GB2551950B

GB2551950B - Energy recovery unit for a vehicle exhaust system

Info

Publication number: GB2551950B
Application number: GB1605852.1A
Authority: GB
Inventors: Hutchins William
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2016-04-06
Filing date: 2016-04-06
Publication date: 2019-11-13
Anticipated expiration: 2036-04-06
Also published as: GB2551950A

Description

ENERGY RECOVERY UNIT FOR A VEHICLE EXHAUST SYSTEM

TECHNICAL FIELD

The present disclosure relates to an energy recovery unit, particularly but not exclusively, for use in a vehicle exhaust system. Aspects of the invention relate to an energy recovery unit, a vehicle exhaust system, and a vehicle.

BACKGROUND

Thermoelectric generators (TEGs) convert heat energy to electrical energy using the Seebeck effect. A typical TEG comprises a plurality of metal plates having high thermal conductivities with thermoelectric materials between them, sandwiched between covers made of a dielectric, substrate material.

It is well-known that vehicle engines are only about 30% efficient, and in normal use generate significant waste heat. Over recent years, TEG devices have been incorporated into vehicle exhaust systems in order to harness waste heat from the exhaust gas. This decreases the load of an electric generator such as an alternator on the engine, in turn improving fuel consumption. A problem associated with using TEGs in this way is that they only operate efficiently over a relatively narrow temperature range - at low temperatures, energy generation is very inefficient; and at high temperatures, the thermoelectric materials are in danger of damage from overheating. In certain scenarios, it has been found that the leading edges of the TEGs may overheat before the majority of the TEG has reached a suitably high temperature for efficient operation to occur. As a result, the hot exhaust air must be diverted away from the thermoelectric materials using bypass valves to prevent damage to the TEG, thereby decreasing the system performance.

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 an energy recovery unit for a vehicle exhaust system, the energy recovery unit including an exhaust gas inlet and an exhaust gas outlet arranged at opposing ends and arrays of thermoelectric generators (TEGs) transversely arranged therebetween, a valve mechanism arranged to direct the exhaust gas in a first direction across the arrays of TEGs from a first side to a second side of the unit and further arranged to direct the exhaust gas in a second, opposite, direction across the TEGs from the second side of the unit to the first side of the unit, and at least one coolant duct in thermal contact with a cold surface of a respective one of the arrays of TEGs; the coolant duct comprising an inlet for influx of coolant and an outlet for outflow of coolant, the inlet being positioned substantially centrally intermediate the first and second sides of the unit; and a flow guide arranged to direct the coolant centrally away from the coolant inlet and along the first and second sides of the energy recovery unit toward the coolant outlet.

In an energy recovery unit, changing the direction of exhaust gas is beneficial to improve electrical generation efficiency and improve longevity of the TEGs. By initially directing the coolant centrally, a more consistent temperature profile can be achieved in the coolant on each side of the energy recovery unit. In this way, regardless as to whether the exhaust gas is directed in the first or the second direction, the cooling profile should be substantially the same thus further improving the efficiency and longevity of the energy recovery unit.

In an embodiment, the coolant inlet is positioned at an exhaust gas inlet end of the energy recovery unit. Such an arrangement is most efficient for cooling purposes since the coolant has its lowest temperature at the coolant inlet with the temperature increasing toward the coolant outlet. The exhaust gas on the other hand has its highest temperature at the exhaust gas inlet end. In this way, the temperature difference is maximised when including the coolant inlet at the same end of the energy recovery unit as the exhaust inlet.

In an embodiment, the coolant inlet and the coolant outlet are positioned at the same end of the energy recovery unit. Such an arrangement provides for improved installation and integration since the coolant reservoir can be provided at the end of the reservoir where both the coolant outlet and coolant inlet are located.

In an embodiment, the coolant outlet comprises a single port.

In an embodiment, the coolant inlet comprises a single port. Employing single ports provides for improved maintainability since there are fewer leakage paths compared to a case where multiple ports are used for the inlet and/or outlet.

In an embodiment, the outlet is provided towards one side of the energy recovery unit.

In one embodiment, the flow guide includes a plurality of coplanar walls defining adjacent flow paths between the inlet and the outlet.

In an embodiment, the flow guide comprises a U-shaped wall having a bend arranged at the coolant inlet and outlet end of the energy recovery unit, the coolant inlet positioned within the U-shaped wall and the coolant outlet positioned outside the U-shaped wall to divide the duct into a central channel directing coolant centrally away from the coolant inlet and opposing side channels directing coolant toward the coolant outlet.

According to a further aspect of the present invention, there is provided an exhaust system for a vehicle comprising the aforementioned energy recovery unit.

According to a further aspect of the present invention, there is provided a vehicle comprising the aforementioned exhaust system.

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 is a schematic block diagram of a vehicle incorporating an energy recovery unit according to an embodiment of the present invention;

Figure 2 is a perspective view of an energy recovery unit, which may be implemented in a vehicle exhaust system, such as that shown in Figure 1;

Figure 3 is a perspective cross-sectional view of the energy recovery unit shown in Figure 2;

Figures 4a-4c are schematic plan views of the energy recovery unit shown in Figure 2, operating in different modes in accordance with various embodiments of the present invention;

Figure 5 is a perspective cross-sectional view of the energy recovery unit shown in Figure 2, incorporating the use of thermocouples, in accordance with an embodiment of the present invention;

Figure 6 is a top cross-section view of an energy recovery unit shown in Figure 2 according to a further embodiment of the present invention;

Figure 7a is a similar view to Figure 6 showing exhaust gas flowing in a first direction through the TEG module;

Figure 7b is a similar view to Figure 6 showing exhaust gas flowing in a second direction through the TEG module;

Figure 8 is a perspective view of an energy recovery unit, implemented in a vehicle exhaust system, in accordance with another embodiment of the present invention; and

Figure 9 is a perspective view of an energy recovery unit, implemented in a vehicle exhaust system, in accordance with a further embodiment of the present invention.

DETAILED DESCRIPTION

Figure 1 is a schematic block diagram of a vehicle 2 which comprises an engine 4 connected to a vehicle exhaust system 6. An energy recovery unit 8 is incorporated in the vehicle exhaust system 6 in accordance with an embodiment of the present invention. The hot exhaust gas from the vehicle exhaust system 6 passes through the energy recovery unit 8 before it is expelled from the vehicle. The energy recovery unit 8 harnesses the heat energy from the exhaust gas passing through it, converting the heat energy into electrical energy using thermoelectric generators.

Figure 2 shows a perspective view of the energy recovery unit 8 of Figure 1. The energy recovery unit 8 comprises a TEG module 20 surrounded by a gas pipe network 22. The gas pipe network 22 comprises an inlet pipe 24 and an outlet pipe 26 disposed at respective opposed ends of the energy recovery unit 8. Two separate bypass ducts 28, 30 flank opposed sides of the TEG module 20 to connect the inlet and outlet pipes 24, 26.

The energy recovery system 8 also comprises an inlet valve (not shown) positioned at the junction 32 of the bypass ducts directly opposite and in the vicinity of the inlet pipe 24, and an outlet valve (not shown) positioned at the junction 34 of the bypass ducts directly opposite and in the vicinity of the outlet pipe 34. The inlet valve and the outlet valve each comprise a valve flap (not shown) which can be rotated to open or close the valve. Movement of each valve flap is controlled by a respective valve actuator 36 which controls the degree and direction of deflection of each valve flap, thereby controlling the direction of exhaust gas flow through the energy recovery unit 8.

In some embodiments, the valve actuators are independently operable, such that one valve may be open to a greater extent than the other. In other embodiments, the valve actuators are operated using a single ‘master’ lever (not shown), enabling both valves to be controlled simultaneously, such that the deflections of the valve flaps mirror one another.

In some modes of operation, the exhaust gas passes from the inlet pipe 24 to the outlet pipe 26 exclusively through the bypass ducts 28, 30, bypassing the TEG module 20 entirely and defining a main gas flow direction. In other modes of operation, some or all of the exhaust gas flows through the TEG module 20 in a cross-flow direction that is substantially orthogonal to the main flow direction. A more detailed description of modes of operation of the energy recovery unit is provided subsequently with reference to Figures 4a-4c.

Figure 3 is a perspective cross-sectional view of the energy recovery system 8 in Figure 2. The energy recovery system comprises a core 40 including a plurality of TEG assemblies 42, in this case two double sided TEG assemblies, and two single sided TEG assemblies. Each double sided TEG assembly comprises first and second opposing arrays 44a, 44b of TEGs. Each TEG assembly 42 comprises a pair of dielectric plates (typically made of a substrate material) that cover a plurality of parallel metal plates with thermoelectric material (for example, semi-conductor materials) between them. Outer faces of the covering plates define heat-exchanging surfaces of the TEG assembly - a hot-side heat-exchanging surface and a cold-side heat exchanging surface. Each array of TEGs 44a, 44b is arranged side by side and has its ‘hot’ side attached to an outer casing 46 of the respective TEG assembly 42 such that its cold side faces the cold side face of the adjacent TEG assembly 42. A separation plate pair 50, separated by a wedge (not shown), is attached to the casing 46 and arranged intermediate to, and separate from, adjacent TEG arrays 44a, 44b. In this way, first and second cold air cooling channels, or coolant ducts, 48a, 48b, are arranged adjacent to or are in part defined by each cold side surface of the TEG arrays 44a, 44b. The plates 50 isolate the respective cold air cooling channels 48a, 48b from one another.

Each TEG casing 46 is enclosed and separate from the adjacent casing 46 of the core 40 such that a transverse exhaust gas passage 52 is provided passing between the casings 46 and from one bypass duct 28 to the other 30, depending on the orientation of the valves flaps 60, 62. The exhaust gas passage 52 also passes on the other side of each casing 46 intermediate the casings 46 and an interior surface of the exterior structure 54 of the energy recovery system 8. A further array of TEGs 44c, 44d is provided on the exterior surface of the structure 54, the TEGs 44c, 44d having their hot side connected to the structure 54 so as to conduct thermal energy from the exhaust gas in the passage 52. Further cooling ducts 48c, 48d are connected to cold sides of the other TEG arrays 44c, 44d.

Figures 4a-4c are schematic plan views of the energy recovery unit of Figure 2, and illustrate different operating modes of the energy recovery unit.

Each operating mode is associated with a different configuration of the inlet and outlet valves. Specifically, each operating mode is defined by the relative proportions of exhaust gas flowing through the TEG module 20 and the bypass ducts 28, 30, which are determined by the degree to which each valve flap 60, 62 is deflected relative to the main flow direction of the exhaust gas, and the directions in which the deflections occur. Three main modes of operation exist - a ‘bypass’ mode, illustrated in Figure 4a; a ‘full flow’ mode, illustrated in Figure 4b; and a ‘feathering’ mode, illustrated in Figure 4c.

In the bypass mode, neither the inlet valve flap 60 nor the outlet valve flap 62 is substantially deflected, and so remain substantially parallel to the main flow direction of the exhaust gas. This allows the exhaust gas to flow unimpeded from the inlet pipe 24 past each side of the inlet valve, into the bypass ducts 28, 30, past the outlet valve and subsequently exit the energy recovery unit through the outlet pipe 26 without entering the TEG module 20 at all.

It is noted that the exhaust gas will not change direction so as to enter an exhaust gas passage 52 of the TEG module 20 unless there is significant resistance to flow along the bypass ducts 28, 30. Therefore in the bypass mode, substantially all of the exhaust gas flows through the bypass ducts 28, 30.

The energy recovery unit 8 is operated in the bypass mode when the TEG module 20 is in danger of overheating. For example, this can occur when the exhaust gas entering the energy recovery unit is at too high a temperature, or when the exhaust gas has been flowing through the TEG module for a prolonged period of time.

In the full flow mode, the inlet valve flap 60 and outlet valve flap 62 are maximally deflected in opposite directions, each extending completely across a mouth of a different one of the bypass ducts 28, 30. This prevents the gas flow from exiting the energy recovery unit 8 from the same bypass duct through which it entered, and so forces all of the exhaust gas through the TEG module 20, as no direct route through either bypass duct 28, 30 from the inlet pipe 24 to the outlet pipe 26 is available for the gas to flow.

For example, as may be seen from the plan view of Figure 4b, the inlet valve flap 60 is deflected maximally downwards, causing the exhaust gas to flow entirely into the upper bypass duct 30; however, as the outlet valve flap 62 is deflected maximally upwards, the exhaust gas cannot exit the energy recovery unit 8 through the outlet pipe 26 directly from the upper bypass duct 30. Instead, the exhaust gas from the upper bypass duct 30 is forced through the exhaust gas passages 52 of the TEG module 20 and into the lower bypass duct 28, in order to reach the outlet pipe 26. The direction of cross-flow through the gas passages 52 of the TEG module 20 may be reversed by reversing the direction of deflection of the input and output valve flaps 60, 62 (as indicated by the dotted lines in Figure 4b).

As a result of efficient heat exchange between the exhaust air and the TEG units 40, and the electrical energy generated from that heat, the exhaust gas cools significantly as it passes though each exhaust gas passage 52. Therefore the leading edges of each TEG unit 40 heat up much more quickly than the rest of the unit.

In an embodiment of the present invention, the direction of deflection of the valve flaps, and hence the direction of cross-flow through the exhaust gas passages 52 of the TEG module 20, is periodically alternated. This prevents overheating of the leading edges of the TEG module 20, thereby prolonging its lifespan.

The performance of the energy recovery unit 8 is also improved as the alternating flow creates a more even temperature profile across each hot-side heat exchanging surface than is achieved with a single direction flow. This means that the bypass mode is used less frequently, and more of the exhaust gas is utilised by the TEG module 20 to generate electricity.

In the feathering mode, shown in Figure 4c, the inlet and outlet valve flaps 60, 62 are deflected to different degrees, with neither bypass duct 28, 30 fully closed. This allows some gas flow through the bypass ducts 28, 30, but creates sufficient resistance to flow to force some of the exhaust gas into the TEG module 20. The feathering mode may therefore be thought of as a combination of the bypass and full flow modes.

For example, as may be seen in Figure 4c, the inlet valve flap 60 is deflected maximally downwards, while the output valve flap 62 remains substantially parallel to the main flow direction. The exhaust gas therefore flows along one of two paths: the first path corresponds to direct flow from the inlet pipe 24 to the outlet pipe 26 through the upper bypass duct 30; the second path corresponds to flow from the upper bypass duct 30, through the gas passages 52 of the TEG module 20 to the lower bypass duct 28, and into the outlet pipe 26.

The degree and direction of deflection of the output valve flap 62 may be varied (as indicated by the dotted line in Figure 4c) depending on the proportion of gas that is intended to flow through the gas passages 52 of the TEG module 20. A greater upwards deflection of the outlet valve flap 62 in Figure 4c results in a higher proportion of gas passing through the TEG module 20.

Although specific valve deflections are shown in Figures 4a to 4c, it should be noted that the functionality of the energy recovery unit 8 would not be substantially affected if the deflections of the inlet valve flap 60 and the outlet valve flap 62 were to be reversed from that which is illustrated. For example, in the feathering mode, it is sufficient for either one of the valve flaps 60, 62 to be maximally deflected in a particular direction, so long as the angle of deflection of the other valve flap remains variable, in order to control the amount of exhaust gas flowing through the TEG module 20.

It should be noted that the presence of independently operable valves would be a useful element of those embodiments where the energy recovery unit operates in the feathering mode, as this would enable precision control of each valve flap. By comparison, the presence of a ‘master’ lever would be a useful addition in those embodiments where the energy recovery unit operates in the bypass or full flow modes, as the degree of deflection of the two valve flaps should ideally mirror each other. The use of the master lever to automate the valve deflections would be particularly useful when periodically alternating flow is required.

Figure 5 shows a perspective cross-sectional view of the energy recovery unit 8 of Figures 2 and 3 in an embodiment of the present invention which incorporates the use of thermocouple devices 68 to measure temperature at spaced locations within the TEG module 20. In this embodiment, a respective thermocouple device 68 is inserted into each end of one of the exhaust gas passages 52 (and positioned at or near where the gas passages 52 feed into the bypass ducts 28, 30), in order to measure the temperate at either end and, and thereby determine the temperature difference across the TEG module 20 (i.e. perpendicular to its main axis).

The temperature measured using the thermocouple devices may be used to determine the best mode of operation of the energy recovery unit. For example, if the temperature of a portion of the thermoelectric generator is measured to be above a certain safe threshold of operation, the valve actuators may automatically change the valve flap deflections so that the energy recovery unit operates in the bypass mode. In another example, when the temperature difference between the two ends of the gas passage is deemed to be greater than a certain pre-defined threshold, the valve actuators will automatically change the direction of deflection of the valve flaps 60, 62. This reverses the direction of cross-flow through the TEG module 20 and re-balances the temperatures within the TEG module 20. This could continue until the thermocouple device indicates that the temperature of the TEG units 40 has reached a safe value, and the valve actuators may then return the energy recovery unit to the full flow or feathering modes.

With reference to Figure 6, the coolant ducts 48a to 48d are shown schematically in a cross-sectional view from above the energy recovery unit 8. The coolant ducts 48a to 48d include an inlet 70 and an outlet 72. Both the inlet 70 and the outlet 72 are provided at the same end of the coolant duct 48. In this embodiment the inlet 70 and outlet 72 are provided at the inlet 24 end of the unit 8. The inlet 70 is a single port like the outlet 72. The outlet 72 is arranged at one side of the recovery unit 8 towards one end of a TEG array (not shown but visible from Figure 3).

The coolant ducts 48a to 48d have a rectangular profile with rounded ends and a U-shaped flow guide 74 free standing within it. The flow guide 74 has a rounded end 76 toward the inlet 24 of the recovery unit 8 and legs 78 extending towards the outlet 26. In this way, the flow guide 74 separates the duct into two ‘warm’ outside channels 80 and a ‘cold’ central channel 82. As a result coolant liquid is able to flow in two directions towards the outlet 72. Firstly, the coolant liquid flows along the ‘cold’ central channel 82. Secondly, the coolant stream splits near the outlet 26 end of the recovery unit 8 and reverses its direction to flow towards the coolant outlet 72 by flowing along the outside channels 80. Accordingly, the outside coolant channels 80 are supplied with coolant of substantially the same temperature.

The impact of causing the coolant to flow in this way is best explained with reference to the exhaust flows as shown in Figures 7a and 7b, which correspond to the modes of operation explained above in relation to Figures 4b. References made to ‘uppermost’, ‘lowermost’, ‘upstream’, ‘downstream’, ‘upwards’, and ‘downwards’ with respect to Figures 7a and b refer to the orientation shown in the figure.

The exhaust flow in Figure 7a shows the configuration corresponding to Figure 4b where the inlet flap is maximally downwards and the outlet flap is maximally upwards. The exhaust gas is thus directed downwards through the TEG arrays (not shown). It can be seen by comparing Figures 7a and 6 that the exhaust gas is substantially transverse to the coolant when flowing along the channels 80, 82. The exhaust gas is cooled by the coolant such that the upstream gas is hotter than the downstream gas. Thermal energy induced into the coolant renders the uppermost channel 80 of coolant liquid, directed toward the outlet 72, hotter than the central and lowermost channels 82, 80.

In contrast, the exhaust flow in Figure 7b shows the configuration corresponding to Figure 4b where the inlet flap is maximally upwards (as in the dotted line position) and the outlet flap is maximally downwards. Accordingly, the exhaust gas flows upwards over the TEG arrays. In this way, the coolant liquid in the lowermost channel 80 experiences the highest degree of thermal energy exchange due to the upstream exhaust gas being hotter than that downstream. In this way, the lowermost column 80 contains the warmest coolant compared to the central and uppermost coolant channels 82, 80.

In this way, regardless as to the direction of exhaust gas flow, the exhaust gas should be cooled to substantially the same extent. In such an arrangement, when TEGs at the end of each array experience temperature extremes, and there is a risk of reduced performance or even damage, the inlet and outlet valve flaps 60, 62, can be reconfigured to reverse the temperature profile of the TEGs at opposing ends of the arrays. The energy recovery unit 8 will thus become more efficient and have a prolonged working life as a result. A further benefit of arranging the inlet 70 and outlet 72 at the same end of the unit 8 is that it leaves space at the other end to accommodate other features of the unit, such as electrical connections to the unit, for example.

Figure 8 shows another embodiment of the present invention comprising an alternative configuration of the TEG module 20 within the energy recovery unit 108. Similarly to the previous embodiment, in the embodiment shown in Figure 8 the TEG module 20 is arranged such that the exhaust gas passages 52 are perpendicular to the direction of the main gas flow through the energy recovery unit 108. In other words, the flow of exhaust gas through the TEG module is in a plane substantially perpendicular to the plane in which the main gas flow through the energy recovery unit occurs.

However, in contrast with the previous embodiment (shown in Figures 2, 3 and 5) in which the bypass ducts 28, 30 are disposed on the sides of the energy recovery unit 108 for horizontal cross flow, in the configuration of Figure 8, the bypass ducts 28, 30 are positioned above and below the TEG module 30, such that any gas flow through the exhaust gas passages 52 of the TEG module 20 is substantially vertical.

Figure 9 shows a further embodiment of the present invention comprising another alternative configuration of the TEG module 20 within the energy recovery unit 208. Similarly to the previous embodiment shown in Figure 8, the TEG module 20 is arranged such that the bypass ducts 28, 30 are positioned above and below the TEG module 20. However, in contrast with the previous embodiments of Figures 2 and 8, in which the exhaust gas passages 52 are arranged perpendicular to the direction of the main gas flow through the energy recovery unit 8; in the configuration of Figure 8, the heat-exchanging surfaces of the TEG units 40 lie in a plane that is parallel to the direction of the main gas flow in the energy recovery unit 108. In the configuration of Figure 9, any gas flow through the exhaust gas passages 52 of the TEG module 20 is therefore substantially vertical and orthogonal to the direction of the main gas flow in the energy recovery unit 208.

In other embodiments of the present invention (not shown), either the inlet pipe 24 or the outlet pipe 26 is arranged so as to be angled at an acute angle (for example, 45°) to the direction of the main gas flow within the energy recovery unit 208. This is in contrast with the previous embodiments where both the inlet pipe 24 and outlet pipe 26 are substantially parallel to one another and to the direction of the main gas flow within the energy recovery unit 208. In some embodiments, both the inlet pipe 24 and the outlet pipe 26 are angled at acute angles to the direction of the main gas flow within the energy recovery unit 208. The pipes may be angled to the same degree (such that they are effectively parallel to one another), or they may be angled to different degrees. This arrangement of inlet and outlet pipes is useful as it increases the flexibility of positioning of the energy recovery unit, allowing it to be mounted in the vicinity of a bend in the exhaust system, and thereby uses the available space efficiently.

Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims.

Claims

1. An energy recovery unit for a vehicle exhaust system, the energy recovery unit comprising an exhaust gas inlet and an exhaust gas outlet arranged at opposing ends of the energy recovery unit and arrays of thermoelectric generators (TEGs) transversely arranged therebetween, the energy recovery unit including a valve mechanism arranged to direct the exhaust gas in a first direction across the TEGs from a first side to a second side of the energy recovery unit and further arranged to direct the exhaust gas in a second, opposite, direction across the TEGs from the second side of the energy recovery unit to the first side of the unit, and further comprising at least one coolant duct in thermal contact with a cold surface of a respective one of the arrays of TEGs; the or each coolant duct comprising an inlet for influx of coolant and an outlet for outflow of coolant, the inlet positioned substantially centrally intermediate the first and second sides of the unit; and a flow guide arranged to direct the coolant centrally away from the coolant inlet and along the first and second sides of the energy recovery unit toward the coolant outlet.

2. The energy recovery unit of claim 1 wherein the coolant inlet is positioned at an exhaust gas inlet end of the energy recovery unit.

3. The energy recovery unit of claim 1 or claim 2 wherein the coolant inlet and the coolant outlet are positioned at the same end of the energy recovery unit.

4. The energy recovery unit of any preceding claim wherein the coolant outlet comprises a single port.

5. The energy recovery unit of any preceding claim wherein the coolant inlet comprises a single port.

6. The energy recovery unit of claim 5 wherein the coolant outlet is provided towards one side of the energy recovery unit.

7. The energy recovery unit as claimed in any preceding claim, wherein the flow guide includes a plurality of coplanar walls defining adjacent coolant flow paths between the inlet and the outlet.

8. The energy recovery unit of claim 7 when dependent on claim 3, wherein the flow guide comprises a U-shaped wall comprising a bend at one end, the coolant inlet being positioned within the U-shaped wall and the coolant outlet being positioned outside the U-shaped wall to divide the coolant duct into a central channel directing coolant centrally away from the coolant inlet and opposing side channels directing coolant towards the coolant outlet.

9. An exhaust system for a vehicle comprising the energy recovery unit of any of claims 1 to 8.

10. A vehicle comprising the exhaust system of claim 9.