Simulation of the operational characteristics of a solid desiccant hybrid air-conditioning system for a greenhouse in hot and humid regions

Greenhouse agriculture can be expected to improve crop yield and quality under intensified climate change. It is often required to effectively control temperature and humidity for crop growth. The high energy demand of air conditioning has posed major challenges to greenhouses in hot and humid regions, thereby highlighting the need for renewable-energy-driven and energy-efficient climate-control solutions. The solar-assisted hybrid dehumidification air-conditioning with solid desiccant can be expected for greenhouse climate control, where the low-grade renewable energy is taken as its driving source. However, agricultural greenhouses are characterized by complex thermal and moisture environments and crop-specific physiological demands. It is still lacking in the dynamic operational characteristics of these systems and their adaptability to greenhouse air-conditioning requirements. Therefore, this study aimed to evaluate the dynamic performance and adaptability of a solar-assisted hybrid dehumidification system for greenhouse climate control under hot-humid conditions. A thermal and moisture load prediction model was developed on the TRNSYS platform. A coupled simulation model was subsequently established to integrate a desiccant wheel with a vapor-compression chiller. Taking the Guangzhou region as the study area, the high demand was observed for greenhouse cooling and dehumidification, according to the annual air temperature and humidity ratio in July. The period was selected from July 1 to July 28 (4344-5016 h) as the representative period. Four typical crops were selected: tomato, cucumber, grape, and sweet pepper. A comparison was conducted on the dynamic distributions of thermal and moisture loads in greenhouses. The results indicated that the ratio of latent heat to sensible heat varied greatly, although the overall variation trends of thermal and moisture loads were consistent among greenhouses with different crops. The crop physiological characteristics also played a decisive role in load composition. A systematic investigation was conducted to examine the effects of key parameters—including fresh air volume, chilled-water outlet temperature, solar collector area, and electric auxiliary heating power—on system performance. The fresh air volume increased the air processed by the desiccant wheel, thereby enhancing its dehumidification. The control performance stabilized at the airflow of 1400 m³/h. The solar collector area was reduced the system’s reliance on auxiliary electric heating, effectively improving the electrical coefficient of performance (COP_el) for the coefficient of performance (COP_sys). The reliability of temperature, relative humidity, and vapor pressure deficit (VPD) remained above 95.0% at the chilled-water outlet temperature below 17 °C. Both temperature–humidity performance coefficients were reduced as the outlet temperature increased and the cooling capacity declined. Electric auxiliary heating power from 4 to 12 kW slightly improved temperature-humidity control. Once the desiccant wheel was already sufficiently regenerated, additional heating contributed little to further dehumidification, while increasing electricity consumption. The COP_el decreased by 23.8%. The system effectively met the thermal and moisture control requirements of greenhouses in five representative hot and humid regions: Guangzhou, Haikou, Xiamen, Changsha, and Nanning with temperature, relative humidity, and VPD reliability all exceeding 90.0%, indicating excellent adaptability. Compared with conventional air-conditioning, the total electricity consumption was reduced by 53.1%, while the COP_sys was improved by 40.4%. A life-cycle operating cost savings of CNY 214000 (1932 CNY/m²) demonstrated excellent energy-saving potential and favorable economic benefits over the service life. Overall, the solar-assisted air-conditioning with solid desiccant can be expected to effectively solve the high energy consumption in greenhouse air-conditioning under hot and humid climates. The findings can provide valuable insights to promote sustainable and low-carbon agriculture.