Daylight greenhouse insulation design to pay attention to what points?

(1) wall

Anshan City Horticultural Research Institute of the wall thickness and thermal insulation of the relationship between the study, the design of three different thicknesses of the earth wall: ① earth wall thickness of 50 centimeters, covered with a layer of plastic film; ② earth wall thickness of 100 centimeters; ③ earth wall thickness of 150 centimeters, the other conditions are the same. The results show that: from early November to early February, ② than ① indoor minimum temperature 0.6 ~ 0.7 ℃; ③ than ② indoor minimum temperature 0.1 ~ 0.2 ℃; indoor maximum temperature difference of 0.2 ~ 0.5 ℃ and 0.1 ~ 0.3 ℃, respectively. It can be seen that as the thickness of the wall increases, the thermal insulation capacity increases, but it does not mean that the thicker the wall the better. In the above test, 100 cm thick earth wall is more than 50 cm thick heat preservation obviously; 150 cm thick earth wall is not much warmer than 100 cm thick, the practical value is not great. Generally speaking, the earth wall to about 1.0 meters is appropriate, the south can be thinner, the north should be thicker. Brick wall to 50 to 60 centimeters is appropriate, there is a better middle compartment.

On the thermal insulation performance of the wall, the main consideration should be the heat absorption, heat storage and thermal conductivity of the wall material. Good thermal insulation of the wall, should be composed of heat absorption, heat storage, but poor thermal conductivity of the material. The heat absorption coefficient of solar radiation is 0.68 W/(m2 -open) for ordinary stone, 0.70-0.77 W/(m2 -open) for red brick; the heat storage coefficient is 23.96-12.10 W/(m2 -open) for stone, 9.65 W/(m2 -open) for red brick, and 10.58 W/(m2 -open) for compacted earth wall; the thermal conductivity coefficient is 3.20-1.13 W/(m 2-open), red brick 0.81 W/(m2-open), and rammed earth 0.93 W/(m2-open). Stone walls and earth walls have a higher heat storage coefficient but less heat absorption and greater heat conduction.

The walls of the solar greenhouse have two kinds of monolithic walls and composite walls. Monolithic wall made of a single earth or brick, stone blocks; heterogeneous composite wall is generally within the outer layer of brick, between the two bricks is padded with thermal insulation materials in the middle of the sandwich, the middle of the sandwich filled with thermal insulation materials such as dry soil, cinder, perlite, and so on. Table 11 can be seen where there are filled with thermal insulation material wall, the lowest temperature in the solarium are not filled with any material than the hollow sandwich wall of the high temperature.

Table 11 different insulation materials wall insulation

In the selection of insulation materials, both to consider its insulation, but also to consider its price, it is best not to use organic material for insulation, organic substances in high temperature, high humidity under the perishable, not only does not play a role in insulation, but also detrimental to the solidity of the wall.

Some studies have pointed out that the outer layer of the wall with aerated bricks than with ordinary bricks with a high thermal insulation effect, the indoor temperature can be correspondingly increased by 0.5 to 0.8 ℃.

The author has been based on the above experiments in Anshan Horticultural Institute, and Yinchuan area, the information obtained in northern China, southern Northeast China, the thermal resistance of the wall is 1.1 ~ 1.6 (m 2 - open) / tile, can basically meet the insulation requirements of daylight greenhouse. Several wall practices given in Table 12 have thermal resistance values ≥ 1.1 (m2 -km) / tile. The order of wall materials in Table 12 is from indoor to outdoor, and the number in parentheses is the thickness in centimeters.

Table 12 Thermal Insulation Properties of Several Types of Walls

In recent years, in order to improve the land utilization rate and reduce the labor input when building walls, people have introduced an insulating material called polystyrene boards on the wall, which is only 5 to 10 centimeters thick and its thermal conductivity is equivalent to that of red bricks of more than 37 centimeters. However, when using polystyrene boards for walls, the polystyrene boards should be placed on the outside of the red bricks or between two layers of red bricks. Because the polystyrene board is only low thermal conductivity, its heat absorption and heat storage capacity is far less than the red brick. A good wall must take into account multiple factors of heat absorption, heat storage and thermal conductivity.

(2) rear roof

The thickness of the rear roof and its projection length affects the thermal insulation capacity of the greenhouse.

First of all, there must be a back roof, according to the research of Ningxia Academy of Agricultural Sciences, no back roof of the simple solar greenhouse, in the indoor heating conditions of a coal stove, the indoor air temperature is maintained between 8.6 to 9.7 ℃, the lowest temperature of 0 ℃ or so, the temperature in the ground of the 10 ~ 20 cm is 7.6 ~ 8.6 ℃; in the same outside meteorological conditions, the greenhouse with a back roof, no heating of the coal stove, the average air temperature can reach 12.3 ~ 8.6 ℃. Under the same external meteorological conditions, the greenhouse with rear roof, without heating by coal stove, the average temperature can reach 12.3-14.4℃, the lowest temperature is 3.0-3.5℃, and the temperature in the ground of 10-20cm is 12.3-14.4℃. This shows that the back of the roof in the insulation does play an important role.

Secondly, the practice of rear roofing, to ensure that the rear roof has a certain thermal resistance value. According to the author's research, the thermal resistance value of the rear roof should be not less than 3.0 ~ 4.0 (m2 - open) / tile. For this reason, in addition to selecting loose, dry, porous materials such as broomcorn millet, straw, dry soil, cinders, etc., but also to ensure that the back of the roof has a certain thickness, in Henan, Shandong, Hebei and other areas of the south, the thickness can be in 30 ~ 40 cm; North China, Northeast China, Inner Mongolia, and other cold regions, the thickness of 60 ~ 70 cm.

In recent years, the practice of rear roofing and some improvements. In the back of the roof on the steel frame first paved a reinforced concrete cement board, on which to put a piece of 5 to 10 centimeters of polystyrene board, polystyrene board covered with a layer of waterproofing materials, such as old plastic film, linoleum, etc., and then poured cinderblock waterproofing on it.

The third is the projected length of the rear roof, the rear roof is long in favor of heat preservation, is not conducive to warming; rear roof is short in favor of warming is not conducive to heat preservation. So the projection length of the rear roof should take into account the two aspects of warmth and heat preservation. According to the author of the thermal effect of the greenhouse that the greenhouse rear roof projection should account for the greenhouse span of 1/4 ~ 1/5. The so-called thermal effect of the greenhouse is the greenhouse warming rate and insulation rate of the product of the thermal effect of the large said warming capacity, insulation capacity are higher (Table 13).

Table 13 rear roof projection and the relationship between the thermal effect of the greenhouse

(3) front roof cover

The front roof is the main heat surface of the greenhouse, the front roof cover can prevent heat dissipation, to achieve the purpose of heat preservation.

Currently in China, the outside of the front roof covering material is mainly grass thatch, paper quilt and so on. Grass thatch is the traditional covering material, it is woven by reed foil, straw, its thermal conductivity is very small, can make the greenhouse at night to reduce heat consumption by 60%, improve the room temperature 1 ~ 3 ℃, but the quality of the grass thatch on the market at present needs to be improved to ensure a certain thickness and density, to choose the maturity of the better reeds, to prolong its service life. Better quality grass thatch, after covering the light surface, the room should not see any bright light. According to the test, a completely dense straw than the loose straw can make the indoor temperature increase 1.0 ~ 2.0 ℃. In cold areas, often in the straw thatched under a layer of 4 to 6 sheets of kraft paper folded into a paper quilt. Thatch under a layer of paper was, not only increased the air spacing layer, and make up for the shortcomings of thatch loose, thus improving the heat preservation. According to the test to increase a layer of 4 kraft paper folded into the paper was, can make the lowest indoor air temperature increased 3.0 ~ 5.0 ℃, increased levels, thermal insulation performance accordingly. Paper was insulation effect is good, but the investment is high, easy to be snow, rain, wet, short life, so many places with the old plastic film instead of paper was, some areas with double grass curtains overlap cover insulation effect is also very good. More cold areas, such as the northeast, Inner Mongolia with quilts as a cover, the room temperature can be increased by 7.0 ~ 8.0 ℃, as high as 10.0 ℃. Shandong and other places with woven bags filled with crushed asbestos, fiber cotton as a covering material, insulation effect is good. In recent years, the development of a new type of thermal insulation, by several materials covered with, the inner layer is a thick non-woven fabric, needle punched cloth and fiber wool, etc., the outer layer is waterproof, anti-aging treatment of thin non-woven fabric, rain silk or aluminized film, good thermal insulation, lightweight, waterproof, durable, but a one-time investment is high.

Another approach is to hang insulation curtains indoors, such as old film, cold yarn, etc., rolled up during the day to let the sunshine into the room, pulled up at night to stop the heat dissipation, generally can make the indoor air temperature increased by 2.0 ℃. Set up a small arch on the cultivation ground, is also a form of indoor cover, the arch inside the temperature can generally increase 1.0 to 3.0 ℃.

(4) cold ditch

Setting up a cold ditch is to prevent lateral heat loss and increase indoor ground temperature. Cold ditch is generally located outside, width 40 ~ 50 cm, depth 40 ~ 60 cm, ditch filled with dry soil, hay or other insulation, can make the indoor 5 cm ground temperature increased by about 4.0 ℃. The cold-proof ditch should be capped to prevent the inflow of rain and snow.

(5) ground cover

Ground cover is an effective measure to improve the ground temperature, ground cover method is mainly laying film, spraying temperature increasing agent on the soil surface, a layer of straw, wheat straw plus horse dung, chicken manure and other heat stuff in the soil at a certain depth. According to some studies, laying a layer of mulch can increase the minimum temperature of the ground by 0.5 ℃; spraying of heat enhancers to increase 2.0 to 2.4 ℃, the heat-enhancing effect of heat enhancers in 3 to 20 days to increase the temperature is obvious, so it is best to spray every 30 days; in the soil at 40 centimeters a layer of straw, can make the root layer of the soil temperature increased by 1.0 to 3.0 ℃, laying a layer of 10 to 15 centimeters thick horse manure, increase the temperature by 2.0 to 4.0℃. Paving horse manure and straw have a certain time limit, paved within 10 days after the effective, 20 days to reach the peak, after the warming effect weakened.

(6) import and export

greenhouse wall should be set up import and export (door), import and export in the east wall is appropriate to prevent the northwest cold wind invasion of the room, to set up a wooden door, and then hang the door curtains insulation. In order to prevent the operator in and out of the greenhouse when the cold wind into the room, should be in the east wall (that is, open the door of the mountain wall) on the east side of an operating room, operating room door to the south, the best in the cold season to hang the door curtain.