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Solar furnaces for high-temperature processing
Solar Furnaces for High-Temperature Processing* F61ix Trombe Research Director, National Scientific Research Center, Paris, France
HE uses of solar energy described in this report involve optical apparatus giving high concentrations of the energy received from direct solar radiation, The total absorption of this energy, concentrated on a relatively small surface or in a relatively small volume, results in high local temperatures. This is the effect obtained in what are incorrectly termed "solar furnaces." This term should, strictly speaking, be applied only to the receiving zone, which may or may not be a furnace, For certain uses of optically concentrated solar radiation, the effect desired is not the temperature rise, which is often very injurious, but the photochemical action, due to photon absorption in certain restricted wavelength regions, Hence, this report will include all uses of highly concentrated solar radiation, whether they employ the total radiation, or only the energy contributed by cerrain wavelength regions. Many of the techniques for collecting convergent solar radiation that are here described are similarly applicable to the exploitation of the thermal effects resulting from the convergence of various types of artificial radiation. These installations, termed "image furnaces" in the United States, are becoming important in research laboratories. Convergent arrays of the type now used to concentrate solar radiation and utilize its photochemical effect might likewise be suggested for certain artificial sources with an abundant emission of wavelengths suitable for photochemical uses. The energy available in direct solar radiation seems so small as to be altogether disproportionate for use in producing high temperatures. Beyond the stratosphere there is about 1350 watts per square meter so available, and at ground level, under optimum conditions, some 1000 watts per square meter. This energy density corresponds roughly to the radiation of a black body at a temperature of only 100 deg C. The spectral distribution of energy, especially that given in Fig. 1 of paper S/66, discloses better prospects. The solar energy corresponds to the radiation of a black body at about 6000 deg K. At ground level it is dispersed, but not de-
T
* General report of r a p p o r t e u r for Session I I I F, United Nations Conference on New Sources of Energy, Rome, Italy, Aug., 1961. 100
graded. The small temperature rise it induces is due to the very small solid angle of 7 × 10-5 steradian through which the sun "lights" the earth. A great increase in this solid angle of illumination would thus be sufficient to obtain a localization of surface energy that could yield high temperatures. This localization is realized in practice at the "focus" of an optical system of very great aperture. The maximum aperture of a paraboloid for the illumination of a plane is given by the ratio D/f = 4, where D is the diameter of the mirror and f the focal length. For illumination of a cylinder or a sphere, the value of D/'f can be still further, and substantially, increased. The high concentrations of radiation required for high temperature operations, or to localize energy for photochemical use, lead to the employment of optical systems under rather severe conditions. The axis of the system must be parallel to the incident radiation. This condition is likewise imperative for maximum incident energy reception at the various hours of the day. Since the apparent position of the sun varies during the day, and is different each day, the receiving system must be constantly oriented if the above conditions are to be satisfied. Two methods have been used for this purpose. In one the optical "concentrator" (parabolic mirror or lens) is turned directly towards the sun; in the other, it is fixed and receives the solar radiation following its axis by means of a moving plane "orienting" mirror. The principal shortcoming of solar radiation for practical uses is its intermittence, whether regular (sequence of days and nights), or accidental, due to interposition of clouds or fogs, screening the terrestrial surface from the sun. In many regions between the tropics and the temperate zone, a solar "breakdown is rare, but the daily intermittence leads one to envisage either operations that can be completed during a daily radiation cycle, or operations involving no thermal mass, which can be rapidly started and stopped. SOLAR MACHINES Direct Receivers, O r i e n t e d Receivers, a n d T h e i r Heliostats Direct reception has the advantage of involving only a single reflection on the mirrors that concentrates the radiation. Various types of direct receivers are given in Solar Energy
Fig. 1 of paper S/16. The principal disadvantage of this type is the fact that the axis of convergence of the radiation is displaced in space, and, consequently, that the focus, located on this axis, is also displaced in space. This arrangement is also unfavorable, since the convergent radiation attacks substances from down to up, or laterally, and it is difficult to perform certain operations, for instance melting, with such apparatus. On the other hand, for operations on solid surfaces, reaction between gases, cracking, etc .... , direct reception is the simplest method. In some assemblies (Meudon apparatus, 1946), the image of the sun is reflected downward by means of a plane mirror placed in the path of the convergent rays. Direct reception can be viewed more favorably for apparatus with a smaller aperture than paraboloids and using lenses. The only solar furnace using lenses that is in operation is at the California Institute of Technology. It moves about its focus, which is itself fixed in space, The reception of solar radiation by a plane orienting mirror, followed by a fixed parabolic mirror, was first put into practice by Corm, and later by the Montlouis Laboratory. This method can now be generalized for most laboratory devices and large-scale installations, It has the considerable advantage of having a fixed focus of orientation, whether horizontal, vertical, or inclined, that can be selected according to the operation being performed. To keep the optical axis of direct receivers pointing to the sun they may be guided either by an astronomical method or by a control mechanism with cells or transistors. Some installations, especially the large solar furnace of Algiers, are mounted on an equatorial axis whose motion is controlled by an astronomical clock, This method has the advantage of holding the axis of the paraboloid in the direction of the sun, even if the solar radiation is briefly intercepted by a cloud. The declination of the mirror is naturally adjusted daily by hand. It would seem that this method of control would be rather expensive for large machines like the one at Algiers. Nevertheless it appears to have been checked for small devices by Peter E. Glaser (paper S/'16). The collector mirrors are often automatically trained on the sun by using an auxiliary lens with its optical axis parallel to that of the paraboloid. A circular screen, located in the focal plane and substantially of the same size as the solar image given by the lens, is surrounded by four control photocells or transistors, operating pairwise to control the two directions in elevation and the two directions in azimuth. A slight displacement of the solar image to outside the screen illuminates one or two of the control photocells. This arrangement has been universally adopted for guidance of direct receivers, or, as we shall see, to control the motion of heliostats. The methods of guidance differ primarily in the conVol. 7, No. 3, 1963
ditions under which the currents from the cells are used to produce the ultimate motion of the mirrors. Heliostats are, in general, guided by a control system (lens, screen, and framing cells) located in the path of the reflected radiation. This control arrangement is fixed, and the direction from the lens to the screen center defines the direction of the reflected radiation. The photocell currents can be used in various ways to cause motion of the heliostat or the direct reception devices. The first Montlouis model controlled electric motors by a series of contacts of decreasing sensitivity, which initiated the operation, in either sense, of asynchronous motors. One adjusted the elevation, the other the azimuth, of the heliostat. Parallel developments, simultaneous in different countries (United States, USSR, France, Japan, and Israel for example) today employ the following schematic arrangement : current from a photocell or phototransistor, electronic amplification of this current, control of the excitation of a dynamo, of Amplidyne type or similar device. Direct current generated by the dynamo runs a direct-current motor in the proper direction. One of these systems controls the changes of elevation, the other the changes of azimuth. This electrical command system permits realization of simultaneous motion of a certain number of heliostats controlled by a master heliostat using mechanical coupling. Electronic coupling does not seem to have been used. Mechanical coupling can insure excellent accuracy of control if the transmission shafts run at adequate rotary speed. Certain devices do not include an electric motor for aligning the machines. The amplified photocell currents act to change the position of distributor slides or servo-valves, admitting oil under pressure into doubleaction hydraulic jacks, one controlling the elevation and the other the azimuth. As before, two elevation photocells, upper and lower, act on the two faces of the same actuating cylinder, while two azimuth photocells, right and left, act on the second cylinder. The axes of rotation of heliostats are in general perpendicular. This system is used to operate the large Montlouis heliostat and will be used for the 63 heliostats to be installed at the 1000-kw solar furnace at d'Odeillo-Font-Romeu. It is also mentioned in paper S/57 of this Conference. Plane Reflections Various experiments have shown that for long-distance reflections, glass surfaces coated on their front or rear faces with metallic reflectors give good results. They appear to be better than reflectors made of polished metal planes. Metallized plastic glass or metMlized plastic film stretched on metal frames do not seem to have been tried for long-distance plane reflectiou. The heliostats described in various papers all use surfaces of metallized glass. The dimensions of the unit lOl
surface are selected on the basis of various factors. The French installations (Montlouis)and the United States Army installation (Natick) of paper S/79, use the reflector on the rear surface after passing through the glass. The thickness and transparency of the glass are thus important for efficiency of the plane reflection. On the other hand, the mounting of the glass must be substantially plane for the various positions assumed by the heliostat. The thickness permitting this result will naturally be greater, the larger the elementary surface itself. Moreover, to avoid losses of light at the glass interfaces, it is better to adopt the maximum dimensions. To give effect to all these factors, the dimensions adopted are 50 by 50 cm by 6 to 7 mm thickhess in the Montlouis installations, and 62 by 62 cm by 0.635 cm at Natick. In these two installations, the reflecting surface consists of a layer of cllemically deposited silver electrolytically coated with copper, and then coated with a protective varnish. At Natick one aluminized adhesive surface is also used. The normal transmission of energy by such reflectors is from 80 to 84 percent, according to the number of glass plates, the vitreous reflections being recovered in the reflected radiation. The behavior of such surfaces in bad weather is excellent. The Montlouis reflectors, in particular, have been exposed for the last ten years to a severe climate, and their reflecting properties are still excellent, Another class of plane reflectors consist of glass surfaces coated on the front face with thermally deposited aluminium. Owing to their high reflectivity, these surfaces were adopted by the authors of paper S/21 for the large solar furnace at Sendal (Japan). The experience they reported includes the following details: a glass mirror 90 by 100 cm, by 10 mm thick, with silvered or aluminized rear face, had a reflection factor of 67 percent for an incident ray of a 15 deg inclination, and one of 56 percent for an incident ray of 40 deg. The aluminium deposit under the same conditions gives 95 and 92 percent respectively. Aluminium surfaces have the important advantage, especially for photochemical reactions, of reflecting solar ultraviolet radiation. On the other hand, their resistance to bad weather is uncertain. This resistance is improved by a protective deposit of silicon monoxide, Another factor to be considered is the resistance of glass to thermal shock and mechanical stress. Experience shows that glass sheets 1 by 1 meter by 1 cm thick are much more fragile, thermally and mechanically, than glass plates 50 by 50 cm but less thick. The alignment of the elementary reflecting planes to obtain a parallel and continuous reflected beam requires individual alignment of each plane supported at three points. Such alignments are in general accomplished with precise topographic instruments, by sighting a point sufficiently distant directly and through the glass, 102
In the use of plane reflection, one must take account of the relative increase in the penumbral zones with increasing distance between the heliostat and the parabolic reflector. These zones act only on the outer contours of the reflected nappes. The surface of the parabolic reflector must be inside this aureole of decreasing energy. The metal framework supporting the reflecting mirrors cannot be described here. Such frameworks must be realized under the desired conditions so as not to form double sheets and must remain plane at the various operating temperatures of the installations. Convergent Reflections The problem of realizing reflecting or refracting surfaces has various solutions: The large aperture necessalT to get appropriate accumulation of energy practically rules out lenses, which are, incidentally, considerably more expensive than mirrors. For small mirrors, the reflecting surface is usually in a single piece. The first laboratory solar furnaces made extensive use of military anti-aircraft mirrors. Such mirrors, 1.5 to 2 meters in diameter, are made either of special water-white glass, silvered on the rear face, or of a polished alloy. Because of the severity of the specifications for their initial use their optical quality is entirely adequate for high concentration of energy. They are valuable for fundamental research with either solar furnaces or image furnaces. Aluminium reflectors can be larger. The largest one in a single piece (3 meters in diameter) was built by Conn at Kansas City. Other aluminium mirrors, made up arrays of metal segments, have been constructed (Algiers). Paper S/16 describes (Fig. 3) a light aluminum mirror 30 feet (about l0 meters) in diameter. Although they do not appear to have the optical qualities of smaller mirrors, especially that of Conn, they do constitute an interesting and economical solution for moderate concentration of useful amounts of energy. The aluminum sheets can be fabricated by drawing on a mould or explosive forming on a mould by the sudden pressure of a gas liberated by an explosion (paper S/16). Another interesting solution for small- and mediumsize equipment is the use of plastics. Parabolic plastic moulds have been produced by solidification of a plastic resin on a mercury surface rotating at constant velocity. The use of aluminized Mylar on a polyurethane mould gives mirrors of good optical quality (Fig. 5, paper S/16). Large installations require separate sectors, individually aligned on a rigid common mounting. The arrangement used at Montlouis and Algiers, which is also mentioned in paper S/21, Fig. 3, can be adopted. It has the advantage that all elements in a single reflecting ring are identical. There are only as many Solar Energy
different reflecting elements as there are rings, thus permitting advantageous duplication in the fabrication and alignment of the elements. The reflecting elements can also be aligned on mounts outlining horizontal and vertical parabolic profiles (Natick installation, paper S/79)--(d'Odeillo - Font - Romeu installation--paper S/48). The problem posed by the design of large solar furnaces with high concentration of energy is that of the unit reflector element, as already mentioned, whether used in concentric rings, or in horizontal alignment on mountings, to form the paraboloid. Each large solar furnace built or now under construction has embodied a different solution in this respect. The first solar furnace built at Montlouis, France, in 1952, which is still in service, comprises elements positioned on concentric rings. Each element supports a number of glass plates individually aligned. These are plane glass plates, but thin enough to be deformed by pressure. They are permanently subjected to pressures in front and behind, giving them the approximate shape of a small sector of the paraboloid to be occupied on the reflecting surface, It has been demonstrated by the Montlouis installation, and now by the larger glass plates for the large 1,0O0-kw solar furnace of Odeillo-Font-Romeu, that glass plates so subjected to permanent stress give at least the same concentration as five plane glass plates adjusted so that their images converge at the focus of the system (see paper S/48). The Montlouis installation, comprising five concentric rings and 3500 glass plates under stress, has five types of reflecting elements. At the Odeillo-Font-Romeu installation, which will have a much larger number of glass elements, the alignment will not be repeated, but will be done on the site. This arrangement makes it possible to construct mirrors giving high concentrations, subject to the obvious condition that the number of glass plates is sufficient. It also has the advantage of holding the glass plates firmly in place on their mountings, thus avoiding all oscillation of the reflected radiation, especially the wind. This arrangement appears to be the most economic for very large solar furnaces, such as the one at OdeilloFont-Romeu, which has a large number of convergent reflecting glass elements. The second installation, the one at Algiers, is 8.40
cost of stamped sheet if the quantity is large enough to justify the cost of the dies. The large solar furnace built at Natick, Massachusetts, by the United States Army, has focusing elements of a peculiar type. There are 180 elementary surfaces, formed of concave mirror segments 62 by 62 cm. They are aligned in successive horizontal profiles on a mounting realizing the paraboloid. Each spherical element was produced by forming, at a suitable temperature, a polished glass plate, against a spherical ceramic mould. The glass face in contact with the ceramic deteriorates partly, but the free face retains its polish and has a good sphericity. These mirrors are aluminized on the front (concave) face and are then individually aligned to cause the reflected rays to converge at the focus (see paper S/79). The large solar furnace under construction at Sendal, Japan (see paper S/21) consists of 181 polished glass surfaces, hot moulded into paraboloidal shape. The concave faces were then thermally aluminized. The merits of this process, which resembles the one used for the Natick solar furnace, are not yet known. It should, nevertheless, be noted that the sectors of the paraboloid should present the same surface finish as the spherical sectors of the Natick installation. Since they are moulded into the parabolic shape correspondlug to their position on the reflecting surface, they probably will give high optical concentrations. With the single exception of the Algiers installation, which directly receives solar radiation and appears to be intended for photochemical studies and gas reactions, all semi-industrial solar furnaces have a heliostat refleeting the sunlight onto a fixed parabolic mirror with a horizontal axis. The only plant whose mirror reflects the radiation in a direction parallel to the celestial axis is that described in S/36, but this paper fails to give the dimensions of the installation. A comparison of the large solar furnaces shows that the plants developed or being developed by the various countries resemble each other rather closely in dimensions and principles of tracking, except for the focusing surfaces themselves (see S/79, S/21 and S/48). The Natick installation includes a large attenuator screen placed in the path of the parallel horizontal radiation. The screens of the French installation (Montlouis) are placed in the path of the convergent radiation.
meters in diameter, and is composed of aluminum sectors produced by stamping on rigid dies. This installation is a direct reception model, and the aluminum surfaces, chemically polished and then anodically oxidized, reflect practically all the solar ultraviolet that reaches the ground. The degree of perfection that can be given by this method is not yet known. The concentration is apparently the same as that yielded by other types. The principal advantage of stamping is the low
THE ENERGY CONCENTRATIONS AT THE FOCUS
l'ol. 7, No. 3, 1963
Solar concentrators, especially the small ones, provide great concentrations of energy. This does not imply that an increase in size is a factor unfavorable to the procurement of high concentrations at the focus (see paper S/5). It would seem, rather, that the concentrations can be very great for single mirrors of great aperture and optically perfect enough, but that the 103
larger models, using sectors, call be realized just as perfectly, provided cost is no object. For very large solar furnaces, the subdivision of the reflecting elements, which is necessarily more advanced, should give energy concentrations as high as those from the smaller ones. For these large installations, it seems that economic conditions of realization and high concentrations of energy are simultaneously possible, The concentrations given by low-power furnaces appear in some cases to be substantial. The 2 meter glass mirrors at Montlouis, France, give experimental energy fluxes in excess of 350 cal per sq cm per sec for an incident energy of 1 kw per sq meter. (1 kw is equivalent to about 240 cal per sq meter per sec.) Table I of paper S/79 gives the measured fluxes for various furnaces. The lens solar furnace at the California Institute of Technology has a flux of 220 cal per sq cm per sec,the Curtiss Wright solar furnace has 600, and the USSR solar furnace, 864. These latter values must obviously correspond to apparatus of very high optical quality. The concentrations of high-power solar furnaces determiued today are clearly below these levels. The large solar furnace of Montlouis gives 0.75 ~o 0.8 kw per sq cm, or a little less than 200 cal per sq cm per sec. The Natick solar furnace (paper S/79) gives a flux of 100 cal per sq cm per sec, but it was not designed to obtain high fluxes, but to realize the most uniform energy possible in the solar spot. It is especially noteworthy that the maximum concentration calculated for the central zone of the furnace should be substantially in agreement with the experimental results. The experimental curve shows that energy uniform to within less than 5 percent can be obtained for a spot 3 cm in diameter, and energy uniform to within l0 percent for a spot 4 cm in diameter. The present Montlouis furnace, which has a focal length of neters, gives a rather similar energy curve, although the central concentralion, several centimeters in diameter, is almost double. Such variations in energy density have only a slight effect on the equilibrium temperatures. We recall the Stefan relation between the total energy E radiated by a black body and its temperature: E = aT 4. The ternperaturc measurements for the plant at Sendal, Japan (paper S/21) are as yet unavailable to us, but it would seem that rather high energy fluxes can be expected. The calculations given in paper S/'48 on the large French solar furnaceundcrconstruction at Odeillo-Font-Romen indicate that, allowing for all the losses, an energy of 1700 watts per sq cm, or over 400 cal per sq cm per sec, should be obtained in the central spot. The equilibrium temperature of the focus, counting only the radiation losses, would be 3900°C for a black body. In this connertion we must stress the fact that the equilibrium ternperatures are not the operating temperatures of solar furnaces. Even for fundamental research, the working temperatures used, for instance, in the neighborhood of 104
3000 deg, are several hundred degrees below the equilibrium temperature. T H E C O L L E C T I O N OF T H E SOLAR E N E R G Y AT T H E FOCUS There are numerous publications on these subjects, especially by the Montlouis Laboratory. A number of methods of collecting the solar energy will be found in S/35 (Figs. 5, 6, 7, 8, 9, 10, l l , 12 and 14). Theywill also be found ill papers S/5, S/16, S/108, S/66,S/52 and S/57. These methods call be reduced to a number of typical operating patterns. Direct Collection
Treatment at the focus, whether on a horizontal, vertical, or inclined surface. A self-supporting body can be treated on a horizontal surface. This is the typical operation in an oxidizing medium, and involves no contamination of the substances treated. The efficiency of this procedure can be improved by displacing the substance being irradiated, so as to obtain belts of melted or fritted products in various shapes. The powdered substance can likewise be spread continuously on the treated surface, which is progressively lowered. In this way one obtains melted rods of substantially the same diameter as the image cast by the optical system. Rotation makes the melted massesmore regular. Direct treatments can likewise be applied to solid substances, for instance to rods held vertically, practically colinear with the focal axis of the optical system. By shifting the rod in the direction of the focal axis, a molten zone can be made to migrate from one end of the rod to the other. This method can also be used when the rod is supported laterally by cooled jaws (see paper S/52, Figs. 4, 5 and 7). Paper S/35, Fig. 12, presents one version of direct treatment permitting treating practically all the substance, without contamination. The material is heated on a metal plate (aluminium or copper) of high thermal conductivity, and vigorously cooled on the bottom. It is advantageous to use relatively thin metal plates (several millimetres thick). All the above methods can be employed in a controlled atmosphere. The use of vacuum requires mechanically resistant and transparent walls. For small heating units, Pyrex glass and silica glass, both of them transparent to solar radiation, are suitable. For large units, the problem of the transparent wall with high mechanical resistance still remains open. The following is an interesting and inexpensive solution. Where the controlled atmosphere remains under ambient pressure, paper S/52, Fig. 3, indicates that the solar energy is admitted through a plastic membrane inflated by a slight overpressure of the gas constituting Solar E~erg!!
the controlled atmosphere. Such containers, of large dimensions and low cost, which are consequently economically expendable for each operation, can be made of thin plastic films of various kinds, According to paper S/35, phenomena of positive photophoresis are observed on transparent walls through which the solar radiation passes, especially in gases like hydrogen, at pressures from several millimeters to 1 cm Hg. This phenomenon does not appear to be connected with the heating of the transparent wall. Cavity Reception Paper S/35, and several others--S/66, S/36, S/5 and S / 1 6 - - m e n t i o n the advantage of treatment in cavities, either to obtain better yields with reflecting substances, or to get more regular variations of temperature in a given operation. The cavities developed (see S/35, Figs. 9 and 10), behave like black bodies, and their inner walls heat up regularly, owing to the numerous internal reflections. They are in general characterized by an internal surface much larger than the outer surface, permitting the penetration of convergent radiation, The scheme of black-body cavity that should be especially emphasized is represented in S/35 (Fig. 10). The substance, held against the furnace walls by the action of the centrifugal force, is treated with no contamination whatsoever. This type of furnace, in particular, affords the advantages of cavity treatment and of direct treatment of a selfsustaining substance. Cavity operations can be conducted in vacuo (for small devices) or in an inert gas. It should be noted that a controlled atmosphere can be realized in centrifugal furnaces by supplying the gas to the furnace interior itself, Treatment in a cavity behaving like a black body finds application in determining the energy distribution of the solar energy on the focal plane. Black-body cavities equipped for calorimetric measurements are used to explore the focal spot at different points of its surface, since the surface of the radiation-access orifice c a n be modified at will by interposing refrigerated s c r e e n s h a v i n g orifices of various diameters, R E S U L T S O B T A I N E D IN T H E H I G H TEMPERATURE FIELD The energy supplied to the focal plane affords the considerable advantage of acting Oil the substances without passing through any intermediate material support. But, as emphasized by S/5, this supply of energy is only on the surface of the materials to be treated, and this may sometimes be a disadvantage. There are other difficulties as well. The energy is supplied to a very small surface. This disadvantage may be alleviated by cavity treatment. Similarly, this method of treatment Vol. 7, No. 3, I963
avoids the temperature inequalities due to a decreasing energy distribution in the focal spot. S/35 gives an excellent analysis of the operational conditions required for various physical measurements and chemical operations. Physical Measurements
Energy Measurements We have already mentioned the principle of calorimetric measurements for the absolute determination of the total incident flux on the various points of the focal spot. It is essential to have "black-body" cavities as perfect as possible. The measure of the incoming energy is given by the heating of a metal mass as a function of the time, or by the temperature difference produced in a current of water of known rate of flow. These experiments have been described previously, and in Paper S/35, by the Montlouis Laboratory, and are also discussed in particular in S/16, S/5 arid S/79. The accuracy according to the authors is ± 5 percent or :t:2 percent. These three papers describe the energy measurements by radiometers (Gardon et al.). One type of such radiometers, including a cooling circuit, is shown in Fig. 9 of S/16. These instruments, which are not absolute and must, of course, be calbrated, allow detailed exploration of the focal spot. Regulationand Redistribution of the Energy--The energy can be regulated on parallel or convergent radiation by means of Venetian blinds (S/79, S/16), or of screens of various shapes interposed after a program (Montlouis). S/79 mentions the use of flux redistributors (see Fig. 10) to give the fluxes a more uniform energy distribution. A great energy decrease is involved in this redistribution. TemperatureMeasurements--These important measurements were made, either on the surfaces, taking their emissivity into account, or on "black body" cavities in centrifugal or other furnaces. In all cases the direct reflection of the solar rays must be avoided, since it may lead to gross errors. a - - T h e first precise t e m p e r a t u r e measurements in solar
furnaces were made by Conn. The principle of his method cornprises the use of two rapidly rntating screens, one cutting off the solar radiation while the other passes the furnace radiation, and vice versa. He uses a disappearing-filament pyrometer to measure the brightness, obviously very low as compared tn the brightness of the source. Using the pyrometric relations and the transmission factor of the rotating screen, he calculates the real t e m p e r a t u r e of the heated surface. b - - P . E. Glaser, in paper S/16, recalls the methods of t e m p e r , d u r e measurement previously published t)3, him. The principle is derived from t h a t of Conn, but the use of photocells and inertialess measuring instruments permits direct temperature measurements during the period of interruption
of the solar radiation. c--M. Foiix--paper S/66--greatly extends the use of inertialess devices and screen methods to the study of the physical transformations of substances. He has found a number of solidification points of refractory oxides heated in rotating cavities (centrifugal furnaces), especially in S/66. Another 105
method mentioned in that paper (see Fig. 2) is to filter the incident solar radiation through materials with narrow but intense absorption bands. Within these bands the temperature measurements are made by the aid of suitable filters. The temperature measurements, especially the measurements of the transformation points of substances, show the great importance of regulating the solar energy with screens, and checking the operations, not with respect to energy, but to the temperature obtained, Emissivity Measurements were previously made by P. E. Glaser. S/66 also mentions several measurements of the selectivity of molten bodies. The measurements of emissivity and reflectivity at various temperatures should be continued, Measurements of thermal properties: (see papers S/66, S/5, S/95 and S/79). a--Specific heat: a set-up for this purpose has been given earlier by P. E. Glaser. Other tests at Montlouis have used "black-body" chambers to heat the product under examination and to measure its temperature before it is placed in the calorimeter, b--Heat of transformation: these very important experiments have not yet been undertaken by the aid of the solar furnace. c--Thermal expansion: This inethod has been utilized at Montlouis. The substances, in the form of rods with pointed ends, are heated in a black-body cavity. The length and ternperature of the specimen are found optically. In this way, in particular, the transformations of zirconium silicate and zirconjure oxide have been followed, d--Thermal conductivity: Experiments are now in progress in various laboratories, but no result appears to have been published yet.
Various physical measurements--It would probably be of interest to make measurements of electrical conductivity, magnetic susceptibility, thermionic emission, and in optical or electron microscopy, while using solar energy or convergent radiation to increase the temperature, Chemical Reactions When we come to industrial applications, we shall discuss reactions that can be exploited in large solar furnaces. In fundamental research, chemical studies in the solar furnace have a number of aspects. .~lelting without a crucible (direct melting or melting in a cavity), in a controlled atmosphere or in the free air, permits the preparation of innumerable binary and ternary mixtures of various refractory compounds, in order to study their phase diagrams and to determine various physical properties. It is to be regretted that the papers do not give more results in this field. The Montlouis Laboratory has studied the electrical conductivity of ZrO2-CaO and ZrO2-La2Oa mixtures, and the properties of various products based on alumina, alumina fl, spinels (A12OaMgO), chromites (Cr2OaMgO), etc. The author of S/57 (,Japan) has studied the melting 106
conditions for various oxides: A1203, MgO, CaO and ZrO. It should also be noted that studies on the conventional refractory oxides especially SiO2 and A12Oa have permitted study of the efficiency of solar furnaces as a function of their power. S/57 concerns a study of the purification of alumina (Al~O:~).I t appears from this paper that operations in vacuo permit good elimination of the impurities. Purification studies on a fairly large scale have been run at the Montlouis Laboratory (S/52), using the methods of distillation, of elimination of the relatively volatile impurities, of fractional crystallization or even segregation, and of zone melting. The great importance of these methods of purification for the future of solar furnaces has now been established. Among the other chemical r~actions accomplished in the solar furnace, which have led to new products, are the results given in papers S/108 and S/36. S/108 mentions the use of high-vacuum technique in the solar furnace, which has led to the development of new methods of treating rare-earth oxides, with special reference to the preparation of europium protoxide. The description of devices of the same kind in S/16, S/5 and S/57 are noted. S/36 mentions the preparation of boron carbide and the attack on stony beryl. This latter operation was accomplished with relative difficulty in the large solar furnace at Algiers, and the author now envisages a fixed-focus plant. The study of chemical reactions in the solar furnace is still entirely inadequate, in view of the research programs that are so extremely promising, which have been drafted by the various laboratories. R E S U L T S IN P H O T O C H E M I S T R Y An initial study performed prior by Justin Coumat at Algiers has shown the advantage of using ultraviolet radiation, concentrated at the focus of an installation with aluminium reflecting surfaces. The concentration of luminous, photochemical energy in the form of convergent radiation seems to have been utilized for the first time in these Algiers experiments. The method described comprises the use of various types of selectors: Filters passing the ultraviolet and reflecting the visible light (thin silver fihn); Filter passing the ultraviolet and absorbing the visible light; Solution absorbing the visible light and passing the ultraviolet (mixture of copper and cobalt sulfates; in the latter, the cooled absorbing solution is placed in the path of the convergent radiation). The ultraviolet energy available in the large Algiers furnace is of the order of 800 watts effective (that is, after filtration). This energy has been utilized, more particularly, in the chlorination of xylene, CsH,0 , and of benzene, in good yield. A second study, on which S/25 comments, wasperformed on a much smaller device, with a total power of several hundred watts. This paper is important for
Solar Energy
the economic future of solar photochemical reactions, and the reader should refer to it. POSSIBLE UTILIZATION OF SOLAR FURNACES While image furnaces seem destined to remain laboratory instruments, the question of the ultimate development of the solar furnace on the industrial scale can be raised. The first remark t h a t anyone can make is encouraging. The efficiency of a solar furnace, which utilizes the energy in its initial form, is a priori considerably higher than that of a solar engine, which must follow a whole cycle of low-efficiency transformations of energy. This reasoning also applies to photochemical transformation. Amortization of invested capital can thus be expected to be considerably faster with solar furnaces than with solar engines. Relying on the conclusions of S/81, another remark can be made. An increase in the power of the "heating unit" solar furnace yields a substantial i m p r o v e m e n t in the efficiency of high-temperature operations. The conclusions of this paper on the t r e a t m e n t of refractories are favorable. The a m o u n t of ultra-refractory oxides melted or fritted per day with a 1,000 kw furnace should reach 2 to 3 metric tons, or 2 to 3 kg per kw per day, while a 50-kw plant produces only 1 kg per kw, or 50 kg per day. The efficiencies observed with small 2-kw units are even smaller. A 1,000 kw plant, installed in a highly insolated zone, should thus produce 1,000 metric tons of refractory products. Still more powerful furnaces (10,000 or 20,000 kw) would not have much higher efficiencies and would be relatively more expensive, since the construction cost of the paraboloid and its mounting increases more rapidly t h a n the power of the installation. One might cite a number of examples of the treatments t h a t have been performed during the last few years with the medium-power solar furnace now operating at Montlouis (paper S/81). The operations in an oxidizing atmosphere are in particular, very appropriate for treating m a n y products based on refractory oxides. These t r e a t m e n t s were first practically applied at the Solar Energy L a b o r a t o r y of Montlouis, where some twelve metric tons of refractory oxides have been fused or treated. These products inelude corundum, magnesium spinel, and calcium zirconate, often prepared in the pure state, sometimes even as monoerystals. Various minerals have also been
Vol. 7, No. 3, 1963
treated at Montlouis b y methods dispensing with refractory walls, with their generally precarious hightemperature corrosion-resistance. Among these treatmerits of minerals, we m a y mention: production of zirconium oxide b y alkaline fusion of zircon with sodium carbonate, the treatment of beryl, or that of tungsten. The future of the solar furnace also appears particularly bright for the preparation of pure products or of products prepared under special conditions (totally oxidizing or totally neutral medium), or of special products, such as monocrystals. I t is appropriate to recall that the operations on a "hot front" m a y also have an industrial future, like the metallurgical operations reported in S/35. Photochemical reactions m a y lead to economically feasible methods of preparation. In view of the fragmentary information available we are not yet able to establish the capital cost, per installed kw, of solar furnaces of various power outputs. PAPERS CONTRIBUTED TO THE SESSION ON A G E N D A I T E M I I I . F "New Techniques and Possibilities in Solar Furnaces", T. Laszlo--S/5 "Industrial Applications--The Challenge to Solar-Furnttee Research", P. Glaser--S/16 "Construction of a Large Solar Furnace", T. Sakurai K. Shishido, O. Kamada and K. Inagaki--S/21 "Chemical Syntheses in the Solar Furnace", R. Marcus and H. Wohlers--S/25 "Conditions de Traitement et Mesures Physiques dams les Fours Solaires", F. Trombe, M. Fofix and C. La Blanchetais -S/35 "Obtention de Carbure de Bore et Traitement Thermique de Mindraux au Four Solaire", G. Vuillard--S/36 "Etude sur les Concentrations l~nergdtiques Donndes par les Miroirs Paraboliques de Trbs Grande Surface", A. LePhat-Vinh--S/48 "Purifications des Substances par Chauffage au Four Solaire", F. Trombe and M. Fo~x--S/52 "Applied Research in a Solar Furnace", T. Noguchi, M. Mizuno, N. Nakayama and H. Hayashi S/57 "Mesure des Tempdratures au Four Solairc", M. Fo~x--S/66 "hnage Quality and Use of the U. S. Army Quartermaster Solar Furnace", E. Cotton, W. Lynch, W. Zagieboylo and J. Davies S/79 "Les Applications Pratiques Actuelles des Fours solaires et leurs Possibilities Economiques de Ddveloppement", F. Trome and M. Fo~x S/81 "Traitement sous Haut Vide au Four Solaire--I)ispositif Exp6rimental, Prdparation et ]~tude de Noveaux Composds des Terres Rares", J. Achard--S/108
107
HE uses of solar energy described in this report involve optical apparatus giving high concentrations of the energy received from direct solar radiation, The total absorption of this energy, concentrated on a relatively small surface or in a relatively small volume, results in high local temperatures. This is the effect obtained in what are incorrectly termed "solar furnaces." This term should, strictly speaking, be applied only to the receiving zone, which may or may not be a furnace, For certain uses of optically concentrated solar radiation, the effect desired is not the temperature rise, which is often very injurious, but the photochemical action, due to photon absorption in certain restricted wavelength regions, Hence, this report will include all uses of highly concentrated solar radiation, whether they employ the total radiation, or only the energy contributed by cerrain wavelength regions. Many of the techniques for collecting convergent solar radiation that are here described are similarly applicable to the exploitation of the thermal effects resulting from the convergence of various types of artificial radiation. These installations, termed "image furnaces" in the United States, are becoming important in research laboratories. Convergent arrays of the type now used to concentrate solar radiation and utilize its photochemical effect might likewise be suggested for certain artificial sources with an abundant emission of wavelengths suitable for photochemical uses. The energy available in direct solar radiation seems so small as to be altogether disproportionate for use in producing high temperatures. Beyond the stratosphere there is about 1350 watts per square meter so available, and at ground level, under optimum conditions, some 1000 watts per square meter. This energy density corresponds roughly to the radiation of a black body at a temperature of only 100 deg C. The spectral distribution of energy, especially that given in Fig. 1 of paper S/66, discloses better prospects. The solar energy corresponds to the radiation of a black body at about 6000 deg K. At ground level it is dispersed, but not de-
T
* General report of r a p p o r t e u r for Session I I I F, United Nations Conference on New Sources of Energy, Rome, Italy, Aug., 1961. 100
graded. The small temperature rise it induces is due to the very small solid angle of 7 × 10-5 steradian through which the sun "lights" the earth. A great increase in this solid angle of illumination would thus be sufficient to obtain a localization of surface energy that could yield high temperatures. This localization is realized in practice at the "focus" of an optical system of very great aperture. The maximum aperture of a paraboloid for the illumination of a plane is given by the ratio D/f = 4, where D is the diameter of the mirror and f the focal length. For illumination of a cylinder or a sphere, the value of D/'f can be still further, and substantially, increased. The high concentrations of radiation required for high temperature operations, or to localize energy for photochemical use, lead to the employment of optical systems under rather severe conditions. The axis of the system must be parallel to the incident radiation. This condition is likewise imperative for maximum incident energy reception at the various hours of the day. Since the apparent position of the sun varies during the day, and is different each day, the receiving system must be constantly oriented if the above conditions are to be satisfied. Two methods have been used for this purpose. In one the optical "concentrator" (parabolic mirror or lens) is turned directly towards the sun; in the other, it is fixed and receives the solar radiation following its axis by means of a moving plane "orienting" mirror. The principal shortcoming of solar radiation for practical uses is its intermittence, whether regular (sequence of days and nights), or accidental, due to interposition of clouds or fogs, screening the terrestrial surface from the sun. In many regions between the tropics and the temperate zone, a solar "breakdown is rare, but the daily intermittence leads one to envisage either operations that can be completed during a daily radiation cycle, or operations involving no thermal mass, which can be rapidly started and stopped. SOLAR MACHINES Direct Receivers, O r i e n t e d Receivers, a n d T h e i r Heliostats Direct reception has the advantage of involving only a single reflection on the mirrors that concentrates the radiation. Various types of direct receivers are given in Solar Energy
Fig. 1 of paper S/16. The principal disadvantage of this type is the fact that the axis of convergence of the radiation is displaced in space, and, consequently, that the focus, located on this axis, is also displaced in space. This arrangement is also unfavorable, since the convergent radiation attacks substances from down to up, or laterally, and it is difficult to perform certain operations, for instance melting, with such apparatus. On the other hand, for operations on solid surfaces, reaction between gases, cracking, etc .... , direct reception is the simplest method. In some assemblies (Meudon apparatus, 1946), the image of the sun is reflected downward by means of a plane mirror placed in the path of the convergent rays. Direct reception can be viewed more favorably for apparatus with a smaller aperture than paraboloids and using lenses. The only solar furnace using lenses that is in operation is at the California Institute of Technology. It moves about its focus, which is itself fixed in space, The reception of solar radiation by a plane orienting mirror, followed by a fixed parabolic mirror, was first put into practice by Corm, and later by the Montlouis Laboratory. This method can now be generalized for most laboratory devices and large-scale installations, It has the considerable advantage of having a fixed focus of orientation, whether horizontal, vertical, or inclined, that can be selected according to the operation being performed. To keep the optical axis of direct receivers pointing to the sun they may be guided either by an astronomical method or by a control mechanism with cells or transistors. Some installations, especially the large solar furnace of Algiers, are mounted on an equatorial axis whose motion is controlled by an astronomical clock, This method has the advantage of holding the axis of the paraboloid in the direction of the sun, even if the solar radiation is briefly intercepted by a cloud. The declination of the mirror is naturally adjusted daily by hand. It would seem that this method of control would be rather expensive for large machines like the one at Algiers. Nevertheless it appears to have been checked for small devices by Peter E. Glaser (paper S/'16). The collector mirrors are often automatically trained on the sun by using an auxiliary lens with its optical axis parallel to that of the paraboloid. A circular screen, located in the focal plane and substantially of the same size as the solar image given by the lens, is surrounded by four control photocells or transistors, operating pairwise to control the two directions in elevation and the two directions in azimuth. A slight displacement of the solar image to outside the screen illuminates one or two of the control photocells. This arrangement has been universally adopted for guidance of direct receivers, or, as we shall see, to control the motion of heliostats. The methods of guidance differ primarily in the conVol. 7, No. 3, 1963
ditions under which the currents from the cells are used to produce the ultimate motion of the mirrors. Heliostats are, in general, guided by a control system (lens, screen, and framing cells) located in the path of the reflected radiation. This control arrangement is fixed, and the direction from the lens to the screen center defines the direction of the reflected radiation. The photocell currents can be used in various ways to cause motion of the heliostat or the direct reception devices. The first Montlouis model controlled electric motors by a series of contacts of decreasing sensitivity, which initiated the operation, in either sense, of asynchronous motors. One adjusted the elevation, the other the azimuth, of the heliostat. Parallel developments, simultaneous in different countries (United States, USSR, France, Japan, and Israel for example) today employ the following schematic arrangement : current from a photocell or phototransistor, electronic amplification of this current, control of the excitation of a dynamo, of Amplidyne type or similar device. Direct current generated by the dynamo runs a direct-current motor in the proper direction. One of these systems controls the changes of elevation, the other the changes of azimuth. This electrical command system permits realization of simultaneous motion of a certain number of heliostats controlled by a master heliostat using mechanical coupling. Electronic coupling does not seem to have been used. Mechanical coupling can insure excellent accuracy of control if the transmission shafts run at adequate rotary speed. Certain devices do not include an electric motor for aligning the machines. The amplified photocell currents act to change the position of distributor slides or servo-valves, admitting oil under pressure into doubleaction hydraulic jacks, one controlling the elevation and the other the azimuth. As before, two elevation photocells, upper and lower, act on the two faces of the same actuating cylinder, while two azimuth photocells, right and left, act on the second cylinder. The axes of rotation of heliostats are in general perpendicular. This system is used to operate the large Montlouis heliostat and will be used for the 63 heliostats to be installed at the 1000-kw solar furnace at d'Odeillo-Font-Romeu. It is also mentioned in paper S/57 of this Conference. Plane Reflections Various experiments have shown that for long-distance reflections, glass surfaces coated on their front or rear faces with metallic reflectors give good results. They appear to be better than reflectors made of polished metal planes. Metallized plastic glass or metMlized plastic film stretched on metal frames do not seem to have been tried for long-distance plane reflectiou. The heliostats described in various papers all use surfaces of metallized glass. The dimensions of the unit lOl
surface are selected on the basis of various factors. The French installations (Montlouis)and the United States Army installation (Natick) of paper S/79, use the reflector on the rear surface after passing through the glass. The thickness and transparency of the glass are thus important for efficiency of the plane reflection. On the other hand, the mounting of the glass must be substantially plane for the various positions assumed by the heliostat. The thickness permitting this result will naturally be greater, the larger the elementary surface itself. Moreover, to avoid losses of light at the glass interfaces, it is better to adopt the maximum dimensions. To give effect to all these factors, the dimensions adopted are 50 by 50 cm by 6 to 7 mm thickhess in the Montlouis installations, and 62 by 62 cm by 0.635 cm at Natick. In these two installations, the reflecting surface consists of a layer of cllemically deposited silver electrolytically coated with copper, and then coated with a protective varnish. At Natick one aluminized adhesive surface is also used. The normal transmission of energy by such reflectors is from 80 to 84 percent, according to the number of glass plates, the vitreous reflections being recovered in the reflected radiation. The behavior of such surfaces in bad weather is excellent. The Montlouis reflectors, in particular, have been exposed for the last ten years to a severe climate, and their reflecting properties are still excellent, Another class of plane reflectors consist of glass surfaces coated on the front face with thermally deposited aluminium. Owing to their high reflectivity, these surfaces were adopted by the authors of paper S/21 for the large solar furnace at Sendal (Japan). The experience they reported includes the following details: a glass mirror 90 by 100 cm, by 10 mm thick, with silvered or aluminized rear face, had a reflection factor of 67 percent for an incident ray of a 15 deg inclination, and one of 56 percent for an incident ray of 40 deg. The aluminium deposit under the same conditions gives 95 and 92 percent respectively. Aluminium surfaces have the important advantage, especially for photochemical reactions, of reflecting solar ultraviolet radiation. On the other hand, their resistance to bad weather is uncertain. This resistance is improved by a protective deposit of silicon monoxide, Another factor to be considered is the resistance of glass to thermal shock and mechanical stress. Experience shows that glass sheets 1 by 1 meter by 1 cm thick are much more fragile, thermally and mechanically, than glass plates 50 by 50 cm but less thick. The alignment of the elementary reflecting planes to obtain a parallel and continuous reflected beam requires individual alignment of each plane supported at three points. Such alignments are in general accomplished with precise topographic instruments, by sighting a point sufficiently distant directly and through the glass, 102
In the use of plane reflection, one must take account of the relative increase in the penumbral zones with increasing distance between the heliostat and the parabolic reflector. These zones act only on the outer contours of the reflected nappes. The surface of the parabolic reflector must be inside this aureole of decreasing energy. The metal framework supporting the reflecting mirrors cannot be described here. Such frameworks must be realized under the desired conditions so as not to form double sheets and must remain plane at the various operating temperatures of the installations. Convergent Reflections The problem of realizing reflecting or refracting surfaces has various solutions: The large aperture necessalT to get appropriate accumulation of energy practically rules out lenses, which are, incidentally, considerably more expensive than mirrors. For small mirrors, the reflecting surface is usually in a single piece. The first laboratory solar furnaces made extensive use of military anti-aircraft mirrors. Such mirrors, 1.5 to 2 meters in diameter, are made either of special water-white glass, silvered on the rear face, or of a polished alloy. Because of the severity of the specifications for their initial use their optical quality is entirely adequate for high concentration of energy. They are valuable for fundamental research with either solar furnaces or image furnaces. Aluminium reflectors can be larger. The largest one in a single piece (3 meters in diameter) was built by Conn at Kansas City. Other aluminium mirrors, made up arrays of metal segments, have been constructed (Algiers). Paper S/16 describes (Fig. 3) a light aluminum mirror 30 feet (about l0 meters) in diameter. Although they do not appear to have the optical qualities of smaller mirrors, especially that of Conn, they do constitute an interesting and economical solution for moderate concentration of useful amounts of energy. The aluminum sheets can be fabricated by drawing on a mould or explosive forming on a mould by the sudden pressure of a gas liberated by an explosion (paper S/16). Another interesting solution for small- and mediumsize equipment is the use of plastics. Parabolic plastic moulds have been produced by solidification of a plastic resin on a mercury surface rotating at constant velocity. The use of aluminized Mylar on a polyurethane mould gives mirrors of good optical quality (Fig. 5, paper S/16). Large installations require separate sectors, individually aligned on a rigid common mounting. The arrangement used at Montlouis and Algiers, which is also mentioned in paper S/21, Fig. 3, can be adopted. It has the advantage that all elements in a single reflecting ring are identical. There are only as many Solar Energy
different reflecting elements as there are rings, thus permitting advantageous duplication in the fabrication and alignment of the elements. The reflecting elements can also be aligned on mounts outlining horizontal and vertical parabolic profiles (Natick installation, paper S/79)--(d'Odeillo - Font - Romeu installation--paper S/48). The problem posed by the design of large solar furnaces with high concentration of energy is that of the unit reflector element, as already mentioned, whether used in concentric rings, or in horizontal alignment on mountings, to form the paraboloid. Each large solar furnace built or now under construction has embodied a different solution in this respect. The first solar furnace built at Montlouis, France, in 1952, which is still in service, comprises elements positioned on concentric rings. Each element supports a number of glass plates individually aligned. These are plane glass plates, but thin enough to be deformed by pressure. They are permanently subjected to pressures in front and behind, giving them the approximate shape of a small sector of the paraboloid to be occupied on the reflecting surface, It has been demonstrated by the Montlouis installation, and now by the larger glass plates for the large 1,0O0-kw solar furnace of Odeillo-Font-Romeu, that glass plates so subjected to permanent stress give at least the same concentration as five plane glass plates adjusted so that their images converge at the focus of the system (see paper S/48). The Montlouis installation, comprising five concentric rings and 3500 glass plates under stress, has five types of reflecting elements. At the Odeillo-Font-Romeu installation, which will have a much larger number of glass elements, the alignment will not be repeated, but will be done on the site. This arrangement makes it possible to construct mirrors giving high concentrations, subject to the obvious condition that the number of glass plates is sufficient. It also has the advantage of holding the glass plates firmly in place on their mountings, thus avoiding all oscillation of the reflected radiation, especially the wind. This arrangement appears to be the most economic for very large solar furnaces, such as the one at OdeilloFont-Romeu, which has a large number of convergent reflecting glass elements. The second installation, the one at Algiers, is 8.40
cost of stamped sheet if the quantity is large enough to justify the cost of the dies. The large solar furnace built at Natick, Massachusetts, by the United States Army, has focusing elements of a peculiar type. There are 180 elementary surfaces, formed of concave mirror segments 62 by 62 cm. They are aligned in successive horizontal profiles on a mounting realizing the paraboloid. Each spherical element was produced by forming, at a suitable temperature, a polished glass plate, against a spherical ceramic mould. The glass face in contact with the ceramic deteriorates partly, but the free face retains its polish and has a good sphericity. These mirrors are aluminized on the front (concave) face and are then individually aligned to cause the reflected rays to converge at the focus (see paper S/79). The large solar furnace under construction at Sendal, Japan (see paper S/21) consists of 181 polished glass surfaces, hot moulded into paraboloidal shape. The concave faces were then thermally aluminized. The merits of this process, which resembles the one used for the Natick solar furnace, are not yet known. It should, nevertheless, be noted that the sectors of the paraboloid should present the same surface finish as the spherical sectors of the Natick installation. Since they are moulded into the parabolic shape correspondlug to their position on the reflecting surface, they probably will give high optical concentrations. With the single exception of the Algiers installation, which directly receives solar radiation and appears to be intended for photochemical studies and gas reactions, all semi-industrial solar furnaces have a heliostat refleeting the sunlight onto a fixed parabolic mirror with a horizontal axis. The only plant whose mirror reflects the radiation in a direction parallel to the celestial axis is that described in S/36, but this paper fails to give the dimensions of the installation. A comparison of the large solar furnaces shows that the plants developed or being developed by the various countries resemble each other rather closely in dimensions and principles of tracking, except for the focusing surfaces themselves (see S/79, S/21 and S/48). The Natick installation includes a large attenuator screen placed in the path of the parallel horizontal radiation. The screens of the French installation (Montlouis) are placed in the path of the convergent radiation.
meters in diameter, and is composed of aluminum sectors produced by stamping on rigid dies. This installation is a direct reception model, and the aluminum surfaces, chemically polished and then anodically oxidized, reflect practically all the solar ultraviolet that reaches the ground. The degree of perfection that can be given by this method is not yet known. The concentration is apparently the same as that yielded by other types. The principal advantage of stamping is the low
THE ENERGY CONCENTRATIONS AT THE FOCUS
l'ol. 7, No. 3, 1963
Solar concentrators, especially the small ones, provide great concentrations of energy. This does not imply that an increase in size is a factor unfavorable to the procurement of high concentrations at the focus (see paper S/5). It would seem, rather, that the concentrations can be very great for single mirrors of great aperture and optically perfect enough, but that the 103
larger models, using sectors, call be realized just as perfectly, provided cost is no object. For very large solar furnaces, the subdivision of the reflecting elements, which is necessarily more advanced, should give energy concentrations as high as those from the smaller ones. For these large installations, it seems that economic conditions of realization and high concentrations of energy are simultaneously possible, The concentrations given by low-power furnaces appear in some cases to be substantial. The 2 meter glass mirrors at Montlouis, France, give experimental energy fluxes in excess of 350 cal per sq cm per sec for an incident energy of 1 kw per sq meter. (1 kw is equivalent to about 240 cal per sq meter per sec.) Table I of paper S/79 gives the measured fluxes for various furnaces. The lens solar furnace at the California Institute of Technology has a flux of 220 cal per sq cm per sec,the Curtiss Wright solar furnace has 600, and the USSR solar furnace, 864. These latter values must obviously correspond to apparatus of very high optical quality. The concentrations of high-power solar furnaces determiued today are clearly below these levels. The large solar furnace of Montlouis gives 0.75 ~o 0.8 kw per sq cm, or a little less than 200 cal per sq cm per sec. The Natick solar furnace (paper S/79) gives a flux of 100 cal per sq cm per sec, but it was not designed to obtain high fluxes, but to realize the most uniform energy possible in the solar spot. It is especially noteworthy that the maximum concentration calculated for the central zone of the furnace should be substantially in agreement with the experimental results. The experimental curve shows that energy uniform to within less than 5 percent can be obtained for a spot 3 cm in diameter, and energy uniform to within l0 percent for a spot 4 cm in diameter. The present Montlouis furnace, which has a focal length of neters, gives a rather similar energy curve, although the central concentralion, several centimeters in diameter, is almost double. Such variations in energy density have only a slight effect on the equilibrium temperatures. We recall the Stefan relation between the total energy E radiated by a black body and its temperature: E = aT 4. The ternperaturc measurements for the plant at Sendal, Japan (paper S/21) are as yet unavailable to us, but it would seem that rather high energy fluxes can be expected. The calculations given in paper S/'48 on the large French solar furnaceundcrconstruction at Odeillo-Font-Romen indicate that, allowing for all the losses, an energy of 1700 watts per sq cm, or over 400 cal per sq cm per sec, should be obtained in the central spot. The equilibrium temperature of the focus, counting only the radiation losses, would be 3900°C for a black body. In this connertion we must stress the fact that the equilibrium ternperatures are not the operating temperatures of solar furnaces. Even for fundamental research, the working temperatures used, for instance, in the neighborhood of 104
3000 deg, are several hundred degrees below the equilibrium temperature. T H E C O L L E C T I O N OF T H E SOLAR E N E R G Y AT T H E FOCUS There are numerous publications on these subjects, especially by the Montlouis Laboratory. A number of methods of collecting the solar energy will be found in S/35 (Figs. 5, 6, 7, 8, 9, 10, l l , 12 and 14). Theywill also be found ill papers S/5, S/16, S/108, S/66,S/52 and S/57. These methods call be reduced to a number of typical operating patterns. Direct Collection
Treatment at the focus, whether on a horizontal, vertical, or inclined surface. A self-supporting body can be treated on a horizontal surface. This is the typical operation in an oxidizing medium, and involves no contamination of the substances treated. The efficiency of this procedure can be improved by displacing the substance being irradiated, so as to obtain belts of melted or fritted products in various shapes. The powdered substance can likewise be spread continuously on the treated surface, which is progressively lowered. In this way one obtains melted rods of substantially the same diameter as the image cast by the optical system. Rotation makes the melted massesmore regular. Direct treatments can likewise be applied to solid substances, for instance to rods held vertically, practically colinear with the focal axis of the optical system. By shifting the rod in the direction of the focal axis, a molten zone can be made to migrate from one end of the rod to the other. This method can also be used when the rod is supported laterally by cooled jaws (see paper S/52, Figs. 4, 5 and 7). Paper S/35, Fig. 12, presents one version of direct treatment permitting treating practically all the substance, without contamination. The material is heated on a metal plate (aluminium or copper) of high thermal conductivity, and vigorously cooled on the bottom. It is advantageous to use relatively thin metal plates (several millimetres thick). All the above methods can be employed in a controlled atmosphere. The use of vacuum requires mechanically resistant and transparent walls. For small heating units, Pyrex glass and silica glass, both of them transparent to solar radiation, are suitable. For large units, the problem of the transparent wall with high mechanical resistance still remains open. The following is an interesting and inexpensive solution. Where the controlled atmosphere remains under ambient pressure, paper S/52, Fig. 3, indicates that the solar energy is admitted through a plastic membrane inflated by a slight overpressure of the gas constituting Solar E~erg!!
the controlled atmosphere. Such containers, of large dimensions and low cost, which are consequently economically expendable for each operation, can be made of thin plastic films of various kinds, According to paper S/35, phenomena of positive photophoresis are observed on transparent walls through which the solar radiation passes, especially in gases like hydrogen, at pressures from several millimeters to 1 cm Hg. This phenomenon does not appear to be connected with the heating of the transparent wall. Cavity Reception Paper S/35, and several others--S/66, S/36, S/5 and S / 1 6 - - m e n t i o n the advantage of treatment in cavities, either to obtain better yields with reflecting substances, or to get more regular variations of temperature in a given operation. The cavities developed (see S/35, Figs. 9 and 10), behave like black bodies, and their inner walls heat up regularly, owing to the numerous internal reflections. They are in general characterized by an internal surface much larger than the outer surface, permitting the penetration of convergent radiation, The scheme of black-body cavity that should be especially emphasized is represented in S/35 (Fig. 10). The substance, held against the furnace walls by the action of the centrifugal force, is treated with no contamination whatsoever. This type of furnace, in particular, affords the advantages of cavity treatment and of direct treatment of a selfsustaining substance. Cavity operations can be conducted in vacuo (for small devices) or in an inert gas. It should be noted that a controlled atmosphere can be realized in centrifugal furnaces by supplying the gas to the furnace interior itself, Treatment in a cavity behaving like a black body finds application in determining the energy distribution of the solar energy on the focal plane. Black-body cavities equipped for calorimetric measurements are used to explore the focal spot at different points of its surface, since the surface of the radiation-access orifice c a n be modified at will by interposing refrigerated s c r e e n s h a v i n g orifices of various diameters, R E S U L T S O B T A I N E D IN T H E H I G H TEMPERATURE FIELD The energy supplied to the focal plane affords the considerable advantage of acting Oil the substances without passing through any intermediate material support. But, as emphasized by S/5, this supply of energy is only on the surface of the materials to be treated, and this may sometimes be a disadvantage. There are other difficulties as well. The energy is supplied to a very small surface. This disadvantage may be alleviated by cavity treatment. Similarly, this method of treatment Vol. 7, No. 3, I963
avoids the temperature inequalities due to a decreasing energy distribution in the focal spot. S/35 gives an excellent analysis of the operational conditions required for various physical measurements and chemical operations. Physical Measurements
Energy Measurements We have already mentioned the principle of calorimetric measurements for the absolute determination of the total incident flux on the various points of the focal spot. It is essential to have "black-body" cavities as perfect as possible. The measure of the incoming energy is given by the heating of a metal mass as a function of the time, or by the temperature difference produced in a current of water of known rate of flow. These experiments have been described previously, and in Paper S/35, by the Montlouis Laboratory, and are also discussed in particular in S/16, S/5 arid S/79. The accuracy according to the authors is ± 5 percent or :t:2 percent. These three papers describe the energy measurements by radiometers (Gardon et al.). One type of such radiometers, including a cooling circuit, is shown in Fig. 9 of S/16. These instruments, which are not absolute and must, of course, be calbrated, allow detailed exploration of the focal spot. Regulationand Redistribution of the Energy--The energy can be regulated on parallel or convergent radiation by means of Venetian blinds (S/79, S/16), or of screens of various shapes interposed after a program (Montlouis). S/79 mentions the use of flux redistributors (see Fig. 10) to give the fluxes a more uniform energy distribution. A great energy decrease is involved in this redistribution. TemperatureMeasurements--These important measurements were made, either on the surfaces, taking their emissivity into account, or on "black body" cavities in centrifugal or other furnaces. In all cases the direct reflection of the solar rays must be avoided, since it may lead to gross errors. a - - T h e first precise t e m p e r a t u r e measurements in solar
furnaces were made by Conn. The principle of his method cornprises the use of two rapidly rntating screens, one cutting off the solar radiation while the other passes the furnace radiation, and vice versa. He uses a disappearing-filament pyrometer to measure the brightness, obviously very low as compared tn the brightness of the source. Using the pyrometric relations and the transmission factor of the rotating screen, he calculates the real t e m p e r a t u r e of the heated surface. b - - P . E. Glaser, in paper S/16, recalls the methods of t e m p e r , d u r e measurement previously published t)3, him. The principle is derived from t h a t of Conn, but the use of photocells and inertialess measuring instruments permits direct temperature measurements during the period of interruption
of the solar radiation. c--M. Foiix--paper S/66--greatly extends the use of inertialess devices and screen methods to the study of the physical transformations of substances. He has found a number of solidification points of refractory oxides heated in rotating cavities (centrifugal furnaces), especially in S/66. Another 105
method mentioned in that paper (see Fig. 2) is to filter the incident solar radiation through materials with narrow but intense absorption bands. Within these bands the temperature measurements are made by the aid of suitable filters. The temperature measurements, especially the measurements of the transformation points of substances, show the great importance of regulating the solar energy with screens, and checking the operations, not with respect to energy, but to the temperature obtained, Emissivity Measurements were previously made by P. E. Glaser. S/66 also mentions several measurements of the selectivity of molten bodies. The measurements of emissivity and reflectivity at various temperatures should be continued, Measurements of thermal properties: (see papers S/66, S/5, S/95 and S/79). a--Specific heat: a set-up for this purpose has been given earlier by P. E. Glaser. Other tests at Montlouis have used "black-body" chambers to heat the product under examination and to measure its temperature before it is placed in the calorimeter, b--Heat of transformation: these very important experiments have not yet been undertaken by the aid of the solar furnace. c--Thermal expansion: This inethod has been utilized at Montlouis. The substances, in the form of rods with pointed ends, are heated in a black-body cavity. The length and ternperature of the specimen are found optically. In this way, in particular, the transformations of zirconium silicate and zirconjure oxide have been followed, d--Thermal conductivity: Experiments are now in progress in various laboratories, but no result appears to have been published yet.
Various physical measurements--It would probably be of interest to make measurements of electrical conductivity, magnetic susceptibility, thermionic emission, and in optical or electron microscopy, while using solar energy or convergent radiation to increase the temperature, Chemical Reactions When we come to industrial applications, we shall discuss reactions that can be exploited in large solar furnaces. In fundamental research, chemical studies in the solar furnace have a number of aspects. .~lelting without a crucible (direct melting or melting in a cavity), in a controlled atmosphere or in the free air, permits the preparation of innumerable binary and ternary mixtures of various refractory compounds, in order to study their phase diagrams and to determine various physical properties. It is to be regretted that the papers do not give more results in this field. The Montlouis Laboratory has studied the electrical conductivity of ZrO2-CaO and ZrO2-La2Oa mixtures, and the properties of various products based on alumina, alumina fl, spinels (A12OaMgO), chromites (Cr2OaMgO), etc. The author of S/57 (,Japan) has studied the melting 106
conditions for various oxides: A1203, MgO, CaO and ZrO. It should also be noted that studies on the conventional refractory oxides especially SiO2 and A12Oa have permitted study of the efficiency of solar furnaces as a function of their power. S/57 concerns a study of the purification of alumina (Al~O:~).I t appears from this paper that operations in vacuo permit good elimination of the impurities. Purification studies on a fairly large scale have been run at the Montlouis Laboratory (S/52), using the methods of distillation, of elimination of the relatively volatile impurities, of fractional crystallization or even segregation, and of zone melting. The great importance of these methods of purification for the future of solar furnaces has now been established. Among the other chemical r~actions accomplished in the solar furnace, which have led to new products, are the results given in papers S/108 and S/36. S/108 mentions the use of high-vacuum technique in the solar furnace, which has led to the development of new methods of treating rare-earth oxides, with special reference to the preparation of europium protoxide. The description of devices of the same kind in S/16, S/5 and S/57 are noted. S/36 mentions the preparation of boron carbide and the attack on stony beryl. This latter operation was accomplished with relative difficulty in the large solar furnace at Algiers, and the author now envisages a fixed-focus plant. The study of chemical reactions in the solar furnace is still entirely inadequate, in view of the research programs that are so extremely promising, which have been drafted by the various laboratories. R E S U L T S IN P H O T O C H E M I S T R Y An initial study performed prior by Justin Coumat at Algiers has shown the advantage of using ultraviolet radiation, concentrated at the focus of an installation with aluminium reflecting surfaces. The concentration of luminous, photochemical energy in the form of convergent radiation seems to have been utilized for the first time in these Algiers experiments. The method described comprises the use of various types of selectors: Filters passing the ultraviolet and reflecting the visible light (thin silver fihn); Filter passing the ultraviolet and absorbing the visible light; Solution absorbing the visible light and passing the ultraviolet (mixture of copper and cobalt sulfates; in the latter, the cooled absorbing solution is placed in the path of the convergent radiation). The ultraviolet energy available in the large Algiers furnace is of the order of 800 watts effective (that is, after filtration). This energy has been utilized, more particularly, in the chlorination of xylene, CsH,0 , and of benzene, in good yield. A second study, on which S/25 comments, wasperformed on a much smaller device, with a total power of several hundred watts. This paper is important for
Solar Energy
the economic future of solar photochemical reactions, and the reader should refer to it. POSSIBLE UTILIZATION OF SOLAR FURNACES While image furnaces seem destined to remain laboratory instruments, the question of the ultimate development of the solar furnace on the industrial scale can be raised. The first remark t h a t anyone can make is encouraging. The efficiency of a solar furnace, which utilizes the energy in its initial form, is a priori considerably higher than that of a solar engine, which must follow a whole cycle of low-efficiency transformations of energy. This reasoning also applies to photochemical transformation. Amortization of invested capital can thus be expected to be considerably faster with solar furnaces than with solar engines. Relying on the conclusions of S/81, another remark can be made. An increase in the power of the "heating unit" solar furnace yields a substantial i m p r o v e m e n t in the efficiency of high-temperature operations. The conclusions of this paper on the t r e a t m e n t of refractories are favorable. The a m o u n t of ultra-refractory oxides melted or fritted per day with a 1,000 kw furnace should reach 2 to 3 metric tons, or 2 to 3 kg per kw per day, while a 50-kw plant produces only 1 kg per kw, or 50 kg per day. The efficiencies observed with small 2-kw units are even smaller. A 1,000 kw plant, installed in a highly insolated zone, should thus produce 1,000 metric tons of refractory products. Still more powerful furnaces (10,000 or 20,000 kw) would not have much higher efficiencies and would be relatively more expensive, since the construction cost of the paraboloid and its mounting increases more rapidly t h a n the power of the installation. One might cite a number of examples of the treatments t h a t have been performed during the last few years with the medium-power solar furnace now operating at Montlouis (paper S/81). The operations in an oxidizing atmosphere are in particular, very appropriate for treating m a n y products based on refractory oxides. These t r e a t m e n t s were first practically applied at the Solar Energy L a b o r a t o r y of Montlouis, where some twelve metric tons of refractory oxides have been fused or treated. These products inelude corundum, magnesium spinel, and calcium zirconate, often prepared in the pure state, sometimes even as monoerystals. Various minerals have also been
Vol. 7, No. 3, 1963
treated at Montlouis b y methods dispensing with refractory walls, with their generally precarious hightemperature corrosion-resistance. Among these treatmerits of minerals, we m a y mention: production of zirconium oxide b y alkaline fusion of zircon with sodium carbonate, the treatment of beryl, or that of tungsten. The future of the solar furnace also appears particularly bright for the preparation of pure products or of products prepared under special conditions (totally oxidizing or totally neutral medium), or of special products, such as monocrystals. I t is appropriate to recall that the operations on a "hot front" m a y also have an industrial future, like the metallurgical operations reported in S/35. Photochemical reactions m a y lead to economically feasible methods of preparation. In view of the fragmentary information available we are not yet able to establish the capital cost, per installed kw, of solar furnaces of various power outputs. PAPERS CONTRIBUTED TO THE SESSION ON A G E N D A I T E M I I I . F "New Techniques and Possibilities in Solar Furnaces", T. Laszlo--S/5 "Industrial Applications--The Challenge to Solar-Furnttee Research", P. Glaser--S/16 "Construction of a Large Solar Furnace", T. Sakurai K. Shishido, O. Kamada and K. Inagaki--S/21 "Chemical Syntheses in the Solar Furnace", R. Marcus and H. Wohlers--S/25 "Conditions de Traitement et Mesures Physiques dams les Fours Solaires", F. Trombe, M. Fofix and C. La Blanchetais -S/35 "Obtention de Carbure de Bore et Traitement Thermique de Mindraux au Four Solaire", G. Vuillard--S/36 "Etude sur les Concentrations l~nergdtiques Donndes par les Miroirs Paraboliques de Trbs Grande Surface", A. LePhat-Vinh--S/48 "Purifications des Substances par Chauffage au Four Solaire", F. Trombe and M. Fo~x--S/52 "Applied Research in a Solar Furnace", T. Noguchi, M. Mizuno, N. Nakayama and H. Hayashi S/57 "Mesure des Tempdratures au Four Solairc", M. Fo~x--S/66 "hnage Quality and Use of the U. S. Army Quartermaster Solar Furnace", E. Cotton, W. Lynch, W. Zagieboylo and J. Davies S/79 "Les Applications Pratiques Actuelles des Fours solaires et leurs Possibilities Economiques de Ddveloppement", F. Trome and M. Fo~x S/81 "Traitement sous Haut Vide au Four Solaire--I)ispositif Exp6rimental, Prdparation et ]~tude de Noveaux Composds des Terres Rares", J. Achard--S/108
107