WO1998028524A1

WO1998028524A1 - Thermionic energy conversion arrangement

Info

Publication number: WO1998028524A1
Application number: PCT/SE1997/002169
Authority: WO
Inventors: Robert Svensson; Leif Holmlid
Original assignee: C.L. Advanced Energy Research Ab
Priority date: 1996-12-20
Filing date: 1997-12-19
Publication date: 1998-07-02
Also published as: SE511239C2; AU5505098A; SE9604816L; SE9604816D0

Abstract

The present invention relates to an arrangement for generating electrical energy using heat emitted from a heat emitting source, said heat emitting source being a combustion engine or a part connected to said combustion engine, wherein said arrangement comprises first and second electrodes spaced from each other and enclosed in a housing, said first electrode being arranged at the source temperature and said second electrode at a drain temperature.

Description

THERMIONIC ENERGY CONVERSION ARRANGEMENT

TECHNICAL FIELD The present invention relates to an arrangement for generating electrical energy using heat emitted from a heat emitting source, said heat emitting source being a combustion engine or a part connected to said combustion engine.

BACKGROUND OF THE INVENTION Modern cars use ever more electric power due to more onboard electric systems, e.g. ABS brakes, active suspension systems, electric windows, chair adjustment systems and electronic engine control systems. Electric power steering may replace the hydraulic ones which is one more large electricity consumer. A modern sedan car may be equipped with an electric generator (generally an alternator) with an output current of maximum 60 - 90 A at 12 V. A belt driven generator has a rather low total efficiency why it is interesting to find alternative solutions for the electricity generation problem.

There are many heat dissipating objects in a normal car powered by a combustion engine. It is necessary to identify the objects with the highest temperatures and heat flows to the ambient. The heat flow from the cylinder walls is rather high, but due to the necessity to maintain the lubrication the wall temperature must be kept low and therefore probably not suitable for thermionic energy converters (TECs). The highest temperature in the combustion engine system is probably in the exhaust pipes close to the cylinder head exhaust ports and in a catalytic converter. The highest exhaust gas temperatures are found close to the exhaust ports, up to 1250 K in high output power conditions from the combustion engine. The temperature at the input pipe of a catalytic converter (Cat) is approximately 200 degrees less than the temperature in the exhaust ports. The temperature out from the Cat is normally 100 degrees higher than the temperature at the input pipe of the Cat. The maximum gas flow is approximately 300 - 450 kg/h.

Catalytic converters are used to reduce the content of harmful emissions in the exhaust gases from the combustion engine. It works with catalytic surface reactions which convert harmful chemical substances to fewer harmful ones. The catalytic conversion of the substances is exothermal reactions which develop heat. One disadvantage with the catalytic converters is that the catalytic reactions are not active below a certain temperature of the catalytic converter cartridge. This means that just after a cold start of the engine the catalytic converter is not working. Decreasing the time for the catalytic converter to reach its working temperature is possible, thus decreasing the amount of harmful emissions. The catalytic converters developed for the near future will probably work at higher temperatures than the present ones. This fact is also favourable concerning their use as energy converter heat sources.

Thermionic energy converters are used to convert thermal energy between 1200 K and 2500 K to electric energy without mechanical moving parts. A thermionic energy converter works as a heat machine above stated source temperature and a drain temperature of typically 600 - 800 K. The converter consists of two electrodes of metal or other appropriate conducting material, one provides at the source temperature, the emitter, and the other at the drain temperature, the collector. The electrodes are located adjacent each other, substantially in a vacuum or at low pressure. The emitter emits a current of electrons to the collector, being held at a higher temperature through supply of thermal energy from the outside, for example from a flame or a catalytic converter cartridge. The electrodes frequently include a part of the external vacuum tight wall of the converter and are separated by insulating material.

PRIOR ART

Novel results related to carbon-covered electrodes in an "open" Thermionic Energy Converter (TEC) have previously been reported, see for example. J. Appl. Phys. 70 (1991) 1489-1492 R. Svensson, L. Holmlid and L. Lundgren, and "Temperature studies and plasma probing of a Rydberg matter collector in a thermionic energy converter" by R. Svensson and L. Holmlid. Paper No. 929430, 27th Intersociety Energy Conversion Engineering Conference, (IECEC 1992. San Diego). Society of Automotive Engineers, Vvarrendale 1992, Vol. 3, p. 537-542. At certain conditions a very high electron current (back current), flowing from the cold collector to the hot emitter, has been observed. This phenomenon has been studied with various methods. which led to the conclusion that a novel form of matter with a very low work function is located on the collector surface and in the inter-electrode gap. The matter consists of highly excited cesium atoms in a condensed form, and is referred to as Rydberg matter (RM . The RM also partially fills the inter-electrode gap and thus gives a smaller inter-electrode distance. The existence of RM has been predicted theoretically by Manykin et. al (E. A. Manykin. M. I. Ozhovan and P. P. Poluektov, Sov. Phys. Dokl. 26 (1981) 974.)

A new concept for thermionic energy converters which allows lower collector temperatures and gives a higher efficiency than the conventional thermionic energy converters is known through US Patent No. 5,578,886 issued to the same inventors. The collector temperature can perhaps be decreased to approximately 400 - 500 K and the efficiency might be increased to 20 - 30%.

OBJECTS OF THE PRESENT INVENTION

The main object of the present invention is to provide a new method and arrangement to generate electrical energy on or close to a heat emitting part of a vehicle, preferably parts in connection with the engine of the vehicle.

Another object of the invention is to obtain a higher total fuel efficiency of a combustion engine powered car by using the waste heat for electricity production and thus omitting the electromechanical generator unit and achieve a shorter heat-up time for the catalytic converter with the aid of a "thermos bottle" effect of the said thermionic energy converter. This concept also allows control of the thermionic energy converter temperature under working conditions with the aid of extracted electric energy.

Yet another object of the present invention is to provide the thermionic energy converter system with an external fuel injection system in order to give extra electricity under low power driving conditions when at the same time much electricity is needed. Instead of having an external fuel injection it is possible to let the engine run with an over stoichiometric mixture, which is combusted in the thermionic energy converters and thus providing the extra needed heat. By using an external mixture chamber, it is also possible to use the thermionic energy converter as an independent heat-electricity source for boats, construction sites, military use etc.

An advantage of the present invention is that due to the very low work function and a smaller effective inter-electrode gap it is possible to increase the efficiency of a TEC from the traditional efficiency degree of 5 - 10 % to 25 - 30 %. It is also possible to run the TEC with lower emitter and collector temperatures which gives the possibility to use a TEC for applications at lower temperatures than what the "traditional" TECs were suitable for.

Above objects and advantages are obtained by the arrangement mentioned in the beginning comprises first and second electrodes spaced from each other and enclosed in a housing, said first electrode being arranged at the source temperature and said second electrode at a drain temperature. Said electrodes are located adjacent each other, substantially in a vacuum or at low pressure. The first electrode is an emitter, which emits a current of electrons to the second electrode being a collector, and held at a higher temperature through supply of thermal energy. In one embodiment said part is an exhaust pipe or an exhaust manifold. In a preferred embodiment said part is a catalytic converter. Preferably, said first electrode at least partly surrounds said heat emitting source and said second electrode at least partly surrounds said first electrode. The electrodes surround the catalytic converter in such a way that a thermos bottle effect around the catalytic converter is achieved. Preferably, said first electrode consist of Ni. said second electrode consist of a group including molybdenum or tungsten and the space between the electrodes contents cesium. One embodiment comprises further set of first and second electrodes arranged in series or in parallel.

The invention also relates to an arrangement for generating electrical energy, preferably in a system including a combustion engine and corresponding exhaust pipes, wherein a catalytic converter is connected to said combustion engine, substantially adapted to purification of exhaust gases from said engine and a thermionic energy converting unit, substantially covering said catalytic converter entirely and including at least one first and second electrodes arranged substantially in parallel and spaced apart from each other. In an preferred embodiment, the thermionic energy converting unit encloses the catalytic converter in such a way that a thermos bottle effect is achieved. Said catalytic converter includes catalyser elements enclosed in a casing. In one embodiment a combustion is carried out in the catalytic converter. The catalytic converter casing includes at least one of the electrodes of the thermionic converter. Preferably, said catalytic converter at leat partly consists of platina, rhodium, metal carbides or perovskites and operates in a temperature range of about 1500 - 1600 K, preferably about 1470 - 1670 K.

The invention further relates to a device for generating electrical energy, said device comprising a catalytic converter unit, a thermionic generator, which entirely covers said catalytic converter, and a mixing chamber having intakes for fuel and air, and an outlet to said catalytic converter. In an embodiment the air is mixed with fuel for later combustion in said catalytic converter.

SPECIFICATION OF THE DRAWINGS

In the following the invention will be described with reference to some preferred embodiments shown on the enclosed drawings, in which:

Fig. 1 is a schematic block diagram, showing the principle of the invention, with a device according to the invention shown in cross-section.

Fig. 2 is a schematic view of cross-section through a catalytic converter equipped with a thermionic energy converter according to the present invention.

Fig. 3 is a schematic cross-section along line A-A of the catalytic converter and the thermionic energy converter in fig. 2. Fig. 4 is an alternative embodiment of a thermionic energy converter.

Figs. 5a and 6a are two configurations of multi thermionic energy convenors.

Figs. 5b and 6b are cross-sections through the multi thermionic energy convenors shown in figs. 5a and 6a.

Fig. 7 shows thermionic energy convenors arranged on the exhaust manifold of an engine. Fig. 8 is a general simplified principle for heat transfer from a gas through a pipe wall.

SPECIFICATION OF THE INVENTION

The principle of the invention is shown schematically in fig. 1. A Thermionic Energy Converter

(TEC) 10 comprising a housing 1 1 and first and second electrodes 12 and 13 arranged on or close to a heat emitting part of an engine, which in this embodiment is an exhaust gas pipe 14. The electrodes. 12 designating the collector and 13 the emitter, are connected to power conditioners and charging devices 15 which are connected to the battery 16 and to the remaining car electric system. The electrodes may preferably be connected to the device 15 through regulating means not shown.

The energy source for electricity generation is to use the waste heat from the car's engine, which generally is as much as 80% of the total energy from the combustion of the gasoline. According to the invention, the best location to tap the excess heat is the Catalytic Converter (Cat) in the exhaust system or perhaps at the exhaust pipes close to the engine. The Cat must be kept within a certain temperature interval. Large amounts of heat are dissipated through the walls of the Cat. A Thermionic Energy Converter (TEC) in coaxial form could conveniently be located around the ceramic cartridge of the Cat. Since the TEC is a rather good heat insulator before it reaches its working temperature the Cat will reach working temperature faster, and the final temperature of it can be controlled better when encapsulated in a concentric TEC arrangement.

The thermionic energy converter 10 and catalytic converter combination 17, hereinafter called TECCAT, can be designed as one thermionic energy converter and one catalytic converter shown in fig. 2. The system could also be designed with more than one TECCAT included.

Here, the collector 12 consists of a metal foil, for example of Ni or another suitable metal. The emitter 13 consists, for example of molybdenum, tungsten or another suitable metal. The catalytic converter cartridge is designated with 18 in the figure. The interelectrode gap (IG) 19 contains, for example cesium or some other suitable substance. A flange 20 is provided as the exhaust gas inlet, which for example is connected to the headers on the exhaust port of an engine (not shown) in a conventional way. The TECCAT unit 17 is enclosed in an outer metal housing 21. The arrow 22 indicates the exhaust gas flow.

According to fig. 4, it is also possible to arrange an external mixing chamber 23 in connection with the catalytic converter whereby fuel can be injected through a fuel injection pipe 24 and air through inlet 25.

When operating, the heat from the catalytic converter cartridge heats the emitter electrode in the thermionic energy converter and the waste heat from the thermionic energy converter is then transported out to the free air by the collector electrode 12. which is located outside the emitter 13. The heat is then converted to electric energy and outputted through conductors 26 as an electric current driven by the voltage between the thermionic energy converter electrodes 12 and 13.

The invention has the advantage that the thermionic energy convener before it has reached its working temperature has a very high vacuum in the electrode gap, for example between 10 ^": and 10^'' mbar which can be considered as perfect vacuum with respect to the heat transfer properties of the thermionic energy converter/housing 21. Thus it acts as a thermos bottle around the catalytic converter which helps the catalytic converter cartridge (CCC) to reach its working temperature much faster than in the case of an ordinary catalytic converter whose enclosure is subjected directly to the ambient air flow. Thus, the amount of harmful emissions is greatly reduced at cold starts of the engine.

It is also possible to obtain more electricity from the system. When the power output from the engine is low, fuel can be injected into the TECCAT which increases its temperature at low power output conditions from the engine. It is then possible to obtain an amount of electricity approaching that in the situation when the engine runs at high power. In such a case it might even be possible to omit the mechanically driven generator and obtain a totally higher efficiency in the electricity production.

Moreover, it is possible to regulate the temperature of the Cat and the TEC by controlling the electrical load of the TEC. The possible working temperatures of present and future Cats appear very suitable for low work function collector TEC, which has been demonstrated to work down to 470 K.

One efficient way to design a Cat-and-TEC combination is to combine the Cat housing with the TEC electrodes, as described above. With this design it is possible to control the Cat temperature by controlling the TEC output power. The TECCAT might be constructed as one cylinder with coaxially placed TECs 28 electrically connected in series in order to increase the output voltage, as shown in figs. 6a and 6b.

Another embodiment is shown in fig. 5a and 5b, in which a few smaller Cat tubes 27 with TECs 28 connected in parallel with respect to the gas stream but with TECs connected electrically in series. In this case the temperature distribution over the TEC units is equal.

The highest temperatures are available close to the exhaust ports. To combine TECs with the exhaust pipes is another solution, according to the invention, at high engine power output conditions to each heat flux calculations are performed with "normal" exhaust pipes. An embodiment illustrating this method is shown in fig. 7. In this embodiment, several TEC units 29 are arranged on each pipe of exhaust manifold connected to exhaust gas ports 30 of an engine (not shown). To increase the heat transfer from the gases to the pipe wall, it is possible to use some kind of porous plug or grid in the exhaust pipes without loosing too much power. However, one objection against this design might be possible difficulties with the metal-ceramic insulator joints due to high static and dynamic mechanical forces on the exhaust pipes. On the other hand, the gas stream is already divided due to the number of cylinders which gives multi-TEC systems in a natural way. For example, a V8 engine might have eight TECs in series with a total output voltage of approximately 6 - 9 V.

The heat flux calculations for the exhaust pipe concept are performed as a standard heat transfer problem where a hot gas delivers its heat to a metal wall and the heat flux in the TECCAT concept is calculated with only radiative heat transfer due to a low heat conductivity in the Cat ceramic cartridge, which is illustrated schematically in fig. 8. In this case, all power calculations are made without any kind of extra fuel mixture injected in the systems.

The calculations show the possible electric power output from a TEC combined with a Cat and a TEC combined with the exhaust pipes under various driving conditions.

The calculations are performed according to the following main parameters for a four-cylinder engine with a volume of approximately 2 litres:

Maximum allowed temperature for the CCC (T _{CCC max} ) is 1270 K. New types capable to handle temperatures up to 1670 K are under development (R. J. Locker, M. J. Schad and C.B. Sawyer, "Hot vibration durability of ceramic preconverters", Automotive

Engineering, March 1996. p. 43-47.)

At normal running conditions T _ccc is between 870 and 970 K and T _{CCC mm} is 570 K.

The dimensions of the CCC are approximately: 100 - 200 mm long (1 _{c cc} ) and approximately 100 - 150 mm in diameter (d _ccc.) - The adiabatic temperature increase T _{gaS 0llI} - _{as m} (ΔT_CCC ) from the Cat inlet to the outlet is 100 K.

The maximum gas flow for the Cat (Q _{n3s mXi} ) is 300 kgΛi. The heat capacity (c )for the exhaust gases can be approximated to the value valid for air.

The temperature of the gas in the exhaust pipes at the cylinder head of an engine (T , ) is

T_{BJ5 in} + 200 K.

Possible candidates for CCC units, in addition to platina and rhodium are metal carbides or perovskites which can work at much higher temperatures. With a small extra fuel mixture injection in the TECCAT, it is then possible to extract very large amounts of power. Table I shows data for a preferred TEC and exhaust pipe concept. Table II shows data for a hypothetical future TECCAT with hi-temperature materials.

Table in. Heat flux and expected TEC electric power output for a 4 cylinder and a 8-cylinder engine with TECs on the exhaust pipes.

4-cyl. 60 187 186 0.08 200 2972 743

4-c l. 90 257 254 0.08 200 4068 1017

4-cyl. 130 342 338 0.08 200 5403 1351

8-cyl. 60 187 186 0.16 200 5944 1486

8-cyl. 90 257 254 0.16 200 8135 2034

8-cvl. 130 342 338 0.16 200 10805 2701

Table II. Heat flux and expected TEC electric power output for TECCATs with various epsilon values and temperatures.

Standard 0.37 0.09 970 870 616 154

Standard 0.73 0.09 970 870 1215 304

Standard 0.37 0.28 970 870 1848 462

Standard 0.73 0.28 970 870 3646 912

Standard 0.37 0.28 1200 1000 6352 1588

Standard 0.73 0.28 1200 1000 12531 3133 enhanced 0.37 0.28 1470 1200 15357 3839 enhanced 0.73 0.28 1470 1200 30300 7575 In the tables: w_m is the gas mean speed, α is the conductive heat transfer coefficient, k is the combined heat transfer coefficient, A is the active pipe area. T - is the temperature of the gas, T -, is the temperature of ^'the wail outer surface (TEC emitter), q is the heat flux, € is the net effective thermal emissivity, T _ccc is the temperature of the CCC, T _R is the radiation receiving cylinder outers surface area

(TEC emitter) and Pel. is the estimated TEC electric output power.

The electric power output is calculated as 25% of the heat flux. These figures are approximate and given as an example.

In the cases with gas-to-wall heat transfer the structure of the pipe wall influence the heats transfer efficiency. It is also assumed that there is no gas temperature gradient graph 31 (fig. 7) parallel with the gas stream. To obtain more accurate data the calculations must be performed with more elaborate formulae where the geometry is taken into account. The gas is also assumed to be air concerning gas constants. The heat flux is assumed to be proportional to the pipe 14 area which also is an approximation, valid only if the heat transfer path parallel to the gas stream is short and the gas is supposed to have no temperature gradient parallel to the gas stream.

In the TECCAT radiation heat transfer case the CCC is assumed to have a constant temperature over its surface. In the multi-TEC case (fig. 6a) the CCCs are supposed to have the same temperature as one large CCC (fig. 5a). Thus, increasing the parameters in the formulae too much might provide unrealistic heat fluxes. As a very rough "rule-of-thumb" one third of the combustion energy in a combustion engine exits the system as thermal energy in the hot gases.

The CCC temp might be kept around 1500 - 1600 K which together with a controlled fuel mixture injection can give a large amount of heat flux at high temperatures. New CCC materials are under development, which can work at temperatures around 1470 - 1670 K. In combination with a high efficiency TEC considerable amounts of electricity might be produced.

The invention is obviously not limited to the described and shown embodiments, and may be varied and modified within the scope of the following claims.

Claims

What we claim is:

1. An anangement for generating electrical energy using heat emitted from a heat emitting source, said heat emitting source being a combustion engine or a part connected to said combustion engine, wherein said arrangement comprises a first and second electrodes spaced from each other and enclosed in a housing, said first electrode being arranged at the source temperature and said second electrode at a drain temperature.

2. The arrangement of claim 1, wherein said electrodes are located adjacent each other, substantially in a vacuum or at low pressure.

3. The arrangement of claim 1 , wherein said first electrode is an emitter, which emits a current of electrons to the second electrode being a collector, and held at a higher temperature through supply of thermal energy.

4. The arrangement of claim 1 , wherein said part is an exhaust pipe.

5. The arrangement of claim 1 , wherein said part is an exhaust manifold.

6. The arrangement of claim 1 , wherein said part is a catalytic converter.

7. The arrangement of claim 1, wherein said first electrode at least partly surrounds said heat emitting source.

8. The arrangement of claim 7, wherein said second electrode at least partly surrounds said first electrode.

9. The arrangement of claim 6, wherein said electrodes surround the catalytic convener in such a way that a thermos bottle effect around the catalytic converter is achieved.

10. The arrangement of claim 1, wherein said first electrode consist ofNi.

1 1. The arrangement of claim 1, wherein said second electrode consist of a group including molybdenum or tungsten.

12. The arrangement of claim 1, wherein the space between the electrodes contents cesium.

13. The arrangement of claim 1 , comprising further set of first and second electrodes arranged in series.

14. The arrangement of claim 1, comprising further set of first and second electrodes arranged in parallel.

15. An arrangement for generating electrical energy, preferably in a system including a combustion engine and corresponding exhaust pipes, wherein a catalytic converter is connected to said combustion engine, substantially adapted to purification of exhaust gases from said engine and a thermionic energy converting unit, substantially covering said catalytic converter entirely and including at least one first and second electrodes arranged substantially in parallel and spaced apart from each other.

16. The arrangement of claim 15, wherein said thermionic energy converting unit encloses the catalytic converter in such a wav that a thermos bottle effect is achieved.

17. The anangement of claim 15, wherein said catalytic converter includes catalyser elements enclosed in a casing.

18. The arrangement of claim 15, wherein said first electrode consist of Ni.

19. The arrangement of claim 15, wherein said second electrode consist of a group including molvbdenum or tungsten.

20. The arrangement of claim 15, wherein the space between the electrodes contains cesium.

21. The arrangement of claim 15, comprising further set of first and second electrodes arranged in series.

22. The arrangement of claim 15, comprising further set of first and second electrodes arranged parallel.

23. The arrangement of claim 15, wherein a combustion is carried out in the catalytic converter.

24. The arrangement of claim 17, wherein the catalytic converter casing includes at least one of the electrodes of the thermionic converter.

25. The arrangement of claim 15, wherein said catalytic converter at leat partly consists of platina, rhodium, metal carbides or perovskites.

26. The arrangement of claim 15, wherein said catalytic converter operates in a temperature range of about 1500 - 1600 K, preferably about 1470 - 1670 K.

27. A device for generating electrical energy, said device comprising a catalytic converter unit. a thermionic generator, which entirely covers said catalytic converter, and a mixing chamber having intakes for fuel and air, and an outlet to said catalytic converter

28. The device of claim 25, wherein the air is mixed with fuel for later combustion in said catalytic converter.