Glass greenhouse refers to the greenhouse with glass as the lighting material, which belongs to a kind of greenhouse. In the cultivation facilities, the glass greenhouse is the form with the longest service life and is suitable for use in a variety of regions and various climate conditions. The industry is divided into non-construction models based on the size of the span and the size of the booth, and is divided into different uses: glass greenhouse for vegetables, glass greenhouse for flower, nursery glass greenhouse, glass greenhouse, research glass greenhouse, three-dimensional glass greenhouse, special-shaped glass greenhouse, leisure glass greenhouse, smart glass greenhouse, etc. Its area and use mode can be freely adjusted by the main greenhouse. The smallest ones are courtyard leisure types, with a large height of more than 10 meters, a span of up to 16 meters, a maximum of 10 meters in the bay, and a smart control of one button. The winter heating problem of glass greenhouses can adopt a variety of heating methods, and its energy consumption costs are centered, and most of them are acceptable.
First: Glasshouse function
Glass greenhouse ventilation
The purpose of glass greenhouse ventilation is mainly to exclude the waste heat of the greenhouse and the moisture in the greenhouse, adjust the air composition in the greenhouse, exclude harmful gases, and make the environmental temperature, humidity and air in the greenhouse suitable for plant growth.
1. Natural ventilation: Glass greenhouses rely on natural ventilation to regulate the indoor environment most of the time. The structural form of large-scale productive glass greenhouses is generally a double-sloping multi-span greenhouse, and the ventilation form is to install ventilation windows on the side walls and the roof ridge. The total ventilation area is not less than 15% of the greenhouse area, preferably more than 30%. When the ridge wind window is opened, the sash should preferably be inclined upward beyond the horizontal plane, and when it is fully opened, it will form a 100 angle with the horizontal plane to obtain a good ventilation effect. The amount of natural ventilation is related to wind speed, wind direction, location of ventilation windows, area of ventilation windows, and temperature difference between inside and outside the greenhouse. When the total area of the ventilation window is 27% of the indoor floor area, if only the ridge ventilation, even if the outdoor wind speed reaches 10 km / h, the ideal ventilation rate of 0.75 per minute cannot be achieved; When the ventilation window is fully opened, the ventilation rate can reach 0.68 times / minute under almost no wind, which is close to the normal recommended 0.75 ventilation / minute.
2. Although forced ventilation glass greenhouses rely on natural ventilation to regulate the environment most of the time, when the summer temperature is high, especially in hot weather where the outdoor temperature exceeds 33 ° C, when natural ventilation alone cannot meet the greenhouse cooling requirements, forced ventilation is used. And in conjunction with other measures to cool down is a commonly used means in production. Forced ventilation is the use of fans to convert electrical energy or other mechanical energy into wind energy, forcing air flow to perform greenhouse ventilation and achieve cooling effects. The theoretical cooling limit of forced ventilation is that the indoor air temperature is equal to the outdoor air temperature. Because the temperature difference between the inside and outside of the greenhouse at this time is zero, and the ventilation volume is infinite, it is impossible in practical applications. Due to mechanical and plant physiological reasons, the general ventilation intensity of the greenhouse is 0.75 times per minute. Five times, the temperature difference between inside and outside the greenhouse can be controlled within 5 ℃.
Agricultural Glass Greenhouses
Second: Glass greenhouse heating
1. Agricultural glass greenhouses in northern China need to be heated in winter, otherwise they cannot be produced in winter. The length of the greenhouse heating time varies. It takes about 5 to 6 months to warm up in the Northeast, and 3 to 5 months in North China. Planting flowers or raising seedlings in the South also requires heating or temporary supplementation.
2. Hot air heating: The hot air heating system consists of a heat source, an air heat exchanger, a fan, and an air supply pipe. The working process is as follows: the air ventilator is heated by the heat provided by the heat source, and a part of the air in the greenhouse is forced to flow through the air heat exchanger with a fan, so that the greenhouse is continuously circulated for heating. The heat source of the hot air heating system can be oil, gas, coal-fired devices or electric heaters, or it can be hot water or steam. Different heat sources have different installation forms for hot air heating equipment. The air heat exchanger of the steam, electric or hot water heating system is installed in the greenhouse and cooperates with the fan to provide hot air directly. Oil and gas-type heating devices are installed in the greenhouse, and the combusted flue gas is discharged into the greenhouse. Coal-fired hot air stoves are generally large and dirty during use, and are generally installed outside the greenhouse. In order to make the hot air evenly distributed in the greenhouse, the hot air is sent into the ventilation pipe by a ventilator.
3. Electric heating: The more common method of electric heating is to bury the geothermal line in the ground to increase the ground temperature. It is mainly used in greenhouses to raise seedlings. Electrical energy is the cleanest and most convenient energy source, but electrical energy is a secondary energy source. It is relatively expensive and can only be used as a temporary heating measure for short-term use.
Third, the energy saving of the greenhouse
The heat loss of the greenhouse is mainly through the following ways: heat conduction through the glass envelope, which can account for 70% to 80% of the total heat loss; º radiation radiation to the sky; »ventilation and heat dissipation; Greenhouse energy saving is to reduce the heat dissipation of the greenhouse. The effective method is to install thermal insulation curtains, which can reduce the heat loss at night. Under the premise of satisfying crop light, it is best to install double-layer light-transmitting materials, whose heat loss can be reduced by 50%. It is also very effective to reduce the heat transfer in the ground by using cold insulation trenches and filling with insulation materials.
Fourth, glass greenhouse cooling
In most parts of China, the summer is hot and the temperature is high. When the outdoor temperature is above 30 ° C, the temperature inside the greenhouse exceeds 40 ° C. If only the ventilation is used and the main temperature in the greenhouse is still above 35 ° C, normal production cannot be performed in the greenhouse, and other cooling methods must be used to reduce the indoor temperature. Greenhouse cooling methods used in production on a daily basis include: 1. Shading and cooling: The use of opaque or low-light-transmitting materials to prevent sunlight from entering the greenhouse. Reduce the temperature of the greenhouse. Due to different shading materials and installation methods, the greenhouse temperature can be lowered by 3 ℃ ～ 10 ℃. Shading methods include indoor shading and outdoor shading. The indoor shading system is a support system made by pulling metal wires or plastic net wires on the framework of the greenhouse, and installing the sun shade net on the supporting film line. Generally use electric control or manual control. The outdoor shading system is to install another shading frame outside the greenhouse frame, and install the shading net on the frame. The outdoor shading net has a good cooling effect. It can directly block the solar energy outside the greenhouse, and all types of shading nets can be used.
3. Evaporative cooling: Evaporative cooling uses the unsaturated nature of air and the latent heat of evaporation of water to cool down. When the moisture in the air does not reach saturation, the water will evaporate into water vapor and enter the air. At the same time as the water evaporates, it absorbs the heat in the air, reduces the temperature of the air, and increases the humidity of the air. In the process of evaporation and cooling, the air flow inside and outside the greenhouse must be ensured, and the high-temperature and high-humidity gases in the greenhouse can be discharged from the greenhouse, and fresh air must be replenished. Therefore, forced ventilation must be adopted. The current methods of evaporative cooling include wet curtains-fan cooling and spray cooling.
3. Roof spray cooling system: It evenly sprays water on the roof of the glass greenhouse to reduce the temperature of the greenhouse. When water flows on the roof of the glass greenhouse, the water exchanges heat with the glass of the greenhouse roof, taking away the heat in the greenhouse, and when the thickness of the water film is greater than 0.2 mm, all the energy of solar radiation is absorbed and carried Go, this is equivalent to shade.
The winter heating problem of glass greenhouses can adopt a variety of heating methods, and its energy consumption costs are centered, and most of them are acceptable. Glass greenhouse structure mainly includes greenhouse foundation, greenhouse steel structure and aluminum alloy structure.
1. Advantages of glass greenhouse:
① Large lighting area and uniform light
② Long use time and high intensity
③ Has strong anti-corrosion and flame retardancy
④ More than 90% of light transmission, and does not decay with time