CA1102194A - Solar energy collection apparatus - Google Patents

Abstract

An evacuated, double wall, tubular solar energy collector having a concave, specular reflecting surface corresponding to a segment of a cylinder positioned at the opposite side of the collector tube from the sun. The reflector is detachably connected to the tube and the tube is engaged by raised spacing points on the reflector to provide the proper spaced relation between them and allow for water drainage along the reflector. Plural units of the collector tube and cylindricallyshaped reflector combination are supported along opposite sides of an elongated manifold for circulation of an energy absorbing media through them. This system improves the efficiency of total energy collection without tracking the sun or focusing the reflected radiation. The tubes are no more than three diameters apart and the cylindrical reflector has its focal line within the absorbing area of the collector tube. The radius of curvature of the reflector surface exceeds the radius of the tube and is defined as a function of the tube diameter and tube spacing.

Description

.94 SOLAR ENERGY COLLECTION APPARATUS

The present invention relates to solar energy collectors.
~ore specifically, the invention provides an improved, efficient tubular, evacuated collector system for providing solar energy as heat in a media for heating, cooling or other purposes.
BACKGROUND OF THE INVENTION
In the Canaaian Patent of co-inventor George R. Mather, Jr., 1,039,132 there is disclosed an array of similar multiple glass solar energy collector tubes connected in a manifold and supported parallel to each other between the sun and a difEuse planar reflecting surface. The tubes are parallel to the reflecting surface and spaced a distance no more than four times the tube outside diameters of the collector tubes from the planar diffuse reflector and spaced apart center-to-center of the tubes a distance no more than four times such outside diameter dimension. Appro~imately half the sunlight falling between the collector tubes and reaching the diffuse reflecting surface is reflected to the tube undersides~

Comparing this array to an array of close packed tubes, the spaced array contains half as much hardware (tubes, etc.) but delivers about the same amount of energy under most operating conditions. Inasmuch as the diffuse reflecting surface of planar design is significantly less expensive than the collector tubes eliminated in the spacing recommended, a highly cost-effective collector system is achieved.
However, this diffuse reflector array has solar intercept efficiencies in the range of 50-60%, dep~nding upon the time of solar day, because approximately half the light incident to the reflecting surface is not reflected to the ~z~9~

lower surface of the paralleI collector tubes. If this ligh* is collected, the resultant intercept efficiency would approach 80%. Additionally, an increase in tube spacing of the array would lead to even lower effective loss coefficients and therefore to further improvements in performance at higher temperatures.
SUMMARY OF THE INVENTION
In the present invention, a non-imaging, cylindrically-contoured, specular reflector is used to collect light other-wise lost in the prior diffuse reflector system. Reflectordesign includes two important considerations" (a) the contoured reflector must collect diffuse as well as beam light efficiently, and (b) the reflector must not require tracking of the sum during the solar day-.
It is therefore an important object of the invention to provide individual non-imaging optical specular reflectors contoured with respect to the tubes and supported at their underside ~away from the sun) of the tube in a precise spacing.

This contoured concave reflector is substantially a cylindrical segment and extends along the length of the absorbing surface in the collector tube. This cylindrical segment reflector has a focal line which does not in general coincide with the tube axis.
A further object of the invention is to provide a cost effectiveness of the special reflector, just described, which will be justified in view of the gain in efficiency of the system.

Another object of the invention is to provide a solar tubular collector system in which extra total light is l~Z~9~

received on the collectors ~rith an improvement in effective loss coefficient, the combinatïon of tube spacing and reflectors of the ;nvention enhance the overall efficiency of operation of the evacuated tubular collectors in total collection of solar energy without sun tracking. While good performance can be achieved with the diffuse planar reflector of the earlier-mentioned Canadian Patent 1,039,132 significant improvements result in the present invention by use of non-imaging specular contoured reflectors.
lQ Best performance is obtained in the preferr~d form of the invention in limiting the relationship of tube spacings on their centers d to the outer diameter of the cover tube of the collector D in the range of a ratio of d/D of slightly more than 1.0 to 3.0 depending upon operating temperature.
Costs tend to decrease fairly rapidly of the hardware in the system with increased tube spacing dimensions d in the array since greater spacings result in less tube hard-ware per effective square foot of collection area. Judging collector cost effectiveness on the basis of cost per unit of energy produced, the most effective design in the use of the invention is at spacings of d/D of about 1.4 to 2.3.
Comparison of the planar, diffuse reflector and the specular cylinder segment reflector on cost does result in energy efficiency increase with the present invention. Overall impact on collector cost effectiveness results in a trade-off of extra cost in the apparatus directly against the enhanced energy output of the collector. At the wider spacings, the extra cost of the reflectors can be counter balanced by a combination of benefits in the enhanced output of energy and lower collector hardware cost per unit of area. In addition, the non-imaging character of the reflector favors non-critical l~,r~

optics of the system such that minor imperfections in the reflector contour can be tolerated with no apprecia~le sacrifice of performance.
As used herein, the term "non-imaging" is intended to means properties o a reflector surface which does not depend upon optical focusing.
The critical design parameters of the apparatus in the invention are determined under the following:
Where: d is center-to-center spacing between adjacent tubular collectors, D is the outer diameter of the cover tube of the collector, and R is the radius of the cylindrically-segmented, contoured reflector.
- R= d

2 ~
The geometric center line or axis of the cylindrically-segmented reflector is generally located along a line extending ; vertically abov~ the center axis of the collector tube by 2Q the distance RD/d. Coincidentally, at d/D o~ 2, the focal line of the reflector coincides with the center axis of the collector. From these design equations the reflector desired for each tube spacing arrangement d and cover tube size D may be determined. It is preferred to place the reflector about the lower part of the tube in a psuedo nesting relationship, the reflector being suspended in its support from the tube.
The reflector is provided with raised, projected (convex~
support points along the reflector surface to space the cover tube and concave reflecting surface a desired distance apart.
The cover tube surface of the collector is thereby also spaced from the concave surface of the reflector to allow for drainage in an installation and prevent ice and d;rt from ~ l~tr~,g~L

accumulating between the two surfaces.
In practice there is an optimum spacing between the reflector and the cover tube surfaces for each center-to-center spacing d and cover tube diameter D.
Thus, in accordance with the present teachings, the solar energy collector apparatus is provided which comprises an elongated manifold, plural elongated tubular solar energy collectors connected to the manifold disposed in parallel, substantially e~ually spaced relationship and depending laterally along one longitudinal side of the manifold with individual elongated concave reflector elements for the collectors, the reflector elements being preshaped from sheet of relatively rigid material and having a light reflecting surface over the concave area of the reflector. Means are provided along the longitudinal edges of adjacent reflectors engaging the latter and interconnecting the adjacent reflectors to each other with spacing means interposed between the reflector surface and the exterior surface of the collector maintaining the two surfaces in spaced relationship, and fastening means provided for suspending a reflector about each collector and in spaced relationship therewith maintained by the spacing means.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a perspective view of the reflectors of the invention assembled on adjacent collector tubes shown in phantom outline, EIGURE 2 is a side elevation, partly broken away, of a collector module incorporating the present invention, FIGURE 3 is an end sectional view of one of the collector tubes taken along line 3-3 on Fig. 1, FIGURE 4 is an enlarged perspective view of a portion of adjacent reflectors and cooperating integral fasteners for connecting them edgewise to each other, FIGURE 5 is a schematic end elevational view of the invention illustrating its operational principles, FIGURE 6 is a chart of operating efficiency of the invention at dif:Eerent operating temperatures over a range of tube spacings and size ratios d/D, FIGURE 7 is a chart of the d/D tube spacing/size relationships versus the distance F between tuhe axis and focal line of the reflector divided by tube diameter D, and FIGURE 8 is a chart of the efficiencies of the energy collection versus the solar energy radiated in the plane of the collectors, the curves comparing performance of the present invention and the diffuse reflector system of the aforementioned Canadian Patent 1,039,132.
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 illustrates a pair of generally cylindrically-shaped, sheek metal reflectors 10 assembled side by side in -6a B

~2~

edgewise connected relationship a~out the lower side of two adjacent glass collector tuhes 11 (sfiown in phantom outline~
as they are a part of the evacuated tubular solar collector apparatus, such as is d;sclosed in U.S. Patent No. 3,952,724 and in Canadian Serial N~o. 259,044, filed August 13, 1976 which is common in ownership with this application. The reflectors lO are shaped of a relatively rigid material, such as a sheet metal. The preferred example is a reflector 10 formed of sheet aluminum that is anodized on the concave surface with high reflectance aluminum to provide a specular (mirror~
finish. The reflector may be made of plastic or organic material, fiberglas or the like and anodized, plated or evaporated with a specular, reflecting coating having a specular reflectance of about 0.85 or higher. The specular surface may be formed by other means; however r the important aspect in such a selection is in the cost effectiveness and from the standpoint of cost and durability, the aluminum sheet and anodized aluminum reflecting surface thereon is the preferred embodiment. The reflector 10 is shaped in its concavity to define a segment of a cylinder having a radius of curvature R (Fig. 5) generated from a line that is along the point Y. In its angular displacement, the cylindrical reflecting surface is an arc segment, in section, that is less than a semi-circle.
Referring to Figs. 2 and 3, the cylindrical reflector 10 is disposed (slung) on the underside of the glass collector tube apparatus, indicated generally at 11, which comprises a transparent glass cover tube 13 and a glass absorber tube 14 of less O.D. than the I.D. of tube 13. The outer surface of the absorber tube 14 is coated with a wave length selective coating 15 to a~sorb solar energy striking that surface.

The interior chamber 12 of the ahsorber tube 14 provides access for an energy absorbing media which may be circulated by a means 12a for exchange o~ the energy to the media and circulating the ener~y laden media in the system for utilizing the collected solar energy. The annular space 16 between the inner surface of cover tube 13 and the coated outer surface o~ tube 14 is evacuated, preferably to a hard vacuum on the order of 10 4 torr. The vacuum in annular space 16 reduces and virtually eliminates conduction and convection losses from the collector. The collector tube 11 is constructed such that the cover tube 13 and absorber tube 14 each have a closed end and an open end.
The open ends of the tubes 13 and 14 are matched and the wall of one of these tubes is hermetically sealed by a glass-to-glass fusion seal with the wall of the other, thus closing the annular space 16. ~he vacuum is pumped through a tubulation at the other closed end of the cover tube and tipped off and fused closed in conventional fashion. The open end of the double-walled tube assembly provides access to the interior of the absorber tube 14. This open end is inserted into a receiving port or receptacle along the side of a manifold 17, such manifold being disclosed in detail in Serial No. 259,044 but shown only in end view on Fig. 2.
The module of the collector apparatus includes multiple collector tubes 11 connected at opposite sides of the manifold and equally spaced apart along its length. Manifold 17 is mounted on a supporting surface 18, such as a sloped roo~
or the like, which parallels the plane of the collectors selected to provide an advantageous solar exposure of the tubes to solar radiation. The system is non tracking.

Manifold 17 includes plural support standards 19 spaced along its length providing for a spacing of the apparatus from surface 18 to allow- for drainage of rain and snow or the like. The outer closed ends of tubes 11 protrude through an apertured bracket 20 and an end support cap 21 is held against the end of each tube 11 and fastened onto bracket 20 by a bayonet fastening means 23. The end support cap 21 includes axial ribs 22 which together with means 23 retain it in the aperture of the bracket.
The reflector 10 is supported along the underside of the cover tube 13, best shown on Fig. 3, in horizontal position and spaced by the upwardly protruding spacing points 24 formed in spaced array along the reflector (see Fig. 1).
Points 24 are made in pairs, at least in 2 locations along the reflector for stability reasons; however, a single spacing point at two locations will provide suitable spacing for drainage. These raised points 24 on the reflector allow for drainage along the reflector underneath the cover tube, thus ice and snow or water will not accumulate. It is preferable 20 the reflector be mounted on the support structure 18 through placement on the bracket-standard arrangement such that the reflector surface has a pi tch for drainage~ The reflector 10 is held on its collector tube 11 by thin, hanger straps or wires 25 which extend from underneath and through an aperture 26 in khe reflector, then over the cover tube 13 and into a like aperture 27 in the reflector at the opposite side of the tube. This hanger 25 is prefera~ly a resilient wire element individual to each reflector-tube set (Fig. 3), however, a continuous wire through a series of these reflector tube 30 sets may be employed. Using the individual spring-wire hangers 25, shown on Fïg. 3, the reversed ends 25Q and 251 ~2~

snap against the underside of reflector 10 and retain their position collectively supporting the reflector 10 in slung fashion in a snug fit against the cover tube 13.
The reflectors 10 mounted on adjacent tubes are fastened together in edgewise fashion by interlocking means best shown on Figs. 1 and 4. Along one side edge 28 of reflector 10 are plural outwardly extending L-shaped tabs 29 shaped integral with the reflector. Along the opposite side and inwardly spaced from edge 30 are corresponding slots 31 through which the tabs 29 may be inserted in unison and when the reflector is shifted axially of the tube and along the elongated slots 31 the tabs engage the other adjacent reflector interconnecting the two. The solar energy collecting apparatus is thus assembled with the inclusion of the reflector improvement of the invention.
Fig. 5 illustrates in a schematic fashion the basic principles upon which the present invention operates. As is known, solar radiation is made up of two primary components. The one component is incident upon the surface of the earth from the position of the sun at any given time and place. This component is a collimated beam of light indicated by the angled line at angle "a" measured from vertical. The second component of total solar energy available is a diffuse-radiation component. This component is not collimated but is available from all angles at a surface, such as cover tube 13. The collimated beam component swings through angle "a" from 90-0 and 0-90 in a corresponding angle at the other side of vertical.
This represents a "solar day". During the solar day, the sun's rays fall upon the intercept area of the tube and to either side of it. The collimated beams of light out of the tube intercept area impinge upon the specular surface of reflector 10. Since the reflector surface is generated as an arc segment of a hollow cy-linder des-cribed about a longitudinal axis Cpoint Y~, the reflector has an optïcal focal line at point X
within the concave cylindrical plane. As is well known in optics, this focal lïne X occurs a-t a point spaced from the central axis that is one-half the radius of curvature (R/2) of the cylindrical reflector surface.
The present invention employs non-imaging properties of the reflector in conjunction with the tubular collector and need not track the position of the sun. The focal line of the reflector should fall within the area encircled by the absorber tube of the collector. Collimated beams directed outside the tube intercept area strike the reflector, and all reflected incident light strikes the collector absorber surface upon reflection. The specular surfaces contemplated have a coefficient of reflection on the order of 0.8 and-higher. Diffuse light is similarly either intercepted or reflected and intercepted by the tubular collector. To obtain improved efficiency of the collector and reflectQr of the invention, there are relationships which interrelate the spacing of the tubes from each other with the diameter of the outer tube utilized, and the radius of curvature of the reflector and the relationship in spacing between the focal line of the reflector and the center axis of the tube collector. It is to this principle the present invention is mainly directed.
The critical spacing features and properties of the reflector~collector tube arrangement will no~ be described, principally with reference to Fig. 5. It is known to mount

3~ a tubular solar collector element with its axis on the focal line of a shaped reflector, such as a focusing or "imaging"

2~9~

parabolic reflector. This requires that the reflector/collector array track the position of the s-un to ensure maximum efficiency o solar radiation collection. The present invention utilizes a non-imaging reflector/collector combination.
As such, the non-imaging reflector has the focal line (X) of the cylindrically-shaped reflector at a dlstance of R/2 from the center of curvature Y, R representing the radius of the cylindrical reflector surface. Since the focal line of this reflector does not in general coincide with the absorber tube center line (see Fig. 5), the reflector is basically non-imaging with respect to the absorber tube. The arrangement also provides for efficient collection of diffuse solar radiation.
The design criteria for the present invention are the following: The axlal center-to-center spaclng between adjacent tubular collectors ls represented as dlmension d on Fig. 5. The cover tube diameter is represented as D. The reflector axis or geometric center ls positioned vertically above the tube axis by the distance equal to RD/d.
The reflector radius, R, is determined by:

R = d 2~ 1-(D/d)2 For large values for d, the gap between the reflector and tube 14 (dimension h on Fig. 5~ becomes too large to be of practical interest. Reflector design is of practical interest over a range of smaller spacings d. The chart of Fig. 6 illustrates collector performance over the range of interest, i.e. d/D values from slightly greater than 1 to 3.

FGr each operating condition wherein a temperature difference Tin-Ta exists, there is an optimum spacing between the reflector and tube and bet~een adjacent tubes. Tin is the temperature of the heat exchange media as it enters the collector, and Ta is the ambient outside temperature. Fig. 6 plots the ratios of d~D versus the per cent efficiency of collection and produces isothermal curves for Tin-Ta in 100 steps from 100F to 400F. The peak efficiencies are obtained overall in the temperatures given utilizing d/D relationships of from 1.4 to 2.3.
The chart on Fig. 7 illustrates the non-imaging function of the reflector in relation to the collector for d/D values of slightly greater than 1.0 to 3.0 in accordance with the design criteria outlined for the invention. On the graph, the plus or minus distance F on Fig. 5 between the center axis of cover tube 13 and the focal line (point X) as a function of tube diameter (D) i5 plotted against the d/D ratio for values 1.0 - 3Ø The focal line X should reside within the cross-sectional area of the absorber tube circumference. Value for F is variable in its transition along the vertical line con-necting through the geometric centers of the tubular collector and reflector from about ~0.3 at d/D of 1.25 through zero at d/D of 2,0 to a ~0.26 at d/D of 3. This curve defines the positioning of the concave reflector 10 and collector 11 relative to each other in achieving the improved efficiencies of energy collection.
EXAMPLE
Fig. 8 is a chart sho~in~ the performance data of a tubular collector and the cylindrically segmented reflector designed in accordance with the parameters of Fig. 6 and 7 and outlined earlier herein compared with the planar diffuse reflector and tubular collector combination described in the z~

aforementi.oned Canadi.an Patent 1,Q39,132. In the tests performed from which.each s-et of the data was obtained, a plurality of tubular collectors, described hereîn, were connected with a manifold which suppli.ed and circulated water through the interi.or chamber 12 of the absorber tubes in series. The tests were made in the spring season at latitude 41.6N
~Toledo, Ohio~ at solar noon and the collectors were mounted in a tilt plane of 45 facing southO Ambient temperature averaged 55F (Ta~ and inlet water temperature varied from 70F-220F (Tin~. The data plotted for the curve "Diffuse Reflector" was obtained using a plain white, flat diffuse.
reflector spaced at the back side (away from the sun~ of the collectors in accordance with the parameters of the disclosure in said Canadian Patent 1,039,132. The curve was drawn to represent the best overall fit of the data plotted. The cylindrical reflectors made in accordance with the present inventi.on, as described above, and having average specular reflectance of approximately 0.85 over their concave surfaces were mounted on the cover tubes of the same set-up of the collectors and manifold as in ~he diffuse reflector test.
The value of d/D used in the test was approximately 2.0, the tube spacing d being identical in each test. The spacing F conformed to Fig. 7. The plot of the data resulted in the curve labelled "Cylindrical Reflector" again drawn to repre~ent the best overall fit to the data. Total insolation, as Sp (sun energy output in the plane of collection~, was measured at approximately a thirty minute interval spanning solar noon and Tin temperatures ~ere recorded by thermocouple in the collectors. The data from each test were recorded after about one-half hour after flow of water was begun in the manifold, IAn element of water requires about one-half z~

hour to move from collector inlet to outlet in the apparatus.
Outlet temperatures were measured one-half hour after inlet temperatures and the insolatï.ons were averaged over the residence time of the water in the collector. The result in the two curves demonstrates the increase in efficiency of total energy collection utilizing the principles of the invention~
The "cylindrical reflector" curve and the "diffuse col-lector" curve each have the same slope, since the spacing between tubes in each test, and therefore the effective loss coefficients, are the same.
The practical range of the spacing in the invention is in the range of d/D 1.25 to 3.0 with the preferred spacing at d/D ratio of 1.4 to 2.3.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A solar energy collector apparatus comprising an elongated manifold, plural elongated tubular solar energy collectors connected to said manifold disposed in parallel, substantially equally spaced relationship and depending laterally along one longitudinal side of said manifold, individual elongated concave reflector elements for said collectors, the reflector elements being preshaped from sheet of relatively rigid material and having a light reflecting surface over the concave area of the reflector, means along the longitudinal edges of adjacent reflectors engaging the latter and interconnecting the adjacent reflectors to each other, spacing means interposed between the reflector surface and the exterior surface of the collector maintaining the two surfaces in spaced relationship, and fastening means for suspending a reflector about each collector and in spaced relationship therewith maintained by said spacing means.

2. The apparatus of claim 1, in which said means at the longitudinal edges of adjacent reflectors intercon-necting them to each other comprise at least one elongated aperture along one longitudinal edge portion of each of the reflectors, and a substantially L-shaped tab corresponding with each said aperture projecting laterally from the opposite longitudinal edge thereof, said tab of one reflector being received in the corresponding aperture of the adjacent reflector and engaging the latter, thereby interconnecting said suspended reflectors to each other.

3. The apparatus of claim 2, in which said reflector is comprised of aluminum sheet and includes an anodized specular reflecting layer over the concave light reflecting surface thereof, there being a plurality of said elongated apertures along said one edge portion of the reflector and a plurality of said tabs along the opposite edge thereof, said tabs being integral with said sheet of the reflector and corresponding with the lateral position of said adjacent apertures.

4. The apparatus of claim 1, in which said spacing means comprises a plurality of integral, raised points in the sheet of said reflector spaced longitudinally thereof.

5. The apparatus of claim 1, in which the sheet reflector is provided with one or more spaced apart pairs of apertures disposed between the said opposite longitudinal edges of the reflector, and the fastening means comprises an elongated thin member extending through each of the apertures of said pair and encircling the tubular collector.