Heat exchange – Regenerator – Heat collector
Reexamination Certificate
1997-06-20
2001-05-08
Atkinson, Christopher (Department: 3743)
Heat exchange
Regenerator
Heat collector
C252S070000, C585S009000, C585S002000
Reexamination Certificate
active
06227285
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
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 instance, in plant for heat generation from solar energy or from fossile energy sources, but in addition also in cooling cycles.
Regarding prior art, reference is made e.g. 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. 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 known, in which context reference is made e.g. 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 22 659. 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 liquid. This causes delays in response of the latent heat accumulator on heat input.
SUMMARY OF THE INVENTION
Starting from the state of the art last referred to, 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 medium should be compatible with environment.
This objective is attained by the object of claim
1
, focussing on modifying the crystal structures by a structure additive, preferably in terms of hollow structures, such as hollow cones. The lamina shape of the crystals known for a paraffin-based heat storage medium from the state of the art will be changed accordingly to hollow cone-type or tubular crystal structures. The modified crystal structure is obtained by use of a crystal nuclei former disposed within the heat storage medium. According to the invention, it has been recognized that by direct modification of the crystal structures of the heat storage medium such as 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 it can be achieved that 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 medium but intersperses very rapidly the entire heat storage medium. It is nearly all of a sudden that thus a response results, i.e. melting of the heat storage medium and, hence, accumulation of heat. As far as previously and hereafter paraffin is mentioned in connection with the invention, that term is to cover paraffinic hydrocarbons such as n-paraffins (liquid) and macroparaffins. Also, it combines particularly so-called intermediate paraffins and microcrystalline waxes. In embodying the invention, it has been intended to have the structure additive homogeneously dissolved in the heat storage medium. In particular, there have been successfully used as structure additives especially those on the basis of polyalkylmethacylates (PAMA) and polyalkylacrylates (PAA), both as individual components and in combination. 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 homogeneous solution in paraffin also in associated form, paraffins can 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 paraffines, 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, suitable as structure additives are also ethylene-vinylacetate copolymers (EVA), ethylen-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 approx. 0.01 percent by weight, and, in particular, up to a proportion of approx. 1 percent by weight shows considerable changes in terms of improvement. A higher dosage might be of 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 event of solid paraffinic hydrocarbons (macroparaffins, intermediate paraffins, microcrystalline waxes) contain liquid components (low-melting 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. The heat storage medium can include an (uncracked) oil portion. It has been found to be of advantage in view of using a heat storage medium of the type hereunder described in more detail if the oil portion is adjusted to between. 0,1 and 10 percent by weight. When introducing in this embodiment heat to the heat storage medium in a solidified state, there results the effect that 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 enlarged paths within the heat storage medium, fostering in addition 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 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 that only chain lengths of a few 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 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 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 of the fact that at least for big-scale production it is not alway
Ahrens Wolfgang
Fieback Klaus
Hildebrand Gunter
Kramer Thomas
Laudi Rolf
Atkinson Christopher
Farber Martin A.
Schumann Sasol GmbH & Co. KG
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