U.S. patent application number 14/399653 was filed with the patent office on 2015-05-21 for heat storage tank with improved thermal stratification.
The applicant listed for this patent is Commissariat a I'energie atomique et aux energies alternatives. Invention is credited to Arnaud Bruch, Raphael Couturier, Jean-Francois Fourmigue.
Application Number | 20150136115 14/399653 |
Document ID | / |
Family ID | 46826656 |
Filed Date | 2015-05-21 |
United States Patent
Application |
20150136115 |
Kind Code |
A1 |
Bruch; Arnaud ; et
al. |
May 21, 2015 |
HEAT STORAGE TANK WITH IMPROVED THERMAL STRATIFICATION
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.
Inventors: |
Bruch; Arnaud; (Sainte
Colombe les Vienne, FR) ; Couturier; Raphael;
(Sassenage, FR) ; Fourmigue; Jean-Francois;
(Fontaine, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Commissariat a I'energie atomique et aux energies
alternatives |
Paris |
|
FR |
|
|
Family ID: |
46826656 |
Appl. No.: |
14/399653 |
Filed: |
May 6, 2013 |
PCT Filed: |
May 6, 2013 |
PCT NO: |
PCT/EP2013/059400 |
371 Date: |
November 7, 2014 |
Current U.S.
Class: |
126/620 ;
165/10 |
Current CPC
Class: |
F28D 20/0039 20130101;
Y02E 10/40 20130101; F24S 60/00 20180501; Y02E 60/142 20130101;
F28D 20/0056 20130101; Y02E 60/14 20130101 |
Class at
Publication: |
126/620 ;
165/10 |
International
Class: |
F24J 2/34 20060101
F24J002/34; F28D 20/00 20060101 F28D020/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 9, 2012 |
FR |
1254229 |
Claims
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.
Description
TECHNICAL FIELD AND PRIOR ART
[0001] The present invention relates to a heat storage tank with
improved thermal stratification.
[0002] 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.
[0003] 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.
[0004] 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).
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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".
[0009] 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.
[0010] 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").
[0011] 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.
[0012] 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.
[0013] 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.
[0014] "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.
[0015] 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: [0016] 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; [0017]
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.
[0018] In real operation, such a storage system has inhomogeneities
and the heat piston is not perfect. These inhomogeneities can stem
from: [0019] 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;
[0020] 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; [0021] poor
distribution of the heat transfer fluid in the static charge.
[0022] 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
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] The elements delimiting the compartments are for example
grates.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] At least one of the supports may comprise a bearing
structure and a slatted structure covered with a metal web
plate.
[0032] The supports are preferably in two parts.
[0033] The solid heat storage elements have advantageously at least
two different particle sizes.
[0034] The layer of heat transfer liquid preferably has a thickness
comprised between 1 cm and 10 cm.
[0035] For example, the envelope is a shell and the height of each
bed is less than the diameter of the envelope.
[0036] 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.
[0037] The first and/or the second collecting and supplying means
advantageously comprise distribution means assuring transversal
homogeneity of the axial velocity of the fluid.
[0038] 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.
[0039] The second supplying and collecting means may be isolated
from the solid heat storage elements.
[0040] Another subject-matter of the present invention is a solar
power plant comprising at least one heat tank according to the
invention.
[0041] The solar power plant may be a Fresnel type solar power
plant or a tower solar power plant.
[0042] The first and the second means for collecting and supplying
the tank may then be connected to a turbine.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] The present invention will be better understood by means of
the description given hereafter and the appended drawings in
which:
[0044] FIG. 1 is a longitudinal sectional view of an example of
embodiment of a heat storage tank according to the invention,
[0045] FIG. 2 is a detailed view of the tank of FIG. 1 represented
schematically illustrating the operation of the tank,
[0046] FIGS. 3A and 3B are top views of an example of embodiment of
supports intended to delimit the compartments in the tank,
[0047] FIG. 3C is a sectional view along the plane A-A of FIG.
3A,
[0048] 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,
[0049] FIG. 5A is a schematic longitudinal sectional view of a tank
according to the invention in which are represented temperature
measurement planes,
[0050] 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,
[0051] 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
[0052] In the description hereafter, the terms "stage" or
"compartment" will be used indiscriminately.
[0053] The terms "lower", "upper", "top" and "bottom" are
considered with respect to the orientation of the tank in FIG.
1.
[0054] In FIG. 1 may be seen a longitudinal sectional view of an
example of heat storage tank according to the invention.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] Moreover, a layer of heat transfer liquid L1, L2, L3 covers
the beds TH1, TH2, TH3 of solid heat storage elements.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] The heat transfer liquid is for example oil or molten salts.
For example, the oil may be Therminol66.RTM. or Jarytherm DBT.RTM.,
this not showing any particular interactions with alluvial rocks,
more generally high temperature synthetic thermal oils may be
suitable in use with alluvial rocks.
[0067] For the sake of simplicity, the "beds of solid heat storage
elements" will be designated hereafter "heat storage beds".
[0068] 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.
[0069] In FIGS. 3A to 3C may be seen details of a support F1
according to an embodiment example.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] In FIG. 4 may be seen an advantageous example of embodiment
of distribution means seen from the top.
[0078] 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.
[0079] 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.
[0080] Other forms of distribution means may be envisaged,
preferably these forms assuring a homogeneous distribution of the
liquids supplying the tank.
[0081] In FIG. 2 may be seen represented schematically an upper
part of a compartment C2 and a lower part of the compartment
C1.
[0082] 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.
[0083] 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.
[0084] 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.
[0085] Preferably, the thickness of the liquid layers is of the
order of 1 cm to 10 cm.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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.
[0090] As an example uniquely, a tank having a low temperature of
150.degree. C. and a high temperature of 300.degree. 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.
[0091] The efficiency of the structure of the tank according to the
invention will now be shown.
[0092] 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.
[0093] The measurements are represented in the graphs of FIGS. 6A
to 6D for each of the compartments C1 to C4 respectively.
[0094] The charge is carried out at the temperature of 170.degree.
C. and the tank is initially entirely at the temperature of
60.degree. C.
[0095] The advance of the thermal front is symbolised by the arrow
Fth in the graphic representations.
[0096] 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.
[0097] 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.
[0098] 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.
[0099] The same phenomenon appears at each of the passages from one
compartment to the other during the passing through of a liquid
layer.
[0100] 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.
[0101] 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.
[0102] 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.
[0103] 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.
[0104] 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.
[0105] 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.
[0106] 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.
[0107] 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.
[0108] The tank according to the invention may be used for storing
the heat of any installation or system producing heat.
[0109] It is particularly suited to use with systems using liquids
having controlled and constant temperatures, such as turbines.
[0110] 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.
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