Development and test of intelligent heating device for double-layer plastic greenhouse in South China

Abstract: Plastic-covered greenhouses have become have become popular as a main method in protected horticulture in China, and their area has increased a lot. The plants were mostly thermophilic crops in greenhouses. These crops are vulnerable to freezing injury. For this reason, it is very important to study how to prevent freezing injury of these crops in greenhouses in winter. In order to develop a new tool for preventing freezing injury in plastic film greenhouses in southern China, a intelligent warming device for plastic-film greenhouses was designed and evaluated. Its structure mainly includes a control circuit board, temperature sensor, fuel tank, ignition system, wick, and other components. After this system was connected to a workstation, its temperature sensor can collect temperature in real time, and send it to micro-controller unit in the control circuit board. After these temperature data were analyzed by a program in the micro controller, a rotating signal was sent to the micro motor that drives the device cover. The cover of the warming device will automatically open until the temperature in plastic film greenhouse decreased to the set starting temperature in the micro control unit, which can be set beforehand. Generally, the set starting temperature was slightly higher than the critical temperature for freezing injury. After the cover opens, the ignition system kindles the asbestos wicks, which absorb fuel from the tank. Thus, the device releases its heat until the temperature increases to the extinction temperature set beforehand. In order to ensure device reliability, the ignition system keeps working until the wick s ignited or the ignition system starts 3 times. The device was made of iron, and it was designed to seal. It is safe for the device to work on any alcohol level in tank. A 7.4V lithium battery was used to power the device. With this power, the device is portable, it can work at any field site without line electricity. In southern China, the battery can supply for a year once fully charged. This device uses alcohol as fuel; its combustion products are only water and CO2. These two substances are not contaminants. Its starting or closing is controlled by the signals from the temperature sensors placed at key positions in the plastic film greenhouse. Based on that design, it can control the heat supply precisely following the variation of greenhouse temperature. Through experiment in which two plastic film greenhouses were heated by different ways in 12 consecutive days，the actual operating expense of the intelligent warming device for plastic film greenhouses was determined. The cost was about 30 Yuan to raise 2-3℃ in a 8 m×40 m greenhouse with 2 plastic films in about 6 hours. This experiment showed that the intelligent warming device has convenient operation, lower cost, rapid warming characteristics, and will satisfy environmental protection objectives. Thus, the warming device offers a powerful tool for defense against freezing injury in plastic film greenhouses in winter.