WO2010080059A1

WO2010080059A1 - Silica aerogel body as isolation in a solar collector system

Info

Publication number: WO2010080059A1
Application number: PCT/SE2010/050003
Authority: WO
Inventors: Leif Gullberg; Christer Westlund
Original assignee: Ab Airglass
Priority date: 2009-01-08
Filing date: 2010-01-04
Publication date: 2010-07-15
Also published as: SE0950004A1; SE533058C2

Abstract

The present invention describes a body for use as insulation in a solar thermal collector system, wherein the body is composed of a silica aerogel material having a density in the range of 100-250 kg/m³ and a heat conductivity λ of less than 0.020 W / (m * K) when measured at a density of 150 kg/m³ and temperature of about 200C. Furthermore is described among others a solar receiver tube 7 for high temperature solar thermal collector systems, comprising an absorber tube 2 for enclosure of a heating medium, two halfpipe-shaped bodies 1 of silica aerogel material according to the invention, which halfpipe-shaped bodies 1 bear against each other to form a symmetrical or non-symmetrical cylinder- shaped insulation which bears against the absorber tube 2 and two protective casings 4 of a transparent glass material, each enclosing a halfpipe-shaped body 1 of silica aerogel material, wherein the protective casings 4 are connected to each other.

Description

SILICA AEROGEL BODY AS I SOLATION IN A SOLAR

COLLECTOR SYSTEM

Field of the invention The present invention relates to a silica aerogel body as the insulation unit of a solar thermal collector system, to such an insulation unit comprising a protective casing and to a solar receiver tube for a high temperature solar thermal collector system. Technical Background It has long been known how to produce silica aerogel in powder form. It is also known how to produce silica aerogel in blocks. For example, it is described in SE 422045 a way to produce silica aerogel in the form of a substantially crack-free, preferably transparent block. The method is carried out by hydrolysis of tetraalkoxysilane, preferably tetrametoxysilane, in an alcohol, preferably methanol, in the presence of a catalyst preferably ammonia, to the formation of an alcogel, the aging for about 10 days and washed with alcohol to remove water. After that the alcogel subsequently is treated in an autoclave by temperature increase to over alcohol critical point, isotherm pressure drop through the discharge of alcohol vapour, and temperature drop. There are several drawbacks to the manner or the method described in

SE 422045. The method is energy-intensive as it is implemented at comparatively high temperatures, such as e.g. final heat treatment at between 500⁰C and 75O⁰C. Furthermore, the process of implementation of the method is not quite easy to keep secure because the removal of the solvent from alcogel and finally out of the autoclave is carried out at relatively high pressures and temperatures, such as for the preferred solvent methanol at about 90 bar and at a final temperature of 275⁰C. Finally, the material produced according to SE 422045 is not optimal for various uses. An example of this is that the heat conductivity specified in SE 422 045 is 0.021 W / (m ^* K) for the obtained aerogel material after evacuation.

In WO2007011988 A2 aerogel composites including organic-inorganic hybride aerogel particles and binders are described. Hybride aerogel mate- rials are materials that include aerogels and other materials. The material described in WO2007011988 A2 is not meant to serve as insulation material in applications where light have to be able to penetrate, which is evident when material include for example polymers, monomers or oligo-isomers. This means that the material is not high-transparent and also not particularly heat resistant or fire resistant. Furthermore, none of the material according to WO2007011988 A2 is super-insulating, and may not be made super-insulating. All of these built in features means that the material according to WO2007011988 A2 is not appropriate as super insulating material in applica- tions where the light penetration is of importance.

WO2007146945 A2 describes flexible aerogel foam composites which includes at least one open cell foam component and at least one aerogel matrix. The material according to WO2007146945 A2 includes at least one component that is not an aerogel type. In this way the material can be made flexible. To make an aerogel material flexibility is virtually impossible without the involvement of other types of material, as shown in WO2007146945 A2. This also involves completely different effects. Even in this case, as in the case of WO2007011988 A2, the material can never be high-transparent, heat-resistant or made super-insulating. It is also known to use silica aerogel as a thermal insulating material in a solar collector. In EP 1 707 897 A1 a solar heat collector panel unit is described in which a heat collector panel and tubes for heat medium is associated with a transparent insulating material. The transparent insulating material may for example be a silica aerogel material obtained by supercritical drying of a wetgel obtained from alkoxysilanes by hydrolysis and polyconden- sation. When producing, an adhesions procedure for integrating a silica aerogel with the heat collecting plate and tubes, which are coated with a selective absorption film, is implemented. At first is produced a solgel for further manufacturing of a wetgel. Ammonium hydroxide is added as a catalyst to the alkoxysilane, which e.g. can be methylsilicate, which is diluted with methanol to produce the solgel. This is then subject to a casting procedure to produce a wetgel. After aging for about one day the wetgel becomes associated with the heat collecting plate and the pipes. Then the gel is released from the mould in which it has grown from solgel to wetgel. Supercritical drying of the wetgel is thereafter implemented, preferably in an alcohol. The form of silica aerogel produced according to EP 1 707 897 A1 , is in rectangular blocks in which the collector and its collector wing and pipes related thereto are contained in the block. The design of the solar collector system is that which is typical for low temperature solar collector systems and typically used for smaller systems, for example as for individual households. In these low temperature solar collector systems the operating temperature normally is below 100⁰C, as for example at 25-75⁰C. One aim of the present invention is to provide an improved silica aerogel material for use as an insulation unit of a solar thermal collector system, especially for high temperature solar thermal collector systems where heating of the media is between 15O⁰C and 500⁰C. Another aim of the present invention is to provide an insulation unit for high temperature solar thermal collector systems comprising the improved silica aerogel material according to the invention. Summary of invention

The first stated purpose above is achieved through a body for use as insulation in a solar thermal collecting system, wherein the body is composed of a silica aerogel material which:

- has a density in the range of 100-250 kg/m³; and

- has a heat conductivity λ of less than 0.020 W / (m * K) when measured at a density of 150 kg/m³ and a temperature of around 2O⁰C, and

- has a sunlight transmittance which is at least 90% at a material thickness of 15 mm.

The silica aerogel material according to the present invention has a very low heat conductivity. At a density of 150 kg/m³ and a temperature of about 2O⁰C and in non-evacuated state, i.e. a state where there is still air left in the pores of the material, the heat conductivity (coefficient of thermal con- ductivity) is below 0.020 W / (m * K), compared with 0.021 W / (m * K) forthe aerogel material described in SE 422 045, the latter in itself measured at a higher density of 240 kg/m³ but yet in evacuated state (free from air in the pores), which in itself implies a much lower heat conductivity than in the non- evacuated state. By comparison, the heat conductivity for vacuum is 0.022 W / (m * K) at room temperature. Brief description of drawing Fig 1 shows a cross section of a solar receiver tube according to the present invention with two symmetrical halfpipe-shaped bodies of silica aerogel material insulating an absorber pipe and each enclosed by a protective transparent glass casing.

Figure 2 shows a cross section of a solar receiver tube according to the present invention with a symmetric halfpipe-shaped body of silica aerogel material as insulation at the bottom, i.e. on the shadow side, while the part where the rays of the sun is to penetrate only is covered with transparent protective outer glass casing.

Figure 3 shows a cross section of a solar receiver tube according to the present invention, with a symmetric halfpipe-shaped body of silica aerogel as insulation on the shadow side and a non-symmetric halfpipe-shaped body of silica aerogel material as insulation on the sunny side, which bodies each are enclosed by a protective transparent glass casing. Specific embodiments of the invention The material according to the present invention may have different properties depending on the solar collector application which the material is intended for. When the material is supposed to have a location where sunlight penetrates, which is most common, the material in accordance with the present invention must have a high transmittance of sunlight. According to a spe- cific embodiment of the present invention, as mentioned above, the body because of this has a sunlight transmittance of at least 90% at a material thickness of 15 mm, such as for example 90-95%, for example at least 92%. Transmittance is, meant in this case, permeability of the entire solar spectrum, i.e. all the light wave lengths the sunlight is composed of. The trans- mittance is measured by dividing the intensity of outgoing light through the intensity of incoming light. A value of the transmittance is to be associated with a certain thickness, since transmittance depends on this. If the material is merely to act as insulation it does not need to have this high transmittance, but may for example be translucent or even opaque by adding other components, such as fiber materials to increase the mechanical strength. Carbon fiber material is for example possible to ad. Possible materials to be mixed in the material according to the present invention, described above, should not be associated with such mixing made in the materials of WO2007011988 A2 or WO2007146945 A2. The adding of polymers, such as according to WO2007011988 A2, means that the material loses heat resistance and the possibility of high transparency or translucency. In addition, such materials can never be super-insulating. In the same way for the materials according to WO2007146945 A2, in which components are added to make the material flexible, this implies that the material cannot be transparent or translucent and that it likewise cannot be made super-insulating. An intrinsic possible property of silica aerogel material according to the present invention is, however, the high transmittance of the material. Since silica aerogel material according to the present invention should be used as insulation in solar thermal collector systems, this is an important property if the material should be placed on the so called sunny side where the sunlight will penetrate through the insulating material. Due to the material's potential high transmittance and low heat conductivity, i.e. high insulation value, the material according to the present invention therefore is suitable as insulation of collectors of various solar thermal collector systems.

There are also other procedural possible measures to implement to enhance certain characteristics that may be of interest for the silica aerogel material according to the present invention. As mentioned above, the evacuation of the material, which means that essentially all the air left in the pores of the material are driven out, is a possible method wherein heat conductivity of the present material can be further improved. According to a specific embodi- ment of the present invention, the body is evacuated and is super-insulating and has a heat conductivity λ which is 0.007 W / (m * K) or less when measured at a density of 150 kg/m³ and temperature of about 20 ° C. This is a value for the heat conductivity that is much lower than for both the vacuum and the material described in SE 422 045. Since the heat conductivity among other things varies with density it might be of interest to regulate the resulting density of the material for some applications. As stated, the density of the silica aerogel material according to the present invention can be between 100 and 250 kg/m3, such as between 120 and 230 kg/m³, but in a number of applications the desired density is between 100 and 200 kg/m³, such as in the range of 125-175 kg/m³, for example at about 150 kg/m³.

The materials according to WO2007011988 A2 or WO2007146945 A2 are not possible to evacuate and make super-insulating. To evacuate mate- rial, such as the present invention, means that the material is enclosed between for example a pair of plates, such as glass plates, and that the air in the material is sucked out. Since the material according to WO2007011988 A2 not is solid but more as a kind of felt carpet it is not possible to carry out an evacuation of the material in this way. Would such an evacuation be carried out, this would flatten the material enormously. The same applies to the material according to WO2007146945 A2. The material according to the present invention is however just a solid and therefore withstand very high widely spread pressures.

The body according to the present invention can also be enclosed in a surrounding transparent material. This is something that can lead to that silica aerogel bodies according to the present invention is easier to handle and transport. According to a specific embodiment of the present invention, a transparent material therefore is surrounding the body, such as when the material is evacuated as described above. According to another embodiment of the invention, the transparent enclosing material is selected from the group consisting of transparent glass materials. If this enclosure is surrounding the body completely, also an evacuated material can be enclosed so that it can be maintained evacuated.

With body according to the present invention is meant a geometric body. What type of geometric body that is wanted in the present invention will naturally depend on the type of solar thermal collector it is supposed to be an isolating unit in. In a specific embodiment of the present invention, the body is a panel-shaped body. A panel according to the present invention is referred to further geometrical shapes, where length and width is greater than the thickness. According to the present invention completely different thicknesses, lengths and widths are possible. The shapes of bodies described according to the present invention is different from the blocks of silica aerogel described in EP 1 707 897. The blocks according to EP 1 707 897 are meant for another type of use than the silica aerogel bodies according to the present invention. Since the blocks under EP 1 707 897 shall surround a collector unit with its collector-wings and tubes to this collector unit, which is typical of devices in a low temperature solar thermal collector system, such as for household use. The panels according to the present invention is however not intended for such solar thermal collector systems, but find use as insulation in larger units in which the media also typically are heated to above 15O⁰C.

The difference between a low temperature solar thermal collector system and a high-temperature solar thermal collector system is that in the latter it is performed a concentrating of the sunlight and its energy. This is performed by means of mirrors, such as e.g. parabola mirrors and/or secondary mirrors, which reflects the sunlight so that it hits a proposed collector area. In contrast, a low temperature solar thermal collector system allows the sunlight to hit directly on the collector and its collector-wings. This implies that in such a system the energy supply of sunlight is relatively not as high, reckoned per area unit, and partly because of that the warming of the used media are at a comparatively lower temperature.

A panel according to the present invention thus finds use in certain types of solar thermal collector systems. These solar thermal collector sys- terns can be very different in appearance, but common to them is that they require isolation of the space in which the collector unit are to receive solar energy. This isolation is today often only of e.g. glass which houses an air space where the collector unit is placed. This means that the heat conductivity is comparatively high because the air itself is around 0.026 W / (m ^* K) at room temperature, which in turn leads to energy losses. By placing a silica aerogel panel according to the present invention inside the glass energy losses can be reduced when the conductivity for the silica aerogel material is at least below 0.020 W / (m * K) at about 20 ° C. In the case where an eva- cuated silica aerogel panel according to the invention is used, which is then enclosed in a transparent material, such as covering and underlying glass panes, as mentioned, a heat conductivity below 0.007 W / (m ^* K) is achieved for the material at about 2O⁰C, which is a significant improvement in compa- rison with only having glass as insulating unit for the collector. It is important to realize that the heat conductivity also depends on the temperature and increases with this. This means that the heat conductivity which is concerned with the use of bodies according to the present invention in solar thermal collector systems, i.e. at temperatures above 15O⁰C and sometimes above 400⁰C, is significantly higher than the heat conductivity when measuring the material at 2O⁰C.

According to another specific embodiment, the body according to the present invention is a halfpipe-shaped body. This variant finds use as insulation of absorber tubes in high temperature solar thermal collector systems. These solar thermal collector systems are as mentioned designed by some form of mirrors reflecting or directing sunlight or sunrays, in this case against an absorber tube which captures the sun heat, thus warming up the medium which flows inside the absorber tube.

Today vacuum tubes of glass are used for the insulation of absorber tubes. The vacuum tubes let through the sun rays, but not with 100% trans- mittance, but rather typically at approximately 96% transmittance, and are tasked with preventing energy losses from the absorber pipes. However, there are problems with using these vacuum tubes to insulate an absorber tube as above. Firstly there is a high risk that there are leaks at the connec- tions between different vacuum tube modules or by cracks in the vacuum tubes. This means that the tubes lose their vacuum and the energy loss is much greater. Secondly, even if the vacuum itself is well insulating one would nevertheless still wish that heat conductivity was even lower. The energy losses in the solar thermal collector systems are, as said, something that one wants to keep as low as possible. The above implies vacuum insulation can be costly in the long run when the efficiency of the absorber tubes decreases in these solar thermal collector systems. The present invention provides an alternative to the vacuum tubes to remedy the above problems. By using halfpipe-shaped silica aerogel bodies according to the present invention as isolation of absorber tubes in high temperature solar thermal collector systems, the heat conductivity can be kept at a very low level. In addition, there is no similar risk of energy losses as for vacuum tubes when leaking. The halfpipe-shaped silica aerogel bodies according to the present invention is suitable for use as isolation of absorber tubes in high temperature solar thermal collector systems in which the media is heated to above 15O⁰C, often up to and above 400°C-500°C.

As is apparent from the description above, the material according to the present invention is heat resisting and can stand at least 400⁰C, for example at least 500⁰C. The material according to the present invention has according to a specific embodiment for example a heat resistance of 600⁰C. Also this property is something that distinguishes the material according to the present invention substantially from the materials according to WO2007011988 A2 and WO2007146945 A2.

It may be important to optimize the thickness of the bodies according to the present invention. This is because a thicker body insulates better, but lets on the other side less light to pass or lowers the transmitted light intensity in comparison to a thinner body. This means that light transmittance decreases. This also means that both the insulation ability and the transmittance have to be looked over simultaneously when an optimized system according to the present invention is designed for a certain application. For the halfpipe- shaped variant can, for example a non-symmetrical variation be possible according to the present invention, where the body is thinner in some places where a lot of sunlight is to penetrate, i.e. at the sun side, while thicker where good insulation is the only important characteristic, namely the shadow side. In addition, an opaque material according to the present invention can be used on the shadow side in combination with glass on the sun side, as isolation of an absorber tube in a high temperature solar thermal collector system.

The present invention also describes the use of silica aerogel bodies as described above for the isolation of a collector unit in a solar thermal collector system. According to one embodiment of the invention there is intended the use of a halfpipe-shaped body of silica aerogel material, for the isolation of a collector unit, such as an absorber tube, in a solar thermal collector system. According to a specific embodiment is referred to the use of a halfpipe-shaped body of silica aerogel material, as insulating by direct contact and enclosure of an absorber tube in a high temperature solar collector system. The absorber tube can be enclosed by silica aerogel bodies according to the present invention in different ways. This can for example be achieved by two identical halfpipe-shaped bodies lying in contact with each other, to form an outer tube enclosing the absorber tube. Furthermore, as mentioned one of these halfpipe-shaped bodies can be non-symmethcally shaped so that it is thinner on the sunny side, where sunlight is to penetrate. Such a halfpipe-shaped body with non-symmetrical cross section should also be considered as a halfpipe-shaped body according to the present invention. Furthermore, the sunny side can be completely free of insulation according to the present invention so that only the remaining part of the absorber tube, i.e. the part not struck by sunlight, is insulated by the material of the present invention. In the latter case the material does not need to be transparent and in this case, insulating and mechanical strength should be optimized so that the heat conductivity is as low as possible and the material becomes sustainable. In this case, for example, fibers can be added to the material according to the present invention. In an application as described above transparent glass is placed on the sunny side.

In addition, the present invention also refers to use of a panel-shaped body of silica aerogel material, for insulating a space surrounding a collector unit of a solar thermal system. The panel according to the present invention is in this case placed as insulation in a position where sunlight will pass through and reach the receiver or sun light collector unit. The panel according to the present invention can be evacuated and enclosed in a transparent material. Thereby the panel can be evacuated and enclosed by glass on all sides. By evacuation, a material with extremely good insulating ability is achieved.

The present invention also describes an insulation unit for a high temperature solar thermal collector system, where the insulation unit includes a halfpipe-shaped body of silica aerogel material according to the invention that at the outside is surrounded by a matching halfpipe-shaped protective casing of a transparent material. This unit is one half of the insulation to an absorber tube. The protective casing may for example be made of a transparent glass material, such as for example a borosilicate glass. The protective casing may also have a female or male along one long side and a female or male along the other long side, the female or male along the long side and the female or male along the other long side can be brought together and locked with a male or female respectively male or females with a further insulation unit with protective casing. What is written above implies that a protective casing for an insulation unit according to the present invention may have a female along one long side and a male along the other long side, as shown in the figures, but may also have a female along the long side and also a similar female along the other long side. Of course, it is also entirely possible that a protective casing according to the present invention has a male along the long side and a male along the other long side. Other possible equivalent designations of the protective casing according to the present invention could for example be "suspension casing", "attachment casing" or "supporting casing". The supporting casings according to the present invention, for example in the form of glass halves, is not only a protection, but keeps the insulating halfpipes in place by holding the glass halves together by suitable clips, seals, etc.

When the protective casings have been brought together and finally have been locked together a cylinder-shaped insulation to an absorber tube is obtained where the silica aerogel material according to the invention, is in contact with the absorber tube as insulation and is enclosed by the transpa- rent protective casing. The present invention therefore also is referred to the use of two insulation units as above, for the enclosure of an absorber tube in a high temperature solar thermal collector system, wherein two halfpipe- shaped bodies of silica aerogel material bear against each other to form a symmetric or non-symmetric cylinder-shaped body of silica aerogel material which encloses and insulates the absorber tube, and wherein the female or the male along the long side and the female or male along the other long side of a protective casing are connected with the male or female along one long side respectively the male or the female along the other long side of another protective casing.

Female or male in one protective casing, and male or female on the other protective casing, can also be security connected to each other using a clamp, such as a metal clip, on both sides of the insulation unit.

As shown above for the device as insulation of an absorber tube these silica aerogel materials usually are not evacuated because they will be in contact with air prior to mounting on a absorber tube. This means consequently that these halves will come as modules and be placed in position around absorber tubes when assembling. It would, according to the invention, however, be possible to produce modules that include evacuated silica aerogel material. These halves are then fully enclosed by glass. The advantage is obviously that it would be possible to obtain very low heat conductivity rates for the silica aerogel material, even as low as down to below 0.007 W / (m * K), at a density of 150 kg/m³ and temperature of about 2O⁰C. On the other hand, it requires a lot of glazing material and handling of this material as well as welding thereof.

The present invention also describes a solar receiver tube for high temperature solar thermal collector systems, comprising: - one absorber tube for enclosure of a heating medium;

- two halfpipe-shaped bodies of silica aerogel material according to the invention, which halfpipe-shaped bodies is in contact with each other to form a symmetrical or non-symmetrical cylinder-shaped insulation that is in contact with the absorber tube; and - two protective casings of a transparent glass material, each surrounding a halfpipe-shaped body of silica aerogel material, wherein the protective casings are connected to each other.

Even in this case protective casings can of course include a female or male along one long side and a female or male along the other long side, wherein a female or male respectively a female or male of a protective casing is attached to a male or female respectively a male or female of the second protective casing and with each connection of male and female furthermore security connected using a clamp, such as a metal clamp. Furthermore, examples of said heating medium are steam, oil or overheated water. Example of preparation

The production of a silica aerogel material according to the present invention can be performed in many different ways. Below is an example of such a production and this should be seen only as an example and not limiting the scope of the present invention. The scope is defined by the claims.

The starting point for this particular example is that a mixture is performed by at least one silane compound and a solvent in the form of an alcohol such as methanol, ethanol or propanol, and that a catalyst is added, for example titanium lactate or ammonia. This is performed under temperature control to form a so-called solgel which by polymerization grows into a wetgel. The wetgel then is further processed through a casting procedure, and extraction in an autoclave where the alcohol is driven out by using supercritical or liquid carbon dioxide. In this way a silica aerogel material is formed which by further heating at for example above 200⁰C can be even more free of alcohol admixture.

In order to provide further explanation of the examples provided above, an explanation of different definitions and terms are given below. A solgel is, by definition, a suspension consisting of a solvent and a sol which then may polymerize and aggregate to form a gel. Polymerization above implies joining to longer chains by chemical reaction and not by any polymer being added to the material. The solvent, in this case, for example, methanol, ethanol or propanol, can then be boiled away and the result is a porous material.

Examples of silane compound used may be tetramethylortosilicate (TMOS), also known as tetramethoxysilane, or tetraethylortosilicate (TEOS), the latter also known as tetraethoxysilane, or mixtures thereof.

There are also other possible solvents than alcohols that generally can be used to produce a silica aerogel material. This is for example ethylaceto- acetate. This is however not something that is desirable because the heating process then must be effected at above 400⁰C to get the silica aerogel material clear again, after at a couple of hundred degrees have been brownish due to the used solvent. In addition, this known process involves that HF is used as a catalyst in the formation of the solgel, which is also not desirable.

In the case of catalysts which can be used according to the present invention, the procedure may be run according to basic as well as acid method. In the acid case are possible alternatives HF, HCI and H₂SO₄, but where HF is the least desirable. However, it is preferable to make use of basic catalysts, such as e.g. ammonia or titaniumlactate. An example of a commercially available titaniumlactate catalyst which can be used in the present procedures, are furthermore TYZOR ®. Among other things, this also differs in comparison with EP 1 707 897 A1 in which ammonium hydroxide is added as a catalyst to the alkoxysilane.

For the manufacture of halfpipe-shaped bodies according to the present invention the mould must have a form adapted to this shape in order to obtain this shape. It will of course be the same case if panels are to be produ- ced. During the polymerization the material shrinks slightly and this is something that should be taken into account in terms of how production is carried out, such as the choice of type of mould and size of mould to obtain silica aerogel material halfpipes that fit around a specific absorber tube. It is also entirely possible that the moulding is made directly into the glass tubes, which then will form the protective casing in insulation units. The shrinkage is again something you have to take into consideration and some adjustment may be needed to the readily polymerized silica aerogel material to get the shape to fit perfectly into the insulation units. This can for example be done by adding bits of silica aerogel material after polymerization so that the design will be complete.

An example of a material for a mould, in this case, is for example glass, but there are other materials that are equally possible.

It could also for example be possible to carry out production by introducing a demoulding step where the wetgel is removed from the moulds before the extraction.

Detailed description of the drawing

Figure 1 shows a solar receiver tube 7 according to the present invention, seen in cross section. The absorber tube 2 is insulated by each one halfpipe-shaped body 1 of silica aerogel material according to the present invention, the bodies 1 located flush against each other, thus forming a symmetrical cylinder-shaped insulation around the absorber tube 2. Respective halfpipe-shaped body 1 is enclosed by a protective casing 4 which is of a transparent glass material. The body 1 and its protective casing 4 together form an insulation unit 3 to the absorber tube 2. The outer protective casings 4, having each one female 5 and male 6, which can be connected, brought together and/or locked with each a male 6 and female 5 of another protective casing. The locking can for example be made by means of a clamp, such as for example a metal clamp, which is not shown in Figure 1.

As shown according to the figures each protective casing 4 has a female 5 along one long side and a male 6 along the other long side. The casings 4 need not to be designed in this way according to the present invention. As mentioned above, a protective casing 4 very well can have a female 5 along the long side and also a female 5 along the other long side. Such a protective casing 4 would then be connected, brought together and/or locked with a protective casing 4 which has a male 6 along its one long side and a male 6 along the other long side. According to an example of a solar receiver tube 7 according to the present invention the upper protective casing 4 of the two casings has a female 5 along the long side and a female 5 along the other long side. This upper protective casing 4 can then be connected, brought together and/or locked with a lower protective casing 4 which has a male 6 along its one long side and another male 6 along its other long side. Such a design according to the present invention could be a useful option when the external cleaning of the solar receiver tube 7 is essential at regular intervals, reflecting that when water is flushed from the top, it runs along the females 5 without any great risk of water entering between the female 5 and male 6 where these are connected to each other.

Figure 2 also shows a solar receiver tube 7 according to the present invention, wherein only the bottom, i.e. the shadow side, has a similar halfpipe-shaped body 1 as in Figure 1. On the sunny side pieces of material are only inserted at the sides below the top protective casing 4 while the area where the sunlight is to penetrate are completely free of insulating silica aerogel material. This is one possible embodiment according to the present invention in accordance with certain applications to maximize the trans- mittance of sunlight and still maintain high insulating capacity of the system. The halfpipe-shaped body 1 on the shadow side does not need to be transpa- rent provided that sunlight will not penetrate in this place. This means that optimization of the material in this case can be made only on insulating ability as high transmittance is not important for this part. This also applies to the lower bodies 1 in Figure 1 and Figure 3 assuming that even in these cases is not intended to that sunlight must pass through these lower bodies 1. If sun- light on the contrary is to pass through these lower bodies 1 the material consequently according to the present invention, of course, ought to have a high transmittance. In Figure 2 the external protective casings 4 are connected to each other as in Figure 1.

In Figure 3 a solar receiver tube 7 is shown in the same way as in Figure 1 and Figure 2, but in this case the upper halfpipe-shaped body 1 is non-symmetrical and as such thinner where sunlight is to penetrate. This is a form according to an embodiment according to the present invention, which as well can cause very high sunlight transmittance and high insulating ability by a low heat conductivity. Conclusions

Using bodies of silica aerogel material according to the present invention in high temperature solar thermal collector systems makes it possible to obtain very good insulating ability by means of the energy losses being low. In addition, a high sunlight transmittance can be obtained. For example half- pipe-shaped bodies according to the present invention are useful for insulation of absorber tubes in high temperature solar thermal collector systems instead of today's vacuum tubes. Examples of how this can be done in accordance with the present invention are shown in Figure 1-3.

Claims

1. Body for use as insulation in a solar thermal collector system, wherein the body is composed of a silica aerogel material, which:

- has a density in the range of 100-250 kg/m³;

- has a heat conductivity λ of less than 0.020 W / (m ^* K) when measured at a density of 150 kg/m³ and a temperature of about 2O⁰C; and - has a sunlight transmittance which is at least 90% at a material thickness of 15 mm.

2. Body according to claim 1 , wherein the body is evacuated and is super- insulating and has a heat conductivity λ which is 0.007 W / (m ^* K) or less when measured at a density of 150 kg/m³ and a temperature of about 20⁰C.

3. Body according to claim 1 or 2, wherein a transparent enclosing material is surrounding the body.

4. Body according to claim 3, wherein the transparent enclosing material is selected from the group consisting of transparent glass materials.

5. Body according to anyone of the claims 1-4, wherein the body is a panel- shaped body.

6. Body according to anyone of the claims 1-4, wherein the body is a halfpipe- shaped body (1).

7. Use of a halfpipe-shaped body (1) of silica aerogel material according to claim 6, for insulating a collector unit in a solar thermal collector system.

8. Use according to claim 7, as insulation by direct contact and enclosure of an absorber tube (2) in a high temperature solar thermal collector system.

9. Use of a panel-shaped body of silica aerogel material according to claim 5, for insulating an area surrounding a collector unit in a solar thermal collector system.

10. Insulation unit (3) for a high temperature solar thermal collector system, comprising a halfpipe-shaped body (1) of silica aerogel material according to claim 6, which halfpipe-shaped body (1) on the outside is surrounded by a matching halfpipe-shaped protective casing (4) of a transparent material.

11. Insulation unit (3) according to claim 10, wherein the protective casing (4) is made of a transparent glass material.

12. Insulation unit (3) according to claim 10 or 11 , wherein the protective casing (4) has a female (5) or male (6) along one long side and a female (5) or male (6) along the other long side, the female (5) or male (6) along the one long side and female (5) or male (6) along the other long side, respectively, may be brought together and locked with a male (6) or female (5) respective male (6) or female (5) of an additional insulation unit (3) with protective casing (4).

13. Use of two insulation units (3) according to claim 12, to enclose an absorber tube (2) in a high temperature solar thermal collector system, wherein two halfpipe-shaped bodies (1) of silica aerogel material bear against each other to form a symmetrical or non-symmetrical cylinder-shaped body of silica aerogel material which enclose and insulate the absorber tube (2), and wherein the female (5) or the male (6) along the one long side and the female (5) or the male (6) along the other long side, respectively, of a protective casing (4) are connected with the male (6) or the female (5) along one long side respective the male (6) or the female (5) along the other long side of another protective casing (4).

14. Use according to claim 13, wherein female (5) or male (6) on the one protective casing (4) and male (6) or female (5) on the other protective casing (4) in addition are security connected to one another by means of a clip.

15. Solar receiver tube (7) for high temperature solar thermal collector systems, comprising:

- one absorber tube (2) for the enclosure of a heating medium;

- two halfpipe-shaped bodies (1) of silica aerogel material according to claim 6, which halfpipe-shaped bodies (1)bear against each other to form a symmetrical or non-symmetrical cylinder-shaped insulation which bear against the absorber tube (2); and

- two protective casings (4) of a transparent glass material, each surrounding one halfpipe-shaped body (1) of silica aerogel material, wherein the protective casings (4) are connected to each other.

16. Solar receiver tube (7) according to claim 15, wherein the protective casings (4) comprise a female (5) or male (6) along one long side and a female (5) or male (6) along the other long side, wherein a female (5) or male (6) respective a female (5) or male (6) of one protective casing (4) are connected to a male (6) or female (5) respective a male (6) or female (5) of the other protective casing (4), and wherein each connection of female (5) and male (6) in addition are security connected with a clip.