TECHNICAL FIELD

The present invention relates to a thermal energy accumulator comprising a container defining a space adapted to contain a heat accumulating substance being solid at room temperature at ambient atmospheric pressure.

BACKGROUND OF THE INVENTION

A prior art thermal energy accumulator 2 is shown in Figs. 9A and 9B. The accumulator 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 12b. The bottom 8 and the interior wall 12b of the mantle 10 define together a space 14. The space 14 is intended to 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 (NaCI), potassium nitrate (N0 ₃ ), sodium nitrate (NaN0 ₃ ) or a mixture of salts, such as a mixture of N0 ₃ and NaN0 ₃ . 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 (Na) or a mixture of eutectic sodium potassium (NaK).

At heating, the heat accumulating substance 20 will melt at temperatures above the boiling 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 solidification, respectively. A similar thermal energy accumulator is known from WO 2011/027309 and is provided with a heat exchanger for utilisation of the accumulated heat.

Another thermal energy accumulator is known from US 2004/0099261 A1 , describing closed containers and an expansion device in the form of metal bellows for taking up contraction and expansion due to thermal growth of the piping containing molten salt. The disadvantage of bellows in general - disregarding the choice of material - is that they may break after an extended period of use. US 2004/0099261 furthermore describes that water is heated to steam by the molten salt via a heat exchanger, and that the produced steam is in turn utilised for producing electricity 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 of potassium nitrate (KN0 ₃ ) sodium nitrate (NaN0 ₃ ) as thermal transfer medium in solar power plants, that the outside surface of the solar receiver may exceed 650°C and that the mixture will range between 560°C at the solar receiver and 290°C at the heat exchanger, and should not sink below 260°C, since the mixture solidifies at about 220°C, while contracting.

In EP 1 873 397 A2, a solar power tower is described, using molten salt containing fluoride for achieving temperatures up to 982°C, while in 2004/0244376 liquid metal is 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 (instead of steam) and used in a Brayton cycle conversion system for producing electricity. It is also stated that it may instead be used for hydrogen production, desalination of water or powering a heat/thermo-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 and WO 2012/168251 all suffer from the disadvantage that the plants are extremely space demanding, since they need a high tower and a very large area surrounding the tower with movable mirrors, so called heliostats, controlled to always reflect sun rays towards 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 tower.

OBJECT OF THE INVENTION

The object of the invention is to provide a 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 and expansion of molten salt or liquid metal in case the temperature falls close to or below the freezing or solidification point of the molten salt or liquid metal to be used and rises again.

SUMMARY OF THE INVENTION

This object has been overcome by the thermal energy accumulator, further having the features that said expansion/contraction device comprises a force actuating means associated with said container for taking up thermal expansion and contraction of the heat accumulating substance at melting and solidification, respectively. Hereby, overflow is avoided at melting of the heat accumulating substance when expanding inside the expansion/contraction device. Furthermore, solidification shrinkage, causing porosities or craters is avoided, in turn causing problems when melting the heat accumulating the substance again. Preferably, said force actuating means is adapted to move together with the heat accumulating substance at melting and solidification, respectively. Hereby is avoided that air enters into the heat accumulating substance, in particular at solidification shrinkage.

Suitably, said container comprises a mantle, a bottom and a lid, together defining said space. Hereby, a container having substantially constant volume is defined.

Preferably, said container is associated with an expansion space, said expansion space being associated with said force actuating means. Hereby, the heat accumulating substance is allowed to expand, yet still being enclosed.

Alternatively, said container comprises a circular cylindrical base portion and a circular cylindrical top portion, the inner diameter of the circular cylindrical base portion being slightly larger than the outer diameter of the circular cylindrical cover portion for allowing the circular cylindrical cover portion to move coaxially in relation to and inside the circular cylindrical base portion in order to actuate force on the heat accumulating substance, said circular cylindrical base portion and said circular cylindrical top portion. Hereby, the space and the expansion space are defined, i.e. the space equals the expansion space.

Preferably, said force actuating means comprises the inherent design of the container. Alternatively, or in addition, said force actuating means comprises one of a spring device, a weight device and a counter-weight device. Suitably, said expansion/contraction device is divided into a first and a second tubing arranged in a telescopic and slidable inter-relationship, said second tubing being connected to the container, said first and second tubings defining said expansion space. Hereby, the expansion/contraction device is allowed to take up expansion and contraction with less need for high strength requirements of the material of the first and second tubing,

Suitably, said first and second tubings are associated with a solar receiver.

Alternatively, said solar receiver is adapted to direct sun rays directly onto a portion of said container, the focal point being adjustable or positioned on said portion or in front of or behind said portion, the material of said portion of the container being chosen such that it withstands the temperature of sun rays. Hereby, the material of the portion of the container can be chosen outgoing from a predetermined temperature of the sunrays of the focal point, and a predetermined melting temperature of the heat accumulating substance to be molten.

Preferably, said container is associated with a utilisation circuit, said utilisation circuit comprising a circulation tubing connected to a heat exchanger associated with said container, said utilisation circuit communicating with a utilisation device. Hereby, the heat accumulated in the molten heat accumulating substance is allowed to heat a fluid also during night hours.

Suitably, said utilisation device comprises an electric generator for generating electricity, a further heat exchanger for production of hot water or heat for an oven. Preferably, said heat accumulating substance is a salt, a mixture of salts, an alkali metal or a mixture of alkali metals.

DRAWING SUMMARY

In the following, the invention will be described in more detail by reference to the enclosed drawings, in which

Fig. 1 illustrates a thermal energy accumulator provided with an annular

expansion/contraction device; Fig. 2 illustrates an alternative thermal energy accumulator provided with a tubular expansion/contraction device; Figs. 3A- 3B illustrate another thermal energy accumulator provided with a telescopic expansion/contraction device;

Figs. 4A- 4B illustrate a thermal energy accumulator provided with an annular

expansion/contraction device and a telescopic expansion/contraction device;

Fig. 5 illustrates a thermal energy accumulator provided with four tubular

expansion/contraction devices and a telescopic expansion/contraction device; Figs. 6A- 6B illustrate a thermal energy accumulator provided with an alternative

expansion device;

Fig. 7 illustrates a thermal energy accumulator provided with yet another alternative expansion device;

Figs 8A- 8B illustrate a thermal energy accumulator provided with another kind of

expansion device;

Fig. 9A illustrates a prior art thermal energy accumulator; and

Fig. 9B is a cross-section along IXB - IXB in Fig. 9A. DETAILED DESCRIPTION

In the following, the use of relevant reference numerals in Figures 9A and 9B referred to above will be used correspondingly.

Figure 1 shows a thermal energy accumulator 2 having a closed container 4 with a bottom, a mantle 10, an exterior wall 12a and a lid 5. The lid 5 may be integrated with or connected to the mantle 10. A double-walled jacket 30 is arranged at a distance from the mantle 10, leaving an annular space 32a without top cover. The mantle 10 is provided with a bottom 8 and is arranged by not shown distance members at a distance from the bottom 9 of the jacket, thus leaving an intermediate space 32b between the bottom of the mantle 10 and the bottom 9 of the jacket 30. Inside the double-walled jacket 30, is arranged a heat exchanger 26 in the form of a tubing 28, being part of an optional utilisation circuit 25. The tubing 28 extends through the jacket and transports a fluid, such as air or water heated by the heat exchanger 26, and is adapted to be connected to a utilisation device 29 (cf. Fig. 4A). It should be noted that the mantle 10 may alternatively be connected directly to the bottom

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 to allow 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 heat accumulating substance 20 and the lid 5. Additionally, the annular space 32a and the intermediate 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 (NaCI), potassium nitrate (N0 ₃ ), sodium nitrate (NaN0 ₃ ) or a mixture of salts, such as a mixture of N0 ₃ and NaN0 ₃ . 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 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 may 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, it would be advisable to adjust the position and thus reduce the temperature of the focal point 37 in order not to burn the lid 5. A control system for adjusting the temperature of the focal point is described in EP 0 418 586.

The manufacturing cost may of course be reduced by setting the focal point manually e.g. by simply moving the lens 35a. Alternatively, 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) towards the 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 alternatively be made of a plastic sheet mounted on e.g. a 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/eeSvHg05fmQ

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 the volume of the container 4 and the expansion constant of the used kind of heat accumulating substance 20.

An optional force actuating means to be arranged inside the space 32a is disclosed in connection with Figs. 4A-4B. Figure 2 shows an alternative thermal energy accumulator 2 provided with an

expansion/contraction device 33 in the form of a hollow pipe 43. The bottom of the pipe 43 is in fluid communication with the interior of the container 4 filled with heat accumulating substance 20, while the top of the pipe 43 is open. The tubing 28 of the utilisation circuit 25 transports a fluid heated by the heat accumulating substance 20 via the heat exchanger 26 to an optional 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 35a in order to position the focal point 37 at a desired position, in particular to avoid wear of the lid 5.

The alternative materials of the lid 5 mentioned above would also be possible to include in this variant. An optional force actuating means to be arranged inside the hollow pipe 43 is disclosed in connection with Figs 5, 6A-6B and 7.

The thermal energy accumulator 2 of Figs. 3A and 3B is provided with a solar receiver 36 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 of focussed sun rays, cf. above. Alternatively, the lenses 40 are adjusted such that the focal 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 may be 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 a first 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 the second 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 heat accumulating 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 force actuating means 50 in the form of a weight device 52. As mentioned above, the heat accumulating substance expands when it melts and contracts when it solidifies. Thus, at low temperatures, the weight device 52 will press the first tubing member 44a towards and into said second tubing member 44b (cf. Fig. 3A) and thus move in the direction of the arrow. Correspondingly, when the heat accumulating substance melts, the first tubing member 44a will move vertically upwards in relation to the second tubing member 44b (cf. Fig. 3B), i.e. in the direction of the arrow.

Thus, the expansion/contraction device 33 comprises the first and second tubing members 44a, 44b and the weight device. As can be understood from Figs. 3A and 3B, the weight device 52 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 be slightly smaller than the inner diameter of the first tubing member 44a. In that case, the weight 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 weight device 52 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 utilisation device 29 (cf. Fig. 4A).

In Figure 4A and 4B another heat accumulating device 2 is shown and is provided with the kind of solar receiver described in connection with Figs. 3A - 3B and the

expansion/contraction device 33 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 is provided. Thus, the container 4 is filled 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, force actuating means 50 in the form of an annular counter-weight device 54 is provided. The annular counter-force device 54 has a higher density than the molten heat accumulating substance 20, but since the annular counter-force device 52 seals against 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. Thus, the annular counter-weight device 52 presses the heat accumulating substance 20 towards the globe 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 annular counter-weight device 52 also depends on other factors, such as the length and diameter of the first and second tubing devices 44a, 44b, in relation to the height 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 alternative expansion/contraction device 33 is shown in Figure 5, according to which four expansion tubings 43 are arranged on top of the container 4 and communicating with the interior of container 4 (cf. Fig. 2). The expansion tubings 43 are provided with a force actuating means 50 in the form of a counter-weight weight device 52 on the free surface of the heat accumulating substance 20. The counter-weight devices 54 are sealingly arranged inside the tubings and have a density higher than that of the molten heat accumulating substance 20. At melting, the counter-weight devices 54 will thus move upwards inside the expansion tubings 43.

Thus, the counter-weight devices 54 press the heat accumulating substance 20 towards the globe 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 counter-weight device 54 also depends e.g. on the length and diameter of the first and second tubing members 44a, 44b, in relation to the height of the expansion tubings 43. Thus, the counter-weight devices 54 may not be needed on the free surface 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 or three 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.

Ajacket 30 and a bottom 9 define a space 27 intended to contain a volume of liquid, such as water surrounding the expansion tubings 43 and the container 4, and thus constituting a 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 any cylindrical form. It may thus have a cross-section or any other polygonal cross-section 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 of a telescopic expansion tubing 43 arranged in communicating relationship with the interior of the container 4, and being provided at its opposite end with a force actuating means 50 in the form of a counter-weight 54. The solar receiver has 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 interior of the container 4, while its opposite end is provided with a force actuating means 50 in the form of a spring means 60 to create a force in a direction towards the container 4, as indicated by arrows. It should be noted that the telescopic expansion tubing 43 may be arranged at any angle in relation to the container 7, since the spring means makes the function of it independent from the influence of gravity.

Fig. 8A and 8B show an alternative thermal energy accumulator 2, having a container 4 comprising a circular cylindrical base portion 70a having a bottom 9 and a mantle 10, and a circular cylindrical top portion 70b having a lid 5 and a mantle 10'. The thermal energy accumulator 2 is provided with any kind of solar receiver (cf. Figs 1 - 3B). The inner diameter of the circular cylindrical base portion 70a is slightly larger than the outer diameter of the circular cylindrical cover portion 70b, such that the circular cylindrical cover portion 70b can be coaxially arranged inside the circular cylindrical base portion 70a. Furthermore, the inner diameter of the circular cylindrical base portion 70a the outer diameter of the circular cylindrical cover portion 70b are chosen such that they are allowed to move in relation to one another along a common axis in a reciprocating manner.

The circular cylindrical cover portion 70b is provided with a force actuating means 50 in the form of a weight 52. The weight 52 may comprise the focal point 37 for the solar receiver of the kind shown in Figs 1 - 2. Of course, in case a solar receiver of the kind shown in Figs 3A-3B is used, the weight 52 may be omitted or reduced in size, depending on the weight of the circular cylindrical cover portion 70b. As already mentioned above, the heat accumulating substance expands during melting.

Thus, the position of the circular cylindrical cover portion 70b in relation to the cylindrical base portion 70a is shown in Fig. 8A, the container 4 being substantially completely filled with molten heat accumulating substance. At solidification, the heat accumulating substance shrinks, causing creation of porosities and/or craters. In order to minimise the creation of porosities or craters during solidification, the weight device 52 moves the circular cylindrical cover portion 70b in relation to the cylindrical base portion 70a, such that the heat accumulating substance is pressed together.

The container 4 according to Figs 6, 7 and 8A- 8B is not provided with a utilisation circuit 25, respectively. When a predetermined temperature of the heat accumulating substance 20 has been reached, the container 4 is removed from the solar heater. Hereby, it is possible to transport the container 4 to a place where the heat is needed for heating or generation of electricity. It should be understood that the portions 70a, 70b may have a cylindrical shape of another cross-section than circular, such as polygonal.

Of course, the embodiments of Figs 1 - 5 may also not be provided with the optional utilisation circuit 25, and thus, the tubing 26, the heat exchanger 28 and the utilisation device may be dispensed with. On the other hand, the container of Figs 6A, 6B, 7 and 8A-

8B may instead be provided with an optional utilisation circuit 25, comprising a heat exchanger 26, a tubing 28 adapted to be connected to a utilisation device 29. Thus, the utilisation circuit may be connected to a utilisation device 29 at after transport of the container 4 from the solar receiver to a site where the heat is needed.

Of course, it would be possible to combine the spring means 60 with a weight device 52 or a counter-weight device 54, depending on the kind of solar receiver used.

Even though the spring means 60 shown in Fig. 7 is a tension spring, it is contemplated that it would be possible to arrange a compression spring instead to act on the telescopic expansion tubing 43 in the direction of the arrows.

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.