GB2117109A

GB2117109A - Solar collector

Info

Publication number: GB2117109A
Application number: GB08307326A
Authority: GB
Inventors: Paul Vandeplas; Lucien Kinnaer; Simonne Heusdains
Original assignee: Centre dEtude de lEnergie Nucleaire CEN
Current assignee: Centre dEtude de lEnergie Nucleaire CEN
Priority date: 1982-03-17
Filing date: 1983-03-16
Publication date: 1983-10-05
Also published as: GB8307326D0; NL8300793A; DE3309252C2; LU84694A1; GB2117109B; DE3309252A1

Abstract

The solar collector comprises a metal absorbing plate 1 which is coated on both sides with a selective- absorbing layer having an absorption coefficient higher than 0.9 and an emissivity lower than 0.1. The plate 1 has an inlet 11 and an outlet for heat- transfer fluid, e.g. water. A housing 2 surrounds the absorbing plate 1 and is filled with a gas having a heat conduction coefficient of 0.015 W/m DEG C. At the most under a pressure lower than the atmospheric pressure. Said housing 2 comprises a frame 3, two translucid panels 4 mounted thereon and spacers 20 between the panels 4 going through openings provided in the plate 1. <IMAGE>

Description

SPECIFICATION Sun collector This invention relates to a sun collector which comprises an absorbing plate, which absorbing plate is provided on the one side at least with a light-absorbing layer, is provided inwardly with at least one flow channel for a heat-transfer fluid and has an inlet and an outlet for said fluid which are connected together by said flow channel, which sun collector further comprises a housing which surrounds the absorbing plate, is partly at least translucent and is filled with a heatinsulating gas.

A sun collector of this kind is described in FR A-2 243 208. The heat-insulating gas does however lie under atmospheric pressure.

This is also the case with the sun collector according to DE-A-2 610 370.

An object of the invention is to provide a sun collector of the above-defined type which has a higher efficiency than said known collectors.

For this purpose, said housing is filled with a gas having a heat conduction coefficient of 0.015 W/m OC at the most under a pressure lower than the atmospheric pressure.

In a particular embodiment of the invention, the gas pressure lies in the range from 10 to 100 millibar.

In a preferred embodiment of the invention, said absorbing plate is coated on both sides thereof with a selective-absorbing layer, said housing is made on either side at least locally from translucent material, and heat-insulating gas under a lower pressure than the atmosphericRressure lies on either side of the absorbing plate between said plate and housing.

Said known sun collectors have a one-sided working. The absorbing plate is covered on the one side with insulating material and the heatinsulating gas lies but on the other side which is provided with a thin heat-insulating layer.

Naturally with a double-sided working, the sun collector may be subjected in the presence of white reflectors or mirror reflectors, to a 50 to 80% high exposure, as compared to a singlesided working collector, both with diffuse light and with direct lighting.

Sun collectors with a double-sided working absorbing plate are known per se, notably from FR-A-2 357 832 (embodiments according to figures 10 and 1 1). The collectors are not however filled with a heat-insulating gas under reduced pressure, which is required to impart to a double-sided working sun collector a higher output than a one-sided working collector.

Other details and advantages of the invention will stand out from the following description of a sun collector according to the invention; this description is only given by way of example and does not limit the invention; the reference numerals pertain to the accompanying drawings.

Figure 1 is a side view of a sun collector according to the invention.

Figure 2 shows a cross-section along line Il-Il in figure 1.

Figure 3 is a front view of the sun collector as shown in figures 1-2, but drawn on a smaller scale and with parts broken-away.

Figure 4 is a cross-section along line IV-lV in figure 3, drawn on a larger scale.

Figure 5 is a cross-section along line V-V in figure 3, drawn on the same scale as figure 4.

In the various figures, the same reference numeals pertain to similar elements.

The sun collector according to figures is comprised of an absorbing plate 1 and a housing 2 therefor.

Said housing 2 is mainly formed by a rectangular frame 3 and two tempered glass panels 4. Said frame 3 is comprised of four mildsteel omega-shaped sections which are facing outwards with opening thereof, and which are soldered together in the corners. Said sections lie with the outer-facing edges thereof opposite the side walls of said panels 4. Said panels 4 are fixedly air-tight glued to frame 3 by means of thiokol and silicone rubber 5.

Said housing 2 is arranged vertically, with the lengthwise direction thereof lying horizontally above a bottom, for example a roof. Said bottom and roof are preferably coated with a reflecting material and next to said sun collector, reflectors may be arranged on said bottom in such a way that said sun collector will receive as much sun light as possible. Said housing 2 is provided on the outer side of the two top corners of the frame 3 thereof with two welded lugs 6. Said lugs 6 are used for gripping the collector as same is moved, for connecting electrodes when electrochemically treating the absorbing plate 1 mounted inside the frame 3 during manufacturing of the collector and for securing said collector, for example to a vertical wall.

On the inner side of frame 3, supports 7 for the absorbing plate 1 are welded in various locations.

Said supports 7 are angle-irons the angle rib of which lies at right angle to the panels 4. With the edges thereof in parallel relationship with said rib, the angle-irons 7 are welded to the inner side of frame 3; the angle thereof is thus facing inwards.

Said angle is crossed by a groove 8 wherein fits an edge of said absorbing plate 1.

In the bottom horizontal section of said frame 3 are further provided two openings 9 and 10 respectively for an inlet 11 and an outlet 12 of said absorbing plate 1.

Said absorbing plate is a flat radiator plate from mild steel or copper which is coated on either side with a selective-absorbing layer having a high absorption coefficient and a low emissivity. The absorbing plate is formed by two rectangular panels 1 3 which are welded together with their edges 14 along the whole circumference thereof. Between the lengthwise edges 14, depressions 1 5 running along the cross-wise direction are provided in the panels 13, said panels 13 being also welded together in the location thereof. In this way, there is formed inside the absorbing plate 1, a feeding space 16 at the top and a discharge space 17 at the bottom, which spaces 16 and 17 communicate through channels 18 which are formed between said depressions 15. With the welded edges 14 thereof said absorbing plate 1 fits into the supports 7.With heat expansion and contraction, the plate 1 is alternately slidable with the edges thereof in the supports 7. Said inlet 11 opens on the feed space 1 6 and runs inside the housing 2, along the absorbing plate 1 upwards from opening 9. In the location of said opening 9, the inlet 11 is provided with a connector 1 9 for a line belonging to a sun-power equipment. The outlet 12 extends between said discharge space 17 and opening 10 and it is also provided in the location of said opening 10 with a connector 1 9 for connecting said outlet 12 to the line of a sunpower equipment. Said connectors 19 are welded to said frame 3, in such a way that they insure a tight sealing of said openings 9 and 10.Said connectors are used as electric contacts during said electro-chemical treatment of the absorbing plate 1.

The selective-absorbing layer the absorbing plate 1 is coated with, is dependent on that metal the panels 13 are made of. Said layer preferably has an absorption coefficient which is higher than 0.9 and an emissivity lower than 0.1. Said layer should be heat-resistant up to temperatures about 3000C. The panels 13 may be made from steel, in which case a suitable absorbing coating is formed by a copper layer which has been subjected to a light chemical oxidizing to flat black CuO2, by zinc chromate oxide over zinc, or by chromium oxide over nickel or copper. The panels 13 may be made from copper, in which case a suitable absorbing layer is a layer from chromium oxide or copper oxide.

The space between the absorbing plate 1 and the housing 2 is a completely tight closed space.

Said space is filled with a gas having a heatconduction coefficient of 0.015 W/m OC under a pressure which is markedly lower than the atmospheric pressure. The gas pressure lies for example between 10 and 100 millibar, and it is preferably about 20 millibar. Suitable gases are: krypton, xenon, hydrocarbon compounds such as C6H6, C6H12, and chlorinated hydrocarbons such as CHCl3,freon and Cm14.

To prevent distortion and even breaking of the glass panels 4 due to the underpressure prevailing inside said housing 2, said panels 4 are retained spaced from one another in said frame 3 by means of spacers 20.

Said spacers 20 are formed by a center body which is of narrowing-expanding shape and by two conical feet connecting thereto, which bear with the great round base thereof against the glass panels 4. Said spacers 20 pass freely with the body thereof through openings 21 in the absorbing plate 1. Said openings 21 lie in the location of a depression 15, which depression is slightly widened in the location of an opening 21.

Consequently said openings 21 do not open on the inner side, that is on the channels 1 8 of said absorbing plate 1. The openings 21 and thus also the spacers 20 arm arranged according to a square pattern. The number of openings 21 and the diameter of the foot bottom of said spacers 20 are dependent on the thickness of said glass panels 4. The cone-shaped feet of the spacers 20 are made from transparent heat-resistant polymethyl metacrylate, polycarbonate or glass.

The center body thereof is made from steel. Said spacers 20 are retained in position by the glass panels 4 themselves which are pressed together by the higher pressure prevailing on the outer side of said housing 2.

Said openings 21 have a diameter which is larger than the cross-section of that portion passing therethrough of said spacers 20, and actually in such a proportion that said spacers do not contact in any way said absorbing plate 1, and the absorbing plate 1 can also expand freely under the action of the temperature changes.

Such expanding may be quite large as the absorbing plate temperature may reach about 3000C under maximum sun radiation and without cooling of said plate. Due to said absorbing plate 1 fitting with the edges thereof with some play into the grooves 8 of said supports 7 and thus being somewhat slidable relative to said supports 7, said supports 7 do not hamper either such expanding.

The sun collector is suitable for the usual heattransfer fluids such as water. Inside the feed space 16, a distributing device connecting to said inlet 11 may be mounted. Due to the sun collector being arranged vertically and the absorbing plate ; being coated on both sides with an absorbing layer, the collector receives relatively much sun power as well with direct radiating as with diffuse light. Said power amount may be further increased by arranging the sun collector at right angle on a white reflector or by directing reflectors towards said sun collector.

Due to the space between said absorbing plate 1 and housing 2 being filled with a gas having a low heat-conduction coefficient and under low pressure, and due to the absorbing plate 1 having substantially no contact with said housing 2, the heat transfer between said absorbing plate 1 and housing 2 is exceedingly low. The efficiency of the sun collector is thereby also very high at high working temperatures. For instance, when using a mild steel absorbing plate which is nickel-plated and provided with a chromium-oxide layer, and which has an absorption coefficient of 0.94 and an emissivity of 0.1, and with a space between said absorbing plate 1 and housing 2 filled with CHCI3 at 20 millibar, the heat-transfer coefficient between said absorbing plate 1 and surrounding air is but 0.9 watt per m2 housing and per OC, which corresponds to a total heat-loss coefficient of 2 watt per m2 absorbing plate and per OC.

It is clear that the sun collector can be used also one-sided, for example on slanting roofs. In such a case the heat losses may be lowered by arranging between the unused sides of the absorbing plate 1 and the glass panel lying opposite, an aluminium film and/or a glass wool mat.

The invention is in no way limited to the abovedescribed embodiment and within the scope of the patent application, many changes may be brought to the described embodiment, notably as regards the shape, the composition, the arrangement and the number of the components being used for embodying the invention.

Claims

1. Sun collector comprising an absorbing plate, which absorbing plate is provided on the one side at least with a light-absorbing layer, is provided inwardly with at least one flow channel for a heattransfer fluid and has an inlet and an outlet for said fluid which are connected together by said flow channel, which sun collector further comprises a housing which surrounds the absorbing plate, is partly at least translucent and is filled with a heat-insulating gas, in which said housing is filled with a gas having a heat conduction coefficient of 0.015 W/m C at the most under a pressure lower than the atmospheric pressure.

2. Sun collector as defined in claim 1, in which the gas pressure lies in the range from 10 to 100 millibar.

3. Sun collector as defined in claim 2, in which the gas pressure is about 20 millibar.

4. Sun collector as defined in any one of claims 1-3, in which said absorbing plate is coated on both sides thereof with a selective-absorbing layer, said housing is made on either side at least locally from translucent material, and heatinsulating gas under a lower pressure than the atmospheric pressure lies on either side of the absorbing plate between said plate and housing.

5. Sun collector as defined in claim 4, in which the housing comprises a frame, two translucid panels mounted thereon lying next to one another, which determine together with the frame a tighly closed space, and spacers which retain both said translucid panels spaced from one another, while the absorbing plate is provided with openings lying completely next to the flow channels thereof for said spacers, openings through which said spacers extend without any contact with said absorbing plate.

6. Sun collector as defined in any one of claims 1-5, in which said absorbing layer has an absorption coefficient which is higher than 0.9 and an emissivity which is lower than 0.1.

7. Sun collector as defined in claim 6, in which said absorbing plate is made from steel and the absorbing layer is comprised of one of the following combinations: copper oxide over copper, zinc chromate oxide over zinc, chromium oxide over nickel, and chromium oxide over copper.

8. Sun collector as defined in claim 6, in which said absorbing plate is made from copper and the absorbing layer is a layer from chromium oxide or copper oxide.

9. Sun collector as defined in any one of claims 1-8, in which said absorbing plate is so mounted as to be free for heat expansion, in said housing.

10. A sun collector substantially as hereinbefore described with reference to the drawings.