GB2023804A

GB2023804A - Solar radiation collectors

Info

Publication number: GB2023804A
Application number: GB7917877A
Authority: GB
Original assignee: MAHRDJURI SABET F
Current assignee: MAHRDJURI SABET F
Priority date: 1978-06-02
Filing date: 1979-05-23
Publication date: 1980-01-03
Also published as: IT1165445B; FR2427561A1; JPS5528487A; GB2023804B; IT7920153A0

Abstract

A solar collector comprises a heat pipe (4) having an absorber plate (5) secured to its evaporator portion. The evaporator portion is surrounded by a glass cover (1). Along the length of the interior of the evaporator portion, a cascade-like arrangement of pockets (101) is provided for receiving condensate. <IMAGE>

Description

SPECIFICATION Solar radiation collectors This invention relates to solar radiation collectors.

A heat tube, as employed in a solar collector, is a device the essential function of which is to transfer and to distribute heat by vaporization and condensation of a working liquid fluid.

The prominent feature of a heat tube lies in that the energy which is required for having both the liquid and the vapour flowing in the presence of the gravity pull and the losses for sliding friction comes exclusively from the heat source; as a result, no external pumping system is necessary. The exploitation of heat tubes in solar collectors is already known (see, for example, the German Patent publication 2646987.4, the U.S.

patent specification 625 650, or the report by ASE in the "Essener Tagung" of February 1977, page 35).

The exploitation of a heattube in collectors of solar radiations has the following defect: When the sky is sunless, the fluid heat-transferring medium is coliected in the bottom end portion of the vaporization compartment. When, subsequently, the sun shines once again, the entire plate of the absorption apparatus and thus also the channels of the vaporizer system are overheated. The working fluid which is in the end bottom portion of the vaporizer channel begins to become vaporized and, during the run towards the condenser, the vapour temperature is increased as a result of the high temperature of the vaporizer channel wall. The condenser cannot perform any vaporization on account of the superheating of the vapors until such time as the vapor temperature has not dropped to the condensation temperature.

This procedure is such as to hamper restarting of the operation, the result being a worse efficiency.

This invention has its basic object to provide an apparatus for collecting solar radiations and having such a structure that, during operation, virtually no delay can be experienced.

This invention relates to an apparatus for collecting solar radiations, which is equipped with a transparent covering means and with a planar absorbing apparatus for capturing solar radiations, said apparatus being configurated with at least a closed channel extended longitudinally and confined by metal walls and enclosing a medium for transferring heat, which medium is capable of being vaporized, the apparatus comprising a vaporization compartment placed in thermal contact with said absorbing apparatus, and a condensation compartment arranged in the space at a higher level during operation, characterized in that said vaporization compartmentS comprises an appropriate number of containers 101, properly spaced apart from each other for the entire longitudinal extension, with an opening area narrower than the transversal internal cross-section 4 of the vaporization compartment for collecting said heat-transferring medium 10 as it is being condensed, said medium flowing backward under the gravity pull towards the vaporization compartment, a further characteristic being that the entire volume of the heat-transferring medium which is a fluid is, at the maximum admissible working temperature of operation of the system, greater than the sum ofthe partial volumes ofsaid containers 101 for collecting said medium.

Contrary to what is disclosed in the above mentioned prior art arrangements for the pumping of the condensed working liquid from the condenser to the vaporization compartment, no wick is necessary, or any capillary cavity, either, due to the fact that the collector is sloping relative to the horizontal line and the condenser is at a level above that of the vaporization compartment, so that, in such a case, the gravity pull as such provides to the feedback to the vaporization compartment (absorbing apparatus of the collector).

The solar collectors are intended to convert the major fraction of the energy of the impinging solar radiations into heat and to transfer said energy with the maximum possible efficiency to a fluid heat transfer medium which, for example, can be water. The solar collectors must have, as far as practicable, the following properties: a) a high absorption power (absorption capacity X 3 0.9) of the absorbing system within the entire span of the solar radiation spectrum (from X = 0.3 X = 2.2 microns), b) a reduced effective emissivity (admission power e S 0.3) of the absorbing system in the thermal radiation range (from X = 3 to X = 30 microns), c) reduced heat losses due to thermal conductivity and to convection, d) areduced thermal capacity, e) a high heat-transferring power from the absorbing system to the fluid heat-transfer fluid medium.

In this specification, the phrase "selective layer" is to be construed as a layer capable of absorbing heat and which presents, for the solar radiations (from 0.3 microns to 2.2 microns) an absorbing power, a > 0.9 and for the thermal radiations (from 3 to 30 microns) an admission power 2 of less than 0.15.

The selective absorption systems of the kind referred to above can be composed, for example, by metallic oxides (such as oxides of nickel, chromium or cobalt) or by metallic sulphides (such as sulphides of iron, chromium, nickel, cobalt lead and others) applied to a metallic base plate.

In addition, a selective absorption layer can be composed by a non-selective absorbing layer which is coated by a doped indium oxide layer.

A few preferred practical embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

In the drawings: Figure 1 is a lengthwise cross-sectional view of a collector apparatus according to the present invention.

Figures 2a, 2b and 2c show a few exemplary embodiments of practical nature of the absorbing compartment of the collector shown in Figure 1, in general cross-sectional transversal view in the position abc-abc of Figure 1.

Figure 3 is a lengthwise cross-sectionai view of a further practical embodiment of the vaporization tubing according to the invention, Figure 3a being a transversal cross-sectional view taken along the line A-B of Figure 3.

Figure 4 is a lenghtwise cross-sectional view of still another practical embodiment of the vaporization tubing according to this invention, and Figure 4a is a transversal cross-sectional view taken along the line C-D of Figure 4, and Figure 5 is a lengthwise cross-sectional view of yet another practical embodiment of vaporization tubing embodying this invention, whereas Figure 5a, which is a transversal cross-sectional view, shows a modification of the embodiment of Figure 5.

The solar collector will be described in more detail hereinafter with reference to the accompanying drawings, like members in the drawings representing like, or analogous component parts.

Figures 2a, 2b, 2c show a number of different embodiments of the absorbing compartment in cross-sectional view as specified above. These absorbing compartments are all composed by a plate 5, and a channel 4, the latter holding the working fluid. The plate 5 consists, for example, of a piece of copper-coated iron sheet on the top face of which a selective absorbing layer 52, is coated. The energy of the solar radiations to be converted into heat is transferred by the iron sheet 5 to the channel 4.

By observing more closely Figures 2a, 2b and 2c, it is seen that in Figure 2a, for example, the plate 5 with the radiation absorbing coating 52 straddles, in its midway region, the top half of the channel 4. In Figure 2b, another modified embodiment is shown, in which the channel 4 and the plate 5 are constructed as an entity by having the plate 5 inwardly and circumferentially bent to form a nearly complete channel 4: the open top slit which is seen, unreferenced in the drawings, can subsequently be sealed by any conventional means, whereafter the absorbing 52, is applied thereto.

Lastly, Figure 2c shows how the plate 5 can be bent in the midway region, 51, to provide roughly one half of the channel 4 for the fluid heat-transfer medium. The bottom half, as viewed in the drawing, of the channel 4 is provided by another narrower plate element 53, which is similarly bent so as to exhibit the half-circle which is necessary for completing the channel 4 aforesaid. The two component parts 51-52 on the one hand, and 53 on the other end, can subsequently be permanently assembled together to complete the channel 4.

Assembly can be made by any conventional means, such as, for example, by co-lamination or welding.

Figure 1, is a lengthwise cross-sectional view of an exemplary typical embodiment of the collector according to the invention, of which, as outlined above, Figures 2a, 2b and 2c show several modifications in transversal cross-sectional views, respectively.

As shown in these Figures, the solar collector generally comprises a tubular covering element made of glass 1, which is closed at one of its ends. The opposite end is closed by melting and united to a shaped metal piece 3.

The union of the glass body 1 to the shaped metal piece 3 takes place with the intermediary of the interposer 2, which is made of a special glass. For example, a lead glass can be used (flint). The shaped metal piece 3 is composed by a material having an expansion coefficient nearly equal to that of the glass 2: for example, ferronickel is a suitable material. The shaping of the piece 3 must fulfil two requirements, which are equally important: 1. Mechanical stability. To this purpose, it is imperative that the shape of the area intended for the union by melting be such that the dimensions be, as far as practicable, identical to those of the tubular body 1.

Thus, thrusts due to expansion will be overcome.

2. The path for the heat must be as lengthy as practicable in order that the heat losses in the conduit may be minimized as far as practicable.

The channel of the plate of the absorbing compartment, channel 4, is closed at either end in a vacuum tight manner, whereas its opposite end is connected to the condenser 11, by the intermediary of a metal piping.

The absorption compartment 5 is held within the tubular cover 1 by means of a planar reflector 7: the latter extends to the ends of the cylindrical shank of the covering tube 1 and transversally of the axis of the tubing and being made of mica (a poor heat conductor) having an aluminum vaporization layer, or the reflector may be a thin aluminum plate.

A getter ring 8, is fastened, moreover, to one end of the absorption compartment.

The piping 4 and the shaped portion 3 are connected to one another in a position 34 in a vacuum tight manner. On the front surface of the glass cylinder, an evacuating channel 9 is provided which is formed by melting and through it the interior of the covering tube 1 can be evacuated so that the residual pressure of the gases may reach a value not exceeding 1 of2 Torr.

Containers such as 101, best seen in Figures 3, 3a, 4, 4a, 5 and 5a are appropriately distributed in a cascade alignment along the entire longitudinal extension of the vaporization tubing 4 and have a generally gutter-like configuration: they are so arranged as to catch the fluid heat-transfer medium 10, which, on being condensed, is fed back by the gravity pull towards the vaporization compartment. The containers such as 101 are to be mounted in correspondence with the bottom generating line of the tube 4.

By this system, at the start of the operation of the installation, it occurs not only that the vaporization from the collector to the bottom end of the vaporizer tubing (4) for the heat-transfer fluid medium 10 can take place, but, in addition, that such a vaporization is evenly distributed along the entire vaporization tubing 4.

In order that the occurrence of localized drying ups may be efficiently prevented, it is necessary that the sum of the volumes of the gutter-like containers 101 does not exceed the volume possessed by the medium 10 of heat transfer at the maximum operative temperature of the installation. Stated briefly and otherwise, the reference temperature is below the critical temperature at which the heat-transfer phenomenon is discontinued.

Figures 3, 3a, 4, 4a and 5 and 5a show modified forms of the vaporization tubing.

In the practical embodiment shown in Figures 3 and 3a, prefabricated containers 101, are inserted at regular intervals, such as from 3 to 4 cm from any container to its next, in the vaporizing tubing 4: for example the containers 101 could snappingly be inserted into the pipes such as 4, provided that the gutterlike pockets 101 themselves are made of a snappingly resilient material. Figure 3a shows, in a somewhat exaggerated manner, the deformation of 101.

In the practical embodiment of Figures 4 and 4a, it can be seen that a sort of metallic tile 102, is slipped in a slanting slot formed through the wall of the tube 4, whereafter sealing in a vacuum tight manner is performed. Preferably, the tile-like metal piece 102 occupies nearly one half of the cross-section of the tube 4 of the vaporizer section. Anyone can see that, by this provsion, a simple gutter-like space 101, analogous to the ones seen in Figures 3 and 3a, is provided. Tiles 102 are analogously positioned at regular intervals of the same order of magnitude (3-4 cm, as before, or so) as in the previous embodiment.

In the alternative embodiment shown in Figures 5 and 5a, gutter-like inwardly directed, slanting, gutter-like jutties 101 are formed by machining, at 103, the pipe 4 at regular intervals so as to form condensatecollecting pockets 101, as before. It will be noted that the gutter 103 of Figure 5a differs a shade from that of Figure 5, but the function is the same and for this reason they are identified by the same reference numeral.

As is known, each heat tube must be used at a temperature which is positively below the critical temperature of the working medium which is employed.

Above the critical temperature, TK a heat tube cannot operate since the working fluid cannot be condensed any more so that no heat transfer can take place. However, since the condenser 11 of the heat tube is outside the collector and inasmuch as the metal piping which connects the condenser 11 to the absorbing compartment 4 has not a great heat transfer capacity, it is not possible that, in such a position, the temperature may substantially exceed the critical temperature. It should be recalled that, at supercritical temperatures, the heat transferthrough a gas layer is considerably reduced.

This is the case which occurs, even if the temperature of the plate 5 of the absorber 4 in the glass tubing 1 rises well above TK. However, since the fluid heat-transfer medium flows around the condenser 11, the temperature is allowed to exceed TK only by a slight extent. If the working fluid medium is so selected that its TK roughly corresponds to the maximum temperature which is permissibie for the assembly, no problem can arise in no-load conditions or in the case of an excess of solar radiation, either, as far as the safety of the system is concerned.

The following Table I lists the fluid working media which can be considered for the production of hot water.

TABLE I Fluid heat-transferring medium Critical Critical temperat. pressure "C bar Freon C318 115 27.8 Freon 12, or (CF2Cl2) 112 41.2 Freon 500, or (CF2C12 + CH3CHF2) 106 44.2 Propane(C3H8) 96.8 43.4 Freon 22, or (CHF2C1) 96.0 49.8 Freon 502 (CHF2C1 + CC1 F2CF3) 82 40.7 Freon 13 B1 or (CBrF3) 67 39.6 For producing hot water, Freon 22, Freon 12 or propane are especiaily suitable.

Arranging an appropriate amount of molecular sieves, (such as sodium and aluminum silicate) within the heatpipe ,for example in the condenser space provides to adsorb the residual gases which are present in the heatpipe and which could otherwise form "a gas plug".

Claims

1. A solar collector comprising a planar metallic absorbing apparatus for capturing solar radiations, and at least a closed channel confined by metallic walls and containing a vaporisable heat-transferring fluid medium, said channel having a vaporization zone placed in heat-transferring contact relationship with said absorbing apparatus, and a condensation zone which, during operation, is at a higher spatial level than said vaporization zone, wherein the vaporization zone has a certain number of containers spaced apart from each other at regular intervals along the entire longitudinal extension of said vaporization zone and having an exposed surface narrower that the internal cross-sectional area of the channel which conveys the heat-transfer medium in the vaporizer, said medium being condensed and being fed back under the only action of the gravity pull towards the vaporization zone, the volume of said heat transferring medium as considered at the maximum admissible temperature for the operation of the solar collector assembly exceeding the sum of the individual volumes of the containers aforesaid.

2. A solar collector according to Claim 1, wherein every container for said heat transfer fluid medium is incorporated with, or slipped into, the vaporization channel of the radiation-absorbing system.

3. A solar collector according to Claim 1, wherein every container is formed on the bottom generating line of the vaporization channel, a metallic sloping tile-like member being inserted into said channel.

4. A solar collector according to Claim 1, wherein every container is formed by an inward projection of the bottom surface of said channel of the vaporizer.

5. A solar collector according to any of the Claims 1 to 4, wherein the vaporizing and condensing channel is shaped as a closed tube placed in heat-transfer contact relationship with the absorbing assembly which has the form of a plate and has, in its portion intended to act as a vaporizer, with containers adapted to collect said heat-transfer fluid medium as it is being condensed by the condenser.

6. A solar collector according to Claims 1 to 4, wherein the channel of the vaporizer is prepared by an appropriate shaping of the plate of the absorbing system.

7. A solar collector according to Claims 1 to 4, wherein the vaporizer channel of the radiation-absorbing system iszformed by two metal plates which are united together.

8. A solar collector according to any of the preceding Claims, characterized in that the heat-transfer fluid medium has a critical temperature TK, which corresponds to the maximum working temperature of the solar heating assembly which is actuated by the condenser portion, and a critical pressure (PK) not exceeding 60 bar.

9. A solar collector according to any of the preceding Claims 1 to 8 inclusive, wherein the absorbing section and the vaporizer are housed in a casing which has been evacuated and which is formed by an optically transparent material

10. Asolar collector substantially as herein described with reference to and as illustrated bythe accompanying drawings.

11. A solar coilector as claimed in claim 10 substantially as herein described with referenceto and as illustrated by Figure 1 of the accompanying drawings.

12. A solar collector as claimed in claim 11 substantially as herein described with reference to and as illustrated by Figure 2a of the accompanying drawings.

13. A solar collector as claimed in claim 11 substantially as herein described with reference to and as illustrated by Figure 2b of the accompanying drawings.

14. A solar collector as claimed in claim 11 substantially as herein described with reference to and as illustrated by Figure 2c of the accompanying drawings.

15. A solar collector as claimed in claim 10 substantially as herein described with reference to and as illustrated by Figures 3 and 3a of the accompanying drawings.

16. A solar collector as claimed in claim 10 substantially as herein described with reference to and as illustrated by Figures 4 and 4a of the accompanying drawings.

17. A solar collector as claimed in claim 10 substantially as herein described with reference to and as illustrated by Figures 5 and 5a of the accompanying drawings.