| With the large-scale construction and extensive application of solar greenhouse in high-altitude and low-temperature areas,the problem of low water temperature of ground irrigation during winter was the key to curb the normal operation of solar greenhouse and the normal growth of crops.How to improve the irrigation water temperature,choose reasonable heating facilities,and control the heating process of irrigation water became the theoretical and technical problem which curbed greenhouse cultivation in wintering period.This paper took the objective problem of low irrigation water temperature in the greenhouse during the wintering period as the starting point,relying on the Shanxi science and technology key project “The key technology research on industrialization of facilities horticulture——The heating-up facility and technology research on irrigation water inside the solar greenhouse in low temperature region”.Based on the large-scale solar greenhouse field tracking and indoor experiment data during the winter,the change characteristics of microclimate and regional meteorological conditions and temporal and spatial variation of irrigation water temperature were analyzed by using the theory of heat thransfer and soil tydrodynamics.The heating pool was selected as the heating facilities with the advantage of well heating effect,small area,well water qualityand safety management and operation.Taking the selected heating facility as the object,CFD software FLUENT is used to simulate the heating process of irrigation water.The real-time forecast model of irrigation water temperature was established under different weather conditions,different initial temperature of irrigation water temperature and different types of heating facilities.The prediction of the heating time of irrigation water during the wintering period in the study area was realized.The appropriate minimum temperature and heating time of irrigation water in different period of winter were put forward.The integrated technology of irrigation water heating system in winter greenhouse was constructed.Scientific value was reflected in the blending of theory of irrigation,heat conduction and facilities agticulture.The research results could effectively promote and enrich the development of solar greenhouse facilities and irrigation management science.The technical method of this paper could be applied to the northern region of China to solve the problem of heating irrigation water temperature in different types of greenhouse under different weather conditions.The main results are as follows:(1)The daily variation of indoor and outdoor air temperature was the diurnal variation of cosine function with 24 h,but there were large differences between the amplitudes of the indoor and outdoor air temperature.The amplitude of indoor air temperature was about 2.0~2.9 times the amplitude of outdoor air temperature.Indoor and outdoor air temperature had a significant linear correlation,but the correlation coefficient was from 0.75 to 0.96 at different periods during the wintering period.The spatial distribution of indoor air temperature had a certain degree of inhomogeneity.The vertical direction distribution was the median altitude temperature> the temperature at the roof and the ground,the temperature difference in the vertical direction during the day could reach 5.2~10°C,and the nighttime temperature difference was small,between 3.0~5.4°C.In the daytime,the horizontal length(east-west)direction distribution was air temperature in the middle> west> east;in the night the distribution was air temperature in the west> middle> east,and the range of change was between 0.3 and 1.8 °C.The horizontal width(north-south)direction distribution was central temperature > temperature near the back wall > temperature near front roof during daytime;temperature near back wall > central temperature > temperature near front roof during the night.The difference of indoor air temperature was the daytime was greater than the nighttime,the temperature difference during the day was between 3.2 and 5.8°C,and the nighttime temperature difference was only 0.8~1.9°C.The daily variation of indoor and outdoor soil temperature was the diurnal variation of cosine function with 24 h.However,the amplitude of the temperature wave gradually decreased with depth,and to a certain depth,the amplitude was substantially zero.The amplitude of indoor ground temperature below 0.4m and the out door temperature below 0.25 m were 0.And there was no obvious daily change in the underlying soil temperature.The amplitude of indoor temperature waves in the same level was greater than outdoors,which was closely related to the indoor temperature variation was greater than the outdoor.(2)During the wintering period,whether ice cover appeared over the river surface water had a great influence on the water temperature.During the non-icing period,the river surface water temperature changed with the air temperature.Air temperature was significantly greater than the water temperature for about 10 hours from 8:00 to 18:00.And in the icing period,water temperature changed with air temperature,but the water temperature was in a relatively stable state,basically maintained at-3.0 ℃ or so.The main reason for the above-mentioned water temperature characteristics was that the large specific heat of the water,the continuous supplies of the lower layer river bed sand and gravel medium,the river surface ice cover and thin air insulation.(3)The shallow underground heating-up reservior was ideal for heating facilities in solar greenhouses during winter.The main source of heat for the irrigation water in the heating-up reservior was the indoor air,the walls of the side and the bottom of the reservior and solar radiation.Whether it was on the surface of water,the side wall or the bottom of the heating-up reservoir,the use rate of air temperature,ground temperature and solar radiation in the underground shallow heating-up reservoir was higher than that of the other two structural types.In terms of warming effect,the underground shallow heating-up reservoir was obviously superior to the other two types of heating-up reservoir.It was suggested to use the underground shallow heating-up reservoir as the main facilities for raising the temperature of solar greenhouse irrigation water in high-altitude and low-temperature areas.(4)For the first time,CFD computational fluid dynamics program was applied to simulate the heating process of greenhouse irrigation water during wintering.The combination of the definite conditions in the solar greenhouse and the CFD computational fluid dynamics solution model was successfully achieved.The prediction of the temperature and temperature rise time of the water heating pool in the solar greenhouse was realized.The cosine function was used to fit daily variations of indoor air temperature and ground temperature as the input boundary conditions.The correlation coefficient between the simulated value and the measured value was above 0.96,the relative error was below 5.46%,and the maximum absolute error was below 1.64°C.The error was within the acceptable range.The simulation predicted that the time required for heating of irrigation water throughout the winter period was between 5 and 51 hours.The boundary conditions were directly related to the average value and amplitude of air temperature and ground temperature,and the initial conditions were directly related to the temperature of water entering the shed,which resulted in that this model could be used to simulate the process of irrigation water temperature change under different condition of weather and initial water temperature in different heating-up reservoir of structures and materials.It could also be applied to simulate the heating process of irrigation water in solar greenhouses with different structures and different soil types,and had a widespread and wide application prospect.(5)The staged irrigation water management model of the solar greenhouse in the wintering period in low temperature area was proposed.During the overwintering period,according to the spatial and temporal distribution characteristics of indoor and outdoor air temperature,ground temperature,temperature of water source,and water temperature entering the shed,staged irrigation word system was proposed including safe irrigation water temperature,heating-up time and suitable irrigation time.The suitable irrigation water temperature for the wintering period was as follows: In half month,the minimum irrigation temperatures after rising in the heating-up reservoir were 8.5,10.8,9.8,9.1,8.0,and 7.5°C from mid-December to mid-March.In January,the temperature was relatively low,and the minimum required irrigation temperature for plants was correspondingly higher,which was 10.8 and 9.8°C,respectively.In half month,the irrigation water warming time was 5,51,47,38,24,8 hours from mid-December to mid-March.Among them,the temperature in January was low,and the irrigation water required the longest time to warm up,which was 51 h and 47 h,respectively.This irrigation water management model could provide strong technical support for the solar greenhouse irrigation management during the wintering season in high altitude and low temperature areas.(6)It was feasible to achieve safe irrigation during wintering period in high-altitude and low-temperature areas.The use of geothermal and solar energy in the process of water supply,water distribution,and irrigation in facility agriculture in high-altitude and low-temperature regions could realize safe irrigation during wintering period.In the process of water intake,the way of using the surface water to convert the groundwater into the submergence wells could effectively utilize the geothermal heat to increase the irrigation water temperature,and the water temperature could be increased by 6.7~9.0°C;During the adjustment of water distribution and regulating pools,the distribution pipelines were buried below a certain depth of the depth of the frozen soil,the depth of the adjustment pool was increased,and the additional insulation facilities above the adjustment pool could effectively use geothermal energy to maintain or increase the irrigation water temperature;In the interior of the solar greenhouse,underground shallow heating-up reservoir could scientifically and efficiently utilize solar energy and geothermically to increase irrigation water temperature.During different periods throughout the wintering period,it could be used for 5 to 51 hours to raise the water temperature to the minimum fillable temperature required by the crop.(7)The integrated winter heating greenhouse irrigation water heating technology was constructed.In the wintering period,suitable surface water for solar greenhouses and safe irrigation needed to be integrated with the engineering and management techniques in the process of water intake,water transportation,heating,and irrigation,that is,organic integration of multiple links and multiple technologies.The main technologies that make up the integrated technology include: The main techniques in the process of water intake included increasing the length of the seepage path and increasing the depth of the submersible well.The main technologies in the process of water distribution and water regulation included increasing the depth of the water distribution pipeline network,selecting a pipe with a large thermal conductivity coefficient,increasing the depth of the regulative water tank,and setting insulation facilities above the water pool.The technologies that could be used to increase the temperature in the heating-up reservoir included high-efficiency absorption of solar energy and geothermal technology,temperature rising process prediction technology with CFD model,and rapid heat conduction technology.The technologies that could be used in the irrigation process included drip irrigation technology,water and fertilizer integration technology,safety irrigation water temperature prediction technology.The technologies that could be used for irrigation management included the technique of determining the irrigation mode and the technology for determining the temperature of the water in the indoor heating-up reservoir.In the course of the study,the differences in the demand for irrigation water temperature between different vegetable cultivation types in the solar greenhouse were not taken into consideration and the indicators of the crop growth process were not monitored.The demand for the minimal irrigation water temperature between different types of vegetables in the greenhouse in different wintering periods and crop growth process indicators should be monitored.In addition,when inputting the boundary conditions of the irrigation water heating model,the cosine function was used to fit the daily variation law of indoor air temperature and ground temperature,which was one of the reasons causing the model error.In the subsequent modeling,the Fourier expansion could be used to fit the daily variations of air temperature and ground temperature in order to achieve higher simulation accuracy. |
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