WO2014149368A1

WO2014149368A1 - Biomass combustion device with a thermoelectric-powered control

Info

Publication number: WO2014149368A1
Application number: PCT/US2014/017710
Authority: WO
Inventors: Charles David STOKES; Michael John MANTINI; Rama Venkatasubramanian
Original assignee: Research Triangle Institute
Priority date: 2013-03-15
Filing date: 2014-02-21
Publication date: 2014-09-25

Abstract

A thermoelectrically-powered combustion control device or a stove add-on unit for insertion into an open door of a combustion chamber. The combustion control device and add-on unit include a reentrant tube entering the opening and extending into an interior region of the combustion chamber inside the interior wall and above the opening, a blower configured to force air through the reentrant tube and inject air into the interior region of the combustion chamber, a thermoelectric element which supplies power to the blower, and a control unit programmed to control power to the blower such that air is injected into the interior region of the combustion chamber above a first predetermined rate which induces turbulent mixing of the air with combustion products produced from burning the fuel and below a second predetermined rate where excess air beyond the second predetermined rate compromises carbon emission.

Description

BIOMASS COMBUSTION DEVICE WITH A THERMOELECTRIC-POWERED

CONTROL

BACKGROUND OF THE INVENTION

Field of the Invention:

The present invention relates to biomass combustion devices, and in particular to cooking stoves capable of burning solid fuels using forced convection to increase burning efficiency and reduce harmful emissions and provide electricity.

Discussion of the Background:

There is an ever increasing need for generating electricity using a variety of biomass combustion devices. Moreover, globally, more than three billion people depend on solid fuels, particularly biomass fuels, for cooking, heating, and lighting. The widespread use of biomass stoves is a leading source of indoor air pollution (IAP). This pollution is not benign.

According to the World Health Organization, IAP from the use of solid fuels in households in the developing world is responsible for nearly 2 million premature deaths each year.

Many stove designs have been developed to address cleaning burning stoves, especially through the use of powered or self-powered forced air convection. U.S. Pat. No. 3,868,943 describes a forced draft, solid fuel portable camping stove including a battery powered fan for forced convection of air to a combustion chamber. The fan is powered by a battery, and air is delivered to the combustion chamber via passages adjacent to the chamber to pre-heat the air. GB 2125160 describes a cooking stove having an air chamber into which air for combustion is drawn from the exterior of the stove either by natural convection or by a hand-operated air pump or combination of both. GB 2081888 describes a solid fuel heating stove in which a motor and fan is positioned within the exhaust flue of the stove driven by a thermocouple positioned on top of the stove. U.S. Pat. No. 5,544,488 describes a fan externally mounted to a heat source for distributing air heated by the heat source about a room, in which the fan is powered by a thermocouple on top of the heat source. U.S. Pat. Appl. Publ. No. 2009/0025703 describes a double wall stove design which reduces smoke emissions and provides for high temperature combustion by way of a thermoelectrically powered fan forcing air into a combustion chamber. U.S. Pat. No. 8,297,271 to Biolite describes a thermoelectric-powered fan providing forced air into a combustion stove through internal channels of a double wall stove design.

However, the improved cook stoves have failed to gain acceptance in the developing world because the new stoves were either too expensive, were not culturally appropriate, required too many behavior changes, or were not sustainable.

Providing reduced emissions for wood burning stove is a problem (which in general) has had been addressed through the use of complicated catalytic converter elements and complex burn boxes. U.S. Pat. No. 4,827,852 describes for example a catalytic wood stove. U.S. Pat. No. 4,862,869 describes for example a low emissions wood burning stove. Both of these patents utilize complicated catalytic converter elements and complex burn boxes.

Indeed, in the '869 patent, a catalytic cell formed a secondary combustion chamber within the stove which was in communication with the primary combustion chamber. A catalytic combustor was disposed in the secondary chamber for catalytically combusting the exhaust from the primary combustion. An exhaust path was formed in the catalytic cell, so as to extend from the cell inlet through the combustor to the cell outlet.

The patents and patent application publications listed above are all incorporated herein by reference in their entirety. SUMMARY OF THE INVENTION

In one embodiment of this invention, there is provided a thermoelectrically-powered combustion control device or a stove add-on unit for insertion into an open door of a combustion chamber. The combustion control device includes a reentrant tube entering the opening and extending into an interior region of the combustion chamber inside the interior wall and above the opening, a blower configured to force air through the reentrant tube and inject air into the interior region of the combustion chamber, a thermoelectric element which supplies power to the blower, and a control unit programmed to control power to the blower such that air is injected into the interior region of the combustion chamber above a first predetermined rate which induces turbulent mixing of the air with combustion products produced from burning fuel and below a second predetermined rate where excess air beyond the second predetermined rate compromises carbon emission.

In one embodiment of this invention, there is provided a stove add-on unit for insertion into an open door of a combustion chamber. The add-on unit includes a reentrant tube entering the opening and extending into an interior region of the combustion chamber inside the interior wall and above the opening, a blower configured to force air through the reentrant tube and inject air into the interior region of the combustion chamber, a

thermoelectric element which supplies power to the blower, and a control unit programmed to control power to the blower such that air is injected into the interior region of the combustion chamber above a first predetermined rate which induces turbulent mixing of the air with combustion products produced from burning fuel and below a second predetermined rate where excess air beyond the second predetermined rate compromises carbon emission.

It is to be understood that both the foregoing general description of the invention and the following detailed description are exemplary, but are not restrictive of the invention. BRIEF DESCRIPTION OF THE FIGURES

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Figure 1 A is a depiction of a fuel burning stove of this invention;

Figure IB is a depiction of the generic combustion chamber add-on device of this invention in an embodiment where the blower or fan is upstream to the heat-sink and depicts optional sensors;

Figure 1 C is a depiction of a fuel burning stove with forced air circulation across the heat sink and into the reentrant tube;

Figure ID is a comparison of indoor particulate matter PM concentrations between different stoves and the fuel burning stove of this invention;

FIG. 2A is a perspective view of a reentrant tube according to the invention providing air to a region above where fuel is placed in a fuel burning stove;

FIG. 2B is another view of a reentrant tube according to the invention providing air to a region above where fuel is placed in a fuel burning stove;

FIG. 2C is another view of a reentrant tube according to the invention providing air to a region above where fuel is placed in a fuel burning stove;

FIG. 2D is a schematic showing an expanded view of three different heat sink/heat pipe configurations;

FIG. 2E is a schematic showing the stacking of a high temperature TE device in series with a low temperature TE device, both in thermal contact to the heat capture probe;

FIG. 2F is a schematic showing is a schematic showing the placement of a high temperature TE device apart from a low temperature TE device; FIG. 2G is a schematic showing a TE device attached to an earthened heat sink;

Figure 3 is a system diagram according to the invention for power management and control of a fuel burning stove;

Figure 4 is a graphical depiction of the particulate matter stove emissions;

Figure 5 is a graphical depiction of CO stove emissions;

Figure 6 is tabular showing of additional operational results obtained with the invention;

Figure 7 is a plot of carbon emission monitoring;

Figures 8A and 8B are charts providing comparative data;

Figure 9 is an exemplary computer system for implementing various embodiments of the control unit of this invention; and

Figure 10 is an exemplary microprocessor for implementing various embodiments of the control unit of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Smokeless woodstoves using fan assistance are known in the art, but investigations by the inventors have identified a significant number of disadvantages with prior cooking stoves. These disadvantages include one or more of: (i) unreliability and a tendency for fans to burn out, (ii) a lack of good heating control, (iii) a cost of manufacture that is incompatible with the under-developed and developing parts of the world, (iv) a less than optimum 'smokeless' performance especially during a warm up phase of the stove, and (v) a requirement for service parts such as replacement batteries that is not ideal for use in remote and underdeveloped parts of the world. This invention addresses deficiencies of prior art devices by providing a fuel burning device (e.g., a wood-burning stove) or a fuel burning stove add-on or insert for insertion into a pre-existing opening in cooking stove.

Figure 1 A is a depiction of a fuel burning stove (e.g., a biomass stove) with forced air convection for improved combustion. With reference to FIG. 1 A, a fuel burning stove generally includes a housing 11 surrounding a combustion chamber having in upper portion an open top end for use as a cooking surface. The housing 11 includes a door 16 exposing a combustion chamber 18. The door (or opening) is typically on a side used for fuel and air supply to the combustion chamber.

When the fuel burning device of FIG. 1 A is configured as an insert (to be discussed in more detail below), this device that can be used with an existing biomass cook stove to enhance combustion by injecting air into the combustion chamber. The device shown in FIG. 1 A includes the following components: a heat conduit 2, a thermoelectric (TE) device 4, a heat sink - heat rejection component 6, a blower 8, a reentrant air injection tube 10, control unit 12 programmed (in hardware or software) to control stove performance, and an optional energy storage 14 such as a battery for startup or extended operation. The fuel burning device is configured as an insert or can be an integral part of a stove having an open door 16 exposing a combustion chamber 18. As an insert, the insert can have its heat conduit 2 and reentrant air injection tube 10 installed in a side opening or a top opening.

The operation of the device includes a power generation component, an air injection component, and a control component.

The power generation component operates by capturing heat from the combustion chamber 18 through the heat conduit 2, transmitting that heat to one or more TE devices 4, rejecting the heat through the heat sink 6 to create a difference in temperature across the TE device 4 which converts heat to electricity. The air injection component operates by drawing air across the heat sink 6 using a blower 8, and forces the air through the reentrant air injection tube 10 into the combustion chamber 18 to a region above where the fuel or biomass is placed. In the air injection process, the air is pre-heated (e.g., in the reentrant air injection tube 10) which enhances combustion by creating flow of pre-heated air into the region above where the fuel or biomass is placed so as not to thermally cool the flame temperature.

The reentrant air injection tube 10 in one embodiment injects air into to a region above where the fuel is placed in order to better mix the combustible species. The pre-heated air (e.g., in the reentrant air injection tube 10) enhances combustion by creating turbulent mixing that leads to more complete combustion of gases in the region above where the fuel or biomass is placed. Turbulent mixing is generated when the rate of air injection into the region above the fuel supply exceeds a threshold for creating the necessary mixing. The inventors have found for a typical "rocket style" stove with a draft that ranges from 25-50 CFM, the air injection flow required to create turbulent mixing is between 1-2 CFM (or 28.3- 56.6 L/min).

Figure 1 A shows a pictorial boundary around components of this invention which form thermoelectric enhanced cookstove add-on (TECA) device 20. This "add-on" device whether permanently installed or not on a commercially manufactured stove or whether added to an existing cook stove includes components such as TE device 4, control unit 12, heat sink 6, blower 8, and reentrant tube 2.

The control component in control unit 12 includes electronics programmed for optional energy conversion, storage, and programmed for regulation the blower 8 throughout the cooking cycle to maintain optimal performance (i.e., reduced carbon emissions for the fuel stock being used). The electronics of control unit 12 can include pressure and other sensors to detect cooking. The electronics of control unit 12 can also include carbon or soot detectors providing feedback to the control unit and sensors to detect cooking. In one example, voltage from the TE device 4 in one embodiment can be used as a surrogate measure (sensor) of combustion chamber temperature, and is used to predict stages in the combustion cycle (startup, boil, simmer, etc.). The control unit 12 can be programmed to adjust the blower output to minimize harmful emissions throughout the stove's operating cycle.

TE device 4 can include any thermoelectric element (or multiple thermoelectric elements) that converts heat energy to electrical energy, such as a thermocouple or Peltier element. Such thermoelectric elements conventionally generate a voltage based on the thermal gradient across the device between a first and second active surface thereof. TE device 4 provides electrical power to the electronics in control unit 12 and blower 8. In use, the blower provides air into reentrant air injection tube 10.

The blower 8 can be placed close to the combustion chamber, can use a low cost motor typically including plastic components, and can be protected from the heat of the stove by an optional heat shield e.g., a reflector such as aluminum foil (not shown).

In one embodiment, blower 8 can be a 1 W brushless DC fan driven by a 1 to 7 V power supply (not shown), compatible with a 5 V motor. In another embodiment, blower 8 can be a 12 V driven fan operated by an 8-14 V power supply. The power supply may be an internally mounted battery accessible from the base of the stove, or may be an external supply where available, or may be a thermoelectric power generation device with electronic components to boost and/or regulate the voltage to the blower.

Figure IB shows an embodiment of a generic combustion chamber add-on device using the thermoelectric enhanced cookstove add-on (TECA) device 20 as the added-on device where a blower or fan 8' cools the heat-sink 6. By cooling the heat sink 6, one side of the thermoelectric element is maintained at a substantially lower temperature than would otherwise be the case, which increases the power output available from TE devices 4 and thus increases the available airflow to the combustion chamber.

The exact mechanisms for cooling the heat sink are not critical and any methods described in the above-noted patents and patent application publications can be used.

Regardless of the cooling configuration, the hot-side of the TE device is attached to a hot (or warm) surface plate 19 to couple heat from a combustion chamber to the TE device. Figure IB also shows optional sensors 20 that can be integrated with the control electronics 12 to provide additional feedback for controlling the air flow to enhancing combustion

performance. These sensors 20 could include a thermocouple or other temperature measurement device, a pressure transducer, a proximity detection device, such as a passive infrared detector, infrared rangefinder, or ultrasonic rangefmder. Specifically, the ability to detect proximity of a person operating the combustion device could provide valuable information to help determine the level of emissions exposure that a person is receiving. Sensors 20 can also include soot detectors, carbon monoxide detectors, and particle detectors.

Figure 1C is a depiction of a fuel burning stove with forced air circulation across the heat sink and into the reentrant tube. In a preferred embodiment, the reentrant tube has a gap covered with a thermally insulative coupling 30 (cover or sleeve) which minimizes heat flow back the heat sink and control electronics. In one embodiment, as shown here, a blower 8' provides air flow across the heat sink and then into the reentrant pipe and forward into the combustion chamber.

Figure ID shows a comparison between a traditional three-stone fire stove with the pot held on the three stones, a standard Envirofit-brand stove made by Envirofit International of Fort Collins Colorado, the enhanced stove of the present invention, and a Philips-brand fan stove made by Phillips Corporation. The results show that the enhanced stove of the present invention was able to reduce the indoor particulate matter (PM) concentration to nearly the level of the best Philips fan stove. In this case, the enhanced stove of the present invention has the advantage of being a continuous-feed stove that does not require the user to remove the pot to add more fuel like the Philips-brand stove, or fundamentally buy a new stove.

Figures 2 A and 2B show different embodiment for the insert described above. More specifically, both of these figures provide respective perspective views of inserts which each include a reentrant tube providing air to a region above where the fuel is placed. The reentrant tube in one embodiment enters the open door near a side of the open door and provides air to a region above where the fuel is placed. The reentrant tube in one

embodiment has a perforated upper section introducing air at circumferential points radially inward to the region above where the fuel is placed (Figure 2B). The reentrant tube in one embodiment introduces the air into a region above where the fuel is disposed. The reentrant tube in one embodiment has a non-perforated section disposed toward an interior wall of the combustion chamber such that air is preheated before being introduced to the region above where the fuel is placed (Figure 2C).

Providing an insert such as the reentrant tubes described above to a pre-existing cook stove serves to reduce harmful emissions. As noted above in the Background, providing reduced emissions for wood burning stove is a problem (which in general) has had to have been addressed through the use of complicated catalytic converter elements and complex burn boxes.

Natural draft cook stoves only generate moderate air flow or draft through natural convection where the heat generated by combustion travels upward through the combustion chamber. The limited air flow or draft generated by natural convection is not enough to create adequate mixing of the incomplete combustion species. By adding air injection into the stove, one is able to accomplish two things. First, oxygen-rich air is added to aid combustion of the solid fuel. Secondly, the additional air flow can be used to enhance mixing of the incomplete combustion species with the flames. This enhanced mixing helps to burn more of the gases to create more complete combustion and fewer harmful emissions (less CO and particulates).

The present inventors have discovered that the onset of improved emissions occurs as the flow rate of air into the combustion flame starts to produce an audible noise considered to be indicative of the onset of air turbulence in the upper part of the combustion chamber. As the flow rate is further increased, the particulates generated from the burning and the CO emissions decrease. However, quite surprisingly, the inventors have found that too much air leads to a subsequent increase in particulate emission from the combustion chamber. It thus is advantageous from both a power-savings and an air quality point of view to operate above the onset of turbulent mixing and below the point where additional degrades or compromises the emissions.

This phenomenon of a preferred range of operation can be measured for each type of stove, and based on those characterizations the control unit can be programmed to operate between those two flow rates: a first predetermined rate which induces turbulent mixing of said air with combustion products produced from burning fuel and below a second predetermined rate where excess air beyond the second predetermined rate compromises carbon emission.

FIG. 2D is a schematic showing an expanded view of three different heat sink/heat pipe configurations. In one case, the heat sink base connects by way of heat pipe to cooling fins connected to the heat pipe. The heat pipe serves to effectively transfer the heat energy further away from the TE devices. In the other case, no heat pipe is used, and the fins are directly connected to the base. Thermally conductive greases and compounds can be used between interfaces of different components. These compounds are frequently organic materials loaded with a metal or other thermally conductive material. In the last case, liquid cooling is provided which might be available from a natural water source or a naturally running water source or in circumstances where there already exists a liquid circulating loop or a pressurized flow of water. The circulation of water could be useful for example beyond cooking applications as a source of heated water.

FIG. 2E is a schematic showing the stacking of a high temperature TE device in series with a low temperature TE device, both in thermal contact to heat conduit 2. The stacking of the TE devices can optimize the power production across a larger temperature differential.

FIG. 2F is a schematic showing is a schematic showing the placement of a high temperature TE device apart from a low temperature TE device. In this case, TE device A is thermally coupled to an upper region of combustion chamber 18. Conversely, TE device B is thermally coupled to a lower region of combustion chamber 18. In one specific example of this embodiment, TE device A would be a high temperature TE device mounted to the inner surface of the combustion chamber wall. This device would employ a heat-pipe heat sink to transmit heat through the insulation around the combustion chamber to an area external to the stove. This approach takes advantage of the use of high temperature TE devices (such as half-Heusler or PbTe/TAGS alloys) that can withstand the higher temperatures of the combustion chamber walls. In this specific example, TE device B would be a low

temperature or mid-temperature TE device that couples to heat that is less effectively transferred from the combustion chamber through a heat collection rod to this low

temperature or mid-temperature TE device.

FIG. 2G is a schematic showing a TE device attached to an earthened heat sink. The TE device is shown connected to a heat conduit 2 extending along the bottom of combustion chamber 18, but heat conduit 2 could be placed to collect heat from other parts of combustion chamber 18. The "earthened" heat sink extends into the sub-surface. In one embodiment, a heat pipe could be used to carry heat from the TE device into the ground. In this configuration, no air flow (no blower) is needed to reject heat. The power could then be used for whatever purpose - recharge batteries, power a light, or power the blower 8 for the reentrant tube.

Accordingly, the devices of this invention represent a number of unique

configurations providing emission control of open air biomass burners. In one embodiment, by way of an add-on device (that is not a part of the stove but made to interface with an existing stove or combustion chamber), emissions are controlled to acceptable levels. In one embodiment, by way of an installed device which is part of the stove, emissions are controlled to acceptable levels.

Accordingly, the add-on device in one embodiment would constitute a stove insert for insertion into an open door, open hole, or top opening of an existing fuel burning stove. The add-on device or the installed device in one embodiment would have a reentrant tube entering the open door, open hole, or top opening, a blower configured to force air through the reentrant tube into the combustion chamber, an optional rechargeable electrical power source which drives the blower, a thermoelectric element which supplies power to the blower and optionally to the rechargeable power source and/or to an external load (e.g. light, radio, or cell phone), and a control unit programmed in hardware or software to control power to the blower such that air flow into the combustion chamber produces minimal carbon soot (particulate) emissions and does not compromise carbon emission.

In one embodiment, the blower would include an electric motor and an impeller forcing air heated through the reentrant tube into the combustion chamber. In one embodiment, the blower would include a speed sensor for use in regulating the flow of injected air.

The reentrant tube in one embodiment would enter the open door, open hole, or top opening near a side of the open door and would provide moving air to the region above where the fuel is placed in order to better mix the combustible species. The reentrant tube in one embodiment would have a perforated section introducing air radially, and/or upwardly, and/or angled inward to the region above where the fuel is placed in order to better mix the combustible species.

The reentrant tube in one embodiment would have a non-perforated section disposed toward an interior wall of the combustion chamber such that air is preheated before being introduced to the region above where the fuel is placed and such that the air is introduced to create circulating flow within the combustion chamber in order to better mix the combustible species. The reentrant tube in one embodiment would be non-fixedly (not permanently) attached to the open door, the open hole, or the top opening of the stove.

The add-on device or the installed device in one embodiment includes a heat conductor disposed between the combustion chamber and the thermoelectric element. The heat conductor makes a first thermal connection to a first side of the thermoelectric element. The add-on device in one embodiment includes a heat sink making a second thermal connection to a second side of the thermoelectric element in order to maintain a temperature differential across the thermoelectric element. The heat sink in one embodiment includes a set of fins to dissipate heat from the thermoelectric element. The heat sink in one embodiment includes a heat pipe to transport heat to a larger finned surface area to more effectively dissipate heat from the thermoelectric element.

In one embodiment, the rechargeable power source is a rechargeable battery. In one embodiment, the rechargeable power source is a capacitor. The control unit is programmed to manage the recharging power to the rechargeable power source. In one embodiment, the control unit is programmed to provide auxiliary power for user consumption.

The add-on device or the installed device in one embodiment includes as part of the control unit a switch for setting the power to the blower to a selectable flow rate control based on a size and configuration of the combustion chamber. The add-on device in one embodiment includes as part of the control unit an algorithm (programmed in software and/or hardware) for controlling the power to the blower to a particular flow rate range based on a size and configuration of the combustion chamber or the type of stove (as described above). The add-on device in one embodiment employs an air-speed sensing signal from the blower to allow closed-loop feedback control over the air flow rate such that if a cooking pot is removed from the stove's top opening, the change in air flow dynamics will not cause the fan to run excessively fast.

The add-on device or the installed device in one embodiment includes a DC-DC converter connected to the thermoelectric element and configured to convert a lower voltage DC power from the thermoelectric element to a regulated higher voltage DC power. The add-on device in one embodiment includes a DC-DC converter and a DC-AC inverter. The DC-DC converter is connected to the thermoelectric element and is configured to convert DC power from the thermoelectric element to a regulated higher voltage DC power. The DC- AC inverter is configured to convert the higher voltage DC power into AC power.

The add-on device or the installed device in one embodiment includes a baffle (for example a solid baffle) disposed below a top surface opening of the combustion chamber to deflect combustion gases and flames from the fuel into un-combusted air entering the combustion chamber from the reentrant tube. The add-on device or the installed device in one embodiment includes a baffle (for example a solid baffle) disposed below a top surface opening of the combustion chamber to confine flames toward the center of the cooking surface in order to maximize heat transfer to the cooking surface. The add-on device or the installed device in one embodiment includes a multiple finned baffle disposed below a top surface opening of the combustion chamber to enhance mixing of combustion gases. The add-on device or the installed device in one embodiment includes for the thermoelectric element a high-temperature thermoelectric material configured to operate at temperatures from room temperature to 800 °C. The high-temperature thermoelectric material can be at least one of the following: Half-Heusler (HH) alloys (n-type

Hf_{0 6}Zr_{0 4}NiSn₀ 99₅Sbo.oo₅ HH alloys and p-type Hf_{0 3}Zr_{0 7}CoSno_.3Sb_0.7/nano-Zr0₂ composites which can attain ZT values of about 1.05 and 0.8 near 900-1000 K, respectively), PbTe alloys, PbTe/TAGS alloys, and SiGe alloys.

Regarding the Half-Heusler (HH) alloys, Half-Heusler (HH) phases are known to be a versatile class of alloys with promising thermoelectric properties. From the perspective of thermal and electronic transport properties, the HH alloys have a relatively high figure of merit (ZT) at 800-1000 K. For example, n-type Hf₀.₆Zro.₄NiSno.99₅Sb₀.₀₀5 HH alloys and p- type Hf_{0 3}Zr₀.₇CoSno.3Sb₀.₇/nano-Zr0₂ composites can attain ZT = 1.05 and 0.8 near 900- 1000 K, respectively. Reported tests performed on p-n couple devices from the HH materials achieved a power generation efficiency of 8.7% for hot-side temperatures of about 700 °C. Thus, HH modules are more robust and produce more power than lower temperature TE modules.

Accordingly, in one embodiment of the invention, a combustion chamber device would be thermally connected to high-temperature HH modules for power generation. In one embodiment of the invention, a combustion chamber device would only be thermally connected to high-temperature HH modules for power generation.

The add-on device in one embodiment can be adjusted or modified during installation to operate optimally with specific stoves. Predetermined flow rate conditions developed for example for experimentation with the specific stove and the convention fuel stock being used can be selected by program at a user interface or by preset pots or control selection devices. The active control over the combustion process differs from previously developed stoves that simply operate in a limited number of states controlled only by a switch.

In one embodiment of the invention, the power management and control algorithms programmed into control unit 12 include options for a microprocessor controller with maximum power point tracking (MPPT). Maximum power point tracking provides electrical impedance matching between the thermoelectric device and the load in order to minimize losses from impedance mismatch. For thermoelectric devices, MPPT control is typically achieved by a continuous cycle that momentarily samples the open-circuit voltage (V_oc) of the TE device (where no load is connected) and adjusts the electrical impedance of the load to keep the under-load voltage output of the TE close to V_oc/2. In doing this, TE power losses due to electrical impedance mismatch are minimized. The matching of electrical impedance can be done through microcontroller adjustments to a DC-DC converter. In one embodiment of this invention, the MPPT control electronics would deliver optimal power to the blower. In one embodiment of the invention, the MPPT control electronics would deliver optimal power to the rechargeable power source. In one embodiment of the invention, a

microprocessor controller would adjust the DC-DC converter to control the blower voltage and control the flow rate of injected air.

Figure 3 is a system diagram for power management and control. Two embodiments are shown where a thermoelectric (TE) device is generating electricity that feeds into DC-DC converter electronics and also into the blower motor. A microprocessor controller is powered by the DC-DC converter or the energy storage device to provide control over the energy storage and blower motor. The motor control electronics can be a DC-DC converter that regulates the input voltage to the blower or a pulse-width modulation control that provides an adjustment to the duty cycle of the blower. In one aspect of this invention, thermoelectric add-on device is able to produce power in excess of 700 mW, which permits the powering of small electronic devices, including the ability to recharge cell phones or power simple household lights.

WORKING EXAMPLES

Bench-scale Prototype: The combustion enhancing reentrant tube insert has been tested as a bench-scale prototype for the Envirofit G-3300 commercial stove and produced the following results

Four separate tests were performed: cold-start with 17.5 L/min air injection, hot-start with 17.5 L/min air injection, hot-start with 25.3 L/min air injection, and hot-start with no air injection (control). The air injection reentrant tube insert had dimensions of 1.25 cm diameter with a length of approximately 25 cm.

This air injection reentrant tube insert was able to reduce black carbon concentrations by 70% and achieve a 55% reduction in particulate matter (PM) emissions over the standard stove's operation (see chart below). This result represents an overall particulate emissions reduction of over 85% as compared to the traditional three-stone fire. Power generation occurred with a thermoelectric device part number 12711-9L31-03 CL from Custom

Thermoelectric, Bishopville, MD, and ranged from 853mW to 1010 mW for the tests while the blower required from 102 mW to 235 mW of power to operate, resulting in a net power gain which could be used to recharge the battery or power auxiliary equipment. Working Prototypes: An integrated prototype has been installed on an Envirofit G- 3300 stove. In addition, an add-on prototype was added to a Kenya Rocket Jiko stove. The data obtained from these prototypes are shown in Figures 4 and 5 and the table below. Figure 4 shows a particulate matter reduction of 42% reduction in PM2.5 concentration and 33% reduction in PM10 concentration for low voltage blower operation. Figure 5 shows a CO reduction of 53% reduction in the Rocket Jiko with the thermoelectric enhanced cookstove add-on (TEC A) device operating with a low air flow (17.5 L/min).

Performance data of the Rocket Jiko are shown in the Table contained in Figure 6.

The TECA device has demonstrated an 85% reduction in toxic emissions for PM and black carbon when utilized with the widely sold "rocket" stove design. The air injection addon has also shown an effective improved stove, with reduced emissions about 50%, reduced black carbon emissions by 70%, and reduced PM emissions by 50%, representing an overall reduction in PM emissions of more than 85% compared to a three-stone stove. Emission measurements of the resulting indoor concentration levels using the MicroPEM™ device has confirmed commensurate reductions in exposure levels, as shown in Figure 7. Details of the MicroPEM™ device are found in PCT/US2012/062167, the entire contents of which are incorporated herein by reference. The MicroPEM™ (Micro Personal Exposure Monitor) is a lightweight device (less than 300 g) that includes a micro-miniature nephelometer for continuous real-time particle concentration monitoring and utilizes integrated filter collection to enable conformation post-analyses and to provide a more accurate picture of personal exposure levels. Simpler real-time sensors such as a carbon monoxide detector could also be used to measure the change in emissions. Figures 8A and 8B are charts providing more comparative data of particulate matter levels for 2.5 micron particles in the indoor air around a traditional "three-stone fire" stove, a Rocket Jiko stove, and a Rocket Jiko using the TECA device of this invention.

This invention thus provides for the following capabilities:

1) An add-on device to enhance performance of existing biomass cook stoves to minimize indoor pollution. Many previous designs specifically required integration with a stove that included modifications and fixedly attaching components to the stove. The advantage of an add-on device is that it can be used with an existing stove without requiring modification of the original stove or physical attachment to the stove.

2) Use of high temperature thermoelectric devices for power generation from a biomass stove - to achieve greater thermal-to-electric conversion efficiency with materials other than Bi₂Te₃. One specific higher temperature thermoelectric material would be Half-Heusler (HH) alloys (n-type Hfo.6Zro.4NiSno.99sSbo.oos alloys and p-type Hf₀.₃Zr_{0 7}CoSn_0.3Sbo.7/nano-Zr0₂ composites), which can attain ZT values of about

1.05 and 0.8 near 900-1000 K, respectively, and thus are capable of operation at temperatures from room temperature to 600 °C. In addition PbTe alloys, PbTe/TAGS alloys, and SiGe alloys also provide enhanced thermal-to-electric conversion efficiency and stability at higher temperatures (above 300°C).

3) Use of intelligent control over the combustion process for a biomass stove - to improve startup and operation of the stove by actively adjusting air flow, to achieve high fuel efficiency and/or to reduce harmful emissions (such as particulate matter and CO). The present inventors have discovered that simply adding air flow to a stove's combustion chamber will not automatically improve combustion to achieve a reduction in harmful emissions. Too much air flow can cause an increase in harmful emissions beyond the nominal operation of the stove. For this reason, feedback control over the air flow can optimize combustion at the different firepower levels associated with higher or lower fuel feed rates or different types of fuel. In one embodiment, the carbon sensor described above (the MicroPEM™ device) provides feedback to control unit 12. 4) Use of active power management for a biomass stove to maximize power generation, energy efficiency, fan power requirements, to minimize indoor pollution such as black carbon concentrations and other particulate matter (PM) emissions with cook stoves as well as for other convenience in outdoor cook stoves.

5) Use of heat pipes for both heat capture from a biomass stove and heat rejection to allow the TE device and or heat sink to be positioned farther away from the heat source (combustion chamber). The use of heat pipes allows the ability to transmit heat collected from the combustion chamber or cooking surface and deliver it to the hot-side of the TE device. This allows the components (TE, blower, electronics, etc.) to be located farther from the heat of the stove which can cause degradation.

6) In another embodiment, heat pipes also provide the capability of

transmitting heat from the cold-side of the TE device to heat rejection fins that create a larger heat rejection surface area for dissipating thermal energy. This increase heat rejection surface allows the cold-side of the TE device to achieve a lower temperature with low air flows across the heat rejection surface. This, in turn, enhances

performance (larger Delta T and power generation) and minimizes degradation to the TE device.

7) Use of sensors or collected electrical data such as, for example, output power from the thermoelectric element and/or power and flow rate of blower to provide a measure of cooking load feedback to allow active control over the cooking process.

8) Pressure sensors as weight detectors can be used to interlock the auxiliary power output from the thermoelectric element when a biomass stove is not being used to cook.

9) A biomass electric generator (not necessarily used for cooking) but which would be used primarily for power generation by converting heat from biomass combustion to electricity.

10) Use of active electric load management for a TE device used in a biomass stove to maximize power generation. Closely matching the electrical impedance of the TE device and the load (blower and/or electronics) can minimize energy losses to make the system more efficient.

10) The integration of data capture and storage components to record stove usage and performance data in a memory of control unit 12. 11) Use of proximity sensors to monitor and record data that can be used to estimate a person's exposure to harmful emissions.

Additional features of this invention include:

1) Air-injection tube made of suitable metal or ceramic and optimally injects at various locations of the combustion chamber;

2) Air-injection tube helps to either create a turbulent or laminar mixing, depending on fuel used or height at which air is injected;

3) The blower can be replaced with an electric motor and fan to move the air;

4) The fuel-feed is neither vertical (top-loading, where it is difficult to add fuel) or horizontal (requiring someone to constantly push the fuel in as it is consumed) but at a suitable inclination (e.g., from 10 degrees to 45 degrees from horizontal) so that the fuel is side-fed with gravity-feed assistance;

5) In the gravity-feed embodiment, the grate holding the fuel on the incline is preferably made of a material such as porcelain steel where the smooth surface, the high temperature corrosion resistance, and the high temperature strength of the porcelain steel are advantages;

6) In the gravity-feed embodiment, ash collection is facilitated, thus keeping the area surrounding the cookstove or combustion chamber cleaner and more attractive to the user;

7) Heat is drawn from the combustion chamber using a metal or ceramic interface material; this can be a plate attached to the side of the combustion chamber, or it can be one rod or a combination of many rods like a multiple-fin that draws heat towards a TE device.

8) One or more TE modules located at various parts; a low-temperature module made of Bi₂Te₃-related materials located at low-temperature sections and a high-temperature module made of half-Heusler materials located at high-temperature sections, etc. Control Unit

FIG. 9 illustrates a computer system 1201 for implementing various embodiments of the control unit of this invention. The computer system 1201 may be used as a processor in the control unit described above to perform any or all of the functions described above. The computer system 1201 includes a bus 1202 or other communication mechanism for communicating information, and a processor 1203 coupled with the bus 1202 for processing the information. The computer system 1201 also includes a main memory 1204, such as a random access memory (RAM) or other dynamic storage device (e.g., dynamic RAM

(DRAM), static RAM (SRAM), and synchronous DRAM (SDRAM)), coupled to the bus 1202 for storing information and instructions to be executed by processor 1203. In addition, the main memory 1204 may be used for storing temporary variables or other intermediate information during the execution of instructions by the processor 1203. The computer system

1201 further includes a read only memory (ROM) 1205 or other static storage device (e.g., programmable read only memory (PROM), erasable PROM (EPROM), and electrically erasable PROM (EEPROM)) coupled to the bus 1202 for storing static information and instructions for the processor 1203.

The computer system 1201 also includes a disk controller 1206 coupled to the bus

1202 to control one or more storage devices for storing information and instructions. The computer system 1201 may also include special purpose logic devices (e.g., application specific integrated circuits (ASICs)) or configurable logic devices (e.g., simple

programmable logic devices (SPLDs), complex programmable logic devices (CPLDs), and field programmable gate arrays (FPGAs)).

The computer system 1201 performs a portion or all of the processing steps (or functions) of this invention in response to the processor 1203 executing one or more sequences of one or more instructions contained in a memory, such as the main memory 1204. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, embodiments are not limited to any specific combination of hardware circuitry and software.

As stated above, the computer system 1201 includes at least one computer readable medium or memory for holding instructions programmed according to the teachings of the invention and for containing data structures, tables, records, or other data described herein. Stored on any one or on a combination of computer readable media, the invention includes software for controlling the computer system 1201, for driving a device or devices for implementing the invention, and for enabling the computer system 1201 to interact with a human user. Such software may include, but is not limited to, device drivers, operating systems, development tools, and applications software. Such computer readable media further includes the computer program product of the invention for performing all or a portion (if processing is distributed) of the processing performed in implementing the invention.

The computer code devices of the invention may be any interpretable or executable code mechanism, including but not limited to scripts, interpretable programs, dynamic link libraries (DLLs), Java classes, and complete executable programs.

The term "computer readable medium" as used herein refers to any medium that participates in providing instructions to the processor 1203 for execution. A computer readable medium may take many forms, including but not limited to, non- volatile media, volatile media, and transmission media.

The computer system 1201 can also includes a display controller and a

communication interface 1213 coupled to the bus 1202. The communication interface 1213 provides a two-way data communication coupling to a network link 1214 that is connected to, for example, a local area network (LAN) 1215, or to another communications network 1216 such as the Internet. For example, the communication interface 1213 may be a network interface card to attach to any packet switched LAN. As another example, the

communication interface 1213 may be an asymmetrical digital subscriber line (ADSL) card, an integrated services digital network (ISDN) card or a modem to provide a data

communication connection to a corresponding type of communications line. Wireless links may also be implemented. In any such implementation, the communication interface 1213 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

These networking and linking capabilities are important for the collection of performance data of the stoves at various locations, providing a valuable tool for survey analysis of biomass cooking and indoor air quality.

The network link 1214 typically provides data communication through one or more networks to other data devices. The signals through the various networks and the signals on the network link 1214 and through the communication interface 1213, which carry the digital data to and from the computer system 1201 maybe implemented in baseband signals, or carrier wave based signals. The baseband signals convey the digital data as unmodulated electrical pulses that are descriptive of a stream of digital data bits, where the term "bits" is to be construed broadly to mean symbol, where each symbol conveys at least one or more information bits. The digital data may also be used to modulate a carrier wave, such as with amplitude, phase and/or frequency shift keyed signals that are propagated over a conductive media, or transmitted as electromagnetic waves through a propagation medium. The computer system 1201 can transmit and receive data, including program code, through the network(s) 1215 and 1216, the network link 1214, and the communication interface 1213. Moreover, the network link 1214 may provide a connection to a mobile device 1217 such as a personal digital assistant (PDA) laptop computer, or cellular telephone. FIG. 10 illustrates a microcontroller system for implementing various embodiments of the control unit of this invention. The microcontroller system 1301 may be used as a processor in the control unit described above to perform any or all of the functions described above. The microcontroller system 1301 includes one or more buses 1302 or other communication mechanism for communicating information, and a reduced instruction set central processing unit (RISC CPU) 1303 coupled with the buses 1302 for processing the information. The microcontroller system 1301 also includes memory 1304, such as volatile random access memory (RAM) or non-volatile Flash or Ferroelectric Random Access Memory (FRAM), coupled to the bus 1302 for storing information and instructions to be executed by processor 1303. In addition, the memory 1304 may be used for storing temporary variables or other intermediate information during the execution of instructions by the processor 1303. The microcontroller system 1301 further includes interfaces for analog peripherals 1305 and digital peripherals 1306. These analog 1305 and digital 1306 peripherals allow interfacing with the electrical components in the overall system, such as a DC-DC converter, sensors, motor control circuitry, etc.

Generalized Aspects of the Invention

The following numbered statements reflect various generalized aspects of this invention.

Statement 1. A thermoelectrically-powered combustion control device comprising: a reentrant tube entering the opening and extending into an interior region of a combustion chamber inside the interior wall (and optionally above the opening); a blower configured to force air through the reentrant tube and inject said air into said interior region of the combustion chamber; a thermoelectric element which supplies power to the blower; and a control unit programmed to control power to the blower such that said air is injected into said interior region of the combustion chamber above a first predetermined rate which induces turbulent mixing of said air with combustion products produced from burning fuel and below a second predetermined rate where excess air beyond the second predetermined rate compromises carbon emission.

Statement 2. The device of statement 1, further comprising a carbon sensor providing feedback to the control unit for adjustment of air flow into the reentrant tube.

Statement 3. The device of statement 1, wherein the carbon sensor comprises a CO detector or a particulate counter.

Statement 4. The device of statement 1, wherein the reentrant tube includes a thermal break disposed along a length of the reentrant tube which restricts heat conduction from the combustion chamber to at least one of the blower, the thermoelectric element, and the control unit.

Statement 5. The device of statement 1, wherein the thermal break comprises a thermally insulative coupling covering a break in the reentrant tube.

Statement 6. The device of statement 1, wherein the reentrant tube comprises a perforated section introducing said air into said interior region.

Statement 7. The device of statement 6, wherein the reentrant tube comprises a non- perforated section connected to the perforated section, the non-perforated section is disposed inside the combustion chamber such that said air is preheated before being introduced to the perforated section.

Statement 8. The device of statement 1, wherein the reentrant tube comprises radially ports which inject said air toward a center of the combustion chamber.

Statement 9. The device of statement 1, further comprising: a heat conductor disposed between the combustion chamber and the thermoelectric element and making a first thermal connection to a first side of the thermoelectric element; and a heat sink making a second thermal connection to a second side of the thermoelectric element in order to maintain a temperature differential across the thermoelectric element.

Statement 10. The device of statement 9, wherein the heat sink comprises a heat pipe connected to a set of fins cooled by air from the blower.

Statement 11. The device of statement 9, wherein the heat sink comprises a set of fins cooled by air from the blower.

Statement 12. The device of statement 9, wherein the heat sink comprises a heat pipe extending to an earthen mount thermally connecting the heat sink to ambient ground temperature.

Statement 13. The device of statement 1, further comprising a rechargeable power source connected to the thermoelectric element.

Statement 14. The device of statement 13, wherein the control unit is configured to provide recharging power to the rechargeable power source.

Statement 15. The device of statement 1, wherein the control unit is configured to provide auxiliary power for user consumption.

Statement 16. The device of statement 1, wherein the control unit comprises a switch for selection of a power to the blower to provide a pre-determined flow rate control based on a size and configuration of the combustion chamber.

Statement 17. The device of statement 1, wherein the control unit is programmed with an algorithm for controlling power to the blower to provide a pre-determined flow rate control based on a size and configuration of the combustion chamber.

Statement 18. The device of statement 1, further comprising a DC-DC converter connected to the thermoelectric element and configured to convert DC power from the thermoelectric element to a higher voltage DC power. Statement 19. The device of statement 1, further comprising: a DC-DC converter and a DC-AC converter; said DC-DC converter connected to the thermoelectric element and configured to convert DC power from the thermoelectric element to a higher voltage DC power; said DC-AC converter configured to convert the higher voltage DC power into AC power.

Statement 20. The device of statement 1, wherein the control unit is programmed with a maximum power point tracking algorithm to provide electrical impedance matching between the thermoelectric device and a load of the blower in order to minimize losses from impedance mismatch.

Statement 21. The device of statement 1, wherein the thermoelectric element comprises plural thermoelectric elements including a first thermoelectric element for operation at a first temperature and a second thermoelectric element for operation at a second temperature higher than the first temperature.

Statement 22. The device of statement 21, further comprising a third thermoelectric element for operation at a third temperature between the first temperature and the second temperature.

Statement 23. The device of statement 21, wherein the second thermoelectric element comprises a thermoelectric material including at least one of the following: Half- Heusler (HH) alloys, PbTe alloys, PbTe/TAGS alloys, and SiGe alloys.

Statement 24. The device of statement 23, wherein the Half-Heusler (HH) alloys comprise n-type Hfo.₆Zro.₄NiSn₀.99₅Sbo.oo₅ alloys and p-type Hf₀.₃Zr₀.₇CoSn_{0 3}Sb₀.₇/nano-ZrO₂ composites.

Statement 25. The device of statement 1, further comprising a proximity sensor for detection of a human in a vicinity of the combustion chamber. Statement 26. The device of statement 1, further comprising a cooking interlock to disable the auxiliary power output when not cooking.

Statement 27. A combustion unit comprising: a combustion chamber which holds fuel for combustion, has an opening for supply of the fuel, and has an interior wall; and the thermoelectrically-powered combustion control device of statements 1-26.

Statement 28. A combustion unit comprising: a combustion chamber which holds fuel for combustion and which has an opening for supply of the fuel; a reentrant tube entering the opening; a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube extending into a region inside the combustion chamber

(optionally above the opening); a thermoelectric element which supplies power to the blower and which includes a Half-Heusler (HH) alloy thermoelectric; and a control unit programmed to control power to the blower.

Statement 29. The unit of statement 28 including any of the elements recited in statements 2-26.

Statement 30. A stove add-on unit for insertion into an opening of a fuel burning device having a combustion chamber which holds fuel for combustion, the unit comprising: a reentrant tube entering the opening; a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube iextending into a region inside the combustion chamber (optionally above the opening); a thermoelectric element which supplies power to the blower and which includes a Half-Heusler (HH) alloy thermoelectric; and a control unit programmed to control power to the blower.

Statement 31. The unit of statement 30 including any of the elements recited in statements 2-26.

Statement 32. A combustion unit comprising: a combustion chamber which holds fuel for combustion and which has an opening for supply of the fuel; a reentrant tube entering the opening; a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube extending into a region inside the combustion chamber

(optionally above the opening); a thermoelectric element which supplies power to the blower; and a control unit programmed to interlock power off to the auxiliary output when the combustion chamber is not being used to cook.

Statement 33. The unit of statement 32 including any of the elements recited in statements 2-26.

Statement 34. A stove add-on unit for insertion into an opening of a fuel burning device having a combustion chamber which holds fuel for combustion, the add-on unit comprising: a reentrant tube entering the opening; a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube extending into a region inside the combustion chamber (optionally above the opening); a thermoelectric element which supplies power to the blower; and a control unit programmed to interlock power off to the auxiliary output when the combustion chamber is not being used to cook.

Statement 35. The unit of statement 34 including any of the elements recited in statements 2-26.

Statement 36. A combustion unit comprising: a combustion chamber which holds fuel for combustion and which has an opening for supply of the fuel; a fuel feed inclined at an angle from 10 degrees to 45 degrees from horizontal so that gravity assists delivery of the fuel into the combustion chamber; a reentrant tube entering the opening; a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube extending into a region inside the combustion chamber (optionally above the opening); a thermoelectric element which supplies power to the blower; and a control unit programmed to control power to the blower. Statement 37. The unit of statement 36 including any of the elements recited in statements 2-26.

Statement 38. A stove add-on unit for insertion into an opening of a fuel burning device having a combustion chamber which holds fuel for combustion, the unit comprising: a fuel feed inclined at an angle from 10 degrees to 45 degrees from horizontal so that gravity assists delivery of the fuel into the combustion chamber; a reentrant tube entering the opening; a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube extending into a region inside the combustion chamber (optionally above the opening); a thermoelectric element which supplies power to the blower; and a control unit programmed to control power to the blower.

Statement 39. The unit of statement 38 including any of the elements recited in statements 2-26.

Statement 40. A combustion unit comprising: a combustion chamber which holds fuel for combustion and which has an opening for supply of the fuel; a reentrant tube entering the opening; a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube extending into a region inside the combustion chamber

(optionally above the opening); a first thermoelectric element for operation at a first temperature and a second thermoelectric element for operation at a second temperature higher than the first temperature; and a control unit programmed to control power to the blower.

Statement 41. The unit of statement 40 including any of the elements recited in statements 2-26.

Statement 42. The unit of statement 41, further comprising a third thermoelectric element for operation at a third temperature between the first temperature and the second temperature. Statement 43. A stove add-on unit for insertion into an opening of a fuel burning device having a combustion chamber which holds fuel for combustion, the unit comprising: a reentrant tube entering the opening; a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube extending into a region inside the combustion chamber (optionally above the opening); a first thermoelectric element for operation at a first temperature and a second thermoelectric element for operation at a second temperature higher than the first temperature; and a control unit programmed to control power to the blower.

Statement 44. The unit of statement 43 including any of the elements recited in statements 2-26.

Statement 45. The unit of statement 44, further comprising a third thermoelectric element for operation at a third temperature between the first temperature and the second temperature.

Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.

Numerous modifications and variations of the invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

CLAIMS:

1. A thermoelectrically-powered combustion control device comprising:

a reentrant tube entering the opening and extending into an interior region of a combustion chamber inside the interior wall;

a blower configured to force air through the reentrant tube and inject said air into said interior region of the combustion chamber;

a thermoelectric element which supplies power to the blower; and

a control unit programmed to control power to the blower such that said air is injected into said interior region of the combustion chamber above a first predetermined rate which induces turbulent mixing of said air with combustion products produced from burning fuel and below a second predetermined rate where excess air beyond the second predetermined rate compromises carbon emission.

2. The device of claim 1, further comprising a carbon sensor providing feedback to the control unit for adjustment of air flow into the reentrant tube.

3. The device of claim 1, wherein the carbon sensor comprises a CO detector or a particulate counter.

4. The device of claim 1, wherein the reentrant tube includes a thermal break disposed along a length of the reentrant tube which restricts heat conduction from the combustion chamber to at least one of the blower, the thermoelectric element, and the control unit.

5. The device of claim 1, wherein the thermal break comprises a thermally insulative coupling covering a break in the reentrant tube.

6. The device of claim 1, wherein the reentrant tube comprises a perforated section introducing said air into said interior region.

7. The device of claim 6, wherein the reentrant tube comprises a non-perforated section connected to the perforated section, the non-perforated section is disposed inside the combustion chamber such that said air is preheated before being introduced to the perforated section.

8. The device of claim 1, wherein the reentrant tube comprises radially ports which inject said air toward a center of the combustion chamber.

9. The device of claim 1, further comprising:

a heat conductor disposed between the combustion chamber and the thermoelectric element and making a first thermal connection to a first side of the thermoelectric element; and

a heat sink making a second thermal connection to a second side of the thermoelectric element in order to maintain a temperature differential across the thermoelectric element.

10. The device of claim 9, wherein the heat sink comprises a heat pipe connected to a set of fins cooled by air from the blower.

11. The device of claim 9, wherein the heat sink comprises a set of fins cooled by air from the blower.

12. The device of claim 9, wherein the heat sink comprises a heat pipe extending to an earthen mount thermally connecting the heat sink to ambient ground temperature.

13. The device of claim 1, further comprising a rechargeable power source connected to the thermoelectric element.

14. The device of claim 13, wherein the control unit is configured to provide recharging power to the rechargeable power source.

15. The device of claim 1, wherein the control unit is configured to provide auxiliary power for user consumption.

16. The device of claim 1, wherein the control unit comprises a switch for selection of a power to the blower to provide a pre-determined flow rate control based on a size and configuration of the combustion chamber.

17. The device of claim 1, wherein the control unit is programmed with an algorithm for controlling power to the blower to provide a pre-determined flow rate control based on a size and configuration of the combustion chamber.

18. The device of claim 1, further comprising a DC-DC converter connected to the thermoelectric element and configured to convert DC power from the thermoelectric element to a higher voltage DC power.

19. The device of claim 1, further comprising:

a DC-DC converter and a DC-AC converter;

said DC-DC converter connected to the thermoelectric element and configured to convert DC power from the thermoelectric element to a higher voltage DC power;

said DC- AC converter configured to convert the higher voltage DC power into AC power.

20. The device of claim 1, wherein the control unit is programmed with a maximum power point tracking algorithm to provide electrical impedance matching between the thermoelectric device and a load of the blower in order to minimize losses from impedance mismatch.

21. The device of claim 1, wherein the thermoelectric element comprises plural thermoelectric elements including a first thermoelectric element for operation at a first temperature and a second thermoelectric element for operation at a second temperature higher than the first temperature.

22. The device of claim 21, further comprising a third thermoelectric element for operation at a third temperature between the first temperature and the second temperature.

23. The device of claim 21, wherein the second thermoelectric element comprises a thermoelectric material including at least one of the following: Half-Heusler (HH) alloys, PbTe alloys, PbTe/TAGS alloys, and SiGe alloys.

24. The device of claim 23, wherein the Half-Heusler (HH) alloys comprise n-type Hf_{0 6}Zr_{0 4}NiSn₀ 995Sb₀.oo₅ alloys and p-type Hfo_.3Zr_{0 7}CoSno.₃Sb₀.7/nano-Zr0₂ composites.

25. The device of claim 1, further comprising a proximity sensor for detection of a human in a vicinity of the combustion chamber.

26. The device of claim 1, further comprising a cooking interlock to disconnect the auxiliary power output when not cooking.

27. A combustion unit comprising:

a combustion chamber which holds fuel for combustion, has an opening for supply of the fuel, and has an interior wall; and

the thermoelectrically-powered combustion control device of Claims 1-26.

28. A combustion unit comprising:

a combustion chamber which holds fuel for combustion and which has an opening for supply of the fuel;

a reentrant tube entering the opening;

a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube extending into a region inside the combustion chamber; a thermoelectric element which supplies power to the blower and which includes a Half-Heusler (HH) alloy thermoelectric; and

a control unit programmed to control power to the blower.

29. The unit of Claim 28 including any of the elements recited in Claims 2-26.

30. A stove add-on unit for insertion into an opening of a fuel burning device having a combustion chamber which holds fuel for combustion, the unit comprising:

a reentrant tube entering the opening;

a blower configured to force air through the reentrant tube into the combustion chamber, the reentrant tube extending into a region inside the combustion chamber;

a thermoelectric element which supplies power to the blower and which includes a Half-Heusler (HH) alloy thermoelectric; and

a control unit programmed to control power to the blower.

31. The unit of Claim 30 including any of the elements recited in Claims 2-26.

32. A combustion unit comprising:

a reentrant tube entering the opening;

a thermoelectric element which supplies power to the blower; and a control unit programmed to interlock power off to the auxiliary output when the combustion chamber is not being used to cook.

33. The unit of Claim 32 including any of the elements recited in Claims 2-26.

34. A stove add-on unit for insertion into an opening of a fuel burning device having a combustion chamber which holds fuel for combustion, the add-on unit comprising:

a reentrant tube entering the opening;

a thermoelectric element which supplies power to the blower; and

a control unit programmed to interlock power off to the auxiliary output when the combustion chamber is not being used to cook.

35. The unit of Claim 34 including any of the elements recited in Claims 2-26.

36. A combustion unit comprising:

a fuel feed inclined at an angle from 10 degrees to 45 degrees from horizontal so that gravity assists delivery of the fuel into the combustion chamber;

a reentrant tube entering the opening;

a thermoelectric element which supplies power to the blower; and a control unit programmed to control power to the blower.

37. The unit of Claim 36 including any of the elements recited in Claims 2-26.

38. A stove add-on unit for insertion into an opening of a fuel burning device having a combustion chamber which holds fuel for combustion, the unit comprising:

a reentrant tube entering the opening;

a thermoelectric element which supplies power to the blower; and

a control unit programmed to control power to the blower.

39. The unit of Claim 38 including any of the elements recited in Claims 2-26.

40. A combustion unit comprising:

a reentrant tube entering the opening;

a first thermoelectric element for operation at a first temperature and a second thermoelectric element for operation at a second temperature higher than the first temperature; and a control unit programmed to control power to the blower.

41. The unit of Claim 40 including any of the elements recited in Claims 2-26.

42. The unit of Claim 41 , further comprising a third thermoelectric element for operation at a third temperature between the first temperature and the second temperature.

43. A stove add-on unit for insertion into an opening of a fuel burning device having a combustion chamber which holds fuel for combustion, the unit comprising:

a reentrant tube entering the opening;

a first thermoelectric element for operation at a first temperature and a second thermoelectric element for operation at a second temperature higher than the first temperature; and

a control unit programmed to control power to the blower.

44. The unit of Claim 42 including any of the elements recited in Claims 2-26.

45. The unit of Claim 44, further comprising a third thermoelectric element for operation at a third temperature between the first temperature and the second temperature.