Anshan Horticultural Research Institute has studied the relationship between wall thickness and thermal insulation, and designed three kinds of soil walls with different thicknesses: ① the soil wall is 50 cm thick and covered with a plastic film; ② The thickness of the earth wall is100 cm; ③ The thickness of the earth wall is 150 cm, other conditions being the same. The results show that:165438+1early October to early February, ② is 0.6 ~ 0.7℃ higher than ① indoor minimum temperature; ③ 0.1~ 0.2℃ higher than ② indoor minimum temperature; The maximum indoor temperature difference is 0.2 ~ 0.5℃ and 0. 1 ~ 0.3℃ respectively. It can be seen that with the increase of wall thickness, the thermal insulation capacity is improved, but it does not mean that the thicker the wall, the better. In the above test, the soil wall with the thickness of 100 cm is more obvious than the soil wall with the thickness of 50 cm. Compared with the soil wall with the thickness of 100 cm, the soil wall with the thickness of 100 cm has little warming range and little practical value. Generally speaking, the earth wall is about 1.0m, which can be thinner in the south and thicker in the north. Brick walls should be 50 ~ 60 cm, and those with intermediate layers are better.
Regarding the thermal insulation performance of the wall, the heat absorption, heat storage and thermal conductivity of the wall material are mainly considered. For the wall with good thermal insulation performance, materials with good heat absorption and storage performance but poor thermal conductivity should be selected. The heat absorption coefficient of ordinary stone is 0.68 w/(m2 k), and that of red brick is 0.70 ~ 0.77 w/(m2 k). The thermal storage coefficient of stone is 23.96 ~12.10w/(m2 khz), that of red brick is 9.65 w/(m2·kHz), and that of compacted earth wall is10.58w/(m2 khz). The thermal conductivity of stone is 3.20 ~1.13w/(m 2khz), red brick is 0.81w/(m 2khz), and compacted soil is 0.93w/(m 2khz). Although stone walls and earth walls have high thermal storage coefficient, they absorb less heat and conduct more heat.
There are two kinds of walls in solar greenhouse: simple wall and composite wall. Simple walls are made of a single soil or brick and stone; Generally, the inner and outer layers of heterogeneous composite walls are all bricks, and there is an interlayer between the two bricks with thermal insulation material in the middle. The insulation materials filled in the interlayer are dry soil, cinder and perlite. As can be seen from the table 1 1, the minimum temperature of the solar greenhouse is higher than that of the hollow sandwich wall filled with thermal insulation materials.
Table 1 1 Wall insulation with different insulation materials
When choosing thermal insulation materials, we should consider both its thermal insulation and its price. It is best not to use organic materials as insulation materials. Organic materials are perishable under high temperature and high humidity, which not only can not play a role in heat preservation, but also damage the firmness of the wall.
Some studies have pointed out that the insulation effect of aerated brick is higher than that of ordinary brick, and the indoor temperature can be increased by 0.5 ~ 0.8℃ accordingly.
According to the experiment of Anshan Horticultural Research Institute and the data of Yinchuan area, it is concluded that the thermal resistance of the wall is1.1~1.6 (m 2k)/watt-hour, which can basically meet the thermal insulation requirements of solar greenhouse. For several wall practices given in table 12, the thermal resistance values are all ≥1.1(m 2k)/watt. Table 12 The order of wall materials is from indoor to outdoor, and the figures in brackets are the thickness in centimeters.
Table 12 Thermal Insulation Performance of Several Walls
In recent years, in order to improve the utilization rate of land and reduce the labor input when building walls, a thermal insulation material called polystyrene board has been introduced into the walls. The thermal conductivity of polystyrene board with a thickness of only 5 ~ 10 cm is equivalent to that of red brick with a thickness of more than 37 cm. But when polystyrene board is used as the wall, it should be placed outside the red brick or between two layers of red brick. Because polystyrene board only has low thermal conductivity, its heat absorption and storage capacity is far less than that of red brick. A good wall must consider many factors such as heat absorption, heat storage and heat conduction.
(2) Rear roof
The thickness of the back roof and its projected length affect the heat insulation capacity of the greenhouse.
First of all, there must be a rear roof. According to the research of Ningxia Academy of Agricultural Sciences, the indoor temperature of simple solar greenhouse without back roof is maintained at 8.6 ~ 9.7℃, the lowest temperature is about 0℃, and the underground temperature of 10 ~ 20 cm is 7.6 ~ 8.6℃ under the heating condition of coal stove. Under the same external meteorological conditions, the average temperature can reach 12.3 ~ 14.4℃, the lowest temperature is 3.0 ~ 3.5℃, and the underground temperature of 10 ~ 20cm is12.3 ~1. This shows that the rear roof does play an important role in heat preservation.
Secondly, the practice of the back top is to ensure that the back top has a certain thermal resistance. According to the author's research, the thermal resistance of the rear roof should be not less than 3.0 ~ 4.0 (m 2k)/watt. Therefore, in addition to choosing loose, dry and porous materials such as straw, straw, dry soil, cinder and so on. It is also necessary to ensure that the rear roof has a certain thickness, which can be 30 ~ 40cm； in Henan, Shandong, southern Hebei and other regions; It is 60 ~ 70 cm thick in cold areas such as North China, Northeast China and Inner Mongolia.
In recent years, the practice of rear roof has been improved. First spread a reinforced concrete cement slab on the steel frame of the back roof, and then spread a 5 ~ 10 cm polystyrene slab on it. Cover the polystyrene board with a layer of old plastic film, linoleum and other waterproof materials, and then pour cinder on it for waterproofing.
Third, the projection length of the rear roof is conducive to heat preservation and is not conducive to temperature rise; A short rear roof is beneficial to heating, but not to heat preservation. Therefore, the projection length of the rear roof should take into account both heating and insulation. According to the thermal effect of the greenhouse, the author thinks that the projection of the roof behind the greenhouse should account for 1/4 ~ 1/5 of the greenhouse span. The so-called thermal effect of greenhouse refers to the product of the heating rate and the heat preservation rate of greenhouse, and the greater the thermal effect means the higher the heating capacity and heat preservation capacity (Table 13).
Table 13 Relationship between Roof Projection and Greenhouse Thermal Effect
(3) Front roof covering
The front roof is the main heat dissipation surface of greenhouse, and the front roof covering can prevent heat loss and achieve the purpose of heat preservation.
At present, the external covering materials of the front roof in China are mainly grass, paper and so on. Straw mat is a traditional covering material, which is woven from reed foil and straw. Its thermal conductivity is very low, which can reduce the heat consumption of greenhouse at night by 60% and increase the room temperature by 1 ~ 3℃. However, the quality of straw mats in the market needs to be improved. In order to ensure a certain thickness and density, mature reed should be selected to prolong its service life. After covering the lighting surface with good quality grass, there should be no light in the room. According to the test, a completely dense grassland can raise the indoor temperature by 1.0 ~ 2.0℃ than a loose grassland. In cold areas, grass is often covered with a paper quilt consisting of 4 ~ 6 pieces of kraft paper. Adding a layer of paper under the straw cover not only increases the air isolation layer, but also makes up for the shortcomings of loose straw cover and improves the thermal insulation. According to the test, adding a layer of paper quilt composed of four pieces of kraft paper can increase the indoor minimum temperature by 3.0 ~ 5.0℃, increase the number of layers and improve the thermal insulation performance accordingly. Although the paper quilt has good thermal insulation effect, it has high investment, easy to get wet by rain and snow, and short service life. Therefore, in many places, old plastic film is used instead of paper quilt, and in some areas, it is covered with double-layer grass curtain to keep warm. In some cold areas, such as Northeast China and Inner Mongolia, using quilts as mulch can increase the room temperature by 7.0 ~ 8.0℃, as high as 10.0℃. In Shandong and other places, woven bags filled with crushed stone cotton and fiber cotton are used as covering materials, which has good thermal insulation effect. In recent years, a new type of thermal insulation quilt has been developed, which is made of several materials. The inner layer is made of thick non-woven fabric, needle punched cloth and fiber cotton, and the outer layer is made of waterproof and anti-aging thin non-woven fabric, rain-proof silk or aluminized film. Good thermal insulation, light weight, waterproof and durable, but high one-time investment.
Another method is to hang thermal insulation curtains indoors, such as old films and cold yarns, which can be rolled up during the day to let sunlight enter the room and pulled up at night to prevent heat dissipation, which can generally increase the indoor temperature by 2.0℃. Erecting a small arch shed on the cultivation land is also a way of indoor covering, and the temperature in the arch shed can generally be increased by 1.0 ~ 3.0℃.
(4) Cold ditch
The purpose of setting cold ditch is to prevent the heat from losing laterally and improve the indoor ground temperature. Cold-proof ditches are generally located outdoors, with a width of 40 ~ 50 cm and a depth of 40 ~ 60 cm. Filling the ditch with dry soil, hay or other heat insulation materials can raise the indoor ground temperature by about 4.0℃ for 5 cm. The cold ditch should be covered to prevent rain and snow from flowing in.
(5) Ground coverage
Ground cover is an effective measure to improve ground temperature. The main method of ground covering is to lay plastic film, spray warming agent on the soil surface, and lay a layer of heat-generating materials such as rice straw, wheat straw plus horse manure and chicken manure in a certain depth in the soil. Studies have shown that laying a layer of plastic film can increase the minimum ground temperature by 0.5℃; Spraying warming agent can raise the temperature by 2.0 ~ 2.4℃, and the warming effect is obvious within 3 ~ 20 days, so it is best to spray it every 30 days. Spreading a layer of straw at 40 cm in the soil can increase the soil temperature in the root layer by 1.0 ~ 3.0℃, and spreading a layer of horse manure with the thickness of 10 ~ 15 cm can increase the temperature by 2.0 ~ 4.0℃. Both horse manure and straw have a certain time limit, which takes effect within 10 day after paving, reaches the peak in 20 days, and then the warming effect weakens.
(6) Import and export
The gables of the greenhouse should be provided with entrances and exits (doors), and the entrances and exits should be located on the Dongshan wall to prevent the northwest cold air from invading the room. Wooden doors should be set up, and then door curtains should be hung for insulation. In order to prevent cold air from pouring into the room when operators enter and leave the greenhouse, an operation room should be set on the east side of Dongshan wall (that is, the gable that opens the door). The door of the operation room faces south, and it is best to hang a curtain in cold season.