Optionally rechargeable batteries are in widespread use in both stationary and portable electronic appliances, such as communications equipment, laptop and handheld computers, all kinds of photographic and video equipment, stereo installations and the like. Disposable batteries and rechargeable batteries of this type often contain environmentally harmful substances, such as compounds containing Cd, Li, Hg, Ag, etc. Environmentally harmful substances of this type form a potential risk to human and animal health. Therefore, batteries of this type are generally collected and processed separately, which requires an expensive logistical system. In addition, substances of this type as mentioned above are relatively expensive. Another drawback is the relatively low energy and power density of batteries of this type. On account of these low densities, in practice the batteries are relatively heavy, which is disadvantageous for the user for the abovementioned mobile applications.

US-A 4,218, 266 has disclosed a battery having a combustion chamber which works using liquid hydrocarbons. A thermoelectric generator (also referred to below as TEG for short) at least partially delimits the combustion chamber. Air which is used for the combustion is heated with the aid of heat from the combustion gases in a heat exchanger. The aim of this measure is to provide a battery with a high efficiency and with low temperatures of the fuel gases discharged.

According to a first aspect, it is an object of the invention to provide a battery which does not use any environmentally harmful substances and which is suitable in particular for use in mobile personal equipment. Another object of the invention is to provide a battery of this type which can be refilled using various readily available fuels. Yet another object of the

invention is to provide a battery of this type having a simple structure which is constructed from relatively inexpensive materials.

For this purpose, the invention provides a thermal battery, in particular with miniaturized dimensions, comprising a combustion chamber, which is delimited partly by an electric generator for converting energy released during combustion into electricity and partly by a thermally insulating wall part which is permeable to oxygen-containing gas and to combustion products in order for the oxygen-containing gas to be introduced and combustion products to be discharged, a burner which is arranged in the combustion chamber and has an inlet for fuel and an ignition means.

The term"with miniaturized dimensions"in the present description means that the dimensions and the weight of the battery according to the invention are such that the battery is suitable for the electronic equipment referred to above, in particular personal portable equipment. As will be clear from the examples and the figures, these dimensions are of the order of magnitude of a few millimetres to a few centimetres.

The battery according to the invention makes use of one or more electric generators, in which the heat which is released during combustion with a flame is converted into electricity. Examples of generators of this type include a thermoelectric generator and a thermophotovoltaic cell (also referred to below as TPV for short), or a combination of the two. In the case of a TEG, energy, in this case heat obtained through combustion of a fuel in the combustion chamber, is transferred from the hot side of the TEG by heat-conducting electrons or ions to the cold side and converted into electricity. No chemical conversion takes place in the TEG. A TEG does not include any moving components and is therefore highly reliable, has a long service life and is maintenance-free. The TEG is compact (2 kW/kg) and therefore suitable for mobile applications. In a TPV, infrared light is converted into electrical energy, once again without any chemical conversion. The cell is protected by a shield which

transmits infrared light, for example a quartz plate for protecting the cell from the heat which is generated during combustion. If a combination of a TPV and a TEG connected in series is used, it is possible for the remaining radiation and heat from the TPV to be passed onto the TEG. The TEG, TPV and the other structural parts and therefore the battery can be produced from inexpensive materials which are commercially available. Electric generators of this type themselves do not have any moving parts, and consequently require little maintenance.

The battery according to the invention comprises a combustion chamber. The combustion chamber defines a hot volume and is partially delimited by at least one electric generator, in which energy (heat and/or light) is converted into electrical energy, and by at least one thermally insulating wall section which is permeable to an oxygen-containing gas, such as air, and to the combustion products obtained after combustion of the fuel with the oxygen-containing gas. It is necessary for the combustion chamber to be thermally insulated, since on the one hand high (flame) temperatures (up to more than 1200°C) are required in the combustion chamber with a view to efficiency. On the other hand, the temperature of the outer side of the battery must be low (< 50°C when used in the open air), since if the surface temperature is high there is a risk of the user burning himself.

Since in the battery according to the invention heat exchange between the hot combustion products and the relatively cool oxygen-containing gas takes place in this permeable wall section, it is possible to reach the high temperatures and therefore a high efficiency. The supply and discharge are effected through diffusion, the stack effect and/or forced flow using a fan through the insulating wall part. The burner for the combustion of fuel is arranged in the combustion chamber and is fed with fuel via an inlet which extends through the wall of the combustion chamber. The burner is provided with an ignition means in order to allow the battery to be"switched on".

Therefore, fuel and oxygen-containing gas are mixed in the combustion chamber.

No environmentally harmful substances are used in the battery according to the invention, meaning that it presents fewer environmental risks. The energy and power density of the battery according to the invention, in which any desired hydrocarbons can be burnt, is high (15-20 times that of conventional Li-ion batteries), even though the conversion coefficient may be low (5-40%). The battery according to the invention works for as long as combustion continues. The combustion can easily be stopped by interrupting the supply of fuel, for example using a shut-off means. The fuel will generally be delivered in handy containers which are quick and easy to connect to the burner inlet, for example using quick-fit coupling means. This is quicker and easier than recharging traditional rechargeable batteries.

According to a preferred embodiment, the thermally insulating wall part comprises a porous material which is able to withstand high temperatures. A material of this type is permeable to the oxygen-containing gas and the combustion products, so that suitable heat exchange can take place between the corresponding gas streams, with the cold incoming gases being preheated by the still warm emerging gases. A preferred, inexpensive material comprises mineral wool, such as rock wool, or open-cell ceramic foam, on account of the insulation and permeability properties being excellent for the abovementioned purposes.

To increase the heat transfer and the heat exchange still further, it is advantageous for a heat conductor made from a thermally conductive material to be provided in the porous material. In practice, if no flow-regulating means are provided, it is usual for the cool oxygen-containing gas to pass inwards through the thermally insulating wall part at a certain distance from the emerging combustion products, for example at the top and bottom sides of a vertical wall. The heat exchange is then low. A heat conductor of this type extending over a wall of this type then ensures efficient heat exchange and transfer.

In order not to restrict the permeability of the thermally insulating wall part, the heat conductor is preferably in the

form of a network of webs which delimit openings. Optionally regular grids, screens, perforated plates and wire gauze made from a material of good conductivity, such as copper, are particularly preferred. An embodiment in which a plurality of heat conductors are arranged at a distance from one another in the thickness direction of the insulating wall part and parallel to the surface of the wall part is even more preferred. On account of this arrangement perpendicular to the temperature gradient, the heat conductors cause little or if any heat loss.

Since the battery does not adopt a fixed position with respect to the outside world during use, and therefore the positions at which the oxygen-containing gas and combustion products pass through the permeable insulation will change, grids arranged in layered form throughout the entire insulation are preferred.

In the case of a combustion chamber which is rectangular in cross section, it is advantageous for the thermally insulating wall part to surround at least that side of the combustion chamber through which the inlet for fuel for the burner passes as well as the opposite side. This achieves an efficient supply and discharge. In the case of a combustion chamber which is rectangular in cross section, it is preferable for the TEG and/or TPV to cover at least two sides which extend between the thermally insulating sides which have just been referred to. If desired, when using a combination of at least one TPV and at least one TEG, it is possible for a reflector which reflects infrared radiation to be arranged behind a TPV which is (partially) transparent to this radiation. This reflector directs the unused radiation to the TEG, where it can be used as heat in the TEG. If desired, it is also possible for the TEG to be provided with a filter for reflecting short-wave radiation which is useful for the TPV (usually with a wavelength in the range from 800-1700 nm) and transmitting long-wave radiation (with a wavelength in the range from 1700-4000 nm). Both measures contribute to increasing the efficiency. The surface area and the porosity of the thermally insulating part, which inter alia determine the diffusion and the stack effect, are advantageously selected in such a way that an excess of oxygen- containing gas is maintained with respect to the fuel which is

supplied in the combustion chamber. An excess of this type is expedient for combustion.

The TEG may be constructed from the known semiconductor materials. It is preferable for the TEG to be constructed from highly efficient (> 15%) thermocouples which may optionally comprise a plurality of layers. When using materials of this type, a high (flame) temperature is required in order to enable a high yield to be achieved. The reliability and service life of thermocouples of this type are high, which is of benefit to the reliability and service life of the battery as a whole.

The burner advantageously comprises a pipe, which at one end is coupled to the inlet for fuel and at the other end comprises a mouthpiece. It is preferable for the mouthpiece to be surrounded by a radiation-emitting body, such as a sleeve made from a material which catalyzes combustion, such as the known"mantles" for camping lamps working, for example, on propane gas.

In this context, it should be noted that in the present description the term"burner"does not mean that the combustion which occurs must be associated with a flame. The combustion may also occur at gauze or sponge material, for example with a ceramic or metal foam structure. As has been stated, a radiation-emitting body may be positioned between the burner and the electric generator (s).

The principle of the battery according to the invention is suitable for all types of fuels, be they gaseous, liquid or solid. The battery is preferably designed in such a way as to be suitable for hydrocarbons, such as propane and butane ("igniter gas"), for which an extensive distribution network is already in place.

Various components may take particular forms depending on the type of fuel.

The inlet is advantageously provided with a main valve for admitting or blocking fuel as required. In the case of a gaseous

fuel, the inlet, in particular the coupling to the gas container, is advantageously provided with a restrictor valve in order to control the quantity of gaseous fuel which is to be burnt.

To burn a gaseous fuel, the ignition means preferably comprises a piezo element. A control device may be provided, so that the piezo element is energized after the main valve has been opened.

If no voltage is detected at the electric generator, in other words no combustion is taking place, the main valve is automatically closed on the basis of a signal from the control device.

To burn a liquid fuel or a molten solid fuel, the ignition means preferably comprises a preheating device, such as an electric preheater. It is advantageous for this preheating device to be accommodated in the sleeve of catalytic material. In this case, the preheating device heats the flowable fuel to above the ignition temperature of the vapour of the fuel. When the fuel has been successfully ignited, the preheating device is switched off.

If the battery according to the invention is intended to burn a liquid fuel, such as oil, or molten solid fuel, such as stearin, the inlet advantageously comprises a tube with capillary, such as a fuse. The capillary sucks up the fuel which is able to flow and is responsible for a continuous supply to the burner. The dimensions of the capillary, in particular the cross section, determine the maximum flow of fuel. The contact between capillary and sleeve may expediently be interrupted for safety reasons, for example by means of an energizable wedge.

Energizing can be effected, for example, by activation of a piezo element, which is deactivated after the combustion has stopped, so that the contact can be restored prior to the subsequent start.

To enable a solid fuel to melt, in a battery according to the invention which is intended for this purpose, there is a container for holding the solid fuel, which container comprises

a coupling for coupling the container to the inlet of the burner, with a heating device for melting the fuel being provided in the vicinity of the said coupling. The heating device, for example an electric heating device, locally melts the solid fuel, such as a fuel stick, and the molten fuel is sucked up through the capillary. In an expedient embodiment, the heating device comprises a plate made from a thermally conductive material, which is thermally conductively connected to the combustion chamber, for example via the metal feed pipe of the burner.

With a view to maintaining a continuous supply of molten fuel, it is preferable for counter-pressure means, such as a counter- pressure spring, for bringing the fuel into contact with the preheating device, to be provided between that side of the container which is located opposite the coupling and the fuel.

A housing, in which the openings required for the supply of oxygen-containing gas, such as air, and discharging combustion products, is advantageously provided in order to protect the insulation and electric generator. A container for fuel may likewise be accommodated within this housing in order to be protected. The housing is advantageously composed of removable parts, for example a case with a lid, so that the container for fuel can easily be replaced. If desired, the housing may be provided with cooling means, such as cooling fins, in order to keep a low surface temperature. Forced cooling, for example by means of a fan, can also be used. Another possible option is cooling by evaporation of water, for example via a mantle which can be wetted with water, for which the water required is extracted from a water container via a capillary.

The battery according to the invention comprises a miniature burner in an insulated combustion chamber. Insulation is required in order to keep the combustion chamber, as a high- temperature reactor, up to temperature (for example 1000°C or more) during operation. Such a high temperature is required in order to be able to obtain a high efficiency, with regard to heat for the TEG (s) and with regard to the light yield in the

case of the TPV (s).

The battery according to the invention may comprise a plurality of combustion chambers, each having a separate burner, for example each having different dimensions, which can each be switched on separately. In fact, a battery of this type comprises a plurality of battery modules according to the invention connected in parallel. This allows the power supplied to be controlled over a wide range. With an embodiment of this type, it is possible to keep the smallest chamber operating continuously as a type of standby for quickly starting up the battery.

If necessary, the battery according to the invention can be combined with other energy sources.

As has already been stated, the battery according to the invention can be used as an energy source for many stationary and mobile applications, such as in the army, outdoors, on (sailing) boats and at other locations far away from populated areas.

The invention is explained below with reference to the appended drawing, in which: Fig. 1 diagrammatically depicts a cross section through an embodiment of a battery according to the invention, working with a gaseous fuel; Fig. 2 diagrammatically depicts an embodiment of a battery according to the invention, working with a liquid fuel; Fig. 3 diagrammatically depicts an embodiment of a battery according to the invention, working with a meltable solid fuel; Fig. 4 shows an illustration of an embodiment of a battery having a plurality of combustion chambers according to the invention; Fig. 5 diagrammatically depicts a 0.5 W TEG battery; and Fig. 6 diagrammatically depicts a 10 W TEG battery with passive cooling.

The figures are not to scale, for the sake of clarity, and are not intended to restrict the invention in any way.

Fig. 1 diagrammatically depicts a cross section through an embodiment of a battery according to the invention, running on a gaseous fuel. The battery comprises a housing, which is not shown here for the sake of clarity. A case-like combustion chamber 10 is located within the housing. In this embodiment, the combustion chamber 10 is in the form of a rectangular case, of which the bottom wall and top wall are each formed by an electric generator 12 and 14, respectively, such as a TEG and/or TPV. The other walls, i. e. the two end walls 16 and 18, as well as the two longitudinal walls 20 (of which only one is visible) consist of a thermally insulating porous material, such as glass wool. A number of copper grids 22 are located at a distance from one another and parallel to the surface of the wall within the porous material. The direction of the heat transfer which occurs in the grid 22 is indicated by a bold arrow 24. A burner 26 is arranged in the combustion chamber 10. The burner comprises a pipe 28 which passes through end wall 16. A mouthpiece 30 at the free end of the pipe 28 is surrounded by a mantle of catalytic material for optimum combustion, with a piezo ignition means therein, these two components being denoted overall by the common reference numeral 32. The other end of the pipe 28 is coupled to a feed hose 36, with a controllable main valve 34 positioned between the two ends. The feed hose 36 is connected to a gas container 40 by means of a bayonet connector with restrictor valve 38.

The battery operates as follows. Gas is fed to the burner 26 through actuation or control of valve 34 and is ignited with the aid of ignition means 32, resulting in flame 42. The quantity of gas is controlled by the throttling valve in connector 38.

Oxygen-containing gas, in this case air, flows from the environment through the porous wall parts into the combustion chamber 10 as a result of diffusion, convection and/or forced by a fan (not shown), while the combustion products flow out of the combustion chamber 10 through the porous walls to the environment, as indicated by a thin arrow 44 at the two end

walls 16 and 18. The hot combustion products heat the copper grids 22 on one side thereof. The heat which is taken up is passed by heat conduction in grids 22 to the other side and transferred to the fresh air flowing in. Furthermore, the TEGs convert heat, and TPVs convert IR radiation, into electricity, which can be tapped off via suitable connections (not shown).

Fig. 2 diagrammatically depicts an-embodiment of a thermal battery according to the invention running on liquid fuel, such as oil. In this figure, components which are identical to those in Fig. 1 are denoted by the same reference numerals. The combustion chamber 10 together with the walls 12,14, 16,18 and 20 delimiting this chamber is constructed in the same way as the combustion chamber shown in Fig. 1. In this embodiment, the burner 26 comprises a mantle in which there is an electrical preheater 32'at the mouthpiece 30. The feed to the burner 26 comprises a hose 36 in which there is a capillary 37, such as a fuse, which extends as far as into the container 40. Instead of the main valve 34, there is a shut-off means with driven wedge 34', which can interrupt the contact between the associated capillary parts 37a and 37b, as shown in this figure.

The battery operates as follows. Oil from the container 40 is drawn through the capillary 37 with the shut-off means 34'open and is transferred to the mouthpiece 30, where the oil is vaporized and ignited with the aid of heater 32'. Air and combustion products diffuse into and out of the combustion chamber 10, respectively, in the same way as has been described with regard to Fig. 1. When the generation of energy is to be stopped, the shut-off means 34'is energized, so that its wedge breaks the contact between the capillary parts 37a and 37b and therefore stops the feed.

Fig. 3 diagrammatically depicts a thermal battery according to the invention which runs on a molten solid fuel, such as stearin. The battery is identical to that shown in Fig. 2, except for the supply of fuel, and consequently identical components are denoted by the same reference numerals. The storage container 40 is filled with solid fuel, for example in

the form of a stick, and at the underside comprises a cover 44, for example with a bayonet catch, with counter-pressure spring 46. The counter-pressure spring 46 presses the solid fuel onto the top side of the container 40, where a thermally conductive plate 48 is provided. Heat is supplied to the plate 48 by the heater 32'via heat conduction through the pipe 28. When combustion takes place for the first time, some of the heat which is released during combustion is transferred via pipe 28 to the plate 48, so that the solid fuel is continuously melted.

A thermally conductive coupling 50 provided with shut-off means 34'with wedge forms a heat bridge between the pipe 28 and plate 48.

Fig. 4 shows an embodiment of a thermal battery according to the invention which comprises three combustion chambers 10 with burners 26 connected in parallel, each of which can be operated separately. The combustion chambers 10 are of various dimensions and therefore have generators of various dimensions. By way of example, the largest delivers 0.3 W, the middle one delivers 0. 15 W and the smallest delivers 0.05 W, which gives the following operating options: 0,0. 05,0. 15,0. 20,0. 30,0. 35, 0.45 and 0.50 W.

Fig. 5 diagrammatically depicts a 0.5 W battery according to the invention, from which the insulation on the front side has been omitted for the sake of clarity.

By way of example, a 0.5 W TEG is 4 mm long and wide. The thickness of this TEG is 5 mm. The thickness of the combustion chamber is 5 mm, and the thickness of the insulation is 6 mm to achieve a good level of performance. Therefore, the height of the battery is 15 mm and its length and width are 16 mm.

Fig. 6 diagrammatically depicts a battery with cooling fins 60 as passive cooling. Components which are identical to those shown in the previous figures are denoted by the same reference numerals.

The following example is given for further illustration, using

butane as the fuel. 1 g of butane delivers 45 kJ of heat. At a TEG efficiency of approx. 14%, this delivers 6 kJ of electricity, i. e. 6000 Ws. In the stand-by state, a mobile telephone uses approximately 0.1 W and can therefore run for approximately 60 000 s = 17 hours on 1 g of butane. When making a call, a mobile telephone uses approximately 1 W and can therefore run for 1.7 hours. A tank of 50 g (30 g of butane) therefore gives sufficient energy for 2 to 4 weeks.

A second aspect of the invention relates to a battery, in particular with miniaturized dimensions, comprising a combustion chamber with burner arranged therein, at least one thermoelectric generator for converting heat which is released during combustion into electricity, and at least one thermophotovoltaic cell for converting radiation into electricity, an inlet for fuel and an inlet for oxygen- containing gas, as well as an outlet for combustion products.

A battery of this type having a combination of at least one TPV and TEG is known from DE-A1-199 19 023. In that publication, this combination is referred to as a"thermoconversion cell". As active elements, this cell comprises at least one thermophotovoltaic cell (also referred to below as TPV for short) and at least one thermoelectric generator (also referred to below as TEG for short). The cell is arranged around a radiation-emitting body. This body delimits a combustion chamber in which a fuel is burnt with oxygen-containing gas. An optical filter which transmits IR radiation in the range from approximately 1000 to 4000 nm is arranged between the radiation- emitting body and thermophotovoltaic element. In an alternative embodiment, this filter is impervious to radiation in the range from approximately 2000 to 4000 nm. This filter is coupled to the hot side of the TEG. The result of this is that on the one hand the heat which harms the TPV is blocked on one side. On the other hand, the heat which has been absorbed by the filter is transferred to the TEG. A radiation reflector may be arranged behind the TPV. To prevent thermal overloading of the TEG, the cell may be provided with a coating which absorbs or reflects IR radiation for radiation in the visible to far IR spectrum at the

location of the TEG.

In the known device, a filter which transmits radiation in the range from approximately 1000 to 4000 nm or a filter which filters out radiation in the range from approximately 2000 to 4000 nm is used for the TPV. A TPV can generally only use short- wave radiation from 800 to approximately 1700 nm efficiently for the generation of electric current. However, a burner or radiation-emitting source with an operating temperature of approximately 1200°C has an emission spectrum ranging from approximately 800 to 4000 nm. Therefore, some of the radiation is not efficiently utilized in this known device.

It is an object of the second aspect of the present invention to provide a battery in which the abovementioned drawback does not occur or occurs to a lesser extent and/or to create a useable alternative.

According to the invention, this object is achieved by virtue of a filter which reflects short-wave radiation being provided between the burner and the thermoelectric generator.

In the battery according to this aspect of the invention, the radiation which is emitted by the burner or radiation-emitting body and which is supposed to reach the TEG is separated into a long-wave radiation component (wavelength > approx. 1700 nm), which is transmitted by the filter to the active element, and a short-wave radiation component (approximately 800-1700 nm), which is reflected to the TPV and can therefore be converted into useful energy. This allows the distribution of the radiation to be efficiently adapted to the different requirements for the TEG and TPV.

It is advantageous for an absorbent element which converts all the radiation transmitted by the filter into heat to be arranged between the said filter and the TEG.

According to a preferred embodiment, a heat shield which transmits radiation is provided around the radiation source. The

heat shield protects TPV elements from the sensible heat.

According to a further development of this, the heat shield comprises a hollow body which is under subatmospheric pressure and is preferably made from quartz as a material that is transparent to radiation.

In a further preferred embodiment, a mirror for reflecting long- wave IR radiation is arranged behind the thermophotovoltaic cell. This produces a battery which is highly efficient.

The battery is advantageously formed in such a manner that the active components of different types are arranged opposite one another.

It is advantageous for a cooling element, for example in the form of cooling fins, along which incoming oxygen-containing gas is guided, to be provided inside the housing of the battery. In this way, this gas is preheated. More particularly, the outer side of the combustion chamber is provided with cooling means of this type. Further heat exchange with the hot combustion gases may also be provided. The fuel is also advantageously preheated by exchange of heat with the hot combustion gases.

Any remaining surfaces of the combustion chamber which are not covered with active elements are advantageously coated with a reflective layer which reflects any remaining radiation back to the burner and/or active elements.

This aspect of the invention is explained below with reference to the appended drawing, in which: Fig. 7 shows a diagrammatic cross section through an embodiment of a battery.

Fig. 7 diagrammatically depicts a battery 100 with a substantially rectangular housing 110. At one end, the housing 110 comprises an inlet 112 for oxygen-containing gas, and at the opposite end it comprises an outlet 114 for combustion gases. In this embodiment, a fan 113, which sucks a forced stream of

oxygen-containing gas out of the environment, is provided in the inlet 112. In this embodiment, a burner 116 is arranged centrally in the middle along the longitudinal axis of combustion chamber 115. In this embodiment, this burner 116 comprises an atomization or injection tube for atomizing or injecting fuel which is supplied. A radiation body 118, such as a catalyst mantle or foam, is arranged around the burner 116 and is in turn surrounded by a heat shield 119 in the form of a vacuum hollow body made from quartz, leaving clear an (annular) space 120, through which combustion gases can flow towards the outlet 114.

The active elements of various types are positioned opposite one another around the heat shield 119. A TEG 122 is located on the left-hand side of the figure. A filter 124, which transmits long-wave radiation and reflects short-wave radiation, and an element 126 which absorbs radiation are arranged between the heat shield 119 and the TEG 122. A TPV 128 is arranged on the right-hand side opposite the TEG 122, with a mirror 130 for reflecting long-wave radiation arranged on the rear side of the transparent TPV. Fuel is supplied via fuel feed 132, and this fuel is heated in a thermally insulating body 134 by the outlet gases which flow towards the outlet in passages passing through the body 134. The air supplied is guided between the combustion chamber and the housing 110 and is preheated by exchange of heat from the combustion with the aid of cooling fins 138 on the outer side of the combustion chamber 115 and also by the combustion gases in heat exchanger 136.