WO2018210389A1

WO2018210389A1 - A heat storage unit

Info

Publication number: WO2018210389A1
Application number: PCT/DK2018/050103
Authority: WO
Inventors: Eva Sass LAURITSEN; Stefano SOPRANI; Kurt Engelbrecht; Ludvig CHRISTENSEN
Original assignee: Seas-Nve Holding A/S
Priority date: 2017-05-15
Filing date: 2018-05-14
Publication date: 2018-11-22

Abstract

The invention relates to a heat storage unit comprising; a heat storage chamber defining within its interior a heat storage volume by one or more side walls extending between a bottom wall and a top wall, and a granular media of granules permitting a fluid to flow there through being disposed within the heat storage chamber, wherein the granular media is divided into at least two horizontal granule beds, each separated by a layer of heat insulating material and wherein the heat storage chamber is provided with a plurality of sets of inlets and outlets arranged such that each granule bed is provided with access to its own set of inlet and outlet, for allowing the fluid to flow there between.

Description

A heat storage unit

TECHNICAL FIELD

The present invention relates to a heat storage unit and a method of operating the heat storage unit.

BACKGROUND ART

Presently, energy system operators ensure that the system is balanced in situations with low power production from wind and solar by using power plants or through interconnectors. In the transition towards the progressing demand toward using renewable energy, the need of cost-effective storage solutions becomes more apparent. Regenerative heat storage units may be used for energy storage in electricity generating plants. But storage solutions that can handle true system level storage for more than a few hours, perhaps up to a week, are presently not available. Regenerative heat storage units go through thermal charging and discharging cycling of thermal media with sensible heat. Sensible heat is heat that effects changes in temperature of material, in contrast to latent heat which effects changes in material phase (for example, solid, liquid and gas). Heat is always sensible heat in the context of this patent. Thermal media is material in which heat can be stored. Thermal charging and discharging cycling goes from a heat charged state, in which a heat storage unit's thermal media is relatively hot, to a heat discharged state, in which this media is relatively cold, with periods of heat charging and discharging in between.

A granule is often understood as a compact particle of solid substance with geologist diameter less than 50mm. A geologist diameter of a particle or granule is the greatest straight linear distance between any two points on its outer surface. Granular solid material and granular media are synonymous and are both piled collections of granules. When a heat storage unit's thermal media is made of a packed bed of granular solid material, heat is transferred to the granular solid material during charging through contact with a relatively hotter heat transfer fluid that flows through (interstitial) passages between the piled granules. During discharging of such a heat storage unit, heat is transferred from the granular solid material (cooling it) through contact with relatively cooler heat transfer fluid flowing through interstitial passages between the granules.

Multiple storage technologies currently exist and are used to some extent, but if storage is to be a commercial factor, cost and energy efficiency are the major challenges.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved heat storage unit which comply with the above-mentioned requirements, e.g. which can store huge amounts of energy at a very low cost and at high efficiency. A further object of the invention is to provide a heat storage unit wherein the operation of the heat storage unit can be adapted to a varying incoming electrical power and/or power demand during discharge, thereby providing a more flexible and efficient operation by e.g. charging when the prices are low on electricity and discharging when the electricity prices are high. Another object of the invention is to provide a heat storage unit allowing for the thermal expansion of the stones during heating.

Yet another object of the invention is to provide a heat storage unit having a minimum of energy waste.

Another object of the invention is to provide a heat storage unit using a low- cost and environmental friendly, non-degradable granular media.

According to the invention, there is provided a heat storage unit comprising; a heat storage chamber defining within its interior a heat storage volume by one or more side walls extending between a bottom wall and a top wall, and a granular media of granules permitting a fluid to flow there through between the granules being disposed within the heat storage chamber, wherein the granular media is divided into at least two horizontal granule beds, each separated by a layer of heat insulating material and wherein the heat storage chamber is provided with a plurality of sets of inlets and outlets arranged such that each granule bed is provided with access to its own set of inlet and outlet, for allowing the fluid to flow there between.

The fluid, i.e. the heat transfer fluid, may be air. The granule bed may have different volumes, i.e. different heat capacities. Alternatively, each granule beds may have the same volume, i.e. the same heat capacity or different heat capacities following from use of different granules materials.

Hereby is provided a heat storage unit constructed in layers giving the possi- bility of operating, i.e. storing heat, in a more flexible and efficient manner. The inclusion of two or more horizontal, or essential horizontal, granule beds separated by sufficient insulation layers so that each granule bed can be operated individually depending on the available incoming electrical power and/or power demand during discharge is an advantage, when it is uncertain whether there is energy enough to charge the whole heat storage volume and/or whether discharge of the whole bed is necessary to fulfil the energy demand. Thereby, since the granule beds may be operated independently of one another, there is provided a way of operating in different amounts of energy and in different time perspectives, both with heat and electricity. This as the construction of the heat storage unit allows for days and week between charging and discharging, i.e. the energy may be charge and discharge independently over time.

Hereby is avoided the need of having the heat storage unit being operated fully charged at all time and/or the need of adding an extra energy source to charge the granular media when sufficient of renewable energy are not available. The amount of renewable energy to be used for charging may continuously vary depending on the season, e.g. on how much the wind is blowing and the sun is shining. Likewise, the energy needed, i.e. the energy consumption, when discharging may continuously vary depending on the season, e.g. on the temperature, the amount of daylight, etc.

The top wall, bottom wall and/or the one or more side walls may comprise two layers of materials, i.e. inner walls facing the inside of the heat storage volume V and a wall insulating layer. The inner walls of the heat storage chamber may be made from porous concrete (cellular / aerated concrete) being capable of withstanding temperatures of preferably at least 600°C-800°C. This as the inner walls may be in direct contact with the granular media and thus exposed to the maximally applied temperature. Optionally the inner walls may be coated on an inside surface with Perlite. This as Perlite is known to withstand temperatures to 1000°C. The inner walls may be insulated by the wall insulating layer, e.g. a mineral wool (Rockwool) insulation material, arranged adjoining an outer surface of the inner walls.

The layer(s) of heat insulating material may be air impervious or substantially air impervious. The layer(s) of heat insulating material may be a mineral wool (Rockwool) insulating material. Alternatively, the layer(s) of heat insulating ma- terial may be an Insulfrax® S Blanket having a high temperature stability of up to 1200°C. Insulfrax® S Blankets may be supplied on rolls having a width of about 610mm and 1220mm, a roll length of about 3.5m, 5m, 7.5m and 14.5m and a thickness of about 13mm, 25mm, 38mm and 50mm.

The layer(s) of heat insulating material is placed within the heat storage cham- ber during packing of the heat storage volume with granular media to avoid open air volume between the granule beds and to provide a preferred path for fluid flow through the granular media over the length of the granule beds.

The heat storage unit may be a High-Temperature Thermal Energy storage unit using electricity from renewables, e.g. from wind and solar, during periods of overproduction to heat up the fluid, i.e. air, to at least 600-900°C, where after the heated fluid is blown through the heat storage chamber comprising the granular media. When electricity is needed again, the airflow may be reversed, and the hot air may be lead through a heat recovery steam generator (HRSG) connected to a traditional power station turbomachinery producing electricity. The charging efficiency may preferably be close to 100%, while the discharging efficiency for electricity may be in the range of at least 30-40% in a traditional steam cycle, alternatively more as in the range of at least 40-50%, depending on the system configuration. The remaining energy not regenerated as electricity may be recovered and used in a district heating system. For the remaining energy used for heating the efficiency may be about 95% due to small conversion losses. Hereby may the granular media be heated to at least 600°C with air using low-cost electricity, the heat may be stored for days or up to a week without significantly loss of energy and be discharged when the elec- tricity prices are high. The energy charge and discharge system may, besides the heat storage unit, use any conventional equipment known in the power generation industry, being considered as being suitable therefor.

In one embodiment the layer of heat insulating material is configured for being placed within the heat storage chamber during packing of the heat storage volume with the granular media. For an often desired controlling, the at least two horizontal granule beds are operatable independently of one another.

The granular media may be a non-degradable material having a grain size of 30-40mm and capable of storing energy at 400°C or more in the granules, preferably between 400°C and 800°C. According to one embodiment, the granules have a volumetric thermal expansion below 3%, preferably below 2.5%, when heated from 0-1000°C and/or the granules have a volumetric thermal expansion below 2%, preferably below 1 .5%, when heated from 0-600°C.

According to one embodiment, the material for the granules is Anorthosite. The advantages of using Anorthosite is that Anorthosite is a low-cost material and it has a low volumetric thermal expansion of about 1 .15% at 600°C and about 2.15% at 1000°C.

Alternatively, magnetite may be used; magnetite has a relatively higher heat capacity.

According to one embodiment, a bent edge portion of the layer of heat insulating material is arranged to overlap with the one or more side walls, i.e. overlap with the inside surface of the one or more side walls.

Hereby a minimum of heat is released form one granule bed to another along the edge of the layer of heat insulating material. This also when the granular media expands and retracts, i.e. the heat insulating material is shifted up and down due to its temperature, as the layer of heat insulating material remains in contact with the side walls.

According to one embodiment, the layer of heat insulating material comprises a top layer and a bottom layer.

According to one embodiment, the top layer overlaps the one or more side walls by extending along the side wall(s) towards the top of the heat storage unit and/or wherein the bottom layer overlaps the one or more side walls by extending along the side wall(s) towards the bottom of the heat storage unit. Besides providing a minimum of heat release between granule beds, dead areas of the granular media along the edge of the layer of heat insulating material that never become fully charged is avoided.

According to one embodiment, the layer of heat insulating material comprises a plurality of heat insulating layer sections arranged in one or more layers in an overlapping and/or shingling manner.

Hereby any commercially available layer of heat insulating material may be used independently of the dimensions the layer is available in. According to one embodiment, an inside surface of at least one of the side walls extends outwardly inclined from the bottom wall to the top wall of the heat storage unit.

Hereby allowing for thermal expansion of the granular media during heating and avoiding increased packing and wearing of the granules (destructive thermal ratcheting) as they can slide to some extent. Destructive thermal ratcheting, being the gradual downward rearrangement (through many thermal charging and discharging cycles) of loose granules of the granular media where the granular media abuts the walls of the heat storage chamber, causing increased thermal contraction stresses on the heat storage chambers walls to the point that these increased stresses cause structural failure of the walls. Downward rearrangement of loose granules during thermal ratcheting may occur by (i) gap spaces opening up between the granular media and the heat storage chamber walls due to differences in the thermal expansions of the granular media and the wall and (ii) loose granules settling downward into gap spaces.

According to one embodiment, the inside surface of the one or more side walls extends outwardly inclined at an angle to the vertical of between 15-45 degrees, preferably between 20-25 degrees, in the direction from the bottom wall towards the top wall of the heat storage unit. According to one embodiment, an inner corner defined by abutting side walls is rounded.

The rounded inner corners provide a laminar fluid flow, e.g. air flow, through the heat storage volume. Turbulence and dead areas of the granular media that never become fully charged is hereby avoided, wherein turbulence may provide energy loss in the charging process.

According to one embodiment, the heat storage chamber comprises a vertical or essentially vertical partitioning wall extending perpendicularly or essentially perpendicularly from an end wall of the heat storage chamber and into the heat storage volume to form a U-shaped heat storage volume, and wherein the plurality of sets of inlets and outlets are arranged on the end wall, the inlets being arranged on one side of the partitioning wall and the outlets being arranged on the other side of the partitioning wall. The end wall may preferably be one of the side walls.

The partitioning wall may be made of a heat insulating material.

The basic construction of the U-shaped heat storage volume where the plurality of sets of inlets and outlets for hot and cold air is arranged on the end wall, e.g. on the same side wall, of the heat storage unit, facilitate proper operation of the heat storage unit, e.g. a uniform fluid flow through and an uniform heat distribution in the heat distribution unit.

The U-shape provides a rounded fluid flow and reduces the presence of parts of the granular media, e.g. corners, not being reached by the fluid flow.

The inside surface of the end wall may extend outwardly inclined at an angle to vertical from the bottom wall to the top wall of the heat storage unit. The inside surface of the end wall may incline outwardly at an angle to vertical from the bottom wall towards the top wall of the heat storage unit with between 15- 45 degrees, preferably 20-25 degrees.

The invention further relates to a method of operating the heat storage unit as described above wherein the granular media is divided into a top granule bed, a first lower granule bed, optimally a second, third and fourth lower granule bed, wherein the top granule bed is heated first, then the first lower granule bed, and optionally then the second, third and fourth lower granule bed in the mentioned order. This is an advantage when the fluid is air. The heat storage unit is hereby operated most efficiently as hot air always rises. During charging, valves at each set of inlets and outlets may be progressively opened and closed to fully charge the heat storage unit - one granule bed at the time, starting from the top granule bed.

When the fluid/air exiting the top granule bed is still hot, it can be recirculated through the first lower granule bed for preheating.

During discharging, valves at each set of inlets and outlets may be progressively opened and closed to fully discharge the heat storage unit - one granule bed at the time, starting from the lower most of the lower granule bed.

When the fluid/air exiting the lower granule bed is not hot enough, e.g. below 400°C, it can be sent to the above hot granule bed being either one of the lower granule beds or the top granular bed.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be explained in further details with reference to the figures showing aspects thereof. Figures 1 a-b illustrate a perspective view and an end view of a heat storage unit according to the invention,

Figures 2a-b illustrate a cross-sectional view of the heat storage unit in figures 1 a-b along the cut A-A in figure 1 b,

Figures 3a-b illustrate a cross-sectional view of the heat storage unit in figures 1 a-b along the cut C-C in figure 2b,

Figure 4 illustrates a heat storage unit according to the invention having rounded inner corners,

Figures 5 and 6 illustrates a heat storage unit according to the invention being packed with multiple granule beds, Figures 7a-b illustrate one overlapping manner heat insulating layer sections may be arranged in, Figure 8 illustrates an energy charge and discharge system comprising a heat storage unit according to the invention, and

Figure 9 shows a graph showing the volumetric thermal expansion (%) depending on the temperature (°C) of different granules material. DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE FIGURES

Figures 1 to 4 illustrate a heat storage unit 1 configured for being packed with multiple horizontal granule beds 21 , 22, 23 of a granular media 20, i.e. horizontal layers of granules material in a heat storage volume V shown in Figure 5 and 6, according to the invention. The heat storage unit 1 , i.e. the heat storage chamber 2, comprises four side walls S, a top wall T and a bottom wall B.

The heat storage chamber walls T, S, B may comprise two layers of materials, i.e. inner walls, facing the inside of the heat storage volume V, and a wall insulating layer 10. The wall insulating layer 10 may be provided to prevent any significant distribution heat loss to the surroundings.

One of the side walls S may be an end wall 8 provided with multiple of sets of inlets 4, 4', 4" and outlets 5, 5', 5" arranged in two vertical or essentially vertical rows, wherein the inlets 4, 4', 4" are arranged in one row and the outlets 5, 5', 5" are arranged in another row. A fluid, i.e. a heat transfer fluid, may flow be- tween the inlets and the outlets during operation of the heat storage unit, e.g. during charging and/or discharging. The inlets 4, 4', 4" may be used as inlets during charging and outlets during discharging or opposite. The outlets 5, 5', 5" may be used as outlets during charging and inlets during discharging or opposite. One or more flow distribution plates 9, see Figure 2a, with a pattern of through- going apertures may define the heat storage volume V together with an inside surface 3 of one or more of the heat storage chamber walls T, S, B. The one or more flow distribution plates 9 may be arranged in-between the inlets 4, 4', 4" and the heat storage volume V and/or in-between the outlets 5, 5', 5" and the heat storage volume V. Alternatively, the inlets 4, 4', 4" and outlets 5, 5', 5" may each comprise a flow distribution plate. The flow distribution plates 9 may provide a more uniform flow of fluid from the inlets and into the granular media 20.

As shown on Figure 2b the inlets 4, 4', 4" and outlets 5, 5', 5" may comprise a first opening 41 , a flow distribution chamber 42 and a second opening 43, wherein the second opening 43 may be provided by the pattern of through- going apertures in the flow distribution plates 9. The inside surfaces 3 of three of the four side walls S, i.e. the side walls S excluding the end wall 8, are outwardly inclining from the bottom wall B to the top wall T of the heat storage unit 1 , allowing for thermal expansion of the granular media 20, see Figure 5 and 6, during heating and avoiding increased packing and wearing of the granules as the thermal expansion may cause the granules to slide to some extent.

As shown in figure 4 inner corners 6 between abutting side walls S are rounded such that turbulence and dead areas of the heat storage unit 1 that never become fully charged is avoided, wherein turbulence may provide energy loss in the charging process. A partitioning wall 7 extending from the end wall 8 shapes the heat storage volume V into a U-shape, so that the presence of parts of the granular media, e.g. corners, not being reached by the fluid flow is reduced.

The arrangement of the inlets 4, 4', 4" and outlets 5, 5', 5" on the same side wall S, i.e. the end wall 8, provides a rounded fluid flow and gather all flow distribution and regulating parts so that they may be easier to access.

Figures 5 and 6 illustrate the heat storage unit 1 according to the invention being packed with multiple, in the presently illustrated embodiment three, horizontal granule beds 21 , 22, 23. The top wall T may be configured as a lid construction, allowing for easy access to the heat storage volume V, e.g. facilitating fast exchange of granular media 20, and eliminating an open space without granular media 20 inside the heat storage volume V. In figure 5 the granule beds 21 , 22, 23 are separated by layers of air impervious heat insulating material 30, 31 , each bed being provided with a set of inlets 4, 4', 4" and outlets 5, 5', 5", allowing for only one of the beds to be operated for heating. A bent edge portion (not shown) of the layers of heat insulating material 30, 31 may be arranged to overlap with the inside surface 3 of one or more of the side walls S to reduce unwanted heat transfer by heated air flowing between the granule beds 21 , 22, 23. The bent edge portion may either overlap with the inside surface 3 of the one or more side walls S by extending along the side wall(s) towards the top wall T of the heat storage unit 1 or by extending along the side wall(s) towards the bottom wall B of the heat storage unit 1. As can be seen, the granules press against the edge portion which is thereby pressed against the side wall to provide for an air flow sealing between the beds.

In figure 6 each heat insulated layer comprises a top layer 30', 31 ' and a bottom layer 30", 31 '. A bent edge portion of the top layer 30', 31 ' overlaps the inside surface 3 of the side walls S by extending along the side wall(s) towards the top wall T of the heat storage unit 1 and a bent edge portion of the bottom layer 30", 31 " overlaps the inside surface 3 of the side walls S by extending along the side wall(s) towards the bottom wall B of the heat storage unit 1 , wherein the granular solid material presses the bent edge portions against the inside surface 3 of the side walls S. An arrangement of such layered layers of heat insulating material provides both a minimum of heat release by heated air flowing between the granule beds 21 , 22, 23, and prevents the presents of dead areas of the granular media along the edge of the layer of heat insulating material that never become fully charged. The layer of heat insulating material as shown in figure 5 and 6 may be divided into heat insulating layer sections 32 and layers of heat insulating layer sections 32, e.g. as shown in Figure 7a-b.

Figures 7a-b illustrate one overlapping manner heat insulating layer sections 32 may be arranged in. Alternatively, the heat insulating layer sections 32 may be arranged in a shingling manner.

Figure 8 illustrates an energy charge and discharge system 100 comprising a heat storage unit 1 according to the invention comprising a top granule bed 21 , a lower granule bed 22 and a second lower granule bed 23. The energy charge and discharge system 100 may preferably be based on the use of air as heat transfer fluid. The energy charge and discharge system 100 comprises a manifold 101 containing ambient air, a plurality of fans 102 configured for blowing the air through a plurality of heaters 103 and towards an inlet collector 104. Air from the inlet collector 104 may be distributed to the heat storage unit 1 during charging and recirculation and to an outlet collector 105 during discharging by opening and closing valves.

During charging, inlet valves VI1 , VI2, VI3 at each inlet and outlet valves V01 , V02, V03 at each outlet may be progressively opened and closed to fully charge the heat storage unit 1 - one granule bed at the time, starting from the top granule bed 21 .

When the air exiting the top granule bed 21 is still hot, it can be recirculated through the first lower granule bed 22 for preheating by opening a first recirculate valve V12 and closing the outlet valve V01 .

When the air exiting the lower granule bed 22 is still hot, it can be recirculated through the second lower granule bed 22 for preheating by opening a second recirculate valve V23 and closing the outlet valve V02.

During discharging, inlet valves VI1 , VI2, VI3 at each inlet and outlet valves V01 , V02, V03 at each outlet may be progressively opened and closed to fully discharge the heat storage unit 1 - one granule bed at the time, starting from the lower most of the lower granule bed, in this case the second lower granule bed 23. During discharging the heaters 103 are turned off, thereby cooler air is supplied to the inlet collector 104. When the air exiting the second lower granule bed 23 is not hot enough, e.g. below 400°C, it can be sent to the hot first lower granular bed 22 by opening second recirculate valve V23 and closing inlet valve VI3. Likewise air not being hot enough may be sent from the first lower granule bed 22 to the top granule bed 21 by opening first recirculate valve V12 and closing inlet valve VI2. Figure 9 shows a graph showing the volumetric thermal expansion (%) depending on the temperature (°C) of different granules material including Anorthosite and magnetite. The graph shows that Anorthosite has a low volumetric thermal expansion of about 1 .15% at 600°C and about 2.15% at 1000°C.

The term "comprises/comprising/comprised of" when used in this specification incl. claims is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Claims

1 . A heat storage unit (1 ) comprising;

- a heat storage chamber (2) defining within its interior a heat storage volume (V) by one or more side walls (S) extending between a bottom wall (B) and a top wall (T), and

- a granular media (20) of granules permitting a fluid to flow there through being disposed within the heat storage chamber (2),

characterised in that the granular media (20) is divided into at least two horizontal granule beds (21 , 22, 23), each separated by a layer of heat in- sulating material (30, 31 ) and in that the heat storage chamber (2) is provided with a plurality of sets of inlets (4, 4', 4") and outlets (5, 5', 5") arranged such that each granule bed (21 , 22, 23) is provided with access to its own set of inlet (4, 4', 4") and outlet (5, 5', 5"), for allowing the fluid to flow there between.

A unit according to claim 1 , wherein the layer of heat insulating material (30, 31 ) is configured for being placed within the heat storage chamber (2) during packing of the heat storage volume (V) with said granular media (20).

A unit according to claim 1 or 2, configured for operation of the granular beds (21 , 22, 23) independently of one another.

A unit according to any of the preceding claims, wherein the granules have a volumetric thermal expansion below 3%, preferably below 2.5%, when heated from 0-1000°C.

5. A unit according to any of the preceding claims, wherein material for the granules is Anorthosite or magnetite. 6. A unit according to any of the preceding claims, wherein a bent edge portion of the layer of heat insulating material (30, 31 ) is arranged to overlap with the one or more side walls (S).

7. A unit according to any of the preceding claims, wherein the layer of heat insulating material (30, 31 ) comprises a top layer (30', 31 ') and a bottom layer (30", 31 ").

8. A unit according to claim 7, wherein the top layer (30', 31 ') overlaps the one or more side walls (S) by extending along the side wall(s) towards the top of the heat storage unit (1 ) and/or wherein the bottom layer (30", 31 ") overlaps the one or more side walls (S) by extending along the side wall(s) towards the bottom of the heat storage unit (1 ).

9. A unit according to any of the preceding claims, wherein the layer of heat insulating material (30, 31 ) comprises a plurality of heat insulating layer sections (32) arranged in one or more layers in an overlapping manner.

10. A unit according to any of the preceding claims, wherein an inside surface (3) of at least one of the side walls (S) extends outwardly inclined from the bottom wall (B) to the top wall (T) of the heat storage unit (1 ). 1 1 . A unit according to claim 10, wherein the inside surface (3) of the one or more side walls (S) extends outwardly inclined at an angle to the vertical of between 15-45 degrees, preferably between 20-25 degrees, in the direction from the bottom wall (B) towards the top wall (T) of the heat storage unit (1 ).

12. A unit according to any of the preceding claims, wherein an inner corner (6) defined by abutting side walls (S) is rounded.

13. A unit according to any of the preceding claims, wherein the heat storage chamber (2) comprises a vertical or essentially vertical partitioning wall (7) extending perpendicularly or essentially perpendicularly from an end wall (8) of the heat storage chamber (2) and into the heat storage volume (V) to form a U-shaped heat storage volume (V), and wherein the plurality of sets of inlets (4, 4', 4") and outlets (5, 5', 5") are arranged on the end wall (8), the inlets (4, 4', 4") being arranged on one side of the partitioning wall (7) and the outlets (5, 5', 5") being arranged on the other side of the partitioning wall (7).

14. Method of operating the heat storage unit (1 ) according any of claims 1 to 13, wherein the granular media (20) is divided into a top granule bed (21 ), a first lower granule bed (22), optimally a second (23), third and fourth lower granule bed, wherein the top granule bed (21 ) is heated first, then the first lower granule bed (22), and optionally then the second (23), third and fourth lower granule bed in the mentioned order.

15. Method according to claim 14, wherein during charging hot fluid exiting the top granule bed is recirculated through the first lower granule bed for preheating and/or hot fluid exiting the first lower granule bed is recirculated through the second lower granule bed for preheating.

16. Method according to claim 14 or 15, wherein the heat storage unit (1 ) is discharged one granule bed (21 , 22, 23) at the time, starting from the lower most of the lower granule beds (22, 23).

17. Method according to claim 16, wherein - during discharging - fluid exiting the lower granule bed (22, 23) and not being hot enough, such as below 400°C, is sent to a hot granule bed being either one of the lower granule beds (22, 23) or the top granular bed (21 ).