US20150136115A1

US20150136115A1 - Heat storage tank with improved thermal stratification

Info

Publication number: US20150136115A1
Application number: US14/399,653
Authority: US
Inventors: Arnaud Bruch; Raphaël Couturier; Jean-François Fourmigue
Original assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2012-05-09
Filing date: 2013-05-06
Publication date: 2015-05-21
Also published as: AU2013258175A1; AU2013258175B2; ZA201408153B; FR2990502B1; FR2990502A1; TN2014000473A1; EP2847535B1; ES2627443T3; EP2847535A1; WO2013167538A1

Abstract

Heat storage tank comprising an envelope (2) with a longitudinal axis (X) filled with a heat transfer liquid and solid heat storage elements, a first longitudinal end provided with first means (10) for collecting and supplying a liquid at a first temperature and a second longitudinal end provided with second means (12) for collecting and supplying a liquid at a second temperature, in which said solid heat storage elements are distributed across three beds (TH1, TH2 and TH3) superposed along the longitudinal axis (X), separated by a layer of liquid (L1, L2 and L3), the heat transfer liquid being capable of flowing from the first longitudinal end to the second longitudinal end.

Description

TECHNICAL FIELD AND PRIOR ART

The present invention relates to a heat storage tank with improved thermal stratification.
Numerous fields and numerous industrial applications implement the storage of heat. The storage of heat enables the valorisation of heat stemming from industrial processes, the recovery of surplus energy or dissociating the moment of production of thermal energy from the use thereof.
As an example, in the CSP field (CSP designating “Concentrated Solar Power”), the surplus heat produced at times of strong sunshine may thus be stored so as to be exploited at the end of the day.
The storage of heat may typically be realised either in the form of sensitive energy (by varying the temperature level of a solid or liquid storage material), in the form of latent energy (by changing the phase of a storage material) or finally in the form of chemical energy (using endothermic and exothermic chemical reactions).
In the case of sensible heat storage, the heat is stored by raising the temperature of a storage material which may be liquid, solid or a combination thereof.
Industrial processes involving a use or a conversion of thermal energy by means of a thermodynamic cycle, for example by the use of a steam turbine, involve overall two temperature levels which are the conditions at the limits of the cycle. It is sought to maintain these two temperature levels as constant as possible in order to obtain optimised operation of the cycle. In fact, as an example, steam turbines, which assure the conversion of thermal energy into electrical energy, have higher efficiency when the input temperature in the turbine is maintained constant at a predefined value. Consequently, storage associated with such systems must thus respect these characteristics and make it possible for example to destore heat at a constant temperature level.
An example of this type of operation is the field of concentrated solar power where a typical storage system consists of two tanks filled with storage fluid at two temperature levels. One of the tanks stores at a constant low temperature and the second storage tank at a constant high temperature. The output temperature of the hot tank is thus constant throughout destorage.
Systems only comprising a single tank containing both the hot fluid and the cold fluid also exist. There then exists thermal stratification within the tank, the hot fluid situated in the upper part and the cold fluid situated in the lower part are then separated by a transition region known as “thermocline”.
The use of a single tank makes it possible to reduce the number of components, such as pumps, valves, etc. and to simplify command-control.
In thermocline type storage, the storage material may be a heat transfer liquid or, advantageously, a mixture of a heat transfer fluid and a cheap solid material. The use of such a solid material furthermore makes it possible to improve the segregation of the hot fluid and the cold fluid while reducing remixing effects. In the latter case, this is then referred to as “dual thermocline” (or “mixed-media thermocline”).
This “dual thermocline” tank has the advantage of reducing the quantity of liquid necessary, given that solid rock type materials are cheap, the total cost is reduced.
In a thermocline tank, in order to take account of density differences and to avoid natural convection movements, the heat transfer fluid is introduced via the top of the tank during storage phases and via the bottom of the tank during destorage phases. Storage is thus characterised by a hot zone at the top of the vessel, a cold zone at the bottom and a transition zone between the two zones known as a thermocline. The principle of this type of heat storage is to create a “heat piston”, that is to say the advance of a thermal front that is as thin as possible and uniform transversally. This makes it possible to maintain constant temperatures during charge and discharge phases.
During charge phases, cold liquid is removed from the tank via the bottom and is heated, for example by passing through a heat exchanger of a solar collector, and then sent back into the tank via the top. During discharge phases, hot liquid is removed from the tank via the top, and is sent for example to the evaporator of a thermodynamic cycle incorporating a turbine, in which it is cooled and is then sent back into the tank via the bottom. During charge and discharge phases the heat piston moves downwards and upwards respectively.
“Dual thermocline” type storage based on a mixture of liquid heat transfer fluid and solid matrix brings into play very low fluid velocities of the order of several mm/s in order to assure the transfer of heat between the fluid and the static charge and to limit inhomogeneities.
Thermocline tanks using a mixture of a liquid heat transfer fluid and a solid matrix are subject to the problem of “thermal ratcheting”: during heating phases, the vessel expands and the solid matrix descends to occupy the freed space. During cooling phases, the vessel contracts and is constrained by the packed bed of rocks. The dimensioning of a vessel for dual thermocline storage must therefore find an equilibrium between:

- mechanical strength, linked to the thermal ratcheting which guides towards rather flat geometries, i.e. large tank diameter and small tank height in order to reduce the height to diameter ratio;
- hydraulics, which guide towards cigar type geometries, i.e. small tank diameter and large height, in order to favour homogeneous distribution of heat transfer fluid and to retain a thin and transversally uniform heat piston.

In real operation, such a storage system has inhomogeneities and the heat piston is not perfect. These inhomogeneities can stem from:

- inhomogeneities in the distribution of the static charge which may be linked to the initial filling of the storage tank (non-homogeneous mixture, segregation of the rock and sand etc.) or to the thermal cycling of the solid matrix which “comes alive” during thermal expansions and contractions of the tank;
- edge effects at the level of the tank wall. These edge effects are of the hydraulic type, since the wall induces inhomogeneity of the static charge, and of the thermal type due to a cold wall in contact with hot fluid in charge phase and a hot wall in contact with cold fluid in discharge phase;
- poor distribution of the heat transfer fluid in the static charge.

These inhomogeneities lead to the appearance of preferential paths, with chimney effects which degrade the heat piston operation and restrict the correct operation of the thermocline. In charge phase, it may happen that there are hot “tongues” progressing in the cold fluid. A high temperature disparity then appears in a transversal plane of the tank. This leads, for example, to the output temperature of the tank during a discharge phase being constant over a smaller time range, which is undesirable for the thermodynamic conversion unit.

DESCRIPTION OF THE INVENTION

It is consequently an aim of the present invention to offer a “dual thermocline” type of heat storage tank having homogeneous transversal temperature distribution, so as to arrive at very stable hot temperature and cold temperature values.
The aim of the present invention is attained by a tank comprising a solid matrix and a heat transfer liquid distributed in several stages in fluid communication, each stage comprising a layer of solid matrix, the layers of solid matrix of two consecutive stages being separated by a layer of heat transfer liquid in which natural convection movements arise in the case of temperature inhomogeneity in a transversal plane. These natural convection movements assure homogenisation of the temperature, which makes it possible to re-establish transversal temperature homogeneity in the beds of solid matrix.
As an example, in charge phase, in each stage, while progressing in the layer of solid matrix from the top to the bottom of the tank transversal temperature inhomogeneities arise in the thermal front. When the thermal front encounters the layer of heat transfer liquid of the lower stage, due to natural convection movements, these transversal inhomogeneities are lessened. Thus, at each passage from one step to the other, the transversal temperature inhomogeneities are lessened, which makes it possible to maintain a heat piston.
In other words, the storage tank is compartmentalised over its height by means of elements capable of allowing the liquid to circulate, in order to create under each element purely liquid zones above solid zones of heat storage material. By virtue of the very low fluid velocities and a solid static charge, the liquid zones thereby created make it possible to reduce inhomogeneities by natural convection mechanisms and thus to “re-initialise” the heat piston at each passage from one compartment to the next.
The elements delimiting the compartments are for example grates.
Preferentially, the solid zones comprise elements with at least two particle sizes, making it possible to reduce the empty spaces of the solid matrix and thus the quantity of heat transfer liquid necessary.
The phenomenon of “thermal ratcheting” is advantageously reduced, since each compartment has a low height with respect to its diameter while assuring a transversally uniform heat piston since the tank has a large height compared to its diameter.
The subject-matter of the present invention therefore is a heat storage tank comprising an envelope with a longitudinal axis filled with a heat transfer liquid and solid heat storage elements, a first longitudinal end provided with first means for collecting and supplying a liquid at a first temperature and a second longitudinal end provided with seconds means for collecting and supplying a liquid at a second temperature, in which said solid heat storage elements are distributed across at least two beds superposed along the longitudinal axis, separated by a layer of heat transfer liquid, the heat transfer liquid being capable of flowing between the first longitudinal end and the second longitudinal end. For example, each bed rests on a support enabling fluid communication.
At least one of the supports may comprise a bearing structure and a slatted structure covered with a metal web plate.
The supports are preferably in two parts.
The solid heat storage elements have advantageously at least two different particle sizes.
The layer of heat transfer liquid preferably has a thickness comprised between 1 cm and 10 cm.
For example, the envelope is a shell and the height of each bed is less than the diameter of the envelope.
The solid heat storage elements may comprise blocks of rocks and sand. The blocks of rock are formed for example from alluvial rocks. The heat transfer liquid is for example a thermal oil.
The first and/or the second collecting and supplying means advantageously comprise distribution means assuring transversal homogeneity of the axial velocity of the fluid.
The envelope may be a shell. The second distribution means may comprise a supply duct extending along the diameter of the shell and distribution ducts extending laterally from the supply duct, said distribution ducts being provided with orifices distributed along the length thereof. Advantageously, the distribution ducts have different lengths such that the contour of the distribution means has substantially the shape of a circle.
The second supplying and collecting means may be isolated from the solid heat storage elements.
Another subject-matter of the present invention is a solar power plant comprising at least one heat tank according to the invention.
The solar power plant may be a Fresnel type solar power plant or a tower solar power plant.
The first and the second means for collecting and supplying the tank may then be connected to a turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood by means of the description given hereafter and the appended drawings in which:

FIG. 1 is a longitudinal sectional view of an example of embodiment of a heat storage tank according to the invention,

FIG. 2 is a detailed view of the tank of FIG. 1 represented schematically illustrating the operation of the tank,

FIGS. 3A and 3B are top views of an example of embodiment of supports intended to delimit the compartments in the tank,

FIG. 3C is a sectional view along the plane A-A of FIG. 3A,

FIG. 4 is a top view of an example of embodiment of a distributor which may be implemented in the tank according to the invention,

FIG. 5A is a schematic longitudinal sectional view of a tank according to the invention in which are represented temperature measurement planes,

FIGS. 5B and 5C are transversal sectional views of the distribution of thermocouples in the heat transfer liquid and in the different planes of the solid bed respectively,

FIGS. 6A to 6D are graphic representations of temperature measurements provided by the thermocouples in the different levels of the different stages of the tank of FIG. 5A.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

In the description hereafter, the terms “stage” or “compartment” will be used indiscriminately.
The terms “lower”, “upper”, “top” and “bottom” are considered with respect to the orientation of the tank in FIG. 1.
In FIG. 1 may be seen a longitudinal sectional view of an example of heat storage tank according to the invention.
The tank comprises a cylindrical envelope 2 with a longitudinal axis X. In the example represented, the tank has a circular section. The longitudinal axis X is intended to be oriented substantially vertically as in the representation of FIG. 1.
The envelope 2 is formed of a shell 4 and two convex bottoms 6, 8 closing the upper and lower longitudinal bottoms respectively of the shell 4.
The tank comprises means for admitting and collecting 10 hot liquid situated in the upper convex bottom 6 of the tank and means for admitting and collecting 12 cold liquid situated in the lower convex bottom 8 in the lower part of the tank.
The inside of the tank is divided into several compartments C1, C2, C3 superposed along the longitudinal axis X. Each compartment C1, C2, C3 comprises an bottom G1, G2, G3 forming support assuring the retention of the solid heat storage elements while enabling fluid communication between the compartments and a bed TH1, TH2, TH3 of solid heat storage elements. Only the bed TH1 is represented by solid elements.
Moreover, a layer of heat transfer liquid L1, L2, L3 covers the beds TH1, TH2, TH3 of solid heat storage elements.
The active volume of the tank does not comprise an empty zone, such that the volume not occupied by the solid elements is filled with the heat transfer fluid. The zone situated above the bed TH1 and delimited by the convex bottom 6 is not filled with liquid and forms a crown for the evacuation of vapours.
In the example represented, the zone situated under the bed TH3 delimited by the lower convex bottom 8 is filled with liquid of a solid material, for example of concrete type. In addition, this makes it possible to reduce the quantity of heat transfer fluid implemented.
The heat storage elements are formed for example of rocks and/or sand. Preferably, the elements have at least two particle sizes thereby assuring good filling and reducing the free spaces for the heat transfer liquid. Advantageously, the solid heat storage elements are formed of blocks of rocks and sand filling the spaces between the rocks.
Each particle size corresponds to a diameter d50 of solid elements, defined as the value for which 50% of the solid elements have a diameter less than d50. The diameter d50 is also designated the median.
Preferably, a factor 10 between the medians of the two particle sizes is chosen which enables the filling of the free space between large rocks by small rocks. For example, the large rocks have a diameter of around 3 cm and the small rocks have a diameter of around 3 mm. The distribution by volume is as follows: around 75% of large rocks and 25% of small rocks by volume.
They may be, for example, alluvial rocks mainly composed of silica. The rocks are chosen as a function of their characteristics linked to the heat storage capacity and to their thermal behaviour (density, specific heat capacity and thermal conductivity) and to their compatibility with the heat transfer liquid, for example the compatibility between the geological nature of the rock and the heat transfer liquid.
The heat transfer liquid is for example oil or molten salts. For example, the oil may be Therminol66® or Jarytherm DBT®, this not showing any particular interactions with alluvial rocks, more generally high temperature synthetic thermal oils may be suitable in use with alluvial rocks.
For the sake of simplicity, the “beds of solid heat storage elements” will be designated hereafter “heat storage beds”.
The supports are thus adapted to support mechanically the heat storage beds, to retain the elements of low particle size, such as sand, and to allow the heat transfer liquid to pass through.
In FIGS. 3A to 3C may be seen details of a support F1 according to an embodiment example.
Advantageously, the support G1 is formed of two half-supports facilitating its mounting in the shell 4. A support in one piece does not go beyond the scope of the present invention.
In FIGS. 3A and 3B may be seen a bearing structure 14 in the shape of a half-disc, and a slatted structure 16 in the shape of a half-disc covered with a grate 18 resting on the bearing structure 14.
The bearing structure 14 is formed of parallel bearing bars 20 secured to one another by cross-pieces 22 and forming a structure in the shape of a half-circle.
In FIG. 3C may be seen a sectional view along the plane A-A of FIG. 3A of the slatted structure 16 and the grate 18.
The grate 18 is for example formed of a metal screen in which the mesh size is such that it assures the retention of the solid elements of the smallest particle sizes.
Each support G1, G2, G3 is suspended in the shell by means of an annular lug 23 lining the inner surface of the shell at the desired height.
The admitting and collecting means 10, 12 preferably comprise an orifice to collect the hot and cold fluid respectively and distribution means to supply the tank with hot and cold fluid respectively.
In FIG. 4 may be seen an advantageous example of embodiment of distribution means seen from the top.
The distribution means 24 comprise a supply duct 26 connected to the external liquid supply and distribution ducts 28 connected to the supply duct and extending transversally with respect thereto. In the example represented, the distribution ducts 28 are perpendicular to the supply duct 26. Each duct is provided with a plurality of distribution orifices assuring a distribution of the liquid along its axis.
The main duct extends advantageously along a diameter of the shell. Also advantageously, the distribution ducts have different lengths as a function of their position along the main duct such that the distribution means cover in a substantially homogeneous manner the entire transversal section of the shell.
Other forms of distribution means may be envisaged, preferably these forms assuring a homogeneous distribution of the liquids supplying the tank.
In FIG. 2 may be seen represented schematically an upper part of a compartment C2 and a lower part of the compartment C1.
The layer of liquid L2 may be seen above the heat storage bed TH2 and below the support G1 which is covered with the heat storage bed TH1.
The arrow F symbolises the natural convection movements that arise in the layer of liquid L2 when it is the site of transversal temperature inhomogeneities.
In the case of transversal temperature inhomogeneities, temperature gradients and thus liquid density gradients arise in the liquid layers, which leads to the appearance of natural convection movements which tend to reduce this gradient.
Preferably, the thickness of the liquid layers is of the order of 1 cm to 10 cm.
It has been observed that for thicknesses less than 1 cm, the overall remixing function is less well assured because the convection cells that are created have a more local effect.
For thicknesses greater than 10 cm, the efficiency of the remixing function is maintained. Conversely, the higher the thickness of the layer of heat transfer liquid the greater the quantity of liquid. Yet the cost of the liquid is high. As a result, a tank with liquid layers having a considerable thickness is economically less interesting. Furthermore, too high thicknesses of liquid layers would result in a too important axial remixing which would reduce the efficiency of the thermocline.
Advantageously, the compartments all have substantially the same height and the same composition in quantity of liquid and in quantity of solid elements so as to assure homogeneous behaviour over the whole height of the tank.
The height of the storage tank is thus “cut up” into several regions of height hi: h1, h2, h3 which can vary from several tens of centimetres to several metres. The height of the beds is in practice chosen so as to conserve a ratio hi/D<1, which makes it possible to reduce the mechanical phenomenon of thermal ratcheting.
As an example uniquely, a tank having a low temperature of 150° C. and a high temperature of 300° C., may comprise a shell having a diameter of 2500 mm, three compartments each comprising a heat storage bed of height equal to 1900 mm and a layer of heat transfer liquid having a thickness of 100 mm.
The efficiency of the structure of the tank according to the invention will now be shown.
For this, a tank with four compartments is considered. Temperature measurements are carried out in the liquid layers L1 to L3 and at different heights in each heat storage bed TH1, TH2, TH3, TH4 and at several points of transversal planes of each bed corresponding to the different heights. The different measurement heights are represented in FIG. 5A. The different measurement points per height are represented in the section of FIG. 5C for the rocks and in the section of FIG. 5B for the liquid. The measurements are performed by means of thermocouples.
The measurements are represented in the graphs of FIGS. 6A to 6D for each of the compartments C1 to C4 respectively.
The charge is carried out at the temperature of 170° C. and the tank is initially entirely at the temperature of 60° C.
The advance of the thermal front is symbolised by the arrow Fth in the graphic representations.
Analysis of the temperature measurements shows that in the upper compartment C1, three groups of curves may be distinguished corresponding to the plane C1-3, to the plane C1-2 and to the plane C1-1. As the thermal front progresses in the shell along the axis X, a temperature inhomogeneity appears: in fact it is observed that the curves are less and less grouped together, which reflects the existence of temperature differences between measurement points situated on a same plane. The inhomogeneity of the temperature thus increases from the plane C1-1 to the plane C1-3.
The passage from the compartment C1 to the compartment 2 results in a stricture of the curves in the plane C2-1 compared to those of the plane C1-3 (FIG. 6A), which signifies a reduction in the inhomogeneity of the temperature in the plane C2-1 compared to the plane C1-3.
The passage by the liquid layer L2 between the beds TH1 and TH2 makes it possible to reduce the spreading out of the group of curves, that is to say to reduce the temperature inhomogeneity.
The same phenomenon appears at each of the passages from one compartment to the other during the passing through of a liquid layer.
In the compartments C3 and C4, only two groups of curves are observed: a first group well compressed together corresponding to the planes C3-1 and C4-1 and a group of spread out curves corresponding to the following two measurement sheets C3-2 and C3-3 and C4-2 and C4-3. This illustrates an inhomogeneity in the tank due to the bed of rocks. Nevertheless, the passage by the layer liquid L4 makes it possible to re-establish temperature homogeneity.
In a tank according to the invention, the reduction of the temperature inhomogeneities during the passage by a uniquely liquid layer has been observed experimentally even in the case of considerable temperature inhomogeneity in a plane situated upstream of the liquid layer. The liquid layer also makes it possible to delay the destabilisation of the thermocline since the temperature dispersion on the planes C2-1, C3-1 and C4-1 is lower than on the plane C1-3.
The tank then has an improved operation which comes close to heat piston operation. The tank according to the invention thus helps in maintaining a constant temperature at the outlet of the tank.
Furthermore, the active proportion of the tank is increased. This is because the reduction of transversal temperature inhomogeneities makes it possible to obtain a greater volume percentage of the tank at constant temperature.
Moreover, thanks to the invention, it is possible to combine the advantages of a low bed height to shell diameter ratio and a high total height to shell diameter ratio.
This is because the segmentation of the bed of solid elements makes it possible to attain, for each compartment, a heat storage bed height to shell diameter ratio less than 1, which makes it possible to reduce the effect of thermal ratcheting and thus assure good mechanical strength. And, simultaneously, the segmentation makes it possible to have a considerable total height of solid element bed and thus a high total height to diameter ratio. Important storage properties in terms of duration and volume of isothermal zone are thereby obtained.
Furthermore, thanks to the invention, it is possible to reduce the thickness of the shell compared to those of the prior art since the thrust linked to the solid storage materials is distributed in the different compartments. Moreover, the phenomenon of packing down during thermal cycles is spread out in the different compartments.
Moreover, thanks to the distribution in compartments, the distribution means situated in the lower bottom of the tank are isolated from the solid heat storage elements, they are then no longer subjected to mechanical stresses linked for example to the packing down of this matrix during thermal cycles.
The tank according to the invention may be used for storing the heat of any installation or system producing heat.
It is particularly suited to use with systems using liquids having controlled and constant temperatures, such as turbines.
The tank according to the present invention is particularly suited to use in a Fresnel type solar power plant to supply a turbine. It may also be used in a tower solar power plant.

Claims

What is claimed is:

1-18. (canceled)

19. Heat storage tank comprising an envelope with a longitudinal axis filled with a heat transfer liquid and solid heat storage elements, a first longitudinal end provided with a first means for collecting and supplying a liquid at a first temperature and a second longitudinal end provided with seconds means for collecting and supplying a liquid at a second temperature, in which said solid heat storage elements are distributed across at least two superposed along the longitudinal axis, separated by a layer of heat transfer liquid, the heat transfer liquid being capable of flowing between the first longitudinal end and the second longitudinal end.

20. Tank according to claim 19, in which each bed rests on a support enabling fluid communication.

21. Tank according to claim 20, in which at least one of the supports comprises a bearing structure and a slatted structure covered with a metal web plate.

22. Tank according to claim 20, in which the supports are in two parts.

23. Tank according to claim 19, in which the solid heat storage elements have at least two different particle sizes.

24. Tank according to claim 19, in which the layer of heat transfer liquid has a thickness comprised between 1 cm and 10 cm.

25. Tank according to claim 19, in which the envelope is a shell and in which the height of each bed is less than the diameter of the envelope.

26. Tank according to claim 19, in which the solid heat storage elements comprise blocks of rocks and sand.

27. Tank according to claim 26, in which the blocks of rock are formed from alluvial rocks.

28. Tank according to claim 19, in which the heat transfer liquid is an oil.

29. Tank according to claim 19, in which the first and/or the second collecting and supplying means comprise a distributor assuring transversal homogeneity of the axial velocity of the fluid.

30. Tank according to claim 29, in which the envelope is a shell and the second distributor comprises a supply duct extending along the diameter of the envelope and distribution ducts extending laterally from the supply duct, said distribution ducts being provided with orifices distributed along the length thereof.

31. Tank according to claim 30, in which the distribution ducts have different lengths such that the contour of the distributor has substantially the shape of a circle.

32. Tank according to claim 1, in which the second supplying and collecting means are isolated from the solid heat storage elements.

33. Solar power plant comprising at least one heat tank according to claim 19.

34. Solar power plant according to claim 33, in which the solar power plant is a Fresnel type solar power plant.

35. Solar power plant according to claim 33, in which the solar power plant is a tower solar power plant.

36. Solar power plant according to claim 33, in which the first and the second means for collecting and supplying the tank are connected to a turbine.