(I) Formation of temperature in the facility Daytime solar rays (wavelength 300-3000nm) are incident on the ground surface through transparent covers such as glass and film, so that the ground receives solar radiant heat and gradually increases soil moisture through conduction. When the temperature is lower than the ground temperature, the ground also releases heat (i.e., through radiation, conduction or convection, turbulence, etc.) to increase the temperature above the surface of the ground. In addition, because the glass or film can prevent some of the long-wave radiation, so that thermal energy is left in the protection facilities, increase the temperature. The warming effect of this transparent cover is called the "greenhouse effect." The greenhouse effect means the ability to obtain or accumulate solar radiation within a facility without artificial heating, so that the temperature in the protection facility is higher than the outside temperature.
The greenhouse effect is caused by two reasons. One reason is that transparent covers such as glass or plastic films allow short-wavelength radiation (320-470nm) to pass through the facility while preventing long-wavelength radiation from being transmitted through the facility. In the atmosphere, another reason is that the protection facilities are semi-enclosed space, and the air exchange inside and outside is weak, so that the accumulated heat is not easily lost.
(b) Daily temperature difference in the facility The temperature difference in the facility means the difference between the highest temperature skin and the lowest temperature in one day. For example, when the ambient temperature is 10°C, the temperature difference in the greenhouse is about 30°C, while the temperature difference in the small arch can reach about 40°C.
There will also be "inversion of temperature" in the facility, which usually occurs after a cloudy day, with a breeze and a clear night. On a sunny and windy night, the surface of the greenhouse is very radiant and heat radiating. Sometimes the temperature inside the shed is inversely lower than the outside temperature. This phenomenon is called “inversion of temperatureâ€. The reason is that the ground surface and the crop body are heated during the day and emit heat to the radiation through the cover during the night, and the heat is exacerbated during the clear and cloudless nights. In addition, under the action of the breeze, outdoor air can supplement heat from the atmosphere by anti-radiation. In the greenhouse, due to the obstruction of the cover, the indoor air can not get this part of the heat, causing the room temperature to be lower than the outside temperature. From October to March of the following year, it is prone to inversion of temperature. Inversion usually occurs in the early hours of the morning. After sunrise, the temperature of the shed increases and the temperature reverses. Some experimental studies have shown that when the inversion occurs, the ground temperature in the facility is still higher than the outside world, so the crop will immediately freeze damage, but the inversion time is long, or the temperature is too low will be a problem. The main factors affecting the daily temperature difference are:
1. The ratio of the soil area s in the heat preservation ratio facility to the surface area W of the covered maintenance structure, ie s/w=p (insulation ratio), the maximum value is 1.0. The smaller the insulation ratio, the greater the surface area of ​​the cover and the maintenance structure, increase the heat exchange area with the outdoor air, and reduce the insulation capacity of the dike. Generally, the insulation ratio of a single greenhouse is 0.5-0.6, and that of a multi-span greenhouse is 0.7-0.8.
2. The smaller the insulation ratio, the smaller the volume of the protection facilities, and the larger the relative coverage area. Therefore, the heat absorption area is large during the day and it is easy to increase the temperature; the heat dissipation surface at night is also easy to cool down, so the daily temperature difference is also large. For example, when the temperature outside the greenhouse is 10°C, the temperature difference in the greenhouse is about 30°C, and the greenhouse can reach 40°C. In the confined condition, the maximum temperature of the small arch can reach 50°C in spring and 40°C in greenhouse.
In facility cultivation, maintaining a certain daily temperature difference is one of the important temperature conditions. In the absence of warming, the daily temperature difference is determined by the radiant heat of the sun and the insulation of the facility (ie, the difference in radiation receipts). Solar radiant heat is constantly changing with conditions such as the height of the sun, latitude, and weather. If these conditions are constant, that is, at a certain latitude and season, the difference in daytime temperature of a facility is mainly determined by its structure and heat preservation. The cover material covers different materials, and the daily temperature difference in the facility is also different. If ethylene is superior to polyvinyl chloride through solar radiation, it is easy to increase its temperature during the day. However, the ability of polyethylene to transmit infrared light is stronger than that of polyvinyl chloride, so it is easy to cool at night. Therefore, the polyethylene insulation performance is poor, and the daily temperature difference in the shed is large. PVC performance is not as good as polyethylene. However, the insulation performance is good, so the temperature difference on the day is small.
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