold surface of the expander. However, it is possible to use other working mediums such as for example R245fa.

According to an embodiment of the invention, the working medium is heated in an evaporator of the WHR-system by means of exhaust gases from a combustion engine. The exhaust gases from a combustion engine contains a lot of heat energy, which usually is supplied to the environment. By means of a WHR-system, it is possible to recover a large part of the heat energy in the exhaust gases. Due to the high temperatures of the exhaust gases, the WHR system may comprise an evaporator manufactured by stainless steel. BRIEF DESCRIPTION OF THE DRAWING

In the following a preferred embodiment of the invention is described, as an example, with reference to the attached drawing, in which: Fig. 1 shows a WHR system according to a first embodiment of the invention and

Fig. 2 shows a WHR system according to a second embodiment of the

invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 shows a combustion engine 2 powering a schematically disclosed vehicle 1. The combustion engine 2 may be a diesel engine. The vehicle 1 may be a heavy vehicle. 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 closed a circuit 4. In this case, the working medium is ethanol. However, it is possible to use other kinds of working mediums such as for example R245fa. The pump 3 circulates the working medium to an evaporator 5. The working medium is heated in the evaporator 5 by exhaust gases directed out the combustion engine 2 in an exhaust line 6. The exhaust line 6 comprises a bypass line 6a and a valve 6b by which it is possible to direct a variable part of the exhaust gases past the evaporator 5. An exhaust line 6 directs exhaust gases from the combustion engine 2. The exhaust line 6 may comprise components which are not indicated in Fig. 1 such as a turbine of a turbo charger and exhaust gas treatment components. The working medium is heated in the evaporator 5 by the exhaust gases to a temperature at which it evaporates. The evaporator is made of stainless steel. The working medium is circulated from the evaporator 5 to an expander 7. The pressurized and heated working medium is expanded in the expander 7. The expander 7 generates a rotary motion which is transmitted, via a suitable mechanical transmission 8a, to a shaft 8b of the power train of the vehicle 1. Alternatively, the expander 7 may be connected to a generator transforming mechanical energy into electrical energy. The electrical energy may be stored in a battery. After the working medium has passed through the expander 7, it is directed to a cooling coil 9 where it may be cooled by a cooling air flow provided by a 10. The fan 10 is driven by an electric motor 11. The electric motor 11 may drive the fan 10 with a variable speed. The speed of the fan 10 and the air flow rate directed to the cooling coil 9 may be changed in a stepless manner. Thus, the cooling capacity of the working medium in the cooling coil 9 may also be changed in a stepless manner.

A temperature sensor 12 senses the temperature of the working medium in a position downstream of the cooling coil 9 and upstream of a condenser 13. The condenser 13 is made by aluminum. A control unit 14 receives information from the temperature sensor 12 and controls the electric motor 11 and the fan 10 in view of this information such that the working medium is cooled to a suitable temperature before it enter the condenser 13. The control unit 10 may also control the operation of the pump 3 and the expander 7. The working medium is cooled in the condenser 13 by coolant circulating in a cooling circuit 15 to a temperature at which it condenses. The coolant may have a temperature of about 70°C. The condensed working medium is directed from the condenser 13 to a tank 16. The pump 3 sucks working medium from the tank 16 and direct it to the evaporator 5. The WHR-system makes it possible to transform thermal energy from the exhaust gases to mechanical energy or electrical energy.

A condenser of a conventional WHR system is made by stainless steel such that it is able to withstand high pressures, high temperatures and a corrosive working medium such as ethanol. However, stainless steel has lower heat transfer properties than aluminum, it is more expensive than aluminum and it has a higher weight. Thus, a condenser of stainless steel with corresponding capacity as a condenser made of aluminum has to be significantly larger, heavier, and more expensive. On the other hand, a condenser of aluminum have a lower resistance to high temperatures and pressures. Furthermore, aluminum is very corrosive to certain a working medium such as ethanol at high temperatures. In order to provide a WHR system comprising a small, lightweight and inexpensive aluminum condenser and a working medium in the form of ethanol, the temperature of the working medium shall not be higher than a maximum allowable temperature. Such a maximum allowable temperature is in this case about 150°C. At higher temperatures of the working medium, there is a great risk of corrosion of aluminum condenser.

During operation of the combustion engine 2, it is desired to maintain a high thermal efficiency in the WHR-system. In order to maintain a high thermal efficiency of the WHR system, the working medium is to be cooled in the condenser 13 to 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 cool of the working medium in the condenser 13 to a condensation pressure just above 1 bar. Consequently, in order to maintain a high thermal efficiency, the control unit 14 may control the temperature and/or the coolant flow in the circuit 15 to the condenser 13 such that the condensation pressure will be just above 1 bar. The working medium ethanol has a condensation temperature of 78°C at the condensation pressure 1 bar. In this case, it is suitable to accomplish a condensation temperature of just above 78°C in the condenser 13. The control unit 14 receives continuously information from the temperature sensor 12 about the temperature of the working medium entering the condenser 13. As soon as the control unit 14 receives information indicating that the working medium has a temperature which is well below 150°C, there is no reason to start the fan 9.

During operating conditions when the control unit 14 receives information from the temperature sensor 12 indicating that the working medium has a temperature close to the maximum allowable temperature, it activates the electric motor 11 and the fan 10 such that it forces a suitable cooling air flow through the cooling coil 9. The cooling air flow cools the working medium such that it obtains a somewhat lower than said maximum allowable temperature. The control unit 14 may control the electric motor 11 and the fan 10 such a variable air flow rate cools the working medium in the cooling coil 9. In this case, it is possible to maintain a suitable temperature of the working medium with a high precision. During operating conditions when the control unit 14 receives information from the temperature sensor 12 indicating that the working medium has temperature well below the maximum allowable temperature, it does not activate the electric motor 11 and the fan 10.

Fig. 2 shows an alternative embodiment of the WHR system. In this case, the working medium is cooled in a heat exchanger 17 by a circulating coolant. The coolant circulates in a circuit 18 connected to a cooling system cooling the combustion engine 2. The coolant may have a temperature of about 90-110°C when it enters the heat exchanger 17. The control unit 14 controls the supply of the coolant to the heat exchanger 17 by means of a valve 19. The heat exchanger 17 is made of stainless steel. The heat exchanger 17 is small in relation to the condenser 13 since the task of the heat exchanger 17 is only to cool overheated gaseous working medium such that it does not have a temperature above 150°C when it enters the condenser 13. Also in this case, it is possible to provide a WHR system comprising aluminium condenser 13.

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