US 20090013990 A1
The invention concerns a concentrating collector comprising a cover (1) transparent to the sun's rays, a cylindro-parabolic reflector (2), a coolant element (3) and a transformer (5) for reducing losses of energy by emissivity, said transformer (5) not being contacted with said coolant element (3).
1. A concentrating solar collector comprising a cover (1) that is transparent to solar rays, a reflector (2) of cylindro-parabolic cross section, said cover (1) and said reflector (2) defining a volume (7), and a heat-transfer element (3), characterised in that it further includes a transformer (5) for reducing the emissivity energy losses, said transformer (5) being covered with a substance having a high thermal emission, in that said transformer (5) absorbs some of the solar radiation and reemits heat into said volume (7) and in that said transformer (5) is not in contact with said heat-transfer element (3).
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The present invention relates to a concentrating solar collector.
The general operating principle of a solar collector is to capture solar radiation to transform it into heat or electricity.
There are two main types of collectors, “flat-plate” collectors and “concentrating” collectors. Flat-plate collectors transform the solar radiation into heat over the entire surface of the collector without concentrating it, whereas concentrating collectors concentrate the solar radiation at the focal point by way of a reflector or optical means using a magnifying glass effect.
Among concentrating solar collectors, the following distinction is made: on the one hand, there are collectors that use a reflector in the shape of a revolving parabola. These collectors concentrate the solar rays at the focal point of the parabola on a Stirling engine, an engine in which the heat is transformed into movement, a movement which is transformed into electricity by means of an alternator.
On the other hand, there are collectors which use a cylindro-parabolic reflector and which concentrate the solar rays onto a converter that enables the solar rays to be transformed into heat. The converter also comprises a tube in which a heat-transfer fluid circulates. This fluid will enable the stored heat to be gathered in a storage reservoir or to retrieve it via a heat exchanger.
The converter must have not only a high level of absorption of the entire spectrum of solar radiation, but also possess a reduced emissivity, i.e. reduced reemission of heat. In addition, in concentrating solar collectors, the surface of the converter is generally weaker than that of the opening of the collector.
Such concentrating solar collectors are well known. The converters used display either a rectangular cross section or a circular cross section, while the reflectors, for their part, can be hemi-cylindrical or cyclo-parabolic.
Documents EP 0 033 054, U.S. Pat. No. 3,321,012 and U.S. Pat. No. 4,024,852 describe solar collectors in which the converter displays a rectangular cross section. As described in document EP 0 35 033 054, the reflector can have the shape of a continuous curve composed of at least five arcs of a circle joining one another tangentially, but it can also have a hemi-cylindrical shape, as described in document U.S. Pat. No. 3,321,012, or else even a cyclo-parabolic shape as described in document U.S. Pat. No. 4,024,852.
Documents U.S. Pat. No. 4,059,094, AT 402 114 and AT 344 375 describe solar collectors in which the converter possesses a circular cross section. As described in documents AT 402 114 and AT 344 375, the reflector can have a hemi-cylindrical shape, or else even a cyclo-parabolic shape as described in document U.S. Pat. No. 4,059,094.
Different solutions have been suggested to improve the efficiency of concentrating solar collectors. Some of these solutions are based on the use of different materials to make the constituent elements of a collector in order to increase the yield of the transfer of solar energy into heat.
Other solutions, as described in documents AT 344 375 and AT 402 114, suggest maximising the quantity of solar radiation received by the collector through using an “absorber”, an auxiliary element that is an integral part of the converter. This absorber captures part of the indirect solar radiation and transfers the supplementary heat, thus absorbed, to the heat-transfer fluid.
Nevertheless, these solutions produce an energy yield that is still much too low and have the disadvantage of not limiting heat losses caused by a phenomenon of heat reemission, particularly at the level of the converter.
The present invention aims to supply a concentrating solar collector that does not present the disadvantages of the prior art.
It aims in particular to supply a concentrating solar collector with improved efficiency. It also aims to supply a solar collector, which can be used in the form of solar panels and which can, in addition, be used to generate electricity, heating, air-conditioning or water desalination.
The expressions “collector”, “cell”, “captor” are used interchangeably in the following account of the invention.
The present invention describes a concentrating solar collector comprising a cover that is transparent to solar rays, a reflector with a cylindro-parabolic cross section, a heat-transfer element and a transformer reducing energy losses through emissivity, said transformer not being in contact with said heat-transfer element.
According to particular embodiments, the invention includes one or several of the following characteristic features:
The present invention in addition describes the use of the solar collector in the form of solar panels.
The present invention also describes the use of the solar collector to generate electricity or heating or air-conditioning or water desalination.
In the concentrating solar collector according to the invention, the solar radiation is concentrated on a heat-transfer element 3 by the intermediary of a longitudinal reflector with a cylindro-parabolic cross section 2.
The solar radiation received by the heat-transfer element 3 is absorbed, however, part of this absorbed energy is reemitted into the volume 7. This reemitted energy can pass through the cover 1, in which case it will be definitely lost, or it can be absorbed by the cover 1 of the solar collector.
The originality of the present invention lies in the use of a transformer 5 enabling part of the solar radiation, preferably the indirect solar radiation, to be used to reduce the energy losses through emissivity and to limit the quantity of radiant energy.
“Radiant energy” means energy emitted during the natural phenomenon called thermal emissivity, a phenomenon during which any material hit by the solar radiation emits an infra-red radiation by cooling down. Thus, radiant energy comprises the energy reemitted by the elements that make up the solar collector, in particular through the heat-transfer element.
The solar collector according to the invention comprises a cover 1, a reflector with a cylindro-parabolic cross section 2, a heat-transfer element 3 as well as a transformer 5.
According to a particular embodiment of the invention, the assembly of the reflector 2 and the heat-transfer element 3 is supported on, or included in, a layer of a thermal insulator 6 resting in a cradle 8 or a frame, on which the cover 1 rests (
The cover 1 is preferably made of a material that is transparent to solar rays, which has a weak degree of reflection and a high transmission. Advantageously, the material is a transparent synthetic material or a solar glass.
“Solar glass” means a glass which has a low iron content and the surface of which is treated so as to reduce the reflection effects. The surface treatment can be achieved through the application of a layer, or film, of Teflon.
Advantageously, the interior surface of the cover 1 can be covered with a substance reflecting the infra-red rays.
The reflector 2 has a cylindro-parabolic cross section and a longitudinal shape. It enables the concentration of the solar radiation to be obtained on the heat-transfer element 3, which is situated at the focal point of the parabola.
The reflector 2 has a highly reflective surface. It preferably has the characteristics of a mirror. With this aim in view, the reflector 2 can be made of a rigid material, for instance glass, on which a layer of a reflecting substance is applied.
Preferably the reflector 2 is made of aluminium.
The heat-transfer element 3, in which a heat-transfer fluid circulates, preferably has a rectangular cross section, a circular cross section, an oval cross section or a square shape. It can be a metal tube or a tube made out of a plastic material resistant to the operating temperatures of the solar collector. Said heat-transfer element is preferably made of copper or aluminium.
The surface of the heat-transfer element 3 is preferably covered, entirely or partially, with a layer of a material with a high absorption of the solar spectrum and a low thermal emission. However, in the case of a partial covering of said heat-transfer element 3, the substance with a high absorption of the solar spectrum and a low thermal emission will have to cover the surface of said element 3, which is turned towards the opening of the parabola.
The heat-transfer element 3 contains a heat-transfer gas or liquid. The heat-transfer gas can be air. The heat-transfer liquid can be water, with or without additive, or, for operating temperatures above 200° C., said fluid can be a stable oil, which does not disintegrate at high temperatures.
Advantageously, the heat-transfer element 3 has the characteristics of a heat pipe.
A heat pipe is a hollow tube containing a heat-transfer fluid under vacuum, which is chosen in function of its condensation/vaporisation temperature. The heat pipe enables the heat to be standardised and displaced as rapidly as possible, thanks to a phase-changing phenomenon. The solar energy concentrated by the reflector within the converter enables the heat-transfer fluid to be vaporised. This energy is then retrieved at the level of a colder zone, in a heat exchanger, for instance, through the condensation of the heat-transfer fluid.
Preferably, the heat-transfer element 3 is arranged inside a vacuum tube 4. Said tube 4 enables energy losses of the heat-transfer element 3 to be limited.
Advantageously, said vacuum tube 4 is made of a transparent material, preferably a solar glass or a borosilicate glass. Said tube 4 can be covered with a layer of a substance that has anti-reflection properties.
The thermal insulation of the solar collector according to the invention is ensured, on the one hand, by the presence of the cover 1 and, on the other hand, by a thermal insulator 6.
The insulator 6 is preferably a polyurethane or polystyrene foam, rock fibre or glass fibre.
The transformer 5 enables the losses of heat due to the emissivity of the heat-transfer element 3 to be diminished and thus plays a role in the insulation of the solar collector by creating a thermal barrier against the infra-red reemissions of the heat-transfer element 3. By capturing part of the solar radiation, the transformer 5 heats up and reemits radiant energy which is transferred to the volume 7.
The transformer 5 is not in contact with the heat-transfer element 3, neither directly nor indirectly. There is no solid-solid contact between said transformer 5 and said heat-transfer element 3. In the embodiment in which the heat-transfer element 3 is arranged inside a vacuum tube 4, the transformer 5 can be in contact with said tube 4 provided that it is not in contact with the heat-transfer element 3.
The transformer 5 is fixed to the cover 1, and/or the reflector 2, and/or the vacuum tube 4. Said transformer 5 is situated in the plane of symmetry of the cross section of the reflector, along the entire length of the solar collector. More precisely, if the heat-transfer element 3, which is located at the focal point of the cylindro-parabolic reflector 2, is situated at the bottom of the parabola, as represented in
The transformer 5 is a good heat conductor and preferably a carbon or silica plate, or else a metal or metal alloy plate or strip. Said metal can for instance be copper, aluminium, silver, gold, tungsten, brass, tin, or an alloy of one or several of the metals cited above.
In a preferred embodiment of the invention, the transformer 5 is made of any material which is covered by a substance with a high thermal emission. Said substance can also have a high absorption of the solar spectrum and a high thermal emission. This can be any darkish substance, preferably of a metallic nature. It can be carbon in any of its forms, soot or graphite for instance, or silica. It can be a metal such as copper, aluminium, anodised aluminium, silver, gold, tungsten, brass, tin, or an alloy of one of several of the metals cited above.
The transformer 5 preferably has a rectangular shape.
Advantageously, the transformer 5 can further comprise lamellae that can be interlinked (
The volume 7 corresponds to the space existing between the reflector and the cover of the collector. Said volume 7 contains a gas or a fluid, capable of absorbing the infra-red radiation.
Preferably, the gas filling said volume 7 is carbon dioxide or air enriched with carbon dioxide or enriched with steam. This could also be one or several CFCs, methane or sulphur hexafluoride.
The pressure of the gas of the volume 7 can be inferior, equal or superior to the atmospheric pressure. To this effect, the solar collector can in addition comprise an opening enabling the fluid to be injected into or evacuated from the volume 7.
The cradle 8, on which the assembly of the cover 1, the reflector 2 and the heat-transfer element 3 is supported, is preferably made of a metal, aluminium for instance, or of wood, or a composite material, or else even a combination of these materials.
The solar collector according to the invention advantageously comprises at least one sun tracking means. Said means enables the course of the sun to be tracked and for the position of the collector to be adjusted in all directions in order to obtain an optimal solar exposure, i.e. in order to enable the collector to adopt an ideal position with regard to the sun with a view to capturing the maximum of energy. This is achieved by mechanical or hydraulic means.
The solar collector according to the invention is preferably used in the form of solar panels (
The solar collector according to the invention is preferably used for the generation of electricity, heating, air-conditioning or for water desalination.