WO1983000916A1

WO1983000916A1 - Solar collector with convection suppression

Info

Publication number: WO1983000916A1
Application number: PCT/AU1982/000150
Authority: WO
Inventors: Scientific And Industrial Research ... Commonwealth
Original assignee: Symons, Jeffery, George; Peck, Malcolm, Kay
Priority date: 1981-09-08
Filing date: 1982-09-07
Publication date: 1983-03-17

Abstract

A flat plate solar collector (10) having a solar absorber panel (20) positioned within a container (12), the container having an opening opposite the panel which opening is closed by a transparent glass cover (18). Convection suppression means (25) is provided in the space between the cover (18) and the panel (20). The convection suppression means (25) comprises an array of thin closely spaced transparent slats (26) which array extends substantially across the full width of the panel (20). The slats (26) are oriented so that when the collector is viewed from above the slats extend with their lengthwise dimensions oriented substantially north-south.

Description

SOLAR COLLECTOR WITH CONVECTION SUPPRESSION

This invention relates to convection suppression means for use with flat plate solar energy collectors mounted at an angle to the horizontal.

It is standard practice to reduce heat losses from a solar absorber panel by placing' it in an insulated box with an open front which is covered by a sheet of glass spaced some distance from the panel. It is recognized, however, that significant thermal losses will still occur, because convection currents circulate between the front surface of the hot panel and the colder glass cover. It has been supposed that these convection currents circulate in the "up-down" direction (i.e. circulating about a horizontal or "cross-wise" axis) . Evacuation of the space between the panel and the cover glass has been proposed to suppress this convection flow,

"' but it is a most costly solution for large-area collectors.

United States Patent No. 4016483 teaches the use of horizontal slats, running cross-wise of the collector and positioned between the absorber panel and the cover glass, to stop the convection currents from circulating in the up-down direction between the panel and glass cover. Because the collector itself is in use oriented to face either north or south, the resultant orientation of the cross-wise -slats is generally east-west. Such slats provide some improvement in performance, but not nearly as much as can be achieved. United States Patent No. 4092977 teaches the further inhibition of the convection current by placing a cellular or honeycomb baffle between the cover glass and the panel. While this provides a marked improvement over the cross-wise slats, the honeycomb structure is difficult and expensive to fabricate from thin material transparent to solar radiation.

This invention is based upon the unexpected finding that a significant improvement in solar collector performance — over and above that provided by the cross-wise slats and at least equivalent to the honeycomb arrangement - can be cheaply and effectively obtained by a system of slats running in the up-down direction of the collector. These slats are thus, in use of the collector, oriented north-south. It could be reasonably expected that this orientation of slats would show no significant -Improvement over a collector with no convection suppression means, because the slats would not then stop the circulation of convection current up and down the panel, but merely divide the current into parallel segments. We have ascertained however that it is simplistic to treat the convection flow which in practice occurs in a collector without convection suppression means as being simply a flow in the up and down direction, as other often more significant flows occur. In particular, for angles of inclination of the collector commonly utilized, the convection flow occurs in "rolls" having axes predominately in the up-down direction, i.e. the flow is predominately cross-wise or east-west in the collector when positioned for use. It is believed that the good performance achieved in accordance with the invention arises in part by inhibiting modes of flow other than in the up-down direction, by direct barrier interposition, and in part by limiting up- down flow by generating viscous shear forces at the interfaces between the slat surfaces and gas between the slats.

More particularly, in one aspect the invention provides a flat plate solar energy collector for

» mounting at an angle inclined to the horizontal so that two opposed edges of a solar absorber panel thereof are then respectively at upper and lower locations, the collector further having a transparent cover arranged in spaced relation to the absorber panel, and convection suppression means located between the cover and the panel; said collector being characterised in that the convection suppression "means comprises thin closely spaced slats of trans¬ parent material extending between said opposed side edges of the absorber panel to form an array of slats which extends substantially across the full width of the panel. The invention also provides a flat plate solar energy collector mounted at an angle inclined to the horizontal so that two opposed side edges of an absorber panel thereof are respectively at upper and lower locations, the collector having a transparent cover arranged in substantially parallel spaced relationship to the absorber panel and having convection suppression means located between the cover and the panel; said collector being'characterised in that the convection suppression means comprises thin closely spaced slats of transparent material extending between said opposed side edges of the absorber panel to form an array of slats which extend substantially across the full width of the panel, said slats being oriented with their lengthwise dimensions generally north-south.

References to north-south directions throughout this specification, including the claims are to be understood as references to directions which are more aligned north-south than east-west.

The invention is further described with reference to the accompanying drawings in which:

Figure 1 is a partly sectioned perspective view of a solar collector constructed in accordance with the invention;

Figure 2 is a graph of efficiency of solar collectors constructed in accordance with the invention - and of a solar collector of conventional form;

Figure 3 is a fragmentary perspective view showing an exemplary manner of forming slats in the collector of Figure 1;

Figure 4 is a fragmentary diagrammatic cross- section of the collector of Figure 1;

OMPI Figure 5 is a cross-section like. igure 4, but showing a modification;

Figure 6 is a cross-section of the collector of

Figure 1 showing its disposition in use; and Figure 7 is a fragmentary plan view showing a modified method of forming slats in the collector of

Figure 1, and

Figure 8 is a cross section like that of Figure 4 but showing a still further modification. The collector 10 shown in Figure 1 has an outer container 12 which has interior insulation 14 along a rectangular base portion thereof. A space 16 is defined within container 12, this being closed at four sides by the container side wall and at the base by the insulation 14. The container 12 is open,-at the face thereof opposite insulation14 , and that face is closed by a transparent glass cover 18. A flat plate solar absorber panel 20 of conventional form is situated in space 16, resting on the insulation 14 to the bottom of "the container 12. . The absorber panel 20 has outlet and inlet header tubes 22, 24 respectively interconnected by riser tubes 27. The header tubes 22, 24 extend exteriorly of the container 12.

In order to permit clearance of moisture . condensing in the space between the absorber panel 20 and cover 18,. the peripheral wall of the collector bounding that space has, at the side thereof intended to be lowermost in use, a series of openings 31 therethrough. The absorber panel 20 extends in generally parallel spaced relationship beneath the cover 18.

O PI In the space 16, between the coyer 18 and absorber panel 20, there is positioned a convection suppression means 25 in the -form of an array of parallel slats 26. These extend with their longer cross-sectional dimensions, marked by the letter "D" in Figure 4, normal to the median planes of the cover 18 and absorber panel 20. Edges 26a of the slats remote from the absorber panel 20 are positioned in touching or closely adjacent relationship with the undersurface of the cover 18 whilst edges 26b ^"of.the slats remote from the cover 18, are positioned a distance, marked "X" in Figure 4, above the surface of the absorber panel 20.

In use, the solar collector 10 is positioned as illustrated by Figures 1 and 6. That is to say, the lengthwise directions of extent of the slats 26 are oriented in a substantially north-south direction and the planes of the cover 18 and absorber panel 20 are arranged at an angle to the horizontal. The last-mentioned angle, designated φ in Figure 6 is selected in accordance with the prevailing latitude of the location at which the collector is installed, but normally is in the range 10° to 70°. Of course, as is customary, the collector is also oriented so that it faces whichever of the northern or southern parts of the sky within which the sun moves, i.e. so as to face in a northerly direction if the collector is in the southern hemisphere and in a southerly direction if the collector is in the northern hemisphere. As shown, openings 31 are lowermost. The slats 26 are formed from.thin transparent material. As particularly shown in Figure 3, they may conveniently be formed by winding thin flexible film material such as 13 micron thickness FEP Teflon (tetrafluorethylene-hexafluoropropylene) strips so as to extend around pegs 30 positioned to either side of the absorber so that the material is tensioned between the pegs. An alternative form of winding such thin flexible film material about posts 30 to form the slats 26 is also shown in Figure 7.

The purpose of the slats 26 is to inhibit convective transfer of heat from the absorber back to the window 18; such heat transfer occurs by convection currents in the air in space 16.

Example 1 - Medium Temperature Collector

A commercial flat plate collector generally used for domestic water heating was modified by incorporation of slats in accordance with this invention. The collector had a single glass cover of window glass and a tube and fin copper absorber panel having a spectrally selective absorber surface. The collector was of approximately the following dimensions - side to side dimensions (in directions normal 'to and parallel to the directions of extent of the slats) : 1220mm x 1315mm rear insulation thickness: 50mm. distance between the absorber panel and cover: 25mm. number of slats: 240. Slat Material: FEP Teflon _ . _ thickness of slat material: 13 micrometer width of slats: 25 mm. spacing between slats 4 mm.

The collector was tested, with the slats oriented north-south, for instantaneous thermal efficiency in accordance with a procedure published in Australian Standard 2535 save that for tests at higher temperatures oil was used as the fluid medium^"to.be heated by the collector rather than water as specified in the Standard at temperatures up-to about 80°C. The results of the test are shown by plot "b" in Figure 2 which also shows the results at plot "c" of a corres¬ ponding test made on the collector before modification. The instantaneous thermal efficiency η is de ined as follows:-

where: F R is the collector heat removal factor

-2 -1 is the overall loss coefficient (W m

T. is the inlet temperature (°C) of liquid passing through the collector

T is the ambient temperature (°C)

(τ α) is the transmittance absorptance product and

-2 G is radiation level (W m ) .

OMP The term F ( τα ) is a measure of the-efficiency

R i under the condition that the inlet temperature is equal to ambient (i.e. _j_ = T_a) . That figure is represented in Figure 2 for the collector for and after modification by the intercepts of the plots "c" and "b" respectively with the ordinate of the graph. It will be observed that while_.F_R ( τα ) decreased from .69 to .68 due to the presence of the slats, the modified collector none the less exhibited improved efficiency over most of the plotted range for the variable T^ - T_a

More particularly heat loss coefficient product -?χ __ which is represented by the slope of the plots "b" and "c", was reduced from 5.00 to 4.05 W K by the provision of the slats. This indicates a substantial reduction in collector heat loss.

Example 2 - High Temperature Collector

A prototype high temperature flat plate collector was constructed comprising a low-iron glass cover with anti-reflection coatings, an absorber with chrome black selective surface and 100mm of back insulation. The absorber was approximately 1700mm wide (in the direction normal to the direction of extent of the slats) and approximately 1000mm wide in the direction parallel to the direction of extent of the slats. The absorber to cover spacing was 70mm whilst. slats were provided between the absorber panel and transparent cover. These were formed from 13 micrometer thick FEP

Teflon and formed from tape having a width of approximately 60mm. The spacing between slats was approximately 5mm. The instantaneous collector

5 efficiency η as defined above was measured at temperatures up to 150°C and is indicated by the line plot "a" in Figure 2. On the same basis as described above, the produc F (τ α ) was found to be 0.90 and the heat loss coefficient F_ ϋ_T was found to be

While, .as described, the slats 26 may be perpendicular to the plane of the cove_^r 18, it is possible to orient them otherwise such as is illustrated in Figure 5 where the upper edges 26a of

15adjacent slats 26 touch and the lower edges 26b of adjacent slats also touch. Thus, whilst in Figure 4, the volumes defined between the slats 26 are substantially rectangular whereas in Figure 5 the volumes are substantially triangular, as viewed in

20cross-section. Figure 8 shows a still further variation where the slats 26 are arranged generally parallel but inclined to the median plane of the cover, such as at the 70° angle shown. ,

Whilst the dimensions of the slats and their

25orientation may be varied, it is preferred that the dimension "D" be in the range of 3 to 15 times the distance between adjacent slats, most preferably about 6 times such distance. The gap designated by reference numeral "X" in Figure 4 may in practice be of

3Qthe order of 10mm. It is preferred that the gap "X" should not exceed 50% of the distance between the panel

20 and cover 18.

Although the described tetrafluoroethylene-hexafluoropropylene or FEP Teflon material has been found particularly satisfactory because of its good transparency, other materials, preferably those having high transparency may be employed.

The collector 10 is of a type having the header tubes 22, 24 parallel, for arrangement as shown in Figure 6 such that tube 22 is uppermost and tube 24 lowermost so that movement of fluid can be achieved with the collector through tube 24, thence^•upwardly through the riser tubes 27 and outwardly through the header tube 22, by natural circulation. Of course the liquid conveying tubes could be otherwise arranged.

OMPI

Claims

1. A flat plate solar energy collector for mounting at an angle inclined to the horizontal so that two opposed edges of a solar absorber panel thereof are then respectively at upper and lower locations, the collector further having a transparent cover arranged in spaced relation to the absorber panel, and convection suppression means located between the cover and the panel; said collector being characterised in that the convection suppression means comprises an array of thin closely spaced transparent slats, the slats extending between said opposed edges of the absorber panel, and the array extending substantially across the full width of the panel.

2. A flat plate solar energy collector as claimed in claim 1 having liquid outlet means for egress of moisture accumulating in said space, said liquid outlet means being positioned adjacent that one of said edges of said absorber panel which is in use at said lower location.

3. A flat plate solar energy collector as claimed in claim 1 or claim 2 wherein.said absorber panel includes tubes for passage of liquid therethrough to effect heating of the liquid in use of the collector, said tubes comprising a pair of opposed header tubes extending parallel to said edges, and one or more riser tubes extending between the header tubes so that said liquid in use flows from one said header tube through said or each said riser tube thence outwardly through the other header tube, said header tubes being transverse to the slats.

4. A flat plate solar energy collector mounted at an angle inclined to the horizontal so that two opposed side edges of an absorber panel thereof are respectively at upper and lower locations, the collector having a transparent cover arranged in spaced relationship to the absorber panel and having convection suppression means located between the cover and the panel; said collector being characterised in that the convection suppression means comprises thin closely spaced slats of transparent material extending between said opposed side edges of the absorber panel to form an array of slats which extends substantially across the full width of the panel, said slats being oriented with their lengthwise dimensions generally north-south.

5. A flat plate solar energy collector as claimed in any preceding claim in which said slats are formed from flexible elements held stretched under tension between supports along the said edges of the absorber panel.

6. A flat plate solar energy collector as claimed in claim 5 wherein said elements are formed from a tape and said supports comprise pegs at said opposed edges, the tape being wound around said pegs.

^■ $OREm

OMPI

7. A flat plate solar energy collector as claimed in any one of claim 4 to 6 wherein said angle is in the range 10 to 70 degrees.

8. A flat plate solar energy collector as claimed in any preceding claim wherein said slats are oriented with their longer cross-sect-ional dimensions substantially orthogonal to the median plane of the absorber panel.

9. A flat plate solar energy collector as claimed in any one of claims 1 to 7 wherein said slats are parallel and oriented with their longer cross-sectional dimensions at an angle to the absorber panel.

10. A flat plate solar energy collector as claimed in claim 8 or claim 9 wherein the longer cross-sectional dimensions of the slats are from 3 ^"to 15 times the distance betwee adjacent slats.

11. A solar energy collector as claimed in any one of claims 1 to 7 preceding claim wherein the slats are arranged with edges of adjacent pairs in contact or closely spaced so as to present a zig-zag configuration when viewed in section transverse to the lengthwise direction of the slats.

12. A flat plate solar energy collector as claimed in any preceding claim wherein lengthwise extending edges of the slats closest the transparent cover are

OMPI positioned in contact-with or immediately-adjacent to the transparent cover.

13. A flat plate solar energy collector as claimed in any preceding claim wherein lengthwise extending edges of the slats closest the absorber panel are immediately adjacent to the absorber panel but spaced therefrom.

14. A flat plate solar energy collector as claimed in any preceding claim wherein said slats are formed from tetrafluoroethylene hexafluoropropylene.

OMPI