US20150361654A1

US20150361654A1 - Construction element having a controllable heat-transfer coefficient u

Info

Publication number: US20150361654A1
Application number: US14/762,518
Authority: US
Inventors: Nikolaus Nestle; Andreas Daiss; Klaus Hahn; Ralf Nörenberg; Johann Martin Szeifert; Elena Khazova; Achim Löffler; Tilmann Kuhn; Christoph Maurer; Thibault PFLUG; Jan Wienold; Andre GLÜCK
Original assignee: BASF SE; Fraunhofer Institut fuer Solare Energiesysteme ISE
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV; BASF SE; Fraunhofer Institut fuer Solare Energiesysteme ISE
Priority date: 2013-01-22
Filing date: 2014-01-17
Publication date: 2015-12-17
Also published as: KR20150109452A; CN105008630B; WO2014114563A1; EP2948600A1; CN105008630A

Abstract

A structural element with a controllable heat transfer coefficient comprises a frame. A first sheet and a second sheet opposite one another are arranged in the frame. The sheets and the frame have the effect of defining a closed-off volume which is filled with at least one gas. At least one two-dimensional element is arranged between the sheets. An upper intermediate space is formed between the two-dimensional element and the frame vertically upwardly and a lower intermediate space is formed between the two-dimensional element in the frame vertically downwardly. A first cavity is between the first sheet and the two-dimensional element. A second cavity is between the two-dimensional element and the second sheet. The cavities are connected via the upper intermediate space and the lower intermediate space such that a convection flows between the cavities via the intermediate spaces. At least one means arranged in the intermediate spaces controls the convection flow.

Description

The present invention relates to a structural element with a controllable heat transfer coefficient U and the use thereof as a wall and/or roof element in buildings or vehicles and to a method for controlling the heat transfer coefficient U in such a structural element.
In construction, the heat transfer coefficient U is a specific characteristic value of a compound unit or building material which in principle indicates the heat insulating properties thereof. The higher the heat transfer coefficient U is, the poorer the heat insulating property of the compound unit or building material.
The heat transfer coefficient U became particularly significant, if not before, when the amended energy-saving order [Energieeinsparverordnung (EnEV)] came into force in Germany in the year 2009, providing that the annual primary energy requirement and the specific transmission heat loss of a building to be erected must be kept within specific limit values. The heat transfer coefficient U is thereby included in the calculation of the transmission heat loss and this in turn is included in the calculation of the primary energy requirement. Furthermore, the energy-saving order prescribes limit values for the heat transfer coefficient U for specific compound units, when they are being replaced in existing buildings or are included in a newly built structure.
A large number of insulating elements that are used for the heat insulating of buildings are known from the prior art. They generally consist of one or more insulating layers of an insulating material (for example foams, expanded polymer materials). Depending on the nature of the insulating material, a protective layer is applied on the outer side of such insulating elements. These insulating elements serve in particular for preventing an outflow of heat from the interior of a building to the outside. At the same time, a heat flow into a building can likewise be reduced. According to the prior art, most insulating elements have fixed insulating properties, that is to say the insulating property can only be controlled by varying the thickness and/or number of insulating elements. However, it is not possible in this way to react flexibly to prevailing temperatures at a given time inside and outside a building.
The use of highly insulating materials has, however, in the meantime led to situations in which the natural outside temperature variation can no longer be used to dissipate the heat introduced into the building during the day by solar irradiation again at night. The energy requirement for active cooling devices is increased by the heat accumulation produced hereby.
There is therefore a need for an insulating element of which the insulating properties are variable. In the prior art, there are a whole host of initial approaches to satisfying this need.
For instance, DE 10 2006 024 067 A1 describes an insulating element which is suitable in particular for the inside and/or outside insulation of buildings. The insulating properties of the insulating element described there can be changed according to the desired inside temperature of the building or according to the outside temperature and/or solar irradiation, in particular by changing the heat transfer coefficient U and/or the reflection properties of the insulating element itself. As a technical solution, the insulating element is in this case provided with an insulating material which can be changed in its position, so that the insulating material used contributes completely, partially or scarcely at all to the insulation of the building. For this purpose, for example, the insulating material may be completely or partially compressed, in order to completely or partially release the heat flow through the insulating element. A major disadvantage of all embodiments of the prior art is that great amounts of material have to be moved or compressed, since the surface area of the element must be substantially filled with or freed from insulating material.
Furthermore, in U.S. Pat. No. 4,058,109 a device for insulating and/or solar heating is disclosed. This device is applied to the facade of an existing building and consists of a transparent panel which is set in front of a wall and thereby encloses a defined space with the wall. Within the defined space, a heat absorber of a closed-cell insulating material is arranged. This heat absorber has openings, so that, depending on the temperature conditions, a convection flow can form within the device described. This is intended on the one hand to achieve a heat insulation of the building by the presence of the insulating material, while on the other hand solar irradiation of the heat absorber is used for heating the volume of gas enclosed in the device and for giving off this heat to a certain extent to the existing wall of the building by way of the convection flow.
A further approach taken in the prior art is described in DE 196 47 567 A1. There, a switchable vacuum insulation, in particular for use for solar energy utilization, is realized, a coarsely porous or coarsely structured insulating material being enclosed in a gastight manner and evacuated. As and when required, this element may be flooded with hydrogen gas, whereby there is within the enclosure an electrically heatable getter material, which is suitable for the adsorption and deadsorption of hydrogen and is enclosed by a heat insulating material, the thermal conductivity of which does not depend, or only a little, on the gas pressure in this structural element.
Furthermore, US 2003/0061776 A1 discloses an insulating system with a variable heat transfer coefficient, which is based on an inflatable structure and thus reacts to a change in the ambient temperature by changing its volume. This allows the rate of heat transfer therethrough to be controlled.
AT 380 946 B1 discloses what is referred to as a heat exchange wall, which substantially consists of an insulating sheet which is surrounded by a system of tubes and in which a gaseous heat transfer medium can circulate, the circulation of which can be automatically shut off as a result of the special design of the system of tubes. An automatic shut-off is not necessarily advisable for an insulating element with switchable insulating behavior, since, depending on the weather situation, the same temperature differences may make strong insulation or else reduced insulation appear advisable. Furthermore, the insulating element described in AT 380 946 B1 is of a comparatively complicated construction, and accordingly can only be produced poorly.
In addition, there are a series of multi-shell wall, window and roof elements, as described for example in EP 0 317 425 A2, FR 2 478 800 A1, EP 2 366 845 B1 and DE 10 2006 037 741 A1. In these elements, a change of the heat flow is achieved by the flow of outside air through an intermediate space between the various shells either being allowed or prevented. The exchange of air through the element also partially takes place with the interior room. All of these approaches share the disadvantage that, when outside air flows through, dust from the air gets into the intermediate space and can lead to undesired contaminants there, which particularly in the case of translucent and transparent elements impair their optical function. When there is an additional exchange of air with the interior room, this hygiene-related problem is additionally exacerbated, since undesired germs or pests can also be entrained by the stream of air.
Finally, FR 2 798 991 A1 presents an element in the case of which the wall is divided into rhomboidal cells, in which it is possible by inclination of an insulating element fitted in them to allow a convection stream to flow around the element or prevent it. On account of the numerous segments and the non-cuboidal outer shape of the individual cells, this element is in turn comparatively complicated to produce.
Although the insulating elements described in the prior art with heat transfer coefficients U that can be controlled within certain limits have advantages over conventional insulating materials, they all entail significant disadvantages with respect to their usability in the building industry and are in some cases extremely complex to manufacture.
It is therefore an object of the invention to provide a novel structural element that minimizes the energy requirement of a building by contributing to controlling the heat balance thereof.
This object is achieved in the case of a structural element of the type mentioned at the beginning by it having a controllable heat transfer coefficient U and being designed as follows:
a structural element (1) with a controllable heat transfer coefficient U comprising

- a frame (7),
- two opposing sheets (3, 5), which are arranged at a distance A from one another in the frame (7), the sheets (3, 5) and the frame (7) having the effect of defining a closed-off volume V, which is filled with at least one gas,
- at least one two-dimensional element (9), the width of which corresponds to the vertical clear width W of the frame (7) and the height of which is less than the clear height H of the frame (7), the two-dimensional element (9) being arranged between the sheets (3, 5) such that it finishes laterally with the inner sides of the frame (7), and an intermediate space (11) being formed between the two-dimensional element (9) and the frame (7) in the vertically upward direction and an intermediate space (13) being formed between the two-dimensional element (9) and the frame (7) in the vertically downward direction,
- a first cavity (15), which is formed between the sheet (3) and the two-dimensional element (9) with a spacing X,
- a second cavity (17), which is formed between the two-dimensional element (9) and the sheet (5) with a spacing Y, the first cavity (15) and the second cavity (17) being in connection via the intermediate space (11) and the intermediate space (13) such that a convection flow can flow between the first cavity (15) and the second cavity (17) via the intermediate space (11) and the intermediate space (13),
- at least one means for controlling the convection flow, which is arranged at least for one of the intermediate spaces (11, 13).

In a first aspect of the present invention, the object of the invention is achieved by a structural element (1) with a controllable heat transfer coefficient U that comprises

- a frame (7),
- a first sheet (3) and a second sheet (5), which are opposite one another and which are arranged at a distance A from one another in the frame (7),
- the first sheet (3), the second sheet (5) and the frame (7) having the effect of defining a closed-off volume V, which is filled with at least one gas,
- at least one two-dimensional element (9), the width of which corresponds to the vertical clear width W of the frame (7) and the height of which is less than the clear height H of the frame (7),
- the two-dimensional element (9) being arranged between the first sheet (3) and the second sheet (5) such that it finishes laterally with the inner sides of the frame (7), and
- an upper intermediate space (11) being formed between the two-dimensional element (9) and the frame (7) in the vertically upward direction and a lower intermediate space (13) being formed between the two-dimensional element (9) and the frame (7) in the vertically downward direction,
- a first cavity (15), which is formed between the first sheet (3) and the two-dimensional element (9) with a spacing X,
- a second cavity (17), which is formed between the two-dimensional element (9) and the second sheet (5) with a spacing Y,
- the first cavity (15) and the second cavity (17) being in connection via the upper intermediate space (11) and the lower intermediate space (13) such that a convection flow can flow between the first cavity (15) and the second cavity (17) via the upper intermediate space (11) and the lower intermediate space 13),
- at least one means for controlling the convection flow, which is arranged for the upper intermediate space (11) and/or for the lower intermediate space (13).

In a second aspect, the aforementioned object is achieved by the use of the structural element (1) according to the invention as a wall and/or roof element in buildings or vehicles.
The third aspect of the present invention achieves the underlying object by a method for controlling the heat transfer coefficient U in a structural element (1) according to the invention, comprising the steps of

- providing a structural element (1),
- absorbing thermal energy by a first sheet (3) or a second sheet (5) on a first side of the structural element (1), whereby the gas filling the volume V is heated in a first cavity (15) or in a second cavity (17) and rises vertically upward,
- opening a vertically upper intermediate space (11) and/or a vertically lower intermediate space (13), whereby a convection flow from one of the cavities (15, 17) through the intermediate space (11) into the other of the cavities (15, 17) is made possible,
- giving off thermal energy by the gas filling the volume V to the first sheet (3) or the second sheet (5) on a second side of the structural element (1), whereby the gas filling the volume V in the other of the cavities (15, 17) cools down and falls vertically downward, so that the convection flow flows from this cavity (15, 17) through the lower intermediate space (13) into one of the cavities (15, 17), the intensity of the convection flow being set by the opening and/or closing of the lower intermediate space (11) and/or of the upper intermediate space (13).

The present invention is based on the realization that the heat transfer through a structural element (1) of the type described can be controlled by forming and regulating an internal convection flow.
It has surprisingly been found that it is possible with the structural element (1) according to the invention to minimize significantly the energy requirement of a building and thus to utilize optimally the prevailing temperatures inside and outside a building and even to do so in a technically simple manner. It is of advantage that, according to the present invention, the heat transfer coefficient U can be controlled according to requirements and independently of the prevailing inside/outside temperatures.
It can thus be achieved by the design according to the invention of the structural element (1) that an intensified discharge of heat from the building is made possible during the cooler nighttime hours, while an insulating effect that conforms to the specifications for adequate heat insulation can be ensured when there are high outside temperatures during the daytime in summer and when there are low outside temperatures in winter.
The present invention is specified more precisely below.
In a first aspect, the present invention relates to a structural element (1) with a controllable heat transfer coefficient U, comprising

- a frame (7),
- a first sheet (3) and a second sheet (5), which are opposite one another and which are arranged at a distance A from one another in the frame (7), the first sheet (3), the second sheet (5) and the frame (7) having the effect of defining a closed-off volume V, which is filled with at least one gas,
- at least one two-dimensional element (9), the width of which corresponds to the vertical clear width W of the frame (7) and the height of which is less than the clear height H of the frame (7),
- the two-dimensional element (9) being arranged between the first sheet (3) and the second sheet (5) such that it finishes laterally with the inner sides of the frame (7), and an upper intermediate space (11) being formed between the two-dimensional element (9) and the frame (7) in the vertically upward direction and a lower intermediate space (13) being formed between the two-dimensional element (9) and the frame (7) in the vertically downward direction,
- a first cavity (15), which is formed between the first sheet (3) and the two-dimensional element (9) with a spacing X,
- a second cavity (17), which is formed between the two-dimensional element (9) and the second sheet (5) with a spacing Y,
- the first cavity (15) and the second cavity (17) being in connection via the upper intermediate space (11) and the lower intermediate space (13) such that a convection flow can flow between the first cavity (15) and the second cavity (17) via the upper intermediate space (11) and the lower intermediate space (13),
- at least one means for controlling the convection flow, which is arranged for the upper intermediate space (11) and/or for the lower intermediate space (13).

In this case, the frame (7) and the opposing sheets (3, 5), i.e. the first sheet (3) and the second sheet (5), form a three-dimensional body, which defines a closed-off volume V. The frame (7) of the structural element (1) according to the invention serves in particular for enclosing and mechanically stabilizing the structural element (1) and for receiving the first and second sheets (3, 5). The design of the first and second sheets (3, 5) is described in more detail below.
The form of the structural element (1) can be freely chosen and adapted to the requirements for its installation position and/or use within wide limits. A preferred embodiment is an approximately cuboid element. But other geometrical forms can also be realized with the structural element (1) according to the invention, depending on the installation situation, for example the basic form of a triangle, a pentagon or the like. Further designs of the frame are defined below.
The structural element (1) comprises at least one two-dimensional element (9), which is arranged substantially centrally such that the internal convection flow around the two-dimensional element (9) is possible, the convection flow being conducted from the side of the structural element (1) on which heat is supplied, through the upper intermediate space (11) to the other side of the two-dimensional element (9), where the convection flow can give off heat to the opposite side, and subsequently flows back through the lower intermediate space (13) to the side of the heat supply. The two-dimensional element (9) consists in particular of an insulating material.
For regulating the internal convection flow, the structural element (1) according to the invention comprises at least one means by which opening and/or closing of one of the intermediate spaces (11, 13) is performed, whereby in turn the convection flow is controlled.
The term “means”, as it is used in the present case, describes on the one hand measures and on the other hand devices by which the convection flow can be controlled. Preferred designs are defined below. If the means are devices, they may be arranged both on and/or in the frame (7) and on and/or in the two-dimensional element (9), in order to control the convection flow in the way according to the invention. Furthermore, the means also include auxiliary structures for achieving the control according to the invention of the convection flow.
The term “structural element”, as it is used here, should be understood for the purposes of the present invention as meaning that the structural element (1) is suitable both for wall surfaces and for roof surfaces. The structural element (1) is self-supporting and can therefore be fitted on its own into a shell of a building as a wall and/or roof element.
The heat transfer coefficient U (formerly also “k value”) describes a heat equalization as a result of a temperature difference between different energy systems. The heat transfer coefficient U is consequently a measure of the rate of heat transfer. The power (amount of energy per unit time) that flows through a surface area of one square meter when the difference in temperature between the air on the two sides of a wall is one Kelvin is given as the heat transfer coefficient U. The heat transfer coefficient U is defined internationally in the standard EN ISO 6946. Its unit of measure is W/(m²·K).
The determination of exact heat transfer coefficients U of different materials is known to a person skilled in the art. It is calculated from the mean value of the heat transfer resistance R_T:
$U = \frac{1}{R_{T}} .$
The required dimensioning values for the heat transfer coefficient U are stipulated in the standards EN 12524 and DIN 4108-4.
It is determined substantially by the thermal conductivity and the thickness of the materials used, but in addition also by heat radiation and convection at the surfaces of the compound unit. The heat transfer coefficient U consequently indicates the rate of heat transfer through a single- or multi-ply layer of material when there are different temperatures on the two sides. In the case of the structural element (1) according to the invention, the heat transfer coefficient U can be varied between a value determined by the insulating materials in the layers contained in the structural element (1) and a value determined by the convection around these.
The term “cavity” is understood as meaning the space that is substantially invariable in its dimensions between a sheet (3, 5), i.e. between the first sheet (3) or the second sheet (5), and a two-dimensional element (9), while “intermediate space” refers to a space between a two-dimensional element (9) and a frame (7) that can be closed in a suitable way.
The references to “vertically upward” and “vertically downward” are to be understood in the context of the present invention as though they relate not only to perpendicularly aligned structural elements (1) but also to structural elements (1) that are arranged at a certain angle with respect to the perpendicular. The reference to “vertically upward” then means that the upper intermediate space (11) is arranged substantially above the lower intermediate space (13), in particular obliquely above it.
It is preferred if the gas filling the volume V is chosen from argon, krypton, xenon, carbon dioxide, hydrocarbons, partially halogenated hydrocarbons, halides of chalcogens and/or pycnogens and mixtures thereof, in order to achieve additional improvements in the insulating effect of the structural element or in the order of magnitude of the heat transfer. The use of polyatomic gases is particularly preferred because of the higher convective thermal conductivity.
In a development of the structural element (1) according to the invention, at least one of the sheets (3, 5), i.e. the first sheet (3) and/or the second sheet (5), is at least partially transparent or translucent.
The design according to the invention of the structural element (1) in which the opposing sheets (3, 5), i.e. the first sheet (3) and/or the second sheet (5), are transparent or at least translucent, and furthermore the two-dimensional element (9) is likewise transparent or at least translucent, allows the structural element (1) also to be used in the form of a partially transparent or at least translucent window element. In particular, the structural element (1) according to the invention of this design is suitable for the replacement of conventionally used glass blocks, as were frequently used in the past for example for stairwells. Here, the structural element (1) according to the invention has the great advantage over conventional glass blocks of good heat insulation with at the same time sufficient light transmittance for the illumination of a stairwell for example.
In a further design of the invention, the structural element (1) may be formed as what is known as an insulating glass unit (IGU). Such an insulating glass unit can be installed in a corresponding modified, conventional window frame structure. Of interest here in particular is the installation of the elements according to the invention in the region of the skylight and the parapet, which can be combined with customary insulating glazing units at eye level. The installation of a structural element (1) according to the invention with translucent two-dimensional elements (9, 9 a, 9 b) in the region of the skylight is accompanied in particular by the advantage that, as a result of the isotropization of the incident radiation in the translucent two-dimensional element (9, 9 a, 9 b), the light that is incident in the region of the skylight can partially reach areas further back in the room than is possible in the case of light incidence through transparent skylight elements.
One design according to the invention of the means mentioned above comprises the vertical displacement or tilting about a horizontal axis of the at least one two-dimensional element (9), so that at least one of the intermediate spaces (11, 13), i.e. the upper intermediate space (11) and/or the lower intermediate space (13), is closed by the two-dimensional element (9) and the convection flow is thereby completely or partially prevented. In this way it is possible to control the convection flow in a simple way just by moving the at least one two-dimensional element (9).
In this case, the means mentioned may also comprise a device for displacing the at least one two-dimensional element (9) that is preferably chosen from servomotors, pneumatic, magnetic or piezoelectric systems, mechanical levers, cables or bimetallic structures. The choice can consequently be made to suit the external conditions of the structural element.
Another design according to the invention of the means mentioned above comprises the changing of the vertical extent of the at least one two-dimensional element (9), so that at least one of the intermediate spaces (11, 13), i.e. the upper intermediate space (11) and/or the lower intermediate space (13), is closed by the two-dimensional element (9) and the convection flow is thereby completely or partially prevented. This design also has the advantage of controlling the convection flow in a simple way just by moving the at least one two-dimensional element (9).
In a further design according to the invention, the means mentioned above may also comprise a closure device for at least one of the upper and lower intermediate spaces (11, 13), which is preferably chosen from flaps, inflatable tubes or bellows, closures in the form of cylinder cocks or displaceable or rotatable wedges. Depending on the dimensional design of the structural element (1) and the materials used for the at least one two-dimensional element (9), it may be advantageous to provide these additional closure devices to allow the convection flow to be effectively controlled. Inflatable bellows of a suitable configuration may also be used for switching the structural element (1) according to the invention automatically into the insulating state when there are very low outside temperatures, as a result of the negative pressure then prevailing in the interior space of the structural element (1). This is of advantage to ensure that there is always adequate insulation even in the event of failure of some other control of the convection at the cold time of year.
If the sheets (3, 5), i.e. the first sheet (3) and/or the second sheet (5), are transparent and the material of these sheets comprises glasses and/or polymers, and also the two-dimensional element or elements (9, 9 a, 9 b) consist(s) of a translucent material, daylight can additionally enter the building through the structural element (1) according to the invention.
In this case, the glasses are preferably chosen from silicate glasses, borosilicate glasses, lead-silicate glasses and/or the polymers are preferably chosen from PET (polyethylene terephthalate), PVB (polyvinyl butyral), EVA (ethylene vinyl acetate), polyolefins, styrenic polymers, polycarbonates, PMMA (polymethyl methacrylate), polyurethanes, PVC (polyvinyl chloride) or mixtures or multilayer systems thereof. In particular, the polymers may be formed as sheets or extruded, blown or cast films or panels. Depending on the application area of the structural element (1) according to the invention, the suitable material is thus available, for example polymers for lightweight applications or special glasses for applications with greater chemical exposure. Furthermore, it is possible to provide one or more layers with specific functions, for example heat protection layers or chromotropic layers.
To allow the structural element (1) also to be used as a light-transmissive window element, apart from using the aforementioned glasses it has proven to be appropriate also to form the at least one two-dimensional element (9) from a translucent material that is preferably chosen from organic, inorganic or hybrid closed-cell or open-cell foams or coated or uncoated textiles.
As an alternative to the aforementioned embodiment, the at least one two-dimensional element (9) may be formed from a mineral, metallic, polymeric and/or bio-organic material.
This is of advantage if the structural element (1) is not intended to be used as a light-transmissive compound unit but is for example exposed to greater mechanical loads (metallic material, fiber-reinforced polymer) or is intended to serve solely for heat insulation (mineral and/or polymeric material). Furthermore, it is possible with this embodiment also to create structural elements (1) that are ecologically particularly compatible (bio-organic materials). In this case, the material used may be open-cell or closed-cell. If in addition the outer first or second sheet (3, 5) of the structural element (1) has been coated or otherwise modified in a suitable way such that it can reflect the incident solar irradiation directly or diffusely, the structural element (1) is responsible for particularly little heating up being caused by the solar irradiation during the day.
To be able to react flexibly to a wide variety of requirements for the structural element (1) according to the invention, it has proven to be advantageous to choose the material of the frame (7) from concrete, gypsum, clays, glasses, natural stones, ceramics, polyamide, polyesters, wood, metals, in particular steel and aluminum and alloys thereof, PVC, polycarbonate, PMMA, styrenic polymers, polyurethanes and fiber composite materials and composite materials of two or more of these materials and also from open-cell or closed-cell foams and fiber boards of synthetic or renewable raw materials. It is particularly preferred if the material of the frame (7) is made so as to be impermeable to gas and/or moisture.
In particular in the case of embodiments of the structural element (1) according to the invention that do not have to be light-transmissive, the aforementioned materials may also be used for one or both sheets (3, 5), i.e. the first sheet (3) and/or the second sheet (5). Materials with a low thermal conductivity should preferably be used. Furthermore, in a further embodiment, the frame (7) may be constructed from photovoltaic elements or solar-thermal elements. Such elements are known to a person skilled in the art and may be made either as opaque elements or as partially translucent structures. They can also be used in such a way that only part of the surface area of the frame is taken up by them.
In a development of the invention, at least the first sheet (3) and/or the second sheet (5) and/or the at least one two-dimensional element (9) may be three-dimensionally structured on the surface. This allows the achievement of optical effects, which for example bring about protection from the glare of directly incident light by modifying the angular distribution of the light radiated and/or by changing the intensity thereof. If a number of two-dimensional elements (9, 9 a, 9 b) are contained in the structural element (1) according to the invention, the light directing effect can be additionally intensified by suitable combinations of two-dimensional elements (9, 9 a, 9 b) with differing angular behavior of the translucence. For example, it has proven to be particularly advantageous to combine a two-dimensional element (9, 9 a, 9 b) with strongly isotropizing translucence on the outer side and a two-dimensional element (9, 9 a, 9 b) with preferred radiation perpendicularly to the element surface with one another in order to intensify the effect of further dividing the light further back into the room. A similar effect as in the case of a three-dimensionally structured surface can be achieved by a combination of two-dimensional elements (9, 9 a, 9 b) with different translucence properties.
In addition, in the non-technical area, a three-dimensional structuring allows creative effects to be achieved. As an alternative to this, the first and/or the second sheets (3, 5) and/or the at least one two-dimensional element (9) may be printed on or coated to achieve the same or similar effects.
In a particularly preferred embodiment, the structural element (1) according to the invention comprises

- a first two-dimensional element (9 a) and a second two-dimensional element (9 b), the width of which respectively corresponds to the vertical clear width W of the frame (7) and the height of which is respectively less than the clear height H of the frame (7),
- the first two-dimensional element (9 a) and the second two-dimensional element (9 b) being arranged between the first sheet (3) and the second sheet (5) such that they each finish laterally with the inner sides of the frame (7), and
- a first upper intermediate space (11 a) and a second upper intermediate space (11 b) being respectively formed between the first two-dimensional element (9 a) and the second two-dimensional element (9 b) and the frame (7) in the vertically upward direction and a first lower intermediate space (13 a) and a second lower intermediate space (13 b) being respectively formed between the first two-dimensional element (9 a) and the second two-dimensional element (9 b) and the frame (7) in the vertically downward direction,
- a first cavity (15), which is formed between the first sheet (3) and the first two-dimensional element (9 a) with a spacing X,
- a second cavity (17), which is formed between the second two-dimensional element (9 b) and the second sheet (5) with a spacing Y,
- a third cavity (23), which is formed between the first two-dimensional element (9 a) and the second two-dimensional element with a spacing Z,
- at least the first cavity (15) and the second cavity (17) being in connection via the first upper intermediate space (11 a) and the second upper intermediate space (11 b) and the first lower intermediate space (13 a) and the second lower intermediate space (13 b) such that a convection flow can flow at least between the first cavity (15) and the second cavity (17) via the first upper intermediate space (11 a) and the second upper intermediate space (11 b) and the first lower intermediate space (13 a) and the second lower intermediate space (13 b).

The convection flow forming thereby flows substantially through the first cavity (15), the first and second upper intermediate spaces (11 a, 11 b), the second cavity (17) and the first and second lower intermediate spaces (13 a, 13 b). A convection flow through the third cavity (23) does not form, or only to a negligible extent.
In a second aspect, the present invention relates to the use of the structural element (1) described above as a wall and/or roof element in buildings or vehicles, in particular in rail vehicles or watercraft. Particularly in rail vehicles with the large ratio thereof between wall surface and volume and long stationary times at locations with high solar irradiation, the need for active cooling can be reduced here.
The third aspect of the present invention that achieves the aforementioned object relates to a method for controlling the heat transfer coefficient U in a structural element (1) described above that comprises the steps of

- providing a structural element (1),
- absorbing thermal energy by a first sheet (3) or a second sheet (5) on a first side of the structural element (1), whereby the gas filling the volume V is heated in a first cavity (15) or in a second cavity (17) and rises vertically upward,
- opening a vertically upper intermediate space (11) and/or a vertically lower intermediate space (13), whereby a convection flow from one of the cavities (15, 17) through the intermediate space (11) into the other of the cavities (15, 17) is made possible,
- giving off thermal energy by the gas filling the volume V to the first sheet (3) or the second sheet (5) on a second side of the structural element (1), whereby the gas filling the volume V in the other of the cavities (15, 17) cools down and falls vertically downward, so that the convection flow flows from this cavity (15, 17) through the lower intermediate space (13) into one of the cavities (15, 17),
- the intensity of the convection flow being set by the opening and/or closing of the lower intermediate space (11) and/or of the upper intermediate space (13).

Further features, advantages and application possibilities emerge from the following description of the preferred exemplary embodiments, which do not however restrict the invention, and the figures. All of the features described here form the subject matter of the invention by themselves or in any desired combination, even independently of how they appear together in the claims or how the claims refer back to preceding claims. In the drawing:

FIG. 1 shows a schematic representation of a structural element in a first embodiment of the invention,

FIG. 2 shows a schematic representation of a structural element in a second embodiment of the invention,

FIG. 2 a shows a view of a detail of the region marked in FIG. 2,

FIG. 3 a shows a simplified depiction of the structural element represented in FIG. 1 with a schematically represented convention flow,

FIG. 3 b shows a simplified depiction of the structural element represented in FIG. 2 with a schematically represented convention flow,

FIG. 4 a shows a simplified depiction of the structural element represented in FIG. 1 with a schematically represented prevented convention flow,

FIG. 4 b shows a simplified depiction of the structural element represented in FIG. 2 with a schematically represented prevented convention flow,

FIG. 5 shows a schematic perspective representation of the structural element represented in FIG. 1,

FIG. 6 shows a schematic representation of a structural element in an embodiment of the invention with reduced flow resistance.

FIG. 1 shows the basic form of a structural element 1 according to the invention. The structural element 1 is constructed by a frame 7, which forms four sides of the element, to be specific the upper side and underside and the side faces. Arranged opposite one another in the frame 7 are two sheets 3, 5, i.e. a first sheet 3 and a second sheet 5, which together with the frame 7 define a closed-off volume. In the interior of the defined volume, a two-dimensional element 9 is arranged such that it respectively finishes laterally with the frame 7 and at the top leaves an upper intermediate space 11 and at the bottom leaves a lower intermediate space 13 with respect to the frame. Furthermore, the two-dimensional element 9 is arranged with a spacing X in relation to the first sheet 3 and with a spacing Y in relation to the second sheet 5.
As represented in the simplified representation of FIG. 3 a, a convection flow can form around the two-dimensional element 9. In the representation of FIG. 3 a, heat is transferred from the second sheet 5 by free convection to the first sheet 3, the free convection being established because the temperature T₂on the left side of the figure is greater than the temperature T₁on the right side of the figure. As a result, the temperature of the gas in the second cavity 17 is higher on average than in the first cavity 15 and the density is correspondingly lower. This produces a pressure difference upstream and downstream of the lower intermediate space 13, which finally sets the convection movement in motion, so that the warmer gas flows out from the second cavity 17 via the upper intermediate space 11 into the first cavity 15, while the colder gas flows via the lower intermediate space 13 into the second cavity 17. Overall, an energy flow from right to left thus takes place.
A preferred embodiment of the invention is schematically represented in FIG. 2. This embodiment has two two- dimensional elements 9 a, 9 b, i.e. a first two-dimensional element 9 a and a second two-dimensional element 9 b, arranged in the defined volume. These elements are arranged in principle in the same way as the two-dimensional element 9 in FIG. 1, but with the difference that they form between them a third cavity 23 with the spacing Z from one another. As is shown in FIG. 3 b, the formation of the internal convection flow is substantially the same as the embodiment represented in FIG. 3 a.
In order to control or prevent the convection flow in one embodiment the two-dimensional element 9 can be moved by suitable means, for example upward, so that it closes the upper intermediate space 11, as is represented in FIG. 4 a. Although the gas volume in the first cavity 15 that has been heated by the heat W1 can rise upward, the formation of a convection flow is not possible due to the closed upper intermediate space 11.
The particular embodiment that is represented in FIG. 2 acts in a similar way when the first and/or second two-dimensional element or elements 9, 9 a, 9 b are displaced such that one of the intermediate spaces 11 a, 11 b, 13 a, 13 b, i.e. the first upper intermediate space 11 a, the second upper intermediate space 11 b, the first lower intermediate space 13 a, the second lower intermediate space 13 b, is closed. One possible configuration is represented in FIG. 4 b, a configuration in which the first two-dimensional element 9 a has been displaced upward, in order to close the first upper intermediate space 11 a, while the second two-dimensional element 9 b has been displaced downward, in order to close the second lower intermediate space 13 b. No convection flow can form in this embodiment either.
Generally, the configurations in FIGS. 3 a and 3 b represent the state in which the structural element 1 has a maximum heat transfer coefficient U, that is to say it makes a maximum heat transfer possible. On the other hand, the configurations in FIGS. 4 a and 4 b represent a state in which the structural element 1 has its minimum heat transfer coefficient U, that is to say offers maximum heat insulation.
FIG. 5 is a perspective representation of the structural element 1 shown in FIG. 1, from which it is clear in particular how the two-dimensional element 9 finishes laterally with the frame 7.
FIG. 6 shows an embodiment of the structural element according to the invention with reduced flow resistance, the reduction in the flow resistance occurring as a result of the rounding 25 of the edges of the two-dimensional element 9 and as a result of the round shaping 27 of the corners of the first and/or second sheets 3, 5. The advantage of such an embodiment is that, with the same temperature difference, a greater convection flow results, and more energy can thereby be transferred, while in the closed case (FIGS. 4 a, 4 b) there is no deterioration of the insulating effect.
Depending on the installation situation of the structural element 1, it is also possible for more than two two- dimensional elements 9, 9 a, 9 b to be provided in the defined volume V. Moreover, it may be of advantage to reduce the third cavity 23 formed between two first and second two- dimensional elements 9 a, 9 b to a minimum, to the extent of the embodiment where the first and second two- dimensional elements 9 a, 9 b are touching.
According to a further embodiment, active convection elements may be integrated into the first cavity 15 and/or into the second cavity 17. “Active convection elements” are understood as meaning for example small rotors that promote the formation of the convection flow and maintain it. As a result, in particular the shift stroke between the side with the higher temperature T₂and the side with the lower temperature T₁is increased.
As becomes clear from the figures, in a reverse of the effect known from the prior art, the structural element 1 according to the invention can in particular be used for the purpose of removing heat from buildings. This may be advantageous for example at the warm time of year. Application of the structural element 1 according to the invention for heat dissipation from industrial constructions is also conceivable.
Depending on the installation situation of the structural element 1 according to the invention, the first and/or second sheets 3, 5 may be provided either in a perpendicular or an inclined configuration. In this way, both wall surfaces and sloping roof surfaces can be formed. The angle of the sloping roof surfaces in relation to the perpendicular is substantially between 0° and 90°, preferably between 5° and 60°. In spite of the sloping position of the structural element 1 according to the invention, the principle of the controllable heat transfer coefficient U, i.e. control obtained by specifically controlling the internal convection flow, is retained.
For use of the structural element 1 according to the invention for flat roof surfaces, only small structural modifications have to be performed, so that the internal convection flow continues to be ensured. Imperative for use in flat roof surfaces is the use of inflatable bellows, slides, flaps and wedges instead of the displacement of the two- dimensional elements 9, 9 a, 9 b, since they would involve a high amount of friction and resulting damage to the two- dimensional elements 9, 9 a, 9 b. Depending on the conditions in which it is used, slightly different dimensioning of the structural elements 1 is possibly necessary.
The structural element 1 according to the invention can accordingly be used as a wall and/or roof element in a shell, without further wall elements or roof elements having to be provided. Of course, the structural element 1 according to the invention can also be used as a classic insulating element for mounting on a facade.
In a further embodiment, the at least one two-dimensional element 9 is formed from a flexible, open-cell foam on a melamine resin basis, which is commercially available under the designation Basotect® (BASF SE). Basotect® displays the same physical properties over a wide temperature range, with at the same time low weight, good heat insulating properties and high sound absorption characteristics. Moreover, Basotect® is flame resistant (without the addition of flame retardants), which makes a structural element 1 according to the invention comprising this material particularly suitable for wall and/or roof elements.
In a specific embodiment, the frame 7 may be provided with light sources (for example LEDs), in order also to use the structural element 1 according to the invention in darkness for interior/exterior lighting. Furthermore, optical and creative effects can be achieved by a diffuser effect of structured sheets 3, 5 and/or structured two- dimensional elements 9, 9 a, 9 b.
The preferred dimensions of the structural element 1 and parts thereof are specified below.
The distance A between the first sheet 3 and the second sheet 5 is <50 cm, preferably <35 cm, particularly preferably between 5 cm and 12 cm. It generally applies that, the higher the structural element 1 is, the wider the first and second cavities 15, 17 have to be chosen in order to bring about a spontaneous convection even when there are small differences in temperature. This ratio of the height of the structural element 1 to the width of the first and second cavities 15, 17 is very sensitive and needs to be set precisely.
In principle, the structural elements 1 according to the invention are not subject to any size limitation. From a practical viewpoint, a height of up to 1.5 m has been found to be appropriate. The width of the elements is substantially limited by the stability of the materials used and is appropriately up to 5 m. For reasons of thermally induced pressure changes, the gas volume enclosed in the structural element 1 should be kept as small as possible.
Dimensions for the structural element 1 according to the invention that have been determined by computer-aided optimization are specified below.
Value ranges (relative) for a first embodiment of the structural element 1:
X/H: relative thickness of the gap between sheet 3 and two-dimensional element 9:
0.001≦X/H≦0.05; preferably: 0.005≦X/H≦0.04
Y/H: relative thickness of the gap between two-dimensional element 9 and sheet 5:
0.001≦Y/H≦0.05; preferably: 0.005≦Y/H≦0.04

if Y<X:

s_O/Y: relative thickness of the gap between the two-dimensional element 9 and the upper frame 7 in the state with a high heat transfer coefficient:
0.3≦s_O/Y≦5; preferably: 0.5≦s_O/Y≦4; particularly preferably: 1≦s_O/Y≦3
s_U/Y: relative thickness of the gap between the two-dimensional element 9 and the lower frame 7 in the state with a high heat transfer coefficient:
0.3≦s_U/Y≦5; preferably: 0.5≦s_U/Y≦4; particularly preferably: 1≦s_U/Y≦3

if Y≧X:

s_O/X: relative thickness of the gap between the two-dimensional element 9 and the upper frame 7 in the state with a high heat transfer coefficient;
0.3≦s_O/X≦5; preferably: 0.5≦s_O/X≦4; particularly preferably: 1≦s_O/X≦3
s_U/X: relative thickness of the gap between the two-dimensional element 9 and the lower frame 7 in the state with a high heat transfer coefficient:
0.3≦s_U/X≦5; preferably: 0.5≦s_U/X≦4; particularly preferably: 1≦s_U/X≦3
H: height of the structural element 1:
0.25 m≦H≦6 m; preferably: 0.5 m≦H≦4 m; particularly preferably: 0.7 m≦H≦3 m
Value ranges (relative) for a second embodiment of the structural element 1:
X/H: relative thickness of the gap between sheet 3 and two-dimensional element 9 a:
0.001≦X/H≦0.05; preferably: 0.005≦X/H≦0.04
Y/H: relative thickness of the gap between two-dimensional element 9 b and sheet 5:
0.001≦Y/H≦0.05, preferably: 0.005≦Y/H≦0.04

if Y<X:

s_O/Y: relative thickness of the gap between the two- dimensional elements 9 a, 9 b and the upper frame 7 in the state with a high heat transfer coefficient:
0.3≦s_O/Y≦5; preferably: 0.5≦s_O/Y≦4; particularly preferably: 1≦s_O/Y≦3
s_U/Y: relative thickness of the gap between the two- dimensional elements 9 a, 9 b and the lower frame 7 in the state with a high heat transfer coefficient:
0.3≦s_U/Y≦5; preferably: 0.5≦s_U/Y≦4; particularly preferably: 1≦s_U/Y≦3

if Y≧X:

s_O/X: relative thickness of the gap between the two- dimensional elements 9 a, 9 b and the upper frame 7 in the state with a high heat transfer coefficient:
0.3≦s_O/X≦5; preferably: 0.5≦s_O/X≦4; particularly preferably: 1≦s_O/X≦3
s_U/X: relative thickness of the gap between the two- dimensional elements 9 a, 9 b and the lower frame 7 in the state with a high heat transfer coefficient:
0.3≦s_U/X≦5; preferably: 0.5≦s_U/X≦4; particularly preferably: 1≦s_U/X≦3
H: height of the structural element 1:
0.25 m≦H≦6 m; preferably: 0.5 m≦H≦4 m; particularly preferably: 0.7 m≦H≦3 m
Value ranges (relative) for a first embodiment with reduced flow resistance:
r/(A−X−Y): relative rounding radius of the two-dimensional element 9:
0≦r/(A−X−Y)≦0.5; preferably: 0.1≦r/(A−X−Y)≦0.5; particularly preferably: 0.25≦r/(A−X−Y)≦0.5
R/A: relative rounding radius of the outer corners:
0≦R/A≦0.5; preferably: 0.1≦R/A≦0.5; particularly preferably: 0.25≦R/A≦0.5
For the distance between the two- dimensional elements 9 a, 9 b which defines the intermediate space 23, a width of 0.003 m to 0.05 m, preferably 0.005 m to 0.04 m, particularly preferably 0.007 m to 0.03 m, has proven expedient.

EXAMPLES

In an experimental setup, the properties of structural element prototypes according to the invention were determined. For the sheets 3, 5, Plexiglass panels with a size of 800×800 mm were used, while the two-dimensional element consisted of a translucent insulating material (non-colored Basotect®). The frame 7 was made out of PVC sheets. The thickness of the prototype was 96 mm. The cavities 15, 17 respectively had a size X, Y of 30 mm.
The test setup was chosen such that a heatable element was pushed in between two identical prototypes of the type described above, while coolable elements were provided on the opposing sides. The heat flow from the heatable element to the coolable elements was measured electrically. The heat flow passing through one of the prototypes is consequently half the heat flow measured as a whole. In this way, the thermal conductivity λ and the heat transfer coefficient U were measured.
In a first measuring setup (I), the size of the upper intermediate space 11 was 60 mm and the size of the lower intermediate space 13 was 0 mm; in a second measuring setup (II), both sizes of the upper and lower intermediate spaces 11, 13 were respectively 30 mm. In a further pair of measuring setups (III) and (IV), the two switching states of a structural element 1 according to the invention have been realized in the configuration with two two- dimensional elements 9 a, 9 b. In the measuring setups, two two- dimensional elements 9 a, 9 b from Basotect® with a thickness of in each case 15 mm have been used. The sizes of the cavities 15 and 17 were in each case 15 mm, the size of the intermediate space 23 was 10 mm. In setup (III), the sizes of the intermediate spaces 11 a and 13 b were 30 mm, the sizes of the intermediate spaces 11 b and 13 a were 0 mm. In setup (IV), the sizes of the intermediate spaces 11 a, 11 b, 13 a and 13 b were in each case 15 mm. The setups (III) and (IV) were additionally measured with CO₂as the filling gas instead of air. These measurements are denoted in the table by IIIb and IVb.
For each measuring setup, two measurements were respectively carried out with a low difference in temperature between the heatable element and the coolable element (measurements 1 and 3) and one measurement was carried out with a high difference in temperature between the heatable element and the coolable element (measurements 2 and 4). The results of the measurements are represented in the table below.


	Temperature difference between
Measuring	both sides of the prototype	U
setup	[K]	[Wm⁻²K⁻¹]

1	I	12.8	1.046
2	I	34.7	1.317
3	II	13.7	2.417
4	II	32.7	2.775
5	III	15	0.71
6	III	29	0.8
7	IV	14	1.44
8	IV	28	1.6
9	IIIb	15	0.65
10	IIIb	29	0.74
11	IVb	14	1.45
12	IVb	28	1.67

Measurements 1 and 3 show that the heat transfer coefficient U is more than doubled if the position of the two-dimensional element 9 is changed while there is substantially the same difference in temperature at the prototypes.
Measurements 2 and 4 confirm that the convection, and consequently also the heat transfer coefficient U, rise with the difference in temperature.
It has been shown with the experimental setup and the measurements that the heat transfer coefficient U in the structural element according to the invention can be controlled.

LIST OF DESIGNATIONS

1 structural element
3 first sheet
5 second sheet
7 frame
9 two-dimensional element
9 a first two-dimensional element
9 b second two-dimensional element
11, 11 a, 11 b upper intermediate space
13, 13 a, 13 b lower intermediate space
15 first cavity
17 second cavity
23 third cavity
25 rounded outer corner
rounded inner corner
H clear height of the frame 7
W clear width of the frame 7
A distance between first sheet 3 and second sheet 5
X spacing of first sheet 3 in relation to two- dimensional element 9, 9 a, 9 b
Y spacing of second sheet 5 in relation to two- dimensional element 9, 9 a, 9 b
S₁thickness of the first sheet 3
S₂, S_2a, S_2bthickness of the two- dimensional element 9, 9 a, 9 b
S₃thickness of the second sheet 5
S_owidth of upper intermediate space 11, 11 a, 11 b
S_uwidth of lower intermediate space 13, 13 a, 13 b
r radius of inner corner
R radius of outer corner

Claims

1.-15. (canceled)

16. A structural element (1) with a controllable heat transfer coefficient U, comprising

a frame (7),

a first sheet (3) and a second sheet (5), which are opposite one another and which are arranged at a distance A from one another in the frame (7),

the first sheet (3), the second sheet (5) and the frame (7) having the effect of defining a closed-off volume V, which is filled with at least one gas,

at least one two-dimensional element (9), the width of which corresponds to the vertical clear width W of the frame (7) and the height of which is less than the clear height H of the frame (7),

the two-dimensional element (9) being arranged between the first sheet (3) and the second sheet (5) such that it finishes laterally with the inner sides of the frame (7), and

an upper intermediate space (11) being formed between the two-dimensional element (9) and the frame (7) in the vertically upward direction and a lower intermediate space (13) being formed between the two-dimensional element (9) and the frame (7) in the vertically downward direction,

a first cavity (15), which is formed between the first sheet (3) and the two-dimensional element (9) with a spacing X,

a second cavity (17), which is formed between the two-dimensional element (9) and the second sheet (5) with a spacing Y,

the first cavity (15) and the second cavity (17) being in connection via the upper intermediate space (11) and the lower intermediate space (13) such that a convection flow can flow between the first cavity (15) and the second cavity (17) via the upper intermediate space (11) and the lower intermediate space (13),

at least one means for controlling the convection flow, which is arranged for the upper intermediate space (11) or for the lower intermediate space (13),

the at least one means consisting in the vertical displacement or tilting about a horizontal axis of the at least one two-dimensional element (9), so that the upper intermediate space (11) or the lower intermediate space (13) is closed by the two-dimensional element (9) and the convection flow is thereby completely or partially prevented, or

the at least one means comprising the changing of the vertical extent of the at least one two-dimensional element (9), so that the upper intermediate space (11) or the lower intermediate space (13) is closed by the two-dimensional element (9) and the convection flow is thereby completely or partially prevented, or

the at least one means comprising a closure device for the upper intermediate space (11) or for the lower intermediate space (13), which is preferably chosen from flaps, inflatable tubes or bellows, closures in the form of cylinder cocks or displaceable or rotatable wedges.

17. The structural element (1) according to claim 16, the first sheet (3) or the second sheet (5) being at least partially transparent or translucent.

18. The structural element (1) according to claim 16, the at least one means also comprising a device (19) for displacing the at least one two-dimensional element (9), preferably chosen from servomotors, pneumatic, magnetic or piezoelectric systems, mechanical levers, cables or bimetallic structures.

19. The structural element (1) according to claim 16, the first sheet (3) or the second sheet (5) being transparent and the material of the first sheet (3) or of the second sheet (5) comprising glasses or polymers.

20. The structural element (1) according to claim 19, the glasses being chosen from silicate glasses, borosilicate glasses, lead-silicate glasses or the polymers being chosen from PET, PVB, EVA, polyolefins, styrenic polymers, polycarbonates, PMMA, polyurethanes, PVC or mixtures or multilayer systems thereof, the polymers being formed as sheets or extruded, blown or cast films.

21. The structural element (1) according to claim 16, the at least one two-dimensional element (9) being formed from a translucent material, chosen from organic, inorganic or hybrid closed-cell or open-cell foams or coated or uncoated textiles.

22. The structural element (1) according to claim 16, the at least one two-dimensional element (9) being formed from a mineral, metallic, polymeric or bio-organic material.

23. The structural element (1) according to claim 16, the material of the frame (7) being chosen from concrete, gypsum, clays, glasses, natural stones, ceramics, polyamide, polyesters, wood, metals, PVC, polycarbonate, PMMA, styrenic polymers, polyurethanes and fiber composite materials and composite materials of two or more of these materials and also from open-cell or closed-cell foams and fiber boards of synthetic or renewable raw materials.

24. The structural element (1) according to claim 16, the first sheet (3) or the second sheet (5) or the at least one two-dimensional element (9) being three-dimensionally structured on the surface.

25. The structural element (1) according to claim 16, comprising

a first two-dimensional element (9 a) and a second two-dimensional element (9 b), the width of which respectively corresponds to the vertical clear width W of the frame (7) and the height of which is respectively less than the clear height H of the frame (7),

the first two-dimensional element (9 a) and the second two-dimensional element (9 b) being arranged between the first sheet (3) and the second sheet (5) such that they each finish laterally with the inner sides of the frame (7), and

a first upper intermediate space (11 a) and a second upper intermediate space (11 b) being respectively formed between the first two-dimensional element (9 a) and the second two-dimensional element (9 b) and the frame (7) in the vertically upward direction and a first lower intermediate space (13 a) and a second lower intermediate space (13 b) being respectively formed between the first two-dimensional element (9 a) and the second two-dimensional element (9 b) and the frame (7) in the vertically downward direction,

a first cavity (15), which is formed between the first sheet (3) and the first two-dimensional element (9 a) with a spacing X,

a second cavity (17), which is formed between the second two-dimensional element (9 b) and the second sheet (5) with a spacing Y,

a third cavity (23), which is formed between the first two-dimensional element (9 a) and the second two-dimensional element with a spacing Z,

at least the first cavity (15) and the second cavity (17) being in connection via the first upper intermediate space (11 a) and the second upper intermediate space (11 b) and the first lower intermediate space (13 a) and the second lower intermediate space (13 b) such that a convection flow can flow at least between the first cavity (15) and the second cavity (17) via the first upper intermediate space (11 a) and the second upper intermediate space (11 b) and the first lower intermediate space (13 a) and the second lower intermediate space (13 b).

26. The use of the structural element (1) according to claim 16 as a wall or roof element in buildings or vehicles.

27. A method for controlling the heat transfer coefficient U in a structural element (1) according to claim 16, comprising the steps of

providing a structural element (1),

absorbing thermal energy by a first sheet (3) or a second sheet (5) on a first side of the structural element (1), whereby the gas filling the volume V is heated in a first cavity (15) or in a second cavity (17) and rises vertically upward,

opening a vertically upper intermediate space (11) or a vertically lower intermediate space (13), whereby a convection flow from one of the cavities (15, 17) through the intermediate space (11) into the other of the cavities (15, 17) is made possible,

giving off thermal energy by the gas filling the volume V to the first sheet (3) or the second sheet (5) on a second side of the structural element (1), whereby the gas filling the volume V in the other of the cavities (15, 17) cools down and falls vertically downward, so that the convection flow flows from this cavity (15, 17) through the lower intermediate space (13) into one of the cavities (15, 17),

the intensity of the convection flow being set by the opening or closing of the lower intermediate space (11) or of the upper intermediate space (13).