Heat exchanger, more particularly microstructure heat exchanger

0001The invention relates to a heat exchanger, in particular a microstructure heat exchanger, according to the preamble of the main claim.

State of the art

0002The storage of heat or Cold can be done in addition to a storage in the form of sensible heat in latent heat, that is, in substances that undergo a phase change in heat or heat dissipation. Such materials are known by the term "phase change material" (PCM). A disadvantage of these materials is that they often have a low thermal conductivity in the solid state, so that in a heat supply or heat dissipation short heat diffusion paths in the solid are required. At the same time, this requires a high density of heat exchanger structures in order to ensure effective heat input or To ensure heat extraction.

0003Since the production of a variety of particular small tube and the joining of these tubes to a heat exchanger in the production is very expensive, so far, the effective thermal conductivity in such materials is often improved by fins between pipes or plates in the heat exchanger. However, this leads to an increased cost and in particular to an additional weight and volume of these heat exchangers, which is disadvantageous, inter alia, when used in vehicles.

0004Further occurs in the illustrated materials that undergo a phase change in heat supply and removal, in practice, always a change in volume (expansion or contraction), resulting in significant mechanical loads within the heat exchanger. Such a volume change is caused, for example, by crystallization or melting.

0005To reduce these loads has already been proposed to compensate for the change in volume by an elastic behavior of the heat exchanger. Thus, for example, in ice storage systems of the company Webasto, Stockdorf, used for trucks plastic pipes with an elastic fold, but have a small heat transfer coefficient.

0006An alternative method is proposed in WO 98/04644, in which a microstructure in the form of an elastic graphite matrix is ​​used, wherein the graphite matrix has pores in which the "phase change" material is located. Although a good heat transfer is ensured by this porous microstructure, its long-term stability has not yet been proven. In addition, no usable cost-effective production technology is available for such a microstructure so far.

0007Another approach for the realization of microstructure heat exchangers with defined fluid guidance through the capillary inner spaces of metallic hollow fiber structures has been proposed in the application DE 199 10 985.0.

0008The object of the present invention is to provide a heat exchanger, in particular a microstructure heat exchanger, which at the same time has high heat transfer coefficients and high stability with respect to changes in volume and at the same time low weight and volume, and which, moreover, is cheap and easy to produce.

Advantages of the invention

0009The heat exchanger according to the invention has the advantage over the prior art that very small pipe diameters or wall thicknesses of the pipes used can be realized thereby, so that a large number of small tubes can now be connected in parallel or in the form of a regular arrangement of tubes in a simple manner , Thus, on the one hand small pipe distances and thus small heat diffusion lengths in the heat exchanger according to the invention can be realized, resulting in a high power density, and on the other hand results due to the small pipe diameter increased stability despite small wall thickness and low weight.

0010The heat exchanger according to the invention has the further advantage that the used, in particular metallic hollow fiber structure is easy and cheap to produce, without a complex process engineering or connection technology for the individual tubes in the hollow fiber structure would be required.

0011It is also advantageous that the heat exchanger according to the invention, despite the use of a material which undergoes a phase change in a heat supply or heat dissipation, has a high heat transfer coefficient.

0012Advantageous developments of the invention will become apparent from the measures mentioned in the dependent claims.

0013Thus, a hollow-fiber structure which has already been proposed in the application DE 199 10 985.0 is particularly advantageously suitable as a metallic hollow fiber structure. It is further advantageous that there are a variety of interconnection options for the micro-tubes used in such a hollow fiber structure, which can be adapted in each case to the respective requirements for the heat exchanger to be produced.

0014For absorption of a change in volume, caused by a phase change induced by heat supply or heat removal, it is further advantageous to attach at least one, but preferably a plurality of elastic shaped bodies to the hollow fiber structure. Alternatively or additionally, however, these elastic shaped bodies can also be contained in the material which undergoes the phase change. In this way it is achieved that the volume change occurring due to the phase change does not lead to mechanical stresses or damage to the heat exchanger, but is absorbed by these elastic moldings. Suitable elastic shaped bodies are, for example, elastic beads, such as polystyrene beads, which are distributed stochastically in the housing or the material or medium which undergoes a phase change. Alternatively or additionally, however, these elastic shaped bodies can also be fastened to the hollow-fiber structure, for example by gluing.

drawings

0015The invention will be explained in more detail with reference to the drawings and the description below. Figure 1 shows a first embodiment of a heat exchanger, Figure 2 shows an alternative embodiment.

embodiments

0016The coupling of heat or decoupling into or out of a medium or a material which undergoes a phase change in the case of heat supply or heat removal takes place in the exemplary embodiments explained below with the aid of a metallic hollow fiber structure in which a first, gaseous or liquid medium flows. For the resulting heat transfer coefficient, in addition to the heat transfer coefficients of the first medium to the hollow fiber structure and the heat transfer coefficient through the hollow fiber structure, especially the heat transfer within the second medium is of crucial importance. The heat transfer coefficient k in a solid phase change material is determined on the one hand by the thermal conductivity λ of the phase change material and on the other hand by the heat diffusion length L. This results<maths id="math0001" num=""><math display="block"><mrow><mtext>k = </mtext><mfrac><mrow><mtext>λ</mtext></mrow><mrow><mtext mathvariant="italic">L</mtext></mrow></mfrac><mtext>,</mtext></mrow></math><img file="EP1156293A2_D0001.tif" /></maths>

0017Since known phase change materials often have a relatively low thermal conductivity λ both in the solid and in the liquid phase, in practice this means that, for a high power density of a heat exchanger to be produced, the distance between the tubes forming the second medium must pass through, in the millimeter or sub-millimeter range must lie. The following table shows examples of some "phase change" materials such as ice, an aqueous salt solution or paraffin typical values ​​for the thermal conductivity λ for a heat transfer coefficient k to be achieved within the heat exchanger to be produced, and the resulting typical average distances between the tubes within a hollow fiber structure. <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" align="left">material</entry><entry namest="col2" nameend="col2" align="left">Thermal conductivity [W / mK]</entry><entry namest="col3" nameend="col3" align="left">Target k-value [W / m² K]</entry><entry namest="col4" nameend="col4" align="left">Mean distance of the tubes [mm]</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">ice</entry><entry namest="col2" nameend="col2" align="left">2.2</entry><entry namest="col3" nameend="col3" align="left">1000</entry><entry namest="col4" nameend="col4" align="left">4.4</entry></row><row><entry namest="col1" nameend="col1" align="left">aqueous salt solution</entry><entry namest="col2" nameend="col2" align="left">0.5</entry><entry namest="col3" nameend="col3" align="left">1000</entry><entry namest="col4" nameend="col4" align="left">1.0</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">paraffins</entry><entry namest="col2" nameend="col2" align="left">0.1</entry><entry namest="col3" nameend="col3" align="left">1000</entry><entry namest="col4" nameend="col4" align="left">0.2</entry></row></tbody></tgroup></table></tables>

0018FIG. 1 illustrates a first exemplary embodiment of a heat exchanger in the form of a microstructure heat exchanger 5. For this purpose, it is provided within a housing 14, which for example has the shape of a plate to arrange a hollow fiber structure 10, which consists of a plurality of tubes 15 which are each connected to a collection tube 12 for supplying a first medium 11, and on the other hand respectively with a collection tube 12 for discharging the tubes 15 and the collection tubes 12 flowing through the first medium 11 are in communication. The first medium 11 is, for example, water, a coolant or oil. In addition, in principle, a gas in question. It is further provided according to FIG. 1 that the housing 14 is filled with a second medium 13 which surrounds the hollow fiber structure 10. This second medium 13 is a phase change material such as water, ice, a saline solution, a molten salt, or a hydrocarbonaceous compound such as a paraffin. The average distance between the tubes 15 in the hollow fiber structure 10 is between 100 microns and 5 mm. The number and the average spacing of the tubes 15 results on the one hand from the thermal conductivity λ of the second medium used and the target for the heat transfer coefficient k in the heat exchanger 5. Furthermore, the heat diffusion length L in the second medium 13 must also be taken into account. The wall thickness of the tubes 15 of the hollow fiber structure 10 is between 100 nm and 100 microns, with wall thicknesses of 500 nm to 5 microns are preferred.

0019The phase change in the second medium 13 can be a phase change from solid to liquid, from liquid to solid, from liquid to gaseous, from gaseous to liquid, from solid to gaseous or from gaseous to solid. Preference is given to a phase change from liquid to solid and vice versa, as occurs for example in the case of water or ice in a cold storage. The heat supply or heat removal from or into the second medium 13 takes place, moreover, in a manner known per se by a temperature difference between the first medium 11 and the second medium 13.

0020For further details on the actual production of the particular metallic hollow fiber structure 10 reference is made to the application DE 199 10 985.0, where such hollow fiber structures are described in detail, and where the manufacturing method used for this purpose is explained. In particular, it should be emphasized that the hollow fiber structure 10 can be realized, for example, in the form of a step-like construction, wherein parallel tubes 15 are connected to form a two-dimensional structure, preferably in so-called "Tichelmanian interconnection". Further, it is possible to carry out the hollow fiber structure 10 in the form of a helical spiral structure, for example, a two-dimensional hollow fiber structure 10 is wound as a helical or spiral spiral according to Figure 1, so that so that a large volume can be filled. Finally, it is also possible to carry out the hollow fiber structure 10 in the form of a tube bundle, wherein the collection tubes 12 are formed for example in the form of tube plates.

0021A second exemplary embodiment will be explained with reference to FIG. 2, which differs from FIG. 1 only in that the hollow fiber structure 10 has a modified construction. Incidentally, the housing 14 according to FIG. 2 may also have a form deviating from a plate.

0022In detail, the hollow fiber structure 10 according to FIG. 2 consists of a plurality of hollow fiber structures 10 arranged one above the other according to FIG. 1, which are connected to one another by means of tubes and / or rods for stabilization. The distance between the individual tube planes according to FIG. 2 is typically 0.5 to 5 mm, the spacing between parallel tubes 15 within a tube plane, wherein a tube plane is formed by a structure according to FIG. 1, is typically also between 0.5 to 5 mm ,

0023In a preferred embodiment of the first embodiment and the second embodiment is further provided that in the second medium 13 at least one, but preferably a plurality of elastic moldings are included. These elastic shaped bodies are, for example, styrofoam beads with typical diameters of 0.5 to 3 mm, which are stochastically distributed within the second medium 13. Alternatively, however, these elastic shaped bodies can also be connected to the hollow fiber structure 10. For this purpose, these elastic moldings are glued stochastically distributed, for example, to the hollow fiber structure 10. In addition to styrofoam, polyurethane foams or other polymer foams are suitable as material for the elastic molded bodies.

0024The illustrated in the embodiments 1 or 2 heat exchangers are particularly suitable for heat storage or cold storage in motor vehicles, in order to smooth load peaks occurring in the starting phase. Furthermore, such heat exchangers are also suitable as latent heat storage in building services, where they can be used in particular as a hot water tank for gas boiler low power or as a heat storage use. In addition, they are suitable as cold storage for reducing peak loads in air conditioning technology.