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17 1 2001 M1j<Transactions of the CSAEVol. 17 No. 1Jan. 2001Article ID: 1002-6819(2001)01-0022-05Theory and Practice of Energy-SavingSolar Greenhouse in ChinaChen Duansheng(College of Horticulture, China Agricultural University, Beijing 100094, China)Abstract: The fundamental structure, performances and application of energy-saving solar greenhouse and itsdevelopment in China were introduced briefly in this paper. The optimal design of structure for this type ofgreenhouse was introduced emphatically, such as the geometrical dimension of various parameters of constructionstructure, the inclination and shape of the lighting surface, the structure of wall body, the structure of back roofand its elevation angle and projection, the outside heat preservation covering materials and the measure ofcontrolling the environment, etc. At thesame time, the advances of theoretical research wereintroduced too, suchas the mathematical model of solar radiation, heat, air moisture, soil temperature and mechanics. All of theseadvances of theoretical research provided basis for the optimal structural design of greenhouse.Key words: China; energy-saving solar greenhouse; advancesCLC number: S625. 2 Document code: AReceived date: 2000-12-01Foundation item: National Natural Science Foundation of China(39830230)Biography: Chen Duansheng(1938- ), male, professor, doctoraladviser, Senior member of the CASE, majored in greenhouseresearch.1 IntroductionSince the mid-1980s, in the district of 32N43N of north part of China, the energy-savingsolar greenhouse has been developed rapidly.According to the statistics, thearea of this type ofgreenhouse reached 200 thousand hm2 by 1999.The energy-saving solar greenhouse was developedfrom the Chinese traditional lean-on greenhouse inAnshan, Liaoning Province. It is composed of 1)North wall and east-west gable against the coolwind and prevent the heat exchange between insideand outside of greenhouse; 2) Front house roof forlighting. During the night, the outside of fronthouse roof is covered by several heat preservationmaterials. In the morning after the air temperatureinside greenhouse rises, the covering materialsshould be rolled up. In the afternoon, when the airtemperature inside greenhouse down to 1718, the covering materials should be put down toprevent loss of heat; 3)Back roof is connected withthe front roof and back wall; 4)Cold-proof ditch toprevent the loss of heat through soil ( Fig. l).1. North wall 2. Front roof 3. Back roof4. Cold-proof ditch 5. Heat preservation materialFig. 1 The solar greenhouseThe energy-saving solar greenhouse is thespecial type of greenhouse, there is no heating oronly a little heating. Its energy resources mainlycome from the solar radiation and depend on therational greenhouse structureand heat conservativetechnology. Even during the severe cold winter theair temperature inside the greenhouse satisfies notonly the growth of the green vegetables such ascelery, Chinese chive, etc, but also the growth offruit vegetables, such as tomato, cucumber, sweatpepper, eggplant, etc, even water melon. Theproductive practice shows that it is possible forabove districts to use this type of greenhouse for22the vegetable production and its benefit is ratherhigh. In general theinvestment of theconstructionand operation is much lower than that of not onlythe large scale modern greenhouse, but also thelean-on heating greenhouse.Since the energy-saving solar greenhouse hascharacteristics of full lighting and tight heatpreservation, during the sunny daytime the 6hours average room light can reach 20000 40000Lux, and in general the minimum roomtemperature can maintain more then 10 in thelight of local meteorological date. During thedaytime under the sunlight the inside maximumtemperature is usually over 25, even higher than30. During the whole day the room airtemperature over 25 may last 2. 5 3. 0 h and thelongest duration may last 5 h. Under such meteor-ological condition and continual improvement ofthe cultural practice, for example, the selection ofthe low temperature resistant variety, grafting,administration of altering temperature, irrigation,and during the continual cloudy or sudden drop intemperature weather to apply the temporarysupplementary heating for short period.Therefore, not only the green vegetable canbe grown, but also the thermophilic gourd vegeta-bles and some fruits and flowers can be grown.Why can the energy-saving solar greenhousetechnology obtain successes in some areas of Chinain which it is severely cold in winter, the minimumair temperature outside door usually lower than- 15 - 10 even than - 20 - 15? It iscommon knowledge that the Chinese climate istypical continental and monsoon climate, there isrich solar radiation resources in winter though thetemperature is very low in the zone of the northpart of China, such as the North China,Northwest China and Northeast China. Thesunshine ratio is more than 50% 60% in winter.The solar radiation can be changed to heat and canbe closed in the greenhouse by the technologies oflighting and heat conservation.2 The Advances of Optimization for Const-ructive StructureIn the earlier stage of developing energy-saving solar greenhouse, people built thegreenhouse depended on experiments only, usingmuch cheaper and simpler materials, for example,the spaceof greenhouse was smaller, span was 5. 4 6. 0 m, ridge height was 2. 6 3. 0 m in Beijingdistrict, the wall was built by brick and concrete,the frames were bamboo or wood commonly, andoutside covering material was straw mat or cattailmat. It is not suitable for operation, not good forperformance of lighting and heat preservation andweak for resistance to weather disasters, etc.Since mid-1980s people do much research work toraise its performances of lighting and heatpreservation and made great progress. Then theenergy-saving solar greenhouse is being continuou-sly perfected.2. 1 The DimensionAfter the energy-saving solar greenhousedevelopment for 10 20 years, its dimension wasenlarging continuously, now the span isdetermined by local climate. In Northeast Chinaand Northwest China the span is about 6. 0 7. 0 mwhere the lowest temperature is less than - 17in winter, in North China the span is about 6. 57. 5 m where the minimum temperature is about- 17 - 12, while the other districts wherethe lowest temperature is higher than that of thespan is more than 7. 5 8. 0 m. The back wall alsois raised correspondingly, reaching 2. 0 2. 4 m.The ridge height depended on the span and localclimate, for example, at the place of latitude 40the span was 7. 0 m, 7. 5 m, 8. 0 m and thecorres-ponding ridge was 2. 9 m, 3. 2 m, 3. 5 m. Thelarger span was, the higher ridge was and thewarmer climate was. These dimensions wereobtained depending on the research on therelationship between theconstructive structureandenvironment of greenhouse.23 1 3:S ? ; i Ll2. 2 The Wall BodyAccording to the study in 1989 1992, it wasdiscovered that during the air temperaturewarming-up inside greenhouse in the daytime, thewall is an endothermic body and during the airtemperature cooling inside greenhouse , the wallbody is an exothermic body so the reasonablestructure of wall body should be 3-layer compositebody consisted of different quality materials. Itsinside layer consists of the materials with strongendothermic, heat preservation capability such asred brick in order to absorb much moreheat in thedaytime and to release much more heat in thenight. Its outside layer consists of the materialswith poor-conducting, exothermic capability suchas the air-entrapping brick. Between them therewas a thermal-protective coating using someinsulation materials such as pearlite, coal cinder,sawdust, dry earth, and polystyrene board, etc. inorder to prevent heat loss.2. 3 The Inclination and Shape of the LightingSurfaceAccording to analyzing environment of solarradiation in greenhouse the optimal inclination ofthe lighting surface was presented, and the archshape ( Fig. 1) was better than the linear ornonlinear, among which the circular-parabolicshape was better than the circular, parabolic, andellipse becauseof thefirst shape not only thedirectsolar radiation energy into the greenhouse wasmuch greater than those of the others but it issuitable for operation, resistance to strong windand was easy to clear away the raindrop.2. 4 The Back RoofThe structure of back roof should be multi-layers composite also, it consists of waterresistance, load-bearing, heat preservation andwater resistance layer from the bottom to roof. Inthe past the load-bearing and heat preservationconsisted of concrete board, dry earth andsorghum stalks, etc. It now consists of compositeboard which was made of polystyrene and resin.The elevation angle of back roof should be morethan the local solar elevation at noon on the wintersolstice day about 35 40, to ensure that thesunshine can reach the whole back wall in thewhole winter. The projection of back roof shouldbe 0. 80 0. 75 of span, because of the largerprojection was ,the stronger sunlight was, but theweaker heat preservation was.2. 5 The Outside Covering MaterialsIn the past theoutsidecovering materials werestraw mat or cattail mat with 5 cm in thickness.These materials were not only heavy but couldbreak and pollute the plastic film. Now we use thelight and good materials of heat preservationinstead of traditional outside covering materials,such as the 3-layer composite pad of heatpreservation. The transfer coefficient of these newmaterials were about 2. 0 W /(m2 K) as same asabove traditional one, energy-saving ratio is about60% .2. 6 The FramePeople used thesteel tubeinstead of bamboo ,wood and concrete to improve the environment oflight and operation. Because it is not necessity touse column and the dimension of frame can besmaller.2. 7 The Measure of Controlling the EnvironmentThere were not any installations of controllingenvironment of greenhousein 10 years ago. Peoplerolled up the outside covering materials in themorning and put it down in the evening bymanpower as well as the ventilation. Now peopledeveloped several kinds of machine to roll up theoutside covering materials and to ventilate. Atsome places people developed monitoring andcontrolling system for greenhouse by computer tomonitor the environment and control it.In general, successful advances on construct-ion of energy-saving solar greenhouse wereobtained, the principal points are as follows: 1)Expanding the space of greenhouse; 2) Using thelight and thin materials; 3 ) Developing themeasures for environment control; 4 ) Theperformances of the greenhouse were continuously24 j<2001 M perfected. All of those advances were obtainedresulting from the theoretical research.3 The Advances of Theoretical ResearchIn the past 10 years, people did much workconcerning theory of construction, structure andenvironment of energy-saving solar greenhouse.People developed several mathematical modelsconcerning the light, heat, soil temperature, airmoisture and mechanics, etc. The objectives ofthese models were to make optimal design andcontrol environment for greenhouse to improve itsperformances. The principal advances were asfollows.3. 1 The Model of Light EnvironmentAccording to meteorology, geometricaloptics, radiology and transparent characteristics ofplastic film, a mathematical model of directradiation environment in greenhouse wasestablished. This model aimed at greatest directsolar radiation available in greenhouse. Using thismodel can obtain the optimal inclination of thelighting surface ( front roof ) at the place ofdifferent latitudes, and can determine optimalshape of the lighting surface if the span and ridgeheight were given.Other model was involved in direct and diffuseradiation, and structure members and pollution ofcover are taken into consideration separately. Onthe basis of this mathematical model, the lightenvironment can be easily evaluated and simulated,on the other hand, optimal parameters of structuresuch as the span, ridge height can be provided.At the same time, the model can be used tocalculate theglobal radiation on the floor, thebackwall as well as the back roof. Calculated resultshows that the proportion of global radiationavailable on the floor was about 50% 60% , theback wall 30% , the back roof 10% 20% . Thiscalculated result is coincided with the practicalmeasurement. This result is very useful for furtheranalysis of thermal environment in greenhouse.3. 2 The Model of Thermal EnvironmentA dynamic thermal model was developedbased on the theory of heat balance in thegreenhouse and theory of similarity. There were 4heat balanceequations in this model, including air,front roof, wall and ground. This model couldpredict the air temperature inside greenhouse,according to air temperature outside door andseveral parameters of constructive structure ofgreenhouse. The result of examines shows that thecalculated value is in good agreement with themeasured one. The mean error was only 1. 6,and the mean relative error is less than 10% .At the same time a comprehensive indexconsisting of 14 single indices such as averageminimum and maximum air temperature insidegreenhouse, was presented by statistic method.We could evaluate the excellence or inferiority ofgreenhouse by this comprehensive index.By using the model and the comprehensiveindex, various parameters of constructive structureof greenhouse such as the relationship between thespan and ridge, the projection length of back roof,the heat resistanceof wall, etc. could besimulatedand evaluated. As a result, the best parameterswere chosen.Other author established the relationshipbetween the local climate and heat resistance ofwall and back roof. So according to heatconductivity of materials, the wall and roofthickness and their structure could be determined.3. 3 The Model of Air HumidityBased on the soil surface energy balanceequation of greenhouse the model of moistureinside greenhouse was developed by using thetheory of similarity. The result of examines showsthat the calculated value is very close to themeasured value, the mean error was about 0. 15kPa and relative error is less than 8% only. Thismodel could be used to predict moisture and dewduration, which provides theoretical reference formoisture regulation, and prevention and control ofplant diseases in greenhouse.25 1 3:S ? ; i Ll3. 4 The Field Model for Simulating SoilTemperatureAn unsteady two-dimensional feild model forsoil temperature was established on the basis oftheory of unsteady heat transfer, using the limiteddifference method. The model was proved to besuccessful and provided a scientific basis forfurther analysis of soil temperature conditions andselection of the soil temperature-raising method ingreenhouse.3. 5 The Model of MechanicsAnalyzing stress and strain in the frame ofgreenhouse, a model to
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