Optimal construction of energy-saving solar greenhouse

The status and characteristics of solar greenhouse in china in agricultural production, solar greenhouse can make full use of solar heat resources, reduce environmental pollution, to a certain extent, to achieve efficient land use. At present, the solar greenhouse in China is developing rapidly, and the construction area has reached nearly 16 × 104 hm2, with considerable economic benefits. With the continuous improvement and development of the construction technology of the solar greenhouse, the scientific and technological content of the solar greenhouse is also increasing, and the energy-saving intelligent solar greenhouse will gradually take shape.
2. Construction principle of energy-saving solar greenhouse
The construction of energy-saving solar greenhouse shall meet the following requirements: First, it shall have good light transmission performance to ensure high utilization rate of sunlight; Second, it is necessary to have good warming and heat preservation performance to ensure faster warming under sunlight; The third is to have good ventilation and moisture removal performance to ensure that the humidity in the solar greenhouse is within reasonable requirements; Fourth, it is necessary to have good windproof performance to ensure that the structure of the solar greenhouse is firm and the service life is long; Fifth, it should be easy to operate, build, manage and invest less.
3 Construction technology of energy-saving solar greenhouse
3.1 Orientation selection of energy-saving solar greenhouse
The orientation selection of energy-saving solar greenhouse mainly refers to the selection of the orientation of energy-saving solar greenhouse according to the actual solar radiation. The orientation of energy-saving solar greenhouse should be north to south, and extended from east to west, so as to ensure long sunshine reception time and high light utilization rate. In places where the temperature is not too low in winter, and the sun rises early, the sunlight is sufficient in the morning, and the selected position is south by east; In places where the temperature is relatively low in winter, and the sun rises relatively late, the sunlight exposure time in the morning is not long, so the orientation should be south by west. The preference of energy-saving solar greenhouse, whether south by west or south by east, should be scientifically and reasonably selected according to the climate, terrain, topography and other conditions on the spot, and the east or west should not exceed 10. If the construction cannot be carried out according to the above requirements due to geographical conditions and other reasons in the actual operation of selecting the orientation, adjustment shall be made according to the following methods and measures. On the construction site of the solar greenhouse, set up a benchmark perpendicular to the ground, observe the projection of the benchmark in the 12:00-12:30 interval in sunny weather, select the shortest projection of the benchmark and make its vertical line. With the vertical foot of the vertical line as the base point, the vertical line rotates at a scientific and reasonable angle. At this time, the rotated vertical line is the reference line in the direction of the back wall, and this reference line establishes the orientation of the energy-saving solar greenhouse. The energy-saving solar greenhouse should not only consider the choice of orientation, but also consider whether the transportation of the selected land is developed, how the soil moisture is, whether it is in a higher terrain, how the soil permeability is, how the flood control and waterlogging prevention capacity is, how the soil fertility is, and so on.
3.2 Structural design of energy-saving solar greenhouse
The structural design elements of energy-saving solar greenhouse include front roof and front roof inclination, rear roof and rear roof inclination, horizontal projection length, span, height, greenhouse area, greenhouse wall thickness, etc. The optimization of these structural design elements can greatly improve the light utilization rate, heat preservation performance, heating speed, wind resistance and service life of solar greenhouse.
3.2.1 Front roof and its inclination
The front roof of the energy-saving solar greenhouse is the daylighting surface. Its shapes include vertical slope type, double fold type, triple fold type and arch circle. The arch circle also has parabola, cycloid, arc and other shapes. At present, there are two types of solar greenhouse commonly used: vertical slope type and parabola type. Due to structural limitations, the vertical slope type solar greenhouse must be pressed with bamboo pole and bound with iron wire. The iron wire passes through the membrane to make holes on the membrane, reducing the heat preservation effect of the solar greenhouse. At the same time, it also forms water droplets on the bamboo pole, which will affect the growth of vegetable crops in the solar greenhouse. Parabolic solar greenhouse has many advantages compared with vertical slope solar greenhouse, such as firm structure, good compression resistance, large lighting area, fast heating speed, high lighting efficiency, good light transmission performance, high light utilization rate, convenient to pull up and put down the thermal insulation cover, convenient to press the film with film pressing rope, parabolic type is not easy to form water droplets, even if there is one, it will flow along the parabola, parabolic type has less snow and is convenient to clean. The inclination angle of the front roof is calculated. According to the calculated inclination angle of the front roof, the appropriate span and height can be selected according to the corresponding relationship between the span, height and inclination angle of the front roof of the solar greenhouse. The inclination angle F of the front roof indicates that the reasonable inclination angle of the front roof should meet the conditions of Formula (1).
3.2.2 Rear roof and its inclination and horizontal projection length
The back roof of the solar greenhouse is designed, which can be flat or slightly arched. The back roof can generate the cold northwest wind in winter in the north, making the cold air flow along the back roof, which has a significant effect on improving the thermal insulation of the solar greenhouse. The rear roof can not only improve the height of the solar greenhouse, increase the angle of the lighting surface, facilitate the sunlight penetration, but also facilitate the placement of the insulation covers (straw mats, quilts, etc.) on the lighting surface after being uncovered. However, the back roof blocks the scattered light in the sky from the north of the solar greenhouse, affecting the light conditions behind the solar greenhouse, resulting in the crops behind the solar greenhouse being significantly inferior to the previous crops in terms of growth, development, yield, quality, etc. In order to ensure the indoor temperature of solar greenhouse in cold winter and balance the advantages and disadvantages, it is necessary to design the rear roof. The angle of inclination of the rear roof, that is, the elevation angle of the rear slope, should be 5 °~8 ° higher than the local sun elevation angle at the noon of the winter solstice. The angle of inclination of the rear roof determines the speed of the cold air flow. Too large the angle of inclination is not conducive to the flow of cold air along the back, and too small the angle of inclination affects the sunlight can not directly shine on the interior of the rear roof, which is not conducive to improving the temperature in the solar greenhouse, and affects the lighting effect behind the solar greenhouse. The solar altitude angle refers to the angle between the solar rays of a certain place on the earth and the surface section connected with the earth center through the place. The sun elevation angle is determined by the latitude of the location. In the area of 35 ° north latitude, the sun elevation angle is 31.6 °, the elevation angle of the back slope should be 36.6 °~39.6 °, and the horizontal projection length of the back roof should be about 1 m to reduce shading.
3.2.3 Height and north-south span of solar greenhouse
The height and north-south span of the solar greenhouse determine the lighting area, the angle of lighting surface and the structural strength of the greenhouse. The height of the solar greenhouse is the distance from the intersection of the front roof and the rear roof (the highest point of the solar greenhouse) to the reference ground. The north-south span is the distance from the back wall of the solar greenhouse to the southernmost end of the front roof framework. The ratio of the height of the solar greenhouse to the north-south span shall be calculated scientifically, and the shape formed by this ratio shall be checked for structural strength through mechanical analysis, with sufficient safety factor. The lighting area and the angle of the lighting surface determined by this ratio (which determines the size of the sunlight incidence angle) should also be scientifically calculated through indicators such as temperature rise, thermal insulation and sunlight utilization rate, so as to make them meet the construction requirements of energy-saving solar greenhouse. The north-south span can be calculated by the formula: north-south span=height of the highest point of the greenhouse × Ctga (a is the angle of the lighting surface)+the projection length of the rear roof (3)
3.2.4 Area of solar greenhouse
Area of solar greenhouse=north-south span of greenhouse × Length of greenhouse. (4)
The area of solar greenhouse shall be designed within a scientific and reasonable range, generally 0.067 hm2~0.034 hm2. The solar greenhouse with reasonable area can enhance the thermal insulation performance, facilitate management and operation, and improve the work efficiency. The reasonable area can effectively reduce the occurrence of cold damage or freezing damage in severe cold weather, and reduce the time for pulling up and putting down the covers in winter. The area parameter of the solar greenhouse is an important economic and energy saving indicator.
3.2.5 Height of back wall
The height of the back wall refers to the distance between the intersection line of the interior surface of the back slope and the interior surface of the back wall of the solar greenhouse and the interior plane of the greenhouse. The height of the back wall can be determined according to Formula (5), and the height of the back wall of the solar greenhouse should generally be 2.5~4.5 m.
h = H-btan α R. (5)
3.2.6 Arch spacing and wall thickness
The arch spacing is determined by arch bar strength, covering material performance and local wind and snow load. The wall thickness of the solar greenhouse is determined by the thickness of the local frozen soil layer, which is generally twice the thickness of the frozen soil layer.
3.3 Covering materials of energy-saving solar greenhouse
The transparent covering material for the daylight surface of the solar greenhouse has the energy saving effect through optimal selection, and also has the characteristics of good light transmission, strong compression and tension resistance, good thermal insulation, and long service life. Polyethylene dripless film, three layers
Extruded composite film, polyethylene drip free light conversion film, ethylene vinyl acetate three-layer co extruded drip free heat preservation and anti-aging film, PVC drip free film and other multi-functional composite films.
3.4 Wall of energy-saving solar greenhouse
The wall of the solar greenhouse not only has the functions of heat preservation, support, closure, etc., but also can store the heat of solar energy in the day, and release the heat stored in the day at night, playing a role in warming the night. The wall can be divided into solid wall and hollow wall, and the hollow wall can be divided into two types: thermal insulation filler material and non thermal insulation filler material.
4 Conclusion
Through the optimization analysis of the orientation selection, structural design, covering materials and foundation walls of the solar greenhouse, the light utilization, heat preservation, moisture removal and wind resistance of the solar greenhouse are improved, and the energy saving effect of the solar greenhouse is improved.