GB2094499A

GB2094499A - Solar Collector

Info

Publication number: GB2094499A
Application number: GB8206164A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-03-04
Filing date: 1982-03-03
Publication date: 1982-09-15
Also published as: FR2501345B1; IT8120127A0; FR2501345A1; GB2094499B

Abstract

A solar collector for domestic usage comprises an axicon-shaped heliostat, with coaxial captor P, a heliostat supporting cradle 10 mounted on a fixed frame in such a manner as to be able to rotate about an axis AP parallel to the earth's polar axis. Said heliostat is pivoted 15 to the cradle 10 so that it can turn perpendicularly to the cradle rotation axis. There are provided means AUM to rotate the cradle for 180 DEG about its axis, as wall as means 7, 8, 5, 12, 13 to pivot the heliostat about its pivot axis for an angle equal to twice the angle between the ecliptic and the earth equatorial planes. These latter means 7, 8, 5, 12, 13 are interlocked with the first means AUM in such a manner that the resulting motion is a combination of the two movements that the heliostat has to carry out. The axicon is described with reference to its profile. <IMAGE>

Description

SPECIFICATION Solar collector This invention generally relates to solar collectors and more particularly to an inexpensive solar collector for domestic uses, having a sun ray concentration element with a geometry, so designed as to optimize its optical/thermal efficiency, which is operated by a continuous sun tracking system.

As it is well known, the solar collectors are devices suitable for concentrating and storing sun energy, as the solar radiation has not an intensity sufficient for the majority of technological uses, at least under the usual receiving conditions. The solar collectors may be divided into two types, namely those in which the sun radiation is focused from large to small heating surfaces, and those wherein the same radiation is collected and stored by means of suitable captors or thermal capacities. The focusing or concentration collectors reach high temperatures and require direct sun rays and clear sky, as well as a given movement possibility to follow the sun motions. The non-focusing collectors usually have flat-plate receiving surfaces, give limited temperatures, are fixed and can operate even during not perfectly clear days.

It is also well known that a heliostat is a mirror which reflects a beam of sun rays, in a fixed direction, not withstanding the apparent sun rotation. This mirror can assume the more suitable shapes to direct the reflected rays on the energy captor element and may be manufactured from any material with high reflection power.

The industrial exploitment of such devices has allowed to manufacture flat-plate solar collectors which are suitable for domestic uses too, as well as heliostats which can be joined together in batteries forming the captor portion of power plants.

It is an object of this invention to provide a solar collector of the linear focusing type, having reduced sizes and suitable for giving an energy efficiency greater than that of a conventional flat-plate collector of an equivalent surface.

It is another object of this invention to provide a solar collector of a linear focusing type which can automatically follow the sun apparent motion without presence of an operator, substantially for an indefinite number of years.

It is a further object of this invention to provide a solar collector, of the type referred to, which has a rather low production cost, in any case at a cost not greater than that of a standard flat-plate collector of equivalent surface, with quite competitive operating costs.

It is still another object of this invention to provide a heliostat having such a geometry as to be not much cumbersome, though having a reflecting surface with a high concentration ratio, and to be reproducible, with a sole model, in a molar manner with different reflecting surfaces, according to the requirements.

Finally, the invention has as a further object to provide an actuating device to impart to the solar collector a cyclic tracking movement of the apparent sun motion, said device being simple in design and therefore operable by an unskilled person, without special interventions, having a substantially unlimited autonomy as well.

Specifically, the solar collector for domestic uses according to this invention is characterized in that it comprises a heliostat having an axicon-shaped geometry with a cylindrical captor coaxial to the axis of the axicon, a heliostat supporting cradle mounted on a fixed frame in such a manner that it can rotate about an axis parallel to the earth polar axis, said heliostat being pivoted to the cradle in such a manner as to be able to turn around about an axis perpendicular to the cradle rotation axis, cradle operating means and heliostat operating means which are interlocked with said cradle operating means in order to impart to the heliostat a combined movement in directions perpendicular to each other, according to a pre-set program.

The invention will be now described in more detail with reference to a preferred embodiment thereof, which is given by way of example only and therefore not intended to limit the scope of the invention, and shown in the attached drawings, wherein: Figure lisa diagram showing the axial distribution of the radiation flux as a function of the heliostat mirror surface according to the invention; Figure 2 is a diagrammatic view of the heliostat axicon geometry; Figure 3 is a plan view of the heliostat according to Figure 2; Figure 4 is a side view diagrammatically showing the solar collector according to this invention; Figure 5 is a view of the motor operating the heliostat supporting cradle; Figure 6 is a detailed view of the heliostat operating means, which is interlocked with the cradle operating means;; Figure 7 is a plan view of the heliostat operating means as shown in Figure 6; Figure 8 is a graph showing the sun declination angle which must be covered by the heliostat motion; Figure 9 is a front view of the cradle with the heliostat and the operating means to obtain heliostat combined motions.

The components of the solar collector according to the invention will be now described in detail.

Concentrator -- The design choice started from a frusto-conical geometry, similar to that presented by Mouchot at the Universal Exhibition in Paris 1878 and analytically examined by M. H.

Cobble (Solar Energy 7/75 - 1963 "Analysis of a Conical Solar Concentrator").

The overall dimensions and limitations of such a geometry relating to a non uniformity of the energy flux on the captor means have induced the inventors to further investigate about the possibilities to combine the advantages thereof with those of a spheric focusing collector, both having a circular aperture angle of 450, to which a maximum interception factor and a minimum dispersion correspond.

As a result (see Figure 2) a reflector is obtained which is formed by the intersection of a cone C having an apex angle of 900 and a radius R with a spherical bowl having the same angle and a radius 'F2-1 R/ (height R .

2 and surface 72-1 R2(72-1), corresponding to =0,103553391 steradians), 4 which is concentric with the circle of the truncated cone minor base and tangent thereto. Said Axicon or spher-axicon (see "Applied Solar Energy" by Meinel and Meinel) having a height R/ and a "caustic" R/2 with superimposition of the spherical effect for a length R.

4 reduces the overall axial sizes of the device as compared to a simple cone.

Captor-With the same captation surface, a double advantage is obtained, in that the concentration ratio is doubled and the radiation losses of captor P are halved. Said captor P is in the form of a cylinder having a radius (r) and a height h=R/2 and an axis coincident with that of the axicon.

Further, this choice allows to have a "Concentration Ratio" #R R c= ## = ## =R/r "SUNS", 2 ##rh 2rR/2 which is a function of the ratio between the radiuses of the sun entrance aperture circle and of the concentrated radiant energy receiving cylinder.

A further advantage is obtained from a better axial distribution of the radiation flux along the caustic. In fact, the law governing the reflected energy flux as a function of the mirror surface radius, is linear and decreasing for the conical mirror and reaches the zero value at the cone apex, while in case of spherical bowl having a circular aperture angle of 450, said law is an epicycloid: (1-2 sin 0 R 4 wherein the aperture angle has its maximum value at 450. From the base of the captor cylinder, i.e. at the tangent circle and for a height of R.

4 the linear flux overlaps the epicycloid one so that the resulting flux follows the law: #2 R[1- ### (1/cos # -2 sin # )] 4 which shows two maxima ====== at both ends Rand R/2, while its minimum value is at R(1-7'2/4) (see Figure 1).

By maintaining the above geometry for the spherical portion of the mirror, but by extending the quadric cone beyond the radius R, a reproducible modular assembly has been designed starting from a single pattern, which permit concentratcrs to be made, having a surface ranging from 2 to 7 m2. The maximum size is limited by the following factors:- Weight and deformability of the structure, Drag to wind, Airing accuracy (time equation), Saving costs of the concentrator/collimator assembly, Optimum interception factor or reflected radiation fraction which is intercepted by the exchange surface.

In view of the above factors, it was experimentally proved that the average optimal practice concentration ratio is about C=20 Suns for the axicon catacaustic of the invention, which is therefore in the range of intermediate temperature solar concentrating collectors.

The basic module has therefore R=100 cm and r=5 cm and the modular function of the average concentration ratio for radiuses greater than #2 R(1- ##)=0.64644661.

4 R is: R2 R2 R C med - - p Suns; 2r h r(2R-1) 2R-1 where R #=#, r which permits the focal tube ====== to be sized as a function of radius R of the axicon, the extreme energy fluxes and the average specific flux of which are listed thereafter.In fact, if the mean concentration is set to the optimal value C=20 Suns it results that R #=2r=####cm (2R-1).10 and the following characteristic parameters are obtained: R2 4-2 R A R 0=2r Cmed= Fmin= spec= Fminimax mq ml cm r(2R-1) 4r Fmax=R/r 2R- I 1.50 0.7 12 20.2/5 10.3/4 20.- 1.3/4 53.3/4 2.- 0.8 11 19.4/5 11.3/4 20.- 1.1/3 58.3/4 2.50 0.9 11 20.1/4 13.- 20.- 1.1/8 65.% 3.- 1.- 10 20.- 13.- 20.- 1.- 65.% 4.- 1.15 10 20.1/8 13.- 23.- 0.885 56.1/2 4.50 1.20 10 20.1/4 13.-- 24.- 0.857 54.% 5.- 1.275 10 21.- 13.- 25.1/2 0.823 51.% 6. 1.40 11 19.7 11.3/4 25.45 0.7 46.% 7. 1.50 11 20.45 11.3/4 27.27 3/4 43% A=surface of axicon R=radius Cmed=average concentration Fmax=maximum flux Fmjn=minimum flux 4ispec=specific flux mq=square meters and ml=linear meters.

In order to track continuously the sun so that the mirror axis AC coincides with the solar versor it is necessary to have the possibility of rotation about two axes to obtain, in a simple and economic manner, the apparent sun motions. Three are the mount types used in astronomy -the altazimuth mount - the meridian mount -the equatorial mount For sake of construction simplicity the equatorial mounting with cradle has been chosen, which has two orthogonal freedom degrees. One is the polar axis AP, which is parallel to the earth rotation axis and therefore is exactly aimed toward the heavenly North pole. The other is the declination axis AD (Figure 6). The concentrator axis AC can rotate about this axis, and the whole rotated about the polar axis with the same angular speed as that of the earth rotation, but in a reverse direction.

This system has the advantage of having a lower number of motions, the hour angle can be simply referred to a watch, while the inclination can be set also once a day, since it changes very slowly.

Automatic universal monitoring device (AUM) (Figures 5 and 6) It is an electromechanical device of great constructive simplicity which combines the two above mentioned motions in a continuous way by taking advantage of the synchronism of the a.c. electric network frequency, requiring, theoretically, only two interventions by year at the solstices in order to select the declination increase or decrease. This mechanism, during the night, automatically resets the aiming for the subsequent sunrise and during one of the two phases causes simultaneously also the daily veriation of the declination of about one quarter of degree in a continuous or discontinuous manner, as desired.

The AUM device (Figures 5 and 6) is made up of a synchronous micromotor (1) (220 Volts 50/60 cycles) having a power in the order of ten watts, ====== controlled by limit microswitches (not shown) for the reversibility, which rotates at the speed of =====/300 RPM and accomplished ====== -= one revolution per hour with a torque of 20-30 m. Newton.meter. through a microreduction gear (ratio 1/15,000). This motor is connected through a nylon ring gear joint (r) to a worm reduction gear (3) (ratio 1/24) with orthogonal axes permanently lubricated ====== which accomplishes one revolution, a day.

The driven shaft (4) of said reduction gear is fastened to the axis AP of cradle (10) coinciding with the polar axis and therefore parallel to the earth axis. This is a power reduction gear, the torque of the driven shaft of which is 5.5 m.kg. to ====== counterpoise stresses caused on the axicon by wind.

In this manner the hourly motion of the cradle at a speed of 1 50/hour from East to West during the day and from West to East during the night is obtained. Any phase difference or current interruption can be readily compensated for by rotating in the direction of advance or delay the ring gear joint (2) one tooth each +2 min or 120, since the ring gear has 30 teeth.

The tilting of the declination axis AD, which can change due to seasonal effect of +23027i=23,450=0.409279709 radians=+EO, is obtained by means of a device of simple design, comprising a threaded rod (5) arranged parallel to the polar axis and rotating within a lead nut (12), fastened to a fork (13), hinged at the apex (6) of the axicon and secured to the cradle (10) through two rigid rods (14), (14'). The axial displacement of said rod (5) is calculated so as to cause a rotation North South or vice versa, of the axicon from a solstice to another one, through an angle which is twice the angle between the ecliptic plane with respect to the earth equator plane, i.e. 2#0=46.90 =46 54' according to the following relation: m=2.n.sin E =2.n.0.39794863=80%.n (Figure 8).

If measures are in mm and the screw pitch is P=1/p, the number of revolutions necessary to get the daily declination change is 80%.n.4 80%.n.4p 8.75% n =8.75% n.p.= revolutions P.365.25 365.25 P This is obtained by means of the conventional system of chain (7) pinion (8), secured to the frame (20) (Figure 4) and having a suitable transmission ratio. Such pinion is tied to the threaded rod through a ratchet wheel (9) that controls reversal of rotation (Figure 7) by means of a selection lever (11) permitting the threaded rod (5) to revolve in the directions (+) (-) depending whether the declination angle has to be increased or decreased. The former case starts at the winter solstice (22nd December) for the winter and spring times, whereas the latter case starts at the summer solstice (21st June) for the summer and autumn times.Therefore the intervention is limited to two times only each year and, if desired, the system can be automatized by means of a simple revolution counter.

The embodiment of said monitoring device without the declination tilting mechanism (5, 6, 7, 8, 9, 11, 12, 13, 14) can be easily applied to the conventional flat-plate collectors. In this case said device has a simple three position hand control and permits the efficiency to be increased of 50%, as will be evident from the following discussion.

The cradle (10) is hinged parallel to the polar axis AP pointed toward the North and tilted with respect to the horizontal plane of an angle corresponding to the latitude of the site. The plane perpendicular to this axis, the plane of rotation, corresponds to the equatorial plane forming with said horizontal plane an angle corresponding to the colatitude of the site: C0 =X/2o By knowing the equatorial co-ordinates, the declination angle and the hourly or azimuthal angle (referred to the true solar time) ====== from the spherical trigonometry we get:: sin h =sin # . sin # +cos # . cos # . cos H Said cradle rotates each day so as to cover a flat angle at the angular velocity of 1 50/h, being the maximum angle of sunlight captation for the concentrator during the six months covering spring and summer times, i.e. between the two equinoxes (21st March-23rd September) and said rotation is accomplished in 12 hours. In the six months covering the autumn and winter times said angle decreases until the winter solstice (22nd December) where the daily time is the shortest and then increases again until the daily time of 12 hours at the spring equinoxe (21st March) is attained and then remains constant.

The law governing the daily time is a function of the latitude Q and declination EO, which, in turn, is bound to the calendar date (progressive number of day=n).

arc. cos. # =tg . # . tg . # =# (azimuth angle) arc . cos. # =tg .23 .27'.tg.# =0.43377512.tg.# =# 12h Vh tday= . =0 13. vi 900 7.5 284+n 284+n # =# .sin(360 ###)=23.45 . sin(360### 365 364 for # =45 : # =# =23 .27'=23 .45; tg .#.## # =arc . cos 23.45 =arc . cos . 0.43377512-64.292622 64.292622 tmin=##=8.5723496h=8h34h20.46 -tmax=24h-tmin=15.4276504h=15h25m39.54s 7.5 where t=time, h=hours, m=minutes and s=seconds.

On the contrary, in the case of the "Automatic hourly Monitoring Device" (i.e. without the vertical swinging motion of the heliostat) since it is a flat-plate solar collector, the sunlight declination angle is the actuai one but, obviously, beyond 12 on the effective surface decreases according to the Lambert's Law Seff=S . cos. # , cos #max -# =23 27' Smin=0.9174077. S

Practical embodiment In order to assure durability and stability during its life time, the axicon is made of fiberglass with orthogonal weave reinforced by a steel rim (17) at the barycenter circumference, to which the pivotpins (15) of the declination angular rotation are welded.In this manner a structure having a high stiffness is obtained, which is weather resistant (wind, bad weather, temperature gradients and the like), the deformation value of which is stable during its life time, so that the interception factor remains about constant, since the system geometry does not change.

The specular surface of the axicon has been adopted after testing various products available on the market, from the simple aluminium foil to the vacuum aluminated adhesive films with or without acrylic protection, such as: Alkor, Scothcal and the like. The best results have been reached, both as to dimensional and time stability, and as to weather resistance, as well as reflecting power, starting from the materials for spatial uses. In this case, however, the cost incidence is too high and it is suitable to reach an economical compromise for home usage, since the efficiency gains are in the order of a few percents only.

A good coating material for concentration solar panels is represented by an acrylic, self-adhesive metalized film with a particular polyester film backing, which is readily removable upon the application.

The application is of extreme simplicity and is carried out by cutting the material in pieces corresponding in area to the lateral cono-quadric surface corresponding to the axicon in order to minimize the waste.

The boiler or absorber P is formed in a very simple manner from a welded steel pipe having a diameter s=100 mm, supported by the inlet and outlet pipes passing through the apex of the axicon and centered by means of struts (1 6) secured to the stiffening steel rim (1 7) by means of tension elements in locations spaced 1200 to each other. In order to provide the black body a non-selective coating for intermediate temperatures has been used (up to 1 500 C) having a high efficiency (absorbance=95%).

The globa! efficiency is given by: reflectancexabsorbancex(direct radiation)x860 Cal/m2. h= 85%x95%x860x555 Cal/m2. h=5,25 Cal/m2. min.

If the interception factor and the radiation losses of the boiler at operating temperature (1 2O0C) are taken into account, one can consider an absorbed energy flux of: j=555x90%=500 Cal/m2. h.

The useful insolation, obtained by subtracting one hour in the morning and another hour in the evening, when the absorbtion by the atmosphere is too strong, is limited by the flat angle of hourly rotation and during the summer is:

SOLIN: 6.1/2hv.s 8.1/2h A#W: 8.1/4h EQUIN: 8.1/4h VEAR: 9.4/5h v. s 12h S8 S: 10.Ohv.s 1.1/3h SOLES: 12.1/4hv.s 151/2h - SS: 111/3h - where SOLIN=Winter Solstice, EQUIN=Winter Equinox, SOLES=Summer Solstice.

These are the average practical values for latitude about 450 which give a sunlight exposure of 80% of the theoretical value. The corresponding average hours of actual insolation taken at a latitude of 450 by the meteorologic stations during the last 1 5 years give 2000 hours/year. Therefore, the average energy capted during one year is: 2000x80%x500=800 K . Cal/m2 that is equivalent to heating 20 m3 of water each year at 550C/m2, or else about 55 litres/m2 day, where from it is easy to know the home requirement of absorbing surface.

Conversely, the safety condition that in the system the boiling temperature is never reached permits the thermal absolute capacity of the absorber/boiler assembly to be determined, which in the most favourable hypothesis of continuous maximum insolation during all the 12 hours pf possible exposure of the concentrator and of zero thermal loss i.e. with a perfect insulation, is: Vmjn=h . W(5-t) 0=12;; 500/(95-1 5)0=600/800=75 litres/m2.

In this case even without daily consumption of hot water the system is thermally balanced before boiling can take place.

The absorber will be connected through a differential thermostat to the heat exchanger (hot water tank), which switches on a motor driven circulation pump when the absorber temperature is higher than 400C.

The cradle (10) of the equatorial mount is formed of square-shaped box steel sections in order to have the maximum stiffness and has an octagonal shape with two longer opposite sides. The cradle (10) supporting frame (20), is similarly formed and the vertical struts (21) and (22) thereof support the bearings (23), (24) spaced from each other of a distance h=d . tg A , where d=distance between center lines of the vertical struts and A =latitude of the site.

From the foregoing the great advantages given by the solar collector for home and industrial usages according to the invention with respect to the conventional solar collectors can be appreciated.

The simplicity of the practical embodiment, which makes use of the synchronism of the frequency of a.c. electric network for operating the reversible synchronous motor combining the two heliostat motions, requires theoretically two interventions only each year at the solstices for selecting the declination increase or decrease.

Of course, while this invention has been shown and described in connection with a preferred embodiment only, it will be apparent that modifications and changes can be made thereto by those skilled in the art and that all the equivalent can be used without departing from the scope of the invention. In particular, the materials herein used can be of other nature than that shown, according to the requirements, within the teachings of the present invention.

Claims

1. Solar collector for home and industrial usages, characterized in that it comprises a heliostat having a particular axicon geometry, provided with cylindrical absorber coaxially disposed with ====== the axicon, ====== a cradle for supporting said heliostat disposed on a fixed frame and susceptible to be rotated about an axis parallel to the earth polar axis, said heliostat being pivotally connected to said cradle and susceptible to be rotated about an axis perpendicular to the cradle rotation axis, cradle operating means and heliostat operating means interlocked with said cradle operating means so as to be able to impart to said heliostat a combined motion in directions perpendicular to each other according to a preset program.

2. Solar collector according to claim 1, characterized in that said heliostat is formed by the intersection of a cone having an aperture angle of 90" and a radius R with a spherical bowl having the same aperture angle and a radius R .7,2, which is concentric with the circumference of the greater base of the frustum ====== and tangent to the latter so that said axicon having a height R . 7,2 and a caustic R/2, superimposes to the spherical effect for a length 2-4 R.

4 thereby reducing the axial overall dimensions of said heliostat.

3. Solar collector according to claim 1, characterized in that said captor is made up of a cylinder having a radius r and a height h=R/2, that forms the boiler or absorber for the solar collector.

4. Solar collector according to claim 1, characterized in that said cradle has a shape assimiiable to an octagon having two longer opposite sides, is pivotally connected to said fixed frame in a tilted position by means of ball bearings or the like so that the longitudinal axis thereof is parallel to the earth polar axis and the tilting angle thereof with respect to the horizontal plane is a function of the latitude of the installation site.

5. Solar collector according to claim 1, characterized in that said heliostat has a metal supporting rim, disposed around the berycenter thereof and having two rotation axes, supported in bearings and disposed at the center of the two longer sides of the cradle, the cylindrical captor or absorber being secured coaxial to the axicon axis by means of adjustable radial struts.

6. Solar collector according to claim 1, characterized in that said cradle operating means comprise a reversible synchronous motor provided with a reduction gear capable of causing, by means of a suitable reduction ratio, one cradle rotation motion by day, said rotation motion being decomposed, by means of suitable limit switches, in two rotation motions of 1 800/day in opposite directions, the movement of the geared motor being time synchronised by the network frequency so as to avoid the use of a clockwork for the operation of the cradle.

7. Solar collector according to claim 1, characterized in that said heliostat operating means comprise a threaded rod engaging in a nut pivotally connected to the axicon apex, said threaded rod having at its free end a pinion engaging in a chain or rack, which is fastened to the collector fixed frame so that a rotation movement of the heliostat about its polar axis causes a rotation movement thereof about an axis perpendicular to said polar axis, said nut having such a screw pitch as to impart to the axicon a rotation movement/day which is twice the solar declination angle.

8. Solar collector according to claim 7, characterized in that a ratchet gear is provided, comprising a free wheel operating in two opposite directions on control of a selection lever, which permits the threaded rod to be rotated in two opposite directions according to the increase or decrease of the declination angle.

9. Solar collector according to claim 1, characterized in that said heliostat is formed of fiberglass reinforced resin, which is coated with a self-adhesive metalised acrylic film having an average reflection power of 85%, whereas said captor or absorber is coated with a non-selective paint having an absorption factor of about 95%.

10. Solar collector according to the preceding claims, characterized in that the concentrator assembly is formed of photovoltaic cells combined with a boiler which takes advantages from the heat to be dissipated within the system, by obtaining both electric and thermal energy, thereby integrally using the sun radiation energy.

11. A solar collector for home and industrial use and substantially as described herein with reference to the accompanying drawings.