FIELD OF INVENTION
The present invention relates to the field of one way heat transfer device and
an improvement particularly to the field of heat reservoir systems. The device can be
placed in between a fluctuating heat source and a heat reservoir system to stop drain
out of heat when heat source temperature is less than reservoir temperature.
BACKGROUND OF INVENTION
In the field of heat reservoir systems (e.g. Batch type solar water heater),
where the heat source is fluctuating in nature, it becomes necessary to place a device
in between them, which will connect or disconnect automatically the source and
reservoir depending on the temperature of both heat source and reservoir, assuring an
one directional heat flow from heat source to heat reservoir. Currently available
devices such as thermal switches relying on differential coefficient of thermal
expansion of different material considers only heat source temperature or reservoir
temperature and connect or disconnects heat source to reservoir only above or below
a preset temperature. Now if heat source temperature suddenly falls below the
reservoir temperature and at that time reservoir temperature is such that it makes the
switch to connect to heat source, than there will be a reverse heat flow from reservoir
to source. Other alternatives presently available (such as one directional heat pipes,
solid state thermal diodes) are either costly, have size and orientation restriction,
show low diodicity or still under development stage. Few examples of prior art are
given below:
Cryogenic Thermal Switch (US 6,276,144 B1): A cryogenic thermal switch
operates on the principle of differential coefficients of thermal expansion of different
materials. A small gap is either closed or opened, on demand raising or lowering the
temperature of two pieces of different materials. As the temperature of the pieces is
raised, the piece having the greater coefficient of thermal expansion increases its
dimensions at a greater rate. This action closes the gap and produces a conduction
heat flow path across the switch. Conversely, when the temperature of the pieces is
lowered, the piece having greater coefficient of thermal expansion shrinks
proportionally faster, thereby opening the gap. The claimed on/off conductance ratio
of this switch is said to be 1000:1. This prior art requires manual operation and thus
not suitable to act as a thermal diode.

Bimetallic thermal switch: In this prior art a bimetallic strip of suitable
dimension is used to connect or disconnect heat source to reservoir depending on the
temperature of the reservoir or the source. It connects or disconnects a heat source to
a heat reservoir when the temperature of the body (heat source or reservoir) exceeds
or falls below a preset temperature. The main problem associated with this prior art is
that it only considers temperature of the connected heat reservoir or source. Suppose
we want to connect a heat source to a heat reservoir using this device. For this
purpose if we use this device in the reservoir side (i.e. it will consider only the
reservoir temperature), it will connect the reservoir to heat source when the reservoir
temperature goes below a preset temperature. At that time if the source temperature is
lower than the reservoir temperature, a reverse heat flow will occur from the reservoir
to heat source draining the heat content of the reservoir. Conversely if we connect the
device to the heat source (i.e. it will consider only the heat source temperature), it will
only connect it to reservoir when heat source temperature exceeds a preset value,
making the system less efficient as it will not connect the heat source to reservoir
even if the reservoir temperature is lower than the heat source temperature. For this
kind of application we need a system which considers the temperature difference
between the heat source and reservoir not the temperature of the heat source or
reservoir temperature alone. A good example of this kind of switch is "Smart Bi-
metallic heat spreader (US 6,278,607 B1).
Magnetomechanical thermal diode: This prior art exploits the temperature
dependent magnetization property of a ferromagnetic material. It consists a hard
magnet (hot side), a metal top surface (cold side) and a ferromagnetic plate held in
between by a linear spring. In normal condition this ferromagnetic plate is pulled by
the hard magnet and touches each other. When heat is applied to the bottom surface,
the ferromagnetic plate losses its magnetic property, hence the spring pulls it upward
connecting it to top surface, where it loses its heat to top surface and regains its
magnetism. The magnetic pull between this plate and hard magnet exceeds the spring
force and it again makes contact with the bottom surface. This ferromagnetic plate
oscillates until the bottom surface has a temperature greater than a threshold value
and transfers heat in small quantity in each cycle. In opposite case when top surface
has higher temperature than the bottom surface, the spring pulling force becomes
more than magnetic pulling force, thus the oscillation ceases. The main problem
associated with this device is its vibration, which makes it unsuitable for vibration
sensitive applications.

Solid-state thermal rectifier: In this device the atomic or molecular vibration
associated with heat is used. Now every material has a resonant frequency depending
upon their molecular structure. When two material segments having different
resonant frequencies connected to each other, it is found that the frequencies of the
materials match one other when a temperature drop (analogous to a voltage drop in an
electric circuit) is introduced in one direction and mismatch one another when the
temperature drop is in the other direction. The net result is that heat can easily flow in
one direction through the sandwich but not the other. The reported forward
conductance is 3% to 11% greater than the reverse conductance. This device is still
under development condition and not ready for practical application yet.
One directional Heat pipe: A one directional hest pipe consists of a sealed
pipe having both hot end and cold end made of high conductivity material such as
copper and a system (e.g. wick) which can produce capillary action, internally
connecting both hot and cold ends. A suitable liquid such as water is kept inside the
pipe at lower pressure. A liquid reservoir is also connected at the evaporator end (hot
end) having no capillary connection to the heat pipe. When heat is applied to the hot
end (evaporator end) liquid inside the evaporator boils and vapor thus produced goes
to the cold end (condenser end) and condenses back to liquid phase. This liquid
comes back to the evaporator end by the capillary action of the wick structure and the
cycle repeats itself. When heat is applied to condenser end, having no liquid at the
condenser no phase transformation can occur and thus heat flow minimizes. One
problem found in this system is that at higher temperature returning liquid tends to
boil off in the wick structure before they reach the evaporator section. Thus one heat
pipe cannot be used for wider range of temperature.
Planer jumping-drop thermal diode: This device consists of a planer vapor
chamber with opposing super hydrophobic and super hydro phallic plates. The plates
are separated by a thermally insulating gasket which also provides a vacuum seal.
When the super hydrophilic surface is heated with respect to the super hydrophobic
one (forward mode), the evaporating water carries heat away from the super
hydrophilic surface and the vapor condenses on the super hydrophobic surface; the
self propelled jumping motion returns the condensed drops back to the evaporator,
completing the circulation of working fluid with phase change heat transfer. When
the super hydrophilic surface is cooler (reverse mode), liquid water is trapped by it
and no phase change heat transfer takes place; heat mainly escapes through
ineffective conduction across the rubber gasket and vapor space. The claimed
diodicity is up to 150±50. This device losses its directional independence at

temperature above 50°C due to production of water bubbles within the device. At
higher temperature in reverse mode heat flow also increases due to convection heat
transfer, limiting its use for wide range of temperature.
Therefore from the above discussion it is evident that we are lacking a thermal
diode design which is low cost, simple in design, silent, have no orientation
restriction, having good diodicity value and can be used for wide range of
temperature conditions.
OBJECT OF THE INVENTION
To produce a thermal diode which is low cost, easy to manufacture, simple in
design, have no orientation restriction but still having good performance values and
which can be used in low cost applications.
STATEMENT OF THE INVENTION
The present invention utilizes two identical bimetallic strips in parallel
relation to each other and having a small gap (10) in between (please refer to Fig 1.),
of which input bimetallic strip (1) is connected to a heat source and the output
bimetallic strip (2) is connected to a heat reservoir or another heat source and both the
bimetallic strips containing inside a evacuated glass enclosure (4). In the surface of
the bimetallic strips and the inside of the glass enclosure a suitable reflective coating
is applied to reduce radiation heat transfer. Now both strips will produce deflection of
its free end depending on its corresponding connected body's temperature. If heat
source temperature is higher than heat reservoir or the second heat source temperature
in such an amount that the deflection of input strip (1) is higher than sum of
deflection of output strip (2) and initial gap between the two strips (10), then the input
strip (1) will touch output strip (2), establishing a heat conduction path between heat
source and heat reservoir. In the reverse case the output strip (2) will produce more
deflection then deflection produced by input strip (1), making it impossible for input
strip (1) to touch output strip (2) and thus no heat flow path will be established
between heat source and heat reservoir. Thus we get a device which allows one way
heat transfer, depending on the temperature difference of the two bodies connected
across it.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG: 1 is a cross-sectional front view of the proposed thermal diode.
FIG: 2 is a top view of the proposed thermal diode.
FIG: 3 is a diagrammatic view illustrating construction of Input bimetallic strip (1).
FIG: 4 is a diagrammatic view illustrating construction of Output bimetallic strip (2).
DETAILED DESCRIPTION
In Fig: 1 and Fig: 2 construction of the proposed bimetallic thermal diode (3)
is shown. In Fig: 1, input bimetallic strip (1) is placed above and parallel to output
bimetallic strip (2) inside a Borosilicate glass enclosure (4), having reflective silver
glazing inside the enclosure surface and in the outside surfaces of the two bimetallic
strips (1) & (2). The fixed end of the bimetallic strips (1) and (2) are connected to
copper conductors (5) and (6) respectively as shown in Fig: 1. The copper conductors
5 and 6 pass through the enclosure (4) wall and a suitable glass copper seal (8) is
formed at this junction. These two seals (8) are vacuum proof and also hold both
conductors in position as shown in Fig: 1. Fig: 2 illustrates top view of the thermal
diode (3). In Fig: 2 both the strips (1) and (2) can be seen making an angle (12) (in
the range of 10° to 30° depending on space available) to each other in the horizontal
plane such a way that strip (l)'s free end is above strip (2)'s free end and having a
small gap(10) in between. The air inside the glass enclosure(4) is evacuated out to
reduce convection heat transfer. An alternative to high vacuum inside the glass
enclosure is keeping an inert gas like helium or nitrogen at around 70 KPa pressure.
Inert gas environment will save the strips from oxidation. High vacuum reduces life
span of the seals but also decreases convection heat loss; therefore the pressure
should be selected according to intended use. (9) refers to the permanent vacuum
plug or seal which is formed on the vacuum port (a small pipe section), which is
melted and sealed after sufficient vacuum is attained. The reflective silver glazing is
applied to reduce radiation heat transfer. The angle (12) between the two strips in
horizontal plane as described in Fig: 2 is also given to reduce the view factor between
the two bimetallic strips, which also reduces radiation heat transfer between the
strips. The thermal conductor (6) is given an "L" shape as shown in Fig: 1, to provide
a free space to bimetallic strips (1) and (2), so that they can deflect freely. This "L"
shape of conductor (6) also increases direct heat flow path between conductor (5) and

conductor (6) through the glass enclosure (4) body, reducing heat conduction from
conductor (5) to conductor (6). In Fig: 3, construction of input bimetallic strip (1) is
illustrated. (1) refers to the input bimetallic strip, where (13) refers to High Expansion
side and (14) refers to Low Expansion side. The High expansion side is made of
copper and low expansion side is made of Invar material. In the free end, a portion of
the copper material is kept extended whose extended length is equal to the breadth of
the bimetallic strip(l), by which a head (15) is formed by bending it over the end of
invar side as shown in Fig: 3. To the input bimetallic strip (l)'s fixed end, a copper
conductor (5) is fitted by brazing or spot welding to the copper side (13). For low
temperature application a thermal grease/tap can be applied at the head (15) for better
conduction.
In Fig: 4, construction of output bimetallic strip (2) is illustrated. (2) refers to
the output bimetallic strip, where (16) refers to High Expansion side and (17) refers to
Low Expansion side. The High expansion side is made of copper and low expansion
side is made of Invar material. In the free end, a portion of the copper material is kept
extended whose extended length is equal to the breadth of the bimetallic strip(2), by
which a head (18) is formed by bending it to high expansion side (16) as shown in
Fig: 4. For rigidity, this head can be brazed or welded to copper side. To the output
bimetallic strip (2)'s fixed end, an L-shaped copper conductor (6) is fitted by brazing
or spot welding to the copper side (16). Outside the enclosure at the both conductor
(5) & (6), holes (7) of suitable size are made to facilitate their connectivity to heat
source or reservoir.
The purpose of this copper heads attached to the free ends of the strips, is to
produce a low thermal resistance path to heat flow when they touch each other.
Copper in the high expansion side is chosen, as it has good coefficient of thermal
expansion and high thermal conductivity.
The length, breadth and thickness of both the input and output bimetallic
strips should be kept same, so that they both form same curvature when both are kept
at a same temperature. The length, breadth and thickness of the bimetallic strip should
be determined according to factors such as sensitivity, heat flow requirement etc.
These values can be well determined by already established procedures in the Bimetal
strip making industry.
The impact of the small gap (10) between Input Bimetallic strip (1) and
Output Bimetallic strip (2) on the thermal diode's (3) working characteristics is that,
it fixes the minimum positive temperature differences between the connected heat

source and heat reservoir, at which the diode will start conduction at the forward
direction i.e. source to reservoir. It can be calculated as follows:

Where,
d= Gap(10) between the bimetalc strips (1&2)
L= Length of the input Bimetallic strip (1)

= Radius of curvature of input Bimetallic strip (1)
t= Thickness of the bimetalc. strip
oA = Coefficient of thermal expansion of the metal in the higher expansion side
= In our strips copper is used. Coefficient of thermal
expansion of copper is 0.000017 per °C
T1= Temperature of the Heat source
T2= Temperature of the Heat reservoir
(T1 -T2 )= Minimum positive temperature difference between the heat,
source and reservoir at which the Diod should start conducting
heat from source to reservoir.
Referring to the Fig: 1, the height (11) of the space provided below output
bimetallic strip can be calculated putting Tl= Maximum working temperature of the
output bimetallic strip (2), T2= Reference temperature of the output bimetallic strip
(2) in equation (ii) given above. The other values should be put as usual. The value of
"r" thus obtained than should be put to equation (i). Here L is the length of the output
bimetallic strip (2). The value of "d" thus obtained is the value of the height (11).
When the Bimetallic thermal diode (3)'s Input conductor (5) is connected to a
heat source and the Output conductor (6) is connected to a heat reservoir or a second
heat source, both strips will produce deflection of its free end depending on its
corresponding connected body's temperature. If heat source temperature is higher

than heat reservoir or the second heat source temperature in such an amount that the
deflection of input strip (1) is higher than sum of deflection of output strip (2) and
initial gap between the two strips (10), then the input strip (1) will touch output strip
(2), establishing a heat conduction path between heat source and heat reservoir. In the
second case if heat source temperature is lower than heat reservoir or the second heat
source temperature the output strip (2) will produce more deflection than deflection
produced by input strip (1), making it impossible for input strip (1) to touch output
strip (2) and thus no heat flow path will be established between heat source and heat
reservoir. Thus we get a device which allows one way heat transfer, depending on the
temperature difference of the two bodies connected across it.

CLAIMS
I claim:-
1. A Bimetallic Thermal Diode (3) comprising of an Input Bimetallic Strip (1),
an Output Bimetallic Strip (2), an Enclosure (4), a Heat Inlet conductor(5) and
a Heat Outlet conductor (6).
2. A Bimetallic Thermal Diode (3) as claimed in Claim 1 wherein the Input
Bimetallic strip(1) is a bimetallic strip having copper at high expansion
side(13) and invar at low expansion side (14), such that an extended part of
the copper strip (13) of length equal to the breadth of the input bimetallic
strip(l), at the free end, folds over the end of the invar strip (14) towards itself
forming a head (15); the said Input Bimetallic strip(l) has the copper strip
(13) at the other end connected to the Heat inlet conductor (5) which is a
copper conductor whose one end holds the Input Bimetallic strip(1) inside the
Enclosure (4) and the other end lies outside the Enclosure (4) for connecting
to a source of heat.
3. A Bimetallic Thermal Diode (3) as claimed in Claim 1 wherein the Output
Bimetallic strip(2) is a bimetallic strip having copper at high expansion side
and invar at low expansion side, such that an extended part of the copper strip
(16) of length equal to the breadth of the input bimetallic strip(2), at the free
end, folds over itself forming a head (18); the said Output Bimetallic strip(2)
has the copper strip (16) at the other end connected to the Heat Outlet
conductor (6) which is an L-shaped copper conductor whose one end holds
the Output Bimetallic strip(2) inside the Enclosure (4) and the other end lies
outside the Enclosure (4) for connecting to a heat reservoir.
4. A Bimetallic Thermal Diode (3) as claimed in Claim 1 wherein the Enclosure
(4) is a sealable Borosilicate glass enclosure containing two glass sealable
holes (8), placed at the opposite ends and opposite corners of the enclosure at
which the Heat inlet conductor (5) and the Heat Outlet conductor (6) are glass
sealed respectively so that they firmly hold their respective bimetallic strips
inside the Enclosure (4) in such a way that the head (15) of the Input
Bimetallic strip(1) is above the head (18) of the Output Bimetallic strip(2) and
the angle (12) between the Input Bimetallic strip(1) and the Output Bimetallic
strip(2) in the horizontal plane is in the range of 10°-30°.

5. A Bimetallic Thermal Diode (3) as claimed in Claim 1 wherein the inside of
the Enclosure (4) is vacuum sealed or filled up with an inert gas like Nitrogen,
Helium, Argon etc. kept at a pressure of around 70 kPa with the use of the
permanent sealable plug (9) and the inner surface of the Enclosure (4) is
silvered.
6. A Bimetallic Thermal Diode (3) as claimed in Claim 1 wherein the Input
Bimetallic strip (1) and the Output Bimetallic strip (2) are equal in length,
breadth and thickness.
7. A Bimetallic Thermal Diode (3) as claimed in Claim 1 wherein the Heat inlet
conductor (5) and the Heat Outlet conductor (6) have holes in them at the
section outside the Enclosure (4) to facilitate their connectivity to heat source
and reservoir respectively.
8. A Bimetallic Thermal Diode (3) as claimed in Claim 1 wherein the permanent
sealable plug (9) is a small extension of the Enclosure (4) in the form of a pipe
to connect to a vacuum pump, which is melted and sealed after sufficient
vacuum is attained.

ABSTRACT

The Bimetallic Thermal Diode(3) utilizes two identical bimetallic strips
placed parallel to each other and one above another , having a small gap(10) in
between, of which top bimetallic strip(1) is connected to a heat source and the bottom
bimetallic strip(2) is connected to a heat reservoir. Now both strips will produce
deflection of its free end depending on its corresponding connected body's
temperature. If heat source temperature is higher than heat reservoir, then the free end
of Input strip(1) will touch output strip(2), establishing a heat conduction path
between heat source and heat reservoir. In the reverse case the output strip(2) will
produce more deflection then deflection produced by Input strip(1), making it
impossible for Input strip(1) to touch output strip(2) and thus no heat flow path will
be established between heat source and heat reservoir. Please refer Fig 1 & 2.