The Study On Surface Heat And Moisture Law Of Air-to-EarthHeat Exchanger In Solar Greenhouse

In recent years,solar greenhouse has made remarkable achievements in the agricultural development of our country,and has become the main form of agricultural production.However,the heat and moisture environment in solar greenhouse is quite complex,and the existing temperature and moisture control facilities are not perfect enough,which limits the advantages of greenhouse.The surface heat and moisture exchange has decisive influence on the environment of greenhouse.Therefore,it is necessary to study the surface heat and moisture law to lay the foundation for the development of low-cost,energy-saving temperature and moisture control facilities to improve the heat and moisture environment regulation technology of solar greenhouse.Firstly,this paper introduces the research background of solar greenhouse and air-to-earth heat exchanger,briefly describes the current research situation of heat and moisture transfer law on the surface at home and abroad.Taking a solar greenhouse as the test platform which in Xiaodian District of Taiyuan,continuously tests the total surface radiation,net radiation,wind speed,soil heat flux,soil temperature,moisture content,air temperature and humidity under different meteorological conditions.A mathmatical model was built up to simulate the dynamic temperature and moisture content of shallow soil in solar greenhouse.The results show that:1.The variation of surface radiation in sunny and overcast days is different.In sunny days,the total radiation changes with the solar height angle,which goes through three stages: the largest in the south,the largest in the middle and the largest in the south.The heat absorption in the central greenhouse is the largest,the net radiation in the middle is greater than that in the south and north.In cloudy days,the total radiation and net radiation in north-south are both the south higher than the north.The maximum total radiation in the sunny day is 12.4 times that of cloudy day,and the maximum net radiation is 21.7 times that of cloudy day.The net radiation and the total radiation in east-west have little influence by the location of the measuring points,mainly by the solar radiation.The effect of solar radiation on indoor is greater than outdoor temperature.The surface air temperature in the middle of greenhouse is higher than other measuring points,and the relative humidity is lower than other measuring points.2.The variation trend of soil temperature and moisture in different depth is consistent,and showed periodic sine curve with time.The range of soil temperature fluctuation showed the law of 1cm > 7cm > 15 cm > 30 cm > 50 cm,that is,the daily temperature range of soil layer closer to the surface was larger than soil layer.With the increase of soil depth,the amplitude of temperature wave decreases and the range of moisture content decreases,the delay time increases gradually,the soil layer of 50 cm depth is little affected by the change of external environment,the temperature and moisture remain constant.3.The distribution of soil temperature in the north-south direction is middle > north > south,the moisture content is south > north > middle,the soil temperature distribution in east-west is that the central soil temperature higher than the eastwest side,the moisture content distribution is just the opposite,whether it is the heat absorption process during the day or the exothermic process at night,the temperature gradient of the central measuring point is larger than the east and west sides.4.The heat transfer in the lateral direction of soil is very little,mainly in the depth direction.The regularity of evapotranspiration in the north-south direction is: south > middle > north,but the evapotranspiration has little difference in eastwest.5.In sunny days,the latent heat is greater than sensible heat transfer.In cloudy days,the sensible heat is greater than latent heat transfer.The change trend of latent and sensible heat in greenhouse is greatly affected by solar radiation,which is consistent with the change of solar radiation.6.According to the change gradient of temperature and moisture content in shallow soil,the soil layer from 0~100cm can be divided into three characteristic layers: variable layer(0 ~ 30cm),slow changing layer(30 ~ 50cm),and stable layer(50 ~ 100cm).