Indoor air temperature and humidity variation of a greenhouse base on field test: A case study in Shandong

. The indoor thermal and humid environment of the solar greenhouse, especially the air humidity, is studied by the field test and psychrometric chart. First, the air temperature and relative humidity, and solar radiation in the greenhouse were tested by field tests. According to the test results, Solar radiation has a great influence on the air temperature and relative humidity in the greenhouse. The air temperature and relative humidity in a greenhouse vary dramatically in the presence of solar radiation. At night, although the indoor air temperature meets the requirements of crop growth, indoor relative humidity is close to 90%, far higher than the suitable relative humidity area (60-65%). Then, the night air condition was analyzed based on the psychrometric chart. Indoor air does not need to be dehumidified, but could be heated to the greenhouse to reduce the relative humidity. Finally, the variation of indoor air humidity ratio was analyzed. Solar radiation and natural ventilation are important factors affecting the variation of the humidity ratio of indoor air in the solar greenhouse. During the test period, solar radiation increases the humidity ratio of indoor air, and natural ventilation reduces the humidity ratio of indoor air in the solar greenhouse.


Introduction
Plants need the appropriate temperature, water, light and nutrients to grow and develop. Greenhouse can provide far better temperature and humidity than outdoor winter environment. Low temperature and high humidity greenhouse environment is not conducive to crop growth, will reduce the crops yield and quality [1]. To increase crop yield, it is very important to create a good heat and humidity environment. In recent years, Solar greenhouse has been widely used in China because of the good heat greenhouse with different vents to study the relationship between ventilation and air humidity [5].
There is much research on the heat and humidity https://doi.org/10.1051/e3sconf/202235603032 E3S Web of Conferences 356, 03032 (2022) ROOMVENT 2022 environment in the solar greenhouse [6][7][8], but the heat and moisture transfer process of the solar greenhouse is complicated due to the factors of crop transpiration and photosynthesis, and soil water evaporation.
The field test was implemented to measure the indoor air temperature, relative humidity, and solar radiation in a solar greenhouse. The transient variation of indoor air temperature, relative humidity, and solar radiation was analyzed based on the test results. The night air condition was explored based on the psychrometric chart, and the variation of the air humidity ratio was also studied.  The measuring points of air temperature, relative humidity, solar radiation in the greenhouse were arranged in the middle of the greenhouse. Fig.2 describes the location of each measurement point. of outdoor relative humidity is opposite to that of temperature. With the increase of temperature, relative humidity decreases and vice versa. The lowest relative humidity is 22%, the highest is 71%, and the difference between day and night is 78%.   According to relevant specifications [9], the suitable temperature zone for tomatoes is 20-30ႏ, with the lowest temperature not less than 8-10ႏ and the highest temperature not higher than 35ႏ. During the test period, the air temperature in the greenhouse is not less than 10ႏ, not higher than 35ႏ. The air temperature can meet the requirements of tomato growth. During the test period, the air temperature in the greenhouse is not less than 10 ,   slowly and remains at about 90%. The optimum relative humidity area of tomatoes is 60-65% [9]. From 20:00 to 8:00 (the next day), the relative humidity is 90%, far higher than the optimum area. Through field tests, it is found that the relative humidity is close to 90% at night, and there is dew condensation on the PVC film surface. By drawing the psychrometric chart, the study analyzes whether the air in the greenhouse needs dehumidification at night. Fig.7 shows the psychrometric chart of the night air state point.

3.Results
The night air state points are 17:00, 21:00, 1:00, 5:00 and 7:00 from December 24 to December 25. It was found that the lowest and highest humidity ratio of these nighttime state points were 7.71 g/kg dry air and 9.31 g/kg dry air , respectively. The lowest and highest humidity ratio in the https://doi.org/10.1051/e3sconf/202235603032 E3S Web of Conferences 356, 03032 (2022) ROOMVENT 2022 optimum area were 8.82 g/kg dry air and 17.62 g/kg dry air . It indicates that the air at night does not need to be dehumidified, but heated inside the solar greenhouse to raise the air temperature so that the air temperature and relative humidity is close to the optimum area.

4.Conclusions
The following conclusions are drawn from the field test of thermal and humid environment of a solar greenhouse and the analysis of psychrometric chart: 1. Solar radiation has a great influence on the air temperature and relative humidity in the greenhouse. The air temperature and relative humidity in a greenhouse vary dramatically in the presence of solar radiation.
2.At night, although the indoor air temperature meets the requirements of crop growth, it is still lower than the optimum temperature zone (20-30ႏ). Indoor relative humidity is close to 90%, far higher than the suitable relative humidity area (60-65%).
3. In this case, indoor air does not need to be dehumidified, but heated to the greenhouse to reduce the relative humidity.
4. The indoor air humidity ratio is higher than outdoor air humidity ratio. Solar radiation and natural ventilation are important factors affecting the variation of humidity ratio in the solar greenhouse. During the test period, solar radiation increased the humidity ratio of indoor air, and natural ventilation can reduce the humidity ratio of indoor air in the solar greenhouse.