A project of Volunteers in Asia by: George Foster and Robert Peart Published by: US Department of Agriculture Office of Communication Washington, D.C. 20250 USA Available from: US Department of Agriculture Office of Communication Washington, D.C. 20250 USA Reproduction of this microfiche document in any form is subject to the same restrictions as those of the original document. AGRICULTURAL RESEARCH SERVICE in cooperation with COOPERATIVE STATE RESEARCH SERVICE and STATE AGRICULTURAL EXPERIMENT STATIONS UNITED STATES DEPARTMENT OF AGRICULTURE and supported by U.S. ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION Agriculture Information Bulletm No. 401 SOLA AIN DRYI PROGRESS AND POTENTIAL George H. Foster and Robert M. Pearl Agriculture Information Bulletin No. 403 Agricultural Research Service In cooperation with Cooperative State Research Service and State Agricultural Experiment Stations UNITED STATES DEPARTMENT OF AGRICULTURE and supported by US. ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION Washington, DC November 1976 Fm sale by the Superlntendent of Documents. U.S. Government Printing Office, Waehington, D.C. 20402 ABSTRACT Drying grain, especially corn, with conventional artificial drying methods requires great quantities of petroleum fuel during a short harvest period. Two systems of drying are used: high-speed, high- temperature batch or continuous-flow and low temperature in- storage drying. Solar energy was studied as an alternative or sup- plemental energy source for low-temperature drying at several different Midwest locations. Adoption of solar grain drying depends on supply and price of petroleum fuel and on competition for scarce fuel for other agri- cultural uses such as powering field operations and manufacturing fertilizer. Key words Grain drying, solar energy, low-temperuture drying, heat storage, supplemental solar heat, high-temperature dry- ing, solar collectors, radiant energy, computer simulation. Names of commerciul companies are used in this publica- tion solely to provide specific information. Mention of them does not constitute a guarantee or warranty of the prod- uct by the U.S. Department of Agriculture or an endorse- ment by the Department over other firms not mentioned. I I USDA policy does not permit discrimination because of age, race, color, national origin, sex, or religion. Any person who believes he or she has been discriminated against in any USDA-related activity should write immediately lo the Secretary of Agriculture, Washington, D.C. 20250. PREFACE This publication reports research in progress and assesses the state of the art of solar grain drying. It is directed toward investigators interested in applying sotar energy, to agriculturists concerned with ap- plied technology, to manufacturers and merchan- dizers of drying equipment-solar and conventional -and to potential users of solar grain drying on farms or wherever crops are dried. Some solar grain drying research has been in progress since the 1950’s (7) (4) (8) (74) (75) (19). However, the increased emphasis placed on solar drying in response to the 1973 fuel crisis and the resulting effort toward U.S. energy self-sufficiency sparked renewed efforts in direct application of solar energy to meet agricultural energy needs. Funding initiated by the National Science Founda- tion in late 1974 and assumed by the U.S. Engery Research and Development Administration (ERDA) in January 1975 has been responsible for much of the renewed research effort. Research on the ap- plication of solar energy to agriculture is managed for ERDA by the Agricultural Research Service (ARS) in cooperation with the Cooperative States Re- search Service, U. S. Department of Agriculture. Some of the funds have been used to expand ARS research in solar grain drying, but most of the money has been distributed to State agricultural experiment stations and other research agencies through research agreements and contracts. Proof of concept tests started late in 1974 in- cluded research at two ARS locations and seven State agricultural experiment stations. The station locations and the investigators in charge include: University of Illinois, Urbana, Gene C. Shove Iowa State University, Ames, Carl J. Bern Kansas State University, Manhattan, Ralph I. Upper University of Minnesota, St. Paul, R. Vance Morey Ohio Agricultural R&D Center, Wooster, Harold M. Keener .Purdue University, W. Lafayette, Ind., Robert M. Peart South Dakota State Univ., Brookings, Mylo A. Hellickson ARS, lowa State University, Ames, Gerald 1. Kline ARS, U.S. Grain Marketing Research Center, Manhattan, Kans., George H. Foster Early in 1975, a computer simulation program was initiated to determine at what locations and under what weather and crop conditions solar grain drying shows the most promise. In addition to some of the institutions already listed, simulation work at the University of Nebraska was inaugu- rated under the leadership of T. L. Thompson. Also funded at the same time was field testing of solar rice drying by ARS at the Texas A&M University Research & Extension Center at Beaumont with D. L. Calderwood, investigator. Later in 1975 three experiment stations, one ad- ditional ARS location, and a research and consult- ing firm were added to the program. The locations and the lead investigators include: Colorado State University, Ft. Collins, Ralph Hansen Michigan Stote University, E. Lansing, F. W. 3c.h. .Arkemo University of Missouri, Columbia, D. B. Brooker ARS, Purdue University, Lafayette, Ind., John R. Barrett Helio Associates, Inc., Tucson, Ariz., A. B. Meinel. Technical papers have been developed on some of the individual projects. The papers hove been presented at professional meetings or published in scientific and trade journals. These are listed in References 6, 72, 73, 76, 77, 78 and 20, page 14. Conversion table-English to metric units To convert from - Foot? Mile” Acre British thermal unit Btu/ft”-hr Foot Horsepower Foot3 Gallon Bushels To Meter’ Kilometer” Hectare Joule Watt/meter” Meter Watt Meter:’ Liter Tonne Multiply by - .0929 2.590 4047 1055 3.152 3048 745.6 .02B3 3.785 .0263 Btu/fts-hr. day, etc. kWh/mete$-hr, etc. .00315 Btu/ftz-day Langley/day .2710 Cfm/bu Meter:‘/min-tonne 1.075 CONTENTS Introduction ........................... 1 Solar energy-availability and history ..... 2 Background ...................... 2 Solar energy availability ............. 2 Solar energy-drying grain ............... 3 Solar collectors and collection efficiencies ... 4 General .......................... 4 Solar collectors used for grain drying ... 5 Collector efficiency ................. 7 Solar drying studies in progress ........... 9 Field tests ......................... 9 Collector design and performance ..... 10 Simulation test results .............. 11 Future of solar grain drying .............. 12 References ............................ 14 bean-growing area where certain disease problems quently requires artificial drying of the crops to develop in the field, requiring early harvest fol- make the system work. Wheat and barley are lowed by artificial dr;Gng. grown ahead of cotton in the Southwest and soy- The growing practice of double-cropping fre- beans follow wheat in the Midwest. §OiAR ENERGY-AVAILABILITY AND HISTORY Background Crops convert and store solar energy by photo- synthesis. Estimates (27) are that energy equivalent to 300 million tons (273 X 10G tonnes) of coal is fixed through photosynthesis each year, an amount equal to about one-seventh of the total energy used annually in the United States. For the other six-sevenths of the energy used, the United States depends on the results from photosynthesis that took place in past ages. Coal has been called black sunshine because it is the result of preserved plant material. Oil is called liquid sunshine because it comes largely from deposits of animal life that ate the plant life that fixed energy from the sun. Ob- viously solar energy has been used almost exclu- siveiy to power the economy-and it continues to do so. However, the reserves that have been ac- cumulated and stored in the form of coal and pe- troleum are disappearing rapidly. Unfortunately, photosynthesis is not an eiTicient process, and little of the total energy from the sun falling on the earth’s surface is converted into plant material. Corn utilizes solar energy through photo- synthesis more efficiently than most other crops. Yet the grain harvested from an acre of corn rep- resents energy equal to only about 1 percent of the solar energy available over the growing sea- son. Estimates (23) show that by collecting and using all of the solar energy available, the total U.S. energy requirement could be met on 4,300 square miles (11,137 km”) or 0.1 percent of the land area in the United States. So why not collect this energy directly and use it more efficiently to meet energy needs? Solar energy has been applied directly in agri- culture for many years. The sun and the wind dry mature crops standing in the fields, in the stack or windrow, or in a ventilated shed or crib. Efforts to collect and concentrate solar energy directly extend back 2,000 years or more. However, considerable investment in equipment is required to collect solar energy in amounts sufficient to replace significant quantities of fossil fuel. A few facts on solar energy availability will document this. 2 Solar energy availability Although the total energy from the sun is im- mense, it is diffuse and often needs to be concen- trated to be used effectively. Solar energy is also intermittent both on a daily basis and on a day-to- day basis, depending on the amount of cloud cover. The energy received from the sun just out- side the earth’s atmosphere (the solar constant) is about 430 Btu/ft’-hr (1,355 W/m2) (3). At the earth’s surface, 290 Bzu/ft’-hr (914 W/m?) is rc- ceived on a surface normal to the sun. However, averaged over day and night, cloudy and bright, solar energy received on a horizontal surface in the United States is about 65 Btu/ft2-hr (205 W/ma) or 1,560 Btu/ft2-day (4.9 kWh/m2-day). Further, this ranges from 2,000 Btu/ft”-day (4.3 kWh/m2-day) in the desert southwest to about 1,300 Btu/ft?-day (4.1 kWh/m2-day) in the centrai Corn Belt (7). The seasonal variation in solar energy must be considered in determining the amount of solar energy available for grain drying. As indicated in figure 1, solar energy available on a horizontal surface peaks in June and July, but most of the drying requirements are for fall maturing crops harvested in September through November. At 40” north latitude (central U.S. Corn Belt), the solar Figure l.-Effect of tilting collector surface to south 50’ above .horizontal an the radiation received during the fall and spring drying seasons in the central Corn Belt. SOLAR GRAIN DRYING PROGRESS AND POTENTIAL George H. Foster and Robert M. Pea& INTRODUCTION Grain drying is an energy intensive agricultural operation that will be increasingly affected by the growing fossil fuel shortage. Of the crops requiring drying, corn uses the most energy. It is the largest grain crop in terms of total production and is nor- mally harvested with mere excess moisture than any other grain crop. Corn matures in the fail and is subject to extensive field losses if not harvested before winter. Corn harvest is normally concen- trated in a few weeks in the fall. Thus, not only is the requirement for energy great, but this de- mand is concentrated during a short period. Energy required for drying corn often exceeds the total amount required for preparing the seed- bed, planting, cultivating, and harvesting the crop. Recent estimates (7 7)” of energy requirements for drying corn are 56 X 1012 Btu (59.1 X 10” kJ), and for rice, 3 X 1 012 Btu (3.2 X 1012 kJ). The fuel equivalent is about 640 million gallons (2.4 X lo9 liter) of LP gas. Liquified petroleum (LP) and natural gas are the principal fuels used for drying. Some fuel oil is used in the larger, nonfarm installations. Electricity use is increasing for low temperature drying on farms. One approach to drying grain involves a high speed, usually high temperature operation, where the grain is held in batches or is passed continu- ously through a special container. The dryer is de- signed for optimum exposure of the grain to the drying air and for easy movement of the grain into and out of the drying chamber. From 90 to 95 per- cent of the energy used in high-speed dryers is sup- plied by fossil fuels. ’ Agricultural engineer, U.S. Grain Marketing Research Center, Agricultural Research Service, Manhattan, Kans., and professor of agricultural engineering, Purdue University, West Lafayette, Ind. 2 Italic numbers in parentheses refers to References, page 14. 1 Another drying process, usually referred to as low-temperature drying, takes place over an ex- tended period while the grain is held in the storage bin. According to recent data from the Corn Belt (2), about one-fifth of the corn is now dried by low- temperature, in-storage drying systems. This system maximizes the use of heat in natural air; solar energy as a source of supplement heat was first used with this method of drying. Rice normally requires drying after harvesting. Most of the crop is dried at commercial drying in- stallations or at rice mills in continuous-flow, heated-air dryers by the multipass method, but a considerable part of the crop in the three produc- tion areas in the United States is dried on the farm in storage bins. Because of new acreage planted to rice in the Arkansas-Mississippi area during the post 2 years and commercial facilities being slow to respond to the increased production, there has been a large increase in on-farm drying in this area. Commercial dryers in California use a com- bination system in which rice is dried to 16 to 18 percent moisture in a continuous-flow, heated-air dryer, then drying is finished with ambient air while the rice is in storage. The other grain crops require drying occasionally. Grain sorghum is normally grown in the drier re- gions of the United States, but some of it is dried in nearly all production areas. Wheat requires dry- ing somewhere in the wheat-producing area al- most every year. The moisture that must be re- moved from wheat is small compared to corn and rice, and the drying usually can be done with natural air. Soybeans, an oilseed crop, do not re- quire drying every year. When the crop does need drying, the amount of moisture removed is small. Exceptions occur in more humid parts of the soy- radiation falling on a horizontal surface on Octo- ber 21 is just about half that falling on the same surface on June 21. However, by tilting the solar collector to the south so that it is normal to the noonday sun, this difference is smaller. The direct radiation normal to the sun on October 21 is 2,454 Btu/ft?-day (7.7 kWh/m’-day) compared to 3,180 Btu/ft?-day (10.0 kWh/m?-day) in June. For the most part, this difference in available solar energy is due to the change in day length from October to June. On a clear day at 40” north latitude, 2,648 Btu/ft’-day (8.4 kWh/ m”-day) is received on a horizontal surface on June 21, but only 610 Btu/ft’- day (1.9 kWh/m?-day) on a vertical surface (3). On October 21, 1,348 Btu/ft?-day (4.2 kWh/m?-day) is received on a horizontal surface and 1,654 Btu/ ft”-day (5.2 kWh/m’-day) on a vertical surface. From the middle of October through the middle of March, more energy can be collected on a vertical surface than on a horizontal surface at this lat- itude. By tilting the collector optimally to the south during the fall drying season-an angle of about 50” with the horizontal in the Corn Belt area- nearly 2,100 Btu/fP-day (6.6 kWh/m’-day) will be received on clear days. By going one step further and making a tracking collector that will follow the sun from sunrise to sunset, the amount received will increase to 2,450 Btu/ft’-day (7.7 kWh/m?- day). The foregoing discussion gives some idea of the solar energy available on an annual average basis and how this varies by geographical location, by time of the year, and by orientation of the collec- tor. Still to be dealt with is the matter of the inter- mittent nature of the availability of solar energy. In the first place, it is only available for 8 to 14 hours each day. The daily fluctuations in solar energy availability is perhaps easier to accommo- date than the situation where sun energy may not he available at all, or is available at low levels, during cloudy or rainy weather. A constant source of energy is not available unless there is some meuns of storing excess energy during sunny weather. The probabilities of receiving various lev- els of solar rcdiation have been calculated for the North Centra! region of the United States, and other locations (5) (7). SOLAR ENERGY-DRYING GRAIN Solar energy is considered more applicable to low-temperature, in-storage drying systems than to high-temperature, high-speed systems. In-storage drying systems require low levels of heat input over extended periods. Such drying methods toler- ate intermittent or variable levels of heat input. In most areas of the Corn Belt, the relative hu- midity of the outdoor air during sunny weather is low enough to dry corn to the desired moisture level without added heat. When solar heat is added in the daytime, continued fan operation during the night when the relative humidity is high helps off- set daytime overdrying and provides more uniform drying through the total grain depth. The overdried grain picks up moisture from the high humidity night air. This reduces overdrying and lowers the air humidity so the grain above the clerdried layer will continue to dry. However, there is insufficient energy stored in the overdried grain to permit dry- ing to proceed during long periods of inclement weather. low-temperature drying is weather dependent and may be least successful during the years when it is needed most-years when the crops mature late or when field drying conditions are poor. In- solation levels (the amount of incoming solar radia- tion) are usually low when field drying conditions are poor. In some areas backup heat systems are necessary. This limits the attractiveness of solar energy because the cost of collecting solar energy must be offset entirely by savings in fuel cost. Where the solar heating system replaces other heating systems, the cost of the solar collector is partially offset by the cost of the heating equip- ment replaced. The feasibility of applying solar energy to high- speed batch and continuous flow drying systems has not been established. Where1 the amount of moisture to be removed is relatively low, as in the case with wheat or soybeans, higher speed solar drying systems employing batch-in-bin drying methods are feasible. This has been demonstrated in solar drying tests with soybeans in Ohio (76). Successful application of solar energy to high- temperature, high-speed, batch or continuous flow- drying systems presents sever:ql problems. Costs of collector systems to provide hi& temperatures (120” to 180°F or 49” to 82°C) are considerably greater than for lower temperature systems. Collection ef- ficiencies are reduced in high-temperature collec- 3 . .- tors unless expensive measures ore taken to limit 10 million Btu/hr (2.1 to 10.6 lo’, kJ. hr) or,d heat losses. higher, depending upon the hourl~~ drying rapacity. To supply the quantity of heat normally used in At a collection efficiency of 50 percent, collectors high-speed, high-temperature dryers, extensive covering nearly 2 acres ;S.;<l hectares) would be areas for deploying the solar collectors are needed. needed to provide 5 rn,,li.:cr 8::~. hr (5.3 1 lo” Heating capacity of these dryers ranges from 2 to kJ!hr). General A simple solar collector is made up of radiant energy transmitting material and an energy ab- sorbing material in a frame or enclosure. The trans- mitting material usually serves as the cover or en- closure. Conventional, flat-plate collectors are shown schematically in figure 2. The cover or glaz- ing is usuo:ly glass or clear plastic. The absorber for air heating collectors may be metal, wood, paper, or plastic, but performs best if it is slightly rough and has a dull, black finish. The bock of the collector and sometimes the sides are insulated to prevent heat losses. In some cases, a bare-plate collector with no cover is used. The absorber may be corrugated or V-shaped to SOLAR RADIATION ABSORBING SURFACE BARE PLATE SOLAR COLLECTOR SOLAR RADIATION TRANSMITTING ABSORBING COVERED PLATE SOLAR COLLECTOR Figure P.--Schematic of bare-plate and covered-plate solar collector for heating air. PN.5189 Figura J.-Commercially available intlated plastic solar collectors used in tests at Purdue University. PN-5190 Figure 4.-Inflated plastic collector used in solar drying tests at the Ohio Agricultural Research and Development Centcr. 4 PN.5191 Figure §.-Commercially available solar collector :. id: of plastic film supported by a wire frame. help trap the incoming solar radiation. The cover should transmit a high percentage of the short wave radiation from the sun. Glass and some plastic materials transmit 90 percent of the sun’s energy. The cover, in addition to transmitting solar radiation, serves to reduce heat loss caused by wind convection and from emission of long wave radiation from the absorber. In some cases, two or more layers of cover material are used. In an air-heating collector, the air is drawn or forced on one or both sides of the absorber. Heat is transferred from the absorber to the air moving over it. In a liquid heating solar collector, fluid is circulated through channels formed in the absorber or through pipes attached :o it. The absorber in liquid systems is usually made of metal to increase the heat transfer from the absorber to the fluid. Solar collectors used for grain drying Plastic collectors either air inflated or supported on a light frame are now available commercially for drying grain. One, a clear, quonset-shaped (hemicylinder) polyvinyl plastic film enclosure, pro- vides about 1,000 ft* (93 m”) of collection area (figs. 3 & 4). Inside the clear plastic is a black plastic absorber, also irrrlated. A separate electrically driven fan is used to inflate the collector and de- liver solar-heated air to the intake of the fan on the drying bin. Another plastic collector has a clear polyethylene cover over a triangular, wire frame with the black polyethylene film absorbing surface forming the floor (fig. 5). Air is drawn through the collector and heated on its way to the fan on the drying bin. A third plastic collector employs two inflated tubes: a black absorber tube inside a slightly larger diameter clear plastic tube (fig. 6). The tubes are inflated either by placing the drying fan at the end farthest from the bin or by a separate fan that de- livers air through the collector to the intake of the drying fan on the bin. The cost of materials for the tube-type solar col- lectors used at the U.S. Grain Marketing Research Center was quite low. Polyethylene plastic (6 mil (.15 mm) thickness) that made up the double-tube collectors cost about 30 cents/ft? ($3.23/m”) of net collector area. Tape for the seams to make tubes from flat sheets plus the straps used for attachment at each end added another 3 to 5 cents/ft’ (32 to 54 cents/m’). This simple collector was fabricated from materials available from a local retail build- ing supplier. Two men made the tubes in 3 or 4 hours. Some additional time was required to make the metal transitions or connections between the fan and the tube and between the tube and the bin. A flat-plate collector through which the drying air was drawn into the drying fan on the bin was PN-5192 PN-5193 Figure 6.-General overhead view (above) and closeup (below) of the tubular collectors used at the U.S. Grain Marketing Research Center, Manhattan, Kans. 5 Figure 7.-Flat-plate collector used in Iowa tests. PN-5194 tested in Iowa. The collector was tilted optimally Another approach to collecting solar energy for toward the sun and had a cover of clear polyethyl- drying grain has been to build the collector into ene plastic film held in position with bowed wood the drying and storage bin. Early tests in South slats or welded wire mesh (fig. 7). The cost of the Dakota used a collector wrapped around the cir- materials for the 250 ft? (23.2 m’) shop-built col- cumference of a round bin, except for the one-third lector was $150. facing the north (fig. 8). Similar approaches were The thickness of the plastic film used with the used in Illinois (fig. 9). Air was drawn between the different collectors varied from 4 to 10 mil (.l to .25 mm). PN-5195 Figure Il.-Solar collectors of different types and materials mounted on circular bin for tests in South Dakota. PN-5196 Figure 9.-Bin wall collector tested in Illinois was mode of clear corrugated fiberglass over a black painted wall. 6 PN-5197 Figure IO.-The roof of this machinery shed in Wisconsin was made into a solar collector and used ta warm air for drying corn in the adjacent bin. collector surface and the bin wall and into the dry- ing fan. Also in Illinois and Wisconsin, the roof or side- wall, or both, of adjacent metal storage structures, usually machinery sheds, were made into bare- plate solar collectors. Air was pulled through these collectors and ducted to a fan on the nearby drying bin (fig. 10). A radiant energy transmitting exterior wall, usually corrugated fiberglass panels, was used in newly constructed livestock shelters and machinery storages to provide covered-plate so!ar collectors for grain drying and for other uses (fig. 11). The size of the collectors used for low-tempera- ture grain drying ranged from 0.10 to 0.75 ft?/bu (0.35 to 2.65 m”/tonne)3 of grain dried. In the batch-in-bin system, from 4 to 8 ft? of collector area was used for each bushel (14 to 28 m’/tonne) of grain. Temperature rise in the drying air from the solar collectors depends largely on the size of the collec- tor, the insolation rate, and the volume of air heated, along with the other factors affecting col- lector efficiency (9). As used in most of the tests with in-storage drying systems, the collectors heated the air a maximum of 5” to 30°F (2.8” to 16.7”C) at noon on a clear sunny day. (The solar heated air was sometimes mixed with larger quantities of out- door air, thus reducing the effective drying air tem- perature.) The average temperature rise for the test period including day and night, cloudy and s Becalqse of the dir7erent grains and bushel weights involved, all conversion from bushel to metric tons was arbitrarily made on the basis of 38 bu/ronne. PN-5198 Figure 1 l.-Solar collector with clear fiberglass cover built into wall and roof of Illinois machinery storage. bright, varied from 1” to 6°F (0.55” to 3.3”C). A typical sunny day pattern of temperature increases from solar and from fan energy in one drying test is shown in figure 12. At the U.S. Grain Marketing Research Center, the amount of solar energy collected in eight tests averaged 620 Btu/ft”-day (2.0 kWh/m”-day). Thus, each 100 ft’ (9.3 m’) of collector provided energy equal to 18.6 kWh of electricity or about 0.67 gal- lon of LP gas each day. Collector EfTiciency Factors affecting collector efficiency are numerous. Some of the more important ones are discussed briefly. Already mentioned was the advantage of I30 35 go- 30 BO- 25 5 u 5oc -4 10 401 I I I I I I I 1 I (45 12 2 4 6 6 IO I2 2 4 6 6 IO 12 M AM hi PM M TIME ISepl.’ 26. 1974) Figure lZ.-Typical sunny day temperatures in drying tests with solar heat. 7 tilting flat-plate collectors toward the south during the fall and winter months. Also, a collector posi- tioned vertically collects more solar energy than one positioned horizontally from November through February. The tubular collector, although stationary, has some of the characteristics of a tracking collector. When oriented north and south, it presents ap- proximately the same optical cross section to the sun from sunrise to sunset. However, as the sun “moves south” after the fall equinox, more energy will be collected by a tubular collector oriented in an east-west direction. When the midday sun strikes the collector surface at an angle of 40” or less, most of the energy is reflected (70). With the cylin- drical collector oriented east and west, the cross section of the collector that is normal to the sun when it is low in the southern sky is the same as when the sun is overhead. In tests conducted in Ohio with the quonset-shaped collector in Novem- ber, the east-west collector consistently produced higher maximum temperature rises and higher heat outputs than the north-south collector. The temperature at which the solar collector operates has a direct effect on collection efficiency: the higher the temperature in the collector, the greater the heat loss. In the tubular collectors tested, about half of the air temperature rise oc- curred in the first 40 feet (12.2 m) of a 1 00-foot (30.5 m) long collector. little temperature change occurred in the last 20 feet (6.1 m) of the collector since the losses from that point were about equal to the energy collected. The losses from the tube also in- creased as the wind speed increased. Performance of uninsulated collectors installed on the ground depends on the amount of heat stored in the soil underneath the collectors. During hot sunny weather, the ground underneath the col- lector is warmed. The heat thus stored in the ground warms the air when no solar energy is available. In Ohio tests conducted in the fall, heat storage in the soil was found to increase overall collection efficiency enough so the uninsulated col- lector supplied about 25 percent more energy than a similar collector insulated from the ground. The collection efficiency reported for the various low-temperature collectors used in the grain drying tests ranged from 12 to 62 percent. The collectors that showed an efficiency of 62 percent over an en- tire test period were small diameter, cylinders or tubes. The lower collection efficiencies reported were from unpainted, bare-metal collectors mounted on the sidewalls of bins. Collection efficiencies were calculated from the quantity of heat collected per unit area expressed as a percentage of the solar energy available, us- ually on a horizontal surface, either as reported by the former U.S. Weather Bureau or as collected at the test site with Weather Bureau type instru- ments and procedures. In Iowa, solar pyranometers were mounted to provide a measure of the solar energy available on the tilted surface of the col- lectors used. When proper corrections are made for collector orientation, sun angles, and other factors affecting collector performance, the collection efh- ciency of low-temperature, uninsulated, solar col- lectors without heat storage is in the 30 to 50 per- cent range. Solar energy may be used directly when it is available or may be stored to b:s used later during the night or during periods of rainy or cloudy weather. Air heated with solar energy can be used to heat pebbles or rocks during sunny weather. The drying air is then moved through the rocks during the night to recover the stored heat and to lower the humidity of the drying air. A collector unit containing a rock heal storage was built and tested at the U.S. Grain Marketing Research Center late in the 1975 drying season (r?~. 13). The collector covered an area of about 300 ft’ (28 m2) and contained 30 tons (27.3 tonnes) of rocks as the storage medium. Air pulled continually through the collector-storage system on a typical sunny day reached maximum temperatures about 7 p.m. and remained above ambient temperature until about 9 a.m. the following day. During the best sunshine hours, 9 a.m. to 4 p.m., the rocks were being reheated, and the temperature of air pulled through the rock bed was below ambient. Thus, the collector-storage system was supplying the most heat during the night when the humidity was high and the least during the day when the air was normally dry. AIR ENTRANCE Figure 13.-Half-section perspective of solar cellecto+- with rock-pile heat storage. 8 - Other important factors in collector performance are reliability and expected life. Plastic collectors are subject to damage from wind, ice, and snow, as well as damage from rodents, farm animals, domestic pets, and vandals. Air-inflated, plastic collectors have been relatively free from wind dam- age while inflated. Rodent and other animal dam- age is also minimal when the collectors are inflated. When collectors are deflated and lying on the ground, livestock or machinery passing over them may punsh holes in the plastic, and dogs may tear the plastic when trying to get at mice under the collectors. Thus, they should be stored when not in use. Wet snow has caused the larger, half-cylinder or quonset-shaped collectors to collapse. The clear, vinyl plastic covers become quite brittle during cold weather and shatter if struck or deformed. Fortu- nately, much of the damage to the plastic collectors can be repaired easily with mending tape. Many plastic collectors are still serviceable after 2 years. The bare-metal collectors used on the sides of bins are not easily damaged when anchored prop- erly to withstand strong winds. Surfaces may need to be recoated with flat, black paint to absorb maximum amounts of solar radiation. Weathered, galvanized metal has proved to be a reasonably effective collecting surface, and painting of bare- metal collectors may not always be important. Unshaded areas to deploy solar collectors are often limited. Some compromise in efficient, grain- handling principles may be required to accommo- date multiple bin arrangements that utilize solar energy for drying. Although bin-mounted collectors take up minimum ground area, the bins must be spaced so they do not shade each other or are shaded by other buildings or trees. SOLAR DRYING STUDIES IN PROGRESS Solar drying studies conducted in 1974 and 1975 fall into three categories: (1) Field testing of the application of solar energy to low-temperature grain drying in storage bins. Solar drying was compared to natural air drying without supplement heat or to low-temperature dry- ing systems using electric heat. (2) Field testing of low-cost, low-temperature, prototype, solar collectors for heating air for grain drying. (3) Evaluating the relative potential for solar grain drying in different geographical areas and for different crop conditions by the use of a mathe- matical simulation model for low-temperature dry- ing. Field Tests Eighteen solar-assisted drying tests were con- ducted with the 1974 crop at eight locations in the North Central region of the United States. One test was with soybeans, two tests were with grain sor- ghum, and 15 tests were with shelled corn. The tests were typical of low-temperature, in-storage drying, although one test approached conditions similar to batch-in-bin drying. Tests were continued at six locations in 1975. All of the grain in the solar drying tests was successfully dried to safe storage moisture levels without significant spoilage. In the late fall of 1974, some tests used supplemental electric heat in addi- 9 tion to solar energy. Drying rates with solar sys- tems were adequate to prevent spoilage and were in the range of typical low-temperature drying results-faster than natural air drying and usually a little slower than similar low-temperature systems with a 24-hour continuous temperature rise of 7” to 10°F (4” to 5.5”C). Final grain-moisture levels were lower in solar tests than in natural-air tests and generally higher than in tests with continuous heat added. In 1974 at the U.S. Grain Marketing Research Center, solar- dried corn averaged 13.2 percent moisture, and corn dried with natural air averaged 14.4 percent moisture after 20 days of drying at 2.5 and 2.8 cfm/bu (2.7 and 3.0 m3/min-tonne). An inflated tube collector with an area of approximately 300 ft2 (28 m2) was used. Typical patterns of drying grain with solar supplemented and natural air are illustrated in figure 14. In Indiana in 1975, with an airflow rate of 2 cfm/bu (2.2 m3/min-tonne) and 0.8 ft” collector per bu (2.8 m3/tonne), the corn initially at 24 percent averaged 16 percent moisture content after 24 days in the solar bin. With 10” F (5.6” C) added continuously by an electric heater in a companion test, the corn averaged 14.6 per- cent moisture after 16 days. Efficiency at which the sensible heat in the dry- ing air was used to remove moisture from grain was calculated for the tests conducted at the U.S. Grain Marketing Research Center. In general, the