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Special glasses and mountings for the utilization of solar energy
cessively. For example, in case of doubt about sunshine conditions on the following day, auxiliary heating (mainly electric) is switched on, in which case, if the following day turns out to be clear or ahnost clear, solar radiation is wasted. Figures based on these considerations show that under local conditions, the minimum auxiliary heating reaches 10 to 15 percent of the total year-round heat. On the other hand, the actual use by the average consumer is much higher and reaches 28 to 30 percent of the total heating. To exploit the maximum solar radiation while reducing auxiliary power consumption to a minimum, as well as make sure of the amount and temperature of the heated water, a special device named by the authors "a solar switch" has been designed, constructed, checked, and used. This switch permits automatic control of the supply of electric energy and operates only during sunshine hours. Of rations possible designs, the " t h e r m o s t a t i c " solar switch was adapted. It comprises a thermostat to which a small solar black collector is soldered. This device is contained in a wooden box with a glass plate on the upper side, without thermal insulation. The switch is actuated by direct solar radiation, and switches the electrical device on directly.
I V - - H e a t Collectors and Solar Cookers Abou-Hussein, M.S.M., "Temperature-Decay Curves in t h e B o x T y p e S o l a r C o o k e r , " U n i t e d N a t i o n s C o n f e r e n c e o n N e w S o u r c e s of E n e r g y , R o m e , 1961, 8 p. In this paper the temperature-decay curve of a solar oven of the box-type is found. The oven itself has all its plane reflectors inside. It is shown that the temperature-decay curve can be taken as a measure of: the initial heat stored, the rate of heat gain or loss, the suitability of the thermal insulation, the possibility of the short-term heat storage and the stability of the operation of the cooker. Some economical and sociological factors are given, indicating the cheapness of the oven and traditions of some underdeveloped countries that hinder its acceptance. Technical improvements relating to minimizing the rate of temperature decay, or increasing the time of short-term heat storage are suggested.
VIII--Thermionic H a t s o p o u l o s , G e o r g e N . , a n d P i e r r e J. B r o s e n s , " S o l a r Heated Thermionic Converter," United Nations C o n f e r e n c e o n N e w S o u r c e s of E n e r g y , R o m e , 1961, 13 p. Illus. After a brief historic review of the development of thermionic conversion, a description of the principles of operation of thermionic engines is given. A discussion of the potential advantages of thermionic engines to be used in converting solar energy into electric power is also given. Experimental results on recently tested engines for use in space applications show that these systems can also be used in the near future for ground applications. A brief description of a ground solar therlnionic conversion system is given. In conclusion, thermionic engines coupled with solar concentrators promise inexpensive, lightweight, maintenance-free generators of power in the range of 100 watts to 100 kilowatts having an over-all efficiency of about 17 percent for an operation life of up to 50,000 hours. F r a n c i a , G i o v a n n i , " A N e w C o l l e c t o r of S o l a r R a d i a n t Energy Theory and Experimental Verification," U n i t e d N a t i o n s C o n f e r e n c e o n N e w S o u r c e s of E n e r g y , R o m e , 1961, 24 p. Illus. The author presents a new type of collector, perhaps economical enough for industrial production, and capable of operating at high efficiencies at temperatures of 4000-5000 degrees C. Experiments last year on three different collectors of this type, and the first results from a collector for a solar Vol. 7, No. 1, 1963
turbine engine, which was completed last March and designed to supply 5 to 7 kilograms per hour of superheated steam al 450 degrees C and 150 atmospheres, appear to confirm the theory. The solution proposed by the author can be schematically described as follows: A frustum of a cone with its axis pointed to the sun and a vertex angle equal to the apparent diameter of the sun is placed in front of a classic blackbody; it does not limit the incoming energy. On the other hand, provided its walls are black and insulated, the frustum does limit the reradiated energy. The analytic development demonstrates that, under extreme conditions, at the limit, the reradiated energy is reduced to that contained in the solid angle occupied by the incoming energy. When a large number of such cylinders are assembled into a beehive structure, the condition that the walls shall be insulated is no longer necessary, since the temperature distributions of two adjacent cylinders can be considered the same. This structure will here be termed absolute blackbody, because of its quality, at the limit, of absorbing all the incoming radiation without reradiating it in a direction different from that of the source. The high yields of these collectors made their application to existing steam engines possible, instead of the reverse situation. Thus a device able to furnish 5-7 kilograms per hour of steam superheated to 400-500 degrees C at 100-150 atmospheres was constructed. It consists of a beehive collector of selective transparency type, formed by about 2000 small glass tubes, placed on a plate suitably prepared and varnished black, so as to absorb almost all the incident radiation. On this collector, with a surface of about 0.50 square meter, the radiant energy from conical aluminum mirrors is concentrated in the geometrical ratio of 16:1. The device, mounted on a sturdy support, provided with the requisite positioning levers, is installed on a small truck and can be easily moved after disassembly of the bulkiest parts. The results can be read directly on an instrument board with thermoelectric thermometers, pressure gauges, etc. The cellular structure of the absolute blackbody may have numerous other applications. As an example, the paper describes a solar furnace for temperatures of the order of 3000 degrees C, consisting of a cone-frustum mirror concentrating the energy on a cylinder placed on its axis and protected by a cellular structure formed, not by a continuous surface, but by tungsten wires 10 to 15 microns in diameter, stretched on frames, and remote from the hot portions. In a different area of application, the property of the cellular structure described has the properties of being cold as well as transparent in one direction. These properties can be usefully applied, under particular conditions, for heat insulators of negligible thermal mass at very high temperatures. P e y c h e s , I., " S p e c i a l G l a s s e s a n d M o u n t i n g s for t h e U t i l i z a t i o n of S o l a r E n e r g y , " U n i t e d N a t i o n s C o n f e r e n c e o n N e w S o u r c e s of E n e r g y , R o m e , 1961, 15p. For the radiation traps utilizing the hothouse effect, the author calls attention to the shape of the absorption edge, usually located near wavelength 2.5 microns, and to the methods of influencing it by modifying the composition of the glass and controlling certain impurities, especially iron with its band at 1.1 microns, or by modifying the thickness of the glass plate. The interval between the longwave end of the solar radiation curve and the shortwave end of the receiver radiation curve diminishes as the receiver temperature is increased, for example, by using semi-concentration techniques. When this is done the conventional radiation traps lose their effectiveness unless the glass used is specifically adapted to the effect desired. Attention is next called to the importance of multiple panes. A triple pane has an effect comparable to what would be given by 3.25 suns incident on the bare collector. The multiple panes now industrially developed are assembled by gas welding or placed in completely hermetic metal or plastic mountings. The efficiency of the collector can also be improved by an extremely thin film of gold deposited on the inner face of the last glass plate. 29
G a u m e r , Roger E., " T h e S p e c t r u m of M a t e r i a l s A v a i l a b l e F o r the U t i l i z a t i o n a n d C o n t r o l of Solar E n e r g y , " U n i t e d N a t i o n s Conference o n New Sources of E n e r g y , R o m e , 1961, 14 p. The task of designing a satellite surface so that it will mainrain an average equilibrium temperature of 20 degrees C in orbit is remarkably similar to that of designing a solar-collecting surface to he used in conjunction with a solar still. Clearly, if a material will withstand the rigors of the space environment, it should be admirably suited for use in terrestrial solar-energy conversion and utilization apparatus. The practical demands of the satellite industry have resulted in a very considerable emphasis upon the development of new materials for radiative thermal control that are stable, reliable, durable, and reasonably economical. It seems probable that much of the information will be of equal value to the designers of practical terrestrial equipment for solar-energy utilization.
IX--Photochemical Processes: tanical, and Inorganic
Biological,
Bo-
F u r u t a , Tok, " M e a s u r i n g L i g h t T o D e t e r m i n e Irr i g a t i o n Needs of P l a n t s , " Southern Florist and Nurseryman, M a y 18, 1962, 8-11 p. Illus. To determine the correlation of solar energy accumulation to actual water use (evapotranspiration), chrysanthemums were grown in a greenhouse following acceptable commercial practices. At intervals between irrigations, soil samples were taken to detern~line the moisture content. At the same time, accumulated solar energy and water evaporation from Livingston atometers were deternlined by the use of an illumination totalizer. H i a t t , A. L., " P h o t o s y n t h e t i c O x y g e n a t i o n Sewage T r e a t m e n t Utilized in W o o d l a n d P l a n t O p e r a t i o n , " Sun at World', T h i r d Q u a r t e r , 1962, 2 p. Illus. The city of Woodland, Calif., recently placed in operation four units of a 12-unit photosynthetic oxygenation sewage treatment plant; the type of raw-sewage lagoon process in which solar energy is used to achieve complete treatment of raw sewage.
XI--Radiation Mechanics and Availability: Insolation, Spectroscopy and Spectral Distribution Y u n g - s e n , C h e n , " T h e Response of the A t m o s p h e r e to the Solar A c t u a l Direct R a d i a t i o n in a L i n e a r M o d e l of the L o n g - R a n g e N u m e r i c a l P r e d i c t i o n , " Scientia Sinica, 2/62 X I ( 2 ) : 2 7 5 - 2 9 1 Illus. A two-level linear model for the long-range numerical prediction is analyzed with a theoretical method commonly used in automatic control systems. The effects of friction, turbulence, and non-adiabatic heating of solar radiation are included in the equations. The responses of the output of the departure of isobaric surface height from that of zonal mean value to the input of the departure of solar radiation from that of zonal mean vahle are investigated. The influences of the variations of basic current, zonal wave numbers, meridional wave numbers, frictional coefficient, and lateral turbulent coefficient on this response are discussed. According to the actual response of the height departure to the solar-radiation departure, the values of basic current, frictional coefficient, and lateral turbulent coefficient in the model can be adjusted to attain the closest possible correspondence with the atmospheric process. F r i e d m a n , H e r b e r t , "Solar R a d i a t i o n , " Astronautics, A u g u s t 1962, 14-25 p. Illus. ( I n c l u d e s article b y Schaefer, H e r m a n n J. " P r o t e c t i o n A g a i n s t the Solar F l a r e , " 24-25 p.) 30
An average yellow star dominates the environment of our space, its radiations both sustaining life and posing the greatest danger to manned space operations. Includes: The Photospheric Spectrum, Structure of the Photosphere and Chromosphere, The Chromospheric Spectrum, The Corona, Solar Activity, Sources of X-ray Emission, Solar Flares, and Future Experiments, plus Mr. Schaeffer's article on Protection. M a n n , A. E. a n d M i c h a e l D u b e y , "Solar S i m u l a t o r s , " Space~Aeronautics, M a r c h , 1962, 67-70 p. Illus. How well a solar simulator works depends on the characteristics of its optical system, the ability of the source lamps to reproduce the solar spectrum and the approximation of the sun's collimation by the intrinsic brightness of these lamps. After describing the solar radiation that must be simulated, this article outlines how the simulation requirements affect the choice of source lamps and the design of the optical systeul. The characteristics of the basic collimating systems are reviewed, and an actual simulator is described as an example.
XII--Economie, Social and Cultural Parameters of Solar Energy Golding, E. W., " P o w e r F r o m Local E n e r g y Resources," U n i t e d N a t i o n s Conference on N e w Sources of E n e r g y , R o m e , 1961, 13 p. To aid development, and to inlprove living conditions, in many remote, thinly-populated areas a supply of energy is a very important requirement, hnportation of this energy, either through electrical transmission or in the form of fuel, is often prohibitively expensive. An alternative, which may be more economical, is the exploitation of local energy resources such as wind energy, solar energy, or organic wastes. These resources have individual characteristics that need special consideration in relation to the possible power loads, and the loads should be planned to match tile unconventional methods of power production. The use of local energy resources in combination may offer the best prospect of success in providing a valuable service to remote communities and experimental schemes should be established to test the possibilities under practical conditions. Mueller, Dr. -Ing. H e r b e r t F., " P r o b l e m s of E n e r g y S u p p l y in U n d e r d e v e l o p e d C o u n t r i e s W i t h Special R e g a r d to New Sources of E n e r g y , " U n i t e d N a t i o n s Conference on New Sources of E n e r g y , R o m e , 1961, 12 p. The technical range of application of the new energy sources is limited. Experiences acquired elsewhere cannot be generalized. Above all, it must be kept in mind that the requirements of the various countries differ widely. It is most difficult to find, in view of the different conditions in the individual countries or areas concerned, a definition fer "underdevelopment" with due regard to the relevant factors, and all efforts to do so have met so far with practically no success.
We have no generally valid criteria for the pattern of the planning of energy supply to underdeveloped countries, which energy sources should be preferred, which degree of energy supply should be provided in the initial stage and which speed of development can be expected. The answers to these questions depend essentially on the dissimilar demands of the
various countries. Peck, E d w a r d C., " D r y i n g 4 / 4 R e d O a k b y Solar H e a t , " Forest Products Journal, X I I : 3 M a r c h , 1962, 103-107 p. (Also described b y Peck, E d w a r d C., i n Sun at Work, T h i r d Quarter, 1962, p. 4 a n d 5). The principle of trapping heat that is absorbed is incorporated into a drying structure, called a predryer, that has been designed by staff members of the U.S. Forest Products Labora-
Solar Energy
I V - - H e a t Collectors and Solar Cookers Abou-Hussein, M.S.M., "Temperature-Decay Curves in t h e B o x T y p e S o l a r C o o k e r , " U n i t e d N a t i o n s C o n f e r e n c e o n N e w S o u r c e s of E n e r g y , R o m e , 1961, 8 p. In this paper the temperature-decay curve of a solar oven of the box-type is found. The oven itself has all its plane reflectors inside. It is shown that the temperature-decay curve can be taken as a measure of: the initial heat stored, the rate of heat gain or loss, the suitability of the thermal insulation, the possibility of the short-term heat storage and the stability of the operation of the cooker. Some economical and sociological factors are given, indicating the cheapness of the oven and traditions of some underdeveloped countries that hinder its acceptance. Technical improvements relating to minimizing the rate of temperature decay, or increasing the time of short-term heat storage are suggested.
VIII--Thermionic H a t s o p o u l o s , G e o r g e N . , a n d P i e r r e J. B r o s e n s , " S o l a r Heated Thermionic Converter," United Nations C o n f e r e n c e o n N e w S o u r c e s of E n e r g y , R o m e , 1961, 13 p. Illus. After a brief historic review of the development of thermionic conversion, a description of the principles of operation of thermionic engines is given. A discussion of the potential advantages of thermionic engines to be used in converting solar energy into electric power is also given. Experimental results on recently tested engines for use in space applications show that these systems can also be used in the near future for ground applications. A brief description of a ground solar therlnionic conversion system is given. In conclusion, thermionic engines coupled with solar concentrators promise inexpensive, lightweight, maintenance-free generators of power in the range of 100 watts to 100 kilowatts having an over-all efficiency of about 17 percent for an operation life of up to 50,000 hours. F r a n c i a , G i o v a n n i , " A N e w C o l l e c t o r of S o l a r R a d i a n t Energy Theory and Experimental Verification," U n i t e d N a t i o n s C o n f e r e n c e o n N e w S o u r c e s of E n e r g y , R o m e , 1961, 24 p. Illus. The author presents a new type of collector, perhaps economical enough for industrial production, and capable of operating at high efficiencies at temperatures of 4000-5000 degrees C. Experiments last year on three different collectors of this type, and the first results from a collector for a solar Vol. 7, No. 1, 1963
turbine engine, which was completed last March and designed to supply 5 to 7 kilograms per hour of superheated steam al 450 degrees C and 150 atmospheres, appear to confirm the theory. The solution proposed by the author can be schematically described as follows: A frustum of a cone with its axis pointed to the sun and a vertex angle equal to the apparent diameter of the sun is placed in front of a classic blackbody; it does not limit the incoming energy. On the other hand, provided its walls are black and insulated, the frustum does limit the reradiated energy. The analytic development demonstrates that, under extreme conditions, at the limit, the reradiated energy is reduced to that contained in the solid angle occupied by the incoming energy. When a large number of such cylinders are assembled into a beehive structure, the condition that the walls shall be insulated is no longer necessary, since the temperature distributions of two adjacent cylinders can be considered the same. This structure will here be termed absolute blackbody, because of its quality, at the limit, of absorbing all the incoming radiation without reradiating it in a direction different from that of the source. The high yields of these collectors made their application to existing steam engines possible, instead of the reverse situation. Thus a device able to furnish 5-7 kilograms per hour of steam superheated to 400-500 degrees C at 100-150 atmospheres was constructed. It consists of a beehive collector of selective transparency type, formed by about 2000 small glass tubes, placed on a plate suitably prepared and varnished black, so as to absorb almost all the incident radiation. On this collector, with a surface of about 0.50 square meter, the radiant energy from conical aluminum mirrors is concentrated in the geometrical ratio of 16:1. The device, mounted on a sturdy support, provided with the requisite positioning levers, is installed on a small truck and can be easily moved after disassembly of the bulkiest parts. The results can be read directly on an instrument board with thermoelectric thermometers, pressure gauges, etc. The cellular structure of the absolute blackbody may have numerous other applications. As an example, the paper describes a solar furnace for temperatures of the order of 3000 degrees C, consisting of a cone-frustum mirror concentrating the energy on a cylinder placed on its axis and protected by a cellular structure formed, not by a continuous surface, but by tungsten wires 10 to 15 microns in diameter, stretched on frames, and remote from the hot portions. In a different area of application, the property of the cellular structure described has the properties of being cold as well as transparent in one direction. These properties can be usefully applied, under particular conditions, for heat insulators of negligible thermal mass at very high temperatures. P e y c h e s , I., " S p e c i a l G l a s s e s a n d M o u n t i n g s for t h e U t i l i z a t i o n of S o l a r E n e r g y , " U n i t e d N a t i o n s C o n f e r e n c e o n N e w S o u r c e s of E n e r g y , R o m e , 1961, 15p. For the radiation traps utilizing the hothouse effect, the author calls attention to the shape of the absorption edge, usually located near wavelength 2.5 microns, and to the methods of influencing it by modifying the composition of the glass and controlling certain impurities, especially iron with its band at 1.1 microns, or by modifying the thickness of the glass plate. The interval between the longwave end of the solar radiation curve and the shortwave end of the receiver radiation curve diminishes as the receiver temperature is increased, for example, by using semi-concentration techniques. When this is done the conventional radiation traps lose their effectiveness unless the glass used is specifically adapted to the effect desired. Attention is next called to the importance of multiple panes. A triple pane has an effect comparable to what would be given by 3.25 suns incident on the bare collector. The multiple panes now industrially developed are assembled by gas welding or placed in completely hermetic metal or plastic mountings. The efficiency of the collector can also be improved by an extremely thin film of gold deposited on the inner face of the last glass plate. 29
G a u m e r , Roger E., " T h e S p e c t r u m of M a t e r i a l s A v a i l a b l e F o r the U t i l i z a t i o n a n d C o n t r o l of Solar E n e r g y , " U n i t e d N a t i o n s Conference o n New Sources of E n e r g y , R o m e , 1961, 14 p. The task of designing a satellite surface so that it will mainrain an average equilibrium temperature of 20 degrees C in orbit is remarkably similar to that of designing a solar-collecting surface to he used in conjunction with a solar still. Clearly, if a material will withstand the rigors of the space environment, it should be admirably suited for use in terrestrial solar-energy conversion and utilization apparatus. The practical demands of the satellite industry have resulted in a very considerable emphasis upon the development of new materials for radiative thermal control that are stable, reliable, durable, and reasonably economical. It seems probable that much of the information will be of equal value to the designers of practical terrestrial equipment for solar-energy utilization.
IX--Photochemical Processes: tanical, and Inorganic
Biological,
Bo-
F u r u t a , Tok, " M e a s u r i n g L i g h t T o D e t e r m i n e Irr i g a t i o n Needs of P l a n t s , " Southern Florist and Nurseryman, M a y 18, 1962, 8-11 p. Illus. To determine the correlation of solar energy accumulation to actual water use (evapotranspiration), chrysanthemums were grown in a greenhouse following acceptable commercial practices. At intervals between irrigations, soil samples were taken to detern~line the moisture content. At the same time, accumulated solar energy and water evaporation from Livingston atometers were deternlined by the use of an illumination totalizer. H i a t t , A. L., " P h o t o s y n t h e t i c O x y g e n a t i o n Sewage T r e a t m e n t Utilized in W o o d l a n d P l a n t O p e r a t i o n , " Sun at World', T h i r d Q u a r t e r , 1962, 2 p. Illus. The city of Woodland, Calif., recently placed in operation four units of a 12-unit photosynthetic oxygenation sewage treatment plant; the type of raw-sewage lagoon process in which solar energy is used to achieve complete treatment of raw sewage.
XI--Radiation Mechanics and Availability: Insolation, Spectroscopy and Spectral Distribution Y u n g - s e n , C h e n , " T h e Response of the A t m o s p h e r e to the Solar A c t u a l Direct R a d i a t i o n in a L i n e a r M o d e l of the L o n g - R a n g e N u m e r i c a l P r e d i c t i o n , " Scientia Sinica, 2/62 X I ( 2 ) : 2 7 5 - 2 9 1 Illus. A two-level linear model for the long-range numerical prediction is analyzed with a theoretical method commonly used in automatic control systems. The effects of friction, turbulence, and non-adiabatic heating of solar radiation are included in the equations. The responses of the output of the departure of isobaric surface height from that of zonal mean value to the input of the departure of solar radiation from that of zonal mean vahle are investigated. The influences of the variations of basic current, zonal wave numbers, meridional wave numbers, frictional coefficient, and lateral turbulent coefficient on this response are discussed. According to the actual response of the height departure to the solar-radiation departure, the values of basic current, frictional coefficient, and lateral turbulent coefficient in the model can be adjusted to attain the closest possible correspondence with the atmospheric process. F r i e d m a n , H e r b e r t , "Solar R a d i a t i o n , " Astronautics, A u g u s t 1962, 14-25 p. Illus. ( I n c l u d e s article b y Schaefer, H e r m a n n J. " P r o t e c t i o n A g a i n s t the Solar F l a r e , " 24-25 p.) 30
An average yellow star dominates the environment of our space, its radiations both sustaining life and posing the greatest danger to manned space operations. Includes: The Photospheric Spectrum, Structure of the Photosphere and Chromosphere, The Chromospheric Spectrum, The Corona, Solar Activity, Sources of X-ray Emission, Solar Flares, and Future Experiments, plus Mr. Schaeffer's article on Protection. M a n n , A. E. a n d M i c h a e l D u b e y , "Solar S i m u l a t o r s , " Space~Aeronautics, M a r c h , 1962, 67-70 p. Illus. How well a solar simulator works depends on the characteristics of its optical system, the ability of the source lamps to reproduce the solar spectrum and the approximation of the sun's collimation by the intrinsic brightness of these lamps. After describing the solar radiation that must be simulated, this article outlines how the simulation requirements affect the choice of source lamps and the design of the optical systeul. The characteristics of the basic collimating systems are reviewed, and an actual simulator is described as an example.
XII--Economie, Social and Cultural Parameters of Solar Energy Golding, E. W., " P o w e r F r o m Local E n e r g y Resources," U n i t e d N a t i o n s Conference on N e w Sources of E n e r g y , R o m e , 1961, 13 p. To aid development, and to inlprove living conditions, in many remote, thinly-populated areas a supply of energy is a very important requirement, hnportation of this energy, either through electrical transmission or in the form of fuel, is often prohibitively expensive. An alternative, which may be more economical, is the exploitation of local energy resources such as wind energy, solar energy, or organic wastes. These resources have individual characteristics that need special consideration in relation to the possible power loads, and the loads should be planned to match tile unconventional methods of power production. The use of local energy resources in combination may offer the best prospect of success in providing a valuable service to remote communities and experimental schemes should be established to test the possibilities under practical conditions. Mueller, Dr. -Ing. H e r b e r t F., " P r o b l e m s of E n e r g y S u p p l y in U n d e r d e v e l o p e d C o u n t r i e s W i t h Special R e g a r d to New Sources of E n e r g y , " U n i t e d N a t i o n s Conference on New Sources of E n e r g y , R o m e , 1961, 12 p. The technical range of application of the new energy sources is limited. Experiences acquired elsewhere cannot be generalized. Above all, it must be kept in mind that the requirements of the various countries differ widely. It is most difficult to find, in view of the different conditions in the individual countries or areas concerned, a definition fer "underdevelopment" with due regard to the relevant factors, and all efforts to do so have met so far with practically no success.
We have no generally valid criteria for the pattern of the planning of energy supply to underdeveloped countries, which energy sources should be preferred, which degree of energy supply should be provided in the initial stage and which speed of development can be expected. The answers to these questions depend essentially on the dissimilar demands of the
various countries. Peck, E d w a r d C., " D r y i n g 4 / 4 R e d O a k b y Solar H e a t , " Forest Products Journal, X I I : 3 M a r c h , 1962, 103-107 p. (Also described b y Peck, E d w a r d C., i n Sun at Work, T h i r d Quarter, 1962, p. 4 a n d 5). The principle of trapping heat that is absorbed is incorporated into a drying structure, called a predryer, that has been designed by staff members of the U.S. Forest Products Labora-
Solar Energy