Abstract:
The present invention relates to forced air cooking stoves, in particular, to efficient cooking stoves for use by the military, those undertaking general camping or other outdoor leisure pursuits, or by larger groups of people for entertaining or humanitarian purposes. The cooking stoves include a fan to aid combustion, and a collar to focus the exhaust gasses onto the cooking vessel. They are designed to make efficient use of any type of fuel, including biomass such as wood, and solid fuel blocks. They are also designed to minimise the soot deposited on the cooking vessel.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to forced air cooking stoves, in particular, to efficient cooking stoves for use by the military, those undertaking general camping or other outdoor leisure pursuits, or by larger groups of people for entertaining or humanitarian purposes. The cooking stoves include a fan to aid combustion, and are designed to make efficient use of any type of fuel, including biomass such as wood, and solid fuel blocks. They are also designed to minimise the soot deposited on the cooking vessel. 
       BACKGROUND TO THE INVENTION 
       [0002]    Cooking food and heating water is a basic human necessity. Where conventional electric or gas devices such as kettles and ovens are not available for this purpose, stoves that burn biomass or other fuels are used. Such stoves are used every day by millions of people in developing countries, as well as by outdoor enthusiasts and military personnel. 
         [0003]    Traditionally biomass, such as wood, charcoal briquettes, peat, and coal etc. has been used as fuel. More recently, solid fuel blocks have been produced, including hexamine blocks, trioxane blocks, solidified methyl decanoate blocks and gelled alcohol packs. In addition to their use by outdoor enthusiasts and military personnel, solid fuel blocks are well suited to humanitarian applications. 
         [0004]    Forcing air through the stove, by means of a fan, can help to burn fuel more quickly, and hence heat food or water in a shorter time. Forced air stoves for use with biomass are known as described in U.S. Pat. No. 3,868,943 and WO 2006/103613. However, the efficiency with which these stoves transfer heat from burning fuel to a cooking vessel is not optimal, and results in a high proportion of lost heat energy. This lost heat energy must be accounted for by burning additional fuel. In the long term this has environmental implications and in the short term means that a greater amount of biomass must be found, which can be very difficult. 
         [0005]    Further, fuel being burnt in prior art forced air stoves often does not completely combust, which leads to dangerous volatile organic compounds (white smoke) and/or soot (black smoke) being produced. The volatile organic compounds and/or soot are deposited on the cooking vessel, which makes it dirty to handle and store, and is unhygienic. 
         [0006]    In addition, prior art forced air stoves have generally not been designed specifically with burning solid fuel blocks in mind and have, instead, been designed to utilise biomass. The efficiency with which these stoves transfer heat from burning solid fuel blocks to a cooking vessel is even less optimal than for biomass. 
         [0007]    There is, therefore, a need to provide a forced air stove that can burn all types of fuel more efficiently, to transfer more heat to the cooking vessel, and reduce the soot deposited on the cooking vessel. The present invention aims to meet this need. 
       SUMMARY OF THE INVENTION 
       [0008]    According to a first aspect, the present invention provides a cooking stove comprising: a combustion chamber which is defined by walls, wherein the walls are closed at one end by a base, and open at the opposing end to define an exhaust outlet, wherein the combustion chamber has one or more air inlets which are defined by apertures in the walls; and a fan configured to force air into the combustion chamber through the one or more air inlets; characterised in that the cooking stove additionally comprises a collar adjacent to the open end of the walls of the combustion chamber to restrict the exhaust outlet, wherein the collar has one or more openings which in total are 20 to 80% smaller in area than the open end of the walls of the combustion chamber. 
         [0009]    The cooking stoves of the present invention advantageously incorporate a collar which significantly restricts the area of the exhaust outlet. The inventors have surprisingly found that this can have the effect of significantly increasing the efficiency of the stove. In tests, incorporating a collar according to the present invention on a forced air stove increased the efficiency of the stove in heating water by around 40%. 
         [0010]    The inventor has also discovered that, by simple positioning of the of the air inlets, the stove can be optimised for burning biomass or for burning solid fuel blocks. This also represents a significant advantage of the present invention over prior art forced air stoves which did not burn solid fuel blocks with an acceptable efficiency. 
         [0011]    The cooking stove may also comprise a fuel delivery chute attached to the stove such that fuel placed on the delivery chute transitions to the combustion chamber. The fuel delivery chute may be reversibly attached. The fuel delivery chute allows fuel to be introduced to the combustion chamber without the user needing to place their hand over the combustion chamber. This is useful when fuel is already being combusted within the stove and the fuel needs to be replenished. There are two main benefits. First, the user is at a reduced risk of a burnt hand. Second, the cooking vessel does not need to be removed in order for the fuel to be added. 
         [0012]    According to a second aspect, the present invention provides a method of heating a cooking vessel using a cooking stove according to the first aspect of the invention, the method comprising the steps of: placing fuel on the base of the combustion chamber and setting the fuel block alight; using the fan to force air through the one or more air inlets in the walls of the combustion chamber; and placing a cooking vessel onto the cooking stove. 
         [0013]    According to a third aspect, the present invention provides a kit comprising a cooking stove according to the first aspect of the invention and one or more solid fuel blocks. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will now be described by way of example only with reference to the accompanying figures, in which: 
           [0015]      FIG. 1  is a schematic perspective view of a prior art forced air cooking stove; 
           [0016]      FIG. 2  is a schematic lower perspective view of an insert that can be placed within a cooking stove to provide a modified combustion chamber; 
           [0017]      FIG. 3  is a schematic vertical cross-section of a forced air cooking stove with an insert; 
           [0018]      FIG. 4  is a schematic top plan view of a collar that can be used in an embodiment of the invention; 
           [0019]      FIG. 5  is a side view of the collar of  FIG. 4 ; 
           [0020]      FIG. 6  is a perspective view of a delivery chute that can be used in an embodiment of the invention; and 
           [0021]      FIG. 7  is a schematic perspective view of a cooking stove that is an embodiment of the invention having a collar and delivery chute; 
           [0022]      FIGS. 8 to 11  are graphs showing temperature in degrees t versus time in minutes for a series of experiments, as described in the Examples section. 
       
    
    
     DESCRIPTION 
       [0023]    According to a first aspect, the present invention provides a combustion chamber. By combustion chamber, we mean the section of the cooking stove where combustion of fuel takes place. The combustion chamber is defined by walls which are closed at one end by a base. The combustion chamber extends in height between the base and the opposing end of the walls which is open to define an exhaust outlet. 
         [0024]    The walls of the combustion chamber may consist of a number of panels that define a prism shape, such as a hexagonal prism, or may consist of a singular continuous panel that defines a cylinder, oval or other smoothly contoured shape, or a combination of these. The walls may be vertical or may be angled so that the cross-sectional area of the combustion chamber varies with height. The base of the combustion chamber is the surface that fuel such as biomass or a solid fuel block is placed on. The base of the combustion chamber is not necessarily the base of the cooking stove. 
         [0025]    The combustion chamber has one or more air inlets which are defined by apertures in the walls. Accordingly, the air inlets are effectively simple holes in the walls of the combustion chamber that air can flow into when the stove is in use. 
         [0026]    The cooking stove of the present invention also comprises a fan configured to force air into the combustion chamber through the one or more air inlets. The air may be delivered from the fan to the one or more air inlets by any suitable means, as known from prior art forced air stoves. By this, we mean that when the fan is operational, i.e. rotating, air is driven from one side of the fan to the other. This generates a positive pressure of air on one side of the fan. The fan is configured so that a pressure differential exists where the air on the outside of the air inlets is a higher pressure than the air inside the combustion chamber, and air is therefore forced into the combustion chamber. Air delivery may be through conduits leading from the fan to each of the air inlets. Alternatively, air delivery may be through the stove having another wall encompassing the combustion chamber which creates an enclosed section where the fan creates a higher air pressure, the air being driven through the one or more air inlets to the combustion chamber. 
         [0027]    As above, an exhaust outlet is provided by the open end of the walls of the combustion chamber. Exhaust outlets allow the exhaust gasses to leave the combustion chamber. The exhaust gasses comprise fully combusted products, typically carbon dioxide and water, oxygen depleted air and potentially fuel that is still combusting. Allowing exhaust gasses out of the combustion chamber allows turnover of air and sustains combustion. 
         [0028]    In contrast to prior art forced air stoves, which typically have parallel or tapered walls, the cooking stove of the present invention comprises a collar adjacent to the open end of the walls of the combustion chamber. By collar, we mean an element that extends inwards from the open end of the walls of the combustion chamber. This has the effect of restricting the exhaust outlet. The collar can be made of any non-flammable material, but is usually metal, such as stainless steel, as is the combustion chamber. It is often substantially disc shaped, and can have a chamfer which can be used to provide a raised inner section. The collar has one or more openings. With one opening, the opening is usually substantially central. The opening is usually round, but could be any convenient shape. With more than one opening, there may be a relatively larger central opening by a plurality of relatively smaller openings. 
         [0029]    By adjacent to the open end of the walls of the combustion chamber, we mean that the collar must be positioned so that there is no gap between the walls and the collar, so that when exhaust gasses exit the combustion chamber, they are forced to travel through the opening or openings in the collar. The collar can be integral, or, preferably, can be attached to the walls of the combustion chamber by any convenient means. It is preferred that the collar is reversibly attachable, so that it can be taken off to load the combustion chamber with fuel, and replaced before the fuel is ignited. 
         [0030]    The inventors have surprisingly found that by significantly restricting the exhaust outlet in a forced air stove, the efficiency of the burn is greatly improved. In particular, the collar of the present invention has one or more openings which in total are 20 to 80% smaller in area than the open end of the walls of the combustion chamber. In other words, the exhaust gasses are focussed through an outlet which is 20 to 80% smaller than it would have been if the collar was not present. 
         [0031]    Restricting the exhaust outlets in such a manner would normally be thought to reduce the efficiency of the burn by choking the flame, and would result in incomplete combustion. However, surprisingly in the present invention, the exhaust outlet can be restricted by up to 80% without compromising complete combustion, i.e. while still burning the fuel cleanly. This has a dramatic effect of the efficiency of the burn and of transfer of heat to the cooking vessel. It is thought that, because the air is under pressure i.e. is being forced into the air inlets, restricting the outlet provides a very concentrated jet of hot exhaust gasses that is unexpectedly more efficient than when the hot exhaust gasses are not focussed in this way. 
         [0032]    Without wishing to be bound by theory, it is thought that, the restriction in exhaust area in stoves of the invention results in faster exhaust gas flow as the fan continues to deliver to the combustion chamber the same volumes of air [mtr 3 /min]. Assuming exhaust gas laminar flow at low velocities then changes to turbulent flow at higher velocities, the oxygen and fuel mixing is therefore further improved. This better mixing results in higher heat energy production, through more complete combustion. Allied to the more complete energy release, in the invention this heat is now ‘concentrated’ by the collar via the now reduced exhaust gas exit area, into a smaller impinged area of the base of the cooking pan. This results in a greater energy concentration per given area in contact with the pan and thus to the contents. Coupled to this, the majority of the hot exhaust gasses being concentrated to a more central area of the cooking pan, now has a longer contact time with the pan due to now having to travel through more of the radial distance to the edge of the pan and so can transfer more of the heat energy into the pan and contents. The overall result is the reduction in time for the contents to reach boil and/or cooking temperatures. 
         [0033]    The size of the opening or openings in the collar, relative to the open ends of the walls of the combustion chamber is important for the effectiveness of the invention. It is thought that where the collar is less than 20% smaller in area than the open end of the walls of the combustion chamber, the improvements in efficiency are not pronounced. Conversely, it is thought that where the openings in the collar are more than 80% smaller in area in total than the open end of the walls of the combustion chamber, exhaust gasses cannot escape quickly enough, and so the fuel starts to undergo incomplete combustion, resulting in lower efficiencies, a yellow flame, and soot on the cooking vessel. 
         [0034]    The cooking stove may also comprise a fuel delivery chute, which can be attached to the stove such that fuel placed on the delivery chute transitions to the combustion chamber. This is particularly important in the present invention because, by restricting the open end of the combustion chamber, the collar makes it more difficult to feed fuel into the combustion chamber, especially once the fuel is alight. The chute may be a simple sheet that the fuel can slide down under gravity. Alternatively, the chute may be horizontal such that fuel is pushed along it into the combustion chamber. The sheet can have side walls to guide the fuel. The fuel delivery chute may also be reversibly attachable, for example, by clipping into the cooking stove such that one end of the delivery chute is held at the top of the combustion chamber. The delivery chute can be attached to the collar. The delivery chute may also have perforations or slots to allow ambient air through. This helps to keep the delivery chute cool to further reduce the risk of burnt hands. 
         [0035]    A cooking vessel support frame is usually provided to support the cooking vessel above the stove while the food or liquid is being heated. The cooking vessel support frame extends above the cooking stove and the cooking vessel is placed on the support. The support may, for example, be a metal framework that holds the cooking vessel at a distance above the combustion chamber. The cooking vessel support frame may comprise a number of tines, such as 3, 4 or 6 tines. These tines may be attached to the cooking stove or cooking stove insert, or may be a separate component that is placed on the cooking stove or cooking stove insert before use. The tines generally extend vertically from the cooking stove to provide separation of the cooking vessel from the cooking stove. The tines may also be curved, so that a section of the tine provides a flat surface to place the cooking vessel on. The cooking vessel support frame may be configured to hold a cooking vessel between 10 mm and 20 mm, preferably 12 mm to 18 mm, more preferably 15 mm, above the top of the combustion chamber. 
         [0036]    The stove of the present invention can be specially adapted for use with biomass, or with solid fuel blocks. Where biomass is used, the air inlets are typically at the level of the fuel, i.e. predominantly in the lower half of the combustion chamber. Hence, a cooking stove adapted to burn biomass will include air inlets in the bottom half of the combustion chamber. In addition, it may also include one or more air inlets in the top half. Biomass may include wood, coal, peat, etc. or may include fuel pellets made from biomass. Such fuel pellets can be made from compressed saw dust or recycled wood clipping. 
         [0037]    However, an alternative approach is beneficial for types of fuel that can vaporise, including solid fuel blocks and liquid fuel. Methyl, ethyl, propyl, and butyl esters of C6-C14 carboxylic acids, which shall be referred to as fatty acid esters (FAEs), are particularly advantageous as the fuel of solid fuel blocks. Of these methyl decanoate is currently the most popular. Hexamine blocks are another type of solid fuel block, but have typically suffered from undergoing incomplete combustion and producing cyanuric combustion products, which have a high odour so can be detected by smell by the user. By using a stove according to the present invention to burn hexamine blocks, complete combustion can be achieved, minimising or even eliminating the presence of cyanuric combustion products deposited on the cooking vessel. Accordingly, the stoves of the present invention are also suitable for use with solid fuel blocks comprising hexamine. 
         [0038]    Compared to embodiments in which the cooking stoves are adapted to burn biomass, with air inlets at fuel height to directly feed the burning fuel with air, a different arrangement of air inlets has been discovered by the inventor, which is adapted to FAE solid fuel blocks. If air is restricted from accessing the burning fuel at fuel height but is supplied to a headspace above the burning fuel, the main combustion occurs in this headspace above fuel height. The inventor has found that this leads to more efficient combustion than burning only the fuel block itself. Accordingly, in an alternative embodiment, the one or more air inlets are positioned at least 30 mm from the base of the combustion chamber, preferably at least 40 mm from the base of the combustion chamber, and more preferably only in the top half of the combustion chamber. In this embodiment, the combustion chamber is substantially free from air inlets below 30 mm from the base of the combustion chamber. This particular arrangement of air inlets has been designed to control the air flow in order to most efficiently combust fatty acid ester (FAE) solid fuel blocks, resulting in more efficient transfer of energy from the fuel to a cooking vessel placed on the stove, and a reduction in the soot and unburned volatile organic compounds that are present in the exhaust fumes that can be deposited on the cooking vessel. As noted above, similar effects are shown with solid fuel blocks comprising hexamine. 
         [0039]    Solid fuel blocks are often 20 mm high. Accordingly, one embodiment of the invention has air inlets at least 30 mm above the base, so that the air inlets would be above the block when placed on the base. This is a key difference from prior art forced air stoves where the air inlets are close to, and at least within 30 mm of, the base, and are designed to be at the level of the fuel. Accordingly, in this embodiment the stove of the present invention operates by forced air being supplied by a fan to the headspace above the burning solid fuel block. It is therefore vaporised fuel from the fuel block that benefits from the supply of air, and the vaporised fuel is therefore fully combusted without aggravating the aggressiveness of the burn of the solid fuel block itself. Surprisingly, even though air is not being supplied directly to the solid fuel block, the ultimate source of the fuel, the flame does not extinguish. The fuel block itself does still undergo combustion, albeit at a rate limited by the amount of oxygen that reaches the fuel block, and vaporises enough FAE or hexamine fuel to sustain the main combustion in the headspace. It is thought that the heat of the flame in the headspace feeds back to the fuel block and drives combustion and vaporisation of the fuel. 
         [0040]    The invention operates by maximising the combustion of vaporised fuel without causing the rate of vaporisation from the solid fuel block to increase to an unmanageable level. Ordinarily, the heat of combustion would vaporise fuel from the fuel source both (a) directly and (b) by driving the convection current that draws fresh air over the burning fuel. In the present invention, this convection current is prevented or severely hindered by not having any air inlets at the level of the fuel in order to limit the rate of vaporisation to a manageable level. Most importantly, full combustion of vaporised fuel occurs. This takes place within the combustion chamber and therefore below any cooking vessel that is placed on the cooking stove. By ensuring that combustion is substantially complete before the combusting fuel exits the combustion chamber, the risk that the flame would cool below the flash point of the fuel is minimised. This would otherwise generate smoke. 
         [0041]    In one particular advantageous embodiment of the invention, an insert is provided that is able to convert a cooking stove designed to burn biomass, i.e. with air inlets close to the bottom, to one that is designed to burn solid fuel blocks, i.e. with air inlets not within 30 mm of the bottom. In this embodiment, the insert can be fitted to provide the base and the walls of the combustion chamber. By insert, we are referring to a device that can be inserted into a cooking stove, for example in to the original combustion chamber of a cooking stove, to form part of, or all of, the combustion chamber in accordance with the present invention. 
         [0042]    Different types of insert are conceivable, but all are capable of modifying a cooking stove for burning biomass so that it becomes a cooking stove in which solid fuel blocks can be burnt. 
         [0043]    Hence, the combustion chamber provided by the insert sits within the original combustion chamber. In this situation, the walls of the insert will have air inlet holes which are at least 30 mm from the base of the insert. The insert should substantially seal the original combustion chamber so that the air delivered to the original combustion chamber is directed through the modified combustion chamber&#39;s air inlets into the modified combustion chamber which is provided by the insert. This type of insert may be configured so that the air inlets sit above the original combustion chamber, within a collar that equalises air pressure around the air inlets. This is particularly useful if the insert is a tight fit within the original combustion chamber and certain regions of the insert and original combustion chamber lie flush against one another. 
         [0044]    Alternatively, the insert may replace the original combustion chamber. In this embodiment, the original combustion chamber is completely removed from the cooking stove, and the insert placed into the cooking stove. 
         [0045]    The insert may be reversibly attachable, which would allow easy inter-conversion between a stove configuration with the insert for burning high energy fuel and a stove configuration without the insert for burning biomass such as wood, coal, peat, etc. 
         [0046]    In all embodiments, (i.e. adapted for biomass or fuel blocks), the one or more air inlets may have a combined surface area of 20 mm 2  to 500 mm 2 , preferably 50 mm 2  to 400 mm 2 , more preferably 80 mm 2  to 350 mm 2 . These dimensions help to ensure an optimal airflow. 
         [0047]    The stove of the present invention can have 4 to 20, preferably 6 to 18, more preferably 8 to 16 air inlets. Where there is more than one air inlet, the air inlets may be spaced evenly around a perimeter of the combustion chamber. Spacing the inlets evenly around such a perimeter helps to ensure that all fuel is combusted and that a steady and even burn is achieved. This also helps to control and limit convection currents within the combustion chamber that could otherwise drive air over the solid fuel block and increase the rate of vaporisation of the fuel. Spacing the air inlets evenly around the perimeter of the combustion chamber helps to ensure that air can be delivered evenly to all sides of the combusting vaporised fuel while also maintaining structural integrity of the cooking stove. 
         [0048]    The cooking stove may further comprise a heat shield configured to protect the fan. A significant amount of heat is generated in the combustion chamber, which will be transmitted to the surroundings. Such heat could cause a fan to malfunction by, for example, melting or charring components of the fan. A heat shield can be provided to protect the fan from the heat of the combustion chamber, particularly infra-red radiative heat. The heat shield should therefore block the line of sight between any part of the combustion chamber and any part of the fan. The heat shield may be made of any non-flammable material that can absorb radiative heat and dissipate that heat to the surroundings, particularly by conduction, such as stainless steel, mild steel, titanium, copper or heat-stable polymeric compounds such as carbon fibre matrix. A particularly preferred material is aluminium, due to cost, resistance to oxidation and excellent heat-sink properties. 
         [0049]    The fan, heat shield and combustion chamber are preferably in a vertical assembly, with the fan at the bottom, combustion chamber at the top, and the heat shield between the fan and combustion chamber. This specific arrangement of fan and heat shield in relation to the combustion chamber is particularly advantageous. Hot air rises, so it is beneficial to have the fan below the combustion chamber to protect it from this convective heat. Heat is also radiated from the combustion chamber, which is why it is preferred to position a heat shield between the combustion chamber and the fan. It is also beneficial to have the fan blowing on the heat shield as this recycles heat that is conducted and radiated from the heat shield to the air being blown through the cooking stove. 
         [0050]    The fan may be electrically driven, such as by a battery, rechargeable battery, capacitor storage device, socket adapted to receive AC or DC electricity, thermoelectric generator, wind-up generator, solar power generator or combination thereof. By this, we mean that the fan comprises an electric motor which transduces electrical energy into rotation of the fan. The electrical energy may be provided by any of the means listed. Furthermore, the rechargeable battery may be recharged by any other source of electricity, including being connected to external AC or DC electricity, a thermoelectric generator, a wind-up generator, a solar power generator, or a combination of these. By having the fan powered by a renewable source such as a thermoelectric generator, wind-up generator or solar panel, which can also store electricity in a rechargeable battery, the stove becomes self-contained. That is, such a cooking stove is not dependent on consumable components such as non-rechargeable batteries and does not need to be close to a mains power outlet. 
         [0051]    Alternatively, the fan may be mechanically driven and this may be by a wind-up spring, kinetic storage device such as a rotating fly-wheel or a means for attaching an external drive shaft or belt drive. By this, we mean that the fan may be driven by mechanically transducing the potential energy of a wound up spring or suspended weight into rotation of the fan. The transduction of energy to the fan may be controlled by means well known in clock making. The energy may be transferred from a source external to the cooking stove, such as a water wheel or suspended weight. Means for powering the fan that do not require electricity are particularly useful in regions where electricity is not readily available, such as in remote regions or disaster relief areas. Equally, mechanical energy could be converted to electrical energy to drive an electric fan or recharge a rechargeable battery. 
         [0052]    The cooking stove may further comprise means for manipulating the speed of the fan. By this, we mean that the cooking stove will have a dial or other control that the user can use to modify the speed of the fan. For an electric fan, this may be through a variable resistor. For a mechanical fan, this may be a means of applying mechanical braking or a constantly variable drive such as a variomatic drive. With such means, the user can adjust the speed of the fan to suit cooking needs conveniently. Surprisingly, full combustion occurs at different speed settings of the fan, and different beneficial effects are seen at these different speeds. For example, a typical FAE solid fuel block comprising methyl decanoate burns freely with a flame temperature of about 600° C. Using a cooking stove according to the invention that has a fast fan configuration, combustion with a flame temperature approaching 1000° C. can be achieved, however, the fuel block is used up more rapidly. By using a slower fan configuration, the flame temperature can be dropped to 800° C., but, surprisingly, the burn duration can be doubled over that of the free burn. This provides an indication of just how much extra energy can be captured from the FAE fuel block by the cooking stove of the invention. Surprisingly, even at very slow fan speeds, the cooking stove of the present invention still operates efficiently. Even though the overall rate of combustion of is slowed, full combustion of the fuel can still occur with the result that no smoke is generated and the flame is not extinguished. It is thought that the present invention allows this effect to occur due to the decreased vaporisation of FAE fuel from the fuel block. This decreased vaporisation is likely to be occurring due to a decreased feedback of heat from the headspace combustion and/or from an increase in the vapour pressure of the vaporised FAE fuel around the fuel block. 
         [0053]    The fan may be driven directly or indirectly by a thermoelectric generator, wherein the thermoelectric generator has a hot side and a cold side, and wherein the hot side is directly or indirectly in thermal contact with the combustion chamber. This exploits the temperature differentials within the stove to generate electricity, making this embodiment of the cooking stove completely self-sustaining when in use. The hot side may be in direct or indirect thermal contact with the heat shield. Furthermore, the cold side of the thermoelectric generator may be cooled by the fan. This further increases the temperature differential across the thermoelectric generator, allowing for an increased generation of electricity. 
         [0054]    The thermoelectric generator may have an additional power outlet suitable for powering a device external to the cooking stove. The external device powered by the stove could, for example, be a light or a charging station for a mobile phone, or computer, or camera etc. This allows a self-sustaining cycle of using the generated heat to power the fan, which in turn increases combustion efficiency. 
         [0055]    The combustion chamber may be reversibly detachable from the fan. By this, we mean that the portion of the cooking stove comprising the surfaces exposed to the burning fuel (i.e. the base and walls of the combustion chamber) can be detached from the remainder of the cooking stove. This is so that these surfaces can be washed without admitting water or cleaning agents to other parts of the cooking stove, in particular, the fan or any components for driving the fan. This also allows the combustion chamber to be replaced with different combustion chambers, for example, combustion chambers tuned to efficiently burn different fuel types. There are a number of features that can be optimised in different combustion chambers, such as air inlet number, size, arrangement and height from the base. 
         [0056]    Any power source of the cooking stove may also be reversibly attachable. This allows consumable components, such as batteries, to be replaced. It also allows the power source to be modular and, for example, a thermoelectric generator may be replaced by a solar power generator. 
         [0057]    The cooking stove may further comprise one or two handles, which allows the stove to be easily moved, even when it is hot from use. The cooking stove may also comprise a section of the perimeter that is free from exhaust outlets such that the stove handle or cooking vessel handle is protected from the heat of combustion. 
         [0058]    The cooking stove may also be substantially as described herein with reference to the accompanying  FIG. 7 . 
         [0059]    According to a second aspect, the present invention provides a method of heating a cooking vessel using a cooking stove as described above. The method comprising the steps of: placing fuel on the base of the combustion chamber and setting the fuel alight; using the fan to force air through the one or more air inlets in the walls of the combustion chamber; and placing a cooking vessel onto the cooking stove. 
         [0060]    As explained above, where biomass is being burnt, the air inlets are included in the bottom half of the stove, whereas when solid fuel blocks are being burnt, the air holes are above the solid fuel block. Normally this means they are at least 30 mm from the base, preferably at least 40 mm from the base, or only in the top half. 
         [0061]    The method may further comprise adjusting the speed of the fan. 
         [0062]    According to a third aspect, the present invention provides a kit comprising a cooking stove as described above, and one or more solid fuel. It is preferred that the one or more solid fuel blocks comprise methyl, ethyl, propyl or butyl esters of a C6 to C14 carboxylic acid or combinations thereof, or hexamine. 
         [0063]    FAE solid fuel blocks typically comprise a methyl, ethyl, propyl or butyl ester of a C6 to C14 carboxylic acid or combination thereof encapsulated in a solid emulsion. Solid fuel blocks comprising methyl decanoate are preferred in the present invention. A solid fuel block comprising an emulsion of methyl decanoate encapsulated in a resin is available on the market under the name “Zip Military Cooking Fuel”®. These fuel blocks comprise about 20% by weight resin/water/emulsifier matrix and about 80% by weight methyl decanoate. A number of sizes of fuel block are available, including a 26 g block that is 42 mm long, 32 mm wide and 20 mm tall, and a 100 g block that is 60 mm long, 60 mm wide and 40 mm tall. 
         [0064]    As noted above, hexamine solid fuel blocks are on the market and are well known to a person working in this field. They are often used by the military. Hexamine is the common name for hexamethylenetetramine or methenamine, which is a heterocyclic organic compound with the formula (CH 2 ) 6 N 4 . This is the main component of hexamine fuel tablets. Tablets currently on the market have block size 45×45×12 mm. Hence, two or more blocks can be stacked on top of one another if required. Traditionally hexamine blocks are not suitable for indoor use, due to the fact they undergo incomplete combustion in prior art stoves. A well known disadvantage is also the unpleasant smell of the partial combustion products. As noted above, by using the stove according to the present invention, these problems can be mitigated. 
         [0065]    Alternatively, other fuels that are similar to FAE fuel blocks may be used. As already mentioned, a FAE fuel block is a high-energy liquid fuel encapsulated in a solid emulsion. Other high-energy hydrocarbons, such as kerosene, gasoline, diesel, and alcohols, may be used. 
       DETAILED DESCRIPTION OF DRAWINGS 
       [0066]      FIG. 1  shows a perspective schematic view of the portable cooking stove according to the prior art. The cooking stove  2  comprises an upper section  4  for mounted on a supportive base  6 . The upper section  4  has a combustion chamber  8  that is made up of six walls  10 . The six walls  10  are arranged such that they define a hexagonal combustion chamber  8 . The exhaust outlet  12  at the top of the combustion chamber  8  can be seen in this drawing. The walls  10  of the combustion chamber  8  have a number of air inlets  14 . The air inlets  14  are apertures in the walls  10  of the combustion chamber  8 . 
         [0067]    Also showing on the upper section are a number of tines  18  which are intended to support cooking vessel. The top portion of the stove is held in position by rivets  16 . 
         [0068]    The upper section  4  also has two handles  20 . The handles  20  are positioned so that they do not heat up when the cooking stove  2  is in use and the cooking stove  2  can therefore be moved without waiting for it to cool. 
         [0069]    The supportive base  6  has a plurality of air inlet holes  22  to admit air into the cooking stove  2 . This air is drawn in by a fan (not shown) and subsequently forced into the combustion chamber  8  through the air inlets  14  in the walls  10  of the combustion chamber  8 . The speed of the fan may be controlled by a control dial  24 . The base  6  is also shown with a USB port  26  and a socket for a power plug  28 . An indicator light  30  allows the use of to easily see when the fan is switched on. 
         [0070]      FIG. 2  shows an insert  40  that can be used, for example, with the cooking stove  2  of  FIG. 1 . The insert  40  comprises a combustion chamber defined by a cylindrical wall  44  having a solid base  42 . The wall  44  of the combustion chamber has a number of air inlet holes  46 . The insert also has a support disk  48  that extends from the cylindrical wall  44 . It can be seen that the support disk has two sections, a relatively higher inner section and a relatively lower outer section. The air inlet holes  46  are above the relatively lower section but below the relatively higher section. The insert can also have tines  50  intended to support a cooking vessel. To use the insert  40 , it would be lowered into a cooking stove such as that in  FIG. 1  until the support disk  48  rested on the top surface of the cooking stove. Such a configuration is shown more clearly in  FIG. 3 . The insert of  FIG. 2  can be used with a collar in accordance with the present invention. 
         [0071]      FIG. 3  shows a schematic cross-sectional view of a cooking stove  60  with the insert  40  of  FIG. 2 . The prior art cooking stove  60  has air inlets  62  to admit air to the cooking stove, the passage of air through the cooking stove  60  to the insert  40  are shown by dashed arrows. The air is drawn in to the cooking stove  60  by the fan  64 . The air is then diverted around a heat shield  66 . The heat shield  66  protects the fan  64  from the heat generated by combustion of fuel. The air then enters the original combustion chamber  68 . The original combustion chamber  68  is defined by walls  70  and a base  72 . The air enters into the original combustion chamber  68  through a number of air inlet holes  74 . The insert  40  has been inserted in to the original combustion chamber  68  to define a new combustion chamber  52  within the cooking stove  60 . The fuel  54  is placed within the combustion chamber  52  of the insert. The insert  40  has a support disk  48  that rests on top of the cooking stove  60 . The support disk  48  creates a substantially air tight seal with the cooking stove  60  so that the forced air is directed into the insert  40  through the air inlet holes  46  of the insert. Furthermore, the support disk  48  has a lower outer section and a raised inner section. The air inlet holes  46  are above the lower outer section of the support disk  48 . 
         [0072]    It can be seen that the insert  40  allows a fuel block to be burnt under conditions where forced air is delivered above the fuel block. Without the insert  40 , it can be seen that burning the fuel block in the original combustion chamber  68  would occur with air being delivered directly to the fuel block  54  through the original air inlet holes  74 . 
         [0073]    The insert  40  also has tines  50  and the cooking stove  60  is shown with a cooking vessel  76  supported on the tines  50 . In this cooking stove, the exhaust gases would exist unrestricted from the walls of the combustion chamber  40 . This is not in accordance with the present invention. 
         [0074]      FIGS. 4 and 5  show a collar  80  for use with a cooking stove. The collar has an opening  82 . The collar  80  and opening  82  are circular in shape. The collar  80  comprises a relatively lower outer section  84  and a relatively higher inner section  86 . This is shown more clearly in  FIG. 5 . The relatively lower section  84  should engage with a cooking stove in a manner that is substantially air tight. 
         [0075]      FIG. 6  shows a fuel delivery chute  90 , which comprises a base  92  with raised outer sections  94 . The raised outer sections  94  are angled inward to guide fuel that is transitioned along the delivery chute  90 . The fuel delivery chute  90  also has a cut out section  96  which allows air to pass through and keeps the outer section of the fuel delivery chute cool. The fuel delivery chute  90  also has a cut out section  98 . This cut out section  98  is generally designed to match the contours of the opening of a combustion chamber. 
         [0076]      FIG. 7  shows a schematic view of the cooking stove to  FIG. 1  in combination with the collar  80  of  FIG. 4  and the fuel delivery chute  90  at  FIG. 6 . The collar  80  has been placed over the exhaust outlet  12  of the cooking stove  2  the delivery chute  90  has been wedged in between the collar  80  and two of the tines  18  of the cooking stove  2 . By wedging the delivery chute  90  in place, the assembly of delivery chute  90 , collar  80  and cooking stove  2  is held tightly together. It can be seen that to add more fuel to the combustion chamber, the fuel can be placed on the fuel delivery chute  90  and guided inwards towards the opening  82  of the collar  80  where it will drop into the combustion chamber  8 . By using the collar  80  the exhaust gas outlet is significantly restricted compared to when the collar is not present. 
       EXAMPLES 
       [0077]    A series of experiments were undertaken to investigate the effectiveness of a collar according to the present invention. 
       Example 1 
       [0078]    In this example a forced air stove was used as shown in  FIG. 1 . Five litres of water was heated from 20° C. to boiling under identical conditions apart from with and without a collar. In particular, in the test 1 the stove was as shown in  FIG. 1 , with no collar. Wood was used as fuel and added to the combustion chamber and set alight. The fan was activated to force air into the stove through the air inlets. The exhaust gases were allowed to exit freely from the walls of the combustion chamber. The results of the experiment are shown in  FIG. 8 . It can be seen that when using the stove of  FIG. 1  with burning wood the stove took about 19 minutes to bring the five litres of water up to boiling. 
         [0079]    As a contrast, in test 2 a collar as shown in  FIGS. 4 and 5  was placed on top of the stove to restrict the exhaust outlet. The collar had a single opening as shown in  FIGS. 4 and 5  which was 55 mm in diameter. The hexagonal walls of the combustion chamber are 100 mm in diameter. Accordingly, the collar restricted the exhaust outlet by about 75% by area. As can be seen in  FIG. 8 , simply adding the collar and keeping all of the other conditions the same reduced the time to bring five litres of water from 20° C. to boiling to around 14 minutes. This represents a very significant improvement. 
       Example 2 
       [0080]    Similar to Example 1, a forced air stove according to  FIG. 1  was used, but in this example with an insert according to  FIG. 2 , and as shown in  FIG. 3 , in order to efficiently burn a methyl ester fuel block. Again, the experiment compared bringing five litres of water from 20° C. to boiling under identical conditions apart from in test 1 there is no collar so the exhaust gases exit freely, compared to the stove according to the invention in test 2 including a collar which restricts the exhaust gases. As in example 1, the collar restricted the exhaust gas outlet area by 75%. As can be seen in  FIG. 9 , using a collar reduced the time to boil to around 17 minutes compared to 21 minutes for the control. 
       Example 3 
       [0081]    Example 3 tests whether or not the collar works in the absence of forced air. In particular, rather than using a forced air stove as in  FIG. 1 , a naturally aspirated stove is used comprising four vertical sides. The stove used is shown in Community design application number 001419428-0001. A methyl ester fuel block is placed in the stove and set alight. The time for five litres of water to reach boiling from 20° C. is measured using the stove alone in test 1, and using the stove plus a collar in test 2. The collar was again, circular in shape and restricted the exhaust outlets by around 75% to just 25% of the original cross-sectional area. As can be seen in  FIG. 10 , the collar has no effect on the boiling time. 
       Example 4 
       [0082]    Similar to Example 3, a methyl ester fuel block is burnt in an alternative naturally aspirated stove, comprising four sides in the shape of a pyramid with the top cut off. The stove used is shown in Community design application number 001419428-0002. Again, the methyl ester fuel block is lit and the time taken for five litres of water to reach boiling measured using the stove alone in test 1, and using the stove with a collar in test 2. The collar restricted the exhaust outlet by 75% to around 25% of the original cross-sectional area. 
         [0083]    As in Example 3, the collar does not make any difference. This shows that the effect of using the collar is specific to forced air stoves and does not apply to naturally aspirated stoves.