US20130068215A1

US20130068215A1 - Method and apparatus for storing and releasing heat by means of a phase change material

Info

Publication number: US20130068215A1
Application number: US13/581,064
Authority: US
Inventors: Anton Neuhäuser; Peter Nitz; Werner Platzer
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2010-02-24
Filing date: 2011-02-08
Publication date: 2013-03-21
Also published as: EP2539661B1; ES2518867T3; WO2011103963A3; DE102010009181A1; CN102803889A; CN102803889B; WO2011103963A2; EP2539661A2

Abstract

A method for storing and releasing heat by means of a phase change material. In said method, a phase change is caused in a first heat exchanging device (4) by supplying heat during a charging process in a storage medium comprising a phase change material in order to store the heat as latent heat in the storage medium, and a phase change is caused in the storage medium while heat is dissipated during a discharging process in the first or another heat exchanging device (2). The invention is characterized in that at least predominantly non-encapsulated phase change material is used as storage medium, the storage medium is fed to the first heat exchanging device (4) as a fluid stream or particle stream during the charging process and is discharged when the phase change has been completed, the storage medium is fed to the first or another heat exchanging device (2) as a fluid stream during the discharging process and is discharged from the heat exchanging device as a fluid stream or particle stream when the phase change has been completed, the storage medium is temporarily stored in a first storage tank (1) following the charging process and/or in the first or another storage tank (3) following the discharging process, and the storage medium is actively conveyed and heat is exchanged during the phase change as the charging process and/or the discharging process take/s place. An apparatus for storing and releasing heat by means of a phase change material is also provided.

Description

BACKGROUND

The invention relates to a method for storing and releasing heat by a phase change material and also such an apparatus.
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 1 816 176 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.

SUMMARY

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 and also an apparatus for storing and releasing heat by a phase change material. Advantageous embodiments of the method according to the invention and of the apparatus according to the invention as described in detail below.
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 invention.
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 KNO₃, NaNO₃, KNO, KNO₃—NaNO₃, KNO₃—LiNO₃. 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.

BRIEF DESCRIPTION OF THE DRAWINGS

Further preferred features and embodiments will be described hereinafter with reference to the figures and the exemplary embodiments. In the figures

FIG. 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

FIG. 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 FIG. 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 NaNO₃.
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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus shown in FIG. 1 further comprises a first heat-exchange device 2 which is fluid-conductingly connected via a line 2 a 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 3 a to a second storage tank 3.
The external circuit for the heat transport fluid is indicated in FIG. 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 FIG. 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 3 a, 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 4 a, 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 4 a 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 4 a, 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 FIG. 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-conductingly connected to the first storage tank 1 on the exit side via a line 1 a. 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 FIG. 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 m³volume 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 FIG. 2 comprises only one storage tank 11 which contains both liquid and solid storage material in granular form. Furthermore, the apparatus shown in FIG. 2 comprises only one heat-exchange device 12. Where not stated otherwise hereinafter, the storage tank 11 is constructed in accordance with the above-described 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 FIG. 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 11 a and a lower line 11 b. 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 11 a, storage medium can be supplied in liquid form and by means of the lower line 11 b, 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 11 b 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 12 b. 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 12 c 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 11 a 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 12 a, 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 FIG. 1, is comminuted by means of a comminutor into a granular form, is returned to the storage tank via the line 12 d, which opens out in the lower region of the storage tank.
The apparatus according to FIG. 2 has the advantage that only one storage tank and only one heat-exchange device are necessary.
In the apparatuses shown in FIG. 1 and FIG. 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.

Claims

1. A method for storing and releasing heat by a phase change material,

wherein in a charging process causing, a phase change 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), causing a phase change in the storage medium with removal of heat,

the storage medium used is at least predominantly unencapsulated phase change material, and

during the charging process, feeding the storage medium to the first heat-exchange device (4) as a fluid stream or as a particle stream and, after the phase change has been completed, removing the storage medium,

during the discharging process, feeding the storage medium as a fluid stream to the first heat-exchange device (2) or another heat-exchange device (2) and, after phase change has been completed, removing the storage medium from the heat-exchange device as a fluid stream or as a particle stream,

temporarily storing the storage medium, after at least one of the charging process or the discharging process in a first storage tank (1), or temporarily storing the storage medium after the charging process in the first storage tank and after the discharging process in another storage tank (3),

and during at least one of the charging process or during the discharging process, during the phase change, at the same time proceeding with active transport of the storage medium and the heat exchange.

2. The method as claimed in claim 1, wherein the active transport during at least one of the charging process or discharging process proceeds mechanically by a motor-driven transport of the heat-exchange device (2, 4) and the heat exchange proceeds at least via at least one of motor-driven elements or stationary elements of the motor driven transport.

3. The method as claimed in claim 2, wherein the active transport during the charging process or discharging process proceeds via a screw conveyor.

4. The method as claimed in claim 1, wherein 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 the particle stream, and during the charging process, the storage medium is fed as the 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, wherein during the discharging process the storage medium is comminuted at least one of during or after the phase change to the particles in the heat-exchange device (2).

6. The method as claimed in claim 5, wherein during the charging process, heat is additionally fed to the storage medium after the phase change for additional storage of sensible heat, and during the discharging process, the sensible heat is removed from the storage medium before the phase change.

7. The method as claimed in claim 1, wherein during at least one of the charging process or the discharging process, the heat exchange proceeds by a heat transport fluid.

8. The method as claimed in claim 7, wherein during at least one of the charging process or discharging process, a phase change of the heat transport fluid is caused.

9. The method as claimed in claim 1, wherein the storage medium has particles having a thermal conductivity greater than that of the phase change material.

10. The method as claimed in claim 1, wherein the storage medium includes admixtures that at least one of favor development of granules on transition from the liquid phase to the solid phase, or prevent the development of a high-volume solid phase.

11. An apparatus for storing and releasing heat by a phase change material, comprising

a storage medium which comprises a phase change material, and at least one first heat-exchange device (2), 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 or further heat-exchange device (4),

the storage medium comprises at least predominantly unencapsulated phase change material, and

the apparatus is formed in such a manner that, during the charging process, the storage medium is fed to the at least one first heat-exchange device as a fluid stream or as a particle stream and after the phase change has been carried out, the storage medium is removed,

during the discharging process, the storage medium is 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 is removed from the heat-exchange device as a fluid stream or as a particle stream,

the at least one heat-exchange device (2, 4) has a transport device to transport the storage medium and is formed in such a manner that, during the phase change, active transport of the storage medium and heat exchange are carried out simultaneously, and

the apparatus comprises at least one storage tank (1, 3) for receiving the storage medium after removal from the at least one first heat-exchange device (2, 4) or the further heat-exchange device (2, 4).

12. The apparatus as claimed in claim 11, wherein at least the transport device 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 at least one of a motor-driven element or a stationary element of the transport device is constructed for the heat exchange.

13. The apparatus as claimed in claim 12, wherein at least the first heat-exchange device (2) comprises a screw conveyor.

14. The apparatus as claimed in claim 13, wherein the first heat-exchange device (2) comprises a comminuter that comminutes the storage medium to particles at least one of during or after the phase change.

15. (canceled)

16. The method as claimed in claim 3, wherein the screw conveyor has at least one hollow screw.

17. The method as claimed in claim 8, wherein 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.

18. The method of claim 8, wherein 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.

19. The apparatus of claim 12, wherein each of the heat-exchange devices comprises in each case one conveyor screw.