FIELD OF THE INVENTION
The present invention generally relates to solar thermal systems such as solar
heat collector, thermal storage system, heat accumulator associated with
effective solar energy utilisation. More particularly, the invention relates to an
array of thermal energy storage elements formed of phase-change materials
and/or ceramic, metallic and polymer materials for enhancing solar thermal
system efficiency.
BACKGROUND OF THE INVENTION
Thermal energy storage is a solution to the intermittent output characteristic
of renewable energy sources. Storing solar thermal energy can allow usage of
solar energy during non-solar hours and also to act as a buffer for generating
uninterrupted heat output during intermittently cloudy weather conditions.
The heat storage media used in the prior art are mainly phase change
materials and few ceramic based materials with means of storage as latent
heat and as sensible heat, respectively.
US 2004/0118229 Al discloses a solar power system capable of storing heat
energy and converting sun light to electrical power; the solar power includes
a solar collection system which gathers and transmits concentrated solar
energy on to a concentrator. A fluid extracts the heat and is transported to a
heat conversion system consisting of plurality of heat exchanger tubes. The
absorptive material may include, for example, castable refractory brick,
graphitic absorbers or heat pipe absorbers. The heat exchanger tubes are of
radiant absorber to liquid type and in this embodiment, are preferably
constructed of Inconel alloy, and may be configured as straight or coiled tube.
Heat exchanger tubes receive thermal energy from the absorption of

concentrated sunlight and transfer the energy into a fluid. The fluid is
retained and flows within the heat exchanger tubes.in this embodiment, the
fluid is a mixture of sodium and potassium nitrate.
US 4343989 A discloses a cast magnesium oxide based structure being
utilized as a heat storage material. The magnesium oxide heat storage
material is cast directly about a source of heat. In another embodiment, a
block of cast magnesium oxide based heat storage material is placed in
contact with a source of heat for raising the temperature and storing heat and
later moved to a different environment for transferring heat to that
environment. The cast structure is produced without the benefit of
conventional ceramic firing processes to form ceramic bonds by the
mechanism of sintering. The cast structures in accordance with this invention
derive their structural and dimensional integrity from the use of calcium
aluminate cement rather than by conventional ceramic processing at
temperatures greater than 1,000 deg.
US 7614397 discloses a method and apparatus for storing, transporting, and
releasing high grade, thermodynamically useful energy for a wide variety of
uses. Solar energy is collected and reflected onto a heat storage container
using a three-mirror reflecting system. This invention involves a method of
heating the heat storage container using a primary, secondary, and tertiary
system, which has a core that is partially comprised of an aluminium alloy and
a metallic shell with a higher melting point than the aluminium alloy contained
within. Once heated, the storage containers can then be transported to
different storage areas in order to heat secondary storage containers or can
be used in processes such as cooking, powering heat engines, water heating,
absorption refrigeration, or drying garbage, waste, or biomass.

US 4265845 A discloses a process for the production of a ceramic heat-
retaining brick based on olivine which comprises shaping and drying a mixture
of granular material and a binding agent. The granular material includes
powdered iron having a particle size of less than 0.1 mm in an amount of
between 5 and 20% by weight of the brick. The brick is subsequently burnt in
an oxidizing atmosphere and the powdered iron is substantially converted to
iron oxide. An object of the invention is to provide a process for the
production of a ceramic heat-retaining brick containing iron oxide based on
olivine using known technology to produce bricks having an increased thermal
storage capacity, preferably using the cheap starting material olivine. .
US 20130269681 A1 discloses a solar thermal storage method capable of
supplying heat of about 1000-1300 K to an external facility for 24 hours. A
solar thermal storage method for storing heat using solar light energy,
comprising a concentrated beam irradiation step of irradiating a reactive
ceramics layer with a concentrated beam obtained by concentrating solar light
to heat the same while moving the reactive ceramics layer formed using
reactive ceramics that transforms from an oxidized form to a reduced form
with a release of oxygen when heated and returns to the oxidized form when
brought into contact with the oxygen; and a thermal storage step of storing
the heat emitted from the reactive ceramics layer in thermal storage means
while bringing the reactive ceramics layer heated in the concentrated beam
irradiation step SI into contact with gas containing the oxygen.
US 20130074826 teaches an Integrated receiver-thermal storage system
comprising a receiver for capturing incoming radiation fused integrally with a
heat storage medium. The receiver is in direct thermal contact with the heat
storage medium, it is actually fused in one device. In a preferred embodiment
the storage medium is a mixture of molten salts and the receiver is a Black
Body Cavity; other embodiments are possible both in terms of receiver and
other storage media (e.g. concrete, metals, aggregates or graphite instead of
salts). In a most preferred embodiment, the black body cavity itself is

immersed within the molten salt; its surface forms part of the containment
vessel of the storage vessel of the molten salt. In this embodiment, the black
body cavity includes fins of high thermal conductivity material in thermal
contact with the heat storage medium which are designed so as to optimally
distribute the heat to the medium. A solar concentrator may be provided at
the opening to further enhance the performance of the system and in most
cases will be an integral part of the design.
US 20120251087 Al describes an improved thermal energy storage materials,
device and system employing the same and related methods. The thermal
energy storage materials may include a phase change material that includes a
metal-containing compound. This invention is directed at methods of
encapsulating thermal energy storage materials, devices containing
encapsulated thermal energy storage materials, and capsular structures for
encapsulating thermal energy storage materials, method for producing a heat
storage device comprising at least partially filling one or more troughs of the
first ply with a thermal energy storage material, wherein the thermal energy
storage material has a temperature greater than about 85° C. and includes a
lithium salt, a sodium salt, a potassium salt, or any combination thereof;
heating the thermal energy storage material for a time and temperature
above the liquids temperature of the thermal energy storage material so that
the thermal energy storage material is essentially free of water; wherein a
blister pack containing a plurality of capsules containing the thermal energy
storage material is formed, wherein the capsules, are in thermal conducting
relation with each other, and the thermal energy storage material is
prevented from escaping from the capsules during operation in their intended
environment.

US 20100212656 describes a thermal energy storage(TES) device including a
vessel housing a continuous volume of a TES media, an input portion, an
output portion, and a plurality of thermal energy transport members
connected to the input portion and/or the output portion. The input portion
receives thermal energy from a thermal energy source. The received thermal
energy is transported by one or more of the thermal energy transport
members to the output portion and/or the TES media for storage. One or
more of the thermal energy transport members connected to the output
portion transport stored thermal energy from the TES media to the output
portion. The output portion is coupled to an external device, such as a Stirling
engine, and configured to transfer thermal energy the external device.
EP 0299903 A2 teaches a heat storage medium comprising a body of parent
metal and an intrinsically cohesive ceramic layer formed integrally with the
metal body and encapsulating said metal body is produced by the directed
oxidation of a body of parent metal outwardly from the surface of said body
to form integrally with the body of parent metal a layer of oxidation reaction
product which encapsulates unreacted parent metal and forms a cavity
resulting from the depletion of parent metal. More particularly, the invention
relates to a method for making heat storage medium by the directed
oxidation of a bulk precursor metal to form a ceramic cover integrally with
and encapsulating unreacted metal, which undergoes a melting and freezing
transformation during service as a heat storage medium. At that temperature,
molten parent metal is reacted with the oxidant outwardly from the surface of
the body of parent metal to form a layer of oxidation reaction product which
initiates containment of the unreacted body of molten parent metal. Molten
parent metal is transported through the encapsulating oxidation reaction
product and into contact with the oxidant at the interface between the
oxidant and previously formed oxidation reaction product, thereby
continuously forming a progressively thicker layer or container of oxidation

reaction product developing outwardly from the surface of the body of parent
metal, and depleting a quantity of the underlying molten parent metal.
US20130111904A1 describes a thermal energy storage and recovery device
which includes a heat exchanger arrangement configured for guiding a flow of
a heat transfer medium between a first end and a second end, and a heat
storage material surrounding the heat exchanger arrangement so that a
thermal interaction region is formed for thermally coupling the heat transfer
medium with the heat storage material. The heat exchanger arrangement is
sealed against the heat storage material so that, when in a first operational
mode, in which the heat storage material is supposed to receive thermal
energy from the heat transfer medium, a compressed gas is usable as the
heat transfer medium for transferring thermal energy from the heat transfer
medium to the heat storage material.
US4222365A discloses a thermal energy storage system and method for
storing substantial quantities of heat for extended periods of time. The
system includes a heat collecting liquid which is in a heat-exchange
relationship with a source of heat or thermal energy, a housing containing a
large volume of particulate material such as rocks for the storage of thermal
energy, a heat transfer gas in a heat-exchange relationship with the rocks
and means for causing the heat collecting liquid and the heat transfer gas to
flow in counter-current, indirect heat-exchange relationship with one another,
the means further includes provisions for reversing the direction of flow of the
liquid and gas for the introduction and removal of heat from a portion of the
body of rock. In a particularly preferred embodiment, the source of heat
comprises a solar heat collector which uses a liquid alkali metal as the heat
collecting liquid and the preferred heat transfer gas comprises air.
EP 2. 278 249 A1 shows a heat storage system for storing solar heat, said
system including a subterranean heat storage for storing heat at
temperatures above 800°C, a collector structure being placed on or above

the ground, said collector structure having at least one reflecting surface or
converging lens for deflecting solar radiation, an absorber body positioned so
as to receive solar radiation deflected by said collector structure and be
heated by said solar radiation, and a solid material heat conductor for
transferring heat from said absorber body to said subterranean heat storage.
All the disclosures discussed above as well as the present industry practice
incorporate large volume of the thermal storage media as separate module.
The thermal storage media refers to various phase storage materials, solid
ceramic materials, various liquid materials etc. However for each of these
thermal storage media, separate enclosure construction is used or suggested
since such thermal storage media needs to be well insulated. Therefore as a
result the thermal storages developed so far are bulky and expensive for
serving the requirement of maintaining steady heat output of the system
during intermittent solar irradiation or fluctuating and cloudy weather
condition.
The above shortcomings are overcome in the present invention, by
incorporating the thermal storage media completely inside the existing
system. This inventive way is such that the thermal storage system has no
extra footprint as it eliminates the need for a separate module and expensive
insulation. In other words, the invented thermal storage elements are, by
design and concept, are invisible externally, yet effectively contribute to
increase efficiency and reducing the cost.
This invention also addresses improving heat transfer efficiency of the solar
thermal system by reducing part of the working fluid which are not
participating or weakly participating in solar heat transfer. The unique design
invented here does not require any additional insulation for storing the
thermal storage elements comprising of phase-change materials and/or
ceramic, metallic and polymer materials.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose thermal storage elements
and their assembly for enhancing solar thermal system efficiency. The
invention thus allows an inexpensive thermal storage to maintain an un-
interrupted heat output of solar thermal system during intermittent and
fluctuating solar irradiation as well as extending the output of the system for
time beyond the sun-set.
Another object of the present invention is to increase the efficiency of the
solar thermal system for the intended application without consuming extra
space.
Yet another object of the present invention is to propose additional thermal
storage elements assembled in the solar thermal system which does not
require extra insulation and thus eliminates consequential thermal loss.
A further object of the invention is to propose a compact solar thermal system
which maximises the efficiency by reducing the components of the system
which are less effective for heat transfer.
SUMMARY OF THE INVENTION
An inventive concept of incorporating thermal storage media in a solar
thermal system is conceived by using the available space inside of the tubular
solar collector for increasing its efficiency. The tubular solar collectors used in
the solar thermal industry are commonly known as Evacuated Tube Collector
(ETC) or Receiver (ETR). These ETC or ETR are typically either 'glass and
glass' or 'glass and metal' construction. The thermal storage media comprising
of various phase storage materials, solid ceramic materials, various liquid
materials, metallic/polymer inclusions and suitable combinations of these
materials commonly referred as Thermal Storage Elements (TSE) in this
patent application are designed to maximise heat transfer and provide
additional storage utilising the existing space within the tubular solar

collectors. A novel concept, design and assembly of the TSE are invented
imparting close thermal contacts between the solar radiation absorbing
surface of the tubular solar collector, the thermal storage elements, thermally
conductive foils and heat collection pipes wherever applicable. Depending on
the application and collector hardware for both 'glass and glass' or 'glass and
metal' construction ETC or ETR, suitable TSE are designed using various
phase change materials, solid ceramic materials and a combination of both.
Ceramic based TSEs are fabricated through ceramic extrusion process
commonly known in the art by designing suitable extrusion dies and using
appropriate ceramic raw materials, binders, plasticizers and additives. The
elements thus fabricated are suitably dried and fired to achieve the desired
properties including dimensional stability and inertness to the heat transfer
working fluid. Phase-change material based TSEs are made by enclosing
suitable phase-change materials in a thin-walled container which can be
accommodated inside the existing tubular solar collectors. The thin-walled
container can be made out of high temperature withstanding polymer, plastic,
metal or ceramic materials, which should be inexpensive and can be moulded
into the shape and size of the space available inside the solar collector.
The invention also uses ceramic/polymer/metal materials which are bonded to
form the TSE with or without phase-change materials by various fabrication
methods such as pressing, slip-casting, gel-casting, injection moulding etc.
Some of the materials composition used might not require high temperature
firing process. Appropriate fabrication method and process of manufacturing
can be adopted aiming to further reducing the TSE cost and provide very
affordable cost effective thermal storage system.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1.Is a cross-section view of an evacuated tube collector assembled with
thermal storage elements in accordance with this invention.
Fig 2. Isometric view of one thermal storage element

DETAIL DESCRIPTION OF THE INVENTION
Fig 1 is the cross section view of invented typical thermal storage elements
(TSE) assembled inside the evacuated tube collector (ETC) in accordance with
this invention. Four thermal storage elements, 1, 2, 3 and 4, respectively and
thermally conductive foil 5 are assembled together before inserting the
assembly (called billet) into the tubular solar collector. The inner tube 7 of the
Evacuated tube collector which absorbs most of the solar energy has its inner
surface which is to be in contact with the working fluid which flows mostly
along the outer surface of the TSE assembly comprising of 1,2,3,4 and 5. In
some of the design of solar thermal system the heat collector tube or heat
pipes are incorporated in order to transfer the solar heat from the ETC to the
respective applications using working fluid. Cavity 6 is designed to fit the
typical heat collector tube or heat pipe and is formed along the peripheral
length of the TSE assembly as shown in Fig 1. The outer tube of the ETC is
shown as 8 in Fig 1 which is fused to the inner tube 7 at both the ends but
separated by a vacuum space 9 along the length of the tube.
In another variation of the invention, element 1 and 3 have been combined to
form one single piece/unit, and element 2 and 4 have been combined to form
another single piece/unit. Yet another variation of the invention all the four
elements 1,2,3 and 4 can be combined and produced as single element. In
some cases where two heat collector tubes are present instead of four as
shown in Fig.l, then only two cavity 6 appropriately sized and positioned will
be provided instead of four. Yet another variation of the invention, where only
one heat collector tube or heat pipe is used instead of four as shown in Fig.l,
then only one cavity 6 appropriately sized and positioned at center or
periphery will be provided instead of four.
Fig 2 is the isometric view of one typical thermal storage element (TSE) 2
with cut design 10 for forming cavity 6 when assembled with other TSE.

However when the heat collector tube or heat pipes are not provided or
needed in the solar thermal system, both the cut designs 10 or one of them
are/is not provided while manufacturing the TSE based on the respective
requirements.
Example 1
An inexpensive ceramic material, Porcelains is used for making one of the
prototype thermal storage elements since it has got adequate strength,
inertness to most of the thermal working fluids and having useful properties
such as high specific heat and high density. The raw materials for
manufacturing porcelains are sourced locally such as clay, quartz and
feldspar, which are also inexpensive. The porcelains batch is mixed as known
in the art and extruded through a die designed to meet the dimensional
requirement of the thermal storage element. Special care is taken with close
process control after the extrusion of the porcelain elements for subsequent
drying and firing stages in order to maintain the straightness and other
dimensional quality. The finished porcelain elements are assembled for this
prototype as 4 pieces per set after suitably wrapping those with 35 micron
thickness Aluminium foils such as one manufactured by M/s Hindalco. The
aluminium foil wrapped thermal storage sets are inserted in the commercially
procured ETC of 1.5 meter length and 33 mm of inner diameter.
Six sets of thermal storage assemblies (billets) each comprising of four
elements and aluminium foil are inserted to each of the ETC tube. 360
storage media elements each of 0.25 meter length were manufactured,
assembled and placed inside the ETC tubes, constituting part of a solar
thermal collector system. This system consisted of 15 ETCs forming part of a
1 m2 solar desalination system located at CTI. After the installation of the
thermal storage elements, the output of the thermal desalination unit has
registered a 40% increase from its normal operation compared to ETC tubes

without any thermal storage elements and the output also found more even
during the non-solar hours.
Example 2
Two adjacent solar collector panels A and B, each having 18 ETCs were
selected. In one panel A, Six sets of thermal storage units each comprising of
two elements were inserted to each of the ETC tube. Cross-section of these
elements were half circles having a groove at the periphery for running
copper pipe. All the 18 ETCs were fitted with 216 thermal storage elements
(TSEs) of required shape and size each of 0.25 meter length. The ETCs of the
second panel B, were kept without any thermal storage elements. Both panels
have common water inlet and therefore have identical inlet water
temperature. Outlet water temperatures of panels A and B were recorded on
24-hour basis over a period spanning few months.


IT
Fig. 3 : Outlet water temperature recordings of two solar panels A and B, with
and without incorporated Ceramic thermal storage elements in the ETCs of
the panels, respectively.
The plot of Fig. 3 shows that for several consecutive days, the temperature
readings of ceramic thermal storage based solar panel are less fluctuating and
consistantly records always lower temperature than that of the solar panel
which are not fitted with the thermal storage elements during the Day time.
Whereas the night temperature readings of the same panels are reverse,
which records higher temperature for thermal storage incorporated solar
panel than that of the normal solar panel without thermal storage.
Table-2: Analysis of typical temperatures recorded during Day and Night time
of Solar panel with and without ceramic thermal storage elements. The solar
panels are connected to a solar adsorption/desorption based air-conditioning
unit.

Analysis of a large number of temperature data points for a few typical days
of the outlet water from solar panels A and B with and without storage units,
respectively statistically confirms the observations of Fig.3. The system shows
a >20C drop of temperature for outlet water from Panel A during Day time,

whereas a rise of >2°C is recorded at Night for the same outlet water from
Panel A compared to outlet water from Panel B. Thus practically invisible
thermal storage elements having no footprints of its own as disclosed in this
patent application confirms storage of heat during a typical Day and releases
the heat during Night in absence of sun power.
Further this invention also maintain steady un-interrupted heat output of the
solar thermal system during intermittent and fluctuating solar irradiation along
with extending the output of the system beyond the sunshine hours. It
further increases the efficiency of the solar thermal system without
necessitating any extra space.
Thermal energy storage elements as described in above examples are also
constructed with inexpensive natural raw materials such as various minerals,
clay, quartz, feldspar, rocks, graphite, metallic powder and waste materials.
Further, TSEs are also made with various phase change materials (such as
wax, molten salts, polymer etc) various liquid materials, metallic/polymer
inclusions and suitable combinations of these materials.

WE CLAIM :
1. An array of thermal energy storage elements formed of phase-change
materials and/or ceramic, metallic and polymer materials for enhancing
solar thermal system efficiency, comprising a plurality of thermal storage
elements formed of natural raw materials, and phase-change materials
fabricated through extrusion process, the fabricated materials being
dried and fired to achieve dimensional stability including inertness to
heat transfer working fluid, the elements integrated within the solar
energy collector or receiver for maintaining steady un-interrupted heat
output of the solar thermal system during intermittent and fluctuating
solar irradiation conditions.
2. The array of thermal energy storage elements as claimed in claim 1,
wherein the various natural raw materials consists of clary, quartz,
feldspar, rocks, graphite, metallic powder and waste materials.
3. The array of thermal energy storage elements as claimed in claim 1,
wherein the various phase change materials consists of wax, molten,
salts, polymer, solid ceramic materials, liquid materials and
metallic/polymer inclusions.
4. The array of thermal energy storage elements as claimed in claim 1,
wherein the elements allow close thermal contacts between the solar
radiation surfaces of the solar energy collector or receiver.

5. The array of thermal energy storage elements as claimed in any of the
proceeding claims, wherein additional space for storage of thermal
energy is created due to the disposal of the elements within the tubular
solar collector/receiver.