DE102009022932B4 - solar collector - Google Patents

Abstract

Solar collector with at least one thermal absorber for absorbing solar radiation (3), which is surrounded by a housing (10), characterized in that the absorber (3) consists of a metallic or metallized, crosslinked 3D foam structure.

Description

The invention relates to a direction independent solar collector with high efficiency, whose application is particularly useful as a thermal collector, for. B. for drying buildings or as a heating support, or offers as a solar concentrator.

Solar collectors are devices that absorb the energy contained in sunlight by means of an absorber, convert it into heat and deliver it to a transmission medium. The recovered heat energy is usually used for the production of hot water or for space heating. Occasionally, the heat energy is also used to generate electrical energy, eg. B. by means of generators that are powered by heat engines used.

Solar collectors use almost the entire radiation spectrum of sunlight; As a result, relatively high efficiencies can be achieved with them. The efficiency of a solar collector is determined superficially by its absorber, but also by the thermal insulation effect of its housing.

Absorbers of solar collectors must absorb the short-wave sunlight well and at the same time emit only longer wavelength heat radiation weakly. In addition, they must allow as complete as possible heat exchange with the heat transfer medium.

The housing of solar collectors, which are provided with a transparent pane for the incidence of light, must be well insulated in order to prevent the recovered heat energy being lost again.

The thermal insulation of the housing is technically already very well developed. The housings of so-called vacuum tube collectors are thermally insulated by means of vacuum insulation. Their panes are made of coated glass, which has a high transparency for the short-wave sunlight and a high reflectivity for long-wave heat radiation.

As absorber so far provided with special coatings aluminum or copper sheets are used. With such sheets, however, neither a maximum absorption of sunlight nor a minimum emission of heat radiation can be achieved. In addition, so no optimal heat exchange with the heat transfer medium is possible.

Due to the fact that the housings of solar collectors are already very effective, further developments of solar collectors are aimed at improved absorbers.

So will in AT 411 104 B an absorber for a solar air collector presented, which consists of longitudinally juxtaposed sheet metal profiles, which are folded so that on the one hand by the sheet metal profiles air ducts are formed and on the other hand, the surface is increased. This is intended to improve the heat transfer of the absorber to the heat transfer medium air. An improvement of the absorption (sunlight) and emission behavior (heat radiation) of the absorber is not achieved.

In DE 44 31 124 A1 describes a thermal solar collector, which consists of transparent, completely filled with a porous absorber glass tubes, which are flowed through by a heat transfer medium. The glass tubes are surrounded by a reflector.

By using a volumetric absorber instead of the usual sheets, an improvement in the efficiency of solar collectors can be achieved per se. However, porous absorbers usually form a very high flow resistance, which must be assumed here since the porous body was not specified in more detail. It follows that only little heat energy is transported per time, which ultimately results in a poor heat exchange.

The DE 38 15 751 C2 describes a solar collector with an absorber in the form of a concertina-like folded metal foil, wherein in the folds of the metal foil thin metal meshes are inserted.

The WO 2009/061 492 A1 describes a carbon fiber composite heat transfer device that may include a high thermal conductivity filler and a foam material.

The EP 0 033 174 A1 describes a collector for converting radiant energy into heat which is in thermal contact with a liquid-absorbing layer, the liquid-absorbing layer having cavities for receiving a liquid.

The invention has for its object to provide a solar collector having an absorber, which shows a high absorption capacity for sunlight and a low emission of heat radiation. In addition, the absorber should allow a good heat exchange to heat transfer media.

This object is achieved by the features of claim 1. Further advantageous embodiments will become apparent from the claims 2 to 19.

The starting point is a solar collector whose absorber is surrounded by a housing. According to the invention, the absorber consists of a metallic, networked 3D structure. It is envisaged that the absorber is preferably made of open-pore metal foam made of aluminum or an aluminum alloy. Particularly suitable are copper and copper alloys, which are characterized by a very high thermal conductivity.

A much cheaper alternative is closed-cell metal foam, which, however, compared to open-cell metal foam one, albeit only slightly, lower efficiency. Both the open-pore and the closed-pore metal foam are made of aluminum or an aluminum alloy. The efficiency of an absorber made of closed-cell metal foam can be increased by applying a layer of open-pore metal foam to the surface of the closed-cell metal foam; Thus, the absorber consists of both open-pore and closed-pore metal foam, the reflection of sunlight being largely prevented by the open-pored metal foam.

A cost-effective, but in particular with respect to the heat exchange deteriorated alternative to the metal foam is open-cell plastic foam (eg., Polyurethane foam) is coated with metal. In the same way, the body may consist of ceramic foam.

It is inventively provided that both the open-pored metal structures and the closed-pore metal structures are coated with doped silicon.

At least for the parts of the surface of the absorber, which face the sun, coatings are provided which absorb the irradiated short-wave solar energy well (high absorption in the solar radiation), but the longer-wave heat radiation generated in the absorber only to a small extent (low emissivity in the area of heat radiation). As coatings black nickel or commercially available coatings are therefore used, the u. a. known under the trade names TiNOX, eta plus and sun select.

The foam structure of the absorber is characterized by a very good ratio of absorption to reflection. The absorption can, depending on the pore size, take place in open-pored metal foam to a depth of 30 mm; Reflection, on the other hand, takes place only to a small extent, since there is no regular reflection surface. The small amount of reflected radiation, moreover, before being able to leave the metal foam, is largely scattered back into the foam structure, i. H. the foam acts as a so-called light trap.

In order to achieve high efficiencies, a good heat exchange between the absorber and the used gaseous (air) or liquid (water, oil) heat transfer medium, which either moves due to convection through the absorber unit or is actively pumped, must take place in the absorber unit. This is best ensured by using an open-pored structure, which on the one hand can be flowed through by heat transfer media (low flow resistance) and on the other hand has a large surface area.

When using closed-cell metal foam, it is intended to provide the absorber with tubes or to impress channels into the foam structure through which the heat transfer medium is pumped. The high density of the closed-cell metal foam allows a higher heat flow from the absorption surface to the heat exchanger. The embossing of the channel structure allows the depth and width of the channels to be varied as desired, thus ensuring optimum removal of the heat from the structure. With correspondingly embossed metal sheets which are introduced into the channel structure, the metal foam is sealed off from the heat transfer medium (eg thermal oil). The metal sheets also protect the metal foam against aggressive heat transfer media (eg chlorinated water in swimming pools) from corrosion.

According to the invention, the absorber is introduced into a housing and covered by a pane of transparent material, preferably glass, designed as window glass or glass with a lens (Fresnel lens), wherein the absorber and the pane are separated by an air gap. The case, depending on the application box or tubular running, is thermally insulated, for. B. by means of foamed glass or vacuum.

According to the invention, the metal foam, which has suitable feed and discharge lines, is flowed through by a fluid, preferably air, but also water, an organic fluid (ORC) or thermal oil. It is envisaged that the sheathed composite metal foam / glass / insulation is vacuum-tight, whereby the heat is transmitted via solid bodies, such as pipes.

In a box-shaped design of the thermal absorber temperatures in the range of 60 to 100 ° C can be achieved in specially designed tubular absorbers, which are used in solar concentrators, a temperature level of 150 to 300 ° C is reached.

The invention will be explained in more detail with reference to eight embodiments; show:

1 a box-shaped solar collector in the lateral section;

2 a solar collector with Fresnel lens in a diagonal top view;

3 a tubular solar collector in longitudinal section;

4 a tubular solar collector in cross section;

5 a solar collector having a plurality of tubular elements arranged in parallel;

6 an absorber consisting of a combination of open-pored and closed-pore metal foam;

7 : an absorber with embossed channels;

8th : a three-dimensional, tetrahedral arrangement of absorbers.

The in 1 illustrated solar collector consists of the sheath 1 , the insulation layer 2 and the open-pored metal foam 3 passing through a gap 4 from the glass pane 5 is disconnected. The heat conduction takes place via the pipes 6 ,

The use of such a solar collector offers, for example, as a cladding element on building walls, where it can also be used here for permanent drainage.

2 shows (on average) a solar collector, in which the disc 5 is designed as a glass plate with Fresnel lens. The metal foam 3 is channel-like 7 spared. He also has the latent memory 8th on.

This solar collector is particularly suitable for the support of heating systems (eg in homes), as by the latent storage 8th a longer-term and largely constant heat release is achieved.

At the in 3 and 4 shown tubular solar collector is the absorber 3 , whose metal foam consists of a CuAl alloy, in an internally mirrored pipe segment 11 , The pipe segment 11 is with a layer of heat-insulating material 2 isolated; Advantageously, it could also be surrounded by an evacuated double-walled tube. The pipe segment 11 and the thermal insulation 2 are for mechanical protection with the sheathing 1 capsuled. At the through the pipe segment 11 , the insulation 1 and the sheath 2 formed housing 10 connecting pieces for the supply and discharge of the heat transfer medium are attached.

The absorber 3 has the shape of a longitudinally cut cylinder, wherein the flat sectional area of the absorber surface 9 forms. Those parts of the surface of the absorber 3 , which face the sun during operation, are provided with a coating known under the trade name "TiNOX", which causes the absorber 3 has a high degree of absorption in the range of solar radiation and a low emissivity in the range of heat radiation.

The absorber 3 is with the three disc-shaped fasteners 12 centered, which consist of metallic solid material and thus act as a heat storage. In addition, the disc-shaped fasteners carry 12 to an improved flow of the metal foam by giving the flow of the medium through the metal foam cylinder a preferred direction and allow a free, turbulent flow along the cylinder surface.

The absorber surface 9 is right in front of the glass 5 which is coated with In ₂ O ₃ , which achieves that the glass pane 5 lets in the incident sunlight, but largely restrains the heat radiation occurring in the solar collector.

In 5 a solar collector is shown, which is composed of a plurality of tubular, parallel and touching arranged elements, which are completely covered with glass. Such or similar constructed collectors open up a variety of design and application possibilities.

An absorber 3 , which is a combination of closed-cell metal foam 13 and open-pored metal foam 14 is, is in 6 shown. The open-pored metal foam 14 is located on the sun-facing side of the absorber 3 , It is thereby achieved that the solar radiation first penetrates deep into the absorber and is finally almost completely absorbed in it. Unlike an absorber 3 made entirely of closed-cell metal foam 13 Thus, the proportion of reflected sunlight can be lowered.

An absorber 3 in which in the closed-cell metal foam channels 7 are imprinted in 7 shown. The metal foam in turn is in contact with a thermal insulation 2 brought. This also has embossed channels 7 , By joining metal foam and thermal insulation 2 one obtains a defined channel structure through which a fluid can flow. The heat transfer within the absorber can be achieved by a free design of the channels 7 be optimized.

In 8th is finally a three-dimensional array of absorbers 3 shown, which is tetrahedral. With such an arrangement of the absorber is achieved that the incident light into the solar collector is used regardless of the direction of incidence with a particularly high efficiency. The tetrahedral arrangement of the absorbers is supported 3 of a three-legged design, sized to allow adults to pass beneath the absorber assembly.

As a further (not shown) embodiment variant is provided to mount a planar solar collector on a tetrahedral support structure, which is rotatably mounted in one of its vertices of its base. As a result, the solar collector can be tracked particularly easily the course of the sun throughout the day. If the supporting structure is mounted floating on a water surface, the solar collector can even be tracked without friction.

It is advantageous to use zeolites or silica gel as fluid and as latent storage. These substances are characterized by absorbing water while releasing heat and releasing it with heat again. This effect can be combined well with a solar collector. In one embodiment variant, water-saturated zeolite or silica gel in powder form is allowed to flow through the absorbers of the solar collector in order to extract the water from it. The dried zeolite or silica gel is then stored without loss of heat. When the zeolite or silica gel is brought back into contact with water, it releases the previously absorbed energy, which can then be used to cover the heat demand of a building. Buildings can be conditioned by installing these materials on the facades of a building.

LIST OF REFERENCE NUMBERS

1

jacket

2

Thermal insulation

3

absorber

4

gap

5

disc

6

pipe

7

channels

8th

latent storage

9

absorber surface

10

casing

11

pipe segment

12

fastener

13

Closed-cell metal foam

14

Open-pored metal foam

Claims (19)

Solar collector with at least one thermal absorber for absorbing solar radiation ( 3 ), of a housing ( 10 ), characterized in that the absorber ( 3 ) consists of a metallic or metallized, cross-linked 3D foam structure. Solar collector according to claim 1, characterized in that the metallic, crosslinked 3D foam structure open-pored metal foam ( 14 ). Solar collector according to claim 1, characterized in that the metallic, crosslinked 3D foam structure closed-cell metal foam ( 13 ). Solar collector according to claim 1, characterized in that the metallic, crosslinked 3D foam structure is a combination of open-pored ( 14 ) and closed-pore ( 13 ) Metal foam is. Solar collector according to one of claims 2 to 4, characterized in that the open-pored ( 14 ) and / or the closed-cell ( 13 ) Metal foam made of aluminum or an aluminum alloy. Solar collector according to claim 2 or 4, characterized in that the open-cell metal foam ( 14 ) consists of copper or a copper alloy. Solar collector according to claim 1, characterized in that the metallized, crosslinked 3D foam structure is provided with a metallic layer open-pore plastic foam. Solar collector according to claim 1, characterized in that the metallized, crosslinked 3D foam structure is provided with a metallic layer ceramic foam. Solar collector according to one of claims 1 to 8, characterized in that the metallic, crosslinked 3D foam structure is coated with doped silicon. Solar collector according to one of claims 1, 2 and 4 to 9, characterized in that the 3D foam structure is an open-pore 3D foam structure, which is directly connected to a memory and / or latent storage. Solar collector according to one of claims 1 to 10, characterized in that at least the sun-facing surface of the absorber ( 3 ) is colored black. Solar collector according to one of claims 1 to 11, characterized in that at least the sun-facing surface of the absorber ( 3 ) is provided with a coating which strongly absorbs short-wave sunlight and weakly emits long-wave heat radiation. Solar collector according to claim 12, characterized in that the absorber ( 3 ) in one with thermal insulation ( 2 ) provided box or tubular housing ( 10 ) and with a transparent disc ( 5 ) and between the absorber ( 3 ) and the disc ( 5 ) A gap ( 4 ), wherein the housing ( 1 ) is provided with inlets and outlets, which serve to pass a heat transfer medium. Solar collector according to claim 13, characterized in that the housing ( 10 ) is an internally mirrored, thermally insulated on the outside metal tube with a circular cross-section, which is provided with a longitudinal slot in which the disc ( 5 ) is inserted, wherein the two ends of the housing ( 10 ) are sealed and provided with connecting pieces for the supply and discharge of the heat transfer medium, and the absorber ( 3 ) is a longitudinally cut cylinder made of metal foam, by means of at least two disc-shaped fastening elements ( 12 ) centered inside the housing ( 10 ) and spaced apart from its inner walls. Solar collector according to claim 13 or 14, characterized in that the housing ( 10 ) coated composite of absorber ( 3 ), Disc ( 5 ) and insulation ( 2 ) is vacuum sealed. Solar collector according to one of claims 13 to 15, characterized in that on the disc ( 5 ) is formed a Fresnel lens. Solar collector according to one of claims 1 to 16, characterized in that at least three absorbers ( 3 ) are spatially arranged to form a three-dimensional structure. Solar collector according to claim 17, characterized in that the spatial arrangement is cuboid. Solar collector according to claim 17, characterized in that the spatial arrangement is tetrahedral.

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