TRANSLATION OF PCT/EP2011/000570
METHOD AND APPARATUS FOR STORING AND RELEASING HEAT BY
MEANS OF A PHASE CHANGE MATERIAL
Description
The invention relates to a method for storing and releasing heat by a phase change
material as per the preamble of claim 1 and also such an apparatus as per the
preamble of claim 11.
For a multiplicity of technical applications, storing heat is necessary or
advantageous. This concerns, for example, the use of renewable energies in solar-
thermal power plants and also cyclic processes in which the efficiency can be
increased by storing surplus heat of a cycle for use in a following cycle.
For efficient storage of heat or cold, in particular heat stores are suitable which
comprise a phase change material (PCM) as storage medium. Such latent heat
stores have the advantage compared with other heat stores that large amounts of
heat can be stored in a narrow temperature range. Compared with conventional
sensible heat stores, using latent heat stores, high energy densities can be achieved
with substantially constant operating temperature. Thus, compared with
conventional heat storage using sensible heat, in typical latent heat stores, via a
temperature change of 10 K in the phase change of the storage medium, a heat
storage density that is ten to twenty fold higher can be achieved. The required
amount of storage material and size of the corresponding apparatuses and
containers are significantly reduced thereby.
A problem in the use of phase change materials is, in particular, the comparatively
low thermal conductivity of the organic or inorganic storage media typically used
(typically 0.5 to 1 W/(m K)). As a result, when the latent heat storage is
implemented on an industrial scale, the problem of inadequate heat transport
between the storage medium and a heat transport fluid used for heat exchange
arises. It is not only, as with other storage systems, overcoming the heat transfer
resistances from the heat transport fluid (optionally via heat-exchange surfaces) to
the storage medium itself, but in addition overcoming comparatively high heat

transfer resistances within the storage volume of the storage medium in order to
utilize the entire storage volume.
It is therefore known to improve the heat transport with the phase change material
by the phase change material being present in microencapsulation in a carrier
liquid. In this case, typically paraffins are used as phase change material, which
paraffins are present in water as capsules having casings made of organic
materials. In this case, the disadvantage of complex production of microcapsules
occurs. In addition, by the use of paraffins and the organic materials, use of such
latent heat stores is only possible below 100°C. Even if a temperature-stable
encapsulation were to succeed, no suitable transport medium seems to be available,
since currently usual heat-transfer media are excluded: water because of excessive
pressure; thermal oil because of expected vigorous reactions with the encapsulated
salts and salt melts, since then no encapsulation would be necessary.
In other previously known apparatuses, the phase change material is stationary. In
this case it is known to encapsulate the phase change material in steel tubes which
are flushed by heat transport fluid for the heat exchange. Likewise it is known to
form from the phase change material and a material having a comparatively higher
thermal conductivity a composite material for developing a latent heat store as
described, for example, in US 2004/0084658 A1. It is additionally known to increase
the heat transfer surface area in the heat exchange by lamellae made of material
having a high thermal conductivity which are in contact with the phase change
material. For example, EP 1816 176 A2 discloses the use of graphite films for
improving the thermal transport and heat transfer properties.
With the previously known apparatuses having a stationary phase change material,
there is the disadvantage that due to the corrosive properties of the phase change
materials typically used, large amounts of high-value materials, such as corrosion-
resistant steel, for instance, are required for forming the heat exchangers, and so in
this case high costs result.
Therefore, the object of the invention is to provide a method for storing and
releasing heat by a phase change material and also to provide such an apparatus
which, compared with the prior art, are less complex and therefore cheaper to
implement. Furthermore, the method according to the invention and the apparatus

according to the invention should be usable in a broad temperature range, in
particular in the temperature range for the heat exchange of relevance for a
multiplicity of applications between 100°C and 300°C or above.
This object is achieved by a method for storing and releasing heat by a phase
change material as per claim 1 and also an apparatus for storing and releasing heat
by a phase change material as per claim 11. Advantageous embodiments of the
method according to the invention may be found in claims 2 to 10 and of the
apparatus according to the invention in claims 12 to 15.
The invention is based in the findings of the applicant that during the previously
known methods and apparatuses, a costly and/or restrictive with respect to the
temperature range production of the storage medium is necessary and/or direct
coupling between the storage capacity on the one hand and the surfaces necessary
for the heat exchange and thereby complexity and size of the heat exchanger on the
other exists. These disadvantages are avoided in the method according to the
invention and the apparatus according to the invention in that there is a spatial
separation between heat exchange with the storage medium and storage of the
storage medium, and in that phase change material is used in unencapsulated form.
In the method according to the invention, during a charging process (also termed
charging process), in a storage medium which comprises a phase change material a
phase change is caused in a heat-exchange device with addition of heat for storing
the heat in the storage medium as latent heat. In a discharging process, a phase
change is caused in the storage medium in a heat-exchange device with removal of
heat.
It is important that the storage medium used is an at least predominantly
unencapsulated phase change material. Furthermore, in the method according to
the invention and the apparatus according to the invention, the storage medium is
fed to the heat exchanger and the storage medium is removed after the phase
change has been completed, in such a manner that there is a spatial separation
between storage of the storage medium and the site of the heat exchange.
In this case, during the charging process, the storage medium is fed as a fluid
stream or as a particle stream to a first heat-exchange device and, after the phase

change has been completed, the storage medium is removed from the heat-exchange
device as a fluid stream. During the discharging process, the storage medium is fed
as a fluid stream to the first heat-exchange device or a further heat-exchange device
and after the phase change has been completed the storage medium is removed
from the heat-exchange device as a fluid stream or as a particle stream. The term
"fluid" in this case and hereinafter comprises substances in liquid and/or gaseous
phase and/or mixtures of substances of liquid and gaseous phases.
The apparatus according to the invention for storing and releasing heat by a phase
change material has a storage medium which comprises a phase change material.
Furthermore, the apparatus according to the invention comprises at least one first
heat-exchange device and is constructed for a discharging process with release of
latent heat of the storage medium by a phase change in the first heat-exchange
device and for a charging process with storage of heat as latent heat in the storage
medium by a phase change in the first heat-exchange device or a further heat-
exchange device. It is important that the storage medium at least predominantly
comprises unencapsulated phase change material, that in the discharging process,
the storage medium can be fed as a fluid stream to the first heat-exchange device,
and after the phase change has been completed, the storage medium can be
removed from the heat-exchange device as a fluid stream or as a particle stream,
and also that in the charging process, the storage medium can be fed to the first
heat-exchange device or a further heat-exchange device as a fluid stream or as a
particle stream, and after the phase change has been completed, the storage
medium can be removed from the heat-exchange device as a fluid stream.
In the method according to the invention and the apparatus according to the
invention, therefore by feeding and removal of the storage medium to and from the
site of the heat exchange, a spatial separation between heat exchange and storage
of the storage medium is provided, in such a manner that, in particular, it is
possible for the storage capacity to be selectable as desired by provision, for
example, of appropriately dimensioned storage tanks for the storage medium,
without an obligatory coupling existing with respect to the dimensioning of the
heat-exchange device. This results in a cost reduction, since the cost-intensive
elements of the heat-exchange device can be optimized, even in the case of a large
heat storage volume merely with respect to an optimum heat exchange.

By using unencapsulated phase change material, previously known PCM materials
can be used, in particular for the temperature range relevant for many applications
between 120°C and 300°C, likewise for higher temperatures, in such a manner that
the temperature restriction resulting when microencapsulated phase change
materials are used is dispensed with. In the method according to the invention and
the apparatus according to the invention, therefore, the storage medium is fed and
removed directly to and from the heat-exchange device.
It is within the scope of the invention that the same heat-exchange device is used
for the charging process and for the discharging process. Likewise, the use of a
plurality of heat-exchange devices is possible. Preferably, at least two heat-
exchange devices are provided, wherein the discharging process is carried out in a
first heat-exchange device and the charging process in a second heat-exchange
device spatially separated therefrom. Hereinafter, the expression "heat-exchange
device" denotes the device assigned to the respective process, regardless of whether
one or more heat-exchange devices are provided for charging and discharging
processes, unless stated otherwise.
Furthermore, in the charging process, in the heat-exchange device during the phase
change, active transport of the storage medium and heat supply proceed
simultaneously and/or during the discharging process in the heat-exchange device
during the phase change, active transport of the storage medium and heat removal
proceed simultaneously. Correspondingly, in the apparatus according to the
invention, at least the first heat-exchange device is constructed in such a manner
that, during the charging process, in the heat-exchange device during the phase
change, active transport of the storage medium and heat supply can be carried out
simultaneously.
In the method according to the invention and the apparatus according to the
invention, by the phase change in the heat-exchange device, the flow properties of
the storage medium change between the gaseous, liquid and/or solid phases. A
further advantage of the invention therefore results from the fact that as described
above, active transport of the storage medium proceeds simultaneously to the phase
change. The necessary transport of the storage medium through the heat-exchange
device is therefore ensured at least during the active transport even when the flow
properties change owing to a phase change. The heat-exchange device of the

apparatus according to the invention therefore combines the properties of a
transport device and the properties of a heat exchanger.
The storage of the storage medium after the charging process and/or after the
discharging process proceeds in the method according to the invention in at least
one storage tank. This provides the spatial separation between storage of the
storage medium and site of the heat exchange, in such a manner that the storage
volume is selectable independently of the design of the heat-exchange device.
Therefore, this provides a decoupling between storage of the storage medium on the
one hand and design of the heat-exchange device, in particular dimensioning of the
heat-exchange surface areas on the other. Correspondingly, the apparatus according
to the invention comprises at least one storage tank for receiving the storage
medium after removal from the first heat-exchange device and/or a further heat-
exchange device.
In a preferred embodiment of the method according to the invention, the storage
medium is stored in a first storage tank after the discharging process and in a
separate second storage tank after the charging process, and so the storage tanks
can be optimized for the respective phase state of the storage medium.
Correspondingly, the apparatus according to the invention preferably comprises two
storage tanks, for storage of the storage medium in a first storage tank after
carrying out the charging process, and in a second storage tank after carrying out
the discharging process.
Likewise, the use of only one storage tank for storage of the storage medium both
after the charging process and after the discharging process is within the scope of
the invention. In this case, preferably, recourse is made to separation of the storage
medium regardless of the phase. Preferably, such a storage tank therefore has feed
lines and outlet lines at different heights, and so the storage medium can be
supplied and removed independently of the density and therefore independently of
the phase.
In any case, the design of the storage tank or storage tanks as heat-insulated
storage tanks is advantageous, in order to reduce heat exchange between the
storage medium and the environment of the storage tank.

Storage tank and heat-exchange device are preferably connected by fluid-conducting
lines.
Preferably, in the method according to the invention the storage medium is actively
transported during the charging process and/or discharging process mechanically by
motor-driven conveying means to the heat-exchange device and the heat exchange
proceeds via the conveying means, in particular the motor-driven elements thereof,
and/or the stationary elements thereof. At least one heat-exchange device of the
apparatus according to the invention preferably therefore has means for conveying
the storage medium and is constructed in such a manner that, during the phase
change, active transport of the storage medium and heat supply and/or heat
removal can be carried out simultaneously, in particular by heat exchange via the
conveying means, preferably the motor-driven elements thereof and/or stationary
elements thereof.
A particularly efficient design of the method according to the invention and the
apparatus according to the invention results therefrom, since the motor-driven
elements and stationary elements of the heat exchanger are directly in contact with
the storage medium. Preferably, the heat exchange therefore proceeds at least via
the motor-driven elements of the heat-exchange device that are directly in contact
with the storage medium. Thereby, the contact necessarily existing between the
motor-driven elements and the storage medium is simultaneously used for heat
exchange, and so no separate surfaces for heat exchange need to be provided or they
can at least be reduced.
A particularly advantageous design of the method according to the invention and
the apparatus according to the invention results from the active transport of the
storage medium during the charging and/or discharging process by a screw
conveyor. The screw conveyor comprises a conveyor screw having a screw thread
arranged on a screw shaft and also a housing at least in part surrounding the
conveyor screw. The housing surrounds the conveyor screw at least in part and is
preferably constructed so as to be cylindrical, covering the conveyor screw over the
entire periphery. Such screw conveyors are known per se for use in methods and
apparatuses outside this specialized field and are described, for example, in
DE 1653872 and DE 288663.

A screw conveyor has the advantage that the storage medium can be transported in
different phases, in particular in the solid and liquid phases by a screw conveyor.
Furthermore, the conveyor screw constructed so as to be able to be driven by motor
of a screw conveyor has a large surface area of the screw thread, which is directly in
contact with the storage medium, and so, in particular, the conveyor screw, in
addition to the transport function, is simultaneously suitable as a heat-exchange
element. This provides in a structurally simple design, the function not only of
transport but also of heat exchange. Preferably, moving and/or non-moving
elements of the screw conveyor have pathways for a heat transport fluid, for
supplying or removing heat. In particular, it is advantageous that the screw shaft of
the conveyor screw of the screw conveyor has pathways for the heat transport fluid,
wherein it is preferred that the shaft of the conveyor screw is constructed as a
hollow cylinder. This design of conveyor screw of a screw conveyor as a hollow screw
is known in applications outside the specialist field and is described, for example, in
DE 288663. Likewise, it is advantageous that the housing of the screw conveyor has
pathways for the heat transport fluid. The use of a screw conveyor simultaneously
as conveying means and as heat-exchange device in the case of latent heat stores
leads to surprisingly structurally simple designs of the apparatus according to the
invention and makes possible, in a simple manner, the spatial separation between
storage of the storage medium and the site of heat exchange.
In the method according to the invention, the storage medium is transported
directly. Preferably, the storage medium therefore does not comprise a carrier
medium, such as water, for example, i.e. preferably no carrier medium is used for
transporting the storage medium.
The method according to the invention and the apparatus according to the invention
are not restricted to certain phase transitions. Thus, the use of the method and the
apparatus for phase change materials and at temperature ranges at which during
the discharging process a phase change proceeds from gaseous to liquid and during
the charging process a phase change proceeds from liquid to gaseous is within the
scope of the invention. Particularly advantageously, the method according to the
invention and the apparatus according to the invention are designed, however, in
such a manner that during the discharging process the storage medium is supplied
in liquid form, a phase change from liquid to solid proceeds and the storage medium
is removed as a particle stream and that during the charging process, the storage

medium is supplied as a particle stream, a phase change from solid to liquid
proceeds, and the storage medium is removed in liquid form.
The solid/liquid phases of the phase change material have the advantage that a
simpler transport of the storage medium in these phases can be carried out. In
particular, when the heat-transfer device is embodied as a screw conveyor,
transport of the storage medium is possible in a simple manner either as a particle
stream or else in liquid form.
Preferably, in the method according to the invention, during the discharging process
in the heat-exchange device during and/or after the phase change of the storage
medium into the solid phase, comminution of the storage medium to particles takes
place. Correspondingly, at least the first heat-exchange device of the apparatus
according to the invention preferably has comminution means and is designed for
comminution of the storage medium to particles during and/or after the phase
change. This ensures that the storage medium is transportable as a particle stream
even after the phase change to the solid phase.
In particular, in the preferred design of the heat exchanger as a screw conveyor, by
structural design of the conveyor screw and/or by design of the surface of the
conveyor screw, comminution of the storage medium to particles can be effected. In
particular, it is advantageous to construct the screw conveyor with a plurality of
adjacently arranged conveyor screws having parallel screw shafts. The screw
conveyor in this case is designed in such a manner that at times a mutual erosion of
the flanks of the conveyor screws proceeds, in such a manner that adhesion of
storage material is prevented. In this case, recourse can be made to previously
known structural designs in which the erosion is achieved by temporary change of
the speed of rotation of at least one screw. This is described in DE 1553134 in the
embodiment having two counter-rotating conveyor screws, of which one conveyor
screw is constructed so as to be right handed and the other conveyor screw is
constructed so as to be left handed. Likewise, the erosion can be achieved in a
manner known per se in the case of co-rotating and identically-handed conveyor
screws, the flanks of which have a spacing in the central position, by changing the
speed of rotation of one or both conveyor screws, as described, for example, in
DE 1653872.

Likewise, it is within the scope of the invention to provide separate comminuting
means, such as, for example, mutually engaging gear-like comminution tools, blade-
like tools, choppers or other elements for comminuting the storage medium in the
heat-exchange device.
Preferably, the first heat-exchange device is constructed in such a manner that the
storage medium is in granular form after the discharging process and optionally
additional comminution.
The design of the heat-exchange device as a screw conveyor is particularly
advantageous, as described hereinbefore. In particular for the discharging process,
the use of a screw conveyor is advantageous on occurrence of the phase transition
from liquid to solid. Therefore, preferably, at least the heat-exchange element used
for the discharging process has a screw conveyor. Likewise, the design of the heat-
exchange device as per other conveying means, such as structural designs that are
known per se, for example, of pumps, in particular an apparatus having gear-
shaped mutually engaging rolls which are arranged transversely to the transport
direction of the storage medium, or differently designed gear pumps, is in the scope
of the inventipn.
A particularly efficient storage and release of heat results from a preferred
embodiment of the method according to the invention and the apparatus according
to the invention in that, during the charging process, heat is additionally fed to the
storage medium after the phase change for additional storage of sensible heat by
the storage medium and correspondingly during the discharging process, sensible
heat is removed from the storage medium before the phase change. Therefore, not
only the storage capacity for latent heat but in addition also the storage capacity for
sensible heat of the storage medium is utilized thereby. The supply and removal of
sensible heat proceeds in a temperature range in which no phase change of the
storage medium proceeds.
The supply and removal of sensible heat preferably proceeds via the same heat
exchanger, via which the latent heat'is supplied to and removed from the storage
medium. As a result, no additional heat exchangers are necessary.

In a further preferred embodiment of the method according to the invention and the
apparatus according to the invention, the sensible heat is supplied and removed by
one or more additional heat exchangers, wherein the heat exchanger in the charging
process is connected downstream of the heat-exchange device for storage of the
latent heat and in the discharging process, is connected upstream of the heat-
exchange device for removing the latent heat. By this means optimization of the
respective heat exchanger for transfer of the latent or sensible heat is possible.
The method according to the invention and the apparatus according to the invention
are preferably constructed for a temperature range in the heat exchange between
100°C and at least 350°C, since a multiplicity of typical applications are in this
temperature range. Equally, applications are known at which higher temperatures,
in particular temperatures up to 500°C, are advantageous. These higher
temperature ranges are also within the scope of the invention when an appropriate
storage medium is selected. Advantageously, the method according to the invention
and the apparatus according to the invention are therefore constructed for
temperatures in the range between 100°C and 500°C. In particular, in the case of
additional storage of sensible heat as described above, higher temperatures, for
example in the range between 100°C and 500°C, are also within the scope of the
invention. The above-mentioned temperature ranges are achievable, in particular,
by using salts as storage medium.
The heat exchange during charging and discharging processes is possible in
principle in a manner known per se. In particular, the supply and/or removal of
heat using a heat transport fluid, preferably using a heat transport gas or a heat
transport liquid or a mixture of liquid and gas is within the scope of the invention.
In particular, the use of thermal oil, water, steam or a water-steam mixture
(saturated steam) as heat transport fluid is advantageous. Likewise, the supply
and/or removal of heat in other ways, for example using radiation, is within the
scope of the invention, in particular the leading of the storage material through the
absorber of a solar collector during the charging process.
The use of a heat transport fluid for the heat exchange in the charging process
and/or the discharging process is in particular advantageous. A particularly
structurally simple design results from the use of a liquid such as, for example,
thermal oil or water, as heat transport fluid.

In a further preferred embodiment of the method according to the invention and the
apparatus according to the invention, during the discharging process, a phase
change of the heat transport fluid from liquid to gaseous is caused by the heat given
off from the storage medium to the heat transport fluid and/or, during the charging
process, a phase change of the heat fluid from gaseous to liquid is caused due to the
heat given off from the heat fluid to the storage medium. By this means, a
particularly efficient transfer and transmission of the stored heat energy of the
latent heat store is achieved. In particular, the above mentioned preferred
embodiment is advantageous on use of the apparatus according to the invention and
the method according to the invention in combination with turbines driven by the
heat-transfer fluid. A further increase in efficiency in the heat transfer using the
heat transport fluid is achieved in a preferred embodiment, in that, during the
discharging process, the heat transport fluid is first vaporized using the heat
released by the storage medium and then the vaporous heat transport fluid is
additionally superheated using the heat released by the storage medium.
As mentioned hereinbelow, in particular the construction of the heat-exchange
device as a screw conveyor is advantageous. Preferably, in this case, the heat carrier
fluid flows through the motor-driven conveyor screw for heat exchange. In
particular, it is advantageous that the heat fluid flows for heat exchange through
not only the conveyor screw, but also the stationary elements of the screw conveyor
which are in direct contact with the storage medium, in particular the housing, in
particular preferably according to the counterflow principle.
Likewise, the configuration of the heat exchanger according to other previously
known embodiments of heat exchangers such as, for example, as an entrained-flow
heat exchanger or as emitter or absorber for radiation is within the scope of the
invention.
Likewise, the supply of heat to the heat-exchange device in the charging process
using a heat-transfer fluid in the vaporous, gaseous or liquid form or by means of a
mixture comprising a plurality of phases is in the scope of the invention, likewise
supplying heat using radiation, in particular solar radiation.

Preferably, the storage medium consists exclusively of phase change material. This
gives an inexpensive configuration of the method and of the apparatus.
An increase in efficiency of the method according to the invention and the apparatus
according to the invention is advantageously achieved in that the storage medium,
in addition to the phase change material, comprises particles having a thermal
conductivity greater than that of the phase change material. By this means, the
overall thermal conductivity of the storage medium is increased and therefore the
efficiency in the heat exchange is increased not only in the charging process but also
in the discharging process. Particularly suitable particles in this case are
nanoparticles or microparticles made of graphite. Preferably, the proportion of these
particles of the total volume of the storage medium is below 10 percent by volume.
A further increase in the efficiency of the method according to the invention and the
apparatus according to the invention is advantageously achieved in that admixtures
are added to the storage medium which favor the formation of granules on
transition from the liquid to the solid phase, or prevent the solidification of the
entire volume, i.e. formation of a high-volume solid phase.
In the method according to the invention and the apparatus according to the
invention, it is possible to use materials that are known per se as phase change
material. Materials which are suitable in particular are salt systems, preferably
binary nitrate salts, nitrate salt mixtures, in particular comprising one or more
substances of the group KN03, NaN03, KNO, KN03-NaN03, KN03-LiN03. The
temperature range between 100°C and 500°C which is relevant in typical
applications is covered thereby.
In the method according to the invention and the apparatus according to the
invention, at least one active transport of the storage medium proceeds during the
phase change in the charging process and/or the discharging process. Preferably,
the apparatus according to the invention comprises one or more additional
transport means which are not designed as a heat-exchange device and are
arranged downstream and/or upstream of the heat-exchange device in the transport
path of the storage medium. This ensures fault-free transport of the storage
medium. In particular, it is advantageous to provide at least one additional
transport means in the transport path of the storage medium in solid, granular

form, since the transport of the granular storage medium is comparatively more
susceptible to interference such as compacting of the storage medium compared
with the transport of the storage medium in liquid or gaseous phase.
The method according to the invention and the apparatus according to the invention
are usable, in particular, for the thermal storage of heat energy, as a thermal store
for solar-heating power plants, in particular in direct steam generation, or as a
thermal store for process heat applications.
Preferably, the storage medium comprises exclusively non-encapsulated phase
change material.
The supply and/or removal of the storage medium as a particle stream in the
advantageous configurations described previously preferably proceed by
transporting the storage medium in granular form, in particular without a carrier
fluid, preferably without a carrier liquid, for the particles.
The transport means of the heat-exchange devices are preferably constructed of
heat-conducting material, in particular of steel, preferably stainless steel. Likewise,
the use of ceramic materials is within the scope of the invention, wherein, however,
they cause higher material costs compared with steel.
The apparatus according to the invention is preferably designed for carrying out the
method according to the invention or an advantageous embodiment thereof.
Likewise, the method according to the invention is preferably designed for being
carried out using an apparatus according to the invention or an advantageous
embodiment thereof.
Further preferred features and embodiments will be described hereinafter with
reference to the figures and the exemplary embodiments. In the figures
Figure 1 shows the schematic representation of a first exemplary embodiment of
an apparatus according to the invention for storing and releasing heat
using a phase change material, wherein the apparatus is part of an
apparatus for converting solar irradiation into electrical energy and
comprises two storage tanks and also two heat-exchange devices and

Figure 2 shows the schematic representation of a second exemplary embodiment of
an apparatus according to the invention for storing and releasing heat
using a phase change material which is a modification of the first
exemplary embodiment and has only one storage tank and also only
one heat-exchange device.
The apparatus of the first exemplary embodiment according to Figure 1 comprises a
first storage tank 1 which contains liquid storage medium. The first storage tank 1
has thermal insulation, such that only slight heat exchange with the environment
takes place. The storage medium consists of the phase change material NaNC>3.
When the storage medium is stored in storage tank 1, the temperature of the
storage medium is above the melting point of the storage medium, i.e. in this case
above 308°C.
The apparatus shown in Fgure 1 further comprises a first heat-exchange device 2
which is fluid-conductingly connected via a line 2a to the storage tank 1. The heat-
exchange device 2 comprises a screw conveyor in which a conveyor screw that is
rotatably mounted and driven using a motor is arranged within a cylindrical
housing. The shaft of the conveyor screw is constructed as a hollow cylinder and is
fluid-conductingly connected to a heat transport fluid circuit. Likewise, in the shell
of the cylindrical housing for the conveyor screw, lines are arranged which are fluid-
conductingly connected to the circuit for a heat transport fluid.
On the exit side of the screw conveyor is arranged a comminution unit which in turn
is connected via a tubular line 3a to a second storage tank 3.
The external circuit for the heat transport fluid is indicated in figure 1 at the heat-
exchange device 2 by arrows.
For carrying out the discharging process, thus, using a control unit (which is not
shown) the conveyor screw of the screw conveyor of the heat-exchange device 2 is set
rotating about the shaft designed as a hollow cylinder, and so, using the screw
conveyor, storage medium is transported out of the storage tank 1 into the heat-
exchange device 2. At the same time, via a pump device that is not shown, supply

and removal of heat transport fluid proceeds according to the abovementioned
arrows, and so heat transport fluid flows through the heat transport fluid circuit of
the heat-exchange device 2. In this case, on the intake side of the screw conveyor,
owing to the lower temperature of the supplied heat transport fluid, compared with
the storage medium, the storage medium is cooled, which leads to a phase change of
the storage medium in the transport screw of the heat-exchange device 2. The heat
released during the phase change is released via the conveyor screw of the screw
conveyor and also the cylindrical housing of the screw conveyor to the heat
transport fluid flowing through these elements, and thus the latter is heated. In the
exemplary embodiment shown in Figure 1, the heat transport fluid is implemented
as water which is supplied to the heat-exchange device 2 in liquid form having a
temperature below the boiling point valid for the prevailing pressure. Due to the
heating in the screw conveyor on account of the released latent heat of the storage
medium, the heat transport fluid undergoes a phase change, and so it leaves the
first heat-exchange device 2 in the form of steam. The steam is converted into
electrical energy by means of a turbine in a further unit which is not shown.
In the screw conveyor of the heat-exchange device 2, therefore a phase change of the
storage medium proceeds from liquid to solid. On account of the transport of the
storage medium during the phase change in the screw conveyor, however, a uniform
solid body does not form, rather, due to the constant further transport, storage
medium particles of different sizes develop. On the exit side of the screw conveyor,
in the heat-exchange device 2, the comminution device is arranged in such a
manner that said particles, after exit from the screw conveyor, pass into the
comminution device and are there comminuted into smaller particles in such a
manner that the storage medium is present in granular form.
Via the tubular connection 3a, the particle stream of the storage medium passes
into the second storage tank 3. The second storage tank 3 is likewise formed in a
heat-insulated manner. The temperature of the storage medium in the storage tank
3 is in the range between the ambient temperature and the melt temperature of the
storage medium, i.e. in this case below 308°C.
For carrying out the charging process, via a further tubular connection 4a, the
storage medium is supplied from the second storage tank 3 as a particle stream to a
second heat-exchange device 4.

The second heat-exchange device 4 likewise comprises a screw conveyor which is
fundamentally identical in structure to the screw conveyor described for the heat-
exchange device 2.
The second heat-exchange device 4 in this case is arranged below the storage tank 3
and the tubular connection 4a opens in the bottom region of the storage tank 3 in
such a manner that using a motor-driven slide valve arranged in the tubular
connection 4a, by means of control via the aforesaid control unit, in a simple
manner, the particle stream from the storage tank 3 into the second heat-exchange
device 4, and thereby on the intake side to the screw conveyor of the second heat-
exchange device 4, is controllable.
Also, the screw conveyor of the second heat-exchange device 4, not only in the
conveyor screw, but also in the cylindrical housing, has pathways of a circuit for a
heat transport fluid. These are fluid-'conductingly connected to a further external
circuit for a heat transport fluid, as indicated by the arrows for the heat-exchange
device 4 in Figure 1.
The second heat transport fluid is also implemented as water. The circuit of the
second heat transport fluid is connected to a solar-heating apparatus in which the
heat transport fluid is vaporized by solar irradiation.
Correspondingly, in the second heat-exchange device 4, a feeding, characterized by
the arrows, of steam into the circuit for the heat transport fluid of the screw
conveyor proceeds, in such a manner that a phase change from solid to liquid of the
storage medium transported in the screw conveyor during the transport proceeds
owing to the heat supplied by the steam and simultaneously, owing to the cooling of
the steam supplied, the heat transport fluid condenses to water. The heat transport
fluid is therefore passed in liquid form from the second heat-exchange device 4 and
to the solar-heating apparatus.
The screw conveyor of the second heat-exchange device 4 is fluid-conduct ingly
connected to the first storage tank 1 on the exit side via a line la. By means of the
screw conveyor of the second heat-exchange device 4, therefore the liquid storage
medium is transported after the charging process into the first storage tank 1.

The screw conveyors of the first heat-exchange device 2 and second heat-exchange
device 4 are each constructed of corrosion-resistant steel, in such a manner that
firstly destruction via the corrosive properties of the storage medium does not take
place and secondly, owing to the high thermal conductivity of the steel, good heat
exchange is ensured between the heat transport fluid and the storage medium.
By the spatial separation of the storage of the storage medium in the storage tanks
1 and 3 on the one hand, and of the heat exchange in the heat-exchange devices 2
and 4, on the other, the dimensions of the screw conveyors are optimized in each
case for an optimum heat exchange under a predetermined conveyor speed.
Independently thereof, the volume of the storage tanks 1 and 3 can be selected as
desired, depending on how high the demand for heat storage capacity is.
In the exemplary embodiment shown in Figure 1 when a 50 MWel turbine is used,
the two storage tanks 1 and 3 each comprise a volume of about 800 m3/h of storage
capacity. That means, for a store having, for example, 7.5 h storage capacity, tanks
each of 6,000 m3 volume are necessary. In comparison therewith, currently designed
salt melt stores based on sensible heat require tanks each having a volume of
14,000 m3.
The particle diameter of the particles of the storage medium after comminution in
the first heat-exchange device 2 is in the range between about 1 mm and 10 mm.
The transport capacity of the screw conveyors of the first heat-exchange device 2
and second heat-exchange device 4 is 500 kg/s for providing heat for a 50 MW
turbine.
The apparatus of the second exemplary embodiment according to Figure 2
comprises only one storage tank 11 which contains both liquid and solid storage
material in granular form. Furthermore, the apparatus shown in figure 2 comprises
only one heat-exchange device 12. Where not stated otherwise hereinafter, the
storage tank 11 is constructed in accordance with the above-de scribed storage tank
1 and the heat-exchange device 12 according to the above-described heat-exchange
device 2.

In contrast to the first exemplary embodiment shown in Figure 1, the storage tank
11 and the heat-exchange device 12 of the second exemplary embodiment each have
two. supply and removal lines for the storage medium.
The storage tank 11 is fluid-conductingly and particle-stream-conductingly
connected to the heat-exchange device 12 via an upper line 11a and a lower line
lib. On account of the differing density of the storage material in liquid and solid
granular form, in the storage tank 11, a spatial separation results between the two
phases, in such a manner that by means of the upper line 11a, storage medium can
be supplied in liquid form and by means of the lower line lib, storage medium can
be supplied in solid granular form to the heat-exchange device 12.
The charging and discharging processes correspond fundamentally to those of the
first exemplary embodiment:
For carrying out the charging process, storage medium is supplied in solid, granular
form via the lower line lib to the heat-exchange device 12. In the heat-exchange
device 12 constructed as a screw conveyor, via a pump device that is not shown, a
supply and removal of heat transport fluid is carried out according to the arrows
12b. A heat transfer from the heat transport fluid to the storage medium proceeds
thereby in the heat-exchange device 12, in such a manner that during the transport
of the storage medium through the conveyor screw, a phase change from solid to
liquid proceeds. The liquid storage medium is returned to the storage tank via the
line 12c which opens out in the upper region of the storage tank 1.
For carrying out the discharging process, liquid storage material is supplied via the
upper line 11a to the heat-exchange device 12 and, by means of a pump device that
is not shown, heat transport fluid is supplied and removed according to the arrows
12a, in such a manner that heat release proceeds from the storage medium to the
heat transport fluid and a phase change of the heat transport fluid from liquid to
solid proceeds during transport through the conveyor screw in the heat-exchange
device 12.
The solid storage material which, similarly to the apparatus shown in Figure 1, is
comminuted by means of a comminutor into a granular form, is returned to the

storage tank via the line 12d, which opens out in the lower region of the storage
tank.
The apparatus according to Figure 2 has the advantage that only one storage tank
and only one heat-exchange device are necessary.
In the apparatuses shown in Figure 1 and Figure 2 in each case the control of the
charging process and discharging process proceeds via a control unit which is
connected in particular to motor drives of the conveyor screws and also to
corresponding motor-actuatable valves and slide valves at the respective exits of the
storage tanks for controlling the transport of the storage medium.

We Claim:
1. A method for storing and releasing heat by a phase change material,
wherein in a charging process, a phase change is caused in a storage medium
which comprises a phase change material by addition of heat in a first heat-
exchange device (4), for storage of the heat in the storage medium as latent
heat, and
in a discharging process, in the first heat-exchange device (2) or another heat-
exchange device (2), a phase change is caused in the storage medium with
removal of heat,
characterized in that,
the storage medium used is at least predominantly unencapsulated phase
change material,
in that, during the charging process, the storage medium is fed to the first
heat-exchange device (4) as a fluid stream or as a particle stream and, after
phase change has been completed, the storage medium is removed,
in that, during the discharging process, the storage medium is fed as a fluid
stream to the first heat-exchange device (2) or another heat-exchange device
(2) and, after phase change has been completed, the storage medium is
removed from the heat-exchange device as a fluid stream or as a particle
stream,
in that the storage medium, after the charging process, is temporarily stored
in a first storage tank (1), and/or after the discharging process, is temporarily
stored in the first storage tank (3) or another storage tank (3),
and in that, during the charging process and/or during the discharging
process, during the phase change, at the same time active transport of the
storage medium and the heat exchange proceeds.
2. The. method as claimed in claim 1,
characterized in that,
the active transport during the charging process and/or discharging process
proceeds mechanically by motor-driven transport means of the heat-exchange
device (2, 4) and the heat exchange proceeds at least via motor-driven
elements and/or stationary elements of the transport means.
3. The method as claimed in claim 2,

characterized in that,
the active transport during the charging process and/or discharging process
proceeds via a screw conveyor, preferably using a screw conveyor comprising
at least one hollow screw.
4. The method as claimed in any one of the preceding claims,
characterized in that,
during the discharging process, the storage medium is fed in liquid form, a
phase change from liquid to solid proceeds, and the storage medium is
removed as a particle stream, and
in that during the charging process, the storage medium is fed as a particle
stream, a phase change from solid to liquid proceeds, and the storage medium
is removed in liquid form.
5. The method as claimed in claim 4,
characterized in that,
during the discharging process the storage medium is comminuted during
and/or after the phase change to particles in the heat-exchange device (2).
6. The method as claimed in any one of claims 4 to 5,
characterized in that,
during the charging process, heat is additionally fed to the storage medium
after the phase change for additional storage of sensible heat, and
in that during the discharging process, sensible heat is removed from the
storage medium before the phase change.
7. The method as claimed in any one of the preceding claims,
in that, during the charging process and/or during the discharging process,
the heat exchange proceeds by a heat transport fluid, preferably by a heat
transport gas or a heat transport liquid or a mixture of liquid and gas.
8. The method as claimed in claim 7,
characterized in that,
during the charging process and/or discharging process, a phase change of the
heat transport fluid is caused, preferably,

in that, during the discharging process, a phase change of the heat transport
fluid from liquid to gaseous is caused by heat given off from the storage
medium to the heat transport fluid
and/or in that during the charging process, a phase change of the heat
transport fluid from gaseous to liquid is caused by the heat given off from the
heat transport fluid to the storage medium.
9. The method as claimed in any one of the preceding claims,
characterized in that,
a storage medium having particles having a thermal conductivity greater
than that of the phase change material is used.
10. The method as claimed in any one of the preceding claims,
characterized in that,
a storage medium having admixtures is used, said admixtures favor
development of granules on transition from the liquid phase to the solid
phase, and/or prevent the development of a high-volume solid phase.
11. An apparatus for storing and releasing heat by a phase change material,
having a storage medium which comprises a phase change material, and
having at least one first heat-exchange device (2), wherein the device is
constructed for a discharging process with release of latent heat of the
storage medium by a phase change in the first heat-exchange device (2) and
for a charging process with storage of heat as latent heat in the storage
medium by a phase change in the first heat-exchange device (4) or other heat-
exchange device (4) of the device,
characterized in that,
the storage medium comprises at least predominantly unencapsulated phase
change material, and
in that the apparatus is formed in such a manner that, during the charging
process, the storage medium can be fed to the first heat-exchange device (4)
as a fluid stream or as a particle stream and after the phase change has been
carried out, the storage medium is removed,
in that, during the discharging process, the storage medium can be fed as a
fluid stream to the first heat-exchange device (2) or further heat-exchange
device (2) and after the phase change has been carried out, the storage

medium can be removed from the heat-exchange device as a fluid stream or
as a particle stream,
in that at least one heat-exchange device (2, 4) has transport means for
transporting the storage medium and is formed in such a manner that,
during the phase change, active transport of the storage medium and heat
exchange can be carried out simultaneously, and
in that the apparatus comprises at least one storage tank (1, 3) for receiving
the storage medium after removal from the first heat-exchange device (2, 4)
and/or a further heat-exchange device (2, 4).
12. The apparatus as claimed in claim 11,
characterized in that,
at least the transport means of the first heat-exchange device (2) comprises a
motor drive and a driven element for transporting the storage medium during
the charging process, and
in that at least the motor-driven element and/or a stationary element of the
transport means is constructed for the heat exchange.
13. The apparatus as claimed in any one of claims 11 to 12,
characterized in that,
at least the first heat-exchange device (2) comprises a screw conveyor,
preferably that each heat-exchange device (2, 4) comprises in each case one
screw conveyor.
14. The apparatus as claimed in any one of claims 11 to 13,
characterized in that,
the first heat-exchange device (2) comprises comminuting means and is
constructed for comminuting the storage medium to particles during and/or
after the phase change.
15. The apparatus as claimed in any one of claims 11 to 14,
characterized in that,
the apparatus is constructed for carrying out a method as claimed in any one
of claims 1 to 11.