SE542593C2 - Waste heat recovery system comprising receiver tank connected to vapour side - Google Patents

Abstract

The present disclosure relates to a waste heat recovery (WHR) system (9) for a vehicle (1). The WHR system comprises a receiver tank (27) having a substantially constant inner volume and being equipped with a second tank inlet (63) arranged in fluid connection with an evaporator outlet line (21).The present disclosure further relates to methods for controlling, starting-up and shutting down such a WHR system, as well as a vehicle comprising such a WHR system.

Description

1 Waste heat recovery system comprising receiver tank connected to vapour side 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 fuelconsumption. This is especially an issue for manufacturers of heavy vehicles, such as trucksand buses. One way of improving engine efficiency and fuel consumption is waste heatrecovery. ln vehicles with combustion engines most ofthe energy from the fuel does notproduce useful work, but instead is dissipated as heat through the exhaust pipes and theengine cooling system. By the use of a waste heat recovery system the waste heat may insteadbe 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 exchangertransferring heat between a heat source, such as exhaust gases, and a working fluid. The heattransfer between the working fluid and the heat source is an exchange of energy resulting in achange in temperature of both the working fluid and heat source. A waste heat recoverysystem may for example be based on a Rankine cycle, or an organic Rankine cycle for lowtemperature heat recovery. Such systems typically comprise a working fluid, a pump forcirculating the working fluid in a circuit, at least one evaporator (heat exchanger), anexpansion device (expander), a condenser and an expansion tank for receiving excess workingliquid. The working fluid in such waste heat recovery system is suitably in a liquid state to startwith. The pump pressurizes the working fluid which is pumped through the evaporator. ln theevaporator, 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 2 resulting vapour is subsequently expanded in the expansion device, e.g. a turbine. By means ofthe expansion device the recovered heat may thereby be converted into mechanical work orelectrical energy. The vapour is thereafter coo|ed in the condenser, such that the working fluidis brought back to its initial liquid state. The condenser is thus typically connected to a coolingsystem, which may be part of the engine cooling system or a separate cooling system. Aftercondensing 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. lt 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-coo|ed, i.e. coo|ed to a temperature belowthe boiling point at the pressure prevailing in the low-pressure side ofthe WHR circuit. Theboiling point may also be referred to as the saturation temperature or condensationtemperature of the working fluid. lf the working fluid is not sufficiently sub-coo|ed, transientincreases in temperature or decreases in pressure may cause undesired boiling or flashing ofthe liquid. This may for example result in cavitation in the working fluid pump, potentiallyleading to damage of pump components. ln order to avoid such problems, the working fluidentering 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 aClausius-Rankine cycle for waste heat recovery. ln the Clausius-Rankine cycle a surge tank isprovided to compensate for volume and/or pressure fluctuations. ln an embodiment, thesurge tank comprises a primary chamber and a secondary chamber, which are divided by amembrane. The primary chamber is in communication with the Clausius-Rankine cycle and thesecondary chamber is in communication with a pressure regulator. This makes it possible tocontrol 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 artwaste heat recovery systems. Expansion tanks comprising a membrane or bladder capable ofbeing actively pressure regulated are complex in design, require a source of pressurizing f|uidsuch 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 ofthe expansiontank is the constant mechanical stress the membrane or bladder material undergoes due toexpansion and contraction ofthe expansion tank volume. Another potential factor is the oftenpoor compatibility between the material of the membrane/bladder, which is often rubber or a synthetic elastomer, and the working f|uid, which is often an organic solvent lt would be advantageous to achieve a waste heat recovery system overcoming, or at leastalleviating, at least some ofthe above mentioned shortcomings. ln particular, it would bedesirable to provide a waste heat recovery system that has an improved longevity, especially with regard to the tank for storing working f|uid. ln order to better address one or more ofthese concerns, a waste heat recovery system for avehicle is provided, the waste heat recovery system having the features defined in the independent claims.The waste heat recovery system comprises: - a working f|uid pump; - an evaporator; - an expander and an expander bypass valve; - a condenser; - a pump outlet line arranged to channel working f|uid from the working f|uid pump to theevaporator; - an evaporator outlet line arranged to channel working f|uid from the evaporator to theexpander; - an expander outlet line arranged to channel working f|uid from the expander to the condenser; 4 - a condenser outlet line arranged to channel working fluid from the condenser to the workingfluid pump; - a receiver tank comprising a first tank inlet and a tank outlet; - a tank inlet line arranged to channel working fluid from a first junction in the condenseroutlet line to the first tank inlet; and - a tank outlet line arranged to channel working fluid from the tank outlet to a second junctionin the condenser outlet line, wherein the second junction is arranged between the first junction and the working fluid pump.

The receiver tank has a substantially constant inner volume or constant inner volume and is equipped with a second tank inlet arranged in fluid connection with the evaporator outlet line.

By utilizing a receiver tank having a constant or during operation substantially constant innervolume, the receiver tank volume is no longer required to contract and expand in order toregulate the WHR system, and the receiver tank may be fully constructed of relatively inelasticmaterials that tolerate prolonged contact with working fluid. The receiver tank may forexample be constructed from metal, such as stainless steel. Due to the receiver tank beingequipped with a second tank inlet connected to the evaporator outlet line, the pressureprevailing in the receiver tank may be controlled by addition of high-pressure vapour from thevapour side of the WHR system. Providing vapour to the receiver tank in this manner leads toan 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 condensationtemperature, and means that the degree of sub-cooling achieved by the condenser may be controlled by the heater.

An expander by-pass line may be arranged to channel working fluid from the evaporatoroutlet line to the second tank inlet. An expander by-pass valve in this case may be arranged ata junction of the expander by-pass line and evaporator outlet line, wherein the expander by-pass valve is arranged to controllably redirect a flow of working fluid from the evaporatoroutlet line to the expander by-pass line. Such an arrangement provides a simple means offurnishing the receiver tank with vaporized working fluid, since many WHR systems alreadyutilize an expander by-pass line. Therefore, no excessive redesign or supplementary components will be required. Arranging the expander by-pass line in this manner also means 5that working fluid redirected to by-pass the expander will not be cooled in the condenser, thus avoiding heat loss from the WHR system during for example system start-up.

The receiver tank may be equipped with a heater arranged to heat working fluid in thereceiver tank. The heater may be used to heat working fluid contained in the receiver tank,thus vaporizing working fluid and raising the pressure in the low-pressure side ofthe WHRsystem. This results in a corresponding rise in condensation temperature ofthe working fluidin the condenser and means that the heater may be used as a further means of controlling subcooling in the WHR system.

The heater may be an electric heater. This provides for a robust and easily regulated means of heating working fluid in the receiver tank.

The heater may comprise a heater inlet adapted to be fluidly connectable with an enginecooling system, and wherein the heater is arranged to facilitate heat transfer from a coolantfluid in the heater to a working fluid in the receiver tank. Thus, the heater may utilize heatfrom the engine cooling system in order to control the WHR system. This heat is typicallyotherwise removed in the radiator, and thus this arrangement may provide a use for heat that otherwise would be more-or-less useless.

The heater may comprise a heater inlet arranged in fluid connection to the evaporator outletline or expander outlet line, and wherein the heater is arranged to facilitate heat transfer fromworking fluid in the heater to working fluid in the receiver tank. Thus, the heater may utilize heat already contained in the working fluid in order to control the WHR system.

A recuperator may be arranged to facilitate heat transfer from working fluid or coolant fluid ina heater return line to working fluid in the pump outlet line. This allows recovery of heat thatotherwise would be lost in the condenser and thus increases the overall efficiency ofthe WHR syste m.

A first controllable flow regulating means may be arranged in the tank inlet line, and a secondcontrollable flow regulating means may be arranged in the tank outlet line. Such flowregulating means allow the receiver tank to be isolated. For example, during shutdown of theWHR 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. 6 Once such a state is achieved, the receiver tank may be isolated by closure ofthe first andsecond flow regulating means, and the working fluid pump may then be switched off to stopworking fluid circulating in the WHR system. Upon further cooling ofthe working fluid, thevaporized working fluid condenses and sub-atmospheric pressures are generated in thereceiver 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 linebetween 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 workingfluid. 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 ofmoisture from the working fluid and ensures predictable and reliable behaviour of the working fluid.

The waste heat recovery 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 a waste heat recoverysystem described herein is provided. The method comprises a step of increasing condensationpressure in the condenser by controllably directing a flow of gaseous working fluid from theevaporator outlet line to the receiver tank via the second tank inlet. By directing a flow ofhigh-pressure vaporized working fluid from the evaporator outlet line to the receiver tank, thecondensation pressure is increased. This provides a corresponding increase in thecondensation temperature and allows the condensation temperature of the working fluid tobe controlled without resorting to the use of working fluid tanks having a variable volume that can be regulated, as known in the prior art. 7 According to a further aspect of the invention, a method for starting up a waste heat recoverysystem described herein is provided. The method comprises a step of, during a start-up phaseof the waste heat recovery system, bypassing the expander and the condenser by controllablydirecting a flow of working fluid from the evaporator outlet line to the receiver tank via thesecond tank in|et. During start-up of the WHR system the evaporator may not providesufficient heat in order to fully vaporize the working fluid. Passing non-va porized working fluidthrough the expander may damage the expander. The method described allows working fluidto bypass the expander, thus avoiding potentially damaging the expander. At the same time,the working fluid bypasses the condenser, meaning that heat is not removed from the workingfluid. This may allow the working fluid in the WHR system to attain a suitable operating temperature considerably quicker during start-up.

According to a yet another aspect of the invention, a method for shutting down a waste heatrecovery system described herein is provided. The method comprises a step of, during ashutdown phase of the waste heat recovery system, isolating the receiver tank by closing thefirst controllable flow regulating means and second controllable flow regulating means, suchthat working fluid contained in a remainder of the waste heat recovery system has atemperature below a condensation temperature ofthe working fluid at ambient pressure. Thisallows the sub-atmospheric pressures generated upon shutdown of the WHR system to belocalised to the receiver tank, where such pressures can be easily accommodated. Thus, WHRsystem components which are much more difficult to engineer to withstand negative pressures, such as the expander, are not required to withstand such pressures.

According to yet a further aspect of the invention, a vehicle comprising a waste heat recovery 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 8 drawings, in which the same reference notations denote similar items in the various diagrams, and in which: Fig.

Fig.

Fig.

Fig.

Fig.

Fig.

Fig.

Fig. schematically illustrates a vehicle according to the invention; schematically illustrates WHR system according to an embodiment ofthe invention; schematically illustrates WHR system according to another embodiment of the invention; schematically illustrates WHR system according to a further embodiment of the invention; schematically illustrates WHR system according to yet another embodiment of the invention; is a flowchart schematically illustrating a method for contro||ing the WHR system according to the invention; is a flowchart schematically illustrating a method for starting up the WHR system according to the invention 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 raising the pressure at the low-pressure side of the WHR system, which includes the condenser and receiver tank. This is done by equipping the receiver tank with second inlet connected to the high-pressure vapour side ofthe WHR system, allowing high-pressure vapour to be transferred to the receiver tank. ln this manner, condensation pressure at the condenser may be controlled and the corresponding condensation temperature is also controlled. 9The waste heat recovery system may be based on a Rankine cycle or organic Rankine cycle. Thewaste heat recovery system comprises a working fluid pump; an evaporator; an expander; acondenser; and a receiver tank equipped with a heater. The working fluid is compressed andpumped as a liquid by the pump to the evaporator. ln the evaporator, the working liquid isheated and vaporised by heat exchange from a heat source passing through the evaporator. Theworking 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 thecondenser. ln this application, the term ”downstream” as applied to the WHR system is definedas 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, dependingon 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 ofthe waste heat recovery system may be of any type known in the art, and may, for example, suitably be electrically driven or mechanically driven. ln the evaporator, the working liquid is heated and vaporised by heat exchange from a heatsource. The eva porator may be ofany 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 vehicleexhaust gases, hot oil from a retarder or hot liquids from the vehicle cooling system. The wasteheat 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 theexpander 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 theexpander without passing through the expander. This may for example be useful during start up and initial operation ofthe 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 fluidback to the liquid phase. lt may be connected to a cooling circuit, which may for example be thestandard engine cooling system, or may be a dedicated cooling system. The cooling power ofthe condenser may be regulated in order to ensure that an appropriate degree of subcooling atan advantageous temperature is obtained. This may be performed by regulating thetemperature of a cooling fluid passing through the condenser, or by regulating the flow ofcooling fluid through the condenser. However, the condenser may not be able to providesufficient cooling in all circumstances, such as for example during full load conditions. Thepresent invention provides a further means of regulating subcooling by regulating pressure inthe 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 innervolume, i.e. it does not possess a membrane capable of varying the inner volume ofthe tank asknown in prior art solutions. Since the receiver tank is not required to vary in volume, it may bemanufactured 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 facilitategravitational phase separation of working fluid into a vapour phase and a liquid phase. This maybe achieved by having the receiver tank outlet located at the bottom of the tank (as orientatedwhen mounted on the vehicle). This ensures that working fluid in the vapour phase is trappedin the receiver tank and is not conveyed through the WHR system towards e.g. the pump. Thereceiver tank inlet may be located high up in the receiver tank (as orientated when mounted onthe vehicle). Liquid working fluid entering the tank thus falls to the bottom of the tank andvaporized working fluid forms a vapour column above the pooled liquid working fluid. Thereceiver tank may contain a desiccant material in order to remove any water entering the wasteheat recovery system. The receiver tank may further comprise a filter arranged to prevent circulation of solids or particulates in the WHR system. 11The first receiver tank inlet and the receiver tank outlet are connected to the condenser outletline, i.e. the conduit channelling working fluid between the condenser and the working fluidpump. The first tank inlet is connected by a receiver inlet line and the tank outlet is connectedby a receiver outlet line arranged downstream ofthe receiver inlet line. Flow regulating means,such as controllable stop valves, may be arranged in the receiver inlet line and receiver outletline. This allows the receiver tank to be isolated when required. For example, during shutdownof the WHR system, the system may be controlled to ensure that only the receiver tank issubjected to sub-atmospheric pressures and that the rest of the system is filled with liquidworking fluid and thus not subject to sub-atmospheric pressures. This facilitates design andconstruction of the WHR system since it means that only the seals of the receiver must bedesigned 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 ofthe condenser outlet line between the junction with the receiver inlet line and the junction withthe 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 containsa volume of liquid working fluid, and may for example normally be approximately half-full withliquid working fluid. This is because the volume of working fluid required to fill the circuit isgreater at lower temperatures than at higher temperatures, and the volume of working fluidshould 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 transfer of vapour from the evaporator outlet line to the receivertank in order to control subcooling of the working fluid from the condenser. By vaporizingworking fluid in the receiver tank, the pressure and/or volume occupied in the receiver tank byvaporized 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 thereceiver tank, thus forcing working fluid into the condenser and resulting in a greater degree ofsubcooling 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 12working fluid. On the contrary, if the condenser is providing excessive subcooling, the pressureprevailing in the receiver tank should be decreased. This may for example be achieved bydirecting a flow of cooled working f|uid from the condenser through the receiver tank in order to lower the temperature in the receiver tank.

Without wishing to be bound by theory, the transfer of vaporized working f|uid to the receivertank may be conceptualised both as a transfer of pressure and as a transfer of energy. |nitia||yupon transfer of vapour the primary effect will be that of an immediate increase in pressure dueto the high-pressure vapour being transferred to the receiver tank. However, over time thetransferred superheated vapour will equilibrate with the liquid working f|uid phase containedin the receiver tank, leading to evaporation of a proportion of the liquid working f|uid and a further increase in condensation pressure. ln order to provide for transfer of vapour from the evaporator outlet line to the receiver tank,the receiver tank is provided with a second tank inlet. This second tank inlet may arranged inf|uid connection with the evaporator outlet line via an expander by-pass line arranged tochannel working f|uid from the evaporator outlet line to the second tank inlet. An expander by-pass valve may be arranged at the junction of the evaporator outlet line and expander by-passline in order to controllably redirect working f|uid from the evaporator outlet line to theexpander by-pass line. This has the advantage that line and valve channelling working f|uid tothe receiver tank may also serve the purpose of a traditional expander by-pass valve and line.Since waste heat recovery systems commonly comprise such a by-pass this means that extracomponents may be avoided. Furthermore, since working f|uid being passed through this by-pass line does not pass through the condenser, heat contained in the working f|uid passingthrough the by-pass line may be conserved, which is of utility during for example start-up ofthe WHR system. ln order to provide for an alternative means of vaporization of the working f|uid contained inthe receiver tank, the receiver tank may also be equipped with a heater. The heater may becontrollably heated by any means known in the art in order to provide heat transfer to theworking f|uid contained in the receiver. This acts to vaporize the working f|uid in the receiverand increase the condensation pressure. Thus, the condensation pressure may be controlled by use of the second tank inlet, the heater, or by a combination of both. 13The heater may for example be an electric heater. Alternatively, the heater may provide heattransfer to the working fluid contained in the receiver tank from a heating medium passingthrough the heater. Thus, the receiver tank may resemble a shell and tube heat exchanger withthe receiver tank corresponding to the shell and the heater corresponding to the tube. Theheating medium may for example be hot coolant fluid from the engine cooling system, such asfrom the radiator return line of the engine cooling system. The heating medium mayalternatively be hot working liquid from the WHR system. The working fluid as heating mediummay for example redirected from the expander outlet line. This has the advantage of using working fluid containing only waste heat as the heating medium.

The WHR system may comprise a recuperator that utilises the heat of the heating mediumleaving the heater in order to preheat the working liquid prior to entering the evaporator. Thisreduces heat loss in the condenser and/or radiator and improves the output of the WHR system.The recuperator may be any type of recuperator known in the art, such as a counter-currentheat exchanger utilizing tuber or plates. The recuperator may be arranged in series with theheater such that heating medium flowing through the recuperator has necessarily previouslyflowed through the heater. However, the recuperator and heater may be arranged such that the heater may be by-passed and the recuperator is still provided with heating medium.

The waste heat recovery system may be configured in an alternative manner or may comprisefurther components as known in the art. For example, the WHR system may comprise sensors,such as temperature and pressure sensors. The WHR system may comprise further valves. TheWHR system may comprise further condensers in order to cool the working fluid in severalstages, 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 may suitably be controlled using a control unit. The control unitmay suitably be connected to the waste heat recovery system and/or the cooling system. Thecontrol unit may suitably be connected to the evaporator, the expander and the pump of thewaste heat recovery system. The control unit may suitably be connected to the cooling pumpand 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 14through 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. lf an engine cooling system is connected to the condenser and/or heater of the waste heatrecovery system, it may suitably comprise a coolant pump arranged to circulate a coolant fluid,a radiator arranged for cooling the coolant fluid, and one or more valve units for controlling theflow of the coolant fluid through the cooling system. The condenser and/or heater may forexample have dedicated loops branching off from the main cooling circuit and controlled by one or more valves or pumps.

The temperature of the working liquid leaving the condenser may be measured directly or itmay be determined virtually. For example, if the temperature and flow of vapour entering thecondenser 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 exemplifyingembodiments and the drawings. However, the invention is not limited to the exemplifyingembodiments discussed herein and/or shown in the drawings, but may be varied within thescope 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 combustionengine 3, and a waste heat recovery system 9 associated with the internal combustion engine3. The vehicle may furthermore comprise a cooling system 71 associated with the internalcombustion engine 3 and connected to the waste heat recovery system 9. The vehicle furthercomprises a transmission 5 connected to the driving wheels 7 ofthe vehicle 1. The vehicle 1 maybe a heavy vehicle, e.g. a truck as herein illustrated or a bus. The vehicle may alternatively be apassenger 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 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; anexpander 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 21connects the evaporator 13 to the expander 15. Expander outlet line 23 connects the expander15 to the condenser 17. Condenser outlet line 25 connects the condenser to the working fluidpump 11, and thus completes the working fluid circuit. The receiver tank has a first inlet 29connected to the condenser outlet line 25 by receiver inlet line 33, and an outlet 31 connectedto the condenser outlet line 25 by receiver outlet line 37. The junction 39 of the receiver outletline 37 with the condenser outlet line 25 is arranged downstream of the junction 35 of thereceiver inlet line 37 with the condenser outlet line 25. Flow regulating means 57 and 59 arearranged 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 heatrecovery system 9 is connected to the engine cooling system 71. Here the condenser coolingsystem is illustrated as a dedicated condenser cooling loop comprising condenser inlet line 107and condenser return line 105, but alternative arrangements are possible. Alternatively, thecondenser 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 fromlow pressure to high pressure by the working fluid pump 11 and enters the evaporator 13. Theworking fluid is thereby heated by the heat source (not shown) connected to the evaporator 13and the working fluid is evaporated. The working fluid vapour is then expanded in the expander15 whereby mechanical work is produced and the temperature and the pressure of the vapouris decreased. The mechanical work may for example be transferred to the transmission 5 ofthevehicle as illustrated, and may thus be used to propel the vehicle. The working fluid vapourthereafter enters the condenser 17 where condensation through heat exchange between thevapour and the cooling fluid of the cooling system 71 brings the working fluid back to its initialliquid state. The working fluid is then transported to receiver tank 27 via tank inlet 29, orconveyed 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 16vapour, and the liquid working fluid harboured in the receiver tank is thus stored in a form readyfor further pumping around the WHR circuit. Overall, the heat source (e.g. exhaust gas) providesthe energy entering the waste heat recovery system 9 and the energy leaves the waste heatrecovery system 9 as mechanical work via the expander 15 and as heat via the cooling system 71. ln some circumstances the condenser 17 is unable to provide sufficient cooling power in orderto sufficiently subcool the working fluid leaving the condenser. The present invention addressesthis problem by allowing vaporized working fluid to be introduced into the receiver tank 27,thus increasing the condensation pressure and condensation temperature at the outlet of the condenser 17.

To this end, the receiver tank 27 is provided with a second tank inlet 63. The second tank inletis arranged in fluid connection with the evaporator outlet line 21 via an expander by-pass line45. An expander by-pass valve 47 is arranged to either permit working fluid flow along theevaporator outlet line 21 to the expander 15, or to redirect working fluid flow to the receivertank 27 via the by-pass line 45. lf working fluid is redirected to the receiver tank 27, thecondensation pressure in the low-pressure side of the WHR system, including at the outlet ofcondenser 17, will be raised. This leads to a higher condensation temperature and a lesser needof cooling power in condenser 17 in order to achieve subcooling, thereby ensuring thatsufficient subcooling is provided by the condenser. The by-pass line 45 also serves the purposeof allowing the expander 15 to be bypassed if required, and such bypass lines are commonlyimplemented in waste heat recovery systems. Thus, the system as illustrated in Figure 2 permitsregulation ofthe condensation pressure ofthe working fluid with a minimum of reconfiguration of existing WHR systems.

Figure 3 schematically illustrates another embodiment of the present invention. ln thisembodiment the receiver tank 27 is equipped with an electric heater 41. The electric heater 41may be used to vaporize working fluid contained in the receiver tank 27, thus providing a further means of controlling condensation pressure in the low-pressure side of the WHR system.

Figure 4 schematically illustrates a further embodiment of the present invention. ln thisembodiment the heater 41 is heated using a heating medium. The heating medium being led through the heater 41 is working fluid obtained from the expander outlet line 23. A heater feed 17line 49 is arranged to channel working fluid from expander outlet line 23 to heater inlet 43. Aheater by-pass valve 51 is arranged at the junction of heater feed line 49 and expander outletline 23 in order to controllably redirect the flow of working fluid from the expander outlet line23 to the heater feed line 49. ln this embodiment the heat being used for the heater 41 is wasteheat, since it would otherwise have been removed from the working liquid by condenser 17.Therefore, the arrangement of heater 41 as depicted in this embodiment does not negatively affect the output of the expander 41.

Figure 5 schematically illustrates yet another embodiment ofthe invention. ln this embodiment, 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 fluidthrough the cooling system, a radiator 73 arranged for cooling the coolant fluid having passedthrough the combustion engine 3, and a radiator return line 75 for channelling coolant fluidfrom the engine to the radiator. The coolant pump 95 may be driven mechanically by thecombustion engine 3, but it may also be electrically driven, or at least controllable. The enginecooling system 71 further comprises a radiator bypass line 101, a first valve unit 97 and a secondvalve unit 99. The first valve unit 97 is suitably arranged to control the flow of coolant fluidthrough the radiator 73 and the radiator bypass line 101 respectively. The second valve unit 99is suitably arranged to control the flow of coolant fluid passing through the condenser 17 of the waste heat recovery system 9.

A heater feed line 49 is arranged to channel coolant fluid from the radiator return line 75 to aninlet 43 of the heater 41. A heater return line 77 is arranged to channel working fluid havingpassed through the heater 41 back to the heater return line 75. A heater feed valve 85 isarranged to controllably redirect coolant fluid flow from the radiator return line 75 to the heaterfeed line 49 as desired. lf coolant fluid is redirected through heater 41, working fluid containedin the receiving tank 27 will be evaporated, and the condensation pressure at the outlet ofcondenser 17 will be raised, leading to a higher condensation temperature and a lesser need of cooling power in condenser 17 in order to achieve subcooling.

Figure 6 is a flowchart schematically illustrating a method for controlling a waste heat recoverysystem according to the invention. Step s601 denotes the start ofthe method. ln step s603 the waste heat recovery system is operated routinely, as described herein; i.e. working fluid is not 18routed via the heater 41. Step s605 denotes a decision: is the condenser providing sufficientsubcooling of the working fluid? This may for example be determined using pressure and/ortemperature sensors located in the waste heat recovery system, or by determining the coolingpower of the condenser. lf the answer is YES, the method returns to step s603. lf the answer isNO the method proceeds to step s607. ln step s607 condensation pressure is increased in thecondenser by controllably directing a flow of working fluid from the evaporator outlet line orexpander outlet line through the heater 41. This has the effect of increasing the condensationtemperature 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 temperatureprevailing in the receiver tank 27 should be decreased. This may for example be achieved bydirecting 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 7 is a flowchart schematically illustrating a method for starting up a waste heat recoverysystem according to the invention. The method is performed during a start-up phase of thewaste heat recovery system. Step s701 denotes the start of the method. ln step s703 theexpander by-pass valve 47 is arra nged to redirect working fluid via by-pass line 45 to the receivertank 27, thus by-passing the expander 25 and condenser 17. Step s705 denotes a decision: doesthe working fluid in evaporator outlet line 21 comprise liquid working fluid, i.e. does aproportion ofthe working liquid remain non-vaporized? |fthe answer is YES, the method returnsto step s703. lf the answer is NO the method proceeds to step s707. ln step s707 the expanderby-pass valve 47 is arranged to allow working fluid to reach the expander 15. Step s709 denotesthe end of the method. By utilizing this method during a start-up phase of the WHR system,non-vaporized working fluid bypasses the expander, avoiding potential damage to theexpander. The working fluid also bypasses the condenser, thus ensuring that heat contained inthe working fluid is not lost to the condenser. ln this manner, the WHR system may quickly bebrought up to operating temperature. Once the WHR system is at an appropriate operatingtemperature, i.e. all working fluid is vaporized in the evaporator, then the working fluid isallowed to be directed through the expander, allowing useful mechanical work to be obtained from the heated working fluid. 19Figure 8 is a flowchart schematically illustrating a method for shutting down a waste heatrecovery system according to the invention. The method is performed during a shutdown phaseof the waste heat recovery system in order to ensure that gaseous working liquid is iso|ated inthe receiver tank 27 and that the rest of the system is filled with liquid working fluid. Liquidgathe rs in the cooler parts ofthe WHR system and va pour gathers in the hotter parts. Therefore,in order to localize vaporized working fluid to the receiver tank 27, working fluid should becirculated and the receiver tank 27 should be heated until it is the hottest part of the WHRsystem. Step s801 denotes the start of the method. ln step s803 working fluid is circulatedaround the waste heat recovery system via second tank inlet 63 in order to heat receiver tank27. Step s805 denotes a decision: does vaporized working fluid remain in the main waste heatrecovery circuit, that is to say all waste heat recovery components with the exception of thereceiver tank? lf the answer is YES, the method returns to step s803. lf the answer is NO themethod proceeds to step s807. ln step s807 the receiver tank is iso|ated by closing the firstcontrollable flow regulating means 57 and second controllable flow regulating means 59, suchthat working fluid contained in the main waste heat recovery circuit has a temperature below acondensation temperature of the working fluid at ambient pressure, i.e. is liquid. The workingfluid now iso|ated 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 providedwith seals able to withstand such low pressures. Other WHR components are much moredifficult to seal effectively against sub-atmospheric pressures, especially the expander, so bylimiting the occurrence of such pressures to the receiver tank, the design and construction ofthe WHR system is simplified. ln step s809 the working fluid pump is shut off. Step s811 denotes the end of the method.

Claims (1)

1. CLAll\/IS A waste heat recovery system (9) for a vehicle (1), the waste heat recovery systemcomprising: a working fluid pump (11); an evaporator (13); an expander (15) and an expander luv-pass valve ifiïfl; a condenser (17); a pump outlet line (19) arranged to channel working fluid from the working fluid pump(jjimto the evaporatorilifi; an evaporator outlet line (21) arranged to channel working fluid from the evaporator (lå)to the expanderilmšj; an expander outlet line (23) arranged to channel working fluid from the expander (mlnågfltothe condenseriljl; a condenser outlet line (25) arranged to channel working fluid from the condenser to theworking fluid pumpjggl; a receiver tank (27) comprising a first tank inlet (29) and a tank outlet (31); a tank inlet line (33) arranged to channel working fluid from a first junction (35) in thecondenser outlet line (Zmšàto the first tank inlet; and a tank outlet line (37) arranged to channel working fluid from the tank outlet (iånifimto asecond junction (39) in the condenser outlet linefimšl, wherein the second junction (nšlíålisarranged between the first junction (åäand the working fluid pumpjimlj;wšaarewcharacteršzecš in that the receiver tank Qähas a substantially constant inner volume and is equipped with a second tank inlet (63) arranged in fluid connection with the evaporator outlet line (21). The waste heat recovery system (Sjlaccording to claim 1, wherein it comprises an expander by-pass line (45) which is arranged to channel working fluid from theevaporator outlet line (Äilto the second tank inletiílafi; and an expander by-pass valve (47) which is arranged at a junction of the expander by-passline and evaporator outlet linejggj, and is arranged to controllably redirect a flow of working fluid from the evaporator outlet line (213 to the expander by-pass line. 10. 2The waste heat recovery system (ålaccording to any one of the preceding claims, whereinthe receiver tank gZJfE is equipped with a heater (41) arranged to heat working fluid in the receiver tank (Zï). The waste heat recovery system (Smäflimaccording to claim 3, wherein the heater '41 is an electric heater. The waste heat recovery system fiåjmaccording to claim 3, wherein the heater (41)comprises a heater in|et (43) adapted to be fluidly connectable with an engine coolingsystem (71), and wherein the heater (41) is arranged to facilitate heat transfer from a coo|ant fluid in the heater (41) to a working fluid in the receiver tank (27). The waste heat recovery system (9) according to claim 3, wherein the heater (41)comprises a heater in|et (43) arranged in fluid connection to the evaporator outlet line(21) or expander outlet line (23), and wherein the heater (41) is arranged to facilitate heat transfer from working fluid in the heater (41) to working fluid in the receiver tank (âï). The waste heat recovery system (gjmaccording to any one of the preceding claims, whereinit comprises a first controllable flow restricting means (57) which is arranged in the tank in|et line_(§¿3_);and a second controllable flow restricting means (59) which is arranged in the tank outlet line The waste heat recovery system (íjšlaccording to any one of the preceding claims, whereinit comprisesa third controllable flow restricting means (61) which is arranged in the condenser outlet line (25) between the first junction (35) and the second junction (39). The waste heat recovery system (íjšlaccording to any one of the preceding claims, whereinthe receiver tank (Zï) is arranged to separate gaseous working fluid from liquid workingfluid, and wherein the receiver tank (27) is arranged to channel liquid working fluid to the tank outlet. The waste heat recovery system (íjšlaccording to any one of the preceding claims, wherein a desiccator medium is contained in the receiver tank (27). 11. 12. 13. 14. 15. 3The waste heat recovery system (ålaccording to any one of the preceding claims, wherein the waste heat recovery system (Ej-Lis a subcooler-free system. A method for controlling a waste heat recovery system (9) according to any one of claims1-11, the method comprising a step of increasing condensation pressure in the condenser(17) by controllably directing a flow of gaseous working fluid from the evaporator outlet line (21) to the receiver tank via the second tank in|et (63). A method for starting up a waste heat recovery system (9) according to any one of claims1-11, the method comprising a step of, during a start-up phase of the waste heat recoverysystem, bypassing the expander (mlmšmåfland the condenser Qflby controllably directing aflow of working fluid from the evaporator outlet line (21) to the receiver tank (Zjjlvia the second tank in|et (63). A method for shutting down a waste heat recovery system (9) according to claim 7, themethod comprising a step of, during a shutdown phase ofthe waste heat recovery system(3), isolating the receiver tank (27) by closing the first controllable flow restricting means(57) and second controllable flow restricting means (59), and regulating by-pass valve (47)or receiver feed valve (51), such that working fluid contained in a remainder of the wasteheat recovery system (QLhas a temperature below a condensation temperature of the working fluid at ambient pressure. A vehicle (1) comprising a waste heat recovery system (9) according to any one of claims 1-11.