GB2093980A

GB2093980A - Solar heat collector

Info

Publication number: GB2093980A
Application number: GB8205447A
Authority: GB
Original assignee: Owens Illinois Inc
Current assignee: OI Glass Inc
Priority date: 1981-02-27
Filing date: 1982-02-24
Publication date: 1982-09-08
Also published as: JPS57155061A; IT1147816B; FR2500918A1; IT8247874A0; NL8200542A; AU8018582A; BE892011A; LU83927A1; IL65020A0; BR8201018A; DE3206624A1

Abstract

A unitary solar energy collector assembly utilizes a plurality of evacuated collector elements (14) and an air-liquid heat exchanger (120) and provides heated liquid to associated devices. The collector elements are disposed in a staggered array on opposite faces of a manifold housing (12) which includes two parallel passageways, a blower (90) and the heat exchanger. Each collector element comprises an elongate double wall glass tube (80, 82) with one open end, the annulus (84) between the walls being evacuated to a high vacuum. A thin wall metal distributor tube (52) having a diameter marginally smaller than the inside diameter of the collector elements and a length marginally longer seats within each of the collector elements. The annulus defined by the distributor tube and inner collector element wall communicates with the proximate passageway and the distributor tube communicates with the distant passageway. <IMAGE>

Description

SPECIFICATION Air-liquid solar collector The invention relates generally to solar collectors and more specifically to a unitary solar energy collector assembly having a plurality of evacuated collector elements and a gas-liquid heat exchanger.

Increasing energy demands coupled with the increasing cost of extraction of fossil fuels has spurred a search for alternative energy sources.

Perhaps the most immediately promising alternative source is solar energy, and the developments and improvements in solar energy collector designs within the last decade have been dramatic. Nonetheless, each physical design such as flat plate or tube, and recovery media type, such as air or water, exhibits certain performance anomalies that invite improvement In evacuated tube collectors which utilize air as a heat recovery medium, a common problem has been uniform distribution of air to the collector elements. Insufficient air flow in certain collector elements due to maldistribution within a central manifold not only results in operation of certain collector elements at elevated temperatures which may affect their service life, but also decreases efficiency of the overall collector assembly.

In the prior art, it is known that non-uniform fluid distribution may generally be improved by increasing the operating pressure of the system and, thus, pressure drop across components of the system. Unfortunately, such a system operating pressure increase can only be achieved with a corresponding increase in energy consumption, i.e., energy input to the air moving components. In a solar energy recovery system, such an energy consumption increase may be greater than the increase in energy recovered as a result of the improved air distribution. Viewed as a thermodynamic system, the overall efficiency of the solar collector may, in fact, lower as a result of the increase in energy consumption associated with achieving the goal of improved air distribution.

This situation suggests that low system pressures and minimal pressure drops may have merit. One such system is disclosed in United States Patent Specification No. 4,016,860.

Another area difficulty in the utilization of solar energy collectors relates to the mode of energy transfer. In many solar collectors utilizing air as the recovery medium, the air is moved from the solar energy collector assembly through ductwork to the location such as a living space, wherein the heat energy is to be utilized. Such ductwork presents several problems. First of all, runs of any appreciable length cause significant pressure drops which aggravate the problems of system energy input and overall efficiency as previously noted. Secondly, energy loss problems to the atmosphere related to the low density of air and the large diameter and circumferential area of such ductwork is significant. Obviously, such ductwork may be, and preferably is, well insulated but this increases the outside diameter of the ductwork.Finally, installation of such large diameter ductwork in existing structures may also present problems which a more compact energy transfer scheme would appear to either minimize or alleviate. An example of a solar energy collector which utilizes such a compact energy transfer scheme is disclosed in United States Patent Specification No. 3,960,136. In the disclosed device, air circulated in a rooftop collector heats a liquid such as water and the water is used as the heat transfer medium.

According to the present invention an air-liquid solar collector comprises a manifold divided into two passageways, a plurality of evacuated solar collector elements extending outwardly from the manifold, an elongate hollow distributor tube disposed within each collector element and spaced from the inner wall thereof, each hollow distributor tube defining an inner passageway communicating with one of the manifold passageways and each collector element defining an outer passageway, by virtue of the space between the collector element inner wall and the elongate distributor tube, communicating with the other of the manifold passageways, the inner and outer passageways of each combined element and tube being in communication with each other at their ends remote from manifold passageways, and an air-liquid heat exchanger disposed in one of said manifold passageways.

Also according to the present invention, an airliquid solar collector comprising a manifold having two adjacent passageways, a plurality of evacuated tube collector assemblies extending outwardly from the manifold, each comprising an elongate collector element having an inner wall and an elongate hollow distributor tube disposed therein and defining an inner passageway within the tube and an outer passageway between the tube and the inner wall of the collector element, the inner passageway of each of the assemblies communicating with one of the two manifold passageways and the outer passageways of each of the assemblies communicating with the other of the two manifold passageways, the inner and outer passageways of each assembly being in communication with one another at their ends remote from the manifold passageways an airliquid heat exchanger disposed within one of the two manifold passageways and means for circulating air through the passageways, the collector assemblies and the heat exchanger.

Further, according to the present invention, an air-liquid solar collector comprises a manifold having an outer sheath, an inner sheath, a baffle disposed generally centrally within the inner sheath and defining two adjacent passageways, an air-liquid heat exchanger disposed in one of those passageways and means for circulating air, a plurality of evacuated tube collector assemblies extending outwardly from the manifold, each assembly comprising an elongate collector element having an inner wall and an outer wall and an elongate hollow distributor tube disposed therein and defining an inner passageway within the tube and an outer passageway between the tube and the inner wall of the collector element, the inner passageway of each of the collector assemblies communicating with one of the two manifold passageways and the outer passageway of each of the assemblies communicating with the other of the two manifold passageways, and the inner and outer passageways of each assembly being in communication one with the other at their ends remote from the manifold.

The invention will now be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a unitary solar energy collector according to the present invention; Figure 2 is an enlarged perspective view of a portion of the heat exchanger utilized in a solar energy collector according to the present invention; Figure 3 is an end elevational view of a unitary solar energy collector according to the present invention mounted upon an inclined surface; Figure 4 is a diagrammatic view of airflow through the manifold and the collector elements of a unitary solar energy collector according to the present invention; Figure 5 is a full sectional view of the manifold of a unitary solar energy collector according to the present invention taken along line 5-5 of Figure 3;; Figure 6 is an enlarged, fragmentary sectional view of a portion of Figure 5 showing the mounting of the distributor tubes in the manifold; and Figure 7 is a fragmentary, sectional view of the manifold of a unitary solar energy collector according to the present invention taken along line 7-7 of Figure 5.

Referring now to Figures 1 and 3, a unitary solar energy collector is generally designated by the reference numeral 10. The solar collector 10 includes a centrally disposed manifold assembly 12 having a plurality of collector elements 14 disposed in a staggered arrangement on opposite faces thereof in a co-planar array. Both the manifold assembly 12 and the collector elements 14 are supported by a generally rectangular frame assembly 16. The frame assembly includes a pair of elongate Z-beams 18 which securely mount and raise the manifold assembly 12 off or above a surface such as a roof 20 to which the collector 10 is-secured. The frame assembly 1 6 also includes a pair of L-shaped collector support beams 22 which are secured generally perpendicularly between the termini of the Zbeams 18.The support beams 22 each include a plurality of U-shaped collector element receiving slots 24 which cradle and support the termini of the collector elements 14 most distant the manifold assembly 12. The frame assembly 16 may be fabricated of material such as galvanized metal, aluminium or similar material. The Zbeams 18 may be secured to the manifold assembly 12 and the L-shaped beams 22 by any suitabie fastening means such as threaded fasteners, rivets, spot welding or other means.

As Figure 3 illustrates, the solar collector 10 may be instailed on the roof 20 of a dwelling or other structure. Preferably, the solar collector 10 is oriented in a southerly direction and at an angle of inclination which optimizes the receipt of solar energy at the latitude of its installation. The collector elements 14 are freestanding, that is, the collector 10 does not include a mirror, reflector or specular reflection device to concentrate the rays of the sun, but rather relies upon diffuse reflection from the surface of the roof 20 or other horizontal or inclined surface on the side of the collector elements 14 opposite the sun to reflect energy thereinto.

Referring now to Figure 5, the manifold assembly 12 is generally rectangular in crosssection and includes an outer sheet metal envelope or sheath 30 and a smaller, inner sheet metal envelope or sheath 32. Between the outer sheet metal envelope 30 and the inner sheet metal envelope 32 are disposed appropriately sized and oriented preformed slabs of insulation 34. The insulation slabs 34 are preferably fabricated of polyurethane-isocyanate or similar material capable of withstanding maximum temperatures of at least as high at 325C Fahrenheit, The uniform thickness of the preformed slab insulation 34 as well as the accurately formed perpendicular edges ensures that the slabs will fit together tightly and fully fill the volume between the outer metal envelope 30 and the inner metal envelope 32.Disposed centrally within the inner metal envelope 32 is a sheet metal baffle 36 which divides the internal volume of the inne; metal envelope 32 along its entire length into an inlet or supply passageway 40 and an outlet or return passageway 42.

Referring now to Figures 5 and 6, the centrally disposed baffle 36 defines a plurality of circular openings 44, each of which receives an annular elastomeric seal member 46. The seal member 46 includes a frusto-conical surface 48 which facilitates insertion of the seal member 46 into the circular opening 44 and an annular reentrant groove 50 disposed about its periphery which securely retains the seal member 46 within one of the circular openings 44. Seated within each of the seal members 46 is a thin wall metal distributor tube 52. The distributor tube 52 is axially restrained within the seal member 46 by an outwardly projecting rib 54 formed in the distributor tube 52 adjacent one end which cooperates with a complementarily configured semicircular depression 56 in the inner surface of the seal member 46. Each of the plurality of distributor tubes 52 is disposed concentrically within one of the collector elements 14 and extends axially beyond the open end of the associated collector element 14 a distance sufficient that it may be secured within the baffle 36 as described. At the opposite end of the distributor tube 52, i.e., the end disposed within the collector element 14, a plurality of, preferably three, outwardly directed ears or tabs 58 maintains the distributor tube 52 in coaxial disposition within the collector element 14.

Axially aligned with and disposed concentrically about each of the distributor tubes 52 are a first circular opening 60 defined by the outer sheet metal envelope 30 and a second circular opening 62 defined by the inner sheet metal envelope 32. A portion of the slab insulation 34 disposed between the openings 60 and 62 is removed to form a circular cavity 64 which receives an annular moulded seal 66. The moulded seal 66 is preferably fabricated of a silicon base elastomer and may be retained in the manifold assembly 12 by the applicaton of a thin layer 68 of a silicon base adhesive coincident with the annulus of contact between the moulded seal 66 and the slab insulation 34.The moulded seal 66 includes an outer lip 70 which functions as a weather seal and further includes a plurality of inwardly directed circumferential triangular ribs 72 which function as a chevron seal to seal against and tightly retain the collector elements 14 in the manifold assembly 12.

As noted above, one of the plurality of collector elements 14 is coaxially disposed about each of the distributor tubes 52. The collector elements 14 are preferably circular in cross-section and are fabricated of glass. Each of the collector elements 14 comprises an outer wall 80 and a smaller diameter, inner wall 82. The walls 80 and 82 define an elongate annular region 84 therebetween which is evacuated to a deep vacuum of approximately 10-4 torr. The vacuum is achieved by withdrawing air from within the region 84 at the tip end of the collector elements 14 and a tabulation 86 thereat is sealed off according to processes well known in the art. The vacuum in the region 84 substantially eliminates conduction and convection losses from the collector elements 14.The inner walls 82 of the collector elements 14 preferably include a solar energy absorbing surface 88. The energy absorbing surface 88 comprises a wavelength selective coating having high absorptivity and low emissivity of 0.1 or lower in the infrared region which can be fabricated by the vacuum deposition of a thin layer (1,000 Angstrons) of aluminium on the outer surface of the inner walls 82 of the collector elements 14. A layer of chromium is then electrically vapourized and deposited over the aluminium substrate as black chrome to a thickness of approximately 1,500 Angstroms.

Alternatively, the surface 88 may be blackened with an over-coating of an infrared energy absorbing material such as magnesium oxide, magnesium fluoride, etc.

Referring now to Figures 5 and 7, the manifold assembly 12 further includes a centrally disposed blower assembly 90. The blower assembly 90 includes a substantially conventional electric motor 92 which is secured by demountable fastening means 94 to the outer metal envelope 30 of the manifold assembly 12. Preferably, the electric motor 92 is suitably weatherproofed.

Alternatively, the motor 92 may be housed in a protective shroud (not illustrated). The electric motor 92 includes an output shaft 96 having a selectively securable coupling member 98 which drives a blower shaft 100 extending through the slab insulation 34 in a lined passageway 102 into the inlet or supply passageway 40. To the end of the shaft 100 disposed within the supply passageway 40 is secured a conventional squirrel cage blower wheel 104. Suitable shrouding 106 secured to the baffle 36 defines an intake aperture 108 which is disposed concentrically with and closely adjacent to the blower wheel 104.

Referring now briefly to Figure 7, the manifold assembly 12 also includes an air dam 110 disposed within the supply passageway 40 between the baffle 36 and the inner sheet metal envelope 32 and a contoured arcuate panel 112.

The air dam 110 and contoured panel 112 both include a symmetrically disposed curved surface 114 which provides uniform distribution of air from the blower wheel 104 into all regions of the inlet or supply passageway 40. Access to the blower wheel 104 is readily provided by fabricating the contoured panel 11 2 and a corresponding outer panel 11 6 which forms a portion of the outer sheet metal envelope 30, as removable sections which may be secured by selectively removable fasteners 11 8.

Referring now to Figures 2 and 5, the manifold assembly 12 also includes an elongate heat exchanger assembly 120 which is disposed within the outlet or return air passageway 42. The heat exchanger assembly 120 includes a pair of parallel, elongate plates 122 which both provide mounting for the heat exchanger assembly 120 between the centrally disposed baffle 36 and the side wall of envelope 32 to close off approximately two-thirds of the width of the outlet or return passageway 42. Sealingly secured to the pair of elongate plates 122 are a pair of parallel, flattened fluid conducting tubes 124. The pair of tubes 124 defines an elongate, rectangular passageway 126 within which is disposed a serpentine heat transfer fin or surface 128.The heat transfer surface 128 is preferably fabricated of a metal such as copper exhibiting good heat transfer practice in order to optimize heat configured according to conventional heat transfer practice in ordr to optimize heat exchange from the air passing through the passageway 126 to the fluid passing through the tubes 1 24. At each end of the flattened tubes 1 24 is a header 1 30 which provides communication and flow distribution between the flattened tubes 124 and a single fluid line 132.At one end of heat exchanger assembly 120, the fluid line 132 defines a reverse bend (not illustrated) and the fluid line 132 is juxtaposed the heat exchanger assembly 1 20 along its length such that both the inlet and outlet fluid lines 132 extend from the same end of the manifold assembly 12 as illustrated in Figure 1.

Referring now to Figure 4, the air circulation within the manifold assembly 10, the collector elements 14, and the distributor tubes 52 will be described. As noted previously, the blower wheel 104 is positioned within the supply passageway 40 and thus the flow of air is supplied thereinto. A portion of the air supplied by the blower wheel 104 enters the plurality of circular passageways defined by the thin wall distributor tubes 52 and travels to the left, outwardly away from the manifold assembly 12. Likewise, a substantially equal portion of the air supplied by the blower wheel 104 enters the annuli defined by the thin wall distributor tubes 52 and the inner surfaces of the inner glass walls 82 of the collector elements 1 6. This air flows to the right, outwardly away from the manifold assembly 12.In both instances, as the flow of air reaches the termini of the distributor tubes 52 and the inner glass walls 82, the direction of flow is reversed. In the first instance, the air flowing within the circular passageway of the distributor 52 begins to flow inwardly toward the return passageway 42 in the annuli defined by the distributor tubes 52 and the inner glass walls 82 of the collector elements 14.

In a converse fashion, the air previously travelling outwardly in the annular regions returns to the passageway 42 in the circular passageway defined by the distributor tubes 52. The air then passes through the passageway 126 of the heat exchanger assembly 120 and gives up the heat energy collected within the collector elements 14 to the heat exchanger surface 128 and ultimately to the fluid flowing through the flattened tubes 124 of the heat exchanger assembly 120. The air is thence drawn through the circular aperture 108, through the blower wheel 104 and recirculates.

Also, with reference to Figure 4, the preferable though not iimiting, spacing between the adjacent collector elements is illustrated. As noted previously, the collector elements 14 are preferably circular in cross-section. Where the diameter of a collector element 14 is given as "T", the optimum density of collector tubes and thus the optimum energy collection efficiency has been found to be achieved when the collector elements 14 are spaced apart a distance approximately equal to "T". Stated differently, the center-to-center spacing between adjacent collector elements is preferably approximately "2T'. It should be noted, however, that this preferred spacing should neither be considered to be limiting of the instant invention nor an absolute which should not be modified.

As noted above, the common approach to improving distribution and heat transfer in conventional air handling systems in the prior art has been to increase system operating pressures.

Unfortunately, attempts to apply this logic to solar heat recovery systems may be accompanied by so great an increase in energy input in the system to provide such increased operating pressures that the overall efficiency of the system is reduced. In the instant solar energy collector 10, the location of pressure drops and thus turbulence and improved heat transfer has been carefully chosen to occur primarily at heat transfer locations and thus to permit operation at exceptionally low air pressure and thus energy input.With regard to the collector elements 14, it should be noted that the cross-sectional areas of the circular passageways defined by the thin wall distributor tubes 52 and the annular passageways defined by the outer wall of the distributor tubes 52 and the inner surfaces of the inner walls 82 of the collector elements 14 are unequal, the crosssectional areas of the former being substantially larger. Such a disparity in cross-sectional areas results in increased flow velocity within the annular regions and accompanying turbulence which disturbs boundary layers adjacent the inner and outer surfaces of the annuli thereby improving heat transfer to the air. By way of example, a thin wall distributor tube 52 having a diameter of approximately 1.25 inches will have an inner cross-sectional area of approximately 1.2 square inches.When positioned concentrically within the inner wall 82 of a collector element 14 having an inside diameter of approximately 1.54 inches, the resulting annulus has a cross-sectional area of approximately .6 square inches or about one-half the cross-sectional area of the inner passageway of the distributor tube 52. Thus, not only is the air more turbulent within the annulus and thus less likely to form insulating boundary layers, but the energy loss and pressure drop associated with the turbulence is incurred precisely at the location of energy input and thus significantly improves energy recovery and overall efficiency.

Further with regard to flow and pressure drop, the construction of the heat exchanger assembly 120 should be noted. The elongate plates 122 close off approximately two-thirds of the crosssectional area of the outlet or return passageway 42, defining the throat of the passageway 126 and throttling the air. Air flow through the passageway 1 26 is thus substantially more rapid and turbulent than air flow in other portions of the manifold assembly 1 2. Thus, again, the location of a pressure drop coincides with a location of energy transfer.

A unitary air-liquid solar energy collector which utilizes a liquid as the ultimate heat transfer medium, also exhibits structural advantages. The open collector array, i.e., the lack of continuous reflector panels and/or protective transparent covers, as well as the cylindrical outer surfaces of the collector elements 14 result in very low aerodynamic drag and virtually eliminate considerations of wind loading. Such reduced drag minimizes the necessity of large, heavy and expensive supporting structures which can significantly increase the overall cost of the solar energy collector system. Furthermore, the use of a gas such as air as the primary heat recovery medium lowers the operating weight of the collector, further reducing the size and expense of associated structural elements.That the secondary and ultimate heat recovery medium is a fluid such as water having a high specific heat also is advantageous. Specifically, the transfer of the recovered solar energy from the collector 10 to the situs of its use within a building may be accomplished by the use of conventional copper or more recently developed plastic pipe which has been well insulated. Typically, such pipe will have an outside diameter of less than one-half inch and will typically be no larger than an inch in diameter when properly insulated. This small diameter makes installation, especially in existing buildings, staightforward and generally uncomplicated inasmuch as the piping can be readily routed through beams, studs and walls without undue complication.

Also with regard to the heat transfer fluid flowing in the tubes 124 of the heat exchanger assembly 120, it should be apparent that water is an immediately attractive choice due to its low cost, availability, safety and high specific heat. An equally apparent drawback to the utilization of water is its susceptibility to a change of phase, i.e., freezing and the attendant volumetric expansion it exhibits during the liquid to solid phase change. It should therefore be understood that other liquids such as glycols and glycol mixtures should be considered for use as a heat recovery fluid in the collector 1 0.

It should also be apparent that the overall size and thus energy collection capability of the unitary airliquid solar energy collector 10 will typically be determined by its application. Nonetheless it is anticipated that the collector 10 will typically include seventy-two collector elements 14 disposed in a pair of staggered arrays of thirty-six elements each. The number as well as the length of the collector elements 14 may vary widely.

Finally, the staggered, alternating arrangement of the distributor tubes 52 and collector elements 14 should be noted. Such an arrangement greatly simplifies the construction of the manifold assembly 12, particularly the baffle 36. In various prior art solar collectors numerous baffles and passageways were required to distribute the air to the heat recovery elements. Such complexity, in addition to increasing the cost of the collector, frequently resulted in poor air distribution. In the instant unitary solar energy collector 10, two parallel, substantially identical passageways 40 and 42 provide direct and uniform air distribution to the collector elements 14 and distributor tubes 52.

Claims

1. An air-liquid solar collector comprising a manifold divided into two passageways, a plurality of evacuated solar collector elements extending outwardly from the manifold, an elongate hollow distributor tube disposed within each collector element and spaced from the inner wall thereof, each hollow distributor tube defining an inner passageway communicating with one of the manifold passageways and each collector element defining an outer passageway, by virtue of the space between the collector element inner wall and the elongate distributor tube, communicating with the other of the manifold passageways, the inner and outer passageways of each combined element and tube being in communication with each other at their ends remote from manifold passageways, and an airliquid heat exchanger disposed in one of said manifold passageways.

2. A solar collector as claimed in claim 1 comprising means for circulating air through the inner and outer passageways, the manifold passageways and the heat exchanger.

3. A solar collector as claimed in claim 1 or 2 comprising a frame structure for supporting the manifold and collector elements extending outwardly therefrom.

4. A solar collector as claimed in claim 1, 2 or 3 wherein the collector elements are disposed in two rows on opposite faces of the manifold in a co-planar array.

5. A solar collector as claimed in any one of claims 1 to 4 wherein the collector elements are cylindrical and are disposed in two rows on opposite sides of said manifold in parallel relationship, the elements of one of said rows being laterally offset with respect to the elements of the other of said rows by a distance approximately equal the element diameter.

6. A solar collector as claimed in any one of claims 1 to 5 wherein the heat exchanger extends substantially the full length of one of the manifold passageways and includes elongate means for closing off a portion of the width of said passageway.

7. An air-liquid solar collector comprising a manifold having two adjacent passageways, a plurality of evacuated tube collector assemblies extending outwardly from the manifold, each comprising an elongate collector element having an inner wall and an elongate hollow distributor tube disposed therein and defining an inner passageway within the tube and an outer passageway between the tube and the inner wall of the collector element, the inner passageway of each of the assemblies communicating with one of the two manifold passageways and the outer passageways of each of the assemblies communicating with the other of the two manifold passageways, the inner and outer passageways of each assembly being in communication with one another at their ends remote from the manifold passageways, an airliquid heat exchanger disposed within one of the two manifold passageways and means for circulating air through the passageways, the collector assemblies and the heat exchanger.

8. An air-liquid solar collector as claimed in claim 7 wherein the collector elements are cylindrical and are disposed in two co-planar rows on opposite sides of the manifold in parallel relationship spaced apart a distance approximately equal to the diameter of the elements, the elements in one of said rows being laterally offset from those in the other of said rows by a distance approximately equal to the element diameter.

9. An air-liquid solar collector as claimed in claim 7 or 8 wherein the air circulating means comprises an electric motor and blower wheel, and the manifold includes means for assisting even distribution of air within at least one of said two manifold passageways.

1 0. An air-liquid solar collector as claimed in claim 7, 8 or 9 wherein the heat exchanger includes a pair of parallel liquid transporting tubes defining an air passageway, means disposed in that air passageway for increasing heat transfer between air flowing in the air passageway and liquid flowing in said tubes, and means for closing off a portion of said one of the two manifold passageways within which the heat exchanger is disposed.

11. An air-liquid solar collector as claimed in any one of claims 7 to 10 wherein the manifold includes an outer sheath and an inner sheath defining a volume therebetween, the volume being occupied by an insulating material, a baffle disposed generally, centrally within the inner sheath and defining the two manifold passageways and a plurality of openings in the baffle for receiving and mounting one end of each of elongate hollow distributor tubes.

1 2. An air-liquid solar collector comprising a manifold haivng an outer sheath, an inner sheath, a baffle disposed generally centrally within the inner sheath and defining two adjacent passageways, an air-liquid heat exchanger disposed in one of those passageways and means for circulating air, a plurality of evacuated tube collector assemblies extending outwardly from the manifold, each assembly comprising an elongate collector element having an inner wall and an outer wall and an elongate hollow distributor tube disposed therein and defining an inner passageway within the tube and an outer passageway between the tube and the inner wall of the collector element, the inner passageway of each of the collector assemblies communicating with one of the two manifold passageways and the outer passageway of each of the assemblies communicating with the other of the two manifold passageways, and the inner and outer passageways of each assembly being in communication one with the other at their ends remote from the manifold.

13. An air-liquid solar collector as claimed in claim 12 wherein the cross-sectional area of the inner passageways is approximately twice the cross-sectional area of the outer passageways.

14. An air-liquid solar collector as claimed in claim 12 or 13 wherein each elongate hollow distributor tube includes means for maintaining it generally coaxially within the inner wall of the associated collector element.

1 5. An air-liquid solar collector as claimed in claim 12, 13 or 14 comprising slab insulation disposed between the outer sheath and inner sheath.

16. An air-liquid solar collector as claimed in any one of claims 12 to 1 5 comprising a frame structure for supporting the manifold and the collector assemblies.

1 7. An air-liquid solar collector as claimed in any one of claims 12 to 1 6 wherein the air circulating means includes an electric motor and blower wheel and the manifold further includes means for assisting even distribution of air within at least one of the two manifold passageways.

18. An air-liquid solar collector as claimed in any one of claims 12 to 1 7 wherein the collector elements are cylindrical and are disposed in two co-planar rows on opposite sides of the manifold in parallel relationship spaced apart a distance approximately equal to the diameter of the elements, the elements in one of the said rows being laterally offset from those in the other of said rows by a distance approximately equal to said diameter.

1 9. An air-liquid solar collector substantially as herein defined with reference to and as illustrated in the accompanying drawings.