WO2008095756A2

WO2008095756A2 - An apparatus for generating rotary power, an engine and a method of generating rotary power

Info

Publication number: WO2008095756A2
Application number: PCT/EP2008/050510
Authority: WO
Inventors: Anthony Osborne Dye
Original assignee: Epicam Limited
Priority date: 2007-02-08
Filing date: 2008-01-17
Publication date: 2008-08-14
Also published as: WO2008095756A3; GB0702466D0; GB2446457A; EP2176518B1; EP2176518A2

Abstract

This invention provides an apparatus and method for generating rotary power, the apparatus comprising a heat exchanger arranged to receive heat from a heat source to which, in use, the apparatus is connected and to transfer heat to a working fluid at high pressure; and, an expander comprising a low friction displacement device for receiving a metered amount of the heated high-pressure working fluid and for retrieving energy from the working fluid by being driven by the heated expanding working fluid.

Description

AN APPARATUS FOR GENERATING ROTARY POWER, AN ENGINE AND A METHOD OF GENERATING ROTARY POWER

The present invention relates to an apparatus for generating rotary power, an engine and a method of generating rotary power.

It is known that energy is lost as heat through the exhaust gases produced by an internal combustion engine during its operation. This energy has been thought of as extremely difficult to retrieve due to its "low-grade" nature, i.e. its temperature and/or pressure are too low to provide a source for energy conversion. Devices have been used for trying to retrieve heat energy from the exhaust flow of an engine and converting this into useful power. Attempts have been made, e.g. by BMW, with the use of a " turbostearner" system which uses waste engine heat to generate steam. However, the complexity of management control and the additional bulk, weight and cost of the required system make it difficult to realise worthwhile energy savings this way.

US-A-7, 080, 512 discloses a heat regenerative engine in the form of a two-stroke piston engine into which super- critical water substance at 1200⁰F (649°C) and 3200 psi pressure (216 bar) is delivered. The water substance is used as the working fluid from which output power is generated by the crankshaft of the engine.

At these conditions, the specific volume of the water substance is approximately 19.68, i.e. its volume is approximately twenty times that of liquid water at ambient temperature. The thermal efficiency of the engine is - -

claimed to be comparable with that of an internal combustion piston engine which suggests that the water can be a suitable substance for use as the working fluid in this type of energy conversion device.

The efficiency of the heat regenerative engine disclosed in US-A-7 , 080 , 512 is limited by the use of pistons and cylinders which, by necessity, are prone to significant friction effects. Thus, although the apparatus disclosed in US-A-7 , 080 , 512 will serve to recover energy and power from exhaust heat, the efficiency is necessarily quite low.

According to a first aspect of the present invention, there is provided an apparatus for generating rotary power, the apparatus comprising a heat exchanger arranged to receive heat from a heat source to which, in use, the apparatus is connected and to transfer heat to a working fluid at high pressure; and, an expander comprising a low friction displacement device for receiving an amount of the heated high-pressure working fluid and for retrieving energy from the working fluid by being driven by the heated expanding working fluid.

The use of a low friction displacement device enables low grade energy to be retrieved and used for generating rotary power. Thus, in applications such as an internal combustion engine the apparatus can be used for retrieving power from the exhaust that would otherwise go to waste. This energy can be returned to the engine, e.g. in the form of a direct power supplement to the engine's output shaft or for driving a supercharger for providing forced aspiration to the engine. In either of these examples, the efficiency of the engine is improved since in the absence of the apparatus this energy would otherwise be wasted.

According to a second aspect of the present invention, there is provided an engine, comprising one or more combustion chambers; and, apparatus for retrieving power from exhaust heat from the engine, the apparatus comprising a first heat exchanger arranged to receive heated exhaust gas from the one or more combustion chambers and to transfer heat from the exhaust gas to a working fluid at high pressure; and, an expander comprising a low friction displacement device for receiving a metered amount of the heated high-pressure working fluid and for retrieving energy from the working fluid by being driven by the heated expanding working fluid.

According to a third aspect of the present invention, there is provided apparatus for generating rotary power, the apparatus comprising a heat exchanger arranged to receive heat from a heat source to which, in use, the apparatus is connected and to transfer heat to a working fluid at high pressure; and, an expander comprising a displacement device for being driven by the expanding working fluid, wherein the expander has a transient chamber of variable volume operative, in use, to receive a metered amount of the heated high-pressure working fluid wherein the expansion ratio of the expander is greater than 50:1.

According to a fourth aspect of the present invention, there is provided a method of generating rotary power from a heat source, the method comprising providing a heat exchanger for transferring heat from the heat source to a working fluid at high pressure; providing the working fluid - -

at high pressure to an expander comprising a low friction displacement device; deriving shaft power from the expander when driven by the expanding hot working fluid.

According to a fifth aspect of the present invention, there is provided an apparatus for retrieving power from exhaust heat from an engine, the apparatus comprising a heat exchanger arranged to receive heated exhaust gas from a said engine and to transfer heat from the exhaust gas to a working fluid at high pressure; and, an expander comprising a low friction displacement device for receiving a metered amount of the heated high-pressure working fluid and for retrieving energy from the working fluid by being driven by the heated expanding working fluid.

The engine could be any suitable type of engine. Examples include internal and external combustion engines. In the case of an internal combustion engine, the engine includes one or more combustion chambers and preferably the engine includes an exhaust manifold for coupling the exhaust gas away from the one or more combustion chambers. In one example, the engine may be a solar engine in which the heat source for the working fluid is a solar concentrator. In other words, the apparatus enables the conversion to shaft power of heat provided from a solar concentrator.

In a preferred example, a lobe rotor and a recess rotor are used to define transient chambers of variable volume. In particular, the expansion cycle of the device is completed in only 90° of rotation with respect to the lobe rotor. Therefore, leakage from the transient chamber can be minimised without the use of mechanical or physical seals. This enables the device to operate with none of the friction normally associated with piston engines. Therefore, the efficiency of the device is significantly greater than that achievable by the engine described in US- A-7,080,512.

According to a further aspect of the present invention there is provided apparatus for retrieving power from exhaust heat from an engine, the apparatus comprising: a heat exchanger arranged to receive heated exhaust gas from a said engine and to transfer heat from the exhaust gas to a working fluid at high pressure; and, an expander comprising a displacement device for being driven by the expanding working fluid, wherein the expander has a transient chamber of variable volume operative, in use, to receive a metered amount of the heated high-pressure working fluid wherein the expansion ratio of the transient chamber of variable volume is greater than 50:1.

Preferably, the expansion ratio is greater than or equal to 100:1. More preferably, the expansion ratio is greater than or equal to 200:1. In other examples, preferably the expansion ratio is greater than or equal to 500:1 or 1000:1.

According to a further aspect of the present invention there is provided a method of recovering power from the heat of exhaust gas from an engine, the method comprising: providing a heat exchanger for transferring heat of the exhaust gas to a working fluid at high pressure; providing the working fluid at high pressure to an expander comprising a low friction displacement device; and deriving - -

shaft power from the expander when driven by the expanding hot working fluid.

Thus, using such a method, power can be derived from the heat of the exhaust gas and returned to the engine output shaft by a suitable connection. If such a method were not used then the recovered power would simply be lost. Thus, an engine operating in conjunction with the method is more efficient than one operating without.

Examples of the present invention will now be described with reference to the accompanying drawings, in which:

Figures 1 to 6 show the end face of each of a recess rotor and a lobe rotor in various stages during an expansion cycle;

Figure 7 shows a schematic representation of a shaped containment wall for use with the rotors of Figures 1 to 6;

Figure 8 shows an example of an end plate for use with the rotors of Figures 1 to 6;

Figure 9 shows a schematic representation of an engine;

Figure 10 shows a schematic representation of an engine; and

Figure 11 shows a schematic representation of an engine . - -

Figure 1 is a schematic view of two rotors from an expander typically used in apparatus for generating rotary power from a heat source, e.g. apparatus for retrieving power from the heat of exhaust gas from an engine. In the following description the apparatus will be described primarily with reference to the retrieval of power from the exhaust heat from an engine such as an internal combustion engine. It will be appreciated that in fact any form of heat source cold be used and the apparatus is suitable in a general sense for generating rotary power using heat from the heat source.

The expander of Figure 1 would typically be arranged within an expander housing which is not shown for the purposes of explanation of the operation of the expander. The housing includes side walls and end walls. One of the end walls includes an injector for injecting a working fluid into the expander as it operates. This wall may be referred to as the injector end wall. The rotors are shown with the injector end wall removed, the rotors being shown in a position early on in an expansion cycle, i.e. 1° (with reference to the lobe rotor) after the start position.

In addition, as will be explained below, to enable the maximum possible volume of a transient chamber of variable volume within the expander to be varied, in some embodiments, a moveable containment wall may also be included.

The expansion cycle begins with the rotors already in a position where the volume of the transient chamber is limited to the clearance volume only, i.e. it is effectively zero. Therefore, whatever the volume reached - -

by the transient chamber at the end of the expansion cycle, the expansion ratio is very large indeed. Preferably it is at least 50:1 and more preferably greater than this, e.g. 100:1 or 200:1 etc.

In the examples shown, the expander device has two expansion rotors 1 and 2. The first rotor 2 has six equiangularly spaced recesses and is therefore referred to herein as a recess rotor. The second rotor 1 has four equiangularly spaced lobes and is referred to herein as a lobe rotor. The rotors 1, 2 are mounted on respective shafts 6 and 7 which are supported in bearings fitted into the end walls (not shown) of the housing. In this particular example, the bearings are geared having a 3:2 speed ratio. Clearly, the required speed ratio depends on the ratio of the number of recesses in the recess rotor to the number of lobes on the lobe rotor.

The ends of the rotors are precisely located so as to form a close fit with the inner surfaces of the end walls of the housing, but without physical contact, i.e. there is only a clearance volume between the mating surfaces. Thus, there is no friction between the end faces (as shown in Figure 1) of the rotors and the walls of the housing to which they are adjacent. Thus, as far as the interaction between the rotors and the end walls of the housing is concerned, the expander is substantially frictionless .

In the examples shown, the cycle of expansion lasts for 90° of rotation of the lobe rotor 1 and 60° of rotation of the recess rotor 2. At the start of the cycle, the interaction between a recess and respective lobe, forms a sliding near contact which leaves only clearance volume - -

between the mating surfaces. Thus, as with the end faces of the rotors, there is no actual physical contact between the rotors and thus the expander as a whole may be referred to as frictionless or low friction.

As the expansion cycle progresses, the mating surfaces of the interacting lobe and recess begin to separate over a section shown in Figure 1 where the rotors are seen in their respective positions 1° (with respect to the lobe rotor shaft) after the start of the expansion cycle.

As can be seen in Figure 1, a new transient chamber of variable volume 3 is beginning to emerge between the lobe and the recess. The position of the injector tip 4 is shown at a location in the end wall immediately opposite the emerging transient chamber 3 towards the tip of the lobe. At this time, the injection of a working fluid such as super-critical water substance takes place directly into the transient chamber 3. Any suitable working fluid could be used.

The super-critical water expands and forces the transient chamber 3 to increase in volume, thereby doing work on the rotors . The work or power can be retrieved from the expander by use of an output shaft connected to the shaft of rotation of, say, the lobe rotor.

As the rotors continue to rotate, the transient chamber 3 enlarges progressively. The increase in volume of the transient chamber is shown in stages at 3° after the start of the cycle (Figure 2) and 10° after the start of the cycle (Figure 3) . - -

As can be seen in Figure 4, the transient chamber 3 enlarges to extend within the containment wall 5 although the lobe and pocket have ceased to interact. In Figure 5, the transient chamber 3 is shown nearing its maximum volume, 85° after the start of the cycle. Figure 6 shows the end point of the expansion cycle, 90° after the start, at which the transient chamber reaches its maximum possible volume .

The rotors function as an expander comprising a low friction displacement device. The low friction displacement device is arranged to receive a metered amount of a heated high-pressure working fluid and is suitable for retrieving energy from the working fluid by being driven by the heated expanding working fluid. It is most preferred that the working fluid is super-critical water and as will be explained below, the energy for heating the water to this condition may be easily retrieved from the hot exhaust gas of a conventional engine, such as an internal combustion engine.

Thus, the expander, being a low friction displacement device, and the apparatus including it is a suitable and convenient apparatus for retrieving the low-grade heat from the exhaust heat produced by an internal combustion engine. Thus , the apparatus can be arranged to return power to the engine directly or indirectly in the form of rotary power, e.g. via an output shaft connected to a main crank shaft of an engine to which the apparatus will be connected in use.

Taking a specific example to illustrate the advantages obtainable using a device such as that shown in Figures 1 to 6, if the maximum volume of the transient chamber 3 - -

shown in Figure 6 is 150cm³ and a charge mass of 0.05 grams of super-critical water substance is injected, then the fully expanded charge will occupy 150cm³ at ambient pressure and a temperature of 375°C. The original volume of the 0.05 grams charge at 1000 bar pressure and 600⁰C, before injection, is 0.134cm³.

If the injection were to take place at such timing that delivery is completed when the transient chamber volume is 0.134cm³, then the expansion ratio would be approximately 1123:1. Thus, the device is extremely efficient. In practice, the expansion ratio is at least 50:1 which is significantly more than is obtainable using a conventional expander e.g such as that shown in US-A- 7,080,512. Preferably the expansion ratio is greater than or equal to 100:1 or 200:1. More preferably an even larger expansion ratio is utilised such as greater than or equal to 500:1 or 1000:1.

The mass of the charge of working fluid delivered to the transient chamber at the start of the expansion cycle, can be optimised for the current operating condition. This is preferably done in such a way that when the maximum volume of the transient chamber is reached at the end of the cycle, then the pressure of the working fluid is close to that of the local ambient pressure. Thus, as the expansion of the working fluid is fully resisted by the cooperating rotors, all of the pressure energy available is converted into shaft power.

Figure 7 shows a perspective view of a sliding containment wall 5, in which an arcuate recess 6 is shaped to communicate with the lobe rotor 1 and an arcuate recess - -

7 is shaped to engage with the pocket rotor 2 during an expansion cycle. The position of a pressure sensor orifice 11 is shown at a location centrally along the apex of the intersection between the two arcuate recesses 6 and 7. This facility enables monitoring of the transient chamber pressure during the latter part of an expansion cycle over the full range of axial travel of the containment wall 5. "Axial" in this sense refers to movement substantially parallel to the axes of rotation of the lobe and recess rotors.

The containment wall 5 may be moved in an axial direction such as to vary the maximum possible volume of the transient chamber of variable volume. Thus, the expansion ratio can be controlled or the maximum possible volume of the transient chamber of variable volume can be varied to take into account a varying mass of charge. If the mass of the charge is varied or variable then by having the ability to vary the maximum possible volume of the transient chamber of variable volume it is possible to vary the expansion ratio and thereby keep it within desired limits .

Figure 8 is an external view of the apparatus viewed from the end wall in which the working fluid injector 4 is located between the shaft 9 of the pocket rotor and the shaft 8 of the lobe rotor. As can be seen, the end wall has an opening suitably shaped for receiving the movable containment wall 5 and for enabling axial movement of the wall 5 so that the maximum possible volume of the transient chamber can be varied as required. - -

Indeed, engines are frequently subject to operating conditions characterised by variable speed and load requirements. The expander described herein is capable of delivering power over a wide range of speeds and loads. However, at all operating conditions, the efficiency of the engine and expander can be optimised by matching the mass of working fluid delivered into the transient chamber of the expander to the maximum possible volume that the transient chamber can have.

Figure 9 is a schematic representation of an internal combustion engine including an expander as described above with reference to Figures 1 to 6. The apparatus may be used to recover reject heat present in the exhaust gas of the engine. The engine includes a heat exchanger 13 which is used to heat water substance prior to injection into the expander. A high pressure pump 14 is provided for pumping water through the heat exchanger 13 and onwards into the expander 16. A second heat exchanger 17 is also provided which serves to preheat water before it is provided to the first heat exchanger 13. A condenser 18 is provided for condensing steam that is exhausted from the expander 16.

The engine comprises a number of combustion chambers 29 from which hot exhaust gases are passed via the exhaust manifold 12 of the engine into the heat exchanger 13. Water is delivered into the cool end of the heat exchanger 13 at high pressure, e.g. at at least 200 bar. Preferably, the water or other working fluid is provided at a pressure of at least 300 bar, or at least 400 bar or at least 500 bar. In one example, the water is provided at 1000 bar pressure. The water is provided via high pressure piping of small bore, by means of a high pressure pump 14. - -

As the water passes through the mesh of coiled piping in the heat exchanger 13, its temperature rises as it gains heat from the exhaust gases. At the hot (delivery) end of the heat exchanger, it will have reached a temperature of, say, 600⁰C. Its pressure remains at 1000 bar and it is now super-critical water substance with a specific volume of 2.67.

When the expander 16 has reached the angular rotational position shown in Figure 1, a signal from a shaft encoder (not shown) which is preferably provided to monitor the angular position of the lobe rotor shaft 8, triggers injection of super-critical water substance directly into the very small volume of the emerging transient chamber 3.

Released from the pressure imposed on it by the high pressure pump 14, the super-critical water immediately expands to the maximum extent possible within the limited volume of the transient chamber. This expansion is fully resisted by the cooperating recess and lobe rotors 2 and 1, which thereby convert the pressure exerted by the expanding super-critical water into shaft power, delivered by the output shaft extension of the lobe rotor 21.

The expander 16 is coupled, in this example, by direct drive on a common shaft 21 with the lobe rotor shaft of a supercharging compressor 27. The supercharging compressor is preferably of a low-friction or frictionless displacement type device much like the expander described above except operating to compress a working fluid (air) instead of operating to be worked upon by an expanding hot substance. This common shaft is preferably coupled to the engine crankshaft via a geared pulley drive 23 or any other suitable connection. Thus, a contribution to overall output power of the engine is made by the heat recovering system by the expander 16, and is delivered via the output shaft extending from the engine crankshaft 25.

The water vapour released at ambient pressure from the transient chamber at the end of each expansion cycle is still very hot. Typically, it might be at approximately 375°C and thus it still carries a significant amount of heat. In this example, recovery of much of this heat then takes place as the water vapour in the form of dry steam passes from the expander 16 into a water/steam heat exchanger 17.

The heat exchanger 17 may be supplied with water, in this example, from a reservoir 20, which enters the heat exchanger at its cool end. Having gained heat from the steam, it leaves at the warm end of the heat exchanger 17, drawn by the intake end of the high pressure pump 14. Thus, the heat recovered from the steam emerging from the expander is re-cycled through the high pressure section of the exhaust heat recovery heat exchanger 13.

Having passed through the heat exchanger 17, the steam then passes into a condenser 18, where it is cooled sufficiently to complete a phase change from vapour to liquid. Liquid water is then collected from the condenser

18 and returned to the reservoir by means of a low pressure pump 19. Cooling for the condenser can be obtained via a cool water supply using the cooling radiator 24 of the - -

engine. Alternatively, a separate cooling radiator can be provided and used if required or desired.

Thus, it can be seen that the apparatus comprising in combination the heat exchanger 13 and the expander 16 serves to provide means by which the heat of exhaust gas produced by the combustion chambers 29 of the engine may be retrieved and converted to shaft power to improve the efficiency of the engine. The use of a low-friction or frictionless expander such as that shown in and described above with reference to Figures 1 to 6, enables the low- grade heat to be recovered. In the absence of such a heat exchanger and expander, the energy of this heat and power derivable from it would merely be wasted.

In one preferred embodiment, apparatus is also provided for the recovery and recirculation via the heat exchanger 13 of heat from the cooling jacket (not shown) around the cylinders and cylinder head of the engine. If considered worthwhile, this heat may be delivered to a point in the heat exchanger 13 at which the temperature has already reached the working temperature of the coolant, i.e. usually about 90⁰C. This would usually require a second high pressure pump to be operating at the same pressure as the high pressure pump 14 operating to pump water through the heat exchanger 13. The proportion of the fuel energy used by a normal IC engine and which is rejected to atmosphere through the cooling radiator is typically about 8% to 10%. This compares with 30% to 40% in the case of heat rejected via the exhaust.

Figure 10 shows a schematic layout for an external combustion engine also utilising an expander and a heat exchanger as shown in and described above with reference to Figure 9. Here, the heat source is a fuel burner (not shown) located in the high pressure heat exchanger 13. The high pressure injection, expansion, steam heat recovery and condensing processes are as indicated above with respect to the internal combustion engine. Heat may also be recovered from the condenser cooling radiator 24 and returned in the air entering the fuel burner. Similarly, further heat recovery can be made from the products of combustion exiting from the burner/high pressure heat exchanger 13 at the outlet 26.

Figure 11 shows a schematic layout for enabling the conversion of heat provided by a solar concentrator to shaft power. Heat is delivered directed from the solar concentrator (not shown) into the body of the high pressure heat exchanger 13. In this example, the heat exchanger may be of solid construction made from good conducting metal or contained liquid. Heat radiated onto the surface of the heat exchanger 13 is thus conducted directly to the water for subsequent injection into the expander 16, as before. Thus, it is clear that any suitable means may be utilised for providing heat to the heat exchanger 13 to enable the heat to be converted into shaft power.

Embodiments of the present invention have been described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention.

Claims

1. Apparatus for generating rotary power, the apparatus comprising: a heat exchanger arranged to receive heat from a heat source to which, in use, the apparatus is connected and to transfer heat to a working fluid at high pressure; and, an expander comprising a low friction displacement device for receiving a metered amount of the heated high- pressure working fluid and for retrieving energy from the working fluid by being driven by the heated expanding working fluid.

2. Apparatus according to claim 1, in which the expander is a rotary expander.

3. Apparatus according to claim 2 , wherein the rotary expander comprises a lobe rotor having one or more lobes and a recess rotor having one or more recesses, the lobe rotor and recess rotor being arranged such that on rotation of the rotors a lobe from the lobe rotor engages with a recess of the recess rotor to define a transient chamber of variable volume and wherein the transient chamber is an expansion chamber for the expansion of the metered dose of working fluid.

4. Apparatus according to claim 2 or 3 , in which the working fluid is super-critical water.

5. Apparatus according to claim 2 or 4, in which the transient chamber of variable volume is also defined in part by a moveable containment wall which is moveable to vary the maximum possible volume of the transient chamber of variable volume.

6. Apparatus according to claim 3 , in which the recess and lobe extend straight in the axial direction.

7. Apparatus according to claim 3, in which the recess and lobe extend helically in the axial direction.

8. Apparatus according t'o claim 6 or 7, comprising a rotary output shaft for delivery of rotary power.

9. Apparatus according to claim 8, wherein the output shaft is coupled to the rotation shaft of the lobe rotor.

10. Apparatus according to any of claims 1 to 9, in which the heat source is one or more of combustion gas generated by an engine and a solar concentrator.

11. An engine, comprising: one or more combustion chambers; and, apparatus for retrieving power from exhaust heat from the engine, the apparatus comprising: a first heat exchanger arranged to receive heated exhaust gas from the one or more combustion chambers and to transfer heat from the exhaust gas to a working fluid at high pressure; and, an expander comprising a low friction displacement device for receiving a metered amount of the heated high- pressure working fluid and for retrieving energy from the working fluid by being driven by the heated expanding working fluid.

12. An engine according to claim 11, wherein the engine is an internal combustion engine comprising an exhaust manifold to conduct exhaust gas away from the one or more combustion chambers to the first heat exchanger.

13. An engine according to claim 11 or 12, comprising: a water source; and, a high pressure pump for pumping water from the water source into the first heat exchanger for heating by the exhaust gas and from there on to the rotary expander.

14. An engine according to claim 13, comprising a rotary output shaft coupled to one of the rotors of the rotary expander for providing a source of power from the rotary expander .

15. An engine according to claim 14, comprising a coupling from the output shaft to provide power to drive either a supercharger or a direct power supplement to the engine's output shaft.

16. An engine according to any of claims 12 to 15, comprising a second heat exchanger for transferring heat from working fluid output from the expander to water whilst it is being coupled to the first heat exchanger.

17. Apparatus for generating roζary power, the apparatus comprising: a heat exchanger arranged to receive heat from a heat source to which, in use, the apparatus is connected and to transfer heat to a working fluid at high pressure; and an expander comprising a displacement device for being driven by the expanding working fluid, wherein the expander has a transient chamber of variable volume operative, in use, to receive a metered amount of the heated high- pressure working fluid wherein the expansion ratio of the expander is greater than 50:1.

18 Apparatus according to claim 17 in which the expander is a rotary expander.

19 A method of generating rotary power from a heat source, the method comprising: providing a heat exchanger for transferring heat from the heat source to a working fluid at high pressure; providing the working fluid ,at high pressure to an expander comprising a low friction displacement device; deriving shaft power from the expander when driven by the expanding hot working fluid.

20. A method according to claim 19, in which the heat source is the hot exhaust gas generated by an internal combustion engine.