Solar heat receiver tube for direct steam generation,

parabolic trough collector with the solar heat receiver tube and use of the parabolic trough collector

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates a solar receiver tube for direct steam generation, a parabolic trough collector with the solar heat receiver tube and a use of the parabolic trough collector. 2. Description of the Related Art

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

The heat transfer fluid with the absorbed thermal energy is piped to a heat exchanger. Water that passes through the heat exchanger becomes steam. The steam runs into a turbine. The turbine is coupled to a generator which produces electricity. Replacing the thermo-oil as the heat transfer fluid by steam (direct steam generation) is an important target. By that operating costs can be reduced. Moreover an efficiency of steam generation is increased by eliminating the step of heat transfer from the thermo-oil to the water.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a solar heat receiver tube for direct steam generation.

It is another object of the invention to provide a parabolic trough collector with the solar 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 solar heat receiver tube for direct steam generation is provided, comprising at least one outer absorber tube with an internal absorber tube space and at least one inner water tube with an internal water tube space for carrying water. The inner water tube is arranged in the internal absorber tube space. The outer absorber tube and the inner water tube are formed and arranged such that solar energy can be

absorbed by the outer absorber tube and absorbed solar energy can be transferred from the outer absorber tube to the inner water tube for the steam generation within the internal water tube space. Inside the water tube liquid water can be

transformed into vaporous water. Additionally a parabolic trough collector is provided, comprising at least one parabolic mirror having a sunlight reflecting surface for concentrating sunlight in a focal line of the parabolic mirror and at least one solar heat receiver tube which is arranged in the focal line of the parabolic mirror .

Finally a use of the parabolic trough collector in a power plant for converting solar energy into electrical energy is disclosed.

Preferably the outer absorber tube is coated by a selective solar coating. By the outer absorber tube solar energy can be absorbed and can be coupled into the inner water tube for the steam generation. In order to maximize an efficiency, with which the energy of the sunlight can be coupled into the water tube, a solar energy absorptive coating (selective solar coating) can be attached on a surface of the outer absorber tube. Such an absorptive coating can provide a high solar absorbance (low solar reflectivity) for wavelengths of solar spectrum (absorption radiation) and a low emissivity (high reflectivity) for infrared radiation. Thereby it is possible, that the solar heat receiver tube comprises different partial surfaces. The solar heat receiver tube comprises a first partial heat receiver tube surface and a second partial heat receiver tube surface. The first partial heat receiver tube surface is formed by a selective solar (absorptive) coating. The second partial heat receiver tube surface is formed by an emission radiation inhibiting coating for inhibiting an emissivity for infrared radiation. For instance such an emission inhibiting coating is a Copper coating. A solar heat receiver tube with such coatings is arranged in the focal line of a parabolic mirror such that the first partial surface with the selective absorptive coating is opposite to the sunlight reflecting surface of the parabolic mirror. The second partial surface with the

emission inhibiting coating is averted to the sunlight reflecting surface of the parabolic mirror.

The inner water tube can be symmetrically located inside the outer absorber tube. The outer absorber tube and the inner water tube are concentrically arranged to each other

(concerning lateral alignments of the tubes) . But preferably the outer absorber tube and the inner water tube are not arranged concentrically to each other. The inner water tube is arranged asymmetrically concerning a space diameter of the outer absorber tube. This is advantageous with respect to asymmetrically distributed sunlight radiation and flux distribution around the outer absorber tube.

In a preferred embodiment the inner water tube is fixed in the internal absorber tube space by at least one fixing element. The fixing element is arranged in the internal space of the outer absorber tube. Preferably the fixing element is a fixing tube with an inner fixing tube space. Both the inner water tube and the fixing tube are arranged inside the outer absorber tube. Thereby the inner water tube and the fixing tube are preferably formed and attached to each other that the water tube is asymmetrically fixed inside the outer absorber tube.

In a preferred embodiment the internal absorber tube space and/or the internal fixing tube space comprise at least one filler material which is selected from the group consisting of a heat transfer material and a heat storage material (heat storage medium) . The filler material comprises high thermal conductivity and/or high thermal capacity. This has the advantage, that thermal energy can be transferred efficiently from the outer absorber tube to the inner water tube.

Additionally, with the heat storage material an inherent thermal inertia is given. By this an interruption in flow, caused for instance by a decrease of solar intensity for a short time, can be eliminated.

The heat transfer material comprises preferably graphite. Other suitable inorganic materials like Alumina (Aluminium Oxide, AI ₂ O ₃ ) are possible, too. The heat storage material is for instance a Nitrate salt of a mixture of different Nitrate salts. Additionally, mixtures of different kinds of heat transfer materials and/or mixtures of different heat storage materials can be used. In order to improve the physical and chemical stability and the thermal characteristics of the solar heat receiver tube and in order to maximize the efficiency of the heat transfer some other measures are additionally implemented. Such measures concern for instance the used materials, e.g.

materials with high thermal conductivity for the tube wall of the inner water tube.

In a preferred embodiment the solar heat receiver tube comprises at least one jacket tube for enveloping at least partially the outer absorber tube. The jacket tube comprises at least one transparent jacket tube wall, which is at least partially transparent for absorption radiation of the

sunlight. The absorption radiation is that sunlight which is absorbed by the outer absorber tube. At least partially transparent is given in the case that a transmission for the absorption radiations is more than 80% and preferably more than 90%. The jacket tube is preferably a glass tube and the jacket tube wall is a glass tube wall. Between the outer absorber tube and the jacket tube wall there is a gap. This gap is evacuated. This means that a gas pressure in the gap is less than 10 ^~2 mbar and preferably less than 10 ^~3 mbar. This has the advantage that a thermal heat transfer away from the absorber tube by convection is reduced. The thermal energy doesn't dissipate and is substantially completely available for the direct steam generation in the water tube.

Alternatively such a result can be reached by filling the gap with a gas which reduces the thermal conductivity in

comparison to air.

In a preferred embodiment at least one gas elimination device for eliminating gas form a vapor phase of the outer absorber space is provided. For instance the device is a pump, located at the ends of the outer absorber tube. Alternatively the device comprises a membrane. For instance the membrane can be used as one way valve allowing a draining of the gas.

Especially the gas elimination device is formed in order to eliminate hydrogen (¾) . One problem by the using of solar heat receiver tubes is the hydrogen permeation from the absorber tube into the evacuated jacket annulus between the absorber tube and the jacket tube wall. In comparison to the conventional thermo-oil technique the problem of the hydrogen permeation is increased by the direct steam generation.

Therefore it is advantageous to eliminate Hydrogen from the absorber tube space. For that a pump can be used (as

mentioned above) . Alternatively the gas elimination device can be a getter for Hydrogen. The getter is located within the absorber tube space. By the elimination of Hydrogen out of the vapor phase of the absorber tube space the permeation of Hydrogen into the if so evacuated gap can be constrained. Hydrogen accumulation in the vacuum doesn't occur. This decreases the damage of having a high thermal loss due to the hydrogen permeation. Following advantages are connected to the invention:

- By using two different tubes (outer absorber tube and inner water tube) a separation between two different and

contradictory requirements is carried out. Each of the tubes can be optimized (materials, size, etc.) based on their specific requirements (inner water tube: hydraulic

requirements; outer absorber tube: optical purposes).

- By the individual optimization of each of the tubes

relatively thin tubes with low tube weights are possible. For instance: By using relatively small diameters for the inner water tube a wall thickness can be reduced. Small wall thicknesses lead to a low weight of the water tube and reduce the costs for the water tube.

- Small diameters of the inner water tube simplify the connections and the piping between adjacent solar collector modules, decrease the piping heat loss and reduce the piping connector costs.

- A bowing effect which is one of the major disadvantages of direct steam generation caused by an unfavorable temperature distribution around the outer absorber tube can be mitigated. - Hydrogen accumulation in the evacuated jacket annulus can be avoided.

- A stable and high efficiency in a wide range of radiation levels can be reached by using optimized water tube and absorber tube diameters. This is advantageous due the impact of different solar radiation through different seasons and different hours of the day. - The whole system comprises an inherent thermal inertia.

BRIEF 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 solar heat receiver tube. Figure 2 shows a cross section of parabolic through collector with the solar heat receiver tube.

DETAILED DESCRIPTION OF THE INVENTION The solar heat receiver tube (DSG receiver) 1 comprises an inner water tube 11 for steam generation and an outer

absorber tube 10. Within the internal water tube space 111 the steam generation can occur. The inner water tube 11 is located in the internal space 101 of the outer absorber tube 11. The outer absorber tube is coated by a selective solar coating (not shown) .

The outer absorber tube 10 and the inner water tube 11 are formed and arranged such that solar energy (solar radiation) can be absorbed by the outer absorber tube 10 and absorbed solar (thermal) energy can be transferred from the outer absorber tube 10 to the inner water tube 11 for the steam generation within the internal water tube space 101. Within the internal absorber tube space 101 there are two fixing tubes 12 with internal fixing tube spaces 121 for asymmetrically fixing the inner water tube 11 in the outer absorber tube space 101.

The internal absorber tube space 101 and the internal fixing tube spaces 121 are filled with graphite and/or other heat transfer materials 13. In an alternative embodiment at least one of the spaces (the internal absorber tube space 101 or the internal fixing tube spaces 121) are filled with a heat storage material like a mixture of different Nitrate salts. In a further embodiment the internal absorber tube space 101 and the fixing tube spaces 121 are filled with both, a heat transfer material and a heat storage material.

The solar heat receiver tube 1 is enveloped by a glass tube (jacket tube) 14 with a glass tube wall 141. The glass tube wall 141 is transparent for the absorption radiations with a transmission of more than 90%. Between the glass tube wall 141 and the absorber tube 10 there is an evacuated jacket annulus 142. A gas pressure of the jacket annulus is about 10 ^~3 mbar.

The heat solar receiver tube 1 is part of a parabolic trough collector 100. The parabolic trough collector 100 comprises at least one parabolic mirror 2 with a sunlight reflective surface 21. By the reflective surface 21 sunlight is

concentrated in the focal line 22 of the parabolic mirror 2.