Patent Application
20080196862
The invention relates to a heat or cold accumulator having a porous body for holding an accumulator medium and having a first heat exchanger which is embedded in the porous body and comprises at least one metal tube, which first heat exchanger can be traversed by a heat transfer medium for charging the accumulator with cold or heat.
Accumulators of said type serve in particular for the auxiliary air-conditioning of utility vehicle cabins, stop-and-go air-conditioning in vehicles whose internal combustion engine is shut down when at a standstill, and for pre-cooling of vehicle interior spaces in order to thereby accelerate the cooling when the vehicle is started.
It is fundamentally possible for accumulators of said type to be used as cold accumulators or as heat accumulators. When used as a cold accumulator, the embedded heat exchanger is generally connected into a compression refrigeration circuit; when used as a heat accumulator, the heat exchanger will be connected into the heating circuit of the vehicle.
A generic accumulator is known from DE 102 42 069 A1. Here, it is provided to introduce the heat exchanger for charging the accumulator as a serpentine-shaped flat tube into an arrangement of porous graphite plates. By virtue of refrigerant flowing through the flat tube heat exchanger, said accumulator is charged as a result of cooling of the accumulator medium held in the graphite plates.
DE 103 18 655 B3 likewise describes an accumulator having a porous body for holding an accumulator medium. Said document describes a possibility for discharging a cold accumulator of said type specifically by means of an overflow of evaporated refrigerant from an evaporator into a tank which surrounds the accumulator. A discharging process of said type is however expedient only with regard to stop-and-go operation; universal use with regard to the main applications of cold accumulators specified above is not possible. The cited disclosure also already proposes to use a porous body composed of metal foam instead of a porous body composed of graphite.
With regard in particular to the graphite matrix provided according to the prior art, it is possible to list numerous positive properties. Graphite has a high water absorption capacity of approximately 5.7 kg H2O/kg graphite. In addition, a high energy density of approximately 600 Wh/kg graphite or 90 Wh/l graphite is provided. The thermal conductivity of graphite is in the range between 5 and 30 W/mK. The mechanical stability is sufficient for applications in the automotive field, and the rigid graphite matrix prevents an expansion of the accumulator during a phase conversion of the accumulator medium.
A disadvantage of an accumulator with a graphite matrix is however that the filling of the accumulator with the accumulator medium is complex since it necessitates the application of a filling pressure. Further properties of the graphite matrix have room for improvement, such as for example the thermal conductivity and the mechanical stability.
The invention is based on the object of refining an accumulator of the generic type in such a way that its thermal properties are at least maintained if not improved, and which can be filled with a heat accumulator medium in a simple manner. A possibility for discharging the accumulator should also be created which can be used universally in all applications.
Said object is achieved by means of the features of claim 1.
Advantageous embodiments of the invention are specified in the dependent claims.
The invention builds on the generic accumulator in that a second heat exchanger, which is provided for discharging the accumulator, is embedded in the porous body, which second heat exchanger comprises at least one metal tube, and in that the porous body is composed at least partially of metal foam, with the metal foam being composed of the same metal as the metal tubes. The accumulator therefore has two heat exchangers, with the one heat exchanger being provided for charging the accumulator and the other heat exchanger being provided for discharging the accumulator. The porous body for holding the heat accumulator medium is composed of aluminum, as are the heat exchanger tubes. It can firstly be stated that metal foam—values are given here and below by way of example for aluminum—has a higher thermal conductivity than graphite, specifically approximately 30 W/mK. This is advantageous for the dynamics of the accumulator. For the same volume, the absorption capacity for the heat accumulator medium and in particular for water is increased, specifically by at least 10%. In this way, the energy density is increased considerably, specifically to 618 Wh/kg metal foam or 100 Wh/l metal foam. In addition, a weight reduction of the accumulator matrix of around 2% is obtained. It is likewise to be mentioned that the filling technology is improved. While a level of process expenditure is involved in the case of an accumulator matrix composed of graphite as the accumulator medium is introduced under a pressure difference generated by evacuation, in the case of a metal foam matrix, it is sufficient merely to dip the latter into the accumulator medium. It is also to be mentioned that a metal foam matrix is yet more stable than the already stable graphite matrix. With regard to the materials used, it is to be mentioned that, on account of the identity of the materials for the metal foam and the metal tubes, there is practically no corrosion potential. Any residual potential can be eliminated by means of suitable treatment of the metal foam matrix. Aside from the fundamentally higher thermal conductivity of the arrangement, it is also to be mentioned that the thermal conductivity between the metal tubes and the metal foam matrix is also improved. Firstly, it is possible to produce good thermal conductivity by means of a suitable form-fitting and/or force-fitting connection between the metal tubes and the metal foam matrix, and secondly, insulating foils which, in the case of a graphite accumulator, are to be provided between the tubes and the graphite for corrosion reasons, and therefore the thermally insulating action of said insulating foils, are dispensed with. With regard to said necessary insulation, it is to be mentioned that the elimination of said insulation also reduces the production expenditure of the metal foam accumulator in relation to graphite accumulators. An advantage, which is not to be underestimated, of the metal foam accumulator over the graphite accumulator is also the better recyclability which results in particular from the uniform material selection.
The invention is advantageously refined in that the metal tubes extend through the metal foam body in a meandering fashion. As a result of the meandering guidance of the metal tubes through the metal foam body, it is possible to provide a large overall area for the heat transfer between the metal tubes and the metal foam matrix.
It is expediently provided that the heat exchangers are of substantially identical construction. It is thus possible to provide two structurally identical or virtually structurally identical heat exchangers both for the charging and for the discharging process, which reduces the production expenditure.
It can be provided that the metal foam body is composed of a plurality of metal foam plates, with tube sections of the heat exchangers running between the plates. It is therefore possible for the tubes to run between two adjacent metal foam plates, with it being possible for cutouts to be provided in the plates, which cutouts are matched to the outer contour of the tubes. Recesses of said type can be milled into the metal foam plates or provided already during the production of the plates. It is also possible to connect half-tubes to the metal foam plates before the latter are joined together, be it by adhesive bonding, pressing or welding, so that as the metal plates are joined together, the half-tubes which are adhered to the respective metal plates are joined to form a complete tube. The different metal foam plates can likewise be adhesively bonded, pressed or connected to one another by means of other suitable methods.
It can likewise be provided that the metal foam body is composed of a plurality of metal foam plates, with tube sections of the heat exchangers being embedded into the plates. In this case, metal foam plates are pre-manufactured with tubes arranged thereon. In order to then generate a large-volume body with a meandering profile of the heat exchanger tubes through said body, plates of said type are placed one above the other, and the tubes already arranged in the plates are then connected to one another in a suitable way outside the plates.
It is likewise possible for the metal foam body to be embodied as a block. If the metal foam body is embodied as a block from the start, it is necessary for openings to be provided or formed therein in order to then insert the tubes into said openings.
The invention is advantageously refined in that each heat exchanger has a plurality of metal tubes, with the first end regions of the metal tubes of the first heat exchanger opening out into a common inflow tube and the second end regions of the metal tubes of the first heat exchanger opening out into a common outflow tube, and the first end regions of the metal tubes of the second heat exchanger opening out into a common inflow tube and the second end regions of the metal tubes of the second heat exchanger opening out into a common outflow tube. The outflow and inflow tubes can therefore be easily connected to the respective lines of cooling, refrigeration or heating circuits.
In this context, it is expediently provided that the inflow tube of the first heat exchanger has inflow openings which are arranged so as to be distributed uniformly with respect to the opening-out points of the first end regions of the first heat exchanger. In the case of a cold accumulator, the first heat exchanger serves for introducing cold from a compression circuit, with the refrigerant being introduced under pressure. In order to ensure as uniform a distribution of the refrigerant as possible between the heat exchanger tubes, it is expedient to provide the uniform distribution of the inflow openings. It can for example be provided to arrange an inflow opening of said type in the direct vicinity of each heat exchanger metal tube.
It is also expedient that individual feed lines are connected to the inflow openings of the inflow tube of the first heat exchanger, which individual feed lines open out into a common feed line.
In this context, it is advantageous for the individual feed lines to have substantially the same diameter and the same length. Said identical length of the feed lines is likewise advantageous with regard to the uniform distribution of the heat carrier medium between the individual heat carrier tubes.
The accumulator according to the invention is advantageously embodied such that the inflow tube of the first heat exchanger is, in operation of the accumulator, arranged higher than the outflow tube of the first heat exchanger. Said arrangement is advantageous in particular when the heat exchanger is connected into a compression refrigeration circuit in order to thereby prevent oil which is present in the refrigeration circuit from being displaced into the heat exchanger or collecting in the latter and in this way reducing the efficiency of the accumulator.
Secondly, it is expediently provided that the inflow tube of the second heat exchanger is, in operation of the accumulator, arranged lower than the outflow tube of the second heat exchanger. It is expedient for the coolant for extracting the heat or cold, that is to say for example brine, to be transported from bottom to top through the accumulator, since the formation of air bubbles in the heat exchanger is prevented in this way.
The invention is advantageously refined in that metal tubes of the heat exchangers are embodied as flat tubes. Flat tubes of said type provide a large surface for the passage of heat between the outer side of the tube and the metal foam matrix. However, round tubes or tubes with some other arbitrary cross section can also expediently be used within the context of the invention. The interior of the tubes can be provided with separating fins, so that a plurality of individual flow paths run through a tube. Said separating fins increase the surface for the passage of heat between the heat carrier medium and the tube.
According to one preferred embodiment of the present invention, it is provided that the accumulator is, for the purpose of cold accumulation, filled with an accumulator medium such as water, paraffin or a mixture of salt hydrates.
It can also be provided that the accumulator is, for the purpose of heat accumulation, filled with salt hydrate or paraffin.
The invention is now explained by way of example on the basis of preferred exemplary embodiments with reference to the appended drawings, in which:
The accumulator 10 is composed of a substantially cuboidal metal foam body 12. Heat exchangers 14, 22 are arranged in said metal foam body 12. The heat exchanger 14 has flat metal tubes 16, 18, 20 and the heat exchanger 22 has the metal tubes 24, 26, 28 which are likewise embodied as flat tubes. The metal foam body 12 shown here by way of example is composed of a plurality of metal foam plates 30, 32, 34, 36, 38, 40, 42, 44 which are arranged one above the other. Sections of the metal tubes 16, 18, 20, 24, 26, 28 of the two heat exchangers 14, 22 are situated between in each case two adjacent plates. In order to ensure a compact construction, the plates are matched to the outer contour of the tubes in the regions in which the flat tubes bear against the plates. With regard to the production process of the accumulator, this can mean that in each case the upper and the lower half-shells of the flat tubes are attached to the plates before the assembly of the metal foam body, so that a complete tube is first formed during assembly. The connection of the tube sections, which are situated between the plates, outside the metal foam body can in this case also be prepared already before the plates are joined together. It can likewise be provided that the meandering heat exchanger is brought into its final form and the accumulator construction is thereafter completed by means of the metal foam plates.
According to further embodiments of the invention which are not illustrated, it is provided that the tubes are simultaneously integrated into metal foam plates. After the individual metal foam plates are joined together, the tubes need then merely be joined together outside the metal foam plates. The resulting construction is then similar to that illustrated in
It can also be seen in
The present invention has been described on the basis of an example of a substantially cuboidal accumulator. The invention is not restricted to this. Other shapes, for example a cylindrical metal foam matrix with heat exchanger tubes arranged therein, likewise fall within the scope of the present invention.
The features of the invention disclosed in the above description, in the drawings and in the claims can be essential to the realization of the invention both individually and also in any desired combination.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 039 672.0 | Aug 2005 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DE2006/001461 | 8/22/2006 | WO | 00 | 3/26/2008 |
