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Patent Searching and Data


Title:
WASTE HEAT RECOVERY FOR FORCED CONVECTION BIOMASS STOVE
Document Type and Number:
WIPO Patent Application WO/2013/015868
Kind Code:
A1
Abstract:
A stove (100) includes a combustion chamber (126) for producing heat, a thermoelectric device (120) thermally coupled to a hot source (114) and a cold source (116), a battery, a fan (118) electrically connected to the thermoelectric device (120) and the battery, and a controller configured to monitor the battery and thermoelectric device (120) and configured to direct the fan (118) be operated from power provided from the battery when power produced from the thermoelectric device (120) is insufficient to power the fan (118).

Inventors:
ZILLMER ANDREW JOHN (US)
CARROLL JOSEPH P (US)
Application Number:
US2012/039023
Publication Date:
January 31, 2013
Filing Date:
May 23, 2012
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
AEROJET GENERAL CO (US)
ZILLMER ANDREW JOHN (US)
CARROLL JOSEPH P (US)
International Classes:
F24B1/00; F02G5/02; F23G5/46; F24B13/00; H02N3/00
Foreign References:
US20070221205A12007-09-27
US5495829A1996-03-05
US6527548B12003-03-04
US6588419B12003-07-08
Attorney, Agent or Firm:
BROWN, Ward (1420 Fifth AvenueSuite 280, Seattle WA, US)
Download PDF:
Claims:
CLAIMS

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A stove, comprising:

a combustion chamber for producing heat;

a thermoelectric device thermally coupled to a hot source and a cold source;

a battery;

a fan electrically connected to the thermoelectric device and the battery; and a controller configured to monitor the battery and thermoelectric device and further configured to direct that the fan be operated from power provided from the battery when power produced from the thermoelectric device is insufficient to power the fan.

2. The stove of Claim 1, further comprising a hot flue gas duct and a cold air inlet duct, and the thermoelectric device is thermally coupled to the hot flue gas duct and the cold air inlet duct.

3. The stove of Claim 1, wherein the thermoelectric device is located on a hot surface of the stove and the cold source is ambient air.

4. The stove of Claim 1, wherein the controller is configured to monitor one or more temperatures, and the controller is configured to determine the thermoelectric device is producing power from one of the monitored temperatures.

5. The stove of Claim 1, wherein the controller is configured to determine a state of charge of the battery, a power consumed by the fan, and a power produced by the thermoelectric device, and when the battery is not fully charged, and the thermoelectric device is producing more power than is consumed by the fan, the controller directs that the battery receive power from the thermoelectric device to charge the battery.

6. The stove of Claim 1, wherein the controller is configured to determine a state of charge of the battery, a power consumed by the fan, and a power produced by the thermoelectric device, and when the battery is fully charged, and the thermoelectric device is producing more power than is consumed by the fan, the controller directs that excess power produced by the thermoelectric device be directed to a resistor. 7. The stove of Claim 1, further comprising a resistor, and the controller is configured to direct the thermoelectric device to dissipate excess power produced by the thermoelectric device to the resistor.

8. The stove of Claim 7, wherein the resistor is a radio, a light, an electric resistor, or any power load.

9. The stove of Claim 1, wherein the controller is configured to monitor one or more temperatures, and the controller is configured to reduce a speed of the fan when a temperature is above a predetermined limit.

10. The stove of Claim 1, wherein the controller is configured to monitor one or more temperatures, and the controller is configured to increase a speed of the fan when a temperature is below a predetermined limit.

11. The stove of Claim 1 , further comprising a hot flue gas duct and a cold air inlet duct and a recuperator that is thermally coupled to the hot flue gas duct and the cold air inlet duct.

12. The stove of Claim 1, further comprising a heating plate and a Stirling engine that is thermally coupled to the heating plate.

13. The stove of Claim 1, wherein the combustion chamber is configured to burn solid fuel.

14. The stove of Claim 1, comprising a combustion chamber configured to burn solid fuel and a combustion chamber configured to burn liquid fuel.

15. A method for operating a stove, comprising:

operating a fan from battery power, wherein the fan forces air to a combustion chamber of the stove;

burning fuel in the combustion chamber;

monitoring a temperature of the stove; and

when the temperature of the stove is at or above a predetermined limit, switching power to operate the fan from battery power to power produced by a thermoelectric device thermally coupled to a hot source and a cold source. 16. A method for operating a stove, comprising:

burning fuel in a combustion chamber of the stove;

monitoring a temperature of the stove;

when the temperature of the stove is at or above a predetermined limit, operating a fan from power produced by a thermoelectric device thermally coupled to a hot source and a cold source; and

operating the fan and charging a battery from power produced by the thermoelectric device.

17. A stove, comprising :

a combustion chamber for producing heat;

a thermoelectric device thermally coupled to a hot source and a cold source;

a battery;

a fan electrically connected to the thermoelectric device and the battery; and a controller configured to direct when the fan is to be operated from power provided from the battery or from power produced from the thermoelectric device.

Description:
WASTE HEAT RECOVERY FOR FORCED CONVECTION BIOMASS STOVE

BACKGROUND

The ability to generate power in remote locations finds many uses for military, civilian, and commercial applications. For military uses, portable power is required for any forward or remote operating bases that cannot tie into an existing power supply grid. Portable power also is useful in civilian applications, for example, during natural disaster relief operations. Portable power is necessary for operating machinery to assist in the disaster relief efforts, such as communications and for illumination, or to provide for basic needs of the population, such as refrigeration of foods and cooking. Portable power also has uses in the commercial realm. For example, portable power is useful when camping or for use when traveling to any location that is not serviced by an existing power grid. Additionally, many residences experience power outages during severe storms, and having backup power sources would be very desirable. One form of power generation relies on liquid fuels to power generators. In some cases, liquid fuels may be unavailable or extremely difficult to obtain. In a mobile application, the liquid fuel also needs to be transported. Biomass, such as wood and other combustibles, provides an alternative to liquid fuels that has the advantage that it may be present on site.

Converting biomass into power has inherent problems. Biomass, for example, can be difficult to combust initially and will take a long time to heat up using natural convection. Accordingly, forced air draft systems could be used, but there needs to be a reliable method of providing forced air flow that could speed the system startup. Also, biomass can be inefficient because it generally produces low temperatures. Accordingly, there needs to be a means for increasing the efficiency of biomass-burning stoves.

Disclosed herein is a stove for power generation that may address one or more of these deficiencies.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

In one aspect of the present invention, a stove has a combustion chamber for producing heat, a thermoelectric device thermally coupled to a hot source and a cold source associated with the stove or its environment, a battery, a fan (such as for selectively blowing combustion air to the combustion chamber), and a controller configured to monitor the battery and thermoelectric device and to direct that the fan be operated from power provided from the battery when power produced by the thermoelectric device is insufficient to power the fan. The stove can include a heating plate, with a Stirling engine thermally coupled to the heating plate.

In additional aspects of the invention, the stove can include a hot flue duct for exhaust gas and a cold air inlet duct through which combustion gas passes. The thermoelectric device can be coupled between the hot flue duct and the cold air inlet duct. Alternatively, the thermoelectric device can be located on a hot surface of the stove and the cold source can be ambient air. A recuperator can be thermally coupled to the hot flue exhaust gas duct and the cold air inlet duct.

In other aspects of the present invention, the controller is configured to monitor one or more temperatures associated with the stove, and the controller is configured to determine the extent to which the thermoelectric device is producing power based on detection of one or more of the temperatures being monitored and/or the controller is configured to change the speed of the fan based on a detected temperature.

In still further aspects of the invention, the controller is configured to determine the state of charge of the battery and/or the power consumed by the fan. The power from the thermoelectric device is directed by the controller depending on the battery's state of charge and power consumed by the fan. For example, if the battery is not fully charged and the thermoelectric device is producing more power than is consumed by the fan, the controller can direct that the battery receive charging power from the thermoelectric device. If the thermoelectric device is producing more power than is consumed by the fan, and the battery is fully charged, excess power produced by the thermoelectric device can be dissipated through a resistor or other load.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a schematic illustration showing one embodiment of a stove according to the present invention; FIGURE 2 is a schematic illustration showing the elements of a power management and distribution system for the stove of FIGURE 1;

FIGURE 3 is a step logic diagram for a method of operating the stove of FIGURE 1;

FIGURE 4 is a diagrammatical illustration of a combustion device with radiative heat transfer and heat recovery;

FIGURE 5 is a diagrammatic illustration of a recuperator that can be used with the combustion device of FIGURE 4; and

FIGURE 6 is a diagrammatic illustration of the recuperator of FIGURE 5, viewed from the right of FIGURE 5.

DETAILED DESCRIPTION

Referring to FIGURE 1, a portable power generating stove 100, in accordance with one embodiment of the invention, is illustrated. The stove 100 includes a combustion chamber 126 suitable to burn biomass, including, but not limited to solid fuels, such as wood, paper, cardboard, plant matter, animal dung, pellets, waste, trash, tires, and other combustibles. Alternatively, the stove 100 may be modified to accept liquid or gas fuels. For example, the stove 100 may be equipped with a suitable burner to burn liquid fuel, or gas fuel. In one embodiment, a combination of solid, liquid and/or gas fuels may be used simultaneously by providing a plurality of combustion chambers. For example, one combustion chamber may be used for burning of solid fuel, and a second combustion chamber may be used for burning liquid fuel.

The combustion chamber 126 and stove 100 may be made generally from metals, with an insulating material between the combustion chamber 126 and stove exterior 102. The stove 100 includes a heating plate 106 (or heat acceptor plate) positioned directly above the combustion chamber 126. The heating plate 106 is heated mainly by radiative and convective heating from combustion and combustion gas products impinging on the heating plate 106. The stove 100 includes a Stirling engine 104, such that the heating plate 106 is thermally coupled to the Stirling engine 104, either directly or through one or more heat exchangers. The specific design of the Stirling engine 104 will dictate the manner of transferring heat from the heating plate 106 to the Stirling engine. As used herein, a Stirling engine is a well known device that operates by cyclical compression and expansion of a working fluid, such as air or other gases. The Stirling engine 104 compresses and expands the working fluid by cyclical cooling and heating of the working fluid using a heat source and a heat sink (cold source). The result is a net conversion of heat energy to mechanical work. In the stove 100, the heating plate 106 is used as the heat source for the Stirling engine, thus powering the Stirling engine, which in turn may power a generator. However, in an alternative embodiment, the hot end of the Stirling engine 104 may be within the combustion chamber 126.

A simple Stirling engine may use a single cylinder (beta-type configuration) with a hot end and a cold end, or two cylinders (alpha-type configuration), where one cylinder is exposed to the heat source and the second cylinder is exposed to the cold source. More complex Stirling engines may aggregate either one or a combination of the simple one or two cylinder designs into a multiplicity of cylinders and complex piston arrangements. A Stirling engine is classified, similar to a steam engine, as an external combustion engine, as heat transfer between the combustion gas and the working fluid occurs through the cylinder wall and no combustion takes place inside of the cylinder. While any Stirling engine may be used as the Stirling engine of the present invention, a suitable Stirling engine is disclosed in U.S. Patent No. 7,134,279, to White et al, which is fully incorporated herein expressly by reference. This patent discloses a double-acting, multi- cylinder, thermodynamically resonant, alpha configuration free-piston Stirling system. The system includes overstroke preventers that control the extent of piston travel to prevent undesirable consequences of piston travel beyond predetermined limits. The overstroke preventers involve controlled work extraction out of the system or controlled work input into the system. The patent discloses that the Stirling engine may be coupled for electrical power generation in which alternating current (AC) power output can be rectified and filtered to provide direct current (DC) power, and that three phase AC power output from a three cylinder module implementation can be converted to DC power with good efficiencies and simple electronics.

The stove 100 includes a flue gas duct 114 for the combustion gases generated in the combustion chamber 126. The stove 100 includes a thermoelectric device 122 placed at a location in proximity or in contact with the flue gas duct 114. The stove 100 includes an air inlet duct 116. The thermoelectric device 122 is also in proximity or in contact with the inlet air duct 116. The stove 100 includes a fan 118 provided at the inlet of the air inlet duct 116. The fan 118 provides forced draft combustion air to the stove 100. The air is forced into air ducts 124 that lead into the combustion chamber 126. The Stirling engine 104 will be able to start faster and reach operating temperatures faster with forced draft combustion air provided by the fan 118.

The thermoelectric device 122 is a well known device. For example, U.S. Patent No. 7,942,010, to Bell et al, which is fully incorporated herein expressly by reference, discloses thermoelectric modules can be used for power generation. In FIGURE 27, for example, Bell et al. discloses a design using thermoelectric modules for generating power. The thermoelectric device 122 generates power based on a temperature gradient. In the embodiments of the instant invention, the difference between the hot flue gas duct 114 and the cold air inlet duct 116 can provide the temperature gradient used by the thermoelectric device 122 The thermoelectric device 122 may include a plurality of modules each one having a P-type and N-type semiconductor. Increasing the number of modules will increase the power output of the thermoelectric device 122 for any given temperature difference. The number of modules will depend on the desired power output from the thermoelectric device 122. Each of the P-type conductors and each of the N-type conductors can be alternately electrically connected to each other in series with electrical conducting shunt elements, which may also serve as thermal conducting shunts from both the hot and cold source, i.e., the hot flue gas duct 114 and the cold air inlet duct 116. However, the shunts should be electrically isolated from the hot flue gas duct 114 and the cold air inlet duct 116. This arrangement can be used to provide a voltage differential that supplies current to power a load.

The hot flue gas duct 114 passes through a recuperator 120 (i.e., a heat exchanger). The recuperator 120 is also in thermal contact with the air inlet duct 116. The recuperator 120 allows heat to be transferred from the combustion gases to the incoming air, thus preheating the air before combustion and providing for more efficient use of the fuel. Recuperators are well known devices for transferring heat from one gas to another and may utilize fin and tube designs, single-pass, multipass, and countercurrent flow patterns. The flue gas is hotter on the hot side before the recuperator 120 than on the cold side after the recuperator 120, and the air will be colder on the cold side before the recuperator 120 than on the hot side after the recuperator 120. Accordingly, this provides several options for thermally coupling the thermoelectric device 122 to provide a temperature differential. In one embodiment, the thermoelectric device 122 can be coupled to the hot side of the hot flue gas duct 114 and the hot side of the cold air inlet duct 116 (this is illustrated in FIGURE 1). In one embodiment, the thermoelectric device 122 can be coupled to the hot side of the hot flue gas duct 114 and the cold side of the cold air inlet duct 116. In one embodiment, the thermoelectric device 122 can be coupled to the cold side of the hot flue gas duct 114 and the hot side cold air inlet duct 116. In one embodiment, the thermoelectric device 122 can be coupled to the cold side of the hot flue gas duct 114 and the cold side cold air inlet duct 116. In other embodiments, the thermoelectric device 122 can be coupled to hot surfaces of the stove 100, as the hot source and ambient air serves as the cold source for the thermoelectric device 122. When the thermoelectric device 122 is placed away from the recuperator 120, such as the outer surface of the stove 100, the performance of the recuperator 120 can be increased due to the removal of the thermoelectric heat path.

The stove 100 includes a plurality of temperature measuring devices 108, 110, and 112. The temperature measuring device 108 may measure the temperature of the hot end of the Stirling engine 104. The temperature measuring device 110 may measure the temperature of the heating plate 106. The temperature measuring device 112 may measure the temperature inside the combustion chamber 126. The temperature measuring devices 108, 110, and 122 are well known devices, such as thermocouples, rated to withstand temperatures in excess of approximately 1000°C. While representative locations of the temperature measuring devices 108, 110, and 112 have been illustrated, it is to be appreciated that these locations are not limiting, and more or less temperature measuring devices may be used, such as on either the hot or cold side of the hot flue gas duct 114 and the hot or cold side of the cold air inlet duct 116. These temperature measuring devices may be used in various control algorithms as further described below.

FIGURE 2 is a schematic illustration showing the elements of a power management and distribution (PMAD) system used for the stove 100. In addition to the features discussed in association with FIGURE 1, the stove 100 includes a battery 206, a resistor 204, a controller 202, and power connection and communications cables connecting the various elements. The elements of the power management and distribution system include the controller 202, the thermoelectric device 122, temperature sensing devices 108, 110, 112, a battery 206, a fan 118, and a resistor 204. The controller 202 is any well-known central processing unit that may be used to perform a series of logic decisions based on several inputs received from the system elements. The thermoelectric device 122 provides power to the system including the controller 202 and fan 118. However, as mentioned above, the thermoelectric device 122 relies on a temperature gradient being produced between the hot flue gas duct 114 and the cold air inlet duct 116. Accordingly, during stove 100 startup and insufficient temperature gradient conditions, the battery 206 is provided for start of the system. The battery 206 can be any type of rechargeable energy storage device, including but not limited to lead acid batteries, liquid electrolyte batteries, gel batteries, absorbed glass mat batteries, and dry batteries, such as nickel cadmium, nickel zinc, nickel metal hydride, and lithium ion. The battery 206 may include instruments for determining the state of charge of the battery. State of charge can be calculated by measuring any one of several parameters of the battery including the electrolyte specific gravity, voltage, current, and temperature. This information is used by the controller 202 for determining the state of charge of the battery 206 and making decisions whether to power the fan 118 from the battery 206 or from the thermoelectric device 122. In one embodiment, if battery state of charge indicates that the battery 206 is charged to capacity, and the thermoelectric device 122 is producing more power than what is being consumed, the excess power generated by the thermoelectric device 122 may be shunted to the resistor 204, where the power is dissipated as heat. However, in the normal operating mode of the stove 100, the battery 206 is being charged by the thermoelectric device 122 and the fan 118 is being powered by the thermoelectric device 122. In other embodiments, the resistor can be any load, such as an electrical load, including, but not limited to radios, lights, heaters, and the like.

The system includes a fan 118, which is capable of being provided with power from the battery 206 as well as from the thermoelectric device 122. The fan 118 may be a variable speed fan that bases its speed on one or more of the stove temperatures 108, 110, and 112. The controller 202 receives the temperature measurements and may make decisions whether to run the fan faster or slower, or stop the fan altogether. Generally, if a decision is made by the controller 202 that a temperature of interest needs to be higher, the fan 118 speed is increased to provide higher burn temperatures. For example, the Stirling engine 104 may operate most efficiently if a certain temperature is achieved. As the stove 100 burns hotter, the fan 118 speed is increased, thereby providing more combustion air. The controller 202 can receive the stove temperatures 108, 110, and 112, and set a corresponding fan 118 speed. The controller 202 also directs which power source is used to power the fan 118 depending on battery 206 state of charge and the thermoelectric device 122. When a temperature gradient is produced, and the controller 202 sensing that the power from the thermoelectric device 122 is sufficient, the controller 202 may direct that power generated from the thermoelectric device 122 is used to power the fan 118. For example, a temperature differential between the hot and cold source of the thermoelectric device 122, a hot temperature, or a voltage may be used to determine when the thermoelectric device 122 is producing the required power. If the controller 202 senses that the thermoelectric device 122 is producing more power than what the fan 118 and system as a whole require, the controller 202 may direct that some of the power produced by the thermoelectric device 122 be used to charge the battery 206. For example, when the controller 202 senses that a temperature has reached a lower limit, the controller 202 assumes that the thermoelectric device 202 is producing sufficient power and closes a switch to connect the thermoelectric device 122 to charge the battery and power the fan. In other embodiments, the fan 118 can simply be connected directly to the battery 206, and the thermoelectric device 122 supplies power to the battery 206. If the controller 202 decides that the thermoelectric device 122 is producing too much power because the battery 206 is fully charged and the fan 118 load is low, either through actual measurements of the battery 206 and fan 118 or through inference from a temperature measurement, the controller 202 may direct that a switch be closed so that the thermoelectric device 122 can dump its power to the resistor 204. The system may have protections to avoid excessive temperature that may damage one or more components. If a high temperature condition is detected, the controller 202 may direct that the fan 118 speed be reduced, or stopped altogether if the high temperature condition has not cleared for more than a specified period of time. In addition to reducing fan load, the thermoelectric device 122 power can be dumped to the resistor 204.

The controller 202 may also sense an overspeed or overtemperature condition.

When an overspeed or an overtemperature condition is detected, the extra power being generated by the thermoelectric device 122 can be dumped to the battery 206 to charge the battery 206 or to the resistor 204 to protect the thermoelectric device 122. A high operating temperature of the stove 100 means that a high delta-T zone allows for less expensive thermoelectric devices to be used. The linking of the fan 118 to flow air past the thermoelectric device 122 allows for increased heat dissipation. The system can start even when the battery 206 is dead. In this condition, the stove 100 can start on natural convection, and soon combustion gases will provide the temperature differential to allow the thermoelectric device 122 to generate electricity for the fan 118, which will then assist in startup and lead to a decrease in the startup time.

FIGURE 3 is a step logic diagram showing one embodiment of a method 300 for starting the stove 100.

In block 304, the controller 202 can determine the state of charge of the battery 206 and based on the state of charge, the controller 202 makes a determination whether the fan 118 can be powered by the battery 206. If the determination in block 304 is YES, the method enters block 306, where the controller 202 can turn on the fan 118 supplied with power from the battery 206, and the stove 100 is started using forced draft air for a faster startup of the stove 100. The fan 118 may have a startup mode designating a proper speed for starting the stove 100.

If the determination in block 304 is NO, the method enters block 310. In block 310, the controller 202 does not turn the fan on, and the thermoelectric device 122 is still not producing power. In this case, the user can start the stove 100 using only natural convection. Once the temperature differential rises to a predetermined value, the controller 202 may determine that the thermoelectric device 122 is producing sufficient power and the controller 202 may determine to start the fan 118 on power produced by the thermoelectric device 122. The controller 202 may rely on a temperature sensed in the combustion chamber 126 or a temperature differential between a hot and cold source coupled to the thermoelectric device 122. Alternatively, the controller may sense a voltage produced by the thermoelectric device 122 to decide when to allow startup of the fan 118.

From block 310, the method enters block 312. In block 312, the controller 202 makes a determination whether the thermoelectric device 122 is producing more power than consumed. In making this determination, the controller 202 may receive inputs of various instruments. For example, the controller 202 may receive the amperage draw from the fan 118, and the amperage supplied by the thermoelectric device 122. To determine whether the thermoelectric device 122 is producing more power that consumed, the controller 202 may receive the amperage from an amp meter that senses the amperage produced by the thermoelectric device 122, and the controller senses the amperage required for the fan 118 to operate. The fan amperage may be predetermined and stored in a correlation table that correlates a fan speed to a fan amperage. The controller may also sense the thermoelectric device 122 voltage via a voltage meter, and the controller 202 may determine whether the thermoelectric device 122 is producing excess power through calculations based on the amperage and/or voltage. Additionally, in some embodiments, the controller 202 senses the state of charge of the battery 206 in making the determination whether or not there is excess power produced by thermoelectric device 122. In another embodiment, the power generation of the thermoelectric device 112 can be provided in a table stored in a memory device within the controller 202. The table includes a correlation of the temperature difference between the hot source and the cold source correlated to a power being supplied by the thermoelectric device 122. Based on measurements such as these, the controller 202 may determine whether the system power requirements exceed the power generation of the thermoelectric device 122.

From block 306, the method enters block 308. The determination of block 308 is similar to the determination made in block 312.

If the determination in either of block 308 and block 312 is NO, the method continues to operate the fan 122 and all other power loads of the system using the thermoelectric device 122. However, if the determination in block 308 or block 312 is YES, signifying that the thermoelectric device 122 is producing more power than is consumed, the method enters block 314. In block 314, the controller 202 directs that the excess power from the thermoelectric device 122 be used to charge the battery 206 while power from the thermoelectric device is also used to power the fan 118.

From block 314, the method enters block 316. In block 316, the controller 202 can determine whether or not the battery 206 is fully charged. Depending on the type of battery, the controller 202 may use the specific gravity of electrolyte, the voltage of the battery, the amperage of the battery, and the temperature of the battery to determine the state of charge of the battery 206. If the determination in block 316 is NO, the controller 202 continues to allow charging of the battery 206. However, if the determination in block 316 is YES, the method enters block 318. In block 318, any excess power not consumed by the fan 118 is dissipated through the resistor 204.

Referring back to FIGURE 1, heat transfer from the combustion chamber 126 to the heating plate 106 can be via radiation, conduction, and convection. The purpose of the heating plate 106 is to act as a means to collect heat to be used in a power conversion device, such as the Stirling engine. Heat transfer is almost minimal through conduction, which would involve heating of the sidewalls of the combustion chamber that touch or contact the heating plate 106. As between radiative heat transfer and convective heat transfer, it has been discovered by the inventors that radiative heat transfer can be more efficient than convective heat transfer. This is because convective heat transfer leads to a large recuperator (heat exchanger) that would typically be located at the top of the stove. From a manufacturing standpoint, this can present a packaging issue, since a large area or volume would be required in order to effectively recover the heat in the recuperator.

In accordance with another embodiment of the invention, a combustion chamber design is provided that increases the radiative heat transfer from a combustion chamber to a heating plate. This combustion chamber design may be used with the stove 100 illustrated in FIGURE 1.

Referring to FIGURE 4, a schematic illustration is provided of a stove 400 with a combustion chamber 404 having angled walls 418 that form a V-shaped combustion chamber 404. The stove 400 may include all the features and operate similar to the stove 100 described in association with FIGURES 1-3. The differences between the stove 100 of FIGURE 1 and the stove 400 of FIGURE 4 will be apparent from the description that follows.

The stove 400 includes a heating plate 410 located directly above the combustion chamber 404. The angled walls 418 increase the view angle of the heating plate 410 and additionally reflect heat from the walls 418 and direct it to the heating plate 410. The flame radiates heat 414 upward so that it directly impinges on the heating plate 410. Additionally, the flame radiates sideways heat 416 which is reflected by the angled walls 418 toward the heating plate 410. The angled walls 418 may be provided on all sides of the combustion chamber 440, or the angled walls 418 may be provided on at least two opposing sides of the combustion chamber 404. The angled walls 418 may be made of metal, such as cast iron.

Combustion gases 408 may exit the stove 400 through a hot flue gas duct 420, provided on one or both sides of the heating plate 410. Forced draft combustion air 406 may enter the combustion chamber 404 through air ducts 422. Ducts 422 may be provided on one, both or all sides of the combustion chamber 404. Ducts 422 may be formed using the opposite side (or surface) of the angled walls 418 facing the combustion chamber.

The angled combustion walls 418 increase the view factor of the heating plate 410. "View factor" is the fraction of all the radiative heat from the flame that strikes the surface of the heating plate, including the reflected heat. The angle and the length of the combustion chamber walls are determined to achieve the greatest amount of reflection toward the heating plate 410 depending on the dimensions of the combustion chamber, and the height and length of the heating plate 410, . The angled combustion chamber walls 418 provide for an increase in reflection of heat 416 to the heating plate 410. Additionally, the angled combustion chamber walls 418 allow for the fire box to be swapped out with a burner. A further advantage of the angled combustion chamber walls 418 is that ash will preferentially fall to the bottom of the combustion chamber 414, thus simplifying the collection and the removal of ash from the combustion chamber 404.

The heating plate 410 may be coupled with any power conversion or chemical process 412 for use in power generation. As described above, one power generating device can be a Stirling engine. In one embodiment, the Stirling engine 104 can operate more efficiently with a higher hot end temperature, such as 850°C. Accordingly, the heating plate 410 should be suitable to withstand temperatures in excess of 850°C. The V-shaped combustion chamber 404 as well as the use of force draft combustion air will be able to achieve such temperatures.

A recuperator 402 can transfer heat from the combustion gases 408 to the combustion air 406, thus increasing performance. In one embodiment, the recuperator 402 is placed around the base of the combustion chamber 404. Locating the recuperator 402 around the stove 400 base leads to better stability due to a wider base and increased mass on the lower section of the stove 400. The combustion gases 408 can be ducted through the recuperator 402 located around the base of the V-shaped combustion chamber 404.

Referring to FIGURE 5, a diagrammatical illustration of one embodiment of a recuperator 402 is shown for the stove illustrated in FIGURE 4, FIGURE 5 illustrates one side of the recuperator 402. However, it is to be appreciated that the opposite side may be constructed similarly. Furthermore, the stove 400 of FIGURE 4 may have a recuperator 402 on one, two, three, or all four sides. The recuperator 402 is shown next to the triangular shaped combustion chamber 404

One representative embodiment of a recuperator 402 includes a shell and tube flow heat exchanger. However, in other embodiments, a recuperator can be a series of flat plates with fins in the gas stream to increase heat transfer. The gas can be arranged to flow countercurrent with respect to the incoming air to the combustion chamber. Countercurrent flow means the hot flue gas and the incoming air flow from opposite directions. In other embodiments, the hot flue gas and air flow is in a cross flow configuration, where flows are 90° offset from each other. In still other embodiments, the flow can be cocurrent where the flow is the same direction for the hot flue gas and the incoming air. Recuperators can have means to increase the amount of surface area to provide for greater amount of heat transfer. For example, fins can be included, in any square, triangular, or other type of geometry.

The recuperator 402 includes an inlet duct 432 for incoming air 406. The duct 432 is fitted with a fan to provide forced air combustion for the stove. The fan may be controlled similarly to the fan 118 shown and described in association with FIGURES 1-3. The incoming air duct 432 leads to the shell side 442 of the recuperator 402. The duct 432 passes through a wall 436 that separates the air 406 from the flue gas 408. The duct 432 then leads into the shell side 442 of recuperator 402. On the shell side, the air 406 may flow across at 90° to the tubes 422 carrying the hot flue gas, and the air 406 may flow countercurrent to the tubes 422 carrying the flue gas. The shell side 442 leads to the air outlet 424 that allows air 406 into combustion chamber 404.

The plate 430 supports one end of the combustion gas tubes 422 at the side where the combustion flue gas enters from the combustion chamber 404. The triangular-shaped combustion chamber 404 is defined by the angled wall 418 which reflects radiative heat to a heating plate 410, as shown in FIGURE 4. Behind the angled wall 418, a manifold 438 is created for the combustion gas to enter one of the plurality of tubes 422. The combustion gases may flow from the combustion chamber 404 through an opening created by the upper end of the angled wall 418 and the slanted wall 430. Combustion gases then flow downwardly through the manifold 438 and into the plurality of tubes 422. After passing through the tubes 422, the hot combustion gases enter a manifold 420 created by the wall 436 separating the air in the shell side 442 from the manifold 420. The manifold 420 directs the combustion gas coming from tubes 422 to the exterior of the recuperator 402 through the flue gas outlet 408.

Referring to FIGURE 6, a view of the recuperator 402 is shown, without the angled combustion chamber wall 418. However, in other embodiments, the wall 430 can be the angled combustion chamber wall. In FIGURE 6, the combustion gas tubes 422 are seen more clearly spaced in an array. The tubes 422 may have fins 426 running longitudinally inside and outside of the tubes 422. The outline 428 denotes the flue gas duct leading to the exterior, and is connected to the manifold 420 that receives the flow of hot flue gas from the exit end of the combustion gas tubes 422. Meanwhile, the incoming air is blocked from entering the flue gas manifold 438 by the wall 430 separating the combustion gas from the incoming air. The incoming air is therefore behind the wall 430, and the wall 430 directs the incoming air downward eventually leading to the air duct 424, which leads to the combustion chamber 404.

While one embodiment of a recuperator 402 is illustrated and described, it should be apparent to those skilled in the art that various design modifications may be made to the recuperator 402 to achieve heat transfer between the hot combustion gases and the incoming air used for combustion.

The V-shaped combustion chamber 404 may be used to burn biomass, liquid, or gas fuel, as described above. Heat is transferred to a heating plate 410 through an increase in radiative heat transfer, both through direct radiation from the burner to the heating plate 410 and reflected heat from the combustion chamber angled walls 418. Heat from the heating plate 410 is then used to generate electricity by a power conversion cycle. One such device that can be used is the Stirling engine described above.

The V-shaped combustion chamber also allows for ash to be collected and removed from the combustion chamber 404. Additionally, a higher temperature burner, such as for liquid or gas, could be placed in the combustion chamber 404 to burn liquid fuel with a higher flame temperature than wood. This would further increase the operating efficiency of the system due to better radiative coupling of the flame to the heating plate 410.

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.