SE539955C2

SE539955C2 - A Solar Thermal Energy Accumulator

Info

Publication number: SE539955C2
Application number: SE1500284A
Authority: SE
Inventors: Ali Mohamed Mansour
Original assignee: Ali Mohamed Mansour
Priority date: 2015-06-23
Filing date: 2015-06-23
Publication date: 2018-02-13
Also published as: WO2016207000A1; CN107923656A; SE1500284A1

Abstract

The present invention relates to a solar thermal energy accumulator comprises a solar receiver (36). According to a first aspect of the invention, a heat accumulating substance (20) has a melting point at a temperature above the boiling point of water at ambient atmospheric pressure. An expansion/contraction device (33) is divided into a first and a second tubing (44a, 44b) arranged in a telescopic and slidable inter-relationship, said first tubing (44a) being connected to said solar receiver (36), said second tubing (44b) being connected to the container (4), the interior of said expansion/contraction device (33) being in communication with said container (4). Said first and second tubings (44a, 44b) are arranged in a substantially vertical relationship to one another, said first tubing (44a) being arranged above the second tubing (44b). According to another aspect of the invention, said expansion/contraction device (33) comprises at least one expansion tubing (43) provided with a ballast device (54) on the free surface of the heat accumulating substance (20), said ballast device (54) being sealingly arranged inside the expansion tubing (43), said ballast device (54) having a density higher than that of the molten heat accumulating substance (20).

Description

A prior art Solar thermal energy accumulator 2 is shown in Figs. 8A and 8B. Theaccumulator has a circular-cylindrical container 4, having a bottom 8 and a mantle 10.The mantle has an exterior Wall 12a and an interior Wall l2b. The bottom 8 and theinterior Wall 12b of the mantle 10 define together a Space 14. The Space 14 is intendedto contain a heat accumulating SubStance 20, Which is solid at room temperature and ambient air pressure.

Such a heat accumulating Substance may be a Salt Such as table Salt (NaCl), potaSSiumnitrate (N 03), Sodium nitrate (NaNO3) or a mixture of SaltS, Such as a mixture of N03and NaNO3. Alternatively, a ﬂuoride Salt is uSed, Such as FLiNaK, FLiBe, FLiNaBe, FLiKBe or combinations thereof.

An alternative heat accumulating Substance may be a pure alkali metal, Such aS Sodium (N a) or a mixture of eutectic Sodium potassium (NaK).

At heating, the heat accumulating SubStance 20 Will melt at temperatureS above theboiling point of Water, 100°C. ThiS alloWS for uSing the melted heat accumulating SubStance for heating Water at leaSt up to itS boiling point.

The container is open in order to allow thermal expansion and contraction of the heat accumulating substance at melting and “ A similar solar ther1nal energy accumulator is known from WO 201 1/0273 09 and is provided with a heat exchanger for utilisation of the accumulated heat.

Another solar ther1nal energy accumulator is known from US 2004/0099261 A1,describing closed containers and an expansion device in the form of metal bellows fortaking up contraction and expansion due to thermal growth of the piping containing molten salt. The disadvantage of bellows in general and disregarding the choice of material, is that they may break after an extended period of use.

.Isßspﬁsßstisvﬁlx s s s s s s s s s s s s s s s s s s s s. -g f US 2004/0099261 furthermore describes that water is heated to steam by the moltensalt via a heat exchanger, and that the produced steam is in tum utilised for producingelectricity by means of a steam turbine generator in a steam Rankine cycle conversion system.

It is also known from the solar power tower of US 2004/0099261 to use a mixture ofpotassium nitrate (KNOg) sodium nitrate (NaNOg) as ther1nal transfer medium in solarpower plants, that the outside surface of the solar receiver may exceed 65 0°C and thatthe mixture will range between 560°C at the solar receiver and 290°C at the heatexchanger, and should not sink below 260°C, since the mixture freezes at about 220°C, while contracting. v ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ “w BOFICIUBQGIC: freezing In EP 1 873 397 A2, a solar power tower is described, using molten salt containingﬂuoride for achieving temperatures up to 982°C, while in 2004/02443 76 liquid metalis used instead of salt. Both variants cause even more thermal growth of the piping containing molten salt or molten metal due to the high temperatures.

In the solar power towers of EP 1 873 397 and US 2004/0244376 air is heated (insteadof steam) and used in a Brayton cycle conversion system for producing electricity. It isalso stated that it may instead be used for hydrogen production, desalination of water or powering a heat/therrno-chemical plant.

In WO 2012/168251 a very complicated solar tower power plant is described using molten salt for heat accumulation and production of electricity.

The solar power towers of US 2004/0099261, EP 1 873 397, US 2004/0244376 andWO 2012/168251 all suffer from the disadVantage that the plants are extremely spacedemanding, since they need a high tower and a Very large area surrounding the towerwith movable mirrors, so called heliostats, controlled to always reﬂect sun raystowards the top of the tower, where a solar collector is arranged. In the “Desertec”plant in Tunisia, about 825 000 large heliostats are mounted on the ground about the 'EOWCIÄ OBJ ECT OF THE INVENTIONThe object of the inVention is to provide a solar thermal energy accumulator overcoming the drawbacks of the prior art solar power towers.The object is in particular to oVercome the problem of thermal contraction andexpansion of molten salt or liquid metal in case the temperature falls close to or below the freezing point of the molten salt or liquid metal to be used and rises again.

SUMMARY OF THE INVENTION i, ~ fí;;;;;;;;;säs;;;¿ív¿;;¿;;"1;¿; ffff Lcrn, Ingen numreríng .ßre .matn0c Mas associated With an This object has.

BOFICIUBQGIC. and is connected to saidthe container, the interior of Which beingin communication With said container. fwaSaVwbmeH is elongatedï __ __ __ __ Preferably, said expansion/contraction device ded. ice is provi ion dev expansion/ contract BOFICIUBQGIC. Suitably, said elongatedexpansion/ contraction device is divided Q into a first and a second tubing arrangedin a telescopic and slidable inter-relationship, said first tubing being ess the first tubing _ _11?ded ï QP evice t a11ast d b rovided With a P ing ai Preferably, s d first tub is nee _ associated With said solar receiver, saidsecond tubing being connected to the relation to said second tubing by gravity. Hereby, no external power source container. Hereby, the expansion/ contraction device is allowedto take up expansion and contractionWith less need for high strength requirements of the material of the first and second tubing, .

\ : Vänster Formaterat: Normal, Indrag0,63 cm Ingen numreríng r BOFICIUBQGICI said first and second tubings are arranged in a substantially vertical relationship to one another, saidfirst tubing being arranged above thesecond tubing Borttaget I being t Altematively, or in addition, said elongated expansion/contraction device comprises an expansion tubing With a free end. :_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ *_ .. -~ f Suitably, the dimensions of the expansion/contraction device are chosen such that it isable to receive a predetermined volume of thermally expanded heat accumulatingsubstance, and Wherein the material of the expansion tubing is chosen such that it Withstands temperatures of molten heat accumulating substance. v, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, __» "V Suitably, said utilisation circuit comprises a circulation tubing connected to a heatexchanger associated With said container, said utilisation circuit communicating Witha utilisation device. Hereby, the heat accumulated in the molten heat accumulating substance is allowed to heat a ﬂuid also during night hours.

Preferably, said utilisation device comprises an electric generator for generating electricity, a further heat exchanger for production of hot Water or heat for an oven.

Suitably, said heat accumulating substance is a salt, a mixture of salts, an alkali metal or a mixture of alkali metals.

Preferably, said container has a bottom and a mantle, said mantle having an inner Wall and an outer Wall, said bottom and said inner Wall defining said space. , ««te¿;a¿æ.;aæ¿+¿¿t.;t;afe¿§f¿ttàafaayaag i T|rnes x c ___________________________________________________________ ___* BOFICIUBQGICI Alteinatively, saidexpansion tubing is provided With aforce device. .

BOFICIUBQGICI Suitably, said expansion/ contraction device comprisesan expansion space, formed by thecontainer and a jacket, together deﬁningsaid expansion space, said expansionspace being in communicatingrelationship With the interior of thecontainer via at least one opening.Hereby, an integrated expansion/ contraction device isachieved. .

¶Preferably, said expansion space isprovided With a Weight device. ¶ BOFICIUBQGICI Preferably, said solarreceiver is adapted to direct sun raysdirectly onto a portion of said container,the focal point being adjustable orpositioned on said portion or in front ofor behind said portion, the material ofsaid portion of the container beingchosen such that it Withstands thetemperature of sun rays. Hereby, thematerial of the portion of the containercan be chosen outgoing from apredeterrnined temperature of thesunrays of the focal point, and apredeterrnined melting temperature ofthe heat accumulating substance to bemolten.¶ DRAWING SUMMARYIn the following, the invention Will be described in more detail by reference to the enclosed draWings, in Which Fig. 1 illustrates a solar thermal energy accumulator provided With an annular expansion/contraction device; Fig. 2 illustrates an alternative solar thermal energy accumulator provided With a tubular expansion/contraction device; Figs. 3A- 3B illustrate another solar thermal energy accumulator provided With a telescopic expansion/contraction device; Figs. 4A- 4B illustrate a solar thermal energy accumulator provided With an annular expansion/contraction device and a telescopic expansion/contraction device; Fig. 5 illustrates a solar thermal energy accumulator provided With four tubular expansion/contraction devices and a telescopic expansion/ contraction device; Figs. 6A- 6B illustrate a solar thermal energy accumulator provided With an alternative expansion device; Fig. 7 illustrates a solar thermal energy accumulator provided With yet another alternative expansion device; Fig. 8A illustrates a prior art solar thermal energy accumulator; and Fig. 8B is a cross-section along VIIIB - VIIIB in Fig. 8A.

DETAILED DESCRIPTION In the following, the use of relevant reference numerals in Figures 8A and 8B referred to above will be used correspondingly.

Figure 1 shows a solar thermal energy accumulator 2 having a closed container 4 with abottom, a mantle 10, an exterior wall 12a and a lid 5. The lid 5 may be integrated with orconnected to the mantle 10. A double-walled jacket 30 is arranged at a distance from themantle 10, leaving an annular space 32a without top cover. The mantle 10 is providedwith a bottom 8 and is arranged by not shown distance members at a distance from thebottom 9 of the jacket, thus leaving an intermediate space 32b between the bottom of themantle 10 and the bottom 9 of the jacket 30. Inside the double-walled jacket 30, isarranged a heat exchanger 26 in the form of a tubing 28, being part of a utilisation circuit25. The tubing 28 extends through the jacket and transports a ﬂuid, such as air or waterheated by the heat exchanger 26, to a utilisation device 29 (cf Fig. 4A).

It should be noted that the mantle 10 may alternatively be connected directly to thebottom 9 of the jacket 30, without need for the intermediate space 32b (cf Fig. 4A).

Close to the bottom 8 of the container 4 openings 34 are provided in the mantle toallow communication between the space 14 and the annular space 32a. Of course, the mantle 10 may be provided with more openings closer to the lid 5.

A heat accumulating substance 20 may be provided to completely fill up the space 14,even though there may be a distance between the uppermost level of the heataccumulating substance 20 and the lid 5. Additionally, the annular space 32a and theintermediate space 32b may be partly filled with the heat accumulating substance 20.

The heat accumulating substance 20 may be a salt such as table salt (NaCl), potassiumnitrate (N03), sodium nitrate (NaNO3) or a mixture of salts, such as a mixture of NO3and NaNO3. Alternatively, a ﬂuoride salt is used, such as FLiNaK, FLiBe, FLiNaBe, FLiKBe or combinations thereof.

Alternatively, the heat accumulating substance 20 may be a pure alkali metal, such as 8 sodium (Na) or a mixture of eutectic sodium potassium (NaK).

In Fig. 1 is also shown schematically a solar receiver 36 having a lens 35a focussing sun-rays 35b to a focal point 37 onto the lid 5 of the accumulator 2.

This may be performed by a simple movable Fresnel lens, as shown on Youtube in the following link: http://voutu.be/drE54ctrHBY According to the film of said link, the Fresnel lens rnay create temperatures up to 2100° C in the focal point.

It is thus preferable that the lid 5 is made of a material that can withstand such high temperatures such as rock, preferably an easily shapeable one.

Even if the lid 5 could be made of a metal alloy withstanding high temperatures, itwould be advisable to adjust the position and thus reduce the temperature of the focalpoint 37 in order not to burn the lid 5. A control system for adjusting the temperatureofthe focal point is described in EP 0 418 586.

The manufacturing cost may of course be reduced by setting the focal point manuallye.g. by simply moving the lens 35a. Altematively, the lens is mounted in such a way that the focal point will always be positioned in front of or behind the lid 5.

The focal point may instead be directed (i.e. without lid as shown in Fig 8A) towardsthe upper surface of the heat accumulating substance 20, or via a transparent or semi-transparent lid, made e. g. of mineral glass or mica. Of course, also in this case, the focal point may be adjusted manually or automatically.

The solar receiver 36 may altematively be made of a plastic sheet mounted on e. g. a 9 square frame, the plastic sheet being filled with water to create an aqua lens, as shown on Youtube in the following link: http://voutu.be/eeSvHgO5frnO When the heat accumulating substance 20 starts melting, it will expand and enter the space 32a through the openings 34.

Thus, the space 32a and openings 34 form together an expansion/contraction device 33.

The width and height of the space 32a in relation to the container 4 depends i.a. on thevolume of the container 4 and the expansion constant of the used kind of heat accumulating substance 20.

Figure 2 shows an alternative solar thermal energy accumulator 2 provided with anexpansion/contraction device 33 in the form of a hollow pipe 43. The bottom of thepipe 43 is in ﬂuid communication with the interior of the container 4 filled with heataccumulating substance 20, while the top of the pipe 43 is open. The tubing 28 of theutilisation circuit 25 transports a ﬂuid heated by the heat accumulating substance 20 via the heat exchanger 26 to a utilisation device 29 (cf Fig. 4A).

Also in this case, a lens 35a focuses sun-rays 35b directly on the top portion or lid 5 of the container 4.

Also in this case, it would be possible to automatically or manually move the lens 35ain order to position the focal point 37 at a desired position, in particular to avoid wear ofthe lid 5.

The alternative materials of the lid 5 mentioned above would also be possible to include in this variant.

The therinal energy accumulator 2 of Figs. 3A and 3B is provided with a solar receiver36 in the form of a spherical lens device 38 provided with a plurality of lenses 40,focussing sun-rays towards a hollow globe 42 made of a material resisting the heat offocussed sun rays, cf above. Altematively, the lenses 40 are adjusted such that thefocal point is positioned in front of or behind the surface of the globe 42, such that it does not become overheated.

It should be noted that instead of the spherical lenses, a plurality of Fresnel lenses maybe provided on the globe 42. Of course, a combination of spherical lenses and Fresnel lenses may be used, side by side or in an optical combination.

The interior of the globe 42 is in communication with a hollow pipe constituted by afirst and a second tubing member 44a, 44b in turn in communication with the interior of the container 4.

The first and second tubing members 44a, 44b are arranged in a substantially vertical,sliding and telescopic relationship. This has been accomplished by an exterior diameter of the first tubing member 44a slightly smaller than the inner diameter of thesecond tubing member 44b. The first tubing member 44a is connected to the globe 42, while the second tubing member 44b is connected to the interior of the container 4.

The container 4 and the first and second tubing member 44a, 44b are filled with a heataccumulating substance 20, while the globe 42 may be partially or completely filled with said substance 20.

The first tubing device 44a is provided with a ballast device 50. As mentioned above,the heat accumulating substance expands when it melts and contracts when it freezes.Thus, at low temperatures, the ballast device 50 will press the first tubing member 44atowards and into said second tubing member 44b (cf. Fig. 3A) and thus move in thedirection of the arrow. Correspondingly, when the heat accumulating substance melts, the first tubing member 44a will move vertically upwards in relation to the second 11 tubing member 44b (cf Fig. 3B), i.e. in the direction of the arrow.

Thus, the first and second tubing members 44a, 44b constitute together an expansion/contraction device 33.

As can be understood from Figs. 3A and 3B, the ballast device 50 also constitutes a stop for the downward movement of the first tubing member 44a.

Of course, the exterior diameter of the second tubing member 44b may instead beslightly smaller than the inner diameter of the first tubing member 44a. In that case,the ballast device cannot constitute a stop member, but a separate device may be provided for that purpose, such as a ring about the second tubing member 44b.

Of course, if the weight of the solar receiver 36 is sufficient, the ballast device 50 may be omitted. Also in that case, a separate stop member may be provided.

As already mentioned above, the tubing 28 transports a heated ﬂuid to a utilisationdevice 29 (cf Fig. 4A).

In Figure 4A and 4B another heat accumulating device 2 is shown and is provided withthe kind of solar receiver described in connection with Figs. 3A- 3B and theexpansion/contraction device described in connection With Fig. 1. In this embodiment,the mantle 10 is connected directly to the bottom 9 of the jacket 30, i.e. no space 32b isprovided. Thus, the container 4 is f1lled With the heat accumulating substance 20, and extends via the opening or openings 34 into the annular space 32a.

However, on top of the heat accumulating substance in the annular space 32a, anannular ballast device 52 is provided. The ballast device 52 has a higher density thanthe molten heat accumulating substance 20, but since the ballast device 52 sealsagainst the peripheral Walls of the annular space 32a, it Will stay a on top of the heat accumulating substance 20 also When it is in its liquid state. 12 Thus, the ballast device 52 presses the heat accumulating Substance 20 towards theglobe 42 in order to avoid problems with overﬂow in connection with maintenance of the globe or the first and second tubing.

However the need for the ballast device 52 also depends on other factors, such as thelength and diameter of the first and second tubing devices 44a, 44b, in relation to theheight of the annular space 32.

The tubing 28 transports steam to a turbine 46 to generate electricity. The turbine 46 is connected to a pump to press condensed steam, i.e. water into the heat exchanger 26.

Alternatively, the tubing may transport steam or hot water to a heat exchanger for heating houses or baking ovens.

Of course, the mantle 10 of the container 4 may be arranged in the way described in connection with Fig. 1, i.e. such that a space 32b is created.

Yet another altemative expansion/contraction device 33 is shown in Figure 5,according to which four expansion tubings 43 are arranged on top of the container 4and communicating with the interior of container 4 (cf. Fig. 2). The expansion tubings43 are provided with a ballast device 54 on the free surface of the heat accumulatingsubstance 20. The ballast devices 54 are sealingly arranged inside the tubings and havea density higher than that of the molten heat accumulating substance 20. At melting, the ballast devices 54 will thus move upwards inside the expansion tubings 43.Thus, the ballast devices 54 press the heat accumulating substance 20 towards theglobe 42 in order to avoid problems with overﬂow in connection with maintenance of the globe 42 of the solar receiver 36 or the first and second tubing devices 44a, 44b.

Again, the need for the ballast device 54 also depends e. g. on the length and diameter 13 of the first and second tubing members 44a, 44b, in relation to the height of theexpansion tubings 43. Thus, the ballast devices 54 may not be needed on the freesurface of the heat accumulating substance, i.e. the expansion tubings 43 may have open ends (cf. Fig. 2).

Of course, the number of expansion tubings 43 may be less than four, i.e. one, two orthree or more than four. Alternatively, the expansion tubings 43 may instead be shaped as one annularly shaped device and be arranged on top of the container 4.

A jacket 30 and a bottom 9 define a space 27 intended to contain a volume of liquid, suchas water surrounding the expansion tubings 43 and the container 4, and thus constitutinga heat exchanger 26. The jacket has openings connected to tubings 28 transporting the heated water to a utilisation circuit 25 having a utilisation device 29 (cf Fig. 4A).

Of course, it would be possible instead to provide the interior of the jacket with a heat exchanger 26 in the form of a tubing 28 to be heated by the water.

It should be noted that the cross-section of the expansion tubings 43 may be of anycylindrical form. It may thus have a cross-section or any other polygonal cross-s ection than rectangular, i.e. triangular, square, pentagonal, hexagonal etc.

The corresponding relates to the container 4 and the jacket 30.

Figures 6A and 6B show an alternative expansion/contraction device 33 in the form ofa telescopic expansion tubing 43 arranged in communicating relationship with theinterior of the container 4, and being provided at its opposite end with a force device56 in the form of a weight. The solar receiver and the utilisation circuit have been omitted, but could of course be of any one of the kinds described above.

Figure 7 shows yet another alternative expansion/contraction device 33 in the form of a telescopic expansion tubing 43 arranged in communicating relationship with the 14 interior of the container 4, while its opposite end is provided with a force device 56 inthe forrn of a spring means 60 to create a force in a direction towards the container 4,as indicated by arroWs. Also in this case, the solar receiver and the utilisation circuit have been omitted, but could be of any one of the kinds described above.

Of course, it Would be possible to combine the spring means 60 With a ballast device 56.

It should be understood that the expansion/contraction devices 33 of Figs 6A, 6B or 7 may be used instead of the expansion/contraction devices 33 shown in Figs. 2 and 5.

It should be noted that the device of Figs. 1 and 2 may be provided with a ballastdevice 52, 54., exfer ifïgn, »L

Claims

1. A solar thermal energy accumulator comprising a container (4) defining a space (14) adapted to contain a heat accumulating Substance (20) being solid at roomtemperature and at ambient atmospheric pressure, said container (4) beingassociated with a solar receiver (36) for receiving sun rays, said container (4)further being associated with a utilisation circuit (25), characterised in that saidcontainer (4) is associated with an expansion/contraction device (33) for taking upthermal expansion and contraction of the heat accumulating substance (20) at melting and freezing, respectively. .

2. A solar thermal energy accumulator according to claim 1,wherein said expansion/contraction device (33) is elongated and is connected to said the container (4), the interior of which being in communication with said container (4)- .

3. A solar thermal energy accumulator according to claim l or 2, wherein said elongated expansion/contraction device (33) is divided into a first and a secondtubing (44a, 44b) arranged in a telescopic and slidable inter-relationship, said firsttubing (44a) being associated with said solar receiver (36), said second tubing (44b) being connected to the container (4). .

4. A solar thermal energy accumulator according to any one of claims 1- 3, wherein said first and second tubings (44a, 44b) are arranged in a substantially verticalrelationship to one another, said first tubing (44a) being arranged above the secondtubing (44b), said first tubing (44a) being provided with a ballast device (50) topress the first tubing in relation to said second tubing (44b) by gravity. .

5. A solar thermal energy accumulator according to any one of the preceding claims, wherein said elongated expansion/contraction device (33) comprises an expansion tubing (43) with a free end. .

6. A solar thermal energy accumulator according to claim 5, wherein said expansion tubing (43) is provided with a force device (54). .

7. A solar thermal energy accumulator according to any one of the preceding claims, wherein said expansion/contraction device (33) comprises an expansion space(32a, 32b), formed by the container (4) and a jacket (30), together defining saidexpansion space (32a, 32b), said expansion space being in communicating relationship with the interior ( 14) of the container (4) via at least one opening (34). .

8. A solar thermal energy accumulator according to claim 7, wherein said expansion space (32a, 32b) is provided with a weight device (52). .

9. A solar thermal energy accumulator according to any one of the preceding claims, wherein the dimensions of the expansion/contraction device (33) are chosen suchthat it is able to receive a predetermined volume of thermally expanded heataccumulating Substance, and wherein the material of the expansion tubing ischosen such that it withstands temperatures of molten heat accumulating substance.

10.A solar thermal energy accumulator according to any one of the preceding claims, wherein said solar receiver (36) is adapted to direct sun rays directly onto a portion(5) of said container (4), the focal point being adjustable or positioned on saidportion (5) or in front of or behind said portion (5), the material of said portion (5)of the container (4) being chosen such that it withstands the temperature of sun rays. 16

11. l1.A solar thermal energy accumulator according to any one of the preceding claims,wherein said utilisation circuit (25) comprises a circulation tubing (28) connectedto a heat exchanger (26) associated with said container (4), said utilisation circuit communicating with a utilisation device (29).

12. l2.A solar thermal energy accumulator according to any one of the preceding claims,wherein said utilisation device (29) comprises an electric generator (46) forgeneratin g electricity, a further heat exchanger for production of hot water or heat for an oven.

13.A solar thermal energy accumulator according to any one of the preceding claims,wherein said heat accumulating substance is a salt, a mixture of salts, an alkali metal or a mixture of alkali metals.

14.A solar thermal energy accumulator according to any one of the preceding claims,wherein said container (4) has a bottom (8) and a mantle (10), said mantle (10)having an inner wall (l2b) and an outer wall (12a), said bottom (8) and said inner wall (l2b) defining said space (14).