HEAT RECEIVER TUBE, METHOD FOR MANUFACTURING THE HEAT

RECEIVER TUBE, PARABOLIC TROUGH COLLECTOR WITH THE RECEIVER TUBE AND USE OF THE PARABOLIC TROUGH COLLECTOR

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a heat receiver and a method for manufacturing the heat receiver tube. More over a parabolic trough collector and a use of the parabolic trough collector are provided.

2. Description of the Related Art

A sun energy collecting unit of a sun field power plant based on the concentrated solar power technique is for instance a parabolic trough collector with parabolic mirrors and a heat receiver tube. The heat receiver tube is arranged in a focal line of the mirrors. By sunlight reflecting surfaces of the mirrors the sunlight is focused to the heat receiver tube, which is filled with a heat transfer fluid, e.g. a thermo-oil or molten salt. Via the heat receiver tube the energy of the sunlight is coupled into the heat transfer fluid. Solar en ^¬ ergy is converted to thermal energy.

In order to maximize an efficiency, with which the energy of the sunlight is coupled into the heat transfer fluid, a solar energy absorptive coating is attached on a surface of the heat receiver tube. Such an absorptive coating commonly comprises a multilayer stack with sequentially deposited thin film layers having different optical characteristics.

An essential overall optical characteristic of the absorptive coating is a high solar absorbance (low solar reflectivity) for wavelengths of solar spectrum (absorption radiation) . Ad- ditionally a low emissivity (high reflectivity) for infrared radiation is essential. Such a coating is called selective solar coating. For the manufacturing of the heat receiver tube the solar absorptive coating is attached on the surface of the heat re ^¬ ceiver tube by a sequential profile of thin films deposition on the surface using a method like sputtering. SUMMARY OF THE INVENTION

It is an object of the invention to provide a heat receiver tube with an energy yield which is improved in comparison to the state of the art.

It is another object of the invention to provide a parabolic trough collector with the heat receiver tube.

A further object of the invention is to provide a use of the parabolic trough collector.

These objects are achieved by the inventions specified in the claims . A heat receiver tube for absorbing solar energy and for transferring absorbed solar energy to a heat transfer fluid which can be located inside a core tube of the heat receiver tube is provided. The core tube comprises at least one first partial core tube surface covered by at least one first solar energy absorptive coating for absorbing a first absorption radiation of a first certain spectrum of the sunlight. The core tube comprises additionally at least one second partial core tube surface covered by at least one second solar energy absorptive coating for absorbing a second absorption radia- tion of a second certain spectrum of the sunlight. An emis ^¬ sion radiation inhibiting coating for inhibiting an emissivity for infrared radiation is deposited on the second solar energy absorptive coating such that the second solar energy absorptive coating is arranged between the second partial core tube surface and the emission radiation inhibiting coating. The first solar energy absorptive coating forms a first partial heat receiver tube surface of the heat receiver tube and the emission radiation inhibiting coating forms a second partial heat receiver tube surface of the heat receiver tube. The radiation inhibiting coating is preferably directly at ^¬ tached to the second solar energy absorptive coating leading to a layer stack arranged on the second partial core tube surface of the core tube. This layer stack consists of the second solar energy absorptive coating and the emission radiation inhibiting coating.

For instance, the first partial surface is formed by a first segment with a first circumference (segment angle) between

90° and 270° whereas the second partial surface is formed by a second segment with a second circumference between 180° and 90° . Additionally a method for manufacturing a heat receiver tube according is disclosed. The method comprises following steps: a) Providing an uncovered core tube for a heat receiver tube with the first partial core tube surface and the second par ^¬ tial core tube surface;

b) Attaching a first solar energy absorptive coating on the first partial core tube surface and attaching a second solar energy absorptive coating on the second partial core tube surface; and

c) Attaching an emission radiation inhibiting coating on the second solar absorptive coating such that the second selec ^¬ tive solar energy coating is arranged between the second partial core tube surface and the emission radiation inhibiting coating . Also provided is a parabolic trough collector comprising at least one parabolic mirror having a sunlight reflecting sur ^¬ face for concentrating sunlight in a focal line of the para ^¬ bolic mirror and at least one heat receiver tube which is ar- ranged in the focal line of the parabolic mirror. The heat receiver tube is arranged in the focal line such that the first partial heat receiver tube surface with the first solar absorptive coating is at least partially located opposite to the sunlight reflecting surface and the second partial heat receiver tube surface with the emission radiation inhibiting coating is at least partially averted to the sunlight re ^¬ flecting surface. Finally a use of the parabolic trough collector in a power plant for converting solar energy into electrical energy is disclosed .

Preferably the first solar energy absorptive coating and the second solar energy absorptive coating form a common solar energy absorptive coating with common physical and chemical characteristics. There is just one kind of solar energy ab ^¬ sorptive coating attached to the lateral area of the core tube. This common solar energy absorptive coating has all over identical chemical and physical characteristics. As a consequence the first absorption radiation of the first the first certain spectrum of the sunlight and the second absorp ^¬ tion radiation of the second certain spectrum of the sunlight are nearly identical. The use of just one kind of solar ab- sorptive coating is advantageous as to the manufacturing of heat receiver tube. It is easier to deposit just one kind of solar energy absorptive coating on the overall core tube sur ^¬ face of the core tube. The concept of the invention is to optimize thermal charac ^¬ teristics of the heat receiver tube by maximizing a coupling of the solar energy (concentrated radiation energy) into the heat receiver tube via the first partial heat receiver tube surface and by minimizing a loss of thermal energy via the second partial heat receiver tube surface. The first solar energy absorptive coating forming the first partial heat re ^¬ ceiver tube surface is designed to absorb solar radiation as much as possible (absorbance more than 97%) . In contrast to that the emissivity via the second partial heat receiver tube is reduced. The heat receiver tube can be arranged in the fo ^¬ cal line of a parabolic mirror such that concentrated solar radiation impinges the first solar absorptive coating of the first partial heat receiver tube surface. The part of the heat receiver tube which is not heated by concentrated solar radiation (i.e. that part which typically faces the sun and is thus subject only to direct solar radiation) is coated by the emission radiation inhibiting coating. The emission ra- diation inhibiting coating is a non-selective coating.

Preferably the first partial surface and/or the second par ^¬ tial surface are aligned along a longitudinal alignment of the heat receiver tube. This characteristic is applied to the first core tube surfaces and/or the second core tube surface, too. The alignment along the longitudinal alignment of the heat receiver tube and the alignment along the longitudinal alignment the core tube, respectively, is advantageous as to an arrangement of the heat receiver tube in the focal line of the parabolic mirror. The coupling of the concentrated radia ^¬ tion energy of the sun into the heart receiver tube is maxi ^¬ mized and the loss of thermal energy of the heat receiver tube is minimized. In a preferred embodiment the first partial heat receiver tube surface comprises a first segment of a lateral area of the heat receiver tube with a first circumference which is selected from the range between 150° and 300° and preferably between 180° and 270°. In a further preferred embodiment the second partial heat receiver tube surface comprises a second segment of the lateral area of the heat receiver tube with a second circumference which is selected from the range between 210° and 60° and preferably between 180° and 90°. These an ^¬ gles are optimized concerning a collector geometry (e.g. RIM angle) .

The emission radiation inhibiting coating is deposited on the second partial heat energy absorptive coating. By the emis- sion radiation inhibiting coating a magnitude of the emissivity of infrared radiation is reduced. The emissivity for in ^¬ frared radiation of the radiation inhibiting coating is less than 30%. Preferably the emission radiation inhibiting coat- ing comprises an emissivity for infrared radiation which is less than 20%.

In a preferred embodiment the emission radiation inhibiting coating comprises a metal which is selected from the group existing of Aluminum, Copper, Silver, Gold and Molybdenum.

Other metals or alloys are possible, too. The emission radia ^¬ tion inhibiting coating can be metallic and therefore sub ^¬ stantially just consist of a metal. For instance, the emis ^¬ sion radiation inhibiting coating is a layer consisting of Copper. Such a coating with Copper blocks a heat radiation (emissivity) on the "upper" part of the heat receiver tube which is impinged upon by direct solar radiation. This strongly reduces the overall heat loss of the heat receiver tube while losing some of the total radiation impinging thereupon.

Preferably at least one of the partial heat receiver tube surfaces forms a contiguous area. The heat receiver tube is arranged in the focal line in parallel to the longitudinal alignment of the mirror. By this the absorption of solar energy is very efficiently. Concentrated solar radiation im ^¬ pinges always the solar absorptive coating of the first par ^¬ tial heat receiver tube surface (intensity about 52 suns) whereas the second partial heat receiver tube surface is not impinged by the concentrated solar radiation (intensity about 0.6 suns) . Very small amount of energy could be wasted while gaining much more in heat losses due to overall emissivity.

The overall ratio of absorption to emissivity of the heat re- ceiver tube is therefore increased even if some of the direct sun radiation is lost. The areas of the first partial heat receiver tube surface and the second partial heat receiver tube surface don't have to have the same extent. The extents of the partial heat receiver tube surfaces are easily opti ^¬ mized as well as their location on the lateral surface of the heat receiver tube (e.g. due to RIM) . In order to improve the physical and chemical stability and the thermal characteristics of the heat receiver tube some other measures are additionally implemented. For instance, the heat receiver tube has an encapsulation which comprises at least one encapsulation wall. This encapsulation wall is at least partially transparent for the first absorption ra ^¬ diation and/or for the second absorption radiation. At least partially transparent is given in the case that a transmis ^¬ sion for the absorption radiations is more than 80% and pref ^¬ erably more than 90%.

The encapsulation is preferably a glass tube and the encapsu ^¬ lation wall is a glass tube wall. Between the heat receiver surface and the encapsulation wall there is a receiver gap. This receiver gap is evacuated. This means that a gas pres- sure in the receiver gab is less than 10 ^~2 mbar and prefera ^¬ bly less than 10 ^~3 mbar. This has the advantage that a ther ^¬ mal heat transfer away from the heat receiver tube with the heat transfer fluid by convection is reduced. The thermal en ^¬ ergy doesn't dissipate and is substantially completely avail- able for the heating of the heat transfer fluid.

For the attaching of at least one of the solar energy absorptive coatings and/or for the attaching of the emission radia ^¬ tion inhibiting coating a thin film deposition technique is used

The thin film deposition technique is preferably selected from the group consisting of atomic layer deposition, chemical vapor deposition and physical vapor deposition. The physical vapor deposition is for instance sputtering.

In order to get structured layers structuring deposition techniques are used. Alternatively a layer can be deposited unstructured and after the deposition a structuring is car- ried out, e.g. by removing deposited material. In a preferred embodiment the attaching of at least one of the solar energy absorptive coatings and/or the attaching of the emission ra ^¬ diation inhibiting coating are carried out with the aid of a mask method. Preferably the first solar energy absorptive coating and the second solar energy absorptive coating form a common, contiguous, the complete core tube covering layer. In this situation the use of a mask method is not necessary. Following advantages are connected to the invention:

- A wider range of available materials is accessible for the second partial heat receiver tube surface of the heat re ^¬ ceiver tube.

- A higher blockage of heat radiation at the non-selective coated part results due to better fitted materials.

- This results in an overall higher ratio of absorption to emissivity of the complete heat receiver tube.

BIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention are produced from the description of exemplary embodiments with reference to the drawing. The drawings are schematic.

Figure 1 shows a cross section of a heat receiver tube and a parabolic through collector with the heat receiver tube.

Figure 2 shows the heat receiver tube on a side.

DETAILED DESCRIPTION OF THE INVENTION Given is a heat receiver tube 1 for absorbing solar energy and for transferring absorbed solar energy to a heat transfer fluid 2 which can be located inside a core tube 10 of the heat receiver tube. The core tube consists of a core tube wall 103 with steel.

The core tube 10 comprises a first partial core tube surface 101 which is covered by a first solar energy absorptive coat ^¬ ing (selective coating) 131 for absorbing a first absorption radiation of a first certain spectrum of the sunlight. The first solar energy absorptive coating is a multilayer arrangement with different layers with different optical char- acteristics.

A second partial core tube surface 102 is covered by a second solar energy absorptive coating 132 for absorbing a second absorption radiation of a second certain spectrum of the sunlight. The physical and chemical characteristics of the first solar energy absorptive coating and the second solar energy absorptive coating are the same. The first solar en ^¬ ergy absorptive coating 131 and the solar energy absorptive coating form a common contiguous solar absorptive coating 13 which is deposited all over the latent area of the core tube surface of the core tube.

An emission radiation inhibiting coating 14 for inhibiting an emissivity for infrared radiation is deposited on the second selective solar energy coating 132 such that the second se ^¬ lective solar energy coating 132 is arranged between the second partial core tube surface 102 and the emission radiation inhibiting coating 14. The emission radiation inhibiting coating consists of Copper. Alternatively the used metal is Aluminium.

The first solar energy absorptive coating 131 forms a first partial heat receiver tube surface 11 of the heat receiver tube 1. The emission radiation inhibiting coating 14 forms a second partial heat receiver tube surface 12 of the heat re ^¬ ceiver tube 1. These partial receiver tube surfaces are aligned along a longitudinal alignment 15 of the heat re ^¬ ceiver tube 1. The first partial heat receiver tube surface 11 forms a first segment 161 of the lateral area 16 of the heat receiver tube 1 with a first circumference 1611 of about 180°. The second partial heat receiver tube surface 12 forms a second segment 162 of the lateral area 16 of the heat receiver tube 1 with a second circumference 1612 of about 180°.

In the figures not shown are following additional structural measures: The heat receiver tube is enveloped in a glass tube with a glass tube wall. The glass tube wall is transparent for the absorption radiations with a transmission of more than 90%. Between the glass tube wall and the receiver sur ^¬ face 16 a receiver gap is located. This receiver gap is evacuated. A gas pressure is about 10 ^~3 mbar.

The heat receiver tube 1 is part of a parabolic trough col ^¬ lector 1000. The parabolic trough collector 1000 comprises at least one parabolic mirror 3 with a sunlight reflective sur- face 31. By the reflective surface 31 sunlight is concen ^¬ trated in the focal line 32 of the parabolic mirror 3.

The heat receiver tube 1 is located in the focal line 32 of the parabolic mirror 3. Thereby the first partial heat re- ceiver tube surface 11 of the heat receiver tube ("lower" part of the receiver tube 1) is arranged opposite to the sunlight reflective surface 31 of the mirror 3. The second partial heat receiver tube surface 12 ("upper" part of the heat receiver tube 1) is averted to the sunlight reflective surface 31 of the mirror 3.

Inside the heat receiver tube a heat transfer fluid 2 is lo ^¬ cated. By the solar energy absorptive coating sunlight is ab ^¬ sorbed and transferred into heat. This heat is transferred to the heat transfer fluid.

The parabolic trough collector is used in a solar power plant for converting solar energy into electrical energy.