TECHNICAL FIELD

The present invention relates to a waste heat recovery system, methods for controlling such a system, and a vehicle comprising such a waste heat recovery system.

BACKGROUND ART

Vehicle manufacturers are today striving to increase engine efficiency and reduce fuel consumption. This is especially an issue for manufacturers of heavy vehicles, such as trucks and buses. One way of improving engine efficiency and fuel consumption is waste heat recovery. In vehicles with combustion engines most of the energy from the fuel does not produce useful work, but instead is dissipated as heat through the exhaust pipes and the engine cooling system. By the use of a waste heat recovery system the waste heat may instead be used to heat various vehicle components or to produce electricity or mechanical work. Such mechanical work may for example be transferred to the driveline and thus be used to propel the vehicle.

A waste heat recovery (WHR) system typically comprises at least one heat exchanger transferring heat between a heat source, such as exhaust gases, and a working fluid. The heat transfer between the working fluid and the heat source is an exchange of energy resulting in a change in temperature of both the working fluid and heat source. A waste heat recovery system may for example be based on a Rankine cycle, or an organic Rankine cycle for low temperature heat recovery. Such systems typically comprise a working fluid, a pump for circulating the working fluid in a circuit, at least one evaporator (heat exchanger), an expansion device (expander), a condenser and an expansion tank for receiving excess working liquid. The working fluid in such waste heat recovery system is suitably in a liquid state to start with. The pump pressurizes the working fluid which is pumped through the evaporator. In the evaporator, the working fluid is heated by heat exchange with a heat source, for example exhaust gases, led through the evaporator. This causes the working fluid to evaporate. The resulting vapour is subsequently expanded in the expansion device, e.g. a turbine. By means of the expansion device the recovered heat may thereby be converted into mechanical work or electrical energy. The vapour is thereafter cooled in the condenser, such that the working fluid is brought back to its initial liquid state. The condenser is thus typically connected to a cooling system, which may be part of the engine cooling system or a separate cooling system. After condensing back to the liquid state, the working fluid may be received in the expansion tank. The working fluid received in the expansion tank is thus ready for further pumping around the WHR circuit.

It is essential that the working fluid after the on the low-pressure side of the WHR circuit, i.e. between the condenser and the pump inlet, is sub-cooled, i.e. cooled to a temperature below the boiling point at the pressure prevailing in the low-pressure side of the WHR circuit. The boiling point may also be referred to as the saturation temperature or condensation temperature of the working fluid. If the working fluid is not sufficiently sub-cooled, transient increases in temperature or decreases in pressure may cause undesired boiling or flashing of the liquid. This may for example result in cavitation in the working fluid pump, potentially leading to damage of pump components. In order to avoid such problems, the working fluid entering the pump must be sufficiently subcooled, i.e. held under conditions sufficiently removed from saturation.

DE 102009050068 A1 describes an internal combustion engine with a cooling circuit and a Clausius-Rankine cycle for waste heat recovery. In the Clausius-Rankine cycle a surge tank is provided to compensate for volume and/or pressure fluctuations. In an embodiment, the surge tank comprises a primary chamber and a secondary chamber, which are divided by a membrane. The primary chamber is in communication with the Clausius-Rankine cycle and the secondary chamber is in communication with a pressure regulator. This makes it possible to control or regulate a condensing pressure and thus a condensation temperature of the Clausius-Rankine cycle working fluid.

There remains a need for improved means for controlling a waste heat recovery system. SUMMARY OF THE INVENTION

The inventors of the present invention have identified a number of shortcomings in prior art waste heat recovery systems. Expansion tanks comprising a membrane or bladder capable of being actively pressure regulated are complex in design, require a source of pressurizing fluid such as compressed air (which is not readily available in all vehicles), and, most importantly, are prone to premature failure. A potential factor in the premature failure of the expansion tank is the constant mechanical stress the membrane or bladder material undergoes due to expansion and contraction of the expansion tank volume. Another potential factor is the often poor compatibility between the material of the membrane/bladder, which is often rubber or a synthetic elastomer, and the working fluid, which is often an organic solvent.

It would be advantageous to achieve a waste heat recovery system overcoming, or at least alleviating, at least some of the above mentioned shortcomings. In particular, it would be desirable to provide a waste heat recovery system that has an improved longevity, especially with regard to the tank for storing working fluid.

In order to better address one or more of these concerns, a waste heat recovery system for a vehicle is provided, the waste heat recovery system having the features defined in the independent claims.

The waste heat recovery system comprises:

- a working fluid pump;

- an evaporator;

- an expander;

- a condenser;

- a pump outlet line arranged to channel working fluid from the working fluid pump to the evaporator;

- an evaporator outlet line arranged to channel working fluid from the evaporator to the expander;

- an expander outlet line arranged to channel working fluid from the expander to the condenser; - a condenser outlet line arranged to channel working fluid from the condenser to the working fluid pump;

- a receiver tank comprising a tank inlet and a tank outlet;

- a tank inlet line arranged to channel working fluid from a first junction in the condenser outlet line to the tank inlet; and

- a tank outlet line arranged to channel working fluid from the tank outlet to a second junction in the condenser outlet line, wherein the second junction is arranged between the first junction and the working fluid pump.

The receiver tank has substantially constant inner volume or a constant inner volume and is equipped with a heater having a heater inlet adapted to be fluidly connectable with an engine cooling system, and wherein the heater is arranged to facilitate heat transfer from a coolant fluid in the heater to a working fluid in the receiver tank.

By utilizing a receiver tank having a constant or during operation substantially constant inner volume, the receiver tank volume is no longer required to contract and expand in order to regulate the WHR system, and the receiver tank may therefore be constructed of robust, relatively inelastic materials that tolerate prolonged contact with working fluid. The receiver tank may for example be constructed from metal, such as stainless steel. Due to the receiver tank being equipped with a heater utilizing coolant fluid from the cooling system as heating medium, working fluid contained in the receiver tank may be vaporized using the heater. Vaporizing working fluid in this manner leads to an increase in pressure in the receiver tank and its environs, including within the condenser. This increase in condensation pressure leads to a corresponding increase in condensation temperature, and means that the degree of sub cooling achieved by the condenser may be controlled by the heater. Note that the heat contained in the coolant fluid would otherwise be lost in the radiator, so the present invention effectively utilizes heat that otherwise would be discarded in order to control the WHR system.

According to another aspect of the present invention, an integrated system for a vehicle is provided, in accordance with the appended claims. The integrated system comprises a waste heat recovery system and an engine cooling system. the waste heat recovery system for the integrated system comprises: - a working fluid pump;

- an evaporator;

- an expander;

- a condenser;

- a pump outlet line arranged to channel working fluid from the working fluid pump to the evaporator;

- an evaporator outlet line arranged to channel working fluid from the evaporator to the expander;

- an expander outlet line arranged to channel working fluid from the expander to the condenser;

- a condenser outlet line arranged to channel working fluid from the condenser to the working fluid pump;

- a receiver tank comprising a tank inlet and a tank outlet;

- a tank inlet line arranged to channel working fluid from a first junction in the condenser outlet line to the tank inlet; and

- a tank outlet line arranged to channel working fluid from the tank outlet to a second junction in the condenser outlet line, wherein the second junction is arranged between the first junction and the working fluid pump.

The engine cooling system for the integrated system comprises:

- a radiator;

- an engine outlet channel arranged to collect coolant fluid exiting an engine; and

- a radiator return line arranged to channel coolant fluid from the engine outlet channel to the radiator.

In the integrated system the receiver tank is equipped with a heater having a heater inlet arranged in fluid connection with the engine outlet channel via a heater feed line. The heater feed line is arranged to channel coolant fluid from the engine outlet channel to the heater inlet. The heater is arranged to facilitate heat transfer from coolant fluid in the heater to working fluid in the receiver tank. The temperature of the coolant fluid in the engine outlet channel is relatively consistent and is hot enough to provide for vaporization of the working fluid in the receiver tank when fed through the heater. Such an integrated system therefore readily permits control of the condensation temperature of the working fluid in the condenser. A heater return line may be arranged to channel coolant fluid from an outlet of the heater to the engine outlet channel or radiator return line. The heater return line may form a junction with the engine outlet channel at a point in closer flow proximity to the radiator return line than a junction of the heater feed line with the engine outlet channel.

A recuperator may be arranged to facilitate heat transfer from coolant fluid in the heater return line to working fluid in the pump outlet line. This allows recovery of heat that otherwise would be lost in the radiator, and thus increases the overall efficiency of the WHR system.

A heater by-pass line may be arranged to channel coolant fluid from the heater feed line to the heater return line, wherein the heater by-pass line forms a junction with the heater return line at a point between the heater and the recuperator. In such a case, a heater by-pass valve may be arranged at a junction of the heater by-pass line and the heater feed line, wherein the heater by-pass valve is arranged to controllably redirect a flow of coolant fluid from the heater feed line to the heater by-pass line. Such an arrangement allows coolant fluid to be directed through the recuperator even when coolant fluid flow though the heater is not desired. This further increase the proportion of waste heat from the engine that may be converted to useful work in the WHR system.

A heater feed valve may be arranged at a junction of the heater feed line and the engine outlet channel, wherein the heater feed valve is arranged to controllably redirect a flow of coolant fluid from the engine outlet channel to the heater feed line. This allows the flow of coolant fluid passing through the heater to be readily controlled without altering the coolant fluid flow to other components in the cooling circuit, such as the engine and/or condenser. Thus, overall control of the WHR is facilitated.

A heater feed pump may be arranged in the heater feed line or the heater return line, wherein the heater feed pump is arranged to controllably direct a flow of coolant fluid from the engine outlet channel to the heater feed line. This allows the flow of coolant fluid through the heater to be controlled independently of the coolant pump and permits a flow of coolant fluid in the heater even when the coolant pump is not circulating coolant fluid in the main cooling circuit.

A recuperator may be arranged to facilitate heat transfer from coolant fluid in the engine outlet channel to working fluid in the pump outlet line. This allows a flow of coolant fluid to be directed through the recuperator independently of any coolant fluid flow though the heater. This further increase the proportion of waste heat from the engine that may be converted to useful work in the WHR system.

A recuperator feed line may be arranged to channel coolant fluid from the engine outlet channel to the recuperator. In such a case, a recuperator return line may be arranged to channel coolant fluid from the recuperator to the engine outlet channel or radiator return line. The recuperator feed line may be arranged to junction with the engine outlet channel at a point downstream of the junction with the heater return line, i.e. in closer flow proximity to the radiator return line than the heater return line. The recuperator return line may be arranged to junction with the engine outlet channel at a point downstream of the junction with the recuperator feed line return line. A recuperator feed pump may be arranged in the recuperator feed line or recuperator return line, wherein the recuperator feed pump is arranged to controllably direct a flow of coolant fluid from the engine outlet channel to the recuperator feed line. Such an arrangement allows a flow of coolant fluid to be directed through the recuperator independently of any coolant fluid flow through the heater or in the main cooling circuit. For example, this permits a flow of coolant fluid through the recuperator even when the coolant pump is not circulating coolant fluid in the main cooling circuit.

A first controllable flow regulating means may be arranged in the tank inlet line, and a second controllable flow regulating means may be arranged in the tank outlet line. Such flow regulating means allow the receiver tank to be isolated. For example, during shutdown of the WHR system, the WHR system may be controlled to ensure that vaporized working fluid is present only in the receiver tank and the rest of the system is filled with liquid working fluid. Once such a state is achieved, the receiver tank may be isolated by closure of the first and second flow regulating means, and the working fluid pump may then be switched off to stop working fluid circulating in the WHR system. Upon further cooling of the working fluid, the vaporized working fluid condenses and sub-atmospheric pressures are generated in the receiver tank. However, the receiver tank is easily designed to withstand such low pressures. Other WHR system components which are more difficult to seal against sub-atmospheric pressures, such as the expander, are therefore protected from exposure to such conditions.

A third controllable flow regulating means may be arranged in the condenser outlet line between the first junction and the second junction. This allows the flow of working fluid to be directed via the receiver tank or directly through the condenser outlet line as desired.

The receiver tank may be arranged to separate gaseous working fluid from liquid working fluid. The receiver tank may be arranged to channel liquid working fluid to the tank outlet.

Such an arrangement ensures that the working fluid channelled to the working fluid pump is always in liquid form and prevents damage to the pump by, for example, cavitation.

A desiccator medium may be contained in the receiver tank. This allows the removal of moisture from the working fluid and ensures predictable and reliable behaviour of the working fluid.

The waste heat recovery system of the integrated system may be a subcooler free system. The ability to regulate condensation pressure using the heater renders a sub-cooler unnecessary.

According to another aspect of the invention, a method for controlling an integrated system as described herein is provided, the method being in accordance with the appended claims. The method comprises a step of increasing condensation pressure in the condenser by controllably directing a flow of coolant fluid from the engine outlet channel through the heater. By directing a flow of coolant fluid from the engine outlet channel through the heater, working fluid contained in the receiver tank is vaporized and the condensation pressure is increased. This provides a corresponding increase in the condensation temperature and allows the condensation temperature of the working fluid to be controlled without resorting to the use of working fluid tanks having a variable volume that can be regulated, as known in the prior art.

According to a further aspect of the invention, a method for shutting down an integrated system as described herein is provided, the method being in accordance with the appended claims. The method comprises a step of, during a shutdown phase of the waste heat recovery system, isolating the receiver tank by closing the first controllable flow regulating means and second controllable flow regulating means, such that working fluid contained in a remainder of the waste heat recovery system has a temperature below a condensation temperature of the working fluid at ambient pressure. This allows the sub-atmospheric pressures generated upon shutdown of the WHR system to be localised to the receiver tank, where such pressures can be easily accommodated. Thus, WHR system components which are much more difficult to engineer to withstand negative pressures, such as the expander, are therefore not required to withstand such pressures.

According to yet another aspect of the invention, a vehicle comprising a waste heat recovery system or integrated system as described herein is provided.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which:

Fig. 1 schematically illustrates a vehicle according to the invention;

Fig. 2 schematically illustrates a WHR system and integrated system according to an embodiment of the invention;

Fig. 3 schematically illustrates a WHR system and integrated system according to

another embodiment of the invention;

Fig. 4 schematically illustrates a WHR system and integrated system according to a further embodiment of the invention;

Fig. 5 is a flowchart schematically illustrating a method for controlling the WHR system according to the invention; and Fig. 6 is a flowchart schematically illustrating a method for shutting down the WHR system according to the invention.

DETAILED DESCRIPTION The present invention is based upon the insight that a waste heat recovery system comprising a receiver tank may be controlled to obtain appropriate subcooling in the condenser by equipping the receiver tank with a heater and using the heater to vaporize working fluid in the receiver tank, thus controlling the condensation pressure at the condenser. The heater operates using hot coolant fluid from the engine cooling system as the heating medium. The waste heat recovery system may be based on a Rankine cycle or organic Rankine cycle. The waste heat recovery system comprises a working fluid pump; an evaporator; an expander; a condenser; and a receiver tank equipped with a heater. The working fluid is compressed and pumped as a liquid by the pump to the evaporator. In the evaporator, the working liquid is heated and vaporised by heat exchange from a heat source passing through the evaporator. The working fluid, now in the gaseous phase, flows to the expander where it is allowed to expand, doing mechanical work. The expanded vapour is then cooled back to liquid again by the condenser. In this application, the term "downstream" as applied to the WHR system is defined as the typical direction of flow of the working fluid in the WHR circuit from the working fluid pump via the evaporator, expander and condenser to the receiver tank. A variety of working fluids may be chosen for use in the waste heat recovery system, depending on the quality of the heat source(s) to be utilised. The working fluid may be water, or may be an organic liquid such as for example ethanol or R-245fa.

The working fluid pump of the waste heat recovery system may be of any type known in the art, and may, for example, suitably be electrically driven or mechanically driven. In the evaporator, the working liquid is heated and vaporised by heat exchange from a heat source. The evaporator may be of any type known in the art, for example a plate heat exchanger. The heat source may be any available source of waste heat in a vehicle, such as the vehicle exhaust gases, hot oil from a retarder or hot liquids from the vehicle cooling system. The waste heat recovery system may have a number of evaporators, each for a separate source of waste heat.

The working fluid is vaporised in the evaporator, and therefore the working fluid arriving at the expander should be in gaseous form. A channel bypassing the expander, equipped with a valve, may be provided in the working fluid circuit in order to direct non-evaporated fluid past the expander without passing through the expander. This may for example be useful during start up and initial operation of the vehicle, or if no mechanical work is needed from the WHR system.

The expander can be of any type known in the art, including but not limited to turbine, screw, scroll, or piston expanders. The mechanical work produced in the expander may be provided to a generator for electricity production, or may be transferred to the vehicle powertrain, e.g. the crankshaft, using for example a clutch or freewheel.

The condenser may be of any type known in the art. The condenser condenses the working fluid back to the liquid phase. It may be connected to a cooling circuit, which may for example be the standard engine cooling system, or may be provided with a dedicated cooling system. The cooling power of the condenser may be regulated in order to ensure that an appropriate degree of subcooling at an advantageous temperature is obtained. This may be performed by regulating the temperature of a coolant fluid passing through the condenser, or by regulating the flow of coolant fluid through the condenser. However, the condenser may not be able to provide sufficient cooling in all circumstances, such as for example during full load conditions. The present invention provides a further means of regulating subcooling by regulating pressure in the low-pressure side of the waste heat recovery system by heating working fluid contained in the receiver tank.

The condensed working fluid is collected in a receiver tank. The receiver tank has a fixed inner volume, i.e. it does not possess a membrane capable of varying the inner volume of the tank as known in prior art solutions. Since the receiver tank is not required to vary in volume, it may be manufactured from strong and stable materials, for example from metals such as stainless steel. This considerably increases the service life of the tank as compared to tanks having for example rubber bladders or membranes. The receiver tank may be configured as commonly known in the art in order to facilitate gravitational phase separation of working fluid into a vapour phase and a liquid phase. This may be achieved by having the receiver tank outlet located at the bottom of the tank (as orientated when mounted on the vehicle). This ensures that working fluid in the vapour phase is trapped in the receiver tank and is not conveyed through the WHR system towards e.g. the pump. The receiver tank inlets may be located high up in the receiver tank (as orientated when mounted on the vehicle). Liquid working fluid entering the tank thus falls to the bottom of the tank and vaporized working fluid forms a vapour column above the pooled liquid working fluid. The receiver tank may contain a desiccant material in order to remove any water entering the waste heat recovery system. The receiver tank may further comprise a filter arranged to prevent circulation of solids or particulates in the WHR system.

The receiver tank inlet and outlet are connected to the condenser outlet line, i.e. the conduit channelling working fluid between the condenser and the working fluid pump. The tank inlet is connected by a receiver inlet line and the tank outlet is connected by a receiver outlet line arranged downstream of the receiver inlet line. Flow regulating means, such as controllable stop valves, may be arranged in the receiver inlet line and receiver outlet line. This allows the receiver tank to be isolated when required. For example, during shutdown of the WHR system, the system may be controlled to ensure that only the receiver tank is subjected to sub-atmospheric pressures and that the rest of the system is filled with liquid working fluid and thus not subject to sub-atmospheric pressures. This facilitates design and construction of the WHR system since it means that only the seals of the receiver must be designed to withstand sub-atmospheric pressure, and that other seals in the system must only protect against leakage outwards from the system due to elevated pressures.

A flow regulating means, such as a controllable stop valve, may be arranged in the section of the condenser outlet line between the junction with the receiver inlet line and the junction with the receiver outlet line. This allow flow of working fluid to be directed to the receiver tank or to bypass the receiver tank as desired.

During normal operation of the waste heat recovery system the receiver tank typically contains a volume of liquid working fluid, and may for example normally be approximately half-full with liquid working fluid. This is because the volume of working fluid required to fill the circuit is greater at lower temperatures than at higher temperatures, and the volume of working fluid should be dimensioned to be capable of always filling the circuit. However, the receiver tank harbours the excess working fluid, thus preventing overfilling of the system.

The present invention utilizes vaporization of the working fluid contained in the receiver tank in order to control subcooling of the working fluid from the condenser. By vaporizing working fluid in the receiver tank, the pressure and/or volume occupied in the receiver tank by vaporized working fluid is increased. Assuming that the receiver tank has a constant volume, this increase in volume of the vapour phase is compensated by lowering the liquid level in the receiver tank, thus forcing working fluid into the condenser and resulting in a greater degree of subcooling of working fluid in the condenser. This effect may be utlilized for example in situations where the condenser is incapable of providing sufficient cooling power to subcool the working fluid. On the contrary, if the condenser is providing excessive subcooling, the pressure prevailing in the receiver tank should be decreased. This may for example be achieved by directing a flow of cooled working fluid from the condenser through the receiver tank in order to lower the temperature in the receiver tank.

In order to provide for vaporization of the working fluid contained in the receiver tank, the receiver tank is equipped with a heater. Heating medium passing through the heater provides heat transfer to the working fluid contained in the receiver, thus acting to vaporize the working fluid in the receiver and increase the condensation pressure. Thus, the receiver tank may resemble a shell and tube heat exchanger with the receiver tank corresponding to the shell and the heater corresponding to the tube.

In the present invention the heating medium passing through the heater is coolant fluid redirected from the engine outlet channel in the engine cooling system. Coolant fluid redirected from this line typically has a relatively consistent temperature and is usually sufficiently hot in order to provide vaporization of the working fluid in the receiver tank. For example, the coolant fluid exiting the engine is typically approximately 95 °C, and the boiling point of ethanol, which is a commonly utilized working fluid in organic Rankine systems, is approximately 78 °C at atmospheric pressure. The engine outlet channel may have a relatively large volume and thus act as a reservoir for hot coolant fluid, from which flows of coolant may be drawn off and returned without adversely impacting upon the fluid dynamics of the cooling system. The engine cooling system may suitably comprise a coolant pump arranged to circulate a coolant fluid, an engine outlet channel arranged to collect hot coolant fluid exiting the engine, a radiator arranged for cooling the coolant fluid, and one or more valve units for controlling the flow of the coolant fluid through the cooling system. The term "downstream" as applied to the engine coolant system is defined as the typical direction of flow of the coolant fluid in the engine cooling circuit from the coolant pump via the engine, engine outlet channel and radiator return line to the radiator. A heater feed line and return line channels coolant fluid to and from the heater. If the condenser is also cooled from the engine cooling system, it may for example be cooled using a dedicated condenser feed loop branching off from the main cooling circuit and controlled by one or more valves or pumps. This condenser fed loop may preferably be supplied with cold coolant fluid, for example coolant fluid having exited the radiator and not having yet passed through the engine.

The WHR system may comprise a recuperator that utilises the heat of the coolant fluid leaving the heater in order to preheat the working liquid prior to entering the evaporator. This reduces heat loss in the radiator and improves the overall conversion of waste engine heat into useful mechanical work. The recuperator may be any type of recuperator known in the art, such as a counter-current heat exchanger utilizing tuber or plates. The recuperator may be arranged in series with the heater such that coolant fluid flowing through the recuperator has necessarily previously passed through the heater. However, the recuperator and heater may be arranged such that hot coolant fluid may always flow through the recuperator, regardless of whether it has previously passed through the heater or not. This may be achieved by providing the engine cooling system with a heater by-pass arrangement (line and valve) allowing the heater to be by passed. This may alternatively be achieved by having separate heater and recuperator feed loops (feed line and return line) running from the engine outlet channel to the heater and recuperator respectively. These feed loops may be equipped with dedicated pumps (heater feed pump and/or recuperator feed pump), thus allowing a flow of coolant fluid through the feed loop even when the coolant pump is not pumping.

The waste heat recovery system and/or engine cooling system may be configured in an alternative manner or may comprise further components as known in the art. For example, the WHR and/or engine cooling systems may comprise sensors, such as temperature and pressure sensors. The WHR and/or engine cooling systems may comprise further valves. The WHR system may comprise further condensers in order to cool the working fluid in several stages, or further expanders in order to expand the working fluid in several stages. However, the present invention results in a lesser need for a dedicated subcooler in the waste heat recovery system and therefore such a component may not be required. The waste heat recovery system and/or engine cooling system may suitably be controlled using a control unit. The control unit may suitably be connected to the waste heat recovery system and/or the cooling system. The control unit may suitably be connected to the evaporator, the expander and the pump of the waste heat recovery system. The control unit may suitably be connected to the coolant pump and any further means of regulating the cooling system, such as further pumps or control valves. The control unit may suitably be connected to any valves controlling the flow of heat source through the evaporator. The control unit may be the engine control unit or may comprise a plurality of different control units. A computer may be connected to the control unit.

The temperature of the working liquid leaving the condenser may be measured directly or it may be determined virtually. For example, if the temperature and flow of vapour entering the condenser is known, and the cooling characteristics of the condenser are known, the temperature of the working fluid leaving the condenser may be readily determined.

The invention will now be described in more detail with reference to certain exemplifying embodiments and the drawings. However, the invention is not limited to the exemplifying embodiments discussed herein and/or shown in the drawings, but may be varied within the scope of the appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate certain features.

Figure 1 schematically illustrates a side view of a vehicle 1 comprising an internal combustion engine 3, and a waste heat recovery system 9 associated with the internal combustion engine 3. The vehicle furthermore comprises a cooling system 71 associated with the internal combustion engine 3 and connected to the waste heat recovery system 9. The vehicle may further comprise a transmission 5 connected to the driving wheels 7 of the vehicle 1. The vehicle 1 may be a heavy vehicle, e.g. a truck as herein illustrated or a bus. The vehicle may alternatively be a passenger car. Furthermore, the vehicle may be a hybrid vehicle comprising an electric machine (not shown) in addition to the combustion engine 3. The vehicle may alternatively be a marine vessel, such as a ship.

Figure 2 schematically shows an integrated system comprising a waste heat recovery system 9 and cooling system 71 associated with a combustion engine 3 of a vehicle 1 according to an embodiment of the invention.

The waste heat recovery system 9 comprises a working fluid pump 11; an evaporator 13; an expander 15, here shown as a turbine; a condenser 17; and a receiver tank 27 for working fluid. Pump outlet line 19 connects the pump 11 to the evaporator 13. Evaporator outlet line 21 connects the evaporator 13 to the expander 15. Expander outlet line 23 connects the expander 15 to the condenser 17. Condenser outlet line 25 connects the condenser to the working fluid pump 11, and thus completes the working fluid circuit. The receiver tank has an inlet 29 connected to the condenser outlet line 25 by receiver inlet line 33, and an outlet 31 connected to the condenser outlet line 25 by receiver outlet line 37. The junction 39 of the receiver outlet line 37 with the condenser outlet line 25 is arranged downstream of the junction 35 of the receiver inlet line 37 with the condenser outlet line 25. Flow regulating means 57 and 59 are arranged in the receiver inlet line 33 and receiver outlet line 37 respectively. A further flow regulating means 61 is arranged in the condenser outlet line 25 between junctions 35 and 39.

The evaporator 13 is arranged for heat exchange between the working fluid and a heat source (not shown) associated with the combustion engine. The condenser 17 of the waste heat recovery system 9 is connected to the engine cooling system 71. Here the condenser cooling system is illustrated as a dedicated condenser cooling loop comprising condenser inlet line 107 and condenser return line 105, but alternative arrangements are possible. Alternatively, the condenser cooling system may be a cooling system entirely separate to the engine cooling system 71.

During routine operation the working fluid in the waste heat recovery system is pumped from low pressure to high pressure by the working fluid pump 11 and enters the evaporator 13. The working fluid is thereby heated by the heat source (not shown) connected to the evaporator 13 and the working fluid is evaporated. The working fluid vapour is then expanded in the expander 15 whereby mechanical work is produced and the temperature and the pressure of the vapour is decreased. The mechanical work may for example be transferred to the transmission 5 of the vehicle as illustrated, and may thus be used to propel the vehicle. The working fluid vapour thereafter enters the condenser 17 where condensation through heat exchange between the vapour and the coolant fluid of the cooling system 71 brings the working fluid back to its initial liquid state. The working fluid is then transported to receiver tank 27 via tank inlet 29, or conveyed directly to working fluid pump 11, depending on the status of flow regulating means 57, 59 and 61. The receiver tank 27 allows liquid working fluid to separate from working fluid vapour, and the liquid working fluid harboured in the receivertank is thus stored in a form ready for further pumping around the WHR circuit. Overall, the heat source (e.g. exhaust gas) provides the energy entering the waste heat recovery system 9 and the energy leaves the waste heat recovery system 9 as mechanical work via the expander 15 and as heat via the cooling system 71.

In some circumstances the condenser 17 is unable to provide sufficient cooling power in order to sufficiently subcool the working fluid leaving the condenser. The present invention addresses this problem by allowing working fluid harboured in receiver tank 27 to be vaporized, thus increasing the condensation pressure and condensation temperature at the outlet of the condenser 17.

To this end, the receiver tank 27 is equipped with a heater 41, and the heater 41 is heated using a flow of hot coolant fluid from the engine cooling system 71.

The engine cooling system 71 comprises a coolant pump 95 arranged to circulate a coolant fluid through the cooling system, an engine outlet channel 74 arranged to collect hot coolant fluid having passed through the combustion engine 3, a radiator 73 arranged for cooling the coolant fluid having passed through the combustion engine 3, and a radiator return line 75 for channelling coolant fluid from the engine outlet channel 74 to the radiator 73. The coolant pump 95 may be driven mechanically by the combustion engine 3, but it may also be electrically driven, or at least controllable. The engine cooling system 71 further comprises a radiator bypass line 101, a first valve unit 97 and a second valve unit 99. The first valve unit 97 is suitably arranged to control the flow of coolant fluid through the radiator 73 and the bypass line 101 respectively. The second valve unit 99 is suitably arranged to control the flow of coolant fluid passing through the condenser 17 of the waste heat recovery system 9. In order to supply hot coolant fluid to the heater 41, a heater feed line 49 is arranged to channel coolant fluid from the engine outlet channel 74 to an inlet 43 of the heater 41. A heater return line 77 is arranged to channel working fluid having passed through the heater 41 back to the radiator return line 75. A heater feed valve 85 is arranged to controllably redirect coolant fluid flow from the engine outlet channel 74 to the heater feed line 49 as desired. If coolant fluid is redirected through heater 41, working fluid contained in the receiving tank 27 will be evaporated, and the condensation pressure at the outlet of condenser 17 will be raised. This leads to a higher condensation temperature and a lesser need of cooling power in condenser 17 in order to achieve subcooling, thereby ensuring that sufficient subcooling is provided by the condenser.

Figure 3 schematically illustrates another embodiment of the present invention. In this embodiment heater return line 77 is routed via a recuperator 55 arranged in the pump outlet line 19 in order to pre-heat working fluid prior to entering evaporator 13. After passing through the recuperator 55, the working fluid is routed onwards towards the radiator return line 75 as previously. This embodiment allows the recovery of heat from the coolant fluid passing through the heater 41 and thus increases the potential output of the waste heat recovery system.

Another feature of this embodiment is the ability to channel coolant fluid between the heater feed line 49 and heater return line 77 without passing through the heater 41. This is achieved by arranging a heater by-pass line 79 between the heater feed line 49 and heater return line 77. A controllable heater by-pass valve 83 directs the flow of coolant fluid through the heater 41 or bypass line 79. Such an arrangement permits the passage of coolant fluid through the recuperator 55 without it necessarily having previously passed through the heater 41.

It can further be seen in this embodiment that instead of a heater feed valve to direct flow to the heater inlet line 49, a heater feed pump 87 is used instead. Having a dedicated pump to provide a flow of coolant fluid through the heater 41 and/or recuperator 55 allows a flow of coolant through the heater and/or recuperator even whenever the coolant pump 95 is switched off.

Figure 4 schematically illustrates a further embodiment of the present invention. In this embodiment, the heater 41 and recuperator 55 are arranged in separate feed loops. The heater 41 is arranged in a heater feed loop comprising a heater feed line 49, heater 41, and heater return line 77. The heater feed loop draws coolant fluid from the engine outlet channel 74 and returns the coolant to the engine outlet channel 74 after having passed through the heater 41. A heater feed pump 87 is arranged in the heater return line 77 to be able to drive the flow of coolant fluid through the heater 41, even if coolant is not circulating in the main coolant loop. The recuperator 55 is arranged in a recuperator feed loop comprising a recuperator feed line 89, recuperator 55, and recuperator return line 91. The recuperator feed loop draws coolant fluid from the engine outlet channel 74 and returns the coolant to the radiator return line 75 after hanving passed through the recuperator 55. A recuperator feed pump 93 is arranged in the recuperator feed line 93 to be able to drive the flow of coolant fluid through the recuperator 55, even if coolant is not circulating in the main coolant loop. By arranging the heater 41 and recuperator 55 each in a separate loop with a dedicated pump, each of these components may be controlled independently of each other and the main cooling circuit.

Figure 5 is a flowchart schematically illustrating a method for controlling a waste heat recovery system according to the invention. Step s601 denotes the start of the method. In step s603 the waste heat recovery system is operated routinely, as described herein; i.e. working fluid is not routed via the heater 41. Step s605 denotes a decision: is the condenser providing sufficient subcooling of the working fluid? This may for example be determined using pressure and/or temperature sensors located in the waste heat recovery system, or by determining the cooling power of the condenser. If the answer is YES, the method returns to step s603. If the answer is NO the method proceeds to step s607. In step s607 condensation pressure is increased in the condenser by controllably directing a flow of coolant fluid from the engine outlet channel 74 through the heater 41. This has the effect of increasing the condensation temperature of the working fluid and ensuring that the condenser may provide sufficient subcooling. Step s609 denotes the end of the method.

On the contrary, if the condenser 17 is providing excessive subcooling, the temperature prevailing in the receiver tank 27 should be decreased. This may for example be achieved by directing a flow of cooled working fluid from the condenser 17 through the receiver tank 27 in order to lower the temperature in the receiver tank 27.

Figure 6 is a flowchart schematically illustrating a method for shutting down a waste heat recovery system according to the invention. The method is performed during a shutdown phase of the waste heat recovery system in order to ensure that gaseous working liquid is isolated in the receiver tank 27 and that the rest of the system is filled with liquid working fluid. Liquid gathers in the cooler parts of the WHR system and vapour gathers in the hotter parts. Therefore, in order to localize vaporized working fluid to the receiver tank 27, working fluid should be circulated and the receiver tank 27 should be heated until it is the hottest part of the WHR system. Step s701 denotes the start of the method. In step s703 working fluid is circulated around the waste heat recovery system via heater 41 in order to heat receiver tank 27. Step s705 denotes a decision: does vaporized working fluid remain in the main waste heat recovery circuit, that is to say all waste heat recovery components with the exception of the receiver tank? If the answer is YES, the method returns to step s703. If the answer is NO the method proceeds to step s707. In step s707 the receiver tank is isolated by closing the first controllable flow regulating means 57 and second controllable flow regulating means 59, such that working fluid contained in the main waste heat recovery circuit has a temperature below a condensation temperature of the working fluid at ambient pressure, i.e. is liquid. The working fluid now isolated in the receiver tank will gradually cool, leading to the generation of sub-atmospheric pressures in the receiver tank. However, the receiver tank may easily be provided with seals able to withstand such low pressures. Other WHR components are much more difficult to seal effectively against sub-atmospheric pressures, especially the expander, so by limiting the occurrence of such pressures to the receiver tank, the design and construction of the WHR system is simplified. In step s709 the working fluid pump is shut off. Step s711 denotes the end of the method.