Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A solid fuel stove with improved combustion
The present invention relates to cooking stoves with improved combustion. In
particular the present invention relates to cooking stoves capable of burning
solid fuels, such
as wood, using forced air circulation in the combustion chamber.
It is estimated that approximately 2.5 billion people in the world burn wood
for cooking. The known stoves and processes are typically inefficient and have
incomplete
combustion, which results in substantial smoke emissions and contributes to
the global
warming process. Many deaths each year may be attributed to such polluted
smoke
emissions. In addition, poor efficiency woodstoves use more natural timber
resource with
consequences for deforestation.
In domestic combustion of wood, air pollution occurs because of incomplete
combustion of the volatiles released from the wood. Volatile organic compounds
are released
from wood at temperatures as low as room temperature, but substantial, rapid
release only
begins when exothermic reactions commence (250 C). The volatiles form a
complex mix of
combustible gases. The ignition temperature of the combustible gas mix is
approximately
600 C. Many of the unburned volatiles released from wood will condense to form
fine
particles when cooled to near-ambient temperatures. This is what we observe as
wood-
smoke. Incomplete combustion of the volatiles occurs for several reasons. When
a batch of
wood is added, the heat from the coals and appliance soon causes gases to be
released from
the wood. When lighting a fire, the burning paper and kindling provide this
heat. If the
volatiles are not exposed to a high temperature source (flame or glowing
charcoal) they will
not ignite and simply pass up the flue/chimney causing pollution. If the gas
does ignite, it can
be quenched if it is cooled by a cold surface (e.g. the metal walls of a cool
firebox) or cool
combustion air. If the combustible gas is not well mixed with air (oxygen) it
will not burn. If
combustion air is reduced to slow the combustion rate, there may be
insufficient oxygen for
complete combustion.
GB 2125160 describes a cooking stove having a combustion air chamber into
which air for combustion is drawn from the exterior of the stove either by
natural convection
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or by a hand-operated air pump or combination of both. The air enters the
combustion
chambers through one or more apertures in the bottom of the chamber.
It is an object of the present invention to provide a solid fuel stove that
allows
a clean combustion process.
According to the present invention a solid fuel woodstove is provided
comprising;
a combustion chamber for containing combustion fuel, which chamber has a
lower side portion for accommodating fuel and an upper side portion;
a blower assembly configured to provide airflow entering the combustion
chamber in operating condition;
guiding means to direct the airflow into the combustion chamber;
wherein the guiding means direct the airflow from the upper side portion to
the
lower side portion.
The stove according to the present invention has proven to give a very clean
combustion process. A cleaner combustion process reduces both the emission of
harmful
combustion gasses, such as carbon monoxide (CO) and condensed volatile organic
compounds. The specific airflow as created by the guiding means allows the
combustible
gasses in the hot combustion chamber to combust completely before eventually
exiting the
combustion chamber and thus improves the cleanliness of the combustion
process. When a
transverse or an upward airflow is established in the combustion chamber there
is a risk of
combustibles gasses being entailed by such airflow outside the chamber before
the
combustible gasses are burned completely. This is especially the case when
such gasses reach
a higher part of the combustion chamber where the temperature is relatively
low. The airflow
according to the invention generates some kind of turbulent air mixture, which
apparently is
very advantageous with relation to a complete combustion. There is more time
for the
combustible gases in the combustion chamber to completely burn before
eventually exiting
the combustion chamber. As it is generally known complete combustion means a
clean
combustion. Detailed measurements with stoves according to the present
invention indicate
reduced levels of residual smoke and volatile organic matter. An additional
advantage is that
flames will not longer touch a cooking vessel disposed on the stove allowing a
significant
reduction of soot levels on the cooking vessels. The guiding means can have a
relatively
simple construction, which does not complicate the design of the stove. Of
course apart from
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the blower assembly there are other sources of air can enter the combustion
chamber, such as
air entering through an (partially) open upper side portion or through
openings in the lower
side portion by natural convection. If the airflow in the combustion is
substantially directed
from the upper side portion to the lower side portion the combustion process
will be very
clean, as was indicated by several experiments.
According to a preferred embodiment the guiding means comprise a
plurality of apertures provided in a wall of the combustion chamber at the
upper side portion.
By providing apertures a simple yet effective guiding means can be obtained.
It is especially
preferred if the upper edge of an aperture is inclined inwardly with relation
to the
combustion chamber and a lower edge of an aperture is inclined outwardly with
relation to
the combustion chamber. This construction is advantageous with relation to
manufacturability in case of a relatively thin wall, because it is easier to
deform the wall
around the apertures. In another preferred embodiment the apertures comprise
drilling holes
having an inclined pitch with relation to the wall. Such apertures are easier
to manufacture in
case of a relatively thick wall.
Furthermore it is preferred if the apertures are substantially evenly
distributed
along the contour of the combustion chamber. This has the advantage that
opposing streams
of air will meet somewhere in the centre of the combustion chamber, which
results in a
turbulent mixture of air while improving the combustion process.
It is also advantageous if each aperture is disposed at a distance from the
upper
side portion. This has the advantage that the flames will not directly contact
cooking utensils
placed on top of the stove, thereby preventing the formation of soot at such
utensils.
According to another preferred embodiment the guiding means establish
airflow at the lower side portion of the combustion chamber. The guiding means
to this end
preferably comprises a plurality of apertures at the lower side portion of the
combustion
chamber. Experiments have proven that addition of airflow at the lower side
portion of the
combustion chamber will assist the gasification of components that are usually
hard to gasify,
such as charcoal. This will benefit the combustion process.
It is preferred when a preheat chamber is disposed around the combustion
chamber, which preheat chamber provides air flow communication from the blower
assembly
into the combustion chamber and comprises an air distributor including heat
reflectors
adapted to reflect heat radiated from the combustion chamber back towards the
combustion
chamber. The air distributor guides the airflow and reflects heat back into
the combustion
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chamber. Air entering the combustion chamber is preheated, while an outer
surface remains
sufficiently cool in operation to be safe to the touch.
In a preferred embodiment a rechargeable electrical power source for driving
the blower assembly and a thermoelectric element are provided, wherein the
thermoelectric
element is configured to provide power to the blower assembly and to the
rechargeable power
source. This gives more freedom with relation to the (electric) power supply
for driving the
blower assembly and makes the woodstove independent from connection to the
main power
grid or to an external battery. Moreover it is very advantageous with respect
to the overall
energy consumption, especially in view of the fact that the wood stoves
according to the
invention typically are powered by a rechargeable energy source, such as a
start-up battery. A
woodstove provided with a thermoelectric element is described in detail in the
non-pre-
published application IB2006/050920, which is incorporated herein by
reference.
It should be acknowledged that the embodiments described above, or aspects
thereof, may be combined.
Embodiments of the present invention will now be described by way of
example and with reference to the accompanying drawings in which:
Fig. 1 shows a perspective view of a solid fuel stove suitable for cooking;
Fig. 2 is a cross section view schematically showing internal detail of the
stove
of figure 1;
Fig. 3 shows a schematic side cross sectional view according to line I-I in
Fig.
2.
With reference to figure 1, a solid fuel stove 10 comprises a substantially
cylindrical housing 11, a combustion chamber 12 formed within an upper portion
of the
housing and having a generally open upper side portion 15 for use as a cooking
surface. The
generally open upper side portion 15 includes a number of support struts 13 or
the like for
supporting cooking utensils such as a pan on the top. The generally open upper
side portion
15 may be at least partially covered by a mesh, grid or other open structure
(not shown) for
further supporting a cooking vessel while still allowing efficient egress of
heat in an upward
direction. The stove 10 preferably is placed on a flat and stable surface 5.
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Opposite to the open upper side portion 15 a lower side portion 17 (shown in
dotted line in Fig. 1) of the combustion chamber 12 provides accommodation for
the solid
fuel. The fuel typically is thrown into the combustion chamber 12 through the
open upper
side portion 15 by hand whenever fresh supply of fuel is needed. It should be
noted that the
5 terms `upper side portion' and `lower side portion' here are only used to be
able to
distinguish between the typical parts of the combustion chamber as it is
positioned in its
normal upright operating position as shown in Fig. 1.
The housing 11 includes a series of air inlets 14 at a lower end thereof for
ingress of air, which is used for forced air convection through the combustion
chamber 12 as
will be described below. The stove 10 preferably is a portable stove and
therefore may be
provided with a removable carrying handle (not shown) which may be attached to
brackets
on the housing 11 (also not shown). A series of upper apertures 23 of the
combustion
chamber 12 are also visible in figure 1.
Figure 2 shows the internal arrangements of a preferred embodiment of the
stove 10. An inner cylindrical wa1121 defines the combustion chamber 12.
Guiding means 40
of the combustion chamber 12 comprises a series of lower apertures or air
outlets 22 and a
series of upper apertures or air outlets 23. The upper apertures are shaped
such that air
flowing into the combustion chamber is directed from the open upper side
portion to the
lower side portion or downwardly with relation to the support surface 5. The
apertures at
least create an airflow component directed from the upper side portion to the
lower side
portion. Next to a downward component the airflow velocity in the combustion
chamber
preferably will also have a velocity component that is directed from the
wa1121 towards the
centre of the chamber.
Considering the air entering the combustion chamber through the upper and
the lower apertures respectively it is preferred if the majority of air enters
through the upper
apertures. A favorable distribution appeared to be 75% of the air entering
through the upper
apertures, while the remaining 25% enters through the lower apertures. This
can easily be
established by choosing an appropriate ratio between the total aperture
surface respectively at
the bottom side portion and lower side portion.
The upper apertures 23 are arranged in 2 rows along the contour of the
combustion chamber 12. Preferably the apertures in a row are substantially
evenly distributed
along this contour. Furthermore it is preferred if apertures in both rows
provide some kind of
alternating perforation as is shown in Figs. 1 and more clearly in Fig. 2.
This has appeared to
be advantageous with relation to the air that egresses from the combustion
chamber.
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Moreover it is favorable with relation to having as many apertures as possible
in a small
surface.
An annular space 18 is formed between the cylindrical wa1121 and the housing
11, which space acts as a preheat chamber. The annular space is filled with an
air distributor
24 which preferably comprises a series of cylindrical metal sheets 24a with
punched out ribs
24b maintaining separation between the sheets to provide air conduits. The
metal sheets 24a
guide the air flow and reflect heat back into the combustion chamber 12,
preheating the air
that enters the combustion chamber through the upper air outlets 23 and
ensuring that the
outer surface of the housing 11 remains sufficiently cool in operation to be
safe to the touch.
The cylindrical metal sheets are held in place by a supporting structure 24c.
At the lower side portion 27 a supporting surface 29 for holding the solid
fuel
is disposed.
The airflow in the combustion chamber is such that a regular combustion
process of solid particles arises. These particles can be any type of solid
material, but
preferably one uses wood. The present invention does not relate to fluidized
bed types of
combustion processes.
The base of the cylindrical vesse121 includes a thermal isolation structure 25
which acts as a heat shield reducing downward radiation of heat towards an
intermediate
chamber 26 and a lower chamber 27 of the housing 11. The intermediate chamber
26 and
lower chamber 27 are separated by a wa1128 having openings (not shown).
Adjacent to these
apertures is mounted a blower assembly 50, preferably having a central motor
52 and integral
outwardly radiating blades 53 forming an impeller to direct air through the
openings in the
wa1128. The central motor 52 is preferably protected by a further heat shield
element 51,
which may be a thin layer of heat reflective material such as aluminium foil
disposed on the
motor. The lower chamber 27 is bounded by the housing 11 which includes the
air inlets 14.
In use, the blower assembly 50 draws air through the air inlets 14, and blows
it
through the openings of the wa1128 into the intermediate chamber 26.
Intermediate chamber
26 acts as a distribution chamber to feed air into the annular space 13 and
the air distributor
24. Air flows between the sheets 24a of the air distributor 24 to warm the air
and direct it to
the lower and upper air inlets 22, 23 of the combustion chamber 12.
In one embodiment, the blower assembly or fan 50 comprises a 1 W brushless
DC fan driven by a 3 to 7 V power supply (not shown), compatible with a 5 V
motor. In
another embodiment, the fan is a 12 V driven by a 6 to 14 V power supply. The
power
supply typically is an internally mounted battery accessible from the base of
the stove.
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Alternatively an external supply may be used, whenever available. Tests have
shown that the
stove 11 is capable of boiling a litre of water in 4 minutes, without
significant soot and
smoke, with a combustion temperature of more than 1000 C. Food may be simmered
at the
lower voltage range or boiled at the higher voltage range thereby providing
good cooking
control.
The intermediate chamber 26 preferably is provided with a thermoelectric
element 31 that has a first active surface in close proximity to the
combustion chamber 12
and a second active surface positioned to receive a cooling draught from the
blower assembly
50. In the preferred arrangement shown, the second active surface of the
thermoelectric
element is in direct thermal association with, or forms part of, a heat sink
arrangement 32
which is cooled by the fan. The first active surface of the thermoelectric
element may be in
close direct contact with a lower wall of the combustion chamber 12, or
isolation structure
25. The thermoelectric element 31 may be embedded into the isolation structure
25 to
increase the temperature available at the first active surface. In view of the
heat shielding
effects of the thermoelectric element 31 and heat sink 32, a separate heat
shield for motor 52
might not be required with this arrangement.
The thermoelectric element 31 is any suitable device 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. The thermoelectric element provides
electrical power
to the blower assembly 50. In use, the blower assembly 50 provides airflow to
the heat sink
32 and thermoelectric element 31 as well as to the air distributor 24. In this
manner, the
second active surface of the thermoelectric element is maintained at a
substantially lower
temperature than would otherwise be the case, which increases the power output
available
from the element, and thus increases the available airflow to the combustion
chamber 12.
An electronic control unit 33 controls the blower assembly or fan and is also
housed in the lower chamber 27, where it is also protected from the heat of
the stove. The
electronic control unit 33 includes a rechargeable battery and a controller
configured to
operate the stove. The thermoelectric element provides electrical power to the
fan 50 and the
rechargeable battery, therewith extending the lifetime of the battery. In the
preferred
embodiment, the electronic control unit is adapted to automatically sequence
through each of
the available modes of the woodstove, such as a start-up or a shutdown mode.
Preferably the
electronic control unit adapts the subsequent steps according to the sensed
operating
conditions, e.g. heat of the fire. A temperature sensor (not shown) may be
used to determine
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the heat of combustion, or this may be deduced from the electrical output of
the
thermoelectric element 31.
Normally the rechargeable battery is only used to supply power in a start-up
phase. In normal operation the battery can then sufficiently be re-charged by
the
thermoelectric element for the next start-up.
Figure 3 shows a cross-section according to line I-I in Fig. 2 showing one of
the apertures in more detail. It shows a part of the cylindrical wall in
detail and illustrates that
an upper edge 61 of the apertures is inclined inwardly with relation to the
combustion
chamber. A lower edge 62 is inclined outwardly with relation to the combustion
chamber.
Preferably the cylindrical wall comprises a heat resistant metallic sheet,
such as stainless
steel. This allows any airflow coming in through the aperture will be directed
downwards, i.e.
from the upper side portion of the combustion chamber to its lower side
portion. This is
especially advantageous in case of a relatively thin wall, wherein it is
easier to deform the
wall around the apertures. When the wall is somewhat thicker the apertures may
comprise
drilling holes having an inclined pitch with relation to the wall. In this
case the wall of the
combustion chamber will remain substantially flat.
An alternative advantageous embodiment is to have guiding means comprising
nozzles having an outlet directed from the upper side portion to the lower
side portion. An
outlet of the nozzle may extend through the wall of the combustion chamber.
Alternatively
such nozzles may be arranged outside the combustion chamber at the upper side
portion
thereof.
When the airflow A in Fig. 3 encounters an airflow B flowing through an
opposite aperture both flows will influence each other while creating a
turbulent air mixture,
which is believed to be torus shaped. Experiments have indicated that the
airflow
significantly contributes to a cleaner combustion process.
The simplest way to create an aperture shown in Fig. 3 is to stick a tool,
such
as a metal rod through an aperture having straight edges. Subsequently one
tilts the tool,
while the part outside the combustion chamber inclines upwards. Herewith one
plastically
deforms the edges of the aperture. Preferably the diameter of the metal rod is
slightly smaller
than the diameter of the aperture. This provides for a simple and reliable
method of creating
appropriate apertures.
In order to develop the required airflow certain sizes and number of apertures
are preferred. Considering the fact that the stove is meant for cooking the
thermal power that
is wanted is in the range of 2-5 kW. That determines, in turn, a combustion
rate in terms of
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gram wood per minute. That, in turn, determines the required airflow. A very
significant
surplus of air is used to ensure clean combustion. After careful experiments
the required
airflow was found to be in the range of 100 Uminute for the low power setting
and 200
1/minute for the high power setting. Approximately 75% of the air is used as
secondary
combustion air.
A larger number of small apertures are preferred over a small number of large
apertures. An optimum was found at 64 apertures of 2.5 mm in diameter.
A small improvement of the combustion properties was found by using two
rows of 32 apertures displaced vertically by a few mm. Each row is arranged at
some
distance from the upper side portion. Preferably both rows are arranged in an
alternating
configuration. This arrangement appeared to result in an even better mixing of
the
combustible gasses with the available air. Typical outer dimensions of the
woodstove are a
height of 30 centimeter and a diameter of 20 centimeter.
The woodstoves according to the present invention is typically applied for in
house cooking, wherein one benefits from the clean combustion process. However
the
woodstove can also be applied outdoors, such as during camping, since it
usually can operate
on (rechargeable) batteries. Another possible field of application is disaster
areas, when
people are in need for fires that are easy to establish with relation to
(emergency) cooking
and providing warmth.
A significant advantage of the design of stove described above is that the fan
is sufficiently protected from the direct source of heat that a cheap mass
produced motor with
plastic components may be used, even though placed at only a short distance
from the
combustion chamber, resulting in a compact stove. Such motors also prove to be
much more
reliable and have a longer design life. Positioning of the motor in the supply
air stream
means that the motor is self-cooling, and also can be conveniently used to
cool the cool side
of the thermoelectric element.
While the invention has been illustrated and described in detail in the
drawings
and foregoing description, such illustration and description are to be
considered illustrative or
exemplary and not restrictive; the invention is not limited to the disclosed
embodiments.
Other variations to the disclosed embodiments can be understood and effected
by those
skilled in the art in practicing the claimed invention, from a study of the
drawings, the
disclosure, and the appended claims. In the claims, the word "comprising" does
not exclude
other elements or steps, and the indefinite article "a" or "an" does not
exclude a plurality. A
single processor or other unit may fulfill the functions of several items
recited in the claims.
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The mere fact that certain measures are recited in mutually different
dependent claims does
not indicate that a combination of these measured cannot be used to advantage.
Any
reference signs in the claims should not be construed as limiting the scope.
The invention relates to a solid fuel stove comprising a combustion chamber
5 (12) for containing combustion fuel and a blower assembly (50) configured to
provide
airflow entering the combustion chamber in operating condition. When guiding
means (40)
establish airflow entering the combustion chamber substantially in a
downwardly direction
the combustion process of the stove is very clean and efficient.
