Psychrometric Evaluation of Air Properties Under the Conditions of Plastic-Covered Greenhouses

Abstract

Plastic-covered greenhouses are widely used in Yemen to cultivate various vegetable crops, often without adequate consideration of their microclimatic conditions. This study evaluated the key microclimatic factors affecting plant growth, including air temperature, relative humidity, humidity ratio, and heat content. Air temperature and relative humidity were measured both inside and outside a 36 meter-long plastic-covered greenhouse. Internal measurements were performed at three locations: the entrance, middle, and end sections. Psychrometric equations were used to calculate the humidity ratio and heat content of air. The results indicated that the air temperature inside the greenhouse was lower than outside during the early and late afternoon hours, with a maximum difference of 3.4°C. However, between 8:00 AM and 3:00 PM, the internal temperature exceeded the external temperature, peaking at 7.5°C higher at 11:00 AM. On average, the internal daytime temperature was 25.9°C (ranging from 8.3°C to 34.9°C), while the nighttime average was 11.8°C (ranging from 8.4°C to 19.1°C). The relative humidity inside the greenhouse remained consistently higher than outside, fluctuating between 32% and 84% throughout the day. The average internal daytime relative humidity was 49.7% (ranging from 32.0% to 86.4%), whereas the nighttime average was 76.1% (ranging from 52.3% to 86.3%). The humidity ratio and heat content remained stable at night, but increased significantly during the daytime. A longitudinal variation was observed, with the greenhouse entrance exhibiting a humidity ratio of 4.3 kg of water per kg of air and a heat content of 12.2 kJ/kg, both lower than those recorded in the middle and end sections. The average internal daytime humidity ratio was 13.2 kg of water per kg of air (ranging from 7.7 to 17.5), while the nighttime average was 8.3 kg of water per kg of air (ranging from 7.6 to 9.5). Similarly, the average internal daytime heat content was 59.8 kJ/kg (ranging from 27.8 to 78.2), while the nighttime average was 33.4 kJ/kg (ranging from 27.9 to 43.3). Based on these findings, this study recommends enhancing greenhouse ventilation through mechanical systems powered by solar panels or by optimizing existing natural ventilation methods.