NZ258561A - Heat storage medium for a latent heat (cold) accumulator which solidifies by forming crystals the structure of which is modified by an additive such as hollow cones accumulator - Google Patents

Abstract

The invention relates to a heat storage medium which forms crystal structures on congealing, like paraffin, for a latent heat store (latent cold store). In order in particular to obtain improved response behaviour when heat is supplied, it is proposed according to the invention that the crystal structures be modified by a structure additive, preferably hollow structures, e.g. hollow cones.

Description

New Zealand No. 258561 International No.

TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: 02.12.1992;19.12.1992;06.03.19Q93; Complete Specification Filed: 29.11.1993 Classification:^) C09K5/06; F28F23/00; F28D20/00; B01J19/30 Publication date: 27 May 1998 Journal No.: 1428 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Heat storage medium Name, address and nationality of applicant(s) as in international application form: ST SPEICHER-TECHNOLOGIE GmbH, a German company of Lindenstrasse 61a, DE-15517 Furstenwalde, Federal Republic of Germany 25856 1 HEAT STORAGE MEDIUM The invention refers in the first place to a heat storage medium for a latent heat accumulator or a latent cold accumulator, such as paraffin, which solidifies by forming crystal structures.

Latent heat accumulators are known to serve for effectivity- increasing temporal uncoupling of heat or cold generation and subsequent heat or cold consumption. Uncoupling permits long, continuous running times of heat or cold generators with high efficiencies and low start-up, shut-down and standstill costs. They are used, for 10 instance, in plant for heat generation from solar energy or from fossil energy sources, but in addition also in cooling cycles.

Regarding prior art, reference is made to DE-A1 27 41 829. It is known therefrom to use for heat storage medium in a latent heat accumulator quantities of paraffin enclosed within a plastic envelope. 15 Those plastic envelopes in turn are in an accumulator vessel filled with water. Heat transfer on such latent heat accumulators is merely by heat conduction through the plastic envelope to the paraffin. Such accumulators are referred to as static accumulators.

Furthermore, so-called dynamic latent heat accumulators are 20 known, in which context reference is made to DD 23 68 62 and DD 28 01 13. As far as prior art is concerned, further reference should be made to DE-A1 41 22 859. In this connection, it is considered of disadvantage that the heat storage medium such as paraffin is only difficult to intersperse by the heat transfer medium, perhaps evaporating water or an alcohol-based 25 liquid. This causes delays in response of the latent heat accumulator on heat input.

It is an objective of the invention to provide a heat storage medium such as paraffin for a latent heat accumulator/latent cold accumulator which will specifically result in an improved response on heat input.

Further, it is also to be borne in mind at this point that the heat storage Intellectual property Office of NZ 16 MAR 1998 RECEIVED 258561 2 medium should be compatible with environment.

This objective is attained by the present invention, focusing on modifying the crystal structures of the heat storage medium by a structure additive, to form hollow structures, such as hollow cones. The laminar 5 shape of the crystals known for a paraffin-based heat storage medium from the state of the art will be changed accordingly to e.g. hollow cone-type or tubular crystal structures.

According to the invention, it has been recognized that by direct modification of the crystal structures of the heat storage medium such as 10 paraffin it is possible to improve decisively the response by the heat storage medium on heat input. Surprisingly, it has been shown that by such a crystal modification the heat storage medium such as paraffin virtually adopts a porous structure. Water vapor, for instance, that forms on heat input not only penetrates the lower area of the heat storage 15 medium but intersperses very rapidly the entire heat storage medium. The resultant response is almost instantaneous, i.e. melting of the heat storage medium and hence, accumulation of heat. As used herein the term 'paraffin' means (in connection with the invention,) paraffinic hydrocarbons such as n-paraffins (liquid) and macroparaffins, and in 20 particular includes so-called intermediate paraffins and microcrystaliine waxes.

In embodying the invention, it has been intended to have the structure additive homogeneously dissolved in the heat storage medium. Structure additives based on polyalkylmethacrylates (PAMA) and 25 polyalkylacrylates (PAA), both as individual components and in combination, have been found particularly successful. Their crystal-modifying effect is produced by the fact that the polymer molecules are built into the growing paraffin crystals and continued growth of this crystal shape is prevented. Due to the presence of the polymer molecules in the 30 homogeneous solution in paraffin also in associated Office of nF 1 6 MAR 1898 RECEIVIB 3 25856 grow up on the special associates. Hollow cones will be formed, which are no longer apt to form networks. Because of the synergistic action of that structure additive on the crystallization behavior of the paraffins, cavity formation and, thus, improved flowability through the heat storage medium paraffin (e.g. for water vapor) as compared to paraffins not so compounded, is achieved.

In general other suitable structure additives are ethylene-vinylacetate copolymers (EVA), ethylene-propylene copolymers (OCP), diene-styrene copolymers, both as individual components and in mixtures, as well as alkylated naphthalins (Paraflow). The proportion of structure additives starts with a fraction of weight percent, realistically at approximately 0.01 percent by weight, and, in particular, up to a proportion of aporoximately 1 percent by weight shows considerable changes in terms of improvement. A higher dosage might be a disadvantage, since a great many small crystallites are formed, which will result in a dense crystal packing and, thus, adversely affect flowability through the heat storage medium. In particular, the proportion of structure additives depends also on the melting temperature of the heat storage medium. With higher-melting heat storage media or higher-melting paraffins, as a rule, a higher weight percentage of structure additives is required than with low-melting ones for achieving the same success. In further embodying the invention it is intended that the heat storage medium in the form of soiid paraffinic hydrocarbons (macroparaffins, intermediate paraffins, microcrystalline waxes) should contain liquid components (low-meltiny n- and isoalkanes as well as naphthenes), a so-called oil portion. As is well known, paraffinic hydrocarbons from vacuum distillate fractions are obtained by various technological separating steps requiring a certain oil portion. It has been found to be of advantage in view of using a heat storage medium of the type here-under described in more detail if the oil portion is adjusted to between 0.1 and 10 percent by Intellectual P Office |>f I6MAIS938 RECPIVPH 258561 weight. In this embodiment, when heating the heat storage medium in a solidified state, the oil portions included in uniform distribution will virtually exude from the heat storage medium and, by gravitation, run off downward. As compared to the said "porosity", this provides for still more 5 enlarged paths within the heat storage medium, assisting rapid penetration by the heat transfer medium. Such an oil portion in the heat storage medium, however, in most instances makes sense only if the heat storage medium has been solidified at room temperature. Furthermore, it is preferable within the scope of the invention to adjust the carbon chain 10 lengths in the paraffin in a carefully directed manner, that is, to arrange for a specific distillation cut so selected that it is comparatively narrow. 'Narrow distillation cut' means a predominance of chain lengths of not more than four figures are comprised, e.g. C 14 through C 16 or C 20 through C 23. Since of course the distillation cut, at least for large-scale 15 production where no very specific measures are taken, is always a result in terms of frequency distribution, the measure previously explained •signifies that in any event by far the largest portion of a given quantity of heat storage medium is made up of the chain lengths comprising but a few figures. In particular, the distillation cut is determined by the melting 20 temperature desired. In addition to that, it has been proved especially advantageous to prefer the even-numbered, normal C chains (n-alkanes). These have, in the isolation referred to, a surprisingly high heat storage capability on phase changes. At this point, it should be taken account that at least for big-scale production it is not always possible (at reasonable 25 cost) to produce the C chains "pure" in terms of even numbers. In any event, it is advantageous to enrich them as far as possible. Another preferred embodiment of the invention provides for boiling bodies or formers of crystal nuclei having such a specific gravity that they are suspended in the heat storage medium. Since the heat storage medium, depending on its liquefied or solidified state, can have Office of NZeity 1 6 MAR 1998 RECEIVED 25856 1 gravities, it is specifically preferred that those boiling bodies or formers of crystal nuclei are adapted to the specific gravity in the liquefied state. Once they are homogeneously distributed there, no segregation will result even on solidification, irrespective of whether or not different specific 5 gravities prevail. This specific gravity can be obtained e.g. by plastic particles or foam glass parts. The boiling bodies or formers of crystal nuclei are preferably relatively small, within the millimeter or fractional range of millimeters, so that they can be very finely distributed in the heat storage medium. It is especially in combination with the above mentioned 10 measure of modifying by a structure builder the crystal structure of the heat storage medium in terms of hollow structures that these bodies are found to be of advantage not only in terms of formers of crystal nuclei during the condensation process but also during boiling within the heat storage medium. The structure of the heat storage medium is virtually 15 adjusted to a porous state and will be interspersed, on heat input when using e.g. water as a heat transfer medium, by water vapor which on the cooler heat storage medium at once recondenses. Steam that follows will again result in boiling processes which then, intensified by the bodies likewise arranged in the heat storage medium in a distributed fashion, will 20 practically (re)occur uniformly and immediately in the heat storage medium. The characterization that the boiling bodies or formers of crystal nuclei are present at least in suspension within the heat storage medium (because of their specific gravity) signifies that there can be provided also other (another group of such) bodies likewise designed as suspended 25 matter within the heat storage medium. This is especially true where the heat transfer medium, e.g. alcohol, has a lower specific gravity than the heat storage medium. Regardless of this, it is preferred within the scope of the invention that also the heat transfer medium contains boiling bodies or formers of crystal nuclei (which normally, however, have therein only a function as boiling bodies). Where the heat transfer medium is water, Intellectual ProjMrty Offloa of NZ 16 MAR 1MB RECEIVED 6 25856 those boiling bodies can be specifically heavier than water and will thus accumulate on the bottom of a corresponding latent heat accumulator (since in dynamic latent heat accumulators preferred for the purpose the lighter medium will normally settle on top of the heavier one). Designing the bodies also in the heat transfer medium as suspended or nearly suspended substances, however, offers advantages, too, in that those bodies, if the heat transfer medium is specifically heavier than the heat storage medium, are hurled especially intensely into the heat storage medium, which further accelerates the desired melting process of the heat storage medium. Even though the functions of creation of crystal nuclei and of suppressing a boiling delay are separate ones, both effects are normally triggered by the same bodies, so that regarding material no distinction needs to be made. Such bodies are contained in the heat storage medium or the heat transfer medium preferably in the order of from 1 through 10 percent by weight. It goes without saying that even fractions of a percent by volume will produce a certain effect. Also, the boiling bodies or crystal nuclei formers can be so designed that based upon their specific gravity they are suspended partially in the heat transfer medium and partially in the heat storage medium. This can be achieved by a practicable design or in that - if e.g. the heat transfer medium (water) is specifically heavier that the heat storage medium (paraffin) - the boiling bodies or the crystal nuclei formers have a specific gravity between that of the heat storage medium and that of the heat transfer medium. Then, correspondingly, those bodies would arrange themselves on the boundary layer between the two media. This can be utilized to the effect that those bodies are designed with oblong stick- or tentacle-type extensions, by which they then project into the heat transfer medium, e.g. by an amount required for compensating their higher weight in relation of the heat storage medium. In this connection the design is preferably made in such a fashion that only the stick- or tentacle-type extensions project into the project into the Intellectual Pi Office of 16 MAR 1998 received 25856 heat transfer medium. For the rest, the crystal nuclei formers/boiling bodies can be so selected by their specific weight that they are either in the heat storage medium (in suspension) or in the heat transfer medium only. In another preferred embodiment the filling medium is provided with 5 an antifoam agent. Antifoam agents are basically known for paraffins or similar media. Reference can be made in this connection to the pertinent literature. In view of a filling medium for a latent heat accumulator or latent cold accumulator, however, such an antifoam agent has particular importance. Foam formation of the heat storage medium will result in 10 locally poorer heat transfer figures on heat transfer surfaces and, hence, reduced heat transfer yields. Further improvement can be achieved here by adding an antifoam agent to the heat storage medium. Antifoaming agents known are based e.g. on silicones, polyalkoxylates, fatty alcohol alkoxylates or carboxylic acid esters. In another embodiment provisions 15 have also been made to the effect that the heat storage medium contains an antioxidant. This will prevent the heat storage medium from aging, e.g. by cracking the carbon chains. With regard to paraffins, antioxidants on the basis of polyalkylated phenols and nitrogen-substituted phenylenediamins are known. Also in this respect reference is made to 20 the literature relating to paraffins.

The invention also provides an additive for a heat storage medium crystallizing by forming crystal structures, such as paraffin, for a latent heat accumulator/latent cold accumulator, which additive includes a structure additive modifying the crystal structures in terms of hollow 25 structures such as hollow cones. For more details regarding the structure additive, reference is made to what has been explained above. The additive can be added to existing heat storage media, especially on a paraffin basis, in order to improve their properties as described above in more detail. Furthermore, the additive can contain boiling bodies, in 30 particular such ones having different specific gravities, which are suitable, Intellectual Property Office of NZ 16 MAR 1998 8 5 S1 8 on the one hand, for being in suspension in the heat storage medium and, on the other hand, to be suspended in the heat transfer medium or to settle. In addition, the additive can also contain an antifoam agent and/or an antioxidant, for the individual components of which reference once 5 again is made to the more detailed description above.

The invention further provides a latent heat accumulator or latent cold accumulator containing a heat storage medium of one of the embodiments previously described. In this connection, however, a specific design in view of the boiling bodies or crystal nuclei formers is still 10 preferred. This one proposes to provide the boiling bodies or crystal nuclei formers as fixed internals in the latent heat accumulator. In particular, the boiling bodies or crystal nuclei formers can be realized by sheet elements, preferably curved sheet elements. These elements have been suitably provided such that they project partially into the area of the 15 heat transfer medium and partially into that of the heat storage medium.

The invention also relates to ihe use of paraffin with acertain oil portion, as previously described, for use in a heat storage medium for a latent heat accumulator.

Hereafter, as an example, a latent heat accumulator with a heat 20 storage medium of the type herein described is explained with reference to the attached drawings in which: Fig. 1 shows an hermetically sealed accumulator vessel with a magnifier-enlarged representation for a diagrammatical illustration of the enveloping structures and boiling bodies/crystal nuclei formers in the heat 25 storage medium; Fig. 2 is a representation according to Fig. 1, with modified boiling bodies/crystal nuclei formers; Fig. 3 is a representation as per Fig. 1 or Fig. 2, showing fixed internals as boiling bodies or crystal nuclei formers.

Fig. 1 illustrates an hermetically closed accumulator vessel Intellectual Property Office of NZ 16 MAR 1998 nr^n\/ rrs I 8 5 consisting especially of a metallic material preferably aluminium. In the condition shown in the drawing the heat storage medium 2 is in a solidified state. That medium is solidified paraffin. In the bottom area of the latent heat accumulator 3 there is substantially pure water 3 as heat 5 transfer medium, whereas the top area 4 of the latent heat accumulator 1 exhibits a low-pressure air space or vacuum.

In the enlargement the crystal structure of the heat storage medium 2 is shown diagrammatically. What can be seen are hollow-cone type structures, which together result in the desired microporosity of the heat 10 storage medium 2.

In the heat transfer medium 3, boiling bodies 5 are arranged, which on heat input effected e.g. by a heat exchanger projecting into the area of the heat transfer medium 3 or by simple heating of the latent heat accumulator 1 at its bottom provide for nearly simultaneous boiling of the 15 water 3. Likewise, other boiling bodies 5 or crystal nuclei formers 6 are arranged in the heat storage medium 2. Because of their specific gravity, the crystal nuclei formers 6 are suspended in the heat storage medium 2 also if the latter is liquefied.

On heat input to the latent heat accumulator 1 at its bottom area, 20 the water 3 starts boiling at a certain temperature which is essentially determined by the vacuum in space 4, and the steam emerging penetrates into the heat storage medium 2. Due to a structure additive homogeneously distributed in the heat storage medium 2, the latter, in its solidified state, is of virtually porous nature, such that the steam can 25 traverse a large surface nearly simultaneously, and, with corresponding suddenness, the heat storage medium 2 experiences a phase change and liquefies. So the steam passes rapidly to the top area 4, where usually, because of heat dissipation, condensation occurs. The water vapor collects in water drops and flows back to the bottom area of the latent heat accumulator 1. If more heat is extracted from the latent heat, Intellectual Property Office of NZ 16 MAR 1998 RECEIVED 1 accumulator in its head area than is put in to its bottom area, the latent heat accumulator discharges and below a certain temperature experiences another phase change (from liquid to solid), whereupon the condition shown in the drawing is reestablished.

Fig. 2 illustrates - essentially in diagrammatic representation -boiling bodies or crystal nuclei formers 6' exhibiting stick- or tentacle-type extensions 7. These can all be somewhat heavier than the crystal nuclei formers 6' themselves, which are here shown as balls. On the whole, the weight of such a body is specifically heavier than that of the heat storage medium 2, but lighter than the weight of the heat transfer medium 3, so that the boiling bodies or crystal nuclei formers will float on the heat transfer medium 3 by utilizing buoyancy. From this explanation it will be evident also that the bodies 6' are composed of different materials, and include parts (e.g. the extensions 7) which consist of a material specifically heavier than the heat transfer medium 3.

It is further shown in Fig. 2 that in addition there exist in the latent heat accumulator 1 other boiling bodies 5 or crystal nuclei formers 6 of the form previously described with reference to Fig. 1.

There result fundamental effects from the extensions 7 in view of quick response of the latent heat accumulator 1. Along the extensions 7, especially if they have a good thermal conductivity, there can rapidly form channels of molten heat storage medium 2, through which the heat transfer medium can flow into other areas of the heat storage medium 2.

A comparable effect is also given in the embodiment represented in Fig. 3, at least as far as the fixed internals 8 provided there project into both the heat transfer medium 3 and the heat storage medium 2. The fixed internals are retained on the accumulator vessel e.g. by holding devices 9.

It is indicated that the fixed internals 8 are preferably curved faces.

There are many other designs of the curved faces possible. Intellectual Property Office of NZ 16 MAR 1998 RECEIVED 11

Claims (26)

CLAIM: 258 561

1. A heat storage medium for use in a latent heat/latent cold accumulator comprising paraffin (as hereinbefore defined), wherein the crystal 5 structure of the heat storage medium is modified by the presence of a structure additive such that in the solid state said heat storage medium crystallizes in the form of hollow structures.

2. A heat storage medium as claimed in Claim 1, wherein the structure 10 additive is homogeneously dissolved in the heat storage medium.

3. A heat storage medium as claimed in Claim 1 or Claim 2 wherein the structure additive belongs to the family of polyalkyl(meth)acrylates. 15

4. A heat storage medium as claimed in any one of the previous claims wherein the structure additive is added to the head storage medium at between 0.01 and 1 percent by weight.

5. A heat storage medium as claimed in any one of the previous claims 20 wherein the heat storage medium includes an oil portion (as hereinbefore defined).

6. A heat storage medium as claimed in Claim 5, wherein the oil portion is between 0.1 and 10 percent by weight.

7. A heat storage medium as claimed in any one of the previous claims wherein the heat storage medium comprises paraffinic hydrocarbons of a narrow distillation cut (as hereinbefore defined). 258561 12

8. A heat, storage medium as claimed in any one of the previous claims wherein the heat storage medium comprises even-numbered n-alkanes. 5

9. A heat storage medium as claimed in any one of the previous claims further including boiling bodies and/or crystal nuclei formers, said boiling bodies and/or crystal nuclei formers having a specific gravity such that they are suspended in the heat storage medium.

10. 10. A heat storage medium as claimed in Claim 9 wherein the boiling bodies and/or crystal nuclei formers of different specific gravities are included.

11. A heat storage medium as claimed in Claim 9 or Claim 10 wherein the 15 boiling bodies and/or crystal nuclei formers are included at a proportion of 1 through 10 percent by weight.

12. A heat storage medium as claimed in any one of Claims 9-11, wherein the boiling bodies and/or crystal nuclei formers are selected from the 20 group consisting of: glass bodies, foam glass bodies, plastic bodies.

13. A heat storage medium as claimed in any one of Claims 9-12, wherein the boiling bodies and/or crystal nuclei formers havo a specific gravity such that they are suspended partly in a heat transfer medium and 25 partly in the heat storage medium.

14. A heat storage medium as claimed in any one of Claims 9-13, wherein the boiling bodies and/or crystal nuclei formers have stick-type extensions (as hereinbefore defined). Intellectual Property 30 Offlca of NZ 16 MAR 1998 Received 258561 13

15. A heat storage medium as claimed in Claim 14 when dependent upon Claim 13, wherein only the stick-type extensions project from the heat storage medium into the heat transfer medium. 5

16. A heat storage medium as claimed in any one of the previous claims further including an antifoam agent.

17. A heat storage medium as claimed in Claim 16 wherein said antifoam agent is present in a proportion of 0.01 - 5 percent by weight. 10

18. A heat storage medium as claimed in Claim 16 or Claim 17, wherein the antifoam agent is selected from the group consisting of silicones, polyalkoxylates, fatty acid alkoxylates, carboxylic acid esters. 15

19. A heat storage medium as claimed in any one of the previous claims further including an antioxidant.

20. A heat storage medium as claimed in Claim 19, wherein said antioxidant is present in a proportion of 0.001 - 0.1 percent by weight. 20

21. A heat storage medium as claimed in any one of the preceding claims wherein said hollow structures include hollow cones,

22. A latent heat accumulator/latent cold accumulator containing a heat 25 storage medium as claimed in anyone of Claims 1-21.

23. A latent heat accumulator/latent cold accumulator containing a heat storage medium as claimed in any one of Claims 1-8, further including boiling bodies and/or crystal nuclei formers comprising fixed surfaces 3 o protruding from the interior of the accumulator vessel. 258561

24. A latent heat accumulator as claimed in Claim 23 wherein the fixed surfaces are curved.

25. A heat storage medium as hereinbefore described with reference to and as shown in the accompanying drawings.

26. A latent heat accumulator/latent cold accumulator substantially as hereinbefore described with reference to and as shown in the accompanying drawings. END OF CLAIMS