Design And Research Of Plant Greenhouse Covering Materials Based On Spectral Separation

As a protective planting model,greenhouse horticulture not only provides protection against outdoor climatic conditions(such as extreme temperature,relative humidity),but it is also the only crop production system that can control the environment in which plants grow.Control ventilation and gas exchange by additional heating or cooling,humidification or dehumidification in a closed greenhouse environment,favorable microclimate conditions for plant growth are created to achieve the goal of efficient production.Greenhouse covering materials are a key factor in controlling the microenvironment inside greenhouse,which not only protects crops from the outdoor environment,it also controls the type of illumination,light transmission,thermal diffusion and the amount of radiation entering the greenhouse.From the spectrum of solar radiation entering the greenhouse,visible light is important for crop growth and photosynthesis,while near infrared and ultraviolet light have great influence on crop physiology,which makes the crop face high temperature stress and ultraviolet stress.Therefore,the solar spectrum can be changed by using specific types of greenhouse covering materials.Covering materials with near infrared shielding are an effective solution for cooling greenhouses in summer,and UV-shielded covering materials can provide an environmentally friendly solution for pest and disease control.However,after studying the present situation of spectral modified covering materials,it is found that there are still some shortcomings in such materials.For covering materials with near infrared shielding,the main problems as described below.First of all,it is usually based on materials with low light transmission coefficient,which affects the normal photosynthesis of plants in areas with short illumination time.Secondly,in some southern countries,due to high-intensity solar radiation and extreme outdoor temperatures,it is difficult to reduce the high temperature in the greenhouse to the physiological needs of plants through such covering materials.Although the gas circulation in the greenhouse can be realized by the exhaust fan,the cooling effect is limited,and the energy waste is also caused.Finally,in the northern countries,due to the short winter light and cold weather,plants in the greenhouse must be supplemented with light and heat to achieve optimal growth conditions.For greenhouse covering materials with ultraviolet shielding performance,the main problems are as follows.The shielding mechanism of such materials for ultraviolet is mainly absorption,although ultraviolet shielding can be achieved by adding an ultraviolet absorber.However,for greenhouse covering materials,this absorption mechanism tends to generate highly reactive free radicals within the material and initiate chemical reactions leading to photodegradation and yellowing of polymers,pigments and dyes.When the polymer is exposed to ultraviolet radiation,the mechanical strength and impact resistance of the covering material will also decrease,leading to accelerated aging.Based on the research of the above problems,this paper proposes corresponding solutions for different types of covering materials.For the near infrared shielded covering material,three problems found to be involved in energy consumption.Since energy consumption has been an important indicator for evaluating the performance of modern greenhouses,a solution combining visible and near infrared spectrally separated cover materials with solar greenhouses is proposed in this paper.Moreover,it is proposed that the optical properties of such materials should meet the design requirements of high transparency in the range of 400 nm to 800 nm and high reflectivity in the range of 800 nm to 2500 nm.In addition,for UV-shielding coating materials,a shielding mechanism based on reflective ultraviolet light is proposed in this paper.The optical properties of these materials should meet the design requirements of high ultraviolet reflectance at 290 nm-400 nm and high visible transparency at 400 nm-800 nm.On the other hand,photonic crystals is an artificial micro/nano structure in which the refractive index(dielectric constant)has a periodic change.Due to its unique characteristics,it can artificially control and manipulate the propagation of light waves to achieve the functions of light guiding,filtering,splitting and wavelength division multiplexing.The control mechanism of photonic crystals for light waves is mainly based on photonic band gap and photon localization.The photonic band gap is considered to be a range of frequencies,and when the incident light wave frequency matches it,the light is reflected and does not pass through the photonic crystals.For photon localization,when magazines or defects are introduced into photonic crystals,photons matching defect frequencies are confined to defect locations and cannot propagate to space.Based on its control mechanism,the main optical control methods of photonic crystals are as follows.The first is to change the geometry(lattice or mid-column size)and material properties(dielectric constant or permeability)of this micro-nano structure.The other is to insert an active material(solid or liquid,colloid,etc.)that is sensitive to external excitation.In fact,the ultimate goal of these two methods of regulation by changing the effective refractive index(dielectric constant)of the material.Because of the complexity of photonic crystals,it is difficult to make qualitative and analytical analysis.In order to understand the interaction between such micro-nanostructures and light waves(electromagnetic waves),it is necessary to simulate the properties of the constituent photonic crystal media and the interaction between the media.The electromagnetic numerical calculation method can accurately simulate the propagation process of electromagnetic waves in various complex media,and apply these calculation methods in the theoretical study of photonic crystals.Therefore,the numerical calculation methods and principles of photonic crystals are studied in this paper,including Transfer Matrix Method(TMM)and Finite-difference time domain(FDTD).Considering the advantages of the FDTD algorithm in dealing with dispersive media(such as graphene,indium tin oxide,metal and superconducting materials),a wide-band optical response can be obtained by solving the structure in time domain.In summary,considering the unique optical control performance of the micro-nano photonic crystals,a design scheme of using the micro-nano photonic crystals as a greenhouse covering material is proposed.And FDTD is used as a theoretical research method for the optical properties of this structure.For the design of solar greenhouse covering materials with near infrared shielding,three micro-nano photonic crystals are proposed,which are(ITO/Metal)NITO,(Graphene/Metal/MgF2)N and(Graphene/YBCO)N respectively.Moreover,the optical properties of the three micro-nano photonic crystals in the visible to near infrared range are numerically simulated by FDTD method.The theoretical analysis results show that the spectral control properties of the(ITO/Metal)NITO structure are closely related to the thickness of the ITO film,the number of cycles and metal material.At ITO layer thickness is 80 nm,the number of cycles is 4,and the metal material is Ag,this structure has a transmittance of more than 95%under visible light,while the reflectance is above 90%in the near infrared range.For the(Graphene/Metal/MgF2)N structure,the optical response in the visible to near infrared range can be influenced by changing parameters such as metal material,metal thickness,period number,and MgF2 thickness.The designed structure is transparent in the visible range,and the reflectivity can reach 100%in most near infrared ranges(1200 nm-2500 nm)when the metal material is Ag,the Ag thickness is 5 nm,the period number is 7,and the MgF2 thickness is 60 nm.For the(Graphene/YBCO)N structure,spectral selectivity is very sensitive to the thickness of superconductor layer,period number and temperature variation,and spectral separation can be achieved by changing these parameters.Moreover,the(Graphene/YBCO)N structure can be transparent in visible light while high-performance shielding in near infrared at the period number is 10,the external temperature is 55 K and the thickness of graphene layer and superconductor layer are 0.34 nm and 50 nm,respectively.For the design of greenhouse covering materials with ultraviolet shielding,a photonic crystals structure composed of ITO,Ag and titanium dioxide was proposed.And the optical properties of the proposed structure-(ITO/Ag/TiO2)N in the ultraviolet to visible range were numerically simulated using the FDTD method.It was found that spectral separation of ultraviolet and visible light can be achieved by changing structural parameters(ITO thickness,Ag thickness,TiO2 thickness and number of cycles).The numerical results show that(ITO/Ag/TiO2)N structure has a reflectance close to 100%in the range of 290 nm to 400 nm and can remain transparent in visible range at the ITO thickness is 60 nm,Ag thickness is 4 nm,TiO2 thickness is 40 nm,and the period number is 8.In summary,the greenhouse covering materials designed in this paper achieve the purpose of spectral separation in their working frequency bands through theory.It also meets the design requirements,thus proving the feasibility of being a greenhouse covering material.In this study,micro-nano photonic crystals is used as greenhouse covering material,which provides a new solution for energy efficient utilization and environmental friendly greenhouse governance.