Design And Thermal Performance Of Greenhouses With Removable Jute Fiber Back Walls

In China,the areas of greenhouses are expanding every year.There are many types of greenhouses in different regions,including solar greenhouses,single span greenhouses and multi-span greenhouses.Most of them are fully passive solar greenhouses,which are used to grow vegetables without any auxiliary heating in the winter.The good thermal insulation properties and cooling performance of greenhouse are necessary for successful crop production throughout the year.For solar greenhouses,the back wall plays an important role in thermal insulation and it has been a pivotal topic in the optimization of solar greenhouse structure.The materials and thickness of the back wall make differences on the microclimate inside solar greenhouse.Numerous studies have been carried out on greenhouses to examine the effects of different wall materials and configurations on thermal performance.Many solar greenhouses are capable of maintain sufficiently high temperatures for the winter production of several major vegetables such as tomato,cucumber and sweet pepper.However,because solar greenhouses are relatively enclosed structures,the high temperatures resulted from poor ventilation make it difficult for crop production to continue during summer.In this study,new design schemes will be introduced using jute fiber boards for the back wall of the solar greenhouses.The jute fiber boards can be installed and dismounted as the inside environment requires.Jute fiber is an excellent natural fiber that is readily available and it has good thermal insulation and can be a promising substitute for the traditional construction materials such as bricks,concrete block.The purpose of this study was to evaluate the thermal performance in the new greenhouses.The results are as follows:1.The moisture content of jute fiber is lower than that of wheat straw under different humidity conditions.The thermal conductivity of jute fiber was 0.09-0.11 W m-1 K-1 and which has good thermal insulation.2.Two new design schemes were presented using jute fiber boards for the back wall of the solar greenhouses.The greenhouses consisted of back wall,back roof,front roof,gables and buffer room.Greenhouses were covered with 0.15 mm thickness of polyethylene film and in the east-west direction,with a net span of 9.0 m(outer span of 9.4 m),ridge height of 3.6 m,length of 50 m and net area of 425 m2.The fully-removable back wall was assembled using jute fiber boards with the size of 1.2×1.2×0.05 m.The boards were wrapped with polyethylene film to form waterproof blocks.The wall with height of 2.6 m and thickness of 400 mm was composed of 50 mm thick jute fiber boards,300 mm thick air space,50 mm thick jute fiber boards,insect-proof nets and polyethylene film from inside to outside.The half-removable back wall had two parts:the removable wall and fixed wall.The fixed wall was built with hollow concrete blocks.In winter and early spring,the back walls were installed;thermal blanket was retracted to the roof top during the daytime,and spread out during the nighttime.In the summer,the thermal blanket was removed;the jute fiber boards in the back wall were dismounted and installed with insect-proof nets to exclude insects.3.For all greenhouses,the soil was the major heat storage for the entire greenhouse and the cover film was major factor in heat loss compared with other surfaces.The results show the released heat of per unit soil area were 0.54,0.55 and 1.07 MJ m-2 inside HRG,FRG and SPG on sunny day,while 0.73,0.64 and 1.11 MJ m-2 on a cloudy day.The higher value inside the SPG was due to larger difference between temperatures of air and soil surface.The results show that the temperatures at different layers and the heat flows of back wall surfaces were significantly affected by indoor,outdoor air temperature and solar radiation,and which had the same diurnal variation as indoor and outdoor air temperatures.Temperature of each point inside the wall was affected by inner surface temperature.Although the block wall had a higher heat storage coefficient than the jute fiber wall,most of energy absorbed by inner wall during the day was lost and not stored and released into the greenhouse at night.The released heat from the back walls was insignificant compared to the soil.As a result,there was no significant difference in the heat released inside the greenhouse overnight between HRG and FRG despite with the different back walls.In summer,with all vents of greenhouses left opened,most of energy was lost by ventilation.4.Results suggest the air temperature in the greenhouse with removable back walls could be maintained above 8.2 ? while the lowest temperature in the SPG was about 0?in winter.During summer the highest indoor air temperature of two greenhouses was under 39.0 ? with improved ventilation while in the SPG was above 45 ?.The tomato yield of new greenhouses were increased significantly than conventional greenhouse in whole year.The removable wall combined with jute fiber boards represents a potential design improvement for the back wall of greenhouses.