Study On The Effect Of Internal Thermal Screens On The Internal Environment Of Greenhouse In Winter And Summer

In recent years,the application of multi-span greenhouse has become increasingly widespread,but due to its high cost and immature technology,China’s multi-span greenhouse still faces many problems in use.In winter,because the greenhouse has no obvious thermal insulation capacity,the temperature difference between indoor and outdoor is small,resulting in high energy consumption for heating.And in summer,influenced by solar radiation,the heat accumulated inside the greenhouse is too much,which is not conducive to crop growth.Therefore,two urgent problems need to be solved are to reduce the energy consumption of greenhouse heating at night in winter by means of internal thermal screens and bubble films multi-layer coverage,and to meet the temperature needs of different crops by regulating the greenhouse environment in summer under different shading conditions.This paper takes Venlo multi-span greenhouse as the research object,and uses the heat balance method to study the influence of internal thermal screens and bubble films on the energy consumption of greenhouse at night in winter;At the same time,under the premise of comprehensive consideration of external shading,internal thermal screens and the installation height of pad and fan,etc.,the influence of different shading conditions on the internal environment of greenhouse is studied by CFD numerical simulation method;It provides some theoretical support for solving the problems of winter insulation and summer cooling of greenhouse.The specific research contents and results are as follows:（1）Taking five kinds of internal thermal screens and bubble films as the test objects,the heat transfer coefficient and spectral transmittance in the wavelength range of 300 nm～2500 nm were measured by hot box and spectrometer.The results show that:the insulation performance of five kinds of internal thermal screens from high to low:GLS75,GLS65,GLS55,GLS85T,GLS55T.The distance between the double-layer bubble film and the internal thermal screen will significantly affect the thermal insulation effect.The greater the distance between the two,the better the thermal insulation effect.Taking the internal thermal screen GLS75 as an example,when the distance between the two is 12 cm,the heat transfer coefficient is only 2.485 W/（m~2·℃）.When the internal thermal screen is covered with single or double-layer bubble film,the reduction range of overall band transmittance is 9.56%～45.26%and 18.49%～57.89%respectively.（2）Based on the principle of heat balance,the effects of internal thermal screens and bubble films on the night temperature of multi-span greenhouse in winter are studied.The night thermal environment models of greenhouse under three different covering conditions are established,and its energy consumption is also analyzed.It is found that under the non-heating mode,the addition of internal tnermal screens can increase the internal temperature of the greenhouse by 2.22℃,and on this basis,the addition of the bubble film can increase the internal temperature of the greenhouse by 5.36℃.When the outdoor environment is 5℃,in order to maintain the internal temperature of the active heating greenhouse at 15℃,additional heat supply of 81 k W,25 k W and 14 k W is required for Covering Condition A,B and C,respectively.Compared with Covering Condition A,the heat loss of enclosure structure under Covering Condition B and C is reduced by47.4%and 76.1%respectively;The total nighttime energy consumption of greenhouse under Covering Condition B and C is reduced by 16.3%and 23.6%respectively,compared with Covering Condition A.The analysis shows that the best heat insulation effect is achieved by adding both internal thermal screens and bubble films inside the greenhouse.（3）The physical model of a Venlo multi-span greenhouse with crops inside is reasonably simplified,and its modeling and grid division are carried out.At the same time,the control equations and various models in CFD numerical model are introduced and selected.Comparing the simulation value with the data in literature^[1],the results show that under the condition of pad-fan cooling system,the maximum relative errors in vertical and horizontal directions are 13.08%and 7.04%respectively.Under the condition of pad-fan cooling system+external shading,the maximum relative errors in the vertical and horizontal directions are 12.91%and 3.49%respectively.After adding external shading,the maximum temperature of canopy decreased by6.1%,but it is still higher than the suitable growth temperature of crops.If necessary,internal thermal screens should be added to improve the cooling effect.（4）Based on the established CFD model,the the internal thermal screen is set as porous jump to further explore the impact of the internal thermal screen on the internal environment of the greenhouse.At the same time,from the perspective of greenhouse structural parameters,the optimization of the installation height of pad and fan under shading conditions is proposed.The results show that after the addition of internal shading insulation curtain in the greenhouse with pad-fan cooling system+external shading condition,the temperature gradient in the vertical direction becomes larger.The temperature difference between the top and bottom of the internal thermal screen under Case A（GLS55T）,Case B（GLS65T）and Case C（GLS85T）conditions is2.1℃～2.5℃,3.6℃～4.1℃and 4.5℃～5.3℃respectively.Under Case C conditions,the temperature gradient below the greenhouse is smoother and the cooling effect is the best.The airflow inside the greenhouse is blocked by the obstruction of the internal thermal screen,resulting in a small airflow speed in the upper part of the greenhouse.The installation height of pad and fan in the existing greenhouse is suitable for crops with canopy height≥1.5 m.When the canopy height is≤1 m,Case 3（the installation height of both pad and fan is 0.6 m）is the most suitable,which can meet the cooling demand of the crop canopy.