Heat exchange assembly for stove

Abstract

A heat exchange assembly 118 for a solid fuel burning stove 100 comprises an inlet 20 receiving combustion exhaust gas, and a heat exchange element 102 which extracts heat from the exhaust gas to form cooled exhaust gas. A ventilation device 104 propels the cooled exhaust gas towards an outlet 22. The heat exchange element may comprise a flue pipe 18. The heat exchange assembly may comprise a plurality of heat exchange elements. The heat exchange assembly may be coupled in-line between a stove and a standard flue liner with the outlet coupled to a chimney or hole in a wall via a tube. Water may be used to cool the exhaust gas to produce heated water which is circulated through a radiator system. A pre-heat exchange section at the inlet may comprise a metal, ceramic, or catalysing scrim. A particulate filter (208, Fig. 3b) may be located at the outlet and the assembly may have further particulate removal, gas cleaning, or carbon dioxide removal means. The ventilation means may be controlled manually, by timers, or remotely, to control or extinguish a fire in the stove. A stove and a method of installation are also claimed.

Description

Field of the Disclosure The present disclosure relates to solid fuel-burning stoves for domestic use. In particular, although not exclusively, the present disclosure relates to heat exchange assemblies for solid fuel-burning stoves and associated installation method. Background Solid fuel-burning stoves such as coal or wood-burning stoves have been a staple for heating and cooking in households for many years. Stoves used for domestic settings typically have a heat power output of up to 5kW for medium sized living areas. Larger domestic units might have a heat power output of up to lOkW. However, traditional solid fuel-burning stoves face significant challenges related to both efficiency and pollution. These issues impact not only the environment but also the overall user experience. The design of conventional wood stoves results in difficulty in efficiently convert fuel into useful heat. This inefficiency results in wasted energy and suboptimal performance. Emissions from wood-burning stoves contribute to air pollution, affecting both indoor and outdoor air quality. Harmful pollutants, such as particulate matter (PM), carbon monoxide (CO), and volatile organic compounds (VOCs), may be released during combustion. The on-going toughening of regulatory standards in some countries, and a growing environmental awareness concerning the use of stoves among the public, underscore the need for cleaner-burning stove technologies. Some aspects of the present invention aim to address these efficiency and pollution issues associated with conventional wood-burning stoves. Further, according to some aspects of the disclosure, it is desirable to: 1) achieve as much as possible of the heat wasted in the flue but using limited resources. 2) retain the features people enjoy by seeing a natural fire. Summary According to a first aspect of the invention, there is provided a heat exchange assembly for a solid fuel-burning stove, the heat exchange assembly comprising: an inlet for receiving exhaust gas from combustion within the solid fuel burning stove; one or more heat exchange elements configured to extract heat for external use from the exhaust gas and form cooled exhaust gas; an outlet for expelling the cooled exhaust gas from the heat exchange assembly; and a ventilation device coupled to the heat exchange assembly, wherein the ventilation device is configured to propel the exhaust gas towards the outlet. The ventilation device may be configured to generate negative pressure to propel the exhaust gas towards the outlet. The solid fuel-burning stove may be a domestic solid fuel-burning stove. The ventilation device may be configured to propel the cooled exhaust gas towards the outlet. The one or more heat exchange elements may be configured to extract most of the heat for external use from the exhaust gases and form cooled exhaust gas. The at least one heat exchanging elements may comprise one or more metal or ceramic flue pipes, wherein the one or more heat exchange elements are configured to recover or extract at least 30%, 40%, 50%, 60%, 70%, 80% or 90% or up to 100% of the heat from the exhaust gas. The exhaust gas may also be referred to as flue gas. The ventilation device may be arranged to cause negative pressure in the at least one flue pipe. The negative pressure resulting from the ventilation device ensures the stove is inherently safe and automatically extinguishes itself or shuts itself down in case of a failure of the ventilation means. The one or more flue pipes may be in any orientation. The heat exchanger and ventilation device may be configured to enable the exhaust the cooled exhaust gas to have a temperature less than 60 degrees centigrade when the inlet is fed with exhaust gas at a temperature of between 250 and 600 degrees centigrade. The one or more metal or ceramic tubes / pipes may be situated in the room; heat being recovered by natural convection and radiation or assisted by ambient air being blown over them by a fan. The one or more metal tubes / pipes are readily available low cost, 1 inch diameter stainless steel exhaust tubes, wound in a loose spiral and fitted underneath the combustion chamber, being replaced and recycled after a long period of use. The heat exchange assembly may comprise a plurality of heat exchange elements placed in succession to cool the exhaust gas. The heat exchanger may comprise a heat pump. The plurality of heat exchange elements may extend within the at least one flue pipe. The plurality of heat exchange elements may extend external to the at least one flue pipe. The first of the plurality of heat exchange elements may be comprised to recover most of the initial, very hot, heat, and another heat exchange element may be configured to recover any residual heat not recovered by the first heat exchange element. The ventilation device is a mechanical or electrical device. The ventilation device comprises a fan powered by one or more of mains electricity, battery, solar cells, or wind power or, once the stove is burning hot, by a Sterling Engine or Peltier Effect device. The assembly may be configured to be coupled in-line between a standard solid fuel burning stove and a standard flue liner. The outlet of the heat exchange assembly may be configured to be coupled to an existing chimney or a hole in the wall via a tube. The flue may have a diameter of between 2 inches and 6 inches or more. The heat exchange mechanism may comprise means to circulate water to cool the exhaust gas. The heated water may be circulated through a radiator system to heat a room. The heat exchange mechanism may further comprise means for replenishing the circulating water and a water outlet for tapping the heated water for use elsewhere, such as in a domestic appliance. The heat exchange assembly may further comprise, at the inlet, a pre heatexchange section comprising any one of a hot metal, ceramic, or catalysing scrim, thereby creating turbulent gas flow conditions that enable seconda ry / tertiary combustion, burning many of the exhaust gas particulates. The heat exchange assembly may further comprise, at the outlet, a particulate filter for filtering out any remaining particles, including micro particles. The cool exhaust gas may be sucked through this filter by the ventilation means. The heat exchange assembly may further comprise further particulate removing, gas cleaning or carbon dioxide removing means. The ventilation means may be configured to be controlled manually, by timers, by feedback mechanisms, or by mobile phone / computer, enabling the fire to be controlled and extinguished directly by the user or by remote control. In another aspect according to the present invention, there is provided a stove for burning solid fuel, comprising: a combustion chamber; and the heat exchange assembly of any preceding claim; wherein the flue inlet is in fluid communication with the combustion chamber to receive the exhaust gas from the combustion chamber. The stove may have the visual, functional and psychological advantages of a standard domestic solid fuel burning stove. The stove may comprise an air inlet. In particular, the combustion chamber may comprise an air inlet. The air inlet may draw air into the combustion chamber from an external source separate from the room in which the stove is situated, thus keeping the stove a closed system. The air inlet may comprise an air inlet tube which may be accommodated by a hole in the wall of the building in which the stove is installed. The stove may be readily movable and may be fitted anywhere in or outside the house; possibly using a secondary paper, plastic, or aluminium foil tube to vent the cool exhaust gas out of the house. The air inlet tube I pipe section and the outlet of the heat exchange assembly may be combined to form a "balanced flue" system with the outgoing exhaust gas preheating the incoming cold air and the incoming cold air cooling the exhaust gas. The outlet may comprise a flue outlet pipe section, and the air inlet pipe section and flue outlet pipe section may be concentric, thus forming a coaxial pipe so as to form a "balanced" flue system with the outgoing exhaust gas pre-heating the incoming cold air and the incoming cold air cooling the exhaust gas. In yet another aspect according to the present invention, there is provided a method of retrofitting a heat exchanger to a stove installation, comprising: removing a section of flue pipe from a stove installation; and coupling the inlet and outlet of the heat exchange assembly to respective openings in the stove installation created by the removal of the section of the flue pipe. The following examples are also disclosed. 1) A heat exchanger is used to extract nearly 100% of the heat from the exhaust gas in a domestic wood burning stove. 2) As example 1 where hot exhaust gas is sucked though long metal pipes. 3) As examples 1 and 2 where the pipes are cooled by an external fan. 4) As example 1 where air and exhaust gas is mechanically sucked through the stove with a small fan or bellows. 5) As the previous examples where the hot exhaust gas is sucked through filter material, such as Stainless Steel scourers and Wire wool. 6) As example 1 where a very fine micro-filter is used to filter out microscopic particles at the cold end of the heat exchanger. 7) As Example 1 where the cooled exhaust gas is sucked through wet scrubbers to remove fine particulates and carbon dioxide. The following clauses are also disclosed. Clause 1. A heat exchange assembly for a solid fuel burning stove, the heat exchange assembly comprising: a flue pipe section having at least one inlet for receiving exhaust gas from combustion within the solid fuel burning stove; a middle section consisting of one or more heat exchanging elements for recovering between 60% and 100% of heat in the exhaust gas; the flue pipe section further having an outlet for expelling the cooled exhaust gas from the flue pipe section by means of a ventilation device coupled to the flue pipe section and configured to generate a negative pressure to propel the exhaust gas towards the outlet of the flue pipe section. 2. As Clause 1 where the ventilation means is fitted in the cooled exhaust gas zone, at the outlet of, or after the flue pipe section, and generates a negative pressure to suck the exhaust gas through the flue pipe and the heat exchanger. 3. As Clauses 1 where the flue can be in any orientation. 4. As any of the previous clauses where the ventilation means is a mechanical or electrical device. 5. As clause 4 where the ventilation means is a fan, powered by one or more of the following - mains electricity, by battery, by solar or wind power or - once the stove is burning hot, by a Sterling Engine or Peltier Effect. 6. As clause 1 where the heat exchange mechanism is an in-line retro-fit between a standard stove and a standard flue liner. 7. As any of the previous clauses where the cooled exhaust gas exits the building through the existing chimney or a hole in the wall via a tube. 8. As clause 1 where the system is "closed"; the air comes through a hole in the wall and enters the combustion chamber via a tube. 9. As clauses 7 and 8 where the air inlet and exhaust gas outlet are combined to form a "balanced" flue, with the outgoing exhaust gas pre-heating the incoming cold air and the incoming cold air cooling the exhaust gas. 10) As any of the previous clauses where the stove has the visual, functional and psychological advantages of a standard domestic solid fuel burning stove. 11) A heat exchanger of clause 1 where the exhaust gas is cooled by water, and the heated water is used for domestic purposes or circulated through a radiator to heat the room. 12) A combustion chamber of clause 1 wherein the fuel burning surface is relatively horizontal and a separate relatively vertical "blind" chimney superstructure that sits on the fuel burning surface, with a relatively air-tight seal between the two; the "blind" chimney being hinged, along one edge of the fuel burning surface, or able to be raised from, and lowered onto the fuel burning surface, using a raising mechanism with a heat resistant handle; the fire being fed air from the room, in a manner similar to a standard solid fuel burning stove, when the "blind" chimney is raised, the system temporarily being "open"; one or more flue pipes, opening relatively high up inside the "blind" chimney (even if the "blind" chimney is raised) sucking exhaust gas upwards from the fire into the chimney cavity, possibly through a secondary burning zone, then downwards through the pipes, either round or through the surface that supports the fire and entering the heat exchanger means outside the combustion chamber. 12A) A combustion chamber of clause 1 wherein the fuel burning surface is flat or curved and relatively horizontal, and a separate relatively vertical, detachable "blind" chimney superstructure that sits on the fuel burning surface, with a relatively air-tight seal between the two. 12B) A combustion chamber of clause 1 wherein the fuel burning surface is relatively horizontal and a separate relatively vertical "blind" chimney superstructure that sits above the fuel burning surface; the "blind" chimney being able to be raised from, and lowered onto the fuel burning surface, using a raising mechanism with a heat resistant handle; the fire being fed air from the room, in a manner similar to a standard solid fuel burning stove, when the "blind" chimney is raised, the system temporarily being "open"; one or more flue pipes, opening relatively high up inside the "blind" chimney (even if the "blind" chimney is raised) sucking exhaust gas upwards from the fire into the chimney cavity, possibly through a secondary burning zone, then downwards through the pipes, either round or through the surface that supports the fire and entering the heat exchanger means outside the combustion chamber. 13) As Clause 1 where more than one heat exchange method is used, the first to recover most of the hot heat, another to recover any residual heat not recovered by the first method. 14) As Clause 1 where there is a pre heat-exchanger section at the inlet of the flue section containing a hot metal, ceramic or catalysing scrim, creating turbulent gas flow conditions that enable secondary / tertiary combustion, burning many of the exhaust gas particulates. 15) As Clause 1 where there is a post heat-exchanger section at the outlet of the exhaust gas section containing a micro-filter for filtering out any remaining particles, including micro particles; cool exhaust gas being sucked through this filter by the ventilation means. 16) As Clause 15 where there are further particulate removing, gas cleaning and carbon dioxide removing means. 17) As all the previous clauses, except clause 6 where the stove is readily movable and can be fitted anywhere in or outside the house; possibly using a secondary paper, plastic or aluminium foil tube to vent the cool exhaust gas out of the house. 18) As all the previous clauses where negative pressure venting ensures the stove is inherently safe and automatically extinguishes itself in case of a failure of the ventilation means. 19) As all the previous clauses wherein the ventilation means is controlled manually, by timers, by feedback mechanisms, or by mobile phone / computer, enabling the fire to be controlled and extinguished directly by the user or by remote control. 20) As Clause 1 where the flue is the heat exchanger, being composed of one or more long metal tubes situated in the room; heat being recovered by natural convection and radiation or assisted by ambient air being blown over them by a fan. 21) As clause 20 where the metal tubes are readily available low cost, 1 inch diameter stainless steel exhaust tubes, wound in a loose spiral and fitted underneath the combustion chamber; being replaced and recycled after a long period of use. 22) As in all the other clauses where the ventilation means is a Cyclonic suction device, with or without a HEPA filter. According to a further aspect of the disclosure, there is provided a heat exchange assembly for a solid fuel burning stove, the heat exchange assembly comprising: a flue pipe section having an inlet for receiving exhaust gas from combustion within the solid fuel burning stove, the flue pipe section further having an outlet for expelling the exhaust gas from the flue pipe section; a ventilation device coupled to the flue pipe section and configured to generate a negative pressure to propel the exhaust gas towards the outlet of the flue pipe section. The flue pipe section may comprise an elongated or coiled section providing a means for exchanging heat between the exhaust gas within the flue pipe section and the surroundings of the flue pipe section. The heat exchange assembly may comprise one or more heat exchange elements in thermal contact with the flue pipe section. The one or more heat exchange elements may comprise radiator plates or cooling fins extending from the flue pipe section to dissipate heat from the heat exchanger. The solid fuel burning stove may further comprise a second ventilation device configured to circulate air over the flue pipe section and / or one or more heat exchange elements. The heat exchange assembly may comprise a plurality of flue pipe sections, each flue pipe section having a respective inlet for receiving exhaust gas from the combustion within the solid fuel burning stove and each flue pipe section further having an outlet for expelling the exhaust gas from the respective flue pipe section. The outlets of the plurality of flue pipe sections may converge to expel exhaust gas from the flue pipe sections via a single ventilation device. The ventilation device may be coupled to the outlet of the flue pipe section. The ventilation device may be arranged to suck the exhaust gas through the flue pipe section. The ventilation device or second ventilation device may comprise a fan, such as an electrical fan, or a bellows. The heat exchange assembly may further comprise or may be configured to receive one or more particulate filters positioned within the flue pipe section. The one or more particulate filters may be positioned between the inlet of the flue pipe section and the one or more heat exchange elements. The one or more particulate filters may be positioned between the one or more heat exchange elements and the ventilation device. The one or more particulate filters may be positioned between the ventilation device and the outlet of the flue pipe section. The one or more particulate filters may comprise a micro-particulate filter or a highly compressed filter. The one or more particulate filters comprises metallic / metal alloy scourers or wire wool or wet scrubbers. According to a further aspect of the disclosure there is provided a stove for burning solid fuel, comprising: a combustion chamber; and the heat exchange assembly, wherein the flue pipe section is in fluid communication with the chamber to receive the exhaust gas from the combustion chamber. The inlet of the flue pipe section may be positioned within the combustion chamber. The flue pipe section may extend downwardly within the chamber and the outlet of the flue pipe section may be situated outside of the combustion chamber. A feature described in relation to a particular example, embodiment, aspect, claim or figure may be provided in combination with any other example, embodiment, aspect, claim or figure. Brief Description of Drawings The disclosure will now be discussed with reference to the drawings, in which - Figure 1 illustrates a schematic diagram of a conventional stove for burning solid fuel; Figure 2 illustrates a schematic diagram of an improved stove for burning solid fuel; Figures 3A to 3D illustrates schematic diagrams of arrangements for improved stoves for burning solid fuel; Figure 4 illustrates a schematic diagram of a further arrangement for a heat exchanger in use on an improved stove; Figure 5 illustrates a schematic diagram of a further arrangement for a heat exchanger in use on an improved stove; Figures 6 and 7 illustrate views of an example of an improved stove; Figure 8 illustrates another example of an improved stove; and Figures 9A and 9B illustrate schematic diagrams of a further arrangement for a heat exchanger in use on an improved stove; Figure 10A illustrates another example of an improved stove without a chimney fitted; and Figure 10B shows the same stove as Figure 10A with the chimney fitted over the fire. Description Figure 1 illustrates a schematic diagram of a stove 10 for burning solid fuel such as wood or coal in a domestic setting. The stove 10 is illustrates in the orientation of its intended use and the use of terms such as "top" and "bottom" shall be construed accordingly. The stove 10 has a combustion chamber 12 for receiving the solid fuel. The combustion chamber 12 has a base plate 14 with an air inlet 16. The stove 10 further comprises a flue pipe 18. The flue pipe 18 has a flue inlet 20 and a flue outlet 22. The flue inlet 20 of the flue pipe 18 is coupled to the top of the combustion chamber 12. The solid fuel may be received through a door (not shown) of the stove 10. In use, the user places combustible solid fuel within the combustion chamber 12 and sets it alight. Air 24 is drawn by the fire from an exterior of the combustion chamber, through the air vent 16 and into the combustion chamber. Within the combustion chamber 12, the air is heated by the combustion process and rises such that it leaves the combustion chamber 12 and enters the flue pipe 18 via the flue inlet 20. Within the flue pipe 18, the exhaust gas from the combustion chamber remain hotter than the ambient air outside of the flue pipe 18. As such, the exhaust gas within the flue travel up the flue pipe 18 from the flue inlet 20 and leave the flue into the external environment via the flue outlet 22. The efficiency of conventional wood burning stoves is typically about 70% to 80% due to the reliance on rising hot gas to suck air through the burning wood. Therefore a 5 KW fire may lose one or two KW of heat up the flue. Some attempt is often made to capture some of this heat by increasing the exposed surface area of the flue pipe and use of Peltier Effect fans to blow the heat from the flue pipe into the room, but there is a limit to how much heat can be extracted before the exhaust gas stops rising, and the fire goes out. The need for an unobstructed flue limits any passive particulate filters that can be placed in the flue pipe. Wood burning stoves also emit the majority of particulates as the fire is starting, especially with damp and green wood. The heat is not strong enough to drive off the water and burn the damp wood without a lot of smoke. The environmental issues concerning wood burning stoves have become so severe that they are being banned in new builds in Scotland from 2025 and may be banned in many other countries soon. Figure 2 illustrates an improved stove 100 according to an aspect of the present disclosure. Corresponding reference numerals are used between the figures in order to identify the components of the stove described previously with respect to figure 1, which in general will not be discussed further. The improved stove 100 of figure 2 differs from the stove described previously with reference to figure 1 in that the improved stove has a modified flue provides a heat exchange assembly 118. The heat exchange assembly 118 comprises a heat exchanger 102 and a ventilation device 104. The heat exchanger 102 and the ventilation device 104 are each coupled to and in fluid communication with the flue pipe 18. For ease of description, in this example the heat exchange assembly 118 is described as being installed within the flue pipe 18. The heat exchange assembly may be provided in a section of flue pipe that is coupled to one or more other flue pipes and / or the combustion chamber. Components of the heat exchange assembly may be provided separately from a flue pipe for retrofitting an existing flue pipe installation. The heat exchanger 102 is provided between the inlet 20 and the outlet 22 of the flue pipe 18. The heat exchanger 102 enables heat within the exhaust gas to be transferred to the ambient environment outside of the flue pipe 18. Removing the heat from the exhaust gas causes the self-propulsion of the exhaust gas to the exterior of the flue to stall. The ventilation device is provided to draw or propel the exhaust gas through the flue pipe 18 so that the cooled exhaust gas maybe expelled from the flue pipe 18 and maintaining fluid flow to allow combustion of the fuel within the combustion chamber 12. In this way, a substantial portion of the heat of the exhaust gas can be recovered whilst allowing combustion to be maintained. The operation of the heat exchange assembly may be assessed by measuring the temperature of the exhaust gases at different points along the assembly. The exhaust gas temperature may be measured by one or more thermocouples or EGT (Exhaust Gas Temperature) sensors mounted in the exhaust gas stream. The proportion of heat energy I thermal energy extracted from the exhaust gas by the one or more heat exchange elements may be estimated by taking temperature measurements at the inlet and outlet of the heat exchange assembly (or before and after the one or more heat exchange elements). A change in internal heat I thermal energy of an ideal gas is directly proportional to the change in temperature of an ideal gas. Therefore, the percentage temperature difference between the exhaust gas temperature at the inlet and outlet of the heat exchange assembly gives an approximate indication of heat energy extracted. A more accurate estimate of the heat I thermal energy extracted from the exhaust gases would account for pressure of the exhaust gases at the inlet and outlet, and the work done by the negative pressure of the ventilation device. For example, the heat exchanger and ventilation device may be configured to enable the exhaust the cooled exhaust gas to have a temperature less than 30 degrees Celsius above the ambient temperature when the inlet is fed with exhaust gas at a temperature of between 200 and 600 degrees Celsius (for example 400 °C) above the ambient temperature. In a preferred embodiment such as that illustrated in figure 2, the ventilation device is provided downstream, with reference to the exhaust gas flow, from the heat exchanger 102. In this way, the exhaust gas is substantially cooled by the time they reach the ventilation device 104 so the thermal requirements of the materials used to construct the ventilation device 104 may be reduced. In this example, the ventilation device 104 is provided adjacent to the outlet 22 of the flue pipe 18. The arrangement of the ventilation device 104 causes negative pressure to be generated upstream of the fan with respect to the combustion gas flow. Instead of relying on the fact that hot air rises, air and exhaust gas can be pulled through the fire with the ventilation device 104, which may be provided by a fan or bellows. The ventilation device 104 may be electrically or mechanically operated. Pulling the exhaust gas through the fire is more failsafe than forcing air through the fire as the former relies on negative pressure in the combustion chamber and the latter relies on positive pressure. With negative pressure, any leakages in the combustion chamber will keep the smoke within the stove, whereas positive pressure will force smoke and gas into the room. The Efficiency of Domestic Wood and Coal burning stoves may be improved by extracting almost all the waste heat in the exhaust gas by means of a heat exchanger in some examples. Exhaust gas is mechanically forced through the heat exchanger, particulate filter and possibly gas scrubber by various means. Various other arrangements of stoves according to the present disclosure are described below with reference to figures 3A to 3D. Figure 3A illustrates a stove 300 that is similar to the stove described previously with reference to Figure 2 and further comprises a filter 206. The filter 206 is provided adjacent to the flue inlet 20 of the flue pipe 18. In this example, the filter 206 is provided between the heat exchanger 102 and the flue inlet 20. Figure 3B illustrates a further stove 200' according to the present disclosure. The stove 200' differs from the stove 200 described previously with reference to figure 3A in that the filter 208' is located closer to the flue outlet 22 of the flue pipe 18. In this example, the filter 208' is provided between the heat exchanger 102 and the ventilation device 104. Figure 3C illustrates a further stove 200" according to the present disclosure. The stove 200" is similar to that described previously with reference to figure 2 and further includes a first filter 206 and a second filter 208. The first filter 206 is arranged as described previously with reference to figure 3A. The second filter 208 is positioned as described previously with reference to figure 3B. Figure 3D illustrates a further stove 300. The stove 300 is similar to the stove described previously with reference to figure 2 except the ventilation device 104' is positioned closer to the flue inlet 20 than the flue outlet 22. In this example, the ventilation device 104' is provided between the flue inlet 20 and the heat exchanger 102. In general, the ventilation device 104' maybe provided at any position along the flue pipe 18. However, the arrangement shown in Figure 3D may be considered to be a sub-optimal arrangement because the ventilation device 104' would be exposed to very high temperatures and therefore have higher requirements for its material properties than in examples in which the ventilation device is provided further downstream and is exposed to cooled exhaust gases. Advantages of mechanically sucking air and exhaust gas through the fire include 1) Nearly all the heat can be extracted from the exhaust gas by means of a heat exchanger. A 5 Watt fan can save 1000 Watts (or more) of heat and reduce greenhouse gas emissions by 20 to 30%. 2) Extracted exhaust gas is only slightly above room temperature when it leaves the heat exchanger, so a heat resistant flue liner is not needed. 3) The flue does not need to be vertically above the stove, but can go in any direction, even downward, and can exit the building though a narrow pipe in the wall. The chimney stack itself can be eliminated, leaving more room in the house. 4) A lot of air is sucked through the fire from the very start, making initial lighting quicker, thereby minimising the amount of particulates produced at the start of the fire. 5) The heat exchanger pipes can initially be routed through the fire, thereby facilitating secondary or tertiary burning of particulates within the pipe. 6) The system can be fully closed, sucking fresh air in from outside the room, via a different inlet pipe or a balanced flue. 7) Use of negative pressure means that the system does not have to be completely airtight, whereas a leaky system would be a problem with positive pressure. 8) the use of an electric fan, to suck air through the system, means that the fire is instantly controllable and can be computer controlled, possibly using positive feedback to control the fire profile. A timer, proximity detector, or remote controller can turn down the fire when no-one is in the room. 9) a more powerful fan can be used to pull the exhaust gas through highly compressed filters, enabling more particulates to be removed from the gas. this can be done in the hot input of the heat exchanger, enabling secondary / tertiary burning, and in the cool output of the exchanger, enabling a washable micro-filter to be used to remove particles of a few microns. 10) the exhaust gas can be bubbled through wet scrubbers to remove very small particles and some of the carbon dioxide from the exhaust gas. 11) more effective particulate removal may enable greener wood to be used. Figure 4 illustrates a schematic diagram of a further arrangement for a heat exchanger in use on an improved stove. The stove of Figure 4 is similar to the stove described previously in relation to figure 3C. The heat exchanger comprises a plurality of heating fins. An optional cooling fan is provided to urge air towards the cooling fins of the heat exchanger. The heat exchanger and ventilation device are provided as an assembly that can be used to retrofit an existing stove installation by replacing a section of the original flue pipe. The heat exchanger and ventilation device may be coupled to an existing flue pipe installation. Figure 5 illustrates a schematic diagram of a further arrangement for a heat exchanger in use on an improved stove. The assembly has an inlet manifold and an outlet manifold and a plurality of pipes extending between the inlet manifold and the outlet manifold providing heat exchange elements of the heat exchanger. A cooling fan may be arranged to urge air towards the heat exchange elements. In this way, the heat exchange assembly can provide a radiator for position within a room to be heated. Figures 6 and 7 illustrate views of an example of an improved stove as demonstrated in Experiment 1. This shows an open system, where air is sucked in through a fire from the local environment. In an open system, ambient air is drawn into the fire from the room. This air is replaced by cold air being drawn into the room through the doors and windows, thereby reducing the heating efficiency of the fire. Smoke and heat from the fire rises into a "blind" flue (also referred to as a "blind" chimney), where it is sucked out, by a 5 Watt electric motor, through four heat resistant exhaust pipes, each 3 metres long. By the time the exhaust gas reaches the motor, nearly all the heat has been lost to the environment through the walls of the tubes, so a heat resistant motor is not needed (a 5 Watt bathroom extractor fan was used for this experiment). The blind chimney is "blind" in the same sense as a "blind alley". The blind chimney may have an enclosed end from which exhaust gases are removed by another flue pipe. A blind chimney may have a window to allow the fire to be visible. The "blind" flue got very hot, as did the first meter of each of the four pipes, releasing their heat into the environment. A separate fan could be used to blow cold air over the "blind" flue or these pipes to increase this heat transfer. The fan may be a Peltier effect fan. To improve the heat recovery from the "blind" flue, by way of radiation and / or convection, the "blind" flue may have a large height, width, or diameter, and / or comprise a corrugated surface or cooling fins fitted to the outside surface of the "blind" flue. Stainless steel scouring pads and stainless steel wire wool can be inserted in these pipes to increase heat transfer, facilitate secondary / tertiary burning and filter out particulates. A very fine washable and reusable micro particle filter (as used in Dyson® vacuum cleaners) can be used at the cold motor end to filter out very small particulates. A setup like this could save over 1000 Watts of heat, at the cost of 5 Watts of electricity. This 5 Watts could come from mains or a battery that is charged by wind power or solar panels. At the cost of increasing the power of the motor to 20 or 30 Watts, more efficient filtering could be used. The exhaust gas could be sucked through a bath of lime water to remove some of the carbon dioxide, or scrubbed in a fine shower of used bathwater, removing many of the particulates and some of the carbon dioxide, forming weak carbonic acid. Figure 8 illustrates another example of an improved stove as demonstrated in Experiment 2 - a retro-fit in-line heat exchanger experiment. Here the exhaust gas is sucked through a stack of metal plates, which absorb heat from the exhaust gas and release it into the room. Filter material, such as Stainless Steel scourers and Stainless Steel wire wool can be stuffed into the flue column to assist heat loss, secondary / tertiary burning and particulate filtration. Figures 9A and 9B illustrate schematic diagrams of a further arrangement for a heat exchanger in use on an improved stove. A closed circuit wood burning stove according to the present disclosure. In a "closed" system, air is directly drawn into the fire from outside the room through a pipe and exits the room through a pipe. There is, therefore, no direct fluid connection between the air in the room and the air in the stove. This leads to greater heating efficiency of the "closed" system. The "Blind" flue can be raised, or hinged open, to feed and clean the fire (this mechanism is not shown). Also, if it is left in the slightly open position, the fire can be seen directly without any smoke escaping. The fuel burning surface is relatively horizontal and a separate relatively vertical "blind" chimney superstructure that sits on the fuel burning surface, with a relatively air-tight seal between the two. The "blind" chimney is hinged, along one edge of the fuel burning surface, or able to be raised from, and lowered onto the fuel burning surface, using a raising mechanism with a heat resistant handle. The "blind" flue may be detachable, able to be lifted away from the fuel burning surface. Alternatively, a surface on which the fire burns may be lowered away from the "blind" chimney. The fire is fed air from below, in a manner similar to a standard solid fuel burning stove, when the "blind" chimney is raised, the system temporarily being "open". One or more flue pipes, opening relatively high up inside the "blind" chimney (even if the "blind" chimney is raised) sucking exhaust gas upwards from the fire into the chimney cavity, possibly through a secondary burning zone, then downwards through the pipes, either round or through the surface that supports the fire and entering the heat exchanger means outside the combustion chamber. Figures IDA and 10B illustrate yet another example of an improved stove as demonstrated in Experiment 3. This shows an open system. Smoke and heat from the fire rises into a "blind" flue, where it is sucked out, by a electric motor, through four heat resistant exhaust pipes, each 3 metres long. In Experiment 3, the exhaust pipes rise up into a "blind" flue or chimney to act as an inlet for the exhaust gases. The pipes have stainless steel scourer / scrim / wire wool stuffed into their inlets to assist heat loss, secondary / tertiary burning and particulate filtration. The exhaust pipes extend down below the fire in a coiled configuration. The electric motor is connected to the outlet end of the pipe. By the time the exhaust gas reaches the motor, nearly all the heat has been lost to the environment through the walls of the tubes, so a heat resistant motor is not needed (a 5 Watt bathroom extractor fan was used for this experiment). In Figure 10A, the "blind" flue / chimney is not fitted over the fire. This demonstrates that relatively little cooled flue exhaust gas is expelled from the exhaust extractor fan without the "blind" chimney fitted over the fire. In comparison, Figure 10B shows the same set up but with the "blind" chimney fitted over the fire. The "blind" chimney allows the pipe inlets to capture a greater proportion of the exhaust gases, thus more exhaust gas will flow through the pipe coils, more heat will be extracted from the hot exhaust gas, and thus more cooled exhaust gas can be seen coming out of the exhaust extractor fan. In use, the "blind" chimney of Figure 10B got very hot, as did the heat exchange pipes, releasing heat into the environment. A separate fan could be used to blow cold air over the "blind" chimney or the pipes to increase this heat transfer. The fan may be a Peltier effect fan. To improve the heat recovery from the "blind" chimney, by way of radiation and / or convection, the "blind" chimney may have a large height, width, or diameter, and / or comprise a corrugated surface or cooling fins fitted to the outside surface of the "blind" chimney. The amount of heat recovered from the "blind" chimney may be further increased by painting the flue black, thereby increasing black body radiation. The amount of heat recovered from the "blind" chimney may be further increased by fitting vertical or vertically angled fins to the outside surface of the "blind" chimney, thereby increasing heat induced updraft, possibly inducing spiral vortices. The effect of said fins is increasing heat loss to the surrounding environment by convection. Further, the "blind" chimney could be coupled to a heat sink, such as a thermally conductive surface, thereby increasing the conducted heat loss to the room.

Claims

1. A heat exchange assembly for a solid fuel-burning stove, the heat exchange assembly comprising:an inlet for receiving exhaust gas from combustion within the solid fuel burning stove;one or more heat exchange elements configured to extract heat for external use from the exhaust gas and form cooled exhaust gas;an outlet for expelling the cooled exhaust gas from the heat exchange assembly; anda ventilation device coupled to the heat exchange assembly, wherein the ventilation device is configured to propel the exhaust gas towards the outlet.

2. The heat exchange assembly of any of claim 1, wherein the at least one heat exchanging elements comprise one or more flue pipes, wherein the one or more heat exchange elements are configured to extract at least 60% of the heat from the exhaust gas.

3. Wherein the heat exchanger and ventilation device are configured to enable the exhaust the cooled exhaust gas to have a temperature less than 60 degrees centigrade when the inlet is fed with exhaust gas at a temperature of between 250 and 600 degrees centigrade.

4. The heat exchange assembly of any preceding claim comprising a plurality of heat exchange elements placed in succession to cool the exhaust gas.

5. The heat exchange assembly of any preceding claim, wherein the ventilation device is a mechanical or electrical device.

6. The heat exchange assembly of claim 5, wherein the ventilation device comprises a fan powered by one or more of mains electricity, battery, solar cells, or wind power or by a Sterling Engine or Peltier Effect device.

7. The heat exchange assembly of any preceding claim wherein the assembly is configured to be coupled in-line between a standard solid fuel burning stove and a standard flue liner.

8. The heat exchange assembly of any preceding claim wherein the outlet is configured to be coupled to an existing chimney or a hole in the wall via a tube.

9. The heat exchange assembly of claim 8, wherein the heat exchange mechanism comprises means to circulate water to cool the exhaust gas.

10. The heat exchange assembly of claim 9, wherein the heated water is circulated through a radiator system to heat a room.

11. The heat exchange assembly of any preceding claim further comprising, at the inlet, a pre heat-exchange section comprising any one of a hot metal, ceramic, or catalysing scrim.

12. The heat exchange assembly of any preceding claim further comprising, at the outlet, a particulate filter for filtering out any remaining particles, including micro particles.

13. The heat exchange assembly of claim 12 comprising further particulate removing, gas cleaning or carbon dioxide removing means.

14. The heat exchange assembly of any preceding claim wherein the ventilation means configured to be controlled manually, by timers, by feedback mechanisms, or by mobile phone / computer, enabling the fire to be controlled and extinguished directly by the user or by remote control.

15. A stove for burning solid fuel, comprising:a combustion chamber; andthe heat exchange assembly of any preceding claim;wherein the flue inlet is in fluid communication with the combustion chamber to receive the exhaust gas from the combustion chamber.

16. The stove of claim 15, wherein the combustion chamber comprises an air inlet.

17. The stove of claim 15 or claim 16, further comprising an air inlet pipe section coupled to the solid fuel burning stove.

18. The stove of claim 17, wherein the outlet comprises a flue outlet pipe section, and wherein the air inlet pipe section and flue outlet pipe section are concentric, thus forming a coaxial pipe so as to form a "balanced" flue system.

19. The stove of any of claims 15-18, wherein the stove has the visual, functional and psychological advantages of a standard domestic solid fuel burning stove.

20. The stove of any of claims 15-19, wherein the stove further comprises a blind chimney section coupled to the combustion chamber.

21. The stove of claim 20, wherein the blind chimney is detachably coupled to the combustion chamber.

22. A method of retrofitting a heat exchanger of any preceding claim to a stove installation, comprising:removing a section of flue pipe from a stove installation; andcoupling the inlet and outlet of the heat exchange assembly to respective openings in the stove installation created by the removal of the section of the flue pipe.

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