Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02643888 2008-11-17
SOLAR POWERED HEAT STORAGE DEVICE
FIELD OF THE INVENTION
The present invention relates to heat storage devices, and more particularly
to a solar powered
heat storage device using phase change materials.
BACKGROUND OF THE INVENTION
Use of solar energy for providing electricity in remote regions, for example,
for outdoor lighting,
lighting of traffic signs, emergency lighting or other electric powered
equipment is often limited
because of performance degradation of the batteries used for storing electric
energy produced by
photovoltaic solar panels during daytime and for providing the same during
absence of sufficient
sunlight. This is particularly a problem in cold regions where energy output
from a battery is
significantly decreased due to exposure of the battery to low temperatures.
Present systems store solar energy by heating a liquid or solid material for
heat storage.
Unfortunately, these systems are of substantial size and weight for storing a
sufficient amount of
heat and are, therefore, impractical for many applications such as, for
example, mobile tower
lights.
Phase Change Materials (PCMs) are substances with high heat of fusion, i.e.
when melting and
solidifying at a phase change temperature PCMs are capable of storing and
releasing large
amounts of heat, respectively. Heat is absorbed when the PCM changes from
solid to liquid and
released when the PCM changes from liquid to solid. When heated in the solid
phase the
temperature of the PCM rises initially as heat is absorbed until the phase
change temperature is
reached. The PCM then absorbs large amounts of heat at a substantially
constant temperature
until all the material is transformed into the liquid phase. During cooling
this process is reversed,
i.e. large amounts of heat are released until all the PCM is solidified.
However, employment of PCMs for heat storage is impeded due to poor thermal
conductivity of
the PCMs in the solid phase, i.e. during cooling the PCMs solidify starting at
container walls and
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preventing effective heat transfer with increasing thickness.
It is desirable to provide a simple and effective heat storage device using
PCMs.
It is also desirable to provide a solar powered system that is capable of
storing solar energy for
heating batteries by employing a heat storage device using PCMs.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a simple and
effective heat storage
device using PCMs.
Another object of the present invention is to provide a solar powered system
that is capable of
storing solar energy for heating batteries by employing a heat storage device
using PCMs.
According to one aspect of the present invention, there is provided a solar
powered heating
system. The heating system comprises a heat storage container made of a heat
conductive
material having a phase change material disposed therein. A heating mechanism
heats the phase
change material to induce a phase change of the same. A solar panel is
connected to the heating
mechanism for capturing solar energy and providing the same to the heating
mechanism. The
heating system further comprises a heating conduit for enabling flow of a
heating fluid there
through such that a heat transfer between the phase change material and the
heating fluid is
enabled. The heating conduit is connected to an enclosure for providing
heating thereto. A
heating fluid actuator induces flow of the heating fluid and a heating fluid
flow control
mechanism controls the flow of the heating fluid through the heating conduit.
According to another aspect of the present invention, there is further
provided a phase change
heater. The phase change heater comprises at least two heat storage containers
made of a heat
conductive material. Each heat storage container has a phase change material
disposed therein,
wherein at least two phase change materials have a different phase change
temperature. A
heating mechanism heats the phase change material to induce a phase change of
the same. The
heating system further comprises a heating conduit for enabling flow of a
heating fluid there
through such that a heat transfer between the phase change materials and the
heating fluid is
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enabled. The heating conduit is connected to an enclosure for providing
heating thereto. A
heating fluid actuator induces flow of the heating fluid and a heating fluid
flow control
mechanism controls the flow of the heating fluid through the heating conduit.
The advantage of the present invention is that it provides a simple and
effective heat storage
device using PCMs.
A further advantage of the present invention is that it provides a solar
powered system that is
capable of storing solar energy for heating batteries by employing a heat
storage device using
PCMs.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the present invention is described below with
reference to the
accompanying drawings, in which:
Figure 1 A is a simplified block diagram of a heating system according to an
embodiment
of the present invention;
Figure 1B is a cross-sectional view along a longitudinal axis of a heat
storage device
according to an embodiment of the present invention;
Figure 1 C is a cross-sectional view perpendicular to the longitudinal axis of
the heat
storage device according to an embodiment of the present invention illustrated
in Figure
113;
Figure 2A is a cross-sectional view along a longitudinal axis of a heat
storage device
according to a preferred embodiment of the present invention;
Figure 2B is a cross-sectional view perpendicular to the longitudinal axis of
the heat
storage device according to the preferred embodiment of the present invention
illustrated
in Figure 2A;
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Figure 2C is a perspective view of the heat storage device according to the
preferred
embodiment of the present invention illustrated in Figures 2A and 2B;
Figure 3A is a simplified block diagram illustrating a tower light according
to a preferred
embodiment of the present invention; and,
Figure 3B is a perspective view of the tower light according to a preferred
embodiment
of the present invention illustrated in Figure 3A.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Unless defined otherwise, all technical and scientific terms used herein have
the same meaning
as commonly understood by one of ordinary skill in the art to which the
invention belongs.
Although any methods and materials similar or equivalent to those described
herein can be used
in the practice or testing of the present invention, the preferred methods and
materials are now
described.
Referring to Figures 1 A to 1 C, a solar powered heating system 100 according
to an embodiment
of the invention is provided. The solar powered heating system 100 stores
solar energy captured
using solar panel 102 in heat storage device 106 using PCMs. The heat storage
device 106
comprises heat storage containers 120A and 120B having a PCM disposed therein.
The heat
storage containers 120A and 120B are made of a heat conductive material such
as, for example,
plastic or metal tubing which is sealable capped on each end for containing
the PCM such as, for
example, wax, in a solid or liquid phase therein. The heat storage containers
are disposed outside
and/or inside a heating conduit 122 made of, for example, plastic or metal
tubing. The cross-
sectional shape of the heating conduit is not limited to a circular cross-
section, as illustrated in
Figure 1 C, but other shapes such as, for example, oval or rectangular shapes
are also applicable.
The heat storage containers 120A and 120B are disposed such that substantially
efficient heat
transfer between the PCM and a heating fluid - for example, air; water; or
glycol - disposed in
the heating conduit 122 and flowing there through is enabled. As illustrated
in Figures 1B and
1 C, the heat storage container 120A is disposed inside the heating conduit
122, while the heat
storage container 120B is disposed on the outside wall of the heating conduit
122 forming, for
example, a helical coil surrounding the heating conduit 122 and being in
thermal contact
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therewith.
Heating fluid actuator 104 - for example, an electrically operated fan or pump
- is connected to
the photovoltaic solar panel 102 (and/or a battery source) for receiving
electrical energy there
from and inducing a flow of the heating fluid. Using heating mechanism 105 -
for example, an
electrical heating element connected to the photovoltaic solar panel 102 - the
heating fluid is
heated. Flow of the heating fluid is controlled using a heating fluid flow
control mechanism
108A and 108B such as, for example, damper valves placed, for example, as
illustrated in Figure
1 A or at an input side and an output side of the heating storage device 106.
In operation solar energy captured by the photovoltaic solar panel 102 is
transformed into
electrical energy and provided to the heating fluid actuator 104 and the
heating mechanism 105
for inducing a flow of the heating fluid and for heating the same. When
passing through the
heating conduit 122, heat energy is transferred from the heating fluid via the
heat storage
containers 120A and 120B to the PCM. The temperature of the PCM increases
until the phase
change temperature is reached at which a substantially large amount of heat
energy is absorbed
until substantially all the PCM is transformed into the liquid phase. During
the reverse process -
for example, for heating an enclosure connected to the solar powered heating
system 100 in
absence of solar energy - the heating mechanism 105 is shut off and the
heating fluid actuator
104 is, for example, battery operated. Heat energy is then transferred from
the PCM to the
heating fluid until substantially all of the PCM is transformed into the solid
phase. It is noted that
further heat energy is stored/released before and after the phase transition
of the PCM, but the
amount of heat energy is relatively small compared to the heat of fusion
stored/released during
the phase transition.
In one embodiment of the present invention, the process of storing/releasing
heat energy is
controlled using controller 112 which is, for example, connected to the
heating fluid actuator
104, the heating mechanism 105, the heating fluid flow control mechanism 108A
and 108B, and
one or more temperature sensors. The controller 112 controls the heating fluid
flow in
dependence upon, for example, the temperature sensor data, data indicative of
electrical energy
provided from the solar panel, and stored threshold data - for example,
desired temperature
range of the enclosure - using, for example, a processor.
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The heat storage containers 120A and 120B are designed such that a sufficient
amount of PCM
is contained therein for storing a predetermined amount of heat energy.
Furthermore, the heat
storage containers 120A and 120B are designed such that an efficient heat
transfer between the
heating fluid and the PCM as well as within the heat storage containers 120A
and 120B is
achieved, for example, by providing a sufficiently large surface area of the
heat storage container
walls in the form of, for example, a long tubing having a relatively small
diameter. Optionally,
heat transfer within the PCM - when in the solid state - is facilitated by
disposing a heat
conductive material such as, for example, a copper wire mesh in contact with
the heat container
walls therein. As is evident, size, shape and number of the heat storage
containers as well as the
heating conduit is variable and determined in dependence upon application
requirements. For
example, the heating conduit 122 has a cross-sectional shape other than
circular such as oval or
rectangular.
Alternatively, the PCM is heated using heating elements that are disposed
within the heat storage
containers or on the outside surface of the heat storage containers and in
thermal contact
therewith.
Further alternatively, the heating element 105 comprises a conduit having
contained therein a
heating fluid such as, for example, glycol, which is circulated to and from a
solar panel for
directly transforming solar energy into heat energy of the heating fluid.
Preferably, but not limited thereto, PCMs having a phase change between a
solid phase and a
liquid phase are employed. Use of PCMs having a phase change between a liquid
phase and a
gaseous phase is limited due to substantial expansion of the PCM when
transformed from the
liquid phase to the gaseous phase.
There are numerous PCMs available having different phase change temperatures
or ranges of
phase change temperatures, some exemplary PCMs are listed in Table I herein
below, but the
invention is not limited thereto.
CM hase Change Temperature C
strowax 27 7
strowax 32 1332
strowax 54 1554
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eeswax 52-64
anolin 38-44
anocerin 11-51
hellac 14-82
)zokerite 52
arnauba 3
andellila 58-74
o'oba
ontan 4-94
araffin 50-57
croc stalline 50-80
i h density ol eth lene 126
ow density ol eth lene 110
ol etrafluoroeth lene 330
0l amide 12-255
alcium chloride hexahydrate 9
odium sulphate decahydrate 32
;odium acetate trihydrate 58
eresine 54-71
s arto wax 13
;oy wax 19-82
ork fats (lard) 0-48
Table 1
Using PCMs, heat energy produced by the heating element 105 is only stored in
a sufficient
amount when the phase change temperature of the PCM has been reached. For
example, using
Astrowax 54 - having a phase change temperature of 54 C - in the heating
system 100, only
one third of the heat generated by the heating element 105 is stored.
Combining use of Astrowax
54 in a first heat storage container with, for example, Astrowax 32 - having a
phase change
temperature of 32 C - in the heating system 100, substantially increases the
portion of the heat
energy that is stored in an efficient manner. Furthermore, use of two or more
PCMs having
different phase change temperatures substantially increases the flexibility of
the heating system
100. For example, it enables design of a system for simultaneously storing
heat energy and
heating an enclosure connected thereto within a predetermined temperature
range. Furthermore,
it enables, for example, the design of a system that is capable of providing
some limited heating
at a lower temperature when the heat storage for providing heating at a higher
temperature is
exhausted and is capable of heat storage for providing limited heating at a
lower temperature
when the solar energy is insufficient for heat storage for providing heating
at a higher
temperature. Preferably, the heat storage container comprising the PCM having
the lowest phase
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change temperature is placed inside the heating conduit 122.
Referring to Figures 2A to 2C, a preferred embodiment of a phase change heater
200 according
to the invention is shown. The phase change heater 200 comprises heat storage
containers 220A
to 220E having different PCMs disposed therein. Preferably, the heat storage
containers 220A
and 220B contain Astrowax 27 - having a phase change temperature of 27 C and
a heat of
fusion of 200 kJ/kG, while one or two of the heat storage containers 220C to
220E contain
Astrowax 32 - having a phase change temperature of 32 C and a heat of fusion
of 200 kJ/kG,
and the remaining one or two of the heat storage containers 220C to 220E
contain Astrowax 42 -
having a phase change temperature of 42 C and a heat of fusion of 200 kJ/kG.
The heat storage
containers 220A to 220E are made of a heat conductive material such as, for
example, plastic
(preferably high density polyethylene) or metal tubing which is sealable
capped on each end for
containing the PCMs in a solid or liquid phase therein.
The heat storage containers 220C to 220E are disposed outside heating conduit
222 forming
three helical coils surrounding the heating conduit 222 and being in thermal
contact therewith.
The heating conduit 222 is made of, for example, plastic (preferably high
density polyethylene)
or metal tubing and has an oval shaped cross-section. The heat storage
containers 220A and
220B are disposed inside the heating conduit 222 oriented substantially
parallel to a longitudinal
axis of the heating conduit 222. The heat storage containers 220A to 220E are
disposed such that
substantially efficient heat transfer between the different PCMs and a heating
fluid - air -
disposed in the heating conduit 222 and flowing there through is enabled.
Plastic mesh 228 is preferably disposed surrounding the heat storage
containers 220C to 220E
for holding the heat storage containers 220C to 220E in contact with the
heating conduit 222.
The heating conduit 222 and the heat storage containers 220A to 220E are
placed in a housing
226, preferably, a zinc galvanized tubular member having a rectangular cross-
section with
insulating materia1224 disposed there between.
Referring to Figures 3A and 3B, a preferred embodiment of a solar powered
mobile tower light
300 according to the invention is shown. The tower light 300 comprises a
photovoltaic solar
pane1302 for capturing solar energy and transforming it into electric energy.
The tower light 300
further comprises: batteries 310 for storing electric energy provided by the
solar pane1302 and
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for providing the same to light 311 during absence of solar energy; solar
powered heating system
100 for receiving electric energy from the solar pane1302 and for storing the
electric energy in
the form of heat energy and for providing the stored heat energy during
absence of solar energy;
ducts 308 for circulating a heating fluid to and from the heating system 100
and for providing the
heat to the batteries 310; and control circuitry 312 connected to the solar
panel, the heating
system 100, the batteries, and the light 311. Preferably, the batteries 310,
the heating system 100,
the ducts 308, and the control circuitry 312 are disposed in an insulated 306
housing 304.
Preferably, the heating system 100 is designed as described above with respect
to Figures 2A to
2C. In operation, solar energy is captured during daytime and provided as
electric energy for
storage in the batteries 310. After the batteries are charged the surplus
electric energy provided
by the solar panel 302 is provided to the heating system 100 for
transformation into heat energy
which is then stored in the PCM(s) thereof. During absence of solar energy the
electricity stored
in the batteries 310 is provided to the light 311, preferably a LED light for
maximum efficiency,
and the heat energy stored in the heating system 100 is used for heating a
heating fluid,
preferably air, which is circulated in the ducts 308 for heating the
batteries. Heating of the
batteries 310 increases their capacity at low ambient temperatures - in
particular, when used in
cold climate regions - i.e. more electric energy is available for enabling
longer lighting periods.
Furthermore, disposing electronic circuits such as the control circuitry 312
within the insulated
306 housing 304 facilitates design of the electric circuitry with respect to
complexity and
robustness.
Optionally, electric energy is provided to the heating system 100 during
charging of the batteries
310 for heating the heating fluid - for example, to a temperature below the
phase change
temperature of the PCM - and subsequently heating the batteries to increase
their capacity for
storing electric energy when exposed to low temperatures during daytime.
The operation of the heating system 100 is controlled using control circuitry
312 and is,
preferably, automated. For example, the control circuitry 312 comprises a
temperature sensor
disposed in the insulated housing 304 for regulating the temperature therein
within a
predetermined temperature range. Furthermore, the control circuitry 312 senses
when the
batteries are charged and then provides the electric energy to the heating
system 100. Optionally,
the control circuitry 312 provides some electric energy to the heating system
100 when the
temperature in the insulated housing is below a predetermined temperature for
efficiently
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charging the batteries.
Optionally, the heating system 100 and the batteries 310 are placed within the
insulated housing
such that a circulation of the heating fluid within the insulated housing is
enabled in absence of
the ducts 308.
Provision of the heating system 100 for heating the batteries enables
provision of a solar
powered tower light 300 as a mobile unit using, for example, trailer 316
having disposed
thereupon the housing 304, the solar panel 302 and the light 311 mounted, for
example, to a
telescopic mast 314.
As is evident, the heating system 100 is employable in a similar fashion in
numerous
applications other than the tower light 300 such as, for example, traffic
signs, traffic lights,
communication equipment, and provision of a heated enclosure, to name a few.
The present invention has been described herein with regard to preferred
embodiments.
However, it will be obvious to persons skilled in the art that a number of
variations and
modifications can be made without departing from the scope of the invention as
described
herein.
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