Method for producing a latent heat accumulator and latent heat accumulator

Abstract

The invention relates to a method for producing a latent heat accumulator of expanded graphite by expanding a graphitic starting material. The invention is characterized in that the graphitic starting material is introduced into a mold which corresponds at least in sections to the negative mold of the latent heat accumulator, and that the graphitic starting material is subsequently expanded in the closed mold.

Claims

1. A method for producing a latent heat accumulator, wherein a phase change material is introduced into a molded body produced by expanding a graphitic starting material, comprising: introducing the graphitic starting material into a mold that predefines a shape of the molded body, closing the mold, expanding the graphitic starting material in the mold by introducing heat, wherein the mold is heated before, or after, the graphitic starting material is introduced, or the energy required for expansion is introduced directly into the graphitic starting material, and after the molded body has been formed, introducing the phase change material into the molded body.

2. The method according to claim 1, wherein the mold is a porous mold.

3. The method according to claim 2, wherein the porous mold comprises a member selected from the group consisting of silicon carbide, graphite, a ceramic, and a sintered metal.

4. The method according to claim 1, wherein the mold comprises ventilation openings and/or is made of a porous or perforated material.

5. The method according to claim 4, wherein the perforated material is a perforated metal.

6. The method according to claim 5, wherein the perforated metal is copper.

7. The method according to claim 1, wherein the mold comprises a closed interior receiving the graphitic starting material that corresponds to a negative mold of the molded body.

8. The method according to claim 1, wherein the phase change material is introduced into the molded body by pressureless infiltration.

9. The method according to claim 8, wherein the pressureless infiltration is a capillary-driven infiltration.

10. The method according to claim 1, wherein the molded body is produced with a recess into which phase change material is introduced.

11. The method according to claim 1, wherein the molded body is stored in liquid phase change material and/or brought floatingly into contact with the phase change material to introduce the phase change material.

12. The method according to claim 1, wherein the graphitic starting material is a graphite salt having a bulk density between 0.1 g/ml and 0.9 g/ml.

13. The method according to claim 12, wherein the bulk density is between 0.3 g/ml and 0.8 g/ml.

Description

(1) In the drawings:

(2) FIG. 1 is a schematic representation of a mold for producing a molded body and

(3) FIG. 2 is a schematic representation of another mold for producing a molded body.

(4) FIG. 1 shows a schematic representation of a mold 10, by means of which a molded body of expanded graphite salt for a latent heat accumulator is to be produced, wherein the interior 12 of the mold 10 defines the outer geometry of the molded body.

(5) In the exemplary embodiment, the mold 12 consists of a bottom part 14, a cover part 16 and a peripheral wall 18 extending therebetween, which can be a hollow cylinder section, for example.

(6) Independently thereof, the bottom part 14, cover part 16 and peripheral wall 18 can have the desired internal geometries that are designed according to the shape of the molded part to be produced.

(7) A defined quantity of graphite salt is introduced into the interior 12.

(8) Usually, a well-ordered, highly crystalline, natural flake graphite is used for producing graphite salt. This is converted into a graphite salt by means of an intercalation agent. The graphite salt is expanded or bloated by a thermal shock treatment. During this process, the intercalation agent escapes. The graphite flakes increase their volume by a factor of up to 400.

(9) The treatment is carried out using acids such that acid radicals, such as sulfates or nitrates, are intercalated between the graphite layers.

(10) After closing the interior 12, the mold 10 is positioned e.g., in a furnace in order to introduce the necessary heat into the graphite salt such that it can expand, wherein the expanded graphite salt fills the entire interior 12 of the mold 10.

(11) FIG. 2 shows another schematic representation of a mold 100, which consists of a pot-shaped bottom part 102 and an upper part 104. As the sectional view shows, the bottom part 102 and the upper part 104 define an interior 112 having a U-shaped cross section and defining the shape of the expanded graphite that is produced. Irrespective of the fact that the cross section of the molded part is considerably reduced in the upper edge area compared to the bottom area, a uniform compaction and homogeneous pore structure of the expanded molded body is achieved based on the teaching of the invention according to which the molding takes place during the expansion of the graphite salt that is to be introduced into the mold 100 and according to which the shape is not achieved after expansion under pressureas in the state of the art.

(12) The expanded molded body remains dimensionally stable as the expanded particles adhere to each other due to adhesive forces and mechanical anchoring (interlocking).

(13) If a mold is to be produced according to the state of the art which corresponds to that shown in FIG. 2, disadvantages would result with respect to pore structure and channel cross sections in the upper area due to the reduction in cross section compared to the bottom area and the different pressures exerted as a result, which in turn have a detrimental effect when the molded body is infiltrated with a phase change material.

(14) An electrically heated, gas- or oil-heated furnace can be used to introduce the heat into the mold 10 or 100. An induction furnace is also an option. Another possibility is that the graphite salt is heated by microwaves.

(15) The amount of graphite salt that to be introduced into the interior 12, 112 depends on the one hand on the desired porosity to be achieved and on the other hand on the temperature at which the expansionalso known as bloatingis to take place.

(16) The material of mold 10, 100 must be selected such that a permeability is ensured for the propellant gas escaping during inflation. Suitable materials are, e.g., inherently porous materials, such as silicon carbide, graphite, ceramics or sintered metal. However, a perforated metal is also an option. It is essential that the through passages are smaller than the graphite salt's grain size.

(17) The graphite salt used can be, e.g., a graphite salt having a bulk density in the range between 0.1 g/ml and 0.9 g/ml. The grain size distribution of the graphite salt can be, e.g.: D10=130 m, D50=240 m and D90=360 m. However, these values are not to be understood as limiting in terms of protection.

(18) Good results can also be achieved if 80% of the grains have dimensions of less than 150 m.

(19) Particularly in industrial production, molded bodies should be produced in a continuous process. For this purpose, corresponding molds can be fed through a continuous furnace.

(20) After expanding the graphite salt and cooling the form 10, 100 the molded body having a defined geometry which is, as mentioned, defined by the internal geometry of the interior 12, 112 is removed. Subsequently, the molded body is preferably impregnated or infiltrated with a phase change material (PCM) by pressureless capillary-driven infiltration. Either a solid phase change material can be applied to the molded part, which is then melted, e.g., in the temperature range up to 150 C. to enable infiltration. Or, it is also possible to immerse the molded body in a liquid phase change material bath or to position it floatingly therein such that pressureless capillary-driven infiltration is enabled.

(21) Suitable phase change materials include in particular materials enabling a phase change in the temperature range between 60 C. and 300 C., especially in the range between 80 C. and 150 C. Preferred materials are waxes, such as paraffin, sugar, alcohol, inorganic salt or salt hydrate.

(22) The ratio of phase change material to expanded graphite should be 10:1 to 2:1.

(23) The gross density of the expanded graphite should be between 0.014 g/cm.sup.3 and 0.79 g/cm.sup.3. The gross density is temperature-dependent. At a temperature of 600 C., for example, the grains or flakes of the graphite salt can increase in volume by a factor of up to 100 and at a temperature of 1,000 C. by a factor of up to 400.

(24) Due to the escaping gases, a loss in mass of the graphite material between 20% and 22% can occur, even if lower values, e.g., 10% are possible.

(25) The following examples are intended to illustrate how the final porosity of the molded body depends on the quantity of graphite salt.

(26) 5 g of graphite salt are filled into a mold having an internal volume of 50 ml. This is kept at a temperature of 600 C. for 15 minutes and then demolded after cooling. Measurements have shown that the body has a density of 0.08 g/cm.sup.3 and a porosity of 96.5%.

(27) In a second experiment, 12 g of graphite salt are filled into the same mold having an internal volume of 50 ml. This is also heated at 600 C. for 15 min. After cooling and demolding, the body has a density of 0.19 g/cm.sup.3 and a porosity of 91.5%. The compressive strength was tested on a corresponding molded body. This resulted in a value of 0.97 N/mm.sup.2.