The invention relates to a vacuum solar collector provided with an absorption plate with channels connected therewith for the supply casu quo discharge of a heat transfer medium, and with a housing substantially formed by a top cover and a bottom cover for forming an evacuable room at least between the top cover and the absorption plate.

It is noted that said channels may also have the shape of a so-called heat pipe.

Such a vacuum solar collector is known from US Patent Specification No. 4,186,723. This known vacuum solar collector contains a flat absorption plate, which is accommodated under vacuum in a housing consisting of a top cover and a bottom cover. These two covers, which are connected together at their longitudinal ends, are curved both in longitudinal and in transverse direction, in such a manner that in longitudinal direction said covers are shaped by sinusoidal curves, whilst in transverse direction peaks of said curves are bent in the shape of paraboloids. The known solar collector thus constructed is able to withstand atmospheric pressures.

One drawback of the known vacuum solar collector is that in particular its top cover must withstand a so-called bending strain caused by the aforesaid atmospheric pres¬ sure, which makes it necessary for the material of said top cover to be rather thick. A disadvantage of the known vacuum solar collector is moreover that the top cover must withstand an extra tensile strain as a consequence of differences in expansion caused by temperature changes between the top cover on the one hand and the bottom cover and the absorption plate on the other hand.

The object of the invention is to provide a vacuum solar collector wherein the thickness of the material of for example the top cover may be small and wherein in particu¬ lar the top cover does not need to withstand the aforesaid extra strain caused by differences in thermal expansion.

In order to accomplish that objective a vacuum solar collector of the kind according to the invention mentioned in the preamble is c_τara ^~ <_rte_±__τ_ϊd irr that? the to cover ^" is substantially formed by at least one wholly concave (when seen from the outside) plate having a cross-section in the shape of a segment of a circle. Said at least one wholly concave (when seen from the outside) plate preferably substantially corresponds with a segment of a circle or of a sphere. The result of this specific shape of the top cover is that it is hardly necessary for the top cover to withstand bending strain, so that it may be thin-walled. The result of this is that it is at the same time effected that the top cover is excellently able to take up diffe¬ rences in expansion occurring in its longitudinal direction caused by temperature changes. By longitudinal direction is meant the direction along the curved surface of the top cover in this connection.

One embodiment of a vacuum solar collector according to the invention is characterized in that the absorption plate can freely expand in a direction of the plate perpendicular to a direction along the curved surface of the top cover. The advantage of this is that the top cover and/or bottom cover is (are) not subjected to any tensile strain in this direction of the plate, as a result of temperature expansion of the absorption plate.

Another embodiment of a vacuum solar collector according to the invention is characterized in that the bottom cover

is also substantially formed by at least one wholly concave (when seen from the outside) plate having a cross-section in the shape of a segment of a circle. Also this at least one wholly concave (when seen from the outside) plate preferably substantially corresponds with a segment of a cylinder or of a sphere. The advantage of this is in particular that there is a symmetric equilibrium of forces on the solar collector as a whole.

Yet another embodiment of a vacuum ^" solar collector accor- ding to the invention is characterized in that the absorp¬ tion plate is present in the housing and that practically the entire inside room bounded by the housing is evacuable.

Yet another embodiment of a vacuum solar collector accor¬ ding to the invention is characterized in that the bottom cover is formed by the absorption plate. In practice this appears to be a very inexpensive version, since a separate bottom cover is not necessary.

Yet another embodiment of a vacuum solar collector accor¬ ding to the invention is characterized in that the channels connected with the absorption plate are in communication with a storage vessel for the heat transfer medium.

Yet another embodiment of a vacuum solar collector accor¬ ding to the invention is characterized in that the top cover is substantially formed by at least two interconnec- ted wholly concave (when seen from the outside) plates, said plates each corresponding with a segment of a cylinder or of a sphere and each being supported, substantially at the location of junctions, by a supporting rib. Because the at least two plates may be more concave (i.e. have a smaller radius of curvature) in that case, the tensile forces occurring therein are smaller, so that the material may be less thick.

Yet another embodiment of a vacuum solar collector accor¬ ding to the invention is characterized in that the bottom cover is also substantially formed by the at least two interconnected wholly concave (when seen from the outside) plates, said plates each corresponding with a segment of a cylinder or of a sphere and each being supported, substan¬ tially at the location of junctions, by a supporting rib.

Yet another embodiment of a vacuum solar collector accor¬ ding to the invention is characterized in that initially a plate is flexible to bending and only corresponds with a segment of a cylinder or of a sphere under the influence of an atmospheric pressure in operating condition. Prior to operation such a concave plate may e.g. have a parabo- lically curved shape, but in operating condition - when the atmospheric pressure is present thereon - it will take a shape corresponding with a segment of a cylinder or of a sphere. Such a plate which is flexible to bending is e.g. made of a foil.

Yet another embodiment of a vacuum solar collector accor- ding to the invention is characterized in that the radius of a cylinder or of a sphere corresponding with said segment of a cylinder or of a sphere is smaller than either 5000 mm, or 1000 mm, or 250 mm.

Yet another embodiment of a vacuum solar collector accor- ding to the invention is characterized in that the top cover and/or the bottom cover is (are) substantially made of plastic material, whether or not reinforced with fibre. Said plastic material is e.g. a thermosetting plastic material (e.g. a polyester) , but preferably it will be a thermoplastic plastic material.

Yet another embodiment of a vacuum solar collector accor¬ ding to the invention is characterized in that the top cover and/or the bottom cover is (are) substantially made of a layer of fibres and a layer of plastic material adhered thereon by means of an adhesive, whereby either one of the layers or the adhesive has a small air permea¬ bility. Said small air permeability is very important, since it must be possible for the solar collector to operate under vacuum for a prolonged period of time.

The invention will be further explained with reference to figures illustrated in a drawing, in which:

figures la and lb show a longitudinal section and a cross- section respectively of a solar collector according to the invention, wherein the top cover and the bottom cover are formed of one wholly concave (when seen from the outside) plate;

figures 2a and 2b show a longitudinal section and a cross- section respectively of a solar collector according to the invention, wherein the top cover and the bottom cover are formed of three wholly concave (when seen from the outside) plates;

figures 2c and 2d show the part of the solar collector boxed by dotted lines in figure 2a, wherein the height of the sides of the solar collector is larger, casu quo smaller than that of supporting ribs;

figure 3 shows the solar collector of figure 2a, with an integrated storage vessel added thereto;

figure 4a shows a longitudinal section of a solar collector according to the invention, wherein the bottom cover is formed by the absorption plate and wherein the top cover

and the absorption plate are formed of three wholly concave (when seen from the outside) plates;

figure 4b shows a detail of the solar collector of figure 4a;

figure 5 shows the solar collector of figure 4a, with an integrated storage vessel added thereto;

figure 6 shows a detail of the solar collector of figure 5;

figure 7 shows a segment of a sphere, with which a concave (when seen from the outside) plate of a solar collector according to the invention may correspond;

figure 8a very diagrammatically shows a perspective view of a solar collector according to the invention; wherein the wholly concave (when seen from the outside) plates each correspond with the segment of a sphere according to figure 7;

figure 8b very diagrammatically shows a partial cross- section of the solar collector of figure 8a;

figure 9 very diagrammatically partly shows a longitudinal section of a solar collector according to the invention, wherein the bottom cover is formed by a bin of aluminium or steel.

Figures la and lb illustrate an absorption plate (l) , conductively connected with channels (2) , which are in communication with collecting channels (3, 4) for the supply casu quo discharge of a transfer medium. A transpa¬ rent top cover (6) and bottom cover (7) are formed of one wholly concave (when seen from the outside) plate, and are

interconnected via sides (5) , as a result of which a balancing of the forces generated by the atmospheric pressures acting on said covers (6, 7) is obtained. The wholly concave (when seen from the outside) plates corres- pond with a segment of a cylinder, i.e. a segment having a cross-section in the shape of a part of a circle. The sides (5) may be made of an insulating material in order to prevent loss of heat, and if necessary their design is as is illustrated in Figure 6. The configuration of the sides (8) is adapted to the radius of the cylinder shape of the covers (6, 7) and may e.g. be made by deforming a pipe of plastic material sawn through in longitudinal direction. Because the absorption plate (1) is loaded in longitudinal direction with tensile forces occurring in the top cover (6) and the bottom cover (7) , said absorption plate must preferably be sufficiently resistant to buck¬ ling.

Figures 2a and 2b show a solar collector corresponding with the one according to Figures la and lb wherein, however, both the top cover (6) and the bottom cover (7) are built up of three wholly concave plates corresponding with a segment of a cylinder. Said wholly concave plates are supported by supporting ribs (9, 10) respectively for the top cover (6) and the bottom cover (7) . Said ribs (9, 10) are supported opposite each other on the absorption plate (1) and are made of a material having a low heat conducting coefficient. If necessary slots may be locally provided in the absorption plate (1) , as a result of which the ribs (9, 10) are supported against one another.

Figures 2c and 2d show the part of the solar collector of Figure 2 which is boxed by dotted lines in said Figure, wherein the height of the side (5) is larger casu quo smaller than the height of a supporting rib (9) , without

the radius of curvature of the covers (6, 7) being diffe¬ rent thereby.

Figure 3 illustrates the same solar collector as the one of Figures 2a and 2b, with the understanding that now an integrated storage vessel (12) is added, whereby the collecting channels (3, 4) are connected with said vessel (12) . When the absorption plate (1) becomes hotter than the fluid in the storage vessel (12) a flow will occur as a result of a so-called therraσsiphoτr action-, and the vessel (12) will be heated. From said vessel (12) heat can be drawn by means of a heat exchanger or by means of ribs at the rear side, with which air is heated.

Figure 4a shows a solar collector according to the inven¬ tion, wherein the bottom cover (7) is formed by the absorption plate (1) , and wherein both the top cover (6) and the absorption plate (1) consist of three wholly concave (when seen from the outside) plates, said plates each again corresponding with a segment of a circle. The pressure forces caused by atmospheric pressure are taken up by the ribs (9) , whilst the rear side is provided with ribs (11) which serve to take up tensile forces and which must therefore be sufficiently resistant to buckling (see Figure 4b) . Said ribs (11) at the same time promote the transfer of heat.

Figure 5 shows the same solar collector as the one in

Figure 4a, with the understanding that now an integrated storage vessel (12) is added.

In connection with the aforementioned differences in expansion as a result of temperature changes the covers must preferably be flexible to bending. In accordance with the invention the top cover (6) and the bottom cover (7) are in that case made of a plastic material, whether or

not reinforced with fibre. The demands that are made are: sufficient resistance to tensile strain, sufficient resistance to creeping and a small air permeability. Moreover, the top cover (6) must be transparent. The cover may also be built up of several layers, which are glued together. For example, a mat of glass fibre or another transparent fibre may be glued on a transparent material or between two layers of a transparent material, such as PPMA, PC, Teflon etc. Said glueing process should prefera- bly take place under vacuu in order to prevent inclusions of air around the fibre. As regards reducing the velocity of diffusion one or more layers may be made of a material having a large resistance to diffusion. Also for the substance which is used as a filling agent of the fibres a composition may be selected having a large resistance to diffusion of air. If necessary the transparent cover may be provided with a transparent coating, which opposes diffusion. The above also applies to the bottom cover (7) , with this distinction that this cover does not have to be transparent. The bottom cover may also be provided with a reflecting layer at the inside and/or at the outside, with which reflecting layer diffusion is opposed and losses of heat at the bottom side are reduced.

The admissible velocity of diffusion is dependent on the degree of sub-atmospheric pressure in the solar collector and the volume of the solar collector that has been evacuated. Besides that a connection may be made with which the sub-atmospheric pressure can be brought up to the required level again from time to time.

Figure 7 illustrates a segment of a sphere, with which the concave (when seen from the outside) plates of the top cover (6) of a solar collector according to Figure 8a correspond. The wholly concave plates of the top cover (6) of Figure 8a corresponding with a segment of a sphere are

supported by supporting ribs (13) substantially at the location of (corner) junctions. In order to take up increased (tensile) strain occurring along edges of the plates of the top cover (6) having the shape of a segment of a sphere a network (14) of e.g. fibre-reinforced strips is provided along said edges.

Figure 8b very diagrammatically illustrates a partial cross-section of the solar collector of Figure 8a, wherein at the same time an absorption plate (1) and a channel for ^" the transfer medium are illustrated.

Figure 9 very diagrammatically partly shows a longitudi¬ nal section of a solar collector according to the inven¬ tion, wherein the bottom cover (7) is formed by a bin of aluminium or steel. On top of a rib (9) is a strip (15) .

It should be clear that, when speaking about the top cover of a solar collector according to the invention being substantially formed by at least one wholly concave plate having a cross-section in the shape of a segment of a circle, this shape does not exactly have to be that of a segment of a circle, but could represent deviations thereof to the maximum extent of 50 %, or 20 % or 5 %.