GB2565818A

GB2565818A - Waste heat recovery and storage system

Info

Publication number: GB2565818A
Application number: GB1713612.8A
Authority: GB
Inventors: Agurto Goya Alan; Garcia Afonso Oscar
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2017-08-24
Filing date: 2017-08-24
Publication date: 2019-02-27
Anticipated expiration: 2037-08-24
Also published as: GB201713612D0; GB2565818B; DE102018213733A1

Abstract

A waste heat recovery and storage system for an internal combustion engine comprises: a first heat exchanger 102 to transfer heat from exhaust gas from an internal combustion engine to the working fluid, an accumulator 112 to store working fluid, a turbine 104 driven by the heated thermal fluid, and a second heat exchanger (condenser) 108. At least one of the accumulator, the first heat exchanger and the second heat exchanger comprises a thermal battery arranged to exchange thermal energy with the working fluid and/or the exhaust gas. The thermal battery stores heat and may comprise phase change material, a sensible heat material or a thermochemical material. By storing thermal energy, the waste heat recovery system may be downsized and operated at a more constant level.

Description

Waste Heat Recovery and Storage System

TECHNICAL FIELD

The present disclosure relates to a waste heat recovery and storage (WHRS) system and particularly, but not exclusively, to a WHRS system for an internal combustion engine (ICE) of a vehicle. Aspects of the invention relate to a WHRS system, to a control system for operating the WHRS system and to a vehicle that includes the WHRS system.

BACKGROUND

As is well-known, internal combustion engines (ICEs) typically convert only around a third of the energy released by a combusted fuel into mechanical work, with the majority of the remaining energy being lost to the environment as heat, in particular through exhaust gasses expelled from the ICE.

Various waste heat recovery (WHR) systems are known for recovering thermal energy contained in exhaust gasses. Such systems are typically disposed upstream of catalytic converters within a vehicle exhaust system, and include, for example, exhaust driven turbochargers. However, the amount of thermal energy that may be recovered by a WHR system positioned upstream of a catalytic converter is limited, as the exhaust gasses must remain sufficiently warm for effective operation of the catalytic converter. Accordingly, opportunities exist for further heat energy recovery downstream of the catalytic converter.

In this respect, Rankine cycle engines have shown effective capability, in a lab setting, for recovering heat energy from exhaust gasses downstream of a catalytic converter during sustained engine loads.

Known Rankine cycle engines use a heat exchanger to transfer thermal energy from exhaust gasses to a working fluid, such as ethanol, to vaporise the working fluid. Gaseous working fluid then passes through an expander, such as a turbine, to drive an electrical generator, with the resulting electrical energy being stored in a vehicle battery. The working fluid then enters a condenser which is connected to a heat sink, such as an engine coolant circuit, to revert the working fluid to a liquid state before it returns to the heat exchanger.

However, under some operating conditions the Rankine cycle engines cannot operate effectively. Such conditions include urban driving, which is characterised by low speed driving with frequent changes in engine load, leading to a correspondingly intermittent supply of thermal energy through the exhaust gasses. In particular, at times of low engine load the exhaust gasses may not be warm enough for the working fluid to vaporise in the heat exchanger.

To cope with the problems associated with transient loads, the working fluid may be pumped in a manner that varies with the exhaust load. As a result, the associated control system becomes highly complicated to accommodate the transient nature of the exhaust gas flow. Moreover, the components of the Rankine cycle engine must be designed to allow for higher working fluid temperatures and increased electrical power in the generator, and so are relatively heavy and expensive.

It is also noted that, in such systems, not all thermal energy extracted from the exhaust gas can be converted to electrical energy when the engine load is high, for example during periods of acceleration. This entails significant heat rejection at the condenser as the expanded working fluid remains at an elevated temperature, thus wasting some of the recovered energy and increasing the demands placed on the coolant circuit.

In another approach, a gas storage tank may be placed between the heat exchanger and the turbine to act as an accumulator, by storing excess gaseous working fluid produced during periods of high engine load. The stored gas can subsequently be released to drive the turbine when the engine load is low. This approach mitigates the problem of excess heat rejection at the condenser during high load and also enables continued operation of the generator during low load. This allows for more steady-state operation of the Rankine cycle engine, and so reduces transient stresses on the Rankine cycle engine components and enables operation within an optimum range for longer periods.

However, the capacity of the storage tank, which is restricted by vehicle package constraints, limits the period for which it can drive the turbine. Moreover, over time the gas contained in the storage tank cools due to heat conduction to the surroundings, in turn reducing the energy that remains available for the turbine and generator to recover. Accordingly, such approaches can only mitigate short-term or high-frequency transient loads.

The present invention has been devised to mitigate or overcome at least some of the above-mentioned problems.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a waste heat recovery and storage system for an internal combustion engine. The waste heat recovery and storage system comprises: a first heat exchanger configured to transfer heat energy from exhaust gas expelled from the internal combustion engine to a working fluid; an accumulator configured to store working fluid received from the first heat exchanger; a heat engine configured to convert thermal energy contained in working fluid exiting the first heat exchanger and the accumulator into mechanical work; and a second heat exchanger configured to transfer heat energy from the working fluid to a heat sink. At least one of the accumulator, the first heat exchanger and the second heat exchanger comprises a thermal battery arranged to exchange thermal energy with the working fluid and/or the exhaust gas.

Advantageously, the accumulator provides quick-release energy storage capability that can be used to meet short-term power demands, whilst the thermal batteries provide longer-term energy storage which allows slow-release of energy through long periods of idling or inactivity. In this manner, the thermal battery is able to satisfy longer term energy demands and heat energy from the thermal battery can be transferred to the accumulator to maintain the supply of usable quick-release energy. As a result, the waste heat recovery and storage system mitigates problems associated with transient exhaust gas temperatures arising from intermittent driving duty cycles, through a combination of short-term and long-term energy storage devices.

Optionally, the waste heat recovery and storage system further includes one or more chambers that at least partially contain the thermal battery whilst fluidly isolating the thermal battery from the working fluid and/or the exhaust gasses.

In an embodiment, the thermal battery includes a phase change material capable of storing heat energy by changing from a first phase to a second phase and releasing the heat energy by changing from the second phase to the first phase.

Additionally, the first phase of the phase change material may be a solid phase

In an embodiment, the phase change material has a latent heat that is greater than the latent heat of the working fluid.

Optionally, the phase change material comprises at least one of: Lithium Nitride; Potassium-Nitrate; or Sodium-Nitrate.

Optionally, the thermal battery includes at least one of a sensible heat material or a thermochemical material.

In an embodiment, the first heat exchanger comprises a boiler.

Optionally, the working fluid comprises organic material. For example, the working fluid may comprise Ethanol.

In an embodiment, the accumulator is capable of storing the working fluid in a liquid state.

Optionally, the accumulator is capable of storing the working fluid in a vaporised state.

In an embodiment, the accumulator is operable as a pressure-drop accumulator and working fluid stored in the liquid state may be vaporised to the vaporised state. Optionally, the pressure-drop accumulator is configured to vaporise at least some of the stored working fluid in dependence on a pressure reduction at the accumulator.

Optionally, the second heat exchanger comprises a condenser, in which case the second heat exchanger may, for example, comprise a reservoir configured to collect liquefied working fluid.

Optionally, the heat sink comprises at least one of the following: an engine coolant system; an intake air heater system; a cabin heating system.

In an embodiment, the waste heat recovery and storage system comprises a first valve that is operable to control flow of working fluid into the accumulator.

Optionally, the waste heat recovery and storage system comprises a second valve that is operable to release working fluid from the accumulator.

In an embodiment, the waste heat recovery and storage system comprises a first bypass configured to allow working fluid to bypass the accumulator. In such embodiments, the waste heat recovery and storage system optionally comprises a third valve that is controllable to adjust the flow of working fluid through the first bypass.

In an embodiment, the waste heat recovery and storage system comprises a second bypass configured to allow working fluid to bypass the heat engine, in which case the waste heat recovery and storage system may comprise a fourth valve that is controllable to adjust the flow of working fluid through the second bypass.

The first valve may, for example, comprise the third valve and the second valve may, for example, comprise the fourth valve.

Optionally, the waste heat recovery and storage system comprises a pressure charging means disposed upstream of the first heat exchanger, the pressure charging means being configured to control a flow of working fluid into the first heat exchanger. The pressure charging means may, for example, comprise a pump.

Optionally, the waste heat recovery and storage system further includes a controller configured to control a mass flow rate of the working fluid.

Optionally, the heat engine comprises a turbine.

Optionally, the waste heat recovery and storage system comprises a generator configured to convert the mechanical work produced by the heat engine into electrical energy.

According to another aspect of the present invention there is provided a control system configured to control the waste heat recovery and storage system as described in a previous aspect of the invention.

Optionally, the control system is configured to operate the waste heat recovery and storage system to: extract heat energy from the exhaust gasses of the internal combustion engine in the first heat exchanger; store heat energy in the accumulator and/or the thermal battery; and convert heat energy from the accumulator and/or the thermal battery into mechanical work at the heat engine.

Optionally, the control system may be configured to characterise heat energy available from the exhaust gasses as dynamic or steady state.

In an embodiment, the control system may be configured to determine a demand for mechanical work at the heat engine.

Optionally, the control system may be configured to determine whether the demand for mechanical work exceeds heat energy and/or pressure energy stored in the accumulator.

In an embodiment, the control system may be configured to determine whether the accumulator is full. Optionally, the control system may be configured to bypass the accumulator if the accumulator is full. In a further option, the control system may be configured to store extra working fluid in the accumulator if the accumulator is not full and there is no demand for mechanical work.

In an embodiment, the control system may be configured to release working fluid from the accumulator if the heat energy available from the exhaust gasses is dynamic, and the demand for mechanical work exceeds heat energy and/or pressure energy stored in the accumulator.

In an embodiment, the control system may be configured to direct working fluid from the first heat exchanger to the heat engine if the heat energy available from the exhaust gasses is steady state, and the demand for mechanical work exceeds heat energy and/or pressure energy stored in the accumulator.

According to another aspect of the present invention there is provided a vehicle comprising the waste heat recovery and storage system as described in a previous aspect of the invention or the control system as described in another aspect of the invention.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows schematically a vehicle including a WHRS system in accordance with the present invention;

Figure 2 illustrates schematically the WHRS system of Figure 1;

Figures 3a and 3b are time charts that plot exhaust power and vehicle speed respectively;

Figure 4 is a graph plotting the power demanded at the output of the WHRS system of Figure 1 compared to the power available from an accumulator of the WHRS system; and

Figure 5 is a logic diagram representing operating modes of the WHRS system of Figure 1.

DETAILED DESCRIPTION

Embodiments of the present invention relate to a waste heat recovery and storage (WHRS) system for a vehicle operating an ICE. Waste heat contained in exhaust gasses exiting the ICE are recovered, and a portion of that recovered energy is stored in a combination of short-term, quick release, energy storage means and long-term, slow release, energy storage means.

The short-term energy storage means comprises an accumulator for storing pressurised vapour, which can be used to power a Rankine cycle engine to recover thermal energy from exhaust gas expelled from the ICE. The accumulator therefore provides quick-release energy that can be used to meet short-term demand in the Rankine cycle engine.

The long-term energy storage means is provided by thermal batteries, such as phase change materials (PCM), that store thermal energy in a slow-release manner through long periods of idling or inactivity in order to satisfy longer term energy demands. For example, heat energy from the long-term energy storage means can be transferred to the short-term energy storage means to maintain the supply of pressurised vapour.

Figure 1 illustrates a hybrid vehicle 1 to provide context for the invention, although it is noted that embodiments of the invention are applicable to any type of vehicle, and indeed more widely may be adapted for use in other environments.

In Figure 1, dashed lines illustrate communication pathways between vehicle systems and directional arrows represent the reuse of exhaust gas energy by each system in the vehicle 1.

The vehicle 1 includes an ICE 2 and an electric motor 4, each of which can provide propulsive power for the vehicle 1. The electric motor 4 can be driven by a battery 6, through an inverter (not shown), under the control of an electronic control unit (ECU) 8. In this manner, the electric motor 4 can provide propulsive power for the vehicle 1 alongside, or separately to, the ICE 2. For example, the ICE 2 and the electric motor 4 may be arranged in a parallel hybrid powertrain layout, which allows either simultaneous or separated power transmission from the ICE 2 and the electric motor 4 to road wheels of the vehicle 1.

The vehicle 1 further includes an exhaust system 10 that evacuates exhaust gasses from combustion chambers of the ICE 2 to the atmosphere. The exhaust system contains a catalytic converter 12, which converts pollutants within the exhaust gasses into more innocuous substances.

In the example shown in Figure 1, a WHRS system 100 is positioned downstream of the catalytic converter 12. The WHRS system 100 operates under the control of the ECU 8 to extract heat energy from the exhaust gasses, as they pass therethrough, and convert that heat energy into electrical energy which may be stored in the battery 6 or used directly to power the electric motor 4. In the downstream position shown in Figure 1, the exhaust gasses have no subsequent use after exiting the WHRS system 100, and so can be reduced to a minimum temperature, in turn maximising energy recovery.

In this instance, the electrical power demand at the WHRS system 100 is determined by an energy management strategy, operated by the ECU 8, which optimises the contributions of propulsive power from the ICE 2 and the electric motor 4 to satisfy the thrust demand with minimal fuel consumption. In other variations, the power demand may depend on the power consumption of other electrical systems of the vehicle 1.

Figure 2 illustrates an embodiment of the WHRS system 100 in more detail. The WHRS system 100 includes a first heat exchange means in the form of a heat exchanger 102 that is configured to transfer heat energy from exhaust gasses in the exhaust system 10 to a working fluid within the WHRS system 100, such as ethanol, to vaporise the working fluid.

The WHRS system 100 further includes a heat engine in the form of a turbine 104 that expands vaporised working fluid received from the heat exchanger 102 to drive an electrical generator 106. In other embodiments the heat engine may take various forms, including that of a reciprocating piston engine, and it may power a mechanical device, such as a flywheel or gearbox, as opposed to the electric generator 106.

A second heat exchange means in the form of a condenser 108 is disposed downstream of the turbine 104 to condense the working fluid before it returns to the heat exchanger 102. The condenser 108 comprises a reservoir for collecting liquefied working fluid, and is connected to a heat sink in the form of a coolant circuit 200, to which heat energy is transferred from the working fluid.

In this embodiment, the coolant circuit 200 corresponds to a vehicle coolant circuit that also delivers coolant fluid to the ICE 2. The coolant circuit 200 circulates a coolant fluid through the ICE 2 of the vehicle 1, a cooler 202 such as a conventional radiator, a coolant pump 204 and the condenser 108 of the WHRS system 100.

A pressure charging means in the form of a pump 110 is provided to pressurise the working fluid and generate circulatory flow within the WHRS system 100.

The skilled reader will appreciate that the components of the WHRS system 100 described thus far correspond to the fundamental elements of an organic Rankine cycle engine, in which the heat exchanger 102 acts as a heat source and the condenser 108 acts as a heat sink.

However, the WHRS system 100 of this embodiment advantageously further includes energy storage components to mitigate the above described problems associated with transient engine loads arising, for example, during urban driving.

Specifically, the WHRS system 100 includes an accumulator in the form of a vapour accumulator 112, which is capable of storing vaporised working fluid under pressure. In addition, thermal batteries are provided, in the form of phase change material in this embodiment, which are installed at various locations within the WHRS system 100 to store heat energy.

In Figure 2, the vapour accumulator 112 is equivalent in principle to a ‘pressure-drop’ variety of steam tank accumulator. As such, the vapour accumulator 112 includes an inlet pipe, an outlet pipe and a storage tank.

In use, the storage tank is initially filled with a volume of working fluid in the liquid state, which is held at pressure. Subsequently, the vapour accumulator 112 is supplied with a pressure charged flow of vaporised working fluid from the heat exchanger 102, which is introduced to a lower portion of the storage tank, through the inlet pipe, to mix with the stored liquid working fluid. The temperature of the mixture rises until it reaches its boiling point, at which point the vapour accumulator 112 enters a ‘charged state’. In the ‘charged state’, the storage tank holds a liquid-vapour equilibrium containing a volume of the working fluid in the liquid state and a comparatively small volume of the working fluid in the vaporised or gaseous state.

The vaporised working fluid contains heat and potential energy that is ready for delivery downstream. The working fluid may be drawn from the vapour accumulator 112 via the outlet pipe, typically under the control of a valve as described in more detail later. The internal pressure of the storage tanks drops as working fluid exits, which lowers the saturation temperature of the remaining working fluid. As a result, some of the stored liquid working fluid vaporises. In this manner, a liquid volume and a vapour volume of the working fluid are held simultaneously in the storage tank and the vaporised volume may be replenished by the liquid volume, or by additional gaseous working fluid received from the heat exchanger 102.

Accordingly, the storage tank provides a replenishable supply of potential energy that may be converted to mechanical work by the turbine 104. However, the replenishment of vaporised working fluid only continues whilst the pressure and/or temperature of the working fluid in the vapour accumulator 112, are at or above the saturation limits.

In the prior art, Rankine cycle engines rely on a supply of gaseous working fluid from the heat exchanger to replace heat energy lost to the surroundings and maintain the pressure and temperature of the working fluid. Moreover, as the vapour accumulator 112 is used during times of low engine load, when little or no gaseous working fluid is received from the heat exchanger 102, it follows that the vapour accumulator 112 can only meet short-term energy demands made by the WHRS system 100.

In this embodiment, the inventors have mitigated the energy storage problems and broadened the capability of the WHRS system 100 by thermally coupling PCM to the vapour accumulator 112, to act as a thermal battery and provide longer term energy storage. As a result, this embodiment mitigates the problems associated with transient exhaust gas temperatures, which arise due to intermittent driving duty cycles, through a combination of short-term and long-term energy storage means.

In other embodiments, alternative forms of thermal battery may be used, including a sensible heat material or a thermochemical material. The suitability of each material to the operational requirements of the WHRS 100 will be taken into account in each case. These alternatives are considered in more detail later.

PCMs gain and store heat energy predominately as they change phase. The heat energy that is gained as the material changes phase is known as latent heat. The latent heat is released when the PCM undergoes the reverse change of phase. This may be a solid-liquid phase change or a liquid-vapour phase change.

In this embodiment, the phase change material must undergo solid-liquid phase changes throughout the operation of the WHRS 100 and so the melting temperature of the selected PCM should be above the boiling point of the working fluid, but within the temperature range of the exhaust gas. Such a characteristic means that the PCM can maintain a vaporised volume of working fluid in the vapour accumulator 112 as the PCM solidifies.

A suitable PCM is selected that is capable of storing sufficient latent heat to maintain a specified volume of vaporised working fluid in the vapour accumulator 112 for a desired duration, for example a period of eight hours. Such a requirement may be established to ensure that the PCM stores sufficient heat energy in a vehicle 1 that is driven to work in the morning and driven home in the evening, for example.

In this case, the latent heat of the PCM should be suitably large to minimise the mass of PCM needed to accumulate sufficient energy to maintain vaporised working fluid over a defined period. Preferably, the latent heat of the PCM exceeds the latent heat of the working fluid.

However, there are many additional factors to consider when selecting a suitable PCM, including the thermal conductivity at solid state, the thermal storing losses and the specific heat capacity at liquid state.

The most suitable PCM for each application will vary significantly depending on the specific operational demands of the WHRS 100. For example, salt-based, paraffinbased and metallic-based PCMs offer different benefits. A metallic PCM tends to have a lower latent heat of fusion but will often provide a dynamic response to temperature changes. Whereas, a PCM with a high latent heat of fusion may allow more energy accumulation but will typically provide slower, less responsive, energy release.

In view of the above requirements, suitable PCMs include Potassium-Nitrate, SodiumNitrate and Lithium Nitride, although many other materials are also possible candidates.

The PCM is contained in one or more PCM chambers 114 that allow heat transfer between the working fluid and the PCM whilst fluidly isolating the PCM from the working fluid. Accordingly, the PCM chambers 114 may be in contact with an external wall of the vapour accumulator 112 or supported internally within the vapour accumulator 112.

PCMs usually exhibit low thermal conductivity and require relatively thin layers of material to ensure effective heat transfer. The one or more PCM chambers 114 that house the PCM must be designed to accommodate such characteristics.

More specifically, the volume of the PCM chambers 114 depends on the amount of PCM needed to retain the desired amount of heat, which varies depending on the PCM’s energy density. Whereas, the shape of the PCM chambers 114 depends on the ability of the PCM to achieve effective heat transfer. For example, long and thin PCM chambers 114, with minimum wall thickness, maximise the ratio of surface area to volume and, in turn, the rate of heat transfer.

The PCM chambers 114 are thermally coupled to the vapour accumulator 112 such that, during ‘charging’, the vapour accumulator 112 fills with high temperature working fluid, which mixes with the stored volume of working fluid and exchanges heat with the PCM. The PCM undergoes a solid-liquid phase change as it gains heat energy, causing it to melt and in doing so absorb thermal energy from the working fluid.

When the supply of heat from the exhaust gasses is diminished or removed, for example during idling or transient periods, the PCM transfers heat energy back to the working fluid in the storage tank as it solidifies. The transfer of heat is able to hold the working fluid at its saturation temperature and the energy stored as latent heat ensures that the PCM is able to transfer heat to the working fluid between idling/inactive periods.

Coupling PCM to the vapour accumulator 112 has various advantages. For example, the PCM reduces thermo-mechanical stresses due to peaks in the transient temperature of the exhaust gasses. In addition, the volume-specific storage capacity of the PCM is typically greater than 200 kWh/m³; this enables smaller storage tanks and helps to reduce the working pressure of the vapour accumulator 112. As a result, the vapour accumulator 112 can be held at least partially filled with working fluid throughout the day and the working fluid can be maintained at elevated temperatures. Consequently, there is a reliable source of energy for the turbine 104 to make use of.

The embodiment shown in Figure 2 makes further use of the heat storage capacity of PCMs by thermally coupling a PCM to the heat exchanger 102. In this application, the PCM provides temperature damping in the heat exchanger and so enhances the sustainability of the heat source.

The PCM can be thermally coupled to the heat exchanger 102 in much the same manner as it is coupled to the vapour accumulator 112. For example, one or more PCM chambers 116 containing a PCM can be supported on or within the heat exchanger 102. Those PCM chambers 116 may also be long and thin, to maximise heat transfer, whilst fluidly isolating the PCM from the working fluid and the exhaust gasses. As a result, the PCM in the heat exchanger 102 is able to absorb heat energy from the exhaust gasses as they pass therethrough, and subsequently release that heat energy to heat the working fluid when the exhaust load is low, for example.

The PCM coupled to the heat exchanger 102 should have a high latent heat of fusion, as for the PCM coupled to the vapour accumulator 112, such that the phase change process accumulates a significant amount of energy as the PCM changes from a solid state to a liquid state.

As with the PCM that is coupled to the vapour accumulator 112, the melting temperature of the PCM coupled to the heat exchanger 102 should be above the boiling point of the working fluid. The melting point of PCM coupled to the heat exchanger 102 should also be within the temperature range of the exhaust gas, which is typically between 200 °C and 700 °C. Indeed, exhaust temperature profiles may be used to design the Rankine system and select an appropriate PCM, as the melting point of the PCM is preferably close to the expected exhaust gas temperature.

In view of the slightly differing requirements, the PCM used on the heat exchanger 102 may not be the same as that used on the vapour accumulator 112, so that the particular PCM used in each location is optimised for the peaks and fluctuations in temperature to which it will be exposed in use. Alternatively, the same PCM may be used throughout the WHRS system 100 for simplicity.

Figures 3a and 3b demonstrate the benefits that can be realised by incorporating PCM in the heat exchanger 102. Figure 3a plots exhaust power on the y-axis and time on the x-axis. In this context, ‘exhaust power’ represents the rate of heat energy loss from the exhaust system to the environment in a vehicle that does not feature a waste heat recovery system. Figure 3b plots vehicle speed on the y-axis against time on the xaxis. When juxtaposed, Figures 3a and 3b show the exhaust power variation with vehicle speed during a driven duty cycle.

The exhaust power governs the variations in the exhaust gas temperatures. Hence, the high-frequency fluctuations in exhaust power shown in Figure 3a are indicative of highly transient exhaust temperatures, and peak temperatures that are excessive and often short lived, as indicated by the circled regions 302. In prior art systems, the components of the Rankine cycle engine must be designed to withstand the peak temperatures, which leads to increased costs and design inefficiencies.

However, in the present embodiment, peak temperatures can be absorbed by the PCM chambers 116 within the heat exchanger 102, allowing working fluid to exit the heat exchanger 102 at more moderate and stable temperatures. This ability is indicated in Figure 3a by a line 304 of constant exhaust power, which represents the ability of the PCM to heat the working fluid to moderate temperatures for long periods.

In this manner, the PCM held in the PCM chambers 116 can store heat energy during high engine load periods and transfer it to the working fluid during transient periods, i.e. when the exhaust gas temperatures are low. Consequently, the heat supplied to the working fluid is more sustainable and there is greater potential for consistent energy production at the turbine 104. The complexity of the WHRS system 100 is also reduced and components can be designed for optimum efficiency due to the enhanced temperature control. This allows better energy utilization and presents component downsizing opportunities.

For example, as the PCM protects the components of the WHRS system 100 from the extremes of exhaust gas temperature, those components can be designed for the more moderate temperatures entailed by the line 304 representing constant exhaust power. This reduces the cost, weight and size of the relevant components and further improves the efficiency of the vehicle 1.

Although not included in the embodiment shown in Figure 2, it is also possible to attach PCM to the condenser 108 to absorb heat energy from the working fluid and aid its return to its liquid state. For example, this may be useful if the temperature of coolant fluid within the coolant circuit 200 is temporarily elevated. Alternatively, or in addition, attaching PCM to the condenser 108 may enable liquid working fluid to exit at a more consistent temperature than is ordinarily possible. Similarly, the rate at which heat energy is rejected to the coolant circuit 200 can be stabilised.

The WHRS system shown in Figure 2 also includes various bypass routes, which create a range of modes of operation. A first bypass means in the form of a first bypass duct 118 provides a bypass route around the vapour accumulator 112, and a second bypass means, in the form of a second bypass duct 120, provides a bypass route around the turbine 104.

The arrangement also includes a set of valves that enable control of the flow of working fluid around the WHRS system 100.

A first valve 122, in the form of a three-way valve, is positioned upstream of the vapour accumulator 112 at the junction between the first bypass duct 118 and the vapour accumulator 112. The first valve 122 controls the flow of working fluid from the heat exchanger 102 into the vapour accumulator 112 and/or through the first bypass duct 118.

A second valve 124, in the form of a four-way valve, is disposed downstream of the vapour accumulator 112 at the junction between the first bypass duct 118, the second bypass duct 120, the vapour accumulator 112 and the turbine 104. The second valve 124 controls the release of vaporised working fluid from the vapour accumulator 112 and/or the first bypass duct 118 to direct that working fluid towards the turbine 104 and/or the second bypass duct 120.

The first and second valves 122, 124 are controlled by the ECU 8 and may be operated in dependence upon feedback measurements from various sensors (not shown). For example, the sensors include pressure, temperature, and volumetric flow rate sensors, used to measure the properties of the working fluid and/or exhaust gasses at various points in the circuit. The measured parameters are used to inform the ECU 8, which controls the system operation in response. The ECU 8 can control the first and second valves 122, 124, the pump 110, the condenser 108 and the heat exchanger 102 to match the exhaust load and meet the instantaneous power demand.

Also provided is a bypass route around the heat exchanger 102 (not shown in figure 2) which allows exhaust gas to bypass the entire WHRS. Flow of exhaust through this bypass route is selectively controlled by a valve (not shown in figure 2) also controlled by the ECU 8. Bypassing the entire WHRS, thereby allowing exhaust gas to flow from upstream of the heat exchanger 102 directly to downstream of the heat exchanger 102, may be beneficial in certain operating states of the WHRS, as will be described below.

The flow of working fluid during operation of the WHRS system 100 will now be described in detail below.

In use, working fluid is collected, in a condensed form, in the reservoir of the condenser 108. The working fluid is drawn from the reservoir by the action of the pump 110 under the control of the ECU 8, which can adjust the flow rate to match the engine load, for example.

The working fluid passes downstream of the pump 110 in a pressure charged state and enters the heat exchanger 102. The working fluid flows through the heat exchanger 102 and exchanges heat with a counter-current flow of exhaust gas received from the ICE 2. The exhaust gasses have already passed through the one or more catalytic converters 12 and thus as much heat as possible can be extracted at the heat exchanger 102.

The heat exchanger 102 vaporises the working fluid by heating it to temperatures at and above the saturation temperature corresponding to the pressure of the working fluid. The ECU 8 operates the pump to control the mass flow rate of working fluid and maximise the heat transfer at the heat exchanger 102.

During normal operation of the ICE 2, working fluid leaves the heat exchanger 102 in a vaporised state and travels downstream. At this point in the cycle, the flow of vaporised working fluid is split between the first bypass duct 118 and the vapour accumulator 112 depending on the state of the vaporised fluid and the power demand at the turbine 104. The control process that governs the relative distribution will be described in greater detail later in relation to Figure 4.

Subsequently, vaporised working fluid is supplied to the turbine 104 from the first bypass duct, from the vapour accumulator 112, or both. At the turbine 104, the working fluid is expanded to perform work on the electric generator 106, which in turn generates electrical energy. The electrical energy is transferred according to the state of charge of the battery 6 and the power demand from the transmission system.

The working fluid cools significantly as it is expanded in the turbine 104, and its pressure may drop below atmospheric pressure. However, downstream of the turbine 104 the working fluid remains in a largely gaseous or vaporised state.

The expanded working fluid travels onwards to the condenser 108 which is connected to the heat sink, namely the coolant circuit 200 in this embodiment. The working fluid passes through the condenser 108 and loses heat energy such that it condenses into a liquid and accumulates in the reservoir, returning to its original condition.

Figure 4 plots vapour power, ‘Ps’, on the y-axis against power demand, ‘Pd’, on the xaxis. The vapour power represents the amount of heat and pressure energy available to be transferred to the turbine 104 from the vapour accumulator 112. The power demand is the required electrical power output at the electric generator 106, which may comprise an electrical power demand from the battery 6 and/or the electric motor 4, for example.

Generally, working fluid can be released from the vapour accumulator 112 to power the turbine 104 when the present driving conditions lead to highly transient exhaust power. Whereas, during sustained periods of high engine load, for example during motorway driving, the working fluid may bypass the vapour accumulator 112 and be fed directly to the turbine 104.

With continued reference to Figure 4, six regions of the graph, which correspond to respective possible control scenarios, are now described to identify various modes in which the WHRS system 100 may be operated.

1.) No power demand; vapour accumulator fully charged, 401

The first region, labelled 401 in Figure 4, relates to a scenario in which the electrical power demand is zero, perhaps because the ICE 2 is acting as the sole supplier of propulsive power or the vehicle 1 is coasting, and auxiliary electrical demands can be met by a main alternator of the vehicle 1. The vapour accumulator 112 is also fully charged, meaning that its storage chamber is filled with a maximum volume of the working fluid, which is stored under pressure at or above the saturation temperature. Accordingly, there is a readily available supply of vaporised working fluid that contains heat and pressure energy.

Under these conditions, the WHRS system 100 may be active or inactive. When inactive, the pump 110 does not generate flow and the circuit is static. However, when active, the ECU 8 may operate the pump 110 to generate flow of the working fluid. To allow circulatory flow, the ECU 8 also operates the first valve 122, to direct working fluid through the first bypass duct 118, and the second valve 124, to direct working fluid from the first bypass duct 118 into the second bypass duct 120 and back to the pump 110.

As a result, working fluid flows around the circuit, bypassing the turbine 104 and the vapour accumulator 112. The pump 110 may be controlled to minimise the flow of working fluid and maximise the working fluid’s temperature as it leaves the heat exchanger 102. This allows faster pressure build up once the power demand returns and primes the WHRS system 100 for maximum power delivery. The PCM allows the WHRS system 100 to maintain this state, when the supply of heat from the exhaust gas is temporarily removed, by transferring heat to the stored volume of working fluid and maintaining the saturation temperature.

Alternatively under these conditions, the bypass route around the heat exchanger 102 (the bypass route not being shown in any of the figures) may be configured to allow exhaust gas to bypass the WHRS entirely. Bypassing the heat exchanger 102 may be useful for controlling back pressure in the exhaust system 10, for example, or to protect components of the WHRS at times when the temperature of exhaust gas upstream of the heat exchanger 102 is very high. The bypass route around the heat exchanger 102 also provides an auxiliary means of regulating the temperature of the working fluid in the WHRS alongside control of the pump 110, in that allowing exhaust gas to bypass the WHRS entirely acts to reduce heat transfer into the heat exchanger 102, in turn lowering the temperature of working fluid exiting the heat exchanger 102.

2. ) No power demand; vapour accumulator not fully charged, 402

In this scenario, represented by region 402 of the graph, the power demand is zero as in scenario 1, but the vapour accumulator 112 is less than fully charged, i.e. not in the ‘charged state’. The ECU 8 operates the first valve 122 to direct fluid flow from the heat exchanger 102 to the vapour accumulator 112 and the second valve 124 is operated to effectively seal the outlet of the vapour accumulator 112. The subsequent supply of working fluid charges the vapour accumulator 112.

3. ) Power demand exceeds vapour power, 403

This scenario, corresponding to region 403 in Figure 4, may arise during transient operation of the ICE 2, for example when switching from low engine load to high engine load with depleted energy reserves in the battery 6. The state of the accumulator is unimportant, as all resources must be directed to responding to the power demand.

In this scenario, the first valve 122 is adjusted to direct the flow from the heat exchanger 102 through the first bypass duct 118, whilst the second valve 124 is operated to supply the turbine 104 with vaporised working fluid from both the first bypass duct 118 and the vapour accumulator 112 until the charge is depleted. In this manner, all of the working fluid that is available in the vaporised form is directed to the turbine 104 for electrical energy production.

The electrical generator 106 may pass the electrical energy directly to the electric motor 4 to provide propulsive power and satisfy the thrust demand as much as possible.

4. ) Power demand is lower than vapour power; vapour accumulator not fully charged, 404

During transient loading, which is represented by a region of the graph labelled 404 in Figure 4, the ECU 8 may, for example, operate the second valve 124 such that the vapour accumulator 112 supplies the turbine 104 with vaporised working fluid in response to the demanded power. Excess working fluid released from the vapour accumulator 112 may be directed through the second bypass duct 120 to bypass the turbine 104, if necessary. Meanwhile, the first valve 122 may be operated such that working fluid leaving the heat exchanger 102 is directed back into the inlet of the vapour accumulator 112 in order to replenish the stored volume.

However, during sustained engine loads, the first valve 122 may be operated to distribute flow between the first bypass duct 118 and the vapour accumulator 112. Meanwhile, the second valve 124 may be operated to supply the turbine 104 from the first bypass duct 118 and simultaneously seal the outlet of the vapour accumulator 112. This restores the charge of the vapour accumulator 112 and supplies the turbine 104 with vaporised working fluid directly from the heat exchanger 102.

5. ) Power demand is less than vapour power; vapour accumulator fully charged,

405

In response to the above defined conditions, which relate to region 405 of Figure 4, the ECU 8 may operate the second valve 124 to discharge working fluid from the vapour accumulator 112 to the turbine 104, satisfying the power demand. The second valve 124 may be suitably adjusted to allow excess working fluid to enter the second bypass duct 120. The first valve 122 can be operated to direct working fluid from the heat exchanger 102 back into the vapour accumulator 112, allowing the vapour accumulator 112 to be refilled with working fluid.

6.) Power demand is equal to the vapour power; vapour accumulator fully charged,

406

In this ideal case, which relates to the point 406 of Figure 4, the ICE 2 may be idling with a negligible supply of heat from the exhaust gas. However, the vapour accumulator 112 is fully charged and the ECU 8 may operate the second valve 124 to supply the turbine 104 with vaporised working fluid from the vapour accumulator 112, satisfying the power demand. The first valve 122 may be operated to return the working fluid to the inlet of the vapour accumulator 112.

The various scenarios described above illustrate how adequate power can be supplied, to meet the demand, throughout a range of engine operating loads by using the various bypass ducts and valves to enhance control of the power delivery.

Figure 5 presents a logic diagram 500 that summarises graphically an example control strategy according to the invention.

The strategy begins by determining 501 whether the ICE 2 is in a cold-start condition in which the ICE has recently been started following a period of ceasing operation during which the engine has cooled to ambient temperature. If this is determined to be the case, the strategy then determines 502 whether or not the vapour accumulator 112 is empty. If the vapour accumulator 112 is found to be empty, then the strategy characterises 503 heat energy flow, from the exhaust gas, as either dynamic or steady state. A dynamic energy flow is defined as an exhaust gas flow that varies, in flow rate and/or temperature, outside threshold limits. Dynamic flows tend to arise during times of transient engine loads such as urban driving, for example. In contrast, if the variations of the exhaust gas properties are within threshold limits, it is characterised as steady-state. The strategy also characterises 503 the heat energy flow if the strategy determines 501 that the ICE 2 is not in a cold-start condition. If the flow is categorised as dynamic, then the WHRS may not be able to reliably deliver energy from the heat exchanger 102 to the turbine 104 in order to meet the power demand, Pd. Therefore, the consideration in this condition may be whether the vapour accumulator 112 can provide energy to meet the power demand and whether or not vapour should be delivered to the accumulator 112.

The strategy then checks 504 whether the power demand, Pd, is zero. If the heat energy flow is categorised as steady state, i.e. not dynamic, the strategy either: checks 505 whether the power demand exceeds the vapour power, Ps, if the power demand is non-zero; or checks 507 if the vapour accumulator 112 is full, if the power demand is zero. If the heat energy flow is categorised as dynamic, the strategy either: checks 506 if the vapour accumulator 112 is empty, if the power demand is non-zero; or checks 507 if the vapour accumulator 112 is full, if the power demand is zero. The strategy also checks 507 if the vapour accumulator 112 is full if the vapour accumulator 112 is not found to be empty and the power demand is non-zero.

The strategy ends by defining five distinct control actions to implement based on the preceding steps, including: directing working fluid from the heat exchanger 102 directly to the turbine 104; accumulating working fluid in the accumulator 112; bypassing the accumulator 112; releasing working fluid from the accumulator 112 to the turbine 104; and releasing working fluid from the accumulator 112 to the condenser 108 in order to reduce the warm-up time of the engine as described below. Some additional conditions are indicated on the diagram 500 to define which control action is selected at the final stage.

Each of the control strategies described above, with reference to Figure 4, can be traced through the logic diagram 500 of Figure 5. As such diagrams will be understood without difficulty by the skilled reader, to avoid unnecessary detail, scenario 1 shall be traced through the logic steps 500, shown in Figure 5, as a worked example.

Control scenario one in which the ICE 2 is either not in a cold-start condition, or is in a cold-start condition in which the accumulator 112 is empty, and there is no power demand and the vapour accumulator 112 is fully charged, begins with identifying, at step 503, that the exhaust load is constant and therefore not a dynamic load. Next, it is determined, at step 504, that there is no power demand from the WHRS system 100, and thereafter the vapour accumulator 112 is determined to be full and in a charged state, at step 507. As a result, the ECU 8 operates the valve controlling the flow of exhaust gases through the bypass route around the heat exchanger 102 (said valve and bypass route not being shown in any of the figures) to allow exhaust gases to bypass the WHRS entirely. Alternatively, the heat exchanger 102 may not be bypassed and the ECU 8 may operate the pump 110 to generate flow of the working fluid. To allow circulatory flow, the ECU 8 also operates the first valve 122, to direct working fluid through the first bypass duct 118, and the second valve 124, to direct working fluid from the first bypass duct 118 into the second bypass duct 120 and back to the pump 110. As a result, working fluid flows around the circuit, bypassing the turbine 104 and the vapour accumulator 112. The pump 110 may be controlled to minimise the flow of working fluid and maximise the working fluid’s temperature as it leaves the heat exchanger 102.

To summarise the benefits of the above described embodiment considered thus far, Rankine cycle engines in the prior art may include a vapour accumulator, but lack the long-term energy storage capabilities provided by the PCM chambers 114, 116. The inventors found that without the PCM chambers 114, 116, part-load operating conditions provide an inadequate supply of heat energy to operate a Rankine cycle engine including an accumulator. The breaks in heat supply cause the stored volume of working fluid to rapidly lose heat to the environment and therefore liquefy. As a result, the supply of vaporised working fluid to the turbine is poorly controlled in such arrangements and often unusable. This reduces the overall efficiency of the vehicle. Adding thermal batteries by way of PCM chambers 114,116 to the vapour accumulator 112 and heat exchanger 102, as described, provides an effective solution to this problem.

In addition to the above described benefits and associated control strategies, the apparatus shown in Figure 2 can also be used in an alternative operating mode to improve vehicle efficiency immediately following a ‘cold-start’.

In this respect, it is noted that the ICE 2 rapidly loses heat to the environment when not in use and so cools to ambient temperature within a short period of ceasing operation, entailing a cold-start at the next ignition. In cold-start conditions, engine oil that lubricates the piston chambers of the ICE 2 must undergo a period of warming to reach its normal operating temperature, before which the oil is relatively cold and hence more viscous, causing increased frictional losses within the ICE 2 during the warming period.

In these conditions, the ICE 2 benefits from the connection of the coolant circuit 200 to the WHRS system 100, as heat energy is recirculated from exhaust gas back to the ICE 2 through the WHRS system 100 and coolant circuit 200.

However, during normal operation the WHRS system 100 is operated to maximise energy recovery from exhaust gas expelled from the ICE 2, meaning that the heat energy transferred to the coolant circuit 200 at the condenser 108 is minimised. Therefore, ordinarily the contribution of the WHRS system 100 to warming of the ICE 2 is minimal.

As the improvement in vehicle efficiency achieved by reducing the warming period is typically greater than that achieved through energy recovery and storage, in an embodiment of the invention the WHRS system 100 is operated in the opposite manner to usual during the warming period, to maximise the heat energy that is delivered to the condenser 108 and on to the ICE 2.

To achieve this, the turbine 104 is bypassed during the warming period by operating the second valve 124, so that the exhaust gasses take the path of least resistance through the second bypass duct 120. This means that hot working fluid exiting the heat exchanger 102 or the vapour accumulator 112 flows directly to the condenser 108, thus maximising the quantity of heat energy that is transferred to coolant fluid within the coolant circuit 200 before it reaches the ICE 2. This in turn increases the temperature of coolant fluid flowing through the ICE 2, thus accelerating its warming.

In other words, this mode of operation enables the WHRS system 100 to be used to return heat energy exiting the ICE 2 through the exhaust gasses back to the ICE 2, through the condenser 108 and the coolant circuit 200.

The capacity of the WHRS system 100 to accelerate warming of the ICE 2 is enhanced by the ability of the PCM in the WHRS system 100 to store heat energy for long periods. In cold-start conditions, the PCM can immediately transfer heat to the working fluid in the WHRS system 100 in addition to that recovered from exhaust gas, thus increasing the temperature of the working fluid that reaches the condenser 108.

For example, PCM may heat working fluid stored in the vapour accumulator 112, or add additional heat to working fluid flowing through the heat exchanger 102.

Accordingly, the use of PCM within the WHRS system 100 enables a further reduction in the warming period for the ICE 2.

Other temperature influenced vehicle systems attached to the condenser 108 may also act as heat sinks and benefit from accelerated warming through operating the WHRS system 100 as described above to act as a heat source. Such vehicle systems may include cabin heating systems or air conditioning units, intake air heater systems, transmission systems and catalytic converters, for example.

As already noted, although PCM is used as a thermal battery in the above described embodiment, in other embodiments alternative forms of thermal battery may be used.

For example, a sensible heat material may be used as a thermal battery instead of, or in addition to, the PCM. Sensible heat materials store heat as their temperature increases and release heat as they cool. Sensible heat materials generally have lower energy density but provide a more dynamic response with a gliding discharge temperature. A suitable sensible heat material might have a large heat capacity and produce relatively low thermal losses in order to act as an effective thermal battery.

More specifically, a suitable sensible heat material might have a heat capacity greater than 40 kWh/m³. Accordingly, certain ceramic materials in particular may prove effective in the WHRS system 100.

In another alternative, a thermochemical material may be used as a thermal battery. Thermochemical materials break down into separate components when exposed to heat. These separate components can then be stored separately to prevent them from reforming the compound until heat is required, at which point the components are reunited to reform the compound and release the stored heat.

Thermochemical materials have very large heat storage capacities and low thermal losses but pose heat transfer problems as they provide heat at different temperatures within different periods. A suitable thermochemical material might have a heat capacity greater than 200 kWh/m³. Suitable thermochemical materials may include metal hydrides and silica gel.

Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims. The person skilled in the art will appreciate that the components of the WHRS system may be suitably modified, repositioned or removed as deemed sensible. For example, a regenerator may sensibly be included in the WHRS system to exchange heat between the expanded working fluid and the pressure charged fluid. Similarly, the ductwork between components may be sensibly modified to allow more tailored control of the working fluid. Other known additions/adjustments to a Rankine cycle engine may suitably be incorporated into the WHRS system without departing from the scope of the present invention.

Claims

1. A waste heat recovery and storage system for an internal combustion engine, the waste heat recovery and storage system comprising:

a first heat exchanger configured to transfer heat energy from exhaust gas expelled from the internal combustion engine to a working fluid;

an accumulator configured to store working fluid received from the first heat exchanger;

a heat engine configured to convert thermal energy contained in working fluid exiting the first heat exchanger and the accumulator into mechanical work; and a second heat exchanger configured to transfer heat energy from the working fluid to a heat sink;

wherein at least one of the accumulator, the first heat exchanger and the second heat exchanger comprises a thermal battery arranged to exchange thermal energy with the working fluid and/or the exhaust gas.

2. The waste heat recovery and storage system as claimed in Claim 1, wherein the thermal battery includes a phase change material capable of storing heat energy by changing from a first phase to a second phase and releasing the heat energy by changing from the second phase to the first phase.

3. The waste heat recovery and storage system as claimed in any preceding claim, wherein the thermal battery includes at least one of a sensible heat material or a thermochemical material.

4. The waste heat recovery and storage system as claimed in any preceding claim, wherein the working fluid comprises organic material.

5. The waste heat recovery and storage system as claimed in any preceding claim, wherein the accumulator is capable of storing the working fluid in a liquid state.

6. The waste heat recovery and storage system as claimed in any preceding claim, wherein the accumulator is capable of storing the working fluid in a vaporised state.

7. The waste heat recovery and storage system as claimed in Claim 6 when dependent on Claim 5, wherein the accumulator is operable as a pressure-drop accumulator and working fluid stored in the liquid state may be vaporised to the vaporised state.

8. The waste heat recovery and storage system as claimed in any preceding claim, wherein the second heat exchanger comprises a condenser.

9. The waste heat recovery and storage system as claimed in any preceding claim, wherein the heat sink comprises at least one of the following: an engine coolant system; an intake air heater system; a heating system.

10. The waste heat recovery and storage system as claimed in any preceding claim, comprising a first valve that is operable to control flow of working fluid into the accumulator.

11. The waste heat recovery and storage system as claimed in any preceding claim, comprising a second valve that is operable to release working fluid from the accumulator.

12. The waste heat recovery and storage system as claimed in any preceding claim, comprising a first bypass configured to allow working fluid to bypass the accumulator.

13. The waste heat recovery and storage system as claimed in Claim 12, comprising a third valve that is controllable to adjust the flow of working fluid through the first bypass.

14. The waste heat recovery and storage system as claimed in any preceding claim, comprising a second bypass configured to allow working fluid to bypass the heat engine.

15. The waste heat recovery and storage system as claimed in Claim 14, comprising a fourth valve that is controllable to adjust the flow of working fluid through the second bypass.

16. The waste heat recovery and storage system as claimed in any preceding claim, comprising a pressure charging means disposed upstream of the first heat exchanger, the pressure charging means being configured to control a flow of working fluid into the first heat exchanger.

17. The waste heat recovery and storage system as claimed in any preceding claim, further including a controller configured to control a mass flow rate of the working fluid.

18. The waste heat recovery and storage system as claimed in any preceding claim, wherein the heat engine comprises a turbine.

19. The waste heat recovery and storage system as claimed in any preceding claim, comprising a generator configured to convert the mechanical work produced by the heat engine into electrical energy.

20. A control system configured to control the waste heat recovery and storage system of any preceding claim.

21. The control system as claimed in Claim 20, configured to operate the waste heat recovery and storage system to:

extract heat energy from the exhaust gasses of the internal combustion engine in the first heat exchanger;

store heat energy in the accumulator and/or the thermal battery; and convert heat energy from the accumulator and/or the thermal battery into mechanical work at the heat engine.

22. The control system as claimed in Claim 21, configured to characterise heat energy available from the exhaust gasses as dynamic or steady state.

23. The control system as claimed in Claim 22, configured to determine a demand for mechanical work at the heat engine.

24. The control system as claimed in Claim 23, configured to determine whether the demand for mechanical work exceeds heat energy and/or pressure energy stored in the accumulator.

25. The control system as claimed in Claim 24, configured to determine whether the accumulator is full.

26. The control system as claimed in Claim 25, configured to bypass the accumulator if the accumulator is full.

27. The control system as claimed in Claim 25 or 26, configured to store extra working fluid in the accumulator if the accumulator is not full and there is no demand for mechanical work.

28. The control system as claimed in any of Claims 24 to 27, configured to release working fluid from the accumulator if the heat energy available from the exhaust gasses is dynamic, and the demand for mechanical work exceeds heat energy and/or pressure energy stored in the accumulator.

29. The control system as claimed in any of Claims 24 to 28, configured to direct working fluid from the first heat exchanger to the heat engine if the heat energy available from the exhaust gasses is steady state, and the demand for mechanical work exceeds heat energy and/or pressure energy stored in the accumulator.

30. A vehicle comprising the waste heat recovery and storage system of any of Claims 1 to 19, or the control system of any of Claims 20 to 29.