U.S. patent application number 13/189257 was filed with the patent office on 2013-01-24 for waste heat recovery for forced convection biomass stove.
This patent application is currently assigned to AEROJET-GENERAL CORPORATION. The applicant listed for this patent is Joseph P. Carroll, Andrew John Zillmer. Invention is credited to Joseph P. Carroll, Andrew John Zillmer.
Application Number | 20130019849 13/189257 |
Document ID | / |
Family ID | 47554874 |
Filed Date | 2013-01-24 |
United States Patent
Application |
20130019849 |
Kind Code |
A1 |
Zillmer; Andrew John ; et
al. |
January 24, 2013 |
WASTE HEAT RECOVERY FOR FORCED CONVECTION BIOMASS STOVE
Abstract
A stove includes 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 configured to
direct the fan be operated from power provided from the battery
when power produced from the thermoelectric device is insufficient
to power the fan.
Inventors: |
Zillmer; Andrew John;
(Citrus Heights, CA) ; Carroll; Joseph P.;
(Cameron Park, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zillmer; Andrew John
Carroll; Joseph P. |
Citrus Heights
Cameron Park |
CA
CA |
US
US |
|
|
Assignee: |
AEROJET-GENERAL CORPORATION
Redmond
WA
|
Family ID: |
47554874 |
Appl. No.: |
13/189257 |
Filed: |
July 22, 2011 |
Current U.S.
Class: |
126/80 |
Current CPC
Class: |
F23M 2900/13003
20130101; F24B 1/02 20130101 |
Class at
Publication: |
126/80 |
International
Class: |
F24C 1/14 20060101
F24C001/14 |
Claims
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
BACKGROUND
[0001] 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.
[0002] 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.
[0003] Disclosed herein is a stove for power generation that may
address one or more of these deficiencies.
SUMMARY
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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
[0009] 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:
[0010] FIG. 1 is a schematic illustration showing one embodiment of
a stove according to the present invention;
[0011] FIG. 2 is a schematic illustration showing the elements of a
power management and distribution system for the stove of FIG.
1;
[0012] FIG. 3 is a step logic diagram for a method of operating the
stove of FIG. 1;
[0013] FIG. 4 is a diagrammatical illustration of a combustion
device with radiative heat transfer and heat recovery;
[0014] FIG. 5 is a diagrammatic illustration of a recuperator that
can be used with the combustion device of FIG. 4; and
[0015] FIG. 6 is a diagrammatic illustration of the recuperator of
FIG. 5, viewed from the right of FIG. 5.
DETAILED DESCRIPTION
[0016] Referring to FIG. 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.
[0017] 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.
[0018] 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. Pat. 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.
[0019] 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.
[0020] The thermoelectric device 122 is a well known device. For
example, U.S. Pat. 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 FIG.
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.
[0021] 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, multi-pass, 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 FIG. 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.
[0022] 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.degree. 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.
[0023] FIG. 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 FIG.
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.
[0024] 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.
[0025] 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.
[0026] FIG. 3 is a step logic diagram showing one embodiment of a
method 300 for starting the stove 100.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] From block 306, the method enters block 308. The
determination of block 308 is similar to the determination made in
block 312.
[0031] 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.
[0032] 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.
[0033] Referring back to FIG. 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.
[0034] 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 FIG. 1.
[0035] Referring to FIG. 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 FIGS. 1-3. The differences between
the stove 100 of FIG. 1 and the stove 400 of FIG. 4 will be
apparent from the description that follows.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.degree.
C. Accordingly, the heating plate 410 should be suitable to
withstand temperatures in excess of 850.degree. C. The V-shaped
combustion chamber 404 as well as the use of force draft combustion
air will be able to achieve such temperatures.
[0040] 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.
[0041] Referring to FIG. 5, a diagrammatical illustration of one
embodiment of a recuperator 402 is shown for the stove illustrated
in FIG. 4, FIG. 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 FIG. 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
[0042] 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.degree. 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.
[0043] 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 FIGS. 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.degree. 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.
[0044] 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 FIG. 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.
[0045] Referring to FIG. 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 FIG. 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
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