Study On A Solar Active Heating And Soil Storage System For Greenhouses

Solar energy is one of the most widely distributed,convenient and clean renewable energies.Mostly areas in China are rich in solar energy resources which provide favorable conditions to the utilization of solar energy.At present,we focus mainly on the solar water heaters in solar thermal utilization field.Technology of greenhouse heating using solar energy develops very slowly,which is far from market.Accordingly, there exists a vast market of prospects for the use of solar energy in greenhouse heating.In this thesis,a new solar active heating system with air collectors collecting and soil heat storage is studied for greenhouses heating.The thesis includes following aspects:1.A new solar active heating system for greenhouses which is comprised of solar air collectors and soil heat storage system has been designed and built.The solar air collecting system is made up of flat-plate air collectors which are laid at two sides of the south-north-trend greenhouse.Soil heat storage system is made of two domain air collecting PVC pipes and some radiator PVC pipes.In this system,outside air is heated by solar air collectors and then pumped into underground pipes.The heat carried by air is transferred through pipes to the soil and stored for use at night.In the night,heat rises slowly to the ground surface to maintain the soil at constant temperature.When the air temperature in the heated greenhouse is lower than a certain one,the fan will work to get the cold air in greenhouse to be heated by the underground hot soil.2.An insulation cover which is made of two PVC films has been designed to reduce thermal loss of the heated greenhouse.Two films are supported by several long and light foam cylindrical tubes to form an air layer of 10 cm thickness.The insulation cover shows good insulation effect which can effectively avoid temperature inversion in contrast to the compared greenhouse which is covered with the same monolayer film.In addition,we design an insulation gate at the inlet of the greenhouse.This attempt can avoid strong convective thermal losses between indoor hot air and outdoor cold air when workers go into the greenhouse on cold days.3.An experiment with the active solar heating system for greenhouses has been performed for days continuously.The emphasis is to study the heating effect of the solar heating system on air and soil temperature in the heated greenhouse.The results show that,in winter of Zhenjiang Jiangsu Province,in a sunny daytime,the air temperature increases about 3.3℃by daylight and 3.8℃at night by an average in the heated greenhouse than that of the greenhouse without heating during the heating experiment.Also the soil temperature is 2.5℃and 2.3℃higher at daytime and night respectively than that the compared greenhouse.The system shows good heat storage performance,which can store 228.9～319.1 MJ heat and 211.9～295.5 W/m~2 by daylight.The experimental results show that using solar air heating system with soil heat storage can increase the air and soil temperatures for greenhouse effectively in cold days.The heating system can storage solar energy at daytime and release heat at night.Also the system has the potential to storage heat for a long time and may be satisfy the needed energy for plants for several cloudy days.The solar active heating system has good prospect for applications in greenhouses.4.Heat transfer mechanism of the soil heat storage is anlynized for active solar greenhouses.On the basis of it,the soil heated model in the heated greenhouse is established and solved the model with numerical analysis method.In addition,the energy budget of unheated solar greenhouse has been analyzed.On the basis of thermal analysis,we establish heat balance equations of the insulation cover,indoor air, indoor moisture and ground surface of the active solar greenhouse.5.The 3-D mathematical models of the soil heat storage system and active solar greenhouse have been established and simplified in this thesis respectively.The boundary conditions are based on hot air temperatre of pipe inlet,solar radiation,sky radiative temperature, external air temperature,wind velocities and soil temperature at 1.0m deep inside.Conservation equations are solved using unsteady simulations by Computational Fluid Dynamics technique and soil temperatures in active solar greenhouse are simulated in time and in space.The simulation results show that:the differences between the simulated soil temperatures at 10cm,20cm,30cm and 40cm depth and the measured values are being no than 2℃respectively.The simulation results are in good agreement with the experimental test.