Design And Studies On Environment Factors Of Super-Span Plastic Covered Tunnel Greenhouse

ABSTRACT:One of the facility agriculture development tendency is large scale, which can expand each facility area,save the material,reduce the construction cost, enhance light transmission rate and improve cultivation benefit.Because of its superior light and heat resources,Huang-Huai-Hai plain is one of the main protected horticultural production area,which serves as a base supporting vegetables in early spring and late autumn for north part of China.Therefore it is very important to promote protected cultivation level in Huang-Huai-Hai plain.First high quality product can be produced to satisfy the market demand and second farmers can get more incomes.Designing and building the large-scale facility,which suits the modernized scale production need,conforms to countryside economic potentiality, conforms to the local climate characteristic,is critical to realize the second time facility agriculture prosperity in Huang-Huai-Hai plain.For the above-mentioned reasons,the author carries on the design of super-span plastic covered tunnel greenhouse.The greenhouse space design was depended on the demands of crop growth and management.The experimental greenhouse was established according to the designing rule of which cushion facility resisting external environment the influence and enhancing the facility use efficiency through increasing the span and length of facility.The multi-layer simple thermal screen were set inside greenhouse in order to reduce heat loss.Microenvironment characteristic and how to regulate temperature and humidity were studied and how to utilize the greenhouse scientifically were put forward based on these researchment.The technical and economic efficiency were evaluated when the greenhouse was applied as cultivating vegetables during early spring and late autumn.The results indicated that:(1)The structural parameters of designed greenhouse were described as followers:the span is 20m,length 100m,shoulder height 1.5m,height 3.5m,,height/span ratio 0.1 and the curve shape circular arch..One row of column was set every 2.5m alongside span direction and one row of column was set every 1.2m alongside length direction. The greenhouse was covered with PVC,and there were two layers of thermal screens at the height of 2.0m and 2.2m above the ground and the two layers of thermal screens hanged by the wall.The total construction cost of 2000 m² greenhouse was 15629.4 Yuan and facility utilization ratio was 94%.(2)Compared with normal-span greenhouse,average air temperature,average air temperature during day and average air temperature during night were enhanced 1.3～1.6℃,0.8～1.2℃,1.8～2.4℃respectively inside super-span greenhouse in November and in March.The super-span had a stronger ability to resist temperature variation outside but its temperature field was less even than that of normal-span greenhouse.Both temperature rising speed and dropping speed of normal-span greenhouse were higher than those of super-span greenhouse in sunny day.The extreme diurnal of air and soil temperature were 2.4℃and 1.4℃in normal-span greenhouse higher than that in super-span greenhouse.The normal-span had higher solar radiation transmission.The solar radiation transmission ratio was 37.5%～73.9% in sunny day and 36.0%～65.0%in cloudy day of super-span greenhouse.(3)It was an efficient way to save energy and the air and soil temperature were enhanced by adding inner covering.The average air and soil temperature during ten days were enhanced 0.6～2.4℃and 0.6～1.8℃,0.5～2.2℃and 1.0～2.9℃,0.6～1.1℃and 0.7～1.8℃by adding the first,the second layer film,and hanging the film by the wall,respectively.The energy -saving ratio was 37.5～40.5%,32.8～33.5%, 15.1～19.0%in double-layer and all around by wail covering,double-layer covering and single covering,respectively.The energy -saving ratio was higher in night cloudy day than that of sunny day.The energy -saving ratio was enhanced and solar radiation transmission ratio reduced by adding more layer covering.The solar radiation transmission ratio was 60～70%in February and March inside greenhouse covered by three layers film.(4)It was showed that daily changes of relative humidity inside super-span greenhouse took on a single valley curve under sunny,cloudy and rainy day.The maximum RH appeared an dawn and the minimum RH at 13:00.The daily changes of absolute humidity inside super-span greenhouse took on a single peak curve.Both RH and absolute humidity were the highest in sunny day and lowest in rainy day according to variation ranges.Super-span greenhouse was at a high RH condition most of time during one day and it was key to reduce the humidity inside greenhouse. It was found that dehumidification effects of plastic mulch and drip irrigation in greenhouses significant,which drip irrigation under plastic mulch had a better dehumidification effects.By using plastic mulch and drip irrigation under plastic mulch techniques,average relative humidity was reduced 3.9%～15.1%,6.5%～23.3% and average absolute humidity reduced 0.1～2.9hPa,0.3～6.1hPa respectively compared to conventional cultivation techniques.The soil temperature and moisture content were enhanced by using these two techniques.(5)The CO₂ environment in super-span greenhouse was surveyed,the results indicated that:Diurnal change curve of CO₂ concentration appeared "U" shape. Canopy photosynthesis was low and CO₂ shortage didn't happen though soil respiration was low because of low soil temperature at seedling stage.CO₂ enrichment was necessary because of high canopy photosynthesis,was affected by weather and canopy situation.CO₂ deficiency was more serious in sunny day than that in cloudy day and it couldn't avoid CO₂ deficiency by air exchange.(6)The main utilization of super-span was during early spring and late autumn.The seedling was transplanted at the beginning of February during early spring cultivation and harvesting time lasted till the last ten days of November.When built a 2000m² greenhouse with double-layer inner thermal covering to cultivate vegetable,the financial internal rate of return(FIRR)is 45.36%,financial net present value(FNPV) is 57900 yuan,payback period of investment(static behavior)was 1.83 years.