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Study On The Light And Thermal Environment Building Of Chinese Solar Greenhouses In The Hexi Corridor And Model Development

Posted on:2021-02-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:X D ZhangFull Text:PDF
GTID:1363330620974696Subject:Horticulture
Abstract/Summary:PDF Full Text Request
Nowadays,developing“Gobi agriculture”has been adopted as an efficient solution for relieving the competition for land availability between grain and vegetables and exploiting the natural resources in the non-arable areas.The cultivation unit of a typical Gobi cultivation system is the Chinese solar greenhouse.The solar greenhouse that follows the green building concept relies entirely on solar energy for lighting and heating requirements of plants.The abundant solar radiation resource ensures the sustainable energy supply to the development of“Gobi agriculture”for the Hexi Corridor.Based on the design theory of traditional solar greenhouse,the facility agriculture research group of Gansu Agricultural University has designed five passive solar greenhouses having different north walls for the Hexi Corridor.These included a solar greenhouse having a flanged-layered wall(FLG),a solar greenhouse having a concrete-layered wall(CLG),a solar greenhouse having a gravel-layered wall(GLG),a solar greenhouse having a concrete hollow block-layered wall(HLG),and a solar greenhouse having an aerocrete brick-layered wall(ALG).The application of these solar greenhouses offers a remarkable opportunity for utilizing the Gobi solar energy resources and improving the growth environment of crops,and thereby creating economic,social,and ecological benefits.However,the present solar greenhouse design is still stumbled by several problems:First,the thermal design of building envelope is focused on heat insulation capacity,rather than the heat storage and preservation capacity;Second,without effectively linking the energy transfer processes within a solar greenhouse system,the light and thermal environment building has not been systematically understood and described;Third,the lack of understanding of the local geographic environment makes the structure design too depend on qualitative experiences.These allow the solar greenhouses designed for the Hexi Corridor based on the present solar greenhouse design code to be not achieve the desired effect and then limit the productivity.To guide the optimization of the structure design,environment regulation,and crop planting of the solar greenhouse in the Hexi Corridor,the dynamic heat transfer process involved in the light and thermal environment building within the solar greenhouse under the periodic external excitation has been analyzed in this paper.By quantitatively describing each stage composing the energy transfer process,a mathematical model of the light and thermal environment for a solar greenhouse was established and validated.By using the model,the light and thermal environment building of a solar greenhouse has been quantitatively depicted,and the periodic heat storage and release performance of the building envelope and the greenhouse system performance has been investigated.The main research work and result are as follows:1.A light environment model of the solar greenhouse was developed.We analyzed the influences of climate conditions,greenhouse shape parameters,materials'optical properties,and solar radiation's propagation modes on the solar greenhouse light environment.Consider the lighting roof as a radiation light-source and expressing the features and links of the solar radiation transmit stages by means of mathematical languages,the light environment building of a solar greenhouse system has been described:First,the influence of the varying slope angle on the beam radiation transmittance values along the roof curve was calculated by establishing the curve equation of the south roof,and then the precise description of the uneven distribution of the light intensity of the greenhouse light-source was achieved according to the dynamic variation of the outside solar radiation environment;Second,the dispersion regularities of the beam and diffuse radiation was illustrated by tracing the sunray paths and calculating the radiation angular distribution,respectively;Third,the interior multi-reflection was reproduced using iterative operation.By calculating step by step,the solar radiation allocation and spatial and temporal distribution in a solar greenhouse can be determined quantitatively.2.A thermal environment model of the solar greenhouse was developed.Development of the model was based on the influence of the solar radiation collected by a solar greenhouse on the energy balance during one operation period(daytime+nighttime)of the greenhouse system:In the first step,the role that each building component played in the thermal environment building and its heat transfer state were analyzed by using onsite measured data,and the links between the entire greenhouse system and the outside environment,and among each component in the greenhouse system was discussed from the point of energy transfer,thereby establishing the energy balance equation of a solar greenhouse system that have the interior air module the at the core.In the second step,the energy-transmitted rule of different heat transfer modes(heat conductive,heat convective,short-wave solar radiation,long-wave heat radiation)in each heat transfer stage was detailed,meanwhile,the characteristics of the interior high-wet air that emits and absorbs long-wave radiation was investigated,and this part of heat exchange has been quantified and then introduced into the energy balance of the greenhouse system;Subsequently,the dynamic heat transfer process of each components of the greenhouse system was analyzed,and the energy balance equation of each heat exchange surface was established based on the heat transfer state of the corresponding component,and thereby obtaining the heat gains of the interior air from each building component.By solving the abovementioned mathematical equations simultaneously,the precise description of the thermal environment evolution of the solar greenhouse was achieved.3.With the developed model,the dynamic building process of the light and thermal environment in the five solar greenhouses having typical structures in the Hexi Corridor can be predicted quantitatively.Taking a greenhouse spatial unit having a length of 1.0 m along the greenhouse's length direction as the research object and the following results were drawn:1)There were no significant differences among the spatial and temporal distributions of solar radiation in the five typical types of solar greenhouses for the Hexi Corridor.The spatial distribution of solar radiation exhibited a decrease in both south-to-north and bottom-to-top directions within the space required for crops(vertical height?2.0 m),and the average illuminance was found to range between 42000 and 51000 lux in the vernal equinox,between54000 and 57200 lux in the summer solstice,between 40000 and 46000 lux in the autumn equinox,between 22000 and 35000 lux in the winter solstice.In the summer solstice,the largest shadow area appeared near the north wall,occupying a width of 1.6 m.2)On a sunny winter solstice day,the solar radiation collected by FLG,CLG,GLG,and HLG(ALG)that was available for the light and thermal environments building were 124.15,118.90,120.49,and 119.56 MJ.There were significant differences among the solar radiation allocations in the five solar greenhouses for the Hexi Corridor:the highest solar radiation availability for the north wall was found in FLG,25.4%,29.4%,and 34.8%larger than that in CLG,GLG,and HLG(ALG),respectively;the difference in the solar radiation availability for the ground corresponding to the five solar greenhouse systems was less than 6.5%.3)During an operation period under clear sunny conditions,the average air temperatures inside FLG,CLG,GLG,HLG,and ALG were higher 26.44,24.77,24.27,22.73,22.95?C than the outside air temperature(fluctuate between-13?C and 0?C).Temperatures below 10?C were observed for HLG and ALG,with the low-temperature condition lasting 8.7 and 9.6 h,respectively.To reduce the unfavorable effect of the low-temperature duration on plant growth,it is necessary to provide the auxiliary heating in the operation of HLG and ALG.4)FLG,CLG,and GLG exhibited better thermal stability than HLG and ALG.During an operation period under clear sunny conditions,the net heat gains of the interior air module in the five types of greenhouses were-0.0057,-0.0059,-0.0054,-0.0070,and-0.0079 MJ/m~3.The heat losses of the interior air in HLG and ALG were higher than other greenhouses,with the average increase of 23.7%and 39.6%,respectively.5)In the thermal environment building of a solar greenhouse,the north wall and ground play a role of regenerator.During one operation period,the effective heat storage/release of the north wall FL,CL,GL,HL,and AL were 18.60,15.62,16.20,6.59,and 4.75 MJ,respectively,and the corresponding heat efficiencies were 79.5%,77.1%,78.5%,60.2%,and65.1%,respectively;the effective heat storage/release of the corresponding ground were19.81,19.89,19.85,24.84,and 26.56 MJ,respectively,and the corresponding heat efficiencies were 77.5%,76.9%,76.6%,80.3%,and 81.1%,respectively.6)During the nighttime,in the five solar greenhouse systems,the north wall FL,CL,GL,HL,and AL compensated the interior air by 46.4%,42.2%,43.1%,19.8%,and 13.1%of the total heat loss,respectively,and the corresponding ground compensated the interior air by51.9%,55.9%,55.0%,77.5%,and 83.8%,respectively.When the north wall HL and AL were unable to provide sufficient heat to the interior air,the ground beneath these two solar greenhouses reached their full heat storage and preservation performance,but the inside thermal environment still cannot be improved.7)The effective solar radiation energy efficiencies of the five solar greenhouse systems were 28.7%,27.7%,27.7%,24.5%,and 24.3%,respectively.FLG was the most effective solar greenhouse with superior lighting,heating,and heat preservation performances and cost-effectiveness,by making full use of the potential of the solar greenhouse to exploit the solar radiation resource of the Gobi areas,which made it the first choice for the solar greenhouse type in“Gobi agriculture”developing.
Keywords/Search Tags:Chinese solar greenhouse, light and thermal environment, available solar radiation, dynamic heat transfer process, periodic heat storage and release performance, mathematical model
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