Patent Application
20230235971
The present invention relates to a heat storage device for sensible heat storage in molten salts.
For storing heat in heat storage systems, in addition to latent heat storage systems and thermochemical heat storage systems, heat storage systems are known that hat perform sensible heat storage.
Heat storage devices for sensible heat storage are known, for example, as so-called liquid salt storage systems in solar thermal power plants. So-called two-tank systems have been implemented on a large scale, in which molten salt heated by solar thermal heating is stored in the so-called hot tank and, in order to discharge the thermal energy, is passed through a heat exchanger into a so-called cold tank. From said cold tank, the cold molten salt is passed through heat exchangers or solar receivers, such as solar tower receivers or parabolic trough receivers, for heating and then stored again in the hot tank. Meanwhile, the temperature remains constant in the respective tank. In such systems, the maximum temperature of the molten salt is limited to approx. 560° C., as otherwise the thermal decomposition of the solar salt is too severe. Solar salt is defined in the literature as a mixture with 60 wt% sodium nitrate and 40 wt% potassium nitrate. During the decomposition of the solar salt, a reaction occurs in the liquid phase and gases are released. The main reactions in the decomposition of the molten salt are a nitrate-nitrite conversion with the release of oxygen and, as temperatures rise, a nitrite-oxide conversion with the release of nitrogen oxides. The release of oxygen is tolerable. However, the toxic nitrogen oxides are to be avoided, as they should not get out of the storage system, but this can only be realized with great effort.
Furthermore, decomposition with nitrogen oxides can lead to an accumulation of oxide ions in the solar salt. The oxide ions can increase the corrosion rate of container materials. Overall, it is therefore desirable to suppress the nitric oxide reaction as far as possible.
A two-tank system also involves a relatively high level of design effort.
In addition to commercial second-tank systems, so-called single-tank systems have also been investigated.
In principle, it is desirable to also store heat at higher temperatures, as this can be used more efficiently during discharge. For this reason, initial investigations have been carried out to increase the temperature of the molten salt to over 560° C.
Overall, the partial pressure of the respective gas phase is decisive for the decomposition of the molten salt. Initial approaches therefore envisage a closed system in which a substantially constant partial pressure of the respective gases is established, which can slow down or reduce further decomposition of the molten salt.
A problem with such closed systems, particularly in the form of two-tank systems, is that a change in volume of both the molten salt and the gas atmosphere above the molten salt in the tank can occur due to the change in temperature during charging and discharging. The liquid salt has a volume increase when heated from 300° C. to 560° C. of approx. 10 vol%. In this temperature range, the gas atmosphere can be subject to a volume change of up to 50%.
In such heat storage tanks, it is also advantageous if they are operated with a maximum overpressure of less than 0.5 bar, since in this case the tanks are not subject to the Pressure Equipment Directive and low wall thicknesses can be used, thus saving design effort and costs.
The technical examples described above are general know-how of the applicant and do not necessarily represent a specific prior art. Moreover, the described systems are not necessarily pre-published.
It is an object of the present invention to provide a heat storage device for sensible heat storage of molten salts, which is of simple constructional design and also enables heat storage of molten salts of more than 560° C.
The heat storage device according to the invention for sensible heat storage in molten salts comprises a heat storage reservoir for receiving molten salt, wherein a separating layer is arranged in the heat storage reservoir for separating a cold region, in which cold molten salt is arranged, and a hot region, in which hot molten salt is arranged, wherein the cold region is arranged below the hot region. The heat storage device further comprises a device for charging and discharging the heat reservoir, which is connected to the cold region and the hot region, and a volume compensation device for compensating for a temperature-related change in volume of the molten salt, wherein the volume compensation device cooperates with the cold region and/or the separating layer. The volume compensation device can be used to advantageously compensate for the temperature-related change in volume of the molten salt that occurs during charging and discharging of the heat storage device. In particular, the volume compensation device can be used to ensure that the heat storage reservoir is completely or almost completely filled in the upper area, and thus in the hot region. When charging and discharging the heat storage reservoir, the cold region, the separating layer and the hot region, or at least part of the separating layer and the hot region, always remain, with the separating layer shifting so that the volume occupied by the hot and cold regions is respectively changed. Since there is always a hot region in which hot molten salt is arranged, even in the discharged state of the heat storage reservoir, a may also be allowed above the hot region in the heat storage reservoir. Since the gas atmosphere is always arranged between the upper walls of the heat storage reservoir and the hot region and thus has a constant temperature, there is no change in the volume of the gas atmosphere during charging and discharging of the heat storage reservoir. Thus, it can also be assumed that a constant partial pressure of the gas phase is present, so that a stability of the hot molten salt is given.
However, the heat storage device according to the invention can be operated particularly advantageously if the volume compensation device ensures that no gas atmosphere or only a very small gas atmosphere is formed above the hot region.
The device for charging and discharging the heat storage reservoir can have a charge pump, a so-called cold pump, which is connected to the cold region when the heat storage device is in a discharged state, and a discharge pump, a so-called hot pump, which is connected to the hot region. During charging, cold molten salt is pumped out of the cold region by means of the cold pump and supplied, for example, to solar receivers of a solar thermal power plant. After heating in the solar receivers, the now hot molten salt is introduced into the hot region. When discharging the heat storage device, hot molten salt is pumped out of the hot region by means of the hot pump and supplied to a heat exchanger. The molten salt cooled by means of the heat exchanger is then supplied to the cold region. Due to the corresponding volume changes of the hot and cold regions, the separating layer moves up and down accordingly.
For example, the separating layer can be a natural separating layer, i.e. that it consists of molten salt, wherein in this case there is a natural temperature layer distribution in the heat storage reservoir. The separating layer may also have a floating installation device, for example a separating plate, whereby thermal insulation of the cold region and the hot region can be achieved in an advantageous manner. It may also be provided that a filler material such as a bulk material, which is typically less expensive than the liquid salt, is introduced in the salt volume. The filler material reduces heat conduction and free convection and a stable separation layer can also be formed. The filler material can be, for example, quartzite sand, silica sand, taconite, basalt, burnt red mud, slags or similar. Such filler material has proven to be particularly advantageous, as it can be flowed through by the molten salt and reduce the costs of the inventory. The heat storage reservoir can be a so-called thermocline tank.
In particular, it may be provided that the molten salt is stored in the heat storage reservoir without pressure, i.e. at atmospheric pressure, or at most with a slight underpressure or overpressure of less than +/-0.5 bar. The volume compensation device cooperates directly with the cold region in particular. In particular, it may be provided that the volume compensation device acts on the molten salt arranged in the cold region or in the separating layer.
Preferably, the volume compensation device comprises a compensation reservoir, the compensation reservoir being connected to the cold region in the discharged state of the heat storage device, wherein a compensation fluid is disposed in the compensation reservoir and cooperates with the cold region for volume compensation.
By providing a compensation fluid, the volume compensation can be carried out in a particularly advantageous manner, for example by introducing the compensation fluid into the heat storage reservoir.
It may be provided that the cold region or the separating layer or the separating layer always occupy, for example, at least 5% of the total volume of the heat storage reservoir.
For example, the compensation reservoir can be connected to the heat storage reservoir by means of a fluid line, the fluid line being connected to a bottom area, for example a bottom plate, with the heat storage reservoir, or to a lower lateral wall area of the heat storage reservoir, the lower lateral wall area of the heat storage reservoir extending across maximum 5% of the height of the heat storage reservoir starting from the bottom area.
Preferably, it is provided that the compensation fluid can be introduced into the cold region or the separating layer for volume compensation. In other words: The compensation fluid is directly introduced into the cold molten salt or the molten salt in the separating layer.
Here, it may be provided that the compensation fluid is cold molten salt. This has the advantage that there is no need for separation between the compensation fluid and the molten salt, so that when the heat storage device according to the invention is discharged, wherein the density of the molten salt arranged in the heat storage reservoir increases, the missing volume can be filled by supplying cold molten salt, so that the heat storage reservoir is always completely or almost completely filled with molten salt. When charging the heat storage device according to the invention, wherein the density of the molten salt in the heat storage reservoir decreases, the excess molten salt can be removed from the cold region or the cold region and the separating layer, respectively.
In doing so, it may be provided that the compensation reservoir is arranged above the heat storage reservoir. This has the advantage that, due to the arrangement of the compensation reservoir above the heat storage reservoir, a slight overpressure can be generated in the heat storage reservoir relative to the surroundings, so that the heat storage reservoir has the desired degree of filling, i.e. either completely filled or at least substantially completely filled. The molten salt in the compensation reservoir is thus forced into the heat storage reservoir by gravity and air pressure for volume compensation. Conversely, during discharging, the cold molten salt can be forced back into the compensation reservoir.
In this respect, it is preferably provided that, when the heat storage device is in the discharged state, the compensation reservoir is connected to the cold region via a riser.
In a preferred embodiment, it is provided that the device for charging and discharging the heat storage device is connected to the cold region via the compensation reservoir when the heat storage device is in the discharged state. In other words: The compensation reservoir can be provided as part of the device for charging and discharging the heat storage reservoir, so that during charging and discharging the corresponding volume compensation is effected by supplying the correspondingly necessary volumes of cold molten salt to the heat storage reservoir via the device for charging and discharging by means of the compensation reservoir or by removing it therefrom, respectively.
In doing so, it may be provided that the device for charging and discharging the heat storage reservoir has a discharge pump connected to the hot region or arranged in the hot region and a charge pump connected to the compensation reservoir or arranged in the compensation reservoir.
Such an arrangement has proved to be particularly advantageous, since, for example, the hot pump can be designed with a relatively short pump shaft, which means that pump costs can be kept low and, in addition, the restriction on the overall height of the heat storage reservoir due to limited realisable pump shaft lengths is eliminated. For example, the compensation reservoir can be designed as a cold salt basin so that atmospheric pressure is applied to the surface of the cold salt. If a connection is provided between the compensation reservoir and the cold region via a riser, the charge pump can also be equipped with a short pump shaft, since the cold molten salt enters the compensation reservoir from the cold region via the riser, so that the charge pump only has to draw in cold molten salt from the compensation reservoir.
In operation, it can also be provided in principle that part of the molten salt in the separating layer is removed from the heat storage reservoir when charging the heat storage device, so that at the end of the charging process the volume compensation device, e.g. the compensation reservoir, is correspondingly connected to the separating layer.
The compensation reservoir can also be connected to a further storage reservoir, which can be a so-called cold tank, wherein the compensation reservoir is connected to the further storage reservoir via an overflow. Thus, the compensation reservoir can be relatively small, since excess cold molten salt can enter the further heat storage reservoir via the overflow. By means of a further pump, the cold molten salt contained in the further storage reservoir can be supplied back into the system. The further storage reservoir can have at least the same volume as the heat storage reservoir, for example, so that the further storage reservoir is also suitable for receiving the entire molten salt, so that the heat storage reservoir can be completely emptied for inspection purposes.
In another embodiment of the heat storage device according to the invention, it may be provided that the compensation fluid is a gas. Since the molten salt used is a major cost factor in systems with heat storage devices, and in the previously described heat storage device in which the compensation fluid is cold molten salt, a certain amount of molten salt is thus always unused, using a gas as a compensation fluid has cost advantages.
In this respect, it is preferably provided that a gas retention device is arranged in the heat storage reservoir, which retains the gas in a predetermined section in the heat storage reservoir after it has been introduced into the cold region or into the separating layer. For example, the gas retention device may be in the form of a plate, such as a curved plate, or a plate having a protruding side edge projecting downwardly from the plate. When the compensation fluid in the form of gas is introduced into the cold region, it traps below the gas retention device so that it remains in this position. When the gas is introduced, it forces the cold molten salt out from under the gas retention device. When the heat storage device according to the invention is charged, the gas is forced back into the compensation reservoir by the cold molten salt. Here, it is advantageous if the compensation reservoir is connected to the cold region by a fluid line, with the fluid line opening into the heat storage reservoir directly below the gas retention device. In this manner, the compensation fluid in the form of gas can be advantageously introduced into and removed from the cold region. The gas retention device can extend from the lower lateral wall region towards the interior of the heat storage reservoir.
The gas retention device in the form of a plate can disturb the temperature stratification in the heat storage reservoir. Therefore, additional devices may be provided in order to stabilize the stratification. For example, an additional diffuser can be provided at the edge region of the gas retention device, through which the stratification is maintained. For example, the gas retention device may circumferentially rest on the wall of the heat storage reservoir and may extend only in one or more sections spaced from the wall to form one or more passages. At the passage or passages, the gas retention device may have the protruding side edge, respectively. Also, the gas retention device may have the diffuser on each of the passage or passages.
It may be provided that the compensation reservoir has a connection to the atmosphere, wherein gas forced out of the heat storage reservoir can be transferred to the atmosphere in the event of temperature-related expansion of the molten salt. The compensation system comprising the compensation reservoir is thus designed as a semi-open system. In the discharging process, the gas can be provided to the compensation reservoir by means of a corresponding gas supply, for example. Unlike the gas phase in the hot region of the storage, the gas phase in the cold region or separating layer of the storage can be substantially free of nitrogen oxides, allowing direct exchange with the environment. Due to the relatively low temperature of the gas, the thermal losses occurring during discharge to the atmosphere are manageable. Furthermore, design requirements for the compensation reservoir and the gas lines connected thereto are relatively low, as these are exposed to relatively low temperatures and relatively small nitrogen oxide pollution.
For example, it may be provided that the compensation reservoir is connected to the atmosphere via a pressure relief valve and to the gas supply via a vacuum valve, so that when the system is charged, with gas being forced into the compensation reservoir, it can be transferred to the atmosphere by means of the pressure relief valve, if necessary. When the heat storage reservoir is discharged, a vacuum is created so that the vacuum valve is opened, allowing the necessary gas to flow into the compensation reservoir.
Instead of a pressure relief valve and a vacuum valve, it is also possible to provide control valves so that a controller controls the discharge of gas to atmosphere and the introduction of gas from the gas supply into the compensation reservoir.
Semi-open systems have the advantage that the compensation reservoir can be relatively small, since it does not have to receive the entire volume of gas required for volume compensation.
It may also be provided that a volume changing device is arranged on the compensation reservoir for changing the volume of the gas. In other words: When it is necessary for the volume compensation device to introduce gas into the heat storage reservoir, the volume changing device increases the volume of the gas and thus decreases the density of the gas. If, during the charging process, the gas is returned from the heat storage reservoir to the compensation reservoir, a reduction in the gas volume and thus an increase in density occurs via the volume changing device. This can be done, for example, by the volume changing device having a heating element and/or a cooling device, wherein the temperature of the gas can be changed by means of the heating element and/or the cooling device to change the volume. In other words: The volume changing device thermally performs the necessary density change of the gas.
For a thermal density and thus volume change of the gas, it may also be provided that the compensation reservoir is connected to a thermal energy storage, for example a regenerator storage, which supplies heat to the gas or extracts heat from the gas, whereby the volume can be changed via the temperature of the gas. The gas can thus be heated or cooled by means of the thermal energy storage.
In addition to a thermal volume change of the gas, a mechanical volume change of the gas is also possible in that the volume changing device has a device for mechanically changing the volume of the gas. This device may include, for example, a blower, compressor, turbine, compactor, or the like.
It is also possible that the compensation reservoir has a variable volume. In this respect, it can be provided, for example, that the compensation reservoir has an elastic, movable or flexible wall or an elastic, movable or flexible wall section. In a charging process of the heat storage reservoir, the gas is forced into the compensation reservoir and, for example, a movable wall or a flexible wall is pressed outward or an elastic wall is pressed outward and stretched accordingly. In the discharging process of the heat storage reservoir, the gas is removed from the compensation reservoir and the elastic, movable or flexible wall is moved back to the previous position. In the exemplary embodiment of a movable wall or a movable wall section, it is particularly possible for the wall or wall section to be displaceable. In principle, it is possible that the flexible or movable wall or the flexible or movable wall section can be driven, for example by means of an active drive or a passive drive, such as an elastic element like a spring or the like. The active drive can, for example, press the movable or flexible wall in such a way that the volume in the compensation reservoir is reduced, so that the gas is forced into the cold region of the heat storage reservoir during the discharging process of the heat storage reservoir. For example, when providing an elastic member on the flexible or movable wall, the elastic member may be compressed when the gas is forced into the compensation reservoir during the charging process of the heat storage reservoir, and during the discharging process of the heat storage reservoir, the elastic member may reform and thus press back the flexible or movable wall so that the gas is forced out of the compensation reservoir.
In the heat storage device according to the invention, it may also be provided that the volume compensation device has a compensation chamber in the cold region or the separating layer, respectively, the compensation chamber having a variable volume, wherein the compensation fluid can be introduced into the compensation chamber. The compensation chamber with the variable volume may have, for example, an elastic, movable or flexible wall or an elastic, movable or flexible wall section. When the heat storage reservoir is discharged, the compensation chamber expands as the compensation fluid is introduced for volume compensation in the heat storage reservoir. When the heat storage reservoir is charged, the compensation chamber is compressed, for example, so that the compensation fluid leaks from the compensation chamber. The movable wall or the movable wall section, respectively, of the compensation chamber can be displaceable, in particular.
In the heat storage device according to the invention, it may also be provided that the heat storage reservoir has a movable wall or a movable wall section. For example, the bottom of the heat storage reservoir can be movable and, for example, can be moved, in particular displaced, by means of a drive device. Thus, the volume compensation device effects a volume change of the heat storage reservoir in which the movable wall or the movable wall section is moved to change the volume.
In the following, the invention is described in more detail with reference to the following figures. In the drawings:
In
The heat storage device 1 has a heat storage reservoir 3 formed as a tank, for example. The heat storage device 1 according to the invention serves for heat storage of sensible heat in molten salts and can be used, for example, for heat storage of thermal energy of a solar power plant.
The heat storage reservoir 3 of the heat storage device 1 according to the invention is designed as a so-called single-tank system, in which three different temperature layers of the molten salt located in the reservoir are present in the heat storage reservoir 3. Said tank is a tank according to the so-called thermocline principle.
For example, the heat storage reservoir 3 is a flat-bottom tank and surrounded by an insulating layer 5.
In the heat storage reservoir 3, the molten salt forms a so-called hot region 7, in which hot molten salt 9 is arranged, and a cold region 11, in which cold molten salt 13 is arranged. Hot molten salt is understood to be molten salt up to approx. 620° C. Cold molten salt is in the range of 300° C. Both the hot and cold molten salt are always liquid. The cold region 11 is arranged below the hot region 7. The hot region 7 is separated from the cold region 11 by a separating layer 15, wherein the separating layer 15 can be formed as a natural separating layer, i.e. also of molten salt. The separation layer 15 may also have appropriate fittings, such as a floating separating plate.
In the exemplary embodiment shown in
A non-illustrated device for charging and discharging the heat storage reservoir 3 removes hot molten salt 9 from the hot region 7 for discharging, with cooled cold molten salt 13 being introduced into the cold region 11. When charging the heat storage device 1 according to the invention, cold molten salt 13 is removed from the cold region 11 and introduced into the hot region 7 after heating. During charging and discharging, the separating layer 15 thus moves up and down accordingly in the heat storage reservoir 3.
Since hot molten salt 9 has a low density and thus a larger volume than the cold molten salt 13, it is necessary to compensate for the volume in the heat storage reservoir 3 in order to keep the thin gas layer 17 as constant as possible or to keep the heat storage reservoir 3 completely filled with molten salt.
Therefore, the heat storage device 1 according to the invention comprises a volume compensation device 19. The volume compensation device 19 has a compensation reservoir 21 which is connected to the cold region 11 in the illustrated state. A compensation fluid is arranged in the compensation reservoir 21, said compensation fluid cooperating with the cold molten salt 13 in the cold region 11 for volume compensation. In the exemplary embodiments illustrated in
In
In
Since the cold molten salt 13 has a greater density than the hot molten salt 9, the cold molten salt 13 occupies a smaller volume in the heat storage reservoir 3, so that the heat storage reservoir 3 would not be completely filled, wherein the supply of cold molten salt by the volume compensation device compensates for the missing volume. Accordingly, the compensation reservoir is largely empty.
As can be seen from
In
The riser 23 is connected to the compensation reservoir 21 via a valve 28. In the discharging process, hot molten salt 9 is supplied to a discharging process 110 by means of the discharge pump 25. The cold molten salt cooled by the discharging process is supplied to the compensation reservoir 21 and, when the valve 28 is open, passes through the riser 23 into the cold region 11 due to the atmospheric pressure acting on the cold molten salt in the compensation reservoir 21.
When charging the heat storage device 1 according to the invention, the cold molten salt from the compensation reservoir 21 is supplied by means of the charge pump 27 to a charging process 120, which can be carried out by a solar thermal power plant, for example. The heated molten salt is supplied to the hot region 7. The hot molten salt 9 in the hot region 7 presses down the separating layer 15 so that cold molten salt 13 from the cold region 11 is forced through the riser 23 into the compensation reservoir 21 with the valve 28 open.
The embodiment shown in
As shown in
The further reservoir 29 can have a size adapted to the heat storage reservoir 3 so that the further reservoir 29 can also be used for emptying the heat storage reservoir 3 for inspection or repair purposes. For this purpose, a drain pump 35 is provided in the heat storage reservoir 3, which is connected to the compensation reservoir 21 via a return line.
In
The substantial difference between the third exemplary embodiment show in
As can be seen from
As can be seen from
As part of the discharging process of the heat storage device 1 according to the invention, the gas 39 must be introduced from the compensation reservoir 21 into the heat storage reservoir 3. For this purpose, it is necessary that a pressure is generated in the compensation reservoir 21 which forces the gas 39 into the heat storage reservoir 3, since the gas below the gas retention device 37 is subject to hydrostatic pressure due to the liquid salt column above it.
When the gas is introduced into the heat storage reservoir 3 as part of the discharging process of the heat storage device 1 according to the invention, gas is supplied to the compensation reservoir 21 by means of an inlet valve 21c. In the exemplary embodiment shown in
In the exemplary embodiment shown in
In the exemplary embodiment shown in
In the exemplary embodiment shown in
In addition to thermal pressure generation for introducing the gas into the heat storage reservoir 3, mechanical pressure generation is also possible.
In
The heat storage device 1 according to the invention has the particular advantage that there is no or only a small volume of gas above the hot molten salt 9, so that the tendency of the salt to decompose is reduced and only slight or no outgassing occurs or increased operating temperatures are made possible, respectively. In this regard, the heat storage reservoir 3 of the heat storage device 1 according to the invention can be operated with an internal pressure that has a difference of less than 500 mbar with respect to the atmosphere, so that the heat storage reservoir 3 is not subject to the Pressure Equipment Directive. If there is a gas phase above the hot region, it has a substantially constant temperature during operation so that no volume compensation is necessary for a volume change of the gas. In this way, the heat storage device 1 according to the invention enables particularly advantageous heat storage in the molten salt.
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| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2020 115 656.1 | Jun 2020 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/063710 | 5/21/2021 | WO |
